JP4902409B2 - Slab type optical waveguide device with optical fuse function - Google Patents

Description

  The present invention relates to a slab type optical waveguide device having a function as an optical fuse.

  Since the development of the first laser, passive optical limiters have been studied, and the concept of protecting the optical sensor from damage induced by the highest intensity of the laser has been tested. The first optical limiter for CW lasers (continuous oscillation lasers) was based on the thermal lens action of a light absorbing bulk liquid. That is, when the imaging system was locally heated, the refractive index decreased, causing “thermal defocus”, and the light beam was out of focus. Other methods have been proposed to limit the pulsed laser source, such as reverse saturable absorption, two-photon free carrier absorption, self-focusing, nonlinear refraction and induced scattering.

  In communications and other systems in medical, industrial and remote sensing applications, a single fiber or waveguide may handle relatively high light intensities from microwatts to several watts. When high intensity (intensity per unit area) is introduced into these systems, many thin coatings, photoadhesives and even the entire material will be exposed to light intensity that exceeds its damage threshold. Another problem is laser safety. The upper limit of the intensity that can be emitted from the fiber into the air is clearly defined. These two problems require passive devices that limit the amount of energy propagating through the fiber / waveguide to an acceptable level. Many attempts have been made to realize optical limiters, primarily for high power laser radiation, high power pulse radiation, and eye safety devices.

  Therefore, several optical limiters have been proposed (Patent Documents 1 and 2).

In Patent Document 1, an input optical transmission element, an output optical transmission element, an optical signal transmitted from the input optical transmission element to the output optical transmission element is disposed between the input optical transmission element and the output optical transmission element. A light intensity limiter including an intensity limiting element for transmission is described. Here, the intensity limiting element contains particles of at least one material that causes a reversible thermal change in response to light exceeding a predetermined light intensity level, thereby changing the light transmission characteristics of the intensity limiting element. Contains light limited solid mixture. The light-limiting solid mixture is composed of light-absorbing particles (for example, selected from the group consisting of polymethyl methacrylate and its derivatives, epoxy resin, glass, sol-gel material, and spin-on glass) dispersed in a light-transparent matrix (for example, And at least one material selected from the group consisting of Ag, Au, Ni, Va, Ti, Co, Cr, C, Re, Si, and SmO ₂ .

Patent Document 2 includes a parametric amplification element that optically parametrically amplifies input signal light, and a wavelength selection element that removes wavelength components other than signal light having a predetermined wavelength from the signal light output from the parametric amplification element. An optical limiter characterized in that is described. Here, the parametric amplifying element is an optical fiber (for example, a dispersion shifted optical fiber or a dispersion flat optical fiber), and the parametric amplifying element is a nonlinear optical material (for example, KTiOPO ₄ , KTiOAsO ₄ , β-BaB ₂ O). ₄ and LiNbO ₃ ), and the wavelength selection element is an optical filter.

On the other hand, research on photonic crystals and optical elements in which liquid crystal is injected attracts attention in 1999, including calculation results (Non-Patent Document 1) and experimental results (Non-Patent Document 2). By injecting liquid crystal into a microcavity (MicroCavity) made of two-dimensional or three-dimensional silicon, and changing the signal from outside (heat, potential difference, polarization, etc.), the alignment and optical characteristics of the liquid crystal can be changed. Research to control is becoming popular. The photonic band shifts as the alignment of the liquid crystal changes due to the phase transition. In addition, the alignment of the liquid crystal is sensitive to the shape and material of the surface, and there is a discussion about the control of the alignment of the liquid crystal in the photonic crystal. Such optical elements have been studied for application as optical switches, high-density information recording, displays, and the like (Non-Patent Documents 3 to 8).
JP 2004-310120 A Japanese Patent Laid-Open No. 11-218792 S. John, et al. PRL 83, 967 K. Yoshino, et al, APL 75, 932 S. Kurihara et al. 'Photochemical switching behavior of azofunctionalized polymer liquid crystal / SiO2 composite photonic crystal' APL 89, 153131 (2006) E. Kosmidou et al. 'Analysis of tunable photonic crystal directional couplers' J. Appl. Phys. 100, 043118 (2006) Ch. Schuller et al. 'Polarization-dependent optical properties of planar photonic crystals infiltrated with liquid crystals' APL 87 121105 (2005) M. Soljacic 'Enhancement of nonlinear effects using photonic crystals' Nature, Vol. 3, 211 (2004) M. Ozaki et al. 'Electrically color-tunable defect mode lasing in one-dimensional phonic-band-gap system containing liquid crystal' APL 82 3593 (2003) G. Mertens et al. 'Two- and three-dimensional photonic crystals made of macroporous silicon and liquid crystals', APL 83, 3036 (2003)

  US Pat. No. 6,057,009 also attempted to use liquid crystal as a limiting material, mainly for high power pulses, but due to the director variation, this material causes more severe noise and distortion than ordinary liquids (paragraph 0007). ), And so far, no optical limiter using liquid crystal has been known.

