Abstract:
One aspect of the invention provides a mode size converter having a first end and a second end. The mode size converter includes: a silicon waveguide having an inverse taper from the first end; and a silicon nitride waveguide having an inverse taper relative to the first end. The silicon nitride waveguide is adjacent and substantially parallel to the silicon waveguide. Another aspect of the invention provides an optical assembly including: a mode size converter as described herein; and a fiber optic optically coupled to the silicon nitride waveguide at the second end of the mode size converter.

Description:
BACKGROUND 
       [0001]    Although significant progress has been made in the fields of silicon-compatible optical interconnect and information processing technology, low loss coupling between optical fiber and high-index contrast single-mode silicon nano-wire waveguide remains a challenge. Mode mismatch between a single-mode silicon wire waveguide and a standard single-mode (SM) fiber depicted in  FIG. 1  is so large that it induces high coupling loss. 
       SUMMARY OF THE INVENTION 
       [0002]    One aspect of the invention provides a mode size converter having a first end and a second end. The mode size converter includes: a silicon waveguide having an inverse taper from the first end; and a silicon nitride waveguide having an inverse taper relative to the first end. The silicon nitride waveguide is adjacent and substantially parallel to the silicon waveguide. 
         [0003]    This aspect of the invention can have a variety of embodiments. The inverse taper of the silicon waveguide can grow from about 0.12 micrometers to about 0.35 micrometers. The silicon nitride waveguide can taper from a width of about 1 μm at the first end to between about 0.6 μm and 0.7 μm at the second end. 
         [0004]    The silicon nitride waveguide can taper from a width of about 1 μm at the first end to about 0.7 μm over a first distance, then from about 0.7 μm to about 0.67 μm over a second distance. The first distance can be about 280 μm and the second distance can be about 180 μm. 
         [0005]    The silicon nitride waveguide can have a height about 200 nm. The silicon waveguide can have a height of about 145 nanometers. The silicon waveguide can have a refractive index n=2.0. 
         [0006]    The mode size converter can further include a polymer waveguide adjacent and substantially parallel to the silicon nitride waveguide. The polymer waveguide can comprise SU-8 or a polyimide. The polymer waveguide can have a refractive index n=1.56 or n=1.57. The polymer waveguide can have a height of about 8 μm and a width of about 8 μm. 
         [0007]    The mode size converter can further include a cladding surrounding the silicon waveguide and the silicon nitride waveguide between the first end and the second end. 
         [0008]    Another aspect of the invention provides an optical assembly including: a mode size converter as described herein; and a fiber optic optically coupled to the silicon nitride waveguide at the second end of the mode size converter. 
         [0009]    Another aspect of the invention provides a mode size converter having a first end and a second end. The mode size converter includes: a silicon waveguide having an inverse taper from the first end adapted and configured for optical coupling with a light source having a first cross-sectional dimension; and a high numerical aperture fiber spliced to a tapered tip of the silicon waveguide. 
         [0010]    This aspect of the invention can have a variety of embodiments. The high numerical aperture fiber can be fusion spliced to the tapered tip of the silicon waveguide. The high numerical aperture fiber can have a core diameter of about 1.8 μm. The high numerical aperture fiber can have a core diameter of about 9 μm. The tapered tip of the silicon waveguide can have a width of about 120 nm. 
         [0011]    The mode size converter can further include a single mode optical fiber coupled to the high numerical aperture fiber. The single mode optical fiber can have a core diameter of about 3 μm. The single mode optical fiber can be fusion spliced to the high numerical aperture fiber. 
         [0012]    Another aspect of the invention provides a mode size converter having a first end and a second end. The mode size converter includes: a silicon waveguide having an inverse taper from the first end adapted and configured for optical coupling with a light source having a first cross-sectional dimension; a polymer waveguide substantially parallel to the silicon waveguide; and a high numerical aperture fiber optically coupled to the polymer waveguide. 
