Abstract:
Disclosed is a dispersion-compensating, Raman optical fiber amplifier used in an optical communications system of the type having an optical transmission block for transmitting wavelength division multiplexed optical signals through a fiber and an optical receiving block for receiving the optical signals through the fiber. The inventive amplifier includes: a circulator for outputting optical signals that are inputted through a first end connected to the fiber to a second end and for outputting optical signals that are reflected through the second end to a third end connected to the fiber; a dispersion-compensating fiber for compensating optical signals passing therethrough and for outputting optical signals after performing a Raman amplification; a pumping light source for pumping the dispersion-compensating fiber and for outputting a pumping light having a designated wavelength; a wavelength selective coupler for outputting the pumping light to the dispersion-compensating fiber; and, a reflector for reflecting the Raman pumped optical signals and for reflecting the optical signals back into the dispersion-compensating fiber.

Description:
CLAIM OF PRIORITY  
         [0001]    This application claims priority to an application entitled, “Dispersion-Compensated Raman Optical Fiber Amplifier,” filed in the Korean Industrial Property Office on Feb. 20, 2002 and assigned Serial No. 02-8955, the contents of which are hereby incorporated by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to optical communications systems and, more particularly, to an optical fiber amplifier disposed between an optical transmission block and an optical receiving block for amplifying signals.  
           [0004]    2. Description of the Related Art  
           [0005]    To keep abreast of the increasing demand for more data, wavelength-division-multiplexing (WDM) optical communication systems have been deployed to meet the increased transmission capacity. In WDM systems, the transmission capacity may be expanded by increasing the number of transmission channels or increasing transmission speed. The transmission speed required in the optical transmission has exponentially grown from 2.5 Gb/s to 10 Gb/s, and it is expected to increase more by newer developments. However, when the transmission speed is greater than 10 Gb/s, the occurrence of dispersion creates serious problems. To this end, a dispersion-compensating fiber (DCF) has been introduced to compensate the dispersion generated during the data transmission. When incorporating the dispersion-compensating fiber, it is necessary to amplify optical signals during transmission in order to compensate for the power loss of optical signals.  
           [0006]    [0006]FIG. 1 is a simplified block diagram illustrating the configuration of a known dispersion-compensating, optical fiber amplifier. As shown in the drawing, the optical fiber amplifier includes a first through fourth isolators  120 ,  160 ,  180 , and  220 ; a first and second pumping light sources  140  and  210 ; a first and second wavelength selective couplers  130  and  200 ; a first and second erbium-doped fibers  150  and  190 ; and, a dispersion-compensating optical fiber  170 .  
           [0007]    In operation, the first isolator  120  passes optical signals inputted in the optical fiber amplifier but blocks (or prevents) backwards-inputted light—for example, the light from the first wavelength selective coupler  130 . The first wave selective coupler  130  couples optical signals from the first isolator  120  with the first pumped light, then outputs them to the first erbium-doped fiber  150 . The first pumping light source  140  pumps the first erbium-doped fiber  150 . For the first pumping light source, a laser diode can be used.  
           [0008]    The first erbium-doped fiber  150  is pumped by the pumping light that is outputted from the first wavelength selective coupler  130 , then outputs the amplified optical signals. The second isolator  160  passes optical signals that are inputted through the first erbium-doped fiber  150  while blocking any backwards-inputted light. The dispersion-compensating optical fiber  170  compensates the optical signal output from the second isolator  160 . The length of the dispersion-compensating, optical fiber  170  is determined in consideration of the transmission distance of the optical signals. Normally, as the transmission distance of the optical signals is increased the degree of dispersion of the optical signals becomes severe. The third isolator  180  passes the optical signal output from the dispersion-compensating fiber  170  and blocks any backwards-inputted light.  
           [0009]    The second erbium-doped fiber  190  is pumped by the pumping light that is inputted through the second wavelength selective coupler  200  and amplifies the optical signal output from the third isolator  180 , then outputs the amplified optical signals. Particularly, the second erbium-doped fiber  190  amplifies the optical signals whose strength has been reduced while passing through the dispersion-compensating fiber  170 . As such, the second wavelength selective coupler  200  couples the optical signal output from the third isolator  180  with the pumping light from the second pumping light source  210  and outputs them to the second erbium-doped fiber  190 . Finally, the fourth isolator  220  passes the optical signal output from the second wavelength selective coupler  200  and blocks any backwards-inputted light.  
