JPH0836196A - Optical switch - Google Patents

Abstract

PURPOSE:To simplify element structure and to easily obtain the best effective refractive index of ferroelectric liquid crystal for the ON/OFF operation of an optical switch by controlling the orientation direction of the ferroelectric liquid crystal according to the intensity of an applied voltage. CONSTITUTION:This optical switch is provided with optical waveguides 11 and 12, an orientation film 13, the ferroelectric liquid crystal 14, transparent electrodes 15 and 16, and an insulating film 17. In the optical switch, the two optical waveguides are set equal in propagation constant; and the light power of one optical waveguide 1 moves to the other optical waveguide 12 when the refractive index of the liquid crystal is large and close to the refractive index of the optical waveguide, but hardly move when the refractive index of the liquid crystal is small. Thus, the ferroelectric liquid crystal has the angle that two different orientation directions contain when a voltage is applied, and the liquid crystal is controlled with the intensity of the applied voltage to perform the switching operation. To control the orientation direction is to control the effective refractive index and the best refractive indexes which are necessary in the ON and OFF state can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device for controlling an optical signal.

[0002]

2. Description of the Related Art An optical switch has a Y-branch type, depending on its shape.
It is divided into a directional coupler type and a cross (X) type. Among them, the directional coupler type optical switch uses a directional coupler that moves optical power by bringing two parallel optical waveguides close to each other. It has the advantage that power can be transferred.

An optical switch of the directional coupler type is LiNbO.
It is divided into those using crystals of inorganic substances such as 3 and those using liquid crystals. An optical switch using liquid crystal has a structure in which liquid crystal is filled between two substrates having an optical waveguide, an electrode, and an orientation control layer, and the refractive index of the liquid crystal is changed by applying an electric field between the electrodes. Then, switching is performed such that light is transferred from one optical waveguide to the other optical waveguide or not. Optical switches using liquid crystals have been reported to use nematic liquid crystals and ferroelectric liquid crystals. Optical switches using nematic liquid crystals have been studied for a long time, but they have problems such as large optical loss in liquid crystals and slow response speed compared to ferroelectric liquid crystals.

FIG. 2 shows an example of a directional coupler type optical switch utilizing a ferroelectric liquid crystal reported in JP-A-5-79906. The ferroelectric liquid crystal 21 is sandwiched between a transparent electrode 22 and a substrate 24 having an optical waveguide 23. The light is switched at the transparent electrode 25 for driving the liquid crystal and travels through one of the upper and lower optical waveguides 23a and 23b. The ferroelectric liquid crystal utilizes the ferroelectricity exhibited in the chiral smectic C phase, the chiral smectic F phase, and the like. These ferroelectric liquid crystals have a spiral structure, but when the cell thickness is thin, the spiral structure is dissolved and two bistable alignment states appear, and a switch is made between these two alignment states depending on the direction of the electric field. It is known to be possible. This is shown in FIG. In the figure, P is the spontaneous polarization of liquid crystal molecules, E is the electric field,
Arrows indicate the respective directions. By applying an electric field in the directions indicated by the solid line and the dotted line in the figure, the directions of the spontaneous polarization of the liquid crystal molecules can be aligned in the directions indicated by the solid line and the dotted line, respectively, and two stable states corresponding to these can be obtained. Since these two stable states are oriented with tilt angles (tilt angles) of + θ and −θ with respect to the smectic layer normal, switching between the two states is performed with an angle of 2θ. Since the refractive index n⊥ in the minor axis direction and the refractive index n∥ in the major axis direction of the liquid crystal are generally different, the effective refractive index for extraordinary light is different between these two stable states. In FIG.
An example in which the smectic layer is substantially perpendicular to the substrate holding the ferroelectric liquid crystal in between is shown, but the smectic layer may have a chevron structure bent in a "<" shape.

It is known that these ferroelectric liquid crystals having a small cell thickness have a memory effect. This means that the two orientation states can be switched by the direction of the applied electric field, but the state is maintained even if the applied electric field is subsequently removed.

