JPH11109304A - Optical coupler - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an excellent optical characteristic and to make coupling efficiency variable. SOLUTION: This optical coupler is constituted so that incident light is coupled with another optical element by a liquid crystal microlens 16 having a liquid crystal layer 11 between a pair of through-hole pattern division electrodes 17, 18 opposite to each other. In such a case, respective electrodes 17, 18 are divided to first-fourth parts 17a, 18a to 17d, 18d by plural circular hole parts 19 and slits 20, 21. By respectively changing voltages applied to respective parts, the focal distance of the liquid crystal microlens 16 is changed freely in an XY plane (substrate surface) vertical to an optical axis. Thus, the coupling efficiency with another optical element is adjusted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an optical coupler including an optical interconnection element used in optical communication, optical measurement, and the like.

[0002]

2. Description of the Related Art Conventionally, a distributed refractive index type flat microlens using an ion exchange technique, a selfoc lens, a diffractive microfresnel lens, and a minute spherical surface have been used as an optical interconnection element of an optical coupler. Various microlenses such as microlenses have been developed and used. Since these microlenses are manufactured using a solid material such as glass, the optical characteristics of the single lens are fixed and cannot be changed. The optical coupler using these microlenses is attached to a predetermined position (for example, so that the focal position of the microlens coincides with the core of the optical fiber) with respect to another optical element such as an optical fiber. .

When the conventional microlens is attached to another optical element such as an optical fiber, if a positional shift occurs due to an error in the attaching operation or the like, the focal position of the microlens is changed. Then, the light beam diameter and the numerical aperture change, the light power density changes, and the coupling efficiency decreases. However, as described above, a conventional microlens made of glass or the like cannot change the focal length, so that it is not possible to recover a decrease in coupling efficiency due to a displacement or the like.

In order to solve the above problems, there is an optical coupler using a liquid crystal microlens instead of a microlens made of a solid material such as glass.

[0005]

The conventional problems when a liquid crystal microlens is used for an optical coupler will be described below.

As a liquid crystal microlens using a liquid crystal material having a positive dielectric anisotropy, a punched pattern electrode having a circular hole and a transparent flat electrode are arranged to face each other, and between the two electrodes. A liquid crystal microlens having a liquid crystal layer interposed has been proposed. In the case of a liquid crystal microlens having such an electrode structure, when a relatively low voltage is applied, an axisymmetric non-uniform electric field centered on the center axis of the circle is generated at a position facing the circular hole. Then, the liquid crystal molecules are re-aligned, the refractive index becomes non-uniform within the circle, and a distribution occurs, and a lens effect occurs. That is, the angle (tilt) of the liquid crystal molecules is different between the vicinity of the center of the circle and the vicinity of the outer periphery, and this causes a difference in the refractive index, which is distributed so that the refractive index increases toward the center of the circle. Acts as a lens.

However, in the thickness direction of the liquid crystal microlens, since the electric field distribution is asymmetric between the hole pattern electrode side and the transparent plane electrode side, the rising direction of the liquid crystal molecules is different, and the disclination for dividing the circular pattern into two parts. Line-shaped linear defects peculiar to the liquid crystal material occur. For this reason, optical characteristics such as light condensing characteristics are significantly impaired, and in that state, it can no longer function as a lens. Further, such a liquid crystal microlens cannot obtain a continuous focus variable characteristic corresponding to a voltage, and thus it is difficult to configure an optical coupler having the variable characteristic.

Therefore, a hybrid alignment type liquid crystal microlens has been proposed in which the initial alignment of the liquid crystal microlens is vertical alignment on the hole pattern electrode substrate side and parallel alignment on the transparent plane electrode side. In this case, although no disclination line was generated, the liquid crystal molecules in the circular region facing the hole were re-oriented in the direction of the axially symmetric non-uniform electric field and radially oriented. In such an alignment state, when linearly polarized light such as a laser is incident, a region where the alignment direction of the liquid crystal molecules and the polarization direction of the incident light are different in the circular region is generated, and the entire incident light transmitted through the circular region is collected. No effect can be obtained. Therefore, the light-collecting efficiency as a lens is poor, and it is not suitable as an optical interconnection element.

