Optical fiber characterization using a nonlinear skirt measurement

Systems and methods include causing transmission of one or more shaped Amplified Spontaneous Emission (ASE) signals, from an ASE source (70), on an optical fiber (58, 60); obtaining received spectrum of the one or more shaped ASE signals from an optical receiver (68) connected to the optical fiber (58, 60); and characterizing the optical fiber (58, 60) based in part on one or more of a nonlinear skirt and a center dip depth in the received spectrum of the one or more shaped ASE signals. The one or more shaped ASE signals can be formed by the ASE source (70) communicatively coupled to a Wavelength Selective Switch (WSS) (62) that is configured to shape ASE from the ASE source to form the one or more shaped ASE signals with one or two or multiple peaks and with associated frequency.

FIELD OF THE DISCLOSURE

The present disclosure generally relates to fiber optic systems. More particularly, the present disclosure relates to systems and methods for optical fiber characterization using a nonlinear skirt measurement.

BACKGROUND OF THE DISCLOSURE

In optical systems, signal transmission performance is significantly impacted by optical fiber dispersion and nonlinearity. It is important to characterize fiber nonlinear parameters for modeling, link budgeting, and performance optimization. Traditionally, fiber characterization for nonlinearity only concerns the propagation coefficient, β (specifically the second derivative, or group velocity dispersion parameter, β2), and the fiber nonlinear coefficient, γ, of a single piece of homogeneous fiber without lumped losses at the input, output, or within the fiber. The Group Velocity Dispersion (GVD) parameter, i.e., β2, and fiber nonlinear coefficient, i.e., γ, are among the most critical fiber characteristics for photonic line system link budget, performance modeling, and optimization. The current fiber nonlinearity measurement can be with a high-power signal having a narrow linewidth or by backing out from a Stimulated Raman Sscattering (SRS) measurement. The dispersion measurement, currently, is commonly implemented by measuring the phase delay difference of modulated signals on different wavelengths. For example, dispersion can also be measured by the differential time delay between two wavelengths such as an Optical Service Channel (OSC) wavelength at 1510 nm and an Optical Time Domain Reflectometer (OTDR) wavelength at 1625 nm.

Disadvantageously, Continuous Wave (CW) signals are single polarization sources by definition. This causes measurement issues in the presence of Polarization Dependent Gain (PDG), Differential Group Delay (DGD), and Polarization Mode Dispersion (PMD) which is significant in real systems. In addition, the practical implementation using CW source(s) for dispersion/nonlinear measurement requires a coherent transponder, transceiver, modem, etc. This requires equipped modules as well as end-to-end communication. This may not be available at or before turn-up. Also, the dispersion measurement using two wavelengths such as 1510 nm and 1625 nm only measures average dispersion at 1568 nm. As optical systems continue to push the limit of bandwidth over fiber, it is critical to get accurate measurements over the entire signal band.

Of note, it is not uncommon that optical fibers in the field include multiple fiber segments in a single “span” due to fiber patch or repair or intentional “glass-through” sites, where in general different fiber types may be used in each segment. Additionally, unknown lumped losses at the input and output of the fiber due to patch panels can make it difficult to determine the nonlinear performance since the exact launch power into the span may be unknown. The lumped losses due to patch panels and between the segments and the transition of fiber types will significantly impact the transmission performance of a fiber span. Disadvantageously, there is no known technique to accurately characterize and model multi-segment/mixed fiber spans with unknown lumped losses in a photonic system.

BRIEF SUMMARY OF THE DISCLOSURE

In an embodiment, a system includes a processor communicatively coupled to an Amplified Spontaneous Emission (ASE) source and an optical receiver, wherein the processor is configured to cause transmission of one or more shaped ASE signals, from the ASE source, on an optical fiber, obtain received spectrum of the one or more shaped ASE signals from the optical receiver connected to the optical fiber, and characterize the optical fiber based in part on one or more of a nonlinear skirt and a center dip depth in the received spectrum of the one or more shaped ASE signals. The one or more shaped ASE signals can be formed by the ASE source communicatively coupled to a Wavelength Selective Switch (WSS) that is configured to shape ASE from the ASE source to form the one or more shaped ASE signals with one or more peaks and with associated frequency. The one or more shaped ASE signals can have two distinct peaks at the transmission with a significant dip at a center frequency, and the received spectrum of the one or more two-peak ASE signals has much less of a dip that the center dip depth. The processor can be further configured to determine a fiber type based on a signature of the one or more of the nonlinear skirt and the center dip depth in the received spectrum of the one or more shaped ASE signals. The optical fiber can be characterized to determine effective Group Velocity Dispersion (GVD) parameter, β2, and fiber nonlinear coefficient, γ.

