A distributed feedback laser, a grating structure and a method for manufacturing the same

Abstract

The application discloses a distributed feedback laser, a grating structure and a preparation method thereof, and comprises the following steps: designing and manufacturing a photoetching plate, providing a substrate, and growing a grating layer on the substrate to form a first epitaxial wafer; forming a photoresist mask structure on the grating layer based on the photoetching plate and by using a photoetching process; etching a plurality of first grating grooves downward in the grating layer by using the photoresist mask structure to form a grating structure; and performing burying on the grating structure to form a second epitaxial wafer. The whole process can control the change curve of the photoresist mask structure pattern of the second phase shift area by accurately controlling the exposure gap between the epitaxial wafer and the photoetching plate and the change amount of the photoetching mask structure pattern period on the photoetching plate, the repeatability of the exposure pattern is good, industrial mass production is facilitated, and the production efficiency is improved and the production cost is reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of laser preparation, in particular to a distributed feedback laser, a grating structure and a preparation method thereof. BACKGROUND

[0002] Semiconductor lasers are the main light source of optical communication networks, including three types of Fabry-Perot (FP laser), distributed feedback laser (DFB) and vertical cavity surface emitting (VCSEL) laser; among them, the distributed feedback laser (DFB) has a Bragg grating built in the semiconductor, which realizes single longitudinal mode selection by using the distributed feedback of light, and has the characteristics of high speed, narrow linewidth and dynamic single mode operation.

[0003] At present, the grating structure of the commercial distributed feedback laser has two types, one is a uniform grating structure, and the other is a phase-shift grating structure. The distributed feedback laser with uniform grating structure is prone to mode hopping, and its side mode suppression ratio and yield are lower than those of the distributed feedback laser with phase-shift grating structure. However, the existing preparation method of phase-shift grating structure usually adopts electron beam exposure and nano-imprinting. However, due to the slow speed of electron beam exposure, it is not suitable for large-scale industrial production. And nano-imprinting is prone to soft film misplacement, resulting in unclear pattern and low yield. SUMMARY

[0004] Therefore, the purpose of the present application is to provide a distributed feedback laser, a grating structure and a preparation method thereof to solve the problems of low production efficiency and low yield in the prior art.

[0005] To achieve the above-mentioned purpose, the first aspect of the present application provides a preparation method of a grating structure of a distributed feedback laser, comprising the following steps:

[0006] S1: designing and making a photoetching plate, wherein the bottom of the photoetching plate has a grating mask structure, the grating mask structure has a plurality of first phase shift regions and first uniform regions distributed at intervals, the grating period of the first phase shift region changes linearly symmetrically, and the grating period of the first uniform region is constant;

[0007] S2: providing a substrate and growing a grating layer on the substrate to form a first epitaxial wafer;

[0008] S3: forming a photoresist mask structure on the grating layer based on the photoetching plate and using a photoetching process;

[0009] S4: etching down the grating layer by using the photoresist mask structure to form a plurality of first grating grooves, so as to form a grating structure, wherein the grating structure has a plurality of second phase shift regions and second uniform regions which are arranged in a spaced manner and have a frequency change relationship with the first phase shift region and the first uniform region on the grating mask structure of the photoetching plate, the grating period of the second phase shift region changes in a symmetrical and nonlinear manner, and the grating period of the second uniform region is constant;

[0010] S5: forming a secondary epitaxial wafer by burying the grating structure.

[0011] Further, the specific method of step S1 is as follows:

[0012] According to the rigorous coupled wave analysis, the duty cycle of the grating mask structure is obtained by adjusting the duty cycle of the grating mask structure on the photoetching plate so that the first-order diffraction is maximum and the zero-order diffraction is minimum during simulation, the length of the first phase shift region and the maximum grating period are determined, and the period of the first uniform region is determined according to the light wavelength of the laser chip;

[0013] A quartz plate is provided, and a plurality of second grating grooves are etched on the lower surface of the quartz plate in a horizontal direction according to the duty cycle of the grating mask structure, the length of the first phase shift region, the maximum grating period and the period of the first uniform region, so as to form a grating mask structure and obtain a photoetching plate.

