JP2006064878A - Optical device and method of controlling light - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize an optical device and a method of controlling light in which a variable light attenuation is given only to a specific wavelength and a semi-fixed operation is possible. <P>SOLUTION: At least either one of two alternative measures are realized: a transmission loss or a reflection loss is generated for light having a wave length in which an asymmetrical mode exists by exciting the asymmetrical mode in the band structure of a photon of a two-dimensional or three-dimensional periodic structure body composed of two or more kinds of media by diagonal incident of external light; or a band gap is generated in light wave effective to at least one of orthogonal polarized waves. Further, the transmission loss is made variable by varying the incident angle by using a stepping motor. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical device mainly used in the field of optical communication, and more particularly to an optical device that functions as an optical attenuator and an optical gain equalizer, and an optical control method.

  In the optical communication field or the like, an optical device that can provide optical attenuation at a specific wavelength may be required. For example, the gain equivalent of an erbium doped optical fiber amplifier (EDFA) can be mentioned.

  In such applications, as described in Non-Patent Document 1, it has been realized by a combination with a filter device such as a long-period fiber Bragg grating, a Fabry-Perot etalon filter, or a planar optical waveguide (hereinafter referred to as Prior Art 1). To do).

  When the transmission path is complicated, the attenuation characteristic of the filter device needs to be variable, and a variable gain equalizer is used. For example, this is realized by combining a waveguide formed with an optical fiber grating described in Non-Patent Document 2 and a liquid crystal to be a clad and controlling an electric field applied to the clad by the liquid crystal (hereinafter referred to as Conventional Technology 2).

  The attenuation characteristics required for the variable gain equalizer are not constantly changing, and once the attenuation characteristics are selected, the same attenuation characteristics are often used for a while, so the attenuation characteristics are changed. In other cases, a semi-fixed operation in which the attenuation characteristic is maintained without external energy supply is desirable.

  On the other hand, as described in Non-Patent Document 3, an attempt to obtain physical properties different from a uniform material depending on the structure is also active. In particular, a two-dimensional or three-dimensional periodic structure having a period of about the wavelength of light composed of two or more kinds of substances called photonic crystals has recently attracted attention.

  The concept of the photonic band that is the origin of the photonic crystal was announced by Non-Patent Document 4, and attracted attention because Non-Patent Document 5 presented important properties such as the localization of photons. The photonic crystal actually manufactured and confirmed to have a photonic band gap is the first structure called Yablonotite (see Patent Document 1), and thereafter, a woodpile structure (see Non-Patent Document 6), OPAL (small size). Various structures such as a spherical arrangement structure (see Non-Patent Document 7), an air hole (see Non-Patent Document 8), a stepped multilayer film (self-cloning type, see Patent Document 2 and Non-Patent Document 9) have been realized. .

  Although there are many phenomena specific to photonic crystals, the photonic band gap is best known. Further, even in a photonic band, a phenomenon that cannot be realized by a conventional crystal by controlling the photonic band structure (super prism phenomenon, see Non-Patent Document 10), super collimator phenomenon (see Non-Patent Document 11), super lens phenomenon (Non-patent Document) Reference 12) was found, and attention was focused on controlling the band structure rather than the photonic band gap. Control of the photonic band structure has created a field called photonic band engineering (see Non-Patent Document 13). Therefore, in this case, the definition of the photonic crystal is not “a periodic structure having a photonic band gap”, but “a two-dimensional or three-dimensional periodic structure having a period of about the wavelength of light composed of two or more substances”. Use "body".

  Photonic crystals cause photonic band folding when photons sense the periodicity of the refractive index (or dielectric constant), as the band structure of electrons changes due to the periodicity of atoms in the crystal. The photonic band structure can be controlled by selecting the refractive index and period. Photonic crystals have various interesting characteristics, but the comparison between photonic crystals and crystals based on periodic arrangement of atoms or molecules is summarized in Non-Patent Document 14. Non-patent document 15 gives a detailed explanation of the origin of the band structure.

  Next, a form known so far for the propagation of light in the photonic crystal will be described.

  The group velocity decreases near the wavelength where the dispersion curve in the propagation region of the photonic crystal turns, but this phenomenon corresponds to the fact that photons propagate with multiple reflections, so the interaction between photons and the medium increases (non- (See Patent Document 16). Moreover, it becomes a standing wave state at the end of the photonic band, and light having a wavelength within the photonic band gap is reflected.

