JP4269788B2 - Reflective light modulator and variable optical attenuator - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to a reflective light modulation element and a variable optical attenuator, and more particularly,The reflection type light modulation element includes a reflection type polarization diffraction grating and a liquid crystal element, and changes the polarization state of the transmitted light of the liquid crystal element by using a control means for controlling the alignment state of the liquid crystal molecules by applying a voltage, thereby reflecting the polarization polarization diffraction. The present invention relates to a reflection type light modulation element that changes a diffraction state by a grating. In addition, the variable optical attenuator is diffracted light by a reflective light modulator.WhenNon-diffracted lightOut ofEither one is received by the sorting means, and the amount of received light changes according to the magnitude of the applied voltage.
[0002]
[Prior art]
  Light modulation elements using liquid crystals are commercialized as liquid crystal displays, and are used in various optical devices such as variable optical attenuators and optical switches that control transmitted light by changing the voltage applied to the liquid crystal layer. . For example, Japanese Patent Application Laid-Open No. 2003-66450 discloses a variable optical attenuator in which a phase plate is integrated with a liquid crystal cell and the amount of transmitted light changes according to the magnitude of an applied voltage. Further, in the same Japanese Patent Application Laid-Open No. 2003-66450, a configuration example in which a polarization diffraction type polarizer is further integrated so that a temperature rise of the liquid crystal layer is small and a stable extinction ratio can be obtained even when high-intensity light is incident. Is disclosed. An example of the configuration of this conventional transmissive light modulation element is shown in FIG. Polarization diffraction in liquid crystal cell 50 with phase plateMold biasThe photon 60 is integrated into a variable optical attenuator 300. The linearly polarized light in the Y-axis direction depends on the voltage applied to the liquid crystal layerWith phase plateThe polarization state after passing through the liquid crystal cell 50 is changed, the linearly polarized light component in the Y-axis direction is diffracted by the polarization diffraction polarizer 60, and the linearly polarized light component in the X-axis direction is transmitted straight. By separating the diffracted light and the straight transmitted light and extracting only the straight transmitted light, a variable attenuator by voltage is obtained.
[0003]
Also, the same Japanese Patent Application Laid-Open No. 2003-66450 discloses that by using one electrode in the liquid crystal cell as a reflective electrode, the thickness of the liquid crystal layer can be halved, leading to lower drive voltage and faster response. ing.
[0004]
In a wavelength division multiplexing optical communication network, an optical switch is required to pass or block light of a predetermined wavelength with respect to the wavelength division multiplexed signal by an external control signal._off/ I_onTherefore, a high extinction ratio of -40 dB or more is required.
[0005]
The conventional variable attenuator shown in FIG. 8 is suitable for an application in which the extinction ratio is adjusted according to the magnitude of the applied voltage, but is stable because the applied voltage range in which a high extinction ratio of −40 dB or more is obtained is narrow. For use as an optical switch that maintains a high extinction ratio, I_offIt was necessary to expand the low voltage range.
[0006]
As a measure for improving the extinction ratio, it is effective to arrange two variable optical attenuators 300 in series. However, there is a problem that the voltage application wiring becomes complicated and increases in size. In particular, in order to make an optical switch or variable optical attenuator that passes or blocks light of a predetermined wavelength in a wavelength multiplexed signal, each pixel is made independent as a pixel electrode structure in which an electrode for applying a voltage to a liquid crystal layer is divided for each wavelength. Therefore, it is difficult to adjust the position of the pixel electrode between the two variable optical attenuators without increasing the size of the variable optical attenuator.
[0007]
In addition, a single variable optical attenuator 300 has a configuration similar to that in which two variable optical attenuators 300 are arranged in series by providing a reflection mirror on the light output side of the transmission type variable optical attenuator 300. Realize. However, when a polarization diffraction type polarizer is used, there is a problem that a high blocking amount cannot be obtained because light diffracted in the forward path is reflected by the reflecting mirror and then diffracted in the return path and is superimposed on straight transmitted light that is not diffracted. It was.
[0008]
[Patent Document 1]
JP 2003-66450 A
[0009]
[Problems to be solved by the invention]
SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and aims to provide a reflection type light modulation element that is excellent in mass productivity and that can stably obtain a high blocking amount and a variable optical attenuator using the reflection type light modulation element. To do.
[0010]
[Means for Solving the Problems]
  The present invention reflects incident lightControl the amount of light emittedA reflective light modulation element,The reflective light modulation element includes a liquid crystal element including a pair of translucent substrates on which transparent electrodes are formed and a liquid crystal layer in the order in which the incident light is incident, and a reflective polarization diffraction grating, The polarization diffraction grating is a blazed diffraction grating having a birefringent birefringent material having a sawtooth cross section and a grating pitch P, or a shape approximating the oblique side of the sawtooth shape of the blazed diffraction grating in a staircase shape. A pseudo blazed diffraction grating having a reflection mirror that reflects the incident light, the incident light having a wavelength of λ, and an incident angle θ greater than zero with respect to the normal direction of the surface of the reflection mirror Incident at (θ> 0), the liquid crystal element changes the polarization state of the incident light incident on the reflective polarization diffraction grating according to the magnitude of the voltage applied to the liquid crystal layer through the transparent electrode. The reflection type polarized light Diffraction grating, the grating pitch P is,
  P = λ / (2 × sin θ),
Further, the first linearly polarized light having the first polarization direction is reflected by the reflection mirror without being diffracted, and the polarization direction orthogonal to the first polarization direction is selected. The traveling direction of the second linearly polarized light diffracted by the second linearly polarized light and reflected by the reflecting mirror is different from the traveling direction of the reflected first linearly polarized light, and the incident light is directed toward the reflecting mirror. Follow the same path asA reflective light modulation element is provided.
