Single transverse mode deep ridge waveguide CW-DFB laser and preparation method thereof

Abstract

The invention relates to the technical field of semiconductor lasers for optical communication, in particular to a single-transverse-mode deep-ridge waveguide CW-DFB laser and a preparation method thereof, and the method comprises the steps: designing a main waveguide and a passive waveguide, coupling a high-order mode of a main ridge to an edge ridge through a coupling resonance principle, not carrying out the current injection of the edge ridge, and finally eliminating a high-order transverse mode of the main ridge; on the basis of increasing the output power of the wide-ridge waveguide, a quantum well region between a main ridge and an edge ridge is etched, and Fe-InP with a lower refractive index than that of an original quantum well is filled in order to ensure that a high-order transverse mode can be coupled to the edge ridge, so that the transverse optical limiting factor of the main ridge is improved, and the threshold current is reduced while the light beam quality is ensured; and the Fe-InP filling material between the main ridge and the side ridge has higher thermal conductivity than the original quantum well, so that the heat dissipation capability of the device can be further improved, the high-temperature saturation current can be increased, the signal distortion can be reduced, and the strict requirement of long-distance communication on wavelength stability can be met.

Description

Technical Field

[0001] This application relates to the field of semiconductor laser technology for optical communication, and in particular to a single transverse mode deep ridge waveguide CW-DFB laser and its fabrication method. Background Technology

[0002] As optical communication systems upgrade to Tb-level bandwidth, there is a need to further suppress signal distortion. One technical approach is to increase the power of the optical transmitter to ensure that the receiver maintains high optical power even after the high-power signal is attenuated by the fiber. Traditional continuous wave distributed feedback (CW-DFB) semiconductor lasers require wide waveguides of over 5μm and ultra-long cavity lengths of 3mm to achieve high-power output. This approach presents a challenge in synergistically optimizing threshold current and power performance. Although this architecture can meet the power requirements of optical communication systems, several key technical constraints exist:

[0003] 1. The incompatibility of performance parameters

[0004] Power-Threshold: Although increasing the ridge width increases the output power, according to the threshold current calculation formula I... th =e(wdL)N th / τ s (where e is the electron charge, w, d, and L are the ridge width, quantum well thickness, and cavity length, respectively, and N is the...) th and τ s (These are the threshold carrier density and the spontaneous emission lifetime of carriers, respectively). As the ridge width increases, the threshold current also increases.

[0005] Efficiency-thermal resistance contradiction: Although the multi-quantum-well structure improves the optical confinement factor, the increased thickness of the active region leads to a longer heat conduction path, and the thermal resistance exceeds 40K / W. The threshold current also increases at a high temperature of 85℃, forming a vicious cycle of "efficiency improvement-heat accumulation-performance degradation".

[0006] 2. Inherent deficiencies in environmental adaptability

[0007] Poor temperature stability: Traditional buried heterojunction BH structures exhibit wavelength shift of 0.12nm / ℃ over a wide temperature range of -40℃ to 85℃. Even with TEC temperature control, additional power consumption is still required, which contradicts the trend of low power consumption in optical modules.

[0008] 3. Collaborative bottlenecks in structural design

[0009] Spatial hole burning effect: The increase in the number of quantum well layers leads to uneven distribution of charge carriers in the vertical direction, and the fluctuation range of the side mode suppression ratio (SMSR) expands under high power, affecting the bit error rate of signal transmission;

[0010] Lateral higher-order modes cause distortion of optical power curve: Wide waveguide structures inevitably generate higher-order modes under high current injection, which leads to distortion of the optical power curve of the fundamental mode and ultimately reduces the output power.

