Abstract:
Provided are an acousto-optic filter and an optical code division multiple access (CDMA) system using the acousto-optic filter. The acousto-optic filer includes: an acousto-optic mode converter (AOMC) converting an optical signal of a specific optical frequency corresponding to a frequency of an electric signal of an optical signal of a first mode having a predetermined optical frequency band; and a mode stripper (MS) stripping an optical signal of the optical signal of the first mode that has been converted to a second mode.

Description:
The present patent application is a Divisional claiming the benefit of application Ser. No. 11/593,909, filed Nov. 6, 2006 now abandoned. 

   CROSS-REFERENCE TO RELATED PATENT APPLICATION 
   This application claims the benefit of Korean Patent Application No. 10-2005-0110359, filed on Nov. 17, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an optical code division multiple access (CDMA) encoder and/or decoder, and more particularly, to an acousto-optic filter used for an optical CDMA decoder and/or encoder and an optical CDMA system using the same. 
   2. Description of the Related Art 
   Conventional wavelength domain optical CDMA encoders and/or decoders mainly include fiber Bragg gratings (FBGs) or general diffraction gratings. 
   Optical CDMA encoders and/or decoders using general diffraction gratings divide light according to a wavelength using the general diffraction gratings, transmit or reflect optical signals having specific wavelengths, and sum the divided optical signals using the general diffraction gratings so as to perform encoding and/or decoding. Thus, the optical CDMA encoders and/or decoders have large volumes and complicated structures. 
   Optical CDMA encoders and/or decoders using FBGs arrange reflected wavelengths of optical filters including the FBGs using arbitrary signs so as to perform encoding and/or decoding. Thus, the optical CDMA encoders and/or decoders include several FBGs. 
   SUMMARY OF THE INVENTION 
   The present invention provides an acousto-optic filter applying several frequencies of an electric signal to arrange a spectrum of a transmitted wavelength with a specific code so as to encode and/or decode an optical signal. 
   The present invention also provides an optical CDMA system using an acousto-optic filter easily adjusting a channel depending on a variation in a frequency of an electric signal. 
   According to an aspect of the present invention, there is provided an acousto-optic filter used for an optical code division multiple access (CDMA) encoder, including: an acousto-optic mode converter (AOMC) converting an optical signal having a specific optical frequency corresponding to a frequency of an electric signal into a second mode, wherein the optical signal is one of signals of a first mode having a predetermined optical frequency band; and an mode stripper (MS) stripping the optical signal of the second mode. 
   According to another aspect of the present invention, there is provided an acousto-optic filter used for an optical CDMA decoder, including: an AOMC converting an optical signal having a specific optical frequency corresponding to a frequency of an electric signal into a second mode, wherein the optical signal is one of signals of a first mode having a predetermined optical frequency band; and a mode divider (MD) outputting the optical signal of the first mode and the optical signal of the second mode to different ports, respectively. 
   According to another aspect of the present invention, there is provided an optical CDMA system using an acousto-optic filter, including: at least one or more optical CDMA encoders outputting only an optical signal except an optical signal having a specific optical frequency corresponding to a frequency of an electric signal using an acousto-optic filter performing acousto-optic mode converting and mode stripping; an optical coupler coupling optical signals output from a plurality of optical CDMA encoders; and an optical CDMA decoder dividing the optical signals received from the optical coupler into a specific optical frequency corresponding to a predetermined frequency of an electric signal and outputting the divided optical signal using an acousto-optic filter performing acousto-optic mode converting and mode stripping. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
       FIG. 1  is a view illustrating an acousto-optic filter used for an optical CDMA encoder according to an embodiment of the present invention; 
       FIG. 2  is a view illustrating a process of applying an electric signal having several frequencies to the acousto-optic filter shown in  FIG. 1 ; 
       FIG. 3A  is a view illustrating a spectrum of light having a wide line width; 
       FIG. 3B  is a view illustrating a spectrum of the light of  FIG. 3A  applied to an acousto-optic filter; 
       FIG. 4  is a view illustrating an acousto-optic filter used for an optical CDMA decoder according to an embodiment of the present invention; and 
       FIG. 5  is a view illustrating an optical CDMA system using an acousto-optic filter according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Hereinafter, an acousto-optic filter used for an optical CDMA encoder and/or decoder and an optical CDMA system according to the present invention will be described in detail with reference to the attached drawings. 
     FIG. 1  is a view illustrating an acousto-optic filter used for an optical CDMA encoder according to an embodiment of the present invention. Referring to  FIG. 1 , an acousto-optic filter  100  used for an optical CDMA encoder includes an acousto-optic mode converter (AOMC)  110  and a mode striper (MS)  120 . 
   If light input to the AOMC  110  is a fundamental mode or a first mode having a light frequency and a frequency of an electric signal input to the AOMC  110  is f 1 , the AOMC  110  outputs a second mode having a light frequency of ω 1 +f 1 . 
   Since the frequency f 1  is very smaller than a frequency ω 1 , an electric frequency component f 1  hardly affects a variation in the light frequency ω 1 . The AOMC  110  and the MS  120  are connected to each other through a dual mode optical fiber. 
   The MS  120  passes a fundamental mode and strips the second mode. Thus, if the light output from the AOMC  110  is input, the MS  120  does not transmit light having a light frequency of ω 1 . 