  The optical limiters described in Patent Documents 1 and 2 are devices designed to have a high transmittance for low-level light input and a low transmittance for high intensity. However, an element having a function of spontaneously blocking transmitted light when the light input exceeds a predetermined level has not been known so far. Furthermore, even if the transmitted light is interrupted once and turned off, there is no known element that can spontaneously cancel the interruption and return to the ON state when the light input falls below a predetermined level. The optical element that can function as such an optical fuse is expected to be used as an element that protects devices installed downstream from high-intensity light.

  Therefore, an object of the present invention is to provide an optical element that can function as the optical fuse as described above.

  The present inventors paid attention to the fact that photonic crystals injected with liquid crystal have excellent points as optical elements, and conducted various studies on elements using photonic crystals injected with liquid crystal. The present invention was completed by finding that a slab type optical waveguide device using a photonic crystal has a function as an optical fuse.

The present invention is as follows.
[1] A slab type optical waveguide in which a one-dimensional photonic crystal and a liquid crystal layer are provided between opposing main surfaces of two light-transmitting substrates , and a temperature adjusting device for adjusting the temperature of the optical waveguide element is provided . element.
[2] The optical waveguide device according to [1], wherein the one-dimensional photonic crystal is made of a grating.
[3] The optical waveguide device according to [1] or 2, wherein the grating is a grating made of chromium, a chromium alloy, gold, tungsten, silicon, or germanium.
[4] The optical waveguide device according to [2] or [3], wherein the grating is formed by arranging wires in parallel at a predetermined interval.
[5] The optical waveguide device according to any one of [1] to [4], wherein the liquid crystal layer is made of a nematic liquid crystal.
[6] The optical waveguide device according to any one of [1] to [5], wherein the light transmissive substrate is a quartz substrate or a glass substrate.
[7] Any one of [1] to [6], further including a spacer that defines an interval between opposing main surfaces of the two light-transmitting substrates between the opposing main surfaces of the two light-transmitting substrates. 2. An optical waveguide device according to 1.
[ 8 ] The optical waveguide device according to any one of [1] to [ 7 ], which is used as an optical fuse.
[ 9 ] (1) A step of causing the excitation light from the input light transmission element to enter the optical waveguide device,
(2) a step of transmitting waveguide mode light generated in the optical waveguide element by the excitation light to the output optical transmission element;
(3) when the incident light to the optical waveguide element exceeds a predetermined light intensity level, including a step of eliminating the waveguide mode light transmitted to the output light transmission element;
The light quantity control method whose said optical waveguide element is an optical waveguide element in any one of [1]-[ 7 ].
[ 10 ] The method according to [ 9 ], wherein the incident light is laser light.
[ 11 ] The method according to [ 10 ], wherein the laser beam has a wavelength range of 300 nm to 300 μm.

  According to the present invention, when the light input exceeds a predetermined level, the transmitted light is spontaneously blocked, and once the transmitted light is blocked and turned off, the light input reaches the predetermined level. When it falls below, the element | device which can cancel | release spontaneously and can return to an ON state can be provided.

[Slab type optical waveguide device]
In the slab type optical waveguide device of the present invention, a one-dimensional photonic crystal and a liquid crystal layer are provided between opposing main surfaces of two light-transmitting substrates.

  The one-dimensional photonic crystal is made of a grating (diffraction grating), and the grating can be, for example, a grating made of chromium, a chromium alloy, gold, tungsten, silicon, or germanium.

  Specifically, the optical waveguide device of the present invention includes a light transmissive substrate provided with a grating on a part or all of one main surface, a light transmissive substrate provided with the grating, and a light transmissive material having no grating. It consists of a liquid crystal layer sandwiched between substrates. That is, the order is one of the light transmissive substrates, the grating on the main surface, the liquid crystal layer, and the other light transmissive substrate.

  The grating may be provided on part or all of one surface of the light transmissive substrate. The grating is suitably made of chromium (Cr) or a chromium alloy. Cr absorbs more light in the visible light region than other commonly used metals. In addition to Cr, if there is a metal that has a large absorption of light in the visible light region, it can be used in the same manner as Cr. For example, gold, tungsten, silicon, germanium, and the like can be cited as materials constituting the grating. The reflection of gold has a low reflectance at a wavelength shorter than 500 nm, and if the grating is designed for a wavelength shorter than 500 nm, light absorption is relatively high, and the effect of the optical fuse of the present invention can be obtained.

