System for multiplexed high resolution measurement of frequency

Abstract

A system for performing high resolution measurement of frequency variations in multimode fiber laser acoustic sensors having an end-pumped active fiber sensor emitting a plurality of lasing modes, wherein precise frequencies of the lasing modes emitted are dependent upon a strain acting on said acoustic sensor. An optical fiber is connected to the active fiber sensor for transmitting the lasing modes. The optical fiber transmits the multiple lasing modes to an optical amplifier fiber where the lasing mode signals are amplified. The amplified lasing modes signals are then fed through a phase modulator connected to the optical fiber for applying a phase generated carrier on the lasing mode signals. The output from the phase modulator is supplied to a receiving interferometer having a predetermined path imbalance for interferometrically analyzing the frequency shifts of each of the lasing mode signals, wherein the predetermined path imbalance is matched with the longitudinal mode spacing between the lasing modes of the active fiber sensor. A demodulation unit is connected to the receiving interferometer for receiving and demodulating the output of the receiving interferometer to determine the frequency shift incurred by the lasing mode signals, wherein the demodulated frequency shift is proportional to the strain acting on the acoustic sensor. In an alternative embodiment, a plurality of active fiber sensors may be pumped by a single laser light source with their outputs multiplexed together and fed through the receiving interferometer for interferometric analysis of all the signals.