Abstract:
An optical splitter, a combiner and a device. The optical splitter comprises a first longitudinal waveguide for receiving an incoming light wave; at least first and second pairs of output waveguides, the output waveguides of each pair being disposed on opposite sides of the first waveguide; wherein each of the output waveguides of each pair comprises a longitudinal portion disposed parallel to the first waveguide and such that optical power is coupled from the first waveguide into the respective longitudinal portions and the longitudinal portions of output waveguides of the first and second pairs are displaced along a length of the first waveguide; wherein each of the output waveguides of each pair further comprises a substantially S-shaped portion continuing from the respective longitudinal portions and such that optical power coupling between the respective S-shaped portions of output waveguides of the first and second pairs is substantially inhibited.

Description:
FIELD OF INVENTION 
     The present invention relates broadly to an optical splitter, to an optical combiner, and to an optical device. 
     BACKGROUND 
     In recent years, integrated optical devices have received tremendous attention as they provide low cost, high density and high data rate devices for a wide range of applications. Integrated optical devices have also been increasingly used for sensing applications because of their inherent characteristics such as high sensitivity, miniaturisation, mechanical stability and immunity to electromagnetic interference. 
     Optical device fabrication processes, which adopt planar CMOS technology from the mature microelectronics industry, offer ease of manufacturing and the possibility of simultaneous detection of several analytes on a single chip. Various methods have been implemented for the design of integrated optical biosensors, such as waveguide surface plasmon resonance, waveguide grating coupler, Mach-Zehnder and nanocavity. 
     For biosensing applications, a number of sensor configurations have been proposed, including Mach-Zehnder interferometer (MZI) and microcavity sensors. 
     However, most existing proposals are limited to single sensing channels. A multiple sensor device with multiple sensing channels based on such proposals typically requires one light source for each sensing channel. 
     In realising multiple sensing channel devices using only a single source, existing optical combiner/splitter structures have a number of disadvantages. For example, in one configuration, arrays of interconnected 3-dB couplers may be used to realise n×N couplers. Such a configuration suffers from disadvantages such as high optical losses and an accumulative size of the overall device. 
     Alternatively, interference based integrated n×N star couplers have been proposed. However, such couplers suffer from high optical losses and complexity of manufacturing. 
     A need therefore exists to provide an optical combiner/splitter structure that seeks to address at least one of the above disadvantages. 
     On the other hand, a need also exists to provide an active optical component for use in optical devices such as sensors or communication devices, the active optical component having improved performance in terms of at least one of attenuation and/or driving power. 
     SUMMARY 
     In accordance with a first aspect of the present invention there is provided a optical splitter comprising a first longitudinal waveguide for receiving an incoming light wave; at least first and second pairs of output waveguides, the output waveguides of each pair being disposed on opposite sides of the first waveguide; wherein each of the output waveguides of each pair comprises a longitudinal portion disposed parallel to the first waveguide and such that optical power is coupled from the first waveguide into the respective longitudinal portions and the longitudinal portions of output waveguides of the first and second pairs are displaced along a length of the first waveguide; wherein each of the output waveguides of each pair further comprises a substantially S-shaped portion continuing from the respective longitudinal portions and such that optical power coupling between the respective S-shaped portions of output waveguides of the first and second pairs is substantially inhibited. 
     The longitudinal portions of the output waveguides of each pair may be disposed at substantially a same distance on the opposite sides of the first way guide. 
     The longitudinal portions of the output waveguides of both pairs may be disposed at substantially the same distance on the opposite sides of the first waveguide. 
     A radius of the S-shaped portions of the output waveguides of each pair may be chosen such that optical transmission losses are reduced compared to an angled alignment of waveguide portions. 
     The optical splitter may be arranged for coupling substantially the same optical power from the light wave into the output waveguides. 
     Substantially all of an input power of the light wave may be coupled into the output waveguides. 
     In accordance with a second aspect of the present invention there is provided an optical combiner comprising a first longitudinal waveguide; at least first and second pairs of input waveguides, the input waveguides of each pair being disposed on opposite sides of the first waveguide; wherein each of the input waveguides of each pair comprises a longitudinal portion disposed parallel to the first waveguide and such that optical power is coupled from the respective longitudinal portions into the first waveguide and the longitudinal portions of input waveguides of the first and second pairs are displaced along a length of the first waveguide; wherein each of the input waveguides of each pair further comprises a substantially S-shaped portion continuing from the respective longitudinal portions for receiving respective incoming light waves and such that optical power coupling between the respective S-shaped portions of input waveguides of the first and second pairs is substantially inhibited. 
     The longitudinal portions of the input waveguides of each pair may be disposed at substantially a same distance on the opposite sides of the first way guide. 
     The longitudinal portions of the input waveguides both pairs may be disposed at substantially the same distance on the opposite sides of the first waveguide. 
     A radius of the S-shaped portions of the input waveguides of each pair may be chosen such that optical transmission losses are reduced compared to an angled alignment of waveguide portions. 
