JP2022043541A - Semiconductor laser and optical device - Google Patents

Abstract

To provide high beam quality and high power laser systems.SOLUTION: A semiconductor laser 100 is equipped with a VCSEL (vertical cavity surface emitting laser) structure 110, and in the semiconductor laser 100, the laser beam 2 is multiply reflected vertically in the VCSEL structure 110 to form a slow light mode wave 4, and the slow light mode wave 4 reciprocates horizontally in the VCSEL structure 110. The effective refractive index neff of the VCSEL structure 110 for the slow light mode wave 4 varies periodically with respect to the direction of propagation of the slow light mode wave 4.SELECTED DRAWING: Figure 1

Description

The present invention relates to a surface emitting semiconductor laser, and more particularly to high speed or high output thereof.

Conventionally, the single wavelength output of a surface emitting laser has been limited to the mW level. If watt-class high-power operation becomes possible, wavelength sweeping light source for optical coherence Tomography (OCT), light source for medium- to long-range optical communication, laser radar (LIDAR) to be installed in automobiles, drones, robots, etc. ) Light source, monitoring system, automatic inspection device at manufacturing site, laser dryer of printer, etc.

The output of the end face emission type laser is limited by the end face destruction. In order to increase the output, a technique for integrating a plurality of end face emitting lasers in an array has been proposed (Non-Patent Documents 1 and 2). In the case of high output by arraying, there is a problem that the beam quality is deteriorated because the wavelengths and phases of the individual end face emitting lasers are not aligned (incoherent).

A technique for increasing the output by increasing the diameter of the VCSEL laser has also been proposed (Non-Patent Document 3). However, even in this case, the problem of incoherence due to arraying cannot be solved, and deterioration of beam quality is unavoidable.

As another approach, high output by a photonic crystal laser has also been proposed (Patent Document 1). This technique requires a submicron microfabrication technique and requires a special process such as regrowth, which makes it difficult to manufacture by a conventional semiconductor laser manufacturing process. In addition, when the area is increased, the beam quality deteriorates due to the non-uniformity of heat distribution.

Patent Document 2 discloses a slow light surface emitting laser amplifier. Although this amplifier can generate high-power and high-beam quality laser light, it requires a single-wavelength seed light source of several mW or more.

Japanese Patent No. 6080941 International release WO2017 / 150382A1

Hirofumi Suga, "Current Status and Future Trends of Higher Output of LD Light Sources", pp. 268-272, Laser Research, May 2008, Jeff Hect, "Higher Brightness of High Power Laser Diodes", pp. 22-24, Laser Focus World Japan 2012.1, Holger Mönch, "VCSEL Array is the State-of-the-art Light for 3D Sensing", pp. 40-41, Laser Focus World Japan 2018.3, Yoshiaki Nakano, "GaAs-based distributed feedback semiconductor laser and its longitudinal mode control", pp. 1554 to 1573, Applied Physics Vol. 58, No. 11 (1989)

The present invention has been made in such circumstances, and one of the exemplary purposes of that aspect is to provide a laser device with high beam quality and high power output.

The semiconductor laser according to one embodiment has a VCSEL (Vertical Cavity Surface Emitting Laser) structure, and the laser light is repeatedly reflected in the VCSEL structure in the vertical direction to form a slow light mode wave, and the slow light mode wave is a VCSEL. It is reciprocating in the horizontal direction of the structure, and the effective refraction coefficient of the VCSEL structure for the slow light mode wave changes periodically with respect to the propagation direction of the slow light mode wave.

It should be noted that an arbitrary combination of the above components or a conversion of the expression of the present invention between methods, devices and the like is also effective as an aspect of the present invention.

According to one embodiment, it is possible to provide a semiconductor laser having high beam quality and high output.

It is a figure which shows the basic structure which concerns on embodiment. It is a figure explaining the effective refractive index n _eff with respect to a slow light mode wave. 3 (a) and 3 (b) are diagrams illustrating a single longitudinal mode in a semiconductor laser. It is a figure explaining the effect of a phase shift region. 5 (a) to 5 (c) are views showing a far-field image of a semiconductor laser. It is a figure which shows the semiconductor laser which concerns on embodiment. It is a figure which shows the relationship between the pitch d of a diffraction grating and the longitudinal mode interval. It is a figure which shows the device length dependence of a net gain difference. It is a figure explaining the emission direction of the beam of the semiconductor laser of FIG. It is a figure which shows the calculation result of the radiation angle θ _m . It is a figure which shows the measurement result of the radiation angle θ _m of the sample of the semiconductor laser actually made. 12 (a) to 12 (d) are diagrams showing a configuration example of a diffraction grating. 13 (a) and 13 (b) are cross-sectional views showing a configuration example of a diffraction grating. It is a figure which shows the relationship (calculation result) of the depth of a grid, and the diffraction efficiency. It is a figure which shows the design example of a diffraction grating. It is a figure which shows the dependence of the duty cycle of the diffraction efficiency. It is a top view which shows the lattice pattern. 18 (a) and 18 (b) are diagrams showing the cross-sectional shape of the grid pattern. It is a figure which shows the diffraction grating which concerns on the modification. 20 (a) to 20 (c) are diagrams illustrating the spectrum of the diffraction grating of FIG. It is a top view of the semiconductor laser which concerns on the modification. It is a figure which shows the optical device which concerns on one Example. It is a figure which shows the optical device which concerns on one Example. It is a figure which shows the optical device which concerns on one Example. It is a figure which shows the optical device which concerns on one Example. It is a figure which shows the optical device which concerns on one Example. It is a figure which shows the optical device which concerns on one Example. It is a figure which shows the optical device which concerns on one Example.

