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We demonstrate the feasibility of optically demodulating and processing the output produced by an
interferometric fiber sensor. A technique for providing the time filtering of an optical signal in the
optical domain based on the use of a doped fiber section is reported.
Optical processing is a field which offers enormous potential for rapid, parallel processing of
information, and is an area which will continue to experience significant growth over the coming
years. Optical sensors have a natural inherent compatibility with this form of processing, which
could be utilized to great advantage for processesing the data from large arrays of sensors. Many
forms of fiber optic sensor, however, produce an optical output which does not vary in a uniform
fashion with the measurand field, and consequently require a linearization circuit. or 'demodulator'
to provide a linear transfer function before a signal could be converted back to the optical domain
and operated on by an optical processor. This is particularly true with interferometric fiber sensors
which exhibit a cosine squared transfer function. To demodulate this form of output, a range of
demodulation schemes have been developed, such as active phase tracking homodyne [1], phase
generated carrier (PGC) passive homodyne [2], and synthetic heterodyne [3J. These techniques
allow the detection of measurand-induced phase shifts down to the irad/'JHz level over a wide range
of frequencies with high linearity and wide dynamic range. Certain forms of interferometric sensors
designed for low frequency measurands, such as magnetometers and electhc field sensors, require
the additional demodulation of a 'dither' induced phase carrier signal which is modulated by the DC
and low frequency measurand information. In such systems, the overall demodulation process
requires several electronic processing blocks after photodetection of the interferometer signal. In
applications where arrays of such sensors are of interest, a significant amount of electronics may be
required to fully demodulate the sensor outputs. We have examined the practicality of using optical
techniques to perform much of the demodulation process, and in this paper demonstrate the
feasibility of this concept.
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