JP2003222915A - Waveguide type liquid crystal optical switch - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical switch which has low electric power consumption and low insertion loss, is inexpensive, and further has no dependence on a polarized wave. <P>SOLUTION: The optical switch is provided with an optical waveguide having a pair of closely arranged cores 1A and 1B, and an optical path is switched over between both the cores. The waveguide type liquid crystal optical switch is provided with a crystal liquid layer 4 in which a space which is covered by the pair of cores or a space which strides over both the cores and covers only the upper face of the cores is filled with a nematic liquid crystal, a first pair of electrodes 2A and 2B which are arranged opposed to the pair of cores in an outer horizontal direction of the cores, a second pair of electrodes which are placed opposite to each other to cover the liquid crystal layer in the direction perpendicular to the first pair of electrodes and controlled independent of the first pair of electrodes, and a clad 3 which integrally encloses the core, the liquid crystal layer, the first pair of electrodes and the second pair of electrodes. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical switch used in, for example, an optical communication system, and more particularly to a waveguide type liquid crystal optical switch for controlling switching of an optical path between waveguides by a liquid crystal.

[0002]

2. Description of the Related Art Optical communication has become widespread in general households in recent years because it can transmit and receive a large amount of information at high speed.
This optical communication is a transmission system using optical fiber,
In order to distribute an optical signal to each terminal, various optical parts such as a fiber type optical coupler, an optical waveguide type optical multiplexer / demultiplexer, an optical multiplexer / demultiplexer, and an optical switch have been developed.

Among these, the optical switch has an optical line switching function and is important as a switch for optical communication. Conventionally, various types of optical switches used for optical communication are known. Among them, a method of using an optical waveguide and switching the propagation path of light by utilizing various physical phenomena is a mechanical movable part. Therefore, it has the advantages of high reliability and high speed. As such a method, for example, a dielectric crystal waveguide such as LiNbO ₃ having an electro-optic effect or an acousto-optic effect, a semiconductor waveguide utilizing carrier injection, a silica-based waveguide utilizing thermo-optic effect, etc. are utilized. Optical switches are known.

In addition, an optical switch using a liquid crystal is also known as an optical switch having a similar optical waveguide. The liquid crystal is
It has an electro-optic effect in a broad sense in which the refractive index is changed by applying an electric field. Further, it has characteristics such that it can be driven at a low voltage, it has high reliability as it has a track record in displays, etc., it has high productivity, and it can be manufactured at low cost. As such a waveguide type liquid crystal optical switch, for example, in Japanese Unexamined Patent Publication No. 5-165068, a single mode optical core pattern having two parallel and partially close coupling portions is formed on a lower clad. Further, there is described a waveguide type liquid crystal optical switch in which a lower electrode is formed on the lower clad of the coupling portion, the coupling portion is filled with oriented liquid crystal, and the glass plate having the upper electrode is sealed.

However, in this waveguide type liquid crystal optical switch, since the liquid crystal and the electrode are formed so as to cover almost the entire lower clad, the refractive index changes in a wide range of the clad and the waveguide mode is disturbed. As a result, a large amount of crosstalk is generated that is fatal to the switch.
In addition, since the coupling core of the waveguide core has its three faces in contact with the liquid crystal, if there are liquid crystal molecules that are not aligned in a predetermined direction, the scattering loss due to the liquid crystal molecules increases. In addition, since the lower electrode exists very close to the coupling portion of the waveguide, the loss due to the lower electrode becomes very large. Further, since the electrodes have a structure in which only a pair of upper and lower electrodes can be arranged, there is a problem that polarization dependence is large.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and in a waveguide type liquid crystal optical switch having the advantages of low power consumption, low price and high reliability, crosstalk and It is an object of the present invention to improve the insertion loss and to provide a waveguide type liquid crystal optical switch having no polarization dependence.

[0007]

In order to achieve the above object, the present invention is an optical switch which comprises an optical waveguide having a pair of adjacent cores and which switches an optical path between the cores. A liquid crystal layer that is filled with nematic liquid crystal in a space sandwiched between the cores or a space that extends over both cores and covers only the upper surface of the cores; Second electrode pair, which is arranged so as to sandwich the liquid crystal layer in a direction orthogonal to the first electrode pair and is controlled independently of the first electrode pair, the core, and the liquid crystal layer. And a cladding that collectively surrounds the first electrode pair and the second electrode pair, and provides a waveguide type liquid crystal optical switch.

