Abstract:
Automatic reduction of optical signal power supplied by an upstream network element by a prescribed amount is achieved by capturing and processing reflected optical energy that is generated within the optical fiber path as a result of a downstream fiber cut, open connector, or other potentially hazardous discontinuity. Generally, the power level of the reflected optical signal is detected and measured in the optical fiber path and the optical signal power supplied by the upstream network element is automatically reduced. The optical signal power may either be reduced by an amount corresponding to the measured reflected optical signal power or may be completely shut off until the faulty condition is resolved. In one illustrative embodiment, an apparatus for automatically reducing or shutting off the optical signal power supplied by an upstream network element includes a directional optical transfer device disposed along the optical fiber path and coupled to the output of a network element, an optical power monitor for measuring the reflected optical energy received via the directional optical coupler as a result of a downstream fiber discontinuity, and control circuitry coupled between the power monitor and the network element to control the optical signal power being supplied by the network element based on the monitored power level of the reflected optical signal. The control circuitry may be analog, digital, or may be implemented using a microprocessor operating under software or firmware program control.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to lightwave communication systems and, more particularly, to techniques for controlling the power level of an optical signal so that harm from the optical signal emanating at a downstream fault in an optical fiber path is substantially reduced. 
     BACKGROUND OF THE INVENTION 
     Retinal and other types of eye injury can occur from inadvertent direct exposure to the optical signals used in present communication systems. Danger is presented by the power and the wavelength of such signals. Generally, these systems operate with signals having relatively high power concentrated in a tiny beam located outside the visible region. 
     Recent developments in optical networking have only heightened existing safety concerns. For example, optical amplifiers and other optical components are now being developed to drive optical signals to even higher output power levels. Multi-wavelength systems are also a concern because the total optical power in the optical fiber is the sum of the powers of the individual wavelength components. Consequently, optical systems having total output power of 20 dBm or more are now being realized as a result of advances in optical amplifier and multi-wavelength optical networking technologies. 
     Because the extent of injury is most likely proportional to the total output power and the time of exposure, it is necessary to quickly shut off or reduce the output power of a network element in the event of a fiber cut, removed connector, or any other discontinuity in the optical path. In prior arrangements, control of upstream elements relies entirely upon downstream elements nearer to the fault. For example, downstream network elements perform fault detection and localization by monitoring the degradation or interruption of the forward propagating signal, i.e., the signal propagating downstream. If such a degradation or interruption is detected, the network control and management system then communicates the necessary supervisory signals to switch off the upstream network element. This scheme is limited in several ways. First, the scheme will only work for faults that occur between the upstream and downstream elements. Secondly, this scheme will fail if, by virtue of the system failure, the downstream element cannot communicate with the upstream element, e.g., if the supervisory channel is lost as a result of the discontinuity in the optical path. Even if this scheme works, there are other issues of added cost and complexity for such control and the possibility of delay in effecting control. 
     SUMMARY OF THE INVENTION 
     Automatic reduction of optical signal power supplied by an upstream network element by a prescribed amount is achieved without the use of downstream control elements by using reflected optical signal power that is generated within the optical fiber path as a result of a downstream fiber cut, open connector, or other potentially hazardous discontinuity. Upon capturing and processing the reflected optical signal power at an upstream position in the optical fiber path, the optical signal power supplied by the upstream network element is automatically reduced. The optical signal power may either be reduced by an amount that is a function of the measured reflected optical signal power or may be completely shut off until the faulty condition is resolved. By using reflected optical signal power within the optical transmission path, the present invention does not require any additional signaling from downstream network elements or from the network control and management system and avoids delay. 