     Substantially all of respective input powers of the respective light waves may be coupled into the first waveguide. 
     In accordance with a third aspect of the present invention there is provided a n optical device comprising a waveguide formed on a substrate, the waveguide comprising a photonic bandgap structure including an optical cavity region; a highly doped cathode region formed along the waveguide and adjacent one side of the optical cavity region of the waveguide; a highly doped anode region formed along the waveguide and adjacent an opposite side of the optical cavity region of the waveguide; wherein the cathode region, the optical cavity region and the anode region form a p-i-n diode structure for controlling an attenuation characteristic of the photonic bandgap structure. 
     In accordance with a fourth aspect of the present invention there is provided a optical device comprising an optical splitter comprising a first longitudinal waveguide for receiving an incoming light wave, at least first and second pairs of output waveguides, the output waveguides of each pair being disposed on opposite sides of the first waveguide, wherein each of the output waveguides of each pair comprises a longitudinal portion disposed parallel to the first waveguide and such that optical power is coupled from the first waveguide into the respective longitudinal portions, and the longitudinal portions of output waveguides of the first and second pairs are displaced along a length of the first waveguide, wherein each of the output waveguides of each pair further comprises a substantially S-shaped portion continuing from the respective longitudinal portions and such that optical power coupling between the respective S-shaped portions of output waveguides of the first and second pairs is substantially inhibited; and a photonic bandgap structure optically connected to each output waveguide of the splitter, each photonic bandgap structure including a microcavity. 
     The optical device may further comprise an optical combiner comprising a first longitudinal waveguide, at least first and second pairs of input waveguides, the input waveguides of each pair being disposed on opposite sides of the first waveguide, wherein each of the input waveguides of each pair comprises a longitudinal portion disposed parallel to the first waveguide and such that optical power is coupled from the respective longitudinal portions into the first waveguide and the longitudinal portions of input waveguides of the first and second pairs are displaced along a length of the first waveguide, wherein each of the input waveguides of each pair further comprises a substantially S-shaped portion continuing from the respective longitudinal portions for receiving respective incoming light waves and such that optical power coupling between the respective S-shaped portions of input waveguides of the first and second pairs is substantially inhibited; and wherein the photonic bandgap structures are formed in respective optical connections between the output waveguides of the splitter and the input waveguides of the combiner. 
     In accordance with a fifth aspect of the present invention there is provided a optical device comprising an optical combiner comprising a first longitudinal waveguide, at least first and second pairs of input waveguides, the input waveguides of each pair being disposed on opposite sides of the first waveguide, wherein each of the input waveguides of each pair comprises a longitudinal portion disposed parallel to the first waveguide and such that optical power is coupled from the respective longitudinal portions into the first waveguide and the longitudinal portions of input waveguides of the first and second pairs are displaced along a length of the first waveguide, wherein each of the input waveguides of each pair further comprises a substantially S-shaped portion continuing from the respective longitudinal portions for receiving respective incoming light waves and such that optical power coupling between the respective S-shaped portions of input waveguides of the first and second pairs is substantially inhibited; and a photonic bandgap structure optically connected to each input waveguide of the combiner, each photonic bandgap structure including a microcavity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings, in which: 
         FIG. 1  shows a schematic diagram of a passive device built in accordance with one embodiment of the invention. 
         FIG. 2  shows a schematic diagram of an optical splitter. 
         FIG. 3  shows a plot of normalised power against cT. 
         FIG. 4  shows a cross sectional view of a photonic bandgap structure. 
         FIG. 5  shows a plot of transmission and Q factor. 
         FIG. 6  shows a table summarising Q factor and transmission values. 
         FIG. 7  shows a schematic diagram of an optical combiner. 
         FIG. 8  shows a plot of normalised power against cT 
         FIG. 9  shows a schematic diagram of an optical demultiplexer built in accordance with one embodiment of the invention. 
         FIG. 10  shows a schematic diagram of an optical multiplexer built in accordance with one embodiment of the invention. 
         FIG. 11  shows a schematic diagram of an active device built in accordance with one embodiment of the invention. 
         FIG. 12  shows across sectional view of a microcavity of an active device. 
         FIG. 13  shows a plot of drive voltage against drive current and a respective change in refractive index, Δn. 
         FIG. 14  shows a plot of attenuation against drive current. 
         FIG. 15  shows a plot of the change in refractive index, Δn, against transient time for an active diode. 
         FIG. 16  shows a schematic diagram of an active phase modulator MZI built in accordance with one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a schematic diagram of a passive sensor device  100  built in accordance with one embodiment of the invention. 
     The passive sensor device  100  has an optical splitter  120  at the input end and an optical combiner  180  at the output end. 
     The passive sensor device  100  shown in  FIG. 1  is of the 1×4 splitter configuration, where the passive sensor device  100  comprises a central waveguide  102  and four waveguides  104 ,  106 ,  108  and  110 . A pair of the waveguides  104 ,  106 ,  108  and  110  is disposed on each side of the central waveguide  102 . A passive photonic bandgap structure  140  is formed in each of the four waveguides  104 ,  106 ,  108  and  110  between the optical splitter  120  and the optical combiner  180 . 