[0011] In existing technologies, to address the high threshold current introduced by large-area lasers while generating high output power, the following solutions are mainly employed: 1. Co-optimization of geometric parameters in the ridge width-cavity length dimension: Increasing the typical ridge waveguide width from 2μm to 7μm can reduce the surface current density, but this requires shortening the cavity length to reduce the threshold current. 2. Enhanced optical confinement design at the quantum well level: Stacking multi-quantum-well (MQW) structures, such as increasing from 3 wells to 7 wells, can improve the optical confinement factor Γ from 0.72 to 0.89, increasing carrier radiative recombination efficiency by 15%. However, at a high temperature of 85℃, the threshold current increment reaches as high as 45mA, and the spatial hole-burning effect caused by uneven carrier distribution among quantum wells expands the SMSR fluctuation range to 20dB. 3. Vertical gradient modulation of doping concentration: Reducing the waveguide layer doping concentration can reduce free carrier absorption loss, thereby reducing the threshold current, but low doping leads to a decrease in electrical injection efficiency and a reduction in output power.

[0012] The above solutions all focus on optimizing a single physical dimension: geometric parameter modulation sacrifices high-frequency response, quantum well stacking exacerbates heat accumulation, and doping optimization worsens electrical performance. Under the comprehensive requirements of "high power + low threshold + wide operating temperature" for optical communication lasers, traditional methods cannot break through the triangular constraint of "threshold current - heat dissipation performance - power stability," and a collaborative optimization mechanism needs to be built from the perspective of waveguide structure innovation. Summary of the Invention

[0013] In view of this, the purpose of this invention is to provide a single transverse mode deep ridge waveguide CW-DFB laser and its fabrication method, so as to solve the problems of excessively high threshold current, low power performance and narrow operating temperature range in the prior art.

[0014] The technical solution of this invention is:

[0015] In a first aspect, embodiments of the present invention provide a single transverse mode deep ridge waveguide CW-DFB laser, comprising:

[0016] The substrate layer has a base epitaxial layer, a secondary epitaxial structure, and a surface functional layer epitaxially grown on top of it. A main waveguide and a passive waveguide are grown on top of the surface functional layer. The base epitaxial layer, the secondary epitaxial structure, and the surface functional layer have a first trench, a second trench, and a third trench formed by etching. The first trench, the second trench, and the third trench are all filled with an Fe-InP filling layer.

[0017] Furthermore, the main waveguide is located above the region between the first trench and the second trench; the passive waveguide is located above the region between the second trench and the third trench.

[0018] Furthermore, the substrate layer is an N-InP substrate, and a basic epitaxial layer is epitaxially grown on the N-InP substrate; the basic epitaxial layer is epitaxially grown sequentially as follows: a first N-InP buffer layer, an N-InGaAsP stop etch layer, a second N-InP buffer layer, an N-InGaAsP lower confinement layer, an InGaAsP lower waveguide layer, an InGaAsP quantum well active region, an InGaAsP upper waveguide layer, a P-InGaAsP upper confinement layer, a P-InP buffer layer, and a P-InGaAsP grating layer.

[0019] Furthermore, a secondary epitaxial structure is epitaxially grown above the P-InGaAsP grating layer, and the secondary epitaxial structure is a P-InP grating buried layer.

[0020] Furthermore, a surface functional layer is epitaxially grown above the P-InP grating buried layer; the surface functional layer is epitaxially grown in sequence with a P-InP interconnect layer, a first P-InGaAsP barrier gradient layer, a second P-InGaAsP barrier gradient layer and a P-InGaAs ohmic contact layer.

[0021] Furthermore, the Fe-InP filling layer is in contact with the stop etching layer.

[0022] Furthermore, the period of the P-InGaAsP grating layer is 202nm, and it is completely covered by the P-InP grating buried layer.

[0023] Secondly, embodiments of the present invention also provide a method for fabricating a single transverse mode deep ridge waveguide laser, comprising the following steps:

[0024] First epitaxy: The basic epitaxial layer is grown sequentially on the N-InP substrate;

[0025] Second epitaxy: A grating pattern is fabricated on the surface of the base epitaxial layer, and a P-InP grating buried layer is grown;

[0026] Third epitaxy: Etching the inter-region between the main waveguide and the passive waveguide, and filling the etched trench with a Fe-InP filling layer;

[0027] Ridge waveguide fabrication: After three epitaxial cycles, a starting wafer is obtained, and a double-ridge structure is formed by photolithography and wet etching; SiO2 insulating layer and metal electrodes are deposited.

