Vibration detection using phase recovery from optical transponders with coherent detection

Abstract

Aspects of the present disclosure describe systems, methods, and structures for vibration detection using phase recovery from an optical transponder via coherent detection. Advantageously, the systems, methods, and structures of the present invention leverage modern digital coherent receiver architectures, in which various adaptive DSP operations are performed to recover the optical phase of a transmitted data track. Because the phase is extracted with low overhead cost, digital coherent transponders can perform vibration detection / monitoring as an adjunct to data transmission. A vibration detection and localization demonstration based on the extraction of optical phase from a payload-carrying telecommunications signal using a coherent receiver in a bidirectional WDM transmission system has been shown and described.

Description

[Technical field]

[0001] The present disclosure generally relates to optical communication systems, methods, and structures using coherent receivers. More specifically, the present disclosure describes vibration detection with phase recovery from a digital optical transponder with coherent detection, enabling the digital coherent transponder to perform vibration detection / monitoring as an auxiliary function of data transmission. [Background technology]

[0002] As is known in the optical communications art, transponders utilizing coherent detection have been widely deployed, where the optical phase is continuously tracked in a digital signal processor (DSP) by a combination of frequency offset compensation (FOC), adaptive time domain equalization (TDE), and carrier phase recovery (CPR). Also, distributed optical fiber sensing (DFOS), including distributed vibration sensing (DVS), has been demonstrated to enhance the functionality of existing optical fiber communications equipment by simultaneously utilizing existing deployed communications optical fibers as sensor systems. Thus, systems, methods, and structures that facilitate utilizing existing communications equipment as sensory systems are a welcome addition to the art. Summary of the Invention

[0003] An advancement in the art is made by aspects of the present disclosure that advantageously provides an optical communications transponder that uses coherent detection to provide vibration detection as an additional function.

[0004] In sharp contrast to the prior art, the systems, methods, and structures of the present disclosure exploit the fact that in a digital coherent receiver, the various adaptive DSP operations performed to recover the transmitted data track the optical phase, which can be extracted at low overhead cost, enabling the digital coherent transponder to perform vibration monitoring as an adjunct to data transmission.

[0005] As an additional advantage, the systems, methods, and structures according to embodiments of the present disclosure can further reduce the effects of laser phase noise compared to conventional systems by replacing conventional external cavity lasers (ECLs) with more stable lasers that exhibit lower frequency noise, and without wavelength dithering for channels that serve the dual purposes of optical transmission and vibration sensing.

[0006] Advantageously, in accordance with further aspects of the present disclosure, the systems and methods of the present invention provide vibration sensing auxiliary functions at a relatively low cost by reusing coherent optical transponders for such functions as compared to building a dedicated transponder solely for vibration sensing. [Brief description of the drawings]

[0007] A more complete understanding of the present disclosure may be realized by reference to the accompanying drawings.

[0008] [Figure 1(A)] FIG. 1 is a schematic diagram illustrating vibration detection using interferometric detection of an ultra-low linewidth laser according to an embodiment of the present disclosure, showing the optical phase detected at the remote end using a different local oscillator (LO) laser. [Figure 1(B)] FIG. 13 is a schematic diagram illustrating vibration detection using interferometric detection of an ultra-low linewidth laser according to an embodiment of the present disclosure, showing the signal reflected at the remote end to enable optical phase detection at the same end using the same laser as the LO.

[0009] [Diagram 2] 1 illustrates determining a location of a point source vibration impinging on an optical cable according to an embodiment of the present disclosure by correlating optical phase in east-west (WE) and east-west (EW) directions;

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[0010] [Figure 3(A)] A standard model of a digital coherent receiver for high-speed optical transmission is shown. [Figure 3(B)] 1 illustrates exemplary digital signal processing (DSP) operations required to recover transmitted bits that jointly track the instantaneous phase of an input signal, according to an embodiment of the present disclosure, in which the highlighted DSP blocks.

