Abstract:
An active photonic crystal device for controlling an optical signal is disclosed. The device includes a planar photonic crystal with a defect waveguide bounded on the top and bottom by an upper cladding region and a lower cladding region. An optical signal propagating in the defect waveguide is confined in the plane of the photonic crystal by the photonic bandgap, and in the direction normal to the photonic crystal by the upper clad region and the lower clad region. The propagation of the optical signal in the defect waveguide is controlled by varying the optical properties at least one of the upper clad region or the lower clad region. The variation of the optical properties of the controllable regions may be achieved using a thermo-optic effect, an electro-optic effect, a stress-optic effect, or a mechano-optic effect, or by moving a material into or out of the controllable region.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority under 35 U.S.C § 119(e) to U.S. Provisional Application No. 60/225,488, filed Aug. 15, 2000, which is incorporated herein by reference; and U.S. Provisional Application No. 60/269,163, filed Feb. 15, 2001, which is incorporated herein by reference. 
     
    
     
         [0002]    BACKGROUND OF THE INVENTION  
           [0003]    1. Field of the Invention  
           [0004]    The present invention relates generally to an optical waveguide structure for an optical communication system, and particularly to a planar photonic crystal waveguide for implementing a variety of optical functions in an optical communication system.  
           [0005]    2. Technical Background  
           [0006]    Photonic crystals are periodic optical materials. The characteristic defining a photonic crystal structure is the periodic arrangement of dielectric or metallic elements along one or more axes. Thus, photonic crystals can be one-, two-, and three-dimensional. Most commonly, photonic crystals are formed from a periodic lattice of dielectric material. When the dielectric constants of the materials forming the lattice are different (and the materials absorb minimal light), the effects of scattering and Bragg diffraction at the lattice interfaces control the propagation of optical signals through the structure. These photonic crystals can be designed to prohibit optical signals of certain frequencies from propagating in certain directions within the crystal structure. The range of frequencies for which propagation is prohibited is known as the photonic band gap.  
           [0007]    An exemplary two dimensional photonic crystal which is periodic in two directions and homogeneous in a third is shown in FIG. 1. More specifically, the photonic crystal  10  is fabricated from a volume of bulk material  12  having a square lattice of cylindrical air-filled columns  14  extending through the bulk material in the z-axis direction and periodic in the x-axis and y-axis directions. For normal theoretical analysis and modeling, the photonic crystal  10  has conventionally been assumed to be homogeneous and infinite in the z-axis direction. In this exemplary figure, the plane of the two dimensional photonic crystal is the xy plane.  
           [0008]    Another exemplary photonic crystal is shown in FIG. 2. The photonic crystal  15  is similar to the photonic crystal  10 , but the cylindrical air-filled columns are disposed in a hexagonal array. A third exemplary two-dimensional photonic crystal is shown in FIG. 3. The photonic crystal  16  is also similar to photonic crystal  10 , but consists of an array of dielectric columns  18  in an air background.  
           [0009]    The propagation of optical signals in these structures is determined by a variety of parameters, including, for example, radius of the columns, pitch (center-to-center spacing of the columns) of the photonic crystal, structural symmetry of the crystal (e.g. square, triangular, hexagonal, rectangular), and refractive indexes, (such as the index of the material of the columns and the index of the bulk material exterior to the columns). FIG. 4 shows the photonic band diagram for a hexagonal array of air-filled columns  14  in a dielectric bulk material  12 . One skilled in the art will appreciate that there is a range of photon frequencies, known as the photonic band gap, for which propagation in the plane of the photonic crystal is prohibited. This photonic band gap, denoted by region  19 , is determined by the structure of the photonic crystal, especially by the parameters listed above.  
           [0010]    A defect can be introduced into the crystalline structure for altering the propagation characteristics and localizing the allowed modes for an optical signal. For example, FIG. 5 shows a two-dimensional photonic crystal  20  made from a dielectric bulk material with a square lattice of air-filled columns  22  and a linear defect  24  consisting of a row of missing air-filled columns. The band diagram for this photonic crystal structure is shown in FIG. 6. The photonic band gap is denoted by the region  30 , while a band of allowed guided modes associated with the defect is denoted by the very thin region  32 . The exact position and shape of the region  32  on the graph of FIG. 6 depends upon the photonic crystal parameters. Physically, this means that while optical signals of a given frequency are prohibited from propagating in the bulk photonic crystal  20 , they may propagate in the defect region  24 . An optical signal, whether a pulse or a continuous wave, traveling in the defect region  24  may not escape into the bulk photonic crystal  20 , and so is effectively waveguided in the defect region  24 . For a given wavevector, the region  32  only encompasses a narrow band of frequencies. Optical signals of a given wavevector must have frequencies within this narrow band in order to be guided in the defect  24 . In the theoretical case of the infinitely thick two-dimensional photonic crystal, light is not confined in the z-axis direction by the photonic crystal structure. While the defect in the above example is a constructed from a row of missing air-filled columns, other defect structures are possible. For example, a defect may consist of one or more columns of a different shape or size than those of the bulk photonic crystal.  
