JPH06112570A - Distributed bragg-reflection type semiconductor laser - Google Patents

Abstract

PURPOSE:To provide a distributed Bragg-reflection laser which can change the mode interval of oscillation and the maximum and minimum values of a selectable mode and can easily make mode selection at the wavelength sweeping time. CONSTITUTION:In the title semiconductor laser provided with a waveguide constituting a light emitting area together with a resonator on a semiconductor substrate, two wavelength control areas 24 and 25 provided with heating means for generating temperature distributions which are repeatedly generated at a fixed period against a distance in the direction of the resonant axis of the waveguide and different from those generated by diffraction gratings 3 are provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tunable distributed Bragg reflection type semiconductor laser with a single mode oscillation and a wide band, which is useful as a light source for coherent optical transmission and optical measurement.

[0002]

2. Description of the Related Art With the increasing amount of information in recent years, a coherent optical transmission system using light as a frequency is being developed as a means of transmitting information by light, and an optical heterodyne system is a promising system as one of the systems. Has been done. According to this method, the interference signal obtained when the signal light on the transmission side and the local light on the reception side are synchronized is treated as an information signal,
It becomes possible to simultaneously transmit a plurality of signal lights having different frequencies on one fiber. To realize this, the performance of the light source used is important. The required performance is that the spectral linewidth is as narrow as possible to put more information in a narrow frequency band, that the wavelength tunable width is wide enough to increase the number of channel settings, and that an accurate signal is obtained. The optical output is higher, and the signal light is A
It is possible to apply the M signal and the FM signal, and the local light can be wavelength-shifted so fast that it can be tuned to the signal light.

As a semiconductor laser that oscillates at a single wavelength, a distributed Bragg reflection type (DBR: Distributed) composed of a light emitting region and a reflector including a diffraction grating.
A Bragg Reflector laser and a distributed feedback (DFB: Distr) equipped with a diffraction grating in the light emitting region.
There is an ibuted feedback laser. In these lasers, the oscillation wavelength is determined by the length of the pitch (cycle) of the diffraction grating and the refractive index of the waveguide, so that the oscillation wavelength can be changed by changing their physical quantities.

From the viewpoint of high practicality, an attempt has been made to produce a wavelength tunable semiconductor laser in which the wavelength is changed by changing the refractive index, and a three-electrode type DBR as shown in FIG.
A laser was developed (prior art 1). This three-electrode type DBR laser has a light emitting region 11 provided on a substrate 1.
The phase control region 12 and the DBR region 13 having the diffraction grating 3 having a constant pitch below the guide layer 4 have independent electrodes 6b, 6c and 6a, respectively. A laser is oscillated by injecting a current into the active layer 2 of the light emitting region 11 to guide the phase control region 12 and the guide layer 4 of the DBR region 13.
A wavelength is shifted by injecting a current to increase the carrier density and changing the refractive index of the waveguide by the plasma effect. This made it possible to obtain a wavelength tunable range of several nm or more with an optical output of several mW or more. However, this method of changing the refractive index by using the plasma effect has a problem in that the fluctuation of the carrier density causes the refractive index to become unstable and the spectral line width to be greatly deteriorated. In addition, since this structure can obtain only a single Bragg wavelength, it is not suitable for wideband wavelength selection, and the wavelength sweep range is 10 n.
It stayed around m.

(Prior Art 2) On the other hand, as a reflector of a semiconductor laser for dramatically increasing the wavelength sweep range, one having a super-periodic structure diffraction grating (hereinafter referred to as SSG) is considered. This will be described in detail below. FIG. 6 shows a schematic view of a reflector having SSG. SS
The basic concept of G is that the region where the pitch of the diffraction grating 3 linearly decreases from Λa to Λb is repeated in the superperiod Λs (see FIG. 7A). Its reflection properties can be predicted using the Fourier transform,
The contents are from wavelength λa = 2neq Λa to λb = 2neq Λb
It has a plurality of high reflection peaks at intervals Δλ = λ0 ² / 2neq λs over the bands up to. On the other hand, S
The relative phase difference Φ based on the SG central wave number β0 = 2π / (Λa + Λb) is expressed as Φ = ∫βdx−∫β0dx (β = π
/ Λ), the SSG can be described as a quadratic curve having a relative phase difference Φ repeated at a long period Λs as shown in FIG. 7B.

