Line monitoring system with heterodyne coherent detection

Abstract

A line monitoring system can include a laser source to emit a probe signal over a first bandwidth; a polarization preserving tap to receive the probe signal and split the probe signal into a first portion and a second portion; a polarization rotator to receive the first portion and send the first portion to a transmission system; a loop tap to receive the second portion and receive a loop signal from the transmission system, wherein the loop signal is derived from the first portion; a photodetector coupled to receive an interference signal from the loop tap, wherein the interference signal is generated by mixing the loop signal with the second portion, wherein the photodetector is arranged to output a power signal based on the interference signal; and a power measurement system to measure the power signal over a second bandwidth comparable to the first bandwidth at a given measurement frequency.

Description

Technical Field

[0001] This disclosure generally relates to the field of optical communication networks, and more particularly to techniques for measuring defects in optical fibers. Background Technology

[0002] Line monitoring systems (LMS) are used to monitor optical fibers such as submarine cables and may employ a high-loss loopback (HLLB) configuration. At each repeater in the system, the HLLB arrangement taps a portion of the arriving probe tone signal and couples the signal back in the opposite direction. The probe signal can be emitted from a laser source (such as an external cavity laser) and combined with an information-carrying channel in the outgoing direction to be transmitted to the submarine cable's transmission system. Due to the HLLB arrangement, the transmission system generates an optical LMS response signal in the outgoing direction.

[0003] Submarine fiber optic communication systems require routine monitoring to ensure their performance and minimize potential service loss by detecting and addressing submerged equipment failures and potential aggressive threats at an early stage. Currently mature monitoring technologies include the use of Line Monitoring Systems (LMS) to monitor the signal peak of the loopback from each submarine repeater and terminal using high-loss loopback (HLLB) technology.

[0004] When a fault occurs along this optical path, the amplitudes of these loopback signals change in the repeaters around the fault location. These changes exhibit different patterns that can be used to identify the fault condition. This fault condition could be caused by changes in fiber span loss, changes in the output power of the optical amplifier-pumped laser, or fiber breakage, among other reasons.

[0005] In known systems, a laser probe signal is generated by a laser source before entering the transmission system, and this laser signal can be broadened to a suitable bandwidth, such as 1 GHz. The broadened signal can then be transmitted as a polarization-maintaining signal to a polarization rotator operating at the desired frequency (such as 1 GHz). The signal can then be modulated using on-off keying (OOK) data before entering the transmission system. A portion of the loop signal can then be filtered out by components such as a wavelength selective switch (WSS). For example, the WSS can reject information-carrying channels located outside the approximately 25 GHz LMS band. This 25 GHz band contains the LMS response signal and system-generated noise, including noise generated by the system amplifier. A photodetector can then be positioned to capture the optical signal generated in this 25 GHz band and generate an electrical signal that includes the LMS probe signal. Due to the relatively low level of the LMS probe signal, the signal-to-noise ratio is relatively low.

[0006] One way to improve the signal-to-noise ratio is to reduce the bandwidth of the fiber optic filter. However, this method requires additional components, and fiber optic filtering is generally difficult with bandwidths less than 1 GHz. This disclosure addresses these and other considerations. Summary of the Invention

[0007] This summary is provided to introduce some conceptual choices in a simplified form, which will be further described in the detailed description below. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to help determine the scope of the claimed subject matter.

[0008] In a first embodiment, the line monitoring system may include a laser source for emitting a detection signal over a first bandwidth; a polarization-maintaining tap for receiving the detection signal and separating it into a first portion and a second portion; a polarization rotator for receiving the first portion and transmitting it to a transmission system. The line monitoring system may further include a loop tap for receiving the second portion and a loop signal from the transmission system, wherein the loop signal is derived from the first portion; a photodetector coupled to receive an interference signal from the loop tap, wherein the interference signal is generated by mixing the loop signal and the second portion, wherein the photodetector is arranged to output a power signal based on the interference signal; and a power measurement system for measuring the power signal at a given measurement frequency over a second bandwidth equivalent to the first bandwidth.

