DFB laser based on three-section grating structure and preparation method thereof

Abstract

This invention relates to the field of semiconductor laser device technology, and more particularly to a DFB laser based on a three-segment grating structure and its fabrication method. The laser comprises, from bottom to top, a substrate, a cladding layer, a waveguide layer, an active region, and a grating functional layer. The grating functional layer forms a ridge waveguide and has electrically isolated trenches, dividing it into three high-order surface grating regions with different periods. Wavelength selection and tuning are achieved through the vernier effect within the grating intervals. The trench region has no capping layer. Each grating region has an independent P-electrode, and an N-electrode is provided on the back side of the substrate, forming independent current injection paths. The fabrication method involves MOCVD epitaxial growth of an epitaxial wafer, followed by photolithography and etching to form the ridge waveguide and electrically isolated trenches. An insulating layer is deposited, electrode windows and independent electrodes are fabricated, and wafer processing and cleaving are completed to obtain a single laser tube. The advantages are: good process compatibility, high integration, stable single-mode output, and suitability for fields such as fiber optic communication and lidar.

Description

Technical Field

[0001] This invention relates to the field of semiconductor laser device technology, and in particular to a DFB laser based on a three-segment grating structure and its fabrication method. Background Technology

[0002] Distributed feedback (DFB) semiconductor lasers possess advantages such as high monochromaticity, wavelength stability, excellent single-mode performance, and ease of integration, making them important light sources in industrial production and military defense. Particularly in cutting-edge fields such as fiber optic communication, lidar, and biomedicine, their tunability provides systems with greater flexibility and performance limits: in wavelength division multiplexing (WDM) networks, they enable dynamic wavelength allocation and routing, significantly increasing network capacity; in lidar and laser medicine, wide-range wavelength scanning is crucial for achieving high-precision spatial ranging and micron-level biological tissue tomographic imaging.

[0003] Current mainstream tunable semiconductor lasers still rely on external cavity structures (such as Littrow or Littman configurations) to achieve a wide tuning range and high side-mode suppression ratio. However, the large number of discrete components leads to large system size, high coupling loss, high manufacturing difficulty, and high cost. In the field of sensing, applications such as environmental monitoring and biosensing require lasers to achieve wide-band wavelength scanning to complete multi-target, high-precision signal identification and quantitative detection. Furthermore, the trend towards portable and miniaturized sensing devices places stringent requirements on the size and integration of lasers. In the medical field, laser diagnostics and laser therapy technologies require matching specific wavelengths to the optical characteristics of different biological tissues. Wide tuning capability is key to achieving precise diagnosis and treatment. Simultaneously, the demands of medical equipment for miniaturization, high stability, and low loss also require lasers to abandon complex external cavity structures. With the surge in demand from applications in sensing, medicine, and other fields, the need for monolithically integrated tunable semiconductor lasers is urgent. Developing highly integrated, low-cost tunable semiconductor lasers has become a current research frontier and a future development direction.

[0004] At present, monolithically integrated tunable semiconductor lasers mainly include DFB laser arrays, three-segment DFB lasers with phase shift regions, tunable distributed Bragg reflector (DBR) lasers, and etched groove tunable lasers. The implementation principles of each structure are as follows: (1) DFB laser array: By integrating multiple parallel DFB laser units with different grating periods to cover a wide band, electronic switching is used to activate DFB lasers in specific bands for coarse tuning, and the temperature of the unit is adjusted by a micro-heating heater to change the refractive index to achieve continuous fine tuning of the wavelength; these parallel optical paths are converged into a single output waveguide through a multi-mode interference coupler (MMI) using the self-imaging effect. (2) Three-segment DFB laser with phase shift region: It consists of two high-order surface grating regions at the front and back and a phase shift region in the middle. All regions are equipped with quantum well layers. Coarse wavelength tuning is achieved by changing the current of the two grating regions, fine wavelength tuning is achieved by changing the current of the phase shift region, and finally, a wide range of wavelength tuning is achieved by the coordinated adjustment of the currents in the three regions. (3) Tunable DBR laser: It consists of three parts: gain region, phase shift region and grating region. Only the gain region is equipped with a quantum well layer, and the other regions are not equipped with a quantum well layer. Coarse wavelength tuning is achieved by changing the current of the grating region, fine wavelength tuning is achieved by changing the current of the phase shift region, and different gains are provided for the laser by changing the current of the gain region. A wide range of wavelength tuning is achieved by the coordinated adjustment of the electrodes in the three regions. (4) Etched Groove Tunable Laser: By etching extremely narrow grooves at both ends of the ridge waveguide to form a structure with gratings at both ends and a ridge waveguide in the middle, different free spectroscopic ranges (FSR) are achieved due to the different spacing between the two grooves. By changing the current in the grating regions at both ends and the gain region in the middle, a wide range of wavelength tuning can be achieved by utilizing the vernier effect.

