Mid infrared optical parametric oscillator
Continuous wave optical parametric oscillators (OPOs) are coherent sources of radiation having unique features, useful for a wide range of applications in high-resolution spectroscopy, frequency metrology, and fundamental physics. We developed a singly-resonant OPO emitting in the range between 2.6 and 4.2 μm, a spectral region of growing interest for precise measurements and accurate tests of fundamental theoretical models. The OPO is based on a periodically poled MgO:LiNbO<sub>3</sub> crystal placed in a bow-tie cavity (Fig. 1) resonant for the signal (1.4 – 1.8 μm) and is pumped at 1064 nm up to 10 W. In the correspondence of the maximum pump power the OPO emits about 1 W of power over the whole tunability range.
As the spectral purity of the source is a crucial issue for high-resolution spectroscopy, we developed different methods to narrow the spectral linewidth of the idler in the mid-infrared region and stabilize the emission frequency of our OPO. In a singly-resonant OPO the idler spectral features depend both on pump and signal frequency fluctuations, which in turn are to a large extent uncorrelated to each other. In a first scheme we pumped the OPO by a narrow-linewidth (40 kHz over 1 ms) cw Yb-doped fibre laser, emitting at 1064 nm, amplified by a Yb-doped fibre amplifier up to 10 W; both pump and signal frequencies have been directly phase-locked to the frequency comb of a NIR-emitting fs mode-locked fibre laser. In this configuration we tested the spectral features of our source by observing sub-Doppler profiles of several CH<sub>3</sub>I rovibrational transitions: we resolved their hyperfine structure (Fig. 2) and determined the absolute frequencies of the multiplet components with an uncertainty of 50 kHz. We estimated an upper limit of 200 kHz for the source linewidth and a long-term stability of 3×10<sup>−12</sup> τ<sup>−1/2</sup> between 1 and 200 s.
Our current activity is focused on further improving the spectral features of the source. We replaced the pump laser by a Nd:YAG laser whose linewidth is narrowed at the sub-kHz level (over 1 ms) against a stable ultra-low-expansion (ULE) Fabry–Pérot cavity and used a frequency comb synthesizer as the transfer oscillator between the pump laser mode and the signal mode resonating in the OPO cavity. As a consequence, the spectral features of the pump laser are transferred to the signal mode, independently of technical fluctuations of the comb frequencies. In turn, the fluctuations of idler mode frequency are reduced to the same order of the pump laser ones. The pump residual frequency noise has been measured against a reference Fabry–Pérot: from the frequency noise power spectral density (PSD) we estimated a sub-kHz pump linewidth, for times shorter than 1 ms. The signal residual frequency noise has been estimated in a similar way, using a second reference cavity. Also in this case, for times shorter than 1 ms we estimated a sub-kHz signal linewidth, resulting in a final sub-kHz linewidth for the idler in the mid-infrared region.