  The grating absorbs and dissipates light by the incident light of the continuous light laser and the excited waveguide. When the amount of heat radiation exceeds a certain value, the waveguide mode is not excited by changing the refractive index of the liquid crystal, the signal is turned off, and the function as an optical fuse is exhibited. At this time, if the temperature setting of the liquid crystal is close to the phase transition, the refractive index of the liquid crystal changes more greatly with a slight temperature difference, so that a faster response can be obtained. In the element of the present invention, as described above, in addition to the switching of the waveguide mode due to the change in the refractive index of the liquid crystal (signal ON and OFF), the switching of the waveguide mode due to the phase transition of the liquid crystal is also possible. However, since the amount of heat required for the phase transition of the liquid crystal is larger than the change in the refractive index of the liquid crystal, the switching and switching of the waveguide mode by the change in the refractive index of the liquid crystal is far superior in sensitivity and responsiveness. Therefore, in the following, switching of the waveguide mode due to a change in the refractive index of the liquid crystal will be described. However, the element of the present invention also operates by switching in the waveguide mode due to the phase transition of the liquid crystal.

  The grating is one in which wires are arranged in parallel at predetermined intervals. The wire width, thickness, and wire spacing can be appropriately set according to the wavelength of light in which the optical waveguide device of the present invention is used. The wavelength of the light used by the optical waveguide element is (assuming that (can be in the range of 300 nm to 300 μm. In this case, the distance between the wires is in the range of the wavelength, and the thickness of the wire is the kind of liquid crystal. However, it is appropriate that the wire width of the wire is about 0.01 (˜0.09 (about 10 / (/ 100).

The liquid crystal layer can be made of, for example, nematic liquid crystal. As the nematic liquid crystal, for example, MBBA: N- (p-methoxybenzylidene) -pn-butylaniline (T _c = 47 ° C.), 5CB: 4`-n-pentyl-4-cyanobiphenyl (T _c = 36 ° C.), PAA: p-azoxyanisole (T _c = 135 ° C.), EBBA: N- (p-ethoxybenzylidene) -pn-butylaniline (T _c = 81 ° C.) and the like. However, it is not intended to be limited to these examples, and any nematic liquid crystal can be used. Furthermore, a mixed liquid crystal of two or more nematic liquid crystals can be used.

  Considering that the response speed increases as the thickness of the liquid crystal layer decreases, and that multimode resonance due to the Fabry-Perot effect is avoided, the thickness of the liquid crystal layer is about the thickness of the incident light wavelength or less. It is preferable. Specifically, it can be in the range of 10 nm to 1000 μm, and preferably in the range of 50 nm to 500 μm.

  The light transmissive substrate can be, for example, a quartz substrate. Other than the quartz substrate, any optical waveguide element can be used as long as it has a predetermined transparency to the light used, for example, a glass substrate or a synthetic resin substrate. Also good. Although there is no restriction | limiting in particular in the thickness of a transparent substrate, For example, it can be set as the range of 0.1-10 mm.

  The optical waveguide device of the present invention can further include a spacer that defines the distance between the opposing main surfaces of the two light-transmitting substrates between the opposing main surfaces of the two light-transmitting substrates. The spacer is not particularly limited as long as it can define the interval between the light transmissive substrates. For example, when the distance between the light-transmitting substrates is submicron, the submicron-sized particles are used as spacers, and the sub-micron-sized particles are arranged between the light-transmitting substrates so that one layer is formed. The predetermined interval can be set. The material of the submicron-sized particles is not particularly limited, but it is appropriate to obtain a chemically or optically inactive substance. Submicron sized particles can be adhered between the particles and with the light transmissive substrate using an adhesive. There is no particular limitation on the adhesive. Any known adhesive can be appropriately selected as long as it is inert to the material constituting the optical waveguide element such as liquid crystal and can satisfactorily bond the light-transmitting substrate and the spacer.

  The optical waveguide device of the present invention can be manufactured without the spacer. Further, a resist (a photoresist or the like) in which a pattern is formed can be used as the spacer.

  The optical waveguide device of the present invention can be provided with a temperature adjusting device for adjusting the temperature of the optical waveguide device. Examples of such a temperature adjusting device include a heater or a Peltier element. It is preferable that the heater or the Peltier element is embedded in a metal block such as aluminum together with an optical element, and the temperature is adjusted by monitoring the metal block with a temperature detector.

