Patent Publication Number: US-7215462-B2

Title: Filter for selectively processing optical and other signals

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application is a continuation of and claims priority to U.S. patent application Ser. No. 10/650,658, entitled “Filter for Selectively Processing Optical and Other Signals,” filed in the U.S. Patent and Trademark Office on Aug. 28, 2003 now U.S. Pat. No. 7,042,657 and having a common inventor as the present document. The entire contents of the above patent application is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention disclosed and claimed herein generally pertains to a multi-section filter for processing optical signals and other signals that can be directed from one filter section to another. More particularly, the invention pertains to filters of the above type, such as lattice filters, that are usefully formed from semiconductor optical amplifier regions (SOARs) coupled together by means of four direction couplers, where the four direction couplers may be implemented by surface grating couplers. 
   2. Discussion of the Background 
   A main tenet of both electrical and optical engineering is the desirability of filtering, sorting and processing information with higher degrees of precision. In electrical engineering, a significant breakthrough in precision filtering and signal processing came with the active filter. In electrical engineering, an active filter is one that includes an electronic gain element. In very early examples of the art, the electronic gain element was a vacuum tube. For the past half century, the electronic gain element has been a transistor. The improvement in filtering precision due to an electronic gain element may be intuitively understood by a simple band pass example. A passive electronic band pass filter may be made from a conductor, a capacitor, and a resistor, and will attenuate frequencies away from resonance more than frequencies near resonance. An active electronic band pass filter that includes a transistor will show improved performance because frequencies near resonance may now be amplified. Active low pass, high pass, matched, and other varieties of electronic filters also show improved performance over their passive counterparts. 
   Currently, there are extensive examples of passive optical filters that act to sort one frequency of light from another, separate bands of frequencies, or preferentially select a set of frequencies from another. For example, a diffraction grating will separate different colors of light into different propagation directions, allowing some to be spatially filtered. Diffraction gratings find wide applications in monochromators and spectraphotometers, as well as in dense wave division multiplexed (DWDM) telecommunications systems. For a second example, a thin film coating filter may be used to greatly reduce or greatly increase the reflected light from an air-glass interface. Anti-reflection (AR) thin film coatings find wide application in camera, telescope and eyeglass lenses. High reflectivity (HR) thin film coatings find wide application in laser mirrors. Thin film filters also find wide application in DWDM telecommunication systems to add, drop and otherwise sort channels. 
   A shortcoming of the optical filters currently known is that they are passive. Current optical filters do not have gain, and thus their performance is limited. For example, the quality factor of a filter is equal to a resonant frequency divided by the uncertainty in that frequency f/(Δf). It is well known that the quality factor of a passive filter is lower than the quality factor of an active filter of the same order. Thus, there is a need for an optical filter that is active and yields higher performance including higher quality factors. This will enhance tunabilty of such filters and provide numerous other benefits. 
   Another shortcoming of the optical filters currently known is that they are manufactured for specific applications. It would be very desirable to provide optical filters that could be readily constructed from combinations of conventional or standardized elements or components. Providing this capability could significantly enhance flexibility in designing optical filters and could also reduce costs associated therewith. 
   SUMMARY OF THE INVENTION 
   The invention is generally directed to a multi-section or multi-stage filter for use in processing optical signals, as well as other signals that can be readily projected or directed from one filter section to another. Thus, filter sections can be respectively positioned in relation to one another so that an output signal from one element can be projected as an input to another section. These characteristics enable filters of the invention to be configured or embodied in numerous forms, to meet many different application requirements. These can include, without limitation, infinite impulse response (IIR) filters, finite impulse response (FIR) filters and both linear and two-dimensional (2D) optical lattice filters, as described hereinafter in further detail. Moreover, respective filter sections can be readily provided with controllable gain and delay, so that embodiments of the invention can be configured as active filters. 
   In important embodiments of the invention, controllable active optical filter sections may be constructed by placing electrodes upon active regions of a semi-conductor material, to form gain regions, or SOARs. Surface gratings are formed in the semi-conductor material between adjacent SOARs, for coupling optical signals therebetween. It is anticipated that standardized arrays of active optical filter sections, having the above features, could be formed with high precision and at reduced cost, using mass production techniques that are well known in the fabrication of semi-conductor devices. A standardized array could then be configured for a particular optical signal processing application, using pole and zero or other conventional design techniques. 
   One useful embodiment of the invention is directed to an active optical filter comprising a filter input component disposed to receive an optical input signal, a filter output component disposed to provide a filtered output signal, an optical output amplifier and at least one optical delay element. A surface grating coupler is positioned between the optical amplifier and each delay element to form a first light transmission path, the first light transmission path having its ends coupled to the filter input and output components, respectively. The active optical filter further comprises a second light transmission path disposed to transmit optical signals without delay from the filter input component to the filter output component. It is anticipated that this embodiment can be adapted to operate in either an IIR or an FIR mode. 
