A broadband fiber laser nonlinear amplification device and its working method

Abstract

This invention discloses a broadband fiber laser nonlinear amplification device and its operating method. The device includes a front-end light source, a first isolator, a nonlinear fiber laser amplification stage, a second isolator, a broadband fiber laser amplification stage, and a compressor connected in sequence. The isolator ensures unidirectional transmission between amplifiers. The picosecond or femtosecond light source at the front end passes through the nonlinear fiber laser amplification, and under the action of strong nonlinearity, the spectrum is broadened to a bandwidth of tens or even hundreds of nanometers. This process gives the pulse linear chirp in the time domain and breaks through the active gain bandwidth limit in the frequency domain, with a chirped pulse width of only tens of femtoseconds. The broadband fiber laser amplification stage utilizes large mode area fiber, which can achieve pulse energy amplification while maintaining the broadband spectrum and linear chirp of the laser. After passing through the compressor composed of grating pairs or prism pairs, the pulse width can still be compressed to tens of femtoseconds.

Description

Technical Field

[0001] This invention relates to the field of fiber laser technology, and in particular to ultrafast fiber laser technology, specifically to a broadband fiber laser nonlinear amplification device and its operating method. Background Technology

[0002] Ultrafast lasers have broad application prospects in both basic science and industry, such as strong-field physics, ultra-high-speed phenomenon detection, high-precision measurement and processing, and biological microscopy imaging. Compared with solid-state lasers, fiber lasers have advantages such as better beam quality, higher conversion efficiency, convenient thermal management, compact structure, and flexible operation. To obtain high-energy, narrow-pulse-width ultrafast lasers, chirped pulse amplification (CPA) technology has become a commonly used solution in femtosecond fiber systems. Currently, fiber CPA systems need to use stretchers to broaden the pulse before amplification to 10 ps to 5 ns; to ensure that the amplified pulse can be compressed to a near-conversion-limited pulse, the nonlinear phase shift accumulated during amplification should be controlled to a low level; due to the gain narrowing effect accompanying amplification, the spectral width of the pulse will narrow, therefore, the pulse width generated by CPA-based ultrafast fiber laser amplification systems is usually above 200 fs. However, many applications require laser pulse widths below 100 femtoseconds. Such applications include high-harmonic generation of extreme ultraviolet pulses, multiphoton microscopy, and broadband mid-infrared optical frequency combs based on pulse self-difference frequencies.

[0003] Therefore, researchers proposed a nonlinear amplification technique. When using gain fiber for amplification, the pulse width is intentionally kept on the picosecond or femtosecond scale to accumulate a large amount of nonlinear phase shift. This results in the amplified pulse having a spectral broadening several times that of the input spectrum and exhibiting near-linear chirp in the time domain. Thus, after compression using a traditional grating pair, ultrafast lasers with pulse widths less than 100 fs can be generated. Currently, nonlinear fiber laser amplification mainly includes self-similar amplification, pre-chirp managed amplification (PCMA), and gain-managed nonlinear amplification. Among these, PCMA combined with large-mode-field fiber technology is currently the main method for obtaining high-energy sub-femtosecond ultrashort pulses. However, the precise pre-chirp adjustment and complex spatial optical path cause fiber laser systems to lose their advantages of flexibility and compactness. For example, in Chinese patent application 202011559678.5, the pre-chirp management module, the dual-pass fiber amplifier module, and the pulse compression module are all constructed using spatial optical paths. All-fiber nonlinear amplification systems often have low output power and pulse energies far below the μJ level, as illustrated in Chinese patent application 201710302907.7. Gain-managed nonlinear amplification, proposed in 2019, relies on nonlinear attractors and can also achieve several-fold spectral broadening and compressed sub-femtosecond ultrashort pulse output (Optica 6, 1328-1333 (2019)). However, the above nonlinear amplification schemes all amplify a narrowband seed light into a broadband spectrum, often requiring processing of the seed laser before amplification. This can be achieved by using pre-chirped components to compress the pulse to the femtosecond level, or by using optical devices such as fiber gratings to filter and shape the seed laser spectrum. These methods result in severe signal light loss, leading to extremely low seed laser energy, making it difficult for the pulse energy after nonlinear amplification to exceed 1 μJ. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies, particularly the difficulty in balancing energy enhancement and system compactness of sub-femtosecond pulses in fiber amplifiers, by providing a broadband fiber laser nonlinear amplification device and its operating method. This device combines the advantages of nonlinear amplification technology and large-mode-area fiber, directly amplifying ultrashort pulses with linear chirp from the nonlinear fiber laser amplification stage through a broadband fiber laser amplification stage. Since the gain spectrum of the signal light in the nonlinear fiber laser amplification stage has already exceeded the active gain bandwidth limit and has a relatively flat upper spectral edge, it is no longer limited by the gain bandwidth in the broadband fiber laser amplification stage. The signal light maintains linear chirp in the time domain, the spectrum can be further broadened in the frequency domain, and the energy is further amplified in the large-mode-area fiber. Notably, the entire system does not require additional dispersion adjustment components or pulse shaping devices, and can directly output sub-femtosecond pulses on the order of microjoules after the compressor. This method significantly reduces system losses while maintaining the compact structure of fiber lasers.

