Abstract:
A method includes modulating lightwaves to provide first and second OFDM signal sidebands at a first polarization direction and first and second OFDM signal sidebands at a second polarization direction, and combining sidebands that are oppositely positioned and joined from the first and second OFDM signal sidebands at each polarization direction to provide a polarization multiplexing OFDM signal.

Description:
[0001]    This application claims the benefit of U.S. Provisional Application No. 61/030,346, entitled “Simultaneous Generation of Centralized Lightwaves and Double/Single Sideband Optical Millimeter-Wave Requiring Only Low frequency Local Oscillator Signals for radio-Over-Fiber Systems”, filed on Feb. 21, 2008, the contents of which is incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Optical networks designed for Ethernet traffic are becoming more important as the dominance of data over voice services increases. Work in both standards committees and research communities have targeted the transport of 100-Gbit/s Ethernet (100 GE) over wide area networks. Orthogonal frequency division multiplexing (OFDM) is a good transmission format for realizing 100 Gbit/s signal transmission. In recent years, a number of different alternatives have OFDM as a promising method to eliminate the need for optical dispersion compensation in long-haul transmission links. Fiber-optic OFDM systems can be realized either with direct detection optical (DDO) or with coherent optical (CO) detection. Recently, several high data rate OFDM transmission experiments have been reported. Up to 52.5 Gbit/s OFDM signal has been generated and transmitted over 4160 km. But due to the limited bandwidth of the analog to digital converter (A/D) and digital to analog converter (D/A), no 100 Gbit/s OFDM signal has been generated. 
         [0003]    The diagrams of  FIGS. 1 and 2  show the architecture to generate over 50 Gbit/s OFDM signal in a publication,  Sander Jansen  et al., 16×52.5-Gb/s, 50-GHz spaced, POLMUX-CO-OFDM transmission over 4,160 km of SSMF enabled by MIMO processing, ECOC 2007: PD. 1. 3. The diagram of  FIG. 1  is directly from the  Sander Jansen  et al. publication and can be reviewed for further details beyond what are necessary here. 
         [0004]    In the  Sander Jansen  et al. technique, each modulator structure consists of two single-ended MZM modulators  202  or MZ to modulate each polarization independently. Subsequently the two POLMUX signals are combined using a polarization beam splitter  208  and the even and odd WDM channels are combined with a 50-GHz inter-leaver. The electrical OFDM channel allocation is illustrated in  FIG. 1 . Two different frequency RF signals  205 ,  206  are mixed with data  1  and data  2 . After the intensity modulator  202 , the electrum spectrum is shown in  FIG. 1 , while the optical spectrum is shown in  FIG. 2 . Due to the optical carrier suppression, the carrier is suppressed. Then optical filter or inter-leaver ( 207 ) is aligned such that the image band of the OFDM signal is rejected. As you can see in  FIG. 2 , only one sideband is employed. Because both sidebands have the same information, one sideband has to be rejected. In this way, only 50 Gbit/s OFDM can be generated due to the limited bandwidth of an A/D converter. 
         [0005]    Accordingly, there is need for a method to generate over 100 Gbit/s OFDM signals with the limited bandwidth for A/D and D/A converter tolerance. 
       SUMMARY OF THE INVENTION 
       [0006]    In accordance with the invention, a method includes modulating lightwaves to provide first and second OFDM signal sidebands at a first polarization direction and first and second OFDM signal sidebands at a second polarization direction, and combining sidebands that are oppositely positioned and joined from the first and second OFDM signal sidebands at each polarization direction to provide a polarization multiplexing OFDM signal. 
         [0007]    In another aspect of the invention, an apparatus includes a modulator for varying lightwaves to provide first and second OFDM signal sidebands at a first polarization direction and first and second OFDM signal sidebands at a second polarization direction; and a polarization beam combiner for combining sidebands that are oppositely positioned and joined from the first and second OFDM signal sidebands at each polarization direction to provide a polarization multiplexing OFDM signal. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0008]    These and other advantages of the invention will be apparent to those of ordinary skill in the art by reference to the following detailed description and the accompanying figures. 
           [0009]      FIGS. 1 and 2  are diagrams illustrating a known technique for generating over 50 Gbit/s in an OFDM signal. 
           [0010]      FIG. 3  is a diagram of an exemplary 100 Gbit/s OFDM optical signal generation for transmission in accordance with the invention. 
           [0011]      FIG. 4  is a diagram of an exemplary reception of 100 Gbit/s OFDM optical signal generated for transmission in accordance with the invention. 