In making an SSG as a prototype, two methods can be used as shown in FIG. Figure 7
For (a), the method of changing the pitch stepwise as shown in FIG. 8 (a) is used, and for FIG. 7 (b), the method shown in FIG.
A method of partially shifting the phase as shown in (b) was used. Regarding the latter phase shift type SSG, F-ma
The reflection characteristics obtained by the trix method are shown in FIG. As described above, in SSG, it is possible to obtain a characteristic having a sharp high reflection peak over a wide band. Similar characteristics can be obtained by a method of changing the pitch stepwise. Here, the experimental results of the phase shift type SSG reflector are shown.
The structure of the prototype reflector is InP substrate 1, 1.3 μm composition InGaAsP guide layer 4 (thickness 0.3 μm), I
The InP clad layer 5 has a three-layer structure composed of the nP clad layer 5 (thickness 1 μm), and is a ridge waveguide processed into a mesa stripe of 2 μm, and AR films (not shown) are applied to both end faces. . The SSG was formed on the InGaAsP guide layer 4 by using the electron beam drawing method. Figure 10
Indicates its transmission characteristics. 10 centered at 1540 nm
It exhibited a characteristic with sharp cut-off peaks at intervals of about 10 nm over the band of 0 nm.

The structure of the element having this SSG will be described with reference to FIG. Since it is extremely difficult to fabricate a structure in which the pitch changes continuously, it is possible to approximate 10 steps of pitches on both sides of the active region by 67.5 μm and 75 μ, respectively.
It has a diffraction grating 3 arranged so as to have a super period of m. Therefore, each SSG is 5.0 and 4.5.
It has high reflection peaks at nm intervals. The wavelength sweep is performed by injecting a current into one of the SSG regions 20 and 21 or by performing uniform injection into both SSG regions 20 and 21. When the high reflection peaks on the longest wave side of the SSGs on both sides are tuned to each other at the time of non-sweep, current is injected into the SSG region 21 on the rear side to the short wave side at every 5.0 nm and the SSG on the front side. In the injection of current into the region 20, after shifting to the high reflection peak on the shortest wave side once,
It shifts to the long wavelength side every 4.5 nm while causing mode jumps.

After tuning the SSG high reflection peaks on both sides, the characteristics become equivalent to those of a normal DBR laser, so that the sweep range can be further expanded by injecting at the same current density on both sides. The active layer 2 is 1.0% compressive strain 4-well MQW (PL wavelength 1.58 μm), and the guide layer 4 with low loss 1.3 μm is butt joint grown on both sides.
SSG was formed on it. As shown in FIG. 12, the wavelength sweep range in the above-described laser having the active layer 2 was expanded by injecting current into the SSGs on both sides. As a result, a wide band sweep characteristic of 63 nm was obtained by the single mode sweep. The spectral line width at this time is about 10 MHz to 30 MHz.

On the other hand, as a reflector having a plurality of reflection peaks like SSG, a sampled grating has been proposed (V. Jayaraman, et.al:Applied Phys).
icsLetters 60, P.2321 (1992)). This means that a uniform diffraction grating with a pitch Λ is formed over the distance Z1 in the resonance axis direction in the waveguide, and no diffraction grating is formed in the waveguide adjacent to the distance Z2. Furthermore, these are super-cycles Z
It has a structure in which the waveguide is repeated with 0 = Z1 + Z2. With this structure, the same function as SSG can be obtained.

[0010]

The above-mentioned SSG,
A semiconductor laser having a so-called super-periodic structure diffraction grating (Prior Art 2) is excellent in that it enables wavelength sweeping in a wide band, but the following problems to be solved remain. First, similar to the conventional current injection type distributed Bragg reflection type laser, the problem that the refractive index becomes unstable due to the fluctuation of the carrier density due to the current injection and the spectrum line width is greatly deteriorated at the time of wavelength sweep has been solved. Absent. Next, since the amount of change in pitch from Λa to Λb cannot be changed and the super-period Λs is fixed,
When performing wavelength sweeping, since there is no choice but to make mode jumps at the same mode interval during wideband sweeping and narrowband sweeping, it is difficult to make delicate sweeping in a narrow band when designing a grating for wideband. Furthermore, the electron beam method is used to fabricate a structure in which the pitch of the diffraction grating is linearly reduced from Λa to Λb, or is reduced stepwise while controlling the phase so as to repeat with a super-cycle of Λs. It is very difficult to carry. Note that the pitch Λ of the diffraction grating and the super-period Z
There is no change in that 0 is fixed, and since only half of the diffraction grating exists, there is also a problem that the reflection efficiency deteriorates.

