All-fiber GHz femtosecond pulse string generation and fiber ring dispersion regulation and control system and method

Abstract

The invention relates to a system and a method for generating, regulating and controlling a GHz femtosecond pulse string, in particular to a system and a method for generating an all-fiber GHz femtosecond pulse string and regulating and controlling fiber ring dispersion, which are used for overcoming the defects that the repetition frequency of a current mode-locked laser pulser is low, and a GHz-level mode-locked oscillator is high in development difficulty and poor in industrial stability. The all-fiber GHz femtosecond pulse string generation and fiber ring dispersion regulation and control system comprises a mode-locked oscillator, a fiber ring and a regulation and control module, the fiber ring is used for generating a pulse string by using a laser pulse generated by the mode-locked oscillator, and the regulation and control module is used for regulating and controlling the pulse string output by the fiber ring and outputting and testing a GHz femtosecond pulse string; the device can overcome the bottleneck that the existing low-repetition-frequency femtosecond machining efficiency is low, achieves high-efficiency and high-quality femtosecond cold machining, and can be applied to GHz high-power femtosecond laser amplification and ultrafast laser precision machining.

Description

TECHNICAL FIELD

[0001] The application relates to a GHz femtosecond pulse train generation and regulation system and method, in particular to an all-fiber GHz femtosecond pulse train generation and fiber ring dispersion regulation system and method. BACKGROUND

[0002] In order to improve the efficiency of femtosecond laser processing and improve the quality of femtosecond laser processing, researchers have carried out research from the processing mechanism level, and believe that the heat diffusion in the processing process mainly occurs in the interval of laser pulses, and the heat diffusion effect is serious, which will cause processing defects such as micro-cracks. In view of the problem of low processing efficiency caused by plasma shielding, researchers found that the use of high repetition rate femtosecond pulse train can not only realize high efficiency processing, but also the high repetition rate femtosecond pulse train has smaller heat diffusion in the processing process due to the extremely short interval of laser pulses (GHz repetition rate, laser pulse interval less than 1 ns), so that the effect of more ideal 'cold processing' can be realized. At the same time, GHz laser pulse can output the required laser pulse energy threshold through energy superposition of femtosecond pulse train, the first N-1 laser pulses are mainly for heat accumulation effect on the material surface, the Nth laser pulse reaches the ablation threshold, and the subsequent laser pulses ablate and remove the material with high efficiency (High power GHz femtosecond laser for ablation efficiency increase, 17th Nordic Laser Materials Processing Conference, Trondheim, NORWAY, 2019, pp.200-207). Such high repetition rate femtosecond pulse train laser can not only achieve high efficiency material removal, but also the single laser pulse energy requirement is not high, so as to reduce the nonlinear effect caused by high single laser pulse energy in the amplification process of ultra-short laser pulse and the damage problem of device, and a larger femtosecond pulse train energy can be obtained through the femtosecond pulse train amplification mode, so as to obtain higher femtosecond laser processing efficiency and processing quality.

[0003] In addition, the heat energy diffused into the target body due to cooling between laser pulses in the processing process is as follows (Ablation-cooled material removal with ultrafast bursts of pulses, Nature, 2016.):

[0004]

[0005] wherein, alpha is the thermal diffusion coefficient, T c -T0 is the temperature rise change caused by single laser pulse acting on the material, tauR is the interval time of laser pulses, τ0 is the thermal relaxation time, m represents the threshold of material ablation after the mth laser pulse, N represents the total number of laser pulses, ΔT represents the instantaneous temperature rise caused by a single laser pulse acting on the material, represents the net temperature rise caused by a single laser pulse when a femtosecond pulse train acts on the material, E P represents the laser pulse energy. Moreover, when Therefore, under the condition that the laser pulse repetition frequency is high enough, "cold processing" can be realized.

[0006] At present, the cavity length of the GHz mode-locked fiber laser directly output by the oscillator is very short (~10 cm in length), and only such a short cavity length can ensure a high repetition frequency of GHz level, but this also limits the selection of fiber length and gain medium; GHz mode-locked fiber lasers usually rely on precise spatial coupling to realize mode-locked output, which limits the possibility of developing the laser into an all-fiber laser, and affects the long-term reliability and stability of the laser system. However, the combination of a low-repetition-frequency all-fiber mode-locked source and a precisely dispersion-compensated fiber ring can realize all-fiber GHz femtosecond pulse train output, but different laser pulses in the femtosecond pulse train have experienced different fiber ring propagation turns, so if the dispersion of the fiber ring is not properly managed (zero dispersion is not achieved), different laser pulses in the femtosecond pulse train will have different laser pulse widths, affecting the consistency and quality of the laser pulses.

[0007] The existing literature reports a structure of a fiber ring for synthesizing GHz femtosecond pulse train output (Active fiber loop for synthesizing GHz bursts of equidistant ultrashort pulses, Optics Express 28(9), 2020, 13059), but does not provide a dispersion regulation method of the fiber ring for generating GHz femtosecond pulse train and a detection method thereof. In the experiment of the literature, whether the dispersion compensation of the fiber ring has reached near-zero dispersion is detected by using a large dispersion (-33.7 ps2) combined with a spatial bulk-chirped Bragg grating compression, and the spatial structure is complex, and the cost of the large-dispersion chirped fiber grating and the bulk-chirped Bragg grating is high, which is not conducive to quickly verifying whether the dispersion of the fiber ring has been precisely compensated. SUMMARY

[0008] The purpose of the present application is to solve the problems of low repetition frequency of the mode-locked laser pulse generator, great difficulty in developing a GHz mode-locked oscillator and poor industrial stability, and to provide an all-fiber GHz femtosecond pulse train generation and fiber ring dispersion regulation system and method.

