WIDE-BANDWIDTH COMB-ASSISTED SPECTROSCOPY IN THE FINGERPRINT REGION AND APPLICATION TO THE ν1 FUNDAMENTAL BAND OF 14N216O

Most spectroscopic data available in databases such as HITRAN are retrieved from FTIR measurements and suffer from uncertainties at the MHz level. Much more accurate data, by up to four orders of magnitude, can be achieved using an optical frequency comb to calibrate the frequency axis of a cw laser source. Actually, in the mid-infrared region, at least beyond 5 $\mu$m, the only available commercial solution for a widely tunable cw laser is represented by extended cavity quantum cascade lasers (EC-QCLs), whose locking to an optical frequency comb has been so far inhibited by a large amount of frequency noise, leading to linewidths of about 20 MHz \footnote{Knabe K., Williams P. A., Giorgetta F., Armacost C. M., Crivello S., Radunsky M., and Newbury N., Opt. Express 20, 12432-12442 (2012)}. In this work we overcome this limitation and describe a spectrometer that relies on the frequency locking of an EC-QCL tunable in the 7.55-8.2 $\mu$m range to a 1.9 $\mu$m Tm fiber comb \footnote{Lamperti M., Alsaif B., Gatti D., Fermann M., Farooq A., and Marangoni M., Sci. Rep. 8,1292 (2018)}. It is applied to the first comb-calibrated direct characterisation of the \nub{1} fundamental band of \chem{N_2O}, specifically of nearly 70 lines in the 1240 – 1310 \wn range, from P(40) to R(31). The spectroscopic constants of the upper state are derived from a fit of the line centers with an average rms uncertainty of 4.8x10$^{-6}$ \wn(144 kHz). The coupling of the spectrometer to a high-finesse optical cavity to the purpose of enhancing its sensitivity and addressing weaker absorbers, is also discussed

Absolute spectroscopy near 7.8 μm with a comb-locked extended-cavity quantum-cascade-laser

We report for the first time the frequency locking of an extended-cavity quantum-cascade-laser (EC-QCL) to a near-infrared frequency comb. The locked laser source is exploited to carry out molecular spectroscopy around 7.8 μm with a line-centre frequency combined uncertainty of ~63 kHz. The strength of the approach, in view of an accurate retrieval of line centre frequencies over a spectral range as large as 100 cm-1, is demonstrated on the P(40), P(18) and R(31) lines of the fundamental rovibrational band of N2O covering the centre and edges of the P and R branches. The spectrometer has the potential to be straightforwardly extended to other spectral ranges, till 12 μm, which is the current wavelength limit for commercial cw EC-QCLs

WIDE-BANDWIDTH COMB-ASSISTED SPECTROSCOPY IN THE FINGERPRINT REGION AND APPLICATION TO THE ν1 FUNDAMENTAL BAND OF 14N216O

Most spectroscopic data available in databases such as HITRAN are retrieved from FTIR measurements and suffer from uncertainties at the MHz level. Much more accurate data, by up to four orders of magnitude, can be achieved using an optical frequency comb to calibrate the frequency axis of a cw laser source. Actually, in the mid-infrared region, at least beyond 5 $\mu$m, the only available commercial solution for a widely tunable cw laser is represented by extended cavity quantum cascade lasers (EC-QCLs), whose locking to an optical frequency comb has been so far inhibited by a large amount of frequency noise, leading to linewidths of about 20 MHz \footnote{Knabe K., Williams P. A., Giorgetta F., Armacost C. M., Crivello S., Radunsky M., and Newbury N., Opt. Express 20, 12432-12442 (2012)}. In this work we overcome this limitation and describe a spectrometer that relies on the frequency locking of an EC-QCL tunable in the 7.55-8.2 $\mu$m range to a 1.9 $\mu$m Tm fiber comb \footnote{Lamperti M., Alsaif B., Gatti D., Fermann M., Farooq A., and Marangoni M., Sci. Rep. 8,1292 (2018)}. It is applied to the first comb-calibrated direct characterisation of the \nub{1} fundamental band of \chem{N_2O}, specifically of nearly 70 lines in the 1240 – 1310 \wn range, from P(40) to R(31). The spectroscopic constants of the upper state are derived from a fit of the line centers with an average rms uncertainty of 4.8x10$^{-6}$ \wn(144 kHz). The coupling of the spectrometer to a high-finesse optical cavity to the purpose of enhancing its sensitivity and addressing weaker absorbers, is also discussed

High accuracy line positions of the ν 1 fundamental band of 14 N 2 16 O

International audienc

Comb-calibrated coherent Raman spectroscopy of molecular hydrogen

International audienceHighly accurate measurements of H2 transition frequencies is fundamental for testing the quantum electrodynamics and physics beyond the standard model [1-3]. However, the retrieval of the un-perturbed line positions is very challenging since it compels to work in low pressure conditions: the achievement of high signal-to-noise ratios is then hindered by the weakness of quadrupole transition moments and by the low molecular density. Alternatively, the distortion of the line profile at higher pressure could be carefully modelled in order to compensate for speed-dependent collisional effects and for the strong Dicke narrowing. High accuracy measurements of the Q(1) transition of the pure H2 1-0 band at 4155.25 cm-1 have been performed from 0.2 to 5 atmosphere using stimulated Raman spectroscopy. An Er:fiber frequency comb has been used to calibrate the frequency difference between the pump and Stokes cw lasers involved in the Raman process. The pump laser emits at 737.8 nm and is kept fixed while the Stokes laser is scanned over 3 GHz around 1064 nm. The two beams are spatially superimposed and travel through a multipass cell filled with H2. Figure 1 (a) displays the line profiles measured at seven different pressures (the measurements at the two lowest pressures are displayed in the inset). As it can be noticed from panel (b) the retrieved widths are in a good agreement with ab-initio values based on H2-H2 quantum scattering calculations. The frequency shift, plotted in panel (c), is proportional to pressure above 1 atm and the retrieved pressure coefficient agrees well with previous results [4]. The strength of the approach which provides high signal-to-noise ratio and frequency accuracy at the same time enables the use of more advanced profile models, such as the Hartmann-Tran profile, for line shape investigation

Supplement 1: Third-generation femtosecond technology

Originally published in Optica on 22 July 2014 (optica-1-1-45