The GVD parameter, β2, and the fiber nonlinear coefficient, γ, can be characterized by a measurement of a shape of the one or more of the nonlinear skirt and the center dip depth as a function of signal wavelength. The one or more shaped ASE signals can include a plurality of shaped ASE signals with a first set of shaped ASE signals utilized to determine launch power for every span in a section to yield an optimum center dip depth, and a second set of shaped ASE signals that sweep at different frequencies across a signal band to determine a corresponding center dip depth at the different frequencies. The optical fiber can be characterized based in part on a shape of the one or more of the nonlinear skirt and the center dip depth and a separate differential delay measurement, to determine the GVD parameter, β2, and fiber nonlinear coefficient, γ. The optical fiber can be characterized based in part on a shape of the one or more of the nonlinear skirt and the center dip depth and a separate Stimulated Raman Scattering (SRS) measurement, to determine the GVD parameter, β2, and fiber nonlinear coefficient, γ. The optical fiber can be characterized based in part on a shape of the one or more of the nonlinear skirt and the center dip depth and a Least Mean Square (LMS) fit, to determine the GVD parameter, β2, and fiber nonlinear coefficient, γ.

In another embodiment, a method includes causing transmission of one or more shaped Amplified Spontaneous Emission (ASE) signals, from an ASE source, on an optical fiber; obtaining received spectrum of the one or more shaped ASE signals from the optical receiver connected to the optical fiber; and characterizing the optical fiber based in part on one or more of a nonlinear skirt and a center dip depth in the received spectrum of the one or more two-peak ASE signals. The one or more shaped ASE signals can be formed by the ASE source communicatively coupled to a Wavelength Selective Switch (WSS) that is configured to shape ASE from the ASE source to form the one or more shaped ASE signals with one or more peaks and with associated frequency. The one or more shaped ASE signals can have two distinct peaks at the transmission with a significant dip at a center frequency, and the received spectrum of the one or more two-peak ASE signals has much less of a dip that the center dip depth. The method can further include determining a fiber type based on a signature of the one or more of the nonlinear skirt and the center dip depth in the received spectrum of the one or more shaped ASE signals.

The optical fiber can be characterized to determine the GVD parameter, β2, and fiber nonlinear coefficient, γ. The one or more shaped ASE signals can include a plurality of shaped ASE signals with a first set of shaped ASE signals utilized to determine launch power for every span in a section to yield an optimum center dip depth, and a second set of shaped ASE signals that sweep at different frequencies across a signal band to determine a corresponding center dip depth at the different frequencies. The optical fiber can be characterized based in part on a shape of the one or more of the nonlinear skirt and the center dip depth and a separate differential delay measurement, to determine the GVD parameter, β2, and fiber nonlinear coefficient, γ. The optical fiber can be characterized based in part on a shape of the one or more of the nonlinear skirt and the center dip depth and a separate Stimulated Raman Scattering (SRS) measurement, to determine the GVD parameter, β2, and fiber nonlinear coefficient, γ. The optical fiber can be characterized based in part on a shape of the one or more of the nonlinear skirt and the center dip depth and a Least Mean Square (LMS) fit, to determine the GVD parameter, β2, and fiber nonlinear coefficient, γ.

In a further embodiment, an Optical Add/Drop Multiplexing (OADM) node includes a Wavelength Selective Switch (WSS) system communicatively coupled to at least a first optical fiber and a second optical fiber; an Amplified Spontaneous Emission (ASE) source connected to the WSS system; a pre-amplifier connected to the WSS system and the first optical fiber; an Optical Channel Monitor (OCM) connected at least to an output of the pre-amplifier; and a processor configured to obtain received spectrum of one or more shaped ASE signals from the OCM, wherein the one or more shaped ASE signals are transmitted over the first optical fiber, and characterize the first optical fiber based in part on a nonlinear skirt shape and a center dip depth in the received spectrum of the one or more shaped ASE signals.