Further, in order to solve these problems,
A liquid crystal microlens having an electrode structure in which two holed pattern electrodes are combined has been proposed. In this case, no disclination was observed when the electric field was applied, and the liquid crystal molecules were not radially arranged.
Good lens characteristics can be obtained as compared with the conventional liquid crystal microlenses described above. However, in this liquid crystal microlens, the movement of the focal point by changing the applied voltage is performed only in the optical axis direction. Therefore, although it is possible to correct the displacement in the direction parallel to the optical axis,
It is not possible to correct the misalignment in other directions, that is, in a plane perpendicular to the optical axis. In order to add an optical axis adjusting function to the optical coupler, it is indispensable to move the focal point three-dimensionally. However, it is difficult to electrically adjust the optical axis using the conventional liquid crystal microlens.

Accordingly, an object of the present invention is to use a liquid crystal microlens having good optical characteristics and change the focal position in both the optical axis direction and the direction perpendicular to the optical axis to change the coupling efficiency. It is an object of the present invention to provide an optical coupler.

[0011]

A feature of the present invention is an optical coupler for coupling incident light to another optical element by a liquid crystal element having a liquid crystal layer between a pair of electrodes facing each other, wherein Is a hole-patterned divided electrode provided with a plurality of circular or elliptical holes and divided into a plurality of portions by slits provided continuously with the holes.

Then, a voltage is independently applied to a plurality of portions of the electrode divided by the slit.

The diameter of the hole or the length of the major axis of the ellipse is 1
mm or less, and the thickness of the liquid crystal layer is preferably １ ／ to 1 times the diameter of the hole or the length of the major axis of the ellipse.

The holes provided in the pair of electrodes have the same shape and the same size, and are provided at positions facing each other.

The liquid crystal layer is made of a nematic liquid crystal or a smectic liquid crystal exhibiting an electroclinic effect.

When the voltage applied between the electrodes changes continuously, the light coupling efficiency changes continuously.

When different voltages are applied to a plurality of portions of the electrode, the light coupling efficiency changes.

According to such a configuration, the focal position of the liquid crystal element can be moved not only in the direction of the optical axis but also in the direction perpendicular to the optical axis according to the voltage applied to each of the divided electrodes.

[0019]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a schematic diagram of a first embodiment of the optical coupler according to the present invention. A liquid crystal microlens (liquid crystal element) using a liquid crystal material is used to realize a variable characteristic of the coupling efficiency. In the embodiment shown in FIG. 1A, a pair of optical fibers 2 is used as another optical element. The liquid crystal microlens 1 is disposed between the optical microlenses 1 and 3 and optically coupled to each other. In the embodiment shown in FIG. 1B, a liquid crystal microlens 1 is disposed between a semiconductor laser array 4 as another optical element and an optical fiber 3 and optically coupled.

Next, the variable characteristics of the liquid crystal microlens 1 used in these optical couplers will be described.

FIG. 2 shows an element structure of the liquid crystal microlens 1. The liquid crystal microlens 1 has a pair of perforated pattern divided electrodes 7 and 8 formed on a pair of substrates 5 and 6 made of glass or the like (see FIG. 3) facing each other at a predetermined interval via a spacer (not shown). Further, an alignment film (not shown) is formed on each of the opposing surfaces (perforated pattern divided electrodes 7, 8). Rubbing is performed on the alignment film as alignment processing. This is a structure in which a liquid crystal layer 11 is provided between both alignment film electrodes. The liquid crystal layer 11 is made of a liquid crystal material such as a nematic liquid crystal (in this embodiment, Merck liquid crystal).
(Made by K15) is sealed between the two alignment films and sealed by a sealing means (not shown).

As shown in FIG. 2, the hole-patterned divided electrodes 7 and 8 are formed by patterning on substrates 5 and 6, and have a circular hole portion 12 provided in a pattern shape. A slit 14 is provided continuously with the portion 12. With the holes 12 and the slits 14, the hole-patterned divided electrode 7 is divided into the first portion 7 a and the second portion 7 b
And the hole pattern dividing electrode 8 is the first
It is divided into a part 8a and a second part 8b. Holes 12 and slits 14 having the same shape and the same size are provided on both electrodes 7 and 8, respectively, and both holes 12 and slits 14 on both electrodes 7 and 8 are opposed to each other. Substrates 5 and 6 are arranged.