[0014] Further, in step S2, the thickness of the grating layer is 30+-2nm.

[0015] Further, the specific method of step S3 is as follows:

[0016] A photoresist layer is coated on the upper surface of the primary epitaxial wafer;

[0017] Exposure conditions and exposure dose are set, the photoresist layer is exposed based on the Talbot effect and by using the photoetching plate, so as to obtain first photoresist columns arranged in a spaced manner in the exposed area and second photoresist columns arranged in a spaced manner in the non-exposed area;

[0018] The exposed photoresist layer is developed to remove the first photoresist columns in the exposed area and form a photoresist mask structure.

[0019] Furthermore, in step S3, the exposure conditions include light intensity, exposure time, exposure gap, and exposure integration distance. The light intensity is 0.3–0.5 μw / cm², the exposure time is 17 ± 1 s, the exposure integration distance is 5 ± 0.5 μm, the exposure gap is calculated based on the distance between the two second phase-shift regions on the grating structure, the wavelength of the exposure light source, and the grating period of the grating mask structure pattern on the photomask, and the exposure dose is 6.6–7 mJ / cm².

[0020] Furthermore, in step S4, the photoresist mask structure forms etching windows at the positions corresponding to the removed first photoresist pillars, exposing the surface of the grating layer. The specific method of step S4 is as follows:

[0021] Dry etching is used to etch the grating layer downward at the etching window to form a first grating groove corresponding to the etching window and grating strips spaced apart from the first grating groove;

[0022] The photoresist mask structure on the grating strip is peeled off to form the grating structure.

[0023] Furthermore, in step S4, when etching the grating layer, the etching depth is greater than the thickness of the grating layer.

[0024] Furthermore, the specific method of step S5 is as follows:

[0025] A P-InP layer and a Zn-doped P-InGaAs ohmic contact layer are sequentially grown on the grating layer to bury the grating structure, thus obtaining a secondary epitaxial wafer.

[0026] A second aspect of the present invention provides a method for fabricating a distributed feedback laser, comprising the following steps:

[0027] P1: The grating structure was prepared using the method described above for fabricating the grating structure of a distributed feedback laser.

[0028] P2: Fabrication of a laser chip based on the grating structure of the distributed feedback laser;

[0029] P3: Test the wavelength and side-mode suppression ratio of the laser chip. When the wavelength and side-mode suppression ratio meet the test conditions, a distributed feedback laser is prepared. Otherwise, repeat all or part of step P1. After changing the exposure conditions and exposure dose during photomask fabrication, or after adjusting the length of the first phase shift region of the photomask and the grating period change, re-prepare the grating structure and its laser chip until the wavelength and side-mode suppression ratio of the laser chip meet the test conditions.

[0030] A third aspect of the present invention provides a distributed feedback laser, comprising a grating structure fabricated using the method described above for fabricating a grating structure of a distributed feedback laser. The grating structure has a plurality of second phase-shifting regions and second uniform regions arranged at intervals. The grating period of the second uniform regions is constant, and the grating period of the second phase-shifting regions is greater than that of the second uniform regions, and the grating period of the second phase-shifting regions exhibits a symmetrical nonlinear variation.

[0031] This invention designs a photomask with a grating period that varies symmetrically and linearly in the first phase-shift region. Based on the principle of diffraction self-imaging, a grating mask structure is formed by exposing a photoresist layer above the grating layer on the photomask. By controlling the exposure dose and exposure gap during the formation of the grating mask structure, the change curve of the pattern in the exposure region is controlled, resulting in a grating structure with a period that varies symmetrically and non-linearly in the second phase-shift region. The exposure speed is much faster than that of electron beam direct writing, which is convenient for large-scale industrial production. The pattern is clear and has good repeatability, which helps to improve the yield and reduce production costs. Attached Figure Description

[0032] Figure 1 This is a flowchart of the fabrication method of the grating structure of the distributed feedback laser in Embodiment 1 of the present invention.

[0033] Figure 2 for Figure 1 A schematic diagram of each step in the fabrication method of the grating structure of a distributed feedback laser.

[0034] Figure 3 The curve showing the change in the grating period of the grating mask structure pattern in step S1.

