Abstract:
Techniques for WDM Mux/DeMux on cable and methods of making the same are described According to one aspect of the present invention, a unit designed to provide multiplexing or demultiplexing (Mux/Demux) functions is implemented on cable. In other words, the Mux/Demux unit is coupled by a multi-fiber cable to a system (e.g., a system rack for router or switch that has multiple pluggable transceiver slots).

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefits of U. S. Provisional Application No. 61/851,114, filed Mar. 4/2013, and entitled “WDM Mux DeMux on Cable and Methods of Making”, which is hereby incorporated by reference for all purposes. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention is generally related to the area of optical communications. In particular, the invention is related to WDM Mux/DeMux on cable and methods of making the same. 
         [0004]    2. The Background of Related Art 
         [0005]    Broadband internet has experienced a compounded annual bandwidth growth rate exceeding 30% over the last decade. The momentum shows no sign of slowing down as wireless broadband, due to the smart phones and portable devices, joins the game. Fibers that facilitate much of such land-line bandwidth supports in the past now challenge device and equipment development to keep up the speed to fill their available bandwidth capacities. In core networks, transport equipment can now support 40 Gbps and 100 Gbps per wavelength using dense wavelength division multiplexing (DWDM) carriers. 400 Gbps and even 1 Tbps per wavelength channel are being discussed and laboratory tried. 
         [0006]    The core network availability of such high capacity data inevitably puts pressure on fiber optical transceivers to keep up with the bandwidth growth so that the Internet routers and switches can be efficiently used. IEEE has released some international standards on 40 Gbps and 100 Gbps Ethernet based transceivers. For the first time in history, we have seen that in order to keep up with the bandwidth growth demand of transceivers, more than a single wavelength are called for duty in order to transmit 40 or 100 Gbps data over 10 km distance via a single mode (SM) fiber. This is largely because even by using various data compression and forward error correction (FEC) techniques, no compact and right power consumed electro-optic modulator can handle such a high bandwidth data single handedly. 4×10 Gbps and 4×25 Gbps formats have been proposed to provide the aggregate data rate of 40 Gbps and 100 Gbps using 4 wavelength channels for transceivers. 
         [0007]    After successful developments of 2.5 Gbps and 10 Gbps single wavelength fiber optic transceivers, the transceiver makers have been getting used to the device form factors, such as small form pluggable (SFP) and quad small form pluggable (QSFP), each of which typically has an electrically pluggable connector at back- and duplex fiber (2 fibers each for input and output) connector adapter port at front-end. Inside the SFP, each of the so-called transmitting optical sub-assembly (TOSA) and receiving optical sub-assembly (ROSA) unit is residing on a printed-circuit board (PCB) that provides power, control and various other supporting functions. While a TOSA typically has a laser and a modulator along with some coupling optics to a fiber output, a ROSA has a photo-detector and a trans-impedance amplifier along with some coupling optics for a fiber input. 
         [0008]    In prior art, a typical fiber optical transceiver architecture is depicted in  FIG. 1 . Traditional TOSA units and ROSA units are very compact and deal with a single wavelength with no multiplexing (Mux) and demultiplexing (DeMux) functionalities needed. To embrace the new standards, the transceiver and TOSA/ROSA makers now are working on integrating WDM Mux/DeMux capabilities in order to offer the complete 40 Gbps and 100 Gbps transceiver capabilities. 
         [0009]      FIG. 2  shows a 4-ch TOSA/ROSA. It has a fiber port and a coupling lens that collimates an optical beam from an incoming fiber. In a zig-zag free space geometry, the collimated beam passes through 4 WDM thin-film filters (some reflected by the mirrors before reaching their destination filters). In a ROSA case, each such a beam is refocused back by its micro lens to one of four photo-detectors. On the contrary, for a TOSA case, optical signals are first generated by the four WDM lasers, through their respective microlenses to be collimated and then traverse through the zig-zag cavity made by the 4 thin-film filters and 3 mirrors that combine these four laser beams into one single beam (hence multiplexing) before being focused into the fiber port. For simplicity, no other electronic part details are shown in  FIG. 2 . One fundamental advantage of this thin-film based Mux/DeMux scheme is that the light power loss is small. Typically the maximum loss will be practically smaller than 1.25 dB. 