[0005] However, all existing monolithically integrated tunable semiconductor lasers have their own technical defects, making it difficult to simultaneously meet the application requirements of high integration, wide tuning range, high spectral uniformity, low fabrication cost and high process feasibility. The specific defects are as follows: (1) DFB laser array: Although the tuning mechanism is relatively simple and the output stability is good, the system structure is complex, the volume is larger and the optical coupling is difficult to achieve; as the number of lasers in the array increases, the device consistency and assembly complexity increase significantly, the yield rate is likely to drop significantly, and the optical path convergence method based on MMI and cascade coupling usually introduces a large additional optical loss. (2) Three-segment DFB laser with phase shift region: Although wavelength tuning can be achieved, when performing wavelength tuning over a large range, there are blank areas that cannot be tuned, resulting in poor spectral uniformity. (3) Tunable DBR laser: Most of them use low-order gratings combined with buried grating technology, which often requires 100-nanometer-level grating fabrication technology and subsequent secondary epitaxial growth, resulting in complicated process flow, high processing difficulty and high manufacturing cost. (4) Tunable laser with etched groove: It is highly sensitive to key dimensional parameters such as the width of the etched groove. Manufacturing deviations can easily lead to mode competition, resulting in multimode output and other problems, affecting the single-mode performance and stability of the device. Therefore, it is urgent to develop a tunable DFB laser that has a wide tuning range, high spectral uniformity, high mode stability, and simple fabrication process with high integration. Summary of the Invention

[0006] To address the aforementioned problems, this invention provides a DFB laser based on a three-segment grating structure and its fabrication method.

[0007] The primary objective of this invention is to provide a DFB laser based on a three-segment grating structure, comprising a substrate, a lower cladding layer, a lower waveguide layer, an active region, an upper waveguide layer, and a grating functional layer stacked sequentially from bottom to top; The grating functional layer comprises, from bottom to top, an upper cladding layer, a capping layer, and an insulating layer; the upper cladding layer, capping layer, and insulating layer form a ridge waveguide; three periodic electrically isolated trenches are formed on the ridge waveguide, and the capping layer is removed and the insulating layer is retained in the electrically isolated trench area; The electrically isolated trench divides the ridge waveguide into a first grating region, a second grating region, and a third grating region arranged sequentially along the optical transmission direction. The first grating region, the second grating region, and the third grating region together constitute three high-order surface grating structures, which achieve wavelength selection and tuning through the vernier effect. Electrode windows are provided on the first grating region, the second grating region, and the third grating region. Each electrode window is provided with a first grating region electrode, a second grating region electrode, and a third grating region electrode that are isolated from each other. The first grating region electrode, the second grating region electrode, and the third grating region electrode are all P electrodes. The substrate has an N electrode on the side away from the lower cladding.

[0008] Preferably, the ridge waveguide has a width of 2~4μm and a height of 1.5~2.5μm.

[0009] Preferably, the spacing between adjacent electrically isolated trenches in the first grating region is d1, the spacing between adjacent electrically isolated trenches in the second grating region is d2, and the spacing between adjacent electrically isolated trenches in the third grating region is d3, and d2 is less than d3 and d1.

[0010] Preferably, the depth of the electrical isolation trench is 0.5~1.5μm.

[0011] Preferably, the insulating layer is made of silicon dioxide; The first grating region electrode, the second grating region electrode, and the third grating region electrode are all Ti / Pt / Au metal layers; The N electrode is an Au / Ge / Ni metal layer.

[0012] Preferably, the lower cladding layer and the lower waveguide layer are n-doped InP-based or GaAs-based materials; the upper cladding layer and the upper waveguide layer are p-doped InP-based or GaAs-based materials.

