JP4595584B2 - Tunable semiconductor laser - Google Patents

Description

  The present invention relates to a semiconductor laser, and more particularly to a wavelength tunable semiconductor laser capable of changing an oscillation wavelength.

  In recent years, with the development of a wavelength division multiplexing (WDM) system, a tunable laser light source capable of changing the oscillation wavelength has been demanded. Various types of laser light sources have been proposed. Here, two conventional techniques are exemplified.

  First, as a first prior art, there is a distributed Bragg reflection type semiconductor laser (hereinafter also referred to as a DBR laser).

  This DBR laser has a problem that mode hopping in which the oscillation wavelength changes discontinuously occurs as the oscillation wavelength is changed.

  This mode hopping is derived from the fact that the oscillation wavelength (longitudinal mode wavelength) of the laser light source takes a discrete value determined by the optical path length of the resonator, and the oscillation wavelength and the currently selected longitudinal mode wavelength. This is a phenomenon in which the oscillation wavelength changes discontinuously to the adjacent longitudinal mode wavelength when the wavelength shift between the two is increased.

  On the other hand, in a DBR laser, a three-electrode DBR laser is known in which the influence of mode hopping is reduced and the oscillation wavelength can be continuously changed over a wider range. This three-electrode DBR laser has a structure in which a phase adjustment region for adjusting the phase of propagating light is provided between an active region and a distributed Bragg reflector region.

  In this three-electrode DBR laser, the current applied to the phase adjustment region is controlled simultaneously with the current applied to the distributed Bragg reflector region. As a result, discontinuous changes in the oscillation wavelength during mode hopping can be suppressed to some extent. Accordingly, the three-electrode DBR laser can change the oscillation wavelength over a range of about 10 nm (see, for example, Patent Document 1).

  As a second conventional technique, there is an external resonator type wavelength tunable laser light source in which a laser light source, an optical deflector, a lens, and a reflective diffraction grating are arranged in series in this order along the light propagation direction. .

  More specifically, in the second prior art, a laser light source and an optical deflector that changes the propagation direction of light generated by the laser light source are monolithically connected.

  The light generated from the laser light source is emitted toward a lens provided outside the laser light source after the emission angle is adjusted by an optical deflector. The light passing through the lens is refracted and enters the reflective diffraction grating at a predetermined incident angle. At this time, the light propagation path (resonator length) from the laser light source to the reflective diffraction grating changes according to the light emission angle.

  Of the light incident on the reflection type diffraction grating, the return light that has been made monochromatic by being reflected at a diffraction angle equal to the incident angle passes through the opposite path (reflection type diffraction grating → lens). Return to the laser source.

In the second prior art, the light emission angle is changed by an optical deflector to make the light monochromatic and at the same time change the length of the light propagation path (resonator length). Thereby, the oscillation wavelength of light is changed without causing mode hopping (see, for example, Non-Patent Document 1).
Japanese Patent Laying-Open No. 2004-273644 (FIG. 3) Oh Kee Kwon et al. “Proposal of Electrically Tunable External-Cavity Laser Diode”, IEEE PHOTOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 8, AUGUST 2004, pp. 1804-1806

  In the first prior art described above, the change range of the oscillation wavelength is about 10 nm, which is never a satisfactory width.

  Further, the second conventional technique described above solves the problem of mode hopping. However, in order to change the oscillation wavelength without causing mode hopping, a lens has to be disposed between the laser light source and the reflective diffraction grating. Therefore, there is a problem that the number of parts constituting the wavelength tunable laser light source increases.

  The present invention has been made in view of such problems. Therefore, the present invention (1) eliminates the mode hopping when changing the oscillation wavelength without increasing the number of components compared to the conventional one, and (3) changes the oscillation wavelength in a wider range than before. An object of the present invention is to provide a tunable semiconductor laser that can be made to operate.

  In order to achieve the above object, the present invention is configured as follows.

  In other words, the wavelength tunable semiconductor laser according to the first aspect includes an active region, an optical waveguide, an optical coupling unit, an optical deflecting unit, a reflective diffraction grating, and a reflective surface.

  The optical coupling part performs optical coupling between the optical waveguide and the active region. The optical waveguide is disposed with the side surface along the light propagation direction in which light propagates facing the incident / exit surface of the active region.

  The optical deflection unit is provided to face the active region with the optical coupling unit interposed therebetween, and includes all or part of the optical waveguide.

  The reflection-type diffraction grating is provided with respect to the optical waveguide with the active region interposed therebetween, and returns light incident from the optical waveguide to the optical waveguide as wavelength-selected return light.

  The reflecting surface is provided on the light emitting end surface orthogonal to the light propagation direction of the optical waveguide. This reflective surface, together with the above-described reflective diffraction grating, constitutes an optical resonator.

  The light deflector described above changes the incident angle of the light traveling from the optical waveguide toward the reflective diffraction grating and the length of the optical path by changing the exit angle of the light exiting from the optical waveguide.

  As described above, according to the wavelength tunable semiconductor laser of the present invention, the light generated in the active region is resonated and emitted as light of a predetermined wavelength in the optical resonator composed of the reflective surface and the reflective diffraction grating. It is taken out from the end face. In this optical resonator, an incident angle and an optical path length (resonator length) of light incident on the reflection type diffraction grating are changed by changing the emission direction of the light from the optical waveguide toward the reflection type diffraction grating by the optical deflection unit. At the same time. In this way, by simultaneously selecting the wavelength that accompanies the change in the incident angle to the reflective diffraction grating and changing the resonator length, mode hopping can be eliminated without providing a lens, and the oscillation wavelength can be adjusted over a wide range. Can be changed.

  According to the tunable semiconductor laser of the second aspect, the reflection type diffraction grating is a separate component from the active region.

  As described above, according to the present invention, the reflection type diffraction grating that requires fine processing can be manufactured separately from the active region by making the reflection type diffraction grating separate from the active region.

  According to the wavelength tunable semiconductor laser of the third aspect, the reflection type diffraction grating is formed in the active region on the end face irradiated with the light incident on the active region from the optical waveguide.

  Thus, according to the present invention, since the reflection type diffraction grating is provided on the end face of the active region, the number of components constituting the wavelength tunable semiconductor laser can be reduced.

  According to the wavelength tunable semiconductor laser of claim 4, the light deflector changes the refractive index of the optical waveguide electro-optically to change the light traveling toward the reflective diffraction grating to the reflective diffraction grating. It is a means for changing the incident angle.

  As described above, according to the present invention, the refractive index of the optical waveguide optically coupled to the active region can be changed using the electro-optic effect by using the optical deflection unit. By adjusting the voltage applied to the optical deflection unit, the emission angle of the light emitted from the optical coupling unit, and hence the incident angle of the light traveling toward the reflective diffraction grating, can be easily adjusted. .

  According to the wavelength tunable semiconductor laser of claim 5, the grating surface of the reflection type diffraction grating is formed in a flat shape, and this reflection type diffraction grating is formed by the method of the light propagation direction of the optical waveguide and the method of the grating surface. It arrange | positions so that a line may make an angle of 45 degrees.

  As will be described in detail later, by arranging a reflection type diffraction grating having a planar grating surface so as to form such an angle, along the optical path in the active region interposed between the optical waveguide and the reflection type diffraction grating. Regardless of the length, mode hopping can be eliminated. That is, under this arrangement condition, the length of the active region along the optical path can be set to an arbitrary length according to the design.

  According to the wavelength tunable semiconductor laser of the sixth aspect, the optical waveguide is further provided with the phase adjustment region for changing the optical path length of the optical path.

  Thus, according to the present invention, since the phase adjustment region is provided in the optical waveguide, the optical path length of the optical path can be changed. Therefore, laser oscillation can be easily achieved at a desired oscillation wavelength by finely adjusting the initial phase.

  According to the wavelength tunable semiconductor laser of claim 7, the reflection type diffraction grating is configured such that the light propagation direction of the optical waveguide and the normal of the grating surface form an angle larger than 0 ° and smaller than 90 °. The blazed gratings intersect each other when the light propagation direction of the light from the optical waveguide toward the reflection type diffraction grating in the state where the incident angle does not change the incident angle is set as the initial direction. A grating groove comprising a first surface parallel to the initial direction and a second surface perpendicular to the initial direction, and a length along the initial direction of the first surface, The natural number times the center wavelength in the active region.

  As described above, the length of the first surface along the initial direction is set to a natural number multiple of the center wavelength of the light generated in the active region in the active region. The intensity distribution of the light emitted from can be made smooth.

  According to the wavelength tunable semiconductor laser of claim 8, the active region, the optical waveguide, and the side surface along the light propagation direction of the active region are opposed to the optical waveguide, and the optical waveguide and the active region are optically coupled. An optical coupling part, an optical deflection part including an optical waveguide provided opposite to the active region across the optical coupling part, and an active region provided across the optical waveguide, Reflective diffraction grating that returns incident light to the active region as wavelength-selected return light and a light emitting end face perpendicular to the light propagation direction of the active region, combined with the reflective diffraction grating. The optical deflector is configured to change the propagation direction of the light from the active region to the reflective diffraction grating, thereby changing the incident angle to the reflective diffraction grating and the length of the optical path. It is characterized by changing.

  Thus, this invention is the structure which replaced the active region and the optical waveguide in the invention of Claim 1. That is, the light generated in the active region is extracted from the light emitting end face as light having a predetermined wavelength by resonating with an optical resonator configured as an optical path between the reflective surface and the reflective diffraction grating. In this optical resonator, the light deflecting unit changes the emission direction of light from the active region toward the reflective diffraction grating, whereby the incident angle of light incident on the reflective diffraction grating and the length of the optical path (resonance). Change the instrument length) at the same time. In this way, by simultaneously selecting the wavelength that accompanies the change in the incident angle to the reflective diffraction grating and changing the resonator length, mode hopping can be eliminated without providing a lens, and the oscillation wavelength can be adjusted over a wide range. Can be changed.

  According to the wavelength tunable semiconductor laser of claim 9, the light propagation direction of the first optical waveguide, the second optical waveguide, and the first optical waveguide having an active region near the end on the light emitting end face side An optical coupling portion that optically couples the first and second optical waveguides with a side surface along the optical waveguide being opposed to the second optical waveguide, and a first optical waveguide provided opposite to the second optical waveguide with the optical coupling portion interposed therebetween The light deflector including the first optical waveguide and the first optical waveguide are sandwiched between the first optical waveguide and the light incident from the first optical waveguide is returned to the first optical waveguide as wavelength-selected return light. A reflection type diffraction grating to be formed, and a reflection surface provided on the light emitting end face of the active region orthogonal to the light propagation direction, and together with the reflection type diffraction grating, constitutes an optical resonator, and an optical deflection unit By changing the emission angle of the light from the first optical waveguide toward the reflective diffraction grating And changes the length of the incident angle and the light path to the reflection type diffraction grating.