[0009] To address the shortcomings of the existing technology, the present invention provides the following technical solution:

[0010] A fully fiber optic GHz femtosecond pulse train generation and fiber ring dispersion control system is characterized by including a mode-locked oscillator, a fiber ring, and a control module.

[0011] The mode-locked oscillator is used to generate laser pulses; the fiber optic ring is used to generate a pulse train using the laser pulses generated by the mode-locked oscillator. The fiber optic ring includes a fiber coupler, a first acousto-optic modulator, a first fiber circulator, a first single-mode gain fiber, a first chirped fiber grating, a pump protector, and a first pump source. The first output of the mode-locked oscillator is connected to the first input of the fiber coupler, and the first output of the fiber coupler is connected to the first port of the first fiber circulator. The second port of the first fiber circulator is interconnected with the first chirped fiber grating through the first single-mode gain fiber. The first pump source, pump protector, and first chirped fiber grating are sequentially connected in output and input. The first chirped fiber grating provides negative dispersion to the laser pulses output from the first single-mode gain fiber and reflects the dispersion-compensated laser pulses back to the first laser source through the first single-mode gain fiber. A fiber circulator enables zero-dispersion transmission and ensures dispersion consistency of the laser pulses participating in the stacking. A first pump source excites doped ions in a first single-mode gain fiber to amplify the laser pulse. The first single-mode gain fiber amplifies the laser pulses passing through it. The third port of the first fiber circulator is connected to the first input of a first acousto-optic modulator. The second input of the first acousto-optic modulator is connected to the second output of a mode-locked oscillator via a first AOM driver. The output of the first acousto-optic modulator is connected to the second input of a fiber coupler. The first acousto-optic modulator controls the number of stacked laser pulses and the repetition frequency of the final pulse train based on the repetition frequency and width of the TTL signal emitted from the internal circuit board of the mode-locked oscillator, and outputs the pulse train through the fiber coupler. The TTL signal is synchronized with the laser pulse sequence generated by the mode-locked oscillator.

[0012] The control module is used to control the pulse train output by the fiber optic ring, outputting and testing GHz femtosecond pulse trains. The control module includes a second acousto-optic modulator, a single-mode amplifier, a second fiber optic circulator, a second chirped fiber grating, an autocorrelation meter, and a second AOM driver. The third output of the mode-locked oscillator is connected to the second input of the second acousto-optic modulator via the second AOM driver. The first input of the second acousto-optic modulator is connected to the second output of the fiber optic coupler. The output of the second acousto-optic modulator is connected to the input of the single-mode amplifier. The second acousto-optic modulator is used to control the output of the fiber optic ring based on the repetition frequency and width of the TTL signal emitted from the internal circuit board of the mode-locked oscillator. The number of laser pulse stacks and the repetition frequency of the final GHz pulse train are used to output a GHz pulse train; the repetition frequency of the second acousto-optic modulator is the same as that of the first acousto-optic modulator; the output of the single-mode amplifier is connected to the first port of the second fiber optic circulator, the second port of the second fiber optic circulator is connected to the second chirped fiber grating, the third port of the second fiber optic circulator is connected to the input of the autocorrelator, the second chirped fiber grating is used to compress the laser pulse width to the femtosecond level, and outputs a GHz femtosecond pulse train through the third port of the second fiber optic circulator, and the autocorrelator is used to measure the duration and shape of the laser pulses in the GHz femtosecond pulse train.

[0013] Furthermore, the mode-locked oscillator includes a resonant cavity and a second pump source; the resonant cavity includes an SESAM, a second wavelength division multiplexer, a polarization-maintaining ytterbium-doped gain fiber, and a chirped fiber Bragg grating connected in sequence to the input and output; the first port of the second wavelength division multiplexer is connected to the SESAM, the second port is connected to the output of the second pump source, and the third port is connected to the chirped fiber Bragg grating through the polarization-maintaining ytterbium-doped gain fiber; the output of the chirped fiber Bragg grating is provided with an isolation beam splitter, the first output of the isolation beam splitter is used to output most of the laser pulses to the first input of the fiber coupler, and the second output is used to output the remaining laser pulses to the internal circuit board of the mode-locked oscillator to generate TTL signals, which are then sent to the first AOM driver and the second AOM driver respectively.

[0014] Furthermore, the single-mode amplifier includes a pump source driver, a third pump source, a first wavelength division multiplexer, and a second single-mode gain fiber connected in sequence to the input and output. The output of the second acousto-optic modulator is connected to the first port of the first wavelength division multiplexer, the output of the third pump source is connected to the second port of the first wavelength division multiplexer, the third port of the first wavelength division multiplexer is connected to the first port of the second fiber circulator through the second single-mode gain fiber, and the first wavelength division multiplexer is used to combine the modulated GHz pulse train with the pump light output from the third pump source and input it into the second fiber circulator.

