WO2016125474A1

WO2016125474A1 - Wavelength tunable laser device and optical coherence tomography apparatus

Info

Publication number: WO2016125474A1
Application number: PCT/JP2016/000455
Authority: WO
Inventors: Takeshi Uchida; Takako Suga; Takeshi Yoshioka
Original assignee: Canon Kabushiki Kaisha
Priority date: 2015-02-05
Filing date: 2016-01-29
Publication date: 2016-08-11
Also published as: JP2016143863A; JP6730783B2

Abstract

A wavelength tunable laser device, including: a first reflector (102); a second reflector (106); and an active layer (140) formed between the first reflector and the second reflector, wherein the wavelength tunable laser device has a gap (104) formed between the active layer and the second reflector, and a length of the gap is changed to change a resonance wavelength, wherein the active layer includes: a first active layer (140); and a second active layer (142) formed above the first active layer and having a peak wavelength of a gain spectrum that is different from a peak wavelength of a gain spectrum of the first active layer, and wherein each of excitation of the first active layer and excitation of the second active layer is controlled depending on the length of the gap.

Description

WAVELENGTH TUNABLE LASER DEVICE AND OPTICAL COHERENCE TOMOGRAPHY APPARATUS

The present invention relates to a wavelength tunable laser device and an optical coherence tomography apparatus using the wavelength tunable laser device.

In recent times, a wavelength tunable laser device that can change a wavelength of laser light emitted therefrom is attracting attention. As the wavelength tunable laser device, a vertical cavity surface emitting laser (VCSEL) device is proposed (NPL 1). In the vertical cavity surface emitting laser device, through displacement of one of two reflectors, a spacing between the two reflectors is changed, thereby changing an oscillation wavelength, that is, a resonance wavelength, of laser light. As a movable portion (movable mechanism) configured to displace a reflector, one to which a micro electro mechanical systems (MEMS) technology is applied has been proposed. A vertical cavity surface emitting laser device to which the MEMS technology is applied is referred to as a MEMS-VCSEL.

The MEMS-VCSEL can continuously change the wavelength. Further, in the MEMS-VCSEL, the movable portion is minute. Therefore, the movable portion can be displaced at high speed, and thus, the wavelength can be changed at high speed. Further, the MEMS-VCSEL has low power consumption. Such characteristics of the MEMS-VCSEL attract great attention thereto.

NPL 1: Connie J. Chang-Hasnain, Fellow, IEEE, "Tunable VCSEL", IEEE Journal on Selected Topics in Quantum Electronics, Vol. 6, No. 6, pp. 978-987, 2000.

However, in the related-art vertical cavity surface emitting laser device, a sufficiently wide wavelength tunable width cannot necessarily be acquired.

It is an object of the present invention to provide a wavelength tunable laser device that can improve a wavelength tunable width thereof, and to provide an optical coherence tomography apparatus using the wavelength tunable laser device.

According to one aspect of an embodiment, there is provided a wavelength tunable laser device, including: a first reflector; a second reflector; and an active layer formed between the first reflector and the second reflector, wherein the wavelength tunable laser device has a gap formed between the active layer and the second reflector, and a length of the gap is changed to change a resonance wavelength, wherein the active layer includes: a first active layer; and a second active layer formed above the first active layer and having a peak wavelength of a gain spectrum that is different from a peak wavelength of a gain spectrum of the first active layer, and wherein each of excitation of the first active layer and excitation of the second active layer is controlled depending on the length of the gap.

According to another aspect of an embodiment, there is provided An optical coherence tomography apparatus, including: a wavelength tunable laser device including: a first reflector; a second reflector; and an active layer formed between the first reflector and the second reflector, wherein the wavelength tunable laser device has a gap formed between the active layer and the second reflector, and a length of the gap is changed to change a resonance wavelength, wherein the active layer includes: a first active layer; and a second active layer formed above the first active layer and having a peak wavelength of a gain spectrum that is different from a peak wavelength of a gain spectrum of the first active layer, and wherein each of excitation of the first active layer and excitation of the second active layer is controlled depending on the length of the gap, the optical coherence tomography apparatus further including: a coherence optical system configured to branch light from the wavelength tunable laser device into radiation light to be radiated to a measurement object and reference light, and to generate coherent light with reflection light of the radiation light radiated to the measurement object and the reference light; an optical detection portion configured to receive the coherent light; and an information acquisition portion configured to acquire information on the measurement object based on a signal from the optical detection portion.

According to the one embodiment of the present invention, each of the excitation of the first active layer and the excitation of the second active layer is controlled depending on the length of the gap between the first reflector and the second reflector. When oscillation is induced at a relatively long wavelength, by exciting an active layer that realizes a large gain in a relatively long wavelength range, reliable oscillation at the relatively long wavelength is realized. On the other hand, when oscillation is induced at a relatively short wavelength, by exciting an active layer that realizes a large gain in a relatively short wavelength range, reliable oscillation at the relatively short wavelength is realized. Therefore, according to the one embodiment of the present invention, the wavelength tunable laser device having a wide wavelength tunable width and an optical coherence tomography apparatus using the wavelength tunable laser can be provided.

Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

FIG. 1A is a plan view for illustrating a wavelength tunable laser device according to a first embodiment of the present invention. FIG. 1B is a sectional view for illustrating a wavelength tunable laser device according to a first embodiment of the present invention. FIG. 2 is a graph for showing a relationship among a gap length, an oscillation wavelength, and modes. FIG. 3 is a graph for showing a relationship between an initial gap length and a wavelength tunable width in the wavelength tunable laser device according to the first embodiment of the present invention. FIG. 4 is a schematic view for illustrating a measuring device according to the first embodiment of the present invention. FIG. 5 is a sectional view for illustrating a wavelength tunable laser device according to a second embodiment of the present invention. FIG. 6 is a sectional view for illustrating a wavelength tunable laser device according to a third embodiment of the present invention. FIG. 7 is a sectional view for illustrating a wavelength tunable laser device according to a fourth embodiment of the present invention. FIG. 8 is a graph for showing a method of driving a wavelength tunable laser device according to a fifth embodiment of the present invention. FIG. 9 is a graph for showing a result of a simulation in a vertical cavity surface emitting laser device according to a reference example. FIG. 10 is a graph for showing a relationship among a gap length, an oscillation wavelength, and modes in a wavelength tunable laser device according to a comparative example.

As described above, in the related-art vertical cavity surface emitting laser device, a sufficiently wide wavelength tunable width cannot necessarily be acquired. In the related-art vertical cavity surface emitting laser device, a sufficiently wide wavelength tunable width cannot necessarily be acquired for the following reasons.

In the vertical cavity surface emitting laser device, by applying a voltage to, for example, a beam-like supporting portion configured to support an upper reflector, the beam-like supporting portion is displaced. When the voltage is applied to the beam-like supporting portion, an electrostatic attractive force is generated to displace the beam-like supporting portion in a direction of reducing a length of a gap existing between the upper reflector and a lower reflector. The beam-like supporting portion is held under a state in which resilience of a spring on the beam-like supporting portion and the electrostatic attractive force are balanced with each other. When the voltage applied to the beam-like supporting portion exceeds a certain value, the resilience of the spring on the beam-like supporting portion and the electrostatic attractive force are not balanced with each other anymore, and the beam-like supporting portion is brought into contact with a member existing below the beam-like supporting portion. In general, when the beam-like supporting portion is displaced by about 1/3 of an initial gap length, the resilience of the spring on the beam-like supporting portion and the electrostatic attractive force are not balanced with each other anymore, and the beam-like supporting portion is brought into contact with a member existing below the beam-like supporting portion. In this way, a displacement of the beam-like supporting portion is limited to about 1/3 of the initial gap length. Such a limitation is referred to as a "1/3 rule". An amount of displacement of the beam-like supporting portion is limited to about 1/3 of the initial gap length, and thus, from the viewpoint of the 1/3 rule, it is preferred that the initial gap length be large.

On the other hand, when the spacing between the lower reflector and the upper reflector is simply changed, transition from one mode to another mode, that is, mode hopping, occurs. The mode hopping occurs due to existence of a plurality of longitudinal modes in a wavelength range in which laser oscillations are allowed. If a longitudinal mode spacing is wider than the wavelength range in which laser oscillations are allowed, the mode hopping does not occur. Specifically, if another longitudinal mode does not exist in the wavelength range in which laser oscillations are allowed, there is no mode to be hopped to, and thus, the mode hopping does not occur. If another longitudinal mode does not exist in the wavelength range in which laser oscillations are allowed, an oscillation wavelength, that is, a resonance wavelength, can be changed over the entire wavelength range in which laser oscillations are allowed. To reduce the spacing between the lower reflector and the upper reflector is a method of increasing the longitudinal mode spacing. Reducing the spacing between the lower reflector and the upper reflector results in reducing the gap length. Therefore, from the viewpoint of inhibiting the mode hopping, it is preferred that the gap length be small.

