Abstract:
A differential delay detection system and method includes an optical splitter to split an incoming optical signal between a first path and a second path. The first path includes a cross-polarization interferometer configured to separately generate polarization independent outputs using split paths and to generate cross-polarization interference outputs, balanced photodetectors to aid in removing cross-polarization beating noise, and a polarization demultiplexer configured to combine the polarization independent outputs and the cross-polarization interference outputs from the cross-polarization interferometer with updated coefficients received from the second path to remove the cross-polarization mixed signals. The second path includes a training signal receiver configured to compute the updated coefficients and output the updated coefficients to the polarization demultiplexer.

Description:
RELATED APPLICATION INFORMATION 
     This application claims priority to provisional application Ser. No. 61/060,702 filed on Jun. 11, 2008, incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present invention relates to optical communications systems, and more particularly to an apparatus and a method for using cross-polarization interferometry in a direct-detection, polarization-multiplexing receiver. 
     2. Description of the Related Art 
     Coherent light may be transmitted in two orthogonal polarizations, where polarization refers to the orientations and relative phases of the electric and magnetic fields that make up the light. Light can be filtered through a polarizer, so that only those photons which have their electric fields along, for instance, a horizontal plane, may pass through. The result is that signals of two different polarizations (e.g., vertically polarized and horizontally polarized) may be transmitted along the same medium without interfering. The signals can then be split through the use of birefringent materials, which have different indices of refraction for light of different polarizations. 
     As the demand for data capacity in high-speed optical transmission systems increases, spectral efficiency (SE, the amount of information which can be transmitted over a given bandwidth) is becoming increasingly important. To achieve high SE, polarization multiplexing (PolMux) is a key technology because it can smoothly work with any modulation formats and can easily double the spectral efficiency from the original SE of the modulation format without PolMux. PolMux accomplishes this by transmitting completely separate signals at the same wavelength, each signal having a polarization that is orthogonal to the other. 
     For PolMux Amplitude Shift Keying (ASK) systems, data is carried by the amplitude of the optical signals on orthogonal polarizations. The signals are combined by a polarization beam combiner (PBC) at the transmitter. At the receiver, the combined optical signals are separated by a polarization beam splitter (PBS) into two orthogonally polarized, optical signals. 
     An obstacle to this process is the fact that polarization does not remain constant in optical fibers. As a signal travels through the fiber, its polarization rotates Because of this effect, the alignment between the PBS and PBC is not guaranteed. Each output of the PBS would be a combined signal from both inputs of the PBC, such that the original signals cannot be immediately extracted. In addition, a crossing-polarization beating noise will be generated between the two signals. Coherent detection is currently the only option at the receiver side which can minimize the crossing-polarization beating noise. 
     However, the coherent detection receiver has drawbacks; both frequency offset and phase offset need to be removed by digital processing which requires large power consumption and complicated system design. Coherent detection also requires a spare narrow line-width laser at the receiver as the local oscillator, which can increase both system cost and complexity. 
     A direct-detection receiver can overcome all of the above issues. There is no frequency offset or phase offset transmitted with the received signal, and a local oscillator laser is not required. However, to realize direct-detection for PolMux-ASK signals, the crossing-polarization beating noise has to be eliminated or avoided before the signal can be correctly detected. The same direct-detection receiver can also work with any other amplitude modulation (AM) systems with PolMux, for example, Quadrature Amplitude Modulation (QAM), On-Off Keying (OOK), etc. 
     SUMMARY 
     A differential delay detection system and method includes a cross-polarization interferometer which receives a polarization multiplexed input signal and is configured to separately generate polarization independent outputs using split paths and to generate cross-polarization interference outputs, balanced photodetectors which filter the output of the cross-polarization interferometer, a training signal receiver which receives the input signal and which is configured to extract a training signal from the input signal and generate updated coefficients representing a rotation of the input&#39;s polarization, and a polarization demultiplexer configured to combine the polarization independent outputs and the cross-polarization interference outputs from the cross-polarization interferometer, after they have been filtered by the balanced photodetectors, with the updated coefficients received from the training signal receiver to extract an original, unrotated signal. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The disclosure will provide details in the following description of preferred embodiments with reference to the following figures wherein: 
         FIG. 1  is a block diagram showing a direct detection receiver for a polarization multiplexing, amplitude signal keying (ASK) system. 