[0009] In another embodiment, a method for measuring a fault in a transmission system may include emitting a probe beam from a laser probe source, the probe beam having a first bandwidth, directing a first portion of the probe beam to the transmission system, and directing a second portion of the probe beam to a loop tap. The method may include receiving a loop signal from the transmission system at the loop tap, the loop signal being based on the first portion of the probe beam, mixing the second portion of the probe beam and the loop signal to generate an interference signal, generating a power signal based on the interference signal, and measuring the power signal at a given measurement frequency over a second bandwidth equivalent to the first bandwidth.

[0010] In another embodiment, a method for measuring a fault in a transmission system may include emitting a probe beam from a laser probe source having a narrow bandwidth, directing a first portion of the probe beam to the transmission system, and directing a second portion of the probe beam to a frequency shifter assembly to form a frequency-shifted beam, and receiving the frequency-shifted beam and a loop signal from the transmission system at a loop tap. Thus, the loop signal may be based on the first portion of the probe beam. The method may include mixing the frequency-shifted beam and the loop signal to generate an interference signal, generating a power signal based on the interference signal, and measuring the power signal at a given measurement frequency over a second bandwidth, which is equivalent to or less than a first bandwidth: the first bandwidth and the sampling rate of the measured power signal.

[0011] In another embodiment, a method for measuring a fault in a transmission system includes: emitting a probe beam from a laser probe source, directing a first portion of the probe beam to the transmission system, directing a second portion of the probe beam to a loop tap, generating an interference signal by combining the second portion of the probe beam with a loop signal from the transmission system, the loop signal being based on the first portion of the probe beam, and converting the interference signal into an electrical power signal at a photodetector. Attached Figure Description

[0012] Figure 1 This is a schematic diagram illustrating an exemplary embodiment of a line measurement system for testing optical fibers according to the present disclosure;

[0013] Figure 2 This is a schematic diagram illustrating an exemplary embodiment of another line measurement system for testing optical fibers according to the present disclosure;

[0014] Figure 3 This is a schematic diagram illustrating an exemplary embodiment of another line measurement system for testing optical fibers according to the present disclosure;

[0015] Figure 4 This is a schematic diagram illustrating an exemplary embodiment of yet another line measurement system for testing optical fibers according to the present disclosure;

[0016] Figure 5 An exemplary process flow is presented;

[0017] Figure 6 Another exemplary process flow is presented;

[0018] Figure 7 This presents yet another exemplary process flow; and

[0019] Figure 8 This is a schematic diagram illustrating another embodiment of a line measurement system for testing optical fibers according to the present invention. Detailed Implementation

[0020] This embodiment, by using a coherent detection method combined with heterodyne mixing and performing filtering over a narrow bandwidth in the electrical domain, can be used to significantly improve the signal-to-noise ratio in LMS systems.

[0021] According to various embodiments of this disclosure, a portion of the light generated by the laser detection source is separated and sent to the transmission system, and added to the loop signal using a tap. This separated portion of the light may interfere with the loop signal, which is detected by a single photodetector. The remainder of the signal processing may be performed by electronic equipment or in software / firmware, and involves electrical power measured at one or more frequencies. Therefore, the bandwidth of the electrical filter used to measure the electrical power may be much smaller than that of an optical filter, for example, at 25 GHz, thus removing a large portion of the noise and resulting in a higher signal-to-noise ratio for the detected LMS signal.

[0022] Figure 1 This is a schematic diagram illustrating an exemplary embodiment of a line measurement system 100 for testing optical fibers according to the present disclosure. The line monitoring system (LMS) 100 includes a laser source 102 for generating a probe signal, shown as a probe beam 120, wherein the probe beam is broadened to a target bandwidth in the range of 25 MHz, such as 10 MHz, 25 MHz, 50 MHz, 100 MHz, or similar values ​​according to various non-limiting embodiments. Considerations for selecting the target bandwidth include the minimum value of the electrical filter used to detect the loop signal. In other words, the value of the target bandwidth of the probe beam can define the minimum bandwidth value used by the electrical filter. In practical designs, this value will be a trade-off between different aspects of the LMS, such as electron velocity, potential nonlinear losses of light within the optical fiber, etc.

