A semiconductor laser with optical mode filtering and a method of manufacturing the same

Abstract

The application relates to a semiconductor laser with light mode filtering and a preparation method thereof, wherein an ohmic contact layer is arranged below a ridge waveguide, shoulder grooves are formed on both sides of the ridge waveguide, and the bottoms of the shoulder grooves are arranged in a P confinement layer; deep grooves are symmetrically arranged on both sides of the ridge waveguide, and the deep grooves are arranged in pairs on the bottoms of the shoulder grooves, and one end of the deep groove is located on a front cavity surface or a rear cavity surface. The technical scheme provided by the application can filter high-order light fields in the laser, can effectively improve the purity of a fundamental mode output, and can optimize light spot quality. The laser structure provided by the application is small in size, and cannot affect a laser threshold and a difficulty of a subsequent packaging process.

Description

Technical Field

[0001] This invention relates to a semiconductor laser with optical mode filtering and its fabrication method, belonging to the technical field of semiconductor lasers. Background Technology

[0002] Since their invention, semiconductor lasers have been a core technology in modern optoelectronic devices, widely used in optical information storage, laser displays, industrial lasers, and medical equipment. Visible-band semiconductor lasers are facing new development opportunities. 650nm semiconductor lasers, with their excellent beam quality, superior cost-effectiveness, and high photoelectric conversion efficiency, have been widely used in laser pointing equipment, laser processing equipment, optical sensing devices, optical communication, and medical devices. To meet new market demands, improving emitted beam quality, enhancing fundamental mode purity, and reducing stray light from higher-order modes have become hot topics in semiconductor laser device research and development and industrial technology development. Traditional ridge waveguide strip semiconductor lasers, due to limitations in manufacturing precision and effects such as spatial hole burning, result in multiple modes in their output beam, leading to poor output beam quality and hindering their applications in fiber coupling and optical communication.

[0003] To address the above issues, existing designs mainly fall into two categories: one is to incorporate a mode filter structure within the laser to compensate for or reduce higher-order modes during laser resonance; the other is to add fine structures to the front and rear cavity surfaces of the laser for laser output filtering. Incorporating a filter structure inside the laser increases resonant losses and reduces current density, thereby significantly decreasing laser efficiency. Cavity surface filter structures are relatively small, placing stringent requirements on photolithography and fabrication processes.

[0004] Chinese patent document CN101895059B discloses a method for fabricating a high-brightness bar-shaped semiconductor laser with a mode filter. Based on a traditional bar-shaped laser, a mode filter region is photolithographically etched onto the P-surface of an epitaxial wafer, creating a device for shaping and homogenizing the laser beam, enabling the bar-shaped semiconductor laser to achieve high-brightness beam transmission with low divergence angle and high beam quality. The mode filter region has a length of 50-500 μm and a width of 4-200 μm; the etching depth of the mode filter region is 0.3-1 μm; a 0.2-0.35 μm thick masking film is sputtered onto the entire epitaxial wafer where the photolithography pattern is formed. Although this design theoretically shapes the beam mode using a large-area filter structure, the large, deeply etched area increases the difficulty of subsequent packaging electrode soldering. Due to the large etched surface, there are no load-bearing structures on both sides of the ridge during the subsequent die soldering process, resulting in greater stress on the packaging and a higher risk of packaging damage, which will severely reduce the laser yield. Furthermore, large-area filtering structures may negatively impact the laser's resonant optical field and reduce its resonant efficiency, thus failing to fundamentally solve the problem of optimizing the beam pattern in mass-produced semiconductor lasers.