           [0010]    As described above, the dispersion-compensating, optical fiber amplifier in the related art increases the total production costs as it uses a highly-priced lengthy dispersion-compensating fiber. Moreover, due to additional power loss of the optical signals, additional amplifying components, such as an erbium-doped fiber, pumping light, or wavelength selective coupler, are needed. Furthermore, to apply the dispersion-compensating amplifier to broadband high-density WDM systems, the amplifier must have a broad and flexible gain bandwidth, small noise figure, and a plurality of isolators for those purposes. This consequently makes the designing process much more complicated and by adding more components the entire production costs of the optical fiber amplifier is increased as well as the volume of the amplifier.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention relates to a dispersion-compensating, Raman optical-fiber amplifier having low production costs and enhanced integration.  
           [0012]    Accordingly, the dispersion-compensating, Raman optical fiber amplifier includes: a circulator for outputting inputted optical signals to a second end through a first end that is connected to the fiber and for outputting reflected optical signals through the second end into a third end that is connected to the fiber; a dispersion-compensating fiber for compensating inputted optical signals and reflected optical signals passing through the circulator, and at the same time for outputting the optical signals after performing a Raman amplification; a pumping means for Raman pumping the dispersion-compensating fiber; and, a reflector for reflecting the Raman pumped optical signals and for reflecting the optical signals back into the dispersion-compensating fiber.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The above features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:  
         [0014]    [0014]FIG. 1 is a schematic diagram of a dispersion-compensating optical fiber amplifier in a related art;  
         [0015]    [0015]FIG. 2 is a schematic diagram of a dispersion-compensating optical fiber amplifier in accordance with a first preferred embodiment of the present invention;  
         [0016]    [0016]FIG. 3 is a schematic diagram of a dispersion-compensating optical fiber amplifier in accordance with a second preferred embodiment of the present invention;  
         [0017]    [0017]FIG. 4 is a schematic diagram of a dispersion-compensating optical fiber amplifier in accordance with a third preferred embodiment of the present invention;  
         [0018]    [0018]FIG. 5 is a schematic diagram of a dispersion-compensating optical fiber amplifier in accordance with a fourth preferred embodiment of the present invention; and, FIG. 6 is a schematic diagram of a dispersion-compensating optical fiber amplifier in accordance with a fifth preferred embodiment of the present invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]    Preferred embodiments of the present invention will be described herein below with reference to the accompanying drawings. For the purpose of clarity and simplicity, well-known functions or constructions are not described in detail as they would obscure the invention in unnecessary detail.  
         [0020]    [0020]FIG. 2 is a schematic diagram of the dispersion-compensating, optical fiber amplifier (also known as a Ramon optical fiber amplifier) in accordance with a first preferred embodiment of the present invention. As depicted in the drawing, the Raman optical fiber amplifier includes an isolator  320 ; a first and second wavelength selective couplers  330  and  370 ; a first and second pumping light sources  340  and  380 ; a first and second erbium-doped fibers  350  and  400 ; a circulator  360 ; a dispersion-compensating fiber  390 ; and, a reflector  410 .  
         [0021]    In operation, the isolator  320  passes optical signals of C-band wavelength bandwidth (Efficiency is the best at 1550 nm) that are inputted into the Raman optical fiber amplifier and prevents backwards-inputted light—that is, the light inputted through the first wavelength selective coupler  330 . The first wavelength selective coupler  330  couples the optical signal output from the isolator  320  to the 980 nm wavelength-pumping light pumped by the first pumping light source  340 , then outputs them to the first erbium-doped fiber  350 . Preferably, a 980/1550 nm wavelength selective coupler is used for the first wavelength selective coupler  330 . The first pumping light source  340  pumps the first erbium-doped fiber  350 . More specifically, the first pumping light source  340  excites erbium ions in the first erbium-doped fiber  350 . A laser diode outputting 980 nm wavelength-pumping light may be used as the pumping light source  340 . The first erbium-doped fiber  350  is pumped by pumping light output from the first wavelength selective coupler  330  and amplifies the optical signal output from the first wavelength selective coupler  330 , then outputs the amplified optical signals.  
         [0022]    The circulator  360  outputs optical signals that have been amplified by the first erbium-doped fiber  350  and inputted through the first end, to the second end, and outputs optical signals inputted through the second end to the third end, which is connected to the fiber  310 .  
