Abstract:
A gain flattening filter is provided to reduce volume and manufacturing cost of a gain flattened optical fiber amplifier, even when a required peak loss exceeds the limit of each dielectric thin film filter. The inventive gain flattening filter includes: a housing having a first opening and a second opening; a first ferrule disposed at one end of the housing, the first ferrule having an opening through which an input of an optical fiber is packaged; a second ferrule disposed at the other end of the housing, the second ferrule having an opening through which an output of the optical fiber is packaged; and, a plurality of thin film filters disposed in sequence between the first and second ferrules for flattening gain of optical signals passing therethrough.

Description:
CLAIM OF PRIORITY  
         [0001]    This application claims priority to an application entitled “Gain flattening filter and gain flattened optical fiber amplifier employing the same” filed in the Korean Industrial Property Office on Mar. 21, 2002 and assigned Serial No. 2002-15252, 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 fiber amplifier and more particularly to a gain flattening filter used in a gain flattened optical fiber amplifier.  
           [0004]    2. Description of the Related Art  
           [0005]    As the recent increase in the quantity of data in geometrical progression requires expansion of the transmission band width of the wavelength division multiplexing optical communication system, it has been widely accepted now that the wavelength division multiplexing optical communication system must be equipped with a gain flattened optical fiber amplifier. A gain flattening filter employed in the gain flattened optical fiber amplifier may be constructed using either active elements or passive elements. Among these two, the utilization of passive elements is accepted more. As passive elements, optical fiber gratings or dielectric thin film filters are typically deployed. However, the gain flattening filters employing the optical fiber gratings are not reliable under variable environmental factors, such as temperature and moisture, thus dielectric thin film filters are a better choice to be used in the gain flattening filter.  
           [0006]    According to the location of the gain flattening filter in the optical power amplifier, the gain flattening filters can be classified into two types: end filters and midway filters. An end filter is located at the rear end of the amplifying medium, while a midway filter is located at the middle portion of the amplifying element. The end filter has a drawback in that it tend to reduce as much gain of the optical fiber amplifier as its own insertion loss, while having an advantage in that it can be designed to limit the peak loss to within 5 dB. In contrast, the midway filter has an advantage in that it compensates for the basic insertion loss of the midway filter in the amplifying element, while having a disadvantage in that it requires a peak loss of at least 7 dB in the design process thereof. There is a limitation in utilizing the dielectric thin film filter as a midway filter as it is difficult to reach a peak loss of at least 6 dB due to the difficulty in its coating process.  
           [0007]    [0007]FIG. 1 is a schematic view illustrating the configuration of a conventional gain flattened, optical fiber amplifier. FIG. 2 is a graphical illustration depicting a loss curve of a first or second gain flattening filter employed in the gain flattened optical fiber amplifier shown in FIG. 1. As shown in FIG. 1, the conventional optical fiber amplifier includes first to third isolators  110 ,  150 , and  210 , first and second pumping light sources  120  and  190 , first and second wavelength selective couplers  130  and  200 , first and second erbium-doped optical fibers  140  and  180 , and first and second gain flattening filters  160  and  170 .  
           [0008]    In operation, the first isolator  110  allows an optical signal inputted to the optical fiber amplifier to pass intact through the first isolator  110 , while intercepting a light inputted in a direction opposite to that of the optical signal—that is, a light inputted from the first wavelength selective coupler  130 . The first wavelength selective coupler  130  combines the optical signal inputted from the first isolator  110  and a pumping light inputted from the first pumping light source  120 , then outputs the combined optical signal to the first erbium-doped optical fiber  140 . The first pumping light source  120  pumps the first erbium-doped optical fiber  140  by exciting erbium ions in the first erbium-doped optical fiber  140 . A laser diode capable of outputting a pumping light may be utilized as the first pumping light source  120 . The first erbium-doped optical fiber  140  is pumped by the pumping light inputted through the first wavelength selective coupler  130 , then amplifies and outputs the optical signal inputted through the first wavelength selective coupler  130 .  
           [0009]    The second isolator  150  allows the optical signal inputted through the first erbium-doped optical fiber  140  to pass intact through the second isolator  150 , while intercepting light inputted in a direction opposite to that of the optical signal. The first and second gain flattening filters  160  and  170  are midway filters and serve to sequentially flatten the gain of the optical signal inputted through the second isolator  150 . Each of the first and second gain flattening filters  160  and  170  includes a dielectric thin film filter. Referring to FIG. 2, note that the peak loss is about 5 dB, as indicated by the loss curve of the first gain flattening filter  160  or the second gain flattening filter  170 . As the gain flattened, optical fiber amplifier requires a peak loss of about 10 dB, the first and second gain flattening filters  160  and  170  each having a loss peak of 5 dB are connected in series with each other in the gain flattened optical fiber amplifier.  
