Objective Chirped pulse amplification is a laser amplification technology widely used in industrial processing, which effectively suppresses the nonlinear effect in the amplification process and ensures the safety of the gain fiber in the amplification process. With the increase of output power, the pulse duration of the CPA output pulse also increases, because the influence of gain-narrowing effect becomes more serious during the amplification process, and the increase of pulse duration will lead to the decrease of the peak power of the output pulse, which in turn affects the processing effect. Shaping the seed spectrum into a saddle spectrum is an effective method to suppress the gain-narrowing, and there have been many experiments using this method to successfully suppress the gain-narrowing. However, few studies have been conducted on the shape of saddle spectrum to better suppress the gain-narrowing, which will affect the time-domain quality and pulse duration of the output pulse, and thus affect the processing effect. Therefore, this paper will investigate which shape of saddle spectrum can better suppress the gain narrowing effect, and the answer to this question is of great significance.

Methods We build a spectral and chirped control device based on liquid crystal spatial light modulator (LC-SLM) and tunable chirped fiber Bragg grating (T-CFBG) and it is applied to the fiber chirped pulse amplification (FCPA) system to realize spectral shaping and pulse chirp control (Fig.1). The seed spectrum is formed into saddle-shaped spectra of different shapes by LC-SLM, and the chirp control was carried out by T-CFBG to explore the effect of inhibiting the gain-narrowing of saddle-shaped seed spectrum with different depression center wavelengths, different depression depths and different depression widths, and then the law under different powers was explored by changing the system output power.

Results and Discussions The saddle-shaped seed spectrum with different depression center wavelengths, different depression depths, and different depression widths are successfully modulated, and the autocorrelation curves after amplification and compression are measured: the wavelength of the depression center have little effect on the pulse duration of the final output pulse (Fig.4). Under different output powers, the greater the depression depth, the narrower the pulse duration after compression (Fig.5). There is an optimal depression width that makes the pulse duration narrowest after compression (Fig.6), and the optimal depression width decreases with the increase of output power at different output powers (Fig.7). Under the guidance of this rule, the femtosecond laser output with an average power of 25 W, a repetition rate of 1 MHz and a pulse width of 197 fs after compression was obtained (Fig.8).

Conclusions The research confirmed the effectiveness of the LC-SLM and T-CFBG in controlling spectral intensity and chirp in FCPA systems, which is crucial for achieving high-quality femtosecond laser pulses. The study identified key parameters for shaping the seed spectrum to suppress gain-narrowing and optimize pulse duration, providing valuable insights for future applications of this technology in scientific research and industrial processes. The system successfully delivered a high-energy femtosecond laser output with improved spectral and temporal characteristics, paving the way for further advancements in the field.