  When an optical waveguide element is used, it is normally in an ON state (a state in which the waveguide mode is excited), and when the optical waveguide element is heated above a certain level, it is in an OFF state (the waveguide mode is excited). The temperature is set so that The temperature at which the optical waveguide element changes from the ON state to the OFF state varies depending on the type of one-dimensional photonic crystal and liquid crystal layer provided in the optical waveguide element. The temperature at which the optical waveguide element changes from the ON state to the OFF state is, for example, such a temperature as described above by using a temperature detector embedded in a metal block with the temperature of the optical waveguide element including the temperature adjusting device. It can be determined by measuring the energy (wavelength) at which the waveguide mode is excited while checking and adjusting. The waveguide mode energy (wavelength) excited when the liquid crystal has a certain refractive index is determined based on the shift of the waveguide mode energy (wavelength) excited when the refractive index of the liquid crystal changes due to heat. If the measured waveguide mode energy is confirmed, the change temperature can be estimated.

  The optical waveguide device of the present invention can be used as an optical fuse that completely blocks incident light.

[Light intensity control method]
The present invention also includes a light amount control method using the optical waveguide element of the present invention. The light quantity control method of the present invention is:
(1) a step of making excitation light from the input light transmission element enter the optical waveguide element;
(2) a step of transmitting waveguide mode light generated in the optical waveguide element by the excitation light to the output optical transmission element;
(3) when the incident light to the optical waveguide element exceeds a predetermined light intensity level, including a step of eliminating the waveguide mode light transmitted to the output light transmission element;
The optical waveguide element is the optical waveguide element of the present invention.

  Incident excitation light to the optical waveguide element enters the optical waveguide element from the light transmitting substrate that is not adjacent to the one-dimensional photonic crystal of the optical waveguide element from the input light transmission element. The input optical transmission element can be, for example, an optical fiber.

  The incident light reaches the one-dimensional photonic crystal after passing through the light-transmitting substrate and the liquid crystal layer. The one-dimensional photonic crystal generates waveguide mode light by excitation light, and the generated waveguide mode light is emitted in a direction parallel to the main surface of the light-transmitting substrate and transmitted to the output light transmission element. Is done. When a standing wave laser that matches the optical characteristics of the one-dimensional photonic crystal is incident as excitation light, the waveguide mode is excited in the liquid crystal layer by Bragg reflection of a one-dimensional photonic crystal, for example, Cr grating, and the waveguide mode is Propagate (this state is signal ON).

  A part of the waveguide mode light generated above is absorbed by the one-dimensional photonic crystal, and the absorbed heat is diffused from the one-dimensional photonic crystal, and a part thereof is absorbed by the liquid crystal layer. Specifically, for example, a grating made of Cr that absorbs light in the visible light region absorbs and dissipates light by incident light of a standing wave laser and an excited waveguide mode. When the incident light exceeds a predetermined light intensity level, the amount of heat dissipated from the one-dimensional photonic crystal exceeds a predetermined level, the amount of heat absorbed by the liquid crystal layer exceeds a predetermined level, and the refractive index of the liquid crystal changes. The energy (wavelength) of the light propagating in the waveguide element changes by the change in the refractive index of the liquid crystal. As a result, the waveguide mode excited in the liquid crystal layer is not excited, and the waveguide mode light transmitted to the output light transmission element disappears (this state is referred to as signal OFF).

  However, since heat is radiated from the entire element to the outside, the amount of heat radiated from the entire element is greater than the amount of heat radiated from the one-dimensional photonic crystal, and the liquid crystal does not reach the change in the refractive index. There is no stop of light propagation in the waveguide mode due to the phase transition. In other words, when the power of the incident excitation light is not at a level where the amount of heat released from the entire element is higher than the amount of heat released from the one-dimensional photonic crystal, the above-described liquid crystal phase transition does not occur and spontaneous switching occurs. Not. In addition, as described above, when excitation light is incident at the required fixed amount of light power, the temperature at which equilibrium is measured is measured, and the wavelength of the waveguide mode that is excited by the refractive index of the liquid crystal at that temperature is selected. To do. Then, the waveguide mode continues to be excited at the required optical power, and the waveguide mode can disappear when more optical power is incident.

  In addition, even if the waveguide mode light is not excited in the liquid crystal layer and the waveguide mode light transmitted to the output light transmission element disappears, the intensity of the incident excitation light decreases, and the amount of heat released from the entire device. If the temperature of the liquid crystal layer is lower than the refractive index change temperature of the liquid crystal and the refractive index of the liquid crystal returns to the original state, the light of the waveguide mode light of the liquid crystal layer returns. Propagation is restored (again, the signal is turned on again).

  The optical waveguide element of the present invention is expected to be used as an element or an optical fuse that protects a device installed downstream from light having an intensity exceeding a predetermined level. Further, when the incident standing wave laser is turned off after being turned off, the liquid crystal returns to the original refractive index again, so that the function as the optical waveguide element is restored. As described above, when the intensity of incident light falls below a predetermined level, the waveguide mode is excited again in the liquid crystal layer, and can be used repeatedly as an optical fuse.

