Method for achieving pulse width less than 50 femtoseconds in polarization-maintaining all-fiber laser

Abstract

A method for achieving a pulse width of less than 50 femtoseconds in a polarization-maintaining all-fiber laser involves an ultrafast laser. In order to solve the problem of wide pulse width in nonlinear amplifying ring mirror passively mode-locked fiber lasers, a chirped mirror is used instead of a traditional return mirror to construct a polarization-maintaining all-fiber laser resonator. In view of the high peak power characteristics of zero-dispersion mode-locked, the length of the compressed fiber outside the cavity is adjusted to achieve direct output of a seed source pulse width of less than 50 femtoseconds. The chirped mirror has a reflectivity R>70% in the wavelength range of 1450-1670nm; a transmittance T>90% at a wavelength of 974nm; and a dispersion>1000fs at a wavelength of 1560nm±30nm. 2 Adjusting the net dispersion of the laser resonant cavity to near zero dispersion not only reduces the difficulty of welding short fiber pigtail devices, but also improves the reliability and stability of the laser with a fully polarization-maintaining structure, achieving the characteristics of narrow pulses and wide spectrum, and can be applied in the fields of additive manufacturing and optical communication systems.

Description

Technical Field

[0001] The invention relates to the technical field of ultrafast lasers, and in particular to a method for realizing a pulse width less than 50 femtoseconds in a polarization-maintaining all-fiber laser. Background Art

[0002] In the application of ultrafast lasers for additive manufacturing, the narrower the pulse width, the higher the processing accuracy. For example, femtosecond lasers can achieve greater processing accuracy than picosecond lasers. In the research of ultrafast lasers, narrow pulse width and high power have always been two important research directions. Before the 1980s, ultrafast lasers based on dyes and crystals as gain media had achieved pulse widths as narrow as a dozen femtoseconds. Fiber lasers have the characteristics of compact structure, good heat dissipation characteristics and high-quality transverse modes. However, some fiber lasers still face the problem of relatively wide pulse widths. Although pulse widths less than 50fs can be achieved based on nonlinear polarization rotation technology, due to the non-polarization-maintaining resonant cavity structure, the stability against the external environment is not as good as the polarization-maintaining structure. In comparison, the method of nonlinear amplifying ring mirror based on polarization-maintaining structure can achieve high-stability laser output, but at present, it is still difficult to achieve a pulse width less than 50fs, especially for all-fiber structures.

[0003] Adjusting the net dispersion in the cavity to near zero dispersion helps to obtain a narrower pulse width. In the 1550nm band, the fiber dispersion is usually negative, so a gain fiber with positive dispersion is required to implement dispersion compensation. If the gain fiber is too long, the reabsorption effect will cause the central wavelength to redshift and make the mode locking unstable. If the gain fiber is too short, it will be difficult to compensate for the negative dispersion in the cavity. In order to implement the all-fiber structure, the devices have their own pigtails that can be fused. Even if the device length is cut to the shortest, the net negative dispersion in the cavity is still large. Taking the PM 1500 fiber from Nufern as an example, even if the pigtail length of all devices is cut to the limit that can be fused, the negative dispersion in the cavity is ~-30000fs. 2 If positive dispersion gain fiber is used, the above gain conditions must be met, and the net dispersion in the cavity is still ~-2000fs 2 . Summary of the invention

[0004] The first purpose of the present invention is to provide a nonlinear amplifying ring mirror mode-locked polarization-maintaining all-fiber laser resonator to solve the problem of wide pulse width in nonlinear amplifying ring mirror passive mode-locked fiber laser.

[0005] The second purpose of the present invention is to provide a method for achieving a pulse width of less than 50 femtoseconds in a polarization-maintaining all-fiber laser. In view of the high peak power characteristics of zero-dispersion mode-locked, based on the polarization-maintaining all-fiber laser resonator, a nonlinear amplifying ring mirror mode-locked technology is used to directly achieve a pulse width of less than 50 fs.

