Abstract:
Disclosed wideband optical fiber amplifier amplifies and outputs wideband optical signal containing C-band and L-band optical signals. The amplifier includes: a first amplification section amplifying the wideband optical signals; a second amplification section amplifying the separated L-band optical signals amplified by the first amplification section; an optical signal coupler combining and outputting the optical signals amplified by the first and second amplification sections; and an optical circulator. A first port of the optical circulator inputs the wideband optical signals from the optical communication network; a second port outputs the inputted wideband optical signals and inputs spontaneous emissions generated from the first amplification section; a third port provides the spontaneous emissions to the second amplification section while inputting the L-band optical signals amplified by the second amplification section; and a fourth port outputs L-band optical signals that were inputted to the third port to the optical signal coupler.

Description:
CLAIM OF PRIORITY 
   This application claims priority to an application entitled “Wideband Optical Fiber Amplifier” filed in the Korean Intellectual Property Office on Sep. 18, 2002 and assigned Ser. No. 2002-56984, the contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a wideband optical fiber amplifier. More particularly, the present invention is directed to a wideband optical fiber amplifier for amplifying optical signals transmitted from an optical communication network under a predetermined condition, thereby compensating for a loss of optical signals or a reduction of optical power. 
   2. Description of the Related Art 
   A conventional optical fiber amplifier, used in an optical communication system, is a device for amplifying various transmitted optical signals. The optical fiber amplifier amplifies optical signals as they are without photoelectric transformation. This allows for a simple, economical construction. The optical fiber amplifier includes an optical fiber doped with at least one rare-earth element, a pumping diode for generating pumping lights, a wavelength selection coupler for providing combination of transmitted optical signals with the pumping lights to the doped optical fiber, and an optical isolator. 
   Various rare-earth elements dopes for the optical fiber may be used, e.g., erbium (Er), praseodymium (Pr), neodymium (Nd) and the like. 
   Optical amplification using the rare-earth element doped optical fiber is carried out through a stimulated emission process. Pumping lights emitted from a pumping diode carry out exciting and ionizing of a rare-earth element doped into optical fiber. As a result, optical signals transmitted into the rare-earth element doped optical fiber are amplified by stimulated emission of the excited ions. 
   With the gradual increase in the amounts of data transmitted through optical communication systems, the demand to broaden transmission bandwidths of optical communication networks has increased. In response to this demand, wideband optical fiber amplifiers, for a wavelength division multiplex in optical communication systems, which can simultaneously make use of C-band optical signals ranged from 1530 nm to 1560 nm and L-band optical signals ranged from 1570 nm to 1600 nm are commonly employed. 
     FIG. 1  shows a construction of a conventional wideband optical fiber amplifier  100 . The wideband optical fiber amplifier  100  includes a C/L splitter  110 , a three-port optical circulator  111 , first and second amplification sections  130  and  140 , optical isolators  113 ,  121  and  125 , and first and second optical signal couplers  115  and  123 . 
   The C/L splitter  110  separates input wideband optical signals, which have been transmitted from an optical communication network to the wideband optical fiber amplifier  100 , into C-band optical signals and L-band optical signals The separated C-band optical signals and L-band optical signals are then transmitted to the first amplification section  130  and the second amplification section  140 , respectively. 
   The three-port optical circulator  111  is provided with a first port, a second port and a third port. The C-band optical signals are received through the first port. The received C-band optical signals are output through the second port to the first amplification section  130 The second port may also receive amplified spontaneous emissions (ASEs) which are generated from the first amplification section  130 . The received ASEs are output through the third port to the second optical coupler  123 . The optical circulator  111  also prevents spontaneous emissions input through the second port or reflected optical signals from being reversely output through the first port, and thus functions as an optical isolator. 
   The first amplification section  130  includes a first pumping diode  131 , a first wavelength selection coupler  132  and a rare-earth element doped optical fiber  133 , acting to amplify the C-band optical signals inputted from the optical circulator  111 . Pumping lights generated from the first pumping diode  131  are combined with the C-band optical signals at the first wavelength selection coupler  132  and then input into the rare-earth element doped optical fiber  133 . A rare-earth element doped into the rare-earth element doped optical fiber  133  is excited by the pumping lights and then amplifies the C-band optical signals under a stimulated emission. The first pumping diode  131  may use a laser diode that outputs pumping lights having a wavelength band of about 980 nm or 1480 nm. Spontaneous emission generated during an optical signal amplification of the first amplification section  130  are input into the second port of the optical circulator  111  and then directed through the third port of the optical circulator  111  to the second amplification section  140 . These spontaneous emissions generated from the first amplification section  130  are supplied as pumping lights for the second amplification section  140 . 