           [0012]      FIG. 5  a diagram of an exemplary 100 Gbit/s OFDM optical signal generation with two RF frequencies for transmission, in accordance with the invention 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    The invention is directed to a method for generating an over 100 Gbit/s OFDM signal due to both sidebands being employed. 
         [0014]      FIG. 3  is a diagram of an exemplary 100 Gbit/s OFDM optical signal generation for transmission, in accordance with the invention, with only one RF frequency.  FIG. 4  is a diagram of an exemplary reception of 100 Gbit/s OFDM optical signal generated for transmission in accordance with the invention.  FIG. 5  a diagram of a modification to the configuration of  FIG. 3  to show 100 Gbit/s OFDM optical signal generation with two RF frequencies for transmission, in accordance with the invention. 
         [0015]    The diagrams of  FIGS. 3 ,  4  and  5  are exemplary configurations using the following optical and electrical components: lightwave source  301 ,  501 , RF frequency  304 ,  505 ,  506 ; electrical mixer  303 ,  304 ,  504 ; optical coupler  306 ,  507 ; intensity modulator  302 ,  502 ; optical filter  305 ,  508 ; and optical polarization beam combiner  307 ,  509 . 
         [0016]    The lightwave  301 ,  501  can be a narrow linewidth laser less than 2 MHz and the intensity modulator generates optical carrier suppression signals. The electrical mixer  303 ,  304 ,  504  up-converts the baseband signal to an RF band. The RF signal  304 ,  505 ,  506  is provided to the electrical mixer so that the base-band can be up-converted. The optical filter  305 ,  508  is realized by an optical interleaver so that only a high or low frequency signal can be passed for each port if the interleaver has two ports. Preferably, the interleaver has two input ports and one output port with sharp edge characteristics. The optical coupler  306 ,  403 ,  507  are preferably 50% to 50% ratio optical couplers that divide the signal into two equal parts. The optical beam combiner or splitter  307 ,  404 ,  509  combines or splits the orthogonal signal. The electrical combiner  503  combines two different frequency RF signals. 
         [0017]    Referring to the diagram of  FIG. 3 , each intensity modulator  302  is driven by the mixed OFDM signal at RF frequency of f  304  by an electrical mixer  303 . The lightwave  301  is split into two parts by an optical coupler  306 . Then the two parts will be split again by the same optical coupler  306 . There are two polarization directions. We assume that the up-subchannel is X polarization direction and the bottom-one is Y polarization direction. Each modulator  302  is operated at carrier suppression OCS mode. After the modulator, the carrier is suppressed. Then for each polarization direction, we use an optical filter  305 , such as an optical inter-leaver to combine the two subchannels. When the interleaver  305  is matched to the wavelength of the input lightwave, we can generate an optical spectrum  308  and  309  as shown in  FIG. 3 . Each one just passes through half of spectrum (right or left). The optical filter  305  plays a key role tin generating the optical spectrum  308  or  309  and this is the main difference from that technique of  FIG. 1  or  2 . For example, in this figure with the invention, only right (black) and blue (left) can pass the interleaver. Then both sidebands can be used to carry the optical signals. After combing the X and Y polarization direction subchannels by an optical polarization beam combiner  307 , we can generate polarization multiplexing OFDM optical signals. 
         [0018]    The diagram of  FIG. 4  shows an exemplary receiver configuration for receiving the 100 Gbit/s OFDM signal generated according to  FIG. 3 . The incoming lightwave is separated into two parts by an optical filter  401 , interleaver or other optical filter. Then the right and left side will be detected by a regular 90 degree polarization-diversity coherent detector which includes a local oscillator LO  402  fed through optical couplers  403 ,  404  to separate coherent detectors  403 . 
         [0019]    The OFDM signal is generated from the D/A converter. Due to the D/A converter bandwidth limitation, the OFDM signal may not be high enough to carry a signal for over 100 Gbit/s signal (the total capacity with all sub-channels). So we need to change  FIG. 3  to  FIG. 5  to add one more RF frequency. Here, two RF frequencies, f 1   505  and f 2   506  are used. They are used to carry the OFDM signal and drive the modulator. The overall architecture is similar to  FIG. 3 , only one more RF frequency is used. From  FIG. 5  we can see that more spectrum components are generated. 
         [0020]    The present invention has been shown and described in what are considered to be the most practical and preferred embodiments. It is anticipated, however, that departures may be made therefrom and that obvious modifications will be implemented by those skilled in the art. It will be appreciated that those skilled in the art will be able to devise numerous arrangements and variations which, not explicitly shown or described herein, embody the principles of the invention and are within their spirit and scope.