         [0023]    Meanwhile, the second wavelength selective coupler  370  couples the optical signal output from the second end of the circulator  360  with the pumping light output from the second pumping light source  380  and then outputs them to the dispersion-compensating fiber  390 . Note that the second wavelength selective coupler  370  also outputs optical signals that are reflected from the dispersion-compensating fiber  390  back into the second end of the circulator  360 . A 1450/1550 nm wavelength selective coupler may be used for the second wavelength selective coupler  370 .  
         [0024]    With continued reference to FIG. 2, the second pumping light source  380  pumps the dispersion-compensating fiber  390  and the second erbium-doped fiber  400 . A laser diode that can output a pumping light with 1450 nm bandwidth may be used as the second pumping light source  380 .  
         [0025]    The dispersion-compensating fiber  390  compensates optical signal output from the second wavelength selective coupler  370  as well as the dispersion of optical signals that are reflected from the second erbium-doped fiber  400 , and at the same time performs Raman amplification on the optical signals before outputting the same. In order to compensate the dispersion corresponding to 80 km of communication, the dispersion-compensating fiber  390  uses a single-mode fiber having a length of 40 km and an insertion loss of 6 dB. In a normal situation where a 80 km dispersion-compensating fiber is required, the present invention can reduce the required length of the dispersion-compensating fiber  390  to be half, as optical signals requiring dispersion compensation pass through the dispersion-compensating fiber  390  to achieve the same effect due to the reflection of signals using the reflector  410 . In addition, the dispersion-compensating fiber  390  performs a Raman amplification on the optical signals that are inputted therethrough. Note that the dispersion-compensating fiber  390  is pumping a 1450 nm wavelength light. Here, the wavelength of the pumping light could be changed depending on the wavelength of the optical signal, and alternatively a designated wavelength-bandwidth pumping light may be implemented.  
         [0026]    The second erbium-doped fiber  400  is pumped by a remaining pumping light output from the dispersion-compensating fiber  390  and serves to amplify the optical signal output from the dispersion-compensating fiber  390 , and finally outputs the amplified optical signals. Moreover, the second erbium-doped fiber  400  amplifies optical signals that are reflected by the reflector  410 . The reflector  410  reflects optical signal output from the second erbium-doped fiber  400  and redirects them back into the second erbium-doped fiber  400 . As for the reflector  410 , a fiber grating that performs a total reflection on the light with a C-band wavelength is preferred.  
         [0027]    [0027]FIG. 3 is a schematic diagram of a dispersion-compensating, optical fiber amplifier in accordance with a second preferred embodiment of the present invention. Similar to before, the Raman optical fiber amplifier includes an isolator  520 , a first and second wavelength selective couplers  530  and  580 , a first and second pumping light sources  540  and  600 , a first and second erbium-doped fibers  550  and  570 , a circulator  560 , a dispersion-compensating fiber  580 , and a reflector  610 . To avoid redundancy, the description for the isolator  520 , the first pumping light source  540 , the first wavelength selective coupler  530 , and the first erbium-doped fiber  550  will be omitted as the construction and operation are essentially the same as that described above with respect to FIG. 2.  
         [0028]    In operation, the circulator  560  receives an amplified signal output from the first erbium-doped fiber  550  via the first end and forwards them to the second end, and at the same time forwards any optical signals received through the second end to the third end, which is connected to the optical fiber  510 .  
         [0029]    The second erbium-doped fiber  570  is pumped by the remaining pumping light inputted through the dispersion-compensating fiber  580 , and at the same time amplifies the optical signal output from the circulator  560 . In addition, the second erbium-doped fiber  570  amplifies optical signals that are reflected and coming from the dispersion compensating fiber  580 . The dispersion-compensating fiber  580  is comprised of a 40 km dispersion-compensating fiber with an insertion loss of 6 dB.  
         [0030]    The second pumping light source  600  pumps the dispersion-compensating fiber  580  and the second erbium-doped fiber  570  via the second wavelength selective coupler  590 . A laser diode is preferred as the pumping light source  600  and outputs the pumping light having a 1450 nm bandwidth. The second wavelength selective coupler  590  couples the optical signals that are reflected by the reflector  610  with the pumping light from the second pumping light source  600 , then outputs them to the dispersion-compensating fiber  580 . At the same time, the second wavelength selective coupler  590  outputs optical signal output from the dispersion-compensating fiber  580  towards the reflector  610 . In the embodiment, a 1450/1550 nm wavelength selective coupler may be used for the second wavelength selective coupler  590 .  