[0015] Furthermore, the length of the first single-mode gain fiber is 48 cm; the first chirped fiber grating is reflective with a dispersion of 0.254 ps / nm; and the first pump source is a single-mode semiconductor laser coupled from an optical fiber with an output wavelength of 976 nm.

[0016] Furthermore, the mode-locked oscillator generates laser pulses with a repetition frequency of 34.8MHz, a power of 5mW, and a spectral width of approximately 12nm; the fiber optic loop transmits in one loop with a transmission length of 572±5cm.

[0017] Meanwhile, this invention provides an all-fiber GHz femtosecond pulse train generation and fiber ring dispersion modulation method, characterized by employing the aforementioned all-fiber GHz femtosecond pulse train generation and fiber ring dispersion modulation system, comprising the following steps:

[0018] Step 1: A laser pulse is generated by a mode-locked oscillator and output to the fiber optic ring. Part of the power of the laser pulse is input into the first fiber optic circulator by the fiber optic coupler. Step 2: The remaining power of the laser pulse is input into the second acousto-optic modulator. Step 4:

[0019] Step 2: In the fiber optic loop, the remaining laser pulses enter the first single-mode gain fiber after passing through the first fiber optic circulator. After being amplified by the first single-mode gain fiber, they are input into the first chirped fiber grating. After the dispersion is compensated by the first chirped fiber grating, they are reflected back to the first single-mode gain fiber. After being amplified again by the first single-mode gain fiber, they are reflected back to the first fiber optic circulator and input into the first acousto-optic modulator. The first acousto-optic modulator controls the number of laser pulses stacked and the repetition frequency of the final pulse train based on the repetition frequency and width of the TTL signal emitted by the circuit board inside the mode-locked oscillator, and outputs the pulse train to the fiber coupler.

[0020] Step 3: Input part of the power of the pulse train into the first fiber optic circulator via the fiber optic coupler, return to step 2, input the remaining power of the pulse train into the second acousto-optic modulator, and execute step 4.

[0021] Step 4: In the control module, the second acousto-optic modulator controls the number of laser pulses stacked and the repetition frequency of the final GHz pulse train by the repetition frequency and width of the TTL signal emitted by the internal circuit board of the mode-locked oscillator, and outputs the GHz pulse train.

[0022] Step 5: The GHz pulse train is amplified by a single-mode amplifier and output to the second fiber optic circulator; the width of the laser pulses in the amplified GHz pulse train is compressed by the second chirped fiber grating and the GHz femtosecond pulse train is output by the second fiber optic circulator.

[0023] Step 6: Measure the duration and shape of the GHz femtosecond pulse train using an autocorrelation analyzer;

[0024] Step 7: Determine whether the width of the laser pulse in the GHz femtosecond pulse train has reached the required stable width. If yes, complete the generation of the all-fiber GHz femtosecond pulse train and the fiber ring dispersion modulation; otherwise, proceed to step 8.

[0025] Step 8: Adjust the dispersion parameters of the first chirped fiber grating in the fiber ring or adjust the fiber ring length, then return to step 1.

[0026] Furthermore, in step 4, the amplification of the GHz pulse train by the single-mode amplifier specifically involves:

[0027] The first wavelength division multiplexer combines the GHz pulse train output from the second acousto-optic modulator with the pump light output from the third pump source and inputs it into the second single-mode gain fiber, which then amplifies the GHz pulse train.

[0028] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0029] (1) The present invention provides an all-fiber GHz femtosecond pulse train generation and fiber ring dispersion control system, comprising a mode-locked oscillator, a fiber ring, and a control module. The fiber ring is used to generate a pulse train using laser pulses generated by the mode-locked oscillator, and the control module is used to control the pulse train output by the fiber ring, outputting and testing the GHz femtosecond pulse train. The present invention can overcome the bottleneck of low repetition rate femtosecond processing efficiency, realize high-efficiency and high-quality femtosecond cold processing, and can be applied to GHz high-power femtosecond laser amplification and ultrafast laser precision processing.

[0030] (2) The present invention provides a method for generating GHz femtosecond pulse trains and controlling the dispersion of fiber rings in an all-fiber optic system, which achieves highly reliable and stable output of GHz femtosecond pulse trains. At the same time, for the precise dispersion compensation of fiber rings, an all-fiber zero dispersion compensation verification method has been developed, which can quickly realize zero dispersion compensation testing. The method is simple and reliable. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of an embodiment of the all-fiber GHz femtosecond pulse train generation and fiber ring dispersion control system of the present invention;

[0032] Figure 2 This is a schematic diagram of the mode-locked oscillator in an embodiment of the present invention;

[0033] Figure 3 This is a schematic diagram illustrating the principle of generating GHz pulse trains using an optical fiber ring in an embodiment of the present invention.

[0034] Figure 4 This is a flowchart of an embodiment of the all-fiber GHz femtosecond pulse train generation and fiber ring dispersion modulation method of the present invention;

[0035] Figure 5 This is a schematic diagram of a GHz femtosecond pulse train containing approximately 50 laser pulses in each GHz femtosecond pulse train according to an embodiment of the present invention;

[0036] Figure 6 This is a schematic diagram of a GHz femtosecond pulse train containing approximately 105 laser pulses in each GHz femtosecond pulse train according to an embodiment of the present invention;

[0037] Figure 7 This is an autocorrelation plot of a stable 446ts femtosecond laser pulse tested in an embodiment of the present invention.

