Directional coupler, micro-ring resonant cavity, laser frequency reference device and system
By integrating a dual-polarization silicon-based microring resonator on a silicon-based optoelectronic chip and utilizing the temperature sensitivity difference between TE and TM polarization modes to form a self-reference effect, the problems of laser frequency stability and low integration are solved, and high-precision laser frequency locking and a highly integrated laser frequency reference device are achieved.
Patent Information
- Application Number
- CN202410295414.5
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-03-14
- Publication Date
- 2025-09-16
AI Technical Summary
In the existing technology, frequency-stable narrow-linewidth lasers are limited by factors such as thermal noise of the laser resonator and environmental interference, making it difficult to achieve long-term frequency stability. In addition, existing on-chip frequency reference cavities have problems such as incompatibility with CMOS processes, large device size, and low integration.
A dual-polarization silicon-based microring resonator is used, and the temperature sensitivity difference between the two orthogonal polarization modes, TE and TM, is used to form a self-reference effect. The self-reference locking of the laser frequency is achieved through a directional coupler and a phase shifter, and is integrated on a silicon-based optoelectronic chip.
It achieves long-term stability of laser frequency, reduces cost and size, improves integration and reliability, and can simultaneously frequency-lock two lasers, making it suitable for high-precision applications such as space gravitational wave detection and optical atomic clocks.
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Figure CN120657550A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to the field of lasers, and more particularly to a directional coupler, a micro-ring resonant cavity, and a laser frequency reference device and system for laser frequency stabilization. Background Art
[0002] In the fields of high-precision coherent detection and precision measurement such as space gravitational wave detection, optical atomic or ion clocks, quantum communication and measurement, and fiber-optic hydrophones, frequency-stable narrow-linewidth lasers are core components. By extending the effective cavity length of the laser and using self-injection locking, narrow-linewidth lasers with extremely low white noise levels can be obtained. However, due to the limitations of factors such as the thermal noise limit of the laser resonant cavity, external environmental temperature fluctuations, mechanical vibrations, and electronic noise, the frequency noise of single-frequency lasers in the low frequency band is high, and the long-term stability of the laser frequency is difficult to guarantee. In order to obtain a frequency-stable single-frequency laser, the laser frequency is usually locked to a frequency reference cavity such as an atomic and molecular absorption cell, a Fabry-Perot resonant cavity, or an optical fiber delay line. However, these frequency reference cavities are large in size and easily interfered with by the external environment. They require expensive vacuum, temperature-insulated, and vibration-insulated packaging, which limits their use scenarios.
[0003] In recent years, silicon-based optoelectronic chips have rapidly advanced from device development to system applications. By heterogeneously integrating III-V gain media with silicon, silicon nitride, and lithium niobate, or through hybrid integration, high-performance, integrated, narrow-linewidth, tunable on-chip light sources have been successfully fabricated. To meet the demands of applications in precision metrology and detection, a monolithic, on-chip frequency reference cavity has become a critical component that urgently needs to be addressed. Current solutions include on-chip gas absorption cells, ultra-long waveguide delay lines, and high-quality (Q) silicon nitride microring cavities. However, these approaches suffer from CMOS process incompatibility, large device size, low integration density, and poor long-term frequency stability.
[0004] Frequency-stable narrow-linewidth lasers have important application value. On-chip light sources based on silicon-based optoelectronic chips are the inevitable choice to broaden the application scenarios of frequency-stabilized lasers, reduce costs and size, and improve reliability. The development of highly integrated, high-stability silicon-based laser frequency reference devices compatible with standard CMOS processes is a shortcoming that needs to be urgently addressed. Summary of the Invention
[0005] The purpose of the present disclosure is to overcome the above-mentioned and / or other problems in the prior art. It provides a laser frequency reference device for laser frequency stabilization, which can use the temperature sensitivity difference between two orthogonal polarization modes to form a self-reference effect. The resonant frequency difference between the two orthogonal polarization modes has a quantitative relationship with the temperature inside the laser frequency reference device.
[0006] Some embodiments of the present disclosure provide a laser frequency reference device for laser frequency stabilization, comprising a first polarization rotation beam splitter, a second polarization rotation beam splitter, a microring resonator, a first photodetector, and a second photodetector, wherein the first polarization rotation beam splitter is configured to receive a first laser in TE mode via a first input waveguide and input the first laser in TE mode to a first end of the microring resonator via a first transmission waveguide; the second polarization rotation beam splitter is configured to receive a second laser in TE mode via a second input waveguide and rotate it to a TM mode and input the second laser in TM mode to the microring resonator via a second transmission waveguide. the second end of the microring resonant cavity; the first laser in TE mode is output from the second end of the microring resonant cavity via the second transmission waveguide to the second polarization rotation beam splitter for beam splitting, and the split first laser is transmitted to the first photodetector via the first output waveguide; the second laser in TM mode is output from the first end of the microring resonant cavity via the first transmission waveguide to the first polarization rotation beam splitter after resonating in the microring resonant cavity, and the second laser in TM mode is rotated into TE mode by the first polarization rotation beam splitter and transmitted to the second photodetector via the second output waveguide.
