A gain spectrum shaping device for a femtosecond fiber amplifier

Abstract

The application provides a gain spectrum shaping device of a femtosecond fiber amplifier, the device comprising a mode-locked fiber oscillator, a one-dimensional adjustable birefringent filter, a pump source, a beam combiner, a gain fiber, a collimator and a grating pair compressor; in a working state, seed light output by the mode-locked fiber oscillator sequentially passes through the one-dimensional adjustable birefringent filter, the beam combiner, the gain fiber, the collimator and the grating pair compressor to obtain shaped target pulse light; wherein the one-dimensional adjustable birefringent filter and the pump source cooperate to adjust to determine the center wavelength of the seed light and make the gain spectrum of the seed light and the gain fiber match. The scheme provided by the application can at least be used to solve the technical problem that the structure and adjustment mode are relatively complex in the nonlinear fiber amplification scheme of the related art.

Description

Technical Field

[0001] This application relates to the field of laser technology, and in particular to a gain spectrum shaping device for a femtosecond fiber amplifier. Background Technology

[0002] Femtosecond fiber lasers (or femtosecond fiber lasers) are considered the next generation of femtosecond lasers due to their advantages such as miniaturization, portability, air cooling, low cost, and high stability. They feature extremely narrow pulse widths, high integration, and ultra-high stability. Femtosecond fiber lasers have a wide range of applications, including long-term and extensive use in fiber optic communications. Furthermore, there is a significant demand for femtosecond lasers in materials micromachining because the extremely short pulse duration of femtosecond lasers allows for the rapid injection of all energy into a very small area, effectively avoiding cracking caused by heat generation during material processing. In particular, high-power, high-energy femtosecond fiber lasers have broad application prospects in basic scientific research, attosecond pulse generation, and laser ranging.

[0003] To produce high-power, high-energy femtosecond laser pulses, femtosecond fiber lasers typically employ a master oscillator plus power amplifier structure. Nonlinear effects are the primary factor limiting pulse energy during amplification. While femtosecond fiber lasers based on traditional chirped pulse amplification techniques can generate femtosecond pulses with energies in the 1 mJ range, these pulses are typically longer than 200 femtoseconds (fs), failing to directly meet the needs of applications requiring lower energy and shorter pulse widths. Therefore, nonlinear amplification techniques have been proposed. Unlike traditional chirped pulse amplification techniques, which reduce nonlinear phase shift by broadening the pulse, nonlinear amplification techniques intentionally maintain the pulse width in the picosecond range during amplification using gain fiber, thus accumulating a significant nonlinear phase shift. This results in a spectral broadening of the amplified pulse that is several times greater than the input spectrum, and it exhibits near-linear chirp in the time domain. Therefore, after compression using traditional grating pairs, shorter near-transform-limited pulses can be generated. Currently, nonlinear fiber amplification techniques can be categorized into three types: 1. self-similar parabolic pulse amplification; 2. pre-chirp-managed amplification; and 3. gain-managed amplification. The following sections describe these three schemes and the corresponding technical problems discovered by the inventors for each scheme:

[0004] 1) Self-similar parabolic pulse amplification. When a femtosecond pulse propagates in a positive dispersion gain fiber amplifier, under suitable conditions, the interaction between dispersion, self-phase adjustment, and gain allows the pulse to gradually evolve into a parabolic pulse during amplification. This pulse has linear chirp and can be easily compressed to below 100 fs. In this scheme, the gain fiber length is typically several meters to ensure that an initial pulse of arbitrary shape asymptotically evolves into a parabolic pulse and subsequently maintains a self-similar propagation state. However, with the gain fiber length typically several meters, higher energy pulses cannot be obtained due to the nonlinear effects of the fiber.

[0005] 2) Prechirped Amplification. This approach introduces prechirped adjustment as a degree of freedom, allowing for the generation of high-quality, broadband femtosecond pulses by jointly adjusting the prechirped amount of the input pulse and the dispersion of the compressor. Because the pulse's spectrum is further broadened during amplification, compression can produce near-transform-limited pulses with shorter widths. This approach typically requires the introduction of grating pairs within the femtosecond fiber laser for prechirped management, increasing structural complexity.