[0006] The invention provides a polarization-maintaining all-fiber laser resonator, comprising a pump source, a polarization-maintaining wavelength division multiplexer, a gain fiber, a polarization-maintaining fiber phase shifter, a polarization-maintaining coupler, a chirp mirror and a compression fiber; the pump source is a semiconductor laser output by a polarization-maintaining fiber, and the polarization-maintaining wavelength division multiplexer comprises a pump end, a common end and a signal end; the pump end of the polarization-maintaining wavelength division multiplexer is connected to a pigtail fiber of a laser, the common end of the polarization-maintaining wavelength division multiplexer is fused to one end of the gain fiber, the other end of the gain fiber is fused to the polarization-maintaining fiber phase shifter, the other end of the polarization-maintaining fiber phase shifter is fused to one arm of a polarization-maintaining coupler, and the other arm of the polarization-maintaining coupler is fused to the signal end of the polarization-maintaining wavelength division multiplexer; the common end of the polarization-maintaining coupler is fused to the chirp mirror of the polarization-maintaining fiber, and the non-common end of the polarization-maintaining coupler is fused to the compression fiber; the generated laser is output out of the cavity through the compression fiber.

[0007] The pump source may be a single-mode semiconductor laser with a wavelength of 974 nm and a maximum output power of 500 mW, which is used to provide energy for the femtosecond laser.

[0008] The polarization-maintaining wavelength division multiplexer uses a polarization-maintaining fiber pigtail, the fiber type is PM1550 fiber, which is used to couple the pump light into the gain fiber.

[0009] The benefit optical fiber is a polarization-maintaining gain optical fiber, doped with Er 3+ ions, used to generate gain in the 1.5μm band.

[0010] The phase shift value of the polarization-maintaining optical fiber phase shifter is π / 2 or π / 4, and a polarization-maintaining optical fiber pigtail is used to introduce the phase shift.

[0011] The polarization-maintaining coupler uses a polarization-maintaining coupler with a splitting ratio of 45:55, which is used to generate interference to form a locking mechanism.

[0012] The parameter index of the chirped mirror is that the reflectivity R>70% in the wavelength range of 1450-1670nm; the transmittance T>90% in the wavelength range of 974nm; the dispersion>1000fs in the wavelength range of 1560nm±30nm. 2 , which is not only used to return the signal light, but also to compensate for the intracavity dispersion.

[0013] The compression optical fiber is a common single-mode optical fiber, a positive dispersion optical fiber, and can be OFS980 optical fiber. The length of the compression optical fiber can be 1 to 3 meters, and is used to compress the pulse width.

[0014] The pigtails of all devices in the laser resonant cavity are polarization-maintaining fibers, which have high stability against external environments.

[0015] The present invention provides a method for realizing a pulse width of less than 50 femtoseconds in a polarization-maintaining all-fiber laser. The method comprises the following steps: firstly constructing the polarization-maintaining all-fiber laser resonator of nonlinear amplifying ring mirror mode locking, adjusting the net dispersion to zero dispersion by using a chirped mirror in the resonant cavity of the laser resonator, using a positive dispersion fiber as a compression fiber outside the resonant cavity to broaden the spectrum, realizing spectrum broadening and pulse compression by regulating the type and length of the compression fiber, and realizing pulse output with a pulse width of less than 50 fs.

[0016] The chirped mirror is used to adjust the net dispersion in the cavity to zero dispersion. The specific steps are: using the chirped mirror to replace the traditional return mirror, the chirped mirror has a reflectivity R>70% in the wavelength range of 1450-1670nm; a transmittance T>90% in the wavelength range of 974nm; and a dispersion>1000fs in the wavelength range of 1560nm±30nm. 2 The tail fiber of the chirped mirror is a polarization-maintaining fiber, which is used to construct a laser resonant cavity. The net dispersion of the laser resonant cavity is close to zero dispersion, and the net dispersion range in the cavity is -2000fs. 2 to 0;

