CN209993863U - A Low Repetition Frequency 1064nm Self-Mode-Locked PM Ytterbium Fiber Laser - Google Patents

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

A Low Repetition Frequency 1064nm Self-Mode-Locked PM Ytterbium Fiber Laser

technical field

The utility model relates to the technical field of ultrafast lasers, in particular to a 1064 nm self-mode-locking and polarization-maintaining ytterbium-doped fiber laser with low repetition frequency.

Background technique

In recent years, fiber lasers have been favored in the field of laser micro-nano processing due to their good beam quality, high energy conversion efficiency, compact structure, and low cost. Ultrafast fiber lasers using rare-earth element-doped fibers as gain media have the advantages of more compact structure, good stability, and high quantum conversion efficiency. Femtosecond-scale fiber lasers have been gradually recognized. also becoming more widespread. Compared with traditional high-power and high-repetition-rate fiber lasers, which can be widely used in industrial processing, low-repetition-rate ultra-short pulse lasers are increasingly used in lidar, biomedical detection, coherence tomography, and micro-nano structure laser processing. Wide range of applications.

Compared with traditional solid-state mode-locked lasers, fiber lasers have unique advantages in the field of mode-locked pulsed lasers due to their nonlinear characteristics, such as dispersion and polarization state. The fiber laser based on the Nonlinear Amplifying Loop Mirror (NALM) mode-locking technology has the characteristics of simple structure and good stability. NALM is an all-fiber mode-locked structure, which generally constitutes a "8"-shaped mode-locked fiber laser. Its mode-locking characteristics can be equivalent to a saturable absorber. This laser was first proposed by Richardson et al. in 1991, and then , in order to ensure the stability of the system output, the full polarization maintaining fiber is used in the oscillator. The full polarization maintaining fiber structure has high system robustness and is minimally affected by the external environment, which is conducive to power amplification outside the oscillator. In 2015, JAN SZCZEPANEK et al. obtained a laser output with a repetition frequency of 15MHz, a single pulse energy of 3.46nJ, and a pulse width of 220fs using a fully polarization-maintaining fiber based on the mode-locking of the mirror in a nonlinear amplification loop. Since NALM mode-locking technology is difficult to achieve self-starting mode-locking, it is often combined with polarization control devices in the loop in practical applications. The polarization state of light in the loop can be adjusted by the polarization device, which can easily achieve stable mode-locking. However, the polarization control device introduced in the loop requires manual adjustment of the polarization controller to achieve mode locking, which not only increases the complexity of the mode locking of the laser, but also reduces the stability of the entire laser. When the external environment changes drastically or occurs When oscillating, the laser will easily lose mode locking. Therefore, if an all-fiber polarization-maintaining structure is realized in the NALM mode-locking technology and the self-mode-locking effect can be achieved, the stability of the fiber laser can be greatly improved.

On the other hand, an important factor for the mode-locking of fiber lasers as passive mode-locking technology is the gain fiber. In the use of the gain fiber of the oscillation cavity of the NALM mode-locking technology, the gain medium of most commercial laser oscillation cavities on the market is an active erbium-doped fiber with a center wavelength of 1030 nm. The ring oscillation cavity of this center wavelength is easy to realize the locking of the seed source. However, in the subsequent amplification system, the laser generated by the mode locking of the seed source has poor compatibility with the double-clad large-mode field ytterbium-doped fiber amplifier commonly used in the market in the gain window of 1060-1100 nm. Therefore, the laser oscillation cavity such as Use active ytterbium-doped fibers with better gain absorption at 1064 nm. Compared with the widely used rare earth elements erbium and neodymium ions, ytterbium ions have the advantages of simple energy level structure, high quantum efficiency, no excited state absorption and large gain bandwidth, etc. As a doping medium, it has gradually attracted widespread attention. and attention, can be used with subsequent amplifier systems for higher output power and pulse energy.