[0013] The second objective of this invention is to provide a method for fabricating a DFB laser based on a three-segment grating structure, specifically including the following steps: S1. A laser epitaxial wafer is fabricated by epitaxial growth on a substrate via metal-organic chemical vapor deposition, forming a lower cladding layer, a lower waveguide layer, an active region, an upper waveguide layer, an upper cladding layer, and a capping layer sequentially from bottom to top. S2. A silicon dioxide hard mask is grown on the upper surface of the laser epitaxial wafer. After the ridge waveguide pattern is transferred by photolithography, the capping layer and the upper cladding layer of the epitaxial wafer are etched to form the ridge waveguide, and the residual silicon dioxide is removed. S3. Re-grow a silicon dioxide hard mask on the surface of the ridge waveguide. After photolithography transfer of three periodic electrically isolated trench patterns, etch the capping layer of the ridge waveguide to form electrically isolated trenches, and sequentially divide the capping layer of the ridge waveguide into the first grating region, the second grating region and the third grating region. Remove the residual silicon dioxide hard mask and uniformly deposit SiO2 on the surface as an insulating layer. S4. Electrode windows are prepared in the first grating region, the second grating region, and the third grating region by photolithography and etching. The insulating layer of the electrode window portion is selectively etched to expose the capping layer. S5. Grow Ti / Pt / Au metal layers, and fabricate mutually isolated first grating region electrodes, second grating region electrodes, and third grating region electrodes at each electrode window using a lift-off process; wherein, the metal layer in the electrode window region is in direct contact with the capping layer, and an ohmic contact is formed by alloying treatment. S6. The substrate is mechanically thinned and chemically mechanically polished. An Au / Ge / Ni metal layer is deposited on the side of the substrate away from the lower cladding to form an N electrode. The N electrode is then alloyed to form an ohmic contact with the substrate, thus producing a laser wafer. S7. The laser wafer is diced and cleaved to form a single laser tube, thus completing the fabrication.

[0014] Preferably, the thickness of the silicon dioxide hard mask is 200~300nm; the alloying treatment is carried out in a nitrogen atmosphere at 300~420℃.

[0015] Preferably, in step S5, a Ti / Pt / Au metal layer is grown by magnetron sputtering or electron beam evaporation.

[0016] Compared with the prior art, the present invention can achieve the following beneficial effects: (1) Good process compatibility: The surface grating structure can be realized through standard photolithography and etching processes, avoiding complex secondary epitaxy and buried grating processes, which significantly reduces the difficulty and cost of fabrication; (2) Wide tuning range: Through the vernier effect of the three-segment grating, only a small change in refractive index is needed to achieve wavelength tuning of nearly 40nm near 1470nm, which meets the requirements of dense wavelength division multiplexing (DWDM) systems for multiple wavelength channels; (3) High mode selectivity: Sufficient feedback gain can only be formed at wavelengths where at least two grating reflection spectrum peaks overlap and there is a sufficient gain difference between the overlapping position and the third grating reflection spectrum, effectively suppressing mode competition. The highest side-mode suppression ratio can reach 42dB, ensuring single longitudinal mode output. (4) Excellent stability: The three grating regions achieve independent current injection through electrical isolation trenches, which avoids current crosstalk and improves the mode stability and long-term working reliability of the laser. In summary, the vernier effect of generating sparse spectra using a three-segment grating achieves a wide tuning range, resulting in higher integration density and reduced system complexity compared to DFB laser arrays. Compared to three-segment DFB lasers with phase-shift regions, it offers a larger tuning range and better spectral uniformity. Furthermore, the use of a high-order surface grating structure, achievable through different i-line lithography techniques, avoids complex nanometer-scale lithography and expensive secondary epitaxy, significantly reducing device fabrication costs. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the cross-sectional structure of a DFB laser based on a three-segment grating structure according to an embodiment of the present invention.

[0018] Figure 2This is a schematic diagram of a DFB laser based on a three-segment grating structure according to an embodiment of the present invention.

[0019] Figure 3 This is a schematic diagram of the cross-sectional structure of a laser epitaxial wafer provided according to an embodiment of the present invention.

[0020] Figure 4 This is an output spectrum diagram of a laser under different injection current combinations according to an embodiment of the present invention.

[0021] Figure 5 This is a graph showing the fitting results of the frequency noise spectrum and linewidth of a laser according to an embodiment of the present invention.

