JP6928824B2 - Optical waveguide structure - Google Patents

Description

The present invention relates to an optical waveguide structure.

In a semiconductor laser element having an optical waveguide structure, a technique is known in which the refractive index of the optical waveguide layer or the diffraction grating layer constituting the optical waveguide structure is changed by heating with a heater to change the laser oscillation wavelength. (For example, Patent Document 1). Patent Documents 2 to 4 disclose techniques for providing a layer or region having low thermal conductivity between an optical waveguide layer and a substrate in order to increase the efficiency of heating by a heater.

International Publication No. 2016/152274 Japanese Unexamined Patent Publication No. 2015-12176 Japanese Patent No. 5303580 U.S. Pat. No. 8,236,589

As disclosed in Patent Documents 2 to 4, it is required to improve the efficiency of heating by the heater. However, when the layer or region having low thermal conductivity is a dielectric layer such as an oxide layer of a semiconductor or a hollow region filled with a gas such as air, the difference in refractive index from the semiconductor is large. Therefore, when these layers and regions are used, the structural design is restricted, for example, they must be provided at a location away from the optical waveguide layer so as not to affect the optical waveguide characteristics of the optical waveguide structure. In some cases.

The present invention has been made in view of the above, and an object of the present invention is to provide an optical waveguide structure in which the heating efficiency by a heater is improved and the structural design is less restricted.

In order to solve the above-mentioned problems and achieve the object, the optical waveguide structure according to one aspect of the present invention includes a substrate, a lower clad layer located on the substrate, and an optical waveguide located on the lower clad layer. A layer, an upper clad layer located on the optical waveguide layer, a heater located on the upper clad layer, and a semiconductor portion located between the substrate and the optical waveguide layer and made of different semiconductor materials. It is characterized by including a sedimentary structure that is deposited so as to be adjacent to each other in the sedimentary direction.

In the optical waveguide structure according to one aspect of the present invention, the semiconductor portion is a semiconductor layer, and the sedimentary structure portion includes a structure in which the semiconductor layers made of different semiconductor materials are alternately laminated, and a total of three or more layers are laminated. It is characterized by being.

The optical waveguide structure according to one aspect of the present invention is characterized in that the thickness of each semiconductor layer is 100 nm or less.

The optical waveguide structure according to one aspect of the present invention is characterized in that the thickness of each semiconductor layer is 50 nm or less.

The optical waveguide structure according to one aspect of the present invention is characterized in that the substrate is made of InP.

The optical waveguide structure according to one aspect of the present invention is characterized in that the semiconductor materials different from each other are two or more selected from InP, GaInAs, GaInAsP, AlInAs, and AlInGaAs.

The optical waveguide structure according to one aspect of the present invention is characterized in that the semiconductor portion forms an undulating interface.

In the optical waveguide structure according to one aspect of the present invention, the semiconductor materials different from each other are one or more selected from InGaAs and InAs, and one or more selected from InP, GaInAs, GaInAsP, AlInAs, and AlInGaAs. It is characterized by being.

The optical waveguide structure according to one aspect of the present invention is further provided with a diffraction grating layer located on the upper clad layer side or the lower clad layer side with respect to the optical waveguide layer.

The optical waveguide structure according to one aspect of the present invention is characterized in that the optical waveguide layer is an active core layer and further includes an electrode for injecting an electric current into the active core layer.

According to the present invention, it is possible to increase the heating efficiency by the heater and to realize an optical waveguide structure with less restrictions on the structural design.

FIG. 1 is a schematic view showing a configuration of a tunable laser device including an optical waveguide structure according to the first embodiment. FIG. 2 is a diagram showing a cross section taken along the line AA and a cross section taken along the line BB of FIG. FIG. 3 is a diagram showing the structure of the sedimentary structure of FIG. FIG. 4 is a schematic view showing the configuration of a tunable laser device including the optical waveguide structure according to the second embodiment. FIG. 5 is a diagram showing a cross section taken along the line CC and a cross section taken along the DD line of FIG. FIG. 6 is a diagram showing the structure of the sedimentary structure of FIG. FIG. 7 is a diagram showing a modified example of the optical waveguide structure.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. The present invention is not limited to this embodiment. Further, in the description of the drawings, the same or corresponding elements are appropriately designated by the same reference numerals. In addition, it should be noted that the drawings are schematic, and the dimensional relationship of each element, the ratio of each element, and the like may differ from the reality. Even between drawings, there may be parts where the relationship and ratio of dimensions are different from each other. In addition, the xyz coordinate axes are appropriately shown in the drawings, and the directions will be described thereby.

(Embodiment 1)
FIG. 1 is a schematic diagram showing a configuration of a tunable laser device including an optical waveguide structure according to the first embodiment. The tunable laser element 100 includes a first optical waveguide portion 10 and a second optical waveguide portion 20 formed on a common base portion S. The base S is composed of, for example, an n-type InP. An n-side electrode 30 is formed on the back surface of the base portion S. The n-side electrode 30 includes, for example, AuGeNi, and makes ohmic contact with the base S.

