JP6019522B2 - Surface emitting laser device - Google Patents

Description

  The present invention relates to a vertical cavity surface emitting laser (VCSEL = Vertical Cavity Surface Emitting Laser) in which the resonance direction of light is perpendicular to a substrate surface, and a reflecting mirror used in the laser. In this specification, the vertical cavity surface emitting laser is simply referred to as a surface emitting laser or a VCSEL in accordance with common usage.

  In recent years, the amount of data transmitted through a network such as the Internet has increased remarkably, and there is an urgent need to develop an optical fiber communication system that is significantly faster than currently used systems. In such a next-generation optical fiber communication system, it is assumed that processing and control such as routing of IP (Internet Protocol) packets that are currently performed electrically are performed as they are as optical signals. If such a photonic packet network without electrical signals is realized, it is expected that not only can communication and transmission be speeded up and increased in capacity, but also a significant reduction in power consumption and downsizing of the device will be possible. .

  An essential and basic device for realizing a photonic packet network is an all-optical type optical switch, an optical buffer memory, an optical flip-flop, and the like. Various types of optical buffer memories have been proposed, and one of them is a polarization bistable surface emission that can switch the polarization state described in Patent Document 1, Non-Patent Documents 1 and 2, etc. There is an optical buffer memory using a laser. These documents also describe two-dimensional array of all-optical shift registers composed of individual polarization bistable surface emitting lasers, optical isolators, optical gates, polarizers, etc., and many polarization bistable surface emitting lasers. Also disclosed is an all-optical shift register that is integrated in a space and inputs and outputs light in parallel in a spatial manner. Furthermore, in Patent Document 2, an optical path is formed so that the output light of a polarization bistable surface emitting laser arranged in a two-dimensional array is vertically input to another adjacent polarization bistable surface emitting laser. An all-optical shift register in which the spatial arrangement of optical elements is devised is disclosed.

  In any case, a basic component in a device such as an optical buffer memory or an all-optical shift register is a polarization bistable surface emitting laser. In general, a surface emitting laser is a resonator in a direction perpendicular to a substrate, which is composed of an active layer formed on a semiconductor substrate and reflecting mirrors provided at the lower and upper portions so as to sandwich the active layer. . In surface-emitting lasers, when polarized light whose vibration direction is orthogonal to the resonator emitting polarized light oscillating in a certain direction is input, the oscillation direction of the oscillating polarized light is the same as the input light. In some cases, the polarization bistability may be maintained such that the oscillating polarization state is maintained even when the input light disappears. The polarization bistable surface emitting laser is a surface emitting laser having this polarization bistability.

  In order to cause high-efficiency laser oscillation in a surface emitting laser, the reflecting mirror needs to have a high reflectance with respect to a target wavelength band. Therefore, a conventional surface emitting laser generally uses a distributed Bragg reflector (DBR = Distributed Bragg Reflector) as a reflecting mirror. For example, FIG. Also in the polarization bistable surface emitting laser disclosed in FIG. 8, DBR by InAlAs / InAlGaAs is used as a reflecting mirror.

  However, since the DBR is formed by laminating a large number of semiconductor thin films, it is inevitable that the DBR becomes thicker to achieve a sufficiently high reflectance. In general, since the active layer is considerably thin in a surface emitting laser, the thickness of the DBR is often a limiting factor for thinning the surface emitting laser. That is, when a conventional DBR is used as a reflecting mirror, there is a problem that it is difficult to reduce the thickness of a polarization bistable surface emitting laser, and it is also difficult to reduce the size of a device such as an optical buffer memory using the laser.

  In addition, in the conventional polarization bistable surface emitting laser, it is necessary to enter a light control signal for switching the polarization in both directions coaxially with the output direction of light from the resonator. Therefore, it is necessary to accurately arrange a large number of optical elements in the space above the semiconductor substrate on which the polarization bistable surface emitting laser is formed. For this reason, even if the polarization bistable surface emitting laser itself can be reduced in thickness and size, it is inevitable that devices such as all-optical shift registers will be increased in size. Furthermore, such a configuration in which a large number of optical elements are spatially arranged has a problem in terms of reliability against mechanical vibration.

  By the way, in recent years, as a reflecting mirror for a surface emitting laser, a high refractive index difference subwavelength grating (HCG = High-index Contrast subwavelength grating) that can realize a high reflectance over a wide wavelength band while having a thickness of a subwavelength order. ) Is attracting attention and is actively researched. For example, Non-Patent Document 3 discloses an HCG in which a stripe structure made of a high refractive index material is formed on a low refractive index layer formed on a semiconductor substrate. Non-Patent Document 3 describes that according to the HCG having such a structure, a high reflectance can be realized with respect to polarized light that vibrates in a direction perpendicular to the extending direction of the stripe structure (that is, the width direction). Non-Patent Document 4 discloses a surface emitting laser using HCG having the stripe structure as described above as a lower reflecting mirror below the active layer.

  However, the HCG described in Non-Patent Document 3 achieves a high reflectance only for one linearly polarized light, and is a polarization bistable surface emitting laser that needs to handle a plurality of linearly polarized lights in different directions. Is not applicable. Similarly, the surface emitting laser described in Non-Patent Document 4 simply emits laser light having one linearly polarized light, and does not correspond to a plurality of linearly polarized lights in different directions.

Japanese Patent No. 4369573 JP 2008-262122 A

Kawaguchi, “Polarized bistable VCSEL and its application to optical buffer memory”, Laser Research, The Laser Society of Japan, Vol. 37, No. 8, August 2009, pp.608-613 Kawaguchi, "Optical signal processing using a bistable semiconductor laser", Laser Research, The Laser Society of Japan, Vol. 40, No. 5, May 2012, pp.369-374 Carlos et al., "Ultrabroadband Mirror Using Low-Index Cladded Subwavelength Grating", I-Triple Photonics Technology Letters (IEEE Photonics) Technology Letters), Vol.16, 2004, pp.518-520 Chung et al., “Silicon-Photonics Light Source Realized by III-V / Si-Grating-Mirror Laser” "Applied Physics Letters, Vol.97, No.15, 2010, pp.15113 Wakabayashi et al., “Reflection characteristics of broadband mirrors using sub-wavelength gratings”, IEICE Technical Report, IEICE, Vol.110, No.437, March 2011, pp.7-12

  The present invention has been made to solve the above-mentioned problems, and a first object of the present invention is to make a polarization bistable surface-emitting laser thin and small, and to be easily manufactured. Is to provide a reflector.

