Patent Publication Number: US-9897770-B2

Title: Fibre stub device and method using butt coupling for receptacled photonic devices

Description:
CLAIM TO DOMESTIC PRIORITY 
     The present application is a continuation of U.S. patent application Ser. No. 14/329,614, now U.S. Pat. No. 9,557,492, filed Jul. 11, 2014, which application is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of photonic optical fibre based devices including those used for data communications, sensing, or other applications, and in particular, to the coupling of a photonic device to an optical fibre through a fibre stub receptacle. 
     BACKGROUND OF THE INVENTION 
     The increased use of photonic devices in many applications is driving the need for reduced cost and improved assembly methods. One of the major difficulties in using photonic devices is the coupling to optical fibres required for transmission from one photonic device to another. Efficient, simple methods of coupling the photonic devices at both ends of the optical link are highly desirable. 
     Existing photonic devices include lasers, detectors, modulators, switches, attenuators, optical multiplexers and de-multiplexers, gratings, couplers and other devices where a function is achieved in a photonic device. Photonic devices are manufactured from a variety of materials including silica, silicon, silicon-Germanium, Indium Phosphide, Gallium Arsenide, Lithium Niobate and other materials that exhibit optical emitting, detection, or guiding properties. 
     Existing methods for coupling photonic devices to optical fibres efficiently include some form of mode matching because the optical waveguides have a different size than the core of an optical fibre. One method of mode matching involves using lenses. The use of lenses for mode matching adds cost and manufacturing complexity to the photonic device. An alternative method of mode matching involves manufacturing a V groove adjacent to the waveguide such that the optical fibre can locate in the V groove and be correctly positioned with respect to the waveguide. The V groove method requires larger photonic devices to provide space for the V groove which increases the cost of the photonic device. Additionally, manufacturing the V grooves requires additional processing steps compared to manufacturing integrated photonic devices without V grooves, which also increases the cost of the photonic device. Another method of mode matching involves producing a tapered region in a waveguide during the manufacturing of the photonic device. Creating a tapered region in the waveguide addresses the issues created because the optical waveguides have a different size than the core of the optical fibre. By using a tapered region in the waveguide for mode matching, it is possible to butt couple the optical fibre to the waveguide and obtain efficient transfer of light between the waveguide and the optical fibre. Butt coupling removes the need for lensing and complicated alignment procedures and is the preferred option for integrated photonic assemblies. Accordingly, a method is required to efficiently and easily butt couple optical fibre to a photonic device using the edge of the waveguide. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an example of a photonic and electronic subassembly; 
         FIG. 2  shows the novel fibre stub assembly where two stubs are co-located in a ceramic sleeve; 
         FIG. 3  shows the novel fibre stub with a singlemode optical connector assembly at one end and a waveguide photonic device at the other end joined by a continuous single piece of fibre; and 
         FIG. 4  shows the novel fibre stub with a multimode optical connector assembly at one end and a waveguide photonic device at the other end joined by a continuous single piece of fibre. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The present invention is described in one or more embodiments in the following description with reference to the figures, in which like numerals represent the same or similar elements. While the invention is described in terms of the best mode for achieving the invention&#39;s objectives, it will be appreciated by those skilled in the art that it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims and their equivalents as supported by the following disclosure and drawings. As employed in the drawings, specification, and claims the term fibre stub includes a short length of fibre encased in a block of material, usually cylindrical such that the fibre is positioned along the central axis of the block. 
     The present application describes a novel fibre stub which allows optical access for UV curing by presenting an interface to the photonic device which is transparent to UV light, thereby enabling the use of UV curable epoxies. Additionally, the novel fibre stub uses standard fibre interfaces. The present invention includes optical transceivers for datacoms. Additionally, the present invention can be used in any situation where an optical fibre is attached to a photonic device by butt coupling a fibre stub. 
     Many photonic devices are used in optical transceivers where optical and electronic functions are combined. The datacom industry has spearheaded the adoption of standards and specifications for optical transceivers. Many optical transceivers are receptacled which means they interface to an optical fibre through a receptacle where an optical connector interface is included in the transceiver. The optical connector interface conforms to a standard commonly known in the industry by acronym for example LC (Lucent Connector), MPO (Multiple fibre Push On), SC (Subscriber Connector), FC (Ferrule Connector), and so on. Optical connector standards enable the optical connectors to interface correctly with the optical fibres. Accordingly, an optical transceiver will specify which standard optical connector interface the optical transceiver is configured to accept. 
