Frequency-locked high-repetition-frequency optical fiber mode-locked laser

Abstract

The invention provides a frequency-locked high-repetition-frequency optical fiber mode-locked laser, which relates to the technical field of lasers and comprises a frequency-stabilized radio frequency source, a frequency-stabilized feedback circuit, a repetition-frequency multiplication module, an optical path adjusting module, a resonant cavity, a wavelength beam combiner and a single-mode pumping source, the frequency stabilization radio frequency source provides a reference frequency and generates dual error signals through the frequency stabilization feedback circuit, the dual error signals are used for precisely adjusting the cavity length of a resonant cavity so as to lock the fundamental frequency repetition frequency and adjusting the relative optical path difference in the multi-stage repetition frequency multiplication module so as to stabilize the frequency multiplication process, and the repetition frequency multiplication module comprises a plurality of repetition frequency multiplication units. The single-mode pumping source is used for carrying out repetition frequency multiplication on an input pulse sequence, and the single-mode pumping source is used for driving a gain medium in the resonant cavity to realize passive mode locking, so that a fundamental frequency mode-locked pulse sequence is generated. According to the invention, phase noise accumulated in the frequency doubling process can be eliminated, double-loop precision locking of the repetition frequency of the output pulse is realized, and the frequency stability and the time sequence precision of the laser are greatly improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of laser, in particular to a frequency-locked high-repetition-rate fiber mode-locked laser. BACKGROUND

[0002] The frequency-locked high-repetition-rate mode-locked laser has the characteristics of ultra-short pulse (picosecond or femtosecond order) and ultra-high repetition rate (hundred MHz to tens of GHz or even higher). Because it is also an equal-interval dense comb in the frequency domain, the frequency-locked high-repetition-rate mode-locked laser has wide applications in optical frequency comb, optical communication, biomedical imaging, etc. For example, the high-repetition-rate optical frequency comb can be connected with microwave frequency and optical frequency, and higher laser repetition rate can achieve higher frequency resolution. In the field of optical communication, the high-repetition-rate pulse sequence can be directly used as a carrier of high-speed data stream to realize Tb / s-level data transmission, and the dense spectral lines in the frequency domain of the high-repetition-rate pulse can be used as an accurate light source of multiple WDM channels to simplify the light source structure and improve the channel stability. In the field of biomedical imaging, high repetition rate allows sufficient average power to be obtained at lower single-pulse energy, thereby reducing the optical damage to the biological sample and achieving faster imaging speed.

[0003] A high-repetition-rate fiber mode-locked laser with a single collimator is disclosed in Chinese Patent No. CN118315906A, which comprises a fiber collimator with a composite structure, a phase shift unit, a polarization beam splitter, a wave plate, a dispersion compensation unit and a mirror. The light output by the fiber collimator with a composite structure successively passes through the phase shift unit, the polarization beam splitter, the wave plate, the dispersion compensation unit and the mirror. The fiber collimator with a composite structure is provided with a wavelength division multiplexer and a polarization beam splitting collimator. One end of the wavelength division multiplexer is connected with a first optical fiber, and the other end of the wavelength division multiplexer is connected with the polarization beam splitting collimator through a second optical fiber and a composite optical fiber. The multi-mode beam combiner combines the output beams of multiple lasers into a single beam to improve the laser power, but generally has problems such as insufficient long-term stability of frequency, large timing jitter, etc. Especially when achieving ultra-high repetition rate by multi-stage external frequency multiplication, there is a lack of precise active control of the relative delay of each stage in the frequency multiplication process, which leads to phase noise accumulation and seriously affects the timing accuracy and spectral purity of the finally output laser. Therefore, it is very necessary to provide a high-repetition-rate fiber mode-locked laser capable of double-loop precise locking and effectively suppressing frequency drift and timing jitter. SUMMARY

[0004] Therefore, the application provides a frequency-locked high-repetition-frequency fiber mode-locked laser, a full-link phase noise suppression system is first constructed by synchronously and paralleling double-loop locking of an oscillator and each frequency multiplier, which not only stabilizes the base frequency, but more importantly, cuts off the noise transmission and accumulation path 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 number, and unprecedented high-multiple and high-fidelity frequency multiplication is realized.

