A method for controlling the output linewidth of a nonlinear picosecond fiber amplifier

Abstract

This invention belongs to the field of nonlinear picosecond fiber amplifiers and discloses a method for controlling the output linewidth of a nonlinear picosecond fiber amplifier. Specifically, based on the target output linewidth, target output power, and parameters of the fiber amplifier module, the dispersion of the pre-chirped module is adjusted by changing the linewidth of the ultrafast femtosecond laser seed source input to the pre-chirped module and / or the length of the passive fiber pigtail in the fiber amplifier module. This results in an output pulse that is closest to the transform limit and has a linewidth closest to the target output linewidth. This invention expands the methods for controlling the linewidth (or pulse width) of nonlinear picosecond fiber amplifiers, providing more flexible linewidth (or pulse width) control capabilities. This invention is expected to overcome the bottleneck of existing nonlinear picosecond fiber amplifiers in linewidth (or pulse width) control, providing a low-cost solution for more versatile nonlinear picosecond fiber amplifiers.

Description

Technical Field

[0001] This invention belongs to the field of nonlinear picosecond fiber amplifiers, and more specifically, relates to a method for controlling the output linewidth of a nonlinear picosecond fiber amplifier. Background Technology

[0002] In industrial applications, narrow-linewidth picosecond lasers have a wide range of uses. For example, in biophotonics, coherent anti-Stokes Raman scattering microscopy based on narrow-linewidth picosecond lasers is well-suited for imaging deep biological tissue cells [J.Phys.Chem.B,108(3):827(2004).]; in gas detection experiments, narrow-linewidth picosecond lasers can significantly improve the resolution during the detection process [Opt.Express 24,11112(2016).]. Therefore, designing a picosecond fiber amplifier that combines high pulse energy and narrow linewidth is particularly important.

[0003] Maintaining a narrow linewidth for the output pulse is challenging due to the unavoidable self-phase modulation (SPM) effect during amplification. In fiber amplifiers, the SPM effect often introduces a significant nonlinear phase shift. The SPM effect introduces spectral broadening with positive chirp in the seed pulse and spectral compression with negative chirp [Appl. Phys. Lett. 63, 1017 (1993)]. Most existing picosecond fiber amplifiers focus on obtaining high peak power output pulses, and therefore often use rare-earth-doped fibers with large mode field radii to circumvent the negative impact of the SPM effect [Chin. Opt. Lett. 17, 071401 (2019)]. Such amplifiers are often referred to as linear amplifiers. Linear amplifiers are well-suited for generating high-power output pulses; however, it is difficult to adjust the output linewidth of such amplifiers, which limits the application range of picosecond fiber amplifiers.

[0004] Existing technologies have reported nonlinear amplifiers that achieve linewidth compression based on the SPM effect of gain fiber and passive fiber. In this approach, a pre-chirped module introduces a negative chirp into the seed pulse output from the seed source module, which is compensated by the positive chirp generated by the SPM effect, resulting in a near-transform-limited pulse at the output. This scheme has been proven applicable to narrow-linewidth picosecond pulse generation [Applied. Physics. B 74, 191 (2002)]. Although existing technologies have reported constructing nonlinear amplifiers using seed source modules, pre-chirped modules, and amplifier modules, they typically ignore the effects of femtosecond seed source linewidth and passive fiber pigtails, leading to output power coupling with output linewidth. This makes independent adjustment of output linewidth and output power impossible, resulting in poor controllability of existing nonlinear amplifiers, often lacking the ability to control the output spectral linewidth (pulse width). This limits the further widespread adoption of nonlinear amplifiers. Summary of the Invention

[0005] To address the aforementioned deficiencies or improvement needs of existing technologies, the present invention aims to provide a method for controlling the output linewidth of a nonlinear picosecond fiber amplifier. This method expands the methods for controlling the linewidth (or pulse width) of nonlinear picosecond fiber amplifiers, thereby achieving greater flexibility in linewidth (or pulse width) control within similar systems. This invention is expected to overcome the bottlenecks in linewidth (or pulse width) control of existing nonlinear picosecond fiber amplifiers, providing a low-cost solution for more versatile nonlinear picosecond fiber amplifiers.

