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

Abstract

The utility model relates to a GHz femtosecond pulse string generation, regulation and control system, in particular to an all-fiber GHz femtosecond pulse string generation and fiber ring dispersion regulation and control system, which is used for overcoming the defects that the repetition frequency of the existing mode-locked laser pulser is low, and the development difficulty of a GHz-level mode-locked oscillator is large and the industrial stability is poor. 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 femtosecond cold machining device can overcome the bottleneck that the existing low-repetition-frequency femtosecond machining efficiency is low, high-efficiency and high-quality femtosecond cold machining is achieved, and the femtosecond cold machining device can be applied to GHz high-power femtosecond laser amplification and ultrafast laser precision machining.

Description

Technical Field

[0001] The utility model relates to a GHz femtosecond pulse train generation and regulation system, in particular to an all-fiber GHz femtosecond pulse train generation and fiber ring dispersion regulation system. Background Art

[0002] In order to improve the efficiency and quality of femtosecond laser processing, researchers have conducted research from the perspective of processing mechanism, and believe that heat diffusion during processing mainly occurs within the interval between laser pulses, and that severe heat diffusion effects can cause processing defects such as microcracks. In response to the problem of low processing efficiency caused by plasma shielding, researchers have found that the use of high-repetition-rate femtosecond pulse trains can not only achieve high-efficiency processing, but also that high-repetition-rate femtosecond pulse trains have smaller heat diffusion during processing due to the extremely short laser pulse interval (less than 1ns at GHz repetition frequency), thus achieving a more ideal "cold processing" effect. At the same time, GHz laser pulses can reach the laser pulse energy threshold required for processing through the energy superposition output of femtosecond pulse trains. The first N-1 laser pulses mainly have a thermal accumulation effect on the surface of the material. The Nth laser pulse reaches the ablation threshold, and the subsequent laser pulses ablate to remove the material efficiently (High power GHz femtosecond laser for ablation efficiency increase, 17th Nordic Laser Materials Processing Conference, Trondheim, NORWAY, 2019, pp. 200-207). Such a high repetition rate femtosecond pulse train laser can not only achieve efficient material removal, but also has low requirements for the energy of a single laser pulse. Therefore, the nonlinear effect caused by the high single laser pulse energy during the amplification of ultrashort laser pulses and the problem of damage to the device can be reduced. Through the femtosecond pulse train amplification method, a larger femtosecond pulse train energy can be obtained, and higher femtosecond laser processing efficiency and processing quality can be obtained.

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

[0004]

[0005] Where α is the thermal diffusion coefficient, T c -T0 is the temperature rise caused by a single laser pulse acting on the material, τ Ris the laser pulse interval time, τ0 is the thermal relaxation time, m represents the threshold of material ablation after the mth laser pulse is incident, 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. Therefore, when the laser pulse repetition frequency is high enough, "cold processing" can be achieved.

[0006] At present, the cavity length of GHz mode-locked fiber lasers based on direct output of oscillators is very short (~10cm in length). Only such a short cavity length can ensure a high repetition rate of GHz level, but this also limits the choice of fiber length and gain medium; GHz mode-locked fiber lasers usually rely on precise spatial coupling to achieve mode-locked output, which limits the possibility of laser development towards all-fiberization and affects the long-term reliability and stability of the laser system. Based on a low-repetition-rate all-fiber mode-locked source combined with a precision dispersion-compensated fiber ring, an all-fiber GHz femtosecond pulse train output can be achieved, but different laser pulses in a femtosecond pulse train experience different fiber ring propagation circles. Therefore, if the dispersion management of the fiber ring is improper (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 the structure of a fiber ring for synthesizing laser pulses to achieve the output of GHz femtosecond pulse trains (Active fiber loop for synthesizing GHz bursts of equidistant ultrashort pulses, Optics Express 28(9), 2020, 13059), but does not provide a dispersion control method and detection method for a fiber ring that generates GHz femtosecond pulse trains. In the experiment of this document, the dispersion compensation of the fiber ring has reached near-zero dispersion. The detection method uses a large dispersion broadening (-33.7ps 2 ) Combined with the spatial volume chirped Bragg grating compression, the spatial structure is complex, and the cost of chirped fiber gratings with large dispersion and volume chirped Bragg gratings is high, which is not conducive to quickly verifying whether the fiber ring dispersion has achieved precise compensation. Summary of the invention

[0008] The purpose of the utility model is to solve the shortcomings of the current low repetition frequency of mode-locked laser pulsers, the difficulty in developing GHz-level mode-locked oscillators, and the poor industrial stability, and to provide an all-fiber GHz femtosecond pulse train generation and fiber ring dispersion control system.