As described above, in the vertical cavity surface emitting laser device, from the viewpoint of the 1/3 rule, it is preferred that the initial gap length be large, and, from the viewpoint of inhibiting the mode hopping, it is preferred that the initial gap length be small. In other words, the vertical cavity surface emitting laser device has a trade-off relationship with respect to the initial gap length.

FIG. 9 is a graph for showing a result of a simulation in a vertical cavity surface emitting laser device according to a reference example. Plots on the dot-and-dash line represent an amount of change in oscillation wavelength when a beam-like supporting portion is displaced by 1/3 of an initial gap length. Specifically, the plots on the dot-and-dash line represent a difference between an oscillation wavelength before the displacement and an oscillation wavelength after the displacement, that is, a wavelength difference in oscillation wavelength between before and after the displacement. Plots on the broken line represent a longitudinal mode spacing corresponding to the initial gap length, that is, a wavelength difference between modes corresponding to the initial gap length.

As can be seen from FIG. 9, as the initial gap length becomes larger, the wavelength difference when the beam-like supporting portion is displaced by 1/3 of the initial gap length becomes larger. On the other hand, as the initial gap length becomes larger, the longitudinal mode spacing corresponding to the initial gap length becomes smaller.

Taking into consideration both the viewpoint of the 1/3 rule and the viewpoint of inhibiting the mode hopping, as can be seen from FIG. 9, the largest wavelength difference is acquired when the initial gap length is about 1.7 μm. When the initial gap length is set to be about 1.7 μm, a wavelength difference of about 70 nm, that is, a wavelength tunable width of about 70 nm can be acquired.

Note that, the result of the simulation shown in FIG. 9 is in a case in which a center wavelength of a wavelength tunable range is about 1,060 nm. In a vertical cavity surface emitting laser device in which the center wavelength of the wavelength tunable range is not 1,060 nm, an optimum value of the initial gap length is not necessarily about 1.7 μm. For example, a vertical cavity surface emitting laser device in an 850 nm range has an oscillation wavelength that is smaller by about 20% than an oscillation wavelength of a vertical cavity surface emitting laser device in a 1,060 nm range. Therefore, a dimension of a resonator of the vertical cavity surface emitting laser device in the 850 nm range (dimension in a direction of reciprocation of light) is smaller by about 20% than a dimension of a resonator of the vertical cavity surface emitting laser device in the 1,060 nm range. It follows that an optimum value of an initial gap length of the vertical cavity surface emitting laser device in the 850 nm range is smaller by about 20% than the optimum value of the initial gap length of the vertical cavity surface emitting laser device in the 1,060 nm range.

Due to this tendency, the following expression holds:

where L_gap is the optimum value of the initial gap length and λ_L is the center wavelength of the wavelength tunable range.

For the reasons described above, in the related-art wavelength tunable laser device, a sufficiently wide wavelength tunable width cannot necessarily be acquired.

As a result of diligent review, the inventor of the subject application has conceived to improve the wavelength tunable width by the following way.

Now, embodiments for carrying out the present invention are described in detail with reference to the drawings.

First Embodiment

A wavelength tunable laser device according to a first embodiment of the present invention and a measuring device using the wavelength tunable laser device are described with reference to FIG. 1A to FIG. 5.

(Wavelength Tunable Laser Device)
First, a wavelength tunable laser device 10 according to this embodiment is described.

The wavelength tunable laser device 10 according to this embodiment is a wavelength tunable laser device to which a MEMS technology is applied, and more specifically, a vertical cavity surface emitting laser device to which the MEMS technology is applied (MEMS-VCSEL).

FIG. 1A is a plan view for illustrating the wavelength tunable laser device according to this embodiment. FIG. 1A is an illustration of the wavelength tunable laser device 10 according to this embodiment when seen from an upper surface side thereof.

As illustrated in FIG. 1A, two supporting portions 121 are arranged on a substrate 101 so as to be apart from each other. Both ends of a beam-like movable portion 122 formed using the MEMS technology are fixed by the supporting portions 121, respectively. The beam-like movable portion 122 is provided for displaceably supporting a reflector 106 to be described below. In this embodiment, the beam-like movable portion 122 forms a mechanism configured to change a spacing between a lower reflector 102 and the upper reflector 106 (movable mechanism, supporting mechanism). In other words, the beam-like movable portion 122 forms a mechanism configured to change a length of a gap between the lower reflector 102 and the upper reflector 106.

A region 123 surrounded by the broken lines in FIG. 1A conceptually represents a region in which light emission occurs.

An electrode 150 configured to inject a current into an active layer 140 (see FIG. 1B) is formed on a first semiconductor layer 141. The electrode 150 is electrically connected to the first semiconductor layer 141. Further, an electrode 151 configured to inject a current into an active layer 142 (see FIG. 1B) is formed on a second semiconductor layer 143. The electrode 151 is electrically connected to the second semiconductor layer 143. Further, an electrode 113 configured to apply a voltage to the beam-like movable portion 122 is formed on the supporting portion 121. The electrode 113 is electrically connected to the beam-like movable portion 122.

FIG. 1A is a sectional view of the wavelength tunable laser device according to this embodiment. FIG. 1B corresponds to a section taken along the line I-I' of FIG. 1A.

As illustrated in FIG. 1B, the reflector (first reflector, lower reflector) 102 is formed on the substrate 101. As the substrate 101, for example, an n-type GaAs substrate is used. As the lower reflector 102, for example, a distributed bragg reflector (DBR) is formed. The lower reflector 102 is formed of, for example, alternately laminated films including thirty pairs of a GaAs layer and an AlAs layer each having an optical thickness of 1/4 λc1. Here, λc1 is a center wavelength of a high reflectivity band of the lower reflector 102, and is, for example, about 1,060 nm in this embodiment. Note that, the term "high reflectivity band of reflector" as used herein means a wavelength band in which the reflector can acquire a sufficient reflectivity for allowing laser oscillations, and specifically means a wavelength band in which the reflector can acquire a reflectivity of 98% or larger. Note that, an electrode (rear surface electrode) 110 is formed on a rear surface (lower surface) of the substrate 101.

The active layer (first active layer) 140 is formed on the lower reflector 102, that is, on the first reflector. The first active layer 140 is an active layer having a quantum well structure in which, for example, an InGaAs layer (not shown) is sandwiched between GaAsP layers (not shown). The InGaAs layer has a thickness of, for example, about 8 nm. In the first active layer 140, the InGaAs layer is set to have an In composition ratio of, for example, 24%. A light emission wavelength corresponding to an energy difference between an excitation level and a ground level in the first active layer 140 is, for example, about 1,050 nm. In other words, a peak wavelength of an excitation spectrum of the first active layer 140 is, for example, about 1,050 nm. The light emission wavelength corresponding to the energy difference between the excitation level and the ground level in the first active layer 140 is smaller than the center wavelength of the high reflectivity bands of the

reflectors

102 and 106 of 1,060 nm. Phosphorus (P) has the action of reducing a lattice constant of GaAs. On the other hand, Indium (In) has the action of increasing the lattice constant of GaAs. Therefore, an accumulative strain is inhibited in the active layer 140 including the InGaAs layer and the GaAsP layers. The first active layer 140 has a conductivity type of, for example, an i type (undoped).

The semiconductor layer (first semiconductor layer) 141 is formed on the active layer 140, that is, on the first active layer. The first semiconductor layer 141 has a conductivity type of, for example, a p type.

The active layer (second active layer) 142 is formed on the first semiconductor layer 141. The second active layer 142 is an active layer having a quantum well structure in which, for example, an InGaAs layer (not shown) is sandwiched between GaAsP layers (not shown). The InGaAs layer has a thickness of, for example, about 8 nm. A basic material of the first active layer 140 and a basic material of the second active layer 142 are the same, but the composition is different. Specifically, the In composition ratio of the InGaAs layer in the first active layer 140 and an In composition ratio of the InGaAs layer in the second active layer 142 are different from each other. In the second active layer 142, the In composition ratio of the InGaAs layer is set to be, for example, 27%. A light emission wavelength corresponding to an energy difference between an excitation level and a ground level in the second active layer 142 is, for example, about 1,070 nm. In other words, a peak wavelength of an excitation spectrum of the second active layer 142 is, for example, about 1,070 nm. The peak wavelength of the excitation spectrum of the second active layer 142 is different from the peak wavelength of the excitation spectrum of the first active layer 140. The light emission wavelength corresponding to the energy difference between the excitation level and the ground level in the second active layer 142 is larger than the center wavelength of the high reflectivity bands of the

reflectors

102 and 106 of 1,060 nm. As described above, P has the action of reducing the lattice constant of GaAs. On the other hand, In has the action of increasing the lattice constant of GaAs. Therefore, the accumulative strain is inhibited also in the second active layer 142 including the InGaAs layer and the GaAsP layers. The second active layer 142 has a conductivity type of, for example, the i type (undoped).

The semiconductor layer (second semiconductor layer) 143 is formed on the second active layer 142. The second semiconductor layer 143 has a conductivity type that is opposite to the conductivity type of the first semiconductor layer 141. In this case, the second semiconductor layer 143 has a conductivity type of, for example, an n type.