[0011] [Figure 4] 11 is a plot illustrating the sampling rate requirement for phase estimation, where the sampling rate 1 / Tθ only needs to be fast enough so that the alias frequency offset does not vary by more than 1 / Tθ, in accordance with an embodiment of the invention.

[0012] [Figure 5(B)] FIG. 13 is a schematic diagram illustrating a vibration experiment with an optical cable suspended from a hammered utility pole, according to an aspect of the present disclosure.

[0013] [Figure 6(A)] According to equation (5)

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[0014] [Figure 7]FIG. 1 is a schematic diagram illustrating a digital phase-locked loop (DPLL) having a loop filter F “z” for extracting vibration-induced phase changes from the optical phase tracked by a coherent receiver, according to an embodiment of the present disclosure.

[0015] [Figure 8] FIG. 2 is a schematic diagram showing an example configuration for determining the location of a point source vibration impinging on an optical cable by correlating the optical phase in the east-west (WE) and east-west (EW) directions. In comparison with FIG. 2, according to an embodiment of the present disclosure, each transponder transmits modulated data and a phase measurement

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[0016] [Figure 9] FIG. 1 is a schematic diagram illustrating an example DSP operation for determining a position of a point source vibration from optical phase tracked by two bidirectional digital coherent transponders, according to an embodiment of the present disclosure.

[0017] [Figure 10] FIG. 1 is a schematic diagram illustrating an experimental setup for demonstrating vibration source localization using a bidirectional digital coherent data link, according to an aspect of the present disclosure.

[0018] [Figure 11(A)] Plots showing the results of vibration localization experiments: (a) Measured phase after PLL for each direction. The inset shows that the WE phase leads the EW phase. (b) Spectrum of the WE phase. (c) Correlation between the WE and EW phases, with maximum correlation at ~480 km. [Figure 11(B)]Plots showing the results of vibration localization experiments: (a) Measured phase after PLL for each direction. The inset shows that the WE phase leads the EW phase. (b) Spectrum of the WE phase. (c) Correlation between the WE and EW phases, with maximum correlation at ~480 km.

[0019] [Figure 12] 1 is a schematic diagram illustrating an example extraction of phase due to vibration from a phase extracted from a coherent receiver, according to an aspect of the present disclosure. FIG.

[0020] [Figure 13(A)] FIG. 1 is a schematic diagram illustrating a bidirectional link according to an aspect of the present disclosure. [Figure 13(B)] FIG. 1 is a schematic diagram illustrating an experimental setup for a bidirectional transponder according to an aspect of the present disclosure.

[0021] Exemplary embodiments are more fully described in the drawings and detailed description, however, embodiments in accordance with the present disclosure may be embodied in various forms and are not limited to the specific or exemplary embodiments set forth in the drawings and detailed description. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] The following merely illustrates the principles of the present disclosure, and it will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the present disclosure and are included within its spirit and scope.

[0023] Furthermore, all examples and conditional terms described herein are intended to be for educational purposes only to aid the reader in understanding the concepts contributed by the inventors to further the principles and techniques of the present disclosure, and should not be construed as being limited to such specifically recited examples and conditions.

[0024] Moreover, all statements herein reciting principles, aspects, and embodiments of the present disclosure, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, such equivalents are intended to include both currently known equivalents as well as equivalents developed in the future, i.e., elements developed that perform the same function, regardless of structure.

[0025] Thus, for example, it will be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the disclosure.

[0026] Unless otherwise specified herein, the figures making up the drawings are not drawn to scale.

[0027] Mechanical vibrations applied to the optical fiber cable induce longitudinal strains in the individual fibers within the optical cable, resulting in a phase shift θ vib It is again noted that one of ordinary skill in the art will understand and appreciate that this can manifest itself in a change in optical path length Δz that can be detected as =(2π / λ)Δz, where λ is the wavelength of the optical signal.