[0009] In order to solve the deficiencies of the above-mentioned prior art, the present invention provides the following technical solutions:

[0010] An all-fiber GHz femtosecond pulse train generation and fiber ring dispersion control system, which is special in that it includes a mode-locked oscillator, a fiber ring and a control module;

[0011] The mode-locked oscillator is used to generate laser pulses; the fiber ring is used to generate a pulse train using the laser pulses generated by the mode-locked oscillator, and the fiber 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 end of the mode-locked oscillator is connected to the first input end of the fiber coupler, and the first output end of the fiber coupler is connected to the first port of the first fiber circulator; the second port of the first fiber circulator is connected to the first chirped fiber grating through the first single-mode gain fiber, and the first pump source, the pump protector and the first chirped fiber grating output and input are connected in sequence; the first chirped fiber grating is used to provide negative dispersion for the laser pulse output by the first single-mode gain fiber, and reflect the dispersion-compensated laser pulse back to the first fiber circulator through the first single-mode gain fiber. A fiber circulator is used to realize zero-dispersion transmission and to make the dispersion of the participating stacked laser pulses consistent; the first pump source is used to excite the doped ions in the first single-mode gain fiber to amplify the laser pulses; the first single-mode gain fiber is used to amplify the laser pulses passing through it; the third port of the first fiber circulator is connected to the first input end of the first acousto-optic modulator, the second input end of the first acousto-optic modulator is connected to the second output end of the mode-locked oscillator through the first AOM drive, and the output end of the first acousto-optic modulator is connected to the second input end of the fiber coupler; the first acousto-optic modulator is used to control the number of laser pulse stacks passing 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, and output 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 optical fiber ring, and output and test the GHz femtosecond pulse train; the control module includes a second acousto-optic modulator, a single-mode amplifier, a second optical fiber circulator, a second chirped optical fiber grating, an autocorrelator and a second AOM driver; the third output end of the mode-locked oscillator is connected to the second input end of the second acousto-optic modulator through the second AOM driver, the first input end of the second acousto-optic modulator is connected to the second output end of the optical fiber coupler, the output end of the second acousto-optic modulator is connected to the input end of the single-mode amplifier, and the second acousto-optic modulator is used to control the repetition frequency and width of the TTL signal sent by 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 calculated, and a GHz pulse train is output; the repetition frequency of the second acousto-optic modulator is the same as the repetition frequency of the first acousto-optic modulator; the output end of the single-mode amplifier is connected to the first port of the second fiber circulator, the second port of the second fiber circulator and the second chirped fiber grating are interconnected, the third port of the second fiber circulator is connected to the input end of the autocorrelator, the second chirped fiber grating is used to compress the laser pulse width to the femtosecond level, and the GHz femtosecond pulse train is output through the third port of the second fiber circulator, and the autocorrelator is used to measure the duration and shape of the laser pulse 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 a SESAM, a second wavelength division multiplexer, a polarization-maintaining ytterbium-doped gain fiber, and a chirped fiber Bragg grating, the output and input of which are connected in sequence; the first port of the second wavelength division multiplexer is connected to the SESAM, the second port is connected to the output end 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 end of the chirped fiber Bragg grating is provided with an isolation beam splitter, the first output end of the isolation beam splitter is used to output most of the laser pulses to the first input end of the optical 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 to generate a TTL signal, which is 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 optical fiber, the output end of which is connected in sequence to the input and output; the output end of the second acousto-optic modulator is connected to the first port of the first wavelength division multiplexer, the output end 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 optical fiber circulator through the second single-mode gain optical fiber, and the first wavelength division multiplexer is used to combine the modulated GHz pulse string with the pump light output by the third pump source and input them into the second optical fiber circulator.

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

[0016] Furthermore, the repetition frequency of the laser pulses generated by the mode-locked oscillator is 34.8 MHz, the power is 5 mW, and the spectrum width is about 12 nm; the transmission length of one loop of the optical fiber ring is 572±5 cm.

[0017] Compared with the prior art, the beneficial effects of the utility model are:

[0018] (1) The utility model discloses 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, wherein 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, and output and test the GHz femtosecond pulse train; the utility model can overcome the bottleneck of low efficiency of low repetition rate femtosecond processing, 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.