[0023] The LMS 100 includes a polarization-maintaining tap 104 configured to receive the probe beam 120, and a polarization rotator 106 configured to receive a first portion 130 of the probe beam 120 and transmit the first portion 130 to the transmission system 132. The polarization-maintaining tap 104 is configured to split the probe beam 120 to output a second portion 122 of the probe beam 120 as a non-polarization-maintaining signal.

[0024] LMS 100 further includes a loop tap 136 for receiving a second portion 122 of the probe beam 120 and a loop signal 134 derived from a transmission system 132 (where channels such as information-bearing channels may be filtered out). LMS 100 further includes a photodetector 142 coupled to receive an interference signal 140 from the loop tap 136, wherein the interference signal 140 is generated by mixing the loop signal 134 and the second portion 122 of the probe beam 120. The photodetector 142 can operate as a known photodetector to output an electrical signal proportional to the intensity of the interference signal 140, shown as a power signal 150. LMS system 100 further includes a power measurement system 152 for measuring the power signal over a second bandwidth, corresponding to the first bandwidth of the output probe beam from laser source 102.

[0025] The LMS 100 may optionally include a first filter amplifier (EDFA) 110 configured to receive the probe beam 120, and a second filter amplifier 112 configured to receive a second portion 122 of the probe beam 120. According to different embodiments, Figure 1 The “LO light” shown (representing the local oscillator light, shown as LO 115 in the figure) should be significantly stronger than the loop signal 134, so some amplification may be required, where the first filter amplifier 110 and the second filter amplifier 112 represent possible locations of optional filter amplifiers.

[0026] Furthermore, LMS 100 may include, individually or in part, an on / off keying (OOK) component 108 of a polarization rotator to modulate a polarization-rotated probe beam using OOK data. According to various embodiments of this disclosure, LMS 100 performs broadening on the probe beam emitted by the laser to facilitate averaging at different optical phases during a single OOK pulse. To perform this averaging, power measurement of the electrical signal (also referred to as the “power signal”) can be performed at the frequency of the polarization rotator (1 GHz in a non-limiting example) and at a bandwidth similar to or the same as the bandwidth of the probe beam 120 (25 MHz in a non-limiting example). To facilitate proper detection of the interference signal 140, power measurement system 152 can average the electrical power at the frequency of polarization rotator 106 in such a way that, in particular, according to various embodiments, the detected OOK pulse has a considerably long duration (100 μs in a non-limiting example) to allow averaging of the electrical power at the polarization rotator frequency over a time length (sampling period) comparable to the pulse duration.

[0027] In the above manner, Figure 1A first embodiment of a method is provided that combines coherent detection with heterodyne mixing and filtering in the electrical domain using a narrow bandwidth. Therefore, this method overcomes the relatively low signal-to-noise ratio of known LMS methods, where the loop signal (e.g., 25 GHz wide) includes the LMS response signal and noise generated in the system. Although Figure 1 The proposed implementation provides a relatively simple method to improve the signal-to-noise ratio in LMS systems, but a potential drawback is that detection is performed using only one polarization of the LO light. This approach can lead to reduced accuracy in systems with large time-varying polarization-dependent losses.

[0028] To address this issue, other embodiments of this disclosure propose additional LMS that include additional components, as described below. Figure 2 This is a schematic diagram illustrating an exemplary embodiment of another line measurement system for testing optical fibers according to the present disclosure. In this example, LMS 200 is shown, including some components similar to those in LMS 100, wherein similar components are labeled as identical. In this embodiment, a dual polarization method with a broadened laser source beam is employed. In this configuration, a polarization rotator 202 is added to the path of a second portion 122 of the probe beam 120 so that the rotated portion 224 returns to the loop tap 136 and is combined with the loop signal 134.