[0007] Some other embodiments of the present disclosure provide a laser frequency reference device for laser frequency stabilization, comprising a first polarization rotation beam splitter, a second polarization rotation beam splitter, a microring resonant cavity, a first photodetector, and a second photodetector, wherein the first polarization rotation beam splitter is configured to: receive a first laser in TE mode via a first input waveguide and input the first laser in TE mode to the first end of the microring resonant cavity via a first transmission waveguide; and receive a second laser in TE mode via a second input waveguide and rotate it to TM mode and input the second laser in TM mode to the first end of the microring resonant cavity via the first transmission waveguide; after the first laser in TE mode resonates in the microring resonant cavity, it is output from the second end of the microring resonant cavity via the second transmission waveguide to the second polarization rotation beam splitter for splitting, and the split first laser is transmitted to the first photodetector via the first output waveguide; after the second laser in TM mode resonates in the microring resonant cavity, it is output from the second end of the microring resonant cavity via the second transmission waveguide to the second polarization rotation beam splitter, the second laser in TM mode is rotated to TE mode by the second polarization rotation beam splitter and transmitted to the second photodetector via the second output waveguide.
[0008] Optionally, the microring resonant cavity includes: a microring waveguide, including a first microring waveguide portion and a second microring waveguide portion forming a loop; a directional coupler, including the first microring waveguide portion and a bus waveguide, the bus waveguide being configured to transmit the first laser and the second laser; and a phase shifter, located around the microring waveguide and configured to adjust the resonant frequency of the microring resonant cavity.
[0009] Optionally, the bus waveguide includes: a first bus waveguide portion optically coupled to the first microring waveguide portion, wherein a gap exists between the first bus waveguide portion and the first microring waveguide portion; a second bus waveguide portion extending from a first end of the first bus waveguide portion in a direction away from the first microring waveguide portion; and a third bus waveguide portion extending from a second end of the first bus waveguide portion in a direction away from the first microring waveguide portion, wherein the second bus waveguide portion and the third bus waveguide portion serve as an end coupling region between the first microring waveguide portion and the bus waveguide.
[0010] Optionally, the microring resonator further includes: a first waveguide coupled between the first transmission waveguide and the bus waveguide; and a second waveguide coupled between the second transmission waveguide and the bus waveguide.
[0011] Optionally, the first waveguide and the second waveguide are single-mode waveguides, the bus waveguide is a multimode waveguide, and the directional coupler further includes: a first spot mode converter, configured to couple the first waveguide to the second bus waveguide portion; and / or a second spot mode converter, configured to couple the second waveguide to the third bus waveguide portion.
[0012] Optionally, the microring waveguide is a multimode waveguide, and the second microring waveguide portion includes a curved region, wherein the curved region has a gradual curvature.
[0013] Optionally, the laser frequency reference device is integrated on an optoelectronic chip.
[0014] Some other embodiments of the present disclosure provide a system for laser frequency stabilization, comprising: a first laser source configured to output a first laser in a TE mode; a second laser source configured to output a second laser in a TE mode; a laser frequency reference device as described above; a first PDH frequency stabilization device configured to lock the frequency of the first laser output by the first laser source to the TE resonance peak of the microring resonant cavity based on the output of the first photodetector; a second PDH frequency stabilization device configured to lock the frequency of the second laser output by the second laser source to the TM resonance peak of the microring resonant cavity based on the output of the second photodetector; a frequency discriminator configured to demodulate the beat frequency fluctuations of the first laser and the second laser and output an error; and a servo controller configured to control a phase shifter in the microring resonant cavity based on the error feedback to lock the equivalent cavity length of the microring resonant cavity.
[0015] Some embodiments of the present disclosure provide a directional coupler, comprising: a first microring waveguide portion; a bus waveguide, comprising: a first bus waveguide portion optically coupled to the first microring waveguide portion, wherein the first bus waveguide portion and the first microring waveguide portion are spaced apart; a second bus waveguide portion extending from a first end of the first bus waveguide portion in a direction away from the first microring waveguide portion; and a third bus waveguide portion extending from a second end of the first bus waveguide portion in a direction away from the first microring waveguide portion, wherein the second bus waveguide portion and the third bus waveguide portion serve as an end coupling region between the first microring waveguide portion and the bus waveguide.
[0016] Some embodiments of the present disclosure provide a microring resonant cavity, comprising: a directional coupler as described above; a microring waveguide, comprising a first microring waveguide portion and a second microring waveguide portion forming a loop; and a phase shifter located around the microring waveguide and configured to adjust the resonant frequency of the microring resonant cavity.
[0017] Optionally, the microring resonator further includes: a first waveguide coupled to the second bus waveguide portion; and a second waveguide coupled to the third bus waveguide portion.
[0018] Optionally, the first waveguide and the second waveguide are single-mode waveguides, the bus waveguide is a multimode waveguide, and the directional coupler further includes: a first spot mode converter, configured to couple the first waveguide to the second bus waveguide portion; and / or a second spot mode converter, configured to couple the second waveguide to the third bus waveguide portion.
[0019] Optionally, the microring waveguide is a multimode waveguide, and the second microring waveguide portion includes a curved region, wherein the curved region has a gradual curvature. BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The present invention may be better understood by describing exemplary embodiments of the present invention in conjunction with the accompanying drawings, in which:
[0021] Figure 1 A schematic diagram of a laser frequency reference device for laser frequency stabilization according to a first exemplary embodiment of the present invention is shown.
[0022] Figure 2 A schematic diagram of a laser frequency reference device for laser frequency stabilization according to a second exemplary embodiment of the present invention is shown.
[0023] Figure 3 A schematic diagram of a directional coupler according to an exemplary embodiment of the present invention is shown.
[0024] Figure 4 The figure shows the simulation diagram of the TE and TM mode power coupling ratios of the directional coupler of the present invention at different coupling spacings and lengths.
[0025] Figure 5 Schematic diagram of the microring waveguide and directional coupler is shown.
