This study aims to develop a high-power green femtosecond laser system with a high beam quality by integrating rod-type photonic crystal fiber amplifiers and coherent beam-combining technology. Continuous advancements in laser technology have focused on realizing femtosecond lasers with high average power, high single-pulse energy, and superior beam quality. This study is significant because high-power green femtosecond lasers are crucial for various applications, including micromachining of wide-bandgap materials, high-quality photonic device processing, extreme ultraviolet generation, pumping optical parametric oscillators, and biomedical imaging. The system design and methodology ensure that the laser maintains high performance and stability, making it a valuable tool for cutting-edge research and technological development.

The system employs rod-type photonic crystal fiber amplifiers and coherent beam combining techniques, along with a lithium triborate (LBO) crystal for nonlinear frequency conversion. To ensure near-diffraction-limited beam quality at this power level, both the thermal management of the amplifier and coupling of the seed light were optimized. The thermal management of the amplifier involved the use of a low water-cooling temperature to mitigate the thermal effects that could degrade the beam quality. Two amplifiers were coherently combined with an efficiency of 95.2%, achieving stable output across different power levels. The coherent beam-combining technique utilizes active phase control to maintain the phase coherence between the beams from the two amplifiers. This involves the use of piezoelectric mirrors and feedback systems to correct the phase errors dynamically, ensuring that the power of the combined beam is stable near its maximum value. Moreover, the use of piezoelectric deflection mirrors ensures automatic alignment of the two beams. Fundamental light was compressed using a transmission diffraction grating compressor. By adjusting the grating angle and spacing and finely adjusting the second-, third-, and fourth-order dispersion parameters of the tunable chirped fiber Bragg grating (T-CFBG), the coherently combined pulses were compressed near the transform limit. The optimized fundamental light was then frequency-doubled in a 2-mm thick LBO crystal. The frequency-doubling process involves optimizing the spot size of the fundamental beam and precisely aligning the LBO crystal to realize efficient second harmonic generation.

The single-channel fiber amplifier realizes a high average power of 130 W with near-diffraction-limited beam quality. This performance is ensured by optimizing the thermal management of the amplifier and the coupling of the seed light. The spectral evolution of the amplifier stages [Fig.2(a)] shows a gradual narrowing of the spectrum, which is a common phenomenon in chirped pulse amplification (CPA) systems. The output power curves of the two rod-type fiber amplifiers [Fig.2(b)] demonstrate their consistent performance. The coherent beam combining the two amplifiers reaches an efficiency of 95.2%, resulting in 238 W fundamental light output. This efficiency is maintained across different output power levels, indicating the stability and reliability of the system [Fig.3(b)]. The residual phase difference of the synthesized light is calculated using λ/30, demonstrating the effectiveness of the phase-locked system [Fig.3(c)]. This indicates that the phase-locked system can effectively suppress the drift in the optical path and phase due to factors such as temperature and air disturbances, thereby maintaining a stable high-power output. After compression with the Treacy compressor, the power corresponds to 210 W and pulse width corresponds to 230 fs. The compressed pulse is close to the transform limit, demonstrating good time-domain quality of the pulse (Fig.4). Frequency doubling in the LBO crystal results in 128 W green laser light with a pulse width of 216 fs and second-harmonic generation efficiency of 61%. The green light power and conversion efficiency versus the fundamental power [Fig.6(a)] demonstrate the effectiveness of the frequency-doubling process. The beam quality of the green light is close to the diffraction limit [Fig.6(b)]. The variations in the intensity noise at each stage of the laser system are investigated. The results indicate that the relative intensity noise of the frequency-doubled light is higher than that of the fundamental-frequency light, and a 13-kHz modulation peak is transmitted during the frequency-doubling process (Fig.8). These results indicate that coherent beam combination significantly enhances the power and energy output of femtosecond lasers. The high efficiency and stable performance of this system make it suitable for various scientific and industrial applications.

The developed high-power green femtosecond laser system combines the advantages of rod-type photonic crystal fiber amplifiers and coherent beam combination technology. By optimizing thermal management and seed light coupling, the system maintains near-diffraction-limited beam quality at high power levels without transverse mode instability (TMI). The coherent combining of the two amplifiers realizes 95.2% efficiency, producing 238 W near-infrared femtosecond laser light at a repetition rate of 1 MHz. The compressed pulses are frequency-doubled in an LBO crystal, yielding 128 W green femtosecond laser output with a pulse width of 216 fs and a peak power of 0.6 GW. This study highlights the potential of coherent beam combination to enhance the output power and energy of femtosecond lasers, with promising applications in research and industry. Future studies can further increase the system output power by improving the single-amplifier performance or adding more amplifier channels, benefiting various applications in both scientific and industrial fields.