JP3982349B2 - Tunable device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a wavelength tunable device, and more particularly to a wavelength tunable device such as a wavelength tunable filter that selects light of a specific wavelength from wavelength multiplexed light and a wavelength tunable laser that oscillates by selecting light of a specific wavelength. .
[0002]
[Prior art]
One of the key devices of the wavelength division multiplexing (WDM) optical transmission system is a wavelength tunable filter that selectively extracts specific wavelength light from a plurality of wavelength multiplexed signal lights, and selectively oscillates specific wavelength light. There is a tunable laser. As a typical example of a conventional wavelength tunable filter, a wavelength tunable filter using multiple reflection in a dielectric multilayer film and a wavelength tunable filter using a Fabry-Perot resonator are known. However, the former makes the wavelength variable by changing the effective film thickness of the dielectric multilayer film by controlling the incident angle of the incident light to the dielectric multilayer film. There is a problem that the structure is complicated and it is difficult to reduce the size and cost, such as requiring a servo mechanism to rotate. It is also difficult to apply an array to optical parallel transmission.
[0003]
On the other hand, as the latter, for example, the one disclosed in JP-A-2000-162516 is known. FIG. 15 is a cross-sectional view showing the configuration of this conventional wavelength tunable filter. The conventional wavelength tunable filter includes optical waveguides 501 and 502 disposed opposite to each other in the optical waveguide substrate 500, and a diaphragm type micromachine 503 disposed between both end faces of these optical waveguides. It arrange | positions between the optical fiber 510 and the optical fiber 511 of an output side. The diaphragm 505 is deformed by an electrostatic force generated by applying a voltage between the electrode 506 and the electrode 507 attached to the main body 504 and the diaphragm 505 constituting the diaphragm type micromachine 503, respectively. As a result, the resonator length of the Fabry-Perot resonator formed by the two half mirrors 508 and 509 made of the dielectric multilayer film changes, and the transmission wavelength becomes variable. This structure has fewer components than the former structure, and can be reduced in size and arrayed.
As a conventional example of a wavelength tunable laser, a rotating type diffraction grating is mounted as an external resonator of a Fabry-Perot laser.
[0004]
[Problems to be solved by the invention]
As described above, the conventional wavelength tunable filter using the Fabry-Perot resonator has the advantage that it has few components and can be reduced in size and arrayed. However, when light is transmitted, the half mirror 508 is used. , 509 are inevitably passed through the main body 504 and the diaphragm 505, and unnecessary transmission loss is caused. In addition, the manufacturing process such as the formation of the half mirrors 508 and 509 and the bonding of the main body 504 and the diaphragm 505 are complicated. Furthermore, in a wavelength tunable filter using a Fabry-Perot resonator, the higher the half mirror reflectivity, the narrower the transmission peak bandwidth, etc. In order to obtain good filter characteristics over the entire wavelength band of light to be used, a broadband high-reflectance mirror capable of covering the wavelength band becomes indispensable. In the future, in addition to the C band (1.55 μm band), the L band (1.58 μm band) and further the S band (1.49 μm band) and the wavelength band used for optical communication are expected to expand. It is extremely difficult to form a mirror having high reflectivity in a wide band extending from the S band to the L band with a conventional dielectric multilayer film.
In addition, since the above-described conventional wavelength tunable laser has a complicated configuration that requires a servo mechanism for rotating the diffraction grating, it is difficult to reduce the size and the cost.
[0005]
The present invention has been made in view of the above problems, and its purpose is to be small, inexpensive, suitable for arraying, and has a high transmission spectrum transmittance in a wide band and a narrow bandwidth. The object is to provide a wavelength tunable filter exhibiting filter characteristics and a wavelength tunable laser that is small, inexpensive, and suitable for arraying.
[0006]
[Means for Solving the Problems]
  In order to achieve the above object, according to the present invention, a resonator composed of a pair of mirrors arranged in parallel and facing each other, and a variable interval mechanism for changing a resonator length by changing a gap between the pair of mirrors, The at least one of the paired mirrors and the spacing variable mechanism are both integrally formed on a substrate, and the mirror surface of the mirror is perpendicular to the substrate surface of the substrate. Formed intoThe variable distance mechanism includes an electrostatically driven microactuator, and includes a fixed electrode fixed on a substrate and a movable electrode integrally formed with any one of the mirrors, and the fixed electrode and the movable electrode are combs. Distance from each comb tooth of the fixed electrode to the two comb teeth of the movable electrode adjacent to both sides of each comb tooth ( Hereinafter, referred to as “intergap gap dimension” ) Are different from each other when no electric field is applied between the fixed electrode and the movable electrode, and by applying an electric field between the fixed electrode and the movable electrode, the narrow inter-gap gap dimension is Narrow intercombination gap dimension when no electric field is applied ( Hereinafter referred to as “minimum inter-gap gap dimension” ) The movable electrode moves in a narrower directionA wavelength tunable device is provided. Preferably, the photonic crystal is composed of a semiconductor and air periodic structure having a photonic band gap in a desired wavelength band with respect to light incident on at least one of the paired mirrors. Preferably, the other of the pair of mirrors is also made of a photonic crystal having a refractive index periodic structure, thereby forming a wavelength tunable filter. Preferably, the other of the pair of mirrors is constituted by a semiconductor chip having an optical amplification function, and one movable mirror functions as an external resonator of the laser.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
Next, embodiments of the present invention will be described with reference to the drawings.
[First Embodiment]
FIG. 1 is a perspective view of a variable wavelength filter according to a first embodiment of the present invention. FIG. 2 is a plan view of the fixed mirror made of the photonic crystal according to the first embodiment. FIG. 3 is a plan view of the variable wavelength filter according to the first embodiment. FIGS. 4A to 4B and 5A to 5C are cross-sectional views in order of steps for explaining one manufacturing method of the first embodiment, and FIGS. (C) is sectional drawing of the order of the process for demonstrating the other manufacturing method of 1st Embodiment.
As shown in FIG. 1, the wavelength tunable filter according to the present embodiment includes a pair of movable electrodes 101A and 101B, four springs 102 that support the movable electrodes, and ends of the four springs. A fixed end 109 provided, two pairs of fixed electrodes 103A, 103B and 104A, 104B, a movable mirror 105 made of a photonic crystal, and a fixed mirror 106 made of a photonic crystal at a position facing the movable mirror 105; , A substrate 107 and an insulating layer 108. Two pairs of fixed electrodes 103A, 103B and 104A, 104B and a fixed mirror 106 made of a photonic crystal are fixed to a substrate 107 to form a fixed portion. The pair of movable electrodes 101A and 101B, the four springs 102 that support the movable electrodes, and the movable mirror 105 made of a photonic crystal are integrally suspended in the air, and the four that support the movable electrodes The movable part is configured while being fixed to the substrate 107 via the fixed end 109 of the spring. The two pairs of fixed electrodes 103A, 103B and 104A, 104B, the fixed mirror 106, and the fixed end 109 of the spring are respectively made of SiO.₂The substrate 107 is fixed to the substrate 107 while being electrically insulated from the substrate 107 by an insulating layer 108 made of, for example. However, when the substrate 107 has an insulating layer on its surface, the insulating layer 108 is not necessarily required. In addition, electrode pads (not shown) are formed on the two pairs of fixed electrodes 103A, 103B and 104A, 104B and the fixed end 109 of the spring, respectively. A voltage is applied to the fixed electrodes 103A, 103B and 104A, 104B, and the fixed end 109 of the spring is grounded.