[0035] Figure 4 This is a schematic diagram showing the correspondence between the first phase shift region and the third phase shift region.

[0036] Figure 5 This is a flowchart of step S3.

[0037] Figure 6 The curve showing the change in the grating period of the photoresist mask structure pattern in step S3.

[0038] Figure 7 This is a flowchart of the fabrication method of the distributed feedback laser according to Embodiment 2 of the present invention.

[0039] Figure 8 for Figure 7 The flowchart for step P1.

[0040] Figure 9 for Figure 7 Another flowchart.

[0041] Figure 10 for Figure 9 A flowchart of another embodiment. Detailed Implementation

[0042] The basic implementation examples are as follows: Figure 1 As shown:

[0043] The following detailed description illustrates the specific implementation method:

[0044] Example 1

[0045] like Figure 1 and Figure 2 The figures shown are a flowchart and a schematic diagram of each step in the fabrication method of the grating structure of the distributed feedback laser in this embodiment. This embodiment uses a distributed feedback laser with a wavelength of 1310 nm as an example, whose grating period is approximately 200 nm. The fabrication method of its grating structure 401 specifically includes the following steps:

[0046] S1: Design and create photomasks.

[0047] First, based on the required grating period and duty cycle of the grating structure 401, determine the length of each first phase shift region, the maximum grating period, the grating period variation, and the grating period of the first uniform region of the grating mask structure 102 on the photomask. Then, provide a quartz plate 101, and etch a plurality of second grating grooves 102a and quartz strips spaced horizontally on the lower surface of the quartz plate 101 according to the length of the first phase shift region and the grating period variation, so as to form a grating mask structure 102 on the lower surface of the quartz plate 101, thereby obtaining a photomask having the grating mask structure 102. In this embodiment, the grating mask structure 102 of the photomask has a plurality of first phase-shifting regions and first uniform regions distributed at intervals. The period of the grating mask structure 102 pattern in the first uniform region is constant, and the period of the grating mask structure 102 pattern in the first phase-shifting region exhibits a symmetrical linear change. Specifically, it first increases the constant grating period value of the first uniform region on one side arithmetically to the maximum grating period, and then decreases it arithmetically to the constant grating period of the first uniform region on the other side (e.g., ...). Figure 3 As shown, the grating period variation curve of the grating mask structure 102 with a first phase shift region is shown. Compared with the first uniform region, the cumulative grating period variation in the first phase shift region is the same as the period variation of a typical λ / 4 phase shift grating.

[0048] When determining the length of the first phase-shift region and the change in grating period, the corresponding duty cycle is determined based on the wavelength and grating period of the grating structure 401 to be fabricated, using process simulation. Specifically, in this embodiment, process simulation is performed based on rigorous coupled-wave analysis, and the duty cycle of the grating mask structure 102 pattern is adjusted so that ±1st order diffraction is maximized and 0th order diffraction is minimized during simulation. The duty cycle of the grating period is obtained, and the length of the first phase-shift region and the maximum grating period on the grating mask structure 102 pattern are determined. Since the period corresponding to the first phase-shift region on the grating mask structure 102 pattern changes linearly with equal arithmetic progression, the change in grating period in the first phase-shift region can be determined based on the length of the first phase-shift region and the maximum grating period. The grating period of the first uniform region is determined according to the emission wavelength corresponding to the fabricated grating structure 401. In this embodiment, when a photomask is prepared using a lithography machine with a wavelength of 266nm, the resulting photomask achieves maximum ±1st order diffraction and minimum 0th order diffraction when the duty cycle of the grating mask structure 102 pattern is 50%. The grating period of the grating mask structure 102 pattern in the first uniform region is 400nm, and the maximum grating period is 600nm. The difference in the grating period variation in the first phase-shift region is (600-400) / L (L is the phase shift from the starting position of the first phase-shift region to the position where the maximum grating period is reached).