[0006] 3) Gain-managed nonlinear amplification. This method optimizes the degree of population inversion in the fiber by controlling the pump intensity and the length of the gain fiber, thereby optimizing the gain spectrum of the amplified fiber. Combined with nonlinear spectral broadening, this generates high-quality femtosecond pulses that can be compressed by grating pairs. This approach requires optimizing both the pump intensity and the length of the gain fiber, and adjusting the length of the gain fiber is quite complex.

[0007] In summary, the nonlinear fiber amplification schemes of related technologies are all quite complex in terms of structure and adjustment methods. Summary of the Invention

[0008] One objective of this application is to provide a gain spectrum shaping device for a femtosecond fiber amplifier, which at least solves the technical problem that the structure and adjustment methods of nonlinear fiber amplification schemes in related technologies are quite complex.

[0009] To achieve the above objectives, some embodiments of this application provide a gain spectrum shaping device for a femtosecond fiber amplifier. The device includes a mode-locked fiber oscillator 1, a one-dimensionally adjustable birefringent filter 2, a pump source 3, a combiner 4, a gain fiber 5, a collimator 6, and a grating pair compressor 7. In operation, the seed light output from the mode-locked fiber oscillator 1 passes sequentially through the one-dimensionally adjustable birefringent filter 2, the combiner 4, the gain fiber 5, the collimator 6, and the grating pair compressor 7 to obtain a shaped target pulse light. The one-dimensionally adjustable birefringent filter 2 and the pump source 3 are adjusted in coordination to determine the center wavelength of the seed light and to match the gain spectrum of the seed light with that of the gain fiber 5.

[0010] Compared to existing technologies, the gain spectrum shaping device for a femtosecond fiber amplifier provided in this application includes a mode-locked fiber oscillator, a one-dimensionally tunable birefringent filter, a pump source, a combiner, a gain fiber, a collimator, and a grating pair compressor. Since the bandwidth and pulse width of the pulse from the mode-locked fiber oscillator narrow after passing through the filter, the one-dimensionally tunable birefringent filter of this device can be used to narrow the spectrum and pulse of the mode-locked fiber oscillator. The narrowed pulse is then amplified in the gain fiber. Because the peak power accumulation during amplification in the gain fiber generates a large amount of nonlinear phase shift, the spectrum of the amplified pulse is broadened to several times that of the input spectrum, thus greatly broadening the spectrum. Since the angle of the one-dimensional birefringent filter is adjustable, the center wavelength of the seed light can be changed by adjusting the angle of the one-dimensional birefringent filter, thereby optimizing the shape of the gain spectrum and ultimately obtaining a shorter, femtosecond-level near-transform-limited pulse. In short, in order to overcome the problems of complex structure and complicated adjustment methods in nonlinear amplification schemes, this scheme only requires placing an angle-adjustable filter in front of the gain fiber to play a role similar to pre-chirp management. At the same time, by adjusting the angle of the filter, the center wavelength of the pulse seed light can be adjusted to optimize the gain spectrum and play the role of gain management nonlinear amplification. The structure of this device is very simple and easy to adjust. Attached Figure Description

[0011] Figure 1 This is an exemplary structural diagram of a gain spectrum shaping device for a femtosecond fiber amplifier provided in an embodiment of this application;

[0012] Figure 2 This is a schematic diagram showing the effect of the gain spectrum shaping device for a femtosecond fiber amplifier before adjustment, as provided in the embodiments of this application.

[0013] Figure 3 This is a schematic diagram showing the effect of adjusting the gain spectrum of a femtosecond fiber amplifier using the gain spectrum shaping device provided in the embodiments of this application. Detailed Implementation

[0014] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0015] The following terms are used in this document.

[0016] Self-phase modulation, or Aelf-Phase Modulation (SPM) for short, refers to the phenomenon where the refractive index of an optical fiber changes as the electric field strength changes, causing a change in the phase of the transmitted signal. This change in the signal's own electric field strength leads to a change in its own phase.

[0017] This application provides a gain spectrum shaping device for a femtosecond fiber amplifier, such as... Figure 1 As shown, the device includes a mode-locked fiber oscillator 1, a one-dimensional adjustable birefringent filter 2, a pump source 3, a bundle combiner 4, a gain fiber 5, a collimator 6, and a grating pair compressor 7.