         [0031]    [0031]FIG. 4 is a schematic diagram of a dispersion-compensating, optical fiber amplifier in accordance with a third preferred embodiment of the present invention. The Raman optical fiber amplifier includes a first and second isolators  720  and  820 , a first and second wavelength selective couplers  730  and  780 , a first and second pumping light sources  740  and  790 , a first and a second erbium doped fibers  750  and  810 , a circulator  760 , a dispersion-compensating fiber  770 , and a reflector  800 . To avoid redundancy, the function of the first isolator  720 , the first pumping light source  740 , the first wavelength selective coupler  730 , and the first erbium-doped fiber  750  will be omitted as they are described in the preceding paragraph with respect to FIG. 2.  
         [0032]    In operation, the circulator  760  receives amplified signal output from the first erbium-dope fiber  750  through the first end and outputs them to the second end. At the same time, the circulator  760  outputs optical signals received therein via the second end and forward them to the third end, which is connected to the fiber  710 . The dispersion-compensating fiber  770  compensates optical signal output from the circulator  760  as well as the optical signals that are reflected from the second wavelength selective coupler  780 . At the same time, the dispersion-compensating fiber  770  performs a Raman amplification on the optical signals passing therethrough. The dispersion-compensating fiber  770  is comprised of a 40 km dispersion-compensating fiber, of which insertion loss is 6 dB, and it is pumped by the pumping light outputted from the second wavelength selective coupler  780 . The second pumping light source  790  pumps the dispersion-compensating fiber  770  and the second erbium-doped fiber  810 . A preferred second pumping source  790  is a laser diode that outputs a 1450 nm-wavelength pumping light.  
         [0033]    The second wavelength selective coupler  780  couples optical signals that are reflected from the reflector  800  with the pumping light that is inputted from the second pumping light source  790 , then outputs them to the dispersion-compensating fiber  770 . Moreover, the second wavelength selective coupler  780  outputs optical signals that are inputted from the dispersion-compensating fiber  770  to the reflector  800 . A preferred second wavelength selective coupler  780  is a 1450/1550 nm wavelength selective coupler.  
         [0034]    The reflector  800  reflects optical signals inputted from the second wavelength selective coupler  780  and forward them back into the dispersion-compensating fiber  770 . As for the reflector  800 , a fiber grating that performs a total reflection on the light with a C-band wavelength is used. The second erbium-doped fiber  810  is pumped by the remaining pumping light inputted through the circulator  760  and amplifies optical signals inputted through the circulator  760 , then finally outputs the amplified optical signals. The second isolator  820  passes 1550 nm wavelength-optical signals inputted from the second erbium-doped fiber  810  and blocks backwards-inputted light.  
         [0035]    [0035]FIG. 5 is a schematic diagram of a dispersion-compensating, optical fiber amplifier in accordance with a fourth preferred embodiment of the present invention. The Raman optical fiber amplifier includes a circulator  920 , a first and second wavelength selective couplers  930  and  970 , a first and second pumping light sources  940  and  980 , an erbium-doped fiber  950 , dispersion-compensating fiber  960 , and a reflector  990 .  
         [0036]    In operation, the circulator  920  forwards optical signals that are inputted through the first end to the second end and outputs optical signals that are inputted through the second end to the third end, which is connected to the fiber  910 . The first pumping light source  940  pumps the erbium-doped fiber  950  and the dispersion-compensating fiber  960 . As for the first pumping light source  940 , a laser diode that outputs 1450 nm bandwidth-pumping light can be used. The first wavelength selective coupler  930  couples optical signals inputted through the circulator  920  with the pumping light from the first pumping light source  940 , then outputs them to the erbium doped fiber  950 . At the same time, the first wavelength selective coupler  930  outputs optical signals inputted from the erbium-doped fiber  950  to the circulator  920 . A preferred first wavelength selective coupler  930  is a 1450/1550 nm wavelength selective coupler.  
