Narrow-linewidth laser based on cooperation of different gratings and misaligned phase shifts

Abstract

This invention relates to the field of semiconductor laser technology, specifically to a narrow linewidth laser based on the synergy of different inner and outer gratings and staggered phase shift, comprising: an n-type substrate; an active region disposed on the n-type substrate; at least one inner grating disposed above or below the active region, wherein the inner grating is a first-order refractive index coupled grating of semiconductor material with a period of Λ1, and the Bragg wavelength of the inner grating is aligned with the center wavelength λ0; and a top grating disposed on the top of the laser, wherein n_eff1 of the inner grating is not equal to n_eff2 of the top grating, and satisfies Λ1 / Λ2=n_eff2 / n_eff1, so that the two gratings have overlapping reflection peaks at λ0. The beneficial effects of this invention are as follows: significantly improved side-mode suppression ratio: the reflection phase difference at the side-mode frequency caused by the dual-period grating leads to destructive interference, achieving an SMSR of 55-60dB, which is superior to the traditional single-grating DFB (45-50dB).

Description

Technical Field

[0001] This invention relates to the field of semiconductor laser technology, specifically to a narrow linewidth laser based on different internal and external gratings and a staggered phase shift synergy. Background Technology

[0002] Semiconductor distributed feedback (DFB) lasers are widely used in fiber optic communication, lidar, quantum key distribution, and other fields due to their advantages such as good single-mode stability, controllable wavelength, and small size. As application scenarios continue to demand higher requirements for coherent detection and high-precision measurement, higher requirements are being placed on the linewidth and side-mode rejection ratio (SMSR) of DFB lasers (linewidth < 100 kHz, SMSR > 50 dB).

[0003] Traditional DFB lasers typically employ a single refractive index-coupled grating (a first-order grating, optionally with a λ / 4 phase-shift structure). Its linewidth is limited by the coupling coefficient κ and cavity length, making further narrowing difficult. Simultaneously, the side-mode suppression ratio is constrained by the grating's sidelobe characteristics, typically only reaching 45-50 dB. To overcome this bottleneck, researchers have proposed dual-grating structures, such as placing two gratings with identical periods in the vertical direction to enhance coupling. However, this method requires precise control of the longitudinal distance between the two gratings (on the order of half a wavelength), which is technically challenging and difficult to mass-produce. Other approaches have attempted to use dual gratings with different periods, but their cooperative mechanism remains unclear, often leading to mode competition or linewidth broadening.

[0004] In addition, existing dual-grating solutions mostly use dual semiconductor gratings, which require three epitaxy processes (lower grating → active region → upper grating → upper cladding), resulting in complex processes, high costs, low yields, and difficulty in industrial mass production.

[0005] In previous research, the inventors discovered that by rationally designing the period ratio and vertical distance between the inner and top gratings, the interference destructive effect of the reflected light from the dual gratings in the side-mode region can significantly improve the SMSR (Supervisory Surge Laser). Simultaneously, the superposition of shallow grooves and double-layer gratings can effectively suppress the spatial hole-burning effect, further narrowing the linewidth. Based on this, the present invention proposes a narrow-linewidth laser based on the synergy of different inner and outer gratings and staggered phase shift. Summary of the Invention

[0006] To address the problems mentioned in the background section, the present invention provides the following technical solution:

[0007] Narrow-linewidth lasers based on different inner and outer gratings and staggered phase shift synergy include:

[0008] n-type substrate;

[0009] Active region disposed on the n-type substrate;

[0010] At least one internal grating is disposed above or below the active region. The internal grating is a first-order refractive index coupled grating of semiconductor material with a period of Λ1. The Bragg wavelength of the internal grating is aligned with the center wavelength λ0.

[0011] A top grating is disposed on top of the narrow linewidth laser based on different inner and outer gratings and staggered phase shift synergy and is combined with the electrode layer. The top grating is a first-order grating with a period of Λ2 and Λ2≠Λ1. The Bragg wavelength of the top grating is also aligned with λ0. The top grating also serves as the p-type electrode of the narrow linewidth laser based on different inner and outer gratings and staggered phase shift synergy.

[0012] The inner grating and the top grating have different effective refractive indices at their respective locations, i.e., n_eff1 of the inner grating ≠ n_eff2 of the top grating, and satisfy Λ1 / Λ2=n_eff2 / n_eff1, so that the two gratings have overlapping reflection peaks at λ0.

[0013] An antireflective film is disposed on the front cavity surface of the narrow linewidth laser based on different inner and outer gratings and staggered phase shift synergy, and a high reflective film is disposed on the rear cavity surface of the narrow linewidth laser based on different inner and outer gratings and staggered phase shift synergy.