DETAILED DESCRIPTION OF THE DISCLOSURE

The present disclosure relates to systems and methods for fiber characterization using a nonlinear skirt measurement. The present disclosure characterizes fiber dispersion and nonlinearity based on the spectral shape of an Amplified Spontaneous Emission (ASE) signal. Of note, the ASE signal is more like a coherent, dual-polarization modulated signal than a single polarization CW source. The ASE signal can explore all states of polarization which averages these effects resulting in a more stable and accurate measurement. Also, the ASE signal is available from common infrastructure in a photonic line system without requiring additional equipment, and this can be utilized on a per section basis prior to system turn-up. For example, next-generation photonic line systems utilize ASE for channel holders and/or can generate ASE via amplifiers. An ASE source can be shaped by a Wavelength Selective Switch (WSS) at Tx. At a corresponding Rx, the nonlinear product can be characterized by the nonlinear skirt, i.e., spectral shape of the ASE signal. In an example, a two-peak Tx signal spectral shape is designed using an ASE source and transmitted over the fiber. Of note, other shapes are also contemplated such as a one-peak Tx signal. At a corresponding Rx, the nonlinear product can be characterized by center dip depth, i.e., the relative power difference between the signal peak and the valley of the overlap area of the nonlinear skirt between the two peaks. As a result, the measurement depends on relative power between the peak and valley of the two-peak signal. Absolute power accuracy and power monitor Wavelength Dependent Loss (WDL) are not critical requirements for the measurement.

In an embodiment, the present disclosure is characterized as in-situ due to utilizing existing common infrastructure in an optical networking system; no specialized hardware is required. In another embodiment, the present disclosure is implemented in test equipment that may be separate from common infrastructure of the optical networking system. The present disclosure measures dispersion and nonlinearity within the signal band (e.g., the C-band such as between about 1528 nm and 1565 nm, and/or the L-band such as between about 1565 nm and 1625 nm) rather than relying on out-of-band measurements and extrapolation. The present disclosure utilizes a relative power measurement, such that measurement can be easily carried out and post-processing is not complicated.

The accurate characterization of fiber dispersion and nonlinearity will help model the exact link performance, which is crucial for link budget and performance optimization. This is a key part of a “plug and play” approach for optical control. A fiber-type determination is currently a manual procedure and has been shown to be a source of system issues in many networks where there is often mis-provisioned fiber type. This disclosure removes the manual effort of fiber type identification and provisioning and the associated potential manual error. Furthermore, fiber-type does not accurately characterize the fiber. Even within each fiber type, there is a range of dispersion and nonlinear coefficient. It is also common in real systems to have mixed fiber types within a single span. This disclosure gives an appropriate value, averaged over the nonlinear-length, for the key performance parameters which can be used in optical control an optimization resulting in better performance and higher capacity.

When an optical signal propagates in a nonlinear medium such as optical transmission fiber, the signal spectrum will be broadened due to the combination of fiber nonlinearity and dispersion. The broadened spectral shape shows a distinct signature for fiber with different characteristics. Consequently, β2and γ can be backed out through measurement and then calculated. This disclosure proposes a process that characterizes β2and γ by measuring the broadened spectrum of a shaped ASE signal.

Additionally, the nonlinear skirt measurement can be used to address the challenge of the multi-segment/mixed fiber spans with unknown lumped losses. Instead of characterizing γ and β2of individual homogeneous fibers and modeling them separately, the nonlinear skirt measurement can be used to characterize any concatenation of fiber spans with or without lumped losses and mixed fiber types where we treat the concatenation of fibers and losses as a single “effective” span of fiber. Because it is an in-situ measurement, any lumped losses and changes of fiber type that the ASE probe experiences is the same as the transmitted signal. The nonlinear parameters backed out from the nonlinear effect on the ASE probe describe the nonlinear behavior of the fiber span including the effects due to the change of fiber types and lumped losses. Therefore, the outputs of the nonlinear skirt measurement can be generalized as effective nonlinear coefficient, γeff, and effective GVD parameter, β2,eff. With the effective parameters, a fiber span with any condition, including multi-segment, heterogeneous spans with arbitrary lumped losses in the general case can be modeled as a single effective span. The proposed effective fiber parameters and effective span modeling process provide a simple procedure for fiber characterization and accurate prediction for transmission performance of any fiber span condition.

The nonlinear skirt measurement provides a measurement approach for the normal fiber nonlinear parameters (fiber nonlinear coefficient, γ, and GVD parameter, β2), the present disclosure also proposes generalized effective parameters (effective fiber nonlinear coefficient, γeff, and effective GVD parameter, β2,eff) that include any lumped losses at patch points as well as change of fiber types over a fiber span. Taking both effective parameters together, it is possible to model any fiber span, mixed fiber or not, patched or not, as if it were a single piece of fiber with no lumped losses. Thus, with the new parameters and modeling approach, any lumped losses and change of fiber types in a single fiber span can be easily taken into account, which greatly simplify fiber characterization measurement and modeling for fiber transmission performance.

An objective of the present disclosure is to replace the physical fiber model with an equivalent fiber for each span which describes fiber span non-linear transmission behavior. Specifically, in an embodiment, the present disclosure includes a generalized procedure for characterizing and modeling nonlinear performance of individual fiber spans regardless of fiber condition, i.e., the procedure inherently deals with arbitrary lumped losses and/or changes of fiber types within the fiber span.