     In one illustrative embodiment, control circuitry is located at an upstream position to capture and process the reflected optical signal that is generated as a result of the downstream fault. The control circuitry may be coupled to a network element, such as a fiber optical amplifier, to control the output power level of the network element in response to the downstream fault. More specifically, the control circuitry generates a control signal and supplies this control signal to the network element to reduce the output power level of the network element accordingly. Alternatively, upon processing the reflected optical signal, the control circuitry may be used to introduce a predetermined amount of loss into the optical fiber to reduce the optical signal power below harmful levels. The control circuitry may be analog, digital, or may be implemented using a microprocessor under software or firmware program control. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     A more complete understanding of the present invention may be obtained from consideration of the following detailed description of the invention in conjunction with the drawing, with like elements referenced with like references, in which: 
     FIG. 1 shows a simplified block diagram of one illustrative lightwave communication system embodying the principles of the present invention; 
     FIG. 2 shows a simplified block diagram of an illustrative fiber optical amplifier arrangement embodying the principles of the present invention; 
     FIG. 3 shows a variation of the embodiment depicted in FIG. 2 useful for achieving complete power reduction; 
     FIG. 4 shows a simplified block diagram of another illustrative fiber optical amplifier arrangement embodying the principles of the present invention; and 
     FIG. 5 shows a variation of the embodiment shown in FIG. 4 useful for achieving complete power reduction. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Although the principles of the invention are particularly applicable to controlling the optical signal power supplied by a fiber optical amplifier, and shall be described in this context, those skilled in the art will understand from the teachings herein that the principles of the invention are also applicable to many other optical components including, but not limited to, semiconductor optical amplifiers, optical transmitters (e.g., laser sources), add/drop multiplexers, cross-connects, or any element that supplies or propagates optical signals along an optical fiber. 
     FIG. 1 shows a typical lightwave communication system that includes an optical transmitter  101 , a network element  105 , and an optical receiver  104 . In this example, network element  105  includes several optical components, such as multiple stages of optical amplifiers  102  and an add/drop multiplexer  103 . In general, network element  105  could be any type of simple or complex arrangement of components. Network element  105  supplies an optical signal having a certain output power level onto optical fiber  115 . The optical signal could either be a multi-wavelength optical signal or a single wavelength optical signal. As shown, downstream cut  110  in optical fiber  115  results in a reflection of an optical signal back towards network element  105 , wherein the reflected optical signal has a power level P R . 
     According to one embodiment of the invention, control element  120 , which is located at an upstream position, captures and processes the reflected optical signal generated within optical fiber  115  as a result of downstream fault  110 . Upon processing the reflected optical signal, control element  120  generates and supplies the appropriate control signal to control the output power of final stage optical amplifier  102 B. In particular, control element  120  may be used to control the pump power being supplied to optical amplifier  102 B which, in effect, shuts off or reduces to a safe level the output power of optical amplifier  102 B. In effect, the optical signal power supplied by optical amplifier  102 B is automatically controlled at an upstream position relative to downstream fault  110 . 
     According to another embodiment illustrated in FIG. 1, the optical signal power may be controlled independent of the particular network element supplying the optical signal. More specifically, upon processing the reflected optical signal, control element  120  introduces a predetermined amount of loss in the fiber path at the upstream position in order to reduce the power level of the optical signal emanating from the fiber cut  110 . This may be accomplished by switching in a lossy element based on the power level P R  of the reflected optical signal. For example, a fiber optic switch could switch the optical signal through a lossy medium, such as an unpumped erbium-doped fiber segment, once the reflected optical signal power exceeds a prescribed threshold. Those skilled in the art will recognize that other techniques may be employed according to the principles of the invention to reduce optical signal power by, for example, introducing the appropriate amount of loss into optical fiber  115 . 
     FIG. 2 shows an illustrative embodiment of the present invention used for controlling the output power level of optical signals from an optical amplifier. More specifically, amplifying element  201  disposed along optical fiber path  202  receives an optical signal and supplies an amplified optical signal downstream along optical fiber path  202 . For uniformity and ease of understanding in the following description, amplifying element  201  is contemplated to be a rare earth-doped optical fiber, such as an erbium-doped fiber (EDF segment). However, it is also contemplated that other suitable rare earth elements may be used, such as praseodymium, neodymium, and the like. 
     In order to provide an amplifying effect, EDF segment  201  is “pumped” with luminous energy using conventional techniques known in the art. As shown in FIG. 2, EDF segment  201  is optically pumped by pump sources  210 , which can be semiconductor laser pump assemblies, such as laser diode pumps or any other suitable pump sources well known in the art. The luminous energy generated by pump sources  210 , also referred to as pump light, has a shorter wavelength than any of the wavelengths in the optical signal (i.e., signal light). Optical couplers  212  are used to couple the pump light emitted by pump sources  210  to optical fiber path  202 . The use of optical couplers  212  for this purpose is also well-known to those skilled in the art. 