[0005] Chinese patent document CN111641104A discloses a semiconductor laser chip structure, including a chip body with a wide strip-shaped current injection electrode. Several current injection separation electrodes are arranged on the wide strip-shaped current injection electrode to form a waveguide compensation structure, thereby reducing the beam divergence angle in the slow axis direction of the wide strip-shaped semiconductor laser and improving its beam quality during high-power operation. By making the width of the channel portion on both sides of the narrowest ridge wider than the channel width of the light emission end portion, it can suppress the variation of the intensity center of the horizontal far-field pattern with changes in light output, emitting a laser beam with a stable far-field pattern. This patent utilizes several current injection separation electrodes to optimize the laser field, and each separation electrode needs to be aligned with the underlying current injection layer, greatly increasing the complexity of the photolithography process. Simultaneously, the large current diffusion region of the injected current leads to a large thermal diffusion region. If a compensation structure is formed by reducing the current density through discrete electrodes, the spacing between the discrete electrodes must be very large, which will severely reduce the current injection efficiency and lower the device efficiency. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a semiconductor laser with optical mode filtering, capable of filtering out higher-order optical fields in the laser, effectively improving the purity of the fundamental mode output and optimizing the beam quality. This design has a small structural size and will not affect the laser threshold or the complexity of subsequent packaging processes.

[0007] In addition, this application also provides a method for fabricating the aforementioned optical mode filtered semiconductor laser.

[0008] The technical solution of this invention is as follows:

[0009] A semiconductor laser with optical mode filtering includes, from bottom to top, a substrate, an N-confinement layer, an N-waveguide layer, a quantum well active layer, a P-waveguide layer, a P-confinement layer, and an ohmic contact layer.

[0010] A ridge waveguide is provided below the ohmic contact layer, and shoulder grooves are formed on both sides of the ridge waveguide. The bottom of the shoulder grooves is located in the P-confining layer.

[0011] The ridge waveguide has symmetrical deep grooves on both sides, and the deep grooves are opened in pairs at the bottom of the shoulder groove. One end of the deep groove is located on the front cavity surface or the rear cavity surface.

[0012] Partial areas of the ohmic contact layer, the surface of the shoulder groove, and the surface of the deep trench are covered with an insulating layer; a P-side metal is provided on the upper surface of the insulating layer.

[0013] Semiconductor lasers typically generate multi-mode output optical fields in the ridge waveguide region, and the more modes a laser contains, the worse the beam quality. Adding deep trenches on both sides of the ridge, with an isolation layer formed within each trench, creates a strong boundary condition by contrasting the refractive index of the isolation layer with that of the epitaxial layer. This restricts the expansion of the ridge waveguide's gain region, preventing the generation of more modes. Furthermore, placing these deep trenches close to the sides of the ridge, within the higher-order mode oscillation cavity region, effectively absorbs and prevents higher-order modes from participating in laser oscillation.

[0014] Adding a deep trench at the end of the ridge waveguide resonator of the semiconductor laser avoids the intracavity loss caused by adding structures inside the cavity, which affects the laser threshold and oscillation efficiency. Setting a pair of deep trenches at the front and rear cavity surfaces allows for mode filtering of the optical field with minimal structural changes. This design helps the laser maintain good operating conditions.

[0015] An insulating layer is placed on the surface of the deep trench. The refractive index difference between the insulating layer and the semiconductor material, and the insulating layer is modified according to the growth process conditions, can effectively filter out high-order mode stray light that deviates from the ridge.

[0016] According to a preferred embodiment of the present invention, two pairs of deep grooves parallel to the ridge waveguide are formed at the bottom of the shoulder groove. The two pairs of deep grooves are symmetrically arranged at both ends of the laser. One end of one pair of deep grooves is located on the front cavity surface, and one end of the other pair of deep grooves is located on the rear cavity surface.

[0017] The deep grooves on the front and rear cavity surfaces are symmetrically arranged and, after cleavage, are located at equal lengths on the front and rear cavity surfaces of the laser to reduce the difficulty of separating the front and rear cavity surface areas in the coating process and the difficulty of the manufacturing process.

[0018] According to a preferred embodiment of the present invention, the distance between the deep trench and the ridge waveguide is 1-10 μm, depending on the size of the laser.