[0026] Figure 6 A schematic diagram of a system for laser frequency stabilization according to an exemplary embodiment of the present invention is shown. DETAILED DESCRIPTION
[0027] The specific embodiments of the present invention will be described below. It should be noted that in the specific description of these embodiments, in order to provide a concise description, this specification cannot provide a detailed description of all the features of the actual embodiments. It should be understood that in the actual implementation of any embodiment, just as in the process of any engineering project or design project, in order to achieve the specific goals of the developer and to meet system-related or business-related restrictions, various specific decisions are often made, and this will also change from one embodiment to another. In addition, it can also be understood that although the efforts made in this development process may be complex and lengthy, for ordinary technicians in the field related to the content disclosed by the present invention, some design, manufacturing or production changes based on the technical content disclosed in this disclosure are just conventional technical means and should not be understood as the content of this disclosure being insufficient.
[0028] Unless otherwise defined, the technical or scientific terms used in the claims and description shall have the usual meaning understood by persons of ordinary skill in the technical field to which the invention belongs. The words "first", "second" and similar terms used in the description and claims of the patent application of the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. Words such as "one" or "a" do not indicate a quantity limitation, but rather indicate the existence of at least one. Words such as "include" or "comprising" mean that the elements or objects appearing before "include" or "comprising" cover the elements or objects listed after "include" or "comprising" and their equivalent elements, and do not exclude other elements or objects. Words such as "connected" or "connected" and similar terms are not limited to physical or mechanical connections, nor are they limited to direct or indirect connections.
[0029] In this disclosure, a structure may be referred to as being "used" or "configured to" perform a certain task even if the structure is not currently being operated. For example, "a laser configured to output laser light" is intended to encompass a laser having components that perform that function during operation, even if the laser is not currently being used (e.g., not connected to a power source).
[0030] In this application, unless otherwise specified, all embodiments and preferred embodiments mentioned herein can be combined with each other to form a new technical solution. In this application, unless otherwise specified, all technical features and preferred features mentioned herein can be combined with each other to form a new technical solution.
[0031] In the description of the embodiments of this application, the term "and / or" is simply a description of the association relationship between associated objects, indicating that three relationships can exist. For example, A and / or B can represent the following three situations: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the character " / " in this document generally indicates that the associated objects are in an "or" relationship.
[0032] A laser frequency reference device for laser frequency stabilization provided by an embodiment of the present invention will be described in detail below with reference to the accompanying drawings.
[0033] refer to Figure 1 , which shows a schematic diagram of a laser frequency reference device 100 for laser frequency stabilization according to a first exemplary embodiment of the present invention. Laser frequency reference device 100 may include a first polarization rotating beam splitter 110, a second polarization rotating beam splitter 120, a microring resonator 130, a first photodetector 140, and a second photodetector 150.
[0034] The first polarization rotation beam splitter 110 may be configured to receive the first laser light 10 in TE mode via the first input waveguide 101 and input the first laser light 10 in TE mode to the first end E1 of the microring resonator 130 via the first transmission waveguide 102 .
[0035] The second polarization rotation beam splitter 120 may be configured to receive the second laser light 20 in TE mode via the second input waveguide 103 and rotate it into TM mode and input the second laser light 20 in TM mode to the second end E2 of the microring resonator 130 via the second transmission waveguide 104 .
[0036] After resonating in the microring resonator 130 , the first laser 10 in TE mode is output from the second end E2 of the microring resonator 130 via the second transmission waveguide 104 to the second polarization rotation beam splitter 120 for splitting. The split first laser 10 (TE mode) is transmitted to the first photodetector 140 via the first output waveguide 105 .
[0037] After resonating in the microring resonator 130, the second laser light 20 in TM mode is output from the first end E1 of the microring resonator via the first transmission waveguide 102 to the first polarization rotation beam splitter 110. The second laser light 20 in TM mode is rotated into TE mode by the first polarization rotation beam splitter 110 and transmitted to the second photodetector 150 via the second output waveguide 106.
[0038] refer to Figure 2 , which shows a schematic diagram of a laser frequency reference device 200 for laser frequency stabilization according to a second exemplary embodiment of the present invention.
[0039] The laser frequency reference device 200 may include a first polarization rotating beam splitter 110 , a second polarization rotating beam splitter 120 , a micro-ring resonator 130 , a first photodetector 140 , and a second photodetector 150 .
[0040] The first polarization rotation beam splitter 110 may be configured to receive the first laser light 10 in TE mode via the first input waveguide 101 and input the first laser light 10 in TE mode to the first end E1 of the microring resonator 130 via the first transmission waveguide 102 .
[0041] The first polarization rotation beam splitter 110 may also be configured to receive the second laser light 20 in TE mode via the second input waveguide 103 and rotate it into TM mode, and input the second laser light 20 in TM mode into the first end E1 of the microring resonator 130 via the first transmission waveguide 102 .
[0042] After resonating in the microring resonator 130, the TE mode first laser 10 can be output from the second end E2 of the microring resonator 130 via the second transmission waveguide 104 to the second polarization rotation beam splitter 120 for beam splitting. The split first laser 10 is transmitted to the first photodetector 140 via the first output waveguide 105.
[0043] After resonating in the microring resonator 130, the second laser light 20 in the TM mode can be output from the second end E2 of the microring resonator 130 via the second transmission waveguide 104 to the second polarization rotation beam splitter 120. The second laser light 20 in the TM mode is rotated into the TE mode by the second polarization rotation beam splitter 120 and transmitted to the second photodetector 150 via the second output waveguide 106.