  As described above, according to the present invention, the light generated in the active region is resonated by the optical resonator configured as the optical path between the reflecting surface and the reflective diffraction grating, so that the light is transmitted as light having a predetermined wavelength. It is taken out from the exit end face. In this optical resonator, the light deflecting unit changes the emission direction of light from the active region toward the reflective diffraction grating, whereby the incident angle of light incident on the reflective diffraction grating and the length of the optical path (resonance). Change the instrument length) at the same time. In this way, by simultaneously selecting the wavelength that accompanies the change in the incident angle to the reflective diffraction grating and changing the resonator length, mode hopping can be eliminated without providing a lens, and the oscillation wavelength can be adjusted over a wide range. Can be changed.

  As described above, according to the configuration of the wavelength tunable semiconductor laser of the present invention, the active region, the optical waveguide having the reflection surface, the optical coupling unit, the optical deflection unit, and the reflection type diffraction grating are provided. As a result, the incident angle and the optical path length of the light traveling from the optical waveguide to the reflective diffraction grating are changed and the return light wavelength-selected by the reflective diffraction grating is fed back to the optical waveguide. Yes.

  As a result, the wavelength tunable semiconductor laser according to the present invention does not use a lens, so that the number of components does not increase compared to the prior art. In addition, the wavelength tunable semiconductor laser of the present invention eliminates mode hopping when changing the oscillation wavelength, and can change the oscillation wavelength in a wider range than before.

  Next, an embodiment of the present invention will be described with reference to FIGS. Each figure is only a schematic illustration to the extent that the present invention can be understood with respect to the shape, size, and arrangement relationship of each component. Moreover, although the preferable structural example of this invention is demonstrated below, the material, numerical condition, etc. of each component are only a preferable example. Therefore, the present invention is not limited to the following embodiments.

(Embodiment 1)
The structure and operation of the wavelength tunable semiconductor laser according to the first embodiment will be described with reference to FIGS. FIG. 1 is a perspective view showing a schematic structure of a wavelength tunable semiconductor laser. FIG. 2 is an enlarged cross-sectional view of a main part taken along line II in FIG. FIG. 3 is a schematic diagram for explaining the structural condition and operation of the wavelength tunable semiconductor laser. FIG. 4 is an enlarged schematic view of the main part schematically showing the vicinity of the reflective diffraction grating of the wavelength tunable semiconductor laser. FIG. 5 is a diagram showing the results of a simulation performed on a wavelength tunable semiconductor laser. FIG. 6 is a diagram schematically showing the intensity distribution of light emitted from the light emitting end face. FIG. 7 is a schematic diagram for explaining the change in the arrangement angle of the reflective diffraction grating. FIG. 8 is a plan view showing a modification of the wavelength tunable semiconductor laser.

  A configuration example shown in FIG. 1 will be described. The wavelength tunable semiconductor laser 10 includes an active region 14, an optical waveguide 16, an optical deflecting unit 20, an optical coupling unit 30, and a reflective diffraction grating, each of which is formed by using a parallel planar plate-like substrate 12 having a rectangular planar shape. 18 is provided. The wavelength tunable semiconductor laser 10 is a monolithic wavelength tunable semiconductor laser in which all components are integrated on a common substrate 12.

  The planar shape of the substrate 12 is a rectangle. The optical waveguide 16 is provided on one end face side facing the longitudinal direction of the substrate 12. Further, the reflective diffraction grating 18 is provided on the other end face side of the substrate 12. At this time, the lattice plane 28 is on the optical waveguide 16 side.

  The active region 14 is a region that generates light by stimulated emission. The active region 14 is provided between the reflective diffraction grating 18 and the optical waveguide 16. The active region 14 includes a substrate 12, an active layer 32 (see FIG. 2) provided on one main surface (upper surface) 12a of the substrate 12, and a p-type cladding layer 34 (see FIG. 2) provided on the active layer 32. 2), electrodes 36 and 38 (see FIG. 2) provided on the p-type cladding layer 34 and on the other main surface (lower surface) 12b of the substrate 12, respectively. In the active region 14, a region functioning as a light propagation path is formed in the active layer 32 between the optical waveguide 16 and the reflective diffraction grating 18.

  The active region 14 has a lateral width (length along the short direction of the substrate 12) gradually narrowing along the direction (longitudinal direction of the substrate 12) from the reflective diffraction grating 18 side toward the optical waveguide 16 side. Go. That is, the active region 14 is formed in a form in which the lateral width is reduced in a substantially tapered shape from the reflective diffraction grating 18 side toward the optical waveguide 16 side in plan view. Therefore, the active region 14 is generally in the form of a triangular block having the same thickness as a reflective diffraction grating 18 to be described later, and the side surface of one side sandwiching the oblique side faces the grating surface 28. In addition, the side surface 14 c on the other side is provided to face the optical waveguide 16. The surface of the active region 14 that faces the optical waveguide 16 and the surface that faces the reflective diffraction grating 18 are flat surfaces that form a surface on which light enters and exits.

  The reflection type diffraction grating 18 is formed in the shape of a right triangle block having the same thickness, and has a configuration in which a grating surface 28 is formed on the block surface of the hypotenuse. The reflective diffraction grating 18 is a separate component from the active region 14. The grating surface 28 of the reflective diffraction grating 18 is orthogonal to the upper surface 12a of the substrate 12, and the respective grating grooves 50 are provided at regular intervals in a direction perpendicular to the upper surface 12a.

  The reflective diffraction grating 18 is arranged with side surfaces on two orthogonal sides parallel to two orthogonal side surfaces of the substrate 12.

  The optical waveguide 16 is a channel-type waveguide formed in a stripe shape, and includes an optical waveguide layer 43 (see FIG. 2) provided on the substrate 12 and a p-type layer 42 provided on the optical waveguide layer 43. (See FIG. 2). The optical waveguide 16 is provided on the upper surface 12 a of the substrate 12 in parallel with the side surface along the longitudinal direction of the substrate 12. From the one end 16b in the longitudinal direction of the optical waveguide 16 to the vicinity of the other end 16a, the side surface 16c of the optical waveguide 16 is opposed to the side surface of the active region 14 opposed thereto, that is, the incident / exit surface 14c. . The end face of the optical waveguide 16 on the one end 16b side faces the active region 14 with a gap. The end face on the other end 16 a side of the optical waveguide 16 is a light emitting end face 22 of the light of the wavelength tunable semiconductor laser 10.

  The optical coupling unit 30 is a component that optically couples the optical waveguide 16 and the active region 14. That is, the optical coupling portion 30 is provided between the side surface 16c parallel to the light propagation direction of the optical waveguide 16, the side surface 14c of the active region 14 facing the side surface 16c at a distance, and both side surfaces 16c and 14c. And an insulating layer 48 (see FIG. 2). The side surface 16 c of the optical waveguide 16 constitutes a surface that emits light whose propagation direction is deflected in the optical waveguide 16, and that receives reflected light from the reflective diffraction grating 18, that is, return light. The optical coupling portion 30 is provided from the other end 16a to a predetermined length W4 along the light propagation direction of the optical waveguide 16. This length W4 is preferably equal to or shorter than the total length of the optical waveguide 16.

  In the optical coupling unit 30, it can be considered that light is coupled with substantially 100% optical coupling efficiency. That is, when light is coupled from the optical waveguide 16 to the active region 14 and when light is coupled from the active region 14 to the optical waveguide 16, it can be considered that there is substantially no reduction in light intensity.

  In the optical waveguide 16, the light reaching the optical coupling portion 30 oozes out from the side surface 16 c to the insulating layer 48 as an evanescent wave, and enters the active layer 32 in the active region 14 through the side surface 14 c. By the way, in the optical coupling part 30, since light is coupled over the entire length W4, the light emitted toward the active region 14 has a width corresponding to the length W4. More specifically, when the emission angle of light emitted from the optical waveguide 16 to the active region 14 is Θ (see FIG. 3), the light emitted toward the active region 14 is the active region in the drawing. 14 has a width of W4sinΘ in a direction orthogonal to the light propagation direction. On the other hand, the light emitted from the active region 14 and reaching the optical coupling unit 30 is similarly incident on the optical waveguide 16 through the optical coupling unit 30.

  The width of the insulating layer 48 (length in the short direction) is set to a width that allows light to be coupled. This width is related to the optical coupling efficiency between the active region 14 and the optical waveguide 16. More specifically, when the width of the insulating layer 48 is wide, in order to achieve 100% optical coupling efficiency between the active region 14 and the optical waveguide 16, the length of the optical coupling unit 30 is increased. There is a need to. Conversely, when the width of the insulating layer 48 is narrow, the length of the optical coupling portion 30 may be short in order to achieve 100% optical coupling efficiency between the active region 14 and the optical waveguide 16. . Here, the optical coupling efficiency refers to the intensity ratio of the light transferred from the optical waveguide 16 (active region 14) to the active region 14 (optical waveguide 16) via the optical coupling unit 30.

  In this way, light can easily propagate between the active region 14 and the optical waveguide 16 via the optical coupling portion 30.

  A region of the optical waveguide 16 corresponding to the optical coupling unit 30 constitutes the optical deflection unit 20. The optical deflection unit 20 includes an optical waveguide layer 43, a p-type layer 42 (see FIG. 2) provided on the optical waveguide layer 43, an electrode 26 (see FIG. 2) provided on the p-type layer, and a substrate. The electrode 38 (refer FIG. 2) provided in the other main surface (lower surface) 12b of 12 is provided. By changing the voltage applied between the electrodes 26 and 38, the refractive index of the region of the optical waveguide layer 43 included in the optical deflection unit 20 is controlled based on the electro-optic effect.

  Hereinafter, this refractive index control will be specifically described.

Now, it is assumed that a predetermined voltage V (including the case where V = 0) is applied between the electrodes 26 and 38. At that time, the refractive index and n _e of the optical waveguide layer 43, the refractive index of the active region 14 is taken as n _act, through the optical coupling portion 30, light emitted from the optical waveguide 16 into the active region 14 emission angle theta of, and a theta _x. At this time, between the exit angle theta _x and the refractive index n _e and n _act it is known that the relationship of cosΘ _x = n _e / n _act is established. This shows that, by controlling the voltage applied to the electrodes 26 and changing the refractive index n _e of the optical waveguide layer 43 corresponding to the light deflection unit 20 and n _e ', because n _act is constant, the emission corner Θ _x is changed to Θ _x '. Therefore, light is emitted from the optical coupling unit 30 to the active region 14 in a state where the emission angles are aligned with Θ _x ′, that is, in a collimated state. In this way, by changing the emission angle Θ, the incident angle α (see FIG. 3) of the light to the reflective diffraction grating 18 also changes.

