High-coherence mid-infrared dual-comb spectroscopy spanning 2.6 to 5.2 μm

Abstract

Mid-infrared dual-comb spectroscopy has the potential to supplant conventional high-resolution Fourier transform spectroscopy in applications that require high resolution, accuracy, signal-to-noise ratio, and speed. Until now, dual-comb spectroscopy in the mid-infrared has been limited to narrow optical bandwidths or to low signal-to-noise ratios. Using a combination of digital signal processing and broadband frequency conversion in waveguides, we demonstrate a midinfrared dual-comb spectrometer that can measure comb-tooth resolved spectra across an octave of bandwidth in the mid-infrared from 2.6-5.2 µm with sub-MHz frequency precision and accuracy and with a spectral signal-to-noise ratio as high as 6500. As a demonstration, we measure the highly structured, broadband cross-section of propane (C3H8) in the 2860-3020 cm -1 region, the complex phase/amplitude spectrum of carbonyl sulfide (COS) in the 2000 to 2100 cm -1 region, and the complex spectra of methane, acetylene, and ethane in the 2860-3400 cm -1 region.Mid-infrared spectroscopy is a powerful technique for the multispecies detection of trace gases with applications ranging from the detection of hazardous materials, to environmental monitoring and industrial monitoring. Compared to the near-infrared, where laser sources are more plentiful, the techniques for measuring mid-infrared spectra are more limited. Mid-infrared spectra are most commonly acquired by Fourier transform spectroscopy (FTS), which provides accurate and high resolution spectra but requires a scanning delay arm and blackbody source leading to large instruments and long acquisition times. Dual-comb spectroscopy (DCS) is a high-performance alternative to conventional FTS providing high resolution, absolute frequency accuracy, fast acquisition times, long interaction lengths, broad bandwidth coverage, and high signal-to-noise ratio [1,2]. The advantages of speed and long path length are of particular relevance to non-laboratory applications, for example in open-path atmospheric monitoring or industrial process monitoring [3][4][5][6]. However, up until now, DCS has only been demonstrated with its full panoply of advantages in the near-infrared, from ~ 1 to 2 µm [7-10]. The near-infrared has much more limited applications compared to the mid-infrared since molecular crosssections are typically 1000 times weaker, if they exist at all. DCS in the mid-infrared has indeed been actively pursued [11][12][13][14][15][16][17][18][19][20][21][22][23][24], yet it is not competitive with high-resolution conventional FTS, limited by the coherence and/or the bandwidth of mid-infrared comb sources.Quantitative broadband mid-infrared DCS requires addressing strong overlapping requirements on the underlying mid-infrared frequency combs: they must produce broad and relatively flat optical spectra while maintaining mutual coherence over the measurement time. Without coherence, adjacent comb teeth blend together, sacrificing orders of magnitude in spectral resolution and obscuring both the frequency and amplit...

Frequency comb spectroscopy

“…Such schemes have been implemented even with free--running or loosely locked lasers 67,68 . They also compensate for the residual fluctuations of stabilized systems 70 . A third trend, which is currently stimulating many creative experiments, is to design dual--comb systems with built--in passive mutual coherence.…”

Dual-comb spectroscopy for high-temperature reaction kinetics

“…The use of two frequency combs, dual-comb spectroscopy, has been applied to high-temperature, nonreacting conditions in a rapid compression machine by Draper et al at 704 µs time-resolution and also by Schroeder et al in the exhaust of a gas turbine at high temperatures and 10-60 s time-resolution [4,8]. Draper Efforts to extend DCS into the 3-20 µm region have involved distributed feedback generation DFG [14], optical parametric oscillators (OPO) [15][16][17], and quantum-cascadelaser (QCL) approaches [6,7,[18][19][20]. Obtaining coherency between two frequency combs is an enabling factor for DCS and DFG approaches can achieve this readily.…”

Terahertz hyperspectral imaging with dual chip-scale combs

“…This scheme eliminates the need for any opto-mechanical movement, which enables high acquisition speeds (μs) and user-friendly operation. Although most of this field primarily involves commercially available fiber-laser based optical frequency combs (OFCs) operating in the nearinfrared, its usefulness has also been demonstrated in both the mid-infrared [19,20] and the THz [21] using nonlinear media for frequency conversion. In 2012, a fundamentally different OFC was demonstrated in the mid-infrared, the quantum cascade laser (QCL) OFC [22], which exploits the nonlinearity of a low-dispersion, electrically pumped, semiconductor gain medium to directly emit comb radiation around an optical frequency defined via careful control of the layered semiconductor growth.…”