(Outline of Embodiment)
Some exemplary embodiments of the present disclosure will be outlined. This overview is a simplified description of some concepts of one or more embodiments for the purpose of basic understanding of embodiments, as a prelude to the detailed description described below, and is an invention or disclosure. It does not limit the size. Also, this overview is not a comprehensive overview of all possible embodiments and does not limit the essential components of the embodiment. For convenience, "one embodiment" may be used to refer to one embodiment (examples or modifications) or a plurality of embodiments (examples or modifications) disclosed herein.

The semiconductor laser according to one embodiment has a VCSEL (Vertical Cavity Surface Emitting Laser) structure, and the laser light is repeatedly reflected in the VCSEL structure in the vertical direction to form a slow light mode wave, and the slow light mode wave is a VCSEL. The structure can be reciprocated horizontally. The effective refractive index of the VCSEL structure for the slow light mode wave changes periodically with respect to the propagation direction of the slow light mode wave.

Generally, when the VCSEL structure is lengthened, the number of vertical modes increases in exchange for high output. On the other hand, according to one embodiment, the slow light mode wave receives a DFB (Distributed Feedback) effect in its propagation direction, and wavelength selection can be realized. As a result, even if the VCSEL structure is lengthened, oscillation in a single mode (single wavelength) or a small number of longitudinal modes equivalent thereto can be realized.

In a general DFB laser, the distribution direction of the refractive index and the propagation direction of the laser beam are the same. On the other hand, in the above structure, the distribution direction of the refractive index coincides with the propagation direction of the slow light mode wave. Since the effective refractive index for the slow light mode wave propagating in the horizontal direction is smaller than the effective refractive index for the laser light, in the above structure, the period (pitch) of the effective refractive index can be increased as compared with the DFB laser. .. This makes it easier to manufacture than a DFB laser.

A phase shift region may be formed substantially in the center of the VCSEL structure in the propagation direction. By providing the phase shift region, oscillation in the adjacent longitudinal mode at both ends of the stopband can be suppressed, oscillation can be promoted at the wavelength at the center of the stopband, and single mode can be enhanced.

A plurality of phase shift regions may be formed with respect to the propagation direction. This makes it possible to average the distribution of light intensity.

In one embodiment, the semiconductor laser may further include a diffraction grating formed on the surface side of the VCSEL structure. In this configuration, the diffraction grating can introduce a periodic change in the effective index of refraction.

At least one of the cross-sectional shape and the planar shape of the lattice pattern of the diffraction grating may be designed so that the reflectance due to the diffraction grating in the center when each lattice pattern is viewed in a plan view is high. This makes it possible to select the basic transverse mode and improve the beam quality.

In one embodiment, the thickness of the grating pattern of the diffraction grating may vary in the width direction of the VCSEL structure.

In one embodiment, the width of the grating pattern of the diffraction grating may be wide at the center of the aperture of the VCSEL structure and narrow at both ends of the aperture.

In one embodiment, additional patterns may be formed along the boundaries of the aperture of the VCSEL structure so as to sandwich the diffraction grating.

In one embodiment, the diffraction grating may include a plurality of regions having different diffraction orders. This makes it possible to selectively oscillate a desired wavelength by the vernier effect by combining a high-order diffraction grating that is easy to process and manufacture.

In one embodiment, the aperture width of the VCSEL structure may change periodically. In this configuration, a periodic change in the effective refractive index can be introduced by a periodic change in the aperture width.

In one embodiment, an end region may be formed adjacent to one end of the VCSEL structure, the upper surface of which is covered with a high reflection mirror and no current is injected. This makes it possible to emit a beam in one direction from the surface of the VCSEL structure.

In one embodiment, the optical device may include any of the semiconductor lasers described above, a high reflection mirror covering the top surface of the semiconductor laser, and an optical amplifier laterally coupled to the semiconductor laser. The optical amplifier includes a VCSEL structure that is continuous with the VCSEL structure of the semiconductor laser and is electrically isolated from the semiconductor laser. In this configuration, high quality and high output seed light is generated in the semiconductor laser, and the seed light is amplified in the optical amplifier, so that high quality and high output can be achieved.

In one embodiment, the optical device may include any of the semiconductor lasers described above, a high reflection mirror covering the top surface of the semiconductor laser, and a beam sweeping device laterally coupled to the semiconductor laser. The beam sweep device includes a VCSEL structure that is continuous with the VCSEL structure of the semiconductor laser. In this configuration, the direction of the beam emitted from the beam sweeping device can be scanned.

In one embodiment, the optical device may include any of the semiconductor lasers described above, a first high reflection mirror covering the top surface of the semiconductor laser, and a modulator laterally coupled to the semiconductor laser. The modulator includes a VCSEL structure that is continuous with the VCSEL structure of the semiconductor laser and is electrically isolated from the semiconductor laser. According to this configuration, high-power laser light can be modulated at high speed in time.

In one embodiment, the optical device may further include a second high reflection mirror overlying the top surface of the modulator and an emission region laterally coupled to the modulator. The emission region includes a VCSEL structure that is continuous with the VCSEL structure of the modulator and is electrically isolated from the modulator. A higher extinction ratio can be achieved by extracting the beam from the emission region adjacent to it rather than from the modulator.