  The light emitting end face 22 constituting the end face 16a of the optical waveguide 16 is a reflecting face 24 on which a reflecting film having a predetermined reflectance (<100%) is formed.

The reflective diffraction grating 18 is disposed on the substrate 12 so as to be separated from the light deflection unit 20 with the active region 14 interposed therebetween. The reflection type diffraction grating 18 is a reflection type plane diffraction grating. The arrangement angle Θ _g of the reflection type diffraction grating 18 (see FIG. 3), that is, the extending direction of the grating surface 28 where the grating grooves 50 between the gratings are formed, that is, the oblique side of the right triangle, and the light propagation direction in the optical waveguide 16 The angle formed by is 45 °. The reflective diffraction grating 18 is a blazed grating in which the cross-sectional shape of the grating groove 50 is serrated, and the grating groove 50 is formed on the grating surface 28 so as to extend in the vertical direction of the drawing.

  Referring to FIG. 4, in the reflective diffraction grating 18, an arbitrary grating groove 50 is composed of a first surface 52 and a second surface 54 that intersect at right angles.

Here, the optical deflecting unit 20, a state that does not change the emission angle theta (incident angle alpha), i.e., in a state where the refractive index n _e of the optical waveguide layer 43 is not changed in the light deflecting section 20, from the optical waveguide 16 The light propagation direction of the light I ₀ toward the reflective diffraction grating 18 is defined as an initial direction, and the acute angle side of the angle formed by the initial direction and the light propagation direction of the optical waveguide 16 is defined as Θ ₀ .

At this time, in the plane parallel to the upper surface 12a of the substrate 12, the extending direction of the first surface 52 is parallel to the initial direction, and the extending direction of the second surface 54 is perpendicular to the initial direction. A lattice groove 50 is formed so that The length S along the initial direction of the first surface 52 is set equal to the center wavelength of the light generated in the active region 14 in the active region 14. Here, when the grating constant of the reflective diffraction grating 18 is d, a relationship of d = S / sin (Θ _g −Θ ₀ ) is established between d and S.

  Here, the “center wavelength in the active region 14” is a peak wavelength in the emission spectrum of light generated in the active region 14 and indicates a wavelength in the active region 14. This peak wavelength is determined by the band gap of the active region 14. In the following description, “the center wavelength of light generated in the active region 14 in the active region 14” is simply referred to as “center wavelength”.

  The reflective diffraction grating 18 is formed as a separate component from the substrate 12 and is attached to the upper surface 12 a of the substrate 12. Therefore, a gap 46 of about several μm exists between the lattice plane 28 and the end face 14 a of the active region 14 facing the lattice plane 28. A non-reflective coating is applied to the upper surface 12 a of the substrate 12 corresponding to the gap 46. This is because light emitted from the end face 14a toward the reflection type diffraction grating 18 and light directed to the end face 14a upon receiving the wavelength selection by the reflection type diffraction grating 18 (also referred to as reflected light or return light) are reflected on the upper surface 12a. This is to prevent a decrease in intensity caused by irregular reflection. The gap 46 is filled with any suitable optical resin according to the design.

  As described above, in the wavelength tunable semiconductor laser 10, the light generated in the active region 14 reciprocates a predetermined number of times through the optical path between the reflective diffraction grating 18 and the reflection surface 24 of the optical waveguide 16. The light is emitted from the light emitting end face 22 as light having a wavelength. That is, an optical resonator is constituted by the optical path between the reflection type diffraction grating 18 and the reflection surface 24 of the optical waveguide 16.

  Next, with reference to FIG. 1 and FIG. 2, details of the size, material, and laminated structure of the wavelength tunable semiconductor laser 10 will be described.

  As the substrate 12, for example, an n-type InP substrate having a thickness of about 300 μm is used. The dimension of the substrate 12 along the light propagation direction of the optical waveguide 16 is, for example, about 600 μm, and the dimension along the direction orthogonal to the light propagation direction of the optical waveguide 16 is, for example, about 50 μm.

  A reflective diffraction grating 18 is provided on one end side of the substrate 12 in the longitudinal direction. The reflective diffraction grating 18 is separate from the substrate 12 and is attached to the substrate 12 by a known method. The reflective diffraction grating 18 is formed, for example, by processing a Si substrate having a thickness of about 2.4 μm by a known method such as photolithography or etching. The length S along the initial direction of the first surface 52 of the reflective diffraction grating 18 is set equal to the center wavelength as described above. Here, the length S is about 0.4 μm, for example.

  The optical waveguide 16 has a length along the light propagation direction of, for example, about 320 μm, and a length (width) in a direction orthogonal to the light propagation direction in the drawing is, for example, about 2 μm. And the length of the optical coupling part 30 shall be about 300 micrometers, for example. In other words, in the optical coupling portion 30, the active region 14 and the optical waveguide 16 face each other over a length of about 300 μm via the insulating layer 48. In addition, the width of the insulating layer 48 in the optical coupling portion 30, that is, the interval between the side surfaces 14c and 16c is, for example, about 1 μm. When the width of the insulating layer 48 (about 1 μm) and the length of the optical coupling portion 30 (about 300 μm) satisfy the above-described conditions, the insulating layer 48 is substantially 100% between the optical waveguide 16 and the active region 14. Optical coupling efficiency is achieved.

The reflection surface 24 of the optical waveguide 16 is, for example, a so-called dielectric multilayer film in which SiO ₂ films and SiN films having different refractive indexes are alternately laminated.

  The length W1 of the active region 14 is about 230 μm, for example. Similarly, the length W2 of the active region 14 is about 580 μm, for example. Further, the length W3 of the active region 14 is, for example, about 10 μm.

  FIG. 2 schematically shows a cross-sectional structure of the active region 14, the optical waveguide 16, and the insulating layer 48.

  The active region 14 has a double heterostructure. That is, the active region 14 includes a substrate 12 as an n-type cladding layer, an active layer 32 made of InGaAsP formed on the substrate 12, and a p-type cladding made of p-type InP formed on the active layer 32. Layer 34. On the p-type cladding layer 34, there is provided an electrode 36 formed by annealing after laminating a Ti film, a Pt film, and an Au film on the p-type cladding layer 34 in this order. An electrode 38 having the same laminated structure as the electrode 36 is provided on the main surface 12 b of the substrate 12. The electrode 38 also serves as an electrode provided on the main surface 12b of the substrate 12 in the optical waveguide 16 described later, and is a common electrode. Then, a forward voltage is applied between the electrodes 36 and 38, whereby the active layer 32 emits light. The center wavelength of light generated in the active layer 32 is about 1.55 μm.

  Here, the film thickness of the active layer 32 is, for example, about 0.4 μm. The film thickness of the p-type cladding layer 34 is, for example, about 2 μm. The thickness of the electrodes 36 and 38 is, for example, about 0.5 μm.

  The optical waveguide 16 constitutes a pin structure. That is, the pin structure has a substrate 12 as an n-type layer, an intrinsic optical waveguide layer 43 made of InGaAsP formed on the substrate 12, and a p-type made of p-type InP formed on the optical waveguide layer 43. And a mold layer 42. In the optical waveguide layer 43, the composition of InGaAsP for confining and guiding light is different from that of the active layer 32. As a result, the band gap of the optical waveguide layer 43 is made larger than that of the active layer 32 so that the light generated in the active layer 32 is not absorbed by the optical waveguide layer 43. More specifically, in the optical waveguide layer 43, the band gap is increased by increasing the composition of the GaAs component of InGaAsP. The film thickness of the optical waveguide layer 43 is equal to that of the active layer 32, and is about 0.4 μm, for example. The thickness of the p-type layer 42 is the same as that of the p-type cladding layer 34, and is about 2 μm, for example.

  The light deflection unit 20 includes an electrode 26 on the upper surface of the pin structure. The electrode 26 has the same structure and film thickness as the electrodes 36 and 38 provided in the active region 14. The other main surface (lower surface) 12b of the substrate 12 is provided with the electrode 38 as the common electrode described above. Unlike the active region 14, a reverse voltage is applied between the electrodes 26 and 38.

  The insulating layer 48 is made of InP made insulating by doping Fe and has a thickness of about 2.4 μm. The insulating layer 48 is disposed on the substrate 12 in contact with the side surfaces 14c and 16c.

  As described above, the forward voltage is applied to the electrodes 36 and 38 of the active region 14, and the reverse voltage is applied to the electrodes 26 and 38 of the optical deflection unit 20. Since the active region 14 and the light deflection unit 20 are electrically insulated by the insulating layer 48, no leakage occurs between the electrode 36 and the electrode 26.

  It should be noted that the expression “the active region 14 generates light” and expressions similar thereto mean the meaning “the active layer 32 generates light”.

  Next, the operation of the wavelength tunable semiconductor laser 10, that is, the oscillation wavelength change that does not cause mode hopping will be described with reference to FIGS.

  When a forward current is applied to the electrodes 36 and 38, light is generated by stimulated emission in the active region 14. Part of the light generated in the active region 14 propagates through the optical waveguide 16 toward the light emitting end face 22 via the optical coupling unit 30. Then, the light reaches the reflection surface 24 and is reflected at the reflection surface 24 with a predetermined reflectance. The light reflected by the reflecting surface 24 propagates through the optical waveguide 16 in the direction of the optical deflection unit 20 (optical coupling unit 30).

The light deflection unit 20 is provided with electrodes 26 and 38, and a predetermined voltage is applied thereto. As a result, the optical waveguide layer 43 in the light deflecting section 20, the refractive index is controlled to n _e. Thus, the light propagating the light waveguide layer 43 included in the optical deflection unit 20, as described above, an output angle Θ from the side surface 16c of the optical waveguide 16 is given by cosΘ = n _e / n _act, the optical coupling The light is emitted from the portion 30 to the active region 14.

  The light incident on the active region 14 enters the reflective diffraction grating 18 at an incident angle α along the path A. The light incident on the diffraction grating 18 is reflected at different diffraction angles depending on the wavelength length. That is, the light is separated by the wavelength by being reflected by the diffraction grating 18.

  Of the light incident on the reflective diffraction grating 18, only the return light selected for the specific wavelength λ is reflected along the path A again by being reflected at the diffraction angle α (= incidence angle α). It returns to and propagates in the direction of the reflecting surface 24.