  An example of an optical switch (optical fuse) using the optical waveguide device of the present invention is shown in FIG. A continuous light (optical signal) having a wavelength matching the grating of the optical waveguide element (sample) is incident on the sample (CW input), and the waveguide mode is excited in the liquid crystal layer. From the side of the optical waveguide element (sample) (in the x-axis direction, see FIG. 1), a signal in the waveguide mode (Signal output) is confirmed by a photodetector (photo detector). (This state is signal ON.) When continuous light of a certain intensity or more is incident, the liquid crystal undergoes a change in refractive index or phase transition, and the excitation energy of the waveguide mode shifts, and the waveguide mode that has been excited. Is no longer excited. (This state is referred to as signal OFF.) Signal ON and OFF can be detected by a photodetector, and incident light can be controlled based on the detection information.

  When continuous light is not incident, the temperature of the liquid crystal decreases and the refractive index or phase of the liquid crystal returns to its original value, so that it can be used again as an optical fuse.

  Conventionally, there have been proposals for optical switching and optical limiters, but the proposal as an optical element that utilizes spontaneous switching and can be reused is unique to the present invention. Conventionally, the optical switching and the optical limiter show the behavior shown in the upper diagram of FIG. 6, whereas the optical waveguide device of the present invention shows the behavior shown in the lower diagram of FIG.

  Hereinafter, the present invention will be described in more detail with reference to examples. The following examples are set forth for purposes of illustration and are not intended to limit the scope of the invention.

Example 1
1) Production of sample The production method of the optical waveguide device of this example is as follows.
A one-dimensional Cr grating (photonic crystal) was fabricated on a single quartz substrate using an electron beam lithography method. The period of the Cr grating was 500 nm. A pre-treatment with concentrated hydrogen peroxide solution (piranha solution) was applied onto a quartz substrate, and a resist for electron beam was applied and spin-coated. After exposure with an electron beam, development was performed, and a Cr thin film having a thickness of about 60 nm was formed at low temperature using plasma sputtering. A Cr grating having a shape in which nano-sized (200 nm) Cr wires are periodically arranged was produced by lift-off (resist peeling). Cr forms a thin film relatively easily by plasma sputtering and can be lifted off. The shape of the grating is suitable for the alignment of nematic liquid crystal and can be mass-produced by photolithography.

  Next, sub-micro size silica particles (provided by Nippon Shokubai Co., Ltd.) were used as spacers and arranged on the substrate on which the grating was fabricated.

  Silica particles were used as spacers to control the gap distance between the two substrates. Since the gap was 1 μm or less, sub-micro size particles were used. After washing the substrate, a silica solution containing silica particles dispersed in water was dropped onto the end side of the substrate surface using a microsyringe. Next, heat was applied from 60 ° C. to 80 ° C. to evaporate the water. The evaporation temperature can be adjusted as appropriate according to the size of the silica particles). It is appropriate to evaporate relatively slowly so that the silica particles do not overlap when the water evaporates.

  A cell was prepared by attaching another pretreated quartz substrate from above with epoxy (obtained from Kyoritsu Chemical Industry) that had been filtered several times to remove suspended matters. Epoxies that have been filtered several times to remove suspended solids are free from fillers, and those having a relatively low viscosity are suitable. A small amount of epoxy is applied to the edge side of the substrate surface on which the cleaned metal grating is fabricated. The epoxy was then UV and heat cured by placing it on a substrate with dispersed silica particles and applying pressure.

The epoxy was cured by UV (15 J / cm ² ) and heat (120 ° C.). After the cell was completed, the gap thickness was confirmed by light interference. A thinner gap is preferable for increasing the response speed in observation. If there is a larger deposit than the silica particles used as the spacer, the desired gap cannot be realized, so the cell is created in a clean room.

The substrate was acid-washed before producing the grating. After preparation of the grating, it was washed with acetone, IPA or the like. The other substrate on which no grating was prepared was subjected to an acid cleaning treatment and then further cleaned with acetone and IPA in a clean room. The clean room is class 100 (100 particles / ft ³ ).

  In this example, an optical waveguide device having a substrate-to-substrate distance (gap) of 5 μm was produced. (See Figure 1)

  Finally, while raising the temperature to a high temperature (80-100 ° C.) and reducing the viscosity of the liquid crystal, a capillary was used to inject the liquid crystal between the cells. The thickness of the liquid crystal layer is determined by the thickness of the cell gap. In this example, 5CB (4-cyano-4′-n-pentylbiphenyl) was used as the liquid crystal. 5CB undergoes a phase transition to a nematic phase at room temperature and an isotropic phase at 35 ° C. After the injection, the liquid crystals were naturally aligned along the Cr grating (Planar orientation). FIG. 1 is an explanatory diagram of the optical waveguide device obtained in this example.