[0008]

DETAILED DESCRIPTION OF THE INVENTION The present invention will be described below in detail with reference to the drawings.

FIG. 1 shows a waveguide type liquid crystal optical switch 1 of the present invention.
0 is a plan view showing one embodiment (however, the upper alignment film 7A
And electrode 6A are omitted), and FIG. 2 is AA of FIG.
FIG. As shown, the waveguide type liquid crystal optical switch 10 is divided into an upper substrate 8A and a lower substrate 8B,
The lower substrate 8B has a pair of cores 1 along its axial direction.
A and 1B are buried. The cores 1A and 1B are close and parallel to each other for a predetermined length in the central portion, and the intervals gradually increase toward both end edges. This close parallel part forms a so-called directional coupler.

The upper substrate 8A and the lower substrate 8B are composed of cores 1A,
1B is formed of a material having a refractive index lower than that of the material forming 1B, for example, (SiO ₂ —TiO ₂ ) having a refractive index of 1.523.
Can be Further, the lower substrate 8B includes the core 1A,
It surrounds 1B and functions as a clad 3. On the other hand, the cores 1A and 1B have, for example, (SiCl ₄ -Ti) having a refractive index of 1.530.
Cl ₄ ).

A groove is formed in the gap between the parallel portions of the cores 1A and 1B from the surface of the lower substrate 8B to a position deeper than the position where the cores 1A and 1B are formed, and the groove is filled with nematic liquid crystal to form a liquid crystal. Forming layer 4. A lower alignment film 7B is formed on the bottom of the groove, and the liquid crystal layer 4 is formed on the lower substrate 8B.
Is sealed with an upper alignment film 7A that covers the entire surface of. Since it is often difficult to form the lower alignment film 7B,
It is also possible to use only the upper alignment film 7A.

The type of nematic liquid crystal forming the liquid crystal layer 4 is not limited and, for example, 4- (4-pentylcyclohexyl) cyanobenzene or the like can be used. Further, both the upper alignment film 7A and the lower alignment film 7B may be known ones, and for example, a rubbing-treated polyamide film or the like can be used.

Outside the cores 1A and 1B, a pair of electrodes (hereinafter referred to as "first electrode pair") 2A and 2B are provided over the entire length of the liquid crystal layer 4 so as to face the cores 1A and 1B.
Is provided. Here, it is preferable that the separation distance D between the cores 1A and 1B and the first electrode pair 2A and 2B be at least half the width W of the cores 1A and 1B. First electrode pair 2
A and 2B can be transparent electrodes such as ITO,
The use of infrared radiation as the transmission medium results in much greater absorption. Therefore, by defining the distance D between the electrode and the core in this way, the loss due to this absorption can be suppressed.

Further, it is orthogonal to the first electrode pair 2A, 2B,
A pair of electrodes (hereinafter, referred to as “second electrode pair”) 6A and 6B are formed so as to cover the liquid crystal layer 4 in the longitudinal direction thereof. In the second electrode pair 6A, 6B, the upper electrode 6A is formed on the upper surface of the upper alignment film 7A, and the lower electrode 6B is formed on the lower alignment film 7B (or the bottom of the groove filling the liquid crystal layer 4). It is formed below by a predetermined distance. This second electrode pair 6A, 6B is also preferably arranged at an interval of at least half the width W of the core 1A, 1B with respect to the core 1A, 1B, and the electrode material is also the first electrode pair 2
Similar to A and 2B, a transparent electrode such as ITO can be used.

Further, the alignment layers 7A and 7B are aligned so as to be orthogonal to the alignment direction of the liquid crystal molecules by the first electrode pair 2A and 2B and the compounding direction of the liquid crystal molecules by the second electrode pair 6A and 6B. The direction is specified. For example, the first
Electrode pair 2A, 2B of the second electrode pair 6
When A and 6B respectively align liquid crystal molecules in the vertical direction on the paper surface, the alignment directions of both alignment films 7A and 7B are defined to be perpendicular to the paper surface.

As described above, the liquid crystal layer 4 has its four surfaces opposed to the electrodes by the first electrode pair 2A, 2B and the second electrode pair 6A, 6B. Also, the first electrode pair 2A, 2B
And the second electrode pair 6A, 6B can be controlled independently of each other, and the alignment state of liquid crystal molecules in the liquid crystal layer 4 can be three-dimensionally controlled. Therefore, it becomes possible to control the refractive index of the TE polarized light and the TM polarized light, and the polarization dependence disappears.

