Compact ultra-narrow linewidth external cavity laser based on top metal reflection grating

Abstract

The invention relates to the technical field of semiconductor lasers, in particular to a compact ultra-narrow linewidth external cavity laser based on a top metal reflection grating, which comprises a gain chip, the left end face of the gain chip is plated with a high reflection film, and the right end face of the gain chip is plated with a first antireflection film; the right side of the gain chip is provided with a metal grating waveguide, the right end face of the gain chip is aligned and attached to the left end face of the metal grating waveguide, the left end face of the metal grating waveguide is plated with a second antireflection film, and the right end face of the metal grating waveguide is plated with a third antireflection film; the gain chip comprises a substrate, the upper part of the substrate is provided with a ridge optical waveguide, and the upper part of the ridge optical waveguide is provided with a metal cover Bragg reflection grating. Through the centimeter-level metal grating and the low-loss coupling structure, light beam quality with 3dB line width less than 750Hz and M2 less than 1.1 is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of semiconductor lasers, in particular to a compact ultra-narrow linewidth external cavity laser based on a top metal reflection grating. Background Art

[0002] DFB lasers were invented in the 1970s to improve the monochromaticity and wavelength stability of lasers. They integrate a grating structure with periodic refractive index changes in the laser cavity, replacing the end mirrors of the traditional FP cavity, and using the Bragg condition to achieve wavelength selection and optical feedback.

[0003] Limitations of existing technologies: 1. The typical linewidth of traditional InP-based DFB lasers in the 1550nm band is 100-200kHz (reference: IEEE J. Sel. Top. Quantum Electron., 2020), which cannot meet the needs of emerging fields such as quantum communication (<1kHz), coherent optical communication (<10kHz) and terahertz photonics (<100Hz).

[0004] 2. The longitudinal mode spacing of DFB laser is Δλ=λ 2 / (2n eff ×L), it can be calculated that when L=300μm, Δλ≈0.01nm, which leads to intensified mode competition.

[0005] 3. The production of DFB lasers requires secondary epitaxial growth, and the process yield is only 65-70% (data source: Photonics Research, 2021). The manufacturing cost is 2-3 times higher than that of external cavity lasers.

[0006] 4. The reflectivity of semiconductor grating is about 93-95%, and additional gain compensation is required, which results in an increase of 15-20% in power consumption. Conflict between beam quality and integration 5. Inner cavity structure M 2 The factor is typically >1.5, resulting in coupling efficiencies <85% in photonic integrated systems (Reference: Optics Express, 2019). Summary of the Invention

[0007] The object of the present invention is to provide a compact ultra-narrow linewidth external cavity laser based on a top metal reflection grating to solve the problems raised in the above background technology.

[0008] To achieve the above objectives, the present invention provides the following technical solution: a compact ultra-narrow linewidth external cavity laser based on a top metal reflection grating, comprising: a gain chip, wherein the left end surface of the gain chip is coated with a high reflective film, and the right end surface of the gain chip is coated with a first anti-reflection film; A metal grating waveguide is provided on the right side of the gain chip, the right end face of the gain chip is aligned and bonded to the left end face of the metal grating waveguide, the left end face of the metal grating waveguide is plated with a second anti-reflection film, and the right end face of the metal grating waveguide is plated with a third anti-reflection film; The metal grating waveguide comprises a substrate, a ridge optical waveguide is arranged on the upper portion of the substrate, and a metal cover Bragg reflection grating is arranged on the upper portion of the ridge optical waveguide.

[0009] As a preferred embodiment of the above technical solution, the emission center wavelength of the gain chip is 1530-1610 nm.

[0010] As a preferred embodiment of the above technical solution, the emission center wavelength of the gain chip is 1550 nm.

[0011] As a preferred embodiment of the above technical solution, the metal cover Bragg reflection grating has a length of 1-3 cm, a period of 220-230 nm, and a surface roughness of less than 0.8 nm; As a preferred embodiment of the above technical solution, the ridge optical waveguide has a ridge width of 3-5 μm, a ridge height of 2-4 μm, and an etching depth of 1-1.5 nm.