[0017] The positive dispersion fiber is used as a compression fiber outside the resonant cavity to broaden the spectrum. The specific steps may be: using the compression fiber at one end of the fusion splice outside the cavity to broaden the spectrum, for mode locking near zero dispersion, exciting nonlinear effects, thereby causing spectrum broadening, and high peak power pulses can directly stimulate self-phase modulation to broaden the spectrum to compress the pulse width without amplification;

[0018] The refractive index distribution of a Gaussian pulse in the time domain is expressed as:

[0019]

[0020] Among them, n0 is the refractive index of the material itself, n2 is the nonlinear refractive index coefficient of the material, I(t) is the light field intensity of the pulse, I0 is the maximum light field intensity of the pulse, τ p is the pulse width;

[0021] The phase shift Φ(t) accumulated by the pulse in the compressed optical fiber over a transmission distance l is expressed as:

[0022]

[0023] Where l is the length of the compressed fiber, c is the speed of light, Φ0 is the linear phase shift of the pulse accumulation, Φ NL is the nonlinear phase shift accumulated by the pulse.

[0024] Taking the derivative of Φ(t) with respect to time t, we get the instantaneous frequency ω(t):

[0025]

[0026] The frequency broadening amount is:

[0027]

[0028] Among them, Δω(t) is the frequency broadening amount, and ω0 is the center frequency. From the duality of the time domain and frequency domain Fourier transform, it can be seen that the periodic modulation of amplitude or phase in the time domain will cause the generation of new frequency components in the frequency domain.

[0029] From the duality of time domain and frequency Fourier transform, it can be seen that the periodic modulation of amplitude or phase in the time domain will cause the generation of new frequency components in the frequency domain, that is, the broadening of the laser spectrum.

[0030] The regulated compression optical fiber uses the OFS980 optical fiber of Nufern Company with positive dispersion at a wavelength of 1550nm as the compression optical fiber with a length of 1 to 3m, and regulates the corresponding pulse width to less than 50fs. At this time, the spectrum width is widened to more than 50nm.

[0031] The present invention uses a mode-locked laser using nonlinear amplifying ring mirror technology. When the pump power reaches the mode-locking threshold, the mode-locking can be self-started, but it is in a multi-pulse state. At this time, reducing the pump power can observe the switch from the multi-pulse state to the single-pulse state, realizing the basic repetition frequency mode-locking. Compared with multi-pulses, this is a stable mode-locking state. The final generated laser is directly output from the non-common end of the coupler to the outside of the cavity.

[0032] The present invention is based on a method for realizing a pulse width of less than 50 fs in a polarization-maintaining all-fiber structure under a common tuning mechanism inside and outside the cavity. Compared with the prior art, the present invention has the following advantages:

[0033] (1) Since the system works near zero dispersion, the nonlinear effect introduced by the passive optical fiber outside the seed source cavity also becomes the dominant introduction to achieve narrow pulses. The width of the seed source output pulse can be tuned over a wide range by regulating the compression optical fiber, such as from 170fs to 50fs. The chirped mirror used has a wavelength of 1560nm±30nm and a dispersion value of >1000fs. 2 , changing the dispersion value compensates for the positive dispersion in the cavity, thereby bringing the net dispersion in the cavity close to 0, thereby jointly achieving a pulse output with a pulse width of <50fs;

[0034] (2) The laser is a fully polarization-maintaining structure, which has strong stability against external environments and improves the reliability and stability of the system;

[0035] (3) The process adopted by the present invention is simple and the device is compact, which is conducive to large-scale production and application.

[0036] (4) The present invention not only reduces the difficulty of welding short pigtail devices, but also improves the reliability and stability of the laser with a fully polarization-maintaining structure. The narrow pulse and wide spectrum characteristics finally achieved make it applicable to the fields of additive manufacturing and optical communication systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 To achieve a polarization-maintaining all-fiber laser resonant cavity structure with a pulse width of less than 50fs.

[0038] Figure 2 Graph showing group delay dispersion and transmittance of the chirped mirror in Example 1.

