Patent Abstract:
An L-band light source is provided that has improved amplifying efficiency and stabilized output power. The L-band light source comprises: a gain medium having first and second sides, and configured to generate an L-band amplified spontaneous emission (ASE); a first pump light source to generate first pumping light; a first wavelength selective coupler to supply the first pumping light to the gain medium; and a first reflector to reflect a part of ASE outputted to the fist side of the gain medium, the first reflector having a predetermined reflection.

Full Description:
CLAIM OF PRIORITY 
     This application claims priority to an application entitled “L-Band Light Source with Improved Amplifying Efficiency and Stabilized Output Power,” filed with the Korean Intellectual Property Office on Dec. 19, 2003 and assigned Ser. No. 2003-93866, 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 module, in particular to an L-band light source. 
     2. Description of the Related Art 
     A light source with a wide wavelength band is needed to measure the optical characteristics employed in optical communication. Moreover, the wavelength band of the optical signals used in optical communication is 1520 nm˜1620 nm when at least one erbium doped fiber amplifier (EDFA) is employed. Thus, a light source capable of measuring optical characteristics of various optical components within such a wavelength band is needed. 
     A wavelength division multiplexing passive optical network (WDM-PON) has recently been highlighted as a technology for a high-speed fiber-to-the-home (FTTH) network. In a WDM-PON attention is paid to the broadband light source that is used along with a wavelength locked Fabry Perot laser diode (FP-LD) in order to accommodate a plurality of subscribers. Existing available broadband light sources mainly employ a white light source or an EDFA outputting amplified spontaneous emission (ASE). However, because white light sources have low output power, they are limited in measuring the optical characteristics of a light source or an optical component for a WDM-PON which requires high output power. In addition, EDFAs are not economical in price. 
     U.S. Pat. No. 6,507,429 issued to Gaelle Ales et al. and entitled “Article Comprising a High Power/Broad Spectrum Superfluorescent Fiber Radiation Source” discloses a broadband source for outputting C-band (1520 nm˜1570 nm) ASE and L-band (1570 nm˜1620 nm) ASE. The broadband light source includes first and second rare earth element doped optical fibers, and an isolator located between the optical fibers. First pumping light from a first pump light source is supplied to the first rare earth element doped optical fiber and second pumping light from second pump light source is supplied to the second rare earth element doped optical fiber. The first rare earth element doped optical fiber has a length longer than that of the second rare earth element doped optical fiber about five times. A reflector reflects ASE inputted from the first rare earth element doped optical fiber, thus assisting generation of L-band ASE in the first rare earth element doped optical fiber. The second rare earth element doped optical fiber conducts functions of amplifying the L-band ASE and generating C-band ASE. As a result, the broadband light source is able to output C-band and L-band ASEs through an output end thereof. 
     However, the typical broadband optical source has poor output efficiency. This is due to the isolator being between the first and second rare earth element doped optical fibers; thus, the C-band ASE outputted to the rear side of the second rare earth element doped optical fiber cannot be used. In addition, if the output power of the first pump light source is changed so as to tune the output power of the L-band ASE (obtained from the first rare earth element doped optical fiber), not only the output power of the L-band ASE but also the output power of the C-band ASE is changed. In contrast, if the output power of the second pump light source is changed so as to tune the C-band ASE (obtained from the second rare earth element doped optical fiber), not only the output power of the C-band ASE, but also the output power of the L-band ASE is changed. Accordingly, since the output powers of the C-band ASE and L-band ASE are affected by one another, it is more difficult to control the output power of the broadband 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 an L-band light source having improved amplifying efficiency and stabilized output power. Thus, the L-band light source is suitable for measuring the characteristics of an optical component or use as a broadband light source for a WDM-PON. 
     In accordance with the principles of the present invention, an L-band light source is provided and includes: a gain medium having first and second sides, and configured to generate an L-band amplified spontaneous emission (ASE); a first pump light source to generate first pumping light; a first wavelength selective coupler to supply the first pumping light to the gain medium; and a first reflector to reflect a part of ASE outputted to the fist side of the gain medium, the first reflector having a predetermined reflection wavelength included in C-band. 
    
    
     
       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 construction of an L-band light source according to a first embodiment of the present invention; 
         FIG. 2  shows a construction of an L-band light source according to a second embodiment of the present invention; 
         FIG. 3  shows a construction of an L-band light source according to a third embodiment of the present invention; and 
         FIG. 4  is a view for illustrating characteristics of output power of the broadband light source shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments 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. 
       FIG. 1  shows a construction of an L-band light source according to a first embodiment of the present invention. The L-band light source  100  comprises a fiber Bragg grating (FBG)  120 , first and second pump light sources  130 ,  135 , first and second wavelength selective couplers (WSCs)  140 ,  145 , a gain medium  150 , and an isolator (ISO)  160 . The fiber Bragg grating  120 , the gain medium  150 , the first and second wavelength selective couplers  140 ,  145  and the isolator  160  are connected in series using a first optical waveguide  110 . The first pump light source  130  is connected in parallel to the gain medium  150  using a second optical waveguide  112  and the second pump light source  135  is connected in parallel to the gain medium  150  using a third optical waveguide  114 . 
