Abstract:
Disclosed is a broadband light source with a direct pumping structure including: a first gain medium for generating an amplified spontaneous emission; and a first pump light source connected in series to the first gain medium, for outputting a first pump light provided in the first gain medium.

Description:
CLAIM OF PRIORITY  
       [0001]     This application claims priority to an application entitled “Broadband light source with direct pumping structure,” filed in the Korean Intellectual Property Office on Dec. 17, 2003 and assigned Serial No. 2003-92365, 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 module, and more particularly to a broadband light source.  
         [0004]     2. Description of the Related Art  
         [0005]     An important consideration in the design and construction wavelength division multiplexing passive optical networks (hereinafter, referred to as WDM-PONs), it is the development of economical broadband light sources. In WDM-PONs, the broadband light source plays an important role in simultaneously accommodating many subscribers together with a wavelength locked fabry-perot laser diode. In addition, in optical communication systems using an erbium doped fiber amplifier (hereinafter, referred to as an EDFA), the broadband light source is inevitably required to measure the optical characteristics in signal wavelength ranges of 1530 nm˜1570 nm and 1570 nm˜1610 nm.  
         [0006]     Conventional broadband light sources generally use a white light source using a halogen lamp or an EDFA which outputs an amplified spontaneous emission (hereinafter, referred to as an ASE), and an edge-emitting light emitting diode (hereinafter, referred to as an EELED), a super luminescent diode (hereinafter, referred to as an SLD). However, since the white light source and the EELED have low power, they are not suitable as light sources for a WDM-PON. Further, since the SLD, which has a relatively high power, is slightly inferior to the EDFA in view of power and bandwidth, it is difficult to use the SLD as a broadband light source of the WDM-PON. While the EDFA has been used as a broadband light source, it is not economical in view of its price.  
         [0007]      FIG. 1  is a diagram showing a construction of a conventional broadband light source. The broadband light source  100  includes a pump laser diode  120 , a wavelength selective coupler (WSC)  130 , an erbium doped fiber (EDF)  140 , and an isolator (ISO)  150 . The wavelength selective coupler  130 , the erbium doped fiber  140 , and the isolator  150  are connected in series to each other by means of a first optical waveguide  110 . The pump laser diode  120  is connected in parallel to the erbium doped fiber  140  by means of a second optical waveguide  115 .  
         [0008]     The pump laser diode  120  outputs a pump light having a predetermined wavelength. The wavelength selective coupler  130  provides the erbium doped fiber  140  with the pump light. The erbium doped fiber  140  is pumped by the pump light to output an ASE through its both ends. The ASE output forwardly from the erbium doped fiber  140  passes through the isolator  150  and is output to an exterior of the broadband light source  100  through an output terminal of the broadband light source  100 . The ASE output rearwardly from the erbium doped fiber  140  passes through the wavelength selective coupler  130  and is input to one end  102  of the broadband light source  100 . The ASE then disappears.  
         [0009]     In the conventional broadband light source  100 , as described above, since the pump light output from the pump laser diode  120  passes through the wavelength selective coupler  130 , it suffers from insertion loss in the wavelength selective coupler  130 . In addition, in order to prevent the ASE reflected from one end  102  of the broadband light source  100  from being input to the wavelength selective coupler  130 , an angled connector having a critical angle must be installed at one end  102  of the broadband light source  100 , or an additional isolator must be installed between one end  102  and the wavelength selective coupler  130 . As should be apparent, these additional elements contribute the manufacturing cost of the conventional broadband light source  100 .  
       SUMMARY OF THE INVENTION  
       [0010]     One aspect of the present invention relates to provide a broadband light source with high power and efficiency, which is suitable for a light source for measuring characteristics of an optical device used in a broadband light source for WDM-PON optical communication or optical communication.  
         [0011]     One embodiment of the present is directed to a broadband light source with a direct pumping structure including a first gain medium for generating an amplified spontaneous emission, and a first pump light source connected in series to the first gain medium, for outputting a first pump light provided in the first gain medium.  
