Silicon-based hybrid integrated external cavity narrow linewidth laser

Abstract

The invention relates to a semiconductor laser, in particular to a silicon-based hybrid integrated external cavity narrow-linewidth laser, which is used for solving the problems of poor integration, insufficient stability, high cost and the like in the existing narrow-linewidth laser technology. The silicon-based hybrid integrated external cavity narrow linewidth laser comprises a semiconductor gain chip, a silicon-based grating and a conical optical fiber. Wherein the semiconductor gain chip is coupled with the incident end face of the grating outer cavity, and the emergent end face of the grating outer cavity is directly coupled with the tapered fiber. By utilizing the grating outer cavity, the photon service life is prolonged, the output line width is narrowed, and the device has the advantages of being narrow in output line width, high in output power, high in process stability, small in size, integrated and the like.

Description

Technical Field

[0001] The invention relates to a semiconductor laser, in particular to a silicon-based hybrid integrated external cavity narrow linewidth laser. Background Art

[0002] Narrow linewidth lasers are widely used in coherent communications, high-precision measurement, optical sensing, lidar, fiber-optic sensing, atomic clock pumping, etc. due to their high coherence and low phase-frequency noise. Their excellent performance makes them important in situations requiring high-precision signal processing and long-distance transmission.

[0003] The existing mainstream narrow linewidth lasers include fiber lasers, external cavity lasers, and distributed feedback (DFB) lasers. Fiber lasers have high output power and stability, but due to their poor performance stability and difficult integration, they are difficult to meet high-demand applications, especially in high-precision coherent communications. Fiber lasers are large in size and difficult to achieve high integration, which limits their widespread use in precision applications.

[0004] Etalon-based external cavity lasers can provide narrower output linewidths, but their manufacturing process is complex, optical path alignment is difficult, and the packaging process is cumbersome, which makes them expensive and difficult to achieve large-scale applications. In addition, due to the complexity of the process and high cost, the popularity of this type of laser is limited.

[0005] Distributed feedback lasers have certain advantages in reducing the output linewidth, but their output linewidth is usually only at the level of hundreds of kHz, and due to the limitations of the resonant cavity length and optical loss, it is difficult to further reduce the output linewidth, so it cannot meet the requirements in some high-precision applications.

[0006] Therefore, although the existing narrow linewidth laser technology has made certain progress, it still faces problems such as large size, poor integration, insufficient stability, and high cost. Summary of the invention

[0007] The purpose of the present invention is to solve the problems of large size, poor integration, insufficient stability, high cost and the like existing in the existing narrow linewidth laser technology, and to provide a silicon-based hybrid integrated external cavity narrow linewidth laser.

[0008] In order to solve the deficiencies of the above-mentioned prior art, the present invention provides the following technical solutions: A silicon-based hybrid integrated external cavity narrow linewidth laser, which is special in that it includes a semiconductor gain chip, a silicon-based grating, and a tapered optical fiber; The semiconductor gain chip is used to emit laser light, and its light emitting surface is coupled with the incident end surface of the grating external cavity; The grating external cavity comprises an optical cavity and a silicon-based grating arranged in the optical cavity. The silicon-based grating is used to reflect and enhance lasers of a specific wavelength by increasing the effective cavity length, while suppressing lasers of other wavelengths, thereby achieving narrow linewidth laser output and feeding back lasers to the semiconductor gain chip; The grating external cavity emission end face is directly end face coupled with the tapered optical fiber input end, and the mode field diameter of the tapered optical fiber input end matches the waveguide mode of the silicon-based grating; the tapered optical fiber is used to couple and shape the laser and output narrow line width laser; The mode field diameter of the tapered optical fiber output end is used to match the subsequent optical system.

[0009] Furthermore, a lens is included, and the semiconductor gain chip is coupled to the grating external cavity through the lens. The design of lens coupling not only reduces coupling loss, but also improves the overall output efficiency of the silicon-based hybrid integrated external cavity narrow linewidth laser.

