Method and apparatus for performing optical time domain reflectometry on an optical fibre

1 . A method for performing optical time domain reflectometry (OTDR) on an optical fibre using a transceiver comprising an optical source and a photodetector having an adjustable decision threshold, wherein the transceiver interfaces to a near end of the optical fibre through an optical plug comprising an optical filter arrangement that prevents backscattered or reflected light from the optical fibre from reaching the photodetector when the optical source outputs optical radiation at a first wavelength used for communicating with a remote transceiver at a far end of the optical fibre, and directs backscattered or reflected light from the optical fibre to the photodetector when the optical source outputs optical radiation at a second wavelength used for performing OTDR, the method comprising:

setting the decision threshold of the photodetector to each of a plurality of decision threshold values and, for each decision threshold value:

transmitting a first signal into the optical fibre, wherein the first signal is at the second wavelength and comprises a sequence of optical radiation pulses based on a coded sequence, wherein the sequence of optical radiation pulses are separated by a fill pattern controlled by a binary sequence of high and low values;

obtaining a measurement signal comprising bit sequences based on optical radiation detected by the photodetector using the decision threshold value, the optical radiation being backscattered or reflected light from transmission of the first signal into the optical fibre; and

comparing the measurement signal with the first signal to obtain a correlation signal for the decision threshold value; and

combining the plurality of correlation signals to obtain an OTDR trace of the optical fibre.

2 . The method according to claim 1 , wherein the coded sequence is based on one or more codewords.

3 . The method according to claim 2 , wherein the one or more codewords comprise pairs of bipolar codewords and the sequence of optical radiation pulses comprises a sequence of pairs of unipolar codewords derived from the pairs bipolar codewords.

4 . The method according to claim 2 , wherein the one or more codewords comprise pairs of complimentary Golay sequences.

5 . The method according to claim 1 , wherein each pulse of the sequence of optical radiation pulses comprises one bit of the coded sequence.

6 . The method according to claim 1 , wherein the fill pattern separating each of the plurality of optical radiation pulses is identical.

7 . The method according to claim 1 , wherein the fill pattern comprises N 0s, where N is an integer number, and M 1s, where M is an integer number and wherein |N−M|≤2.

8 . The method according to claim 1 , wherein the fill pattern comprises an equal number of 1s and 0s.

9 . The method according to claim 1 , wherein the fill pattern comprises a binary sequence of alternate 1s and 0s.

10 . The method according to claim 1 , wherein the transceiver is a small form-factor pluggable (SFP) transceiver.

11 . The method according to claim 1 , wherein the first signal comprises AC signals.

12 . The method according to claim 1 , wherein a time taken to transmit the fill pattern is longer than a round trip time of the optical fibre.

13 . An apparatus for performing optical time domain reflectometry (OTDR) on an optical fibre, the apparatus comprising:

a transceiver comprising a transmitter configured to output optical radiation into the optical fibre and a photodetector configured to detect optical radiation incoming from the optical fibre, the photodetector having an adjustable decision threshold;

an optical plug comprising an optical filter arrangement that prevents backscattered or reflected light from the optical fibre from reaching the photodetector when the transmitter outputs optical radiation at a first wavelength used for communicating with a remote transceiver at a far end of the optical fibre, and directs backscattered or reflected light from the optical fibre to the photodetector when the transmitter outputs optical radiation at a second wavelength used for performing OTDR; and

processing circuitry configured to;

set the decision threshold of the photodetector to each of a plurality of decision threshold values and, for each decision threshold value:

transmit a first signal into the optical fibre, wherein the first signal is at the second wavelength and comprises a sequence of optical radiation pulses based on a coded sequence, and wherein the sequence of optical radiation pulses are separated by a fill pattern controlled by a binary sequence of high and low values;

obtain a measurement signal comprising bit sequences based on optical radiation detected by the photodetector using the decision threshold value, the optical radiation being backscattered or reflected light from transmission of the first signal into the optical fibre; and

compare the measurement signal with the first signal to obtain a correlation signal for the decision threshold value; and

combine the plurality of correlation signals to obtain an OTDR trace of the optical fibre.