           [0011]    Additionally, the crystal structure can be composed of several photonic crystal regions having different parameters, in which case the defect is located at the border between the two regions. Such a structure is shown in FIG. 7, in which the photonic crystal structure  40  has a first photonic crystal region  42  and a second photonic crystal region  44 . In the example of FIG. 8, in the first region  42  the cylindrical columns have radius R 1  and are arranged with a pitch P 1 . In the second region, the photonic crystal structure has different parameters, with a column radius of R 2  and a pitch of P 2 . This photonic crystal also has a photonic bandgap, with the possibility of a defect mode for allowing propagation of an optical signal. Because of this defect mode phenomenon and its dependence on the photonic crystal parameters, it is possible to control the propagation of an optical signal in a defect waveguide by controlling the parameters associated with the photonic crystal regions.  
           [0012]    Since an optical signal propagating in a defect waveguide is prohibited from propagating in the bulk photonic crystal, it must follow the waveguide, regardless of the shape of the defect waveguide. An advantage of such a structure is that waveguides with a very small bend radius on the order of several wavelengths or even less are expected to have a very low bend loss, since an optical signal is prohibited from escaping the defect waveguide and propagating in the surrounding photonic crystal. FIG. 8 shows the results of a simulation of propagation in a 2D photonic crystal wherein substantially all of the optical signal successfully navigates a 90° bend with a radius of curvature smaller than the wavelength of the optical signal. Likewise, waveguide splitters and combiners are expected to have low radiation losses. FIG. 9 shows a 180° splitter in which nearly 100% transmission is achieved with the optical signal from the input guide  60  perfectly split into the two branches  62  and  64 . In this case, a pair of small columns was added in order to reduce the small fraction of light that was backreflected into the input guide  60 .  
           [0013]    In-plane confinement by a photonic crystal defect waveguide can be confined with refractive confinement in the dimension normal to the photonic crystal to provide a defect channel waveguide. This is most commonly achieved by providing a thin slab of a two-dimensional photonic crystal (known as a planar photonic crystal) having a defect waveguide with lower refractive index materials above and below the photonic crystal waveguide. For example, FIG. 10 shows the structure of a planar photonic crystal defect waveguide  70  with a core layer  71 , an underclad layer  72 , and an overclad layer  74 , all of which include a photonic crystal structure defining the defect channel waveguide. For use herein, an effective refractive index of a material is defined as the volume average refractive index of that material. In order to provide vertical confinement, the effective refractive index of the core layer  71  is higher than the effective refractive indices of the underclad layer  72  and the overclad layer  74 . This structure may be made by etching an array of columnar holes into a slab waveguide containing a core layer, an underclad layer, and an overclad layer.  
           [0014]    An example of an alternative structure appears in FIG. 11. In this case, only the higher effective refractive index core layer  81  has the photonic crystal structure; the underclad  82  and the overclad  84  are homogeneous. In this structure, which may be fabricated by bonding a thin slab of material containing the 2D photonic crystal structure to a substrate, the substrate serves as the underclad, and the overclad is air Alternative structures have been envisioned wherein a free-standing planar photonic crystal is clad on both sides by air, or wherein both the underclad and overclad are a dielectric material.  
           [0015]    [0015]FIG. 12 shows another alternative structure, having both the core layer  90  and the underclad layer  92  patterned with a two dimensional photonic crystal structure, and a homogeneous overclad  91 . This structure can be made by etching an array of columnar holes into a slab waveguide having an optically homogeneous core layer deposited onto a optically homogeneous underclad layer. In both of these alternative architectures, the upper cladding may be air, or it may be a layer of dielectric material.  
           [0016]    In all three architectures, an optical signal is constrained in the defect waveguide vertically by total internal reflection, and horizontally by the photonic band gap. Passive waveguiding has been predicted by optical simulations and demonstrated in experimental systems in all three architectures. Calculations for a planar photonic crystal waveguide have been described in Kuchinsky et al., “3D localization in a channel waveguide in a photonic crystal with 2D periodicity,” Optics Communications 175, p. 147-152 (2000), which is hereby incorporated by reference. The calculation method uses a numerical solution of the full vector Maxwell equations, in which the electromagnetic modes are expanded in a sum of plane waves. This approach is well suited to periodic photonic crystals. When the physical system lacks periodicity, for example as in the z-direction of a bulk photonic crystal or the transverse direction of a defect waveguide, then a supercell is employed in which a periodic array of crystals or waveguides is considered. The artificial repeat distance of this supercell is kept large enough to avoid unwanted calculation artifacts. The supercell method is a standard approach that allows periodic band structure computer codes to salve nonperiodic systems. Solution of the full vector Maxwell equations required, as the simpler scalar approximation gives incorrect results due to the large dielectric/air index. Propagation through sharp defect waveguide bends has also been predicted and experimentally demonstrated.  
