Semiconductor laser cavity surface damage threshold improving method and semiconductor laser

Abstract

The application discloses a kind of semiconductor laser cavity surface damage threshold promotion method and semiconductor laser, the semiconductor laser cavity surface damage threshold promotion method is applied to straight waveguide, end emission, electrically pumped semiconductor laser.This method is set gradually variable waveguide section on the side of laser emission end face, gradually variable waveguide section is connected with the optical propagation direction end of straight waveguide section, its width gradually widens to the larger exit end face width from the equal width with straight waveguide section along optical propagation direction;Etching electrically isolated groove between gradually variable waveguide section and straight waveguide section, groove depth reaches the cladding of semiconductor laser;Current injection area is limited in straight waveguide section simultaneously, so that gradually variable waveguide section has no current direct injection.This method can significantly reduce the heat density generated by optical absorption in cavity surface area and the heat generated by current diffusion, significantly improve the optical damage threshold of cavity surface, effectively solve the problem of material ablation and permanent damage of device caused by cavity surface overheating.

Description

Technical Field

[0001] This invention relates to the field of semiconductor laser device design technology, and in particular to a method for improving the cavity surface damage threshold of a semiconductor laser and a semiconductor laser. Background Technology

[0002] Semiconductor lasers, with their advantages of small size, light weight, and high integrability, have been widely used in communications, industry, and medical fields. With the continuous upgrading of high-end equipment industries, the market demand for the output power of semiconductor lasers is constantly increasing. However, surface-cavity optical damage remains the most significant failure mode for high-power semiconductor lasers, severely restricting further improvements in device output power and reliability.

[0003] In existing straight-waveguide, end-emitting, electrically pumped semiconductor lasers, surface overheating is the direct cause of surface material ablation and permanent device damage. This is especially true in long-wavelength infrared quantum cascade lasers, where surface coatings exhibit strong absorption, which is currently the primary factor limiting power increases. Specifically, this thermal runaway process is mainly driven by a dual-source coupling of photothermal and electrothermal effects. On the one hand, there is a significant photothermal effect in the cavity surface region. The cavity surface formed by cleaving of the laser has inherent surface defect states, and the coating material deposited on the cavity surface to improve emission coupling also exhibits optical absorption. When the laser propagates to the emitting end face in a conventional straight waveguide with a uniform cross-section, the optical power density at the cavity surface is extremely high due to the small cross-sectional area of ​​the straight waveguide. The high-density optical field is strongly absorbed at the cavity surface and converted into heat energy, leading to a sharp increase in the local temperature of the cavity surface.

[0004] On the other hand, the cavity surface region suffers from severe electrothermal effects. In traditional semiconductor laser structures, the current injection region typically extends to the area near the emitter facet. Even in some existing schemes that employ simple tapered waveguides to expand the mode field, the waveguide width increases near the facet. Without effective physical isolation of the current injection, the injected current easily diffuses laterally within the semiconductor material (i.e., current dispersion), spreading to the region near the cavity surface. This current dispersion and continuous injection near the cavity surface generate severe Joule heating, further exacerbating heat accumulation in the cavity surface region.

[0005] The aforementioned photothermal and electrothermal effects superimpose on the cavity surface region, leading to excessive heat accumulation. More seriously, the increase in cavity surface temperature causes bandgap contraction in the semiconductor material, further increasing the optical absorption coefficient in the cavity surface region. This increased light absorption generates even more heat, creating a vicious positive feedback loop of "temperature increase - bandgap contraction - intensified absorption - further temperature increase," i.e., thermal runaway. When the accumulated heat exceeds the material's tolerance limit, it will directly ablate the cavity surface material, causing catastrophic optical damage and resulting in permanent device failure.

[0006] To address the issue of optical damage at the cavity surface, existing technologies primarily employ methods such as cavity surface coating, surface passivation, quantum well hybridization, and non-absorption window design. However, these methods have significant limitations: they are highly dependent on laser type and operating wavelength, and have poor applicability to novel semiconductor lasers such as quantum cascade lasers and interband cascade lasers; in the long-wave infrared band, the optical absorption of commonly used coating materials increases sharply, significantly reducing their effectiveness; and single techniques offer limited improvement in the cavity surface damage threshold, failing to meet the growing demands of high-power applications. More importantly, most of these existing technologies rely on a "passive heat dissipation" approach, attempting to remove heat through external means after it has already been generated, rather than controlling the heat generation mechanism in the cavity surface region at its source, thus failing to fundamentally solve the problem of cavity surface overheating.

[0007] Therefore, there is an urgent need for a universal method that is independent of specific laser types, operating wavelengths, and material systems. This method can effectively improve the optical damage threshold of the cavity surface of semiconductor lasers by suppressing photo-heating and electro-heating effects in the cavity surface region at the source, thereby solving the problems of cavity surface material ablation and permanent device damage caused by cavity surface overheating. Summary of the Invention

[0008] The embodiments of the present invention provide a method for improving the cavity surface damage threshold of a semiconductor laser and a semiconductor laser, aiming to solve the technical problem of cavity surface overheating leading to ablation of cavity surface material and permanent damage to the device in the prior art.

[0009] In a first aspect, the present invention provides a method for improving the cavity surface damage threshold of a semiconductor laser, applicable to a straight waveguide, end-emitting, electrically pumped semiconductor laser. The semiconductor laser includes a straight waveguide section and an emitting end face located at one end of the straight waveguide section. The method includes: setting a gradient waveguide section on one side of the emitting end face; connecting the gradient waveguide section to the end of the straight waveguide section in the light propagation direction, wherein the end of the gradient waveguide section in the light propagation direction has an exit end face, the width of the gradient waveguide section gradually widens along the light propagation direction from the same width as the straight waveguide section to the width of the exit end face, and the width of the exit end face is greater than the width of the straight waveguide section; etching an electrically isolated groove between the gradient waveguide section and the straight waveguide section, the etching depth of the electrically isolated groove reaching the cladding of the semiconductor laser; and confining the current injection region within the straight waveguide section.

[0010] Secondly, the present invention provides a semiconductor laser, which is fabricated using the semiconductor laser cavity surface damage threshold enhancement method described above. The semiconductor laser includes: a straight waveguide section; a tapered waveguide section located on the optical output end side of the straight waveguide section, the width of the tapered waveguide section gradually widening from the same width as the straight waveguide section to the width of the output end face along the optical propagation direction; and an electrically isolated groove disposed between the tapered waveguide section and the straight waveguide section, the etching depth of the electrically isolated groove reaching the cladding; wherein, the current injection region is confined within the straight waveguide section, and no current is directly injected into the tapered waveguide section.