As described above, a laminate 103 including the first active layer 140, the first semiconductor layer 141, the second active layer 142, and the second semiconductor layer 143 is formed on the lower reflector 102.

The beam-like supporting portion (beam) 122 formed using the MEMS technology is located above the laminate 103. As described above, the beam-like movable portion 122 is provided for displaceably supporting the upper reflector 106. A gap 104, that is, an air gap 104, exists between the laminate 103 and the movable portion 122. In other words, the gap 104 exists between the lower reflector 102 and the upper reflector 106. Therefore, the movable portion 122 is freely displaceable in a direction of a normal to a main surface of the substrate 101. The gap 104 has a length (height) of, for example, about 3.8 μm under a state in which no voltage is applied to the beam-like movable portion 122. Specifically, in this embodiment, the gap 104 has a length that is larger than 1.7 μm under the state in which no voltage is applied to the beam-like movable portion 122, that is, under a state in which the spacing between the lower reflector 102 and the upper reflector 106 is not changed. In other words, the length of the gap 104 under the state in which no voltage is applied to the beam-like movable portion 122 is larger than 1.7 μm. Further, the length of the gap 104 under the state in which no voltage is applied to the beam-like movable portion 122 is larger than 1.6 times the center wavelength of the high reflectivity bands of the

reflectors

102 and 106. Note that, the length of the gap 104 is also referred to as an air gap length. As described above, both the ends of the beam-like movable portion 122 are respectively fixed by the two supporting portions (fixing portions, holding portions) 121 formed on the substrate 101. In other words, the beam-like movable portion 122 is a both-ends fixed beam (double supported beam) having the ends being fixed by the supporting portions 121, respectively. Note that, here, a case in which the length of the gap 104 under the state in which no voltage is applied is about 3.8 μm is described as an example, but the length of the gap 104 under the state in which no voltage is applied is not limited to about 3.8 μm, and may be set as appropriate.

As described above, the electrode 150 configured to inject a current into the first active layer 140 is formed on the first semiconductor layer 141. Further, as described above, the electrode 151 configured to inject a current into the second active layer 142 is formed on the second semiconductor layer 143.

The reflector (second reflector, upper reflector) 106 is formed on the beam-like movable portion 122. The upper reflector 106 is displaceably supported by the beam-like movable portion 122. As the upper reflector 106, for example, a DBR is formed. The upper reflector 106 is formed of, for example, alternately laminated films including ten pairs of an SiO₂ layer and a TiO₂ layer each having an optical thickness of 1/4 λc2. Here, λc2 is a center wavelength of the high reflectivity band of the upper reflector 106, and is, for example, about 1,060 nm in this embodiment.

A resonator 12 of the wavelength tunable laser device 10 according to this embodiment includes the lower reflector 102, the first active layer 140, the second active layer 142, the upper reflector 106, and the mechanism 122 configured to change the spacing between the lower reflector and the upper reflector. The gap 104 having a length that changes along with a change in spacing between the lower reflector 102 and the upper reflector 106 exists between the lower reflector 102 and the upper reflector 106. In this way, the wavelength tunable laser device 10 according to this embodiment is formed.

FIG. 2 is a graph for showing a relationship among the gap length, the oscillation wavelength, and modes. The horizontal axis denotes the length of the gap 104. The vertical axis denotes the oscillation wavelength of laser light.

As can be seen from FIG. 2, under the state in which no voltage is applied to the beam-like movable portion 122, the length of the gap 104 is, for example, about 3.8 μm. When the length of the gap 104 is about 3.8 μm, there exists a mode in which resonance occurs at a wavelength of, for example, about 1,098 nm. This resonance mode (longitudinal mode) is herein referred to as Mode B. As can be seen from FIG. 2, when the length of the gap 104 is about 3.8 μm, there exists another mode in which resonance occurs at a wavelength shorter than 1,098 nm. This resonance mode is herein referred to as Mode A.

When the spacing between the reflector 102 and the reflector 106 is reduced so that the length of the gap 104 is, for example, about 3.0 μm, the oscillation wavelength in Mode B is, for example, about 1,007 nm. As can be seen from FIG. 2, when the length of the gap 104 is about 3.0 μm, there exists still another mode in which resonance occurs at a wavelength longer than 1,007 nm. This resonance mode is herein referred to as Mode C.

According to this embodiment, a plurality of modes, that is, two or more modes, exist in a range in which the oscillation wavelength of laser light can be changed. More specifically, in this embodiment, the three modes exist in the range in which the oscillation wavelength of laser light can be changed. Therefore, when the spacing between the lower reflector 102 and the upper reflector 106 is simply changed, transition from Mode B to other modes: Mode A and Mode C, that is, mode hopping, may occur.

Therefore, according to this embodiment, by controlling excitation of the first active layer 140 and excitation of the second active layer 142 depending on the length of the gap between the lower reflector 102 and the upper reflector 106, mode hopping is prevented and oscillation in Mode B is maintained in a wide wavelength band.

Specifically, when oscillation is induced at a relatively long wavelength, a current is injected into the second active layer 142 in which a gain is acquired in a relatively long wavelength range. When a current is injected into the second active layer 142, the second active layer 142 is excited, and a gain is acquired at the relatively long wavelength. Therefore, reliable oscillation at the relatively long wavelength is realized.

On the other hand, when oscillation is induced at a relatively short wavelength, a current is injected into the first active layer 140 in which a gain is acquired in a relatively short wavelength range. When a current is injected into the first active layer 140, the first active layer 140 is excited, and a gain is acquired at the relatively short wavelength. Therefore, reliable oscillation at the relatively short wavelength is realized.

Both the center wavelength λc1 of the high reflectivity band of the lower reflector 102 and the center wavelength λc2 of the high reflectivity band of the upper reflector 106 are 1,060 nm. In this case, when resonance is induced at a wavelength longer than 1,060 nm that is the center wavelengths λc1 and λc2 of the high reflectivity bands of the reflector 102 and the reflector 106, the second active layer 142 that realizes a large gain in a wavelength range longer than the center wavelengths λc1 and λc2 is excited. On the other hand, when resonance is induced at a wavelength shorter than 1,060 nm that is the center wavelengths λc1 and λc2 of the high reflectivity bands of the

reflectors

102 and 106, the first active layer 140 that realizes a large gain in a wavelength range shorter than the center wavelengths λc1 and λc2 is excited.

For example, the length of the gap 104 corresponding to 1,060 nm that is the center wavelengths λc1 and λc2 of the high reflectivity bands of the

reflectors

102 and 106 is, for example, about 3.4 μm. Therefore, when the length of the gap 104 is, for example, 3.4 μm or larger, the second active layer 142 that realizes a large gain in a wavelength range longer than the center wavelengths λc1 and λc2 is excited. On the other hand, when the length of the gap 104 is, for example, smaller than 3.4 μm, the first active layer 140 that realizes a large gain in a wavelength range shorter than the center wavelengths λc1 and λc2 is excited. The strength of the excitation of the second active layer 142 when the length of the gap 104 is equal to or larger than a predetermined value is higher than the strength of the excitation of the second active layer 142 when the length of the gap 104 is smaller than the predetermined value. Further, the strength of the excitation of the first active layer 140 when the length of the gap 104 is equal to or larger than the predetermined value is lower than the strength of the excitation of the first active layer 140 when the length of the gap 104 is smaller than the predetermined value.

The length of the gap 104 can be changed by changing a voltage applied to the beam-like movable portion 122. In other words, the length of the gap 104 can be changed by changing a voltage applied to the electrode 113. Note that, the length of the gap 104 is changed by changing the voltage applied to the beam-like movable portion 122 because, by changing the voltage applied to the beam-like movable portion 122, an electrostatic attractive force that acts on the beam-like movable portion 122 is changed.

The voltage applied to the electrode 113, which sets the length of the gap 104 to be, for example, 3.4 μm, can be determined in advance. Therefore, when the voltage applied to the electrode 113 is a voltage with which the length of the gap 104 is 3.4 μm or larger, the second active layer 142 that realizes a large gain in a wavelength range longer than the center wavelengths λc1 and λc2 is excited. In other words, when the voltage applied to the electrode 113 is equal to or smaller than the predetermined voltage, the second active layer 142 that realizes a large gain in a wavelength range longer than the center wavelengths λc1 and λc2 is excited. On the other hand, when the voltage applied to the electrode 113 sets the length of the gap 104 to be smaller than 3.4 μm, the first active layer 140 that realizes a large gain in a wavelength range shorter than the center wavelengths λc1 and λc2 is excited. In other words, when the voltage applied to the electrode 113 is larger than the predetermined voltage, the first active layer 140 that realizes a large gain in a wavelength range shorter than the center wavelengths λc1 and λc2 is excited.