In one embodiment, the optical device may be a coupled resonator slow light laser comprising a semiconductor laser and an external resonator laterally coupled to the semiconductor laser. The external cavity may include a VCSEL structure that is continuous with the VCSEL structure of the semiconductor laser and is electrically isolated, and a high reflectance mirror that covers the upper surface of the VCSEL structure. This makes it possible to achieve high output, high single-mode stability, high modulation speed, and the like.

In one embodiment, the optical device may include a semiconductor laser and a MEMS (Micro Electro Mechanical Systems) mirror formed across the top surface of the semiconductor laser and the air gap so that the length of the air gap can be controlled. ..

(Embodiment)
Hereinafter, the present invention will be described with reference to the drawings based on the preferred embodiments. The same or equivalent components, members, and processes shown in the drawings shall be designated by the same reference numerals, and duplicate description thereof will be omitted as appropriate. Further, the embodiment is not limited to the invention, but is an example, and all the features and combinations thereof described in the embodiment are not necessarily essential to the invention.

(Basic configuration)
FIG. 1 is a diagram showing a basic configuration according to an embodiment. The semiconductor laser 100 is a surface emitting laser including a substrate 102 and a VCSEL (vertical cavity surface emitting laser) structure 110. The VCSEL structure 110 has a laminated structure in which the core (gain region) 112 is sandwiched between the lower DBR (Distributed Bragg Reflector) layer 116 and the upper DBR layer 118. The core 112 includes an active layer 113 and an oxidative stenosis layer 114. The oxidative stenosis layer ₁₁₄ defines the width W of the aperture 120.

The semiconductor laser 100 includes an electrode 122 connected to the upper surface of the VCSEL structure 110 and an electrode 124 connected to the bottom surface of the VCSEL structure 110. When a drive circuit (not shown) is connected between the electrodes 122 and 124 and a current is injected into the core 112 of the VCSEL structure 110, the laser beam 2 is vertical in the VCSEL structure 110 between the upper DBR layer 118 and the upper DBR layer 116. A slow light mode wave 4 is formed in which multiple reflections occur in the direction (z direction) and slowly propagate in the horizontal direction (long direction of the VCSEL structure 110, that is, the x direction). The slow light mode wave 4 is folded back at the end of the VCSEL structure 110, reciprocates in the horizontal direction (x direction), and is amplified. Then, a part of the laser beam 2 is emitted from the aperture 120 on the upper surface of the VCSEL structure 110 as the emitted light 6.

FIG. 2 is a diagram illustrating an effective refractive index n _eff with respect to the slow light mode wave 4.

Consider the slow light mode wave 4 pointing to the right. The wave vector k of the laser beam 2 having the wavelength λ is represented by the equation (1).
k = 2πn _m / λ… (1)

Further, when the resonance wavelength (also referred to as cutoff wavelength) stable in the vertical direction (z direction) of the VCSEL structure 110 is λ _c , the wave vector k _c with respect to the resonance wavelength λ _c is expressed by the equation (2). To.
k _c = 2πn _m / λ _c … (2)

For the slow light mode wave, the slow light propagation constant β _SL corresponding to the wave vector can be conceived, and it is expressed by the equation (3) using the effective refractive index n _eff .
β _SL = 2πn _eff / λ… (3)

Further, the equation (4) holds for the three wave vectors.
k ² = kc ² + β _SL ² … (4)

By substituting the equations (1) to (3) into the equation (4) and rearranging them, the equation (5) is obtained.
_{n eff} = nm x √ (1- (λ / λ _c ) ² )… (5)
This is the effective index of refraction for slow light mode waves.

Assuming that the incident angle of the laser beam 2 is θ _i and the emission angle is θ _r , the equation (6) holds.
sinθ _r = _nm sinθ _i ... (6)

Further, the equation (7) holds.
sinθ _i = β _SL / k… (7)

From the equations (5) to (7), the emission angle _θr satisfies the equation (8).
sin θ _r = n _m √ (1- (λ / λ _c ) ² )… (8)

Return to FIG. In the present embodiment, the effective refractive index n _eff of the VCSEL structure 110 with respect to the slow light mode wave 4 changes periodically with respect to the propagation direction (x direction) of the slow light mode wave 4. For example, for the effective refractive index n _eff , the first value n _eff1 and the second value n _eff2 are alternately repeated, and the period (pitch) of the refractive index change is Λ.

The periodic structure of the effective refractive index n _eff can be designed by the laminated structure, and the main parameters are the core 112, the lower DBR layer 116, the upper DBR layer 118, the material of the resonance wavelength adjusting layer (not shown), and the thickness. , Aperture width _Wap and the like are exemplified.

A phase shift region 130 may be formed in the center of the VCSEL structure 110 in the propagation direction. In the phase shift region 130, the pitch Λ of the refractive index change may be given a phase shift of an odd multiple of λ / 4 (λ / 4 × (2n-1), n is a natural number).

The above is the configuration of the semiconductor laser 100. Next, the operation will be described.