[0011]
  Also, the reflective polarization gratingIs a refractive index n in the concave portion of the blazed diffraction grating or the concave portion of the pseudo-blazed diffraction grating. _s The refractive index n _s Is the ordinary refractive index n of the birefringent material _o The reflective light modulation element is substantially equal to the above. In addition, the compoundRefractive materialIsProvided is a reflection type light modulation element made of a polymer liquid crystal.
[0012]
  Further, between the liquid crystal element and the reflective polarization diffraction grating,BirefringenceA transmissive polarization diffraction grating is arranged,The transmissive polarization diffraction grating includes the incident light,Diffracts the first linearly polarized light; andAboveSecond linear polarizationStraight throughHaveAboveThe reflective light modulation element described above is provided.
[0013]
  The transmission type diffraction grating is a polymer liquid crystal.The cross sectionSaw bladeformConditionAlternatively, it has a shape that approximates the oblique side of the sawtooth shape like a staircase, and the longitudinal direction of the reflective polarization diffraction grating and the longitudinal direction of the transmission diffraction grating are orthogonal to each otherThe above reflection type light modulation element is provided.
[0014]
  further,Return light that emits light in a predetermined wavelength band, receives return light that is reflected and travels in the same direction as the light exit direction, as signal light, and travels by being reflected in a direction different from the light exit direction An optical system that does not receive light, and the reflection type light modulation element that is connected to control means for applying a voltage through an AC power source in the optical path of the light emitted from the optical system.A variable optical attenuator,ApplyReceiving light according to the voltage levelOf the signal lightProvided is a variable optical attenuator characterized in that the amount of light is adjusted.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
[First Embodiment]
FIG. 1 is a side view showing a configuration example of a reflective light modulation device according to a first embodiment of the present invention.
Transparent electrodes 8 and 9 are respectively formed on one side of the translucent substrates 5 and 6, and an alignment film (not shown) that is aligned in the same direction is formed thereon, and is formed into a cell by using the sealing material 10. Has been. Here, the alignment treatment of the alignment film makes an angle of 45 degrees with respect to the X axis in the XY plane. In addition, the ordinary refractive index n in the cell_o(LC) and extraordinary refractive index n_e(LC) (n_o(LC) <n_e(LC)) nematic liquid crystal is injected to form the liquid crystal layer 7, and the alignment direction of the liquid crystal molecules, that is, the extraordinary light refractive index n is parallel to the translucent substrate and forms an angle of 45 degrees with respect to the X axis._eA liquid crystal element 30 with uniform (LC) is obtained.
[0016]
  Further, after forming an alignment film (not shown) aligned in the X-axis direction on the reflection mirror surface of the translucent substrate 4 having the reflection mirror 3 formed on one side, and applying a liquid crystal monomer solution thereon. Ordinary refractive index n in which the alignment direction of liquid crystal molecules is aligned in the X-axis direction by irradiating ultraviolet rays_o(PLC) and extraordinary refractive index n_e(PLC) (n_o(PLC) <n_e(PLC)) polymer liquid crystal layer is formed. Furthermore, the cross section of the polymer liquid crystal layer is sawn by photolithography and reactive ion etching.toothThe blazed diffraction grating 1 is processed into a linear shape in the X-axis direction.
[0017]
Next, ordinary light refractive index n_oIsotropic refractive index n substantially equal to (PLC)_sThe reflective polarizing diffraction grating 20 is obtained by filling the transparent adhesive 2 in the concave portion of the blazed diffraction grating 1 and adhering and fixing it to the translucent substrate 5. In this way, the reflection type light modulation element 100 in which the reflection type polarization diffraction grating 20 composed of a diffraction grating using a polymer liquid crystal as a birefringent material and the liquid crystal element 30 are integrated is obtained.
[0018]
Here, the birefringence (difference between the ordinary light refractive index and the extraordinary light refractive index) of the liquid crystal layer is expressed by Δn (LC) = n_e(LC) -n_oAssuming (LC), the thickness d (LC) of the liquid crystal layer is adjusted so that the retardation value Δn (LC) × d (LC) of the liquid crystal layer 7 becomes λ / 2 with respect to incident light of wavelength λ. .
[0019]
  Further, the grating height d (PLC) of the blazed diffraction grating 1 made of polymer liquid crystal is set so that the + 1st order diffraction efficiency is maximized with respect to the incident light. That is, the birefringence of the polymer liquid crystal is expressed by Δn (PLC) = n_e(PLC) -n_oIf (PLC), Δn (PLC) × d (PLC) is approximately λ / 2. In FIG. 1, the cross section is a sawtoothShows a blazed diffraction grating 1 in the shape of a sawtoothA pseudo blazed diffraction grating in which the hypotenuse portion of the shape is approximated by a multi-step staircase may be used.
[0020]
  When light incident on the reflective light modulation element 100 at an incident angle θ with respect to the reflective mirror 3 is reflected as + 1st order diffracted light by the reflective polarizing diffraction grating 20 and emitted in the same direction as the incident light, the blazed diffraction grating 1 The grating pitch P is set to P = λ / (2 × sin θ), and the light incident / exit surface is a saw of the blazed diffraction grating 1toothIt is in the YZ plane that is the cross section of the shape.
[0021]
In addition, the incident angle θ of the incident light with respect to the reflective light modulation element 100₁And the outgoing angle of outgoing light θ₂May be different. In that case, the grating pitch P of the blazed diffraction grating 1 is set to P = λ / (sin θ₁+ Sinθ₂).
[0022]
  FIG. 2 is a side view showing another configuration example of the reflective light modulation element according to the first embodiment of the present invention. As in the configuration example of the reflective light modulation element 110 shown in FIG. 2, a blazed diffraction grating 1 made of a polymer liquid crystal layer is formed on a translucent substrate 5, and the ordinary light refractive index n_oIsotropic refractive index n substantially equal to (PLC)_sTranslucent substrate in which a transparent mirror 2 is filled in a recess of a blazed diffraction grating 1 and a reflecting mirror 3 is formed4It is good also as a structure which adheres and fixes to. 2, the same reference numerals as those in FIG. 1 denote the same elements.