[0012] As a preferred embodiment of the above technical solution, the material of the ridge optical waveguide includes but is not limited to Si3N4, single crystal silicon and lithium niobate.

[0013] As a preferred embodiment of the above technical solution, a back photodetector chip is provided on the high-reflection film side of the gain chip.

[0014] As a preferred embodiment of the above technical solution, a TEC temperature control module is provided below the gain chip and the metal grating waveguide.

[0015] The present invention provides a compact ultra-narrow linewidth external cavity laser based on a top metal reflection grating, which has the following beneficial effects: 1. The traditional sandwich-type DFB semiconductor grating is replaced by a metal-capped Bragg reflection grating. Its control accuracy and structural flatness are better than those of the semiconductor etched grating, ensuring the purity of the Bragg reflection spectrum and the narrow linewidth performance of the laser.

[0016] 2. No need for secondary epitaxy, simplifying the process.

[0017] 3. The metal cover Bragg reflection grating also serves as an electrode layer, so there is no need to make separate electrodes. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a structural schematic diagram of the present invention; Figure 2 for Figure 1 Top view of .

[0019] Figure 3 Schematic diagram of the structure of the metal grating waveguide in the present invention.

[0020] Figure 4 for Figure 3 Top view of .

[0021] In the figure: 1. Gain chip; 11. High-reflection film; 12. First anti-reflection film; 2. Metal grating waveguide; 21. Substrate; 22. Back ridge optical waveguide; 23. Metal cover Bragg reflection grating; 24. Second anti-reflection film; 25. Third anti-reflection film; 3. Back photodetector chip; 4. TEC temperature control module. DETAILED DESCRIPTION

[0022] The technical solutions in the embodiments of the present invention will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the present invention.

[0023] It should be noted that the PID control algorithm is implemented as follows: 1. Proportional (P) section: Calculates the error between the set temperature and the actual temperature and generates a control signal proportional to the error based on the proportional coefficient (Kp). For example, if the set temperature is 25°C, the current actual temperature is 24°C, the error is 1°C, and Kp = 10, the proportional output is 10 × 1 = 10.

[0024] 2. Integration (I) section: The error is accumulated and an integral control signal is generated based on the integral coefficient (Ki) to eliminate static error. The integral time constant (Ti) determines the strength of the integral effect. For example, if the error lasts for 1°C for 10 seconds and Ki = 0.1, the integral output is 0.1 × 1 × 10 = 1.

[0025] 3. Differentiation (D) section: Calculates the rate of change of the error and generates a differential control signal based on the differential coefficient (Kd) to predict the error trend and reduce overshoot and oscillation. The differential time constant (Td) determines the strength of the differential action. For example, if the error suddenly changes from 1°C to 2°C and Kd = 5, the differential output is 5×(2-1) / Δt, where Δt is the sampling time interval.

[0026] 4. Total Control Signal: The outputs of the proportional, integral, and differential components are summed to generate the total control signal, which is used to regulate the TEC current. The total control signal = proportional output + integral output + differential output. This signal is typically output to the TEC's driver circuit via PWM (pulse width modulation) or other methods.

[0027] 5. Initial parameter setting: Set the initial Kp, Ki, and Kd values ​​based on experience or reference to parameters of similar systems. For example, in some semiconductor laser temperature control systems, the initial parameters can be set to Kp=10, Ki=0.1, and Kd=5.

[0028] 6. Observe system response: During actual operation, observe the system's response to changes in the temperature set point, including overshoot, settling time, and steady-state error. For example, if the system exhibits a large overshoot after the set temperature, Kp may be set too large.

[0029] 7. Parameter Adjustment: Adjust PID parameters based on the response. If the system responds slowly, increase Kp appropriately; if there is static error, increase Ki; if oscillation occurs, increase Kd. This process may require multiple iterations until the system achieves satisfactory control. For example, reducing Ki can reduce the integral action and thus reduce oscillation.

[0030] In a specific implementation, TEC is connected to PID to realize TEC temperature control module 4, with temperature stability of ±0.003°C and wavelength stability of <±0.05pm / °C.