[0006]

As an example of the directional coupler type optical switch using the ferroelectric liquid crystal, there is the above-mentioned Japanese Patent Laid-Open No. 5-79906. In this case, the refractive index of the liquid crystal other than the driving portion is used. It is necessary to apply a voltage for controlling In the directional coupler type optical switch using liquid crystal, it is necessary to control the effective refractive index of the liquid crystal not only in the portion that drives the liquid crystal but also in the portion that does not drive the liquid crystal. Because the attenuation of light in the liquid crystal cannot be ignored, it is desirable that the driving portion is as narrow as possible, but it is very difficult to accurately inject the liquid crystal only into the driving portion of 1 mm or less, for example. Therefore, it is inevitable to inject the liquid crystal into the portion other than the driving portion, but if the refractive index of the liquid crystal in this portion cannot be accurately controlled, it may be possible to accurately control the refractive index of the liquid crystal in the driving portion. Optical switches cannot be designed.

In the case of Japanese Patent Laid-Open No. 5-79906, the bistable memory state when no voltage is applied is used to control the refractive index of the liquid crystal in the non-driving portion. In this case, in order to align the liquid crystal in one of the bistable states, a voltage must be applied at least once, and it is necessary to attach a transparent electrode also to a portion where the liquid crystal is not driven. Therefore, the device structure and the driving circuit are complicated. In addition, this transparent electrode has the adverse effect of attenuating the light passing through the waveguide. Usually, the transparent electrode is often used for liquid crystal displays and the like, but in most cases, light only passes through the film of the transparent electrode, and therefore is only attenuated by the thickness of the transparent electrode and can be almost ignored. However, when a transparent electrode is directly attached on the waveguide, the light is attenuated by the amount corresponding to the width of the electrode, so that the light attenuation by the transparent electrode has a great influence. Therefore, it is desirable that the number of transparent electrodes is as small as possible. However, when utilizing a bistable memory state, it is necessary to attach transparent electrodes to at least the region where the liquid crystal is injected, resulting in a large light loss.

Further, in order to optimize the effective refractive index of the liquid crystal of the driving part in the ON state (when a voltage is applied) and the OFF state (when no voltage is applied), the alignment direction between the two states should be optimized. Strict conditions are imposed on the angle formed and the refractive indices of the liquid crystal molecules in the major axis direction and the minor axis direction, but in JP-A-5-79906, the angle formed by the two alignment directions in the ON state and the OFF state is fixed by the liquid crystal material. Therefore, there is also a problem that the liquid crystal materials that can be used are considerably limited. Therefore, according to the present invention, the area of the transparent electrode is reduced to suppress light attenuation, the device structure is simplified, and the effective refractive index of the ferroelectric liquid crystal that is optimal for turning on and off the optical switch can be easily obtained. It is intended to provide a means.

[0009]

The present invention has been made to solve the above-mentioned problems, and has at least an optical waveguide having a clad layer, an electrode film, and an orientation control layer. An optical switch comprising an optical directional coupler filled with a ferroelectric liquid crystal, wherein the ferroelectric liquid crystal has two different orientation directions when a voltage is applied, and the angle is controlled by the intensity of the applied voltage. An optical switch that performs switching by doing so is provided.

Further, the ferroelectric liquid crystal provides an optical switch which has a unique orientation direction when no voltage is applied and turns on or off when no voltage is applied.

Furthermore, the two substrates sandwiching the ferroelectric liquid crystal provide an optical switch which is asymmetrically oriented.

The ferroelectric liquid crystal is an optical switch in which the refractive index n⊥ in the minor axis direction is less than or equal to the refractive index of the cladding layer of the optical waveguide, and at the same time, the refractive index anisotropy Δn is 0.1 or more. It is provided.

Further, the ferroelectric liquid crystal provides an optical switch driven by a voltage value for obtaining an effective refractive index which is optimum for an on or off state.

