| Keywords [eng] |
ultrashort pulses, chirped fiber Bragg gratings, group delay ripples, spectral ripples, temporal contrast, pulse temporal quality, fiber-based seed, femtosecond laser, CPA, FCPA, CFBG |
| Abstract [eng] |
Ultrashort laser pulses with femtosecond durations are critical in both fundamental research and applied fields, ranging from high-intensity laser physics to precision materials processing. Besides pulse duration, temporal pulse quality or temporal contrast (TC) – a ratio of the main pulse to pre/post-pulses – may be of high importance. To achieve high temporal quality, modern hybrid femtosecond laser systems often combine multiple amplification stages to enhance both power and pulse temporal and spatial quality. In fiber chirped-pulse amplification (FCPA) systems, the TC degradation typically arises from a combination of nonlinear phase accumulation and uncompensated higher-order dispersion. Nonlinearities, primarily driven by the Kerr effect, occur during high-gain amplification when peak intensities are sufficient to induce self-phase modulation. Simultaneously, spectral and temporal distortions may originate from fabrication-based imperfections in dispersive optical components. Consequently, a comprehensive temporal characterization is essential for isolating and mitigating these distinct contrast-degrading mechanisms within hybrid chirped pulse amplification (CPA) architectures. In this work, the output of a hybrid CPA laser system was characterized using high-dynamic-range third-order autocorrelation. The initial configuration exhibited a TC of approximately 10-4, with the main pulse containing 98% of the total energy relative to the measured intensity background. By replacing the standard-quality CFBG with a high-quality component featuring reduced spectral and group delay ripples, the TC was improved 2-3 times. Simultaneously, the energy fraction of the pulse background relative to the main peak decreased by a factor of 1.5, indicating that the CFBG response function has a significant influence on the quality of the compressed pulse. Additionally, post-compression via a multi-pass cell module yielded 56 fs pulses, a sixfold reduction in duration, with a TC of 9·10-5, improving nearly in proportion to the applied compression ratio of 6. High-dynamic-range characterization using the third-order autocorrelator and the frequency-resolved optical gating (FROG) techniques confirmed that using a high-quality CFBG reduced the post-compressed pulse background energy fraction by a factor of 2, a slightly greater change compared to the CPA output. Finally, application of external Kerr-lens filtering provided a modest TC improvement, while substantial operational trade-offs were observed. A large accumulated nonlinear phase shift caused spectral broadening and splitting, while strong spatial filtering severely degraded the beam quality. Resulting spectral and spatial distortions may limit the direct application of the examined contrast enhancement method and require further refinement for practical implementation. Overall, the results highlight that CFBG quality is an important aspect in FCPA architectures; thus, for superior temporal contrast, CFBG-related distortions should be minimized. |