2) Confirmation of sample characteristics As the first process of observation, white light (halogen light) was perpendicularly incident on the surface of the device, and the value of the wavelength at which the waveguide was formed by the transmitted light was confirmed. At this time, the device was set to a nematic phase temperature slightly below the phase transition temperature. In addition, the polarizing plate of white light for observation of the waveguide was adjusted by the TE mode and the TM mode. That is, the TE waveguide mode is a waveguide mode polarized in the coaxial direction (y-axis direction, see FIG. 1) with the grating. The TM mode is a waveguide mode that is polarized in the vertical direction of the grating (the electric field is in the x and z planes and the magnetic field is in the y-axis direction, see FIG. 1). By polarizing incident light in the x-axis or y-axis, the TM waveguide mode or the TM waveguide mode can be excited between the substrates (liquid crystal layer).

  FIG. 2 shows a white light transmission spectrum. This is a result of measuring the angle between the sample and the incident light every 2 ° from −10 ° to + 10 °. It can be confirmed that the waveguide mode is excited by the value of the excitation energy (wavelength) in the grating. Incident light is polarized in the y-axis direction (see FIG. 1, parallel to the grating) for TE waveguide mode measurement.

  In this example, since the TE mode waveguide was remarkably observed, all observations were conducted using the TE mode (see FIG. 2).

  The element was embedded in a thermo bath (not shown in FIG. 1) for temperature control. This thermobus (temperature controller) can be adjusted every other degree Celsius. Next, the temperature of the device was adjusted to 31 to 34 ° C. so that the temperature of the liquid crystal in the device was higher than the temperature at which the liquid crystal transitioned to the isotropic phase. And the waveguide formed in the isotropic phase was confirmed. The white transmission spectrum is shown in (a-1, a-2) of FIG. Incident light is perpendicular to the sample. (A-1) The sample temperature is set to 31 degrees Celsius below the liquid crystal (nematic to isotropic phase) phase transition temperature, and (a-2) the sample temperature is set to 34 degrees Celsius, where the liquid crystal is oriented in the isotropic phase. Has been. In (a-1) and (a-2), a shift of 50 nm or more could be measured.

  Next, instead of white light, the waveguide element was excited with a continuous light laser. As the continuous light laser, a laser capable of modulating the wavelength with visible and near infrared light after releasing the picosecond lock of TSUTNAMI manufactured by SpectraPhysics was used. The wavelength was adjusted to the wavelength on the high wavelength side (797.73 nm) of the TE waveguide mode formed in the nematic liquid crystal layer. The wavelength width of the excitation light is 1 nm or less. First, the phase transition of liquid crystal with respect to standing wave incidence was confirmed.

  Fig. 4 shows numerical calculations based on the ideal alignment state of the (b-1, b-2) liquid crystal in Fig. 3. (B-1) is a numerical result of a transmission spectrum in which the liquid crystal is aligned along the grating. (B-2) Numerical transmission spectrum assuming that the liquid crystal is isotropically oriented. In (a) and (b), the waveguide mode was excited to the same extent, and the spectral shift due to the liquid crystal phase transition of 50 nm or more was measured to the same extent. Therefore, it was shown that the liquid crystal is ideally aligned and can be controlled by adjusting the temperature.

  Using white light as probe light, transmitted light was observed at an angle of about 10 degrees with the surface of the element. In the waveguide (d-1) observed immediately after the excitation light (power, 157 mW) is incident and the waveguide (d-2) that has been incident for about 10 minutes, before the liquid crystal undergoes phase transition (nematic phase) After the phase transition with the waveguide (c-1)), the same spectrum as that of the waveguide in the isotropic phase (c-2) was confirmed (see FIGS. 3 (c) and 3 (d)).

  (C-1, c-2) white light (probe light) transmission spectrum of FIG. Incident light is incident on the sample at an angle of 10 °. (C-1) The sample temperature was adjusted to 31 ° C. at which the liquid crystal is in the nematic phase. (C-2) The sample temperature was adjusted to 34 ° C. where the liquid crystal is in an isotropic phase. This is a measurement of spontaneous switching of a waveguide mode with respect to (d-1, d-2) pump light (737.79 nm excitation light). The peak near the center of 737.79 nm is scattered light of pump light (excitation light). Similar to (c), the white transmission spectrum was incident on the sample at an angle of 10 °, and the transmission spectrum was measured. (D-1) A white light (probe light) transmission spectrum immediately after the pump light is incident on the sample. The sample temperature was set to 31 ° C. where the liquid crystal was considered to be in the nematic phase. (D-2) This is a white light (probe light) transmission spectrum measured after pump light was kept on the sample for 10 minutes without changing the sample temperature. In (d-1, d-2), the spectrum based only on excitation light without temperature adjustment equivalent to the shift of the transmission spectrum (c-1, c-2) when the temperature is adjusted and the liquid crystal undergoes phase transition. Shift was measured. Therefore, (d-1, d-2) indicates that the phase transition was spontaneously caused in the liquid crystal by the excitation light, and the shift of the waveguide mode was measured.

