Hybrid integrated narrow linewidth frequency conversion laser

Abstract

The application relates to the technical field of lasers, in particular to a hybrid integrated narrow-line-width frequency conversion laser, which comprises a gain chip and a lithium niobate external cavity; the lithium niobate external cavity comprises, in sequence, a mode spot converter, a transmission waveguide provided with a phase shift part, a double-micro-ring resonator and a Sagnac ring mirror; the lithium niobate external cavity further comprises a periodical polarization waveguide, which is located between the Sagnac ring mirror and the double-micro-ring resonator or is connected to one end of the Sagnac ring mirror away from the double-micro-ring resonator, and is used for realizing frequency conversion; the lithium niobate external cavity further comprises two comb-shaped electrodes which are symmetrically arranged on the two sides of the periodical polarization waveguide. The application is at least beneficial to improving the performance of the narrow-line-width frequency conversion laser.

Description

Technical Field

[0001] This invention belongs to the field of laser technology, and particularly relates to a hybrid integrated narrow linewidth frequency conversion laser. Background Technology

[0002] Narrow-linewidth frequency-converting lasers (NLLs), characterized by low frequency noise, high coherence, and frequency stability, generate second harmonic generation (SHG) waves using nonlinear crystals. They possess the capability to output in near-visible and even visible light bands, making them highly valuable for applications in quantum communication, quantum precision measurement, and atomic manipulation. For instance, in quantum key distribution (QKD) technology, precise frequency selection of photons is crucial to ensure high-fidelity quantum state generation, thereby improving overall key distribution performance. NLLs can achieve photon emission with high phase and frequency stability, providing more accurate photon frequency and phase control, thus improving the signal-to-noise ratio and helping to reduce the bit error rate and increase transmission distance in QKD systems. In precision measurement fields such as atomic clocks, NLLs benefit from their ability to precisely align with specific atomic transition wavelengths, improving atomic excitation efficiency while minimizing atomic transition frequency fluctuations caused by phase noise of the excitation source, thus enhancing the accuracy and long-term stability of atomic clocks.

[0003] Currently, the main implementation schemes for narrow-linewidth frequency-conversion lasers include: using a distributed feedback laser (DFB) or a distributed Bragg reflector (DBR) as the narrow-linewidth active laser supply, and a periodically polarized lithium niobate (PPLN) waveguide as the frequency conversion section. The narrow-linewidth active laser supply section and the PPLN waveguide are connected by an erbium-doped fiber amplifier to compensate for coupling loss. A significant problem with this approach is its low integration level, which fails to meet the demands for miniaturization and portability in fields such as quantum precision measurement. Furthermore, its wide spectral linewidth and the coupling between multiple components result in poor resistance to environmental interference. Another approach utilizes a reflective semiconductor optical amplifier (RSOA). Using an amplifier or DFB laser as the active component, and combining Rayleigh backscattering of an all-pass lithium niobate microring cavity for self-injection locking to achieve linewidth narrowing, frequency conversion is achieved with the periodic polarization part inside the ring. However, the Rayleigh scattered light has certain frequency and intensity fluctuations compared to the incident light, which places high demands on the quality factor of the microring cavity. Therefore, it is not conducive to large-scale production for lithium niobate, a material that is difficult to etch.

[0004] Therefore, current discrete component frequency conversion lasers suffer from low integration, poor linewidth performance, and stringent quality factor requirements for self-injection locking based on Rayleigh scattering. Summary of the Invention

[0005] In view of this, the present invention aims to provide a hybrid integrated narrow linewidth frequency conversion laser, which at least helps to improve the performance of narrow linewidth frequency conversion lasers.

[0006] To achieve the above objectives, the technical solution created by this invention is implemented as follows:

[0007] This invention provides a hybrid integrated narrow-linewidth frequency conversion laser, comprising: a gain chip, one end of which outputs initial fundamental frequency light, and the other end of which is coated with a high-reflectivity mirror; a lithium niobate external cavity, the lithium niobate external cavity including a mode converter, a transmission waveguide with a phase shift section, a dual microring resonator, and a Sagnac ring mirror connected in sequence. The mode converter is used to couple the gain chip to the transmission waveguide, the transmission waveguide is used to transmit light between the gain chip and the dual microring resonator, the phase shift section is used to phase-modulate the light in the transmission waveguide, and the dual microring resonator is used to perform narrowband filtering on the initial fundamental frequency light output from the gain chip to form the fundamental frequency light. The c-ring mirror and the high-reflectivity mirror form a resonant cavity. The Sagnac ring mirror reflects a portion of the received light back to the gain chip to achieve frequency stabilization, and outputs the other portion of the received light. The lithium niobate external cavity also includes a periodically polarized waveguide, which is used to achieve frequency conversion. The periodically polarized waveguide is located between the Sagnac ring mirror and the dual micro-ring resonator. The output end of the Sagnac ring mirror outputs frequency-doubled light, or the periodically polarized waveguide is connected to the end of the Sagnac ring mirror away from the dual micro-ring resonator, and the output end of the periodically polarized waveguide outputs frequency-doubled light. The lithium niobate external cavity also includes two comb-shaped electrodes, which are symmetrically arranged on both sides of the periodically polarized waveguide.

[0008] Furthermore, the feature is that the end of the gain chip that outputs the initial fundamental frequency light has an anti-reflection film.

