Abstract:
A bidirectional fiber-optic transceiver with lower optical loss than previous beam-splitters or fiber couplers and a method of implementing the same. Preferably, the transceivers can operate in both directions using a single-wavelength of electromagnetic radiation.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates generally to fiber optic transceivers and, more particularly, to fiber optic transceivers that both transmit and receive optical signals using the same optical port.  
       BACKGROUND OF THE INVENTION  
       [0002]     Fiber optic links are quite useful for transferring high bandwidth data between components that may be quite distant from each other. Examples of these links include data communication trunks, intranets, and (even lately) flight control systems. With regard to the use of fiber aboard aircraft, and other mobile platforms, fiber optic technology allows large amounts of targeting, navigation, communication, command, and control data to be shared with the pilot thereby allowing the pilot increased situational awareness.  
         [0003]     Generally, fiber technology provides a higher bandwidth capability than conventional, copper wire systems. However, fiber technology has been constrained in its use because the fiber optic transceivers currently available can transmit or receive data on one fiber, but can not do both on that one fiber without incurring significant performance penalties (e.g. loss of bandwidth or signal strength) or complicating the design of the transceivers and the overall system.  
         [0004]     Thus, for every bidirectional communication need in which simplicity and performance are desired, the transceivers at the ends of the link must have one optical port for transmitting data signals and a second optical port for receiving data signals. Further, the duplication of parts (primarily optical fibers and connectors) extends along the length of the link. Each direction of the link therefore requires a complete set of cables and connectors when only one cable assembly is used with transceivers that transmit and receive optical signals from a single port.  
         [0005]     If either performance or system simplicity can be sacrificed, then existing bidirectional fiber optic links that are more complex or have more loss can be used. These links fall into two categories. The first category of bidirectional link uses two different wavelengths (one for each direction) and dichroic mirrors (mirrors that transmit or reflect based on the wavelength) or, perhaps, a wavelength splitter to combine and split the signals traveling in the two directions with low loss. This approach has the disadvantages of requiring multiple wavelengths for the same link and of complicating the system configuration. More particularly, on one side of the link, the transmitter operates at a first wavelength with the receiver at the other end also operating at the first wavelength. For the return link, the transmitter operates at a second wavelength with the receiver operating at the second wavelength. Thus, for each bidirectional link, two pairs of transmitters and receivers must be supplied with each pair operating at separate distinct wavelengths.  
         [0006]     The second type of bidirectional link uses free space or fiber beamsplitters at each end to manage (combine or separate) the incoming and outgoing optical signals. This type of link uses only a single wavelength, but it wastes half the available optical power at the transmitter (at one end of the link) and another half of the available optical power at the receiver (at the other end of the link). Thus, only 25% of the available transmitted power can be received at best. As a result, the signal to noise ratio, or the bandwidth, of the link decreases accordingly.  
       SUMMARY OF THE INVENTION  
       [0007]     It is in view of the above problems that the present invention was developed. The invention provides bidirectional fiber optic transceivers and couplers.  
         [0008]     A number of benefits occur from being able to transmit fiber optic signals from, and to, a fiber optic device over a single fiber. In particular, the number of interconnects and cables are halved. This significantly improves the cost, weight, and reliability of the overall fiber optic system. In addition, certain types of Built-In Test (BIT) functions that were impractical before the conception of the present invention can be implemented that further improve system and link performance.  
         [0009]     A co-owned, co-pending U.S. patent application Ser. No. 10/788,987, entitled BIDIRECTIONAL, CO-LOCATED LASER/DETECTOR, filed on Feb. 27, 2004, and incorporated herein as if set forth in full, discloses bidirectional fiber optic links that can be operated over a single cable assembly by using co-located sources and optical detectors. U.S. Pat. No. 5,894,534 and entitled Fiber Optic “T” Coupler Single Path Transceiver and U.S. Pat. No, 5,809,187 and entitled Multi-Port Network Using Passive Optical Couplers, both assigned to The Boeing Company disclose additional fiber optical couplers and are incorporated herein as if set forth in full. The present invention provides improved bidirectional source/detectors by, inter alia, allowing the use of off-the-shelf detectors and transmitters. Further, the source/detectors (or transceivers) provide more efficient use of available signal power resulting in higher signal-to-noise ratios than previously available with bidirectional source/detectors.  
