Abstract:
A multimode fiber optical fiber transmission system includes an improved configuration for launching a single mode long wavelength transmission signal into existing multimode optical fiber networks. More specifically, the invention utilizes new single mode long wavelength VCSEL devices to realize a novel transmitter/transceiver for multimode fiber links where offset launch with controlled mode conditioning is achieved without the use of a mode conditioning patchcord, and in some embodiments, without the use of any collecting or focusing elements.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS AND CLAIM OF PRIORITY 
       [0001]    This application claims the priority benefit of U.S. Provisional Patent Application No. 60/472,278, filed May 21, 2003, entitled “MULTIMODE FIBER OPTICAL FIBER TRANSMISSION SYSTEM WITH OFFSET LAUNCH SINGLE MODE LONG WAVELENGTH VERTICAL CAVITY SURFACE EMITTING LASER TRANSMITTER”. 
         [0002]    This application is a divisional application of U.S. patent application Ser. No. 10/851,484, filed May 22, 2004 (currently pending). 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    The present invention relates to optical fiber transmission systems and more particularly to an improved configuration for launching a single mode long wavelength transmission signal into existing multimode optical fiber networks. 
         [0004]    Much of the installed based of optical fiber consists of multimode fiber. The problem addressed by this invention is that a significant amount of the installed multimode fiber base consists of inferior quality fiber where the bandwidth associated with the Over-Fill Launch (OFL) condition is not realized. In fact, approximately 10% of installed fiber can suffer bandwidth collapse with long wavelength laser transmitters when launched on axis or with a typical offset of 3 to 7 μm for a multimode fiber connector. This has been a serious problem in the past, and has been the focus of many experts in this area for the past 15 years. As early as 1991, the US Patent to Yoshizawa U.S. Pat. No. 5,077,815 disclosed the general concept of launching the transmission signal down the multimode fiber offset from the optical axis of the core. Yoshizawa utilized a single mode optical fiber coupled to a laser diode, or LED, for launching the transmission signal into the core of the multimode fiber. The single mode fiber was butt-coupled to the end face of the multimode fiber with the core of the singe mode fiber offset from the optical axis of the multimode fiber. Alternate configurations using lenses to focus the optical radiation from the single mode fiber to the multimode fiber were also disclosed. The theoretical basis for the realized improvement in bandwidth was believed to be a preferential excitation of higher order mode groups as opposed to lower order mode groups. 
         [0005]    In 1995 the US Patent to Haas No. U.S. Pat. No. 5,416,862 identified that adding an angle to either a center launch or an offset launch would further preferentially excite higher order mode groups traveling in the multimode fiber and would also increase bandwidth. 
         [0006]    The most recent work in this area can be found in the US Patent to Cunningham et al U.S. Pat. No. 6,064,786 which describes a theoretical model that distinguishes preferential excitation of only mid-order modes. The primary configurations claimed in Cunningham use a long wavelength (1300 nm) Fabry Perot edge emitter diode, and require the use of a multimode collecting fiber for collecting optical radiation from the diode. Low order mode groups are excited in the collecting multimode fiber and are launched into a conducting multimode fiber offset from the optical axis of the conducting multimode fiber where they preferentially excite mid-order mode groups. One alternate configuration disclosed in the Cunningham &#39;768 patent briefly describes the experimental testing of a short wavelength (850 nm) VCSEL operating in a single transverse mode and launching the VCSEL radiation into the multimode fiber offset from the optical axis of the core. However, practical use of the system was discounted due to power limitations of the VCSEL. The short wavelength VCSEL described in Cunningham would had to have been operated at a low power in order for it to lase in only a single transverse mode. For the most part, the &#39;768 Cunningham patent seems to set forth an explanation of the theoretical reasons “why” offset launch provides better bandwidth. 
         [0007]    Another practical solution for providing an offset launch into multimode fiber communication systems has been addressed by the GbE standards bodies by specifying a mode conditioning patchcord that is used to guarantee a known offset in the range 17 to 23 um for 62.5 um multimode fiber and 10 to 16 um for 50 um multimode fiber where the reduced number of excited modes guarantees satisfactory performance. The mode conditioning patchcord is an extra component used between a conventional single mode fiber transmitter (transceiver) and the multimode fiber system that guarantees the optimum launch condition. Further details of the multimode fiber patch cord configurations can be found in the US Patent to Cunningham U.S. Pat. No. 6,304,352. 