           [0017]    Active devices may be based on planar photonic crystal defect channel waveguides. For example, an actively controllable Y junction is shown in FIG. 13. The Y junction has an input waveguide  94 , a first output waveguide  95 , and a second output waveguide  96 . The output waveguides are modified by the presence of controllable lattice sites  98  located in the regions  97  of the output waveguides near the branch point and comprising cylindrical columns formed of a ferrite material to which a variably controllable external electromagnetic field may be applied. The locations of the controllable lattice sites conform to the column and row positions of the surrounding lattice region and in effect form an extension of the lattice. Control of the controllable lattice sites  98  is effected such as to vary the refractive index of the ferrite material, and therefore the propagation characteristics of the defect waveguides. The presence of the controllable lattice sites can in effect be turned on or off in variable number to thereby variably control the effective apertures of the output waveguides  95  and  96 . This is represented in FIG. 13 by showing only those controlled lattice sites  98  which are turned “on” and which in this example are shown only in the second output waveguide  96 . The amount of optical signal coupled into the second output waveguide  96  is thereby controllable by setting the number of sites which are turned on, the remainder of the optical signal being diverted into the second waveguide  96 . Active photonic crystal materials and devices with bandgaps in the near infrared, however, would be difficult to fabricate using ferrite materials.  
           [0018]    It is also possible to externally control the propagation of an optical signal in a planar photonic crystal defect channel waveguide by varying the refractive index of the bulk material of the planar photonic crystal. The externally applied control may be one of a number of available options including the application of local heating, the injection of electrical current into a semiconductor bulk material, or other suitable optically, electromagnetically or electromechanically induced effects. The photonic crystal lattice is substantially unaffected by this control and continues to serve as a means of confining the optical signal within the waveguide so as to pass through the controlled dielectric region. These types of devices are unattractive in that the photonic crystal must be formed in a thermo-optically, electro-optically, or mechano-optically active material, limiting the choice of device materials and fabrication processes.  
           [0019]    Accordingly, photonic crystal waveguide devices which can perform a wide variety of optical transformations and are amenable to a wide variety of materials and manufacturing processes are desired.  
         SUMMARY OF THE INVENTION  
         [0020]    One aspect of the present invention relates to a planar photonic crystal defect waveguide device in which an optical signal is confined in the plane of the photonic crystal by the photonic bandgap, and in the direction normal to the photonic crystal by the upper clad region and the lower clad region, wherein the propagation of light in the waveguide is controlled by varying the optical properties of either the upper clad region or the lower clad region or both.  
           [0021]    Another aspect of the present invention relates to a method for controlling an optical signal propagating in such a photonic crystal defect channel by changing the optical properties of either the upper clad region and the lower clad region or both.  
           [0022]    Another aspect of the present invention relates to an optical device for controlling an optical signal including a planar photonic crystal structure having a defect waveguide, an upper clad region continuous with the top surface of the defect waveguide, and a lower clad region continuous with the bottom surface of the defect waveguide, wherein at least one of the upper clad region or the lower clad region is a controllable region having a controllable optical property sufficient to modify the optical signal.  
           [0023]    Another aspect of the present invention relates to a method for controlling an optical signal including providing a device including a planar photonic crystal structure having a defect waveguide, an upper clad region continuous with the top surface of the defect waveguide, and a lower clad region continuous with the bottom surface of the defect waveguide, wherein at least one of the upper clad region or the lower clad region is a controllable region having a controllable optical property sufficient to modify the optical signal; launching the optical signal into the defect waveguide; and controlling the optical property of at least one of the upper clad region or the lower clad region so as to effect a change in the propagation of the optical signal in the defect waveguide.  
           [0024]    The device of the present invention results in a number of advantages. Active planar photonic crystal defect waveguide devices may be designed and fabricated with well-defined guiding characteristics in all three dimensions, and may have modes with zero group velocity. The active planar photonic crystal defect waveguides may be fabricated by standard semiconductor manufacturing techniques. The devices of the present invention do not derive their activity from an active photonic crystal waveguide core, and so may be made from photonic crystals of any standard passive waveguide material. The refractive indices of the upper clad region or the lower clad region or both may be varied in many ways, including by a thermo-optic effect, an electro-optic effect, a mechano-optic effect, or by physically introducing a different material into the clad region. The devices of the present invention may affect various optical transformations, including attenuation, modulation, and switching, all with the reduced device size afforded by the efficiency of tight photonic crystal waveguide bends.  
           [0025]    Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the invention as described in the written description and claims hereof, as well as the appended drawings.  
           [0026]    It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework to understanding the nature and character of the invention as it is claimed.  
