Method and apparatus for suppression of crosstalk and noise in time-division multiplexed interferometric sensor systems

Abstract

Unwanted signal components in time-division multiplexed (TDM) systems may lead to crosstalk and noise if these pulses overlap with signal pulses from an interrogated sensor. The crosstalk and noise are dominated by interference between the signal pulses from the interrogated sensor and the unwanted signal components and can be greatly reduced by suppressing this interference signal. The unwanted signal components may include overlapping pulses originating from different sets of interrogation pulses (repetition periods). Modulating the phase or frequency between the repetition periods so that the unwanted interference signal does not appear at frequencies from which the phase of the interrogated sensor is demodulated suppresses this interference. Other unwanted signal components include leakage light during dark periods of the duty cycle of an interrogation signal. Modulating the phase difference between the interrogation signal and the leakage light suppresses the interference between the leakage light and the interrogation signal.

1 . An interferometric sensor system, comprising:
 an optical network having multiple optical pathways between a transmitter unit and a receiver unit, wherein pairs of optical pathways form sensor interferometers, each sensor interferometer having a sensor imbalance;   an optical source in the transmitter unit for generating optical signals comprising a sequence of optical pulses during pulse transmission time slots that are selected from a plurality of transmission time slots with remaining transmission time slots providing dark transmission time slots without pulse generation, wherein each transmission time slot comprises one of a plurality of transmission time intervals-the plurality of transmission time intervals divided into a sequence of time-division multiplexing (TDM) repetition periods-from each TDM repetition period located at the same position relative to the start of each TDM repetition period; and   a modulator disposed in the transmitter unit and configured to modulate a phase of the optical signals between at least one of the pulse transmission time slots and the dark transmission time slots before the optical signals reach the optical network such that unwanted interference signal components reaching the receiver unit are distributed to frequency bands that do not affect a demodulated sensor signal.   
   
   
       2 . The system of  claim 1 , wherein the unwanted interference signal components comprise unwanted interference between a signal of one of the pulse transmission time slots and a signal of one of the dark transmission time slots. 
   
   
       3 . The system of  claim 1 , wherein the unwanted interference signal components comprise unwanted interference between a signal of one of the pulse transmission time slots and a signal of another one of the pulse transmission time slots. 
   
   
       4 . The system of  claim 1 , wherein the modulator is configured to modulate the phase of the optical signals to provide a shift in frequency of interference between a signal of one of the pulse transmission time slots and a signal of any other one of the transmission time slots away from frequency bands used for the demodulated sensor signal. 
   
   
       5 . The system of  claim 1 , wherein the modulator is configured to modulate a phase or a frequency of the optical pulses between TDM repetition periods to provide a shift in frequency of interference between a delayed signal of one of the TDM repetition periods and a signal of any other one of the TDM repetition periods away from frequency bands used for the demodulated sensor signal. 
   
   
       6 . The system of  claim 1 , wherein the modulator is configured to suppress noise due to leakage light during the dark transmission time slots. 
   
   
       7 . A method of interrogating sensor interferometers of an optical network comprising multiple optical pathways from a transmitter unit to a receiver unit, wherein pairs of optical pathways form the sensor interferometers, each sensor interferometer having a sensor imbalance, the method comprising:
 defining a sequence of time-division multiplexing (TDM) repetition periods;   producing an optical signal containing optical pulses within a portion of a TDM repetition period, wherein corresponding optical pulses in different TDM repetition periods have the same position relative to the start of the TDM repetition period, the remaining portion of the TDM repetition period containing leakage light;   varying a phase difference of the optical pulses between consecutive TDM repetition periods at the inverse of a sub-carrier frequency;   phase modulating the optical signal such that interference between the optical pulses and the leakage light will not contain frequency components in a frequency band centered at the sub-carrier frequency;   transmitting the optical signal to the optical network;   receiving an interference signal generated by reflections of the optical signal from the sensor interferometers in the optical network; and   extracting a portion of the interference signal in the frequency band centered at the sub-carrier frequency such that the portion of the interference signal does not contain the frequency components due to the interference between the optical pulses and the leakage light.   
   
   
       8 . The method of  claim 7 , wherein the portion of the TDM repetition period has corresponding edges separated by a length of time equal to the sensor imbalance.