Methods and assemblies for using electrostriction to characterize properties of optical fiber

Abstract

Systems and method include a detector communicatively coupled to a fiber span; and processing circuitry connected to the detector and configured to digitally sample and process an output of the detector, detect phase changes in the output, and identify an electrostrictive response of the fiber span based on the detected phase changes and based on a dependence of the detected phase changes with frequency. A property of the fiber span can be determined on the electrostrictive response. The property of the optical fiber can include one or more of optical fiber material type, optical fiber material property, optical fiber area, optical fiber geometry, optical fiber condition, optical fiber stress and strain, optical fiber temperature. and optical fiber radiation exposure.

Claims

1. A system comprising: a detector communicatively coupled to a fiber span; and processing circuitry connected to the detector and configured to digitally sample and process an output of the detector, detect phase changes in the output, and identify an electrostrictive response of the fiber span based on the detected phase changes and based on a dependence of the detected phase changes with frequency.

2. The system of claim 1, further comprising a local oscillator (LO) or laser communicatively coupled to the detector for combination with an output of the fiber span.

3. The system of claim 1, wherein the detector is configured to detect an intensity modulated tone injected into the fiber span.

4. The system of claim 3, wherein the intensity modulated tone is provided on a service channel and extracted and provider to the detector.

5. The system of claim 1, wherein the detector is configured to detect one of a phase modulated waveform injected into the fiber span and a data carrying waveform injected into the fiber span.

6. The system of claim 1, wherein the processing circuitry is further configured to determine a property of the fiber span based on the electrostrictive response.

7. The system of claim 6, wherein the property of the optical fiber includes one or more of optical fiber material type, optical fiber material property, optical fiber area, optical fiber geometry, optical fiber condition, optical fiber stress and strain, optical fiber temperature. and optical fiber radiation exposure.

8. The system of claim 1, wherein the processing circuitry is configured to identify the electrostrictive response based on a measurement of nonlinear interaction of a modulated waveform either with itself through self-phase modulation or with another propagating field through cross phase modulation.

9. A non-transitory computer-readable medium having computer readable code stored thereon for programming a processor to perform functions comprising: digitally sampling and processing an output of a detector communicatively coupled to a fiber span, detecting phase changes in the output, and identifying an electrostrictive response of the fiber span based on the detected phase changes and based on a dependence of the detected phase changes with frequency.

10. The non-transitory computer-readable medium of claim 9, wherein the detector is also communicatively coupled to a local oscillator (LO) or laser for combination with an output of the fiber span.

11. The non-transitory computer-readable medium of claim 9, wherein the detector is configured to detect an intensity modulated tone injected into the fiber span.

12. The non-transitory computer-readable medium of claim 11, wherein the intensity modulated tone is provided on a service channel and extracted and provider to the detector.

13. The non-transitory computer-readable medium of claim 9, wherein the detector is configured to detect one of a phase modulated waveform injected into the fiber span and a data carrying waveform injected into the fiber span.

14. The non-transitory computer-readable medium of claim 9, wherein the computer readable code is configured to further program the processor to perform functions comprising determining a property of the fiber span based on the electrostrictive response.

15. The non-transitory computer-readable medium of claim 14, wherein the property of the optical fiber includes one or more of optical fiber material type, optical fiber material property, optical fiber area, optical fiber geometry, optical fiber condition, optical fiber stress and strain, optical fiber temperature. and optical fiber radiation exposure.

16. The non-transitory computer-readable medium of claim 9, wherein the identifying the electrostrictive response is based on a measurement of nonlinear interaction of a modulated waveform either with itself through self-phase modulation or with another propagating field through cross phase modulation.

17. A method comprising: digitally sampling and processing an output of a detector communicatively coupled to a fiber span, detecting phase changes in the output, and identifying an electrostrictive response of the fiber span based on the detected phase changes and based on a dependence of the detected phase changes with frequency.

18. The method of claim 17, wherein the detector is also communicatively coupled to a local oscillator (LO) or laser for combination with an output of the fiber span.

19. The method of claim 17, wherein the detector is configured to detect one of an intensity modulated tone injected into the fiber span, a phase modulated waveform injected into the fiber span, and a data carrying waveform injected into the fiber span.