[0011] Compared with the prior art, the beneficial effects of the present invention are: In the technical solution of this invention, the method for improving the cavity surface damage threshold of semiconductor lasers is applied to straight waveguide, end-emitting, electrically pumped semiconductor lasers. This method involves setting a tapered waveguide section on one side of the laser's emitting end face, connecting the tapered waveguide section to the end of the straight waveguide section along the light propagation direction. The width of the tapered waveguide section gradually widens from the same width as the straight waveguide section to a larger width at the emitting end face along the light propagation direction. An electrically isolated groove is etched between the tapered waveguide section and the straight waveguide section, with the groove depth reaching the cladding of the semiconductor laser. Simultaneously, the current injection region is confined within the straight waveguide section, preventing direct current injection into the tapered waveguide section. This method significantly reduces the heat density generated by optical absorption and the heat generated by current diffusion in the cavity surface region, significantly improving the cavity surface optical damage threshold and effectively solving the problems of material ablation and permanent device damage caused by cavity surface overheating. Attached Figure Description

[0012] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0013] Figure 1 This is the main flowchart of the semiconductor laser cavity surface damage threshold improvement method according to an embodiment of the present invention; Figure 2 This is a flowchart of a first sub-implementation of the semiconductor laser cavity surface damage threshold enhancement method according to an embodiment of the present invention; Figure 3 This is a flowchart of a second sub-implementation of the semiconductor laser cavity surface damage threshold enhancement method according to an embodiment of the present invention; Figure 4 This is a flowchart of a third sub-implementation of the semiconductor laser cavity surface damage threshold enhancement method according to an embodiment of the present invention; Figure 5 This is a flowchart of the fourth sub-implementation of the semiconductor laser cavity surface damage threshold improvement method according to the present invention; Figure 6 This is a flowchart of the fifth sub-implementation of the semiconductor laser cavity surface damage threshold improvement method according to the present invention; Figure 7 This is a schematic diagram of the top-view structure of a semiconductor laser prepared by the method for improving the cavity surface damage threshold of a semiconductor laser according to an embodiment of the present invention. Figure 8 This is a schematic diagram of the structure of a semiconductor laser obtained from a side cross-section view (the upper electrode is transparent at the cross-section) prepared by the method for improving the cavity surface damage threshold of a semiconductor laser according to an embodiment of the present invention. Figure label explanation: 100. Straight waveguide section; 200. Gradient waveguide section; 210. Emerging end face; 300. Electrically isolated groove; 410, Top electrode; 420, Insulating layer; 430, Cap layer; 440, Cladding layer; 450, Active region; 460, Substrate; 500, Current Injection Window; 600, antireflective coating; 700, high reflectivity coating. Detailed Implementation

[0014] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0015] It should be noted that all directional indications (such as up, down, left, right, front, back, etc.) in the embodiments of the present invention are only used to explain the relative positional relationship and movement of each component in a certain specific posture (as shown in the figure). If the specific posture changes, the directional indication will also change accordingly.

[0016] In order to solve the technical problem of cavity surface ablation and permanent damage to the device caused by overheating of the cavity surface of semiconductor lasers in the prior art, this invention proposes a method to improve the cavity surface damage threshold of semiconductor lasers.

[0017] Reference Figures 1 to 8 The semiconductor laser cavity surface damage threshold enhancement method of the present invention is applied to a straight waveguide, end-emitting, electrically pumped semiconductor laser, wherein the semiconductor laser includes a straight waveguide section 100 and an emitting end face located at one end of the straight waveguide section 100. For ease of illustration, in Figure 8 The upper electrode 410 of the semiconductor laser is made transparent from a side cross-sectional view, making the current injection window 500 visible.

[0018] The method for improving the cavity surface damage threshold of semiconductor lasers of the present invention includes: S110. A gradient waveguide section 200 is provided on one side of the transmitting end face; S120. Connect the gradient waveguide section 200 to the end of the straight waveguide section 100 in the light propagation direction. The end of the gradient waveguide section 200 in the light propagation direction is provided with an exit end face 210. The width of the gradient waveguide section 200 gradually widens along the light propagation direction from the same width as the straight waveguide section 100 to the width of the exit end face 210. The width of the exit end face 210 is greater than the width of the straight waveguide section 100. S130, an electrically isolated groove 300 is etched between the gradient waveguide section 200 and the straight waveguide section 100, and the etching depth of the electrically isolated groove 300 reaches the cladding 440 of the semiconductor laser. S140. The current injection region is confined within the straight waveguide section 100.

[0019] A method for improving the cavity surface damage threshold of semiconductor lasers is applied to straight waveguide, end-emitting, electrically pumped semiconductor lasers. The semiconductor laser includes a straight waveguide section 100 and an emitting end face located at one end of the straight waveguide section 100. To suppress the thermal accumulation effect at the emitting end face, a tapered waveguide section 200 is provided on one side of the emitting end face. The tapered waveguide section 200 is connected to the end of the straight waveguide section 100 along the light propagation direction. The width of the tapered waveguide section 200 gradually widens along the light propagation direction from the same width as the straight waveguide section 100 to a width greater than the width of the emitting end face 210. In practice, the tapered waveguide section 200 can be formed by patterning the epitaxial wafer of the semiconductor laser using photolithography and etching processes. When the laser propagates in its fundamental transverse mode in the straight waveguide section 100 to the tapered waveguide section 200, the optical mode field gradually widens as the width of the tapered waveguide section 200 increases, resulting in a larger beam cross-sectional area. This significantly reduces the optical power density at the emitting end face, effectively suppressing the photothermal effect caused by optical absorption at the emitting end face and alleviating the problem of photothermal runaway at the cavity surface. It is important to emphasize that a wider tapered waveguide section 200 is not necessarily better. If the width of the emitting end face 210 of the tapered waveguide section 200 is too large, the transverse dimension of the waveguide will change drastically within a finite length, destroying the waveguide's ability to confine the optical field laterally. This makes it very easy to excite higher-order transverse modes within the tapered waveguide section 200. Once higher-order transverse modes are excited, the beam quality of the output laser will be significantly degraded, manifested as far-field distortion, increased beam divergence angle, and unstable mode competition, thereby affecting the laser's focusing performance and coupling efficiency. Therefore, the width expansion of the gradient waveguide section 200 should be controlled within a reasonable range that can effectively reduce the optical power density at the output end face 210 while maintaining stable oscillation of the fundamental transverse mode.

[0020] An electrically isolated groove 300 is etched between the tapered waveguide section 200 and the straight waveguide section 100. The etching depth of the electrically isolated groove 300 reaches the cladding 440 of the semiconductor laser. The electrically isolated groove 300 can be etched in the interface region between the tapered waveguide section 200 and the straight waveguide section 100 using dry etching processes such as inductively coupled plasma etching. The etching depth of the electrically isolated groove 300 needs to reach the cladding 440 of the semiconductor laser to ensure effective blocking of the lateral diffusion path of charge carriers. The depth design of the electrically isolated groove 300 is a trade-off between isolation capability and waveguide optical loss. If the etching is too deep, it will introduce additional waveguide optical loss; if the etching is too shallow, it will not effectively constrain current dispersion. The function of the electrically isolated groove 300 is to physically cut off the lateral diffusion channel of charge carriers inside the semiconductor material, weaken the heat accumulation at the emitter facet caused by current dispersion, and further increase the cavity surface damage threshold.

[0021] The current injection region is confined within the straight waveguide section 100. First, an insulating layer 420, which can be made of silicon dioxide, is placed over the waveguide of the semiconductor laser. Then, a current injection window 500 is etched onto the insulating layer 420 corresponding to the straight waveguide section 100, allowing the upper electrode 410 to contact the semiconductor capping layer 430 through the current injection window 500, thereby achieving selective current injection. The insulating layer 420 corresponding to the tapered waveguide section 200 remains intact, without a current injection window 500. Through the physical shielding of the insulating layer 420, the current injection region is strictly confined within the straight waveguide section 100, blocking direct current injection into the tapered waveguide section 200 at its source. The absence of a current injection window 500 on the tapered waveguide section 200, in conjunction with the electrically isolated groove 300, completely eliminates the additional electrothermal heating problem that might be introduced by the increased width of the tapered waveguide section 200.