         [0010]     In a first preferred embodiment, the present invention provides an optical coupler including a port, an objective lens and an optical source. The objective lens and the port are spaced apart along the principal axis of the lens. A first portion of the objective lens is optically coupled with the port and receives a first optical signal from the port. The source is coupled with a second portion of the lens (which may overlap an outer periphery of the first portion of the lens) and transmits a second optical signal to the second portion of the lens. From there the objective lens couples the second optical signal to the port. Preferably, the source includes a laser, possibly coupled via a fiber pigtail and a lens to collimate the output radiation, or the fiber may be omitted with the use of a laser with a pre-aligned collimating lens. Optionally, a detector may be included to detect the first signal. A second lens may also be optically coupled between the objective lens and the detector.  
         [0011]     In a second preferred embodiment, the present invention provides a method of transmitting and receiving optical signals. The method includes spacing apart an objective lens (that has a first and a second portion that can overlap) and an optical port along a principal axis of the objective lens. The first portion of the objective lens is coupled with the port to receive a first optical signal. Also, the method includes optically coupling an optical source with the second portion of the objective lens so that, when the source transmits an optical signal to the objective lens, the lens couples the transmitted signal to the port. A collimated optical source illuminates the second portion of the objective lens, and a detector is used to detect the first optical signal.  
         [0012]     In a third preferred embodiment, the present invention provides an optical coupler that includes three ports and a lens. The lens is optically coupled with the first port and receives electromagnetic radiation from the first port that is traveling in a first direction from the first port toward the second port. Also, the lens is optically coupled with the third port and receives electromagnetic radiation traveling generally in a second direction from the third port to the first port. Further, the third port is configured in such a manner that the electromagnetic radiation from the third port is either overlapping, adjacent to, or set apart, from the electromagnetic radiation traveling in the first direction.  
         [0013]     Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The accompanying drawings, which are incorporated in and form a part of the specification, illustrate exemplary embodiments of the present invention and together with the description, serve to explain the principles of the invention. In the drawings:  
         [0015]      FIG. 1  illustrates a conventional bidirectional fiber optic system; and  
         [0016]      FIG. 2  illustrates a fiber optic transceiver constructed in accordance with the principles of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]     Referring to the accompanying drawings in which like reference numbers indicate like elements,  FIG. 1  illustrates a conventional bidirectional fiber optic system. Generally, the coupler assemblies of the illustrated system each include a beam-splitter and three lenses to couple optical signals to and from the coupler. Optionally, the coupler assemblies that are shown can be implemented in fiber.  
         [0018]     The system  10  includes two optical devices  12  and  14  and a link with one fiber  18 . The device  12  has a transmitter  20  for sending optical signals (i.e. electromagnetic radiation or “light”) to a receiver  22 , which is located in the device  14 , while the device  14  has another transmitter  24  for sending optical signals to another receiver  26  in device  12 . The signals are coupled to/from the transmitters  20  and  24  and the receivers  22  and  26  via coupler assemblies  27  and pass in/out of the transceiver package via connectors  28  on each of the devices  12  and  14 . Thus, each of the devices necessarily includes one port at device connector  28 . The signals travel along the fiber  18  independent of which transmitter  20  or  24  is transmitting the signal. The fiber  18  also necessarily includes a number of connectors  30  that allow, among other things, the fiber  18  to be mechanically and optically coupled to the transceivers  12  and  14 . The connectors  30  also allow the fiber  18  to pass through bulkheads  32  and other obstructions between the devices  12  and  14 . Thus, generally, for the required fiber  18  of the system  10  the system  10  must include connectors  28  and  30  along with, of course, the fiber  18  itself. Accordingly, the higher the number of fibers  16  and  18  required for the system  10 , the higher the weight and complexity of the system  10 . Bidirectional links are preferred because they require half the fibers and connectors of conventional unidirectional links.  
