This invention relates to optical transmission systems and, in particular, to an Optical Time Domain Reflectometer (OTDR) for optical fiber verification and characterization.
Optical Time Domain Reflectometers (OTDRs) have been widely used for verification of performance characteristics of optical networks. To obtain OTDR measurements, a series of OTDR pulses are injected into optical fibers under test, and returned light from the optical fibers is measured as a function of time. Using the OTDR measurements, fiber problems (e.g., fiber loss, fiber cut) can be localized. Conventionally, standalone OTDR equipment with dedicated components have been utilized to perform the OTDR measurements. However, the dedicated components lead to increased size and cost, and limit the capability to easily deploy and test fiber spans. Integration of OTDR with other optical components may meet these challenges. However, the integration of OTDR with other optical components is difficult because noise from the other components may interfere with the OTDR measurements. Therefore, there is a need to provide improved OTDR functionality.
In accordance with an aspect of the disclosure there is provided a device having a transmitter configured to generate an OTDR-modulated optical supervisory channel (OSC) signal by applying an Optical Time Domain Reflectometer (OTDR) modulation to an optical supervisory channel (OSC) signal using an OTDR signal; and transmit the OTDR-modulated OSC signal through an optical fiber. The device also includes an OTDR module configured to generate the OTDR signal, to monitor a returned light from the optical fiber, to determine transmitter noise compensation information, and to generate OTDR trace information using the noise compensation information and the modified returned light.
In accordance with another aspect of the disclosure there is provided a method that entails applying an Optical Time Domain Reflectometer (OTDR) modulation to an optical supervisory channel (OSC) signal to generate an OTDR-modulated OSC signal, transmitting through an optical fiber the OTDR-modulated OSC signal, monitoring a returned light signal from the optical fiber, and determining transmitter noise compensation information to generate OTDR trace information using the noise compensation information and the monitored returned light signal.
In accordance with another aspect of the disclosure there is provided a non-transitory computer readable memory containing instructions which when executed by a processor cause the device to apply an Optical Time Domain Reflectometer (OTDR) modulation to an optical supervisory channel (OSC) signal using an OTDR signal to generate an OTDR-modulated OSC signal; transmit the OTDR-modulated OSC signal through an optical fiber, monitor a returned light signal from the optical fiber, and determine transmitter noise compensation information to generate OTDR trace information using the transmitter noise compensation information and the monitored returned light signal.
For a more complete understanding of this disclosure, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description.
Optical devices having integrated OTDR and related methods for compensating for transmitter noise are described below, by way of example only, with reference to
In one implementation, the optical device 110 generates an OTDR signal and applies an OTDR modulation to an OSC signal using the OTDR signal. The OTDR signal may use any known codes, such as complementary Golay codes. The OSC signal with the OTDR modulation is transmitted from the optical device 110 and is fed into an optical fiber under test (e.g., optical fiber 120). The optical device 110 uses the OSC signal with the OTDR modulation to monitor fiber loss events in the optical fiber under test. The events may include Fresnel reflection and/or Rayleigh backscattering. The optical device 110 generates OTDR trace information using a returned light signal r(t) from the optical fiber under test. The OTDR trace information includes the trace of the power of the returned light signal r(t). The OTDR trace provides information quantifying the extent of Fresnel reflection and Rayleigh backscattering. The OTDR circuitry is configured to compensate for transmitter noise (e.g., OSC data, OSC laser noise) to generate the OTDR trace information. The OSC data rate (e.g. 155 Mbps) is higher than the OTDR bit rate (e.g. 1 Mbps). Each OTDR bit thus may contain some number of OSC data bits. OSC data behaves like noise for OTDR. In the implementation, the optical device 110 measures the transmitter optical signal that is output from the transmitter to implement the transmitter noise compensation.
