The electromagnetic spectrum consists of all the possible wavelengths (and associated frequencies) of electromagnetic radiation. With conventional spectroscopy techniques, individual frequencies are scanned one at a time, providing information about molecular content. A frequency comb, the discovery of which led to the Nobel Prize in 2005, is a light spectrum producing spectral lines just like the teeth of a comb corresponding to numerous frequencies. The METROCOMB (Femtosecond comb optical parametric oscillators for high-resolution spectroscopy in the mid-infrared) project brought together a consortium of international experts in the burgeoning technology fields of ultrafast lasers, stabilised frequency combs and optical parametric oscillators. Project members successfully developed an optical parametric oscillator frequency comb laser, achieving an unprecedented degree of detection sensitivity and accuracy. Known as a frequency comb laser, the new laser operates in the infrared region, in which conventional frequency comb lasers cannot operate. It therefore has the potential to transform spectroscopy in the molecular fingerprint region, delivering real-time imaging with molecular identification, detecting trace chemicals and offering medical breath analysis. The optical parametric oscillator establishing the near-infrared femtosecond comb was pumped by a mode-locked semiconductor laser (VECSEL). Instead of producing light of just one wavelength, the frequency comb produced several different wavelengths, forming a set of evenly spaced frequencies. These laser pulses were ultrashort, separated by nanosecond intervals. Project members succeeded in enhancing the stability of the combs in the 1 to 4.5 µm wavelength region. Pulse compression in the VECSEL configuration was performed in two stages. First, an extra-cavity pulse compressor using two diffraction gratings was used to reduce the pulse duration of the primary laser output beam. In the second stage, the team used a five-metre long polarisation-maintaining fibre imposing self-phase modulation on the output beam which broadened the spectrum. After this, the spectrally broadened pulse was compressed once again keeping pulse durations under 300 fs. The high-power, robust femtosecond comb incorporated into a compact, portable and affordable laser configuration developed in METROCOMB could revolutionise industrial and environmental monitoring as well as medical diagnosis and security systems.
Molecular fingerprinting, frequency comb, laser, infrared, spectroscopy, metrology