[0028] θ vib One way to measure is to launch a probe pulse into an optical fiber and detect the Rayleigh backreflection (backscatter) using distributed acoustic sensing (DAS). This method achieves high spatial resolution, but its reach is typically limited by the weak nature of the Rayleigh scattering coefficient, resulting in a low optical signal-to-noise ratio (OSNR).

[0029] Recently, a forwarding approach using ultra-low linewidth lasers has been proposed to detect seismic waves in undersea cables. In this scheme, an unmodulated continuous wave (CW) laser is launched into the target optical fiber. At the remote end of the fiber, the received signal is coherently detected using another ultra-low linewidth laser as a local oscillator (LO). The optical phase extracted from the interferometric detection has the following form:

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[0030] where Δf is the frequency offset between the transmitter (Tx) and the LO laser, Φ Tx (t) and Φ L0 (t) are the phase noise of the Tx laser and the LO laser, respectively, and θ vib (t) is the optical phase shift caused by the accumulated longitudinal strain of the fiber, θ n (t) is the equivalent phase noise caused by the amplified spontaneous noise (ASE) of all amplifiers.

[0031] Note that at the far end, the CW signal can be reflected back down another fiber to the near end, and the same transponder can perform coherent detection using the same laser for both the outgoing signal and the LO. The goal of vibration detection is to find the exact time from θ(t) to θ vib (t) is extracted. In equation (1), θ vib If there exists a frequency range in which the spectrum of (t) is greater than all other terms, vibration detection is possible.

[0032] Furthermore, if the vibration is properly modeled as a point source, it is possible to infer its location by launching CW signals from both ends and correlating the optical phase measured in the east-west and west-west directions.

[0033] 1(A) and 1(B) are schematic diagrams illustrating vibration detection using interferometric detection of an ultra-low linewidth laser according to an embodiment of the present disclosure, where FIG. 1(A) shows the optical phase detected at a remote end using a different local oscillator (LO) laser, and FIG. 1(B) shows the signal reflected at the remote end to enable the optical phase to be detected at the same end using the same laser as the LO.

[0034] FIG. 2 illustrates determining the location of a point source vibration impinging on an optical cable according to an embodiment of the present disclosure by correlating optical phase in the east-west (WE) and east-west (EW) directions;

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[0035] The aforementioned approach, which requires transmitting a CW tone over a dedicated channel, has drawbacks. First, the dedicated channel no longer carries data. Second, the CW tone must be transmitted at low power so as not to induce nonlinear effects in the data-carrying channel (if it is present on the same fiber) and so as not to exceed the Brillouin threshold. This latter factor is usually the more stringent requirement. Power limitations reduce the OSNR of the receiver, which results in a lower θ vib There may be a noisy estimate of (t).

[0036] 3(A) and 3(B) show a standard model of a digital coherent receiver for high-speed optical transmission in FIG. 3(A) and FIG. 3(B) show typical digital signal processing (DSP) operations required to recover transmitted bits that jointly track the instantaneous phase of the input signal, in accordance with an embodiment of the present disclosure, with highlighted DSP blocks.

[0037] With reference to the diagram, it can be seen that the optical front-end of a coherent receiver includes a dual-polarized 90° hybrid that combines the received signal with a local oscillator (LO) laser signal. This is followed by a balanced photodetector, whose output includes baseband electrical signals corresponding to the in-phase and quadrature components of the polarizations of the two received signals. The baseband electrical signals are low-pass filtered and then sampled by an analog-to-digital converter (ADC) at a rate of 1 / T. If the sampling rate is greater than twice the highest frequency component of the baseband electrical signal, the digitized waveform retains all the information of the input optical signal.

[0038] In the DSP operation shown in the figure, the digitized waveform is first resampled to a rate synchronous with the baud rate of the modulated data (usually twice the baud rate), and then compensated for receiver imperfections, which are usually constant. Three adaptive components that together track the optical phase θ(t) are highlighted.

