Abstract:
Optical apparatus and optical switching methods that provide optical high data rate switching at a wavelength or packet level using optical tone addressing. Optical signal routing is a result of optically induced total internal reflection at the intersection of an X-junction waveguide structure. The total internal reflection effect is controlled by a high intensity optical pump beam separate from the optical data signal. Total internal reflection may result from a free-carrier induced change in refractive index, which is a nonlinear effect found in common III-V semiconductors and selected polymers. Optical switching networks may be formed using cascaded pluralities of optical waveguide switches.

Description:
BACKGROUND  
         [0001]    The present invention relates generally to optical switches and switching methods, and more particularly, to X-junction waveguide optical switches, switch networks and optical switching method that permit optical high data rate switching at a wavelength or packet level using optical tone addressing.  
           [0002]    Conventional all-optical nonlinear material approaches to implementing optical switches include (1) Fabry-Perot waveguide switches and retro-reflectors, (2) coaxially illuminated directional coupler waveguide switches, and (3) microring resonators. Other similar electronically actuated switches are known by the inventors to exist.  
           [0003]    It is an objective of the present invention to provide for X-junction waveguide optical switches, switch networks and optical switching method that improve upon capabilities of conventional optical switching devices.  
         SUMMARY OF THE INVENTION  
         [0004]    To meet the above and other objectives, the present invention provides for apparatus and methods that enable all-optical high data rate switching at the wavelength or packet level using optical tone addressing. More particularly, the present invention provides for an all-optical waveguide switch, which is a device that directs guided optical (data) signals as a result of optical control signal inputs, without the need for conversion between electrical and optical domains. Optical switching networks may be formed using cascaded pluralities of optical waveguide switches. Optical switching methods are also provided.  
           [0005]    Optical signal routing in the present invention occurs using optically induced total internal reflection (TIR) at the intersection of an X-junction waveguide structure. The total internal reflection effect is controlled by a high intensity optical pump beam separate from the optical data signal. Total internal reflection can be the result of a free-carrier induced change in refractive index, which is a nonlinear effect found in common III-V semiconductors or selected polymers, for example. The all-optical switch, switch fabric and switching methods are compatible with optical tone addressing network schemes. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which:  
         [0007]    [0007]FIG. 1 is a schematic of an exemplary optical switch in accordance with the principles of the present invention;  
         [0008]    [0008]FIG. 2 illustrates activation of the present X-junction waveguide switch using a high intensity pump beam;  
         [0009]    [0009]FIG. 3 illustrates a schematic of an exemplary 4×4 cascaded all-optical switch fabric or network in accordance with the principles of the present invention; and  
         [0010]    [0010]FIG. 4 is a flow diagram that illustrates an exemplary optical switching method in accordance with the principles of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0011]    Referring to the drawing figures, FIG. 1 is a schematic of an exemplary optical switch  10  in accordance with the principles of the present invention. The exemplary optical switch  10  shown in FIG. 1 has a 1-input-by-2-output (1×2) port configuration. FIG. 2 illustrates activation of the X-junction waveguide switch  10  using a high intensity pump beam  17  (comprising an optical carrier), such as may be provided by a vertical cavity surface emitting laser  18 , for example.  
         [0012]    As is shown in FIG. 1, the exemplary optical switch  10  comprises a plurality of single mode optical ridge waveguides  11  intersecting at an angle θ g  between the waveguides  11  at an X-junction  12 . The ridge waveguides  11  have a two-dimensional effective refractive index n r , while cladding  13  in a surrounding field area  14  has two-dimensional refractive index n c  such that n r &gt;n c . An optical signal propagates with a mode effective index n m , which is a weighted average of n r  and n c . An interaction region  15  illustrated by the dashed box in the center of the X-junction is where optical switching action occurs. The interaction region  15  may be activated by a high intensity pump beam  17  that illuminates the top-side of the interaction region  15  as is shown in FIG. 2.  
         [0013]    Operation of the exemplary optical switch  10  shown in FIG. 1 follows, with reference also to FIG. 2. An optical signal is launched into an input port (Input 1) of one of the two waveguides  11  (the top left waveguide  11  in the case of FIG. 1). The optical signal travels down the single mode waveguide  11  until it reaches the X-junction  12  in the center. If the interaction region  15  is illuminated by the high intensity pump beam  17 , the refractive index in the affected region  15  decreases by an amount An. The induced lower refractive index in the illuminated interaction region  15  creates a reflection plane (FIG. 1) for the incoming optical signal. Thus, the optical signal is totally internally reflected and directed towards the port labeled Output 1. If the optical pump beam  17  is absent, then no refractive index change occurs in the interaction region  15 , and the input optical signal continues through the X-junction  12  unaffected, exiting via the port labeled Output 2.  
         [0014]    The reflection angle (θ r ) and the angle (θ g ) between the waveguides  11  are limited by the critical angle of reflection, determined by Snell&#39;s law of reflection. Geometrically, the optical switch  10  shown in FIG. 1 is designed such that θ g =2θ r , A larger θ g  reduces crosstalk between the output ports (Output 1, Output 2).  
         [0015]    The interaction region  15  contains material with the capability of producing a negative refractive index change (Δn&lt;0) when illuminated by high intensity light output by the high intensity pump beam  17 . The index change as a result of optical pumping is known to be a nonlinear phenomena. The governing equation is:  
         Δ n=I   p   ·n   2    
         [0016]    where I p  is the optical pump intensity and n 2  is the nonlinear coefficient of the material comprising the interaction region  15 .  