[0044] In some embodiments of the present invention, the microring resonator 130 may be a dual-polarization silicon-based microring resonator configured to simultaneously support TE and TM orthogonal polarization modes.
[0045] The laser frequency reference device 110 or 200 of the present invention can create a self-referencing effect by utilizing the temperature sensitivity difference between two orthogonal polarization modes. Because there is a one-to-one correspondence between the resonant frequency difference between the two orthogonal polarization modes and the temperature within the laser frequency reference cavity, the laser frequency reference device 110 or 200 can achieve long-term stability of the resonant cavity reference frequency through real-time demodulation and feedback control of the effective cavity length of the microring resonator 130.
[0046] According to an exemplary embodiment of the present disclosure, a micro-ring resonator is provided. Figure 1 or Figure 2 As shown, the microring resonator 130 may include a microring waveguide 31, a directional coupler 32, and a phase shifter 33. Figure 3 , which shows a schematic diagram of a directional coupler according to an exemplary embodiment of the present invention.
[0047] Combine Figure 1-Figure 3 As can be seen, the microring waveguide 31 may include a first microring waveguide portion 31A and a second microring waveguide portion 32B forming a loop. The first microring waveguide portion 31A may be a section of the microring waveguide 31 located inside the directional coupler 32, while the second microring waveguide portion 31B may be a section of the microring waveguide 31 located outside the directional coupler 32.
[0048] The directional coupler 32 may include a first microring waveguide portion 31A and a bus waveguide 320. The bus waveguide 320 may be configured to transmit the first laser light 10 and the second laser light 20. It will be appreciated that the region adjacent to the bus waveguide 320 and the microring waveguide 31 may form a coupling region, i.e., in the coupling region, at least a portion of the laser light transmitted in the bus waveguide 320 may be coupled to the microring waveguide 31, or at least a portion of the laser light transmitted in the microring waveguide 31 may be coupled to the bus waveguide 320.
[0049] When the accumulated phase shift of the light field after one cycle along the microring waveguide 31 is an integer multiple of 2π, the light in the microring resonant cavity 130 meets the resonance condition, and the resonant center frequency can be described as
[0050]
[0051] Where c is the speed of light in vacuum, n eff is the effective refractive index, and L is the cavity length of the microring resonator.
[0052] Lasers in the TE and TM polarization modes cover different areas in the optical waveguide and have different waveguide confinement factors. The effective refractive index and group refractive index of the two are different, so the two orthogonal polarization modes of light have different resonant center frequencies and free spectral ranges in the microring resonator 130.
[0053] The phase shifter 33 can be located around the microring waveguide 31 and configured to adjust the resonant frequency of the microring resonant cavity 130. The phase shifter 33 can be used to feedback stabilize the equivalent cavity length of the microring resonant cavity 130 and can be located above or near the microring waveguide 31. For example, the phase shifter 33 can be in the form of a thermo-optic phase shifter, an elastic-optic phase shifter, or an electro-optic phase shifter. The thermo-optic phase shifter can include a resistive material such as titanium nitride or aluminum. The elastic-optic phase shifter can include a piezoelectric material such as aluminum nitride. The electro-optic phase shifter can include lithium niobate and a metal electrode. In some embodiments, the phase shifter 33 can include one or more phase shifters of the same form. In other embodiments, the phase shifter 33 can also include multiple phase shifters of different forms. For example, if the phase shifter 33 includes not only a thermo-optic phase shifter but also an elastic-optic phase shifter or an electro-optic phase shifter, then the elastic-optic phase shifter or the electro-optic phase shifter needs to be designed so that the elastic-optic effect or the electro-optic effect has the same ability to change the cavity length of the microring resonant cavity 130 as the thermo-optic effect. In this way, any physical quantity in the three feedbacks can have an equal feedback effect on the absolute resonant frequency of the microring resonant cavity 130, that is,
[0054] In some embodiments of the present invention, the laser frequency reference device 100 or the laser frequency reference device 200 can be integrated on a chip (e.g., an optoelectronic chip) as a small-volume, surface-mount packaged on-chip integrated laser frequency reference cavity. In this way, the equivalent cavity length of the microring resonator 130 may only be affected by the thermo-optical effect and thermal expansion effect caused by temperature fluctuations, while the external stress interference is relatively weak. Since the TE and TM modes have different mode field distribution areas (i.e., mode field confinement factors) in the waveguide, the effective refractive index, group refractive index, and effective thermo-optic coefficient of the two modes are different, and the resonant frequency difference between the TE and TM modes has a one-to-one correspondence with their respective resonant center frequencies. As long as the resonant frequency difference is locked, the TE and TM reference frequencies of the dual-polarization silicon-based microring resonator 130 can be locked.
[0055] In some embodiments of the present invention, the bus waveguide 320 may include a first bus waveguide portion 321, a second bus waveguide portion 322, and a third bus waveguide portion 323. The first bus waveguide portion 321 may be disposed adjacent to the first microring waveguide portion 31A and configured to be optically coupled with the first microring waveguide portion 31A.
[0056] The second bus waveguide portion 322 can extend from the first end of the first bus waveguide portion 321 in a direction away from the first microring waveguide portion 31A. The third bus waveguide portion 323 can extend from the second end of the first bus waveguide portion 321 in a direction away from the first microring waveguide portion 31A. In this way, the second bus waveguide portion 322 and the third bus waveguide portion 323 can serve as the end coupling region between the first microring waveguide portion 31A and the bus waveguide 320. In this way, the coupling region between the bus waveguide 320 and the microring waveguide 31 can be formed between the first bus waveguide portion 321 and the first microring waveguide portion 31A.