           [0010]    The second erbium-doped optical fiber  180  is pumped by a pumping light inputted through the second wavelength selective coupler  200 , then amplifies and outputs the optical signal inputted from the second wavelength selective coupler  200 . The second wavelength selective coupler  200  outputs the optical signal inputted from the second pumping light source  190  to the second erbium-doped optical fiber  180  and outputs the optical signal inputted from the second erbium-doped optical fiber  180  to the third isolator  210 . The third isolator  210  allows the optical signal inputted through the second wavelength selective coupler  200  to pass intact through the third isolator  210 , while intercepting a light inputted in a direction opposite to that of the optical signal.  
           [0011]    As described above, in the case where the gain flattened optical fiber amplifier requires a peak loss exceeding the limit of each dielectric thin film filter, a plurality of gain flattening filters, each including a dielectric thin film filter, must be connected in series with each other in the conventional gain flattened, optical fiber amplifier, thereby increasing the volume and manufacturing cost of the entire gain flattened optical fiber amplifier.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention overcomes the above-described problems, and provides additional advantages, by providing a gain flattened optical fiber amplifier, which can reduce the volume and manufacturing cost found in the prior art even when a required peak loss exceeds the limit of each dielectric thin film filter.  
           [0013]    Accordingly, there is provided a gain flattening filter employed in an optical fiber amplifier which includes a housing having a first opening and a second opening; a first ferrule disposed at one end of the housing, the first ferrule having an opening through which an input of an optical fiber is packaged; a second ferrule disposed at the other end of the housing, the second ferrule having an opening through which an output of the optical fiber is packaged; and, a plurality of thin film filters disposed in sequence between the first and second ferrules for flattening gain of optical signals passing therethrough.  
           [0014]    In accordance with another aspect of the present invention, there is provided a gain flattened erbium-doped optical fiber amplifier comprising: a first erbium-doped optical fiber amplifying and outputting an inputted optical signal; a second erbium-doped optical fiber re-amplifying and outputting the optical signal having been amplified; and, a gain flattening filter. The gain flattening filter comprises: a housing having a hole formed in and through the housing; a first ferrule inserted in a first end of the housing, the first ferrule having a hole through which an input-side optical fiber is packaged; a second ferrule inserted in a second end of the housing, the second ferrule having a hole through which an output-side optical fiber is packaged; and, a plurality of thin film filters disposed between the first and second ferrules, each of the thin film filters having a predetermined loss curve.  
           [0015]    According to another aspect of the invention, the first and second ferrules comprises end surfaces opposed to each other and the respective end surface is inclined at a predetermined angle. Meanwhile, the gain flattening filter further includes a first lens disposed between the first ferrule and the thin film filters, the first lens collimating light emitted through one end of the input of the optical fiber; and, a second lens disposed between the thin film filters and the second ferrule, the second lens collecting light having been transmitted through the thin film filters, wherein the first lens comprises one end surface opposed to the first ferrule, and the second lens comprises one end surface opposed to the second ferrule, the end surfaces of the first and second lenses being inclined at a predetermined angle, respectively. The housing is shaped as a hollow cylinder. Further, the first ferrule is shaped as a hollow cylinder with a predetermined diameter substantially equal to a diameter of the optical fiber. The first and the lenses are one of a graded-index rod lens having a predetermined pitch and a refractive index. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The above 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:  
         [0017]    [0017]FIG. 1 is a schematic view illustrating the configuration of a conventional gain flattened optical fiber amplifier;  
         [0018]    [0018]FIG. 2 is a graph depicting a loss curve of a first or second gain flattening filter employed in the gain flattened optical fiber amplifier shown in FIG. 1;  
         [0019]    [0019]FIG. 3 is a schematic view illustrating the configuration of a gain flattened optical fiber amplifier according to a preferred embodiment of the present invention;  
         [0020]    [0020]FIG. 4 is a sectional view of a gain flattening filter shown in FIG. 3;  
         [0021]    [0021]FIG. 5 is a graph depicting a loss curve of the gain flattening filter shown in FIG. 3; and  
         [0022]    [0022]FIG. 6 is a sectional view of a gain flattening filter according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]    Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. For the purpose of clarity and simplicity, a detailed description of well-known functions and configurations incorporated herein will be omitted as it may make the subject matter of the present invention rather unclear.  
