JP5580068B2 - Optical device - Google Patents

Description

  The present invention relates to an optical device that controls propagation characteristics of an optical fiber.

  An optical switch that removes a part of the cladding of an optical fiber to turn light on and off is known (Patent Document 1). In this optical switch, one of an on-side switch body and an off-side switch body acting as an optical medium is selectively disposed around the exposed core. The refractive index of the on-side switch body is lower than the refractive index of the core, and the refractive index of the off-side switch body is higher than the refractive index of the core. A liquid such as ethylene tetrafluoride is used for the on-side switch body, and ferromagnetic fine particles are used for the off-side switch body. On / off switching is performed by moving the ferromagnetic fine particles with a magnet.

  A technique is known in which an optical waveguide in which a pair of cores having a rectangular cross section are embedded in a substrate acting as a clad is operated as an optical switch (Patent Document 2). The substrate between the pair of cores is removed, and this portion is filled with a liquid crystal material. Switching is performed by controlling the alignment of the liquid crystal material.

Japanese Utility Model Publication No.59-66203 JP 2003-222915 A

  In the optical switch that moves the ferromagnetic fine particles, the switching speed is limited by the moving time of the ferromagnetic fine particles. For this reason, it is difficult to increase the switching speed. In the structure in which the core is embedded in the substrate, the core embedded in the substrate and the core of the optical fiber must be connected.

  The objective of this invention is providing the optical apparatus which can control the propagation characteristic of an optical fiber at high speed.

According to one aspect of the invention,
An optical fiber in which the clad surrounds the core and the clad is removed in a part of the length direction;
A liquid crystal material arranged so as to surround a portion of the optical fiber where the cladding is removed, and expressing a cholesteric blue phase;
An electrode for applying an electric field for vertically aligning liquid crystal molecules in the liquid crystal material,
An optical device is provided that switches the liquid crystal molecules between a vertically aligned nematic phase and a cholesteric blue phase.

  The optical properties of the liquid crystal material differ between a state in which a cholesteric blue phase is developed and a state in which an electric field is applied by an electrode. This changes the propagation characteristics of the optical fiber. The response speed can be increased compared to the case where the optical medium is mechanically moved.

(1A) is a cross-sectional plan view of the optical device according to Example 1, and (1B) is a cross-sectional view taken along one-dot chain line 1B-1B of (1A). (2A) is a schematic diagram of a cross section of the liquid crystal material and the core when no voltage is applied, and (2B) is a schematic diagram of a cross section of the liquid crystal material and the core when a voltage is applied. (3A) and (3Ba) are plan views in the course of manufacturing the optical device according to Example 1, and (3Bb) is a cross-sectional view taken along one-dot chain line 3Bb-3Bb in (3Ba). (3Ca) and (3Da) are plan views in the course of manufacturing the optical device according to the first embodiment, and (3Cb) and (3Db) are cross-sectional views taken along the alternate long and short dash line 3Cb-3Cb and (3Da), respectively. It is sectional drawing in dashed-dotted line 3Db-3Db. (3E) and (3F) are cross-sectional views in the course of manufacturing the optical device according to Example 1. (4A) is a cross-sectional plan view of the optical device according to Example 2, and (4B) is a cross-sectional view taken along one-dot chain line 4B-4B in (4A).

  FIG. 1A is a plan sectional view of the optical device according to the first embodiment. FIG. 1B is a cross-sectional view taken along one-dot chain line 1B-1B in FIG. 1A. A cross-sectional view taken along one-dot chain line 1A-1A in FIG. 1B corresponds to FIG. 1A.

  In a partial region 23 in the length direction of the optical fiber 20 in which the core 21 is covered with the clad 22, the clad 22 is removed and the core 21 is exposed. For example, the core 21 has a diameter of 10 μm, and the optical fiber 20 including the cladding 22 has a diameter of 30 μm. A general optical fiber having a cladding outer diameter of about 125 μm may be used. The space on the side of the core 21 in the region 23 where the cladding is removed is filled with the liquid crystal material 18. The liquid crystal material 18 is sandwiched between the first substrate 10 and the second substrate 12 that are arranged in parallel to each other. For the first substrate 10 and the second substrate 12, for example, a glass substrate having a thickness of 0.7 mm is used. A plastic substrate may be used instead of the glass substrate.

