Abstract:
A Raman optical-fiber amplifier used in a wavelength-division-multiplexing optical communication system is provided. The inventive amplifier includes an optical transmitter for transmitting wavelength-division-multiplexed optical signals through an optical fiber and an optical receiver for receiving the optical signals through the optical fiber, and further comprises an erbium-doped fiber which Raman-amplifies and outputs optical signals inputted through the first end of the erbium-doped fiber amplifier connected with the optical fiber; a pumping source which outputs pump light with a predetermined wavelength so as to Raman-pump the erbium-doped fiber; and, a wavelength-selective coupler which outputs the pump light to be introduced into the erbium-doped fiber.

Description:
CLAIM OF PRIORITY  
         [0001]    This application claims priority to an application entitled “RAMAN OPTICAL FIBER AMPLIFIER USING ERBIUM DOPED FIBER,” filed in the Korean Industrial Property Office on May 17, 2002 and assigned Serial No. 2002-27327, the contents of which are hereby incorporated by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to an optical communication system, and more particularly to a Raman optical-fiber amplifier disposed between an optical transmitter and an optical receiver in an optical communication system.  
           [0004]    2. Description of the Related Art  
           [0005]    Recently, a higher demand for more data has forced the increase of transmission capacity in wavelength-division-multiplexing optical communication systems (WDM optical communication systems). One way to increase transmission capacity is to increase the number of transmission channels or the transmission rate. The transmission rate has now been improved a great deal and ranges from 2.5 Gb/s to 10 Gb/s, but more efforts are being made to further increase the transmission rate. Known methods for increasing the transmission capacity includes the parallel coupling of a conventional C-band erbium-doped fiber amplifier (C-band EDFA) with a L-band erbium-doped fiber amplifier (L-band EDFA), and the use of a new amplification medium, such as a tulium-doped fiber and Rare-earth doped fiber amplifiers, have disadvantages in that the available amplification band is narrow and the noise factor tends to be high. As a solution to these limitations, research efforts have been focusing on the Raman optical-fiber amplifier.  
           [0006]    [0006]FIG. 1 shows the construction of a conventional Raman optical-fiber amplifier. The Raman amplifier comprises a first to a fourth isolator  120 ,  160 ,  180 ,  220 ; a first and a second pumping source  140  and  200 ; a first and a second wavelength-selective coupler  130  and  190 ;  
           [0007]    an erbium-doped fiber  150 ; a connector  170 ; and, a dispersion-compensation fiber 210.  
           [0008]    The first isolator  120  permits optical signals inputted into the Raman optical-fiber amplifier to pass without filtration while shutting out backward light—i.e., light emitted from the first wavelength-selective coupler  130 . The first wavelength-selective coupler  130  couples optical signals emitted from the first isolator  120  and pumps light with a 980 nm or 1,480 nm wavelength emitted from the first pumping source  140 , then outputs them to be introduced into the erbium-doped fiber  150 .  
           [0009]    The first pumping source  140  forward pumps the first erbium-doped fiber  150  by exciting erbium ions. A laser diode that outputs pump light with a 980 nm or 1,480 nm wavelength can be used as the first pumping source  140 . As such, the erbium-doped fiber  150  is forward pumped by pump light emitted from the first wavelength-selective coupler  130  and amplifies and outputs optical signals emitted from the first wavelength-selective coupler  130 . The second isolator  160  permits optical signals emitted from the erbium-doped fiber  150  to pass without filtration while shutting out backward light.  
           [0010]    The connector  170  serves to connect the erbium-doped fiber amplification portion  230  at the front stage thereof and the Raman optical-fiber amplification portion  240  at the back stage thereof—i.e., an optical fiber  110  connected with the erbium-doped fiber amplification portion  230  and an optical fiber  110  connected with the Raman optical-fiber amplification portion  240 . The connector  170  is provided with a circular hall therein.  
           [0011]    The third isolator  180  permits optical signals emitted from the connector  170  to pass without filtration while shutting out backward light. The second wavelength-selective coupler  190  couples optical signals emitted from the third isolator  180  and Raman pump light emitted from the second pumping source  200  then outputs them to be introduced into the dispersion-compensation fiber  210 . The second pumping source  200  Raman-pumps the dispersion-compensation fiber  210 . A laser diode that outputs Raman pump light with a 1,450 nm band wavelength can be used as the second pumping source  200 . The fourth isolator  220  permits optical signals emitted from the connector  170  to pass without filtration while shutting out backward light.  
