Abstract:
The present invention discloses a receiver for optical system, which provides improved performance due to implementation of multiple parallel analog-to-digital converters. Such configuration allows reducing the data speed processing thus improving bit-error-rate. Each channel of the WDM communications system consists of a set of orthogonal spectral bands. These bands are modulated via orthogonal frequency division multiplexing (OFDM) technique using M-PSK modulation format. At the receiver side, the incoming optical beam is split into a set of parallel branches. Each branch is mixed with a local oscillator beam having a spectrum within one sub-band of the WDM channel. In the preferred embodiment these beams are mixed in 90-degrees optical hybrid, which is followed by a set of balanced photodetectors. The baseband of each sub-band signal is converted into a digital signal using ADC. This allows the implementation of a series of lower-speed ADCs working in parallel instead of one high-speed ADC for the data recovery from the incoming optical signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit of the provisional application No. 61/315,434 filed Mar. 19, 2010; it is also a continuation-in-part of U.S. patent applications Ser. No. 12/045,765 filed Mar. 11, 2008, Ser. No. 11/679,376 filed Feb. 27, 2007, Ser. No. 11/938,655 filed Nov. 12, 2007, and Ser. No. 12/696,957 filed Jan. 29, 2010, which is CIP of Ser. No. 12/418,060 filed Apr. 3, 2009, all of which applications are fully incorporated herein by reference. 
     
    
     FIELD OF INVENTION 
       [0002]    An optical receiver and a communications system are disclosed for simultaneous processing of a series of data flows. The disclosed system and method are applicable to any kind of communications and signal processing schemes and especially important for very high rate data transmission such as 1 Tbit/s communications. In particular this invention addresses data processing in optical communications systems and methods that utilize coherent detection technique, WDM M-PSK transmission and optical orthogonal frequency division multiplexing (OFDM). 
       BACKGROUND OF THE INVENTION 
       [0003]    Data transmission in dense WDM communications system with orthogonal frequency division multiplexed channels has been discussed by the same inventors entity in the parent application U.S. Ser. No. 12/045,765 and U.S. Pat. No. 7,693,428. In optical OFDM systems each WDM channel the optical carrier is directly modulated by a complex RF signal that can be construed as a linear combination of M separate digitally modulated RF signals at frequencies f m  such that f m =m/T, where T is the period of modulation. Thus the total symbol rate of the transmitted information is M/T. In the text we shall refer to the frequencies f m  as “subcarriers”. 
         [0004]    Optical OFDM system demonstrates robustness to fiber chromatic dispersion and polarization mode dispersion (PMD) thus allowing to achieve the best performance. 
         [0005]    In modern optical communication systems, a coherent detection technique is implemented, which provides improved sensitivity compared with traditional direct detection schemes. Typically coherent detection is used with phase-shift-keying (PSK) data transmission. The present invention is also focused on M-PSK, and in the preferred embodiment, QPSK (quadrature PSK) data transmission. However this does not limit the scope of the invention, and various types of data modulation can benefit from the disclosed invention. 
         [0006]    In a coherent receiver, the QPSK incoming optical signal is mixed with a strong local oscillators to produce in-phase (I) and in-quadrature (Q) outputs. I and Q components of the output optical signal are converted into electrical signals by a set of photodetectors. In the preferred configuration four balanced photodetectors are used to recover QPSK encoded data. 
         [0007]    Data transmission multiplexing light of two orthogonal polarizations via the same optical channel allows doubling the data rate. At the receiver side, the orthogonal polarizations are split by a polarization beam splitter, and the light of each orthogonal polarization is detected separately. 
         [0008]    There is still a need to increase the transmission rate and provide a reliable detection scheme to improve the high-rate data processing at the receiver side, and the present invention addresses this problem thus allowing achieving more reliable systems operating at higher data rates. 
       SUMMARY OF THE INVENTION 
       [0009]    A number of architectural solutions have been proposed in order to reduce the data rate at the receiver, to split high rate flow into a number of parallel branches and process lower rate signal in each branch digitally. In particular this approach is useful in systems that use standard ADCs with a sampling rate close to the Nyquist rate (e.g 25 Gsps). 
         [0010]    The WDM system of the present invention includes multiple channels, each channel being able to transmit up to 1 TBit/s data stream. It is achieved by forming the channel bandwidth as a set of non-overlapping spectral bands being orthogonal to each other. Transmitting of such spectral bands does not require a guard band between them thus achieving high spectral efficiency and better utilization of the fiber band. In the preferred embodiment the set of spectral bands is formed by an optical comb generator. The data is embedded using OFDM with M-PSK modulation format, in the preferred embodiment it is QPSK format (quadrature phase shift keying). Orthogonal frequency division multiplexing of the signal in each spectral band allows achieving high data rate transmission and improved robustness to the spectral dispersion and PMD. 
