Patent Abstract:
A wideband erbium-doped optical fiber amplifier is disposed among an optical fiber through which a first and second band-band optical signals (for example, the C-band and L-band) are transmitted and forms a first optical path and a second optical path parallel to each other. The wideband erbium-doped optical fiber amplifier comprising a first amplifying section disposed on the first optical path, including a first erbium-doped optical fiber to amplify the first-band optical signals, a filter to gain-flatten the amplified first-band optical signals, wherein a reflected portion of the first band optical signal by the filter is directed to the second optical path; and a second amplifying section disposed on the second optical path, having a second erbium-doped optical fiber to amplify received second-band optical signals, wherein the reflected first-band optical signal is used to pump the second erbium-doped optical fiber.

Full Description:
CLAIM OF PRIORITY 
   This application claims priority to an application entitled “Gain-flattened wideband erbium-doped optical fiber amplifier,” filed in the Korean Intellectual Property Office on May 17, 2003 and assigned Ser. No. 2003-31402, 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 fiber amplifier, and more particularly to a wideband erbium-doped optical fiber amplifier for amplifying C-band and L-band optical signals. 
   2. Description of the Related Art 
   An optical fiber amplifier is an apparatus used in an optical transmission system to amplify optical signals without optoelectric conversion. Accordingly, the optical fiber amplifier has a simple and economic construction. Such an optical fiber amplifier includes (1) a gain medium optical fiber, (2) a pumping light source necessary in optical pumping, (3) a wavelength division multiplexing (WDM) optical coupler for coupling an optical signal and pumping light to the gain medium optical fiber, and (4) an optical isolator for passing forward light and intercepting backward light. 
   The optical signal is amplified through an induced discharge of rare-earth elements, such as erbium, added to the gain medium optical fiber. Specifically, the pumping light excites the rare-earth element ions added to the gain medium optical fiber. Thereafter, the optical signal incident to the gain medium optical fiber is amplified through the induced discharge of the excited ions. In current ultrahigh speed WDM optical transmission systems, a wavelength band of 1.55 μm is widely used along with erbium-doped optical fiber amplifiers suitable for amplifying such a wavelength band. WDM optical technology is capable of simultaneously transmitting a plurality of channels with different wavelengths using a single-core optical fiber. WDM optical technology, researches are actively seeking wider transmission bands, for example by using optical signals not only the C-band, having a wavelength band of 1525 to 1565 nm, but also the L-band, having a wavelength band of 1570 to 1610 nm. In particular, researchers are seeking a wideband erbium-doped optical fiber amplifier (which is one of core elements of a WDM optical communication system) that can amplify not only C-band optical signals but also L-band optical signals. 
   A typical C-band erbium-doped optical fiber amplifier utilizes a population inversion of 70 to 100%. This produces non-uniform gain characteristics (according to wavelengths) for the C-band erbium-doped optical fiber amplifier. Usually, the C-band erbium-doped optical fiber amplifier has the highest gain at a wavelength of 1530 nm and has the lowest gain at a wavelength of 1560 nm. Various gain-flattening methods are used, since the C-band erbium-doped optical fiber amplifier has non-uniform gain characteristics. Conventional gain-flattening methods include a method employing an optical filter, a method employing a Fabry-Perot filter, a method employing a Mach-Zender interferometer, a method employing a dielectric thin film, and a method employing an optical Fiber Bragg Grating (FBG), etc. In such gain flattening methods, a filter designed to have a loss spectrum that is opposite to the gain spectrum of the C-band erbium-doped optical fiber amplifier is used, thereby obtaining a uniform gain regardless of wavelengths. Among the various gain flattening methods described above, the method employing an optical Fiber Bragg Grating is generally utilized. 
   An optical fiber grating is an optical fiber element having optical fiber cores each of which has a periodically changing refractivity. They either reflect or eliminate optical signals (channels) of specific wavelengths from multi-wavelength optical signals incidented to the optical fiber grating. Optical fiber gratings may be classified into long period (reflection type) and short period (elimination type) optical fiber gratings. In the short period optical fiber grating, optical fiber cores have a refractivity changing in a period of several hundreds nanometers (which is generally called “grating period”). Optical fiber mode coupling occurs between a forward mode and a backward mode, thereby reflecting only a channel of a specific wavelength from an incidented multi-wavelength optical signal. In contrast, in the long period optical fiber grating, a grating period is several hundreds micrometers. Optical fiber mode coupling occurs between two forward modes, thereby eliminating only a channel of a specific wavelength from an incidented multi-wavelength optical signal. A transmission (reflection) spectrum of an optical fiber grating can be properly adjusted according to the grating period, grating intensity, grating length, and refractivity distribution. 