The basic configuration of the semiconductor laser 100 is a structure similar to that of a normal VCSEL in which a quantum well is sandwiched between DBRs. Unlike a normal waveguide, in this semiconductor laser 100, the wave vector (propagation constant β _SL ) maintains a low group velocity leakage waveguide mode (slow light mode) that is close to the wave vector k of the laser beam 2. This allows the slow light mode wave 4 to wave back and forth in the right and left directions in the figure. As can be seen from FIG. 2, the emitted light of the semiconductor laser 100 is emitted in symmetrical directions by the slow light mode wave 4 pointing to the right and the slow light mode wave 4 pointing to the left.

3 (a) and 3 (b) are diagrams illustrating the single longitudinal mode in the semiconductor laser 100.

When a periodic structure having an effective refractive index n _eff is not introduced, as shown in FIG. 3A, a plurality of longitudinal modes oscillate simultaneously in the vicinity of the wavelength at which the net gain is maximized. In particular, if the VCSEL structure 110 is lengthened, the number of longitudinal modes (number of oscillation wavelengths) increases in exchange for higher output.

On the other hand, in the semiconductor laser 100 of FIG. 1, the slow light mode wave 4 is produced by periodically changing the effective refractive index n _eff with respect to the slow light mode wave 4 in the waveguide direction (x direction). Receives DFB (Distributed Feedback) effect in the propagation direction. As a result, as shown in FIG. 3B, the longitudinal mode outside the stopband does not obtain a sufficient net gain, while the only longitudinal mode inside the stopband obtains a sufficient net gain. One-mode oscillation is possible. As a result, even if the VCSEL structure is lengthened, oscillation can be realized in a single mode or a small number of longitudinal modes equivalent thereto, and both high output and single mode can be achieved. The stop band can be designed according to the periodic structure, and a wavelength away from the resonance wavelength (cutoff wavelength) λ _c can be selected.

FIG. 4 is a diagram illustrating the effect of the phase shift region 130. The left side of FIG. 4 shows a wide wavelength range, and the right side of FIG. 4 shows a spectrum expanded in the wavelength direction in the stop band. Within the stopband, there are multiple adjacent longitudinal modes. If the phase shift region 130 is not provided, the central oscillation mode cannot be obtained, and oscillation occurs in the adjacent longitudinal mode at both ends of the stop band. On the other hand, by providing the phase shift region 130, as shown on the right side of FIG. 4, oscillation in the adjacent longitudinal mode at both ends of the stop band is suppressed, and oscillation in the center oscillation mode of the stop band is promoted. It is possible to enhance the single mode.

Next, a far-field image of the semiconductor laser 100 will be described. 5 (a) to 5 (c) are views showing a far-field image of a semiconductor laser. The horizontal axis shows the emission angle θ _r , and the vertical axis shows the radiation intensity. FIG. 5 (a) shows a far-field image of a VCSEL having no periodic structure. When the periodic structure is not formed, the emitted beam is distributed in the vicinity of the vertical (θ = 0) and the angle corresponding to the vicinity of the gain spectrum peak.

On the other hand, when a periodic structure is formed and the mode is set to a single mode, a plurality of orders of diffracted light are emitted as shown in FIG. 5 (b).

By optimizing the periodic structure and the peripheral structure of the semiconductor laser 100, a unidirectional radiation pattern can be realized as shown in FIG. 5 (c).

The semiconductor laser 100 of FIG. 1 has an advantage that it is easy to manufacture as compared with a general DFB laser. This point will be explained.

In a general DFB laser, the distribution direction of the refractive index and the propagation direction of the laser beam are the same. Therefore, the equation (9) is formed between the center wavelength λ _m of the stopband and the pitch Λ of the periodic structure. It holds.
λ _m = 2 ・ Λ ・ n / m… (9)
When m = 1, λ _m = 850 nm, and n = 2.35, Λ = 0.1 μm, and it is necessary to form a fine structure.

On the other hand, in the semiconductor laser 100 using the slow light mode wave, the equation (10) holds between the center wavelength λ _m of the stop band and the pitch Λ of the periodic structure.
λ _m = 2 ・ Λ ・ n _eff / m… (10)
m represents the diffraction order.

Since the effective refraction factor n _eff for the slow light mode wave is smaller than the refraction factor n for the laser light, the pitch Λ required to obtain the center wavelength λ _m of the same stop band is usually the pitch Λ required for the semiconductor laser 100 of FIG. The pitch Λ of the effective refraction factor can be made larger than that of the DFB laser. Specifically, in the semiconductor laser 100 of FIG. 1, the pitch Λ of the periodic structure can be expanded to more than twice that of a normal DFB laser, and in a more preferable example, to about 5 times (0.5 micron). .. Since the semiconductor laser 100 does not require a fine structure like a normal DFB laser, it can be manufactured by ordinary sputtering lithography.

The present disclosure extends to various devices and methods ascertained from the basic configuration of FIG. 1 or derived from the above description, and is not limited to a specific configuration. Hereinafter, more specific configuration examples and examples will be described not to narrow the scope of the present invention but to help understanding the essence and operation of the invention and to clarify them.

FIG. 6 is a diagram showing a semiconductor laser 100A according to an embodiment. A diffraction grating 140 including a plurality of lattice patterns 142 is formed on the upper surface of the VCSEL structure 110, and the above-mentioned periodic structure having an effective refractive index n _eff is introduced by the diffraction grating 140. The pitch of the grid pattern 142 is d. The grid pattern 144 near the center is different in size from the other grid patterns 142 and functions as the phase shift region 130 in FIG.