[0014] The narrow linewidth laser based on different inner and outer gratings and staggered phase shift is an InP-based laser with a center wavelength λ0=1550nm. The inner grating period Λ1=235-245nm, the top grating period Λ2=280-320nm, and the period ratio Λ2 / Λ1=1.15-1.35.

[0015] As a preferred embodiment of the above technical solution, the vertical distance Δz between the inner grating and the top grating is 1.5-2.5μm. This distance ensures that the phase difference between the reflected light from the two gratings is close to an odd multiple of π within a range of ±100 GHz from the center frequency, resulting in destructive interference. This leads to a higher side-mode suppression ratio and a narrower laser linewidth than that of a single grating structure.

[0016] As a preferred embodiment of the above technical solution, the internal grating is a uniform grating without phase shift, and the top grating has a λ0 / 4 phase shift structure at the middle of its length direction.

[0017] As a preferred embodiment of the above technical solution, at least one λ0 / 6 phase shift structure is provided in the internal grating, and the spacing between the two grating segments after phase shift is 5λ0 / 12, and the top grating is a uniform periodic grating without phase shift.

[0018] As a preferred embodiment of the above technical solution, the top grating is a metal grating, which is formed into an alloyed ohmic contact through rapid thermal annealing and has a thickened gold layer electroplated on its surface to reduce contact resistance and enhance mechanical strength; the rapid thermal annealing conditions are 380-420℃ for 30-60s, and the thickness of the electroplated thickened gold layer is 1-2μm; the sidewall tilt angle of the metal grating is 80°-85°.

[0019] As a preferred embodiment of the above technical solution, the internal grating is disposed within 100 nm above or below the active region, and the vertical distance Δz between the internal grating and the top grating is 1.5-2.5 μm.

[0020] As a preferred embodiment of the above technical solution, a temperature control module is also included, which includes a TEC thermoelectric cooler and a temperature sensor, with a temperature control accuracy of ±0.01℃.

[0021] As a preferred embodiment of the above technical solution, the narrow linewidth laser based on different inner and outer gratings and staggered phase shift synergy is fabricated by the following steps:

[0022] S1. Active region and grating support layer are epitaxially grown on n-type substrate by MOCVD;

[0023] S2. An internal grating with a period of Λ1 is fabricated by electron beam lithography and ICP etching.

[0024] S3. Continue MOCVD epitaxial growth of the cladding layer;

[0025] S4. The back waveguide is formed by photolithography and ICP etching;

[0026] S5. A top grating with a period of Λ2 is prepared on the top of the back waveguide by electron beam lithography, metal evaporation and lift-off processes. The top grating also serves as a p electrode.

[0027] S6. Perform rapid thermal annealing on the top grating to form alloyed ohmic contacts;

[0028] S7. Thicken the gold layer on the surface of the top grating using an electroplating process;

[0029] S8. Thin the n-type substrate and deposit the n-type electrode;

[0030] S9. Cleavage the cavity surface and coat it with a high-reflection film and an anti-reflection film;

[0031] S10, integrates temperature control module and is packaged.

[0032] As a preferred embodiment of the above technical solution, the photolithography accuracy of the inner grating is ±2nm, and the etching depth accuracy is ±5nm; the photolithography accuracy of the top grating is ±5nm, and the inner grating and the top grating do not require sub-nanometer alignment.

[0033] This invention provides a narrow linewidth laser based on the synergy of different internal and external gratings and misaligned phase shift, which has the following advantages compared with the prior art:

[0034] The side-mode suppression ratio is significantly improved: the interference cancellation is caused by the reflection phase difference at the side-mode frequency through the dual-period grating, and the SMSR can reach 55-60dB, which is better than the traditional single-grating DFB (45-50dB).

[0035] The laser linewidth is significantly narrowed: the measured linewidth in Example 1 is 42kHz, which is better than that of traditional DFB (50-100kHz); the second technical solution is theoretically expected to reach 1-5kHz (further process optimization is required), which can meet the application requirements of coherent communication, high-precision FMCWLiDAR and other applications that are extremely sensitive to linewidth.

[0036] The spatial hole burning effect is effectively suppressed (Examples 2 and 3): The superposition of shallow grooves and double-layer light field reduces the standard deviation of longitudinal light field distribution by more than 40%, significantly weakens the spatial non-uniformity of carrier density, and ensures single-mode stability and narrow linewidth characteristics under high power.