[0038] The annotations in the attached figures are explained as follows:

[0039] 1-Mode-locked oscillator, 011-First output of mode-locked oscillator, 012-Second output of mode-locked oscillator, 013-Third output of mode-locked oscillator, 0011-Semiconductor saturable absorber mirror (SESAM), 0012-Second wavelength division multiplexer, 0013-Second pump source, 0014-Polarization-maintaining ytterbium-doped gain fiber, 0015-Chirped fiber Bragg grating, 0016-Isolation beam splitter;

[0040] 100-Fiber Optic Ring;

[0041] 2-Fiber optic coupler; 21-First input terminal of fiber optic coupler; 22-First output terminal of fiber optic coupler; 23-Second output terminal of fiber optic coupler; 24-Second input terminal of fiber optic coupler; 3-First acousto-optic modulator; 31-First input terminal of first acousto-optic modulator; 32-Second input terminal of first acousto-optic modulator; 33-Output terminal of first acousto-optic modulator; 4-First fiber optic circulator; 41-First port of first fiber optic circulator; 42-Second port of first fiber optic circulator; 43-Third port of first fiber optic circulator; 5-First single-mode gain fiber; 6-First chirped fiber grating; 7-Pump protector; 8-First pump source; 9-Second acousto-optic modulator 91-Second Acousto-Optic Modulator First Input Terminal; 92-Second Acousto-Optic Modulator Second Input Terminal; 10-First Wavelength Division Multiplexer; 101-First Wavelength Division Multiplexer First Port; 102-First Wavelength Division Multiplexer Second Port; 103-First Wavelength Division Multiplexer Third Port; 11-Third Pump Source; 12-Pump Source Driver; 13-Second Single-Mode Gain Fiber; 14-Second Fiber Circulator; 141-Second Fiber Circulator First Port; 142-Second Fiber Circulator Second Port; 143-Second Fiber Circulator Third Port; 15-Second Chirped Fiber Bragg Grating; 16-Autocorrelator; 17-First AOM (Acousto-Optic Modulator) Driver; 18-Second AOM Driver. Detailed Implementation

[0042] The present invention will be further described below with reference to the accompanying drawings and exemplary embodiments.

[0043] Reference Figure 1 A full-fiber GHz femtosecond pulse train generation and fiber ring dispersion control system includes a mode-locked oscillator 1, a fiber ring 100, and a control module.

[0044] The fiber optic ring 100 is used to generate pulse trains. The fiber optic ring 100 includes a fiber optic coupler 2, a first acousto-optic modulator 3, a first fiber optic circulator 4, a first single-mode gain fiber 5, a first chirped fiber grating 6, a pump protector 7, and a first pump source 8.

[0045] The control module is used to control the pulse train output by the fiber optic ring 100, outputting and testing GHz femtosecond pulse trains. The control module includes a second acousto-optic modulator 9, a first wavelength division multiplexer 10, a third pump source 11, a pump source driver 12, a second single-mode gain fiber 13, a second fiber optic circulator 14, a second chirped fiber grating 15, an autocorrelator 16, and a second AOM driver 18. Among them, the third pump source 11, the pump source driver 12, the first wavelength division multiplexer 10, and the second single-mode gain fiber 13 constitute a single-mode amplifier.

[0046] The first output terminal 011 of the mode-locked oscillator is connected to the first input terminal 21 of the fiber optic coupler. The first output terminal 22 of the fiber optic coupler is connected to the first port 41 of the first fiber optic circulator through a single-mode polarization-maintaining fiber. The second output terminal 23 of the fiber optic coupler is connected to the first input terminal 91 of the second acousto-optic modulator. The beam splitting ratio of the fiber optic coupler 2 is 50:50. The fiber optic coupler 2 is used to input 50% of the power of all coupled laser pulses into the second acousto-optic modulator 9. The laser pulses with 50% power are input into the fiber optic ring 100 (the first fiber optic circulator 4).

[0047] The second port 42 of the first fiber optic circulator is interconnected with the first chirped fiber grating 6 via the first single-mode gain fiber 5. The first pump source 8, pump protector 7, and the first chirped fiber grating 6 are connected in sequence for input and output. The first chirped fiber grating 6 provides negative dispersion to the laser pulse output from the first single-mode gain fiber 5 and reflects the dispersion-compensated laser pulse back to the first fiber optic circulator 4 via the first single-mode gain fiber 5, thereby achieving zero-dispersion transmission and ensuring dispersion consistency among the laser pulses participating in the stacking. The first single-mode gain fiber 5 is used to amplify the laser pulse passing through it. The length of the first single-mode gain fiber 5 is 48 cm. The first chirped fiber grating 6 is reflective and has a parameter of 0.254 ps / nm.

[0048] The first pump source 8 is used to excite doped ions in the first single-mode gain fiber 5 to amplify the laser pulse. The first pump source 8 is a single-mode semiconductor laser with an output wavelength of 976nm.