[0019] (2) The utility model provides an all-fiber GHz femtosecond pulse train generation and fiber ring dispersion control system, which realizes highly reliable and stable output of GHz femtosecond pulse trains. At the same time, for the precise dispersion compensation of the fiber ring, an all-fiber zero dispersion compensation verification method is developed, which can quickly realize zero dispersion compensation testing. The method is simple and reliable. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a structural schematic diagram of an embodiment of the utility model of the all-fiber GHz femtosecond pulse train generation and fiber ring dispersion control system;

[0021] Figure 2 This is a schematic diagram of the structure of the mode-locked oscillator in the embodiment of the utility model;

[0022] Figure 3 This is a schematic diagram of the principle of generating a GHz pulse train by an optical fiber ring in an embodiment of the utility model;

[0023] Figure 4 It is a flow chart of the working principle of the embodiment of the utility model;

[0024] Figure 5 A schematic diagram of a GHz femtosecond pulse train in which each GHz femtosecond pulse train contains about 50 laser pulses in an embodiment of the present invention;

[0025] Figure 6A schematic diagram of a GHz femtosecond pulse train in which each GHz femtosecond pulse train contains about 105 laser pulses in an embodiment of the present invention;

[0026] Figure 7 This is the autocorrelation diagram of the stable 446fs femtosecond laser pulse tested in the embodiment of the present utility model.

[0027] The following are the descriptions of the reference numerals:

[0028] 1-mode-locked oscillator, 011-first output end of the mode-locked oscillator, 012-second output end of the mode-locked oscillator, 013-third output end of the 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;

[0029] 100-fiber ring;

[0030] 2-fiber coupler, 21-first input end of fiber coupler, 22-first output end of fiber coupler, 23-second output end of fiber coupler, 24-second input end of fiber coupler; 3-first acousto-optic modulator, 31-first input end of first acousto-optic modulator, 32-second input end of first acousto-optic modulator, 33-output end of first acousto-optic modulator; 4-first fiber circulator, 41-first port of first fiber circulator, 42-second port of first fiber circulator, 43-third port of first fiber 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-the first input end of the second acousto-optic modulator, 92-the second input end of the second acousto-optic modulator; 10-the first wavelength division multiplexer, 101-the first port of the first wavelength division multiplexer, 102-the second port of the first wavelength division multiplexer, 103-the third port of the first wavelength division multiplexer; 11-the third pump source; 12-the pump source drive; 13-the second single-mode gain fiber; 14-the second fiber circulator, 141-the first port of the second fiber circulator, 142-the second port of the second fiber circulator, 143-the third port of the second fiber circulator; 15-the second chirped fiber grating; 16-the autocorrelator; 17-the first AOM (Acousto-Optic Modulator, acousto-optic modulator) drive; 18-the second AOM drive. DETAILED DESCRIPTION

[0031] The present invention will be further described below in conjunction with the accompanying drawings and exemplary embodiments.

[0032] Reference Figure 1An all-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.

[0033] The fiber ring 100 is used to generate a pulse train, and 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.

[0034] The control module is used to control the pulse train output by the fiber ring 100, output and test the GHz femtosecond pulse train; 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 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.

[0035] The first output end 011 of the mode-locked oscillator is connected to the first input end 21 of the optical fiber coupler, the first output end 22 of the optical fiber coupler is connected to the first port 41 of the first optical fiber circulator through a single-mode polarization-maintaining optical fiber, the second output end 23 of the optical fiber coupler is connected to the first input end 91 of the second acousto-optic modulator, the splitting ratio of the optical fiber coupler 2 is 50:50, and the optical fiber coupler 2 is used to input 50% of the power of all laser pulses after coupling into the second acousto-optic modulator 9, and the laser pulses with 50% power are input into the optical fiber ring 100 (the first optical fiber circulator 4).

[0036] The second port 42 of the first fiber circulator is interconnected with the first chirped fiber grating 6 through the first single-mode gain fiber 5, and the first pump source 8, the pump protector 7 and the output and input of the first chirped fiber grating 6 are connected in sequence; the first chirped fiber grating 6 provides negative dispersion for the laser pulse output by the first single-mode gain fiber 5, and reflects the dispersion-compensated laser pulse back to the first fiber circulator 4 through the first single-mode gain fiber 5, so as to realize zero-dispersion transmission and realize the dispersion consistency of the laser pulses involved in the stacking; the first single-mode gain fiber 5 is used to amplify the laser pulse passing through it, and the length of the first single-mode gain fiber 5 is 48 cm. The first chirped fiber grating 6 is reflective, and the parameter is 0.254 ps / nm.