的研究团队通过在放大器中引入声光调制器(AOM)，对振荡器输出光选频后再放大的方法，在重复频率900kHz、脉冲宽度500fs将单脉冲能量提高到了100μJ。2017年，Song Huanyu等利用声光调制器将NPR锁模振荡器输出重复频率降到1MHz，通过啁啾脉冲放大得到24fs压缩脉宽，1μJ单脉冲能量的脉冲输出。但接入外部声光调制器件的方法不仅会使种子源激光在放大过程中引入部分损耗，也使激光器失去了结构的紧凑性，增加激光器结构的不稳定性。因此，若能够在激光器振荡腔中实现激光的低重复频率输出，则可大大简化激光器的复杂结构，增强激光器整体结构的紧凑性。The current mainstream fiber lasers are mostly mode-locked lasers with a high repetition rate of 100MHz and above. In order to apply lasers to research fields that require a lower laser repetition rate, such as radar and biological detection, a laser pulse with a low repetition rate is required. This usually requires the step of reducing the laser repetition frequency by subjecting the seed source pulse output from the oscillation cavity to an external acousto-optic modulation (AOM) technique before power amplification. In 2007,

By introducing an acousto-optic modulator (AOM) into the amplifier, the oscillator output light is frequency-selected and then amplified, and the single-pulse energy is increased to 100 μJ at a repetition frequency of 900 kHz and a pulse width of 500 fs. In 2017, Song Huanyu et al. used an acousto-optic modulator to reduce the output repetition frequency of the NPR mode-locked oscillator to 1MHz, and obtained a pulse output with a compressed pulse width of 24fs and a single pulse energy of 1μJ through chirped pulse amplification. However, the method of connecting an external acousto-optic modulation device will not only introduce partial loss in the amplification process of the seed source laser, but also make the laser lose the compactness of the structure and increase the instability of the laser structure. Therefore, if the low repetition frequency output of the laser can be realized in the laser oscillation cavity, the complex structure of the laser can be greatly simplified, and the compactness of the overall structure of the laser can be enhanced.

Utility model content

In view of this, in order to solve the above problems in the prior art, the utility model proposes a low-repetition self-mode-locked polarization-maintaining ytterbium-doped fiber laser based on NALM mode-locking technology and a center wavelength of 1064 nm, which has high energy conversion efficiency and system. Strong compatibility and simple structure.

The present utility model solves the above-mentioned problems through the following technical means:

A low repetition frequency 1064nm self-mode-locking polarization-maintaining ytterbium-doped fiber laser, including a NALM mode-locking, a full-polarization-maintaining laser oscillation cavity, and a pulse test device. One end is connected to the NALM mode-locking, and the other end of the full polarization maintaining laser oscillation cavity is connected to the pulse testing device.

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

Further, the NALM mode locking includes a first 980 diode pump, a first 1064nm wavelength division multiplexer, a first gain fiber and a first common single-mode fiber; the pump light of the first 980 diode pump enters the first The 1064nm wavelength division multiplexer, that is, the signal light and the pump light are in the same direction, and the forward pumping method is adopted. The forward direction of the first 1064nm wavelength division multiplexer is connected to the first gain fiber in the ring. The first gain fiber is NALM mode locking provides the gain of optical signal power and phase; the reverse of the first 1064nm wavelength division multiplexer is connected to the first ordinary single-mode fiber, and the first ordinary single-mode fiber is used to control the self-generated light generated in the NALM mode locking. Phase modulation effects.