[0023]
Next, the operation of the reflective light modulation device 100 shown in FIG. 1 will be described with reference to FIG. Hereinafter, description will be given with reference to FIG. A voltage is applied to the liquid crystal layer 7 through the AC power supply 11 connected to the transparent electrodes 8 and 9, and the alignment state of the liquid crystal molecules changes according to the voltage.
[0024]
  Applied voltage V₀When (= 0V), the orientation of the liquid crystal molecules is transparent.LightnessThe retardation value of the liquid crystal layer 7 is approximately λ / with respect to incident light that is parallel to the substrate and is aligned in a direction that forms an angle of 45 degrees with respect to the X axis and that has a wavelength λ and a polarization direction within the YZ plane. 2 As a result, as shown in FIG. 3A, the polarization direction of the light having the wavelength λ transmitted through the liquid crystal element 30 is rotated by 90 degrees to be changed to the second linearly polarized light in the X-axis direction. Is incident on. The linearly polarized light in the X-axis direction is the extraordinary refractive index n of the blazed diffraction grating 1 made of a polymer liquid crystal._eIn order to cope with (PLC), a refractive index difference Δn (PLC) corresponding to the birefringence of the polymer liquid crystal is generated between the blazed diffraction grating 1 and the transparent adhesive 2, and the + 1st order diffracted light is maximum. It becomes the polarization diffraction grating 20, and returns from the reflection type light modulation element 100 through the same path as the incident light. Here, the reflected light of the second linearly polarized light in the X-axis direction diffracted by the reflective polarization diffraction grating 20 is transmitted through the liquid crystal element 30 again, so that the polarization direction becomes outgoing light in the YZ plane.
[0025]
On the other hand, an applied voltage V (> V) sufficient to align most liquid crystal molecules in the electric field direction.₀), The retardation value of the liquid crystal layer 7 becomes substantially zero, the polarization direction of incident light after passing through the liquid crystal element 30 does not change, and the first linearly polarized light is reflected while maintaining the polarization direction in the YZ plane. The light enters the polarization diffraction grating 20. The polarization direction at this time is the ordinary refractive index n of the polymer liquid crystal layer._o(PLC) and isotropic refractive index n of transparent adhesive 2_sTherefore, the light is transmitted through the blazed diffraction grating 1 without being diffracted. As a result, as shown in FIG. 3B, the light is reflected by the reflecting mirror 3 and reciprocates in the polymer liquid crystal layer, and is emitted from the reflective light modulation element 100 in a direction different from the incident light.
[0026]
In the reflective light modulation element 100, a high extinction ratio can be realized at a low voltage by canceling out the retardation value of the liquid crystal layer 7 remaining when a voltage is applied, using a phase plate. For example, in FIG. 1, a phase plate made of a polymer liquid crystal having an orientation orthogonal to the orientation direction of the liquid crystal molecules of the liquid crystal layer 7 is formed on one surface of the translucent substrate 6 of the liquid crystal element 30, and the retardation value thereof is The thickness of the polymer liquid crystal layer may be the same as the retardation value of the remaining liquid crystal layer 7. Instead of a phase plate made of a polymer liquid crystal, a phase plate made of quartz or uniaxially stretched polycarbonate may be used.
[0027]
Here, the return light emitted in the same direction as the incident light is received as signal light on the light emitting side of the reflective light modulation element 100, and an optical system that does not receive the return light emitted in the direction different from the incident light is used. The voltage applied to the transparent electrodes 8 and 9 is V₀Voltage variable optical attenuator that can change the amount of signal by changing from V to V₀By switching between V and V, the optical switch is turned on / off.
[0028]
FIG. 4 shows a configuration example of an optical system in the case where the reflection type light modulation element 100 of the present invention is used as a variable optical attenuator or an optical switch by voltage. The light emitted from the optical fiber 14 is collected by the lens 13 and enters the polarizer 12.
Polarizer 12 is rutile (T_iO₂) A birefringent crystal, such as a crystal, processed so that the light incident surface and the light output surface form an angle. The light path of polarized light in the YZ plane and polarized light in the X-axis direction perpendicular to it is separated and propagated in the crystal. Thus, the crystal orientation is determined, and the light enters the reflective light modulation element 100. Here, for example, in the incident light to the reflection type light modulation element 100, the polarization in the X-axis direction is perpendicularly incident on the reflection mirror surface, and the polarization in the YZ plane is incident on the reflection mirror surface at an incident angle θ. The angle between the light incident surface and the light exit surface of the polarizer 12 is set.
[0029]
The voltage applied to the transparent electrodes 8 and 9 in FIG.₀FIG. 4 (a) shows the propagating light rays. Of the incident light to the reflective light modulation element 100, the polarized light in the X-axis direction is transmitted through the liquid crystal element to become the first linearly polarized light in the YZ plane, and is not diffracted by the reflective polarization diffraction grating. The light is incident and reflected vertically, and is collected on the optical fiber 14 through the reflective light modulator 100, the polarizer 12, and the lens 13 along the same optical path as the incident light. Also, the polarized light in the YZ plane is transmitted through the liquid crystal element to become the second linearly polarized light in the X-axis direction, and is mainly + 1st-order diffracted by the reflective polarization diffraction grating, and the reflective light modulation element in the same optical path as the incident light The light is condensed on the optical fiber 14 through 100, the polarizer 12, and the lens 13. That is, the light emitted from the optical fiber 14 propagates back to the same optical fiber 14 regardless of its polarization.
[0030]
  Further, when the voltage applied to the transparent electrodes 8 and 9 is V, the propagating light beam is illustrated.4Shown in (b). Of the incident light of the reflection type light modulation element 100, the extraordinary light polarization is transmitted through the liquid crystal element while being extraordinary light polarization and is diffracted by the reflection type polarization diffraction grating. Even after passing through the polarizer 12 and the lens 13, the light is not condensed on the optical fiber 14. Further, the ordinary light polarized light is transmitted through the liquid crystal element as the ordinary light polarized light and is not diffracted by the reflective polarization diffraction grating, but is specularly reflected by the reflecting mirror surface and reflected by a light path different from the incident light. And the lens 13, the light is not condensed on the optical fiber 14. That is, the light emitted from the optical fiber 14 does not return to the same optical fiber 14 regardless of the polarization direction.