[0005] The objective of this invention is achieved through the following technical solution: a broadband fiber laser nonlinear amplification device, comprising a front-end light source, a first isolator, a nonlinear fiber laser amplification stage, a second isolator, a broadband fiber laser amplification stage, and a compressor connected in sequence; the front-end light source is an ultrafast fiber laser source of picosecond or femtosecond voltammetry; the first and second isolators are used to ensure unidirectional transmission between amplifiers; the nonlinear fiber laser amplification stage is composed of a semiconductor pump source, a combiner or wavelength division multiplexer, and a gain fiber, used to achieve drastic spectral broadening by utilizing the strong nonlinear effect induced by ultrashort pulses in the fiber; the broadband fiber laser amplification stage is used to amplify the energy of the broadband ultrafast laser input from the previous stage using the gain fiber and a high-power pump source; under the action of a high nonlinear phase shift, the spectrum is broadened while maintaining linear chirp in the time domain; the compressor is used to provide negative dispersion, dechirping and compressing the linearly chirped pulse to a near-transform-limited pulse width.

[0006] Furthermore, the first isolator and the second isolator are implemented using fiber optic isolators or circulators.

[0007] Furthermore, the first isolator, the nonlinear fiber laser amplification stage, the second isolator, and the broadband fiber laser amplification stage are all connected by fiber optic fusion splices.

[0008] Furthermore, the semiconductor pump source, combiner or wavelength division multiplexer, and gain fiber are all spliced ​​using fiber optics; the nonlinear fiber laser amplification stage utilizes the strong nonlinear effect induced by ultrashort pulses in the fiber to achieve drastic spectral broadening under the action of fiber gain, dispersion, and self-phase modulation, generating broadband ultrafast lasers with bandwidths of tens or even hundreds of nanometers. At this time, the pulse has linear chirp in the time domain and is compressible; it can employ amplification mechanisms such as self-similar amplification, pre-chirp-managed amplification, and gain-managed nonlinear amplification.

[0009] Furthermore, the output end of the gain fiber in the broadband fiber laser amplification stage is collimated by two convex lenses or a collimator, and output in the spatial optical path after passing through a dichroic mirror.

[0010] Furthermore, the broadband fiber laser amplification stage is composed of a gain fiber and a pump source. Without propagating in the spatial optical path, the compressor uses fiber gratings, negative dispersion fibers, and hollow fibers as fiber dispersion compensation elements. These are fused together at the output end of the broadband fiber laser amplification stage to achieve an all-fiber structure.

[0011] Furthermore, the gain fiber in the broadband fiber laser amplification stage adopts a rod-shaped photonic crystal fiber amplification stage, and the two ends of the fiber are spatially coupled bidirectional pumping, forward pumping, or backward pumping to achieve energy enhancement of ultrashort pulses.

[0012] Furthermore, the compressor is composed of grating pairs or prism pairs.

