An acoustic emission sensor was designed for partial discharges (PD), constructed, calibrated, and tested. The device is
based on an interferometric fiber-optic probe and is able to detect PD ultrasound emission at 150 kHz. It may be placed
next to the discharge source, thus overcoming the difficulties of acoustic attenuation. The device works in the
appropriate bandwidth for narrowband acoustic detection of PD activity, like the PZT transducers mounted on the
exterior of the transformer tank. It represents a simple and cheap alternative for detecting acoustic emission, susceptible
of being used in a multi-channel optical configuration and able to provide information for locating the source.
The sensor is an optical fiber coil exposed to the ultrasonic waves that is interrogated with an all-fiber Mach-Zehnder
interferometer. We report first the calibration at the natural frequency of the coil (20 kHz) and at the main frequency of
the application (150 kHz), and compared with the response of PZT transducers. The sensitivity decays with the
frequency, but it is comparable with the PZT sensitivity by placing the sensor next to the source, which is possible with
the immersed approach (or embedded). A certain range of compensation is obtained at low frequencies with a feed-back
loop. A second feed-back loop with electronic resonance around 150 kHz is used in order to improve the sensitivity.
Thus, the conditioning circuit provides directly the amplified optical phase signal. Results of acoustic emission with both
frequencies simultaneously are presented.