JP3189198B2 - Waveguide type optical device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide-type optical device having no polarization dependence and comprising a liquid crystal optical element inserted in an optical waveguide.

[0002]

2. Description of the Related Art In a silica-based optical waveguide, an optical switching element using a thin film heater in a Mach-Zehnder interferometer has been developed (for example, A. Sugita et al., Tran. IEICE.
vol.E73, pp105-109). This device utilizes the temperature dependence of the refractive index of the optical waveguide, so that a low-loss optical switch can be obtained. However, since a heater is used for heating, power consumption is large.

On the other hand, it has been proposed to use a liquid crystal in order to impart functionality to an optical waveguide. For example, an optical switch in which a gap is provided in an optical waveguide formed on a substrate and a liquid crystal element is inserted can be considered (Japanese Patent Application No. 5-3).
No. 33258). However, there is a disadvantage that the optical switch exhibits polarization dependence due to the anisotropy of the liquid crystal itself.

[0004]

In a waveguide type optical device in which an optical waveguide and a liquid crystal element are combined, there is a possibility that the device has polarization dependence as described above. Generally, when applied to a communication system, if a device has polarization dependency, it cannot be used as it is. The purpose of the present invention is
An object of the present invention is to provide a waveguide type optical device having no polarization dependency by using a liquid crystal element having birefringence.

[0005]

In order to achieve the above object, the present invention relates to a Mach-Zehnder interferometer type optical switch comprising two thin liquid crystal elements horizontally and uniaxially (homogeneously) aligned on a liquid crystal substrate. In which the liquid crystal alignment directions are orthogonal to each other. It differs from the prior art in that there is no polarization dependence while using liquid crystal, and that a thin film heater with large power consumption is not used.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention is a waveguide type optical device having an optical waveguide formed on a substrate, wherein the waveguide type optical device has two input optical waveguides. A waveguide, a first directional coupler connected to the input optical waveguide, two output optical waveguides, a second directional coupler connected to the output optical waveguide, and the two directions A symmetric Mach-Zehnder optical modulator composed of a first optical waveguide and a second optical waveguide having the same length for connecting a sexual coupler, wherein the first optical waveguide has a transparent substrate having a transparent electrode. A liquid crystal optical element in which a nematic liquid crystal having a positive dielectric anisotropy is uniaxially oriented horizontally and sandwiched between a transparent substrate and a transparent substrate, and the liquid crystal orientation direction is horizontal to the substrate of the optical waveguide, and the optical waveguide is guided. It is inserted so as to be perpendicular to the wave direction, and the second The optical waveguide, the liquid crystal optical element similar to the liquid crystal optical element, so that the liquid crystal alignment direction becomes the substrate surface and perpendicular to said optical waveguide, characterized in that it is inserted.

A second embodiment of the present invention is characterized in that, in the first embodiment, the transparent substrate constituting the liquid crystal optical element is a polymer film.

According to a third embodiment of the present invention, in the first or second embodiment, the polarization dependence caused by an error in manufacturing an optical waveguide formed on a substrate is limited to the liquid crystal. It is characterized by being eliminated by fine adjustment of the applied voltage.

As described above, the waveguide type optical device of the present invention eliminates the polarization dependence by using two liquid crystal elements orthogonal to each other. That is, for so-called TM light having a polarization perpendicular to the substrate, one of the liquid crystal elements operates to delay the phase. Therefore, the same phase difference can be given to both the TM light and the TE light, and it is not necessary to control the polarization direction of the incident light for the operation as the optical switch. Further, since a nematic liquid crystal is used as the liquid crystal, the magnitude of the birefringence can be continuously controlled by an electric field. Therefore, it is possible to compensate for an optical path difference caused by an error in manufacturing the optical waveguide by finely adjusting the voltage applied to the liquid crystal.

Further, the waveguide type optical device of the present invention uses a liquid crystal element whose refractive index can be controlled by an electric field, instead of the conventionally used thin film heater. Therefore, the power consumption can be extremely small.

[0011]

EXAMPLES Hereinafter, the present invention will be described in detail with reference to Examples, but the present invention is not limited to these Examples.

(Embodiment 1) An embodiment of an optical waveguide type optical device according to the present invention is illustrated in FIG. In FIG. 1, the first directional coupler 4 and the second directional coupler 10 are 3 dB.
It is a coupler. For example, light is incident on the first input optical waveguide 2, the liquid crystal elements 7 and 8 are inserted into the gap 9 formed in the optical waveguide substrate, and an electric field is applied to the liquid crystal elements 7 and 8 to cause birefringence with respect to the guided light. By changing the size, the outgoing light can be switched between the first output optical waveguide 11 and the second output optical waveguide 12.

