JP2007518365A - Method and apparatus for monitoring a local optical submarine optical transmission device in operation using COTDR - Google Patents

Abstract

Methods and apparatus are provided for obtaining status information regarding optical transmission paths. The method generates a COTDR survey signal having a defined wavelength, an optical traffic signal on an optical transmission path (306) having a length corresponding to that used in a local seabed demand application, and Begin by communicating the COTDR survey signal. The defined wavelength of the COTDR survey signal is separated from the wavelength at which the optical traffic signal is located at a distance at least equal to a predetermined guard frequency band. The backscattered and / or reflected portion of the COTDR survey signal in which state information about the optical path is embodied is received through the optical path. The backscattered and / or reflected portion of the COTDR survey signal is detected to obtain state information.

Description

Description of related applications

  This application was filed on Jan. 7, 2004, and is assigned to US Provisional Patent Application No. 60 / 535,135 claims priority benefits.

  This application is entitled “Method and Apparatus for Obtaining Status Information on an Active Optical Transmission Line” and is filed on March 5, 2004. Related to 10 / 794,178.

This application is also named “Submarine Optical Transmission with Unified Design Optical Amplifiers” and is filed on June 17, 2004, in US patent application no. Also related to 10 / 870,327.

The present invention relates generally to optical transmission devices, and in particular to coherent optical time domain reflectometry used to detect faults in the optical transmission path of an optical power transmission device comprised of multiple spans of fiber and optical amplifier It relates to the use of an arrangement that allows (COTDR).

  A typical long-range optical transmission device includes a pair of unidirectional optical fibers that support optical signals traveling in opposite directions. The optical signal is attenuated over a long distance. Therefore, optical transmission lines typically include automatic repeaters spaced along the transmission lines at appropriate distances from each other to restore signal power lost due to fiber attenuation. The automatic repeater includes an optical amplifier. The automatic repeater also includes an optical isolator that limits the propagation of the optical signal in a single direction.

  It is important to monitor the health of the system over long distance optical transmission lines. For example, the monitor can detect optical fiber cable faults or interruptions, local increases in attenuation due to sharp bends in the cable, or degradation of optical components. Amplifier performance must also be monitored. For long-range submarine cables, there are two basic approaches for active monitoring: monitoring performed by an automatic repeater with results transmitted to a land station via a telemetry channel, and A land-based monitor in which special signals are sent over the line, accepted and analyzed for performance data. Coherent optical time domain reflectometry (COTDR) is one land-based technique used to detect optical transmission device failures remotely. In COTDR, a light pulse is emitted into the optical fiber and the backscatter signal returning to the launch end is monitored. In the presence of discontinuities such as fiber obstructions or adhesions, the amount of backscatter generally changes, and such changes are detected in the monitored signal. Backscatter and reflection also arise from discrete elements such as couplers, which create a unique signature. The health or performance of the link is determined by comparing the monitored COTDR with a reference record. New peaks and other changes in the monitored signal level represent changes in the fiber path that usually indicate a failure.

  When COTDR is used in a plurality of span transmission lines in which individual spans are connected by an automatic repeater, an optical isolator located downstream of each automatic repeater first emits a backscatter signal by an optical pulse. Complicating factors occur that prevent them from being returned along the same fiber. To solve this problem, each auto repeater includes a bi-directional coupler that couples the auto repeater to a similar coupler in the fiber extending to the opposite side, and thus for backscattered light. Provides an optical path to the COTDR unit. In most DWDM links that use such a return path, there is also a filter immediately after the coupler, so only the COTDR signal is connected to the return path, so the signal from one fiber is connected onto the return path fiber. If so, avoid the interference that occurs. In this way, the signal generated by backscattering and the reflection of the COTDR pulse emitted to one fiber are coupled onto the oppositely extending fiber returned to the COTDR unit for analysis.

