Multi-wavelength tunable laser and method of manufacturing the same

Abstract

The application relates to a multi-wavelength tunable laser and a preparation method thereof. The multi-wavelength tunable laser comprises a two-dimensional sampling grating with an hourglass-shaped profile prepared on a grating layer of a laser; and the transverse duty cycle of each order grating in the two-dimensional sampling grating is determined based on the effective coupling coefficient and the reflectivity of the output wavelength of the order grating, so as to ensure the single longitudinal mode characteristic of the laser, improve the output power, and stabilize the quality of the output light beam.

Description

Technical Field

[0001] This application relates to the field of semiconductor technology, and in particular to a multi-wavelength tunable laser and its fabrication method. Background Technology

[0002] DFB (Distributed Feedback) lasers, with their excellent single-mode characteristics, narrow linewidth, and high reliability, have become the core light source in fields such as optical communication, coherent optical detection (such as lidar), high-precision spectral absorption sensing, and microwave photonics.

[0003] However, as technologies in fields such as optical communication and coherent optical detection evolve towards higher speeds, longer distances, and larger capacities, the core light sources involved in these technologies not only need to maintain their single-mode characteristics (high spatial beam quality) but also extremely high output power. If the output power of a single DFB laser is increased while the width of the active region is enlarged (i.e., a wide emission region structure), the waveguide width can easily exceed the single-mode cutoff size, exciting higher-order transverse modes. This can also lead to spatial hole burning and multifilamentation, causing a sharp deterioration in beam quality, failure of single-mode characteristics, and ultimately, unstable laser output.

[0004] How to ensure the single-mode characteristics of DFB lasers while improving output power and stabilizing the quality of output beams has become an urgent technical challenge. Summary of the Invention

[0005] Based on this, a DFB laser and its fabrication method are provided to solve the technical problem that DFB lasers cannot improve output power and stabilize output beam while ensuring single longitudinal mode characteristics.

[0006] In a first aspect, embodiments of this application provide a multi-wavelength tunable laser, comprising:

[0007] A two-dimensional sampling grating with an hourglass-shaped profile is fabricated on the grating layer of the laser;

[0008] The lateral duty cycle of each order of a two-dimensional sampling grating is determined based on the effective coupling coefficient of that order of grating and the reflectivity of the output wavelength.

[0009] In one embodiment, the laser further includes: a substrate, a buffer layer, a lower cladding layer, an active layer, a grating layer, a ridge waveguide, a passivation insulating layer, and an n-type electrode and a p-type electrode located on the back side of the substrate and the passivation insulating layer, respectively.

[0010] In one embodiment, the two-dimensional sampling grating has a laterally gradient duty cycle, and the variation law of the lateral length of the two-dimensional sampling grating satisfies a hyperbolic function, a Gaussian function, or a parabolic function.

[0011] In one embodiment, the shape of the vacant portion between every two periods in the two-dimensional sampling grating is an elliptical structure. The minor axis of the elliptical structure is greater than 0.5 times the sampling period of the two-dimensional sampling grating and not greater than 0.8 times the grating ridge width. The sampling period length of the two-dimensional sampling grating is 22.79 μm.

[0012] In one embodiment, the seed grating of the two-dimensional sampling grating has a period of 227.9 nm and a waveguide length of 228 μm.

[0013] Secondly, embodiments of this application also provide a method for fabricating a multi-wavelength tunable laser, comprising:

[0014] A buffer layer, a lower cladding layer, an active layer, and a grating layer are grown sequentially on the substrate.

[0015] A two-dimensional sampling grating with an hourglass-shaped profile is fabricated in the grating layer. The lateral duty cycle of each order grating in the two-dimensional sampling grating is determined based on the effective coupling coefficient of that order grating and the reflectivity of the output wavelength.

[0016] In one embodiment, a two-dimensional sampling grating with an hourglass-shaped profile is fabricated in the grating layer, including:

[0017] Photoresist is coated on the grating layer, and one-dimensional high-frequency interference fringes are formed by dual-beam holographic interference exposure technology. The period of the one-dimensional high-frequency interference fringes is 227.9nm.

[0018] A two-dimensional sampling latent image is obtained by using ultraviolet stepping lithography to perform secondary exposure based on a two-dimensional REC mask. The two-dimensional reconstruction equivalent chirped REC mask includes a periodically repeating elliptical structure.