     The single fibre connectors use a fibre stub which is a length of optical fibre held in an accurately dimensioned ceramic stub and polished at both ends to create a good optical surface. The ceramic stub is optionally held in an accurately dimensioned ceramic sleeve which also accepts the incoming optical connector with the transmission fibre attached. The second ceramic sleeve co-locates the optical fibres for efficient coupling from the transmission fibre to the optical transceiver. 
     Inside the transceiver, the polished fibre stub is interfaced to the waveguide of the photonic device, using butt coupling with an optical fibre. A tapered region in the waveguide is provided for mode matching within the photonic device edge. However, in conventional fibre stubs, the optical fibres are surrounded by materials which are opaque to UV light. Many photonic device waveguides are also opaque to UV light. Accordingly, the junction between the optical fibre and the waveguide is not accessible to UV light. Thus, conventional fibre stubs lack the ability to utilize UV curable epoxies for improving butt coupling of optical fibres to waveguides. UV curable epoxies used between the optical fibre and the waveguide improve butt coupling of optical fibres to waveguides. The UV curable epoxy technique of the present invention provides stability and refractive index matching for the optical fibre, thereby reducing optical reflections associated with refractive index mismatch and improving butt coupling between the optical fibre and the waveguide.  FIG. 1  shows one example of a photonic device  10 . Input/output (IO) pads  11  are formed on base substrate  14 . Active electrical circuitry  12  is formed on base substrate  14 . Optical transmitters  16  and  18  convert electrical signals from electrical circuitry  12  on semiconductor device  10  into optical signals. Optical transmitters  16  and  18  include silicon lasers, silicon germanium lasers, indium phosphide lasers, LEDs, or other suitable photon emitting devices. The optical signals leave optical transmitters  16  and  18  and enter waveguides  20  and  22 , respectively. Waveguides  20  and  22  include silicon, silica, indium phosphide, Gallium Arsenide, Lithium Niobate, or other suitable optical medium. Optical receiver  24  converts an optical signal into an electrical signal for use by electrical circuitry  12  on semiconductor device  10 . Optical receiver  24  includes silicon germanium, InGaAs, silicon, or other optoelectronic material. Waveguide  26  transports an optical signal from waveguide  28  to optical receiver  24 . Waveguides  26  and  28  include silicon, silica, indium phosphide, Gallium Arsenide, Lithium Niobate, or other suitable optical medium. Once the optical signal is converted to an electrical signal, the electrical signal is available for use by electrical circuitry  12 . Waveguides  20  and  22  transport optical signals from optical transmitters  16  and  18  to waveguide  28 . Waveguide  28  and optical fibre  30  meet at junction  32 . Waveguide  28  is butt coupled to optical fibre  30  at junction  32 . Butt coupling is a method of joining two optical fibres or an optical fibre to a waveguide. When joining optical fibre  30  to waveguide  28 , the end of the optical fibre is polished and the optical fibre is aligned to the connection point on the waveguide. When an air gap exists between optical fibre  30  and waveguide  28 , the light passes from the optical fibre to the air, and then from the air to the waveguide. When light passes from optical fibre  30  to the air, a portion of the light reflects back into the fibre due to the difference in index of refraction between the optical fibre core medium and air. Similarly, when light passes from the air to waveguide  28 , a portion of the light reflects back into the air due to the difference in index of refraction between the waveguide and air. One way to reduce these reflections is to allow the end of optical fibre  30  and the connection point of waveguide  28  to come into direct physical contact. Another way to reduce the reflections is to surround the end of optical fibre  30  and the connection point of waveguide  28  with a gel or epoxy having a refractive index matching the refractive indices of the optical fibre and the waveguide. When the end of optical fibre  30  and the connection point of waveguide  28  are surrounded by a material with matching refractive index, the light does not encounter a boundary between two refractive indices as it travels between the optical fibre and the waveguide across the connection. Optical fibre  30  and waveguide  28  are aligned to ensure the optical signal propagates from the optical fibre into the waveguide properly. A fibre stub helps ensure proper alignment of the optical fibre by holding the optical fibre in place in the center of a ferrule.  