Abstract:
A broadband light source includes: at least two semiconductor optical active devices for generating TE polarized lights of different wavelength bands; an optical coupler for dividing each of the TE polarized lights input from each of the semiconductor optical active devices into two TE polarized lights, and outputting the two TE polarized lights, the two TE polarized lights including a first TE polarized light and a second TE polarized light; a first optical line for transmitting the first TE polarized light, while maintaining a polarization mode of the first TE polarized light; a second optical line for converting the second TE polarized light into a TM polarized light; a polarization beam combiner for combining the first TE polarized light and the TM polarized light to generate a polarization-independent light; and a band separator for separating and outputting the polarization-independent light according to wavelength bands.

Description:
CLAIM OF PRIORITY  
       [0001]     This application claims to the benefit under 35 U.S.C. 119 of an application entitled “Wavelength-Division-Multiplexed Passive Optical Network,” filed in the Korean Intellectual Property Office on Jan. 3, 2005 and assigned Serial No. 2005-111, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a semiconductor optical device, and more particularly to an active-type semiconductor optical device capable of generating a broad wavelength band light.  
         [0004]     2. Description of the Related Art  
         [0005]     In general, broadband light sources capable of generating a broad band light include an erbium-doped optical fiber amplifier and a semiconductor optical active device which may be used to generate wavelength-locked optical signals in a wavelength-division-multiplexed optical network or may be used as a spectrum-sliced light source. In the wavelength-division-multiplexed optical network, an optical signal having a distinct wavelength is allocated to each of multiple optical network units so as to be used in an optical communication. That is, according to the wavelength-division-multiplexed optical network, light of a specific wavelength band is divided into a plurality of channels having different wavelengths, and data is loaded in a corresponding channel to be transmitted. Therefore, in the wavelength-division-multiplexed optical network, the wider the wavelength band of a light becomes, the easier the expansion of lines becomes. However, the erbium-doped optical fiber amplifier has limited wavelengths and has a problem in that it has high fabrication costs and large volume. The erbium-doped optical fiber amplifier stably generates a high-power polarization-insensitive light, but is largely limited in selection of a wavelength band and range as compared with the semiconductor optical active device.  
         [0006]     In contrast, the semiconductor optical active device has advantages in that the wavelength band and range can be simply selected and miniaturization is possible, but has drawbacks in that it has relatively poor characteristics in output, polarization, and spectrum. However, a light generated from the semiconductor optical active device has a large polarization dependence, which limits the application in an optical communication network.  
         [0007]     In order to decrease the polarization dependence of the semiconductor optical active device, a method of controlling the polarization component of light by applying strain to a bulk active structure instead of a quantum-well active structure has been proposed. However, it is practically impossible to precisely control the polarization mode of light, and also it is impossible to completely eliminate all polarization dependence without causing a decrease in the yield of products due to inherent characteristic of the semiconductor devices. Accordingly, the high polarization dependence of the semiconductor optical active device has many disadvantages.  
       SUMMARY OF THE INVENTION  
       [0008]     Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art and provides additional advantages, by providing e a broadband light source capable of eliminating a polarization dependence of light having a broad wavelength band generated from a semiconductor optical active device.  
         [0009]     In one embodiment, there is provided a broadband light source comprising: at least two semiconductor optical active devices for generating TE polarized lights of different wavelength bands; an optical coupler for dividing each of the TE polarized lights input from each of the semiconductor optical active devices into two TE polarized lights, and outputting the two TE polarized lights, the two TE polarized lights including a first TE polarized light and a second TE polarized light; a first optical line for transmitting the first TE polarized light, while maintaining a polarization mode of the first TE polarized light; a second optical line for converting the second TE polarized light into a TM polarized light; a polarization beam combiner for combining the first TE polarized light and the TM polarized light to generate a polarization-independent light; and a band separator for separating and outputting the polarization-independent light according to wavelength bands. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     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:  
         [0011]      FIG. 1  is a block diagram illustrating a configuration of a broadband light source according to a first embodiment of the present invention;  
         [0012]      FIG. 2  is a block diagram illustrating a configuration of a broadband light source according to a second embodiment of the present invention; and  
         [0013]      FIG. 3  is a block diagram illustrating a passive optical network including a broadband light source according to a third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0014]     Hereinafter, embodiments according to 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 obscure the subject matter of the present invention.  
         [0015]      FIG. 1  is a block diagram illustrating the configuration of a broadband light source according to a first embodiment of the present invention.  
         [0016]     As shown, the broadband light source  100  includes two or more semiconductor optical active devices  111  and  112 , an optical coupler  120 , first and second optical lines  121  and  122 , a polarization beam combiner  130 , a band separator  140 , and a single-mode optical fiber  131 . The single-mode optical fiber  131  is located between the polarization beam combiner  130  and the band separator  140  and transmits a polarization-independent light to the band separator  140 .  
         [0017]     The semiconductor optical active devices  111  and  112  may include a superluminescent diode (SLD), a semiconductor optical amplifier (SOA), or other equivalent optical device capble of generating a light of a specific polarization mode. The semiconductor optical active devices  111  and  112  generate TE polarized lights having wavelength bands different from each other.  