  The time between pulse firing and reception of the backscatter signal is proportional to the distance along the fiber to the source of backscatter, thus allowing placement of the obstacle. Accordingly, in order to obtain an unambiguous return signal, the duty cycle of the pulses must be greater than those of the individual round trip travel times of the transmission line. In order to obtain a high spatial resolution, pulses are typically short in duration (eg, between a few microseconds and tens of microseconds) and bright (eg, a maximum of tens of milliwatts) to obtain a good signal-to-noise ratio. output).

  The two features mentioned before the COTDR pulse, which is a powerful and low duty cycle, is generally the use of COTDR when the power transmission is in operation (ie when it carries customer traffic). Make it unacceptable. This is because strong COTDR pulses can interact with channels that support traffic that has undergone four-wave mixing (FWM) or cross-phase modulation (XPM). Furthermore, XPM from the customer traffic channel can widen the COTDR pulse width sufficient to remove a significant amount of its energy from the original signal bandwidth. Since the COTDR receiver has a fairly narrow bandwidth, some power of the COTDR signal is lost when it crosses the receiver, thereby lowering its optical signal-to-noise ratio (OSNR) and significantly compromising COTDR sensitivity. The problems caused by FWM and XPM can be mitigated by positioning the COTDR at a wavelength that is sufficiently far from the nearest signal wavelength. Proper separation will generally depend on details of the distribution map, system length and customer traffic signal level.

  Another reason, including problems with the use of COTDR in operation, is that COTDR pulses cause gain fluctuations that cause temporary behavior of the optical amplifier. This in turn creates a signal bearing channel. This effect is generally known as cross gain coupling. Optical amplifiers generally use erbium as an active element that provides gain. Since the duty cycle of the COTDR pulse (for any transmission span of realistic length) is longer than the lifetime of the erbium ion in the excited state that defines the amplifier's characteristic response time, the optical amplifier treats the COTDR pulse as a transient phenomenon To do. (If Raman optical amplifiers or semiconductor optical amplifiers are used, such temporary behavior also occurs because they have characteristic lifetimes of femtoseconds and nanoseconds, respectively.) For example, a COTDR pulse with a transmission span of 500 km The round trip travel time is approximately 5 milliseconds, while the lifetime of erbium is approximately 300 microseconds. Since the time between COTDR pulses is much greater than the response time of the optical amplifier, the presence of the COTDR pulse along with the traffic causes the amplifier to behave temporarily. The temporal behavior of the optical amplifier caused by the COTDR pulse represents itself as a gain reduction and gain tilt change that adversely affects system efficiency.

  In accordance with the present invention, a method and apparatus is provided for obtaining status information regarding an optical transmission path. The method includes a COTDR survey signal having a defined wavelength and carrying an optical traffic signal, and a COTDR over an optical transmission path having a length corresponding to that used in local seabed demand applications Start by generating a survey signal. The defined wavelength of the COTDR survey signal is separated from the wavelength at which the optical traffic signal is located by a distance equal to at least a predetermined guard frequency band. The backscattered and / or reflected portion of the COTDR survey signal that materializes state information about the optical path is received through the optical path. The backscattered and / or reflected portion of the COTDR survey signal is detected to obtain state information.

  According to one aspect of the invention, the length of the optical transmission path is less than about 5,000 km.

  According to another aspect of the invention, the predetermined guard frequency band is equal to or greater than about 200 GHz.

  According to yet another aspect of the invention, the COTDR survey signal is a pulse signal.

  According to yet another aspect of the invention, the COTDR survey signal includes a saturation signal to reduce gain adjustment.

