Frequency-locked high-repetition-rate fiber mode-locked laser

Abstract

The application provides a frequency-locked high-repetition-frequency fiber mode-locked laser, and relates to the technical field of lasers.The frequency-locked high-repetition-frequency fiber mode-locked laser comprises a frequency-stabilized radio frequency source, a frequency-stabilized feedback circuit, a repetition frequency multiplication module, an optical path adjustment module, a resonant cavity, a wavelength combiner and a single-mode pump source.The frequency-stabilized radio frequency source provides a reference frequency and generates a double error signal by using the frequency-stabilized feedback circuit.The double error signal is used for precisely adjusting the resonant cavity length to lock the fundamental frequency repetition frequency and adjusting the relative optical path difference in the repetition frequency multiplication module to stabilize the frequency multiplication process.The repetition frequency multiplication module comprises a plurality of repetition frequency multiplication units and is used for multiplying the repetition frequency of an input pulse sequence.A single-mode pump source is used for driving a gain medium in the resonant cavity to realize passive mode locking, so as to generate a fundamental frequency mode-locked pulse sequence.The application can help to eliminate the accumulated phase noise in the frequency multiplication process, realize double-loop precise locking of the output pulse repetition frequency, and greatly improve the frequency stability and timing accuracy of the laser.

Description

Technical Field

[0001] This invention relates to the field of laser technology, and more particularly to a frequency-locked high-repetition-rate fiber mode-locked laser. Background Technology

[0002] Frequency-locked high-repetition-rate (HPR) mode-locked lasers (HLS) are characterized by ultrashort pulses (picosecond or femtosecond range) and ultra-high repetition rates (hundreds of MHz to tens of GHz or even higher). Due to their densely packed, equally spaced comb-like structure in the frequency domain, HLS have wide applications in optical frequency combs, optical communications, and biomedical imaging. For example, high-repetition-rate optical frequency combs can link microwave and optical frequencies, and higher laser repetition rates can achieve higher frequency resolution. In optical communications, high-repetition-rate pulse sequences can be directly used as carriers for high-speed data streams, achieving Tb / s-level data transmission. The dense spectral lines in the frequency domain of high-repetition-rate pulses can serve as precise light sources for multiple WDM channels, simplifying the light source structure and improving channel stability. In biomedical imaging, high repetition rates allow for sufficient average power at lower single-pulse energies, reducing optical damage to biological samples and enabling faster imaging speeds.

[0003] Chinese Patent Publication No. CN118315906A discloses a high repetition rate fiber mode-locked laser with a single collimator, comprising a composite fiber collimator, a phase shifting unit, a polarization beam splitter, a waveplate, a dispersion compensation unit, and a mirror. The light output from the composite fiber collimator sequentially passes through the phase shifting unit, the polarization beam splitter, the waveplate, the dispersion compensation unit, and the mirror. The composite fiber collimator includes a wavelength division multiplexer (WDM) and a polarization beam splitter collimator. One end of the WDM is connected to a first fiber; the other end of the WDM is connected to the polarization beam splitter collimator via a second fiber and a composite fiber. Multimode beam combiners combine the output beams of multiple lasers into a single beam to increase laser power, but they generally suffer from insufficient long-term frequency stability and significant timing jitter. Especially when achieving ultra-high repetition rates through multiple external frequency doubling stages, the lack of precise active control over the relative delays of each stage during frequency doubling leads to phase noise accumulation, severely affecting the timing accuracy and spectral purity of the final output laser. Therefore, it is essential to provide a high-repetition-rate fiber mode-locked laser that can achieve precise dual-ring locking and effectively suppress frequency drift and timing jitter. Summary of the Invention

[0004] In view of this, this invention proposes a frequency-locked high-repetition-rate fiber mode-locked laser. By synchronously and in parallel using a dual-loop locking mechanism for the oscillator and each frequency multiplier, a full-link phase noise suppression system is constructed for the first time. This not only stabilizes the fundamental frequency, but more importantly, it cuts off the propagation and accumulation path of noise in the frequency multiplication link, so that the timing accuracy of the final output laser no longer deteriorates significantly with the increase of the frequency multiplication level, achieving unprecedented high-magnification and high-fidelity frequency multiplication.

[0005] This invention provides a frequency-locked high-repetition-rate fiber mode-locked laser, comprising a frequency-stabilized RF source, a frequency-stabilized feedback circuit, a repetition rate multiplication module, an optical path adjustment module, a resonant cavity, a wavelength combiner, and a single-mode pump source, wherein... The frequency-stabilized radio frequency source is electrically connected to the frequency-stabilized feedback circuit, and the frequency-stabilized radio frequency source is used to output a frequency reference signal to the frequency-stabilized feedback circuit; The frequency stabilization feedback circuit is electrically connected to the frequency repetition rate multiplication module and the optical path adjustment module, respectively. The frequency stabilization feedback circuit is used to generate a first error signal for adjusting the optical path adjustment module and a second error signal for adjusting the frequency repetition rate multiplication module based on the frequency reference signal. The frequency repetition rate multiplier module is connected to the resonant cavity through the wavelength beam combiner. The frequency repetition rate multiplier module includes multiple frequency repetition rate multiplier units. The frequency repetition rate multiplier units are used to multiply the repetition frequency of the pulse sequence input to the current frequency repetition rate multiplier unit. The optical path adjustment module is connected to the resonant cavity, and the optical path adjustment module is used to precisely adjust the cavity length of the resonant cavity according to the first error signal; The single-mode pump source is connected to the wavelength combiner. The single-mode pump source is used to pump the gain medium in the resonant cavity to achieve passive mode-locking through nonlinear absorption in the resonant cavity, thereby generating a fundamental frequency mode-locked pulse sequence with a repetition frequency from the resonant cavity.