         [0013]    This aspect of the invention can have a variety of embodiments. The high numerical aperture fiber can have a core diameter of about 1.8 μm. The high numerical aperture fiber can have a core diameter of about 3 μm. 
         [0014]    The mode size converter can further include a single mode optical fiber coupled to the high numerical aperture fiber. The single mode optical fiber can have a core diameter of about 9 μm. The single mode optical fiber can be fusion spliced to the high numerical aperture fiber. 
         [0015]    Another aspect of the invention provides a mode size converter having a first end and a second end. The mode size converter includes: a silicon waveguide having an inverse taper from the first end adapted and configured for optical coupling with a light source having a first cross-sectional dimension; a single mode optical fiber proximate to and substantially coaxial with a tapered tip of the silicon waveguide; and an optical element interposed between the tapered tip of the silicon waveguide and the single mode optical fiber. The optical element can be selected from the group consisting of: a gradient index coating, a graded index fiber, a ball lens, and a self-written polymer waveguide. 
         [0016]    This aspect of the invention can have a variety of embodiments. The gradient index coating can include a plurality of dielectric layers applied to the tip of the single mode optical fiber. The single mode optical fiber can have a core diameter of about 9 μm. 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0017]    For a fuller understanding of the nature and desired objects of the present invention, reference is made to the following detailed description taken in conjunction with the figure wherein: 
           [0018]      FIG. 1  depicts the mode mismatch between silicon wire waveguides and standard single-mode optical fibers; 
           [0019]      FIG. 2  depicts a mode size converter according to an embodiment of the invention; 
           [0020]      FIG. 3  depicts a mode size converter according to an embodiment of the invention; 
           [0021]      FIG. 4  depicts a mode size converter according to an embodiment of the invention; 
           [0022]      FIGS. 5A-5C  depict a mode size converters according to embodiments of the invention; 
           [0023]      FIG. 6  depicts a mode size converter according to an embodiment of the invention; and 
           [0024]      FIG. 7  depicts a mode size converter according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Aspects of the invention provide mode size converters. Such aspects are particularly useful for chip-to-fiber coupling in silicon photonics devices and reduce optical coupling loss due to mode mismatch between silicon nano-wire waveguide and standard single-mode fiber effectively. Aspects of the invention can be fabricated/assembled in an automated and cost-effective way, and have potential to reduce overall cost of photonic integrated circuits packaging. 
       Mode Size Converters Incorporating Inverse Taper Silicon and Silicon Nitride Waveguides 
       [0026]    One aspect of the invention provides a mode size converter  200  including a silicon waveguide  202  having an inverse taper from a first end, a silicon nitride (Si 3 N 4 ) waveguide  204  having an inverse taper from the first end, the silicon nitride waveguide substantially parallel to the silicon waveguide  202 , and a polymer waveguide  206  applied over the silicon nitride waveguide. 
         [0027]    The inverse taper of the silicon waveguide  202  can grow from about 0.12 μm to about 0.35 μm over about a taper length of about 50 μm. 
         [0028]    The tapered silicon nitride waveguide  204  can be deposited on top of the silicon waveguide. The tapered silicon nitride waveguide  204  can have a height of about 0.2 μm. In one embodiment, the silicon nitride waveguide  204  first tapers from about 0.67 μm to about 0.7 μm over a taper length of about 180 μm, then tapers from about 0.7 μm to about 1 μm over a taper length of about 280 μm. 
         [0029]    Fundamental transverse (TE) mode in the silicon waveguide  202  will be adiabatically transferred into the silicon nitride waveguide  204  through the inversed silicon taper structure first. Silicon nitride waveguide  204  can have a refractive index (n) of about 1.98 or 2.00. 
