This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. P2014-36586 filed on Feb. 27, 2014, the entire contents of which are incorporated herein by reference.
An embodiment described herein relates to a directional coupler and a multiplexer and demultiplexer. The embodiment relates to in particular a directional coupler which can be miniaturized used for optical waves, terahertz (THz) waves, or millimeter waves, and a multiplexer and demultiplexer to which such a directional coupler is applied.
In recent years, for THz wave band (0.1 THz to 10 THz) positioned in intermediate frequencies between electromagnetic waves and optical waves, studies of applicabilities of ultra high-speed wireless communications, sensing, imaging, etc. have become active, and there has been expected its practical application. However, since THz-wave systems are composed of large-sized and three-dimensional structured components under the current circumstances, large-sized and expensive configurations are required for such THz-wave systems. In order to miniaturize the whole of such systems, implementation of THz-wave integrated circuits (ICs) integrating devices is indispensable.
Utilization of technologies of both of an optical wave region and an electric wave region can be considered as fundamental technologies of the THz-wave ICs. However, optical components, e.g. lenses, mirrors, are composed of large-sized and three-dimensional structured components, and therefore are not suitable for the integration. Moreover, it is becoming difficult to produce hollow metal waveguides used in the electric wave region due to its fine three-dimensional structure. Furthermore, a waveguide loss in planar metallic-transmission lines is increased as effect of metallic absorption is increased.
As a fundamental technology of THz-wave ICs, there has been studied applicability of a two dimensional photonic crystal (2D-PC) slab where outstanding progress is seen in the optical wave region.
Moreover, there has been studied resonant and waveguiding line defect modes in an electromagnetic 2D band-gap (BG) slab structure for millimeter wave frequency bands.
Moreover, there has been realized multiplexers and demultiplexers using minute resonators in a wavelength-order size, in minuteness and integration of optical devices with the PC having a periodic refractive index profile.
Furthermore, in directional couplers using the PC, coupling length is miniaturized up to approximately wavelengths until now.
It is theoretically difficult to operate the multiplexer and demultiplexer using the resonator in broader bandwidths. Moreover, the sizes of ordinary optical multiplexers and demultiplexers are approximately several millimeters. Moreover, the optical multiplexer and demultiplexer using conventional micro PC directional couplers have narrower operational bands in a crossed state, such as approximately 0.2% of an operational frequency, and a degree of signal separation between a bar state and the crossed state is also as insufficient, such as less than 10 dB.
The embodiment provides a directional coupler which has a wide-band and high degree of signal separation and can be miniaturized, used for optical waves, THz waves, or millimeter waves, and a multiplexer and demultiplexer to which such a directional coupler is applied.
According to one aspect of the embodiment, there is provided a directional coupler comprising: a two dimensional photonic crystal slab; lattice points periodically arranged in the two dimensional photonic crystal slab, the lattice points configured to diffract optical waves, terahertz waves, or millimeter waves in photonic bandgap frequencies in photonic band structure of the two dimensional photonic crystal slab in order to prohibit existence in a plane of the two dimensional photonic crystal slab; a first two dimensional photonic crystal waveguide disposed in the two dimensional photonic crystal slab and formed with a line defect of the lattice points; a second two dimensional photonic crystal waveguide formed of a line defect of the lattice point in the two dimensional photonic crystal slab, mode coupling of the second two dimensional photonic crystal waveguide being realized to the first two dimensional photonic crystal waveguide; and a directional coupling unit disposed between the first two dimensional photonic crystal waveguide and the second two dimensional photonic crystal waveguide, the directional coupling unit including lattice points between waveguides, the size of the lattice points between waveguides is smaller than that of the lattice point.
According to another aspect of the embodiment, there is provided a multiplexer and demultiplexer comprising such a directional coupler.
According to the embodiment, there can be provided the directional coupler which has the wide-band and high degree of signal separation and can be miniaturized, used for optical waves, THz waves, or millimeter waves, and the multiplexer and demultiplexer to which such a directional coupler is applied.
Next, a certain embodiment will now be described with reference to drawings. In the description of the following drawings, the identical or similar reference numeral is attached to the identical or similar part. However, it should be noted that the drawings are schematic and the relation between thickness and the plane size and the ratio of the thickness of each component part differs from an actual thing. Therefore, detailed thickness and size should be determined in consideration of the following explanation.