[0037] The first pump source 8 is used to excite the doped ions in the first single-mode gain fiber 5 to amplify the laser pulses. The first pump source 8 is a single-mode semiconductor laser with fiber-coupled output, and the output wavelength is 976 nm.

[0038] The third port 43 of the first optical fiber 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 optical fiber coupler; the first acousto-optic modulator 3 is used to control the number of laser pulse stacks passing through it and the repetition frequency of 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 control the number of laser pulse stacks passing through it and the repetition frequency of the final pulse train according to the complex frequency and width, and output the pulse train through the optical fiber coupler 2; the TTL signal is synchronized with the laser pulse sequence generated by the mode-locked oscillator 1.

[0039] 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.

[0040] The output and input of 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 a resonant cavity of the mode-locked oscillator 1; the first port of the second wavelength division multiplexer 0012 is connected to the SESAM 0011 through the polarization-maintaining ytterbium-doped single-mode fiber, and the SESAM 0011 is used to start and maintain the mode-locked operation to generate a stable laser pulse, the second port is connected to the output end of the second pump source 0013 through the polarization-maintaining ytterbium-doped single-mode fiber, and the third port is connected to the input end of the polarization-maintaining ytterbium-doped gain fiber 0014 through the polarization-maintaining ytterbium-doped single-mode fiber, and the second pump source 0013 is used to provide the necessary energy for the polarization-maintaining ytterbium-doped gain fiber 0014 through the wavelength division multiplexer 0012 to stimulate light amplification; the polarization-maintaining ytterbium-doped gain fiber 0014 is used to amplify the optical signal passing through it; the chirped fiber Bragg grating The grating 0015 is used to compensate for the positive dispersion in the cavity and provide a resonant cavity feedback oscillation loop as an output coupler; the output end of the chirped fiber Bragg grating 0015 is provided with an isolation beam splitter 0016, and the isolation beam splitter 0016 is used to prevent the reverse propagation of the laser pulse. 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 a TTL signal.

[0041] By changing the length of the polarization-maintaining ytterbium-doped single-mode fiber in the resonant cavity of the mode-locked oscillator 1, different intracavity dispersions are achieved, and stable mode locking with different repetition frequencies is obtained. The mode-locked output repetition frequency is about 34 MHz.

[0042] Reference Figure 3 , the method of generating pulse train by fiber ring 100 is completely based on the all-fiber structure, with very high stability and reliability, and is an effective technical path for generating pulse train. Among them, the formed laser pulse interval is obtained by the difference between the cavity length of the resonant cavity of the mode-locked oscillator 1 and the length of the fiber ring 100, where T0 is the laser pulse interval of the mode-locked oscillator 1, according to the repetition frequency of 34MHz, so the corresponding laser pulse interval T0 is 29.4ns, the cavity length is 3.04m, and T1 is the generated laser pulse interval, so when the length of the fiber ring 100 differs from the cavity length of the resonant cavity of the mode-locked oscillator 1 by ΔL, when the laser is transmitted along the fiber ring 100 for a circle, the generated laser pulse interval T1 = ΔL×n / c, where n is the refractive index of the quartz fiber (~1.45), c is the speed of light in a vacuum (3×108m / s), when ΔL is controlled to be 20.689cm, the laser pulse interval is 1ns, and the corresponding repetition frequency is 1GHz. By appropriately reducing ΔL, the laser pulse repetition frequency can be greater than 1 GHz, and even a very high repetition frequency of THz level can be achieved as needed. After the output pulse train passes through the second acousto-optic modulator 9 with synchronization signal delay, the pulse train is selected to obtain a GHz pulse train with a repetition frequency of 50kHz-100kHz, where T2 is the interval of the GHz pulse train. Calculated at 100kHz, the value of T2 is 10μs.

[0043] 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 through 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 pulse stacks passing through it and the repetition frequency of the final GHz pulse train according to the repetition frequency and width of the TTL signal sent by 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 the repetition frequency of the first acousto-optic modulator 3.

