We have implemented optical absorption spectroscopy in which a Ti:sapphire pumped femtosecond optical parametric oscillator based on periodically poled lithium niobate was used as a broadband source to directly acquire a mid-infrared absorption spectrum of methane gas. Fourier-transform spectroscopy was performed using the idler output from the optical parametric oscillator to directly acquire spectra spanning over 600 nm (14.4 THz or 480 cm−1) with around 3 nm (78 GHz or 2.6 cm−1) resolution. This approach combines the advantages of spectroscopy using broadband thermal sources with the high power and excellent beam quality of a mode-locked laser source.

Acquisition was carried out using a custom-made scanning interferometer (right), scanned at 5 Hz and time-calibrated to a delay resolution of around 100 as using a parallel HeNe laser

interferometer. The scan range of the interferomete was around 18 ps and, as one arm was scanned, interferograms at the idler and the HeNe laser wavelengths were detected simultaneously on a PbSe photoconductor (OPO, idler) and a Si photodiode (HeNe). The interferograms were digitized on separate channels of a deep memory oscilloscope

(Agilent Infinium) and the data set size used was approximately 250 000 samples.  Typical data are shown below.

By using OPO cavity-length tuning a total of 9 independent spectra

were recorded and concatenated with

excellent registration. A full spectrum spanning 600 nm

(14.4 THz) from 3.25 to 3.85 µm.

The figure opposite shows (top) the methane spectrum constructed from the six individual spectral transmission measurements and (bottom) the EPA calculated methane spectrum with (inset) longer wavelength absorption features shown magnified 50 times

The advantage of broadband laser light over thermal sources is its spatial coherence which allows it to be tightly focused and efficiently coupled into an optical fibre. We have carried out spectroscopy inside a photonic bandgap fibre (opposite) that guided at 3.3µm, corresponding to the methane absorption lines.

The figure below shows two spectra recorded for the core containing only nitrogen (thick line) and a methane:nitrogen mixture (thin line). The complicated structure of the reference spectrum originates from the transmission properties of the fiber used. In (b) we present the experimental transmission profile obtained by taking the ratio of these two spectra (thick line) and we compare this with the corresponding calculated transmission spectrum for a 5:95 methane:nitrogen mixture (thin line). The calculated spectrum was based on data from the Hitran database [9] and simulated a measurement with a resolution of 3.3 nm which is close to our estimated experimental resolution of 3.1 cm-1.

The differences between the observed and calculated spectra may arise because we were operating in a spectral region in which the fiber transmission profile contained steep edges, and modulations at frequencies close to those in the methane spectrum.