     The first pump light source  130  outputs first pumping light, and the first and second pump light sources  130 ,  145  may each incorporate a laser diode outputting light having a wavelength of 980 nm or 1480 nm. 
     The first wavelength selective coupler  140  is located between the fiber Bragg grating  120  and the gain medium  150 . The first wavelength selective coupler  140  supplies pumping light to the gain medium  150 . 
     The second pump light source  125  outputs second pumping light. The second wavelength selective coupler  145  is located between the gain medium  150  and the isolator  160 . The second wavelength selective coupler  145  supplies the second pumping light to the gain medium  150 . 
     The gain medium  150  is located between the first and second wavelength selective couplers  140 ,  145  and has a length suitable for generating L-band ASE. The gain medium  150  is controlled to have a relatively long length. Thus, it generates ASE in a wavelength band of 1520 nm˜1620 nm. In addition, the C-band (1520 nm˜1570 nm) ASE in the generated ASE is absorbed while progressing within the gain medium  150 . As a result, the gain medium  150  serves to amplify the L-band (1570 nm˜ASE 1620 nm) with a lower output power generated at the end of the gain medium  150 . For example, the gain medium  150  may incorporate an EDF having a length of about 50 m. The gain medium  150  outputs ASE to a first and second side, hereinafter, front and rear sides, thereof as it is pumped by the first and second pumping light. The ASE outputted to the front side of the gain medium  150  passes the second wavelength selective coupler  145  and the isolator  160 . Then the ASE is outputted to the outside through the output end  104  of the L-band light source  100 . The ASE outputted to the rear side of the gain medium  150  passes the first wavelength selective coupler  140 . Then, the ASE is inputted into the fiber Bragg grating  120 . 
     The fiber Bragg grating  120  is located between a terminal end  102  of the L-band light source  100  and the first wavelength selective coupler  140 . The fiber Bragg grating  120  has a predetermined reflection wavelength and reflects a part of the inputted rear side ASE to the gain medium  150 . The rear side ASE reflected from the fiber Bragg grating  120  passes the first wavelength selective coupler  140 . Then, the rear side ASE is inputted into the gain medium  150 , thus pumping the gain medium  150 . The ASE having passed the fiber Bragg grating  120  is inputted into the terminal end of the L-band light source  100  and disappears. The fiber Bragg grating  120  may have a reflection wavelength of 1560 nm. 
     In order to prevent the rear side ASE reflected from the terminal end  102  of the broadband light source  100  from being inputted into the first wavelength selective coupler  140 , an angled connector may be provided at the terminal end  102  of the broadband light source  100 . Alternatively, an additional isolator may be installed between the terminal end  102  and the fiber Bragg grating  120 . It is also possible to form a reflecting body which reflects about 4% of the rear side ASE. This can be accomplished by cutting an end surface of the first optical waveguide  110  vertically to the progressing direction of the rear side ASE, whereby the reflected C-band ASE can improve the output power of the L-band ASE. 
     The isolator  160  is located between the gain medium  150  and the output end  104  of the broadband light source  100 . The isolator  160  passes the front side ASE inputted from the gain medium  150  and blocks light progressing in the opposite direction. 
       FIG. 4  is a view for illustrating output characteristics of the broadband light source shown in  FIG. 1 .  FIG. 4  shows output spectrum  430  of the broadband light source  100  and output spectrum  430  obtained after removing the fiber Bragg grating  120  from the broadband light source  100 . The fiber Bragg grating  120  has a wavelength of 1560 nm, and the reflected spectrum  410  of the fiber Bragg grating  120  is shown in the drawing. It can be seen that the L-band ASE is efficiently amplified after the gain medium  150  is pumped with reflected light having a wavelength of 1560 nm. At this time, the amplified intensity of L-band ASE may be varied depending on the power of the reflected light. If the power of the reflected light is too high, the reflected light takes the energy of the L-band ASE and the reflected light may be amplified whereas the power of the L-band ASE may decrease. As a result, the gain medium  150  may be placed in a saturated condition in a predetermined power range. 
       FIG. 2  shows a construction of an L-band light source according to a second embodiment of the present invention. The L-band light source  200  comprises first and second fiber Bragg gratings  220 ,  225 , first and second pump light sources  230 ,  235 , first and second wavelength selective couplers  240 ,  245 , a gain medium  250 , and an isolator  260 . 
     The first pump light source  230  outputs first pumping light. The first wavelength selective coupler  240  is located between the first fiber Bragg grating  220  and the gain medium  250 . The first wavelength selective coupler  240  supplies the first pumping light to the gain medium  250 . 
     The second pump light source  235  outputs second pumping light. The second wavelength selective coupler  245  is located between the gain medium  250  and the second fiber Bragg grating  225 . The second wavelength selective coupler  245  supplies the second pumping light to the gain medium  250 . 
