Abstract:
A dual wavelength optical coupler is disclosed which provides either bidirectional or unidirectional communication at different wavelengths over a single fiber. The coupler utilizes a dichroic filter and a set of three lenses to achieve dual wavelength communication. Additional blocking filters may be incorporated to decrease the degree of optical crosstalk present in the system.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical data link dual wavelength coupler and, more particularly, to such a coupler which provides either bidirectional or unidirectional communication over a single optical fiber. 
     2. Description of the Prior Art 
     In optical communication systems, the need may arise to multiplex different wavelength signals onto a single fiber. For example, wavelength multiplexing is one method for achieving full bidirectional transmission on a single fiber. In its simplest form, a bidirectional system may comprise two stations, S1 and S2, which transmit information at wavelengths λ 1  and λ 2 , respectively. Thus, station S1 needs a transmitter which operates at λ 1  and a receiver which is tuned to wavelength λ 2 . Station S2, obviously, has the opposite requirements. Each station also needs a duplexing element to inject both wavelengths onto the single transmitting fiber. Although simple in theory, such an arrangement is cumbersome in implementation. For example, each station comprises a separate transmitter, receiver, and duplexer. Therefore, some sort of optical coupling must also be provided, for example, by using optical waveguides. Such coupling requires many expensive and time-consuming adjustments to achieve optimum alignment. Additionally, the optical losses attributed to this coupling, including attachment between the duplexer, fiber, transmitter and receiver, may degrade the overall performance of the station to an unacceptable level. 
     An alternative to this straight-forward implementation is disclosed in U.S. Pat. No. 4,592,619 issued to E. Weidel on Jun. 3, 1986. Weidel discloses an optical coupling element utilizing a variety of microoptic elements with spherical and plane surfaces for collimating, focusing and redirecting transmitted/received lightwaves. Although an improvement over the prior art, the Weidel arrangement utilizes at least one optical element which must be traversed twice by a received light signal. Further, Weidel is necessarily limited to providing coupling between both a transmitter and receiver to an optical fiber. However, there exist situations wherein a pair of transmitters, operating at different wavelengths, must be coupled over the same fiber (unidirectional transmitter). 
     Thus, a need remains in the prior art for a dual wavelength optical coupler which is robust in design and is capable of operating in either a bidirectional mode (transmitter and receiver) or unidirectional mode (two transmitters or two receivers). 
     SUMMARY OF THE INVENTION 
     The need remaining in the prior art is addressed by the present invention which relates to an optical data link dual wavelength coupler and, more particularly, to such a coupler which provides either bidirectional or unidirectional communication over a single optical fiber. 
     In accordance with the teachings of the present invention, dual wavelength coupling is achieved utilizing a set of three lenses and a dichroic filter, all held in a precision die-cast housing with the active devices. 
     In one exemplary embodiment of the present invention, the coupler may be used a bidirectional transceiving device, including an LED operating at a first wavelength and a PIN receptive to a different wavelength. 
     In an alternative embodiment, the coupler may be used as a unidirectional device, including either a pair of LEDs at different wavelengths or a pair of PINs at different wavelengths. 
     It is an aspect of the present invention to avoid active alignment of the components forming the coupler. By careful choice of the lenses, alignment tolerances may be minimized to the extent that the filter and lenses may be merely placed in their proper locations within the housing. 
     Another aspect of the present invention is to provide a coupler design which is flexible enough to be utilized with a number of different lenses, as well as different transmitting and receiving wavelengths. 
     Other and further aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Referring now to the drawings, where like numerals represent like parts in several views: 
     FIG. 1 is illustrative of the optics utilized in an exemplary bidirectional (transmitting/receiving) coupling arrangement of the present invention; 
     FIG. 2 is illustrative of the optics utilized in an exemplary unidirectional (transmitting) coupling arrangement of the present invention; 
     FIG. 3 is a graph illustrating alignment sensitivity of a transmitting device including the optics of FIG. 1, as compared with a conventional buttcoupled prior art scheme; 
     FIG. 4 illustrates an exemplary coupling arrangement of the present invention, including the optics discussed with respect to FIG. 1 as formed in an exemplary low-cost, die-cast housing; and 
     FIG. 5 illustrates an exemplary optical data link assembly including a dual wavelength multiplexer formed in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates the basic optics of a bidirectional coupling arrangement of the present invention. As shown, the arrangement comprises a first element, or lens 10, a dichroic filter 12, a first spherical lens 14 and a second spherical lens 16. In operation, an incoming lightwave signal L IN , at wavelength λ 2 , is coupled to lens element 10 which operates to transform the incoming lightwave to a collimated beam. Although element 10 is illustrated as a GRIN lens, other types of lenses (e.g. spherical, plano-convex) may also be used. The collimated beam then strikes dichroic filter 12 which is configured so as to allow the incoming lightwave at wavelength λ 2  to pass through unaffected (i.e., filter 12 is transparent to wavelength λ 2 ). This collimated incoming signal is then refocused by the first spherical lens 14 for reception by a light sensitive device, for example, a PIN diode (not shown). 