[0044] The pump source driver 12, the third pump source 11 and the output and input of 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 optical fiber circulator through the second single-mode gain optical fiber 13. The first wavelength division multiplexer 10 is used to combine the modulated laser pulse with the pump light output by the third pump source 11 and input them into the second optical fiber circulator 14; the second port 142 of the second optical fiber circulator is interconnected with the second chirped fiber grating 15, and the third port 143 of the second optical fiber circulator is connected to the input end 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 optical fiber circulator. The autocorrelator 16 is used to measure the duration and shape of the laser pulse in the GHz femtosecond pulse train.

[0045] Another important parameter involved in the GHz femtosecond pulse train generation technology is the zero dispersion management of the optical fiber ring 100, wherein zero dispersion control is the key to ensure that each laser pulse in the GHz femtosecond pulse train can achieve a consistent femtosecond output. In order to achieve zero dispersion output, the dispersion introduced by the optical fiber is calculated by the length of the optical fiber ring 100, and the first chirped fiber grating 6 is used to compensate for the optical fiber dispersion. At the same time, after the output of the optical fiber ring 100, due to the low energy of the laser pulse, a first-level single-mode amplifier (second single-mode gain fiber 13) is built to increase the power. The amplified laser pulse is connected to the second chirped fiber grating 15 through the second optical fiber circulator 14, and the laser pulse is compressed to the femtosecond level, thereby 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 means that the dispersion of the optical fiber ring 100 that generates the GHz femtosecond pulse train is not 0. Therefore, it is necessary to adjust the dispersion parameters of the first chirped fiber grating 6 in the optical fiber ring 100 or adjust the length of the optical fiber ring 100, and iterate repeatedly until the laser pulse width in the GHz femtosecond pulse train reaches the required stable width, thereby achieving the purpose of zero dispersion transmission of the optical fiber ring 100.

[0046] Reference Figure 4 The working principle of the utility model includes the following steps:

[0047] Step 1, the mode-locked oscillator 1 generates a laser pulse and outputs it to the optical fiber ring 100, a part of the power of the laser pulse is input into the first optical fiber circulator 4 through the optical fiber coupler 2, and step 2 is performed, the remaining power of the laser pulse is input into the second acousto-optic modulator 9, and step 4 is performed;

[0048] The laser pulse has a repetition frequency of 34.8 MHz, a power of 5 mW, and a spectrum width of about 12 nm;

[0049] Step 2, in the optical fiber ring 100, the laser pulse is sequentially input into the first optical fiber circulator 4 and the first single-mode gain optical fiber 5, amplified by the first single-mode gain optical fiber 5 and input into the first chirped optical fiber grating 6, dispersion compensated by the first chirped optical fiber grating 6 and reflected back into the first single-mode gain optical fiber 5, amplified again by the first single-mode gain optical fiber 5 and reflected back into the first optical fiber circulator 4, and input into the first acousto-optic modulator 3; the first acousto-optic modulator 3 controls the number of laser pulse stacks passing 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 to the optical fiber coupler 2;

[0050] The transmission length of one loop of the optical fiber ring 100 is controlled to be 572 cm, and the interval between laser pulses in the pulse train after transmission reaches 1.08 GHz;

[0051] Step 3, the fiber coupler 2 inputs part of the power of the pulse train into the first fiber circulator 4, returns to step 2, inputs the remaining power of the pulse train into the second acousto-optic modulator 9, and executes step 4;

[0052] Step 4: In the control module, the second acousto-optic modulator 9 controls the number of laser pulse stacks passing 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 a GHz pulse train; Figure 5 , Figure 6 Schematic diagram of two GHz femtosecond pulse trains containing different numbers of laser pulses for each GHz femtosecond pulse train;

[0053] Step 5: The first wavelength division multiplexer 10 combines the GHz pulse train output by the second acousto-optic modulator 9 and the pump light output by the third pump source 11, and then inputs the combined pulses 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 pulse in the amplified GHz pulse train, and the second fiber circulator 14 outputs a GHz femtosecond pulse train;

[0054] Step 6, measuring the duration and shape of the GHz femtosecond pulse train by the autocorrelator 16;

[0055] Step 7, judging whether the width of the laser pulse in the GHz femtosecond pulse train reaches the required stable width, if so, completing the all-fiber GHz femtosecond pulse train generation and the dispersion control of the optical fiber ring 100, otherwise executing step 8;

[0056] Step 8: Adjust the dispersion parameter 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.