[0039] Figure 3 This is the autocorrelation curve of the 49fs pulse obtained in Example 1 using the invention. DETAILED DESCRIPTION

[0040] For a better understanding of the present invention, the present invention is further described below in conjunction with embodiments, but the scope of protection claimed by the present invention is not limited to the scope represented by the embodiments.

[0041] The embodiment of the present invention provides a polarization-maintaining all-fiber laser resonator, comprising a pump source 1, a polarization-maintaining wavelength division multiplexer 2, a gain fiber 3, a polarization-maintaining fiber phase shifter 4, a polarization-maintaining coupler 5, a chirped mirror 6 and a compression fiber 7; the pump source 1 is a semiconductor laser outputted by a polarization-maintaining fiber, and the polarization-maintaining wavelength division multiplexer 2 comprises a pump end, a common end and a signal end; the pump end of the polarization-maintaining wavelength division multiplexer 2 is connected to a pigtail of the laser, the common end of the polarization-maintaining wavelength division multiplexer 2 is fused to one end of the gain fiber, the other end of the gain fiber 3 is fused to the polarization-maintaining fiber phase shifter 4, the other end of the polarization-maintaining fiber phase shifter 4 is fused to one arm of the polarization-maintaining coupler 5, and the other arm of the polarization-maintaining coupler 5 is fused to the signal end of the polarization-maintaining wavelength division multiplexer 2; the common end of the polarization-maintaining coupler 5 is fused to the chirped mirror 6 of the polarization-maintaining fiber, and the non-common end of the polarization-maintaining coupler 5 is fused to the compression fiber; the generated laser 8 is output out of the cavity through the compression fiber.

[0042] The pump source 1 may be a single-mode semiconductor laser with a wavelength of 974 nm and a maximum output power of 500 mW, which is used to provide energy for the femtosecond laser.

[0043] The polarization-maintaining wavelength division multiplexer 2 uses a polarization-maintaining optical fiber pigtail, and the optical fiber type is PM1550 optical fiber, which is used to couple the pump light into the gain optical fiber.

[0044] The benefit optical fiber 3 is a polarization-maintaining gain optical fiber, doped with Er 3+ ions, used to generate gain in the 1.5μm band.

[0045] The phase shift value of the polarization-maintaining fiber phase shifter 4 is π / 2 or π / 4, and a polarization-maintaining fiber pigtail is used to introduce the phase shift.

[0046] The polarization-maintaining coupler 5 uses a polarization-maintaining coupler with a splitting ratio of 45:55, which is used to generate interference to form a locking mechanism.

[0047] The parameter index of the chirped mirror 6 is that the reflectivity R>70% in the wavelength range of 1450-1670nm; the transmittance T>90% in the wavelength range of 974nm; the dispersion>1000fs in the wavelength range of 1560nm±30nm. 2 , which is not only used to return the signal light, but also to compensate for the intracavity dispersion.

[0048] The compression optical fiber 7 is a common single-mode optical fiber, a positive dispersion optical fiber, and can be OFS980 optical fiber. The length of the compression optical fiber 7 can be 1 to 3 meters, and is used to compress the pulse width.

[0049] The pigtails of all devices in the laser resonant cavity are polarization-maintaining fibers, which have high stability against external environments.

[0050] Example 1

[0051] This embodiment realizes ultrashort pulse laser output with a pulse width of 49 fs in a polarization-maintaining all-fiber ultrafast laser. A chirped mirror is used inside the laser resonant cavity to adjust the net dispersion to zero, and a positive dispersion fiber is used outside the cavity to broaden the spectrum, which together achieves pulse output with a pulse width of 49 fs.