[0039] In operation, the frequency offset compensation (FOC) unit rotates the signal by the estimated frequency offset Δf between the transmitter laser and the LO laser.

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[0040] After frequency-domain equalization for chromatic dispersion (CD), the signal passes through a "butterfly-structured" adaptive time-domain equalizer (TDE), which compensates for polarization rotation and polarization-mode dispersion.

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[0041] In formula (3),

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[0042]

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[0043] are the values ​​of the adaptive equalizer used to recover symbol n, for each partition, and

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[0044] Finally, the equalizer output is rotated by carrier phase recovery (CPR) as follows:

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[0045] The output of the CPR is passed through a soft-decision forward error correction (SD-FEC) decoder and a hard-decision FEC (HD-FEC) decoder to recover the transmitted bits.

[0046] In Figure 3, the three DSP components that collectively track the instantaneous optical phase are highlighted and their operation is shown in Equation (2-4). Phase tracked by the combination of FOC, TDE, and CPR

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[0047] The overhead cost of computing equation (5) is only moderate, involving additions, phase multiplications, and angle calculations. Furthermore, mechanical vibrations and laser phase noise are slow processes compared to the baud rate of the data modulated signal. Thus, equation (5) reduces to 1 / T θ <<1 / T, and is calculated infrequently.

[0048] FIG. 4 illustrates an embodiment of a 1 / T alias frequency offset in accordance with the present invention. θ To avoid fluctuations exceeding the sampling rate 1 / T θ 1 is a plot showing the sampling rate requirement for phase estimation, where k is only required to be sufficiently fast.

[0049] With reference to this figure, in equation (1), we consider the phase change due only to the laser frequency offset and the effects of phase noise. The spectrum of θ(t) is approximately Lorentzian and centered on the instantaneous frequency shift Δf. If we sample θ(t) at twice the maximum frequency offset, the Nyquist criterion is met,

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[0050] FIG. 5 is a schematic diagram illustrating a vibration experiment with an optical cable suspended from a hammered utility pole, according to an embodiment of the present disclosure.

[0051] 6(A) and 6(B) are plots, and FIG. 6(A) shows that according to Eq.

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[0052] FIG. 7 is a schematic diagram illustrating a digital phase-locked loop (DPLL) having a loop filter F “z” for extracting vibration-induced phase changes from the optical phase tracked by a coherent receiver, in accordance with an embodiment of the present disclosure.

[0053] Referring to the figures, note that Fig. 6(A) shows an example of the optical phase of the interference product between two narrow linewidth lasers, where the signal laser passes through an optical cable suspended on a hammered utility pole as shown in Fig. 5. According to equation (1), the sampled optical phase θ[n] is the sum of the phase due to the carrier frequency offset, the laser phase noise, and the change in optical path length due to vibration.

[0054] From Figure 6(A), we can see that the first two terms dominate the phase measurement. The vibration-induced phase component θ vib To recover [n], as shown in FIG.

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[0055] For the second order DPLL,

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[0056] FIG. 6(B) is a diagram of FIG. 6(A).

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[0057] The DPLL output can be passed through a bandpass filter (BPF), whose passband frequency depends on the characteristics of the vibration and how the vibration induces phase changes in the optical fiber. In the hammering experiments mentioned above, filtering between 200 Hz and 12 kHz was found to be a good compromise between a high signal-to-noise ratio of the vibration signature and the wide bandwidth required to accurately localize the vibration position using the bidirectional setup described in the next section. For buried optical cables, the ground tends to act as a low-pass filter, which drowns out high vibration frequencies, so the frequency range of the BPF may need to be narrowed to maintain a high SNR of the vibration signature.

[0058] 8 is a schematic diagram illustrating an exemplary configuration for determining the location of a point source vibration impinging on an optical cable by correlating the optical phase in the east-west (WE) and east-west (EW) directions. Compared to FIG. 2, according to an embodiment of the present disclosure, each transponder transmits modulated data and phase measurements.