[0006] Based on the above technical solutions, preferably, the repetition rate multiplication unit includes an optical coupler, a photodetector, an adjustable attenuator, a first collimator, a second collimator, a retroreflector, a first piezoelectric ceramic, and a first linear displacement platform, wherein... The optical coupler is connected to the photodetector, one end of the adjustable attenuator, and the first collimator. The first collimator collimates the light pulses split from the optical coupler and couples them to the second collimator through the retroreflector. The other ends of the second collimator and the adjustable attenuator are both connected to the input of the second coupler for outputting a multiplied pulse sequence. The output of the second coupler is connected to another frequency multiplication unit. The retroreflector is disposed on the surface of the first piezoelectric ceramic. The side of the first piezoelectric ceramic facing away from the retroreflector is fixedly connected to the first linear displacement platform, and the first piezoelectric ceramic is electrically connected to the frequency stabilization feedback circuit.

[0007] Based on the above technical solutions, preferably, the optical path adjustment module includes a second linear displacement platform, a second piezoelectric ceramic, and a saturable absorber semiconductor. The second piezoelectric ceramic is disposed on one side of the second linear displacement platform, and the saturable absorber semiconductor is disposed on the side of the second piezoelectric ceramic opposite to the second linear displacement platform. The second piezoelectric ceramic is electrically connected to the frequency stabilization feedback circuit.

[0008] More preferably, the resonant cavity forms an optical resonant circuit, including the saturable absorber semiconductor as a first end mirror, the fiber grating as a second end mirror, and a focusing lens, a third collimator, and a single-mode gain fiber sequentially disposed between the first end mirror and the second end mirror, wherein the focusing lens, the third collimator, and the single-mode gain fiber are located on the same axis.

[0009] More preferably, the frequency stabilization feedback circuit is used to transmit the second error signal, which is relatively delayed in the frequency doubling process, to the first piezoelectric ceramic in each frequency doubling unit to adjust the relative optical path difference in each frequency doubling unit. The frequency stabilization feedback circuit is also used to transmit the first error signal, which stabilizes the fundamental frequency of the oscillator, to the second piezoelectric ceramic to adjust the cavity length of the resonant cavity to lock the fundamental repetition frequency.

[0010] More preferably, the repetition rate multiplication unit includes a probe input path, a fixed delay path, and an adjustable delay path, wherein, The detection input path includes the optical coupler and the photodetector, and the detection input path is used to directly detect the frequency input to the repetition rate multiplier unit; The fixed delay path includes the optical coupler and the adjustable attenuator. The fixed delay path is used to compensate for the optical power imbalance caused by differences in device insertion loss and coupling efficiency. The adjustable delay path includes the optical coupler, the first collimator, the retroreflector, and the second collimator. The retroreflector adjusts the optical path through the first piezoelectric ceramic and the first linear displacement platform so that the outgoing pulse of the adjustable delay path is inserted between any two adjacent pulses in the outgoing pulse of the fixed delay path.

[0011] More preferably, the single-mode pump source includes a single-mode fiber-coupled semiconductor laser with a center wavelength of 976nm, and the output power of the single-mode pump source is 1W.

[0012] More preferably, the center reflection wavelength of the fiber grating is 1030 nm, and the fiber grating has high reflectivity for pulses with a wavelength of 1030 nm.

[0013] More preferably, the single-mode gain fiber is a single-mode ytterbium-doped fiber, and the absorption coefficient of the single-mode gain fiber for the pump light output from the single-mode pump source is 140 dB / m.

[0014] More preferably, the wavelength combiner has high transmittance for pulses with an incident wavelength of 976 nm and high reflectivity for pulses with an incident wavelength of 1030 nm.

[0015] The frequency-locked high-repetition-rate fiber mode-locked laser provided by this invention has the following advantages over the prior art: (1) By generating a reference signal through a frequency-stabilized RF source and a feedback circuit, a dual closed-loop control system is formed for the first time by combining cavity length locking and multiplication delay locking. This ensures that the output pulse is strictly synchronized with the RF reference source without the need for external optical parametric mode-locking devices, significantly reducing the risk of frequency drift and mode jump. It can also continuously fine-tune the optical path of the resonant cavity, effectively suppress cavity length fluctuations and significantly reduce time-domain jitter. The frequency multiplication module compensates for the phase noise accumulated during the frequency multiplication process, further improving the timing stability of the pulse train. At the same time, the multi-level frequency multiplication module can multiply the repetition rate of the fundamental frequency mode-locked pulse. While ensuring the stability of passive mode-locking in a single cavity, it achieves high-multiplication and wideband repetition rate expansion, thereby improving the frequency stability and timing accuracy of the laser.

[0016] (2) By incorporating an optical coupler and a photodetector in the input path, the pulse repetition rate of the current frequency multiplier unit can be read online, and the frequency drift information can be fed back to the frequency locking control loop in a timely manner, which greatly improves the frequency locking accuracy and dynamic response speed of the frequency multiplication stage. Furthermore, an adjustable attenuator is introduced in the fixed delay path to ensure that the fixed delay output pulse and the adjustable delay pulse are strictly matched in power, effectively reducing the amplitude noise and phase jitter caused by power imbalance. At the same time, in the adjustable delay path, the pulse optical path can be adjusted in two stages, coarse and fine, to ensure that the adjustable delay pulse is accurately inserted at any position between the two pulses in the fixed delay path, thereby obtaining ideal time complementarity and equal interval multi-path output. This three-path structure can realize arbitrary time slot insertion within each stage of the frequency multiplier, which greatly reduces insertion jitter and accumulated phase noise. The high repetition rate pulse train output is not only frequency stable, but also has excellent time domain and frequency domain purity, meeting the requirements for ultra-low timing jitter and high coherence. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 A schematic diagram of the frame of a frequency-locked high-repetition-rate fiber mode-locked laser provided by the present invention; Figure 2 This is a schematic diagram of the repetition rate multiplication module provided by the present invention.