United States Patent No. 5172267 Japanese Patent No. 3325825 Japanese Patent No. 3288976 Japanese Patent No. 2539563 Edited by Shoichi Sudo, “Erbium-doped fiber amplifier”, p113, Optronics, 1999 Yeralan et.al., “Switchable Bragg grating devices for telecommunications applications”, Optical Engineering, August2002, Vol. 41 Edited by Ryoichi Yamamoto, multilayer thin film and material development, CMC Publishing, 1986 Ohtaka, PHYSICAL REVIEW B, VOL19, NO10, pp5057-5067,15 MAY 1979 Physical Review Letters, May 1987, vol58, pp2059-2062 S.Noda et.al., New realization method for three-dimensional photonic crystal in optical wavelength region.Jpn.J.Appl.Phys., Vol.35, pp.L909-L912 (1996) H. Miguez, et.al., Photonic crystal properties of packed submicrometric SiO2 spheres, Appl. Phys. Lett., 71, pp. 1148-1150 (1997) T. Baba and T. Matsunami, Electron. Lett., 31, 1776 (1995) S. Kawakami, “Fabrication of submicromatre 3D periodic structures composed of Si / SiO2”, ELECTRONICS LETTERS, 3rd July 1997, vol33, No.14 H. Kosaka et al., Phys. Pev. B58, R10096 (1998) H. Kosaka, T. Kawashima, A. Tomita, M. Notomi, T. Tamamura, T. sato, and S. Kawakami, “Self-collimating phenomena in photonic crystals,” Appl. Phys. Lett., Vol. 74, no.9, pp. 1212-1214, 1 March 1999 H. Kosaka, T. Kawashima, A. Tomita, M. Notomi, T. Tamamura, T. Sato, and S. Kawakami, Appl. Phys. Lett. 74 (1999) 1212 Supervised by Shojiro Kawakami, “Photonic crystal technology and its application”, pp.221-228, CMC Publishing, 2002 John.D.Joannopoulos, Robert D.Meade and Joshua N.Winn, Photonic Crystals, Appendixes A, PRINCETON UNIVERSITY PRESS, 1995 Kazuo Otaka, “Characteristics of light in a periodic field, interaction between light and matter in a periodic field”, 27th Winter Workshop organized by the Optical Society of Japan (Applied Physics Society) of photonic crystals and ultrafine periodic structures Optics Proceedings, 2001 Supervised by Shojiro Kawakami, “Photonic crystal technology and its applications”, pp.222-223, CMC Publishing, 2002 Supervised by Tadahiro Omi, “New Semiconductor Manufacturing Process and Materials”, Chapter 3, Chapter 4, CMC Corporation, May 2000 Yoshino Katsumi, Takeda Hiroyuki, “Basics and Applications of Photonic Crystals”, pp90-93, Corona Inc., 2004 Supervised by Shojiro Kawakami, Photonic crystal technology and its application, Chapter 3, CMC Publishing, 2002 Takashima Kawashima, Yasuo Ohtera, Nao Sato, Shojiro Kawakami, "Fabrication of 2-D Photonic Crystal Polarization Separation Elements and Their High Performance", IEICE Technical Committee on Optical Electronics, OPE99-109, December 1999

  The various optical attenuators described in Prior Art 1 are difficult to give variable optical attenuation only to a specific wavelength. Although prior art 2 can give variable optical attenuation only to a specific wavelength, semi-fixed operation is impossible, so it is necessary to constantly apply an electric field or heat, and there is a large risk associated with operation.

  Furthermore, the above-described documents do not disclose any phenomenon in the case where the photonic crystal is irradiated with incident light, and do not suggest a device using the characteristics when incident light is irradiated.

  The present inventors have discovered a novel phenomenon in which light loss, light attenuation, etc. occur only at a specific wavelength when light is incident on the photonic crystal obliquely, and a device using the phenomenon is proposed. At the same time, a method for controlling the novel phenomenon is proposed.

  The present inventors have also found that according to the proposal, the problems due to the above-described prior arts 1 and 2 can be solved.

  Therefore, an object of the present invention is to provide an optical device utilizing the fact that light loss, light attenuation, and the like occur at a specific wavelength when a photonic crystal is irradiated with incident light.

  Another object of the present invention is to provide a method of controlling light using a photonic crystal that attenuates a specific wavelength.

  According to the present invention, the periodic structure is composed of two or more kinds of media, has a periodic refractive index, and has a photon band structure, and the periodic structure has a three-dimensional period. In the Cartesian coordinates x, y, and z, the electromagnetic field distribution is present in two directions in the x and z directions or in three directions in the x, y, and z directions, and the symmetry plane that includes the axis extending in the z direction. In an optical device that can exhibit an antisymmetric mode and non-evanescent incident light on the periodic structure is incident at an incident angle greater than 0 ° with respect to the z direction, for example, Embodiments (1) to (10) are obtained.

(1) The periodic structure imparts transmission loss or reflection loss to light having a wavelength in which the electromagnetic field distribution of the incident light has an antisymmetric distribution.
(2) The periodic structure has an effective photon band for at least one of polarized waves orthogonal to light having a wavelength in which the electromagnetic field distribution of the incident light has an anti-symmetric distribution. Create a gap.
(3) The periodic structure has a mode in which an electromagnetic field distribution is antisymmetric with respect to a symmetry plane including an axis extending in the z direction, and at least one symmetrical mode of orthogonal polarized waves It is.
(4) The periodic structure emits light having a wavelength in which the mode exists in which the electromagnetic field distribution is antisymmetric among the incident light incident at an incident angle larger than 0 ° with respect to the z direction. Radiate out of the optical axis.
(5) The incident angle of the incident light to the periodic structure is relatively set so as to change the transmission loss for light having a wavelength in which the electromagnetic field distribution of the incident light is antisymmetric. It further has means for changing.
(6) It further has a polarization separation element and a polarization rotator arranged on the light incident side of the periodic structure, and the periodic structure has two beams having substantially the same polarization state. Incident.
(7) The plurality of periodic structures are arranged in a direction from the incident port to the emission port of the optical device.
(8) The plurality of periodic structures have different periods in the x direction or the y direction.
(9) The plurality of periodic structures have the same z-direction period.
(10) The incident angles of light with respect to the plurality of periodic structures can be independently controlled, and the incident angles with respect to the periodic structures can be independently controlled with the x, y, and z directions as axes. is there.

  According to the present invention, a photonic crystal is used to excite the antisymmetric mode in the photon band structure of the photonic crystal by obliquely entering non-evanescent incident light on the photonic crystal. A and / or B below: (A) A transmission loss or a reflection loss is caused in the light having the antisymmetric mode in the incident light. (B) The antisymmetric in the incident light. An optical control method characterized in that an effective light wave bandgap for at least one of orthogonally polarized waves is generated in light having a wavelength in which a mode exists.

