An ultra-narrow linewidth tunable wavelength external cavity laser based on planar metal grating

Abstract

The application relates to the technical field of semiconductor lasers, in particular to an ultra-narrow linewidth tunable wavelength external cavity laser based on a planar metal grating, which comprises a gain chip, the left and right end faces of the gain chip are respectively coated with a high reflection film and a first antireflection film; a planar metal grating waveguide is arranged on the right side of the gain chip, the right end face of the gain chip is aligned and attached to the left end face of the planar metal grating waveguide, and the left and right end faces of the planar metal grating waveguide are respectively coated with a second antireflection film and a third antireflection film; the planar metal grating waveguide comprises a grating waveguide substrate, a back ridge optical waveguide is arranged on the upper portion of the grating waveguide substrate, a planar metal grating is arranged on the upper portion of the back ridge optical waveguide, and a top covering layer is arranged on the upper portion of the planar metal grating. Through the centimeter-level metal grating and the low-loss coupling structure, the application realizes a 3dB linewidth of less than 700Hz and a beam quality of less than 0.9. 2 ​

Description

Technical Field

[0001] This invention relates to the field of semiconductor laser technology, specifically to an ultra-narrow linewidth tunable wavelength external cavity laser based on a planar metal grating. Background Technology

[0002] DFB lasers were developed in the 1970s to improve the monochromaticity and wavelength stability of lasers. They achieve wavelength selection and optical feedback by integrating a periodically varying refractive index grating structure into the laser cavity, replacing the end-face mirrors of traditional FP cavities, and utilizing the Bragg condition.

[0003] Limitations of existing technology:

[0004] 1. DFB lasers require secondary epitaxial growth, with a process yield of only 65-70% (data source: Photonics Research, 2021), and the manufacturing cost is 2-3 times higher than that of external cavity lasers.

[0005] 2. Semiconductor grating etching: Due to the anisotropy of the crystal lattice, there are many irregularities in the surface structure after photolithography, which affects and increases the linewidth of the laser.

[0006] 3. An additional grating etching process is added, increasing costs.

[0007] 4. The reflectivity of semiconductor gratings is approximately 93-95%, requiring additional gain compensation, which increases power consumption by 15-20%.

[0008] 5. Internal cavity structure M 2 The factor is typically >1.5, and the coupling efficiency in photonic integrated systems is <85% (Reference: Optics Express, 2019). Summary of the Invention

[0009] The purpose of this invention is to provide an ultra-narrow linewidth tunable wavelength external cavity laser based on a planar metal grating to solve the problems mentioned in the background art.

[0010] To achieve the above objectives, the present invention provides the following technical solution: an ultra-narrow linewidth tunable wavelength external cavity laser based on a planar metal grating, comprising: a gain chip, wherein a high-reflection film is deposited on the left end face of the gain chip, and a first anti-reflection film is deposited on the right end face of the gain chip;

[0011] A planar metal grating waveguide is provided on the right side of the gain chip. The right end face of the gain chip is aligned and attached to the left end face of the planar metal grating waveguide. A second anti-reflection film is deposited on the left end face of the planar metal grating waveguide, and a third anti-reflection film is deposited on the right end face of the planar metal grating waveguide.

[0012] The plane metal grating waveguide comprises a grating waveguide substrate, a back ridge optical waveguide is arranged on the upper portion of the grating waveguide substrate, a plane metal grating is arranged on the upper portion of the back ridge optical waveguide, and a top cover layer is arranged on the upper portion of the plane metal grating.

[0013] As a preferred form of the above technical solution, the emission center wavelength of the gain chip is 1530-1610 nm.

[0014] As a preferred form of the above technical solution, the emission center wavelength of the gain chip is 1550 nm.

[0015] As a preferred form of the above technical solution, the length of the plane metal grating is 1-3 cm, the period is 220-230 nm, and the surface roughness is less than 0.8 nm.

[0016] As a preferred form of the above technical solution, the ridge width of the back ridge optical waveguide is 3-5 μm, the ridge height is 2-4 μm, and the etching depth is 1-1.5 nm.

[0017] As a preferred form of the above technical solution, the material of the back ridge optical waveguide comprises but is not limited to Si3N4, monocrystalline silicon and lithium niobate.

[0018] As a preferred form of the above technical solution, a TEC temperature control module is arranged below the gain chip and the plane metal grating waveguide.

[0019] As a preferred form of the above technical solution, the top cover layer is an epitaxially grown SiO2 layer.

[0020] The application provides an ultra-narrow linewidth tunable wavelength external cavity laser based on a plane metal grating, which has the following beneficial effects:

[0021] 1. No etching is needed, the process flow is simplified, and the controllable precision and flatness of the grating structure are improved.

