Abstract:
Optical communication systems and methods using coherently combined optical beams are disclosed. A representative system includes a first data source for sending first data at a first frequency of a first optical beam to a first aperture, and at a second frequency of a second optical beam to a second aperture. The system further includes a second data source for sending second data at a third frequency of a third optical beam to the first aperture, and at a fourth frequency of a fourth optical beam to the second aperture. The system also includes a first interleaver of the first aperture configured to interleave the first data at the first frequency and the second data at the third frequency; and a second interleaver of the second aperture configured to interleave the first data at the second frequency and the second data at fourth frequency.

Description:
TECHNICAL FIELD 
       [0001]    The disclosed embodiments generally relate to the field of optical communication, and more specifically to transmitting beams of light from multiple data sources into one or more apertures. In some embodiments, the beams of light are transmitted at different frequencies (i.e., using non-coherent transmission) through the atmosphere to reduce or eliminate the effects of signal fading caused by atmospheric turbulence. 
       BACKGROUND 
       [0002]    Errors in optically transmitted data are caused by a number of different factors, including distortion of optical signals in the air. In free-space optical communications systems that propagate optical signals through air, turbulence can be a significant source of channel impairment. For example, anomalous refraction of an optical beam (e.g., scintillation) can be caused by small-scale fluctuations in air density that result from temperature or pressure gradients along the path of the optical beam. These atmospheric fluctuations can cause frequency-nonselective fades in the optical beam&#39;s power. The fade process has a correlation time which is typically much longer than the duration of a typical symbol in the optical beam, therefore reducing the signal-to-noise ratio of the data stream. 
         [0003]    To reduce the effects of optical beam fading, some conventional technologies apply channel equalization and forward error correction (FEC) coding at the physical layer. Channel equalization is used to reduce the inter-symbol interference that is induced by band-limiting in the receiver or channel. Forward error correction at the physical layer introduces a structured redundancy on the transmitted symbol sequence that can be exploited at the receiver to correct errors in recovering the transmitted data due to channel impairments. However, the complexity associated with encoding and decoding a physical layer with a FEC code increases with the length of the codeword. For example, in high data rate systems, a codeword should span multiple channel coherence times to enable recovery of the symbols lost due to optical beam fading. However, such a codeword would be prohibitively complex to handle in many practical situations. 
         [0004]    With other conventional technologies, lost data may be retransmitted from a transmitter to a receiver upon detecting data loss (e.g., a dropped data frame). However, in many cases, the additional round-trip latency caused by the re-transmission requests and the need for an additional feedback channel make these technologies impractical or undesirable. 
         [0005]    Another conventional approach to mitigate fading relies on spatial diversity. Since turbulence has a transverse correlation length r 0 , if two optical source beams are separated by a distance D, then their fades will become statistically independent when D&gt;&gt;r 0 . Therefore, turbulence-induced errors in the optical beam (e.g., scintillation) are sufficiently non-correlated for optical beams that are spaced sufficiently apart. A conventional technology that utilizes spatial diversity to mitigate turbulence is called multi-beaming. The multi-beaming technique includes sending the same symbol along different paths separated by D, where D&gt;&gt;r 0 , such that different paths experience statistically independent fades and phase offsets. In this scenario, the total received signal intensity is the sum of several independent optical beams, each characterized by independent fading processes. As a result, the total received signal will thus have a smaller optical beam fading. However, this approach is only suitable when information is encoded by intensity, but is not applicable when the optical phase carries information. Accordingly, there remains a need for improved technique for the transmission of optical data at high data rates and low latency of the data transmission. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a schematic diagram illustrating a system for transmitting optical data transmission through the atmosphere in accordance with various embodiments. 
           [0007]      FIG. 2  is a schematic diagram illustrating a system for transmitting optical data by deinterleaving at a receiver (RX) in accordance with various embodiments. 
           [0008]      FIG. 3  is a schematic diagram of a system for converting optical data at the RX in accordance with various embodiments. 
