Robust all-PM fiber few-cycle pulse source at 1560 nm and its second-harmonic generation

We demonstrate a robust and compact all-polarization-maintaining (all-PM) fiber laser system delivering few-cycle pulses at 1560 nm and its second-harmonic generation (SHG) at 780 nm. The system features a simplified architecture comprising a PM seed oscillator, a PM fiber power amplifier, and a periodically poled lithium niobate (PPLN) based frequency doubling module. Benefiting from the all-PM fiber integration, the source exhibits superior environmental immunity and long-term reliability. Experimental results show that the amplified pulses at 1560 nm reach a minimum duration of ∼25.4 fs, corresponding to approximately 4.9 optical cycles, with an average power of 139 mW and a pulse energy of 3.4 nJ. The radio-frequency (RF) spectrum at the fundamental repetition rate exhibits a signal-to-noise ratio of ∼106 dB, confirming a stable mode-locking state. The integrated relative intensity noise (RIN) over 10 Hz-1 MHz is 0.0108% at the maximum pump power, and the root-mean-square power fluctuation is 0.17% over 10 h under free-running operation. Furthermore, PPLN-based second-harmonic generation produces 780-nm pulses with a deconvolved duration of ∼55 fs. The high-fidelity temporal and noise characteristics, validated by full-field reconstruction and comprehensive relative intensity/phase noise analysis, indicate that this robust all-PM fiber source can be a promising alternative for precision frequency metrology, nonlinear imaging, and ultrafast spectroscopy.