  According to the present invention, a relative angle of the incident light with respect to the photonic crystal body is changed using a photonic crystal body having a predetermined photon band structure and predetermined incident light. As a result, a light control method characterized by changing the transmission loss or reflection loss of the incident light can be obtained.

  According to the present invention, variable optical attenuation can be given only to a specific wavelength, almost arbitrary attenuation characteristics can be obtained, the attenuation width (dynamic range) of each wavelength is large, and half Since fixed operation is possible, there is no need to constantly apply an electric field or heat, and the risks and costs associated with operation are small.

  An optical device according to the present invention has a photonic crystal body having a predetermined photon band structure, and when a predetermined non-evanescent incident light is incident on the photonic crystal body, the photonic crystal of the incident light By changing the relative angle with respect to the body, it is possible to arbitrarily change the transmission loss or reflection loss of incident light.

  Hereinafter, the optical device and the light control method according to the present invention will be described in detail with reference to some examples.

  A first embodiment of an optical attenuator as an optical device according to the present invention will be described below with reference to FIGS.

  1A to 1D are schematic views showing the configuration and operation of an optical attenuator according to the present invention.

  1A, this optical attenuator includes an optical fiber 101, a quarter-wave plate 102, a half-wave plate 103, a lens 104, a two-dimensional photonic crystal 105, 2 The two-dimensional photonic crystals 105 and 106 include a two-dimensional photonic crystal 106, a lens 107, and an optical fiber 108, and the incident angle of the beam is variable. A linearly polarized wave can be obtained by using a combination of the quarter wavelength plate 102 and the half wavelength plate 103.

  Similarly to the optical attenuator shown in FIG. 1A, the optical attenuator shown in FIGS. 1B to 1D also has an optical fiber 101, a quarter wavelength plate 102, and a half wavelength plate 103. A lens 104, a two-dimensional photonic crystal 105, a two-dimensional photonic crystal 106, a lens 107, and an optical fiber 108. At least one of the two-dimensional photonic crystals 105 and 106 is included. Is angled with the x direction in FIG. 2 to be described later as the central axis.

  FIG. 2 is a schematic diagram of the two-dimensional photonic crystals 105 and 106.

  The structure of FIG. 2 is a structure called a self-cloning photonic crystal disclosed in Patent Document 3, and is named because it was first produced by a construction method called a self-cloning method. The self-cloning method forms a multidimensional periodic structure while preserving the substrate shape by performing multilayer film deposition under optimized conditions on the substrate processed into irregularities, triangular wave shapes, sine curve shapes, etc. Technology. A self-cloning photonic crystal that is a thin film is particularly suitable for increasing the area. The self-cloning photonic crystal is a multilayer structure in the z direction, and the high refractive index medium layer (which may be a low refractive index medium layer) included in the unit cell is substantially V-shaped in the x direction. It is.

  FIG. 3 is a side view of a substrate used for the two-dimensional photonic crystals 105 and 106.

  The material of the substrate used for the two-dimensional photonic crystals 105 and 106 is fused silica, and the substrate 401 has rectangular irregularities (hereinafter referred to as a substrate pattern) with a pitch of 0.5 μm by a lithography process by EB exposure and dry etching. Is formed.

Further, as shown in FIG. 4, a triangular wave shaped shaping layer is formed by depositing a SiO ₂ film 402 on the substrate 401 by an rf bias sputtering method under an appropriate condition. As described above, the a-Si: H film and the SiO ₂ film are alternately formed for 14 periods while maintaining the triangular wave shape formed on the substrate, thereby forming a two-dimensional refractive index periodic structure. On the photonic crystal portion, an antireflection film made of niobium pentoxide (hereinafter referred to as Nb ₂ O ₅ ) and SiO ₂ is similarly laminated by the self-cloning method.

As shown in FIG. 6, the unit cell of the photonic crystal in Example 1 has a period of 500 nm in the direction orthogonal to the Z axis and the thickness of the hydrogenated amorphous silicon (hereinafter a-Si: H) film in the Z direction. Is 233 nm, the thickness of the SiO ₂ film is 350 nm, and the period in the Z direction is 14 periods. The apex angle is 100 degrees. The unit cell shown in FIG. 6 is the most basic configuration of a self-cloning photonic crystal, and is made of two materials having different refractive indexes, is rectangular in shape, symmetrical with respect to the Z axis, and has a high refractive index portion. Since it has an inverted V shape, it is hereinafter referred to as a two-dimensional two-material symmetric V-shaped rectangular unit cell in this case.

Next, physical properties of the material will be described. SiO ₂ has a refractive index of 1.46. On the other hand, a-Si: H has Eg = 1.7 eV and a refractive index of 3.4. Fused quartz is also made of SiO ₂ and has a refractive index of 1.46. a-Si: H is formed by bonding dangling bonds of amorphous silicon and hydrogen atoms to terminate them. As the substrate, fused quartz is used.

  In Example 1, a-Si: H was used as the high refractive index material in the photonic crystal, but any material having a refractive index n = 2 or higher can be used. However, as the refractive index ratio of the material increases, the influence of the photonic band structure can be obtained with a smaller number of periods.

Next, a manufacturing method will be described. The structure shown in FIG. 2 is produced by rf bias sputtering (rf sputtering with an effect of sputter etching) according to the self-cloning method. When the a-Si: H film is stacked, the target is silicon, the deposition gas is a mixed gas of argon and hydrogen, and the gas pressure is 0.3 Pa to 1.0 Pa. The target for laminating the SiO ₂ film is quartz, the deposition gas is a mixed gas of argon and oxygen, and the gas pressure is 0.3 Pa to 1.0 Pa. The bias to be applied is on the order of several tens of volts although the optimum value depends on other film forming conditions and on the apparatus. In general, the substrate heating temperature in the thin film process is desirably 0.3 times or more the melting point of the material. In Example 1, the substrate heating temperature is 570K.