   A further embodiment of the invention is directed to an active optical lattice filter for selectively processing an optical input signal. The filter comprises a plurality of active lattice sections, or gain blocks, spaced apart from one another in a linear array, and a surface grating coupler positioned between each pair of adjacent gain blocks in the array. Each of the gain blocks is disposed to receive an optical signal as an input from one of its adjacent gain blocks, to transmit a portion of the received input to its other adjacent gain block, and to reflect the remainder of the received input. Each of the gain blocks is provided with controllable gain and delay characteristics, respectively selected to produce an output from the linear lattice array comprising an IIR when the input signal comprises a single optical pulse. 
   Yet another embodiment, in its most general form, is directed to a 2D lattice filter disposed to selectively process a received input signal. The 2D filter comprises a plurality of gain blocks, each gain block disposed to receive, process and project specified signals, the gain blocks being grouped into one or more filter sections for the lattice filter. A number of 2D lattice couplers, each associated with gain blocks in at least one of the sections, are each positioned to exchange specified signals directed along a first axis with one of its associated gain blocks, and to exchange specified signals directed along a second axis orthogonal to the first axis with another of its associated gain blocks. Usefully, the lattice filter output is an IIR, when the received input signal comprises a single pulse. In a preferred embodiment, the input signal comprises a single optical pulse, each of the lattice couplers comprises a crossed grating coupler, and the gain blocks have controllable gain and delay characteristics. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the present invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a schematic diagram showing a single stage IIR active optical filter using surface grating couplers in accordance with an embodiment of the invention. 
       FIG. 2  is a schematic diagram showing a multi-stage IIR active optical filter that includes the single stage filter shown in  FIG. 1 . 
       FIG. 3  is a schematic diagram showing a single stage FIR active optical filter using surface grating couplers in accordance with a further embodiment of the invention. 
       FIG. 4  is a schematic diagram illustrating use of semiconductor material disposed to conduct optical signals and form SOARs, in order to implement embodiments of the invention. 
       FIG. 5  is a diagramillustrating optical signal flow in gain block, or latticesection, of an active optical lattice filter. 
       FIG. 6  is a diagram showing signal flow in a gain block of an optical lattice filter constructed in accordance with an embodiment of the invention. 
       FIG. 7  is a schematic diagram showing a lattice filter including the gain block of  FIG. 6  and constructed from semiconductor material as described in connection with  FIG. 4 . 
       FIG. 8  is a schematic diagram showing the signal flow in a four direction coupler. 
       FIG. 9 . is a schematic diagram showing a stage or section of a 2D active optical lattice filter using the 2D coupler of  FIG. 8 . 
       FIG. 10  is a schematic diagram showing a 2D lattice filter formed of multiple stages of the type shown in  FIG. 9 . 
       FIG. 11  is a schematic diagram showing a top view of a four direction coupler implemented as crossed beamsplitters. 
       FIG. 12  is a schematic diagram showing a top view of a four direction coupler implemented as a crossed surface grating. 
       FIG. 13  is a schematic diagram showing a top view of a four direction coupler implemented as a photonic crystal. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
   Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, preferred embodiments of the present invention are described. 
   Referring to  FIG. 1 , there is shown a first order or single stage active optical filter  10 , constructed in accordance with an embodiment of the invention. Filter  10  has an input component, comprising a surface grating coupler  12  disposed to receive optical input signals, and an output component likewise comprising a surface grating coupler  14 . Optical signals are projected directly from surface grating coupler  12  to surface coupling  14  along a light transmission path  16 , which is configured to avoid delaying light signals passing therethrough. 
   Each of the grating couplers  12  and  14  usefully comprises a component of a grating surface emitter (GSE) photonic integrated circuit (IC). A surface grating coupler of this type may be fabricated by conventional techniques, wherein a series of grooves or ridges are formed in the surface of an active region of semiconductor material carrying laser light. Light is projected in both directions through the grating coupler, wherein the amount of light passing in each direction is determined by the depth and spacing of respective grooves. A grating coupler may also be configured to couple light in two orthogonal directions. Thus, a surface grating coupler receiving a single optical input signal may provide two optical output components in directions orthogonal to one another. Alternatively, a grating coupler that furnishes a single optical output signal may receive two optical input components from different directions, which may be orthogonal to one another. 
   Referring further to  FIG. 1 , there are shown the above properties of surface grating couplers used in constructing optical filter  10 . That is, output grating coupler  14  receives a single optical input signal from transmission path  16 , but provides two optical output components, one comprising the filter output and the other being coupled to the input end of a light transmission path  18 .  FIG. 1  shows light transmission path  18  comprising optical delay elements  20  and  22 , an optical amplifier  24  and surface grating couplers  26  and  28 . Grating coupler  26  is positioned between delay element  22  and the output side  24   a  of optical amplifier  24 , and grating coupler  28  is positioned between delay element  20  and the input side  24   b  of optical amplifier  24 . The output of delay element  22 , which is the output end of light transmission path  18 , is supplied as an input signal to gradient coupler  12 , together with the filter input signal, to collectively provide the optical signal transmitted along path  16 . 