[0008]
In this configuration, a microactuator comprising a pair of movable electrodes 101A and 101B, four springs 102 and a fixed end 109 for supporting them, and two pairs of fixed electrodes 103A and 103B and 104A and 104B is used. The wavelength-multiplexed incident signal light incident on the movable mirror 105 is controlled by driving the movable mirror 105 in the horizontal direction in the figure and controlling the gap (that is, the resonator length) between the movable mirror 105 and the fixed mirror 106. λ₁~ Λ_nAmong them, the outgoing signal light λ having a desired wavelength from the fixed mirror 106_iIs selectively transmitted. As a result, it is possible to realize a good filter characteristic with a high transmission spectrum transmittance and a narrow half-value width in a wide band. In FIG. 1, only two or three comb teeth are drawn for each fixed electrode and each movable electrode for simplification of drawing, but an arbitrary number of combs are drawn depending on the required driving force. The number of teeth is selected.
[0009]
Next, the operation of each component of the wavelength tunable filter according to the present invention will be described in order. First, a pair of movable electrodes 101A and 101B, four springs 102 supporting the movable electrodes, a fixed end 109 thereof, and two pairs of fixed electrodes 103A and 103B and 104A and 104B illustrate the movable mirror 105. The microactuator for driving in the left-right direction is configured. The pair of movable electrodes 101A, 101B and the two pairs of fixed electrodes 103A, 103B and 104A, 104B are all in a comb-like shape, and the movable electrode 101A is fixed electrodes 103A, 104A and the movable electrode 101B is fixed. The electrodes 103B and 104B are arranged so as to mesh with each other. Both the movable electrodes 101A and 101B are grounded via a spring 102 supporting each movable electrode and an electrode pad formed on the fixed end 109 thereof.
[0010]
By simultaneously applying the same voltage to the fixed electrodes 103 </ b> A and 103 </ b> B, the movable portion is driven in a direction approaching the fixed mirror 106. On the other hand, by simultaneously applying the same voltage to the fixed electrodes 104A and 104B, the movable part is driven in a direction away from the fixed mirror 106. Further, when a voltage is applied to the fixed electrodes 103A and 103B, the fixed electrodes 104A and 104B are grounded. Conversely, when a voltage is applied to the fixed electrodes 104A and 104B, the fixed electrodes 103A and 103B are grounded. The
[0011]
In order to displace the movable part to the left and right, the gap between the comb teeth of the movable electrode and the comb teeth of the fixed electrode is asymmetric on the left and right of each comb tooth except for the central comb tooth of the movable electrode. By making it asymmetrical, a stronger electrostatic attractive force acts between the comb teeth having a narrower gap, and the movable electrode is drawn in a direction in which the gap is further narrowed. In this case, if attention is paid to only one comb tooth of the fixed electrode (movable electrode), one of the comb teeth of the movable electrode (fixed electrode) on both sides is made as close to the fixed electrode (movable electrode) as possible, and the other is The sum of the electrostatic attraction becomes larger as far as possible from the fixed electrode (movable electrode). However, increasing the interval between the comb teeth reduces the number of comb teeth that can be formed within the unit interval, and acts to reduce the spatial efficiency. After all, if the electrode formation area is constant, the electrostatic force acting on one comb tooth and the unit length can be formed in order to maximize the volumetric efficiency (generated force per unit volume) of the generated force. The product with the number of comb teeth may be maximized. The volume efficiency of the generated force is maximized when the amount of any one of the comb tooth spacing of the narrower gap, the comb tooth spacing of the wider gap, the ratio of both, and the width of the comb teeth is determined. The other two quantities are also determined. Further, when the fixed electrode is considered as a reference, if the distance between the comb teeth of the two movable electrodes adjacent to the comb teeth of the fixed electrode is smaller, the movable electrode is positioned on the right side. Conversely, if the distance between the right movable electrode and the comb teeth is smaller, the movable electrode is displaced to the left.
[0012]
Instead of providing two pairs of fixed electrodes, it is possible to cover the entire stroke of the movable electrode by using only one pair of fixed electrodes. In the present embodiment, the fixed electrode is divided into the right drive and the left drive for the following reason. In other words, the comb teeth at the center of the movable electrode (in FIG. 1, the comb teeth at the center of the movable electrode 101A or the movable electrode 101B) have a narrower spacing between the comb teeth of either of the left and right fixed electrodes. Thus, it can be used for either right driving or left driving by contributing to both the left and right fixed electrodes. Therefore, it becomes advantageous from the point of volume efficiency. In particular, when the number of comb teeth of the movable electrode is small, the effect becomes relatively large.
Since the electrostatic attractive force that pulls the movable electrode 101A toward the near side of the drawing and the electrostatic attractive force that pulls the movable electrode 101B in the depth direction of the drawing cancel each other, the movable portion is not displaced in these directions.
[0013]
Next, the movable mirror 105 and the fixed mirror 106 made of a photonic crystal will be described. In recent years, a structure in which materials having different dielectric constants and therefore refractive indexes are periodically arranged with a period of one-dimensional, two-dimensional, or three-dimensional wavelength has been fabricated, and the behavior of electrons in semiconductors is similar In addition, material structures that control the behavior of light or electromagnetic waves have attracted attention. Such a structure is called a photonic crystal, and its behavior is explained using the concept of a photonic band corresponding to the energy band of electrons in a semiconductor. This photonic band has a frequency region where light or electromagnetic waves cannot have a specific mode, that is, a photonic band gap. In general, the photonic band gap opens over a wider band as the refractive index modulation of a dielectric having a periodic structure becomes stronger. Thus, since the photonic crystal essentially has optical wavelength selectivity, it can be said that the photonic crystal has a structure suitable for application to a filter having wavelength selectivity, that is, a wavelength tunable filter.
[0014]
However, it is extremely difficult to realize three-dimensional refractive index modulation having a period of the order of the optical wavelength. If a mirror composed of a microactuator and a photonic crystal is formed in a batch by micromachining (micromachine technology) represented by anisotropic etching or the like as in the present invention, it is more one-dimensional than three-dimensional photonic crystal. Alternatively, the two-dimensional photonic crystal has better process consistency. As a two-dimensional photonic crystal, for example, a structure in which cylindrical holes are arranged in a triangular lattice in a structural material has good consistency with a micromachining process, and furthermore, a Brave lattice corresponds to a hexagonal lattice. It is preferable in that it can generate a wavelength region exhibiting high reflectivity in any direction in the plane. In the present embodiment, a mirror is formed using a one-dimensional photonic crystal.
[0015]
Next, taking a fixed mirror as an example, the structure of a mirror made of a photonic crystal will be described. FIG. 2 is a plan view of a fixed mirror made of the one-dimensional photonic crystal of the present embodiment. A movable mirror made of a one-dimensional photonic crystal has the same structure.
The fixed mirror 106 made of a photonic crystal is made of a one-dimensional photonic crystal made of a periodic structure of a structural material layer 120 in which rectangular holes are formed and an air layer 121 that fills the holes. Each layer thickness d_iIs the center wavelength in vacuum of the operating band of the wavelength tunable filter, and the refractive index of each layer is n_iAs shown in equation (1).
d_i= Λ / 4n_i                      ... (1)
[0016]
As a result, the center wavelength λ of the operating band of the wavelength tunable filter becomes the maximum transmission wavelength when a resonator is configured. However, the subscript i indicates the structural material layer (S) and the air layer (A). That is, the fixed mirror 106 using the one-dimensional photonic crystal of FIG. 2 has the following periodic structure including four air layers 121.
Structural material layer (d_S= Λ / 4n_S) / Air layer (d_A= Λ / 4) /
Structural material layer (d_S= Λ / 4n_S) / Air layer (d_A= Λ / 4) /
Structural material layer (d_S= Λ / 4n_S) / Air layer (d_A= Λ / 4) /
Structural material layer (d_S= Λ / 4n_S) / Air layer (d_A= Λ / 4) /
Structural material layer (d_S= Λ / 4n_S)
However, the refractive index of air: n_A= 1. In FIG. 2, a structure including four air layers 121 is employed as described above, but it is needless to say that the number of cycles is several.
