Abstract:
Disclosed is a wide band optical fiber amplifier for amplifying C and L-band optical signal components having an economical configuration, high amplification efficiency while exhibiting a low noise figure. The amplifier includes first and second isolators; first, second, and third amplification units; a distributor; a gain flattening filter; and first and second reflectors. The amplifier receives C and L band optical signals and process the signals by: amplifying C and L band signals; gain flattening the only C band signal twice; amplifying C and L band signals for the second time; splitting the C band signal from L band signal; subjecting L band signal to be amplified three more times; and combining resulting C and L band signals.

Description:
CLAIM OF PRIORITY 
   This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. § 119 arising from an application entitled, WIDE BAND OPTICAL FIBER AMPLIFIER, earlier filed in the Korean Industrial Property Office on Mar. 11, 2002, and there duly assigned Serial No. 2002-12835. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to optical communication systems, and more particularly to an optical fiber amplifier disposed between an optical transmission block and an optical receiver block in an optical communication system. 
   2. Description of the Related Art 
   To keep abreast of exponentially growing data demand, wavelength division multiplexing (WDM) optical transmission systems will be required to have expanded transmission bandwidth range. In this regard, a number of active research efforts are ongoing in conjunction with wide band systems that use both the C-band (Conventional band, 1,530 to 1,560 nm) and the L-band (Long band 1,570 to 1,600 nm). 
   In a conventional optical fiber transmission system, an optical fiber amplifier is adapted to perform an amplification function of transmitted and/or received optical signals. These amplifiers are typically doped with a rare-earth element such as erbium as one example. Such erbium-doped fiber amplifiers have a total bandwidth on the order of about 30 nm. This bandwidth is insufficient to amplify both the C and L-band requiring approximately twice the bandwidth. Raman amplifiers or tellulite-based erbium-doped fiber amplifiers, have a wider amplification bandwidth capable of amplifying both the C and L-bands. However, Raman amplifiers have limited practical utility because they require a high pumping power. Also, tellulite-based erbium-doped fiber amplifiers are based on amplifying techniques which have not been thoroughly proven. 
   In light of the aforementioned drawbacks, new research is being conducted to develop a wide band erbium-doped fiber amplifier capable of amplifying both the C and L-band based on a conventional silica-based erbium-doped fiber amplifier. However, one drawback of most wide band erbium-doped fiber amplifiers is that they have a configuration in which independent C and L-bands are coupled in parallel. 
     FIG. 1  is a diagram illustrating the configuration of a known wide band optical fiber amplifier. As shown in the drawing, the conventional wide optical band fiber amplifier includes first and second isolators  110  and  250 , a splitter  120 , first through third wavelength selective couplers  140 ,  190 , and  220 , first through third pumping light source  150 ,  200 , and  230 , first and second erbium-doped fibers  160  and  210 , a gain flattening filter  170 , and a combiner  240 . 
   In operation, the first isolator  110  allows light inputted to the wide band optical fiber amplifier to pass in a forward direction through the wide band optical fiber amplifier, but blocks light flowing in the opposite direction. 
   The splitter  120  splits the input optical signal into a C-band optical signal and an L-band optical signal, and guides the C-band optical signal to flow along a first path  130  while guiding the L-band optical signal to flow along a second path  180 . 
   The first wavelength selective coupler  140  couples the C-band optical signal outputted from the splitter  120  with a pumping light outputted from the first pumping light source  150 , and outputs the resultant amplified C-band optical signal to the first erbium-doped fiber  160 . 
   The first pumping light source  150  outputs a pumping light having a wavelength of 980 nm in order to pump the first erbium-doped fiber  160 . That is, to excite the erbium ions in the first erbium-doped fiber  160 . 
   The gain flattening filter  170  serves to flatten the gain of the C-band component of the input optical signal. 
   The second wavelength selective coupler  190  couples the L-band optical signal received from the splitter  120  with a pumping light received from the second pumping light source  200 , and outputs the resultant optical signal to the second erbium-doped fiber  210 . 
   The second pumping light source  200  outputs a pumping light having a wavelength of 980 nm in order to forwardly pump the second erbium-doped fiber  210 . 
   The third wavelength selective coupler  220  sends the amplified L-band optical signal output from the second erbium-doped fiber  210  to the combiner  240  in a forward direction while independently sending a pumping light outputted from the third pumping light source  230  to the second erbium-doped fiber  210  in a backward direction. 
   The third pumping light source  230  outputs a pumping light having a wavelength of 1,480 nm in order to backwardly pump the second erbium-doped fiber  210 . 
