Abstract:
A free space wavelength duplexed system includes first and second terminals. The first terminal includes an optical transmitter and an optical receiver. The optical receiver has a telescope, an optical to electrical converter and an optical amplifier coupled between the telescope and the optical to electrical converter. A method includes the steps of receiving a received optical signal through a telescope, diverting the received optical signal in an optical duplexer into an optical amplifier, and transmitting a transmit optical signal through the optical duplexer to the telescope. Another method includes the steps of receiving plural received optical signals through a telescope, diverting the plural received optical signals in an optical duplexer into an optical amplifier, separating the plural amplified optical signals by wavelength, and transmitting plural transmit optical signals at distinct wavelengths through the optical duplexer to the telescope.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a free space optical communication link, and in particular, the invention relates to techniques to spread error sources over time intervals and provide redundant channels to reduce the effects of fading.  
           [0003]    2. Description of Related Art  
           [0004]    Known optical communication systems rely on optical fibers between transmitter and receiver. However, to establish a system network require obtaining right of ways and installation of fiber, a time consuming and expensive process.  
           [0005]    Free space optical communication systems are fundamentally different than fiber optic systems in that fiber is typically used to transport data between node points. Distances are more limited with free space due to atmospheric attenuation or man-made or natural obstacles. In particular, in free space, the media is air and subject to atmospheric disturbances such as fog, rain and resulting fades. Please refer to our earlier United States patent application with Robert Miller, U.S. Ser. No. 09/640,576 filed Aug. 17, 2000, entitled “Free Space Optical Communication Link with Diversity,” to be deemed incorporated by reference as to its entire contents  
           [0006]    Koh and Davidson (“Interleaved Concatenated Coding For The Turbulent Atmospheric Direct Detection Optical Communication Channel”,  IEEE Transactions On Communications , Vol. 37, No. 6, June 1989, pages 648-651) discuss how the direct detection atmospheric optical communication channel is characterized by strong fading of the received laser light intensity caused by random variations in the index of refraction encountered by laser light variations as it propagates through the channel.  
           [0007]    In addition, the Jet Propulsion Laboratory of the California Institute of Technology published in November of 1998 a Technical Support Package on Multiple-Beam Transmission For Optical Communication in November 1998 as NASA Tech Brief, Vol. 22, No. 11 from a JPL New Technology Report NPO-20384. This NASA Tech Brief describes how superposition of mutually incoherent beams would reduce deleterious effects of atmospheric turbulence.  
         SUMMARY OF THE INVENTION  
         [0008]    It is an object to the present invention to provide a free space wavelength duplexed optical communication link that is reduces the effects of fading.  
           [0009]    This and other objects are achieved in a free space wavelength duplexed system that includes first and second terminals. The first terminal includes an optical transmitter and an optical receiver. The optical transmitter provides multiplexing. The optical receiver has an optical system that functions similarly to a telescope for collecting light emitted by the corresponding transmitter, an optical to electrical converter and an optical amplifier coupled between the optical system and the optical to electrical converter.  
           [0010]    In an alternative embodiment, a method includes the steps of receiving a received optical signal through a telescope, diverting the received optical signal in an optical splitter into an optical amplifier, and transmitting a transmit optical signal through the optical splitter to the telescope.  
           [0011]    In another alternative embodiment, a method includes the steps of receiving plural received optical signals through a telescope, diverting the plural received optical signals in an optical splitter into an optical amplifier, separating the plural amplified optical signals by wavelength, and transmitting plural transmit optical signals at distinct wavelengths through the optical splitter to the telescope.  
           [0012]    The receiver includes diversity reception means to optimally combine the received signals. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0013]    The invention will be described in detail in the following description of preferred embodiments with reference to the following figures wherein:  
         [0014]    [0014]FIG. 1 is a schematic of an optical communication system incorporating the present invention;  
         [0015]    [0015]FIG. 2 is a block diagram of a first embodiment of the present invention;  
         [0016]    [0016]FIG. 3 is a block diagram of a second embodiment of the present invention;  
         [0017]    [0017]FIG. 4 is a schematic diagram of a telescope according to the present invention; and  
         [0018]    [0018]FIG. 5 is a block diagram of an encoder section of a transmitter according to the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0019]    In FIG. 1, communication system  10  includes a plurality of nodes, depicted as nodes  12 ,  14 ,  16  and  18 , that may be located on the tops of tall buildings in metropolitan areas and on towers elsewhere. Each node is coupled to a network control system that includes central controller  20 , land lines  22  and one or more radio towers  24 . Radio towers  24  communicate with the nodes over wireless links  26 . The control system may advantageously include a typical cellular telephone system, controller  20  (located at a convenient location) and cell phone transceiver  46  at each node to direct the operation of communication system  10 .  