[0019] Explanation of reference numerals in the attached figures: 1. Frequency-stabilized RF source; 2. Frequency-stabilized feedback circuit; 3. Frequency repetition rate multiplier module; 31. Frequency repetition rate multiplier unit; 311. Optical coupler; 312. Photodetector; 313. Adjustable attenuator; 314. First collimator; 315. Second collimator; 316. Retroreflector; 317. First piezoelectric ceramic; 318. First linear displacement platform; 4. Optical path adjustment module; 41. Second linear displacement platform; 42. Second piezoelectric ceramic; 43. Saturable absorber semiconductor; 5. Resonant cavity; 51. Focusing lens; 52. Third collimator; 53. Single-mode gain fiber; 54. Fiber grating; 6. Wavelength combiner; 7. Single-mode pump source. Detailed Implementation

[0020] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0021] like Figure 1 As shown, this invention provides a frequency-locked high-repetition-rate fiber mode-locked laser, comprising a frequency-stabilized radio frequency source 1, a frequency-stabilized feedback circuit 2, a repetition rate multiplication module 3, an optical path adjustment module 4, a resonant cavity 5, a wavelength combiner 6, and a single-mode pump source 7, wherein... The frequency stabilization RF source 1 is electrically connected to the frequency stabilization feedback circuit 2. The frequency stabilization RF source 1 is used to output a frequency reference signal to the frequency stabilization feedback circuit 2.

[0022] In this embodiment, the frequency-stabilized RF source 1 and the frequency-stabilized feedback circuit 2 constitute a frequency locking and stabilization control module, used to stabilize the fundamental frequency of the oscillator and the relative delay of each multiplication unit, ensuring frequency locking of the final output high-repetition-rate laser. The frequency-stabilized RF source 1 provides a highly stable frequency reference signal, the frequency of which is the repetition frequency of the high-repetition-rate fiber mode-locked laser. The frequency-stabilized feedback circuit 2 is used to transmit the second error signal of the relative delay in the stabilization frequency multiplication process to the first piezoelectric ceramic 317 in each repetition-rate multiplication unit 31, so as to adjust the relative optical path difference in each repetition-rate multiplication unit 31. The frequency-stabilized feedback circuit 2 is also used to transmit the first error signal of the oscillator fundamental frequency to the second piezoelectric ceramic 42, so as to adjust the cavity length of the resonant cavity 5 to lock the fundamental repetition frequency.

[0023] The frequency stabilization feedback circuit 2 is electrically connected to the frequency repetition rate multiplication module 3 and the optical path adjustment module 4, respectively. The frequency stabilization feedback circuit 2 is used to generate a first error signal for adjusting the optical path adjustment module 4 and a second error signal for adjusting the frequency repetition rate multiplication module 3 based on the frequency reference signal.

[0024] In this embodiment, the frequency repetition multiplier module 3 is connected to the resonant cavity 5 through the wavelength beam combiner 6. The frequency repetition multiplier module 3 includes multiple frequency repetition multiplier units 31. The frequency repetition multiplier unit 31 is used to multiply the repetition frequency of the pulse sequence input to the current frequency repetition multiplier unit 31.

[0025] like Figure 2As shown, the frequency repetition rate multiplication unit 31 includes an optical coupler 311, a photodetector 312, an adjustable attenuator 313, a first collimator 314, a second collimator 315, a retroreflector 316, a first piezoelectric ceramic 317, and a first linear displacement platform 318. The optical coupler 311 is connected to one end of the photodetector 312, one end of the adjustable attenuator 313, and one end of the first collimator 314. The first collimator 314 collimates the light pulse split from the optical coupler 311 and passes it through the retroreflector 316. 16 is coupled to the second collimator 315. The other ends of the second collimator 315 and the adjustable attenuator 313 are both connected to the input of the second coupler used to output the multiplied pulse sequence. The output of the second coupler is connected to another frequency multiplication unit 31. The retroreflector 316 is disposed on the surface of the first piezoelectric ceramic 317. The side of the first piezoelectric ceramic 317 away from the retroreflector 316 is fixedly connected to the first linear displacement platform 318. The first piezoelectric ceramic 317 is electrically connected to the frequency stabilization feedback circuit 2.

[0026] Optical coupler 311 divides the input mode-locked pulse sequence into three paths. Two paths are used for delay, and one path samples and detects the repetition frequency of the laser incident from the previous stage (or resonant cavity 5). There are two optical delay paths, one of which is a fixed delay path. This fixed delay path includes an adjustable optical attenuator to precisely balance the intensity of each optical pulse to optimize the superposition effect. The other path is an adjustable delay path, which collimates the light in this optical fiber to free space and couples the laser in free space back into the optical fiber.