The above is the configuration of the semiconductor laser 100A. FIG. 7 is a diagram showing the relationship between the pitch d of the diffraction grating 140 and the longitudinal mode interval. The center wavelength of the stopband is given by the equation (10), and the smaller the pitch d (= Λ) of the diffraction grating 140, the wider the longitudinal mode interval (stopband interval). The pitch d may be designed so that one longitudinal mode is included in the gain band.

FIG. 8 is a diagram showing the device length dependence of the net gain difference. The net gain difference is the difference in gain between the vertical mode in the center of the stop band (oscillation mode in FIG. 4) and the adjacent vertical mode. The horizontal axis is the device length L, which corresponds to the length of the aperture 120 in the x-direction.

The order was set to m = 2,4,8,16,20, and the device length L was selected and calculated so that kL = 2. From this result, it can be seen that even with an elongated device of L> 3 mm, a gain difference of 10 cm ^-1 is obtained, and it is possible to oscillate in a long and single mode.

FIG. 9 is a diagram illustrating an emission direction of the beam of the semiconductor laser 100A of FIG. The emission direction of the m-th order diffracted light is expressed as θ _m . The wavelength of the slow light mode wave is λ _SL , and the resonance wavelength is λ _c . Assuming that the refractive index on the waveguide side (inside the VCSEL structure 110) below the diffraction grating 140 is n _wg and the refractive index above the diffraction grating 140 is n', the following relational expression holds.
n'sinθ _m -n _wg sinθ _i = λ _SL · m / d
m = 0, ± 1, ± 2, ...
cos θ _i = λ _SL / λ _c

FIG. 10 is a diagram showing the calculation result of the radiation angle θ _m . In the calculation, λ _c = 871 nm, n _wg = 3.5, n'= 1, and d = 5 μm.

FIG. 11 is a diagram showing the measurement results of the radiation angle θ _m of the sample of the semiconductor laser 100A actually produced. By providing the diffraction grating 140, a far-field image as shown in FIG. 5B can be obtained.

Subsequently, the configuration of the diffraction grating 140 will be described. The lattice pattern of the diffraction grating 140 may be formed by etching a high refractive index material, may form a low refractive index material by the inversion pattern thereof, or may form a groove on the surface.

12 (a) to 12 (d) are diagrams showing a configuration example of the diffraction grating 140. In FIGS. 12A and 12B, the lattice pattern 142 forming the diffraction grating 140 is formed convexly. In FIG. 12A, the phase shift region 130 is formed by increasing the width of the grid pattern 144. In FIG. 12B, the phase shift region 130 is formed by partially expanding the distance between the grid pattern 142 and the grid pattern 142.

In FIGS. 12 (c) and 12 (d), the lattice pattern 142 forming the diffraction grating 140 is formed concave. In FIG. 12 (c), the phase shift region 130 is formed by increasing the width of the grid pattern 144, and in FIG. 12 (d), the distance between the grid pattern 142 and the grid pattern 142 is partially expanded. , The phase shift region 130 is formed.

13 (a) and 13 (b) are cross-sectional views showing a configuration example of the diffraction grating 140. In the example of FIG. 13A, the diffraction grating 140 is formed by forming the lattice pattern 142 (144) on the uppermost film 118a (for example, the GaAs layer) of the upper DBR layer 118. A dielectric film 119 such as a SiO _x or SiN _x film may be formed as a protective film on the upper side of the film 118a. Alternatively, the dielectric film 119 may be omitted and used as air.

In the example of FIG. 13B, a dielectric film 117a such as SiN _x or SiO _x is added above the upper DBR layer 118, and a lattice pattern 142 is formed on the dielectric film 117a. A diffraction grating 140 is formed on the protective film. A dielectric film (buffer layer) 117b such as SiN _x or SiO _x may be inserted between the dielectric film 117a and the upper DBR layer 118.

Dry etching, wet etching, nanoimprint, FIB (Focused Ion Beam) and the like can be used to form the diffraction grating 140. Further, as the shape of the lattice patterns 142 and 144, in addition to a rectangle, a semicircle, a sine wave, a triangle, a blazeed grating, or the like can be adopted.

FIG. 14 is a diagram showing the relationship (calculation result) between the depth of the grid and the diffraction efficiency. The calculation was performed for the case where the GaAs layer was etched to form the diffraction grating 140, as shown in FIG. 13 (a). The duty cycle of the diffraction grating (ratio of the length of the concave portion to the convex portion) is 50%. The horizontal axis is the etching depth, and the 0th-order, 1st-order, and 2nd-order diffraction efficiencies are shown.

From this calculation result, it can be seen that it is possible to select an arbitrary diffraction order m with the etching depth as a parameter. As an example, at an etching depth of about 20 nm, the 0th-order diffracted light is dominant, but when the etching is deeper, the ratio decreases and the ratio of the 1st-order diffracted light increases.

FIG. 15 is a diagram showing a design example of a diffraction grating. The depth D is 60 nm and the duty cycle is 50%. The period d is 500 nm for the first-order diffraction grating, 1000 nm for the second-order diffraction grating, 1500 nm for the third-order diffraction grating, and 2000 nm for the fourth-order diffraction grating. The length L _shift of the grid pattern 144 corresponding to the phase shift region 130 is λ / 4 in the 1st, 3rd, and 4th orders, but takes a value different from λ / 4 as in the 2nd order. In some cases.

