KR20110077225A - All optical logic device based on multimode interference by photonic crystal structure - Google Patents

Abstract

PURPOSE: A device for an electric light logic based on a multi-mode interference is provided to operate various logic gates by having improve signal processing speed. CONSTITUTION: An element for an electrical light logic includes as follows. An multi mode interference device(100). An input unit(200) inputs an input light signal to the multi mode interference device. The input unit has a plurality of input path(210-1,210-2,210-3). An output unit outputs an output optical signal generated from the multimode interference device to external side by receiving the signal. The output unit has a plurality of output path(310-1,310-2,310-3).

Description

All-optical logic device based on multimode interference by photonic crystal structure

The present invention relates to an all-optical logic device based on multi-mode interference by a photonic crystal structure, and more particularly, to form an optical waveguide by the photonic crystal structure, and to output an all-optical light whose output is determined by multimode interference of an input optical signal. It relates to a logic element.

Recently, as the research on the ultra-high speed optical network is actively conducted, there is a need for a device capable of processing optical signals at a higher speed than in the prior art. However, the conventional electric device did not sufficiently process the optical signal because its processing speed is very slow compared to the optical signal. As a result, researches on optical devices capable of processing all signals as light without using an electric signal are being actively conducted. In particular, in order to implement an optical-based optical communication system such as a transistor-based system, it is necessary to develop an optical logic device that is small in size, integratable, easy to manufacture, and low in reliability.

Currently, research on optical logic devices has a basic limitation such as large size and very few functions. They are largely divided into two based on integrated optics, one of which uses nonlinear optics. This method has the advantage of being capable of amplitude modulation, performing the function of logic circuits between different wavelengths, and not considering the phase of the signal. However, there are disadvantages in that materials used for using nonlinear optics are expensive and are difficult to apply to optical circuits and optical chips in integrated systems.

Another work on optical logic devices is the use of multimode interferometers. In this case, the design of the multi-mode interferer is made of a general semiconductor material, thereby reducing the cost, the structure thereof is very simple, and the desired logic circuit can be designed by configuring several devices in multiple stages. However, optical logic devices using multi-mode interferometers studied to date have a disadvantage that their size is about 1 mm, which does not sufficiently take advantage of the integration. Since there is a limit to the miniaturization of the logic element, the length of the region through which light travels is lengthened, thereby delaying the speed of signal processing.

As described above, the necessity of the development of the optical logic element is minimized so that the optical logic element is integrated and applied to various applications, and the processing speed is improved.

SUMMARY OF THE INVENTION The present invention has been made in an effort to provide a multi-mode interference-based all-optical logic device having a photonic crystal structure that can be integrated with a minimized size, has an improved signal processing speed, and can operate with various logic gates. have.

In order to achieve the above technical problem, the multi-mode interference-based all-optical logic device by the photonic crystal structure according to the present invention, the multi-mode for outputting the output optical signal generated by causing the multi-mode interference to the input optical signal The multi-mode interferer includes a defect in which the periodicity of the refractive index of the photonic crystal structure in which the refractive index is periodically changed is removed by a predetermined width from one end of the input optical signal to the other end of the output optical signal. Multimode waveguides are formed.

According to the multi-mode interference-based all-optical logic device by the photonic crystal structure according to the present invention, by providing a multi-mode interferometer formed of the photonic crystal structure, the size of the device is reduced by more than 10 times compared to the conventional all-optical logic device to facilitate integration On the other hand, the travel distance of the optical signal is shortened, which can significantly improve the speed of signal processing. In addition, at least one phase of the plurality of input optical signals is modulated to be input to the multi-mode waveguide so that the shape of the output optical signal can be changed, and as a single element, various logic gates can be operated.

Hereinafter, with reference to the accompanying drawings will be described in detail a preferred embodiment of the multi-mode interference-based all-optical logic device by the photonic crystal structure according to the present invention.

1 is a diagram showing the configuration of a preferred embodiment of a multi-mode interference-based all-optical logic device by a photonic crystal structure according to the present invention.

Referring to FIG. 1, an all-optical logic device according to the present invention includes a multi-mode interferer 100, an input unit 200 for inputting an input optical signal to the multi-mode interferer 100, and an output generated by the multi-mode interferer 100. An output unit 300 for receiving an optical signal and outputting it to the outside.

