External clock synchronized solid mode-locked laser and digitized frequency-locked phase modulation device and method

Abstract

An external clock synchronization solid-state mode-locked laser and a digital frequency-locked and phase-modulated device and method, comprising a mode-locked laser resonant cavity, a cavity length adjusting system, a frequency locking system, a phase modulation system and an external clock signal source; the mode-locked laser resonant cavity is used for generating a mode-locked laser pulse sequence; the cavity length adjusting system is used for processing an electrical signal converted from the pulse optical signal, and after frequency multiplication with the external clock signal respectively, through mixing with a local clock, analog-digital conversion, phase discrimination, digital-analog conversion, filtering and amplification, a feedback signal is output to drive the cavity length adjusting system, so as to realize the frequency locking function with the external clock. The phase modulation system can high-precisely adjust the phase difference between the laser sequence and the external clock signal source within one pulse period. The application realizes the digital synchronization of the mode-locked laser and the external clock, and has the automatic phase modulation function of the laser pulse sequence, can realize the high-precision arbitrary adjustment of the laser pulse time delay, and is suitable for the fields of laser inertial confinement fusion, time-frequency signal transmission and quantum entanglement.

Description

Technical Field

[0001] The present invention relates to a solid mode-locked laser, in particular to an external clock synchronized solid mode-locked laser and a digital frequency-locked phase modulation device and method. Background Art

[0002] Solid-state mode-locked lasers have a simple structure, minimal nonlinear phase shift, and a high signal-to-noise ratio, making them suitable as seed sources for ultrashort pulse laser devices. After passing through stretchers, amplifiers, and compressors, they can output high-peak-power laser pulses, making them suitable for numerous applications. Experimental research in fields such as pump detection, strong-field physics, precision machining, and inertial confinement fusion requires precisely synchronized multiple laser pulses. However, because the oscillation processes of different laser resonators are independent, parameters such as the repetition frequency and time delay of the laser pulse train have no fixed relationship. Synchronization technology aims to align the frequency, timing, and time delay between different laser pulses. To achieve this goal, large laser devices also require synchronization of the laser pulses with a clock system or timing transmitter to facilitate timing distribution and implement active control methods such as high-precision electro-optical modulation. Therefore, synchronizing mode-locked lasers with an external clock is a key technical approach for high-power laser devices.

[0003] Taking inertial confinement fusion laser device as example, it is necessary to control multiple different types of laser pulses (nanosecond pulses, picosecond pulses) to arrive at the target pellet simultaneously or maintain accurate relative time delay. Typical passive synchronization technology refers to the laser pulses generated by two lasers being overlapped in time on the same nonlinear crystal to produce nonlinear signals, and feedback control lasers to realize synchronization technology [document 1, Miura T, Nagaoka H, ​​Takasago K, et al. " Active synchronization of two mode-locked lasers with optical cross correlation ". Applied Physics B, 2002, 75: 19-23.]. This technology has a simple structure, but cannot realize synchronization between 3 and above lasers. Using a unified external clock as a reference benchmark, and by feedback control of a piezoelectric element to lock the cavity length of a mode-locked laser resonant cavity through a phase-locked loop circuit, this type of active synchronization technology can realize synchronization between 3 and above laser systems.

[0004] Typically, synchronization devices for mode-locked lasers only require that the target system maintain consistent repetition frequency with an external clock, but lack the ability to automatically and precisely control the relative time delay (phase) between the optical pulse signal and the external clock signal. To improve synchronization accuracy, the optoelectronic conversion signal of the mode-locked laser sequence must be frequency-multiplied or harmonized before being locked to the frequency-multiplied external clock. However, this frequency-locking technique suffers from phase randomness, meaning that the actual mode-locked pulse sequence locks to any frequency-division period in the frequency-multiplied clock sequence, hindering subsequent synchronization control. In 2008, Wei Zhiyi's research group at the Institute of Physics, Chinese Academy of Sciences [Patent 1, Wei Zhiyi, Wang Peng, Zhao Huan, et al. "Precision Active Synchronization Device for Different Ultrashort Pulse Lasers." Beijing: CN101599610B, 2011-01-26.] employed two phase-locked loops in parallel. After phase modulation was first achieved in the first phase-locked loop corresponding to the fundamental frequency, high-precision synchronization was achieved using the second phase-locked loop corresponding to the 12th harmonic. Furthermore, sum-frequency optical feedback was used to feed back to the third phase-locked loop, further improving synchronization accuracy. The control logic of the phase-locked loop of this device is relatively complex, and because it uses nonlinear crystals to generate sum-frequency light to improve synchronization accuracy, its application scenarios are limited to two mode-locked lasers. It cannot achieve precise synchronization with an external clock and has no automatic phase adjustment function. In the same year, Kim [Reference 2, Kim J, Cox J, Chen Jet al. "Drift-free femtosecond timing synchronization of remote optical and microwave sources" Nature Photon 2, 733–736 (2008).] built a clock synchronization system based on the balanced optical cross-correlation (BOC) method. This also has application scenario limitations and lacks phase adjustment function. However, the synchronous phase modulation device using extracavity delay control has disadvantages such as complex optical path structure and small adjustment range [Reference 3, Yang H, Han B, Shin J, et al. “10-fs-level synchronization of photocathode laser with RF-oscillator for ultrafast electron and X-ray sources”. Scientific Reports, 2017, 7(1): 1-7.].