[0028] Furthermore, the etching in the third epitaxial step is performed using ICP dry etching, and the etching endpoint is controlled by the N-InGaAsP stop etching layer.

[0029] Furthermore, the wet etching process uses a 0.5% (w / v) freezing point bromine aqueous solution and a 7% (w / w) freezing point HCl solution.

[0030] This invention, based on a wide-ridge waveguide structure, fully considers the performance in terms of output power, single-mode characteristics, and threshold current, and proposes a high thermal conductivity Fe-InP filled double-ridge waveguide structure, achieving a technological breakthrough through the following three synergistic innovations:

[0031] 1. Eliminate high-order transverse molds by adding side ridges.

[0032] The design incorporates a main waveguide ridge and a passive waveguide side ridge. By using the coupling resonance principle, the higher-order modes of the main ridge are coupled to the side ridges. No current is injected into the side ridges, thus eliminating the higher-order transverse modes of the main ridge.

[0033] 2. Output power and threshold current optimization

[0034] Based on increasing the output power of the wide-ridge waveguide, the quantum well region between the main ridge and the side ridge is etched away. At the same time, in order to ensure that the higher-order transverse modes can be coupled to the side ridge, Fe-InP with a lower refractive index than the original quantum well is filled to improve the transverse optical confinement factor of the main ridge, thereby ensuring beam quality while reducing the threshold current.

[0035] 3. Enhanced thermal management

[0036] The Fe-InP filling material located between the main ridge and the side ridge has a higher thermal conductivity than the original quantum well, which can further improve the heat dissipation capacity of the device and increase the high-temperature saturation current. Attached Figure Description

[0037] Figure 1 This is a schematic diagram of the structure after the first extension;

[0038] Figure 2 This is a schematic diagram of the structure after the second extension;

[0039] Figure 3 This is a schematic diagram of the structure with an Fe-InP filling layer after the third epitaxy;

[0040] Figure 4 This is a schematic diagram of a ridge waveguide structure;

[0041] Figure 5 This is a schematic diagram of the optical field of a single-ridge wide waveguide;

[0042] Figure 6 This is a schematic diagram of the optical field of a double-ridged waveguide with an Fe-InP filling layer.

[0043] In the figure, 001 is the N-InP substrate; 002 is the first N-InP buffer layer; 003 is the N-InGaAsP stop etch layer; 004 is the second N-InP buffer layer; 005 is the N-InGaAsP lower confinement layer; 006 is the InGaAsP lower waveguide layer; 007 is the InGaAsP quantum well active region; 008 is the InGaAsP upper waveguide layer; 009 is the P-InGaAsP upper confinement layer; 010 is the P-InP buffer layer; 011 is the P-InGaAsP grating layer; 012 is the P-InP grating buried layer; 013 is the P-InP connection layer; 014 is the first P-InGaAsP barrier gradient layer; 015 is the second P-InGaAsP barrier gradient layer; 016 is the P-InGaAs ohmic contact layer; 017 is the Fe-InP filling layer; 101 is the main waveguide; and 102 is the passive waveguide. Detailed Implementation

[0044] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0045] The terms "first" and "second," etc., used in the specification and claims herein are used to distinguish different objects, not to describe a specific order of objects. In the description of embodiments of this application, unless otherwise stated, "a plurality of" means two or more, for example, "a plurality of processing units" means two or more processing units, "a plurality of elements" means two or more elements, etc.

[0046] In the embodiments of this application, the terms "exemplary" or "for example" are used to indicate that something is an example, illustration, or description. Any embodiment or design that is described as "exemplary" or "for example" in the embodiments of this application should not be construed as being more preferred or advantageous than other embodiments or designs. Specifically, the use of the terms "exemplary" or "for example" is intended to present the relevant concepts in a specific manner.