         [0020]    The nodes are configured into a network by a plurality of point-to-point links of which link  30  is typical. Each link, as depicted by link  30 , includes a bidirectional (e.g., duplex) free space optical channel. However, in any network, there may be one or more links with only a unidirectional channel.  
         [0021]    Each node includes at least one outdoor unit  40  (hereinafter ODU), and typically a plurality of ODUs. For example, eight ODUs  40  are depicted in FIG. 1 on the top of a building at node  12 . Each ODU is coupled to switch circuit  42  through respective cables  44 . Each ODU couples free space optical signals received over link  30  into cable  44 , and propagates optical signals in a fiber in cable  44  as free space optical signals over link  30 . Switch circuit  42  is controlled by controller  20  through cell phone transceiver  46 . Typically switch circuit  42  and cell phone transceiver  46  are part of an indoor unit (IDU).  
         [0022]    The free space optical channel (hereinafter FSOC) can transmit at a super high bandwidth that no other wireless technology can offer. However, the FSOC is subject to transmission beam fades due to atmospheric turbulence. Some or all of the transmission beam fades can be reduced by use of delay and diversity techniques which include redundant transmission beams and wavelengths within the optical or electrical path through a free space optical communication system to overcome transmission errors due to atmospheric fade.  
         [0023]    In FIG. 2, a free space optical channel (as in link  30  of FIG. 1) includes transmitter  80  and receiver  90 . Transmitter  80  includes electrical multiplexer  81  to drive laser transmitter  82  as input to optical amplifier  84  to couple an optical signal through fiber connection  86  to optical emitting telescope  88 . Electrical multiplexer  81  accepts multiple independent data signals and combines them into a single signal to modulate laser transmitter  82 . Laser transmitter  82  is preferably, but not necessarily, a properly driven laser diode. Optical amplifier  84  is preferably, but not necessarily, an erbium doped fiber amplifier (EDFA). Optical amplifier  84  receives the optical signal from laser transmitter  82  and provides an amplified optical signal at the desired power level to fiber connector  86 . Fiber connector  86  is preferably, but not necessarily, a single mode optical fiber to deliver the amplified optical signal from optical amplifier  84  (usually part of the indoor unit) to optical emitting telescope  88  (usually part of the outdoor unit). Optical emitting telescope  88  propagates the optical signal through free space (the atmosphere) from a node that includes optical emitting telescope  88  to the node that includes optical (light collecting) telescope  91 .  
         [0024]    Receiver  90  includes optical telescope  91  coupled to optical duplexer  92 . The optical signal that passes out of the receive port of optical duplexer  92  is captured by conical fiber collector  93 . Preferably, conical fiber collector  93  is surrounded by four quadrant detectors (e.g., photo diodes) that are sensed electrically to adjust the point of optical telescope  91 , if necessary; however, conical fiber collector  93  collects the optical signal for transmission in a fiber to optical filter  94 . Optical filter  94  ensures that only the desired receive optical band is passed to optical amplifier  95 ; thereby eliminating any back scatter from optical transmitter  99 . Optical amplifier  95  is preferably, but not necessarily, an erbium doped fiber amplifier (EDFA). The output of optical amplifier  95  is coupled through optical attenuator  96  to optical detector  97  (e.g., a photo diode). Optical attenuator  96  senses the optical power level and receives a command to adjust the amount of attenuation to ensure that optical detector  97  is always operated at an optimal power operating point. Dectector  97  converts the optical signal into an electrical signal that is demultiplexed in demultiplexer  98 . Demultiplexer  98  is the conjugate of multiplexer  81  of transmitter  80 .  
         [0025]    The link from transmitter  80  to receiver  90  is one direction of a duplex channel. To implement the other direction, wavelength duplexing is used (transmission in one direction is at a wavelength that is different from the transmission in the other direction). This feature enables optical filter  94  to block back scatter form optical transmitter  99 . The reverse direction transmission originates at optical transmitter  99 . Optical transmitter  99  preferably includes all of the individual elements described as multiplexer  81 , laser transmitter  82 , optical amplifier  84  and fiber connection  86 . The optical output of optical transmitter  99  enters the transmitter port of optical duplexer  92 , and from there is propagated to an optical system for collecting light from a regional light source, hereinafter, a telescope  91  for transmission to telescope  88 . Optical duplexer  92  may be any device for providing a return channel path as well as a forward channel path merge functionality. An optical duplexer (that corresponds to optical duplexer  92 ) is coupled between telescope  88  and fiber  86  so that a receive optical signal may be processed in a way that corresponds to the way described with respect to receiver  90 .  
