Pulse-to-pulse spectral noise in a spontaneous parametric down conversion light source

The presence of pronounced chemical signatures of several important molecules and gases in the mid-infrared (mid-IR) spectral region has enabled numerous spectroscopic applications in diverse fields such as environmental monitoring and the life sciences. This has motivated the development of new, efficient light sources and detectors in this spectral region. In this work, we report on the development and characterization of a simple, low-cost, tunable mid-IR light source, covering the 1.45 µm to 3.5 µm spectral region, based on spontaneous parametric down conversion (SPDC) in MgO doped periodically poled lithium niobate (MgO:PPLN). The spectral coverage can be easily extended to 5 µm simply by choosing a crystal with an appropriate poling period. A low repetition rate, passively Q-switched laser provides high-energy pump pulses at 1.03 µm for the SPDC source, resulting in the generation of signal and idler fields. The source is characterized in terms of pulse-to-pulse spectral stability of the generated signal field. Interestingly, a high pulse-to-pulse energy stability does not necessarily lead to high spectral stability. Pulse-to-pulse spectral stability is measured at different pump pulse energies, showing improved spectral stability when operated well above the SPDC threshold.