Abstract:
Disclosed is a hybrid broadband optical source comprising an amplified spontaneous emission (ASE) light source module to generate ASE, a gain medium to amplify the ASE, a pump light source to generate pump light, and a wavelength selective coupler to supply the pump light to the gain medium.

Description:
CLAIM OF PRIORITY 
   This application claims priority to an application entitled “Hybrid Broadband Light Source,” filed with the Korean Intellectual Property Office on Dec. 19, 2003 and assigned Serial No. 2003-93857, 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 communication device, in particular to a broadband light source. 
   2. Description of the Related Art 
   In constructing wavelength division multiplexing passive optical networks (WDM-PON), which are technology candidates for high-speed fiber-to-the-home (FTTH) applications, a low-price broadband light source are required. Together with a wavelength locked (Fabry Perot laser diode (FP-LD), such a broadband light source plays an important role in concurrently accommodating a plurality of subscribers. In addition, when such a broadband light source is employed in an optical communication system incorporating an erbium doped fiber amplifier (EDFA), it is also needed to measure optical characteristics of the communication components in predetermined signal wavelength bands (e.g. 1530 nm˜1570 nm, 1570 nm˜1610 nm). Existing available broadband light sources mainly employ a white light source incorporating a halogen lamp, an EDFA outputting amplified spontaneous emission (ASE), an edge-emitting light emitting diode (EELED), or a super luminescent diode (SLD). However, white light sources and EELEDs have low output power and are not suitable as a light source for a WDM-PON and SDLs as they output relatively high power. Thus, white light sources and EELEDs are insufficient for use as broadband light sources for a WDM-PON as compared to EDFAs. In fact, EDFAs are commercialized as a broadband light source but are required to have a higher output power over a wide wavelength band when employed as a light source for a WDM-PON. Disadvantageously, however, their construction becomes complicated in order to meet with the above requirements, and thus are not economical in price. 
     FIG. 1  shows a diagram of a broadband light source according to the prior art. The broadband light source  100  comprises first and second erbium doped optical fibers  140 ,  145 , first and second pump laser diodes  120 ,  125 , first and second wavelength selective couplers (WSCs)  130 ,  135 , a band-pass filter (BPF)  160 , and first and second isolators (ISOs)  150 ,  155 . The first wavelength selective coupler  130 , the first erbium doped optical fiber  140 , the first isolator  150 , the band-pass filter  160 , the second erbium doped optical fiber  140 , the second wavelength selective coupler  135 , and the second isolator  155  are connected in series using a first optical waveguide. In addition, the second isolator  155  is connected in parallel to the first erbium doped optical fiber  140  using a second optical waveguide. The second pump laser diode  114  is connected in parallel to the second erbium doped optical fiber  145  using a third optical waveguide. 
   The first pump laser diode  120  outputs first pump light. 
   The first wavelength selective coupler  130  is located between a terminal end of the broadband light source  100  and the first erbium doped optical fiber. The first wavelength selective coupler  130  supplies the first pump light to the first erbium doped optical fiber  140 . 
   The first erbium doped optical fiber  140  is located between the first wavelength selective coupler  130  and the first isolator  150 . The first erbium doped optical fiber  140  outputs ASE to the front and rear sides as it is pumped by the first pump light. The ASE outputted to the rear side of the first erbium doped optical fiber  140  passes the first wavelength selective coupler  130 . Then, the ASE is inputted into the terminal end  102  and disappears. The ASE outputted to the front side of the first erbium doped optical fiber  140  passes the first isolator  150  and the band-pass filter  160 , and the ASE is inputted into the second erbium doped optical fiber  145 , thus being amplified. Thereafter, the ASE passes the second wavelength selective coupler  135  and the second isolator  155 , and the ASE is outputted to the outside through an output end  104  of the broadband light source  100 . 
   The first isolator  150  is located between the first erbium doped optical fiber  140  and the band-pass filter  160 . The first isolator  150  passes the ASE inputted from the first erbium doped optical fiber and blocks light progressing in the opposite direction. 
   The band-pass filter  160  is located between the first isolator  150  and the second erbium doped optical fiber  145 . The band-pass filter  160  limits the bandwidth of the ASE passing the first isolator  150  in a wavelength band of 1541 nm˜1559 nm in such a manner that high output power can be obtained in the wavelength band. 
   The second pump laser diode  125  outputs second pump light. 
   The second wavelength selective coupler  135  is located between the second erbium doped optical fiber  145  and the second isolator  155 . The second wavelength selective coupler  135  supplies the second pump light to the second erbium doped optical fiber  145 . 
   The second erbium doped optical fiber  145  is located between the band-pass filter  160  and the second wavelength selective coupler  135 . The second erbium doped optical fiber  145  amplifies and outputs the ASE having passed the band-pass filter  160 . 
   The second isolator  155  is located between the second wavelength selective coupler  135  and the output end  104  of the broadband light source  100 . The second isolator  155  passes the ASE having passed the second wavelength selective coupler  135  and blocks light progressing in the opposite direction. 
     FIG. 2  is a graph showing ASE spectrums in regard to positions of the broadband light source shown in  FIG. 1 .  FIG. 2  shows first spectrum  210  in position A, second spectrum  220  in position B, and third spectrum  230  in position C. The first spectrum  210  has lower output power as compared to the third spectrum  220 , and the ASE in position C serves as a seed for the second erbium doped optical fiber  145 . 
   However, such a broadband optical source has a number of limitations, including employing expensive components for generating ASE serving as a seed, thus it is not economical. In addition, the ASE is not effective because the portion of the spectrum beyond a predetermined wavelength band is removed in the band-pass filter  160 . 
   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 broadband light source which can obtain high output power in a selective wavelength band in an economical and effective manner. 
   In accordance with the principles of the present invention, a hybrid broadband optical source is provided and includes: an amplified spontaneous emission (ASE) light source module to generate ASE, a gain medium to amplify the ASE, a pump light source to generate pump light, and a wavelength selective coupler to supply the pump light to the gain medium. 