The input unit 200 includes a plurality of input waveguides 210-1, 210-2, 210-3, and the following 210 which are passages for inputting the plurality of input optical signals to the multi-mode interferometer 100 when there are a plurality of input optical signals. And the output unit 300 includes a plurality of output waveguides 310-1, 310-2, 310-3, hereinafter referred to as 310, which are passages respectively outputted from the multi-mode interferer 100 when there are a plurality of output optical signals. It is provided. Hereinafter, the present invention will be described in detail with a representative embodiment in which there are three input waveguides 210 and three output waveguides 310 as shown in FIG. 1.

In addition, the form of the input optical signal and the output optical signal is represented using 1 or 0. For example, if the input optical signal is represented as (1, 1, 0), the input optical signal is input to the input waveguides 210-1 and 210-2 corresponding to A and B of the three input waveguides 210, respectively. It means that the input optical signal is not input to the input waveguide 210-3 corresponding to C. The same applies to the output optical signal.

The multi-mode interferer 100 outputs at least one output optical signal generated by causing multi-mode interference to the input optical signal to the output unit 300. In addition, the multi-mode interferometer 100 has a multi-mode waveguide having a defect in which the periodicity of the refractive index of the photonic crystal structure in which the refractive index is periodically changed is removed by a predetermined width from one end of the input optical signal to the other end of the output optical signal ( 110 is formed.

Photonic crystal refers to a material in which dielectrics are periodically arranged. In the present invention, the photonic crystal structure forming the multi-mode interferer 100 may be formed by periodically arranging two dielectric materials having different refractive indices, and as shown in FIG. 1, air holes may be periodically formed in the substrate. This may be a formed structure. In general, materials having a crystal structure have a periodic potential due to a regular arrangement of atoms or molecules constituting the material, which affects the movement of electrons. An important phenomenon that occurs is the formation of a bandgap (bandgap), the semiconductor device using such a bandgap.

This concept applies equally to photons, where the dielectric acts as a potential for the photons. Even in the case of photons, when a dielectric acting as a potential is periodically arranged, a band gap is formed like an electron, and this is called a photonic bandgap to distinguish it from an electric bandgap.

FIG. 2A illustrates a representative two-dimensional photonic crystal structure formed by an array of air holes formed to have a rectangular lattice structure in silicon (Si), and FIG. 2B illustrates a plane wave development method for the photonic crystal structure of FIG. 2A. The bandgap diagram obtained.

Referring to FIG. 2B, it can be seen that a relatively large photon bandgap exists within a normalized frequency range of 0.245 to 0.265 (normalized frequency is a / λ, where a is a lattice constant of the photonic crystal and λ is a wavelength). have. If the wavelength is set to 1.55 μm to suit a small propagation loss, the value of a is determined to be 0.4 μm. Accordingly, the radius of the air hole constituting the photonic crystal has a value of 0.15 μm.

When an optical signal having a frequency belonging to the photon bandgap is incident on the photonic crystal, it is totally reflected without propagation into the medium regardless of the incident direction. In this case, if a local line defect is broken artificially in the photonic crystal, an optical waveguide in which the optical signal incident to the line defect is guided to a space smaller than the wavelength by the photon bandgap can be made. This is the multi-mode waveguide 110 of the multi-mode interferer 100 shown in FIG. 1.

Hereinafter, multi-mode interference occurring in the optical waveguide will be described. Multi-mode interferer 100 is a waveguide designed to support various interference modes (generally three or more). In order to inject an optical signal into the multi-mode waveguide 110 of the multi-mode interferer 100 and to obtain an optical signal output from the multi-mode waveguide 110, waveguides serving as passages are disposed at both ends of the multi-mode waveguide 110. Should be. These devices are defined as N × M multimode interference couplers. Where N and M are the number of input and output waveguides, respectively.

In the multi-mode interference region, the light field Ψ (x, y, z) may be expressed as Equation 1 below.

Where z is the propagation direction of the optical signal, x and y are the vertical and vertical directions, C _v is the field simulation coefficient, Ψ _v (x, y) is the mode field function, n (x, y) is the refractive index, β _v Is the mode propagation constant, and v = 0,1,2,... , m-1 is the number of interference modes supported by the multi-mode waveguide 110.

The beat length of the two lowest interference modes is expressed by Equation 2 below.

Where L _π is the bit length, λ is the wavelength, W _e is the lateral penetration depth, W _M is the width of the multi-mode interference, and generally W _e ≒ W _M.