     It should also be noted that although pump sources  210  are shown in a hybrid bi-directional pump arrangement, other known pump arrangements can also be used without departing from the spirit and scope of the present invention. For example, EDF segment  201  may be pumped using a co-propagating pump configuration (forward pumping) or, alternatively, using a counter-propagating pump configuration (backward pumping), both of which are well-known in the art. For additional background on these pumping arrangements, see U.S. Pat. No. 5,218,608, Optical Fiber Amplifier, issued to Aoki and herein incorporated by reference. 
     As shown, an optical isolator  215  can also be included, if desired, prior to EDF segment  201 . This optional optical isolator  215  can be advantageously used to protect against the undesirable backscattering or back reflection of optical signals which may cause damage to upstream components (e.g., lasers) or which may adversely affect the operation of the upstream components. 
     Importantly, a directional optical transfer device  220  is disposed along optical fiber path  202  and coupled on the output side of EDF segment  201 . Directional optical transfer device  220  can be any suitable device for capturing and transferring optical energy in a directional manner, such as a multi-port optical circulator, a passive optical coupler, and the like. For the embodiments shown in FIGS. 2 and 3, directional optical transfer device  220  will be referred to as optical circulator  220 . As shown, optical circulator  220  includes an input port  221  for receiving the amplified optical signal from EDF segment  201 , an output port  222  for supplying the amplified optical signal along downstream optical fiber path  202 , and a monitor port  223 . 
     In operation, pump sources  210  optically pump EDF segment  201 , which in turn supplies the amplified optical signal as an output. The amplified signal exits EDF segment  201  and enters input port  221  of optical circulator  220 . Using a clockwise directional transfer implementation as an example configuration, optical circulator  220  circulates the amplified optical signal or signals via output port  222  onto downstream optical fiber path  202 . In a typical scenario, a fiber cut, open connector, or other discontinuity problem (referred hereinafter as downstream fault  210 ) occurs along optical fiber path  202  at a point downstream from EDF segment  201 . Downstream fault  210  would cause a reflection of the optical signal having a power level P R  back towards output port  222  of optical circulator  220 . 
     Upon entering output port  222 , the reflected optical signal would exit from optical circulator  220  via monitor port  223 . Photodetector  230  is coupled to monitor port  223  to receive the reflected signal. Photodetector  230  could be any suitable means known to those skilled in the art (e.g., photodiode) for detecting optical energy and converting the optical signal to an electrical signal. The electrical signal from photodetector  230  is processed through a reflected power monitor  231  which relates the photocurrent of photodetector  230  to the power level of the reflected optical signal in its electrical form. Suitable circuitry for reflected power monitor  231  is also well-known. 
     Control circuitry is coupled between reflected power monitor  231  and pump sources  210  to provide the necessary control of the optical signal power supplied by EDF segment  201 . Control circuitry may comprise analog electrical circuitry, such as inverting amplifier  235 , which is used to generate an output signal having a voltage level that is inversely related to that of the reflected optical signal. The output signal from inverting amplifier  235  is then provided to pump controller/driver  211  which adjusts the bias circuitry of pump sources  210  in order to achieve a desired output level of EDF segment  201 . More specifically, in the presence of downstream fault  210  occurring in optical fiber path  202 , inverting amplifier  235  generates the inverted voltage signal of the reflected optical signal and pump controller/driver  211 , in response to the output from inverting amplifier  235 , effects the necessary reduction in pump power supplied by pump sources  210  to EDF segment  201 . 
     By using the analog control circuitry described above, the present invention can be used to control the pump power of EDF segment  201  in a continuous and revertive mode without the need for a manual or controller-based reset capability. It should be noted that the analog circuitry shown and described herein is intended to represent just one possible implementation. As such, other known components may be used without departing from the spirit and scope of the present invention. 