[0022] 2. The reflectivity of the plane metal grating is greater than 99.5% (Cr / Au structure), which is significantly higher than that of a conventional etched semiconductor grating (reflectivity of about 93-95%).

[0023] 3. The flatness of the plane metal grating is less than 0.08 nm RMS (detected by a white light interferometer), and the reflection filtering efficiency is significantly improved.

[0024] 4. A top cover layer is arranged on the upper portion of the plane metal grating, the top cover layer is an SiO2 layer, the problem of mismatching of secondary epitaxial lattices of semiconductors is solved, diffraction light is constrained, and light energy overflow is reduced.

[0025] 5. The external cavity structure provides the freedom of a grating waveguide length much greater than the length of the gain chip, so that a longer effective cavity length and filtering space are possible, thereby further reducing the linewidth.

[0026] 6. The material of the back ridge waveguide includes but not limited to Si3N4, single crystal silicon and lithium niobate, the absorption coefficient <0.08 dB / cm (measured data) at 1550 nm wavelength, the effective refractive index n eff =2.025±0.005 (ellipsometer test). BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 is a structural schematic diagram of the present application;

[0028] Figure 2 is a top view of Figure 1 .

[0029] Figure 3 is a structural schematic diagram of the planar metal grating waveguide in the present application.

[0030] Figure 4 is a top view of Figure 3 .

[0031] In the figure: 1, gain chip; 11, high reflection film; 12, first anti-reflection film; 2, planar metal grating waveguide; 21, grating waveguide substrate; 22, back ridge waveguide; 23, planar metal grating; 24, second anti-reflection film; 25, third anti-reflection film; 26, top cover layer; 3, TEC temperature control module. DETAILED DESCRIPTION

[0032] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application.

[0033] It should be noted that the PID control algorithm implementation method is as follows:

[0034] 1. Proportional (P) part: calculate the error between the set temperature and the actual temperature, and generate a control signal proportional to the error according to the proportional coefficient (Kp); for example, if the set temperature is 25℃, the current actual temperature is 24℃, the error is 1℃, and Kp=10, then the proportional output is 10x1=10.

[0035] 2. Integral (I) part: accumulate the error, and generate an integral control signal according to the integral coefficient (Ki) to eliminate the static error; the integral time constant (Ti) determines the strength of the integral action; for example, if the error 1℃ lasts for 10 seconds, and Ki=0.1, then the integral output is 0.1x1x10=1.

[0036] 3. Differential (D) part: calculates the rate of change of error, generates a differential control signal according to the differential coefficient (Kd), predicts the trend of error change, and reduces overshoot and oscillation; the differential time constant (Td) determines the strength of the differential action; for example, if the error suddenly changes from 1℃ to 2℃, and Kd=5, the differential output is 5×(2-1) / Δt, where Δt is the sampling time interval.

[0037] 4. Total control signal: the outputs of the proportional, integral and differential parts are added to obtain the total control signal, which is used to adjust the current of the TEC. Total control signal = proportional output + integral output + differential output. This signal is usually output to the driving circuit of the TEC through PWM (pulse width modulation) or other ways.

[0038] 5. Initial parameter setting: set the initial Kp, Ki and Kd values according to experience or reference to similar system parameters. For example, in some semiconductor laser temperature control systems, the initial parameters can be set as Kp=10, Ki=0.1, Kd=5.

[0039] 6. Observe system response: observe the system's response to temperature set value changes during actual operation, including overshoot, settling time and steady-state error, etc.; for example, if the system has a large overshoot after setting the temperature, it means that Kp may be set too large.

[0040] 7. Parameter adjustment: adjust the PID parameters according to the response. If the system response is slow, Kp can be increased appropriately; if there is a static error, Ki can be increased; if there is oscillation, Kd can be increased. This process may need to be iterated several times until the system achieves satisfactory control effect. For example, reduce the value of Ki to reduce the integral action and thus reduce oscillation.

[0041] In a specific implementation, the TEC is connected to the PID to realize a TEC temperature control module 3, with temperature stability of ±0.003℃ and wavelength stability of <±0.05pm / ℃.

[0042] Example 1

[0043] As shown in Figures 1-4 the embodiment, a super-narrow linewidth tunable wavelength external cavity laser based on a planar metal grating includes: a gain chip 1, a high reflection film 11 is coated on the left end face of the gain chip 1, and a first anti-reflection film 12 is coated on the right end face of the gain chip 1;

[0044] A planar metal grating waveguide 2 is arranged on the right side of the gain chip 1, the right end face of the gain chip 1 is aligned and attached to the left end face of the planar metal grating waveguide 2, a second anti-reflection film 24 is coated on the left end face of the planar metal grating waveguide 2, and a third anti-reflection film 25 is coated on the right end face of the planar metal grating waveguide 2.