           [0009]      FIG. 4  is a schematic diagram of a system for compensating for delays at a transmitter (TX) in accordance with various embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    Embodiments are directed to non-coherent combining light beams emitted by multiple data sources and received by multiple transmitting (TX) apertures. The received data-carrying optical beams may be non-coherently combined at their respective TX apertures because the optical beams have different wavelengths, frequencies, phases, or polarization angles. The combined optical beams can be sent through the atmosphere to a receiving (RX) aperture. Because the combined optical beams arrived to the RX aperture along paths having statistically uncorrelated turbulence the optical beams may also have statistically uncorrelated signal fading, therefore improving signal-to-noise ratio of the optical beams reconstructed at the RX aperture. In some embodiments, the optical beams at individual TX apertures can be combined using an optical multiplexer (MUX) and dense wavelength division multiplexing (DWDM), resulting in improved data throughput from the TX apertures to the RX aperture. 
         [0011]    Briefly described, various embodiments use arrangements for non-coherently sending optical beams from multiple (e.g., two or more) TX apertures to one or more RX apertures. For example, a first data source may send multiple optical beams at different frequencies, wavelengths, phases or polarization angles to the corresponding TX apertures through optical fiber or the atmosphere. The multiple optical beams emitted by the first data source may carry same data. A second data source can also send data to the same set of TX apertures using multiple optical beams at another set of frequencies, wavelengths, phases or polarization angles. More data sources may similarly be employed to, for example, match the number of data sources to the number of the TX apertures. Therefore, in some embodiments, each TX aperture combines the incoming data from several data sources at different frequencies, and sends the data to a receiving (RX) aperture. In some embodiments, the optical beams propagate from the TX apertures to the RX aperture through the atmosphere over relatively long distances (e.g., kilometer scale), and are therefore exposed to scintillating effects of the turbulence. However, in at least some embodiments, the optical beams originating from different TX apertures may be spatially separated enough to be exposed to statistically non-correlated optical fading effects. Therefore, combining the optical light beams that carry the same data from different TX apertures along different paths may reduce the optical fading effects (e.g., the symbol loss). 
         [0012]    In some embodiments, an interleaver (a multiplexer or MUX) at the TX aperture may interleave the optical beams arriving from multiple data sources based on, e.g., first-in-first-out (FIFO) method. In some embodiments, routing data from multiple data sources to multiple TX apertures may create timing inaccuracies among the optical beams arriving to the TX aperture or among the TX apertures, because of, for example, different paths of the optical beams. Therefore, in at least some embodiments, data rates can be synchronized using FIFOs (e.g., for a coarse adjustment) and phase-locked-loops (PLLs) (e.g., for a fine adjustment). The TX apertures may combine several frequencies of light that correspond to the frequencies of light sent by the data sources. In some embodiments, the TX apertures may use dense wavelength division multiplexing (DWDM) to combine optical beams and to send data to the RX aperture at a higher data rate. 
         [0013]      FIG. 1  is a schematic diagram illustrating optical data transmission through the atmosphere in accordance with various embodiments. In the illustrated system  1000 , TX apertures  201 - 204  combine data from four data sources  101 - 104 , and send optical beams through the atmosphere  250  to an RX aperture  300 . In some embodiments, each of the four data sources  101 - 104  sends data-carrying optical beams  150  to the TX apertures  201 - 204 . For example, the TX aperture  203  receives optical beams at wavelengths λ 13  from data source  101 , λ 23  from data source  102 , λ 33  from data source  103 , and λ 43  from data source  104 . In the illustrated example, each TX aperture receives data from four data sources, but other combinations of the data sources and the TX apertures are also possible. For example, the number of the TX apertures may be greater than the number of the data sources for additional reduction of data fading caused by turbulence. In at least some embodiments, the optical beams  150  can be generated by lasers or light emitting diodes (LEDs), and may be transferred to the TX apertures  201 - 204  through the atmosphere or optical fiber. In some embodiments, the optical beams  150  propagate over a relatively short distance (e.g., less than several meters) from the data sources  101 - 104  to the TX apertures  201 - 204 . 
         [0014]    In some embodiments, the optical beams  150  are multiplexed at the TX apertures  201 - 204  before sending the multiplexed optical beams  221 - 224  through the atmosphere  250 . For example, the TX aperture  202  may multiplex optical beams at wavelengths λ 12  from data source  101 , λ 22  from data source  102 , λ 32  from data source  103 , and λ 42  from data source  104 , and then send a combined optical beam  222  at wavelengths λ 12 -λ 42  through the atmosphere  250  to the RX aperture  300 . An example of a set of optical beam frequencies (in THz) for a sample combination of data sources and apertures is shown in Table 1 below. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Data Source 
                   
