TECHNICAL FIELD

[0001] This disclosure relates generally to the detection of low frequency vibrations using laser interferometry. More particularly it pertains to systems, methods, and structures for low frequency vibration (i.e., seismic) detection exhibiting an improved laser phase noise tolerance through the use of fiber turnarounds.

BACKGROUND

[0002] As is known, the detection of low frequency vibrations using ultra-stable laser interferometry is difficult, due in part to laser phase noise, particularly 1/f technical noise, which can swamp (overwhelm) any low frequency vibration(s) that one is trying to detect. A conventional method is to use an ultra-stable laser having a linewidth on the order of 1 Hz, taking special care to isolate the laser chamber mechanically and thermally to reduce the 1/f technical noise. Lasers exhibiting such characteristics are extremely expensive, and mechanical isolation may be operationally difficult depending upon an environment in which it is operating.

SUMMARY

[0003] An advance in the art is made according to aspects of the present disclosure directed to laser interferometric systems, methods, and structures exhibiting superior laser phase noise tolerance particularly in seismic detection applications.

[0004] In sharp contrast to the prior art, systems, methods and structures according to aspects of the present disclosure advantageously relax laser requirements by exhibiting a novel configuration wherein the same laser which generates an outgoing signal is coherently detected using the same laser as local oscillator. According to further aspects of the present disclosure, the use of fiber turnarounds allows for the cancellation and/or mitigation of undesired mechanical vibration. BRIEF DESCRIPTION OF THE DRAWING

[0005] A more complete understanding of the present disclosure may be realized by reference to the accompanying drawing in which:

[0006] FIG. 1 shows a schematic diagram of an illustrative prior art configuration for detecting mechanical vibrations on an optical cable using ultra-stable laser interferometry;

[0007] FIG. 2 shows a phasor diagram showing the impact of additive white

Gaussian noise on vibration detection using interferometric phase;

[0008] FIG. 3 shows a plot / spectrum of laser phase noise, additive white

Gaussian noise and ambient mechanical vibration showing the condition on detecting the presence of seismic wave(s);

[0009] FIG. 4 shows schematic diagram of an illustrative improved configuration for detecting seismic vibration(s) using ultra-stable laser interferometry according to aspects of the present disclosure wherein a same laser that generates an outgoing signal also provides local oscillator allowing for mitigation of low-frequency laser phase noise;

[0010] FIG. 5 shows a plot of simulation data comparing the phase spectra using the conventional, prior-art method and improved method according to aspects of the present disclosure;

[0011] FIG. 6 shows schematic diagram of an illustrative improved configuration for rejecting common-mode mechanical vibration(s) in an undersea fiber cable application that uses ultra-stable laser interferometry according to aspects of the present disclosure wherein a fiber turnaround is placed just beyond the shore in the undersea application to mitigate undesired near shore vibration(s); and [0012] FIG. 7 shows schematic diagram of an illustrative improved configuration for rejecting common-mode mechanical vibration(s) in an undersea fiber cable application that uses ultra-stable laser interferometry according to aspects of the present disclosure wherein a fiber turnaround is placed just before the shore at a remote end in the undersea application to mitigate undesired near shore vibration(s) at that remote end.

[0013] The illustrative embodiments are described more fully by the Figures and detailed description. Embodiments according to this disclosure may, however, be embodied in various forms and are not limited to specific or illustrative embodiments described in the drawing and detailed description.

DESCRIPTION

[0014] The following merely illustrates the principles of the disclosure. 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 disclosure and are included within its spirit and scope.

[0015] Furthermore, all examples and conditional language recited herein are intended to be only for pedagogical purposes to aid the reader in understanding the principles of the disclosure and the concepts contributed by the inventor(s) to furthering the art and are to be construed as being without limitation to such specifically recited examples and conditions.

[0016] Moreover, all statements herein reciting principles, aspects, and embodiments of the disclosure, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. [0017] 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.

[0018] Unless otherwise explicitly specified herein, the FIGs comprising the drawing are not drawn to scale.