FIG. 16 is a diagram showing the duty cycle dependence of diffraction efficiency. If the grating depth is shallow at D = 20 nm, the duty cycle dependence of the 0th order diffracted light efficiency is low regardless of the order, and the higher order diffracted light is suppressed in any duty cycle. I understand.

Subsequently, examples and modifications related to the diffraction grating 140 will be described.

FIG. 17 is a plan view showing a grid pattern. FIG. 17 shows some variations of the grid pattern. PTN1 is a general rectangle. PTN2 and PTN3 have a rhombic shape or an elliptical shape, and may have a shape wider toward the center of the aperture. In PTN4, an additional pattern extending in the propagation direction of the slow light mode wave is formed at the boundary of the aperture so as to sandwich the rectangular grid pattern.

According to the shapes PTN2 to PTN4, the reflectance due to the diffraction grating at the center when each lattice pattern is viewed in a plane is increased. As a result, the reflectance of the basic transverse mode in which the electric field strength at the center is large increases, so that the basic transverse mode can be selected and the single transverse mode can be enhanced.

Further, when the shapes PTN2 to PTN4 are adopted, the aperture width can be widened as compared with the case where the rectangular pattern PTN1 is used. Increasing the aperture width contributes to higher output.

18 (a) and 18 (b) are diagrams showing the cross-sectional shape of the grid pattern. As shown in FIG. 18A, in the lattice pattern 142 (144) formed in the concave portion, the thickness (depth) thereof may change with respect to the width direction (y direction) of the VCSEL structure 110. Specifically, the thickness is the largest at the center in the width direction, and the thickness is thinner at both ends.

As shown in FIG. 18B, the thickness (height) of the convexly formed lattice pattern 142 (144) may also change with respect to the width direction (y direction) of the VCSEL structure 110. Specifically, the thickness is the largest at the center in the width direction, and the thickness is thinner at both ends.

When a diffraction grating is formed as shown in FIGS. 18 (a) and 18 (b), the reflectance of the basic transverse mode diffraction grating having a large central electric field strength distribution increases, so that the basic transverse mode can be selected. It is possible to enhance the transverse mode.

FIG. 19 is a diagram showing a diffraction grating 140 according to a modified example. The diffraction grating 140 includes a plurality of regions 140A and 1402B having different diffraction orders, in other words, different pitches Λ of changes in the refractive index. A grid pattern 144 corresponding to the phase shift region 130 is formed at the boundary between the plurality of regions 140A and 140B.

20 (a) to 20 (c) are diagrams illustrating the spectrum of the diffraction grating of FIG. FIG. 20A shows the spectral intensity of the entire diffraction grating 140. 20 (b) and 20 (c) show the spectral intensities _R1 and _R2 of the first region 140A and the second region 140B, respectively. In the first region 140A, as shown in FIG. 20B, there are a plurality of longitudinal modes that can be guided in the band of the gain spectrum at wavelength intervals that are inversely proportional to the lattice period Λ ₁ . On the other hand, in the second region 140B, as shown in FIG. 20 (c), there are a plurality of longitudinal modes that can be guided in the band of the gain spectrum at wavelength intervals that are inversely proportional to the lattice period Λ ₂ .

The spectral intensity of the entire diffraction grating 140 including the first region 140A and the second region 140B is the product of the spectral intensity R ₁ of the first region 140A and the spectral intensity R ₂ of the second region 140B. Therefore, the so-called vernier effect selects the longitudinal mode common to the first region 140A and the second region 140B.

In general, the lower the lattice order, in other words, the shorter the lattice period Λ, the more difficult it is to manufacture the diffraction grating. According to the configuration of FIG. 19, by combining a plurality of regions having a high lattice order, that is, having a long lattice period Λ, manufacturing can be facilitated and the single longitudinal mode characteristic can be improved.

In the examples so far, one phase shift region 130 is formed with respect to the propagation direction, but a plurality of phase shift regions may be formed. In a structure having a single phase shift region, the intensity of light in the propagation direction (longitudinal direction) becomes maximum in the vicinity of the phase shift region and is attenuated outward. Therefore, in the vicinity of the phase shift region, the carrier density due to stimulated emission decreases (spatial hole burning), which may deteriorate the efficiency of the laser and the beam quality. On the other hand, by dividing the phase shift region into a plurality of parts, the light intensity distribution in the longitudinal direction can be further flattened and these problems can be alleviated.

Subsequently, a modification of the periodic structure having an effective refractive index of n _eff will be described. In the examples so far, the diffraction grating 140 is formed on the upper side of the VCSEL structure 110 as a periodic structure having an effective refractive index n _eff , but this is not the case.

FIG. 21 is a plan view of the semiconductor laser 100B according to the modified example. In this semiconductor laser 100B, the width _Wap of the aperture 120 changes periodically with respect to the waveguide direction of the slow light mode wave. As a result, the effective refractive index n _eff changes periodically.

In order to control the width _Wap of the aperture 120, a mesa structure in which the mesa width W _mesa changes periodically may be formed, and then the aperture 120 may be formed by oxidative stenosis. As a result, the width of the aperture 120 can have a periodic structure.

(Composite device / functional device)
The semiconductor laser 100 can be used as a high-power laser by itself, but it can be further increased in power by integrating with other devices, or another function can be added. can. Hereinafter, such composite devices and functional devices will be described.