FIG. 2  illustrates a novel fibre stub  40 . Fibre stub  40  includes a UV transparent output ferrule  42 . UV transparent output ferrule  42  includes glass, fused quartz, fused silica, sapphire, alumina, single crystal Al2O3, calcium fluoride, magnesium fluoride, plastic, or other suitable UV transparent material. The center of UV transparent output ferrule  42  contains hole  44 . Hole  44  is sized to accommodate an optical fibre. UV transparent output ferrule  42  includes endface  46  and endface  48 , opposite endface  46 . Fibre stub  40  includes an input ferrule  50 . Input ferrule  50  includes ceramic zirconia or other suitable materials. The center of input ferrule  50  contains hole  52 . Hole  52  is sized to accommodate an optical fibre. Hole  52  is aligned with hole  44  such that a continuous optical fibre passes through holes  44  and  52 . Input ferrule  50  includes endface  54  and endface  56 , opposite endface  54 . Epoxy  60  is disposed between UV transparent output ferrule  42  and input ferrule  50 . Sleeve  62  is disposed over UV transparent output ferrule  42  and input ferrule  50 . Sleeve  62  includes zirconia ceramic, aluminum, phosphor bronze, or other suitable materials. Sleeve  62  centers and aligns UV transparent output ferrule  42  and input ferrule  50 . Housing  64  houses sleeve  62 . Housing  64  includes neck protrusions  66 . Housing  64  includes stainless steel, aluminum, phosphor bronze, or other suitable materials. Sleeve  68  is disposed over input ferrule  50  and receptacle  72 . Sleeve  68  includes zirconia ceramic, aluminum, phosphor bronze, stainless steel, or other suitable materials. Housing  70  houses sleeve  68  and receptacle  72 . Housing  70  includes stainless steel, aluminum, phosphor bronze, or other suitable materials. Receptacle  72  is sized to accommodate a portion of a standard optical connector, such as LC connector  84 , as shown in  FIG. 3 . Optical fibres contained within fibre optic cables terminate in standard optical connectors, such as LC connectors, ST connectors, SC connectors, FC connectors, MT connectors, or other standard optical fibre terminations. If a cable contains more than one optical fibre, the individual fibres are broken out such that each optical fibre terminates in a useful connection, such as provided by LC connectors, ST connectors, SC connectors, FC connectors, MT connectors, or other standard optical fibre connections. Photonic devices  10  incorporating optical components have receptacles, such as receptacle  72 , sized to accommodate one or more connectors, including standard optical connectors such as such as LC connectors, ST connectors, SC connectors, FC connectors, and MT connectors. The receptacles allow for the transmission of modes of light from the core of the optical fibre terminated in the connector to waveguide  28  of photonic device  10  coupled to fibre stub  40 .  FIG. 3  shows a portion of LC connector  84  disposed in receptacle  72 . LC connector  84  includes pigtail  86 . Pigtail  86  is a singlemode fibre (SMF) optical cable. Sleeve  68  is disposed over input ferrule  50  and the portion of LC connector  84  inserted into receptacle  72 . Sleeve  68  centers and aligns input ferrule  50  and the portion of LC connector  84  inserted into receptacle  72 . Optical fibre  74  passes through hole  44  in UV transparent output ferrule  42  and hole  52  in input ferrule  50 . Optical fibre  74  is a SMF. Optical fibre  74  is continuous from endface  54  of input ferrule  50  to endface  46  of UV transparent output ferrule  42 . Epoxy  60  secures optical fibre  74  in input ferrule  50  and UV transparent output ferrule  42 . The end of optical fibre  74  at endface  54  of input ferrule  50  and the end of optical fibre  74  at endface  46  of UV transparent output ferrule  42  are polished. Fibre stub  40  aligns input ferrule  50  and the portion of LC connector  84  inserted into receptacle  72  in position for butt coupling optical fibre  74  with the end of the optical fibre contained in LC connector  84 . Sleeve  68  aligns the end of the optical fibre contained in LC connector  84  with the end of optical fibre  74  at endface  54 . The end of the optical fibre contained in LC connector  84  connects with the end of optical fibre  74  at endface  54 . Accordingly, the junction between the optical fibre contained in LC connector  84  and optical fibre  74  is at the center of endface  54  of input ferrule  50 . Fibre stub  40  aligns UV transparent output ferrule  42  and the end of optical fibre  74  in position for butt coupling with the connection point of waveguide  38 . The connection point of waveguide  38  connects with the end of optical fibre  74  at endface  46 . A UV curable epoxy  80  is applied to endface  46  of UV transparent output ferrule  42  prior to butt coupling the end of optical fibre  74  to the connection point of waveguide  38 . UV curable epoxy  80  has a refractive index selected to match the refractive indices of optical fibre  74  and waveguide  38 . UV light source  82  illuminates UV light through UV transparent output ferrule  42  to cure UV curable epoxy  80  disposed on endface  46  of the UV transparent output ferrule joining the UV transparent output ferrule and the end of optical fibre  74  to the connection point of waveguide  38 . UV light from UV light source  82  is able to cure UV curable epoxy  80  which is index matched to optical fibre  74  and waveguide  38  through UV transparent output ferrule  42 . UV curable epoxy  80  improves performance of signal transmission across the junction at endface  46  by having a refractive index selected to match the refractive indices of optical fibre  74  and waveguide  38 . UV curable epoxy  80  is cured to permanently bond UV transparent output ferrule  42  and the end of optical fibre  74  to the connection point of waveguide  38  and improve performance of signal transmission across the junction at endface  46 . If ferrule  42  were ceramic, the ferrule