         [0011]    At the receiver side the light of one channel is split by intensity into a number of branches. The signal in each branch is mixed in a coherent receiver with a local oscillator signal with a wavelengths in one of N spectral bands (N&gt;1) of the channel. The output electrical signal are digitized using standard ADCs with a sampling rate of &gt;25 Gsps. 
         [0012]    The receiver, the method of data receiving and the system of data transmission and receiving are the objects of the present invention. The system includes multiple channels, each channel consisting of N spectral bands, and the light of each spectral band is modulated with data via orthogonal frequency multiplexing (OFDM) using M-PSK format. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The present invention may be understood by reference to the following detailed description of the preferred embodiment of the present invention, illustrative examples of specific embodiments of the invention and the appended figures in which: 
           [0014]      FIG. 1 . General scheme of channel structure in a WDM OFDM system and method of the present invention. 
           [0015]      FIG. 2  A block diagram of an OFDM M-PSK communications system. 
           [0016]      FIG. 3  A block diagram of an OFDM M-PSK communications system operating in two polarizations. 
           [0017]      FIG. 4  A data encoding block in OFDM QPSK communications system. 
           [0018]      FIG. 5  An OFDM unit structure. 
           [0019]      FIG. 6  A coherent optical receiver for OFDM communications system: (a) with 90-degrees optical hybrid, (b) with 120-degrees optical hybrid. 
           [0020]      FIG. 7  A receiver unit according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0021]    This invention solves the problem of the data recovery in very high data rate signal. We describe the main approaches using the example of optical communications up to 1 Tbit/s rate in one wide-WDM channel, however the approach is applicable to any type of schematics with data processing, such as optical chemical sensing, LADAR, image processing and others. 
         [0022]      FIG. 1  shows schematics of DWDM channels in optical communications. Each WDM channel contains a number of spectral bands, each having subcarriers (OFDM) modulated with data.  FIG. 1   b  shows one WDM channel with N spectral bands, N=10. These spectral bands are orthogonal to each other, and in the preferred embodiment are formed by a comb generator producing a set of orthogonal teeth. These orthogonal bands do not require frequency guard band and can be separated and combined without mutual interference. The absence of the guard bands allows achieving very high spectral efficiency in such system. This system is not equivalent to WDM OFDM communications, where each channel width is equal to the width of spectral band (shown as 50 Ghz in  FIG. 1  as an example). In WDM system with such narrow channel, each channel will require a guard band and must be distant from the neighbor channel. The proposed arrangement has an advantage over standard WDM channel structure. 
         [0023]      FIG. 1  ( a ) shows two channels of WDM system.  FIG. 1  ( b ) illustrates the structure of one channel consisting of N spectral bands.  FIG. 1(   c ) shows OFDM subcarriers of each spectral band of  FIG. 1  ( b ). 
         [0024]      FIG. 2  illustrates a point-to-point OFDM data transmission system in one WDM channel using coherent detection. In a transmitter  1  a digital data stream  2  enters an OFDM encoder  3 , which outputs two analog signals  4  and  5  (I and Q) driving an optical modulator  6 . The modulator  6  applies the modulation to a light beam  7  emitted by a light source  8 . The signal  9  transmitted via an optical link  10  is received by coherent receivers  11 . Local oscillator optical signal  12  coming from a light source  13  enters the coherent receiver  11  and interferes with the optical signal  14 . The receiver  11  includes an optical hybrid  15 , which is a 90-degrees optical hybrid in the preferred embodiment. In another embodiment it is a 120-degrees optical hybrid. Output optical signals  17 - 20  from the optical hybrid enter a photodetector unit  16  with at least four balanced photodetectors. I and Q electrical outputs  21 ,  22  from the photodetector unit enter a set of A/D converters  23 , followed by a digital signal processing (DSP) unit  26 . The output signal  27  can be used for the further processing or display. A control line  28  provides a control signal for the OFDM encoder to adjust the modulation signal to comply with the transmission characteristics. The components of the optical receiver  11  will be described in more details in the following paragraphs. 
         [0025]    In another embodiment, the receiver  11  is a polarization diversity receiver ( FIG. 3 ), and it further comprises the following elements. The signal is received by coherent receivers  11 H and  11 V after splitting by a polarization beam splitter  29  into two beams  30 H and  30 V with orthogonal polarization. Local oscillator optical signals  12 H and  12 V having H and V polarization state coming from a local oscillator light source  13  enter the coherent receivers  11 H and  11 V and interfere with optical signals  30 H,  30 V having the corresponding H and V polarization states. Each of the receivers  11 H and  11 V includes an optical hybrid and a set of photodetectors; it will be described in more details in the following paragraphs. Each of the receivers outputs two electrical signals  21 H,  22 H and  21 V,  22 V, converted into digital signals in  23 , followed by a digital signal processing unit  26 . Output signals  27  represent a series of the decoded data streams that can be displayed or transformed into any format for further presentation and use. 