[0045] It should be noted that the material parameters of the gain chip 1 are: InP / InGaAsP quantum well structure, lasing wavelength 1530-1610nm, threshold current <28mA (25℃), slope efficiency 0.85W / A.

[0046] In specific implementation, the right end face of the gain chip 1 is aligned and attached to the left end face of the planar metal grating waveguide 2 to form an external cavity structure, and the total length L of the external cavity is cavity =L wg +800μm, and the free spectral range is >50GHz.

[0047] The planar metal grating waveguide 2 comprises a grating waveguide substrate 21, a back ridge optical waveguide 22 is arranged on the upper part of the grating waveguide substrate 21, a planar metal grating 23 is arranged on the upper part of the back ridge optical waveguide 22, and a top cover layer 26 is arranged on the upper part of the planar metal grating 23.

[0048] As a preferred embodiment of the above technical solution, the emission center wavelength of the gain chip 1 is 1550nm.

[0049] As a preferred embodiment of the above technical solution, the length of the planar metal grating 23 is 1cm, the period is 220nm, and the surface roughness is <0.8nm.

[0050] As a preferred embodiment of the above technical solution, the ridge width of the back ridge optical waveguide 22 is 3μm, the ridge height is 2μm, and the etching depth is 1nm.

[0051] In specific implementation, the back ridge optical waveguide 22 adopts a metal layer with a Cr / Au double-layer structure, and the thickness of the metal layer is 150nm, of which Cr is 30nm and Au is 120nm.

[0052] As a preferred embodiment of the above technical solution, the material of the back ridge optical waveguide 22 includes but is not limited to Si3N4, monocrystalline silicon and lithium niobate.

[0053] In specific implementation, the absorption coefficient of Si3N4 is <0.08dB / cm, the refractive index n is 2.025±0.005; the absorption coefficient of monocrystalline silicon is 0.1-0.2dB / cm, the refractive index n is 3.47; and the absorption coefficient of lithium niobate is <0.05dB / cm, the refractive index n is 2.28.

[0054] As a preferred embodiment of the above technical solution, a TEC temperature control module 3 is arranged below the gain chip 1 and the planar metal grating waveguide 2.

[0055] In the embodiment, the TEC is selected and closely attached to the heat sink or chip of the semiconductor laser for real-time temperature monitoring. The TEC is installed to heat or cool the semiconductor laser, and the TEC is usually used in cooperation with the heat sink to ensure that the heat can be effectively dissipated to the environment.

[0056] The TEC is connected with the PID feedback control module to realize accurate control of the temperature.

[0057] It should be noted that the TEC temperature control module 3 selects an integrated micro-TEC (TEC1-12703), and the temperature fluctuation is <±0.003℃ (closed-loop control).

[0058] As a preferred embodiment of the above technical solution, the top cover layer 26 is an epitaxially grown SiO2 layer, which can constrain diffracted light and reduce light energy overflow.

[0059] Embodiment 2

[0060] As shown in Figures 1-4 , in this embodiment, a super-narrow linewidth tunable wavelength external cavity laser based on a planar metal grating includes: a gain chip 1, a high reflection film 11 is coated on the left end face of the gain chip 1, and a first anti-reflection film 12 is coated on the right end face of the gain chip 1.

[0061] A planar metal grating waveguide 2 is arranged on the right side of the gain chip 1, the right end face of the gain chip 1 is aligned and attached to the left end face of the planar metal grating waveguide 2, a second anti-reflection film 24 is coated on the left end face of the planar metal grating waveguide 2, and a third anti-reflection film 25 is coated on the right end face of the planar metal grating waveguide 2.

[0062] It should be noted that the material parameters of the gain chip 1 are: InP / InGaAsP quantum well structure, lasing wavelength 1530-1610nm, threshold current <28mA (25℃), and slope efficiency 0.85W / A.

[0063] In the embodiment, the right end face of the gain chip 1 is aligned and attached to the left end face of the planar metal grating waveguide 2 to form an external cavity structure, and the total length L cavity = L wg +800μm, and the free spectral range is >50GHz.

[0064] The planar metal grating waveguide 2 includes a grating waveguide substrate 21, a back ridge optical waveguide 22 is arranged on the upper part of the grating waveguide substrate 21, a planar metal grating 23 is arranged on the upper part of the back ridge optical waveguide 22, and a top cover layer 26 is arranged on the upper part of the planar metal grating 23.