The first optical waveguide portion 10 includes an optical waveguide 11, a laminated portion 12, a p-side electrode 13, a microheater 14 made of Ti, and two electrode pads 15. The first optical waveguide portion 10 has an embedded structure, and the optical waveguide 11 is formed so as to extend in the z-direction in the laminated portion 12. The laminated portion 12 has a function of a clad portion with respect to the optical waveguide 11.

The p-side electrode 13 is arranged on the laminated portion 12 along a predetermined portion of the optical waveguide 11. A SiN protective film, which will be described later, is formed on the laminated portion 12, and the p-side electrode 13 is in contact with the laminated portion 12 via an opening formed in the SiN protective film. The microheater 14 is arranged along a predetermined portion of the optical waveguide 11 on the SiN protective film of the laminated portion 12. Each electrode pad 15 is arranged on the SiN protective film of the laminated portion 12 and is electrically connected to the microheater 14. The microheater 14 generates heat by being supplied with an electric current via each electrode pad 15.

2 (a) and 2 (b) are views showing a cross section taken along line AA and a cross section taken along line BB of FIG. 1, respectively.

As shown in FIG. 2A, in the AA line cross section, the laminated portion 12 has a lower clad layer 12a on the n-type InP substrate constituting the base portion S. In the lower clad layer 12a, the first layer 12a1 made of n-type InP, the sedimentary structure portion 12ab made of n-type semiconductor, and the second layer 12a2 made of n-type InP are arranged in this order along the y direction of FIG. It has a structure laminated with.

An active core layer 11a as an optical waveguide layer, which is an optical waveguide 11 in the AA line cross section, is laminated on the lower clad layer 12a. Further, an upper clad layer 12b made of p-type InP is laminated on the active core layer 11a. The upper part of the lower clad layer 12a, the active core layer 11a, and the upper clad layer 12b have a striped mesa structure having a width (for example, 2 μm) suitable for optical waveguide of 1.55 μm band light in a single mode by etching or the like. It has become. Both sides of the striped mesa structure (in the left-right direction of the paper surface) have an embedded structure having a current blocking structure composed of a p-type InP embedded layer 12c and an n-type InP current blocking layer 12d. Further, a p-type semiconductor layer 12e is laminated on the upper clad layer 12b and the embedded structure. The p-type semiconductor layer 12e is composed of an upper clad layer 12ea made of p-type InP and a contact layer 12eb made of p-type InGaAs laminated on the upper clad layer 12ea and forming the uppermost layer of the laminated portion 12. .. The p-type semiconductor layer 12e is provided from at least directly above the upper clad layer 12b over the embedded structures on both sides thereof. A SiN protective film 16 is formed on the laminated portion 12 so as to cover the laminated portion 12. The p-side electrode 13 is composed of AuZn, is formed on the contact layer 12eb along the active core layer 11a, and is in ohmic contact with the contact layer 12eb through the opening 16a of the SiN protective film 16. There is. With the above configuration, it is possible to inject a current from the n-side electrode 30 and the p-side electrode 13 into the active core layer 11a.

The active core layer 11a has a multiple quantum well structure including a plurality of well layers and a plurality of barrier layers that are alternately laminated, and a lower and upper light confinement layers that sandwich the multiple quantum well structure from above and below. It emits light by injecting an electric current. The well layer and the barrier layer constituting the multiple quantum well structure of the active core layer 11a are made of GaInAsP having different compositions, and the emission wavelength band from the active core layer 11a is a 1.55 μm band in this embodiment. The lower light confinement layer is composed of n-type GaInAsP. The upper light confinement layer is made of p-type GaInAsP. The bandgap wavelengths of the lower and upper light confinement layers are set to be shorter than the bandgap wavelengths of the active core layer 11a.

Further, as shown in FIG. 2B, in the cross section of the line BB, the lamination portion 12 has the active core layer 11a and the upper clad layer 12b, the optical waveguide layer 11ca, and the spacer in the structure shown in FIG. 2A. It has a structure in which the layer 12f and the diffraction grating layer 11cc are replaced. Further, the microheater 14 is provided on the SiN protective film 16 along the optical waveguide layer 11ca, the spacer layer 12f, and the diffraction grating layer 11cc. The optical waveguide layer 11ca, the spacer layer 12f, and the diffraction grating layer 11cc are the optical waveguide 11 in the BB line cross section. In the laminated portion 12, the active core layer 11a and the optical waveguide layer 11ca are arranged in series along the z direction.

The optical waveguide layer 11ca is made of GaInAsP. The bandgap wavelength of the optical waveguide layer 11ca is preferably shorter than the bandgap wavelength of the active core layer 11a, for example, 1.2 μm.

The diffraction grating layer 11cc is a layer including a sampled diffraction grating (Sampled Grating) provided along the optical waveguide layer 11ca in the vicinity of and directly above the optical waveguide layer 11ca with the spacer layer 12f made of p-type InP interposed therebetween. It is formed as a DBR (Distributed Bragg Reflector) type diffraction grating layer. That is, the diffraction grating layer 11cc is located on the upper clad layer 12ea side with respect to the optical waveguide layer 11ca. The diffraction grating layer 11cc has a structure in which a sampled diffraction grating is formed in the p-type GaInAsP layer along the z direction, and the groove of the diffraction grating is embedded with InP. In the diffraction grating layer 11cc, the lattice spacing of the diffraction grating is constant but sampled, thereby exhibiting a substantially periodic reflection response with respect to the wavelength. The bandgap wavelength of the p-type GaInAsP layer of the diffraction grating layer 11cc is preferably shorter than the bandgap wavelength of the active core layer 11a, for example, 1.2 μm.

