CN209993863U

CN209993863U - Low-repetition-frequency 1064nm self-mode-locking polarization-maintaining ytterbium-doped fiber laser

Info

Publication number: CN209993863U
Application number: CN201920086135.2U
Authority: CN
Inventors: 郭亮; 梁纯佳; 欧尚明; 隋庆林
Original assignee: South China Normal University
Current assignee: South China Normal University
Priority date: 2019-04-24
Filing date: 2019-04-24
Publication date: 2020-01-24
Anticipated expiration: 2029-04-24

Abstract

The utility model discloses a 1064nm of low repetition frequency is from mode locking polarization-preserving ytterbium-doped fiber laser, including a NALM mode locking, a full polarization-preserving laser oscillation chamber and a pulse testing arrangement, adopt "8 style of calligraphy" structure, the one end and the NALM mode locking of full polarization-preserving laser oscillation chamber are connected, and the other end and the pulse testing arrangement of full polarization-preserving laser oscillation chamber are connected. The utility model discloses a NALM mode locking technique and the long oscillation chamber more than hectometre of structure of protecting partially entirely can directly realize lower repetition frequency's pulse output under the higher pulse energy condition, need not to adopt AOM to reduce pulse repetition frequency, has strengthened the compactedness and the stability of laser instrument structure. Meanwhile, a 1064nm active gain fiber doped with Yb ions is used in the oscillation loop, and is compatible with the current mainstream ytterbium-doped fiber amplifier, so that good pulse quality output of a laser is ensured, and finally 1064nm femtosecond laser is output.

Description

Low-repetition-frequency 1064nm self-mode-locking polarization-maintaining ytterbium-doped fiber laser

Technical Field

The utility model relates to an ultrafast laser technical field, concretely relates to 1064nm of low repetition frequency is from mode locking polarization maintaining ytterbium doped fiber laser.

Background

In recent years, fiber lasers are favored in the field of laser micro-nano processing by virtue of the characteristics of good beam quality, high energy conversion efficiency, compact structure and low cost. The ultrafast fiber laser using the rare earth element doped fiber as the gain medium has the advantages of more compact structure, good stability, high quantum conversion efficiency and the like, the femtosecond level fiber laser is gradually approved, and the application requirement in high-precision micromachining is more and more extensive. Compared with the traditional high-power high-repetition-frequency optical fiber laser, the low-repetition-frequency ultra-short pulse laser can be widely applied to the field of industrial processing, and has more and more extensive application in the fields of laser radar, biomedical detection, coherence tomography, micro-nano structure laser processing and the like.

Compared with the traditional solid mode-locked laser, the fiber laser has unique advantages in the field of mode-locked pulse lasers due to the nonlinear characteristics of the fiber laser, such as dispersion, polarization state and the like. The fiber laser based on Nonlinear Amplification Loop Mirror (NALM) mode locking technology has the characteristics of simple structure and good stability. NALM is a mode locking structure of all optical fibers, generally forms an 8-shaped mode locking optical fiber laser, the mode locking characteristic of the NALM can be equivalent to a saturable absorber, the laser is firstly proposed by Richardson and the like in 1991, and then, in order to ensure the stability of system output, the fully-polarization-maintaining optical fiber is adopted in an oscillator, so that the system robustness of the fully-polarization-maintaining optical fiber structure is high, the influence of the external environment is extremely small, and the power amplification outside the oscillator is facilitated. In 2015, JAN SZCZEPANEK and the like obtain laser output with the repetition frequency of 15MHz, the single pulse energy of 3.46nJ and the pulse width of 220fs by adopting a full-polarization-maintaining optical fiber based on nonlinear amplification loop reflector mode locking. The NALM mode locking technology is difficult to realize self-starting mode locking, so that the NALM mode locking technology is usually combined with a polarization control device in a loop in practical application at present, and the polarization state of light in the loop is adjusted through the polarization device, so that stable mode locking can be easily realized. However, a polarization controller introduced into the loop needs to be manually adjusted to realize mode locking, which not only increases the complexity of mode locking of the laser, but also reduces the stability of the whole laser. Therefore, if an all-fiber polarization maintaining structure is realized in the NALM mode locking technology and the effect of self-mode locking can be achieved, the stability of the fiber laser can be greatly improved.