     One advantage of using the passive sensor device  100  is that only one light source (broad-band or tunable laser) and one detector is required, whereas conventional devices require individual light sources and detectors for each arm. As such, the passive sensor device  100  arrangement can provide cost savings. 
     The passive sensor device  100  has a 1×4 splitter configuration. However, it is possible to realise a 1×N splitter configuration with the appropriate design consideration, in different embodiments. 
     In one embodiment, the passive sensor device  100  has a semiconductor on insulator (SOI) structure, where silicon Si is used for the waveguides  102 ,  104 ,  106 ,  108  and  110 . The waveguides  102 ,  104 ,  106 ,  108  and  110  are formed on an insulating SiO 2  layer which is formed on a Si substrate. 
     The form of the waveguides  102 ,  104 ,  106 ,  108  and  110  depends on the manufacturing material and technology and can consist of any one or more of the following shapes: ridge, channel and/or strip. 
       FIG. 2  shows a schematic diagram of the optical splitter  120  of  FIG. 1 . 
     In the context of  FIG. 2 , the four waveguides  104 ,  106 ,  108  and  110  are more commonly referred to as splitter waveguides. The four splitter waveguides  104 ,  106 ,  108  and  110  have an S-bend arrangement in their respective regions  104   a ,  106   a ,  108   a  and  110   a  approaching the central waveguide  102 . The radii of the S-bends, for instance R and R 1  of the splitter waveguides  104  and  106  respectively, are designed for minimum optical power loss, which is dependent on the splitter waveguides width w and the refractive index contrast ΔN of the splitter waveguides material. 
     The S-bend arrangement provides several advantages over a waveguide that has a straight alignment. For instance, the S-bend arrangement achieves an optical splitter with a more narrow width, thus producing an overall more compact device. Also, light travelling through the waveguide with an angled arrangement of straight waveguide portions has greater transmission loss compared to the S-bend arrangement in the example embodiment. 
     Substantially straight waveguides (for instance portions L 2  and L 1  of the splitter waveguides  108  and  110  respectively) are incorporated at the beginning of each of the S-bend arrangements  104   a ,  106   a ,  108   a  and  110   a  to couple optical power from the central waveguide  102  into the splitter waveguides  104 ,  106 ,  108  and  110 . Optical coupling strength gradually decreases from the coupling regions  102   a  and  102   b  towards the output ends of the splitter waveguides  104 ,  106 ,  108  and  110  of the optical splitter  120 . 
     The input ends of the four splitter waveguides  104 ,  106 ,  108  and  110  are separated a distance Lgap, from the central waveguide  102 . On the other hand, a distance Lsep separates the output ends of each adjacent waveguide in the optical splitter  120 . The end separation distance Lsep is selected to fulfill the criteria of zero power transfer between adjacent waveguides. 
     In operation, optical powers  112  with broad wavelengths are injected into the central waveguide  102 . The coupling regions  102   a  and  102   b  is such that optical energy will be split substantially equally between each of the four splitter waveguides  104 ,  106 ,  108  and  110 . The amplitude and phase of the optical field in each of the four splitter waveguides  104 ,  106 ,  108  and  110  is influenced by several waveguide parameters at the coupling regions  102   a  and  102   b . These waveguide parameters include the waveguide width w, gap Lgap, height (not shown), refractive index difference (not shown) and the interactive length L i  of the four splitter waveguides  104 ,  106 ,  108  and  110 , where i is an integer. 
     The process of manufacturing the optical splitter  120  in one example embodiment utilises recticle (masks) writing for the splitter pattern using an electron-beam pattern generator. The written pattern is then transferred through photolithography. A SOI wafer is first coated with photoresist, followed by the transfer of the pattern through photolithograpy. Anisotropic etching is performed to remove silicon in the exposed areas, hence creating the Si waveguides of the optical splitter  120  on the wafer. This is followed by stripping the remaining resist from the wafer. 
       FIG. 3  shows a plot  300  of normalised power against cT, where cT is the unit of time used in the computational calculation, where c is the speed of light in vacuum 
     The plot  300  illustrates simulation results of the normalised power in each of the four splitter waveguides  104 ,  106 ,  108  and  110  ( FIG. 2 ) of the optical splitter  120  ( FIG. 2 ) having a silicon-on-insulator (SOI) structure with the following parameters: 
     w=500 nm, si-overlayer=220 nm, Lgap=0.1 μm, L 1 =3.1 μm, L 2 =8 μm, Lsep=0.7 μm 
     The Finite Differential Time Domain (FDTD) Method was used to observe the power transfer effect occurring in the optical splitter  120  ( FIG. 2 ). This method is useful for analysis of small cross sectional wave-guiding structures. First, modal analysis is used to solve the fundamental mode in a photonic wire. This fundamental mode is then used as a launch file to solve for the optical splitter structure in question. This way, the loss of the structure in question is computed due to device configuration and not the loss inherent by the waveguide structure caused by modal mismatch. 
     Numerals  302  and  304  indicate overlapping curves for the simulation results obtained from the splitter waveguides  104 ,  110  and  106 ,  108  ( FIG. 2 ) respectively. In other words, the curves for the pair  306  of waveguides  104 ,  110  ( FIG. 2 ) overlap, and the curves for the pair  308  of waveguides  106 ,  108  ( FIG. 2 ) overlap.  FIG. 3  illustrates that each of the four splitter waveguides  104 ,  106 ,  108  and  110  ( FIG. 2 ) has an output normalised power of approximately 0.22. The result obtained agrees well with calculation based on parallel coupling using coupled mode theory. 
     Coupled mode theory specifies that when two waveguides are brought close together, optical modes of each waveguides either couple or interfere with each other. The coupled mode equations are given by: 
     