Further, as described above, the microheater 14 generates heat by being supplied with an electric current via each electrode pad 15, and heats the diffraction grating layer 11cc. By changing the amount of supplied current, the temperature of the diffraction grating layer 11cc changes, and its refractive index changes.

Here, the sedimentary structure 12ab will be described more specifically. The sedimentary structures 12ab are located between the base S and the optical waveguide layer (optical waveguide layer 11ca and active core layer 11a). FIG. 3 is a diagram showing the structure of the sedimentary structure 12ab. The sedimentary structure 12ab includes a structure in which semiconductor layers 12ab1 and semiconductor layers 12ab2, which are semiconductor portions made of different semiconductor materials, are alternately laminated so as to be adjacent to each other in a total of three or more layers in the deposition direction. In the first embodiment, the semiconductor layer 12ab1 is composed of n-type GaInAs having a composition lattice-matched with InP, and the thickness thereof is 10 nm. The semiconductor layer 12ab2 is made of n-type InP and has a thickness of 10 nm. Then, in the sedimentary structure portion 12ab, the semiconductor layers 12ab1 and the semiconductor layers 12ab2 are alternately laminated in a total of 49 layers. The semiconductor layer 12ab1 has a total of 25 layers, and the semiconductor layer 12ab2 has a total of 24 layers. The thickness of the sedimentary structures 12ab is 490 nm.

Returning to FIG. 1, the second optical waveguide portion 20 will be described. The second optical waveguide portion 20 includes a two-branch portion 21, two arm portions 22, 23, a ring-shaped waveguide 24, and a microheater 25 made of Ti.

The two-branch portion 21 is composed of a 1 × 2 type bifurcated waveguide including a 1 × 2 type multimode interference type (MMI) waveguide 21a, and the two port side is connected to each of the two arm portions 22 and 23. At the same time, the 1-port side is connected to the 1st optical waveguide portion 10 side. One ends of the two arm portions 22 and 23 are integrated by the two-branch portion 21, and the two arm portions 22 and 23 are optically coupled to the diffraction grating layer 11cc.

Both the arm portions 22 and 23 extend in the z direction and are arranged so as to sandwich the ring-shaped waveguide 24. The arm portions 22 and 23 are close to the ring-shaped waveguide 24, and both are optically coupled to the ring-shaped waveguide 24 with the same coupling coefficient κ. The value of κ is, for example, 0.2. The arm portions 22 and 23 and the ring-shaped waveguide 24 form a ring resonator filter RF1. Further, the ring resonator filter RF1 and the two-branch portion 21 form a reflection mirror M1. The microheater 25 has a ring shape and is arranged on a SiN protective film formed so as to cover the ring-shaped waveguide 24. The microheater 25 generates heat when an electric current is supplied to heat the ring-shaped waveguide 24. By changing the amount of supplied current, the temperature of the ring-shaped waveguide 24 changes, and its refractive index changes.

The two branch portions 21, the arm portions 22, 23, and the ring-shaped waveguide 24 all have a high-mesa waveguide structure in which the optical waveguide layer 20a made of GaInAsP is sandwiched between the lower clad layer and the upper clad layer. .. Similar to the lower clad layer 12a, the lower clad layer includes a first layer made of n-type InP, a sedimentary structure portion made of an n-type semiconductor and having the configuration shown in FIG. 3, and a second layer made of n-type InP. , Have a structure in which they are laminated in this order. The upper clad layer is made of p-type InP.

Further, a microheater 26 is arranged on a part of the SiN protective film of the arm portion 23. The region of the arm portion 23 below the microheater 26 functions as a phase adjusting portion 27 that changes the phase of light. The microheater 26 generates heat when an electric current is supplied, and heats the phase adjusting unit 27. By changing the amount of supplied current, the temperature of the phase adjusting unit 27 changes, and its refractive index changes.

The first optical waveguide portion 10 and the second optical waveguide portion 20 constitute an optical resonator C1 composed of a diffraction grating layer 11 cc, which is a set of wavelength selection elements optically connected to each other, and a reflection mirror M1. doing.

In the tunable laser element 100, the diffraction grating layer 11cc generates a first comb-shaped reflection spectrum having substantially periodic reflection characteristics at substantially predetermined wavelength intervals. On the other hand, the ring resonator filter RF1 generates a second comb-shaped reflection spectrum having a reflection characteristic that is substantially periodic at predetermined wavelength intervals. The second comb-like reflection spectrum has a half-value full-width peak narrower than the half-value full width of the peak of the first comb-like reflection spectrum, and has a substantially period at a wavelength interval different from the wavelength interval of the first comb-like reflection spectrum. Reflective characteristics. However, considering the wavelength dispersion of the refractive index, it should be noted that the spectral components are not strictly at equal wavelength intervals.

To exemplify the characteristics of each comb-shaped reflection spectrum, the wavelength interval (free spectrum region: FSR) between the peaks of the first comb-shaped reflection spectrum is 373 GHz in terms of the frequency of light. The wavelength interval (FSR) between the peaks of the second comb-shaped reflection spectrum is 400 GHz in terms of the frequency of light.