  Next, it supplements about the film-forming method. The rf bias sputtering method and the ECR sputtering method are known as a self-cloning photonic crystal production method, but depending on the condition setting, rf magnetron sputtering and sputtering or ion gun that do not apply a bias to a pyramid shaped substrate. A self-cloning photonic crystal can also be formed by a combination of etching such as RIE.

  It supplements about the formation method of the unevenness | corrugation on a board | substrate. A process combining lithography and etching by electron beam (hereinafter referred to as EB), etching, a process combining lithography and etching by light or X-ray exposure, or nanoimprinting can be used. In addition, although it cannot be applied to a transparent material substrate up to the ultraviolet region, such as quartz, a substrate made of an opaque material in the ultraviolet region or a film made of an opaque material in a sufficiently thick ultraviolet region laminated on a transparent substrate. Irregularities on the substrate can be formed even in a process combining interference exposure and etching. In Example 1, since the shape accuracy of the unevenness is excellent and etching is easy, a process combining lithography and etching by EB is adopted for the fused quartz substrate. Non-patent document 17 describes lithography and etching.

  Next, it supplements about formation of the shaping layer of a triangular wave shape. The triangular wave shape can be formed by depositing a film by rf bias sputtering on a rectangular substrate pattern as shown in FIG. 3 under appropriate conditions, or on a rectangular substrate pattern as shown in FIG. The film is formed by depositing a film by rf sputtering under various conditions and then shaping it by sputter etching.

  The two-dimensional two-substance V-shaped rectangular unit cell shown in FIG. 6 has a shape that is most easily produced by the self-cloning method, and can cope with a case where the stacking period in the Z direction exceeds 100 periods. Conversely, if the stacking period in the Z direction is about 10 periods, a photonic crystal with a unit cell having a shape as shown in FIG. 7 can be realized.

  Next, the periodic structure will be described. Example 1 employs a structure called a self-cloning type, but the change in transmission loss derived from the photonic band structure that characterizes Example 1 can also be realized by a photonic crystal employing another structure.

  Another important item is that a large-area photonic crystal can be easily manufactured, and the self-cloning type is optimal as a periodic structure that satisfies these.

  Here, from the refractive index of each material and the shape of the unit cell, it has the band structure shown in FIGS.

  The even symmetric mode is a mode in which the electric field intensity distribution is symmetric with respect to the central axis as shown in FIG. 10, and the antisymmetric mode is a mode in which the electric field intensity distribution is antisymmetric with respect to the central axis as shown in FIG. reference).

  For the analysis, a calculation program using a finite difference time domain method (FDTD method) was used. The photonic crystal analysis method is described in detail in Non-Patent Document 19. The band structure is calculated under an infinite period.

  Here, the self-cloning photonic crystal actually produced and the unit cell of FIG. 6 are not completely coincident with each other, such as the self-cloning photonic crystal actually produced has a slightly curved shape, The calculation results of the band structure using the unit cell of the approximate shape formed by the self-cloned photonic crystal actually produced and the optical characteristics derived from the even symmetric mode almost coincide. Actually, the Z direction is 14 periods, but it has been confirmed that the optical characteristics derived from the band structure of the even symmetric mode almost coincide with the calculation results even under such conditions.

  Hereinafter, the measurement data and comparison data of the two-dimensional photonic crystal of Example 1 will be shown, and the effect of Example 1 will be described.

  First, the direction is defined. As shown in FIG. 12, the direction parallel to the groove formed in the self-cloning type two-dimensional photonic crystal film is x direction, and the direction perpendicular to the groove formed in the self-cloning type two-dimensional photonic crystal film is y. The direction.

  In FIG. 13, the two-dimensional photonic crystal 106 in FIG. 1 is removed, and the two-dimensional photonic crystal 105 is fixed at 0 degree around the y axis, and 0 degree (perpendicular incidence) around the x axis is 2 degrees, 5 degrees, 10 degrees. The transmission loss wavelength characteristic with respect to the TM polarized wave when rotated by 20 degrees is shown. Referring to FIG. 13, it can be seen that there is no loss at normal incidence and that the loss increases at 2 ° and 5 °.

  In FIG. 14, when the two-dimensional photonic crystal 106 in FIG. 1 is removed and only the two-dimensional photonic crystal 105 is used, the x-axis rotation is fixed at 10 degrees and the y-axis rotation is 0 degrees (normal incidence) 2 degrees. The transmission loss wavelength characteristic with respect to the TM polarized wave when rotated by 5 degrees is shown. According to FIG. 14, it can be seen that when combined with rotation around the x axis, the influence of rotation around the y axis also appears and can be used to adjust the attenuation.

  FIG. 15 shows a case where the two-dimensional photonic crystal 106 in FIG. 1 is removed and the two-dimensional photonic crystal 105 is rotated by 2 degrees, 5 degrees, and 10 degrees around the x axis, and the y axis is fixed at 2 degrees. The transmission loss wavelength characteristic with respect to TM polarization is shown.

  Incidentally, by rotating the two-dimensional photonic crystal 106 around the z-axis, the same amount of transmission loss with respect to the TM polarization can be given to the entire region from 1480 nm to 1560 nm. This is because the two-dimensional photonic crystal 106 also has a function as a polarizer.

  As described above, it is possible to realize a variable optical attenuator having a large attenuation width only in a specific wavelength band in which an antisymmetric mode exists as shown in FIG. 16 by obliquely entering the two-dimensional photonic crystal. Recognize.