[0027] In this embodiment, by incorporating an optical coupler 311 and a photodetector 312 into the detection input path, the pulse repetition rate entering the current frequency multiplication unit can be read online, and the frequency drift information can be fed back to the frequency locking control loop in a timely manner, greatly improving the frequency locking accuracy and dynamic response speed during the frequency multiplication stage. Furthermore, an adjustable attenuator 313 is introduced in the fixed delay path to ensure strict power matching between the fixed delay output pulse and the adjustable delay pulse, effectively reducing amplitude noise and phase jitter caused by power imbalance. At the same time, in the adjustable delay path, coarse / fine two-stage adjustment of the pulse optical path is realized, ensuring that the adjustable delay pulse is accurately inserted at any position between the two pulses in the fixed delay path, thereby obtaining ideal time-complementary, equally spaced multi-path output. This three-path structure can achieve arbitrary time slot insertion within each stage of the frequency multiplier, greatly reducing insertion jitter and accumulated phase noise. The high repetition rate pulse train output not only has stable frequency but also excellent time-domain and frequency-domain purity, meeting the requirements for ultra-low timing jitter and high coherence.

[0028] Retroreflector 316 is used to reflect the free-space beam. The incident and reflected beams are parallel and aligned with the displacement axis. A first piezoelectric ceramic 317 (PZT) drives retroreflector 316 for precise displacement, acting as a feedback actuator to compensate for optical path jitter caused by the environment and achieve laser repetition rate locking. A first linear displacement platform 318 physically connects the first piezoelectric ceramic 317 and retroreflector 316, used for coarse adjustment of the optical path of the adjustable delay path. After reflection by the retroreflector, the beam is coupled back to the optical fiber through a second collimator 315. The optical path difference between the two optical fiber paths is set to be equal to half the input pulse period to achieve temporal interleaving of the pulses.

[0029] Furthermore, the frequency multiplication unit 31 includes a detection input path, a fixed delay path, and an adjustable delay path. The detection input path includes an optical coupler 311 and a photodetector, used to directly detect the frequency input to the frequency multiplication unit 31. The fixed delay path includes an optical coupler 311 and an adjustable attenuator 313, used to compensate for optical power imbalance caused by device insertion loss and coupling efficiency differences. The adjustable delay path includes an optical coupler 311, a first collimator 314, a retroreflector 316, and a second collimator 315. The retroreflector 316 adjusts the optical path length through a first piezoelectric ceramic 317 and a first linear displacement platform 318, allowing the output pulse from the adjustable delay path to be inserted between any two adjacent pulses in the output pulse from the fixed delay path. Subsequent stages of the optical coupler 311 recouple and superimpose the beams from different delay paths, outputting a pulse sequence with multiplied repetition frequency, and continue to perform splitting and sampling. The final optical coupler 311 no longer performs splitting, only sampling to detect its repetition frequency. The photodetector 312 is used to monitor the repetition frequency of the internal or output signal of the frequency multiplier unit. Its output signal is input to the frequency stabilization feedback circuit 2 and mixed with the reference signal of the frequency stabilization RF source 1.

[0030] In this embodiment, the optical path adjustment module 4 is connected to the resonant cavity 5. The optical path adjustment module 4 is used to precisely adjust the cavity length of the resonant cavity 5 according to the first error signal. The optical path adjustment module 4 includes a second linear displacement platform 41, a second piezoelectric ceramic 42, and a saturable absorption semiconductor 43. The second piezoelectric ceramic 42 is disposed on one side of the second linear displacement platform 41, and the saturable absorption semiconductor 43 is disposed on the side of the second piezoelectric ceramic 42 opposite to the second linear displacement platform 41. The second piezoelectric ceramic 42 is electrically connected to the frequency stabilization feedback circuit 2.

[0031] The second piezoelectric ceramic 42 is mechanically connected to the saturable absorber semiconductor 43 (SESAM) for precise and rapid adjustment of the oscillator cavity length. The second linear displacement platform 41 supports the second piezoelectric ceramic 42 and the saturable absorber semiconductor 43, providing a wide range of coarse cavity length adjustment capabilities. The saturable absorber semiconductor 43 serves as an end mirror of the resonant cavity 5 and enables passive mode-locking initiation and maintenance.

[0032] Furthermore, the resonant cavity 5 forms an optical resonant circuit, including a saturable absorber semiconductor 43 as a first end mirror, a fiber grating 54 as a second end mirror, and a focusing lens 51, a third collimator 52, and a single-mode gain fiber 53 sequentially disposed between the first end mirror and the second end mirror. The focusing lens 51, the third collimator 52, and the single-mode gain fiber 53 are located on the same axis.

[0033] In this embodiment, the focusing lens 51 is used to focus the light output from the third collimator 52 onto the saturable absorber semiconductor 43 to increase the laser power density on the saturable absorber semiconductor 43, facilitating passive mode-locking. The third collimator 52 is used to collimate the light in the optical fiber into a spatial beam, facilitating the length adjustment of the resonant cavity 5. The fiber grating 54 serves as the other end mirror or output coupling mirror of the resonant cavity 5 and defines the laser oscillation wavelength. The center reflection wavelength of the fiber grating 54 is 1030 nm, and the fiber grating 54 has high reflectivity for pulses with a wavelength of 1030 nm. The single-mode gain fiber 53 is a single-mode ytterbium-doped fiber, and the absorption coefficient of the single-mode gain fiber 53 for the pump light output from the single-mode pump source 7 is 140 dB / m.

[0034] A single-mode pump source 7 is connected to a wavelength combiner 6. The single-mode pump source 7 pumps the gain medium in the resonant cavity 5 to achieve passive mode-locking through nonlinear absorption in the resonant cavity 5, thereby generating a fundamental frequency mode-locked pulse sequence with a repetition frequency. The single-mode pump source 7 includes a single-mode fiber-coupled semiconductor laser with a center wavelength of 976 nm and an output power of 1 W. The wavelength combiner 6 has high transmittance for pulses with an incident wavelength of 976 nm and high reflectivity for pulses with an incident wavelength of 1030 nm.