         [0030]    A polymer waveguide  306  (e.g., SU-8 with n=1.57, or ULTRADEL 9120D with n=1.56) can be applied over silicon nitride waveguide  304 . (The chemical structure of ULTRADEL 9120D is provided in Y. Liu, Investigation of Polymer Waveguides for Fully Embedded Board-level Optoelectronic Interconnects (May 2004) (Ph.D. dissertation, The University of Texas at Austin), available at http://repositories.lib.utexas.edu/handle/2152/2072.) Polymer waveguide  306  can have a width of about 8 μm and a height of about 8 μm in order to be mode matched to single mode fiber (SMF-28). Optical mode that was transferred from silicon waveguide to silicon nitride waveguide will then be transferred into the polymer waveguide through the nitride waveguide taper, and eventually coupled to single mode fiber. 
         [0031]    An outer cladding (e.g., a cladding having a refractive index n of about 1.5 or about 1.54. Suitable materials include EPO-TEK® OG113 epoxy available from Epoxy Technology, Inc. of Billerica, Mass. and ULTRADEL 9020D polyimide, the chemical structure of which is provided in T.C. Kowalczyk et al., Guest-Host Crosslinked Polyimides for Integrated Optics (1995), available at http://www.osti.gov/scitech/biblio/94010 and Y. Liu, Investigation of Polymer Waveguides for Fully Embedded Board-level Optoelectronic Interconnects (May 2004) (Ph.D. dissertation, The University of Texas at Austin), available at http://repositories.lib.utexas.edu/handle/2152/2072. 
         [0032]    The silicon waveguide  202  and silicon nitride waveguide  204  can be formed on top of one or more substrates. For example, the silicon waveguide  202  can be embedded within an oxide layer  208 . In one embodiment, oxide layer  208  has a height of about 145 nm. Oxide layer  208  can be formed over a buried oxide (BOX) layer  210 , which can have a thickness of about 2 μm and a refractive index n=1.45. BOX layer can be formed over a silicon handle wafer  212 . Silicon handle wafer  212  can have a refractive index n=3.50. 
         [0033]    This mode size converter scheme can be fabricated through CMOS (complimentary metal-oxide-semiconductor) compatible processing to ensure low cost, and will be especially useful in silicon photonics chips that use nitride layer as upper cladding of Si waveguide. 
         [0034]    Embodiments of converter  200  achieve a coupling length of 500 μm and a coupling efficiency of greater than 85% (88% in some embodiments) with a 2 dB tolerance around +/−2 μm. 
       Mode Size Converters Incorporating Inverse Tapers and High Numerical Aperture (NA) Fibers 
       [0035]    Referring now to  FIG. 3 , another aspect of the invention provides still another mode size converter  300  based on a silicon waveguide  302  having an inverse taper and a combination of high numerical aperture (NA) fiber  320  (e.g., having a core diameter of about 3 μm) and a single mode fiber  318  (e.g., SMF-28 fiber having a 9 μm core). The high NA fiber  320  can have a numerical aperture (NA) of about 0.35, a core diameter of about 1.8 μm, and a field diameter of about 3.3 μm at a wavelength of about 1310 nm. Suitable high NA fiber  320  is available under model number UHNA3 from Nufern of East Granby, Conn. 
         [0036]    An inversed taper first expands the mode in the silicon waveguide  302 . Due to limited cladding thickness of the silicon waveguide  302  structure, the mode field diameter at the inversed taper tip can not be expanded large enough to match a SMF-28 fiber. This aspect of the invention applies a small-core, high NA, single mode fiber  320  with up to 3 μm core diameter to first match the mode of the inversed taper. 
         [0037]    Silicon waveguide  302  can terminate in a tip having a width of about 120 nm. 
         [0038]    The small-core fiber  320  can be connected to a standard single mode fiber (SMF-28)  318  by fusion splicing. By applying multiple sparks, the fused region between two fibers will form a relatively smooth transition region that can reduce transition loss. This design provides a cost-effective way to convert mode size from a silicon nano-wire waveguide with relatively thin cladding to a SMF-28 fiber, without the added complication of modifying the waveguide structure itself. 
         [0039]    Embodiments of this aspect of the invention provide improved coupling efficiency, a short spot size converter (SSC), and relatively simple fabrication. Additionally, nitride layer etching is not required. 
