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. 61/169,361, entitled “Parallel Digital Coherent Detection Using Symmetrical Optical Interleaver And Direct Optical Down Conversion”, filed on Apr. 15, 2010, the contents of which is incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    In high-capacity, long-haul fiber data transmission, digital signal processing (DSP) after signal detection can greatly improve transmission performance by providing compensation against fiber impairments such as chromatic dispersion (CD) and polarization mode dispersion (PMD). Together with polarization diversity coherent detection, DSP also grants polarization division multiplexing (PDM) to double the transmitted data rate. To implement DSP for post detection processing, analog-to-digital converters (ADCs) at high sampling rate are needed in the coherent receiver design. In reality, the signal bandwidth that can be supported by electronic ADC is much lower than the E-O modulator bandwidth at the transmitter. Currently, the state of the art is about 20 GHz for electronic ADC bandwidth, while 40 GHz bandwidth E-O modulators are already commercially available. To fully utilize the potential of post-detection DSP at high data rates, a new ADC technology is required to fill the bandwidth gap. 
         [0003]    Parallel processing of incoming high BW signals is often used to achieve high ADC sampling rates. Electronic time-interleaved ADCs have synchronized track-and-hold circuitry, which has to cover the entire input signal BW (state of the art ˜20 GHz), on each parallel sampling path. Parallel processing in frequency domain, which can reduce the BW requirement of each sampling path, has been proposed to reach higher sampling rates. We proposed recently a photonic filter bank (PFB) structure using orthogonal filter design to allow digital perfect reconstruction in theory. In order to sample the high frequency tributary of the incoming signal, filter bank methods need RF electronic frequency down converters. The need for rf down conversion not only increases system complexity and cost (a typical phase/polarization diversity receiver requires a total of four converters), but it may also degrade system performance because of the difficulty of analog wideband processing in electronic domain. In terms of filter design, it is desirable to have sharp filter roll-offs because of the limited ADC BW in each parallel sampling paths. In PFB structure, sharp roll-offs are challenging to implement using orthogonal filter design because many optical taps are required. 
         [0004]    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 
       [0005]    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. 
         [0006]    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 
         [0007]    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. 
           [0008]      FIG. 1  is a diagram illustrating a PFB architecture with two parallel ADC sampling paths, in accordance with the invention 
           [0009]      FIG. 2  is a diagram illustrating details of the front end of the digital coherent scheme, in accordance with the invention. 
           [0010]      FIG. 3  is a diagram of transformations of an optical signal by the PFB architecture, in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    The invention is directed to an inventive PFB architecture that uses an optical interleaver as a two-way filter bank. The optical interleaver exhibits a symmetrical spectral response about the optical carrier frequency, which will automatically translate the two filtered optical signal to low-frequency and high-frequency tributaries after O-E conversion. Optical interleavers can be easily designed to have very sharp roll-offs so that the filtered tributaries will fit into the sampling BW of electronic ADC. Another aspect of the PFB architecture is a direct optical down conversion scheme, which uses two synchronized optical local oscillators (LO) during coherent detection located with respect to the high-frequency tributary in such a way that RF down converters are no longer required. With this new invention, we can nearly double the supported signal BW for digital coherent detection. 
         [0012]      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. 
         [0013]    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 . 
         [0014]    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. 
         [0015]    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 interleaver 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. 
         [0016]    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 . 
         [0017]    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. 
         [0018]    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.