As described above, the light emission wavelength corresponding to the energy difference between the excitation level and the ground level in the second active layer 142 is, for example, about 1,070 nm. In other words, the peak wavelength of the excitation spectrum of the second active layer 142 is, for example, about 1,070 nm. When the second active layer 142 is excited through injection of a current into the second active layer 142, a relatively large gain can be acquired in a wavelength range of, for example, about 1,060 nm to about 1,100 nm. In this case, no current is injected into the first active layer 140, and thus, absorption is induced in the first active layer 140. The light emission wavelength corresponding to the energy difference between the excitation level and the ground level in the first active layer 140 is smaller than the light emission wavelength corresponding to the energy difference between the excitation level and the ground level in the second active layer 142. As described above, in the first active layer 140, the light emission wavelength corresponding to the energy difference between the excitation level and the ground level is about 1,050 nm. In other words, the peak wavelength of the excitation spectrum of the first active layer 140 is, for example, about 1,050 nm. Therefore, oscillation in a wavelength range around 1,050 nm is inhibited by the first active layer 140. Therefore, when a current is injected into the second active layer 142 without injecting any current into the first active layer 140, oscillation in Mode B can be induced with reliability in the wavelength range of about 1,060 nm to about 1,100 nm. Therefore, according to this embodiment, mode hopping to Mode A can be prevented with reliability.

On the other hand, as described above, the light emission wavelength corresponding to the energy difference between the excitation level and the ground level in the first active layer 140 is about 1,050 nm.

In other words, the peak wavelength of the gain spectrum of the first active layer 140 is about 1,050 nm. With a sufficiently high strength of the excitation of the first active layer 140, a gain on a shorter wavelength side can be sufficiently large. This is realized by the quantum well having a rectangular density of states and a large difference in band gap between an In_0.24GaAs quantum well layer and a GaAsP barrier layer. When the first active layer 140 is excited through injection of a current into the first active layer 140, a relatively large gain can be acquired in a wavelength range of, for example, about 1,007 nm to about 1,060 nm. Therefore, when a current is injected into the active layer 140 without injecting any current into the active layer 142, oscillation in Mode B can be induced with reliability in the wavelength range of about 1,007 nm to about 1,060 nm, and mode hopping to Mode C can be prevented with reliability.

According to this embodiment, the oscillation wavelength of laser light is swept with, for example, the center wavelengths λc1 and λc2 of the high reflectivity bands of the

reflectors

102 and 106 being a center of the sweep.

It is preferred that a wavelength difference between Mode A and Mode B, that is, a longitudinal mode spacing between Mode A and Mode B, be larger than 30 nm. Further, it is preferred that a wavelength difference between Mode B and Mode C, that is, a longitudinal mode spacing between Mode B and Mode C, be larger than 30 nm. In an ordinary active layer such as a bulk or a quantum well, it is difficult to sufficiently reduce a width of the gain spectrum with respect to 30 nm. Therefore, when the longitudinal mode spacing is 30 nm or smaller, it is difficult to control the

active layers

140 and 142 so that one of the plurality of longitudinal modes is selected. Therefore, it is preferred that the longitudinal mode spacing be larger than 30 nm.

However, the above-mentioned numerical value of 30 nm is a numerical value in the case of a wavelength tunable laser device of a 1,060 nm range. The numerical value of 30 nm comes from an energy distribution of carriers in the quantum well, and, at room temperature, the numerical value is about 30 nm when the wavelength is 1,060 nm. Therefore, with regard to a wavelength tunable laser device of a wavelength band other than 1,060 nm, it is appropriated that not the wavelength difference but an energy difference is used in the description. An energy difference corresponding to a wavelength difference of 30 nm in the 1,060 nm range is 33 meV. Therefore, it is preferred that the longitudinal mode spacing be larger than a wavelength difference corresponding to the energy difference of 33 meV in the oscillation wavelength.

In this way, according to this embodiment, when oscillation is induced at a relatively long wavelength, by exciting the second active layer 142 that realizes a large gain in a relatively long wavelength range, reliable oscillation at the relatively long wavelength is realized. On the other hand, when oscillation is induced at a relatively short wavelength, by exciting the first active layer 140 that realizes a large gain in a relatively short wavelength range, reliable oscillation at a relatively short wavelength is realized. Therefore, according to this embodiment, a wavelength tunable laser device that can prevent mode hopping with reliability and has a wide wavelength tunable width can be acquired. Note that, in this embodiment, two active layers having different gains are used, but three or more active layers having different gains may also be used.

(Result of Evaluation)
Next, a result of evaluation of the wavelength tunable laser device according to this embodiment is described.

FIG. 10 is a graph for showing a relationship among the gap length, the oscillation wavelength, and the modes in a wavelength tunable laser device according to a comparative example. The comparative example changes the oscillation wavelength by simply changing the spacing between the lower reflector and the upper reflector. Specifically, according to the comparative example, current injection into the active layer is not adjusted depending on the length of the gap between the lower reflector and the upper reflector.

In the comparative example, the initial gap length is about 1.5 μm. When the initial gap length is 1.5 μm, and the beam-like supporting portion is displaced by 1/3 of the initial gap length, the gap length becomes 1.0 μm.

As can be seen from FIG. 10, when the gap length is changed from 1.5 μm to 1.0 μm, the oscillation wavelength is changed by about 68 nm. When the gap length is smaller than 1.0 μm, the 1/3 rule is not satisfied, and the resilience of the spring on the beam-like supporting portion and the electrostatic attractive force may not be balanced with each other. Thus, it is not preferred that the gap length be smaller than 1.0 μm. Therefore, in the wavelength tunable laser device according to the comparative example, the wavelength tunable width is about 68 nm.

On the other hand, according to this embodiment, excitation of the first active layer and excitation of the second active layer are controlled depending on the length of the gap between the lower reflector and the upper reflector, and thus, mode hopping can be prevented with reliability. Therefore, according to this embodiment, there is no restriction that is due to the longitudinal mode spacing as represented by the plots on the broken line in FIG. 9. Therefore, according to this embodiment, under the state in which no voltage is applied to the beam-like supporting portion 122, the length of the gap 104 can be considerably larger than 1.6 times the center wavelengths λc1 and λc2 of the high reflectivity bands of the

reflectors

102 and 106. According to this embodiment, the initial length of the gap 104 can be considerably large, and thus, a length that is about 1/3 of the initial length of the gap 104 is sufficiently large. Therefore, according to this embodiment, the oscillation wavelength, that is, the resonance wavelength, can be changed in an extremely wide wavelength range.

When the length of the gap 104 is changed, for example, from 3.8 μm to 3.0 μm, as can be seen from FIG. 2, in a wavelength range of, for example, about 1,007 nm to about 1,098 nm, the oscillation wavelength can be changed without occurrence of mode hopping from Mode B to other modes: Mode A and Mode C. The wavelength difference between the wavelength of 1,098 nm and the wavelength of 1,007 nm is 91 nm. Specifically, according to this embodiment, a wavelength tunable width of, for example, about 91 nm can be acquired.

FIG. 3 is a graph for showing a relationship between the initial gap length and the wavelength tunable width in the wavelength tunable laser device according to this embodiment. The horizontal axis denotes the initial gap length and the vertical axis denotes the wavelength tunable width.

As can be seen from FIG. 3, when the initial length of the gap 104 is set to be 3.8 μm, the wavelength tunable width can be about 91 nm.

When the oscillation wavelength is changed from 1,098 nm to 1,007 nm, the length of the gap 104 is changed from 3.8 μm to 3.0 μm. The difference between the length of 3.8 μm and the length of 3.0 μm, that is, the amount of change in length of the gap 104, is about 0.8 μm. The amount of change in length of the gap 104 of 0.8 μm is sufficiently small with respect to 1/3 of the initial length of the gap 104. Therefore, even when the oscillation wavelength is changed from 1,098 nm to 1,007 nm, a sufficient margin to the 1/3 rule is secured.

As described above, according to this embodiment, a wavelength tunable laser device having an extremely wide wavelength tunable width can be acquired.

(Measuring Device)
Next, a measuring device using the wavelength tunable laser device according to this embodiment is described with reference to FIG. 4. FIG. 4 is a schematic view for illustrating the measuring device according to this embodiment.

Note that, as an example, a case is herein described in which the wavelength tunable laser device 10 according to this embodiment is used for a light source portion 801 of an optical coherence tomography (OCT) apparatus, but the present invention is not limited thereto. The wavelength tunable laser device 10 according to this embodiment is not limited to the light source portion 801 of the optical coherence tomography apparatus, and can be used for various purposes. The optical coherence tomography apparatus in which the wavelength tunable laser device is used for the light source portion 801 eliminates the need for a spectroscope, and can thus acquire a tomographic image with low loss in light amount and a high S/N ratio.

As illustrated in FIG. 4, a measuring device (OCT device) 8 according to this embodiment includes the light source portion 801, a coherence optical system 802, an optical detection portion 803, and an information acquisition portion 804.

The wavelength tunable laser device 10 according to this embodiment is used for the light source portion 801.

The information acquisition portion 804 includes a Fourier transformer (not shown). The phrase "the information acquisition portion 804 includes a Fourier transformer" herein means that the information acquisition portion 804 has the function of performing a Fourier transform on data input thereto, and the mode of the Fourier transformer is not particularly limited. As an example, the information acquisition portion 804 includes an arithmetic portion (not shown) and the arithmetic portion has the function of performing a Fourier transform. More specifically, the arithmetic portion is a computer including a central processing unit (CPU), and the computer runs an application having the function of performing a Fourier transform. As another example, the information acquisition portion 804 includes a Fourier transform circuit having the function of performing a Fourier transform.