FIG.1Ais a graph10A of an optical signal and associated signal broadening over 100 km in Lambda Shifted (LS) fiber illustrating a one-peak signal.FIG.1Bis a graph10B similar toFIG.1Aillustrating a two-peak signal. A spectral shape of the Tx signal is illustrated by line12in the graph10A,10B, and the Rx signal after a 100 km LS span is illustrated by a line14in the graph10A,10B. Of note, the graphs10A,10B illustrate the Tx signal and the Rx signal on the same graph for illustration purposes to highlight the difference in the spectral shapes based on transmission over an optical fiber. The nonlinear broadening effect due to the transmission will result in a skirt shape on the Rx signal spectrum, a.k.a. nonlinear skirt18. That is, when an optical signal is transmitted over fiber, the signal spectrum will be broaden due to the combination of fiber nonlinearity and dispersion, and the received signal shape is different after different type of fibers with different characteristics. Therefore, an LMS fit and the like can be performed to determine the various fiber parameters.

InFIG.1B, it is observed that the gap between the two peaks, marked as a center dip depth16in the graph10B, becomes shallower after transmission due to the broadening effect. It is discovered as outlined in this disclosure that β2and γ can be characterized by measuring center dip depth as a function of signal wavelength.

Of note, the present disclosure performs fiber characterization (e.g., fiber type determination, β2and γ measurements, etc. based on sending an ASE signal that is spectrally shaped and measuring the received spectral shape, in particular, the nonlinear skirt. In an example, a two-peak signal as shown inFIG.1Bis utilized where an ASE signal from an ASE source is shaped to form the two peaks. The two-peak signal simplifies measurement and post-processing by providing a relative measurement between the peak and valley of the two peaks. Those of ordinary skill in the art will appreciate it is possible to characterize fiber based on the nonlinear skirt shape of a one-peak signal as shown inFIG.1Abased on LMS fit of the shape of the nonlinear skirt. Further, the present disclosure contemplates other shapes including multiple peaks.

FIG.2Ais a graph of measured results for received spectrum in LS fiber over 100 km, andFIG.2Bis a graph of measured results for received spectrum in Non-Dispersion Shifted Fiber (NDSF) over 40 km.FIG.3Ais a graph of simulated results for received spectrum in Truewave (TW) Classic fiber,FIG.3Bis a graph of simulated results for received spectrum in LS fiber,FIG.3Cis a graph of simulated results for received spectrum in NDSF fiber;FIG.3Dis a graph of simulated results for received spectrum in Large Effective Area Fiber (LEAF),FIG.3Eis a graph of simulated results for received spectrum in enhanced LEAF (eLEAF), andFIG.3Fis a graph of simulated results for Dispersion Shifted Fiber (DSF).

Specifically, the measured/experimental results inFIG.2and the simulated results inFIG.3illustrate the change of the center dip depth16sweeping the signal across the C-band. Distinct signatures are observed for different fiber type with different characteristics. Of note, fiber type can be identified from the change of the center dip depth16over wavelength.

Furthermore, an important phenomenon is discovered that given the fiber to be transmitted, the normalized center dip depth over wavelength does not change with different signal launching power. This can be explained by a Gaussian Noise (GN) model, where the nonlinear product of fiber transmission is expressed as[1]

The normalized shape of GNLI(ν) is

Depth(β2,P0,γ,Ls,α)=10⁢log⁢10⁢(gRX(vc)gNLI(v=0,β2,Ls,α))+10⁢log⁢10⁢(1627⁢γ2⁢P02)=D~⁢epth⁡(β2,Ls,α)+A⁡(γ,P0)+C(3)
where νcis the relative center frequency of the each of the two peaks, as marked inFIG.1B. The first term in Eq (3) is the normalized center dip depth, denoted as {tilde over (D)}epth(β2,Ls,α); the second term is the offset, denoted as A(γ,P0), and C=10 log 10( 16/27).

FIG.4Ais a graph of measurement of center gap depth versus frequency versus β2for LS fiber, andFIG.4Bis a graph of a measurement of center gap depth versus frequency versus β2for NDSF fiber. Given the fiber span under test, i.e., Leffand γ are approximately constant over wavelength, the normalized center dip depth as a function of β2, {tilde over (D)}epth|Leff(β2), remains unchanged with different P0, while the offset, A(P0), changes twice as fast as P0. Note, the corresponding β2at each frequency on the top X-axis ofFIGS.4A and4Bis computed from measured dispersion and dispersion slope at 1550 nm.