   The second erbium-doped fiber  210  is pumped in both the forward and backward directions by a forward pumping light source received via the second wavelength selective coupler  190  and a backward pumping light source, received via the third wavelength selective coupler  220 . The L-band optical signal is amplified by the second erbium-doped fiber  210  via the first wavelength selective coupler  190 . 
   The combiner  240  combines the amplified C-band optical signal with the L-band optical signal, and outputs the combined C and L-band optical signal to the second isolator  250 . 
   The second isolator  250  forwardly outputs the combined C and L-band optical signals applied thereto while blocking light flowing in the opposite direction. 
   One drawback of the conventional wide band optical fiber amplifier of the prior art, having the configuration above, is that the pumping light sources  150 ,  200 ,  230  require high power while exhibiting a low amplification efficiency. A further drawback of the amplifier of  FIG. 1  is that since the splitter  120 , which exhibits an insertion loss of about 0.7 dB, is arranged upstream from the erbium-doped fibers a reduction in noise figure occurs. 
   SUMMARY OF THE INVENTION 
   The present invention relates to a wide band erbium-doped fiber amplifier having an economical configuration and a high amplification efficiency while exhibiting a low noise figure. 
   Accordingly, the wide band optical fiber amplifier used in an optical transmission system, comprises: a first amplifying unit configured for amplifying an optical signal having C band and L-band optical signal component; a distributor coupled to the first amplifying unit configured for receiving the amplified C and L-band optical signal components via a first path; a gain flattening filter coupled to the distributor for receiving the amplified C and L band optical signal components and for gain flattening only the amplified C band optical signal component; a first reflector coupled to the gain flattening filter for reflecting the amplified gain flattened C band optical signal component and the amplified L band optical signal component back into the distributor, via the gain flattening filter, the first reflector causing the gain flattened optical signal to be gain flattened a second time; a second amplifying unit coupled to the distributor for receiving and amplifying the twice gain flattened amplified C band optical signal component and the amplified L-band optical signal component; the distributor further configured for receiving the amplified C and L band optical signal components and for splitting the amplified C and L band optical signal components into a separate amplified C and L band optical signal components; a third amplifying unit coupled to the distributor and configured for amplifying the separate amplified L-band signal component; a second reflector coupled to the third amplifying unit configured for reflecting the separate amplified L band signal component back to the distributor via the second amplifying unit; the distributor further configured for combining the amplified separate amplified L band signal component with the separate amplified C band signal component and for outputting the combined signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which: 
       FIG. 1  is a view illustrating the configuration of a conventional wide band optical fiber amplifier according to the prior art; 
       FIG. 2  is a view illustrating a wide band optical fiber amplifier according to a first embodiment of the present invention; 
       FIG. 3  is a view illustrating a wide band optical fiber amplifier according to a second embodiment of the present invention; and 
       FIG. 4  is a view illustrating a wide band optical fiber amplifier according to a third embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Now, preferred embodiments of the present invention will be described in detail with reference to the annexed drawings. In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear. 
     FIG. 2  illustrates the configuration of a wide band optical fiber amplifier according to a first embodiment of the present invention. As shown in  FIG. 2 , the wide band optical fiber amplifier includes first and second isolators  310  and  500 , first through third amplifying units  540 ,  550 , and  470 , a distributor  560 , a gain flattening filter  370 , and first and second reflecting units  380  and  480 . 
   The first isolator  310  allows light inputted to the wide band optical fiber amplifier to pass in a forward direction, while preventing (blocking) light from flowing in a backward direction. 
   The first amplifying unit  540  includes a first wavelength selective coupler  320 , a first pumping light source  330 , and a first erbium-doped fiber  340 . 
   The first wavelength selective coupler  320  couples C and L-band optical signal components received from the first isolator  310  with a pumping light outputted from the first pumping light source  330 , and outputs a coupled C and L-band optical signal components to the first erbium-doped fiber  340 . 
   The first pumping light source  330  outputs a pumping light having a wavelength of 980 nm in order to pump the first erbium-doped fiber  340 . That is, to excite the erbium ions in the first erbium-doped fiber  340 . 
   The first erbium-doped fiber  340  is pumped by the pumping light received from the first pumping light source  330  via the first wavelength selective coupler  320 , thereby amplifying the C and L-band optical signal components received via the first wavelength selective coupler  320 . The first amplifying unit  540  outputs the C and L-band optical signal components to a distributor unit  560  as will be described. 