  A second object of the present invention is a surface emitting laser capable of miniaturizing devices such as an optical switch and an all-optical shift register and ensuring high reliability against mechanical vibration. Is to provide a device.

  The inventors of the present application pay attention to the fact that HCG as described in Non-Patent Documents 3 and 4 can achieve a high reflectance over a wide wavelength band, and can realize a high reflectance without depending on polarization. Research and development of reflectors by is underway. Among them, the inventors of the present application have come up with a method for extending a conventional HCG having a one-dimensional periodic structure to a two-dimensional periodic structure, and further considering the ease and cost of mounting on an actual surface emitting laser. Thus, the present invention has been made through repeated studies based on simulation calculations and the like.

That is, the laser reflecting mirror used in the present invention for solving the above problems is a high refractive index difference grating (HCG) used for forming a resonator together with an active layer that generates light in a surface emitting laser. ) Reflector by structure,
a) a low refractive index layer having a first refractive index and occupying a planar area;
b) It is arranged so as to be in contact with one surface of the low refractive index layer, and is made of a material having a second refractive index larger than the first refractive index, and is 90 ° with respect to an axis orthogonal to the low refractive index layer. A grid-like two-dimensional lattice structure having rotational symmetry of
And the first refractive index and the second refractive index so that the reflectance is not less than a predetermined value over a target wavelength band with respect to light having an arbitrary polarization incident along the axis direction. Parameters including a refractive index, a thickness of the low refractive index layer, a thickness of the two-dimensional grating structure, a pitch of the grating of the two-dimensional grating structure, and a grating width are adjusted.

In this laser reflecting mirror, the low refractive index layer can be made of various materials having a first refractive index smaller than the second refractive index. For example, in addition to a material generally used in a semiconductor process such as SiO ₂ (silicon oxide), air having a smaller refractive index can be used. When the low refractive index layer is a layer made of air, the reflecting mirror has an air bridge structure using a spacer layer.

In this laser reflector, the two-dimensional lattice structure arranged in contact with one surface of the low refractive index layer has a rotational symmetry of 90 ° with respect to an axis orthogonal to the low refractive index layer. Have Therefore, the incident conditions are the same regardless of whether the light incident along the axis is polarized light Ex (0 °) or polarized light Ey (90 °) orthogonal thereto, and equivalent reflection characteristics can be obtained. That is, it becomes a polarization-independent reflecting mirror that achieves a high reflectance regardless of polarization.

Similar to this laser reflector, the one described in Non-Patent Document 5 is known as an HCG obtained by extending the HCG having a one-dimensional periodic structure disclosed in Non-Patent Documents 3 and 4 to a two-dimensional periodic structure. Yes. However, the HCG having a two-dimensional periodic structure described in Non-Patent Document 5 has a structure in which a large number of regular rectangular prisms made of a high refractive index material are arranged in a two-dimensional lattice pattern on a low refractive index layer. Therefore, an air bridge structure in which the low refractive index layer is an air layer cannot be adopted. In order to increase the reflectance in HCG, the difference between the refractive index of the material constituting the low refractive index layer (first refractive index) and the refractive index of the material constituting the two-dimensional lattice structure (second refractive index) is large. Is more advantageous. Air has a significantly lower refractive index than materials such as SiO ₂ that can be considered other than air. Therefore, it is a great advantage to adopt an air bridge structure when considering mounting on a surface emitting laser, particularly application to an upper reflecting mirror disposed above the active layer of the surface emitting laser.

The present invention was made in order to solve the aforementioned problems is a surface emitting laser device using the laser reflecting mirror,
Resonance comprising a semiconductor substrate, a lower reflecting mirror formed on the semiconductor substrate, an active layer for generating light disposed on the lower reflecting mirror, and an upper reflecting mirror formed on the active layer A surface emitting laser device comprising:
At least the lower reflecting mirror is formed in a low refractive index layer occupying a region having a first refractive index and extending in a planar shape, and one semiconductor layer formed on the semiconductor substrate, and the low refractive index It is arranged so as to be in contact with one surface of the layer, and is made of a material having a second refractive index larger than the first refractive index, and has a rotational symmetry of 90 ° with respect to an axis orthogonal to the low refractive index layer. A grid-like two-dimensional lattice structure, and for the light having an arbitrary polarization incident along the direction of the axis, the reflectance over a target wavelength band is not less than a predetermined value, Parameters including the first refractive index, the second refractive index, the thickness of the low refractive index layer, the thickness of the two-dimensional grating structure, the pitch of the grating of the two-dimensional grating structure, and the grating width are adjusted. Reflector with high refractive index difference grating structure I can,
An optical waveguide for guiding light to the semiconductor layer in which the two-dimensional lattice structure is formed is arranged in a grid shape so that the two-dimensional lattice structure is located at an intersection,
From the resonator including the two-dimensional grating structure, polarized light beams having different vibration directions can be transmitted to the optical waveguides extending in different directions .

In the surface emitting laser device according to the second aspect of the present invention, it is preferable to use the above laser reflecting mirror for both the lower reflecting mirror and the upper reflecting mirror, but the laser reflecting mirror is used as the lower reflecting mirror, and the other is DBR or the like. Reflectors having other structures may be used. Since the reflecting mirror with the HCG structure is thinner than the DBR, according to the surface emitting laser device of the present invention, the surface emitting laser device itself can be made thin by thinning the resonator.

A surface-emitting laser device according to the present invention is an optical waveguide coupling type surface-emitting laser in which a resonator is directly connected to an optical waveguide, and the light generated in the resonator and confined in the resonator is passed through the upper reflecting mirror. In addition to emitting light to space, it is possible to send light through the lower reflector to the optical waveguide coupled thereto. Therefore, for example, by adopting a configuration in which a plurality of surface emitting lasers are connected through an optical waveguide, light confined in a resonator of a certain surface emitting laser is separated from another through the optical waveguide instead of through the upper space. It can propagate to the surface emitting laser and be transferred to the resonator of the surface emitting laser. That is, it is not necessary to use the upper space when transmitting the light output between the plurality of surface emitting lasers.