Light that is output from the light source portion 801 passes through the coherence optical system 802 to be coherent light including information on an object 812 to be measured (measurement object), and the coherent light is received by the optical detection portion 803.

Note that, the optical detection portion 803 may be a photodetector of a differential detection type or may be a photodetector of a simple intensity monitor type.

Information on a time waveform of intensity of the coherent light received by the optical detection portion 803 is output from the optical detection portion 803 to the information acquisition portion 804.

The information acquisition portion 804 acquires information on the object 812 (for example, information on a tomographic image thereof) by acquiring a peak value of the time waveform of the intensity of the received coherent light and performing a Fourier transform.

In the following, from light emission from the light source portion 801 to acquisition of the information on the tomographic image of the object 812 to be measured is described in detail.

Light emitted from the light source portion 801 passes through a fiber 805, enters a coupler 806, and then, is branched into radiation light that passes through a fiber 807 for radiation light and reference light that passes through a fiber 808 for reference light. As the coupler 806, a 3dB coupler that is a coupler having a branching ratio of 1:1 is used. The coupler 806 can operate in a single mode in a wavelength band of light emitted from the light source portion 801.

The radiation light propagating through the fiber 807 passes through a collimator 809 to be collimated light and is reflected by a mirror 810. Light reflected by the mirror 810 passes through a lens 811 to be radiated to the object 812, and is reflected by respective layers in a depth direction of the object 812.

On the other hand, the reference light propagating through the fiber 808 passes through a collimator 813 and is reflected by a mirror 814.

At the coupler 806, coherent light is generated with the reflection light from the object 812 and the reflection light from the mirror 814. The coherent light generated in this way passes through a fiber 815, passes through a collimator 816 to be condensed, and is received by the optical detection portion 803.

The information on the intensity of the coherent light received by the optical detection portion 803 is converted into electrical information such as a voltage, and is sent to the information acquisition portion 804.

The information acquisition portion 804 processes the data on the intensity of the coherent light, specifically, performs a Fourier transform, to acquire information on the tomographic image. The data on the intensity of the coherent light on which a Fourier transform is performed is data sampled at equal wavenumber intervals in general, but the present invention is not limited thereto, and the data may be data sampled at equal wavelength intervals.

The information on the tomographic image acquired in this way is sent from the information acquisition portion 804 to an image display portion 817 and can be displayed as an image.

Note that, by scanning with the mirror 810 in a direction perpendicular to an incident direction of the radiation light, a three-dimensional tomographic image of the object 812 to be measured can also be acquired.

Further, although not illustrated, intensity of light emitted from the light source portion 801 may be successively monitored and data thereof may be used for correcting an amplitude of a signal for indicating the intensity of the coherent light.

The information acquisition portion 804 can control the light source portion 801 via an electric circuit 818. As described above, according to this embodiment, when the wavelength tunable laser device is oscillated at a relatively long wavelength, the second active layer 142 that acquires a gain in a relatively long wavelength range is excited. On the other hand, when the wavelength tunable laser device is oscillated at a relatively short wavelength, the first active layer 140 that acquires a gain in a relatively short wavelength range is excited. The excitation of the first active layer 140 and the excitation of the second active layer 142 can be controlled by, for example, the information acquisition portion 804. Specifically, the information acquisition portion 804 can function as a control portion configured to control the excitation of the first active layer 140 and the excitation of the second active layer 142. The information acquisition portion (control portion) 804 controls the excitation of the first active layer 140 and the excitation of the second active layer 142 through appropriate application of a voltage to the

electrodes

150 and 151 via the electric circuit 818. Further, the information acquisition portion (control portion) 804 displaces the beam-like movable portion 122 through appropriate application of a voltage to the electrode 113 via the electric circuit 818, thereby changing the oscillation wavelength of laser light emitted from the wavelength tunable laser device.

Note that, as an example, a case is herein described in which the information acquisition portion 804 controls the excitation of the first active layer 140 and the excitation of the second active layer 142, but the present invention is not limited thereto. The excitation of the first active layer 140 and the excitation of the second active layer 142 may be controlled using a control portion (not shown) other than the information acquisition portion 804. Further, a control portion other than the information acquisition portion 804 may control a voltage to be applied to the electrode 113 connected to the beam-like movable portion 122, thereby changing the oscillation wavelength of laser light emitted from the wavelength tunable laser device. In this case, the control portion other than the information acquisition portion 804 may be formed in the light source portion 801 or may be formed separately from the light source portion 801.

According to this embodiment, the measuring apparatus 8 is useful for, for example, acquiring information on a tomographic image of a living body such as an animal or a human being in the field of ophthalmology, dentistry, dermatology, and the like. Note that, information on a tomographic image of a living body includes not only the tomographic image itself of the living body but also a numerical data necessary for acquiring the tomographic image of the living body. In particular, an object to be measured by the measuring apparatus 8 according to this embodiment is a fundus of a human body, and the measuring apparatus 8 according to this embodiment is suitable for acquiring information on a tomographic image of the fundus.

As described above, the wavelength tunable laser device 10 according to this embodiment has an extremely wide wavelength tunable width. Therefore, by using the wavelength tunable laser device 10 according to this embodiment, the measuring apparatus 8 that can acquire a high-resolution tomographic image can be realized.

Note that, as an example, a case is herein described in which the wavelength tunable laser device 10 according to this embodiment is used as a light source of the measuring apparatus 8, but the present invention is not limited thereto. For example, the wavelength tunable laser device 10 according to this embodiment may be used as a light source for optical communication, or may be used as a light source for optical measurement.

Second Embodiment

A wavelength tunable laser device according to a second embodiment of the present invention is described with reference to FIG. 5. FIG. 5 is a sectional view for illustrating the wavelength tunable laser device according to this embodiment. The same components as in the wavelength tunable laser device and the like according to the first embodiment illustrated in FIG. 1A to FIG. 4 are represented by the same reference numerals, and description thereof is omitted or simplified.

The wavelength tunable laser device according to this embodiment excites a second active layer 242 by radiating pumping light thereto.

As illustrated in FIG. 5, according to this embodiment, the second active layer 242 is formed between the first semiconductor layer 141 and the second semiconductor layer 143. The second active layer 242 is excited when light of a predetermined wavelength is radiated thereto. According to this embodiment, light emitted from a pumping light source 820 (pumping light) is radiated to the second active layer 242. As the pumping light source 820, for example, an edge-emitting semiconductor laser device can be used. The wavelength of pumping light emitted from the pumping light source 820 is, for example, about 670 nm. A barrier layer of the second active layer 242 functions as an absorbing layer configured to absorb pumping light to generate carriers. The composition of the barrier layer of the second active layer 242 is, for example, Al_0.25GaAs.

According to this embodiment, the second active layer 242 can be excited by light emitted from the pumping light source 820, and thus, the electrode 151 (see FIG. 1B) configured to inject a current into the second active layer 242 is not formed.

According to this embodiment, it is not necessary to form the electrode 151 (see FIG. 1B) configured to inject a current into the second active layer 242, and thus, the number of electrodes to be formed on the substrate 101 can be reduced. The number of electrodes required to be formed on the substrate 101 is small, and thus, the degree of difficulty in manufacturing is low and the number of processing steps is small. Therefore, according to this embodiment, manufacturing yield can be improved and manufacturing costs can be reduced.

According to this embodiment, when oscillation is induced at a relatively long wavelength, light is radiated from the pumping light source 820 to the second active layer 242 that acquires a gain in a relatively long wavelength range. When light is radiated to the second active layer 242, the second active layer 242 is excited, and a gain is acquired at the relatively long wavelength. Therefore, reliable oscillation at the relatively long wavelength is realized.

On the other hand, when oscillation is induced at a relatively short wavelength, a current is injected into the first active layer 140 that acquires a gain in a relatively short wavelength range. When a current is injected into the first active layer 140, the first active layer 140 is excited, and a gain is acquired at the relatively short wavelength. Therefore, reliable oscillation at the relatively short wavelength is realized.

Therefore, also according to this embodiment, mode hopping can be prevented with reliability and the oscillation wavelength can be changed in an extremely wide wavelength range.

Note that, the pumping light source 820 may be controlled by the information acquisition portion (control portion) 804, or may be controlled by a control portion formed separately from the information acquisition portion 804.

As described above, the second active layer 242 may be excited by radiating light thereto. According to this embodiment, similarly to the case of the wavelength tunable laser device according to the first embodiment, the oscillation wavelength can be changed in an extremely wide wavelength range.

Third Embodiment

A wavelength tunable laser device according to a third embodiment of the present invention is described with reference to FIG. 6. FIG. 6 is a sectional view for illustrating the wavelength tunable laser device according to this embodiment. The same components as in the wavelength tunable laser device and the like according to the first and second embodiments illustrated in FIG. 1A to FIG. 5 are represented by the same reference numerals, and description thereof is omitted or simplified.