On the other hand, an important factor for realizing mode locking of a fiber laser as a passive mode locking technique is a gain fiber. In the use of gain fiber of an oscillation cavity in NALM mode locking technology, the gain medium of most commercial laser oscillation cavities in the market at present is active erbium-doped fiber with 1030nm central wavelength, the ring oscillation cavity with the central wavelength is easy to realize the mode locking of a seed source, but the compatibility of laser generated by the mode locking of the seed source and a double-cladding large-mode-field ytterbium-doped fiber amplifier commonly used in the market at a 1060-1100nm gain window in a subsequent amplification system is poor, so that the laser oscillation cavity is used as the active ytterbium-doped fiber with better gain absorption at a 1064nm wavelength. Compared with the prior art that the rare earth elements such as erbium ions and neodymium ions are widely used, the ytterbium ion doped medium has the advantages of simple energy level structure, high quantum efficiency, no excited state absorption, larger gain bandwidth and the like, is gradually paid attention and paid attention to as a doped medium, and can be used in combination with a subsequent amplifier system to obtain higher output power and pulse energy.

Most of the currently mainstream fiber lasers are high repetition frequency mode-locked lasers with the frequency of 100MHz or above, and a low repetition frequency laser pulse is needed for applying the lasers to the research fields such as radar, biological detection and the like which need lower laser repetition frequency requirements. This usually requires that the seed source pulse output from the oscillator cavity be subjected to an external acousto-optic modulation technique before power amplificationAn Operation (AOM) to implement the step of reducing the repetition rate of the laser. In the year of 2007, it was shown that,

by introducing an acousto-optic modulator (AOM) into an amplifier and amplifying output light of an oscillator after frequency selection, the research team of (1) improves the single-pulse energy to 100 muJ at the repetition frequency of 900kHz and the pulse width of 500 fs. In 2017, Song Huanganyu and the like use an acousto-optic modulator to reduce the output repetition frequency of an NPR mode-locked oscillator to 1MHz, and obtain 24fs compressed pulse width and pulse output with 1 muJ single-pulse energy through chirped pulse amplification. However, the method of accessing the external acousto-optic modulation device not only can lead the seed source laser to introduce partial loss in the amplification process, but also can lead the laser to lose the structural compactness and increase the structural instability of the laser. Therefore, if the low repetition frequency output of the laser light can be realized in the laser oscillation cavity, the complex structure of the laser can be greatly simplified, and the compactness of the entire structure of the laser can be enhanced.

SUMMERY OF THE UTILITY MODEL

In view of this, in order to solve the above-mentioned problem among the prior art, the utility model provides a low repetition frequency self-mode-locking polarization-maintaining ytterbium-doped fiber laser based on NALM mode-locking technology, center wavelength are at 1064nm, and energy conversion efficiency is high, and system compatibility is strong, simple structure.

The utility model discloses an above-mentioned problem is solved to following technical means:

a1064 nm self-mode-locking polarization-maintaining ytterbium-doped fiber laser with low repetition frequency comprises an NALM mode-locking structure, a full polarization-maintaining laser oscillation cavity and a pulse testing device, wherein an 8-shaped structure is adopted, one end of the full polarization-maintaining laser oscillation cavity is connected with the NALM mode-locking structure, and the other end of the full polarization-maintaining laser oscillation cavity is connected with the pulse testing device.

Further, the device connecting the NALM mode-locked and the full polarization-maintaining laser oscillation cavity is a 2 x 2 coupler with a splitting ratio of 60: 40.

Further, the NALM mode locking comprises a first 980 diode pump, a first 1064nm wavelength division multiplexer, a first gain fiber and a first common single-mode fiber; pumping light pumped by the first 980 diode enters the first 1064nm wavelength division multiplexer, namely, the signal light and the pumping light are in the same direction, a forward pumping mode is adopted, the forward direction of the first 1064nm wavelength division multiplexer is connected with a first gain optical fiber in the ring, and the first gain optical fiber provides gains of optical signal power and phase for NALM mode locking; the first common single-mode fiber is reversely connected with the first 1064nm wavelength division multiplexer and used for controlling a self-phase modulation effect generated by light in NALM mode locking.

