Method for regulating and controlling output line width of nonlinear picosecond optical fiber amplifier

Abstract

The invention belongs to the field of nonlinear picosecond optical fiber amplifiers, and discloses a method for regulating and controlling the output line width of a nonlinear picosecond optical fiber amplifier, in particular to a method for regulating and controlling the output line width of the nonlinear picosecond optical fiber amplifier according to the target output line width and target output power of an optical fiber amplifier module and parameters of the optical fiber amplifier module. And by changing the line width of the ultrafast femtosecond laser seed source input to the pre-chirp module and / or the length of a passive optical fiber pigtail in the optical fiber amplifier module, the dispersion amount of the pre-chirp module is adjusted so as to obtain an output pulse which is closest to the transformation limit and the line width of which is closest to the target output line width. The method for regulating and controlling the line width (or pulse width) of the nonlinear picosecond optical fiber amplifier can be expanded, and the freer line width (or pulse width) regulation and control capability can be obtained. The nonlinear picosecond fiber amplifier is expected to break through the bottleneck of the existing nonlinear picosecond fiber amplifier in the aspect of line width (or pulse width) regulation and control, and a low-cost scheme is provided for the nonlinear picosecond fiber amplifier with higher applicability.

Description

Technical Field

[0001] The present invention belongs to the field of nonlinear picosecond optical fiber amplifiers, and more specifically, relates to a method for regulating the output line width of a nonlinear picosecond optical fiber amplifier. Background Art

[0002] In industrial applications, narrow linewidth picosecond lasers are widely used. For example, in biophotonics, coherent anti-Stokes Raman scattering microscopy based on narrow linewidth picosecond lasers is very suitable for imaging deep biological tissue cells [J.Phys.Chem.B,108(3):827(2004).]; in gas detection experiments, narrow linewidth picosecond lasers can greatly improve the resolution of the detection process [Opt.Express 24,11112(2016).]. Based on this, it is particularly important to design a picosecond fiber amplifier with both high pulse energy and narrow linewidth.

[0003] The difficulty in maintaining the narrow linewidth of the output pulse lies in the inevitable self-phase modulation effect (SPM) during the amplification process. In fiber amplifiers, the SPM effect often introduces a large amount of nonlinear phase shift. The SPM effect introduces spectral broadening when the seed pulse is positively chirped and spectral compression when it is negatively chirped [Appl.Phys.Lett.63,1017(1993)]. Most of the existing picosecond fiber amplifiers focus on obtaining high-peak power output pulses, so they often use rare-earth-doped optical fibers with large mode field radius to avoid the negative effects of the SPM effect [Chin.Opt.Lett.17,071401(2019)]. Such amplifiers are often referred to as linear amplifiers. Linear amplifiers are very suitable for generating high-power output pulses. However, it is difficult for such amplifiers to adjust the output linewidth of the pulses, which limits the application range of picosecond fiber amplifiers.

[0004] In the prior art, a nonlinear amplifier has been reported, which realizes linewidth compression based on the SPM effect of the gain fiber and the passive fiber, wherein the pre-chirp module introduces a certain negative chirp to the seed pulse output by the seed source module, and the positive chirp generated by the SPM effect compensates for it, thereby obtaining a near-conversion limit pulse at the output end. This scheme has been proven to be applicable to the generation of narrow linewidth picosecond pulses [Applied. Physics. B 74, 191 (2002)]. Although the related prior art has reported the construction of a nonlinear amplifier by using a seed source module, a pre-chirp module and an amplifier module, since they usually ignore the influence of the femtosecond seed source linewidth and the passive fiber pigtail, the output power is coupled with the output linewidth, and the output linewidth and output power cannot be adjusted independently. The existing nonlinear amplifier has poor control capability and often does not have the ability to control the output spectral linewidth (pulse width), which limits the further promotion of nonlinear amplifiers. Summary of the invention

[0005] In view of the above defects or improvement needs of the prior art, the purpose of the present invention is to provide a method for regulating the output linewidth of a nonlinear picosecond fiber amplifier, aiming to expand the method of regulating the linewidth (or pulse width) of a nonlinear picosecond fiber amplifier, so as to obtain a more free linewidth (or pulse width) regulation capability in similar systems. The present invention is expected to break through the bottleneck of the existing nonlinear picosecond fiber amplifier in terms of linewidth (or pulse width) regulation, and provide a low-cost solution for nonlinear picosecond fiber amplifiers with higher applicability.