Of course, the part from which the relation and ratio of a mutual size differ also in mutually drawings is included. Moreover, the embodiment described hereinafter merely exemplifies the apparatus and method for materializing the technical idea; and the embodiment does not specify the material, shape, structure, placement, etc. of each component part as the following. The embodiment may be changed without departing from the spirit or scope of claims.
As shown in
Although the directional coupler 20 includes 2D-PC waveguides 141, 142 separated for the distance amount of two rows of lattice points, for example, a detailed configuration will be mentioned below (in
The input/output interface 60 is a coupler from free space, and is composed of a grating coupler composed of a one-dimensional PC, for example. The input/output interface 60 can also be composed by using 2D-PC.
The detector 18R can be composed of a THz wave receiver on which a Resonant Tunneling Diode (RTD) etc. is mounted, or a Schottky Barrier Diode (SBD), for example.
The transmitter (light source) 18T can be composed of a THz wave transmitter on which RTD etc. is mounted, or a semiconductor laser. In this case, the following are applicable as materials of the semiconductor laser, for example. That is, for example, GaInAsP/InP based materials are applicable in the case of wavelengths of 1.3 μm to 1.5 μm; InGaAs/GaAs based materials are applicable in the case of an infrared light with a wavelength of 900 nm; GaAlAs/GaAs based or GaInNAs/GaAs based materials are applicable in the case of an infrared light/near-infrared light with wavelengths of 800 to 900 nm; GaAlInAs/InP based materials are applicable in the case of wavelengths of 1.3 μm to 1.67 μm; AlGaInP/GaAs based materials are applicable in the case of a wavelength of 0.65 μm; and GaInN/GaN based materials are applicable in the case of a blue light.
The multiplexer and demultiplexer 30 according to the embodiment can propagate optical waves, THz waves, or millimeter waves.
As shown in
The 2D-PC waveguide 14, 141, 142, 14R, 14T are disposed on the 2D-PC slab 12 and is formed of a line defect of the lattice points 12A.
The directional coupler is a device for extracting a signal propagated in a specific direction in a transmission line, and has frequency selectivity slowly than that of a resonator.
As shown in
As shown in
The directional coupler 20 according to the embodiment has a large operational band, and can secure sufficient degree of signal separation, and can be miniaturized as explained below in detail, with respect to the above-mentioned theoretic configuration. Moreover, the directional coupler 20 can propagate the optical waves, THz waves, or millimeter waves.
The multiplexer and demultiplexer has a signal processing function for switching a path of light and a path of electromagnetic wave in accordance with the frequencies (wavelengths). In the directional coupler according to the embodiment can be miniaturized and integrated by applying the 2D-PC.
Although
There will be mainly explained a structural example of one input port and two output port for the sake of simplifying the detailed structure of the PC slab, but it is also possible to configure to provide both of multi-input ports and multi-output ports.
There will now be explained a design procedure of the multiplexer and demultiplexer 30 to which the directional coupler 20 according to the embodiment is applied, with reference to
Step (a): firstly, the directional coupler 20 is designed using a PB diagram so that broader bandwidths and small operation can be achieved as much as possible, as shown in
Step (b): next, the design of the directional coupler 20, or the input waveguide 14(I) and the output waveguide 14(02) is changed so that a band of the input waveguide 14(I) and the output waveguide 14 (02) is matched to that of the directional coupler 20.
The above-mentioned steps (a) and (b) are fundamentally required as the multiplexer and demultiplexer 30.
Step (c): ideally, it is an operation to be output only to the port OP2 at one side in a certain frequency, but actually, an output component to another port OP1 also exist. In order to reduce an excessive output to another port OP1 and to improve a degree in separation (ratio between the output to main port OP2 and the output to another port OP1), the design is changed so that an interrupt of the signal propagation to the output waveguides 14 except for main port OP2(01) can be achieved. Thereby, the signal separative performance of the multiplexer and demultiplexer 30 can be further improved.
As shown in
The following configurations are adopted for the directional coupler 20 according to the embodiment.
(a) The separation distance between the 2D-PC waveguides 141, 142 is formed by inserting the lattice points in two rows between the PC waveguides so that the even mode and the odd mode occur in the 2D-PC waveguides 141, 142 portions are coupled to each other and the mode spacing becomes as large as possible. In this case, the holes between waveguides (lattice points) arranged at two rows are illustrated with reference numeral 12S.