[0057] As a self-referenced laser frequency reference cavity, the microring resonator 130 needs to meet the following conditions: (1) ensure that the TE and TM modes oscillate simultaneously, which is the basis for the frequency reference cavity to achieve the self-reference effect; (2) ensure that both the TE and TM modes have a sufficiently high quality factor in the cavity, since the frequency discrimination capability of the frequency reference cavity is proportional to the quality factor; (3) ensure that both the TE and TM modes have a suitable extinction ratio to ensure that the subsequent PDH frequency stabilization has a high signal-to-noise ratio. A suitable power coupling ratio is the key to achieving the above three conditions. It is necessary to ensure that both the TE and TM modes operate near the critical coupling state. In a multimode silicon waveguide with a large aspect ratio, the TE mode has a high mode field confinement factor, and the mode field is mainly concentrated inside the silicon waveguide. The TM mode has a low mode field confinement factor, and the mode field diverges above and below the silicon waveguide. According to the coupled mode theory of directional couplers, under the same coupling spacing and coupling length, the coupling efficiency of the TM mode is much higher than that of the TE mode. Under conventional coupling methods, it is impossible to simultaneously ensure that the two orthogonal polarization modes operate near the critical coupling state. The present invention takes into account the high coupling efficiency of the TM mode and allows the TM mode to couple back and forth in the coupling region so that appropriate residual light remains in the microring waveguide 31 for resonance.
[0058] In the directional coupler 32 of the present invention, the first bus waveguide section 231 has a length L, and a spacing D is defined between the first bus waveguide section 231 and the first microring waveguide section 31A. By adjusting at least one of the spacing D between the first bus waveguide section 231 and the first microring waveguide section 31A and the length L of the first bus waveguide section 231, the transverse magnetic mode (TM) can be coupled back and forth within the directional coupler 230, while a small portion of the remaining light remains in the first microring waveguide section 31A. This allows for a configurable power coupling ratio between the TE and TM modes. In the present invention, the term "back and forth coupling" means that the TM laser light can be fully coupled from the first bus waveguide section 231 to the first microring waveguide section 31A within the coupling region, and then coupled back from the first microring waveguide section 31A to the first bus waveguide section 231, retaining a predetermined portion of the TM laser light to resonate within the microring waveguide 31. In this manner, an appropriate power coupling ratio can be achieved for both the TE and TM modes, ensuring that both modes operate near the critical coupling state simultaneously.
[0059] See also Figure 4 , which shows the simulation diagram of the TE and TM mode power coupling ratios of the directional coupler of the present invention at different coupling spacings and lengths. Figure 4 The left side shows the curve of the TE and TM mode power coupling ratio as the coupling region length (μm) changes when the coupling spacing = 300nm; Figure 4The right side shows a graph of the TE and TM mode power coupling ratio as a function of the coupling region length (μm) for a coupling spacing D = 400 nm. As shown in the figure, the desired coupling efficiency for the TM mode and the desired coupling efficiency for the TE mode can be determined by selecting the coupling spacing (i.e., spacing D) and the coupling region length (i.e., length L), respectively, so that the two orthogonal polarization modes operate near the critical coupling state.
[0060] In some embodiments of the present invention, the microring resonator 130 may further include a first waveguide 131 and a second waveguide 132. Figure 1 or Figure 2 As shown, the first waveguide 131 may be coupled between the first transmission waveguide 102 and the bus waveguide 320 of the directional coupler 32. The second waveguide 132 may be coupled between the second transmission waveguide 104 and the bus waveguide 320 of the directional coupler 32.
[0061] In some embodiments of the present invention, the first waveguide 131 and the second waveguide 132 can be single-mode waveguides, while the bus waveguide can be a multimode waveguide. In this case, the directional coupler 32 can further include a first spot mode converter 324 and / or a second spot mode converter 325 for adiabatically transitioning the multimode waveguide to the single-mode waveguide. The first spot mode converter 324 can be configured to couple the first waveguide 131 to the second bus waveguide portion 322. The second spot mode converter 325 can be configured to couple the second waveguide 132 to the third bus waveguide portion 323.
[0062] In some embodiments of the present invention, the microring waveguide 31 may be a multimode waveguide. Figure 5 , which shows a second microring waveguide portion 31B of the microring waveguide 31 located outside the directional coupler 32. The second microring waveguide portion 31B may include a curved region BR. In some embodiments, the microring waveguide 31 may be a racetrack type, and thus the second microring waveguide portion 31B may include two curved regions BR, such as Figure 5 As shown. The curved region BR can have a gradual curvature. For example, the curved region BR can adopt Euler bending, Bessel bending, or other curvature-gradient bending methods. In a multimode wide waveguide, the contact between the light field and the waveguide sidewall is reduced, which can effectively reduce the waveguide loss. However, the cost is that multimode bending requires a larger turning radius, otherwise it will cause great radiation loss, mode mismatch loss, and mode crosstalk. The use of Euler bending, Bessel bending, or other curvature-gradient bending methods can effectively reduce the bending loss of the microring, avoid the excitation of high-order modes, and effectively reduce the equivalent bending radius of the microring, thereby improving the integration of the device.