[0018] In operation, the seed light output by the mode-locked fiber oscillator 1 passes sequentially through the one-dimensional adjustable birefringent filter 2, the bundler 4, the gain fiber 5, the collimator 6, and the grating pair compressor 7 to obtain the shaped target pulse light.

[0019] The one-dimensional adjustable birefringent filter 2 and the pump source 3 are adjusted in coordination to determine the center wavelength of the seed light and to match the gain spectrum of the seed light and the gain fiber 5.

[0020] In some embodiments of this application, the seed light output from the mode-locked fiber oscillator 1 is input to one end of the combiner 4 through the one-dimensional adjustable birefringent filter 2. One end of the pump source 3 is connected to one end of the combiner 4, the other end of the combiner 4 is connected to one end of the gain fiber 5, the other end of the gain fiber 5 is connected to one end of the collimator 6, and the spatial light output from the other end of the collimator 6 is output after passing through the grating and acting on the compressor 7.

[0021] It should be noted here that there is no actual connection between the mode-locked fiber oscillator 1 and the one-dimensional adjustable birefringent filter 2, nor between the collimator 6 and the grating pair compressor 7. The seed light output by the mode-locked fiber oscillator 1 is essentially spatial light, which travels through the air to the one-dimensional adjustable birefringent filter 2. Similarly, the light output by the grating pair compressor 7 is essentially spatial light, which travels through the air to the grating pair compressor 7. Specifically, this manifests as follows: Figure 1 As shown by the dashed line in the image, Figure 1 The solid lines in the diagram represent optical fibers.

[0022] In some embodiments of this application, such as Figure 1 As shown, the device also includes an isolator 8; the other end of the gain fiber 5 is connected to one end of the collimator 6 through the isolator 8.

[0023] In some embodiments of this application, the mode-locked fiber oscillator 1 has a center wavelength of 1030 nm, a repetition frequency of 36 MHz, a pulse width of 3 ps, a spectral width of 13 nm, and a smooth, approximately parabolic spectral shape. In some other examples, the center wavelength of the mode-locked fiber oscillator 1 is 1550 nm, 1950 nm, etc., which are not specifically limited here.

[0024] In some embodiments of this application, the bandwidth of the one-dimensionally adjustable birefringent filter 2 is 2.5 nm.

[0025] In some embodiments of this application, the one-dimensional adjustable birefringent filter 2 is placed at a Brewster angle.

[0026] In some embodiments of this application, the pump source 3 may specifically be a laser diode. For example, the pump source 3 may specifically be a multimode laser diode with a center wavelength of 915nm and a maximum power of 30W.

[0027] In some embodiments of this application, the gain fiber 5 is specifically a 1.5m long multimode gain fiber. In some examples, the multimode gain fiber can be of type PLMA-YDF-10 / 125-M, with an absorption coefficient of 4.95dB / m@976nm.

[0028] In some embodiments of this application, the grating pair compressor 7 is specifically a grating pair compressor with a capacity of 1000 L / mm.

[0029] In some embodiments of this application, the device is applied to a femtosecond fiber laser.

[0030] The following describes the usage of the gain spectrum shaping device for the femtosecond fiber amplifier provided in the embodiments of this application:

[0031] The mode-locked fiber oscillator 1 is turned on, and the mode-locked fiber oscillator 1 outputs seed light. The seed light is filtered by the one-dimensional adjustable birefringent filter 2 to narrow the spectrum and pulse of the seed light. The filtered seed light then enters the combiner 4. At the same time, the pump source 3 can be turned on to provide pump light, which also enters the combiner 4. After being combined by the combiner, the seed light is amplified by the gain fiber 5, and the spectrum is gradually broadened during the amplification process. The output light amplified by the gain fiber 5 enters the grating pair compressor 7 through the collimator 6 to obtain the shaped target pulse light.

[0032] The one-dimensional tunable birefringent filter and the pump source are adjusted in conjunction to determine the center wavelength of the seed light and to match the gain spectrum of the seed light and the gain fiber.