[0037] High process tolerance and no need for three epitaxy cycles: The two grating periods are independently controlled, eliminating the need for sub-nanometer alignment. The internal grating lithography accuracy is ±2nm, and the top grating lithography accuracy is ±5nm, comparable to conventional DFB processes. Compared to the dual-semiconductor grating solution (requiring three epitaxy cycles), this solution only requires two epitaxy cycles, reducing costs by 20%-30% and improving yield.

[0038] Electrode integration ensures reliable contact: The top grating also serves as the p-electrode, forming an alloyed ohmic contact via RTA, with a contact resistivity <1×10⁻⁶. -5 Ω·cm 2 The electroplated thick gold layer and sloping sidewall design significantly improve mechanical strength and withstand ultrasonic cleaning and bonding processes.

[0039] Metal absorption loss is negligible: the metal grating is located at the evanescent wave tail, and the additional absorption loss is <0.5cm. -1 The impact on laser efficiency is negligible.

[0040] Good thermal stability: Integrated ±0.01℃ temperature control module, typical wavelength temperature coefficient of 0.085nm / ℃, wavelength drift after temperature control <0.001nm, ensuring stable output with narrow linewidth. Attached Figure Description

[0041] Figure 1This is a front view structural diagram of Embodiment 1 of the present invention;

[0042] Figure 2 This is a front view structural diagram of Embodiment 2 of the present invention;

[0043] Figure 3 This is a front view structural diagram of Embodiment 3 of the present invention;

[0044] Figure 4 This is a schematic diagram of the light field superposition scene of two gratings with different periods (Λ2≠Λ1) in this invention.

[0045] In the figure: 1. n-type substrate; 2. active region; 3. internal grating; 4. top grating; 5. high-reflection film; 6. anti-reflection film. Detailed Implementation

[0046] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0047] Example 1: Interference-destructive dominant type (multilayer grating cascade structure)

[0048] Based on a narrow-linewidth laser with different inner and outer gratings and staggered phase shift synergy, this InP-based laser has a center wavelength of λ0 = 1550 nm and employs a multi-layer DFB grating cascade structure. The core technology is to achieve destructive interference to improve the Smart Stream Resonance (SMSR). The specific structure is as follows:

[0049] The structure of the three-layer internal grating 3 above the active region 2:

[0050] The first DFB grating layer (closest to the active region 2): Located above the quantum well layer, it is a first-order refractive index coupled grating of semiconductor material, composed of alternating InP and InGaAsP, with a layer thickness of 20nm, a period Λ1=238nm, divided into two segments, with a λ0 / 6 phase shift in the middle of the grating length (after the phase shift, the two grating segments are 5λ0 / 12 apart); the etching depth is 40nm, and the duty cycle is 0.5;

[0051] The second DFB grating (located above the first DFB grating): is a first-order refractive index coupled grating made of semiconductor material. Its material and layer thickness are the same as the first layer. The period Λ1 = 238 nm is divided into three segments. A λ0 / 6 phase shift is made at 1 / 3 and 2 / 3 of the grating length, respectively, and it is staggered from the first grating (offset of 59.5 nm, Λ1 / 4).

[0052] The third DFB grating above (located above the second DFB grating above): is a first-order refractive index coupled grating made of semiconductor material. Its material and layer thickness are the same as the first layer. The period Λ1=238nm is divided into four segments. A λ0 / 6 phase shift is made at 1 / 4, 1 / 2 and 3 / 4 of the grating length, and it is staggered with the second grating.

[0053] Similarly, if there are more upper DFB grating layers, each layer has one more grating segment than the lower layer, and each segment has a λ0 / 6 phase shift.

[0054] The structure of the three-layer internal grating 3 below the active region 2:

[0055] Located below the quantum well layer, symmetrical to the grating structure above, with the same material, layer thickness, and period as the grating above:

[0056] The first DFB grating at the bottom (closest to the active region): the phase shift is the same as the phase shift of the first grating at the top plus 1;

[0057] The second DFB grating below (located below the first DFB grating below): the phase shift is the amount of the phase shift of the second grating above plus 1;

[0058] The third DFB grating below (located below the third DFB grating below): the phase shift is the phase shift of the third grating above plus 1;

[0059] Similarly, the number of phase shifts in each lower grating layer is one more than the corresponding layer above it.