In the tunable laser element 100, in order to realize laser oscillation, one of the peaks of the first comb-shaped reflection spectrum and one of the peaks of the second comb-shaped reflection spectrum can be superposed on the wavelength axis. ing. In such superposition, at least one of the microheater 14 and the microheater 25 is used to heat the diffraction grating layer 11cc by the microheater 14 and change the refractive index by the thermooptical effect. The comb-shaped reflection spectrum is moved and changed as a whole on the wavelength axis, and the ring-shaped waveguide 24 is heated by the microheater 25 to change its refractive index to change the second comb-shaped reflection spectrum on the wavelength axis. This can be achieved by doing at least one of moving and changing the whole thing above.

On the other hand, in the tunable laser element 100, there is a resonator mode by the optical resonator C1. In the tunable laser element 100, the resonator length of the optical resonator C1 is set so that the interval between the resonator modes (longitudinal mode interval) is 25 GHz or less. As for the wavelength of the resonator mode of the optical resonator C1, the phase adjusting unit 27 is heated by using the microheater 26 to change its refractive index, and the wavelength of the resonator mode is moved as a whole on the wavelength axis. This can be fine-tuned. That is, the phase adjusting unit 27 is a part for actively controlling the optical path length of the optical resonator C1.

In the wavelength variable laser element 100, when a current is injected from the n-side electrode 30 and the p-side electrode 13 into the active core layer 11a to cause the active core layer 11a to emit light, the peak of the spectral component of the first comb-shaped reflection spectrum, the first The peak of the spectral component of the second comb-shaped reflection spectrum and one of the resonator modes of the optical resonator C1 are configured to oscillate the laser at a matching wavelength, for example, 1550 nm, and output the laser beam L1.

Further, in the tunable laser element 100, the laser oscillation wavelength can be changed by utilizing the vernier effect. For example, when the diffraction grating layer 11cc is heated by the microheater 14, the refractive index of the diffraction grating layer 11cc increases due to the thermooptical effect, and the reflection spectrum of the diffraction grating layer 11cc (first comb-shaped reflection spectrum) becomes a long wave as a whole. Shift to the side. As a result, the peak of the first comb-shaped reflection spectrum in the vicinity of 1550 nm is not overlapped with the peak of the reflection spectrum (second comb-shaped reflection spectrum) of the ring resonator filter RF1, and is present on the long wave side. It overlaps another peak of the second comb reflection spectrum (eg, near 1556 nm). Further, by tuning the phase adjusting unit 27 to fine-tune the resonator mode and superimposing one of the resonator modes on the two comb-shaped reflection spectra, laser oscillation in the vicinity of 1556 nm can be realized.

Here, as described above, in the tunable laser element 100, for example, the diffraction grating layer 11 cc is heated by the microheater 14 in order to realize laser oscillation and change the laser oscillation wavelength. Sedimentary structures 12ab are provided to increase the heating efficiency of the microheater 14.

As described above, in the sedimentary structure portion 12ab, a semiconductor layer 12ab1 made of GaInAs and a semiconductor layer 12ab2 made of InP are alternately laminated in a total of 49 layers. As a result, only 50 interfaces between GaInAs and InP are formed in the first layer 12a1, the sedimentary structure 12ab, and the second layer 12a2. Since GaInAs and InP, which are semiconductor materials different from each other, have different dispersion relations of phonons propagating in the crystal, the phonons advanced to these interfaces are reflected or scattered. As a result, the thermal conductivity in the vertical direction at the interface decreases. Since the effect of lowering the heat conduction occurs at each interface of the sedimentary structure, the effect of lowering the heat conduction can be obtained more strongly in the laminated structure portion 12ab composed of 49 layers. Therefore, since the dissipation of the heat given by the microheater 14 to the base S side is suppressed by the presence of the sedimentary structure 12ab, the amount of heat that the microheater 14 should generate in order to bring the diffraction grating layer 11cc to a desired temperature. Can be made smaller. That is, the sedimentary structure 12ab can increase the heating efficiency of the microheater 14.

Further, since the sedimentary structure portion 12ab is made of a semiconductor material, the difference in refractive index from the optical waveguide layer (optical waveguide layer 11ca and active core layer 11a) made of the semiconductor material is smaller than that in the case of the dielectric layer or the hollow region. Therefore, even if the sedimentary structure portion 12ab is arranged closer to the optical waveguide layer than the dielectric layer or the cavity region, the influence on the optical waveguide characteristics of the optical waveguide layer is small, and the desired optical waveguide characteristics are realized. Structural design restrictions can be reduced. For example, since the sedimentary structure portion 12ab can be arranged closer to the optical waveguide layer than the dielectric layer or the cavity region while realizing the desired optical waveguide characteristics, the microheater 14 which is a heat source or the heating is performed. It can also be approached to the power diffraction grating layer 11 cc. As a result, the heating efficiency of the microheater 14 can be increased more effectively.

As described above, the sedimentary structure portion having the same structure as the sedimentary structure portion 12ab is also provided in the lower clad layer of the second optical waveguide portion 20. Therefore, the heating efficiency of the microheaters 25 and 26 is also improved.

