Abstract:
An optical device includes at least two photonic bandgap crystal (PBG) stacks that are each comprised of alternating layers of high and low index materials. A defect region is formed in a cavity region between the at least two photonic bandgap crystal stacks so as to provide the properties needed to reflect light received by the optical device.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to the field of photonic bandgap crystals, and in particular to forming compact and simple devices using photonic bandgap crystals. 
   A photonic bandgap crystal (PBG) has been widely investigated recently due to its unique property. It is well known that by employing a PBG structure, high reflection is easily achieved by sandwiching a defect layer between two PBG stack layers. It is possible to form a cavity mode with a high Q (quality factor) using a PBG structure, where a specific wavelength can be transmitted and other wavelengths are reflected. There have been various optical devices formed using PBG structures in integrated optics, but most of these devices are limited to two-dimensional optical waveguide structures by which light can be guided along a waveguide or optical fiber. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the invention, there is provided an optical device. The optical device includes at least two photonic bandgap crystal (PBG) stacks that are each comprised of alternating layers of high and low index materials. A defect region is formed in a cavity region between the at least two photonic bandgap crystal stacks so as to provide the properties needed to reflect light received by the optical device. 
   According to another aspect of the invention, there is provided a method of forming an optical device. The method includes forming at least two photonic bandgap crystal (PBG) stacks that are each comprised of alternating layers of high and low index materials. A defect region is formed in a cavity region between the at least two photonic bandgap crystal stacks so as to provide the properties needed to reflect light received by the optical device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a schematic diagram of a modulator in accordance with the invention;  FIG. 1B  is a graph demonstrating the properties of the inventive modulator;  FIG. 1C  is a graph demonstrating the properties of the inventive modulator when the wavelength matches the defect mode; 
       FIG. 2  is a schematic diagram of an optical amplifier in accordance with the invention; 
       FIG. 3  is a schematic diagram of a de-multiplexer in accordance with the invention; 
       FIG. 4  is a schematic diagram of a time domain multiplexer in accordance with the invention; 
       FIGS. 5A-5F  is a schematic diagram illustrating the steps associated with forming a PBG structure; 
       FIG. 6  is a schematic diagram demonstrating the top view of a PBG device. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The invention utilizes PBG structures to form three dimensional device structures, such as modulators, amplifiers, and multiplexers. Given the apparent limitations of the present state of the technology, forming three dimensional PBG structures greatly enhances the ability to use optical technology. Moreover, the size of a standard PBG structure will allow one to incorporate this technology in integrated systems without hindering the performance of such systems. 
     FIG. 1A  shows a modulator  2  that includes a PBG structure  4  that is integrated into an optical waveguide  6  by placing the PBG structure  4  perpendicular to the optical waveguide  6 . A defect layer  8  is sandwiched-between two PBG stacks  10 ,  12  on both sides. Moreover, the defect layer  8  having a λ/2n thickness and made from electro-optic materials or non-linear optical materials, where n is refractive index of the materials. Note that the PBG stacks  10 ,  12  are comprised of alternating layers of high  14  and low  16  index materials. The high  14  and low  16  index layers each have a thickness of λ/4n. In addition, the high  14  and low  16  index materials can be comprised of materials, such as Si and SiO2, respectively, or Si and SiN. In this embodiment, each PBG stack  10 ,  12  have three sets of alternating high and low index layers, however, in other embodiments that number can vary. 
   By changing the refractive index of the defect layer by applying bias or the like, this device works as a modulator or a switch having a defect mode, as is shown in  FIG. 1B . If the wavelength matches the defect mode as shown in  FIG. 1C , the light can go through the PBG structures without significant loss. By changing the refractive index of the defect layer slightly by applying a bias, the defect mode shifts, and the light is blocked by a PBG stop band. 
   An optical amplifier can be formed using a PBG structure  20  that is perpendicular to an optical waveguide  28 , as shown in  FIG. 2 . A defect layer  22  is sandwiched between two PBG stacks  24 ,  26  on both of its sides. Also, the defect layer  22  has a thickness of λ/2n and doped with amplifying material, such as Erbium or the like. The PBG stacks  24 ,  26  comprise alternating layers of high  30  and low  32  index materials. The high  30  and low  32  index layers have a thickness of λ/4n, and can be comprised of Si and SiO2, respectively, or Si and SiN. 
   A PBG structure usually has a high quality factor (Q). The high amplifying effect is achieved since the path length of the light can be expressed by the “quality factor* λ/2”. If it is assumed that the layers of the PBG structures used in this embodiment have a thickness of 0.5 mm and Q is 1,000, then the actual path length will be 0.5 μm*1,000=500 μm. Therefore, a long amplifying path length using a compact device can be achieved. 