[0005] The application provides a frequency-locked high-repetition-frequency fiber mode-locked laser, comprising 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, wherein, The frequency-stabilized radio frequency source is electrically connected with 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-stabilized feedback circuit is electrically connected with the repetition frequency multiplication module and the optical path adjustment module respectively, and the frequency-stabilized 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 repetition frequency multiplication module based on the frequency reference signal; The repetition frequency multiplication module is connected with the resonant cavity through the wavelength combiner, and the repetition frequency multiplication module comprises a plurality of repetition frequency multiplication units, and the repetition frequency multiplication unit is used to multiply the repetition frequency of a pulse sequence input into the current repetition frequency multiplication unit; The optical path adjustment module is connected with 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 with the wavelength combiner, and the single-mode pump source is used to pump a gain medium in the resonant cavity to realize passive mode locking through nonlinear absorption in the resonant cavity, so as to generate a base frequency mode-locked pulse sequence with a repetition frequency from the resonant cavity.

[0006] On the basis of the above technical scheme, preferably, the repetition frequency multiplication unit comprises 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 with the photodetector, one end of the adjustable attenuator and the first collimator respectively, the first collimator collimates the light pulse branched by the optical coupler and couples the light pulse into the second collimator through the retroreflector, the second collimator and the other end of the adjustable attenuator are both connected to the input end of a second coupler for outputting a multiplied pulse sequence, the output end of the second coupler is connected with another frequency multiplication unit, the retroreflector is arranged on the surface of the first piezoelectric ceramic, the side of the first piezoelectric ceramic away from the retroreflector is fixedly connected with the first linear displacement platform, and the first piezoelectric ceramic is electrically connected with the frequency stabilization feedback circuit.

[0007] On the basis of the above technical scheme, preferably, the optical path adjustment module comprises a second linear displacement platform, a second piezoelectric ceramic and a saturable absorption semiconductor, the second piezoelectric ceramic is arranged on one side of the second linear displacement platform, the saturable absorption semiconductor is arranged on the side of the second piezoelectric ceramic away from the second linear displacement platform, and the second piezoelectric ceramic is electrically connected with the frequency stabilization feedback circuit.

[0008] More preferably, the resonant cavity forms an optical resonant loop, comprising the saturable absorption 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 optical fiber arranged in sequence between the first end mirror and the second end mirror, the focusing lens, the third collimator and the single-mode gain optical fiber are located on the same axis.

[0009] More preferably, the frequency stabilization feedback circuit is used for transmitting the second error signal relatively delayed by the stable frequency multiplication process to the first piezoelectric ceramic in each frequency multiplication unit, so as to adjust the relative optical path difference in each frequency multiplication unit, and the frequency stabilization feedback circuit is also used for transmitting the first error signal for stabilizing the fundamental frequency of the oscillator to the second piezoelectric ceramic, so as to adjust the cavity length of the resonant cavity to lock the basic repetition frequency.

[0010] More preferably, the frequency multiplication unit comprises a detection input path, a fixed delay path and an adjustable delay path, wherein, The detection input path comprises the optical coupler and the photodetector, and the detection input path is used for directly detecting the frequency input into the frequency multiplication unit; The fixed delay path comprises the optical coupler and the adjustable attenuator, and the fixed delay path is used for compensating the optical power imbalance caused by the device insertion loss and the coupling efficiency difference; The adjustable delay path comprises the optical coupler, the first collimator, the retroreflector and the second collimator, the retroreflector is adjusted in optical path by 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 of the outgoing pulse of the fixed delay path.

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

[0012] Further preferably, the fiber grating has a central reflection wavelength of 1030 nm, and the fiber grating has high reflectivity to pulses with a wavelength of 1030 nm.

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

[0014] Further preferably, the wavelength combiner has high transmittance to incident pulses with a wavelength of 976 nm, and the wavelength combiner has high reflectivity to incident pulses with a wavelength of 1030 nm.

[0015] The frequency-locked high-repetition-frequency fiber mode-locked laser provided by the application has the following beneficial effects relative to the prior art: (1) The reference signal is generated by the frequency-stabilized radio frequency source and the feedback circuit, the cavity length locking and the multiplication delay locking are combined to form a double-closed-loop control system for the first time, the output pulse and the radio frequency reference source are strictly synchronized without external optical parametric mode-locked devices, the frequency drift and mode jump risk are greatly reduced, the resonant cavity optical path can be continuously fine-tuned, the cavity length fluctuation is effectively suppressed, the time-domain jitter is significantly reduced, the phase noise accumulated in the frequency multiplication process is compensated by the multi-stage repetition frequency multiplication module, the time sequence stability of the pulse train is further improved, the repetition rate of the base frequency mode-locked pulse can be multiplied by the multi-stage repetition frequency multiplication module, the repetition rate expansion with high multiples and wide bands is realized while the stability of the single-stage intracavity passive mode-locked is ensured, and the frequency stability and time sequence precision of the laser are improved.