As described above, the tunable laser element 100 including the optical waveguide structure according to the first embodiment has high heating efficiency by the microheaters 14, 25, and 26, and has a structural design for realizing optical waveguide characteristics. It is less constrained.

The tunable laser element 100 can be manufactured by the following process. First, on the n-type InP substrate constituting the base S, the lower clad layer 12a and the lower clad layer in the second optical waveguide portion 20 and the active core are used by the metal organic chemical vapor deposition (MOCVD) method. The layer 11a and the upper clad layer 12b are sequentially deposited.

Subsequently, after depositing the SiN film on the entire surface, the SiN film is patterned. Then, the SiN film is etched as a mask to remove all the active core layer 11a and the upper clad layer 12b in the region other than the region where the active core layer 11a remains in the first optical waveguide portion 10. Further, using the SiN film mask as it is as a selective growth mask, the optical waveguide layer 20a, the spacer layer 12f, and the p-type InGaAsP layer to be the diffraction grating layer 11cc in the optical waveguide layer 11ca and the second optical waveguide portion 20 by the MOCVD method, the upper part. A part of the clad layer 12ea is sequentially deposited.

Subsequently, after removing the mask of the SiN film, the SiN film is deposited on the entire surface, and then the SiN film in the region forming the diffraction grating layer 11 kb is patterned with a diffraction grating. Then, the SiN film is etched as a mask to form a lattice groove serving as a diffraction grating in the p-type InGaAsP layer, and all the p-type InGaAsP layers at positions other than the position where the diffraction grating layer 11 kb are formed are removed.

Subsequently, after removing the mask of the SiN film, the p-type InP layer is re-grown on the entire surface. After that, a SiN film is newly deposited, and patterning is performed so as to have a pattern corresponding to the optical waveguide 11 in the first optical waveguide portion 10 and the optical waveguide layer 20a in the second optical waveguide portion 20. Then, this SiN film is etched as a mask to form a striped mesa structure in the first optical waveguide portion 10 and a mesa structure in the second optical waveguide portion 20. At this time, the etching depth is set so that the sedimentary structure portion 12ab of the lower clad layer 12a and the sedimentary structure portion of the lower clad layer 20 in the second optical waveguide portion 20 are not etched. At this time, the regions corresponding to the two branch portions 21, the arm portions 22, 23, and the ring-shaped waveguide 24 are etched in the shape of a wide region including them.

Subsequently, using the SiN film mask used in the immediately preceding step as a selective growth mask, the p-type InP embedded layer 12c was placed on the exposed lower clad layer 12a and the lower clad layer in the second optical waveguide portion 20 by the MOCVD method. , N-type InP current blocking layer 12d is sequentially deposited to form an embedded structure. Subsequently, after removing the mask of the SiN film, the p-type InP layer and the contact layer 12eb, which are the remaining portions of the upper clad layer 12ea and the upper clad layer in the second optical waveguide portion 20, are used on the entire surface by the MOCVD method. Are sequentially deposited. Subsequently, a step of removing the contact layer 12eb other than the region where the active core layer 11a remains is performed. Subsequently, after depositing a SiN film on the entire surface, patterning of the shape of the first optical waveguide portion 10 and patterning of the waveguide corresponding to the two-branch portions 21, the arm portions 22, 23, and the ring-shaped waveguide 24 are performed. Then, etching is performed using this SiN film as a mask to form the shape of the first optical waveguide portion 10 and the high-mesa waveguide structure in the second optical waveguide portion 20. This etching is performed, for example, to a depth reaching the base S. After that, the SiN protective film 16, the n-side electrode 30, the microheaters 14, 25, 26, the electrode pad 15, and the wiring pattern are formed. Finally, the substrate is opened in a bar shape in which a plurality of tunable laser elements 100 are lined up, and the end faces of the first optical waveguide portion 10 and the end faces of the arm portions 22 and 23 are coated with an antireflection film, and then each tunable laser The wavelength tunable laser element 100 is completed by separating the elements for each element 100.

(Embodiment 2)
FIG. 4 is a schematic view showing the configuration of a tunable laser device including the optical waveguide structure according to the second embodiment. The tunable laser element 100A has a configuration in which the first optical waveguide portion 10 is replaced with the first optical waveguide portion 10A in the tunable laser element 100, and the microheater 26 is deleted. Hereinafter, the configuration of the first optical waveguide portion 10A will be specifically described.

The first optical waveguide portion 10A includes an optical waveguide 11A, a laminated portion 12A, a p-side electrode 13A, and microheaters 14A and 17A made of Ti. The first optical waveguide portion 10A has an embedded structure, and the optical waveguide 11A is formed so as to extend in the laminated portion 12A in the z direction. The laminated portion 12A has a function of a clad portion with respect to the optical waveguide 11A.

The p-side electrode 13A is arranged on the laminated portion 12A so as to be along a predetermined portion of the optical waveguide 11A. A SiN protective film is formed on the laminated portion 12A, and the p-side electrode 13A is in contact with the laminated portion 12A through an opening formed in the SiN protective film. The microheater 17A is arranged along a predetermined portion of the optical waveguide 11A on the SiN protective film of the laminated portion 12A. The microheater 14A is arranged along the p-side electrode 13A on the SiN protective film of the laminated portion 12.

