Abstract:
An optical device provides optical routing functions, such as switching or redirecting of optical signals. The device utilizes one or more control light beams, which couple through a top surface of a planar substrate (via relatively small control windows) into one or more preselected regions of optical channels formed in the substrate. The presence of a control light beam at a control window increases the refractive index of the nonlinear optical medium of a portion of a channel. The portion of the channel includes a structure that functions as an on/off filter to reflect or transmit an optical signal propagating in the channel in a manner that is responsive to the intensity of the control light beam applied to the portion of the channel. In some embodiments, the optical channels interrupt a 2D PBG structure, which functions as a boundary for the optical channels.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to optical devices that use non-linear optical mediums.  
         BACKGROUND OF THE INVENTION  
         [0002]    In optical waveguides used in optical data transmission and optical laser cavities, light propagates in one spatial direction. These waveguides use total internal reflection at an interface between two media with relatively higher and lower refractive indices to direct the light. Total internal reflection causes the light to propagate in the medium with the higher refractive index.  
           [0003]    Periodic dielectric structures can also be used to direct light propagation. In periodic dielectric structures, light propagation is analogous to electron propagation in a crystal. If the wavelength of the light is of the order of the dimensions of the lattice, a photonic bandgap (PBG) forms. The PBG is a wavelength range in which photons cannot propagate through a periodic dielectric structure. If incident light&#39;s wavelength is in the PBG, the incident light is reflected off the periodic dielectric structure rather than transmitted through the structure. Period dielectric structures whose lattice lengths are of the order of wavelengths of near infrared or visible light are often referred to as PBG structures. Light with a wavelength in a PBG can propagate down a narrow channel in a PBG structure.  
           [0004]    Proposals exist for using PBG structures to make optical cavities. An article entitled “Two-Dimensional Photonic Band-Gap Defect Mode Laser” by  0 . Painter et al. appearing in the 11 Jun. 1999 issue of  Science  (p. 18 et seq.) describes the formation of a laser cavity in a two-dimensional (2D) a 2D PBG structure. The laser cavity is fabricated in a group III-V crystalline semiconductor and uses a channel in a 2D PBG structure and a defect to form the laser cavity.  
         SUMMARY OF THE INVENTION  
         [0005]    One aspect of the present invention relates to an integrated optical switch that includes a planar waveguide with a one-dimensional (1D) optical waveguide therein. The 1D waveguide has a specific interaction region that defines a filter. The filter is, at least in part, made of a nonlinear optical medium and is controllable by externally introduced control light. Changing the intensity of the control light causes optical switching by changing the index of refraction of the nonlinear medium so that the filter changes between first and second states. The filter transmits light propagating in the 1D waveguide in the first state and reflects light propagating in the 1D waveguide in the second state.  
           [0006]    In some embodiments, the 1D waveguide is formed from a PBG structure and a channel therein. The PBG structure causes the channel to function as a 1D waveguide in which a selected wavelength range of light propagates. The channel is formed of a nonlinear optical medium and includes a resonant cavity formed of a periodic array of holes with a defect. Switching results from applying a control light beam that changes the index of refraction of the medium in the cavity and thus, the resonant frequency of the cavity.  
           [0007]    Exemplary switches use PBG structures in III-V semiconductors, e.g., semiconductors comprising elements from the group consisting of gallium (Ga), arsenic (As), indium (In), and phosphorus (P), and wavelengths of control light that generate carrier densities in these semiconductors. The presence of carrier densities strongly modifies the medium&#39;s index of refraction and the resonant frequencies of optical cavities therein. These exemplary switches are able to switch light with wavelengths between about 0.9 and 1.65 μm.  
           [0008]    Various embodiments of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIGS. 1A and 1B show respective top and side views of an integrated optical switch;  
         [0010]    [0010]FIG. 1C is an embodiment of the switch of FIGS. 1A-1B based on a PBG structure;  
         [0011]    [0011]FIG. 1D illustrates how the reflectivities of filters used in the switches of FIGS. 1A-1C depend on the intensity of a control light beam;  
         [0012]    [0012]FIG. 1E is a flow chart for a method of operating the integrated optical switches of FIGS. 1A-1C;  
         [0013]    [0013]FIG. 2 is a top view of an embodiment of a 1×2 optical switch; and  
         [0014]    [0014]FIG. 3 is a top view of an embodiment of a 1×N optically controllable coupler. 
