JP2013047763A - Periodically structured reflection mirror and optical element using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To implement a reflection mirror which has low loss and strong wavelength selectivity for diverging and converging light waves and has high reflectivity, with a simple structure in the field of microphotonic elements.SOLUTION: A high-refractive index thin film layer having refractive-index periodic structures is formed on a substrate having the same curvature as a light wave to be controlled to achieve low loss, strong wavelength selectivity, and high reflectivity. A guided mode resonance phenomenon that light diffracted into a guided mode by periodic structures and propagated in the high-refractive index layer is resonantly reflected to the incidence side is utilized. When a curvature radius of the substrate is equal to or less than about 5 times the intervals of periodic structures, reflectivity at a resonance wavelength can be kept at 90% or more. Minimum requirements for operation of this structure are only a substrate made of a low-refractive index medium and a diffraction grating layer made of a high-refractive index medium, so that an element can be implemented with a much simpler configuration than a dielectric multilayer reflection mirror or the like.

Description

  The present invention relates to a reflection mirror having a form of a periodic structure thin film formed on a transparent substrate among curved mirrors.

  When irregularities are provided on a flat substrate such as glass at regular intervals and a plane wave is incident on the substrate from the outside, light is diffracted in a direction determined by the interval and wavelength of the irregularities and the refractive index of the substrate. Such an element is called a surface diffraction grating. As a similar structure, there is an element that diffracts light by unevenness or modulation of refractive index formed in a high refractive index thin film on a substrate. This is called a thin film type diffraction grating. In a thin-film diffraction grating, it is known that the incident light power is reflected by 100% when the relationship between the average refractive index of the thin film and the gap between the gratings and the incident angle and wavelength of the external incident light is satisfied. ing. This phenomenon is called guided mode resonance (Guided-Mode Resonance). Since the wavelength range in which guided mode resonance occurs is generally narrow, such a structure has been studied since the 1980s as a form for realizing a highly reflective reflector with sharp wavelength selectivity (non-patented). Document 1, Non-Patent Document 2, Non-Patent Document 3).

  In other words, the thin-film diffraction grating can be regarded as an optical waveguide having a periodic structure formed on a substrate. When the propagation constant β of light propagating through the optical waveguide satisfies the relationship of βΛ≈2π with respect to the lattice interval Λ, the guided light is diffracted and goes out as external light above and below the waveguide. This phenomenon has also been used as an element for inputting and outputting a plane wave coming from the outside to the optical waveguide, that is, a grating coupler (Non-patent Document 4). Light incident from the outside is diffracted by the grating, converted into a waveguide mode of the optical waveguide, and propagates through the thin film layer. This mode is again diffracted by the grating and goes out. When the phase of the light emitted from each part of the periodic structure becomes the same phase, the intensity of the external light is enhanced as a whole, and 100% reflection is observed. This is why this phenomenon is called guided mode resonance. The thin film type diffraction grating is sometimes called a “resonant mode grating” (Resonant Grating) particularly when it is used for the purpose of generating guided mode resonance. The resonance mode grating can be constituted most simply by forming a single high refractive index periodic waveguide layer on a low refractive index substrate. That is, a 100% reflecting mirror can be realized with an extremely simple configuration.

  However, all the resonance mode gratings that have been researched and developed have been formed on a flat substrate. Therefore, in order for the grating to act as a reflecting mirror, the external light needs to be a plane wave. In other words, the application of the resonant mode grating has been limited to an optical device using a plane wave.

  However, at present, there are overwhelmingly more cases using non-planar wavefronts in industrial fields related to optical engineering such as measurement, communication, and information processing. For example, in the field of photovoltaic power generation, sunlight is converted into a convergent wave using a Fresnel lens or a parabolic reflector. In the field of communications, light propagating in an optical fiber is expressed by superposition of diverging or converging cylindrical waves. Further, the reflector of the resonator such as a gas laser has a minute curvature to prevent light from diffusing in the resonator. In the above-described grating coupler, it is a practical application to guide the radiated light from the end face of the optical fiber to the optical waveguide. In this case, the light from the optical fiber can be said to be a wave having a curved wavefront.