[0016] (2) By building-in optical coupler and photoelectric detector in the detection input path, the pulse repetition rate entering the current frequency multiplication unit can be read on-line, and the frequency drift information can be fed back to the frequency locking control loop in time, so that the frequency locking precision and dynamic response speed in the frequency multiplication stage are greatly improved. An adjustable attenuator is introduced in the fixed delay path to ensure strict matching of the power of the fixed delay output pulse and the adjustable delay pulse, effectively reducing the amplitude noise and phase jitter caused by power imbalance. Meanwhile, in the adjustable delay path, coarse / fine two-stage adjustment of the optical path of the pulse is realized to ensure that the adjustable delay pulse is accurately inserted at any position between the two pulses of the fixed delay path, so that ideal time complementary and equal-interval multi-output is obtained. The three-path structure can realize arbitrary time slot insertion in each frequency multiplier, greatly reducing the insertion jitter and cumulative phase noise of the pulse insertion, and the output high repetition rate pulse train not only has stable frequency, but also has excellent time domain and frequency domain purity, meeting the demand for ultra-low timing jitter and high coherence. BRIEF DESCRIPTION OF DRAWINGS

[0017] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, below the drawings needed to be used in the embodiments or prior art description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative effort on the basis of these drawings.

[0018] Figure 1 A frame schematic diagram of a frequency-locked high repetition rate fiber mode-locked laser is provided. Figure 2 A structure schematic diagram of a repetition rate multiplication module is provided.

[0019] Explanation of reference signs: 1, frequency-stabilized radio frequency source; 2, frequency-stabilized feedback circuit; 3, repetition rate multiplication module; 31, repetition rate multiplication unit; 311, optical coupler; 312, photoelectric detector; 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 absorption semiconductor; 5, resonant cavity; 51, focusing lens; 52, third collimator; 53, single-mode gain optical fiber; 54, fiber grating; 6, wavelength combiner; 7, single-mode pump source. DETAILED DESCRIPTION

[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 repetition frequency 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 with the photodetector 312, one end of the adjustable attenuator 313, and the first collimator 314 respectively, the first collimator 314 collimates the light pulses split by the optical coupler 311 and couples them into the second collimator 315 through the retroreflector 316, the second collimator 315 and the other end of the adjustable attenuator 313 are both connected to the input end of a second coupler for outputting a multiplied pulse sequence, the output end of the second coupler is connected with another repetition frequency multiplication unit 31, the retroreflector 316 is arranged 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 with the first linear displacement platform 318, and the first piezoelectric ceramic 317 is electrically connected with the frequency stabilization feedback circuit 2.

[0026] The optical coupler 311 divides the input mode-locked pulse sequence into three paths, two of which are used for delay, and one of which is used for sampling and detecting the laser repetition frequency of the previous stage (or the resonant cavity 5). Two optical delay paths are provided, one of which is a fixed delay path. The fixed delay path includes an adjustable optical attenuator, which is used to accurately balance the intensities of the light pulses in each path to optimize the superposition effect. The other is an adjustable delay path, which collimates the light in the optical fiber and outputs it to free space, and couples the laser in free space back into the optical fiber.

[0027] In this embodiment, by embedding the optical coupler 311 and the photodetector 312 in 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 time, greatly improving the frequency locking accuracy and dynamic response speed in the frequency multiplication stage. An adjustable attenuator 313 is introduced in the fixed delay path to ensure that the power of the fixed delay output pulse and the adjustable delay pulse is strictly matched, effectively reducing the amplitude noise and phase jitter caused by power imbalance. Meanwhile, in the adjustable delay path, coarse / fine two-stage adjustment of the pulse optical path is realized 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 complementary and equal-interval multi-path output. This three-path structure can realize arbitrary time slot insertion in each frequency multiplier, greatly reducing the insertion jitter and cumulative phase noise of pulse insertion, and the output high repetition rate pulse train not only has stable frequency, but also has excellent time domain and frequency domain purity, meeting the demand for ultra-low timing jitter and high coherence.