     
    
     DETAILED DESCRIPTION  
       [0015]    [0015]FIGS. 1A and 1B show respective top and side views of a planar structure  8  that forms an integrated 1×1 optical switch. In the planar structure  8 , propagating light is confined to a center layer by total internal reflection off upper and lower outer layers  6 . The planar structure  8  includes a 1D optical channel  12  made of a nonlinear optical medium, e.g., a semiconductor including elements from the group consisting of Ga, As, In, and P or another group III-V semiconductor. The channel  12  and boundaries  13  form a 1D optical waveguide that guides light from an optical input  14  to an optical output  16 . The channel  12  includes an optical filter  18  that transmits channel light in a selected wavelength range and reflects channel light in other wavelength ranges. The wavelength selectivity of the filter  18  is controllable by control light introduced into the filter  18  through a control access window  19 . The control access window  19  is located on the top face of the planar structure  8 . The channel  12  responds to changes in the intensity of control light in the filter  18  as an optical switch.  
         [0016]    The 1D optical waveguide includes a propagation medium in channel  12  and boundaries  13  that laterally confine light to propagate in the medium of the channel  12 . In one embodiment, the channel  12  is formed of a group III-V semiconductor and lateral boundaries  13  are formed of one or more dielectric layers. The dielectric layers have an index of refraction that is lower than that of the semiconductor of the channel  12 . Thus, the lateral layers confine light to propagate along the channel  12  by total internal reflection. In some embodiments, channel  12  has a lateral width that varies with distance from the input  14 .  
         [0017]    [0017]FIG. 1C shows one embodiment of structure  8  of FIGS. 1A-1B in which channel  12  is a group III-V semiconductor medium and lateral boundaries  13  are periodic arrays of identical scattering objects  20  located in the same semiconductor medium. Thus, the boundaries  13  form a PBG structure that is interrupted by the channel  12 , i.e., channel  12  is free of the objects  20 . The objects  20  have a different index of refraction than the semiconductor medium. Exemplary objects  20  include cylindrical holes that transverse planar structure  8  and inclusions in the planar structure  8 .  
         [0018]    The PGB structure laterally confines light to propagate in channel  12  by coherent diffraction from the array of objects  20 . Furthermore, the PBG structure restricts light propagation in channel  12  to a selected wavelength range. Light in other wavelength ranges reflects off side faces of planar structure  8  instead of entering into the channel  12 .  
         [0019]    Referring to FIGS. 1A-1C, filter  18  includes an array of regularly spaced identical objects  24  and a defect  26  in the array. The objects  24  have an index of refraction that differs from the index of refraction of the surrounding medium and thus, scatter light propagating in channel  12 . Exemplary objects  24  include inclusions or holes that traverse planar structure  8 . An exemplary defect  26  is a larger or smaller separation between two sequentially adjacent objects  24  in channel  12  than the separation between other sequentially adjacent objects  24  of the filter  18 . Another exemplary defect  26  is one object  24  that is larger or smaller than the other objects  24  of the filter  18 .  
         [0020]    Defect  26  separates the array of objects  24  into two smaller arrays  22 ,  23  that are sequentially adjacent in channel  12 . Exemplary separations between arrays  22 ,  23  are about ¼ to 2 times the wavelength of light propagating in channel  12 . Together the defect  26  and the smaller arrays  22 ,  23  function like a resonant optical cavity for light propagating in channel  12 . The smaller arrays  22 ,  23  are distributed reflectors for the resonant cavity. The resonant cavity allows a narrow range of wavelengths of light to be transmitted and thus, functions as a band-pass filter  18 .  
         [0021]    [0021]FIG. 1D illustrates how the spectral reflectivities (or transmissivities) of filters  18 , shown in FIGS. 1A-1C, depend on the control light intensities introduced into the filters  18 . The reflectivities depend on the control light intensity, because the filters  18  are made in a nonlinear optical medium whose index of refraction depends on light intensities therein.  
         [0022]    [0022]FIG. 1D shows that control light intensities A and B produce center wavelengths λ and λ′, respectively, for the passband of filters  18 . Control light of intensity A puts the filter  18  in a state that transmits wavelengths propagating in channel  12 . Control light of intensity B shifts the filter&#39;s center wavelength so that light of the original wavelength λ is now outside the filter&#39;s passband. Thus, control light of intensity B puts the filter  18  in a state that reflects light propagating in the channel  12 .  
         [0023]    The size of the shift, i.e., |λ-λ′|, to the center wavelength of the filter  18  depends on the nonlinear optical medium, the wavelength of the control light, and the intensity change to the control light. For a semiconductor medium, wavelengths that correspond to energies above the bandgap produce electron and hole carrier densities and thus, cause relatively larger shifts to refractive indexes and to the center wavelength of the filters constructed in such mediums. Nevertheless, even for the optically responsive III-V semiconductors, moderate changes in the intensity of the control light only generate about a 1 percent shift to the index of refraction and a comparable shift to the center wavelength of a filter constructed in such a medium. For a 1 percent shift in refractive index, the reflectivity needs a Q of about 100 or more if shifted and unshifted passbands of filter  18  are to not significantly overlap. Herein, Q is the inverse of the full width of the reflectivity curve at half maximum.  