FIG. 1 shows the structure of the optical switch according to the present invention. Here, 11 and 12 are optical waveguides, 13 is an alignment film, and 14
Is a ferroelectric liquid crystal, 15 and 16 are transparent electrodes, and 17 is an insulating film. In this optical switch, the propagation constants of the two optical waveguides are made equal, and when the refractive index of the liquid crystal is large and close to the refractive index of the optical waveguide, the optical power of one optical waveguide 11 is changed to the other optical waveguide. When the liquid crystal has a low refractive index (12) (on state), the optical power does not easily move (off state).

[0015]

[Function] It is well known as described above that a ferroelectric liquid crystal having a small cell thickness exhibits bistability when a voltage is applied, but it is considered that the angle formed by these two stable orientation directions cannot be changed. It was being done. However, depending on the conditions, it was found that the angle formed by these two orientation directions can be adjusted to some extent by the electric field strength (Japanese Patent Laid-Open No. 4-1376).
7). FIG. 4 shows the voltage and tilt angle θ reported therein.
Shows the relationship with. The tilt angle is an angle formed by the long axis of the liquid crystal molecule and the layer normal, and the angle formed by the alignment directions of the two states of the switch is 2θ, as shown in FIG. Therefore,
This data shows that the angle between the two orientations can be controlled by voltage. Incidentally, Japanese Patent Laid-Open No. 4-1
In 3767, θ is changed by changing the value of the rectangular wave voltage.

The fact that the orientation direction can be controlled means that the effective refractive index can be controlled, so that it becomes easy to obtain the optimum refractive index required in the ON state and the OFF state.

Further, in general, when the applied voltage is removed, the ferroelectric liquid crystal has two memory states corresponding to the two stable states at the time of voltage application by the memory effect as described above. It is also possible to reduce (give monostable) to. To give monostability, for example,
There is a method in which an upper substrate and a lower substrate that sandwich the liquid crystal layer have asymmetric alignment treatments in which the alignment directions are the same but different, or only one substrate is subjected to the alignment treatment. By utilizing the monostable state when no voltage is applied, the monostable orientation direction can be turned on or turned on.

FIG. 5 shows the relationship between the effective refractive index of liquid crystal, the coupling efficiency and the coupling length. The effective refractive index ne of the liquid crystal is φ, where φ is the angle between the long axis of the liquid crystal molecule and the incident direction of the light beam polarized parallel to the substrate surface.

[0019]

[Equation 4]

(N / |: Refractive index in the long axis direction of liquid crystal molecule n
⊥: refractive index in the minor axis direction of the liquid crystal molecule), and takes a value in the range of n⊥ ≦ ne ≦ n‖ by controlling the alignment direction.

Coupling efficiency is the rate at which optical power is transferred from one optical waveguide to the other optical waveguide, and is the length at which 100% of light is transferred when the coupling efficiency is one.

Therefore, when the bond length when the refractive index of the liquid crystal is ne and the width (electrode width) of the portion having the refractive index of ne coincide with each other, the coupling efficiency becomes the maximum value 1 and the refraction of the coupling length of ne is performed. The coupling efficiency decreases as it deviates from the width of the portion having the ratio (electrode width). FIG. 5 shows the result when the electrode width is 0.5 mm, which corresponds to the bond length at the refractive index where the bond length is the minimum.

The most important point in designing the present optical switch is control of the refractive index of the liquid crystal.

In order to maximize the on / off ratio, ne in the on state and the off state may be selected by the following procedure. Note that this is the procedure when the monostable orientation direction when no voltage is applied is in the off state.

Now, let the refractive indices of the core portion and the cladding layer of the waveguide be n1 and n2, respectively, and let the thickness of the core portion be t and the thickness of the liquid crystal layer be d. The wavelength of the incident light is λ.

The coupling coefficient κ is

[0027]

[Equation 1]

Where the bond length L is

[0029]

[Equation 2]

Is given by

The total efficiency R is determined by the ratio of L to the electrode width w, and if L = aw,

[0032]

(Equation 3)

It is represented by

Therefore, when a = 1, that is, L = w, R has a maximum value of 1. At this time, it is desirable that w is as small as possible. Therefore, it is preferable to select ne such that κ is as large as possible in Expression 1 and match w with L at this time.

