JP3752538B2 - Optical coupling device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to, for example, an optical coupling device used in a receiving device when receiving an optical space propagation signal from a moving body.
[0002]
[Prior art]
When performing spatial optical communication with a distant moving body, light propagating in space is coupled to a photodetector, an optical fiber, a leading waveguide, or the like via a telescope. In this case, distortion of the wavefront, vibration of the device, and the like cause the focal point to be scattered and the focal point fluctuate on the condensing surface of the received light, which adversely affects efficient optical coupling.
[0003]
For this reason, an apparatus incorporating a rotating mirror or the like as shown in FIG. 12 is used, and the rotation angle is controlled to suppress the scattered range of the focal point, thereby improving the efficiency and stabilization of optical coupling. The apparatus of FIG. 12 generates a control signal based on the function of measuring received light and data acquired from the measured light, and controls a mirror, a photoelectric effect element, and the like based on the control signal, and controls an optical fiber and a lead. It is the structure which guides correctly to a waveguide. For this reason, the apparatus becomes complicated and large, and a time delay required for controlling the moving part of the rotary mirror from the measurement of light occurs, and the limit of the response speed is on the order of kilohertz (milliseconds).
[0004]
Moreover, there exists patent document 1 as what made the structure which does not have said control part etc. by devising a condensing system. Patent Document 1 relates to an optical remote control receiver used in an optical space transmission system that performs device control such as lighting and data communication between terminals by spatial transmission using an optical signal in an indoor space such as an office or home. This is because an optical remote control receiver that can reduce the effects of optical noise by efficiently installing a filter that limits the incidence of optical noise to the optical remote control receiver and that also ensures a sufficient receivable distance. It is disclosed. This is a light receiving device composed of a wide-angle light-receiving lens that can receive an optical signal transmitted from a transmitter in a wide range, a light-receiving element that receives the optical signal condensed by the wide-angle light-receiving lens, and a signal amplification circuit. In an optical remote control receiver comprising a module and a filter for blocking optical noise due to illumination light, etc., a light quantity adjusting filter for attenuating the optical noise is inserted in the incident optical path of the optical noise such as illumination light Remote control receiver.
[0005]
Patent Document 2 discloses a distance measuring device that provides a compact multi-point distance measuring device without reducing distance measuring accuracy even for a wide angle of view. The light emitting system and the light receiving system are arranged in the base length direction, and a plurality of spot lights are projected from the light projecting system toward the distance measuring object side along the base length direction. In a multi-point distance measuring device that receives spot light reflected on the distance measuring object side by the position detection type light receiving system and measures the distance to the distance measuring object, the light receiving system includes a light receiving lens. And a light receiving element having a light receiving portion that receives light that has passed through the light receiving lens, and a reflecting member that reflects the spot light of the light emitting portion toward the light receiving portion.
[0006]
Patent Document 3 discloses a reflection measuring device that accurately measures the distance to the object to be measured in a wide angle range and is small and lightweight. First, the light receiving lens of the reflection measuring apparatus is arranged in a blade shape by repeatedly circulating an annular short focal portion, intermediate focal portion, and long focal portion having different focal lengths from the lens outer peripheral portion toward the lens central portion. It is a circular focus Fresnel lens. The short focal portion, the middle focal portion, and the long focal portion have longer focal lengths in this order. The short focal point collects the refracted light from the light receiving lens at the center of the light receiving element when the incident angle of the incident light with respect to the light receiving lens is 0 ·, and the middle focal point of the light receiving element when the incident angle is 4 ·. The refracted light beam is condensed at the center, and the long focal point condenses the refracted light beam at the center of the light receiving element when the incident angle is 8 ·. Thereby, the received light quantity characteristic of the light receiving element with respect to the incident angle is flattened.
[0007]
Patent Document 4 discloses an optical coupling element and an optical coupling method capable of converting the spot size of the incident side light beam and the spot size of the output side light beam by orders of magnitude. This is because an optical coupling element that couples optical waveguide elements having different spot sizes has a modulation structure with a periodic refractive index at intervals similar to the wavelength of light used by the optical waveguide element. By using crystals, the spot size at the exit end is converted to a size different from the spot size at the entrance end. In this disclosure, light propagates through a photonic crystal waveguide provided in the photonic crystal, which is different from the present invention in this respect.
[0008]
Non-Patent Document 1 describes a super collimator using a photonic crystal. In this super collimator, light is collected at the point of incidence on the photonic crystal and travels while being collected in the photonic crystal, but is scattered when emitted from the photonic crystal. This is different from the present invention.