[0047] Key parameter design:

[0048] The main waveguide supports N≥2 modes. The mode overlap rate between the higher-order modes in the main waveguide and the fundamental mode in the passive waveguide, as well as the resonance sensitivity, must be considered simultaneously. Therefore, the ridge width range W of the main waveguide 101 is... m The ridge width ranges from 4µm to 10µm; the passive waveguide 102 has a ridge width range of W. pThe distance S between the main waveguide 101 and the passive waveguide 102 is 2um-6um. The quantum well layer between the main ridge and the side ridge is etched away. The etching stops at the N-InGaAsP stop etching layer 003 and is filled with a high thermal conductivity Fe-InP filling layer 017.

[0049] Please refer to Figures 1-4 This embodiment proposes a method for fabricating a single transverse mode deep ridge waveguide CW-DFB laser, which employs a secondary epitaxial process combined with photolithography etching technology. The fabrication process includes the following four steps:

[0050] S1. First extension (basic structure growth)

[0051] Process conditions: MOCVD equipment, substrate is N-InP filled layer 001;

[0052] Growth sequence:

[0053] The first N-InP buffer layer 002 has a thickness of 500 nm and is used to improve the flatness of the substrate surface.

[0054] N-InGaAsP stop etch layer 003: thickness 15nm, wavelength 960nm-860nm, doping concentration 2×10⁻⁶ 18 cm -3 ;

[0055] Second N-InP buffer layer 004: 1600 nm thick, doping concentration 2 × 10⁻⁶ 18 cm -3 ;;

[0056] N-InGaAsP lower confinement layer 005: thickness 100nm, wavelength 1000nm, doping concentration 1×10⁻⁶ 18 cm -3 ;

[0057] InGaAsP waveguide layer 006: thickness 50nm, wavelength 1057nm;

[0058] InGaAsP quantum well active region 007: InGaAsP multi-quantum-well structure, quantum barrier thickness 10nm, wavelength 1120nm, quantum well thickness 5nm, wavelength 1120nm.

[0059] InGaAsP waveguide layer 008: thickness 25nm, wavelength 1057nm;

[0060] P-InGaAsP confinement layer 009: thickness 75nm, wavelength 1000nm, doping concentration 1×10⁻⁶ 18 cm -3 ;

[0061] P-InP buffer layer 010: thickness 80nm;

[0062] P-InGaAsP grating layer 011: 30nm thick, used to form a distributed feedback DFB grating;

[0063] Formed after one extension Figure 1 The basic stacked structure shown has the P-InGaAs grating layer 011 located on top.

[0064] S2. Second epitaxy (fabrication of grating buried layer)

[0065] Photolithography process: A grating pattern with a period of 202nm is fabricated on the surface of the P-InGaAsP grating layer 011 by holographic exposure or electron beam exposure;

[0066] Epitaxial growth:

[0067] P-InP grating buried layer 012: 300 nm thick, doped with a doping concentration of 1×10⁻⁶, deposited using a pulse deposition method. 18 cm -3 Completely covers the P-InGaAsP grating layer 011;

[0068] P-InP connection layer 013: 300nm thick, connecting P-InP grating buried layer 012 with first P-InGaAsP barrier gradient layer 014 and second P-InGaAsP barrier gradient layer 015.

[0069] First P-InGaAsP barrier gradient layer 014, second P-InGaAsP barrier gradient layer 015: each layer is 50 nm thick, and the concentration of the first P-InGaAsP barrier gradient layer 014 is 1 × 10⁻⁶. 18 cm -3 The concentration of the second P-InGaAsP barrier gradient layer is 5 × 10⁻⁶. 19 cm -3 ;

[0070] P-InGaAs ohmic contact layer 016: 200 nm thick, doping concentration 5 × 10⁻⁶ 19 cm -3 This reduces contact resistance.

[0071] Formed after secondary extension Figure 2 The buried grating structure shown.

[0072] S3. Third epitaxy (Preparation of Fe-InP filled layer)

[0073] 1. Starting wafer: obtained after secondary epitaxy;

[0074] 2. PECVD SiO2 masking: A layer of SiO2 is grown on the cleaned wafer using PECVD;

[0075] 3. Photoresist deposition: The masked wafer is fixed on the spin coater's turntable. Photoresist is uniformly coated onto the wafer surface in liquid form;

[0076] 4. Photoresist Patterning: After coating, the wafer undergoes soft baking, usually on a hot plate, to remove solvents from the photoresist and make it more stable. Before the photoresist cures, the area between the double ridges is exposed to ultraviolet light. After exposure, the wafer is immersed in a developer to remove the exposed photoresist and form the desired pattern. After development, the wafer typically undergoes hard baking to enhance the mechanical strength and corrosion resistance of the photoresist, preparing it for subsequent etching or deposition steps.