In the waveguide type liquid crystal optical switch 10 configured as described above, the voltage applied to the first electrode pair 2A, 2B and the second electrode pair 6A, 6B is adjusted to control the liquid crystal molecules of the liquid crystal layer 4. By controlling the orientation, it is possible to switch the optical path between the cores 1A and 1B.
For example, as shown in FIG. 3A, the optical signals 5A and 5B are
Are transmitted to the liquid crystal layer 4 from the respective cores on the incident side and are further transmitted to the other core, or the optical signal transmitted to the other core is transmitted to the liquid crystal layer 4 again as shown in FIG. 3B. Then, the light can be further moved to the core on the incident side.
In the following description, the case where the incident optical signal is emitted from the same core is called the “ON state” (FIG. 3B), and the case where the incident optical signal is emitted from the other core is the “OFF state”. (FIG. 3 (A)).

Generally, the change in the refractive index of the liquid crystal depends on the core 1A,
Since the difference in refractive index between the material forming 1B and the material forming the cladding 3 is several tens of times larger, the above switching can be reliably performed with a small voltage. Further, the liquid crystal layer 4 is in contact with the cores 1A and 1B only on both side surfaces thereof, has no crosstalk, and has a small loss.

Incidentally, the above switching mode is based on the liquid crystal layer 4
The case where the nematic liquid crystal used in the above shows a positive change in the refractive index and the case where the nematic liquid crystal shows a negative change in the refractive index is opposite.

In the present invention, various changes can be made by utilizing the large change in the refractive index of the liquid crystal as described above. For example, as shown in FIG. 4 in a switching mode, each optical signal can be input and output by the same core without being transmitted through the liquid crystal layer 4 to be in the “ON state”.

Further, the structure itself can be changed. For example, as shown in the sectional view similar to FIG. 2 in FIG. 5, the bottom of the liquid crystal layer 4 can be extended to the vicinity of the center of the cores 1A and 1B. As a result, the filling amount of nematic liquid crystal can be reduced.

Further, as shown in a similar sectional view in FIG.
The liquid crystal layer 4 can be formed so as to cover only the upper surfaces of the cores 1A and 1B. The lower alignment film 7B of the liquid crystal layer 4 is
Although it may be in contact with the upper surfaces of the cores 1A and 1B, when there is a concern about the optical influence of the lower alignment film 7B, there is a very small amount between the upper surfaces of the cores 1A and 1B and the lower alignment film 7B. A gap may be provided.

The waveguide type liquid crystal optical switch 10 configured as described above is manufactured, for example, as follows. still,
Here, the waveguide type liquid crystal optical switch shown in FIGS. 1 and 2 is exemplified.

First, a clad material (eg, SiO ₂ —TiO ₂ ) is deposited up to the position where the lower electrode 6B of the second electrode pair is formed by the CVD method or the like. Then, a thin film made of an electrode material (for example, ITO) is formed and patterned to form the lower electrode 6B. Then core 1
The clad material is redeposited to the formation positions of A and 1B, and the core material (for example, Si
Cl ₄ —TiCl ₄ ) is deposited to a predetermined thickness and patterned to form cores 1A and 1B. Next, the lower substrate 8B is formed by further depositing a clad material by the CVD method so as to have a predetermined thickness.

Next, a vertical groove is formed in parallel to the outside of the cores 1A and 1B of the lower substrate 8B, and an electrode material (for example,
ITO) is filled to form the first electrode pair 2A, 2B. After that, a groove having a predetermined depth is formed along the gap between the parallel portions of the cores 1A and 1B, and a nematic liquid crystal (for example, 4-
(4-pentylcyclohexyl) cyanobenzene) is filled, and the entire surface of the lower substrate 8B is covered with the upper alignment film 7A (for example, a rubbing-treated polyamide film) to seal the liquid crystal layer 4. In addition, in order to obtain the structure shown in FIG.
Is formed so that the upper surfaces of the cores 1A and 1B are exposed.

Then, the upper electrode 6A (for example, IT) of the second electrode pair is formed at a position of the upper alignment film 7A facing the liquid crystal layer 4.
An O film) is formed, and a clad material is deposited on the O film to form the upper substrate 8A, whereby the waveguide type liquid crystal optical switch 10 is completed.