[0031]
The liquid crystal element 30 may be a reflective light modulation element in which at least one of the transparent electrodes 8 and 9 is composed of a plurality of divided pixel electrodes and a voltage can be applied independently for each electrode. At this time, in FIG. 1, it is preferable that the pixel electrode has a pattern that is divided in the X-axis direction and not divided in the Y-axis direction.
[0032]
  4 shows an example in which the polarizer 12 processed with a birefringent crystal is used in combination with the reflective light modulation element 100, a polarization diffraction polarizer may be used as the polarizer 12. FIG. 5 is a side view showing the operation of the optical attenuator when the polarization diffraction type polarizer 22 is used on the light incident side of the reflection type light modulation element 100. (A) is applied voltage V₀(B) shows a state where the applied voltage is V. Here, a polarization diffraction type polarizer 22 composed of a transmission type blazed diffraction grating is used as the polarizer 12. The polarization diffractive polarizer 22 is a diffraction grating made of the same polymer liquid crystal as the blazed diffraction grating 1 of the reflective light modulator 100, but the grating pitch is doubled to 2P, and the normal refractive index and extraordinary light of the polymer liquid crystal are used. The orientation direction of the polymer liquid crystal molecules is 90 degrees different from that of the blazed diffraction grating 1 so that the refractive index is reversed. Also sawtoothCompared with the blazed diffraction grating 1 of the reflective light modulation element 100, the polarization direction of the oblique side of the cross-sectional shape is reversed and the polarization diffractive polarizer 22 maximizes the + 1st order diffraction of the transmission blazed diffraction grating. The grating height of a blazed diffraction grating composed of a molecular liquid crystal layer is doubled.
[0033]
Applied voltage V₀In this case, as shown in FIG. 5A, the polarized light in the X-axis direction that travels straight through the polarization diffraction type polarizer 22 is incident and reflected perpendicularly to the reflection mirror of the reflection type light modulation element 100 and returns to the original path. To do. On the other hand, the polarized light in the YZ plane + 1st order diffracted by the polarization diffraction type polarizer 22 is + 1st order diffracted by the reflective polarization diffraction grating of the reflective light modulation element 100 and passes through the lens 13 to the optical fiber 14. Return the route.
[0034]
In addition, when the applied voltage is V, as shown in FIG. 5B, the X-axis direction polarized light that travels straight through the polarization diffraction type polarizer 22 is diffracted by the reflection type polarization diffraction grating of the reflection type light modulation element 100 and light. It is not focused on the fiber 14. On the other hand, the polarized light in the YZ plane + 1st order diffracted by the polarization diffraction type polarizer 22 is obliquely incident on the reflection mirror of the reflection type light modulation element 100 and is specularly reflected, so that it is not condensed on the optical fiber 14. That is, it functions as a voltage variable optical attenuator and an optical switch as in the case shown in FIG.
[0035]
  4 and 5Voltage variable lightIn the figure showing the action of the attenuator, the case where the polarized light in the YZ plane and the polarized light in the X-axis direction orthogonal thereto are incident, but as shown in FIGS. 1 to 3, only the polarized light in the YZ plane is present. It may be incident.
[0036]
In an optical switch and a variable optical attenuator used in a wavelength division multiplexing optical communication network, it is possible to stably realize a high extinction ratio of −40 dB or more, that is, I_offIt is important to make the level sufficiently small.
[0037]
I_offThe level is the amount of light returning on the same optical path as the incident light of the reflective light modulation element 100. In FIG. 1, the first straight line in the YZ plane that enters the reflective polarizing diffraction grating when the retardation value of the liquid crystal layer 7 is zero. This is the amount of light whose polarization is + 1st order diffracted and returned on the same optical path as the incident light. Here, the extinction ratio of the reflective polarization grating is I_off+ 1st order diffracted light generated with respect to incident light of first linearly polarized light (ordinary light polarization of polymer liquid crystal) related to the level, and I_onThis corresponds to the light amount ratio with the + 1st order diffracted light generated with respect to the incident light of the second linearly polarized light (abnormal light polarization of the polymer liquid crystal) related to the level. That is, the ordinary refractive index n of the polymer liquid crystal of the blazed diffraction grating 1_oIsotropic refractive index n of (PLC) and transparent adhesive 2_sAnd + 1st order diffracted light is not generated with respect to the incident light of ordinary light polarization, and in principle I_offThe level is zero and the extinction ratio is infinite. However, n_o(PLC) and n_s+ 1st order diffracted light is generated with respect to the first linearly polarized light due to a slight difference from the above, causing the extinction ratio of the reflective polarization diffraction grating to deteriorate.
[0038]
  [Second Embodiment]
  FIG. 6 is a side view showing a configuration example of the reflective light modulation element 200 according to the second embodiment of the present invention. By improving the extinction ratio of reflective polarization gratings,Stable and high extinction ratioTo obtain a reflective light modulation element 100Realizebe able to.
[0039]
Compared to the reflective light modulation element 100 shown in FIG. 1, a polymer liquid crystal layer is processed into a blazed diffraction grating 15 between the reflective polarization diffraction grating 20 and the liquid crystal element 30, and the transparent adhesive 2 The difference is that a transmissive polarization diffraction grating 40 filled with a concave portion is formed. 6, the same reference numerals as those in FIG. 2 represent the same elements.