         [0024]    To produce a 1 percent refraction index shift in a semiconductor medium formed of Ga, As, In, and P, control light is chosen to have a wavelength whose energy is close to that of the electronic bandgap of the Ga, As, In, and P based semiconductor, e.g., the energy is equal to about 0.1 and 0.5 electron volts (eV) plus the energy of the electronic bandgap. This choice for the wavelength enables absorption of most of control light in the interior of a thin channel  12 , a channel whose thickness is about 0.3 to 1.5 microns. Such a channel is adapted to use in a structure  8  that switches wavelengths of about 1.55 microns-wavelengths that correspond to energies slightly below the electronic bandgap.  
         [0025]    For filter  18  of FIGS. 1A-1C to produce an optical switch, the change in refractive index produced by control light, should switch the filter  18  between reflective and transmissive states for light propagating in channel  12 . For available center wavelength shifts of about 1 percent, such state change to the filter  18  usually requires that the filter  18  have a high Q. The filter  18  can have a high Q if both arrays  22 ,  23  have 3-5 or more objects  24  serially spaced along channel  12  and if separations between serially adjacent objects in the arrays  22 ,  23  is about ¼ to 2 times the filter&#39;s center wavelength.  
         [0026]    [0026]FIG. 1E is a flow chart for a method  30  of operating the integrated optical switches of FIGS. 1A-1C. At an initial time, a request to block optical transmissions through the switch&#39;s channel  12 , is received by a switch controller (step  32 ). In response to the request, the controller adjusts the control light intensity in filter  18  to cause filter  18  to reflect light propagating in the channel  12  (step  34 ). The control light is introduced into the filter  18  through transparent control access window  19  of planar structure  8 . In response to the control light intensity, the filter&#39;s spectral response shifts so that the filter  18  is in the reflective state described above. While maintaining the same control light intensity, an input optical signal is received at optical input  14  of the channel  12  (step  36 ). In response to the control light, the filter  18  reflects the input optical signal back towards the input  14  (step  38 ). At a later time, a new request to transmit optical transmissions through the channel  12  is received by the controller (step  40 ). In response to the new request, the controller readjusts the control light intensity to a new value that causes the filter  12  to transmit light propagating in the channel  12  (step  42 ). While maintaining the intensity of the control light, an input optical signal is received at an optical input  14  of the channel  12  (step  44 ). In response to the new control light intensity, the channel  12  transmits the input optical light through the filter  12  to an output  16  of the channel  12  (step  46 ).  
         [0027]    The optically controllable filter  18  and channel  12  of FIGS. 1A-1C can be used to make more complex optical switches.  
         [0028]    [0028]FIG. 2 illustrates a 1×2 optical switch  50  that includes a single input optical channel  52  and a pair of output optical channels  54 ,  56 . Exemplary channels  52 ,  54 ,  56  are located in a 2D PBG structure (not shown). The 2D PBG structure is itself located in a planar structure  58  analogous to structure  8  of FIGS. 1A-1C. The PBG structure includes a 2D periodic array of identical objects whose index of refraction differ from that of the nonlinear optical media of planar structure  58 , e.g., the objects may be holes through the planar structure  58 . The channels  52 ,  54 ,  56  interrupt the 2D array of the PBG structure. The PBG structure coherently diffracts light received at input  51  thereby causing a range of wavelengths of the input light to propagate along the channels  52 ,  54 ,  56 . The light propagates down input channel  52  and thereafter amplitude splits to propagate along output channels  54  and  56 . The planar structure  58  confines light propagation in the direction normal to the structure&#39;s plane by total internal reflection.  
         [0029]    The 1×2 switch also includes arrays  64 ,  66  of objects in specific regions of both output channels  54 ,  56 . The arrays  64 ,  66  function as resonant optical cavities analogous to the array of objects  24 , shown in FIGS. 1A-1C. The arrays  64 ,  66  function as optical filters with optically controllable transmissivities to light propagating in associated channels  54  and  56 . The arrays  64 ,  66  are independently controlled by the intensities of control light beams introduced into the arrays  64 ,  66  via transparent control access windows  67 ,  68  in the top surface of planar structure  58 . The intensities of control light beams switch arrays  64 ,  66  between transmissive and reflective states to selectively switch input light to output channels  54 ,  56 .  