The on / off ratio is best when the above state (R = 1) is in the on state, but it may be in the on state when R ≧ 0.7. At this time, assuming that the state where R = 0.1 is the off state, the off state is obtained when a ≧ 5 from Equation 3. Therefore, from Equations 1 and 2, n
Among e (n⊥ ≦ ne ≦ n /), it is only necessary to select the off-state ne such that L ≧ 5w. Since the coupling efficiency R decreases as the coupling length L increases, the on / off ratio improves.

Here, it is assumed that the liquid crystal other than the driving portion has a sufficiently low refractive index, and light does not migrate in this region. As the refractive index of the liquid crystal other than the driving portion becomes higher, the coupling length becomes shorter, the light transfer cannot be ignored, and the on / off ratio decreases.

The present invention facilitates the optimization of the refractive index by controlling the orientation direction of the liquid crystal by the voltage intensity when a voltage is applied and utilizing the monostability when no voltage is applied, and simplifies the device structure. And low power consumption is possible. The method will be described below. First, orientation is performed by using an orientation means such as rubbing so that the monostable orientation direction (FIG. 7, 71) when no voltage is applied is turned off or on. Next, when a voltage is applied, the ferroelectric liquid crystal is caused to move in two bistable orientation directions (FIGS. 7, 72, 7) depending on the direction of the electric field.
Take 3). Here, by adjusting the strength of the applied voltage, the angle 2θ formed by the two alignment directions changes,
It is possible to adjust the refractive index to the maximum (on state) or the minimum (off state) in the coupling efficiency in one orientation direction (72 in FIG. 7). However, when the liquid crystal is driven, AC drive must be performed without fail, so that the other bistable orientation direction (FIG. 7, 73) is also taken. Since the coupling efficiency in this orientation direction needs to be as small (off state) or large (on state) as the coupling efficiency when no voltage is applied, further fine adjustment may be necessary.

FIG. 6 shows an example of drive waveforms. In this figure, when the monostable orientation direction when no voltage is applied is in the off state, it is turned on when + V is applied, and turned off when -V is applied and when no voltage is applied. . At this time, the on / off ratio can be maximized by adjusting the magnitude of V and the direction of initial alignment (rubbing direction, etc.).

If the monostable orientation direction when no voltage is applied is in the off state, the power consumption is effectively reduced in the optical switch in which the time in the off state is longer than the time in the on state (normally off). Can be lowered. On the other hand, in the case of an optical switch in which the time in the ON state is longer than the time in the OFF state (normally OFF), if the monostable orientation direction when no voltage is applied is designed to be in the ON state, Low power consumption is possible.

In order to optimize the refractive index, certain conditions are necessary for the refractive index of the liquid crystal molecules themselves. The condition for the refractive index of the liquid crystal varies depending on the tilt angle and the refractive index of the optical waveguide, but the refractive index n⊥ in the minor axis direction is equal to or less than the refractive index of the cladding layer of the optical waveguide, and the refractive index anisotropy Δn ( =
It is necessary that n ∥-n ⊥) is 0.1 or more.

[0041]

Embodiments of the present invention will be described below with reference to the drawings. In this embodiment, SCE-8 (n.ltoreq. = 1.64, n.perp. = 1.49) manufactured by Merck Ltd. was used as the ferroelectric liquid crystal material 14. The tilt angle of SCE-8 was controllable in the range of 0 to 10 V / μm. The optical waveguides 11 and 12 had a core having a refractive index of 1.53, a cladding layer having a refractive index of 1.52, and a thickness of The one with 4 μm was used. The electrode width of the transparent electrodes 15 and 16 is optimized, and the on / off ratio is optimized to be 0.5 m.
m. Further, the PVA alignment film 13 was rubbed on one side so that the alignment direction when no voltage was applied was aligned with the incident direction of light. 7 and 8 show the traveling direction of the light beam and the alignment direction of the liquid crystal in the off state and the on state of the switch, respectively. The wavelength of incident light is 780 nm,
The polarization direction of light is parallel to the substrate surface.