  The optical waveguide can also be used for transmission of a light control signal for switching the polarization of the surface emitting laser light output. If optical waveguides are used for both the optical output transmission and the optical control signal transmission of each surface emitting laser, there is no need to place an optical element in the space above the surface emitting laser, and the device can be made significantly thinner and thinner. Miniaturization can be achieved. On the other hand, the structure of the optical waveguide becomes considerably complicated, making it difficult to manufacture. Therefore, the optical waveguide is used for transmission of the optical output of each surface emitting laser, and the light control signal for switching the polarization of the light confined in the resonator of the laser device is transmitted from the outside of the upper reflecting mirror in the direction of the axis. You may make it inject along.

  For example, in an optical buffer memory using a polarization bistable surface emitting laser, an optical control signal for switching the polarization of the light output of each surface emitting laser needs to be incident on the resonator. If used, the light control signal can be incident from the upper space, and the light output can be extracted using the optical waveguide. As a result, when configuring devices such as an optical buffer memory and an all-optical shift register using the same, it is possible to reduce the number of optical elements disposed in the upper space as compared with the prior art, which is advantageous for downsizing these devices. It is. Also, the reliability with respect to mechanical vibration of such devices is improved by reducing the number of spatially arranged optical elements.

In the surface-emitting laser device according to the present invention, when a plurality of surface-emitting lasers are arranged on the optical waveguide, a plurality of surface-emitting lasers are arranged according to the polarization state confined in the resonator of each surface-emitting laser. The optical coupling state changes. Therefore, for example, a plurality of surface emitting lasers arranged on a linearly extending optical waveguide are optically coupled and injection-locked so that the wavelengths of specific polarized light oscillated by each surface emitting laser are aligned and a high output array is obtained. A laser can be realized. In addition, by switching the light control signal incident on some of the surface emitting lasers and switching the polarized light oscillated by the surface emitting laser, the number of surface emitting lasers that are strongly coupled on the optical waveguide changes, and the light passes through the optical waveguide. The output (energy) of the extracted light can be changed.

According to the surface emitting laser device of the present invention , light is selectively output from one surface emitting laser to one of the optical waveguides extending in two directions according to the polarization of the light control signal. An optical switch can be realized. Furthermore, a shift register function for transferring light confined in the resonator of one surface emitting laser to any of a plurality of adjacent surface emitting lasers according to the polarization of the light control signal is realized. be able to.

Further, as described above, the laser reflecting mirror used in the present invention can adopt an air bridge structure in which the low refractive index layer is an air layer. Therefore, in the surface emitting laser device according to the present invention, the upper reflection is performed. The laser reflecting mirror is used as a mirror, and has a structure having an air bridge structure in which a low refractive index layer located between the two-dimensional grating structure and the active layer included in the reflecting mirror is an air layer. can do.

  In the manufacturing process of a semiconductor laser, it is relatively difficult to form an air bridge structure in which a low refractive index layer is an air layer by selectively etching a part of a layer in which crystals are grown at once for a large number of lasers. Easy. Therefore, it is possible to increase the reflectance of the upper reflecting mirror and suppress the laser oscillation efficiency while suppressing an increase in cost.

According to the laser reflecting mirror used in the present invention, it is possible to realize a polarization-independent reflecting mirror having a high reflectance even if it is made thinner than the conventional DBR. Further, according to the laser reflecting mirror used in the present invention, it is possible to adopt an air bridge structure in which the air layer is a low refractive index layer, thereby making it easier to manufacture a reflecting mirror exhibiting a high reflectivity. Is possible. Furthermore, unlike the HCG having a two-dimensional periodic structure described in Non-Patent Document 5, etc., the laser reflector used in the present invention allows a current for laser oscillation to flow through the entire two-dimensional grating structure. Therefore, it is advantageous for efficient oscillation and has an advantage of good heat dissipation because of its large area.

According to the surface emitting laser device according to the present invention using such a laser reflecting mirror, it is possible to realize a polarization bistable surface emitting laser that is thinner than the conventional one. In addition , since the structure is a waveguide coupling type, it is possible to reduce the number of optical elements disposed in the space above the surface emitting laser, and devices such as an optical buffer memory and an all-optical shift register using the surface emitting laser. Can be made thinner and smaller. Further, since the number of spatially arranged optical elements is reduced, it is strong against mechanical vibration and the reliability is improved. In addition, the optical coupling between the optical outputs of multiple surface emitting lasers can be controlled by polarization, so that various logic operations and stable buffering can be performed while maintaining high light density. Is possible.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic perspective view of a polarization-independent HCG that is an embodiment of a laser reflector used in the present invention. The figure which shows the simulation result of the reflection spectrum in an example of polarization independent HCG shown in FIG. 1 is a schematic perspective view of an embodiment of a polarization bistable surface emitting laser using a laser reflecting mirror according to the present invention. FIG. FIG. 4 is a schematic perspective view of the polarization bistable surface emitting laser shown in FIG. 3 arranged two-dimensionally. Operation | movement explanatory drawing in the structure which has arrange | positioned the polarization bistable surface emitting laser shown in FIG. 3 one-dimensionally. Operation | movement explanatory drawing in the structure which has arrange | positioned the polarization bistable surface emitting laser shown in FIG. 3 two-dimensionally. The figure which shows an example of a structure of the two-dimensional shift register using the polarization bistable surface-emitting laser shown in FIG. FIG. 4 is a diagram showing another example of the configuration of a two-dimensional shift register using the polarization bistable surface emitting laser shown in FIG. 3. FIG. 4 is an operation timing chart of a 1-bit memory of a shift register using the polarization bistable surface emitting laser shown in FIG. 3. SEM observation image of the prototype polarization-independent HCG. The figure which shows the calculation result of the power reflection spectrum of the prototype polarization-independent HCG. The figure which shows the measurement result of the reflectance spectrum of the prototype polarization-independent HCG. The figure which shows the polarization direction dependence of the reflectance in wavelength 1.45 [micrometer], 1.50 [micrometer], and 1.55 [micrometer] of the prototype polarization-independent HCG. FIG. 3 is a schematic cross-sectional view of a polarization bistable surface emitting laser assumed for calculating optical output characteristics. The figure which shows the calculation result of the light intensity and electromagnetic field distribution in Monitor X (and Monitor Y) in FIG. The figure which shows the result of having calculated the power ratio of the optical waveguide output with respect to the surface emitting output in a laser oscillation state with respect to the distance d from the center point of a mesa part. The figure (a) which shows the calculation result of the electromagnetic field distribution of the whole calculation area | region at the time of distance d = 6 [micrometer], and the electromagnetic field distribution (solid line) in the Si layer center, and the 0th-order mode distribution in a mesa part The figure which shows a calculation result (b).