         [0012]     Another embodiment of the present invention is directed to a broadband light source including a first gain medium disposed on a first optical waveguide and a first pump light source connected in series to the first gain medium. The first gain medium generates a first amplified spontaneous emission in accordance with a first pump light provided by the first pump light source. The source also includes a second gain medium disposed on a second optical waveguide and a second pump light source connected in series to the second gain medium. The second gain medium generates a second amplified spontaneous emission in accordance with a second pump light provided by the second pump light source. The source also includes a wavelength selective coupler for connecting the first optical waveguide to a third optical waveguide, connecting the second optical waveguide to the third optical waveguide, and outputting the first and second amplified spontaneous emissions to the third optical waveguide. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The above and other aspects, features and embodiments of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0014]      FIG. 1  is a diagram showing the construction of a conventional broadband light source;  
         [0015]      FIG. 2  is a diagram showing the construction of a broadband light source with a direct pumping structure according to a first embodiment of the present invention;  
         [0016]      FIG. 3  is a diagram showing the construction of a broadband light source with a direct pumping structure according to a second embodiment of the present invention;  
         [0017]      FIG. 4  is a diagram showing the construction of a broadband light source with a direct pumping structure according to a third embodiment of the present invention;  
         [0018]      FIG. 5  is a diagram showing the construction of a broadband light source with a direct pumping structure according to a fourth embodiment of the present invention; and  
         [0019]      FIG. 6  is a graph showing a comparison between power of the broadband light source shown in  FIG. 1  and power of the broadband light source shown in  FIG. 2 . 
     
    
     DETAILED DESCRIPTION  
       [0020]     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 configuration incorporated herein will be omitted when it may obscure the subject matter of the present invention.  
         [0021]      FIG. 2  is a diagram showing the construction of a broadband light source with a direct pumping structure according to a first embodiment of the present invention. The broadband light source  200  includes a pump laser diode (pump LD)  220  connected in series by means of an optical waveguide  210 , a gain medium (GM)  230 , and an isolator  240 .  
         [0022]     The pump laser diode  220  is installed at a one (e.g., left) end of the broadband light source  200  and outputs a pump light having a predetermined wavelength.  
         [0023]     The gain medium  230  is connected in series to the pump laser diode  220  and is pumped by the pump light to output an ASE forwardly and rearwardly from the gain medium  230 . The ASE output forwardly from the gain medium  230  passes through the isolator  240  and is output to an exterior of the broadband light source  200  through an output terminal  202  of the broadband light source  200 . The ASE output rearwardly from the gain medium  230  is input to the pump laser diode  220  and then disappears. The gain medium  230  may include a rare-earth ion doped fiber or a rare-earth ion doped planar waveguide.  
         [0024]     The gain medium  230  may generate an ASE having a wavelength range of 1520 nm˜1620 nm when using an erbium doped fiber. It is also noted that the gain medium  230  may generate an ASE having a wavelength range of 1520 nm˜1570 nm, when density inversion in the erbium doped fiber is increased, by shortening the length of the erbium doped fiber or increasing the power of a pump light. In contrast, the gain medium  230  may generate an ASE having a wavelength range of 1570 nm˜1620 nm when the density inversion in the erbium doped fiber is reduced by lengthening the length of the erbium doped fiber. Furthermore, the gain medium  230  may generate an ASE having a wavelength range of 1450 nm˜1510 nm when using a thulium doped fiber (TDF), and may generate an ASE having a wavelength range of 1270 nm˜1330 nm when using a praseodymium doped fiber (PDF). In order to obtain an ASE having a desired wavelength, a gain medium capable of obtaining a high gain spectrum in a corresponding wavelength range and a pump light source capable of exciting the gain medium are used. In this way, a wavelength range of the broadband light source  200  is not limited to a specific wavelength range, but may include various wavelength ranges.  
         [0025]     The isolator  240  is disposed between the gain medium  230  and the output terminal  202  of the broadband light source  200 . It passes the ASE input from the gain medium  230 , and blocks light progressing in a direction reverse to an input direction of the ASE.  