Further, the full polarization maintaining laser oscillation cavity includes a first isolator, a second 980 diode pump, a second 1064nm wavelength division multiplexer, a second gain fiber, a second ordinary single-mode fiber, and a 2nm bandpass filter. The second 980 diode pump, the second 1064 wavelength division multiplexer and the second gain fiber form the reverse amplifier; the first isolator is connected to the 60:40 2× 2 The output end of the coupler, its function is to maintain the unidirectional transmission of the output light of the NALM mode locking, and to isolate the reverse light transmission that may be caused by the input of the reverse amplifier, so as to avoid the interference of the reverse light on the loop beam and affect the laser Mode-locking, at the same time play the role of protecting the device; the reverse amplifier connected with the first isolator adopts the way of reverse pumping, and the second 980 diode is pumped through the second 1064nm wavelength division multiplexer with the second gain Optical fiber connection; the output light of the NALM mode-locking is amplified by the reverse amplifier to obtain the same light intensity, and the nonlinear dispersion is accumulated at the same time; the output end of the reverse amplifier is the other end of the second 1064nm wavelength division multiplexer Connected to the second ordinary single-mode fiber, the self-phase modulation generated by light transmission in the second ordinary single-mode fiber can realize the self-mode locking of the laser through phase matching with the dispersion generated by the aforementioned structure; at the same time, the second ordinary single-mode fiber The length of the optical fiber lengthens the oscillation cavity of the laser. According to the inverse relationship between the laser repetition frequency and the cavity length of the oscillation cavity, the repetition frequency of the laser can be reduced, and the laser pulse output of low repetition frequency can be realized; the second ordinary single-mode fiber connects a center Bandpass filter with a wavelength of 1064nm and a bandwidth of 2nm; splitting with a 1x2 coupler with a splitting ratio of 10:90 between the 2nm bandpass filter and a 60:40 2x2 coupler; signal After the light passes through the annular cavity for one week, 10% of the light in the 10:90 1×2 coupler is used as the output, and the remaining 90% of the light is used as the input for the next round of iterations.

Further, the model used for the first gain fiber and the second gain fiber is CorActive Yb401-PM.

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

Further, the structure of the 60:40 2×2 coupler is an input end, a 60% beam splitting end, a 40% beam splitting end and an output end; the light from the 60:40 2×2 The input end of the 2 coupler enters, and the 2×2 coupler of 60:40 splits the input light into 60:40 light from the 60% split end and the 40% split end, respectively, and enters the NALM mode locking; 60% split The beam end is connected to the first gain fiber of the forward pump, so that the counterclockwise signal light entering the NALM mode locking is first amplified counterclockwise, and then passes through the single-mode fiber to obtain a larger nonlinear phase shift; 40% points The bundle end is connected to the first ordinary single-mode fiber. The clockwise beam first passes through the first ordinary single-mode fiber and then undergoes gain amplification, which also accumulates a certain amount of nonlinear phase shift, but it is less than the nonlinear phase shift at the 60% end. ; The counterclockwise beam at the 60% split end and the clockwise beam at the 40% split end will return to the 60:40 2×2 coupler for interferometric modulation after one revolution in the NALM mode locking, because the two beams are different The light intensity and phase of the 60:40 reflectivity in the 2×2 coupler achieve a stronger modulation depth.

Further, the pulse detection device includes a second isolator, a 50:50 1×2 coupler, a 20G broadband oscilloscope and a spectrometer; the second isolator is connected to a 10:90 1×2 coupling of the full polarization-maintaining laser oscillation cavity The 10% output end of the oscillating cavity is used to prevent the reverse transmission of the output light due to the influence of other conditions, which will affect the mode-locking effect in the oscillating cavity and protect the oscillating cavity; the 50:50 1×2 coupler is connected to the second isolation After the laser detector, split the output beam into two beams of equal intensity and input them into a 20G broadband oscilloscope and a spectrometer respectively; the 20G broadband oscilloscope is used to test the mode-locked waveform and repetition frequency of the laser, and the spectrometer is used to measure the mode-locked spectrum.

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

The utility model can obtain self-starting mode locking and directly output lower repetition frequency, avoid the loss of energy and frequency signal caused by the seed laser in the AOM modulation process, maintain high reliability and stability, and can be compatible with subsequent Pulse power amplifier system has better compatibility. The designed all-fiber laser is less affected by the use environment and is more compact and simple.

The output center wavelength of the utility model is 1064 nm, and the existing commercial lasers are mostly 1030 nm, and this wavelength is incompatible with the existing large mode field amplifier of 1064 wavelength. The active gain fiber used in the laser has a higher energy conversion rate at the wavelength of 1064, so the output of the seed laser has better compatibility with the large mode field fiber amplifier.

The output of the utility model has a low repetition frequency of 1.14MHz and a high chirp pulse energy output. Compared with the 10-100MHz pulse generated by the previous mode-locking technology, the frequency often needs to be reduced by the acousto-optic modulation system before amplification. The output pulse does not need to be down-converted by AOM, which can better maintain the pulse quality in the amplification process and enhance the compactness of the fiber laser.