First, an embodiment of a laser reflector used in the present invention will be described with reference to the accompanying drawings. FIG. 1 is a schematic perspective view of the laser reflecting mirror of this embodiment.

The reflecting mirror 10 is a high refractive index difference sub-wavelength grating (HCG) in which a two-dimensional periodic structure made of a high refractive index material has a top-view girder shape. Specifically, the reflecting mirror 10 includes a semiconductor substrate 11 made of a material such as silicon (Si) and a low refractive index material (refractive index n _L ) on the semiconductor substrate 11 in the Z-axis direction in FIG. A low-refractive-index layer 12 and a two-dimensional lattice structure 13 having a top-view girder shape made of a high-refractive-index material (refractive index _ng ) formed on the low-refractive-index layer 12. The thickness of the low refractive index layer 12 is T _L , and the thickness of the two-dimensional lattice structure 13 is T _g . In both the X-axis direction and the Y-axis direction, the width of the lattice portion of the two-dimensional lattice structure 13 is W, and the pitch of the lattice portion is Λ. That is, the two-dimensional lattice structure 13 has a rotational symmetry of 90 ° around an axis orthogonal to the semiconductor substrate 11 (axis extending in the Z-axis direction). Therefore, as shown in FIG. 1, the reflecting mirror 10 is a HCG having a reflection characteristic independent of the polarization direction of light incident along the Z axis from above, that is, polarization-independent HCG. Two orthogonal linearly polarized light Ex (vibrates in the X-axis direction) and Ey (vibrates in the Y-axis direction) have the same reflectance.

In order to confirm the characteristics of the polarization-independent HCG shown in FIG. 1, the parameters are as follows: Λ = 556 [nm], W = 178 [nm], T _g = 270 [nm], T _L = 630 [nm] , N _g = 3.5, n _L = 1, the reflection spectrum was obtained by computer simulation. The result is shown in FIG. Here, MEEP which is typical finite difference time domain (FDTD = Finite-Difference Time-Domain) method software was used for the calculation. As boundary conditions, a periodic boundary condition is used in the in-plane direction, and an absorbing boundary condition (PML = Perfectly Matched Layer) is used in the direction orthogonal to the plane, and the incident plane wave is incident from above along the Z axis in FIG. I did it.

As is clear from FIG. 2, a high reflectance band exists in a wavelength range of about 80 [nm] centering on a wavelength of 0.97 [μm]. This is comparable to the high reflectivity bandwidth that can be achieved with DBR. Further, the reflectance in the high reflectance band exceeds 99.5%, and it can be said that the reflection characteristics are sufficient as a reflecting mirror for a surface emitting laser. On the other hand, for example, the thickness of the DBR described in Non-Patent Document 1 is 6 [μm], whereas the thickness T _g of the two-dimensional lattice structure 13 in the above example is 270 [nm] and is extremely thin. . That is, the laser reflecting mirror of the present embodiment can be made extremely thin while realizing the reflection characteristics equivalent to those of the conventional DBR.

  Next, a polarization bistable surface emitting laser using the polarization-independent HCG described above will be described as an example of the surface emitting laser apparatus according to the present invention. FIG. 3 is a schematic perspective view of the polarization bistable surface emitting laser of this embodiment. In this polarization bistable surface emitting laser 30, polarization-independent HCG is used as two reflecting mirrors sandwiching the active layer from above and below.

That is, in the SOI (Silicon on Insulator) substrate 20 having a structure in which the SiO ₂ layer 22 is inserted between the Si substrate 21 and the surface Si layer 23, the SiO ₂ layer 22 is lowered on the surface Si layer 23. A two-dimensional grating structure 37 constituting the lower reflecting mirror, that is, a polarization-independent HCG is formed as a refractive index layer. Further, the surface Si layer 23 other than the portion where the polarization bistable surface emitting laser 30 is formed is partially removed, and the remaining surface Si layer 23 propagates light generated by laser oscillation. A substantially rectangular optical waveguide 24 is formed.

On the surface Si layer 23, an n-type indium / phosphorus (n-InP) layer 32, indium / phosphorus / indium / gallium, in that order (from the Si substrate 21 side) with the SiO ₂ layer 31 in between. An arsenic / phosphorus (InP / InGaAsP) layer 33 and a p-type indium / phosphorus (p-InP) layer 34 are provided so that a very thin semiconductor layer is sandwiched between both sides by a layer having a larger band gap than the MQW. An active region having a (Multiple Quantum Well) structure is formed. An air layer 36 using a p-type indium / gallium / arsenic / phosphorus (p-InGaAsP) layer 35 as a spacer is formed on the p-InP layer 34, and the air layer 36 is formed on the low refractive index layer. As shown, a two-dimensional lattice structure 38 is formed by a p-type indium phosphorus (p-InP) layer constituting the upper reflecting mirror.

  As shown in the figure, each layer provided above the lower half of the n-InP layer 32 is processed so as to have a substantially square shape when viewed from above, and its side surface is covered with a buried portion 39 made of polyimide. . Then, the p-electrode 40 is provided where the p-InP layer, which is the two-dimensional lattice structure 38 above the embedded portion 39, is exposed, and the n-electrode 41 is provided where the n-InP layer 32 is exposed. Thus, one polarization bistable surface emitting laser 30 is formed. Current is injected from the p electrode 40 and the n electrode 41 into the active region of the MQW structure. As a result, the active region has an optical gain, and laser oscillation occurs between the two-dimensional grating structure 37 that is the lower reflecting mirror and the two-dimensional grating structure 38 that is the upper reflecting mirror.