[0019] The grating layer is developed, etched, stripped of resist, and cleaned to obtain a two-dimensional sampling grating.

[0020] In one embodiment, the minor axis of the elliptical structure is greater than 0.5 times the sampling period of the two-dimensional sampling grating and not greater than 0.8 times the ridge width of the grating, and the sampling period length of the two-dimensional sampling grating is 22.79 μm.

[0021] In one embodiment, the preparation method further includes:

[0022] An upper cladding layer is fabricated above the grating layer;

[0023] Etch the upper cladding to fabricate the ridge waveguide;

[0024] Fabrication of a passivated insulating layer covering the ridge waveguide;

[0025] n-type electrodes and p-type electrodes were fabricated on the back side of the substrate and on the passivation insulating layer, respectively.

[0026] In one embodiment, the variation law of the transverse length of the two-dimensional sampling grating satisfies a hyperbolic function, a Gaussian function, or a parabolic function, and the period of the seed grating of the two-dimensional sampling grating is 227.9 nm, and the waveguide length is 228 μm.

[0027] This application provides a multi-wavelength tunable laser that uses a two-dimensional sampling grating with an hourglass profile. Due to its laterally gradient duty cycle, it achieves ultra-wide wavelength tuning, constructs a flattened multi-wavelength comb spectrum, and effectively suppresses higher-order modes in the lateral direction, thereby improving the lateral single-mode nature of the beam. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of this application or the conventional technology, the drawings used in the description of the embodiments or the conventional technology will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 A schematic flowchart of the method for fabricating a DFB laser using REC technology provided in the embodiments of this application;

[0030] Figure 2 This application provides a schematic diagram of a two-dimensional sampling grating for a multi-wavelength tunable laser.

[0031] Figure 3 This is a schematic diagram comparing the structures of a uniform sampling grating and a two-dimensional sampling grating according to embodiments of this application;

[0032] Figure 4 This is a schematic diagram of an elliptical mask plate used for the two-dimensional REC mask in an embodiment of this application;

[0033] Figure 5 This is a comparison of the reflectance spectra of a uniform sampling grating and a two-dimensional sampling grating in TE1 mode in the embodiments of this application.

[0034] Figure 6 This is a comparison of the reflectance spectra of a uniform sampling grating and a two-dimensional sampling grating in the fundamental mode in the embodiments of this application.

[0035] Figure 7 This is a schematic flowchart illustrating a method for fabricating a multi-wavelength tunable laser, as provided in an embodiment of this application. Detailed Implementation

[0036] To facilitate understanding of this application, it will be described in more detail below. However, it should be understood that this application can be implemented in many different forms and is not limited to the embodiments or examples described herein. Rather, these embodiments or examples are provided to provide a more thorough and complete understanding of the disclosure of this application.

[0037] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein in the specification of this application is for the purpose of describing particular implementations or embodiments only and is not intended to be limiting of this application. The optional scope of the term "and / or" as used herein includes any one of two or more of the related listed items, as well as any and all combinations of the related listed items, including any two related listed items, any more related listed items, or a combination of all related listed items.

[0038] To achieve high power output in DFB lasers, it is often necessary to increase the width of the active region (or ridge waveguide) to improve the catastrophic optical mirror damage threshold and output power. However, increasing the width of the ridge waveguide introduces higher-order transverse modes, which can lead to unstable laser output, deterioration of beam quality, and kinking of the photocurrent curve, resulting in the actual usable output power being far lower than expected.

[0039] If a multi-wavelength tunable laser based on a one-dimensional sampling grating is used to increase the output power, the reflection spectrum envelope of the one-dimensional sampling grating is constrained by the Fourier transform law and must strictly follow the sinc function distribution. This will cause the multi-wavelength comb spectrum of the multi-wavelength tunable laser to exhibit a pattern of extremely high reflectivity at the central peak and sharp attenuation at the edge peaks. Consequently, during multi-wavelength or bandwidth tuning operations, the edge channel will experience a sudden drop in lasing power or even oscillation stoppage due to insufficient optical feedback.