[0019] FIG. 1 shows a schematic diagram of an illustrative prior art configuration for detecting mechanical vibrations on an optical cable using ultra-stable laser interferometry. As may be observed in that figure, a continuous-wave (CW) laser having a linewidth of approximately 1 Hz generates laser light that is launched into an optical fiber located inside a submarine cable. The submarine cable link may extend over multiple spans, wherein the laser light signal is periodically amplified by inline optical amplifiers. At a receiver, the laser light output signal is coherently detected using a second CW laser as local oscillator (LO).

[0020] Those skilled in the art will understand and appreciate that the complex- valued baseband photocurrent takes the form of: where R is the responsivity of the photoreceiver, P _r and P _LO are the powers of the received signal and LO at the input of the coherent receiver, respectively. It is assumed that the lasers included in both interrogators - are operating and outputting light at substantially the same wavelength λ _L = λ _k = λ, where φ _L (t) and φ _R (t) are phase noises of the left (L) interrogator (transmitter) and right (R) interrogator (receiver - local oscillator), respectively, n(t) is a photocurrent arising from all the noise sources in the system, including amplified spontaneous emission (ASE) of all the inline amplifiers, as well as any shot noise and any thermal noise in the coherent receiver. [0021] The variable of interest is φ _vib (t) which is the optical phase arising from cumulative strain along the fiber, and takes the form of: where L _tot is the length of the optical cable, and Δ _∈ (z) is the tensile strain at position z along the fiber optic cable.

[0022] A usual method of recovering φ _vib (t) is to take the unwrapped phase of the photocurrent: where φ _AWNG (t) is the angle formed by the projection of n(t) onto a vector perpendicular to

[0023] FIG. 2 shows a phasor diagram showing the impact of additive white

Gaussian noise on vibration detection using interferometric phase. In the absence of seismic vibration, φ(t) varies over only a narrow range due to phase noise and AWGN. In presence of seismic vibration, φ(t) is dominated by φ _vib (t), and its excursion may reach hundreds to thousands of radians depending on the strength of the vibration.

[0024] Due to signal propagation delay, the onset of rapid phase variation will only be apparent to the receiver at time t _R = t +(L — z _vib )/(c/n _eff ), where t is the instance at which the seismic wave first impinges the optical cable at position z _vib from the transmitter, which is the closest point to the epicenter. n _eff is the effective index of the fiber. To determine z _vib , a bidirectional link can be implemented: suppose an identical system as above propagates from right to left as shown in FIG. 1. [0025] The onset of rapid phase variation will occur for the L interrogator at t _L = t + z _vib /(c/n _eff ). The time difference t _R — t _L = ( L _tot — 2z _vib )/(c/n _eff ) can then be used to infer z _vib since L _tot is known. Using bidirectional transmission for multiple submarine cables allows determination of the epicenter of an earthquake by triangulation. Ultrastable laser interferometry relies on: (i) ultra-narrow linewidth lasers, and (ii) clock synchronization of all interrogators, which may be achieved by synchronization with respect to global positioning system (GPS).

[0026] One key to determining the onset of rapid phase variation is that φ _vib (t) can be distinguished from cumulative noise sources φ _L (t) — φ _R (t) + φ _AWNG (t) Performance can be analyzed in the frequency domain as shown in FIG. 3, which shows a plot / spectrum of laser phase noise, additive white Gaussian noise and ambient mechanical vibration showing the condition on detecting the presence of seismic wave(s).

[0027] Laser frequency noise can be modeled as a two-sided power spectral density (PSD) of: where Δv is the Lorentzian linewidth (in Hz) arising from spontaneous emission inside the laser cavity. At low frequencies, however, a laser’s frequency spectrum is usually dominated by “technical” noise, which can be interpreted as slow drift of the laser’s center frequency. The fitting parameter is the frequency at which frequency noise due to technical noise becomes equal to spontaneous emission. Since laser phase φ _PN (t) = is the integral of frequency noise, the two-sided PSD of laser phase noise is: while the two-sided PSD of φ _AWNG (t) is: where η is the signal-to-noise (SNR) ratio of the received signal.