           [0027]    The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s) of the invention, and together with the description serve to explain the principles and operation of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]    The various advantages of the present invention will become apparent to one skilled in the art by reading the following specification and appended claims, and by referencing the following drawings in which:  
         [0029]    [0029]FIGS. 1, 2 and  3  are perspective views showing exemplary prior art two-dimensional photonic crystal structures;  
         [0030]    [0030]FIG. 4 is a graph showing the band structure of the exemplary prior art two-dimensional photonic crystal structure of FIG. 1;  
         [0031]    [0031]FIG. 5 is a perspective view showing an exemplary prior art two-dimensional photonic crystal structure with a linear defect;  
         [0032]    [0032]FIG. 6 is a graph showing the band structure of the exemplary prior art two-dimensional photonic crystal structure of FIG. 5;  
         [0033]    [0033]FIG. 7 is a perspective view showing exemplary prior art two-dimensional photonic crystal structures having two different photonic crystal regions separated by a linear defect waveguide;  
         [0034]    [0034]FIG. 8 is a diagram showing propagation of an optical signal through a sharp 90° bend in a prior art two-dimensional photonic crystal defect waveguide;  
         [0035]    [0035]FIG. 9 is a diagram showing propagation of an optical signal through a 180° splitter in a prior art two-dimensional photonic crystal defect waveguide;  
         [0036]    [0036]FIG. 10 is a perspective view of a prior art planar photonic crystal defect waveguide with a planar photonic crystal core and photonic crystal underclad and overclad layers;  
         [0037]    [0037]FIG. 11 is a perspective view of a prior art planar photonic crystal defect waveguide with planar photonic crystal core and homogenous underclad and overclad layer;  
         [0038]    [0038]FIG. 12 is a perspective view of a prior art planar photonic crystal defect waveguide with planar photonic crystal core and underclad layer, and a homogeneous overclad layer;  
         [0039]    [0039]FIG. 13 is a top view of a prior art active planar photonic crystal defect waveguide Y junction;  
         [0040]    [0040]FIG. 14 is a perspective view of a general planar photonic crystal defect waveguide device;  
         [0041]    [0041]FIG. 15 is a perspective view of a planar photonic crystal defect waveguide device with a movable slab of material in a rest switch state;  
         [0042]    [0042]FIG. 16 is a perspective view of a planar photonic crystal defect waveguide device with a movable slab of material in an actuated switch state;  
         [0043]    [0043]FIG. 17 is a cross-sectional view of a planar photonic crystal defect waveguide device with a movable slab of material coupled to a MEMS actuator in a rest switch state;  
         [0044]    [0044]FIG. 18 is a cross-sectional view of a planar photonic crystal defect waveguide device with a movable slab of material coupled to a MEMS actuator in an actuated switch state;  
         [0045]    [0045]FIG. 19 is a top view of a planar photonic crystal defect waveguide device with a movable slab of material coupled to a MEMS actuator;  
         [0046]    [0046]FIG. 20 is a cross-sectional view of a thermo-optic planar photonic crystal defect waveguide device;  
         [0047]    [0047]FIG. 21 and  22  are cross-sectional views of electro-optic planar photonic crystal defect waveguide devices;  
         [0048]    [0048]FIG. 23 is a cross-sectional view of a mechano-optic planar photonic crystal defect waveguide device;  
         [0049]    [0049]FIG. 24 is a top view of a planar photonic crystal defect waveguide Mach-Zehnder interferometer;  
         [0050]    [0050]FIG. 25 is a top view of a planar photonic crystal defect waveguide directional coupler;  
         [0051]    [0051]FIG. 26 is a top view of a planar photonic crystal defect waveguide variable optical attenuator;  
         [0052]    [0052]FIG. 27 is a top view of a Y-shaped planar photonic crystal defect waveguide;  
         [0053]    [0053]FIG. 28 is a perspective view of a planar photonic crystal defect waveguide 1×2 switch with a movable slab of material in a rest switch state; and  
         [0054]    [0054]FIG. 29 is a perspective view of a planar photonic crystal defect waveguide 1×2 switch with a movable slab of material in a actuated switch state. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0055]    Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts.  
         [0056]    Referring now to FIG. 14, a perspective view of a general planar photonic crystal defect waveguide device is shown. The device consists of a thin slab  100  of planar photonic crystal material, having, for example, air-filled columns  14  disposed in a hexagonal array in a bulk material  12 . Alternatively, the geometry of the array may be square, triangular, rectangular, or more complex, depending on the desired in-plane photonic band gap. The bulk material may be any material transparent to the wavelengths of the optical signal. For example, the planar photonic crystal bulk material may be doped silica, undoped silica, silicon, a polymeric organic material, a organic/inorganic hybrid material, an inorganic glass (especially chalcogenide glass), and III-V semiconductor materials such as gallium arsenide. The planar photonic crystal may be made by any method used in the art, such as photolithographic patterning followed by etching. There is a defect waveguide  102  in the photonic crystal slab  100 . A lower clad region  104  is a volume of space contiguous with the bottom surface of the defect waveguide  102 . This lower clad region  104  is on the order of microns thick, and encompasses the evanescent wave of an optical signal propagating in the defect waveguide  102 . The lower clad region  104  may contain any material, including a substrate material, a deposited layer, or air, which may or may not itself be patterned with a photonic crystal structure. An upper clad region  106  is a volume of space contiguous with the top bottom surface of the defect waveguide  102 . This upper clad region  106  is also on the order of microns thick, and encompasses the evanescent wave of an optical signal propagating in the defect waveguide  102 . The upper clad region may contain any material, including a substrate material, a deposited layer, or air, which may or may not itself be patterned with a photonic crystal structure. In operation of the device, the propagation of an optical signal is controlled by varying an optical property of the lower clad region  104 , the upper clad region  106 , or both clad regions  104  and  106 .  