[0009] Furthermore, the dual microring resonator includes a first microring resonator, a second microring resonator, and an intermediate waveguide. The first microring resonator includes a first straight waveguide and a first ring waveguide. The second microring resonator includes a second straight waveguide and a second ring waveguide. The input end of the first straight waveguide is connected to the transmission waveguide, and the output end of the second straight waveguide is connected to the input end of the Sagnac ring mirror or the input end of the periodically polarized waveguide. The intermediate waveguide is located between the first ring waveguide and the second ring waveguide, and is located on the side of the first ring waveguide away from the first straight waveguide, and on the side of the second ring waveguide away from the second straight waveguide.

[0010] Furthermore, the radii of the first ring waveguide and the second ring waveguide are different.

[0011] Furthermore, a first thermoelectric electrode is provided on both the first and second ring waveguides.

[0012] Furthermore, the periodically polarized waveguide is a straight waveguide, and a second thermoelectric electrode is disposed on the periodically polarized waveguide.

[0013] Furthermore, each comb-shaped electrode includes a main body and multiple extensions connected to each other. The main body is arranged parallel to the periodically polarized waveguide, and the multiple extensions are arranged at intervals along the extension direction of the main body on the side of the main body facing the periodically polarized waveguide.

[0014] Furthermore, the radii of both the first and second ring waveguides are not less than 80 μm.

[0015] Furthermore, the polarization period of the comb-shaped electrode , where λ ω n is the wavelength of the fundamental frequency light. 2ω n represents the effective refractive index of the frequency-doubled light in the lithium niobate waveguide. ω The effective refractive index and polarization period of the fundamental frequency light in the lithium niobate waveguide. It is equal to twice the width of the extension; the polarization duty cycle of the comb electrode is 50%.

[0016] Furthermore, the theoretical linewidth Δv of the hybrid integrated narrow linewidth frequency conversion laser satisfies:

[0017] ;

[0018] in, , ;

[0019] in, ;

[0020] in, ;

[0021] Where Δv0 is the linewidth of the Fabry-Perot cavity in the equivalent resonant cavity model, F 2 Let A be the narrowing factor, B be the first factor, and r be the second factor. eff (ω) represents the reflectivity of the rear-end mirror of the Fabry-Perot cavity in the equivalent resonant cavity model, and α H τ0 is the linewidth broadening factor of the gain chip, τ0 is the round-trip time of light in the active region of the gain chip, j is the imaginary number, and t0 is the linewidth broadening factor of the gain chip. transition For coupling loss, t passive For passive waveguide loss, κ m R is the coupling coefficient between the ring waveguide and its adjacent straight waveguide. m Let α be the radius of the ring waveguide. m Let βp be the amplitude propagation loss constant of the ring waveguide, m = 1 for the first ring waveguide, m = 2 for the second ring waveguide, βp be the effective propagation constant of the lithium niobate waveguide, and κ be the amplitude propagation loss constant of the ring waveguide. c Let κ be the coupling coefficient of the Sagnac ring mirror. c Determined based on coupling length and coupling gap.

[0022] Compared with existing technologies, this invention achieves the following beneficial effects: To address the problems of low integration, poor linewidth performance, and stringent quality factor requirements of Rayleigh scattering-based self-injection locking in discrete component frequency-converting lasers, this invention provides a hybrid integrated narrow-linewidth frequency-converting laser. This hybrid integrated narrow-linewidth frequency-converting laser is based on a hybrid integrated structure of a gain chip and a lithium niobate external cavity. Specifically, the initial fundamental frequency light (broadband light) output from the gain chip is coupled into the lithium niobate external cavity through a mode converter. After phase adjustment by the phase shift section and narrowband filtering by a dual micro-ring resonator, it is then filtered using a Sagnac ring resonator. The mirror achieves self-injection locking through optical feedback. Specifically, there are two implementation methods: In the first method, part of the light is fed back to the gain chip via the Sagnac ring mirror to achieve external cavity feedback frequency stabilization, and the remaining light is frequency-doubled and converted through a periodically polarized waveguide composed of a straight waveguide. In the second method, the beam after narrowband filtering is frequency-doubled and converted through a periodically polarized waveguide composed of a straight waveguide before entering the Sagnac ring mirror. Part of the light is fed back to the gain chip via the Sagnac ring mirror to achieve external cavity feedback frequency stabilization, and the remaining light is output as frequency-doubled light. Both of these methods can achieve narrow linewidth laser output at a specific wavelength. Compared to discrete component structures, the hybrid integrated narrow-linewidth frequency conversion laser of this invention employs a hybrid integrated structure that significantly improves the laser's integration density and enhances its resistance to environmental interference. Furthermore, the feedback method using a Sagnac ring mirror offers more controllable feedback light intensity compared to Rayleigh backscattering. Additionally, by placing a second thermoelectrode on the periodically polarized waveguide, and utilizing the change in quasi-phase matching conditions caused by the thermo-optic effect in conjunction with the vernier effect brought about by the dual micro-ring resonator, the wavelength of the output beam can be tuned within a certain range. By controlling the phase shift portion and the first thermoelectrode on the ring waveguide, precise alignment of specific wavelengths can be achieved, thereby providing a highly integrated, narrow-linewidth frequency conversion laser for fields such as quantum communication. Attached Figure Description

[0023] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0024] Figure 1 A schematic diagram of the hybrid integrated narrow linewidth frequency conversion laser described in the embodiments of the present invention;

[0025] Figure 2 This is an enlarged schematic diagram of a portion of the structure in the hybrid integrated narrow linewidth frequency conversion laser described in the embodiment of the present invention. Detailed Implementation

[0026] Analysis revealed that existing narrow-linewidth frequency-converting lasers are mostly composed of discrete components. Specifically, silicon-based filtered external-cavity narrow-linewidth lasers and lithium niobate frequency-doubled chips are typically fabricated separately and then integrated together, making miniaturization difficult. Furthermore, the discrete components result in poor resistance to environmental interference. In addition, since the frequency doubling efficiency of the lithium niobate frequency-doubled chip is directly related to the power of the fundamental frequency light, the power loss caused by the coupling process is clearly detrimental to improving the conversion efficiency of the second harmonic.