   In forming a de-multiplexer (demux) device  40 , a selective number of PBG structures  42 ,  44 , and  46  are placed at an angle to their respective input  48 ,  50 , and  52  and output waveguides  70 ,  72 ,  74 , and  54 , as shown in  FIG. 3 . Note that input signal  59  is comprised of wavelengths  62 ,  64 ,  66 , and  68 . Each PBG structure has a defect layer whose thickness is λ s /2n, where λ s  varies depending on its target wavelength and n is the index of the materials used in the defect layers. Moreover, each PBG structure  42 ,  44 , and  46  has a different thickness to pick up different specific wavelengths  62 ,  64 ,  66 , and  68 . Therefore, a wavelength  62 ,  64 ,  66 , or  68  which matches the defect mode of the PBG structure  62 ,  64 ,  66 , or  68  can pass through and be outputted to an output waveguide  70 ,  72 ,  74 , or  54 . The other wavelengths are reflected and guided into an input waveguide  50  or  52 , thus allowing light with various wavelengths to be de-multiplexed. 
   In forming a time division multiplexer (TDM) device  80 , several PBG structures  82 ,  84 , and  86  are placed at a tilted angle, in this case 45 degrees, between their respective input  88 ,  90 , and  92  and output  94 ,  96 ,  98 , and  100  optical waveguides, as shown in  FIG. 4 . Note that the input signal  105  is comprised of wavelengths  108 ,  110 ,  112 , and  114 . Each PBG structure  82 ,  84 , and  86  includes a reflected waveguide  102 ,  104 , and  106  that guides light that has been reflected from the PBG structure. The reflected waveguides  102 ,  104 , and  106  act as an input waveguide to a successive PBG structure by providing the reflected light as input. 
   Moreover, each PBG structure  82 ,  84 , and  86  has a defect layer  116 ,  118 , and  120  whose thickness is λ/2n, where n is the index of the materials used in the defect layers  116 ,  118 , and  120 , respectively. The difference between the PBG structures  42 ,  44 , and  46  described in the demux device and that used in the TDM device is that all the PBG structures  82 ,  84 , and  86  have defect layers that are similarly sized. In this embodiment, each defect layer  116 ,  118 , and  120  is made either from electro-optic material or non-linear optical material. When the switch is on or bias is applied to the defect layer, the light that is reflected goes to the next PBG structure by way of its reflected waveguide  102 ,  104 , and  106 . By applying electric bias to each PBG structure  102 ,  104 , and  106 , a light signal is distributed. 
     FIG. 5A-5H  shows the steps taken to form the PBG structures described herein.  FIG. 5A  shows the initial construction of the waveguide structure  130  that is to be coupled to a PBG structure. The waveguide structure  130  includes two cladding regions  132 ,  136  and a core  138 , and is formed on a substrate  140 , such as Si or the like. Moreover, the core  138  can be comprised of high index materials, such as Si, SiN, or SiON. The cladding regions  132 ,  136  can be comprised of SiO 2  or the like. Afterwards, a deep etch process is used to etch a certain section of the waveguide structure  132  to form an air hole  142 , as shown in  FIG. 5B . Note that the formation of the air hole  142  has created two distinct waveguide portions  144 ,  146 . These portions  144 ,  146  can be used to form an input and output waveguide to be coupled with a PBG structure. Standard techniques for deep etching can be used to form the air hole  142 . 
     FIG. 5C  shows the deposition of a high index  148  and low index  150  layers to form a PBG stack  152 . The high  148  and low index  150  layers are deposited using chemical vapor deposition (CVD) to stack the high  148  and low index  150  layers where low pressure chemical deposition (LPCVD) is preferred since LPCVD usually has better step coverage. After making the PBG stack  152 , a defect layer  154  is formed by depositing in the cavity region  156  of the PBG stack  152  the necessary materials using a CVD technique, such as LPCVD or the like, as shown in  FIG. 5D . If necessary, planarization can be used to remove the excess layers of the PBG stack  152  and defect layer  156 , as shown in  FIG. 5E . If necessary, it is possible to add a top PBG layer  158  after planarizing the surface to achieve better light confinement, as shown in  FIG. 5F . By employing this fabrication steps, we don&#39;t have to use fine lithography system which is usually quite expensive and leads to high production cost. Only the air hole is required to realize these PBG devices. 
     FIG. 6  shows the top view of the inventive PBG device  160 . The PBG device  160  includes high index layers  162 , low index layers  164 , and a defect layer  166 . Moreover, the PBG device  160  is coupled to a waveguide  168 . The sidewalls  170 ,  172  of the PBG device  160  are also fabricated with the PBG layers  162 ,  164 . In this arrangement, light is confined laterally. 
   The present invention makes it possible to obtain compact and simple devices based on the same simple structure. These devices can be integrated with an optical waveguide using CMOS compatible processes. The ease of using the invention to make PBG structures without significant cost provides a clear advantage over other prior art techniques. 
   Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.