  The inventor of the present invention found that COTDR technology could be used in submarine optical power transmission devices while the system is in operation when the power transmission device is of a type that leads to so-called local seabed demand. I realized that. Local seabed demand lies approximately between certain demands of short-range (also known as “Festoon” markets) “without automatic repeaters” and long-range overseas repeaters. Systems that are short range or have no automatic repeater use a link that is powered and without in-line amplification (hence the term "not" in the repeater is used). Short range links generally rely on high optical signal launch power from land to overcome any inherent loss of the line. Systems without automatic repeaters are generally limited to links about 250 km long. The maximum upper limit of 400-450 km is actually observed because it overtakes the line gain where line loss proportional to distance is available, the ability to release more power to the line, and the system's ability to resolve the received optical signal. Is done. By comparison, the trading portion of long-range submarine demand, including system lengths of about 5,000 km or more, uses extremely sophisticated transmission methods to maximize bandwidth capacity and system area.

  The present invention recognizes the above-mentioned problems of conventional COTDR deployments by recognizing that the state in which an in-service COTDR monitor is implemented is compatible with the system length, particularly for those used in local submarine demand. Overcome the limitations. The major problems that normally occur when using a COTDR in operation can be overcome. As noted above, these problems include the degradation of the COTDR signal due to traffic-carrying signals as a result of nonlinear effects that give rise to spectral broadening and the inevitable loss of coherence. In addition, the presence of the COTDR signal weakens the traffic-carrying signal due to damage due to optical signal-to-noise ratio and / or gain adjustment effects.

  Since the COTDR monitor of the present invention can be used in operation, pump drop, increased localized fiber loss in the cable, as well as failures that occur in loss of delivery, such as cable disconnection and auto repeater failures Faults such as fiber aging and loop back breakage can be located. Through normal monitoring, it should be possible to monitor the performance of both the fiber and the automatic repeater in the transmission path. It should also be possible to observe trends through normal monitors for the purpose of identifying or predicting potential failures before they occur. Since automatic repeater telemetry is not required to monitor pump fault positioning and amplifier performance, the complexity of the submarine facility is reduced. Furthermore, the inventive COTDR monitoring technology has the advantage of providing additional information about fiber performance not available from automatic repeater telemetry.

  FIG. 1 shows a simplified block diagram of an exemplary local submarine optical power transmission using the Dense Wavelength Division Multiplexing (DWDM) of the present invention. The power transmission device serves to transmit a plurality of optical channels over a pair of unidirectional optical fibers 306 and 308 between terminals 310 and 320 located remotely with respect to each other. Terminals 310 and 320 each include a transceiver unit (not shown). The transmission unit generally includes a series of encoders and digital transmitters coupled to a wavelength division multiplexer. For each DWDM channel, the encoder is alternately coupled to an optical source that is coupled to the wavelength division multiplexer. Similarly, the receiving unit includes a series of decoders, a digital receiver and a wavelength division demultiplexer. Each terminal 310 and 320 includes a COTDR unit 305 and 307, respectively.

The optical amplifiers 312 are positioned along the fibers 306 and 308 so as to amplify the optical signals as they propagate along the transmission path. An optical amplifier is an optical amplifier doped with rare earths, such as an erbium doped fiber amplifier using erbium as a gain medium. As shown in FIG. 1, a pair of rare earth-doped optical amplifiers that support signals traveling in opposition are often housed in a single unit known as an automatic repeater 314. Transmission path consisting of optical fiber 306-308 is divided into transmission span ₃₃₀ 1 -330 ₄ which is connected by an automatic repeater 314. While only three automatic repeaters 314 are represented in FIG. 1 for clarity of discussion, the present invention covers all lengths with many additional (or fewer) sets of such automatic repeaters. It should be well understood by those skilled in the art to discover the application of this transmission path. The optical isolator 315 is located downstream of the optical amplifier 220 to remove the backward propagating light and to remove multiple path interferences.

  Each automatic repeater 314 includes a coupler arrangement that provides an optical path for COTDR. In particular, the signal generated by reflection and scattering of the interrogation signal on fiber 306 between adjacent automatic repeaters enters coupler 318 and is coupled onto fiber 308 that extends to the opposite side via coupler 322. . The COTDR signal then travels with data about the optical fiber 308. Since COTDR 307 functions in a similar manner to generate COTDR signals that are reflected and scattered onto the fiber, they are returned back to COTDR 307 along optical fiber 306. The signal returned to COTDR is then used to provide information about the loss characteristics of each span.