   In one method of employing a long period optical fiber grating for flattening the gain of the C-band erbium-doped optical fiber amplifier, the long period optical fiber grating is first designed to have a transmission spectrum opposite to the gain spectrum of the C-band erbium-doped optical fiber amplifier. Then it is inserted into the C-band erbium-doped optical fiber amplifier, thereby enabling the gain to be uniform regardless of the wavelengths. This method does not require a separate additional optical element since there is no reflected optical signal. However, this method has a number of shortcomings including having a spectrum characteristic that is very sensitive to temperature. In order to overcome such temperature sensitivity, another method employing a chirped optical fiber grating (or Chirped Fiber Bragg Grating; CFBG) has been proposed. This method has a short period optical fiber gratings. The CFBG has a grating with a grating period that changes linearly or non-linearly in a longitudinal direction of the grating. In this method, the CFBG is designed with a reflection spectrum opposite to the gain spectrum of the C-band erbium-doped optical fiber amplifier. Then, it is inserted into the erbium-doped optical fiber amplifier, thereby enabling the gain to be uniform. However, this method requires an additional optical element such as an optical isolator in order to prevent an optical signal reflected by the CFBG from coupling and interfering with a forward optical. 
   When compared to a C-band erbium-doped optical fiber amplifier, an L-band erbium-doped optical fiber amplifier shows no difference in the pumping light source. 
   However, it is about five to ten times longer, since the L-band erbium-doped optical fiber amplifier utilizes population inversion of about 40%. Further, an article entitled “Flat gain erbium-doped fiber amplifier in 1570 nm–1600 nm region for dense WDM transmission systems”, OFC &#39;97, vol. PD3, 1997, by M. Fukushima, Y Tashiro, and H. Ogoshi, has shown that the gain flattening characteristic of an L-band erbium-doped optical fiber amplifier is improved through co-pumping by auxiliary pumping light source of the C-band (1530, 1550, or 1570 nm) wavelength together with an existing high power LD light source of 980 or 1480 nm However, such a method requires a separate exterior light source as an auxiliary pumping light source. 
     FIG. 1  illustrates a conventional wide band erbium-doped optical fiber amplifier. The conventional erbium-doped optical fiber amplifier  100  is disposed on an external optical fiber  110  and includes a first and a second amplifying section  170  and  180  and a first and a fifth WDM coupler  121  and  125  for connecting the first and second amplifying section  170  and  180  in parallel to each other. 
   The first WDM coupler  121  divides an optical signal of 1550 and 1580 nm wavelength bands received through the external optical fiber  110  into optical signals of a 1550 nm wavelength band (C-band) and a 1580 nm wavelength band (L-band). Then it outputs the C-band optical signal to a first optical path and the L-band optical signal to a second optical path. 
   The first amplifying section  170  includes a first and a second isolator  131  and  132 , a first pump LD  141 , a second WDM coupler  122 , a first erbium-doped optical fiber  151 , and a chirped optical fiber grating  160 . Each of the first isolator  131  and the second isolator  132  intercepts backward light such as Amplified Spontaneous Emission (ASE) noise outputted from the first erbium-doped optical fiber  151 . The first pump LD  141  outputs a first pumping light having a wavelength of 980 nm or 1480 nm. The second WDM coupler  122  is interposed between the first isolator  131  and the second isolator  132 . It couples the C-band optical signal having passed the first isolator  131  with the first pumping light inputted from the first pump LD  141 . Then, it outputs the coupled light. The first erbium-doped optical fiber  151  experiences a population inversion (is pumped) by the first pumping light that has passed the second isolator  132 . It also amplifies the C-band optical signal that has passed the second isolator  132 . The chirped optical fiber grating  160  gain-flattens the C-band optical signal received from the first erbium-doped optical fiber  151 . 
