Ultra-narrow linewidth DBR+DFB laser based on external two different metal gratings

Abstract

The application relates to the technical field of semiconductor lasers, in particular to an ultra-narrow linewidth DBR+DFB laser based on external two different metal gratings, which comprises a monolithic InP substrate, a semiconductor gain chip arranged on the upper surface of the monolithic InP substrate, wherein the semiconductor gain chip comprises a multi-quantum well active layer, a DFB active waveguide section and a DBR passive waveguide section; a first metal grating is arranged on the top of the DFB active waveguide section; a second metal grating is arranged on the top of the DBR passive waveguide section; the second metal grating simultaneously serves as a p-type electrode; and the DBR passive waveguide section is transparentized through quantum well hybridization treatment. The application realizes a low-cost, mass-produced ultra-narrow linewidth laser through simple one-time epitaxy, top surface metal grating process and quantum well hybridization treatment, and fills the technical blank.

Description

Technical Field

[0001] This invention relates to the field of semiconductor laser technology, specifically to an ultra-narrow linewidth DBR+DFB laser based on two externally different metal gratings. Background Technology

[0002] Ultra-narrow linewidth lasers are core light sources in fields such as coherent optical communication, high-precision fiber optic sensing, lidar, and quantum key distribution. Traditional external cavity lasers (ECLs) use volume gratings or fiber gratings as external cavity feedback elements. Although they can achieve linewidths in the kHz or even sub-kHz range, they suffer from drawbacks such as complex structure, large size, sensitivity to vibration, high coupling loss, high packaging cost, and limited output power (typically <20mW). These limitations make it difficult to meet the demands of modern optoelectronic systems for miniaturization, high integration, and low cost.

[0003] Among on-chip integrated semiconductor lasers, distributed feedback (DFB) lasers and distributed Bragg reflection (DBR) lasers are the two most common structures. Conventional DFB lasers achieve single-mode operation through an internal grating, with linewidths typically ranging from 100 kHz to 1 MHz. Limited by cavity length (generally < 500 μm) and grating coupling strength (κL ≈ 1-2), further narrowing the linewidth is difficult. Traditional DBR lasers use passive gratings as mirrors, which can extend the effective cavity length, but require complex secondary epitaxy or quantum well disordering processes. Furthermore, single-sided DBR structures are prone to mode hopping and multimode problems. Cascaded structures combining DFB and DBR have been attempted, but typically require complex multiple epitaxy processes and precise wavelength alignment, resulting in high fabrication difficulty. Additionally, the identical periods of the two grating segments lead to thermal detuning during operation, limiting linewidth improvement.

[0004] In the prior art, metal gratings have been used as electrodes and feedback elements in DFB lasers, but no DBR+DFB laser scheme has been found that integrates two top surface metal gratings with different periods and different groove depths on the same chip and uses the thermo-optic effect to dynamically compensate for periodic detuning. Summary of the Invention

[0005] To address the problems mentioned in the background section, the present invention provides the following technical solution:

[0006] Ultra-narrow linewidth DBR+DFB lasers based on two different external metal gratings include:

[0007] Monolithic InP substrate;

[0008] A semiconductor gain chip disposed on the upper surface of the monolithic InP substrate, the semiconductor gain chip comprising a multi-quantum-well active layer, a DFB active waveguide section, and a DBR passive waveguide section;

[0009] The top of the DFB active waveguide section is provided with a first metal grating, the first metal grating having a period of 226 nm, a groove depth of 30-60 nm, and a λ_B at its center. DFB / 4 wavelength phase shift, λ_B DFB =1550.36nm, used for mode-locking;

[0010] The top of the passive waveguide section of the DBR is provided with a second metal grating. The second metal grating has a period of 231nm, a groove depth of 80-120nm, and no phase shift. The second metal grating also serves as a p-type electrode, covering the entire passive region.

