JP2012093584A - Optical coupler, optical scanner and spectrometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical coupler which can be easily manufactured at low cost.SOLUTION: An optical coupler 1 includes a quartz substrate 10, and metal films 11, 12 made of Ag, which are respectively formed on both surfaces of the quartz substrate 10. The metal film 11 is formed on one surface 10a of the quartz substrate 10, and the metal film 12 is formed on a whole area of the other surface 10b of the quartz substrate 10. Multiple holes 13 penetrating the metal layer 11 are formed in the metal layer 11. A cross section of the holes 13 on a surface parallel to the main surface of the quartz substrate 10 is a circular shape. A diameter of the holes 13 is 150 nm. The holes 13 are arranged in a square lattice shape, and a gap between the adjacent holes 13 is 500 nm. In the optical coupler 1, a part of incident light 14 in the quartz substrate 10 can be emitted as branch light 15 in a direction of 111° with respect to a propagation direction of the incident light 14.

Description

  The present invention relates to an optical coupler for branching light, and particularly relates to an optical coupler using surface plasmon resonance. The present invention also relates to an optical scanning device and a spectroscopic device using the optical coupler.

  As an optical coupler for branching light, a grating type optical coupler as in Patent Documents 1 and 2 has been known. For example, the optical coupler of Patent Document 1 has a structure in which a grating (diffraction grating) is provided on the surface of an optical waveguide.

  Further, there are Patent Documents 3 to 5 as optical scanning devices that change the radiation direction of light without depending on a mechanical method. The optical scanning devices of Patent Documents 3 and 4 use an optical waveguide made of an electro-optic crystal and a diffraction grating. By applying a voltage to the optical waveguide and changing the refractive index, the action of the diffraction grating is changed. This is a device that changes the radiation direction of light. In addition, the optical scanning device of Patent Document 5 is a device that uses the parallel stripes generated by applying a voltage to the liquid crystal as a diffraction grating and changes the light emission direction by changing the interval between the parallel stripes depending on the voltage. is there.

  Further, as a spectroscopic device for decomposing light for each wavelength, one using a diffraction grating as in Patent Documents 6 and 7 is known. The radiation angle of the light emitted from the diffraction grating has a wavelength dependency, which enables spectroscopy. However, since the wavelength dependence of the radiation angle of the light separated by the diffraction grating is small, the wavelength of the radiation angle of light can be obtained by using a multilayer structure in Patent Document 6 and using a photonic crystal in Patent Document 7. The dependency is increased and the resolution is increased.

JP-A-11-281831 JP2009-80449 JP-A-7-28101 JP-A-8-76153 JP 7-261204 A JP 2002-18026 A JP 2004-125603 A

  However, the grating type optical coupler is difficult to manufacture, and the manufacturing cost is high.

  In addition, in the optical scanning devices as in Patent Documents 3 to 5, it is necessary to apply a very large voltage in order to greatly change the light emission direction. In addition, it is difficult to grow a large electro-optic crystal, and therefore it is difficult to change the radiation angle of light having a large beam diameter.

  Further, in the spectroscopic devices such as Patent Documents 6 and 7, the diffraction grating is large, and it is difficult to downsize the device.

  Therefore, an object of the present invention is to realize an optical coupler that can be easily manufactured at low cost. Another object is to realize an optical scanning device capable of changing the radiation angle of light having a large beam diameter. Another object is to realize a compact spectroscopic device.

  According to a first aspect of the present invention, there is provided an optical coupler that performs demultiplexing / multiplexing of light, an optical waveguide made of a dielectric that propagates light therein, and an optical waveguide on the optical waveguide, the thickness direction of which is the light within the optical waveguide. A metal film made of a metal arranged so as to be perpendicular to the propagation direction of the metal, and the metal film is provided with a plurality of through holes periodically arranged in a two-dimensional manner. The diameter and interval are equal to or less than the wavelength of light, and the metal is a metal that exhibits surface plasmon resonance at the wavelength of light to be guided.