  A sealing member 15 disposed between the first substrate 10 and the second substrate 12 seals the space filled with the liquid crystal material 18. The seal member 15 is in contact with the side surface of the core 21 between the end portion of the clad 22 and the space filled with the liquid crystal material 18. The refractive index of the seal member 15 is lower than the refractive index of the core 21. For this reason, the seal member 15 acts as a clad of the optical fiber 20.

  A first electrode 11 and a second electrode 13 are formed on the inner surfaces of the first substrate 10 and the second substrate 12, respectively. For example, an indium tin oxide (ITO) film having a thickness of 150 nm is used for the first electrode 11 and the second electrode 13. The first electrode 11 and the second electrode 13 are led out from the space closed by the seal member 15 in a direction perpendicular to the axial direction of the optical fiber 20. The direction in which the first electrode 11 is derived (downward in FIG. 1A) is opposite to the direction in which the second electrode 13 is derived. That is, on one side of the optical fiber 20 (the lower side of the optical fiber 20 in FIG. 1A), the first electrode 11 and the seal member 15 overlap, and on the other side (the upper side of the optical fiber 20 in FIG. 1A), The second electrode 13 and the seal member 15 overlap. When the three members of the seal member 15, the first electrode 11, and the second electrode 13 overlap, the overlapped portion causes deterioration. In Example 1, since the area | region where these three members overlap does not exist, degradation can be suppressed.

  The drive power supply 25 applies an alternating drive voltage to the first electrode 11 and the second electrode 13 via the switch 26.

  The liquid crystal material 18 has, for example, positive dielectric anisotropy and refractive index anisotropy, and develops a cholesteric blue phase (hereinafter simply referred to as “blue phase”) within an operating temperature range when no voltage is applied. . As the liquid crystal material 18, it is preferable to use a material containing a polymer network and expressing a polymer-stabilized blue phase. The polymer stabilized blue phase will be described in detail later.

The blue phase is a liquid crystal phase that appears between a chiral nematic phase having a relatively short helical pitch and an isotropic phase, and is optically isotropic. It is known that there are three types of blue phases called blue phase I, blue phase II, and blue phase III from the low temperature side. Blue phase I has body-centered cubic symmetry, blue phase II has simple cubic symmetry, and blue phase III has isotropic symmetry. The ordinary refractive index of the liquid crystal material 18 n _o, the extraordinary ray refractive index n _e. The blue phase is optically isotropic and its refractive index is (2n _o + n _e ) / 3.

FIG. 2A schematically shows a cross section of the liquid crystal material 18 and the core 21 when no voltage is applied. The liquid crystal material 18 exhibits a blue phase. The refractive index (2n _o + _ne ) / 3 of the blue phase is lower than the refractive index of the core 21. For this reason, the liquid crystal material 18 acts as a clad. Therefore, the light propagating along the core 21 on one side of the region 23 where the cladding 22 is removed propagates as it is in the region 23 where the cladding 22 is removed, and reaches the core 21 on the other side. The state when no voltage is applied is referred to as a “transmission state”.

FIG. 2B schematically shows a cross section of the liquid crystal material 18 and the core 21 when a voltage is applied. The liquid crystal material 18 is aligned to be in a homeotropic alignment state. The liquid crystal material of the homeotropic alignment state 18 is the refractive index anisotropy with the ordinary refractive index n _o, and the extraordinary ray refractive index n _e. The extraordinary ray refractive index _ne is higher than the refractive index of the core 21. Therefore, components affected by the extraordinary refractive index n _e of the light propagating along the core 21, from leaking from the core 21 in the liquid crystal material 18. For this reason, the light propagating in the optical fiber 20 is attenuated in the region 23 where the cladding 22 is removed. The state when the voltage is applied is referred to as an “attenuation state”.