[0013] A method for operating the aforementioned broadband fiber laser nonlinear amplification device includes: the front-end light source passes through a first isolator to prevent backlighting from the amplifier from damaging the seed source; through the nonlinear effect induced in the nonlinear fiber laser amplification stage, the spectral width is drastically broadened to several or tens of times the input spectrum, while the evolution of the nonlinear attractor causes the ultrashort pulse to have an approximately linear chirp in the time domain, and the pulse is compressed to a near-transform-limit pulse width through linear compression; then, it passes through a second isolator and directly enters the broadband fiber laser amplification stage, where the broadband laser is further amplified in a large mode area fiber, while mitigating the accumulation of harmful nonlinear phase shifts; finally, the pulse is compressed in the time domain by a pulse compression device to obtain a high-energy ultrashort pulse with a pulse width of less than 100 femtoseconds.

[0014] The beneficial effects of this invention are:

[0015] 1. This system has very low requirements for the front-end light source. It only requires that the output spectrum be narrow and the pulse width be on the order of picoseconds or femtoseconds. There are no requirements for pulse energy, repetition frequency, or chirp. Mode-locked fiber oscillators or ultrafast fiber amplifiers operating in the positive dispersion region can be used. There are many general-purpose laser products on the market that can meet the requirements at low cost.

[0016] 2. This system has strong versatility and scalability. The selection of the nonlinear fiber laser amplification stage allows for considerable flexibility; existing techniques such as self-similar amplification, pre-chirped amplification, and gain-managed nonlinear amplification can all achieve the function of the nonlinear fiber laser amplification stage. The selection of the gain fiber for the broadband fiber laser amplification stage also offers considerable flexibility, requiring only that the fiber's mode area is not less than that of the previous amplification stage. Furthermore, the choice between adopting an all-fiber structure can be made based on actual application requirements.

[0017] 3. This broadband fiber laser nonlinear amplification device and its operating method directly utilize the pulses from the nonlinear amplification stage for energy amplification. Compared with existing technologies, this amplification process eliminates the need for pre-chirped / pre-compressed components and pulse shapers, significantly simplifying the amplification system, reducing losses during amplification, and improving amplification efficiency. The compressor performs time-domain compression on the pulses, resulting in ultrashort pulse outputs with pulse widths below 100 femtoseconds and high energy and peak power. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of the overall structure of the present invention;

[0020] Figure 2 This is a diagram of the experimental apparatus for Embodiment 1 of the present invention;

[0021] Figure 3 These are the output spectra of each stage in Embodiment 1 of the present invention, wherein, Figure 3 (a) in the figure is the spectrum of the picosecond light source in Example 1; Figure 3 In the diagram, (b) shows the output spectra of the nonlinear fiber laser amplification stage (dashed line) and the broadband fiber laser amplification stage (solid line);

[0022] Figure 4 This is the final output pulse width measured by the autocorrelation instrument in Embodiment 1 of the present invention; wherein, the dashed line is the Gaussian fitting curve, and the result shows that the compressed pulse width is 76fs.

[0023] In the figure, the components are: front-end light source 101, first isolator 105, nonlinear fiber laser amplification stage 102, second isolator 106, broadband fiber laser amplification stage 103, compressor 104, fiber isolator 201, semiconductor laser 202, combiner 203, gain fiber 204, fiber isolator 205, cladding light filter 206, photonic crystal fiber 207, first convex lens 208, dichroic mirror 209, second convex lens 210, high-power semiconductor laser 211, first reflector 212, grating pair 213, and second reflector 214. Detailed Implementation

[0024] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0025] like Figure 1 As shown, the present invention provides a broadband fiber laser nonlinear amplification device and operating method. The device includes a front-end light source 101, a first isolator 105, a nonlinear fiber laser amplification stage 102, a second isolator 106, a broadband fiber laser amplification stage 103, and a compressor 104 connected in sequence.

[0026] The front-end light source 101 is a picosecond or femtosecond ultrafast fiber laser source, which can be a mode-locked fiber oscillator or an ultrafast fiber amplifier operating in the positive dispersion region, and its output pulse sequence should be picosecond or femtosecond pulses; the output end of the front-end light source 101 is connected to the input end of the first isolator 105.

