EXTINCTION RATIO FREE PHASE SENSITIVE OPTICAL TIME DOMAIN REFLECTOMETRY BASED DISTRIBUTED ACOUSTIC SENSING SYSTEM

Abstract

A phase sensitive optical time domain reflectometry based distributed acoustic sensing system eliminating a degradation in a sensing performance encountered due to a finite extinction ratio of optical elements used to generate optical pulses is provided. A classical Optical Time Domain Reflectometer (OTDR) concept and a phase-OTDR concept are merged to generate an optic pulse for an interrogation with commercially available optic modulators. Characteristics of a light inside a fiber optic cable carry properties of both classical OTDR and phase-OTDR systems. The proposed solution does not require any modifications in a receiver part of the phase-OTDR systems and the proposed solution is used for any type of phase-OTDR system structure.

Claims

1. A phase sensitive optical time domain reflectometry based distributed acoustic sensing system eliminating a degradation in a sensing performance encountered due to a finite extinction ratio of optical elements, comprising: a coherent laser source generating a coherent Continuous Wave (CW) laser with a narrow linewidth for detecting vibration sources near a sensing cable, a first gain and filter block amplifying the coherent CW laser to adjust a power and filtering the coherent CW laser to be injected into the sensing cable to eliminate possible undesired frequency components introduced during a power adjustment, an optic pulse generator shaping the coherent CW laser into narrow optic pulses with a desired pulse shape generated by a Radio Frequency (RF) signal pulse generator, a circulator injecting an optic signal to the sensing cable and forwarding back reflected Rayleigh scattering signals coming from the sensing cable to a third gain and filter block, the third gain and filter block adjusting a power level of the optic signal and filtering out the possible undesired frequency components on a returned Rayleigh backscattered signal, a receiver block measuring an optic power of the returned Rayleigh backscattered signal, digitizing a measured analog optic power and processing a digitized optic signal power, the phase sensitive optical time domain reflectometry based distributed acoustic sensing system further comprising; an incoherent laser source generating an incoherent CW laser with a large linewidth for degrading a coherence interference effect or a coherence length for a light injected into the sensing cable when the optic pulse generator is in an OFF state, a second gain and filter block adjusting a power of an incoherent light and filtering the incoherent light, wherein a resulting incoherent light has a wide linewidth with a power of the resulting incoherent light is similar to a power of a coherent leakage light injected when the optic pulse generator is in the OFF state, a combiner combining the optic signal coming from the optic pulse generator and the second gain and filter block and forwarding the optic signal to the circulator.

2. The phase sensitive optical time domain reflectometry based distributed acoustic sensing system according to the claim 1, wherein an optic pulse, p(t), injected into a fiber optic cable is represented as follows: $\begin{array}{cc}p\left(t\right)=& \underset{k=-\infty }{\overset{\infty }{.Math.}}\left[f\left(t\right)\Pi \left(\frac{t-{\mathrm{kT}}_p}{T_w}\right)+{ϵ}_{1}f\left(t\right)\left(1-\Pi \left(\frac{t-{\mathrm{kT}}_p}{T_w}\right)\right)+{ϵ}_{2}g\left(t\right)\right]\\ =& \underset{k=-\infty }{\overset{\infty }{.Math.}}\left[\left(1-{ϵ}_1\right)f\left(t\right)\Pi \left(\frac{t-{\mathrm{kT}}_p}{T_w}\right)+{ϵ}_1\left(f\left(t\right)+g\left(t\right)\right)+\left({ϵ}_2-{ϵ}_1\right)g\left(t\right)\right],\end{array}$ wherein f(t) and g(t) are CW optic signals generated from the coherent laser source and the incoherent laser source, respectively; ∈.sub.1 and ∈.sub.2 are extinction ratio values of optic paths, respectively; T.sub.P is a pulse repetition period and Π(.) represents a rectangular pulse, wherein the rectangular pulse is defined as: $\Pi \left(\frac{t}{T_w}\right)=\{\begin{array}{cc}1,& 0\le t\le T_w\\ 0,& \mathrm{otherwise}.\end{array}$

3. The phase sensitive optical time domain reflectometry based distributed acoustic sensing system according to the claim 2, wherein the optic pulse p(t) is simplified as follows: $p\left(t\right)=\underset{k=-\infty }{\overset{\infty }{.Math.}}\left[f\left(t\right)\Pi \left(\frac{t-{\mathrm{kT}}_p}{T_w}\right)+ϵ\left(f\left(t\right)+g\left(t\right)\right)\right],$ when extinction ratios are similar and ∈=∈.sub.1≈∈.sub.2<<1, thus the phase sensitive optical time domain reflectometry based distributed acoustic sensing system becomes an interrogation unit using the optic pulse with two states, comprising a coherent ON state and an incoherent OFF state.