[0060] Top grating 4 structure:

[0061] The top grating 4 is a metal-semiconductor composite grating located on the top surface of the ridge waveguide. A phase shift of λ0 / 4 is performed in the middle of the length of the metal grating (after the phase shift, the spacing between the two grating segments becomes λ0 / 2). The specific parameters are as follows:

[0062] Material: Ti / Pt / Au multilayer structure (Ti 15nm, Pt 20nm, Au 105nm), which is then thickened to 1.5μm by subsequent electroplating;

[0063] Structure: The top contact layer (200nm InGaAs) is etched with an InGaAs groove depth of 70nm. Then the entire grating is filled with metallic gold, which simultaneously and continuously covers the peaks and valleys of the grating.

[0064] Period: Λ2=300nm, effective refractive index n_eff2≈2.58, period ratio Λ2 / Λ1≈1.26, satisfying Λ1 / Λ2=n_eff2 / n_eff1;

[0065] Other: Sidewall tilt angle 83°, lithography accuracy ±5nm, etching depth accuracy ±5nm.

[0066] Key distinctions:

[0067] The “semiconductor DFB grating” buried deep inside the waveguide is a semiconductor grating layer with alternating InP and InGaAsP materials, with a thickness of 20nm. The raw material is InGaAsP, which is then grooved and etched through. The grating grooves and grating ridges are filled and completely covered by InP. The upper and lower layers of the grating layer are made of InP material.

[0068] The groove depths of the two different gratings are clearly distinguishable:

[0069] 1) Metal grating (top contact layer): The InGaAs groove depth is 70nm, and then the entire grating is filled with gold, and the grating ridge and groove are continuously covered with gold.

[0070] 2) Semiconductor DFB grating (inside the waveguide): located in the InP / InGaAsP layer, with a thickness of 20nm (shallow etching, not involving the top surface contact layer), and an etching depth of 40nm.

[0071] The two gratings have different effective refractive indices (n_eff1≈3.25, n_eff2≈2.58), satisfying the period ratio relationship, so that the Bragg wavelengths are precisely aligned with the center wavelength λ0=1550nm; the vertical distance between the two gratings Δz=2.0μm, ensuring a significant improvement in the side-mode suppression ratio.

[0072] The laser fabricated in this embodiment has the following test results: at an operating temperature of 25℃ and an injection current of 250mA, the output power is 42mW, the linewidth is 42kHz, and the side-mode rejection ratio is 58dB; after 1000 hours of continuous operation, the output power decreases by 2.5%, the linewidth changes by 8%, and there is no mode skipping phenomenon; within an ambient temperature range of 15-35℃, the device performance fluctuates by <4%, and the stability is good.

[0073] Example 2: Coupled Enhancement and SHB Suppression Type (Multilayer Grating Cascade Structure)

[0074] This is an InP-based laser with a center wavelength of λ0 = 1550 nm, based on different inner and outer gratings and staggered phase shift synergy. It adopts a multi-layer DFB grating cascade structure, which achieves enhanced coupling to narrow the linewidth and suppress the spatial hole burning effect. The specific structure is as follows:

[0075] The structure of the three-layer internal grating 3 above the active region 2:

[0076] The first layer of DFB grating (closest to the active region 2): a phase-shift-free uniform semiconductor grating, composed of alternating InP and InGaAsP, with a layer thickness of 15nm, a period Λ1=235nm, a groove depth of 15nm (shallow groove), and a duty cycle of 0.5.

[0077] The second DFB grating above: Located above the first DFB grating, it has the same material and layer thickness as the first layer, with a period Λ1=235nm, and one more phase shift than the first layer. Each segment has a λ0 / 6 phase shift and is staggered from the first grating.

[0078] The third DFB grating above: Located above the second DFB grating, it has the same material and layer thickness as the first layer, with a period Λ1=235nm, and one more phase shift than the second layer. Each segment has a λ0 / 6 phase shift and is staggered from the second grating.

[0079] And so on.

[0080] The structure of the three-layer internal grating 3 below the active region 2:

[0081] Located below the quantum well layer, it is symmetrical to the grating structure above it. Each layer has one more phase shift than the corresponding layer above it, and its material, layer thickness, and period are the same as the grating above it.

[0082] Top grating 4 structure:

[0083] The top grating 4 is a metal-semiconductor composite grating located on the top surface of the ridge waveguide. A phase shift of λ0 / 4 is performed in the middle of the length of the metal grating (after the phase shift, the spacing between the two grating segments becomes λ0 / 2). The specific parameters are as follows:

[0084] Material: Ti / Pt / Au multilayer structure (Ti 15nm, Pt 20nm, Au 105nm), which is then thickened to 1.2μm by subsequent electroplating;

[0085] Structure: The top contact layer (200nm InGaAs) is etched with an InGaAs groove depth of 60nm. Then the entire grating is filled with metallic gold, which simultaneously and continuously covers the peaks and valleys of the grating.