  The laser light generated in the resonator formed by the active region and the upper and lower reflecting mirrors is polarized light Ey (90 °) whose vibration direction is the Y-axis direction and polarized light Ex (0 °) whose vibration direction is the X-axis direction. ) And either. As described above, the polarization is switched according to the polarization of another light control signal incident from above along the Z axis. This is the same as the conventional polarization bistable surface emitting laser. One of the features of the polarization bistable surface emitting laser of the present embodiment is that the resonator of the polarization bistable surface emitting laser 30 is coupled to the optical waveguides 24x and 24y, and therefore the light confined in the resonator. Is output upward along the Z axis, and the optical output is also output to the optical waveguides 24x and 24y.

  In this embodiment, the optical waveguide 24x is formed so as to extend in the X-axis direction, and the optical waveguide 24y is formed so as to extend in the Y-axis direction, and the polarization bistable surface emitting laser 30 is provided at the intersection. Yes. Therefore, as shown in FIGS. 6, 7 and 8, optical waveguides 24x and 24y orthogonal to each other are formed in a top view girder shape, and polarization bistable surface emitting lasers 30 (in FIG. 6, 30A1, 30A2,...) May be provided as a polarization bistable surface emitting laser array structure.

  FIG. 4 is a schematic perspective view showing a state in which two polarization bistable surface emitting lasers 30A and 30B are arranged on the optical waveguide 24x extending in the X-axis direction. As shown in FIGS. 3 and 4, the optical waveguide 24x is strongly coupled to the polarized light Ey (90 °) whose vibration direction is the Y-axis direction, while the optical waveguide 24y is the polarized light Ex ( (0 °) Therefore, for example, in FIG. 4, when the polarization in the resonator of the polarization bistable surface emitting laser 30A is Ey (90 °), a part of the polarization is output with a large energy to the optical waveguide 24x. On the other hand, almost no output is made to the optical waveguide 24y and it can be ignored. Conversely, when the polarization in the resonator of the polarization bistable surface emitting laser 30A is Ex (0 °), a part of the polarization is output with a large energy to the optical waveguide 24y, whereas Almost no output to the optical waveguide 24x is negligible.

  That is, when attention is paid to the optical waveguide 24x or 24y extending in one of the X-axis direction and the Y-axis direction, the light output from the polarization bistable surface emitting laser 30A to the optical waveguide 24x or 24y depends on the polarization. It will be turned on and off. If attention is paid to both the optical waveguides 24x and 24y, the light output from the polarization bistable surface emitting laser 30A to the optical waveguides 24x and 24y is switched according to the polarization. As described above, since the polarized light oscillated by the polarization bistable surface emitting laser 30 is switched by the polarization of the light control signal supplied from the outside, the light output to the optical waveguide 24 as described above is turned on / off and the light is output. Switching between the optical waveguides 24x and 24y can be controlled by polarization of a light control signal incident on the polarization bistable surface emitting laser 30.

  FIG. 5 is an operation explanatory diagram of a configuration example in which the polarization bistable surface emitting laser shown in FIG. 3 is arranged one-dimensionally. In this example, a plurality of polarization bistable surface emitting lasers 30A, 30B,... Are arranged on one optical waveguide 24x extending in the X-axis direction. As shown in FIG. 5A, when the polarization oscillated by the polarization bistable surface emitting laser 30A is Ey (90 °), light is output from the polarization bistable surface emitting laser 30A to the optical waveguide 24x. Then, by being input to the adjacent polarization bistable surface emitting laser 30B, the polarization of the surface emitting laser 30B is switched to the same Ey (90 °) as that of the preceding surface emitting laser 30A. Such polarization switching is sequentially performed, and the polarization of all the polarization bistable surface emitting lasers 30A, 30B,. On the other hand, as shown in FIG. 5B, when the polarization of each polarization bistable surface emitting laser 30A is Ex (0 °), no light is output from the polarization bistable surface emitting laser 30A to the optical waveguide 24x. . Therefore, when the polarization in the polarization bistable surface emitting laser 30A is Ex (0 °), an optical signal is not transmitted to the adjacent polarization bistable surface emitting laser 30B.

  In addition, the polarization in all the polarization bistable surface emitting lasers 30A, 30B,... Is not only aligned to Ey (90 °), but also the polarization of all the polarization bistable surface emitting lasers 30A, 30B,. A synchronization phenomenon occurs, and the wavelengths of oscillation light of all the polarization bistable surface emitting lasers 30A, 30B,... Can be made the same. If the wavelengths of light output from the polarization bistable surface emitting lasers 30A, 30B,... To the optical waveguide 24x are the same, light emitted from each resonator in the Z-axis direction (that is, in free space) is added. As a result, a high-power laser beam can be obtained.

  FIG. 6 is an operation explanatory diagram in a configuration example in which the polarization bistable surface emitting laser shown in FIG. 3 is two-dimensionally arranged. In this example, a plurality of polarization bistable surface emitting lasers 30A1, 30B1,... Are arranged at the intersections of the optical waveguide 24x extending in the X-axis direction and the optical waveguide 24y extending in the Y-axis direction. As shown in FIG. 6A, when the polarized light oscillated by the polarization bistable surface emitting laser 30A1 arranged on one optical waveguide 24x in the X-axis direction is Ey (90 °) The other polarization bistable surface emitting lasers 30B1, 30C1,... Are switched to the same polarization by the light injection. At this time, no light is output to the optical waveguide 24y in the Y-axis direction.