[0028] The reflector 316 is used to reflect the free space light beam, the incident and reflected light beams of the reflector are parallel and parallel with the displacement axis. The first piezoelectric ceramic 317 (PZT) drives the reflector 316 to perform fine displacement, as a feedback actuator to compensate the optical path jitter caused by the environment, and to realize the laser repetition frequency locking. The first linear displacement platform 318 is physically connected to the first piezoelectric ceramic 317 and the reflector 316, and is used to coarsely adjust the optical path of the adjustable delay path. After the light beam is reflected by the reflector, it is coupled back to the optical fiber through the second collimator 315. The optical path difference of the two optical fiber paths is set to be equal to half of the input pulse period, so as to realize the time staggered superposition of the pulses.

[0029] Further, the repetition frequency multiplication unit 31 includes a probe input path, a fixed delay path and an adjustable delay path. The probe input path includes an optical coupler 311 and a photodetector, and is used to directly probe the frequency input into the repetition frequency multiplication unit 31. The fixed delay path includes an optical coupler 311 and an adjustable attenuator 313, and is used to compensate the optical power imbalance caused by the device insertion loss and the coupling efficiency difference. The adjustable delay path includes an optical coupler 311, a first collimator 314, a reflector 316 and a second collimator 315. The reflector 316 adjusts the optical path through the first piezoelectric ceramic 317 and the first linear displacement platform 318, so that the output pulse of the adjustable delay path is inserted between any two adjacent pulses of the output pulse of the fixed delay path. The subsequent optical couplers 311 re-couple and superimpose the light beams passing through different delay paths, output the repetition frequency multiplied pulse sequence, and continue to branch and sample. The last optical coupler 311 no longer branches, and only samples the repetition frequency. The photodetector 312 is used to monitor the repetition frequency of the internal or output signal of the frequency multiplication unit, and the output signal is input into the frequency stabilization feedback circuit 2 and mixed with the reference signal of the frequency stabilization radio frequency source 1 for comparison.

[0030] In the embodiment, the optical path adjustment module 4 is connected to the resonant cavity 5, and the optical path adjustment module 4 is used to finely 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 arranged on one side of the second linear displacement platform 41, the saturable absorption semiconductor 43 is arranged on the side of the second piezoelectric ceramic 42 away from the second linear displacement platform 41, and the second piezoelectric ceramic 42 is electrically connected to the frequency stabilization feedback circuit 2.

[0031] The second piezoelectric ceramic 42 is mechanically connected with a saturable absorbing semiconductor 43 (SESAM) for precisely and rapidly adjusting the length of the oscillator cavity. The second linear displacement platform 41 is used to carry the second piezoelectric ceramic 42 and the saturable absorbing semiconductor 43 to provide a large range of coarse adjustment of the cavity length. The saturable absorbing semiconductor 43 serves as one end mirror of the resonant cavity 5 and realizes passive mode-locking start and maintenance.

[0032] Further, the resonant cavity 5 forms an optical resonant loop, which includes the saturable absorbing semiconductor 43 as a first end mirror, the fiber grating 54 as a second end mirror, and the focusing lens 51, the third collimator 52 and the single-mode gain fiber 53 arranged in sequence 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 being located on the same axis.

[0033] In the embodiment, the focusing lens 51 is used to focus the light output by the third collimator 52 onto the saturable absorbing semiconductor 43 to improve the laser power density on the saturable absorbing semiconductor 43, facilitating passive mode-locking. The third collimator 52 is used to collimate the light in the optical fiber to output a spatial light beam, facilitating the length adjustment of the resonant cavity 5. The fiber grating 54 serves as another 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 to the pulse 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 to the pump light output by the single-mode pump source 7 is 140 dB / m.

[0034] The single-mode pump source 7 is connected with the wavelength combiner 6. The single-mode pump source 7 is used to pump the gain medium in the resonant cavity 5 to realize passive mode-locking through nonlinear absorption in the resonant cavity 5, so as to generate a base frequency mode-locked pulse sequence with a repetition frequency from the resonant cavity 5. The single-mode pump source 7 includes a single-mode fiber-coupled semiconductor laser with a center wavelength of 976 nm, and the output power of the single-mode pump source 7 is 1 W. The wavelength combiner 6 has high transmittance to the incident pulse with a wavelength of 976 nm, and high reflectivity to the incident pulse with a wavelength of 1030 nm.