         [0057]    An exemplary planar photonic crystal defect waveguide device is shown in FIG. 15. A planar photonic crystal slab  110  with a defect waveguide  111  is formed on a substrate  112 , preferably using known semiconductor manufacturing techniques. The substrate  112  is transparent to the wavelength of interest and of a lower effective refractive index than effective refractive index of the photonic crystal slab  110 . In this case, the lower clad region  104  contains substrate material. Depending on the fabrication process, the material in the lower clad region  104  may or may not be patterned with the photonic crystal structure. As fabricated, the upper clad region  106  contains air. As part of the present invention, the defect mode, and therefore the propagation of an optical signal in the defect waveguide  111 , may be changed by changing the refractive index of the upper clad region  106 . One technique for achieving this is to position a plain slab of material  114  near but outside of the upper clad region  106 . The material of the slab  114  may be any material with a desired effective refractive index, for example doped silica, undoped silica, silicon, a polymeric organic material, a organic/inorganic hybrid material, an inorganic glass, and Ill-V semiconductor materials such as gallium arsenide. The effective refractive index of the upper clad region  106  in the rest switch state shown in FIG. 15 is that of air. In the actuated switch state shown in FIG. 16, the slab  114  is moved into the upper clad region, which increases the effective refractive index of the upper clad region  106  by an amount depending on the effective refractive index of the slab  114  and its protrusion into the upper clad region  106 . The change in effective refractive index of the upper clad region will perturb the defect mode, modifying the propagation of the optical signal through the defect waveguide  111 . The closer the slab  114  is placed to the top surface of the defect waveguide  111 , the larger the perturbation of the defect modes will be. This controllable perturbation of the defect modes will serve to modify the propagation of an optical signal through the planar photonic crystal defect waveguide  111 . As such, actuation of the slab  114  controls the propagation of an optical signal in the defect waveguide  111 .  
         [0058]    One technique for positioning and moving the slab  114  is the use of a mechanical actuator, such as, for example, a microelectromechanical (MEMS) actuator. An example of such a device is shown in FIGS. 17, 18 and  19 . FIG. 17 is a side view of the exemplary device in a rest switch state. Built onto a substrate  112  is a ground electrode  115 , an underclad layer  116 , and a planar photonic crystal  110  having a defect waveguide  111  having an effective refractive index greater than that of the underclad layer  116 . Coupled to this is a MEMS device, having a beam  117  with an electrode  118  and the slab  114  deposited thereon. The beam  117  is connected to the substrate  112  through a pair of cantilever arms  119  and a supporting structure  120 . A voltage controller  121  is able to place an electric potential between the electrodes  118  and  115 . In the rest switch state shown in FIG. 17, wherein substantially no electric potential exists between the electrodes  115  and  118 , the slab  114  is outside of the upper clad region  106 , so the effective refractive index of the upper clad region  106  is that of air. FIG. 18 shows an actuated switch state wherein an electric potential is placed between the electrodes. In this state, the beam  117  is attracted to the ground electrode  115 , moving the slab  114  into the upper clad region  106 . The distance of penetration into the upper clad region will depend on the potential difference between the electrodes, as will be appreciated by a person of skill in the art. The cantilever arms are sufficiently flexible to allow the beam to deflect an amount sufficient to bring the slab substantially in contact with the surface, as shown in FIG. 18. As described above, the closer the slab  114  is placed to the top surface of the defect waveguide  111 , the larger the perturbation of the defect modes will be. This device may be constructed, for example, by separately fabricating the photonic crystal waveguide and the MEMS structure using procedures familiar to one of skill in the art, followed by mating of the two structures. FIG. 19 shows a top view of the mated structures, with the MEMS structure on top of the photonic crystal waveguide structure. This view shows the cantilevered arms  119  as pairs of S-shaped structures. As the slab  114  may have no features, the alignment tolerance of the mating step is low. Other methods of MEMS device actuation, such as piezoelectric and thermal actuation may be used in this device. The person of skill in the art will appreciate that other art-recognized techniques for building MEMS-waveguide structures may be used advantageously here. As the size of the of the slab  114  is not critical provided it is large enough to encompass the evanescent wave of an optical signal propagating in the defect waveguide  111 , larger actuating devices may be used in this invention. For example, a piezoelectric actuator may be used in the device instead of the MEMS actuator described above.  