         [0026]    In FIG. 3, a free space optical channel (as in link  30  of FIG. 1) includes transmitter  50  and receiver  60 . Transmitter  50  includes first optical transmitter  52  and second optical transmitter  54 . The input signal IN is divided to independently and simultaneously excite first and second optical transmitters  52  and  54 . Typically, each optical transmitter is a laser diode but may include other high speed modulated electro-optical devices such as light emitting diodes (LEDs). First optical transmitter  52  transmits the input signal carried on wavelength λ1, and second optical transmitter  54  transmits the input signal carried on wavelength λ2. Transmitter  50  further includes optical combiner  56  and optical telescope  58  to transduce the optical signals from the outputs of first and second optical transmitters  52  and  54  into free space optical beams directed in the direction of receiver  60 .  
         [0027]    Receiver  60  includes optical telescope  62  to transduce the free space optical beams received from transmitter  50  into an optical signal (typically contained in an optical fiber) that is supplied through optical amplifer  63  (e.g., erbium doped fiber amplifier) to wavelength demultiplexer  64 . Wavelength demultiplexer  64  separates wavelength division multiplex optical signals into an optical signal carried on wavelength λ1, and an optical signal carried on wavelength λ2. The optical signal carried on wavelength λ1 is detected in optical-to-electrical converter  66 , and the optical signal carried on wavelength λ2 is detected in optical-to-electrical converter  68 . The optical-to-electrical converters may be, for example, photodiodes, avalanche photodiodes, phototransistors or photogates. The detected outputs of converters  66  and  68  are combined in diversity combiner  70 , and the combined signal is output as signal OUT.  
         [0028]    The optical transmitter and optics of FIG. 3 includes a dual channel (or plural channel) arrangement that converts the input signal into redundant optical signals at different wavelengths before optically sending two beams (or plural beams) to the receiver. In this way, an optical transceiver optically modulates a signal onto redundant channels at different wavelengths. At the receiver end, the free space signal is converted to an optical signal in a fiber and amplified in optical amplifier  63 . Prior art transmission systems do not transmit multiple beams at correspondingly distinct wavelengths while optically amplifying the received signal.  
         [0029]    At the receiver, signal levels of the optical signals at wavelengths λ1 and λ2 are monitored and used to optimally combine the received signals.  
         [0030]    In FIG. 4, transmit telescope  120  includes two input fibers carrying optical signals at two different wavelengths (λ1 and λ2). The two optical signals at different wavelengths are combined or summed in a device for combining light channels, such as a biconic taper fiber or combiner or coupler,  122  and the combined signal is divided or split in signal splitter  124 , for example, to provide approximately ½(λ1+λ2) in each path. From each end  126  of the dividing coupler, a multi-wavelength beam is launched and focused by optical system, such as a lens or lens and mirror system,  128  on a distant receiving light collector or telescope  130 . Although the transmit telescope collimates the outgoing beams to a desired degree of divergence, there will be some small dispersion of the beam that results in an overlap area. Optical lens  132  of receive telescope  130  is positioned in the overlap area so that lens  132  receives the superimposed beams. Lens  132  focuses the overlapped beams into, for example, a multimode fiber, conical taper, or other device  134  which collects the optical signal as a multimode signal for further processing.  
         [0031]    The laser transmitter of FIG. 2 or  3  may be replaced with a wavelength division transmitter of FIG. 5. In FIG. 5, the transmitter includes encoding section  150 . Encoding section  150  includes multiplexer  152  to multiplex together plural diverse input signals and provide a serial bitstream at its output. Then, in serial to parallel converter  154  the serial bitstream is converted into plural parallel signals (a predefined number of signals) to be processed. Each parallel signal is process in parallel section  160  that includes forward error correction encoder  162  (an FEC encoder or other redundance error correction encoder), bit interleaver  164  and a laser transmitter  166  (e.g., a laser diode or other laser source). For example, an output of FEC encoder  162  might be a signal organized in a block made of  8  bytes with each byte having  8  bits. Interleaver  164  might take the first bit of each byte before taking the second bit of each byte. In this way, errors are spread out over the time it takes to transmit the block in order to “whiten” the effect of an error and make it easier for a FEC code to correct for the error. Each interleaved signal is then converted into an optical signal on a distinct, predefined wavelength and the optical signals are combined in optical combiner  156  (e.g., coupler  122  of FIG. 4), and the combined signal is amplified in optical amplifier  158  (e.g., an erbium doped fiber amplifier, EDFA) before being sent to a transmitter telescope (see FIG. 2 or  3 ). Prior art does not use this arrangement for an optical transmitter, and thus is unable to tolerate deep fades (&gt;30 dB) that last for tens of milliseconds.  
         [0032]    Having described preferred embodiments of a novel free space optical communications link (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. For example, various techniques of sending redundancy information and redistributing information over the time slot for a block of data may be combined to whiten and limit the effects of fading. It is therefore to be understood that changes may be made in the particular embodiments of the invention disclosed which are within the scope and spirit of the invention as defined by the appended claims.  
         [0033]    Having thus described 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.