[0022] The method for improving the cavity surface damage threshold of a semiconductor laser provided by this invention effectively reduces the optical power density at the emitting end face and suppresses photothermal effects by setting a gradually widening gradient waveguide section 200 on one side of the emitting end face. Furthermore, by etching an electrically isolated groove 300 with a cladding depth of 440mm between the gradient waveguide section 200 and the straight waveguide section 100, and strictly confining the current injection region within the straight waveguide section 100, the direct injection of charge carriers to the emitting end face is completely blocked, and lateral diffusion to the vicinity of the emitting end face is weakened, thus suppressing the electrothermal effect. The gradient waveguide section 200, the electrically isolated groove 300, and the current injection region confinement work synergistically to simultaneously eliminate photothermal and thermoelectric effects from the perspective of heat source control, breaking the positive feedback of thermal runaway caused by increased light absorption due to rising cavity surface temperature. This significantly improves the cavity surface optical damage threshold, output power, and long-term operational stability of the semiconductor laser.

[0023] In one embodiment, the width expansion profile of the gradient waveguide segment 200 is one of linear tapered, parabolic, exponential, or piecewise linear gradient.

[0024] The width extension profile of the gradient waveguide section 200 can be constructed by selecting one of the following: linear tapered, parabolic, exponential, or piecewise linear gradient, depending on the specific operating wavelength, material system, and beam quality requirements of the semiconductor laser.

[0025] A linear tapered transition refers to a waveguide segment 200 whose sidewall boundaries extend in a straight line with a constant slope along the direction of light propagation, and whose width increases uniformly with the propagation distance. The linear tapered profile is easy to fabricate and is suitable for applications where the requirements for mode field stretching smoothness are relatively relaxed and a rapid reduction in cavity surface power density is needed.

[0026] A parabolic gradient refers to the width of the gradient waveguide segment 200 varying according to a quadratic function along the direction of light propagation. The broadening rate of the gradient waveguide segment 200 is slower at the beginning and gradually accelerates at the end. The parabolic profile can effectively reduce scattering loss during waveguide mode conversion and is suitable for high-power devices that are sensitive to optical losses.

[0027] An exponential gradient refers to the fact that the width of the gradient waveguide section 200 increases exponentially along the light propagation direction, resulting in the smoothest mode field expansion process. The exponential profile can maximally suppress the excitation of higher-order transverse modes, making it suitable for precision optical systems with extremely high requirements for beam quality and single-mode stability.

[0028] Piecewise linear gradient refers to dividing the gradient waveguide segment 200 into multiple continuous sub-intervals, with each sub-interval employing a linear broadening at a different slope. Local optimization of the mode field transition is achieved through step-wise slope adjustment. Piecewise linear gradient profiles are suitable for complex waveguide designs that require a balance between low loss and high-order mode suppression.

[0029] In practical implementation, designers first establish an optical transmission model based on the emission wavelength of the target semiconductor laser and the fundamental mode field distribution of the straight waveguide section 100 using the beam propagation method or the finite element method. They then simulate the mode field evolution by substituting linear conical, parabolic, exponential, or piecewise linear gradient contours, comparing and analyzing the mode matching efficiency, higher-order mode excitation threshold, and end-face power density distribution under each contour. After selecting the optimal contour based on the simulation results, the corresponding contour pattern is transferred to a photoresist mask using electron beam lithography or deep ultraviolet lithography. Subsequently, an etching process is used to etch the semiconductor cap layer, cladding layer, and epitaxial layer along the mask pattern, forming a gradient waveguide section 200 with a target width broadened contour.

[0030] Through contour optimization and process control in this embodiment, the gradient waveguide section 200 can achieve smooth transition and effective broadening of the optical field mode while ensuring stable transmission of the laser fundamental mode, further reducing the peak optical power density at the emitting end face, and avoiding mode mismatch and additional scattering loss caused by abrupt contour changes.

[0031] In one embodiment, the length of the gradient waveguide segment 200 is determined in the following manner: S151. Based on the current diffusion equation, combined with the tensor conductivity of each layer of the semiconductor laser material and the boundary conditions of the electrically isolated groove 300, calculate the current density attenuation distribution along the light propagation direction. S152. The length of the gradient waveguide section 200 is set as: the distance from the electrically isolated groove 300 along the light propagation direction until the current density decays to a preset percentage below the injected current density.

[0032] In the semiconductor laser cavity surface damage threshold improvement method of the present invention, the length of the gradient waveguide section 200 is calculated and optimized based on the carrier transport physical model.

[0033] The lateral diffusion behavior of charge carriers in each layer of semiconductor laser material is quantitatively analyzed by current diffusion equation, and the safe gradient distance that can effectively suppress electrothermal effect is determined by combining the physical boundary conditions of the electrically isolated groove 300.

[0034] In practical implementation, a two-dimensional or three-dimensional electrical simulation model of the semiconductor laser epitaxial structure is first established. The current diffusion equation is introduced into the model as the basis for calculating carrier transport. The current diffusion equation is as follows: ; In this equation, This represents the vector representing the current density distribution at any location inside a semiconductor laser. This represents the tensor conductivity of each layer of the semiconductor laser material, including a capping layer 430, a cladding layer 440, an active region 450, and a substrate 460. It is used to characterize the anisotropic conductivity under different crystal orientations, doping concentrations, and temperature conditions. For spatial gradient operators, This refers to the spatial voltage distribution inside the device.

[0035] By numerically solving the equation using the finite element method or the finite difference method, the spatial distribution map of the current density within the regions of the straight waveguide section 100, the electrically isolated groove 300, and the tapered waveguide section 200 can be obtained.

[0036] During the solution process, boundary conditions must be strictly set to match the actual device fabrication process. These boundary conditions include the cutoff position of the current injection window 500 above the straight waveguide section 100, the etching depth and lateral position of the electrically isolated groove 300, and the interface continuity conditions between the semiconductor layers. The etching depth of the electrically isolated groove 300 reaches the cladding layer 440, which in the simulation model manifests as a sharp drop in conductivity in this region or as an insulating boundary, thereby forcibly altering the carrier diffusion path. Combining the above boundary conditions and tensor conductivity parameters, the current density attenuation distribution curve along the light propagation direction is calculated. This current density attenuation distribution curve visually reflects the attenuation trend of carrier lateral diffusion from the straight waveguide section 100 to the tapered waveguide section 200.

[0037] Based on the calculated current density attenuation distribution, the length of the tapered waveguide section 200 is set to the distance from the electrically isolated groove 300 along the light propagation direction until the current density attenuates to below a preset percentage of the injected current density. The preset percentage is typically set according to the semiconductor laser's requirements for electrothermal suppression, preferably below 10%, and more preferably below 6%. When the length of the tapered waveguide section 200 meets this condition, it means that at the output cavity surface of the tapered waveguide section 200, the laterally diffused carrier concentration has dropped to an extremely low level, and the electrothermal effect is effectively suppressed. Designers can directly read the coordinate position corresponding to the preset percentage from the simulated attenuation curve; the distance between this coordinate and the starting position of the electrically isolated groove 300 is the optimal design length of the tapered waveguide section 200. It should be noted that the length of the tapered waveguide section 200 is not necessarily better the longer it is. Since the tapered waveguide section 200 is electrically levitated, there is no current injection and no gain compensation in its region. As the light field propagates within it, optical losses continuously occur due to sidewall scattering, free carrier absorption, and intrinsic material absorption. If the length of the tapered waveguide section 200 is too long, the propagation path of the light field within it will be too long, significantly increasing the accumulated waveguide optical losses. This leads to an increase in the laser's threshold current and a decrease in output power and electro-optical conversion efficiency. Therefore, the length of the tapered waveguide section 200 should be selected as short as possible while ensuring that the current density decays to below a preset percentage and effectively suppresses the electrothermal effect, thus achieving a balance between suppressing electrothermal effects and optical losses.