         [0026]     In an embodiment in which the pump laser diode  220  is a fabry-perot laser diode and when a wavelength range of the ASE generated by the gain medium  230  belongs to a wavelength range of a gain spectrum of the pump laser diode  220 , the ASE input to the pump laser diode  220  is resonated on the inside of the pump laser diode  220  and may be then output. The ASE output from the pump laser diode  220  is amplified by the gain medium  230  and may be shown as a ripple on an entire ASE output spectrum of the broadband light source  200 . Further, if the power of the ASE input to the pump laser diode  220  is large, the interior of the pump laser diode  220  may be damaged. When a fabry-perot laser diode having a wavelength range of 980 nm generally used as the pump laser diode  220  is used and the erbium doped fiber is used as the gain medium  230 , a large difference exists between an output wavelength (980 nm band) of the pump laser diode  220  and a wavelength range (1550 nm band) of the ASE generated by the gain medium  230 . Further, since an optical waveguide in the pump laser diode  220  has a core size relatively smaller than that of the optical waveguide  210 , ASE mostly disappears before it is input to a resonator in the pump laser diode  220 . In this regard, large problem does not occur. However, when a fabry-perot laser diode having a wavelength range of 1480 nm is used as the pump laser diode  220 , an additional isolator may be provided between the pump laser diode  220  and the gain medium  230  in order to prevent a ripple from occurring. Currently, in manufacturing the fabry-perot laser diode having the wavelength range of 1480 nm, an isolator is provided at its output side and may be packaged. In this case, a separate isolator is not required.  
         [0027]      FIG. 6  is a graph showing a comparison between power of the broadband light source shown in  FIG. 1  and power of the broadband light source shown in  FIG. 2 . The two broadband light sources have used erbium doped fibers having the lengths equal to each other and pump laser diodes of 980 nm. The broadband light source according to the first embodiment of the present invention has power level improved by about an average of 2 dB in a wavelength range of 1537 nm˜1560 nm, in comparison with the power level of the conventional broadband light source.  
         [0028]      FIG. 3  is a diagram showing the construction of a broadband light source with a direct pumping structure according to a second embodiment of the present invention. The broadband light source  300  includes a pump laser diode  320  connected in series by means of an optical waveguide  310 , a first isolator  330 , a gain medium  340 , and a second isolator  335 .  
         [0029]     The pump laser diode  320  is installed at one end of the broadband light source  300  and outputs a pump light having a predetermined wavelength.  
         [0030]     The first isolator  330  is disposed between the pump laser diode  320  and the gain medium  340 . It passes the pump light input from the pump laser diode  320  and blocks light progressing in a direction reverse to an input direction of the pump light.  
         [0031]     The gain medium  340  is disposed between the first isolator  330  and the second isolator  335 , and is pumped by the pump light to output an ASE forwardly and rearwardly from the gain medium  340 . The ASE output forwardly from the gain medium  340  passes through the second isolator  335  and is output to an exterior of the broadband light source  300  through an output terminal  302  of the broadband light source  300 . The ASE output rearwardly from the gain medium  340  is input to the first isolator  330  and then disappears.  
         [0032]     The second isolator  335  is disposed between the gain medium  340  and the output terminal  302  of the broadband light source  300 . It passes the ASE input from the gain medium  340 , and blocks light progressing in a direction reverse to an input direction of the ASE.  
         [0033]      FIG. 4  is a diagram showing the construction of a broadband light source with a direct pumping structure according to a third embodiment of the present invention. The broadband light source  400  includes a first gain medium  430 , a second gain medium  435 , a first isolator  440 , a second isolator  445 , a first pump laser diode  420 , a second pump laser diode  425 , and a wavelength selective coupler  450 . The first gain medium  430 , the second gain medium  435 , the first isolator  440 , the second isolator  445 , the first pump laser diode  420 , and the wavelength selective coupler  450  are connected in series to each other by means of a first optical waveguide  410 . The second pump laser diode  425  is connected in parallel to the second gain medium  435  by means of a second optical waveguide  415 .  