[0028] From FIG. 3, it is apparent that vibration φ _vib (t) can be detected provided there exists frequency components over which its power spectral density is higher than the sum of laser phase noise and AWGN: Furthermore, from FIG. 3, it is possible to improve vibration detection SNR by either: (i) increasing the PSD of φ _vib (t), (ii) decreasing the PSD of φ _PN (t), or decreasing the PSD of φ _AWNG (t)·

[0029] We shall now show and describe in greater detail, systems, methods, and structures according to aspects of the present disclosure advantageously employ fiber turnarounds to improve laser phase noise tolerance in low frequency (seismic) detection using ultra-low linewidth interferometry.

[0030] As we have previously noted, a conventional, prior-art configuration includes a transmitter and a receiver positioned at opposite ends of a submarine optical fiber link, such that the transmitter laser and local oscillator laser are two different lasers.

[0031] In sharp contrast, systems, methods, and structures according to aspects of the present disclosure incorporate a fiber turnaround at the far end of the submarine optical fiber link such that the outgoing (transmitted) signal is routed back to the same transmitter side of the submarine fiber optic cable thereby advantageously allowing a single laser to serve as transmitter and local oscillator in a coherent interferometry arrangement. It will be appreciated by those skilled in the art that such an arrangement helps reduce phase noise at low frequencies where seismic vibration energy is concentrated, thereby allowing increased sensitivity, increased sensing range, and/or the use of less expensive lasers in the interrogators. [0032] Examples of fiber turnarounds incorporated into a submarine cable may be observed in FIG. 4, FIG. 6, and FIG. 7.

[0033] FIG. 4 shows schematic diagram of an illustrative improved configuration for detecting seismic vibration(s) using ultra-stable laser interferometry according to aspects of the present disclosure wherein a same laser that generates an outgoing signal also provides local oscillator allowing for mitigation of low-frequency laser phase noise.

[0034] As shown in that figure are two interrogators positioned at opposite ends of an undersea optical cable. Each of the interrogators - L Interrogator and R Interrogator - include a laser light source operating at a specific wavelength for the L Interrogator and R Interrogator that are in optical communication with the undersea optical cable. Along with the laser(s), each of the interrogators include a coherent receiver which is in optical communication with the optical cable. As shown further, the optical cable includes at least a pair of separate optical fibers that are configured directionally - one from the L Interrogator to the R Interrogator - and conversely one from the R Interrogator to the L Interrogator.

[0035] Advantageously, we improve the performance of detecting φ _vib (t) by using the system architecture as shown illustratively in FIG. 4 which reduces the spectral density of phase noise. With continued reference to that figure, we note that at the R interrogator, instead of simply coherently detecting a L-R signal, such signal is transmitted to a far end (R) of the optical cable and at least a portion of that signal is routed back to the L interrogator via a different fiber in the same optical cable. The routed back L-R signal is coherently detected by the L interrogator, which allows the same laser to be used to both generate the outgoing signal and as local oscillator.

[0036] The phase of the received signal is thus φ _L,Rx (t) = φ _L,Tx (t - T _rt ) + φ _AWNG (t), where T _rt = 2 _neff L/c is the round-trip delay, while φ _L,LO (t) = φ _L,Tx (t) Their coherent beat product produces a phase of φ(t) = φ _L (t — T _rt ) — φ _L (t) + φ _AWNG (t)

[0037] Ignoring the contribution by ASE noise, this phase has a Fourier transform of:

[0038] Thus PSD of phase noise in this scheme is:

[0039] As compared with the original scheme, which produces a phase noise PSD of [rad ² /Hz] assuming transmit and LO lasers are independent but have the same phase noise characteristics, the current scheme according to aspects of the present disclosure has a multiplication factor of At low frequencies where so phase noise is suppressed. This is intuitive since at modulation frequencies lower than the round-trip frequency 1/T _rt , the return signal’s phase will be almost identical to the outgoing signal’s phase Low frequency modulation on the laser’s phase is canceled by delay interferometry.

[0040] FIG. 5 shows a plot of simulation data comparing the phase spectra using the conventional, prior-art method and improved method according to aspects of the present disclosure. With reference to that figure, one line shows the spectrum of φ(t) using the first scheme when the transmitter and receiver (LO) lasers are different, while the other line shows the spectrum of φ(t) using a scheme according to the present disclosure - illustratively when the LO is the transmit laser signal delayed by 10 ms, corresponding to a cable length of ~1000 km. In this simulation, all lasers are modeled to have Lorentzian linewidth of 10 Hz and a corner frequency of 100 Hz, below which technical noise dominates. [0041] Phase noise suppression is observed for frequencies below ≈ 1/2πT _rt =

16 Hz. At 1 Hz and 10 Hz, the phase noise suppression ratio are -24 dB and -4 dB, respectively. Since seismic vibration has its energy concentrated mostly between 1 and 10 Hz, the proposed scheme improves laser phase noise tolerance by the same magnitude.

[0042] It is possible to further extend the concept of using a fiber return to reject common-mode mechanical vibration, such as near landing stations in submarine links where mechanical vibration is strongest near the shore. As those skilled in the art will appreciate, to detect a seismic vibration on the ocean floor, it may be necessary to cancel the strong vibrations near the shore.

[0043] FIG. 6 shows schematic diagram of an illustrative improved configuration for rejecting common-mode mechanical vibration(s) in an undersea fiber cable application that uses ultra-stable laser interferometry according to aspects of the present disclosure wherein a fiber turnaround is placed just beyond the shore in the undersea application to mitigate undesired near shore vibration(s).

[0044] We note that in the configuration shown in FIG. 6, two frequency tones are generated from the same seed laser. One tone propagates to the other end of the cable and returns via a different fiber in the same cable. The other tone is turned around at an output of a first optical amplifier immediately after a landing station, which is at distance L _T from the L interrogator.

[0045] The optical phase due to mechanical strain at the two frequencies are: Note that φ _vib,1 (t), is the undesired phase due to cumulative mechanical strain between the transmitter and the shore. φvib,1(t) has the same form as the first term of φ _{vib, 2} (t) apart from a scaling factor equal to the ratio of their wavelengths λ ₁ /λ _2. It is possible to coherently detect both tones and retrieve their phases φ ₁ (t) = φ _vib,1 (t) + φ _Tx (t —

2n _eff L _T /c) — φ _Tx (t) + φ _{AWGN ,1} (t) and φ ₂ (t) = φ _vib,2 (t) + φ _Tx (t — 2n _eff L/c) — φ _Tx (t) + φ _{AWGN ,2} (t) _· The difference φ(t) = φ ₂ (t) — ( λ ₁ /λ ₂ )φ ₁ (t) will be free of undesirable landing station vibration φ _vib,1 (t) Note this method potentially increases the impact of phase error due to laser phase noise and ASE noise, but if vibration near the landing station, φ _vib,1 (t), is the dominant source of interference over seismic frequencies of interest as shown in the configuration of FIG. 3, this method will reduce its impact.

[0046] Note that suppression of near-shore vibration is possible because the two tones traveled in the same fiber cores (both outbound and inbound directions), so they experience identical mechanical strains. Note further that turning the outbound signal around just before the landing station at the remote end will also mitigate mechanical noise the far-end landing station.

[0047] FIG. 7 shows schematic diagram of an illustrative improved configuration for rejecting common-mode mechanical vibration(s) in an undersea fiber cable application that uses ultra-stable laser interferometry according to aspects of the present disclosure wherein a fiber turnaround is placed just before the shore at a remote end in the undersea application to mitigate undesired near shore vibration(s) at that remote end.

[0048] As noted previously, such configuration allows the interrogator to perform coherent detection using the same laser as that which generated the outgoing signal. Thought of alternatively, we thus convert laser interferometry using two independent lasers into delay interferometry. As shown by Eq. (7), phase noise is suppressed at low frequencies where seismic vibration energy is concentrated, relaxing the requirement on the phase noise spectrum of the laser. The use of fiber turnarounds in FIG. 6 and FIG. 7 also allows cancellation (and/or mitigation) of undesired near-shore mechanical vibration caused by ships, machinery, etc. [0049] While we have presented this disclosure using some specific examples, those skilled in the art will recognize that our teachings are not so limited. Accordingly, this disclosure should be only limited by the scope of the claims attached hereto.