[0011] The DBR passive waveguide section is made transparent through quantum well hybridization, and its absorption coefficient for lasing wavelength is less than 2 cm⁻¹. -1 The quantum well hybridization process includes: depositing a phosphorus-containing silicon oxide film with a thickness of 80-120 nm and a phosphorus atom percentage of 2%-5% on the surface of the passive waveguide section of the DBR using PECVD; followed by phosphorus ion implantation at an implantation energy of 200-360 keV and a dose of 1×10⁻⁶. 13 -1×10 14 cm -2 Then, rapid hot annealing was performed.

[0012] An electrically isolated region is formed between the DFB active waveguide segment and the DBR passive waveguide segment through proton injection.

[0013] When the DFB active waveguide section is in continuous operation, the temperature rises by ΔT due to the Joule heating effect, which increases its effective refractive index and causes a redshift in the Bragg wavelength. This redshift compensates for the difference in room temperature wavelength caused by the difference in the physical period of the two gratings, so that the two Bragg wavelengths are dynamically aligned at the steady-state operating point.

[0014] The output end face of the ultra-narrow linewidth DBR+DFB laser based on two different external metal gratings is coated with an anti-reflection film, with a reflectivity of <0.5%.

[0015] The operating temperature range of the ultra-narrow linewidth DBR+DFB laser based on two different external metal gratings is 15°C to 35°C.

[0016] When the ultra-narrow linewidth DBR+DFB laser based on two different external metal gratings is working, the DFB active waveguide section provides gain and wavelength locking, and the DBR passive waveguide section provides narrowband high-reflection feedback, forming a composite resonant cavity, achieving a laser linewidth of 5-10kHz and a room temperature single-mode continuous wave output power of ≥40mW.

[0017] As a preferred embodiment of the above technical solution, the length of the DFB active waveguide section is 2 mm, and its coupling coefficient κ≈8 cm. -1κL=1.6 to suppress the spatial burning effect.

[0018] As a preferred embodiment of the above technical solution, the length of the DBR passive waveguide segment is 2mm, and the coupling coefficient κ≈35cm. -1 κL_g=7, reflectivity R=tanh 2 (κL_g)=tanh 2 (7)≈0.9999, 3-dB reflection bandwidth Δλ≈0.3nm.

[0019] As a preferred embodiment of the above technical solution, the physical period difference ΔΛ between the first and second metal gratings is 5nm, and the Bragg wavelength difference at room temperature is Δλ_B0 = 2n_eff・ΔΛ, where n_eff ≈ 3.2, resulting in a calculated Δλ_B0 ≈ 32nm (theoretical value). In reality, due to the difference in effective refractive index between the two sections (n_eff ≈ 3.43 for the DFB active waveguide section and n_eff ≈ 3.35 for the DBR passive waveguide section), the room temperature wavelength difference is approximately 16nm. The temperature rise ΔT of the DFB active waveguide section under continuous operation satisfies: 2Λ_DFB・(dn / dT)・ΔT = actual required compensation, where dn / dT = 2.5 × 10⁻⁶. -4 K -1 Λ_DFB=226nm, the actual compensation amount is about 0.8nm, and we can solve for ΔT≈7K; in actual devices, ΔT=5-10K, which can be aligned by thermo-optical compensation.

[0020] As a preferred embodiment of the above technical solution, the absorption coefficient α of the quantum well hybridization of the DBR passive waveguide section is less than 2cm. -1 The single-pass optical loss of a 2mm length is ≤4.34×α×L≈1.7dB, which can be compensated by the gain of the DFB active waveguide section. The mode gain of the DFB active waveguide section is estimated to be 2-4dB under an injection current of 200-300mA, which is sufficient to cover the loss of the DBR passive waveguide section and the end-face output loss.

[0021] As a preferred embodiment of the above technical solution, the materials of the first metal grating and the second metal grating are Ti / Au or Ni / Au, the bottom Ti layer has a thickness of 20-50nm, the top Au layer has a thickness of 200-500nm, and the second metal grating is connected to an additionally thickened parallel metal electrode to reduce the long-distance resistance to below 5Ω.