[0063] As an example, the laser frequency reference device that can be integrated on a chip in a conventional 220nm thick silicon waveguide can have the following advantages: (1) Using a high-Q micro-ring resonator on a silicon substrate as a laser frequency reference cavity, the micro-ring resonator is based on a multi-mode curvature gradient waveguide, and the intrinsic Q value of the TE and TM modes can reach 10 6 (2) A directional coupler is specially designed in the coupling area of the microring resonator to solve the problem of large difference in coupling efficiency between the two orthogonal polarization modes: the TM mode travels back and forth twice in the coupler, and a small amount of light remains inside the microring resonator. By controlling the spacing and length of the coupling area, the power coupling ratio of the TE mode and the TM mode can be arbitrarily adjusted. Based on this technology, TE and TM dual-polarization oscillation is achieved, and at the same time, 2×10 6 The above load Q value and extinction ratio are greater than 6dB; (3) the temperature sensitivity difference between the two orthogonal polarization modes is used to form a self-reference effect. The resonant frequency difference between the two orthogonal polarization modes has a quantitative relationship with the temperature in the laser frequency reference cavity. The temperature sensitivity can reach 3.05GHz / K. When the frequency discrimination accuracy is in the kHz level, the temperature fluctuation of the μK level inside the reference cavity can be demodulated. With the help of a phase shifter, the absolute frequency jitter of the microring resonant cavity can be locked to the kHz level; (4) It can be used to simultaneously stabilize the laser frequency of two lasers. The relative frequency difference between the two lasers is locked at the same time. It can be used in application scenarios where the absolute frequency and relative frequency of the laser need to be locked at the same time, such as as the seed light of a carbon dioxide detection lidar.
[0064] According to another exemplary embodiment of the present disclosure, a system for laser frequency stabilization is also provided. Figure 6 , which shows a schematic diagram of a system 600 for laser frequency stabilization according to an exemplary embodiment of the present invention.
[0065] The system 600 for laser frequency stabilization may include a first laser source 610 and a second laser source 620. The first laser source 610 may be configured to output a first laser 10. The second laser source 620 may be configured to output a second laser 20. For example, the first laser 10 and the second laser 20 may both include a transverse electric mode (TE).
[0066] The system 600 for laser frequency stabilization may further include the laser frequency reference device 100 or 200 as described above. Figure 6 The system 600 including the laser frequency reference device 200 is taken as an example for description.
[0067] The first polarization rotation beam splitter 110 may be configured to receive the first laser light 10 from the first laser source 610 via the first input waveguide 101 and input the first laser light 10 in TE mode to the first end E1 of the microring resonator 130 via the first transmission waveguide 102 .
[0068] The first polarization rotation beam splitter 110 may also be configured to receive the second laser light 20 from the second laser source 620 via the second input waveguide 103 and rotate it into a TM mode and input the TM mode second laser light 20 into the first end E1 of the microring resonator 130 via the first transmission waveguide 102 .
[0069] After resonating in the microring resonator 130, the TE mode first laser 10 can be output from the second end E2 of the microring resonator 130 via the second transmission waveguide 104 to the second polarization rotation beam splitter 120 for beam splitting. The split first laser 10 is transmitted to the first photodetector 140 via the first output waveguide 105.
[0070] After resonating in the microring resonator 130, the second laser light 20 in the TM mode can be output from the second end E2 of the microring resonator 130 via the second transmission waveguide 104 to the second polarization rotation beam splitter 120. The second laser light 20 in the TM mode is rotated into the TE mode by the second polarization rotation beam splitter 120 and transmitted to the second photodetector 150 via the second output waveguide 106.
[0071] The system 600 for laser frequency stabilization may further include a first PDH frequency stabilization device 630 and a second PDH frequency stabilization device 640. The first PDH frequency stabilization device 630 may be configured to lock the frequency of the first laser light 10 output by the first laser source 610 to the TE resonance peak of the microring resonator 130 based on information about the first laser light 10 output by the first photodetector 140. The second PDH frequency stabilization device 640 may be configured to lock the frequency of the second laser light 20 output by the second laser source 620 to the TM resonance peak of the microring resonator 130 based on information about the second laser light 20 output by the second photodetector 150.
[0072] The system 600 for laser frequency stabilization may further include a frequency discriminator 650 and a servo controller 660. The frequency discriminator 650 may be configured to demodulate the beat frequency fluctuations of the first laser 10 and the second laser 20 and output an error. The servo controller 660 may be configured to control the phase shifter 33 in the microring resonator 130 based on error feedback to lock the effective cavity length of the microring resonator 130.
[0073] The system 600 for laser frequency stabilization of the present invention can simultaneously perform laser frequency stabilization on two lasers, and the relative frequency difference between the two lasers is simultaneously locked. It can be used in application scenarios where both the absolute and relative frequencies of the lasers need to be locked simultaneously, such as as a seed light for a carbon dioxide detection laser radar. Specifically, the system 600 for laser frequency stabilization of the present invention can lock the frequencies of the first laser output by the first laser and the second laser output by the second laser to the TE and TM resonance peaks of the dual-polarization silicon-based microring resonator 130 respectively through PDH frequency stabilization technology. At this time, the beat frequency of the first laser and the second laser is the resonance frequency difference between the TE and TM of the microring resonator. The fluctuation of the beat frequency reflects the temperature fluctuation inside the microring resonator, and the beat frequency has a one-to-one correspondence with the internal temperature of the reference device. The beat frequency fluctuation is demodulated by the frequency discriminator 650 and the error is output to the servo controller 660, which feedback controls the phase shifter 33 to lock the equivalent cavity length of the dual-polarization silicon-based microring resonator 130, thereby achieving long-term stability of the absolute resonance frequency of the microring resonator 130. At this time, the frequencies of the first laser and the second laser are simultaneously stable.