         [0019]      FIG. 2  illustrates an optical source/detector (or transceiver)  110  constructed in accordance with the principles of the present invention. The exemplary optical source/detector  110  is shown as being coupled to an optical fiber  112  (shown with cladding  113 ) and includes a first objective lens  114 , a second lens  116 , an optical detector  118 , a transmitter  120 , a fiber pigtail  122 , a collimating lens  123 , and a housing to maintain the alignment between the various optical components. Generally, the components  114 ,  116 ,  118 ,  120 ,  123 , and  122  serve to couple electromagnetic radiation (i.e. “light”) from the fiber  112  to the detector  118  (in a first direction) and serve to couple light from the transmitter  120  to the fiber  112  (in a second direction). Lenses  114  and  116  are spaced apart and define a collimated volume  124  between themselves. The lenses  114  and  116  also define a longitudinal axis  126  of the source/detector  110  that is co-located with the principle axis of the lenses  114  and  116 . The fiber  112  and the detector  118  are held in spaced apart relationship with the lenses  114  and  116 , respectively, toward opposite ends of the source/detector  110 . The proximal end of the fiber pigtail  122  is coupled to the transmitter  120  with the distal end of the fiber pigtail  122  with collimating lens  123  held in a fixed relationship to the objective lens  114  such that the fiber pigtail  122  with collimating lens  123  directs the light upon the lens  114  and, preferably, upon an outer periphery of the objective lens  114 . Preferably, the radiation from collimating lens  123  is substantially parallel to the longitudinal axis  126  (see  FIG. 2 ).  
         [0020]     In operation, an optical signal  128  (i.e. electromagnetic radiation or “light”) exits the fiber  112 , enters the source/detector  110 , and diverges about an axis generally parallel to the longitudinal axis  126 . The divergence of the received light  128  as it enters the source/detector  110  and seen in  FIG. 2  is generally how visible, or invisible, electromagnetic radiation behaves when it exits the end of an optical fiber such as the fiber  112  (shown with a diameter of “d 1 ” and cladding  113 ) and is no longer guided by the fiber. Nonetheless, the received light  128  generally travels in the first direction until it encounters the lens  114 . The lens  114  refracts most of the incident light  128  so that the light is collimated (or has very much less divergence) as it travels through the collimated volume  124  between the lenses  114  and  116 . The received light  128  then encounters the lens  116  which again refracts the received light  128  into a converging cone  132  after the second lens  116  to focus the light upon the detector  118 . Of course the detector  118  (e.g. a photodiode that operates in the visible, near infrared, or infrared spectrum or an equivalent device) detects the received light  128  and preferentially generates an electric signal that is representative of the signal conveyed by the received light  128 .  
         [0021]     In the other direction, the source/detector  110  transmits optical signals to the fiber  112 . The transmitter  120  (e.g. a light emitting diode, a laser, or an equivalent device) generates the outbound optical signal (i.e. transmitted light  134 ) which is then coupled into the fiber pigtail  122 . The transmitted light  134  travels through the fiber pigtail  122  and exits from the collimating lens  123  at the distal end of the pigtail  122 . The collimating lens  123  collimates the transmitted light  134  from the laser  120  (or fiber pigtail  122 ) with the received light  128 . From the collimating lens  123 , the transmitted light  134  encounters the first lens  114  (preferably the outer periphery of the first lens  114 ) and is thereby refracted and directed toward the fiber  112 . Upon encountering the fiber  112 , the transmitted light  134  begins traveling along the fiber  112 .  
         [0022]     The various components of the source/detector  110  are preferably arranged so that in the collimated volume  124  (between the collimating lens  123  and the first lens  114 ) the transmitted light  134  and the received light  128  are substantially parallel. The arrangement shown in  FIG. 2  therefore allows the light  128  and  134  to focus on detector  118  and the fiber core  112  respectively.  
         [0023]     Referring still to  FIG. 2 , further details of the paths that the received and transmitted light  128  and  134  take through the source/detector  110  are shown. For instance, the received light  128  exits the fiber  112  and begins diverging as it travels from the end of the fiber  112  to the objective lens  114 . The received light  128  then encounters the objective lens  114  and is preferably refracted in such a manner that the received light  128  is collimated or no longer diverges (much) once it is beyond the objective lens  114 . Thus, as the received light  128  leaves the objective lens  114  it travels in parallel with the longitudinal axis  126  of the source/detector  110 . Upon encountering the second lens  116 , the received light  128  is refracted again such that it converges on the detector  118  with an intensity almost the same as when the received light  128  left the fiber  112 , having incurred minimal loss. Accordingly, the detector  118  will perform more advantageously as it receives a higher signal level than it would have received with previously available devices.  