[0027] The first isolator 105 and the second isolator 106 can be implemented using fiber optic isolators or circulators.

[0028] The output end of the first isolator 105 is connected to the input end of the nonlinear fiber laser amplifier stage 102 via fiber optic fusion splicing.

[0029] The nonlinear fiber laser amplification stage 102 comprises a semiconductor pump source, a combiner or wavelength division multiplexer, and a section of gain fiber, all of which are spliced ​​using fiber optics. Utilizing the strong nonlinear effect induced by ultrashort pulses in the fiber, the stage achieves a dramatic spectral broadening effect under the influence of nonlinear effects such as fiber gain, dispersion, and self-phase modulation, generating broadband ultrafast lasers with bandwidths of tens or even hundreds of nanometers. The pulses exhibit linear chirp in the time domain and are compressible. Amplification mechanisms such as self-similar amplification, pre-chirp-managed amplification, and gain-managed nonlinear amplification can be employed.

[0030] The output of the nonlinear fiber laser amplification stage 102 is connected to the input of the second isolator 106; the output of the second isolator 106 is connected to the input of the broadband fiber laser amplification stage 103. All of the above devices employ fiber fusion splicing.

[0031] The broadband fiber laser amplification stage 103 utilizes a gain fiber with a large mode area and a high-power pump source to further amplify the energy of the broadband ultrafast laser input from the previous stage. Because the laser from the previous stage is directly injected without additional loss, the pulse energy of the previous stage is tens or hundreds of nanojoules. Under the action of a highly nonlinear phase shift, the spectrum is further broadened while maintaining linear chirp in the time domain. At this point, the pulse has high energy, and the output end of the gain fiber is tangent at an 8-degree angle. It is collimated by two convex lenses or a collimator, and then directly output in the spatial optical path after passing through a dichroic mirror.

[0032] The compressor 104 provides appropriate negative dispersion, dechirping and compressing linearly chirped pulses to near-transform-limit pulse widths. It can be a grating pair or a prism pair.

[0033] In particular, the broadband fiber laser amplification stage 103 consists only of a gain fiber with a large mode area and a pump source. Without entering the spatial optical path for propagation, the compressor 104 can use fiber gratings, negative dispersion fibers, and hollow fibers as fiber dispersion compensation elements. By fusion splicing at the output end of the broadband fiber laser amplification stage 103, the entire experimental device can be made entirely of fiber.

[0034] The working principle of this invention is as follows: The front-end light source in this invention is a picosecond or femtosecond light source with a narrow spectral bandwidth. A first isolator prevents backlighting from the amplifier from damaging the seed source. Because picosecond or femtosecond pulses have relatively high peak power, they easily induce strong nonlinear effects in the nonlinear fiber laser amplification stage, causing the spectral width to be drastically broadened to several or tens of times the input spectrum. Simultaneously, the evolution of the nonlinear attractor gives the ultrashort pulse an approximately linear chirp in the time domain. The pulse can be compressed to near-transform-limit pulse width using simple linear compression. When the pulse directly enters the broadband fiber laser amplification stage after passing through the second isolator, since no additional shaping devices are introduced, the small amount of chirp introduced by the very short passive fiber has almost negligible impact on the signal light. Therefore, the broadband laser is further amplified in the large mode area fiber, while the large mode area alleviates the accumulation of harmful nonlinear phase shifts. The pulse maintains good compressibility while effectively amplifying its energy. Finally, by using commercially available pulse compression devices to compress the pulse in the time domain, a high-energy ultrashort pulse with a pulse width of less than 100 femtoseconds can be obtained.