     The gain medium  250  is located between the first and second wavelength selective couplers  240 ,  245  and has a length suitable for generating L-band ASE. The gain medium  250  outputs ASE to the front and rear sides thereof as it is pumped by the first and second pumping light. The ASE outputted to the front side of the gain medium  250  passes the second wavelength selective coupler  245 . Then, the ASE is inputted into the second fiber Bragg grating  225 . The ASE outputted to the rear side of the gain medium  250  passes the first wavelength selective coupler  240 . Then, the ASE is inputted into the first fiber Bragg grating  220 . 
     The first fiber Bragg grating  220  is located between a terminal end  202  of the L-band light source  200  and the first wavelength selective coupler  140 . The first fiber Bragg grating  220  has a predetermined reflection wavelength and reflects a part of the inputted rear side ASE toward the gain medium  250 . The rear side ASE reflected from the first fiber Bragg grating  220  passes the first wavelength selective coupler  240 . Then, the rear side ASE is inputted into the gain medium  250 , thus pumping the gain medium  250 . The rear side ASE having passed the first fiber Bragg grating  220  is inputted into the terminal end  202  of the L-band light source  200  and disappears. The first fiber Bragg grating  220  may have a reflection wavelength of 1560 nm. 
     The second fiber Bragg grating  225  is located between the second wavelength selective coupler  245  and the isolator  160  and has a predetermined reflection wavelength included in the C-band. The second fiber Bragg grating  225  reflects a part of the inputted front side ASE toward the gain medium  250 . The front side ASE reflected from the second fiber Bragg grating  225  passes the second wavelength selective coupler  245 . Then the front side ASE is inputted into the gain medium  250 , thus pumping the gain medium  250 . The ASE having passed the second fiber Bragg grating  225  passes the isolator  260  and then the ASE is outputted to the outside through the output end  204  of the L-band light source  200 . The second fiber Bragg grating  225  may have a wavelength of 1550 nm. If the reflection wavelengths of the first and second fiber Bragg gratings  220 ,  225  are the same as one another and the reflected ASEs are not sufficiently absorbed within the gain medium  250 , they may form a resonance structure and cause oscillation. Therefore, it is possible to make the first and second fiber Bragg gratings  220 ,  225  have different wavelengths. 
     The isolator  260  is located between the second fiber Bragg grating  225  and the output end  204  of the broadband light source  200 . The isolator  260  passes the front side ASE having passed the second fiber Bragg grating  225  and blocks light progressing in the opposite direction. 
       FIG. 3  shows a construction of an L-band light source according to a third embodiment of the present invention. The L-band light source  300  comprises a reflector  320 , a fiber Bragg grating  360 , first and second pump light sources  330 ,  335 , first and second wavelength selective couplers  340 ,  345 , a gain medium  350 , and an isolator  370 . 
     The first pump light source  330  outputs first pumping light. The first wavelength selective coupler  340  is located between the reflector  320  and the gain medium  350 . The first wavelength selective coupler  340  supplies the first pumping light to the gain medium  350 . 
     The second pump light source  335  outputs second pumping light. The second wavelength selective coupler  345  is located between the gain medium  350  and the fiber Bragg grating  360 . The second wavelength selective coupler  345  supplies the second pumping light to the gain medium  350 . 
     The gain medium  350  is located between the first and second wavelength selective couplers  340 ,  345  and has a length suitable for generating L-band ASE. The gain medium  350  outputs the ASE to the front and rear sides thereof as it is pumped by the first and second pumping light. The ASE outputted to the front side of the gain medium  350  passes the second wavelength selective coupler  345 . Then, the ASE is inputted into the fiber Bragg grating  360 . The ASE outputted to the rear side of the gain medium  350  passes the first wavelength selective coupler  340 . Then, the ASE is inputted into the fiber Bragg grating  360 . 
     The reflector  320  is provided at a terminal end of the L-band light source  300 . The reflector  320  reflects the inputted rear side ASE toward the gain medium  350 . The ASE reflected from the reflector  320  passes the first wavelength selective coupler  340 . Then the ASE is inputted into the gain medium  350 , thus pumping the gain medium  350 . 
     The fiber Bragg grating  360  is located between the second wavelength selective coupler  345  and the isolator  370  and has a predetermined reflection wavelength included in C-band. The fiber Bragg grating  360  reflects a part of the inputted front side ASE toward the gain medium  350 . The ASE reflected from the second fiber Bragg grating  360  passes the second wavelength selective coupler  345 . Then, the front side ASE is inputted into the gain medium  350 , thus pumping the gain medium  350 . The ASE having passed the fiber Bragg grating  360  passes the isolator  370 . Then, the ASE is outputted to the outside through the output end  304  of the L-band light source  300 . 
     The isolator  370  located between the fiber Bragg grating  360  and the output end  304  of the broadband light source  300 . The isolator  370  passes the front side ASE having passed the fiber Bragg grating  360  and blocks light progressing in the opposite direction. 
     Advantageously, an L-band light source according to the present invention reuses a part of ASE generated in a gain medium as pumping light by employing a fiber Bragg grating. 
     Accordingly, amplifying efficiency is increased and output power is stabilized. The present invention also enables fabrication of (1) an expanded broadband light source and (2) a light source for measuring an optical characteristic of an optical component, needed in a wavelength division multiplexing passive optical network to be developed in earnest in the future. 
     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