[0022] Figure label: 1. Substrate; 2. Lower cladding layer; 3. Lower waveguide layer; 4. Active region; 5. Upper waveguide layer; 6. Upper cladding; 7. Cap layer; 8. Insulation layer; 9. P electrode; 10. N electrode; 101. First grating region; 102. Second grating region; 103. Third grating region; 104. Electrically isolated trench; 901. First grating region electrode; 902. Second grating region electrode; 903. Third grating region electrode. Detailed Implementation

[0023] In the following description, embodiments of the invention will be described with reference to the accompanying drawings. In the description below, the same modules are denoted by the same reference numerals. Where the same reference numerals are used, their names and functions are also the same. Therefore, their detailed description will not be repeated.

[0024] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and do not constitute a limitation thereof.

[0025] This invention provides a DFB laser based on a three-segment grating structure, wherein a substrate, a lower cladding layer, a lower waveguide layer, an active region, an upper waveguide layer, and a grating functional layer are stacked sequentially from bottom to top; The grating functional layer consists of an upper cladding, a capping layer, and an insulating layer from bottom to top; the upper cladding, capping layer, and insulating layer form a ridge waveguide (etched down to the upper cladding, without exposing the upper waveguide layer); three periodic electrically isolated trenches are formed on the ridge waveguide, and the electrically isolated trenches are groove structures formed by etching away the capping layer at the corresponding position of the ridge waveguide. The capping layer is removed and the insulating layer is retained in the area of ​​the electrically isolated trench. Specifically, the ridge waveguide width is 2~4μm and the height is 1.5~2.5μm; the electrical isolation trench depth is 0.5~1.5μm; in some embodiments, the ridge waveguide height is 2μm and the electrical isolation trench depth is 1μm. The electrically isolated trench divides the ridge waveguide into a first grating region, a second grating region, and a third grating region arranged sequentially along the optical transmission direction. The capping surfaces of the first grating region, the second grating region, and the third grating region are all deposited with insulating layers. The upper cladding, upper waveguide layer, active region, lower waveguide layer, and lower cladding corresponding to the first grating region, the second grating region, and the third grating region remain intact and continuous, together forming a three-segment high-order surface grating structure. Specifically, the lengths of the first, second, and third grating regions are set according to specific requirements. In practical applications, the first and third grating regions can be set to approximately 1 mm, and the length of the second grating region is approximately 500 μm, so that the length of a single tube of the laser is approximately 2.5 mm. The electrical isolation trenches divide each grating region into multiple independent rectangular grating units. The spacing between adjacent electrical isolation trenches in the first grating region is d1, the spacing between adjacent electrical isolation trenches in the second grating region is d2, and the spacing between adjacent electrical isolation trenches in the third grating region is d3. Furthermore, d2 is less than both d3 and d1, meaning d2 should be minimized. Electrode windows are provided on the first grating region, the second grating region, and the third grating region. Each electrode window is provided with a first grating region electrode, a second grating region electrode, and a third grating region electrode that are isolated from each other. The first grating region electrode, the second grating region electrode, and the third grating region electrode are all P electrodes and form ohmic contact with the corresponding grating region. The first, second, and third grating regions are grating structures with different periods, and wavelength selection and tuning are achieved through the vernier effect among them. Specifically, the first, second, and third grating regions are high-order grating structures with different periods. Each grating segment corresponds to a set of comb-shaped Bragg reflection spectra, and its peak wavelength is determined by both the grating period and the effective refractive index. Due to the slight difference in the periods of the three grating segments, their free spectral ranges (FSRs) also have slight differences. The lasing wavelength of the laser must simultaneously satisfy the Bragg condition of the three grating segments, that is, only at wavelengths where the peak values ​​of the reflection spectra of at least two grating segments coincide, and where there is a sufficient gain difference between the coincidence position and the reflection spectrum of the third grating segment, can sufficient feedback gain be formed to generate single-mode lasing. When the effective refractive index of a certain grating region is adjusted by independently injecting current, the reflection spectrum of that grating segment will drift as a whole. Due to the slight difference in the FSRs of the three grating segments, the coincidence point of the reflection peaks will move to the next coincidence position. By finely adjusting the injection current of the three grating regions, the coincidence point can be continuously moved within a wide wavelength range, thereby utilizing the vernier effect between the three gratings to achieve a wide range of wavelength selection and tuning. An N-electrode is also provided on the side of the substrate away from the lower cladding.