         [0008]    A further improved solution to this problem has been found and is the basis for the present invention. The fiber bandwidth demonstrated for the present invention is far better than predicted, and this has opened up a new commercial opportunity for data links on multimode fiber by extending the distances and data transfer speeds that can be accommodated. 
       SUMMARY OF THE INVENTION 
       [0009]    In recent years, significant effort has gone into developing Long Wavelength Vertical Cavity Surface Emitting Lasers (VCSEL&#39;s). One long wavelength VCSEL material system specifically in current development is based on InGaAsN technology, but other material systems are also possible. These long wavelength VCSEL&#39;s now realize the low drive current and ease of packaging benefits previously realized with shorter wavelength VCSEL structures and are a significant technical advance from the prior art shorter wavelength VCSELs. The new long wavelength VCSEL&#39;s have been designed with efficient coupling into single mode fiber (SMF) in mind and have small, substantially circular emission spots in the range of 3 to 10 μm diameter and more preferably 6 to 7 μm diameter. These VCSEL devices also have a small numerical aperture (˜0.1) that provides the opportunity for precise alignment and direct butt coupling to single mode fibers in some circumstances. The near parallel optical emission profile of these devices enables efficient optical coupling into single mode fiber where the core diameter is typically 9 um. Furthermore, the long wavelength VCSELs have been specifically designed to operate in a single longitudinal mode at high power levels and are now ready to replace the traditional Fabry Perot edge emitters commonly used at long wavelengths such as 1300 nm. 
         [0010]    The present invention brings together the new single mode long wavelength VCSEL devices with the previous work in offset launch to realize a novel and unique transmitter/transceiver device for multimode fiber links where offset launch with controlled mode conditioning is achieved without the use of a mode conditioning patchcord, and in some embodiments, without the use of any collecting or focusing elements. 
         [0011]    Experimental evidence from testing of the present invention has been collected and the results show that bandwidths considerably in excess of the overfill launch bandwidth of the multimode fiber has been demonstrated, and in fact, GbE (1.25 Gb/s) data rates have been transmitted over 2.8 km of multimode fiber, which is a factor of 5 better than previously thought. 
         [0012]    The ideal offset launch has been found experimentally to be 20 um on 62.5 um MMF and this has been reliably achieved by translating the VCSEL device laterally from the center axis of the multimode fiber by a fixed amount and then fixing the VCSEL in place. This is an easy process in manufacture because of the VCSEL package configuration, and negates the need for a mode conditioning patchcord. The emission size of the long wavelength VCSELs, which were developed for single mode fiber applications, is fortuitously ideal for providing a restricted offset launch on multimode fibers. This novel application of long wavelength VCSEL technology to the problem of limited multimode fiber bandwidth now enables the manufacture of very low cost transmitters capable of distances previously unattainable. 
         [0013]    Accordingly, among the objects of the instant invention are: 
         [0014]    the provision of a new way of achieving improved fiber bandwidth on multimode fiber using long wavelength VCSEL devices; 
         [0015]    the provision of a new launching technique which removes the need for a mode conditioning patchcord specified in the standards; 
         [0016]    the provision of such a new configuration wherein the long wavelength VCSEL could have a 1300 nm emission wavelength but which could equally be any wavelength in the long wavelength communications band (1100 nm to 1700 nm); 
         [0017]    the provision of a low cost assembly technique for guaranteeing the optimum offset launch condition into multimode fiber systems; and 
         [0018]    the provision of such techniques which enhance the bandwidth of multimode fiber and therefore achieve longer transmission distances than previously possible on multimode fiber, for example, &gt;2 km at GbE but whilst this is an example of what has been achieved, other data rates and distances are possible (e.g. 10 GbE over 300 m). 
