Abstract:
A method and apparatus for detecting if an optical module has been disconnected from a fiber span or if there has been a break in the span, and for automatically reducing the output signal level of the optical module such that the output signal level is within an acceptable safety limit. Also disclosed is a system and technique for automatically resetting a Raman pump unit once the source of an optical leak has been located and addressed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/284,737, filed on Apr. 17, 2001 which application is hereby incorporated herein by reference in its entirety. 
    
    
     STATEMENTS REGARDING FEDERALLY SPONSORED RESEARCH 
     Not applicable. 
     1. Field of the Invention 
     This invention relates generally to optical amplifiers and more particularly to a system and method for auto-shutdown of distributed Raman amplifiers in optical communications systems 
     2. Background of the Invention 
     As is known in the art, distributed Raman amplifiers in optical communications systems function by injecting a high-power optical beam into the transmission fiber. Energy is transferred from the Raman pump laser to the signals as they propagate in the fiber. Stimulated Raman scattering is an intensity dependent process, hence the optical power requirement of the Raman pump laser increases as the optical gain and bandwidth of the amplifier are increased. 
     The Raman pump laser is injected into the transmission fiber and the communication system is considered as a closed system such that during normal operation there is no risk of human exposure to dangerous optical power levels. However, there can exist a significant safety hazard and risk of exposure to high optical powers from the amplifier if there is a break in the transmission fiber or the Raman pump laser is disconnected from the transmission fiber while the unit is active. 
     In order to meet US and European safety regulations, as well as to significantly reduce any exposure risk, it would be desirable if it was possible to detect if amplifier was disconnected from the fiber span or if there was a break in the span, and automatically reduce it&#39;s output power level to an acceptable safety limit. 
     It would, therefore, be desirable to provide a system and technique for detecting if an optical module has been disconnected from a fiber span or if there has been a break in the span, and for automatically reducing the output signal level of optical module such that the output signal level is within an acceptable safety limit. 
     It would also be desirable to provide a system and technique for automatically resetting a Raman pump unit once the source of the optical leak has been located and addressed. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a technique for detecting if an optical module has been disconnected from the fiber span or if there has been a break in the span, and for automatically reducing the output of an amplifier such that the output is within an acceptable safety limit is provided. 
     In one embodiment, for a counter-propagating distributed Raman amplifier, where the signals and Raman pumps are travelling in opposite several criteria can be used. For an amplifier providing gain to the C- and L signal bands, the a break in the transmission line can be detected by: (1) a loss of signals in both the C-band and L-band or (2) a high amount of Raman pump light backreflected in the Raman pump module. 
     The loss of signal can be determined by measuring the optical power as tapped from the main transmission path. It is preferred that the C and L-bands first be demultiplexed before detecting the signal power. A loss of signal in a given band would be determined if the signal power level falls below a set minimal threshold level. To determine a break anywhere in the fiber span, this level should be set at a power above the Raman generated ASE at measured at the Raman pump module Lower power levels will restrict the detection of a fiber break to a limited section of the transmission span. This may be acceptable, as this will allow monitoring of a portion of the fiber span where the Raman pump power is greatest. 
     The high backreflection criteria is based on the fact that when the Raman pump module is disconnected from the transmission span, ˜4% of the pump light will be reflected back into the module if a flat polished connector is used When the module is connected to the span, typically less than 0.2% of the light is backreflected into the module due to Rayleigh scattering in the fiber. The power threshold level for shutdown should be set between 4% and 0.5% of the nominal operating Raman pump power level. 
     If either criteria 1 or 2 is detected, the module should automatically turn off in a time period short enough such that the maximum permitted optical exposure does not exceed ANSI safety limits. 