[0038] In practical engineering implementation, designers first obtain the doping concentration and carrier mobility data of each epitaxial layer through material growth parameters, and construct a tensor conductivity matrix accordingly. Then, the semiconductor laser geometry is imported into electrical simulation software, and the current injection window 500 cutoff boundary and the insulating boundary of the electrically isolated groove 300 are set. The solver is run to obtain the voltage and current density distributions. After extracting the current density attenuation data along the light propagation direction, a corresponding length value is truncated according to the target preset percentage, and this length value is directly applied to the subsequent photolithography mask layout design. After the device fabrication is completed, the current diffusion range of the actual semiconductor laser can be verified by micro-area electrical probe testing or infrared thermal imaging technology to ensure a high degree of agreement between the measured attenuation curve and the simulation prediction, thereby ensuring the accuracy and repeatability of the 200-length gradient waveguide section design.

[0039] In one embodiment, the width of the exit end face 210 is determined in the following manner: S160. By simulating the propagation behavior of the fundamental transverse mode in the gradient waveguide section 200 using the beam propagation method or the finite element method, the maximum width of the semiconductor laser that maintains fundamental transverse mode oscillation within the operating current range is selected as the width of the output end face 210.

[0040] In the implementation of the method for improving the cavity surface damage threshold of semiconductor lasers, the selection of the width of the output end face 210 needs to achieve a precise balance between reducing the cavity surface optical power density and maintaining beam quality. If the width of the output end face 210 is too small, the broadening effect of the tapered waveguide section 200 on the optical field mode field will be limited, the optical power density at the output end face cannot be effectively reduced, and the effect of improving the cavity surface thermal damage threshold will be limited. If the width of the output end face 210 is too large, the change in the lateral dimension of the tapered waveguide section 200 will destroy the lateral confinement condition of the waveguide, which will easily excite higher-order transverse modes, leading to beam quality degradation, far-field distribution distortion, and unstable mode competition.

[0041] In practice, designers first establish an optical transmission model including a straight waveguide section 100 and a tapered waveguide section 200 based on the epitaxial structure parameters of the semiconductor laser and the waveguide geometry. The propagation behavior of the laser's fundamental transverse mode in the tapered waveguide section 200 is then numerically simulated using the beam propagation method or the finite element method. During the simulation, the refractive index distribution of each layer of the semiconductor laser material, the operating wavelength, and the width broadening profile of the tapered waveguide section 200 are used as input parameters. Reasonable boundary conditions are set to simulate the constraint effect of the optical field at the interface between the waveguide sidewall and the cladding 440. By gradually adjusting the initial setting value of the width of the output end face 210, the simulation software can output the evolution trajectory of the optical field modes, the change in the effective refractive index of the modes, and the excitation threshold of higher-order modes under different width conditions.

[0042] After acquiring simulation data, the fundamental mode stability was verified based on the actual operating characteristics of the semiconductor laser across the entire operating current range. Since changes in the injection current cause dynamic changes in the carrier concentration and refractive index of the active region 450, thus affecting the transverse confinement factor and mode competition relationship of the waveguide, repeated simulations or dynamic mode analysis are necessary at different operating current points. The maximum width at which the semiconductor laser maintains fundamental transverse mode oscillation within the operating current range was selected as the width of the output end face 210. The criteria for determining the maximum width are: throughout the entire range from the threshold current to the maximum safe operating current, the optical field energy remains highly concentrated in the fundamental transverse mode, the power proportion of higher-order transverse modes is lower than the engineering-allowed stray mode suppression ratio requirement, and the far-field distribution maintains a stable single-lobe morphology. After determining the maximum width, it was directly applied to the photolithographic mask layout design to ensure that the physical dimensions at the end of the gradient waveguide section 200 are consistent with the simulation optimal value.

[0043] During the actual device fabrication and testing phases, the design effectiveness of the output end face 210 width can be verified through near-field spot imaging and far-field angular distribution measurement. Test results show that the output end face 210 width determined by this method can maximize the expansion of the optical field cross-sectional area while ensuring stable output of the fundamental transverse mode, thereby reducing the optical power density at the output end face to below the safe threshold.

[0044] Taking a high-power long-wavelength infrared quantum cascade laser with an operating wavelength of approximately 10 micrometers as an example, the ridge width of the straight waveguide section 100 is set to 14 micrometers. Simulation using the beam propagation method reveals that when the width of the output end face 210 remains at 14 micrometers, although the fundamental transverse mode propagates stably, the surface power density is not effectively reduced. When the width of the output end face 210 is widened to 22 micrometers, the fundamental transverse mode can still propagate stably within the tapered waveguide section 200, and the surface power density is significantly reduced. However, when the width of the output end face 210 is further widened to 33 micrometers, higher-order transverse modes begin to be excited within the tapered waveguide section 200, leading to a deterioration in the output beam quality. Therefore, the width of the output end face 210 is determined to be 22 micrometers, meaning the width of the tapered waveguide section 200 is linearly and gradually widened from 14 micrometers to 22 micrometers.

[0045] In one embodiment, confining the current injection region within the straight waveguide section 100 specifically includes: S141. An insulating layer 420 is covered over the waveguide of the semiconductor laser; S142. Etch a current injection window 500 on the insulating layer 420 corresponding to the straight waveguide section 100 so that the upper electrode 410 contacts the semiconductor capping layer 430 through the current injection window 500. S143. The insulating layer 420 corresponding to the gradient waveguide section 200 remains intact.

[0046] Specifically, a dense silicon dioxide layer 420 is grown on the waveguide surface of the semiconductor laser using plasma-enhanced chemical vapor deposition (PECVD). A current injection window 500 is etched onto the insulating layer 420 corresponding to the straight waveguide section 100. In practice, the pattern of the current injection window 500 is defined on the silicon dioxide insulating layer 420 above the straight waveguide section 100 using photolithography. Then, the silicon dioxide insulating layer 420 within the current injection window 500 area is removed using etching, exposing the underlying semiconductor capping layer 430. Next, metal electrode material is deposited so that the upper electrode 410 directly contacts the semiconductor capping layer 430 through the current injection window 500, forming an ohmic contact, thereby enabling current injection from an external power source into the active region 450 inside the straight waveguide section 100. Finally, the insulating layer 420 corresponding to the gradient waveguide section 200 remains intact. During the photolithography process of defining the current injection window 500, the silicon dioxide insulating layer 420 above the gradient waveguide segment 200 is not exposed or etched, ensuring that the insulating layer 420 corresponding to the gradient waveguide segment 200 remains intact. Because the silicon dioxide insulating layer 420 has extremely high resistivity, maintaining the integrity of the insulating layer 420 physically and completely blocks the electrical connection between the upper electrode 410 and the semiconductor material of the gradient waveguide segment 200, preventing direct current injection into the gradient waveguide segment 200.