[0005] Therefore, an externally clocked mode-locked laser with high-precision frequency locking and automatic phase modulation functions will help improve the synchronization control accuracy and stability of multi-beam laser devices. Summary of the Invention

[0006] To address the current challenges of current development, the present invention aims to provide a solid-state mode-locked laser and digital frequency-locked phase modulation device and method synchronized with an external clock. The device utilizes a mode-locked laser resonator, a cavity length adjustment system, a frequency-locking system, a phase modulation system, and an external clock signal source. The mode-locked laser resonator generates a sequence of mode-locked laser pulses. The cavity length adjustment system includes a piezoelectric actuator and a piezoelectric displacement stage for adjusting the cavity length of the mode-locked laser resonator. The frequency-locking system processes the electrical signal converted from the pulsed optical signal and, after frequency multiplication with the external clock signal, outputs a feedback signal that drives the cavity length adjustment system after mixing with a local clock, analog-to-digital conversion, phase detection, digital-to-analog conversion, filtering, and amplification, achieving frequency locking with the external clock. At the detection end, the phase modulation system precisely adjusts the phase difference between the laser sequence and the external clock signal source within a single pulse cycle by controlling the delay of the laser pulse sequence and phase modulation of a reference clock signal. The present invention realizes digital synchronization between the mode-locked laser and the external clock, and has the function of automatic phase adjustment of the laser pulse sequence, which can realize high-precision and arbitrary adjustment of the laser pulse delay, and is suitable for fields such as laser inertial confinement fusion, time-frequency signal transmission and quantum entanglement.

[0007] The technical solutions of the present invention are as follows:

[0008] A solid-state mode-locked laser and a digital frequency-locked phase-modulation device synchronized with an external clock, characterized by comprising a mode-locked laser resonant cavity, a cavity length adjustment system, a frequency-locking system, a phase-modulation system, and an external clock signal source:

[0009] The cavity length adjustment system includes a piezoelectric actuator and a piezoelectric displacement stage; the frequency locking system includes a first frequency multiplier, a second frequency multiplier, a local clock, a first mixer, a second mixer, a first A / D converter, a second A / D converter, an FPGA phase detector, a D / A converter, a filter, an amplifier, a power divider, a comparator, and an alarm; the phase modulation system includes a fiber coupler, a first high-speed photodetector, a fiber delay, a second high-speed photodetector, a counter, a master controller, and a phase shifter;

[0010] The connection relationship of the above components is as follows:

[0011] The piezoelectric actuator and the piezoelectric displacement stage are arranged in the mode-locked laser resonant cavity. The output monitoring end of the mode-locked laser resonant cavity is connected to the input end of the optical fiber coupler. The optical fiber coupler has a first output port and a second output port. The first output port of the optical fiber coupler, the optical fiber delay, the second high-speed photodetector, and the frequency multiplier are connected in sequence; the second output port of the optical fiber coupler is connected in sequence via the first high-speed photodetector, the counter, and the main controller; the main controller has four output ports: the first output port is connected to the first input port of the phase shifter; the second output port of the main controller is connected to the second input port of the optical fiber delay; the third output port of the main controller is connected to the second input port of the FPGA phase detector; the fourth output port of the main controller is connected to the first input port of the alarm; the second input port of the counter is connected to the first input port of the external clock source. The output port is connected; the output end of the second high-speed photodetector is connected in sequence via the first frequency multiplier, the first output port of the first mixer, the first A / D converter and the first input port of the FPGA phase detector; the output port of the phase shifter, the second frequency multiplier, the first input port of the second mixer, the second A / D converter, and the second input port of the FPGA phase detector are connected in sequence; the second input port of the first mixer and the second input port of the second mixer are respectively connected to the output port of the local clock; the FPGA phase detector, D / A converter, filter, amplifier and power divider are connected in sequence; the output end of the power divider is respectively connected to the input ends of the piezoelectric actuator and the comparator; the first output port and the second output port of the comparator are respectively connected to the second input port of the piezoelectric displacement stage and the alarm; the second output port of the external clock signal source is connected to the second input port of the phase shifter.