(b) The radius r′ of the circular holes between the waveguides is set to 0.23 time of the period a so that the propagation constant of the even mode and odd mode may become constant over the broader frequency ranges as possible.
(c) In order to be matched to the operational band of the 2D-PC waveguide 14 of the input port (port P1), the waveguide width of the 2D-PC waveguide 142 is formed to be narrowed only 0.15a so that the whole dispersion curve of the directional coupling unit 50 is be moved to the higher-frequency side. In this case, the waveguide width of the 2D-PC waveguide 142 is formed to be narrowed only 0.3a at first, and then as a result of which the width of the 2D-PC waveguide 141 is formed to be narrowed only 0.15a as mentioned below, thereby finally is formed to be narrowed only 0.15a, up to 0.3a-0.15a.
(d) In order to improve a degree in separation between the bar state (ports P1 to P2) and the crossed state (ports P1 to P3), the width of the 2D-PC waveguide 141 connected from the directional coupling unit 50 to the port P2 is formed to be narrowed only 0.15a to form a mode gap to the port P2 in the frequency band of cross operation.
Calculation of a propagation mode in a wavenumber direction of the arrow shown in
According to the directional coupler 20 according to the embodiment, since the wavenumber difference Δk can be made smaller in order to be constantly held over the broader bandwidths of frequency difference Δf, it is possible to realize broader bandwidths and miniaturizing of the directional coupler for optical waves, THz waves, or millimeter waves.
In the directional coupler according to the embodiment, the even mode and the odd mode are generated by forming two waveguides to be adjacent to each other, in order to use an interference effect between the even mode and the odd mode.
In the directional coupling, as shown in
Although the even mode and the odd mode are generated by coupling two waveguide modes, a degree of decoupling of a state in the mode (frequency and wavenumber) is proportional to a strength of coupling of two waveguides. That is, the degree of decoupling is equal to the wavenumber difference Δk and the frequency difference Δf between the even mode and the odd mode, as shown in
In the directional coupler according to the embodiment, the degree of decoupling is increased and thereby miniaturizing and broader bandwidth thereof are possible, as the strength of coupling of two 2D-PC waveguides 141, 142 becomes stronger. This is because the coupling length LC is proportional to the inverse number of wavenumber difference Δk (1/Δk).
Moreover, as shown in
However, as shown in
A dispersion relation which is a relationship between these frequencies f and the wavenumber k is obtained with the PB diagram. In the PC, the dispersion relation can be flexibly adjusted by using a structural parameter, and coupling between the waveguide modes can be strengthened since optical confinement to the waveguide is strong.
The 2D-PC slab 12 includes a dielectric plate structure having 2D periodic structure. In the 2D-PC slab 12, PBG in which the electromagnetic mode cannot exist appears by the design thereof. Furthermore, the waveguide mode can be introduced in the PBG by disturbing the periodic structure, and thereby a low-loss waveguide in a micro region equal to or less than the wavelength size thereof can be realized.
In this case, the bandwidth of PBG depends on a refractive index of dielectrics, and therefore high-refractive index materials are preferable to be adapted therefor.
Materials of the 2D-PC slab 12 applicable to the directional coupler 20 according to the embodiment may be formed with semiconducting materials.
Since the directional coupler according to the embodiment can propagate the optical waves, THz waves, or millimeter waves, it can apply the following as the semiconducting materials. More specifically, silicon (Si), GaAs, InP, GaN, etc. are applicable thereto, and GaInAsP/InP based, GaInAs/GaAs based, GaAlAs/GaAs based or GaInNAs/GaAs based, GaAlInAs/InP based, GaAlInP/GaAs based, GaInN/GaN based materials, etc. are applicable thereto. In particular, high resistivity Si has a high refractive index in the THz wave bands, and therefore there is little material absorption.
In addition, the lattice point for resonator 12A may be formed as an air hole, or may be filled up with a semiconductor layer differing in the refractive index, for example. For example, the lattice point may be formed by a GaAs layer filled up with a GaAlAs layer.