[0040]
  The blazed diffraction grating 15 is formed by forming an alignment film (not shown) aligned in the Y-axis direction on the surface of the translucent substrate 5, applying a liquid crystal monomer solution thereon, and then irradiating it with ultraviolet rays or the like. Same ordinary refractive index n as blazed diffraction grating 1_o(PLC) and extraordinary refractive index n_eHowever, a polymer liquid crystal layer in which the alignment direction of liquid crystal molecules is aligned in the Y-axis direction is formed. That is, the alignment direction of the polymer liquid crystal is perpendicular to the blazed diffraction grating 1 and the blazed diffraction grating 15. Furthermore, the cross section of the polymer liquid crystal layer is sawn by photolithography and reactive ion etching.toothThe blazed diffraction grating 15 is processed into a linear shape in the Y-axis direction. In FIG. 6, the saw is cut in the YZ plane so that the cross-sectional shape can be seen.toothAlthough the cross section of the shape is shown, in order to suppress the stray light caused by the multiple diffracted light of the blazed diffraction grating 1 and the blazed diffraction grating 15 as will be described later, it is actually a saw.toothThe cross section of the shape is processed so as to be an XZ plane perpendicular to the paper surface.
[0041]
  The grating height D (PLC) of the blazed diffraction grating 15 made of polymer liquid crystal is set so that the + 1st-order diffraction efficiency is maximized with respect to incident light. That is, the birefringence of the polymer liquid crystal is expressed by Δn (PLC) = n_e(PLC) -n_oIf (PLC), Δn (PLC) × D (PLC) = λ. In FIG. 6, the cross section is a sawtoothShows a blazed diffraction grating 15 in the shape of a sawtoothA pseudo blazed diffraction grating in which the hypotenuse portion of the shape is approximated by a multi-step staircase may be used.
[0042]
Further, since the diffracted light of the transmissive polarization diffraction grating 40 composed of the blazed diffraction grating 15 is not used as signal light, it is sufficient that the diffraction pitch is a diffraction angle at which the diffracted light does not overlap the signal light.
[0043]
Next, the effect of improving the extinction ratio of the reflective light modulation element 200 by adding the transmissive polarization diffraction grating 40 will be described.
Of the light incident on the transmissive polarization diffraction grating 40, the second linearly polarized light that has been transmitted through the liquid crystal element 30 and becomes polarized in the X-axis direction is diffracted to the transmissive polarization diffraction grating 40 because it is ordinary light polarization. + 1st order diffracted light is generated because of the extraordinary light polarization with respect to the reflective polarization diffraction grating 20, and is reflected by the reflection mirror 3 and returns along the same path as the incident light. The light is transmitted through the straight line 40 and emitted from the reflective light modulation element 200 as signal light. At this time, the less diffracted light and the diffracted light other than the + 1st order diffracted light of the transmissive polarizing diffraction grating 40 and the + 1st order diffracted light of the reflective polarizing diffraction grating 20,_onSince it does not cause a decrease in level, it is preferable that the amount is small.
[0044]
  In addition, the first linear polarization transmitted through the liquid crystal element 30 and becomes polarized light in the XZ plane.Light isThe transmission type polarization diffraction grating 40 is diffracted because it is extraordinary light polarization, and the reflection type polarization diffraction grating 20 is ordinary light polarization, so most of the light is reflected by the reflection mirror 3 without being diffracted. Then, the light is diffracted again by the transmissive polarization diffraction grating 40 and emitted from the reflective light modulation element 200 through a path different from the incident light, and is not superimposed on the signal light. At this time, n_o(PLC) and n_s+ 1st order diffracted light generated by the reflective polarization diffraction grating 20 due to a slight difference from the diffracted light is diffracted by the transmissive polarization diffraction grating 40 (shown by a dotted line in FIG. 6), and is emitted out of the YZ plane to become signal light. Can not realize superposition, so a high extinction ratio can be realized. Further, since the grating shape of the transmission type polarization diffraction grating 40 is the blazed diffraction grating 15, the multiplexed light diffracted by the forward path (front) and the return path (rear) of the transmission polarization diffraction grating 40 before and after the reflection by the reflection mirror 3. Of the diffracted light, the amount of light superimposed on the signal light has a very low value.
[0045]
As a result, by adopting the configuration of the reflection type light modulation element 200 of the present invention in which the transmission type polarization diffraction grating 40 is laminated on the reflection type polarization diffraction grating 20, I_offSince the level is stabilized and becomes a sufficiently small value, an optical switch or a variable optical attenuator using a voltage that realizes a high extinction ratio of −40 dB or more can be obtained. In particular, a high extinction ratio can be ensured in a wide applied voltage margin with respect to the wavelength bandwidth of incident light used in the wavelength division multiplexing optical communication network.
[0046]
Moreover, since the space | interval of the reflective mirror 3 and the liquid crystal layer 7 can be made small by using the thin translucent board | substrate 5, the several pixel electrode by which at least one of the transparent electrodes 8 and 9 of the liquid crystal element 30 was divided | segmented Even in this case, leakage of light transmitted through the pixel electrode to the adjacent electrode is reduced in the forward path and the backward path of the liquid crystal element, and crosstalk can be suppressed.
[0047]
When the blazed diffraction grating 15 is a rectangular grating by increasing the distance between the transmission polarization diffraction grating 40 and the reflection polarization diffraction grating 20 and distributing the spatial grating pattern of the transmission polarization diffraction grating 40. However, it is possible to avoid the multiple diffracted light diffracted in the forward path and the return path of the transmission type polarization diffraction grating 40 from being superimposed on the signal light.
[0048]
In addition, in the reflective light modulation elements 100 (FIG. 5) and 200 (FIG. 6), the reflected light generated at the interface between the translucent substrates 5 and 6 and the medium having a different refractive index is reflected on the surface of the translucent substrate. By forming fine irregularities, it becomes diffused light, and stray light superimposed on signal light can be reduced. Alternatively, the reflective polarizing diffraction grating 20 in which the liquid crystal element 30 and the reflective polarizing diffraction grating 20 or the transmissive polarizing diffraction grating 40 are integrated may be separately manufactured and integrated using an adhesive.
