Research Projects

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 Comb-based Fourier transform spectroscopy with sub-nominal resolution

The resolution of traditional Fourier transform spectroscopy (FTS), based on incoherent light sources, is limited by the maximum delay range of the interferometer and the acquisition of high-resolution spectra implies long measurement times and large instrument size. This limit is overcome when the incoherent light source is replaced with an optical frequency comb and the nominal resolution is matched precisely to the comb repetition rate. Using a proper sampling approach, the intensities of the comb lines are measured precisely and one measurement yields sampling points spaced by the repetition rate of the comb. For denser sampling point spacing, the measurement is repeated with comb modes tuned to different positions, and all measurements are interleaved. The resulting spectral resolution is limited by the width of the comb lines. Using comb-based FTS allows measuring absorption lines narrower than the nominal (optical path-limited) resolution without ringing effects from the instrumental lineshape and reduces the acquisition time and interferometer length by orders of magnitude​.
We use the comb-based FTS technique in the near- and mid-infrared wavelength range for high precision measurements of entire molecular bands, e.g. the v1+v3 band of CO2 at 1.57 um or the nu4 band of CH3I at 3.3 um.

Sub-Doppler double-resonance spectroscopy using a comb probe

Double-resonance (DR) spectroscopy is a powerful tool for assignment of highly excited energy levels. It provides a way to use an already assigned transition to unambiguously identify the lower or upper state quantum numbers of measured spectra. In optical-optical DR spectroscopy a saturating pump laser transfers the population of a single quantum state into another state, and a weaker probe laser measures transitions from/to the selectively populated/de-populated states. When a monochromatic pump is used, only a narrow velocity group of molecules is excited, and the resulting probe transitions are free of Doppler broadening.
We use a high-power 3.3 µm continuous wave optical parametric oscillator as a pump and a 1.67 µm comb as a probe to detected sub-Doppler DR transitions in methane. The comb probe spectra are recorded using a Fourier transform spectrometer with comb-mode limited resolution. With pump tuned to 9 different transitions in the v3 fundamental band, we detected 36 ladder-type transitions to the 3v3 overtone band region, and 18 V-type transitions to the 2v3 overtone band. The accuracy of the center frequencies was ~1.7 MHz, limited by the frequency stability o the pump. 
The ladder-type probe transitions allowed the first verification of the accuracy of theoretical predictions from highly vibrationally excited states, needed e.g. to model the high-temperature spectra of exoplanets. We compared the center frequencies and relative intensities of these ladder-type transitions to theoretical predictions from the TheoReTS and ExoMol line lists, demonstrating good agreement with TheoReTS.
We recently implementing an enhancement cavity for the probe to increase the increase the absorption sensitivity, improve the frequency precision, and detect a larger number of probe transitions with high signal to noise ratio. 

Continuous-filtering Vernier spectroscopy

Continuous-filtering Vernier spectroscopy is a cavity-enhanced technique that allows the acquisition of broadband spectra with medium to high resolution in few tens of ms using a compact and robust detection system. The principle of CF-VS is the generation of tunable frequency filters for the comb using an external cavity, which also acts as a sample cell. The cavity length is tuned to introduce a small mismatch between the cavity free spectral range (FSR) and comb repetition rate, frep. As a result, the cavity resonances form a series of frequency filters that transmit groups of comb lines through the cavity. These filters, called Vernier orders (VO), can be continuously swept across the entire bandwidth of the comb by scanning either the cavity FSR or the comb frep. A diffraction grating positioned after the cavity separates the consecutive VOs and its rotation is synchronized with the spectral scan of the selected VO, keeping the beam spatially fixed on a detector. In this scheme, the entire spectrum in cavity transmission, with bandwidth limited by the comb source, can be recorded using a single detector positioned after the grating.
We implemented this technique in the MIR for detection of atmospheric species and in the NIR for time resolved detection of water and OH radical in a flame.

Cavity-enhanced complex refractive index spectroscopy

Optical cavities provide high sensitivity to molecular absorption and dispersion since the frequency, width and amplitude of the cavity modes are modified in the vicinity of molecular transitions. Moreover, the broadening and frequency shift of the modes are directly proportional to the imaginary (absorption) and the real (dispersion) parts of the molecular index of refraction, respectively, but independent of the cavity parameters, e.g. mirror reflectivity and cavity length. Thus, measuring the mode broadening and shift allows calibration-free assessment of the complex refractive index of molecular transitions. Using an Er:fiber frequency comb and a Fourier transform spectrometer, we performed cavity-enhanced complex refractive index spectroscopy (CE-CRIS) over 15 THz of bandwidth and retrieved the complex refractive index of three combination bands of CO2 between 1525 nm and 1620 nm. Broadband CE-CRIS opens up for precision spectroscopy of the complex refractive index of several molecular bands simultaneously.

Spectroscopy of high-temperature water and OH radical in a flame

We demonstrated for the first time cavity enhanced comb spectroscopy in a combustion environment by detecting broadband water and OH spectra in a premixed methane/air flat flame. From these measurements, we derived an experimental water line list with absolute line intensities at 1950 K. Line assignment was made using a combination of empirically known energy levels and predictions from the POKAZATEL variational line list.

Photoacoustic spectroscopy

In photoacoustic spectroscopy the absorption of modulated light is detected via measurement of the acoustic wave created during periodic non-radiative deexcitation of the molecules. This technique allows sensitive detection in a sample volume of a few mL. We reported the first photoacoustic detection scheme based on an optical frequency comb (OFC-PAS). A Fourier transform spectrometer is used to modulate the intensity of the exciting comb source at frequencies determined by its scanning speed, and a cantilever-enhanced photoacoustic cell operating in a non-resonant mode provides broadband frequency response. We demonstrated the broadband and the high-resolution capabilities of OFC-PAS by measuring the rovibrational spectra of the fundamental C-H stretch band of CH4, with no instrumental line shape distortions. A limit of detection of 0.8 ppm of methane in N2 was obtained (200 s measurement time, 1 GHz spectral resolution, 1000 mbar total pressure) for an exciting power spectral density of 42 mW/cm-1. OFC-PAS extends the capability of optical sensors for multispecies trace gas analysis in small sample volume with high resolution and selectivity.

Faraday rotation spectroscopy

Spectral interference is one of the major problems of analytical spectroscopy in e.g. combustion environments, where spectra are congested and sufficiently accurate models for background constituents, such as H2O and CO2, are often lacking. Faraday rotation spectroscopy, which relies on probing the opto-magnetic properties of paramagnetic species subjected to an external magnetic field, allows interference-free detection of paramagnetic species, such as NO, NO2, OH and HO2. The insusceptibility of diamagnetic species (e.g. H2O and CO2) to the Faraday effect infers that interferences from these species are efficiently suppressed.
We implemented the FRS detection scheme in our mid-infrared optical frequency comb-based Fourier transform spectrometer to obtain a broadband high-resolution spectroscopy system for interference-free measurement of entire ro-vibrational bands of paramagnetic species. In the first demonstration we measured the spectrum of the entire Q- and R-branches of NO at 5.2-5.4 µm.