[0031] Example 1 like Figure 1-Figure 4 As shown, in this embodiment, a compact ultra-narrow linewidth external cavity laser based on a top metal reflection grating includes: a gain chip 1, wherein the left end surface of the gain chip 1 is coated with a high reflective film 11, and the right end surface of the gain chip 1 is coated with a first anti-reflection film 12; A metal grating waveguide 2 is provided on the right side of the gain chip 1. The right end face of the gain chip 1 is aligned with the left end face of the metal grating waveguide 2. The left end face of the metal grating waveguide 2 is plated with a second anti-reflection film 24, and the right end face of the metal grating waveguide 2 is plated with a third anti-reflection film 25. In a specific implementation, the right end face of the gain chip 1 is aligned with the left end face of the metal grating waveguide 2 to form an external cavity structure. The total length of the external cavity is L cavity =L wg +800μm, free spectral range >50GHz.

[0032] The metal grating waveguide 2 includes a substrate 21 , a ridge optical waveguide 22 is provided on the substrate 21 , and a metal cover Bragg reflection grating 23 is provided on the ridge optical waveguide 22 .

[0033] As a preferred embodiment of the above technical solution, the emission center wavelength of the gain chip 1 is 1550 nm.

[0034] As a preferred embodiment of the above technical solution, the metal cover Bragg reflection grating 23 has a length of 1 cm, a period of 220 nm, and a surface roughness of less than 0.8 nm; As a preferred embodiment of the above technical solution, the ridge optical waveguide 22 has a ridge width of 3 μm, a ridge height of 2 μm, and an etching depth of 1 nm.

[0035] In a specific implementation, the ridge optical waveguide 22 uses a metal layer with a Cr / Au double-layer structure, and the thickness of the metal layer is 150 nm, including 30 nm of Cr and 120 nm of Au.

[0036] As a preferred embodiment of the above technical solution, the material of the ridge optical waveguide 22 includes but is not limited to Si3N4, single crystal silicon and lithium niobate.

[0037] In a specific implementation, Si3N4 has an absorption coefficient of less than 0.08 dB / cm and a refractive index n=2.025±0.005; single crystal silicon has an absorption coefficient of 0.1-0.2 dB / cm and a refractive index n=3.47; and lithium niobate has an absorption coefficient of less than 0.05 dB / cm and a refractive index n=2.28.

[0038] As a preferred embodiment of the above technical solution, a back photodetector chip 3 is provided on the side of the high reflective film 11 of the gain chip 1 .

[0039] In a specific implementation, the back photodetector chip 3 is connected to a PID feedback control module.

[0040] As a preferred embodiment of the above technical solution, a TEC temperature control module 4 is provided below the gain chip 1 and the metal grating waveguide 2 .

[0041] In practice, a TEC is used to closely attach to the heat sink or chip of a semiconductor laser for real-time temperature monitoring. A TEC is installed to heat or cool a semiconductor laser and is typically used in conjunction with a heat sink to ensure that heat is effectively dissipated into the environment.

[0042] The TEC is connected to the PID feedback control module to achieve precise temperature control.

[0043] Example 2 like Figure 1-Figure 4 As shown, in this embodiment, a compact ultra-narrow linewidth external cavity laser based on a top metal reflection grating includes: a gain chip 1, wherein the left end surface of the gain chip 1 is coated with a high reflective film 11, and the right end surface of the gain chip 1 is coated with a first anti-reflection film 12; A metal grating waveguide 2 is provided on the right side of the gain chip 1. The right end face of the gain chip 1 is aligned with the left end face of the metal grating waveguide 2. The left end face of the metal grating waveguide 2 is plated with a second anti-reflection film 24, and the right end face of the metal grating waveguide 2 is plated with a third anti-reflection film 25. In a specific implementation, the right end face of the gain chip 1 is aligned with the left end face of the metal grating waveguide 2 to form an external cavity structure. The total length of the external cavity is L cavity =L wg +800μm, free spectral range >50GHz.

[0044] The metal grating waveguide 2 includes a substrate 21 , a ridge optical waveguide 22 is provided on the substrate 21 , and a metal cover Bragg reflection grating 23 is provided on the ridge optical waveguide 22 .

[0045] As a preferred embodiment of the above technical solution, the emission center wavelength of the gain chip 1 is 1530 nm.