FIG.5is a graph of a simulation of center dip depth versus β2with different launching power.FIG.6is a graph of a simulation of center dip depth versus β2with different gamma, γ. Note, the shape of the β2versus the center dip depth trace remain approximately unchanged when center dip depth is greater than about 15 dB. The center dip depth trace versus β2versus measure can provide the fiber type and dispersion slope. The shape of the center dip depth versus β2changes at different fiber lengths, but is unchanged with different gamma, γ at the same fiber length. For detection, the shape of the center dip depth versus β2can be stored at various different fiber lengths for simple comparisons. The simulation results inFIG.5demonstrate that, with fixed γ and Leff, {tilde over (D)}epth|Leff(β2) remains approximately unchanged with different P0, when the absolute value of Depth is larger than ˜15 dB. Depth gets distorted with high P0, when the in band nonlinear product is no longer negligible.FIG.6demonstrates that with fixed P0and Leff, {tilde over (D)}epth|Leff(β2) remains approximately unchanged, while absolute value of Depth changes twice as fast as the change of γ.

Accordingly, the relative center dip depth change over wavelength can be a distinct indicator of the type of fiber (e.g., NSDF, LS, TWc, LEAF, eLEAF, DSF, etc.). No absolute power information is required;

FIG.7is a network diagram of an optical section50with associated equipment for providing a nonlinear skirt measurement. The optical section50is a segment in an optical network between Optical Add/Drop Multiplexer (OADM) nodes52,54. The optical section50can be referred to as an Optical Multiplex Section (OMS), and one aspect of each optical section50is the spectral load is identical over the entire section. A real implementation of an optical network can include multiple optical sections50in a mesh, ring, linear, hub and spoke, etc. architecture. The nonlinear skirt measurement can be performed on a per optical section basis. The optical section50can also include intermediate optical line amplifier nodes56. Further, a practical implementation of the optical section50includes two optical fibers58,60for bidirectional communication. The nonlinear skirt measurement is performed on each optical fiber58,60separately.

The OADM nodes52,54include a Wavelength Selective Switch (WSS)62that faces the optical fibers58,60. The WSS62forms an optical degree that faces the optical fibers58,60. In this example, a single degree is illustrated at each of the OADM nodes52,54. Of course, practical implementations may include multiple degrees, each facing a different optical section50. The WSS62is configured to add/drop spectrum to/from the degrees and/or locally. Each OADM node52,54includes a post-amplifier64on the transmit side and a pre-amplifier66on the receive side. The amplifiers64,66can be Erbium-Doped Fiber Amplifiers (EDFAs). Also, Raman amplifiers may be used as well in addition to EDFAs. The OADM nodes52,54also include an Optical Channel Monitor (OCM)68(a.k.a. Optical Power Monitor (OPM), etc.) which is an optical receiver connected (e.g., by a tap) to an output of each of the amplifiers64,66. The OCM68can have two receivers to simultaneously monitor each of the optical fibers58,60or a switch to allow a single receiver to monitor one of the optical fibers58,60at a time.

The OADM nodes52,54also can include an ASE source70coupled to the WSS62. In newer optical line systems, the ASE source70can be used to fill unused spectrum to reduce power optimization time. Here, the ASE source70provides so-called channel holders used to fill the optical spectrum on the optical section50so that it always appears to have a full-fill configuration. Such an approach significantly reduces capacity change time.

Also, the OADM nodes52,54can include other components72such as an OTDR, an OSC, a polarimeter, and a processor. Again, the components72are in-situ, i.e., part of the OADM nodes52,54. Also, in another embodiment, the components72are implemented in a test device or system, such as separate from the OADM nodes52,54. In an embodiment, the components72can provide a differential delay measurement, an SRS measurement, a fiber58,60effective length (Leff) measurement, etc. For example, these measurements are described in commonly-assigned U.S. patent application Ser. No. 15/986,396, filed May 22, 2018, and entitled “Optical fiber characterization measurement systems and methods,” the contents of which are incorporated by reference herein. Further, the processor can be used to obtain the measurement data and perform various data analyses described herein. Also, the intermediate optical line amplifier nodes56include in-line optical amplifiers72.

As described herein, the fiber nonlinear skirt measurement can be performed with the ASE source70and the WSS62causing a two-peak signal to be transmitted on the optical fibers58,60and received by the OCM68. Referring back toFIGS.1A and1B, the graphs10A,10B illustrate the transmitted spectral shape in line12. This spectral shape can be achieved through configuration of the ASE source70and the WSS62. The received spectral shape in line14is received by the OCM68with the corresponding center dip depth16a function of the fiber58,60.