  On the other hand, as shown in FIG. 6B, the polarized light oscillated by the polarization bistable surface emitting laser 30A1 arranged on one optical waveguide 24y in the Y-axis direction is Ex (0 °). , The polarizations of the polarization bistable surface emitting lasers 30A1, 30A2,... Are aligned at Ex (0 °). At this time, no light is output to the optical waveguide 24x in the X-axis direction. Therefore, in this two-dimensional array configuration, the transmission direction of the optical signal can be switched by aligning the polarizations of all the polarization bistable surface emitting lasers arranged on one optical waveguide 24x or 24y. . Similarly to the one-dimensional array configuration described above, the surface emitting laser can be obtained by aligning the polarizations of all the polarization bistable surface emitting lasers arranged on one optical waveguide 24x or 24y to any one and aligning the oscillation wavelength. It is possible to obtain a high-power laser beam by aligning the phase of the light emitted directly from the resonator to the free space.

  7 and 8 are diagrams showing a configuration example of a two-dimensional shift register using the polarization bistable surface emitting laser shown in FIG. The 1-bit memory constituting the shift register has a memory unit for holding data. Data input to the memory unit (Data (90 °)) and reset input (Reset ( 0 °)) and a set input (Set (90 °)) for setting the memory contents of the memory unit. Here, the polarization bistable surface emitting laser is used as an all-optical memory unit that records 1-bit information (digital binary information of “0” or “1”) as a polarization state. In the configuration example shown in FIG. 7, the data signal light input, the reset light input, and the set light input are all performed through the optical waveguide, and the configuration example shown in FIG. 8 is the data signal light input and the reset light. The input is performed through the optical waveguide, and the set light input is given from the upper space of the polarization bistable surface emitting laser, and the basic operation is common.

FIG. 9 is an operation timing chart of the 1-bit memory of the shift register. In the data signal light, set light having polarization Ey (90 °) is input in accordance with a bit to be recorded in the memory unit. In the data signal light, binary “0” is polarization Ex (0 °), and binary “1” is polarization Ey (90 °). The data signal light and the set light are combined, and the light having the intensity obtained by adding the two light intensities is input to the resonator of the polarization bistable surface emitting laser. As shown in FIG. 9, when it is desired to record bit a ₄ in the data signal light with the oscillation polarization of the polarization bistable surface emitting laser being Ex (0 °), the input timing of bit a ₄ is When the set light is input in synchronization, the intensity of the combined polarization Ey (90 °) exceeds the switching threshold. As a result, the oscillation polarization is switched from Ex (0 °) to Ey (90 °). That is, the data value “1” is recorded as the oscillation polarization Ey (90 °).

When reset light having polarization Ex (0 °) whose light intensity is set so as to exceed the switching threshold is input, the oscillation polarization is switched from Ey (90 °) to Ex (0 °). In this state, when it is desired to record the bit b ₄ in the data signal light, if the set light is input in synchronization with the input timing of the bit b ₄ , the data signal light is zero at this time, so that it is multiplexed. The intensity of polarized light Ey (90 °) does not exceed the switching threshold. As a result, the oscillation polarization is maintained at Ex (0 °), and the data value “0” is recorded as the oscillation polarization Ex (0 °). In this way, the oscillation polarization of the polarization bistable surface emitting laser is determined as Ex (0 °) or Ey (90 °) depending on whether the data signal light is “0” or “1”.

  In the two-dimensional shift register shown in FIG. 7 and FIG. 8, the polarization bistable surface emitting lasers are arranged at the intersections of the optical waveguides assembled in a cross beam shape, and are set independently for each polarization bistable surface emitting laser. Since optical input is possible, the data value is selectively selected according to the state of oscillation polarization from the polarization bistable surface emitting laser (laser located at the upper left in FIGS. 7 and 8) in which data is recorded first. Can be transferred to a polarization bistable surface emitting laser adjacent to the X axis direction or the Y axis direction.

  The shift register operation is common to both the configurations of FIGS. 7 and 8. However, if the set light input is input from above the surface emitting laser as shown in FIG. There is an advantage that other elements such as an optical gate can be easily incorporated. On the other hand, in terms of thinning and miniaturization of the apparatus, as shown in FIG. 7, it is better to be able to input / output all light including set light input on the substrate.

Next, the structure of the polarization-independent HCG actually fabricated by the inventors of the present application and the measured characteristics thereof will be described. Table 1 shows the structural parameters of the prototype wavelength-independent HCG.
As a procedure for producing HCG, first, an electron beam resist is applied to the surface of an SOI substrate, a predetermined HCG pattern is formed on the resist using an electron beam drawing apparatus, and then reactive ion etching is performed to perform two-dimensional. A void portion of the lattice structure was formed. An SEM observation image of the prototype polarization-independent HCG is shown in FIG.

  FIG. 11 is a diagram illustrating a result of calculating a power reflection spectrum of a prototype polarization-independent HCG using a finite difference time domain method. As boundary conditions for the calculation, periodic boundary conditions were used in the in-plane direction of the two-dimensional lattice structure, and absorbing boundary conditions were used in the direction perpendicular to the plane. As described above, the structure of the polarization-independent HCG has 90 ° rotational symmetry about an axis orthogonal to the plane, so that in principle, the high polarization as shown in FIG. A reflectance wavelength band should be realized.

  FIG. 12 is an actual measurement result of the reflectance spectrum of the prototype polarization-independent HCG. In this measurement, light from a light source having a wide wavelength band was passed through a linear polarizer, then condensed to a diameter of about 100 [μm], and incident on a polarization-independent HCG. Then, the reflected light was introduced into an InGaAs array spectrometer, and the reflectance spectrum was measured while rotating the polarization-independent HCG at an interval of 30 ° with respect to the incident linearly polarized light. As can be seen from FIG. 12, a high reflectivity close to 100% is obtained in the wavelength band of 1.43 [μm] to 1.58 [μm].

FIG. 13 shows the results of summarizing the polarization direction dependence of the reflectance at each wavelength of 1.45 [μm], 1.50 [μm], and 1.55 [μm]. There is a fluctuation of about 10% in the power reflectance at each wavelength. This variation can be presumed to be mainly due to the difference between the rotation center of the stage on which the polarization-independent HCG is mounted and the position of the sample. Note that the decrease in reflectance on the longer wavelength side than the wavelength of 1.58 [μm] observed in FIG. 12 is caused by the decrease in sensitivity of the spectrometer used for measurement (that is, measurement limitations). It can be assumed that this is not due to a decrease in power reflectivity of the prototype polarization-independent HCG itself.
From the above measurement results, it can be concluded that the polarization dependence of the reflectance of the prototype polarization-independent HCG is sufficiently small. As a result, it was confirmed that a broadband high reflectivity characteristic independent of polarization in the wavelength band of 1.55 [μm] can be realized.