[0027] In summary, this invention provides a hybrid integrated narrow-linewidth frequency conversion laser, abandoning the traditional method of coupling a silicon-based filtered external cavity narrow-linewidth laser with a lithium niobate frequency doubling chip. Instead, it simultaneously designs a dual-micro-ring resonator and a periodically polarized waveguide on a single thin-film lithium niobate wafer to achieve narrow-linewidth second harmonic output. Considering the difficulty of etching lithium niobate material, the dual-micro-ring resonator employs two ring waveguides with relatively large radii (radius not less than 80 micrometers). The larger radius of the ring waveguides not only reduces scattering loss during light propagation, but also expands the free spectral region due to the dual-micro-ring structure of the resonator. After the fundamental frequency light filtered by the dual-micro-ring resonator enters the Sagnac ring mirror, a portion of the light... The feedback gain chip achieves optical negative feedback frequency stabilization, while another portion of the optical output can be frequency-doubled via a periodically polarized waveguide. This invention provides a hybrid integrated narrow-linewidth frequency-converting laser that simultaneously achieves laser linewidth narrowing and second-harmonic conversion on the lithium niobate external cavity. Furthermore, a second thermoelectrode enables tuning of the second harmonic within a certain wavelength range. Compared to discrete device combinations, this invention offers higher integration and stronger resistance to environmental interference. Moreover, compared to forming the periodic polarization portion within the ring, this invention sets the periodic polarization portion outside the ring, making the design of the dual-microring resonator more flexible and solving the problem of difficulty in achieving frequency-doubled optical tuning in lasers using periodically polarized lithium niobate for frequency conversion. Since the second-harmonic conversion efficiency is proportional to the square of the periodic polarization length, setting a periodically polarized waveguide independent of the dual-microring resonator in the lithium niobate external cavity allows for a longer periodic polarized waveguide to compensate for the decrease in conversion efficiency caused by lower fundamental frequency optical power, contributing to higher-power second-harmonic output.

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and do not constitute a limitation thereof.

[0029] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0030] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," and "counterclockwise," etc., indicating orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on this invention. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, features defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0031] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art will understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0032] The invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0033] refer to Figure 1 and Figure 2This invention provides a hybrid integrated narrow-linewidth frequency conversion laser, comprising: a gain chip 1, one end of which outputs initial fundamental frequency light, and the other end of which is coated with a high-reflectivity mirror; a lithium niobate external cavity 2, which includes a mode converter 3, a transmission waveguide 11 with a phase shift section 4, a dual microring resonator, and a Sagnac ring mirror 8 connected in sequence. The mode converter 3 is used to couple the gain chip 1 to the transmission waveguide 11, the transmission waveguide 11 is used to transmit light between the gain chip 1 and the dual microring resonator, the phase shift section 4 is used to phase-modulate the light in the transmission waveguide 11, and the dual microring resonator is used to perform narrowband filtering on the initial fundamental frequency light output from the gain chip 1 to form the fundamental frequency light. The Sagnac ring mirror 8... The Sagnac ring mirror 8 and the high-reflectivity mirror form a resonant cavity. The Sagnac ring mirror 8 reflects a portion of the received light back to the gain chip 1 to achieve frequency stabilization, and outputs the other portion of the received light. The lithium niobate external cavity 2 also includes a periodically polarized waveguide 10, which is used to achieve frequency conversion. The periodically polarized waveguide 10 is located between the Sagnac ring mirror 8 and the dual micro-ring resonator. The output end of the Sagnac ring mirror outputs frequency-doubled light. Alternatively, the periodically polarized waveguide 10 is connected to the end of the Sagnac ring mirror 8 away from the dual micro-ring resonator, and the output end of the periodically polarized waveguide outputs frequency-doubled light. The lithium niobate external cavity 2 also includes two comb-shaped electrodes 12, which are symmetrically arranged on both sides of the periodically polarized waveguide 10.

[0034] It should be noted that the Sagnac ring mirror 8 serves as the rear end face of the equivalent external resonant cavity. With the periodic polarized waveguide 10 connected to the end of the Sagnac ring mirror 8 furthest from the dual micro-ring resonator, the Sagnac ring mirror 8 is used to feed back a portion of the light filtered by the dual micro-ring resonator to the gain chip 1. The remaining light is transmitted to the periodic polarized waveguide 10, which performs frequency conversion on the light beam by frequency doubling and outputs frequency-doubled light.

