Optical: systems and elements – Optical amplifier – Correction of deleterious effects
Reexamination Certificate
1998-05-08
2001-04-24
Moskowitz, Nelson (Department: 3662)
Optical: systems and elements
Optical amplifier
Correction of deleterious effects
C359S199200, C359S199200
Reexamination Certificate
active
06222668
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is directed to methods and devices for detecting a loss of signal in an optical transmission system, and more particularly, to detection of a loss of signal and the inputs of bidirectional optical amplifiers.
2. Background Art
In an optical transmission system, a loss of input signal alarm is an important tool for determining that an optical connector has been disconnected, a cable has been broken, removed, or introduces a high loss.
Moreover, in optical amplified systems, the reflection of a significant portion of the light leaving via a given fiber may cause problems with detection of the loss of the input signal on that fiber. The outgoing light, that is then reflected, could be amplified signal and amplified spontaneous emission (ASE), in the case of a bidirectional system, or could be just ASE, in the case of a unidirectional system. Or, the outgoing light could be a combination of signals and ASE from both directions in the case where there are more complex optical path reflections. If the reflected outgoing light could be distinguished from the desired input signal, then appropriate alarms or control actions could be initiated.
In bidirectional optical amplifier applications, whenever both connectors are open with the pump laser in both directions being “on”, reflections can cause Q-switching and oscillations, despite the optical isolators that may be present. The oscillation path can involve more than one optical amplifier in the system and be quite complex. Prevention of this condition is desirable, as this phenomena can cause damage to the amplifier.
If a fiber is accidentally disconnected from one particular input of the amplifier, the amplifier should be shut off in that direction to minimize the risk of self-oscillation and Q-switching. In addition, shutting off the laser pump must occur very fast, before the other fiber can be disconnected.
For this reason, it is advantageous for a bidirectional amplifier to have a fast method of detecting a loss of signal (LOS) due to a disconnected fiber, in order that the amplifier can be shut off quickly in the corresponding direction.
Measurement of the strength of reflections is presently done with an optical time domain reflectometer (OTDR) that sends strong short pluses of light down a fiber and measures the signal returned to determined LOS conditions from sudden increases in reflected power levels. This is an accurate method, but the OTDR is a relatively large and expensive piece of test equipment that can not be easily used while there is traffic on the fiber. Also, as an OTDR is a separate device from the amplifier, there is no direct means of shutting off the amplifier in a particular direction when the reflection condition is detected.
Another prior art method for detecting reflections is to measure the amount of DC light reflected back via a four port coupler. However, this method cannot be used in bidirectional systems.
Techniques such as disclosed in U.S. patent application Ser. No. 08/588,776 (O'Sullivan et al.) filed Jan. 18, 1996 and assigned to Northern Telecom Limited) can be used to measure reflected power on an in-service link at the amplifier site. This method however, can take in the order of a few seconds to provide accurate results following a change in conditions at an input port, leaving enough time for the second input port to be disconnected following the first, resulting in above mentioned undesired mode of operation.
There is a need to provide a fast means for detecting LOS in a transmission system equipped with bidirectional optical amplifiers, irrespective if a data signal is present or absent.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a fast means for detecting a LOS at the input of a bidirectional optical amplifier, which alleviates totally or in part of the drawbacks of the prior art.
It is another object of the invention to detect when one connector of a bidirectional optical amplifier has been disconnected and to shut-off quickly the pump laser in the corresponding direction for preventing Q-switching and self oscillation, should the other connector be disconnected.
Accordingly there is provided a detector fast signalling a loss of signal (LOS) condition at a forward input port of a bidirectional optical amplifier of a bidirectional optical transmission system with optical fiber amplifiers, comprising means for monitoring the forward input port, corresponding to a forward transmission channel, to obtain a first alarm signal, means for monitoring a forward output port of the amplifier, corresponding to the forward transmission channel, to obtain a second alarm signal, means for monitoring a reverse output port of the amplifier, corresponding to a reverse transmission channel, to obtain a third alarm signal, and means for processing the first, second and third alarm signals for generating the LOS signal.
The invention further comprises a fast loss of signal (LOS) detector for a bidirectional optical transmission system with an optical amplifier, an add/drop bidirectional optical service channel, and an optical band-pass filter connected at a reverse service output port of the amplifier, comprising means for monitoring the reverse service output port, to produce a service channel alarm whenever a forward service signal is detected at the reverse service output port, and means for processing the service channel alarm for generating a LOS signal.
The invention also comprises a fast loss of signal (LOS) detector for a bidirectional optical transmission system with an optical amplifier connected to a forward and a reverse transmission channels, comprising an optical tap arranged at an input port of the amplifier corresponding to the forward channel, for diverting a fraction of a forward optical signal traveling over the forward channel, an optical band-pass filter connected to the optical tap for retaining a reflection component present in the fraction based on the frequency of the reverse channel, means for converting the reflected component into a reflected voltage, and means for producing a LOS signal whenever the reflected voltage is higher than a threshold.
Further there is provided a method for detecting a loss of signal in an optically amplified transmission system, comprising, at an optical amplifier site, the steps of monitoring an input forward optical signal on a forward channel to produce a first alarm signal whenever the input forward optical signal increases over a first threshold, monitoring an output forward optical signal on the forward channel to produce a second alarm signal whenever the output forward optical signal drops under a second threshold, monitoring an output reverse optical signal on a reverse channel to produce a third alarm signal whenever the output reverse optical signal drops under a third threshold, and processing the first, second and third alarm signals and accordingly declaring a loss of signal (LOS) condition.
Advantageously, the present invention provides a method and apparatus which is an inexpensive addition to an optical amplifier module and gives a good accuracy in identifying the LOS. Being built into the equipment, it does not significantly disturb the traffic, and can be continuous or remotely monitored.
In addition, some of the components necessary for detecting LOS may be already present at the amplifier site, so that minimal HW may be needed.
REFERENCES:
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patent: 5442472 (1995-08-01), Skrobko
patent: 5701195 (1997-12-01), Akikama
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patent: 6064501 (2000-05-01), Roberts et al.
patent: 6075629 (2000-06-01), Al-Salameh et al.
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Dutrisac Mathieu
Habel Richard Achile
Keys Robert
Mok Franz Li Hsien
Weslowski Jeffrey Alan
Diaconescu Aprilia U.
Moskowitz Nelson
Nortel Networks Limited
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