b) overlapping optical spectra of two combs (black and green) and down converted multi-heterodyne in the RF domain (red), which can be evaluated with electronic equipment. 1: a) dual comb setup schematic with signal and local oscillator comb mixing on a single detector element. Its commercial success mostly builds on its exceptionally low noise levels in short measurement times. As such, it was successfully employed for numerous applications such as protein dynamics studies, combustion analysis, or remote explosives detection. It is optimized for high-speed measurements with microsecond time resolution and below 10 -4 cm -1 spectral resolution. These radio frequency signals can be measured by conventional electronics.Ĭurrently, QCL dual-comb spectroscopy is commercially available as a table-top instrument for research applications. The mixing of the two waves on the detector produces a multi heterodyne signal, where every pair of optical lines produces a signal at a distinct frequency in the radio frequency domain. Both light sources have a set of equidistant emission lines, though the distance between the lines is different for the two sources (see right hand side of Fig. 1: two frequency comb laser sources are superimposed, and the spectrum is measured on a single detector. The operation principle of dual-comb spectroscopy is illustrated in Fig. Its concept and the associated innovation potential were honored with the physics Nobel Prize in 2005. This ground-breaking invention opens the way for integration of frequency comb spectroscopy in laboratory and also everyday life applications.ĭual frequency comb spectroscopy is a seminal new way of measuring optical spectra. A quantum cascade laser frequency comb comprises all important features of a mid-infrared frequency comb on a single semiconductor chip of a few square-millimeters in size. Recently, a novel kind of mid-infrared frequency comb source based on quantum cascade lasers was demonstrated. Until a few years ago, frequency comb sources in the mid-infrared spectral region were bulky and delicate contraptions, a long way from the small and robust instruments desired in process analytical applications. These limitations of single color DFB QCLs and external cavity QCLs for measurements of oil in water content are resolved by employing dual frequency comb spectroscopy, which enables broad band spectral measurements without the complicated mechanical system employed in external cavity QCLs. With external cavity QCLs this issue is resolved, since the laser wavelength can be tuned by several hundreds of inverse cm, however, the simultaneous detection of the background and sample is not possible. Dual-Comb spectroscopy for oil detection Background:ĭetection of oil with single color Distributed Feedback Quantum Cascade Laser (DFB QCL) laser requires a reference sample and/or an auxiliary laser with different color for background determination.