[0074] The above describes in detail a laser frequency reference device and system for laser frequency stabilization according to an exemplary embodiment of the present invention. The present invention proposes a laser frequency reference device that can be integrated on a chip, which supports orthogonal polarization mode oscillation and has a high quality factor and extinction ratio. The laser frequency reference device of the present invention can have two layouts. In the first layout, as shown in FIG. Figure 1 As shown, the first laser 10 and the second laser 20 enter the laser frequency reference device 100 in opposite directions, and both laser beams are in transverse electric mode (TE); the first laser 10 enters the first polarization rotation beam splitter 110 through the first input waveguide 101, and the output light is still in TE mode, and enters the dual-polarization silicon-based micro-ring resonant cavity 130 through the first transmission waveguide 102; after the dual-polarization silicon-based micro-ring resonant cavity 130 resonates, it is emitted through the second transmission waveguide 104 and enters the second polarization rotation beam splitter 120 for beam splitting, and the output light is transmitted through the first The output waveguide 105 enters the first photodetector 140; synchronously, the second laser 20 enters the second polarization rotation beam splitter 120 through the second input waveguide 103, and the output light is rotated into the TM mode, and enters the dual-polarization silicon-based micro-ring resonator 130 through the second transmission waveguide 104; after the dual-polarization silicon-based micro-ring resonator 130 resonates, it is emitted through the first transmission waveguide 102 and enters the first polarization rotation beam splitter 110 for beam splitting, and the output light enters the second photodetector 150 through the second output waveguide 106. In the second layout, as shown in FIG. Figure 2As shown, the first laser 10 and the second laser 20 enter the on-chip integrated laser frequency reference device 200 in the same direction, and both laser beams are in transverse electric mode (TE); the first laser 10 enters the first polarization rotation beam splitter 110 through the first input waveguide 101, and the output light is still in TE mode, and enters the dual-polarization silicon-based micro-ring resonant cavity 130 through the first transmission waveguide 102; after the dual-polarization silicon-based micro-ring resonant cavity 130 resonates, it is emitted through the second transmission waveguide 104 and enters the second polarization rotation beam splitter 120 for beam splitting. The laser beam 20 enters the first photodetector 140 through the first output waveguide 105; synchronously, the second laser beam 20 enters the first polarization rotation beam splitter 110 through the second input waveguide 103, and the outgoing light is rotated into the TM mode and enters the dual-polarization silicon-based microring resonant cavity 130 through the first transmission waveguide 102; after the dual-polarization silicon-based microring resonant cavity 130 resonates, it exits through the second transmission waveguide 104 and enters the second polarization rotation beam splitter 120 for beam splitting, and the outgoing light enters the second photodetector 150 through the second output waveguide 106.
[0075] The present invention exploits the temperature sensitivity difference between two orthogonal polarization modes to create a self-referencing effect. The resonant frequency difference between the two orthogonal polarization modes is quantitatively related to the temperature within the laser frequency reference device. After demodulating the internal temperature fluctuations of the device, the absolute frequency of the microring resonator is locked using a phase shifter feedback. The present invention implements a dual-polarization, high-Q microring resonator on a silicon substrate. The microring resonator is constructed based on a multimode curvature gradient waveguide. A specially designed directional coupler is incorporated into the coupling region of the microring resonator. The TM mode travels back and forth through the coupler twice, leaving a small amount of light remaining within the microring resonator. By controlling the spacing and length of the coupling regions, the power coupling ratio between the TE and TM modes can be arbitrarily controlled. Based on this technology, dual-polarization TE and TM mode oscillation is achieved, while simultaneously achieving high load Q and a large extinction ratio. The present invention enables a highly integrated on-chip laser frequency reference cavity. In addition to the core dual-polarization microring resonator component, the chip also integrates a polarization rotation beam splitter and a photodetector. This significantly improves the integration and reliability of the laser frequency reference cavity and the entire frequency stabilization system, facilitating integration with an on-chip single-frequency light source. The present invention can simultaneously perform laser frequency stabilization on two lasers, and the absolute frequencies of the two lasers and the relative frequency difference between them are locked at the same time, which can be used in application scenarios where the absolute frequency and relative frequency of the lasers need to be locked at the same time.
[0076] It should be understood that the above description is illustrative and not restrictive. For example, the above embodiments (and / or their aspects) can be used in combination with each other. In addition, without departing from the scope of the present invention, many modifications can be made to adapt specific conditions or materials to the teachings of the various embodiments of the present invention. Although the size and type of materials described herein are used to define the parameters of the various embodiments of the present invention, the various embodiments are not meant to be restrictive, but rather exemplary embodiments. Upon reading the above description, many other embodiments will be apparent to those skilled in the art. Therefore, the scope of the various embodiments of the present invention should be determined with reference to the appended claims, and the full range of equivalent forms claimed for protection by these claims.
Claims
1. A laser frequency reference device for laser frequency stabilization, comprising a first polarization rotating beam splitter, a second polarization rotating beam splitter, a microring resonator, a first photodetector, and a second photodetector. wherein the first polarization rotation beam splitter is configured to receive a first laser beam in TE mode via a first input waveguide and input the first laser beam in TE mode into a first end of the microring resonator via a first transmission waveguide; The second polarization rotation beam splitter is configured to receive the second laser light in TE mode via a second input waveguide and rotate it into TM mode, and input the second laser light in TM mode into the second end of the microring resonator via a second transmission waveguide; After the first laser light in the TE mode resonates in the microring resonator, it is output from the second end of the microring resonator via the second transmission waveguide to the second polarization rotation beam splitter for beam splitting, and the split first laser light is transmitted to the first photodetector via the first output waveguide; After resonating in the microring resonator, the second laser in the TM mode is output from the first end of the microring resonator via the first transmission waveguide to the first polarization rotation beam splitter. The second laser in the TM mode is rotated into the TE mode by the first polarization rotation beam splitter and is transmitted to the second photodetector via the second output waveguide.