[0017]
As a characteristic required for the structural material layer 120 of the wavelength tunable filter of the present invention, first, a material capable of high aspect ratio processing by micromachining can be mentioned. Next, when a hole is formed in the structural material layer 120 to form a photonic crystal structure, the refractive index n of the structural material layer 120 is set so that the photonic band gap opens over a wide wavelength region._SIs preferably sufficiently higher than the refractive index of air. This is a characteristic indispensable for the photonic crystal to function as a broadband high reflectivity mirror. In view of the above, as the material of the structural material layer 120 of the wavelength tunable filter according to the present invention, it has been already sufficiently proven in the field of semiconductor microfabrication technology, and Si having a high refractive index is determined to be suitable. Is done. Examples of the material equivalent to Si include GaAs and InP.
[0018]
A pair of mirrors made of a photonic crystal forming a resonator functions as a perfect mirror for wavelength light in the photonic band gap. The photonic band gap opens over a wider band as the refractive index modulation of the dielectric having the periodic structure is stronger. For example, in the one-dimensional photonic crystal having a periodic structure of the structural material layer 120 and the air layer 121 as shown in FIG. 2, the structural material layer 120 is formed of a high refractive index material such as Si. Depends on the design of the periodic structure (how much the thickness of each layer is), but generally, a photonic band gap of about several hundred nm to 1000 nm is opened around the center wavelength λ. Therefore, the mirror made of the photonic crystal in the wavelength tunable filter of the present invention exhibits a high reflectance over a wide band. In a tunable filter using a Fabry-Perot resonator, the reflectance of this mirror is a very important factor, and the higher the reflectance of the mirror, the narrower the half width of the transmission spectrum can be made.
[0019]
Next, a Fabry-Perot resonator composed of a movable mirror 105 made of a photonic crystal and a fixed mirror 106 made of a photonic crystal will be described. In FIG. 1, from the left side, a plurality of incident signal lights λ wavelength-multiplexed on a movable mirror 105 integrated with the movable electrodes 101A and 101B.₁~ Λ_nIn the wavelength band within the photonic band gap, the wavelength λ satisfying the equation (2)_mOnly the signal light is resonantly transmitted in the right direction of the fixed mirror 106 to become output signal light.
λ_m= 2L / m (2)
Here, L represents a gap between the right side surface of the movable mirror 105 made of photonic crystal and the left side surface of the fixed mirror 106 made of photonic crystal, and m represents an integer. At this time, the outgoing signal light that is resonantly transmitted only propagates in the air except that it passes through the photonic crystal, and does not pass through any unnecessary substances such as a substrate. Further, since the transmission distance is as short as the light wavelength, the transmission loss can be kept extremely low. Furthermore, as described above, since the mirror exhibits a high reflectance over a wide band, the half-value width of the transmission spectrum of the resonator becomes extremely narrow.
Also, at this time, an electrode is formed on a part of a portion where light does not pass between the right side surface of the movable mirror 105 made of photonic crystal and the left side surface of the fixed mirror 106 made of photonic crystal, and the capacitance therebetween is monitored. Then, it is also possible to sense a change in the gap between the mirrors.
[0020]
Next, a method for manufacturing the wavelength tunable filter according to the present embodiment will be described. FIG. 3 is a plan view of the wavelength tunable filter according to the present embodiment, and FIGS. 4A to 5C are taken along line AA for explaining one manufacturing method of the present embodiment. FIG. 3 is a sectional view in the order of the steps (however, as in FIG. 1, a part of the comb teeth of the electrode is omitted in FIG. 3). The wavelength tunable filter according to the present embodiment is collectively formed on an SOI substrate using a micromachining technique.
First, an SOI substrate 140 having a surface index (110) in which an SOI layer 141 is formed on a Si substrate 143 via a buried oxide film 142 is prepared, and the back surface is polished to a thickness of about several tens to 100 μm. A thermal oxide film 130 having a thickness slightly smaller than the thickness of the buried oxide film 142 is formed on the back surface thereof, that is, on the surface on the Si substrate 143 side, and then a normal photolithography technique and an RIE technique are used. Then, the thermal oxide film 130 is patterned to remove the movable portion of the wavelength tunable filter, that is, the fixed end 109 of the spring, and the thermal oxide film 130 on the portions to be the spring 102, the movable electrodes 101A and 101B, and the movable mirror 105. An opening is formed [FIG. 4 (a)]. At this time, it is desirable to form the opening so as to protrude about 0.5 μm outside each side of the movable part. Next, on the surface of the SOI substrate 140, that is, on the surface on the SOI layer 141 side, the movable electrodes 101A and 101B, the spring 102, the fixed end 109 of the spring, the fixed electrodes 103A, 103B, 104A and 104B, and the structural material of the movable mirror 105 A resist film 131 is formed to cover the layer 120 and the portion to be the structural material layer 120 of the fixed mirror 106 (FIG. 4B). Next, after forming a protective resist 132 on the entire surface of the SOI substrate 140, anisotropic etching is performed on the back surface of the SOI substrate 140 using a KOH aqueous solution with the thermal oxide film 130 as a mask. As a result, a deep groove 133 can be formed at a high aspect ratio in the opening of the thermal oxide film 130, that is, the portion that becomes the movable portion of the wavelength tunable filter [FIG. 5A]. In this anisotropic etching, the buried oxide film 142 serves as an etching stopper. Next, in order to separate the bottom surface of the movable part from the substrate, the buried oxide film 142 in the groove 133 is removed. In this method, either the following wet etching or dry etching is used. When wet etching is used, etching is performed with an HF aqueous solution. In this case, as the wet etching progresses, the etching of the buried oxide film 142 proceeds toward the fixed electrode and the portion serving as the fixed end of the spring. For example, the thickness of the buried oxide film 142 is 2 μm, and the movable electrode and the fixed electrode. If the distance between and is 1.5 μm, the width of the fixed electrode is 4 μm, and the fixed end of the spring is about 6 μm □, the wet etching time is adjusted to fix the four springs that support the fixed electrode and the movable electrode. It is possible to leave an oxide film layer having a width sufficient to fix the end to the substrate. In this step, the thermal oxide film 130 on the back surface of the substrate is also removed. In the case of dry etching, C having a large selection ratio with Si₃F₈Or C₃F₈H₂The buried oxide film 142 in the trench 133 is removed by using a gas added with hydrocarbon gas or the like. Through the above steps, the bottom surface of the movable portion of the wavelength tunable filter of the present embodiment is separated from the Si substrate 143 and exposed in the air in the groove 133 [FIG. 5B].
[0021]
Next, the protective resist 132 protecting the surface of the SOI substrate 140 is removed, and RIE etching is performed on the SOI layer 141 using the exposed resist film 131 as a mask. This RIE etching is performed until the buried oxide film 142 is removed, but there is no problem even if the etching proceeds to the Si substrate 143 beyond the buried oxide film 142. Through the above steps, each component of the wavelength tunable filter of the present embodiment is formed. Finally, a resist film 131 on the surface of the SOI layer 141 is formed on the O layer.₂By removing by ashing, the manufacturing process of the present embodiment is completed (FIG. 5C). The electrode pads (not shown) for applying a voltage to the fixed electrodes 103A, 103B, 104A, 104B and the fixed end 109 of the spring are formed, for example, by forming a resist film on the surface of the SOI substrate 140 during the above-described process. Prior to the step of forming, a step of depositing Al in a region to be a pad portion is placed, and then covered with a resist film 131, and the subsequent steps described above are advanced.
[0022]
Next, the operation of the wavelength tunable filter according to the present embodiment will be specifically described.
The wavelength tunable filter of the present embodiment was prototyped using an SOI substrate 140 having a SOI layer thickness of 4 μm and a buried oxide film thickness of 2 μm. The main structural parameters of the microactuator part in the prototype wavelength tunable filter are described below. The structural material is Si. The structure is the same as the basic structure of FIG. The ratio of the narrower gap to the wider gap between the comb teeth of the movable electrode and the comb teeth of the fixed electrode was 1: 2.5. In the prototype variable wavelength filter, the number of comb teeth was increased from the number of comb teeth shown in FIG. 1 in order to increase the generated force.
