Abstract:
An optical device may include a fiber or waveguide that is intended to be coupled to another waveguide having a significantly larger/smaller cross-sectional size. As such, two wedge shaped waveguides may be positioned atop a tapered waveguide. The dual wedge design may allow for shorter taper lengths and smaller final waveguide cross section dimensions.

Description:
FIELD OF THE INVENTION  
       [0001]     Embodiments of the present invention relate to waveguides, and more particularly, to tapered optical waveguides.  
       BACKGROUND INFORMATION  
       [0002]     Efficient light coupling between an optical fiber and a silicon waveguide is highly desired for silicon based photonic device and circuit applications. Due to the high refractive index contrast of silicon waveguide systems, obtaining good fiber-silicon waveguide coupling is very challenging particularly for small silicon rib waveguides.  
         [0003]     Often is the case that an optical device includes a fiber or waveguide that is intended to be coupled to another waveguide having a significantly larger/smaller cross-sectional size. For example, a planar lightwave circuit (PLC) can have a waveguide on the order of four microns in height to be coupled to an optical fiber with a diameter of about ten microns. One way to couple a port of a relatively large waveguide to a port of a significantly smaller waveguide is by forming a tapered waveguide structure to couple the two waveguides.  
         [0004]     In one type of taper, similar to that shown in U.S. Pat. No. 6,108,478 to Harpin et al., the taper at one end has a height or diameter of about the same size as a larger waveguide to which it is to be coupled. At the other end, the taper typically comes to a point. The sides of the taper are typically straight so that the taper has a wedge-like shape, with the wider part of the taper being at the end of the waveguide. This end of the taper is used to couple the taper to the larger waveguide. The interior end of the taper serves as a termination, which along with the narrowing shape of the taper helps force light to propagate from the wide end of the taper to the smaller waveguide (or from the smaller waveguide to the wide end of the taper).  
         [0005]      FIG. 1  shows a tapered rib waveguide, sometimes referred to as a “cheese wedge” taper waveguide  100  similar to that shown in Harpin et al, mentioned above. The waveguide  100  may be formed on a silicon-on-insulator (SOI) substrate comprising an insulation layer  102  and a silicon layer  104 . The waveguide  100  generally comprises a tapered section  106  and a final waveguide or rib section  108 , shown divided by illustrative line  111 . The tapered section  106  comprises a lower taper  110  and an upper, generally wedge shaped taper  112 . The upper taper  112  and lower taper  110  include an input facet  114  which may be integrally formed. The lower taper  110  gradually tapers down over length “L” to match the size of an output waveguide  116  in section  108 . The upper taper  112  may taper to a point  118  to be generally wedge shaped. This type of waveguide taper  100  may be used to provide high coupling efficiency (coupling loss &lt;1 dB/facet) between a standard fiber (with a modal diameter of ˜9 μm) coupled at the input facet  114  and silicon waveguide  116  with a width or height of ˜4-5 μm.  
         [0006]     As the refractive index contrast is larger for silicon waveguides as compared to optical fibers, a larger taper input facet  114  may be needed for better coupling. For example, as shown in  FIG. 1 , if a larger input facet  114  of 13×13 μm is called for, it may be difficult to obtain efficient coupling to waveguides  116  smaller than W×H=2.5×2.5 μm with a reasonable taper length (e.g., L=1-2 mm).  
         [0007]     Referring to  FIG. 2  there are shown modeling results for taper loss for a 2 mm long taper (L) for different final rib waveguide sizes (W)  116 . The graph shows that the loss for the standard cheese wedge taper  100  increases with decreasing the final waveguide dimension. When the waveguide dimension W is smaller than 2.5×2.5 μm, the loss is larger than 1 dB/facet. Further, loss increases quickly with further decreasing the final waveguide  116  dimension. Since these small waveguides (1-2.5 μm) are often used for the high density silicon photonic integrated circuits and for better laser diode to silicon waveguide coupling, such losses may be unacceptable.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a plan view of a standard cheese wedge taper waveguide;  
         [0009]      FIG. 2  is a graph illustrating taper loss for a 2 mm long taper for two different taper input cross section dimensions;  
         [0010]      FIG. 3  is a plan view of a dual cheese wedge taper waveguide according to embodiments of the invention;  
         [0011]      FIG. 4  is a simulation of light propagation through the dual cheese wedge taper shown in  FIG. 3 ;  
         [0012]      FIG. 5  is a graph comparing taper loss for various taper lengths;  
         [0013]      FIGS. 6A, 6B ,  6 C, and  6 D are diagrams illustrating one process for forming a dual cheese wedge taper waveguide;  
         [0014]      FIG. 7  is a top view of a dual cheese wedge taper waveguide; and  
         [0015]      FIG. 8  is an exemplary system using the dual cheese wedge taper.  
     
    
     DETAILED DESCRIPTION  
       [0016]     In optical systems it may be challenging to efficiently couple light into and out of a chip. Particularly difficult may be the coupling of light from a standard optical fiber or external light source to a silicon waveguide owing to the differences in size, shape and refractive indices (n) between typical optical fibers and silicon waveguides. For example, a single-mode fiber core (n=1.5) usually has a diameter of 8 μm with a symmetric mode, while a silicon waveguide (n=3.5) is typically only a few micrometers in width with an asymmetric mode.  
         [0017]     One method to address the aforementioned disparities may be to use a waveguide taper according to embodiments of the invention. Referring to  FIG. 3 , there is shown a plan view of a “dual cheese wedge” tapered waveguide  300 . The waveguide  300  may be formed on a silicon-on-insulator (SOI) substrate comprising an insulator layer  302  and a silicon layer  304 . The waveguide  300  generally comprises a tapered section  306  and an output waveguide section  308 , shown divided by illustrative line  311 . The tapered section  306  comprises a lower taper  310 , a first, generally wedge shaped taper (first wedge)  312  on top of the lower taper  310 , and a second generally wedge shaped taper (second wedge)  313  on top of the first wedge  312 .  