[0049] In this embodiment, the photomask is exposed using electron beam direct writing technology. Specifically, firstly, an electron beam resist layer and an electron beam photoresist layer 301 are sequentially formed on the lower surface of the quartz plate 101. Then, using electron beam direct writing technology, a grating mask structure 102 pattern is formed on the electron beam photoresist layer 301 according to the length of the first phase shift region and the grating period variation. Next, the electron beam resist layer and the quartz plate 101 are etched using wet etching or dry etching methods according to the grating mask structure 102. The etching thickness of the quartz plate 101 is less than the thickness of the quartz plate 101, so as to form a second grating groove 102a at the bottom of the quartz plate 101. Finally, the electron beam photoresist layer 301 and the electron beam resist layer on the quartz strip are removed to obtain a photomask with a grating mask structure 102 at the bottom.

[0050] S2: Growth to form a primary epitaxial layer.

[0051] Specifically, a substrate 201 is provided, and a grating layer 202 is grown on the substrate 201 to form a primary epitaxial wafer. In this embodiment, the substrate 201 can be an InP substrate 201, and the growth thickness of the grating layer 202 is 30±2nm.

[0052] S3: Form a photoresist mask structure 302 on a primary epitaxial wafer.

[0053] Specifically, a photoresist layer 301 is coated and formed on the primary epitaxial wafer, and the photoresist layer 301 is exposed under preset exposure conditions and exposure dose to obtain a photoresist mask structure 302 pattern. Finally, the exposed photoresist mask structure 302 pattern is developed to form a photoresist mask structure 302 on the primary epitaxial wafer.

[0054] like Figure 4 As shown, during the exposure of the photoresist layer 301, due to diffraction, a first phase-shifting region on the photomask will form two third phase-shifting regions corresponding to the first corresponding region on the photoresist mask structure 102 after exposure. The photoresist mask structure 102 has third phase-shifting regions and third uniform regions arranged at intervals. The period corresponding to the third phase-shifting region exhibits a symmetrical nonlinear change, and its maximum period is half of the maximum grating period of the first phase-shifting region. The period corresponding to the third uniform region is constant, and its period is half of the grating period of the first uniform region.

[0055] like Figure 5 As shown, step S3 includes the following sub-steps:

[0056] S301: A photoresist layer 301 is uniformly coated on the upper surface of the primary epitaxial wafer (i.e., the upper surface of the grating layer 202) and the photoresist layer 301 is baked.

[0057] S302: Position the grating mask structure 102 of the photomask downwards opposite the photoresist layer 301, align the photomask with the primary epitaxial wafer, adopt the Gaussian mode of the deep ultraviolet lithography machine with the Talbot effect, set the corresponding exposure conditions and exposure dose, use the photomask as a mask, control the primary epitaxial wafer to move up and down in a direction perpendicular to the photomask, the total distance of its movement is 5μm, the distance of movement is greater than the distance of two Talbot cycles, so as to expose the photoresist layer 301; so that a first photoresist pillar located in the exposure area and a second photoresist pillar located in the non-exposure area are formed in the photoresist layer 301 at intervals. In this embodiment, the exposure conditions include light intensity, exposure time, exposure gap, and exposure integration distance. The light intensity is 0.3–0.5 μw / cm², the exposure time is 17 ± 1 s, the exposure integration distance is 5 ± 0.5 μm, and the exposure dose is 6.6–7 mJ / cm². The exposure gap is calculated based on the distance between the two second phase-shift regions on the grating structure, the wavelength of the exposure light source, and the grating period of the grating mask structure pattern on the photomask. Please refer to the reference. Figure 4 In the diagram, Mask represents a photomask, and Wafer represents a substrate (or slab).

[0058] Gap = W / tanθ (1)

[0059] Where Gap is the exposure gap; 2W is the distance between the two second phase-shifting regions; θ is the angle between the diffraction direction of the first phase-shifting region and the direction perpendicular to the photomask, which is related to the wavelength of the exposure light source of the lithography machine and the grating period of the first uniform region on the photomask. The angle θ between the diffraction direction of the first phase-shifting region and the direction perpendicular to the photomask is calculated by the following formula:

[0060] θ=arcsin(λ / P) (2)

[0061] Where λ is the wavelength of the exposure light source of the lithography machine (i.e., the incident light wavelength), and P is the grating period of the first uniform region on the photomask. In this embodiment, the calculated exposure gap is 112±1μm.