Description of drawings

In order to illustrate the technical solutions in the embodiments of the present invention more clearly, the following briefly introduces the accompanying drawings used in the description of the embodiments. Obviously, the drawings in the following description are only some implementations of the present invention. For example, for those of ordinary skill in the art, other drawings can also be obtained based on these drawings without any creative effort.

Fig. 1 is the module design flow chart of the low repetition frequency 1064nm self-mode-locked polarization-maintaining ytterbium-doped fiber laser of the utility model;

Fig. 2 is the system schematic diagram of the low repetition frequency 1064nm self-mode-locking and polarization-maintaining ytterbium-doped fiber laser of the present invention;

3 is a schematic diagram of 2×2 ports of the 40:60 coupler 5 connecting the NALM and the main loop structure of the present invention;

4 is a schematic diagram of the laser pulse repetition frequency measured by the 20G bandwidth oscilloscope 15 of the utility model;

5 is a schematic diagram of a single laser pulse measured by the 20G bandwidth oscilloscope 15 of the utility model;

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.

Among them, 1. 980 diode pump (LD1); 2. 1064 wavelength division multiplexer 1 (WDM-1); 3. Gain fiber CorActive Yb 401-PM; 4. Ordinary single-mode fiber Nufern PM-980; 5. 60:40 2×2 coupler; 5-1, input end; 5-2, 60% split end; 5-3, 40% split end; 5-4, output end; 6, isolator (ISO -1); 7, 980 diode pump (LD2); 8, 1064 wavelength division multiplexer (WDM); 9, gain fiber CorActive Yb 401-PM; 10, ordinary single-mode fiber Nufern PM-980; 11, 2nm 12, 10:90 1×2 coupler; 13, isolator (ISO-2); 14, 50:50 1×2 coupler; 15, 20G broadband oscilloscope; 16, spectrometer.

Detailed ways

In order to make the above objects, features and advantages of the present invention more clearly understood, the technical solutions of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be pointed out that the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, those of ordinary skill in the art can obtain without creative work. All other embodiments of the present invention belong to the protection scope of the present invention.

Example 1

The internal structure design of a low repetition frequency 1064nm self-mode-locked polarization-maintaining ytterbium-doped fiber laser is shown in Figure 1. The laser consists of three parts and adopts a "figure-of-eight" structure, that is, a NALM structure, a main loop oscillation cavity and a pulse test module. The seed source uses all polarization-maintaining fiber components, so that the laser can still work very stably under external environmental disturbances.

Figure 2 is a schematic diagram of a low-repetition frequency 1064nm self-mode-locked and polarization-maintaining ytterbium-doped fiber laser system of the utility model. The NALM ring structure consists of diode pump 1 (LD-1), 1064nm wavelength division multiplexer 2 (WDM-1), gain fiber 3, and a section of ordinary single-mode fiber 4 (SMF). The diode pump 1 (LD-1) pump light enters the wavelength division multiplexer 2 (WDM-1), that is, the signal light and the pump light are in the same direction, and the forward pumping method is adopted. The forward direction of the wavelength division multiplexer 2 (WDM-1) is connected to a gain fiber 3 with a length of 1m in the loop. The model used is CorActive Yb 401-PM. The gain fiber 3 provides light for the NALM loop. Gain of signal power and phase. The reverse connection of the wavelength division multiplexer 2 (WDM-1) is a common single-mode fiber 4 with a length of 1m. The fiber used is Nufern PM-980. The common single-mode fiber 4 is mainly used to control the optical Self-phase modulation effects generated in NALM loops.