[0033] In some examples, the process of coordinating the adjustment of the one-dimensional adjustable birefringent filter and the pump source is as follows:

[0034] By increasing the pump power of pump source 3, the seed light, after being combined by the combiner, undergoes power amplification through gain fiber 5 due to self-phase modulation, and the spectrum gradually broadens during amplification. When the pump power is 1W, the spectral width is greater than 20nm, and the center wavelength is redshifted to 1035nm. At this point, the spectrum exhibits an asymmetrical shape, with some modulation at the top, and incompressible substrates appear on both sides of the directly compressed pulse. Adjusting the angle of the one-dimensional adjustable birefringent filter 2, observation shows that when the center wavelength of the seed light passing through the one-dimensional adjustable birefringent filter 2 is greater than 1035nm, the spectrum exhibits a relatively flat shape, the compressed pulse shape is cleaner, and the pulse width is 114fs.

[0035] Meanwhile, during the adjustment of the angle of the one-dimensional adjustable birefringent filter 2, since the pump power of the pump source 3 remains constant and the fiber length of the gain fiber 5 also remains constant, the gain fiber 5 is in a state with a certain number of inverted particles, and the gain fiber 5 has a specific gain spectrum. If the center wavelength of the seed light and the gain spectrum do not match, due to the gain shaping effect, the shape of the amplified spectrum will be relatively poor, specifically exhibiting obvious spectral modulation and asymmetry. Therefore, the gain fiber 5, in a state with a certain number of inverted particles, has a specific center wavelength of seed light suitable for nonlinear amplification, thereby obtaining a relatively flat amplified spectrum.

[0036] Furthermore, after determining the center wavelength of the seed light that matches the gain spectrum of the gain fiber 5, and keeping the angle of the one-dimensionally adjustable birefringent filter 2 fixed, if the pump power of the pump source 3 is further increased, the spectrum can be broadened to a greater extent, but the spectral morphology deteriorates accordingly. At this point, if the angle of the one-dimensionally adjustable birefringent filter 2 is adjusted simultaneously, the amplified spectral morphology will be optimized, resulting in a narrower near-transform-limited pulse.

[0037] As can be seen, in this embodiment, using a one-dimensionally adjustable birefringent filter 2 to determine the center wavelength not only serves a similar function to pre-chirp management, accelerating the nonlinear amplification process of the pulse and broadening the spectrum, but also optimizes the gain spectrum by adjusting the angle of the one-dimensionally adjustable birefringent filter 2, making the gain spectra of the seed light and the gain fiber match, similar to the function of gain-managed nonlinear amplification. This combines the advantages of pre-chirp management and gain-managed nonlinear amplification, resulting in a simple and easily adjustable structure. For different pump powers of the pump source 3, only the one-dimensionally adjustable birefringent filter 2 needs to be adjusted to optimize the gain spectrum and obtain the shaped target pulse light, which can be a femtosecond pulse near the transform limit.

[0038] For details, please refer to [link / reference]. Figure 2 and Figure 3 As shown, Figure 2 For example, the pulse pattern before adjusting the angle of the one-dimensionally adjustable birefringent filter 2 in some cases, Figure 3 The image shows the pulse pattern after adjusting the angle of the one-dimensional adjustable birefringent filter 2 in some of the examples. 20 can be considered as the pulse curve shape measured by the instrument, and 10 is the pulse curve shape obtained by fitting. The higher the overlap between 10 and 20, the better the pulse curve shape. The comparison of the two images shows that the pulse shape after adjusting the angle of the one-dimensional adjustable birefringent filter 2 is better, and the pulse is smoother, resulting in a superior technical effect.

[0039] It is not difficult to see that, compared with related technologies, the gain spectrum shaping device for a femtosecond fiber amplifier provided in this application includes a mode-locked fiber oscillator, a one-dimensionally adjustable birefringent filter, a pump source, a combiner, a gain fiber, a collimator, and a grating pair compressor. Since the bandwidth and pulse width of the pulse from the mode-locked fiber oscillator narrow after passing through the filter, the one-dimensionally adjustable birefringent filter of this device can be used to narrow the spectrum and pulse of the mode-locked fiber oscillator. The narrowed pulse is then amplified in the gain fiber. Because the peak power accumulation in the gain fiber during amplification generates a large amount of nonlinear phase shift, the spectrum of the amplified pulse is broadened to several times that of the input spectrum, thus greatly broadening the spectrum. Since the angle of the one-dimensional birefringent filter is adjustable, the center wavelength of the seed light can be changed by adjusting the angle of the one-dimensional birefringent filter, thereby optimizing the shape of the gain spectrum and ultimately obtaining a near-transform-limited pulse with a shorter width, on the order of femtoseconds.