[0029] In this embodiment, the polarization of the second portion 122 of the probe beam 120 is rotated by the polarization rotator 202 at a target frequency Ω (e.g., 100 MHz). Detection can be arranged to occur at a monitoring frequency corresponding to the frequency difference in the polarization rotator, or alternatively, it can be arranged to occur at the sum of the frequencies of the polarization rotator. Similarly, in Figure 2 In this embodiment, the detection bandwidth can be arranged to be similar to the broadened bandwidth of the probe beam 120 output by the laser source 102. Note that the noise reduction expected for both single-polarization and dual-polarization is proportional to the ratio of the bandwidth of the original optical filter to the bandwidth of the electrical filter. In a non-limiting example, this ratio can be calculated as the ratio of 25 GHz to 25 MHz, which has a value of 30 dB.

[0030] The aforementioned laser broadening method benefits from the possibility of truly broadening the laser, i.e., the laser phase should be a random process. Such a device may be difficult to implement because traditional broadening methods involve modulating the laser current or a mirror, which, while generating a broadening signal, may itself be an oscillation of the laser's center optical frequency. In this case, the phase of the laser beam output is not random, and averaging the optical phase may introduce artifacts. Therefore, Figure 1 and 2If the method is implemented using traditional broadening components, it may effectively reduce the signal-to-noise ratio, but it may generate artifacts in the detection signal that need to be considered.

[0031] In order to solve Figure 1 and Figure 2 Potential problems with the implementation methods Figure 3 An LMS organized based on a slightly different approach is described. In order to... Figure 3 This method works, and depending on the implementation, the laser probe beam can be a narrowband beam, or it can be like... Figure 1 and 2 The bandwidth is slightly widened as in the embodiments described herein. As used herein, "narrowband" can refer to the bandwidth of the laser probe beam at output without applying widening to the beam. As an example, a bandwidth less than 1 MHz wide can be considered narrowband. In this example, LMS 300 is shown, which includes some components similar to those in LMS 100 and LMS 200, where similar components are labeled as the same. In this embodiment, LMS 300 includes a frequency shifter 302 arranged to shift the frequency of a second portion 122 of the probe beam 120. Detection of the interference signal 340 can be performed based on a frequency difference (while in some embodiments, the sum of frequencies can be used). In this embodiment, the bandwidth of the power signal 350 should be comparable to the bandwidth of the probe beam 120, or comparable to the bandwidth of the OOK signal output by the polarization rotator 106, whichever signal bandwidth is larger. Because... Figure 1 and 2 In this embodiment, the bandwidth of the laser and the bandwidth of the OOK signal can be smaller than the bandwidth in the bandwidth-enhanced laser, so this method can generate higher sensitivity.

[0032] Figure 4 This is a schematic diagram illustrating an exemplary embodiment of another line measurement system for testing optical fibers according to the present disclosure. Figure 4 LMS 400 indicates Figure 3 A dual-polarization variant is arranged. The LMS 400 includes an intermediate tap (shown as tap 410) that receives the second part 122 and outputs signals 412 and 414, respectively, received by frequency shifters 422 and 424. A frequency shifter is a device that shifts the frequency of optical light by a given value Ω. Before being combined at the loop tap 136, the frequency-shifted signals 432 and 434 are output by frequency shifters 422 and 424. These two output signals are combined at a polarization beam combiner (e.g., Figure 4The PBC 436 (shown in the diagram) generates a combining signal 438. This component combines two single-polarized beams at the input into a single fiber at the output. Similarly, in some embodiments, this polarization beam combiner can be a standard device, such as a commercially available one. Optionally, a third filter amplifier 442 can be positioned between the PBC 436 and the loop tap 136.

[0033] exist Figure 4 In one embodiment, the detection at PD 142 occurs for two polarizations at two different frequencies, and the average electrical power for each frequency (1GHz-Ω1, 1GHz-Ω2) is summed to obtain an electrical LMS response signal that is insensitive to time-varying PDL.

