Chip integrated ultra-narrow linewidth laser based on micro FP (Fabry-Perot) cavity

Abstract

The invention discloses a chip integrated ultra-narrow linewidth laser based on a micro Fabry-Perot (FP) cavity. The chip integrated ultra-narrow linewidth laser comprises a laser chip, an on-chip reflection conversion loop, a coupling lens and the micro FP cavity, wherein the on-chip reflection conversion loop consists of a waveguide, a first phase shifter, an adjustable coupler, a second phase shifter and a waveguide grating; the core is that feedback light of the FP cavity is purified by utilizing a reflection conversion loop; specifically, laser is divided into two paths through an adjustable coupler in a loop, one path enters a miniature FP cavity, and the other path is reflected by a waveguide grating. Through accurate adjustment, non-resonant light directly reflected from the front end of the FP cavity and reflected light of the waveguide grating are subjected to destructive interference in the direction of returning to the laser chip, so that noise is filtered out, pure FP cavity resonant light is fed back to the laser chip, and the laser line width is narrowed by using the self-injection locking principle. Meanwhile, constructive interference occurs between the two paths of light at the output end, and output of narrow-linewidth laser is achieved.

Description

Technical Field

[0001] This invention relates to the field of integrated optics and narrow linewidth laser source technology, and in particular to a chip-integrated ultra-narrow linewidth laser based on a micro FP cavity. Background Technology

[0002] Narrow-linewidth lasers possess advantages such as high spectral purity, good coherence, and low phase noise, playing a crucial role in precision metrology, coherent optical communication, quantum computing, and optical sensing. Developing high-quality on-chip integrated light sources is essential for advancing the development of photonic integrated chips, and is of great significance for promoting the miniaturization and arraying of quantum computing and lidar, as well as the development of precision metrology equipment such as portable optical clocks.

[0003] Currently, common technical approaches to achieving on-chip integrated narrow-linewidth laser output involve heterogeneous or hybrid integration of semiconductor laser chips and silicon-based photonic chips. This includes using integrated Bragg gratings or specially structured waveguides for internal cavity optical feedback, and employing on-chip integrated microcavities such as micro-ring cavities or waveguide-type FP cavities to achieve self-injection locking. However, the former approach is limited by the properties of semiconductor materials and the precision of the resonant cavity, making it difficult to achieve laser linewidths below the kHz level. The latter approach, on the other hand, is limited by the thermal noise of the resonant cavity itself and the achievable precision, resulting in limited linewidth narrowing capabilities.

[0004] On the other hand, bulk high-precision FP cavities (including micro FP cavities) can provide higher precision and lower thermal noise. However, because the front face of a micro FP cavity strongly reflects laser frequency components that do not meet the resonance condition, its reflected signal contains a large number of non-resonant components, making it difficult to directly use its high-Q resonant light as a feedback signal to achieve self-injection locking. To address this, existing solutions typically use the transmission signal or higher-order mode of the micro FP cavity to achieve self-injection locking, or use techniques such as Pound-Drever-Hall (PDH) for frequency locking. However, these solutions often require complex optical path structures and alignment methods, or complex supporting circuits, resulting in a large system size, which to some extent restricts the compact integration of micro FP cavities with laser chips. Summary of the Invention

[0005] The purpose of this invention is to provide a chip-integrated ultra-narrow linewidth laser based on a micro-FP cavity, addressing the limitations of existing on-chip integrated narrow linewidth lasers and the difficulty in achieving high integration with traditional FP cavity-based narrow linewidth lasers. This invention introduces an on-chip reflection transformation circuit to perform interference filtering on the reflected signal from the micro-FP cavity, suppressing non-resonant reflections and extracting high-Q resonant light for self-injection locking. This allows for direct utilization of the high-Q characteristics of the micro-FP cavity, enabling the integration of the micro-FP cavity with a laser chip and achieving on-chip ultra-narrow linewidth laser output.