[0035] Please continue reading. Figure 1The frequency stabilization feedback circuit 2 divides the output frequency of the frequency stabilization RF source 1. The detected laser repetition frequency (or its multiplied higher harmonics) and the 1 / 2N division of the reference signal output by the frequency stabilization RF source 1 are compared for frequency and phase discrimination. An error control signal is generated based on the comparison result. The error signal used to stabilize the fundamental frequency of the oscillator is applied to the second piezoelectric ceramic 42 in the optical path adjustment module 4, and the fundamental repetition frequency of the laser is locked by precisely adjusting the cavity length of the resonant cavity 5. The error signal used to stabilize the relative delay of the frequency multiplication process is applied to the first piezoelectric ceramic 317 in each repetition rate multiplication unit 31, and the time accuracy of pulse superposition is locked by precisely adjusting the relative optical path difference within the corresponding multiplication stage, thus suppressing the relative delay drift caused by environmental disturbances.

[0036] A single-mode pump source 7 drives a single-mode gain fiber 53 to emit light. Within a resonant cavity 5 composed of a saturable absorber semiconductor 43 and a fiber grating 54, passive mode-locking is achieved through the nonlinear absorption of the saturable absorber semiconductor 43, generating a repetition frequency of [frequency missing]. f A fundamental frequency mode-locked pulse sequence of 0. The frequency-stabilized RF source 1 and the frequency-stabilized feedback circuit 2 compare the detected repetition frequency with a reference RF source. The frequency-stabilized feedback circuit 2 drives the second piezoelectric ceramic 42 to precisely control the cavity length, setting the repetition frequency to 0. f 0 is stably locked at the target value.

[0037] The fundamental frequency mode-locked pulse enters the first-stage repetition rate multiplier unit 31. Optical coupler 311 splits it into two paths: one with a fixed delay, and the other with an adjustable delay path controlled by the first piezoelectric ceramic 317 and the first linear displacement stage. Coarse adjustment is performed by adjusting the first linear displacement stage, and fine adjustment is performed by driving the first piezoelectric ceramic 317 through the frequency stabilization feedback circuit 2, ensuring that the optical path difference between the two paths is precisely stabilized at a value that allows the pulses to be interleaved and superimposed. The combined output repetition frequency is 2. f A pulse sequence of 0. If a multi-stage frequency multiplication unit 31 exists, then the output of the previous stage is 2(N-1)× f The zero pulse sequence is input to the next stage, and the frequency multiplication process is repeated. The final output repetition frequency is 2N× f A high-repetition-rate pulse sequence of 0.

[0038] In this embodiment, not only is the cavity length of the mode-locked oscillator precisely locked by piezoelectric ceramic feedback, but also the relative delay path within each cascaded frequency multiplication unit 31 is configured with independent piezoelectric ceramic precision feedback locking. This distributed, multi-point precision locking mechanism can effectively compensate for optical path drift caused by factors such as ambient temperature changes and mechanical vibration in various parts (oscillator cavity length, relative delay of each frequency multiplication unit), ensuring that even after multiple frequency multiplications, the final output high repetition rate laser pulse sequence still has extremely high frequency stability and timing accuracy. This solves the technical problem of difficulty in guaranteeing stability at high frequency multiplication rates in existing technologies, and enables frequency-locked high repetition rate laser output.

[0039] In one example, the mode-locked laser oscillator in this embodiment consists of an optical path adjustment module 4, a resonant cavity 5, a wavelength beam combiner 6, and a single-mode pump source 7. The mode-locked laser oscillator adopts a linear cavity structure to generate a stable, low-noise fundamental frequency. f 0-mode-locked pulse sequence. Its specific structure and working process are as follows: A single-mode fiber-coupled semiconductor laser with a center wavelength of 976 nm was selected as the single-mode pump source 7, with an output power of 1 W. The output fiber of the single-mode pump source 7 was coupled into a section of single-mode ytterbium-doped fiber through a wavelength combiner 6. This wavelength combiner 6 was designed to have high transmittance at 976 nm and high reflectivity near 1030 nm (the laser signal wavelength). The single-mode ytterbium-doped fiber served as the gain medium, absorbing the pump light and amplifying the signal light circulating in the cavity through stimulated emission. The length of the single-mode ytterbium-doped fiber was approximately 25 cm, and the overall cavity length, including the passive fiber and the spatial optical path, was approximately 40 cm, corresponding to a laser repetition rate of 266 MHz.

[0040] The resonant cavity 5 includes two end mirrors. One end mirror is composed of a fiber Bragg grating (FBG), which is etched onto a single-mode fiber. The center reflection wavelength of the FBG is set around 1030 nm, and its reflectivity is approximately 98%. The FBG acts as a high-reflectivity mirror within the cavity and also serves as a filter, limiting the center wavelength and spectral width of the laser oscillation. The transmission end of the FBG serves as part of the laser's output. In the linear cavity of this embodiment, the FBG typically functions as a high-reflectivity mirror. The other end mirror is composed of a saturable absorptive semiconductor 43. To couple the light from the fiber to the FBG and back, a free-space coupling structure is used, i.e., a third collimator 52 is connected to the end of the fiber to collimate the fiber mode into a parallel beam. This parallel beam passes through a focusing lens 51 (e.g., an aspherical lens with a focal length of a few millimeters), focusing the beam onto the effective absorption region of the saturable absorptive semiconductor 43. The saturable absorptive semiconductor 43 exhibits intensity-dependent absorption characteristics, with high absorption at low light intensity and a saturated decrease in absorption at high light intensity. This saturable absorption characteristic helps to promote the formation and stabilization of ultrashort pulses and achieve passive mode-locking. The saturable absorption semiconductor 43 has a saturation flux of 20 μJ / cm², corresponding to the cavity power.