[0086] Period: Λ2=280nm, effective refractive index n_eff2≈2.58, period ratio Λ2 / Λ1≈1.19, satisfying Λ1 / Λ2=n_eff2 / n_eff1;

[0087] Other: Sidewall tilt angle 80°, lithography accuracy ±5nm, etching depth accuracy ±5nm.

[0088] Key distinctions:

[0089] The “semiconductor DFB grating” buried deep inside the waveguide is a semiconductor grating layer with alternating InP and InGaAsP materials, with a thickness of 15nm. The raw material is InGaAsP, which is then grooved and etched through. The grating grooves and grating ridges are filled and completely covered by InP. The upper and lower layers of the grating layer are made of InP material.

[0090] The groove depths of the two different gratings are clearly distinguishable:

[0091] 1) Metal grating (top contact layer): The InGaAs groove depth is 60nm, and then the entire grating is filled with gold, and the grating ridge and groove are continuously covered with gold.

[0092] 2) Semiconductor DFB grating (inside the waveguide): located in the InP / InGaAsP layer, with a thickness of 15nm (shallow etching, not involving the top surface contact layer), and an etching depth of 15nm.

[0093] The two gratings have different effective refractive indices and satisfy the period ratio relationship, so that the Bragg wavelengths are precisely aligned with the center wavelength λ0=1550nm; the vertical distance between the two gratings Δz=1.5μm, the standard deviation of the longitudinal light field distribution is reduced by 42%, and the spatial hole burning effect is effectively suppressed.

[0094] The laser prepared in this embodiment was tested as follows: at an operating temperature of 25℃ and an injection current of 250mA, the output power was 39mW, the linewidth was 55kHz, and the side-mode rejection ratio was 56dB; after 1000 hours of continuous operation, the output power decreased by 2.8%, the linewidth changed by 9%, and the stability was good.

[0095] Example 3: Fusion type (multilayer grating cascade structure)

[0096] This is an InP-based laser with a center wavelength of λ0=1550nm, based on different internal and external gratings and a staggered phase shift. It adopts a multi-layer DFB grating cascade structure to achieve a triple effect: interference destructive phase shift to improve the SMSR (Supervisory Signal-Reduction Ratio), enhanced coupling to narrow the linewidth, and shallow grooves + double-layer stacking to suppress the spatial hole burning effect. The specific structure is as follows:

[0097] The structure of the three-layer internal grating 3 above the active region 2:

[0098] The first DFB grating layer above (closest to the active region 2): a phase-shift-free uniform semiconductor grating (shallow groove), composed of alternating InP and InGaAsP, with a layer thickness of 30nm, a period Λ1=245nm, a groove depth of 20nm, and a duty cycle of 0.5.

[0099] The second DFB grating above: located above the first layer, with the same material and layer thickness as the first layer, period Λ1=245nm, one more phase shift than the first layer, each segment is a λ0 / 6 phase shift, and it is staggered from the first layer grating;

[0100] The third DFB grating above: located above the second layer, with the same material and layer thickness as the first layer, period Λ1=245nm, one more phase shift than the second layer, each segment is a λ0 / 6 phase shift, and it is staggered from the second layer grating;

[0101] And so on, with more layers, the number of phase shifts in each layer increases progressively.

[0102] The structure of the three-layer internal grating 3 below the active region 2:

[0103] Located below the quantum well layer, it is symmetrical to the grating structure above it. Each layer has one more phase shift than the corresponding layer above it, and its material, layer thickness, and period are the same as the grating above it.

[0104] Top grating 4 structure:

[0105] The top grating 4 is a metal-semiconductor composite grating, possessing: a λ0 / 4 phase shift structure; a period Λ2 satisfying Λ1 / Λ2=n_eff2 / n_eff1; and a vertical distance Δz=2.5μm. Specific parameters are as follows:

[0106] Material: Ti / Pt / Au multilayer structure (Ti 15nm, Pt 20nm, Au 105nm), which is then thickened to 2.0μm by subsequent electroplating;

[0107] Structure: The top contact layer (200nm InGaAs) is etched with an InGaAs groove depth of 80nm. Then the entire grating is filled with metallic gold, which simultaneously and continuously covers the peaks and valleys of the grating.

[0108] Period: Λ2=320nm, effective refractive index n_eff2≈2.58, period ratio Λ2 / Λ1≈1.30, satisfying Λ1 / Λ2=n_eff2 / n_eff1;

[0109] Other: Sidewall tilt angle 85°, lithography accuracy ±5nm, etching depth accuracy ±5nm.