[0006] To achieve the above objectives, the present invention provides a chip-integrated ultra-narrow linewidth laser based on a micro FP cavity. The laser includes: a laser chip, an on-chip reflection conversion circuit, a coupling lens, and a micro FP cavity; wherein the on-chip reflection conversion circuit includes: a waveguide, a first phase shifter disposed on the waveguide, an adjustable coupler, a second phase shifter, and a waveguide grating.

[0007] The output terminal of the laser chip is optically coupled to the input terminal of the on-chip reflection conversion circuit. The laser generated by the laser chip is coupled into the on-chip reflection conversion circuit, transmitted through the waveguide to the tunable coupler, and split into a first optical path and a second optical path at the tunable coupler: the first optical path is output from the first output terminal of the on-chip reflection conversion circuit through the waveguide, coupled into the miniature FP cavity after mode matching by the coupling lens; the miniature FP cavity provides high Q-value resonant feedback for the laser that meets the resonance condition, and couples the resonant light and the non-resonant reflected light generated by the front end face back to the on-chip reflection conversion circuit through the coupling lens, and returns to the tunable coupler through the waveguide; the second optical path is transmitted through the waveguide to the waveguide grating, and reflected back to the tunable coupler by the waveguide grating.

[0008] The second phase shifter is disposed on the waveguide reflected back to the tunable coupler from the micro FP cavity, and is used to adjust the phase of the resonant light and the non-resonant reflected light from the micro FP cavity. By adjusting the coupling ratio of the tunable coupler and cooperating with adjusting the phase of the second phase shifter, the non-resonant reflected light and the light reflected back from the waveguide grating achieve destructive interference at the end of the tunable coupler facing the laser chip, thereby suppressing the energy of the non-resonant reflected light at that port, and extracting the narrow-linewidth FP cavity resonant light after interference filtering as self-injection locked feedback light, which is transmitted back to the laser chip in the reverse direction to achieve laser linewidth narrowing. The first phase shifter is disposed on the waveguide between the laser chip and the tunable coupler, and is used to adjust the feedback phase of the self-injection locked feedback light, so that the feedback light matches the phase of the cavity mode inside the laser chip, thereby optimizing the self-injection locked state and the linewidth narrowing effect.

[0009] At the other output of the adjustable coupler, the two reflected beams from the micro FP cavity and the waveguide grating are designed to achieve constructive interference at this port and are transmitted through the waveguide to the second output of the on-chip reflection transformation circuit to output a narrow linewidth laser signal, thereby realizing resonant feedback extraction and narrow linewidth laser output within the same on-chip structure.

[0010] Preferably, the micro FP cavity includes a first reflecting surface and a second reflecting surface disposed opposite to each other. The first reflecting surface is the front end face facing the coupling lens, and the second reflecting surface is concave, forming a plano-concave cavity structure. The first and second reflecting surfaces are coated with a highly reflective dielectric film at the laser chip output wavelength, enabling the laser to form a high-precision resonance within the micro FP cavity. In one embodiment, the precision F of the micro FP cavity can be approximately expressed by the following formula: R1 and R2 are the reflectivities of the first and second reflective surfaces at the calculated wavelengths, respectively; the substrate materials of the first and second reflective surfaces and the supporting materials between them are made of low thermal expansion materials, such as ultra-low thermal expansion glass materials, to improve the temperature stability of the cavity length and resonant frequency.

[0011] Preferably, the micro FP cavity is fabricated using vacuum bonding, which keeps the hollow cavity between the first and second reflective surfaces in a vacuum state, reducing the change in the refractive index of the air inside the cavity caused by changes in the external environment and reducing the impact on the thermal noise stability of the micro FP cavity.

[0012] Preferably, the adjustable coupler is an integrated optical 2×2 coupler, specifically a Mach-Zehnder interferometer coupler or a directional coupler; the first phase shifter and / or the second phase shifter are thermo-optical phase shifters, electro-optical phase shifters or carrier injection phase shifters; the waveguide grating is a Bragg grating or a reflection grating based on a periodic refractive index modulation structure, and its central reflection wavelength matches the target resonant wavelength of the micro FP cavity.

