Patent Publication Number: US-6671435-B2

Title: Wireless optical communications without electronics

Description:
This is a Divisional of pending U.S. patent application Ser. No. 09/623,902 filed on Sep. 11, 2000, now U.S. Pat. No. 6,366,723, which is a 371 of PCT/IL99/00500, filed Sep. 14, 1999, which claims the benefit of Provisional application Ser. No. 60/100,632, filed Sep. 16, 1998. 
    
    
     FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates to wireless communications systems in general, and more particularly to optical wireless communications systems. 
     Medved et al., in U.S. Pat. No. 5,818,619, which is incorporated by reference for all purposes as if fully set forth herein, teach a wireless communications system for linking different parts of an optical communications network. Each part of the network is provided with one or more optical communications network interface units and with universal converter units that are optically coupled to their respective network interface units. Each universal converter unit includes an airlink transmitter, an airlink receiver, a fiber optic receiver and a fiber optic transmitter. The fiber optic receiver receives outgoing optical signals from the network interface unit and transforms these optical signals to electronic signals. These electronic signals are sent to the airlink transmitter, where these electronic signals are transformed back to optical signals and transmitted as such into free space. The airlink receiver receives optical signals that were transmitted into free space by another universal converter unit and transforms these incoming optical signals into electronic signals. These electronic signals are sent to the fiber optic transmitter, which transforms these electronic signals back to optical signals that are sent to the network interface unit via a fiber optic cable. The network interface units and the universal converter units are operated in pairs, with each member of the pair being a portion of a different optical communications network or of a different part of the same optical communications network. The airlink transmitter of each universal converter unit is aimed at the airlink receiver of the other universal converter unit to enable exchange of optical signals between the two optical communications network or between the two parts of the same optical communications network. 
     The wireless communications system of Medved et al. is intended for use in an optical communications network in which signals are encoded in a single carrier wavelength. Recently, optical communications networks based on dense wavelength division multiplexing (DWDM) have been introduced. In a DWDM network, several carrier wavelengths are multiplexed on the same optical fiber. The data transmission rate available using DWDM would overwhelm the electronics of the universal converter units of Medved et al. In any case, the various carrier wavelengths would have to be demultiplexed, and a separate network interface unit and universal converter unit would be needed for each carrier wavelength. 
     There is thus a widely recognized need for, and it would be highly advantageous to have, a system for linking two parts of an optical communications network that are remote from each other in a way that facilitates the exchange of DWDM optical signals. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided an optical device including: (a) a multimode optical waveguide having a proximal end and a distal end; (b) a single mode optical waveguide having a distal end; (c) a mechanism for optically coupling the distal end of the single mode optical waveguide to the proximal end of the multimode optical waveguide; and (d) imaging optics, optically coupled to the distal end of the multimode optical waveguide. 
     According to the present invention there is provided an optical transmitter, including: (a) a common input optical waveguide; (b) a plurality of transmitter optical waveguides, each transmitter optical waveguide having a distal end; (c) for each transmitter optical waveguide, imaging optics, optically coupled to the distal end of the each transmitter optical waveguide; and (d) a mechanism for optically coupling the common input optical waveguide to the transmitter optical waveguides. 
     According to the present invention there is provided an optical receiver, including: (a) a common output optical waveguide; (b) a plurality of receiver optical waveguides, each receiver optical waveguide having a distal end; (c) for each receiver optical waveguide, imaging optics, optically coupled to the distal end of the each receiver optical waveguide; and (d) a mechanism for optically coupling the common output optical waveguide to the receiver optical waveguides. 
     According to the present invention there is provided an optical transceiver including: (a) a transmitter optical waveguide having a distal end; (b) transmitter imaging optics, having a transmitter optical axis, optically coupled to the distal end of the transmitter optical waveguide; (c) a plurality of receiver optical waveguides, each receiver optical waveguide having a distal end; and (d) for each receiver optical waveguide, receiver imaging optics, having a receiver optical axis, optically coupled to the distal end of the each receiver optical waveguide, the transmitter optical axis and the receiver optical axes all being substantially parallel. 