More specifically, in the two liquid crystal elements 7 and 8 inserted into the optical waveguide, the orientation direction of each liquid crystal molecule is a homogeneous orientation parallel to the substrate surface, one perpendicular to the substrate surface. ing. FIG. 2 is a schematic diagram showing this state viewed from the side of the optical waveguide. When an electric field is applied to the liquid crystal elements 7 and 8, a force acts to orient the liquid crystal molecules in the direction of the optical waveguide, and as shown in FIG. 2, the magnitude of the optical anisotropy felt by the guided light decreases.
When the liquid crystal is completely oriented in the optical waveguide direction, the optical anisotropy due to the liquid crystal disappears.

Let us consider a case where light is incident on the first input optical waveguide 2. The incident light is equally divided by the first directional coupler 4 and enters the respective liquid crystal elements 7 and 8. Assuming that light having an electric vector perpendicular to the substrate surface is TE light, the refractive index n felt by TM light incident on the first liquid crystal element 7 in the absence of an electric field.
The refractive index nTE1 felt by TM1 and TE light is nTM1 = n‖, nTE1 = n⊥ (where n⊥ is the refractive index in the long axis direction of the liquid crystal molecule, and n 、 is the refractive index in the short axis direction) The refractive index nTM2 felt by the TM light and the refractive index nT felt by the TE light incident on the second liquid crystal element 8 in the absence of an electric field.
E2 is nTM2 = n⊥, nTE2 = n‖ If the thickness of the two liquid crystal elements is equal to d, the liquid crystal element 7
And between the TM light incident on the liquid crystal element 8

[0015]

## EQU1 ## A phase difference of R (TM) = (nTM1-nTM2) .d = (n‖-n⊥) ・ d is generated. Similarly, during TE light,

[0016]

## EQU2 ## A phase difference of R (TE) = (nTE1−nTE2) · d = (n⊥−n‖) · d is generated. If n‖> n⊥, the phase of light passing through the first optical waveguide 5 is delayed for TM light, and the phase of light passing through the second optical waveguide 6 is delayed for TE light. become. However, the magnitude is | n‖−n⊥ | · d
Is equal to This phase difference R (TM) = R (TE) is the wavelength λ of the guided light,

[0017]

When the relationship of R = (m + ／) · λ (m is an integer of 0 or more) is satisfied, the emitted light appears only in the first output optical waveguide of FIG. vice versa,

[0018]

When the relationship of R = m · λ (m is an integer of 0 or more) is satisfied, 100% of the light is obtained from the second output optical waveguide 6. By applying an electric field, the above relational expression can be selected by apparently reducing the magnitude of n‖, so that the optical switch can be operated as an optical switch for switching outgoing light.

As described above, in the configuration of the present invention, R (T
Since the absolute values of M) and R (TE) can be made equal, light can be switched without depending on the polarization direction of the incident light. It should be noted that even when apparently reducing the magnitude of n‖ by applying an electric field, nTM1 = nTE2, nTM2
= NTE1. For this reason, it is possible to operate not only as an optical switch but also as an optical intensity modulator, and in this case, an element having no polarization dependence can be obtained.

In this embodiment, an example is shown in which a polyimide film provided with an ITO electrode is used as the substrate 1 and a commercially available nematic liquid crystal is used as the liquid crystal material.

The polyimide film used for the substrate 1 needs to be transparent. For example, a fluorinated polyimide having a structure represented by the formula (1) as a repeating unit can be suitably used.

[0022]

[Outside 1]

Although a normal glass substrate can be used as a substrate for the liquid crystal elements 7 and 8, the thickness of the glass is usually about 0.7 mm, which increases the thickness of the entire element. In that case, the width of the groove (gap 9) for insertion into the optical waveguide also increases, and the light loss increases. Thin glass can be used, but is extremely difficult to handle. Therefore, it is desirable to use a polymer film as a thin substrate having a small insertion loss.

The liquid crystal elements 7 and 8 to be used are obtained by the same manufacturing method as that of a normal liquid crystal element. However, since the liquid crystal has a homogeneous alignment, the direction of the alignment should be parallel between the upper and lower substrate surfaces. A commercially available nematic liquid crystal, for example, ZLI2293 manufactured by Merck Co., Ltd. is injected by utilizing the capillary phenomenon, and is gradually cooled to obtain a thin liquid crystal optical element for an optical waveguide. At this time, the thickness of the element is several μm to several tens μm.

Optical anisotropy Δ of used liquid crystal ZLI2293
Since n (= n‖−n⊥) is 0.13, assuming that the thickness of the liquid crystal is 10 μm, the optical path difference R0 between the two optical waveguides when there is no electric field is TM light and TE light.