[0062] S303: The exposed epitaxial wafer is immersed in a developing solution to develop the exposed photoresist layer 301, thereby removing the first photoresist pillars in the exposed area and forming a photoresist mask structure 302. The photoresist mask structure 302 has etching windows 302a at the positions of the removed first photoresist pillars, which expose the surface of the grating layer 202.

[0063] In this embodiment, during the exposure process, the diffracted light field at the midpoint between the light-transmitting area (i.e., the area corresponding to the second gate groove 102a) and the opaque area (i.e., the area corresponding to the quartz strip) on the photoresist layer 301 will cause the photoresist at the corresponding position on the photoresist layer 301 to receive the same exposure dose. This results in the formation of a frequency doubling of the photoresist mask structure 302 image on the photoresist layer 301. That is, after exposure and development, the period of the third uniform region on the photoresist mask structure 302 is equal to the period of the grating mask structure on the photoresist layer 301. The maximum period of the grating in the first uniform region of the photoresist mask structure 102 is half of the grating period of the first phase-shift region, which is 200 nm. The maximum period of the third phase-shift region of the photoresist mask structure 302 is half of the maximum grating period of the first phase-shift region, which is 300 nm. Since the period corresponding to the third phase-shift region of the photoresist mask structure 302 exhibits a symmetrical nonlinear variation, it can be concluded that the period corresponding to the third phase-shift region of the photoresist mask structure 302 exhibits a symmetrical nonlinear variation between 200 nm and 300 nm (relationship curve as shown in Figure 102). Figure 6 (As shown).

[0064] S4: Etch the first grating groove 401a to form the grating structure 401.

[0065] Specifically, using the photoresist mask structure 302, the grating layer 202 and the substrate 201 layer are etched downwards at the etching window 302a using a wet or dry method, forming a first gate trench 401a corresponding to the etching window 302a and a grating strip corresponding to the second photoresist pillar. The first gate trench 401a and the grating strip are spaced apart to form a grating structure 401. Then, the photoresist mask structure 302 (i.e., the second photoresist pillar) on the grating strip is peeled off to form the grating structure 401. In this embodiment, the etching of the grating layer 202 is over-etched, that is, the etching depth is greater than the thickness of the grating layer 202. The etching depth is preferably 30-50 nm.

[0066] The grating structure 401 has several spaced-apart second phase-shift regions and second uniform regions that exhibit frequency doubling variations with the first and second phase-shift regions on the grating mask structure 102. The grating period of the second phase-shift regions exhibits a symmetrical nonlinear variation, while the grating period of the second uniform regions is constant. The second phase-shift regions correspond to the third phase-shift regions, and their number is twice that of the first phase-shift regions. The maximum grating period of the second phase-shift region is half the maximum grating period of the first phase-shift region, i.e., 300 nm. Correspondingly, the grating period of the second phase-shift regions also exhibits a symmetrical nonlinear variation between 200 nm and 300 nm. Similarly, the third uniform region corresponds to the second uniform region, and its grating period is half the grating period of the first uniform region, i.e., 200 nm.

[0067] S5: Bury the grating structure 401 to form a secondary epitaxial wafer.

[0068] A P-InP layer 501 and a Zn-doped P-InGaAs ohmic contact layer 502 are grown sequentially above the grating structure 401 of the primary epitaxial wafer obtained in step S4, and the grating structure 401 is buried to obtain a secondary epitaxial wafer.

[0069] The fabrication method of the grating structure of the distributed feedback laser in this embodiment is based on the principle of diffraction self-imaging. It uses a photomask with a first phase shift region to expose a photoresist layer 301 above the grating layer 202 to form a photoresist mask structure 302. By controlling the exposure dose and exposure gap when forming the photoresist mask structure 302, the change curve of the exposure area pattern is controlled, so that a grating structure 401 including a second phase shift region with a symmetrical nonlinear change in grating period is obtained. The exposure speed is much higher than the exposure speed of electron beam direct writing, which is convenient for large-scale industrial production, and the pattern is clear and has good repeatability.