[0023]
Comb length: 50 μm
Comb width: 4 μm
Number of comb teeth of movable electrodes 101A and 101B: 11 each
Number of comb teeth of fixed electrodes 103A and 103B: 6 for each
Number of comb teeth of fixed electrodes 104A and 104B: 6 each
Narrow gap between comb teeth of movable electrode and comb teeth of fixed electrode: 1.6 μm
Wide gap between the comb teeth of the movable electrode and the comb teeth of the fixed electrode: 4.0 μm
Spring constant of the spring supporting the movable electrode: 60.4 N / m
[0024]
The structure of the mirror made of the photonic crystal is a one-dimensional photonic crystal structure as shown in FIG. The periodic structure is as follows, with the center wavelength of the operating band being λ = 1.55 μm and including four air layers.
Si layer (110 nm) / Air layer (390 nm) /
Si layer (110 nm) / Air layer (390 nm) /
Si layer (110 nm) / Air layer (390 nm) /
Si layer (110 nm) / Air layer (390 nm) /
Si layer (110 nm)
The main structural parameters of the Fabry-Perot resonator section are as follows.
Structural material layer height: 4 μm
Initial gap between opposing mirrors: 6.2 μm
The wavelength tunable filter according to the present embodiment is an ultra-small element that fits in a square of about 200 μm if it is limited to the size of the chip itself, and has a structure suitable for arraying.
[0025]
When the dynamic characteristics of the microactuator part in the wavelength tunable filter of the present embodiment were examined with a microscope type laser Doppler vibrometer, the movable electrodes 101A and 101B were moved from the initial position when a voltage of 50 V was applied to the fixed electrodes 103A and 103B. It was found that when a voltage of 200 nm was applied in the direction in which the mirror interval was reduced and 50 V was applied to the fixed electrodes 104A and 104B, the movable electrodes 101A and 101B were translated 200 nm from the initial position in the direction in which the mirror interval was increased. Further, when the frequency response spectrum of the microactuator was examined by sweeping the frequency of the driving voltage, it was found that the resonance frequency was about 230 kHz.
[0026]
Next, the filter characteristics of the variable wavelength filter of the present embodiment were examined. When wavelength multiplexed light was made incident on the variable wavelength filter and the wavelength of the transmitted light was identified, the transmission center wavelength when no voltage was applied to the fixed electrode was 1.55 μm as designed. As the voltage value applied to the fixed electrodes 103A and 103B was increased, the resonator length was decreased and the transmission center wavelength was decreased, and was 1502.0 nm when 50 V was applied. On the other hand, when the voltage value applied to the fixed electrodes 104A and 104B was increased, the transmission center wavelength was increased as the resonator length was increased, and was 1597.4 nm when 50 V was applied. Further, when the wavelength spectrum of the transmitted light was examined using a wavelength tunable light source, the transmittance of the resonant transmission spectrum was 95% within a variable width of about 100 nm around 1.55 μm when no voltage was applied to the fixed electrode. The above is shown. The half width of the spectrum observed at that time was about 0.1 nm.
[0027]
Next, another method for manufacturing the wavelength tunable filter according to the first embodiment will be described with reference to FIG. The wavelength tunable filter in this embodiment is collectively formed on a single Si substrate by using the micromachining technique according to this manufacturing method. 6A to 6C are sectional views of the wavelength tunable filter according to the present embodiment in the order of steps along the line AA in the plan view shown in FIG.
First, a resist pattern 152 is formed on the surface of the Si substrate 151. Using this resist pattern 152 as a mask, a portion to be the fixed electrodes 103A, 103B, 104A, 104B, a portion to be the structural material layer 120 of the fixed mirror 106, a spring The other region is etched by several μm while leaving the portion where the fixed end 109 is formed (FIG. 6A). Next, the resist pattern is formed again, and the movable electrode 101A, 101B of the wavelength tunable filter, the four springs 102, the fixed end 109 of the spring, the fixed electrodes 103A, 103B, 104A, 104B, and the structural material layer of the movable mirror 105 Then, a resist pattern 153 is formed to cover a portion that becomes the structural material layer of the fixed mirror 106. Etching is performed by several μm to several tens of μm using the resist pattern 153 as a mask [FIG. In this etching, anisotropic etching such as RIE is used to obtain a high aspect ratio structure. Next, an SiN layer having a thickness of several tens of nm is formed at the bottom of the etched region with the resist pattern 153 remaining. This becomes an etching stop layer in a later step. Through the above steps, a structure in which the upper surface of the movable part region is lower by several μm than the upper surface of the fixed part region that will later become the bottom of the wavelength tunable filter is formed on the Si substrate.
[0028]
Next, the processed surface of the Si substrate 151 and a separately prepared glass substrate 154 are bonded using a well-known anodic bonding technique. Next, after polishing the back surface of the Si substrate 151 to a thickness of about several tens of μm, the SiN etching stop layer is etched to expose each part of the wavelength tunable filter formed by the previous etching, The manufacturing process of the wavelength tunable filter of this embodiment is completed.
[0029]
[Second Embodiment]
FIG. 7 is a plan view of a wavelength tunable filter according to the second embodiment of this invention. FIG. 7 is a layout diagram of the entire device, which is drawn based on a detailed design considering practicality. FIG. 8 is a detailed plan view of the main part of the wavelength tunable filter in FIG. 7 (region surrounded by a broken line in FIG. 7). 7 and 8, the same reference numerals as the first two digits are assigned to the same parts as those in the first embodiment shown in FIG. 1, and duplicate descriptions are omitted as appropriate. The conceptual perspective view of the present embodiment is the same as that of the first embodiment shown in FIG.
As shown in FIGS. 7 and 8, the wavelength tunable filter according to the present embodiment supports a pair of movable electrodes 201A and 201B and the movable electrodes, similarly to the wavelength tunable filter according to the first embodiment. Four springs 202, fixed ends 209 provided at respective ends of the four springs, two pairs of fixed electrodes 203A, 203B and 204A, 204B, a movable mirror 205 made of a photonic crystal, And a fixed mirror 206 made of a photonic crystal located at a position facing the movable mirror 205. Two pairs of fixed electrodes 203A, 203B and 204A, 204B and a fixed end 209 of the spring are respectively connected to the electrode take-out portion 220.₁, 220₂Is formed, and the electrode extraction portion 220 is formed.₁A drive voltage is applied to the two pairs of fixed electrodes 203A, 203B and 204A, 204B via the electrode take-out portion 220.₂The fixed end 209 of the spring is grounded via.
[0030]
As shown in FIG. 8, the wavelength tunable filter according to the present embodiment includes the comb teeth of the movable electrodes 201A and 201B and the fixed electrodes 203A and 203B when a large external force is applied or an excessive voltage is applied to the electrodes. A stopper 221 is provided to prevent a collision with the comb teeth of 204A and 204B or a collision between the movable mirror 205 and the fixed mirror 206. In addition, the electrode take-out unit 220 can monitor the displacement of the movable electrode.₃A gap 222 between the end of each of the electrodes and the ends of the movable electrodes 201A and 201B forms a capacitive displacement sensor. In the present embodiment, in order to allow collimated light to enter / exit the resonator formed by the movable mirror 205 and the fixed mirror 206, a diameter of 125 μm is provided on each of the light incident side and the light emitting side. The collimation fibers are arranged so as to face each other with the resonator interposed therebetween. Therefore, the wavelength tunable filter of this embodiment is provided with a deep groove (fiber insertion groove) for inserting a collimation fiber. 7 and 8, the number of comb teeth of each fixed electrode is six, and the number of comb teeth of each movable electrode is eleven.