  This experiment confirmed that the excitation light (steady wave of 157 mW) was absorbed by the one-dimensional Cr photonic crystal, dissipated heat, and the liquid crystal phase transition spontaneously occurred.

3) Operation check The difference in the phase transition speed of the liquid crystal in the device due to the difference in the excitation light power intensity was measured. The wavelength of the excitation light was 797.73 nm, the same as in the previous experiment. A He—Ne laser was used as the probe light. Similar to the above-described experiment, the probe light transmission is measured from an angle of about 10 degrees with respect to the surface of the element. In the transmission measurement of this experiment, the probe light was passed between the two polarizing plates, and the element was placed between the two polarizing plates. In order to minimize the noise of the scattered light from the excitation light, the intensity of the transmitted light was measured with a photo detector after passing through the spectroscope. The two polarizing plates were installed so that the intensity of the probe transmitted light was minimized (near the zero value) when the liquid crystal temperature in the device was raised to the isotropic phase from the outside. The results are shown in FIG.

  FIG. 4 shows the measurement result of the speed of liquid crystal phase transition due to the difference in luminous intensity of excitation light (737.79 nm). A state in which liquid crystal by excitation light spontaneously undergoes phase transition is shown. A sample was placed between two polarizing plates and measured by its transmission intensity. The sample temperature is set to 34 ° C. where the liquid crystal is oriented in the isotropic phase, and the two polarizing plates are set perpendicular to each other. When the liquid crystal is in the isotropic phase, the transmission intensity is set to a minimum and near zero. When the liquid crystal has a directionality, the transmitted light can be measured even if the two polarized light beams are at right angles because of the birefringence. First, the liquid crystal is set to a nematic phase and 31 ° C. The transmitted light intensity at that time is normalized by the transmitted light intensity at that time. When the excitation light intensity was 79 mW, the transmitted light intensity did not become zero. As the excitation light intensity was increased, it was confirmed that the liquid crystal transitioned after 535 milliseconds at a maximum of 157 mW. (Refer to the table in FIG. 4.)

  A liquid crystal phase transition could not be observed for 79 mW excitation light. The probe transmitted light increases once from the initial value due to steady incidence of 13 msec or more for 102 mW excitation light, 630 msec or more for 130 mW excitation light, and 353 msec for 157 mW excitation light. Then, it attenuated with time near zero. It was measured that the phase transition of the liquid crystal in the device did not occur with the excitation light of 79 mW, but the speed of the phase transition increased as the intensity of the excitation light increased. The excitation light is set to a wavelength on the high wavelength side of the waveguide excited in the nematic phase. When the waveguide excited in the nematic phase changes to the isotropic phase, the refractive index of the liquid crystal changes greatly, so that it switches to the lower wavelength side by 50 nm or more (see FIG. 2). Therefore, the waveguide that has been excited on the high wavelength side in the nematic phase is not excited.

  As shown in FIG. 4, the switching speed depends on the intensity of the excitation light, but a maximum of 353 milliseconds or more was measured in this example. The intensity of the excitation light at this time is an intensity that can be easily obtained with a diode laser or the like, and is therefore suitable for an optical switch for preventing light leakage and avoiding irradiation with intense light.

  Note that the shape of the Cr grating produced in this example is generally similar to the polymer that is rubbed when the liquid crystal is planarized. This application enables a nematic liquid crystal layer that is more concisely aligned than liquid crystal alignment using two-dimensional and three-dimensional photonic crystals.

  In general, two-dimensional and three-dimensional photonic crystals are used to study optical switches by injecting liquid crystals into photocavity microcavities. A nematic liquid crystal is an aggregate fluid of a rod-like (one-dimensional shape). Since there are a plurality of equivalent combinations of liquid crystal orientations in the two-dimensional and three-dimensional photonic crystals, it is difficult to control the liquid crystal in a specific direction. In the one-dimensional grating, the liquid crystal is almost certainly aligned along the grating. As a result, the optical switch seen when the liquid crystal undergoes phase transition is also maximized efficiently. Therefore, switching (shift of 50 nm or more) at a larger value than that observed in the past was observed.