   The distributor  560  receives the amplified C and L-band optical signal components from the first amplifying unit  540 . The distributor  560  includes a circulator  360 , a splitter  430 , a combiner  510 , and third and fourth wavelength selective couplers  420  and  440 . 
   The circulator  360  receives the amplified C and L-band optical signal components via a first path  350  at a first port of the circulator and outputs the amplified C and L-band signals at a second port of the circulator. The circulator  360  also outputs the C and L-band optical signal components, received through the second circulator port, at a third circulator port thereof, and outputs the L-band optical signal component, received through the third port, at a fourth port thereof. 
   The circulator  360  also may be required to prevent an amplified spontaneous emission (ASE) generated from the second and third erbium-doped fibers  410  and  470  from flowing backward into the first erbium-doped fiber  340 , thereby avoiding a degradation in amplification efficiency. 
   The splitter  430  guides the C-band optical signal component output from the third port of the circulator  360  to flow along a fifth path  490 , while guiding the L-band optical signal component to flow along a third path  460 . 
   The second isolator  500  allows the C-band optical signal component received from the splitter  430  to pass forward therethrough, while blocking light flowing in the opposite direction. 
   The third and fourth wavelength selective couplers  420  and  440  perform a pumping operation for the third amplifying unit  470  (i.e., the third erbium amplifier) by use of a residual pumping light outputted from the second amplifying unit  550 . That is, the third wavelength selective coupler  420  is arranged between the second amplifying unit  550  and the splitter  430  to output the residual pumping light outputted from the second amplifying unit  550  to a sixth path  450 . The sixth path  450  coupling the second amplifying unit  550  with the fourth wavelength selective coupler  440 . The fourth wavelength selective coupler  440  being arranged between the splitter  430  and the third amplifying unit  470  to output, to the third amplifying unit  470 , the residual pumping light received via the sixth path  450 . 
   The combiner  510  combines the L-band optical signal component received via a fourth path  520  with the C-band optical signal component received via the fifth path  490 , and outputs the combined optical signal via a seventh path  530  to the optical receiver (not shown). The gain flattening filter  370  serves to flatten the gain of the C-band component of the optical signal received via the second port of the circulator  360  two times, and a second time by virtue of the first reflector  380 . Accordingly, the maximum insertion loss of the gain flattening filter  370  required for gain flattening can be reduced by half. The first reflecting unit  380  reflects the C and L-band optical signal components received from the gain flattening filter  370  so that the reflected optical signal components are re-applied to the gain flattening filter  370 . 
   The third amplifying unit  560  comprises a third erbium-doped fiber  470 . The third erbium-doped fiber  470  is pumped by a pumping light that is received via the fourth wavelength selective coupler  440 , thereby amplifying the L-band optical signal component received from the splitter  430  and the L-band optical signal component re-applied thereto. 
   The second reflecting unit  480  reflects the L-band optical signal component received from the third erbium-doped fiber  470  so that the reflected L-band optical signal component is re-applied to the third erbium-doped fiber  470 . 
   The second amplifying unit  550  includes a second wavelength selective coupler  390 , a second pumping light source  400 , and a second erbium-doped fiber  410 . 
   The second wavelength selective coupler  390  couples C and L-band optical signal components received via the third port of the circulator  360  with a pumping light received from the second pumping light source  400 , and outputs the resultant optical signal to the third erbium-doped fiber  410 . 
   The second pumping light source  400  outputs a pumping light having a wavelength, of 480 nm in order to pump the second erbium-doped fiber  410 . That is to excite the erbium ions in the second erbium-doped fiber  410 . 
   The second erbium-doped fiber  410  is pumped by the pumping light received via the second wavelength selective coupler  390 , thereby amplifying the C and L-band lights received via the second wavelength selective coupler  390 . 