[0006] To achieve the above-mentioned purpose, according to a first aspect of the present invention, a method for regulating the output linewidth of a nonlinear picosecond fiber amplifier is provided. The nonlinear picosecond fiber amplifier targeted by the method comprises an 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 a seed pulse and input an ultrafast femtosecond laser seed source into the pre-chirp module; the pre-chirp module is used to widen the ultrafast femtosecond laser seed source pulse into a picosecond pulse; the fiber amplifier module is used to amplify the picosecond pulse and simultaneously realize spectral linewidth compression based on the nonlinear effect of the gain fiber and the passive fiber, and output it; the method is to adjust the dispersion amount of the pre-chirp module by changing the linewidth of the ultrafast femtosecond laser seed source input into the pre-chirp module according to the target output linewidth, target output power and parameters of the fiber amplifier module, so as to obtain an output pulse that is closest to the conversion limit and whose linewidth is closest to the target output linewidth.

[0007] According to a second aspect of the present invention, a method for regulating the output linewidth of a nonlinear picosecond fiber amplifier is provided. The nonlinear picosecond fiber amplifier targeted by the method comprises an 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 a seed pulse and input an ultrafast femtosecond laser seed source into the pre-chirp module; the pre-chirp module is used to widen the ultrafast femtosecond laser seed source pulse into a picosecond pulse; the fiber amplifier module is used to amplify the picosecond pulse and simultaneously realize spectral linewidth compression based on the nonlinear effect of the gain fiber and the passive fiber, and output it; the method is based on the target output linewidth of the fiber amplifier module, the target output power and the parameters of the fiber amplifier module, by changing the length of the passive fiber pigtail in the fiber amplifier module, thereby adjusting the dispersion amount of the pre-chirp module to obtain an output pulse that is closest to the conversion limit and whose linewidth is closest to the target output linewidth.

[0008] According to a third aspect of the present invention, a method for regulating the output linewidth of a nonlinear picosecond fiber amplifier is provided. The nonlinear picosecond fiber amplifier targeted by the method comprises an 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 a seed pulse and input an ultrafast femtosecond laser seed source into the pre-chirp module; the pre-chirp module is used to widen the ultrafast femtosecond laser seed source pulse into a picosecond pulse; the fiber amplifier module is used to amplify the picosecond pulse and simultaneously realize spectral linewidth compression based on the nonlinear effect of the gain fiber and the passive fiber, and output it; the method is based on the target output linewidth of the fiber amplifier module, the target output power and the parameters of the fiber amplifier module, by changing the linewidth of the ultrafast femtosecond laser seed source input to the pre-chirp module and changing the length of the passive fiber pigtail in the fiber amplifier module, thereby adjusting the dispersion amount of the pre-chirp module to obtain an output pulse that is closest to the conversion limit and whose linewidth is closest to the target output linewidth.

[0009] As a further preferred embodiment of the present invention, the line width of the ultrafast femtosecond laser seed source input to the pre-chirp module is greater than the output line width of the optical fiber amplifier module.

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

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

[0012] As a further preferred embodiment of the present invention, the gain fiber in the optical 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 optical fiber amplifier module has a pulse width of 1-10 picoseconds and a line width of 0.1-2 nm.

[0014] Compared with the prior art, the above technical solutions conceived by the present invention can achieve the following

[0015] Beneficial effects:

[0016] (1) The present invention controls the output linewidth of the nonlinear picosecond fiber amplifier 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. Different from the nonlinear amplification system reported in the prior art, which ignores the influence of the femtosecond seed source linewidth and the passive fiber pigtail, resulting in the output power of the nonlinear amplification system being coupled with the output linewidth and unable to be independently controlled, the present invention controls 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-chirp module and / or the length of the passive fiber pigtail in the fiber amplifier module, so that the nonlinear picosecond fiber amplifier allows the output spectral linewidth (pulse width) to be arbitrarily adjusted, and the output power and the output linewidth can be independently controlled. A method for controlling the output linewidth of a nonlinear picosecond fiber amplifier proposed in the present 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, in particular, the dispersion amount of the pre-chirp module can be adjusted by changing the linewidth of the input ultrafast femtosecond laser seed source, and a scheme of a narrow linewidth picosecond laser closest to the conversion limit can be obtained at the output port. This scheme is easy to implement, so that the nonlinear picosecond fiber amplifier can more freely control the spectral linewidth (pulse width) of the output pulse, thereby increasing the scope of application of the picosecond nonlinear fiber amplifier in practical applications.