[0031]
In this configuration, as in the first embodiment, a pair of movable electrodes 201A and 201B, four springs 202 and a fixed end 209 that support them, and two pairs of fixed electrodes 203A, 203B and 204A, The movable mirror 205 is driven by moving the movable mirror 205 in the left-right direction in the figure using a microactuator composed of 204B and controlling the gap (that is, the resonator length) between the movable mirror 205 and the fixed mirror 206. Outgoing signal light having a desired wavelength is selectively transmitted from the fixed mirror 206 among the wavelength-multiplexed incident signal light incident on the light. As a result, it is possible to realize good filter characteristics with a high transmission spectrum transmittance and a narrow bandwidth in a wide band.
[0032]
The function of each component of the wavelength tunable filter according to the present embodiment is the same as that of the first embodiment, but the left driving fixed electrode pair 204A, 204B and the right driving fixed electrode pair 203A, The voltage applied to 203B is V, respectively.₊, V₋The bias voltage V is constant.₀And a variable voltage control voltage V, V₊= V₀+ V, V₋= V₀Displacement response to the applied voltage by driving the movable electrode by differentially applying a voltage to −V, the left driving fixed electrode pair 204A, 204B and the right driving fixed electrode pair 203A, 203B. It is also possible to improve the linearity of. The effect becomes more prominent when the displacement is small.
[0033]
Next, the structure of the mirror in the present embodiment will be described. The mirror made of a photonic crystal is composed of a one-dimensional photonic crystal having a periodic structure of a structural material layer in which rectangular holes are formed, and an air layer filling the holes, together with the movable mirror 205 and the fixed mirror 206. . Layer thickness d of each structural material layer and air layer_SAnd d_AIs the refractive index of each layer n_SAnd n_AThe center wavelength in vacuum of the operating band of the wavelength tunable filter is set as λ, and is determined as shown in Equation (3)
d_i= Uλ / 4n_i                      ... (3)
Here, u is an odd number, and may take a different value in each layer. The subscript i indicates a structural material layer (i = S) and an air layer (i = A).
[0034]
As a result, a photonic band gap with λ as the center wavelength is formed, and the one-dimensional photonic crystal functions as a high reflectance mirror in the operating band of the wavelength tunable filter. The larger the value of u, the easier the fabrication, but the wavelength band showing the photonic band gap becomes narrower. The movable mirror 205 and the fixed mirror 206 using the one-dimensional photonic crystal in the present embodiment have the following periodic structure including four air layers.
Structural material layer (d_S= U_Sλ / 4n_S) / Air layer (d_A= U_Aλ / 4) /
Structural material layer (d_S= U_Sλ / 4n_S) / Air layer (d_A= U_Aλ / 4) /
Structural material layer (d_S= U_Sλ / 4n_S) / Air layer (d_A= U_Aλ / 4) /
Structural material layer (d_S= U_Sλ / 4n_S) / Air layer (d_A= U_Aλ / 4) /
Structural material layer (d_S= U_Sλ / 4n_S)
However, the refractive index of air: n_A= 1. In the present embodiment, as described above, a structure including four air layers is employed, but it is needless to say that the period may be several.
As the structural material layer of the wavelength tunable filter according to the present invention, as in the first embodiment, Si having already a sufficient record in the field of semiconductor microfabrication technology and having a high refractive index was used. .
[0035]
Next, a method for manufacturing the wavelength tunable filter according to the present embodiment will be described. FIG. 9A to FIG. 11D are cross-sectional views in the order of steps for explaining the manufacturing method of the present embodiment. FIG. 9A to FIG. 11D show the cross section taken along line BB in FIG. The wavelength tunable filter of this embodiment is formed in a lump by a surface process using a two-mask using an SOI substrate as shown below. With the first mask, patterning is performed on a resonator composed of a one-dimensional photonic crystal mirror, which is a component of the wavelength tunable filter of the present embodiment, and an electrostatically driven microactuator. Further, in the second mask, patterning of a fiber insertion groove for arranging two collimation fibers having a diameter of 125 μm so as to sandwich the resonator on the light incident side and the light emitting side is performed.
[0036]
First, a P-type SOI substrate 240 having a surface index (110) in which an SOI layer 241 is formed on a Si substrate 243 via a buried oxide film 242 is prepared [FIG. 9A], and a concentration of 10 is formed on the wafer surface.²⁰cm^-3A boron diffusion layer 244 having the same degree is formed (FIG. 9B). Next, a thermal oxide film 245 is formed on the wafer surface (FIG. 9C). Next, photolithography is performed using a first mask on which a resonator composed of a one-dimensional photonic crystal mirror and an electrostatically driven microactuator are patterned, and a first resist layer 246 is formed on the thermal oxide film 245. Patterning is performed (FIG. 9D). At this time, the mask and the SOI wafer 240 are aligned so that the mirror surfaces of the movable mirror 205 and fixed mirror 206 of the wavelength tunable filter coincide with the (111) plane of the SOI layer 241. Next, the thermal oxide film 245 is patterned by RIE (Reactive Ion Etching) technique [FIG. 10A], and then the first resist layer 246 is removed [FIG. 10B]. Next, the second resist layer 247 is thickly coated on the thermal oxide film 245, and the second resist layer 247 is patterned by photolithography using a second mask in which the fiber insertion groove is patterned. [FIG. 10 (c)]. Next, etching of the SOI layer 241 [FIG. 10D], etching of the buried oxide film 242 [FIG. 10E], and etching of the Si substrate 243 [FIG. 11A] are sequentially performed by the RIE technique. A shallow fiber insertion groove is formed. Next, after removing the second resist layer 247 (FIG. 11B), the SOI layer 241 is etched by the RIE technique using the thermal oxide film 245 as a mask, and then the movable surface functioning as a mirror surface by the KOH aqueous solution. The side walls of the mirror 205 and the fixed mirror 206 are further flattened (FIG. 11C). The principle of this side wall flattening is as follows. The (111) plane of the SOI layer 241 aligned to coincide with the sidewall functioning as the mirror plane has a very low etching rate with respect to the KOH aqueous solution. On the other hand, the unevenness present on the sidewall has a high etching rate with respect to the aqueous KOH solution because various crystal faces are exposed. The difference in etching rate between the two significantly improves the flatness of the side wall. Finally, the thermal oxide film 245 and the buried oxide film 242 in the movable part are removed with an aqueous HF solution (FIG. 11D), the movable part is released, and the manufacturing process of the wavelength tunable filter of this embodiment is completed. Complete.
In this embodiment, the metal electrode pad is not formed. However, the metal pad can be formed by depositing a metal film through a metal mask after the step of FIG. . Further, as shown in FIG. 9B, after forming a boron diffusion layer on the SOI layer surface, a metal electrode pad is formed by a lift-off method or a deposition / photo-etching method, and an oxide film serving as an etching mask is formed thereon. May be formed by a CVD method. Alternatively, in the step shown in FIG. 9C, instead of forming the thermal oxide film 245, a metal film to be an electrode pad is deposited, and the SOI layer and the Si substrate are etched using this as a mask. Also good.
[0037]
As described above, each component of the wavelength tunable filter according to the present embodiment is formed. When removing the buried oxide film 242, as shown in FIG. 8, it should be fixed to the Si substrate 243 via the buried oxide film 242 such as the fixed electrodes 203A, 203B, 204A, 204B, the fixed mirror 206, and the fixed end 209. The portion and the movable electrodes 201A, 201B and the spring 202 to be completely released from the Si substrate 243 have a considerable difference in the width and area of the structure. If adjusted appropriately, only the movable electrodes 201A and 201B and the spring 202 can be released from the Si substrate 243. If the widths of the comb teeth of the fixed electrode and the movable electrode are equal, the comb teeth portion of the fixed electrode is released together with the release of the movable electrode from the substrate. However, as shown in FIG. If the rigidity is increased by making it wider than the width, even if a voltage is applied to the fixed electrode and an electrostatic force is applied between the comb teeth of the fixed electrode and the comb teeth of the movable electrode, the spring only deforms. Therefore, there is no problem in releasing the comb teeth of the fixed electrode from the substrate.