[0026] In some embodiments, the substrate is any one of the following materials: n-type InP, n-type GaAs, InGaAs, GaSb, etc. The lower cladding and lower waveguide are made of n-doped InP-based or GaAs-based materials. The lower cladding is used to provide vertical optical field confinement and current conduction. The refractive index of the lower waveguide is slightly higher than that of the lower cladding but lower than that of the active region, and is used to confine the optical field to the vicinity of the active region. The active region is a quantum well layer used to provide optical gain. The emission wavelength is determined by the material composition and the thickness of the quantum well. An InGaAsP / InGaAsP multiple quantum well (MQW) structure is adopted, with a well layer thickness of about 8~10nm and a barrier layer thickness of about 10~15nm. The upper cladding and upper waveguide layer are p-doped InP-based or GaAs-based materials, corresponding to the lower cladding and lower waveguide layer, respectively. The upper and lower cladding layers together form a vertical optical field confinement structure, and the upper and lower waveguide layers together form a symmetrical waveguide structure to optimize the optical field distribution. The capping layer is used to reduce the contact resistance of the P-side electrode and form a good ohmic contact. The insulating layer is made of silicon dioxide; The first, second, and third grating region electrodes are Ti / Pt / Au metal layers; the N electrode is an Au / Ge / Ni metal layer.

[0027] The fabrication method of the DFB laser based on the three-segment grating structure described above specifically includes the following steps: S1. A laser epitaxial wafer is fabricated by epitaxial growth on a substrate using metal-organic chemical vapor deposition (MOCVD), forming a lower cladding layer, a lower waveguide layer, an active region, an upper waveguide layer, an upper cladding layer, and a capping layer sequentially from bottom to top. The active region employs a multi-quantum-well structure to provide optical gain, and the thickness and doping concentration of each epitaxial layer are optimized according to the target wavelength and mode characteristics.

[0028] S2. A silicon dioxide hard mask is grown on the upper surface of the laser epitaxial wafer by plasma-enhanced chemical vapor deposition (PECVD); a pre-designed ridge waveguide pattern is transferred to a photoresist layer using photolithography, and then the pattern is transferred to the silicon dioxide hard mask; subsequently, using the silicon dioxide hard mask as a mask, the capping layer and the upper cladding layer of the epitaxial wafer are etched, with the etching depth controlled to the upper cladding layer to ensure that the upper waveguide layer is not exposed, thus forming the ridge waveguide.

[0029] S3. Re-grow a silicon dioxide hard mask on the surface of the ridge waveguide; transfer the three periodic electrically isolated trench patterns to the photoresist layer using ultraviolet lithography, and then transfer the patterns to the silicon dioxide hard mask through etching; using the silicon dioxide hard mask as a mask, etch the capping layer of the ridge waveguide to form electrically isolated trenches, and sequentially divide the capping layer of the ridge waveguide into the first grating region, the second grating region and the third grating region along the light transmission direction; after removing the residual silicon dioxide hard mask, deposit silicon dioxide as an insulating layer on the surface.

[0030] S4. Through-hole electrode windows are prepared in the first grating region, the second grating region, and the third grating region. The insulating layer of the electrode window portion is selectively etched to expose the capping layer surface. The pattern of the laser electrode is prepared again in the electrode window region by photolithography and development to define the position and shape of the three electrode segments.

[0031] S5. On the P-side of the laser, Ti / Pt / Au metal layers are grown sequentially by magnetron sputtering or electron beam evaporation; excess metal is removed by lift-off process, and mutually isolated first grating region electrodes, second grating region electrodes, and third grating region electrodes are prepared at the electrode windows of the first grating region, second grating region, and third grating region, respectively; wherein the metal layer in the electrode window region is in direct contact with the capping layer and is alloyed.

[0032] S6. The substrate is mechanically thinned and then chemically mechanically polished (CMP) to ensure a flat substrate surface. An Au / Ge / Ni metal layer is grown on the N-side of the substrate by electron beam evaporation to form an N-electrode. Alloying is then performed to make an ohmic contact between the N-electrode and the substrate, thus obtaining a complete laser wafer.

[0033] S7. The laser wafer is diced to separate into individual chips, and then a laser single tube with a resonant cavity is formed by cleaving process, thus completing the fabrication of a DFB laser based on a three-segment grating structure.