         [0026]    Obviously the system can operate in bi-directional configuration with data transmission in both directions. In this case light sources, located at each end of the link, have double functions. Each light source generates the beam for the data transmission by the transmitter  1  and, at the same time, it provides the local oscillator signal for the receiver  11 . 
         [0027]    A variety of the data modulation formats can be used in the system and method disclosed in the present invention. In one embodiment a quadrature phase shift keying modulation format (QPSK) is implemented. In the preferred embodiment the modulator  6  is a Mach-Zehnder Interferometer (MZI) electro-optic modulator. In the preferred embodiment shown in  FIG. 4  QPSK data is embedded in the system using two separate data modulators, which are the parts of the optical modulator  6 . One modulator  31  is used for I component and another modulator  32  is for Q component of the data stream. The optical beam  7  is split by the splitter  33  into two beams  34  and  35 , modulated and then combined together by the combiner  36  forming the output beam  9 . A phase shift of 90-degrees is introduced by a phase shifter  37  in one of the beams  38  or  39 . The output beam  9  is transmitted to the receiver via optical link. The optical link can be a fiber link or a free-space link. 
         [0028]    In the preferred embodiment the QPSK modulator is an integrated device as disclosed in U.S. patent applications Ser. Nos. 11/679,378 and 10/613,772 by the same inventive entity. 
         [0029]      FIG. 5  shows an embodiment of the OFDM encoder  3 . A serial data stream  2  is converted into a parallel sub-carrier data stream  47  in a serial-to-parallel converter  48 . In OFDM, the sub-carrier frequencies are chosen so that the sub-carriers are orthogonal to each other, meaning that cross-talk between the sub-channels is eliminated and inter-carrier guard bands are not required. Parallel output data stream  47  enters a QPSK data encoder  48 . Two parallel output signals  49  and  50  correspond to the I and Q parts of the QPSK signals of each subcarrier. Inverse Fast Fourier Transform is applied in an IFFT unit  51  to the data streams  49  and  50 . Then the phase shift is introduced to the signals  52  and  53  in a non-linearity compensation unit  54 . A cyclic prefix is added to the signals  56 ,  57  at a prefix unit  58 ; the cyclic prefix takes a few last symbols of each data block and repeats them at the beginning of the next block. The purpose is to make the scheme resistant to chromatic dispersion. Two sub-carriers may experience differential delay up to the length of prefix, but the orthogonality between the sub-carriers will be preserved and the data will be recovered at the receiver. The data streams  59 ,  60  are converted in a parallel-to-serial converter  61 , followed by conversion of  62 ,  63  into analog signals in a D/A converter  64 . The analog I and Q signals  4  and  5  are applied to the optical modulator  6  as shown in  FIG. 1 . 
         [0030]      FIG. 6(   a ) illustrates an embodiment of the coherent receiver  11  to be used to recover QPSK data. The incoming signal  14  is input into an optical hybrid  15 , which is a 90-degrees optical hybrid in the preferred embodiment. The 90-degrees hybrid has four couplers  71 ,  72 ,  73 ,  74  and a phase shifter  75 . The structure of the 90-degrees optical hybrid  15  is disclosed in detail in co-pending U.S. patent application Ser. No. 11/610,964, incorporated herein by reference. The incoming signal  14  is mixed with the local oscillator optical signal  12  producing four output optical signals  17 - 20 . A set of four balanced photodetectors  80 - 83  is used to convert the signals  17 - 20  into electrical domain. I and Q electrical outputs  21  and  22  are digitized in the A/D converter  23 . The data is recovered in digital signal processing unit using Fast Fourier Transform such as described in our previous patent application U.S. Ser. No. 12/045,765. 
         [0031]    In another embodiment the optical hybrid is a 120-degrees optical hybrid shown in  FIG. 6  ( b ). The structure and performance of the 120-degrees optical hybrid is disclosed in details in U.S. Pat. No. 4,732,447 by Wright and in U.S. Pat. No. 7,085,501 by Rickard. 120-degrees optical hybrid  90  has three inputs  24 ,  91 ,  21  and three outputs  92 ,  93 ,  94 . The output signals  92 - 94  pass through three detector diodes  95 ,  96 , and  97  as illustrated. In the signal processing unit  34  the electrical signals  98 ,  99 , and  100  are split into two signal paths each. Each of these six signals is mixed with a signal from a local oscillator so as to create phase differences between said six signal paths. These six signals are combined in two groups of three so as to create an in phase and a quadrature channels in a 120-degrees hybrid processing unit  101 . The transmitted data is recovered from the in phase and quadrature signals. 