[0110] Key distinctions:

[0111] The “semiconductor DFB grating” buried deep inside the waveguide is a semiconductor grating layer with alternating InP and InGaAsP materials, with a thickness of 30nm. The raw material is InGaAsP, which is then grooved and etched through. The grating grooves and grating ridges are filled and completely covered by InP. The upper and lower layers of the grating layer are made of InP material.

[0112] The groove depths of the two different gratings are clearly distinguishable:

[0113] 1) Metal grating (top contact layer): The InGaAs groove depth is 80nm, and then the entire grating is filled with gold, and the grating ridge and groove are continuously covered with gold.

[0114] 2) Semiconductor DFB grating (inside the waveguide): located in the InP / InGaAsP layer, with a thickness of 30nm (shallow etching, not involving the top surface contact layer), and an etching depth of 20nm.

[0115] The laser prepared in this embodiment has the following test results: at an operating temperature of 25℃ and an injection current of 250mA, the output power is 45mW, the linewidth is 48kHz, and the side-mode rejection ratio is 59dB; after 1000 hours of continuous operation, the output power decreases by 2.2%, the linewidth changes by 7%, and the stability is excellent, achieving triple technical effects at the same time.

[0116] The following common features apply to the above three embodiments:

[0117] The chip structure of the narrow linewidth laser based on different internal and external gratings and staggered phase shift is 1550nm InP-based;

[0118] n-type substrate 1: n-type InP, thickness 150μm, Si doping concentration 8×10⁻⁶ 18 cm -3 .

[0119] n-type graded refractive index confinement layer (n-GRIN SCH): InGaAsP, thickness 0.25 μm, bandgap wavelength λg=1.18μm, parabolic graded refractive index.

[0120] Active region 2: 4 multiple quantum wells (MQWs), with AlGaInAs as the well material and InGaAsP as the barrier material. The well width is 7nm, the barrier width is 11nm, and the peak gain wavelength is 1550nm.

[0121] p-type graded refractive index confinement layer (p-GRIN SCH): InGaAsP, thickness 0.25 μm.

[0122] p-type top cladding: InP, thickness 1.8μm.

[0123] Back waveguide: 2.5μm width, 2.8μm height.

[0124] Internal grating 3: Located within the p-type GRIN SCH layer or at the bottom of the p-type upper cladding, at a distance of 2≤50nm from the active region, made of InGaAsP, a first-order refractive index coupled grating, with a period Λ1=235-245nm (typically 238nm), a grating depth of 30-50nm (typically 40nm), and a duty cycle of 0.5.

[0125] Top grating 4: Located on the top surface of the spine waveguide, it is a Ti / Pt / Au multilayer structure (Ti 15nm, Pt 20nm, Au 105nm), a first-order grating with a period Λ2 = 280-320nm (typically 300nm), a grating depth of 60-80nm (typically 70nm), and a duty cycle of 0.5-0.6. Top grating 4 is a metallic grating and also serves as a p-type electrode. Alloyed ohmic contacts are formed through rapid thermal annealing (RTA, 380-420℃, 30-60s, nitrogen atmosphere), with a contact resistivity <1×10⁻⁶. -5 Ω·cm 2 Electroplating thickens the gold layer (1-2μm) to reduce resistance and enhance mechanical strength. The metal grating sidewalls are tilted at an angle of 80°-85°.

[0126] Temperature control module: The TEC thermoelectric cooler is integrated with a high-precision thermistor, with a temperature control accuracy of ±0.01℃.

[0127] Cavity surface coatings: Rear cavity surface high reflectance coating (HR, reflectance > 98%), front cavity surface antireflectance coating (AR, reflectance < 2%).

[0128] Key parameter design basis:

[0129] The effective refractive index of the internal grating 3 is n_eff1≈3.25 (1550nm), Λ1=λ0 / (2·n_eff1)=238nm.

[0130] The effective refractive index of the top grating 4 is n_eff2≈2.58 (finite element simulation), Λ2=λ0 / (2·n_eff2)=300nm.

[0131] The period ratio Λ2 / Λ1=300 / 238≈1.26=n_eff1 / n_eff2, ensuring that the Bragg wavelengths of both gratings are precisely aligned to 1550nm.

[0132] The vertical distance Δz = 1.5-2.5 μm makes the phase difference Δφ = 4π·n_eff·Δz·Δν / c at the side mode frequency (deviating from the center frequency ±100 GHz, corresponding to a wavelength shift of about ±0.8 nm) close to an odd multiple of π, thus achieving interference cancellation.