[0040] To address the aforementioned technical issues, this application provides a two-dimensional sampling grating structure with a gradually varying lateral duty cycle. By generating a series of equally spaced reflection peaks through longitudinal sampling, the duty cycle of the lateral space is adjusted to achieve ultra-wide wavelength tuning, constructing a flattened multi-wavelength comb spectrum. Simultaneously, it effectively suppresses higher-order lateral modes and improves the lateral single-mode nature of the beam.

[0041] See Figure 1 As shown, an optional embodiment of this application provides a method for fabricating a DFB laser using REC (reconstruction-equivalent chirp) technology, comprising:

[0042] S101: The first epitaxial layer is grown on the substrate to obtain a wafer.

[0043] The first epitaxial layer includes: a buffer layer, a lower cladding layer, an active layer, and a grating layer.

[0044] Step S101 can use techniques such as metal-organic chemical vapor deposition or molecular beam epitaxy to grow the underlying epitaxial structure.

[0045] S102: Photoresist is uniformly coated on the wafer surface, and the wafer is exposed using dual-beam holographic interference exposure technology to form one-dimensional high-frequency interference fringes.

[0046] S103: A two-dimensional REC mask with a lateral gradient duty cycle is used to perform secondary exposure on the wafer, and a two-dimensional sampling latent image is superimposed on the photoresist.

[0047] For example, a two-dimensional REC mask with a lateral gradient duty cycle includes at least one elliptical gradient periodic mask pattern, the widths of the lateral and longitudinal axes of each elliptical gradient periodic mask pattern being determined based on the desired reflection spectrum of the grating, the mode light source, and monitor parameters.

[0048] S104: Place the wafer, which has undergone secondary exposure, into the developing solution.

[0049] S105: Remove residual photoresist and clean to eliminate etching damage to obtain a two-dimensional sampling grating.

[0050] S106: Based on MOCVD technology, a cladding material is used to seamlessly fill the two-dimensional sampling grating on the wafer.

[0051] S107: Ridge waveguide etching, passivation insulating layer and metallized electrode are prepared on the upper cladding.

[0052] In this embodiment, the Fourier space of the grating is reconstructed by a sampling grating with irregular horizontal spacing. The light energy originally concentrated in the central mode is redistributed to other order (wavelength) modes, so that more wavelength modes can obtain sufficient light feedback, thereby showing an increase in the number of modes or a broadening of the comb spectrum.

[0053] See Figure 2 As shown in the figure, this application provides a multi-wavelength tunable laser, on which a two-dimensional sampling grating with an hourglass-shaped profile is prepared on the grating layer.

[0054] The lateral length of each grating segment gradually decreases from both ends of the grating towards the center, reaching its minimum value at the center. The variation of the lateral length follows a hyperbolic function, a Gaussian function, or a parabolic function.

[0055] In terms of reflection spectral characteristics, the sampling grating exhibits a multiple resonance peak morphology, and the spacing between adjacent reflection peaks is inversely proportional to the sampling period. The refractive index perturbation function of the sampling grating is a superposition combination of numerous standard uniform grating refractive index modulation functions with different periods; correspondingly, its reflection spectrum can also be regarded as the product of the overlapping and compounding of multiple uniform grating reflection spectral lines.

[0056] The transverse duty cycle of each order grating in the two-dimensional sampling grating is determined based on the effective coupling coefficient of that order grating and the reflectivity of the output wavelength, wherein the reflectivity of the output wavelength may include, but is not limited to, the TE1 mode or the fundamental mode.

[0057] For the m-th order grating, its effective coupling coefficient can be obtained, but is not limited to, through the following process.

[0058] The period of the m-th order refractive index modulation satisfies:

[0059] Formula (1)

[0060] Formula (2)

[0061] Where m represents the diffraction order of the m-th grating. Characterizing the period of the m-th order grating, P represents the period of the seed grating, and P represents the period of the sampling grating.

[0062] Formula (3)

[0063] in, Indicates the Bragg wavelength. Indicates the effective refractive index, Indicates the period of the seed grating. Let m be the diffraction order of the m-th grating.

[0064] Using the Bragg condition, the center wavelength of the m-th order grating can be derived:

[0065] Using the Bragg condition, the center wavelength of the m-th order grating can be derived:

[0066] Formula (4)

[0067] in, The wavelength at the center of the m-th order grating is represented. The center wavelength of the seed grating is represented by , and P represents the period of the sampling grating. Indicates the effective refractive index, This indicates the period of the seed grating.