         [0032]    The above description of the 120-degrees optical hybrid is presented as an illustration of its possible structure and performance. Obviously various modifications can be made by a person skilled in the art. The present invention is not limited to one particular example, but comprises a variety of possible embodiments. 
         [0033]    It is an object of the present invention to provide improved spectral efficiency and system performance at high bit rates. Let us consider an example, which is not limiting the invention: each channel spacing ( FIG. 1 , a) being 500 GHz and each channel containing ten spectral bands spaced 50 GHz apart. Each band ( FIG. 1 , a) contains multiple, for example 100, OFDM subcarrier signals ( FIG. 1 , c). 
         [0034]    In the preferred embodiment an output of a mode-locked laser, which operates at 50 GHz and produces teeth separated by 50 GHz, is split by an AWG (Arrayed Waveguide Grating) into WDM channels. Each channel is spanning 500 GHz and contains 10 lines of the laser output. 
         [0035]    Ten teeth of each channel are split by a second AWG (a fine AWG) or, in other embodiment, by a set of MZI (Mach-Zehnder Interferometers) interleavers. Each tooth get modulated by an OFDM signal synthesized using inverse FFT of 100 subchannels of 250 Msym/s each. 
         [0036]    Ten modulated bins are combined together forming one WDM channel signal spanning 500 GHz and carrying 1000 of 250 Msym/s OFDM subchannels. 
         [0037]    All wide-WDM channels then combined using a specially designed AWG and send through the fiber. At the receiver side, the incoming signal first de-MUX into separate channels using another AWG. Each WDM channel  14  then split into N branches by intensity (N=10 in  FIG. 7 ). The light in each branch is mix with a local oscillator beam having a wavelength within the n-th spectral band (2≦n≦N). In the preferred embodiment this local oscillator beam is one tooth of mode-locked laser  131 . The mixing is performed by optical hybrids  11   a - 11   d  in  FIG. 7 , and the hybrid outputs are digitized by a set of standard ADCs  121 - 124 , which is followed by the digital signal processing (including FFT) and the data recovery. As we previously showed in the parent application Ser. No. 11/695,920, optical hybrids have a function of selecting from the incoming signal only that spectral part, which corresponds to the spectral band of the local oscillator. The result of the incoming signal and the local oscillator signal interference is processed, and the data is recovered, which was embedded in the part of the incoming signal, which spectral band corresponds to the spectral band of the local oscillator. 
         [0038]    The total transmission capacity of one channel 1 TBit/s, which is achieved by imposing 250 Gsymbol/s OFDM signal multiplied by 2 polarizations and also multiplied by 2 bit/s of QPSK. 
         [0039]    The channel selection from the whole multichannel system may be performed in two steps. First the incoming multi-channel signal  14  is split by a polarization beam splitter  104  into light beams with orthogonal (V and H) polarization states. Then the light of one polarization state is spectrally split by an AWG  105  into a set of channels.  FIG. 7  shows data recovery from one channel  106 ; the rest of the channels  107  have similar receivers. 
         [0040]    In  FIG. 7  the incoming optical signal  14  is split by intensity by a splitter  110  into N branches. For simplicity, the  FIG. 7  shows only four branches  111 ,  112 ,  113 ,  114  each serving as inputs for the receivers  11   a - d . In the receivers these signals are mixed with the local oscillator beam  13   a - 13   d,  correspondingly. The output baseband signals  115 ,  116 ,  117 ,  118  are converted into digital signals by A/D converters  121 ,  122 ,  123 ,  124  operating at 25 Gsamples/s rate. In real system signals in all spectral bands are detected. 
         [0041]    The local oscillator beam  130  contains a set of spectral bands, all being orthogonal to each other. In one embodiment, a comb generator is used as a local oscillator. A variety of comb generator schematics may be found in literature, see for example U.S. Pat. No. 4,989,201 by B. Glance or U.S. Pat. No. 7,239,442 by M. Kourogi et al. The local oscillator beam is split by the spectral splitter  132  (which is preferably another AWG) into the LO channels  133 - 136 . 
         [0042]    Such configuration allows processing of the incoming high data rate signal and recovering the data using lower rate ADCs  121 - 124 . For example, 250 GSym/s incoming signal being split into 10 spectral bands, can be recovered using 25 Gsamples/s ADCs. 
         [0043]    The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in the light of the above teaching. The described embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.