[0133] Top grating 4 absorption loss analysis:

[0134] Top grating 4 is located at the top of the spine waveguide, in the evanescent wave tail region of the optical field distribution. Finite element simulations show that for the 1550nm waveguide mode, the overlap factor of the optical field at the top metal is <0.5%, and the metal absorption coefficient is approximately α_metal ≈ 5 × 10⁻⁶. 4 cm -1(The real part of the complex refractive index of Au at 1550 nm is approximately 0.5, and the imaginary part is approximately 5.) It is estimated that the additional absorption loss is <0.5 cm⁻¹. -1 The impact on threshold current and output power is negligible. Meanwhile, by optimizing the duty cycle (0.6) and stripe width (approximately 180 nm) of the top grating 4, the interaction between the metal and the light field is further reduced.

[0135] Spatial hole burning effect suppression mechanism (unique to Examples 2 and 3):

[0136] Traditional phase-shifted DFB lasers exhibit high photon density concentration near the phase shift point, leading to excessive local carrier consumption (spatial hole burning effect), causing linewidth broadening and mode switching. This invention suppresses this effect through a dual mechanism:

[0137] Shallow grooves reduce single-layer coupling strength: the groove depth of the inner grating 3 is only 10-20nm, and the groove depth of the top grating 4 is 30-50nm. The low coupling strength avoids excessive accumulation of photon density in local areas.

[0138] Light field homogenization of double-layer uncorrelated gratings: The two gratings are located at different vertical heights and have different physical periods. Their diffracted light fields form complex interference superposition in the cavity, which reduces the standard deviation of the longitudinal light field distribution by more than 40% and makes the carrier consumption more uniform.

[0139] Preparation method (Example 1 is the preferred embodiment; other methods can be adjusted accordingly):

[0140] Step 1: Epitaxial growth. On an n-type InP substrate, an n-GRINSCH layer, active region 2, p-GRINSCH layer, and first p-type upper cladding layer (thickness 0.9 μm) are grown sequentially using MOCVD.

[0141] Step 2: Fabrication of the internal grating 3. The grating pattern is defined using electron beam lithography, followed by ICP dry etching (Cl2 / Ar, etching depth 40±3nm) to form a grating with a period of 238nm and a duty cycle of 0.5. If a λ / 4 phase shift is required, a grating-free region with a length of 119nm is designed in the center of the cavity. The photoresist mask is then removed.

[0142] Step 3: Secondary epitaxy. Continue MOCVD growth of the second p-type cladding layer (0.9 μm thick), planarize the surface, and cover it with the internal grating 3.

[0143] Step 4: Ridge waveguide formation. The ridge pattern is defined using i-line lithography, and ICP etching is performed onto the surface of the p-GRINSCH layer to form a ridge waveguide with a width of 2.5 μm and a height of 2.8 μm.

[0144] Step 5: Fabrication of Top Grating 4. A grating pattern (300 nm period, 0.6 duty cycle) is defined on top of the spine waveguide using electron beam lithography. Etching parameters are controlled to maintain the grating sidewall tilt angle at 80°-85°. Ti (15 nm), Pt (20 nm), and Au (105 nm) are deposited sequentially using electron beam evaporation. After lift-off, top grating 4 is formed. Top grating 4 is a metallic grating and also serves as the p-electrode.

[0145] Step 6: Alloying and Electroplating Thickening. The chip was rapidly thermally annealed in a nitrogen atmosphere (RTA, 400℃, 45s). Subsequently, a gold layer of 1.5μm was thickened on the grating surface using an electroplating process.

[0146] Step 7: Back electrode fabrication. Thin the substrate to 150 μm and electron beam evaporate Ti / Pt / Au (50 / 50 / 200 nm) on the back side as the n electrode.

[0147] Step 8: Cavity surface treatment. Cleavage a cavity length of 2mm, deposit an anti-reflective film (reflectivity < 2%) on the front cavity surface, and deposit a high-reflective film (reflectivity > 98%) on the rear cavity surface.

[0148] Step 9: Temperature control integration and packaging. The chip is eutectic soldered onto the TEC thermoelectric cooler, the thermistor is attached to the sidewall of the chip, packaged in a TO-56 socket, connected to the temperature control circuit and calibrated to 25±0.01℃.

