Mid-IR frequency combs: generation and applications

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 [1]. 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.

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Fig. 1. A ring-type subharmonic OPO cavity pumped by an ultrafast Tm-fiber laser.

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Fig. 2. The OPO ‘engine’ that contains orientation-patterned GaAs as nonlinear crystal.

Doubly resonant OPOs operating at degeneracy which we develop in our group are a special class of synchronously pumped OPOs, which combine low pump threshold with an exceptionally broad bandwidth [2]. In addition, they are phase- and frequency locked to the ultrafast pump laser [3,4]. Low intracavity dispersion, combined with an enormous OPO acceptance bandwidth near degeneracy point, results in extremely broad instantaneous mid-IR bandwidths that can be more than one octave wide [5,6].

Applications of such mid-IR combs include environmental monitoring, real-time analysis of chemical /bio threats and explosives, trace molecular detection, study of dynamic processes (e.g. combustion) and medical breath analysis. Because of their unique coherence properties, frequency combs are also ideal tools for studying quantum optics phenomena such as quantum entanglement, photon squeezing, and random number generation.

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Fig. 3. Frequency comb spanning 2.6-7.5 µm, produced as a subharmonic of a Tm-fiber laser [6].

Fig. 4. The usable range of a subharmonic frequency comb, as compared to the most prominent molecular resonances.

[1] Jun Ye and Steven T. Cundiff (eds.), Femtosecond Optical Frequency Comb: Principle, Operation, and Applications, Springer, 2005.

[2] K.L. Vodopyanov, S.T. Wong, R.L. Byer, Infrared frequency comb methods, arrangements and applications, US Patent 8,384,990 B2 (2010).

[3] N. Leindecker, A. Marandi, R. L. Byer, K. L. Vodopyanov, Broadband degenerate OPO for mid-infrared frequency comb generation, Opt. Express 19, 6296-6302 (2011).

[4] A. Marandi, N. Leindecker, V. Pervak, R.L. Byer, K. L. Vodopyanov, Coherence properties of a broadband femtosecond mid-IR optical parametric oscillator operating at degeneracy, Opt. Express 20, 7255-7262 (2012).

[5] N. Leindecker, A. Marandi, R.L. Byer, K. L. Vodopyanov, J. Jiang, I. Hartl, M. Fermann, and P. G. Schunemann, Octave-spanning ultrafast OPO with 2.6-6.1 µm instantaneous bandwidth pumped by femtosecond Tm-fiber laser, Opt. Express 20, 7047-7053 (2012).

[6] V. O. Smolski, H. Yang, S. D. Gorelov, P. G. Schunemann, and K. L. Vodopyanov, Coherence properties of a 2.6-7.5 µm frequency comb produced as a subharmonic of a Tm-fiber laser, Opt. Lett. 41, 1388-1391 (2016). pdf