[0070] Example 2

[0071] like Figures 7-8The diagram shows a flowchart of the fabrication method for the distributed feedback laser in this embodiment. In this embodiment, the grating structure 401 of the distributed feedback laser is fabricated based on the fabrication method of the grating structure of the distributed feedback laser in Embodiment 1. Specifically, the fabrication method of the distributed feedback laser in this embodiment includes the following steps:

[0072] P1: The grating structure 401 of the distributed feedback laser was fabricated and buried to obtain a secondary epitaxial wafer.

[0073] Specifically, the grating structure 401 and its secondary epitaxial wafer for a distributed feedback laser were prepared using the preparation method of Example 1. For example... Figure 9 As shown, step P1 includes the following sub-steps:

[0074] P101: Design and manufacture photomasks.

[0075] First, based on the required grating period and duty cycle of the grating structure 401, determine the length of each first phase shift region, the maximum grating period, the grating period variation, and the grating period of the first uniform region of the grating mask structure 102 on the photomask. Then, provide a quartz plate 101, and etch a plurality of second grating grooves 102a and quartz strips spaced horizontally on the lower surface of the quartz plate 101 according to the length of the first phase shift region and the grating period variation, so as to form a grating mask structure 102 on the lower surface of the quartz plate 101, thereby obtaining a photomask having the grating mask structure 102.

[0076] P102: Growth to form a primary epitaxial layer.

[0077] Specifically, a substrate 201 is provided, and a grating layer 202 is grown on the substrate 201 to form a primary epitaxial wafer. In this embodiment, the substrate 201 can be an InP substrate 201, and the growth thickness of the grating layer 202 is 30±2nm.

[0078] P103: Forming a photoresist mask structure 302 on a primary epitaxial wafer.

[0079] A photoresist layer 301 is coated and formed on the primary epitaxial wafer. The photoresist layer 301 is exposed under preset exposure conditions and exposure dose to obtain a photoresist mask structure 302 pattern. Finally, the exposed photoresist mask structure 302 pattern is developed to form a photoresist mask structure 302 on the primary epitaxial wafer.

[0080] P1031: A photoresist layer 301 is uniformly coated on the upper surface of the primary epitaxial wafer (i.e., the upper surface of the grating layer 202) and the photoresist layer 301 is baked.

[0081] P1032: Position the grating mask structure 102 of the photomask downwards opposite the photoresist layer 301, align the photomask with the primary epitaxial wafer, adopt the Gaussian mode of the deep ultraviolet lithography machine with the Talbot effect, set the corresponding exposure conditions and exposure dose, use the photomask as a mask, control the primary epitaxial wafer to move up and down in a direction perpendicular to the photomask, the total distance of its movement is 5μm, the distance of movement is greater than the distance of two Talbot cycles, so as to expose the photoresist layer 301.

[0082] P1033: The exposed epitaxial wafer is immersed in a developing solution to develop the exposed photoresist layer 301, thereby removing the first photoresist pillars in the exposed area and forming a photoresist mask structure 302. The photoresist mask structure 302 forms etching windows 302a at the positions of the removed first photoresist pillars, which expose the surface of the grating layer 202.

[0083] P104: Etch the first grating groove 401a to form the grating structure 401.

[0084] Using the photoresist mask structure 302, the grating layer 202 and the substrate layer 201 are etched downward at the etching window 302a using a wet or dry method to form a first grating groove 401a corresponding to the etching window 302a and a grating strip corresponding to the second photoresist pillar. The first grating groove 401a and the grating strip are spaced apart to form a grating structure 401. Then, the photoresist mask structure 302 (i.e. the second photoresist pillar) on the grating strip is peeled off to form the grating structure 401.

[0085] P105: Bury the grating structure 401 to form a secondary epitaxial wafer.

[0086] A P-InP layer 501 and a Zn-doped P-InGaAs ohmic contact layer 502 are grown sequentially above the grating structure 401 of the primary epitaxial wafer obtained in step S4, and the grating structure 401 is buried to obtain a secondary epitaxial wafer.

[0087] In this embodiment, steps P101 to P105 correspond one-to-one with steps S1 to S5 in Embodiment 1. For details, please refer to the description of Embodiment 1. This embodiment will not repeat the details.

[0088] P2: Laser chip fabricated based on grating structure 401.