         [0037]    The erbium-doped fiber  950  is pumped by pumping light that is inputted through the first wavelength selective coupler  930  in a forward direction and the remaining pumping light that is inputted through the dispersion-compensating fiber  960  in a reverse direction. Moreover, the erbium-doped fiber  950  amplifies optical signals that are reflected and outputted from the dispersion-compensating fiber  960 .  
         [0038]    The dispersion-compensating fiber  960  compensates optical signal output from the erbium-doped fiber  950  and from the second wavelength selective coupler  970 . Meanwhile, the dispersion-compensating fiber  960  performs a Raman amplification on the optical signals before outputting the same. The dispersion-compensating fiber  960  is comprised of a 40 km dispersion-compensating fiber, of which insertion loss is 6 dB, and it is pumped by the pumping light that is inputted through the second wavelength selective coupler  970 .  
         [0039]    The second wavelength selective coupler  970  couples optical signals that are reflected from the reflector  900  with the pumping light from the second pumping light source  980  and outputs optical signals inputted from the dispersion-compensating fiber  960  to the reflector  990 . A preferred second wavelength selective coupler  970  is a 1450/1550 nm wavelength selective coupler. The second pumping light source  980  pumps the dispersion-compensating fiber  960 , and a preferred second pumping source  980  is a laser diode that outputs a 1450 nm-wavelength pumping light. The reflector  990  reflects optical signals inputted from the second wavelength selective coupler  970  and forwards them back into the dispersion-compensating fiber  960 . As for the reflector  990 , a fiber grating that performs a total reflection on the light with a C-band wavelength is used.  
         [0040]    [0040]FIG. 6 is a schematic diagram of a dispersion compensating, optical fiber amplifier in accordance with a fifth preferred embodiment of the present invention. The Raman optical fiber amplifier includes a circulator  1020 , a wavelength selective coupler  1030 , a pumping light source  1040 , a dispersion-compensating fiber  1050 , an erbium-doped fiber  1060 , and a reflector  1070 .  
         [0041]    In operation, the circulator  1020  outputs optical signals that are inputted through the first end to the second end and outputs optical signals that are inputted through the second end to the third end connected to the fiber  1010 . The wavelength selective coupler  1030  couples optical signals inputted through the circulator  1020  with the pumping light from the first pumping light source  1040 , then outputs them to the dispersion-compensating fiber  1050 . A preferred wavelength selective coupler  1030  is a 1450/1550 nm wavelength selective coupler. The pumping light source  1040  pumps the dispersion-compensating fiber  1050  and the erbium-doped fiber  1060 . As for the pumping light source  1040 , a laser diode that outputs 1450 nm bandwidth—pumping light can be used.  
         [0042]    The dispersion-compensating fiber  1050  compensates optical signals that are inputted through the wavelength selective coupler  1030  and the dispersion of optical signals that are reflected back through the erbium-doped fiber  1060 . At the same time, the dispersion-compensating fiber  1050  performs a Raman amplification on the optical signals reflected back from the erbium-doped fiber  1060  before outputting the same. The dispersion-compensating fiber  105  is comprised of a 40 km dispersion-compensating fiber, of which insertion loss is 6 dB, and it is pumped by the pumping light that is inputted through the wavelength selective coupler  1030 .  
         [0043]    The erbium-doped fiber  1060  is pumped by the remaining pumping light that is inputted through the dispersion-compensating fiber  1050  and amplifies optical signals inputted through the dispersion-compensating fiber  1050  and optical signals that are reflected by the reflector  1070 . The reflector  1070  reflects optical signals inputted from the erbium-doped fiber  1060  back into the erbium-doped fiber  1060 . As for the reflector  1070 , a fiber grating that performs a total reflection on the light with a C-band wavelength is used.  
         [0044]    In conclusion, the dispersion-compensating, Raman optical fiber amplifier embodying the principles of the present invention can reduce the total production costs and improve integration of the amplifier, by reducing the length of the dispersion-compensating fiber with the use of a circulator and a reflector and by enabling the dispersion-compensating fiber to perform the Raman amplification. In addition, the dispersion-compensating, Raman optical fiber amplifier according to the present invention has other merits; for example, a relatively broad and a flexible gain bandwidth, and a low noise figure can be achieved using the inventive configuration explained herein. Furthermore, the dispersion-compensating, Raman optical fiber amplifier according to the present invention does not require any complicated design or separate components, and as a result thereof, a total production costs and volume of the optical fiber amplifier can be greatly reduced.  
         [0045]    While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.