5 (a) and 5 (b) are views showing a cross section taken along the line CC and a cross section taken along the DD line of FIG. 4, respectively.

As shown in FIG. 5A, in the CC line cross section, the laminated portion 12A has a lower clad layer 12Aa on the n-type InP substrate constituting the base portion S. The lower clad layer 12Aa has a structure in which a first layer 12Aa1 made of n-type InP, a sedimentary structure portion 12Aab made of n-type semiconductor, and a second layer 12Aa2 made of n-type InP are laminated in this order.

An active core layer 11Aaa as an optical waveguide layer, which is an optical waveguide 11A in the CC line cross section, is laminated on the lower clad layer 12Aa. Further, on the active core layer 11Aaa, a diffraction grating layer 11Ab composed of a sampled diffraction grating provided along the active core layer 11Aaa near and directly above the active core layer 11Aaa via a spacer layer 12Af made of p-type InP. Are laminated. The diffraction grating layer 11Ab is formed as a DFB (Distributed FeedBack) type diffraction grating layer. The upper part of the lower clad layer 12Aa, the active core layer 11Aaa, the spacer layer 12Af, and the diffraction grating layer 11Aab are made into a width (for example, 2 μm) suitable for optical waveguide of 1.55 μm band light in a single mode by etching or the like. It has a striped mesa structure. Both sides of the striped mesa structure (in the left-right direction of the paper surface) have an embedded structure having a current blocking structure composed of a p-type InP embedded layer 12Ac and an n-type InP current blocking layer 12Ad. Further, a p-type semiconductor layer 12Ae is laminated on the diffraction grating layer 11Ab and the embedded structure. The p-type semiconductor layer 12Ae is composed of an upper clad layer 12Aea made of p-type InP and a contact layer 12Aeb formed of p-type InGaAs laminated on the upper clad layer 12Aea and forming the uppermost layer of the laminated portion 12A. The p-type semiconductor layer 12Ae is provided at least from directly above the diffraction grating layer 11Ab to the embedded structures on both sides thereof. A SiN protective film 16A is formed on the laminated portion 12A so as to cover the laminated portion 12A. The p-side electrode 13A is configured to include AuZn, is formed on the contact layer 12Aeb along the active core layer 11Aaa, and is in ohmic contact with the contact layer 12Aeb via the opening 16Aa of the SiN protective film 16A. .. With the above configuration, it is possible to inject a current from the n-side electrode 30 and the p-side electrode 13A into the active core layer 11Aaa. Further, the microheater 14A is arranged along the p-side electrode 13A on the SiN protective film 18A provided in the laminated portion 12A to insulate the p-side electrode 13A and the microheater 14A.

Like the active core layer 11a, the active core layer 11Aaa has a multiple quantum well structure and lower and upper light confinement layers, and emits light by injecting an electric current. Further, the diffraction grating layer 11Ab has a configuration in which a sampled diffraction grating is formed in the p-type GaInAsP layer along the z direction and the groove of the diffraction grating is embedded with InP, similarly to the diffraction grating layer 11cc. The diffraction grating layer 11Ab exhibits a reflection response that is substantially periodic with respect to the wavelength.

On the other hand, as shown in FIG. 5 (b), in the DD line cross section, the lamination portion 12A attaches the active core layer 11Aaa to the phase adjusting portion 11Ab which is an optical waveguide layer made of InGaAsP in the structure shown in FIG. 5 (a). It has a structure in which the diffraction grating layer 11Ab and the spacer layer 12Af are replaced with an upper clad layer 12Ab made of p-type InP, and the contact layer 12Aeb is deleted. In order to reduce the light loss in the phase adjusting unit 11Ab and effectively confine the light, the bandgap wavelength of the phase adjusting unit 11Ab is preferably shorter than the bandgap wavelength of the active core layer 11Aaa, for example, at 1.3 μm or less. be.

The microheater 14A generates heat by being supplied with an electric current, and heats the diffraction grating layer 11Ab. By changing the amount of supplied current, the temperature of the diffraction grating layer 11Ab changes, and its refractive index changes.

The first optical waveguide section 10A and the second optical waveguide section 20 constitute an optical resonator C2 composed of a diffraction grating layer 11Ab, which is a set of wavelength selection elements optically connected to each other, and a reflection mirror M1. doing.

In the wavelength variable laser element 100A, similarly to the wavelength variable laser element 100, when a current is injected from the n-side electrode 30 and the p-side electrode 13A into the active core layer 11Aaa to cause the active core layer 11Aaa to emit light, the diffraction grating layer 11Aab is generated. The peak of the spectral component of the first comb-shaped reflection spectrum generated, the peak of the spectral component of the second comb-shaped reflection spectrum generated by the ring resonator filter RF1, and one of the resonator modes of the optical resonator C2 matched. It is configured to oscillate the laser at a wavelength, for example, 1550 nm, and output the laser beam L1. The wavelength of the resonator mode of the optical resonator C2 is determined by heating the phase adjusting unit 11Ab using the microheater 17A to change its refractive index and moving the wavelength of the resonator mode as a whole on the wavelength axis. Can be fine-tuned.