         [0019]    Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         [0020]    In the drawings which illustrate the best modes presently contemplated for carrying out the present invention: 
           [0021]      FIG. 1  is an end view of a multimode fiber (MMF) showing the VCSEL illuminating a spot offset from the optical axis of the MMF; 
           [0022]      FIG. 2A  is a schematic view of a first embodiment with a 1300 nm VCSEL and lens coupled to the multimode fiber; 
           [0023]      FIG. 2B  is a schematic view illustrating exit of the transmission signal from the multimode fiber and collection of the signal by a wide angle photodetector; 
           [0024]      FIG. 3  is a cross-sectional view of a transmitter optical sub-assembly (TOSA) package constructed in accordance with the present invention; 
           [0025]      FIG. 4  is a perspective view of a GigaBit Interface Converter (GBIC) Module including the TOSA package illustrated in  FIG. 3 ; 
           [0026]      FIGS. 5A through 5H  are plan views showing assembly of the GBIC module; 
           [0027]      FIG. 6  is a perspective view of multimode fiber pairs terminated with SC connector plugs; 
           [0028]      FIG. 7  is a perspective view of a communication system showing connection of the GBIC module with a router device and connection of the multimode fiber pair with the GBIC; 
           [0029]      FIG. 8  is a schematic view of a second embodiment with a 1300 nm VCSEL butt coupled to a multimode fiber; 
           [0030]      FIG. 9  is a schematic view of a third embodiment with a 1300 nm VCSEL and angular lens coupled to a multimode fiber; and 
           [0031]      FIGS. 10A-10C  are graphical illustrations of eye diagrams showing signal strength at 2800 m for (A) Offset single mode VCSEL launch, (B) on axis Fabry-Perot laser launch and, (C) 850 nm multimode VCSEL on axis launch. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0032]    Referring now to the drawings, a multimode optical fiber communication system including an offset launch long wavelength VCSEL transceiver constructed in accordance with the present invention is illustrated and generally indicated at  10  in  FIGS. 1 through 7 . As will hereinafter be more fully described, the instant multimode optical fiber communication system is operative for communicating data between two remote nodes in a computer network. Each node in the network includes a router device generally indicated at  12  in  FIG. 7 . Each router  12  is typically configured to accept one or more transceiver devices  10  (See  FIGS. 4-7 ) for transmitting data to and receiving data from another location(s). Each transceiver  10  includes a transmitter and a receiver operative with a pair of optical communication fibers  14  thus forming a bidirectional data link. The present invention is specifically directed to a novel offset launch transmitter that permits two router devices  12  to communicate at higher speeds and over longer distances utilizing an installed base of multimode fibers  14 . 
         [0033]    Turning to  FIGS. 4-7 , there is shown a GBIC form factor communication system. The GBIC form factor is standardized in the industry and is well known in the art.  FIG. 4  illustrates a GBIC transceiver module  10  comprising a housing generally indicated at  16  having a first end  18  and a second end  20  and a fiber connector structure  22  at the first end  18  for directly receiving connectorized ends  24  of the multimode fibers  14 . The GBIC form factor utilizes a standardized SC fiber connector system as illustrated in  FIG. 6 . The individual fibers  14  are terminated with a ferrule  26 , an outer shroud  24  and a latching structure  30  for selectively engaging with the fiber connector structure  22  on the housing  16  of the transceiver module  10 . 
         [0034]    Step-by-step assembly of the GBIC module  10  is illustrated in  FIGS. 5A-5H . The transceiver module  10  comprises upper and lower housing parts  32 ,  24  respectively ( FIGS. 4 and 5A ), a latch structure  36  including latch tabs  38  at the first end  18  within the fiber connector structure  22  ( FIG. 5B ), a circuit card  40  mounted within said housing  16  ( FIG. 5E ), a transmitter optical subassembly  42  mounted to the circuit card  40  adjacent the fiber connector structure  22  ( FIGS. 5C-5E ), a receiver optical subassembly  44  mounted to the circuit board  40  adjacent the fiber connector structure  22  ( FIGS. 5C-5E ), and an electrical connector  46  at the second end  20  of the housing  16  ( FIG. 5E ), the electrical connector  46  being connected to the circuit card  40  and protruding from the second end  20  of the housing  16  for selectively connecting the circuit card  40  with a mating receptacle (not shown) mounted inside the router  12 . 
         [0035]    It is noted there are many form factors for optoelectronic transceiver configurations (GBIC, SFF, SFP, XFP etc), and those skilled in the art will appreciate that the GBIC form factor is not critical to the invention. 
