**2.1. Optoelectronic gate**

Let us consider features of constructing a universal gate, which is based on the modulation of a light flux by an electric field [8-11], and when only a single transition of energy (between light and electric signal), the source of a light flux is shared by many gates. The scheme of such a gate is presented in Figure 1. It has the following main components: a light modulator LM controlled by the electric field, EC converter of a light signal to an electric signal (e.g. a photoelectric transducer), energy storage or a load element LE (capacitance for dynamic circuits). The element controls the intensity of the luminous flux transmitted by LM, and with the help of the light flux entering the optical input of EC.

The most optimal physical and technical solution, as follows from [8-11], is obtained when the light modulator is electro-optical, the energy converter is photovoltaic and the energy storage is electrostatic. Such a decision is mainly due to the consideration of energy. In this sense, the use of magneto-optical and acousto-optic light modulators is limited by a high energy consumption and by the complexity of the direct energy conversion (photomagnetic or photoacoustic).

**Figure 1.** Flowchart of universal gate

The state of a gate is determined by the ability of the light modulator to pass a light flux coming on its input (state "0" or "1"). This state is unambiguously related to the amount of energy stored, which, in turn, is determined by the intensity of the light flux entering the energy converter (entrance gate). According to Figure 1 a gate in the dynamic mode operates as follows:


Assume that the modulator lets the light flux pass when the amount of energy stored in LE, and does not let the light pass otherwise. Let us also assume that when EC receives a light flux composed of the streams X1, X2,..., Xn, such energy in the load element is released. Then the light flux on the output of the modulator Y (at the moment of reading) will be the Pierce logical function:

**Figure 2.** Universal gate with optical connections

58 Optical Communication

or photoacoustic).

**Figure 1.** Flowchart of universal gate

"erasing information");

operates as follows:

**2.1. Optoelectronic gate** 

**2. The physical basis of implementation of 3D optical logic circuits** 

LM, and with the help of the light flux entering the optical input of EC.

Let us consider features of constructing a universal gate, which is based on the modulation of a light flux by an electric field [8-11], and when only a single transition of energy (between light and electric signal), the source of a light flux is shared by many gates. The scheme of such a gate is presented in Figure 1. It has the following main components: a light modulator LM controlled by the electric field, EC converter of a light signal to an electric signal (e.g. a photoelectric transducer), energy storage or a load element LE (capacitance for dynamic circuits). The element controls the intensity of the luminous flux transmitted by

The most optimal physical and technical solution, as follows from [8-11], is obtained when the light modulator is electro-optical, the energy converter is photovoltaic and the energy storage is electrostatic. Such a decision is mainly due to the consideration of energy. In this sense, the use of magneto-optical and acousto-optic light modulators is limited by a high energy consumption and by the complexity of the direct energy conversion (photomagnetic

The state of a gate is determined by the ability of the light modulator to pass a light flux coming on its input (state "0" or "1"). This state is unambiguously related to the amount of energy stored, which, in turn, is determined by the intensity of the light flux entering the energy converter (entrance gate). According to Figure 1 a gate in the dynamic mode

supply of energy from the energy source (voltage pulse) to LE at the time t0 (operation

supply of a light flux on the input of the transmitter PE at the time t0 + Δt (operation

 supply of a light flux on the input of LM at the time t0 + 2Δt (operation "read information"). Assume that the modulator lets the light flux pass when the amount of energy stored in LE, and does not let the light pass otherwise. Let us also assume that when EC receives a light

"information recording"), where Δt is the duration of a cycle;

This function forms a complete basis of Boolean functions, so the element proposed can be considered universal.

The electric functional scheme of the gate is shown in Figure 2, where K is the key, V is the power supply voltage. The gate implements the Pierce function and it operates by cycles according to the following general description.

*cycle 1 -* "erasing information", key K is in the position 1, and capacitance C is being charged by the source when the photodetector FP is illuminated.

*cycle 2 -* "storing" K is in position 2 (see Figure 2, a), the light flux with intensity J1 or J2 (depending on the level of illumination corresponding to signal "1" or "0", i.e. whether there is at least one of Xi not equal to "0") comes to the optical input of the gate (photodetector FP). According to the level of illumination the capacitance C is discharged with the time constants *RTC*, or *RCC*, and the *RTC* >> *RCC*, where *RT* is dark resistance of FP, and *RC* is its resistance under illumination.

*cycle 3* - "reading", key K is in position 2 (see Figure 2, b) and the light signal passes to the optical input of the modulator. The light flux corresponding to "0" light is obtained from the output if signal "1" arrived at the input in the second cycle. And vice versa, signal "1" with a high intensity of light is on the output of LM if there was signal "0" in the previous cycle.

The storage time of information received by the element is proportional to the value of *RTC*. However, we can create a dynamic memory by inclusion of two gates so that the output of one of them be connected to the input of another and vice versa, as shown in Figure 3. Its storage time is limited only by the time of maintaining the voltage on the power supply.