       
         
           
             
               
                 
                   
                     
                       ⅆ 
                       A 
                     
                     
                       ⅆ 
                       z 
                     
                   
                   = 
                   
                     
                       - 
                       j 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       k 
                       12 
                     
                     ⁢ 
                     B 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       exp 
                       ⁡ 
                       
                         [ 
                         
                           
                             - 
                             
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                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     β 
                                     2 
                                   
                                   - 
                                   
                                     β 
                                     1 
                                   
                                 
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                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       ⅆ 
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                       z 
                     
                   
                   = 
                   
                     
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                       j 
                     
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       k 
                       21 
                     
                     ⁢ 
                     A 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       exp 
                       ⁡ 
                       
                         [ 
                         
                           
                             - 
                             
                               j 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     β 
                                     2 
                                   
                                   - 
                                   
                                     β 
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                           ⁢ 
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                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
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     where β is the propagation constant. The solutions, with the assumption of β 1  and β 2  are greater than zero, can be expressed as:
 
 A ( z )=└ a   1   e   −jqz   +a   2   e   −jqz ┘ exp(− jδz )  (3)
 
 B ( z )=└ b   1   e   −jqz   +b   2   e   −jqz ┘ exp( jδz )  (4)
 
     where q is an unknown parameter to be determined. Constants a 1 , a 2 , b 1 , and b 2  should satisfy the initial conditions:
 
 a   1   +a   2   =A (0)  (5)
 
 b   1   +b   2   =B (0)  (6)
 
     Substituting Eqs. (3) and (4) into Eqs. (1) and (2) and applying the initial conditions from Eqs. (5) and (6) yield the following equations: 
     
       
         
           
             
               
                 
                   
                     A 
                     ⁡ 
                     
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                   = 
                   
                     
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                                     ( 
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                     ⁢ 
                     
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                       } 
                     
                     ⁢ 
                     
                       exp 
                       ⁡ 
                       
                         ( 
                         
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                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     where q is given by q=√{square root over (κ 2 +δ 2 )} and k is the coupling coefficient. 
     If light is coupled into the adjacent waveguide only at z=0, this yields the conditions of A(0)=A 0  and B(0)=0. The optical power flow along the z-direction is given by: 
     
       
         
           
             
               
                 
                   
                     
                       P 
                       a 
                     
                     ⁡ 
                     
                       ( 
                       z 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
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                         2 
                       
                       
                         
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                             A 
                             o 
                           
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                         2 
                       
                     
                     = 
                     
                       1 
                       - 
                       
                         F 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           
                             sin 
                             2 
                           
                           ⁡ 
                           
                             ( 
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                   9 
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                         2 
                       
                       
                         
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                         ⁡ 
                         
                           ( 
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                   ( 
                   10 
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     where F is the maximum power-coupling efficiency, defined by 
     
       
         
           
             
               
                 
                   F 
                   = 
                   
                     
                       
                         ( 
                         
                           κ 
                           q 
                         
                         ) 
                       
                       2 
                     
                     = 
                     
                       1 
                       
                         1 
                         + 
                         
                           
                             ( 
                             
                               δ 
                               / 
                               κ 
                             
                             ) 
                           
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
       FIG. 4  shows a cross sectional view of one of the four passive photonic bandgap structures  140  of  FIG. 1 . 
     The passive photonic bandgap structure  140  has a SOI structure. The passive photonic bandgap structure  140  comprises a silicon layer  402  formed above an insulating SiO 2  layer  406 . 
     The silicon layer  402  has arrays of 1-D periodic air holes  404  formed in the silicon layer  402 , with a microcavity  410  breaking the periodicity of the airholes. The airholes that are immediately adjacent to the microcavity  410  are given the reference numeral  408 . 
     The microcavity  410  acts as a defect within the silicon layer  402  that causes a state that is analogous to the formation of a level within a semiconductor bandgap. In this manner, the silicon layer  402  acts as a photonic crystal waveguide with a defect in the mid section. The light confinement capability of the microcavity  410  induces a strong electric field in the microcavity  410  which is stronger than the electric field inside the remainder of the silicon layer  402 . By removing the cladding (not shown) above the microcavity  410 , the induced electric field of the microcavity  410  enhances the effective cross section of analytes that are present above the surface of the photonic bandgap device  100  ( FIG. 1 ) when the photonic bandgap device  100  ( FIG. 1 ) is used as a biosensor. Thus, when an analyte is present in the vicinity of the microcavity  410 , optical light being transmitted through the microcavity  140  becomes attenuated. A drop in the output power of the photonic bandgap device  100  ( FIG. 1 ) thus indicates that analytes are detected. As one microcavity  410  exists along each of the four splitter waveguides  104 ,  106 ,  108  and  110  ( FIG. 1 ), the passive sensor device  100  ( FIG. 1 ) can be applied, e.g. as a multiple sensing arm biosensor using only one light source and detector pair. By choosing each microcavity  410  to exhibit a different transmission wavelength, one broadband light source can be placed at the input side and a detector in the form of a spectrum analyser can be placed at the output side to monitor the response of the respective wavelengths along each respective sensing arm. 
     A Bragg mirror-like response is present on both sides  406   a  and  406   b  of the microcavity  410 . The air holes  408  that are immediately adjacent to the microcavity  410  have a different diameter than the diameter of the remaining air holes  404 . The air holes  408  are commonly referred to as the innermost air holes. Unlike conventional photonic crystal waveguides, where the air holes are etched only into the silicon layer, the air holes  404  and  408  are etched a depth within the SiO 2  layer  402 . This “etch-down” design improves the transmission property of the photonic crystal waveguide  402 . This “etch-down” design also prevents the wave travelling inside the silicon layer  402  from leaking into the SiO 2  layer  406 . In preventing wave leakage, the “etch-down” design was found as effective as the known “air-bridge” design, where in the “air-bridge” design an air gap is formed beneath the microcavity structure. However, the “etch-down” design is simpler to fabricate. 
     The formation of the airholes  404  and  408  are to limit the wave vector factor inside the photonic crystal waveguide  402 . The airholes  404  and  408  also produce an evanescent field that facilitates coupling of the waveguide modes in the photonic crystal waveguide to the microcavity  410 . 
     The symbols used to demarcate the various dimensions of the photonic crystal waveguide  402  are summarised as follows: a 0  is the length of the microcavity  410 ; a is the period of the air holes; d 0  is the diameter of the two air holes  408  immediately adjacent to the microcavity  410 ; d is the diameter of the remaining air holes  404 ; w is the width of the photonic crystal waveguide  402 ; t 1  is the thickness of the photonic crystal waveguide  402 ; t 2  is the etching depth of the SiO 2  layer  406 ; and h is the etching depth of the air holes  404  and  408 . 
     From running simulations using a 3-D FDTD package from CST Microwave Studio, it was found that an optimum etching depth h for the air holes  404  and  408  was 400 nm, which produced a Q factor of around 320, for a 0 =295 nm, a=180 nm, d 0 =80 nm, d=68 nm, w=475 nm, t 1 =400 nm and t 2 =400 nm. The Q factor is a measure of the strength of the relative linewidth. The 0 factor is the ratio of the resonance frequency, v and the (full width at half maximum) bandwidth dv of the resonance, i.e.
 