[0049] The third port 43 of the first fiber optic circulator is connected to the first input terminal 31 of the first acousto-optic modulator, and the second output terminal 012 of the mode-locked oscillator is connected to the second input terminal 32 of the first acousto-optic modulator through the first AOM driver 17; the output terminal 33 of the first acousto-optic modulator is connected to the second input terminal 24 of the fiber optic coupler; the first acousto-optic modulator 3 is used to control the number of laser pulses stacked through it and the repetition frequency and the final pulse train according to the repetition frequency and width of the TTL (Transistor-Transistor Logic) signal emitted by the internal circuit board of the mode-locked oscillator 1, and outputs the pulse train through the fiber optic coupler 2; the TTL signal is synchronized with the laser pulse sequence generated by the mode-locked oscillator 1.

[0050] Reference Figure 2 The mode-locked oscillator 1 is used to generate laser pulses through mode-locking technology. The mode-locked oscillator 1 is an all-fiber femtosecond laser based on SESAM, including a semiconductor saturable absorber mirror (SESAM) 0011, a second wavelength division multiplexer 0012, a second pump source 0013, a polarization-maintaining ytterbium-doped gain fiber 0014, and a chirped fiber Bragg grating 0015.

[0051] The SESAM 0011, the second wavelength division multiplexer 0012, the polarization-maintaining ytterbium-doped gain fiber 0014, the chirped fiber Bragg grating 0015, and the isolation beam splitter 0016 are sequentially connected to form the resonant cavity of the mode-locked oscillator 1. The first port of the second wavelength division multiplexer 0012 is connected to the SESAM 0011 via a polarization-maintaining ytterbium-doped single-mode fiber. The SESAM 0011 is used to initiate and maintain mode-locking operation, generating stable laser pulses. The second port is connected to the output of the second pump source 0013 via a polarization-maintaining ytterbium-doped single-mode fiber. The third port is connected to the input of the polarization-maintaining ytterbium-doped gain fiber 0014 via a polarization-maintaining ytterbium-doped single-mode fiber. The second pump source 0013 provides the necessary energy to the polarization-maintaining ytterbium-doped gain fiber 0014 through the wavelength division multiplexer 0012 to excite optical amplification. The ytterbium-doped gain fiber 0014 is used to amplify the optical signal passing through it. The chirped fiber Bragg grating 0015... The fiber Bragg grating 0015 is used to compensate for intracavity positive dispersion and serves as an output coupler to provide a resonant cavity feedback oscillation circuit. An isolation beam splitter 0016 is provided at the output end of the chirped fiber Bragg grating 0015. The isolation beam splitter 0016 is used to prevent the back propagation of laser pulses. The first output end of the isolation beam splitter 0016 is used to output most of the laser pulses to the first input end 21 of the fiber coupler, and the second output end is used to output the remaining laser pulses to the internal circuit board of the mode-locked oscillator 1 to generate TTL signals.

[0052] By changing the length of the polarization-maintaining ytterbium-doped single-mode fiber in the resonant cavity of mode-locked oscillator 1, different intracavity dispersions are achieved, resulting in stable mode-locking with different repetition frequencies. The mode-locked output repetition frequency is approximately 34MHz.

[0053] Reference Figure 3 The method for generating pulse trains using fiber ring 100 is entirely based on an all-fiber structure, exhibiting very high stability and reliability, and is an effective technical path for pulse train generation. The laser pulse interval is obtained by the difference between the cavity length of the mode-locked oscillator 1 and the length of the fiber ring 100. T0 is the laser pulse interval of the mode-locked oscillator 1. With a repetition frequency of 34MHz, the corresponding laser pulse interval T0 is 29.4ns, and the cavity length is 3.04m. T1 is the generated laser pulse interval. Therefore, when the length of the fiber ring 100 differs from twice the cavity length of the mode-locked oscillator 1 by ΔL, the generated laser pulse interval T1 = ΔL × n / c after the laser travels one revolution along the fiber ring 100, where n is the refractive index of the silica fiber (~1.45), and c is the speed of light in vacuum (3 × 10⁸ m / s). When ΔL is controlled to be 20.689cm, the laser pulse interval is 1ns, corresponding to a repetition frequency of 1GHz. By appropriately reducing ΔL, a laser pulse repetition frequency greater than 1 GHz can be achieved, and even extremely high repetition frequencies at the THz level can be achieved as needed. After the output pulse train passes through a second acousto-optic modulator 9 with a synchronization signal delay, the pulse train is selected to obtain a GHz pulse train with a repetition frequency of 50 kHz to 100 kHz. T2 is the interval of the GHz pulse train. Calculated at 100 kHz, the value of T2 is 10 μs.

[0054] Reference Figure 1 The third output terminal 013 of the mode-locked oscillator is connected to the second input terminal 92 of the second acousto-optic modulator via the second AOM driver 18. The output terminal of the second acousto-optic modulator 9 is connected to the first port 101 of the first wavelength division multiplexer. The second acousto-optic modulator 9 controls the number of laser pulses stacked and the repetition frequency of the final GHz pulse train based on the repetition frequency and width of the TTL signal emitted from the internal circuit board of the mode-locked oscillator 1, and outputs a GHz pulse train. The repetition frequency of the second acousto-optic modulator 9 is the same as that of the first acousto-optic modulator 3.