[0049]
In the first and second embodiments, the applied voltage V₀Although the operation between V and V has been described, other applied voltage or the alignment state of the liquid crystal according to the applied voltage may be changed.
The liquid crystal alignment may be twist alignment, vertical alignment, or hybrid alignment. In addition to nematic liquid crystals, smectic liquid crystals, cholestic liquid crystals, or other liquid crystal materials may be used as long as the polarization state of transmitted light changes according to the change in the alignment state of the liquid crystals.
[0050]
In addition, as an example of the control means for changing the alignment state of the liquid crystal, an AC voltage applied between the transparent electrodes has been shown. However, instead of the voltage, the alignment state of the liquid crystal is changed using a control means such as a magnetic field, temperature, or light. May be.
Further, although an example in which a reflective polarizing diffraction grating made of a diffraction grating processed into a sawtooth cross section using a polymer liquid crystal as a birefringent material has been described, a birefringent material such as a dielectric crystal may be used. . In addition, by providing a spatial distribution of tilt alignment of liquid crystal molecules in the thickness direction within the grating pitch, a blaze-type diffraction grating against extraordinary light polarization with the same polymer liquid crystal layer thickness that is not processed into a sawtooth cross-sectional shape It is possible to have the same function.
[0051]
As a reflection mirror, a metal film such as aluminum, gold, silver, etc.₂O₅And TiO₂High refractive index dielectric film such as SiO and SiO₂A dielectric multilayer film mirror in which low-refractive-index dielectric films such as these are alternately laminated can be formed by vapor deposition or sputtering.
[0052]
As a material for forming the transparent electrode, an oxide film such as ITO or a metal foil film such as aluminum or chromium can be used, but it is preferable to use an ITO film from the viewpoint of conductivity, light transmittance, durability, and the like. .
[0053]
Changing the polarization state means converting linearly polarized light incident on the liquid crystal element into elliptically polarized light including circularly polarized light and linearly polarized light. In the above embodiment, the voltage V₀Shows a case where the polarization direction of linearly polarized light is rotated by 90 degrees.
[0054]
Further features of the reflective light modulation element and the variable optical attenuator of the present invention will be specifically described with reference to the following embodiments.
[0055]
【Example】
This embodiment will be described with reference to a cross-sectional view of the reflective light modulation element 200 shown in FIG. A transparent electrode 8 made of an ITO film formed on one side of a glass substrate which is a light-transmitting substrate 5 is linear in the Y-axis direction by photolithography and etching (linear in the X-axis direction so that the cross-sectional shape can be visually recognized). The transparent electrode 9 made of an ITO film is formed on one surface of a glass substrate which is a light-transmitting substrate 6 so as to form a pixel electrode shape having an electrode width of 100 μm and an electrode interval of 5 μm. An alignment film (not shown) is obtained by applying a polyimide film on the transparent electrodes 8 and 9 and performing an alignment process in a direction that forms an angle of 45 degrees with respect to the X axis.
[0056]
  Next, spacer for cell gap controlTheSealed on one side of a translucent substrate 6 (glass substrate) on which an alignment film is formed using a mixed sealing materialMaterialAn empty cell is produced by printing and applying 10 and solidifying by pressure on the light-transmitting substrate 5 (glass substrate). stickerMaterialAfter injecting liquid crystal from an injection port (not shown) provided in a part of the liquid crystal, the injection port is sealed to complete the liquid crystal element 30. The liquid crystal used here has an ordinary refractive index n for a wavelength λ = 1.56 μm._o(LC) = 1.49 and extraordinary light refractive index n_e(LC) = 1.65 and the cell gap is d (LC) = 5.5 μm. At this time, the retardation value of the liquid crystal layer 7 is 0.88 μm.
[0057]
Although not shown in FIG. 6, a phase plate made of a polymer liquid crystal having a normal light refractive index direction coincident with the extraordinary light refractive index direction of the liquid crystal layer and a retardation value of 0.1 μm is separated from the translucent substrate 6. And a transparent substrate (not shown). When no voltage is applied to the transparent electrodes 8 and 9, the retardation value of the entire liquid crystal element 30 in which the phase plate and the liquid crystal layer 7 are integrated is 0.78 μm, and λ / 2 wavelength with respect to the wavelength λ = 1.56 μm. It corresponds to a board. At this time, the incident light having the polarization in the YZ plane is transmitted through the liquid crystal element 30 and the polarization direction is changed to the outgoing light having the polarization in the X-axis direction.
[0058]
By applying a voltage to the transparent electrodes 8 and 9 and changing the liquid crystal molecules of the liquid crystal layer 7 from the parallel alignment to the vertical alignment with respect to the translucent substrate, the retardation value of the liquid crystal layer is reduced, and the applied voltage is 6V. Although 1 μm remains, by integrating the phase plate and the liquid crystal layer 7, the phase plate cancels this residue and the overall retardation value becomes zero, and the incident light is transmitted through the liquid crystal element 30 and the polarization state remains unchanged. The light is emitted as it is.
[0059]
  Next, an alignment film (not shown) is obtained by applying a polyimide film on one surface of the light-transmitting substrate 5 of the liquid crystal element 30 with a phase plate and performing an alignment process in the Y-axis direction. A liquid crystal monomer solution is applied thereon, then irradiated with ultraviolet rays, and an ordinary refractive index n_o(PLC) = 1.55 and extraordinary refractive index n_e(PLC) = 1.75, and a polymer liquid crystal layer having a film thickness of 6.8 μm and aligned in the Y-axis direction is formed. Furthermore, the cross section is sawn against the polymer liquid crystal layer by photolithography and reactive ion etching.toothProcessed into a pseudo blazed diffraction grating 15 having a linear shape in the Y-axis direction (displayed as a linear shape in the X-axis direction so that the cross-sectional shape can be visually recognized) at a depth of 6.8 μm, which is approximated by an 8-step staircase shape. .