[0013] Preferably, the waveguide grating adopts an apodized grating design to improve the reflection bandwidth and sidelobe suppression ratio, ensuring a certain degree of laser frequency tunability during actual use. This improves the fabrication process tolerance of the device and allows the output frequency of the laser chip to be finely adjusted within a certain range to match the resonant wavelength of the miniature FP cavity.

[0014] Preferably, the on-chip reflection conversion circuit, the first phase shifter, the adjustable coupler, the second phase shifter, and the waveguide grating are all integrated on the same photonic chip; the coupling lens is a microlens or a graded refractive index lens, used to achieve mode matching between the on-chip waveguide mode and the spatial mode of the micro FP cavity; the laser chip is a semiconductor laser chip, including a distributed feedback laser chip, a distributed Bragg reflection laser chip, or an FP-type laser chip.

[0015] Preferably, the input and output ends of the on-chip reflection transformation circuit are designed with on-chip mode converters: at the input end of the on-chip reflection transformation circuit, the mode converter ensures mode matching between the laser chip and the waveguide; at the first output end of the on-chip reflection transformation circuit, the submicron-level mode field size of the waveguide is pre-amplified to the micron level by the on-chip mode converter and combined with the coupling lens to achieve a better mode matching effect with the micro FP cavity; at the second output end of the on-chip reflection transformation circuit, the on-chip mode converter is designed to ensure mode matching with the optical path (such as optical fiber, other integrated optical modules, etc.) that subsequently uses the narrow linewidth laser output.

[0016] One or more technical solutions provided in this invention have at least the following technical effects or advantages:

[0017] This invention constructs a reflection transformation circuit on-chip, consisting of an adjustable coupler, a waveguide grating, and a second phase shifter, to coherently modulate and interfere with the reflected signal from the front end of a micro FP cavity. This enables the non-resonant reflected light to achieve destructive interference at the port facing the laser chip, thereby suppressing the interference of non-resonant components on the laser chip. Furthermore, it extracts high-purity, high-Q resonant feedback light from the reflected signal, making the reflected signal of the micro FP cavity usable for self-injection locking.

[0018] This invention uses a bulk micro FP cavity as a high-Q reference cavity and optically couples it with an on-chip reflection transformation circuit through a coupling lens. While taking into account the high precision and low thermal noise characteristics of the micro FP cavity, it introduces a highly integrable on-chip interference structure to process the reflected signal, avoiding the dependence on complex optical paths and large-volume systems that are traditionally locked by only using transmission signals or PDH circuits.

[0019] The laser chip, on-chip reflection conversion circuit, coupling lens, and micro FP cavity of this invention can achieve a highly integrated packaging structure through a hybrid integration approach, making it suitable for mass production. It can be used as an on-chip integrated ultra-narrow linewidth laser source and applied to scenarios requiring high coherence and narrow linewidth light sources, such as precision metrology, coherent optical communication, quantum information, and lidar. Attached Figure Description

[0020] Figure 1This is a schematic diagram of a chip-integrated ultra-narrow linewidth laser based on a micro-FP cavity, according to an embodiment of the present invention.

[0021] Figure 2 This is a schematic diagram of the micro FP cavity structure provided in an embodiment of the present invention. Detailed Implementation

[0022] To facilitate understanding of the present invention, a more complete description of this application will be given below with reference to the accompanying drawings, which illustrate preferred embodiments of the invention. However, the invention can be implemented in many different forms and is not limited to the embodiments described herein; rather, these embodiments are provided to enable a more thorough understanding of the disclosure of the present invention.

[0023] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains; the terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to limit the invention; the term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0024] Example 1, Figure 1 This is a schematic diagram of a chip-integrated ultra-narrow linewidth laser based on a micro FP cavity according to an embodiment of the present invention. It includes a laser chip (1), an on-chip reflection conversion circuit (2), a micro FP cavity (3), and a coupling lens (4). The on-chip reflection conversion circuit is disposed on the photonic chip and the circuit configuration includes: an input port (201) optically coupled to the laser chip 1, a waveguide (202) for transmitting optical signals between various functional devices, a first phase shifter (203) disposed on the waveguide 202 and located between the laser chip 1 and the tunable coupler, a tunable coupler (204) disposed on the waveguide 202 for realizing tunable distribution of optical power and interference synthesis, a second phase shifter (205) disposed on the optical path returning from the micro FP cavity 3, a waveguide grating (206) disposed on the waveguide 202, a first output terminal (207) optically connected to the coupling lens 4, and a second output terminal (208) for outputting narrow linewidth laser signals.