[0035] With the periodically polarized waveguide 10 positioned between the Sagnac ring mirror 8 and the dual micro-ring resonator, the initial fundamental frequency light output from the gain chip 1 undergoes narrowband filtering by the dual micro-ring resonator before directly entering the periodically polarized waveguide 10 for frequency conversion. Then, the Sagnac ring mirror 8 feeds back a portion of the light to the gain chip 1, with the remaining light output as frequency-doubled light. Since the power of the second harmonic is proportional to the square of the power of the fundamental frequency light, placing the frequency conversion part (periodicly polarized waveguide 10) within the equivalent external resonant cavity is beneficial for obtaining higher-power frequency-doubled light. Although the fundamental frequency light fed back from the Sagnac ring mirror 8 undergoes a second frequency doubling process through the periodically polarized waveguide 10, resulting in a reduction in the intensity of the feedback light, according to the adiabatic chirp reduction theory, the reflectivity of the equivalent rear end face will decrease, thereby reducing the optical negative feedback coefficient. The linewidth of the fundamental frequency light may be correspondingly broadened, but a higher-power frequency-doubled light output will be obtained.

[0036] It should be noted that the waveguides of all components on the lithium niobate external cavity provided in this invention are lithium niobate waveguides, and the periodically polarized waveguide 10 is a periodically polarized lithium niobate waveguide. Lithium niobate (LiNbO3), due to its high second-order nonlinear coefficient and wide optical transparency range (approximately 420 nm to 5.2 μm), can achieve efficient second-harmonic generation. As a high-performance optical crystal, it is one of the important materials required for manufacturing narrow-linewidth frequency-conversion lasers. It should be noted that the efficient second-harmonic generation using lithium niobate employs quasi-phase-matching technology, changing the polarization direction of the ferroelectric domains in the crystal through periodic polarization, thereby periodically compensating for the phase mismatch caused by dispersion and improving the second-harmonic generation efficiency.

[0037] Analysis revealed that the frequency doubling efficiency of the periodically polarized waveguide 10 is proportional to the fundamental frequency power and also proportional to the square of the length of the periodically polarized waveguide. Therefore, a high-output-power gain chip 1 and a longer periodically polarized waveguide 10 can achieve higher conversion efficiency. At the same time, the fundamental frequency light with a narrower linewidth can also ensure that the frequency-doubled light has better coherence. This enables a hybrid integrated narrow-linewidth frequency conversion laser with dual-wavelength output in the near-visible and communication bands.

[0038] In some embodiments, in a hybrid integrated narrow linewidth frequency conversion laser, the initial fundamental frequency light with a center wavelength of 1560nm output by gain chip 1 can be converted into a frequency-doubled light output with a wavelength of 780nm. In practical use, by changing the radius of the ring waveguide in the dual micro-ring resonator and the properties of the periodically polarized waveguide, it can be extended to achieve frequency-doubled light output of other wavelengths, thus making it applicable to applications in multiple fields.

[0039] In some embodiments, the output power of gain chip 1 is not less than 50 milliwatts. This helps to ensure that the fundamental frequency light has high power.

[0040] For the periodically polarized waveguide 10 connected to the end of the Sagnac ring mirror 8 furthest from the dual microring resonator, in some embodiments, the linewidth of the fundamental frequency light input to the periodically polarized waveguide 10 is in the range of 5kHz to 10kHz, the linewidth of the frequency-doubled light output from the periodically polarized waveguide 10 is in the range of 10kHz to 20kHz, the power of the fundamental frequency light input to the periodically polarized waveguide 10 is in the range of 1mW to 10mW, the power of the frequency-doubled light output from the periodically polarized waveguide 10 is in the range of 1mW to 2mW, and the length of the periodically polarized waveguide 10 is in the range of 500nm to 600nm.

[0041] For the configuration where the periodically polarized waveguide 10 is located between the Sagnac ring mirror 8 and the dual microring resonator, the linewidth of the fundamental frequency light input to the periodically polarized waveguide 10 is in the range of 10kHz to 20kHz, the linewidth of the frequency-doubled light output from the Sagnac ring mirror 8 is in the range of 20kHz to 40kHz, the power of the fundamental frequency light input to the periodically polarized waveguide 10 is in the range of 5mW to 10mW, the power of the frequency-doubled light output from the Sagnac ring mirror 8 is in the range of 2mW to 4mW, and the length of the periodically polarized waveguide 10 is in the range of 500nm to 600nm.

[0042] In some embodiments, the gain chip 1 is a gain chip made of group III-V materials. In some examples, the center wavelength of the gain chip 1 can be 1560 nm, and the gain spectrum of the gain chip 1 is a box-shaped spectrum with a center wavelength around 1560 nm.

[0043] In some embodiments, the waveguide at the end of the gain chip 1 coupled to the template converter is bent at an angle of about 7° to reduce end-face reflection and suppress noise caused by the Fabry-Perot effect of the cavity itself of the gain chip 1.

[0044] In some embodiments, wide wavelength, wide transparency range and high second-order nonlinear coefficient (χ) are employed. 2 The lithium niobate material is used as the lithium niobate outer cavity 2. The lithium niobate outer cavity 2 includes an insulating layer and a lithium niobate layer located on the insulating layer. The top of the lithium niobate layer can be SiO2 as a buried oxide layer to avoid contamination of the lithium niobate waveguide and improve the light confinement capability.

[0045] The mode converter is used to improve the integral value of the light spot overlap between the gain chip 1 and the transmission waveguide 11 of the lithium niobate external cavity 2, reduce the coupling loss caused by the mismatch of light spot size, and ensure that the light input into the lithium niobate external cavity 2 has high power.