[0019] The distance between the deep trench and the ridge waveguide can be adjusted according to the actual laser ridge waveguide size design and processing technology. According to waveguide theory analysis, the closer the ridge waveguide is to the deep trench, the stronger the suppression effect on higher-order modes, but at the same time, the higher the precision requirements of the etching process and photolithography alignment process.

[0020] According to a preferred embodiment of the invention, the bottom of the deep trench passes through the N-waveguide layer, and the bottom of the deep trench is located in the N-confining layer.

[0021] The trench depth must exceed the active region, with the bottom extending beyond the N-waveguide layer. This trench structure cuts off the active region, effectively limiting ridge current diffusion, reducing the gain range, and filtering higher-order modes. Furthermore, to maintain a symmetrical optical field distribution within the vertical epitaxial layers of the semiconductor laser, the N-waveguide and P-waveguide layers must be parametrically symmetrical; therefore, the bottom of the trench must extend beyond the N-waveguide layer. Based on this, an air isolation layer and a high refractive index deviation are formed within the trench. The presence of the air isolation layer effectively suppresses the number of laser modes generated in the ridge region. For higher-order laser modes, the high refractive index deviation also provides a filtering effect, effectively improving the beam quality.

[0022] According to the present invention, the length of the deep trench is preferably 10-70 μm; more preferably, the length of the deep trench is 50 μm.

[0023] When the trench length is less than 10µm, the filtering effect is weak; if the trench length is too long, the ratio of the laser filter structure to the entire resonant cavity length will increase, increasing the laser resonant cavity loss and reducing the laser efficiency. At the same time, long trenches require higher precision in photolithography overlay.

[0024] According to the present invention, the width of the deep trench is preferably 1-20 μm; more preferably, the width of the deep trench is 7.5 μm.

[0025] The trench width is determined by the etching time. Since both the width and depth of the trench are directly proportional to the etching time during the etching process, a narrow trench will prevent the etching depth from penetrating the N-waveguide layer. This design has a high redundancy for excessively large trench widths; however, the trench width can be appropriately increased while ensuring the distance between the ridge and the trench edge.

[0026] According to a preferred embodiment of the invention, the top of the deep groove is semi-circular.

[0027] According to a preferred embodiment of the present invention, the insulating layer is made of SiO2, Si3N4, or other transparent insulating media.

[0028] The fabrication method of the above-mentioned optical mode filtered semiconductor laser includes the following steps in sequence:

[0029] (1) An N-confinement layer, an N-waveguide layer, a quantum hydrazine active region, a P-waveguide layer, a P-confinement layer and an ohmic contact layer were sequentially grown on the substrate using the MOCVD method.

[0030] (2) Etching forms shoulder grooves;

[0031] (3) A deep groove is etched at the bottom of the shoulder groove;

[0032] (4) Grow an insulating layer;

[0033] (5) Remove the insulating layer on the ridge waveguide by photolithography, etching or stripping to leave a contact area for the next step of P-side ohmic contact;

[0034] (6) Prepare P-side metal by vapor deposition or other methods, and remove excess metal at the laser edge by peeling or etching.

[0035] (7) After substrate thinning, preparation of back metal, alloy treatment, and cleavage, a semiconductor laser chip is finally formed.

[0036] The beneficial effects of this invention are as follows:

[0037] 1. This invention provides a semiconductor laser with optical mode filtering. Deep grooves are symmetrically arranged on both sides of the ridge waveguide in the shoulder slot, which can filter the output optical mode of the ridge waveguide semiconductor laser, effectively suppress the oscillation and transmission of higher-order modes generated in the ridge region, filter the higher-order optical modes that interfere with the quality of the laser spot, thereby filtering stray light fields and realizing the pure output of the fundamental mode spot.