[0052] First, a nonlinear amplifying ring mirror-locked polarization-maintaining all-fiber laser resonator is constructed. Figure 1 As shown, the pump source is composed of a semiconductor laser 1 outputted by a polarization-maintaining optical fiber with a wavelength of 974nm, and the maximum output power is 500mW. It is fused to the pump end of the polarization-maintaining wavelength division multiplexer 2 to excite the polarization-maintaining erbium-doped gain fiber 3 fused to the common end of the polarization-maintaining wavelength division multiplexer. The other end of the polarization-maintaining erbium-doped gain fiber 3 is fused to the polarization-maintaining optical fiber phase shifter 4, which can provide a π / 2 phase shift. The other end of the polarization-maintaining optical fiber phase shifter 4 is fused to the 45% arm of the 45:55 polarization-maintaining coupler 5, and the 55% arm of the polarization-maintaining coupler 5 is fused to the signal end of the polarization-maintaining wavelength division multiplexer 2. The common end of the polarization-maintaining coupler 5 is fused to the chirped mirror 6 of the polarization-maintaining optical fiber, and the dispersion value of the chirped mirror 6 in the wavelength range of 1525 to 1600nm is about +1000fs. 2 , in the wavelength range of 1500-1600nm, the reflectivity is 80%, and the total thickness of the film is 17.76μm. The generated laser 8 is output from the cavity through the compression fiber. The test transmittance curve and group velocity dispersion are as follows Figure 2 shown.

[0053] One end of the passive compression fiber 7 is fused outside the cavity to broaden the spectrum; the non-common end of the polarization-maintaining coupler 5 is fused with the compression fiber 7. The chirped mirror 6 is used to replace the traditional return mirror. The tail fiber of the chirped mirror is a polarization-maintaining fiber. The laser resonant cavity is constructed with it. The net dispersion of the laser resonant cavity is close to zero dispersion, and the net dispersion range in the cavity is -2000fs. 2 to 0.

[0054] For mode locking near zero dispersion, since the pulse width is very narrow, even if the seed source output power is only a few mW, a sufficiently high peak power will stimulate nonlinear effects, thereby causing spectral broadening. The refractive index distribution of a Gaussian pulse in the time domain is expressed as:

[0055]

[0056] Among them, n0 is the refractive index of the material itself, n2 is the nonlinear refractive index coefficient of the material, I(t) is the light field intensity of the pulse, I0 is the maximum light field intensity of the pulse, τ p is the pulse width;

[0057] The phase shift Φ(t) accumulated by the pulse in the compressed optical fiber over a transmission distance l is expressed as:

[0058]

[0059] Where l is the length of the compressed fiber, c is the speed of light, Φ0 is the linear phase shift of the pulse accumulation, Φ NL is the nonlinear phase shift accumulated by the pulse.

[0060] Taking the derivative of Φ(t) with respect to time t, we get the instantaneous frequency ω(t):

[0061]

[0062] The frequency broadening amount is:

[0063]

[0064] Among them, Δω(t) is the frequency broadening amount, and ω0 is the center frequency. From the duality of the time domain and frequency domain Fourier transform, it can be seen that the periodic modulation of amplitude or phase in the time domain will cause the generation of new frequency components in the frequency domain.

[0065] Since the system works near zero dispersion, the nonlinear effect introduced by the compression fiber 7 (passive fiber) outside the seed source cavity becomes the dominant factor in achieving narrow pulses. By adjusting the compression fiber 7, the width of the seed source output pulse can be tuned over a wide range. The compression fiber 7 used in the embodiment adopts the OFS980 fiber of Nufern Company, which provides a dispersion of 1.8 fs at 1550 nm. 2 / mm, the length used is 170~200cm.

[0066] When the power of the 974nm laser pump source is directly turned on to 300mW, the system can achieve mode-locked self-starting, and the oscilloscope monitors the multi-pulse state. When the pump power is reduced to 235mW, the laser switches back to single-pulse operation. Due to the compression fiber outside the cavity, a pulse width of 49fs can be obtained. Figure 3This is the autocorrelation curve of the test. It can be seen that the pulse has no base, the corresponding mode-locked spectrum has obvious soliton sidebands, and the 3dB spectrum width is about 57nm.

[0067] Since the optical fiber used in the cavity and the passive device pigtail are both polarization-maintaining optical fibers, the laser system of the present invention can not only achieve an ultra-short pulse width, but also greatly improve the stability and reliability of the system against the external environment, so that it can be used in extreme environments.