[0046] The phase shift section 4 can be an electrically controlled phase shift section, which modulates the phase of the light in the transmission waveguide 11 through the electro-optic effect to ensure that the wavelength of the beam output from the transmission waveguide 11 to the dual micro-ring resonator is aligned with the micro-ring vernier mode. The phase shift section 4, together with the first thermoelectrode 7 on the micro-ring resonator, can achieve wavelength tuning.

[0047] The free spectral region of a microring resonator is directly related to the radius of the microring. Therefore, by designing two microrings with different circumferences, two microrings with different free spectral regions can be obtained, so that the first microring resonator and the second microring resonator in the dual microring resonator have different free spectral ranges, thereby realizing the vernier effect. In some embodiments, the dual microring resonator includes a first microring resonator 5, a second microring resonator 6, and an intermediate waveguide 63. The first microring resonator 5 includes a first straight waveguide 51 and a first ring waveguide 52. The second microring resonator 6 includes a second straight waveguide 61 and a second ring waveguide 62. The input end of the first straight waveguide 51 is connected to the transmission waveguide 11, and the output end of the second straight waveguide 61 is connected to the input end of the Sagnac ring mirror 8 or the input end of the periodically polarized waveguide. The intermediate waveguide 63 is located between the first ring waveguide 52 and the second ring waveguide 62, and the intermediate waveguide 63 is located on the side of the first ring waveguide 52 away from the first straight waveguide 51, and on the side of the second ring waveguide 62 away from the second straight waveguide 61.

[0048] In some embodiments, the radii of the first ring waveguide 52 and the second ring waveguide 62 are different, thereby achieving a vernier effect, expanding the overall free spectral range while increasing the gain difference between adjacent modes, thus giving the laser better frequency stability. The two ring waveguides of the dual micro-ring resonator can utilize the vernier effect to improve the side-mode suppression ratio and free spectral range.

[0049] In some embodiments, the radius of the first ring waveguide 52 and the radius of the second ring waveguide 62 are both not less than 80 μm.

[0050] In some embodiments, a first thermoelectrode 7 is provided on both the first ring waveguide 52 and the second ring waveguide 62. The first thermoelectrode 7 generates a temperature change by passing a current, thereby inducing a thermo-optical effect on the ring waveguide to alter its refractive index and adjust the effective optical path. Combined with the vernier effect, wavelength tuning within a certain range can be achieved, thus aligning the wavelength of the light output from the dual micro-ring resonator with the wavelength of the fundamental frequency light required to generate the second harmonic. In other words, the first thermoelectrode 7 allows the frequency of the fundamental frequency light to be tuned within a certain range, thereby aligning it with the wavelength of the fundamental frequency light required to generate the second harmonic. This helps mitigate resonant wavelength drift caused by process errors and facilitates precise alignment with the wavelength requirements corresponding to the polarization periodic waveguide, ultimately producing high-quality frequency-doubled light.

[0051] In some embodiments, the frequency conversion achieved by the periodically polarized waveguide 10 is to output the received fundamental frequency light after frequency doubling.

[0052] In some embodiments, the gain chip 1 has an anti-reflection film at one end where it outputs the initial fundamental frequency light.

[0053] In some embodiments, the periodically polarized waveguide 10 is a straight waveguide, and a second thermoelectrode 9 is disposed on the periodically polarized waveguide 10. The second thermoelectrode 9 changes the refractive index of the periodically polarized waveguide 10 through the thermo-optic effect, thereby changing the quasi-phase matching condition and realizing the tuning of the second harmonic within a certain wavelength range.

[0054] In some embodiments, the second thermal electrode 9 may be a NiCr thermally modulated electrode.

[0055] It should be noted that the periodically polarized waveguide 10 is a lithium niobate waveguide that achieves domain inversion by using polarization. In this way, the polarization period of the periodically polarized waveguide 10 can be designed according to the wavelength and waveguide structure to meet the quasi-phase matching condition. The comb-shaped electrode 12 is used to achieve domain inversion of the periodically polarized waveguide 10.

[0056] In some embodiments, each comb electrode 12 includes a main body portion 14 and a plurality of extension portions 13 connected to each other. The main body portion 14 is arranged parallel to the periodically polarized waveguide 10, and the plurality of extension portions 13 are arranged at intervals along the extension direction of the main body portion 14 on the side of the main body portion 14 facing the periodically polarized waveguide 10.

[0057] In some embodiments, the main body has a protrusion on the side away from the periodically polarized waveguide. The protrusion is a contact portion designed for applying power during polarization. Since the main body itself is relatively thin and it is not easy to contact the electrodes of the external power supply, two protrusions with larger areas are designed to contact the electrodes of the external power supply.

[0058] In some embodiments, the design of the comb electrode 12 is based on the quasi-phase matching principle, and the polarization period of the comb electrode 12 is... The polarization period is equal to twice the width of the extension, and in this invention, the width of each extension is equal, that is, the polarization period is equal to twice the width of each extension, where λ ω n is the fundamental frequency wavelength. 2ω n represents the effective refractive index of the frequency-doubled light in the lithium niobate waveguide. ωThe effective refractive index of the fundamental frequency light in the lithium niobate waveguide is given. For the x-cut lithium niobate layer, the x-crystal axis of lithium niobate is perpendicular to the surface of the lithium niobate layer, and the extension direction of the main body 14 is perpendicular to the z-crystal axis of lithium niobate. This makes the polarization electric field applied by the comb electrode 12 parallel to the spontaneous polarization direction of the lithium niobate crystal. In addition, the polarization duty cycle of the comb electrode 12 can be 50% to ensure the best conversion effect. The polarization duty cycle of the comb electrode 12 is 50%, which means that the width of the extension 13 is equal to the spacing width between adjacent extensions 13.