The main loop of the oscillator cavity (right) consists of an isolator 6 (ISO-1), an inverse amplifier 7-9, an ordinary single-mode fiber 10, a 1060nm wavelength bandpass filter 11 with a bandwidth of 2nm, and a 90:10 1× 2 couplers 12. The isolator 6 is connected to the output end 5-4 of the 60:40 2×2 coupler 5. Its function is to maintain the unidirectional transmission of the output light of the NALM, and to isolate the reverse optical transmission that may be caused by the input of the reverse amplifier. Avoid the interference of the reverse light on the loop beam and affect the laser mode locking, and at the same time play the role of protecting the device. The inverting amplifier connected with the isolator 6 adopts the mode of inverting pumping. The diode pump 7 (LD-2) is connected to the gain fiber 9 of CorActiveYb 401-PM with a length of 1 m after passing through the 1064 nm wavelength division multiplexer 8 (WDM-2). The output light of the NALM is amplified by the reverse amplifier to obtain the same light intensity and accumulate nonlinear dispersion at the same time. The output end of the reverse amplifier, that is, the other end of the wavelength division multiplexer 8 (WDM-2) is connected to a 170m passive fiber 10, the fiber model Nufern PM980-XP, the self-phase generated by the light transmission in the passive fiber 10 The modulation can achieve self-mode locking of the laser through phase matching with the dispersion generated by the aforementioned structure; at the same time, the 170m fiber length lengthens the laser oscillation cavity. According to the inverse relationship between the laser repetition frequency and the oscillation cavity cavity length, the laser repetition rate can be made The frequency is reduced to achieve low repetition rate laser pulse output. The passive fiber 10 is connected to a band-pass filter 11 with a center wavelength of 1064 nm and a bandwidth of 2 nm. A 1×2 coupler 12 with a splitting ratio of 10:90 is used for splitting between the bandpass filter 11 and the 60:40 coupler 5 . After the signal light passes through the ring cavity for one cycle, 10% of the light in the coupler 12 is used as the output, and the remaining 90% of the light is used as the input for the next round of iterations.

The device connecting the NALM ring to the main ring is a 2x2 coupler5 with a split ratio of 60:40. As shown in FIG. 3 , the 2×2 coupler 5 is a connector connecting the main loop and the NALM loop, and the structure is an input end 5-1, a 60% split end 5-2, a 40% split end 5-3 and Outputs 5-4. The light of the main loop enters from the input end 5-1 of the coupler 5, and the coupler splits the input light into 60:40 light from the 60% beam splitting end 5-2 and the 40% beam splitting end 5-3 respectively. Enter the NALM loop. The 60% beam splitting end 5-2 is connected to the forward pumped amplifier gain fiber 3, so that the counterclockwise signal light entering the NALM is first amplified counterclockwise and then passes through the single-mode fiber to obtain a larger nonlinear phase shift amount ; 40% beam splitting end 5-3 is connected to ordinary single-mode fiber 4 (SMF). The clockwise beam first passes through single-mode fiber 4 and then passes through gain amplification, which also accumulates a certain amount of nonlinear phase shift. The amount of nonlinear phase shift is small. The counterclockwise beam at the 60% split end 5-2 and the clockwise beam at the 40% split end 5-3 will return to the coupler for interferometric modulation after one revolution in the NALM ring, because the two beams are different Intensity and phase enable the reflectivity in the coupler to obtain a stronger modulation depth.

A pulse detection device is connected to the 10% output of the 10:90 coupler 12 in the oscillating cavity. The pulse detection device consists of an isolator 13 (ISO-2), a 1×2 coupler 14 of 50:50, a 20G wideband oscilloscope 15 and a spectrometer 16 . The isolator 13 (ISO-2) is connected to the 10% output end of the 10:90 coupler 12 of the main loop of the oscillation cavity. Its function is to prevent the output light from being reversed due to the influence of other conditions, which will affect the mode locking effect in the oscillation cavity. , to protect the oscillatory cavity. The 50:50 coupler 14 is connected to the isolator 13 (ISO-2), and the output beam is split into two beams of equal intensity, which are respectively input to the 20G broadband oscilloscope 15 and the spectrometer 16. The 20G broadband oscilloscope 15 is used to test the laser of the mode-locked waveform and repetition rate, the spectrometer 16 is used to measure the mode-locked spectrum.