[0038] 2. Using the solution provided by this invention, the proportion of semiconductor laser chips with pure far-field beam fundamental mode is increased from the original 14% to 75.8%. Attached Figure Description

[0039] Figure 1 A schematic cross-sectional view of a semiconductor laser provided by the present invention;

[0040] Figure 2 This is a top view of the ridge waveguide, shoulder groove, and deep trench;

[0041] Figure 3 The image shows the spot pattern before the improvement provided by the technical solution in this application;

[0042] Figure 4 The improved spot pattern is obtained by adopting the technical solution provided in this application.

[0043] 1. Ohmic contact layer, 2. Ridge waveguide, 3. Shoulder groove, 4. Insulating layer, 5. P-waveguide layer, 6. Quantum well active layer, 7. P-plane metal, 8. Deep trench, 9. N-waveguide layer, 10. N-confining layer, 11. GaAs substrate, 12. Shoulder region. Detailed Implementation

[0044] The present invention will be further described below with reference to the embodiments and accompanying drawings, but is not limited thereto.

[0045] Example 1

[0046] A semiconductor laser with optical mode filtering, such as Figure 1As shown, it includes, from bottom to top, a substrate, an N-confinement layer 10, an N-waveguide layer 9, a quantum well active layer 6, a P-waveguide layer 5, a P-confinement layer, and an ohmic contact layer 1;

[0047] A ridge waveguide 2 is disposed below the ohmic contact layer 1, and shoulder grooves 3 are formed on both sides of the ridge waveguide 2; the bottom of the shoulder grooves 3 is disposed in the P-confining layer;

[0048] The ridge waveguide 2 has symmetrical deep grooves 8 on both sides, and the deep grooves 8 are opened in pairs at the bottom of the shoulder groove 3. One end of the deep groove 8 is located on the front cavity surface or the rear cavity surface; and the surface of the shoulder groove 3 and the surface of the deep groove 8 are covered with an insulating layer 4.

[0049] A portion of the ohmic contact layer 1 is made electrically connected to the P-side metal 7 by removing the insulating layer 4 through an overlay windowing process; the remaining upper surfaces of the structure are all provided with the P-side metal 7.

[0050] The semiconductor laser in the ridge waveguide 2 region typically exhibits multi-mode oscillation output; the more modes it contains, the worse the output beam of the semiconductor laser. Deep trenches 8 are added to both sides of the ridge waveguide 2, forming an isolation layer within the trenches 8. The insulating layer 4 within the isolation layer effectively limits the expansion of the gain region below the ridge waveguide 2, preventing the gain region from expanding outwards and generating more modes.

[0051] Adding a deep trench 8 at the end of the resonant cavity of the ridge waveguide 2 of the semiconductor laser avoids the intracavity loss caused by adding structures inside the cavity, which affects the laser threshold and oscillation efficiency. Setting a pair of deep trenches 8 at the front cavity surface and the rear cavity surface can achieve mode filtering of the optical field with minimal structural changes. This design is beneficial for the laser to maintain good operating conditions.

[0052] The deep trench 8 is not sensitive to depth. During the etching process of the deep trench 8, since the depth and width of the deep trench 8 are directly proportional, if the etching time exceeds the set value, the etching depth of the deep trench 8 will penetrate through the N-confining layer 10 to reach the GaAs substrate 11. At the same time, the distance between the deep trench 8 and the edge of the ridge waveguide 2 structure will be less than the set value of 5 μm. According to waveguide theory analysis, the closer the distance between the deep trench 8 and the edge of the ridge waveguide 2 structure, the fewer higher-order modes there are. Therefore, exceeding the etching distance is beneficial for mode suppression and filtering. In summary, this design can provide a larger process window for the etching process, which is conducive to industrial implementation.

[0053] Example 2

[0054] The optical mode filtering semiconductor laser differs from the optical mode filtering semiconductor laser provided in Example 1 in that:

[0055] like Figure 2As shown, two pairs of deep grooves 8 parallel to the ridge waveguide 2 are opened at the bottom of the shoulder groove 3. The two pairs of deep grooves 8 are symmetrically arranged at both ends of the laser. One end of one pair of deep grooves 8 is located on the front cavity surface, and one end of the other pair of deep grooves 8 is located on the rear cavity surface.