[0041] The saturable absorber semiconductor 43 is precisely fixed on the second piezoelectric ceramic 42. The saturable absorber semiconductor 43 has a nanometer-scale displacement and fast response capability (59kHz level). The saturable absorber semiconductor 43 is mounted on a manually or electrically controlled second linear displacement platform 41. The second linear displacement platform 41 provides a 1 cm coarse adjustment range for initially setting the approximate cavity length to obtain a frequency close to the target fundamental frequency. f The mode-locking pulse is 0. The second piezoelectric ceramic 42 is used to receive the feedback voltage signal from the frequency locking and stabilization control module, and to perform precise and rapid cavity length fine-tuning to actively stabilize the repetition frequency of the laser. f 0.

[0042] When the pump power reaches the mode-locking threshold, the intracavity noise pulse undergoes nonlinear absorption by the saturable absorbable semiconductor 43, amplification by the single-mode gain fiber 53, and filtering by the fiber Bragg grating, ultimately forming a stable mode-locked pulse sequence. Simultaneously, the repetition frequency of this pulse sequence is stabilized at the fundamental frequency of 260MHz by the second linear displacement platform 41 and the stable control of the second piezoelectric ceramic 42. This pulse sequence is then output through the wavelength combiner 6 and used as the input signal for the subsequent frequency multiplication unit 31.

[0043] To achieve GHz-level high repetition rates without significantly shortening the oscillator cavity length, the repetition rate multiplier module 3 employs cascaded repetition rate multiplier units 31. Each unit achieves a 2x frequency multiplication. For example... Figure 2As shown, taking the first-stage multiplication unit as an example, its structure and working principle are as follows: A pulse sequence with a fundamental frequency of 266MHz from a mode-locked laser oscillator is input to the first optical coupler 311 via optical fiber. Optical coupler 311 uses a 1x3 coupler, where the weaker output is directly used to probe the input frequency, and the other two outputs are fed into subsequent delay paths: a fixed delay path and an adjustable delay path, respectively.

[0044] One optical pulse travels through a predetermined length of optical fiber, forming a fixed-delay path. To precisely control the consistency of the pulse intensity after the two pulses are superimposed, an adjustable optical attenuator is connected in series in this path. By adjusting the attenuator, the optical power imbalance between the two pulses caused by differences in device insertion loss, coupling efficiency, etc., can be compensated.

[0045] Another optical pulse enters an adjustable delay path. The core purpose of this path is to introduce a precisely controllable delay, which needs to be actively stabilized. The fiber optic signal is first output to free space through the first collimator 314. After propagating a certain distance in free space, the fiber optic signal is reflected by the retroreflector 316. The retroreflector 316 is characterized in that, regardless of the incident angle, the reflected beam always propagates parallel to and in the opposite direction to the incident beam. The retroreflector 316 is mounted on the first piezoelectric ceramic 317, which also has nanometer-level precision displacement capability and fast response characteristics. The first piezoelectric ceramic 317 and the retroreflector 316 mounted on it are integrally mounted on a manually or electrically controlled first linear displacement platform 318, which is used to coarsely adjust the length of the free space optical path. The beam reflected back by the retroreflector 316, after propagating in free space, is recoupled back into the fiber by the second collimator 315.

[0046] The optical path difference (ΔL) between the fixed delay path and the adjustable delay path is precisely set to be equal to the input pulse period T0 = 1 / f The optical path length corresponding to half of 0 is ΔL = c × (T0 / 2) = 5.769 cm. The approximate ΔL is achieved by coarsely adjusting the first linear displacement platform 318, and then the first piezoelectric ceramic 317 is finely adjusted and locked by the frequency locking and stabilization control module to ensure that the optical path difference is accurately stabilized at the required value, so that the pulse from the adjustable path is precisely inserted between two adjacent pulses from the fixed path.

[0047] Two pulse sequences with different delays are recoupled and superimposed at the second optical coupler 311 (the optical coupler of subsequent stages, or the output coupler of the first stage). Due to the precise half-cycle delay, the two pulses are staggered in time, forming a repetition frequency of 2. fA new pulse sequence of 0=520MHz is generated. This sequence serves as the output of this stage multiplier unit. If a higher repetition frequency is required, the output of the first-stage repetition rate multiplier unit 31 can be increased by 2... f A zero-pulse sequence is fed as input into a second, identically structured frequency multiplier unit 31. The second frequency multiplier unit 31 also has an optical path difference corresponding to half the input pulse period, i.e., ΔL2 = 2.8845 cm. The combined output repetition frequency is 4. f A pulse sequence with a repetition rate of 0. Similarly, through N cascaded repetition rate multiplier units 31, a repetition rate of 2 can ultimately be obtained. N × f A high repetition rate pulse sequence of 0. Each repetition rate multiplier unit 31 requires an independent first piezoelectric ceramic 317 and corresponding feedback control to stabilize its internal relative delay.

[0048] A photodetector 312 is installed at one port of the optocoupler 311 of each repetition rate multiplier unit 31. This photodetector 312 is used to monitor the repetition frequency of the current stage output pulse sequence in real time and send the detected electrical signal to the frequency locking and stabilization control module. A photodetector 312 must also be installed at the output of the last stage repetition rate multiplier unit 31 to monitor the final output frequency.