[0009]
However, if these condensing systems are devised to have a structure that does not have a moving part such as a rotating mirror, the light incident on the condensing system cannot be efficiently collected in the photoelectric conversion element, and the light Is only partially used.
[0010]
The manufacturing method of such a photonic crystal is already known. For example, a method for producing a three-dimensional photonic crystal by self-organization of polymer fine particles described in Non-Patent Document 2 or a three-dimensional laser It can be produced by a photonic crystal production method using an ultraviolet curable resin by microfabrication or a self-cloning method described in Non-Patent Document 3.
[0011]
[Patent Document 1]
Japanese Patent Laid-Open No. 7-298374 [Patent Document 2]
JP-A-8-14886 [Patent Document 3]
Japanese Patent Laid-Open No. 9-21874 [Patent Document 4]
JP 2001-4869 A [Non-Patent Document 1]
Kosaka, “Photonic crystal optics starting with Super Prism”, O plus E, December, 1560-1569, 1999 [Non-Patent Document 2]
Misawa, “New formation method of three-dimensional organic photonic crystal”, O plus E, December, pp. 1549-1553, 1999 [Non-patent Document 3]
Sato, “Preparation of Dielectric Photonic Crystals and Applied Devices”, O plus E, December, 1554-1559, 1999 [0012]
[Problems to be solved by the invention]
As described above, conventional optical coupling devices for guiding a spatial light propagation signal to a photoelectric conversion element include those having a control unit and those having a control unit, but those having a control unit have a low response speed, I couldn't cope with fast changes. In addition, in the case where the condensing system is devised without the control system, the utilization efficiency of the received light is low.
[0013]
The present invention has been proposed in view of the above. An optical coupling device capable of increasing the efficiency of use of received light by devising a condensing system when guiding an optical space propagation signal to a photoelectric conversion element. The purpose is to provide.
[0014]
[Means for Solving the Problems]
In order to achieve the above object, according to a first aspect of the present invention, a light condensing system is provided in front of a light condensing element using a photonic crystal as viewed from the traveling direction of light, and the photonic crystal is used for the photonic crystal. The light condensing element constitutes the optical coupling device, and for a part of the light incident surface, the trajectory of the light incident from the opposite part of the region intersects inside the light condensing element. having a first intersection, also its a region is a trajectory having a second intersection which intersects again after exiting, condensing element having the region configured by the photonic crystal, said And a light detector or an optical waveguide provided in the vicinity of the second intersection, and the light is applied to the light condensing system to collect the light on the light detector or the optical waveguide.
[0015]
In addition to the features of the first invention described above, the photonic crystal has a Brillouin in the wave number space in order to prevent incident light from being emitted in various directions. The zone is characterized by having a single closed iso-frequency surface exhibiting a negative refractive index.
[0016]
In addition to the characteristics of the first or second invention described above, the third invention is the easiest to produce a photonic crystal having a two-dimensional periodic structure. Is characterized by comprising a photonic crystal having a two-dimensional periodic structure.
[0017]
In addition, the fourth aspect of the present invention is described above because two photonic crystals having a two-dimensional periodic structure can be condensed in a dot shape by arranging the periodic structures so that the periodic structures are perpendicular to each other. In addition to the features of the first or second invention, the first and second light condensing elements are used, and the light condensing elements are composed of a photonic crystal having a two-dimensional periodic structure. Projections of the periodic structures of the photonic crystals intersect each other at right angles.
[0018]
In addition to the features of the first, second, third, or fourth invention described above, the fifth invention has higher light collection characteristics by condensing with a convex lens or concave mirror in advance. The distance dependency between the light source and the light condensing system at the condensing point position of only the light condensing system is offset by the distance dependency between the light source and the photonic crystal at the light condensing point position of only the light condensing element. It is characterized by that.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic diagram of a configuration for introducing an optical signal from a distant light source A into an optical fiber. Although not shown in the figure, the optical signal from a distant light source is normally incident on the photonic crystal through a telescope. The reason why the photonic crystal is used here is to use its optical characteristics. Light incident on the photonic crystal is refracted in the direction corresponding to the negative refractive index, propagates through the photonic crystal, and also propagates in the direction corresponding to the negative refractive index when emitted. Due to this effect, the light emitted from the light source at point A in FIG. This light condensing action depends on the wavelength of light and is determined by the periodicity of the photonic crystal, and therefore has wavelength selectivity. For this reason, only light of a specific wavelength can be collected, and light of other wavelengths that become noise can be selected. Moreover, even if the position of the light source A changes, the position of the condensing point B does not change so much. This change can be made smaller than when the light is collected using a lens.