The shape of the optical waveguide does not have to be a flat plate-like dielectric substrate, and may be a circular rod shape, a fiber shape, or the like. The dielectric may be a material having translucency with respect to the light to be guided, and is an inorganic material such as SiO ₂ , Al ₂ O ₃ , Si ₃ N ₄ , ZnO, TiO ₂ , ZrO ₂ , Si, and GaAs. In addition to the material, a light-transmitting resin material such as an acrylic resin can be used. Furthermore, an electro-optic crystal such as LiNbO ₃ , LiTaO ₃ , KTP, KTN may be used. If an electro-optic crystal is used as the material of the optical waveguide, the branching direction of the light can be changed by changing the refractive index by changing the voltage applied to the electro-optic crystal, thereby realizing an optical scanning device. it can. Furthermore, the optical waveguide may be a translucent liquid held in a casing. In this case, it is possible to change the light branching direction by replacing the liquid with a liquid having a different refractive index, or by changing the orientation of the liquid crystal by a magnetic field or an electric field.

  The angle formed between the thickness direction of the metal film and the light propagation direction inside the optical waveguide does not have to be strictly perpendicular, and may be an angle that is nearly perpendicular to the extent that the operation and effect of the present invention can be obtained. . For example, it may be in the range of 70 to 110 °.

  Although it is desirable that the optical waveguide and the metal film are in direct contact with each other for reasons such as a reduction in optical loss, the optical waveguide and the metal film may be separated via a translucent material or the like as long as the effects and advantages of the present invention are obtained.

  The material of the metal film is arbitrary as long as it is a material that causes surface plasmon resonance with respect to the wavelength of light to be used. For example, Al, Ag, Au, Cu, or the like can be used.

  The diameter and interval of the through holes may be equal to or less than the wavelength of light propagated inside the optical waveguide in order to cause surface plasmon resonance on the metal film surface. In this range, the diameter, interval, and shape of the through holes, the two-dimensional arrangement of the plurality of through holes, the length of the through holes (the thickness of the metal film), etc., the resonance frequency of surface plasmon resonance, Design according to the light branching direction. Examples of the shape of the through hole include a cylinder, a prism, a truncated cone, a truncated pyramid, and a hemispherical shape. The arrangement of the plurality of through holes is, for example, a square lattice shape or a triangular lattice shape. The thickness of the metal film is desirably in the range of 0.01 to 1.0 times the wavelength of light propagating inside the optical waveguide. Within this range, the branching loss of the optical coupler can be made sufficiently small. It is also possible to design the metal film to operate as a left-handed material.

  A second invention is an optical coupler according to the first invention, wherein the plurality of through holes are arranged in a square lattice pattern.

  A third invention is an optical coupler according to the first or second invention, wherein the metal film is made of Al, Ag, Au, or Cu.

  A fourth invention includes the optical coupler according to any one of the first to third inventions and a power supply device, wherein an optical waveguide of the optical coupler is made of an electro-optic crystal, and the electro-optic crystal is formed by the power supply device. The optical scanning device is characterized in that a voltage is applied to the light source and the radiation angle of the branched light is controlled by controlling the voltage.

  A fifth invention comprises the optical coupler according to any one of the first to third inventions, and a light receiving element positioned at a predetermined distance from the branched light emission side of the optical coupler. Device.

  According to the first invention, a part of the light guided in the optical waveguide is absorbed by surface plasmon resonance on the optical waveguide side surface of the metal film in which the through hole is formed, and the plasmon is propagated to the side surface of the through hole. The light can be branched by emitting light again from the opening opposite to the dielectric side of the hole. Also, the light can be multiplexed by the reverse operation. The optical coupler according to the first aspect of the present invention does not require any processing or the like on the optical waveguide itself and has a very simple configuration, so that it can be easily manufactured at low cost. In addition, according to the optical coupler of the first invention, light having a large beam diameter can be easily branched.

  Further, as in the second invention, the plurality of through holes can be arranged in a square lattice shape, and the optical coupler of the present invention can be manufactured more easily.

  Further, as in the third aspect of the invention, by using Al, Ag, Au, or Cu as the material of the metal film, light can be efficiently absorbed by surface plasmon resonance.

  According to the fourth aspect of the invention, it is possible to realize an optical scanning device that can easily change the radiation angle of light having a large beam diameter.

  In addition, according to the fifth aspect of the present invention, a spectroscopic device that is smaller than a conventional diffraction grating type spectroscopic device can be realized.