         [0010]    Another prior art that does not use thin-film filter as WDM mux-demux mechanism works with a planar lightwave circuit (PLC). Shown in  FIG. 3  is a typical PLC based combiner that acts as a 4 to 1 WDM input combiner. There are no filters or mirrors employed but the price to pay for the simplicity is that any of the laser input will have to suffer at least 75% (or 6 dB) minimum power loss due to basic physics constraints of combing SM optical power using a splitter/combiner or non-multiplexing scheme. The PLC can also be used to make a WDM multiplexer. The advantage of using a PLC chip with butt coupling directly to a laser or photodetector chip on one end and to a fiber on the other end is its relative simplicity. The PLC is a mass-production friendly technology. However, the limitation is the high power coupling loss, not only just for the &gt;6 dB fundamental combing loss in the PLC combiner case, but also for the WDM Mux/DeMux scheme that also suffers severe waveguide bending loss associated with the high refractive index material for PLC waveguides. For example, using a silicon material for the PLC, the butting oriented coupling at both front and back ends of the silicon PLC chip together with its inherent waveguide absorption and bending loss will mount to be &gt;5 dB. Silica may be used and has a matching refractive index to glass fiber to form PLC waveguides on chip. However, such a chip will inevitably have much larger chip area unless a chip designer plans to endure severe bending loss (e.g., &gt;5 dB if making the chip as compact as its silicon counterparts). 
         [0011]      FIG. 4  shows a parallel optics transceiver for 40 Gbps for multi-mode (MM) fibers based on vertical cavity surface emitting laser (VCSEL) array technology. VCSEL is directly modulatable to ˜10 Gbps and very inexpensive comparing to distributed feedback (DFB) lasers, thus suitable for various short reach links (e.g., &lt;300 meters). Many applications can afford using parallel ribbon fibers (MM fibers) to link between some equipment racks inside a data or cloud center).  FIG. 4  shows that TOSA and ROSA for the VCSEL based transceiver have 4 fiber ports. Such TOSA and ROSA due to not having Mux/DeMux components on board (see  FIG. 5 ) are much compact and straightforward than their SM counterpart shown in  FIG. 2 . As far as the current transceiver types are concerned, QSFP, XFP, CXP, CFP, CFP-2 form factor standards can support array fiber connector interfaces such as MPO or MTP connectors that allow up to 12 fibers in a single row on a multi-fiber ferrule.  FIG. 6  shows a typical MPO/MTP array fiber connector. An exemplary QSFP optical transceiver accepts this array fiber connector format. An MPO/MTP connector ensures all fibers to be connected with a small ˜0.35 dB insertion loss because it uses a high precision guide pin structure on the two ends of the rectangular shaped fiber ferrule. 
         [0012]    In a brief summary, the above mentioned prior arts illustrate several important facts:
       Transceivers must incorporate multi-wavelengths in the future for the era where the data rate needs to be in excess of 40 Gbps for a SM fiber reach of &gt;2 km.   Many current transceiver standards shall be capable of accepting MTP/MPO fiber array connectors.   Incorporating multi-wavelength Mux/DeMux inside transceivers will cost the already tight real estate inside a transceiver regardless of using either thin-film or PLC based Mux/DeMux technology.       
 
         [0016]    Accordingly, there is a need for improved techniques that separate the multi-wavelength Mux/DeMux functionality out of the main transceiver body. This separation allows the TOSA and ROSA community to continue to focus on active component (e.g., lasers, modulators, photo-detectors) integration management which itself sees increasing complexity that demands more space inside a transceiver. The separated MDOC is a purely passive component, thus the makers thereof in future can focus on making this type of passive devices more compact, reliable and lower in cost. 
       SUMMARY OF THE INVENTION 
       [0017]    This section is for the purpose of summarizing some aspects of the present invention and to briefly introduce some preferred embodiments. Simplifications or omissions may be made to avoid obscuring the purpose of the section. Such simplifications or omissions are not intended to limit the scope of the present invention. 
         [0018]    In general, the present invention is related to WDM Mux/DeMux on cable and methods of making the same. According to one aspect of the present invention, a unit designed to provide multiplexing or demultiplexing (Mux/Demux) functions is implemented on cable. In other words, the Mux/Demux unit is coupled by a multi-fiber cable to a system (e.g., a system rack for router or switch that has multiple pluggable transceiver slots). The system comprises: a pluggable active module having a plurality of multi-wavelength transmitting optical sub-assembly (TOSA) units and receiving optical sub-assembly (ROSA) units, each having multiple parallel wavelength channels and a multi-fiber connector adapter port; at least a passive optical unit housed in an enclosure, wherein the passive optical unit is provided to perform multiplexing or demultiplexing (Mux/Demux) functions and facilitate Mux/DeMux of the wavelength channels; and a multi-fiber cable provided to couple the passive optical unit to the pluggable active module. 