[0012] Furthermore, the piezoelectric actuator can selectively drive any reflective mirror in the resonant cavity; the piezoelectric displacement stage can selectively drive the cavity mirror or the output coupling mirror of the resonant cavity.

[0013] Furthermore, the comparator in the frequency locking system is set with upper and lower voltage thresholds; and the FPGA phase detector is constructed by a Field Programmable Gate Array (FPGA).

[0014] Furthermore, the phase modulation system can adjust the phase difference (also known as relative time delay) between the laser sequence and the external clock signal source with high precision within one pulse cycle by controlling the delay of the laser pulse sequence and modulating the phase with the reference clock signal at the detection end.

[0015] Furthermore, the external clock source can be selected from an external clock system or a timing transmitter, etc.; the applicable frequency range of the external clock source is on the order of tens of megahertz to several gigahertz, and the difference range between the external clock source and the intrinsic frequency of the mode-locked laser resonant cavity is on the order of ±200kHz; the period of the external clock source is T, and after being multiplied N times by a frequency multiplier, each multiplication sub-period is T / N.

[0016] Furthermore, the phase modulation method of the external clock synchronized solid-state mode-locked laser and the digital frequency-locked phase modulation device is characterized by comprising the following steps:

[0017] 1) The external clock signal source inputs clock signals to the counter and the phase shifter respectively. After the counter receives the signal output by the mode-locked laser resonator from the first high-speed photodetector, it calculates the time delay signal between the signal output by the mode-locked laser resonator and the clock signal output by the external clock signal source. The counter inputs the time delay signal and the clock signal to the master controller.

[0018] 2) The FPGA phase detector determines the frequency difference and phase difference between the external clock signal source and the mode-locked laser resonant cavity, and outputs an error signal which is sequentially converted by the FPGA phase detector, the D / A converter, the filter, and the amplifier, and then transmitted to the power divider for power distribution. When the voltage signal received by the comparator is within the upper and lower thresholds set by the comparator, step 3) is executed; when the voltage signal exceeds or falls below the threshold set by the comparator, step 4) is executed.

[0019] 3) The position of the piezoelectric displacement stage remains unchanged, and only the piezoelectric actuator is used to track and adjust the cavity length, and the comparator outputs a frequency locking signal to the alarm;

[0020] 4) The comparator outputs an unlock warning signal to the alarm device and enters a relock mode: the comparator output voltage drives the piezoelectric displacement stage to increase or decrease by one step, and then the position of the piezoelectric displacement stage remains unchanged. One step is approximately half of the total stroke of the piezoelectric actuator.

[0021] 5) Under the action of the FPGA phase detector, filter, and amplifier, the piezoelectric actuator is driven through the power divider to adjust the return stroke to the middle position of the total stroke, and the process returns to step 2) to lock the laser pulse frequency with the external clock signal. The comparator then re-outputs a frequency lock signal to the alarm device.

[0022] 6) The external clock source frequency locking function enters the real-time tracking and adjustment working mode, and returns to step 2);

[0023] 7) The master controller receives the time delay signal output by the counter, determines the deviation Δt between the time delay signal and the preset value, and if the deviation Δt≠0, phase adjustment is required, and the real-time frequency locking (i.e., steps 2)-5) is suspended. The master controller outputs a loss of lock warning signal to the alarm device;

[0024] 8) The master controller first determines the magnitude of the deviation Δt. If the magnitude of the deviation is within the adjustable range (T1) of the optical fiber delay device, a signal is transmitted to the optical fiber delay device for adjustment, and the process proceeds to step 11). Otherwise, the master controller first calculates the number of frequency doubling cycles n required to adjust the laser pulse sequence, and transmits the signal to the phase shifter, and the process proceeds to step 9).

