Methods and apparatus for OFDR interrogator monitoring and optimization
Example embodiments add an optical amplifier to a multi-channel, continuously swept OFDR measurement system, adjust amplified swept laser output power between rising and falling laser sweeps, and/or utilize portions of a laser sweep in which OFDR measurements are not typically performed to enhance the integrity of the OFDR measurement system, improve the performance and quality of OFDR measurements, and perform additional measurements and tests.
1. A multi-channel optical interrogation system comprising:
a tunable laser;
a modulator to inject a ripple signal into laser light generated by the tunable laser;
an optical interferometric network and associated detection and acquisition electronics collectively configured to provide multiple measurement channels,
wherein the optical interferometric network comprises multiple reference paths associated with the multiple measurement channels,
wherein the optical interferometric network is configured to, when receiving the laser light carrying the ripple signal, produce multiple output signals, corresponding to laser light that has traversed the multiple reference paths, that carry the ripple signal, and
wherein the detection and acquisition electronics measure the multiple output signals to produce channel signals for the multiple measurement channels that carry the ripple signal; and
data processing circuitry configured to:
process the channel signals to produce respective channel phases of the ripple signal for the multiple measurement channels,
calculate, from the channel phases, one or more phase differences corresponding to one or more differences in optical and electrical delays between the multiple measurement channels, and
correcting for the one or more phase differences.
2. The multi-channel optical interrogation system of claim 1 , wherein the tunable laser is driven in accordance with a drive signal, and wherein the modulator is configured to generate a modulation signal for combination with the drive signal to thereby add the ripple signal into the laser light.
3. The multi-channel optical interrogation system of claim 2 , wherein:
the modulator includes a processor-controlled digital to analog converter (DAC) configured to drive a voltage-controlled oscillator (VCO) to generate an output signal for use as the modulation signal; or
the modulator includes a numerically-controlled oscillator (NCO) configured to generate a digital output having a most significant bit (MSB), and a filter configured to filter a clock signal corresponding to the MSB to generate a filtered clock signal for use as the modulation signal.
4. The multi-channel optical interrogation system of claim 1 , wherein:
the optical interferometric network comprises a measurement branch for coupling to an optical sensor comprising multiple optical fiber cores, the measurement branch including multiple measurements paths for coupling to respective cores of the multiple optical fiber cores;
the tunable laser is driven to generate swept laser light for measurement of the optical sensor;
the optical interferometric network is configured to, when receiving the swept laser light from the tunable laser while the optical sensor is coupled to the measurement branch, output multiple combined signals corresponding to reflected swept laser light from the multiple optical fiber cores interfered with swept laser light that has traversed respective reference paths of the multiple reference paths;
the detection and acquisition circuitry is configured to measure the multiple combined signals to produce sensor measurement data for the multiple measurement channels; and
the data processing circuitry is configured to correct for the one or more phase differences in optical and electrical delays between the multiple measurement channels when processing the sensor measurement data.
5. The multi-channel optical interrogation system of claim 4 , wherein the data processing circuitry is further configured to compute a shape of the optical sensor based on the processed sensor measurement data.
6. The multi-channel optical interrogation system of claim 4 , wherein:
the data processing circuitry is further configured to monitor the channel phases to detect errors induced due to changes in the optical and electrical delays.
7. The multi-channel optical interrogation system of claim 4 , wherein the ripple signal has a frequency outside a frequency range associated with the measurement of the optical sensor.
8. The multi-channel optical interrogation system of claim 4 , wherein:
the tunable laser is driven, during measurement of the optical sensor, to generate a rising sweep where a laser light frequency increases from a lower optical frequency to a higher optical frequency of a measurement tuning range, a falling sweep where the laser light frequency decreases from the higher optical frequency to the lower optical frequency, and a turnaround portion transitioning between the rising and falling sweeps; and
the modulator is controlled to add the ripple signal to the swept laser light during the turnaround portion.
9. The multi-channel optical interrogation system of claim 4 , wherein the ripple signal has a frequency within a frequency range associated with the measurement of the optical sensor.