  Further, when measurement was performed with an integrating sphere to examine the occurrence of transmission loss, as shown in FIG. 17, an increase in the amount of reflected light commensurate with the increase in transmission loss with respect to the TM polarization, and similar to scattered light and diffracted light. The generation of abnormal beams in the propagation direction was observed. Hereinafter, the abnormal beams in the propagation direction similar to the scattered light and diffracted light are collectively referred to as emission to the outside of the optical axis.

  The aforementioned incident angle dependency will be described from the band structure. FIG. 18 is a band diagram when the incident angle is 10 degrees around the x-axis, and it can be seen that a band gap exists in the wavelength range of the antisymmetric mode wavelength at the time of vertical incidence. However, since the band structure is defined with an infinite number of periods, the band gap is not fully opened in 14 periods, and some components of incident light are scattered as the group velocity decreases, resulting in the aforementioned characteristics. .

FIG. 19 shows a case where the two-dimensional photonic crystal 105 in FIG. 1 is removed and the two-dimensional photonic crystal 106 is rotated by 5 degrees, 10 degrees, and 15 degrees around the x axis, and the y axis is fixed at 5 degrees. The transmission loss wavelength characteristic with respect to TM polarization is shown. The two-dimensional photonic crystal 106 is formed by alternately stacking 14 cycles of a-Si: H with a thickness of 220 nm and SiO ₂ with a thickness of 330 nm on the same substrate as the two-dimensional photonic crystal 105. It can be seen from FIG. 19 that the antisymmetric mode wavelength can be changed by the configuration in the thickness direction of the photonic crystal.

  Furthermore, at the same incident angle, the transmission loss varies depending on the number of periods of the photonic crystal as shown in FIG. FIG. 20 shows 12 cycles and 55 cycles of a two-dimensional photonic crystal composed of a material having a refractive index of about 2.4 and a refractive index of about 1.5 and a layer having a refractive index of about 1.5 and a thickness of 230 nm. The transmittance with respect to TM polarization at an incident angle of 5 degrees.

  As is apparent from FIG. 20, the transmission loss of the wavelength where the antisymmetric mode exists can be increased by increasing the number of periods.

  Next, the principle of an optical attenuator using a plurality of photonic crystals will be described. By arranging a plurality of types of photonic crystals having attenuation in the wavelength range of several nm in series and independently adjusting the incident angle, the attenuation curve can be adjusted almost freely over a wide wavelength range. For example, as shown in FIG. 21, an attenuation curve for correcting the gain curve of an erbium-doped optical fiber amplifier is also obtained.

  22 and 23 show typical characteristics of the optical attenuator shown in FIG. FIG. 22 shows transmission loss when the two-dimensional photonic crystal 105 is rotated 5 degrees around the y axis and 15 degrees around the x axis, and the two dimensional photonic crystal 106 is rotated 2 degrees around the y axis and 10 degrees around the x axis. TM in the case where the wavelength dependency 2201 and the two-dimensional photonic crystal 105 are rotated 5 degrees around the y axis and 0 degrees around the x axis, and the two-dimensional photonic crystal 106 is rotated about 2 degrees around the y axis and 0 degrees around the x axis. This is the transmission loss wavelength dependency 2202 for the polarization. In the latter case, it is flat with almost no loss, and in the former case, the characteristics of adding the transmission loss of each two-dimensional photonic crystal are obtained.

  FIG. 23 shows transmission loss when the two-dimensional photonic crystal 105 is rotated 5 degrees around the y axis and 5 degrees around the x axis, and the two dimensional photonic crystal 106 is rotated 2 degrees around the y axis and 10 degrees around the x axis. Wavelength dependence 2301 and the case where the two-dimensional photonic crystal 105 is rotated 5 degrees around the y axis and 10 degrees around the x axis, and the two dimensional photonic crystal 106 is rotated about 2 degrees around the y axis and 5 degrees around the x axis. This is the transmission loss wavelength dependency 2302 for the TM polarization. In both cases, the characteristics of adding the transmission loss of each two-dimensional photonic crystal are obtained.

  A second embodiment of the present invention will be described with reference to FIG.

  FIG. 24 illustrates an optical coupling system including a fiber collimator including a single mode optical fiber 2401 and a lens 2402, and a fiber collimator including a single mode optical fiber 2412 and a lens 2411, two-dimensional photonic crystals 2406 and 2407, and polarization state control. It is a schematic diagram of the variable optical attenuator which consists of a means. The photonic crystals 2406 and 2407 are the same as the photonic crystals 105 and 106 of the first embodiment.

  The feature of the second embodiment is that it is a variable optical attenuator whose attenuation characteristic does not depend on the incident polarization state.

  The operation of the configuration of FIG. 24 will be described. After light in a randomly polarized state is emitted from the single mode optical fiber 2401, it is converted into a parallel beam by the lens 2402. Next, the beams are separated into two orthogonally polarized beams by the walk-off crystal polarization separating element 2403, and the polarizations of the two beams by the half-wave plate 2404 and the half-wave plate 2405 whose c-axis directions are 45 degrees different from each other. The state is converted to TM polarization for the two-dimensional photonic crystal 2406 and the two-dimensional photonic crystal 2407. Next, after the two-dimensional photonic crystal 2406 and the two-dimensional photonic crystal 2407 are given attenuation amounts according to their respective characteristics, the half-wave plates 2408 and 1/2 wavelengths whose c-axis directions are different from each other by 45 degrees The polarization states of the two beams are converted into orthogonal polarization states by the plate 2409, are combined by the walk-off crystal polarization separation element 2410, enter the single mode optical fiber 2412 through the lens 2411, and are output. "Polarization separation-> polarization conversion (two beams having the same polarization state)-> some process having polarization dependence-> polarization conversion (two beams having polarization states orthogonal to each other)-> polarization synthesis" This process is disclosed in Patent Document 4 as means for realizing a polarization-independent device.