[0068] Formula (5)

[0069] Refractive index The periodic change of the seed grating's period Λ denoted by z, which represents the refractive index of the seed grating, and z represents the transverse distance of the grating along the direction of light transmission.

[0070] Formula (6)

[0071] ∆λ represents the wavelength spacing of the reflection peaks of the sampling grating. The center wavelength of the seed grating is represented by , and P represents the period of the sampling grating. Indicates the effective refractive index.

[0072] In a waveguide, the effective coupling coefficient K is the reflectivity at a specific output wavelength (i.e., the m-th order supermode). m :

[0073] K m The grating shape function S(x,y) can be determined by the transverse overlap integral between the light field distribution E(x,y) at that wavelength and the grating shape function S(x,y), and the formula is as follows:

[0074] Formula (7)

[0075] K represents the m-th Fourier coefficient of the grating at the lateral y-position. m The effective coupling coefficient K represents the reflectivity of the m-th output wavelength. m , Let represent the light field distribution in the transverse y-direction, indicating the reflectivity of the m-th order output wavelength. For a two-dimensional sampling grating with a transversely graded duty cycle, the duty cycle varies in the transverse direction, which means... It is a function of spatial position y. By distributing the lateral duty cycle, the Fourier coefficients of higher-order modes can obtain a larger integral weight in the lateral direction, physically flattening the reflection spectrum envelope. This allows even the originally very weak higher-order supermodes to achieve high reflectivity, thus supporting a larger number of longitudinal modes with balanced intensity within the same resonant cavity. For modes of different wavelengths, in order to find the grating periodic region with the maximum coupling efficiency, the energy center can be slightly shifted laterally in the lateral direction (i.e., the main energy of light of different wavelengths is concentrated in different duty cycle regions in the lateral direction). This physically isolates and demultiplexes the originally stacked multi-wavelength longitudinal modes in the lateral direction.

[0076] See Figure 3 As shown in (a), the structure of the uniform sampling grating has a sampling period of P and a seed grating period of Λ. Figure 3In (b), by finely designing the spatial distribution of the grating duty cycle in the transverse direction (using Gaussian, hyperbolic, and parabolic distribution designs), the two-dimensional sampling grating can effectively change the overlapping integral weight of Fourier coefficients of different orders in the transverse direction, breaking the constraints of the traditional sinc function. This achieves the flattening of the reflection spectrum envelope and, in DFB lasers with large ridge widths, helps to suppress higher-order transverse modes and improve transverse single-mode performance.

[0077] When preparing a two-dimensional REC mask with a laterally gradient duty cycle, simulation software can be used to model and simulate the structure of a uniform sampling grating and a sampling grating with a laterally gradient duty cycle.

[0078] For example, the ridge width W of the DFB laser is selected to be 8 μm, the period Λ of the seed grating in the grating region is 227.9 nm, the period P of the sampling grating is 22.79 μm, and the duty cycle of the uniform grating is 0.5. The refractive index of the waveguide substrate material is 3.47, and the refractive index of the waveguide material is 3.48.

[0079] During the transverse single-mode verification process, the incident mode of the selected light source was TE1 mode, with a wavelength range of 1.5μm-1.8μm. A monitor was placed at each end of the grating waveguide to monitor the reflection and transmission spectra, respectively.

[0080] The specific simulation modeling steps are as follows:

[0081] Step 1: Construct a set of uniform seed gratings with a period Λ of 227.9 nm as the substrate, with a waveguide length of approximately 228 μm, and approximately 1000 sets of gratings.

[0082] Step 2: On the basis of the uniform seed grating, a set of large-period gratings with a sampling period of P are superimposed to construct a uniform sampling grating. Every 100 sets of sub-gratings constitute a large-period grating, and the length of its sampling period P is 22.79 μm.