[0020]
As an example of a photonic crystal having a simple structure, a photonic crystal having a two-dimensional lattice structure in which silicon square prisms are arranged in a square shape in the air will be described below. A cross section of the size of the unit cell is shown in FIG. Here, d is a lattice constant, and the cross section of the prism has a size of a × a. Similar to a normal crystal, in general, a photonic crystal has a characteristic of a finite region surrounded by a thick solid line connecting three symmetry points Γ, X, and M shown in FIG. The dispersion characteristics of crystals of infinite size can be represented.
[0021]
FIG. 3A is a diagram showing dispersion characteristics of the photonic crystal with respect to an electric field that vibrates in the height direction of the prism. Here, it is assumed that the wavelength of light is λ, and the values of d and a are 0.85λ and 0.37λ, respectively. In order to examine an equal frequency surface or an equal wave number surface, attention is paid to d / λ = 0.85 drawn by a broken line in FIG. As shown in FIG. 3B, the wave number surface is a circle having a radius of 0.1 centered on the Γ point. A large circle with a radius = 0.85 indicates the wavefront of air. In this case, the photonic crystal can be regarded as a uniform medium, and since the inclination at the intersection with the broken line of the dispersion curve is negative, the refractive index can be considered negative.
[0022]
The refraction phenomenon of such a photonic crystal has been clarified by performing an electromagnetic field propagation analysis by computer simulation. In this simulation, it is assumed that the photonic crystal is irradiated with a plane wave inclined by 3 degrees counterclockwise from the vertical direction of the boundary between the crystal and air. An example of the result is shown in FIG. FIG. 4 is a snapshot of the electric field calculated by the FDTD method (FiniteDifference Time Domain). Since the calculation is performed by limiting the width of the plane wave, an unnecessary wave is generated around the edge of the plane wave, but the negative refraction phenomenon can be sufficiently observed.
[0023]
As shown in FIG. 5, when the point light source A is placed in front of the photonic crystal plate causing the negative refractive index phenomenon, all the light incident on the crystal passes through the point B and gathers again at the point C. Here, it is assumed that the boundary between the air and the crystal is given by a straight line and is parallel to each other. It is also assumed that the straight line connecting points A, B and C is perpendicular to the crystal boundary. For this reason, when the crystal boundary is drawn so as to be convex toward the light source, the perpendicular of each point converges behind the crystal. Compared with the irradiation range of the focal point spread by strain, the range is smaller behind the crystal. This effect shows that this device acts as a field expander (FOVE) in a light receiver placed behind the crystal.
[0024]
Next, the photonic crystal having the curved surface shape will be described. As an example of this, FIG. 6 shows a schematic diagram of FOVE using a photonic crystal of constant thickness having the same center and a boundary between curves with radii r1 and r2. This crystal has the detailed structure and optical characteristics shown in FIGS. In this example, it is assumed that three point light sources are respectively placed at the positions of -h, 0, and h in the y coordinate, and the photonic crystal is placed at x = L. In this configuration, the result of the computer simulation when the propagation angle of the light emitted from each light source is set to −3 degrees to 3 degrees will be described. In this simulation, ray tracing was performed by standardizing h = 1 and giving L = 6.7, r1 = 20, and r2 = 18.7. FIG. 7 shows the propagation angle of each ray. Although the propagation angle of light when entering or exiting the photonic crystal changes greatly, the propagation angle of the light beam after passing through the photonic crystal is smaller than the angle entering the photonic crystal. There are few. The simulation result is shown in FIG. In this figure, the right figure shows the actual scale ratio, but the left figure shows the details of the light rays, so that the interval in the y-axis direction is expanded and displayed. The small circles in the figure represent the intersections between the rays and the crystal boundaries. As can be seen from FIG. 8, a complete focus cannot be obtained by the crystal having the shape shown in FIG. 6, but all the rays that have passed through the crystal are in the position of x = 27.0 indicated by W in FIG. Y = −0.31 to 0.31. The value of the amount of light at the point W is about 9.6% of the area where light is incident on the photonic crystal, and it can be seen that the light is sufficiently condensed.
[0025]
It can be seen that the light detection sensitivity can be improved by providing a light detector at the above-mentioned condensed portion. Further, by providing the optical fiber light introducing portion in this portion, the optical signal propagated through the space can be efficiently taken into the optical fiber.
