Abstract:
An optical device and method for bidirectional communication in both high and low bit rate applications simultaneously using a polarization-sensitive beam splitting cube and optical port assemblies. The device provides optical isolation for a transmitting laser optic source and provides optical reflection of an optical beam to a receiver, simultaneously, over a single fiber optic link.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates generally to bidirectional fiber optic communication systems and, more particularly, to an optical device which may be configured as an optical isolator and circulator for use in both high and low bit rate applications in such fiber optic systems. 
     2. Description of the Prior Art 
     Communication service providers are experiencing significant consumer demands to accommodate additional bandwidth in optically-based communications systems and the demand is ever-increasing. Today&#39;s optical communication systems and networks field rising consumer demands for e-mail, video, multimedia, data and voice-data transmission requirements across a variety of communication protocols. In the future, all indications are that the use of fiberoptic networks will become even more prevalent as a preferred medium for transferring information as the marketplace for wide-band services matures. It is anticipated that additional services such as enhanced pay-per-view, video-on-demand, interactive television and gaming, image networking, video telephony, CATV and ISDN switching services will be depend on and be substantial users of such systems. Because capacity is a critical parameter for system viability, bidirectional systems are desirable when the increased capacity or other attributes afforded by the bidirectional fiber is required. Enabling bidirectional use of installed and developing fiber in fiberoptic systems will permit communication service provider to gain additional utility from limited system resources. 
     Lasers are employed in numerous applications, particularly within fiberoptic communications networks, in which the laser emits an information-carrying light signal to an optical fiber which transmits the light signal to a photodetector for further processing. Typically, the optical signal propagates in one direction over a signal optical fiber. In a bidirectional fiber optic configuration, an optical signal propagates in both directions over an optical fiber. However, due to the sensitivities of these systems even a small amount of reflected light will cause instability to the laser source in terms of its power and frequency characteristics. 
     To reduce some of the problems of reflected light, optical circulators and isolators, non-reciprocal devices, may be installed at each end of a fiber link in a system thereby enabling the bit carrying capacity of an existing unidirectional fiber optic link. The use of Faraday isolators in optic systems is well known as an integral component for removing reciprocal light based on the use of polarizers which are rotated by 45 degrees relative to each other on either side of a magnetic medium, as optical isolators passes a signal in the forward direction from a first optical port to a second optical port. Optical circulators are employed in bidirectional systems for multiplexing the forward and reverse paths of an optical light source, such as a laser. Optical circulators provide a non-reciprocal coupling of light between two fiber paths, based on the Faraday rotation of light, as light is treated differently depending on its entry port into an optical port. 
     As is generally known, an optical circulator is a non-reciprocal optical device which allows the passage of light from a first port to a second port while a reverse optical signal into the second port is transmitted in totality to a third port; similar transmissions continue for remaining ports thusly creating a circulating operation. Effectively, any two consecutive ports of an optical circulator are an optical isolator as signals are transmitted only one way. 
     FIG. 1 is illustrative of the operation employing optical circulators to provide simultaneous, bidirectional communication in a single fiber optic link. In FIG. 1, optical circulators  100  and  200 , each comprised of ports  10 ,  20 , and  30 , are installed at opposite ends of a fiber optic link  150 . Communication transmitters  110  and  210  are connected to each port  10 , communication receivers  120  and  220  are connected to each port  30 , and the fiber optic link  150  is connected between ports  20  at each optical circulator. Light entering port  10  exits the optical circulator at port  20  as directionally indicated  160 . Light that enters the optical circulator at port  20  exits at port  30  as directionally indicated  170 . Light travels bidirectionally across a single fiber  150  as directionally indicated  180 . 
     Using traditional beam splitting plates in bidirectional systems may result in substantial reductions in light intensity each time a light beam passes through a beam splitting plate (e.g. on the order of 50% loss). Typically these “ping-pong” type of fiber communications occur at low speeds. Additionally, there is an increase in noise to the system due to insertion loss, cross-talk and coupling loss. With the advent of long-haul applications, these reflective problems may cause the communication to fail; similarly, low speed rates are not well-suited for long-haul application. Wavelength dependent beam splitter cubes and dichroic mirrors are known in the field. U.S. Pat. No. 5,210,643 (Fuji et al.) discloses a wave combining apparatus having dichroic mirrors for combining laser beams having the same direction of oscillation by waveform division, respectively, and a polarizing beam splitting prism for combining the first and second resultant beams into a single combined waveform. However, the separation and recombination of optical beams as described in this reference is suited only for low-speed applications. 
