Abstract:
Disclosed is a gain-controllable wideband optical fiber amplifier for amplifying both C-band and L-band optical signals, which comprises: a first amplifying section configured to (1) be pumped in at least one direction, (2) amplify both C-band and L-band optical signals and (3) output an amplified spontaneous emission; an optical attenuator for attenuating the power of the spontaneous emission; and a second amplifying section pumped by the attenuated spontaneous emission to secondarily amplify the amplified L-band optical signals.

Description:
CLAIM OF PRIORITY 
   This application claims priority to an application entitled “Gain-Controllable Wideband Optical Fiber Amplifier,” filed in the Korean Intellectual Property Office on May 20, 2003 and assigned Serial No. 2003-32061, the contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an optical transmission system, and more particularly to a wideband optical fiber amplifier which amplifies both C-band and L-band optical signals for an optical transmission system. 
   2. Description of the Related Art 
   With the amount of data growing at an explosive rate recently, there is a large demand to broaden the transmission bandwidth of Wavelength Division Multiplexing (WDM) optical transmission systems. Accordingly, research is now active in a wideband transmission system using both C-band having a wavelength in the range of 1530 nm to 1560 nm and L-band having a wavelength in the range of 1568 nm to 1610 nm. In optical transmission systems, an erbium-doped fiber amplifier (EDFA) (a fiber amplifier doped with the rare-earth element erbium) is generally used as an optical fiber amplifier for amplifying optical signals. This amplifier works only over a bandwidth of about 30 nm in the C-band and the L-band. Although a Raman fiber amplifier (RFA) has a sufficiently broad bandwidth to amplify both C-band and L-band optical signals, it requires a high pump power to obtain a desired gain. Therefore, an EDFA is more generally used as a wideband optical fiber amplifier. However, most EDFAs have a parallel structure for separately amplifying C-band optical signals and L-band optical signals. 
     FIG. 1  shows a conventional wideband optical fiber amplifier. Optical fiber amplifier  100  is connected to an external optical fiber  110  and comprises first and second amplifying sections  160  and  170 . The optical fiber amplifier  100  also includes first and second wavelength selective couplers (WSCs)  121  and  122  for connecting first and second amplifying sections  160  and  170  in a parallel structure. 
   First wavelength selective coupler  121  divides optical signals having wavelengths of 1550 to 1590 nm, which are inputted through external optical fiber  110 , into 1550 nm wavelength band (C-band) signals and 1590 nm wavelength band (L-band) signals, and outputs C-band optical signals to first amplifying section  160  and L-band optical signals to second amplifying section  170 . 
   First amplifying section  160  includes first and second optical isolators (ISOs)  131  and  132 , first and second pump laser diodes (LDs)  141  and  142 , third and fourth wavelength selective couplers  123  and  124 , and a first erbium-doped optical fiber  151 . First and second optical isolators  131  and  132  block a light emitted in the backward direction, such as a noise of amplified spontaneous emission (ASE) from first erbium-doped optical fiber  151  and a reflected light. First pump laser diode  141  outputs a first pump light having a 980 nm wavelength. Third wavelength selective coupler  123  outputs the first pump light and the C-band optical signals to first erbium-doped optical fiber  151 . Second pump laser diode  142  outputs a second pump light having a 1480 nm wavelength. Fourth wavelength selective coupler  124  outputs the second pump light to first erbium-doped optical fiber  151  and passes the amplified C-band optical signals. First erbium-doped optical fiber  151  is pumped in both directions by the first and second pump lights, thereby amplifying and outputting the inputted C-band optical signals. 
   Second amplifying section  170  includes third and fourth optical isolators  133  and  134 , third and fourth pump laser diodes  143  and  144 , fifth and sixth wavelength selective couplers  125  and  126 , and a second erbium-doped optical fiber  152 . Third and fourth optical isolators  133  and  134  block light emitted in the backward direction, such as an ASE noise outputted from second erbium-doped optical fiber  152  and a reflected light. Third pump laser diode  143  outputs a third pump light having a 980 nm wavelength. Fifth wavelength selective coupler  125  outputs the third pump light and the L-band optical signals to second erbium-doped optical fiber  152 . Fourth pump laser diode  144  outputs a fourth pump light having a 1480 nm wavelength. Sixth wavelength selective coupler  126  outputs the fourth pump light to second erbium-doped optical fiber  152  and passes the amplified L-band optical signals. Second erbium-doped optical fiber  152  is pumped in both directions by the third and fourth pump lights, thereby amplifying and outputting the inputted L-band optical signals. 
   Second wavelength selective coupler  122  couples the C-band optical signals and L-band optical signals received from first and second amplifying sections  160  and  170 , respectively, and outputs the coupled signals through external optical fiber  110 . 