Further, the full polarization-maintaining laser oscillation cavity comprises a first isolator, a second 980 diode pump, a second 1064nm wavelength division multiplexer, a second gain fiber, a second common single-mode fiber, a 2nm band-pass filter and a 10:90 1 × 2 coupler; the second 980 diode pump, the second 1064 wavelength division multiplexer and the second gain fiber form a reverse amplifier; the first isolator is connected with the output end of the 2 multiplied by 2 coupler of 60:40, and is used for keeping the output light unidirectional transmission of NALM mode locking, isolating the reverse light transmission possibly caused when the reverse amplifier is input, avoiding the influence of the reverse light on the loop light beam to influence the mode locking of the laser, and playing a role of protecting devices; the reverse amplifier connected with the first isolator adopts a reverse pumping mode, and a second 980 diode pump is connected with a second gain optical fiber after passing through a second 1064nm wavelength division multiplexer; the output light of NALM mode locking is amplified by a reverse amplifier to obtain equal light intensity, and nonlinear dispersion is accumulated; the output end of the reverse amplifier, namely the other end of the second 1064nm wavelength division multiplexer, is connected with a second common single-mode fiber, and self-phase modulation generated by light transmission in the second common single-mode fiber can realize self-mode locking of the laser through phase matching with dispersion generated by the structure; meanwhile, the optical fiber length of the second common single-mode optical fiber lengthens the oscillation cavity of the laser, and the repetition frequency of the laser can be reduced according to the inverse relation of the laser repetition frequency and the oscillation cavity length, so that the laser pulse output with low repetition frequency is realized; the second common single-mode optical fiber is connected with a band-pass filter with the central wavelength of 1064nm and the bandwidth of 2 nm; splitting between a 2nm bandpass filter and a 60:40 2 x 2 coupler with a 1 x 2 coupler splitting ratio of 10: 90; after the signal light passes through the ring cavity for one circle, 10% of the light in the 10:90 1 × 2 coupler is output, and the remaining 90% of the light is input for the next iteration.

Further, the first gain fiber and the second gain fiber are made of Coractive Yb 401-PM.

Further, the model of the first common single-mode optical fiber and the second common single-mode optical fiber is Nufern PM-980.

Further, the structure of the 60:40 2 coupler is an input end, a 60% splitting end, a 40% splitting end and an output end; the light of the oscillation cavity of the full polarization-maintaining laser enters from the input end of the 2 × 2 coupler of 60:40, and the 2 × 2 coupler of 60:40 splits the input light into 60:40 light is output from a 60% splitting end and a 40% splitting end respectively and enters an NALM mode locking; the 60% beam splitting end is connected with a first gain optical fiber of a forward pump, so that the anticlockwise signal light entering the NALM mode locking is amplified anticlockwise firstly and then passes through the single mode optical fiber to obtain a larger nonlinear phase shift amount; the 40% beam splitting end is connected with the first common single mode fiber, and the clockwise light beam firstly passes through the first common single mode fiber and then is subjected to gain amplification to accumulate a certain amount of nonlinear phase shift, but the amount of the nonlinear phase shift is less than that of the nonlinear phase shift at the 60% end; the 60% beam-splitting end anticlockwise light beam and the 40% beam-splitting end clockwise light beam are transmitted for one circle in NALM mode locking and then return to the 60:40 2 coupler for interference modulation, and the reflectivity in the 60:40 2 coupler obtains stronger modulation depth due to different light intensity and phase of the two light beams.

Further, the pulse detection device comprises a second isolator, a 50:50 1X 2 coupler, a 20G broadband oscilloscope and a spectrometer; the second isolator is connected with the 10% output end of the 1 multiplied by 2 coupler of the 10:90 of the oscillation cavity of the full polarization-maintaining laser, and has the functions of preventing the mode locking effect of the oscillation cavity from being influenced by reverse transmission of light caused by the influence of other conditions when the output light is output and protecting the oscillation cavity; a 50:50 1 multiplied by 2 coupler is connected behind the second isolator, and the output light beam is split into two beams of light with equal light intensity which are respectively input into a 20G broadband oscilloscope and a spectrometer; the 20G broadband oscilloscope is used for testing the mode locking waveform and the repetition frequency of the laser, and the spectrograph is used for measuring the mode locking spectrum.