         [0019]    One of the advantages, objects and benefits in the present invention is to alleviate the pressure on the TOSA/ROSA and transceiver manufacturers to provide more spaces with passive optical functions such as Mux/DeMux. This is done by separating out those functions out of a multi-wavelength transceiver to a unit on cable. 
         [0020]    The present invention may be implemented as a method, an apparatus or a part of a system. According to one embodiment, the present invention is an optical device comprising: an enclosure including optical components to provide multiplexing or demultiplexing (Mux/Demux) functions; and a cable having first and second ends, the first end coupled with an interface to be accommodated into one of multiple pluggable transceiver slots in a system rack, the second end coupled with a first end of the enclosure, wherein a second end of the enclosure is coupled with at least one optical fiber. In general, the optical components are fixed to a substrate housed in the enclosure. 
         [0021]    In one embodiment, the enclosure is a ruggidized cage to accommodate the unit with two transition areas that are provided in the enclosure to isolate fibers from experiencing undesired forces. 
         [0022]    Other objects, features, and advantages of the present invention will become apparent upon examining the following detailed description of an embodiment thereof, taken in conjunction with the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
           [0024]      FIG. 1  shows a typical fiber optical transceiver architecture; 
           [0025]      FIG. 2  shows a 4-ch TOSA/ROSA; 
           [0026]      FIG. 3  shows a typical PLC based combiner that acts as a 4-to-1 WDM input combiner; 
           [0027]      FIG. 4  shows a parallel optics transceiver for 40 Gbps for multi-mode (MM) fibers based on vertical cavity surface emitting laser (VCSEL) array technology; 
           [0028]      FIG. 5  shows that TOSA and ROSA, due to not having Mux/DeMux components on board, are much compact and straightforward than their SM counterpart shown in  FIG. 2 ; 
           [0029]      FIG. 5  shows a tap filter packaged as a 3-port optical device according to one embodiment of the present invention; 
           [0030]      FIG. 6  shows a typical MPO/MTP array fiber connector; 
           [0031]      FIG. 7  shows a typical 19″ system rack  700  for router or switch that has multiple pluggable transceiver slots; 
           [0032]      FIG. 8  shows one embodiment of the present invention housed in an enclosure that is environmentally hardened and provided with a boot on either side to ensure ribbon fiber and duplex cables can withstand normal cable handling stress, such as side pull, retention and cable flex; 
           [0033]      FIG. 9  shows an embodiment based on thin-film technology for the Mux/DeMux functionality; 
           [0034]      FIG. 10  shows how the Mux/DeMux functionality is implemented by a single block; 
           [0035]      FIG. 11  shows one embodiment having a Mux/DeMux unit with fiber feed-through on each of two sides of a substrate; 
           [0036]      FIG. 12  shows, in one embodiment, an extra cage is provided to form a ruggedized package that protects the Mux/DeMux unit with an additional enclosure; 
           [0037]      FIGS. 13-15  show respectively several embodiments of the invention, where the Mux/Demux enclosures can take the form of a rectangular, circular or hexagonal in cross section; and 
           [0038]      FIG. 16  and  FIG. 17  further show respectively some variations of the crimp tubes for cables, both dual fiber and multi-fiber types. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0039]    In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will become obvious to those skilled in the art that the present invention may be practiced without these specific details. The description and representation herein are the common means used by those experienced or skilled in the art to most effectively convey the substance of their work to others skilled in the art. In other instances, well-known methods, procedures, components, and circuitry have not been described in detail to avoid unnecessarily obscuring aspects of the present invention. 
         [0040]    Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Further, the order of blocks in process flowcharts or diagrams representing one or more embodiments of the invention do not inherently indicate any particular order nor imply any limitations in the invention. 
         [0041]    Embodiments of the present invention are discussed herein with reference to  FIGS. 7-17 . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these figures is for explanatory purposes as the invention extends beyond these limited embodiments. 
         [0042]    Referring now to the drawings, in which like numerals refer to like parts throughout the several views,  FIG. 7  shows a typical 19″ system rack  700  for router or switch that has multiple pluggable transceiver slots. Each of the slots  702  may accommodate a QSFP or other pluggable transceiver type as long as it accepts MPO/MTP array fiber connector. According to one embodiment of the present invention, instead of making a full functional on-board 40 Gbps, 100 Gbps or 400 Gbps transceiver with multi-wavelength Mux/DeMux, a transceiver is implemented in an enclosure on a cable. As shown in  FIG. 7 , a transceiver in the enclosure  704  is implemented on a cable with an MPO/MTP connector interface, hence Mux/DeMux on cable (MDOC). The transceiver  704  includes some integrated active components such as SM laser diodes (LDs), photo-diodes (PDs), and amplifiers. In one embodiment, the necessary SM Mux/DeMux functions are separated out into an independent compact enclosure hung on a MPO/MTP cable that contains the equal number of fibers providing the links between a pluggable transceiver for the system rack  700  and the Mux/DeMux. The Mux/DeMux optical outputs for SM fibers are provided on the other side of the enclosure in a duplex cable form. The cable is terminated by a duplex fiber connector/adapter pair so that a SM fiber duplex cable of up to 2 km length can be plugged into it. 