[0011]

Means for solving the above problems will be described below. First, in order to prevent the deterioration of the spectral line width due to the injection of current, the wavelength is controlled by the temperature change due to heating. Next, in a distributed Bragg reflection type laser having a waveguide 23 that forms a resonator with the light emitting region 11 on a semiconductor substrate, a diffraction grating that is repeated with a constant period in the resonance axis direction of the waveguide is used. ,
A first wavelength control region 2 having heating means periodically arranged to generate a first temperature distribution which is repeated with a constant cycle with respect to distance in the resonance axis direction of the waveguide.
4 further includes a diffraction grating which is repeated with a constant period in the resonance axis direction of the waveguide with respect to the distance, and a second diffraction grating which is repeated with a constant period in the resonance axis direction of the waveguide with respect to the distance. Has invented a structure with a second wavelength control region 25 having heating means arranged periodically to produce a temperature distribution of

[0012]

According to the distributed Bragg reflection type semiconductor laser of the present invention, the wavelength control region having the diffraction grating having a uniform pitch is heated by the heating means to form a periodic temperature distribution in the waveguide, The effective period of can be modulated. As a premise, since the wavelength control is performed by the temperature change due to heating, deterioration of the spectral line width does not occur.
The Bragg wavelength of the diffraction grating with the pitch Λ is λ = 2nΛ
Since the equivalent refractive index n changes with temperature, the Bragg wavelength of each diffraction grating is distributed as a plurality of peaks in the band from λa corresponding to the maximum temperature Ta to λb corresponding to the minimum temperature Tb in the temperature distribution. To do. The temperature distribution is Λ
If it has a period of s, the reflection peaks having the diffraction grating will be distributed at intervals of Δλ = λ0 ² / 2nΛs.

[0013]

(First Embodiment) FIG. 1 shows a first embodiment of the present invention. 1A is a cross-sectional view of the device of the present invention taken along a waveguide, and FIG. 1B is a top view of the device. The distributed Bragg reflection type semiconductor laser of the present invention is manufactured by the following procedure.

First, 1.55 μ is formed on the p-type InP substrate 1.
An active layer 2 made of m-band InGaAsP is grown.

Next, the active layer 2 other than the portion to be the light emitting region 11 is removed by etching, and the DBR regions 13 on both sides of the light emitting region are subjected to Bragg reflection of light having a wavelength of 1.55 μm at a pitch of 2420 angstroms. A grating 3 is formed, and a 1.3 μm band InGaAsP guide layer 4 is grown on the grating 3.

After that, the cladding layer 5 made of n-type InP
Are grown over the entire surface of the light emitting region 11 and the DBR region 13, and a mesa-shaped waveguide having a width of 1.5 μm is formed by etching so as to control the lateral mode, and n is formed on both sides of the mesa-shaped waveguide. Re-grow the InP and p-InP current blocking layers (not shown).

Further, a p-type electrode 7 is provided on the substrate 1 side, DB
N-type electrodes 6a and 6b are formed on the cladding regions 5 of the R region 13 and the light emitting region 11, respectively. Although there are a plurality of DBR regions 13 across the light emitting region 11, one DBR region (hereinafter referred to as the first wavelength control region 24) and the other DBR region (hereinafter referred to as the second wavelength control region 25). ), The super-cycle Λs is changed so that a single oscillation mode can be selected. Electrodes 6 of the first wavelength control region 24 and the second wavelength control region 25
A heating electrode 1 is formed on the upper part of a through an insulating film 8 of SiO2.
Au thin film stripes 9 having 0a and 10b are formed, respectively. In the first wavelength control region 24, the thin film stripe 9 is provided along the waveguide 23 at intervals of 150 μm.
A portion mixed with Ti having a width of μm is formed and alloyed to form the heating resistance portion 22 (shown by hatching in FIG. 1).
In the second wavelength control region 25, the thin film stripe 9 has a width of 10 μm at intervals of 165 μm along the waveguide 23.
A portion mixed with i is formed and alloyed to form the heat generating resistance portion 22 (shown by hatching in FIG. 1). The heating resistor portion 22 corresponds to the heating means of the present invention.