   Each of the delay elements  20  and  22  delays light passing therethrough by a known delay period, and optical amplifier  24  is provided with controllable gain. Usefully, this may be implemented by means of associated adjustable voltage controls. In one embodiment, optical amplifier  24  and delay elements  20  and  22  comprise further components of the GSE photonic IC described above. In such arrangement, optical amplifier  24  includes a gain region of the semiconductor active region that is in adjacent relationship with an electrode. Gain through the gain region may then be controlled by adjusting the voltage applied to the electrode. It will be observed that the active gain region can be located between the grating couplers  26  and  28  by forming them in the surface of the semiconductor material, as described above, at positions respectively adjacent to the gain region. Construction of components of a GSE photonic IC is described hereinafter in further detail, in connection with  FIG. 4 . 
   In accordance with the invention, it has been recognized that by judicious adjustments of the gain provided by amplifier  24 , the comparatively simple active optical filter  10  shown in  FIG. 1  can be readily adapted to perform numerous filtering tasks. For example, the filter  10  of  FIG. 1  can be adapted to generate an IIR  32  in response to a single optical input pulse  30 . Moreover, by adjustment of amplifier  24  the filter  10  can be tuned to operate at a specified frequency. Alternatively, filter  10  can be programmed to transmit only optical signals lying in a passband of specified bandwidth. 
   Referring to  FIG. 2 , there is shown the single stage filter  10  of  FIG. 1  combined with a number of additional light transmission paths  18 , to form a multi-stage or higher order optical filter  33 . It will be readily apparent that an optical filter  33  of virtually any order can be formed as a GSE photonic IC, by simply repeating steps used in forming respective components of the single order optical filter  10 . 
   Referring to  FIG. 3 , there is shown a single order active optical filter  34  formed of components respectively similar to those of filter  10 , described above. Thus, optical filter  34  has an input component  36  and an output component  38 , each comprising a surface grating coupler, and a light transmission path  40  disposed to carry light signals from grating coupler  36  to grating coupler  38  without delay. Active optical filter  34  is further provided with a light transmission path  42 .  FIG. 3  shows light transmission path  42  comprising optical delay elements  44  and  46 , an optical amplifier  48  and surface grating couplers  50  and  52 . Grating coupler  50  is positioned between delay element  44  and the input side  48   a  of optical amplifier  48 , and grating coupler  52  is positioned between delay element  46  and the output side  48   b  of amplifier  48 . 
   Referring further to  FIG. 3 , there is shown filter input component  36  providing two optical output components, one coupled through light transmission path  40  to filter output component  38 , and the other coupled to delay element  44 , at the input end of light transmission path  42 . The output end of path  42  is coupled as a second input to filter output component  38 , which combines the inputs thereto to provide the overall filter output. As with filter  10 , the gain of output amplifier  48  of filter  34  is controllable, as are the delays of delay elements  44  and  46 . By judicious adjustments of its gain and delays, optical filter  34  can be adapted to generate an output comprising an FIR  53 , when single optical pulse  51  is applied to the filter input. 
   It is anticipated that components of optical filter  34  can be constructed as respective components of a GSE photonic IC. It is anticipated further that one or more additional light transmission paths  42  can be connected to single order filter  34 , similar to the configuration shown in  FIG. 2 , to construct an FIR optical filter of higher order. 
   Referring to  FIG. 4 , there is shown a GSE photonic IC  54  of a type which may be used to construct or implement embodiments of the invention. IC  54  comprises an active region  56 , formed of semiconductor material such as AlGaInP, supported upon a substrate  58  formed of GaAs. As is known by those of skill in the art, light  55  projected from a surface emitting laser light can be directed to move along the active region  56  in the direction of the arrows, that is, rightward or leftward as viewed in  FIG. 4 . 
   Referring further to  FIG. 4 , there are shown electrodes  60  adjoining active region  56 , at selected locations. If an electrode  60  is operated to apply a voltage to an adjacent portion of active regions  56 , a light component or optical signal passing though the portion can be made to experience either a positive or negative gain. Thus, such portions of the active region comprise gain regions  62 . It will be observed that gain regions  62  may advantageously be used to implement optical amplifiers such as amplifiers  24  and  48  shown in  FIGS. 1 and 3 , respectively. 
     FIG. 4  further shows gratings  64  formed in the surface of active region  56 . As described above, the amount of light traveling in each direction through a grating  62  is determined by the depth and spacing of its respective grooves. A surface grating  64  may function as a coupler or interface for optical signals passing into or out of an adjacent amplifier gain region  62 . The depth and spacing of respective grooves  64   a  of a grating  64  may be configured to regulate the passage and direction of light therethrough as desired. A grating coupler or interface  64  can comprise a number of parallel grooves. Alternatively, a surface grating can comprise one or more trenches formed in active region  56 , or can comprise a crossed grating. A crossed grating is described hereinafter, in connection with  FIG. 8 . 