[0010] Furthermore, the light-emitting surface and non-light-emitting surface of the semiconductor gain chip are coated with an anti-reflection film and a high-reflection film respectively. The design of the anti-reflection film and the high-reflection film ensures the optimization of the light feedback inside the semiconductor gain chip and reduces unnecessary light loss.

[0011] Furthermore, the reflectivity of the anti-reflection film on the light-emitting surface of the semiconductor gain chip is ≤0.01%, and the reflectivity of the high-reflection film on the non-light-emitting surface is ≥90%.

[0012] Furthermore, the silicon-based grating is one of a 1st-order grating, a 2nd-order grating, a 3rd-order grating, a 4th-order grating, and a 5th-order grating. The order of the grating affects its coupling coefficient. By selecting a suitable order and optimizing the design (such as duty cycle and etching depth), precise control of the reflectivity and the resonant cavity mode distribution can be achieved, thereby achieving a narrower laser output linewidth.

[0013] Furthermore, the incident end face and the output end face of the grating external cavity are coated with an anti-reflection film with a reflectivity of ≤0.01%. The purpose is to increase the coupling efficiency with the semiconductor gain chip, improve the overall output power of the silicon-based hybrid integrated external cavity narrow linewidth laser, and narrow the linewidth.

[0014] Furthermore, a low-loss SiO2 film or Si3N4 film with a thickness of ≥500 nm is deposited on the surface of the silicon-based grating to protect the silicon-based grating and reduce transmission loss.

[0015] Furthermore, the SiO2 film or Si3N4 film is prepared by low-pressure chemical vapor deposition.

[0016] Furthermore, a TEC (Thermoelectric Cooler) is integrated below the semiconductor gain chip and the grating external cavity.

[0017] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention is a silicon-based hybrid integrated external cavity narrow linewidth laser, which introduces a silicon-based grating at the end face of a semiconductor gain chip to extend the length of the grating external cavity and improve the photon lifetime, thereby achieving the purpose of narrowing the linewidth. At the same time, the integration of the semiconductor gain chip and the grating external cavity can ensure the low noise and high power output of the present invention.

[0018] (2) The silicon-based hybrid integrated external cavity narrow linewidth laser of the present invention has small size, high stability, simple preparation method and low cost, and provides an ideal on-chip narrow linewidth light source for future large-scale silicon photonic integrated systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 1 is a schematic structural diagram of a first embodiment of a silicon-based hybrid integrated external cavity narrow linewidth laser according to the present invention; Figure 2 Schematic diagram of the structure of the silicon-based grating in the first embodiment of the present invention; Figure 3 This is a schematic diagram of the output line width test results of the first embodiment of the present invention; Figure 4 This is a power-current curve diagram of the first embodiment of the present invention.

[0020] The reference numerals are as follows: 1-semiconductor gain chip; 2-lens; 3-grating external cavity; 4-tapered optical fiber; 5-silicon-based grating; 6-waveguide substrate; 7-intermediate layer; DETAILED DESCRIPTION

[0021] The present invention will be further described below in conjunction with the accompanying drawings and exemplary embodiments.

[0022] Embodiment 1 Reference Figure 1 A silicon-based hybrid integrated external cavity narrow linewidth laser comprises a semiconductor gain chip 1, a lens 2, a grating external cavity 3, and a tapered optical fiber 4. The grating external cavity 3 comprises an optical cavity and a silicon-based grating 5 arranged in the optical cavity.

[0023] The light-emitting surface of the semiconductor gain chip 1 is coupled to the incident end face of the grating external cavity 3 through a lens 2, and the lens 2 is used to adjust the output mode field of the semiconductor gain chip 1 to match the waveguide mode of the silicon-based grating 5 to reduce the coupling loss; the output end face of the grating external cavity 3 is directly end-face coupled to the input end of the tapered optical fiber 4 to further improve the integration of the device, and the mode field diameter of the input end of the tapered optical fiber 4 matches the waveguide mode of the silicon-based grating 5, and the mode field diameter of the output end is larger than that of the input end.

