Courtesy of Nathalie Picqué
The frequency comb—a spectrum of phase-coherent, evenly spaced laser lines—has found applications that extend far beyond its initial purpose as a ruler in frequency space. Most recently, it has emerged as an exciting tool to extend the limits of molecular spectroscopy.
OSA Fellow Nathalie Picqué, who specializes in frequency comb spectroscopy at the Max-Planck Institute of Quantum Optics, discussed recent advances in the field during the third visionary talk of the all-virtual FiO+LS 2020 conference on Tuesday, 15 September. She described the evolution of the technique, its advantages over traditional spectroscopy methods and progress toward lab-on-a-chip technology.
Picqué began her session by reviewing the basics of frequency comb spectroscopy. Most of the time, a frequency comb is generated by a mode-locked laser that emits a train of short pulses. The spectrum of a single pulse, as analyzed by a spectrometer, appears as a very broad continuum. If two pulses are analyzed, however, they will interfere and show a broad spectrum with a periodic pattern.
“If we now have a spectrometer in which many pulses can interfere, this periodic pattern thins out, and the spectrometer output is made of narrow laser lines which are evenly spaced,” said Picqué. “But this is only possible if the waveform at the output of the laser has been very precisely controlled in timing and in phase.”
Even small random fluctuations in timing and phase will cause the laser lines to broaden and prevent the generation of frequency combs. With improvements in pulsed laser technology, the possibilities of frequency comb spectroscopy became apparent to a growing number of researchers.
A dramatic improvement
In its simplest form, known as direct frequency comb spectroscopy, a frequency comb as a broadband light source illuminates an absorbing sample. A spectrometer analyzes the transmission spectrum, which is a frequency comb with an imprint of the sample’s absorption features.
“This is a field that is particularly relevant to spectroscopy of molecules because molecules usually have a very broad and dense spectrum,” Picqué said. “So broad-spectral-bandwidth spectroscopy is a very useful tool for these complex spectra.”
Her work has demonstrated that frequency combs dramatically enhance the performance of existing spectrometers, such as scanning Michelson Fourier transform interferometers or dispersers. In particular, the resolution, accuracy, acquisition time and acquisition speed for broad-spectral-bandwidth linear absorption spectroscopy can all be dramatically improved.
The power of two
The field has grown considerably over the last 15 years, according to Picqué, who showed a slide depicting the evolutionary tree of frequency combs. While frequency measurements with frequency combs led to better atomic clocks and precision spectroscopy measurements, frequency comb spectroscopy has also branched into alternate forms and applications.
Picqué’s research focuses on a branch known as dual-comb spectroscopy, a comb-enabled approach to Fourier transform interferometry with the additional advantage of no moving parts. It employs two frequency combs that emit a slightly different repetition frequency, where one is used to interrogate the sample while the other acts as a local oscillator. The two beams are combined with a beam splitter that leads into a single photodiode, and the resulting signal is Fourier transformed to reveal the spectrum.
“The recording consists of measuring the interference signal between the two frequency combs as a function of time,” she said. “So when the pulses [from both combs] overlap, there is a strong interference signal. When they do not overlap, there is no interference signal.”
Applications of frequency comb spectroscopy are still in the exploration phase, but possibilities include environmental sensing, analytical chemistry and precision spectroscopy of atomic transitions. Picqué’s lab is currently exploring the idea of lab-on-a-chip technology for broadband gas-phase spectroscopy, a field which has remained inaccessible up to this point.
During her talk, she described a different approach to on-chip frequency comb generation that employs a low-noise III-V-semiconductor-on-silicon mode-locked laser on a photonic chip. With dual-comb spectroscopy, Picqué and her colleagues recorded high-resolution sensitive multiplexed spectra with resolved comb lines in times as short as 5 microseconds. In addition, the device was able to perform carbon monoxide detection within 1 millsecond.
Such work paves the way for broad, dense networks of spectrometers that perform real-time, compact environmental sensing of greenhouse gases, industrial pollution or motor-vehicle emissions.