[0038]
Next, the operation of the wavelength tunable filter according to the present embodiment will be specifically described.
The wavelength tunable filter of the present embodiment was prototyped using an SOI substrate 240 having a SOI layer thickness of 40 μm and a buried oxide film thickness of 2 μm. The main structural parameters of the microactuator part in the prototype wavelength tunable filter are described below. The structural material is Si. The structure is the same as that shown in FIGS. The ratio of the narrower gap to the wider gap between the movable electrode comb teeth and the fixed electrode comb teeth was 1: 2.8.
[0039]
The length of the part where the comb teeth face each other: 50 μm
Comb width: 4 μm
Number of comb teeth of movable electrodes 201A and 201B: 11 each
Number of comb teeth of fixed electrodes 203A and 203B: 6 each
Number of comb teeth of fixed electrodes 204A and 204B: 6 for each
Narrow gap between comb teeth of movable electrode and comb teeth of fixed electrode: 1.5 μm
The wide gap between the comb teeth of the movable electrode and the comb teeth of the fixed electrode: 4.2 μm
Spring constant of the spring supporting the movable electrode: 218 N / m
[0040]
The structure of the mirror made of the photonic crystal is a one-dimensional photonic crystal structure as shown in FIG. The periodic structure is as follows, with the center wavelength of the operating band being λ = 1.55 μm and including four air layers.
Si layer (996 nm) / Air layer (1163 nm) /
Si layer (996 nm) / Air layer (1163 nm) /
Si layer (996 nm) / Air layer (1163 nm) /
Si layer (996 nm) / Air layer (1163 nm) /
Si layer (996 nm)
The main structural parameters of the Fabry-Perot resonator section are as follows.
Structural material layer height: 40 μm
Initial gap between opposing mirrors: 1.55 μm
The wavelength tunable filter according to the present embodiment is a microminiature element that fits in a square of about 1 mm if it is limited to the size of the chip itself, and has a structure suitable for arraying.
[0041]
When the dynamic characteristics of the microactuator part in the wavelength tunable filter of the present embodiment were examined with a microscope type laser Doppler vibrometer, the movable electrodes 201A and 201B were in the initial positions when a voltage of about 28 V was applied to the fixed electrodes 203A and 203B. From the above, it was found that the movable electrodes 201A and 201B were translated by 200 nm from the initial position in the direction of increasing the mirror interval when a voltage of 200 nm was applied in the direction of reducing the mirror interval and 28V was applied to the fixed electrodes 204A and 204B. Further, when the frequency response spectrum of the microactuator was examined by sweeping the frequency of the drive voltage, it was found that the resonance frequency was about 60 kHz.
[0042]
Next, the spectral characteristics of the wavelength tunable filter of the present embodiment were examined. In the optical system at the time of spectral characteristic evaluation, one collimation fiber having a diameter of 125 μm was placed on the light incident side and the light emission side so as to sandwich the resonator. The beam diameter at the beam waist of the collimation fiber is about 20 μm, and the working distance is 300 μm. Under the above optical system, the wavelength of the light incident on the tunable filter was swept to measure the spectral characteristics, and the center wavelength of the transmission spectrum when no voltage was applied to the fixed electrode was as designed. 1.55 μm. As the voltage value applied to the fixed electrodes 203A and 203B was increased, the resonator length decreased and the center wavelength of the transmission spectrum was shortened, and was about 1475 nm when 28 V was applied. On the other hand, when the voltage value applied to the fixed electrodes 204A and 204B was increased, the center wavelength of the transmission spectrum became longer as the resonator length increased, and was about 1605 nm when 28V was applied. Further, in the wavelength variable band from 1475 nm to 1605 nm, the transmission loss of the transmission spectrum was within −0.8 dB, and the half width was 0.06 nm or less. The bandwidth (−25 dB bandwidth) at a level where the gain was reduced by 25 dB from the peak of the transmission spectrum was 0.8 nm or less.
[0043]
[Third Embodiment]
FIG. 12 is a perspective view of a wavelength tunable filter according to the third embodiment of the present invention. FIG. 13 is a plan view of a wavelength tunable filter according to the third embodiment. Here, FIG. 12 is a conceptual diagram briefly showing only the minimum components required in the wavelength tunable filter of the present embodiment, and FIG. 13 is drawn based on a detailed design considering practicality. It is a device layout diagram.
As shown in FIG. 13, the wavelength tunable filter in the present embodiment is horizontally symmetric with respect to the CC line, and the left half structure is the same as that of the wavelength tunable filter of the second embodiment shown in FIG. 8. It is the same as the structure. 12 and 13, the same reference numerals in the last two digits are attached to the same components as those in the first and second embodiments shown in FIGS. 1 and 8, and the right half of the components Is denoted by “′” after the reference symbol, and redundant description is omitted. The fixed mirror 306 is common to the left and right. In FIG. 12, the substrate and the insulating layer are omitted, and in FIG. 13, the electrode pad portion is omitted.
[0044]
The wavelength tunable filter in the present embodiment has a structure in which the movable mirrors 305 and 305 'are opposed to each other on both the left and right sides of the fixed mirror 306 made of a photonic crystal, that is, a structure having two cavities. In a Fabry-Perot resonator, a structure having only one cavity as in the first and second embodiments is called a single cavity structure, whereas a structure having a plurality of cavities as in this embodiment is Although referred to as a multi-cavity structure, by adopting this multi-cavity structure, the rectangularity of the transmission spectrum can be remarkably improved as compared with the single-cavity structure. Improving the rectangularity of the transmission spectrum is beneficial in the wavelength division multiplexing (WDM) optical transmission system, because the bandwidth per channel can be increased while suppressing crosstalk between adjacent channels in the wavelength region.
The wavelength tunable filter of the present embodiment can be manufactured by the same manufacturing method as the wavelength tunable filter of the second embodiment.
[0045]
As shown in FIG. 13, the mirror made of a photonic crystal has a one-dimensional photonic crystal structure as in the case of the second embodiment. The periodic structures of the movable mirrors 305 and 305 ′ are exactly the same as those of the second embodiment, and include the following four air layers so that the center wavelength of the operating band is λ = 1.55 μm. The structure was
Si layer (996 nm) / Air layer (1163 nm) /
Si layer (996 nm) / Air layer (1163 nm) /
Si layer (996 nm) / Air layer (1163 nm) /
Si layer (996 nm) / Air layer (1163 nm) /
Si layer (996 nm)
[0046]
In addition, the periodic structure of the fixed mirror 306 has the following structure including nine air layers so that the center wavelength of the operating band is λ = 1.55 μm.
Si layer (996 nm) / Air layer (1163 nm) /
Si layer (996 nm) / Air layer (1163 nm) /
Si layer (996 nm) / Air layer (1163 nm) /
Si layer (996 nm) / Air layer (1163 nm) /
Si layer (996 nm) / Air layer (1163 nm) /
Si layer (996 nm) / Air layer (1163 nm) /
Si layer (996 nm) / Air layer (1163 nm) /
Si layer (996 nm) / Air layer (1163 nm) /
Si layer (996 nm) / Air layer (1163 nm) /
Si layer (996 nm)
[0047]
The main structural parameters of the resonator section are as follows.
Structural material layer height: 40 μm
Initial gap between opposing mirrors: 1.55 μm
[0048]
When the dynamic characteristics of the microactuator part in the wavelength tunable filter of the present embodiment were examined using a microscope type laser Doppler vibrometer, the same dynamic characteristics as in the case of the second embodiment were obtained. First, when a voltage of about 28V is applied to the fixed electrodes 303A and 303B, the movable electrodes 301A and 301B are 200 nm in a direction in which the mirror interval is reduced from the initial position, and when a voltage of 28V is applied to the fixed electrodes 304A and 304B, 301B was found to translate 200 nm from the initial position in the direction in which the mirror spacing increases. Similarly, when a voltage of about 28 V is applied to the fixed electrodes 303A ′ and 303B ′, the movable electrodes 301A ′ and 301B ′ have a voltage of 200 nm in the direction in which the mirror interval is reduced from the initial position, and a voltage of 28 V is applied to the fixed electrodes 304A ′ and 304B ′. When applied, the movable electrodes 301A ′ and 301B ′ were found to translate 200 nm from the initial position in the direction in which the mirror spacing increases. Further, when the frequency response spectrum of the microactuator was examined by sweeping the frequency of the drive voltage, it was found that the main resonance was about 60 kHz for both the movable mirrors 305 and 305 '.
