Method and OTDR apparatus for optical cable defect location with reduced memory requirement

1. A method for locating defects in an optical cable, the method comprising:

transmitting a first optical signal at a first location of the optical cable;

sampling optical reflections at the first location of the optical cable at a first sample rate over a first time range following transmission of the first optical signal to obtain a first set of sample data corresponding to reflections of the first optical signal;

identifying a first time in the first time range corresponding to a suspected defect location of the optical cable based at least partially on a temporal correlation of the first set of sample data with the transmitted first optical signal;

transmitting a second optical signal at the first location of the optical cable;

selectively sampling optical reflections at the first location of the optical cable at a second sample rate over a second time range following transmission of the second optical signal to obtain a second set of sample data corresponding to reflections of the second optical signal, the second time range including the first time, the second time range being less than the first time range, and the second sample rate being greater than the first sample rate; and

identifying at least one defect location of the optical cable based at least partially on a temporal correlation of the second set of sample data with the transmitted second optical signal.

2. The method of claim 1 , wherein the first optical signal is transmitted with a first pulse width, wherein the second optical signal is transmitted with a second pulse width, and wherein the first pulse width is greater than the second pulse width.

3. The method of claim 1 , wherein the second sample rate is greater than or equal to a constant K times the first sample rate, and wherein the constant K is greater than 1.

4. The method of claim 1 , comprising identifying multiple defect locations (DL 1 , DL 2 ) of the optical cable based at least partially on the temporal correlation of the second set of sample data with the transmitted second optical signal.

5. The method of claim 1 , comprising:

identifying a second time in the first time range corresponding to a second suspected defect location of the optical cable based at least partially on the temporal correlation of the first set of sample data with the transmitted first optical signal;

selectively sampling optical reflections at the first location of the optical cable at the second sample rate in a third time range following transmission of the second optical signal to obtain a third set of sample data corresponding to reflections of the second optical signal, the third time range including the second time, and the third time range being less than the first time range; and

identifying a second defect location of the optical cable based at least partially on a temporal correlation of the third set of sample data with the transmitted second optical signal.

6. The method of claim 1 , wherein the first optical signal is transmitted with a first pulse width, wherein the second optical signal is transmitted with a second pulse width, and wherein the second pulse width is greater than or equal to 2 divided by the second sample rate.

7. The method of claim 1 , wherein the second sample rate is greater than or equal to twice the speed of light in the optical cable divided by a target defect location resolution.

8. The method of claim 6 , wherein the first pulse width is greater than the second pulse width.

9. The method of claim 7 , wherein the first optical signal is transmitted with a first pulse width, wherein the second optical signal is transmitted with a second pulse width, and wherein the second pulse width is greater than or equal to 2 divided by the second sample rate.

10. The method of claim 9 , wherein the first pulse width is greater than the second pulse width.

11. An optical time domain reflectometer (OTDR) system, comprising:

a transmit circuit operative to generate a transmit signal defining at least one pulse;

an optical source operative to transmit optical signals at a first location of an optical cable according to the transmitted signal from the transmit circuit;

an optical sensor operative to receive optical reflections at the first location of the optical cable and to generate a receive signal according to the received optical reflections;

a receive circuit operative to sample the receive signal at a selectable sample rate and to provide sample data corresponding to the optical reflections received by the optical sensor;

an electronic memory operative to store the sample data; and

at least one processor operatively coupled with the transmit circuit, the receive circuit, and the electronic memory, the at least one processor operative to:

provide at least one transmit control signal or value to cause the transmit circuit and the optical source to generate a first optical signal at the first location of the optical cable,

provide at least one receive control signal or value to cause the receive circuit to sample the receive signal at a first sample rate over a first time range following transmission of the first optical signal to obtain a first set of sample data corresponding to reflections of the first optical signal,

temporally correlate the first set of sample data with the transmitted first optical signal,

identify a first time in the first time range corresponding to a suspected defect location of the optical cable based at least partially on the temporal correlation of the first set of sample data with the transmitted first optical signal;

provide at least one further transmit control signal or value to cause the transmit circuit and the optical source to generate a second optical signal at the first location of the optical cable,

provide at least one further receive control signal or value to cause the receive circuit to sample the receive signal at a second sample rate over a second time range following transmission of the second optical signal to obtain a second set of sample data corresponding to reflections of the second optical signal, the second time range including the first time, the second time range being less than the first time range, and the second sample rate being greater than the first sample rate,

temporally correlate the second set of sample data with the transmitted second optical signal, and

identify at least one defect location of the optical cable based at least partially on the temporal correlation of the second set of sample data with the transmitted second optical signal.