  In a configuration that requires a variable optical attenuator in actual optical communication, light propagates in a random polarization state in the path, so that polarization independence is necessary as in the second embodiment. Although the configuration is simple, PDL (polarization dependence of loss) can be easily suppressed to 0.1 dB or less.

  A third embodiment of the present invention will be described with reference to FIG.

  FIG. 25 shows a variable optical attenuator having a fiber collimator including a single mode optical fiber 2501 and a lens 2502, a fiber collimator including a single mode optical fiber 2508 and a lens 2507, and three-dimensional photonic crystals 2503 to 2506. is there.

  A feature of the third embodiment is that it is a variable optical attenuator whose attenuation characteristic does not depend on the incident polarization state by using a self-cloning type three-dimensional photonic crystal as shown in FIG. In the case of a three-dimensional photonic crystal, the rotation axis is set to 45 degrees with respect to the unevenness of the photonic crystal as shown in FIG.

  In the case of a two-dimensional photonic crystal, the polarization dependency is large, but in the case of a three-dimensional photonic crystal, the polarization dependency is almost canceled if it is a square lattice type. The third embodiment has a simpler configuration than that of the second embodiment, and is suitable for reducing the price.

  A fourth embodiment of the present invention will be described with reference to FIGS. 28 (a) and 28 (b).

  FIGS. 28A and 28B show fiber collimators 2801 and 2082 that form light incident and output ports, two-dimensional photonic crystals 2813 to 2817, and stepping motors 2818 to 28 that are rotation mechanisms for the respective two-dimensional photonic crystals. 2822, rutile walk-off crystal polarization separation elements 2804 and 2810, quartz half-wave plates 2805, 2806, 2808, and 2809, two-dimensional photonic crystal polarization separation element 2807, PMD compensation plates 2803 and 2811, It is the side view and top view which show the structural example of the variable optical attenuator which consists of the optical spectrum analyzer 2812.

  In Example 4, “polarization dispersion imparting → polarization separation → polarization conversion (two beams having the same polarization state) → variable attenuation mechanism of a specific wavelength by the rotation mechanism of a two-dimensional photonic crystal → reflection (partial transmission) ) → Polarization dispersion compensation → Variable attenuation mechanism of specific wavelength by rotation mechanism of two-dimensional photonic crystal → Polarization conversion (two beams having two orthogonal polarization states) → Polarization synthesis → Polarization dispersion compensation ” It is carried out.

  In addition, the partially transmitted component is incident on the optical spectrum analyzer 2812 through a process of “polarization conversion (two beams having polarization states orthogonal to each other) → polarization synthesis → polarization dispersion compensation” on another path. To do.

  The feature of the fourth embodiment is that the light attenuation mechanism by the photonic crystal is transmitted twice by the reflection mechanism, so that the required light attenuation can be obtained at a smaller incident angle when the same photonic crystal is used. is there. Further, a two-dimensional photonic crystal polarization separation element 2807 is used as the reflection mechanism, and the transmittance and the transmittance can be selected by selecting an angle formed between the polarization direction of incident light and the axial direction of the two-dimensional photonic crystal polarizer. Reflectivity can be selected.

  Further, by arranging the fiber collimator 2801 and the fiber collimator 2802, which require a drawing space, on one side, there is an effect of space saving.

  The two-dimensional photonic crystals 2813 to 2817 are five types of two-dimensional photonic crystals having different periods in the x direction and having a common film thickness, and each has an attenuation of about 8 nm to 12 nm. Covers the entire C band of optical communication (1525 nm to 1565 nm). It is disclosed in Non-Patent Document 20 that the antisymmetric mode wavelength shifts when the film thickness configuration is common and the period in the x direction is different, and by simultaneously laminating four types of photonic crystals, Since the period, the refractive index of each layer, and the angle of the apex of the unit cell are common, it is easy to keep the intervals of the antisymmetric mode wavelengths of the four types of photonic crystals within a certain range, and the four types of photonic crystals The cost is lower than that when separately manufacturing.

  Only two photonic crystals having an antisymmetric mode near 1535 nm are used. 1535 nm has a gain peak of the erbium dome optical fiber amplifier.

The two-dimensional photonic crystal 2813 has an x-direction period of 516 nm, the two-dimensional photonic crystal 2814 has an x-direction period of 518 nm, the two-dimensional photonic crystal 2815 has an x-direction period of 520 nm, and the two-dimensional photonic crystal 2816 has an x-direction period x The period of the direction is 522 nm, the two-dimensional photonic crystal 2817 has a period of 524 nm in the x direction, the thickness of the SiO ₂ layer forming the periodic structure is 330 nm, and the a-Si: H layer is 210 nm.

  Further, the same photonic crystal 2813 as the photonic crystal polarization separation element 2807 can be used. However, since the incident light beam is perpendicularly incident, optical attenuation due to the antisymmetric mode does not occur.

  Although not shown in FIG. 28, a control board, a case, and the like for the case and the stepping motors 2818 to 2822 are added.

  A fifth embodiment of the present invention will be described with reference to FIG.

  FIG. 29 is a diagram illustrating a configuration example of a variable optical attenuator including a rotation mechanism for a photonic crystal and a housing 2902. FIG. The basic configuration is the same as that of the second embodiment, and a tap coupler 2909 and an optical spectrum analyzer 2911 are added. Further, the same two-dimensional photonic crystal and stepping motor as the variable optical attenuator shown in FIG. 28 are used.