  In this way, the light of wavelength λ reciprocates the optical path consisting of the optical waveguide 16 and the path A a plurality of times. Whenever light having a wavelength λ propagates along the path A through the active region 14, the light is amplified by stimulated emission, and is eventually emitted from the light emitting end face 22 as light having the wavelength λ.

  At this time, the light oscillation wavelength λ is a longitudinal mode wavelength determined by the optical path length of the optical path composed of the optical waveguide 16 and the path A, that is, the sum of the optical path lengths of the optical waveguide 16 and the path A (hereinafter also referred to as the resonator length T). And a wavelength selected by the diffraction angle α of the reflective diffraction grating 18 (hereinafter also referred to as a selected wavelength).

  As described in the background art section, the longitudinal mode wavelength is a discrete value. On the other hand, the selected wavelength is a value that continuously changes by changing the diffraction angle α. Therefore, the actual oscillation wavelength λ is the longitudinal mode wavelength closest to the selected wavelength selected by the diffraction angle α. That is, when the resonator length T is constant, the oscillation wavelength λ does not continuously change even if the selected wavelength is continuously changed. The oscillation wavelength λ only shifts discontinuously to the adjacent longitudinal mode wavelength. This discontinuous transition in wavelength is mode hopping.

  In order to continuously change the oscillation wavelength λ without causing mode hopping, it is necessary to change the resonator length T according to the change in wavelength of the selected wavelength at the same time as changing the selected wavelength. Good. That is, by changing the resonator length T, the longitudinal mode wavelength itself in the order is changed while keeping the longitudinal mode order constant.

  The wavelength tunable semiconductor laser 10 of this embodiment also changes the selected wavelength and the resonator length T at the same time. That is, as the diffraction angle α increases (the emission angle Θ increases), the selected wavelength increases. At the same time, since the path A, that is, the resonator length T is also increased, the longitudinal mode wavelength is increased without changing the order of the longitudinal mode.

  The same applies to the case where the diffraction angle α is reduced (the emission angle Θ is reduced). As a result, the selected wavelength is shortened, and the resonator length T is shortened (path A is shortened) as the selected wavelength is shortened. Therefore, the longitudinal mode wavelength is shortened while maintaining a constant longitudinal mode order.

  In this way, the wavelength tunable semiconductor laser 10 can continuously change the oscillation wavelength λ without causing mode hopping.

  Next, structural conditions of the wavelength tunable semiconductor laser 10 will be described mainly with reference to FIG. FIG. 3 is a schematic diagram showing the main part of FIG. 1 together with dimensions and angles.

  First, each symbol used in FIG. 3 will be described.

One end 16b of the optical waveguide 16 and the grating surface 28 on which the grating grooves of the reflective diffraction grating 18 are formed are separated by a distance L _g . That is, the distance between the one end 16b of the optical waveguide 16 measured along the light propagation direction of the optical waveguide 16 and the grating surface 28 of the reflective diffraction grating 18 is L _g .

Arrangement angles of the reflecting type diffraction element 18, i.e., the angle of the extending direction of the grating surface 28 grating grooves 50 are formed, the light propagation direction of the optical waveguide 16 is a theta _g.

  The emission angle of the light I emitted from the light deflecting unit 20 toward the reflective diffraction grating 18 is Θ. Here, the emission angle Θ indicates the acute angle side of the angle formed by the light propagation direction of the optical waveguide 16 and the light I propagation direction.

  Let L be the distance by which the light I propagates until it reaches the point P of the reflective diffraction grating 18. That is, let L be the geometric length of the path A in the active region 14.

  Further, the incident angle when the light I is incident on the point P of the reflective diffraction grating 18 is α. Here, the incident angle α is an angle formed between the normal line of the reflective diffraction grating 18, that is, the normal line of the grating surface 28 and the propagation direction of the light I.

  A diffraction angle when the light I is reflected and diffracted in the direction of the path A at the point P is α. Here, the diffraction angle α is an angle formed between the normal line of the reflective diffraction grating 18, that is, the normal line of the grating surface 28, and the light propagation direction reflected and diffracted in the direction of the path A at the point P. That is, the incident angle and the diffraction angle are equal to each other and α.

  Further, d is the grating constant of the reflective diffraction grating 18.

Further, the refractive index of the activated region 14 is n _act . Further, the length along the light propagation direction of the optical waveguide 16 to L _e, the refractive index n _e.

  In the wavelength tunable semiconductor laser 10 having such a structure, the following equation (1) is established from the diffraction conditions in the Littrow arrangement, where the wavelength λ of the light diffracted at the diffraction angle α at the light incident point P.

mλ / n _act = 2dsinα (1)
(However, m is a positive integer.)
From the equation (1), the change Δα of the diffraction angle α due to the minute change Δλ of λ is given by the following equation (2).

mΔλ / n _act = Δα2d cos α (2)
When the formula (2) is divided by both sides of the formula (1), the following formula (3) is obtained.

Δλ / λ = Δα / tan α (3)
Geometrically, the following equation (4) holds between the emission angle Θ and the diffraction angle α.

_{α = π / 2-Θ g} + Θ ··· (4)
From the equation (4), the relationship between the minute α change Δα and the emission angle Θ change ΔΘ is given by the following equation (5).

Δα = ΔΘ (5)
Further, from the equation (4), the following equations (6) and (7) are established.

sin α = cos (Θ−Θ _g ) (6)
cos α = −sin (Θ−Θ _g ) (7)
By substituting the equations (5) to (7) into the equation (3), the following equation (8) is obtained.

Δλ / λ = −tan (Θ−Θ _g ) ΔΘ
= Tan (Θ _g −Θ) ΔΘ (8)
On the other hand, the length L of the path A is geometrically expressed by the following equation (9).

L = L _g / (cos Θ−sin Θ / tan Θ _g ) (9)
In the equation (9), the change ΔΘ in the emission angle Θ due to the minute change ΔL in the length L is given by the following equation (10).

ΔL = L _g (sin Θ + cos Θ / tan Θ _g ) / (cos Θ−sin Θ / tan Θ _g ) ² × ΔΘ
= L (sinΘ + cosΘ / tanΘ g) / (cosΘ-sinΘ / tanΘ g) × ΔΘ
_{= Lcos (-Θ g + Θ)} / sin (Θ g -Θ) × ΔΘ
= L / tan (Θ _g −Θ) × ΔΘ (10)
By the way, the resonance condition in the optical resonator formed between the reflecting surface 24 and the point P of the reflective diffraction grating 18 is given by the following equation (11) using the resonator length T of the optical resonator.

kT = qπ (11)
(Where k represents the wave number (= 2π / λ) of light of wavelength λ, and q represents a positive integer (longitudinal mode order +1))
Here, the resonator length T, since given by _{T = n act L + n e} L e, from (11), the following (12) is obtained.

_{k (n act L + n e} L e) = qπ ··· (12)
Here, when the emission angle Θ is changed, the diffraction angle α is changed to change the wave number k by the minute change amount Δk in the equation (12). For this minute change Δk, the resonance condition (12) can be transformed into the following expression (13).

_{Δk (L + n e L e} ) + k (n act ΔL + Δn e L e) = Δqπ ··· (13)
(However, [Delta] L is a minute change of L, [Delta] n _e is a small change in the n _e, [Delta] q is the change in q, represent respectively)
Here, in order that mode hopping does not occur even if the wave number k (wavelength λ) changes, Δq = 0 in the equation (13). Thus, the following equation (14) is obtained.

_{Δk (n act L + n e} L e) + k (ΔL + Δn e L e) = 0 ··· (14)
By the way, when the left side of the equation (14) is expanded and arranged using the equation (12), the following equation (15) is obtained.

^{Δkqπ / k 2 + ΔL + Δn} e L e = 0 ··· (15)
Further, by substituting Δk = −k ² / 2π × Δλ, which is a relationship obtained by differentiating both sides of k = 2π / λ, into the equation (15), the following equation (16) is obtained.

_{-Δλq / 2 + ΔL + Δn e} L e = 0 ··· (16)
By substituting the equations (8) and (10) into the equation (16), the following equation (17) is obtained.
_{ΔΘ (-qλtan (Θ g -Θ)} / 2 + L / tan (Θ g -Θ)) + Δn e L e = 0 ··· (17)
The following equation (18) is obtained by modifying the equation (17).

_{(L + n e L e)} tan (Θ g -Θ) -L / tan (Θ g -Θ) = Δn e L e / ΔΘ ··· (18)
Meanwhile, according to the simulation by the inventors have conducted, in (18), if Θ is small (approximately 10 ° or less), and the refractive index change [Delta] n _e and ΔΘ is the put to be in approximately proportional relation Became clear. From this, the right side becomes a certain constant. According to the simulation described above, this constant is a value about 50 times the wavelength λ.

Therefore, (18) so as to satisfy the formula, L, L _e, theta _g, by determining the theta and n _e, while as the mode hopping does not occur, thereby the oscillation wavelength λ continuously change .

  Furthermore, in order to prevent the equation (18) from depending on the length L of the path A, that is, the length of the active region 14, it is sufficient that the term including L can be eliminated from the equation (18). That is, the following equation (19) may be established.

L tan (Θ _g −Θ) = L / tan (Θ _g −Θ) (19)
The condition of the equation (19) is established by Θ _g −Θ = π / 4. When Θ is sufficiently small (about 5 ° or less), Θ _g ≈π / 4 (45 °) can be approximated. Therefore, the angle formed by the direction in which the grating surface 28 of the reflective diffraction grating 18 extends and the light propagation direction of the optical waveguide 16 is π / 4, that is, the normal line of the grating surface 28 and the light of the optical waveguide 16. If the reflective diffraction grating 18 is arranged so that the angle formed with the propagation direction is π / 4, the length of the active region 14 (the length L of the path A) when the emission angle Θ is sufficiently small. Regardless of this, it is possible to continuously change the oscillation wavelength λ while preventing mode hopping.

  Next, a simulation performed for the wavelength tunable semiconductor laser 10 will be described with reference to FIG.

  This simulation shows that the oscillation wavelength λ of light is changed by changing the refractive index of the optical waveguide layer 43 of the optical deflection unit 20. In FIG. 5, the vertical axis represents light intensity (arbitrary unit), and the horizontal axis represents light wavelength (μm).

  Of the three graphs depicted in the figure, the central graph (solid line) I shows the case where the refractive index of the optical waveguide layer 43 is not changed in the optical deflection unit 20. The graph (dashed line) II on the left shows a case where the refractive index of the optical waveguide layer 43 is slightly decreased. The right graph (one-dot chain line) III shows a case where the refractive index of the optical waveguide layer 43 is slightly increased.

