  The ability of detecting harmful chemicals is an important safety requirement
for drinking water systems. An apparatus for in-water chemical sensing based on
the absorption of evanescent waves generated by a quantum cascade laser array
propagating in a silver halide optical fiber immersed into water is
demonstrated. We present a theoretical analysis of the sensitivity of the
system and experimentally characterize its real-time response and spectroscopic
detection for injection of a sample chemical (ethanol) in a tube containing
water.