Further, in the tunable laser element 100A, the laser oscillation wavelength can be changed by utilizing the vernier effect, similarly to the tunable laser element 100.

Here, the sedimentary structure 12Ab will be described more specifically. The sedimentary structures 12Aab are located between the base S and the optical waveguide layer (phase adjusting part 11Ab and active core layer 11Aaa). FIG. 6 is a diagram showing the structure of the sedimentary structure 12Ab. The sedimentary structure portion 12Aab includes a convex portion 12Aab1 which is a semiconductor portion made of different semiconductor materials and a cover layer 12Aab2. The sedimentary structure 12Aab includes a structure in which the convex portion 12Aab1 and the cover layer 12Aab2 formed so as to cover the convex portion 12Aab1 are alternately deposited so as to be adjacent to each other in the deposition direction. In the second embodiment, the convex portion 12Aab1 is formed of n-type InAs and has a shape like a quantum dot or a quantum dash, and its height is 10 nm. InAs is a semiconductor material that has a certain degree of lattice mismatch with respect to InP, and can be three-dimensionally grown on InP or a semiconductor lattice-matched with the InP. The cover layer 12Aab2 is made of n-type InP, and its thickness (thickness from the surface to the surface indicating the height of the convex portion 12Ab1 shown by the broken line) is 10 nm. Then, in the sedimentary structure portion 12Aab, the layer formed by the convex portion 12Aab1 and the cover layer 12Aab2 are alternately laminated in a total of 50 layers. The convex portion 12Aab1 has a total of 25 layers, and the cover layer 12Aab2 has a total of 25 layers. The sedimentary structure 12Ab has a thickness of 500 nm.

Even in the tunable laser element 100A, for the purpose of realizing laser oscillation and changing the laser oscillation wavelength, for example, the diffraction grating layer 11Ab is heated by the microheater 14A, and the phase adjusting unit 11Ab is heated by the microheater 17A. Sedimentary structures 12Abs are provided to increase the heating efficiency of the microheaters 14A and 17A.

As described above, in the sedimentary structure portion 12Aab, a convex portion 12Aab1 made of InAs and a cover layer 12Aab2 made of InP are alternately laminated in a total of 50 layers. As a result, only 50 interfaces between InAs and InP are formed. Further, the convex portion 12Ab1 makes the interface a three-dimensional interface with undulations. Since the dispersion relation of phonons propagating in the crystal is different between InAs and InP and the interface is undulating, the phonons that have advanced to the interface are more strongly reflected or scattered. As a result, the thermal conductivity in the deposition direction decreases. Therefore, the conduction of heat applied by the microheaters 14A and 17A to the base S side is suppressed by the presence of the sedimentary structure 12Aab, so that the diffraction grating layer 11Ab and the phase adjusting part 11Ab are brought to a desired temperature. The amount of heat that the heaters 14A and 17A should generate can be reduced. That is, the sedimentary structures 12Ab can increase the heating efficiency of the microheaters 14A and 17A.

Further, since the sedimentary structure portion 12Ab is made of a semiconductor material, restrictions on structural design for realizing desired optical waveguide characteristics in the optical waveguide layer can be reduced.

Further, since the sedimentary structure portion 12Aab is made of a conductive semiconductor material, a current can be injected into the active core layer 11Aaa even if it is arranged between the n-side electrode 30 and the active core layer 11Aaa.

Further, in the second embodiment, a sedimentary structure portion having the same structure as the sedimentary structure portion 12Ab is also provided in the lower clad layer of the second optical waveguide portion 20. Therefore, the heating efficiency of the microheater 25 is also improved.

As described above, the tunable laser element 100A including the optical waveguide structure according to the second embodiment has high heating efficiency by the microheaters 14A, 17A, 25, and has a structural design for realizing optical waveguide characteristics. It has less restrictions and is also suitable for injecting a current into the active core layer 11Aaa.

In the first embodiment, the diffraction grating layer 11cc is on the microheater 14 side (upper clad layer 12ea side) of the optical waveguide layer 11ca, but the first optical waveguide having the optical waveguide structure of the modified example shown in FIG. 7 is provided. Like the part 10B, the diffraction grating layer 11cc may be on the base S side (lower clad layer 12a side) of the optical waveguide layer 11ca.

Further, in the first embodiment, the semiconductor layers 12ab1 and the semiconductor layers 12ab2 made of different semiconductor materials in the sedimentary structure 12ab are GaInAs and InP, respectively, but are selected from InP, GaInAs, GaInAsP, AlInAs, and AlInGaAs. Two types are preferable because the composition for lattice matching with the InP substrate can be selected. Further, the semiconductor materials different from each other are not limited to two types, and may be three or more types. For example, semiconductor layers made of InP, GaInAs, and GaInAsP may be laminated so that semiconductor layers of the same type do not overlap to form a sedimentary structure. Further, in the first embodiment, the thicknesses of the semiconductor layer 12ab1 and the semiconductor layer 12ab2 are both 10 nm, but the thickness is not particularly limited. However, the effect of reflecting or scattering phonons to reduce the thermal conductivity increases as the number of interfaces in the semiconductor portion made of different semiconductors increases, so the number of interfaces per unit thickness in the semiconductor deposition direction is increased. In the above, the thickness of the semiconductor layer is preferably 100 nm or less, and more preferably 50 nm or less. Further, in order to produce the semiconductor layer with good reproducibility and to exert the effect of reflecting or scattering phonons at the interface, it is preferable that the semiconductor layer is thick to some extent, and the thickness is preferably 3 nm or more, more preferably 5 nm or more. Further, if the semiconductor layers made of different semiconductor materials are alternately laminated in a total of three or more layers, a plurality (two or more) interfaces of the semiconductor layers having different phonon dispersion relations are formed, which is preferable. The number of more preferable interfaces is 10 to 100.