FIG.8is a flowchart of a measurement process100. The measurement process100can be implemented in the optical section50. The measurement process100starts with an initial scan at 1550 nm. At the beginning of a network section, e.g., at the upstream OADM, the measurement process100includes setting up the WSS62to shape the ASE source70to create the two-peak signal at 1550 nm (step102).

For every span i=1˜N in the optical section, starting with i=1 (step104), the measurement process100includes setting the optical amplifiers in a power mode for all spans (this power mode setting only needs to be done once, not necessarily for each iteration) and setting signal launching power at a reference power level, P0reffor span i, for example P0ref=15 dBm, and setting the rest of the spans at a much lower launching power, for example (P0ref−15) (step106). The measurement process100includes reading the OPM68at the downstream OADM, and recording the center dip depth of the received signal as Depthref|span=i(step108). The span count is incremented and steps104-110are repeated until the end of the section (step110).

The signal broadening effect characterized by Depthref|span=iis mainly generated by span i with high signal launching power. The purpose of steps104-110is to find the launching power for every span to yield the center dip depth around an optimum center dip depth, Depthopt. Depthoptis found when the PSD of the nonlinear product at the center gap is much higher than line EDFA ASE noise, while the corresponding in-band nonlinear product is still negligible compared to the signal. Depthoptdepends on the width of the signal and the gap of the two peaks. For example, when both the signal and gap width is 50 GHz, Depthopt=15 dB. Since the absolute level of Depth changes twice as fast as P0, the launching power for span i=1˜N is computed by
P0|span=i=P0ref+(Depthref|span=i−Depthopt)/2 (dBm)  (4)

The next step is to sweep the signal through the signal band at X points with the optimum signal launching powerP0|span=ifrom Eq. (4). In the upstream OADM, for every wavelength to be tested (step112), the measurement process100includes setting up the WSS/ASE source to create the two-peak signal at wavelength x, x=1˜X (step114).

For every span i=1˜N in section (step116), the measurement process100includes setting the optical amplifiers in power mode for all spans (this power mode setting only needs to be done once, not necessarily for each iteration), and setting launch power of span i to P0|span=i, such that the center dip depth due to the signal broadening effect of span i is around the pre-defined Depthopt. For the rest of the spans j=1˜N, j≠i, set launch power to
P0|span=i=P0ref(Depthref|span=j−(Depthopt+20))/2 (dBm)  (5)
such that the center dip depth due to the signal broadening effect of span=j is about 20 dB less than Depthopt, and is deemed as negligible in the signal spectrum at the end of section (step118).

For every span i and wavelength x, the measurement process100includes measuring the center dip depth of the received signal with the OCM at the downstream OADM and recorded as Depthmeas|span=i,wvl=x(step120). Steps112-120are repeated for all spans and wavelengths (steps122,124). Finally, the measurement process100has all required data measurements and can proceed to data analysis which is described in processes200A,200B,200C.

FIG.9Ais a flowchart of a data analysis process200A which also uses a differential delay measurement,FIG.9Bis a flowchart of a data analysis process200B which uses an SRS measurement, andFIG.9Cis a flowchart of a data analysis process200C which sweeps dispersion, dispersion slope and γ. Each of the processes200A,200B,200C can be performed in a processor at the OADM nodes52,54, in a management system, or the like.

InFIG.9A, the data analysis process200A utilizes dispersion data at 1568 nm from a differential delay measurement such as from an OSC at 1510 nm and an OTDR at 1625 nm. This dispersion data is utilized in addition to data from the measurement process100.

The data analysis process200A, for each span i=1˜N at each measured wavelength, x=1˜X (step202), includes loading measurement results from center dip depth measurement Depthmeas|wvl-x,span-i, for each span i=1˜N at each measured wavelength, x=1˜X (step204). The data analysis process200A includes getting the measured dispersion at 1568 nm from the differential delay measurement, denoted as β2|wvl=1568,span=i(step208); as well as the effective length, Leff, from an OTDR measurement (step206). Then, the data analysis process200A includes loading the pre-computed normalized center dip depth as a function β2at the measured Leff|span-i, denoted as {tilde over (D)}epth|Leff(β2).

Next, the data analysis process200A includes scaling {tilde over (D)}epth|Leff(β2) to Depthmeas|wvl-x,span-i, by extrapolating Depthmeas|wvl-x,span-ito Depth|wvl-1568,span-i(step210) and finding A to satisfy Eq. (6) (step212):
Depthmeas|wvl=1568,span=i={tilde over (D)}epth|Leff(β2|wvl=1568,span=i)+A+c(6)

Next, the data analysis process200A, from Eq. 3, includes determining γ is by
γ=100.1*(A/2-P0|span=i)(7)
and β2of the fiber at each measured wavelength, β2|wvl=x,span=i, by
β2|wvl=x,span=i={tilde over (D)}epth|Leff−1(Depthmeas|wvl=x,span=i−A−c)  (8)
Where {tilde over (D)}epth|Leff−1( ) is the inverse function of {acute over (D)}epth|Leff( ) (step214). The data analysis process200A loops through every span in the section (step216).