4. The phase sensitive optical time domain reflectometry based distributed acoustic sensing system according to the claim 1, wherein the optic pulse generator is an acousto-optic modulator or an electro-optic modulator or a Kerr medium.

5. The phase sensitive optical time domain reflectometry based distributed acoustic sensing system according to the claim 1, wherein the optic power is adjusted by using an Erbium Doped Fiber Amplifier in the first gain and filter block, the second gain and filter block and the third gain and filter block.

6. The phase sensitive optical time domain reflectometry based distributed acoustic sensing system according to the claim 1, wherein the receiver block consists of a photo detector measuring the optic power, an analog to digital converter and a processor processing the digitized optic signal power.

7. The phase sensitive optical time domain reflectometry based distributed acoustic sensing system according to the claim 1, comprising a photo detector block consists of a single photo detector or multiple photo detectors for measuring the returned Rayleigh backscattered signal.

8. The phase sensitive optical time domain reflectometry based distributed acoustic sensing system according to the claim 1, wherein the receiver block is in a form of a direct detection, a heterodyne detection, a homodyne detection or a receiver format used in a distributed acoustic sensing.

9. The phase sensitive optical time domain reflectometry based distributed acoustic sensing system according to the claim 1, wherein the incoherent laser source is a combination of multiple CW lasers with the narrow linewidth shifted with optical frequency shifters.

10. The phase sensitive optical time domain reflectometry based distributed acoustic sensing system according to the claim 1, wherein the incoherent laser source is a combination of multiple CW lasers with the narrow linewidth shifted with optical phase shifters.

11. The phase sensitive optical time domain reflectometry based distributed acoustic sensing system according to the claim 9, wherein the optical frequency shifters shift a laser frequency with a fixed value or a shifting value changed with an external signal source.

12. The phase sensitive optical time domain reflectometry based distributed acoustic sensing system according to the claim 1, wherein the sensing cable is a fiber optic cable.

13. The phase sensitive optical time domain reflectometry based distributed acoustic sensing system according to the claim 1, wherein the second gain and filter block is realized with the first gain and filter block combined with the optic pulse generator and the RF signal pulse generator, wherein the RF signal pulse generator is always in the OFF state.

14. A phase sensitive optical time domain reflectometry based distributed acoustic sensing method eliminating a degradation in a sensing performance encountered due to a finite extinction ratio of optical elements, comprising the steps of; generating a coherent CW laser with a narrow linewidth for detecting vibration sources near a fiber optic cable, adjusting a power of the coherent CW laser and filtering to eliminate possible undesired frequency components introduced during a power adjustment, shaping the coherent CW laser into narrow optic pulses by an optic pulse generator with a desired pulse shape, generating an incoherent CW laser with a large linewidth for degrading a coherence interference effect or a coherence length for a light injected into the fiber optic cable when the optic pulse generator is in an OFF state, adjusting a power of an incoherent light and filtering the incoherent light, wherein a resulting incoherent light has a wide linewidth with a power of the resulting incoherent light is similar to a power of a coherent leakage light injected when the optic pulse generator is in the OFF state, combining optic signals coming from optic paths with a combiner and forwarding the optic signals to a circulator, injecting the optic signals to the fiber optic cable and forwarding back reflected Rayleigh scattering signals coming from the fiber optic cable to a gain and filter block, adjusting a power level of the optic signals and filtering out the possible undesired frequency components on returned Rayleigh backscattered signal, measuring an optic power of the returned Rayleigh backscattered signal, digitizing a measured analog optic power and processing a digitized optic signal power.

15. The phase sensitive optical time domain reflectometry based distributed acoustic sensing system according to the claim 6, comprising a photo detector block consists of a single photo detector or multiple photo detectors for measuring the returned Rayleigh backscattered signal.

16. The phase sensitive optical time domain reflectometry based distributed acoustic sensing system according to the claim 10, wherein the optical phase shifters shift a laser phase with a fixed value or a shifting value changed with an external signal source.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] FIG. 1 illustrates the flow diagram of currently used classical phase-OTDR architecture in prior art.

[0035] FIG. 2 shows optical pulses injected into the fiber optic cable, where P.sub.max and P.sub.min are the maximum and minimum optical power level that is injected into the fiber optic cable, respectively and T.sub.W and T.sub.P are pulse width and pulse repetition period, respectively.

[0036] FIG. 3 is the diagram of extinction ratio free phase sensitive optical time domain reflectometry based distributed acoustic sensing system.