[0022] As a preferred embodiment of the above technical solution, the isolation resistance of the electrical isolation region is >2MΩ, and the additional optical loss is <0.8dB.

[0023] As a preferred embodiment of the above technical solution, the side-mode rejection ratio is ≥60dB and the relative intensity noise is <-140dB / Hz@10GHz.

[0024] As a preferred embodiment of the above technical solution, a high thermal conductivity heat sink (SiC or diamond) is used for packaging, and the thermal resistance from the DFB active waveguide section to the heat sink is designed to be ≤20K / W, so as to ensure that the temperature rise of the active region of the DFB active waveguide section is 5-10K under an injection current of 200-300mA.

[0025] As a preferred embodiment of the above technical solution, the fabrication method of the ultra-narrow linewidth DBR+DFB laser based on two different external metal gratings includes the following steps:

[0026] S1. Using a traditional FP epitaxial wafer, an n-type InP lower cladding layer, a multi-quantum well active layer, and an InP upper waveguide layer are sequentially grown on a single InP substrate.

[0027] S2. Perform quantum well hybridization on the passive waveguide section of the DBR, define the DBR region by photolithography, and deposit a 50nm thick SiN layer. x A protective layer was then deposited, followed by PECVD deposition of a 100 nm thick SiOP film with a 3% P content, and phosphorus ion implantation (300 keV, 5 × 10⁻⁶ m² / h). 13 cm -2） Rapid thermal annealing at 780℃ for 90 seconds to remove SiOP and SiN films. x Protective layer;

[0028] S3. Proton injection (150keV, 3×10⁻⁶) into the electrically isolated region. 13 cm -2 Anneal at 750℃ for 30 seconds;

[0029] S4. A metal layer (Ti / Au20 / 200nm) is deposited on the top surface of the ridge waveguide. Segmented metal gratings are formed by electron beam lithography and etching, and a 0.3μm transition region is set at the junction of two grating segments to avoid field splicing errors: DFB active waveguide segment period 226nm, groove depth 45nm, center λ_B DFB / 4 phase shift; DBR passive waveguide section period 231nm, groove depth 100nm, no phase shift, length 2mm; at the same time, a thickened electrode (Au thickness 500nm) is fabricated in parallel with the second metal grating to reduce resistance;

[0030] S5. Electrical isolation is achieved by removing the metal at the electrode gap through photolithography and etching;

[0031] S6. Thin the substrate to 80μm, deposit AuGeNin type electrodes on the back side, and alloy anneal;

[0032] S7, the chip is cleaved, with an Al2O3 antireflection coating of <0.5% on the output end face and no coating on the rear end face; it is packaged with a high thermal conductivity heat sink (SiC), and the thermal resistance from the DFB active waveguide section to the heat sink is controlled to ≤20K / W, thus completing the device.

[0033] This invention provides an ultra-narrow linewidth DBR+DFB laser based on two different external metal gratings. Through a simple one-step epitaxy, top-surface metal grating process and quantum well hybridization, a low-cost, mass-producible ultra-narrow linewidth laser is achieved, filling a technological gap. Compared with existing technologies, it has the following advantages:

[0034] 1. Linewidth 5-10kHz, meeting the requirements of coherent communication, high-precision sensing and other fields;

[0035] 2. Output power ≥ 40mW, no space burn-in holes;

[0036] 3. Automatic thermal and optical compensation, requiring no additional tuning components;

[0037] 4. No secondary epitaxy is required; compatible with standard DFB process.

[0038] 5. With a total length of only 4mm, the chip is small, low-cost, and has good heat dissipation;

[0039] 6. Side mode suppression ratio ≥60dB, relative intensity noise <-140dB / Hz;

[0040] 7. The metal grating electrode has low resistance and low power consumption. Attached Figure Description

[0041] Figure 1 This is a top view of the structure of the present invention;

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

[0043] Figure 3 This is a schematic diagram of the right-side structure of the present invention.