12. The OTDR system of claim 11 , wherein the receive circuit comprises:

a signal conditioning circuit operative to provide an output signal based on the received signal from the optical sensor;

an analog to digital converter circuit operative to sample the output signal of the signal conditioning circuit at the selectable sample rate and to provide the sample data; and

a memory operative to store the sample data from the analog to digital converter.

13. The OTDR system of claim 11 , wherein the at least one processor is operative to provide the at least one receive control signal or value and the at least one further receive control signal or value to set the second sample rate of the receive circuit to be greater than the first sample rate.

14. The OTDR system of claim 11 , wherein the at least one processor is operative to provide the at least one further receive control signal or value to set the second sample rate of the receive circuit to be greater than or equal to twice the speed of light in the optical cable divided by a target defect location resolution.

15. The OTDR system of claim 11 , wherein the at least one processor is operative to provide the at least one transmit control signal or value and the at least one further transmit control signal or value to set the first pulse width to be greater than the second pulse width.

16. The OTDR system of claim 11 , wherein the at least one processor is operative to:

provide the at least one transmit control signal or value to cause the transmit circuit and the optical source to generate the first optical signal with a first pulse width; and

provide the at least one further transmit control signal or value to cause the transmit circuit and the optical source to generate the second optical signal with a second pulse width, the second pulse width being greater than or equal to 2 divided by the second sample rate.

17. The OTDR system of claim 16 , wherein the at least one processor is operative to provide the at least one transmit control signal or value and the at least one further transmit control signal or value to set the first pulse width to be greater than the second pulse width.

18. An integrated circuit for an optical time domain reflectometer (OTDR) system, the integrated circuit comprising:

a transmit circuit operative to generate a transmit signal defining at least one pulse for controlling an optical source to transmit optical signals;

a receive circuit operative to sample receive signals from an optical sensor receiving optical reflections at a selectable sample rate and to provide sample data corresponding to the optical reflections;

an electronic memory operative to store the sample data; and

at least one processor operatively coupled with the transmit circuit, the receive circuit, and the electronic memory, the at least one processor operative to:

provide at least one transmit control signal or value to cause the transmit circuit and the optical source to generate a first optical signal at a first location of an optical cable,

provide at least one receive control signal or value to cause the receive circuit to sample the receive signal at a first sample rate over a first time range following transmission of the first optical signal to obtain a first set of sample data corresponding to reflections of the first optical signal,

identify a first time in the first time range corresponding to a suspected defect location of the optical cable based at least partially on a temporal correlation of the first set of sample data with the transmitted first optical signal;

provide at least one further transmit control signal or value to cause the transmit circuit and the optical source to generate a second optical signal at the first location of the optical cable,

provide at least one further receive control signal or value to cause the receive circuit to sample the receive signal at a second sample rate over a second time range following transmission of the second optical signal to obtain a second set of sample data corresponding to reflections of the second optical signal, the second time range including the first time, the second time range being less than the first time range, and the second sample rate being greater than the first sample rate, and

identify at least one defect location of the optical cable based at least partially on the temporal correlation of the second set of sample data with the transmitted second optical signal.

19. The integrated circuit of claim 18 , wherein the at least one processor is operative to provide the at least one transmit control signal or value and the at least one further transmit control signal or value to set the first pulse width to be greater than the second pulse width.

20. The integrated circuit of claim 18 , wherein the at least one processor is operative to provide the at least one further receive control signal or value to set the second sample rate of the receive circuit to be greater than or equal to twice the speed of light in the optical cable divided by a target defect location resolution.