[0035] Please continue to refer to Figure 1The frequency stabilization feedback circuit 2 divides the output frequency of the frequency stabilization RF source 1, and the detected laser repetition frequency (or its multiplied higher harmonic) and the 1 / 2N division of the output reference signal of the frequency stabilization RF source 1 are respectively frequency and phase detected. According to the comparison result, an error control signal is generated. The error signal for stabilizing the oscillator base frequency is applied to the second piezoelectric ceramic 42 in the optical path adjustment module 4, and the basic repetition frequency of the laser is locked by precisely adjusting the cavity length of the resonant cavity 5. The error signal for stabilizing the relative delay of the frequency multiplication process is applied to the first piezoelectric ceramic 317 in each level of the repetition frequency multiplication unit 31, and the time precision of the pulse superposition is locked and the relative delay drift caused by environmental disturbance is suppressed by precisely adjusting the relative optical path difference in the corresponding multiplication level.

[0036] The single-mode pump source 7 drives the single-mode gain optical fiber 53 to emit light, and in the resonant cavity 5 composed of the saturable absorption semiconductor 43 and the fiber grating 54, passive mode locking is realized through the nonlinear absorption of the saturable absorption semiconductor 43, and a base frequency mode-locked pulse sequence with a repetition frequency of 100 MHz is generated. The frequency stabilization RF source 1 and the frequency stabilization feedback circuit 2 compare the detected repetition frequency with the reference RF source, and the second piezoelectric ceramic 42 is driven by the frequency stabilization feedback circuit 2 to accurately control the cavity length, so that the repetition frequency of 100 MHz is stably locked at the target value. f f The base frequency mode-locked pulse enters the first level of the repetition frequency multiplication unit 31, and the optical coupler 311 divides it into two paths, one of which passes through a fixed delay, and the other of which passes through an adjustable delay path controlled by the first piezoelectric ceramic 317 and the first linear displacement table. Coarse adjustment is performed by adjusting the first linear displacement table, and fine adjustment is performed by driving the first piezoelectric ceramic 317 by the frequency stabilization feedback circuit 2, so that the optical path difference of the two paths is accurately stabilized at a value that enables the pulses to be interleaved and superimposed. After being combined, a pulse sequence with a repetition frequency of 200 MHz is output. If there are multiple levels of the repetition frequency multiplication unit 31, the 2(N-1)×100 MHz pulse sequence output by the upper level is input to the lower level, and the above frequency multiplication process is repeated, and finally a high repetition frequency pulse sequence with a repetition frequency of 2N×100 MHz is output.

[0037] The base frequency mode-locked pulse enters the first level of the repetition frequency multiplication unit 31, and the optical coupler 311 divides it into two paths, one of which passes through a fixed delay, and the other of which passes through an adjustable delay path controlled by the first piezoelectric ceramic 317 and the first linear displacement table. Coarse adjustment is performed by adjusting the first linear displacement table, and fine adjustment is performed by driving the first piezoelectric ceramic 317 by the frequency stabilization feedback circuit 2, so that the optical path difference of the two paths is accurately stabilized at a value that enables the pulses to be interleaved and superimposed. After being combined, a pulse sequence with a repetition frequency of 200 MHz is output. If there are multiple levels of the repetition frequency multiplication unit 31, the 2(N-1)×100 MHz pulse sequence output by the upper level is input to the lower level, and the above frequency multiplication process is repeated, and finally a high repetition frequency pulse sequence with a repetition frequency of 2N×100 MHz is output. f f f

[0038] ​​​​In the embodiment, not only the cavity length of the mode-locked oscillator is precisely locked by piezoelectric ceramic feedback, but also the relative delay path inside each cascaded frequency multiplication unit 31 is also precisely locked by independent piezoelectric ceramic feedback. This distributed and multi-point precise locking mechanism can effectively compensate the optical path drift of each part (oscillator cavity length, relative delay of each frequency multiplication unit) caused by environmental temperature changes, mechanical vibration and other factors, ensuring that even after multi-frequency multiplication, the final output high-frequency laser pulse sequence still has extremely high frequency stability and timing accuracy, solving the technical problem that the stability of the prior art is difficult to guarantee at high frequency multiplication, and realizing frequency-locked high-frequency laser output.