   
     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  is a diagram of a broadband light source according to the prior art; 
       FIG. 2  is a graph showing ASE spectrums in regard to positions of the broadband light source shown in  FIG. 1 ; 
       FIG. 3  is a diagram of a hybrid broadband light source according to a first embodiment of the present invention; 
       FIG. 4  is a diagram of a hybrid broadband light source according to a second embodiment of the present invention; and 
       FIG. 5  is a graph showing ASE spectrums in regard to positions of the broadband light source shown in  FIG. 4 . 
   

   DETAILED DESCRIPTION 
   Hereinafter, preferred 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. 3  is a diagram of a hybrid broadband light source according to a first embodiment of the present invention. The broadband light source  300  comprises an ASE light source module  320 , first and second isolators  330 ,  335 , a gain medium  340 , a pump light source  350 , and a wavelength selective coupler  360 . The ASE light source module  320 , the first isolator  330 , the gain medium  340 , the wavelength selective coupler  360 , and the second isolator  335  are connected in series using a first optical waveguide  310 . The pump light source  350  is connected in parallel to the gain medium  340  using a second optical waveguide  315 . 
   The ASE light source module  320  is installed at a terminal end of the broadband light source  300 . The ASE light source module  320  generates and outputs ASE. The ASE light source module  320  is a semiconductor ASE light source of a single module and may incorporate an ASE light source fabricated from a low-price semiconductor including a low-price EELED or SLD having the desired wavelength band. 
   The first isolator  330  is located between the ASE light source module  320  and the gain medium  340 . The first isolator passes the ASE inputted from the ASE light source module  320  and blocks light progressing in the opposite direction. 
   The pump light source  350  outputs pump light and may incorporate a laser diode with a wavelength of 980 nm or 1480 nm. 
   The wavelength selective coupler  360  is located between the gain medium  340  and the second isolator  335 . The wavelength selective coupler  360  supplies the pump light to the gain medium  340 . 
   The gain medium  340  is located between the first isolator  330  and the wavelength selective coupler  360 . The gain medium  340  amplifies and outputs the ASE having passed the first isolator  330  as it is pumped by the pump light. The ASE amplified by and outputted from the gain medium  340  passes the wavelength selective coupler  360  and the second isolator  335 . Then, the ASE is outputted to the outside through an output end  302  of the broadband light source  300 . When a thulium doped fiber (TDF) is used as the gain medium  340 , it is possible to obtain ASE which has high output power in a wavelength band of 1450 nm˜1510 nm. When a praseodymium doped fiber (PDF) is used, it is possible to obtain ASE which has high output power in a wavelength band of 1270 nm˜1330 nm. Accordingly, in order to obtain ASE in a desired wavelength band, a gain medium having a high gain spectrum within a corresponding wavelength band and a pump light source capable of exciting the gain medium need be used. That is, because the broadband light source  300  is able to employ all the conventionally applicable gain mediums, the wavelength band of the broadband light source  300  is expandable over an entire available wavelength band without being limited to a certain band. 
   The second isolator  335  is located between the wavelength selective coupler  360  and the output end of the broadband light source  300 . The second isolator  335  passes the ASE having passed the wavelength selective coupler  360  and blocks light progressing in the opposite direction. 
     FIG. 4  is a diagram of a hybrid broadband light source according to a second embodiment of the present invention. The broadband light source  400  comprises an ASE light source module  420 , a band-pass filter  440 , a gain medium  450 , a pump light source  460 , a wavelength selective coupler  470 , and a second isolator  435 . The ASE light source module  420 , the band-pass filter  440 , the gain medium  450 , the wavelength selective coupler  470 , and the second isolator  435  are connected in series using a first optical waveguide  410 . The pump light source  460  is connected in parallel to the gain medium  450  using a second optical waveguide  415 . 
   The ASE light source module  420  is installed at a terminal end of the broadband light source  300 . The ASE light source module  420  generates and outputs ASE. At an output end of the ASE light source module  420 , a first isolator  430  is directly integrated, and the first isolator  430  passes the ASE inputted to the first isolator and blocks light the opposite direction. 
   The band-pass filter  440  is located between the ASE light source module  420  and the gain medium  450 . The band-pass filter  440  limits the bandwidth of the ASE spectrum generated light source module  420  as the wavelength band of 1530 nm˜1570 μm, and the gain medium  450  can amplify this wavelength band ASE effectively. Thus, high output power can be obtained. 
   The pump light source  460  outputs pump light. 
   The wavelength selective coupler  470  is located between the gain medium  450  and the second isolator  435 . The wavelength selective coupler  470  supplies the pump light to the gain medium  450 . 
   The gain medium  450  is located between the band-pass filter  440  and the wavelength selective coupler  470 . The gain medium  450  amplifies and outputs ASE having passed the band-pass filter  440 , as it is pumped by the pump light. The ASE amplified by and outputted from the gain medium  450  passes the wavelength selective coupler  470  and the second isolator  435 . Then, the ASE is outputted through the output end of the broadband light source  400 . 
     FIG. 5  is a graph of ASE spectrums in regard to positions of the broadband light source shown in  FIG. 4 .  FIG. 5  shows first spectrum  510  in position D, and second spectrum  520  in position E. It can be seen that the ASE of position D having the first spectrum  510  appears high output power in the wavelength band of 1540 nm˜1560 nm by passing the band-pass filter  440  and the gain medium  450 . 
   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.