[0149] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A narrow-linewidth laser based on the synergy of different inner and outer gratings and misaligned phase shift, characterized in that, include: n-type substrate (1); Active region (2) disposed on the n-type substrate (1); At least one internal grating (3) is disposed above or below the active region (2). The internal grating (3) is a first-order refractive index coupled grating of semiconductor material with a period of Λ1. The Bragg wavelength of the internal grating (3) is aligned with the center wavelength λ0. The top grating (4) is disposed on the top of the narrow linewidth laser based on different inner and outer gratings and staggered phase shift synergy and is combined with the electrode layer. The top grating (4) is a first-order grating with a period of Λ2 and Λ2≠Λ1. The Bragg wavelength of the top grating (4) is also aligned with λ0. The top grating (4) also serves as the p-type electrode of the narrow linewidth laser based on different inner and outer gratings and staggered phase shift synergy. An antireflective film (6) is disposed on the front cavity surface of the narrow linewidth laser based on different inner and outer gratings and staggered phase shift synergy, and a high reflective film (5) is disposed on the rear cavity surface of the narrow linewidth laser based on different inner and outer gratings and staggered phase shift synergy. The narrow linewidth laser based on different inner and outer gratings and staggered phase shift is an InP-based laser with a center wavelength λ0=1550nm. The inner grating (3) has a period Λ1=235-245nm, and the top grating (4) has a period Λ2=280-320nm. The period ratio Λ2 / Λ1=1.15-1.

35.

2. The narrow linewidth laser based on different inner and outer gratings and staggered phase shift synergy as described in claim 1, characterized in that: The vertical distance Δz between the inner grating (3) and the top grating (4) is 1.5-2.5 μm.

3. The narrow linewidth laser based on different inner and outer gratings and staggered phase shift synergy as described in claim 1, characterized in that: The inner grating (3) is a uniform grating without phase shift, and the top grating (4) has a λ0 / 4 phase shift structure in the middle of its length direction. After phase shift, the distance between the two grating segments is λ0 / 2.

4. The narrow linewidth laser based on different inner and outer gratings and staggered phase shift synergy as described in claim 1, characterized in that: The inner grating (3) is provided with at least one λ0 / 6 phase shift structure. After phase shift, the distance between the two grating segments is 5λ0 / 12. The top grating (4) is a uniform periodic grating without phase shift.

5. The narrow linewidth laser based on different inner and outer gratings and staggered phase shift synergy according to claim 1, characterized in that: The top grating (4) is a metal grating. The metal grating is formed by rapid thermal annealing to form alloyed ohmic contacts and is electroplated with a thickened gold layer on its surface. The rapid thermal annealing conditions are 380-420℃ and 30-60s. The thickness of the electroplated thickened gold layer is 1-2μm. The sidewall tilt angle of the metal grating is 80°-85°.

6. The narrow linewidth laser based on different inner and outer gratings and staggered phase shift synergy according to claim 1, characterized in that: The inner grating (3) is located within 100 nm above or below the active region, and the vertical distance Δz between the inner grating (3) and the top grating (4) is 1.5-2.5 μm.

7. The narrow linewidth laser based on different inner and outer gratings and staggered phase shift synergy according to claim 1, characterized in that: It also includes a temperature control module, which comprises a TEC thermoelectric cooler and a temperature sensor.

8. The narrow linewidth laser based on different inner and outer gratings and staggered phase shift synergy according to claim 7, characterized in that, Its preparation method includes the following steps: S1. An active region (2) and a grating support layer are epitaxially grown on an n-type substrate (1) by MOCVD. S2. An internal grating (3) is prepared by electron beam lithography and ICP etching, with a period of Λ1; S3. Continue MOCVD epitaxial growth of the cladding layer; S4. The back waveguide is formed by photolithography and ICP etching; S5. A top grating (4) with a period of Λ2 is prepared on the top of the back waveguide by electron beam lithography, metal evaporation and lift-off process. The top grating (4) also serves as a p electrode. S6. Perform rapid thermal annealing on the top grating (4) to form an alloyed ohmic contact; S7. The gold layer is thickened on the surface of the top grating (4) by electroplating. S8. Thin the n-type substrate (1) and deposit the n-type electrode; S9. Cleaving the cavity surface and depositing a high-reflection film (5) and an anti-reflection film (6); S10, integrates temperature control module and is packaged.

9. The narrow linewidth laser based on different inner and outer gratings and staggered phase shift synergy according to claim 8, characterized in that, The lithography accuracy of the inner grating (3) is ±2nm, the etching depth accuracy is ±5nm, the grating depth is 30-50nm, and the duty cycle is 0.5; the lithography accuracy of the top grating (4) is ±5nm, the etching depth is 60-80nm, and the duty cycle is 0.5-0.6; the inner grating (3) and the top grating (4) do not require sub-nanometer alignment.

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