Further, the above-mentioned waveguide type liquid crystal optical switch 1
0 can be configured to connect an even number thereof and to have a pair of input ends and a pair of output ends as a single optical switch as a whole. In the present invention, a configuration in which an even number of waveguide type liquid crystal optical switches 10 are connected to function as a single optical switch as a whole is defined as a "double gate type optical switch".

FIG. 7 shows an example of such a double gate type optical switch, and is a diagram schematically showing a double gate type optical switch composed of four waveguide type liquid crystal optical switches 10. The illustrated double-gate type optical switch includes one output end c of the first waveguide type liquid crystal optical switch 10A, one input end e of the second waveguide type liquid crystal optical switch 10B, and the first waveguide type optical switch. The other output end d of the liquid crystal optical switch 10A, one input end m of the fourth waveguide type liquid crystal optical switch 10D, one output end k of the third waveguide type liquid crystal optical switch 10C and the second waveguide. Type liquid crystal optical switch 10B, the other input end f, third waveguide type liquid crystal optical switch 10C
Of the other output end l of the fourth waveguide type liquid crystal optical switch 10
The other input end n of D is connected by an optical waveguide 20. Further, one input end a of the first waveguide type liquid crystal optical switch 10A is connected to the first input port (IN
1), one input end j of the third waveguide type liquid crystal optical switch 10C is set as the second input port (IN2), and one output end g of the second waveguide type liquid crystal optical switch 10B is set as the first
Output port (OUT1) of the second waveguide type liquid crystal optical switch 10B, and the other output end p of the fourth waveguide type liquid crystal optical switch 10D is used as a second output port (OUT2). The other output end h is terminated as a dummy port 1, and the other output end o of the fourth waveguide type liquid crystal optical switch 10D is terminated as a dummy port 2.

Also in this double gate type optical switch, following the above-mentioned waveguide type liquid crystal optical switch (see FIG. 3), the optical signal incident from the first input port IN1 is emitted from the second output port OUT1. The optical signal incident from the second input port IN2 is transmitted to the second output port OUT2.
The state of being emitted from the "ON state", conversely, the optical signal incident from the first input port IN1 is output to the second output port OUT.
A state in which an optical signal emitted from the second input port IN2 and emitted from the second input port IN2 is emitted from the first output port OUT1 is referred to as an “OFF state”. Further, in the ON state or the OFF state, the ratio between the intensity of the optical signal to be emitted from each output port and the intensity of the optical signal (stray light) that should not be emitted is defined as an "extinction ratio".

In the above double gate type optical switch,
To turn on, all waveguide type liquid crystal optical switches 10
Turn on A to 10D. As a result, the optical signal incident on the first input port IN1 exits from the output end c of the first waveguide type liquid crystal optical switch 10A and enters the input end e of the second waveguide type liquid crystal optical switch 10B. Output end g,
That is, the light is emitted from the first output port OUT1. that time,
Even if stray light exists, the first waveguide type liquid crystal optical switch 10
In A, the light is emitted from the output end d and reaches the dummy port 2, and in the second waveguide type liquid crystal optical switch 10B, it reaches the dummy port 1 and is not emitted from the respective original output ports, thus realizing a high extinction ratio. it can.

On the other hand, to turn it off, all the waveguide type liquid crystal optical switches 10A to 10D may be turned off. In that case as well, stray light is emitted from the dummy port 1 or the dummy port 2.

Further, as shown in FIG. 8, two waveguide type liquid crystal optical switches 10 can be connected to form a single optical switch as a whole. In this configuration, one output end c of the first waveguide type liquid crystal optical switch 10A and one input end e of the second waveguide type liquid crystal optical switch 10B are connected by the optical waveguide 20. Of the waveguide type liquid crystal optical switch 10A is used as a first input port IN1, and the other input end f of the second waveguide type liquid crystal optical switch 10B is used as a second input port IN2. One output end g of the waveguide type liquid crystal optical switch 10B is used as a first output port OUT1, and the first waveguide type liquid crystal optical switch 10A is used.
The other output terminal d of is used as the second output port OUT2. The other output end h of the second waveguide type liquid crystal optical switch 10B is used as a dummy port.

