Incoherent broad-band cavity-enhanced absorption spectroscopy

Incoherent broad band cavity enhanced absorption spectroscopy (IBBCEAS), sometimes called broadband cavity enhanced extinction spectroscopy (IBBCEES), 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.When the optical cavity is illuminated by an incoherent broadband light source like the white light of a lamp or LED, the mode structure of the cavity intensity can be neglected. Consider a cavity of length d formed by two identical high reflectivity mirrors(R1 = R2 = R > 99.9%) with losses L, which is continuously excited with incoherent light of intensity Iin. For an empty resonator with L = 0, the time integrated transmitted intensity I0 is given byA basic IBBCEAS setup consists of an incoherent light source, collimation optics, the absorber of interest and a detector. The incoherent source of 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. Light transmitted through the cavity is detected using a suitable detector, for example, a monochromator / charge coupled device (CCD) combination interfaced with a computer. 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 Reff(λ) can be determined by:Fourier Transform Incoherent Broadband Cavity Enhanced Spectroscopy (FT-IBBCEAS) is a variant of IBBCEAS which uses a Fourier transform spectrometer/photodiode instead of the conventional monochromator/CCD in order to establish a spectrum. In this case, the absorption is determined from the Fourier Transform of the intensity of light escaping the cavity. The combination of a Fourier transform spectrometer allows for high spectral resolution to be achievable, however, at the expense of good temporal resolution, making the technique less suitable for kinetic studies. On the other hand, the approach provides an improvement to conventional Fourier Transform spectroscopy for gas applications where small sample volumes are required (e.g. for discharges, combustion plasmas, flames or chemical flow reactors).Since its development in 2003, IBBCEAS has been used with a wide variety of incoherent light sources, including arc lamps, LEDs, SLEDs and supercontinnum sources.