Femtosecond lasers function as highly precise "optical scalpels," playing an irreplaceable role in precision machining, medical surgery, spectral detection, and scientific research. Particularly in the 2 μm wavelength band, these lasers cover multiple molecular vibrational energy levels and overlap with absorption peaks of various amino compounds and biological tissues. Consequently, their application demands are especially urgent in fields like non-metallic material processing and biomedical engineering.
However, amplifying weak femtosecond seed lasers to high power is exceptionally difficult. The key challenge lies in the intense nonlinear interactions between the femtosecond pulse's extremely high optical intensity and the amplifying medium during amplification. Additionally, severe thermal effects at high repetition rates can degrade beam quality, cause pulse distortion, and even damage optical components. Existing solutions primarily employ chirped pulse amplification (CPA) technology, which involves first temporally broadening the pulse (reducing peak power), amplifying the laser energy to a certain level, and then compressing it back. However, this system is complex, expensive, and bulky. Therefore, the ability to eliminate the broadening and compression steps and achieve "direct amplification" of 2 μm femtosecond pulses while maintaining a simple, compact structure and strong power handling capability has become a research hotspot in the field of amplification technology.
Femtosecond Laser Amplifier Based on "Discrete" SCF
Recently, researchers including Wang Jianlei and Zhao Yongguang from the State Key Laboratory of Crystal Materials at Shandong University proposed an innovative B-integral (nonlinear phase shift) management strategy. By employing a discrete single-crystal fiber (SCF) configuration in the power amplification stage, they successfully achieved direct amplification of 2 μm femtosecond pulses at high repetition rates. The system achieved femtosecond laser output with an average power exceeding 56 W at a 75.45 MHz repetition rate, demonstrating exceptionally high optical-to-optical extraction efficiency (>55%) and near-diffraction-limited beam quality (M² < 1.2). The study demonstrates that the discrete SCF layout significantly reduces cumulative nonlinear phase shift, effectively suppressing detrimental nonlinear effects and ensuring stable spectral and temporal evolution during amplification. This simple, compact, and efficient approach enables amplification of 2 μm ultrashort pulses at MHz to kHz repetition rates, opening new avenues for achieving high average/peak power and exhibiting immense potential for modern nonlinear photonics applications.
The structure of this Ho:YAG SCF amplification system, as shown in Figure 1, comprises a laser seed source, a preamplifier stage, and an amplifier stage (consisting of three series-connected 0.5% doped Ho:YAG SCFs). The laser seed source delivers an average power of 0.45 W at 2091 nm, with a temporal pulse width of 360 fs and a repetition rate of 75.45 MHz. After passing through the preamplifier stage and the tandem SCF power amplifier stage, the average power increases to 56.3 W, and the temporal pulse broadens to 778 fs. The spectral characteristics and temporal evolution of the final output pulse from the entire amplification system are shown in Figure 2.
Figure 1 Schematic of the Ho:YAG SCF amplification system
Figure 2 Spectral and temporal evolution of the Ho:YAG SCF amplification system
In conventional amplification techniques, direct amplification of femtosecond pulses suffers from pulse distortion and beam degradation due to self-focusing effects triggered by strong nonlinear phase shifts. This limitation confines bulk/fiber amplifiers to operating only within the picosecond pulse range. This necessitates chirped pulse amplification (CPA) systems based on pulse stretching and compression. While optical parametric chirped pulse amplification (OPCPA) systems can achieve milliwatt-level pulse energy at kHz repetition rates, thermal effects limit improvements in average power and efficiency. While fiber-based CPA systems offer distinct advantages in high average power and high beam quality, their output energy/peak power is constrained by nonlinear effects and optical damage. Consequently, existing technologies struggle to simultaneously optimize the three key performance metrics: power, repetition rate, and pulse width. This study innovatively proposes a discrete Ho:YAG SCF series configuration. By segmentally interrupting the continuous accumulation path of nonlinear phase shift, it reduces the total B-integral of the amplification system. This approach balances SCF length with self-focusing length, thereby mitigating self-focusing risks. By employing a discrete single-crystal fiber structure, this work successfully overcomes the long-standing challenges of nonlinear effect suppression and efficiency enhancement in 2 μm femtosecond laser amplification. It achieves significant breakthroughs in laser performance through a highly efficient, structurally simplified amplification scheme.
This work demonstrates a direct amplification technique for 2 μm femtosecond lasers, providing a novel technical pathway for developing compact, efficient, and high-performance 2 μm ultrafast lasers. Future efforts will integrate pulse selection and post-compression techniques to pursue higher single-pulse energy and shorter pulse widths.