[0018] (3) Based on the present invention, in particular, by changing the length of the passive optical fiber pigtail in the optical fiber amplifier module, the dispersion amount of the pre-chirp module can be adjusted, and a scheme of a narrow linewidth picosecond laser closest to the conversion limit can be obtained at the output port. This scheme is easy to implement, so that the nonlinear picosecond optical fiber amplifier can more freely control the spectral linewidth (pulse width) of the output pulse, thereby increasing the scope of application of the picosecond nonlinear optical fiber amplifier in practical applications.

[0019] The nonlinear picosecond fiber amplifier with adjustable linewidth obtained based on the present invention is helpful to achieve the output of picosecond pulses with narrow linewidth close to the conversion limit, with a typical output linewidth covering 0.1-2nm and a typical output pulse width covering 1-10 picoseconds. The present invention provides a potential way to break through the bottleneck of existing picosecond fiber amplifiers in spectral regulation, and provides a standardized and practical design scheme for the design and application of narrow linewidth picosecond fiber amplifiers. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a structural schematic diagram of a method for realizing output linewidth control of a nonlinear picosecond fiber amplifier in an embodiment of the present invention.

[0021] Figure 2The variation of the output pulse line width obtained at the output port is obtained by changing the line width of the input ultrafast femtosecond laser seed source and the length of the passive fiber pigtail in the fiber amplifier module based on the numerical simulation of the parameters in the embodiment of the present invention, thereby adjusting the dispersion of the pre-chirp module. Figure 2 (a) shows the curves of the output linewidth (i.e., half-maximum full width linewidth) of the nonlinear picosecond fiber amplifier and the dispersion of the pre-chirp module as the linewidth (i.e., input linewidth) of the ultrafast femtosecond laser seed source input to the pre-chirp module changes; Figure 2 (b) shows the variation curves of the output linewidth (ie, half-maximum full width linewidth) of the nonlinear picosecond fiber amplifier and the dispersion of the pre-chirp module as the length of the passive optical fiber pigtail (ie, pigtail length) in the fiber amplifier module changes.

[0022] Figure 3 The narrow line width near-transformation limit picosecond pulse spectrum and intensity autocorrelation obtained based on the parameters in the embodiment of the present invention. The "input line width" in the figure refers to the line width of the input ultrafast femtosecond laser seed source input from the ultrafast femtosecond laser seed source module to the pre-chirp module. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present 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 diagram of the method for realizing output linewidth control of a nonlinear picosecond fiber amplifier provided in this embodiment is as follows: Figure 1 As shown. Similar to the nonlinear amplifier reported in the prior art, it includes: an ultrafast femtosecond laser seed source module, a pre-chirp 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 light source. Among them, the ultrafast femtosecond laser seed source module is used to generate a seed pulse; the pre-chirp module is used to broaden the ultrafast femtosecond laser seed source pulse into a picosecond pulse; the optical fiber amplifier module is used to amplify the seed light and realize 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, in order to verify the regulatory effect of the newly discovered passive pigtail of the present invention, the length of the passive pigtail can be set to be more than 0.2m.

[0026] Similar to the nonlinear amplifiers reported in the prior art, the fiber amplifier has good output beam quality, compact size, and excellent heat dissipation. The nonlinear fiber amplifier based on SPM effect-induced compression can compress the spectrum simultaneously during the amplification process, so that the mature ultrafast femtosecond fiber mode-locked laser can be used as the seed source. Therefore, this embodiment intends to use a nonlinear fiber amplifier to amplify the ultrafast femtosecond seed light to obtain a narrow linewidth near-conversion-limited picosecond pulse.

[0027] Taking the ytterbium-doped gain fiber as an example, the device for regulating the output linewidth of the nonlinear picosecond fiber amplifier obtained based on the present invention includes an 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; the pre-chirp module widens the femtosecond seed pulses to picoseconds by introducing group velocity dispersion; the fiber amplifier module amplifies the low-power seed pulses through rare-earth-doped fiber, and realizes spectral linewidth compression based on the nonlinear effect of the gain fiber and the passive fiber pigtail. The specific pre-chirp amount of the pre-chirp module is optimized to obtain a unique output pulse that is closest to the conversion limit at the output port.