The wavelength tunable laser device according to this embodiment uses radiation of light not only for exciting a second active layer 342 but also for exciting a first active layer 340.

As illustrated in FIG. 6, the first active layer 340 is formed between the lower reflector 102 and the first semiconductor layer 141. The first active layer 340 is set to have an In composition ratio of, for example, about 24%. A light emission wavelength corresponding to an energy difference between an excitation level and a ground level in the first active layer 340 is, for example, about 1,050 nm. A barrier layer of the active layer 340 is formed of, for example, Al_0.05GaAs.

The second active layer 342 is formed between the first semiconductor layer 141 and the second semiconductor layer 143. The second active layer 342 is set to have an In composition ratio of, for example, 27%. A light emission wavelength corresponding to an energy difference between an excitation level and a ground level in the second active layer 342 is, for example, about 1,070 nm. A barrier layer of the second active layer 342 is formed of, for example, Al_0.25GaAs.

According to this embodiment, light emitted from a pumping light source 821 (pumping light) is radiated to the second active layer 342 and the first active layer 340. When the first active layer 340 is excited, for example, pumping light of a relatively long wavelength is radiated thereto. In this case, pumping light that is used for exciting the first active layer 340 has a wavelength of, for example, about 808 nm. On the other hand, when the second active layer 342 is excited, for example, pumping light of a relatively short wavelength is radiated thereto. In this case, pumping light that is used for exciting the second active layer 342 has a wavelength of, for example, about 670 nm. These two kinds of pumping light are incident from a side on which the beam-like movable portion 122 is formed, that is, from above the substrate 101, so as to be coaxial with each other. As described above, according to this embodiment, two kinds of pumping light having different wavelengths are radiated as appropriate. As a light source of the two kinds of pumping light having different wavelengths, similarly to the case of the second embodiment, for example, an edge-emitting semiconductor laser device or the like can be used.

Pumping light emitted from the pumping light source 821 first reaches the second active layer 342 existing above the first active layer 340. Most part of the second active layer 342 is occupied by the barrier layer thereof, and the barrier layer of the second active layer 342 has a band gap of, for example, about 1.73 eV. Energy of photons of pumping light having a wavelength of 670 nm is higher than energy of the band gap of the barrier layer of the second active layer 342. Therefore, when pumping light having a wavelength of 670 nm is radiated, the pumping light having a wavelength of 670 nm is mainly absorbed by the second active layer 342. Therefore, when pumping light having a wavelength of 670 nm is radiated, the second active layer 342 is mainly excited. Pumping light having a wavelength of 670 nm is sufficiently absorbed by the second active layer 342, and thus, the first active layer 340 existing below the second active layer 342 is hardly excited.

Energy of photons of pumping light having a wavelength of 808 nm is lower than energy of the band gap of the barrier layer of the second active layer 342. Therefore, pumping light having a wavelength of 808 nm is not absorbed by the second active layer 342 and passes through the second active layer 342. Energy of photons of pumping light having a wavelength of 808 nm is higher than energy of a band gap of the barrier layer of Al_0.05GaAs of the first active layer 340. Therefore, when pumping light having a wavelength of 808 nm is radiated, the pumping light having a wavelength of 808 nm is absorbed by the barrier layer of the first active layer 340. Therefore, when pumping light having a wavelength of 808 nm is radiated, the first active layer 340 is excited.

As described above, according to this embodiment, by appropriately radiating two kinds of pumping light having different wavelengths, the

active layers

340 and 342 can be selectively excited.

In this way, light radiation may be used not only for exciting the second active layer 342 but also for exciting the first active layer 340. According to this embodiment, similarly to the case of the wavelength tunable laser device according to the first embodiment, the oscillation wavelength can be changed in an extremely wide wavelength range.

Fourth Embodiment

A wavelength tunable laser device according to a fourth embodiment of the present invention is described with reference to FIG. 7. FIG. 7 is a sectional view for illustrating the wavelength tunable laser device according to this embodiment. The same components as in the wavelength tunable laser device and the like according to the first to third embodiments illustrated in FIG. 1A to FIG. 6 are represented by the same reference numerals, and description thereof is omitted or simplified.

As illustrated in FIG. 7, according to this embodiment, the semiconductor layer 141 (see FIG. 1B) does not exist between the first active layer 140 and the second active layer 142. According to this embodiment, the second active layer 142 is directly formed on the first active layer 140. A semiconductor layer 443 is formed on the second active layer 142. The semiconductor layer 443 has a conductivity type of, for example, the p type. An electrode 450 is formed on the semiconductor layer 443.

According to this embodiment, the substrate 101 has a conductivity type of, for example, the n type, the

active layers

140 and 142 have a conductivity type of the i type (undoped), and the semiconductor layer 443 has a conductivity type of, for example, the p type. In short, according to this embodiment, the conductivity type is an n-i-p type. In other words, according to this embodiment, two different quantum wells (active layers) 140 and 142 exist in the i-type region between the n-type region 101 and the p-type region 443.

Even when one electrode 450 is formed above the

active layers

140 and 142, by using an overflow of carriers as described below, the gain spectrum can be controlled.

When a voltage is applied to the electrode 450, electrons injected from the n-type region 101 and positive holes injected from the p-type region 443 reach the i-type region (undoped region) formed of the active layer 140 and the active layer 142. The mobility of the positive holes is lower than the mobility of the electrons, and thus, carriers tend to accumulate in one of the i-

type regions

140 and 142 closer to the p-type region 443. Therefore, when an injected current is relatively small, a lot of carriers accumulate in one of the i-

type regions

140 and 142 closer to the p-type region 443, that is, the second active layer 142, and a large gain is generated in the second active layer 142. A large gain is generated in the second active layer 142 in which the peak wavelength of the gain spectrum is relatively long, and thus, when the injected current is relatively small, reliable oscillation at a relatively long wavelength is realized.

On the other hand, when the injected current is relatively large, carrier are saturated in the second active layer 142, and sufficient carriers flow into the first active layer 140 as well. A gain larger than a gain generated in the second active layer 142 is generated in the first active layer 140. A large gain is generated in the first active layer 140 in which the peak wavelength of the gain spectrum is relatively short, and thus, when the injected current is relatively large, reliable oscillation in a relatively short wavelength range is realized.

According to this embodiment, the first active layer 140 and the second active layer 142 are respectively located so as to correspond to locations of antinodes of a standing wave of laser light. Specifically, the first active layer 140 and the second active layer 142 are respectively located so as to correspond to locations of antinodes of the standing wave generated when oscillation is induced at 1,060 nm that is the center wavelength of the wavelength tunable range (high reflectivity band). The first active layer 140 and the second active layer 142 are located so as to correspond to locations of adjacent antinodes of the standing wave, respectively. The first active layer 140 and the second active layer 142 are located so as to correspond to locations of antinodes of the standing wave, respectively, for the purpose of acquiring a sufficient overlap of light.

When the

active layers

140 and 142 are located so as to correspond to locations of antinodes of the standing wave of laser light, respectively, it is preferred that a center location of a quantum well group forming the

active layers

140 and 142 be equal to a maximum value of antinodes of the standing wave. However, in actuality, a deviation occurs due to a manufacturing error or the like.

There is a node at the midpoint between two adjacent antinodes of the standing wave. Therefore, a node-to-node range (range between one node and a node next to the one node) of one standing wave can be represented as a range of 1/2 (location of the one node) to 3/2 (location of the node next to the one node), where a spacing between one antinode and an antinode next to the one antinode is represented by 1. Therefore, a condition for the first active layer 140 to be located in one node-to-node range of the standing wave and for the second active layer 142 to be located in a node-to-node range next to the one node-to-node range is as follows. Specifically, the condition for the first active layer 140 and the second active layer 142 to be located as described above can be represented as a range of 1/2 to 3/2 of the spacing between antinodes of the standing wave with reference to the location of the antinode corresponding to the first active layer 140.

Further, the first active layer 140 and the second active layer 142 may be located so as to respectively correspond to locations of antinodes of the standing wave, which are apart from each other. In this case, the location of the second active layer 142 can be represented as a location apart from a location, at which the node-to-node range of the standing wave in which the first active layer 140 exists has the maximum value, by a distance of 1/2 or more of the spacing between antinodes of the standing wave.

The wavelength tunable laser device 10 according to this embodiment changes the oscillation wavelength by changing magnitude of an injected current, and thus, output of laser light is changed depending on a change in amount of the injected current. When stable laser light is required, a semiconductor optical amplifier (SOA) (not shown) may be arranged in a stage subsequent to the wavelength tunable laser device 10 according to this embodiment. A semiconductor optical amplifier is arranged in a stage subsequent to the wavelength tunable laser device 10 and an amplification factor of the semiconductor optical amplifier is controlled as appropriate, thereby being capable of finally emitting laser light as stable output from the semiconductor optical amplifier.