   In conventional wideband optical fiber amplifiers as explained above, the second amplifying section for amplifying L-band optical signals has a low amplification efficiency and thus requires the second erbium-doped optical fiber to be long. Also, the second amplifying section requires a higher pump power and has a high noise factor in the L-band. In addition, conventional wideband optical fiber amplifiers require a complicated electric control circuit to control amplification gain, because it uses a large number of pump laser diodes. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention has been made to reduce or overcome the above-mentioned problems occurring in the prior art. One object of the present invention is to provide a wideband optical fiber amplifier that has a high amplification efficiency and a low noise factor, and which can easily control an amplification gain. 
   In accordance with the principles of the present invention, a gain-controllable wideband optical fiber amplifier is provided a first and second wavelength band optical signals, comprising: a first amplifying section configured to (1) be pumped in at least one direction, (2) amplify both the first and second wavelength band optical signals and (3) output an amplified spontaneous emission; an optical attenuator for attenuating the power of the amplified spontaneous emission; and a second amplifying section configured to be pumped by the attenuated spontaneous emission to secondarily amplify the amplified second wavelength band optical signals. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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 conventional wideband optical fiber amplifier; and 
       FIG. 2  shows a wideband optical fiber amplifier according to a preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Hereinafter, a preferred embodiment of the present invention will be described with reference to the accompanying drawings. For the purposes of clarity and simplicity, a detailed description of known functions and configurations incorporated herein will be omitted as it may make the subject matter of the present invention unclear. 
   The wideband optical fiber amplifier according to the present invention includes circulators (CIRs) and wavelength selective couplers (WSCs), each comprising a plurality of ports. Supposing that a circulator or a wavelength selective coupler is provided with a particular drawing reference numeral “###”, the n th  port of the circulator or the wavelength selective coupler will be provided and depicted with drawing reference numeral “###n”. 
     FIG. 2  shows the configuration of the wideband optical fiber amplifier according to the preferred embodiment of the present invention. The amplifier  200  includes first and second circulators  221  and  222 , first and second amplifying sections  270  and  290 , an optical attenuator (ATT)  280  and first and second wavelength selective couplers  241  and  242 . 
   First circulator  221  has first to third ports  2211  to  2213 . An optical signal inputted to an upper port is outputted to an adjacent lower port. First port  2211  of first circulator  221  is connected to an external optical fiber  210 . Second port  2212  is connected to the first amplifying section  270 , while third port  2213  is connected to optical attenuator  280 . First circulator  221  outputs optical signal having at least two wavelength band optical signals, such as C-band optical signals having a 1550 nm wavelength and L-band optical signals having a 1590 nm wavelength, which have been inputted to first port  2211 , to second port  2212 . Also, first circulator  221  outputs an amplified spontaneous emission inputted to second port  2212  to third port  2213 . 
   First amplifying section  270  connected to second port  2212  of the first circulator  221  includes a first pump light source  231 , a third wavelength selective coupler  243 , a first amplifying optical fiber  251  and a first optical isolator  260 . 
   First pump light source  231  outputs a first pump light of 980 nm. Laser diodes can be used for the first pump light source and second and third pump light sources. 
   Third wavelength selective coupler  243  has first to third ports  2431  to  2433 . First port  2431  is connected to the second port  2212  of the first circulator  221 . Second port  2432  is connected to the first amplifying optical fiber  251 . Third port  2433  is connected to the first pump light source  231 . Third wavelength selective coupler  243  couples the inputted C-band and L-band optical signals to the first pump light and outputs the coupled signals to first amplifying optical fiber  251 . 
   First amplifying optical fiber  251  is pumped in the forward direction by the first pump light. Also, first amplifying optical fiber  251  outputs an amplified spontaneous emission going in the opposite direction to the optical signals. The amplified spontaneous emission is inputted to second port  2212  of first circulator  221 . First circulator  221  outputs the inputted spontaneous emission to third port  2213 . First amplifying optical fiber  251  can be an erbium-doped optical fiber. 
   First optical isolator  260  is disposed between the first amplifying optical fiber  251  and first wavelength selective coupler  243 . First optical isolator  260  passes the inputted C-band and L-band optical signals, while blocking a light traveling in the backward direction. 
   First wavelength selective coupler  241  has first to third ports  2411  to  2413 . First port  2411  is connected to first optical isolator  260 . Second port  2412  is connected to second wavelength selective coupler  242 . Third port  2413  is connected to second amplifying section  290 . First wavelength selective coupler  241  outputs the C-band optical signals, among the inputted C-band and L-band optical signals, to second port  2412 , and the L-band optical signals to third port  2413 . 
   Optical attenuator  280  is disposed between third port  2213  of first circulator  221  and first port  2221  of a second circulator  222 . Since the transmissivity varies depending on the applied current, optical attenuator  280  transmits the inputted spontaneous emission according to a preset transmissivity. 
