B. Cavity-Enhanced Absorption Spectroscopy (CEAS) CEAS is also known as ICOS (integrated cavity output spectroscopy). Developed in 1998 (Engeln et al.; O Keefe et al.) In cavity ringdown spectroscopy, a pulse bounces backwards and forwards between two mirrors. The measurement of interest is the time decay of the pulse in the cavity. What happens if we use a cw-laser, that is, one with a continuous output? The measurement of the transmitted intensity through an optical cavity forms the basis of cavityenhanced absorption spectroscopy.
Light in an optical cavity: Consider how a plane wave interacts with a Fabry-Perot etalon: Resonant conditions occur when the wave reproduces itself after each round trip. Wavelengths matching the resonant condition are: λ q = 2L q where q is the axial mode number. The axial modes of the cavity are then
The influence of mirror reflectivity The intensity of light transmitted through a cavity with mirrors of reflectivity R is given by the Airy functions: Some (surprising) conclusions: Most wavelengths are reflected The cavity is perfectly transmitting at certain wavelengths irrespective of the mirror reflectivity!!! As the reflectivity increases, the transmitting wavelengths get narrower Scanning a laser slowly shows transmission as a function of wavelength.
The separation between adjacent transmitting wavelengths is known as the free spectral range, ν fsr : ν fsr = c 2nL where n is the refractive index of the medium ( 1 for gases) Cavities are also characterised by their finesse, F: F ν fsr π = = Δν 1 R R E.g., What is the finesse of cavities with mirrors of 99% and 99.99% reflectivity? F 99% = π 0.99 / (1 0.99) = 313 F 99.9% = π 0.999 / (1 0.999)= 3140 (Higher finesse implies that the transmitting part of the cavity is narrower.) Light transmitted through the cavity includes long pathlengths through the etalon use in absorption spectroscopy?
Experimental approach: With a laser and a stable optical cavity: 1. scan the laser continuously, recording intensity as a function of wavelength. Initially, the spectrum has a lot of mode structure. 2. adjust the cavity length, changing the axial modes of the cavity 3. repeat (1) and (2) over the same spectral region until the mode structure is averaged out, leaving a beautiful spectrum. The cavity modes can be shifted by adjusting the length: thermal expansion of the cavity piezo modulation of the cavity length or minimised using a suitable filter to smear out mode structure.
The absorption coefficient in the cavity is then: 1 ΔI αc = (1 R) L I compared to 1 ΔI α sp = L I0 for a single pass system (i.e., no optical cavity). The sensitivity in the cavity is thus enhanced by a factor of (1 R). That is, an equivalent decrease in the transmitted intensity, ΔI/I, corresponds to a smaller absorption coefficient when using an optical cavity than in a single pass absorption experiment. E.g., Compare the absorption coefficients of a 1% decrease in transmitted intensity in a cavity of 100 cm for (a) a single pass experiment, (b) an optical cavity with 99.9% reflective mirrors Single pass: α sp = (100 cm) -1 (0.01 / 1.00) = 10-4 cm -1 Cavity: α c = (100 cm) -1 (0.01 / 0.99) (0.001) = 10-7 cm -1
A real CEAS experiment: (M. Staak, PhD 2005) ECDL = Extended cavity diode laser (tunable) Wavelength measurement: etalon relative wavelength measurement NH 3 cell absolute reference lines wavelength meter identify wavelength region
Infrared spectra of (2 mbar) formaldehyde of extremely weak combination bands comprising 3 to 5 normal modes (e.g., ν 3 +2ν 4 +ν 5 at 6636 cm -1 ): Over 8000 absorption lines identified between 6550 cm -1 and 6800 cm -1 with a resolution of about 0.001 cm -1 The Doppler-broadened linewidth (~0.015 cm -1 ) at room temperature is clearly resolved.
Broadband cavity-enhanced absorption spectroscopy (BBCEAS) Enhanced absorption in a cavity does not require a laser; a broadband source will also work. Broadband (white light) sources: arc lamps, LEDs, incandescent bulbs. In BBCEAS a broadband source is used together with a monochromator and CCD a lot of spectral information simultaneously Experimental system: High reflectivity mirrors PC CCD
Drawbacks compared to laser-based methods: - Lower spectral density - More difficult to image light - Lower resolution (determined by monochromator) Advantages: - Much more information collected at once: a 1024 column CCD is effectively 1024 individual detectors at separate wavelengths! - Cheaper, more robust, lower power - Mode structure is not resolved Results: 1.1 ppb NO 3