   The C-band optical signals amplified by the first amplification section  130  are then input through the optical isolator  113  into the first optical signal coupler  115 . 
   The L-band optical signals separated by the C/L splitter  110  are input through the optical isolator  121  and the second optical signal coupler  123  into the second amplification section  140 . The separated L-band optical signals are then amplified by the second amplification section  140 . 
   The second optical coupler  123  combines the separated L-band optical signals with the spontaneous emissions input from the optical circulator  111 , and then inputs the combined results into the second amplification section  140 . As mentioned above, the spontaneous emissions generated from first amplification section  130  are supplied as pumping lights for the second amplification section  140 . 
   The second amplification section  140  includes second and third pumping diodes  141   a  and  141   b,  second and third wavelength selection couplers  143   a  and  143   b,  and a rare-earth element doped optical fiber  145 , acting to amplify the separated L-band optical signals. 
   Pumping lights generated from the second and third pumping diodes  141   a  and  141   b  are input through each of the wavelength selection couplers  143   a  and  143   b  into the rare-earth element doped optical fiber  145 . As noted above, the spontaneous emissions generated from the first amplification section  130  are used as pumping lights for amplifying the L-band optical signals. A rare-earth element doped into the rare-earth element doped optical fiber  145  is excited by the pumping lights and then amplifies the L-band optical signals under a stimulated emission. Each of the second and third wavelength couplers  143   a  and  143   b  is provided on the corresponding input and output sides of the second amplification section  140 , inputting the pumping lights into the rare-earth element doped optical fiber  145  from both sides of the second amplification section  140 . 
   The L-band optical signals amplified by the second amplification section  140  are input through the optical isolator  125  into the first optical signal coupler  115 . 
   The first optical coupler  115  combines the C-band optical signals and the L-band optical signals amplified by each of the fist and second amplification sections  130  and  140 , respectively, and then outputs the combined results to the optical communication network (not shown). 
   However, as mentioned above, since the conventional wideband optical fiber amplifier uses an additional optical signal coupler to separate wideband optical signals input first from the optical communication network into C-band optical signals and L-band optical signals and then to amplify each of the separated two band optical signals, it has various problems. For example, a noise figure as well as an insertion loss is increase. In addition, the cost of manufacture is increased due to the increased number of components to be constructed. 
   Accordingly, there is an need in the art for an improved optical signal amplification system. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and an object of the present invention is to provide a wideband optical fiber amplifier for improving optical signal amplification quality by lowering a noise figure and an insertion loss. 
   An other object of the present invention is to provide a wideband optical fiber amplifier for saving manufacturing expenses by having a reduced number of constructed components. 
   In one embodiment, a wideband optical fiber amplifier for amplifying and outputting wideband optical signals is provided, having C-band optical signals and L-band optical signals, which are input from an optical communication network. The amplifier includes; 
   a first amplification section for amplifying and outputting wideband optical signals consisting of C-band optical signals and L-band optical signals and for outputting amplified spontaneous emissions generated in process of amplifying the wideband optical signals; 
   a C/L splitter for separating the wideband optical signals amplified by the first amplification section into C-band optical signals and L-band optical signals and then outputting the separated optical signals; 
   a second amplification section for amplifying and outputting the L-band optical signals outputted form the C/L splitter; 
   an optical signal coupler for combining the C-band optical signals output from the C/L splitter with the L-band optical signals amplified by the second amplification section and then outputting the combined optical signals; and 
   an optical circulator having a first port for causing the wideband optical signals from the optical communication network to be inputted, a second port for causing the inputted wideband optical signals to be outputted and for causing spontaneous emission generated form the first amplification section to be inputted, a third port for causing the spontaneous emissions to be provided as pumping lights for the second amplification section and for causing the L-band optical signals amplified by the second amplification section to be inputted and a fourth port for causing the L-band optical signals inputted into the third port to be outputted to the optical signal coupler. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  shows a construction of a conventional wideband optical fiber amplifier; 
       FIG. 2  shows a construction of a wideband optical fiber amplifier according to a first embodiment of the present invention; and 
       FIG. 3  shows a construction of a wideband optical fiber amplifier according to a second embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying 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  shows a construction of an wideband optical fiber amplifier  200  according to a first embodiment of the present invention. As shown in  FIGS. 2 , the wideband optical fiber amplifier  200  includes a four-port optical circulator  210 , first and second amplification sections  230  and  240 , an optical isolator  211 , a C/L splitter  213  and an optical signal coupler  215 . 