This resistance is 0.02 W and 0.
Figure 2 shows the temperature distribution in the waveguide when heat of 04 W is generated.
Shown in. The vertical axis represents temperature change and the horizontal axis represents position, but the origin of the horizontal axis is arbitrary. When the size of the heating resistor portion 22 is made extremely small in this way, only the Bragg wavelength in an extremely limited region near the heating resistor portion 22 is extremely shifted, and the temperature of the remaining portion is almost constant. Tb (0.02W
In this case, a value of 48 ° C. is taken, and a region having the corresponding Bragg wavelength of λb is formed. This is the same function as forming a sampled grating DBR in which a region with a diffraction grating having a Bragg wavelength of λb and a region without a diffraction grating repeat from the beginning with a superperiod Λs (corresponding to a heater arrangement interval). have. Further, by changing the temperature, λb can be changed to shift the entire oscillating mode group without changing the mode interval.

On the other hand, if the size of the heating resistor portion 22 is increased to generate a rectangular wave-like temperature distribution that becomes constant at high temperature and low temperature, the pitch described in the prior art 2 is stepped. It has the same function as that of producing a diffraction grating that changes in a shape, and by changing the temperature difference, the selectable mode range can be set freely. That is, if the temperature difference is increased, the mode can be selected in a wider range. In this example, the super-period Λ determined by the heater interval
Since s is fixed, the spacing between each mode remains constant.

Further, a waveguide in a range from a position directly below the center position of the heat generating resistor portion 22 to Λs / 2 in the resonance axis direction (if the positional relationship in the temperature distribution is shown, the black position of the black and white pattern in FIG. 2, Or a white position), a diffraction grating having a uniform pitch can be formed, and the pitch is continuously changed and has a large super-period, which is theoretically described in the prior art 2. A wavelength control region having a repeating super-periodic structure is effectively formed. In this case as well, the high and low temperatures correspond to an overall uniform wavelength shift of the oscillation mode group,
The magnitude of the temperature difference corresponds to the determination of the range of the maximum wavelength and the minimum wavelength that can be oscillated. On the other hand, with respect to the structure of the heat generating resistor portion, the same result can be obtained by reducing the thickness of Au only in the portion where the resistance is desired to be increased, regardless of alloying.

(Second Embodiment) Next, a second embodiment of the present invention will be described with reference to FIG. FIG. 3 is a top view of the device, the waveguide 23 is shown by a broken line, and the diffraction grating is not shown.
The Au thin film resistor 26 having the heating electrodes 10a and 10b as heating means in the first wavelength control region 24 and the second wavelength control region 25 is provided with the electrode 6 on the waveguide 23 via the insulating film 8.
The first wavelength control region 24 and the second wavelength control region 25 are arranged on the upper surface a in a direction orthogonal to the waveguide 23 at intervals of 150 μm and at intervals of 165 μm. If ask cause the same heating all of the thin film resistor 26, the same effect as the first embodiment can occur, if the heating has another fixed period, such as every other place or two, a temperature distribution super The period can also be changed to narrow the oscillation mode interval. If the oscillation mode interval is narrow, delicate wavelength sweeping in a narrow band becomes extremely easy. Also in this embodiment, the waveguide in the range from just below the center position of the thin film resistor 26 to Λs / 2 in the resonance axis direction (if it is shown by the positional relationship in the temperature distribution, the black position of the black and white pattern in FIG. Or, it is represented by a white position.), A wavelength control region having a super-periodic structure in which the pitch changes continuously and repeats with a large super-period is effective. Is formed.

