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Power spectral characteristic of a microchip Nd:YAG laser subjected to frequency-shifted optical feedback

We systematically investigated the power spectrum of a Nd:YAG laser with external frequency-shifted feedback and identified three factors dominating the spectrum, namely, the feedback level, the pumping level of the laser diode (LD), and the shifted frequency introduced in the external cavity. For very weak feedback, the laser power spectrum presents two peaks at frequencies of Ω and ωr, which are the shifted frequency and relaxation oscillation frequency, respectively. When the feedback level is increased to an intermediate level, the laser presents a series of nonlinear effects to cause harmonic and parametric oscillation; for strong feedback, only harmonic peaks are observed in the spectrum. The impact of the pumping levels and the amount of frequency shifting are also experimentally investigated. Especially, even at a weak feedback level, when the frequency Ω approaches ωr, strong nonlinear effects still appear in laser dynamics to make the laser power present only harmonic oscillation. A theoretical analysis is provided which agrees well with the experimental results.