Subsequently, as shown in FIGS. 3 and 4, the optical output of the polarization bistable surface emitting laser having a structure in which the resonator is coupled to the optical waveguide through the two-dimensional grating structure constituting the lower reflecting mirror. The examination result by the simulation calculation of characteristics will be described.
FIG. 14 is a schematic cross-sectional view of the polarization bistable surface emitting laser assumed here. In this polarization bistable surface emitting laser, the polarization-independent HCG as described above is used as the lower reflecting mirror, while DBR is used as the upper reflecting mirror. In FIG. 14, the same reference numerals are given to the portions corresponding to the components in the structure shown in FIGS. 3 and 4. That is, in the SOI substrate 20 having a structure in which the SiO ₂ layer 22 is inserted between the Si substrate 21 and the surface Si layer 23, a two-dimensional lattice structure 37, that is, polarization-independent HCG and An optical waveguide 24 is formed, and an active region is disposed on the optical waveguide 24 via a SiO ₂ bonding layer 31. On top of that, DBR is arranged instead of HCG. The DBR disposed between the SiO ₂ bonding layer 31 and the active region is for preventing the deterioration of the active region that occurs during device fabrication.

The number of periods of the polarization-independent HCG grating corresponding to one polarization bistable surface emitting laser is 6, and the width of the optical waveguide is 5.3 [μm]. The other structural parameters were optimized so that the reflectivity was maximized at a wavelength of 1.55 [μm] and the resonance wavelength was 1.55 [μm]. Specifically, the parameters of polarization-independent HCG are: Λ = 884 [nm], W = 178 [nm], T _g = 400 [nm], DC (ratio of Si portion per cycle of HCG) = 0.32, T _L1 = 1000 [nm], T _L2 = 540 [nm]. The active region is composed of nine quantum wells and InP spacers composed of InGa _0.24 As _0.82 P (5.4 [nm]) / InGa _0.22 As _0.48 P (8.6 [nm]), and gain is not considered. The DBR below the active region is composed of 2.5 pairs of InP / InGa _0.34 As _0.74 P, and the upper DBR is composed of 8.5 pairs of InP / InGa _0.34 As _0.74 P and 6 pairs of Al _0.16 GaAs / Al _0.90 GaAs. Further, the incident light is Ex (0 °) linearly polarized light and a Gaussian plane wave, and the total width at which the intensity becomes 1 / e ² is 3 [μm]. The y-axis direction orthogonal to the paper surface of FIG. 14 has the same structure as that shown in FIG.

  The output of the optical waveguide 24 is output from the start point of the optical waveguide 24 by Monitor X arranged 10 [μm] apart in the X-axis direction and Monitor Y (not shown) arranged 10 [μm] apart in the axial direction. Observed. On the other hand, the calculation region is defined by the absorption boundary condition indicated by a thick dashed line in FIG. 14, and the size of the calculation mesh is 30 [nm]. However, in a region of 30 nm or less such as a quantum well, a mesh corresponding to the thickness of each film layer is arranged. For the calculation, FullWAVE manufactured by Synopsys, which is a commercially available three-dimensional finite difference time domain simulator, was used.

  FIG. 15 is a diagram showing the calculation results of the light intensity and the electromagnetic field distribution at the observation positions Monitor X and Monitor Y. The light intensity is normalized by the maximum value of intensity in Monitor Y, and the electromagnetic field amplitude is normalized by the maximum values of Ex and Hz in Monitor Y. From this figure, it is understood that when Ex (0 °) polarized light is input, the light intensity propagating through the optical waveguide in the Y-axis direction is stronger than the light intensity propagating through the optical waveguide in the X-axis direction. As a result of the numerical calculation, the power of the optical waveguide output in the Y-axis direction is 4.5 times larger than that in the X-axis direction. This difference in power indicates that an optical waveguide to be output by polarized light can be selected. This simulation also revealed that light propagated in a mode close to the TE mode in both optical waveguides.

  In addition, according to the inventors' subsequent investigation, the power ratio between the optical waveguide output in the Y-axis direction and the optical waveguide output in the X-axis direction is set to 1: by making the optical waveguide close to a single mode. It was found that it was possible to expand from 4.5 to about 1:12. By realizing such a large power ratio, switching and selection between the optical waveguide in the Y-axis direction and the optical waveguide in the X-axis direction can be performed appropriately and efficiently. It can be said that it is effective for a device using such switching and selection.

In addition, the coupling efficiency to the optical waveguide in a polarization bistable surface emitting laser having a similar structure was also obtained by simulation calculation. The structure of the polarization bistable surface emitting laser assumed here is similar to that shown in FIG. 14, and an HCG and an optical waveguide are formed on the Si layer of the SIO substrate, and an SiO ₂ bonding layer is formed thereon. The active area is arranged. The DBR above it is 20 sets of structures. The parameters of the polarization-independent HCG are Λ = 632 [nm], T _g = 443 [nm], DC = 0.6, T _L1 = 1425 [nm], T _L2 = 470 [nm]. Each of these parameters is a result of optimization so that the reflectance is maximized at a wavelength of 1.55 [μm] and the resonance wavelength is 1.55 [μm]. The active region is composed of seven quantum wells and InP spacers made of In _0.76 Ga _0.24 As _0.82 P _0.18 (5.4 [nm]) / In _0.78 Ga _0.22 As _0.48 P _0.52 (8.6 [nm]), and has a refractive index. Are 3.51, 3.33, and 3.17, respectively. Al _0.16 Ga _0.84 As / Al _0.90 Ga _0.10 As DBRs (refractive indices are 3.34 and 2.96, respectively) leave the bottom five layers to form a mesa portion having a 10 [μm] □ shape when viewed from above. The starting point of the optical waveguide is defined by a distance d from the center position of the mesa portion.

