All-polarization-maintaining divided pulse fiber oscillator mode-locked with the optical Kerr effect

In this Letter, we investigate a Yb-doped mode-locked fiber oscillator that uses coherent pulse division and recombination to avoid excessive nonlinear phase shifts. The mode locking mechanism of the laser is based on the accumulation of a differential nonlinear phase between orthogonal polarization modes in the polarization-maintaining fiber segment. The inserted coherent pulse divider, based on YVO₄ crystals rotated successively by 45°, enables stable and undistorted mode-locked steady states. The output pulse energy is increased from 89 pJ in the nondivided operation by ≈6.5 dB to more than 400 pJ with three divisions. A measurement of amplitude fluctuations reveals a simultaneous broadband noise suppression of up to ≈9 dB in the frequency range from 10 kHz to 2 MHz.