[0056] One side of the shoulder groove 3 is the ridge waveguide 2, and the other side is the shoulder region 12.

[0057] The deep grooves on the front and rear cavity surfaces are symmetrically arranged and, after cleavage, are located at equal lengths on the front and rear cavity surfaces of the laser to reduce the difficulty of separating the front and rear cavity surface areas in the coating process and the difficulty of the manufacturing process.

[0058] Depending on the size of the laser, the distance between the deep trench 8 and the ridge waveguide 2 is 1-10 μm;

[0059] The distance between the deep trench 8 and the ridge waveguide 2 can be adjusted according to the actual size design and processing technology of the ridge waveguide 2 of the laser. According to waveguide theory analysis, the closer the ridge waveguide 2 is to the deep trench 8, the stronger the suppression effect on higher-order modes, but at the same time, the higher the precision requirements of the etching process and photolithography alignment process.

[0060] The bottom of the deep trench 8 passes through the N waveguide layer 9, and the bottom of the deep trench 8 is located in the N confinement layer 10.

[0061] The depth of trench 8 must exceed the active region, with the bottom of trench 8 extending beyond the N-waveguide layer 9. The trench 8 structure cuts off the active region, effectively limiting ridge current diffusion, reducing the gain range, and filtering higher-order modes. Furthermore, to maintain a symmetrical optical field distribution within the vertical epitaxial layers of the semiconductor laser, the N-waveguide layer 9 and P-waveguide layer 5 need to be parametrically symmetrical; therefore, the bottom of trench 8 must extend beyond the N-waveguide layer 9. Based on this, an air isolation layer is formed within trench 8, and the semiconductor material exhibits a high refractive index deviation. The presence of the air isolation layer effectively suppresses the number of laser modes generated in the ridge region. For the higher-order laser modes that are formed, the high refractive index deviation also provides a filtering effect, thereby effectively improving the beam quality.

[0062] The length of deep trench 8 is 10-70 μm.

[0063] When the length of the deep trench 8 is less than 10µm, the filtering effect is weak; if the length of the deep trench 8 is too long, the ratio of the laser filter structure to the entire resonant cavity length will increase, increasing the laser resonant cavity loss and reducing the laser efficiency. At the same time, long trenches require higher precision in photolithography overlay.

[0064] The width of deep trench 8 is 1-20 μm.

[0065] The width of the deep trench 8 is determined by the etching time. Since the width and depth of the deep trench 8 are directly proportional to the etching time during the etching process, a narrow deep trench 8 will prevent the etching depth from penetrating the N-waveguide layer 9. This design has a high redundancy for an excessively large deep trench 8 width. While ensuring the distance between the ridge and the edge of the deep trench 8, the width of the deep trench 8 can be appropriately increased.

[0066] The top of deep groove 8 is semi-circular.

[0067] The insulating layer 4 is made of SiO2, Si3N4, or other transparent insulating media.

[0068] Using the technology provided in this embodiment, the proportion of semiconductor laser chips with pure far-field beam fundamental mode is increased from the original 14% to 75.8%.

[0069] In contrast, a semiconductor laser that does not have a deep groove 8 in the shoulder slot 3, but whose other parameters are the same as in this embodiment, such as... Figure 3 As shown, before improvement, the beam spot edge exhibited side modes, and higher-order modes of the beam spot remained a significant issue. After adopting the optimization scheme of this invention, the optimized beam spot pattern is as follows: Figure 4 As shown, the edge side mold is significantly reduced.

[0070] Example 3

[0071] The optical mode filtering semiconductor laser differs from the optical mode filtering semiconductor laser provided in Example 2 in that:

[0072] The length of deep trench 8 is 50 μm.

[0073] The length of the deep trench 8 can be adjusted according to the actual filtering effect of the laser. The longer the length, the longer the effective area of ​​the deep trench and the better the filtering effect. However, it will also increase the loss of the laser waveguide resonator and the processing difficulty.