[0047] In one embodiment, the method for improving the cavity surface damage threshold of a semiconductor laser further includes: S161. An antireflection coating 600 is deposited on the exit end face 210 of the gradient waveguide section 200, wherein the material of the antireflection coating 600 includes any one of Y2O3, Al2O3, or TiO2; and / or, S162. A high-reflectivity film 700 is deposited on the rear end face of the straight waveguide section 100 away from the transmitting end face. The high-reflectivity film 700 is a Y2O3 / Au / Ti multilayer film structure.

[0048] A high-reflectivity film 700 is deposited on the rear face of the straight waveguide section 100 away from the emitting end face. After the epitaxial wafer of the semiconductor laser, which has been fabricated, is cleaved or etched to form a flat cavity surface, a thin film deposition process such as electron beam evaporation, magnetron sputtering, or ion beam deposition is used for end face coating treatment. For the emitting end face 210 of the gradient waveguide section 200, the antireflection film 600 is deposited to reduce the light reflectivity of the emitting end face 210, thereby reducing the standing wave effect of the light field at the emitting end face and improving the light extraction efficiency. The material of the antireflection film 600 includes any one of Y2O3, Al2O3, or TiO2. It should be noted that the material selection of the antireflection film 600 should match the operating wavelength of the semiconductor laser. Different materials have significant differences in optical transparency and refractive index characteristics at different wavelengths. A material or combination of materials with low optical absorption at the operating wavelength and good matching with the material of the emitting end face 210 and the refractive index of air should be selected. For example, Al2O3 has excellent transparency in the visible to near-infrared band, making it suitable for lasers in the visible and near-infrared bands; TiO2 has a high refractive index, making it suitable for the visible band or as a high refractive index layer for antireflection coatings in the near-infrared band; Y2O3 has low optical absorption and good refractive index matching characteristics in the long-wave infrared band, making it particularly suitable for long-wave infrared quantum cascade lasers.

[0049] Taking a long-wave infrared quantum cascade laser as an example, Y₂O₃ is preferentially selected as the material for the antireflection coating 600 due to its low optical absorption and excellent refractive index matching characteristics in the long-wave infrared band. The thickness of the antireflection coating 600 is set to 800 nanometers to achieve the best antireflection effect. For the rear end face of the straight waveguide section 100, which is far from the emitting end face, a high-reflection coating 700 is deposited to efficiently reflect the backward-propagating laser beam back into the resonant cavity, thereby significantly improving the single-end output power of the output end face 210 of the tapered waveguide section 200.

[0050] The high-reflectivity film 700 is a Y2O3 / Au / Ti multilayer film structure. Specifically, a Y2O3 thin film is first deposited on the rear end face as an optical transition layer and adhesion layer, followed by an Au thin film as a core high-reflectivity metal layer, and finally a Ti thin film is deposited on the outside of the Au thin film as an anti-oxidation protective layer. It should be noted that, in addition to its optical transition and adhesion functions, the core function of the Y2O3 thin film is to achieve electrical isolation. The rear end face exposes both the upper and lower electrodes of the semiconductor laser. If the Au thin film directly covers the entire rear end face, it would connect the upper and lower electrodes, causing a short circuit. Therefore, an insulating Y2O3 thin film is first deposited on the rear end face to isolate the subsequently deposited Au thin film from the upper and lower electrodes, thereby avoiding short circuits. Furthermore, in addition to its anti-oxidation protection function, the Ti thin film mainly serves as a solder diffusion barrier layer. During the flip-chip bonding process, the solder tends to diffuse upwards along the Au film and cover the entire rear end face. However, Ti does not readily react with the solder, effectively preventing the solder from diffusing along the Au film and avoiding short circuits or contamination caused by the solder covering the end face.

[0051] By depositing an antireflection film 600 of a specific material on the emitting end face 210 of the gradient waveguide section 200 and a high reflectivity film 700 of a Y2O3 / Au / Ti multilayer film structure on the rear end face of the straight waveguide section 100, the reflectivity of the emitting end face 210 is effectively reduced, the standing wave accumulation of the light field at the emitting end face is reduced, and the optical power is concentrated and extracted from a single end through the high reflectivity of the rear end face.

[0052] In one embodiment, the method for improving the cavity surface damage threshold of a semiconductor laser further includes: S171. The waveguide of the semiconductor laser is fabricated into a buried heterojunction ridge structure; S172. The processed semiconductor laser chip is flip-mounted onto the diamond heat sink with the epitaxial layer facing down.

[0053] The waveguide of the semiconductor laser is fabricated into a buried heterojunction ridge structure. Specifically, a ridge-shaped waveguide mesa is etched onto the epitaxial wafer of the semiconductor laser using photolithography and etching processes. Subsequently, a semi-insulating material layer is grown on both sides of the ridge-shaped waveguide mesa using a secondary epitaxial growth technique, completely burying both sides of the ridge-shaped waveguide mesa to form a buried heterojunction ridge structure. The buried heterojunction ridge structure not only provides excellent lateral optical field confinement and current confinement, but also improves lateral thermal conduction through the semi-insulating material layers on both sides.

[0054] The processed semiconductor laser chip is flip-chip bonded to a diamond heat sink with the epitaxial layer facing down. Specifically, a metallization layer is prepared on the bonding surface between the epitaxial layer of the semiconductor laser chip and the diamond heat sink, and indium solder is used as the bonding medium to attach and bond the semiconductor laser chip to the diamond heat sink with the epitaxial layer facing down. Since the active region 450 of the semiconductor laser is close to the surface of the epitaxial layer, flip-chip bonding with the epitaxial layer facing down can greatly shorten the physical distance for heat generated in the active region 450 to be conducted to the diamond heat sink. Combined with the extremely high thermal conductivity of the diamond heat sink, efficient heat conduction is achieved.

[0055] In one embodiment, the electrical isolation groove 300 is located 20 μm to 30 μm after the current injection window 500 cutoff position.

[0056] During the operation of the semiconductor laser, charge carriers injected into the straight waveguide section 100 diffuse laterally into the uninjected region due to the concentration gradient and the built-in electric field. If the electrically isolated groove 300 is too close to the cutoff position of the current injection window 500, the charge carriers encounter physical obstruction before sufficient decay, which may lead to localized charge carrier accumulation and intensified nonradiative recombination, resulting in additional heat accumulation at the edge of the electrically isolated groove 300. If the electrically isolated groove 300 is too far from the cutoff position of the current injection window 500, the lateral diffusion range of the charge carriers is too large, and some charge carriers may have already reached the region of the tapered waveguide section 200, weakening the suppression effect of the electrically isolated groove 300 on the electrothermal effect. The distance range of 20μm to 30μm provides a sufficient safety buffer for the natural decay of the charge carrier concentration, allowing the charge carriers to decay to a low level before reaching the electrically isolated groove 300, thereby ensuring that the electrically isolated groove 300 can achieve efficient lateral current blocking with minimal etching depth and optical disturbance.

[0057] In practice, designers first determine the precise cutoff coordinates of the current injection window 500 through photolithographic mask layout design. Then, offsetting 20μm–30μm backward along the light propagation direction, the etching start position of the electrically isolated groove 300 is defined. A mask pattern corresponding to the position is formed on the surface of the semiconductor epitaxial wafer using deep ultraviolet lithography or electron beam lithography, followed by etching. During etching, the etching depth must be monitored in real time to ensure that the bottom of the electrically isolated groove 300 penetrates the capping layer 430 and reaches the cladding layer 440, while strictly controlling the sidewall morphology to avoid excessive tilting or undercutting. After etching, the actual distance between the electrically isolated groove 300 and the cutoff position of the current injection window 500 is measured and verified using a scanning electron microscope to ensure that the deviation between the device structure and the design layout is controlled within the sub-micron range.