  Next, a mechanism for changing the incident angle with respect to the two-dimensional photonic crystal will be described.

  Two-dimensional photonic crystals 2812 to 2816 are connected to stepping motors 2817 to 2821 and rotated. The rotation angle of the stepping motor is adjusted by a computer via a control board. The attenuation is adjusted by selecting a data table in which the rotation angle of each photonic crystal (or the rotation amount of the stepping motor) is recorded in the recording area of the computer 2913 in advance in the computer 2913 from the detection result of the optical spectrum analyzer 2911. An instruction is transmitted from 2913 to the control board 2912.

  In the fifth embodiment, the stepping motors 2817 to 2821 are used because the movement amount is large and a semi-fixed operation is possible, which is excellent in that power is not consumed except during movement. The optical attenuator using the antisymmetric mode of the photonic crystal according to the present invention has a low required accuracy of the incident angle accuracy, and the loss due to the misalignment of the photonic crystal other than the incident angle hardly occurs. The stepping motor is sufficient. Moreover, although it is necessary to detect a rotation angle with a rotary encoder, a servo motor can also be used.

  A sixth embodiment of the present invention will be described with reference to FIGS.

  FIG. 30 is a band diagram when the two-dimensional photonic crystal 105 is incident at an angle of about 10 degrees around the y-axis. In FIG. 9, the TE polarized wave is at the wavelength where the TE polarized antisymmetric mode band exists. There is a band.

  Here, the TE polarization band in the vicinity of the wavelength of 1.45 μm is derived from the antisymmetric mode of the TE polarization in FIG. 9, and the TM polarization mode existing in the wavelength of 1.3 μm or more is the TM polarization in FIG. Derived from the even symmetry mode.

  The TE polarized band near the wavelength of 1.45 μm in FIG. 30 can be excited by an external plane wave. In addition, since there is a band gap with respect to the TE polarized wave except for the wavelength of 1.45 μm in the wavelength range of 1.3 μm to 1.6 μm, the TE polarized wave has a wavelength range of 1.3 μm to 1.6 μm. Most of the light is reflected except for a wavelength of 1.45 μm. An example is shown in FIG.

  FIG. 31 shows measurement of transmittance, reflectance, and other off-axis radiation (including scattering diffraction) when TE polarized light is incident on the two-dimensional photonic crystal 105 obliquely about 10 degrees around the y axis. It is data. The wavelength at which the antisymmetric mode exists is deviated from the band diagram in the vicinity of 1520 nm because the refractive index of the a-Si: H film deviates from the value used for the calculation of the band diagram. According to FIG. 31, the transmittance increases only in the vicinity of 1520 nm, the reflectance decreases, and off-axis radiation occurs. The off-axis radiation increases as the incident angle increases.

  Note that when TM polarized light is incident obliquely around the y-axis, there is almost no change in transmission loss, as can be inferred from the band structure.

  FIG. 32 is a schematic diagram of a variable optical attenuator including an optical coupling system using fiber collimators 3201 and 3202 and two-dimensional photonic crystals 3203 to 3206. In particular, FIG. 32A shows a case where the incident angle to each two-dimensional photonic crystal is 10 degrees, and FIG. 32B shows a case where the incident angle to each two-dimensional photonic crystal is 15 degrees.

  The feature of the variable optical attenuator shown in FIG. 32 is that the reflected light due to the band gap of the photonic crystal is used.

  The operation of the configuration of FIG. 32 will be described. The collimated beam emitted from the fiber collimator 3201 is in a polarization state of TE polarization when perpendicularly incident on the two-dimensional photonic crystals 3203 to 3206.

  First, a part of the light incident on the two-dimensional photonic crystal 3203 is lost due to transmission, scattering or the like at a wavelength at which the antisymmetric mode exists, but the other part is reflected. Next, the light is reflected by the two-dimensional photonic crystals 3204 to 3206 and reaches the fiber collimator 3202 in the same manner. Here, if all the incident angles with respect to the two-dimensional photonic crystals 3203 to 3206 are the same, the light reaches the fiber collimator 3202 even when the incident angles are changed, and thus the operation as a variable optical attenuator is possible.

  While the present invention has been described in terms of several embodiments, it will be apparent to those skilled in the art that the present invention can be variously modified within the scope of the claims.

(A)-(d) is a schematic diagram which shows the structure and operation | movement of the optical attenuator by Example 1 of this invention. It is a schematic diagram of a two-dimensional photonic crystal. 3 is a side view of a substrate used for the two-dimensional photonic crystal 105. FIG. It is the block diagram which formed the shaping layer into the board | substrate of FIG. FIG. 4 is a configuration diagram in which a multilayer film is formed. It is a side view of a unit cell. It is a side view of a unit cell which can be replaced. It is a band figure of a symmetric mode. It is a band figure of an antisymmetric mode. It is a symmetrical mode schematic diagram. It is an antisymmetric mode schematic diagram. It is a figure which shows the definition of an axis | shaft. It is a figure which shows a transmission loss wavelength characteristic. It is a figure which shows a transmission loss wavelength characteristic. It is a figure which shows a transmission loss wavelength characteristic. It is a schematic diagram of the incident angle dependence of the transmission loss wavelength characteristic. It is a figure which shows the wavelength characteristic of a transmittance | permeability, a reflectance, and a scattering rate. It is a band figure at the time of entering obliquely 10 degree | times around the x-axis. FIG. 6 is a diagram showing characteristics of a two-dimensional photonic crystal 106. It is a figure which shows the number dependence of transmission loss. It is a schematic diagram which shows an example of an attenuation characteristic. It is a figure which shows the typical characteristic of an optical attenuator. It is a figure which shows the typical characteristic of an optical attenuator. It is a figure which shows the structure of Example 2 of this invention. It is a figure which shows the structure of Example 3 of this invention. It is a perspective view of a self-cloning 3D photonic crystal. It is a front view which shows the rotation direction of three-dimensional PhC. (A) And (b) is the side view and top view which show the structure of Example 4 of this invention. It is a figure which shows the structure of Example 5 of this invention. It is a band figure at the time of entering 10 degree | times diagonally around a y-axis with respect to a two-dimensional photonic crystal. FIG. 6 is a measurement data diagram of transmittance, reflectance, and other off-axis radiation (including scattering diffraction) when TE polarized light is incident on the two-dimensional photonic crystal obliquely about 10 degrees around the y axis. It is a schematic diagram of a variable optical attenuator comprising an optical coupling system using a fiber collimator and a two-dimensional photonic crystal.

