Low-Jitter, Narrow-Bandwidth, Single Longitudinal Mode Passively Q-Switched Nd:YAG Ring Laser Oscillator with External Feedback

Fourier-limited, also called single-frequency or single-longitudinal-mode (SLM) nanoseconds lasers at 1 μm are key elements for several applications, such as optical communications, data storage and high-resolution spectroscopy. Some special application such as high spectral resolution LIDAR [1], require very high spectral purity (tens of MHz bandwidth), resulting in long pulse durations in the range of several tens of ns. Microchip lasers with large free spectral range (FSR) comparable to the active medium gain bandwidth (about 160 GHz for Nd:YAG) allow easy SLM selection, but with much shorter pulses. Recently, we demonstrated a Cr:YAG Passively Q-Switched (PQS) Nd:YVO 4 ring laser oscillator delivering 50-ns, 80μJ SLM pulses at 0.1–10 kHz repetition rate employing a specifically designed intracavity etalon to enhance SLM operation [2]. For SLM PQS operations, Nd:YAG offers some advantages if compared to Nd:YVO4. Namely, the higher saturation fluence relaxes the fulfilment of the second PQS threshold condition, the ∼2 times longer fluorescence lifetime enhances energy storage and the narrower gain bandwidth favours SLM operation.