               
             
          
           
               
                 Aperture 
                 1 
                 2 
                 3 
                 4 
               
               
                   
               
               
                 1 
                 194.4 
                 194.35 
                 194.3 
                 194.25 
               
               
                 2 
                 194.2 
                 194.15 
                 194.1 
                 194.05 
               
               
                 3 
                 194.0 
                 193.95 
                 193.9 
                 193.85 
               
               
                 4 
                 193.8 
                 193.75 
                 193.7 
                 193.65 
               
               
                   
               
             
          
         
       
     
         [0015]    For the embodiment illustrated in Table 1, the frequencies of the optical beams emitted by the same data source are 50 MHz apart. For example, the data source  1  (e.g., data source  101  in  FIG. 1 ) can emit four optical beams in the range of 193.8-194.4 THz, with the difference between adjacent frequencies being 0.2 THz or 200 MHz. As explained above, in at least some embodiments, the data source  101  sends the same data on the four optical beams (e.g., the wavelength λ 11  of a waveform  111  sent to TX aperture  201 ). In the embodiment illustrated in Table 1, the aperture  2  can receive data from data source  1  at 194.2 THz, from data source  2  at 194.15 THz, from data source  3  at 194.1 THz, and from data source  4  at 194.05 THz. Therefore, the frequencies of the four optical beams received by the aperture  2  are 0.05 THz or 50 MHz apart. In at least some embodiments, the different frequencies of the optical beams enable combining the optical beams (and the symbols embedded in the optical beams) using, for example, DWDM before sending the combined beam from any of the TX apertures to the RX aperture. For the example illustrated in Table 1, the frequency offset for the DWDM for any of the apertures is 50 MHz (i.e., four optical beams offset by 50 MHZ for a total frequency spectrum of 200 MHz allocated per an aperture). Other combinations of the numbers of apertures and data sources, and their corresponding frequencies are possible. 
         [0016]    The TX apertures  201 - 204  may send their corresponding optical beams  221 - 224  to the RX aperture  300 . As explained above, each optical beam  221 - 224  may include multiple optical beams at different wavelengths that are wavelength-division multiplexed, and sent along the same path to the RX aperture  300  (e.g., a set of wavelengths λ 11 , λ 21 , λ 31  and λ 41  in a waveform  211  sent from TX aperture  201 ). For example, the aperture  4  may emit the optical beam  224  that combines four wavelengths: λ 14 , λ 24 , λ 34  and λ 44  from the optical beams received from the four data sources  101 - 104 . Using the example illustrated in Table 1, the data source  4  would emit a wavelength-division multiplexed optical beam that includes the frequencies 193.8 THz, 193.75 THz, 193.7 THz and 193.65 THz. In at least some embodiments, the optical beams  221 - 224  may be sufficiently apart such that they experience a statistically uncorrelated beam fading. As a result, the incidence of symbol loss due to beam fading may be reduced. In some embodiments, a distance between the RX aperture and the TX apertures may be several hundred meters or several kilometers. 
         [0017]    In some embodiments, the RX aperture  300  sends the received optical beams  221 - 224  to a deinterleaver (deMUX)  350  through, for example, an optical fiber. In some embodiments, the deinterleaver  350  may deinterleave the optical beams  221 - 224  back to or close to the frequencies/wavelengths of the optical beams  150  sent by the data sources  101 - 104  (e.g., λ 11 -λ 44 ). The deinterleaver  350  can route the deinterleaved optical beams to data sinks  401 - 404  through optical fiber or the atmosphere. For example, in one embodiment, the deinterleaver  350  can send optical beams using a set of wavelengths λ 11 , λ 12 , λ 13  and λ 14  in in a waveform  361  to the data sink  401 . Additionally, the deinterleaver  350  can send optical beams to the data sink  403  at the wavelengths λ 31 , λ 32 , λ 33  and λ 34 . As a result, in the illustrated embodiment, the data sink  403  receives the data sent from the data source  103 . As explained above, if the optical beams  221 - 224  are spaced apart enough to experience statistically uncorrelated fading, then the optical beams received by the data sink  403  at the wavelengths λ 31 , λ 32 , λ 33  and λ 34  may be summed (or otherwise combined) to reduce or eliminate the symbol loss caused by the optical beam fade. Analogously, the data sinks  401 ,  402  and  404  may receive the optical beams that were sent by the data sources  101 ,  102  and  104 , respectively. The deinterleaving of the optical beams in the deMUX  350  is described in more detail with reference to  FIG. 2  below. 
         [0018]      FIG. 2  is a schematic diagram illustrating optical data deinterleaving at the receiver (RX) in accordance with various embodiments. In the illustrated embodiment, the TX apertures  201 - 204  send combined optical beams  221 - 224  to the RX aperture  300 . In some embodiments, each optical beam  221 - 224  may be generated using a MUX that applies DWDM on the optical beams received from the data sources. The RX aperture  300  may send the received combined optical beams  221 - 224  to the deinterleaver (deMUX)  350 . In some embodiments, the deinterleaver  350  may include multiple stages, e.g., a deinterleaver  350   a  in the first stage and deinterleavers  350   b  and  350   c  in the second stage. For example, the deinterleaver  350  may deinterleave the incoming optical beams into two optical beams: an optical beam  351  that includes optical beams sent from data sources  101  and  102 , and an optical beam  352  that includes optical beams sent from data sources  103  and  104 . In some embodiments, the optical beam  351  is received by the deinterleaver  350   b  in the second stage, and is further deinterleaved into two optical beams; an optical beam  353  that includes the optical beams sent from the data source  1  at the wavelengths λ 11 , λ 12 , λ 13  and λ 14 , and an optical beam  354  that includes the optical beams sent from the data source  2  at the wavelengths λ 21 , λ 22 , λ 23  and λ 24 . Analogously, the optical beam  352  may be deinterleaved into an optical beam  355  that includes the optical beams sent from the data source  3  at the wavelengths λ 31 , λ 32 , λ 33  and λ 34 , and an optical beam  356  that includes the optical beams sent from the data source  4  at the wavelengths λ 41 , λ 42 , λ 43  and λ 44 . Other combinations of deinterleaving the incoming optical beams are also possible. For example, in some embodiments a single deinterleaver may be used. In other embodiments, three or more stages of deinterleaving may be used. 
         [0019]      FIG. 3  is a schematic diagram illustrating optical data conversion at the RX in accordance with various embodiments. In some embodiments, the deinterleaver  350  may deinterleave the incoming optical beams into four optical beams  353 - 356 , each respectively including the optical beams from one of the data sources  101 - 104 . For example, the optical beam  353  may include four optical beams at the wavelengths λ 11 , λ 12 , λ 13  and λ 14  sent by the data source  101  and received by the data sink  401 . The optical beam  353  may be converted to electrical signals in a converter  411  (e.g. a photo diode). For at least some DWDM schemes, the resulting electrical signals (e.g., corresponding to symbols in the optical beam) may be reconstructed as: 
         [0000]        S (λ 1 ( t ))= S (λ 11 ( t ))+ S (λ 12 ( t−τ   2 ))+ S (λ 13 ( t−τ   3 ))+ S (λ 14 ( t−τ   4 ))   (Equation 1)
 