[0040] It will be apparent to those skilled in the art that this application is not limited to the details of the exemplary embodiments described above, and that this application can be implemented in other specific forms without departing from the spirit or essential characteristics of this application. Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of this application is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be embraced within this application. No reference numerals in the claims should be construed as limiting the scope of the claims. Furthermore, it is clear that the word "comprising" does not exclude other units or steps, and the singular does not exclude the plural. Multiple units or devices recited in the apparatus claims may also be implemented by a single unit or device in software or hardware. The terms "first," "second," etc., are used to indicate names and do not indicate any particular order.

Claims

1. A gain spectrum shaping device for a femtosecond fiber amplifier, characterized in that, The device includes a mode-locked fiber oscillator (1), a one-dimensional adjustable birefringent filter (2), a pump source (3), a bundler (4), a gain fiber (5), a collimator (6), and a grating pair compressor (7). In the working state, the seed light output by the mode-locked fiber oscillator (1) passes sequentially through the one-dimensional adjustable birefringent filter (2), the bundler (4), the gain fiber (5), the collimator (6), and the grating pair compressor (7) to obtain the shaped target pulse light. The one-dimensional adjustable birefringent filter (2) and the pump source (3) are adjusted in coordination to determine the center wavelength of the seed light and to match the gain spectrum of the seed light and the gain fiber (5). The seed light output from the mode-locked fiber oscillator (1) is input to one end of the combiner (4) through the one-dimensional adjustable birefringent filter (2). One end of the pump source (3) is connected to one end of the combiner (4). The other end of the combiner (4) is connected to one end of the gain fiber (5). The other end of the gain fiber (5) is connected to one end of the collimator (6). The spatial light output from the other end of the collimator (6) is output after passing through the grating and acting on the compressor (7). The bandwidth of the one-dimensional tunable birefringent filter (2) is 2.5 nm; The one-dimensional adjustable birefringent filter (2) and the pump source (3) are adjusted in coordination, specifically including: The pump source is controlled to increase the pump power, so that the seed light after being bundled by the combiner is amplified in the gain fiber and the spectrum is gradually broadened during the amplification process. During the adjustment of the angle of the one-dimensional adjustable birefringent filter, the pump power of the pump source and the fiber length of the gain fiber are kept constant, so that the center wavelength of the seed light is matched with the gain spectrum of the gain fiber, and a flat amplified spectrum is obtained. After determining the center wavelength of the seed light that matches the gain spectrum of the gain fiber (5), the angle of the one-dimensional adjustable birefringent filter (2) is kept fixed. When the pump source (3) continues to increase the pump power, so that the spectrum is further broadened but the spectral shape deteriorates, the angle of the one-dimensional adjustable birefringent filter (2) is adjusted to optimize the amplified spectral shape and obtain a near-transform limit pulse.

2. The apparatus according to claim 1, characterized in that, The device also includes an isolator (8); The other end of the gain fiber (5) is connected to one end of the isolator (8) and the collimator (6).

3. The apparatus according to claim 1, characterized in that, The mode-locked fiber oscillator (1) has a center wavelength of 1030nm, a repetition frequency of 36MHz, a pulse width of 3ps, a spectral width of 13nm, and a smooth, approximately parabolic spectral shape.

4. The apparatus according to claim 1, characterized in that, The one-dimensional adjustable birefringent filter (2) is placed at Brewster angle.

5. The apparatus according to claim 1, characterized in that, The pump source (3) is specifically a laser diode.

6. The apparatus according to claim 1, characterized in that, The gain fiber (5) is specifically a 1.5m long multimode gain fiber.

7. The apparatus according to claim 1, characterized in that, The grating pair compressor (7) is specifically a grating pair compressor with a capacity of 1000 L / mm.

8. The apparatus according to any one of claims 1 to 7, characterized in that, The device is used in femtosecond fiber lasers.

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