[0089] Based on the grating structure 401 and its secondary epitaxial wafer of the distributed feedback laser prepared in step P1, the laser chip is prepared according to the general fabrication process of distributed feedback lasers, which will not be described in detail in this embodiment.

[0090] P3: Test the laser chip.

[0091] The wavelength and side-mode rejection ratio (SMR) of the laser chip are tested. When the wavelength and SMR meet the test conditions, a distributed feedback laser is fabricated; otherwise, all or part of step P1 is repeated until the wavelength and SMR of the laser chip meet the test conditions. In this embodiment, the wavelength of the laser chip is 1310 nm, the SMR is 45 dB under room temperature testing conditions, and the SMR is 40 dB under high temperature testing conditions.

[0092] Specifically, step P3 includes the following sub-steps:

[0093] P301: Test the wavelength and side-mode rejection ratio of the laser chip.

[0094] P302: Determine whether the wavelength and side-mode suppression ratio both meet the test conditions. If yes, proceed to step P303; otherwise, return to step P1032. Adjust the exposure gap and exposure dose in the exposure conditions to change the duty cycle of the grating period, and then re-expose the photomask to re-fabricate the grating structure 401.

[0095] like Figure 10 As shown, as another optional method in this embodiment, if the wavelength and side-mode suppression ratio of the laser chip still cannot meet the test conditions after the grating structure 401 is re-fabricated, the process can return to step P101, change the length of the first phase shift region of the photomask, re-fabricate the photomask, and then re-fabricate the grating structure 401.

[0096] P303: A distributed feedback laser was fabricated.

[0097] The fabrication method of the distributed feedback laser in this embodiment is based on the diffraction effect. Several segments of second phase shift regions with symmetrical nonlinear changes in grating period are prepared in the grating structure 401 of the distributed feedback laser. While satisfying the characteristics of high speed, narrow linewidth and dynamic single-mode operation of the distributed feedback laser, it can also realize large-scale industrial production with high yield, thereby reducing production costs.

[0098] Example 3

[0099] The distributed feedback laser of this embodiment is prepared using the fabrication method of the distributed feedback laser of Embodiment 2, which includes a grating structure 401 prepared using the fabrication method of the grating structure of the distributed feedback laser of Embodiment 1.

[0100] Specifically, the distributed feedback laser of this embodiment includes a grating structure 401. The grating structure 401 has a plurality of spaced second phase-shifting regions and second uniform regions. The grating period of the second uniform region is constant, and the grating period of the second phase-shifting region is greater than that of the second uniform region. The period difference between the grating period of each position in the second phase-shifting region and the grating period of the second uniform region is an indefinite value. The grating period of the second phase-shifting region increases from the starting position to the maximum grating period according to a predetermined curve in the second phase-shifting region, and then decreases to the ending position of the second phase-shifting region according to the predetermined curve, so that the grating period of the second phase-shifting region exhibits a symmetrical nonlinear change.

[0101] The distributed feedback laser of this embodiment has a grating structure 401 obtained by exposure, development and etching based on the principle of diffraction self-imaging. By precisely controlling the exposure gap between the epitaxial wafer and the photomask during the preparation process, the repeatability of the photoresist mask structure 302 pattern after exposure is good and the exposure speed is fast, which is conducive to improving production efficiency and reducing production costs.