     If the Raman pump unit is on and there is any detectable signal power in either the C-band, L-band, or the supervisory channel, the Raman pump unit should override the shutdown circuitry and reset to the operating power level for a time period not to exceed the maximum permitted exposure. At the end of the time period, the shutdown circuitry should be re-enabled. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing features of this invention, as well as the invention itself, may be more fully understood from the following description of the drawings in which: 
     FIG. 1 is a block diagram of an optical amplifier module; and 
     FIG. 2 is a flow diagram of a process for detecting if an optical module has been disconnected from a fiber span or if there has been a break in the span, and for automatically reducing the output signal level of the optical module such that the output signal level is within an acceptable safety limit. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, an amplifier  10  having an input  10   a  and an output  10   b  includes a pump signal combiner  12  having a first port  12   a  coupled to the amplifier input  10   a  and a second port  12   b  coupled to a tap  14  at a first input  14   a . A tap output  14   b  is coupled to the amplifier output  10   b . The combiner  12  is a wavelength selective combiner as is generally known. 
     Tap  14   b  couples a portion of the optical signal along an optical path  15  to a power monitor  16  at a first input  16   a . Power monitor  16  measures the optical signal portion provided thereto and provides a control signal along a control signal path  17   a  to an input  18   a  of a shutdown decision circuit  18 . In one exemplary embodiment, the control signal corresponds to an electrical signal. 
     The shutdown decision circuit  18  is coupled to a control terminal of a pump laser  20 . An output of the pump laser  20  is coupled through a second tap  22  to the pump-signal combiner  12  such that the Raman pump laser signal is injected into the transmission fiber as is generally known to produce an output signal having a relatively high power level at the optical module output port  10   b.    
     In response to the signal level of the portion of the optical signal coupled via tap  14  to the power monitor  16  being less than a first reference signal level, the control signal provided to the decision circuit  18  has a first signal characteristic. In response to the signal level of the portion of the optical signal coupled via tap  14  to the power monitor  16  being greater than the first reference signal level, the control signal provided to the decision circuit  18  has a second signal characteristic which is different than the first signal characteristic. 
     In response to the decision circuit  18  receiving the control signal having the first signal characteristic, the decision circuit  18  provides a control signal to the Raman pump laser  20  which stops the operation of the Raman pump laser  20  In response to the decision circuit  18  receiving the control signal having the second signal characteristic, the decision circuit  18  provides a control signal to the Raman pump laser  20  which maintains the operation of the Raman pump laser  20 . 
     Tap  22  is disposed such that a portion of any signal of appropriate wavelength reflected back toward the Raman pump laser is coupled via tap  14  to the power monitor  16 . If the signal level is less than a second reference signal level, the control signal provided to the decision circuit  18  along path  17   b  has a first signal characteristic. In response to the signal level of the portion of the optical signal coupled via tap  22  to the power monitor  16  being greater than the second reference signal level, the control signal provided to the decision circuit  18  has a second signal characteristic which is different than the first signal characteristic. 
     In response to the signal level of the portion of the optical signal coupled via tap  22  to the power monitor  16  being less than a first reference signal level, the control signal provided to the decision circuit  18  has a first signal characteristic In response to the signal level of the portion of the optical signal coupled via tap  14  to the power monitor  16  being greater than the first reference signal level, the control signal provided to the decision circuit  18  has a second signal characteristic which is different than the first signal characteristic. 
     In response to the decision circuit  18  receiving the control signal having the first signal characteristic, the decision circuit  18  provides a control signal to the Raman pump laser  20  which stops the operation of the Raman pump laser  20 . In response to the decision circuit  18  receiving the control signal having the second signal characteristic, the decision circuit  18  provides a control signal to the Raman pump laser  20  which maintains the operation of the Raman pump laser  20 . 
     Typically the fiber plant (which is underground) produces signals which are provided to the input  10   a  of the optical module  10  and which preferably propagate to the optical module output port  10   b . The optical module includes a tap  14  which couples to the power monitor  16  a relatively small portion of the signal propagating to the optical module output port  10   b.    