[0046] As a preferred embodiment of the above technical solution, the metal cover Bragg reflection grating 23 has a length of 3 cm, a period of 230 nm, and a surface roughness of less than 0.8 nm; As a preferred embodiment of the above technical solution, the ridge optical waveguide 22 has a ridge width of 5 μm, a ridge height of 4 μm, and an etching depth of 1.5 nm.

[0047] In a specific implementation, the ridge optical waveguide 22 uses a metal layer with a Cr / Au double-layer structure, with a thickness of 250 nm, including 70 nm of Cr and 180 nm of Au.

[0048] As a preferred embodiment of the above technical solution, the material of the ridge optical waveguide 22 includes but is not limited to Si3N4, single crystal silicon and lithium niobate.

[0049] In a specific implementation, Si3N4 has an absorption coefficient of less than 0.08 dB / cm and a refractive index n=2.025±0.005; single crystal silicon has an absorption coefficient of 0.1-0.2 dB / cm and a refractive index n=3.47; and lithium niobate has an absorption coefficient of less than 0.05 dB / cm and a refractive index n=2.28.

[0050] As a preferred embodiment of the above technical solution, a back photodetector chip 3 is provided on the side of the high reflective film 11 of the gain chip 1 .

[0051] In a specific implementation, the back photodetector chip 3 is connected to a PID feedback control module.

[0052] As a preferred embodiment of the above technical solution, a TEC temperature control module 4 is provided below the gain chip 1 and the metal grating waveguide 2 .

[0053] In practice, a TEC is used to closely attach to the heat sink or chip of a semiconductor laser for real-time temperature monitoring. A TEC is installed to heat or cool a semiconductor laser and is typically used in conjunction with a heat sink to ensure that heat is effectively dissipated into the environment.

[0054] The TEC is connected to the PID feedback control module to achieve precise temperature control.

[0055] Example 3 like Figure 1-Figure 4 As shown, in this embodiment, a compact ultra-narrow linewidth external cavity laser based on a top metal reflection grating includes: a gain chip 1, wherein the left end surface of the gain chip 1 is coated with a high reflective film 11, and the right end surface of the gain chip 1 is coated with a first anti-reflection film 12; A metal grating waveguide 2 is provided on the right side of the gain chip 1. The right end face of the gain chip 1 is aligned with the left end face of the metal grating waveguide 2. The left end face of the metal grating waveguide 2 is plated with a second anti-reflection film 24, and the right end face of the metal grating waveguide 2 is plated with a third anti-reflection film 25. In a specific implementation, the right end face of the gain chip 1 is aligned with the left end face of the metal grating waveguide 2 to form an external cavity structure. The total length of the external cavity is L cavity =L wg +800μm, free spectral range >50GHz.

[0056] The metal grating waveguide 2 includes a substrate 21 , a ridge optical waveguide 22 is provided on the substrate 21 , and a metal cover Bragg reflection grating 23 is provided on the ridge optical waveguide 22 .

[0057] As a preferred embodiment of the above technical solution, the emission center wavelength of the gain chip 1 is 1610 nm.

[0058] As a preferred embodiment of the above technical solution, the metal cover Bragg reflection grating 23 has a length of 2 cm, a period of 225 nm, and a surface roughness of less than 0.8 nm; As a preferred embodiment of the above technical solution, the ridge optical waveguide 22 has a ridge width of 4 μm, a ridge height of 3 μm, and an etching depth of 1.2 nm.

[0059] In a specific implementation, the ridge optical waveguide 22 uses a metal layer of a Cr / Au double-layer structure with a thickness of 200 nm, including 40 nm of Cr and 160 nm of Au.

[0060] As a preferred embodiment of the above technical solution, the material of the ridge optical waveguide 22 includes but is not limited to Si3N4, single crystal silicon and lithium niobate.

[0061] In a specific implementation, Si3N4 has an absorption coefficient of less than 0.08 dB / cm and a refractive index n=2.025±0.005; single crystal silicon has an absorption coefficient of 0.1-0.2 dB / cm and a refractive index n=3.47; and lithium niobate has an absorption coefficient of less than 0.05 dB / cm and a refractive index n=2.28.