[0025] In this embodiment, the laser chip 1 can be a semiconductor laser chip, such as a distributed feedback laser (DFB) chip, a distributed Bragg reflector (DBR) laser chip, or an FP-type laser chip. The laser chip 1 is optically coupled to the input port 201 of the on-chip reflection conversion circuit 2 via edge coupling or vertical coupling.

[0026] The laser generated by laser chip 1 is coupled into on-chip reflection conversion circuit 2, and then transmitted in waveguide 202 to tunable coupler 204, where it is split into two paths:

[0027] The first optical path: The light is transmitted along waveguide 202 to the first output terminal 207 of the on-chip reflection conversion circuit 2. After being output from the first output terminal 207, it undergoes mode conversion via coupling lens 4 and is coupled into the micro FP cavity 3. The micro FP cavity 3 provides high-Q resonant feedback for lasers that meet the resonance conditions, causing them to form narrow-linewidth resonant reflected light at the front end face after multiple round trips within the cavity. Simultaneously, the front end face of the micro FP cavity 3 generates non-resonant reflections of frequency components that do not meet the resonance conditions. These resonant lights, together with the non-resonant reflected lights, are coupled back to the first output terminal 207 via coupling lens 4 and re-enter the on-chip reflection conversion circuit 2, then transmitted back to the adjustable coupler 204 in waveguide 202.

[0028] The second optical path: the light is transmitted along the waveguide 202 to the waveguide grating 206, where the waveguide grating 206 reflects the incident light and generates an optical signal that returns to the adjustable coupler 204.

[0029] The second phase shifter 205 is positioned on the optical path returning from the miniature FP cavity 3 to adjust the phase of the resonant light and the non-resonant reflected light. By adjusting the coupling ratio of the adjustable coupler 204 and coordinating with the phase adjustment of the second phase shifter 205, the non-resonant reflected light from the miniature FP cavity 3 forms destructive interference with the optical signal reflected back by the waveguide grating 206 at the end of the adjustable coupler 204 facing the laser chip 1, thereby suppressing the energy of the non-resonant reflected light at this port. By appropriately setting the operating points of the adjustable coupler 204 and the second phase shifter 205, the residual light at this port is mainly narrow-linewidth FP cavity resonant light. This resonant light is used as self-injection locked feedback light and transmitted back to the laser chip 1, achieving self-injection locking and linewidth narrowing of the laser chip 1.

[0030] The first phase shifter 203 is disposed on the waveguide 202 between the laser chip 1 and the tunable coupler 204, and is used to adjust the overall feedback phase of the self-injection locked feedback light. By adjusting the first phase shifter 203, the feedback light is matched with the internal cavity mode of the laser chip 1 in phase, which is beneficial to achieving a stable self-injection locked state and further optimizing the linewidth narrowing effect.

[0031] At the other output of the adjustable coupler 204, the two optical signals from the micro FP cavity 3 and the waveguide grating 206 are designed to achieve constructive interference at this port, thereby obtaining power-enhanced narrow-linewidth resonant light at this port. This optical signal is transmitted through the waveguide 202 to the second output 208 of the on-chip reflection conversion circuit 2, and is output from the second output 208 as the narrow-linewidth laser output of this embodiment.

[0032] In some embodiments, in order to achieve mode matching between the laser chip 1, the on-chip reflection transformation circuit 2 and the subsequent optical path, an on-chip mode field converter structure can be provided at the input terminal 201 and / or the output terminals 207 and 208 of the on-chip reflection transformation circuit 2, for matching the mode field size and numerical aperture between the submicron waveguide mode and the external fiber mode, free space mode or other integrated optical module modes.

