A research team from the Laser Spectroscopy Division of the Max Planck Institute of Quantum Optics, led by Dr Nathalie Picqué, has recorded fine spectroscopic “fingerprints” of molecular gases like ethylene or acetylene in the main mid-infrared region using mode-locked femtosecond lasers.
Experimental dual-comb spectrum showing 82,000 resolved comb lines recorded in half an hour (a). The molecular fingerprints, here of ethylene, are well sampled (b) by the comb, while the individual comb lines show a shape (c) exactly following the theoretical expectation. (Image credit: Max Planck Institute of Quantum Optics)
Molecular spectroscopy is an important tool for examining the structure and dynamics of matter, and also for identifying and recognizing a variety of substances. It finds innumerable applications from basic research in biology, chemistry, astronomy, or physics to industrial process control, medical diagnostics, or environmental monitoring.
Dual-comb spectroscopy performed using two lasers having somewhat different pulse repetition rates is evolving as a robust tool for quick, accurate, and sensitive broadband spectroscopy of molecules. Similar to common Fourier-transform spectroscopy, it requires only a single fast photo-detector; however, it does not have any restrictions on recording speed and resolving power as inflicted by mechanically moving parts.
However, the two frequency comb sources face the challenge of maintaining mutual optical phase coherence while taking data. Any instability can make averaging the “interferograms” in the detector signal impossible eventually, which is done to enhance the signal-to-noise ratio, or to acquire usable spectra even under light-starved conditions. In reality, it is hard to achieve this target. Elaborate laser stabilization schemes have earlier been used to demonstrate mutual coherence times reaching 1 second.
Presently, Zaijun Chen, a doctoral student, has been able to realize mutual coherence times beyond 30 minutes: he rectifies phase fluctuations of the laser pulses once they leave the laser using an acousto-optic feed-forward servo control scheme. This feed-forward method has first been refined in the near-infrared spectral region using mode-locked erbium-doped femtosecond lasers, as reported in
In more recent experiments, currently described in the
Proceedings of the National Academy of Sciences of the United States of America, the researchers have used nonlinear difference frequency generation to create frequency combs with long mutual coherence times in the more significant 3-µm mid-infrared spectral region, where most molecules exhibit strong and characteristic absorption spectra.
The new instrument without moving parts measures mid-infrared high-resolution spectra, over a broad span. All the spectral elements are simultaneously measured, leading to spectra with precise molecular line shapes and direct calibration of the frequency scale to an atomic clock.
Zaijun Chen, Doctoral Student, Max Planck Institute of Quantum Optics
Thus, it has opened up new interesting possibilities for precision molecular spectroscopy across broad spectral bandwidths. “
The 3-µm spectral region is ideally suited for the sensitive detection of hydrocarbons as well as oxygen- or nitrogen-containing organic compounds. Therefore many questions relevant to fundamental and applied spectroscopy may be addressed,” concluded Zaijun Chen.