  There is a need for a reflecting mirror that can efficiently convert the propagation direction of a wave having a curved wavefront that converges and diverges in a wide field of optical engineering. For example, the reflecting mirror can be simply configured by using a metal surface such as aluminum or gold, and is often used in practice. On the other hand, there are problems that it is difficult to increase the reflectance to 90% or more, heat is generated due to light absorption, and the reflectance is lowered due to oxidation. Moreover, it can be said that the wavelength selectivity is gradual depending on the application. The reflecting mirror could also be realized by forming a dielectric multilayer film on the curved surface. However, in this case, for example, in order to obtain a reflectance of 90% or more, there is a problem in terms of manufacturing cost, such as generally requiring a film number of several tens of layers or more.

P. Vincent and M. Neviere, “Corrugated dielectric waveguides: A numerical study of the second-order stop bands”, Applied Physics vol. 20, pp. 345-351, 1979. D. Rosenblatt, A. Sharon, and A. A. Friesem, “Resonant grating waveguide structures”, IEEE Journal of Quantum Electronics, vol. 33, no. 11, pp. 2038-2059, 1997. Y. Ding and R. Magnusson, “Resonant leaky-mode spectral-band engineering and device applications”, Optics Express, vol. 12, no. 23, pp. 5661-5674, 2004. G. Roelkens, D. V. Thourhout, and R. Baets, “High efficiency Silicon-on-Insulator grating coupler based on a poly-Silicon overlay”, Optics Express, vol. 14, no. 24, pp. 11622-11630, 2006.

  Thus, in various fields of optical engineering, there is a known structure that can achieve a reflectivity close to 100% with a simple structure, despite the need for a highly reflective mirror for convergent and divergent waves. It wasn't. Since the resonance mode grating that can be the only candidate works as a reflector only for plane waves, the application destination must be remarkably limited. The present invention aims to solve this problem.

で表される範囲の値であることを特徴としている。 In order to solve the above problems, the present invention provides:
A structure having a periodic structure with a refractive index of at least one dimension in the in-plane direction,
It consists of a thin film formed on a substrate with curvature,
When the interval Λ in the direction along the curved surface of the periodic structure is λ, the refractive index of the substrate is n ₁ , and the refractive index of the thin film is n ₂ ,

It is the value of the range represented by.

The present invention
An optical waveguide device having a high refractive index thin film formed on a substrate surface as a waveguide layer,
The shape of the element is circular or oval,
A periodic structure of refractive index is formed on the outer periphery,
The interval Λ in the direction along the outer periphery of the periodic structure is expressed by the relational expression according to claim ₁ , where n _{1 is} the refractive index of the cladding of the optical waveguide, n _{2 is} the refractive index of the core, and λ is the operating wavelength. It is characterized by having a range of values.

The present invention
An optical fiber type optical waveguide element,
The core located in the center of the fiber is concentrically surrounded by a periodic structure made of a medium with a high refractive index.
When the interval in the direction along the circumference of the periodic structure is Λ, the refractive index of the cladding outside the periodic structure is n ₁ , the refractive index of the medium constituting the periodic structure is n ₂ , and the operating wavelength is λ Further, it has a value in a range represented by the relational expression described in claim 1.

  The present invention provides a resonant mode grating that operates as a high-efficiency reflector for waves having a finite curvature, such as divergent waves and convergent waves, by forming a diffraction grating on a substrate with curvature.

  FIG. 1 shows a conceptual diagram of a resonance mode grating of the conventional type, and FIG. The form of the periodic structure may be physical irregularities on the thin film as shown in this figure, or may be a refractive index modulation formed on the thin film by ultraviolet irradiation or ion implantation. The curvature applied to the grating is made to coincide with the curvature of the wavefront of the light wave to be incident thereon. That is, each grating is excited in phase by the incident wave.

  Provided is a method of constructing a reflector having a simple structure, which has a reflectance of almost 100%, has wavelength selectivity, and can reflect a light wave having a curved wavefront. By forming a high refractive index thin film having a refractive index periodic structure on a substrate having a curvature, it acts as a reflecting mirror for a light wave having the same wavefront as that curvature. In principle, the resonant reflection effect occurs when there is at least one thin film. Therefore, a curved reflector can be realized with a simple structure.