Compared with the prior art, the beneficial effects of the utility model include at least:

the utility model discloses can obtain the lower repetition frequency of direct output when self-starting mode locking, avoid the energy and the frequency signal loss that seed laser caused in AOM modulation process, keep higher reliability and stability to can have better compatibility with subsequent pulse power amplification system. The designed all-fiber laser is less influenced by the use environment and is more compact, simpler and more convenient.

The utility model discloses output center wavelength is at 1064nm, and current commercial laser instrument is many at 1030nm, and this wavelength is incompatible with the big mode field amplifier of current 1064 wavelength, the utility model relates to a 1064nm of low repetition frequency has higher energy conversion at 1064 wavelengths from the used active gain optic fibre of mode locking polarization maintaining ytterbium doped fiber laser, consequently this kind of seed source laser output has better compatibility to big mode field fiber amplifier.

The utility model discloses the output has the pulse energy output of 1.14 MHz's low repetition rate and high chirp, compares 10 ~ 100 MHz's that mode locking technique produced in the past pulse often need fall the frequency through acousto-optic modulation system before enlargeing, and the output pulse of this oscillator need not to fall the frequency through AOM, can keep the pulse quality in the amplification process and strengthened fiber laser's compactedness better.

Drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the description of the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings without creative efforts.

FIG. 1 is a block diagram of the design process of a 1064nm self-mode-locked polarization-maintaining ytterbium-doped fiber laser with low repetition frequency according to the present invention;

FIG. 2 is a system diagram of a low repetition frequency 1064nm self-mode-locked polarization maintaining ytterbium-doped fiber laser of the present invention;

fig. 3 is a schematic 2 x 2 port diagram of a 40:60 coupler 5 of the present invention connecting a NALM to a main loop structure;

fig. 4 is a schematic diagram of the repetition frequency of the laser pulse measured by the 20G bandwidth oscilloscope 15 according to the present invention;

fig. 5 is a schematic diagram of a laser single pulse measured by the 20G bandwidth oscilloscope 15 of the present invention;

fig. 6 is a schematic diagram of the mode-locked pulse frequency domain logarithmic spectrum (large) and linear spectrum (small) measured by the spectrometer 16 of the present invention.

Wherein, 1, 980 diode pumping (LD 1); 2. 1064 wavelength division multiplexer 1 (WDM-1); 3. a gain fiber Coractive Yb 401-PM; 4. a common single mode fiber Nufern PM-980; 5. 60:40 2 x 2 coupler; 5-1, input end; 5-2, 60% of beam splitting end; 5-3, 40% beam splitting end; 5-4, output end; 6. an isolator (ISO-1); 7. 980 diode pumping (LD 2); 8. 1064 Wavelength Division Multiplexer (WDM); 9. a gain fiber Coractive Yb 401-PM; 10. a common single mode fiber Nufern PM-980; 11. a 2nm band pass filter; 12. 10:90 1 × 2 coupler; 13. an isolator (ISO-2); 14. a 50:50 1 × 2 coupler; 15. a 20G broadband oscilloscope; 16. a spectrometer.

Detailed Description

In order to make the above objects, features and advantages of the present invention more comprehensible, embodiments accompanying the drawings are described in detail below. It should be noted that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments, and all other embodiments obtained by those skilled in the art without any inventive work are within the scope of the present invention.

Example 1

The internal structure design of a 1064nm self-mode-locking polarization-maintaining ytterbium-doped fiber laser with low repetition frequency is shown in figure 1, and the laser is composed of three parts, namely an NALM structure, a main loop oscillation cavity and a pulse test module, and adopts an 8-shaped structure. The seed source uses a fully polarization maintaining fiber element so that the laser can still work very stably under external environmental disturbance.