     An additional monitoring tap  240  can be coupled to optical fiber path  202  to support a forward signal monitoring function, typically referred to as performance monitoring. The use of passive optical couplers as monitoring taps is well-known. In general, optical amplifiers sometimes include an optical tap on the output side for tapping off a fraction of the amplified signal in order to monitor the performance of the optical amplifier (e.g., performance monitoring based on output power) as well as the integrity of the outgoing signal (e.g., power level, signal to noise ratio, wavelength, etc.). By way of example, the optical tap can be a passive optical coupler which taps off a fraction, e.g., 1%-10%, of the output signal. Monitoring tap  240  includes a first port for receiving the amplified signal, a second port for coupling a portion of the amplified signal to downstream optical fiber path  202 , and a third port coupled to pump controller/driver  211  via photodetector  241  for tapping a fraction of the incoming amplified signal from the first port for performance monitoring as described above. 
     A network control and management system is normally used in lightwave communication systems to carry out specified control and management functions. As previously described, prior art systems utilize the network control and management system as an integral part of the scheme for controlling the output power of fiber optical amplifiers. In particular, prior art systems utilize supervisory and/or maintenance signals generated through the network control and management system to control the pump power of upstream elements in response to downstream faults. By contrast, the present invention uses the reflected optical power of the optical signal within the transmission path itself to effect the necessary control of output power from EDF segment  201 . Accordingly, the embodiments of the present invention do not rely on supervisory signals from network control and management system  250  as do the prior systems. More specifically, optical signal power supplied by EDF segment  201  is adjusted automatically in accordance with the principles of the present invention without signaling from downstream elements via network control and management system  250 . Network control and management system  250  is shown in FIG. 2 (dotted lines) only to illustrate the coupling that may exist for carrying out the other normal control and management functions of the system. 
     According to the principles of the invention, a variable power reduction capability can be provided that corresponds to the amount of reflected optical signal power generated as a result of a discontinuity in the downstream fiber path. As is well-known, the power level of the reflected optical signal will vary as a function of the proximity of the discontinuity in optical fiber path  202  to EDF segment  201 . For example, a fiber cut in close proximity to EDF segment  201  will result in a higher reflected power level and thus would require a proportionally higher reduction of pump power from pump sources  202 . Accordingly, the present invention can be used to maintain safe output power levels in order to comply with applicable technical and safety standards and, most importantly, to protect maintenance personnel from injury. 
     As compared with prior art arrangements, another apparent and significant advantage of the previously described embodiments is the absence of an optical isolator coupled on the output side of EDF segment  201 . For example, an optical isolator is not required at the output side of EDF segment  201  in the embodiment shown in FIG. 2 because optical circulator  220  itself protects against any backscattering effects from the reflected optical signal. In particular, the reflected power entering output port  222  is circulated to monitor port  223  and not to original input port  221 . Consequently, optical circulator  220  provides an inherent isolator function without the need for additional components. 
     FIG. 3 shows another embodiment of the present invention which may be advantageously used when it is desirable to implement a complete shutdown of optical signal power supplied by EDF segment  201 . Because the embodiment shown in FIG. 3 is a variation of the embodiment depicted in FIG. 2, the description of the relationships and functions for like elements having like reference numerals in FIG. 2 apply equally to those in FIG.  3  and will not be re-stated here for reasons of brevity. In particular, the variation depicted in FIG. 3 relates to the control circuitry coupled between reflected power monitor  231  and pump controller/driver  211  that provides the necessary control of the optical signal power supplied by EDF segment  201 . Here, the control circuitry comprises discrete logic elements, namely comparator  335  and flip-flop device  336 . 
     In operation, the power level of the reflected optical signal is measured in reflected power monitor  231 , as previously described, and provided as a first input to comparator  335 . Comparator  335  compares the power level of the reflected optical signal with a predetermined reference value supplied as a second input to comparator  335 . When the reflected optical signal power exceeds the reference level, comparator  335  generates an appropriate output to flip-flop  336 . In response, flip-flop  336  generates an appropriate output signal to disable pump sources  210  via pump controller/driver  211 , effectively shutting down EDF segment  201 . This arrangement is not automatically revertive in that the system would have to be reset manually or by a controller after the discontinuity in the fiber path is repaired or otherwise removed. 
     It should be noted that the digital circuitry shown and described herein is intended to represent just one possible implementation for the digital control circuitry. As such, other suitable digital circuitry can be used without departing from the spirit and scope of the present invention. For example, a set-reset (S-R) flip-flop is shown, but other conventional logic elements may be equally effective in carrying out the desired function. Additionally, those skilled in the art will understand from the teachings herein that other alternatives are available to provide the pump power control functions performed by the revertive analog circuitry depicted in FIG. 2 or the discrete shutoff logic in FIG.  3 . By way of example, the analog functions of inverting amplifier  235  or the discrete functions of comparator  335  and flip-flop  336  may be carried out by microprocessors and associated software or firmware control. 