         FIG. 2  is a block diagram showing detail on the cross-polarization interferometer, 
         FIG. 3  is a block diagram showing detail on the ASK polarization demultiplexer. 
         FIG. 4  is a block/flow diagram showing a method by which polarization multiplexed optical signals can be received. 
         FIG. 5  is a block/flow diagram showing a method for cross-polarization interferometry. 
         FIG. 6  is a block/flow diagram showing a method for extracting an original ASK signal from four polarization interference signals. 
         FIG. 7  shows an example rotation matrix used to calculate coefficients for extracting ASK signals from polarization interference signals. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present principles provide differential delay detection for polarization multiplexing (PolMux) systems, and in particular for PolMux-amplitude shift keying (ASK) systems. The differential delay detection can simplify a receiver design, improve system reliability and reduce cost by simplifying signal processing for a frequency offset and a phase offset and removing an expensive narrow line-width local oscillator laser. 
     Through simulation, it has been proven that the cross-delay differential detection in accordance with the present principles can recover polarization rotation very well. As such, PolMux-ASK systems can benefit from using cross-delay differential detection instead of coherent detection, resulting in lower cost and complexity. In accordance with an illustrative embodiment, a 4-path butterfly cross-polarization differential delay (Mach-Zehnder) interferometer is employed to process the differential detection when signals are transmitted at two orthogonal polarizations. Two additional cross-polarization differential delay outputs can help to remove the cross-polarization mixed signals generated by the necessarily random split of the polarized signals at the receiver. 
     Embodiments described herein may be entirely hardware, entirely software or including both hardware and software elements. In a preferred embodiment, the present invention is implemented in hardware but may include software components, which may include but are not limited to firmware, resident software, microcode, etc. 
     Referring now in detail to the figures in which like numerals represent the same or similar elements and initially to  FIG. 1 , an exemplary system is shown which receives PolMux-ASK signals. The incoming beam  100  is split by a non-polarizing beam splitter  102 . One signal is used as a training signal for determining coefficients  112  at block  110  which will allow recovery of the polarizations (described in greater detail below). The other signal serves as input to a cross-polarization interferometer  104 . 
     The cross-polarization interferometer produces four output optical signal pairs. These pairs comprise two versions of each interfered combination, one delayed with respect to the other. For the sake of simplicity,  FIG. 1  only shows four signals leaving the cross-polarization interferometer.  FIG. 2 , shows that the four output signals depicted in  FIG. 1  are each made up of two separate signals. 
     The signals from the interferometer include additional, unnecessary information which is referred to as the cross-polarization beating noise. This noise is removed at the balanced photodetectors  106 . Each photodetector  106  takes an optical signal pair as inputs, and produces an electrical signal G 1 -G 4  with the cross-polarization beating noise removed. The photodetectors  106  accomplish this by outputting the difference in power between the pair of signals. 
     The signals G 1 -G 4  then enter the polarization demultiplexer  108 , which uses the coefficients  112  produced by block  110  to reconstruct the original polarized signals  100   a  and  100   b . Training signals are periodically sent containing a distinctive sequence of symbols. These training signals allow for the calculation of updated coefficients  112  responsive to changing transmission conditions as described below and shown in  FIG. 7 . Each of the signals  100   a  and  100   b  is then sent to a traditional ASK receiver  114 . 
     Referring now to  FIG. 2 , an exemplary cross-polarization interferometer  104  is shown. The input signal  200  is split in a polarizing beam splitter (PBS)  202 , producing two polarized signals, X′ and Y′. The polarization orientation of PBS  202  is arbitrary, because the polarization of the original signal will have rotated by an unknown amount. The two polarized signals X′ and Y′ are then split again in beam splitters  204 . One X′ signal is rotated by  902  in polarization controller  205 , producing an X′ signal with the same polarization as Y′. Each of the four signals is then split again and analyzed using delay differential interferometry. The interferometer uses one-tap delays  206  to delay signals with respect to one another. These delays can be any duration as long as the duration of the ASK symbols is an integer multiple of the delay duration. For best performance, the delay should be equal to or less than one quarter of the ASK symbol duration. The delays should additionally maintain wavelength level synchronization. 