 Q  factor= v/dv   (12)
 
     Varying the etching depth h from 400 nm to 550 nm did not provide any obvious improvement, where the power transmission at resonance remained almost unchanged. 
     The transmission property for the photonic bandgap structure  140  was simulated using the 3-D FDTD package from CST Microwave Studio. An etching depth h of 400 nm was used. The simulation was conducted using diameter d 0  values 60, 70, 80 and 90 nm for the air holes  408 , and respective length a 0  values 275, 285, 295, 305 and 315 nm for the microcavity  410 . The results of the simulation are shown in  FIG. 5 , for a=180 nm, d 0 =80 nm, d=68 nm, w=475 nm, t 1 =400 nm and t 2 =400 nm. 
       FIG. 5  shows a plot  502  of the transmission  506  and Q factor  508  against the diameter d 0  of the innermost air holes  408  of the photonic bandgap structure  140  of  FIG. 4 .  FIG. 5  also shows how varying the length a 0  of the photonic bandgap structure  140  ( FIG. 4 ) affects both the transmission  506  and the Q factor  508  characteristics of the photonic bandgap structure  140  ( FIG. 4 ). Graphs  520 ,  521 ,  522 ,  523  and  524  show the range of transmission  506  and factor  508  values for varying the diameter d 0  for cavity lengths a 0  of 315, 305, 295, 285 and 275 nm respectively. 
     From  FIG. 5 , it can be observed that the factor  508  decreases when the length a 0  increases. This is because the optical mode is not as well confined in the microcavity  410  ( FIG. 4 ) when the length a 0  increases. Transmission  506  characteristics improve when the length a 0  is increased. The Q factor  508  reduces when the diameters d 0  are reduced. When the diameter d 0  is reduced, the optical throughput also increases. Thus, the transmission  506  and Q factor  508  characteristics share an inverse relationship. 
     The above simulation results are summarised in Table  600  shown in  FIG. 6 . Both the transmission  506  values and the Q factor  508  values of  FIG. 5A  are tabulated in table  520  against their respective lengths a 0  and diameters d 0 . The transmission  506  values are placed in brackets in table  600 . It will be appreciated, from table  600 , that a range of device parameters can be chosen to obtain a high Q factor  508  and high transmission  506  properties by tuning the microcavity length a 0  and the air holes diameters d 0 . For example, when length a 0 =295 nm and diameter d 0 =80 nm, we obtain a Q factor  508  of around 325 and a corresponding transmission  506  of around 90%. Another example would be when length a 0 =275 nm and diameter d 0 =70 nm, we obtain a Q factor  508  of around 333 and a corresponding transmission  506  of around 91%. 
     For a sub-micrometer height of 400 nm, high scattering loss is dominated by the microcavity  410  ( FIG. 4 ) waveguide sidewall roughness. The documents “Low loss ultra-small branches in a silicon photonic wire waveguide,”  IEICE Trans. Electron ., vol. E85-C, pp. 1033-1038, 2002 by Sakai et al. and “Effect of size and roughness on light transmission in a Si/SiO 2 , by Lee et al. discuss the relationship between waveguide losses and waveguide dimensions to enable the design and fabrication of a waveguide in SOI with minimal loss, where 0.1 dB/cm transmission loss was demonstrated for a device with a Si layer height of 200 nm in the latter document. As such, it is feasible to use a 400 nm thick Si layer (t i ) for the microcavity  410 . The results discussed above pertain to TE polarization optical signals in the example embodiments. However, polarisation independent SOI waveguides can also be formed, e.g. based on the technique described in S. P. Chan, C. E. Png, S. T. Lim, G. T. Reed, V. M. N. Passaro, “Single Mode and Polarisation Independent Silicon-on-Insulator Waveguides with Small Cross Section”, IEEE J. of Lightwave Tech. 23, 2103-2111 (2005). Polarization independent waveguides may be incorporated in different embodiments for fabrication of polarization independent devices. 
       FIG. 7  shows a schematic diagram of the optical combiner  180  of  FIG. 1 . It will be appreciated that the optical combiner  180  has an inverse configuration to the optical splitter  120  of  FIG. 2 . Thus, the optical combiner  180  has corresponding parameters to that of the optical splitter  120  ( FIG. 2 ). 
     In the context of  FIG. 7 , the four waveguides  104 ,  106 ,  108  and  110  are more commonly referred to as combiner waveguides. The four combiner waveguides  104 ,  106 ,  108  and  110  have an S-bend arrangement in their respective regions  104   b ,  106   b ,  108   b  and  110   b  approaching the output portion of the central waveguide  102 . The radii of the S-bends, for instance R and R 1  of the combiner waveguides  110  and  108  respectively, are designed for minimum optical power loss, which is dependent on the combiner waveguides width w and the refractive index contrast ΔN of the combiner waveguide material. 
     Substantially straight waveguides (for instance portions LO 2  and LO 1  of the splitter waveguides  106  and  104  respectively) are incorporated at the end of each of the S-bend arrangements  104   b ,  106   b ,  108   b  and  110   b  to couple optical power from each of the splitter waveguides  104 ,  106 ,  108  and  110  into the central waveguide  102 . 