         [0040]    Referring now to  FIG. 4 , another aspect of the invention interposes a low index waveguide  422  (e.g., a waveguide having a suitable refractive index between about 1.45 and about 1.6) between silicon waveguide  402  and high NA fiber  420 . 
         [0041]    The low index waveguide  422  can be fabricated from a polymer such as SU-8 and can have a refractive index n=1.57. In one embodiment, the low index waveguide  422  can have a width WI of about 3 μm, a height HI of about 3 μm, and a length L of about 220 μm. 
         [0042]    The silicon waveguide  402  can have a tapered width Wt of about 120 nm, from a taper length Lt of about 170 μm. (Other dimensions for silicon waveguide  402  can be as described herein.) 
         [0043]    Embodiments of this invention achieve 89% coupling (0.5 dBM loss) with a 2 dBm tolerance around ±0.9 μm. Embodiments of this aspect of the invention provide improved coupling efficiency, use a short spot size converter (SSC), and avoid direct contact between the tip of the silicon waveguide  402  taper with the high NA fiber  420 . Additionally, nitride layer etching is not required. 
       Mode Size Converters Incorporating Gradient Index (GRIN) Coatings 
       [0044]    Referring now to  FIGS. 5A-5C , another aspect of the invention provides a mode size converter  500  between a silicon waveguide  502  and a single mode fiber  518  (e.g., SMF-28) using a gradient index (GRIN) coating  524   a  (e.g., a silica coating) or a graded index fiber  524   b,    524   c.    
         [0045]    Light confined in a typical silicon waveguide will diverge quickly once exiting the confinement region. 
         [0046]    Referring to  FIG. 5A , multiple layers of dielectric material  524  with specially designed refractive index contrast will act like a GRIN lens  524   a  on a fiber tip, and the diverged light can be coupled into the fiber  518  with reduced insertion loss. The layer closest to the silicon waveguide will typically have the highest refractive index. 
         [0047]    Referring to  FIGS. 5B and 5C , a graded index fiber can either have an abrupt or tapered interface, respectively. 
       Mode Size Converters Incorporating Ball Lenses 
       [0048]    Referring now to  FIG. 6 , another aspect of the invention provides a similar mode size conversion  600  to that presented in  FIG. 5 , but instead of using a potentially-costly GRIN coating  524 , a silica ball lens  626  is applied to collimate the beam from silicon waveguide  602  using its front surface, and refocus the beam into the single mode fiber  618  using its back surface. Both the lens and the fiber can be accurately aligned in either a V-groove or a trench. 
       Mode Size Converters Incorporating Self-Written Polymer Waveguides 
       [0049]    Referring now to  FIG. 7 , another aspect of the invention provides a mode size converter  700  coupling a silicon waveguide  702  to fiber  718  utilizing self-written polymer waveguide material  728  (available from Norland Products of Cranbury, N.J.). By applying UV curable resin  728  between the silicon waveguide  702  and the optical fiber  718 , and launching UV light from both directions, a mode matching region will be created in the polymer material  728  that can reduce coupling loss between the silicon waveguide  702  and the fiber  718 . To achieve better coupling, the silicon waveguide  702  can include an inversed taper structure as described and depicted herein. 
       EQUIVALENTS 
       [0050]    The functions of several elements may, in alternative embodiments, be carried out by fewer elements, or a single element. Similarly, in some embodiments, any functional element may perform fewer, or different, operations than those described with respect to the illustrated embodiment. Also, functional elements (e.g., modules and the like) shown as distinct for purposes of illustration can be incorporated within other functional elements, separated in different hardware, or distributed in a particular implementation. 
         [0051]    While certain embodiments according to the invention have been described, the invention is not limited to just the described embodiments. Various changes and/or modifications can be made to any of the described embodiments without departing from the spirit or scope of the invention. Also, various combinations of elements, steps, features, and/or aspects of the described embodiments are possible and contemplated even if such combinations are not expressly identified herein.