         [0059]    An additional embodiment of the invention is shown in FIG. 20. A photonic crystal slab  110  with a defect channel waveguide  111  is formed on an undercladding layer  112 , preferably using known semiconductor manufacturing techniques. The undercladding layer  112  is of a lower effective refractive index than the effective refractive index of the planar photonic crystal  110 , and may be the substrate, or may be a layer deposited on the substrate. Depending on the fabrication process, the undercladding layer  112  may or may not be patterned with the photonic crystal structure. Here, the lower clad region  104  is completely filled by this undercladding layer  112 . A slab of material  122  is fixed in the upper clad region  106 , preferably substantially in contact with the top surface of the planar photonic crystal slab. The material of the slab  122  is chosen to have optical properties that may be varied by some means of control. For example, the slab  122  may consist of a material with a substantial thermo-optic coefficient, such as for example a polymer, an inorganic glass, or an organic-inorganic hybrid material. In this case, the means of control would preferably be a heater  124  disposed near the surface of the thermo-optic slab opposite that in contact with the planar photonic crystal slab. The heater  124  is coupled to a suitable controller  125 . The heater may be in contact with the slab  122 , or may be operatively coupled to the slab  122  through air or some other thermally conductive material. For example, a suitable heater  124  may be constructed by the deposition of a thin layer of metal, such as aluminum, on the thermo-optic layer. Passing a current through the metal will resistively heat the metal, and the heat will be transferred from the heater  124  to the thermo-optic slab  122 , changing the effective refractive index of the thermo-optic slab and therefore the propagation of an optical signal in the defect waveguide  111 . In an alternative embodiment, the upper clad region is filled with an inert fluid chosen to have substantial thermo-optic coefficient.  
         [0060]    Alternatively, the slab  122  may consist of a material with a substantial electro-optic coefficient, as shown in FIG. 21. Examples of materials with a substantial electro-optic coefficient include lithium niobate, electro-optic polymers, and liquid crystal composites. In the case of an electro-optic material, the means of control would be a pair of electrodes  132  and  134  connected to a voltage source  136  and positioned so as to place an electric field in the upper clad region when the electrodes are biased at different electrical potentials. For example, the electrodes  132  and  134  may be situated the surface of the electro-optic slab opposite that in contact with the planar photonic crystal slab, with one electrode  132  disposed over a photonic crystal region  136  on one side of the defect waveguide  111 , and the other electrode  134  disposed over a photonic crystal region  138  on the other side of the defect waveguide  111 . Biasing the electrodes at different electric potentials with a voltage controller  140  coupled to the electrodes  132  and  134  will create an electric field, denoted by the lines  139 , in the overclad region. Alternatively, the electrodes may be placed as illustrated in FIG. 22, with a top electrode  142  situated on the surface of the electro-optic slab opposite that in contact with the photonic crystal slab and disposed over the defect waveguide, and a bottom electrode  144  disposed underneath the defect waveguide. Both the top electrode  142  and the bottom electrode  144  are preferably outside of the lower clad region  104  and upper clad region  106 . Placing a voltage across the electrodes with a voltage controller  150  coupled to the electrodes  142  and  144  will create an electric field, denoted by the lines  149 , in the upper clad region  106 . In both examples, the electrodes  132 ,  142 ,  134  and  144  may be fabricated from a thin layer of deposited metal such as gold, silver, or chrome; from a conductive oxide such as indium tin oxide; or from a doped semiconductor such as n-doped silicon. One of ordinary skill in the art will realize that the two different electrode configurations exemplified in FIG. 21 and FIG. 22 may be combined in series or in parallel in a device to reduce or enhance effects due to the polarization state of the optical signal. In the devices shown in FIGS. 21 and 22, a voltage is placed across the electrodes, causing the electro-optic slab to change in effective refractive index, thus modifying the propagation of an optical signal in the defect waveguide  111 .  
         [0061]    Alternatively, the slab  122  may consist of a material with a substantial stress-optic coefficient, as shown in FIG. 23. Materials with a substantial stress-optic coefficient have a substantial change in refractive index when they are subject to a stress, and include, for example, inorganic glasses and polymers, and especially main chain liquid crystalline polymers. The slab  122  is coupled to an actuator  154  that serves to place a stress on the slab of material  122 . The actuator  154  is coupled to a controller  156 . In this case, the slab  122  is preferably not in direct contact with the planar photonic crystal slab to avoid the mechanical transfer of stress to the planar photonic crystal slab  110  itself, but is as close as possible so as to maximize the volume of the upper clad region  106  that is filled with the slab  122 . Alternatively, a material with a substantial mechano-optic coefficient may be employed in the slab  122  of this device. A mechano-optic material undergoes a change in refractive index with a change in dimension. This material may be, for example, a material with a glass transition temperature below 10° C., such as poly(dimethylsiloxane). In both the stress-optic and the mechano-optic case, actuating the material causes a controllable change in effective refractive index of the slab  122 , and modifies the propagation of an optical signal in the defect waveguide  111 . In both cases, the actuator  154  may be, for example, a piezoelectric actuator.  