[0028] based on Figure 1 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 widened to picoseconds in the pre-chirp module. In the amplifier module, the power is amplified while the spectral linewidth is gradually compressed, and a narrow linewidth picosecond pulse output close to the conversion limit is obtained at the output port. Figure 2 is based on Figure 1 Under the numerical simulation model established by the flowchart, the line width of the input ultrafast femtosecond laser seed source and / or the length of the passive fiber pigtail in the fiber amplifier module are changed to adjust the dispersion of the pre-chirp module, and the output pulse line width changes obtained at the output port. Figure 2 The numerical simulation results show that by reducing the input seed source spectral linewidth or increasing the passive pigtail length in the amplifier module, the output near-conversion limit picosecond pulse spectral linewidth will gradually decrease, and correspondingly, the pulse width under near-conversion limit conditions will gradually increase. Based on this, we have obtained a method to control the output pulse linewidth (pulse width) in a picosecond fiber amplifier.

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

[0030] The above embodiments are only examples. For example, other rare-earth-doped gain fibers also have similar regulation effects.

[0031] It will be easily understood by those skilled in the art that 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 and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for regulating the output linewidth of a nonlinear picosecond fiber amplifier, wherein the nonlinear picosecond fiber amplifier targeted by the method comprises an 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 a seed pulse and input the ultrafast femtosecond laser seed source into the pre-chirp module; the pre-chirp module is used to widen the ultrafast femtosecond laser seed source pulse into a picosecond pulse; the optical fiber amplifier module is used to amplify the picosecond pulse and realize spectral linewidth compression based on the nonlinear effect of the gain fiber and the passive fiber, and output it; it is characterized in that the method is based on the target output linewidth, target output power and parameters of the optical fiber amplifier module, by changing the linewidth of the ultrafast femtosecond laser seed source input to the pre-chirp module, thereby adjusting the dispersion amount of the pre-chirp module to obtain an output pulse that is closest to the transformation limit and whose linewidth is closest to the target output linewidth.

2. A method for regulating the output linewidth of a nonlinear picosecond fiber amplifier, wherein the nonlinear picosecond fiber amplifier targeted by the method comprises an 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 a seed pulse and input the ultrafast femtosecond laser seed source into the pre-chirp module; the pre-chirp module is used to widen the ultrafast femtosecond laser seed source pulse into a picosecond pulse; the optical fiber amplifier module is used to amplify the picosecond pulse, and at the same time realize spectral linewidth compression based on the nonlinear effect of the gain fiber and the passive fiber, and output it; it is characterized in that the method is based on the target output linewidth, target output power and parameters of the optical fiber amplifier module, by changing the length of the passive optical fiber pigtail in the optical fiber amplifier module, thereby adjusting the dispersion amount of the pre-chirp module to obtain an output pulse that is closest to the transformation limit and whose linewidth is closest to the target output linewidth.

3. A method for regulating the output linewidth of a nonlinear picosecond fiber amplifier, wherein the nonlinear picosecond fiber amplifier targeted by the method comprises an 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 a seed pulse and input the ultrafast femtosecond laser seed source to the pre-chirp module; the pre-chirp module is used to widen the ultrafast femtosecond laser seed source pulse into a picosecond pulse; the optical fiber amplifier module is used to amplify the picosecond pulse and realize spectral linewidth compression based on the nonlinear effect of the gain optical fiber and the passive optical fiber, and output it; it is characterized in that the method is based on the target output linewidth, target output power and parameters of the optical fiber amplifier module, by changing the linewidth of the ultrafast femtosecond laser seed source input to the pre-chirp module and changing the length of the passive optical fiber pigtail in the optical fiber amplifier module, thereby adjusting the dispersion amount of the pre-chirp module to obtain an output pulse that is closest to the transformation limit and whose linewidth is closest to the target output linewidth.

4. The method according to any one of claims 1 to 3, characterized in that: The line width of the ultrafast femtosecond laser seed source input to the pre-chirp module is greater than the output line width of the optical fiber amplifier module.

5. The method according to any one of claims 1 to 3, characterized in that: The ultrafast femtosecond laser seed source input to the pre-chirp module has a pulse width of 100-1000 femtoseconds and a line width of 1-30nm.

6. The method according to any one of claims 1 to 3, characterized in that: The pre-chirp module is used to apply anomalous dispersion, and the chirp amount applied to the seed pulse can be changed by changing the dispersion amount of the pre-chirp module.

7. The method according to any one of claims 1 to 3, characterized in that: The gain optical fiber in the optical fiber amplifier module is a rare-earth-doped gain optical fiber.

8. The method according to any one of claims 1 to 3, characterized in that: The output pulse output by the optical fiber amplifier module has a pulse width of 1-10 picoseconds and a line width of 0.1-2nm.

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