[0025] 9) The phase shifter first shifts the frequency-multiplied clock signal by 0.k frequency-multiplied cycles (corresponding to a time domain delay of 0.k*T / N, 0 <k<1);

[0026] 10) The phase modulation process of the laser pulse sequence returns to step 2) and is achieved by adjusting the cavity length of the mode-locked laser resonator back and forth. However, this does not change the final frequency of the mode-locked laser resonator, that is, the cavity length of the resonator remains unchanged before and after the phase modulation. The piezoelectric actuator and piezoelectric displacement stage introduce a spatial delay during the reciprocating motion, thereby achieving back-and-forth adjustment of the mode-locked laser sequence in the time domain.

[0027] 11) The fiber delay device controls the laser pulse sequence to move 0.k-1 frequency doubling period ((0.k-1)*T / N), and repeats step 10) to achieve a delay adjustment of 1 frequency doubling period (1*T / N);

[0028] 12) Repeat steps 9)-11) n times to adjust n frequency doubling periods (n*T / N), and then use the optical fiber delay device to accurately adjust the delay amount Δt-n*T / N;

[0029] 13) When the deviation Δt = 0, the phase adjustment is completed. After the phase adjustment is completed, the preset value is updated to the real-time delay signal, the real-time frequency locking function is restored, and the process returns to step 2).

[0030] Compared with existing technologies, this invention offers the following advantages: a simple structure, digital synchronization of a mode-locked laser with an external clock, a wide frequency adjustment range, high precision, and automatic phase adjustment of laser pulse trains, enabling highly precise and arbitrary adjustment of laser pulse delay. It has broad application prospects in fields such as laser inertial confinement fusion, time-frequency signal transmission, and quantum entanglement. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a schematic diagram of the structure of the solid-state mode-locked laser and digital frequency-locked phase modulation device synchronized with the external clock of the present invention.

[0032] Figure 2 This is a schematic diagram of a connection between a mode-locked laser resonant cavity (1) and a cavity length adjustment system (2) according to an embodiment of the present invention.

[0033] Figure 3 This is a schematic diagram of a mode-locked laser pulse signal locked to a certain frequency division period after the external clock signal is multiplied by 32. DETAILED DESCRIPTION

[0034] The following describes the implementation of this solution in detail with reference to the diagrams.

[0035] like Figures 1 to 3 As shown in the figure, the present embodiment shows a solid-state mode-locked laser and a digital frequency-locked phase-modulation device and method for external clock synchronization. The device includes a mode-locked laser resonator 1, a cavity length adjustment system 2, a frequency-locking system 3, a phase-modulation system 4, and an external clock signal source 5.

[0036] The cavity length adjustment system 2 includes a piezoelectric actuator 201 and a piezoelectric displacement stage 202. The frequency locking system 3 includes a first frequency multiplier 301, a second frequency multiplier 302, a local clock 303, a first mixer 304, a second mixer 305, a first A / D converter 306, a second A / D converter 307, an FPGA phase detector 308, a D / A converter 309, a filter 310, an amplifier 311, a power divider 312, a comparator 313, and an alarm 314. The phase modulation system includes a fiber coupler 401, a first high-speed photodetector 402, a fiber delay 403, a second high-speed photodetector 404, a counter 405, a master controller 406, and a phase shifter 407.

[0037] The connection relationship of the above components is as follows:

[0038] The piezoelectric actuator 201 and the piezoelectric displacement stage 202 are arranged in the mode-locked laser resonant cavity 1. The output monitoring end of the mode-locked laser resonant cavity 1 is connected to the input end of the fiber coupler 401. The fiber coupler 401 has a first output port and a second output port. The first output port of the fiber coupler 401, the fiber delay 403, the second high-speed photodetector 404, and the frequency multiplier 301 are connected in sequence; the second output port of the fiber coupler 401 is connected in sequence via the first high-speed photodetector 402, the counter 405, and the main controller 406; the main controller 406 has four output ports: the first output port is connected to the first input port of the phase shifter 407; the second output port of the main controller 406 is connected to the second input port of the fiber delay 403; the third output port of the main controller 406 is connected to the second input port of the FPGA phase detector 308; the fourth output port of the main controller 406 is connected to the first input port of the alarm 314; the second input port of the counter 405 is connected to the first output port of the external clock source 5 The output end of the second high-speed photodetector 404 is connected in sequence via the first frequency multiplier 301, the first output port of the first mixer 304, the first A / D converter 306 and the first input port of the FPGA phase detector 308; the output port of the phase shifter 407, the second frequency multiplier 302, the first input port of the second mixer 305, the second A / D converter 307, and the second input port of the FPGA phase detector 308 are connected in sequence; the second input port of the first mixer 304 and the second input port of the second mixer 305 are connected in sequence. They are respectively connected to the output ports of the local clock 303; the FPGA phase detector 308, D / A converter 309, filter 310, amplifier 311 and power divider 312 are connected in sequence; the output end of the power divider 312 is respectively connected to the piezoelectric actuator 201 and the input end of the comparator 313; the first output port and the second output port of the comparator 313 are respectively connected to the piezoelectric displacement stage 202 and the second input port of the alarm 314; the second output port of the external clock signal source 5 is connected to the second input port of the phase shifter 407.