         [0024]    [0024]FIG. 3 is a schematic view illustrating the configuration of a gain flattened, optical fiber amplifier according to a preferred embodiment of the present invention. FIG. 4 is a sectional view of the gain flattening filter shown in FIG. 3, and FIG. 5 is a graphical illustration depicting a loss curve of the gain flattening filter shown in FIG. 3.  
         [0025]    As shown in FIG. 3, the gain flattened, optical fiber amplifier according to the embodiment of the present invention includes first to third isolators  310 ,  360 , and  390 , a pumping light source  320 , first to third wavelength selective couplers  330 ,  350  and  370 , first and second erbium-doped optical fibers  340  and  380 , and a gain flattening filter  500 .  
         [0026]    In operation, the first isolator  310  allows an optical signal inputted to the optical fiber amplifier to pass intact through the first isolator  310 , while intercepting a light inputted in a direction opposite to that of the optical signal, that is, a light inputted from the first wavelength selective coupler  330 . The first wavelength selective coupler  330  combines the optical signal inputted from the first isolator  310  and pumping light inputted from the pumping light source  320  and outputs the combined optical signal to the first erbium-doped optical fiber  340 .  
         [0027]    The pumping light source  320  pumps the first erbium-doped optical fiber  340  by exciting erbium ions in the first erbium-doped optical fiber  340 . In this embodiment, a laser diode capable of outputting a pumping light with a predetermined wavelength may be utilized as the pumping light source  320 . The first erbium-doped optical fiber  340  is pumped by the pumping light inputted through the first wavelength selective coupler  330 , then amplifies and outputs the optical signal inputted through the first wavelength selective coupler  330 .  
         [0028]    The second wavelength selective coupler  350  separates the remnant pumping light and the optical signal inputted from the first erbium-doped optical fiber  340  from each other, then outputs the separated optical signal to the second isolator  360  and the separated remnant pumping light to the third wavelength selective coupler  370 . The second isolator  360  allows the optical signal inputted from the second wavelength selective coupler  350  to pass intact through the second isolator  360 , while intercepting light inputted in a direction opposite to that of the optical signal. The gain flattening filter  500  is a midway filter and sequentially flattens the gain of the optical signal inputted through the second isolator  360 .  
         [0029]    Referring to FIG. 4, the gain flattening filter  500  includes a housing  510 , first and second ferrules  520  and  570 , first and second lenses  530  and  560 , and first and second thin film filters  540  and  550 . The housing  510  is shaped like a hollow cylinder having a hole with a predetermined diameter and forms an external appearance of the gain flattening filter  500 . The first ferrule  520  is also shaped like a hollow cylinder having a hole with a predetermined diameter similar to the diameter of an optical fiber  400 . The first ferrule  520  has an end surface opposed to the first lens  530 , which is inclined at a predetermined angle, so as to minimize the signal-to-noise ratio due to the internal reflection. The inclined end surface of the first ferrule  520  is inserted into one end of the housing  510  while being opposed to the first lens  530 , and the optical fiber  400  optically connected with the second isolator  360  is inserted in the hole of the first ferrule  520 .  
         [0030]    The first lens  530  collimates the optical signal emitted through one end of the optical fiber  400  inserted in the first ferrule  520  and packaged in the housing  510 , so that the first lens  530  is disposed between the first ferrule  520  and the first thin film filter  540 . As for the first lens  530 , a graded-index rod lens (GRIN) having a predetermined pitch and a refractive index, which varies in the longitudinal direction of the lens, may be employed. The first lens  530  has an end surface opposed to the first ferrule  520 , which is inclined at a predetermined angle, so as to minimize the signal-to-noise ratio due to internal reflection. The first and second thin film filters  540  and  550  are disposed between the first lens  530  and the second lens  560  and sequentially flatten the gain of the optical signal collimated by the first lens  530 . Referring to FIG. 5, which shows the entire loss curve formed by superposing the loss curves of the first and second thin film filters  540  and  550 , it is noted that the peak loss has a value of about 10 dB.  
         [0031]    The second lens  560  collects the optical signal emitted through the second thin film filter  550  and packaged in the housing  510 , so that the second lens  560  is disposed between the second thin film filter  550  and the second ferrule  570 . As for the second lens  560 , a graded-index rod lens having a predetermined pitch and a refractive index, which varies in the longitudinal direction of the lens, may be employed. The second lens  560  has an end surface opposed to the second ferrule  570 , which is inclined at a predetermined angle, so as to minimize the signal-to-noise ratio due to internal reflection.  