[0058] Furthermore, the positioning of the electrically isolated groove 300 in this embodiment forms a mutually verifying design closed loop with the current density attenuation calculation in the aforementioned embodiment. During the simulation phase, designers can input the current diffusion equation with an initial boundary condition of 20μm to 30μm to verify whether the current density attenuation curve meets the preset percentage threshold requirement at this spacing. During the fabrication testing phase, the current distribution and temperature field distribution in the areas before and after the electrically isolated groove 300 can be detected using micro-area electrical probe scanning or infrared thermal imaging technology to confirm that the lateral diffusion of charge carriers is effectively suppressed and no significant waveguide scattering loss is introduced. Through the above process control and position optimization, the electrically isolated groove 300 achieves precise truncation of the charge carrier diffusion path within a safe distance of 20μm to 30μm, providing a clean optical transmission environment for the tapered waveguide section 200, further consolidating the improvement effect of the cavity surface optical damage threshold.

[0059] In one embodiment, the semiconductor laser cavity surface damage threshold enhancement method of the present invention is applied to any one of quantum cascade lasers, quantum well lasers, interband cascade lasers, or quantum dot lasers; the operating wavelength of the semiconductor laser includes the ultraviolet to far-infrared band; and the operating mode of the semiconductor laser includes continuous wave operating mode or pulsed operating mode.

[0060] For the semiconductor laser cavity surface damage threshold enhancement method of the present invention, the gradient waveguide section 200 setting, electrically isolated groove 300 etching, and current injection region restriction schemes in the aforementioned embodiments are all based on the physical mechanism of waveguide geometry manipulation and carrier lateral transport blocking, and do not depend on the band structure or emission transition type of a specific semiconductor material. Therefore, this method can be directly applied to any of quantum cascade lasers, quantum well lasers, interband cascade lasers, or quantum dot lasers. In actual implementation, regardless of whether the semiconductor laser adopts an interband transition mechanism or an interband transition mechanism, designers can construct the gradient waveguide section 200 and electrically isolated groove 300 on one side of the emitting end face according to the same process route, and restrict the current injection region within the straight waveguide section 100 by patterning the insulating layer 420, without requiring any modification to the core active region 450 structure or material growth process.

[0061] For semiconductor lasers operating in the ultraviolet to far-infrared band, this method optimizes cavity surface thermal management across the band by adjusting the width ratio of the gradient waveguide section 200 and the etching depth of the electrically isolated groove 300. In the ultraviolet or visible light bands, semiconductor lasers typically use gallium nitride (GaN)-based materials. The width ratio of the gradient waveguide section 200 needs to be scaled accordingly based on the optical field mode size at shorter wavelengths, and the etching depth of the electrically isolated groove 300 needs to be precisely matched to the cladding 440 position of the GaN epitaxial layer. In the mid-infrared or long-infrared bands, semiconductor lasers often use indium phosphide (IPT) or gallium arsenide (GaAs)-based materials. The width of the exit face 210 of the gradient waveguide section 200 needs to be appropriately increased to match the diffraction limit at longer wavelengths, and the etching process of the electrically isolated groove 300 needs to be adapted to the etching selectivity of the corresponding material. Since the core of this method lies in reducing the optical power density at the output end face 210 and blocking the lateral diffusion of charge carriers, its physical mechanism is independent of the absolute value of photon energy. Therefore, it can achieve full-band coverage without redesigning the active region 450 for a specific wavelength or introducing complex band engineering.

[0062] In both continuous-wave and pulsed operating modes, this method provides a stable enhancement of the cavity surface damage threshold. In continuous-wave mode, the semiconductor laser operates in a long-term steady-state thermal equilibrium, resulting in significant thermal accumulation in the cavity surface region. The tapered waveguide section 200 continuously broadens the optical mode field, reducing the end-face optical power density. The electrically isolated groove 300 and the current injection confinement structure continuously block the lateral diffusion of charge carriers; their synergistic effect effectively suppresses positive feedback from thermal runaway during continuous operation. In pulsed operating mode, the semiconductor laser is subjected to high peak power and transient thermal shocks, making the cavity surface region susceptible to micro-area ablation due to the superposition of instantaneous photoabsorption and Joule heating. This method significantly improves the semiconductor laser's tolerance to transient thermal shocks by pre-lowering the baseline end-face optical power density and eliminating carrier dispersion during the pulse rise edge. Designers can fine-tune the length of the tapered waveguide section 200 and the position parameters of the electrically isolated groove 300 according to pulse width, repetition frequency, and peak power requirements, ensuring that the semiconductor laser maintains high beam quality and long lifespan even in pulsed operating mode.

[0063] In practical engineering applications, for different types of semiconductor lasers, designers only need to adjust the geometric proportions of the gradient waveguide section 200 and the etching mask size of the electrically isolated groove 300 according to the target wavelength and material system during the layout design stage. Subsequent photolithography, etching, insulating layer 420 deposition, and electrode fabrication processes remain completely consistent. After device fabrication, performance comparison tests were conducted on quantum cascade lasers, quantum well lasers, interband cascade lasers, or quantum dot laser samples in continuous wave and pulsed operating modes using a spectrometer, power meter, and infrared thermal imager. The test results show that the semiconductor lasers fabricated using this method exhibit significantly improved cavity surface optical damage thresholds across the entire ultraviolet to far-infrared wavelength range without introducing additional mode jumps or efficiency degradation. This embodiment fully verifies the cross-platform compatibility and engineering feasibility of this method as a general-purpose cavity surface thermal management solution.

[0064] This invention also discloses a semiconductor laser, which is fabricated using the cavity surface damage threshold enhancement method described in the above embodiments. The semiconductor laser includes: a straight waveguide section 100; a gradient waveguide section 200 located on one side of the optical output end of the straight waveguide section 100, the width of the gradient waveguide section 200 gradually widening along the optical propagation direction from the same width as the straight waveguide section 100 to the width of the output end face 210; and an electrically isolated groove 300 disposed between the gradient waveguide section 200 and the straight waveguide section 100, the etching depth of the electrically isolated groove 300 reaching the cladding 440; wherein, the current injection region is confined within the straight waveguide section 100, and no current is directly injected into the gradient waveguide section 200.

[0065] The core structure of the semiconductor laser of the present invention includes, along the light propagation direction, a straight waveguide section 100, an electrically isolated groove 300, and a tapered waveguide section 200. The straight waveguide section 100 serves as the transmission channel for the laser fundamental mode. An insulating layer 420 covers the straight waveguide section 100, and a current injection window 500 is formed on the insulating layer 420, allowing the upper electrode 410 to form an ohmic contact with the semiconductor capping layer 430 only through the current injection window 500, thereby strictly limiting the current injection area within the straight waveguide section 100. The tapered waveguide section 200 is located on the light output side of the straight waveguide section 100. The width of the tapered waveguide section 200 gradually widens along the light propagation direction from a starting point equal to the width of the straight waveguide section 100, until it reaches a width at the output end face 210 that is greater than the width of the straight waveguide section 100. The electrically isolated groove 300 is etched between the gradient waveguide section 200 and the straight waveguide section 100. The etching depth of the electrically isolated groove 300 penetrates the upper semiconductor structure and reaches the cladding 440, forming a barrier to the lateral diffusion of charge carriers at the physical level.