InFIG.9B, the data analysis process200B incorporates an SRS measurement. The data analysis process200B, for each span i=1˜X at each measured wavelength x=1˜X (step220); starts with loading results from center dip depth measurement, Depthmeas|wvl-x,span-i(step222), loading γ|span-ifrom SRS measurement (step226), and pre-computed {tilde over (D)}epth|Leff(β2) at OTDR measured Leff|span-i(step224). The scaling factor A can be found by (step228)
A=2P0|span=i+20 log 10(γ|span=i)  (9)

Then, β2of the fiber at each measured wavelength, β2|wvl=x,span=i, is found by Eq (8) (step230). The data analysis process200B loops through every span in the section (step232).

InFIG.9C, the data analysis process200C sweeps dispersion, dispersion slope and γ to scale {tilde over (D)}epth|Leff(β2) to a guessed Depthguess(γ,β2(λ)) based on Eq. (3). The data analysis process200C, for each span i=1˜N at each measured wavelength x=1˜X (step240), starts with loading results from center dip depth measurement, Depthmeas|wvl-x,span-i(step242), and pre-computed {tilde over (D)}epth|Leff(β2) at OTDR measured Leff|span-i(step244). The data analysis200C sweeps dispersion, dispersion slope and γ to scale {tilde over (D)}epth|Leff(β2) to a guessed Depthguess(γ,β2(λ)) (step246). By comparing Depthguess(γ,β2(λ)) with the measured Depthmeas|wvl-x, γ|span-iand β2|wvl=x,span=iare found at Least Mean Square (LMS) fit (steps248,250). The data analysis process200C loops through every span in the section (step252).

In an embodiment, the measurement technique described herein with a two-peak ASE signal can be used to characterize optical amplifiers. For example, next-generation optical amplifiers with higher output power can have tighter Four-Wave Mixing (FWM) specifications. The various measurement techniques described herein work well to characterize this effect. For example, conventionally, the FWM specification of amplifiers was verified in the factory and lab using a specified measurement technique with two CW lasers (which also means single polarization) that had to be polarization controlled (scanned) to search for a peak interaction, with fixed spacing between the CW lasers which could not be widened because of polarization evolution, such that the walk-off could not be measured. This approach was only valid for FWM tones of 50 GHz narrow single polarization sources. This is not representative of the Nonlinear (NL) impact on wider, dual-polarization, coherent transceivers or modems.

The measurement technique employed here uses two-peaks of ASE and has the following benefits 1) ASE more accurately represents spectrally shaped dual-polarization coherent signals, 2) this eliminates the strong polarization hole burning that contaminates the CW approach, 3) the peak separation can be varied to get a direct measurement of walk-off, and 4) it is easy to create multiple peaks to get a full-fill representative G(length) FWM measurement.

FIG.10is a block diagram of a system300for characterizing optical amplifiers302,304utilizing the two-peak ASE signal. The system300includes the ASE source70which is connected to the optical amplifiers302,304, such as via WSSs306,308(or a single WSS). An Optical Spectrum Analyzer (OSA)310can be selectively connected to an output of the amplifier302and the amplifier304.

A measurement process utilizing the system300can include creating a two-peak ASE signal with the ASE source70and the WSSs306,308, sending the two-peak ASE signal to the amplifiers302,304, and measuring the outputs of the amplifiers302,304with the OSA310. Here, and in the section50, the WSSs can be configured to set attenuation to even the power of the two-peaks in the two-peak ASE signal.

First, the optical amplifier302can have its Total Output Power (TOP) adjusted to normalize signal power to a first frequency tested (e.g., 187000 GHz) and an OSA trace can be recorded. Next, the optical amplifier304can have its Total Output Power (TOP) adjusted to normalize signal power to a first frequency tested (e.g., 187000 GHz) and an OSA trace can be recorded after calibration. The OSA trace can be recorded at the output of the optical amplifier304with stepping signal frequent and TOP of either amplifier302,304.

FIG.11Ais a graph of optical spectrum in the system300before subtracting ASE,FIG.11Bis a graph of optical spectrum in the system300after subtracting ASE, andFIG.11Cis a graph of FWM level versus signal center frequency.