           [0012]    As seen from the above description, the conventional optical-fiber amplifier comprises two amplification portions  230  and  240 —i.e., the erbium-doped fiber amplification portion 230 at the front stage of the connector  170  and the Raman optical-fiber amplification portion  240  at the back stage of the connector  170 . For this reason, price competitiveness is lowered due to the requirement of multiple optical components. In addition, the increased total volume causes poor integration.  
         SUMMARY OF THE INVENTION  
         [0013]    Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a dispersion-compensated, Raman optical-fiber amplifier, in which the total manufacturing cost is inexpensive and the degree of integration is simplified compared to the prior art.  
           [0014]    In accordance with one aspect of the present invention, provision of a Raman optical-fiber amplifier used in a wavelength-division-multiplexing optical communication system is provided and includes an optical transmitter and an optical receiver, wherein the optical transmitter transmits wavelength-division-multiplexed optical signals through an optical fiber and the optical receiver receives the optical signals through the optical fiber. The Raman optical-fiber amplifier comprises an erbium-doped fiber for amplification and outputs optical signals inputted through the first end of the erbium-doped fiber; a pumping source that outputs pump light with a predetermined wavelength so as to Raman-pump the erbium-doped fiber; and, a wavelength-selective coupler that outputs the pump light to be introduced into the erbium-doped fiber.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The above features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0016]    [0016]FIG. 1 is a view showing the construction of a conventional optical-fiber amplifier;  
         [0017]    [0017]FIG. 2 is a view showing the construction of a Raman optical-fiber amplifier using an erbium-doped fiber in accordance with a first preferred embodiment of the present invention;  
         [0018]    [0018]FIG. 3 is a view showing the construction of a Raman optical-fiber amplifier using an erbium-doped fiber in accordance with a second preferred embodiment of the present invention; and,  
         [0019]    [0019]FIG. 4 is a view showing the construction of a Raman optical-fiber amplifier using an erbium-doped fiber in accordance with a third preferred embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    In the following description, for purposes of explanation rather than limitation, specific details are set forth such as the particular architecture, interfaces, techniques, etc., in order to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments, which depart from these specific details. For purposes of simplicity and clarity, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail.  
         [0021]    [0021]FIG. 2 shows the construction of a Raman optical-fiber amplifier using an erbium-doped fiber in accordance with a first preferred embodiment of the present invention. The Raman optical-fiber amplifier comprises a first and a second isolator  320  and  360 , an erbium-doped fiber  330 , a wavelength-selective coupler  340 , and a pumping source  350 .  
         [0022]    In operation, the first isolator  320  permits optical signals inputted into the optical-fiber amplifier to pass without filtration while shutting out the backward light—i.e., light emitted from the erbium-doped fiber  330 . Then, the erbium-doped fiber  330  Raman-amplifies and outputs optical signals emitted from the wavelength-selective coupler  340 . The erbium-doped fiber  330  is backwardly Raman-pumped by pump light with a 1,450 nm wavelength in case the optical signals with a 1,550 nm wavelength are passed there-through, and by pump light with a 1,480 nm wavelength in case the optical signals with a 1,580 nm wavelength are passed there-through. That is, the erbium-doped fiber  330  is backwardly Raman-pumped by pump light with a wavelength depending on the wavelength of the optical signals. In the case that the optical signals have channels of different wavelengths, the erbium-doped fiber can be backwardly Raman-pumped by pump light with different wavelengths. The intensity of pump light according to the wavelengths is preferably adjusted to output optical signals depending on the channels.  
         [0023]    The erbium-doped fiber  330  has several km lengths to obtain a sufficient Raman gain and has a pump light absorption rate of less than 1.0 dB/km by setting an erbium concentration to a sufficiently low level. Where the erbium-doped fiber  330  with the above properties is backwardly Raman-pumped, amplification by erbium ions, one constituent of the erbium-doped fiber  330  (an amplification principle of a conventional erbium-doped fiber amplifier) and amplification by the vibration energy of silica, another constituent of the erbium-doped fiber  330  (an amplification principle of a Raman optical-fiber amplifier) are generated at the same time.  
         [0024]    A Raman gain in a narrow wavelength band behaves according to the following equation 1:  
                       G        (   λ   )       =     exp        (         g        (   λ   )         A   eff            P   p          L   eff       )         ,             L   eff     =       1     α   p            (     1   -            -     α   p          L         )                     Equation                 1                               
 
         [0025]    wherein, g(A) represents a Raman-gain coefficient of an amplification medium, P p  represents a pump light power, A eff  represents an effective area of the pump light, L eff  represents an effective length of an amplification medium, L represents the total length of an amplification medium, and α p  represents a loss value.  