[0033] In some embodiments, the adjustable coupler 204 may employ an integrated optical 2×2 coupler structure, such as a Mach-Zehnder interferometer (MZI) coupler or a directional coupler; the first phase shifter 203 and / or the second phase shifter 205 may employ thermo-optic, electro-optic, or carrier injection phase shifters to achieve adjustable phase control; the waveguide grating 206 may employ a Bragg grating or a reflection grating based on periodic refractive index modulation, wherein its central reflection wavelength is preferably matched with the target resonant wavelength of the micro FP cavity 3.

[0034] In some preferred embodiments, the waveguide grating 206 may adopt an apodized Bragg grating design to improve the reflection bandwidth and sidelobe suppression ratio, increase the device fabrication process tolerance, and at the same time enable the output frequency of the laser chip 1 to be finely adjusted within a certain range to match the resonant wavelength of the micro FP cavity 3.

[0035] In this embodiment, the first phase shifter 203, the adjustable coupler 204, the second phase shifter 205, and the waveguide grating 206 in the on-chip reflection conversion circuit 2 are preferably integrated on the same photonic chip; the coupling lens 4 can be a microlens or a graded refractive index lens (GRIN lens) to achieve efficient mode matching between the on-chip waveguide mode and the spatial mode of the micro FP cavity 3; the laser chip 1 and the on-chip reflection conversion circuit 2 can be packaged on the same substrate in a hybrid integration manner.

[0036] like Figure 2 As shown in the diagram, this embodiment provides a structural schematic of the micro FP cavity 3. The micro FP cavity 3 includes a first reflecting surface 301 and a second reflecting surface 302 arranged opposite to each other, and a hollow cavity 304 is formed between them by a support structure 303. The first reflecting surface 301 is the front end face facing the coupling lens 4, and is preferably a planar reflecting surface; the second reflecting surface 302 is preferably a concave reflecting surface, and the two together constitute a plano-concave FP cavity.

[0037] High-reflectivity dielectric films 311 and 312 are respectively disposed on the optical working surfaces of the first reflecting surface 301 and the second reflecting surface 302, providing high reflectivity at the working wavelength corresponding to the output wavelength of the laser chip 1 to ensure high precision of intracavity resonance. In one implementation, the precision F of the micro FP cavity 3 can be approximately expressed by the following formula:

[0038]

[0039] Wherein, R1 and R2 are the reflectivities of the first reflecting surface 301 and the second reflecting surface 302 at the calculated wavelength, respectively. The miniature FP cavity 3 can achieve high precision by reasonably selecting the reflectivity parameters. High precision corresponds to high Q-value resonance, which is beneficial for realizing narrow linewidth feedback.

[0040] The first reflective surface 301 and the second reflective surface 302 can be respectively fabricated on a low thermal expansion substrate material, such as an ultra-low thermal expansion glass material. The two are mechanically supported and connected at a certain cavity length position by a support structure 303. The support structure 303 can be a ring support frame or other suitable mechanical support structure, and its material is preferably also a low thermal expansion material to improve the temperature stability of the cavity length and resonant frequency.

[0041] In a preferred embodiment, the fabrication process of the micro FP cavity 3 can employ vacuum bonding, bonding the first reflective surface 301 and the second reflective surface 302 in a vacuum environment. This creates a near-vacuum state in the hollow cavity 304 formed between them, thereby reducing changes in the refractive index of the air inside the cavity caused by external environmental variations and minimizing the impact on the thermal noise stability of the micro FP cavity. The micro FP cavity 3 can be fabricated using microfabrication, dry etching, coating, and bonding processes. Its size is significantly smaller than that of a traditional bulk FP cavity, facilitating the formation of a compact external cavity structure with the on-chip reflection conversion circuit 2 via a coupling lens 4.