[0049] The structure and working principle of the frequency locking and stabilization control module are as follows: The frequency locking and stabilization control module uses a highly stable frequency-stabilized RF source 1 to provide a frequency of f ref The reference signal. The frequency of this reference signal. f ref Set as the high repetition frequency of the final desired output f ref = 2 N × f 0.

[0050] Frequency stabilization feedback circuit 2 receives a reference signal from the frequency stabilization RF source. f ref The reference signal of the frequency-stabilized RF source 1 is divided to obtain... f ref , f ref / 2… f ref / 2 N The laser repetition frequency signal is received from the photodetector at the output of the mode-locked laser oscillator and the photodetector 312 at the output of each repetition rate multiplier unit 31 (which are respectively...). f 0,2 f 0,…,2 N f0). The detected oscillator base frequency signal f 0det (or one of its higher harmonics) and f ref / 2 N Frequency discrimination is performed. The comparison result generates an error voltage signal V. err0 .

[0051] For the k class( k =1 to N) Frequency multiplication unit 31, frequency stabilization feedback circuit 2 will affect the reference frequency f ref Perform (N- k The frequency is divided by two to obtain... f ref / 2 (N-k) Then, the detected k-th level output frequency signal f kdet (or its harmonics) and f ref / 2 (N -k) Frequency and phase discrimination is performed to generate the corresponding error voltage signal V. errk The generated error voltage signal V err0 V err1 ,…,V errN After appropriate loop filtering and amplification, V is applied to the corresponding piezoelectric ceramics. err0 The second piezoelectric ceramic 42 is driven to precisely adjust the cavity length of the resonant cavity, thereby controlling the fundamental frequency. f 0 locked in f ref / 2 N .

[0052] Error voltage signal V errk ( k =1 to N) drive the first k The first piezoelectric ceramic 317 in the first-stage frequency multiplication unit precisely adjusts the optical path of the adjustable delay path inside this stage, stabilizes the relative delay of the two pulses, and ensures the accuracy and stability of pulse interleaving and superposition, thereby achieving the desired effect. k Stage output frequency locked at f ref / 2 (N-k) Through this distributed, multi-point precision feedback locking mechanism, not only the oscillator's fundamental frequency is... f The frequency repetition rate is precisely locked, and the relative delay in each frequency doubling process is also independently and precisely locked. This allows the entire system to effectively resist disturbances caused by environmental temperature fluctuations, mechanical vibrations, and other factors to the optical path length of each part, ensuring that the final output repetition frequency F = 2N × 0. f 0 has extremely high frequency stability.

[0053] The overall operation process is as follows: Start the single-mode pump source 1 and adjust the pump power to bring the oscillator into a stable passive mode-locked state. Use the second linear displacement platform 41 to coarsely adjust the cavity length to achieve the fundamental frequency. f 0 is close to the target value f ref / 2 (N-k) For each repetition rate multiplication unit, the length of the adjustable delay path is coarsely adjusted using its internal first linear displacement plateau 318. The repetition frequency of the output signal of that stage is observed to bring it close to the target multiplication value (2). f 0,4 f 0,…). Simultaneously, the adjustable optical attenuator 313 balances the intensity of the two light sources to obtain a frequency-doubled pulse sequence with uniform amplitude. The frequency locking and stabilization control module is activated, and the frequency stabilization feedback circuit 2 begins operation. It compares the frequency of each detection point with the reference frequency (or its division), generating an error signal to drive all piezoelectric ceramics, and the system enters a closed-loop locked state. In the closed-loop locked state, the system can automatically compensate for optical path drift caused by environmental disturbances, and maintain a stable output frequency of F= for a long period. f ref A high-repetition-rate, high-precision mode-locked pulse laser whose frequency stability is determined by the frequency-stabilized RF source 1.

[0054] By generating a reference signal through a frequency-stabilized RF source 1 and a feedback circuit, a dual closed-loop control system is formed for the first time, combining cavity length locking and multiplication delay locking. This ensures that the output pulse is strictly synchronized with the RF reference source without the need for external optical parametric mode-locking devices, significantly reducing the risk of frequency drift and mode jump. Furthermore, it can continuously fine-tune the optical path of the resonant cavity 5, effectively suppressing cavity length fluctuations and significantly reducing time-domain jitter. The frequency multiplication module 3 compensates for the phase noise accumulated during the frequency multiplication process, further improving the timing stability of the pulse train. At the same time, the multi-stage frequency multiplication module 3 can multiply the repetition rate of the fundamental frequency mode-locked pulse, achieving high-multiplication and wideband repetition rate expansion while ensuring the stability of passive mode-locking within a single cavity, thereby improving the frequency stability and timing accuracy of the laser.

[0055] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Similarly, the terms "an" or "a" and similar terms do not indicate a quantity limitation, but rather indicate the presence of at least one. The terms "connected" or "linked" and similar terms are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. "Up," "down," "left," "right," etc., are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship also changes accordingly.