[0044] In the figure: 1. Monolithic InP substrate; 2. Semiconductor gain chip; 21. Multi-quantum well active layer; 22. DFB active waveguide section; 221. First metal grating; 23. DBR passive waveguide section; 231. Second metal grating; 24. Electrically isolated region. Detailed Implementation

[0045] 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.

[0046] Example 1

[0047] The core structure and parameter design of the ultra-narrow linewidth DBR+DFB laser based on two externally different metal gratings are as follows:

[0048] In this embodiment, a single InP substrate 1 is used, the DFB active waveguide section 22 is 2mm long, and the DBR passive waveguide section 23 is 2mm long.

[0049] The physical period of the first metal grating 221 at the top of the DFB active waveguide section 22 is Λ_DFB=226nm, and the physical period of the second metal grating 231 at the top of the DBR passive waveguide section 23 is Λ_DBR=231nm, with a difference of 5nm.

[0050] At room temperature, the effective refractive index n_eff of the DFB active waveguide segment 22 DFB ≈3.43, Effective refractive index n_eff of DBR passive waveguide segment 23 DBR ≈3.35, the two Bragg wavelengths are respectively:

[0051] λ_B DFB =2×3.43×226=1550.36nm,

[0052] λ_B DBR =2×3.35×231=1547.70nm,

[0053] The wavelength difference Δλ_B0 ≈ 2.66 nm (the actual difference is slightly different due to waveguide dispersion, but it is within the range that can be thermally compensated).

[0054] Thermo-optical dynamic compensation mechanism

[0055] When a current (typically 200-300mA) is injected into the DFB active waveguide section 22 during continuous operation, the active region experiences a temperature rise ΔT due to Joule heating. The thermo-optical coefficient of the InP-based material is dn / dT ≈ 2.5 × 10⁻⁶. -4 K -1 The effective refractive index change of the DFB active waveguide section 22 is Δn = (dn / dT)・ΔT, and its Bragg wavelength redshift is:

[0056] Δλ_shift=2・Λ_DFB・Δn=2・Λ_DFB・(dn / dT)・ΔT.

[0057] Let Δλ_shift = Δλ_B0 (approximately 2.66 nm), then the required temperature rise can be obtained:

[0058] ΔT=Δλ_B0 / (2・Λ_DFB・dn / dT)=2.66 / (2×226×2.5×10 -4 )≈23.5K.

[0059] In actual devices, ΔT = 5-10K is insufficient to fully compensate for the 2.66nm static detuning. However, it should be noted that the 5nm difference in the periods of the two grating segments is a theoretical design value. The actual effective refractive index difference can be finely adjusted by changing the groove depth and metal thickness, reducing the room temperature wavelength difference to within 1nm.

[0060] This embodiment optimizes the groove depth (45nm for DFB active waveguide section 22 and 100nm for DBR passive waveguide section 23) and metal thickness to effectively control the room temperature wavelength difference within 0.8-1.2nm, requiring a temperature rise of approximately 7-10K, which matches the actual temperature rise of the device. Therefore, through proper design, the thermo-optical effect can effectively compensate for periodic mistuning, achieving dynamic wavelength alignment.

[0061] To achieve the required temperature rise, proper thermal management is necessary. The heat power P_heat generated by the DFB active waveguide section 22 is P_heat = I・V - P_out. At 250mA, 1.8V, and an output of 42mW, P_heat ≈ 0.41W. If the thermal resistance is R_th, then the temperature rise ΔT = R_th × P_heat. Taking ΔT = 8K (corresponding to a 0.95nm redshift), the required thermal resistance R_th = 8 / 0.41 ≈ 19.5K / W. Therefore, in this embodiment, the thermal resistance from the DFB active waveguide section 22 to the heat sink is designed to be ≤20K / W. In practice, this thermal resistance value can be achieved by thinning the InP substrate to 80μm and using AuSn eutectic solder to solder the chip onto the SiC heat sink.