[0006] To achieve the above objectives, according to a first aspect of the present invention, a method for controlling the output linewidth of a nonlinear picosecond fiber amplifier is provided. The nonlinear picosecond fiber amplifier includes an ultrafast femtosecond laser seed source module, a pre-chirped module, and a fiber amplifier module. The ultrafast femtosecond laser seed source module generates a seed pulse and inputs it to the pre-chirped module. The pre-chirped module broadens the ultrafast femtosecond laser seed source pulse into a picosecond pulse. The fiber amplifier module amplifies the picosecond pulse and simultaneously compresses the spectral linewidth based on the nonlinear effect between the gain fiber and the passive fiber, and outputs the pulse. The method adjusts the dispersion of the pre-chirped module by changing the linewidth of the ultrafast femtosecond laser seed source input to the pre-chirped module, based on the target output linewidth, target output power, and parameters of the fiber amplifier module, to obtain an output pulse that is closest to the transform limit and has a linewidth closest to the target output linewidth.

[0007] According to a second aspect of the present invention, a method for controlling the output linewidth of a nonlinear picosecond fiber amplifier is provided. The nonlinear picosecond fiber amplifier includes an ultrafast femtosecond laser seed source module, a pre-chirped module, and a fiber amplifier module. The ultrafast femtosecond laser seed source module generates a seed pulse and inputs the ultrafast femtosecond laser seed source to the pre-chirped module. The pre-chirped module broadens the ultrafast femtosecond laser seed source pulse into a picosecond pulse. The fiber amplifier module amplifies the picosecond pulse and simultaneously compresses the spectral linewidth based on the nonlinear effect of the gain fiber and the passive fiber, and outputs the pulse. The method adjusts the dispersion of the pre-chirped module by changing the length of the passive fiber pigtail in the fiber amplifier module, based on the target output linewidth, target output power, and parameters of the fiber amplifier module, to obtain an output pulse that is closest to the transform limit and has a linewidth closest to the target output linewidth.

[0008] According to a third aspect of the present invention, a method for controlling the output linewidth of a nonlinear picosecond fiber amplifier is provided. The nonlinear picosecond fiber amplifier includes an ultrafast femtosecond laser seed source module, a pre-chirped module, and a fiber amplifier module. The ultrafast femtosecond laser seed source module generates a seed pulse and inputs the ultrafast femtosecond laser seed source to the pre-chirped module. The pre-chirped module broadens the ultrafast femtosecond laser seed source pulse into a picosecond pulse. The fiber amplifier module amplifies the picosecond pulse and simultaneously compresses the spectral linewidth based on the nonlinear effect of the gain fiber and the passive fiber, and outputs the pulse. The method adjusts the dispersion of the pre-chirped module by changing the linewidth of the ultrafast femtosecond laser seed source input to the pre-chirped module and changing the length of the passive fiber pigtail in the fiber amplifier module, based on the target output linewidth, target output power, and parameters of the fiber amplifier module, to obtain an output pulse that is closest to the transform limit and has a linewidth closest to the target output linewidth.

[0009] As a further preferred embodiment of the present invention, the ultrafast femtosecond laser seed source input to the pre-chirped module has a linewidth greater than the output linewidth of the fiber amplifier module.

[0010] As a further preferred embodiment of the present invention, the ultrafast femtosecond laser seed source input to the pre-chirped module has a pulse width of 100-1000 femtoseconds and a linewidth of 1-30nm.

[0011] As a further preferred embodiment of the present invention, the pre-chirping module is used to apply anomalous dispersion, and the amount of chirping applied to the seed pulse can be changed by changing the amount of dispersion of the pre-chirping module.

[0012] As a further preferred embodiment of the present invention, the gain fiber in the fiber amplifier module is a rare-earth-doped gain fiber.

[0013] As a further preferred embodiment of the present invention, the output pulse output by the fiber amplifier module has a pulse width of 1-10 picoseconds and a linewidth of 0.1-2 nm.