The optical device 110 may include one or more other components, such as one or more programmable on-board modules, a combination of software components and hardware components, an OSC receiver module, other test or diagnostic components, user interfaces, and components for online monitoring of performance characteristics of optical fibers. The optical device 110 may allow various operation modes of the optical device 110, OSC applications, OTDR applications, and data processing schemes. The user of the optical device 110 may set at least one of parameters for OSC applications and parameters for OTDR applications, including parameters of the OTDR measurement (e.g., modulation index/ratio, pulse sequences, pulse width) via a user interface or network communications. The optical device 110 may be an embedded Optical Time Domain Reflectometer (eOTDR) embedded into an OSC small-form factor pluggable (SFP) transceiver that may be configured to continuously monitor fibers.
In
The optical module 220 includes a beam-splitter 222 (or a directional coupler). The beam-splitter 222 is used for OSC and OTDR applications. In one example, the beam-splitter 222 is a Wavelength Division Multiplexing (WDM) beam-splitter. The beam-splitter 222 is communicatively coupled to the optical fiber 210 and a transmitter (denoted as “Tx” in
The transmitter module 200 includes OTDR circuitry for OTDR applications. At least a part of the OTDR circuitry may be integrated into the transmitter 230 or the optical module 220. The OTDR circuitry includes an OTDR measurement module 240 (denoted as “OTDR” in
The OTDR circuitry includes a detector 224 configured to monitor/detect the waveform of the returned light signal r(t) (i.e. light that reflects, scatters or otherwise returns back to the reflector due to Fresnel reflection and Rayleigh backscattering or any other phenomena) from the optical fiber under test. The returned light signal r(t) is directed to the detector 224 via the beam-splitter 222. In one example, the detector 224 includes a photo-detector for measuring the power of the returned light signal r(t). The returned light signal r(t) is substantially proportional to the transmitted optical signal. The OTDR signal is extractable from the returned light signal by using the correlation operation. Data representing the measurement of the returned light signal r(t) may be recorded in a memory, such as a memory in the OTDR measurement module 240.
The OTDR circuitry optionally includes a monitor 226 configured to monitor/detect the transmitter (“Tx”) waveform of the transmitter optical signal output from the transmitter 230, which is used for the transmitter noise compensation. The transmitter optical signal is directed to the monitor 226 via the beam-splitter 222. In one example, the monitor 226 includes a photo-detector for measuring the power of the transmitter optical signal. Data representing the measurement of the transmitter optical signal may also be recorded in a memory, such as the memory in the OTDR measurement module 240.
The OTDR measurement module 240 includes a data processor 244. The data processor 244 is communicatively coupled to the detector 224 and the monitor 226. The data processor 244 is configured to implement the OTDR measurement, including generation of OTDR trace information. The data processor 244 implements the transmitter noise compensation using the monitored transmitter optical signal. In one implementation of the transmitter noise compensation, the data processor 244 computes a modified code signal c′(t) and recovers a fiber loss function using the modified code signal c′(t), as described in detail below. The data processor 244 may be configured to generate and visually display the OTDR trace information using the fiber loss function.
The operation of the transmitter module 200 is controlled using a controller 250. The controller 250 may be an OSC controller for OSC applications with integrated OTDR circuitry. The controller 250 may control various parameters of the transmitter module 200, such as operation modes, OSC applications, or OTDR applications including OTDR measurements and OTDR modulations (e.g. timing, code sequences, pulse width, modulation index/ratio).
In one implementation, the transmitter module 200 has a plurality of operating modes, such as an OSC mode and a correlation mode. In the OSC mode, OSC data without the OTDR modulation is transmitted from the transmitter module 200. In the correlation mode, OSC data with the OTDR modulation is transmitted from the transmitter module 200. In this particular implementation, the detector 224 and the monitor 226 operate only in the correlation mode. In another implementation, the transmitter module 200 can operate in the correlation mode but not in the OSC mode.