20. The method of claim 17, further comprising determining a property of the fiber span based on the electrostrictive response.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The present disclosure is illustrated and described herein with reference to the various drawings, in which like reference numbers are used to denote like method steps/assembly components, as appropriate, and in which:

[0016] FIG. 1 is a plot of the real and imaginary components of the electrostrictive response of non-dispersion shifted fiber (NDSF), where the calculated response is normalized such that the real component attains a maximum value of 1.7.Math.10.sup.20 W/m.sup.2;

[0017] FIG. 2 is a plot of the frequency dependences of the real part of nonlinear refractive index due to electrostriction for various fiber types;

[0018] FIG. 3 is a schematic diagram illustrating one exemplary embodiment of an assembly for performing coherent detection using a local oscillator (LO) such that the frequency dependence of the nonlinear refractive index can be determined and fiber type distinguished in accordance with the present disclosure;

[0019] FIG. 4 is a series of plots of (top) the power spectrum of the field, A(t), for a 100 MHz intensity modulated tone and (bottom) the power spectrum of the intensity, I(t)|A(t)|.sup.2, that would correspond to the measured power spectrum without a LO (100% modulation index);

[0020] FIG. 5 is a plot of the power spectrum of the heterodyne beat note between a received field and the LO following square law detectionthe intermodulation frequency (IF) is 178 MHz; the solid line is the spectrum that would be observed following linear propagation through the channel; the dots correspond to the spectrum following nonlinear propagation; the squares correspond to the spectrum following nonlinear propagation at the tone frequencies (IF+/178 MHz); and the circles are at the second harmonic of the tone frequency (IF+/2*178 MHz);

[0021] FIG. 6 is a series of plots of the second harmonic to fundamental in the power spectrum of I(t) as a function of tone frequency for various fiber types;

[0022] FIG. 7 is a plot of n.sub.2e contrast normalized to peak contrast as a function of acoustic mode order;

[0023] FIG. 8 is a series of plots of (top) the ratio of second harmonic to fundamental of I(t) as a function of tone frequency for various launch powers and (bottom) the corresponding normalized n.sub.2e contrast being relatively invariant with respect to changes in power;

[0024] FIG. 9 is a series of plots of (top) the ratio of second harmonic to fundamental of I(t) as a function of tone frequency for a 6 dBm tone propagating through various lengths of NDSF fiber and (bottom) the corresponding normalized n.sub.2e contrast being relatively invariant with respect to changes in span length;

[0025] FIG. 10 is a schematic diagram illustrating another exemplary embodiment of an assembly for performing intensity detection on a compensated link using a LO such that the frequency dependence of the nonlinear refractive index can be determined and fiber type distinguished in accordance with the present disclosure;

[0026] FIG. 11 is a plot of the normalized n.sub.2e contrast for a compensated span, as well as for similar lengths of the two fiber types used to construct the span; the compensated span exhibiting an inflection at around mode 12; and

[0027] FIG. 12 is a series of plots of (top) the phase noise correlation induced by a modulated pump on a quadrature phase shift keyed (QPSK) probe channel showing the peak and valley at the tone frequency and (bottom) the difference between the peak and valley in noise correlation for NDSF and TWC as a function of tone frequency

DETAILED DESCRIPTION OF THE DISCLOSURE

[0028] Again, the electrostrictive response of an optical fiber can be measured from the frequency dependence of the nonlinear phase shift induced through processes such as SPM and XPM, and is proportional to n.sub.2e(). The strength of n.sub.2e() is governed by the overlap integral between the radial acoustic wave eigen functions and the transverse distribution of the optical field in the fiber. The oscillations in n.sub.2e() diminish to extinction when the spatial period of the associated acoustic modes drops below the fiber spot size. The contrast of the modes of n.sub.2e(), and particularly the frequency of extinction, is indicative of the type of fiber under test, for example. A full derivation of n.sub.2e() is provided in the relevant art. The simulation results described herein below all assume a step index fiber geometry with a peak real component of 1.7.Math.10.sup.20 W/m.sup.2 as taken from the relevant art. Recent work puts this value at 0.6.Math.10.sup.20 W/m.sup.2. Differences in the peak value rescale the results shown, but do not change the frequency dependence.

[0029] The present disclosure provides that the contrast of the frequency dependent response, which is defined as (maxmin)/(max+min) in the vicinity of an eigenmode resonance and as a function of eigenmode order, can be used to differentiate between fiber types and determine other fiber properties. Differences caused by overall scale factors or non-step index fiber geometries aid in fiber type classification, for example. Various methodologies for measuring n.sub.2e in deployed fibers are provided herein accordingly.

