Project Alma Mater

Absorption of Light, Macro algae and Atmosphere

 

Marie Curie Intra European Fellowship (IEF)

 

Project No: 3012109

 

Persons in Charge: Dr. Ranjini Raghunandan, Dr. Andy Ruth

 

 

 

Background Methodology Outreach Project Output

 

          IBBCEAS FT-IBBCEAS Off-Axis CEAS

 

 

 

Incoherent Broadband Cavity Enhanced Absorption Spectroscopy (IBBCEAS)

 

IBBCEAS measures the transmission of light intensity through a stable optical cavity consisting of high reflectance mirrors (typically R>99.9%). The technique is realized using incoherent sources of radiation e.g. Xenon arc lamps, LEDs or supercontinuum (SC) lasers, hence the name.

 

Typically in IBBCEAS, the wavelength selection of the transmitted light takes place after the cavity by either dispersive or interferometric means. The light is either directly focused onto the entrance slit of amonochromator and imaged onto a charged coupled device (CCD) array via a dispersive optical element (e.g. a reflection grating) or imaged onto the entrance aperture of a conventional interferometer. The spectrum is reconstructed taking the Fourier transform of the recorded interferogram.

 

 

 

 

Similar to other cavity enhanced spectroscopic techniques, in IBBCEAS, the transmission signal strength is measured with and without the absorber of interest present inside the cavity ( I(λ) and I₀(λ)respectively). From the ratio of the wavelength-dependent transmitted intensities, the effective reflectivity of the mirrors R_eff(λ) and the sample path length per pass d inside the cavity, the sample's extinction coefficient α(λ) is calculated as:

 

 

 

 

 

 

 

 

The sensitivity (smallest achievable α for a given sample) increases for large mirror reflectivities and large path lengths in the cavity, which is maximal, if d equals the cavity length.(1-Reff) includes all unspecified losses per pass (e.g. scattering or diffraction losses) other than the losses due to the limited reflectivity of the cavity mirrors.

 

To obtain quantitative results, the reflectivity of the mirrors must be accurately determined. This is usually accomplished by measuring the reflectivity as a function of wavelength using known concentrations of a calibration sample inside the cavity. By knowing the number density n (molecules/cm³) and wavelength-dependent absorption cross-section of the calibration sample, the effective reflectivity R_eff(λ) can be determined by:

 

 

 

 

 

where σ(λ) is the known absorption cross-section of the gas and d is the length of the cavity.

 

The incoherent radiation is spectrally filtered to match the bandwidth of the high reflectivity cavity mirrors. The filtered light is passively coupled into a stable optical cavity formed by two mirrors. Due to the high reflectivity of the mirrors effective absorption path lengths can reach a few kilometres. The spectral resolution of IBBCEAS depends solely on the way the wavelength of the transmitted light through the cavity is selected. Dispersive methods (typically grating spectrographs with multichannel detectors) enable high time resolution but limited selectivity, while interferometric approaches permit a high spectral resolution but a limited time resolution.