  FIG. 2 shows one embodiment of COTDR units 305 and 307. As shown, the COTDR unit 400 includes a COTDR survey signal generator 402, an optical homodyne detection type optical receiver 404, and a signal processor 406. The optical homodyne detection type optical receiver 404 includes an optical fiber coupler 410, an optical receiver 412, an electric amplifier 414, and a low-pass filter 416. The branch port of the optical fiber coupler 410 and the branch port of the optical fiber coupler 418 are connected to each other.

  In operation, backscattered and reflected COTDR signals are received on either optical fiber 306 or 308 (shown in FIG. 1), fed to COTDR 400, and received by optical homodyne detection type optical receiver 410. In the optical homodyne detection type optical receiver 410, the search light scattered backward is mixed by the optical fiber coupler 418 with the oscillation light branched from the search signal generator 402, and square principle detection is performed by the optical receiver 412. Received and converted to a baseband signal having luminance information on the survey pulse. The optoelectronically converted baseband signal derived from the survey signal is amplified by an electrical amplifier 414 and its noise content is reduced by a low pass filter 416. Then, the signal processor 406 calculates the loss characteristic of the optical fiber from the arrival position of the homodyne detection signal and the reflection position of the inspection signal on the optical fiber and the level of the homodyne detection signal. The method of measuring an optical fiber using a probe optical signal is an optical time domain reflectometer (COTDR) method by an interferometric method.

Referring now to the drop to the COTDR signal, the COTDR signal is detected interferometrically at the receiver and passes through an electrical filter so that the spectral broadening (ie traffic) due to non-linear interaction with the signal channel is clear. Causes weakening of the COTDR signal. The most significant degradation of the COTDR signal arises from cross phase modulation (XPM), which causes the signal channel to induce a step adjustment on the COTDR signal. According to Tin-Kwanchan et al., "Dispersive Fiber Cross-Phase Modulation: Theoretical and Experimental Investigation of the Effect of Tuning Frequency" (IEEE Optical Communications Technical Report 6, 1994), the induced phase ΔΦ _XPM is It can be written to clearly show the dependence on the wavelength separation, which is Δλ of the jamming channel.

Where α is fiber loss, D is dispersion, Ω is the modulation rate of the jamming signal, and ΔΦ ₀ is a component of the induced phase that depends on the jamming channel and the power P of the system length L. Using this relationship, the required protection frequency band between the COTDR signal and the DWDM signal can be estimated. It should be noted that increasing the dispersion or guard frequency band (Δλ) reduces the adverse conditions of cross-phase modulation, and increasing the signal power or system length increases the adverse conditions.

  FIG. 1 shows the adverse conditions of the COTDR signal measured as a function of the guard frequency band to the nearest DWDM signal for a 1400 km system. For 1000-2000 km systems, a protection frequency band with a spacing of 200 GHz is sufficient, and for longer systems, a larger protection frequency band is required.

  Referring to the degradation of the DWDM signal channel, the COTDR signal weakens the DWDM signal through reduction of the optical signal-to-noise ratio, gain adjustment effects and non-linear interactions.

  The addition of the COTDR signal effectively reduces the total power available for the DWDM signal, thus creating a disadvantageous condition for OSNR. Measurements show that very modest COTDR power is sufficient and this adverse condition is therefore small.

  The non-linear degradation of the DWDM signal channel caused by the COTDR signal is much less than that of COTDR caused by the presence of nearby DWDM signals. This is because the coherent detection used by COTDR is more sensitive to nonlinear phase distortion than the direct detection method used for DWDM signals.

  The gain adjustment effect is very serious and the system length increases. This occurs because the pulsed COTDR signal in the outgoing path adjusts the gain of the EDFA amplifier. Reduction can be controlled by reducing the COTDR signal level. Unfortunately, this only works for shorter systems (<1000 km) where less COTDR power is required. For longer systems, it is necessary to use a method that eliminates the COTDR gain adjustment.