First, when no voltage is applied, the liquid crystal molecules exhibit monostability and are oriented as shown by 71 in FIG. 7a. The angle θm formed with the layer normal was 10.5 °. At this time, as shown in FIG. 7b, the light that has passed through the optical waveguide 11 is not switched and travels in the waveguide as it is, and is turned off. At this time, the effective refractive index of the liquid crystal had a minimum value of 1.49, and the coupling efficiency (transmission rate of light from the waveguide 11 to the waveguide 12) was 5.1%. Next, as shown in FIG.
When the liquid crystal is oriented to be in the ON state as in 72, the light that has passed through the optical waveguide 11 as shown in FIG. 8B moves to the optical waveguide 12 at the electrode 16. In this embodiment,
The applied voltage was 15 V, and the angle θ1 formed by the alignment direction 71 in the ON state and when no voltage was applied was 29.9 °. At this time, the effective refractive index of the liquid crystal sensed by light was 1.523, and the coupling efficiency was 98%.

Since the liquid crystal needs to be driven by an alternating current, it is necessary to apply a voltage having a polarity opposite to that of the ON state. In this case, the alignment direction is 73. At this time, the angle θ2 formed with the orientation direction when no voltage is applied is 8.8 °, the coupling efficiency is 2.1%, and the state is off. The on / off ratio can be further improved by optimizing the refractive index, the alignment direction, and the applied voltage of the optical waveguide and the liquid crystal.

[0044]

Since the alignment direction of the ferroelectric liquid crystal, that is, the effective refractive index can be controlled by the strength of the applied voltage, it is possible to easily obtain the optimum effective refractive index for turning on and off the optical switch for more liquid crystal materials. Is possible. By utilizing the monostable orientation state when no voltage is applied as the ON state or the OFF state, it is possible to simplify the drive waveform and reduce the power consumption. Further, since the area of the transparent electrode can be reduced, light attenuation can be suppressed, and the electrode shape can be simplified, so that the drive circuit can be simplified.

[Brief description of drawings]

FIG. 1 is a diagram illustrating a structure of an optical switch of the present invention.

FIG. 2 is a diagram showing an example of a conventional optical switch using a ferroelectric liquid crystal.

FIG. 3 is a conceptual diagram of a ferroelectric liquid crystal switch.

FIG. 4 is a graph showing that the orientation direction can be controlled by a voltage.

FIG. 5 is a diagram showing a relationship between an effective refractive index of a ferroelectric liquid crystal and a coupling efficiency and a coupling length.

FIG. 6 is a diagram showing an example of drive waveforms.

FIG. 7 is a diagram illustrating the principle of the optical switch of the present invention.

FIG. 8 is a diagram illustrating the principle of the optical switch of the present invention.

[Explanation of symbols]

11, 12, 23 Optical waveguide 13 Alignment film 14, 21 Ferroelectric liquid crystal 15, 16, 22, 25 Transparent electrode 17 Insulating film 24 Substrate 71 Alignment direction of liquid crystal when no voltage is applied 72, 73 Orientation direction

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location G02F 1/1337 505

Claims (5)

[Claims] 1. An optical switch comprising an optical directional coupler having at least an optical waveguide having a clad layer, an electrode film, and an orientation control layer and having a ferroelectric liquid crystal filled between a pair of substrates. The ferroelectric liquid crystal is different when a voltage is applied.
An optical switch having an angle formed by two orientation directions and performing switching by controlling this angle by the intensity of an applied voltage. 2. The optical switch according to claim 1, wherein the ferroelectric liquid crystal has a unique orientation direction when no voltage is applied, and is turned on or off when no voltage is applied. 3. The optical switch according to claim 1, wherein the two substrates sandwiching the ferroelectric liquid crystal are asymmetrically oriented. 4. The ferroelectric liquid crystal has a refractive index n⊥ in the minor axis direction that is not more than the refractive index of the cladding layer of the optical waveguide, and at the same time has a refractive index anisotropy Δn of 0.1 or more. The optical switch according to claim 1, 2, or 3. 5. The optical switch according to claim 1, 2, 3, or 4, wherein the ferroelectric liquid crystal is driven by a voltage value that obtains an effective refractive index optimum for an on or off state. .