     The output ends of the four combiner waveguides  104 ,  106 ,  108  and  110  are separated a distance LOgap, from the central waveguide  102 . On the other hand, a distance LOsep separates the beginning portions of each adjacent waveguide in the optical combiner  180 . The separation distance LOsep is selected to fulfill the criteria of zero power transfer between adjacent waveguides. 
       FIG. 8  shows a plot  800  of normalised power against cT. The plot  800  illustrates results, using the FDTD simulation method, of the total normalised output power from the central waveguide  102  of the optical combiner  180  of  FIG. 8 . The simulated optical combiner  180  ( FIG. 7 ) had a silicon-on-insulator (SOI) structure with the following parameters: 
     w=500 nm, si-overlayer=220 nm, Lgap=0.1 μm, L 1 =3.1 μm, L 2 =8 μm, Lsep=0.7 μm 
     The FDTD simulation was conducted to investigate the effectiveness of the optical coupling between the four combiner waveguides  104 ,  106 ,  108  and  110  and the central waveguide  102 . Curve  802  shows that the output from the optical combiner  180  has a normalised power of 0.95. 
     It will be appreciated that the optical splitter and the optical combiner discussed above can be used in a number of device applications, including, e.g. in optical multiplexers and optical demultiplexers. 
       FIG. 9  shows a schematic diagram of an optical demultiplexer  900  built in accordance with one embodiment of the invention. 
     The optical demultiplexer  900  shown in  FIG. 9  is of the 4 channel configuration. However, it will be appreciated that it is possible to realise an N=4, 6, 8, 10, . . . etc channel configuration with the appropriate design consideration, in different embodiments. The optical demultiplexer  900  has an optical splitter  920  having a central waveguide  902  and four waveguides  904 ,  906 ,  908  and  910 . A photonic bandgap structure  940  is formed in each of the four waveguides  904 ,  906 ,  908  and  910 . 
     Optical energy injected into the central waveguide  902  will be equally split into each of the four waveguides  904 ,  906 ,  908  and  910 . The photonic bandgap structure  940  acts as an optical filter for the respective channel wavelengths. 
       FIG. 10  shows a schematic diagram of an optical multiplexer  1000  built in accordance with one embodiment of the invention. 
     The optical multiplexer  1000  shown in  FIG. 10  is of the 4 channel configuration. However, it will be appreciated that it is possible to realise an N=4, 6, 8, 10, . . . etc channel configuration with the appropriate design consideration, in different embodiments. The optical multiplexer  1000  has an optical combiner  1080  having a central waveguide  1002  and four waveguides  1004 ,  1006 ,  1008  and  1010 . A photonic bandgap structure  1040  is formed in each of the four waveguides  1004 ,  1006 ,  1008  and  1010 . 
     The photonic bandgap structure  1040  acts as an optical filter for the respective channel wavelengths. Resulting optical energy that is transmitted from each photonic bandgap structure  1040  on each of the four waveguides  1004 ,  1006 ,  1008  and  1010  will be combined into the central waveguide  1002 . 
     In another application, the optical splitter and combiner can be used in an active attenuation device  1100  as shown in  FIG. 11  for one embodiment of the invention. 
     The active attenuation device  1100  has an optical splitter  1120  at the input end and an optical combiner  1180  at the output end. 
     The active attenuation device  1100  shown in  FIG. 11  is of the 1×4 splitter configuration, where the active attenuation device  1100  comprises a central waveguide  1102  and four waveguides  1104 ,  1106 ,  1108  and  1110 . A pair of the waveguides  1104 ,  1106 ,  1108  and  1110  is disposed on each side of the central waveguide  1102 . An active photonic bandgap structure  1140  is formed in each of the four waveguides  1104 ,  1106 ,  1108  and  1110  between the optical splitter  1120  and the optical combiner  1180 . 
     The active attenuation device  1100  has a 1×4 splitter configuration. However, it is possible to realise a 1×N, N=4, 6, 8, 10, . . . etc splitter configuration with the appropriate design consideration, in different embodiments. 
     It will be appreciated that the optical splitter  1120  and the optical combiner  1180  have similar structures, dimensions and fabrication methodology to both the optical splitter  120  and the optical combiner  180  of  FIG. 1  respectively. The active photonic bandgap structure  1140  includes a photonic bandgap structure similar to the photonic bandgap structure  140  of  FIG. 1 , but the microcavity  1140   a  of the active photonic bandgap structure  1140  has adjacent cathode and anode regions  1140   b  and  1140   c.    
       FIG. 12  shows a cross sectional view of a microcavity  1140   a  of the active attenuation device  1100  of  FIG. 11 . 
     The cathode and anode regions  1140   b  and  1140   c , and the microcavity  1140   a  are formed above a SiO 2  layer  1206 , where both the cathode region  1140   b  and the anode region  1140   c  are adjacent to and in contact with the microcavity  1140   a . The SiO 2  layer  1206  is formed above a Si substrate  1246 . 
     In one example embodiment, fabrication of the p+ (anode) and n+ (cathode) contacts  1140   c ,  1140   b  will now be described. Initially, the designated n+ region is covered by deposition of a 500 nm oxide layer by oxidation using plasma enhanced chemical vapour deposition (PECVD), and exposing only the designated p+ region through patterning and etching. 
