Abstract:
A hub for use in a passive optical network (PON) includes a transmission fiber on which an information-bearing optical signal is received, a double-cladded, rare-earth doped fiber located along the transmission fiber for imparting gain to the information-bearing optical signal, and a combiner having an output coupled to the transmission fiber and a plurality of inputs. The output is coupled to the transmission fiber such that optical energy at pump energy wavelengths but not signal wavelengths are communicated therebetween. At least one pump source is optically coupled to one of the inputs of the combiner for providing optical pump energy to the double-cladded, rare-earth doped fiber. An optical splitter is also provided. The optical splitter has an input coupled to the transmission fiber for receiving an amplified, information-bearing optical signal and a plurality of outputs for directing portions of the amplified, information-bearing optical signal to remote nodes in the PON.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to passive optical networks, and more particularly to a passive optical network having a hub or head end that employs a cladding pumped erbium doped optical amplifier. 
   BACKGROUND OF THE INVENTION 
   Optical networks that employ passive architectures are often referred to as Passive Optical Networks (PONs). Such networks use some form of passive component such as an optical star coupler or a static wavelength router and thus do not have any active switching elements. A primary advantage of a PON is its reliability, ease of maintenance and the fact that the field-deployed network does not need to be powered. Accordingly, PONs are often used as access networks by cable TV and telecommunications providers for the purpose of distributing their services from their facility to the customer premises (e.g., a home or business). 
     FIG. 1  shows the architecture of a PON in its most generalized form. The PON  100  includes a hub  102 , remote nodes  104  that are deployed in the field, and network interface units (NIUs)  106 . The hub  102 , remote nodes  104  and NIUs  106  are in communication with one another over optical fiber links. If the PON  100  is a telecommunications network, hub  102  is a central office. If the PON  100  is a CATV network, hub  102  is generally called a head end. The NIUs  106  may be terminal equipment located on the customer premises or they may serve multiple customers, in which case the NIUs  106  simply provide another level in the network hierarchy below the remote nodes. 
     FIG. 2  shows a portion of a conventional PON  200  that is sometimes employed in a cable TV system. PON  200  includes a head end  202  having a driver amplifier  204 , a 1×N splitter  206  and a high power optical amplifier  208  that is coupled to one of the outputs of splitter  206 . As explained below, additional optical amplifiers (not shown) may be coupled to the remaining outputs of the splitter  206  as the capacity of the network is increased. Finally, the output of the high power optical optical amplifier  208  is coupled to an input of a second 1×N splitter  210 . Each output from the splitter  210  is coupled to a remote node  212 , which may be located in the field or on customer premises. 
   In operation, driver amplifier  204  typically receives an optical signal with about 1-4 mw of power and provides an amplified optical signal with about 100 mw of power to the 1×N splitter  206 . If 1×N splitter  206  is an 1×8 splitter, high power optical amplifier  208  receives an optical signal with about 10-12 mw of power, after losses in the splitter are taken into account. In turn, the high power optical amplifier  208  provides an optical signal to the second splitter  210 . 
   Driver amplifier  204  and high power amplifier  208  are generally rare-earth doped fiber amplifiers that use rare-earth ions as the active element. The ions are doped in a fiber core and pumped optically to provide gain. While many different rare-earth ions can be used to provide gain in different parts of the spectrum, erbium-doped fiber amplifiers (EDFAs) have proven to be particularly attractive because they are operable in the spectral region where optical loss in the fiber is minimal. Because of the electronic structure of the erbium ion, EDFAs can be pumped with optical energy at a wavelength of 980 nm or 1480 nm. Driver amplifier  204  is typically supplied with pump energy at 980 nm to achieve a lower noise figure and high power amplifier  208  is generally supplied with pump energy at 1480 nm to achieve higher output power (at the expense of an increase in noise relative to the driver amplifier  204 ). 
   One advantage of the arrangement shown in  FIG. 2  is its scalability. That is, as demand for service grows, additional high power optical amplifiers can be added to the remaining unused outputs of the 1×N splitter  206 . The driver amplifier  204  and splitter  206  are generally located in a common chassis and the high power optical amplifiers are modules that plug into the chassis. Thus, increasing capacity simply requires the provision of additional modules into the chassis. Moreover, capacity can be increased in this manner without any interruption in service. This arrangement is also highly reliable and requires minimal upfront cost. One disadvantage of this arrangement, however, is that as demand continues to grow, the increasing number of high power amplifier modules that are required makes the head end increasingly expensive. 