   Referring to  FIG. 5 , there is shown a transfer function diagram depicting the optical signal flow in a lattice filter section, or gain block  66 , of an active optical lattice filter. A lattice filter comprises a number of gain blocks placed end to end in series. From the overall transfer function of a lattice filter, computed from all the gain blocks collectively, the filter operating characteristics can be determined. Each gain block has a controllable optical amplifier to amplify optical signals passing therethrough. A component of the optical signal will be transmitted into the next following gain block, and another component will be reflected back, by the interface  66   d  with the next following gain block. Thus, the transfer diagram of  FIG. 5  shows two light paths, a light path  66   a  for transmitted light signal T k (z), traveling through the gain block  66  in a forward direction, and a light path  66   b  for reflective light signal R k (z), traveling through gain block  66  in the opposing direction.  FIG. 5  further shows transmissive light path  66   a  provided with an optical amplifier  70   a  having a gain G, and a delay element  68   a  having an impedance Z −1/2 . Element  68   a  represents the delay of light traveling from interface  66   c  to interface  66   d , where interfaces  66   c  and  66   d  are the boundaries between gain block  66  and the preceding and the next following gain blocks, respectively, of the corresponding lattice filter. Reflective light path  66   b  is similarly provided with an optical amplifier  70   b  and a delay element  68   b.    
   Referring further to  FIG. 5 , gain block  66  is shown to have an optical input signal T k−1 (z) from the preceding filter section. A component of the optical input signal is transmitted past interface  66   c  as t k−1 , and another component is reflected by interface  66   a  as r k−1 . In like manner, R k (z) produces t k−1  and −r k−1  upon encountering interface  66   c .  FIG. 5  shows t k−1  and −r k−1  combined to generate T k (z), and shows t k−1  and −r k−1 , combined to generate R k−1 (z). These operations are represented in  FIG. 5  by summers  72   a  and  72   b , respectively. Similarly, t k  and −r k  are combined to generate T k+1 (z), and t k  and −r k  are combined to generate R k (z). These operations are represented by summers  74   a  and  74   b , respectively. Further description of an optical lattice filter can be found in commonly owned U.S. patent application Ser. No. 09/432,352, filed Nov. 2, 1999. 
   Referring to  FIG. 6 , there is shown a lattice filter section or gain block  76  constructed in accordance with the invention, wherein gain block  76  generally operates in like manner as gain block  66 , described above, and has similar flow paths. Thus, gain block  76  includes a transmissive light path provided with an optical amplifier and a delay element, and is also provided with a reflective light path provided with an optical amplifier and a delay element. Each of these elements is similar to the corresponding element shown in  FIG. 5 . However,  FIG. 6  additionally shows gain block  76  coupled to a surface grating coupler  82 , to transfer optical signals at its input side, and also coupled to surface grating coupler  84 , to transfer optical signals at its output side. More generally, a surface grating coupler is placed between each two adjacent gain blocks or lattice sections of an active optical lattice filter, to transfer optical signals between the adjacent sections. 
   Referring to  FIG. 7 , there is shown an active optical lattice filter  86  that includes a gain block  76  and grating couplers  82  and  84 , as described in connection with  FIG. 6 . Lattice filter  86  is constructed from semiconductor material, as described above, as a GSE photonic IC. Thus, filter  86  includes a substrate  88   a , and an active region  88   b  disposed to conduct or pass optical signals. 
   Referring further to  FIG. 7 , there are shown electrodes  90   a–c  placed upon active region  88   b , in spaced apart relationship. Electrodes  90   a–c  generate gain regions  92   a–c , respectively, in the active region as described above.  FIG. 7  shows gain region  92   b  providing the amplification for lattice section  76 , that is, the amplification represented in  FIG. 6  by amplifiers  80   a  and  80   b .  FIG. 7  further shows grating couplers  82  and  84  formed in the surface of active region  88   b , in the spaces between gain region  92   b  and gain regions  92   a  and  92   c , respectively. Filter  86  usefully comprises an IIR filter. 
   Referring to  FIG. 8  there is shown the signal flow in a four direction coupler. A defining feature of the traditional lattice structure is the presence of an interface, depicted in  FIG. 8  by components  94   a–d , where a portion of the signal is reflected and a portion is transmitted. In several embodiments of the invention disclosed herein, an interface is constructed that routes fractions into four directions rather than the two of reflection and transmission. This four direction coupler may be realized through a photonic crystal, a crossed grating structure or through crossed beam splitters. Based on the teachings in this specification, it will be appreciated by those skilled in the art that these technologies may also be employed to create five, six, and higher direction couplers, and that these may be used to construct more complex two dimensional lattice filters. Herein, five and higher direction couplers are referred to generically as multi-direction couplers. 
   Referring further to  FIG. 8  there is shown the signal flow of a four direction coupler. The coupler may support up to four input signals, and will yield four output signals. Herein, the four ports are referred to as N, S, E and W. For each port there is a reflection coefficient ρ, a transmission coefficient τ, a right handed coupling coefficient, α, and a left handed coupling coefficient, β. Consequently the four direction coupler is characterized by as many as 16 parameters. 
   In physical systems, the four dimensional coupler of  FIG. 8  must conserve energy. Accordingly, constraints are derived on the α&#39;s, β&#39;s, ρ&#39;s and τ&#39;s imposed by energy conservation according to field theory, an approach most pertinent to the photonic realization of this filter. To conserve energy, the sum of the output powers must equal the sum of the input powers. If a single input, E W   in  is applied at the west port, then there will be four output signals, E W   out , E N   out , E E   out , and E S   out . Since the powers are proportional to the square of the total fields, the first condition imposed by energy conservation is:
 