[0077] 5. Etching: Next, ICP dry etching is used to etch down to the N-InGaAsP stop etching layer 003, forming the first trench, the second trench and the third trench within the etching range;

[0078] 6. Tertiary epitaxy: Fe-InP is filled into the first, second, and third trenches formed by etching via MOCVD as Fe-InP filling layer 017;

[0079] Formed after three extensions Figure 3 The structure shown is 017 with an Fe-InP filling layer.

[0080] S4. Ridge waveguide fabrication:

[0081] 1. Starting wafer: obtained after three epitaxial growth cycles;

[0082] 2. PECVD SiO2 masking: A layer of SiO2 is grown on the cleaned wafer using PECVD;

[0083] 3. Photoresist Patterning: After coating, the wafer undergoes soft baking, typically performed on a hot plate, to remove solvents from the photoresist, making it more stable. Before the photoresist cures, the wafer is exposed to ultraviolet light, and the double-ridge waveguide pattern is transferred onto the photoresist using a mask. After exposure, the wafer is immersed in a developer to remove the exposed photoresist, forming the desired pattern. After development, the wafer typically undergoes hard baking to enhance the mechanical strength and corrosion resistance of the photoresist, preparing it for subsequent etching or deposition steps.

[0084] 4. Ridge Etching: Next, wet etching with freezing point bromine aqueous solution and freezing point HCl solution is used to remove the material under the photoresist, forming a ridge structure;

[0085] 5. Photoresist removal: Photoresist is removed using chemical solvents;

[0086] 6. Oxide deposition: A SiO2 insulating layer is grown by PECVD to achieve electrical isolation between the p electrode and InP, while reducing dangling bonds on the ridge waveguide surface.

[0087] 7. Photoresist deposition: Photoresist is deposited again;

[0088] 8. Photoresist patterning: Patterning is performed again;

[0089] 9. Oxide etching: Oxides are etched using hydrofluoric acid solution to form p-electrode windows;

[0090] 10. Photoresist removal: Remove the photoresist again;

[0091] 11. p-contact metal deposition: Depositing metal compounds onto the front side of a wafer using magnetron sputtering or electroplating processes;

[0092] 12. Photoresist deposition: Photoresist is deposited again on the front side of the wafer;

[0093] 13. Photoresist patterning: Patterning is performed again;

[0094] 14. Metal etching: Excess metal is etched using a metal etching solution or HF solution to form electrode patterns;

[0095] 15. Photoresist removal: Remove the photoresist again;

[0096] 16. Flexible metal deposition / patterning: Depositing metal compounds onto wafers using magnetron sputtering or electroplating processes;

[0097] 17. Wafer thinning: The substrate is thinned by about 120um through mechanical grinding;

[0098] 18. Metal deposition on the back of the wafer: Metal compounds are deposited onto the wafer using magnetron sputtering or electroplating processes.

[0099] The double-ridge waveguide structure after the ridge waveguide is fabricated is as follows: Figure 4 As shown.

[0100] The beneficial effects of the single transverse mode deep ridge waveguide CW-DFB laser and its fabrication method provided in this embodiment are as follows:

[0101] In W m =7.5um, W p With parameters S = 4.0µm and S = 3.2µm:

[0102] Please see Figure 5A structure with only one ridge will exhibit higher-order modes. With current injection, mode competition will cause the optical power current curve to distort, affecting the final output power.

[0103] Please see Figure 6 The double-ridge waveguide structure with Fe-InP filling layer can filter out the higher-order films of the main ridge through coupling resonance by the side ridge without current injection, ensuring the laser operates in single mode. At the same time, the lateral optical confinement factor is increased and the threshold current is reduced. The actual measurement shows that, compared with the single-ridge structure, the double-ridge waveguide structure with Fe-InP filling layer can reduce the threshold current to 140mA (a decrease of 22%) and increase the saturation current at 85℃ to 350mA (an increase of 30%).

