To reduce the amplification factor requirements of the pre-amplification system in inertial confinement fusion laser drivers, simplify the system structure, and lower engineering complexity and development costs. It is suggested to use a dual-pass ytterbium-doped single-mode fiber amplifier in the front end to enhance the output energy to the μJ level. At this energy level, nonlinear effects are insufficient to disrupt the single-mode transmission conditions of the fiber, ensuring beam quality. Additionally, under pump powers of hundreds of mW, severe thermal effects do not occur, facilitating thermal management in the front-end system. However, the impact of energy enhancement on pulse temporal waveform distortion and contrast requires computational evaluation.

A dual-pass ytterbium-doped single-mode fiber amplification scheme was designed, incorporating filters and circulators as reflective units to effectively suppress the re-reflection of out-of-band spontaneous emission noise into the amplifier. In the scheme, co-directional pumping in the first pass improved the signal-to-noise ratio, while backward pumping in the second pass enhanced laser output capability. Then, numerical simulations were conducted using a rate equation model that accounted for upper- and lower-level population densities, signal light, pump light, and broadband spontaneous emission. By tracking the temporal evolution of population inversion density to address gain recovery between dual-pass amplification stages, the study focused on dual-pass amplification capabilities, gain saturation characteristics, and amplified pulse waveform variations under the influence of forward and backward spontaneous emission.

Compared to previous studies on dual-pass ytterbium-doped fiber amplification, this paper presented a systematic investigation of dual-pass amplification characteristics based on multi-wavelength ytterbium-doped fiber rate equations while considering broadband spontaneous emission. Computational results (Fig.2~Fig.4, and Fig.10) demonstrated that under 200 mW effective pump power, using commercial single-mode ytterbium-doped fiber and standard optical components, the amplified output energy exceeded 1 μJ for input optical pulses with 2 ns~3 ns pulse width and 1 nJ energy, achieving a pulse peak-to-leading-edge spontaneous emission noise floor contrast ratio better than 10^5. Compared to scenarios without insertion loss, the dual-pass gain difference decreased from 9.1 dB to 8.3 dB, both below the total insertion loss of 11 dB. This indicated that gain saturation effects become weaker with increasing pump power when insertion loss existed, and the dual-pass gain did not decrease synchronously with insertion loss. Input and reflection-stage losses received partial compensation through gain saturation, while output-stage component losses directly attenuated amplified pulse energy. Computational results (Fig.5~Fig.7) revealed that the forward amplified spontaneous emission spectrum peaks at 1029 nm, determined jointly by the interplay of emission and absorption cross-section spectra. The reverse amplified spontaneous emission spike at 1053 nm arises because forward amplified spontaneous emission at this wavelength is reflected and re-amplified at the amplifier’s right end. The non-uniform upper-level population density distribution along the fiber length results in unequal gains for spontaneous emission sources during forward and reverse amplification. The final amplified spontaneous emission intensity ratio depends on this population distribution. Both forward and reverse amplified spontaneous emission intensities increase with pump power but are suppressed to varying degrees when optical pulses are injected. Computational results (Fig.9 and Fig.10) showed that the first amplification stage operated in a small-signal regime with negligible pulse distortion, while gain saturation primarily occurred in the second stage. Based on peak power values and theoretical nonlinear coefficients of single-mode fiber, the core refractive index variation induced by nonlinear effects was estimated at 10^ - 6. This variation is three orders of magnitude smaller than the core-cladding refractive index difference (numerical aperture, 0.12), ensuring single-mode transmission throughout the amplification process and effectively suppressing higher-order mode excitation.

The results showed that under 200 mW effective pump power, achieving μJ-level pulse energy output was feasible, and boosting the front-end system’s output energy to the μJ level using a dual-pass ytterbium-doped single-mode fiber amplifier was viable. In the amplifier structure shown in Fig. 1, circulator 1 and filter 2 should preferably be components with lower insertion loss. Study provides valuable insights for optimizing front-end systems in high-power laser facilities.