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[0059] With the above described technique for measuring vibration-induced optical phase, it is possible to build a bidirectional link as shown in Figure 8, which is similar to the configuration of Figure 2, except for the fact that the two transponders transmit modulated data rather than unmodulated tones. As will be understood and appreciated by those skilled in the art, the receiver of each transponder estimates the vibration-induced phase change as an aid to data demodulation and detection. If the vibration is properly modeled as a point source, its location can be estimated by correlating the phase measured by the transponder in each direction. Let the distance between the vibration source and the "west" transponder be z E-W Let z be the distance between the vibration source and the "East" transponder. W-E The phase measured by the transponder is

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[0060] FIG. 10 is a schematic diagram illustrating an experimental setup to demonstrate vibration source localization using a bidirectional digital coherent data link according to an embodiment of the present disclosure. The "point source" vibration is the same as the optical cable suspended on a utility pole shown previously in FIG. 5. In the east-west direction, a 6×80 km span of standard single mode fiber (SSMF) with all Raman amplification is placed in front of the suspended cable, and in the west-east direction, a 6×80 km span of SSMF is placed after the suspended cable. The transponder transmits DP-16QAM at 32 Gbaud (raw data rate 128 Gb / s), and the EW and WE signals are detected by a dedicated receiver, which can also track the optical phase according to the operation already described in equation (2-5).

[0061] Figures 11(A) and 11(B) are plots showing the results of the vibration localization experiment: (a) the phase after the PLL measured for each direction, with the inset showing that the WE phase leads the EW phase, (b) the spectrum of the WE phase, and (c) the correlation between the WE and EW phases, with maximum correlation at ~480 km.

[0062] Figure 11(A) shows the phase measured at the EW and WE transponders, and the relative delay between the two measurements can be clearly seen.

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[0063] Vibration detection and localization based on extracting the optical phase from the DSP element of a coherent receiver is demonstrated with bidirectional transmission of a 200 Gb / s DP-16QAM WDM channel over 380 km of installed field fiber.

[0064] In recent years, there has been a trend to use optical fiber infrastructure for environmental monitoring. Distributed acoustic sensing (DAS) is one of the distributed optical fiber sensing (DFOS) techniques that offers high vibration sensitivity and localization accuracy. However, DAS relies on Rayleigh backscattering and is incompatible with existing communication infrastructure due to the unidirectional operation of in-line amplifiers. Tins limit the operating range of DAS, making it difficult to extend to wide-area deployment. Recently, vibration detection based on forward transmission using state of polarization (SOP) extracted from digital coherent transponders has been demonstrated. However, it is difficult to localize vibration using SOP.

[0065] Another forward transmission technique has been demonstrated using a laser interferometer with ultra-low phase noise. The use of optical phase has higher sensitivity and bandwidth than SOP, and location is possible by correlating the phase measured by interferometers at both ends of a bidirectional link [4], but the location capability has only been demonstrated in laboratory experiments so far, and in addition, implementing a laser interferometer requires the use of dedicated sensing equipment to transmit and receive continuous wave (CW) tones, and the sensing channel cannot be used for data transmission.

[0066] Herein, we implement an operational cable vibration monitoring scheme based on the optical phase recovered by a coherent communications transponder, where the phase is already tracked by an adaptive digital signal processing (DSP) element in the receiver. This method enables the communications transponder to perform vibration monitoring as an auxiliary function, with small DSP overhead and requiring only the use of a low phase noise laser in the sensing transponder.

[0067] We demonstrate the scheme over a 380 km field fibre link, demonstrating that the optical phase extracted from a coherent transponder has similar performance to that achieved using a cw signal, and demonstrating the detection and localisation of a variety of events.

[0068] In a coherent receiver, the optical phase is tracked by a frequency offset compensator (FOC), an adaptive time-domain equalizer, and carrier phase recovery (CPR).

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[0069] where ΩΔ is the digital frequency of the FOC.