         [0043]      FIG. 8  shows a detailed view of one embodiment of an MDOC  800 . The Mux/DeMux device  801  housed in an enclosure is environmentally hardened and is provided with a boot  802  on either side to ensure ribbon fiber and duplex cables can withstand normal cable handling stress, such as side pull, retention and cable flex. While the ribbon fiber side of the cable  804  is terminated by an MPO/MTP connector  806 , the duplex cable side  808  is terminated by two individual fiber connectors or their duplex counterpart  810  that integrate two connectors into one. It is also possible to directly integrate the two single fiber connectors and its mating adapter into a single unit to serve as pigtailed duplex fiber adapter. 
         [0044]    Depending in implementation, the Mux/DeMux can be a thin-film based or PLC based or a combination of the two technologies.  FIG. 9  shows an embodiment  900  based on thin-film technology for the Mux/DeMux functionality. Four WDM thin-film filters  902  are used, each for a different WDM channel is fixed by epoxy on a substrate that could be made of metal, ceramic or glass material. Three mirrors  904  are aligned to receive and reflect respective beams to such four filters  904  in the well-known zig-zag geometry. Three mirrors  904  they are also fixed to the substrate using the epoxy. Five collimators  906  with four on one end of the substrate while the other one is on its opposite end are also aligned and epoxy fixed (may be with some additional structure components such as glass wedges, rods, blocks) on the same substrate. Shown in  FIG. 9  is only one side of the substrate due to the other side is hidden in the drawing.  FIG. 10  shows how this Mux/DeMux functionality is implemented by a single block. One notable difference from the  FIG. 8  architecture is that the eight fibers from the transceiver side cable are putting into one common buffered cable instead of keeping them in a ribbon form. 
         [0045]    Since the Mux/DeMux functioning block must be environmentally hardened, the packaging of it shall be considered to isolate moisture from entering and destroying the thin-film filters, mirrors and collimators. In one embodiment, a dual-sided substrate is inserted into a tube-like enclosure, where each side end cap is a fiber feed-through unit having the right number of holes for fiber access. The tube also needs to have sliding guides in the middle so that the edges of the dual-sided block can follow the guides to slide in. This sliding structure prevents the micro optic components from touching on any part of the tube walls.  FIG. 11  shows one embodiment having a Mux/DeMux unit with fiber feed-through on each of two sides of the substrate. This enclosing structure shown in  FIG. 11  can be epoxy sealed, soldered or even laser welded so that moisture would not enter and degrade the performance of each individual functioning component inside. 
         [0046]    One of the features for an environmentally hardened device is that it must stand for more handling stress. In particular, fiber cables of the MDOC contemplated in one embodiment of the present invention must withstand various kinds of cable retention forces. Thus, in one embodiment, an extra cage is provided to form a ruggedized package that protects the Mux/DeMux unit with an additional enclosure shown in  FIG. 12 . The sealed Mux/Demux body is embedded into the slots of two halves of the ruggidized enclosure. To isolate fibers from taking undesired forces, each of the two cable crimp tubes is used to shield cable retention forces. The crimp tubes are embedded into the ruggidized box and they lock up cable Kevlar fabrics that act as structure members of the cable. With this extra protection, any external pull forced that put on the cables will not be transmitted to the optical fibers.  FIGS. 13-15  show respectively several embodiments of the invention, where the Mux/Demux box enclosures can take the form of a rectangular, circular or hexagonal in cross section.  FIG. 16  and  FIG. 17  further show respectively some variations of the crimp tubes for cables, both dual fiber and multi-fiber types. 
         [0047]    The present invention has been described in sufficient details with a certain degree of particularity. It is understood to those skilled in the art that the present disclosure of embodiments has been made by way of examples only and that numerous changes in the arrangement and combination of parts may be resorted without departing from the spirit and scope of the invention as claimed. Accordingly, the scope of the present invention is defined by the appended claims rather than the foregoing description of embodiments.