[0039] In one example, the mode-locked laser oscillator in the embodiment is composed of an optical path adjustment module 4, a resonant cavity 5, a wavelength combiner 6, and a single-mode pump source 7, and adopts a linear cavity structure to generate stable and low-noise base frequency f 0mode-locked pulse sequence. The specific structure and working process are as follows: A single-mode fiber-coupled semiconductor laser with a center wavelength of 976 nm is selected as the single-mode pump source 7, and the output power of the single-mode pump source 7 is 1 W. The output fiber of the single-mode pump source 7 is coupled into a section of single-mode ytterbium-doped fiber through the wavelength combiner 6, which is designed to have high transmittance at 976 nm and high reflectivity near 1030 nm (laser signal wavelength). The single-mode ytterbium-doped fiber acts as a gain medium, absorbs pump light, and amplifies the circulating signal light in the cavity through stimulated radiation. The length of the single-mode ytterbium-doped fiber is about 25 cm, and the total cavity length including passive fibers and spatial optical paths is about 40 cm, corresponding to a laser repetition frequency of 266 MHz.

[0040] The resonant cavity 5 includes two end mirrors, one of which is composed of a fiber Bragg grating, which is inscribed on a single-mode fiber, the center reflection wavelength of the fiber Bragg grating is set near 1030 nm, the reflectivity of the fiber Bragg grating is about 98%, the fiber Bragg grating acts as a high-reflectivity mirror in the cavity and also functions as a filter to limit the center wavelength and spectral width of the laser oscillation. The transmission end of the fiber Bragg grating serves as a partial output of the laser. In the linear cavity of the present embodiment, the fiber Bragg grating generally functions as a high-reflectivity mirror. The other end mirror is composed of a saturable semiconductor absorber 43. In order to couple the light in the fiber to the fiber Bragg grating and return it, a free-space coupling structure is adopted, i.e., the end of the fiber is connected to a third collimator 52, which collimates the fiber mode into a parallel light beam. The parallel light beam passes through a focusing lens 51 (for example, an aspheric lens with a focal length of a few millimeters), and is focused onto the effective absorption region of the saturable semiconductor absorber 43. The saturable semiconductor absorber 43 has an optical intensity-dependent absorption characteristic, i.e., the absorption rate is high at low light intensity, and the absorption rate is saturated and reduced at high light intensity. This saturable absorption characteristic helps to promote the formation and stabilization of ultrashort pulses, and realizes passive mode locking. The saturable semiconductor absorber 43 has a saturation flux of 20 μJ / cm², which corresponds to the in-cavity power.

[0041] The saturable semiconductor absorber 43 is precisely fixed on the second piezoelectric ceramic 42, which has a nanoscale displacement stroke and a fast response capability (59 kHz level). The saturable semiconductor absorber 43 is integrally installed on a second linear displacement platform 41 which is manually or electrically controlled. The second linear displacement platform 41 provides a coarse adjustment range of 1 cm, which is used to initially set the approximate cavity length to obtain mode-locked pulses close to the target fundamental frequency f 0. f 0.

[0042] When the pump power reaches the mode-locking threshold, the in-cavity noise pulses undergo nonlinear absorption by the saturable semiconductor absorber 43, amplification by the single-mode gain fiber 53, and filtering by the fiber Bragg grating, and finally form a stable mode-locked pulse train. At the same time, the second linear displacement platform 41 and the stable control second piezoelectric ceramic 42 are used to stabilize the repetition frequency of the pulse train at 260 MHz, which is the fundamental frequency pulse train. The pulse train is output through the wavelength combiner 6 as the input signal of the subsequent repetition frequency multiplication unit 31.

[0043] In order to obtain a high repetition frequency of GHz level without significantly shortening the oscillator cavity length, the repetition frequency multiplication module 3 adopts a cascaded repetition frequency multiplication unit 31. Each unit realizes a 2-fold frequency multiplication. As Figure 2The first stage multiplication unit is shown as an example, its structure and working principle are as follows: The pulse sequence of the base frequency 266MHz from the mode-locked laser oscillator is input into the first optical coupler 311 through an optical fiber. The optical coupler 311 uses a 1x3 coupler, in which the weaker one is directly used for detecting the input frequency, and the other two enter the subsequent delay paths. They enter the fixed delay path and the adjustable delay path respectively.

