Combing through air with light

Some amazing things can be done with light: it can be generated with a variety of characteristics, it can be manipulated very precisely for industry and manufacturing applications, and the way it propagates through matter can tell us a great deal about natural and artificial materials. When light takes the form of so-called frequency combs, it may be used to calibrate the spectrometers used in telescopes to study distant planets, perform spectroscopic analyses of molecules produced by industrial and environmental processes, and to build LIDAR devices with ultra-high precision.

In the research group founded by Professor Ursula Keller in the Institute for Quantum Electronics, an active area of work is that of dual-comb spectroscopy (DCS), a powerful optical technique that greatly advanced the capability of gas absorption spectroscopy applied to environmental monitoring, among others. In their latest publication, which has just appeared in the journal Nature Communications, researchers from the Keller group succeeded in realising a compact and highly sensitive dual-comb spectrometer based on a specifically designed optical parametric oscillator (OPO) and an improved detection method. The optical system can be used to scan both short-wave (from 1300nm to 1670nm) and mid-infrared wavelengths (between 2700nm and 5000nm), offering a controllable measurement range that can be tuned according to the absorption spectra of the studied gases.

Previously, the mid-infrared range could be studied with DCS at a great experimental cost: for example, the light sources for the spectrometers had to be kept stable over time with bulky electronics. Yet this frequency range harbours tremendous potential for future applications. "The mid-infrared range is also known as the molecular fingerprint region, because many molecules strongly absorb light at these wavelengths," explains doctoral student Carolin Bauer, who's the first author of the publication. Bauer and her co-authors have now shown a way to probe this important portion of the electromagnetic spectrum that combines high sensitivity with a comparatively simpler setup.

One comb, two combs

A frequency comb is a light source made of light pulses that are evenly spaced in time and consist of a superposition of different frequencies. In frequency space, its frequency components are equidistant from each other, just like the teeth of a comb – hence its name. Frequency combs are used in spectroscopy to study light signals with frequencies that are too high to be detected with conventional electronics. In frequency comb spectroscopy, a light input is mixed with a frequency comb to create an output in the form of a beat signal that can be detected and spectrally analysed to learn about the original light signal.

Dual-comb spectroscopy relies on light pulses from two frequency combs with slightly different spacings between their prongs. For example, a gas imprints characteristic patterns on the optical lines of the frequency comb that passes through it: mixing these comb lines with the second comb signal produces output microwave comb lines that can be measured directly with an oscilloscope to draw conclusions about the absorption characteristics and molecular concentrations of the gas.

Combs and cavities

The dual-comb spectrometer built by Bauer and co-workers has two optical cavities at its core, that of the pump laser and that of the parametric oscillator. The solid-state laser producing the frequency combs is an yttrium aluminium garnet (YAG) laser doped with ytterbium, designed to emit two spatially separated light beams at near-infrared frequencies within a single cavity. In a second stage, the two frequency combs synchronously pump – that is, excite – the optical parametric oscillator that affords wavelength tunability to the spectrometer. The nonlinear crystal in the OPO converts each input near-infrared frequency comb into two output light beams at longer wavelengths, referred to as signal and idler beams. As with the laser source, the signal and idler pairs from the two combs are spatially separated in the OPO cavity. This so-called spatial multiplexing within the two optical cavities, that is, the ability to produce co-propagating beams in the light source given by the laser and the OPO, is a distinctive feature of the research team's system. It reduces the complexity of the spectrometer and helps to achieve the high sensitivity reported in the paper.

Gas detection

A new scheme for light detection, known as cross-comb spectroscopy (CCS), constitutes the final crucial component of the DCS system presented by Bauer and colleagues. The principle of CCS was only introduced last year by researchers at the California Institute of Technology in the US. For their demonstration, the team adapted the original CCS method so that their OPO acts as both light source and detector. Specifically, one idler beam coming from the OPO propagates through the absorbing gas sample and is then sent back to the OPO: in the crystal the idler interacts with the signal beam from the other comb and, through another nonlinear process known as up-conversion, creates the output signal that can be detected and analysed to learn more about the considered gas sample. Unlike the approach put forward by the team at the California Institute of Technology, this scheme doesn't require an additional external light source for the up-conversion process.

As a proof of concept, Bauer and colleagues used their dual-comb spectrometer to detect methane in its natural concentration of 2 parts per million in ambient air over a 3-metre path length. The system required as little as 10 milliseconds for this measurement; most important, its demonstrated sensitivity was shown to be close to the fundamental physical limits given by the available laser power. The complexity of the experimental setup is a known challenge in conventional dual-comb spectroscopy, as is the limited sensitivity of the measurements. Thanks to their spatially multiplexed, wavelength-tunable light source and intra-cavity detection through CCS, the researchers showed that high-sensitivity gas absorption DCS is possible over a large spectral range in the infrared region.

Translated from German by Gaia Donati