Moreover, it is possible to adapt as the lattice point (hole) 12A not only the structure where the hole of air is formed, but the structure where (a part of) the hole is filled up with a low-refractive index (low-dielectric constant) medium. Polymeric materials, e.g. Teflon, fluorine contained resin, a polyimide, acrylic, polyester, an epoxy resin, a liquid crystal, a polyurethane, etc. are applicable to the low-refractive index (low-dielectric constant) medium, for example. As a low-refractive index (low-dielectric constant) medium, dielectrics, e.g. SiO2, SiN, SiON, an alumina, and a sapphire, are also applicable, for example. Moreover, porous bodies, e.g. an aerogel, etc. are also applicable to the low-refractive index (low-dielectric constant) medium.
Moreover, not only the semiconductor materials but also the high-refractive index medium can be applied, as the materials of the 2D-PC slab 12. For example, magnesium oxide (MgO) is applicable to the 2D-PC slab 12 since the refractive index in the THz wave band becomes approximately 3.1 which is high dielectric (insulator).
The 2D-PC slab 12 applicable to the directional coupler according to the embodiment can be formed with a silicon, for example. Furthermore, as shown in
According to the electromagnetic field simulation result of the relationship between the lattice constant a of the lattice points 12A and the PGB frequency which are periodically arranged in the 2D-PC slab 12, the PGB frequency band can be varied to higher frequency by making the lattice constant small. For example, the PGB frequency band is appeared ranging from approximately 0.9 to approximately 1.1 THz in the lattice constant a=80 μm, ranging from approximately 0.31 THz to approximately 0.38 THz in the lattice constant a=240 μm (experiment structure), and ranging from approximately 0.10 THz to approximately 0.12 THz in the lattice constant a=750 μm.
Moreover, handling frequency bands are not limited to the THz wave band, but a general optical waves are also included. In this case, as the 2D-PC slab 12, the lattice constant a of the lattice points 12A is miniaturized, and thereby the operating wavelength may be set as ranging from approximately 1 μm to 2 μm bands, and the lattice constant is set as ranging from approximately 250 nm to approximately 500 nm, etc., for example. Moreover, the diameter and the depth of the lattice points 12A are respectively approximately 200 nm and approximately 300 nm, for example. The numerical examples can be appropriately changed according to materials, a wavelength, etc. to compose the 2D-PC slab 12. For example, in the 2D-PC slab 12 to which GaAs/GaAlAs based materials are applied, the wavelength is approximately 200 nm to approximately 400 nm.
(Variation of Dispersion Relationship in the Case where the Row Numbers Between Waveguides is Different from One Another)
Next, there will now be explained a method to vertically narrow the mode spacing. The row number of holes between the 2D-PC waveguides 141, 142 is varied in order to adjust the strength of coupling between two 2D-PC waveguides 141, 142.
In the directional coupler according to the embodiment,
The coupling strength between the waveguide modes becomes strong as the space between the 2D-PC waveguides 141, 142 becomes narrow, as shown in
The mode coupling cannot be obtained in the example of inserting the lattice points 12A(1) in one row between the 2D-PC waveguides 141, 142, as shown in
It is proved that the mode spacing vertically narrows as the row number between the line defects are increased, in the variation of dispersion property at the time when the row number of holes for separating two waveguides is changed. At this time, if the row number is one, the sufficient coupling can be realized since the size of the mode spacing is too great. Moreover, if three rows thereof are used, the mode spacing becomes narrow, but the sufficient bands cannot be fully secured. That is, the row number is optimally two since the sufficient mode coupling can be realized and the sufficient bands can secured.
In the directional coupler 20 according to the embodiment,
In the directional coupler 20 according to the embodiment, the strength of the coupling between the 2D-PC waveguides 141, 142 can be varied also by varying the radius r of holes between the 2D-PC waveguides 141, 142.
As shown with the arrow R in
In
As shown with the arrow S in
In the directional coupler 20 according to the embodiment, the waveguide band between the 2D-PC waveguides 141, 142 is adjustable by adjusting the waveguide width, and the hole diameter, period, and refractive index of the PC slab, etc. For example, if a semiconducting material which composing the PC slab 12 is GaxIn1-xAsyP1-y, the refractive index can be changed by changing the composition ratios x and y.