[0055] Pump source driver 12, third pump source 11, and the second port 102 of the first wavelength division multiplexer are connected in sequence. The third port 103 of the first wavelength division multiplexer is connected to the first port 141 of the second fiber circulator through the second single-mode gain fiber 13. The first wavelength division multiplexer 10 is used to combine the modulated laser pulse with the pump light output from the third pump source 11 and input it into the second fiber circulator 14. The second port 142 of the second fiber circulator is connected to the second chirped fiber grating 15. The third port 143 of the second fiber circulator is connected to the input of the autocorrelator 16. The second chirped fiber grating 15 is used to compress the laser pulse width to the femtosecond level and output a GHz femtosecond pulse train through the third port 143 of the second fiber circulator. The autocorrelator 16 is used to measure the duration and shape of the laser pulse in the GHz femtosecond pulse train.

[0056] Another important parameter involved in this GHz femtosecond pulse train generation technology is the fiber ring 100 zero-dispersion management, in which zero-dispersion control is the key to ensuring that each laser pulse in the GHz femtosecond pulse train can achieve a consistent femtosecond output. To achieve zero-dispersion output, the dispersion introduced by the fiber is calculated based on the length of the fiber ring 100, and the fiber dispersion is compensated using the first chirped fiber grating 6. Simultaneously, since the laser pulse energy is low after the output of the fiber ring 100, a single-mode amplifier (second single-mode gain fiber 13) is built to boost the power. The amplified laser pulse is then connected to the second chirped fiber grating 15 through the second fiber circulator 14, compressing the laser pulse to the femtosecond level and increasing the peak power of the laser pulse entering the autocorrelator 16. By observing the stability of the laser pulse width measured by the autocorrelator 16, if the laser pulse width fluctuates greatly, it indicates that the dispersion of the fiber ring 100 generating the GHz femtosecond pulse train is not zero. Therefore, it is necessary to adjust the dispersion parameters of the first chirped fiber grating 6 in the fiber ring 100 or adjust the length of the fiber ring 100. This process is iterated repeatedly until the laser pulse width in the GHz femtosecond pulse train reaches the required stable width, thus achieving the goal of zero-dispersion transmission of the fiber ring 100.

[0057] Reference Figure 4 A method for generating GHz femtosecond pulse trains and controlling fiber ring dispersion in an all-fiber optic cable includes the following steps:

[0058] Step 1: Mode-locked oscillator 1 generates laser pulses and outputs them to fiber optic ring 100. Part of the power of the laser pulses is input to the first fiber optic circulator 4 via fiber optic coupler 2. Step 2 is executed. The remaining power of the laser pulses is input to the second acousto-optic modulator 9. Step 4 is executed.

[0059] The laser pulse has a repetition frequency of 34.8 MHz, a power of 5 mW, and a spectral width of approximately 12 nm.

[0060] Step 2: In the fiber optic ring 100, the laser pulse is sequentially input into the first fiber optic circulator 4 and the first single-mode gain fiber 5. After being amplified by the first single-mode gain fiber 5, it is input into the first chirped fiber grating 6. After the first chirped fiber grating 6 compensates for dispersion, it is reflected back to the first single-mode gain fiber 5. After being amplified again by the first single-mode gain fiber 5, it is reflected back to the first fiber optic circulator 4 and input into the first acousto-optic modulator 3. The first acousto-optic modulator 3 controls the number of laser pulses stacked and the repetition frequency of the final pulse train according to the repetition frequency and width of the TTL signal emitted by the internal circuit board of the mode-locked oscillator 1, and outputs the pulse train to the fiber optic coupler 2.

[0061] The transmission of the optical fiber ring 100 in one loop has a transmission length controlled to ~572cm, and the laser pulse interval within the transmitted pulse train reaches 1.08GHz.

[0062] Step 3: Input part of the power of the pulse train into the first fiber optic circulator 4 via fiber optic coupler 2, return to step 2, input the remaining power of the pulse train into the second acousto-optic modulator 9, and execute step 4.

[0063] Step 4: In the control module, the second acousto-optic modulator 9 controls the number of laser pulses stacked and the repetition frequency of the final GHz pulse train by the repetition frequency and width of the TTL signal emitted by the internal circuit board of the mode-locked oscillator 1, and outputs the GHz pulse train. Figure 5 , Figure 6 A schematic diagram of two GHz femtosecond pulse trains, each containing a different number of laser pulses;

[0064] Step 5: The first wavelength division multiplexer 10 combines the GHz pulse train output from the second acousto-optic modulator 9 with the pump light output from the third pump source 11 and inputs it into the second single-mode gain fiber 13. The second single-mode gain fiber 13 then amplifies the GHz pulse train and outputs the amplified GHz pulse train to the second fiber circulator 14. The second chirped fiber grating 15 compresses the width of the laser pulses in the amplified GHz pulse train and outputs the GHz femtosecond pulse train from the second fiber circulator 14.

[0065] Step 6: Measure the duration and shape of the GHz femtosecond pulse train using an autocorrelation instrument 16;

[0066] Step 7: Determine whether the width of the laser pulse in the GHz femtosecond pulse train has reached the required stable width. If so, complete the generation of the all-fiber GHz femtosecond pulse train and the dispersion control of the fiber ring 100. Otherwise, proceed to step 8.

[0067] Step 8: Adjust the dispersion parameters of the first chirped fiber grating 6 in the fiber ring 100 or adjust the length of the fiber ring 100, and then return to step 1.