     According to the present invention there is provided a wireless communications system, including: (a) a transmitter optical waveguide having a proximal end and a distal end; (b) transmitter imaging optics, optically coupled to the distal end of the transmitter optical waveguide; (c) at least one receiver optical waveguide having a proximal end and a distal end; (d) for each at least one receiver optical waveguide, receiver imaging optics optically coupled to the distal end of the at least one receiver optical waveguide; and (e) an optical communication network interface unit, optically coupled to the proximal ends of the transmitter optical waveguide and of the at least one receiver optical waveguide, for transmitting optical signals to the transmitter optical waveguide and for receiving optical signals from the at least one receiver optical waveguide. 
     According to the present invention there is provided an optical transceiver including: (a) a transmitter optical waveguide having a distal end; (b) transmitter imaging optics, having a transmitter optical axis, optically coupled to the distal end of the transmitter optical waveguide; and (c) an airlink receiver having a receiver optical axis substantially parallel to the transmitter optical axis. 
     According to the present invention there is provided a wireless communication system, including: (a) a transmitter optical waveguide having a proximal end and a distal end; (b) transmitter imaging optics, optically coupled to the distal end of the transmitter optical waveguide; (c) an airlink receiver; (d) a converter unit, electrically coupled to the airlink receiver; and (e) an optical communication network interface unit, optically coupled to the proximal end of the transmitter optical waveguide and to the converter unit, for transmitting optical signals to the transmitter optical waveguide and for receiving optical signals from the converter unit. 
     According to the present invention there is provided an optical device including: (a) an optical fiber having a distal end; and (b) a FC/APC fiber optic connector serving as a reflection-suppressing interface between the distal end and a rarefied optical medium. 
     According to the present invention there is provided a wireless system for transmitting wavelength-multiplexed optical signals from a first location to a second location, including: (a) an optical transmitter, at the first location, the optical transmitter including a multimode input optical waveguide for receiving the optical signals; and (b) an optical receiver, at the second location, for receiving the optical signals from the optical transmitter. 
     According to the present invention there is provided a method for exchanging optical signals between two parts of an optical network, including the steps of: (a) providing each part of the network with: (i) a network interface unit, and (ii) a transceiver including: (A) transmitter imaging optics, (B) at least one transmitter optical waveguide for optically coupling the network interface unit to the transmitter imaging optics, (C) receiver imaging optics, and (D) at least one receiver optical waveguide for optically coupling the network interface unit to the receiver imaging optics; and (b) aiming the transceivers so that at least part of the optical signals emerging from the transmitter imaging optics of a first the transceiver are intercepted by the receiver imaging optics of a second the transceiver and so that at least part of the optical signals emerging from the transmitter imaging optics of the second transceiver are intercepted by the receiver imaging optics of the first transceiver. 
     According to the present invention there is provided a method for exchanging optical signals between two parts of an optical network, including the steps of: (a) providing each part of the network with: (i) a network interface unit, and (ii) a transceiver including: (A) transmitter imaging optics, (B) at least one transmitter optical waveguide for optically coupling the network interface unit to the transmitter imaging optics, (C) an airlink receiver, and (D) a converter unit, electrically coupled to the airlink receiver and optically coupled to the network interface unit; and (b) aiming the transceivers so that at least part of the optical signals emerging from the transmitter imaging optics of a first the transceiver are intercepted by the airlink receiver of a second the transceiver and so that at least part of the optical signals emerging from the transmitter imaging optics of the second transceiver are intercepted by the airlink receiver of the first transceiver. 
     The basic idea of the present invention is to eliminate the conversion of optical signals in the universal converter unit to electronic signals and then back to optical signals. Instead, the outgoing optical signals, from one network interface unit in one part of the optical communications network, are launched directly into free space and are received directly by another network interface unit in another part of the optical communications network. 