[0060]
  Next, an alignment film (not shown) that is aligned in the X-axis direction is formed on the surface of the reflection mirror 3 of the glass substrate, which is the translucent substrate 4 having the reflection mirror 3 formed on one side, and on that. The same liquid crystal monomer as the blazed diffraction grating 15 is used, and in the same process, the liquid crystal molecules are aligned in the X-axis direction.toothIt is processed into a pseudo blazed diffraction grating 1 having a linear shape in the X-axis direction whose shape is approximated by an eight-step staircase shape. Here, the height of the pseudo blazed diffraction grating 1 in which the hypotenuse is approximated by a staircase is set to 3.4 μm. The reflection mirror 3 is made of a high refractive index dielectric film Ta.₂O₅And low refractive index dielectric film SiO₂30 or more layers are alternately laminated so that the optical film thickness becomes approximately λ / 4, and a multilayer film reflection mirror having a reflectance of 99.9% or more in a wavelength band from 1.48 μm to 1.62 μm is used.
[0061]
Further, ordinary light refractive index n_oIsotropic refractive index n substantially equal to (PLC)_s= 1.56 transparent adhesive material 2 is used to fill the concave portions of the pseudo blazed diffraction grating 1 and the pseudo blazed diffraction grating 15 and to adhere to and fix to the translucent substrates 4 and 5, thereby reflecting polarization. The diffraction grating 20 and the transmissive polarization diffraction grating 40 are obtained. In this way, the reflective light modulation element 200 is manufactured. At this time, the second linearly polarized light in the X-axis direction passes straight through the transmissive polarization diffraction grating 40 without being diffracted, and 80% or more is + 1st-order diffracted by the reflective polarization diffraction grating 20. Further, 80% or more of the first linearly polarized light in the YZ plane is + 1st-order diffracted by the transmission type polarization diffraction grating 40 and passes straight through the reflection type polarization diffraction grating 20.
[0062]
Light is incident on the reflective light modulation element 200 at an incident angle θ = 5 ° with respect to the Z axis, which is the normal direction of the reflecting mirror surface in the YZ plane. Incident light is incident on each pixel electrode of the liquid crystal element 30 with light having different wavelengths in the wavelength band from 1.48 μm to 1.62 μm, and the diffraction angle of the + 1st order diffracted light with respect to the incident light wavelength corresponding to each pixel electrode. The grating pitch of the pseudo blazed diffraction grating 15 of the reflective polarization diffraction grating 20 is distributed from 8.5 μm to 9.3 μm so as to be the same as the incident angle θ.
[0063]
The voltage V applied to the liquid crystal layer 7 is adjusted for each pixel electrode through an AC power source connected to the transparent electrodes 8 and 9. When the polarized light in the YZ plane (first linearly polarized light) is transmitted through the liquid crystal element 30 with a phase plate, it changes to polarized light in the X-axis direction (second linearly polarized light) substantially orthogonal at V = 0, and polarized at around V = 6V. The state does not change.
[0064]
Therefore, the light in the wavelength band from 1.48 μm to 1.62 μm incident on each pixel electrode of the reflective light modulation element 200 is diffracted by the reflective polarization diffraction grating 20 and reflected in the same angle direction as the incident light. Most of the light emitted from the type light modulation element 200 is almost zero at the applied voltage V = 0, but is almost zero at around the applied voltage V = 6V. Further, in the intermediate region where the applied voltage V is 0 and 6V, the light quantity is intermediate.
[0065]
That is, an optical switch or variable attenuator with a high extinction ratio voltage can be obtained by combining with an optical system that separates outgoing light having the same angle as incident light and outgoing light having a different angle. For example, after wavelength-multiplexed light in a wavelength band from 1.48 μm to 1.62 μm emitted from an optical fiber is dispersed by a diffraction grating, it is made incident on the reflective light modulation element 200 at an incident angle θ using a lens and each pixel. Condensed on the electrode.
[0066]
  As a result, the light emitted from the reflective light modulation element 200 in the same angle direction as the incident light is propagated back to the optical fiber, but the other light is blocked, so that a high extinction ratio of −40 dB or more is stabilized. Realized by optical switch or variablelightIt becomes an attenuator. In addition, since the distance between the reflecting mirror and the electrode can be reduced, optical signal crosstalk between pixels can be suppressed even with a pixel electrode structure.
[0067]
In FIG. 7, when the voltage is applied to the liquid crystal layer of the reflection type light modulation element 200 of the present invention from 0 V to 10 V, the same optical fiber reflects the reflection type light modulation element in the light emitted from the optical fiber. It is a graph which shows the signal light efficiency which returns to. A high extinction ratio of -40 dB or more can be obtained in a wide voltage application range of 4.6 V to 10 V or more.
[0068]
For comparison, the signal light efficiency with respect to the applied voltage when the conventional variable optical attenuator 300 is used is shown by a solid line in FIG. An extinction ratio of -40 dB or higher is obtained in a narrow applied voltage range from 6.0 V to 6.4 V.
[0069]
【The invention's effect】
As described above, in the reflection type light modulation element of the present invention, the liquid crystal element that changes the polarization state of incident light according to the change in the alignment state of the liquid crystal molecules and the birefringent material formed on the reflective substrate surface are sawtooth-shaped. And a reflection type polarization diffraction grating that reflects the first linearly polarized light and reflects the second linearly polarized light is integrated, and thus is compact and has a high extinction ratio. A reflective light modulation element is obtained. Further, when the alignment state of the liquid crystal is changed by voltage, the distance between the reflection mirror and the voltage application electrode can be reduced, so that optical signal crosstalk between pixels can be suppressed even in the pixel electrode structure.
[0070]
Further, by using a polymer liquid crystal having birefringence as the reflective polarization diffraction grating, it is possible to accurately process a sawtooth cross-sectional shape even with a fine grating pitch, so that high signal light diffraction efficiency can be obtained.
[0071]
In addition, a transmissive polarization diffraction grating that diffracts the first linearly polarized light and transmits the second linearly polarized light in the polarization direction orthogonal to the first linearly polarized light without diffracting is disposed between the liquid crystal element and the reflective polarization diffraction grating. As a result, a higher extinction ratio can be obtained.
