Abstract: The invention is a method for obtaining a measure of the light propagation time difference for two light-propagating-media paths. The first step consists of generating two substantially-identical frequency-modulated light waves whereby the frequency of the light waves is offset from a reference frequency by a different frequency increment for each basic time interval in each of a plurality of groups of three or more basic time intervals. Each frequency increment is the sum of a specified increment and a frequency-modulation-error increment. The frequency-modulation-error increments associated with the specified increments are independent of each other and unknown. The second step consists of feeding the two light waves into the entry points of two light-propagating-media paths having a light propagation time difference and obtaining a combination light wave by summing the light waves emerging from the exit points of the two light-propagating-media paths.

Abstract: Leakage optical signals injected into a time division multiplexed sensor system are modulated during off periods in the duty cycle in the sensor interrogation signal. The leakage light is modulated such that its unwanted contributions to the sensor output signals are displaced in frequency from the base band frequency. This frequency displacement greatly reduces unwanted noise power in the received signals.

Abstract: A phase modulation control apparatus is taught in combination with an interferometer excited by a coherent light source providing a light beam to the interferometer having first and second optical paths. The interferometer senses a physical quantity such as pressure. The interferometer is coupled to provide an optical signal containing information providing a measure of the physical quantity via first and second interfering beams to a detector. The detector provides a composite output signal. The invention phase modulation control apparatus has a modulation means responsive to a modulating frequency signal and a phase modulation control signal to provide a frequency modulating factor signal for frequency modulating at least a portion of the light beam to provide a phase modulation between the first and second interfering beams at a carrier frequency. The composite output signal has at least four harmonic signals referenced to the carrier frequency.

Abstract: A switch-selectable calibration signal is applied to an interferometric sensor channel of fiber optic sensor array for monitoring its operation. A calibration signal is applied at a single physical location to determine the channel scale factors and proper operability of multiple sensors at remote locations in the array. An optical signal input to each interferometric sensor channel in the array, and a compensating interferometer receives optical signals output from the fiber optic sensor array. A signal is applied to the compensating interferometer to form an output signal that simulates signals output from each interferometric sensor in the array. These signals are processed to determine the scale factor of each interferometer in the fiber optic sensor array.