In this simulation, the calculation is performed under the assumptions listed below. That is, (1) the reflective diffraction grating 18 having a length S along the initial direction of the first surface 52 of 0.4 μm and an arrangement angle Θ _g of 45 ° is used. (2) The length of the optical coupling unit 30 along the light propagation direction is 350 μm, the width is 2 μm, the refractive index of the optical waveguide layer 43 is 3.4, and the refractive indexes of the p-type layer 42 and the substrate 12 are equal to 3 .385. (3) There is a difference in refractive index between the active region 14 and the optical waveguide layer 43, and by applying a predetermined offset voltage to the electrodes 26 and 38, the emission angle Θ is 5 ° in advance. (4) The intensity is obtained assuming that the light is extracted from the light emitting end face 22 at the stage where the optical path composed of the optical waveguide 16 and the path A is reciprocated once. The other conditions are assumed to be the same as those of the wavelength tunable semiconductor laser 10 described above.

In FIG. 5, the graph II on the left side of the drawing shows that the refractive index of the optical waveguide layer 43 is decreased by 2.8 × 10 ⁻³ by making the voltage applied to the electrodes 26 and 38 smaller than the offset voltage. This corresponds to reducing the emission angle Θ by 0.02 rad (about 1.2 °). At this time, the peak wavelength is about 1.481 μm.

  Similarly, in the graph I in the center of the drawing, the refractive index change of the optical waveguide layer 43 is set to 0 by keeping the voltage applied to the electrodes 26 and 38 at the offset voltage. At this time, the peak wavelength is about 1.492 μm.

In the graph III on the right side of the drawing, the refractive index of the optical waveguide layer 43 is increased by 2.6 × 10 ⁻³ by making the voltage applied to the electrodes 26 and 38 larger than the offset voltage. This corresponds to increasing the emission angle Θ by 0.02 rad (about 1.2 °). At this time, the peak wavelength is about 1.503 μm.

  Thus, by changing the emission angle Θ within a range of ± 1.2 °, the oscillation wavelength λ of light can be changed over about 20 nm. Further, the longitudinal mode order was constant when the oscillation wavelength λ changed over about 20 nm. That is, mode hopping did not occur.

In this simulation, the refractive index change width (−2.8 × 10 ^{−3 to} 2.6 × 10 ⁻³ ) of the light deflecting unit 20 is relative to the refractive index (3.4) of the optical waveguide layer 43. It remains at about ± 0.1%. This is because the refractive index of the light deflecting unit 20 is controlled by the electro-optic effect. If the refractive index control of the light deflection unit 20 is performed using the free carrier plasma effect by current injection, the refractive index change width can be widened to about ± 1%. In this case, the change width of the light oscillation wavelength λ can be set to the order of 100 nm.

  In each graph, the wavelength distribution is broad (wide). As described in Assumption (4), this is because the light is extracted at the stage of one round trip of the optical path. This is because it is not sufficiently monochromatic. In an actual case, the light exits from the light exit end face 22 after making many rounds of the optical path, so that the monochromization is sufficiently advanced and the wavelength distribution becomes narrower.

  As described above, according to the wavelength tunable semiconductor laser 10 of this embodiment, the light generated in the active region 14 is used as an optical path (the optical waveguide 16 and the path A) between the reflecting surface 24 and the reflective diffraction grating 18. By resonating with the configured optical resonator, light having a predetermined oscillation wavelength λ is extracted from the light emitting end face 22. In this optical resonator, the light deflecting unit 20 changes the emission angle Θ of light directed from the optical waveguide 16 to the reflective diffraction grating 18, whereby the incident angle α of light incident on the reflective diffraction grating 18, and The optical path length (resonator length T) of the optical path (optical waveguide 16 and path A) is simultaneously changed. In this way, by simultaneously performing the wavelength selection caused by the change of the incident angle α to the reflection type diffraction grating 18 and the change of the resonator length T, mode hopping can be eliminated and a wide range can be achieved without providing a lens. The oscillation wavelength λ can be changed.

  In the first prior art, in order to continuously change the oscillation wavelength, the current applied to the phase adjustment region must be controlled simultaneously with the current applied to the distributed Bragg reflector region. The control was complicated. However, according to the wavelength tunable semiconductor laser 10, it is possible to change the incident angle α of the light to the reflection type diffraction grating 18 and hence the oscillation wavelength λ with a simple operation of controlling the voltage applied to the optical deflecting unit 20. it can.

  Further, the reflection type diffraction grating 18 is arranged so that the angle formed by the direction in which the grating surface 28 of the reflection type diffraction grating 18 extends and the light propagation direction of the optical waveguide 16 is π / 4. As a result, mode hopping can be eliminated regardless of the length L along the optical path (path A) of the active region 14 interposed between the light deflection unit 20 and the reflective diffraction grating 18. As a result, the active region 14 having a gain (the length L of the path A) can be increased, so that the light output can be increased.

  Further, as shown in FIG. 4, in this embodiment, the length S along the initial direction of the first surface 52 of the reflective diffraction grating 18 is set to a value equal to the center wavelength. Thereby, although the reason is not clear, the intensity distribution of the light emitted from the light emitting end face 22 can be a smooth distribution close to a Gaussian distribution.

  This will be described with reference to FIG. 6A and 6B are simulation results of the intensity distribution of the light emitted from the light emitting end face 22. 6A and 6B, the vertical axis represents the light intensity (arbitrary unit), and the horizontal axis represents the distance (arbitrary unit) along the thickness direction of the optical waveguide 16 with the light center as the origin. ). FIG. 6A shows a case where the length S along the initial direction of the first surface 52 is equal to the center wavelength. FIG. 6B shows a case where the length S along the initial direction of the first surface 52 is different from the center wavelength.

  In FIG. 6A, the intensity distribution is a smooth distribution close to a Gaussian distribution, whereas in FIG. 6B, the rough shape of the intensity distribution shows the same tendency as in FIG. 6A. However, fine irregularities can be seen throughout the graph.

  From the simulation, it has been clarified that this tendency persists when the distance from the light emitting end face 22 is in the range of 100 to 300 μm.

  From another simulation, when the length S along the initial direction of the first surface 52 of the reflective diffraction grating 18 is set to a natural number multiple of the center wavelength, the light intensity distribution is similar to the above. It became clear that it could be a smooth distribution close to the Gaussian distribution.

  As described above, the length S along the initial direction of the first surface 52 of the reflective diffraction grating 18 is most preferably practically a natural number times the center wavelength. However, this is not intended to limit the length S to a natural number times the center wavelength. This is because even if the length S is different from a natural number multiple of the center wavelength, the wavelength can be changed without causing mode hopping. That is, the length S may be a value different from a natural number multiple of the center wavelength as long as the unevenness of the intensity distribution of the light emitted from the light emitting end face 22 can be practically allowed.

In this embodiment, the arrangement angle Θ _g of the reflective diffraction grating 18 is 45 °. However, if the _ne and _Le are adjusted so as to satisfy the condition of the equation (18), the arrangement angle Θ _g The angle Θ _g is not limited to 45 °, and may be an angle selected within a range greater than 0 ° and less than 90 °.

The arrangement angle Θ _g will be described in detail with reference to FIG. In FIG. 7, the same components as those in FIGS. 1 and 3 are denoted by the same reference numerals, and the description thereof is omitted.

For example, as shown in FIG. 7A, consider the case where the arrangement angle Θ _g is smaller than 45 °, that is, the case where the reflection type diffraction grating 18 is arranged more laid down. In this case, the light I _a having _a constant width W4sin Θ emitted from the optical coupling unit 30 is applied to the reflection diffraction grating 18 over a wide irradiation area a. That is, the light I _a is diffracted by a larger number of grating grooves than when the arrangement angle Θ _g of the reflective diffraction grating 18 is 45 °. As a result, in one diffraction, the light I _a is monochromatized more than when the arrangement angle Θ _g is 45 °. On the other hand, the oscillation wavelength λ of the light I _a can be changed only within a narrow range. In summary, when the arrangement angle Θ _g of the reflective diffraction grating 18 is smaller than 45 °, it is advantageous for monochromatic light, but the oscillation wavelength λ can be changed only within a narrow range.

As shown in FIG. 7B, when the arrangement angle Θ _g is larger than 45 °, the light I _b is irradiated onto the reflective diffraction grating 18 over a narrow irradiation area b, and the changeable range Θ _b of the emission angle Θ _b. Will grow. Accordingly, in the same discussion as described above, when the arrangement angle Θ _g of the reflective diffraction grating 18 is larger than 45 °, it is disadvantageous for monochromatic light, but the oscillation wavelength λ changes in a wide range. be able to.

Therefore, the arrangement angle Θ _g of the reflective diffraction grating 18 can be selected in accordance with the design in consideration of the monochromatic light and the changeable range of the oscillation wavelength λ.

  Further, in this embodiment, the optical coupling portion 30 has a length W4 of 300 μm and the insulating layer 48 has a width of 1 μm. This dimension is an optimum dimension in the wavelength tunable semiconductor laser 10 of this embodiment. However, when the dimensions of each part of the wavelength tunable semiconductor laser 10 are changed, it is preferable that the length W4 of the optical coupling unit 30 and the width of the insulating layer 48 are also set to appropriate values according to the design.

  For example, when the length W4 of the optical coupling unit 30 is increased, the width W4sinΘ of the light emitted from the optical coupling unit 30 toward the active region 14 is increased. That is, the irradiation area of the light incident on the reflective diffraction grating 18 is widened. As a result, in the reflection type diffraction grating 18, light is diffracted by a larger number of grating grooves, which is advantageous for monochromatic light. In this case, the width of the insulating layer 48 can be increased in order to obtain 100% optical coupling efficiency. As a result, the distance between the active region 14 and the optical waveguide 16 is widened, so that manufacturing is easy in terms of dimensions. However, in this case, an increase in the size of the wavelength tunable semiconductor laser 10 is inevitable.

  On the other hand, when the length W4 of the optical coupling unit 30 is shortened, the irradiation area of the light in the reflection type diffraction grating 18 becomes narrow due to the discussion opposite to the above, which is disadvantageous for making the light monochromatic. In addition, since the width of the insulating layer 48 must be reduced, it is difficult to manufacture in terms of dimensions. However, the tunable semiconductor laser 10 can be reduced in size.

  Therefore, it is preferable that the length W4 of the optical coupling portion 30 and the width of the insulating layer 48 are set to appropriate values according to the design in consideration of the advantages and disadvantages described above.