[0072]
In particular, by using a diffraction grating obtained by processing a polymer liquid crystal having birefringence as a transmission polarization diffraction grating into a sawtooth cross-sectional shape, multiple diffracted light diffracted by the transmission polarization diffraction grating in the forward path and the return path is I_offSince it does not overlap with the level, I can be stable over a wide applied voltage range_offThe level can be maintained at a low value. As a result, an optical switch or an optical attenuator suitable for use in a wavelength division multiplexing optical communication network or the like is obtained.
[0073]
In addition, by adding a separation means for separating the light diffracted by the reflective polarizing diffraction grating that exits the reflective light modulation element and the light that is not diffracted and receiving only one of the emitted light, the magnitude of the applied voltage is increased. Thus, a variable optical attenuator that adjusts the amount of emitted light according to the above is obtained.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing a configuration example of a reflective light modulation element according to a first embodiment of the present invention.
FIG. 2 is a cross-sectional view showing another configuration example of the reflective light modulation element according to the first embodiment of the present invention.
3 is a cross-sectional view showing the operation when light is incident on the reflective light modulation element shown in FIG. 1, (a) maximum + 1st order diffraction reflection state when no voltage is applied, and (b) mirror reflection when voltage is applied. Status.
4 is a cross-sectional view for explaining the operation of an optical attenuator when a polarizer made of a birefringent crystal is used on the light incident side of the reflective light modulation element shown in FIG. 1, (a) state when no voltage is applied. , (B) shows the state when a voltage is applied.
5 is a cross-sectional view showing the operation of an optical attenuator when a polarization diffraction type polarizer is used on the light incident side of the reflective light modulation element shown in FIG. 1, (a) state when no voltage is applied, (b) ) State when voltage is applied.
FIG. 6 is a cross-sectional view showing a configuration example of a reflective light modulation element according to a second embodiment of the present invention.
FIG. 7 is a graph showing applied voltage and signal light efficiency in the case of using the reflective light modulation device of the example of the present invention (circle line indicates an example, solid line indicates a comparative example of a conventional configuration).
FIG. 8 is a cross-sectional view showing a configuration example of a conventional transmissive light modulation element.
[Explanation of symbols]
1, 15: Blaze diffraction grating
2: Transparent adhesive
3: Reflection mirror
4, 5, 6: Translucent substrate
7: Liquid crystal layer
8, 9: Transparent electrode
10: SealMaterial
11: AC power supply
12: Polarizer
13: Lens
14: Optical fiber
22, 60: Polarization diffraction type polarizer
20: Reflective polarization diffraction grating
30: Liquid crystal element
40: Transmission polarization grating
50: Liquid crystal cell with phase plate
100, 110, 200: reflection type light modulation element

Claims (6)

The incident light is reflected, a reflective optical modulation element that controls the amount of light emitted,
The reflective light modulation element is in the order in which the incident light enters.
A liquid crystal element comprising a pair of translucent substrates on which a transparent electrode is formed and a liquid crystal layer;
A reflective polarization grating, and
The reflective polarizing diffraction grating is a blazed diffraction grating having a birefringent birefringent material with a sawtooth cross section and a grating pitch P, or a stepwise approximation of the sawtooth-shaped oblique side portion of the blazed diffraction grating. A quasi-blazed diffraction grating having the shape formed above, and a reflection mirror that reflects the incident light,
The incident light has a wavelength of λ and is incident at an incident angle θ (θ> 0) larger than zero with respect to the normal direction of the surface of the reflecting mirror.
The liquid crystal element changes a polarization state of the incident light incident on the reflective polarization diffraction grating according to a magnitude of a voltage applied to the liquid crystal layer through the transparent electrode,
In the reflective polarization diffraction grating, the grating pitch P is
P = λ / (2 × sin θ),
Further, the first linearly polarized light having the first polarization direction is reflected by the reflection mirror without being diffracted, and the polarization direction orthogonal to the first polarization direction is selected. Diffracting second linearly polarized light having
The traveling direction of the second linearly polarized light reflected by the reflecting mirror, reflected the a and a direction different from the first traveling direction of the linearly polarized light, characterized Rukoto Tado the same path as the incident light toward the reflecting mirror side Reflective light modulation element. The reflective polarization diffraction grating has a refractive index n in a recess of the blazed diffraction grating or a recess of the pseudo-blazed diffraction grating. _s With transparent adhesive
Refractive index n _s Is the ordinary refractive index n of the birefringent material _o The reflective light modulation element according to claim 1, which is substantially equal to: Reflective optical modulation element prior Kifuku refractive material according to claim 1 or claim 2 consisting of liquid crystal polymer. A transmissive polarization diffraction grating having birefringence is disposed between the liquid crystal element and the reflective polarization diffraction grating,
The transmission type polarizing diffraction grating, out of the incident light, the first linearly polarized light diffraction vital, reflective light according to 請 Motomeko 1-3 any one that spend straight permeability the second linearly polarized light Modulation element. The transmissive diffraction grating is made of polymer liquid crystal, cross-section saw-tooth shape or has an approximate shape of the hypotenuse portion of the sawtooth stepwise,
The reflective light modulation element according to claim 4, wherein a longitudinal direction of the reflective polarization diffraction grating and a longitudinal direction of the transmission diffraction grating are orthogonal to each other. Return light that emits light in a predetermined wavelength band, receives return light that is reflected and travels in the same direction as the light exit direction, as signal light, and travels by being reflected in a direction different from the light exit direction An optical system that does not receive light,
The optical path of the light emitted from the optical system, Ru and a reflection-type light modulation element according to any one of claims 1 to 5, which is connected to the control means for applying a voltage through an AC power source variable An optical attenuator,
The variable optical attenuator, wherein the amount of the signal light received is adjusted according to the magnitude of the applied voltage.

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