[0024] The substrate of the semiconductor gain chip 1 is InP, and the gain medium is InGaAsP. In other embodiments, the gain medium may also be AlGaInAs. The semiconductor gain chip 1 may have a ridge structure or a buried structure. The light-emitting surface and the non-light-emitting surface of the semiconductor gain chip 1 are respectively coated with an anti-reflection film and a high-reflection film. The reflectivity of the anti-reflection film is ≤0.01%, and the reflectivity of the high-reflection film is ≥90%.

[0025] Silicon-based grating 5 Figure 2 , which is prepared on a silicon-based waveguide with a ridge structure, the silicon-based waveguide includes a waveguide substrate 6 (SiO2), an intermediate layer 7 (Si), the top surface of the intermediate layer 7 is etched with periodic grooves to form a silicon-based grating 5, the silicon-based grating 5 is one of the first-order grating, the second-order grating, the third-order grating, the fourth-order grating, and the fifth-order grating; in this embodiment, the silicon-based grating 5 adopts a Bragg grating, and the Bragg grating is a third-order grating (the third-order diffraction peak is used to reflect the light of the target wavelength). The third-order grating achieves a good balance between reflection efficiency, manufacturing process tolerance and the size of the grating external cavity 3.

[0026] The purpose of narrowing the output linewidth of the silicon-based hybrid integrated external cavity narrow linewidth laser can be achieved by optimizing the duty cycle, etching depth, length and other design parameters of the silicon-based grating 5, optimizing the coupling coefficient and reflectivity of the silicon-based grating 5, and comprising the following steps: Step 1: Determine the target operating wavelength λ of the silicon-based hybrid integrated external cavity narrow linewidth laser B and the effective refractive index n of the silicon-based waveguide eff ; Step 2, determining the period Λ of the silicon-based grating 5 by Bragg conditions; Step 3, preliminarily select the duty cycle of the silicon-based grating 5 (e.g., 50%), and use an optical simulation tool (e.g., Lumerical FDTD Solutions) for simulation optimization; Step 4, determine the range of the etching depth h of the silicon-based grating 5 (eg, 100-500 nm), and simulate and adjust to balance the reflectivity R and loss L' of the silicon-based grating 5; L'=1−T−R Where T is the transmittance, which is the ratio of the grating transmitted light power to the incident light power; Step 5: Determine the length L of the silicon-based grating 5 and calculate the reflectivity R=tanh 2 (κL), so that κL is within an ideal range (such as 1 to 3), κ is the coupling coefficient of the silicon-based grating 5, and κL is the normalized length of the silicon-based grating 5; Step 6: verify the reflection bandwidth Δλ of the silicon-based grating 5 to ensure that it matches the highest gain wavelength of the semiconductor gain chip 1 .

[0027] The incident end face and the exit end face of the grating external cavity 3 are both coated with an anti-reflection film, and the reflectivity of the anti-reflection film is ≤0.01%, which plays a role in reducing the coupling loss between the silicon-based grating 5 and the semiconductor gain chip 1 and the tapered optical fiber 4, and can improve the output power of the silicon-based hybrid integrated external cavity narrow linewidth laser and reduce the output linewidth of the silicon-based hybrid integrated external cavity narrow linewidth laser.

[0028] The coupling of the silicon-based grating 5 and the tapered optical fiber 4 instead of the single-mode optical fiber is to further improve the coupling efficiency and the output power of the silicon-based hybrid integrated external cavity narrow linewidth laser, and also to reduce the output linewidth of the silicon-based hybrid integrated external cavity narrow linewidth laser.

[0029] The surface of the silicon-based grating 5 is prepared with a low-loss SiO2 film by low-pressure chemical vapor deposition, with a thickness of ≥500nm, which plays a role in protecting the silicon-based grating 5. Low loss refers to low absorption loss and low scattering loss. In other embodiments, Si3N4 film can also be used instead of SiO2 film.