[0034] Although not specifically shown, in some embodiments... Figure 4 The technique can be achieved by using two polarization shifters and two detectors to perform measurements under two polarizations.

[0035] Further embodiments of this disclosure

[0036] In implementations where LO light may require amplification, an amplifier such as an erbium-doped filter amplifier (EDFA) can be used at the marked locations shown in the above figure. It is likely that only one EDFA is needed. However, EDFAs themselves generate broadband noise that can negatively impact performance. Therefore, some filtering may be required before the LO light from the second portion 122 is received by the detector. In this case, we can place a filter that blocks most of the ASE (amplified spontaneous emission) light outside the LO bandwidth. This filter can be a separate device, or it can be the same filter that filters out the transmission path (typically a WSS, which is not shown separately in the transmission system shown in the figure). In this case, the filter (or WSS) can be positioned between PD 142 and the loop tap 136, which combines the second portion 122 with the LMS response optical signal of the loop (i.e., the loop signal).

[0037] Typically, the location of the aforementioned TAP will be defined by considerations such as cost and performance. Therefore, according to some embodiments, multiple components may be placed between the TAP and the PD, and between the output of transmission system 132 and the TAP. A key consideration is the ratio between the LO signal (second part 122) and the total loop signal in the filtered optical bandwidth. According to embodiments of this disclosure, the LO light received by the tap that combines the LO light with the loop signal from transmission system 132 should be significantly larger (e.g., greater than 15 dB) than the loop signal in the filtered optical bandwidth received by the PD. Other practical considerations, such as the optimal total power to the PD, are also part of the design considerations, and additional optical components such as VOAs, optical filters, taps, splitters, and EDFAs may be required in the loop path.

[0038] Figure 5 An exemplary process flow 500 is illustrated. In block 502, a probe beam is emitted from a laser probe source. In some embodiments, the laser probe source may be an external cavity laser. The probe beam may be broadened in some embodiments and may be a narrowband beam in other embodiments.

[0039] In block 504, a first portion of the probe beam is guided to a transmission system, where the first portion is manipulated and conducted through the transmission system to generate a loop signal. For example, the first portion can be sent to the transmission system via a polarization rotator, and then the transmission system generates the loop signal.

[0040] In block 506, the second portion of the probe beam is directed to the loop tap. For example, the second portion of the probe beam may be separated from the first portion at the polarization-maintaining tap.

[0041] In block 508, an interference signal is generated by combining a first portion of the probe beam with a loop signal from the transmission system, wherein the loop signal is based on the first portion of the probe beam. For example, the loop signal can be derived from the first portion of the probe beam after it has been conducted through a transmission system with a filtered carrier channel.

[0042] In block 510, the interference signal is converted into an electrical power signal at a photodetector. The electrical power signal can be measured at one or more different frequencies, depending on the embodiment. For example, in various embodiments, the bandwidth of the electrical filter is much smaller than the optical bandwidth of 25 GHz, thus eliminating a large portion of any additional noise.

[0043] Figure 6 An exemplary process flow 600 is illustrated. In block 602, a probe beam is emitted from a laser detection source. This probe beam may be characterized by a first bandwidth, wherein the probe beam is broadened in some embodiments and may be a narrowband beam in other embodiments.

[0044] In block 604, a first portion of the probe beam is guided to the transmission system. In one example, the first portion can be transmitted to the transmission system via a polarization rotator, which operates at a predetermined frequency, such as 1 GHz.

[0045] In block 606, a second portion of the probe beam is sent to a second polarization rotator. For example, a polarization-maintaining tap can separate the first and second portions of the probe beam and send the second portion to the second polarization rotator.

[0046] In block 608, the second part is received at the loop tap after passing through the second polarization rotator.

[0047] In block 610, a loop signal is received from the transmission system, wherein the loop signal is based on a first portion of the probe beam. The loop signal can be derived from the first portion of the probe beam after being conducted through the transmission system with a filtered carrier channel. The loop signal can be received at a loop tap where a second portion of the probe beam is received.