[0059] In some embodiments, the gain chip 1 is a high-power gain chip with a quantum well structure, which has a low linewidth broadening factor and a wide amplified spontaneous emission spectrum output characteristics. One end of the gain chip 1 is coated with a high-reflectivity film with a reflectivity greater than 90% as the rear end face of the resonant cavity, and the other end of the gain chip 1 serves as the coupling side with the lithium niobate external cavity 2. A curved waveguide design is used in conjunction with an antireflection film to reduce coupling loss, while reducing the noise introduced by the FP (Fabry-Pérot) effect from the two end faces of the gain chip 1 itself.

[0060] In some embodiments, the lithium niobate outer cavity 2 can be made of He + Lithium niobate layers are fabricated using ion implantation. Specifically, He can be implanted into the lithium niobate layer. + Ionic bulk lithium niobate material is bonded to a silicon dioxide insulating layer. After polishing, a high-quality lithium niobate-on-insulator substrate is obtained. A partial-thickness lithium niobate layer on the lithium niobate-on-insulator substrate is then etched to form a lithium niobate waveguide. The etching of the lithium niobate waveguide can employ traditional silicon photonics etching methods, such as reactive ion dry etching. In some cases, the thickness of the lithium niobate layer on the insulating layer before etching can range from 580 nm to 620 nm, and the etching depth to form the lithium niobate waveguide can range from 290 nm to 310 nm.

[0061] It should be noted that the Sagnac ring mirror 8, serving as the rear end face of the equivalent resonant cavity, needs to balance the intensity of the feedback light and the intensity of the output light to ensure that the fundamental frequency light has a narrower linewidth and higher output power, thereby improving the conversion efficiency and spectral purity of the second harmonic. Therefore, the coupling coefficient of the Sagnac ring mirror 8 needs to meet specific requirements to achieve narrow linewidth, high-power second harmonic output. The lithium niobate external cavity 2 provided by this invention simultaneously includes a dual micro-ring resonator (filtering section) and a periodically polarized waveguide 10 (frequency conversion section). By setting a reasonable coupling coefficient, the Sagnac ring mirror can output the narrow linewidth, high-power fundamental frequency light required for high frequency and high conversion efficiency, thereby realizing a highly integrated, environmentally resistant narrow linewidth frequency conversion laser.

[0062] Specifically, considering the practical application of quantum precision measurement requires laser linewidths on the order of kHz, and to balance the power of the fundamental frequency light input to the periodically polarized waveguide 10, the Sagnac ring mirror 8 is designed using the equivalent resonant cavity model of an external cavity laser and the adiabatic chirp reduction theory. Specifically, the reflection coefficient of the Sagnac ring mirror 8 affects the magnitude of the complex amplitude reflectivity of the dual micro-ring resonator, thus affecting the reflection coefficient of the effective rear mirror of the equivalent resonant cavity. For an external cavity semiconductor laser, its linewidth will be subject to a compression narrowing factor F. 2 The effect of narrowing factor F 2 Closely related to the reflection coefficient of the effective mirror in the aforementioned equivalent resonant cavity, under the equivalent resonant cavity model, the passive external cavity can be equivalent to a mirror with a reflection coefficient of r. eff The rear end mirror of the Fabry-Perot cavity with a reflectivity r (ω) eff (ω) is a function of the angular frequency ω of the fundamental light. The linewidth of the equivalent Fabry-Perot cavity is defined as Δv0, and the linewidth broadening factor of gain chip 1 is defined as α. H Then the theoretical linewidth Δv of the final laser satisfies:

[0063] ;

[0064] in, , ;

[0065] Where τ0 is the round-trip time of light in the active region of gain chip 1, and j is the imaginary number;

[0066] in addition, ;

[0067] in, ;

[0068] Among them, t transition For coupling loss, t passive For passive waveguide loss, κ m R is the coupling coefficient between the ring waveguide and its adjacent straight waveguide. m Let α be the radius of the ring waveguide. m Let β be the amplitude propagation loss constant of the ring waveguide. Here, the ring waveguide represents the first ring waveguide 52 when m equals 1, and the second ring waveguide 62 when m equals 2. p κ is the effective propagation constant of the lithium niobate waveguide. c The coupling coefficient κ is the coupling coefficient of the Sagnac ring mirror 8. Based on the coupling length and coupling gap, the coupling coefficient κ can be obtained. c ,refer to Figure 2The coupling length L refers to the length of the parallel portion between the second straight waveguide 61 and the output waveguide of the Sagnac ring mirror 8. The coupling gap D refers to the gap between the second straight waveguide 61 and the output waveguide of the Sagnac ring mirror 8 within the parallel portion between them. The first factor A reflects the noise dilution effect brought about by the passive external cavity length, while the second factor B reflects the frequency stabilization effect achieved by the optical negative feedback effect. Overall, the narrowing factor F... 2 This comprehensively demonstrates the contribution of introducing an external cavity to narrowing the laser linewidth.