[0062] Temperature stability and compensation strategy:

[0063] When the ambient temperature changes, both the DFB active waveguide section 22 and the DBR passive waveguide section 23 exhibit synchronous drift due to the thermo-optical effect. Based on the thermo-optical coefficient of InP bulk material (dn / dT≈2.5×10⁻⁶),... -4 K -1 The theoretically calculated Bragg wavelength temperature coefficient is Δλ / ΔT = 2・Λ・dn / dT ≈ 2 × 231 × 2.5 × 10⁻⁶. -4 =0.1155nm / ℃. The effective refractive index temperature coefficient (dN_eff / dT) of the actual waveguide is affected by waveguide dispersion and carrier injection, and the measured value is approximately 0.08nm / ℃ (statistical value in the embodiment of this patent). Therefore, when the ambient temperature changes by 10℃, the two Bragg wavelengths drift synchronously by about 0.8nm, but the relative detuning remains unchanged.

[0064] To ensure stable operation of the device within an ambient temperature range of 15-35℃, this embodiment employs a two-stage temperature control strategy:

[0065] a) The chip substrate temperature is controlled within ±0.5℃ of the set value by using a thermoelectric cooler (TEC), eliminating most of the ambient temperature drift;

[0066] b) Sub-nanometer fine wavelength alignment is achieved by finely adjusting the injected current of the DFB active waveguide section 22 (which generates a temperature rise of about 0.5K per 10mA). This current adjustment range of ±20mA can compensate for residual temperature fluctuations of the substrate.

[0067] Therefore, an operating ambient temperature range of 15-35℃ is reasonable, and there is no need to significantly change the injection current.

[0068] Thermal crosstalk analysis: The heat generated by the DFB active waveguide segment 22 will be conducted through the substrate to the DBR passive waveguide segment 23, causing the temperature of the DBR passive waveguide segment 23 to rise by about 1-3K, and its Bragg wavelength to redshift by 0.12-0.36nm. Since the DBR reflection bandwidth is about 0.3nm, this thermal crosstalk will not cause the two segments to completely mismatch and can still maintain partial overlap.

[0069] DBR passive waveguide segment 23 narrowband filtering and linewidth compression

[0070] The length of DBR passive waveguide segment 23 is L_g=2mm, and the coupling coefficient κ≈35cm. -1 Then κL_g = 7. Reflectivity R = tanh 2 (7)≈0.9999, close to 100%. 3-dB reflection bandwidth Δλ≈0.8・λ_B 2 / (n_g・L_g), where the group refractive index n_g≈3.2, and Δλ≈0.3nm is calculated.

[0071] The linewidth compression factor is approximately (1 + κL_g). 2 =64, the typical intrinsic linewidth of DFB is 100kHz, and the theoretical compressed linewidth is ≈1.6kHz. Considering actual noise (1 / f noise, thermal noise, residual AM noise), a linewidth of 5-10kHz can be achieved in engineering.

[0072] Loss and gain compensation

[0073] After quantum well hybridization, the absorption coefficient α of DBR passive waveguide segment 23 is < 2cm. -1 The single-pass optical loss for a 2mm length is approximately 1.7dB (4.34×α×L). Adding waveguide propagation loss (approximately 0.5dB) and metal absorption loss (approximately 0.5dB), the total loss is approximately 2.7dB. The DFB active waveguide section 22 uses a weakly coupled grating (κ=8cm). -1 (κL=1.6), mode gain estimation: take material gain 500cm -1 If the light confinement factor Γ = 0.5%, then the mode gain g_mode = 2.5cm. -1 The gain in single-pass mode is 2.5cm. -1 ×0.2cm=0.5Np, which translates to approximately 0.5×4.34≈2.2dB in dB. Considering the feedback provided by the approximately 30% natural cleavage reflectivity of the rear end face, the actual threshold gain requirement is reduced. Therefore, the 2.2dB mode gain is sufficient to compensate for the 2.7dB DBR loss and end face output loss. If a higher constraint factor or a slightly longer DFB active waveguide section 22 (2.5mm) is used, the gain margin is even more ample.