     Therefore, the need exists for an apparatus which improves over the light intensity losses, virtually eliminates ghost images resulting from reflections, and is able to communicate in both high and low bit rate applications at a reasonably low-cost. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, the optical device of the present invention is an optical isolator and circulator comprising a Faraday rotator, a polarizer and a polarizing sensitive beam splitting cube to provide optical isolation for the transmitting laser source and at the same time reflect the optical beam to the receiver. 
     The Faraday rotator rotates linearly polarized light by forty-five degrees such that the rotation may be clockwise or counterclockwise; the Faraday rotator is a traditional Faraday rotator requiring a magnetic field or preferably, is a latched-Faraday rotator which does not utilize a magnetic field. For the traditional Faraday rotator, the presence of an additional magnet ring having a different direction of magnetic field may be required depending on the configuration employed. In the present invention, a first Faraday rotator rotates linearly polarized light counterclockwise forty-five degrees, while a second Faraday rotator rotates linearly polarized light clockwise forty-five degrees in the same reference frame as to the first source. 
     The Faraday rotator and polarizer may preferably be integrated into a generic bidirectional module capable of transmitting, receiving or both. In this preferred arrangement, the module requires isolation from both specular reflections from fiber, connectors, and transmitted source signals from other emitters or transmitter devices. 
     In a further preferred arrangement, the bidirectional module comprises an isolator, a circulator, or both. 
     The polarizing beam splitting cube preferably is comprised of two triangular prisms connected (i.e. affixed) together by a dielectric film or coating. Undesirable light reflections from the cube&#39;s faces are eliminated by orienting the cube about the dielectric film&#39;s normal axis so that light is incident with nonzero incidence angle at the cube&#39;s faces. Preferably, a broadband coating is applied to the cube faces to virtually eliminate reflected light. In an additional preferred embodiment, an absorbing filter is optically cemented on the back face of the cube to absorb stray light. 
     It is an object of the present invention to provide an apparatus which enables bidirectional transmission and reception of both high and low bit rate applications, simultaneously. 
     It is another object of the present invention to provide an apparatus which reduces reflective light signals from laser optic sources. 
     It is another object of the present invention to provide an apparatus which is economically suited for long-haul communication systems. 
     As will be more readily appreciated from the detailed description that follows, the present invention offers a number of advantages not previously achieved in the prior art. For example, the present invention offers both transmission and receiving capabilities through a single fiber for both low and high bit rate applications. 
     These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with its various features and advantages, can be readily understood from the following more detailed description taken in conjunction with the Figures herein. 
     FIG. 1 is a block diagram of a bidirectional communication system employing an optical circulator at each end of a single fiber optic link. 
     FIG. 2 is a diagrammatic view of the beam splitting cube which provides optical isolation and reflects optical beams. 
     FIG. 3 is a diagrammatic view of the optical isolator and circulator according to the present invention, showing the paths of light rays and directional flow of optical beams throughout the system. 
     FIG. 4 depicts diagrammatic views of a preferred embodiment of the invention having transceivers integrated with the beam splitting cube. 
     FIG. 5 is a diagrammatic view of the optical isolator and circulator according to the present invention comprising a plurality of beam splitting cubes. 
     FIG. 6 depicts diagrammatic views of a preferred embodiment of the invention having a plurality of beam splitting cubes integrated with transceivers. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The preferred embodiment of the present invention and its advantages are best understood by referring to the drawings, like numerals being used for like and corresponding parts of the various drawings. 
     FIG. 2 depicts a diagrammatic view of the beam splitting cube  300  which provides optical isolation and optical reflection of light beams relative to the polarization of such light beams. 
     A first optic light source  310  preferably having a polarized counterclockwise rotation of forty five degrees relative to the horizontal axis  500  of the first light source enters the cube  300  passing through a first face of the cube  320  directed towards the cube interface  330  wherein the interface is coated with an optical coating  340 . In a preferred embodiment, the cube is coated with an anti-reflective coating (not shown), the interface coating is a broadband coating for the range of 1300-1600 nm, and the first optic light source is a polarized laser source. 
     Approximately 99% of the first optic light source is transmitted at the interface and is directed to a second face of the cube  350 , wherein the transmitted portion  360  of the first light source is emitted from the second face  350 . The remaining 1% non-transmitted portion of the first optic light source is isolated from the laser source due to the presence of the Faraday rotator which prevents the light from traveling in the source light&#39;s direction. 
     A second optic light source  400  preferably having a polarized clockwise rotation of forty five degrees relative to the horizontal axis  500  of the first light source enters the cube  300  passing through a second face of the cube  350  directed towards the cube interface  330  wherein approximately 99% of the second optic light source is transmitted by reflection to a third face of the cube  370  and the transmitted portion  410  of the second light source is emitted from the third face  370 . 