[0014] Compared with the prior art, the above-described technical solutions conceived in this invention can achieve the following results.

[0015] Beneficial effects:

[0016] (1) This invention regulates the output linewidth of a nonlinear picosecond fiber amplifier by changing the linewidth of the input ultrafast femtosecond laser seed source and / or the length of the passive fiber optic pigtail in the fiber amplifier module. Unlike existing nonlinear amplification systems that ignore the influence of the femtosecond seed source linewidth and passive fiber optic pigtail, resulting in the output power of the nonlinear amplification system being coupled with the output linewidth and unable to be independently controlled, this invention regulates the output linewidth (pulse width) of the nonlinear picosecond fiber amplifier by changing the linewidth of the ultrafast femtosecond laser seed source input to the pre-chirped module and / or the length of the passive fiber optic pigtail in the fiber amplifier module. This allows the nonlinear picosecond fiber amplifier to arbitrarily adjust the output spectral linewidth (pulse width), and the output power and output linewidth can be independently controlled. The method for regulating the output linewidth of a nonlinear picosecond fiber amplifier proposed in this invention can provide a new design scheme for designing nonlinear picosecond fiber amplifiers with different target output linewidths (pulse widths).

[0017] (2) Based on the present invention, the dispersion of the pre-chirped module can be adjusted by changing the linewidth of the input ultrafast femtosecond laser seed source, thereby obtaining a narrow linewidth picosecond laser at the output port that is closest to the transformation limit. This scheme is easy to implement, allowing the nonlinear picosecond fiber amplifier to more freely control the spectral linewidth (pulse width) of the output pulse, thus increasing the applicability of the picosecond nonlinear fiber amplifier in practical applications.

[0018] (3) Based on the present invention, the dispersion of the pre-chirped module can be adjusted by changing the length of the passive fiber pigtail in the fiber amplifier module, thereby obtaining a narrow linewidth picosecond laser at the output port that is closest to the transformation limit. This scheme is easy to implement, allowing the nonlinear picosecond fiber amplifier to more freely control the spectral linewidth (pulse width) of the output pulse, thus increasing the applicability of the picosecond nonlinear fiber amplifier in practical applications.

[0019] The nonlinear picosecond fiber amplifier with adjustable linewidth obtained based on this invention helps to achieve near-transform-limited picosecond pulse output with narrow linewidths, typically covering an output linewidth of 0.1-2 nm and a typical output pulse width of 1-10 picoseconds. This invention provides a potential way to overcome the bottleneck of existing picosecond fiber amplifiers in spectral control, and provides a standardized and practical design scheme for the design and application of narrow-linewidth picosecond fiber amplifiers. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of a method for controlling the output linewidth of a nonlinear picosecond fiber amplifier according to an embodiment of the present invention.

[0021] Figure 2This invention relates to the variation of the output pulse linewidth at the output port, obtained through numerical simulation of parameters in embodiments of the present invention, by changing the linewidth of the input ultrafast femtosecond laser seed source and the length of the passive fiber pigtail in the fiber amplifier module, thereby adjusting the dispersion of the pre-chirped module. Figure 2 Figure (a) shows the curves of the output linewidth (i.e., full width at half maximum) of the nonlinear picosecond fiber amplifier and the dispersion of the prechirped module as a function of the linewidth (i.e., the input linewidth) of the ultrafast femtosecond laser seed source input to the prechirped module. Figure 2 Figure (b) shows the curves of the output linewidth (i.e., full width at half maximum) of the nonlinear picosecond fiber amplifier and the dispersion of the prechirped module as a function of the length (i.e., the length of the passive fiber pigtail in the fiber amplifier module).