[0030] The mode field diameter at the input end of the tapered optical fiber 4 matches the waveguide mode of the silicon-based grating 5, and has low coupling loss. At the same time, the output end of the tapered optical fiber 4 has a larger mode field diameter for matching the subsequent optical system. A TEC can be added under the semiconductor gain chip 1 and the grating external cavity 3 to actively control the temperature of the silicon-based hybrid integrated external cavity narrow linewidth laser to improve thermal stability and ensure the long-term stability of the output frequency and output linewidth.

[0031] The working principle of the present invention is as follows: Step 1: optical amplification process; The gain medium in the semiconductor gain chip 1 stimulates electron transition through carrier injection, generates stimulated emission photons, and outputs laser light to the lens 2; the light-emitting surface is coated with an anti-reflection film to ensure that the laser light is output with low reflectivity; the non-light-emitting surface is coated with a high-reflection film to reflect the laser light back to the gain medium of the semiconductor gain chip 1; Step 2: Optical coupling; The lens 2 collects the laser and couples it to the grating external cavity 3. The function of the lens 2 is to reduce the divergence angle of the laser, reduce the coupling loss, and improve the transmission efficiency of the laser. Step 3: Wavelength selection and feedback; The grating external cavity 3 enhances the laser of a specific wavelength by reflection, while suppressing the laser of other wavelengths, thereby achieving narrow line width laser output; At the same time, the laser is fed back to the gain medium of the semiconductor gain chip 1 to form a stable laser oscillation loop, further improving the coherence of the laser; Step 4: Optical signal output; The laser output from the grating external cavity 3 enters the tapered optical fiber 4 through end face coupling. The tapered optical fiber 4 shapes and efficiently couples the laser to ensure that the laser output has a high-quality beam and low loss. The output wavelength of this embodiment is 1260-1560nm.

[0032] Figure 3 The output line width test result of this embodiment is shown in Figure 1. The measured wavelength is 1550 nm. The output line width of this embodiment is 4.898 kHz. The horizontal axis is the frequency range (Frequency), in Hz, which represents the distribution range of the noise frequency, from 1 Hz to 2×10 7 Hz, covering the low frequency, medium frequency and high frequency range. The vertical axis represents the optical frequency noise density (Optical Frequency Noise), the unit is Hz 2 / Hz, that is, the frequency noise intensity at different frequencies. The frequency noise density in the figure shows a low noise characteristic as a whole, indicating that the spectrum of this embodiment is pure.

[0033] Figure 4 The power-current curve diagram of this embodiment, the horizontal axis is the current, the unit is mA, the vertical axis is the power, the unit is mW; the current threshold (Threshold Current, I th ) is 60 mA, indicating that the semiconductor gain chip 1 has good gain efficiency, indicating that the design of the device can effectively achieve stimulated emission under laser oscillation conditions; the width of the linear output region is large (60-800 mA), showing the stability of this embodiment in a wide current range; at a driving current of 800 mA, the fiber output power is as high as 14.5 mW, indicating the potential of this embodiment in miniaturized and high-power applications.

[0034] Embodiment 2 A silicon-based hybrid integrated external cavity narrow linewidth laser comprises a semiconductor gain chip 1, a silicon-based grating 5, and a tapered optical fiber 4. The grating external cavity 3 comprises an optical cavity and a silicon-based grating 5 arranged in the optical cavity.

[0035] The light-emitting surface of the semiconductor gain chip 1 is directly coupled with the incident end face of the grating external cavity 3 to improve the integration and further reduce the size of the device. The output mode field of the semiconductor gain chip 1 matches the waveguide mode of the silicon-based grating 5. The grating external cavity 3 output end face is directly end face coupled with the input end of the tapered optical fiber 4, and the mode field diameter of the input end of the tapered optical fiber 4 matches the waveguide mode of the silicon-based grating 5, and the output end has a larger mode field diameter than the input end.

