Mid-infrared subharmonic optical parametric oscillator (OPO) produces frequency comb with one-and-half-octave-wide instantaneous band and superior temporal coherence, suitable for real-time trace molecular detection.
Optical parametric oscillators (OPOs) have long been recognized as a versatile means of producing optical output in important spectral regions unreachable by laser sources. The mid-IR (> 2.5 µm) is one such region, rich in spectroscopic information but underpopulated by convenient laser lines. In a typical OPO, a laser pumps a suitable optical material having second-order nonlinear susceptibility. When combined with an appropriate resonator for optical feedback, the OPO splits photon into two photons (signal and idler) with longer wavelengths. The oscillation wavelength is tuned by adjusting the parameters of the resonator or nonlinear material. With their broad tunability OPOs are used extensively for mid-IR spectroscopy. Quantum cascade lasers (QCLs) now offer a tantalizing alternative to OPOs, although with somewhat smaller tuning range. It is challenging however, for both OPOs and QCLs, to be tuned in a precise and continuous fashion, preserving narrow-linewidth single-longitudinal-mode operation for precision spectroscopic measurements.
Fourier Transform (FT) spectroscopy is a nice mathematical trick that helps evade this limitation. As originally proposed by Michelson more than a century years ago, one can perform high-resolution spectroscopy even with a broadband source. To retrieve the whole optical spectrum one just needs to interfere an optical beam with its time-delayed replica and then take a Fourier transform of the detector signal vs. time delay dependence.
Optical frequency combs appear to be an ideal instrument for FT spectroscopy. The broadband and coherent nature of frequency combs – both in frequency (a manifold of equally spaced narrow spectral lines) and in time (a strictly periodic train of pulses with stable carrier-envelope phase) – has allowed already a revolution in precision metrology and high-resolution visible-UV spectroscopy . A gold rush for creating broadband mid-IR frequency combs began a decade ago with a number of techniques applied, such as supercontinuum generation, optical rectification, difference-frequency generation, OPOs, microresonators, and quantum cascade lasers.