[0057] The number of femtosecond pulse trains selected in the experiment is about 800, and the repetition frequency of the femtosecond pulse train is 100kHz. The stability of the laser pulse width in the GHz femtosecond pulse train measured by the autocorrelator is observed. If the laser pulse width fluctuates greatly, it means that the dispersion of the optical fiber ring 100 that generates the GHz femtosecond pulse train is not 0. Therefore, it is necessary to adjust the dispersion parameters of the first chirped fiber grating 6 in the optical fiber ring 100 or adjust the length of the optical fiber ring 100. Repeat the iteration until the laser pulse width in the GHz femtosecond pulse train reaches the required stable width, thereby achieving the purpose of zero dispersion transmission of the optical fiber ring 100.

[0058] Figure 7 This is the autocorrelation diagram of a tested laser pulse, and the full width at half maximum (FWHM) of the laser pulse is obtained by Lorentz fitting to be about 446 fs.

Claims

1. An all-fiber GHz femtosecond pulse train generation and fiber ring dispersion control system, characterized in that: It comprises a mode-locked oscillator (1), an optical fiber ring (100) and a control module; The optical fiber ring (100) comprises an optical fiber coupler (2), a first acousto-optic modulator (3), a first optical fiber circulator (4), a first single-mode gain optical fiber (5), a first chirped optical fiber grating (6), a pump protector (7) and a first pump source (8); the first output end (011) of the mode-locked oscillator is connected to the first input end (21) of the optical fiber coupler, the first output end (22) of the optical fiber coupler is connected to the first port (41) of the first optical fiber circulator; the second port (42) of the first optical fiber circulator is connected to the first single-mode gain optical fiber (5), the ... The optical fiber (5) and the first chirped fiber grating (6) are connected to each other, and the first pump source (8), the pump protector (7) and the first chirped fiber grating (6) output and input are connected in sequence; the third port (43) of the first optical fiber circulator is connected to the first input end (31) of the first acousto-optic modulator, the second input end (32) of the first acousto-optic modulator is connected to the second output end (012) of the mode-locked oscillator through the first AOM driver (17), and the output end (33) of the first acousto-optic modulator is connected to the second input end (24) of the optical fiber coupler; The control module comprises a second acousto-optic modulator (9), a single-mode amplifier, a second optical fiber circulator (14), a second chirped optical fiber grating (15), an autocorrelator (16) and a second AOM driver (18); the third output end (013) of the mode-locked oscillator is connected to the second input end (92) of the second acousto-optic modulator through the second AOM driver (18); the first input end (91) of the second acousto-optic modulator is connected to the second output end (23) of the optical fiber coupler; the output end of the second acousto-optic modulator (9) is connected to the input end of the single-mode amplifier; the repetition frequency of the second acousto-optic modulator (9) is the same as the repetition frequency of the first acousto-optic modulator (3); the output end of the single-mode amplifier is connected to the first port (141) of the second optical fiber circulator, the second port (142) of the second optical fiber circulator is connected to the second chirped optical fiber grating (15), and the third port (143) of the second optical fiber circulator is connected to the input end of the autocorrelator (16).

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) comprises a resonant cavity and a second pump source (0013); the resonant cavity comprises a SESAM (0011), a second wavelength division multiplexer (0012), a polarization-maintaining ytterbium-doped gain fiber (0014), and a chirped fiber Bragg grating (0015), the output and input of which are connected in sequence; the first port of the second wavelength division multiplexer (0012) is connected to the SESAM (0011), the second port is connected to the output end of the second pump source (0013), and the third port is connected to the chirped fiber Bragg grating (0015) through the polarization-maintaining ytterbium-doped gain fiber (0014); the output end of the chirped fiber Bragg grating (0015) is provided with an isolation beam splitter (0016), the first output end of the isolation beam splitter (0016) is connected to the first input end (21) of the fiber coupler, and the second output end is respectively connected to a first AOM driver (17) and a 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 comprises a pump source driver (12), a third pump source (11), a first wavelength division multiplexer (10), and a second single-mode gain optical fiber (13) whose output and input are connected in sequence; the output end of the second acousto-optic modulator (9) is connected to the first port (101) of the first wavelength division multiplexer, the output end of the third pump source (11) is connected to the second port (102) of the first wavelength division multiplexer, and the third port (103) of the first wavelength division multiplexer is connected to the first port (141) of the second optical fiber circulator via the second single-mode gain optical fiber (13).

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 48 cm; the first chirped fiber grating (6) is reflective and has a dispersion of 0.254 ps / nm; and the first pump source (8) is a single-mode semiconductor laser with fiber-coupled output and an output wavelength of 976 nm.

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

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