         [0017]    III-V semiconductors and certain polymers are known to exhibit large values of n 2 . Exemplary III-V semiconductor materials include bulk and multi-quantum well compositions of In x Ga 1-x As y P 1-y /InP and Al x Ga 1-x As/GaAs, for example. Exemplary polymers include polydiacetylenes derived from 2-methyl-4-nitroaniline (PDAMNA), for example. Semiconductors, in particular, can exhibit large nonlinearities due to optically generated free-carriers. The free-carriers can affect the index of refraction via band filling or by the plasma effect. Response times may be tailored (by adjusting material characteristics and properties) to be very fast (on the order of picoseconds). Tailoring may be achieved by doping the semiconductor with donor or acceptor atoms. An input optical signal is provided that has a low enough intensity so as to not activate a nonlinear effect in the waveguides  11 .  
         [0018]    It is possible to create the optical switch  10  entirely out of the same uniform material. In this switch  10 , the interaction region  15  is comprised of the same material as the rest of the optical switch  10 . In this type of switch  10 , a finite pump beamwidth makes it necessary to accurately define the reflection plane, since all the material is subject to a Δn when it is top-side illuminated.  
         [0019]    [0019]FIG. 2 is an enlarged view of the X-junction region  12  when observed at an oblique angle. A cross-hatched region  16  in FIG. 2 depicts a photo-lithographically defined metal mask  16  on top of the optical ridge waveguide  11 . The purpose of the metal mask  16  is to block the pump beam  17  from illuminating the X-junction region  12  where the refractive index should remain unchanged. In other words, the exposed area within the pump beamwidth defines the interaction region  15 , as pictured in FIG. 2, and Δn will only occur within the area of the pump beamwidth that is not masked by the metal mask  16 .  
         [0020]    The all-optical switch  10  is completely passive, and no electronics are needed to power it. In addition, if optical tone addressing is used as a network architecture, no controlling electronics or routers are required when the switch  10  is used as a core wavelength router. Optical-to-electrical conversions are also eliminated as a result. In general, the all-optical switch  10  reduces the cost, power and size of high speed wavelength routers/optical cross connects.  
         [0021]    Optical waveguides  11  are typically very small, particularly semiconductor waveguides  11  designed for telecommunication wavelengths (on the order of a couple micrometers). This means the interaction regions  15  are small. Therefore, it is possible to cascade many of the 1×2 devices shown in FIG. 1 compactly into a larger matrix on a single integrated circuit chip. FIG. 3 is a schematic of a 4×4 cascaded all-optical switch fabric  20 , matrix  20  or network  20  in accordance with the principles of the present invention. Scaling up to larger matrices  20  is simple and straightforward.  
         [0022]    Note that the 4×4 example of the optical switch fabric  20  shown in FIG. 3 is non-blocking, wherein any input port can route signals to any output port, provided the output port is not already occupied. With such a layout, particularly for higher port counts, a matrix of vertical cavity surface emitting lasers (VCSELs) may be suspended and aligned above the plane of the cascaded switch fabric  20  to provide the required pump beams  17 . An array of micro-lenses may be fabricated to tight tolerances and positioned directly between the lasers and the integrated circuit chip comprising the switch fabric  20  to maintain alignment between the many pump beams  17  and their corresponding interaction regions  15 . This aids in packaging of the optical switch  10  and switch matrix  20 .  
         [0023]    The potential for greater than 100 GHz optical switching exists as a result of using the present optical switch  10 . This is a result of how fast the nonlinear effect is, particularly in III-V semiconductors, for example. Synthesized polymers, such as conjugated poly(silylenearylene) and poly(aryleneethylenesilylene) in polymer matrices, for example, may also by used to fabricate even faster optical switches  10 .  
         [0024]    Referring to FIG. 4, it is a flow diagram that illustrates an exemplary optical switching method  30  in accordance with the principles of the present invention. The exemplary optical switching method comprising the following steps.  
         [0025]    First and second single mode optical ridge waveguides  11  are provided  31  that intersect at a predetermined angle to form an X-junction  12  comprising an interaction region  15 , each waveguide having an input port and an output port. A mask  16  is disposed  32  on top of a portion of the optical ridge waveguides in the X-junction region. An optical signal is launched  33  into an input port of one of the waveguides. The interaction region is selectively illuminated  34  using a high intensity pump beam  17  to route the optical signal to a selected one of the output ports.  
         [0026]    The present invention provides for optical data routing that is optically actuated (as opposed to electronically actuated) by carrier-induced (pump beam) total internal reflection at the intersection of a completely passive, compact X-junction waveguide structure  12 . The present invention is completely passive, very compact and is easily scalable, whereas conventional waveguide switches are not, especially electronic switches. The present invention has a larger wavelength range of operation compared to conventional narrow band reflectors, waveguide couplers and ring resonators, for example.  
         [0027]    Thus, X-junction waveguide optical switches, switch networks and optical switching method have been disclosed. It is to be understood that the described embodiments are merely illustrative of some of the many specific embodiments which represent applications of the principles of the present invention. Clearly, numerous and other arrangements can be readily devised by those skilled in the art without departing from the scope of the invention.