   The four-port optical circulator  210  has a first port, a second port, a third port and a fourth port. Wideband optical signals input through the first port from an optical communication network (not shown) are output through the second port to the first amplification section  230 . The optical circulator  210  also causes amplified spontaneous emissions (ASEs), which are generated in the optical signal amplification process of the first amplification section  230  that are input through the second port, to be output to the third port. L-band optical signals (as discussed below) which are amplified by the second amplification section  240  that are input through the third port to be output to the fourth port. Accordingly, the optical circulator  210  is designed so that wideband optical signals input through the first port are output to the second port, spontaneous emissions input through the second port are output to the third port, and the L-band optical signals input through the third port are output to the fourth port. At the same time, the optical circulator  210  blocks optical signals each of which are transmitted reverse to its advancing direction to reduce amplification efficiency, thereby functioning as an optical isolator. 
   The first amplification section  230  includes a first pumping diode  231 , a first wavelength selection coupler  232  and a rare-earth element doped optical fiber  233 , that function to amplify wideband optical signals input from the optical circulator  210 . Pumping lights generated from the first pumping diode  231  are combined with the wideband optical signals at the first wavelength selection coupler  232  and then input into the rare-earth element doped optical fiber  233 . A rare-earth element doped into the rare-earth element doped optical fiber  133  is excited by the pumping lights and then amplifies the wideband optical signals in process of a stimulated emission. A laser diode which outputs pumping lights having a wavelength of about 980 nm or 1480 mn may be used as the first pumping diode  231  mentioned above, Spontaneous emissions generated in the optical signal amplification process of the first amplification section  230  are input into the second port of the optical circulator  210  and then directed through the third port of the optical circulator  210  to the second amplification section  240 . These spontaneous emissions generated from the first amplification section  130  are supplied as pumping lights for the second amplification section  240 . 
   The wideband optical signals amplified by the first amplification section  230  are input through the optical isolator  211  into the C/L splitter  213 . 
   The optical isolator  211  blocks optical signals advancing opposite to the direction in which the wideband optical signals advance between the first amplification section and the C/L splitter  213 . For example, the optical isolator  211  allows optical signals input into the second port of the optical isolator  211  to be directed to the third port, but prevents optical signals input into the second port of the optical isolator  211  from being directed to the first port. 
   The C/L splitter  213  separates amplified optical signals into C-band optical signals and L-band optical signals to output the separated optical signals. The C-band optical signals separated by the C/L splitter  213  are input into the optical signal coupler  215 , while the L-band optical signals are input into the second amplification section  240 . 
   The second amplification section  240  includes a second pumping diode  241 , a second wavelength selection coupler  242  and a rare-earth element doped optical fiber  243 , amplifying the L-band optical signals separated by and inputted from the C/L splitter  213 . The spontaneous emissions generated from the first amplification section  230  as well as pumping lights generated from the second pumping diode  241  are input through the second wavelength selection coupler  242  into the rare-earth element doped optical fiber  243 . A rare-earth element doped into the rare-earth element doped optical fiber  243  is excited by the spontaneous emissions and the pumping lights, and then amplifies the L-band optical signals in a stimulated emission process. It should be understood that the spontaneous emissions generated from the first amplification section  230  are input into the second amplification section  240  in a direction opposite to that in which the L-band optical signals advance. The pumping lights generated from the second pumping diode  241  are also input in a direction opposite to that in which the L-band optical signals advance. In that regard, the pumping lights input into the rare-earth element doped optical fiber  243  can be input in a direction identical or opposite to the direction in which the amplified optical signals advance. All of the above is also true to the first amplification section  230 . 
   The L-band optical signals amplified by the second amplification section  240  are input into the third port of the optical circulator  210  and then are directed through the fourth port of the optical circulator  210  to the optical signal coupler  215 . 
   The optical signal coupler  215  combines the C-band optical signals and the L-band optical signals amplified by each of the fist and second amplification sections  230  and  240 , respectively and then causes the combined results to be outputted to the optical communication network. 
   Consequently, the wideband optical signals input into the optical circulator  210  are primarily amplified by the first amplification section  230 , C-band optical signals and L-band optical signals are separated, and then secondarily amplified by the second amplification section  240 . The spontaneous emissions generated from the first amplification section  230  are input through the optical circulator  210  into the second amplification section  240  in a direction opposite to that in which the L-band optical signals advance, thereby acting as pumping lights. 