[0065] As a preferred embodiment of the above technical solution, the emission center wavelength of the gain chip 1 is 1530nm.

[0066] As a preferred embodiment of the above technical solution, the planar metal grating 23 has a length of 3cm, a period of 230nm, and a surface roughness of <0.8nm.

[0067] As a preferred embodiment of the above technical solution, the back ridge waveguide 22 has a ridge width of 5μm, a ridge height of 4μm, and an etching depth of 1.5nm.

[0068] In a specific implementation, the back ridge waveguide 22 adopts a Cr / Au double-layer metal layer with a thickness of 250nm, of which Cr is 70nm and Au is 180nm.

[0069] As a preferred embodiment of the above technical solution, the material of the back ridge waveguide 22 includes, but is not limited to, Si3N4, single crystal silicon, and lithium niobate.

[0070] In specific implementations, Si3N4 has an absorption coefficient of <0.08dB / cm and a refractive index n=2.025±0.005; monocrystalline silicon has an absorption coefficient of 0.1-0.2dB / cm and a refractive index n=3.47; and lithium niobate has an absorption coefficient of <0.05dB / cm and a refractive index n=2.28.

[0071] As a preferred embodiment of the above technical solution, a TEC temperature control module 3 is disposed below the gain chip 1 and the planar metal grating waveguide 2.

[0072] In practice, a TEC (Heat Transfer Device) is selected and closely attached near the heat sink or chip of the semiconductor laser for real-time temperature monitoring. The TEC is installed to heat or cool the semiconductor laser; it is typically used in conjunction with a heat sink to ensure that heat is effectively dissipated into the environment.

[0073] The TEC is connected to a PID feedback control module to achieve precise temperature control.

[0074] It should be noted that the TEC temperature control module 3 is an integrated micro TEC (TEC1-12703), with temperature fluctuation < ±0.003℃ (closed-loop control).

[0075] As a preferred embodiment of the above technical solution, the top cover layer 26 is an epitaxially grown SiO2 layer, which can confine diffracted light and reduce light energy leakage.

[0076] Example 3

[0077] like Figures 1-4 As shown, in this embodiment, an ultra-narrow linewidth tunable wavelength external cavity laser based on a planar metal grating includes: a gain chip 1, wherein a high reflectivity film 11 is deposited on the left end face of the gain chip 1, and a first antireflection film 12 is deposited on the right end face of the gain chip 1.

[0078] The gain chip 1 is provided with a planar metal grating waveguide 2 on the right side, the right end face of the gain chip 1 is aligned and attached to the left end face of the planar metal grating waveguide 2, the left end face of the planar metal grating waveguide 2 is coated with a second antireflection film 24, and the right end face of the planar metal grating waveguide 2 is coated with a third antireflection film 25.

[0079] It should be noted that the material parameters of the gain chip 1 are: InP / InGaAsP quantum well structure, lasing wavelength 1530-1610 nm, threshold current <28 mA (25℃), and slope efficiency 0.85 W / A.

[0080] In specific implementation, the right end face of the gain chip 1 is aligned and attached to the left end face of the planar metal grating waveguide 2 to form an external cavity structure, and the total length L of the external cavity is 800-1000 μm. cavity =L wg +800 μm, and the free spectral range is >50 GHz.

[0081] The planar metal grating waveguide 2 comprises a grating waveguide substrate 21, a back ridge optical waveguide 22 is arranged on the upper portion of the grating waveguide substrate 21, a planar metal grating 23 is arranged on the upper portion of the back ridge optical waveguide 22, and a top cover layer 26 is arranged on the upper portion of the planar metal grating 23.

[0082] As a preferred embodiment of the above technical solution, the emission center wavelength of the gain chip 1 is 1610 nm.

[0083] As a preferred embodiment of the above technical solution, the length of the planar metal grating 23 is 2 cm, the period is 225 nm, and the surface roughness is <0.8 nm.

[0084] As a preferred embodiment of the above technical solution, the ridge width of the back ridge optical waveguide 22 is 4 μm, the ridge height is 3 μm, and the etching depth is 1.2 nm.

[0085] In specific implementation, the back ridge optical waveguide 22 adopts a metal layer with a Cr / Au double-layer structure, and the thickness of the metal layer is 200 nm, in which the thickness of Cr is 40 nm, and the thickness of Au is 160 nm.

[0086] As a preferred embodiment of the above technical solution, the material of the back ridge optical waveguide 22 includes but is not limited to Si3N4, monocrystalline silicon, and lithium niobate.