In one implementation, the transmitter 230 is configured to generate a an OTDR-modulated OSC signal having a waveform described as (1+OTDR(t))*OSC(t) in which * is the multiplication operator, and the OTDR modulation OTDR(t) is superimposed on OSC(t), as shown by way of example in
The amplitude modulation OTDR(t) reduces the effective amplitude of the OSC modulation of the OSC data which, in turn, may degrade the OSC performance (e.g. its fault-detecting sensitivity). The transmitter module 200 therefore sets the OTDR modulation OTDR(t) to balance the relative signal strengths of the OTDR component and the OSC component to avoid degradation of the OSC component by the OTDR component. In one implementation in which the transmitter 223 generates the OTDR-modulated OSC signal (1+OTDR(t))*OSC(t), the transmitter module 200 adjusts or normalizes the amplitude of OTDR(t). For example, the amplitude of OTDR(t) is adjusted or normalized such that an OTDR modulation ratio K is equal to or less than a threshold. In one implementation, the OTDR modulation ratio is defined as K=[max (OTDR(t))−min (OTDR(t))]×100, where the amplitude is measured in units such that the peak-to-peak amplitude of OSC(t)=1. A large K value means that the OTDR signal OTDR(t) is stronger relative to the original OSC signal OSC(t) and may thus interfere with the OSC operation. Accordingly, the OTDR modulation ratio K may be adjusted such that OSC performance (sensitivity) is insubstantially deteriorated by the presence of the OTDR signal. When OSC is chosen such that the “1” level in amplitude is 1, and the “0” level in amplitude is 0, the OSC “1” level becomes 0.8 to meet the OTDR modulation ratio K=20%. Therefore, the effective amplitude is reduced to 0.8 from 1. In one example, the OTDR modulation ratio K is set to a value that is less than 50%, such as 20%-30%. Setting the OTDR modulation rate K, i.e. normalization of OTDR(t), may be configured by the controller 250, the OTDR circuitry (e.g., data processor 244), user interfaces or a combination thereof.
In the embodiment depicted by way of example in
Referring to
One implementation of the transmitter noise compensation technique is described in detail below. For comparison purposes, it is assumed that there is no transmitter noise, and an OTDR transmitter sends out a code signal c(t) that correlates with itself to produce a delta function δ(t) (i.e., c(t)*c(t)=δ(t): * is the correlation operator). Some suitable examples of the code signal c(t) include: complementary Golay codes, biorthogonal codes, simplex codes, CCPONS (Complementary Correlated Prometheus Orthonormal Sequence). The noise resulting from transmission through the fiber can be represented by the fiber loss function ƒ(z) or ƒ(t), where t=z/(vg/2) is the return time in the fiber, and vg is the group velocity in the fiber. The returned light signal r(t) can be expressed by the convolution of c(t) and ƒ(t) (omitting the scale factor) as shown in Equation (1):
r(t)=c(t)ƒ(t)=∫0tc(τ)ƒ(t−τ)dτ (1)
where is the convolution operator.
The fiber loss function can be recovered by the correlation operation of the code signal c(t) and the returned light signal r(t) as follows:
c(t)*r(t)=c(t)*(c(t)ƒ(t))=(c(t)*c(t))ƒ(t))=δ(t)ƒ(t)=ƒ(t) (2)
where * is the correlation operator.
Taking into account transmitter noise, represented by the function d(t), the OTDR transmitter transmits a signal c(t)d(t) instead of c(t) where d(t)≠1. The returned light signal r(t) is expressed by the convolution of c(t)d(t) and ƒ(t), as described in Equation (3):
r(t)=(c(t)d(t)ƒ(t)) (3)
Using r(t) as expressed in Equation (3), the convolution of c(t) and the returned light signal r(t) is described as:
c(t)*r(t)=c(t)*((c(t)d(t))ƒ(t))=(c(t)*(c(t)d(t)))ƒ(t) (4)
c(t)d(t) in Equation (4) can be described in Equation (5):
c(t)d(t)=c(t)(1+d(t)−1)=c(t)+c(t)(d(t)−1) (5)
As a result, c(t)*r(t) in Equation (4) can be expressed as:
c(t)*r(t)=ƒ(t)+{c(t)*[c(t)(d(t)−1)]}ƒ(t) (6)
In Equation (6), c(t)*(c(t)d(t))≠δ(t) because d(t)≠1. Due to the last term in Equation (6) the transmitter noise reduces the dynamic range over which the OTDR measurement can be reliably used to detect a fault or imperfection in the fiber. The last term in Equation (6) contributes to a noise floor on the OTDR trace.