Next, the operation principle of the liquid crystal microlens 1 will be described using a molecular orientation model shown in FIG.

In the liquid crystal microlens 1 having the structure described above, the liquid crystal molecules 13 in a state where no voltage is applied
FIG. 3 is a sectional view showing the state of FIG. 3 and a graph showing the refractive index.
(A) is a cross-sectional view showing a state of the liquid crystal molecules 13 in a state where a relatively low voltage is applied, and a graph showing a refractive index.
FIG. 3 (b) is a cross-sectional view showing a state where different voltages are applied to the first portion and the second portion of the electrode, and a graph showing the refractive index is shown in FIG. 3 (c).

As shown in FIG. 3A, when no electric field is applied, all the liquid crystal molecules 13 remain aligned in the liquid crystal layer 11 in parallel to the substrates 5 and 6, and Since the refractive index is uniform and a refractive index distribution does not occur within a certain circular region, there is no light condensing effect and no lens effect can be obtained.

On the other hand, when a voltage is applied as shown in FIG. 3B, the liquid crystal molecules 13 between the electrodes 7 and 8 rise. At the same time, the liquid crystal molecules 13 near the electrodes 7 and 8 in the circular region, that is, near the outer periphery of the circular region also rise. However, in the vicinity of the center of the circular region, the electric field is weak and the electric field is weak, so that it does not rise almost in parallel. At this time, the electric field is continuously weakened (voltage is non-uniform) from the outer periphery of the circular region toward the center, and the rising angle of the liquid crystal molecules 13 is continuously reduced. According to the rising angle, a state in which the refractive index continuously changes in the circular region is realized. With such a refractive index distribution, a lens effect (light collecting effect) is obtained.

As shown in FIG. 3 (c), the voltage V1 applied to the first portions 7a, 8a of the electrodes 7, 8 is different from the voltage V2 applied to the second portions 7b, 8b. Is shown. In this example, V1> V2. In this case, the liquid crystal molecules 13 rise as in the state of FIG. 3B, but the first portions 7a and 8a having a high voltage in the circular region.
Is closer to the second portions 7b and 8b having a lower voltage, the rising angle of the liquid crystal molecules 13 is larger. If the rising angles of the liquid crystal molecules 13 are different, a difference occurs in the refractive index. The rising angle of the liquid crystal molecules does not change continuously and uniformly from the center of the circular region toward the periphery as shown in FIG.
The voltage shifts from the center of the circular region to the lower voltage. That is, when this liquid crystal microlens is considered as a normal convex lens, the center of the convex surface is shifted from the center of the circle to the lower voltage side. As a result, the focal position moves in a plane perpendicular to the optical axis direction. The light incident on the circular area is deflected from the optical axis and collected.

The present applicant has observed the state of interference fringes in a circular region when the voltage is variously changed using the liquid crystal microlens 1 having the above-described configuration. The applied voltage V1 to the first portions 7a, 8a is kept constant at 2.00 V, and the second portions 7b, 8a
8b is 1.80 V, 2.00 V, 2.2
The state of interference fringes in each case was observed at 0 V and 2.50 V. The result is shown in FIG.

In the case of FIG. 4A, the second parts 7b, 8b
Is 1.80 V, and the first portion 7a,
8a is lower than the applied voltage V1. Then, the center of the interference fringes is slightly shifted toward the second portions 7b and 8b where the voltage is low.

In the case of FIG. 4B, the second parts 7b, 8b
And the applied voltage V1 of the first portions 7a and 8a.
(2.00 V), and the center of the interference fringe coincides with the center of the circular region.

In the case of FIG. 4C, the second parts 7b, 8b
Is 2.20 V, and the first portion 7a,
8a is higher than the applied voltage V1. Then, the center of the interference fringes is slightly shifted toward the first portions 7a and 8a where the voltage is low.

In the case of FIG. 4D, the second parts 7b, 8b
Is further increased to 2.50 V, and the center of the interference fringe is further shifted toward the lower voltage second portions 7b and 8b.