As shown in
Conversely, the width of the 2D-PC waveguides 141, 142 is enlarged, the radius r′ of the hole 12S between waveguides is reduced, the period a is enlarged, and the refractive index of the materials of the 2D-PC slab 12 is enlarged, and thereby the even mode and the odd mode of the PB diagram are moved to the lower-frequency side. Accordingly, although the operational band of the directional coupler 20 may not be matched to the original operational band of the input waveguide, it is adjustable in the operational band of the input/output waveguide in the directional coupling unit 50 or the directional coupler 20 according to the directional coupler 20 according to the embodiment.
As shown in
As shown in
As shown in
That is, it was confirmed that the even mode and the odd mode are converted at every 4 periods, 8 periods, 12 periods, 16 periods . . . to one another, and the coupling length LC becomes constant with 4a at a band which is approximately 10 GHz, and the similar result as the band calculation result shown in
Moreover, as shown in
As shown in
In the directional coupler according to the comparative example, as shown in
The 2D-PC slab 12 which is a sample of the directional coupler 20 is composed of a silicon substrate in approximately 200 μm thick, and the period a of the lattice points 12A is approximately 240 μm.
As shown in
As shown in
As shown in
The transmittances with regard to the port P1 to port P2 (bar state), the port P1 to port P3 (crossed state), and the port P2 to port P3 are respectively measured by changing connection between the waveguide and the adiabatic mode converter. Examples of system of measurement of the port P1 to port P3 (crossed state) are shown in
In the experimental result shown in
As clearly from the experimental result shown in
A configuration to connect the directional coupler in parallel may be adopted as a method of achieving further broader bandwidths.
The configuration to connect the directional coupler according to the embodiment in parallel for the purpose of broader bandwidth is schematically illustrated as shown in
Moreover,
As shown in
Since the operational frequency f is determined with the period a of PC and the waveguide width, the further broader bandwidth can be achieved by forming the parallel connection structure in which the period a or the waveguide width is changed.
In the directional couplers 201, 202, 203, there are obtained signals respectively having the operational frequency f1 and operational band B1, the operational frequency f2 and operational band B2, and the operational frequency f3 and operational band B3 from the ports P31, P32, P33, via the 2D-PC waveguides 1431, 1432, 1433 in the crossed state branched from the 2D-PC waveguides 1411, 1412, 1413 in the bar state. The directional couplers 201, 202, 203 are composed as well as the above-mentioned directional coupler 20 according to the embodiment.
In this case, the mode coupling of the 2D-PC waveguides 1411, 1421 is realized, and the lattice points between waveguides arranged in two rows are arranged between the 2D-PC waveguides 1411, 1421. The radius r′ of holes of the lattice points between waveguides is set as 0.23a1, for example, so that the propagation constant of the even mode and the odd mode may become constant over the broader frequency ranges.
In order to match the 2D-PC waveguide 14 to the operational band at the side of port P1 from the directional coupling unit, the width of the 2D-PC waveguide 1421 is formed to be narrowed compared with the width formed with the line defect of lattice point so that the whole dispersion curve of the directional coupling unit may be moved to the higher-frequency side. For example, the width of the 2D-PC waveguide 1421 is formed to be narrowed for 0.15 time of the period a1.
In order to increase the degree of signal separation in the bar state between the port P1 and the port P2, and in the crossed state between the port P1 and the port P31, the width of the 2D-PC waveguide 1411 at the side of the port P2 from the directional coupling unit is formed to be narrowed. For example, the width of the 2D-PC waveguide 1411 is formed to be narrowed for 0.15 time of the period a1.
Similarly, the mode coupling of the 2D-PC waveguides 1412, 1422 is realized, and the lattice points between waveguides arranged in two rows are arranged between the 2D-PC waveguides 1412 and 1422. The radius r′ of holes of the lattice points between waveguides is set as 0.23a2, for example, so that the propagation constant of the even mode and the odd mode may become constant over the broader frequency ranges.
In order to match the 2D-PC waveguide 1412 to the operational band at the side of port P1 from the directional coupling unit, the width of the 2D-PC waveguide 1422 is formed to be narrowed compared with the width formed with the line defect of lattice point so that the whole dispersion curve of the directional coupling unit may be moved to the higher-frequency side. For example, the width of the 2D-PC waveguide 1422 is formed to be narrowed for 0.15 time of the period a2.