In this double gate type optical switch,
To turn on, the first waveguide type liquid crystal optical switch 1
0A and the second waveguide type liquid crystal optical switch 10B are both O
Set to N state. As a result, the optical signal incident on the first input port IN1 is transmitted to the first waveguide type liquid crystal optical switch 10
The light exits from the output end c of A, enters the input end e of the second waveguide type liquid crystal optical switch 10B, and exits from the output end g, that is, the first output port OUT1. At that time, if stray light exists, it reaches the other output end d of the first waveguide type liquid crystal optical switch 10A and is emitted from the second output port OUT2. Therefore, the extinction ratio is in the case of the waveguide type liquid crystal optical switch alone. Will be the same as

On the other hand, to turn it off, both the first waveguide type liquid crystal optical switch 10A and the second waveguide type liquid crystal optical switch 10B are turned off. As a result, both the stray light of the optical signal that has entered the first input port IN1 and the second input port IN2 is output from the dummy port 2.

As described above, in the double gate type optical switch composed of the two waveguide type liquid crystal optical switches 10A and 10B, the number of the waveguide type liquid crystal optical switches is small and the cost is low. Since it is not possible to output from the input port IN2 to the second output port OUT2, it is necessary to have a configuration in which the second output port OUT2 is not used in the ON state.

The double-gate type optical switch is preferably constructed by connecting all the waveguide type liquid crystal optical switches on the same substrate by connecting them with optical waveguides, but the waveguide type liquid crystal optical switches are individually formed. Can also be configured to be connected by an optical fiber.

[0037]

As described above, according to the present invention,
As compared with the conventional waveguide type liquid crystal optical switch, a high performance waveguide type liquid crystal optical switch with low insertion loss and no polarization dependence can be provided.

[Brief description of drawings]

FIG. 1 is a plan view showing an embodiment of a waveguide type liquid crystal optical switch of the present invention.

FIG. 2 is a sectional view taken along line AA of FIG.

FIG. 3 is a plan view showing an example of an optical path switching state in the waveguide type liquid crystal optical switch shown in FIG.

FIG. 4 is a plan view showing another example of an optical path switching state in the waveguide type liquid crystal optical switch shown in FIG.

FIG. 5 is a cross-sectional view showing another embodiment of the waveguide type liquid crystal optical switch of the present invention.

FIG. 6 is a sectional view showing still another embodiment of the waveguide type liquid crystal optical switch of the present invention.

FIG. 7 is a plan view showing a double gate type optical switch configured by connecting four waveguide type liquid crystal optical switches of the present invention.

FIG. 8 is a plan view showing a double-gate type optical switch configured by connecting two waveguide type liquid crystal optical switches of the present invention.

[Explanation of symbols]

1A, 1B core 2A, 2B First electrode pair 3 clad 4 Liquid crystal layer 6A, 6B Second electrode pair 7A, 7B Alignment film 8A upper substrate 8B lower substrate 10 Waveguide type liquid crystal optical switch 20 optical waveguide

Claims (5)

[Claims] 1. An optical switch comprising an optical waveguide having a pair of adjacent cores and switching an optical path between the cores, the space being sandwiched between the pair of cores, or straddling both cores, and only the upper surface of the cores. A liquid crystal layer having a space filled with nematic liquid crystal, a first electrode pair arranged to face the cores in a horizontal direction outward of the pair of cores, and a direction orthogonal to the first electrode pair. A second electrode pair, which is arranged so as to sandwich the liquid crystal layer and is controlled independently of the first electrode pair, and the core, the liquid crystal layer, the first electrode pair, and the second electrode pair are collectively included. A waveguide type liquid crystal optical switch, comprising: a clad surrounding the waveguide. 2. The liquid crystal layer has an alignment film on at least one surface facing the second electrode pair, and the alignment direction of the liquid crystal molecules by the alignment film is the same as the alignment direction of the liquid crystal molecules by the first electrode pair. 2. The waveguide type liquid crystal optical switch according to claim 1, which is orthogonal to the alignment direction of liquid crystal molecules by the second electrode pair. 3. The waveguide type liquid crystal optical switch according to claim 1, wherein each electrode is separated from the core at an interval of at least half the width of the core. 4. The liquid crystal layer has a columnar shape extending parallel to the optical axis of the optical waveguide.
Item 2. A waveguide type liquid crystal optical switch according to item. 5. The waveguide type liquid crystal optical switch according to claim 1, comprising two or four waveguide type liquid crystal optical switches, and one waveguide type liquid crystal optical switch output end and the other waveguide type liquid crystal optical switch. The input end of the liquid crystal optical switch is connected by an optical waveguide or an optical fiber, and as a whole, two input ends, two output ends,
A double-gate optical switch having one or two dummy output terminals.