  FIG. 2 shows the disadvantageous condition of average Q per DWDM channel caused by COTDR signal for COTDR with pulsed COTDR and saturation signal that removes gain adjustment. The results shown below are for an 850 km system.

  These results indicate that pulsed COTDR causes conditions that are highly detrimental to the DWDM delivery channel. In this case, an active monitor using COTDR could not be used unless COTDR power was reduced. If a saturation signal is included, the gain adjustment effect from the pulsed COTDR is gone and the adverse conditions in the system for active COTDR operation may be ignored.

FIG. 2 shows a simplified block diagram of a power transmission device using the COTDR arrangement of the present invention. FIG. 3 is a block diagram illustrating one embodiment of a COTDR arrangement configured in accordance with the present invention. FIG. 5 is a graph showing adverse conditions for COTDR performance for the closest data channel of 1400 km system length. FIG. 6 is a graph showing the effect of COTDR pulsing in adverse conditions of system Q. FIG.

Claims (16)

A method for obtaining status information about an optical transmission path, said method generating a COTDR survey signal having a defined wavelength; having a length corresponding to that used in local submarine demand applications Transmission of an optical traffic signal and a COTDR survey signal over an optical transmission path, wherein a defined wavelength of the COTDR survey signal is such that the optical traffic signal is at least equal to a predetermined guard frequency band To receive backscattered and / or reflected portions of the COTDR survey signal in which the state information about the optical path is embodied; and to obtain the state information; A method comprising the step of detecting the backscattered and / or reflected part of the COTDR survey signal.
2. The method of claim 1, wherein the length of the optical transmission path is less than about 5,000 km.
2. The method of claim 1, wherein the predetermined guard frequency band is about 200 GHz or higher.
2. The method of claim 1, wherein the COTDR survey signal is a pulse signal.
The method of claim 1, wherein the COTDR search signal includes a saturation signal to reduce gain adjustment.
5. The method of claim 4, wherein the optical traffic signal is located at one or more wavelengths sufficiently away from the frequency band occupied by the cw probe signal to reduce FWM and XPM. In which both the quality and COTDR sensitivity of the are maintained at an acceptable level.
2. The method of claim 1, wherein the transmission path includes at least one optical amplifier located therein.
A bidirectional optical power transmission device including first and second terminals interconnected by at least first and second unidirectional optical transmission paths having at least one automatic repeater therein; A method using COTDR, the method comprising: generating a COTDR survey signal having a defined wavelength; transmitting an optical traffic signal on the first optical transmission path and the COTDR survey signal. Wherein the first and second optical transmission paths each have a length corresponding to those used in local submarine demand applications, wherein the optical traffic signal is at least a predetermined protection The defined wavelength of the COTDR search signal is separated from wavelengths located by a distance equal to the frequency band; status information about the first optical path is Receiving backscattered and / or reflected portions of the COTDR survey signal to be embodied through the second optical path; and the backscattering and / or reflection of the COTDR survey signal to obtain the state information. A method comprising the step of detecting a given part.
9. The method of claim 8, wherein the at least one automatic repeater includes a rare earth doped optical amplifier through which the optical survey signal is transmitted.
9. The method of claim 8, further comprising transmitting the returned COTDR signal from the first optical path over the optical path to the second optical return path.
11. The method of claim 10, wherein the optical return path is located at the automatic repeater.
9. The method of claim 8, wherein the state information includes a discontinuity in the first optical path that causes optical attenuation.
9. The method of claim 8, wherein the length of the optical transmission path is less than about 5,000 km.
9. The method of claim 8, wherein the predetermined guard frequency band is about 200 GHz or higher.
9. The method of claim 8, wherein the COTDR survey signal is a pulse signal.
9. The method of claim 8, wherein the COTDR search signal includes a saturation signal to reduce gain adjustment.

Images