         [0034]     The first pump laser diode  420  is installed at one end of the broadband light source  400  and outputs a first pump light having a predetermined wavelength.  
         [0035]     The first gain medium  430  is disposed between the first pump laser diode  420  and the first isolator  440 , and is pumped by the first pump light to output an ASE forwardly and rearwardly from of the first gain medium  430 . The ASE output forwardly from the first gain medium  430  passes through the first isolator  440 , is input to the second gain medium  435 , and is amplified by the second gain medium  435 . The ASE then passes through the wavelength selective coupler  450  and the second isolator  445  and is output to an exterior of the broadband light source  400  through an output terminal  402  of the broadband light source  400 . The ASE output rearwardly from the first gain medium  430  is input to the first pump laser diode  420  and then disappears.  
         [0036]     The first isolator  440  is disposed between the first gain medium  430  and the second gain medium  435 . It passes the first pump light input from the first pump laser diode  420 , and blocks light progressing in a direction reverse to an input direction of the first pump light.  
         [0037]     The second pump laser diode  425  outputs a second pump light having a predetermined wavelength.  
         [0038]     The wavelength selective coupler  450  is disposed between the second gain medium  435  and the second isolator  445  and provides the second gain medium  435  with the second pump light.  
         [0039]     The second gain medium  435  is disposed between the first isolator  440  and the wavelength selective coupler  450 , and is pumped by the second pump light to amplify the input ASE, which will be output.  
         [0040]     The second isolator  445  is disposed between the wavelength selective coupler  450  and the output terminal  402  of the broadband light source  400 . It passes the ASE input from the first gain medium  430 , and blocks light progressing in a direction reverse to an input direction of the ASE.  
         [0041]     When erbium doped fibers having lengths different from each other are uses as the first gain medium  430  and the second gain medium  435 , ASEs in a C-band (1530 nm˜1570 nm) and an L-band (1570 nm˜1610 nm) may be simultaneously obtained.  
         [0042]     For instance, when the first gain medium  430  has a length of 50 m and the second gain medium  435  has a length of 10 m, the first gain medium  430  generates an L-band ASE, and the second gain medium  435  amplifies the L-band ASE and simultaneously generates an C-band ASE  
         [0043]      FIG. 5  is a diagram showing the construction of a broadband light source with a direct pumping structure according to a fourth embodiment of the present invention. The broadband light source  500  includes a first to a fifth optical waveguide  510 ,  512 ,  514 ,  516 , and  518 , a first to a fourth pump laser diode  520 ,  522 ,  524 , and  526 , a first to a fourth gain medium  530 ,  532 ,  534 , and  536 , a first to a third isolator  540 ,  542 , and  544 , and a first to a third wavelength selective coupler  550 ,  552 , and  554 .  
         [0044]     The first pump laser diode  520 , the first and the second gain medium  530  and  532 , the first isolator  540 , and the first wavelength selective coupler  550  are connected in series to each other by means of the first optical waveguide  510 . The second pump laser diode  522  is connected in parallel to the second gain medium  532  by means of the fourth optical waveguide  516 .  
         [0045]     The first pump laser diode  520  is installed at a first end of the broadband light source  500  and outputs a first pump light having a predetermined wavelength.  
         [0046]     The first gain medium  530  is disposed between the first pump laser diode  520  and the first isolator  540 , and is pumped by the first pump light to output a first ASE forwardly and rearwardly from of the first gain medium  530 . The first ASE output forwardly from the first gain medium  530  passes through the first isolator  540 , is input to the second gain medium  532 , and is amplified by the second gain medium  532 . The first ASE then passes through the first wavelength selective coupler  550  and is input to the third wavelength selective coupler  554 . The first ASE output rearwardly from the first gain medium  530  is input to the first pump laser diode  520  and then disappears.  