[0022] Figure 3 The narrow-linewidth near-transform-limited picosecond pulse spectrum and intensity autocorrelation are obtained based on the parameters in the embodiments of this invention. The "input linewidth" in the figure refers to the linewidth of the input ultrafast femtosecond laser seed source, which is input from the ultrafast femtosecond laser seed source module to the pre-chirped module. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0024] Example 1

[0025] The structural schematic diagram of the method for implementing nonlinear picosecond fiber amplifier output linewidth control provided in this embodiment is shown below. Figure 1 As shown, similar to nonlinear amplifiers reported in the prior art, it includes: an ultrafast femtosecond laser seed source module, a pre-chirped module, and an optical fiber amplifier module; the optical fiber amplifier module includes a rare-earth-doped gain fiber, a passive optical fiber pigtail, and a pump source. The ultrafast femtosecond laser seed source module is used to generate seed pulses; the pre-chirped module is used to broaden the ultrafast femtosecond laser seed source pulses to picosecond pulses; the optical fiber amplifier module is used to amplify the seed light and simultaneously achieve linewidth compression based on the spectral compression effect. However, unlike the prior art, in this embodiment, the passive pigtail is no longer a factor that needs to be ignored. On the contrary, to verify the newly discovered control effect of the passive pigtail, the length of the passive pigtail can be set to 0.2m or more.

[0026] Similar to nonlinear amplifiers reported in the prior art, fiber amplifiers possess excellent output beam quality, compact size, and superior heat dissipation. Nonlinear fiber amplifiers based on SPM-induced compression can simultaneously compress the spectrum during amplification, allowing the use of mature ultrafast femtosecond fiber mode-locked lasers as seed sources. Therefore, this embodiment aims to utilize a nonlinear fiber amplifier to amplify ultrafast femtosecond seed light to obtain narrow-linewidth near-transform-limited picosecond pulses.

[0027] Taking ytterbium-doped gain fiber as an example, the device for controlling the output linewidth of a nonlinear picosecond fiber amplifier based on this invention includes an ultrafast femtosecond laser seed source module, a pre-chirped module, and a fiber amplifier module. The ultrafast femtosecond laser seed source module generates a seed pulse; the pre-chirped module broadens the femtosecond seed pulse to picoseconds by introducing group velocity dispersion; the fiber amplifier module amplifies the low-power seed pulse using rare-earth-doped fiber, and simultaneously compresses the spectral linewidth based on the nonlinear effect between the gain fiber and the passive fiber pigtail. By optimizing the specific pre-chirped amount of the pre-chirped module, a unique output pulse closest to the transform limit is obtained at the output port.

[0028] based on Figure 1 Based on the flowchart, we established a numerical simulation model to simulate the pulse evolution process in a nonlinear picosecond fiber amplifier. Figure 1 As shown, the ultrafast femtosecond pulse output by the seed source module is broadened to picoseconds in the pre-chirped module. In the amplifier module, the power is amplified while the spectral linewidth is gradually compressed, resulting in a narrow linewidth picosecond pulse output at the output port that is close to the transform limit. Figure 2 Based on Figure 1 The numerical simulation model established by the flowchart shows the change in output pulse linewidth at the output port by adjusting the dispersion of the pre-chirped module by changing the linewidth of the input ultrafast femtosecond laser seed source and / or the length of the passive fiber pigtail in the fiber amplifier module. Figure 2 Numerical simulation results show that reducing the input seed source spectral linewidth or increasing the length of the passive pigtail in the amplifier module gradually reduces the output near-transform-limited picosecond pulse spectral linewidth, and correspondingly, gradually increases the pulse width under near-transform-limit conditions. Based on this, we have obtained a method for controlling the output pulse linewidth in a picosecond fiber amplifier.

[0029] based on Figure 2 Based on the results, we designed an experiment and measured the spectrum and intensity autocorrelation of the output pulse. The parameters used were: the ultrafast femtosecond seed linewidth output by the seed source module was 11.76 nm (corresponding to...). Figure 3 (a) and 8.56nm (corresponding to Figure 3 (c) and Figure 3(e) in the above, all have a power of 62.5mW; the pre-chirped module uses a transmission grating pair to provide anomalous dispersion; the amplifier module uses ytterbium-doped fiber for gain, with a length of 2.5m, an absorption coefficient of 4.95dB / m for the 976nm pump laser, and a passive pigtail length of 0.2m (corresponding to...). Figure 3 (a) and Figure 3 (c) and 1.2m (corresponding to) Figure 3 (e)). By changing the linewidth of the ultrafast femtosecond seed source and the length of the passive pigtail in the amplifier module, the measurement results at the output port showed that the average power of the amplified output pulses in the three experimental groups was 6.1W, and the spectral linewidths were 0.87nm, 0.76nm, and 0.57nm, respectively, achieving a maximum spectral compression of 15 times compared to the seed light pulse (8.56nm ÷ 0.57nm = 15); the time-domain pulse widths under the Gaussian pulse assumption were 3.4ps, 4.4ps, and 5.2ps, respectively. Taking 5.2ps as an example, the calculated time-bandwidth product was 0.83, which is approximately 1.9 times that of the transform-limited pulse (the time-bandwidth product of the transform-limited pulse is a constant of 0.441).