[0048] In block 612, the second portion of the probe beam is mixed with the loop signal to generate interference, such as in a loop tap.

[0049] In block 614, a power signal is generated from the interference signal, such as by using a single photodetector.

[0050] In block 616, the power signal is measured over a second bandwidth equivalent to the first bandwidth. In a non-limiting example, the first bandwidth could be approximately 25 MHz, and the second bandwidth could also be approximately 25 MHz.

[0051] Figure 7 An exemplary process flow 700 is illustrated. In block 702, a probe beam is emitted from a laser probe source. The probe beam is characterized by a narrow bandwidth.

[0052] In block 704, a first portion of the probe beam is guided to the transmission system. In one example, the first portion can be transmitted to the transmission system via a polarization rotator, which operates at a predetermined frequency, such as 1 GHz.

[0053] In block 706, a second portion of the probe beam is sent to the frequency shifter assembly to form a frequency-shifted beam.

[0054] In block 708, a frequency-shifted beam and a loop signal from the transmission system are received at the loop tap, wherein the frequency of the loop signal is based on a first portion of the probe beam.

[0055] In block 710, the frequency-shifted beam and the loop signal are mixed to generate an interference signal.

[0056] In block 712, a power signal is generated based on the interference signal. This power signal can be generated at a photodetector, wherein the detection is based on the frequency difference of the signal, or an addition to the frequency of the signal.

[0057] In block 714, the power signal is measured over a second bandwidth. The second bandwidth can be chosen to be comparable to the larger of a narrow bandwidth or a sampling rate for measuring the power signal.

[0058] In summary, this embodiment provides an apparatus and technique in which an LMS system is provided such that a portion of the light generated from a laser detection source is separated and transmitted to a transmission system, and added to the loop signal using a tap changer. This separated portion of the light may interfere with the loop signal, wherein this interference is detected by a single photodetector. Figure 8 A general arrangement is provided, wherein the arrangement 800 shown typically includes, with Figure 1 The same components, with similar components marked as identical. This arrangement 800 includes a pre-transmission block 802, which, according to known LMS systems, is processed before the loop signal is combined with the second portion 122 of the probe beam 120. Therefore, the pre-transmission block 802 may include components other than the aforementioned polarization / OOK block.

[0059] As used herein, an element or step referred to in the singular and used with the words “a” or “an” should be understood to not exclude plural elements or steps unless such exclusion is explicitly cited. Furthermore, references to “one embodiment” in this disclosure are not intended to exclude the existence of additional embodiments that also include the features cited.

[0060] While this disclosure relates to certain embodiments, many modifications, alterations, and variations of the described embodiments are possible without departing from the scope and domain of this disclosure, as defined in the appended claims. Therefore, this disclosure is not limited to the described embodiments, but has the full scope defined by the language of the claims and their equivalents.

Claims

1. A line monitoring system, comprising: A laser source, used to emit a detection signal over a first bandwidth; A polarization-maintaining tap is used to receive the detection signal and to separate the detection signal into a first part and a second part; A polarization rotator is used to receive a first portion of the detection signal and transmit the first portion to a transmission system; A loop tap is used to receive a second portion of the detection signal and to receive a loop signal from the transmission system, wherein the loop signal is derived from a first portion of the detection signal; A photodetector, coupled to receive an interference signal from the loop tap, wherein the interference signal is generated by mixing a second portion of the loop signal and the detection signal, and the photodetector is arranged to output a power signal based on the interference signal; and A power measurement system for measuring the power signal at a given measurement frequency over a second bandwidth equivalent to the first bandwidth.

2. The line monitoring system according to claim 1, wherein, The polarization rotator further includes an on-off keying (OOK) component for modulating the first portion using OOK data.

3. The line monitoring system according to claim 1, further comprising a second polarization rotator for receiving a second portion of the detection signal, for applying rotation and sending the rotated second portion to the loop tap.