   Accordingly, it would be desirable to provide a scalable passive optical network whose capacity can be increased in a relatively inexpensive manner. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, a hub is provided for use in a passive optical network (PON). The hub includes a transmission fiber on which an information-bearing optical signal is received, a double-cladded, rare-earth doped fiber located along the transmission fiber for imparting gain to the information-bearing optical signal, and a combiner having an output coupled to the transmission fiber and a plurality of inputs. The output is coupled to the transmission fiber such that optical energy at pump energy wavelengths but not signal wavelengths are communicated therebetween. At least one pump source is optically coupled to one of the inputs of the combiner for providing optical pump energy to the double-cladded, rare-earth doped fiber. An optical splitter is also provided. The optical splitter has an input coupled to the transmission fiber for receiving an amplified, information-bearing optical signal and a plurality of outputs for directing portions of the amplified, information-bearing optical signal to remote nodes in the PON. 
   In accordance with one aspect of the present invention, the PON is CATV access network. 
   In accordance with another aspect of the invention, the rare-earth doped fiber is an erbium doped fiber. 
   In accordance with another aspect of the invention, the rare-earth doped fiber is an erbium and yttrium doped fiber. 
   In accordance with another aspect of the invention, the pump source is a multimode pump source. 
   In accordance with another aspect of the invention, an initial optical amplifier is also provided. The initial optical amplifier provides the information-bearing optical signal onto the transmission fiber. 
   In accordance with another aspect of the invention, a plurality of pump sources are respectively coupled to the plurality of inputs of the combiner. 
   In accordance with another aspect of the invention, a hub is provided for use in a passive optical network (PON) The hub includes a transmission fiber on which an information-bearing optical signal is received, a double-cladded, rare-earth doped fiber located along the transmission fiber for imparting gain to the information-bearing optical signal, and a combiner having an output coupled to the transmission fiber and a plurality of inputs. The output of the combiner is coupled to the transmission fiber such that optical energy at pump energy wavelengths but not signal wavelengths are communicated therebetween. At least one integrated pump source/splitter module is optically coupled to one of the inputs of the combiner for providing optical pump energy to the double-cladded, rare-earth doped fiber. A first optical splitter has an input receiving the amplified, information-bearing optical signal from the doped fiber and a plurality of outputs. At least one of the outputs is coupled to the integrated pump source/splitter module. A second optical splitter has an input coupled to a splitter output of the integrated pump source/splitter and a plurality of outputs for directing portions of the amplified, information-bearing optical signal to remote nodes in the PON. 
   In accordance with another aspect of the invention, a hub is provided for use in a passive optical network (PON). The hub includes a transmission fiber on which an information-bearing optical signal is received, a double-cladded, rare-earth doped fiber located along the transmission fiber for imparting gain to the information-bearing optical signal, and a combiner having an output coupled to the transmission fiber and a plurality of inputs. The output of the combiner is coupled to the transmission fiber such that optical energy at pump energy wavelengths but not signal wavelengths are communicated therebetween. At least one integrated pump source/splitter module is optically coupled to one of the inputs of the combiner for providing optical pump energy to the double-cladded, rare-earth doped fiber. A first variable ratio coupler (VRC) has first and second outputs and an input receiving the amplified, information-bearing optical signal from the doped fiber. A first of the outputs of the VRC is coupled to a splitter input of the integrated pump source/splitter module. An optical splitter has an input coupled to a splitter output of the integrated pump source/splitter module and a plurality of outputs for directing portions of the amplified, information-bearing optical signal to remote nodes in the PON. 
   In accordance with another aspect of the invention, a hub is provided for use in a passive optical network (PON). The hub includes a transmission fiber on which an information-bearing optical signal is received, a double-cladded, rare-earth doped fiber located along the transmission fiber for imparting gain to the information-bearing optical signal, and a combiner having an output coupled to the transmission fiber and a plurality of inputs. The output of the coupler is coupled to the transmission fiber such that optical energy at pump energy wavelengths but not signal wavelengths are communicated therebetween. At least one integrated pump source/splitter module is optically coupled to one of the inputs of the combiner for providing optical pump energy to the double-cladded, rare-earth doped fiber. The module includes a pump source, a first optical splitter and a first 1×N optical switch having N inputs, where N is an integer greater than or equal to two, and an output coupled to an input of the optical splitter. A second 1×N optical switch has N outputs and an input receiving the amplified, information-bearing optical signal from the doped fiber. (N−1) second optical splitters each having an input respectively coupled to one of the outputs of the second optical switch, wherein each of the second optical splitters i, where i=1 to (N−1), have i+1 outputs, wherein each input of the first optical switch is sequentially coupled to an output of a different one of the second optical splitters. A third optical splitter has an input coupled to a splitter output of the first optical splitter in the integrated pump source/splitter module and a plurality of outputs for directing portions of the amplified, information-bearing optical signal to remote nodes in the PON. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows the high-level architecture of a conventional PON. 