         [0000]    where S(λ 1 (t)) is a reconstructed signal from the data source  101  corresponding to time t, S(λ 11 (t)) is signal sent by the data source  101  at time t using wavelength λ 11 , S(λ 12 (t−τ 2 )) is a signal sent by the data source  101  at time t using wavelength λ 12 , etc. Generally, the time offsets τ 2 , τ 3  and τ 4  can be selected to account for the interleaving time offsets of the DWDM schemes. Furthermore, the non-coherency of the optical beams (e.g., the light beams having different wavelengths) enables the transmission and summing of the optical beams at the receiver. In at least some embodiments, a sufficient spatial separation of the optical beams results in lower statistical coherence in optical beam fading that improves symbol recovery when the optical beams are combined using Equation 1. In at least some embodiments, a digital equalizer  412  may adjust amplitudes of the signals S. 
         [0020]      FIG. 4  is a schematic diagram illustrating delay compensation at the transmitter (TX) in accordance with various embodiments. In some embodiments, routing the optical beams from the data sources to different TX apertures may result in uneven arrival times at the TX apertures. For example, one optical beam may travel over a longer distance and therefore be delayed with respect to another optical beam. In at least some embodiments, the delays of the optical beams may correspond to a fraction of a baud (symbol per second), therefore being difficult to adjust for using, for example, optical fiber splicing. In at least some embodiments, an electronic compensation may be used to reduce or eliminate the delays between the optical beams. In the illustrated system  4000 , the data source  101  sends optical beams to FIFOs  211 - 214 . In at least some embodiments, each FIFO can receive optical beams from additional data sources, e.g., from data sources  102 - 104 . In some embodiments, the FIFOs may be connected to a common data clock  205  for synchronizing the operation of the FIFOs. For example, the common data clock  205  may clock data from data sources to the FIFOs, therefore providing at least a coarse synchronization of the optical beam arrival. In some embodiments, the PLLs  221 - 224  may provide a fine synchronization of the optical beam arrival by independently adjusting the phase of the optical beams interleaving to a sub-baud level. The synchronized interleaved optical beams from the data sources (only one data source is illustrated) may be sent to the TXs  201   a - 204   a  and the apertures  201   b - 204   b.  In at least some embodiments, the combination of the coarse and fine synchronization (adjustment) improves signal-to-noise ratio at the RX apertures. In some embodiments, the coarse synchronization may be used, while the fine synchronization is not used. 
         [0021]    From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the scope of the invention. For example, in some embodiments the optical beams may propagate through a vacuum, or a combination of air and vacuum. In some embodiments, the optical beams may have frequency that is not visible, for example, frequency higher than that of visible light. In some embodiments, multiple RX apertures may be used. For example, one RX aperture may receive optical beams from a subset of TX apertures, while the remaining optical beams are received by another RX aperture. Accordingly, the invention is not limited, except as by the appended claims.