The figure which shows the conventional resonance mode grating | lattice which consists of a diffraction grating thin film formed on the flat board | substrate. The figure which shows the resonance mode grating | lattice of this invention formed on the board | substrate with a curvature. The figure which shows the reflection spectrum of the resonance mode grating | lattice with a curvature of this invention. Some curves correspond to different curvature radii. The figure which shows the relationship between a curvature radius and Q value of a resonance spectrum in the resonance mode grating | lattice with a curvature of this invention. The conceptual diagram of a grating coupler using the resonant mode grating with a curvature of this invention. The conceptual diagram of the circular resonator using the resonant mode grating with a curvature of this invention. The conceptual diagram of the optical fiber which has arrange | positioned the resonance mode grating | lattice with a curvature of this invention as a 1st clad.

  The form of a thin film diffraction grating on a curved surface according to the present invention will be described using the conceptual diagram shown in FIG. Here, as shown in the figure, a structure in which the radius of curvature is constant for each location on the thin film will be taken up. It is assumed that the refractive index distribution is uniform in the z direction, where z is the coordinate axis perpendicular to the plane of the paper, r is the coordinate axis in the radial direction from the center of curvature, and θ is the coordinate axis in the circumferential direction. The diffraction grating layer is niobium pentoxide (wavelength 400 nm to 2000 nm and the refractive index is about 2.1 to 2.2), and the substrate shown outside the diffraction grating layer is fused silica (wavelength 400 nm to 2000 nm and the refractive index is about 1). .44 to 1.45). The side closer to the center than the diffraction grating layer is air. The thickness of the diffraction grating layer is set to such a thickness that at most about three waveguide modes exist at the design wavelength when the unevenness is leveled and regarded as an aperiodic structure type slab waveguide. Specifically, it is 1/9 of the thickness of the waveguide layer. Further, the radius of curvature, which is the distance from the center of curvature to the waveguide layer, is set to be at least four times the lattice spacing Λ.

  Reflection spectra when TM waves that spread concentrically from the center of curvature (waves having only the z component and the magnetic field having the r and θ components) are incident on several structures with different curvature radii. This is shown in FIG. In the structure of FIG. 2, the thickness of the diffraction grating layer and the depth of the unevenness were made equal to the values shown in FIG. The left end of FIG. 3 also shows the reflection spectrum when a plane wave is vertically incident on a flat resonance mode grating. It can be seen that in the structure having a radius of curvature greater than the value indicated by A in the figure, the shape of the reflection spectrum is almost the same as the flat structure. This A corresponds to a structure in which 50 lattices are arranged on the circumference. When the radius of curvature becomes relatively small with respect to the lattice spacing Λ, the maximum value of the reflectivity also decreases, but at least 30 gratings are arranged on the circumference (in this case, the radius of curvature is about 4. It can be seen that the peak reflectance is 50% or more up to the radius of 8Λ. In the range where the reflectance is kept close to 100%, the spectrum width is about the same as the flat structure.

  FIG. 4 shows the maximum reflectance and the resonance Q value (corresponding to the ratio of the center wavelength to the wavelength width) with respect to the radius of curvature. Thus, it can be seen that the reflectance and the Q value are kept equal to the original flat structure at a certain radius of curvature.

  In the above example, a diffraction grating layer composed of one niobium pentoxide layer is directly formed on a fused quartz substrate, and the surface is exposed to air. However, materials and structures necessary to generate such a guided mode resonance phenomenon are not limited to this. For example, the diffraction grating layer has Si (refractive index of about 3.5 at a wavelength of 1300 nm to 2000 nm), etc. Ta2O5 (having a wavelength of 400 to 2000 nm and a refractive index of 2.1 to 2.2) can also be used. The substrate may be quartz glass (refractive index = 1.52 or so). Further, third and fourth layers called spacer layers may be provided above and below the waveguide layer. Further, the depth of the unevenness may be greater than or equal to the value shown above. Further, the shape of the unevenness does not have to be a rectangular wave shape, and may be a sine wave shape, for example. Further, deep irregularities may be formed to such an extent that the waveguide layer is divided.

  Here we show the application of this structure to a grating coupler. FIG. 5 is a conceptual diagram thereof. That is, an optical waveguide layer is formed on a certain plane inside the substrate. Above this, a resonance mode lattice layer having a constant curvature is formed. A cylindrical divergent wave coming from the outside of the substrate is coupled to the grating and starts to propagate in the circumferential direction. A part of the propagating light is coupled from the grating to the lower waveguide layer and extracted as a signal into the lower waveguide layer. On the other hand, it is also conceivable that the light is first propagated in the lower waveguide layer. That is, the guided light in the lower waveguide layer is coupled to the diffraction grating layer at a position closest to the diffraction grating layer, and a part of the light is further converted into a cylindrical wave-like convergent wave and emitted upward. This wave continues to converge toward the center of curvature and is collected near the center (focal point) of curvature. In this example, the local curvature is constant. However, if the curvature depends on the location on the lattice, that is, if the lattice is formed in a parabolic shape, the divergent wave from the focal point is converted into parallel light. Can be operated as a parabolic reflector.