[0044] One of the light pulses passes through an optical fiber of a predetermined length to form the fixed delay path. In order to accurately control the consistency of the pulse intensity after superposition, an adjustable optical attenuator is connected in series in this path. By adjusting the attenuator, the imbalance of the optical power caused by the device insertion loss and the coupling efficiency difference can be compensated.

[0045] The other light pulse enters the adjustable delay path. The core purpose of this path is to introduce an accurately controllable delay, and the delay needs to be actively stabilized. The optical fiber signal is first output to free space through the first collimator 314. After propagating in free space for a distance, it is reflected by the reflector 316. The reflector 316 has the characteristic that the reflected beam always propagates in the opposite direction parallel to the incident beam regardless of the incident angle. The reflector 316 is installed 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 reflector 316 installed thereon are installed on a first linear displacement platform 318 which is manually or electrically controlled, and the first linear displacement platform 318 is used for coarse adjustment of the length of the free space optical path. The light beam reflected by the reflector 316 is coupled back into the optical fiber after propagating in free space.

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

[0047] The two pulse sequences with different delays are superimposed at the second optical coupler 311 (the output coupler of the first stage, or the optical coupler of the subsequent stage). Due to the accurate half-period delay, the two pulses are staggered in time, forming a pulse sequence with 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. 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 shown below: Start the single-mode pump source 1, and adjust the pump power to make the oscillator enter a stable passive mode-locked state. Coarse adjust the cavity length using the second linear displacement platform 41, so that the fundamental frequency f 0is close to the target value f ref / 2 (N-k) For each stage of the frequency multiplication unit, use the first linear displacement platform 318 inside to coarsely adjust the length of the adjustable delay path, and observe the repetition frequency of the output signal of the stage, so that it is close to the target frequency multiplication value (2 f 0,4 f 0,…). At the same time, adjust the adjustable optical attenuator 313 to balance the light intensity of the two paths, so as to obtain a sequence of amplitude-uniform frequency-multiplied pulses. Start the frequency locking and stabilization control module, and the frequency stabilization feedback circuit 2 starts to work, compares the frequencies of the detection points with the reference frequency (or its frequency division), generates an error signal to drive all piezoelectric ceramics, and the system enters a closed-loop locking state. In the closed-loop locking state, the system can automatically compensate for the optical path drift caused by environmental disturbances, and stably output a frequency of F= f ref = F

[0054] The reference signal is generated by the frequency stabilization RF source 1 and the feedback circuit, which first forms a double-closed-loop control system of cavity length locking and multiplication delay locking, ensures that the output pulses are strictly synchronized with the RF reference source without external optical parametric modulation devices, greatly reduces the risk of frequency drift and mode jump, and can continuously fine-tune the optical path of the resonant cavity 5, effectively suppresses the cavity length fluctuation and significantly reduces the time-domain jitter, the frequency multiplication module 3 compensates for the accumulated phase noise in the frequency multiplication process, further improves the timing stability of the pulse train, and at the same time, the multi-stage frequency multiplication module 3 can multiply the repetition rate of the fundamental frequency mode-locked pulses, while ensuring the stability of the single-stage intracavity passive mode-locked state, realizes high-multiple and wide-band repetition rate expansion, and thus improves the frequency stability and timing accuracy of the laser.

[0055] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of this application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and / or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. As used herein, the term "if' can be construed to mean "when" or "upon" or "in response to the happening of," unless explicitly indicated to the contrary. As used herein, the term "or" is synonymous with "and / or," unless the context clearly indicates otherwise.

[0056] The application has been described herein in relation to particular embodiments, which are in no way meant to be limiting of the application but rather are meant to be illustrative thereof. Any modifications, equivalents, improvements, and the like not described above are intended to be within the scope of the application.

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 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 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).

3. 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).

4. A frequency-locked high-repetition-rate fiber mode-locked laser as described in claim 3, 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.

5. A frequency-locked high-repetition-rate fiber mode-locked laser as described in claim 3, 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.

6. A frequency-locked high-repetition-rate fiber mode-locked laser as described in claim 2, 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.

7. 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.

8. A frequency-locked high-repetition-rate fiber mode-locked laser as described in claim 4, 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.

9. A frequency-locked high-repetition-rate fiber mode-locked laser as described in claim 4, 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.

10. 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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