[0074] Weakly Coupled DFB Design and Its Spatial Burn-in Suppression

[0075] This embodiment uses a weakly coupled DFB grating (κ=8cm). -1 (κL=1.6), this is an optimized design for a 2mm long cavity. Conventional DFB lasers typically use κL≈1-2, while for short cavities (e.g., 300μm), κ≈30-60cm is required. -1 For a 2mm long cavity, maintaining κL=1.6 only requires κ≈8cm. -1 This design has the following advantages:

[0076] Weak coupling ensures moderate standing wave intensity within the cavity, avoiding the spatial hole-burning effect under high power.

[0077] The λ / 4 phase-shift structure can still provide a narrow-band transmission peak under weak coupling, ensuring single-mode selectivity;

[0078] Experiments show that when κL=1.6, the threshold gain of the DFB active waveguide segment 22 is approximately 4cm. -1 With an injection current of 250mA, a mode gain of >2dB can be obtained, which is sufficient to compensate for the loss of the passive waveguide section 23 of the DBR.

[0079] Current density reliability: In this embodiment, the metal grating strip is 2μm wide and 200nm thick. Under an injection current of 250mA, without parallel electrodes, the grating strip current density reaches as high as 62.5A / cm². 2 This could lead to electromigration risks. To address this, this embodiment connects a thickened Au electrode (500 nm thick, 50 μm wide) in parallel above the metal grating, ensuring that the vast majority of the current (>90%) flows through the thickened electrode, reducing the actual current density of the grating strip to a negligible level and guaranteeing long-term reliability.

[0080] Low resistance design of metal grating electrodes

[0081] The metal grating itself has a stripe width of approximately 2μm and a thickness of 200nm; a 2mm stripe has a resistance of approximately 15-20Ω. To reduce the series resistance, this embodiment connects a thickened Au electrode (500nm thick, 50μm wide) in parallel above the metal grating. Through windowed contact, the total resistance is reduced to below 5Ω. With an injection current of 200-300mA, the metal voltage drop is <1.5V, which does not affect laser performance.

[0082] Proton injection isolation loss description

[0083] In this embodiment, a 150keV proton injection is used to form the electrically isolated region, and the annealing conditions are 750°C for 30 seconds. This condition can partially repair lattice damage, but a small number of point defects may still remain, resulting in an additional optical loss of approximately 0.5-0.8 dB in the isolated region. This loss is within the total loss budget (<2 dB) of the DBR passive waveguide section 23 and has no substantial impact on device performance; therefore, a conservative value of additional optical loss <0.8 dB is adopted.

[0084] Preparation method:

[0085] S1. Using a conventional FP epitaxial wafer, an n-type InP lower cladding layer, a multi-quantum well active layer 21, and an InP upper waveguide layer are sequentially grown on a single InP substrate 1.

[0086] S2. Perform quantum well hybridization on the passive waveguide section 23 of the DBR, define the DBR region by photolithography, and deposit a 50nm thick SiN layer. x A protective layer was then deposited, followed by PECVD deposition of a 100 nm thick SiOP film with a 3% P content, and phosphorus ion implantation (300 keV, 5 × 10⁻⁶ m² / h). 13 cm -2） Rapid thermal annealing at 780℃ for 90 seconds to remove SiOP and SiN films. x Protective layer;

[0087] S3. Proton injection (150keV, 3×10⁻⁶) into the electrically isolated region. 13 cm -2 Anneal at 750℃ for 30 seconds;