   The second amplifying section  180  includes a third isolator  133 , a second and a third pump LD  142  and  143 , a third and a fourth WDM coupler  123  and  124 , and a second erbium-doped optical fiber  152 . The second pump LD  142  intercepts backward light such as ASE noise outputted from the second erbium-doped optical fiber  152 . The second pump LD  142  outputs a second pumping light having a wavelength of 980 nm or 1480 nm. The third WDM coupler  123  couples the L-band optical signal that has passed the third isolator  133  with the second pumping light received from the second pump LD  142 . Then it outputs the coupled light. The third pump LD  143  outputs a third pumping light having a wavelength of 1550, 1530 or 1570 nm. The fourth WDM coupler  124  couples the L-band optical signal inputted from the third WDM coupler  123  with the second and third pumping lights. Then it outputs the coupled light. The second erbium-doped optical fiber  152  experiences a population inversion by the second and third pumping lights received from the fourth WDM coupler  124 . It also amplifies the L-band optical signal received from the fourth WDM coupler  124 . 
   The fifth WDM coupler  125  couples the C-band and L-band optical signals received through the first and second optical paths. Then it outputs them through the external optical fiber  110 . 
   The first erbium-doped optical fiber  151  and the second erbium-doped optical fiber  152  have similar construction. The second erbium-doped optical fiber  152  has a length larger than that of the first erbium-doped optical fiber  151 . Further, each of the first and second erbium-doped optical fibers  151  and  152  has a forward pumping construction in which the received optical signal and the pumping light progress in the same direction. However, each of them may have a backward pumping construction in which the inputted optical signal and the pumping light progress in opposite directions, if necessary. 
   As described above, the conventional wideband erbium-doped optical fiber amplifier  100  has gain flattening characteristics of not only the C-band but also the L-band optical signal. However, the conventional wideband erbium-doped optical fiber amplifier  100  has a number of limitations, including (1) that the first amplifying section  170  must include the second isolator  132  which is an additional element for preventing generation of backward ASE noise and (2) the second amplifying section  180  requires the second pump LD  142  as a separate and auxiliary pumping light source. 
   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 gain-flattened wideband erbium-doped optical fiber amplifier which does not require a separate pumping light source. Consequently, enabling a simpler and lower-cost optical fiber amplifier. 
   In accordance with the principles of the present invention, a wideband erbium-doped optical fiber amplifier is disposed among an optical fiber through which a first and second wavelength-band optical signals (for example, the C-band and L-band) are transmitted and forms a first optical path and a second optical path parallel to each other is provided, the amplifier including a first amplifying section disposed on the first optical path, including a first erbium-doped optical fiber to amplify the first-band optical signals, a filter to gain-flatten the amplified first-band optical signals, wherein a reflected portion of the first band optical signal by the filter is directed to the second optical path; and a second amplifying section disposed on the second optical path, having a second erbium-doped optical fiber to amplify received second-band optical signals, wherein the reflected first-band optical signal is used to pump the second erbium-doped optical fiber. 

   
     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  illustrates a conventional wideband erbium-doped optical fiber amplifier; 
       FIG. 2  illustrates a gain-flattened wideband erbium-doped optical fiber amplifier according to a first embodiment of the present invention; 
       FIGS. 3 to 7  are graphs for describing the output characteristics of the erbium-doped optical fiber amplifier shown in  FIG. 2 ; and 
       FIG. 8  illustrates a gain-flattened wideband erbium-doped 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 with reference to the accompanying drawings. For the purposes of clarity and simplicity, well-known functions or constructions are not described in detail as they would obscure the invention in unnecessary detail. 
     FIG. 2  illustrates a gain-flattened wideband erbium-doped optical fiber amplifier according to a first embodiment of the present invention. The wideband erbium-doped optical fiber amplifier  200  is disposed on an external optical fiber  210 . It includes a first and a second amplifying section  280  and  290 , and a first and a fifth WDM coupler  221  and  225  for connecting the first and the second amplifying section  280  and  290  in parallel to each other. 
   The first WDM coupler  221  divides an optical signal of 1550 and 1580 nm wavelength bands received through the external optical fiber  210  into optical signals of a first wavelength band (for example 1550 nm, the C-band) and a second wavelength band (for example 1580 nm, the L-band). It then outputs the first or C-band optical signal to a first optical path and the second or L-band optical signal to a second optical path. 