[0034] Example 1 See Figures 1-2 This embodiment provides a DFB laser based on a three-segment grating structure, wherein a substrate 1, a lower cladding layer 2, a lower waveguide layer 3, an active region 4, an upper waveguide layer 5, an upper cladding layer 6, a capping layer 7, and an insulating layer 8 are stacked sequentially from bottom to top. The upper cladding layer 6, the capping layer 7, and the subsequently deposited insulating layer 8 form a ridge waveguide; three periodic electrical isolation trenches 104 are formed on the ridge waveguide; the electrical isolation trenches 104 divide the ridge waveguide into a first grating region 101, a second grating region 102, and a third grating region 103 arranged sequentially along the optical transmission direction; Electrically isolated trenches 104 divide the first grating region 101, the second grating region 102, and the third grating region 103 into multiple independent rectangular grating units. The spacing between adjacent electrically isolated trenches 104 within the first grating region 101 is 44.7 μm, the spacing between adjacent electrically isolated trenches 104 within the second grating region 102 is 9.2 μm, and the spacing between adjacent electrically isolated trenches 104 within the third grating region 103 is 42.4 μm. The different spacing of the electrically isolated trenches 104 results in differences in the grating period of each grating region, which in turn causes subtle differences in the free spectral range (FSR) of the reflection spectrum of each grating region, enabling wavelength selection and tuning under the vernier effect. Electrode windows (penetrating the corresponding insulating layer and cover layer) are provided on the first grating area 101, the second grating area 102, and the third grating area 103. Each electrode window is provided with a first grating area electrode 901, a second grating area electrode 902, and a third grating area electrode 903 that are isolated from each other. An N-electrode 10 is provided on the side of the substrate 1 away from the lower cladding 2.

[0035] Laser fabrication methods include: S1. On substrate 1, a metal-organic chemical vapor deposition (MOCVD) epitaxial growth is performed to form, from bottom to top, a lower cladding layer 2, a lower waveguide layer 3, an active region 4, an upper waveguide layer 5, an upper cladding layer 6, and a capping layer 7, as follows: Figure 3 Laser epitaxial wafers were fabricated; the active region 4 adopted a multi-quantum well structure to provide optical gain, and the thickness and doping concentration of each epitaxial layer were optimized according to the target wavelength and mode characteristics.

[0036] S2. A silicon dioxide hard mask (200-300 nm thick) is grown on the upper surface of the laser epitaxial wafer using plasma-enhanced chemical vapor deposition (PECVD). A pre-designed ridge waveguide pattern is transferred to a photoresist layer using ultraviolet lithography, and then transferred to the silicon dioxide hard mask using reactive ion etching (RIE). Subsequently, using the silicon dioxide hard mask as a mask, the capping layer 7 and the upper cladding layer 6 of the epitaxial wafer are etched using inductively coupled plasma (ICP), with the etching depth controlled up to the upper cladding layer 6 to ensure that the upper waveguide layer is not exposed, forming a ridge waveguide with a width of 3 μm. Finally, the residual silicon dioxide hard mask is removed using buffered oxide etchant (BOE).

[0037] S3. A new silicon dioxide hard mask is grown on the surface of the ridge waveguide. The three-segment periodic electrical isolation trench pattern is transferred to the photoresist layer by ultraviolet lithography, and then the pattern is transferred to the silicon dioxide hard mask by RIE etching. Using the silicon dioxide hard mask as a mask, the capping layer 7 of the ridge waveguide is etched by ICP to form electrical isolation trenches. The capping layer 7 of the ridge waveguide is sequentially divided into the first grating region, the second grating region and the third grating region along the light transmission direction. After removing the residual silicon dioxide hard mask by BOE, silicon dioxide is deposited as an insulating layer 8 on the surface of the capping layer 7 to achieve electrical isolation between electrodes.

[0038] S4. Using ultraviolet lithography and buffer oxide etching (BOE) processes, through electrode windows are fabricated on the insulating layer 8 and capping layer 7 of the first grating region 101, the second grating region 102, and the third grating region 103, exposing the surface of the capping layer 7; again, using ultraviolet lithography and development processes, laser electrode patterns are fabricated in the electrode window region to define the position and shape of the three electrode segments.

[0039] S5. On the P-side of the laser, Ti / Pt / Au metal layers are sequentially grown by magnetron sputtering or electron beam evaporation. Excess metal is removed by lift-off process, and mutually isolated first grating region electrodes, second grating region electrodes, and third grating region electrodes are prepared at the electrode windows of the first grating region, second grating region, and third grating region, respectively. Subsequently, alloying treatment is performed in a nitrogen atmosphere at 300~360℃ to form good ohmic contact between each grating region electrode and the corresponding grating region, thereby reducing contact resistance.