  Thus, by applying a voltage between the first electrode 11 and the second electrode 13, the propagation characteristic of the optical fiber 20 can be changed. Switching between the transmission state and the attenuation state is performed at a speed at which the alignment state of the liquid crystal molecules changes. For this reason, compared with the case where an optical medium is moved mechanically, switching can be performed at high speed.

  In the optical communication system, the region 23 from which the cladding is removed can be operated as an optical switch. It is also possible to operate as a relatively low-speed optical modulator. When the optical fiber 20 is used for transmission of illumination light, the region 23 from which the cladding is removed can be operated as a dimmer. By collecting the light leaking from the optical fiber 20 in the region 23 where the cladding is removed, it is possible to function as an optical branching unit.

  With reference to FIGS. 3A to 3F, a method of manufacturing an optical device according to the first embodiment will be described.

  As shown in FIG. 3A, the first electrode 11 is formed on a part of the surface of the first substrate 10, and the second electrode 13 is formed on a part of the surface of the second substrate 12. Patterning of the first electrode 11 and the second electrode 13 made of ITO can be performed using a well-known photolithography technique. For etching ITO, for example, aqua regia mixed acid or ferric chloride can be used. After forming the electrodes, the first substrate 10 and the second substrate 12 are cleaned. In the cleaning step, for example, brush cleaning using an alkaline detergent, pure water cleaning, air blowing, ultraviolet irradiation, and infrared drying are performed in this order. High pressure spray cleaning or plasma cleaning may be used.

  As shown in FIG. 3Ba, a seal member 15 is formed on the surface of the first substrate 10. FIG. 3Bb is a cross-sectional view taken along one-dot chain line 3Bb-3Bb in FIG. 3Ba. For the seal member 15, for example, an ultraviolet curable sealant is used. As shown in FIG. 1B, since the optical fiber 20 has a function of adjusting the distance between the first substrate 10 and the second substrate 12, it is not necessary to add a gap control agent to the sealing agent. For the application of the sealing agent, for example, a screen printing method, a dispenser method, or the like can be used.

  As shown in FIG. 3Ba, the seal member 15 is disposed so as to surround a region where the liquid crystal material is disposed. If the seal member 15 is disposed in a portion overlapping the core, the seal member 15 spreads in the plane when the optical fiber is mounted. In order to prevent the seal member 15 from spreading, a groove 15A in which the seal member 15 is not disposed is provided in a portion overlapping the core of the optical fiber. Note that the entire coating amount of the seal member 15 may be reduced, and the seal member 15 may be disposed in a portion overlapping the core. By reducing the coating amount, the in-plane spread of the seal member 15 when the optical fiber is mounted is suppressed.

  As shown in FIG. 3Ca, the liquid crystal material 18 is dropped on the first substrate 10 in the region surrounded by the seal member 15. A dispenser method can be used for dropping the liquid crystal material 18. Hereinafter, a method for adjusting the liquid crystal material 18 will be described.

Nematic liquid crystal composition JC1041-XX (manufactured by Chisso Corporation) and 4-cyano-4′-pentylbiphenyl (manufactured by Merck Corporation) are mixed in an equimolar ratio. Refractive index anisotropy Δn of JC1041-XX is 0.142, ordinary refractive index _{n o} is 1.507. Refractive index anisotropy Δn of 4-cyano-4'-pentyl-biphenyl is 0.184, ordinary refractive index _{n o} is 1.534. To this mixed liquid crystal composition, ZLI-4572 (manufactured by Merck & Co., Inc.) is added as a chiral agent so as to be 5.6 mol%.

  A monofunctional photopolymerizable monomer and a bifunctional photopolymerizable monomer are mixed at a molar ratio of 70:30 to prepare a mixed monomer. As the monofunctional monomer, 2-ethylhexyl acrylate (manufactured by Sigma-Aldrich Japan) can be used, and as the bifunctional monomer, RM257 (manufactured by Merck) can be used. A photopolymerization initiator 2,2-dimethoxy-2-phenylacetophenone (DMPAP) is added so as to be 5 mol% based on the mixed monomer.