     The p+ region is realised with the growth of 400 nm of highly doped Si in the exposed region by selective epitaxy at 1×10 20  atoms/cm 3  (Boron). In instances where the epitaxy equipment used has a limited target concentration (e.g. can only reach up to 1×10 19  atoms/cm 3 ), an implantation step of Boron at an appropriate dosage to increase the net dopant concentration to 1×10 20  atoms/cm 3  can be included. The p+ region is then protected with 500 nm silicon dioxide using PECVD. 
     The silicon dioxide at the designated n+ region is then etched off. The n+ region is realised with the growth of 400 nm of highly doped Si in the exposed region by selective epitaxy at 1×10 20  atoms/cm 3  (Arsenic and Phosphorous). In instances where the epitaxy equipment used has a limited target concentration (e.g. can only reach up to 1×10 19  atoms/cm 3 ), and an implantation step of Arsenic or Phosphorus at an appropriate dosage to increase the net dopant concentration to 1×10 20  atoms/cm 3  can be included. The n+ region is then protected with 500 nm silicon dioxide using PECVD. 
     Where dopant implantation is performed in the fabrication of the n+ region, the p+ region, or both, an appropriate annealing step may be required to restore the crystal lattice to its original state and to activate the implanted carriers. 
     Next, a layer of 300 nm oxide is deposited over the wafer and then planarised using chemical mechanical polishing (CMP) in preparation of metallisation using deposition contact lithography and dry etching, followed by an alloy and anneal at 350° C. for 30 mins, to segregate the anode and cathode regions. 
     It is noted that the fabrication order of the n+ and p+ regions can be interchanged to increase flexibility in different embodiments. 
     Collectively, the anode region  1140   c , the microcavity  1140   a  and the cathode region  1140   b  form an active p-i-n diode  1248 . The active diode  1248  provides optical switching capability via the free carrier injection effect. This involves biasing the active diode  1248  so that carriers are injected into the microcavity  1140   a  which causes attenuation of the optical signal being transmitted through the microcavity  1140   a.    
     In the embodiment shown in  FIG. 12 , the cathode region  1140   b  and the anode region  1140   c  are highly doped regions with constant doping concentrations of 10 20  cm −3 . The SiO 2  layer  1206  and the active diode  1248  each have a thickness of around 400 nm. The cathode region  1140   b  and the anode region  1140   c  have a width of around 500 nm each, while the microcavity  1140   a  has a width of around 475 nm. The microcavity  1140   a  has a length a 0 . 
     The dc and transient characteristics of the active diode  1248  was investigated using a 2-D ATLAS device simulation package from SILVACO. The results are discussed with reference to  FIGS. 13 ,  14  and  15 . 
       FIG. 13  shows a plot  1300  of drive voltage (V) against drive current (I) and a respective change in refractive index, Δn. 
     Curve  1302  shows the I-V relationship for the active diode  1248  of  FIG. 12 . From curve  1302 , it is observed that in the OFF state, i.e. when there is no carrier injection into the active diode  448 , there is no change in the refractive index, Δn. In the ON state, designated here as where N e  (concentration of electrons)=N h  (concentration of free holes)≈3×10 17  cm −3 , there is a change in refractive index. Δn, of approximately 10 −3 . The applied current and voltage required to achieve the ON state are 0.92 volts and 0.0879 μA respectively, corresponding to an applied power of around 80.9 nW. 
     Curve  1304  shows the static performance of the active diode  1248  of  FIG. 12 , which indicates the change in refractive index, Δn, and hence phase change, varies nonlinearly with the applied current. One factor, which contributes to the nonlinearity of the change in phase versus current density relationship, is the sublinear dependence of the change in concentration of the free holes, ΔN h , with the change in refractive index, Δn. As the active diode  1248  is driven harder, more free carriers are injected into the intrinsic region (the microcavity  1140   a ) of the active diode  1248 . This increase in the concentration of the previously intrinsic region results in an increase in the Auger recombination rate (at injected carrier concentrations much greater than 10 17  cm −3  the Auger recombination becomes the dominant recombination process). This results in a reduced carrier lifetime in the intrinsic region. The active diode  1248  has thus to be driven harder to achieve an equivalent refractive index change, Δn, than at lower drive powers. An increase in the recombination rate will result in the active diode  1248  becoming a faster switching device, i.e. a reduction in the rise and fall times of the active diode  1248 . 
       FIG. 14  shows a plot of attenuation (dB) against drive current (μA). Curve  1402  shows the attenuation experienced by an optical signal propagating through the microcavity  1140   a  of  FIG. 12  of length a 0  275 nm. From  FIG. 14 , it is observed that attenuation increases as the active diode  1248  is driven harder. 
     For example, by driving the device to produce an injection level off 3.9×10 8 , which corresponds to an applied voltage of around 1.05V, the attenuation is given by: 
     