[0083] Figure 4 A schematic diagram illustrating the use of an elliptical mask for a 2D REC mask. The expression for an elliptical mask is: Here, 'a' represents the length of the major axis of the ellipse, 'b' represents the length of the minor axis, and 'x' and 'y' represent the x and y coordinates of each point on the ellipse, respectively. In each major period P, the origin of the elliptical mask is set at 3 / 2P of each major period. Assume the longitudinal distance from point i on the grating region to the origin of the elliptical mask is 'd'. i Once d i Satisfies the elliptic expression, d i This represents the absolute value of point i. At this point, the horizontal duty cycle of point i is 2 × |z|. i |. For example Figure 4If the lateral duty cycle of point A is 2×|z A The horizontal duty cycle of point C is 2×|z C |, The horizontal duty cycle of point B is the largest, which is 2a, the major axis of the ellipse.

[0084] In this embodiment, the grating structure exhibits a lateral duty cycle variation that follows the changes at various points on the elliptical mask, satisfying the elliptical equation and showing a distribution trend where the duty cycle gradually increases from both ends of the grating towards the center, with the center having the largest duty cycle. Using the elliptical mask as the sampling area, the non-grating region of each sampling cycle is covered on the uniformly sampled grating structure. By controlling the lengths of the major and minor axes of the elliptical mask, the lateral characteristics of the grating are adjusted, thereby producing a gradual change in the lateral duty cycle of the grating region.

[0085] During the simulation process, the simulation area, mode light source, and monitor parameters are set to calculate and solve the reflection spectrum of the sampling grating.

[0086] Figure 5 The incident light source is set as follows: TE1 mode is used as the incident light source, with a wavelength range of 1.5μm-1.8μm. Figure 4 According to Figure 3 The reflection spectra of the simulated grating structures are compared between uniform sampling gratings and sampling gratings with laterally gradient duty cycles.

[0087] contrast Figure 5 The reflection spectrum comparison diagrams in (a) and (b) show that, under the same conditions (same mode light source settings), the reflectance of the sampling grating with a gradient duty cycle in the transverse direction is an order of magnitude lower than that of the uniform sampling grating. Therefore, the sampling grating with a gradient duty cycle in the transverse direction of this embodiment has a suppression effect on the first-order transverse mode, and can improve transverse single-mode performance.

[0088] Figure 6 The intermediate mode light source is set as follows: the fundamental mode is used as the incident mode, with a wavelength range of 1.5μm-1.8μm. Figure 5 According to Figure 3 The simulation results show a comparison of the reflection spectra of a uniform sampling grating and a sampling grating with a laterally gradient duty cycle. Under the same conditions (same mode light source settings), the sampling grating with a laterally gradient duty cycle excites more light beams in the spectrum compared to the uniform sampling grating.

[0089] If the axial spacing of the elliptical structure covering the vacant portion (non-grating region) of the sampling grating is increased in the longitudinal direction, then even with a larger difference in the duty cycle of the sampling grating in the transverse direction, more longitudinal modes are excited by the reflection spectrum of the grating. See also Figure 5 As shown, Figure 5Examples (b), (c), and (d) all use a sampling grating with a laterally gradient duty cycle under the same mode light source settings; the only difference is from... Figure 5 From (b) to (d), the longitudinal axial distance of the lateral duty cycle gradient grating increases sequentially. Figure 5 Figures (b), (c), and (d) show the simulated reflectance spectra of the longitudinal axial spacing of the lateral duty cycle graded grating at 4 μm, 5 μm, and 5.5 μm, respectively. Figure 5 As can be seen from the comparison of the reflection spectra of (b), (c), and (d), the intensity difference between the main peak and the two side peaks of the reflection spectrum becomes smaller. This allows the originally extremely weak high-order supermode to also obtain high reflectivity, thereby supporting more longitudinal modes with balanced intensity in the same resonant cavity. Therefore, a wider tunable multi-wavelength spectrum can be obtained, and the intensity between each wavelength is also more uniform and flat.

[0090] See Figure 7 As shown in the embodiments of this application, a method for fabricating a multi-wavelength tunable laser is also provided, comprising:

[0091] S701: A buffer layer, a lower cladding layer, an active layer, and a grating layer are grown sequentially on the substrate;

[0092] S702: A two-dimensional sampling grating with an hourglass-shaped profile is fabricated in the grating layer. The lateral duty cycle of each order grating in the two-dimensional sampling grating is determined based on the effective coupling coefficient of that order grating and the reflectivity of the output wavelength.

