Intensity noise suppression in pulsed lasers via combining relaxation oscillation with nonlinear frequency conversion

This paper reports a method for reducing the relative intensity noise (RIN) of actively Q-switched pulsed laser by combining the relaxation oscillation (RO) and nonlinear optical frequency conversion (NOFC). When a RO is introduced by slightly lowering the diffraction loss of acousto-optic (AO) Q-switch, the initial population inversion per pulse is stabilized to a constant value by the RO process. The RIN between pulses decreases from 4.90% to 2.48% by optimizing the diffraction loss of AO Q-switch and the RO duration. On this basis, the NOFC is introduced into the cavity by a noncritical phase-matched nonlinear crystal LiB₃O₅ (LBO) to further suppress the intensity fluctuation arising from different longitudinal-mode beating with random phases, and the RIN further decreases to 1.30%, corresponding to a pulse energy stability of 0.09%. This is the lowest RIN between pulses in large energy Q-switched lasers, to the best of our knowledge. The proposed method paves the way to effectively suppress the RIN of pulsed lasers, which will help us to explore what we believe to be many new physical phenomena due to their high peak power and available manipulation in time dimension.