Moreover, in order to increase the degree of signal separation in the bar state between the port P1 and the port P2, and in the crossed state between the port P1 and the port P32, the width of the 2D-PC waveguide 1412 at the side of the port P2 from the directional coupling unit is formed to be narrowed. For example, the width of the 2D-PC waveguide 1412 is formed to be narrowed for 0.15 time of the period a2.
Similarly, the mode coupling of the 2D-PC waveguides 1413, 1423 is realized, and the lattice points between waveguides arranged in two rows are arranged between the 2D-PC waveguides 1413 and 1423. The radius r′ of holes of the lattice points between waveguides is set as 0.23a3, for example, so that the propagation constant of the even mode and the odd mode may become constant over the broader frequency ranges.
In order to match the 2D-PC waveguide 1413 to the operational band at the side of port P1 from the directional coupling unit, the width of the 2D-PC waveguide 1423 is formed to be narrowed compared with the width formed with the line defect of lattice point so that the whole dispersion curve of the directional coupling unit may be moved to the higher-frequency side. For example, the width of the 2D-PC waveguide 1423 is formed to be narrowed for 0.3 time of the period a3.
Moreover, in order to increase the degree of signal separation in the bar state between the port P1 and the port P2, and in the crossed state between the port P1 and the port P33, the width of the 2D-PC waveguide 1413 at the side of the port P2 from the directional coupling unit is formed to be narrowed. For example, the width of the 2D-PC waveguide 1413 is formed to be narrowed for 0.15 time of the period a3 of the lattice points.
In the directional couplers 201, 202, 203 connected thereto in parallel, the following relationships are realized between the operational frequencies f1, f2, f3 and the periods a1, a2, a3 of the lattice points:
f
2=(a1/a2)f1,f3=(a2/a3)f2 (1)
B
2=(f2/f1)B1,B3=(f3/f2)B2 (2)
Also in the directional couplers in multi stage connected in parallel, the similar relationships as the equations (1) and (2) are realized between adjacent directional couplers.
In the directional coupler according to the embodiment, as mentioned above, due to the configuration of the directional couplers 201, 202, 203 in three-stage connected in parallel, the operational bands B1, B2 B3 are set up just to be connected on the frequency characteristics on the relationship between the operational band B and the operational frequency f, or are set up to be respectively larger than 0% and smaller than 100%, and thereby the operational band can be enlarged by connecting in parallel.
The periods of lattice points 12A in the 2D-PC slab 12 composing the directional couplers 201, 202 are respectively a1, a2. In this case, the periods a1 and a2 are respectively approximately 240 μm and approximately 235 μm, as a detailed numerical example, for example. Moreover, in order to reduce an influence of reflection in a junction interface between the directional couplers 201, 202, as shown in
In the directional couplers 201, 202, the 2D-PC waveguides 1431, 1432 in the crossed state are branched from the 2D-PC waveguides 1411, 1412 in the bar state between the port P1 and the port P2, and the 2D-PC waveguides 1431(R), 1432(R) in the crossed state are branched from the 2D-PC waveguides 1411(R), 1412(R) in the bar state between the port P3 and the port P4. The 2D-PC waveguides 1431, 1432 in the crossed state are coupled with the 2D-PC waveguides 1431(R), 1432(R) in the crossed state at a center portion, and the structure shown in
In this case, the mode coupling of the 2D-PC waveguides 1411, 1421 is realized, and the lattice points between waveguides arranged in two rows are arranged between the 2D-PC waveguides 1411, 1421. The radius r′ of holes of the lattice points between waveguides is set as 0.23a1, for example, so that the propagation constant of the even mode and the odd mode may become constant over the broader frequency ranges.
In order to match the 2D-PC waveguide 1411 to the operational band at the side of port P1 from the directional coupling unit, the width of the 2D-PC waveguide 1421 is narrowed compared with the width formed with the line defect of lattice point so that the whole dispersion curve of the directional coupling unit may be moved to the higher-frequency side. For example, the width of the 2D-PC waveguide 1421 is formed to be narrowed for 0.15 time of the period a1.
Moreover, in order to increase the degree of signal separation between the bar state and the crossed state, the width of the 2D-PC waveguide 1411 at the side of the second port P2 from the directional coupling unit is formed to be narrowed. For example, the width of the 2D-PC waveguide 1411 is formed to be narrowed for 0.15 time of the period a1.