     FIGS. 4 and 5 illustrate other embodiments of the present invention in which optical circulator  220  from FIGS. 2 and 3 has been replaced with passive optical coupler  420 . Because the embodiments shown in FIGS. 4 and 5 are variations of the embodiments depicted in FIGS. 2 and 3, respectively, the description of the relationships and functions for like elements in FIGS. 2 and 3 apply equally to those in FIGS. 4 and 5 and will not be re-stated here for reasons of brevity. 
     As shown in FIG. 4, four-port passive optical coupler  420  is disposed along optical fiber path  202  at a point downstream from EDF segment  201 . It should be noted that passive optical coupler  420  may be implemented using any of a number of conventional fiber coupler devices known to those skilled in the art. As an example, passive optical coupler  420  can be the same type of optical coupler device used for monitoring tap  240  (FIGS. 2,  3 ). The basic principles of operation of optical coupler  420  are the same as those previously described for monitoring tap  240  (FIGS. 2,  3 ), except that optical coupler  420  uses four ports instead of three ports. Using conventional optical coupler devices, it is well-known that the modification to use four ports instead of three has minimal impact with regard to cost or optical loss. 
     As shown, the first three ports of optical coupler  420  are coupled in a similar manner as that previously described for monitoring tap  240  in FIGS. 2 and 3. Namely, a first port  421  is used for receiving the amplified optical signal from EDF segment  201 , a second port  422  is used for coupling a major portion of the amplified signal to downstream optical fiber path  202 , and a third port  423  is coupled to pump controller/driver  211  via photodetector  241  to tap a fraction of the incoming amplified signal from the first port for performance monitoring in the same way as that previously described for the embodiments shown in FIGS. 2 and 3. Additionally, a fourth port  424  of optical coupler  420  is coupled to photodetector  230  which is further coupled to reflected power monitor  231  as in the FIGS. 2 and 3. 
     In contrast to optical circulator  220  (FIGS.  2  and  3 ), optical coupler  420  is a passive device and, as a result, optical isolator  425  may be needed to prevent any undesirable backscattering or back reflection of optical energy. The remaining elements shown in FIGS. 4 and 5 are the same as those described for the previous embodiments. Because only a fraction of the reflected optical signal power is tapped off at port  424 , the amount of reflected optical signal power measured by reflected power monitor  231  will typically be less in this embodiment than that reflected through optical circulator  220  (FIGS. 2 and 3) since optical circulator  220  circulates substantially all the reflected optical energy to reflected power monitor  231 . Additionally, if the fiber cut is located at a greater distance downstream from EDF segment  201 , the reflected optical energy could be even less. As such, more sensitive monitoring may be required in this embodiment. 
     FIG. 5 represents a combination of the embodiments shown in FIGS. 3 and 4, wherein passive optical coupler  420  is used in place of optical circulator  220  as previously described in FIG. 4, and comparator  335  and flip-flop  336  are used in place of inverting amplifier  235  as previously described in FIG.  3 . 
     It will be understood that particular embodiments described above are only illustrative of the principles of the present invention, and that various modifications could be made by those skilled in the art without departing from the spirit and scope of the present invention. For example, although optical circulators and passive optical couplers were described in the above embodiments, those skilled in the art will recognize that other suitable components or circuitry may be used for capturing and transferring the reflected optical energy generated from a downstream fiber fault. Similarly, the particular implementation of the control circuitry for processing the reflected optical energy can be modified without departing from the principles of the present invention. 
     As previously described, the principles of the present invention may also be advantageously used to control optical signal power supplied by other optical components even though the above embodiments were described only in the context of fiber optical amplifiers. For example, the present invention can be used to control the output power levels of semiconductor optical amplifiers by controlling the electrical current that is supplied to “pump” the semiconductor device. The present invention can also be used to reduce or shut off power from sources and transmitters (e.g., in a transmit terminal) in response to downstream faults in a fiber path. Accordingly, the scope of the present invention is limited only by the claims that follow.