[0030] The above embodiments are merely examples; for instance, other rare-earth-doped gain fibers also exhibit similar modulation effects.

[0031] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for independently regulating the output linewidth of a nonlinear picosecond fiber amplifier, the nonlinear picosecond fiber amplifier comprising a ultrafast femtosecond laser seed source module, a pre-chirp module and a fiber amplifier module, wherein, The ultrafast femtosecond laser seed source module is used to generate seed pulses and input ultrafast femtosecond laser seed sources into the pre-chirped module; the pre-chirped module is used to broaden the ultrafast femtosecond laser seed source pulses into picosecond pulses; the fiber amplifier module includes a gain fiber, a passive fiber pigtail, and a pump source, used to amplify the picosecond pulses, and simultaneously achieve spectral linewidth compression based on the nonlinear effect of the gain fiber and the passive fiber, and output the pulses; the method is characterized in that, according to the target output linewidth, target output power, and parameters of the fiber amplifier module, the length of the passive fiber pigtail in the fiber amplifier module is changed to adjust the dispersion of the pre-chirped module, so as to obtain an output pulse that is closest to the transform limit and whose linewidth is closest to the target output linewidth, thereby achieving independent control of output power and output linewidth, and the output linewidth can be arbitrarily adjusted.

2. A method for independently regulating the output linewidth of a nonlinear picosecond fiber amplifier, the nonlinear picosecond fiber amplifier comprising a ultrafast femtosecond laser seed source module, a pre-chirp module and a fiber amplifier module, wherein, The ultrafast femtosecond laser seed source module is used to generate seed pulses and input ultrafast femtosecond laser seed sources into the pre-chirped module; the pre-chirped module is used to broaden the ultrafast femtosecond laser seed source pulses into picosecond pulses; the fiber amplifier module includes a gain fiber, a passive fiber pigtail, and a pump source, used to amplify the picosecond pulses, and simultaneously achieve spectral linewidth compression based on the nonlinear effect of the gain fiber and the passive fiber, and output the pulses; the method is characterized in that, according to the target output linewidth, target output power, and parameters of the fiber amplifier module, the dispersion of the pre-chirped module is adjusted by changing the linewidth of the ultrafast femtosecond laser seed source input to the pre-chirped module and changing the length of the passive fiber pigtail in the fiber amplifier module, so as to obtain an output pulse that is closest to the transform limit and whose linewidth is closest to the target output linewidth, thereby achieving independent control of output power and output linewidth, and the output linewidth can be arbitrarily adjusted.

3. The method of claim 1 or 2, wherein, The ultrafast femtosecond laser seed source input to the pre-chirped module has a linewidth greater than the output linewidth of the fiber amplifier module.

4. The method of claim 1 or 2, wherein, The ultrafast femtosecond laser seed source input to the pre-chirped module has a pulse width of 100-1000 femtoseconds and a linewidth of 1-30nm.

5. The method of claim 1 or 2, wherein, The pre-chirped module is used to apply anomalous dispersion, and the amount of chirp applied to the seed pulse can be changed by changing the amount of dispersion of the pre-chirped module.

6. The method of claim 1 or 2, wherein, The gain fiber in the fiber amplifier module is a rare-earth-doped gain fiber.

7. The method of claim 1 or 2, wherein, The output pulse from the fiber amplifier module has a pulse width of 1-10 picoseconds and a linewidth of 0.1-2 nm.

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