       FIG. 2  shows a portion of a conventional PON that is sometimes employed in a cable TV system. 
       FIG. 3  shows a hub for use in a PON constructed in accordance with the present invention. 
       FIGS. 4-8  show various alternative embodiments of the hub constructed in accordance with the present invention. 
       FIGS. 9-11  shows the arrangement of the switches employed in the hub depicted in  FIG. 8  as additional two pump/splitter modules are inserted into the head end. 
       FIG. 12  shows yet another alternative embodiment of the hub constructed in accordance with the present invention. 
   

   DETAILED DESCRIPTION 
   The present inventor has recognized that the hub of a passive optical network (PON) can be reduced in cost by replacing the conventional EDFAs that serve as the high power amplifiers with cladding pumped EDFAs. As explained below, cladding pumped EDFAs can use less expensive, multimode pump sources instead of the more expensive single mode pump sources required by conventional EDFAs. 
   Cladding pumped EDFAs overcome a problem that arises in a conventional EDFA when attempting to increase their output power by increasing the pump power with which they are supplied. Generally, the pump source is a laser diode. A common way of increasing the output power of the laser diode is to increase its emitting area. This makes it possible to increase the power without increasing the power density at the output facet of the device. Unfortunately, the resulting broad-area laser diode is multimode, and its output is no longer sufficiently coherent to be coupled into a single-mode fiber. Such a diode can, however, be coupled into a multimode fiber to provide an essentially incoherent, high power multimode source. 
   A typical cladding-pumped fiber device comprises a single-mode core and a plurality of cladding layers. The inner cladding surrounding the core is typically a silica cladding of large cross-sectional area (as compared to the core) and high numerical aperture. It is usually non-circular (rectangular or star-shaped) to ensure that the modes of the inner cladding will have good overlap with the core. The outer cladding is commonly composed of a low refractive index polymer. The index of the core is greater than that of the inner cladding which, in turn, is greater than the outer cladding. 
   A major advantage of the cladding pumped fiber is that it can convert light from low-brightness, multimode sources into light of high brightness in a single mode fiber. Light from low brightness sources such as diode arrays can be coupled into the inner cladding due to its large cross-sectional area and high numerical aperture. Such multimode sources have the advantage of being significantly less expensive than single mode pump sources. 
     FIG. 3  shows a hub for use in a PON constructed in accordance with the present invention. While for purposes of illustration only the hub will be sometimes referred to as a head end for a PON employed in a CATV network, those of ordinary skill in the art will recognize that the invention could also serve as the hub for any PON in a telephone access system, for example. In comparison to  FIG. 2 , the head end  302  in  FIG. 3  replaces the high power optical amplifiers  208  with a cladding pumped EDFA that comprises a single, double-cladded doped fiber  308  and one or more pump sources  309  (only one of which is shown in  FIG. 3 ), which advantageously may be multimode pump sources. A combiner  314  is used to couple the pump energy from the pump sources  309  to signal fiber  311 . The doped fiber  308 , which is located at an intermediate point along the signal fiber  311 , amplifies the signal using the pump energy it receives from the pump sources  309  via the combiner  314 . An isolator  307  is provided between the driver amplifier  304  and the doped fiber  308  to prevent counter-propagating pump energy from reaching the driver amplifier  304 . 
   The combiner  314  may be built into the backplane of the head end chassis. In this embodiment of the invention the modules that are inserted into the chassis are simply pump source modules rather than optical amplifier modules. 
   One important advantage of the hub  302  shown in  FIG. 3  is that as the demand for service grows, the service provider only needs to add additional pump sources  309  to the available input ports of the combiner  314  to achieve an incremental increase in output power. This leads to a cost reduction for two reasons. First, unlike in  FIG. 2 , in  FIG. 3  only a pump source needs to be added whereas in  FIG. 2  a complete optical amplifier (the pump source and the doped fiber) needs to be added to achieve an incremental increase in output power. Second, pump sources  309  may be multimode pumps, which are less expensive than the single mode pump sources required by the hub in  FIG. 2 . 