FIG.12is flowchart of a process400for fiber characterization. The process400includes causing transmission of one or more shaped Amplified Spontaneous Emission (ASE) signals, from an ASE source, on an optical fiber (step402); obtaining received spectrum of the one or more shaped ASE signals from the optical receiver connected to the optical fiber (step404); and characterizing the optical fiber based in part on one or more of a nonlinear skirt and a center dip depth in the received spectrum of the one or more two-peak ASE signals (step406). The process400can also include determining a fiber type based on a signature of the one or more of the nonlinear skirt and the center dip depth in the received spectrum of the one or more shaped ASE signals (step408). In another embodiment, the process400can have the step402be completed separately and the obtaining step404can be performed responsive to another device causing the transmission. For example, the process400here could be performed by a management system or the like that receives the spectrum of the one or more shaped ASE signals. The transmission or the causing of such transmission can be performed by an optical network element or node. The received spectrum by the optical network element or node can be provided to the processor or other apparatus implementing the process400.

The one or more shaped ASE signals can be formed by the ASE source communicatively coupled to a Wavelength Selective Switch (WSS) that is configured to shape ASE from the ASE source to form the one or more shaped ASE signals with one or more peaks and with associated frequency. The one or more shaped ASE signals can have two distinct peaks at the transmission with a significant dip at a center frequency, and the received spectrum of the one or more two-peak ASE signals has much less of a dip that the center dip depth. The optical fiber can be characterized to determine chromatic dispersion, β2, and fiber nonlinear coefficient, γ. The chromatic dispersion, β2, and the fiber nonlinear coefficient, γ can be characterized by a measurement of a shape of the one or more of the nonlinear skirt and the center dip depth as a function of signal wavelength.

The one or more shaped ASE signals can include a plurality of shaped ASE signals with a first set of shaped ASE signals utilized to determine launch power for every span in a section to yield an optimum center dip depth, and a second set of shaped ASE signals that sweep at different frequencies across a signal band to determine a corresponding center dip depth at the different frequencies. The optical fiber can be characterized based in part on a shape of the one or more of the nonlinear skirt and the center dip depth and a separate differential delay measurement, to determine group velocity dispersion parameter, β2, and fiber nonlinear coefficient, γ. The optical fiber can be characterized based in part on a shape of the one or more of the nonlinear skirt and the center dip depth and a separate Stimulated Raman Scattering (SRS) measurement, to determine group velocity dispersion parameter, β2, and fiber nonlinear coefficient, γ. The optical fiber can be characterized based in part on a shape of the one or more of the nonlinear skirt and the center dip depth and a Least Mean Square (LMS) fit, to determine group velocity dispersion parameter, β2, and fiber nonlinear coefficient, γ.

FIG.13is a flowchart of a process420for effective fiber parameter characterization and effective span modeling. First, the in-situ nonlinear skirt measurement is performed (step422). Next, the calculation of the fiber parameters are the same described herein, but instead of characterizing the normal fiber parameters, β2and γ, this step calculates the generalized effective parameters, β2,effand γeff, to cover any effects from change of fiber types and lumped losses (step424). The effective parameters can be applied to the effective span model for transmission performance estimation (step426). The effective span performance is modeled by span incremental Signal to Noise Ratio, ΔSNRspan, as expressed in Eq. (10):

1Δ⁢SNRspan=1SNRlinear+1SNRNL(10)
SNRlinearis linear Signal to Noise Ratio (SNR) from Amplifier ASE, which is a function of total span loss αtotincluding fiber loss and lumped loss, output power of the EDFA from the previous fiber span, P0, and amplifier noise figure, NF.

All the parameters for backing out SNRlinearare readily available in photonic systems. For example, P0can be read from the EDFA output power monitor; αtotcan be calculated from the delta between P0and the input power of the downstream EDFA; measurement of EDFA NF is characterized during manufacturing of EDFAs and is usually stored and accessible in the memory of EDFA card, or statistical data would be available from the EDFA manufacturer.

The effective parameters, β2,effand γeff, are used in the calculation of nonlinear SNR, SNRNL, which could take a form similar to that shown in Eq. (11):

where gNL(●) is the normalized power spectral density of nonlinear noise which can be obtained by different methods, for example GN model as described in Eq (1) It is also noticed that lumped losses information is not required for SNRlinearand SNRNL, because it is already included in αtot, β2,effand γeff.

Based on Eq. (10)˜(11), the relationship between ΔSNRspanand P0can be plotted as illustrated inFIG.14. Therefore, in step428, the span transmission performance can be optimized by configuring P0at the maximum ΔSNRspan. The estimated transmission performance can be obtained at the same time.