     To facilitate the direct exchange of optical signals between the network interface units, each network interface unit is provided with an optical transceiver, based on a transceiver unit that is used either as a transmitter unit or a receiver unit. A basic transceiver unit has an optical fiber terminating at one end of a cylindrical housing and imaging optics at the other end of the housing. The optical fiber is provided with a mechanism, such as a FC/APC, for suppressing reflections at the fiber-air interface. When the transceiver unit is used as a transmitter unit, optical signals launched from the end of the optical fiber are collimated by the imaging optics into a collimated beam. When the transceiver unit is used as a receiver unit, the imaging optics focus optical signals that they intercept onto the end of the optical fiber. Preferably, the optical fiber is a multimode optical fiber so that the beam launched from the optical fiber in transmitter mode has an adequately large divergence angle. 
     The transmitter units and the receiver units are used in clusters, to overcome scintillation. In a compound transmitter that includes several transmitter units, the optical fibers of the transmitter units are connected to a common input optical fiber by a splitter. In a compound receiver that includes several receiver units, the optical fibers of the receiver units are connected to a common output optical fiber by a combiner. For transmission over distances greater than several hundred meters, it is necessary to amplify the optical signals input to the transmitter, using an optical amplifier such as an erbium-doped fiber amplifier or a semiconductor fiber amplifier. In a compound transmitter, one optical amplifier may be provided for the common input optical fiber, or each transmitter unit may be provided with its own optical amplifier. In the latter case, because the input and output of an optical amplifier is via a single mode optical fiber, a mechanism such as a FC/APC is provided for coupling the single mode output of each optical amplifier to the multimode optical fiber of the respective transmitter unit. 
     To facilitate aiming, the transmitter and receiver units of a transceiver are aligned mutually so that all their optical axes are parallel. 
     The common output optical fiber of a compound receiver preferably is a multimode fiber. In case the network interface unit is designed to receive single mode optical input, the common output optical fiber is provided with a passive adapter, such as a graded index lens or a collimator, for coupling the common output optical fiber to the network interface unit. Similarly, the multimode optical fiber of a single-unit receiver is provided in such a case with a similar passive adapter. 
     As an alternative to all-optical reception, a transceiver of the present invention may include an airlink receiver and a fiber optic transmitter, as in the prior art universal converter unit. The transmitter of the transceiver remains all-optical. 
     In the application of the present invention to the exchange of DWDM signals, each network interface unit preferably includes a demultiplexer for demultiplexing the DWDM signals. 
     Although the examples of the present invention described herein are based on optical fibers, it is to be understood that the scope of the present invention includes optical waveguides generally. The wavelengths of the optical signals that fall within the scope of the present invention include infrared, viable, and ultraviolet wavelengths, although the preferred wavelengths are those that are commonly used for optical communication: wavelengths in the neighborhood of 850 nm, wavelengths in the neighborhood of 1330 nm and wavelengths in the neighborhood of 1550 nm. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
     FIG. 1 shows two schematic axial cross sections of two variants of a transceiver unit of the present invention; 
     FIG. 2 is a schematic depiction of a system of the present invention; 
     FIG. 3 shows three different transceiver unit cluster configurations; 
     FIG. 4 shows a variant of a transmitter cluster that includes optical amplifiers; 
     FIG. 5 is a schematic depiction of an alternate transceiver of the present invention; 
     FIG. 6 is a partial schematic depiction of a system of the present invention for exchanging DWDM optical signals. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is of an optical communications system which can be use to link two widely separated parts of an optical communications network. Specifically, the present invention can be used to exchange DWDM signals between the two parts of the network. 
     The principles and operation of optical communications according to the present invention may be better understood with reference to the drawings and the accompanying description. 