   Operational Description of the First Embodiment 
   The operation of the wide band fiber amplifier having the above described configuration will now be described with reference to  FIG. 2 . C and L-band 
   The C and L-band optical signal components are inputted to the wide band optical fiber amplifier. The C and L-band signals first pass through the first isolator  310 , and then enter the first amplifying unit  540 . In the first amplifying unit  540 , the C and L-band optical signal components are input to the first wavelength selective coupler  320  which couples the C and L-band signals with the pumping light output from the first pumping light source  330  to amplify the C and L-band signals in the first erbium-doped fiber  340 . After being amplified in the first erbium-doped fiber  340 , the amplified C and L-band optical signal components are then applied to a distributor  560 . In the distributor  560 , the amplified C and L-band signals are applied to the first port of the circulator  360 . The circulator  360  outputs the amplified C and L-band signals at the second port, at which point only the C band component is twice subjected to a gain flattening procedure in the gain flattening filter  370 . This process occurs twice as a consequence of the signals being reflected by the first reflector  380 . Subsequently, the twice gain flattened C band optical signal component and L-band optical signal component, output from the gain flattening filter  370  are re-applied to the second port of the circulator  360 . The circulator  360  then outputs the C and L-band optical signal components at the third port of the circulator  360 , so as to apply the C and L-band optical signal components to the second erbium-doped fiber  410  via the second wavelength selective coupler  390 . After being amplified in the second erbium-doped fiber  410 , the C and L-band optical signal components are applied to the splitter  430  via the third wavelength selective coupler  420 . The splitter  430  splits the C and L-band signals and guides the C-band optical signal component along the fifth path  490  while guiding the L-band optical signal component along the third path  460 . The L-band optical signal component is then amplified two times in the third erbium-doped fiber  470  by virtue of the second reflecting unit  480  and then re-applied to the second erbium-doped fiber  410  after passing through the fourth wavelength selective coupler  440 , the splitter  430 , and the third wavelength selective coupler  420 , in this order. The second erbium-doped fiber  410  then re-amplifies the L-band optical signal component, and then outputs the re-amplified L-band optical signal component. The re-amplified L-band optical signal component is then re-applied to the third port of the circulator  360  via the second wavelength selective coupler  390 . The circulator  360 , upon receiving the re-amplified L-band optical signal component at the third port, outputs the L-band optical signal component through the fourth port of the circulator  360  along a fourth path  520 . 
   The combiner  510  combines the C-band optical signal component received via the fifth path  490  with the re-amplified L-band optical signal component received via the fourth path  520 , and outputs the combined optical signal along the seventh path  530  to an optical receiver (not shown). 
   Second Embodiment 
     FIG. 3  illustrates the configuration of a wide band optical fiber amplifier according to a second embodiment of the present invention. As shown in  FIG. 3 , the wide band optical fiber amplifier includes first and second isolators  610  and  750 , first through third amplifying units  785 ,  660 , and  795 , a gain flattening filter  650 , a distributor  790 , and a reflecting unit  740 . 
   The first isolator  610  allows light inputted to the wide band optical fiber amplifier to pass forward through the wide band optical fiber amplifier, while blocking light flowing in the opposite direction. 
   The first amplifying unit  785  includes a first wavelength selective coupler  620 , a first pumping light source  630 , and a first erbium-doped fiber  640 . 
   The first wavelength selective coupler  620  couples C and L-band optical signal components received from the first isolator  610  with a pumping light received from the first pumping light source  630 , and outputs the coupled C and L-band optical signal components to the first erbium-doped fiber  640 . 
   The first pumping light source  630  outputs a pumping light having a wavelength of 980 nm in order to pump the first erbium-doped fiber  640 . That is, to excite the erbium ions in the first erbium-doped fiber  640 . 
   The first erbium-doped fiber  640  is pumped by the pumping light received from the first pumping light source  630  via the first wavelength selective coupler  620 , thereby amplifying the C and L-band lights received via the first wavelength selective coupler  620 . 
   The gain flattening filter  650  flattens the gain of only the C-band component in the C and L-band optical signal components received from the first erbium-doped fiber  640 . 
   The third amplifying unit  795  includes a second wavelength selective coupler  710 , a second pumping light source  720 , and a third erbium-doped fiber  730 . 
   The second wavelength selective coupler  710  couples the L-band optical signal component received via the second port of the circulator  690  with a pumping light received from the second pumping light source  720 , and outputs the resultant optical signal to the third erbium-doped fiber  730 . 
   The second pumping light source  720  outputs a pumping light having a wavelength of 1,480 nm in order to pump the third erbium-doped fiber  730 . That is, to excite the erbium ions in the third erbium-doped fiber  730 . 
   The third erbium-doped fiber  730  is pumped by the pumping light received from the second pumping light source  720  via the second wavelength selective coupler  710 , thereby amplifying the L-band optical signal component received via the second wavelength selective coupler  710 . 
   The reflecting unit  740  reflects the L-band optical signal component received from the third erbium-doped fiber  730  so that the reflected L-band optical signal component is re-applied to the third erbium-doped fiber  730 . 
   The distributor  790  includes a splitter  680 , a circulator  690 , and a combiner  770 . 