  In this embodiment, the light deflection unit 20 controls the refractive index of the optical waveguide layer 43 using the electro-optic effect. However, the refractive index may be controlled using, for example, an acousto-optic effect or a free carrier plasma effect. In particular, when the free carrier plasma effect is used, since the refractive index change width can be about ± 1% as described above, the oscillation wavelength λ of light can be changed in a wider range.

  In this embodiment, a blazed grating is used as the reflective diffraction grating 18. However, the reflection type diffraction grating 18 is not limited to a blazed grating, and a relief type diffraction grating having a cross-sectional shape other than a sawtooth shape or a refractive index modulation type diffraction grating may be used.

  In addition, for example, a reflective film made of a dielectric multilayer film in which dielectric films having different refractive indexes are alternately laminated may be provided on the surface of the reflective diffraction grating 18 where the grating grooves 50 are formed. By doing in this way, the reflectance of the light in the reflection type diffraction grating 18 can be raised.

  In addition, it is assumed that the wavelength tunable semiconductor laser 10 of this embodiment is used in an optical communication wavelength band (1.3 to 1.5 μm). Therefore, the active region 14 has a double hetero structure in which n-type InP (n-type cladding layer), InGaAsP (active layer 32), and p-type InP (p-type cladding layer 34) are stacked in this order. It was. However, the structure of the active region 14 is not limited to this. For example, a stacked structure composed of n-type AlGaAs (n-type cladding layer), GaAs (active layer), and p-type AlGaAs (p-type cladding layer), n-type A laminated structure composed of ZnSSe (n-type cladding layer), ZnCdSe (active layer), and p-type ZnSSe (p-type cladding layer), or n-type GaN (n-type cladding layer), InGaN (active layer), and p-type Any of the laminated structures made of GaN (p-type cladding layer) may be selected according to the oscillation wavelength.

  In this embodiment, the active region 14 faces the one end 16 b of the optical waveguide 16. This facing portion is not an essential component, and can be shaped appropriately according to the design. For example, as shown in FIG. 8A, the above-described facing portion can be removed, and the active region 14 and the optical waveguide 16 can be configured to face each other only in the optical coupling portion 30 so as to be capable of optical coupling.

  Further, in this embodiment, the shape of the active region 14 in a plan view is substantially tapered, but the active region 14 hinders the propagation of light from the optical coupling portion 30 to the reflective diffraction grating 18. Otherwise, there is no particular limitation on the shape. For example, as shown in FIG. 8B, the active region 14 may be substantially rectangular in plan view.

  Further, in this embodiment, the distance between the side surfaces 14c and 16c, that is, the width of the insulating layer 48 is set to 1 μm regardless of the place. That is, the side surfaces 14c and 16c are arranged in parallel. However, the side surfaces 14c and 16c are not necessarily arranged in parallel, and may be arranged so that the width gradually increases from one end 16b of the optical waveguide 16 to the other end 16a. That is, the width of the insulating layer 48 may be gradually increased from the one end 16b to the other end 16a of the optical waveguide 16. Also in this way, practically sufficient optical coupling efficiency can be obtained.

  In this embodiment, the reflection type diffraction grating 18 is attached to the substrate 12 separately from the active region. However, the reflection type diffraction grating 18 may be integrated with the substrate 12.

(Embodiment 2)
Next, the structure of the wavelength tunable semiconductor laser according to the second embodiment will be described with reference to FIG. FIG. 9 is a partially cutaway perspective view showing a schematic structure of the wavelength tunable semiconductor laser according to the second embodiment.

  The wavelength tunable semiconductor laser 70 according to the second embodiment is the same as the wavelength tunable semiconductor laser 10 according to the first embodiment except that the reflective diffraction grating 72 is provided on the end face 14a of the active region 14. Therefore, in FIG. 9, the same components as those in FIG. 1 are denoted by the same reference numerals, and the description thereof is omitted as appropriate.

  In the wavelength tunable semiconductor laser 70, the reflection type diffraction grating 72 is formed on the end surface 14a in the active region 14 where the light incident from the optical waveguide 16 to the active region 14 is irradiated.

More specifically, the reflective diffraction grating 72 has an outer angle formed with an extension line of the base when the side where the optical coupling unit 30 is provided is the base in the substantially triangular active region 14 in plan view. it is provided on the end face 14a corresponding to the sides a theta _g. That is, unlike the first embodiment, the reflective diffraction grating 72 and the active region 14 are integrated.

Further, on the surface of the reflective diffraction grating 72, that is, the end face 14a, a reflective film 78 is formed so as to cover at least the active layer 32. The reflection film 78 is, for example, a so-called dielectric multilayer film in which SiO ₂ films and SiN films having different refractive indexes are alternately stacked.

The reflection type diffraction grating 72 is a reflection type plane diffraction grating, and the arrangement angle Θ _g , the grating constant d, the arrangement of the first surface 74 and the second surface 76, and the length S of the first surface 74 are as follows. This is the same as the first embodiment.

  The wavelength tunable semiconductor laser 70 according to the second embodiment has the same function and effect as the wavelength tunable semiconductor laser 10 according to the first embodiment, and the reflection type diffraction grating 72 is integrated with the active region 14. It is possible to reduce the number of parts constituting the 70 and to reduce the size of the wavelength tunable semiconductor laser 70.

  In the first embodiment, the light incident on the active region 14 from the optical coupling unit 30 once goes out of the active region 14 and reaches the gap 46 (see FIG. 1) before reaching the reflective diffraction grating 18. Had to pass. Therefore, as described above, there is a possibility that light intensity is reduced due to irregular reflection at the upper surface 12a in the gap 46. However, in the second embodiment, since the reflection type diffraction grating 72 is formed on the end face 14a, the light does not come out from the active region 14, so that the reduction of the light intensity can be suppressed.

The wavelength tunable semiconductor laser 70 according to the second embodiment can be modified in the same manner as the wavelength tunable semiconductor laser 10 according to the first embodiment. More specifically, the arrangement angle Θ _{g of} the reflective diffraction grating 72, the length S of the first surface 74 of the reflective diffraction grating 72, the length W4 of the optical coupling unit 30, the width of the insulating layer 48, the optical deflection unit 20 The refractive index control method, the composition and laminated structure of the active region 14, the shape of the active region 14, and the width of the insulating layer 48 can be changed in the same manner as in the first embodiment.

(Embodiment 3)
Next, the structure and operation of the wavelength tunable semiconductor laser according to the third embodiment will be described with reference to FIG. FIG. 10 is a perspective view showing a schematic structure of the wavelength tunable semiconductor laser according to the second embodiment.

  The tunable semiconductor laser 60 of the third embodiment is the same as the tunable semiconductor laser 10 of the first embodiment except that the phase adjustment region 62 is provided in the optical waveguide 16. Therefore, in FIG. 10, the same components as those in FIG. 1 are denoted by the same reference numerals, and the description thereof is omitted as appropriate.

  In the wavelength tunable semiconductor laser 60, a phase adjustment region 62 is provided on the light emitting end face 22 side of the optical waveguide 16. More specifically, the phase adjustment region 62 is provided in the optical waveguide 16 at a position that does not overlap the optical coupling unit 30. Note that the laminated structure in the phase adjustment region 62 is the same as that of the optical deflection unit 20.

  The phase adjustment region 62 includes a phase adjustment electrode 64 on the upper surface of the p-type layer 42. The phase adjusting electrode 64 is disposed with a space sufficient to ensure electrical insulation from the electrode 26 of the light deflection unit 20. Another phase adjustment electrode (not shown) is also provided at a position corresponding to the phase adjustment electrode 64 on the other main surface (lower surface) 12 b of the substrate 12. These two phase adjusting electrodes have the same laminated structure as the electrode 26.

  In the phase adjustment region 62, by adjusting the voltages applied to the two phase adjustment electrodes, the refractive index of the optical waveguide layer 43 is controlled by the electro-optic effect, and as a result, the optical path length of the optical waveguide 16 is changed. .

  The wavelength tunable semiconductor laser 60 according to the third embodiment has the same effect as that of the wavelength tunable semiconductor laser 10 according to the first embodiment, and is provided with the phase adjustment region 62, so that the voltage applied to the phase adjustment electrode is The optical path length of the optical path through which the light propagates can be changed. Therefore, the initial phase of light emitted from the optical coupling unit 30 to the active region 14 can be finely adjusted.

  More specifically, when the change in the emission angle Θ of the light emitted from the optical coupling unit 30 to the active region 14 is increased, the nonlinear change in the initial phase of the light cannot be ignored. That is, when the change in the output angle Θ is small, only the structure of the tunable semiconductor laser 60 configured to linearly change the length L of the path A according to the change in the output angle Θ, It is possible to compensate for changes in the initial phase of light. However, when the angle change of the emission angle Θ becomes large, it becomes impossible to compensate for the change of the initial phase of light with this structure alone.

  Therefore, by providing the phase adjustment region 62 in the optical waveguide 16, the optical path length of the optical waveguide 16 is finely adjusted to compensate for a change in the initial phase of nonlinear light. Thereby, the wavelength tunable semiconductor laser 60 can continuously change the oscillation wavelength λ without causing mode hopping. Further, since the degree of phase adjustment at this time is slight, precision is not required unlike the first prior art.

The wavelength tunable semiconductor laser 60 of the third embodiment can be modified in the same manner as the wavelength tunable semiconductor laser 10 of the first embodiment. More specifically, the arrangement angle Θ _{g of} the reflective diffraction grating 18, the length S of the first surface 52 of the diffraction grating 18, the length W 4 of the optical coupling unit 30, the width of the insulating layer 48, and the refraction in the optical deflection unit 20. The rate control method, the composition and laminated structure of the active region 14, the shape of the active region 14, and the width of the insulating layer 48 can be changed in the same manner as in the first embodiment.

  In this embodiment, the phase adjustment region 62 is provided adjacent to the optical deflection unit 20, but an interval may be provided between the phase adjustment region 62 and the optical deflection unit 20.

(Embodiment 4)
Next, the structure and operation of the wavelength tunable semiconductor laser according to the fourth embodiment will be described with reference to FIG. FIG. 11 is a perspective view showing a schematic structure of the wavelength tunable semiconductor laser according to the fourth embodiment.

  In other words, the tunable semiconductor laser 80 according to the fourth embodiment has a structure in which the active region 14 and the optical waveguide 16 are interchanged in the tunable semiconductor laser 10 according to the first embodiment. Therefore, in FIG. 11, the same components as those in FIG. 1 are denoted by the same reference numerals, and the description thereof is omitted as appropriate.

