Abstract

Photonic bandgap (PBG) fibers offer a good solution for gas sensing because more than 98% of the light transmitted is confined within their air core. In previous works, we have demonstrated an all-silica photonic bandgap (PBG) fiber capable of low-loss (0.95dB m^-1) transmission in the mid-infrared region and its application to methane sensing in the range from 3.0 to 3.3μm using a thermal light source and a spectrophotometer [1]. An alternative method, based on Fourier transform infrared (FTIR) spectroscopy, offers a higher resolution, and is presented in this paper. The experimental arrangement is shown in Figure 1 (left). The light source is an optical parametric oscillator (OPO) based on a 1 mm-thick periodically-poled lithium niobate (PPLN) crystal and idler resonant around 3.2μm. Accurate tuning was provided by a piezo-electric actuator situated in the translation stage of the cavity end mirror. The spectral measurement and processing were performed using a FTIR spectrometer consisting of a scanning Michelson interferometer. The idler beam was sent through the interferometer and all wavelengths except the idler were removed with a Ge filter. Light leaving the Michelson interferometer was coupled into an 80cm-long PBG fiber using a ZnSe lens, and the output from this fiber detected using a PbSe photodiode. This PBG fiber, of core diameter 40μm and pitch of 6.5μm, although similar to the one used in [1], was designed to transmit over the full range of fundamental absorption peaks of methane around 3.2μm. Both ends of the fibers were mounted inside specially-designed gas cells. These allowed light to be coupled in and out of the fiber through CaF₂ windows, whilst providing a means to fill the fiber core with a precisely controlled mixture of methane and nitrogen. The transmission spectra were calculated from a Fourier transform of the interferogram acquired in a single scan of the interferometer mirror. The scanning mirror was mounted on a linear actuator running at a frequency of 0.5Hz with a usable range of 3.6 mm corresponding approximately to a resolution of 3nm.

© 2007 IEEE