Optical Logic Gates Using Frequency Plane Filtering
Technique

FIELD OF THE INVENTION

This invention relates to optical devices functioning as all-optical logic gates. A digital device performing its computation with photons as opposed to the traditional electron based computation system will be much faster because of photons’ speed of light.

BACKGROUND OF THE INVENTION

In recent years, with increase of the demand for high bandwidth computation, the speed limits of the electronic devices are challenged and Optical Computer may enter the industrial market within a decade. This situation encourages the researchers worldwide, to contribute in the field of all-optical computation, primarily by building the fundamental blocks of the optical processor, one of which is optical logic gates.
While there may be many proposed designs of optical logic gates1-2 , their implementations are complex. We intend to invent all-optical logic gates in a simpler and cost-effective way.

SUMMARY OF THE INVENTION

The present invention describes an all-optical method to obtain the basic logic operations, namely AND, OR and NOT. The basic designs of these gates are comprised of some low-cost components, which can also be used in chips. These components include a couple of LEDs along with sinusoidal gratings as sources, small lenses, beam combiner and spatial filter. The two states of the binary logic are described by the presence and absence of the light. Binary logic 1 means light is ON, implying there is entry of light, and logic 0 means light is OFF, implying there is no entry of light.
In case of AND and OR gate, two inputs comes through two perpendicularly aligned Sinusoidal gratings, which after getting combined by a beam combiner, gets imaged by a lens. The light emanating from that image then passes through a passive 4f set-up, which contains a spatial filter at its frequency plane. The output of the gate comes from the output plane of the 4f set-up.

In case of AND gate, the frequency plane filter allows an (1, 1) order of the composite grating’s spectrum; whereas for OR gate the spatial filter allows the (0, 0) order of the spectrum.
In case of NOT gate with single input, a variant of the previous set-up is used to enter data. The single input comes through one sinusoidal grating and passes through the beam combiner, while another plane beam (without any grating) reaching the beam combiner, remains always ON. In the subsequent 4f set-up, the first spherical lens is replaced with a cylindrical lens, which produces a one dimensional intensity variation in case of high input of the NOT gate. The frequency plane spatial filter is placed on the first minimum of that intensity pattern, so that it produces a low value at the output. When in input of NOT gate is off, in the frequency plane of the 4f set-up we get a continuous line instead of the intensity variation; and therefore at the output it produces a high value.

EMBODIMENTS
Figure 1 shows the schematic diagram of the proposed all-optical AND GATE. Light from sinusoidal input gratings A 11 and B 12 is combined in a Beam Combiner 13 and then was passed through a Double Convex lens 14 and imaged as C 15. Then from C, the light is passed through a 4f setup containing a Double Convex lens 16 and a spatial filter (a corner slit at a position of (1,1) spectral order ) 17 in its frequency plane. The full spectrum obtained in this plane is a 3×3 optical impulse array 18. The light from the spatial filter then passes through another Double Convex lens 19 and the final output of the gate 21 is received on the screen 20.
Figure 2 shows the schematic diagram of the proposed all-optical OR GATE. Light from sinusoidal input gratings A 22 and B 23 is combined by a Beam Combiner 24 and then is passed through a Double Convex lens 25 and imaged as C 26. Then from C, the light is passed through a 4f setup containing a Double Convex lens 27 and a spatial filter (a center slit at the position of (0,0) spectral order) 28 in its frequency plane. The full spectrum obtained in this plane is a 3×3 optical impulse array 29. The light from the spatial filter then passes through another Double Convex lens 30 and the final output of the gate 32 is received on the screen 31.
Figure 3 shows the schematic diagram of the proposed all-optical NOT GATE. An always ON light 33 and light from Input Grating A 34 is combined by a Beam Combiner 35 and then is passed through a Double Convex lens 36 and imaged as C 37. Then from C, the light is passed through a 4f setup containing a Cylindrical Lens 38 and a spatial filter (placed on the first minimum of that intensity pattern) 39 in its frequency plane. The output is in the form of a 3×1 optical impulse array 40. The light from the spatial filter then passes through another Double Convex lens 41 and the final output 43 is received on the screen 42.
DESCRIPTION OF THE INVENTION
In the arrangement of AND gate, two Sinusoidal gratings A and B, along with two input beams are used as sources. Inputs from gratings A and B are introduced into a beam combiner, wherein they incident on the double convex lens to form the image C. The light then goes on to get Fourier Transformed optically to form a 3x3 optical impulse array. Essentially it looks like optical dots on the screen.
For executing the logic AND, an appropriate spatial filter is chosen that allows only one corner optical dot corresponding to the (1, 1) spectral order. It makes the light emerge out of the filter only if both the sources A and B are ON. The light from this filter, which is essentially a point

source, again gets Fourier Transformed to give the final output on the screen. In case any of the sources are absent, then there is no illumination on the screen.
For executing the logic OR, the spatial filter is chosen to allow only the central optical dot corresponding to the (0,0) spectral order, so that light emerges out of the filter if either of the sources A and B are ON. All other components are same as used for AND gate.
In the NOT gate arrangement, a cylindrical lens is used instead of the spherical lens Fourier Transformer. There are one input A to the system from a single sinusoidal grating and another input to the beam combiner which is always ON.
When the input A is not there, the light from the control source enters the system, and a uniform vertical spectrum is formed on the first screen after passing through the cylindrical lens. A spatial filter is used as before, which allows the light to pass through it from a particular region. Hence the output screen is illuminated, suggesting logic 1.
When input A is there, then the spectrum on the first screen becomes discontinuous. The filter is selected, such that the slit is at a discontinuity of the spectrum. Hence light does not pass through it, and the output screen remains dark, suggesting logic 0.
Hence the purpose of an optical inverter is served.

According to our design, the basic components of these two gates are same, except the positions of the frequency plane filters.
The mathematical representations of the operations are discussed below.

CLAIMS

1. An optical AND gate comprising of two input gratings, a beam combiner, and an optical 4f setup along with a spatial filter and three double convex lenses has been proposed.
2. An optical OR gate comprising of two input gratings, a beam combiner, and an optical 4f setup along with a spatial filter and three double convex lenses has been proposed.
3. An optical NOT gate comprising of one input grating, a beam combiner, and an optical 4f setup along with a spatial filter, two double convex lenses and one cylindrical lens has been proposed.
4. The Input gratings according to claim 1, 2 and 3 consist of horizontal and vertical sinusoidal gratings.
5. The 4f set-up according to claim 1 and 2 is achieved using two double convex lenses of same focal length f placed at 2f distance. At the back focal plane of the first lens the Optical Fourier Transform was achieved. This shows the spatial frequency distribution of the source and hence called frequency plane.
6. The spatial filter according to claim 1, consists of an opaque screen with a small opening at a position corresponding to the (1, 1) order of the 3x3 impulse array at frequency plane.
7. The spatial filter according to claim 2, consists of an opaque screen with a small opening at a position corresponding to the (0, 0) order of the 3x3 impulse array at frequency plane.

8. The spatial filter according to claim 3, consists of an opaque screen with a small slit at a position corresponding to the first minimum of the 3x1 impulse array at frequency plane.
9. The impulse array according to claim 6, 7 and 8 is formed by the Fourier transform of the beam of light emerging from the combined grating.