As described above, according to this embodiment, by controlling magnitude of a current injected via the electrode 450, the excitation of the first active layer 140 and the excitation of the second active layer 142 are controlled. Also according to this embodiment, the excitation of the first active layer 140 and the excitation of the second active layer 142 can be controlled, and thus, mode hopping can be prevented with reliability and the oscillation wavelength can be changed in an extremely wide wavelength range. According to this embodiment, the number of electrodes required to be formed on the substrate 101 is small, and thus, the degree of difficulty in manufacturing is low and the number of processing steps is small. Therefore, according to this embodiment, manufacturing yield can be improved and manufacturing costs can be reduced.

Fifth Embodiment

A wavelength tunable laser device according to a fifth embodiment of the present invention is described with reference to FIG. 8. FIG. 8 is a graph for showing a method of driving the wavelength tunable laser device according to this embodiment. The same components as in the wavelength tunable laser device and the like according to the first to fourth embodiments illustrated in FIG. 1A to FIG. 7 are represented by the same reference numerals, and description thereof is omitted or simplified.

The wavelength tunable laser device according to this embodiment has a structure similar to, for example, that of the wavelength tunable laser device 10 according to the first embodiment. However, according to this embodiment, a balance between the excitation of the first active layer 140 and the excitation of the second active layer 142 is adjusted to intentionally cause mode hopping from Mode A to Mode B.

According to this embodiment, a voltage applied to the electrode 113 configured to displace the beam-like movable portion 122 is gradually raised from 0 V, and a current is injected into the second active layer 142 until the voltage applied to the electrode 113 reaches a predetermined voltage. The predetermined voltage is, for example, a voltage at which the oscillation wavelength is 1,060 nm that is the center wavelength of the wavelength tunable range. When a current is injected into the second active layer 142, the second active layer 142 is excited, and a gain is acquired at a relatively long wavelength. Therefore, when oscillation is induced at a wavelength longer than 1,060 nm that is the center wavelength of the wavelength tunable range, the oscillation is induced in Mode B. While a current is injected into the second active layer 142, no current is injected into the first active layer 140, and thus, the length of the gap 104 is reduced under a state in which the strength of the excitation of the second active layer 142 is higher than the strength of the excitation of the first active layer 140.

When the voltage applied to the electrode 113 configured to displace the beam-like movable portion 122 reaches the predetermined voltage, the current injection into the second active layer 142 is stopped, and a current is injected into the first active layer 140. In other words, according to this embodiment, when the voltage applied to the electrode 113 reaches the predetermined voltage, the current injection is switched from into the second active layer 142 to into the first active layer 140. Then, oscillation in Mode A on a shorter wavelength side is more liable to be induced than Mode B, and transition from Mode B to Mode A, that is, mode hopping, occurs. When the mode hopping from Mode B to Mode A occurs, the oscillation wavelength is, for example, about 1,007 nm. The peak wavelength of the gain spectrum of the first active layer 140 is set to be sufficiently low with respect to 1,060 nm that is the center wavelength of the wavelength tunable range, and thus, the mode hopping from Mode B to Mode A can occur with reliability.

After the mode hopping occurs, the voltage applied to the electrode 113 configured to displace the beam-like movable portion 122 is gradually lowered. When the voltage applied to the electrode 113 is gradually lowered, the oscillation in Mode A is maintained. When the voltage applied to the electrode 113 becomes 0 V, the oscillation wavelength is about 1,050 nm. While a current is injected into the first active layer 140, no current is injected into the second active layer 142, and thus, the length of the gap 104 is increased under a state in which the strength of the excitation of the first active layer 140 is higher than the strength of the excitation of the second active layer 142.

After this, when the current injection is switched from into the first active layer 140 to into the second active layer 142, the oscillation wavelength is about 1,098 nm. In other words, when the current injection is switched from into the first active layer 140 to into the second active layer 142, the oscillation wavelength returns to the initial state. The peak wavelength of the gain spectrum of the second active layer 142 is set to be sufficiently high with respect to 1,060 nm that is the center wavelength of the wavelength tunable range, and thus, the mode hopping from Mode A to Mode B can occur with reliability.

As described above, according to this embodiment, the mode hopping is intentionally caused. The wavelength tunable width acquired by reciprocation of the beam-like movable portion 122 according to this embodiment is similar to the wavelength tunable width acquired in the first embodiment. Therefore, the wavelength tunable laser device may be operated as in this embodiment. According to this embodiment, after the mode hopping occurs, the voltage applied to the electrode 113 configured to displace the beam-like movable portion 122 is gradually lowered, and thus, an amount of displacement of the beam-like movable portion 122 is about a half of that in the case of the first embodiment. According to this embodiment, the voltage applied to the beam-like movable portion 122 may be low, and thus, power consumption and the like can be achieved.

Modified Embodiments

The present invention is not limited to the embodiments described above, and various modifications thereof are possible.

For example, in the fifth embodiment, a case in which the mode hopping is intentionally caused in the wavelength tunable laser device according to the first embodiment is described as an example, but the mode hopping may be intentionally caused also in the wavelength tunable laser devices according to the second to fourth embodiments. The two

active layers

140 and 142 different from each other are formed also in the wavelength tunable laser devices according to the second to fourth embodiments. Therefore, also in the wavelength tunable laser devices according to the second to fourth embodiments, by appropriately controlling the balance between the excitation of the active layer 140 and the excitation of the active layer 142, the gain spectrum can be controlled as appropriate as in the fifth embodiment. Therefore, the wavelength tunable laser devices according to the second to fourth embodiments can also be operated as in the fifth embodiment. Further, the method of intentionally causing the mode hopping is not limited to appropriately adjusting the balance between the excitation of the active layer 140 and the excitation of the active layer 142. For example, the peak wavelength of the gain spectrum of the first active layer 140 according to the fifth embodiment may be set to be lower than that in the case of the first embodiment. Through setting of the peak wavelength of the gain spectrum of the first active layer 140 to be lower, the mode hopping from Mode B to Mode A can occur with reliability. Further, the current injection may be switched from into the second active layer 242 to into the first active layer 140 at a wavelength slightly higher than the center wavelength of the high reflectivity bands of the

reflectors

102 and 106. This also enables occurrence of the mode hopping from Mode B to Mode A with reliability.

Further, in the embodiments described above, the wavelength tunable laser devices in which the center wavelength of the wavelength tunable range is 1,060 nm, that is, the wavelength tunable laser devices of the 1,060 nm range, are described as examples, but the present invention is not limited thereto. For example, the present invention may be applied to a wavelength tunable laser device in which the center wavelength of the wavelength tunable range is 850 nm, that is, a wavelength tunable laser device of the 850 nm range. When the center wavelength of the wavelength tunable range is set to be a wavelength different from 1,060 nm in the second embodiment or the third embodiment, the wavelength of pumping light may be set as appropriate depending on band gaps of the respective components.

Further, in the embodiments described above, a case in which the

active layers

140 and 142 have the quantum well structures is described as an example, but the structures of the

active layers

140 and 142 are not limited to the quantum well structures. For example, the

active layers

140 and 142 may be active layers having other structures such as a bulk or a quantum dot.

Further, in the embodiments described above, a case in which DBRs are used as the

reflectors

102 and 106 is described as an example, but the present invention is not limited thereto. For example, high-index contrast subwavelength gratings (HCGs) or the like may be used as the

reflectors

102 and 106.

Further, in the second embodiment, a case in which the excitation of the second active layer 242 is controlled by pumping light radiation and the excitation of the first active layer 140 is controlled by current injection is described as an example, but the present invention is not limited thereto. For example, the excitation of the second active layer 242 may be controlled by current injection and the excitation of the first active layer 140 may be controlled by pumping light radiation. Specifically, by controlling a current injected into one of the first active layer 140 and the second active layer 242 depending on the length of the gap between the first reflector 102 and the second reflector 106, the excitation of the one of the first active layer 140 and the second active layer 242 may be controlled. Further, by controlling light radiation depending on the length of the gap between the first reflector 102 and the second reflector 106, the excitation of another one of the first active layer 140 and the second active layer 242 may be controlled.

Further, in the embodiments described above, a case in which the oscillation wavelength is swept with the center wavelengths λc1 and λc2 of the high reflectivity bands of the

reflectors

102 and 106 being the center of the sweep is described as an example, but the present invention is not limited thereto. For example, the center wavelengths λc1 and λc2 of the high reflectivity bands of the

reflectors

102 and 106 and the center wavelength of the band in which the oscillation wavelength is changed may be different from each other.

Further, in the embodiments described above, a case in which the center wavelength λc1 of the high reflectivity band of the first reflector 102 and the center wavelength λc2 of the high reflectivity band of the second reflector 106 are the same is described as an example, but the present invention is not limited thereto. For example, the center wavelength λc1 of the high reflectivity band of the first reflector 102 and the center wavelength λc2 of the high reflectivity band of the second reflector 106 may be different from each other.