[0037] FIG. 4 shows CW laser with large linewidth constructed by multiple optic frequency shifters.

[0038] FIG. 5 shows CW laser with large linewidth constructed by multiple optic phase shifters.

PART REFERENCES

[0039] 10. Coherent laser source

[0040] 20. First gain and filter block

[0041] 30. Optic pulse generator

[0042] 40. Radio frequency (RF) signal pulse generator

[0043] 50. Combiner

[0044] 60. Circulator

[0045] 70. Sensing cable

[0046] 80. Incoherent laser source

[0047] 90. Second gain and filter block

[0048] 100. Third gain and filter block

[0049] 110. Photo detector

[0050] 120. Analog to digital converter (ADC)

[0051] 130. Processor

[0052] 140. Frequency shifter

[0053] 150. Phase shifter

[0054] 160. Shifted signal combiner

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0055] The invention is a phase sensitive optical time domain reflectometry based distributed acoustic sensing system that eliminates the degradation in sensing performance encountered due to the finite extinction ratio of the optical elements. In the system, a coherent laser source (10) generates CW laser with narrow linewidth (smaller than 100 Hz) for detecting vibration sources near a sensing cable (70). The phase-OTDR systems are based on measuring the changes in phase of the returned Rayleigh scattering signal and this measurement is possible when the light injected into the sensing cable (70) is coherent. Sensing cable (70) is a telecommunications grade fiber optic cable or any other type of fiber optic cable that can be used for remote sensing applications.

[0056] The first gain and filter block (20) amplifies and filters the laser in order to satisfy the power and coherency criteria of the light to be injected into the cable (70). The block (20) contains an optical amplifier/attenuator to adjust the power of the light to be injected into the cable (70). Optic power can be adjusted by using EDFA. The amplified signal is then filtered to remove undesired signal components that are due to the non-ideal behavior of the amplifier. Several different optical filter designs can be used in this block (20).

[0057] The optic pulse generator (30) shapes the CW laser into the narrow optic pulses 100 ns) with the desired pulse shape generated by RF signal pulse generator (40) for interrogation. The critical point in pulse generation is to obtain high extinction ratio, which is the ratio of the optic power within pulse (the ON state) and the optic power outside pulse (the OFF state). Optic pulse can be generated by using Acousto-optic Modulator (AOM) or Electro-optic Modulator (EOM) or any other optical component such as Kerr medium. The shape of the optic pulse can vary based on the application, i.e. rectangular pulse, saw tooth pulse, etc.

[0058] On the other path, an incoherent laser source (80) generates CW laser with large linewidth for degrading the undesired coherent interference effect for the leakage light injected into the cable when optic pulse generator (30) is in OFF state. The incoherent laser source (80) can be constructed with: [0059] Laser sources that are commonly used in classical OTDR applications but not used in phase-OTDR applications due to the large linewidth, [0060] The combined multiple CW lasers with narrow linewidth as shown in FIG. 4. Frequency shifters (140) can shift the source laser frequency with a fixed value or shifting value can be changed with an external signal source. [0061] The combination of multiple different phase shifted versions of a single CW laser with narrow linewidth such in FIG. 5. Phase shifters (150) can shift the source laser phase with a fixed value or shifting value can be changed with an external signal source. [0062] All of the other possibilities for generating large linewidth laser can also be used throughout this design.

[0063] The second gain and filter block (90) amplifies/attenuates and filters the incoherent CW laser coming from the incoherent laser source (80) to satisfy the power and incoherency criteria. This part contains an optical amplifier/attenuator to adjust the power of the incoherent light injected into the sensing cable (70) such that its power is similar to the power of the coherent leakage light injected when the optic pulse generator (30) is in OFF state. Several different optical filter designs can be used in this block (90). This filter block should shape the frequency spectrum of the light injected into the sensing cable (70) such that the resulting incoherent light has a wide linewidth with a total power that is similar to the power of the coherent leakage light injected when the optic pulse generator (30) is in OFF state. For example, gain and filter block (90) can be constructed as a combination of first gain and filter block (20) combined with optic pulse generator (30) and RF signal pulse generator (40), where RF signal pulse generator (40) is always in OFF state.