     In the interferometer  104 , one X′ signal is interfered with a delayed version of itself, and similarly one Y′ signal is interfered with a delayed version of itself. Then one rotated X′ signal is interfered with a delayed Y′ signal and one Y′ signal is interfered with a delayed, rotated X′ signal. The interference takes place at polarization-maintaining beam combiners  208 , producing four output signals, G 1 -G 4 . 
       FIG. 3  shows greater detail on an exemplary ASK polarization demultiplexer  108 . Each of the outputs from the cross-polarization interferometer  104  passes through a balanced photodetector  106 . Each output is then multiplied in multipliers  308  by a coefficient calculated in training signal receiver  110 . The training signal receiver  110  calculates the coefficients b 11 −b 14  using a low-speed analog-to-digital converter and digital signal processing, as described below and as shown in  FIG. 7 . The four signals are then combined in adders  310  to produce a single signal. By taking the absolute value  312  of the sum signal and passing it through a low pass filter  314 , the original ASK signal on one of the polarizations is produced as signal  100   a . The same technique, using different coefficients, can be used to extract the orthogonal ASK signal and output it as signal  100   b . In this manner, both of the original ASK signals, having been transmitted on a common medium using orthogonal polarizations, can be recovered. 
       FIG. 4  illustrates a method for demultiplexing polarization-multiplexed ASK signals. The incoming signal is received at block  402 . The signal is then split into orthogonal polarizations at block  404 . As noted above, the particular axes used to split the signals into orthogonal polarizations are arbitrary. The signals are then processed using balanced photodetectors at block  408 . The interferometry and photodetector steps serve to eliminate the cross-polarization beating noise. Block  410  calculates the coefficients which allow the extraction of the ASK signals from the arbitrarily polarized signals. The final demultiplexing takes place at block  412 , after which the signals can be interpreted using traditional ASK receivers. As a result, two separate ASK signals may be received using a shared frequency, effectively doubling the throughput. 
       FIG. 5  illustrates a method for cross-polarization interferometry. At block  502 , two signals (X′ and Y′) are received having orthogonal polarizations. Each signal is split in block  504 , and one of the X′ signals is rotated by ninety degrees in block  506 . The four signals are then further split in block  507 . The rotated X′ signal and one of the Y′ signals are then cross-coupled in block  508 , each being added to a delayed version of the other. This cross-coupling allows recapture of the polarization information stored in the interfered signals. The unrotated X′ signal and the remaining Y′ signal are then self-coupled to a delayed version of themselves in block  510  to produce polarization independent signals. The four resultant polarization interference signals are then output at block  512 . 
       FIG. 6  illustrates a method for extracting an original ASK signal from four polarization interference signals. At block  602 , the four signals are received as input. Each signal is then multiplied by a coefficient estimated from a training signal at block  604 . The procedure for calculating said coefficients is described below and shown in  FIG. 7 . The signals are then added together at block  606 . Block  608  then takes the absolute value of the summed signal. The signal is then low-pass filtered at block  610 . The signal is then output as one of the original ASK signals in block  612 —the particular coefficients used in block  604  determine which of the two original signals is output. 
     Referring to  FIG. 7 , an example of transmitting signal sequences  70  and receiving training signal sequences  72  is illustratively depicted along with a 4×4 channel estimation matrix  74 . The example of  FIG. 7  uses a 2-level ASK scheme, which can encode one bit with every symbol, but higher-order schemes which can encode more bits per symbol are also contemplated. There are two different training signals (X pol:0,1; Y pol:1,0) transmitted periodically. Upon receiving the training signals, a 4×4 channel estimation matrix  74  will be estimated using the latest set of training signals (a 11 , a 21 , a 12 , and a 22 ) with the previous set. Then, the coefficients for polarization demultiplexing can be updated by finding the inverse matrix of the 4×4 channel estimation matrix  74 . The ASK polarization demultiplexer  108  uses updated coefficients  110  to recover the received signals back to the ASK signal which can be detected directly by a traditional ASK receiver  114 . The ASK polarization demultiplexing can be performed with either digital or analog signal processing. 
     Having described preferred embodiments of a system and method (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments disclosed which are within the scope and spirit of the invention as outlined by the appended claims. Having thus described aspects of the invention, with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.