As described above, when different voltages are applied to the divided electrodes, the center moves toward the lower voltage, and the focal position also moves accordingly. When this is applied, the focal position can be moved in a plane (XY plane) perpendicular to the optical axis by adjusting the voltage applied to both parts of the electrode. FIG. 5 shows the state of movement of the focal position. According to this, the voltage V1 of the first portions 7a, 8a
Is fixed to 2.00 V, and the voltage V of the second portions 7b and 8b is fixed.
2 in the range of 1.80 to 2.50 V, V2
It can be seen that the focal position can be moved by about ± 10 μm in the Y-axis direction, centering on the case of = 2.00 V.

Next, a second embodiment shown in FIG. 6 will be described. In the present embodiment, the cutout pattern divided electrode 1
A circular hole 19 is provided in each of the slits 7 and 18. The slit 2 is continuous with the hole 12 and extends in the X direction and the Y direction.
0 and 21 are provided. By the holes 19 and the slits 20 and 21, the hole-patterned divided electrode 17 is divided into a first portion 17a, a second portion 17b, a third portion 17c,
The hole-dividing pattern divided electrode 18 is divided into four parts, that is, a fourth part 17d, and the first part 18a and the second part 18
b, a third portion 18c, and a fourth portion 18d. Holes 19 and slits 20, 2 of the same shape and size are provided in both electrodes 17, 18, respectively.
1 are provided, and the holes 19 and the slits 20 and 21 of the electrodes 17 and 18 are arranged so as to face each other.

The applicant used the liquid crystal microlens 16 of the second embodiment to observe the state of interference fringes in a circular region when the voltage was changed variously. First part 17
a, the applied voltage V1 of 18a is fixed at 2.00 V,
The applied voltage V4 of the fourth portions 17d and 18d is set to 1.80.
V, 2.00 V, 2.50 V, and the second portion 17
b, 18b and the third portions 17c, 18c
Is applied voltage V3, V2 = V3 = (V1 + V4) / 2
It was set so that it might become the relation of. FIG. 7 shows the state of interference fringes in each case.

In the case of FIG. 7A, the fourth portion 17d, 1
The applied voltage V4 of 8d is 1.80 V, and the second portion 1
7b, 18b and the third portions 17c, 18
c is 1.90 V and the first portion 1
It is smaller than the applied voltage V1 of 7a and 18a. The center of the interference fringes is slightly shifted in the lower right direction in the drawing (the direction of the fourth portion).

In the case of FIG. 7B, the applied voltage V
1, V2, V3, and V4 are all the same (2.00 V), and the center of the interference fringe coincides with the center of the circular region.

In the case of FIG. 7C, the fourth portion 17d, 1
8d is 2.50 V, and the second portion 1
7b, 18b and the third portions 17c, 18
c is 2.25 V and the first portion 1
It is higher than the applied voltage V1 of 7a and 18a. Then, the center of the interference fringes is slightly shifted in the upper left direction in the drawing (the direction of the first portion where the voltage is low).

As in the first embodiment, when a different voltage is applied to the divided electrodes, the center moves to the lower voltage, and the focal position also moves accordingly. In the present embodiment, since the electrode is divided into four parts, by adjusting the voltage applied to each part, the focal position can be freely moved in both the XY directions within a plane (XY plane) perpendicular to the optical axis. it can. FIG. 8 shows the state of movement of the focal position. According to this, the voltage V of the first portions 17a and 18a
1 is fixed to 2.00 V, and the fourth portions 17d, 18d
Is changed in the range of 1.80 to 2.60 V,
Assuming that the applied voltage V2 of the second portions 17b and 18b and the applied voltage V3 of the third portions 17c and 18c are set to an intermediate value (V1 + V4) / 2 between V1 and V2, the focal position is in the X and Y directions. Each can move about ± 5 μm. Of course, the applied voltages V2 and V3 can be freely set independently of V1 and V4, and the focal position can be freely moved in the XY plane perpendicular to the optical axis. In addition,
In the first and second embodiments, the outer shape of the liquid crystal microlenses 1 and 16 is a square having a side of 116.4 μm, and the width of the slits 14, 20 and 21 is 30 μm or less.