[0036] In this embodiment, the silicon-based grating 5 adopts a Bragg grating, and the Bragg grating is a 5-order grating. The 5-order grating has a large processing tolerance and a short processing cycle, and is suitable for low-cost applications.

[0037] A low-loss Si3N4 film with a thickness of ≥500nm is prepared on the silicon-based grating 5 by low-pressure chemical vapor deposition, which plays a role in protecting the silicon-based grating 5. Low loss refers to low absorption loss and low scattering loss. At the same time, compared with SiO2 film, the refractive index difference of the grating region using Si3N4 film is lower, which makes it easy to prepare a grating with a lower coupling coefficient, and can achieve a narrower output line width.

[0038] A TEC is integrated below the semiconductor gain chip 1 and the grating external cavity 3 to maintain the temperature stability of the coupling region and prevent the influence of temperature drift on the lasing wavelength.

[0039] The rest of the configuration of this embodiment is the same as that of the first embodiment.

[0040] Those skilled in the art will easily understand that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A silicon-based hybrid integrated external cavity narrow linewidth laser, characterized in that: It comprises a semiconductor gain chip (1), a grating external cavity (3), and a tapered optical fiber (4); The semiconductor gain chip (1) is used to emit laser light, and its light emitting surface is coupled with the incident end surface of the grating external cavity (3); The grating external cavity (3) comprises an optical cavity and a silicon-based grating (5) arranged in the optical cavity, wherein the silicon-based grating (5) is used to reflect and enhance laser light of a specific wavelength by increasing the effective cavity length, while suppressing laser light of other wavelengths, thereby achieving narrow line width laser output, and feeding back the laser light to the semiconductor gain chip (1); The grating external cavity (3) output end face is directly end face coupled with the input end of the tapered optical fiber (4), and the mode field diameter of the input end of the tapered optical fiber (4) matches the waveguide mode of the silicon-based grating (5); the tapered optical fiber (4) is used to couple and shape the laser and output narrow line width laser; The mode field diameter at the output end of the tapered optical fiber (4) is used to match a subsequent optical system.

2. The silicon-based hybrid integrated external cavity narrow linewidth laser according to claim 1, characterized in that: It also comprises a lens (2), and the semiconductor gain chip (1) and the incident end face of the grating external cavity (3) are coupled via the lens (2).

3. The silicon-based hybrid integrated external cavity narrow linewidth laser according to claim 1 or 2, characterized in that: The light-emitting surface and the non-light-emitting surface of the semiconductor gain chip (1) are respectively coated with an anti-reflection film and a high-reflection film.

4. The silicon-based hybrid integrated external cavity narrow linewidth laser according to claim 3, characterized in that: The reflectivity of the anti-reflection film on the light-emitting surface of the semiconductor gain chip (1) is ≤0.01%, and the reflectivity of the high-reflection film on the non-light-emitting surface is ≥90%.

5. The silicon-based hybrid integrated external cavity narrow linewidth laser according to claim 3, characterized in that: The silicon-based grating (5) is one of a 1st-order grating, a 2nd-order grating, a 3rd-order grating, a 4th-order grating, and a 5th-order grating.

6. The silicon-based hybrid integrated external cavity narrow linewidth laser according to claim 5, characterized in that: The incident end face and the output end face of the grating external cavity (3) are both plated with an anti-reflection film with a reflectivity of ≤0.01%.

7. The silicon-based hybrid integrated external cavity narrow linewidth laser according to claim 6, characterized in that: A SiO2 film or Si3N4 film with a thickness of ≥500 nm is deposited on the surface of the silicon-based grating (5).

8. The silicon-based hybrid integrated external cavity narrow linewidth laser according to claim 7, characterized in that: The SiO2 film or Si3N4 film is prepared by low-pressure chemical vapor deposition.

9. The silicon-based hybrid integrated external cavity narrow linewidth laser according to claim 7, characterized in that: TEC is integrated below the semiconductor gain chip (1) and the grating external cavity (3).

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