Theoretical analysis shows that Ψ (x, y, z + L) corresponds to an image of the form Ψ (x, y, z) at the z + L position, depending on the C _v and mode phase factor. do. Therefore, the light field is repeated when the value of L satisfies the single image condition of Equation 3 below.

In 3x3 multimode interference, the asymmetrical input optical signal emitted from the cross port is 5π / 6, 2π / 3 and π, respectively, producing three self images. Thus, phase modulation at selected points within the interval of multi-mode interference where such self-images occur may change the output image.

Finally, the coupler length L, that is, the length of the multi-mode waveguide 110 may be given by Equation 4 below.

Where N is an image of the input field.

The all-optical logic element according to the present invention shown in FIG. 1 can pass up to three input optical signals and up to three output optical signals. Thus, the multimode interferer 100 can cause 3x3 multimode interference.

As described above, the size of the all-optical logic device according to the present invention can be remarkably minimized by using the multi-mode interferometer 100 having the multi-mode waveguide 110 formed by the defect whose periodicity is removed from the photonic crystal structure. have. Specifically, all-optical logic devices using multi-mode interferometers studied to date are designed to be about 1 mm in size, which is not preferable for integration. However, the all-optical logic device having the multi-mode interferometer 100 of the photonic crystal structure according to the present invention can be downsized from 1/10 to up to 1/40, so it is easy to integrate. Furthermore, the reduction in the size of the device reduces the travel distance of the optical signal, thereby showing a remarkable improvement in the processing speed of the optical signal. In the exemplary embodiment of the all-optical logic element according to the present invention, the specific size of each part will be described in detail later.

Up to three input optical signals input to the multi-mode interferer 100 of the all-optical logic device shown in FIG. 1 and up to three output optical signals output from the multi-mode interferometer 100 may be provided at one end of the multi-mode waveguide 110. The optical waveguide-type device coupled to the other end may be input to or output from the multi-mode interferer 100. In this case, at least one phase of the plurality of input optical signals may be set to be different from the remaining input optical signals.

As described above, phase modulation of the input optical signal changes the output optical signal. The shape of the output optical signal is determined by the multi-mode interference generated in the multi-mode interferometer 100 for the plurality of input optical signals. When at least one phase of the plurality of input optical signals is different from other input optical signals, the interference pattern is general. It is different from the case.

In order to change the phase of at least one of the plurality of input optical signals, the phase of the input optical signal input to the multi-mode waveguide 110 or the plurality of input waveguides 210 may be input differently from the beginning. However, by inputting a plurality of input optical signals having the same wavelength, phase, and polarization, respectively, at the inlet of the plurality of input waveguides 210 and adjusting the lengths of the input waveguides 210, the result is input to the multi-mode waveguide 110. The phase of the input optical signal may be changed.

Meanwhile, the input unit 200 and the output unit 300 may be formed in a photonic crystal structure, such as the multimode interferer 100, and may be integrally coupled to the multimode interferer 100. That is, as shown in FIG. 1, the input unit 200 coupled to one end of the multi-mode interferer 100 is a defect in which the periodicity of the refractive index of the photonic crystal structure is removed in the same direction as the longitudinal direction of the multi-mode interferer 100. A plurality of input waveguides 210 are formed, and in the output unit 300 coupled to the other end of the multi-mode interferer 100, the periodicity of the refractive index of the photonic crystal structure is the same as the length direction of the multi-mode interferer 100. A plurality of output waveguides 310 formed of the defects removed by the second transistor are formed. As such, the input unit 200 and the output unit 300 may be formed to have a photonic crystal structure like the multi-mode interferer 100, thereby minimizing the size of the all-optical logic device.

In this case, the widths of the input waveguide 210 and the output waveguide 310 may be set to the same width as the periodicity of the refractive index of the photonic crystal structure is removed by one column, and the adjacent input waveguide 210 or the output waveguide 310 may be formed. The spacing therebetween may be set to a width corresponding to at least three columns of the photonic crystal structure. Accordingly, the width of the multi-mode waveguide 110 is also set such that the plurality of input waveguides 210 or output waveguides 310 may be connected to the multi-mode waveguide 110.

When the input unit 200 is formed in the photonic crystal structure as described above, the lengths of the plurality of input waveguides 210 may be determined by adjusting the number of periods in the photonic crystal structure. For example, in FIG. 1, the input waveguide C 210-3 is formed longer by one period than the input waveguides A and B 210-1 and 210-2, and the length of the input waveguide 210 is long. The difference modulates the phase of the input optical signal and changes the output optical signal. In addition, the length difference between the plurality of input waveguides 210 may be appropriately set according to the type of the desired output optical signal.