ρ 2   W +α 2   W +τ 2   W +β 2   W =1  (1)
 
   Three more conditions emerge by applying a single input at each of the north, east and south ports. These are:
 
ρ 2   N +α 2   N +τ 2   N +β 2   N =1  (2)
 
ρ 2   E +α 2   E +τ 2   E +β 2   E =1  (3)
 
and
 
ρ 2   S +α 2   S +τ 2   S +β 2   S =1  (4)
 
   Two more energy conservation conditions follow from applying two input signals to opposite ports. Applying inputs to the west and east ports yields:
 
ρ W τ E +ρ E τ W +α W β E +α E β W =0  (5)
 
   Similarly, applying inputs to the north and south ports yields:
 
ρ N τ S +ρ S τ N +α N β S +α S β N =0  (6)
 
   Four final energy conservation conditions follow from applying two inputs to adjacent ports. Applying inputs to the west and north ports yields:
 
ρ W α N +ρ N β W +τ W β N +τ N α W =0  (7)
 
   Similarly applying inputs to the north and east ports yields:
 
ρ N α E +ρ E β N +τ N β E +τ E α N =0  (8)
 
   Similarly applying inputs to the east and south ports yields:
 
ρ E α S +ρ S β E +τ E β S +τ S α E =0  (9)
 
   Similarly applying inputs to the south and west ports yields:
 
ρ S α W +ρ W β S +τ S β W +τ W α S =0  (10)
 