Fig. 2 is a diagram of a 1064nm self-mode-locking polarization-maintaining ytterbium-doped fiber laser system with low repetition frequency. The NALM ring structure comprises a diode pump 1(LD-1), a 1064nm wavelength division multiplexer 2(WDM-1) and a gain fiber 3, and a section of common single mode fiber 4 (SMF). The diode pump 1(LD-1) pump light enters the wavelength division multiplexer 2(WDM-1), i.e. the signal light and the pump light are in the same direction, and a forward pumping mode is adopted. The wavelength division multiplexer 2(WDM-1) is connected in the forward direction to a gain fiber 3 of length 1m inside the loop, of the type CorActive Yb401-PM, said gain fiber 3 providing the NALM loop with gain in optical signal power and phase. The wavelength division multiplexer 2(WDM-1) is reversely connected with a section of common single mode fiber 4 with the length of 1m, the type of the fiber is Nufern PM-980, and the common single mode fiber 4 is mainly used for controlling the self-phase modulation effect of light generated in an NALM loop.

The main loop (right) of the oscillation cavity consists of an isolator 6(ISO-1), an inverting amplifier 7-9, a common single mode fiber 10, a 1060nm wavelength band-pass filter 11 with a bandwidth of 2nm and a 90:10 1 x 2 coupler 12. The isolator 6 is connected with the output end 5-4 of the 2 x 2 coupler 5 of 60:40, and has the functions of keeping the output light of the NALM in one-way transmission, isolating reverse light transmission possibly caused when the reverse amplifier is input, avoiding the influence of the reverse light on loop light beams to influence laser mode locking and simultaneously playing a role in protecting devices. The inverting amplifier connected to the isolator 6 is pumped in the opposite direction. The diode pump 7(LD-2) is connected to a gain fiber 9 of CorActiveYb 401-PM with a length of 1m after passing through a 1064nm wavelength division multiplexer 8 (WDM-2). The output light of NALM is amplified by inverse amplifier to obtain equal light intensity and accumulate non-linear dispersion. The output end of the reverse amplifier, namely the other end of the wavelength division multiplexer 8(WDM-2) is connected with a passive optical fiber 10 of 170m, the type of the optical fiber is Nufern PM980-XP, the self-phase modulation generated by the transmission of light in the passive optical fiber 10 can realize the self-mode locking of the laser through the phase matching with the dispersion generated by the structure; meanwhile, the 170m optical fiber length lengthens the oscillation cavity of the laser, and the repetition frequency of the laser can be reduced according to the inverse proportion relation of the laser repetition frequency and the oscillation cavity length, so that the laser pulse output with low repetition frequency is realized. The passive optical fiber 10 is connected with a band-pass filter 11 with the center wavelength of 1064nm and the bandwidth of 2 nm. A 1 x 2 coupler 12 with a splitting ratio of 10:90 is used for splitting between the band pass filter 11 and the 60:40 coupler 5. After the signal light passes through the annular cavity for one revolution, 10% of the light in the coupler 12 is output, and the remaining 90% of the light is input for the next iteration.

The device connecting the NALM loop and the main loop is a 2 x 2 coupler 5 with a splitting ratio of 60: 40. As shown in fig. 3, the 2 × 2 coupler 5 is a connector for connecting the main loop and the NALM loop, and has a structure of an input terminal 5-1, a 60% splitting terminal 5-2, a 40% splitting terminal 5-3, and an output terminal 5-4. The light of the main loop enters from the input 5-1 of the coupler 5, which splits the input light into 60:40 light is output from a 60% beam splitting end 5-2 and a 40% beam splitting end 5-3 respectively and enters the NALM loop. The 60% beam splitting end 5-2 is connected with the amplifier gain optical fiber 3 of the forward pumping, so that the anticlockwise signal light entering the NALM is amplified anticlockwise firstly and then passes through the single mode optical fiber to obtain larger nonlinear phase shift quantity; the 40% beam splitting end 5-3 is connected with a common single mode fiber 4(SMF), and a certain amount of nonlinear phase shift is accumulated after the clockwise light beam passes through the single mode fiber 4 and is amplified by gain, but the amount of the nonlinear phase shift is less than that of the 60% end. The counterclockwise light beam of the 60% beam splitting end 5-2 and the clockwise light beam of the 40% beam splitting end 5-3 are transmitted in the NALM ring for one circle and then return to the coupler for interference modulation, and the reflectivity in the coupler obtains stronger modulation depth due to different light intensity and phase of the two light beams.

A pulse detection device is connected to the 10% output of the 10:90 coupler 12 in the oscillator cavity. The pulse detection device consists of an isolator 13(ISO-2), a 50:50 1 × 2 coupler 14, a 20G broadband oscilloscope 15 and a spectrometer 16. The isolator 13(ISO-2) is connected with the 10% output end of the oscillator cavity main loop 10:90 coupler 12, and has the function of preventing the mode locking effect of the oscillator cavity from being influenced by reverse transmission of light caused by the influence of other conditions when the output light is transmitted, so that the oscillator cavity is protected. The 50:50 coupler 14 is connected after the isolator 13(ISO-2) and splits the output beam into two equal intensity beams which are input to the 20G broadband oscilloscope 15 and the spectrometer 16, respectively. A 20G broadband oscilloscope 15 is used to test the mode-locked waveform and repetition frequency of the laser, and a spectrometer 16 is used to measure the mode-locked spectrum.

In the experiment, the pumping power of the oscillation cavity is adjusted, and when the power of the NALM and the power of the pumping diode pump 1(LD-1) and the power of the pumping diode pump 7(LD-2) in the main loop are respectively 130mW and 180mW, stable single-pulse mode locking is formed, and the repetition frequency of the pulse is 1.14MHz as shown in FIG. 4. The single pulse shape can be detected using a photodetector and displayed on a 20G broadband oscilloscope 15, as shown in FIG. 5, with a full width at half maximum (FWHM) of the pulse of 91.25 ps. It is worth mentioning that the power of the two pumps can be adjusted within the range of 90-130mW and 160-180mW at the same time without affecting the single-pulse mode-locking state. The spectrometer 16 was used to collect the spectrum of the output pulse, the log and linear spectra (inclusive) of which are shown in fig. 6, with a center wavelength of 1064nm and a spectral width of 10.4 nm. It can be seen from the linear spectrogram that the edges of the spectrum appear steep in shape, since the cavity is modelocked at full positive dispersion.

The above-mentioned embodiments only represent some embodiments of the present invention, and the description thereof is specific and detailed, but not to be construed as limiting the scope of the present invention. It should be noted that, for those skilled in the art, without departing from the spirit of the present invention, several variations and modifications can be made, which are within the scope of the present invention. Therefore, the protection scope of the present invention should be subject to the appended claims.

Claims

1. A1064 nm self-mode-locking polarization-maintaining ytterbium-doped fiber laser with low repetition frequency is characterized by comprising an NALM mode locking device, a full polarization-maintaining laser oscillation cavity and a pulse testing device, wherein an 8-shaped structure is adopted, one end of the full polarization-maintaining laser oscillation cavity is connected with the NALM mode locking device, and the other end of the full polarization-maintaining laser oscillation cavity is connected with the pulse testing device.

2. The low repetition frequency 1064nm self-mode-locked polarization-maintaining ytterbium-doped fiber laser of claim 1, wherein the device connecting the NALM mode-locked and the full polarization-maintaining laser oscillation cavity is a 2 x 2 coupler with a splitting ratio of 60: 40.

3. The low repetition frequency 1064nm self-mode-locked polarization maintaining ytterbium-doped fiber laser of claim 2, wherein the NALM mode-lock comprises a first 980 diode pump, a first 1064nm wavelength division multiplexer, a first gain fiber, and a first common single mode fiber; pumping light pumped by the first 980 diode enters the first 1064nm wavelength division multiplexer, namely, the signal light and the pumping light are in the same direction, a forward pumping mode is adopted, the forward direction of the first 1064nm wavelength division multiplexer is connected with a first gain optical fiber in the ring, and the first gain optical fiber provides gains of optical signal power and phase for NALM mode locking; the first common single-mode fiber is reversely connected with the first 1064nm wavelength division multiplexer and used for controlling a self-phase modulation effect generated by light in NALM mode locking.