         [0032]    The second ferrule  570  is shaped like a hollow cylinder having a hole with a predetermined diameter similar to the diameter of the optical fiber  400 . The second ferrule  570  has an end surface opposed to the second lens  560 , which is inclined at a predetermined angle, so as to minimize the signal-to-noise ratio due to the internal reflection. The inclined end surface of the second ferrule  570  is inserted into the other end of the housing  510  while being opposed to the second lens  560 , and the optical fiber  400  optically connected with the third wavelength selective coupler  370  is inserted in the hole of the second ferrule  570 .  
         [0033]    With continued reference to FIG. 3, the third wavelength selective coupler  370  combines the optical signal inputted from the gain flattening filter  500  and pumping light inputted from the second wavelength selective coupler  350  and outputs a combined optical signal to the second erbium-doped optical fiber  380 . The second erbium-doped optical fiber  380  is pumped by a pumping light inputted through the third wavelength selective coupler  370 , then amplifies and outputs the optical signal inputted from the third wavelength selective coupler  370 . The third isolator  390  allows the optical signal inputted through the second erbium-doped optical fiber  380  to pass intact through the third isolator  390 , while intercepting light inputted in a direction opposite to that of the optical signal.  
         [0034]    [0034]FIG. 6 is a sectional view of a gain flattening filter according to another embodiment of the present invention. The gain flattening filter  600  includes a housing  610 , first and second ferrules  620  and  680 , first and second lenses  630  and  670 , and first to third thin film filters  640 ,  650 , and  660 . The housing  610  is shaped like a hollow cylinder having a hole with a predetermined diameter and forms an external appearance of the gain flattening filter  600 . The first ferrule  620  is also shaped like a hollow cylinder having a hole with a predetermined diameter similar to the diameter of an input-side optical fiber  690 . The first ferrule  620  has an end surface opposed to the first lens  630 , which is inclined at a predetermined angle, so as to minimize the signal-to-noise ratio due to the internal reflection. The inclined end surface of the first ferrule  620  is inserted into one end of the housing  610  while being opposed to the first lens  630 , and the input-side optical fiber  690  is inserted in the hole of the first ferrule  620 .  
         [0035]    The first lens  630  collimates the optical signal emitted through one end of the input-side optical fiber  690  and packaged in the housing  610 , so that the first lens  630  is disposed between the first ferrule  620  and the first thin film filter  640 . As for the first lens  630 , a graded-index rod lens having a predetermined pitch and a refractive index, which varies in the longitudinal direction of the lens, may be employed. The first lens  630  has an end surface opposed to the first ferrule  620 , which is inclined at a predetermined angle, so as to minimize the signal-to-noise ratio due to internal reflection. The first to third thin film filters  640 ,  650 , and  660  are disposed between the first lens  630  and the second lens  670  while being spaced at predetermined intervals apart from each other, and sequentially flatten the gain of the optical signal collimated by the first lens  630 .  
         [0036]    The second lens  670  collects the optical signal emitted through the third thin film filter  660  and packaged in the housing  610 , so that the second lens  670  is disposed between the third thin film filter  660  and the second ferrule  680 . As for the second lens  670 , a graded-index rod lens having a predetermined pitch and a refractive index, which varies in the longitudinal direction of the lens, may be employed. The second lens  670  has an end surface opposed to the second ferrule  680 , which is inclined at a predetermined angle, so as to minimize the signal-to-noise ratio due to internal reflection.  
         [0037]    The second ferrule  680  is shaped like a hollow cylinder having a hole with a predetermined diameter similar to the diameter of an output-side optical fiber  700 . The second ferrule  680  has an end surface opposed to the second lens  670 , which is inclined at a predetermined angle, so as to minimize the signal-to-noise ratio due to the internal reflection. The inclined end surface of the second ferrule  680  is inserted into the other end of the housing  610  while being opposed to the second lens  670 , and the output-side optical fiber  700  is inserted in the hole of the second ferrule  680 .  
         [0038]    As described above, in a gain flattening filter according to the present invention, a plurality of thin film filers are packaged in a housing, so that various required values of peak losses can be accomplished by a single device. Also, a gain flattened optical fiber amplifier having the gain flattening filter can solve the problems of the prior art—that is, the increase of the volume and manufacturing cost of the conventional gain flattened optical fiber amplifier due to the multiple gain flattening filters.  
         [0039]    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.