  The liquid crystal material 18 is prepared by adding the mixed monomer so as to be 8 mol% with respect to the mixed liquid crystal composition containing the chiral agent. It is also possible to use materials other than those exemplified as the liquid crystal composition, chiral agent, photopolymerizable monomer, and photopolymerization initiator. However, the photopolymerizable monomer preferably includes both a monofunctional monomer and a bifunctional monomer.

  As shown in FIG. 3Da, a portion of the cladding 22 of the optical fiber 20 is removed. 3Db is a cross-sectional view taken along one-dot chain line 3Db-3Db in FIG. 3Da. For removing the clad 22, for example, hydrofluoric acid or buffered hydrofluoric acid can be used. The optical fiber 20 is mounted on the first substrate 10. The exposed portion of the core 21 is accommodated in the groove 15 </ b> A of the seal member 15 and penetrates the liquid crystal material 18. The end surface of the clad 22 is in contact with the outer side surface of the seal member 15. For example, the refractive index of the core 21 is 1.6, and the refractive index of the cladding 22 is 1.57.

  As shown in FIG. 3E, the second substrate 12 is overlaid on the first substrate 10. At this time, the seal member 15 is crushed and slightly spreads in the in-plane direction. Further, the seal member 15 also enters the space below and above the exposed core 21, and the space in which the liquid crystal material 18 is confined is sealed. The optical fiber 20 plays a role of adjusting the distance between the first substrate 10 and the second substrate 12. The step of overlaying the second substrate 12 on the first substrate 10 may be performed in a vacuum. The first substrate 10, the second substrate 12, the liquid crystal material 18, and the like are heated to cause the liquid crystal material 18 to develop a blue phase.

As shown in FIG. 3F, the liquid crystal material 18 is irradiated with ultraviolet rays in a wavelength region of 365 nm in a state where a blue phase is developed. By the ultraviolet irradiation, the photopolymerizable monomer in the liquid crystal material 18 is polymerized and the seal member 15 is cured. The ultraviolet intensity is, for example, 30 mW / cm ² . In the ultraviolet irradiation process, for example, an irradiation period of 1 second and a non-irradiation period of 10 seconds are alternately repeated 10 times, and then continuous irradiation for 3 minutes is performed. The intensity of ultraviolet rays is not limited to the above range. However, if the intensity of ultraviolet light is weakened, it is necessary to lengthen the irradiation time.

  Polymerization of the photopolymerizable monomer forms a polymer network in the liquid crystal material 18. When the polymer network is formed, the temperature range in which the liquid crystal material 18 develops a blue phase is expanded. In Example 1, the liquid crystal material 18 exhibits a blue phase in a wide temperature range of −5 ° C. to 60 ° C. The temperature range in which the blue phase is developed can be expanded depending on the materials used, the mixing ratio, the polymerization conditions, and the like. A blue phase including a polymer network and having an expanded temperature range of the blue phase is referred to as a “polymer stabilized blue phase”.

The refractive index of the liquid crystal material 18 in a state where the blue phase is expressed is 1.574, which is lower than the refractive index 1.6 of the core 21. For this reason, the liquid crystal material 18 acts as a clad. Ordinary refractive index _{n o} and the extraordinary ray refractive index _{n e} of the liquid crystal material 18 of the homeotropic alignment state, respectively 1.521 and 1.683. For extraordinary refractive index n _e is higher than the refractive index of the core 21, the light affected by the extraordinary refractive index n _e is not confined to the core 21, from leaking into the liquid crystal material 18.

  When a driving voltage of 200 V is applied under the condition of room temperature, the response time to change from the blue phase to the homeotropic alignment state is about 30 μs, and the response time to change from the homeotropic alignment state to the blue phase Was about 15 μs. This response time is shorter than the time for mechanically moving the optical medium disposed around the exposed core 21. For this reason, the transmission state and the attenuation state can be switched at high speed.