     Use of polarizing beam splitter cube virtually eliminates the light intensity losses of traditional beam splitting plates, resulting in 50-87% more light in the final image. Another benefit of using polarizing beam splitting cube is the virtual elimination of ghost images from reflected light. 
     FIG. 3 depicts a diagrammatic view of the optical isolator and circulator in a preferred embodiment of the present invention. 
     A first light source  600 , which is preferably a polarized laser source, is transmitted through a polarizer  610 , a Faraday rotator  620 , and optionally an analyzer  630 , wherein the combination  640  of the polarizer, Faraday rotator and optional analyzer are aligned relative to one another such that in a preferred embodiment, the combination is a Faraday isolator  640 , and is emitted as a first rotated light source  650 . In a preferred embodiment the Faraday isolator  640  is part of a first transceiver device  700  which is capable of transmitting and receiving polarized light beams and comprises a first transmitter  601  and first receiver  602 . 
     The first rotated light source  650 , preferably rotated counterclockwise forty five degrees, is directed to the beam splitting cube  300  and passes through an interface  330  having a broadband optical coating  340  wherein a first emitted beam  660 , having approximately 99% of the first rotated light source intensity, is directed to a second receiving means  800 , which in a preferred embodiment the receiving means is part of a second transceiver  810  which is capable of transmitting and receiving polarized light beams, and further comprises a second transmitter  901 . 
     A second light source  900 , which is preferably a polarized laser source, is transmitted through a polarizer  910 , a Faraday rotator  920  and optionally an analyzer  930  wherein the combination of the polarizer, the Faraday rotator and the optional analyzer are aligned relative to one another and a first light source  600  such that in a preferred embodiment, the combination is a Faraday isolator  940 , and is emitted as a second rotated light source  950  wherein the second clockwise rotated light source is preferably at a polarization of forty five degrees. 
     The second rotated light sources  950  is directed to the beam splitter cube  300  and is reflected from the interface  330 , wherein a reflected beam  960  having approximately 99% of the second rotated light source intensity is directed towards a third receiving means  1000  which is positioned perpendicular to the first transmitted beam  650  and parallel to the cube  300 . 
     In a preferred embodiment, the first light source and the second light source transmit respective light sources simultaneously. 
     In another preferred embodiment, a fiber means  1100  connects the first transceiver  700 , the second transceiver  810 , the beam splitting cube  300 , and the third receiving means  1000 . 
     FIG. 4 depicts a preferred embodiment of the present invention having a beam splitting cube integrated with transceiver devices. FIG. 4 a  shows the beam splitting cube  300  integrated with a first transceiver device  700 . A first rotated light  650  passes through the cube and is directed to a second transceiver device  810 . The second transceiver device  810  transmits a second rotated light source  950  to the integrated cube  300  and a reflected beam  960  results which is received by the first transceiver  700 . Optionally, the beam splitting cube may be integrated with the second transceiver so the second transceiver receives both the first rotated light  650  and the reflected beam  960 . 
     FIG. 4 b  shows an optional fiber connection  1500  in conjunction with an integrated cube. The fiber connection  1500  provides a communications path between each of the transceivers excluding the cube. 
     FIG. 4 c  shows a cube  300  integrated with both a first and second transceivers  700 ,  810 , and an optional fiber path  1510  for a resulting reflected beam. 
     FIG. 5 depicts a further preferred embodiment of the present invention wherein a plurality of beam splitting cubes  300 ,  301  and arranged to permit bidirectional communications. In FIG.  5  a first transmitted light source  650  to pass through a first beam splitting cube  300  and a first emitted beam  660  is directed away from the cube  300 . 
     A second transmitted light source  950  is emitted from a second transceiver device  810  and is thereafter directed to a second beam splitting cube  301  wherein a first reflected beam  302  is emitted and is thereafter directed to a first beam splitting cube  300  such that the resulting emitted beam  1200  is reflected from the first beam splitting cube  300  and is thereafter directed to a receiving means  602  of the first transceiver device  700 . FIG. 5 depicts two beam splitting cubes by way of example and not of limitation such that a plurality of cubes may further be arranged for bidirectional communication. 
     In a further preferred embodiment, a fiber means  1250  connects the first transceiver  700 , the second transceiver  810 , and the first  300  and second  301  beam splitting cubes. 
     FIG. 6 depicts the cubes  300 ,  301  integrally arranged with transceivers  700 ,  810 , and having optical light paths traveling in a similar manner as previously discussed for FIG.  5 .