       
         
           
             
               
                 
                   
                     
                       
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     This is equivalent to 244 dB/cm, which is in turn equivalent to 6.72e-3 dB for the microcavity  1140   a  ( FIG. 12 ) length a 0  of 275 nm, or 1.34e-2 dB for a microcavity  1140   a  ( FIG. 12 ) length a 0  of 550 nm. Such attenuation is about 5 orders of magnitude greater than existing attenuators, such as existing fiber based photonic bandgap attenuators or Si rib or strip waveguide based attenuators. It will also be appreciated that attenuation can be increased by lengthening the microcavity  1140   a  ( FIG. 12 ). 
       FIG. 15  shows a plot  1500  of the change in refractive index, Δn, against transient time for the active diode  1248  of  FIG. 12 . The plot  1500  is used to evaluate the dynamic performance of the active diode  1248  ( FIG. 12 ) by investigating the switching speed exhibited by a transient modelling solution. Both the anode region  1140   c  ( FIG. 12 ) and the cathode region  1140   b  ( FIG. 12 ) were first zero biased  1502  for 10 ns, followed by a step increase  1504  to a voltage V π  for 200 ns, and a subsequent step decrease  1506  to a voltage 0V. V π  is the voltage corresponding to a 180° phase shift and is about 0.92V from  FIG. 13 . The rise time, t r , is defined as the time required for the induced phase shift to change from 10% to 90% of the maximum value. Likewise, the fall time, t f , is defined as the time required for the induced phase shift to change from 90% to 10% of the maximum value. For the active diode  1248  ( FIG. 12 ), the rise and fall times were determined to be t r =0.215 ns and t f =0.043 ns respectively. 
     The dynamic optical absorption introduced by switching the device ON (step increase  1204 ) corresponds to N e =N h ≈3×10 17  cm −3 . Using equation (14),
 
Δ a=Δa   e   +Δa   h =8.5×10 −18 (Δ N   e )+6.0×10 −18 (Δ N   h )  (14)
 
     where ΔN e  (cm −3 ) is the electron concentration change; ΔN h  (cm −3 ) is the hole concentration change; Δa e  (cm −1 ) is the absorption coefficient variation due to ΔN e ; and Δa h  (cm −1 ) is the absorption coefficient variation due to ΔN h , this injection of both electrons and holes translate to an additional absorption loss of approximately 4.35 cm −1  (i.e. 18.9 dB/cm). This results in a dynamic optical absorption of approximately 0.0005 dB if the microcavity  1140   a  ( FIG. 12 ) length a 0  is around 275 nm. 
     The optical attenuation can be improved by optimising the position of the dopant contact windows for both the anode region  1140   c  ( FIG. 12 ) and the cathode region  1140   b  ( FIG. 12 ). The active diode  1248  ( FIG. 12 ) switching performance can also be improved, without changing its physical dimensions, by overdriving during the rise and fall times (steps  1204  and  1206  respectively). However, overdriving has the disadvantage of increasing the complexity of the active attenuation device  1100  ( FIG. 11 ). 
     In another application, phase modulation can be introduced into a waveguide arm.  FIG. 16  shows a schematic diagram of an active phase modulator MZI  1600  built in accordance with one embodiment of the invention. 
     The active phase modulator MZI  1600  has an optical splitter  1620  having a central waveguide  1602  and four waveguides  1604 ,  1606 ,  1608  and  1610 . An active photonic bandgap structure  1640  is formed in each of the four waveguides  1604 ,  1606 ,  1608  and  1610 . 
     The output portion of the waveguides  1604  and  1606  are joined so that interference between the optical field transmitted in each waveguide  1604  and  1606  occurs at region  1650 . It will be appreciated that interference also occurs in the same manner at region  1652  for the waveguides  1608  and  1610 . 
     Photodetectors  1654  can be placed to measure the outputs from the coupling regions  1650  and  1652 . While  FIG. 16  shows an active phase modulator MZI  1600 , it will also be appreciated that a passive configuration can also be designed into another embodiment, where the active structure  1640  is replaced with a passive photonic bandgap structure  140 , such as  FIG. 1 , for sensing applications. 
     Embodiments of the photonic bandgap devices described herein can be applied in different operations. 
     The photonic bandgap devices can be deployed in optical communications, which require devices that are low cost, have a high density and provide high data transmission rates. The devices can also be used for biosensing and optical interconnects. 
     Example embodiments can provide an integrated electronic chip with more waveguide arms in addition to the central waveguide. By closely packing more waveguide arms, multiple tests could be performed simultaneously. The size and sensitivity of the photonic bandgap devices facilitates nanoparticle detection and detection of sensitive analytes. 
     Other possible applications include diagnostic toolkits to test for GO/NOGO, as well as optical multiplexers and demultiplexers. GO/NOGO applications can be used to determine whether a functional response is met, e.g. serving as an indicator that represent a positive or negative result.