  In the first embodiment, the dropping injection method (ODF method) is used for filling the liquid crystal material 18, but a vacuum injection method may be used. When the vacuum injection method is used, the seal member 15 provided with the injection port is cured before filling with the liquid crystal material. After curing, the liquid crystal material 18 is injected into the seal member 15 from the injection port, and finally the portion through which the injection port and the core pass is closed with a sealant. When the vacuum injection method is applied, a thermosetting sealant can be used as the seal member 15.

  The liquid crystal material 18 is not limited to that exemplified in the first embodiment. As the liquid crystal material 18, another liquid crystal material that exhibits a collective blue phase having a refractive index lower than that of the core 21 can be used. When refractive index anisotropy is manifested when voltage is applied, at least one refractive index component is preferably larger than the refractive index of the core 21. By becoming larger than the refractive index of the core 21, light propagating along the core 21 leaks out of the core 21.

  Since the structure of the optical device according to the first embodiment is a hermetically sealed type, the optical device is hardly affected by the environment in which the device is installed. For example, since it is hardly affected by dust or the like, it can be used indoors.

  In the first embodiment, an example in which the liquid crystal material 18 is sandwiched between a pair of flat plates is shown, but the structure for confining the liquid crystal material 18 is not limited to the illustrated example. For example, the liquid crystal material 18 may be filled in a cylinder such as a quadrangular prism or a cylinder that surrounds the exposed core. In addition, after the liquid crystal material 18 and the exposed core are arranged in the casing in which one surface of the hexahedron is opened, the opened one surface may be closed with a lid.

  FIG. 4A is a plan sectional view of the optical device according to the second embodiment. FIG. 4B is a cross-sectional view taken along one-dot chain line 4B-4B in FIG. 4A. A cross-sectional view taken along one-dot chain line 4A-4A in FIG. 4B corresponds to FIG. 4A. Hereinafter, description will be made by paying attention to differences from the optical device according to the first embodiment shown in FIGS. 1A and 1B, and description of the same configuration will be omitted.

  In the first embodiment, the seal member 15 is in contact with the exposed portion of the core 21, but in the second embodiment, the seal member 15 is in contact with the clad 22. For this reason, the liquid crystal material 18 is disposed around the core 21 over the entire length direction of the exposed portion of the core 21. In this configuration, the seal member 15 does not affect the propagation of light in the optical fiber 20. For this reason, the freedom degree of selection of the sealing member 15 increases.

  Although the present invention has been described with reference to the embodiments, the present invention is not limited thereto. It will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made.

DESCRIPTION OF SYMBOLS 10 1st board | substrate 11 1st electrode 12 2nd board | substrate 13 2nd electrode 15 Seal member 18 Liquid crystal material 20 Optical fiber 21 Core 22 Clad 23 The area | region 25 from which the clad was removed 25 Drive power supply 26 Switch

Claims (4)

An optical fiber in which the clad surrounds the core and the clad is removed in a part of the length direction;
A liquid crystal material arranged so as to surround a portion of the optical fiber where the cladding is removed, and expressing a cholesteric blue phase;
An electrode for applying an electric field for vertically aligning liquid crystal molecules in the liquid crystal material,
An optical device for switching the liquid crystal molecules between a vertically aligned nematic phase and a cholesteric blue phase.   The liquid crystal material when the refractive index of the liquid crystal material expressing a cholesteric blue phase is lower than the refractive index of the core of the optical fiber, and anisotropy is developed in the refractive index by applying an electric field to the liquid crystal material The optical device according to claim 1, wherein a component having the highest refractive index is higher than a refractive index of the core. Wherein the refractive index anisotropy of the liquid crystal material is positive, the ordinary refractive index of the liquid crystal material and n _o, when the extraordinary refractive index was n _e, n _e is higher than the refractive index of the core of the optical fiber The optical device according to claim 1, wherein (n _e + 2n _o ) / 3 is lower than a refractive index of a core of the optical fiber. further,
A pair of substrates disposed on both sides of the portion where the cladding of the optical fiber is removed and sandwiching the liquid crystal material;
A sealing member disposed between the substrates and sealing a space where the liquid crystal material is disposed, and the electrodes are formed on inner surfaces of the pair of substrates. 4. The optical device according to any one of 3.

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