Objective Compared with traditional solid-state lasers, such as slats and wafers, femtosecond fiber lasers have many advantages, including compact structures, good beam quality, stable systems, and good heat dissipation. They are widely used in various application fields, particularly in the field of fine material processing. The current femtosecond fiber laser is mainly used to reduce the problems of nonlinearity and device damage in the amplification process using the chirped pulse amplification technology to obtain a femtosecond laser with its peak power and narrow pulse width meeting processing requirements. Owing to the increase in the energy demand of an industrial fiber laser, the amplification classes also gradually increase, rendering the narrowing effect due to an uneven gain medium increasingly severe and influencing the final spectral width. Thus, its occurrence increases the compressed pulse width limit, reduces the pulse peak power, and severely impacts the thermal diffusion in processing. Currently, the gain narrowing problem of a femtosecond fiber laser can be solved using self-phase modulation, a new amplifying stage system, or dielectric layer filters. However, these approaches can only form fixed spectral modulation, which cannot solve the gain narrowing problem in different amplification or laser systems with different power requirements of the same amplification system. Therefore, programmable devices are proposed to achieve the controllability of spectral modulation. Among them, the liquid crystal spatial light modulator, a common element for beam shaping and pulse shaping, exhibits higher regulating precision than the general acousto-optic filter. For a fiber laser seed source, its spectral width is only a few nanometers or tens of nanometers; the liquid crystal spatial light modulator is more suitable for fiber laser spectral modulation. This study focuses on the characteristics of the spatial light modulator and the requirement of a chirp pulse amplification system for spectral modulation. The solutions of a spectral shaping system are proposed based on the two-dimensional (2D) intensity-type liquid crystal spatial light modulator for spectral modulation. The chirped pulse amplification system eliminates the gain narrowing problem and achieves a narrower output pulse duration.

Methods Herein, the 2D intensity-type liquid crystal spatial light modulator was used for spectral modulation. First, the seed source is amplified using the single-mode amplification system and enters the spectral shaping system, which comprises the polarization-splitting prism, grating, and the intensity of reflection-type liquid crystal spatial light modulator to perform various spectral modulation shapes and verify the programmability and high precision of the spectral shaping system. Then, the subsequent multi-mode amplification and main amplification systems with an optical fiber aperture of 25 μm were established. The shape required for spectral shaping, which can be compared with the initial spectrum function to achieve a 2D grayscale map, was obtained using the step-by-step Fourier transform method combined with reverse operation. Next, the grayscale image was loaded into the spatial light modulator, and the spectral widths before and after the addition of the grayscale image were compared to investigate the influence of the shaping system on the spectra and the reasons for the spectral shape formation. Finally, the near-limited output pulse duration of the femtosecond laser was obtained by compressing an appropriate distance by the 1450 line/mm grating, and the pulse duration was measured using an autocorrelation instrument to determine the optimization results of pulse duration.

Results and Discussions The spectrum can be reshaped into flat-top, central-depression, triangle, continuous- depression, and other shapes using the spectral shaping system (Fig.4). Generally, as the spectral shaping becomes more complex and its shaping effect becomes worse, the modulation of the liquid crystal void generated using the liquid crystal spatial light modulator becomes increasingly obvious. After adding the main amplification system, the spectral width before entering the amplification system is reduced by approximately 1.5 nm (Fig.5) and the gain narrowing effect is evident. The grayscale map obtained using reverse operation was loaded on the 2D liquid crystal spatial light modulator to obtain the super-Gaussian spectral shape (Fig.6) after spectral shaping and amplification. The full width at half maximum of this spectrum is approximately 9.5 nm. The spectral width is approximately 2.5 nm, and the corresponding limited pulse duration is reduced from 222 fs to approximately 164 fs. Finally, the pulse duration of 170 fs, close to the limit, can be obtained after the laser with spectral shaping is compressed through the grating. Compared with that of direct compression, the pulse duration is decreased by 86 fs. Moreover, the pulse shape has a side lobe owing to the liquid crystal spacing problem of the liquid crystal spatial light modulator (Fig.7).

Conclusions Results show that the spectral shaping system can realize the arbitrary spectral modulation using the liquid crystal spatial light modulator. It can also set the spectral bandpass characteristics by regulating the function of different spectral component gains and realize dynamic matching using the subsequent amplification spectrum. Thus, this system is suitable for different conditions of gain spectrum modulation demand and can achieve a pulse duration close to the limit to meet the narrow output pulse duration and peak power of a femtosecond laser. Moreover, it achieves the purpose of efficient material cold working and satisfies the high requirements of the thermal effect for femtosecond laser applications. However, the improved results can be obtained using a modulator with a higher filling rate and smaller clearance for spectral shaping.