     Referring now to the drawings, FIG. 1 illustrates two variants,  10 A and  10 B, of a basic transceiver unit  10  of the present invention. Variants  10 A and  10 B are illustrated schematically, in axial cross section. Both variants are based on a substantially cylindrical housing  12 , at the distal end  14  of which are imaging optics, represented as a lens  18 , and at the proximal end  16  of which is a multimode optical fiber  20  whose distal end  22  is terminated in a FC/APC  24 . Variant  10 B includes, in addition, a single mode optical fiber  28 , optically coupled at the distal end  30  thereof to the proximal end  26  of multimode fiber  20  by a FC/APC  32 . 
     Imaging optics  18  define an optical axis  34 . When unit  10  is used as a transmitter, optical signals emerge from distal end  22  of optical fiber  20  as a divergent beam of light that is collimated by imaging optics  18  to propagate as a collimated beam of light in the direction defined by optical axis  34 . When unit  10  is used as a receiver, unit  10  is aimed so that imaging optics  18  intercept a portion of an incoming beam of light that carries optical signals. Imaging optics  18  focuses the incoming light onto distal end  22  of optical fiber  20 . The focal length of imaging optics  18  is adapted to the divergence angle of optical fiber  20 . 
     Typically, optical fibers  20  and  28  are made of optically pure glass. Single mode optical fiber  28  typically has a core diameter of 9 microns. Multimode optical fiber  20  typically has a core diameter of 50, 62.5 and 100 microns, most preferably 100 microns. The purpose of FC/APC  24  is to suppress reflections at the air-glass interface at distal end  22  of optical fiber  20 . This use of an FC/APC to suppress reflections at an interface between a solid optical fiber and a rarefied optical medium such as air constitutes an independent aspect of the present invention. The purpose of FC/APC  32  is to connect, and to optically couple, optical fibers  20  and  28 . An FC/APC is particularly convenient for this purpose because both optical fibers  20  and  28  typically have the same cladding diameter, 125 microns. 
     FIG. 2 is an illustrative schematic depiction of a system of the present invention, for linking two parts of an optical communications network, represented by two network interface units  58 L and  58 R. Each part of the optical communications network is provided with a transceiver  36  that includes a transmitter  38  and a receiver  40 . Transmitter  38  includes a cluster of three transceiver units  10  configured as transmitter units  42 . Each transmitter unit  42  includes a transmitter optical fiber  50  and a transmitter optical axis  46 . Receiver unit  40  includes a cluster of three transceiver units  10  configured as receiver units  44 . Each receiver unit  44  includes a receiver optical fiber  52  and a receiver optical axis  48 . In variant A of transmitter unit  42 , transmitter optical fiber  50  is multimode optical fiber  20 . In variant B of transmitter unit  42 , transmitter optical fiber  50  is the combination of multimode optical fiber  20  and single mode optical fiber  28 , coupled by FC/APC  32 . Similarly, in variant A of receiver unit  44 , receiver optical fiber  52  is multimode optical fiber  20 , and in variant B of receiver unit  44 , receiver optical fiber  52  is the combination of multimode optical fiber  20  and single mode optical fiber  28 , coupled by FC/APC  32 . As noted below, it is preferred that receiver units  44  be variants A of transceiver units  10 . Transmitter optical fibers  50  are optically coupled, at proximal ends  51  thereof, to the distal end  65  of a common input optical fiber  64 , by a splitter  54 . Receiver optical fibers  52  are optically coupled, at proximal ends  53  thereof, to the distal end  67  of a common output optical fiber  66 . Common input optical fiber  64  is optically coupled, at the proximal end  61  thereof, to a fiber optic transmitter  60  of network interface unit  58 . Common output optical fiber  66  is optically coupled, at the proximal end  63  thereof, to a fiber optic receiver  62  of network interface unit  58 . 