         [0062]    It will be apparent to those of ordinary skill in the pertinent art that modifications and variations may be made to the controllable planar photonic crystal defect waveguides of the disclosed examples without departing from the spirit or scope of the invention. For example, in the aforementioned examples, it is the effective refractive index of the material in the upper clad region  106  that is controlled to modify the propagation of an optical signal in the defect waveguide. The effective refractive index of the material in the lower clad region  104  may likewise be controlled to effect a modification of optical signal propagation. For example, the substrate itself may be disposed in the lower clad region and may be made of a material with a substantial electro-optic or thermo-optic coefficient. It may also be desirable to control the refractive indices of both the lower clad region  104  and the upper clad region  106  in order to reduce the interaction length necessary for a desired modification of an optical signal. For example, the photonic crystal slab may be disposed between layers of electro-optic or thermo-optic materials. Alternatively, the planar photonic crystal defect waveguide as fabricated may have air in both the upper clad region  106  and the lower clad region  104 , and have movable slabs  114  above the upper clad region  106  and below the lower clad region  104  that may be actuated into the clad regions, changing the effective refractive indices of the regions and effecting a modification of an optical signal in the defect waveguide  111 . In all cases, the control of the upper clad region  106  and the lower clad region  104  may be independent or concerted. The control of the effective refractive index of the upper clad region  106 , the lower clad region  104 , or both, may arise from effects other than those disclosed above. For example, a photorefractive effect may be used to control the effective refractive index of the clad regions and thereby effect a change in the propagation in the defect waveguide  111 .  
         [0063]    The aforementioned examples have provided a series of planar photonic crystal defect waveguide elements wherein the propagation of an optical signal is controlled by varying the optical properties of the upper clad region  106 , the lower clad region  104 , or both. As will be understood by the person of ordinary skill in the art, these controllable elements can be combined with other waveguide elements to construct integrated optic devices with a variety of functions. For example, a Mach-Zehnder interferometer device may be constructed with at least one of the arms being contiguous with a controllable region as described in the examples above. An example of such a structure is shown in FIG. 24. In this top view, the boundaries of the photonic crystal defect waveguide are shown by solid lines. A planar photonic crystal defect waveguide structure  160  is provided with an input defect waveguide  161 , an optical power splitter  162  at which the input waveguide is separated into a first defect waveguide arm  163  and a second defect waveguide arm  164 . An optical power combiner  165  recombines the defect waveguides  163  and  164  into an output defect waveguide  166 . While the optical power splitter  162  and combiner  165  are shown here as Y-shaped junctions, the person of skill in the art will recognize that they may also be directional couplers. The second defect waveguide arm  164  is contiguous with a controllable region  167  as disclosed above, which is provided for perturbing the defect mode, thereby varying the optical path length of the second waveguide  164 . As is well understood in the art, the difference in optical path length between the waveguides  163  and  164  controls the interference of the optical signals propagating in those waveguides upon recombination, and therefore the intensity of the optical signal in the output waveguide  166 .  
         [0064]    Another example of an integrated optic device using a controllable photonic crystal waveguide is illustrated in FIG. 25. In this top view, the boundaries of the photonic crystal defect waveguide  111  are shown by solid lines. In this example, a 2×2 switch is made from a pair of defect waveguides  170  and  172  arranged in the well-known directional coupler configuration. In the coupling region, the defect waveguide  170  is contiguous with a controllable region  173 , and the defect waveguide  172  is contiguous with a controllable region  174 . In the example of FIG. 17, these waveguides may be controlled, for example, by an electro-optic polymer slab essentially in contact with the photonic crystal slab  110 . The controllable regions  173  and  174  may both be controlled by the same means; in this example, an electric field between an electrode above the electro-optic polymer slab and an electrode below the lower clad region serves to modulate the effective refractive index of the electro-optic polymer, and thereby control the coupling ratio between the output waveguides  175 .  
         [0065]    An exemplary embodiment of a variable optical attenuator is shown in FIG. 26. In this top view, the boundaries of the photonic crystal defect waveguide  111  are shown by solid lines. The variable optical attenuator includes a defect waveguide  176 , which is contiguous with a controllable region  177 . In this example, the controllable region is contiguous with an area of the bulk planar photonic crystal as well as the defect waveguide. In a rest state (e.g. unactuated MEMS device, no electric field, no heat), the optical signal is prohibited from propagating in the bulk planar photonic crystal, and is thereby confined to the defect waveguide. In an actuated state (e.g. actuated MEMS device, applied electric field, applied heat), the photonic band structure is perturbed, and a fraction of the optical signal is allowed to couple into the bulk photonic crystal, thereby attenuating the optical signal in the defect waveguide. As the person of skill in the art will understand, the attenuation is controlled by the magnitude of the actuation, the photonic crystal structure, and the length of interaction of the defect waveguide  176  and the controllable region  177 .  
         [0066]    Referring now to FIG. 27, a top view of a 1×2 photonic crystal switch  180  is shown in accordance with a preferred embodiment of the present invention. Switch  180  includes a planar photonic crystal slab  181 . The planar photonic crystal slab  181  includes a first photonic crystal zone  182 , a second photonic crystal zone  183 , and a third photonic crystal zone  184 . Each of the photonic crystal zones  182 ,  183  and  184  preferably has different parameters associated therewith, such as the geometry of the crystal structure and/or the index of refraction of the materials forming the photonic crystal lattice in that zone. FIG. 27 further illustrates the differences in geometry in that the cylinders  14  or rods  18  forming the crystal structure in each zone have differing radii and pitch parameters. More specifically, the cylinders  14  in the third zone  184  have the largest radius, the cylinders  14  in the second zone  183  have the smallest radius, and the cylinders  14  in the first zone  182  have a radius between that of the first and third zone cylinders.  
