Multi-watt red nanosecond frequency-doubled Raman-shifted fiber laser

We present a nanosecond-pulsed 655 nm laser source based on frequency-doubling a Raman-shifted fiber laser. At a repetition rate of 1.5 MHz, the source generates an average power of 3.3 W, corresponding to a pulse energy of 2.2 μJ, with a pulse duration of 1.8 ns. The fundamental Raman-shifted fiber laser operating at 1310 nm has a novel configuration where the first Raman shift is performed in an Yb-doped fiber amplifier and the second Raman shift is performed in a phosphosilicate fiber. Both Raman shifting stages are seeded with narrow linewidth CW signals, enabling the temporal properties of an amplified 1064 nm modulated laser diode to be transferred to narrow-band light at 1310 nm with very high conversion efficiency. The resulting micro-Joule-level, nanosecond pulses at 1310 nm are frequency-doubled to 655 nm in a double LBO crystal setup with a conversion efficiency of 51%. The multi-Watt, micro-Joule-level red pulses have near diffraction limited beam quality (M² ≤ 1.04), making this source ideally suited to biomedical imaging applications such as super-resolution and photoacoustic microscopy.