Further, in the first embodiment, a case in which no current is injected into the second active layer 142 when a current is injected into the first active layer 140 and no current is injected into the first active layer 140 when a current is injected into the second active layer 142 is described as an example, but the present invention is not limited thereto. For example, when the oscillation is induced at a relatively long wavelength, the current injected into the second active layer 142 may be set to be larger than the current injected into the first active layer 140. Further, when the oscillation is induced at a relatively short wavelength, the current injected into the first active layer 140 may be set to be larger than the current injected into the second active layer 142. In other words, the magnitude relationship between the current injected into the first active layer 140 and the current injected into the second active layer 142 may be adjusted as appropriate, which may be similarly applied to the second to fifth embodiments. Specifically, when the oscillation is induced at a relatively long wavelength, the gain in the second

active layer

142, 242, or 342 may be set to be relatively large with respect to the gain in the first

active layer

140 or 340. On the other hand, when the oscillation is induced at a relatively short wavelength, the gain in the first

active layer

140 or 340 may be set to be relatively large with respect to the gain in the second

active layer

142, 242, or 342.

Further, in the embodiments described above, an active layer that realizes a large gain in a relatively short wavelength range is used as the first

active layer

140 or 340 and an active layer that realizes a large gain in a relatively long wavelength range is used as the second

active layer

142, 242, or 342, but the present invention is not limited thereto. An active layer that realizes a large gain in a relatively long wavelength range may be used as the first

active layer

140 or 340 and an active layer that realizes a large gain in a relatively short wavelength range may be used as the second

active layer

142, 242, or 342. In this case, when the oscillation is induced at a relatively long wavelength, by exciting the first

active layer

140 or 340 that realizes a large gain in a relatively long wavelength range, reliable oscillation at the relatively long wavelength is realized. On the other hand, when the oscillation is induced at a relatively short wavelength, by exciting the second

active layer

142, 242, or 342 that realizes a large gain in a relatively short wavelength range, reliable oscillation at the relatively short wavelength is realized.

Further, in the embodiments described above, a case in which the upper reflector 106 is arranged on the beam-like movable portion 122 is described as an example, but the present invention is not limited thereto. For example, the upper reflector 106 may be fixed to a lower surface side of the beam-like movable portion 122.

Further, in the embodiments described above, a case in which the beam-like movable portion 122 and the upper reflector 106 are separate from each other is described as an example, but the present invention is not limited thereto. For example, the upper reflector 106 may also serve as the beam-like supporting portion.

Further, in the embodiments described above, a case in which both the ends of the beam-like movable portion 122 are fixed, that is, a case in which the beam-like movable portion 122 is a both-ends fixed beam, is described as an example, but the present invention is not limited thereto. For example, the beam-like movable portion 122 may be a cantilever having one end being fixed.

Further, in the embodiments described above, a case in which the spacing between the lower reflector 102 and the upper reflector 106 is changed by the beam-like movable portion 122 is described as an example, but the present invention is not limited thereto. Various mechanisms (movable mechanism, supporting mechanism) that can change the spacing between the lower reflector 102 and the upper reflector 106 can be used as appropriate.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Application No. 2015-021300, filed February 5, 2015, which is hereby incorporated by reference herein in its entirety.

8 measuring device
10 wavelength tunable laser device
12 resonator
101 substrate
102 lower reflector
103 laminate
104 gap, air gap
106 upper reflector
110, 113 electrode
122 beam-like movable portion, movable mechanism
140 first active layer
141 first semiconductor layer
142 second active layer
143 second semiconductor layer
150, 151 electrode

Claims

A wavelength tunable laser device, comprising:
a first reflector;
a second reflector; and
an active layer formed between the first reflector and the second reflector,
wherein the wavelength tunable laser device has a gap formed between the active layer and the second reflector, and a length of the gap is changed to change a resonance wavelength,
wherein the active layer includes:
a first active layer; and
a second active layer formed above the first active layer and having a peak wavelength of a gain spectrum that is different from a peak wavelength of a gain spectrum of the first active layer, and
wherein each of excitation of the first active layer and excitation of the second active layer is controlled depending on the length of the gap.
The wavelength tunable laser device according to claim 1, further comprising:
a first semiconductor layer formed between the first active layer and the second active layer;
a second semiconductor layer of a conductivity type opposite to a conductivity type of the first semiconductor layer, the second semiconductor layer being formed on the second active layer;
a first electrode electrically connected to the first semiconductor layer; and
a second electrode electrically connected to the second semiconductor layer,
wherein the excitation of the first active layer is controlled by controlling a current injected into the first active layer via the first electrode depending on the length of the gap, and
wherein the excitation of the second active layer is controlled by controlling a current injected into the second active layer via the second electrode depending on the length of the gap.
The wavelength tunable laser device according to claim 1, further comprising:
a semiconductor layer connected to one of the first active layer and the second active layer; and
an electrode electrically connected to the semiconductor layer,
wherein the excitation of the one of the first active layer and the second active layer is controlled by controlling a current injected into the one of the first active layer and the second active layer via the electrode depending on the length of the gap, and
wherein the excitation of another one of the first active layer and the second active layer is controlled by controlling light radiation depending on the length of the gap.
The wavelength tunable laser device according to claim 1,
wherein the excitation of one of the first active layer and the second active layer is controlled by radiating light having a first wavelength depending on the length of the gap, and
wherein the excitation of another one of the first active layer and the second active layer is controlled by radiating light having a second wavelength different from the first wavelength depending on the length of the gap.
The wavelength tunable laser device according to claim 1, further comprising:
a semiconductor layer formed on the second active layer; and
an electrode electrically connected to the semiconductor layer,
wherein the each of the excitation of the first active layer and the excitation of the second active layer is controlled by controlling magnitude of a current injected via the electrode depending on the length of the gap.
The wavelength tunable laser device according to any one of claims 1 to 5,
wherein strength of the excitation of one of the first active layer and the second active layer when the length of the gap is equal to or larger than a predetermined value is higher than the strength of the excitation of the one of the first active layer and the second active layer when the length of the gap is smaller than the predetermined value, and
wherein strength of the excitation of another one of the first active layer and the second active layer when the length of the gap is equal to or larger than the predetermined value is lower than the strength of the excitation of the another one of the first active layer and the second active layer when the length of the gap is smaller than the predetermined value.
The wavelength tunable laser device according to any one of claims 1 to 5,
wherein, when the gap is reduced, strength of the excitation of one of the first active layer and the second active layer is higher than strength of the excitation of another one of the first active layer and the second active layer, and
wherein, when the gap is increased, the strength of the excitation of the another one of the first active layer and the second active layer is higher than the strength of the excitation of the one of the first active layer and the second active layer.
The wavelength tunable laser device according to any one of claims 1 to 7,
wherein one of the peak wavelength of the gain spectrum of the first active layer and the peak wavelength of the gain spectrum of the second active layer is longer than a center wavelength of a high reflectivity band of one of the first reflector and the second reflector; and
wherein another one of the peak wavelength of the gain spectrum of the first active layer and the peak wavelength of the gain spectrum of the second active layer is shorter than the center wavelength of the high reflectivity band of the one of the first reflector and the second reflector.
The wavelength tunable laser device according to any one of claims 1 to 8, wherein the first reflector and the second reflector form a resonator having a longitudinal mode spacing smaller than both of the high reflectivity band of the first reflector and the high reflectivity band of the second reflector.
The wavelength tunable laser device according to any one of claims 1 to 9,
wherein an oscillation wavelength of the wavelength tunable laser device is swept in a wavelength range including 1,060 nm, and
wherein the first reflector and the second reflector form a resonator having a longitudinal mode spacing larger than 30 nm.
The wavelength tunable laser device according to any one of claims 1 to 10, wherein the first reflector and the second reflector form a resonator having a longitudinal mode spacing larger than a wavelength difference corresponding to an energy difference of 33 meV in an oscillation wavelength.
The wavelength tunable laser device according to any one of claims 1 to 11,
wherein an oscillation wavelength of the wavelength tunable laser device is swept in a wavelength range including 1,060 nm, and
wherein the length of the gap under a state in which a spacing between the first reflector and the second reflector is not changed is larger than 1.7 μm.
The wavelength tunable laser device according to any one of claims 1 to 12, wherein the length of the gap under the state in which the spacing between the first reflector and the second reflector is not changed is larger than 1.6 times the center wavelength of the high reflectivity band of the one of the first reflector and the second reflector.
An optical coherence tomography apparatus, comprising:
a wavelength tunable laser device including:
a first reflector;
a second reflector; and
an active layer formed between the first reflector and the second reflector,
wherein the wavelength tunable laser device has a gap formed between the active layer and the second reflector, and a length of the gap is changed to change a resonance wavelength,
wherein the active layer includes:
a first active layer; and
a second active layer formed above the first active layer and having a peak wavelength of a gain spectrum that is different from a peak wavelength of a gain spectrum of the first active layer, and
wherein each of excitation of the first active layer and excitation of the second active layer is controlled depending on the length of the gap,
the optical coherence tomography apparatus further comprising:
a coherence optical system configured to branch light from the wavelength tunable laser device into radiation light to be radiated to a measurement object and reference light, and to generate coherent light with reflection light of the radiation light radiated to the measurement object and the reference light;
an optical detection portion configured to receive the coherent light; and
an information acquisition portion configured to acquire information on the measurement object based on a signal from the optical detection portion.