         [0036]    Referring back to  FIG. 1 , the multimode fibers  14 , are part of an installed base of multimode fibers. Each of the multimode fibers  14  comprises a core  48  and an external cladding layer  50 . The multimode fiber  14  may typically have either a 50 μm core or a 62.5 μm core. In  FIG. 1 , the radius of the core  48  is indicated as R, the diameter of the beam spot  52  is indicated as d, and the offset dimension of the beam spot  52  from the optical axis  54  of the multimode fiber  14  is indicated as X. 
         [0037]    The source of optical radiation for the offset launch transmitter comprises a single mode long wavelength VCSEL generally indicated at  56 . For purposes of the present invention, long wavelength is generally defined as operating in the 1300 nm optical communication window. However, it is to be understood that the present invention is applicable to all communication windows in the long wavelength band generally ranging from 1000 nm to 1700 nm. 
         [0038]    Generally speaking, the VCSEL device  56  preferably has a beam spot diameter d of approximately 6-7 μm. However, the beam spot  52  may range in diameter from about 3 μm to about 9 μm. As discussed previously, the long wavelength VCSEL&#39;s as utilized in the present invention were originally designed for launching optical radiation into the smaller core (˜9 μm) of a single mode fiber. They are generally constructed to emit a small circular spot, as opposed to an elliptical spot for edge emitters, and have a small (˜0.1) numerical aperture (NA), as opposed to a larger NA (˜0.4) for edge emitters. More specific details of the VCSEL structure and operation are disclosed in co-pending U.S. patent application Ser. Nos. 10/122,707 entitled “Long Wavelength Vertical Cavity Surface Emitting Laser”, (US Patent Publication No. 2002/0150135) and 10/613,652 entitled “Method of Self Aligning an oxide aperture with an annular intra-cavity contact in a long wavelength VCSEL” (US Patent Publication No. 2004/0058467), the contents of which are both incorporated herein by reference. 
         [0039]    Turning to  FIGS. 2A and 2B , a schematic diagram of a first embodiment of the offset launch configuration is illustrated wherein the VCSEL transmission signal  58  is offset launched into the core  50  of the multimode fiber  14  with the help of a lens  60  to focus the laser output. The lens  60  operates to focus and size the launch beam  52  at the offset launch point on the end face of the fiber  14 . In this embodiment, both the VCSEL  56  and the lens  60  are positioned offset from the fiber axis  54 . 
         [0040]    The ideal offset launch has been found experimentally to be 20 um on 62.5 um MMF and this has been reliably achieved by translating the VCSEL device  56  laterally from the center axis  54  of the multimode fiber  14  by a fixed amount and then fixing the VCSEL  56  in place. This is an easy process in manufacture and negates the need for a mode conditioning patchcord. 
         [0041]    Turning to  FIG. 3 , the VCSEL  54  and lens  60  are assembled into a TO-38 package generally indicated at  62 . The TO-38 package  62  including a header  64 , and a can enclosure  66 . The VCSEL  56  is mounted onto the header  64  and provided with conventional electrical contacts  68  (See also  FIGS. 5C-5G ) that extend through the header  64  for connection with the circuit card  40 . The lens  60  is hermetically sealed at the top of the can enclosure  66 . The VCSEL  56  is aligned with the optical axis of the lens  50  which defines the optical axis of the TO-38 package  62 . 
         [0042]    Still referring to  FIG. 3 , the transmitter optical subassembly  42  comprises an annular base  70  having an upper surface  72  and a lower surface  74 , and a receptacle  76  having a first end  78  for receiving the connectorized end  24  of a multimode fiber  14  and further having a second end  80  which is received in assembled relation with the upper surface  72  of the base  70 . A weld sleeve  82  is affixed to the upper surface  72  of the base  70  for slidably receiving the second end  80  of the receptacle  76 . During assembly, the TO-38 package  62  is received within a recess  84  in the bottom surface  74  of the base  70 . The recess  84  is slightly larger in diameter than the header  64  of the TO-38 package  62  so that the TO-38 package  62  can be laterally translated within the base  70  in the x-y plane to provide the proper offset alignment with the core  48  of the multimode fiber  14  to be received in the first end  78  of the receptacle  76 . During this alignment, the TO-38 package  62  is also provided with a slight angle (0-10°) along the z-axis to reduce back reflection of the optical radiation (transmission signal) off the end surface of the multimode fiber  14 . Once a rough alignment of the TO-38 package  62  is completed, the TO-38 header  64  is secured to the base  70 , and a fine alignment is completed by translation of the receptacle assembly  76  relative to the base  70 . 