[0042] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. For those skilled in the art, the present invention can have various modifications and variations. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A chip-integrated ultra-narrow linewidth laser based on a micro-FP cavity, characterized in that, include: The laser chip, on-chip reflection conversion circuit, coupling lens, and miniature FP cavity; wherein, the on-chip reflection conversion circuit includes: waveguide, first phase shifter disposed on the waveguide, adjustable coupler, second phase shifter, and waveguide grating; The output terminal of the laser chip is optically coupled to the input terminal of the on-chip reflection conversion circuit. The laser generated by the laser chip is coupled into the input terminal of the on-chip reflection conversion circuit, transmitted through the waveguide to the tunable coupler, and split into a first optical path and a second optical path at the tunable coupler. The first optical path is output from the first output terminal of the on-chip reflection conversion circuit via the waveguide, coupled into the micro FP cavity after mode matching by the coupling lens, and the micro FP cavity provides high Q-value resonant feedback for the laser that meets the resonance condition. It also couples the resonant light and the non-resonant reflected light generated by the front end of the micro FP cavity back to the on-chip reflection conversion circuit via the coupling lens, and returns to the tunable coupler via the waveguide. The second optical path is transmitted through the waveguide to the waveguide grating and reflected back to the tunable coupler by the waveguide grating. The second phase shifter is disposed on the waveguide reflected back to the tunable coupler from the micro FP cavity, and is used to adjust the phase of the resonant light and the non-resonant reflected light from the micro FP cavity. By adjusting the coupling ratio of the tunable coupler and the phase of the second phase shifter, the non-resonant reflected light and the light reflected back from the waveguide grating achieve destructive interference at the end of the tunable coupler facing the laser chip, so as to suppress the interference of the non-resonant reflected light on the laser chip, and extract the narrow linewidth FP cavity resonant light as self-injection locked feedback light and transmit it back to the laser chip. The first phase shifter is disposed on the waveguide between the laser chip and the tunable coupler, and is used to adjust the feedback phase of the self-injection locked feedback light. At the other output end of the tunable coupler, the two light paths from the micro FP cavity and the waveguide grating achieve constructive interference, and are transmitted through the waveguide to the second output end of the on-chip reflection conversion circuit to output narrow linewidth laser light.

2. The chip-integrated ultra-narrow linewidth laser based on a micro-FP cavity according to claim 1, characterized in that, The miniature FP cavity includes a first reflective surface and a second reflective surface arranged opposite to each other. The first reflective surface is the front end face facing the coupling lens, and the second reflective surface is a concave surface. The first reflective surface and the second reflective surface form a plano-concave cavity structure.

3. The chip-integrated ultra-narrow linewidth laser based on a micro-FP cavity according to claim 2, characterized in that, The substrate material of the first and second reflective surfaces, as well as the supporting material disposed between the first and second reflective surfaces, are made of low thermal expansion materials, including ultra-low thermal expansion glass materials.

4. The chip-integrated ultra-narrow linewidth laser based on a micro-FP cavity according to claim 1, characterized in that, The adjustable coupler is an integrated optical 2×2 coupler, specifically a Mach-Zehnder interferometer coupler or a directional coupler.

5. The chip-integrated ultra-narrow linewidth laser based on a micro-FP cavity according to claim 1, characterized in that, The first phase shifter and / or the second phase shifter are thermo-optical phase shifters, electro-optical phase shifters, or carrier injection type phase shifters.

6. The chip-integrated ultra-narrow linewidth laser based on a micro-FP cavity according to claim 1, characterized in that, The waveguide grating is a Bragg grating or a reflection grating based on a periodic refractive index modulation structure, and its central reflection wavelength matches the target resonant wavelength of the micro FP cavity.

7. The chip-integrated ultra-narrow linewidth laser based on a micro-FP cavity according to claim 1, characterized in that, The on-chip reflection conversion circuit, the first phase shifter, the adjustable coupler, the second phase shifter, and the waveguide grating are all integrated on the same photonic chip.

8. The chip-integrated ultra-narrow linewidth laser based on a micro-FP cavity according to claim 1, characterized in that, The coupling lens is a microlens or a GRIN lens, used to achieve mode matching between the waveguide mode of the on-chip reflection transformation circuit and the spatial mode of the micro FP cavity.

9. The chip-integrated ultra-narrow linewidth laser based on a micro-FP cavity according to claim 1, characterized in that, The laser chip is a semiconductor laser chip, including a distributed feedback laser chip, a distributed Bragg reflection laser chip, or an FP-type laser chip.

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