[0062] As a preferred embodiment of the above technical solution, a back photodetector chip 3 is provided on the side of the high reflective film 11 of the gain chip 1 .

[0063] In a specific implementation, the back photodetector chip 3 is connected to a PID feedback control module.

[0064] As a preferred embodiment of the above technical solution, a TEC temperature control module 4 is provided below the gain chip 1 and the metal grating waveguide 2 .

[0065] In practice, a TEC is used to closely attach to the heat sink or chip of a semiconductor laser for real-time temperature monitoring. A TEC is installed to heat or cool a semiconductor laser and is typically used in conjunction with a heat sink to ensure that heat is effectively dissipated into the environment.

[0066] The TEC is connected to the PID feedback control module to achieve precise temperature control.

[0067] Comparative Example 1 Conventional DFB laser.

[0068] Comparative Example 2 Commercial external cavity lasers.

[0069] The lasers provided in the above three embodiments and two comparative examples were tested for various technical indicators. The test results are shown in the following table: As can be seen from the table above, the present invention provides a compact ultra-narrow linewidth external cavity laser based on a top metal reflection grating, which achieves a 3dB linewidth of <750Hz and M through a centimeter-level metal grating (length 1-3cm) and a low-loss coupling structure (total loss <0.2dB). 2 The device boasts a beam quality of less than 1.1. It utilizes low-absorption waveguide materials (absorption coefficient less than 0.08dB / cm), integrates a high-precision temperature control system (±0.003°C), and offers wavelength stability of ±0.05pm / °C. It is suitable for applications in quantum communications, coherent optical communications, and photonic integrated chips.

[0070] It should be noted that the gain chip 1 is also suitable for other emission wavelengths such as 1310 nm, 1650 nm, and even 905 nm, as long as the light-emitting structure and material of the gain chip match the grating constant of the grating waveguide.

[0071] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A compact ultra-narrow linewidth external cavity laser based on a top metal reflection grating, comprising a gain chip (1), characterized in that: The left end surface of the gain chip (1) is plated with a high-reflection film (11), and the right end surface of the gain chip (1) is plated with a first anti-reflection film (12); A metal grating waveguide (2) is provided on the right side of the gain chip (1), the right end face of the gain chip (1) is aligned and bonded to the left end face of the metal grating waveguide (2), the left end face of the metal grating waveguide (2) is plated with a second anti-reflection film (24), and the right end face of the metal grating waveguide (2) is plated with a third anti-reflection film (25); The metal grating waveguide (2) comprises a substrate (21), a ridge optical waveguide (22) is provided on the upper portion of the substrate (21), and a metal cover Bragg reflection grating (23) is provided on the upper portion of the ridge optical waveguide (22).

2. The compact ultra-narrow linewidth external cavity laser based on a top metal reflection grating according to claim 1, characterized in that: The emission center wavelength of the gain chip (1) is 1530-1610 nm.

3. The compact ultra-narrow linewidth external cavity laser based on a top metal reflection grating according to claim 2, characterized in that: The emission center wavelength of the gain chip (1) is 1550 nm.

4. The compact ultra-narrow linewidth external cavity laser based on a top metal reflection grating according to claim 1, characterized in that: The metal cover Bragg reflection grating (23) has a length of 1-3 cm, a period of 220-230 nm, and a surface roughness of less than 0.8 nm.

5. The compact ultra-narrow linewidth external cavity laser based on a top metal reflection grating according to claim 4, characterized in that: The metal cover Bragg reflection grating (23) has a length of 2 cm, a period of 225 nm, and a surface roughness of less than 0.8 nm.

6. The compact ultra-narrow linewidth external cavity laser based on a top metal reflection grating according to claim 1, characterized in that: The ridge optical waveguide (22) has a ridge width of 3-5 μm, a ridge height of 2-4 μm, and an etching depth of 1-1.5 nm.

7. The compact ultra-narrow linewidth external cavity laser based on a top metal reflection grating according to claim 6, characterized in that: The ridge optical waveguide (22) has a ridge width of 4 μm, a ridge height of 3 μm, and an etching depth of 1.2 nm.

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