         [0043]    Generally speaking, the VCSEL  56  and lens  60  are cooperatively positioned within the receptacle  76  to direct the transmission signal  58  onto the core  48  of the multimode fiber  14  offset from the optical axis  54  of the core  48  when the connectorized transmitting end  24  of the multimode fiber  14  is received in the fiber connector structure  22 . 
         [0044]    The receiver optical subassembly  44  ( FIG. 5C ) is virtually identical to the transmitter optical subassembly  42 , with the exception of the VCSEL  56  being replaced by a wide-angle photodetector  86  capable of collecting all of the transmission signal  58  exiting the terminal end of the multimode fiber  14  (See  FIG. 2B ). A separate illustration of the detailed construction of the receiver optical subassembly  44  is not believed to be necessary for an understanding of the invention. 
         [0045]    Experimental evidence has been collected which shows that bandwidths considerably in excess of the OFL bandwidth of the MMF has been demonstrated and in fact, GbE (1.25 Gb/s) data rate has been transmitted over 2.8 km of multimode fiber which is a factor of 5 better than previously thought. In this regard,  FIGS. 10A-10C  are graphical illustrations of eye diagrams showing experimental evidence of signal strength at 2800 m for (A) offset single mode VCSEL launch (present invention), (B) on axis Fabry-Perot laser launch and, (C) 850 nm multimode VCSEL on axis launch. 
         [0046]    In a second embodiment as shown in  FIG. 8 , the VCSEL  56  is mounted normal to the end surface of the fiber  14 , and the transmission beam  58  is launched directly into the core  48  at an offset of approximately 20 μm from the optical axis  54  of the core  48 . In this particular embodiment, the transmission signal  58  is launched directly into the core  48  of the multimode fiber  14  without an intermediate collecting element, i.e. lens  60 . More specifically, the TO-38 package  62  is provided with a window (not shown) rather than the lens at the top of the can enclosure  66 , and the TO-38 package  62  is mounted immediately adjacent to the end surface of the multimode fiber  14 , i.e. eliminating the extra distance required for the focal length of the lens  60 . This arrangement is advantageous for simplifying manufacture of the TO-38 package  62  as there is no need to optically align a lens  60  with the VCSEL  56 . Furthermore, the length of the entire transmitter optical subassembly  42  would be shortened considerably. A separate illustration of the lenless TOSA  42  is not believed to be necessary for an understanding of the invention. 
         [0047]    Turning to  FIG. 9 , a third embodiment is disclosed wherein the VCSEL  56  and the lens  60  are positioned on axis and the launch beam is offset launched at an angle into the core by directing the launch beam  58  at an angle from the lens  60 . The transmitter optical assembly  42  in this embodiment would be identical to that disclosed above in the first embodiment. However, the optical axis of the TO-38 package  62  would be fixed in alignment with the optical axis  54  of the core  48  of the multimode fiber  14 , and then angled along the z-axis to provide both the offset launch and the angled entry of the beam  58  into the core  48 . 
         [0048]    It is noted that the angular launch can also be achieved by butt coupling the VCSEL in an orientation angled to the end surface of the multimode fiber. The angled butt couple launch thus provides a fourth alternative configuration. 
         [0049]    It can therefore be seen that the present invention provides a unique and novel long wavelength transmission system that makes use of the existing installed base of multimode fiber yet increases transmission distance and bandwidth. For these reasons, the instant invention is believed to represent a significant advancement in the art that has substantial commercial merit. 
         [0050]    While there is shown and described herein certain specific structure embodying the invention, it will be manifest to those skilled in the art that various modifications and rearrangements of the parts may be made without departing from the spirit and scope of the underlying inventive concept and that the same is not limited to the particular forms herein shown and described except insofar as indicated by the scope of the appended claims.