[0064] The combiner (50) combines the optic signal coming from the optic pulse generator (30) and the second gain and filter block (90) and forwards the combined optic signal to a circulator (60). The circulator (60) injects the combined optic signal to the sensing cable (70) and forwards back reflected Rayleigh scattering signals coming from the sensing cable (70) to the third gain and filter block (100). The third gain and filter block (100) adjusts the power level of the returned Rayleigh backscattered signal and filters out the undesired frequency components in the returned Rayleigh backscattered signal for obtaining the best performance in the receiver block. The receiver block consists of a photo detector (110), an ADC (120) and a processor (130). The receiver block can be in the form of direct detection, heterodyne detection, homodyne detection or any other receiver format that can be used in distributed acoustic sensing applications. The photo detector (110) measures the optic power of the returned Rayleigh backscattered signal as an electrical signal (voltage or current). The photo detector block (110) may consist of a single photo detector or multiple photo detectors for measuring the returned Rayleigh backscattered optical signal based on the preferred embodiment. The ADC (120) digitizes the measured analog optic power to be processed by the processor (130). The single or multiple ADC (120) may be used for measuring based on the preferred embodiment.

[0065] In the present invention, only the optic pulse generation mechanism of the phase-OTDR architecture is modified and the receiver side is not changed as shown in FIG. 3. A second path is added into the transmitter side to change the characteristics of the optical pulse injected into the sensing cable (70) for interrogation. On the second path, incoherent laser source (80) is used instead of the highly coherent one with the center frequency same as the coherent laser source (10) in the first path. In a possible implementation, the other optical and electronic parts may be kept identical to the first path. The optical signals generated at these two paths are combined by the combiner (50) and fed into the sensing cable (70). In this configuration, the AOM in the first path is driven by very narrow 100 ns) electronic pulse with ON and OFF state and the AOM in the second path is driven continuously with OFF state. Therefore, while the first path generates optical pulse, the second path generates only the leakage optical signal with a much higher linewidth as compared to the optic signal in the first path.

[0066] The optic pulse injected into the sensing cable (70) can be represented as

[00001]$\begin{array}{cc}p\left(t\right)=& \underset{k=-\infty }{\overset{\infty }{.Math.}}\left[f\left(t\right)\Pi \left(\frac{t-{\mathrm{kT}}_p}{T_w}\right)+{ϵ}_{1}f\left(t\right)\left(1-\Pi \left(\frac{t-{\mathrm{kT}}_p}{T_w}\right)\right)+{ϵ}_{2}g\left(t\right)\right]\\ =& \underset{k=-\infty }{\overset{\infty }{.Math.}}\left[\left(1-{ϵ}_1\right)f\left(t\right)\Pi \left(\frac{t-{\mathrm{kT}}_p}{T_w}\right)+{ϵ}_1\left(f\left(t\right)+g\left(t\right)\right)+\left({ϵ}_2-{ϵ}_1\right)g\left(t\right)\right],\end{array}$

where f(t) and g(t) are the CW optic signals generated from highly coherent and incoherent laser sources (10, 80), respectively; ∈.sub.1 and ∈.sub.2 are the extinction ratio values of optic paths (AOMs in first and second path), respectively. The pulse repetition period is denoted as T.sub.P and Π(.) represents a rectangular pulse that is defined as

[00002]$\Pi \left(\frac{t}{T_w}\right)=\{\begin{array}{cc}1,& 0\le t\le T_w\\ 0,& \mathrm{otherwise}.\end{array}$

[0067] Note that, when the AOMs have similar low extinction ratios, i.e., ∈.sub.1≈∈.sub.2<<1, the injected optic signal p(t) can be simplified as

[00003]$p\left(t\right)=\underset{k=-\infty }{\overset{\infty }{.Math.}}\left[f\left(t\right)\Pi \left(\frac{t-{\mathrm{kT}}_p}{T_w}\right)+ϵ\left(f\left(t\right)+g\left(t\right)\right)\right],$

where ∈=∈.sub.1≈∈.sub.2. In that case, inserting incoherent laser signal g(t) to the normal phase-OTDR signal can be interpreted as contaminating the coherent leakage light with incoherent light. Since ∈<<1, the incoherent contamination does not alter the sensing performance of the system in the ON state of the optic pulse. On the other hand, it significantly changes the characteristics of the back reflected signal due to the leakage light injected into the fiber optic cable in the OFF state of the optic pulse. In the OFF state, while all the back reflected signals are almost the same frequency when g(t) does not exist, the back reflected signals are in random frequencies in a wide range when the g(t) is also injected. Then, our proposed system architecture becomes an interrogation unit using optic pulse with two states, i.e., coherent ON state and incoherent OFF state. The coherent ON state is the desired optic signal that is highly sensitive to the small vibrations near or on the fiber optic cable. On the other hand, the incoherent OFF state is unavoidable and the undesired leakage signal due to the AOMs or EOMs with finite extinction ratio that is insensitive to the small vibrations near or on the fiber optic cable. Therefore, in the proposed system configuration, the small vibrations that are on the outside the area of the interrogating region with ON state of the optic pulse do not affect the sensing performance as compared to the fully coherent phase-OTDR system.

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