[0069] Therefore, different coupling coefficients κ can be obtained using the above formula. c The corresponding narrowing factor F 2 By using different narrowing factors F 2 The coupling coefficient κ can be obtained. c The corresponding theoretical linewidth Δv is used to determine the appropriate coupling coefficient κ based on the actual application linewidth requirements. c Specifically, different coupling coefficients κ can be obtained using the aforementioned formula. c The compression factor F under 2 The correspondence between the theoretical linewidth Δv0 and the equivalent Fabry-Perot cavity is obtained, showing the relationship between the theoretical linewidth Δv and the coupling coefficient κ. c The changing curve is used to obtain the coupling coefficient κ corresponding to the linewidth required for practical applications. c Considering process errors and limitations, the coupling coefficient κ can be obtained through simulation. c The corresponding coupling length L and coupling gap D are shown below.

[0070] Taking quantum optics and precision detection applications as an example, the near-visible light bandwidth is required to be on the order of kHz. Considering that the second harmonic linewidth is twice the bandwidth of the fundamental frequency light, even higher requirements are placed on the bandwidth of the fundamental frequency light. This is achieved through the aforementioned narrowing factor F. 2 The calculation method can be used to deduce the fundamental frequency beam width required to meet the kHz requirement for the octave beam width. Then, considering the variation range of the coupling coefficient of the Sagnac ring mirror 8 due to manufacturing errors and the second harmonic power required in actual applications, the coupling coefficient κ that ensures the octave beam width meets application requirements under the maximum error can be obtained. c This ensures that the linewidth of the output second harmonic meets the application requirements, while maximizing the power of the fundamental frequency light in the input periodically polarized waveguide 10, thereby achieving higher frequency conversion efficiency.

[0071] For the periodically polarized waveguide 10, this invention designs a relatively long periodically polarized waveguide 10 to achieve a higher second harmonic conversion efficiency. When the polarization period is determined, once the fundamental frequency light deviates from the wavelength value set by the polarization period, it will cause a rapid decrease in the second harmonic conversion efficiency. In order to enable the frequency-doubled light to achieve a certain range of tuning function while satisfying a high conversion efficiency, a second thermoelectrode 9 is covered on the periodically polarized waveguide 10. The thermo-optic effect is used to change the refractive index of the periodically polarized waveguide 10 for the fundamental frequency light and the frequency-doubled light. Since the quasi-phase matching condition of the periodic polarization is directly related to the refractive index of the periodically polarized waveguide 10 for the fundamental frequency light and the refractive index for the frequency-doubled light, the periodically polarized waveguide 10 set in this way can achieve tuning of the frequency-doubled light within a certain range while ensuring that the second harmonic conversion efficiency remains at a high level.

[0072] In some embodiments, the first thermal electrode 7 and the second thermal electrode 9 are both made of NiCr, and the phase shift portion 4 and the comb electrode 12 can be made of gold. Specifically, after forming a lithium niobate layer with a lithium niobate waveguide, photoresist can be applied to the lithium niobate layer, and then electron beam lithography can be used to define the position of the phase shift portion 4 and the position of the comb electrode 12. Then, gold deposition is performed, and after removing the photoresist, a silicon dioxide protective layer with a thickness of about 1 μm is covered on the surface of the lithium niobate layer and the gold electrode. Then, NiCr is evaporated to form the first thermal electrode 7 and the second thermal electrode 9. After opening the comb electrode 12, a high-voltage pulsed electric field is applied in the insulating liquid to realize the domain inversion of the periodically polarized waveguide 10. The polarization period of the periodically polarized waveguide 10 can be set according to the wavelength of the target output beam. The quasi-phase matching principle is used to realize efficient second harmonic generation. An output mode converter can also be provided at the end of the periodically polarized waveguide 10 to facilitate coupling output.

[0073] The narrow-linewidth frequency-converting laser, integrating a hybrid III-V group gain chip and a lithium niobate external cavity 2, provided in the above embodiments, simultaneously achieves narrowband filtering and frequency conversion functions on a single lithium niobate external cavity 2, exhibiting high integration and stronger resistance to environmental interference. Furthermore, the external ring frequency doubling scheme allows for more flexible design of the dual micro-ring resonators, and the periodically polarized waveguide 10 can achieve a longer length, contributing to improved second harmonic conversion efficiency and resulting in higher-power frequency-doubled light output. Additionally, the use of a second thermoelectrode to achieve thermo-optical effects altering the phase-matching conditions of the periodically polarized waveguide 10 enables tuning of the frequency-doubled light within a certain wavelength range. Moreover, the use of a dual micro-ring resonator paired with the external cavity of the Sagnac ring mirror 8 for narrowband filtering allows for adjustment of the coupling coefficient of the Sagnac ring mirror according to actual application needs, resulting in a device more suitable for the application requirements. This provides a promising integrated narrow-linewidth frequency-doubled laser for the field of quantum precision measurement.

[0074] In summary, this invention provides a hybrid integrated narrow-linewidth frequency conversion laser with high conversion efficiency and externally tunable frequency doubling. It simultaneously achieves linewidth narrowing and frequency conversion functions on a single thin-film lithium niobate chip. Addressing the technical challenge of low frequency conversion efficiency, the length of the periodic polarization section and the coupling coefficient of the Sagnac ring mirror 8 were carefully considered. Furthermore, the thermo-optical effect was utilized to achieve tuning of the frequency-doubled light within a certain wavelength. This hybrid integrated narrow-linewidth frequency conversion laser improves the design flexibility of the dual micro-ring resonator (filter section) and exhibits high operational stability, making it promising for applications in quantum precision measurement and other fields.

