Cavity enhanced absorption spectroscopy using a broadband prism cavity and a supercontinuum source

We report the design and construction of a cavity enhanced absorption spectrometer using broadband Brewster's angle prism retroreflectors and a spatially coherent 500 nm to $>$1.75 $\mu $m supercontinuum excitation source. Using prisms made from fused silica an effective cavity reflectivity of $>$99.99{\%} at 1.064 $\mu $m was achieved. A proof of principle experiment was performed by recording the cavity enhanced absorption spectrum of the weak b-X (1$\leftarrow $0) transition of molecular oxygen at 14529 cm$^{-1}$ and the fifth overtone of the acetylene C-H stretch at 18430 cm$^{-1}$. CCD frames were integrated for 150 sec and 30 sec, with 3 frames (each 100 cm$^{-1}$ wide) and 1 frame (266 cm$^{-1}$ wide) required to observe the O$_{2}$ and C$_{2}$H$_{2}$ spectra, respectively. A rms noise equivalent absorption ($\alpha _{min})$ of 7.21x10$^{-8}$ cm$^{{\-}1 }$Hz$^{{\-}1/2}$ and 1.28x10$^{{\-}7}$ cm$^{-1 }$Hz$^{{\-}1/2}$ with full width half maximum line widths of 0.18 cm$^{-1}$ and 0.44 cm$^{-1}$ was achieved for the molecular oxygen band and acetylene overtone.