         [0026]    As seen from Equation 1, provided that g(λ), P p  and L eff  are constant, a Raman gain is inversely proportional to A eff . The smaller the core of an amplification medium to be used (the core of the optical fiber), the smaller the A eff . As a result, a Raman gain is increased.  
         [0027]    Generally, A eff  of a communication optical fiber (monomode optical fiber) is 70 μm 2 , A eff  of a dispersion-transition fiber is 50 μm 2 , A eff  of a dispersion-compensation fiber is 20 μm 2 , and A eff  of an erbium-doped fiber is 15 to 20 μm 2 . In this respect, where an erbium-doped fiber is used as an amplification medium, a similar Raman gain to that of a dispersion-compensation fiber used as the amplification medium of a Raman optical-fiber amplifier can be obtained.  
         [0028]    The wavelength-selective coupler  340  permits optical signals emitted from the erbium-doped fiber  330  to pass without the filtration and outputs pump light emitted from the pumping source  350  to be introduced into the erbium-doped fiber  330 .  
         [0029]    The pumping source  350  backwardly Raman-pumps the erbium-doped fiber  330 . A laser diode which outputs pump light with a predetermined wavelength can be used as the pumping source  350 . The pump light may have wavelengths in the 1,425 nm to 1,520 nm range to cover the total amplification band (1,525 m to 1,620 nm) of the erbium-doped fiber  330 . Lastly, the second isolator  360  permits optical signals emitted from the wavelength-selective coupler  340  to pass without the filtration while shutting out backward light.  
         [0030]    [0030]FIG. 3 is a view showing the construction of a Raman optical-fiber amplifier using an erbium-doped fiber in accordance with a second preferred embodiment of the present invention. The Raman optical-fiber amplifier comprises a first and a second isolator  420  and  460 , an erbium-doped fiber  450 , a wavelength-selective coupler  430 , and a pumping source  440 . The construction of the Raman optical-fiber amplifier is the same as that shown in FIG. 2 except that the wavelength-selective coupler  430  and the pumping source  440  are disposed between the erbium-doped fiber  450  and the first isolator  420  to Raman-pump the erbium-doped fiber  450  in a forward direction. Therefore, a description of the construction, of the Raman optical-fiber amplifier shown in FIG. 3 will be omitted to avoid redundancy.  
         [0031]    [0031]FIG. 4 is a view showing the construction of a Raman optical-fiber amplifier using an erbium-doped fiber in accordance with a third preferred embodiment of the present invention. The Raman optical-fiber amplifier comprises a first and a second isolator  520  and  580 ; an erbium-doped fiber  550 ; a first and a second wavelength-selective coupler  530  and  560 ; and, a first and a second pumping source  540  and  570 .  
         [0032]    The first isolator  520  permits optical signals inputted into the optical-fiber amplifier to pass without the filtration while shutting out backward light—i.e., light emitted from the wavelength-selective coupler  530 . The erbium-doped fiber  550  Raman-amplifies and outputs optical signals emitted from the wavelength-selective coupler  530 . The erbium-doped fiber  550  has several km lengths to obtain a sufficient Raman gain and has a pump-light absorption rate of less than 1.0 dB/km by setting an erbium concentration to a sufficiently low level. The first wavelength-selective coupler  530  couples optical signals emitted from the first isolator  520  and pump light emitted from the first pumping source  540 , and outputs them to be introduced into the erbium-doped fiber  550 . The first pumping source  540  forward Raman-pumps the erbium-doped fiber  550 . The pump light may have wavelengths in the 1,425 nm to 1,520 nm range to cover the total amplification band (1,525 nm to 1,620 nm) of the erbium-doped fiber  550 . The second wavelength-selective coupler  560  permits optical signals emitted from the erbium-doped fiber  550  to pass without the filtration and outputs pump light emitted from the second pumping source  570  to be introduced into the erbium-doped fiber  550 . The second pumping source  570  backward Raman-pumps the erbium-doped fiber  550 . The pump light may have wavelengths in the 1,425 nm to 1,520 nm range to cover the total amplification band (1,525 nm to 1,620 nm) of the erbium-doped fiber  550 . The second isolator  580  permits optical signals emitted from the second wavelength-selective coupler  560  to pass without the filtration while shutting out backward light.  
         [0033]    As apparent from the above description, the Raman optical-fiber amplifier according to the present invention combines a conventional erbium-doped fiber amplification portion and a conventional Raman optical-fiber amplification as one unit. Therefore, a low-cost amplifier can be fabricated and improved integration can be accomplished due to a reduced number of construction devices. Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions, and substitutions are possible without departing from the scope and spirit of the invention as disclosed in the accompanying claims.