Similarly, the mode coupling of the 2D-PC waveguides 1412, 1422 is realized, and the lattice points between waveguides arranged in two rows are arranged between the 2D-PC waveguides 1412 and 1422. The radius r′ of holes of the lattice points between waveguides is set as 0.23a2, for example, so that the propagation constant of the even mode and the odd mode may become constant over the broader frequency ranges.
In order to match the 2D-PC waveguide 1412 to the operational band at the side of the first port P1 from the directional coupling unit, the width of the 2D-PC waveguide 1422 is formed to be narrowed compared with the width formed with the line defect of lattice point so that the whole dispersion curve of the directional coupling unit may be moved to the higher-frequency side. For example, the width of the 2D-PC waveguide 1422 is formed to be narrowed for 0.15 time of the period a2.
Moreover, in order to increase the degree of signal separation between the bar state and the crossed state, the width of the 2D-PC waveguide 1412 at the side of the second port P2 from the directional coupling unit is formed to be narrowed. For example, the width of the 2D-PC waveguide 1412 is formed to be narrowed for 0.15 time of the period a2.
Similarly, the mode coupling of the 2D-PC waveguides 1411(R), 1421(R) is realized, and the lattice points between waveguides arranged in two rows are arranged between the 2D-PC waveguides 1411(R), 1421(R). The radius r′ of holes of the lattice points between waveguides is set as 0.23a1, for example, so that the propagation constant of the even mode and the odd mode may become constant over the broader frequency ranges.
In order to match the 2D-PC waveguide 1411 to the operational band at the side of the port P3 from the directional coupling unit, the width of the 2D-PC waveguide 1421(R) is formed to be narrowed compared with the width formed with the line defect of lattice point so that the whole dispersion curve of the directional coupler may be moved to the higher-frequency side. For example, the width of the 2D-PC waveguide 1421(R) is formed to be narrowed for 0.15 time of the period a1.
Moreover, in order to increase the degree of signal separation between the bar state and the crossed state, the width of the 2D-PC waveguide 1411(R) at the side of the port P4 from the directional coupling unit is formed to be narrowed. For example, the width of the 2D-PC waveguide 1411(R) is formed to be narrowed for 0.15 time of the period a1.
Similarly, the mode coupling of the 2D-PC waveguides 1412(R), 1422(R) is realized, and the lattice points between waveguides arranged in two rows are arranged between the 2D-PC waveguides 1412(R), 1422(R). The radius r of holes of the lattice points between waveguides is set as 0.23a2, for example, so that the propagation constant of the even mode and the odd mode may become constant over the broader frequency ranges.
In order to match the 2D-PC waveguide 1411(R) to the operational band from the directional coupling unit, the width of the 2D-PC waveguide 1422(R) is formed to be narrowed compared with the width formed with the line defect of lattice point so that the whole dispersion curve of the directional coupling unit may be moved to the higher-frequency side. For example, the width of the 2D-PC waveguide 1422(R) is formed to be narrowed for 0.15 time of the period a2.
Moreover, in order to increase the degree of signal separation between the bar state and the crossed state, the width of the 2D-PC waveguide 1412(R) at the side of the port P4 from the directional coupling unit is formed to be narrowed. For example, the width of the 2D-PC waveguide 1412(R) is formed to be narrowed for 0.15 time of the period a2.
As clearly from a simulation result of the transmission spectrum, −10 dB band can be extended to approximately 12 GHz by parallelizing.
Moreover,
It is proved that, in the case of the frequency f=0.32 THz, as shown in
It is proved that, in the case of the frequency f=0.33 THz, as shown in
It is proved that, in the case of the frequency f=0.34 THz, as shown in
As shown in
The adiabatic mode converters 101, 102, 103 are provided with a taper shape so that a tip part thereof becomes thin as being separated from the edge face of the 2D-PC slab 12, in the planar view of the 2D-PC slab 12. In this case, the side surface of the taper shape may include an inclined surface. Moreover, the side surface of the taper shape may include a curved surface. Moreover, the side surface of the taper shape may include a stepped surface.
Moreover, the adiabatic mode converters 101, 102, 103 may include a conical shape so that the tip part becomes thinner as being distanced from the edge face of 2D-PC slab 12.