   It should be noted that while the pump source  309  and combiner  314  are arranged in  FIG. 3  so that the doped fiber  308  is counter-pumped, the invention also contemplates a similar arrangement in which the doped fiber  308  is co-pumped. 
   One disadvantage of the head end  302  shown in  FIG. 3  arises because all the optical signals are amplified by the cladding pumped EDFA on the signal fiber  311  before the signal reaches the splitters  306  and  310 . As a result, as additional pump sources  309  are added to the system, the power of the optical signals being transmitted to all the customers via splitters  306  and  310  is increased. In contrast, if an optical amplifier  208  is added in  FIG. 2  to increase capacity, the power level of the signals being amplified by the previously installed optical amplifiers  208  will be unchanged. Accordingly, whenever pump sources are added in the embodiment of the invention shown in  FIG. 3 , the customer will need to make appropriate adjustments to the network such as rearranging splitter loss to accommodate the increase in signal power. As result, the PON will necessarily experience some operational downtime whenever capacity is increased in this manner. This problem is overcome with the embodiment of the invention depicted in  FIG. 4 . 
   In  FIGS. 3 and 4 , as well as the figures that follow, like elements are denoted by like reference numerals. In  FIG. 4  pump energy is supplied from the pump sources  309  to doped fiber  308  in the same manner as in  FIG. 3 . However, in  FIG. 4  a 1×N splitter  316  is also provided, which has an input port that receives the amplified optical signals from the doped fiber  308 . Also, pump source  309  now includes an integrated splitter  313  located in the same module as the pump source  309 , thereby forming a pump/splitter module  315 . That is, in this embodiment of the invention the modules that are inserted into the chassis are pump/splitter modules rather than the pump modules employed in  FIG. 3 . In this embodiment of the invention both the combiner  314  and the splitter  316  may be located in the backplane of the head end chassis. 
   As shown, one of the output ports of the splitter  316  on which a portion of the optical signal is now carried is coupled to an input of the splitter in the pump/splitter module  315 . Likewise, as additional pump/splitter modules  315  are added as the demand for capacity increases the pump/splitter modules  315  will be coupled to an unused input of the combiner  314  (for supplying pump energy) and to an unused output of the splitter  316  (to receive a portion of the optical signal). In this way the power level of the signals provided by previously installed ones of the pump/splitter module  315  will not be affected when capacity is increased. 
     FIG. 5  shows another embodiment of the invention that avoids the need for the splitter  316  used in  FIG. 4  while still maintaining the power level of the signals provided by previously installed ones of the pump/splitter module  315  when capacity is increased. As shown, the amplified optical signals received from doped fiber  308  on signal fiber  311  are directed to a pump/splitter module  315  of the type depicted in  FIG. 4 . Pump/splitter module  315  includes pump source  316  and splitter  313 . Of course, in  FIG. 5  as more pump/splitter modules  315  are added to unused output ports of the combiner  314 , the power level of the amplified optical signals received by each of the pump/splitter modules  315  will increase. This problem can be overcome by adding a variable ratio coupler (VRC)  318  to the input of the splitter  313  in the pump/splitter modules  315 . The VRC  318  has an input that receives the amplified optical signals from signal fiber  311  and two outputs. The first output directs a portion of the amplified optical signals to the splitter  313  and the second output directs the remaining portion of the amplified optical signals to a tap fiber  319  for elimination. The VRC  318  has a variable coupling ratio, which determines the distribution of power between its two outputs. In this way the signal power provided to the splitter  313  can be maintained at a constant level even as additional pump/splitter modules  315  are added to the head end. This is accomplished simply by adjusting the coupling ratio of the variable ratio coupler  318  so that any excess optical power is directed to the tap fiber  319 . While the PON will still experience some downtime in order to adjust the coupling ratio of the VRC  318  when additional pump/splitter modules  315  are added, presumably this downtime can be kept to a minimum and will be less than the downtime experienced with the embodiment of the invention shown in  FIG. 3 . 
   One problem with all the aforementioned embodiments of the invention is that there is a substantial waste of optical power through the unused output ports of the splitters (i.e., the unused output port of splitters  306  in  FIG. 3 ; the unused output ports of splitter  316  in  FIG. 4 ; and the tap fiber  319  in  FIG. 5 ).  FIG. 6  shows yet another embodiment of the invention that avoids such wastage by reusing excess optical power that arises when additional pump/splitter modules  315  are added. 