[0039] like Figure 2As shown, a mode-locked laser resonator 1 is used to generate a train of mode-locked laser pulses. The mode-locked laser resonator 1 includes a pump light source 101, lenses 102 and 103, a saturable absorber 104, a concave mirror 105, a plane mirror 106, a plane mirror 107, a concave mirror 108, a gain medium 109, a dichroic mirror 110, a GTI mirror 111, a GTI mirror 112, a plane mirror 113, an output coupling mirror 114, a semi-transparent mirror 115, and a plane mirror 116. The resonator has a length of 1929.012 mm and a corresponding repetition frequency of 77.76 MHz. A cavity length adjustment system 2, comprising a piezoelectric actuator 201 and a piezoelectric displacement stage 202, is used to adjust the cavity length of the mode-locked laser resonator. The piezoelectric actuator 201 drives the concave mirror 105 in the resonator 1, while the piezoelectric displacement stage 202 drives the saturable absorber 104 in the resonator 1. Under the control of the frequency-locking system 3 and the phase-modulation system 4, the high-precision, high-bandwidth, small-stroke piezoelectric actuator 201 and the large-stroke piezoelectric displacement stage 202 work in conjunction to accurately adjust the cavity length of the mode-locked laser resonant cavity 1 over a wide range, thereby achieving synchronization of the laser output pulse frequency with an external clock.

[0040] The comparator 313 in the frequency locking system 3 is configured with upper and lower voltage thresholds; the FPGA phase detector 308 is constructed using a Field Programmable Gate Array (FPGA).

[0041] The phase modulation system can adjust the phase difference (also known as relative time delay) between the laser sequence and the external clock signal source with high precision within one pulse cycle by controlling the delay of the laser pulse sequence and phase modulation with the reference clock signal at the detection end.

[0042] The external clock source 5 can be an external clock system or a timing transmitter, etc.; the applicable frequency range of the external clock source 5 is on the order of tens of megahertz to several gigahertz, and the difference range between the intrinsic frequency of the external clock source 5 and the mode-locked laser resonant cavity 1 is on the order of ±200 kHz; the period of the external clock source 5 is approximately 12.86 ns, and after 32 times of multiplication by the frequency multiplier 302, each multiplication sub-period is approximately 0.4019 ns.

[0043] The following describes a phase modulation method for a solid-state mode-locked laser and a digital frequency-locked phase modulation device synchronized with an external clock, including the following steps:

[0044] 1) The external clock signal source 5 inputs clock signals to the counter 405 and the phase shifter 407 respectively. After the counter 405 receives the signal output by the mode-locked laser resonator 1 from the first high-speed photodetector 402, it calculates the time delay signal between the signal output by the mode-locked laser resonator 1 and the clock signal output by the external clock signal source 5. The counter 405 inputs the time delay signal and the clock signal to the master controller 406.

[0045] 2) The FPGA phase detector 308 determines the frequency difference and phase difference between the external clock signal source 5 and the mode-locked laser resonator 1, and outputs an error signal that is sequentially converted by the FPGA phase detector 308, converted by the D / A converter 309, filtered by the filter 310, and amplified by the amplifier 311 before being transmitted to the power divider 312 for power distribution. The piezoelectric actuator 201 has a typical operating voltage range of 0-130V and a stroke of 0-5μm. When the voltage signal received by the comparator 313 is within the upper and lower threshold ranges set by the comparator 313 (30V-100V), the process proceeds to step 3); when the voltage signal exceeds a preset upper threshold of 117V or is lower than a preset lower threshold of 13V (exceeding 90% of the stroke of the piezoelectric actuator 201 / lowering 10% of the stroke of the piezoelectric actuator 201), the process proceeds to step 4);

[0046] 3) The position of the piezoelectric displacement stage 202 remains unchanged, and only the piezoelectric actuator 201 performs tracking and adjustment of the cavity length, and the comparator 313 outputs a frequency locking signal to the alarm 314;

[0047] 4) The comparator 313 outputs an unlock warning signal to the alarm 314 and enters a relock mode: the comparator 313 outputs a voltage to drive the piezoelectric displacement stage 202 to increase or decrease by one step, and then the position of the piezoelectric displacement stage 202 remains unchanged. One step is approximately 0.4 times the total stroke of the piezoelectric actuator 201.