     Transmitter units  42 L and receiver units  44 L are mounted so that optical axes  46 L and  48 L all are parallel. Similarly, transmitter units  42 R and receiver units  44 R are mounted so that optical axes  46 R and  48 R all are parallel. In use, transceiver  36 L is aimed at transceiver  36 R, so that the collimated beams of light emitted by transmitter units  42 L are at least partly intercepted by receiver units  44 R and so that the collimated beams of light emitted by transmitter units  42 R are at least partly intercepted by receiver units  44 L. Optical signals transmitted by network interface unit  58 L via transmitter  60 L are conveyed, via optical fibers  64 L and  50 L and splitter  54 L, to transmitter units  42 L, where these optical signals are launched into free space, as collimated beams of light, towards transceiver  36 R. At transceiver  36 R, the optical signals received by receiver units  44 R are conveyed, via optical fibers  52 R and  66 R and combiner  56 R, to receiver  62 R of network interface unit  58 R. Meanwhile, optical signals transmitted by network interface unit  58 R via transmitter  60 R are conveyed, via optical fibers  64 R and  50 R and splitter  54 R, to transmitter units  42 R, where these optical signals are launched into free space, as collimated beams of light, towards transceiver  36 L. At transceiver  36 L, the optical signals received by receiver units  44 L are conveyed, via optical fibers  52 L and  66 L and combiner  56 L, to receiver  62 L of network interface unit  58 L. 
     Clusters of transmitter units  42  are used in transmitter  38 , and clusters of receiver units  44  are used in receiver  40 , to overcome scintillation. FIG. 3 shows transverse views of three different configurations of transceiver units  10  in clusters. FIG. 3A shows three transceiver units  10  in a triangular configuration. FIG. 3B shows four transceiver units  10  in a square configuration. FIG. 3C shows seven transceiver units  10  in a hexagonal configuration. 
     Kostal et al., in U.S. Pat. No. 4,960,315, teaches a similar system, for temporarily bridging a break in an optical fiber network. Because Kostal et al. base their system on single transmitter and receiver units and on single mode optical fibers, they require an elaborate feedback mechanism to keep their transceivers aimed at each other. This feedback mechanism is not needed in the present invention, because the use of clusters of transmitter units and receiver units compensates for scintillation and beam wander, and because multimode optical fibers  20  of the present invention have wider divergence angles (order of 2 milliradians) than the very narrow divergence angles of the single mode optical fibers used by Kostal et al. 
     The system of FIG. 2 is adequate for linking two parts of an optical communications network that are separated by distances up to several hundred meters. For communications across greater distances, the transmitted optical signals must be amplified. FIG. 4 shows a variant  38 ′ of transmitter  38  that includes optical amplifiers  70  and  71  for this purpose. In variant  38 ′, each transmitter unit  42  is provided with its own optical amplifier  70 . Each optical amplifier  70  is optically coupled to a respective transmitter optical fiber  50  at proximal end  51  of transmitter optical fiber  50  and to a respective optical amplifier input optical fiber  72  at the distal end  74  of optical amplifier input optical fiber  72 . Optical amplifier input optical fibers  72  are optically coupled at the proximal ends  76  thereof to distal end  65  of common input optical fiber  64  by splitter  54  and a fourth, common optical amplifier  71 . Common optical amplifier  71  is optically coupled to common input optical fiber  64  at distal end  65  thereof and to an optical amplifier output fiber  78  at the proximal end  82  thereof, and the distal end  80  of optical amplifier output fiber  78  is optically coupled to splitter  54 . Note that optical amplifier  71  is optional. The optical fibers leading into and out of an optical amplifier  70  must be single mode optical fibers. Therefore, transmitter units  42  must be variants B of transceiver units  10 . 
     Optical amplifiers  70  typically are erbium-doped fiber amplifiers or semiconductor optical amplifiers. 