[0068] Example 2

[0069] Similar to Example 1, the difference is that the positive dispersion value of the chirped mirror is increased, and the length of the extracavity compression fiber is adjusted to achieve a narrower pulse width in the polarization-maintaining all-fiber ultrafast laser:

[0070] For the chirped mirror parameters, at a wavelength of 1560nm±30nm, the dispersion is improved to +1500fs 2 , in the wavelength range of 1500-1600nm, its reflectivity R = 90%. In addition, the compression fiber for spectrum broadening outside the cavity uses Nufern's OFS980 fiber, the length of which is increased to 200-300cm, and the pulse can be further compressed to ~40fs.

[0071] By utilizing the above improvements, the net dispersion of the polarization-maintaining laser resonator is further close to 0, the self-phase modulation has an effect on spectrum broadening, and pulse compression can be further achieved. Therefore, the present invention can flexibly control system parameters, thereby achieving stable ultrashort pulse mode-locked laser output. In addition, this environmentally stable laser design will expand its use under extreme conditions.

[0072] The present invention uses a chirped mirror to replace the traditional return mirror to construct a fully polarization-maintaining laser resonant cavity. In view of the high peak power characteristics of zero-dispersion mode-locked mode, the length of the compressed optical fiber outside the cavity is adjusted to achieve the direct output of the seed source with a pulse width of less than 50 femtoseconds. The chirped mirror has a wavelength range of 1450 to 1670 nm, and its reflectivity R>70%; its transmittance T>90% at a wavelength of 974 nm; and a dispersion>1000 fs at a wavelength of 1560 nm±30 nm. 2 . In the nonlinear amplifying ring mirror mode locking, the net dispersion in the cavity is achieved near zero dispersion through dispersion compensation. For the mode-locked laser near zero dispersion, since the peak power of the narrow pulse is high enough, the nonlinear effect of the passive optical fiber outside the cavity can be stimulated without amplification. Therefore, regulating the type and length of the compression optical fiber can further achieve spectral broadening and pulse compression. The present invention combines the common regulation technology inside and outside the cavity to directly obtain a pulse width of less than 50fs in the polarization-maintaining laser seed source, which will promote the application of this laser light source in additive manufacturing and optical communication systems.

[0073] The above embodiments are only preferred embodiments of the present invention and cannot be considered to limit the scope of the present invention. All equivalent changes and improvements made within the scope of the present invention should still fall within the scope of the present invention.

Claims

1. A polarization-maintaining all-fiber laser resonator based on nonlinear amplifying ring mirror mode locking, characterized in that The invention comprises a pump source, a polarization-maintaining wavelength division multiplexer, a gain fiber, a polarization-maintaining fiber phase shifter, a polarization-maintaining coupler, a chirp mirror and a compression fiber; the pump source is a semiconductor laser outputted by a polarization-maintaining fiber, and the polarization-maintaining wavelength division multiplexer comprises a pump end, a common end and a signal end; the pump end of the polarization-maintaining wavelength division multiplexer is connected to the pigtail of the laser, the common end of the polarization-maintaining wavelength division multiplexer is fused to one end of the gain fiber, the other end of the gain fiber is fused to the polarization-maintaining fiber phase shifter, the other end of the polarization-maintaining fiber phase shifter is fused to one arm of the polarization-maintaining coupler, and the other arm of the polarization-maintaining coupler is fused to the signal end of the polarization-maintaining wavelength division multiplexer; the common end of the polarization-maintaining coupler is fused to the chirp mirror of the polarization-maintaining fiber, and the non-common end of the polarization-maintaining coupler is fused to the compression fiber; the generated laser is output out of the cavity through the compression fiber.

2. A polarization-maintaining all-fiber laser resonator based on nonlinear amplifying loop mirror mode locking as claimed in claim 1, characterized in that The phase shift value of the polarization-maintaining optical fiber phase shifter is π / 2 or π / 4, and a polarization-maintaining optical fiber pigtail is used.