[0040] S6. Mechanically thin the substrate 1 to a thickness of 100~150μm, and then perform chemical mechanical polishing (CMP) to ensure the surface of the substrate 1 is flat. On the N side of the substrate 1, grow an Au / Ge / Ni metal layer by electron beam evaporation to form an N electrode. Alloying is performed in a nitrogen atmosphere at 350~420℃ to form an ohmic contact between the N electrode and the substrate 1, thus obtaining a complete laser wafer.

[0041] S7. The laser wafer is diced to separate into individual chips, and then a laser single tube with a resonant cavity is formed by cleaving process, thus completing the fabrication of a DFB laser based on a three-segment grating structure.

[0042] Laser testing and performance characterization: Before laser testing, the prepared laser tube was first soldered onto an aluminum nitride (AlN) heat sink using indium solder to ensure good heat dissipation performance. Then, the first grating region electrode 901, the second grating region electrode 902, the third grating region electrode 903 and the N electrode 10 were connected to the pins of the test circuit board through gold wire bonding to achieve independent electrical injection of the three electrodes.

[0043] During testing, a thermoelectric cooler (TEC) was used to stabilize the laser chip temperature at 24–26°C to eliminate the influence of temperature on the refractive index and lasing wavelength. An automated current control system was used to independently scan the electrode currents in the three grating regions, with a current adjustment range of 0–100 mA and a step size of 1 mA. Simultaneously, the laser's output light was coupled into a high-resolution spectrometer via a single-mode fiber for spectral analysis, and the output spectral data under different current combinations were recorded in real time. The currents of the three electrode regions were scanned independently, and the spectral data corresponding to all current combinations were saved. Then, all spectral data were analyzed to select the spectra that met the single-longitudinal-mode lasing conditions, and the corresponding three-current combinations were recorded.

[0044] This invention has been verified through simulation and experiments, achieving a wide wavelength tuning range of nearly 40nm around 1470nm, covering the 1455~1495nm band. Figure 4 As shown, under the optimal current combination, the laser achieves a maximum side-mode rejection ratio (SMSR) of 42dB, with stable single-longitudinal-mode output and excellent mode selectivity.

[0045] Figure 5 This is a graph showing the frequency noise spectrum and linewidth fitting results of the laser in an embodiment of the present invention. The curves in the graph represent the measured frequency noise spectrum, the dots represent the measured linewidth values ​​at the corresponding frequency points, and the diagonal lines represent the linewidth fitting lines. As can be seen from the graph, the laser of the present invention has an integrated linewidth as low as approximately 5 kHz in the 1470 nm band, exhibiting low frequency noise and excellent phase noise performance.

[0046] The key technical points and advantages of this invention are as follows: The laser consists of three gratings with different periods. Wavelength selection and tuning are achieved through the vernier effect between the multiple gratings, thereby expanding the tunable wavelength range while improving mode selectivity and mode stability, reducing the probability of mode hopping, and exhibiting good spectral uniformity. Unlike traditional low-order refractive index coupled grating structures or etched trench tuning structures, this invention uses shallow electrically isolated trenches to achieve current limiting and segmented injection control, effectively suppressing lateral current diffusion while reducing additional optical losses and achieving higher gain contrast. Furthermore, the high-order surface grating structure eliminates the need for complex and costly electron beam lithography and secondary epitaxial processes, significantly reducing fabrication difficulty and improving process feasibility.

[0047] It should be understood that the various forms of processes shown above can be used to reorder, add, or delete steps. For example, the steps described in this invention disclosure can be executed in parallel, sequentially, or in different orders, as long as the desired result of the technical solution disclosed in this invention can be achieved, and this is not limited herein.