[0026]

## EQU5 ## R0 = 0.13 × 10 = 1.3 μm. Assuming that the wavelength of the light to be used is 1.3 μm, R0
Is an integer multiple of the wavelength, the output of the interferometer will appear at the second output optical waveguide 12.

When an electric field is applied to the two liquid crystal elements to reduce the apparent optical anisotropy Δn to 0.065, the optical path difference between the two optical waveguides 5 and 6 becomes smaller for both the TM light and the TE light.

[0028]

R = 0.065 × 10 = 0.65 μm, which is の of the wavelength of light. Therefore, at this time, the output of the interferometer appears on the first output optical waveguide 11. That is, it is understood that the output light can be switched by controlling the voltage applied to the liquid crystal. Even if the optical anisotropy of the liquid crystal used or the wavelength of the light used is different, the above conditions can be satisfied by controlling the thickness of the liquid crystal or the voltage applied to the liquid crystal.

In the production of an actual optical waveguide,
The lengths of the two optical waveguides of the interferometer may not be completely equal and may be slightly different. Even in such a case, if the phase difference caused by the difference in the length of the optical waveguide is smaller than the phase difference of the liquid crystal element, the optical path difference of the optical waveguide is compensated by applying a larger electric field to one of the liquid crystal elements. be able to. When the manufacturing accuracy of the optical waveguide is low, the compensation range can be increased by increasing the thickness of the liquid crystal and the phase difference of the liquid crystal element.

By inserting the liquid crystal element into the two optical waveguides as described above, an optical switch having no polarization dependence and low power consumption can be obtained.

In this embodiment, the directional coupler is used as the 3 dB coupler. However, it is apparent that a similar optical switch can be formed by using a Y-branch optical waveguide instead.

[0032]

As described above, the waveguide type optical device of the present invention eliminates polarization dependency by using two liquid crystal elements so as to be orthogonal to each other. That is, for so-called TM light having a polarization perpendicular to the substrate, one of the liquid crystal elements operates to delay the phase. Therefore, TM light, TE
The same phase difference can be given to any light, and it is not necessary to control the polarization direction of the incident light for operation as an optical switch. Further, since a nematic liquid crystal is used as the liquid crystal, the magnitude of the birefringence can be continuously controlled by an electric field. Therefore, it is possible to compensate for an optical path difference caused by an error in manufacturing the optical waveguide by finely adjusting the voltage applied to the liquid crystal.

Further, the waveguide type optical device of the present invention uses a liquid crystal element whose refractive index can be controlled by an electric field instead of the thin film heater conventionally used. Therefore, the power consumption can be extremely small.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a waveguide type optical device according to the present invention.

FIG. 2 is a schematic diagram showing a change in the alignment direction of liquid crystal molecules accompanying the application of an electric field, as viewed from the side of the optical waveguide.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 substrate of optical waveguide 2 first input optical waveguide 3 second input optical waveguide 4 first directional coupler 5 first optical waveguide 6 second optical waveguide 7 first liquid crystal element 8 second liquid crystal Element 9 Gap 10 Second directional coupler 11 First output optical waveguide 12 Second output optical waveguide

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-3-118511 (JP, A) JP-A-62-183406 (JP, A) JP-A-7-191294 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) G02F 1/13 G02F 1/01 G02F 1/065 G02F 1/313

Claims (3)

(57) [Claims] 1. A waveguide type optical device having an optical waveguide formed on a substrate, wherein the waveguide type optical device includes two input optical waveguides and a first optical waveguide connected to the input optical waveguide. Directional coupler, two output optical waveguides, a second directional coupler connected to the output optical waveguide, and a first directional coupler having the same length connecting the two directional couplers. A symmetric Mach-Zehnder optical modulator including an optical waveguide and a second optical waveguide, wherein the first optical waveguide includes a nematic liquid crystal having a positive dielectric anisotropy between transparent substrates having transparent electrodes. A liquid crystal optical element sandwiched between a transparent substrate and a uniaxial orientation is inserted so that the liquid crystal orientation direction is horizontal to the substrate of the optical waveguide and perpendicular to the waveguide direction of the optical waveguide. The second optical waveguide includes the liquid crystal optical element; Like liquid crystal optical element, so that the liquid crystal alignment direction becomes the substrate surface and perpendicular to the optical waveguide, the inserted waveguide type optical device, wherein a is the. 2. The waveguide type optical device according to claim 1, wherein the transparent substrate constituting the liquid crystal optical element is a polymer film. 3. The liquid crystal display according to claim 1, wherein the polarization dependency caused by an error in the fabrication of the optical waveguide formed on the substrate is eliminated by finely adjusting the voltage applied to the liquid crystal. 2. The waveguide type optical device according to 1.