Example 2
Example 1 is an example of switching based on the phase transition of the liquid crystal, but in this example, an example of switching based on a change in the refractive index of the liquid crystal is shown. The optical waveguide element used is the same as that of the first embodiment. A schematic diagram of the experimental apparatus is shown in FIG.

(A) Confirmation experiment of presence / absence of waveguide mode in liquid crystal layer.
(B) An experiment to observe changes in the liquid crystal layer at the same time.

(A) The sample was set to an initial temperature, T = Tc-1: nematic phase and near the phase transition temperature. The excitation light (continuous light) is set to the waveguide mode energy (wavelength in this case) excited at that time. The temporal response of the waveguide mode propagating perpendicular to the excitation light (from the side of the sample) was measured. The results are shown in FIG. The shutter was opened at time 0 seconds, at which time the waveguide mode was measured. However, the waveguide mode disappeared after 25 milliseconds. The power of the waveguide mode at this time is higher than the value in the situation where the shutter is closed. This is thought to be because some of the excitation light transmitted through the liquid crystal layer in the sample caused total internal reflection in the quartz substrate and propagated in the substrate. In order to remove this background noise, the side surface of the substrate is painted black, but it is thought that light leaks a little.

(B) At the same time, the change of the liquid crystal layer at the same position of the sample was measured. The result is shown in FIG. In contrast to the disappearance of the waveguide mode within 25 seconds, the liquid crystal does not undergo phase transition within this measurement time (transmitted light is not zero). However, the light transmitted through the layer is greatly changed. This represents a change in the refractive index of the liquid crystal. It is shown in (a) and (b) that the waveguide mode energy is well confined, so that even a slight change in the refractive index of the liquid crystal causes the waveguide mode energy to shift and not be excited.

  The present invention is useful in the field of optical transmission.

The explanatory view of the optical waveguide device of the present invention produced in the example is shown. The upper left is a photograph of a Cr grating sample taken with SEM. A white light transmission spectrum is shown. (A-1, a-2) It is a white transmission spectrum. (B-1, b-2) These are numerical calculation results assuming the ideal alignment state of the liquid crystal. (C-1, c-2) Transmission spectrum of white light (probe light). It is a measurement result of the spontaneous switching of the waveguide mode with respect to (d-1, d-2) pump light (737.79 nm excitation light). The measurement result of the speed | rate of a liquid crystal phase transition by the difference in the luminous intensity of excitation light (737.79 nm) is shown. Explanatory drawing of the optical switch using the optical waveguide element of this invention. The upper diagram of FIG. 6 shows the behavior of the optical switching and optical limiter conventionally, and the lower diagram of FIG. 6 shows the behavior of the optical waveguide device of the present invention. The schematic of the experimental apparatus used in Example 2 is shown. The experimental result of Example 2 is shown. (A) is an experimental result for confirming the presence / absence of a waveguide mode in the liquid crystal layer, and (b) is an experimental result for observing a change in the liquid crystal layer performed simultaneously.

Claims (11)

A slab type optical waveguide device in which a one-dimensional photonic crystal and a liquid crystal layer are provided between opposing main surfaces of two light-transmitting substrates , and a temperature adjusting device for adjusting the temperature of the optical waveguide device is provided. The optical waveguide device according to claim 1, wherein the one-dimensional photonic crystal is made of a grating. The optical waveguide device according to claim 1 or 2, wherein the grating is a grating made of chromium, a chromium alloy, gold, tungsten, silicon, or germanium. The optical waveguide device according to claim 2 or 3, wherein the grating has wires arranged in parallel at predetermined intervals. The optical waveguide device according to claim 1, wherein the liquid crystal layer is made of a nematic liquid crystal. The optical waveguide device according to claim 1, wherein the light transmissive substrate is a quartz substrate or a glass substrate. The spacer which prescribes | regulates the space | interval of the main surface which the said 2 light transmissive board | substrates oppose is further provided between the main surfaces which the said 2 light transmissive board | substrates oppose. Optical waveguide element. The optical waveguide device according to any one of claims 1 to 7 , which is used as an optical fuse. (1) a step of making excitation light from the input light transmission element enter the optical waveguide element;
(2) a step of transmitting waveguide mode light generated in the optical waveguide element by the excitation light to the output optical transmission element;
(3) when the incident light to the optical waveguide element exceeds a predetermined light intensity level, including a step of eliminating the waveguide mode light transmitted to the output light transmission element;
Light quantity control method the optical waveguide device is an optical waveguide element according to any one of claims 1-7. The method according to claim 9 , wherein the incident light is a laser beam. The method according to claim 10 , wherein the laser beam has a wavelength range of 300 nm to 300 μm.