In the experiment, the pump power of the oscillation cavity was adjusted. When the powers of the pump diode pump 1 (LD-1) and diode pump 7 (LD-2) in the NALM and the main loop were 130mW and 180mW, respectively, a stable The single-pulse mode-locking is shown in Figure 4, and the pulse repetition frequency is 1.14MHz. A single pulse waveform can be detected using a photodetector and displayed on a 20G wideband oscilloscope 15, as shown in Figure 5, and the pulse's full width at half maximum (FWHM) is 91.25ps. It is worth mentioning that the two pump powers can be adjusted in the range of 90-130mW and 160-180mW at the same time without affecting the single-pulse mode-locking state. Use the spectrometer 16 to collect the spectrum of the output pulse. The logarithmic spectrum and the linear spectrum (included) of the pulse are shown in FIG. 6 , the center wavelength of the pulse is 1064 nm, and the spectral width is 10.4 nm. As can be seen from the linear spectrogram, the edges of the spectrum appear sharply shaped, which is due to the mode-locking of the oscillating cavity under total positive dispersion.

The utility model can obtain self-starting mode locking and directly output lower repetition frequency, avoid the loss of energy and frequency signal caused by the seed laser in the AOM modulation process, maintain high reliability and stability, and can be compatible with subsequent Pulse power amplifier system has better compatibility. The designed all-fiber laser is less affected by the use environment and is more compact and simple.

The output center wavelength of the utility model is 1064 nm, and the existing commercial lasers are mostly 1030 nm, and this wavelength is incompatible with the existing large mode field amplifier of 1064 wavelength. The active gain fiber used in the laser has a higher energy conversion rate at the wavelength of 1064, so the output of the seed laser has better compatibility with the large mode field fiber amplifier.

The output of the utility model has a low repetition frequency of 1.14MHz and a high chirp pulse energy output. Compared with the 10-100MHz pulse generated by the previous mode-locking technology, the frequency often needs to be reduced by the acousto-optic modulation system before amplification. The output pulse does not need to be down-converted by AOM, which can better maintain the pulse quality in the amplification process and enhance the compactness of the fiber laser.

The above-mentioned embodiments only represent several embodiments of the present invention, and the descriptions thereof are specific and detailed, but should not be construed as limiting the scope of the present invention. It should be pointed out that for those of ordinary skill in the art, some modifications and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent for this utility model shall be subject to the appended claims.

Claims (8)