[0066] In the actual fabrication process, the semiconductor laser of this invention first forms an active region 450, a cladding layer 440, and a capping layer 430 sequentially on a substrate 460 using an epitaxial growth process. Subsequently, photolithography and etching processes are used to define the lateral contours of the straight waveguide section 100 and the tapered waveguide section 200, and an electrically isolated groove 300 is etched at the interface between the straight waveguide section 100 and the tapered waveguide section 200. The etching depth of the electrically isolated groove 300 is precisely controlled to the position of the cladding layer 440, ensuring effective blocking of the lateral migration of charge carriers to the tapered waveguide section 200. After waveguide etching is completed, an insulating dielectric layer is deposited on the surface of the semiconductor laser, and a current injection window 500 is opened only in the corresponding region of the straight waveguide section 100 using photolithography and selective etching processes, while the insulating dielectric layer above the tapered waveguide section 200 remains intact without windows. Finally, a metal deposition process is used to form the upper electrode 410 and the lower electrode, completing the ohmic contact and electrode interconnection of the semiconductor laser. After the structure is fabricated, the gradient waveguide section 200 is in an electrically floating state, with no direct current injection, and only undertakes the function of optical field mode broadening.

[0067] When the semiconductor laser of this invention is powered on, the injected current only forms effective carrier transitions or recombination emission within the active region 450 of the straight waveguide section 100, and the generated laser fundamental mode is transmitted to the tapered waveguide section 200 along the light propagation direction. Since the width of the tapered waveguide section 200 gradually increases along the light propagation direction, the optical mode field broadens smoothly during transmission, resulting in a significant reduction in the optical power density reaching the output end face 210, directly weakening the optical absorption heat generation in the output end face 210 region. Simultaneously, the synergistic effect of the electrically isolated groove 300 and the complete insulating layer 420 completely blocks the lateral diffusion of carriers from the straight waveguide section 100 to the tapered waveguide section 200, eliminating non-radiative heat generation and Joule heating effects near the output end face 210. The straight waveguide section 100, the electrically isolated groove 300, and the gradient waveguide section 200 form a tightly coupled optical-electrical-thermal management architecture in space, which simultaneously suppresses photothermal and electrothermal effects from the source, breaking the vicious cycle of bandgap shrinkage, increased absorption coefficient, and thermal runaway positive feedback caused by the rise in cavity surface temperature in traditional semiconductor lasers.

[0068] Based on the above embodiments, it can be seen that the semiconductor laser of the present invention can be widely used in different device platforms such as quantum cascade lasers, quantum well lasers, interband cascade lasers or quantum dot lasers, with an operating wavelength covering the ultraviolet to far-infrared band, and is compatible with continuous wave operating mode and pulsed operating mode.

[0069] To further verify the effectiveness and superiority of the semiconductor laser cavity surface damage threshold enhancement method provided by the present invention, this embodiment provides a specific experimental verification case based on a long-wavelength infrared quantum cascade laser. This experimental case applies the cavity surface damage threshold enhancement method in the aforementioned embodiment.

[0070] First, the growth of a semiconductor laser epitaxial wafer is performed. This experimental example uses strain-compensated Al0. 64 In0. 36 As / Ga0. 417 In0. 583 In an As material system, epitaxial layers are sequentially grown on an n-type InP substrate 460 using gas-source molecular beam epitaxy. The epitaxial structure, from bottom to top, includes: an n-type InP substrate 460, an active region 450 containing 50 cascaded stages, a 4-micrometer-thick low-doped InP cladding layer 440, and a 1-micrometer-thick high-doped InP capping layer 430. The high aluminum composition creates a high potential barrier in the quantum well system, effectively suppressing carrier leakage at high temperatures.

[0071] Subsequently, the epitaxial wafer is processed using device fabrication techniques to form a buried heterojunction ridge structure comprising a straight waveguide section 100, a gradient waveguide section 200, and an electrically isolated groove 300. Specific parameters and processes are as follows: the ridge waveguide mesa is defined using photolithography and etching processes, wherein the width of the active region of the straight waveguide section 100 is set to 14 micrometers. The overall lateral width of the straight waveguide section 100 is equal to the sum of the width of the active region and the width of the semi-insulating InP material buried on both sides of the active region and other redundant portions, typically reaching the level of several hundred micrometers.

[0072] A silicon dioxide insulating layer 420 is deposited above the waveguide, and a current injection window 500 is etched on the insulating layer 420 corresponding to the straight waveguide section 100, so that the upper electrode 410 contacts the highly doped InP capping layer 430 through the current injection window 500. The current injection window 500 is cut off in the region near the emitter facet. 25 micrometers after the cutoff position of the current injection window 500, an electrically isolated groove 300 with a depth of 2 micrometers is etched using an etching process, reaching the InP cladding layer 440, to completely block the lateral diffusion of charge carriers. After the electrically isolated groove 300, a graded waveguide section 200 with a length of 100 micrometers is formed. The width of the active region of the graded waveguide section 200 linearly widens from 14 micrometers to the width of the emitter facet 210 of 22 micrometers along the light propagation direction. The silicon dioxide insulating layer 420 above the graded waveguide section 200 remains intact, with no direct current injection. Finally, the ridge waveguide platform is buried on both sides by secondary epitaxial growth of semi-insulating InP material to form a buried heterojunction structure.

[0073] During the cleaving and coating stage, the processed wafer is cleaved into independent chips with a cavity length of 5 mm. An 800 nm thick Y2O3 antireflection film 600 is deposited on the emitting end face 210 of the graded waveguide section 200 to enhance the emission coupling of long-wave infrared light; a Y2O3 / Au / Ti multilayer high reflectivity film 700 is deposited on the rear end face of the straight waveguide section 100 away from the emitting end face to achieve high power output from a single end face.

[0074] During the packaging stage, the semiconductor laser chip is flip-chip bonded to a patterned diamond heat sink with the epitaxial layer facing down using indium solder, and the diamond heat sink is then mounted on a water-circulating microchannel cooler.

[0075] During the performance testing phase, the cooling water temperature was set to 15°C, and the semiconductor laser was tested in continuous wave operating mode.

[0076] Test results show that for a device with an emission wavelength of 10 micrometers, the threshold current density is 1.52 kA / cm². 2 The single-endpoint continuous wave output power reaches 2 watts, and the single-endpoint output brightness is as high as 2.0 MW·cm. -2 ·sr -1 This brightness value is more than twice that of existing conventional Fabry-Perot cavity structures and ordinary conical structure devices without an electrically isolated groove 300. The maximum electro-optical conversion efficiency of the device reaches 6%. In far-field testing, the full width at half maximum (FWHM) divergence angle in the transverse far field remained stable at 17.25° across the entire current range. The device consistently maintained fundamental transverse mode oscillation without any higher-order mode excitation or beam deflection, demonstrating that the 22-micron output end face width 210 perfectly maintained beam quality while reducing optical power density.

[0077] The above test results show that the long-wave infrared quantum cascade laser using the scheme of this invention has achieved significant breakthroughs in all performance indicators.