   As should be appreciated, the wideband optical fiber amplifier according to first embodiment of the present invention uses the four-port optical circulator, so that the number of components such as the optical signal coupler, the optical isolator, etc., can be decreased. 
     FIG. 3  shows a construction of a wideband optical fiber amplifier  300  according to a second embodiment of the present invention. The wideband optical fiber amplifier  300  includes a four-port optical circulator  310 , first and second amplification sections  330  and  340 , an optical isolator  311 , a C/L splitter  313  and an optical signal coupler  315 . 
   With respect to amplification of the wideband optical signals, the wideband optical fiber amplifier  300  according to this embodiment is similar to that of the wideband optical fiber amplifier  200 discussed above, in which the wideband optical signals are input into the optical circulator  310 , initially amplified by the first amplification section  330 , and separated into C-band optical signals and L-band optical signals, and then only L-band optical signals are amplified again by the second amplification section  340 . Further, the spontaneous emissions generated from the first amplification section  330  are input through the optical circulator  310  into the second amplification section  340  in a direction opposite to that in which the L-band optical signals advance, thereby acting as pumping lights. 
   However, the first and second amplification sections  330  and  340  of the second embodiment are different from that of the first embodiment in each construction. 
   The first amplification section  330  includes first and second pumping diodes  331   a  and  331   b,  first and second wavelength selection couplers  333   a  and  333   b  and a rare-earth element doped optical fiber  335 . The first pumping diode  331   a  generates pumping lights for amplifying the wideband optical signals input into the first amplification section  330  and supplies the pumping lights to an input side of rare-earth element doped optical fiber  335  via the first wavelength selection coupler  333   a.  The second pumping diode  331   b  generates pumping lights for amplifying the wideband optical signals and supplies the pumping lights to an output side of rare-earth element doped optical fiber  335  via the second wavelength selection coupler  333   b.    
   The second amplification section  340  includes third and fourth pumping diodes  341   a  and  341   b,  third and fourth wavelength selection couplers  343   a  and  343   b  and a rare-earth element doped optical fiber  345 . The third pumping diode  341   a  generates pumping lights for amplifying the wideband optical signals input into the second amplification section  340  and supplies the pumping lights to an input side of the rare-earth element doped optical fiber  345  via the third wavelength selection coupler  343   a.  The fourth pumping diode  341   b  generates pumping lights for amplifying the wideband optical signals and supplies the pumping lights to an output side of the rare-earth element doped optical fiber  345  via the fourth wavelength selection coupler  343   b.  In addition, the second amplification section  340  uses the spontaneous emissions, which are generated from the first amplification section  330  and then input through the optical circulator  310 , as pumping lights. On an output side of the second amplification section  340 , the spontaneous emissions are input through the third wavelength selection coupler  343   b  into the rare-earth element doped optical fiber  345 . 
   Referring now to the first embodiment shown in  FIG. 2 , it should be understood that pumping lights in the first amplification section  230  are input in same direction as the direction of advancement of the optical signals on the input side of the rare-earth element doped optical fiber  233 , but pumping lights in the second amplification section  240  are inputted in a direction opposite to the direction of advancement of the optical signals on the output side of the rare-earth element doped optical fiber  233 . 
   Referring now to the second embodiment shown in  FIG. 3 , it should be understood that pumping lights in the respective first and second amplification sections  330  and  340  are input on the respective input and output sides of the rare-earth element doped optical fibers  335  and  345 . 
   In this regard, it is possible for pumping lights to excite at least one rare-earth element to be input in a direction identical or opposite to the direction of advancement of the optical signals or in both directions. Moreover, when the pumping lights are input in both directions, as in the second embodiment, the optical fiber amplifiers provide an increased amplification output. 
   As mentioned above, the wideband optical fiber amplifier according to the second embodiment of the present invention not only has a decreased number of components, including the optical signal coupler, the optical isolator, etc., by employment of the four-port optical circulator, but also creates increased output from the optical fiber amplifier by using the bilateral transmission pumping lights. 
   While the invention has been shown and described with reference to certain preferred embodiments 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. 
   As seen form the above, the wideband optical fiber amplifier according to preferred embodiments of the present invention receives wideband optical signals from the optical communication network and at the same time functions as an optical isolator in the process of optical signal amplification using the four-port optical circulator, so that the number of components such as the optical signal coupler, the optical isolator, etc., can be decreased. Therefore, both a noise figure and an insertion loss depending on the use of a plurality of components can be improved, expenses for manufacturing the wideband optical fiber amplifier can be saved, and the dimensions of the wideband optical fiber amplifier can be reduced.