[0087] In specific implementation, the absorption coefficient of Si3N4 is <0.08 dB / cm, the refractive index n is 2.025±0.005; the absorption coefficient of monocrystalline silicon is 0.1-0.2 dB / cm, the refractive index n is 3.47; and the absorption coefficient of lithium niobate is <0.05 dB / cm, and the refractive index n is 2.28.

[0088] As a preferred embodiment of the above technical solution, a TEC temperature control module 3 is disposed below the gain chip 1 and the planar metal grating waveguide 2.

[0089] In practice, a TEC (Heat Transfer Device) is selected and closely attached near the heat sink or chip of the semiconductor laser for real-time temperature monitoring. The TEC is installed to heat or cool the semiconductor laser; it is typically used in conjunction with a heat sink to ensure that heat is effectively dissipated into the environment.

[0090] The TEC is connected to a PID feedback control module to achieve precise temperature control.

[0091] It should be noted that the TEC temperature control module 3 is an integrated micro TEC (TEC1-12703), with temperature fluctuation < ±0.003℃ (closed-loop control).

[0092] As a preferred embodiment of the above technical solution, the top cover layer 26 is an epitaxially grown SiO2 layer, which can confine diffracted light and reduce light energy leakage.

[0093] Comparative Example 1

[0094] Traditional DFB laser.

[0095] Comparative Example 2

[0096] Commercial external cavity laser.

[0097] The lasers provided in the three embodiments and two comparative examples were tested for various technical indicators, and the test results are shown in the table below:

[0098]

[0099] As shown in the table above, the ultra-narrow linewidth tunable wavelength external cavity laser provided by this invention achieves a 3dB linewidth <700Hz and MHz through a centimeter-scale metal grating (length 1-3cm) and a low-loss coupling structure (total loss <0.2dB). 2 The beam quality is <0.9. The device uses low-absorption waveguide material (absorption coefficient <0.08dB / cm), integrates a high-precision temperature control system (±0.003℃), and achieves wavelength stability of ±0.04pm / ℃. It is suitable for quantum communication, coherent optical communication, and photonic integrated chips.

[0100] It should be noted that the gain chip 1 is also suitable for other emission center wavelengths such as 1310nm, 1650nm, or even 905nm, as long as the light-emitting structure and material of the gain chip match the grating constant of the grating waveguide.

[0101] While embodiments of the application have been shown and described, it is to be understood that the embodiments described are merely exemplary of the principles and application of the present application. Numerous modifications and adaptions can be effected without departing from the spirit and scope of the present application, which is not limited to the exact construction and arrangement described. It is intended, therefore, to cover all modifications and adaptions that fall within the scope of the claims and their equivalents.

Claims

1. An ultra-narrow linewidth tunable wavelength external cavity laser based on a planar metal grating, comprising a gain chip (1), characterized in that: The gain chip (1) has a high reflectivity film (11) deposited on its left end face and a first antireflection film (12) deposited on its right end face. A planar 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 attached to the left end face of the planar metal grating waveguide (2). A second anti-reflection film (24) is deposited on the left end face of the planar metal grating waveguide (2), and a third anti-reflection film (25) is deposited on the right end face of the planar metal grating waveguide (2). The planar metal grating waveguide (2) includes a grating waveguide substrate (21), a back ridge waveguide (22) is disposed on the upper part of the grating waveguide substrate (21), a planar metal grating (23) is disposed on the upper part of the back ridge waveguide (22), and a top cover layer (26) is disposed on the upper part of the planar metal grating (23).

2. The ultra-narrow linewidth tunable wavelength external cavity laser based on a planar metal grating according to claim 1, characterized in that: The top cover layer (26) is an epitaxially grown SiO2 layer.

3. The ultra-narrow linewidth tunable wavelength external cavity laser based on a planar metal grating according to claim 1, characterized in that: A TEC temperature control module (3) is disposed below the gain chip (1) and the planar metal grating waveguide (2).

4. The ultra-narrow linewidth tunable wavelength external cavity laser based on a planar metal grating according to claim 1, characterized in that: The materials of the back ridge waveguide (22) include, but are not limited to, Si3N4, single crystal silicon and lithium niobate.

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

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

7. The ultra-narrow linewidth tunable wavelength external cavity laser based on a planar metal grating according to claim 1, characterized in that: The planar metal grating (23) has a length of 1-3 cm, a period of 220-230 nm, and a surface roughness of <0.8 nm.

8. The ultra-narrow linewidth tunable wavelength external cavity laser based on a planar metal grating according to claim 4, characterized in that: The planar metal grating (23) has a length of 2cm, a period of 225nm, and a surface roughness of <0.8nm.

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