4. The low repetition frequency 1064nm self-mode-locked polarization-maintaining ytterbium-doped fiber laser of claim 3, wherein the oscillation cavity of the full polarization-maintaining laser comprises a first isolator, a second 980 diode pump, a second 1064nm wavelength division multiplexer, a second gain fiber, a second common single mode fiber, a 2nm band-pass filter, and a 10:90 1 x 2 coupler; the second 980 diode pump, the second 1064 wavelength division multiplexer and the second gain fiber form a reverse amplifier; the first isolator is connected with the output end of the 2 multiplied by 2 coupler of 60:40, and is used for keeping the output light unidirectional transmission of NALM mode locking, isolating the reverse light transmission possibly caused when the reverse amplifier is input, avoiding the influence of the reverse light on the loop light beam to influence the mode locking of the laser, and playing a role of protecting devices; the reverse amplifier connected with the first isolator adopts a reverse pumping mode, and a second 980 diode pump is connected with a second gain optical fiber after passing through a second 1064nm wavelength division multiplexer; the output light of NALM mode locking is amplified by a reverse amplifier to obtain equal light intensity, and nonlinear dispersion is accumulated; the output end of the reverse amplifier, namely the other end of the second 1064nm wavelength division multiplexer, is connected with a second common single-mode fiber, and self-phase modulation generated by light transmission in the second common single-mode fiber can realize self-mode locking of the laser through phase matching with dispersion generated by the structure; meanwhile, the optical fiber length of the second common single-mode optical fiber lengthens the oscillation cavity of the laser, and the repetition frequency of the laser can be reduced according to the inverse relation of the laser repetition frequency and the oscillation cavity length, so that the laser pulse output with low repetition frequency is realized; the second common single-mode optical fiber is connected with a band-pass filter with the central wavelength of 1064nm and the bandwidth of 2 nm; splitting between a 2nm bandpass filter and a 60:40 2 x 2 coupler with a 1 x 2 coupler splitting ratio of 10: 90; after the signal light passes through the ring cavity for one circle, 10% of the light in the 10:90 1 × 2 coupler is output, and the remaining 90% of the light is input for the next iteration.

5. The low repetition frequency 1064nm self-mode-locked polarization maintaining ytterbium doped fiber laser of claim 4, wherein the first and second gain fibers are of the type Coractive Yb 401-PM.

6. The low repetition frequency 1064nm self-mode-locked polarization maintaining ytterbium doped fiber laser of claim 4, wherein the first and second common single mode fibers are of the type Nufern PM-980.

7. The 1064nm self-mode-locked polarization-maintaining ytterbium-doped fiber laser with low repetition frequency as claimed in claim 4, wherein the 60:40 2 coupler has the structure of an input end, a 60% beam splitting end, a 40% beam splitting end and an output end; the light of the oscillation cavity of the full polarization-maintaining laser enters from the input end of the 2 × 2 coupler of 60:40, and the 2 × 2 coupler of 60:40 splits the input light into 60:40 light is output from a 60% splitting end and a 40% splitting end respectively and enters an NALM mode locking; the 60% beam splitting end is connected with a first gain optical fiber of a forward pump, so that the anticlockwise signal light entering the NALM mode locking is amplified anticlockwise firstly and then passes through the single mode optical fiber to obtain a larger nonlinear phase shift amount; the 40% beam splitting end is connected with the first common single mode fiber, the light beam in the clockwise direction firstly passes through the first common single mode fiber and then is subjected to gain amplification, the nonlinear phase shift is accumulated, and the nonlinear phase shift amount is less than that of the 60% end; the 60% beam-splitting end anticlockwise light beam and the 40% beam-splitting end clockwise light beam are transmitted for one circle in NALM mode locking and then return to the 60:40 2 coupler for interference modulation, and the reflectivity in the 60:40 2 coupler obtains stronger modulation depth due to different light intensity and phase of the two light beams.

8. The low repetition frequency 1064nm self-mode-locked polarization-maintaining ytterbium-doped fiber laser of claim 4, wherein the pulse detection device comprises a second isolator, a 50:50 1 x 2 coupler, a 20G broadband oscilloscope and a spectrometer; the second isolator is connected with 10% of the output end of a 10:90 1 multiplied by 2 coupler of the oscillation cavity of the full polarization-maintaining laser, and has the functions of preventing the mode locking effect of the oscillation cavity from being influenced by the reverse transmission of light caused by the influence of conditions when the output light is output and protecting the oscillation cavity; a 50:50 1 multiplied by 2 coupler is connected behind the second isolator, and the output light beam is split into two beams of light with equal light intensity which are respectively input into a 20G broadband oscilloscope and a spectrometer; the 20G broadband oscilloscope is used for testing the mode locking waveform and the repetition frequency of the laser, and the spectrograph is used for measuring the mode locking spectrum.