Frequency combs

Frequency comb spectroscopy, in particular dual-comb spectroscopy, is a novel technique that is suitable for fast, precise and broadband measurements on a large number of absorbers. For example, it is possible to determine the concentration of several gases in less than 1 millisecond using one single measurement. Dual-comb spectroscopy provides great flexibility for the best compromise between measurement speed, measurement accuracy and the number of substances to be measured. At Fraunhofer IPM it is our goal to make this technology easily accessible and usable for process measurement technology. Therefore, our research within the COSPA project focuses on frequency comb generation, frequency conversion and parallel, fast measurement of several gas components in one sample. In particular, we focus on maximum flexibility for the measurement of gases in the mid-infrared range, since gas molecules in this wavelength range exhibit strong and characteristic spectral lines.

Frequency combs: Optical rulers

A frequency comb (FC) is a laser light source that emits light using a large number of adjacent spectral lines with equal and well-defined frequency spacing. Hence, the name frequency comb. A frequency comb is used to measure the frequency of electromagnetic radiation very precisely, so that it is also referred to as an »optical ruler«. The development of frequency combs began more than 20 years ago and is considered groundbreaking. Its leading inventors, Prof. Theodor Hänsch of the Max Planck Institute for Quantum Physics and his US colleague John Hall, received the 2005 Nobel Prize in Physics for their work on optical frequency comb generators.

Dual comb systems: multi-heterodyne spectrometers

The use of two frequency combs allows for a multi-heterodyne measurement procedure in which one comb is superimposed with a second one. If the spectral lines of one comb are offset from the spectral lines of the other, a beat or periodic change in intensity occurs, which can be evaluated in a similar way to Fourier transform spectroscopy. As a result, a single photodetector takes the place of a classical spectrometer. This is a further advantage for gas spectroscopy, since the measurement method avoids the limitations of classical spectrometers and permits a freely selectable frequency resolution in the range of a few hundred MHz. In comparison, typical widths of gas absorptions under atmospheric conditions lie in the single-digit GHz range.

Research focus: Production of dual combs

The construction of a single frequency comb is complex because it requires many highly controllable components. These control requirements become more demanding for the dual comb technique due to the additional need to synchronize the two single combs. Frequency combs that can be derived from a common source are of particular interest for applications outside the laboratory, since synchronization is considerably simplified here. Against this background, we generate two frequency combs via electro-optical modulation from a single continuous wave laser and spectrally widen them in a further step via nonlinear effects in a fiber.

Research focus: Conversion to mid-infrared

Frequency combs suitable for gas spectroscopy are typically generated in the near infrared and can therefore only be applied to gases with absorptions in this spectral range. Based on the current state of technology, it is therefore unavoidable to convert these frequency combs into the desired spectral range using non-linear processes such as difference frequency generation. In spectroscopy, the mid-infrared region is of particular importance because many molecules have characteristic spectral lines in this spectral range. Drawing on many years of experience in the generation of tunable laser sources and the nonlinear conversion of various light sources, we are developing a conversion module for the MIR range that is capable of converting a single or two superimposed frequency combs into any desired spectral range from 3 µm to 5 µm.

Research focus: Multi-component analysis

The high frequency resolution and the large spectral range which can be covered in one single measurement are promising when it comes to separating the absorption of different sample components and performing an isolated concentration determination. Therefore, we investigate the possibilities of simultaneously measuring the concentration of several gases whose characteristic spectral lines overlap, and of determining different gases which absorb in different spectral ranges, by temporally separated measurement.