[0030] Referring now specifically to FIG. 3, in one exemplary embodiment, fiber type or other property or environment sensing within a given fiber span 10 can be performed by injecting an intensity modulated tone 12 into a service (or other) channel 14 that is multiplexed onto the fiber span 10 along with (optionally) other traffic carrying channels 15. The intensity modulated tone 12 is generated by a first laser 13 and injected into the service channel 14 at a first amplifier 16, for example, such as an erbium doped fiber amplifier (EDFA) or the like. Following propagation through the fiber span 10, the intensity modulated tone 12 is extracted from the service channel 14 at a second amplifier 18, for example, such as another EDFA or the like. The intensity modulated tone 12 is then combined with a LO 20 and directed onto a photodiode 22. The heterodyne beat note is digitally sampled and processed. It should be noted that a laser 13 within a given amplifier 16,18 can be used for the receiver LO 20 for a detector 22 and can also generate a modulated waveform for propagation through the next span. Detected phase changes in the intensity modulated tone 12 are in part attributable to nonlinear effects. The dependence of the observed phase changes with tone modulation frequency are observed to help identify the part of the phase change that is attributable to electrostriction. It should be noted that homodyne detection where the modulation index is reduced to less than 100% can also be utilized, in which case an LO 20 at the receiver is not required. It is also noted that the intensity modulated tone could be replaced with other types of modulated waveforms such as phase modulated waveforms or those carrying data. The service channel 14 can be an optical service channel (OSC) which is used for communications between amplifier sites.

[0031] The power spectrum of the field (top) and intensity (bottom) of an exemplary 100 MHz intensity modulated waveform is shown in FIG. 4. During propagation, the nonlinear response of the fiber causes an intensity dependent phase shift in the laser field, which can be observed as an increase in the field power spectral density at the second harmonic of the tone frequency (shifted by the IF). Detection of this phase shift requires a coherent reference. For this example, the coherent reference is provided by the LO, A.sub.LO(t), which combines with the propagated field, A(t), in the photodiode 22, which measures the intensity I(t)(t)+A.sub.LO(t)|.sup.2. The intensity, I(t), contains mixing products of the propagated field with the LO. The ratio of the fundamental to second harmonics of the tone frequencies in the power spectrum of I(t) depends on the strength of the nonlinear interaction during propagation. FIG. 5 is a plot of the power spectrum of the heterodyne beat note between a received field and the LO following square law detectionthe IF is 2.5 GHz; the solid line is the spectrum that would be observed following linear propagation through the channel; the dots correspond to the spectrum following nonlinear propagation; the squares correspond to the spectrum following nonlinear propagation at the tone frequencies (IF+/178 MHz); and the circles are at the second harmonic of the tone frequency (IF+/2*178 MHz).

[0032] The tone frequency, ft, and its harmonics are visible on either side of the IF. The strength of this nonlinear interaction is modulated by the magnitude of n.sub.2e at the tone frequency. The ratio of the second harmonic to the fundamental tone powers, as a function of tone frequency, is plotted for four different fiber types in FIG. 6. In all cases, a 6 dBm launch power was used and the n.sub.2e response modeled with a stepped index fiber with core radius defined by A.sub.eff and a peak electrostrictive response of 1.7.Math.10.sup.20 W/m.sup.2.

[0033] The ratio of the fundamental to second harmonic tone powers are modulated by the frequency dependent n.sub.2e response. The peaks correspond to the frequencies of eigenmodes of the transverse acoustic waves induced in the fiber.

[0034] For a given eigen mode, m, the n.sub.2e contrast, v(m), is defined in terms of the ratio of fundamental to second harmonic tone powers (c.f. FIG. 6) as the ratio v (m)=(maxmin/(max+min). The n.sub.2e contrast for NDSF, large effective area fiber (LEAF), Vascade EX3000, and TWC, all with a launch power of +6 dBm, is shown in FIG. 7. The n.sub.2e contrast is unique for each of the fiber types tested. It is noted that the detected second harmonic tone power is a few percent of the fundamental and, while this is a small signal, it is also narrow band and periodic, which allows for relatively simple detection.

[0035] The simulation alluded to herein above for the case of NDSF fiber was repeated for three different launch powers, 0 dBm, 3 dBm, and 6 dBm. As expected, the ratio of fundamental to second harmonic power increases with increasing launch power, as shown in FIG. 8 (top). The normalized contrast, as shown in FIG. 8 (bottom), is relatively invariant to changes in power.

[0036] The simulation alluded to herein above for the case of NDSF fiber was repeated for four different span lengths, 20 km, 40 km, 60 km, and 80 km, using a +6 dBm launch power for all cases. The normalized contrast, as shown in FIG. 9 (bottom), is relatively invariant to changes in fiber length.