     Although the transfer of optical signals from a single-mode optical fiber to a multimode optical fiber is energetically efficient, this is not the case for transfer of optical signals from a multimode optical fiber to a single mode optical fiber. Therefore, it is preferred that receiver units  44  be variants A of transceiver units  10 , and that optical fibers  52  and  66  be multimode optical fibers. If receiver  62  of a network interface unit  58  is configured to receive single mode input, receiver  62  must be provided with a passive adapter  68 , as shown in FIG. 2 for network interface unit  58 L, to provide efficient optical coupling of common output optical fiber  66  to receiver  62 . Examples of suitable passive adapters  68  include graded index lenses and collimators. 
     FIG. 5 shows an alternate transceiver  84  of the present invention, optically coupled to network interface unit  58 . Transmitter  60  of network interface unit  58  is optically coupled by a transmitter optical fiber  86  to a transmitter  38  of the type discussed above. Receiver  62  of network interface unit  58  is optically coupled by a receiver optical fiber  88  to a converter unit  90  that is substantially identical to TXU  20  of U.S. Pat. No. 5,818,619. Converter unit  90  is in turn electronically coupled to an airlink receiver  92  by a suitable connector  96 . Optical signals intercepted by airlink receiver  92  are converted to electronic signals and relayed to converter unit  96 , which converts the electronic signals back to optical signals, as described in U.S. Pat. No. 5,818,619. 
     In FIG. 5, dashed line  46  represents the optical axis of a single transmitter unit  42 , if transmitter  38  includes only one transmitter unit  42 , or the parallel optical axes of all the transmitter units  42  of transmitter  38 , if transmitter  38  includes more than one transmitter unit  42 . Airlink receiver  92  also has an optical axis, indicated by reference numeral  94 . Optical axes  46  and  94  are parallel. 
     Transceiver  84  is used in the same way as transceiver  36  to link two different parts of an optical communications network. 
     The systems of the present invention may be substituted for the system described in U.S. Pat. No. 5,818,619 in any of the applications of the latter system. The systems of the present invention also may be used to exchange DWDM signals between two widely separated parts of an optical communications network. FIG. 6 is a partial schematic illustration of a system of the present invention configured for this purpose. At the transmitting location, transmitter  38 L receives DWDM optical signals from transmitter  60 L of network interface unit  58 L via an optical fiber  64 L and launches those signals as a collimated light beam  104  towards receiver  40 R at the receiving location. Preferably, transmitter  38 L is one of the variants of a transmitter of the present invention that includes one or more optical amplifiers  70 , as discussed above, and optical fiber  64 L is a single mode optical fiber. Receiver  40 R is optically coupled to a demultiplexer  98  by a multimode optical waveguide  104 . Demultiplexer  98  directs each of the incoming carrier wavelengths to a respective channel that includes a detector  100  for converting the optical signals carried on that carrier wavelength to electronic signals and an amplifier  102  for amplifying the electronic signals from the respective detector  100 . 
     The definition of a network interface unit  58 , as understood in the context of the present invention, is broader than in U.S. Pat. No. 5,818,619. In particular, network interface unit  58  may be an RF-optical transceiver, such as the SAT-LIGHT 2000 transceiver available from Foxcom Ltd. of Jerusalem, Israel, that is used for converting RF analog signals to optical signals and vice versa. Conventionally, these optical signals are exchanged between two separate locations via optical fibers. The present invention enables these transceivers to be used to exchange optical signals between two separate locations without laying optical fibers between the two locations. One important application of this is in cellular telephony. It often is desirable to locate a cellular telephony base station antenna at a considerable distance from the other base station hardware. The present invention allows this to be done without laying optical fibers between the base station and the base station antenna, thereby allowing enhanced flexibility in the siting of the base station antenna. 
     As is well known to those skilled in the art, the preferred optical amplifiers  70  and  71  for a transmitter  38 ′ intended for the transmission of RF analog signals are not the preferred optical amplifiers  70  and  71  that are used for digital applications such as DWDM. The optical amplifiers  70  and  71  that are used in analog applications must have enhanced linearity. Erbium-doped fiber amplifiers of suitable linearity are commonly used in CATV applications. 
     While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.