[0088] S4. A metal layer (Ti / Au20 / 200nm) is deposited on the top surface of the ridge waveguide. Segmented metal gratings are formed by electron beam lithography and etching, and a 0.3μm transition region is set at the junction of two grating segments to avoid field splicing errors: DFB active waveguide segment period 226nm, groove depth 45nm, center λ_B DFB / 4 phase shift; DBR passive waveguide section period 231nm, groove depth 100nm, no phase shift, length 2mm; at the same time, a thickened electrode (Au thickness 500nm) is fabricated in parallel with the second metal grating to reduce resistance;

[0089] S5. Electrical isolation is achieved by removing the metal at the electrode gap through photolithography and etching;

[0090] S6. Thin the substrate to 80μm, deposit AuGeNin type electrodes on the back side, and alloy anneal;

[0091] S7, the chip is cleaved, with an Al2O3 antireflection coating of <0.5% on the output end face and no coating on the rear end face; it is packaged with a high thermal conductivity heat sink (SiC), and the thermal resistance from the DFB active waveguide section to the heat sink is controlled to ≤20K / W, thus completing the device.

[0092] Test conditions:

[0093] At a substrate temperature of 25℃, the injected currents in the DFB passive waveguide section 23 are 200mA, 250mA, and 300mA, with voltages of 1.7V, 1.8V, and 1.9V, respectively.

[0094] The test results are as follows:

[0095] 200mA: Output power 35mW, linewidth 8.2kHz, SMSR 61dB, temperature rise 6.0K, redshift 0.71nm;

[0096] 250mA: Output power 42mW, linewidth 6.5kHz, SMSR 62dB, temperature rise 8.1K, redshift 0.96nm;

[0097] 300mA: Output power 48mW, linewidth 5.1kHz, SMSR 63dB, temperature rise 10.2K, redshift 1.21nm.

[0098] At a 250mA operating point, after 1000 hours of continuous operation, the output power attenuation is <3%, the linewidth variation is <10%, and there is no mode skipping phenomenon. Environmental temperature variation tests (15-35℃) show that the TEC automatically adjusts the substrate temperature to 25℃, with device performance fluctuations of <5%.

[0099] 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. An ultra-narrow linewidth DBR+DFB laser based on two different external metal gratings, characterized in that, include: Monolithic InP substrate (1); The semiconductor gain chip (2) disposed on the upper surface of the monolithic InP substrate (1) includes a multi-quantum well active layer (21), a DFB active waveguide section (22) and a DBR passive waveguide section (23). The top of the DFB active waveguide section (22) is provided with a first metal grating (221), the first metal grating (221) has a period of 226nm, a groove depth of 30-60nm, and a λ_B at the center position. DFB / 4 wavelength phase shift, λ_B DFB =1550.36nm; The DBR passive waveguide section (23) is provided with a second metal grating (231) at the top. The second metal grating (231) has a period of 231nm and a groove depth of 80-120nm. The second metal grating (231) also serves as a p-type electrode. The DBR passive waveguide section (23) is made transparent through quantum well hybridization, and its absorption coefficient for lasing wavelength is less than 2 cm⁻¹. -1 The quantum well hybridization process includes: depositing a phosphorus-containing silicon oxide film with a thickness of 80-120 nm and a phosphorus atom percentage of 2%-5% on the surface of the passive waveguide section (23) of the DBR using PECVD; then performing phosphorus ion implantation at an implantation energy of 200-360 keV and a dose of 1×10⁻⁶. 13 -1×10 14 cm -2 Then, rapid hot annealing was performed. An electrically isolated region (24) is formed between the DFB active waveguide section (22) and the DBR passive waveguide section (23) through proton injection. The output end face of the ultra-narrow linewidth DBR+DFB laser based on two different external metal gratings is coated with an anti-reflection film, with a reflectivity of <0.5%. The operating temperature range of the ultra-narrow linewidth DBR+DFB laser based on two different external metal gratings is 15°C to 35°C.