[0048] 5) Under the action of the FPGA phase detector 308, the filter 310, and the amplifier 311, the piezoelectric actuator 201 is driven through the power divider (312) to adjust the return stroke to the middle position of the total stroke, and the process returns to step 2) to achieve locking of the laser pulse frequency with the external clock signal, and the comparator 313 re-outputs the frequency locking signal to the alarm 314;

[0049] 6) The frequency locking function of the external clock source 5 enters the working mode of real-time tracking and adjustment, and returns to step 2);

[0050] 7) The master controller 406 receives the delay signal output by the counter 405 and determines the deviation Δt between the delay signal and the preset value. If the deviation Δt ≠ 0, phase adjustment is required, and the real-time frequency locking (i.e., steps 2) to 5) is suspended. The master controller 406 outputs a loss of lock warning signal to the alarm 314;

[0051] 8) The master controller 406 first determines the magnitude of the deviation Δt. If the magnitude of the deviation is within the adjustable range (0.5 ns) of the fiber delay 403, a signal is transmitted to the fiber delay 403 for adjustment, and the process proceeds to step 11). Otherwise, the master controller 406 first calculates the number of frequency doubling cycles n required to adjust the laser pulse sequence, and transmits the signal to the phase shifter 407, and the process proceeds to step 9).

[0052] 9) The phase shifter 407 first shifts the frequency-multiplied clock signal by 0.9 frequency-multiplied periods (corresponding to a time domain delay of approximately 0.3617 ns);

[0053] 10) The phase modulation process of the laser pulse sequence returns to step 2) and is achieved by adjusting the cavity length of the mode-locked laser resonator 1 back and forth. However, this does not change the final frequency of the mode-locked laser resonator 1, that is, the cavity length of the resonator remains unchanged before and after the phase modulation. The piezoelectric actuator 201 and the piezoelectric displacement stage 202 introduce a spatial delay during the reciprocating motion, thereby achieving back and forth adjustment of the mode-locked laser sequence in the time domain.

[0054] 11) The fiber delay device 403 controls the laser pulse sequence to shift by -0.1 DOF period (corresponding to a time domain delay of approximately -0.0402 ns), and repeats step 10) to achieve a delay adjustment of 1 DOF period (approximately 0.4019 ns).

[0055] 12) Repeat steps 9)-11) n times to achieve n frequency doubling cycles (approximately n*0.4019 ns), and then use the optical fiber delay device (403) to accurately adjust the delay amount Δt-n*0.4019 ns;

[0056] 13) When the deviation Δt = 0, the phase adjustment is completed. After the phase adjustment is completed, the preset value is updated to the real-time delay signal, the real-time frequency locking function is restored, and the process returns to step 2).

[0057] Fiber delay 403 offers high-precision, small-range adjustment characteristics, with a spatial resolution on the order of μm (corresponding to an adjustment accuracy of approximately 3 fs) and an adjustment range of 0.5 ns. The period of the mode-locked laser pulse sequence in this example is approximately 12.86 ns, and using fiber delay 403 alone would not achieve a phase modulation effect covering a complete period. Phase shifter 407 has an adjustment range sufficient to cover a complete period of external clock source 5. The synergistic effect of fiber delay 403 and phase shifter 407 allows this device to precisely adjust the phase difference between the laser sequence and the external clock signal source within a single pulse period.

[0058] The present invention's externally clocked, solid-state mode-locked laser and digital frequency-locked phase modulation method offer innovations over traditional externally clocked lasers, achieving digital synchronization of the mode-locked laser with an external clock. The method boasts a wide frequency adjustment range and high precision, and includes automatic phase modulation of the laser pulse sequence, enabling highly precise and arbitrary adjustment of the laser pulse delay. The present invention not only automatically adjusts the laser delay by presetting it before startup but also adjusts the delay in real time during operation. The phase modulation process is fully digital, eliminating the need for manual adjustment. The present invention boasts a simple structure and a wide range of applications, including laser inertial confinement fusion, time-frequency signal transmission, and quantum entanglement.