**Figure 3.** A dynamic memory cell

When constructing digital devices based on these optical gates the interlayer connections are implemented by placing pairs of "modulator - photodetector" in different layers-planes so that the optical output light modulator of one gate is geometrically aligned with the photodetector optical input of another gate.

### **2.2. A sample of constructing a dynamic memory cell**

Such a cell is intended for storing information in the electric form on capacitors and transferring information between cells by pulsed optical signals, while a direct transfer of charge does not take place. The scheme of the cell is depicted in Figure 3.

Assume that a bit of information (e.g. "0") is written in the upper gate 1. The switch K1 was shot to pin 2 and PD1 was closed in order to do so.

*Cycle 1.* "Erasing" of information in lower gate 2 by charging the capacitance C*2* under lit PD*<sup>2</sup>* and the switch K*2* shot to pin 1 of the power source with voltage *V*.

*Cycle 2.* The reading of information from element 1 by the light signal Ird1. The switches K1 and K2 are in position 2. The output signal from LM1 is the input writing signal for element 2.

*Cycle 3.* The "erasing" of information in element 1. The switch K1 is in position 1 and the capacitance C1 is connected to the power source with voltage *V* and is charging.

*Cycle 4.* The reading of information from element 2 by the light signal Ird2. The switches K1 and K2 are in position 2. The output signal from LM2 is the input writing signal for element 1.

The information in a cell is represented by a pair of light signals ("0", "1") or by voltages taken from pins of capacitances. It can be kept as long as needed as a result of iterative repetition of cycles 1-4. If "1" was written to a cell in the initial state, it would have kept a couple of "1", "0".

Similarly, other schemes of optical elements can be built to implement the basic logic functions: repeater, inverter, AND, NOT-AND, OR, NOT-OR, sum by modulo 2, implication. The analysis of transfer characteristics for such elements is presented in [10].

The described principle of implementation of inter-layer communications allows creating functionally flexible devices by replacing electric logical communications by the optical ones and by using 3D structures of gates. A computational device would contain a minimum number of elements and electric connections if the following rules of its creation are followed: parallel electric circuits are used to supply power to elements and parallel logical links are done using optical channels.

60 Optical Communication

**Figure 3.** A dynamic memory cell

2.

couple of "1", "0".

photodetector optical input of another gate.

shot to pin 2 and PD1 was closed in order to do so.

**2.2. A sample of constructing a dynamic memory cell** 

charge does not take place. The scheme of the cell is depicted in Figure 3.

and the switch K*2* shot to pin 1 of the power source with voltage *V*.

When constructing digital devices based on these optical gates the interlayer connections are implemented by placing pairs of "modulator - photodetector" in different layers-planes so that the optical output light modulator of one gate is geometrically aligned with the

Such a cell is intended for storing information in the electric form on capacitors and transferring information between cells by pulsed optical signals, while a direct transfer of

Assume that a bit of information (e.g. "0") is written in the upper gate 1. The switch K1 was

*Cycle 1.* "Erasing" of information in lower gate 2 by charging the capacitance C*2* under lit PD*<sup>2</sup>*

*Cycle 2.* The reading of information from element 1 by the light signal Ird1. The switches K1 and K2 are in position 2. The output signal from LM1 is the input writing signal for element

*Cycle 3.* The "erasing" of information in element 1. The switch K1 is in position 1 and the

*Cycle 4.* The reading of information from element 2 by the light signal Ird2. The switches K1 and K2 are in position 2. The output signal from LM2 is the input writing signal for element 1.

The information in a cell is represented by a pair of light signals ("0", "1") or by voltages taken from pins of capacitances. It can be kept as long as needed as a result of iterative repetition of cycles 1-4. If "1" was written to a cell in the initial state, it would have kept a

Similarly, other schemes of optical elements can be built to implement the basic logic functions: repeater, inverter, AND, NOT-AND, OR, NOT-OR, sum by modulo 2, implication. The analysis of transfer characteristics for such elements is presented in [10].

capacitance C1 is connected to the power source with voltage *V* and is charging.

An illustration of the functionality of a logic gate with optical connections is presented in [10, 11] using samples constructing 1D and 2D shift registers, switch and matrix processor for parallel image processing. The processor consists of a control block and a program controlled cellular automaton. The control block stores a program and fetches its instructions. The cellular automaton performs information storage and processing [11]. A cell in cellular automaton transforms information by changing its own state and those states of its neighbors. Any transformation is represented as a sequence of elementary transformations. Each elementary transformation is defined by the contents of a command that comes to a cell from the control unit. The transformation of information in a cellular automaton is performed simultaneously by all cells.

The following conclusions can be drawn from the analysis of the functioning of the described devices (primarily the matrix processor), the specific features of their design and comparison with microelectronic devices, capable of performing similar functions.


A breadboard construction of a cell of optic dynamic memory is created in order to demonstrate the possibility of practical implementation of optoelectronic gates [11]. It consists of two optoelectronic logic gates. Lithium niobate crystals are used as optoelectronic modulators.

The study of the dynamic memory model has made possible to draw the following conclusions:

