  An all-optical detection method for the detection of acoustic pressure waves
is demonstrated. The detection system is based on a stripped (bare) single-mode
fiber. The fiber vibrates as a standard cantilever and the optical output from
the fiber is imaged to a displacement-sensitive optical detector. The absence
of a conventional microphone makes the demonstrated system less susceptible to
the effects that a hazardous environment might have on the sensor. The sensor
is also useful for measurements in high temperature (above $200^{\circ}$C)
environments where conventional microphones will not operate. The
proof-of-concept of the all-optical detection method is demonstrated by
detecting sound waves generated by the photo-acoustic effect of NO$_2$ excited
by a 455 nm LED, where a detection sensitivity of approximately 50 ppm was
achieved.