Claims

1. A solid-state mode-locked laser and digital frequency-locked phase modulation device synchronized with an external clock, characterized in that The invention comprises a mode-locked laser resonant cavity (1), a cavity length adjustment system (2), a frequency-locked system (3), a phase modulation system (4) and an external clock signal source (5): The cavity length adjustment system (2) includes a piezoelectric actuator (201) and a piezoelectric displacement stage (202); the frequency locking system (3) includes a first frequency multiplier (301), a second frequency multiplier (302), a local clock (303), a first mixer (304), a second mixer (305), a first A / D converter (306), a second A / D converter (307), an FPGA phase detector (308), a D / A converter (309), a filter (310), an amplifier (311), a power divider (312), a comparator (313) and an alarm (314); the phase modulation system includes a fiber coupler (401), a first high-speed photodetector (402), a fiber delay (403), a second high-speed photodetector (404), a counter (405), a master controller (406) and a phase shifter (407); The connection relationship of the above components is as follows: The piezoelectric actuator (201) and the piezoelectric displacement stage (202) are arranged in the mode-locked laser resonant cavity (1); the output monitoring end of the mode-locked laser resonant cavity (1) is connected to the input end of the optical fiber coupler (401); the optical fiber coupler (401) has a first output port and a second output port; the first output port of the optical fiber coupler (401), the optical fiber delay device (403), the second high-speed photodetector (404), and the frequency multiplier (301) are connected in sequence; the second output port of the optical fiber coupler (401) is connected via the first high-speed photodetector (402), the counter (405), and the master controller (406) are connected in sequence; the master controller (406) has four output ports: the first output port is connected to the first input port of the phase shifter (407); the second output port of the master controller (406) is connected to the second input port of the optical fiber delay device (403); the third output port of the master controller (406) is connected to the second input port of the FPGA phase detector (308); the fourth output port of the master controller (406) is connected to the first input port of the alarm (314); the second input port of the counter (405) is connected to the first output port of the external clock signal source (5) The output end of the second high-speed photodetector (404) is connected in sequence to the first input port of the FPGA phase detector (308) via the first frequency multiplier (301), the first output port of the first mixer (304), the first A / D converter (306); the output port of the phase shifter (407), the second frequency multiplier (302), the first input port of the second mixer (305), the second A / D converter (307), and the second input port of the FPGA phase detector (308) are connected in sequence; the second input port of the first mixer (304) and the second input port of the second mixer (305) are connected to the first input port of the first mixer (304) and the second input port of the second mixer (305) respectively. The output port of the local clock (303) is connected; the FPGA phase detector (308), D / A converter (309), filter (310), amplifier (311) and power divider (312) are connected in sequence; the output end of the power divider (312) is respectively connected to the piezoelectric actuator (201) and the input end of the comparator (313); the first output port and the second output port of the comparator (313) are respectively connected to the piezoelectric displacement stage (202) and the second input port of the alarm (314); the second output port of the external clock signal source (5) is connected to the second input port of the phase shifter (407).

2. The solid-state mode-locked laser and digital frequency-locked phase modulation device synchronized with an external clock according to claim 1, characterized in that: The piezoelectric actuator (201) can selectively drive any reflective mirror in the resonant cavity; and the piezoelectric displacement stage (202) can selectively drive the cavity mirror or the output coupling mirror of the resonant cavity.

3. The external clock synchronized solid-state mode-locked laser and digital frequency-locked phase modulation device according to claim 1, characterized in that: The comparator (313) in the frequency locking system (3) is configured to set upper and lower voltage thresholds; and the FPGA phase detector (308) is constructed using a field programmable gate array.

4. The external clock synchronized solid-state mode-locked laser and digital frequency-locked phase modulation device according to claim 1, characterized in that: The phase modulation system can adjust the phase difference between the laser sequence and the external clock signal source with high precision within one pulse cycle by controlling the delay of the laser pulse sequence and modulating the phase with the reference clock signal at the detection end, which can also be called the relative time delay.

5. The external clock synchronized solid-state mode-locked laser and digital frequency-locked phase modulation device according to claim 1, characterized in that: The external clock signal source (5) can be selected from an external clock system or a timing transmitter; the applicable frequency range of the external clock signal source (5) is in the order of tens of megahertz to several gigahertz, and the difference range between the intrinsic frequency of the external clock signal source (5) and the mode-locked laser resonant cavity (1) is in the order of ±200 kHz; the period of the external clock signal source (5) is T, and after being multiplied N times by the frequency multiplier (302), each multiplication sub-period is T / N.