[0068] The experiment selected approximately 800 femtosecond pulse trains with a repetition frequency of 100 kHz. The stability of the laser pulse width in the GHz femtosecond pulse trains was observed using an autocorrelator. If the laser pulse width fluctuated greatly, it indicated that the dispersion of the fiber ring 100 generating the GHz femtosecond pulse trains was not zero. Therefore, it was necessary to adjust the dispersion parameters of the first chirped fiber grating 6 in the fiber ring 100 or adjust the length of the fiber ring 100. This process was iterated repeatedly until the laser pulse width in the GHz femtosecond pulse trains reached the required stable width, thus achieving the goal of zero-dispersion transmission of the fiber ring 100.

[0069] Figure 7 The autocorrelation plot of a laser pulse was obtained, and the full width at half maximum (FWHM) of the laser pulse was found to be approximately 446 fs by Lorentz fitting.

Claims

1. A system for generating GHz femtosecond pulse trains and controlling fiber ring dispersion using an all-fiber optic cable, characterized in that: It includes a mode-locked oscillator (1), an optical fiber ring (100), and a control module; The mode-locked oscillator (1) is used to generate laser pulses; the fiber ring (100) is used to generate a pulse train using the laser pulses generated by the mode-locked oscillator (1). The fiber ring (100) includes a fiber coupler (2), a first acousto-optic modulator (3), a first fiber circulator (4), a first single-mode gain fiber (5), a first chirped fiber grating (6), a pump protector (7), and a first pump source (8). The first output terminal (011) of the mode-locked oscillator is connected to the first input terminal (21) of the fiber coupler, and the first output terminal (22) of the fiber coupler is connected to the first port (41) of the first fiber circulator. The second port (42) of the first fiber circulator is connected to the first single-mode gain fiber. The optical fiber (5) is interconnected with the first chirped fiber grating (6), and the first pump source (8), pump protector (7), and first chirped fiber grating (6) are connected in sequence for input and output. The first chirped fiber grating (6) is used to provide negative dispersion for the laser pulse output from the first single-mode gain fiber (5), and to reflect the dispersion-compensated laser pulse back to the first fiber circulator (4) through the first single-mode gain fiber (5), thereby achieving zero-dispersion transmission and dispersion consistency of the laser pulses participating in the stacking. The first pump source (8) is used to excite the doped ions in the first single-mode gain fiber (5) to amplify the laser pulse. The first single-mode gain fiber (5) is used to amplify the laser pulse passing through it. The third port (43) of the first fiber optic circulator is connected to the first input terminal (31) of the first acousto-optic modulator, the second input terminal (32) of the first acousto-optic modulator is connected to the second output terminal (012) of the mode-locked oscillator through the first AOM driver (17), and the output terminal (33) of the first acousto-optic modulator is connected to the second input terminal (24) of the fiber optic coupler; the first acousto-optic modulator (3) is used to control the number of laser pulses stacked through it and the repetition frequency of the final pulse train according to the repetition frequency and width of the TTL signal emitted by the internal circuit board of the mode-locked oscillator (1), and outputs the pulse train through the fiber optic coupler (2); the TTL signal is synchronized with the laser pulse sequence generated by the mode-locked oscillator (1); The control module is used to control the pulse train output by the fiber optic ring (100), and output and test the GHz femtosecond pulse train; The control module includes a second acousto-optic modulator (9), a single-mode amplifier, a second fiber optic circulator (14), a second chirped fiber grating (15), an autocorrelator (16), and a second AOM driver (18). The third output terminal (013) of the mode-locked oscillator is connected to the second input terminal (92) of the second acousto-optic modulator via the second AOM driver (18). The first input terminal (91) of the second acousto-optic modulator is connected to the second output terminal (23) of the fiber optic coupler. The output terminal of the second acousto-optic modulator (9) is connected to the input terminal of the single-mode amplifier. The second acousto-optic modulator (9) is used to control the number of laser pulses stacked and the final number of pulses passing through it based on the repetition frequency and width of the TTL signal emitted by the internal circuit board of the mode-locked oscillator (1). The repetition frequency of the GHz pulse train is used to output the GHz pulse train; the repetition frequency of the second acousto-optic modulator (9) is the same as that of the first acousto-optic modulator (3); the output of the single-mode amplifier is connected to the first port (141) of the second fiber optic circulator, the second port (142) of the second fiber optic circulator is connected to the second chirped fiber grating (15), the third port (143) of the second fiber optic circulator is connected to the input of the autocorrelator (16), the second chirped fiber grating (15) is used to compress the laser pulse width to the femtosecond level, and output the GHz femtosecond pulse train through the third port (143) of the second fiber optic circulator, and the autocorrelator (16) is used to measure the duration and shape of the laser pulse in the GHz femtosecond pulse train.