[0093] The lateral duty cycle of a two-dimensional sampling grating can be obtained, but is not limited to, through the following steps:

[0094] Determining the period Λ of the seed grating based on the Bragg wavelength:

[0095] Formula (8)

[0096] Formula (9)

[0097] in, Indicates the Bragg wavelength. Indicates the effective refractive index, Δλ represents the period of the seed grating, P represents the sampling period, and ∆λ represents the wavelength spacing of the reflection peaks of the two-dimensional sampling grating.

[0098] Formula (10)

[0099] k(z) represents the effective coupling coefficient of the two-dimensional sampling grating. This represents the maximum effective coupling coefficient of the grating. This represents the lateral width of the two-dimensional sampling grating. The ridge width of the ridge waveguide is represented by z, and the z-th period is represented by z.

[0100] For example, the two-dimensional REC mask of the two-dimensional sampling grating is elliptical in shape, and its longitudinal minor axis 2b can be obtained by the following formula:

[0101] 2b=W max -W min , formula (11)

[0102] W min W represents the waveguide width at the narrowest point (center point) of the hourglass shape of the two-dimensional sampling grating. max This represents the waveguide width at the widest point (edge ​​point) of the hourglass shape of the two-dimensional sampling grating.

[0103] Formula (12)

[0104] W represents the lateral width of the two-dimensional sampling grating. max Let 2b represent the waveguide width at the widest point (edge) of the hourglass shape of the two-dimensional sampling grating, 2b represent the longitudinal minor axis length of the two-dimensional REC mask (i.e., the elliptical mask), a represent half of the transverse major axis 2a of the two-dimensional REC mask, and z represent the transverse distance of the grating along the optical transmission direction, where z∈[-a,a]. The transverse major axis 2a is not less than half of the sampling period P, i.e., 1 / 2P.

[0105] An hourglass-shaped two-dimensional sampling grating can be fabricated by adjusting the major horizontal axis and minor vertical axis of the two-dimensional REC mask.

[0106] In an optional embodiment of this application, the multiple output wavelengths (e.g., 5 or 9 channels) need to have the same reflectivity. Therefore, the target reflectance spectrum needs to exhibit a flat-top characteristic in the frequency domain. According to Fourier transform theory, the flat-top characteristic in the frequency domain is mapped to the spatial domain for each channel to obtain the effective coupling coefficient k(z) of the grating. In a single sampling period P, k(z) is large at both ends and small in the middle of the sampling segment, thereby enhancing the reflectivity of the edge channels, thus ensuring that the multiple wavelengths output by multiple channels have the same reflectivity.

[0107] In this embodiment, the grating structure adjusts the effective coupling coefficient in the lateral space, causing the energy center to shift laterally in the lateral direction, resulting in an increase in the energy of the side peaks, thereby narrowing the energy gap with the main peak and obtaining a flatter comb spectrum.

[0108] A two-dimensional sampling grating with an hourglass-shaped profile can be used. Figures 1 to 2 The corresponding preparation process can also be carried out through the following steps:

[0109] Photoresist is coated on the grating layer, and one-dimensional high-frequency interference fringes are formed by dual-beam holographic interference exposure technology. The period of the one-dimensional high-frequency interference fringes is 227.9nm.

[0110] A two-dimensional sampling latent image is obtained by using ultraviolet stepping lithography to perform secondary exposure based on a two-dimensional REC mask. The two-dimensional reconstruction equivalent chirped REC mask can be a periodically repeating elliptical structure.

[0111] The grating layer is developed, etched, stripped of resist, and cleaned to obtain a two-dimensional sampling grating.

[0112] In one embodiment, the minor axis of the elliptical structure is greater than 0.5 times the sampling period of the two-dimensional sampling grating and not greater than 0.8 times the ridge width of the grating, and the sampling period length of the two-dimensional sampling grating is 22.79 μm.

[0113] In one embodiment, the preparation method further includes:

[0114] S703: Prepare an upper cladding layer above the grating layer;

[0115] S704: Etching the upper cladding to fabricate the ridge waveguide;

[0116] S705: Fabrication of a passivated insulating layer covering the ridge waveguide;

[0117] S706: n-type electrodes and p-type electrodes are fabricated on the back side of the substrate and the passivation insulating layer, respectively.