Moreover, the adiabatic mode converters 101, 102, 103 may include a quadrangular pyramid shape so that the tip part becomes thinner as being distanced from the edge face of 2D-PC slab 12.
Moreover, the adiabatic mode converters 101, 102, 103 may include a wedge-like shape so that the tip part becomes thinner as being distanced from the edge face of 2D-PC slab 12.
Moreover, the adiabatic mode converters 101, 102, 103 may include a wedge-like shape so that the tip part becomes thinner as being distanced from the edge face of 2D-PC slab 12.
Moreover, the adiabatic mode converters 101, 102, 103 may include a stairs-like shape so that the tip part becomes thinner as being distanced from the edge face of 2D-PC slab 12.
Moreover, the adiabatic mode converters 101, 102, 103 may be protected with a resin layer.
The adiabatic mode converters 101, 102, 103 can be inserted into the waveguide line. In this case, a waveguide flange arranged at an edge face of the 2D-PC slab 12 may be in contact with the edge face. The waveguide flange arranged at the edge face of the 2D-PC slab 12 may be separated from the edge face.
Furthermore, the edge face of the 2D-PC slab 12, where the adiabatic mode converters 101, 102, 103 are arranged, includes a gap between the waveguide flanges arranged at the edge face of the 2D-PC slab 12, in a peripheral part of the adiabatic mode converters 101, 102, 103, and may be separated from the waveguide flange. If there is such a gap, since the waveguide flange is arranged so as to be separated from the edge face of the 2D-PC slab 12, a surface mode of the THz input wave can be controlled.
In particular, in order to control the surface mode, it is preferable to set the gap distance WG>wavelength/3, where WG is a gap distance.
Although the detailed structure is omitted, the edge face of the 2D-PC slab 12 where the adiabatic mode converters 101, 102, 103 are arranged, includes a gap between the waveguide flanges arranged at the edge face of the 2D-PC slab 12, in the peripheral part of the adiabatic mode converters 102, 102, 103, in an example shown in
As shown in
The directional coupler 20 according to the modified example 1 includes the adiabatic mode converters 101, 102, 103 composed of the tapered structure to which the 2D-PC waveguide extended, in the ports P1, P2, P3, as shown in
As shown in
In the multiplexer and demultiplexer 30 to which the directional couplers according to modified examples 1-4 of the embodiment is applied, there is realized a method of entering focused light with a lens into the edge face as an input/output. Alternatively, a method of outputting and inputting from free space via the input/output interface 60 composed of the PC is also realized as well as
In the 2D-PC slab 12 applicable to the directional coupler 20 and the multiplexer and demultiplexer 30 according to the embodiment,
The lattice point for forming resonant-state may be arranged in any one selected from the group consisting of a square lattice, a rectangular lattice, a face-centered rectangle lattice, and a triangular lattice.
Moreover, the lattice point 12A is arranged in a square lattice or a rectangular lattice, and can resonate the electromagnetic wave in a Γ point (gamma point), an X point, or an M point in the PB structure of the 2D-PC slab 12, in the PC slab plane.
Moreover, the lattice point 12A is arranged in a face-centered rectangle lattice or a triangular lattice, and can resonate the electromagnetic wave in a Γ point, an X point, or an J point in the PB structure of the 2D-PC slab 12, in the PC slab plane.
Moreover, the lattice points 12A may be provided with any one of the polygonal shape, circular shape, oval shape, or ellipse shape.
As mentioned above, according to the embodiment, there can be provided the directional coupler which has the wide-band and high degree of signal separation and can be miniaturized, used for optical waves, THz waves, or millimeter waves, and the multiplexer and demultiplexer to which such a directional coupler is applied.
In particular, since the directional coupler of the present invention can be miniaturized, it is applicable to broad applicable fields, e.g. filters, switches, power monitors, distribution of power, etc., besides the multiplexer/demultiplexer.
As explained above, the embodiment has been described, as a disclosure including associated description and drawings to be construed as illustrative, not restrictive. This disclosure makes clear a variety of alternative embodiments, working examples, and operational techniques for those skilled in the art.
Such being the case, the embodiment covers a variety of embodiments, whether described or not.
Number | Date | Country | Kind |
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2014-036586 | Feb 2014 | JP | national |