         [0047]     The first isolator  540  is disposed between the first gain medium  530  and the second gain medium  532 . It passes the first ASE input from the first gain medium  530 , and blocks light progressing in a direction reverse to an input direction of the first ASE.  
         [0048]     The second pump laser diode  522  outputs a second pump light having a predetermined wavelength.  
         [0049]     The first wavelength selective coupler  550  is disposed between the second gain medium  532  and the third wavelength selective coupler  554 , and provides the second gain medium  532  with the second pump light.  
         [0050]     The second gain medium  532  is disposed between the first isolator  540  and the first wavelength selective coupler  550 , and is pumped by the second pump light to amplify the first ASE to be output. The first ASE then passes through the first wavelength selective coupler  550  and is input to the third wavelength selective coupler  554 .  
         [0051]     The third pump laser diode  524 , the third and the fourth gain medium  534  and  536 , the second isolator  542 , and the second wavelength selective coupler  552  are connected in series to each other by means of the second optical waveguide  512 . The fourth pump laser diode  526  is connected in parallel to the fourth gain medium  536  by means of the fifth optical waveguide  518 .  
         [0052]     The third pump laser diode  524  is installed at a second end of the broadband light source  500  and outputs a third pump light having a predetermined wavelength.  
         [0053]     The third gain medium  534  is disposed between the third pump laser diode  524  and the second isolator  542 , and is pumped by the third pump light to output a second ASE forwardly and rearwardly from of the third gain medium  534 . The second ASE output forwardly from the third gain medium  534  passes through the second isolator  542 , is input to the fourth gain medium  536 , and is amplified by the fourth gain medium  536 . The second ASE then passes through the second wavelength selective coupler  552  and is input to the third wavelength selective coupler  554 . The second ASE output rearwardly from the third gain medium  534  is input to the third pump laser diode  524  and then disappears.  
         [0054]     The second isolator  542  is disposed between the third gain medium  534  and the fourth gain medium  536 . It passes the second ASE input from the third gain medium  534 , and blocks light progressing in a direction reverse to an input direction of the second ASE.  
         [0055]     The fourth pump laser diode  526  outputs a fourth pump light having a predetermined wavelength.  
         [0056]     The second wavelength selective coupler  552  is disposed between the fourth gain medium  536  and the third wavelength selective coupler  554 , and provides the fourth gain medium  536  with the second pump light.  
         [0057]     The fourth gain medium  536  is disposed between the second isolator  542  and the third wavelength selective coupler  554 , and is pumped by the fourth pump light to amplify the input second ASE to be output. The second ASE then passes through the second wavelength selective coupler  552  and is input to the third wavelength selective coupler  554 .  
         [0058]     The third wavelength selective coupler  554  connects the first optical waveguide  510  to the third optical waveguide  514 , and connects the second optical waveguide  512  to the third optical waveguide  514 . The third wavelength selective coupler  554  outputs the input first and second ASEs to the third optical waveguide  514 .  
         [0059]     The third isolator  544  is installed on the third optical waveguide  514  in order to be disposed between the third wavelength selective coupler  554  and an output terminal  502  of the broadband light source  500 . Further, the third isolator  544  passes the input first and second ASEs and blocks light progressing in a direction reverse to an input direction of the first and the second ASE. The first and the second ASE passing through the third isolator  544  is output to an exterior of the broadband light source  500  through the output terminal  502  of the broadband light source  500 .  
         [0060]     As described above in relation to the first embodiment, a broadband light source with a direct pumping structure can obtain improved power output by connecting a pump laser diode to a gain medium in series. In addition, such a broadband light source can be more economically manufactured by reducing the number of optical devices.  
         [0061]     It will also be appreciated that broadband light sources contribute to a major portion of the total cost of a WDM-PON system using a wavelength locked fabry-perot laser diode. Therefore, when a broadband light source employs a basic structure or an applied structure according to various embodiments of the present invention, the broadband light source enables construction of an economic optical subscriber network.  
         [0062]     While the invention has been shown and described with reference to certain 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.