     An outgoing signal L OUT  is provided by a light source, an LED (not shown) for example, operating at a different wavelength λ 1 . As shown in FIG. 1, outgoing signal L OUT  passes through second spherical lens 16 so as to form a collimated beam. The outgoing collimated beam then strikes dichroic filter 12, which is chosen to reflect this particular wavelength λ 1 . Dichroic filter 12 thus redirects the outgoing beam L OUT  onto lens element 10 which focuses beam L OUT  and couples the beam into the core region of the optical fiber (not shown). 
     As stated above, the coupler of the present invention may be utilized in a unidirectional arrangement, that is, with a pair of transmitting devices or a pair of receiving devices. FIG. 2 illustrates an exemplary coupler for use with a pair of transmitting devices, LEDs, for example. Similar to the arrangement of FIG. 1, this coupler comprises a set of three lenses and a dichroic filter. In this example, however, light is transmitted in only one direction--out of the coupler and into an associated transmission fiber. In operation, a first LED (not shown) produces a first light signal L A  at a wavelength λ A . Light signal L A  is then collimated by a first lens 140, shown as a spherical lens, and subsequently passed through a dichroic filter 120. Dichroic filter 120 is chosen to allow this particular wavelength to pass therethrough with a minimal amount of reflection (i.e., filter 120 is transparent to wavelength λ A ). The collimated light signal L A  then passes through a second lens 100, shown here as a plano-convex lens, where the light is focused before being coupled to the core region of the transmission fiber (not shown). 
     In a similar fashion, a second LED (not shown) produces a second light signal L B  at a different wavelength λ B . The light signal L B  is subsequently collimated by a third lens 160, illustrated as another spherical lens, and passed to dichroic filter 120. Light signal L B , as shown in FIG. 2, is reflected by dichroic filter 120 into plano-convex lens 100, where it is focused into the associated fiber core region. 
     In order to provide a robust coupler, the optics must be chosen so as to achieve coupling between the fiber and a variety of different active transmitting and receiving devices. Obviously, each active device has different physical and optical characteristics, so that a different lens system would be necessary for optimum coupling using each different combination. The arrangement of the present invention, while not being optimal for any given pair of active transmitting/receiving devices, does provide acceptable performance. 
     One particular LED which may be used as a transmitting device with the coupling arrangement of the present invention is a 870 nm LED which typically has a flat surface with a 30 μm diameter active area. It has been determined that a 1.0 mm diameter sapphire ball may be used as second spherical lens 16,160 to provide the necessary collimation of the 870 nm lightwave. A high index material such as sapphire (n=1.75) is desirable since it provides a relatively high NA with moderate spherical aberration. Additionally, sapphire is known to be very hard and resistant to environmental attack. In order for the beam produced by this LED to couple through a lens element 10, such as a GRIN lens FIG. 1, the beam presented to dichroic filter 12 must be as collimated as possible. Thus, for an NA of 0.4, the separation between the LED and lens 16 must be less than 50 μm, if a 1.0 mm diameter sapphire ball is used as lens 16. For most applications, this separation is too small, that is, it cannot provide for the necessary wire bond connections to the LED surface. If the diameter of lens 16 is scaled to 2.0 mm, however, the lens-LED separation may be doubled (to 100 μm), providing sufficient room to perform the LED wire bonding operations. 
     Once the specific choice for spherical lens 16 has been made, the appropriate collimating element 10 must be chosen so as to provide the desired magnification and coupling. For example, the utilization of a quarter-pitch GRIN lens comprising a diameter of approximately 1.8 mm with a 2.0 mm sapphire spherical lens results in an LED magnification of 1.6× at the fiber. The utilization of a quarter-pitch 1.8 mm GRIN lens as collimating element 10 also provides for a significant increase in coupling as compared to a straight-forward butt-coupled arrangement. FIG. 3 illustrates this comparison, showing both axial and transverse alignment measurements. In particular, the approximate coupled power associated with the aligned butt-coupled arrangement is approximately -13.4 dBm, as compared with approximately -12 dBm for the lensed system of the present invention. 