[0074] Example 4

[0075] The optical mode filtering semiconductor laser differs from the optical mode filtering semiconductor laser provided in Example 2 in that:

[0076] The width of deep trench 8 is 7.5 μm.

[0077] Example 5

[0078] The method for fabricating a semiconductor laser with optical mode filtering provided in any one of the embodiments 1-4 includes the following steps in sequence:

[0079] (1) An N-confinement layer 10, an N-waveguide layer 9, a quantum hydrazine active region, a P-waveguide layer 5, a P-confinement layer and an ohmic contact layer 1 were sequentially grown on a substrate using the MOCVD method.

[0080] (2) Etching forms shoulder groove 3;

[0081] (3) A deep groove 8 is etched at the bottom of the shoulder groove 3;

[0082] (4) Growth of insulating layer 4;

[0083] (5) Remove the insulating layer 4 on the ridge waveguide 2 by photolithography, etching or stripping to leave a contact area for the next step of P-side ohmic contact;

[0084] (6) Prepare the P-side metal 7 by vapor deposition or other methods, and remove the excess metal at the edge of the laser by peeling or etching.

[0085] (7) After substrate thinning, preparation of back metal, alloy treatment, and cleavage, a semiconductor laser chip is finally formed.

Claims

1. A semiconductor laser with optical mode filtering, characterized in that, The laser comprises, from bottom to top, a substrate, an N-confinement layer, an N-waveguide layer, a quantum well active layer, a P-waveguide layer, a P-confinement layer, and an ohmic contact layer. A ridge waveguide is disposed below the ohmic contact layer, with shoulder grooves formed on both sides of the ridge waveguide. The bottom of the shoulder grooves is located within the P-confinement layer. Two pairs of deep trenches parallel to the ridge waveguides are formed at the bottom of the shoulder grooves, symmetrically positioned at both ends of the laser. One end of one pair of trenches is located at the front cavity surface, and one end of the other pair is located at the rear cavity surface. The bottom of each deep trench passes through the N-waveguide layer and is located within the N-confinement layer. Partial areas of the ohmic contact layer, the surface of the shoulder groove, and the surface of the deep trench are covered with an insulating layer; a P-side metal is provided on the upper surface of the insulating layer.

2. The semiconductor laser with optical mode filtering according to claim 1, characterized in that, The distance between the deep trench and the ridge waveguide is 1-10 μm.

3. A semiconductor laser with optical mode filtering according to claim 1, characterized in that, The length of the deep trench is 10-70 μm.

4. A semiconductor laser with optical mode filtering according to claim 3, characterized in that, The length of the deep trench is 50 μm.

5. A semiconductor laser with optical mode filtering according to claim 1, characterized in that, The width of the deep trench is 1-20 μm.

6. A semiconductor laser with optical mode filtering according to claim 5, characterized in that, The width of the deep trench is 7.5 μm.

7. A semiconductor laser with optical mode filtering according to claim 1, characterized in that, The top of the deep trench is semi-circular.

8. A semiconductor laser with optical mode filtering according to claim 1, characterized in that, The insulating layer is made of SiO2 and Si3N4.

9. The method for fabricating a semiconductor laser with optical mode filtering as described in any one of claims 1-8, characterized in that, The steps are as follows: (1) An N-confinement layer, an N-waveguide layer, a quantum hydrazine active region, a P-waveguide layer, a P-confinement layer and an ohmic contact layer were sequentially grown on the substrate using the MOCVD method. (2) Etching forms shoulder grooves; (3) A deep groove is etched at the bottom of the shoulder groove; (4) Grow an insulating layer; (5) Remove the insulating layer on the ridge waveguide; (6) Preparation of P-faced metals; (7) After substrate thinning, preparation of back metal, alloy treatment, and cleavage, a semiconductor laser chip is finally formed.

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