3. A polarization-maintaining all-fiber laser resonator based on nonlinear amplifying loop mirror mode locking as claimed in claim 1, characterized in that The chirped mirror's pigtail is a polarization-maintaining fiber. The chirped mirror's parameter indicators are: reflectivity R>70% in the wavelength range of 1450-1670nm; transmittance T>90% in the wavelength range of 974nm; dispersion>1000fs in the wavelength range of 1560nm±30nm. 2 .

4. A polarization-maintaining all-fiber laser resonator based on nonlinear amplifying ring mirror mode locking as claimed in claim 1, characterized in that The compression optical fiber adopts positive dispersion optical fiber, and the length of the compression optical fiber is 1 to 3 meters.

5. A method for achieving a pulse width of less than 50 femtoseconds in a polarization-maintaining all-fiber laser, characterized in that First, a nonlinear amplifying ring mirror-locked polarization-maintaining all-fiber laser resonator as claimed in claim 1 is constructed, and a chirped mirror is used in the resonant cavity of the laser resonator to adjust the net dispersion to zero dispersion. A positive dispersion fiber is used as a compression fiber outside the resonant cavity to broaden the spectrum. By adjusting the type and length of the compression fiber, spectrum broadening and pulse compression are achieved, and a pulse output with a pulse width less than 50 fs is achieved.

6. A method for achieving a pulse width of less than 50 femtoseconds in a polarization-maintaining all-fiber laser as claimed in claim 5, characterized in that The specific steps of adjusting the net dispersion to zero dispersion by using a chirped mirror are: using a chirped mirror to replace a traditional return mirror, the chirped mirror has a reflectivity R>70% in the wavelength range of 1450-1670nm; a transmittance T>90% in the wavelength range of 974nm; and a dispersion>1000fs in the wavelength range of 1560nm±30nm. 2 ; Use it to construct a laser resonant cavity, the net dispersion of the laser resonant cavity is close to zero dispersion, and the net dispersion range in the cavity is -2000fs 2 to 0.

7. A method for achieving a pulse width of less than 50 femtoseconds in a polarization-maintaining all-fiber laser as claimed in claim 5, characterized in that The specific steps of using positive dispersion fiber as compression fiber to broaden the spectrum outside the resonant cavity are: using the compression fiber fused at one end outside the cavity to broaden the spectrum, locking the mode near zero dispersion, stimulating nonlinear effect, thereby causing spectrum broadening, and directly stimulating self-phase modulation of high peak power pulses without amplification to broaden the spectrum and compress the pulse width; The refractive index distribution of a Gaussian pulse in the time domain is expressed as: Among them, n0 is the refractive index of the material itself, n2 is the nonlinear refractive index coefficient of the material, I(t) is the light field intensity of the pulse, I0 is the maximum light field intensity of the pulse, τ p is the pulse width; The phase shift Φ(t) accumulated by the pulse in the compressed optical fiber over a transmission distance l is expressed as: Where l is the transmission distance of the pulse in the compressed optical fiber, c is the speed of light, Φ0 is the linear phase shift accumulated by the pulse, Φ NL is the nonlinear phase shift accumulated by the pulse; Taking the derivative of Φ(t) with respect to time t, we get the instantaneous frequency ω(t): The frequency broadening amount is: Among them, Δω(t) is the frequency broadening amount, and ω0 is the center frequency. From the duality of Fourier transform in time domain and frequency domain, it can be seen that periodic modulation of amplitude or phase in time domain will cause the generation of new frequency components in frequency domain.

8. A method for achieving a pulse width of less than 50 femtoseconds in a polarization-maintaining all-fiber laser as claimed in claim 6, characterized in that The pigtails of all devices in the laser resonant cavity are polarization-maintaining optical fibers, which have high stability against external environment.

9. The method for achieving a pulse width of less than 50 femtoseconds in a polarization-maintaining all-fiber laser according to claim 5, characterized in that: A pulse width of less than 50fs and a 3dB spectral bandwidth of greater than 50nm are achieved in a polarization-maintaining all-fiber structure.

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