[0026]
According to this photonic crystal, even if the incident angle changes as shown in FIG. 9, it can be condensed near the condensing point in FIG.
[0027]
The photonic crystal having the characteristics shown in FIG. 3 is prepared by, for example, a method for producing a photonic crystal using an ultraviolet effect resin by the above-described three-dimensional laser microfabrication. The condensing shape in this case is linear.
[0028]
Making the above-mentioned linear condensing shape into a dot shape can be realized by using two photonic crystals that are shifted by 90 degrees and used as shown in FIG.
[0029]
Further, when the light source moves away from the photonic crystal, the condensing point in FIG. 1 also has a characteristic of moving away from the photonic crystal. Since this characteristic is opposite to that of a convex lens or concave mirror, as shown in FIG. 11, when combined with a convex lens or concave mirror, the position of the condensing point does not change even if the distance between the light source and the photonic crystal changes. can do.
[0030]
【The invention's effect】
Since this invention consists of an above-described structure, there can exist an effect which is demonstrated below.
[0031]
In the first invention, since the optical coupling device is configured by the condensing element using the photonic crystal, the fluctuation of the condensing point can be suppressed even if the light source moves laterally with respect to the condensing element.
[0032]
In the second invention, in the Brillouin zone of the wave number space, since the photonic crystal having a single closed iso-frequency surface exhibiting a negative refractive index is used, the light dissipation can be suppressed.
[0033]
In the third invention, since a photonic crystal having a two-dimensional periodic structure is used, manufacturing is easy.
[0034]
In the fourth invention, two photonic crystals having a two-dimensional periodic structure are arranged so that the periodic structures are perpendicular to each other. Easy to type in.
[0035]
In the fifth invention, in combination with a convex lens or a concave mirror, it is possible to suppress the movement of the condensing point in the vertical direction as seen from the traveling direction of the light, and to suppress the movement of the condensing point. Control can be simplified.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing the principle of the present invention.
2A is a diagram showing the structure of a photonic crystal used in the present invention, and FIG. 2B is a diagram showing a symmetry point.
FIG. 3A is a diagram showing dispersion characteristics of a photonic crystal, and FIG. 3B is a diagram showing a constant wave number surface.
FIG. 4 is a diagram illustrating an example of a result of electromagnetic field propagation analysis;
FIG. 5 is a diagram showing a locus of light incident on a photonic crystal.
FIG. 6 is a diagram showing a trajectory of light incident on a photonic crystal having a gentle curve at the boundary of the crystal.
FIG. 7 is a diagram showing a propagation angle of light rays incident on a photonic crystal.
FIG. 8 is a diagram showing a trajectory simulation result of light entering and exiting a photonic crystal.
FIG. 9 is a schematic diagram showing a condensing point when an incident angle is changed.
FIG. 10 is a schematic diagram showing an optical coupling device using two photonic crystals.
FIG. 11 is a schematic diagram showing an optical coupling device using a photonic crystal combined with a convex lens.
FIG. 12 is a schematic diagram showing a configuration of a conventional receiving device for a spatial light propagation signal from a moving body.

Claims (4)

A condensing system is provided in front of the condensing element using the photonic crystal as seen from the traveling direction of light,
For a partial region of the light incident surface on the photonic crystal, the locus of light incident from the opposite portion of the region has a first intersection where the light converging element intersects, and A region that is a locus having a second intersection that intersects again after the emission, and is provided in the vicinity of the light-collecting element having the above-described region composed of the photonic crystal and the second intersection. An optical detector or an optical waveguide,
The distance dependency between the light source and the condensing system for the condensing point position of the condensing system is the distance between the light source and the condensing system for the condensing point position of the condensing element using the photonic crystal. Set to offset the dependency,
An optical coupling device, wherein the light condensing system is irradiated with light and condensed on the photodetector or the optical waveguide.   2. The optical coupling device according to claim 1, wherein the photonic crystal has a single closed iso-frequency surface exhibiting a negative refractive index in the Brillouin zone of the wave number space.   3. The optical coupling device according to claim 1, wherein the condensing element is composed of a photonic crystal having a two-dimensional periodic structure.   The first and second light condensing elements composed of photonic crystals having a two-dimensional periodic structure are used, and the projections of the periodic structures of the respective photonic crystals intersect each other at right angles. The optical coupling device according to claim 1 or 2.

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