   The four cases of three inputs and the one case of four inputs do not yield any new constraints, and thus yield no new information. 
   Energy conservation equations (1) through (10) must be satisfied for a physically real coupler and will also assure stability in a passive network comprised of these couplers. We also note that these equations reduce to their two port equivalents for the case of α=β=0. However, equations (1) through (10) need not be the only constraints on the 16 parameters. The coupler may obey other constraints such as symmetry, and these additional constraints may further limit the number of allowable sets of coefficients. For example, a solution to equations (1) through (10) under the condition for perfect symmetry is α=β=τ=½, ρ=−½ for each port. This example illustrates the implication of equations (1) through (10) that the solution set includes at least one negative coupling coefficient per port, and this may be interpreted as a required phase shift of a coupled wave. 
   Equations (1) through (10) are algebraically nonlinear, and hence their simultaneous solution is not necessarily straight-forward. While there are many approaches to their solution, one convenient approach is to first select the values for three coefficients of a first port, then select the values for two coefficients of a second port, and finally select the value for one coefficient of a third port. This approach will usually yield eight possible solution sets from which one may be chosen based on additional constraints or design preferences. 
   Referring to  FIG. 9 , there is shown a further embodiment of the invention, wherein crossed gratings as described above are used to construct a lattice section  96  for a 2D active optical lattice filter.  FIG. 9  more particularly shows the lattice section  96  comprising four gain blocks  98 , each providing two paths for optical signal flow in opposing directions, as described above in connection with the gain blocks of  FIGS. 5 and 6 .  FIG. 9  further shows section  96  including four crossed grating couplers  100 , each positioned or interspersed between two adjacent gain blocks  98  in order to form a closed loop or path for the flow of optical signals. That is, each crossed grating coupler  100  is positioned to exchange signals with one gain block  98  through a first one of its faces, and to exchange signals with the other gain block  100  through another of its faces, orthogonal to the first face. As further shown by  FIG. 9 , one or both of the remaining faces of each crossed grating coupler  100  is available to exchange signals with other lattice sections  96 , or to receive or project filter input or output signals, respectively. 
   Referring to  FIG. 10 , there are shown a number of 2D lattice sections  96  joined to form a 2D active optical lattice filter  102 . It will be readily apparent that sections  96  may be added continually to filter  102  as desired, along each of two dimensions.  FIG. 10  further depicts two opposing optical signal flowpaths of each gain block  98  as paths  98   a  and  98   b , respectively. 
   Referring now to  FIG. 11 , there is shown a schematic diagram of a top view of a four direction coupler implemented as crossed beamsplitters. This is a preferred embodiment of the four direction coupler in both a bulk optics implementation and an integrated architecture. In a bulk optics implementation the crossed beamsplitters are realized by thin film filters on a flat substrate or a prism substrate or by pellicle beamsplitters. In a wafer or die scale integrated optics architecture a preferred method of fabricating the crossed beamsplitters is by etching trenches  104  into the semiconductor substrate  101 , as specifically shown by  FIG. 11 . As is well known in the art the etching may be performed by a focused ion beam or by photolithography followed by reactive ion etching. The thickness of the trenches and the depth of the trenches, in particular the degree to which it reaches the waveguide region, control the relative coupling coefficients. 
     FIG. 12  is a schematic diagram showing a top view of a four direction coupler implemented as a crossed surface grating. This is a preferred embodiment of the four direction coupler in an integrated architecture. In a wafer or die scale integrated optics architecture a preferred method of fabricating the crossed surface grating is by etching a repetitive grating pattern  106  into the semiconductor substrate  108 . As is well known in the art the etching may be performed by a focused ion beam or by photolithography followed by reactive ion etching. The photolithography may be performed by a mask or by multiple beam interference of a coherent (laser) source. The pitch, profile and the depth of the grating, in particular the degree to which it reaches the waveguide region, control the relative coupling coefficients. For example a deeper grating in one direction than the other will lead to a stronger coupling in the first direction than the second. 
     FIG. 13  is a schematic diagram showing a top view of a four direction coupler implemented as a photonic crystal. This is a preferred embodiment of the four direction coupler in an integrated architecture. In a wafer or die scale integrated optics architecture a preferred method of fabricating the photonic crystal is by etching a photonic crystal pattern  110  into the semiconductor substrate  112 . As is well known in the art the etching may be performed by a focused ion beam or by photolithography followed by reactive ion etching. The photolithography may be performed by a mask or by multiple beam interference of a coherent (laser) source. The particular pattern and the depth of the photonic crystal pattern, in particular the degree to which it reaches the waveguide region, control the relative coupling coefficients. 
   Obviously, many other modifications and variations of the present invention are possible in light of the above teachings. The specific embodiments discussed herein are merely illustrative, and are not meant to limit the scope of the present invention in any manner. It is therefore to be understood that within the scope of the disclosed concept, the invention may be practiced otherwise then as specifically described.