3 is a diagram illustrating a comparison of the shape change of an output signal before and after phase modulation of an input optical signal. Referring to FIG. 3, one length of three input waveguides 210 is designed to be longer than the length of the other two input waveguides 210 to modulate the phase of the input optical signal. Accordingly, when all three input optical signals inputted as (1,0,1) have the same phase, the output optical signal is obtained as (0,1,0), but is input to one input waveguide 210. By modulating the phase of the input optical signal, an output optical signal such as (1,1,0) is obtained. As described above, in the present invention, the phase of the input optical signal can be adjusted to obtain a desired output optical signal.

4 is a view showing a simulation result performed to determine the shape of the output optical signal according to the shape of the various input optical signal in the all-optical logic device according to the present invention.

The all-optical logic element for the simulation is designed to pass an optical signal with a wavelength of 1.55 μm, and the air hole radius is set to 150 nm, the lattice constant is 400 nm, the silicon refractive index is 3.478, and the air hole refractive index is 1.0. The propagation loss of the waveguide formed by removing the periodicity of the refractive index is 1.6 dB / mm. In addition, the multi-mode waveguide 110 is designed to have a width of 4.8 μm and a length of 64 μm, and a distance between two neighboring input waveguides 210 or output waveguides 310 is 1.2 μm. The all-optical logic device will have a length of 70 μm, including an input length of 3.2 μm.

This corresponds to a size smaller than 1/10 compared with the conventional all-optical logic device having a size of 700 μm or more. Furthermore, it is possible to minimize the thickness of the all-optical logic element to within several μm, preferably to 1 μm. The reason for minimizing the size of the device is that the multi-mode interferer 100 has a smaller size than a general waveguide by implementing the photonic crystal structure. Therefore, it is desirable to implement not only the multi-mode interferer 100 but also the input unit 200 and the output unit 300 by the photonic crystal structure in order to minimize device size.

Each part size of the above-mentioned all-optical logic element is a value that can be obtained when the device is miniaturized to the maximum, and can be made by changing the size of each part according to the shape of the desired output optical signal.

The patterns shown in FIGS. 4A to 4G illustrate a path through which multi-mode interference occurs between input optical signals input from the input waveguide 210 on the left side and output through the output waveguide 310 on the right side. . First, referring to FIGS. 4A to 4C, when only one input optical signal is input, an output optical signal is output through all three output waveguides 310. Next, referring to FIGS. 4D through 4F, when two input optical signals are input, two output optical signals are generated, and as shown in FIG. 4G, three input waveguides 210 are provided. When the input optical signal is input through both, the output optical signal is obtained only in the output waveguide II 310-2.

As described above, in the all-optical logic device according to the present invention, when a plurality of input optical signals are input, at least one phase may be modulated to obtain an output different from that of the existing output optical signal. Can function as a gate.

Table 1 below shows the shape of the input / output optical signal when the all-optical logic device according to the present invention performs the function of the OR logic gate.

input
Light signal
A 0 0 0 One 0 One One One B 0 0 One 0 One 0 One One C 0 One 0 0 One One 0 One Output optical signal (Ⅱ) 0 One One One One One One One

Referring to Table 1, an output optical signal is obtained through the output waveguide II 310-2 in all cases except when no input optical signal is input through the A, B, and C input waveguides 210. . Therefore, when the output waveguide II 310-2 is designated as an output port, the all-optical logic element according to the present invention corresponds to an OR logic gate.

Next, when an input optical signal input to the input waveguide B 210-2 is designated as a control optical signal and an output waveguide I 310-1 or III 310-3 is designated as an output port, according to the present invention, The all-optical logic element may operate as a NOT logic gate for the input optical signal input to the input waveguides A 210-1 or C 210-3. The results are shown in Table 2 and Table 3 below.

Input optical signal A 0 One Control light signal B One One Output optical signal (Ⅰ) One 0

Input optical signal C 0 One Control light signal B One One Output optical signal (Ⅲ) One 0

Similarly, when the input optical signal input to the input waveguide A 210-1 is designated as the control optical signal and the output waveguides I 310-1 or III 310-3 are designated as output ports, The all-optical logic element may operate as a NOT logic gate for the input optical signal input to the input waveguide B 210-2 or C 210-3. The results are shown in Tables 4 and 5 below.