[0056] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A frequency-locked high-repetition-rate fiber mode-locked laser, characterized in that, It includes a frequency-stabilized RF source (1), a frequency-stabilized feedback circuit (2), a repetition rate multiplier module (3), an optical path adjustment module (4), a resonant cavity (5), a wavelength combiner (6), and a single-mode pump source (7), among which, The frequency-stabilized radio frequency source (1) is electrically connected to the frequency-stabilized feedback circuit (2), and the frequency-stabilized radio frequency source (1) is used to output a frequency reference signal to the frequency-stabilized feedback circuit (2); The frequency stabilization feedback circuit (2) is electrically connected to the frequency repetition rate multiplication module (3) and the optical path adjustment module (4) respectively. The frequency stabilization feedback circuit (2) is used to generate a first error signal for adjusting the optical path adjustment module (4) and a second error signal for adjusting the frequency repetition rate multiplication module (3) based on the frequency reference signal. The frequency multiplication module (3) is connected to the resonant cavity (5) through the wavelength beam combiner (6). The frequency multiplication module (3) includes multiple frequency multiplication units (31). The frequency multiplication unit (31) is used to multiply the repetition frequency of the pulse sequence input to the current frequency multiplication unit (31). The frequency repetition rate multiplication unit (31) includes an optical coupler (311), a photodetector (312), an adjustable attenuator (313), a first collimator (314), a second collimator (315), a retroreflector (316), a first piezoelectric ceramic (317), and a first linear displacement platform (318), wherein, The optical coupler (311) is connected to the photodetector (312), one end of the adjustable attenuator (313), and the first collimator (314) respectively. The first collimator (314) collimates the light pulse split from the optical coupler (311) and couples it to the second collimator (315) through the retroreflector (316). The other ends of the second collimator (315) and the adjustable attenuator (313) are both connected to the input end of the second coupler used to output the multiplied pulse sequence. The output end of the second coupler is connected to another frequency multiplication unit (31). The retroreflector (316) is disposed on the surface of the first piezoelectric ceramic (317). The side of the first piezoelectric ceramic (317) facing away from the retroreflector (316) is fixedly connected to the first linear displacement platform (318), and the first piezoelectric ceramic (317) is electrically connected to the frequency stabilization feedback circuit (2). The optical path adjustment module (4) is connected to the resonant cavity (5), and the optical path adjustment module (4) is used to precisely adjust the cavity length of the resonant cavity (5) according to the first error signal; The single-mode pump source (7) is connected to the wavelength combiner (6). The single-mode pump source (7) is used to pump the gain medium in the resonant cavity (5) to achieve passive mode-locking through nonlinear absorption in the resonant cavity (5), thereby generating a fundamental frequency mode-locked pulse sequence with a repetition frequency by the resonant cavity (5).

2. A frequency-locked high-repetition-rate fiber mode-locked laser as described in claim 1, characterized in that, The optical path adjustment module (4) includes a second linear displacement platform (41), a second piezoelectric ceramic (42), and a saturable absorption semiconductor (43). The second piezoelectric ceramic (42) is disposed on one side of the second linear displacement platform (41), and the saturable absorption semiconductor (43) is disposed on the side of the second piezoelectric ceramic (42) away from the second linear displacement platform (41). The second piezoelectric ceramic (42) is electrically connected to the frequency stabilization feedback circuit (2).

3. A frequency-locked high-repetition-rate fiber mode-locked laser as described in claim 2, characterized in that, The resonant cavity (5) forms an optical resonant circuit, including the saturable absorber semiconductor (43) as the first end mirror, the fiber grating (54) as the second end mirror, and a focusing lens (51), a third collimator (52), and a single-mode gain fiber (53) sequentially disposed between the first end mirror and the second end mirror. The focusing lens (51), the third collimator (52), and the single-mode gain fiber (53) are located on the same axis.

4. A frequency-locked high-repetition-rate fiber mode-locked laser as described in claim 2, characterized in that, The frequency stabilization feedback circuit (2) is used to transmit the second error signal, which is relatively delayed in the frequency doubling process, to the first piezoelectric ceramic (317) in each frequency doubling unit (31) to adjust the relative optical path difference in each frequency doubling unit (31). The frequency stabilization feedback circuit (2) is also used to transmit the first error signal, which stabilizes the fundamental frequency of the oscillator, to the second piezoelectric ceramic (42) to adjust the cavity length of the resonant cavity (5) to lock the fundamental repetition frequency.

5. A frequency-locked high-repetition-rate fiber mode-locked laser as described in claim 1, characterized in that, The frequency repetition rate multiplication unit (31) includes a probe input path, a fixed delay path, and an adjustable delay path, wherein, The detection input path includes the optical coupler (311) and the photodetector, and the detection input path is used to directly detect the frequency input to the repetition rate multiplier unit (31); The fixed delay path includes the optical coupler (311) and the adjustable attenuator (313), and the fixed delay path is used to compensate for the optical power imbalance caused by the difference in device insertion loss and coupling efficiency; The adjustable delay path includes the optical coupler (311), the first collimator (314), the retroreflector (316), and the second collimator (315). The retroreflector (316) adjusts the optical path through the first piezoelectric ceramic (317) and the first linear displacement platform (318) so that the outgoing pulse of the adjustable delay path is inserted between any two adjacent pulses in the outgoing pulse of the fixed delay path.

6. A frequency-locked high-repetition-rate fiber mode-locked laser as described in claim 1, characterized in that, The single-mode pump source (7) includes a single-mode fiber-coupled semiconductor laser with a center wavelength of 976nm, and the output power of the single-mode pump source (7) is 1W.

7. A frequency-locked high-repetition-rate fiber mode-locked laser as described in claim 3, characterized in that, The fiber grating (54) has a center reflection wavelength of 1030 nm and the fiber grating (54) has high reflectivity for pulses with a wavelength of 1030 nm.

8. A frequency-locked high-repetition-rate fiber mode-locked laser as described in claim 3, characterized in that, The single-mode gain fiber (53) is a single-mode ytterbium-doped fiber, and the absorption coefficient of the single-mode gain fiber (53) for the pump light output from the single-mode pump source (7) is 140dB / m.

9. A frequency-locked high-repetition-rate fiber mode-locked laser as described in claim 1, characterized in that, The wavelength combiner (6) has high transmittance for pulses with an incident wavelength of 976 nm and high reflectivity for pulses with an incident wavelength of 1030 nm.

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