   The first amplifying section  280  includes a first isolator  231 , a first pump LD  241 , a second WDM coupler  222 , a first erbium-doped optical fiber  251 , a circulator  260 , and a filter  270 . 
   The first isolator  231  intercepts backward light such as ASE noise outputted from the first erbium-doped optical fiber  251 . 
   The first pump LD  241  outputs a first pumping light having a wavelength of 980 nm or 1480 nm. A laser diode (LD) or a light emitting diode (LED) may be employed as the first pump LD  241 . 
   The second WDM coupler  222  couples the C-band optical signal that has passed the first isolator  231  with the first pumping light received from the first pump LD  241 . It then outputs the coupled light. 
   The first erbium-doped optical fiber  251  experiences a population inversion (is pumped) by the first pumping light received from the second WDM coupler  222 . It also amplifies the C-band optical signal received from the second WDM coupler  222 . 
   The circulator  260  has three ports, a first port through a third port. The circulator  260  receives light through an upper port and outputs the received light through adjacent lower ports. Specifically, the first port of the circulator  260  is connected with the first erbium-doped optical fiber  251 . The second port of the circulator  260  is connected with the filter  270 . The third port of the circulator  260  is connected with the second amplifying section  290 . In the circulator  260 , the C-band optical signal received through the first port is outputted through the second port. The filtered C-band optical signal received through the second port is outputted through the third port. 
   The filter  270  is designed to have a transmission spectrum characteristic opposite to the gain spectrum characteristic of the first erbium-doped optical fiber  251 . In the gain spectrum, a non-uniform portion (i.e. the filtered C-band optical signal) is reflected by the filter  270 . The reflected C-band optical signal is inputted to the second port of the circulator  260  as a second pumping light. 
   The second amplifying section  290  includes a second isolator  232 , a second pump LD  242 , a third and a fourth WDM coupler  223  and  224 , and a second erbium-doped optical fiber  252 . 
   The second isolator  232  intercepts backward light such as ASE noise outputted from the second erbium-doped optical fiber  252 . 
   The third WDM coupler  223  couples the L-band optical signal that has passed the second isolator  232  with the second pumping light received from the circulator  260 . It then outputs the coupled light. 
   The second pump LD  242  outputs a third pumping light having a wavelength of 1550, 1530 or 1570 nm. An LD or LED may be employed as the second pump LD  242 . 
   The fourth WDM coupler  224  couples the L-band optical signal received from the third WDM coupler  223  with the second and third pumping lights. It then outputs the coupled light. The second erbium-doped optical fiber  252  experiences a population inversion (is pumped) by the second and third pumping lights received from the fourth WDM coupler  224 . It also amplifies the L-band optical signal received from the fourth WDM coupler  224 . 
   The fifth WDM coupler  225  couples the C-band and L-band optical signals received from the first and second optical paths with each other. It then outputs them through the external optical fiber  210 . 
   Although each of the first and second erbium-doped optical fibers  251  and  252  has a forward pumping construction in the present embodiment, they may have either a forward pumping construction or a backward pumping construction. In the erbium-doped optical fiber amplifier  200 , the gain of the first amplifying section  280  is first flattened using the filter  270 . Thereafter, the C-band optical signal reflected by the filter is supplied to the second erbium-doped optical fiber  252  as an auxiliary second pumping light. Consequently, the erbium-doped optical fiber amplifier  200  of the present invention has a simpler construction, as well as enabling a competitive price. 
     FIGS. 3 to 7  are graphs for describing output characteristics of the erbium-doped optical fiber amplifier  200  shown in  FIG. 2 .  FIG. 3  shows a gain spectrum of the first erbium-doped optical fiber  251  which has a maximum gain value in a short wavelength region of the spectrum.  FIG. 4  shows a transmission spectrum of the filter  270  which has a minimum gain value in a short wavelength region of the spectrum.  FIG. 5  shows a gain spectrum of the first amplifying section  280  that is gain-flattened by the filter  270 .  FIG. 6  shows a gain spectrum of the second amplifying section  290  that is gain-flattened by employing the C-band optical signal reflected by the filter  270  as the auxiliary second pumping light.  FIG. 7  shows a gain spectrum of the erbium-doped optical fiber amplifier  200  in which both the C-band optical signal and the L-band optical signal are gain-flattened by the filter  270 . 