(Third Embodiment) Next, a third embodiment of the present invention will be described with reference to FIG. FIG. 4 is a perspective view of the element. This embodiment is different from the first or second embodiment in which a heating resistor 22 or a thin film resistor 26 arranged periodically is used to generate a temperature distribution, and the first and second wavelength control regions 24 and 25 are different. The current blocking layers 27 and 28 are sawtooth-shaped when viewed from above to generate a temperature distribution. That is,
By removing the portions on both sides of the waveguide that have almost no influence on the confinement of light by etching to a depth of about 5 μm, a heat conduction distribution is given to the waveguide in the resonance axis direction, and a constant period is provided in the resonance axis direction. The temperature distribution is generated. Further, it is natural that the same effect as that of the second embodiment can be obtained by individually controlling each heating means as in the second embodiment.

Although all of the embodiments shown in FIGS. 1 to 4 have four repetition periods, the number is not limited to four, and at least two may be used. The heating means used in the present invention is a heating means in a broad sense,
It is a general term for means for inducing a temperature distribution in the waveguide, and is not limited to a heater, heat conduction distribution, or the like. Also,
In each figure, the scale of the vertical axis is set arbitrarily,
The fine structure of the element such as the diffraction grating is particularly emphasized. Although not mentioned in these three examples, a current is injected into the first wavelength control region 24 and the second wavelength control region 25 by using the electrode 6a in a state where the temperature distribution is generated by the heating means in a broad sense. However, it is also possible to perform wavelength sweep by the plasma effect. However, as described above, deterioration of the spectral line width naturally occurs. on the other hand,
In the wavelength sweep by heat, the spectral line width was always maintained at a value around 1 MHz.

[0024]

The following effects are obtained with the present invention. First of all, the problem that the refractive index becomes unstable due to the fluctuation of the carrier density during the current injection method and the spectral line width is greatly deteriorated has been solved. Next, by changing the amount of change in the pitch of the diffraction grating within one super period and by changing the size of the super period, the oscillation mode interval and the maximum and minimum values of the selectable modes can be changed. It became possible to select the mode at the time of wavelength sweep. Furthermore, since the manufacturing method of changing the pitch of the diffraction grating is not adopted, it is possible to use the electron beam method instead of the manufacturing method.
It is manufactured by a simple method such as a light flux interference exposure method. In addition, there is no reduction in reflection efficiency as in the sampled grating method.

[0025]

[Brief description of drawings]

FIG. 1 is a sectional view and a top view showing a first embodiment of the present invention.

FIG. 2 is a diagram showing a temperature distribution in the waveguide according to the first embodiment of the present invention.

FIG. 3 is a top view showing a second embodiment of the present invention.

FIG. 4 is a perspective view showing a third embodiment of the present invention.

FIG. 5 is a sectional view of a conventional distributed Bragg reflection type semiconductor laser.

FIG. 6 is a schematic view of a so-called SSG reflector.

FIG. 7 is a diagram showing so-called SSG pitch and phase.

FIG. 8 is a diagram showing a pitch and a phase of the produced SSG.

FIG. 9 is a diagram showing a so-called SSG reflection characteristic.

FIG. 10 is a diagram showing a so-called SSG transmission characteristic.

FIG. 11 is a perspective view of a device provided with so-called SSG.

FIG. 12 is a diagram showing expansion of a so-called SSG wavelength sweep range.

[Explanation of symbols]

 1 substrate 2 active layer 3 diffraction grating 4 guide layer 5 clad layer 6a electrode 6b electrode 6c electrode 7 p-type electrode 8 insulating film 9 thin film stripe 10a heating electrode 10b heating electrode 11 light emitting region 12 phase control region 13 DBR region 20 SSG Region 21 SSG Region 22 Heating Resistor 23 Waveguide 24 First Wavelength Control Region 25 Second Wavelength Control Region 26 Thin Film Resistor 27 Current Blocking Layer 28 Current Blocking Layer.

Claims (1)

[Claims] 1. A distributed Bragg reflection type semiconductor laser having a light emitting region and a waveguide constituting a resonator on a semiconductor substrate, wherein the distributed Bragg reflection type semiconductor laser is repeated at a constant cycle in the resonance axis direction of the waveguide. A first wavelength control region (24) having a diffraction grating and heating means periodically arranged to generate a first temperature distribution; and a constant period with respect to distance in the resonance axis direction of the waveguide. Bragg distribution characterized in that it comprises a second wavelength control region (25) having a diffraction grating and heating means periodically arranged to produce a second temperature distribution, Reflective semiconductor laser.