   The splitter  680  splits the optical signal received from the second erbium-doped fiber  660  into respective C and L-band optical signal components, and sends the C-band optical signal component to a fifth path  760  while sending the L-band optical signal component to the circulator  690 . 
   The second isolator  750  allows the C-band optical signal component received from the splitter  680  to pass in a forward direction while blocking light flowing in the opposite direction. 
   The circulator  690  outputs the L-band optical signal component, received at its first port, to a second path  700  through its second port. The circulator  690  simultaneously outputs the L-band optical signal component, received at its second port, to a fourth path  765  through its third port. 
   The combiner  770  combines the L-band optical signal component received via the fourth path  765  with the C-band optical signal component received via the fifth path  760 , and outputs the combined optical signal to a third path  780 . 
   The second amplifying unit  660  comprises the second erbium-doped fiber  660 . 
   The second erbium-doped fiber  660  is pumped by a pumping light received via the gain flattening filter  650 , thereby amplifying the C and L-band optical signal components received from the gain flattening filter  660 . 
   The operation of the wide band fiber amplifier having the above described configuration will now be described. C and L-band 
   Operational Description of the Second Embodiment 
   In operation, when C and L-band optical signal components are inputted to the wide band optical fiber amplifier, the C and L-band signals are first pass through the first isolator  610 , and then the first wavelength selective coupler  620 , after which the C and L-band signals then enter the first erbium-doped fiber  640 . After being amplified in the first erbium-doped fiber  640 , the C and L-band optical signal components are applied to the gain flattening filter  650  which flattens the gain of only the C-band component of the applied optical C and L-band signals. Thereafter, the C and L-band optical signal components are amplified by the second erbium-doped fiber  660 , and then applied to the splitter  680 . This splitter  680  splits the optical signal applied thereto into C and L-band optical signal components respectively, and outputs the C-band optical signal component to the fifth path  760  while outputting the L-band optical signal component to the circulator  690 . The circulator  690  outputs the L-band optical signal, received thereto, through its second port. This L-band optical signal is amplified twice in the third erbium-doped fiber  730  by virtue of the reflecting unit  740 , and thereafter applied to the second port of the circulator  690  via the second wavelength selective coupler  710 . The circulator  690  outputs the L-band optical signal, received at its second port, to the fourth path  765  through its third port. The combiner  770  combines the L-band optical signal received via the fourth path  765  with the C-band optical signal received via the fifth path  760 , and outputs the combined optical signal. 
   Third Embodiment 
     FIG. 4  illustrates the configuration of a wide band optical fiber amplifier according to a third embodiment of the present invention. As shown in  FIG. 4 , the wide band optical fiber amplifier includes first and second isolators  810  and  930 , first and second amplifying units  985  and  995 , a gain flattening filter  940 , a distributor  990 , and a reflecting unit  920 . C and L-band 
   In operation, when the C and L-band optical signal components are inputted to the wide band optical fiber amplifier, the C and L-band signals first pass through the first isolator  810 , and then the first wavelength selective coupler  820 , after which the C and L-band signals enter the first erbium-doped fiber  840 . After being amplified in the first erbium-doped fiber  840 , the C and L-band optical signal components are applied to the splitter  860 . The optical signal applied to the splitter  860  are split into C and L-band optical signal components respectively, whereby the C-band optical signal is output via the fifth path  950  and the L-band optical signal is output to the circulator  870 . The circulator  870  outputs the L-band optical signal, received thereto, through its second port. The L-band optical signal output from the second circulator port is then amplified twice in the second erbium-doped fiber  910  by virtue of the reflecting unit  920 , and then applied to the second port of the circulator  870  via the second wavelength selective coupler  890 . The circulator  870  outputs the twice amplified L-band optical signal, received at its second port, to the fourth path  970  through via the third port of the circulator. The combiner  960  combines the L-band optical signal received via the fourth path  970  with the C-band optical signal received via the fifth path  950 , and outputs the combined optical signal. 
   In conclusion, the wide band optical fiber amplifier according to the present invention provides advantages including making it possible to reduce the power of a pumping light, reducing the length of an erbium-doped fiber, and achieving an enhancement in amplification efficiency by virtue of the signal light passing through an amplifying unit two times as a consequence of a reflecting unit. Moreover, the wide band optical fiber amplifier of the present invention can achieve a minimization in noise figure by reducing the maximum insertion loss of its gain flattening filter by use of the reflecting unit. 
   While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment, but, on the contrary, it is intended to cover various modifications within the spirit and scope of the appended claims.