[0035] Example 1 illustrates the structure of a broadband fiber laser nonlinear amplification device according to the present invention. See also... Figure 2 This is a high-energy ultrashort pulse fiber amplification device. It includes: a picosecond light source, a nonlinear fiber amplification stage, a broadband fiber laser amplification stage, and a pulse compressor. The picosecond light source has a repetition frequency of 10 MHz, and its spectrum is as follows: Figure 3As shown in (a), the center wavelength is 1040 nm, the bandwidth is 2.3 nm, the pulse width is 2 ps, the average power is 13 mW, and the pulse energy is 1.3 nJ. A fiber isolator 201 with a core / cladding ratio of 10 / 125 μm is used, with its output connected to the input of a nonlinear fiber laser amplifier. A semiconductor laser 202 with a center wavelength of 976 nm is injected through the fiber pump arm of a combiner 203, undergoing gain-managed nonlinear amplification in a 4.5 m long gain fiber 204 (core / cladding ratio of 10 / 125 μm, absorption coefficient 5 dB / m@976 nm), resulting in a significant broadening of the spectrum towards longer wavelengths (e.g., ...). Figure 3 As shown by the dashed line in (b) of the image, the average power is amplified to 500mW, the pulse energy is 50nJ, and the dechirped pulse width is measured to be 66fs using a compressor. The amplified signal light enters the broadband fiber laser amplification stage through a fiber isolator 205 with a core / cladding ratio of 10 / 125μm. The pumping method is back-pumping. The high-power semiconductor laser 211 is collimated by the first convex lens 208 and the second convex lens 210 and then coupled into the photonic crystal fiber 207. A cladding light filter 206 is fused to the front end of the photonic crystal fiber. The signal light is further amplified to 30W in the 3m long photonic crystal fiber 207 (core / cladding ratio of 40 / 200μm, absorption coefficient of 10dB / m@976nm). The output spectrum is as follows: Figure 3 As shown by the solid line in (b), the signal light is separated by a 976nm / 1064nm dichroic mirror 209 and enters the compressor. The laser first passes over the first reflector 212 without passing through it, then passes through a 1000 lines / nm grating pair 213, and is reflected back by the second reflector 214. By adjusting the second reflector 214, the height of the reflected laser is slightly lowered, and it hits the mirror surface of the first reflector 212. After passing through the grating pair 213 a second time, the signal light is subjected to appropriate negative dispersion, the pulse is compressed in the time domain, the chirped pulse is extracted, and the final output pulse width is measured to be 76 fs using an autocorrelation meter. Figure 4 As shown. The final output power is 27W, corresponding to a diffraction efficiency of 90%, and the pulse energy is 2.7μJ.

[0036] In Embodiment 2, the nonlinear fiber laser amplification stage 102 includes: a low-power pump source, a wavelength division multiplexer, and a section of single-clad single-mode ytterbium-doped fiber. The broadband fiber laser amplification stage 103 includes: a high-power pump source, a combiner, and a section of large-mode-area ytterbium-doped fiber. The compressor 104 is a large-mode-area ytterbium-doped fiber chirped fiber grating with a consistent core / cladding ratio, providing negative dispersion matched to the amplification stage. Under these conditions, the present invention can realize an all-fiber structure.

[0037] In Example 3, the second isolator 106 is a spatial optical isolator, and the gain fiber in the broadband fiber laser amplification stage 103 can be a rod-shaped photonic crystal fiber amplification stage. The two ends of the fiber are spatially coupled bidirectional pumping, forward pumping, or backward pumping, which can achieve ultrashort pulses with higher power and higher energy than in Example 1.

[0038] In summary, this invention provides a broadband fiber laser nonlinear amplification device and its operating method, which has a flexible and compact structure and versatility. It can output high-energy sub-100 femtosecond pulses with low cost and simple structure, and has important application prospects in fields such as strong field physics, high-precision measurement and processing, and biological microscopic imaging.

[0039] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

[0040] The above embodiments are only used to illustrate the design concept and features of the present invention, and their purpose is to enable those skilled in the art to understand the content of the present invention and implement it accordingly. The protection scope of the present invention is not limited to the above embodiments. Therefore, all equivalent changes or modifications made based on the principles and design ideas disclosed in the present invention are within the protection scope of the present invention.