         [0067]    The three photonic crystal zones  182 ,  183  and  184  define a Y-shaped defect waveguide junction which is used in a 1×2 coupler. As shown, the Y-shaped defect waveguide junction includes an input waveguide section  185 , a first or upper output waveguide  186 , and a second or lower output waveguide  187 . Each section of the waveguide is formed by a defect or channel between the photonic crystal structures  182 ,  183  and  184 .  
         [0068]    In this example, the parameters (e.g. geometry, index) of the first photonic crystal zone  182  and the third photonic crystal zone  184  are selected such that an optical signal can always propagate between these zones along the input waveguide section  185 . More specifically, this is accomplished by creating a defect band in the frequency vs. wave vector band diagram associated with the structure. An example is that shown in association with FIG. 4. Additionally, some or all of the zones  182 ,  183  and  184  have parameters that can be switched from a first value to a second value for similarly creating a defect mode that can be alternately switched for the first output waveguide  186  and the second output waveguide  187 . As such, when an optical signal is allowed to propagate along the first output waveguide  186 , the optical signal is prohibited from propagating along the second output waveguide  187 . Conversely, when an optical signal is allowed to propagate along the second output waveguide  187 , an optical signal is prohibited from propagating along the first output waveguide  186 . As will be appreciated, by changing the defect mode, an optical signal traveling along the input waveguide  185  can be alternately switched between one of the first output waveguide  186  and the second output waveguide  187  for creating a 1×2 optical switching device.  
         [0069]    The defect mode may be changed in a variety of ways, such as by changing the defect mode associated with one or more of the defect waveguides  185 ,  186  and  187 . One technique for changing the defect mode is to position a slab  188  of material over the top surface  189  of the planar photonic crystal structure  181 . The material of the slab  188  may be any material with a desired effective refractive index, for example doped silica, undoped silica, silicon, a polymeric organic material, a organic/inorganic hybrid material, an inorganic glass, and III-V semiconductor materials such as gallium arsenide. As shown in the rest switch state of FIG. 28, when the slab  188  is separated from the top surface  189  so as to be outside of the upper clad region  106 , a first defect mode is created, and an optical signal will propagate for example only through the first output waveguide  186 . As shown in the actuated switch state of FIG. 29, when the slab  188  is placed very near or in contact with the top surface  189  so as to substantially fill the upper clad region  106 , a second defect mode is created, and light will propagate for example only through the second output waveguide  187 . It will be appreciated that positioning the slab so as to only partially fill the upper clad region  106  may route only a portion of the optical signal to the first output waveguide  186 , with the balance of the optical signal being routed to the second optical waveguide. It will also be appreciated that the controllable region is contiguous with at least one of the defect waveguides  185 ,  186 , and  187 . One technique contemplated for positioning and moving the slab  188  is the use of a microelectromechanical (MEMS) actuator, as described in connection with FIGS. 17 and 18. As disclosed above, other actuators may be used. As such, the actuation of the slab  188  controls the switching function of the 1×2 optical switch  180 . As is appreciated by one of skill in the art, the effective refractive index of the upper clad region  106  may alternatively be changed by many the other methods disclosed herein.  
         [0070]    It will be apparent to those of ordinary skill in the pertinent art that the devices described in the above examples may be modified to be operative through control of the upper clad region  106 , the lower clad region  104 , or both clad regions  104  and  106  without departing from the spirit or the scope of this invention. It will likewise be apparent to those of ordinary skill that other devices dependent on modification of the propagation characteristics of a planar photonic crystal defect waveguide by a change in the optical properties of the upper clad region  106 , the lower clad region  104 , or both may be contemplated without departing from the spirit or scope of the invention.  
         [0071]    As described above and understood by the skilled artisan, the function of a planar photonic crystal defect waveguide device is highly dependent on the wavelengths of the optical signal propagating therethrough. This property may be used advantageously in connection with this invention to make devices with wavelength-dependent functionality. For example, the variable optical attenuator of FIG. 26 may be designed to attenuate a first wavelength of an optical signal, while leaving a second wavelength undisturbed. These devices may be designed using the calculation methods known in the art as well as those described in the Optics Communications paper incorporated by reference herein.  
         [0072]    As previously noted, the optical devices of the present invention may be employed for implementing a variety of optical switching functions in an optical communication system, including optical fiber communications switching modules and equipment, optical computing, optical sensor arrays, antennae arrays, and other applications where optical waveguides, optical fibers, or other guided or partially-guided light signal transmission media are utilized to route light signals for voice, data, and other information-carrying purposes.  
         [0073]    The foregoing discussion discloses and describes exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.