Input optical signal B 0 One Control light signal A One One Output optical signal (Ⅰ) One 0

Input optical signal C 0 One Control light signal A One One Output optical signal (Ⅲ) One 0

On the other hand, if the input optical signal input to the input waveguide C (210-3) is designated as the control optical signal and the output waveguide I (310-1) as the output port, the all-optical logic device according to the present invention is also a NAND logic gate It can work. That is, when the all-optical logic element according to the present invention operates as the NAND logic gate, the output optical signal according to the input optical signal is shown in Table 6 below.

Control light signal C One One One One input
Light signal B 0 One 0 One A 0 0 One One Output optical signal (Ⅰ) One One One 0

Finally, when the input optical signal input to the input waveguide C 210-3 is designated as the control optical signal and the output waveguide III 310-3 is designated as the output port, the all-optical logic device according to the present invention operates as a NOR logic gate. Done. This is because the NAND logic gate and the control optical signal described above are the same and only the output waveguide 310 designated as the output port is different. Operation to the NOR logic gate is shown in Table 7 below.

Control light signal C One One One One input
Light signal B 0 One 0 One A 0 0 One One Output optical signal (Ⅲ) One 0 0 0

As described above, by designating an input optical signal input to any one of the plurality of input waveguides 210 as a control optical signal and designating any one of the plurality of output waveguides 310 as an output port of a logic gate. It is possible to make the all-optical logic device according to it operate with the desired logic gate. In addition, the operation of the various logic gates described above is merely an embodiment of the all-optical logic device according to the present invention, and the operation of the logic logic gates may be operated by different logic gates through the control of the size of each part and the designation of the control signal and the output port. You may.

As described above, the all-optical logic device according to the present invention has a silicon-based multi-crystal interferometer 100 having a photonic crystal structure, the size of which is significantly smaller than that of the conventional all-optical logic device, resulting in integration and low-cost mass production. This is possible. In addition, as the size of the device is minimized, the optical signal travel distance is also shortened, which may result in a significant improvement in signal processing speed.

Although the preferred embodiments of the present invention have been shown and described above, the present invention is not limited to the specific preferred embodiments described above, and the present invention belongs to the present invention without departing from the gist of the present invention as claimed in the claims. Various modifications can be made by those skilled in the art, and such changes are within the scope of the claims.

1 is a view showing the configuration of a preferred embodiment for a multi-mode interference-based all-optical logic device by a photonic crystal structure according to the present invention,

2A and 2B are diagrams each showing a representative two-dimensional photonic crystal structure and a bandgap diagram obtained by the planar wave unfolding method;

3 is a view showing a comparison of the shape change of the output signal before and after phase modulation of the input optical signal, and

4 is a view showing a simulation result performed to determine the shape of the output optical signal according to the shape of the various input optical signal in the all-optical logic device according to the present invention.

Claims (6)

An all-optical logic element having a multi-mode interferer for outputting an output optical signal generated by causing multi-mode interference to an input optical signal, The multi-mode interferer is formed with a multi-mode waveguide formed by a defect in which the periodicity of the refractive index of the photonic crystal structure in which the refractive index is periodically changed is removed by a predetermined width from one end where the input optical signal is input to the other end where the output optical signal is output. All-optical logic element, characterized in that. The method of claim 1, And at least one phase of a plurality of input optical signals input to one end of the multi-mode interferer is different from the remaining input optical signals. The method of claim 1, A plurality of input waveguides formed of defects in which the periodicity of the refractive index of the photonic crystal structure is removed in the same direction as the longitudinal direction of the multimode interferer is formed, and are integrally coupled to one end of the multimode interferer to input the input into the multimode waveguide. An input unit for inputting an optical signal; And A plurality of output waveguides formed of defects in which the periodicity of the refractive index of the photonic crystal structure is removed in the same direction as the longitudinal direction of the multimode interferer are formed, and are integrated at the other end of the multimode interferer facing one end to which the input unit is coupled. And an output unit coupled to the all-optical logic element. The method of claim 3, And at least one length of the plurality of input waveguides is different from the length of the remaining input waveguides to modulate the phase of an input optical signal input to the multi-mode waveguide. 5. The method according to any one of claims 1 to 4, And the photonic crystal structure is a structure in which a plurality of air holes are regularly arranged. 5. The method according to any one of claims 1 to 4, And an logic gate of any one of OR, NOT, NAND, and NOR logic gates according to an input pattern of the input optical signal.

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