[0037] Compensated links consist of spans of positive and negative dispersion fiber. This example considers one span of a composite link that is comprised of 40.6 km of Vascade L1000 (A.sub.eff100 m.sup.2) 10 followed by 24.3 km of Vascade S1000 (A.sub.eff27 m.sup.2) 11, as shown in FIG. 10. The harmonic contrast for the span (65 km combined) is plotted along with results for 65 km of L1000 @ 6 dBm as well as for 65 km of S1000 @ 0 dBm. The power for a span of pure S1000 fiber is lowered in consideration of the fact that a 6 dBm launch power would greatly exceed the nominal launch power for such a fiber given its small effective area. Results are shown in FIG. 11, where it is observed that relatively high n.sub.2e contrast is maintained for higher order harmonics in the smaller core S1000 fiber as compared with the L1000 results. Results for the composite span show an inflection at around order 12, indicating that one can readily distinguish between pure and composite spans.

[0038] Thus, an intensity modulated tone can excite the frequency dependent n.sub.2e() response of a fiber and the response can be used to distinguish between fiber types. The response can be measured with a photodiode 22 (FIGS. 3 and 10) through a heterodyne measurement with a LO 20, for example. The modulated tone 12 can be generated with a directly modulated laser 13, and a single laser 21 located within each amplifier can serve as a source as well as the LO 20. The measured n.sub.2e contrast is shown to be largely independent of launch power and span length.

[0039] The n.sub.2e() response of a fiber can also be measured through the XPM interaction between an intensity modulated (pump) waveform propagating at one wavelength and a (probe) waveform propagating at a different wavelength. In this example, the probe consists of a dual polarization QPSK waveform located on ITU channel 51 and the pump is located on channel 50. Interaction with the pump induces correlated phase noise on the probe that oscillates with the period of the pump waveform. By measuring the peak to valley ratio as a function of the tone frequency, one can extract a measure of the n.sub.2e response.

[0040] FIG. 12 shows the phase noise correlation measured on QPSK symbols following propagation with an intensity modulated pump. The difference between the peak and the valley as a function of tone frequency is shown in the bottom plot. The ratio reflects the frequency dependence of n.sub.2e and the oscillations extend to higher frequencies for TWC as compared with the result for NDSF, as expected, given the smaller core size of TWC.

[0041] The ability to measure phase noise correlation requires the functionality to measure received symbols from a burst and compare the measured symbols with associated symbols synthesized from their associated estimated (Pre-forward error correction, Pre-FEC) or, advantageously, error free (Post FEC) bits to determine through subtraction the noise field at each symbol.

[0042] The pump/probe method described herein above could be of interest because it can be implemented as a special mode for coherent transceivers deployed in a link. The pump waveform can be injected into a service (or other) channel in one of the spans of the link or the noise correlation can be measured for co-propagating pump and probe waveforms as the pump is successively blocked at different add/drop or equalization sites.

[0043] In general, the frequency dependent n.sub.2e response of a link can be useful for characterizing the nonlinear propagation characteristics of the link, for example, it could be used to determine optical power or to aid in determining the nonlinear c-matrix coefficients for operation of a fiber optic communication system. Measurements of the noise power as a function of frequency can be useful for separating the contribution of nonlinear noise, which is modulated by the n.sub.2e response from amplified spontaneous emission (ASE) noise that is not modulated. The normalized contrast can be used as an input into a neural network or other type of fitting algorithm to classify the fiber type. Detection of the normalized contrast for a span may be possible with an arrangement similar to that described herein above, where the intensity modulated tone is generated with a reduced modulation index, e.g., 0.1, and homodyne detection is employed using a square law detector without a LO. It is possible to measure linear effects, such as local dispersion, through the square law detection of modulated tones with or without the aid of the LO.

[0044] Thus, the present disclosure provides methods and assemblies for measuring the electrostrictive response of an optical fiber, and the use of those measurements to characterize properties or the environment of the fiber, such as the fiber type. Most persons working in telecommunications who are familiar with electrostriction recognize it by its manifestation in Stimulated Brillouin Scattering (SBS), wherein photons are scattered by phonons they produce through electrostriction. The fact that electrostriction introduces a frequency dependence in the efficiency of SPM and XPM is much less well known, and there have been no attempts to exploit this dependence for measuring the properties of a fiber. Because the electrostrictive response is closely tied to the details of the fiber geometry, it is useful for measuring fiber properties. Several fiber measurement techniques are based on observing the SBS efficiency, which can be related to properties such as temperature and strain in the fiber. These techniques look at the back-scattering efficiency of photons by acoustic phonons. The techniques require high power narrow band lasers and, in most cases, spatial resolution is achieved through time gating of counter propagating waves on polarization maintaining fiber. Fiber dispersion can also be measured by observing the phase shift between tones that propagate with a known frequency separation. The optical path length of a fiber can be measured using optical time domain reflectometry (OTDR). Finally, fiber dispersion can be estimated by comparing the differences in propagation delays for pulses propagating through the same fiber at different wavelengths. Measurements based on SBS, however, usually require high power narrow linewidth lasers and in many cases require polarization maintaining fiber. SBS based measurement techniques are difficult to adapt to deployed telecommunications fiber. By combining OTDR and dispersion measurements, it may be possible to infer fiber type. However, it would be difficult to identify the composition of a composite link that combines segments of different fiber types, for example, by measuring the path length and cumulative dispersion of the link.

[0045] Thus, the techniques of the present disclosure advantageously rely on measuring the frequency dependence of the electrostrictive response. Uncertainty regarding the type of fiber installed between amplifier sites, for example, is one of the main factors preventing optimal provisioning of a link. It is desirable to have a low-cost method whereby a pair of amplifiers could determine the type of fiber that is installed in between them. Other fiber properties may likewise be determined.

[0046] As stated earlier the details of the fiber geometry, along with the material acoustic wave velocities, define resonance conditions for the transverse acoustic waves. The fiber geometry and or acoustic wave velocities may be modified through the application of mechanical stress or strain to the fiber, changes in temperature or changes in the structure of the glass through radiation exposure, doping, aging etc. The sensitivity of the n.sub.2e response to fiber geometry and acoustic wave velocities may be exploited to sense characteristics of the fibers environment such as optical fiber routing and optical fiber spooling. Those skilled in the art will recognize that the methods and assemblies described herein for the example of fiber type sensing may be adapted in a strait forward manner for detecting other fiber properties or environmental conditions which modify the fiber geometry and or acoustic wave frequencies as evidenced through changes in n.sub.2e() response of the fiber.

[0047] In the foregoing descriptions, the characterization of fiber properties was described with reference to optical fiber used by a fiber optic communication system. For example, fiber optic communication systems can include Dense Wavelength Division Multiplexed (DWDM) and other types of systems which provide data transmission in local, metro, regional, long-haul, and/or submarine networks. The systems and methods described herein can be incorporated and performed through optical network elements and associated equipment therein. The measured fiber properties or environment can advantageously be used during operation of such systems for various aspects as described herein. Those skilled in the art will recognize the fiber optic communication system is an example use case. The approach described herein can be used in other systems using optical fibers such as for temperature sensing and the like.

[0048] It will be appreciated that some embodiments described herein may include one or more generic or specialized processors (one or more processors) such as microprocessors; Central Processing Units (CPUs); Digital Signal Processors (DSPs): customized processors such as Network Processors (NPs) or Network Processing Units (NPUs), Graphics Processing Units (GPUs), or the like; Field Programmable Gate Arrays (FPGAs); and the like along with unique stored program instructions (including both software and firmware) for control thereof to implement, in conjunction with certain non-processor circuits, some, most, or all of the functions of the methods and/or systems described herein. Alternatively, some or all functions may be implemented by a state machine that has no stored program instructions, or in one or more Application Specific Integrated Circuits (ASICs), in which each function or some combinations of certain of the functions are implemented as custom logic or circuitry. Of course, a combination of the aforementioned approaches may be used. For some of the embodiments described herein, a corresponding device in hardware and optionally with software, firmware, and a combination thereof can be referred to as circuitry configured or adapted to, logic configured or adapted to, etc. perform a set of operations, steps, methods, processes, algorithms, functions, techniques, etc. on digital and/or analog signals as described herein for the various embodiments.

[0049] Moreover, some embodiments may include a non-transitory computer-readable storage medium having computer readable code stored thereon for programming a computer, server, appliance, device, processor, circuit, etc. each of which may include a processor to perform functions as described and claimed herein. Examples of such computer-readable storage mediums include, but are not limited to, a hard disk, an optical storage device, a magnetic storage device, a ROM (Read Only Memory), a PROM (Programmable Read Only Memory), an EPROM (Erasable Programmable Read Only Memory), an EEPROM (Electrically Erasable Programmable Read Only Memory), Flash memory, and the like. When stored in the non-transitory computer readable medium, software can include instructions executable by a processor or device (e.g., any type of programmable circuitry or logic) that, in response to such execution, cause a processor or the device to perform a set of operations, steps, methods, processes, algorithms, functions, techniques, etc. as described herein for the various embodiments.

[0050] Although the present disclosure is illustrated and described herein with reference to preferred embodiments and specific examples thereof, it will be readily apparent to those of ordinary skill in the art that other embodiments and examples can perform similar functions and/or achieve like results. All such equivalent embodiments and examples are within the spirit and scope of the present disclosure, are contemplated thereby, and are intended to be covered by the following non-limiting claims for all purposes.