Claims

1. A method for fabricating a grating structure for a distributed feedback laser, characterized in that, Includes the following steps: S1: Design and fabricate a photomask, wherein the bottom of the photomask has a grating mask structure, the grating mask structure has a plurality of first phase shift regions and first uniform regions distributed at intervals, the grating period of the first phase shift regions exhibits a symmetrical linear change, and the grating period of the first uniform regions is constant. S2: Provide a substrate and grow a grating layer on the substrate to form a primary epitaxial wafer; S3: Based on the photomask, a photoresist mask structure is formed on the grating layer using a photolithography process; S4: Using the photoresist mask structure, a plurality of first grating grooves are etched downwards into the grating layer to form a grating structure. The grating structure has a plurality of second phase-shifting regions and second uniform regions that are spaced apart and have frequency-doubled variations with the first phase-shifting region and the first uniform region on the grating mask structure of the photomask. The grating period of the second phase-shifting region exhibits a symmetrical nonlinear variation, while the grating period of the second uniform region is constant. S5: Bury the grating structure to form a secondary epitaxial wafer; The specific method of step S3 is as follows: a photoresist layer is coated on the upper surface of the primary epitaxial wafer, exposure conditions and exposure dose are set, the photoresist layer is exposed based on the Talbot effect and using the photomask to obtain a first photoresist pillar located in the exposure area and a second photoresist pillar located in the non-exposure area, which are spaced apart; the exposed photoresist layer is developed, and the first photoresist pillar in the exposure area is removed to form a photoresist mask structure. The exposure conditions include light intensity, exposure time, exposure gap, and exposure integration distance. The light intensity is 0.3–0.5 μw / cm², the exposure time is 17 ± 1 s, the exposure integration distance is 5 ± 0.5 μm, the exposure gap is calculated based on the distance between the two second phase-shift regions on the grating structure, the wavelength of the exposure light source, and the grating period of the grating mask structure pattern on the photomask, and the exposure dose is 6.6–7 mJ / cm².

2. The method for fabricating a grating structure for a distributed feedback laser according to claim 1, characterized in that, The specific method for step S1 is as follows: Process simulation was performed based on rigorous coupled-wave analysis. By adjusting the duty cycle of the grating mask structure on the photomask, the ±1st order diffraction of the photomask was maximized and the 0th order diffraction was minimized during the simulation. The duty cycle of the grating mask structure was obtained, the length of the first phase shift region and the maximum grating period were determined, and the period of the first uniform part was determined based on the emission wavelength of the laser chip. A quartz plate is provided. Based on the duty cycle of the grating mask structure, the length of the first phase shift region, the maximum grating period, and the period of the first uniform region, a plurality of second grating grooves arranged at intervals along the horizontal direction are etched on the lower surface of the quartz plate to form a grating mask structure, thereby obtaining a photomask.

3. The method for fabricating a grating structure for a distributed feedback laser according to claim 1, characterized in that, In step S2, the thickness of the grating layer is 30±2nm.

4. The method for fabricating a grating structure for a distributed feedback laser according to claim 1, characterized in that, In step S4, the photoresist mask structure forms etching windows at the positions corresponding to the removed first photoresist pillars, exposing the surface of the grating layer. The specific method of step S4 is as follows: Dry etching is used to etch the grating layer downward at the etching window to form a first grating groove corresponding to the etching window and grating strips spaced apart from the first grating groove; The photoresist mask structure on the grating strip is peeled off to form the grating structure.

5. The method for fabricating a grating structure for a distributed feedback laser according to claim 1, characterized in that, In step S4, when etching the grating layer, the etching depth is greater than the thickness of the grating layer.

6. The method for fabricating a grating structure for a distributed feedback laser according to claim 1, characterized in that, The specific method for step S5 is as follows: A P-InP layer and a Zn-doped P-InGaAs ohmic contact layer are sequentially grown on the grating layer to bury the grating structure, thus obtaining a secondary epitaxial wafer.

7. A method for fabricating a distributed feedback laser, characterized in that, Includes the following steps: P1: The grating structure is prepared by the method for preparing the grating structure of the distributed feedback laser as described in any one of claims 1 to 6; P2: Fabrication of a laser chip based on the grating structure of the distributed feedback laser; P3: Test the wavelength and side-mode suppression ratio of the laser chip. When the wavelength and side-mode suppression ratio meet the test conditions, a distributed feedback laser is prepared. Otherwise, repeat all or part of step P1. After changing the exposure conditions and exposure dose during photomask fabrication, or after adjusting the length of the first phase shift region of the photomask and the grating period change, re-prepare the grating structure and its laser chip until the wavelength and side-mode suppression ratio of the laser chip meet the test conditions.

8. A distributed feedback laser, characterized in that, The invention includes a grating structure fabricated using a method for fabricating a grating structure of a distributed feedback laser as described in any one of claims 1 to 6. The grating structure has a plurality of second phase-shift regions and second uniform regions arranged at intervals. The grating period of the second uniform region is constant, and the grating period of the second phase-shift region is greater than that of the second uniform region, and the grating period of the second phase-shift region exhibits a symmetrical nonlinear variation.

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