10. A method of operating an optical interrogation system configured to provide multiple measurement channels, the method comprising:
generating laser light carrying a ripple signal;
providing the laser light carrying a ripple signal to an optical interferometric network of the optical interrogation system;
measuring multiple output signals, corresponding to laser light that has traversed multiple respective reference paths of the optical interferometric network, that carry the ripple signal to produce channel digital signals for the multiple measurement channels that carry the ripple signal;
processing the channel digital signals to produce channel phases for the multiple measurement channels, and
calculating, based on the channel phases, one or more phase differences corresponding to one or more differences in optical and electrical delays between the multiple measurement channels, and
correcting for the one or more phase differences.
11. The method of claim 10 , wherein generating the laser light carrying a ripple signal comprises generating a modulation signal and driving a laser in accordance with a drive signal combined with the modulation signal.
12. The method of claim 10 , further comprising:
generating swept laser light for measurement of an optical sensor coupled to a measurement branch of the optical interferometric network, the optical sensor comprising multiple optical fiber cores coupled to multiple respective measurement paths associated with the multiple measurement channels,
providing the swept laser light to the optical interferometric network;
measuring multiple combined signals corresponding to reflected swept laser light from the multiple optical fiber cores interfered with swept laser light that has traversed respective reference paths of the multiple reference paths to produce sensor measurement data for the multiple measurement channels; and
processing the sensor measurement data, wherein the processing comprises the correcting for the one or more differences in the optical and electrical delays between the multiple measurement channels.
13. The method of claim 12 , further comprising:
monitoring the channel phases to detect errors induced due to changes in the optical and electrical delays.
14. The method of claim 12 , wherein the ripple signal has a frequency outside a frequency range associated with measurements of the optical sensor.
15. The method of claim 12 , wherein:
generating the swept laser light comprises generating a rising sweep where a laser light frequency increases from a lower optical frequency to a higher optical frequency of a measurement tuning range, a falling sweep where the laser light frequency decreases from the higher optical frequency to the lower optical frequency, and a turnaround portion transitioning between the rising and falling sweeps; and
the ripple signal is added to the swept laser light during the turnaround portion.
16. The method of claim 12 , wherein the ripple signal has a frequency within a frequency range associated with the measurement of the optical sensor.
17. The method of claim 16 , further comprising:
repeating the generating, providing, measuring, and processing steps for multiple frequencies of the ripple signal, the multiple frequencies covering the frequency range associated with the measurement of the optical sensor.
18. An optical interrogation system configured to provide multiple measurement channels, the system comprising:
generating means for generating laser light carrying a ripple signal;
providing means for providing the laser light carrying a ripple signal to an optical interferometric network of the optical interrogation system and for producing multiple output signals, corresponding to light that has traversed multiple respective reference paths of the optical interferometric network, that carry the ripple signal;
measuring means for measuring the multiple output signals to produce channel digital signals for the multiple measurement channels that carry the ripple signal; and
processing means for processing the channel digital signals to produce channel phases of the ripple signal for the multiple measurement channels, for calculating, from the channel phases, one or more phase differences corresponding to one or more differences in optical and electrical delays between the multiple measurement channels, and for correcting for the one or more phase differences.
19. The optical interrogation system of claim 18 , wherein:
the generating means further generates swept laser light;
the optical interrogation system further comprises:
means for providing the swept laser light to multiple optical fiber cores of an optical sensor, and
means for interfering reflected laser light from the multiple optical fiber cores with laser light that has traversed respective reference paths of the multiple reference paths to produce multiple combined signals;
the measuring means further measure the multiple combined signals to produce sensor measurement data for the multiple measurement channels; and
the processing means further process the sensor measurement data, wherein the processing comprises the correcting for the one or more differences in the optical and electrical delays between the multiple measurement channels.
20. The optical interrogation system of claim 19 , wherein:
the generating means further adds a further ripple signal into the swept laser light; and
the processing means further processes the sensor measurement data to produce further channel phases for the multiple measurement channels based on the further ripple signal, and monitors the further channel phases for changes in at least one of the delays.