2. The all-fiber GHz femtosecond pulse train generation and fiber ring dispersion control system according to claim 1, characterized in that: The mode-locked oscillator (1) includes a resonant cavity and a second pump source (0013); the resonant cavity includes an SESAM (0011), a second wavelength division multiplexer (0012), a polarization-maintaining ytterbium-doped gain fiber (0014), and a chirped fiber Bragg grating (0015) connected sequentially to the input and output; the first port of the second wavelength division multiplexer (0012) is connected to the SESAM (0011), the second port is connected to the output of the second pump source (0013), and the third port is connected to the polarization-maintaining ytterbium-doped gain fiber. A fiber (0014) is connected to a chirped fiber Bragg grating (0015); an isolation beam splitter (0016) is provided at the output end of the chirped fiber Bragg grating (0015). The first output end of the isolation beam splitter (0016) is used to output most of the laser pulses to the first input end (21) of the fiber coupler, and the second output end is used to output the remaining laser pulses to the internal circuit board of the mode-locked oscillator (1) to generate TTL signals, which are respectively sent to the first AOM driver (17) and the second AOM driver (18).

3. The all-fiber GHz femtosecond pulse train generation and fiber ring dispersion control system according to claim 1 or 2, characterized in that: The single-mode amplifier includes a pump source driver (12), a third pump source (11), a first wavelength division multiplexer (10), and a second single-mode gain fiber (13) connected in sequence from input to output. The output of the second acousto-optic modulator (9) is connected to the first port (101) of the first wavelength division multiplexer. The output of the third pump source (11) is connected to the second port (102) of the first wavelength division multiplexer. The third port (103) of the first wavelength division multiplexer is connected to the first port (141) of the second fiber circulator through the second single-mode gain fiber (13). The first wavelength division multiplexer (10) is used to combine the modulated GHz pulse train with the pump light output from the third pump source (11) and input it into the second fiber circulator (14).

4. The all-fiber GHz femtosecond pulse train generation and fiber ring dispersion control system according to claim 3, characterized in that: The length of the first single-mode gain fiber (5) is 48cm; the first chirped fiber grating (6) is reflective with a dispersion of 0.254ps / nm; the first pump source (8) is a single-mode semiconductor laser coupled from an optical fiber with an output wavelength of 976nm.

5. The all-fiber GHz femtosecond pulse train generation and fiber ring dispersion control system according to claim 4, characterized in that: The mode-locked oscillator (1) generates laser pulses with a repetition frequency of 34.8MHz, a power of 5mW, and a spectral width of approximately 12nm; the fiber optic ring (100) transmits in one loop with a transmission length of 572±5cm.

6. A method for generating GHz femtosecond pulse trains and controlling fiber ring dispersion in an all-fiber optic cable, characterized in that, The all-fiber GHz femtosecond pulse train generation and fiber ring dispersion control system described in claim 1 includes the following steps: Step 1: A laser pulse is generated by the mode-locked oscillator (1) and output to the fiber optic ring (100). Part of the power of the laser pulse is input to the first fiber optic circulator (4) by the fiber optic coupler (2). Step 2 is executed. The remaining power of the laser pulse is input to the second acousto-optic modulator (9). Step 4 is executed. Step 2: In the fiber optic ring (100), the remaining laser pulses enter the first single-mode gain fiber (5) after passing through the first fiber optic circulator (4). After being amplified by the first single-mode gain fiber (5), the pulses are input into the first chirped fiber grating (6). After the first chirped fiber grating (6) compensates for dispersion, the pulses are reflected back to the first single-mode gain fiber (5). After being amplified again by the first single-mode gain fiber (5), the pulses are reflected back to the first fiber optic circulator (4) and input into the first acousto-optic modulator (3). The first acousto-optic modulator (3) controls the number of laser pulses stacked and the repetition frequency of the final pulse train according to the repetition frequency and width of the TTL signal emitted by the internal circuit board of the mode-locked oscillator (1), and outputs the pulse train to the fiber coupler (2). Step 3: Input part of the power of the pulse train into the first fiber optic circulator (4) through the fiber optic coupler (2), return to step 2, input the remaining power of the pulse train into the second acousto-optic modulator (9), and execute step 4. Step 4: In the control module, the second acousto-optic modulator (9) controls the number of laser pulses stacked through it and the repetition frequency of the final GHz pulse train according to the repetition frequency and width of the TTL signal emitted by the internal circuit board of the mode-locked oscillator (1), and outputs the GHz pulse train. Step 5: The GHz pulse train is amplified by a single-mode amplifier and output to the second fiber optic circulator (14); the width of the laser pulse in the amplified GHz pulse train is compressed by the second chirped fiber grating (15), and the GHz femtosecond pulse train is output by the second fiber optic circulator (14). Step 6: Measure the duration and shape of the GHz femtosecond pulse train using an autocorrelation analyzer (16); Step 7: Determine whether the width of the laser pulse in the GHz femtosecond pulse train has reached the required stable width. If so, complete the generation of the all-fiber GHz femtosecond pulse train and the dispersion control of the fiber ring (100). Otherwise, proceed to step 8. Step 8: Adjust the dispersion parameters of the first chirped fiber grating (6) in the fiber ring (100) or adjust the length of the fiber ring (100), and then return to step 1.

7. The all-fiber GHz femtosecond pulse train generation and fiber ring dispersion modulation method according to claim 6, characterized in that: In step 4, the amplification of the GHz pulse train by the single-mode amplifier specifically involves: The GHz pulse train output from the second acousto-optic modulator (9) and the pump light output from the third pump source (11) are combined by the first wavelength division multiplexer (10) and then input into the second single-mode gain fiber (13), and the GHz pulse train is amplified by the second single-mode gain fiber (13).

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