  That is, the wavelength tunable semiconductor laser 80 includes an active region 82 provided on one end face 12c facing the longitudinal direction of the substrate 12, a reflective diffraction grating 18 provided on the other end face 12d side, and an active region 82. And an optical waveguide 87 provided between the reflective diffraction grating 18.

  The active region 82 is a rectangular component having a rectangular cross-sectional shape perpendicular to the light propagation direction and a planar shape that is long along the light propagation direction. That is, the active region 82 is a channel type. The active region 82 is arranged near the corner of the rectangular substrate 12. In a plan view, the side along the longitudinal direction of the active region 82 (hereinafter also referred to as the light propagation direction) is parallel to the longitudinal side of the substrate 12. In addition, one end face 82 a orthogonal to the light propagation direction of the active region 82 is flush with the end face 12 c of the substrate 12. This end face 82a is a light emitting end face 86. The active region 82 has a stacked structure similar to that of the active region 14 of the first embodiment. The active region 82 generates light by stimulated emission and can propagate the generated light in the longitudinal direction.

  The reflection type diffraction grating 18 has a block shape with a right triangle shape in plan view, and is provided on the end surface 12d side of the substrate 12, that is, on the side opposite to the end surface 12c on the side where the active region 82 is provided. A side surface corresponding to the oblique side of the reflective diffraction grating 18 is a grating surface 28, and a grating groove 50 is formed on the grating surface 28 as in the first embodiment. The lattice plane 28 extends at an angle larger than 0 ° and smaller than 90 ° with the light propagation direction of the active region 82.

  The optical waveguide 87 is a component having a substantially trapezoidal planar shape, and is interposed between the reflective diffraction grating 18 and the active region 82. The optical waveguide 87 includes an optical waveguide layer 84 provided on the main surface 12 a of the substrate 12 and a p-type layer 85 provided on the optical waveguide layer 84 and having the same planar shape as the optical waveguide layer 84.

  More specifically, the optical waveguide 87 has substantially the same shape as the substrate 12 except that regions corresponding to the reflective diffraction grating 18 and the active region 82 are cut out in a plan view. The optical waveguide 87 is a planar optical waveguide. The end surface 84 a of the optical waveguide layer 84 on the reflective diffraction grating 18 side and the grating surface 28 face each other with a minute gap 88 therebetween. Further, the active region 82 is arranged so as to be fitted into the notch 96 corresponding to the active region 82 of the optical waveguide 87. In addition, the optical waveguide 87, more specifically, the optical waveguide layer 84, propagates light incident on the optical waveguide 87 from an optical coupling unit 30 (to be described later) toward the reflective diffraction grating 18, and reflects the diffraction. The return light reflected by the grating 18 propagates toward the optical coupling unit 30.

  In the notch 96, the side surface 87b of the optical waveguide 87 is opposed to the side surface 82b in the longitudinal direction of the active region 82 with a minute gap 90 therebetween, and an insulating layer similar to that of the first embodiment is formed in the gap 90. 48 is provided. The optical coupling unit 30 is configured by the side surface 87b, the side surface 82b, and the insulating layer 48. As in the first embodiment, the optical coupling unit 30 performs optical coupling between the optical waveguide 87 (the optical deflection unit 92) and the active region 82.

  In the optical waveguide 87, an optical deflection unit 92 is provided in a region facing the active region 82 with the optical coupling unit 30 interposed therebetween. The optical deflecting unit 92 is a part of the optical waveguide 87 and is distinguished from the optical waveguide 87 by the presence of the electrode 94 for controlling the refractive index of the optical waveguide layer 84. That is, the light deflection unit 92 has a structure in which the electrode 94 having a predetermined shape is provided on the p-type layer 85 of the optical waveguide 87.

  The electrode 94, that is, the light deflecting portion 92 has a right triangle shape in plan view, and the right angle corner portion is disposed so as to coincide with the end portion on the end face 12 c side of the optical coupling portion 30. Further, one base 94b sandwiching the right angle is arranged so as to coincide with a ridge line formed by the side surface 87b and the upper surface of the optical waveguide 87. The hypotenuse 94a facing the right-angled corner of the light deflection section 92 is inclined so that its height gradually increases from the end face 12d side of the substrate 12 toward the end face 12c.

  As in the first embodiment, the light deflecting unit 92 controls the refractive index of the optical waveguide layer 84 included in the light deflecting unit 92, thereby allowing the light emitted from the light deflecting unit 92 to the reflective diffraction grating 18. The exit angle, and therefore the incident angle of light on the reflective diffraction grating 18 and the length of the optical path are changed.

  As described above, in the wavelength tunable semiconductor laser 80, the reflection surface 24 and the reflection type diffraction grating 18 constitute an optical resonator.

  The operation of the wavelength tunable semiconductor laser 80 will be briefly described below.

  The light generated in the active region 82 is coupled to the light deflection unit 92 through the optical coupling unit 30 in the same manner as in the first embodiment. By controlling the refractive index of the light deflector 92 (the optical waveguide layer 84), the light is emitted from the light deflector 92 toward the reflective diffraction grating 18 after adjusting the emission angle. Of the light incident on the reflective diffraction grating 18, the light reflected at a specific diffraction angle is returned as light selected for a specific wavelength along the reverse path to the above, the optical waveguide 87, the light deflection unit. Returning to the active region 82 through the optical coupling unit 92 and the optical coupling unit 30. The light that has returned to the active region 82 is reflected by the reflecting surface 24, follows the above-described path again, and reaches the reflective diffraction grating 18. In this manner, the laser beam having a specific wavelength oscillates by the light reciprocating several times by the optical resonator composed of the reflection surface 24 and the reflection type diffraction grating 18.

  Thus, according to the wavelength tunable semiconductor laser 80 of this embodiment, the light generated in the active region 82 resonates with an optical resonator configured as an optical path between the reflective surface 24 and the reflective diffraction grating 18. Thus, light having a predetermined oscillation wavelength is extracted from the light emitting end face 86. In this optical resonator, the light deflecting unit 92 changes the emission angle of the light directed toward the reflective diffraction grating 18 to thereby change the incident angle of light incident on the reflective diffraction grating 18 and the optical path length (resonance) of the optical path. Change the instrument length) at the same time. In this way, by simultaneously performing the wavelength selection caused by the change of the incident angle to the reflection type diffraction grating 18 and the change of the resonator length, the mode hopping is eliminated without providing a lens, and oscillation is performed in a wide range. The wavelength can be changed.

  For the same reason as in the first embodiment, according to the wavelength tunable semiconductor laser 80, the incident angle of light to the reflective diffraction grating 18 can be controlled by a simple operation of controlling the voltage applied to the light deflecting unit 92. Therefore, the oscillation wavelength can be changed.

  In this embodiment, the planar shape of the light deflection unit 92 is a right triangle, but the planar shape of the light deflection unit 92 controls the emission angle of light emitted toward the reflective diffraction grating 18. If possible, it is not limited to a right triangle shape. For example, the entire surface of the optical waveguide 87 may be used as the light deflection unit 92. That is, the electrode 94 may be provided on the entire surface of the optical waveguide 87. By doing so, it is possible to change the refractive index of the light deflector 92 at a response speed that is practically acceptable.

  In this embodiment, the shape of the optical waveguide 87 in a plan view is substantially trapezoidal. However, the optical waveguide 87 hinders the propagation of light from the optical coupling unit 30 to the reflective diffraction grating 18. Otherwise, there is no particular limitation on the shape.

Further, the wavelength tunable semiconductor laser 80 of the fourth embodiment can be modified in the same manner as the wavelength tunable semiconductor laser 10 of the first embodiment. More specifically, the arrangement angle Θ _{g of} the reflective diffraction grating 18, the length S of the first surface of the reflective diffraction grating 18, the length W 4 of the optical coupling unit 30, the width of the insulating layer 48, and the light deflection unit 92 The refractive index control method, the composition of the active region 82, and the laminated structure can be changed in the same manner as in the first embodiment.

(Embodiment 5)
Next, the structure and operation of the wavelength tunable semiconductor laser according to the fifth embodiment will be described with reference to FIG. FIG. 12 is a perspective view showing a schematic structure of the wavelength tunable semiconductor laser according to the fifth embodiment.

  In other words, in the wavelength tunable semiconductor laser 100 according to the fifth embodiment, in the wavelength tunable semiconductor laser 60 according to the third embodiment, the region corresponding to the active region 14 is the second optical waveguide 104 having no gain, and the optical waveguide 16 is used. In this structure, the first optical waveguide 102 is provided, and an active region 108 is provided at the end of the first optical waveguide 102 on the light emitting end face 106 side. Therefore, in FIG. 12, the same components as those in FIG. 10 are denoted by the same reference numerals, and the description thereof is omitted as appropriate.

  That is, the wavelength tunable semiconductor laser 100 includes a first optical waveguide 102 provided on one end face 12c facing the longitudinal direction of the substrate 12, a reflective diffraction grating 18 provided on the other end face 12d side, And a second optical waveguide 104 provided between the one optical waveguide 102 and the reflective diffraction grating 18.

  The first optical waveguide 102 has a structure in which the active region 108, the phase adjustment region 62, and the light deflecting unit 20 are integrally arranged in series in the direction from the end surface 12c of the substrate 12 toward the end surface 12d. The first optical waveguide 102 is a component having a rectangular cross-sectional shape orthogonal to the light propagation direction and a rectangular shape having a long planar shape along the light propagation direction. That is, the first optical waveguide 102 is a channel type.

  The active region 108 is provided on the substrate 12, the active layer 112 provided on the main surface 12 a of the substrate 12, the p-type cladding layer 114 provided on the active layer 112, and the p-type cladding layer 114. And an electrode (not shown) provided on the other main surface (lower surface) 12b of the substrate 12. As in the case of the first embodiment, the electrode provided on the main surface 12b is a common electrode that also serves as the electrode provided on the main surface 12b in the phase adjustment region 62 and the optical deflection unit 20. In the active region 108, the end face 108 a on the end face 12 c side of the substrate 12 is a light emitting end face 106. In addition, a reflection film is provided on the end surface 108 a, and the reflection film constitutes the reflection surface 120. Note that the stacked structure in the active region 108 is the same as that in the active region 14 of Embodiment 1 including the electrode 116.

  The phase adjustment region 62 is provided between the active region 108 and the light deflection unit 20 in the first optical waveguide 102. Since the phase adjustment region 62 is the same as that of the third embodiment, the description thereof is omitted. Note that the phase adjustment region 62 is disposed with an interval sufficient to ensure electrical insulation between the phase adjustment region 62 and the active region 108 and between the optical deflection unit 20.