FIG. 3 is a diagram illustrating a configuration of the optical coupler 1 according to the first embodiment. The figure shown about the arrangement pattern of the hole 13. FIG. FIG. 6 is a diagram illustrating a configuration of an optical scanning device 2 according to a second embodiment. FIG. 5 is a diagram illustrating a configuration of a spectroscopic device 3 of Example 3. The graph which showed the relationship between a wavelength and the radiation angle of branched light.

  Hereinafter, specific examples of the present invention will be described with reference to the drawings. However, the present invention is not limited to the examples.

  FIG. 1 is a diagram illustrating the configuration of the optical coupler 1 according to the first embodiment. The optical coupler 1 includes a flat quartz substrate 10 (corresponding to the optical waveguide of the present invention) and Ag metal films 11 and 12 formed on both surfaces of the quartz substrate 10 respectively. Hereinafter, as shown in FIG. 1, the x-axis and the y-axis are parallel to the main surface of the quartz substrate 10, and the z-axis is in a direction perpendicular to the main surface. The metal film 11 is formed on one surface 10 a of the quartz substrate 10, and the metal film 12 is formed on the entire surface of the other surface 10 b of the quartz substrate 10. The thicknesses of the metal films 11 and 12 are 50 nm and 100 nm, respectively. One metal film 11 is formed with a plurality of holes 13 penetrating the metal film 11. The cross section of the hole 13 in a plane parallel to the main surface of the quartz substrate 10 is circular. The diameter of the hole 13 is 150 nm. FIG. 2 is a diagram showing an arrangement pattern of the holes 13. As shown in FIG. 2, the holes 13 are arranged in a 1 mm square region in a square lattice pattern periodically in the x-axis direction and the y-axis direction perpendicular to the x-axis direction, and the interval between the holes 13 is 500 nm. is there.

  The metal plate 12 is provided to efficiently confine light inside the quartz substrate 10 by reflection, and is not necessarily required.

  The optical coupler 1 of Example 1 was manufactured as follows. First, a metal film 12 made of Ag was formed on the entire surface of one surface of the quartz substrate 10 by vapor deposition. Next, a resist was applied on the quartz substrate 10 and was exposed using an electron beam by an electron beam drawing apparatus, thereby forming a resist having a pattern in which circular dots were arranged in a square lattice pattern. Then, a metal film made of Ag is formed on the resist and the quartz substrate 10 by vapor deposition, and unnecessary portions of the metal film are removed by lift-off, so that a plurality of holes 13 are formed in a square lattice shape. 11 was formed. As described above, the optical coupler of Example 1 can be easily manufactured at low cost by performing the deposition and lift-off of the metal film 11 on the surface of the quartz substrate 10 without processing the quartz substrate 10 itself that is an optical waveguide. . The hole 13 in the metal film 11 may be formed by dry etching or wet etching.

  Next, the operation of the optical coupler of Example 1 will be described. First, He—He laser light (wavelength 633 nm) is incident from the side surface of the quartz substrate 10, and the x-axis direction (that is, the direction parallel to the main surface of the quartz substrate 10 and the periodic arrangement direction of the holes 13). To propagate.

  A part of the incident light 14 propagating through the quartz substrate 10 is the surface 10a on the quartz substrate 10 side of the metal film 11, and in the region 10c where the holes 13 are arranged, the incident light 14 and the metal film 11 are caused by surface plasmon resonance. The surface plasmon on the surface is combined and absorbed by the metal film 11. Here, the holes 13 whose diameter is 150 nm smaller than the wavelength of light (633 nm) are arranged in a square lattice at intervals of 500 nm smaller than the wavelength of light, so that the resonance wavelength of surface plasmon resonance is changed to the wavelength of light. In the vicinity, the surface plasmon and the incident light 14 are efficiently coupled.

  Surface plasmon polariton, which is a combination of incident light 14 and surface plasmon generated on the surface of the metal film 11 on the quartz substrate 10 side by surface plasmon resonance, is a direction in which the side surface of the hole 13 is opposite to the quartz substrate 10 (FIGS. 1 and 2). In the positive z-axis direction). Then, light (branched light 15) is emitted again after being separated from the surface plasmons at the opening of the hole 13 opposite to the quartz substrate 10 side. At this time, the radiation angle of the branched light 15 was 111 ° with respect to the propagation direction (x-axis direction) of the incident light 14 in the zx plane. The reason why the branched light 15 is emitted in the direction exceeding 90 ° is considered to be that the metal film 11 provided with the holes 13 arranged in a square lattice is operating as a left-handed material.