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[0070] The vibration components can be extracted according to FIG. 12, where:

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[0071] Field tests were conducted on an in-service cable with fiber pair connections from Long Beach Island (LBI) in New Jersey to Seaside Heights (SH) in the north, Somers Point (SP) in the south, and an 11 km spur. Since vibration localization requires a bidirectional link for phase correlation and time synchronization, both transponders were collocated at LBI to configure a fiber link with bidirectional signals propagating in clockwise (CW) and counterclockwise (CCW) directions, as shown in Figure 13(A). Since same-fiber bidirectional transmission is susceptible to Rayleigh backscattering and reflections from connectors / splices, the CW direction transmits only on even channels (192.20-195.30 THz@100 GHz spacing) and the CCW direction transmits only on odd channels (192.15-195.25 THz@100 GHz spacing). The length of the loop is 378.8 km, of which approximately 25% is overhead cable suspended from utility poles and 75% is buried cable.

[0072] The link includes nine spans of standard single-mode fiber (SSMF) with losses ranging from 14 to 22 dB, with each span followed by a bidirectional DPLL BPF AEQ repeater consisting of two erbium-doped fiber amplifiers (EDFAs) and a circulator.

[0073] To mitigate cumulative double reflections, optical interleavers were inserted in the amplifier mid-stage of the LBI and SP. The transponder pair setup is shown in Figure 13(B). Two low-phase-noise NKT-X15 lasers at 193.4 (CW) and 193.3835 THz (CCW) were used as seed lasers. The CCW laser is downshifted by 33.5 GHz (193.35 THz) before data modulation, so that it is aligned to the 50G grid. A 4-channel 64 GSa / s digital-to-analog converter (DAC) modulated a 32 Gbaud 16QAM signal to the laser via an I / Q modulator. After polarization multiplexing using delay and polarization combining circuits, each DP-16QAM signal is multiplexed with 31 other 100 GHz-spaced channels, emulated by splitting and flattening the ASE source with a wavelength selective switch (WSS). The CW and CCW signals are sent to the link via a circulator. The launch power at each span was roughly optimized using the GN model.

[0074] At the receiver, the CW signal is mixed with a CCW laser acting as a local oscillator (LO) in a dual-polarized optical hybrid, followed by balanced optical detection, while for the CCW signal, the LO is obtained by downshifting the CW laser by 33.5 GHz. Since the 32 Gbaud DP-16QAM centered on a 16.5 GHz intermediate frequency (IF) between the seed lasers fits one side of a 80 GSa / s Keysight digital sampling oscilloscope (DSO) with a 33 GHz bandwidth, heterodyne detection was used to recover the field. This allowed both the CW and CCW direction signals to be recovered by the same 4-channel DSO, ensuring time synchronization. An optical bandpass filter (OBPF) was used to remove noise in the image band. Due to imperfect image band rejection and reduced effective number of bits (ENOB) at high frequencies, the performance of heterodyne detection is -0.5 dB worse than homodyne detection.

[0075] The DSO was operated in "sequential capture mode" with a frame rate of 200 kHz. For each frame, 10 ns (320 symbols) were acquired. The adaptive DSP

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[0076] The DSO captured a maximum of 131,072 frames, so at 200 kHz

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[0077] The results were:

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[0078] Similarly, when the cable was vibrated by hitting the utility pole at B in (Figure 3(A)), the correlation peaked at +0.96 ms. Both utility poles were hit simultaneously about every 0.5 seconds, and the correlation was determined when the hits happened to overlap. This result shows that it is possible to detect multiple vibration events.

[0079] As one skilled in the art will appreciate, the ability to detect / locate vibration events depends on phase SNR and bandwidth. To be detectable, there must be a frequency at which the event exceeds ambient levels due to laser phase noise and ambient vibrations. Striking the pole from which the overhead cable is suspended with a hammer will produce vibrations up to 1-3 kHz above ambient frequencies, depending on the force used and the length of the cable. We also tested an intrusion scenario where the cable loop outside the LBI building connecting the spur section to the south section is struck, and a tampering scenario where the patch cord at the output of the EDFA connecting the north and south cables is struck. Intrusion will have a lower bandwidth than tampering, as the rigid cable mute higher frequencies than the patch cord.