2. The ultra-narrow linewidth DBR+DFB laser based on two different external metal gratings according to claim 1, characterized in that: The length of the DFB active waveguide segment (22) is 2 mm, and its coupling coefficient κ≈8 cm. -1 , κL=1.

6.

3. The ultra-narrow linewidth DBR+DFB laser based on two different external metal gratings according to claim 1, characterized in that: The length of the DBR passive waveguide segment (23) is 2 mm, and the coupling coefficient κ≈35 cm. -1 κL_g=7, reflectivity R=tanh 2 (κL_g)=tanh 2 (7)≈0.9999, 3-dB reflection bandwidth Δλ≈0.3nm.

4. The ultra-narrow linewidth DBR+DFB laser based on two different external metal gratings according to claim 1, characterized in that: The absorption coefficient α of the DBR passive waveguide section (23) after quantum well hybridization is < 2cm. -1 The single-pass optical loss for a 2mm length is ≤4.34×α×L≈1.7dB.

5. The ultra-narrow linewidth DBR+DFB laser based on two different external metal gratings according to claim 1, characterized in that: The first metal grating (221) and the second metal grating (231) are made of Ti / Au or Ni / Au.

6. The ultra-narrow linewidth DBR+DFB laser based on two different external metal gratings according to claim 1, characterized in that: The isolation resistance of the electrical isolation region (24) is >2MΩ, and the additional optical loss is <0.8dB.

7. The ultra-narrow linewidth DBR+DFB laser based on two different external metal gratings according to claim 1, characterized in that: Side-mode rejection ratio ≥60dB, relative intensity noise <-140dB / Hz@10GHz.

8. The ultra-narrow linewidth DBR+DFB laser based on two different external metal gratings according to claim 1, characterized in that: High thermal conductivity heat sink packaging is adopted. The thermal resistance from the DFB active waveguide section (22) to the heat sink is designed to be ≤20K / W to ensure that the temperature rise of the active region of the DFB active waveguide section (22) is 5-10K under an injection current of 200-300mA.

9. The ultra-narrow linewidth DBR+DFB laser based on two different external metal gratings according to claim 1, characterized in that, Its preparation method includes the following steps: S1. Using a conventional FP epitaxial wafer, an n-type InP lower cladding layer, a multi-quantum well active layer (21) and an InP upper waveguide layer are sequentially grown on a single InP substrate (1). S2. Quantum well hybridization is performed on the passive waveguide section (23) of the DBR. The DBR region is defined by photolithography, and a SiN layer with a thickness of 50 nm is deposited. x A protective layer was then deposited, followed by PECVD deposition of a 100 nm thick SiOP film with 3% phosphorus content. Phosphorus ion implantation was performed, and rapid thermal annealing at 780°C for 90 seconds was used to remove the SiOP film and SiN. x Protective layer; S3. Proton implantation is performed on the electrically isolated region (24), followed by annealing at 750°C for 30 seconds; S4. A metal layer is deposited on the top surface of the ridge waveguide, and a segmented metal grating is formed by electron beam lithography and etching. A 0.3 μm transition region is set at the junction of the two grating segments to avoid field splicing errors. The DFB active waveguide segment (22) has a period of 226 nm, a groove depth of 45 nm, and a center λ_B. DFB / 4 phase shift; DBR passive waveguide section (23) with period 231nm, groove depth 100nm, no phase shift, and length 2mm; at the same time, a thickened electrode is fabricated in parallel with the second metal grating (231) to reduce resistance; S5. Electrical isolation is achieved by removing the metal at the electrode gap through photolithography and etching; S6. Thin the substrate to 80μm, deposit AuGeNin type electrodes on the back side, and alloy anneal; S7, the chip is cleaved, the output end face is coated with an Al2O3 antireflection film with a reflectivity of <0.5%, and the rear end face is not coated; a high thermal conductivity heat sink is used for packaging, and the thermal resistance from the DFB active waveguide section (22) to the heat sink is controlled to ≤20K / W, thus completing the device.

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