6. A phase modulation method using the external clock synchronized solid-state mode-locked laser and the digital frequency-locked phase modulation device according to claim 1, characterized in that The following steps are involved: 1) The external clock signal source (5) inputs clock signals to the counter (405) and the phase shifter (407) respectively; after the counter (405) receives the signal output by the mode-locked laser resonant cavity (1) from the first high-speed photodetector (402), it calculates the time delay signal between the signal output by the mode-locked laser resonant cavity (1) and the clock signal output by the external clock signal source (5); and the counter (405) inputs the time delay signal and the clock signal to the master controller (406); 2) The FPGA phase detector (308) determines the frequency difference and phase difference between the external clock signal source (5) and the mode-locked laser resonant cavity (1), and outputs an error signal which is sequentially converted by the FPGA phase detector (308), the D / A converter (309), filtered by the filter (310), and amplified by the amplifier (311) before being transmitted to the power divider (312) for power distribution. When the voltage signal received by the comparator (313) is within the upper and lower threshold ranges set by the comparator (313), step 3) is entered; when the voltage signal exceeds or is lower than the threshold range set by the comparator (313), step 4) is entered; 3) The position of the piezoelectric displacement stage (202) remains unchanged, and only the piezoelectric actuator (201) performs tracking adjustment of the cavity length, and the comparator (313) outputs a frequency locking signal to the alarm (314); 4) the comparator (313) outputs a lock-out warning signal to the alarm (314) and enters a re-locking mode: the comparator (313) outputs a voltage to drive the piezoelectric displacement stage (202) to increase or decrease a step length, and then the position of the piezoelectric displacement stage (202) remains unchanged, and a step length is approximately half of the total stroke of the piezoelectric actuator (201); 5) Under the action of the FPGA phase detector (308), the filter (310), and the amplifier (311), the piezoelectric actuator (201) is driven through the power divider (312) to adjust the return stroke to the middle position of the total stroke, and the process returns to step 2) to achieve locking of the laser pulse frequency with the external clock signal, and the comparator (313) re-outputs the frequency locking signal to the alarm (314); 6) The frequency locking function of the external clock signal source (5) enters the working mode of real-time tracking and adjustment, and returns to step 2); 7) The master controller (406) receives the time delay signal output by the counter (405), determines the deviation Δt between the time delay signal and the preset value, and if the deviation Δt≠0, phase adjustment is required, and the real-time frequency locking, i.e., steps 2)-5), is suspended, and the master controller (406) outputs a loss of lock warning signal to the alarm device (314); 8) The master controller (406) first determines the size of the deviation Δt. If the size of the deviation is within the adjustable range (T1) of the optical fiber delay (403), the signal is transmitted to the optical fiber delay (403) for adjustment, and the process proceeds to step 11). Otherwise, the number of frequency doubling cycles n required to adjust the laser pulse sequence is first calculated, and the signal is transmitted to the phase shifter (407), and the process proceeds to step 9). 9) The phase shifter (407) first shifts the frequency-multiplied clock signal by 0.k frequency-multiplied periods, and the corresponding time domain delay is 0.k*T / N, 0 <k<1; 10) The phase modulation process of the laser pulse sequence returns to step 2) and is achieved by adjusting the cavity length of the mode-locked laser resonant cavity (1) back and forth, but this does not change the final frequency of the mode-locked laser resonant cavity (1), that is, the cavity length of the resonant cavity remains unchanged before and after the phase modulation; the piezoelectric actuator (201) and the piezoelectric displacement stage (202) introduce a spatial delay amount during the back-and-forth motion, so that the mode-locked laser sequence can be adjusted back and forth in the time domain; 11) The optical fiber delay device (403) controls the laser pulse sequence to move 0.k-1 frequency doubling period ((0.k-1)*T / N), and repeats step 10), thereby achieving a delay adjustment of 1 frequency doubling period (1*T / N); 12) Repeating steps 9)-11) n times to achieve n frequency doubling periods (n*T / N) adjustment, and then using the optical fiber delay device (403) to accurately adjust the delay amount Δt-n*T / N; 13) When the deviation Δt=0, the phase adjustment is completed, and the preset value after the phase adjustment is completed is updated to the real-time delay signal, the real-time frequency locking function is restored, and the process returns to step 2).

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