[0118] In one embodiment, the variation law of the transverse length of the two-dimensional sampling grating satisfies a hyperbolic function, a Gaussian function, or a parabolic function, and the period of the seed grating of the two-dimensional sampling grating is 227.9 nm, and the waveguide length is 228 μm.

[0119] In the process of preparing the sampling grating structure in this application embodiment, REC technology is used to prepare the sampling grating structure, which eliminates the expensive and extremely slow electron beam exposure. Holographic exposure can cover the entire wafer in one go, and ultraviolet step lithography can expose a die in a few seconds. Therefore, multi-wavelength two-dimensional DFB laser arrays can be mass-produced to obtain multi-wavelength tunable lasers.

[0120] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0121] The above embodiments are merely illustrative of several implementation methods of this application, and their descriptions are relatively specific and detailed. However, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application.

Claims

1. A multi-wavelength tunable laser, characterized in that, include: A two-dimensional sampling grating with an hourglass-shaped profile is fabricated on the grating layer of the laser; The lateral duty cycle of each order grating in the two-dimensional sampling grating is determined based on the effective coupling coefficient of that order grating and the reflectivity of the output wavelength.

2. The laser according to claim 1, characterized in that, Also includes: The substrate, a buffer layer, a lower cladding layer, an active layer, a grating layer, a ridge waveguide, a passivation insulating layer, and an n-type electrode and a p-type electrode located on the back side of the substrate and the passivation insulating layer, respectively.

3. The laser according to claim 1, characterized in that, The two-dimensional sampling grating has a gradually changing duty cycle in the lateral direction, and the variation law of the lateral length of the two-dimensional sampling grating satisfies a hyperbolic function, a Gaussian function, or a parabolic function.

4. The laser according to claim 1, characterized in that, The shape of the vacant portion between every two periods in the two-dimensional sampling grating is an elliptical structure. The minor axis of the elliptical structure is greater than 0.5 times the sampling period of the two-dimensional sampling grating and not greater than 0.8 times the ridge width of the grating. The sampling period length of the two-dimensional sampling grating is 22.79 μm.

5. The laser according to claim 1, characterized in that, The seed grating of the two-dimensional sampling grating has a period of 227.9 nm and a waveguide length of 228 μm.

6. A method for fabricating a multi-wavelength tunable laser, characterized in that, include: A buffer layer, a lower cladding layer, an active layer, and a grating layer are grown sequentially on the substrate. A two-dimensional sampling grating with an hourglass-shaped profile is fabricated in the grating layer. The lateral duty cycle of each order grating in the two-dimensional sampling grating is determined based on the effective coupling coefficient of that order grating and the reflectivity of the output wavelength.

7. The preparation method according to claim 6, characterized in that, Fabricating a two-dimensional sampling grating with an hourglass-shaped profile in the grating layer includes: Photoresist is coated on the grating layer, and one-dimensional high-frequency interference fringes are formed by dual-beam holographic interference exposure technology. The period of the one-dimensional high-frequency interference fringes is 227.9 nm. A two-dimensional sampling latent image is obtained by secondary exposure using ultraviolet stepping lithography based on a two-dimensional reconstruction equivalent chirped REC mask, wherein the two-dimensional REC mask includes a periodically repeating elliptical structure. The grating layer is developed, etched, stripped of resist, and cleaned to obtain a two-dimensional sampling grating.

8. The preparation method according to claim 7, characterized in that, The minor axis of the elliptical structure is greater than 0.5 times the sampling period of the two-dimensional sampling grating and not greater than 0.8 times the ridge width of the grating. The sampling period length of the two-dimensional sampling grating is 22.79 μm.

9. The preparation method according to claim 6, characterized in that, Also includes: An upper cladding layer is prepared above the grating layer; The upper cladding is etched to fabricate a ridge waveguide; A passivated insulating layer is prepared to cover the ridge waveguide; n-type electrodes and p-type electrodes were fabricated on the back side of the substrate and on the passivation insulating layer, respectively.

10. The preparation method according to claim 6, characterized in that, The variation law of the transverse length of the two-dimensional sampling grating satisfies a hyperbolic function, a Gaussian function, or a parabolic function. The seed grating of the two-dimensional sampling grating has a period of 227.9 nm and a waveguide length of 228 μm.

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