[0049]
Next, using the same optical system as in the second embodiment, the spectral characteristics of the wavelength tunable filter of this embodiment were examined. When spectral characteristics were measured by sweeping the wavelength of light incident on the wavelength tunable filter, the center wavelength of the transmission spectrum in a state where no voltage was applied to the fixed electrode was 1.55 μm as designed. As the voltage value applied to the fixed electrodes 303A, 303B, 303A ', and 303B' is increased, the resonator length is decreased and the center wavelength of the transmission spectrum is shortened, which is about 1475 nm when 28V is applied. On the other hand, when the voltage value applied to the fixed electrodes 304A, 304B, 304A ′, 304B ′ is increased, the center wavelength of the transmission spectrum increases with an increase in the resonator length, which is about 1605 nm when 28V is applied. . In the wavelength tunable band from 1475 nm to 1605 nm, the transmission loss of the transmission spectrum was within −1.5 dB, and the half width was 0.06 nm or less. Further, the -25 dB bandwidth was 0.4 nm or less. When the -25 dB bandwidth / half-width ratio is compared between the case of this embodiment and the case of the second embodiment, the case of this embodiment is about half that of the case of the second embodiment. It is. Therefore, the wavelength tunable filter in the present embodiment has a rectangular transmission spectrum compared to the wavelength tunable filter of the single cavity resonator in the second embodiment because the resonator structure is a double cavity structure. Is clearly improved.
[0050]
[Fourth Embodiment]
FIG. 14 is a perspective view of a wavelength tunable laser according to the fourth embodiment of the present invention. FIG. 14 briefly shows only the minimum components necessary for the wavelength tunable laser according to the present embodiment. As shown in FIG. 14, the tunable laser according to the present embodiment has a structure in which a laser chip 461 is mounted instead of the fixed mirror of the tunable filter according to the first or second embodiment. 14, components equivalent to those in the first embodiment shown in FIG. 1 are given the same reference numerals in the last two digits, and redundant description is omitted.
The laser chip 461 and the movable mirror 405 constitute a Fabry-Perot laser, and the resonator length is about 300 μm. An antireflection film is formed on the cleavage surface of the laser chip 461 facing the movable mirror 405, and the laser chip 461 is mounted at a position separated by about 3 μm from the movable mirror 405. With such a configuration, a wavelength tunable laser using the movable mirror 405 as an external resonator can be obtained.
[0051]
Except for the laser chip 461, the wavelength tunable laser according to the present embodiment is manufactured in the same manner as the wavelength tunable filter according to the second embodiment in both the resonator length variable mechanism including the external resonance mechanism and the electrostatic drive microactuator It can be produced by a method. The periodic structure of the movable mirror 405 made of a photonic crystal is exactly the same as that of the second and third embodiments including four air layers so that the center wavelength of the operating band is λ = 1.55 μm. is there. The height of the structural material layer and the width of the portion of the movable mirror 405 where the one-dimensional photonic crystal is formed are 40 μm.
[0052]
When the dynamic characteristics of the microactuator part in the wavelength tunable filter of the present embodiment were examined using a microscope type laser Doppler vibrometer, the same dynamic characteristics as in the case of the second embodiment were obtained. When a voltage of about 28V is applied to the fixed electrodes 403A and 403B, the movable electrodes 401A and 401B apply 200 nm from the initial position in a direction in which the distance between the movable mirror 405 and the laser chip 461 decreases, and a voltage of 28V is applied to the fixed electrodes 404A and 404B. At that time, it was found that the movable electrodes 401A and 401B translate 200 nm from the initial position in the direction in which the distance between the movable mirror 405 and the laser chip 461 increases. Further, when the frequency response spectrum of the microactuator was examined by sweeping the frequency of the drive voltage, it was found that the main resonance of the movable mirror 405 was about 60 kHz.
[0053]
Simultaneously with the evaluation of the dynamic characteristics of the microactuator portion described above, an operating current of 25 mA was passed through the laser chip 461, and the oscillation characteristics of the wavelength tunable laser of this embodiment were examined. As a result, when focusing on a specific oscillation mode among the longitudinal modes of the Fabry-Perot laser, it becomes clear that the oscillation wavelength shifts by about 0.4 nm when the movable mirror 405 is moved ± 200 nm as described above. It was.
The fourth embodiment is a combination of the second embodiment and the laser chip. However, a laser chip may be arranged instead of the fixed mirror of the third embodiment. . In this case, antireflection films are formed on both end faces of the laser chip.
[0054]
Although the present invention has been described based on the preferred embodiments, the wavelength tunable filter and the wavelength tunable laser of the present invention are not limited to the above-described embodiments, and the gist of the present invention is changed. Wavelength tunable filters in which various changes are made within the range not included are also included in the scope of the present invention. For example, the number of comb teeth of the movable electrode and the fixed electrode can be changed according to the required electrostatic attraction. The four springs that support the movable electrode are not limited to leaf springs, but may be springs having a multiple-folded structure in which the spring constant is reduced when it is desired to increase the amount of displacement of the movable electrode. The structural material is not limited to Si, and a material having a high refractive index such as GaAs or InP can be used. The structure of the mirror is not limited to the one-dimensional photonic crystal, and a two-dimensional photonic crystal structure in which cylindrical holes are arranged in a triangular lattice arrangement or a tetragonal lattice arrangement in the structural material may be used. Furthermore, the thermal oxide film used as the etching mask is not limited to the thermal oxide film, but may be a silicon nitride film or a laminate of a silicon oxide film and a silicon nitride film.
[0055]
【The invention's effect】
As described above, in the wavelength tunable filter according to the present invention, (1) the mirror surfaces of a pair of opposing mirrors constituting the resonator of the Fabry-Perot resonator type tunable filter are formed perpendicular to the substrate. The mirror is a photonic crystal composed of a high-refractive index semiconductor and a periodic structure of air. (2) Characteristically, there is no unnecessary substance such as a substrate other than the mirror composed of the photonic crystal in the light propagation path. Therefore, according to the present invention, it is possible to realize a wavelength tunable filter that exhibits good filter characteristics with a high transmission spectrum in a wide band and a narrow bandwidth.
[0056]
Further, the wavelength tunable filter and the wavelength tunable laser according to the present invention are: (4) Fabry-Perot resonance, wherein the mirror constituting the resonator has a one-dimensional photonic crystal structure composed of a high refractive index semiconductor and a periodic structure of air. Since the resonator length variable mechanism is composed of an electrostatically driven microactuator, (5) the microactuator structural material is the same high refractive index semiconductor as the photonic crystal structural material. The cavity length variable mechanism and the photonic crystal structure can be collectively formed by micromachining based on semiconductor microfabrication technology. Therefore, according to the present invention, it is possible to realize a small and inexpensive wavelength tunable filter and wavelength tunable laser.
[Brief description of the drawings]
FIG. 1 is a perspective view of a wavelength tunable filter according to a first embodiment of this invention.
FIG. 2 is a plan view of the fixed mirror according to the first embodiment of the present invention.
FIG. 3 is a plan view of the wavelength tunable mirror according to the first embodiment of this invention.
FIG. 4 is a partial cross-sectional view in the order of steps for explaining the manufacturing method according to the first embodiment of the present invention.