   Second circulator  222  has first to third ports  2221  to  2223 . First port  2221  is connected to optical attenuator  280 . Second port  2222  is connected to second amplifying section  290 . Third port  2223  is connected to first port  2421  of second wavelength selective coupler  242 . Second circulator  222  outputs the attenuated spontaneous emission, which has been inputted to first port  2221 , to second port  2222 . Also, second circulator  222  outputs the secondarily-amplified L-band optical signals, which have been inputted to second port  2222 , to third port  2223 . 
   Second amplifying section  290  is disposed between third port  2413  of first wavelength selective coupler  241  and second port  2222  of second circulator  222 . Second amplifying section  290  includes second and third pump light sources  232  and  233 , fourth and fifth wavelength selective couplers  244  and  245 , and a second amplifying optical fiber  252 . 
   Second pump light source  232  outputs a second pump light of 980 nm. 
   Fourth wavelength selective coupler  244  has first to third ports  2441  to  2443 . First port  2441  is connected to second port  2222  of second circulator  222 . Second port  2442  is connected to second amplifying optical fiber  252 . Third port  2443  is connected to second pump light source  232 . Fourth wavelength selective coupler  244  couples the attenuated spontaneous emission to the second pump light and outputs the coupled light to second amplifying optical fiber  252 . Also, fourth wavelength selective coupler  244  outputs the secondarily-amplified L-band optical signals, which have been inputted to second port  2442 , to first port  2441 . 
   Third pump light source  233  outputs a third pump light of 1480 nm. 
   Fifth wavelength selective coupler  245  has first to third ports  2451  to  2453 . First port  2451  is connected to second amplifying optical fiber  252 . Second port  2452  is connected to third port  2413  of first wavelength selective coupler  241 . Third port  2453  is connected to third pump light source  233 . Fifth wavelength selective coupler  245  couples the amplified L-band optical signals to the third pump light and outputs the coupled signals to second amplifying optical fiber  252 . 
   Second amplifying optical fiber  252  is pumped in the backward direction by the attenuated spontaneous emission and the second pump light, while being pumped in the forward direction by the third pump light. Accordingly, second amplifying optical fiber  252  secondarily amplifies and outputs the amplified L-band optical signals. In other words, the L-band optical signals are amplified twice once by each of the first and second amplifying sections  270  and  290 . Second amplifying optical fiber  252  can be an erbium-doped optical fiber. 
   Second wavelength selective coupler  242  has first to third ports  2421  to  2423 . First port  2421  is connected to third port  2223  of second circulator  222 . Second port  2422  is connected to second port  2412  of first wavelength selective coupler  241 . Third port  2423  is connected to external optical fiber  210 . Second wavelength selective coupler  242  couples the secondarily-amplified L-band optical signals, which have been inputted to first port  2421 , to the C-band optical signals, which have been inputted to second port  2422 , and outputs the coupled signals to third port  2423 . 
   Optical attenuator  280  performs the following two functions. 
   It is possible to control the gain of first amplifying section  270  by controlling the power of the first pump power supplied to first amplifying optical fiber  251 . The power of an amplified spontaneous emission outputted from the first amplifying optical fiber  251  is changed with the power variation of the first pump light. If the amplified spontaneous emission is supplied to second amplifying section  290  without any change, the gain of the second amplifying section  290  will be influenced by a change in gain of the first amplifying section  270 . Therefore, the optical attenuator  280  eliminates such an influence. This is the first function of the optical attenuator  280 . 
   While the gain of first amplifying section  270  can be controlled by the control of the power of the first pump light, the gain of second amplifying section  290  can be controlled by the control of transmissivity of optical attenuator  280 . Therefore, it is not necessary to control the second and third pump power sources included in second amplifying section  290  by a complicated process of providing an additional control circuit, setting a new algorithm, or the like. It is possible to easily control the gain of second amplifying section  290  by controlling optical attenuator  280  only. This is the second function of optical attenuator  280 . 
   Wideband optical fiber amplifier  200  pumps second amplifying optical fiber  252  which amplifies L-band optical signals with an amplified spontaneous emission in the C-band, thereby having a higher amplification efficiency. In addition, wideband optical fiber amplifier  200  can greatly reduce the noise factor in the L-band, as compared to conventional optical fiber amplifiers, because it amplifies C-band optical signals and L-band optical signals together through first amplifying optical fiber  251 . 
   As described above, the gain-controllable wideband optical fiber amplifier of the present invention can obtain a higher amplification efficiency by pumping an amplifying optical fiber for amplifying only L-band optical signals with an amplified spontaneous emission in the C-band. At the same time, the wideband optical fiber amplifier of the present invention can reduce the noise factor in the L-band by amplifying C-band and L-band optical signals together through an amplifying optical fiber for pre-amplification. 
   In addition, the gain-controllable wideband optical fiber amplifier according to the present invention can easily control the gain of L-band optical signals by controlling the power of the spontaneous emission inputted to an amplifying optical fiber, which amplifies only L-band optical signals, using an optical attenuator. 
   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.