  Moreover, light can be multiplexed by the reverse operation to the above. That is, by irradiating the surface of the metal film 11 opposite to the quartz substrate 10 side with a He—He laser beam from a direction forming 111 ° with respect to the metal film 11, the quartz film 10 is parallel to the metal film 11. Light propagating in any direction can be made incident, and light can be multiplexed.

  As described above, the optical coupler 1 of Embodiment 1 has a very simple configuration, but can realize a function of branching and multiplexing light. Moreover, since the structure is simple, it can be manufactured inexpensively and easily. In addition, since the quartz substrate 10 itself, which is an optical waveguide, is not processed, light having a large beam diameter can be easily branched.

In the optical coupler 1 according to the first embodiment, the quartz substrate 10 is used. However, the optical substrate 1 is made of a dielectric that can guide light therein and has a light-transmitting property with respect to the guided light. If so, the material and shape are arbitrary. As the material, in addition to SiO ₂ , for example, an inorganic optical material such as Al ₂ O ₃ , ZnO, TiO ₂ , ZrO ₂ , LiNbO ₃ , Si, or GaAs, or a translucent resin material such as an acrylic resin may be used. it can. Further, the shape is not limited to a flat plate, but may be a cylindrical shape, a fiber shape, or the like. In the case of a cylindrical shape or a fiber shape, a metal film provided with holes along the circular side surface may be formed.

  In the optical coupler 1 of Example 1, Ag is used as the metal film 11 in which the holes 13 are formed. However, any material may be used as long as it is a material that causes surface plasmon resonance with respect to the wavelength of light to be guided. be able to. For example, in addition to Ag, Au, Al, Cu, or the like can be used.

  Further, the diameter, interval, shape, length (thickness of the metal film 11) and arrangement of the holes 13 are not limited to those shown in the optical coupler of Example 1, and the diameter and interval of the holes 13 are quartz substrates. It is arbitrary as long as it is less than the wavelength of the light guided to the inside 10 and the holes 13 are two-dimensionally periodic array. In this range, the radiation angle of the branched light 15 and the branching loss can be arbitrarily designed by designing the diameter, interval, shape, and arrangement of the holes 13. The shape of the hole 13 is, for example, a prism, a truncated cone, a truncated pyramid, a hemispherical shape, etc. in addition to a cylinder. The arrangement of the holes 13 is, for example, a triangular lattice shape in addition to the square lattice shape.

  Further, in the optical coupler 1 of the first embodiment, the quartz substrate 10 and the metal film 11 are in direct contact with each other, but they may be separated via a translucent material or the like. However, direct contact is desirable because the configuration can be simplified and optical loss can be reduced.

As shown in FIG. 3, the optical scanning device 2 of Example 2 includes an optical coupler 22 in which the quartz substrate 10 in the optical coupler 1 of Example 1 is replaced with an electro-optic crystal substrate 20 made of LiNbO _3, and an electro-optic crystal substrate. And a power supply device 21 that applies a voltage to the power supply 20.

As a material of the electro-optic crystal substrate 20, in addition to LiNbO ₃ , electro-optic crystals such as LiTaO ₃ , KTP, and KTN can be used.

  The optical coupler 22 has the same configuration as that of the optical coupler 1 except that the quartz substrate 10 of the optical coupler 1 of the first embodiment is replaced with the electro-optic crystal substrate 20. The optical coupler 22 branches light by the same operation as the optical coupler 1 of the first embodiment. That is, a part of the light guided into the electro-optic crystal substrate 20 is absorbed by the surface of the metal film 11 on the electro-optic crystal substrate 20 side by surface plasmon resonance, and light is again emitted from the surface of the metal film 11 on the opposite side. The light is branched by radiating.

  The power supply device 21 controls the refractive index of the electro-optic crystal substrate 20 by controlling the voltage applied to the electro-optic crystal substrate 20. Since the wave number of the light propagating through the electro-optic crystal substrate 20 changes due to the change in the refractive index, the relationship between the wave number and the diameter and interval of the holes 13 also changes. As a result, the radiation angle of the branched light branched by the optical coupler 2 also changes.