  Here, application to a circular resonator for an optical integrated circuit is shown. FIG. 6 is a conceptual diagram thereof. That is, a high-refractive-index microstructure is disposed concentrically on an optical integrated circuit substrate. This arrangement interval Λ is set so that the relationship βΛ≈2π holds as described above, where β is the propagation constant of the waveguide mode that circulates in the circumferential direction of the concentric structure.

  Light irradiated from the outside near the center of this structure forms divergent light within the substrate surface. This diverging wave is reflected by the ring-shaped periodic structure and returns to the center again. A part of the light is converted into a mode that circulates in the ring, and is extracted to the outside through an optical waveguide disposed closer to the ring.

  Here, application to an optical fiber guide is shown. FIG. 7 is a conceptual diagram thereof. That is, a first clad having a periodic structure made of a high refractive index medium is concentrically surrounding the core. The outside is a second cladding made of a low refractive index medium.

  When light propagating through the core obliquely enters the first cladding, it is reflected with a reflectance close to 100% when its wavelength is close to the resonance wavelength of the periodic structure. As a result, the light of the wavelength forms a waveguide mode that propagates in a zigzag manner in the core. In this type of optical fiber, the wavelength of light allowed to be guided and the propagation angle with respect to the axis of the core are limited to a very narrow range of values determined from the resonance conditions of the periodic structure. In other words, guided / non-guided waves are sensitive to slight bending of the fiber and changes in refractive index due to temperature changes. Therefore, it can be applied to highly sensitive stress sensors, distributed temperature sensors, and the like.

  By using the thin film diffraction grating structure with curvature or the periodic waveguide structure of the present invention, it is possible to realize a reflector having a high reflectivity with respect to a light wave having a curved wavefront with a simple configuration. This structure is a wavelength-selective high-efficiency optical concentrator in the solar energy field, a grating coupler that guides light from an external optical fiber to an optical waveguide on an optical integrated circuit, a circular laser resonator, and optical communication in the optoelectronic field. And optical fiber sensors for high-sensitivity stress and temperature measurement in the field of optical sensing.

101 A diffraction grating layer made of a high refractive index medium or a core layer 102 of a waveguide 102 A substrate 501 made of a low refractive index medium A curved diffraction grating layer 502 A waveguide layer 503 of an optical integrated circuit formed on a flat substrate surface Substrate 504 Linear Focal point 601 Periodic structure waveguide 602 concentrically arranged on a flat substrate Optical extraction waveguide 701 Optical fiber core 702 First clad 703 with high refractive index medium Second clad

Claims (3)

A structure having a periodic structure with a refractive index of at least one dimension in the in-plane direction,
It consists of a thin film formed on a substrate with curvature,
When the interval Λ in the direction along the curved surface of the periodic structure is λ, the refractive index of the substrate is n ₁ , and the refractive index of the thin film is n ₂ ,

A structure having a value in the range represented by An optical waveguide device having a high refractive index thin film formed on a substrate surface as a waveguide layer,
The shape of the element is circular or oval,
A periodic structure of refractive index is formed on the outer periphery,
The interval Λ in the direction along the outer periphery of the periodic structure is expressed by the relational expression according to claim ₁ , where n _{1 is} the refractive index of the cladding of the optical waveguide, n _{2 is} the refractive index of the core, and λ is the operating wavelength. An optical waveguide device characterized by having a value in a range to be controlled. An optical fiber type optical waveguide element,
The core located in the center of the fiber is concentrically surrounded by a periodic structure made of a medium with a high refractive index.
When the interval in the direction along the circumference of the periodic structure is Λ, the refractive index of the cladding outside the periodic structure is n ₁ , the refractive index of the medium constituting the periodic structure is n ₂ , and the operating wavelength is λ An optical waveguide element having a value in a range represented by the relational expression according to claim 1.

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