  Further, the incident light source has an intensity distribution of the 0th order mode of the mesa portion, and this is disposed on the active region. Similarly to FIG. 14, the surface emitting output of the surface emitting laser and the output of the optical waveguide are X from the position of Monitor O, which is 500 [nm] away from the DBR top layer in the Z-axis direction above the DBR, and from the start point of the optical waveguide. Observed at the position of Monitor C 5 [μm] away in the axial direction. The calculation area is defined by the absorbing boundary condition, and the size of the calculation mesh is 50 [nm]. However, a mesh corresponding to the thickness of each film layer is arranged in a region of 50 nm or less such as a quantum well. As in the above example, FullWAVE manufactured by Synopsys was used for the calculation.

  FIG. 16 is a diagram showing the power ratio of the optical waveguide output to the surface emission output in the laser oscillation state with respect to the distance d. The gain is set to 1.3 times the threshold gain at each distance d. The output of the optical waveguide increases at a short distance d from around 5 [μm] where the oscillation mode and the optical waveguide begin to overlap. It can be seen that at the distance d = 6 [μm], the mesa portion and the optical waveguide do not overlap, but light is coupled to the optical waveguide. This is because the confinement in the X-axis direction is weak when the laser oscillation mode is at the HCG position, and is slightly wider than the width of the mesa portion.

17A shows the calculation result of the electromagnetic field | Hy | ² distribution in the entire calculation region when the distance d = 6 [μm], and FIG. 17B shows the Hy distribution (solid line) at the center of the Si layer and the top view. The calculation result of the zeroth-order mode distribution (dotted line) of the mesa part of 10 [μm] □ is shown. From FIG. 17B, it can be seen that the Hy distribution inside the HCG is slightly wider than the 0th order mode of the mesa portion.
From these results, in the optical waveguide coupled polarization bistable surface emitting laser studied here, the laser output in the perpendicular direction and the output to the optical waveguide are determined by the intensity of the resonance mode at the starting point of the optical waveguide. It was confirmed that the output to the optical waveguide of about 1 to 2% can be obtained by matching the position of the end of the portion with the position of the start point of the optical waveguide with an accuracy of about 1 [μm]. This also reveals that the device using the polarization bistable surface emitting laser as described above can sufficiently propagate the output through the optical waveguide.

  It should be noted that any of the above-described embodiments is an example of the present invention, and it should be understood that modifications, additions, and the like as appropriate within the scope of the present invention are included in the scope of the claims of the present application.

10 ... reflector 11 ... semiconductor substrate 12 ... low-refractive index layer 13 ... two-dimensional grating structure 20 ... SOI substrate 21 ... Si substrate 22 ... SiO ₂ layer 23 ... surface Si layer 24 ... optical waveguide 30 ... polarization bistable surface emitting Laser 31 ... SiO ₂ layer 32 ... n-InP layer 33 ... InP / InGaAsP layer 34 ... p-InP layer 35 ... p-InGaAsP layer 36 ... air layers 37, 38 ... two-dimensional lattice structure 39 ... buried portion 40 ... p Electrode 41 ... n electrode

Claims (5)

Resonance comprising a semiconductor substrate, a lower reflecting mirror formed on the semiconductor substrate, an active layer for generating light disposed on the lower reflecting mirror, and an upper reflecting mirror formed on the active layer A surface emitting laser device comprising:
At least the lower reflecting mirror is formed in a low refractive index layer occupying a region having a first refractive index and extending in a planar shape, and one semiconductor layer formed on the semiconductor substrate, and the low refractive index It is arranged so as to be in contact with one surface of the layer, and is made of a material having a second refractive index larger than the first refractive index, and has a rotational symmetry of 90 ° with respect to an axis orthogonal to the low refractive index layer. A grid-like two-dimensional lattice structure, and for the light having an arbitrary polarization incident along the direction of the axis, the reflectance over a target wavelength band is not less than a predetermined value, Parameters including the first refractive index, the second refractive index, the thickness of the low refractive index layer, the thickness of the two-dimensional grating structure, the pitch of the grating of the two-dimensional grating structure, and the grating width are adjusted. Reflector with high refractive index difference grating structure I can,
An optical waveguide for guiding light to the semiconductor layer in which the two-dimensional lattice structure is formed is arranged in a grid shape so that the two-dimensional lattice structure is located at an intersection,
An optical waveguide coupling type surface emitting laser device characterized in that it is possible to transmit polarized light beams having different vibration directions from the resonator including the two-dimensional grating structure to the optical waveguides extending in different directions . The surface-emitting laser device according to claim 1 ,
As the upper reflecting mirror, a reflecting mirror having the high refractive index difference grating structure is used, and an air layer is a low refractive index layer located between the two-dimensional grating structure included in the reflecting mirror and the active layer. A surface emitting laser device having a bridge structure. The surface-emitting laser device according to claim 1 or 2 ,
Said light control signal for switching the polarization of the light confined in the cavity surface emitting laser device being characterized in that so as to incident along the direction of the axis from the outside of the upper reflector. The surface-emitting laser device according to any one of claims 1 to 3 is arranged in a two-dimensional array, and the optical waveguide is arranged so that the two-dimensional grating structure included in each surface-emitting laser device is located at an intersection. A surface emitting laser unit arranged in a shape,
A plurality of surface-emitting laser devices disposed on the optical waveguide that are linearly extended in the optical waveguide are optically coupled and injection-locked to align the wavelengths of specific polarizations oscillated by the plurality of surface-emitting laser devices. A surface emitting laser unit characterized in that a high output laser is taken out. The surface-emitting laser device according to any one of claims 1 to 3 is arranged in a two-dimensional array, and the optical waveguide is arranged so that the two-dimensional grating structure included in each surface-emitting laser device is located at an intersection. A surface emitting laser unit arranged in a shape,
A plurality of surface emitting lasers disposed on the optical waveguide extending linearly in the optical waveguide according to a light control signal incident on each surface emitting laser device to switch the polarization of light confined in the resonator By switching the polarized light oscillating in some of the surface emitting laser devices, the number of surface emitting laser devices that are strongly coupled on the optical waveguide is changed to change the output of the extracted light. A surface emitting laser unit characterized by the above.