Optical fiber monitoring by detection of polarization variations

• US 5,258,615 A
• Filed: 09/03/1992
• Issued: 11/02/1993
• Est. Priority Date: 08/03/1990
• Status: Expired due to Fees

• Current Assignee: GPT Ltd.
• Original Assignee: GPT Ltd.
• Inventors: Thorley, Anthony M.
• Primary Examiner(s): Dzierzynski, Paul M.
• Assistant Examiner(s): Beyer, James
• Application Number:
  US07/939,421
• Time in Patent Office: 425 Days
• Field of Search: 250/227.17, 250/225, 356/364, 356/365, 356/366, 356/367, 369/13, 359/192, 359/156, 359/122
• US Class Current: 250/227.17
• CPC Class Codes: G01M 11/33 with a light emitter being disposed at one fibre or waveguide end-face, and a light receiver at the other end-face

Abstract

The invention concerns apparatus for monitoring optical fibers by detecting variations in the polarization of light being transmitted through the fibers. In one embodiment the optical fibre carries non-coherent light and the incoming light is split by a passive splitter (2) connected to a polarization beam splitter (5). Variations in the output of this polarization beam splitter are detected, amplified and filtered so that they can be compared with a preset reference voltage. A counter (12) is used to detect variations. Another embodiment relates to the detection of polarization variations in coherent systems.

Claims

1. In a communications system for transmitting data carried by light having a polarization along an optical fiber having a data transmission characteristic, an apparatus for monitoring a disturbance to, and a change in the data transmission characteristic of, the optical fiber, said apparatus comprising:

detector means for detecting variations in the polarization of the light carrying the data caused by a physical disturbance to the optical fiber, and for generating a polarization fluctuation signal indicative of the disturbance and a change in the data transmission characteristic, thereby indicating potential faulty data transmission, said detector means including means for sampling non-coherent light transmitted along the optical fiber to obtain a non-coherent light sample signal, and a polarization beam splitter for converting the sample signal to the polarization fluctuation signal having a variable light power; and

means for screening out variations in the polarization fluctuation signal caused by natural disturbances to the optical fiber to prevent such natural disturbances from interfering with data transmission.

2. The apparatus as recited in claim 1, further comprising processor means including transducer means for converting the polarization fluctuation signal of variable power to an electrical signal of variable voltage.

3. The apparatus as recited in claim 2, wherein the screening means includes a filter for filtering the electrical signal to screen out voltage variations caused by natural disturbances to the fiber.

4. The apparatus as recited in claim 3, wherein the processor means includes comparator means for comparing the filtered signal to a preset reference voltage, and for generating a comparator output signal having a state that changes due to the comparison of the filtered signal with the reference voltage.

5. The apparatus as recited in claim 4, wherein the processor means includes counter means for counting the number of changes in state of the comparator output signal, timer means for timing a predetermined time period during which the counter means is operational, and alarm means for triggering an alarm when the number of changes in state of the comparator output signal counted during said predetermined time period exceeds a preset value.

6. In a communications system for transmitting data carried by light having a polarization along an optical fiber having a data transmission characteristic, an apparatus for monitoring a disturbance to, and a change in the data transmission characteristic of, the optical fiber, said apparatus comprising:

detector means for detecting variations in the polarization of the light carrying the data caused by a physical disturbance to the optical fiber, and for generating a polarization fluctuation signal indicative of the disturbance and a change in the data transmission characteristic, thereby indicating potential faulty data transmission, said detector means including means for splitting coherent light transmitted along the optical fiber to obtain a first horizontally polarized signal and a first vertically polarized signal;

means for screening out variations in the polarization fluctuation signal caused by natural disturbances from interfering with data transmission; and

processor means including an oscillator means for generating an oscillator light signal, and means for splitting the oscillator light signal to obtain a second horizontally polarized signal and a second vertically polarized signal.

7. The apparatus as recited in claim 6, wherein the processor means includes means for mixing the first and second horizontally polarized signals to obtain a combined horizontally polarized signal, and means for mixing the first and second vertically polarized signals to obtain a combined vertically polarized signal.

8. The apparatus as recited in claim 7, wherein the processor means includes a first receiver for receiving the combined horizontally polarized signal, and for generating a first receiver output signal;

and a second receiver for receiving the combined vertically polarized signal, and for generating a second receiver output signal; and

combiner means for combining the receiver output signals and for generating an output compensation signal.

9. In a communications system for transmitting data carried by light having a polarization along an optical fiber having a data transmission characteristic, an apparatus for monitoring a disturbance to, and a change in the data transmission characteristic of, the optical fiber, said apparatus comprising:

detector means for detecting variations in the polarization of the light carrying the data caused by a physical disturbance to the optical fiber, and for generating a polarization fluctuation signal indicative of the disturbance and a change in the data transmission characteristic, thereby indicating potential faulty data transmission, said detector means including coupler means for coupling coherent light transmitted along the optical fiber to a receiver;

means for screening out variations in the polarization fluctuation signal caused by natural disturbances to the optical fiber to prevent such natural disturbances from interfering with data transmission; and

processor means including oscillator means for generating an oscillator light signal having a polarization and control means for controlling the state of polarization of the oscillator light signal to match the polarization of the coherent light transmitted along the fiber.

10. The apparatus as recited in claim 9, wherein the control means includes means for generating an alarm when the polarization fluctuation signal exceeds predetermined criteria.