     FIG. 6  is similar to  FIG. 5  except that in  FIG. 6  two pump/splitter modules  315   1  and  315   2  are shown. Also, in  FIG. 6 , the tap fiber  319  of the first pump/splitter module  315   1  is coupled to the input of the VRC  318  of the second pump/splitter module  315   2 . In this way excess optical signal power that is unused by the first pump/splitter module  315   1  can be used by the second pump/splitter module  315   2 . Likewise, if a third pump/splitter module  315  (not shown in  FIG. 6 ) is added to an unused output port of combiner  314 , excess optical signal power that is not used by the second pump/splitter module  315   2  can be used by the third pump/splitter module by coupling the tap fiber  319  of the second pump splitter module  315   2  to the input of the VRC  318  of the third pump/splitter module. By establishing in this manner a daisy chain between the tap fiber output of each VRC  318  with the input of the VRC  318  in the successive coupler/splitter module  315 , the amount of signal power that goes unused can be substantially reduced or even eliminated. 
     FIG. 7  shows a further enhancement of the present invention that may be employed in connection with either of the embodiments shown in  FIGS. 5 and 6 . In this embodiment the coupling ratio of the VRCs  318  are automatically adjusted when the optical signal power varies as a result of adding (or removing) pump/splitter modules  315 , thereby reducing operational downtime. As shown, the pump/splitter modules  315  include a photodiode  322  that receives, via a tap  320 , a small portion of the optical signal power being directed from the output of the VRC  318  to the input of the splitter  313 . The photodiode  322  monitors the power level of the optical signal being directed to the splitter  313  and sends an electrical reference signal to a controller  324 . The VRC  318  is connected to the controller  324  so that control signals sent from the controller to the VRC  318  varies the coupling ratio of the VRC  318 . The controller  324  is programmed (via software, firmware, hardware, or any combination thereof) to adjust the coupling ratio of the VRC  318  so that the photodiode  320  always detects the same amount of optical power. In this way, if an additional pump/splitter module  315  is added to the head end so that the power level of the optical signals being monitored by the photodiode  320  increases, the controller  324  in any given one of the previously installed pump/splitter modules  315  will send a control signal to its VRC  318  to adjust the coupling ratio so that the optical power being monitored is reduced, while the optical power being directed by the VRC  318  to the tap fiber is increased. This change in the coupling ratio will result in more excess optical power being directed from the tap fiber  319  of each pump/splitter module  315  to its subsequent pump/splitter module. 
   One limitation of the embodiments of the invention shown in  FIGS. 6 and 7  is that if one of the pump/splitter modules  315  becomes inoperable, all subsequent downstream modules  315  will be adversely impacted because they will not receive the optical signal from the inoperable module  315 . This problem is overcome with the embodiment of the invention shown in  FIG. 8  in which two 1×N optical switches  330  and  340  are employed. While in the particular embodiment of the invention depicted in  FIG. 8  N is equal to 8, those ordinary skill in the art will recognize that N may be any integer greater than 2. As shown, optical switch  330  is located in the head end chassis and receives at its input the amplified optical signal from the doped fiber  308 . In addition to the optical switch  330 , N splitters  350   1 ,  350   2 , . . .  350   N  are also provided in the head end chassis. It should be noted that while for simplicity of presentation reference numeral  350   1  is referred to as a splitter, it is actually a single fiber, which for purposes herein may be considered a splitter with a single input and output port. 
   Each splitter  350   i  has i output ports. For instance, splitter  350   4  has 4 output ports and splitter  350   8  has 8 output ports. As further shown in  FIG. 8 , the input port of each splitter  350  is sequencially coupled to the output ports of the 1×N optical switch  330 . Thus, in operation, when optical switch  350  is switched to its fifth output, for instance, the amplified optical signal is directed to the input of the five-port splitter  350   5 . 
   1×N optical switch  340  is located in the pump splitter/module  315 . Optical switch  340  is arranged so that its N input ports are sequentially coupled to an output of the N splitters  350  (i.e., input port j of optical switch  340  is coupled to an output of splitter  350   j ). The output of optical switch  340  is coupled to the input of the splitter  313  that is integrated with pump/splitter module  315 . 