[0080] In this field test, CCW and CW signals were propagated on the same fiber since only one fiber pair was available for testing. In a real-world scenario, bidirectional signals travel on different fibers in the same cable, so their phases will be less correlated than same-fiber propagation. In-house tests were performed prior to the field test. In a "back-to-back" transmission through a cable suspended on a utility pole that was struck with a hammer, the correlation measured for propagation on different fibers was 95%, slightly lower than the 99% for same-fiber propagation.

[0081] Finally, to verify that the setup is compatible with data transmission, a standard bit error rate (BER) sweep was performed on all 64 channels (32 in each direction). The transmitter is reconfigured with two groups of three external cavity lasers (ECLs) modulated by two IQMs in Figure 13(B), interleaved in an ababab fashion with 100 GHz spacing, and then multiplexed with the ASE load channel. The CW and CCW directions were measured separately. All channels achieved a BER below the SD-FEC limit.

[0082] Here we disclose and demonstrate for the first time vibration detection and localization based on extraction of optical phase from payload-carrying communication signals using a coherent receiver in a bidirectional WDM transmission system. We detected real-world vibration events such as utility pole strikes, intrusions, and fiber tampering over 380 km of deployed field fiber. While the accuracy of the localization depends on the bandwidth of the vibration event above ambient levels, in the disclosed system and method, the coherent telecommunications transponder has low DSP overhead and can perform vibration monitoring as an auxiliary function using a low phase noise laser in the sensing transponder.

[0083] At this point, the present disclosure has been presented using several specific examples, but those skilled in the art will recognize that the present teachings are not so limited. Accordingly, the present disclosure should be limited only by the scope of the claims appended hereto.

Claims

1. A system for simultaneously performing coherent optical fiber transmission and vibration sensing, comprising: an optical fiber transmission medium; a transceiver in optical communication with the optical fiber transmission medium, the transceiver comprising: a coherent optical receiver responsive to the received optical signal and a local oscillator signal for detecting a carrier of the received optical signal; a digital signal processor configured to recover digital data carried in the received optical signal, track the optical phase of the received optical signal, and determine vibration-induced distortions of the optical fiber transmission medium; said recovering said digital data and said determining said vibration-induced distortion of said optical fiber transmission medium are performed simultaneously; The system, wherein the digital signal processor is configured to track the optical phase of the received optical signal by combining the embedded phase with adaptive digital signal processing elements including frequency offset compensation, adaptive time domain equalization, and carrier phase recovery.

2. 2. The system of claim 1, wherein the transceiver comprises a phase-locked loop followed by a bandpass filter to isolate the vibration-induced phase change from any other sources of phase change including frequency drift and laser phase noise.

3. 3. The system of claim 2, wherein the transceiver determines the vibration-induced phase change at a rate less than a baud rate of the received optical signal.

4. 4. The system of claim 3, further comprising a second transceiver located at an opposite end of the optical fiber transmission medium from the transceiver, the two transceivers configured to transmit data modulated optical signals to each other, the two transceivers independently determining vibration-induced phase changes from optical signals transmitted by the other transmitter.

5. 5. The system of claim 4, wherein the two transceivers independently determine a location of vibrations occurring at a position along the optical fiber transmission medium by correlating each transceiver's determination of the vibration-induced phase change.

6. 6. The system of claim 5, wherein an estimate of the vibration-induced phase is made by each transceiver and applied to an amplifier equalizer prior to correlation.

7. 7. The system of claim 6, wherein the optical fiber transmission medium includes at least two separate optical fibers over which a bidirectional link is established.

8. The system of claim 7 , wherein at least a portion of the two optical fibers coexist as part of a common optical fiber cable.

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