FIG. 22 is a diagram showing an optical device 200A according to an embodiment. The optical device 200A includes a semiconductor laser 100 and a termination region 210. In the termination region 210, the semiconductor laser 100 and the VCSEL structure 110 are continuous, are optically coupled in the lateral direction (the waveguide direction of the slow light mode wave), and are electrically isolated. An insulating region 212 is formed in the vicinity of the boundary between the semiconductor laser 100 and the terminal region 210. The upper surface of the end region 210 is covered with a high reflection mirror (shielding structure) 214. The high reflection mirror 214 may be a dielectric multilayer mirror or a metal mirror. During the operation of the optical device 200A, no current is injected into the termination region 210.

As described above, since the semiconductor laser 100 having a diffraction grating loaded on its surface as shown in FIG. 6 has a symmetrical structure in the propagation direction, the emitted beam is divided into two symmetrical angles. By introducing a structure in which the surface is covered with a high reflectance mirror 214 without injecting current at the end of the semiconductor laser 100 as in the optical device 200A of FIG. 22, reflection at the end can be suppressed and unidirectional. Beam emission is possible.

FIG. 23 is a diagram showing an optical device 200B according to an embodiment. The optical device 200B includes a semiconductor laser 100 and an optical amplifier 220. In the semiconductor laser 100 and the optical amplifier 220, the VCSEL structure 110 is continuously formed in the lateral direction (the waveguide direction of the slow light mode wave). The optical amplifier 220 is formed with an insulating region 222 and a drive electrode 224. The upper surface of the semiconductor laser 100 is covered with a high reflection mirror 150. The high reflection mirror 150 may be a dielectric multilayer mirror or a metal mirror. The slow light mode wave 4 generated inside the semiconductor laser 100 is coupled to the optical amplifier 220.

The high output and single longitudinal mode slow light mode wave 4 generated by the semiconductor laser 100 is coupled to the optical amplifier 220. The optical amplifier 220 amplifies the slow light mode wave 4 and outputs it as a higher output light 8.

FIG. 24 is a diagram showing an optical device 200C according to an embodiment. The optical device 200C includes a semiconductor laser 100 and a beam sweeping device 230. The configuration of the beam sweep device 230 is similar to that of the optical amplifier 220 in FIG. The semiconductor laser 100 is configured so that the wavelength λ of the slow light mode wave 4 can be controlled. In order to control the wavelength λ, the VCSEL structure 110 may be configured with a variable longitudinal resonator length.

The emission angle _θr of the emission beam 8 of the beam sweep device 230 is expressed by the equation (8) as described above.
sin θ _r = n _m √ (1- (λ / λ _c ) ² )… (repost 8)
λ _c is the resonance wavelength of the beam sweep device 230. Therefore, the emission angle _θr can be swept by controlling the wavelength λ of the slow light mode wave 4.

FIG. 25 is a diagram showing an optical device 200D according to an embodiment. The optical device 200D includes a semiconductor laser 100 and an optical modulator 240. In the semiconductor laser 100 and the light modulator 240, the VCSEL structure 110 is continuous and is coupled in the lateral direction. The light modulator 240 has an insulating region 242 and a modulation electrode 244. By reverse-biasing the modulation electrode 244 and switching the drive current, the intensity (on, off) of the emitted beam 8 can be switched.

FIG. 26 is a diagram showing an optical device 200E according to an embodiment. The optical device 200E includes a semiconductor laser 100, an optical modulator 240, and an emission region 250. The semiconductor laser 100, the light modulator 240, and the emission region 250 have a continuous VCSEL structure 110 and are coupled in the lateral direction.

The upper surface of the light modulator 240 is covered with a high reflection mirror 246. The intensity of the slow light mode wave 4E waveguideed in the light modulator 240 varies depending on the modulation signal applied to the modulation electrode 244. This slow light mode wave 4E is coupled to the emission region 250 and is emitted from the upper surface thereof. The intensity of the emitted beam 8 in the emitted region 250 is switched according to the modulated signal.

FIG. 27 is a diagram showing an optical device 200F according to an embodiment. The optical device 200F includes a semiconductor laser 100 and at least one external resonator 260. In this example, the two external resonators 260 are optically coupled laterally to the semiconductor laser 100 and electrically isolated.

More specifically, in the semiconductor laser 100 and the external resonator 260, the respective VCSEL structures 110 are continuously formed. An insulating region 262 is formed in the external resonator 260, and the upper surface thereof is covered with a high reflection mirror 266. The feedback adjusting electrode 264 may be formed on the external resonator 260.

By laterally connecting the external resonator 260 to the semiconductor laser 100, a coupled resonator slow light laser can be realized, and high output, high single-mode stability, high modulation speed, and the like can be realized.

FIG. 28 is a diagram showing an optical device 200G according to an embodiment. The optical device 200G includes a semiconductor laser 100 and a MEMS (Micro Electro Mechanical Systems) mirror 270. The MEMS mirror 270 is, for example, a dielectric multilayer mirror, and is formed on the upper surface side of the semiconductor laser 100 with an air gap 274 interposed therebetween. The reflectance of the MEMS mirror 270 is designed to be lower than 100%, and the light 6 transmitted through the MEMS mirror 270 can be taken out to the outside.

For example, the air gap 274 may be formed by forming the sacrificial layer 276 on the same substrate as the semiconductor laser 100 and partially removing the sacrificial layer 276.

The MEMS mirror 270 is a composite device of a mirror and an actuator. A drive electrode 272 is formed on the MEMS mirror 270, and the length of the air gap 274, in other words, the length of the resonator in the vertical direction of the semiconductor laser 100 can be controlled according to the drive signal.