         [0024]     In the other direction, the transmitted light  134  exiting the fiber pigtail  122  and collimator  123  diverges minimally until it encounters the lens  114 . After passing through the lens  114  and being refracted accordingly, the transmitted light converges as it travels across the space between the lens  114  and the fiber  112 . As shown in  FIG. 2 , the received light  128  is incident on a first portion  140  of the objective lens  114 . The transmitted light  134  is incident on another portion  142  of the objective lens  1   14  that is preferably on the outer periphery of the lens  1   14 , or on the outer periphery of the first portion  140 . Further, as shown, the two portions  140  and  142  of the objective lens  114  on which the received and transmitted light  128  and  134  are incident may overlap.  
         [0025]     Because the received light  128  has expanded when it reaches the objective lens  114 , any shadow cast by the fiber pigtail  122  and collimating lens  123  on the detector  118  (and second lens  116 ) will reduce the overall intensity of the received light  128  at the detector  118  by a fraction of the amount that the shadow would have reduced the intensity had the received light  128  not been expanded. In one embodiment the reduction in intensity of the received light  128  at the detector is predicted to be a mere 0.5 dB. Moreover, because the fiber pigtail  122  and collimating lens  123  direct the transmitted light  134  almost directly away from the detector  118 , very little of the transmitted light  134 , if any, is reflected toward the detector  118 . Thus, even though the collimated source (consisting of fiber pigtail  122  and collimating lens  123 ) and the detector  118  share a common port (the coupling with the fiber  112 ) to transmit and receive signals, the dark current of the detector  118  can be lower than with previous devices and the transmitted signal-to-noise ratio and/or bandwidth can be higher. The fiber pigtail  122  could be configured so that the transmitted light  134  is incident on the objective lens  114  adjacent to, or spaced apart from, the portion  140  where the received light  128  is incident (provided that the objective lens  114  is able to direct the transmitted light  134  to the fiber  112 ).  
         [0026]     To assemble the source/detector  110  shown in  FIG. 2 , a housing (not shown) is fabricated to mechanically couple the fiber  112  to the housing. In addition, the housing holds the lens  114  spaced apart from the end of the fiber  112  by the distance “I 1 ” which is selected such that the light exiting from the fiber  112  will expand to no more than about the size of the lens  114  after traveling through the distance “I 1 ”. The housing also holds the second lens  116  far enough from the first lens  114  such that the fiber pigtail  122  and collimating lens  123  can be inserted between the two lenses  114  and  116 . Of course, the housing also holds the detector  118  and the second lens  116  spaced apart by a distance that is selected such that the intensity of the received light  128  will be about the same at the detector  118  as it was when it exited the fiber  112 . In addition, the housing preferably ensures that the fiber  112 , the two lenses  114  and  116 , and the detector  118  lie along a common axis, which is coincident with the principal axes of the lenses  114  and  116 . Also, the housing ensures that the fiber pigtail  122  and collimating lens  123  assembly is oriented so as to direct the transmitted light  134  onto the portion  142  of the objective lens  114  (and subsequently from there to the fiber  112 ). Of course, the fiber pigtail  122  can lead out of the housing to be coupled to an external light source such as laser  120 . With the source/detector  110  assembled in such a manner, light from the port (e.g. received light  128 ) illuminates and is collimated by the lens  114  and collimated light from the lens (e.g. transmitted light  134 ) is focused into the port by the lens  114 . Also, preferably, a second portion of the lens  114  is coupled with a collimated signal from an optical source (e.g. transmitted light  134 ). The coupling of the lens  114  with the port, and with the source, can be done in such a manner that the first portion  140  and the second portion  142  of the lens  114  complement each other.  
         [0027]     In view of the foregoing, it will be seen that the several advantages of the invention are achieved and attained. In particular, bidirectional optical couplers and transceivers have been provided that can possess increased signal-to-noise ratios or even higher bandwidths than previously available transceivers. Moreover, the devices and methods provided by the present invention can result in a lower dark current associated with the detector because the transmitted light does not reflect toward the detector. Also, because the devices allow bidirectional optical communications with a single unitary port, the number of connectors and fibers required for a bidirectional communication link is greatly reduced. Therefore, the present invention provides a more efficient single wavelength method to implement bidirectional communications.  
         [0028]     The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated.  
         [0029]     As various modifications could be made in the constructions and methods herein described and illustrated without departing from the scope of the invention, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative rather than limiting. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the claims and their equivalents.