Mid-infrared subharmonic optical parametric oscillators (OPO) produce frequency comb light 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 a 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. Quantumcascade 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 narrowlinewidth single-longitudinal-mode operation for precision spectroscopic measurements.
Fourier Transform (FT) spectroscopy is a nice mathematical trick that helps evade this limitati on. As originally proposed by Michelson more than a century 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 versus time delay dependence. The gold rush for creating broadband mid-IR frequency combs – laser sources with the output consisting of manifold of equally spaced phase locked spectral lines – began a decade ago. A number of techniques were used that include mode-locked lasers, optical rectification, difference – frequency generation, OPOs, whispering gallery microresonators, and QCLs. Doubly resonant OPOs operating at degeneracy, pioneered by the group of Konstantin Vodopyanov, are a special class of synchronously pumped OPOs that combine low pump threshold with an exceptionally broad oscillation bandwidth. These devices can produce a broadband frequency comb centered at twice the pump wavelength. Most importantly, they inherit the coherence properties of the pump laser. Vodopyanov and his team have demonstrated broadband mid-IR combs in a number of degenerate (subharmonic) OPO systems using both periodically poled lithium niobate (PPLN) and orientation-patterned gallium arsenide (OP-GaAs) crystals as the nonlinear optical material. These were combined with a variety of pump sources including erbium-fiber (1.56 μm) thulium-fiber (2 μm), Cr:ZnSe (2.45 μm) and Cr:ZnS (2.35 μm) modelocked lasers. With the thulium-fiber pump, a world-record mid-IR frequency comb spectral span of 2.6 to 7.5 μm was achieved in 2015. Such frequency comb is especially suitable for massively parallel, simultaneously broadband and high-resolution spectroscopy, based on the Fourier transform principle.
The most attractive approach is dual-comb (or multi-heterodyne) spectroscopy, when two phase-locked frequency combs having a small offset in the comb spacing are combined and used for interrogating molecular spectral signatures. Operating in the middle of the spectroscopically important “fingerprint” region, where OH, CH, CO, and NH bonds show their strongest vibrational signatures has allowed sensitive (down to part-per billion concentration) real-time (~1 sec) and massively parallel (about a million spectral points) trace gases measurements through their effect on the OPO spectrum. Also, there is an on-going research in Vodopyanov’s lab (DARPA SCOUT program) – on creating a chip-scale broadband mid-IR comb for the realtime and real-world molecular sensing.