  In a state where no voltage is applied to the electro-optic crystal substrate 20 by the power supply device 21, the radiation angle of the branched light of the optical coupler 22 is the light propagation direction inside the electro-optic crystal substrate 20 (on the main surface of the electro-optic crystal substrate 20. It was 92 degrees with respect to (parallel direction). Next, when a voltage was applied to the electro-optic crystal substrate 20 by the power supply device 21 to change the refractive index of the electro-optic crystal substrate 20 by 1%, the radiation angle of the branched light changed by 0.2 °.

  As described above, in the optical scanning device 2 according to the second embodiment, the optical coupler 22 is controlled by controlling the refractive index of the electro-optic crystal substrate 20 by applying a voltage to the electro-optic crystal substrate 20 of the optical coupler 22 by the power supply device 21. The radiation angle of the 22 branched lights can be controlled. The optical coupler 22 is not processed on the electro-optic crystal substrate 20 itself, which is an optical waveguide. Therefore, the optical scanning device according to the second embodiment can easily branch light having a large beam diameter.

  In Example 2, an electro-optic crystal is used as the optical waveguide, but other structures may be used as long as the refractive index of the optical waveguide is changed. For example, by using a housing that holds a translucent liquid as an optical waveguide and switching to a liquid with a different refractive index, or by changing the orientation of the liquid crystal by a magnetic field or an electric field using liquid as a liquid crystal, It is also possible to change the light emission angle.

  As shown in FIG. 4, the spectroscopic device 3 of the third embodiment is separated from the optical coupler 1 of the first embodiment by 5 mm from the optical coupler 1 via the air by the spacer 6 on the radiation side of the branched light from the optical coupler. It is comprised by CCD5 arrange | positioned. Instead of the CCD 5, other light receiving elements such as CMOS may be used.

  In the optical coupler 1 of the first embodiment, the radiation angle of the branched light differs depending on the wavelength of the light incident on the quartz substrate 10, so that the branched light radiation side of the optical coupler 1 as in the spectroscopic device 3 of the third embodiment. Spectrometer rules are possible by arranging the CCD 5 in the configuration.

  FIG. 5 is a graph showing the relationship between the wavelength of light guided to the optical coupler 1 and the radiation angle from the optical coupler 1. As shown in the graph of FIG. 5, it can be seen that the radiation angle of the branched light increases almost linearly at a wavelength of 500 to 800 nm. Moreover, it turns out that the radiation angle is changing greatly with 95-135 degrees. Further, it can be seen from the graph of FIG. 5 that when the distance from the optical coupler 1 to the light receiving device 5 is 5 mm, a resolution of about 1 nm wavelength per 15 μm can be obtained on the CCD 5.

  As described above, the spectroscopic device of the third embodiment can be downsized as compared with the conventional diffraction grating type spectroscopic device.

  The optical coupler of the present invention can be used for optical communication and the like. The optical scanning device of the present invention can be used for a laser printer or the like. The spectroscopic device of the present invention can be used for various spectrometer rules.

1, 22: Optical coupler 2: Optical scanning device 3: Spectroscopic device 5: CCD
6: Spacer 10: Quartz substrate 11, 12: Metal film 13: Hole 20: Electro-optic crystal substrate 21: Power supply device

Claims (5)

In an optical coupler that performs demultiplexing and multiplexing of light,
An optical waveguide made of a dielectric that propagates light inside;
A metal film made of a metal on the optical waveguide and disposed such that a film thickness direction is perpendicular to a propagation direction of the light inside the optical waveguide;
Have
The metal film is provided with a plurality of through-holes periodically arranged two-dimensionally,
The diameter and interval of the through holes are not more than the wavelength of the light,
The metal is a metal that exhibits surface plasmon resonance at the wavelength of the light to be guided.
An optical coupler characterized by that.   The optical coupler according to claim 1, wherein the plurality of through holes are arranged in a square lattice pattern.   The optical coupler according to claim 1, wherein the metal film is made of Al, Ag, Au, or Cu. An optical coupler according to any one of claims 1 to 3 and a power supply device,
The optical waveguide of the optical coupler is made of an electro-optic crystal,
An optical scanning device characterized in that a voltage is applied to the electro-optic crystal by the power supply device, and the voltage is controlled to control an emission angle of branched light branched by the optical coupler. An optical coupler according to any one of claims 1 to 3,
A light receiving element positioned on the radiation side of the branched light branched by the optical coupler and spaced apart from the optical coupler by a predetermined distance;
A spectroscopic device comprising:

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