[0078] Furthermore, in practical applications, it was found that extending the semiconductor laser cavity surface damage threshold enhancement method and semiconductor laser scheme of this invention to single-frequency devices combined with distributed feedback gratings also achieved excellent results: for devices with an emission wavelength of 8 micrometers, the threshold current density was 1.29 kA / cm². 2 The single-end-face continuous wave output brightness reached a record 5.0 MW·cm. -2 ·sr -1 The device emits at a wavelength of 9 micrometers and has a threshold current density of 1.44 kA / cm². 2 The single-end-face continuous wave output brightness reaches 2.2 MW·cm². -2 ·sr -1 .

[0079] The method for improving the cavity surface damage threshold of semiconductor lasers of the present invention can be widely used in professional fields with strict requirements for the upper limit of output power and extremely high requirements for long-term reliability. The following uses two typical scenarios, namely high-speed satellite-to-ground laser communication and high-power industrial laser processing, as examples for illustration.

[0080] In high-speed space-to-ground laser communication scenarios, this method can be used to enhance the power of long-wavelength infrared quantum cascade laser sources and ensure link stability. Without relying on long-wavelength infrared end-face coatings that easily generate strong optical absorption, the device achieves the following cavity surface thermal management through physical structure optimization: first, by setting a gradually widened optical field mode by a gradient waveguide section 200 to reduce the cavity surface optical power density; then, by using an electrically isolated groove 300 etched to the cladding layer 440 to block the lateral diffusion of charge carriers; and finally, by combining this with a current injection window 500 confinement design in the insulating layer 420, end-face electrothermal heating is completely eliminated. This process not only significantly overcomes the power roll-off bottleneck caused by cavity surface thermal runaway in long-wavelength infrared lasers but also enhances the anti-interference capability of space-to-ground communication terminals in complex space environments. Because long-wavelength infrared quantum cascade lasers have a long emission wavelength and high transmittance in the atmosphere, and atmospheric turbulence has a relatively small impact on communication links in this band, it helps reduce signal packet loss and improve the transmission reliability of the "space-air-ground integrated" network.

[0081] In industrial high-power laser processing applications, this method can be used for high-brightness laser output from high-power semiconductor laser pump sources or direct processing light sources. For example, in thick plate welding and high-speed cladding of metal materials, a semiconductor laser array modified by the method of this invention is used as the core light source. By independently configuring a graded waveguide section 200 and an electrically isolated groove 300 in each light-emitting unit, the photothermal and electrothermal effects in the cavity surface region are suppressed respectively. This breaks through the power limit of traditional straight waveguide structures, which are prone to catastrophic optical damage, thereby outputting a stable laser beam with higher power and higher brightness. The factory can not only achieve deeper penetration and faster processing speed, but also significantly extend the service life and maintenance-free period of the laser pump source, enhancing the production efficiency and economic benefits of high-end equipment manufacturing lines.

[0082] As can be seen from the above applications, the method of the present invention supports universal, low-cost, and high-reliability end-face thermal management optimization in scenarios with high power requirements and severe cavity surface heat accumulation, such as satellite-to-ground communication and industrial processing, and has good practical value and development prospects.

[0083] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and these modifications or substitutions should all be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for improving the cavity surface damage threshold of a semiconductor laser, applied to a straight waveguide, end-emitting, electrically pumped semiconductor laser, wherein the semiconductor laser includes a straight waveguide section and an emitting end face located at one end of the straight waveguide section, characterized in that, The method includes: A gradient waveguide section is provided on one side of the transmitting end face; The gradient waveguide segment is connected to the end of the straight waveguide segment in the light propagation direction. The end of the gradient waveguide segment in the light propagation direction is provided with an exit end face. The width of the gradient waveguide segment gradually widens from the same width as the straight waveguide segment to the width of the exit end face along the light propagation direction. The width of the exit end face is greater than the width of the straight waveguide segment. An electrically isolated groove is etched between the gradient waveguide section and the straight waveguide section, and the etching depth of the electrically isolated groove reaches the cladding of the semiconductor laser. The current injection region is confined within the straight waveguide section.

2. The method for improving the cavity surface damage threshold of a semiconductor laser according to claim 1, characterized in that, The width expansion profile of the gradient waveguide segment is one of linear conical, parabolic, exponential, or piecewise linear gradient.

3. The method for improving the cavity surface damage threshold of a semiconductor laser according to claim 1, characterized in that, The length of the gradient waveguide segment is determined in the following way: Based on the current diffusion equation, combined with the tensor conductivity of each layer of the semiconductor laser material and the boundary conditions of the electrically isolated groove, the current density attenuation distribution along the light propagation direction is calculated. The length of the gradient waveguide section is set as: the distance from the electrically isolated groove along the light propagation direction to when the current density decays to a preset percentage below the injected current density; The current diffusion equation is as follows: ； For current density distribution, For tensor conductivity, For gradient operators, This represents the voltage distribution.

4. The method for improving the cavity surface damage threshold of a semiconductor laser according to claim 1, characterized in that, The width of the exit end face is determined in the following way: By simulating the propagation behavior of the fundamental transverse mode in the graded waveguide section using the beam propagation method or the finite element method, the maximum width at which the semiconductor laser maintains fundamental transverse mode oscillation within the operating current range is selected as the output end face width.

5. The method for improving the cavity surface damage threshold of a semiconductor laser according to claim 1, characterized in that, The confinement of the current injection region within the straight waveguide section specifically includes: An insulating layer is covered over the waveguide of the semiconductor laser; A current injection window is etched on the insulating layer corresponding to the straight waveguide section, so that the upper electrode contacts the semiconductor cap layer through the current injection window. The insulating layer corresponding to the gradient waveguide section remains intact.

6. The method for improving the cavity surface damage threshold of a semiconductor laser according to claim 1, characterized in that, The method further includes: An antireflection coating is deposited on the outgoing end face of the gradient waveguide section; A high-reflectance film is deposited on the rear end face of the straight waveguide section away from the transmitting end face.

7. The method for improving the cavity surface damage threshold of a semiconductor laser according to claim 1, characterized in that, The method further includes: The waveguide of the semiconductor laser is fabricated into a buried heterojunction ridge structure; The processed semiconductor laser chip is flip-chip bonded to a diamond heat sink with the epitaxial layer facing down.

8. The method for improving the cavity surface damage threshold of a semiconductor laser according to claim 3, characterized in that, The electrical isolation groove is located 20 μm to 30 μm after the current injection window cutoff position.

9. The method for improving the cavity surface damage threshold of a semiconductor laser according to any one of claims 1 to 8, characterized in that, The semiconductor laser is any one of a quantum cascade laser, a quantum well laser, an interband cascade laser, or a quantum dot laser; the operating wavelength of the semiconductor laser includes the ultraviolet to far-infrared band; the operating mode of the semiconductor laser includes continuous wave operating mode or pulsed operating mode.

10. A semiconductor laser, characterized in that, The semiconductor laser is fabricated using the semiconductor laser cavity surface damage threshold enhancement method as described in any one of claims 1 to 9, and the semiconductor laser comprises: straight waveguide section; A gradient waveguide section is located on the optical output end side of the straight waveguide section. The width of the gradient waveguide section gradually increases from the same width as the straight waveguide section to the width of the output end face along the optical propagation direction. An electrically isolated groove is disposed between the tapered waveguide section and the straight waveguide section, and the etching depth of the electrically isolated groove reaches the cladding. The current injection region is confined within the straight waveguide section, while the tapered waveguide section has no direct current injection.

Images