1. a 1064nm self-mode-locking polarization-maintaining ytterbium-doped fiber laser of low repetition frequency, is characterized in that, comprises a NALM mode-locking, a full polarization-maintaining laser oscillation cavity and a pulse testing device, adopts " 8 figure " structure, One end of the oscillating cavity of the all-polarization-maintaining laser is mode-locked to the NALM, and the other end of the oscillating cavity of the all-polarization-maintaining laser is connected to the pulse testing device. 2. the 1064nm self-mode-locking polarization-maintaining ytterbium-doped fiber laser of low repetition frequency according to claim 1, is characterized in that, the device connecting NALM mode-locking and full polarization-maintaining laser oscillation cavity is that the beam splitting ratio is 60:40 2x2 couplers. 3. The 1064nm self-mode-locking and polarization-maintaining ytterbium-doped fiber laser of claim 2, wherein the NALM mode-locking comprises the first 980 diode pump, the first 1064 nm wavelength division multiplexer, The first gain fiber and the first ordinary single-mode fiber; the pump light pumped by the first 980 diode enters the first 1064nm wavelength division multiplexer, that is, the signal light and the pump light are in the same direction, and the forward pumping method is adopted. The forward direction of a 1064nm wavelength division multiplexer is connected to the first gain fiber in the ring, and the first gain fiber provides optical signal power and phase gain for NALM mode locking; the reverse connection of the first 1064nm wavelength division multiplexer The first common single-mode fiber, the first common single-mode fiber is used to control the self-phase modulation effect generated in the NALM mode locking of light. 4. The low repetition frequency 1064nm self-mode-locked polarization-maintaining ytterbium-doped fiber laser according to claim 3, wherein the full polarization-maintaining laser oscillation cavity comprises a first isolator, a second 980 diode pump, a second Two 1064nm wavelength division multiplexers, second gain fiber, second ordinary single-mode fiber, 2nm bandpass filter, and 10:90 1×2 coupler; second 980 diode pump, second 1064 wavelength division The multiplexer and the second gain fiber form a reverse amplifier; the first isolator is connected to the output end of the 60:40 2×2 coupler, and its function is to maintain the unidirectional transmission of the output light of the NALM mode-locking, and isolate the reverse amplifier. The reverse light transmission that may be caused when inputting to the amplifier avoids the interference of the reverse light on the loop beam and affects the laser mode locking, and at the same time plays the role of protecting the device; the reverse amplifier connected to the first isolator adopts the reverse direction In the pumping method, the second 980 diode is pumped through the second 1064nm wavelength division multiplexer and then connected to the second gain fiber; the output light of the NALM mode-locking is amplified by the reverse amplifier to obtain the same light intensity. At the same time, the nonlinear dispersion is accumulated; the output end of the inverse amplifier, that is, the other end of the second 1064nm wavelength division multiplexer, is connected to the second ordinary single-mode fiber, and the self-phase modulation generated by the light transmission in the second ordinary single-mode fiber passes through Phase matching with the dispersion generated by the aforementioned structure can realize the self-mode locking of the laser; at the same time, the fiber length of the second common single-mode fiber lengthens the laser oscillation cavity. According to the inverse relationship between the laser repetition frequency and the oscillation cavity cavity length, it can make The repetition frequency of the laser is reduced to realize the laser pulse output of low repetition frequency; the second ordinary single-mode fiber is connected to a bandpass filter with a center wavelength of 1064nm and a bandwidth of 2nm; A 1×2 coupler with a beam splitting ratio of 10:90 is used for splitting between the ×2 couplers; after the signal light passes through the ring cavity for one week, 10% of the light in the 10:90 1×2 coupler is used as the output, and the rest 90% of the light is used as the input for the next round of iterations. 5 . The 1064 nm self-mode-locked polarization-maintaining ytterbium-doped fiber laser with low repetition frequency according to claim 4 , wherein the model adopted by the first gain fiber and the second gain fiber is CorActive Yb 401-PM. 6 . 6. the 1064nm self-mode-locking polarization-maintaining ytterbium-doped fiber laser of low repetition frequency according to claim 4, is characterized in that, the model that described first common single-mode fiber and the second common single-mode fiber adopt is Nufern PM- 980. 7 . The 1064 nm self-mode-locked polarization-maintaining ytterbium-doped fiber laser of claim 4 , wherein the 60:40 2×2 coupler has an input end and a 60% beam splitter end. 8 . , 40% beam splitting end and output end; the light of the full polarization maintaining laser oscillation cavity enters from the input end of the 60:40 2×2 coupler, and the 60:40 2×2 coupler splits the input light into 60: The light of 40% enters the NALM mode locking from the 60% beam splitting end and the 40% beam splitting end respectively; the 60% beam splitting end is connected to the first gain fiber of the forward pump, so that the counterclockwise signal entering the NALM mode locking The light is amplified counterclockwise first and then passes through the single-mode fiber to obtain a larger nonlinear phase shift; 40% of the split ends are connected to the first ordinary single-mode fiber, and the clockwise beam first passes through the first ordinary single-mode fiber and then passes through The gain amplification also accumulates the nonlinear phase shift, but it is less than the nonlinear phase shift at the 60% end; the counterclockwise beam at the 60% split end and the clockwise beam at the 40% beam split end will be transmitted for one revolution in the NALM mode locking. Going back to the 60:40 2×2 coupler for interferometric modulation, the reflectivity in the 60:40 2×2 coupler obtains a stronger modulation depth due to the different light intensities and phases of the two beams. 8 . The low repetition frequency 1064nm self-mode-locked polarization-maintaining ytterbium-doped fiber laser according to claim 4 , wherein the pulse detection device comprises a second isolator, a 50:50 1×2 coupler, a 20G Broadband oscilloscope and spectrometer; the second isolator is connected to the 10% output end of the 10:90 1×2 coupler of the full polarization-maintaining laser oscillation cavity, and its function is to prevent the output light from being reversely transmitted due to the influence of the conditions. It affects the mode locking effect and protects the oscillating cavity; the 50:50 1×2 coupler is connected after the second isolator, and the output beam is split into two beams of equal light intensity, which are respectively input to a 20G broadband oscilloscope and a spectrometer; 20G broadband An oscilloscope is used to test the mode-locked waveform and repetition rate of the laser, and a spectrometer is used to measure the mode-locked spectrum.

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