   In other words, the AOMC  110  does not frequency and mode convert all light frequencies or wavelengths but converts only light having specific light frequencies corresponding to the frequency of an electrical signal input to the AOMC  110  into a second mode. The MP  120  is hardly affected by a light frequency of light. 
   Thus, a notch filter can be formed using the AOMC  110  and the MS  120  shown in  FIG. 1 . The frequency of the electric signal input to the AOMC  110  can be adjusted so as to realize an acousto-optic tunable filter (AOFT)  100 . 
     FIG. 2  is a view illustrating a process of applying an electric signal having several frequencies to the acousto-optic filter shown in  FIG. 1 . Referring to  FIG. 2 , if an electric signal having several frequencies is applied to the AOMC  110  simultaneously, only an optical signal having an optical frequency corresponding to each of the frequencies of the electric signal is frequency and mode converted. 
   In other words, if light having optical frequencies of ω 1 , ω 2 , . . . , and ω n  and an electric signal having frequencies of f 1 , f 2 , f 4 , f 5 , . . . , and f n  are input, the light having optical frequencies of ω 1 , ω 2 , . . . , and ω n  is converted into light having optical frequencies of ω 1 +f 1 , ω 2 +f 1 , . . . , and ω n +f 1 , and a mode is converted into a second mode. 
   If light having a broadband wavelength as shown in  FIG. 3A  is input to the AOMC  110  of the acousto-optic filter shown in  FIG. 1  and an electric signal having frequencies of f 1 , f 2 , f 4 , f 5 , . . . , f n  are applied to the AOMC  110 , a transmission characteristic as shown in  FIG. 3B  can be obtained. 
     FIG. 4  is a view illustrating an acousto-optic filter used for an optical CDMA decoder according to an embodiment of the present invention. Referring to  FIG. 4 , an acousto-optic filter  400  used for an optical CDMA decoder includes an AOMC  410  and a mode divider (MD)  420 . 
   If light input to the AOMC  410  is a fundamental mode having an optical frequency of ω 1  and a frequency of an electric signal input to the AOMC  410  is f 1 , the AOMC  410  converts the input light into an optical frequency of ω 1 +f 1  and a mode into a second mode. 
   The frequency f 1  is very smaller than the frequency ω 1  as described with reference to  FIG. 1 , and thus the frequency f 1  can be disregarded. 
   The MD  420  divides the mode into a fundamental mode and the second mode and then outputs them to different ports. In other words, if the MD  420  receives the light from the AOMC  410 , the MD  420  outputs light having the optical frequency of ω 1  to a port  430  and light having a different optical frequency to a port  440 , 
   Similar to the MS  120  shown in  FIG. 1 , the MD  420  is hardly affected by an optical frequency. Thus, the AOMC  410  and the MD  420  can be constituted as shown in  FIG. 4  so as to be used as decoders. 
     FIG. 5  is a view illustrating an optical CDMA system using an acousto-optic filter according to an embodiment of the present invention. Referring to  FIG. 5 , the optical CDMA system includes a plurality of optical CDMA encoders  500 - 1 ,  500 - 2 , and  500 - n  including acousto-optic filters  520  as shown in  FIG. 1  and a plurality of optical CDMA decoders  550 - 1 ,  550 - 2 , and  550 - n  including acousto-optic filters as shown in  FIG. 4   
   Light output from broadband light sources  510  is input to the acousto-optic filter  520  including the AOMC  110  and the MS  120 . 
   If electric signals having frequencies (for example, f 1 , f 2 , f 3 , and f 6 ) corresponding to specific channels are summed and then to applied to the acousto-optic filters  520 , only optical signals having optical frequencies except optical frequencies ω 1 , ω 2 , ω 3 , and ω 6  corresponding to f 1 , f 2 , f 3 , and f 6  are transmitted so as to perform encoding in a wavelength region. Different frequencies are respectively allocated to the encoders  500 - 1 ,  500 - 2 , and  500 - n . Light encoded and output from the encoders  500 - 1 ,  500 - 2 , and  500 - n  is coupled by an optical coupler  530  and then incident on the decoders  550 - 1 ,  550 - 2 , and  550 - n.    
   In a case where a decoder has a complementary code to a frequency allocated to an encoder, each of balanced photodetectors (BPDs)  570  detects light signals from two output ports of each of the acousto-optic filters  560  and outputs the signal corresponding to the power difference between two light signals. 
   In a case where the decoder does not have the complementary code to the frequency allocated to the encoder, light is divided and then incident on the two ports of each of the BPDs  570 , because matched and mismatched signals with AOMC  410  characteristics are extracted to the first mode and second mode output ports, respectively. Thus, a signal transmitted from another channel is offset so that the BPDs  570  having a balanced detector structure does not detect signals. As a result, an optical CDMA system having a simple structure can be constituted 
   As described above, according to the present invention, a simple encoder and/or decoder can be constituted using an acousto-optic filter in a wavelength domain optical CDMA. Also, a variable encoder and/or decoder can be constituted to adjust an optical transceiver channel so as to improve efficiency of communication traffic of an optical communication system. 
   Differently from a conventional optical CDMA encoder and/or decoder controlling an optical element, a frequency of an electric signal applied to an acousto-optic filter can be adjusted to adjust a channel. Thus, the simple encoder and/or decoder are advantageous in terms of cost and operation. As a result, the simple encoder and decoder can be very useful to an optical communication network and optical communications requiring security. 
   While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.