Explanation of symbols

101 optical fiber 102 quarter-wave plate 103 half-wave plate 104 lenses 105 and 106 two-dimensional photonic crystal 107 lens 108 optical fiber 401 substrate 402,502,504 SiO ₂ film 501,503,505 a-Si: H Film 2401 Single mode optical fiber 2402 Lens 2403, 2410 Walk-off crystal polarization separation element 2404, 2405, 2408, 2409 Half wave plate 2406, 2407 Two-dimensional photonic crystal 2411 Lens 2412, 2501 Single mode optical fiber 2502 Lens 2503 2506 3D photonic crystal 2507 Lens 2508 Single mode optical fiber 2801, 2802 Fiber collimator 2803, 2811 PMD compensator 2804, 2810 Rutile Walk-off crystal polarization separation element 2805, 2806, 2808, 2809 half-wave plate 2807 Two-dimensional photonic crystal polarization separation element 2812, 2911 Optical spectrum analyzer 2813-2817 Two-dimensional photonic crystal 2818-2822 Stepping motor 2901 Fiber collimator 2902 Enclosure Body 2903, 2908 Walk-off crystal polarization separation element 2904-2907 1/2 wave plate 2909 Tap coupler 2910 Fiber collimator 2912 Control board 2913 Computer 3201, 3202 Fiber collimator 3203-3206 Two-dimensional photonic crystal

Claims (12)

It is composed of two or more types of media, has a periodic structure having a refractive index periodic structure, and a photon band structure,
In the periodic structure, in the three-dimensional orthogonal coordinates x, y, and z, the period exists in two directions in the x and z directions or three directions in the x, y, and z directions, and in the z direction. It is possible to exhibit a mode in which the electromagnetic field distribution is antisymmetric with respect to a symmetry plane including the extending axis,
Non-evanescent incident light on the periodic structure is incident at an incident angle greater than 0 ° with respect to the z direction,
The optical device according to claim 1, wherein the periodic structure gives a transmission loss or a reflection loss to light having a wavelength in which the electromagnetic field distribution of the incident light is antisymmetric. It is composed of two or more types of media, has a periodic structure having a refractive index periodic structure, and a photon band structure,
In the periodic structure, in the three-dimensional orthogonal coordinates x, y, and z, the period exists in two directions in the x and z directions or three directions in the x, y, and z directions, and in the z direction. It is possible to exhibit a mode in which the electromagnetic field distribution is antisymmetric with respect to a symmetry plane including the extending axis,
Non-evanescent incident light on the periodic structure is incident at an incident angle greater than 0 ° with respect to the z direction,
The periodic structure generates an effective photon bandgap for at least one of orthogonally polarized waves in the light having a wavelength in which the electromagnetic field distribution of the incident light is antisymmetric. An optical device characterized in that   The periodic structure includes a mode in which an electromagnetic field distribution is antisymmetric with respect to a symmetry plane including an axis extending in the z direction, and at least one symmetry mode of orthogonally polarized waves. Item 3. The optical device according to Item 1 or 2.   The periodic structure emits light having a wavelength in the mode in which the electromagnetic field distribution is antisymmetric among the incident light incident at an incident angle larger than 0 ° with respect to the z direction. The optical device according to claim 1, wherein the optical device is radiated to a light source.   Means for relatively changing an incident angle of the incident light to the periodic structure in order to change a transmission loss with respect to light having a wavelength of the mode in which the electromagnetic field distribution of the incident light is antisymmetric. The optical device according to claim 1, 3, or 4. A polarization separation element and a polarization rotator disposed on the light incident side of the periodic structure;
The optical device according to claim 1, wherein two beams having substantially the same polarization state are incident on the periodic structure.   The optical device according to claim 1, comprising a plurality of the periodic structures arranged in a direction from the incident port to the output port of the optical device.   The optical device according to claim 7, wherein the plurality of periodic structures have different periods in the x direction or the y direction.   The optical device according to claim 7, wherein the plurality of periodic structures have the same period in the z direction. The incident angle of light with respect to the plurality of periodic structures can be independently controlled, and
The optical device according to any one of claims 7 to 9, wherein an incident angle with respect to each of the periodic structures can be independently controlled with respect to the x direction, the y direction, and the z direction. Using photonic crystals,
By exciting the antisymmetric mode in the photonic crystal structure of the photonic crystal body by obliquely incident non-evanescent incident light on the photonic crystal body, the following A and / or B:
(A) Transmission loss or reflection loss is caused in the light having the antisymmetric mode in the incident light. (B) Polarization orthogonal to the light having the antisymmetric mode in the incident light. An effective light wave band gap for at least one of the above is generated. By using a photonic crystal body having a predetermined photon band structure and a predetermined incident light, a relative angle of the incident light with respect to the photonic crystal body is changed to thereby change the incident light. A light control method characterized by changing transmission loss or reflection loss.

Images