         [0018]     The optical coupler  120  divides each of the TE polarized lights input from the semiconductor optical active devices  111  and  112  into two TE polarized lights, then outputs the divided two TE polarized lights (including a first TE polarized light and a second TE polarized light) to corresponding paths, respectively. The optical coupler  120  may include a 2×2 optical coupler.  
         [0019]     The first and second optical lines  121  and  122  may include a polarization-maintaining optical fiber. The first optical line  121  transmits the first TE polarized light, which is one of the divided TE polarized lights, while maintaining the polarization mode of the first TE polarized light. The second optical line  122  includes a polarization-maintaining optical fiber, which is rotated 90° about the axis of the TE polarization. Therefore, the second optical line  122  converts the second TE polarized light, which is the other of the two divided TE polarized lights, into a TM polarized light, and outputs the TM polarized light.  
         [0020]     The polarization beam combiner  130  combines each of the TE polarized lights and each of the TM polarized lights, which have been input through the first and second optical lines  121  and  122 , thereby generating polarization-independent lights having different wavelength bands. That is, the TE polarized light and TM polarized light of relevant wavelength bands output from the semiconductor optical active devices  111  and  112 , respectively, are combined with each other to generate broadband lights, which have different wavelength bands.  
         [0021]     The lights obtained through the combination are transmitted to the band separator  140  through the single-mode optical fiber  131 . The band separator  140  separates and outputs the received light according to their wavelength bands.  
         [0022]      FIG. 2  is a block diagram illustrating the configuration of a broadband light source according to a second embodiment of the present invention.  
         [0023]     As shown, the broadband light source  200  includes two or more semiconductor optical active devices  211  and  212 , an optical coupler  220 , first and second optical lines  221  and  222 , a polarization beam combiner  230 , a band separator  240 , and a single-mode optical fiber  231 . The single-mode optical fiber  231  is located between the polarization beam combiner  230  and the band separator  240 , and transmits a polarization-independent light to the band separator  240 .  
         [0024]     The semiconductor optical active devices  211  and  212  generate TM polarized lights having wavelength bands different from each other and output the generated TM polarized lights to the optical coupler  220 . The optical coupler  220  divides each of the TM polarized lights into two TM polarized lights, then outputs the two divided TM polarized lights (including a first TM polarized light and a second TM polarized light) to the first and second optical lines  221  and  222 , respectively.  
         [0025]     The first optical line  221  includes a polarization-maintaining optical fiber which is rotated 90° about the axis of the TM polarization. The first optical line  221  converts the first TM polarized light, which is one of the two divided TM polarized lights, into a TE polarized light, and outputs the converted TE polarized light to the polarization beam combiner  230 . In contrast, the second optical line  222  outputs the second TM polarized light, which is the other of the two divided TM polarized lights, to the polarization beam combiner  230 , while maintaining the polarization mode of the TM polarized light.  
         [0026]     The polarization beam combiner  230  combines each TM polarized light and each TE polarized light of relevant wavelength bands, which have been input through the first and second optical lines  221  and  222 , to generate broadband lights having different broad wavelength bands. The light obtained through the combination are separated and transmitted through the band separator  240   
         [0027]      FIG. 3  is a block diagram illustrating a passive optical network including a broadband light source according to a third embodiment of the present invention. The passive optical network  300  includes a central office  310 , a plurality of optical network units  340 , and a remote node  330  located between the central office  310  and the optical network units  340 . The central office  310  generates wavelength-locked downstream optical signals and also detects upstream optical signals, and each of the optical network units  340  receives a downstream optical signal of a relevant wavelength. The remote node  330  and the central office  310  are linked by a main optical fiber, and each of the optical network units  340  is linked to the remote node  330  by a local optical fiber.  
         [0028]     The central office  310  includes a broadband light source  320 , a plurality of downstream light sources  312 , a plurality of upstream optical detectors  311 , a plurality of wavelength selection combiners  313 , a multiplexer/demultiplexer  314 , and an optical switch  315 .  
         [0029]     The broadband light source  320  includes two or more semiconductor optical active devices  312  and  322 , an optical coupler  323 , first and second optical lines  326  and  327 , a polarization beam combiner  324 , and a band separator  325 .  
         [0030]     The semiconductor optical active devices  321  and  322  generate TE polarized lights having wavelength bands different from each other and output the generated TE polarized lights to the optical coupler  323 .  
         [0031]     The optical coupler  323  includes two input ports and two output ports. The optical coupler  323  divides each of the TE polarized lights into two TE polarized lights and outputs the two divided TE polarized lights (including a first TE polarized light and a second TE polarized light) to the first and second optical lines  326  and  327  connected to relevant output ports, respectively.  