Claims

1. A broadband fiber laser nonlinear amplification device, characterized in that, The system comprises a front-end light source, a first isolator, a nonlinear fiber laser amplification stage, a second isolator, a broadband fiber laser amplification stage, and a compressor, connected in sequence. The front-end light source is an ultrafast fiber laser source with a picosecond or femtosecond wavelength range. The first and second isolators ensure unidirectional transmission between amplifiers and are achieved through fiber isolators or circulators. The nonlinear fiber laser amplification stage consists of a semiconductor pump source, a combiner or wavelength division multiplexer, and a gain fiber. It achieves spectral broadening through amplification mechanisms such as self-similar amplification, pre-chirped amplification, or gain-managed nonlinear amplification, utilizing the strong nonlinear effect induced by ultrashort pulses in the fiber to achieve dramatic spectral broadening. The broadband fiber laser amplification stage uses the gain fiber and a high-power pump source to amplify the energy of the broadband ultrafast laser input from the previous stage. Under the influence of a high nonlinear phase shift, the spectrum is broadened while maintaining linear chirp in the time domain. The compressor is used to provide negative dispersion, dechirping pulses with linear chirp and compressing them to near-transform-limit pulse widths.

2. The broadband fiber laser nonlinear amplification device according to claim 1, characterized in that, The output of the front-end light source is connected to the input of the first isolator, the output of the first isolator is connected to the input of the nonlinear fiber laser amplification stage, the output of the nonlinear fiber laser amplification stage is connected to the input of the second isolator, the output of the second isolator is connected to the input of the broadband fiber laser amplification stage, and the output of the broadband fiber laser amplification stage is connected to the input of the compressor. The first isolator, the nonlinear fiber laser amplification stage, the second isolator, and the broadband fiber laser amplification stage are all connected by fiber optic fusion splices.

3. The broadband fiber laser nonlinear amplification device according to claim 1, characterized in that, The semiconductor pump source, combiner or wavelength division multiplexer, and gain fiber are all spliced ​​using fiber optics. The nonlinear fiber laser amplification stage utilizes the strong nonlinear effect induced by ultrashort pulses in the fiber to achieve drastic spectral broadening under the action of fiber gain, dispersion, and self-phase modulation, generating broadband ultrafast lasers with bandwidths of tens or even hundreds of nanometers. At this time, the pulse has linear chirp in the time domain and is compressible. It can employ amplification mechanisms such as self-similar amplification, pre-chirp-managed amplification, and gain-managed nonlinear amplification.

4. The broadband fiber laser nonlinear amplification device according to claim 1, characterized in that, The output of the gain fiber in the broadband fiber laser amplification stage is collimated by two convex lenses or a collimator, and then output in the spatial optical path after passing through a dichroic mirror.

5. The broadband fiber laser nonlinear amplification device according to claim 1, characterized in that, The broadband fiber laser amplification stage consists of a gain fiber and a pump source. Without entering the spatial optical path for propagation, the compressor uses fiber gratings, negative dispersion fibers, and hollow fibers as fiber dispersion compensation elements. These are fused together at the output end of the broadband fiber laser amplification stage to achieve an all-fiber structure.

6. The broadband fiber laser nonlinear amplification device according to claim 1, characterized in that, The gain fiber in the broadband fiber laser amplification stage adopts a rod-shaped photonic crystal fiber amplification stage. The two ends of the fiber are spatially coupled bidirectional pumping, forward pumping, or backward pumping to achieve energy enhancement of ultrashort pulses.

7. The broadband fiber laser nonlinear amplification device according to claim 1, characterized in that, The compressor is composed of grating pairs or prism pairs.

8. A method for operating the broadband fiber laser nonlinear amplification device according to claim 1, characterized in that, include: The front-end light source passes through the first isolator to prevent backlight from the amplifier from damaging the seed source; The nonlinear effects induced in the nonlinear fiber laser amplification stage dramatically broaden the spectral width to several or tens of times that of the input spectrum. Simultaneously, the evolution of the nonlinear attractor gives the ultrashort pulse an approximately linear chirp in the time domain. The pulse is compressed to a near-transform-limit pulse width through linear compression. It then passes through a second isolator and directly enters the broadband fiber laser amplification stage, where the broadband laser is further amplified in a large mode area fiber while mitigating the accumulation of harmful nonlinear phase shifts. Finally, the pulse is compressed in the time domain by a pulse compression device to obtain a high-energy ultrashort pulse with a pulse width of less than 100 femtoseconds.

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