METHOD AND APPARATUS FOR SUPPRESSION OF CROSSTALK AND NOISE IN TIME-DIVISION MULTIPLEXED INTERFEROMETRIC SENSOR SYSTEMS

• US 20080018904A1
• Filed: 10/02/2007
• Published: 01/24/2008
• Est. Priority Date: 02/11/2005
• Status: Abandoned Application

• Current Assignee: Optoplan AS (Nokia Corporation)
• Original Assignee: Optoplan AS (Nokia Corporation)
• Inventors: Ronnekleiv, Erlend, WAAGAARD, OLE
• Application Number:
  US11/866,032
• Publication Number:
  US 20080018904A1
• Time in Patent Office: Days
• US Class Current: 356/478
• CPC Class Codes: G01D 5/35312 using a Fabry Perot
• CPC Class Codes: G01D 5/3539 using time division multiplexing

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.

Claims

1. 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 plurality of transmission time intervals;

dividing the plurality of transmission time intervals into a sequence of time-division multiplexing (TDM) repetition periods;

transmitting optical pulses from the transmitter unit during a portion of the transmission time intervals such that corresponding optical pulses are positioned equally in each TDM repetition period; and

modulating a frequency of the optical pulses between different TDM repetition periods 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 method of claim 1, wherein modulating the frequency of the optical signals comprises:

applying a frequency shift between each TDM repetition period; and

resetting the frequency shift to an initial frequency after a predetermined number of TDM repetition periods.

3. The method of claim 2, wherein the frequency shift is reset after at least k_max repetition periods, wherein k_max=└

T_f/T┘

, where Tis a period of one of the TDM repetition periods and T_f is a maximum difference in delay from the transmitter unit to the receiver unit between a sensor pathway and any other pathway from the transmitter unit to the receiver unit that gives rise to unwanted interference at the receiver unit.

4. The method of claim 2, wherein the frequency shift is larger than a bandwidth of the receiver unit.

5. The method of claim 1, wherein modulating the frequency comprises at least one of using a frequency shifter and tuning the frequency of a light source for producing the optical pulses.

6. The method of claim 1, wherein the unwanted interference signal components comprise unwanted interference between a delayed signal of one of the optical pulses in a first TDM repetition period and a signal of a corresponding optical pulse in a second TDM repetition period.

7. The method of claim 1, wherein the unwanted interference signal components comprise unwanted interference between a delayed signal of one of the optical pulses in a first TDM repetition period and a signal of a non-corresponding optical pulse in the first or a second TDM repetition period.

8. 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 for generating a sequence of pulses during a sequence of time-division multiplexing (TDM) repetition periods such that corresponding optical pulses are positioned equally in each TDM repetition period; and

a modulator configured to modulate a frequency of the pulses between different TDM repetition periods such that unwanted interference signal components reaching the receiver unit are distributed to frequency bands that do not affect a demodulated sensor signal.

9. The system of claim 8, wherein the unwanted interference signal components comprise unwanted interference between a delayed signal of one of the optical pulses in a first TDM repetition period and a signal of a corresponding optical pulse in a second TDM repetition period.

10. The system of claim 8, wherein the unwanted interference signal components comprise unwanted interference between a delayed signal of one of the optical pulses in a first TDM repetition period and a signal of a non-corresponding optical pulse in the first or a second TDM repetition period.

11. The system of claim 8, wherein the modulator is configured 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.

12. The system of claim 11, wherein the shift in frequency is larger than a bandwidth of the receiver unit.

13. The system of claim 8, wherein the modulator comprises a Bragg cell.

14. The system of claim 8, wherein the modulator is configured to apply a frequency shift between each TDM repetition period and to reset the frequency shift to an initial frequency after a predetermined number of TDM repetition periods.

15. The method of claim 14, wherein the frequency shift is reset after at least k_max repetition periods, wherein k_max=└

T_f/T┘

, where Tis a period of one of the TDM repetition periods and T_f is a maximum difference in delay from the transmitter unit to the receiver unit between a sensor pathway and any other pathway from the transmitter unit to the receiver unit that gives rise to unwanted interference at the receiver unit.

16. 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; and

a tunable optical source for generating a sequence of pulses during a sequence of time-division multiplexing (TDM) repetition periods such that corresponding optical pulses are positioned equally in each TDM repetition period, wherein the optical source is configured to modulate a frequency of the pulses between different TDM repetition periods such that unwanted interference signal components reaching the receiver unit are distributed to frequency bands that do not affect a demodulated sensor signal.

17. The system of claim 16, wherein the unwanted interference signal components comprise unwanted interference between a delayed signal of one of the optical pulses in a first TDM repetition period and a signal of a corresponding optical pulse in a second TDM repetition period.

18. The system of claim 16, wherein the unwanted interference signal components comprise unwanted interference between a delayed signal of one of the optical pulses in a first TDM repetition period and a signal of a non-corresponding optical pulse in the first or a second TDM repetition period.

19. The system of claim 16, wherein the optical source is configured to apply a frequency shift between each TDM repetition period and to reset the frequency shift to an initial frequency after a predetermined number of TDM repetition periods.

20. The system of claim 16, wherein the receiver unit comprises a demodulation unit for producing the demodulated sensor signal and at least one of a digital filter in the demodulation unit or an analog receiver filter for removing the unwanted interference signal components.