5 is a cross-sectional view in order of steps in the step that follows the step of FIG. 4, for describing one manufacturing method of the first embodiment of the present invention. FIG.
FIG. 6 is a cross-sectional view in order of steps for explaining another manufacturing method of the first embodiment of the present invention.
FIG. 7 is a plan view of a wavelength tunable filter according to a second embodiment of this invention.
8 is a plan view of a main part of the wavelength tunable filter of FIG.
FIG. 9 is a partial cross-sectional view in order of the steps for explaining the manufacturing method according to the second embodiment of the present invention.
FIG. 10 is a partial cross-sectional view in order of the steps in the step subsequent to the step in FIG. 9 for describing the manufacturing method according to the second embodiment of the present invention.
11 is a cross-sectional view in order of the steps, following the step in FIG. 10, for explaining the manufacturing method according to the second embodiment of the present invention;
FIG. 12 is a perspective view of a wavelength tunable filter according to a third embodiment of the present invention.
13 is a plan view of the wavelength tunable filter in FIG. 12. FIG.
FIG. 14 is a perspective view of a wavelength tunable laser according to a fourth embodiment of the present invention.
FIG. 15 is a cross-sectional view of a conventional wavelength tunable filter.
[Explanation of symbols]
101A, 101B, 201A, 201B, 301A, 301B, 301A ', 301B', 401A, 401B Movable electrode
102, 202, 302, 302 ', 402 Four springs supporting the movable electrode
103A, 103B, 104A, 104B, 203A, 203B, 204A, 204B, 303A, 303B, 304A, 304B, 303A ', 303B', 304A ', 304B', 403A, 403B, 404A, 404B Fixed electrode
105, 205, 305, 305 ', 405 Movable mirror made of photonic crystal
106, 206, 306 Fixed mirror made of photonic crystal
107 substrate
108 Insulation layer
109, 209, 309, 309 ', 409 Fixed ends of four springs that support the movable electrode
120 Structural material layer
121 Air layer
130 Thermal oxide film
131 resist film
132 Protective resist
133 groove
140, 240 SOI substrate
141, 241 SOI layer
142, 242 buried oxide film
143, 243 Si substrate
151 Silicon substrate
152, 153 resist pattern
154 glass substrate
220₁, 220₂, 220₃  Electrode extraction part
221, 321, 321 'stopper
222, 322, 322 'spacing
244 Boron diffusion layer
245 Thermal oxide film
246 First resist layer
247 Second resist layer
461 Laser chip
500 Optical waveguide substrate
501, 502 Optical waveguide
503 Diaphragm type micromachine
504 body
505 Diaphragm
506, 507 electrode
508, 509 half mirror
510, 511 Optical fiber

Claims (16)

In a wavelength tunable device having a resonator formed of a pair of mirrors arranged opposite to each other in parallel and an interval variable mechanism that changes a resonator length by changing a gap between the pair of mirrors,
At least one of the pair of mirrors and the variable spacing mechanism are integrally formed on the substrate, and the mirror surface of the mirror is formed perpendicular to the substrate surface of the substrate,
The interval variable mechanism is configured by an electrostatically driven microactuator, and includes a fixed electrode fixed on a substrate, and a movable electrode integrally formed with any of the mirrors,
The fixed electrode and the movable electrode are formed in a comb-teeth shape, and the distance from each comb tooth of the fixed electrode to the two comb teeth of the movable electrode adjacent to both sides of each comb tooth ( (Hereinafter referred to as “inter-gap gap dimension” ) is different when no electric field is applied between the fixed electrode and the movable electrode, and an electric field is applied between the fixed electrode and the movable electrode. The movable electrode moves in a direction in which the narrow inter-gap gap dimension is narrower than the narrow inter-gap gap dimension when no electric field is applied ( hereinafter referred to as the `` minimum inter-gap gap dimension '' ). A tunable device.   2. The photonic crystal according to claim 1, wherein the mirror formed on the substrate is a photonic crystal composed of a semiconductor and an air periodic structure having a photonic band gap in a desired wavelength band with respect to incident light. Tunable device.   The wavelength variable device according to claim 1, wherein the interval variable mechanism is configured by a micromachine. The fixed electrode is composed of two types of fixed electrodes that move the movable electrode in two different directions (hereinafter referred to as “positive direction driving fixed electrode” and “negative direction driving fixed electrode”). The wavelength tunable device according to claim 1. The comb teeth at the center of the movable electrode are sandwiched between the comb teeth at the fixed electrode end for positive direction driving and the comb teeth at the fixed electrode end for negative direction driving, and the comb teeth at the center of the movable electrode When the electric field between the fixed electrode and the movable electrode is not applied, the inter-gap gap dimension between the positive-direction driving fixed electrode end and the negative-direction driving fixed electrode end comb-tooth is both 5. The wavelength tunable device according to claim 4, which has a minimum inter-gap gap dimension. Constant bias voltage V o Between the positive electrode driving fixed electrode, the negative electrode driving fixed electrode, and the movable electrode, respectively, using a bias voltage source that outputs a variable voltage and a control voltage source that outputs a variable voltage control voltage V, respectively. , V o + V, V o 6. The wavelength tunable device according to claim 4, wherein a voltage of −V is applied. The wavelength tunable device according to claim 1, wherein the movable electrode is supported on the substrate by a fixed end of a spring that supports the movable electrode. 8. The substrate according to claim 1, wherein the substrate is formed of a semiconductor, and both the movable electrode and the fixed electrode are formed of a semiconductor layer formed on the substrate via an insulating layer. The wavelength tunable device according to any one of the above. The wavelength tunable device according to claim 8, wherein the insulating layer under the movable electrode and the movable mirror is removed. 10. The wavelength tunable device according to claim 8, wherein the substrate, the insulating layer, and the semiconductor layer constitute an SOI substrate having the semiconductor layer as an SOI layer. 11. The semiconductor device according to claim 8, wherein the semiconductor layer is a silicon layer having a (110) plane as a surface, and a mirror surface of the resonator is a (111) plane of the silicon layer. Tunable device. 11. The wavelength tunable device according to claim 1, further comprising a capacitance type displacement sensor that detects displacement of the movable electrode. A wavelength tunable device having a resonator comprising a pair of mirrors arranged in parallel and a spacing variable mechanism for changing the length of the resonator by changing a gap between the pair of mirrors. In the vice
  At least one of the pair of mirrors and the variable spacing mechanism are integrally formed on the substrate, and the mirror surface of the mirror is formed perpendicular to the substrate surface of the substrate,
  Both of the paired mirrors are photonic crystals composed of a periodic structure of semiconductor and air having a photonic band gap in a desired wavelength band with respect to incident light,
  2. A wavelength tunable device, wherein a plurality of resonators connected to the interval variable mechanism are arranged in series in a light propagation path. The wavelength tunable device according to claim 13, wherein a fixed mirror is used in common, and mirrors each having the interval varying mechanism are arranged in front and behind the fixed mirror. One of the paired mirrors has the interval variable mechanism and the other is a semiconductor chip having an optical amplification function, and the mirror having the interval variable mechanism functions as an external resonator of a laser. Item 13. The wavelength tunable device according to any one of Items 1 to 12. In a wavelength tunable device having a resonator formed of a pair of mirrors arranged opposite to each other in parallel and an interval variable mechanism that changes a resonator length by changing a gap between the pair of mirrors,
At least one of the paired mirrors and the interval variable mechanism are both integrally formed on the substrate, and the mirror surface of the mirror is formed perpendicular to the substrate surface of the substrate,
Both of the paired mirrors have the above-described variable distance mechanism, and one-dimensional or two-dimensional structures composed of a periodic structure of a semiconductor and air having a photonic band gap in a desired wavelength band with respect to light incident on both of the mirrors. A wavelength tunable device comprising a semiconductor chip which is a photonic crystal and has an optical amplification function between the pair of mirrors, and the pair of mirrors functions as an external resonator of a laser.

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