   When, as in  FIG. 8 , only a single pump/splitter module  315  is employed, optical switch  330  located in the head end chassis is switched to its first output position so that the amplified optical signal is directed to splitter (i.e. optical fiber)  350   1 . Likewise, optical switch  340  located in pump/splitter module  315  is switched to its first input position so that it received the amplified optical signal form splitter  350   1  and directs it to the splitter  313 . Optical switches  330  and  340  may be configured annually, or alternatively, they may be configured automatically using microprocessor control. 
     FIG. 9  shows the arrangement of switches  330  and  340  when two pump/splitter modules  315   1  and  315   2  are inserted into the head end. As shown, when the second module  315   2  is added, optical switch  330  is switched to its second output position so that the amplified optical signal is directed to splitter  350   2 . In addition, optical switch  340   1  in module  315   1  and optical switch  340   2  in module  315   2  are both switched to their respective second input positions. In this way the portions of the amplified optical signal that are split between the two outputs of splitter  350   2  are received by pump/splitter modules  315   1  and  315   2 , respectively. The optical switches  340   1  and  340   2 , in turn, direct the optical signals to their respective splitters  313   1  and  313   2 . 
     FIGS. 10-11  shows the arrangement of switches  330  and  340  as additional pump/modules  315  are added. For example, in  FIG. 10 , three pump/splitter modules  315   1 ,  315   2 , and  315   3  are employed. In this case optical switch  330  is switched to its third output position and optical switches  340   1 ,  340   2 , and  340   3  are switched to their third input position. Similarly, in  FIG. 11 , four pump/modules  315  are employed and the switches  330  and  340   1 ,  340   2 ,  340   3 , and  340   4  are all in their fourth position. 
   As previously mentioned, one important advantage of the embodiment of the invention shown in  FIGS. 8-11  is that if any given pump/splitter module  315  were to fail, the remaining pump/splitter modules would be unaffected. Another advantage of this embodiment of the invention arises if one of the splitters  350  should fail. For example, referring to  FIG. 11 , assume splitter  350   4  fails so that the optical signals cannot be transmitted from splitter  350   4  to the modules  315   1 ,  315   2 ,  315   3 , and  315   4 . In this case power can be restored to the modules by switching optical switch  330  and optical switches  340   1 ,  340   2 ,  340   3  and  340   4  to their fifth position. In this way the inoperable splitter  350   4  is bypassed and instead splitter  350   5  is used. Of course, in this configuration less optical signal power is directed to each of the modules  340  since the power is now being split five ways instead of four ways. Nevertheless, this is generally a minor sacrifice compared to the failure of all the pump/splitter modules, which would otherwise occur if the five-way splitter were not used to compensate for the failed four-way splitter. 
   One problem with the embodiment of the invention shown in  FIGS. 8-11  is that if optical switch  330  should fail, none of the pump/splitter modules  315  will receive any signal power.  FIG. 12  shows another embodiment of the invention that overcomes this problem by providing a degree of redundancy. Specifically, a second optical switch  360  is located in the head end chassis to which the optical signal can be diverted by a VRC  362  in the event that the optical switch  330  should fail. VRC  362  has an input that receives the amplified optical signal from doped fiber  308  along signal fiber  311 . VRC  362  has two outputs that are respectively coupled to the inputs of optical switches  330  and  360 . A series of VRCs  380  are also provided, which interconnect the outputs of switches  330  and  360  to the splitters  350 , respectively. As shown, the VRCs  380  have two inputs and a single output. The outputs of optical switch  330  are respectively coupled to one of the inputs of the VRCs  380 . Similarly, the outputs of optical switch  360  are respectively coupled to the other input of the VRCs  380 . The outputs of the VRCs  380  are respectively coupled to the inputs of the splitters  350 . Although not shown in  FIG. 12 , the splitters  350  are connected to the pump/splitter modules  315  in the previously described manner. 
   Instead of VRCs  362  and  380 , other switching elements may be employed, such as a 1×2 switch, for example. One advantage of a VRC over a 1×2 switch, however, is that the insertion loss of the VRC is lower. 
   In operation, the coupling ratio of VRC  362  is adjusted so that all the signal power is directed to optical switch  330 . Likewise, the coupling ratio of VRCs  380  are adjusted so that all the signal power arriving from the outputs of switch  330  and received on the first input of the VRCs  380  are directed to the splitters  350 . Should optical switch  330  fail, the coupling ratio of VRC  362  is adjusted so that all the signal power is directed to optical switch  360 , Likewise, the coupling ratio of VRCs  380  are adjusted so that all the signal power arriving from the outputs of switch  360  and received on the second input of the VRCs  380  are directed to the splitters  350 .