2. A laser frequency reference device for laser frequency stabilization, comprising a first polarization rotating beam splitter, a second polarization rotating beam splitter, a microring resonator, a first photodetector, and a second photodetector. wherein the first polarization rotating beam splitter is configured as: receiving a first laser beam in TE mode via a first input waveguide and inputting the first laser beam in TE mode into a first end of the microring resonator via a first transmission waveguide; and receiving a second laser beam in TE mode via a second input waveguide and rotating it into a TM mode, and inputting the second laser beam in TM mode into the first end of the microring resonator via the first transmission waveguide; After the TE mode first laser resonates in the microring resonator, it is output from the second end of the microring resonator via the second transmission waveguide to the second polarization rotation beam splitter for beam splitting, and the split first laser is transmitted to the first photodetector via the first output waveguide; After resonating in the microring resonator, the second laser in the TM mode is output from the second end of the microring resonator via the second transmission waveguide to the second polarization rotation beam splitter. The second laser in the TM mode is rotated into the TE mode by the second polarization rotation beam splitter and is transmitted to the second photodetector via the second output waveguide.
3. The laser frequency reference device according to claim 1 or 2, characterized in that: The microring resonator comprises: A microring waveguide comprising a first microring waveguide portion and a second microring waveguide portion forming a loop; a directional coupler comprising the first microring waveguide portion and a bus waveguide, the bus waveguide being configured to transmit the first laser light and the second laser light; and A phase shifter is located around the microring waveguide and is configured to adjust the resonant frequency of the microring resonant cavity.
4. The laser frequency reference device according to claim 3, wherein: The bus waveguide comprises: a first bus waveguide portion optically coupled to the first microring waveguide portion, wherein a gap is provided between the first bus waveguide portion and the first microring waveguide portion; a second bus waveguide portion extending from a first end of the first bus waveguide portion in a direction away from the first microring waveguide portion; and a third bus waveguide portion extending from the second end of the first bus waveguide portion in a direction away from the first microring waveguide portion, wherein the second bus waveguide portion and the third bus waveguide portion serve as an end coupling region between the first microring waveguide portion and the bus waveguide.
5. The laser frequency reference device according to claim 4, wherein: The microring resonator further comprises: a first waveguide coupled between the first transmission waveguide and the bus waveguide; and The second waveguide is coupled between the second transmission waveguide and the bus waveguide.
6. The laser frequency reference device according to claim 5, wherein: The first waveguide and the second waveguide are single-mode waveguides, and the bus waveguide is a multi-mode waveguide. The directional coupler further comprises: a first spot converter configured to couple the first waveguide to the second bus waveguide portion; and / or A second spot converter is configured to couple the second waveguide to the third bus waveguide section.
7. The laser frequency reference device according to claim 3, wherein: The microring waveguide is a multimode waveguide, and the second microring waveguide portion includes a curved region, wherein the curved region has a gradual curvature.
8. The laser frequency reference device according to claim 1 or 2, characterized in that: The laser frequency reference device is integrated on an optoelectronic chip.
9. A system for laser frequency stabilization, comprising: A first laser source is configured to output a first laser in TE mode; a second laser source configured to output a second laser in TE mode; The laser frequency reference device according to any one of claims 1 to 8; a first PDH frequency stabilization device, configured to lock the frequency of the first laser output by the first laser source to the TE resonance peak of the microring resonator based on the output of the first photodetector; a second PDH frequency stabilization device, configured to lock the frequency of the second laser light output by the second laser source to the TM resonance peak of the microring resonator based on the output of the second photodetector; a frequency discriminator configured to demodulate the beat frequency fluctuations of the first laser and the second laser and output an error; as well as The servo controller is configured to control a phase shifter in the microring resonant cavity based on the error feedback to lock the equivalent cavity length of the microring resonant cavity.
10. A directional coupler comprising: a first microring waveguide portion; Bus waveguide, including: a first bus waveguide portion optically coupled to the first microring waveguide portion, wherein a gap is provided between the first bus waveguide portion and the first microring waveguide portion; a second bus waveguide portion extending from a first end of the first bus waveguide portion in a direction away from the first microring waveguide portion; and a third bus waveguide portion extending from the second end of the first bus waveguide portion in a direction away from the first microring waveguide portion, wherein the second bus waveguide portion and the third bus waveguide portion serve as an end coupling region between the first microring waveguide portion and the bus waveguide.
11. A microring resonator, comprising: The directional coupler according to claim 10; A microring waveguide, comprising the first microring waveguide portion and the second microring waveguide portion forming a loop; as well as A phase shifter is located around the microring waveguide and is configured to adjust the resonant frequency of the microring resonant cavity.
12. The microring resonator according to claim 11, wherein: The microring resonator further comprises: a first waveguide coupled to the second bus waveguide portion; and A second waveguide is coupled to the third bus waveguide portion.
13. The microring resonator according to claim 12, wherein: The first waveguide and the second waveguide are single-mode waveguides, and the bus waveguide is a multi-mode waveguide. The directional coupler further comprises: a first spot converter configured to couple the first waveguide to the second bus waveguide portion; and / or A second spot converter is configured to couple the second waveguide to the third bus waveguide section.
14. The microring resonator according to claim 11, wherein: The microring waveguide is a multimode waveguide, and the second microring waveguide portion includes a curved region, wherein the curved region has a gradual curvature.