         [0018]     The lower taper  310 , and the wedges  312  and  313 , may be integrally formed and comprise an input/output facet  314  at one end. The lower taper  310  gradually tapers down over length “L” to match the size of an output waveguide rib  316  in section  308 . The first wedge  312  may taper to a tip  318  to be generally wedge shaped. The second wedge  313  may be smaller than the first wedge  312  and also taper to a tip  320 .  
         [0019]     A top view of first wedge  312  may resemble an isosceles triangle having its lateral sides being generally equal lengths. Likewise, a top view of the second wedge  313  may also resemble an isosceles triangle having its lateral sides being generally equal lengths, typically shorter than the lateral sides of the first wedge  312 . The base of the first wedge  312  and the base of the second wedge  313  may generally be as wide as the wide end of the lower taper  310  and arranged to form the input facet  314 .  
         [0020]     According to embodiments, the waveguide taper  300  may allow for a reduction in coupling loss through an adiabatic modal transformation and may also be used to increase the alignment tolerance of other optical devices, such as lasers. In operation, a light beam travels through the waveguide  300  entering at facet  314 . The optical mode of the beam may be gradually squeezed from the second wedge  313  into the first wedge  312  and then into the lower taper  310  and into the output waveguide  316 . Parameters for this transition include the length (L) of the taper (the longer the length L the more slowly the mode may be transformed resulting in lower loss) and the taper tip widths  318  and  320 . In order to reduce optical losses associated with the finite size of the tip widths, the tips  318  and  320 , shown axially aligned with the rib  316 , may be designed such that the minimum width is smaller than the wavelength of light to be transmitted in the waveguide  300 .  
         [0021]      FIG. 4  shows a simulation of light propagation through the dual cheese wedge taper  300  shown in  FIG. 3 . In this particular simulation, the input facet  314  is 10×13 μm, the size of the final waveguide  316  is 1×1 μm (i.e. W=1 μm) and the width of the wedge tips  318  and  320  is 0.3 μm. As illustrated a light beam enters the input facet  314 . Thereafter the beam may be gradually squeezed from the second wedge  313  into the first wedge  312  and then into the lower taper  310  and into the output waveguide  316 . In this simulation, for a single mode fiber (SMF) to a 1×1 μm waveguide, the loss is shown to be only about 0.9 dB/facet. Without the taper  300  the coupling loss is shown to be about 14 dB/facet.  
         [0022]     Thus, according to embodiments of the invention, such low losses (e.g. 0.9 dB/facet) may be obtained with taper lengths (L) as short as 2 mm. The above results may not be attainable with a standard cheese wedge, such as that shown in  FIG. 1  even when the taper length (L) is as long as 10 mm.  FIG. 5  is a graph showing simulated loss for a standard cheese wedge taper with an input facet  114  sized at 13×13 μm, tip width (W) of 0.5 μm for final waveguide (WG) sizes of 1.5×1.5 μm and 2.5×2.5 μm. As shown, with the standard cheese wedge even if the taper length (L) is extended to 10 mm (10,000 μm) taper losses as low as 0.9 dB/facet may not be achieved for 1.5×1.5 μm final waveguides.  
         [0023]      FIGS. 6A-6D  illustrate a process for forming a dual cheese wedge taper according to embodiments of the invention.  FIG. 6A  shows a side view of a single cheese wedge taper including a silicon substrate  600  having an oxide layer  602 . The lower taper  604  and final waveguide rib  605  may comprise a single silicon layer over the oxide  602 . A fist wedge  606 , also comprising silicon may be positioned over the lower taper  604 .  
         [0024]     In  FIG. 6B , an oxide deposition layer  608  is formed over the rib waveguide  605  and the first wedge  606 . The height of the oxide layer  608  over the first wedge may be roughly as high as the height of the first wedge  606 . Thereafter, the oxide layer  608  may be polished to form a smooth top surface.  
         [0025]     In  FIG. 6C , the oxide is patterned such as with a mask (not shown) and a portion is etched  610  over the first wedge  606 . Finally, in  FIG. 6D  silicon is regrown. This may be grown as a single crystal or may be poly-silicon to form the second wedge  612 . Using silicon re-growth and etching may provide for better control of critical dimension (CD) in the final waveguide. Thereafter, the top surface may be polished and the oxide layer  608  removed.  
         [0026]      FIG. 7  shows a top view of the dual cheese wedge taper waveguide according to embodiments of the invention. As illustrated, the first wedge  606  may be larger than the second wedge  612  positioned over top. The lower taper  604  may be larger yet and taper into the rib  605 .  
         [0027]      FIG. 8  illustrates a system  800  in which a waveguide taper  804  according to embodiments of the present invention can be used. System  800  includes an optical signal source  801  connected to one end of an optical fiber  802 . The other end of optical fiber  802  is connected to a PLC  803  that includes a taper  804 . Taper  804  may be fabricated according to one of the embodiments described above. For example, when the taper is implemented as shown in the embodiment of  FIG. 7 , the wide end of the taper  804  would be used to connect PLC  803  to the end of optical fiber  802 . In one embodiment, PLC  803  may be implemented in an integrated circuit. Other embodiments may have one or more other tapers (not shown) that are similar in structure to taper  804 .  
         [0028]     The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the embodiments to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible, as those skilled in the relevant art will recognize. These modifications can be made to embodiments of the invention in light of the above detailed description.  
         [0029]     The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Rather, the following claims are to be construed in accordance with established doctrines of claim interpretation.