The output of a normal tunable MEMS VCSEL is several mW or less, but by introducing the semiconductor laser 100 having the above-mentioned slow light laser structure, high output, high single-mode stability, large continuous wavelength sweep width, etc. can be achieved. You can expect it. By realizing an output of several tens of mW, it can be applied to high spatial resolution tomographic image measurement of living organisms such as OCT (Optical Coherence Tomography).

Although the present invention has been described using specific terms and phrases based on the embodiments, the embodiments merely indicate the principles and applications of the present invention, and the embodiments are defined in the claims. Many modifications and changes in arrangement are permitted within the range that does not deviate from the idea of the invention.

100 ... semiconductor laser, 102 ... substrate, 110 ... VCSEL structure, 112 ... core, 113 ... active layer, 114 ... oxide constriction layer, 116 ... lower DBR layer, 118 ... upper DBR layer, 120 ... aperture, 122, 124 ... electrodes , 130 ... Phase shift region, 140 ... Diffraction grating, 142, 144 ... Grating pattern, 150 ... High reflection mirror, 2 ... Laser light, 4 ... Slow light mode wave, 200 ... Optical device, 210 ... Termination region, 212 ... Insulation Region, 214 ... High reflection mirror, 220 ... Optical amplifier, 230 ... Beam sweep device, 240 ... Optical modulator, 244 ... Modulation electrode, 250 ... Ejector, 260 ... External resonator, 270 ... MEMS mirror.

Claims (17)

A VCSEL (Vertical Cavity Surface Emitting Laser) structure is provided, and laser light is repeatedly reflected in the VCSEL structure in the vertical direction to form a slow light mode wave, and the slow light mode wave reciprocates in the horizontal direction of the VCSEL structure. It is possible and
A semiconductor laser characterized in that the effective refractive index of the VCSEL structure with respect to the slow light mode wave changes periodically with respect to the propagation direction of the slow light mode wave. The semiconductor laser according to claim 1, wherein a phase shift region is formed substantially in the center of the VCSEL structure in the propagation direction. The semiconductor laser according to claim 2, wherein a plurality of the phase shift regions are formed with respect to the propagation direction. The semiconductor laser according to any one of claims 1 to 3, further comprising a diffraction grating formed on the surface side of the VCSEL structure. The claim is characterized in that at least one of the cross-sectional shape and the planar shape of the lattice pattern of the diffraction grating is designed so that the reflectance by the diffraction grating at the center when each lattice pattern is viewed in a plan view is high. 4. The semiconductor laser according to 4. The semiconductor laser according to claim 4, wherein the thickness of the lattice pattern of the diffraction grating changes in the width direction of the VCSEL structure. The semiconductor laser according to claim 4, wherein the width of the lattice pattern of the diffraction grating is wide at the center of the aperture of the VCSEL structure and narrow at both ends of the aperture. The semiconductor laser according to claim 4, wherein an additional pattern is formed along the boundary of the aperture of the VCSEL structure so as to sandwich the diffraction grating. The semiconductor laser according to any one of claims 4 to 8, wherein the diffraction grating includes a plurality of regions having different diffraction orders. The semiconductor laser according to any one of claims 1 to 3, wherein the aperture width of the VCSEL structure changes periodically. The semiconductor laser according to any one of claims 1 to 10, wherein an upper surface thereof is covered with a high-reflection mirror and a terminal region in which no current is injected is formed adjacent to one end of the VCSEL structure. The semiconductor laser according to any one of claims 1 to 11.
A high-reflection mirror that covers the upper surface of the semiconductor laser,
An optical amplifier laterally coupled to the semiconductor laser,
Equipped with
The optical amplifier includes a VCSEL structure continuous with the VCSEL structure of the semiconductor laser, and is electrically isolated from the semiconductor laser. The semiconductor laser according to any one of claims 1 to 11.
A high-reflection mirror that covers the upper surface of the semiconductor laser,
A beam sweeping device laterally coupled to the semiconductor laser,
Equipped with
The beam sweep device is an optical device including a VCSEL structure continuous with the VCSEL structure of the semiconductor laser. The semiconductor laser according to any one of claims 1 to 11.
A first high-reflection mirror that covers the upper surface of the semiconductor laser,
A modulator coupled laterally to the semiconductor laser,
Equipped with
The modulator includes a VCSEL structure continuous with the VCSEL structure of the semiconductor laser, and is an optical device characterized by being electrically isolated from the semiconductor laser. A second high-reflection mirror that covers the upper surface of the modulator,
An emission region laterally coupled to the modulator,
Further prepare
The optical device according to claim 14, wherein the emission region includes a VCSEL structure continuous with the VCSEL structure of the modulator and is electrically isolated from the modulator. The semiconductor laser according to any one of claims 1 to 11.
An external resonator laterally coupled to the semiconductor laser,
Equipped with
The external resonator is
A VCSEL structure that is continuous with the VCSEL structure of the semiconductor laser and is electrically isolated.
A high reflectance mirror covering the upper surface of the VCSEL structure and
An optical device characterized by including. The semiconductor laser according to any one of claims 1 to 11.
A MEMS (Micro Electro Mechanical Systems) mirror formed so as to separate the upper surface of the semiconductor laser from the air gap and having a controllable length of the air gap.
An optical device characterized by being equipped with.

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