In one implementation of the transmitter noise compensation, the transmitter noise is compensated by computing an effective modified code signal c′(t) expressed as Equation (7) and then recovering the fiber function as expressed by Equation (8):
c′(t)=c(t)+g′(t) (7),
c′(t)*r(t)=ƒ(t) (8)
To determine g′(t) in Equation (7), g(t) is defined as described in Equation (9):
g(t)=c(t)d(t)−c(t) (9)
where c(t) is the original transmitter optical signal, c(t)d(t) is the monitored waveform of the output from the transmitter.
Using Equation (7), the left term in Equation (8) can be expressed as follows:
c′(t)*r(t)=(c(t)+g′(t))*r(t) (10)
Due to the transmitter noise d(t), the returned light signal r(t) in Equation (10) can be expressed by Equation (3). Thus, the right term of Equation (10) is expressed as shown in Equation (11):
The first term of Equation (11) represents the effect of the transmitter noise. The transmitter noise is thus compensated when the condition expressed in Equation (12) is met:
c(t)*g(t)+g′(t)*g(t)+g′(t)*c(t)=0 (12)
The modified code c′(t) is determined based on g′(t) that meets the condition expressed by Equation (12). In one implementation, the condition (12) is solved in the frequency domain by using a Fourier transformation, as shown in Equation (13):
C(ω)G*(ω)+G′(ω)G*(ω)+G′(ω)C*(ω)=0 (13)
where the superscript * in Equation (13) indicates the complex conjugate, g(t) is a known function and computed by Equation (9), G(ω) is the Fourier transform of g(t), G′(ω) corresponds to the frequency domain variable of g′(t), and C(ω) corresponds to the frequency domain variable of c(t).
Using Equation (13), G′(ω) is obtained by Equation (14) as follows:
The impact of the transmitter noise may be increased by the so-called end reflection at zero distance due to cross-talk inside the OSC module with integrated OTDR (e.g., transmitter module 200 of
Referring to
Any processing of the present disclosure may be implemented by causing a processor (e.g., a general purpose CPU inside a computer system) in a computer system to execute a computer program. In this case, a computer program product can be provided to a computer or a mobile device using any type of non-transitory computer readable media. The computer program product may be stored in a non-transitory computer readable medium in the computer or the network device. Non-transitory computer readable media include any type of tangible storage media. Examples of non-transitory computer readable media include magnetic storage media (such as magnetic tapes, hard disk drives, flash memory etc.), optical magnetic storage media (e.g. magneto-optical disks), compact disc read only memory (CD-ROM), compact disc recordable (CD-R), compact disc rewritable (CD-R/W), digital versatile disc (DVD), Blu-ray™ disc (BD), and semiconductor memories (such as mask ROM, programmable ROM (PROM), erasable PROM), flash ROM, and RAM). The computer program product may also be provided to a computer or a network device using any type of transitory computer readable media. Examples of transitory computer readable media include electric signals, optical signals, and electromagnetic waves. Transitory computer readable media can provide the program to a computer via a wired communication line (e.g. electric wires and/or optical fibers) or a wireless communication link.
The words “during”, “while”, and “when” as used herein relating to circuit operation are not exact terms that mean an action takes place instantly upon an initiating action but that there may be some small but reasonable delay, such as a propagation delay, between the reaction that is initiated by the initial action. Additionally, the term “while” means that a certain action occurs at least within some portion of a duration of the initiating action. The use of the word “approximately” or “substantially” means that a value of an element has a parameter that is expected to be close to a stated value or position. However, as is well known in the art, there are always minor variances that prevent the values or positions from being exactly as stated. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments described herein are capable of operation in other sequences than described or illustrated herein.
While several embodiments have been provided in the present disclosure, it may be understood that the disclosed systems and methods might be embodied in many other specific forms without departing from the scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented. A number of variations and modifications can be made without departing from the scope of the invention as defined in the claims.
This application is a continuation of PCT Application No. PCT/CN2015/074306 filed on Mar. 16, 2015, which application is hereby incorporated herein by reference.
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Number | Date | Country | |
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Number | Date | Country | |
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Parent | PCT/CN2015/074306 | Mar 2015 | US |
Child | 14872484 | US |