would be opaque to UV light. Accordingly, UV light from UV light source  82  would be unable to penetrate a ceramic ferrule and cure UV curable epoxy  80 . Because ferrule  42  is comprised of UV transparent material, UV light from UV light source  82  penetrates UV transparent output ferrule  42  and cures UV curable epoxy  80  disposed on endface  46 . UV curable epoxy  80  fills the interface between the end of optical fibre  74  and the connection point of waveguide  38  at the junction at endface  46  and includes a refractive index selected to match the refractive indices of the optical fibre and the waveguide. Fibre stub  40  including UV transparent output ferrule  42  improves return loss and reduces insertion loss at the junction between optical fibre  74  and waveguide  38 . UV transparent output ferrule  42  allows UV light from UV light source  82  to cure UV curable epoxy  80 . UV curable epoxy  80  fills the interface between the end of optical fibre  74  and the connection point of waveguide  38  at the junction at endface  46  and includes a refractive index selected to match the refractive indices of the optical fibre and the waveguide. Return loss is the amount of signal that is reflected back toward the signal source by a component, such as a junction, due to a refractive index mismatch. The use of index matched UV epoxy  80  at the junction of optical fibre  74  and waveguide  38  reduces refractive index mismatch, which improves return loss. Insertion loss is a comparison of signal power at the point the incident energy, or mode, strikes the junction and the signal power at the point it exits the junction. The use of index matched UV epoxy  80  at the junction of optical fibre  74  and waveguide  38  reduces refractive index mismatch, which means less of the optical signal is reflected back at the junction. If less of the signal is reflected back at the junction, then more of the signal continues past the junction. Accordingly, a reduction in refractive index mismatch reduces insertion loss. Thus fibre stub  40  including UV transparent output ferrule  42  improves return loss and reduces insertion loss at the junction between optical fibre  74  and waveguide  38  by allowing UV light from UV light source  82  to penetrate UV transparent output ferrule  42  and cure index matched UV curable epoxy  80  disposed at the junction at endface  46  between optical fibre  74  and waveguide  38 . Additionally, use of fibre stub  40  including UV transparent output ferrule  42  allows UV light from UV light source  82  to cure UV curable epoxy  80 . UV curable epoxy  80  securely aligns waveguide  38  and optical fibre  94  to improve signal transmission across the junction. 
       FIG. 4  shows the end of waveguide  38  butt coupled to fibre stub  40  in an alternate embodiment for applications requiring a multimode fibre (MMF). Sleeve  62  is disposed over UV transparent output ferrule  42  and input ferrule  50 . Sleeve  62  centers and aligns UV transparent output ferrule  42  and input ferrule  50 . Optical fibre  94  passes through hole  44  in UV transparent output ferrule  42  and hole  52  in input ferrule  50 . UV transparent output ferrule  42  includes glass, fused quartz, fused silica, sapphire, alumina, single crystal Al2O3, calcium fluoride, magnesium fluoride, plastic, or other suitable UV transparent material. Input ferrule  50  includes ceramic zirconia, composite plastic polymers, or other suitable materials. Optical fibre  94  is an MMF. Optical fibre  94  is continuous from endface  54  of input ferrule  50  to endface  46  of UV transparent output ferrule  42 . Epoxy  60  secures optical fibre  94  in input ferrule  50  and UV transparent output ferrule  42 . Ends of optical fibre  94  at endface  54  of input ferrule  50  and endface  46  of UV transparent output ferrule  42  are polished. Sleeve  68  is disposed over input ferrule  50  and receptacle  96 . Sleeve  68  includes zirconia ceramic, aluminum, phosphor bronze, stainless steel, or other suitable materials. Receptacle  96  is sized to accommodate a portion of a standard optical connector, such as LC connector  92 . A portion of LC connector  92  is inserted into receptacle  96 . Sleeve  68  centers and aligns input ferrule  50  and the portion of LC connector  92  inserted into receptacle  96 . LC connector  92  includes optical cable  90 . Optical cable  90  is an MMF. Fibre stub  40  aligns input ferrule  50  and the portion of LC connector  92  inserted into receptacle  96  in position for butt coupling the end of optical fibre  94  with the end of the optical fibre terminated in LC connector  92 . Sleeve  68  aligns the end of the optical fibre terminated in LC connector  92  with the end of optical fibre  94  at endface  54 . The end of the optical fibre terminated in LC connector  92  connects with the end of optical fibre  94  at endface  54 . Accordingly, the junction between the optical fibre in LC connector  92  and optical fibre  94  is at the center of endface  54  of input ferrule  50 . Fibre stub  40  aligns UV transparent output ferrule  42  and the end of optical fibre  94  in position for butt coupling with the connection point of waveguide  38 . The connection point of waveguide  38  connects with the end of optical fibre  94  at endface  46 . Accordingly, the junction between waveguide  38  and optical fibre  94  is at the center of endface  46  of UV transparent output ferrule  42 . 