     FIG. 8  illustrates a gain-flattened wideband erbium-doped optical fiber amplifier according to a second embodiment of the present invention. The wideband erbium-doped optical fiber amplifier  300  is disposed on an external optical fiber  310 . It includes a first and a second amplifying section  380  and  390  and a first and a fifth WDM coupler  321  and  325  for connecting the first and the second amplifying section  380  and  390  in parallel to each other. The erbium-doped optical fiber amplifier  300  has a construction similar to that of the erbium-doped optical fiber amplifier  200  shown in  FIG. 2 , except for the pumping structure of the second amplifying section  390 . 
   The first WDM coupler  321  divides an optical signal of 1550 and 1580 nm wavelength bands received from the external optical fiber  310  into optical signals of a 1550 nm wavelength band (C-band) and a 1580 nm wavelength band (L-band). Then it outputs the C-band optical signal to a first optical path and the L-band optical signal to a second optical path. 
   The first amplifying section  380  includes a first isolator  331 , a first pump LD  341 , a second WDM coupler  322 , a first erbium-doped optical fiber  351 , a circulator  360 , and a filter  370 . 
   The first isolator  331  intercepts backward light such as ASE noise outputted from the first erbium-doped optical fiber  351 . 
   The first pump LD  341  outputs a first pumping light having a wavelength of 980 nm or 1480 nm. An LD or LED may be employed as the first pump LD  341 . The second WDM coupler  322  couples the C-band optical signal that has passed the first isolator  331  with the first pumping light received from the first pump LD  341 . It then outputs the coupled light. 
   The first erbium-doped optical fiber  351  experiences a population inversion by the first pumping light received from the second WDM coupler  322 . It also amplifies the C-band optical signal received from the second WDM coupler  322 . 
   The circulator  360  has three ports, a first port through a third port. The circulator  360  receives light through an upper port and outputs the received light through adjacent lower ports. Specifically, the first port of the circulator  360  is connected with the first erbium-doped optical fiber  351 . The second port of the circulator  360  is connected with the filter  370 . The third port of the circulator  360  is connected with the second amplifying section  390 . In the circulator  360 , the C-band optical signal received from the first port is outputted through the second port. The filtered C-band optical signal received from the second port is outputted through the third port. 
   The filter  370  is designed to have a transmission spectrum characteristic opposite to the gain spectrum characteristic of the first erbium-doped optical fiber  351 . In the gain spectrum, a non-uniform portion (i.e. the filtered C-band optical signal) is reflected by the filter  370 . The reflected C-band optical signal is inputted to the second port of the circulator  360  as a second pumping light. 
   The second amplifying section  390  includes a second isolator  332 , a second pump LD  342 , a third and a fourth WDM coupler  323  and  324 , and a second erbium-doped optical fiber  352 . 
   The second isolator  332  intercepts backward light such as ASE noise outputted from the second erbium-doped optical fiber  352 . 
   The second pump LD  342  outputs a third pumping light having a wavelength of 1550, 1530 or 1570 nm. An LD or LED may be employed as the second pump LD  342 . 
   The fourth WDM coupler  324  couples the L-band optical signal that has passed the second isolator  332  with the third pumping light. It then outputs the coupled light. 
   The third WDM coupler  323  outputs the second pumping light received from the circulator  360  to the second erbium-doped optical fiber  352 . It also allows the L-band optical signal received from the second erbium-doped optical fiber  352  to pass intact through the third WDM coupler  323 . 
   The second erbium-doped optical fiber  352  experiences a population inversion by the third pumping light received from the fourth WDM coupler  324  and the second pumping light received from the third WDM coupler  323 . It also amplifies the L-band optical signal received from the fourth WDM coupler  324 . In this manner, the second erbium-doped optical fiber  352  is pumped forward by the third pumping light and backward by second pumping light. 
   The fifth WDM coupler  325  couples the C-band and L-band optical signals received from the first and second optical paths. It then outputs them through the external optical fiber  310 . 
   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.

Technology Classification (CPC): 7