[0048] The specific embodiments described above do not constitute a limitation on the scope of protection of this invention. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A DFB laser based on a three-section grating structure, characterized in that: It includes a substrate, a lower cladding layer, a lower waveguide layer, an active region, an upper waveguide layer, and a grating functional layer, which are stacked sequentially from bottom to top; The grating functional layer comprises, from bottom to top, an upper cladding layer, a capping layer, and an insulating layer; the upper cladding layer, capping layer, and insulating layer form a ridge waveguide; three periodic electrically isolated trenches are formed on the ridge waveguide, and the capping layer is removed and the insulating layer is retained in the electrically isolated trench area; The electrically isolated trench divides the ridge waveguide into a first grating region, a second grating region, and a third grating region arranged sequentially along the optical transmission direction. The first grating region, the second grating region, and the third grating region together constitute three high-order surface grating structures, which achieve wavelength selection and tuning through the vernier effect. Electrode windows are provided on the first grating region, the second grating region, and the third grating region. Each electrode window is provided with a first grating region electrode, a second grating region electrode, and a third grating region electrode that are isolated from each other. The first grating region electrode, the second grating region electrode, and the third grating region electrode are all P electrodes. The substrate has an N electrode on the side away from the lower cladding.

2. The DFB laser based on a three-section grating structure according to claim 1, characterized in that: The ridge waveguide has a width of 2~4μm and a height of 1.5~2.5μm.

3. The DFB laser based on a three-section grating structure according to claim 1, characterized in that: The spacing between adjacent electrically isolated trenches in the first grating area is d1, the spacing between adjacent electrically isolated trenches in the second grating area is d2, and the spacing between adjacent electrically isolated trenches in the third grating area is d3, with d2 being less than d3 and d1.

4. The DFB laser based on a three-section grating structure according to claim 1, characterized in that: The depth of the electrical isolation trench is 0.5~1.5μm.

5. The DFB laser based on a three-section grating structure according to claim 1, characterized in that: The insulating layer is made of silicon dioxide; The first grating region electrode, the second grating region electrode, and the third grating region electrode are all Ti / Pt / Au metal layers; The N electrode is an Au / Ge / Ni metal layer.

6. The DFB laser based on a three-section grating structure according to claim 1, characterized in that: The lower cladding layer and the lower waveguide layer are n-doped InP-based or GaAs-based materials; the upper cladding layer and the upper waveguide layer are p-doped InP-based or GaAs-based materials.

7. A method for fabricating a DFB laser based on a three-section grating structure, characterized in that: Specifically, the steps include the following: S1. A laser epitaxial wafer is fabricated by epitaxial growth on a substrate via metal-organic chemical vapor deposition, forming a lower cladding layer, a lower waveguide layer, an active region, an upper waveguide layer, an upper cladding layer, and a capping layer sequentially from bottom to top. S2. A silicon dioxide hard mask is grown on the upper surface of the laser epitaxial wafer. After the ridge waveguide pattern is transferred by photolithography, the capping layer and the upper cladding layer of the epitaxial wafer are etched to form the ridge waveguide, and the residual silicon dioxide is removed. S3. Re-grow a silicon dioxide hard mask on the surface of the ridge waveguide. After photolithography transfer of three periodic electrically isolated trench patterns, etch the capping layer of the ridge waveguide to form electrically isolated trenches, and sequentially divide the capping layer of the ridge waveguide into the first grating region, the second grating region and the third grating region. Remove the residual silicon dioxide hard mask and uniformly deposit SiO2 on the surface as an insulating layer. S4. Electrode windows are prepared in the first grating region, the second grating region, and the third grating region by photolithography and etching. The insulating layer of the electrode window portion is selectively etched to expose the capping layer. S5. Grow Ti / Pt / Au metal layers, and fabricate mutually isolated first grating region electrodes, second grating region electrodes, and third grating region electrodes at each electrode window using a lift-off process; wherein, the metal layer in the electrode window region is in direct contact with the capping layer, and an ohmic contact is formed by alloying treatment. S6. The substrate is mechanically thinned and chemically mechanically polished. An Au / Ge / Ni metal layer is deposited on the side of the substrate away from the lower cladding to form an N electrode. The N electrode is then alloyed to form an ohmic contact with the substrate, thus producing a laser wafer. S7. The laser wafer is diced and cleaved to form a single laser tube, thus completing the fabrication.

8. The method of claim 7, wherein the method further comprises: forming a first grating in the first section of the DFB laser; forming a second grating in the second section of the DFB laser; and forming a third grating in the third section of the DFB laser. The thickness of the silicon dioxide hard mask is 200~300nm; the alloying treatment is carried out in a nitrogen atmosphere at 300~420℃.

9. The method of claim 7, wherein the method further comprises: forming a first grating in the first section of the DFB laser; forming a second grating in the second section of the DFB laser; and forming a third grating in the third section of the DFB laser. Step S5 involves growing a Ti / Pt / Au metal layer by magnetron sputtering or electron beam evaporation.

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