         [0032]     The first and second optical lines  326  and  327  may include a polarization-maintaining optical fiber. The first optical line  326  transmits the first TE polarized light, which is one of the two divided TE polarized lights, while maintaining the polarization mode of the first TE polarized light. In contrast, the second optical line  327  includes a polarization-maintaining optical fiber which is rotated 90° about the axis of the TE polarization. Therefore, the second optical line  327  converts the second TE polarized light, which is the other of the two divided TE polarized lights, into a TM polarized light, and outputs the TM polarized light.  
         [0033]     The polarization beam combiner  324  combines each of the TE polarized lights and each of the TM polarized lights, which have been input through the first and second optical lines  326  and  327 , to generate and output downstream and upstream lights having different wavelength bands.  
         [0034]     The band separator  325  separates and outputs each of the downstream and upstream lights, which have been input through a single-mode optical fiber.  
         [0035]     The broadband light source  320  may include a light source for generating TM polarized lights of different wavelength bands. That is, from the generated TM polarized lights, downstream and upstream polarization-independent lights having different wavelength bands can be generated in the same way as in the case of the TE polarized lights.  
         [0036]     The above-mentioned broadband light source may be applied to the case in which the semiconductor optical active devices  321  and  322  generate TM polarized lights having different wavelength bands. In this case, each of the generated TM polarized lights is input to the optical coupler  323 . The optical coupler  323  divides each of the TM polarized lights into two TM polarized lights and outputs the two divided TM polarized lights to the first and second optical lines  326  and  327 .  
         [0037]     The first optical line  326  outputs an input TM polarized light to the polarization beam combiner  324 , while maintaining the mode of the input TM polarized light. The second optical line  327  converts an input TM polarized light into a TE polarized light and outputs the converted TE polarized light to the polarization beam combiner  324 . That is, the second optical line  327  includes a polarization-maintaining optical fiber which is rotated 90° about the axis of the TE polarization, thereby converting input TM polarized lights into TE polarized lights. Thereafter, the broadband light source combines the TE and TM polarized lights with upstream and downstream lights having different wavelength bands.  
         [0038]     The multiplexer/demultiplexer  314  divides the downstream light into a plurality of downstream channels having different wavelengths and outputs the divided lights to corresponding downstream light sources. Also, the multiplexer/demultiplexer  314  demultiplexes upstream optical signals, which has been multiplexed and input from the remote node  330 , and outputs the demultiplexed upstream optical signals to corresponding upstream optical detectors  311 . In addition, the multiplexer/demultiplexer  314  multiplexes and outputs downstream optical signals wavelength-locked by the downstream light sources  312  to the remote node  330 .  
         [0039]     Each of the downstream light sources  312  generates a downstream optical signal wavelength-locked by a corresponding downstream channel, and each of the upstream optical detectors  311  detects an upstream optical signal having a corresponding wavelength from among the demultiplexed upstream optical signals.  
         [0040]     The optical switch  315  is located on the main optical fiber disposed between the multiplexer/demultiplexer  314  and the remote node  330 , and is connected to the broadband light source  320 . The optical switch  315  inputs and outputs multiplexed downstream and upstream optical signals. That is, the optical switch  315  outputs the downstream light to the multiplexer/demultiplexer  314 , and outputs the upstream light to the remote node  330 .  
         [0041]     The remote node  330  divides the upstream light into a plurality of upstream channels having different wavelengths and outputs the divided light to corresponding optical network units  340 , respectively. The remote node  330  multiplexes and outputs wavelength-locked upstream optical signals, which have received from the optical network units  340 , to the central office  310 . In addition, the remote node  330  demultiplexes and outputs multiplexed downstream optical signals, which have been input through the optical switch  315 , to corresponding optical network units  340 . The remote node  330  includes a multiplexer/demultiplexer  331 , which may include an arrayed waveguide grating or a wavelength division multiplexing (WDM) filter.  
         [0042]     Each of the optical network units  340  includes a wavelength selection combiner  343 , an upstream light source  342 , and a downstream optical detector  341 . The wavelength selection combiner  343  outputs an upstream channel of a relevant wavelength to the upstream light source  342 , and outputs an upstream optical signal wavelength-locked by the uplink channel in the upstream light source  342  to the remote node  330 . In addition, the wavelength selection combiner  343  outputs a downstream optical signal of a relevant wavelength to the downstream optical detector  341 .  
         [0043]     The upstream light source  342  may include a Fabry-Perot laser and generates an upstream optical signal wavelength-locked by an upstream channel of a relevant wavelength. The downstream optical detector  341  may include a photo diode and detects a downstream optical signal of a relevant wavelength.  
         [0044]     As described above, the broadband light source according to the present invention can generate a polarization-independent light which is not influenced by the polarization property of the semiconductor optical active device, and enables the semiconductor optical active device to easily generate broadband lights having different wavelength bands. Furthermore, since the broadband light source according to the present invention can be applied to the semiconductor optical active device, which can be easily integrated and manufactured in a compact size, it is easy to optimize and miniaturize the volume and configuration of a system to which the broadband light source is applied.  
         [0045]     While the present 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. Accordingly, the scope of the invention is not to be limited by the above embodiments but by the claims and the equivalents thereof.