     The Raman pump laser  20  provides a pump signal having a wavelength different than the wavelength of the signals provided by the fiber plant and as is generally known, the Raman pump laser injects the pump signal into the fiber plant in a direction which is opposite to the direction of the signal provided by the fiber plant. As shown in FIG. 1, the pump signal provided by the Raman pump laser  20  is coupled through the tap  22  and the pump-signal combiner  12  to the fiber plant. The pump signal combiner  12  combines the pump signal with the fiber plant signal to produce an amplified signal at the optical module output port  10   b.    
     In operation, the circuit operates to detect a break in the fiber as follows. Assuming that there is a break in the fiber  11 , then the fiber  11  will have a reflection characteristic Thus, in this case, the pump signal provided by the Raman pump laser  20  is coupled through the tap  22  and the pump-signal combiner  12  until the pump signal reaches the discontinuity caused by the break in the fiber  11 . At this point, at least a portion of the Raman pump signal is reflected off the discontinuity back through the combiner  12  and toward the Raman pump laser  20 . 
     The tap  22  thus couples a portion of the back-reflected signal to the power monitor  16 . The power monitor  16  then measures the back-reflected signal and compares the signal level of the back-reflected signal to a reference signal level. 
     FIG. 2 is a flow diagram showing the processing performed by portions of system  10  (FIG. 1) to detect unsafe operating conditions and to automatically reduce amplifier output power to power levels within acceptable safety limits. This can include of course automatically stopping operation of the amplifier. The rectangular elements (typified by element  26  in FIG.  2 ), are herein denoted “processing blocks” and represent computer software instructions or groups of instructions. The diamond shaped elements (typified by element  38  in FIG.  2 ), are herein denoted “decision blocks,” represent computer software instructions, or groups of instructions which affect the execution of the computer software instructions represented by the processing blocks. 
     Alternatively, the processing and decision blocks represent steps performed by functionally equivalent circuits such as a digital signal processor circuit or an application specific integrated circuit (ASIC). The flow diagrams do not depict the syntax of any particular programming language. Rather, the flow diagrams illustrate the functional information one of ordinary skill in the art requires to fabricate circuits or to generate computer software to perform the processing required to perform backup and restore operations in accordance with the present invention. It should be noted that many routine program elements, such as initialization of loops and variables and the use of temporary variables are not shown. It will be appreciated by those of ordinary skill in the art that unless otherwise indicated herein, the particular sequence of steps described is illustrative only and can be varied without departing from the spirit of the invention. Thus, unless otherwise stated the steps described below are unordered meaning that, when possible, the steps can be performed in any convenient or desirable order 
     Turning now to FIG. 2, the process of detecting unsafe operating conditions and to automatically reduce amplifier output power to power levels within acceptable safety limits begins by comparing a reference signal to an amplifier output signal as shown in decision block  26 . If the amplifier signal level is less than the reference signal level, then processing proceeds to step  28  in which the amplifier output signal is reduced such that it is within acceptable safety limits. In one embodiment, the amplifier output may be reduced by shutting down a Raman pump module. The pump module should be shut down quickly enough to prevent any harmful effects. 
     If the amplifier signal level is not greater than the reference signal level, then processing proceeds to decision block step  30  in which a reflected power signal level is compared to a reference signal level. If the reflected power signal level is greater than the reference signal level, then processing again proceeds to step  28  in which the amplifier output signal is reduced such that it is within acceptable safety limits. If the amplifier signal level is not greater than the reference signal level, then processing proceeds to decision block  32  in which a determination is made as to whether the amplifier is operating. If the amplifier is operating, then processing returns to decision block  26  and steps  26 - 32  are repeated. If the amplifier is not operating, processing then ends. 
     Having described the preferred embodiments of the invention, it will now become apparent to one of ordinary skill in the art that other embodiments incorporating their concepts may be used. It is felt therefore that these embodiments should not be limited to disclosed embodiments but rather should be limited only by the spirit and scope of the appended claims. 
     All publications and references cited herein are expressly incorporated herein by reference in their entirety.