Further, in the second embodiment, the convex portions 12Aab1 and the cover layer 12Aab2 made of different semiconductor materials in the sedimentary structure portion 12Aab are InAs and InP, respectively, but the convex portions are lattice-mismatched with InP to some extent. The semiconductor material may be one or more selected from InGaAs and InAs, and the cover layer may be one or more selected from InP, GaInAs, GaInAsP, AlInAs, and AlInGaAs. Further, the thickness of the layer formed by the convex portion (height of the convex portion) or the thickness of the cover layer is preferably 100 nm or less, more preferably 50 nm or less, and 3 nm or more. Is preferable, and 5 nm or more is more preferable. Further, in the second embodiment, each convex portion is discrete and has an island shape, but a plurality of convex portions may be connected to form a layer having undulations on the upper surface.

Further, in the first and second embodiments, the diffraction grating is a sampled diffraction grating, but the type of the diffraction grating is not limited to this, and a superstructure grating or a superimposed grating may be used.

Moreover, the present invention is not limited by the above-described embodiment. The present invention also includes a configuration in which the above-mentioned components are appropriately combined. For example, as the sedimentary structure, a structure in which the semiconductor layers of the first embodiment are laminated and a structure including a convex portion and a cover layer may be combined. Further, further effects and modifications can be easily derived by those skilled in the art. Therefore, the broader aspect of the present invention is not limited to the above-described embodiment, and various modifications can be made.

10, 10A, 10B First optical waveguide 11, 11A Optical waveguide 11a, 11Aaa Active core layer 11ca, 20a Optical waveguide layer 11cab, 11Ab Diffraction grating layer 11Ab, 27 Phase adjustment unit 12, 12A Laminated part 12a, 12Aa Lower clad layer 12a1, 12Aa1 1st layer 12b, 12Ab, 12ea, 12Aea Upper clad layer 12a2, 12Aa2 2nd layer 12ab, 12Aab Stacked structure part 12Ab1 Convex part 12Aab2 Cover layer 12c, 12Ac p type InP embedded layer 12d, 12Ad n type Blocking layer 12e, 12Ae p type semiconductor layer 12eb, 12Aeb contact layer 12f, 12Af Spacer layer 12ab1, 12ab2 Semiconductor layer 13, 13A p side electrode 14, 14A, 17A, 25, 26 Microheater 15 Electrode pad 16, 16A, 18A SiN Protective film 16a, 16Aa Opening 20 Second optical waveguide 21 2 Branch 21a MMI waveguide 22, 23 Arm 24 Ring-shaped waveguide 30 n-side electrode 100, 100A Waveguide variable laser element C1, C2 Optical resonator L1 laser Optical M1 Reflecting Mirror RF1 Ring Resonator Filter S Base

Claims (7)

With the board
The lower clad layer located on the substrate and
The optical waveguide layer located on the lower clad layer and
The upper clad layer located on the optical waveguide layer and
The heater located on the upper clad layer and
Sedimentary structures located between the substrate and the optical waveguide layer, in which semiconductor parts made of different semiconductor materials are deposited so as to be adjacent to each other in the deposition direction.
With
On the upper clad layer, an electrode for injecting a current into the optical waveguide is not located at the position of the heater.
The semiconductor portion is a semiconductor layer and
The sedimentary structure
A structure in which the semiconductor layers made of different semiconductor materials are alternately laminated is included, and phonons related to heat applied from the heater side to the interface of the laminated semiconductor layers are reflected or scattered from the heater to the substrate. Suppresses dissipated heat,
The thickness of each semiconductor layer is 5 nm to 50 nm.
An optical waveguide structure characterized in that the number of interfaces formed by the laminated semiconductor layers is 10 to 100. The optical waveguide structure according to claim 1, wherein the substrate is made of InP. The optical waveguide structure according to claim 1 or 2, wherein the semiconductor materials different from each other are two or more kinds selected from InP, GaInAs, GaInAsP, AlInAs, and AlInGaAs. The optical waveguide structure according to claim 1, wherein the semiconductor portion forms an undulating interface. The fourth aspect of claim 4, wherein the semiconductor materials different from each other are one or more selected from InGaAs and InAs, and one or more selected from InP, GaInAs, GaInAsP, AlInAs, and AlInGaAs. Optical waveguide structure. The optical waveguide structure according to any one of claims 1 to 5, further comprising a diffraction grating layer located on the upper clad layer side or the lower clad layer side with respect to the optical waveguide layer. The optical waveguide structure according to any one of claims 1 to 6, wherein the optical waveguide layer is an active core layer and further includes an electrode for injecting an electric current into the active core layer.

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