         [0030]    In particular, introducing a selected control light intensity at a wavelength slightly above the bandgap of the medium (e.g., 1.2-1.3 μm for some crystalline semiconductors formed of Ga, As, In, and P) into window  67  changes the refractive index in array  64 . The new refractive index causes the array  64  to reflect light received from input channel  52  into channels  52 ,  56  and to stop light from propagating into channel  54 . Some embodiments position one or more optical scattering objects  59 , e.g., holes, near the intersection between channels  54  and  56  to increase the percentage of the reflected light that ends up in the output channel  56 .  
         [0031]    A control light beam can also be introduced into window  68  to cause array  66  to become reflective to light received from input channel  52 . Then, light is reflected by the array  66  into channels  52 ,  54  instead of propagating through channel  56 .  
         [0032]    [0032]FIG. 3 is a top view of a planar structure  70  that functions as a 1×N optical switch. The structure  70  includes an input optical channel  72  and N output optical channels  74   1 - 74   N . Each output optical channel  74   1 - 74   N  includes a filter  76   1 - 76   N  that is controllable by an independent control light beam and is analogous to filter  18  of FIGS. 1A-1C. The intensities of individual control light beams determine whether the associated filters  76   1 - 76   N  are transmissive or reflective to light in the associated channels  76   1 - 76   N  and thereby control routing of an optical signal from input channel  72  through the switch.  
         [0033]    Exemplary switches are the planar structure  70  with an embedded 2D PBG structure. The optical scattering objects of the PBG structure, e.g., holes traversing the planar waveguide, are absent from channels  74   1 - 74   N .  
         [0034]    Each filter  76   1 - 76   N  may be formed of an array of objects analogous to the array used in filter  18  of FIGS. 1A-1C. Accordingly, the presence and/or absence of control light beams associated with individual filters  76   1 - 76   N  will control the propagation of an input light through the output channels  76   1 - 76   N . For example, when all control light beams are “off”, each channel a filter  76   1 - 76   N  exhibits essentially identical propagation characteristics and input light will propagate to each output channel  76   1 - 76   N . Alternatively, if N−1 control light beams are activated, the N−1 output channels  76   1 - 76   N  associated with the control light beams become reflective to input light. Then, these channels  76   1 - 76   N  reflect the input light back along input optical channel  72  and the remaining output optical channel. Thus, for this configuration, only one output channel allows the input optical signal to propagate therethrough.  
         [0035]    In other embodiments of structure  70 , additional optical filters (not shown) are positioned along locations on output channel  74   N  between intersections with the other output channels  76   1 - 76   N−1  The additional filters are similar to filter  18  of FIGS. 1A-1C and operated by separate control light beams to provide more routing control.  
         [0036]    In the above-described switches, switching speeds are a sum of turn-on and turn-off times. The switching times depend both on properties of the bulk medium, e.g., properties of III-V semiconductors, and on the forms of the arrays of objects  24  and defects  26  used in the controllable optical filters  18 ,  64 ,  66 ,  76   1 - 76   N . More particularly, carrier densities of electrons and holes induced by the control light beams fix the refractive indexes in the controllable optical filters  18 ,  64 ,  66 ,  76   1 - 76   N . Thus, the turn-on times are only limited by the time needed to generate the control optical pulses that produce the needed carrier densities. On the other hand, turn-off times are limited by the times needed to recombine the same electron and hole carriers. Carrier recombination rates depend both on intrinsic properties of the bulk medium and on surface processes. Surface contributions to recombination rates depend both on surface areas and on surface properties, i.e., for surfaces of the objects  24  making up the controllable filters  18 ,  64 ,  66 ,  76   1 - 76   N . The greater the number of objects  24  in a filter  18 ,  64 ,  66 ,  76   1 - 76   N , the greater the actual surface contribution to recombination and the shorter the turn off time becomes. Nevertheless, shortening the turn-off time produces a proportional increase in the power that must be supplied by the optical control beam to maintain the switch in the same state. Thus, switching speeds will be limited by the power budget available to maintain the switch in a particular configuration.  
         [0037]    Various embodiments of the optical switches shown in FIGS. 1A-1C,  2 , and  3  include integrated optical amplifiers. The amplifiers provide gain that offsets signal attenuation caused by signal propagation through a semiconductor media that has injected charge carriers. Exemplary amplifiers include electrical contacts positioned across the semiconductor media of the output channels and voltage sources connected to the contacts. The voltage sources pump carrier densities in the output channels, and the pumped carrier densities amplify output signals through stimulated emission. The use of such amplifiers to offset attenuation is known to those of skill in the art.  
         [0038]    Other embodiments of the invention will be apparent to those skilled in the art in light of the specification, drawings, and claims of this application.