4. The line monitoring system of claim 1, further comprising a frequency shifter assembly arranged to receive the second portion and apply a frequency shift to the second portion, and transmit the second portion to the loop tap, and in, The frequency shifter assembly includes a first frequency shifter and a second frequency shifter, and the line monitoring system further includes: An intermediate tap, configured to receive a second portion from the polarization-maintaining tap, output a first segment of the second portion to the first frequency shifter, and output a second segment of the second portion to the second frequency shifter; and A polarization beam combiner is used to receive a first frequency-shifted beam from the first frequency shifter and a second frequency-shifted beam from the second frequency shifter, and outputs the second portion as a combined beam to the loop tap. The intermediate tap is the second polarization-maintaining tap.

5. The line monitoring system according to claim 1, further comprising at least one filter amplifier configured to intercept at least a portion of the detection signal between the laser source and the loop tap, and The given measurement frequency corresponds to two different frequencies.

6. The line monitoring system according to claim 1, wherein, The detection signal includes a bandwidth in the range of 25MHz.

7. The line monitoring system according to claim 1, wherein, The power signal includes a frequency of approximately 25 MHz to 5 GHz, and the power measurement system is configured to perform measurements within a sampling period of 50 μs to 500 μs.

8. The line monitoring system according to claim 1, wherein, The rotation frequency of the polarization rotator is approximately 1 GHz.

9. The line monitoring system according to claim 1, wherein, The given measurement frequency corresponds to the frequency of the polarization rotator.

10. The line monitoring system according to claim 3, wherein, The given measurement frequency corresponds to: The difference between the rotation frequency of the polarization rotator and the rotation frequency of the second polarization rotator; or The rotation frequency of the polarization rotator is the sum of the rotation frequency of the second polarization rotator.

11. A method for measuring faults in a transmission system, comprising: A detection beam is emitted from a laser detection source, the detection beam having a first bandwidth; The first portion of the probe beam is guided through the polarization rotator and then to the transmission system; The second portion of the probe beam is guided to the loop tap; At the loop tap, a loop signal from the transmission system is received, the loop signal being based on a first portion of the probe beam; The second portion of the probe beam is mixed with the loop signal to generate an interference signal; A power signal is generated based on the interference signal; and The power signal is measured at a given measurement frequency on a second bandwidth equivalent to the first bandwidth.

12. The method according to claim 11, wherein, The first part of the guidance includes: The first portion is guided through the polarization rotator; and The first portion is modulated using an on-off keying (OOK) component for transmission to the transmission system, and The detection beam includes a bandwidth in the range of 25MHz. The measurement of the power signal includes measurements at frequencies ranging from approximately 25 MHz to 5 GHz and sampling periods ranging from 50 μs to 500 μs. The guiding of the second part includes guiding the second part through the second polarization rotator before the second part is received at the loop tap.

13. A method for measuring faults in a transmission system, comprising: A detection beam is emitted from a laser detection source, the detection beam having a first bandwidth; The first portion of the probe beam is guided through the polarization rotator and then to the transmission system; The second portion of the probe beam is directed to the frequency shifter assembly to form a frequency-shifted beam; The frequency-shifted beam and a loop signal from the transmission system are received at the loop tap joint, the loop signal being based on a first portion of the probe beam; The frequency-shifted beam is mixed with the loop signal to generate an interference signal; A power signal is generated based on the interference signal; and At a given measurement frequency, the power signal is measured over a second bandwidth, which is equivalent to the first bandwidth or to the greater of the following: the first bandwidth and the sampling rate at which the power signal is measured.

14. The method according to claim 13, wherein, The second part that guides the probe beam includes: The first segment of the second part of the probe beam is frequency-shifted by the first frequency shifter to form the first frequency-shifted beam; The second segment of the second portion of the probe beam is frequency-shifted using a second frequency shifter to form a second frequency-shifted beam; and The first frequency-shifted beam and the second frequency-shifted beam are combined to form a frequency-shifted beam.

15. The method according to claim 14, wherein, The measurement of the power signal includes measurements at a first frequency of the first frequency-shifted beam and at a second frequency of the second frequency-shifted beam.

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