[0075] It should be understood that the various forms of processes shown above can be used to reorder, add, or delete steps. For example, the steps described in this invention disclosure can be executed in parallel, sequentially, or in different orders, as long as the desired result of the technical solution disclosed in this invention can be achieved, and this is not limited herein.

[0076] The specific embodiments described above do not constitute a limitation on the scope of protection of this invention. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A hybrid integrated narrow linewidth frequency conversion laser, characterized in that, include: A gain chip, one end of which outputs initial fundamental frequency light, and the other end of which is coated with a high-reflectivity mirror; The lithium niobate external cavity includes a mode converter, a transmission waveguide with a phase shift section, a dual micro-ring resonator, and a Sagnac ring mirror connected in sequence. The mode converter couples the gain chip to the transmission waveguide, the transmission waveguide transmits light between the gain chip and the dual micro-ring resonator, the phase shift section modulates the phase of the light in the transmission waveguide, the dual micro-ring resonator performs narrowband filtering on the initial fundamental frequency light output from the gain chip to form the fundamental frequency light, and the Sagnac ring mirror and the high-reflectivity mirror form a resonant cavity. The Sagnac ring mirror reflects a portion of the received light back to the gain chip to achieve frequency stabilization. The other portion of the received optical output includes a dual microring resonator comprising a first microring resonator, a second microring resonator, and an intermediate waveguide. The first microring resonator comprises a first straight waveguide and a first ring waveguide. The second microring resonator comprises a second straight waveguide and a second ring waveguide. The input end of the first straight waveguide is connected to the transmission waveguide, and the output end of the second straight waveguide is connected to the input end of the Sagnac ring mirror or the input end of the periodically polarized waveguide. The intermediate waveguide is located between the first ring waveguide and the second ring waveguide, and is located on the side of the first ring waveguide away from the first straight waveguide, and on the side of the second ring waveguide away from the second straight waveguide. The lithium niobate external cavity also includes a periodically polarized waveguide, which is used to realize frequency conversion. The periodically polarized waveguide is located between the Sagnac ring mirror and the dual micro-ring resonator. The output end of the Sagnac ring mirror outputs frequency-doubled light. Alternatively, the periodically polarized waveguide is connected to the end of the Sagnac ring mirror away from the dual micro-ring resonator, and the output end of the periodically polarized waveguide outputs frequency-doubled light. The lithium niobate outer cavity also includes two comb-shaped electrodes, which are symmetrically arranged on both sides of the periodically polarized waveguide.

2. The hybrid integrated narrow linewidth frequency conversion laser according to claim 1, characterized in that, The gain chip has an anti-reflection coating at one end where it outputs the initial fundamental frequency light.

3. The hybrid integrated narrow linewidth frequency conversion laser according to claim 1, characterized in that, The radius of the first ring waveguide is different from the radius of the second ring waveguide.

4. The hybrid integrated narrow linewidth frequency conversion laser according to claim 1, characterized in that, Both the first ring waveguide and the second ring waveguide are provided with a first thermoelectric electrode.

5. The hybrid integrated narrow linewidth frequency conversion laser according to claim 1, characterized in that, The radius of both the first and second ring waveguides is not less than 80 μm.

6. The hybrid integrated narrow linewidth frequency conversion laser according to claim 1, characterized in that, The periodically polarized waveguide is a straight waveguide, and a second thermoelectric electrode is disposed on the periodically polarized waveguide.

7. The hybrid integrated narrow linewidth frequency conversion laser according to claim 6, characterized in that, Each of the comb-shaped electrodes includes a connected main body and a plurality of extensions. The main body is arranged parallel to the periodically polarized waveguide, and the plurality of extensions are arranged at intervals along the extension direction of the main body on the side of the main body facing the periodically polarized waveguide.

8. The hybrid integrated narrow linewidth frequency conversion laser according to claim 7, characterized in that, The polarization period of the comb-shaped electrode , where λ ω n is the wavelength of the fundamental frequency light. 2ω n represents the effective refractive index of the frequency-doubled light in the lithium niobate waveguide. ω The effective refractive index and polarization period of the fundamental frequency light in the lithium niobate waveguide. Equal to twice the width of the extension; The polarization duty cycle of the comb-shaped electrode is 50%.

9. The hybrid integrated narrow linewidth frequency conversion laser according to claim 1, characterized in that, The theoretical linewidth Δv of the hybrid integrated narrow linewidth frequency conversion laser satisfies: ； in, , ; in, ; in, ; Where Δv0 is the linewidth of the Fabry-Perot cavity in the equivalent resonant cavity model, F 2 Let A be the narrowing factor, B be the first factor, and r be the second factor. eff (ω) represents the reflectivity of the rear-end mirror of the Fabry-Perot cavity in the equivalent resonant cavity model, and α H τ0 is the linewidth broadening factor of the gain chip, τ0 is the round-trip time of light in the active region of the gain chip, j is the imaginary number, and t0 is the linewidth broadening factor of the gain chip. transition For coupling loss, t passive For passive waveguide loss, κ m R is the coupling coefficient between the ring waveguide and its adjacent straight waveguide. m Let α be the radius of the ring waveguide. m Let β be the amplitude propagation loss constant of the ring waveguide. If m equals 1, it represents the first ring waveguide; if m equals 2, it represents the second ring waveguide. p κ is the effective propagation constant of the lithium niobate waveguide. c Let κ be the coupling coefficient of the Sagnac ring mirror. c Determined based on coupling length and coupling gap.

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