  Since the optical deflection unit 20 is the same as that of the first embodiment, the description thereof is omitted.

  The optical coupling unit 30 includes a side surface 102c that is parallel to the light propagation direction of the first optical waveguide 102, a side surface 104c of the second optical waveguide 104 that faces the side surface 102c at a distance from the side surface 102c, and both side surfaces 102c and 104c. And an insulating layer 48 provided.

  The second optical waveguide 104 is a planar optical waveguide. The laminated structure is the same as that of the optical waveguide 16 of the first embodiment except that the electrode 26 (see FIG. 2) and the electrode 38 provided on the other main surface (lower surface) 12b of the substrate 12 are not provided. is there. More specifically, the second optical waveguide 104 includes an optical waveguide layer 122 provided on the substrate 12 and a p-type layer 124 provided on the optical waveguide layer 122. In other words, the second optical waveguide 104 has no gain, and has a function of guiding light from the optical coupling unit 30 toward the reflective diffraction grating 18 or in the opposite direction. The second optical waveguide 104 has the same shape as that of the active region 14 in the first embodiment when limited to a planar shape.

  Since the reflective diffraction grating 18 is the same as that of the first embodiment, the description thereof is omitted.

  The operation of the wavelength tunable semiconductor laser 100 is that light is generated in the active region 108 provided at the end of the first optical waveguide 102 and that the second optical waveguide 104 has no gain. Except for this, the third embodiment is the same as the third embodiment. Therefore, the description is omitted.

  The light generated in the active region 108 propagates through the first optical waveguide 102 and reaches the light deflecting unit 20 through the phase adjustment region 62. In the optical deflection unit 20, the refractive index of the optical waveguide layer 43 is adjusted by the voltage applied to the electrode 26. Thus, the light is coupled to the optical coupling unit 30 after the emission angle is adjusted, and is emitted toward the reflective diffraction grating 18. The emitted light propagates through the second optical waveguide 104 and enters the reflective diffraction grating 18. Of the light incident on the reflective diffraction grating 18, the light reflected at a specific diffraction angle is used as return light selected for a specific wavelength along the reverse path to the second optical waveguide 104 and the light. It returns to the active region 108 through the coupling unit 30 and the light deflection unit 20. In this manner, the laser beam having a specific wavelength oscillates by the light reciprocating several times by the optical resonator including the reflection surface 120 and the reflection type diffraction grating 18.

  As described above, according to the wavelength tunable semiconductor laser 100 of this embodiment, the light generated in the active region 108 resonates with an optical resonator configured as an optical path between the reflecting surface 120 and the reflective diffraction grating 18. As a result, light having a predetermined oscillation wavelength is extracted from the light emitting end face 106. In this optical resonator, the light deflecting unit 20 changes the light emission angle toward the reflection type diffraction grating 18 to change the incident angle of the light incident on the reflection type diffraction grating 18 and the optical path length (resonance). Change the instrument length) at the same time. In this way, by simultaneously performing the wavelength selection caused by the change of the incident angle to the reflection type diffraction grating 18 and the change of the resonator length, the mode hopping is eliminated without providing a lens, and oscillation is performed in a wide range. The wavelength can be changed.

  Further, by providing the phase adjustment region 62 in the first optical waveguide 102, the optical path length of the first optical waveguide 102 is finely adjusted to compensate for a change in the initial phase of nonlinear light. Thereby, the wavelength tunable semiconductor laser 100 can continuously change the oscillation wavelength without causing mode hopping. Further, since the degree of phase adjustment at this time is slight, precision is not required unlike the first prior art.

The wavelength tunable semiconductor laser 100 according to the fifth embodiment can be modified in the same manner as the wavelength tunable semiconductor laser 10 according to the first embodiment. More specifically, the arrangement angle Θ _{g of} the reflective diffraction grating 18, the length S of the first surface of the reflective diffraction grating 18, the length of the optical coupling unit 30, the width of the insulating layer 48, and the refraction in the optical deflection unit 20. The rate control method, the composition of the active region 108, and the stacked structure can be modified in the same manner as in the first embodiment.

1 is a perspective view showing a schematic structure of a wavelength tunable semiconductor laser according to a first embodiment. It is a principal part expanded sectional view along the II line | wire of FIG. FIG. 3 is a schematic diagram for explaining the structural condition and operation of the wavelength tunable semiconductor laser according to the first embodiment. FIG. 3 is an enlarged schematic view of a main part schematically showing the vicinity of a reflective diffraction grating of the wavelength tunable semiconductor laser according to the first embodiment. (A) And (B) is a figure which shows the simulation result performed about the wavelength tunable semiconductor laser of Embodiment 1. FIG. (A) And (B) is a figure which shows typically the intensity distribution of the light radiate | emitted from the light emission end surface of the wavelength variable semiconductor laser of Embodiment 1. FIG. (A) And (B) is a schematic diagram with which it uses for description of the arrangement | positioning angle change of the reflection type diffraction grating of the wavelength variable semiconductor laser of Embodiment 1. FIG. (A) And (B) is a top view which shows the modification of the wavelength tunable semiconductor laser of the wavelength tunable semiconductor laser of Embodiment 1. FIG. FIG. 5 is a partially cutaway perspective view showing a schematic structure of a wavelength tunable semiconductor laser according to a second embodiment. FIG. 6 is a perspective view showing a schematic structure of a wavelength tunable semiconductor laser according to a third embodiment. FIG. 6 is a perspective view showing a schematic structure of a wavelength tunable semiconductor laser according to a fourth embodiment. FIG. 10 is a perspective view showing a schematic structure of a wavelength tunable semiconductor laser according to a fifth embodiment.

Explanation of symbols

10, 60, 70, 80, 100 Wavelength tunable semiconductor laser 12 Substrate 12a, 12b Main surface 14, 82, 108 Active region 14a, 12c, 12d, 82a, 84a, 108a End surface 14c, 16c, 82b, 84b, 87b, 102c , 104c Side surface 16 Optical waveguide 16a Other end 16b One end 18, 72 Reflective diffraction grating 20, 92 Light deflector 22, 86, 106 Light exit end surface 24, 120 Reflective surface 26, 36, 38, 94, 116 Electrode 28 Grid surface 30 Optical coupling portion 32, 112 Active layer 34, 114 p-type cladding layer 42, 85, 124 p-type layer 43, 84, 122 Optical waveguide layer 46, 88, 90 Gap 48 Insulating layer 50 Lattice groove 52, 74 First surface 54, 76 Second surface 62 Phase adjustment region 64 Phase adjustment electrode 78 Reflective film 94a Slope 94b Bottom 96 Notch 102 First Optical waveguide 104 second waveguide

Claims (9)

An active region, an optical waveguide,
An optical coupling portion that optically couples the optical waveguide and the active region with a side surface in the light propagation direction of the optical waveguide facing the active region;
An optical deflecting unit provided facing the active region across the optical coupling unit and including the optical waveguide;
A reflective diffraction grating that is provided across the active region with respect to the optical waveguide and returns light incident from the optical waveguide to the optical waveguide as wavelength-selected return light;
A light emitting end surface of the optical waveguide that is orthogonal to the light propagation direction, and a reflective surface that forms an optical resonator in combination with the reflective diffraction grating;
The optical deflection unit changes an incident angle to the reflection type diffraction grating and a length of an optical path by changing an emission angle of light from the optical waveguide toward the reflection type diffraction grating. Semiconductor laser.   2. The wavelength tunable semiconductor laser according to claim 1, wherein the reflective diffraction grating is a separate component from the active region.   2. The wavelength tunable semiconductor laser according to claim 1, wherein the reflection type diffraction grating is formed on an end surface of the active region where light incident on the active region from the optical waveguide is irradiated. .   The wavelength tunable semiconductor according to any one of claims 1 to 3, wherein the light deflection unit is configured to change the incident angle by changing a refractive index of the optical waveguide electro-optically. laser.   The reflective diffraction grating has a grating surface formed in a planar shape, and is arranged such that the light propagation direction of the optical waveguide and a normal line of the grating surface form an angle of 45 °. The wavelength tunable semiconductor laser according to claim 1, wherein:   6. The wavelength tunable semiconductor laser according to claim 1, wherein a phase adjustment region for changing an optical path length of the optical path is further provided in the optical waveguide. The reflective diffraction grating is a blazed grating arranged such that the light propagation direction of the optical waveguide and the normal of the grating surface form an angle larger than 0 ° and smaller than 90 °,
When the light propagation direction of the light from the optical waveguide toward the reflective diffraction grating in a state where the light deflection unit does not change the incident angle is set as an initial direction,
The blazed grating comprises a grating groove comprising a first surface parallel to the initial direction and a second surface perpendicular to the initial direction intersecting each other;
The length along the initial direction of the first surface is a natural number multiple of the center wavelength of light generated in the active region in the active region. The wavelength tunable semiconductor laser according to any one of the above. An active region, an optical waveguide,
An optical coupling portion that optically couples the optical waveguide and the active region with a side surface along the light propagation direction of the active region facing the optical waveguide;
An optical deflecting unit provided facing the active region across the optical coupling unit and including the optical waveguide;
A reflective diffraction grating that is provided across the optical waveguide with respect to the active region and returns light incident from the active region to the active region as wavelength-selected return light;
A reflection surface that is provided on a light emitting end surface of the active region that is orthogonal to the light propagation direction and constitutes an optical resonator in combination with the reflective diffraction grating;
The wavelength tunable device characterized in that the light deflection unit changes an incident angle to the reflection type diffraction grating and a length of an optical path by changing a propagation direction of light from the active region toward the reflection type diffraction grating. Semiconductor laser. A first optical waveguide having an active region near the end on the light emitting end face side, a second optical waveguide,
An optical coupling portion for optically coupling the first and second optical waveguides with a side surface of the first optical waveguide along the light propagation direction facing the second optical waveguide;
An optical deflecting unit including the first optical waveguide provided opposite to the second optical waveguide with the optical coupling unit interposed therebetween;
A reflection type provided with the second optical waveguide sandwiched with respect to the first optical waveguide and returning light incident from the first optical waveguide to the first optical waveguide as wavelength-selected return light A diffraction grating,
A reflection surface that is provided on a light emitting end surface of the active region that is orthogonal to the light propagation direction and constitutes an optical resonator in combination with the reflective diffraction grating;
The light deflection unit changes an incident angle to the reflection type diffraction grating and a length of an optical path by changing an emission angle of light directed from the first optical waveguide toward the reflection type diffraction grating. Tunable semiconductor laser.

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