     UV curable epoxy  80  is applied to endface  46  of UV transparent output ferrule  42  prior to butt coupling the end of optical fibre  94  to the connection point of waveguide  38 . UV curable epoxy  80  has a refractive index selected to match the refractive indices of optical fibre  94  and waveguide  38 . UV light source  82  illuminates UV light through UV transparent output ferrule  42  to cure UV curable epoxy  80  disposed on endface  46  of the UV transparent output ferrule permanently joining the UV transparent output ferrule and the end of optical fibre  94  to the connection point of waveguide  38 . UV light from UV light source  82  is able to cure UV curable epoxy  80  which is index matched to the optical fibre and the waveguide through UV transparent output ferrule  42 . UV curable epoxy  80  improves performance of signal transmission across the junction at endface  46  by having a refractive index selected to match the refractive indices of optical fibre  94  and waveguide  38 . UV curable epoxy  80  is cured to permanently bond UV transparent output ferrule  42  and the end of optical fibre  94  to the connection point of waveguide  38  and improve performance of signal transmission across the junction at endface  46 . If ferrule  42  were ceramic, the ferrule would be opaque to UV light. Accordingly, UV light from UV light source  82  would be unable to penetrate a ceramic ferrule and cure UV curable epoxy  80 . Because ferrule  42  is comprised of UV transparent material, UV light from UV light source  82  penetrates UV transparent output ferrule  42  and cures UV curable epoxy  80  disposed on endface  46 . UV curable epoxy  80  fills the interface between optical fibre  94  and waveguide  38  at the junction at endface  46  and includes a refractive index selected to match the refractive indices of the optical fibre and the waveguide. Fibre stub  40  including UV transparent output ferrule  42  improves return loss and reduces insertion loss at the junction between optical fibre  94  and waveguide  38 . UV transparent output ferrule  42  allows UV light from UV light source  82  to cure UV curable epoxy  80 . UV curable epoxy  80  fills the interface between optical fibre  94  and waveguide  38  at the junction at endface  46  and includes a refractive index selected to match the refractive indices of the optical fibre and the waveguide. Return loss is the amount of signal that is reflected back toward the signal source by a component, such as a junction, due to a refractive index mismatch. The use of index matched UV epoxy  80  at the junction of optical fibre  94  and waveguide  38  reduces refractive index mismatch, which improves return loss. Insertion loss is a comparison of signal power at the point the incident energy, or mode, strikes the junction and the signal power at the point it exits the junction. The use of index matched UV epoxy  80  at the junction of optical fibre  94  and waveguide  38  reduces refractive index mismatch, which means less of the optical signal is reflected back at the junction. If less of the signal is reflected back at the junction, then more of the signal continues past the junction. Accordingly, a reduction in refractive index mismatch reduces insertion loss. Thus fibre stub  40  including UV transparent output ferrule  42  improves return loss and reduces insertion loss at the junction between optical fibre  94  and waveguide  38  by allowing UV light from UV light source  82  to penetrate the UV transparent output ferrule and cure index matched UV curable epoxy  80  disposed at the junction at endface  46  between the optical fibre and the waveguide. Additionally, use of fibre stub  40  including UV transparent output ferrule  42  allows UV light from UV light source  82  to cure UV curable epoxy  80 . UV curable epoxy  80  securely aligns waveguide  38  and optical fibre  94  to improve signal transmission across the junction. 
     While one or more embodiments of the present invention have been illustrated in detail, the skilled artisan will appreciate that modifications and adaptations to those embodiments may be made without departing from the scope of the present invention as set forth in the following claims.