[0104] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications and substitutions should be covered within the scope of the claims of the present invention. Technical aspects, shapes, and structures not described in detail in this invention are all well-known technologies.

Claims

1. A single transverse mode deep ridge waveguide CW-DFB laser, characterized in that, include: The substrate layer has a base epitaxial layer, a secondary epitaxial structure and a surface functional layer epitaxially grown on top of it. A main waveguide (101) and a passive waveguide (102) are grown on top of the surface functional layer. The base epitaxial layer, the secondary epitaxial structure and the surface functional layer have a first trench, a second trench and a third trench formed by etching. The first trench, the second trench and the third trench are all filled with a Fe-InP filling layer (017).

2. The single transverse mode deep ridge waveguide CW-DFB laser according to claim 1, characterized in that, The main waveguide (101) is located above the region between the first trench and the second trench; the passive waveguide (102) is located above the region between the second trench and the third trench.

3. A single transverse mode deep ridge waveguide CW-DFB laser according to claim 1, characterized in that, The substrate layer is an N-InP substrate (001), and a basic epitaxial layer is epitaxially grown on the N-InP substrate (001); the basic epitaxial layer sequentially grows a first N-InP buffer layer (002), an N-InGaAsP stop etch layer (003), a second N-InP buffer layer (004), an N-InGaAsP lower confinement layer (005), an InGaAsP lower waveguide layer (006), an InGaAsP quantum well active region (007), an InGaAsP upper waveguide layer (008), an P-InGaAsP upper confinement layer (009), a P-InP buffer layer (010), and a P-InGaAsP grating layer (011).

4. A single transverse mode deep ridge waveguide CW-DFB laser according to claim 3, characterized in that, A secondary epitaxial structure is epitaxially grown above the P-InGaAsP grating layer (011), and the secondary epitaxial structure is a P-InP grating buried layer (012).

5. A single transverse mode deep ridge waveguide CW-DFB laser according to claim 4, characterized in that, A surface functional layer is epitaxially grown above the P-InP grating buried layer (012); the surface functional layer is epitaxially grown in sequence with a P-InP interconnect layer (013), a first P-InGaAsP barrier gradient layer (014), a second P-InGaAsP barrier gradient layer (015) and a P-InGaAs ohmic contact layer (016).

6. A single transverse mode deep ridge waveguide CW-DFB laser according to claim 3, characterized in that, The Fe-InP filling layer (017) is in contact with the N-InGaAsP stop etching layer (003).

7. A single transverse mode deep ridge waveguide CW-DFB laser according to claim 4, characterized in that, The P-InGaAsP grating layer (011) has a period of 202nm and is completely covered by the P-InP grating buried layer (012).

8. A method for fabricating a single transverse mode deep ridge waveguide laser, characterized in that, Includes the following steps: First epitaxy: The basic epitaxial layer is grown sequentially on the N-InP substrate (001); Second epitaxy: A grating pattern is fabricated on the surface of the base epitaxial layer, and a P-InP grating buried layer (012) is grown; Third epitaxy: Etching the inter-region between the main waveguide (101) and the passive waveguide (102), and filling the etched trench with a Fe-InP filling layer (017); Ridge waveguide fabrication: After three epitaxial cycles, a starting wafer is obtained, and a double-ridge structure is formed by photolithography and wet etching; SiO2 insulating layer and metal electrodes are deposited.

9. A method for fabricating a single transverse mode deep ridge waveguide laser according to claim 8, characterized in that, The etching in the third epitaxial step is performed using ICP dry etching, and the etching endpoint is controlled by the N-InGaAsP stop etching layer (003).

10. A method for fabricating a single transverse mode deep ridge waveguide laser according to claim 8, characterized in that, The wet etching process uses a 0.5% (w / v) freezing point bromine aqueous solution and a 7% (w / v) freezing point HCl solution.

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