     Another LED choice for use with the coupling arrangement of the present invention is a 1300 nm LED. Unlike the 870 nm LED discussed above, an exemplary 1300 nm LED may include a lensed surface to optimize butt-coupled power. It has been determined that, on average, coupling to lensed LEDs cannot be improved with additional lenses. Therefore, this particular LED (or any similar lensed LED) will perform adequately with the GRIN lens/spherical lens arrangement chosen for use with the 870 nm flat LED. In fact, to reduce the cost of a system using this lensed LED, a 2.0 mm glass ball (n=1.52) may be used as spherical lens 16 in place of the sapphire ball discussed above; the same lens holder may then be utilized regardless of the specific LED included in the system. It is to be understood that a flat surfaced 1300 nm LED including requisite lensing, may also be utilized in the inventive arrangement. 
     In order to provide bidirectional communication, the dual wavelength multiplexer of the present invention must also provide coupling between the communication fiber and a photodetecting device by producing an image of the fiber at the photodetector surface. One such photodetecting device employed in this type of arrangement in an InGaAs PIN photodiode which has an active spot size of only 75 μm in diameter. Additionally, it has been determined that for optimal packaging, the image should be at least 0.8 mm behind spherical lens 14. Considering the use of the 1.8 mm GRIN lens as element 10, the beam radius at the output of lens 10 will be approximately 0.55 mm for a ray exiting the fiber with an angle of 16.8° (0.29 NA). The utilization of a 3.0 mm glass ball as spherical lens 14, in combination with the 1.8 mm GRIN lens, yields a paraxial focus 0.75 mm behind the lens. This results in a image spot size of 72 μm in diameter, a close match to the spot size of the InGaAs PIN. 
     Another choice for a photodetecting element in this arrangement is a silicon PIN. The coupling of this diode to the fiber is somewhat simpler than that of the InGaAs diode, since a Si PIN has a relatively large spot size (150-200 μm). Therefore, the 3.0 mm glass lens described above would be more than sufficient for use with this particular PIN. In order to simplify the manufacture, the same 3.0 mm lens could be used, regardless of which photodetecting device is employed. 
     An exemplary dual wavelength transceiving multiplexer sub-assembly 20 is illustrated in FIG. 4. Sub-assembly 20 consists of a die cast housing 22, fabricated of zinc, for example. Zinc is a preferred material for this housing since it is relatively inexpensive, yet may be precisely formed to provide the required alignment of the inventive lens system. Referring to FIG. 4, a fiber ferrule 24 housing an optical fiber 26 is inserted into an opening 28 in housing 22. GRIN lens 10 is also positioned in opening 28 so as to abut ferrule 24. A first slot 30 in housing 22 is formed to hold dichroic filter 12 at the predetermined angle required to pass the incoming lightwave signal L IN  and reflect the outgoing lightwave signal L OUT . For example, an angle of 30° may be used. A blocking filter 32, positioned in a second slot 34 in the path of the incoming lightwave, may be inserted to further decrease optical crosstalk by blocking the outgoing lightwave radiation at the wavelength λ 1  from reaching the PIN. Spherical lens 14, a 3.0 mm glass ball, is inserted in an opening 36 in housing 22 so as to be aligned with filter 12 and optional filter 32. A PIN 38, mounted on a header 40 is then inserted in opening 36 behind lens 14. Although active alignment of lens 10, filter 12 and lens 14 is not required, active alignment of PIN 38 is necessary. However, such alignment is performed in conventional data link assembly processes and is not difficult. 
     The transmission portion of sub-assembly 20 consists of an LED 42, mounted on a header 44 being positioned in an opening 46 in housing 22. Opening 46 must be formed in housing 22 so as to provide alignment between LED 42 and dichroic filter 12. As with PIN 38, active alignment of LED 42 is necessary. Spherical lens 16 is positioned in a narrowed portion of opening 46 between LED 42 and filter 12. As shown in FIG. 4, a third slot 48 is formed in housing 22 between lens 16 and filter 12 to allow for the insertion of an additional blocking filter (not shown). This blocking filter, similar to blocking filter 32 included in the received signal path, may be utilized to decrease the crosstalk between the transmitted and received signals by preventing any portion of incoming signal L IN  at wavelength λ 2  from reaching the LED. 
     An exemplary dual wavelength multiplexer 50 including sub-assembly 20 of FIG. 4 is shown, in an exploded view, in FIG. 5. Dichroic filter 12 is shown in phantom to illustrate the directions of the various signal paths for incoming signal L IN  and outgoing signal L OUT . LED header 44 and PIN header 40 are also illustrated in FIG. 5. As shown, sub-assembly 20 is inserted onto a circuit board 52 including a transmitter integrated circuit 54 and receiver integrated circuit 56. A cover 58 is used to encapsulate the coupler, where fiber ferrule 24 mates with an appropriate connector 60 formed on cover 58.