As described above, in the liquid crystal microlenses 1 and 16 of the present invention, the focal position changes continuously in a plane perpendicular to the optical axis according to the change in the applied voltage. Therefore, an optical coupler in which the liquid crystal microlenses 1 and 16 are combined with another optical element such as an optical fiber, as shown in FIG.
The optical axis of the transmitted light can be directed to any direction such as 22a, 22b, 22c. Accordingly, even when the focal position of the liquid crystal microlens 1 is not aligned with another optical element due to an assembly error or the like, the focal position can be adjusted to an arbitrary position on another optical element by adjusting the applied voltage. And the coupling efficiency can be increased accordingly.

As described above, in order to use a liquid crystal microlens for an optical coupler, it is necessary that the liquid crystal microlens 1 has practically sufficient performance with respect to optical characteristics and the like. This will be described below.

First, the applicant of the present invention has found that the shape and size of the holes 12, 19 of the perforated pattern electrodes 7, 8, 17, 18 and the thickness of the liquid crystal layer 11 have a significant meaning in the characteristics of the liquid crystal microlens. It was found experimentally.

That is, in order to use a liquid crystal microlens as an optical interconnection element of an optical coupler, it is necessary to reduce aberrations such as astigmatism and spherical aberration. Must be circular or oval, not. Further, in the case of coupling with a light source having a relatively small aberration emitted from an optical fiber or the like, circular holes 12, 1 as in the above embodiment are used.
9 and a light source having large astigmatism such as a semiconductor laser, an elliptical hole (not shown) is provided.
It is desirable to correct the aberration of the light source by using the liquid crystal microlenses to perform the coupling with high efficiency.

In this embodiment, the refractive index distribution is in an ideal square distribution state as shown in FIG. 3, but the size of the hole (the diameter or the length of the long axis), the diameter or the length of the hole, Depending on the ratio between the length of the long axis and the thickness of the liquid crystal layer, the refractive index distribution may not be squared. On the other hand, if the hole is too large, the response time from the application of the voltage to the rise of the liquid crystal molecules to fulfill the function as a lens becomes long, which is not practically suitable. Therefore, the size of the hole (the diameter d in the case of a circle: see FIG. 2; the length of the major axis in the case of an ellipse) needs to be 1 mm or less. Further, when the liquid crystal layer is too thick, light is scattered due to fluctuation of liquid crystal molecules at the time of applying a voltage, thereby decreasing the transmittance and deteriorating the optical characteristics. Therefore, as a range suitable for practical use, the thickness t of the liquid crystal layer
(See FIG. 2) is the diameter d of the hole or the long axis length (1 mm
It is necessary to be in the range of 1/4 to 1 times as compared with the following.

Further, it is preferable that the slit width be 30 μm or less so that a disclination line or the like is not generated in the circular region due to a leakage electric field when a voltage is applied, thereby adversely affecting the liquid crystal molecule alignment.

It should be noted that the liquid crystal material is not limited to the nematic liquid crystal used in the present embodiment, and it is also possible to use a smectic liquid crystal exhibiting an electroclinic effect.

[0047]

According to the present invention, by changing the applied voltage, the focal position of the liquid crystal element can be moved not only in the optical axis direction but also in the XY plane perpendicular to the optical axis.
After the optical coupler is assembled, the focal position can be adjusted to obtain the optimum coupling efficiency. And by limiting the size of the hole of the hole pattern electrode and the thickness of the liquid crystal layer,
Good optical characteristics suitable for practical use can be achieved.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an optical coupler of the present invention.

FIG. 2 is a perspective view illustrating a liquid crystal element used in the optical coupler according to the first embodiment of the present invention.

FIGS. 3A and 3B are a cross-sectional view illustrating movement of liquid crystal molecules when no voltage is applied and when a voltage is applied to the liquid crystal element according to the first embodiment, and a diagram illustrating a refractive index distribution.

FIG. 4 is a plan view illustrating an interference image of the liquid crystal element according to the first embodiment.

FIG. 5 is a diagram showing a focal position shift of the liquid crystal element of the first embodiment.

FIG. 6 is a perspective view illustrating a liquid crystal element used in an optical coupler according to a second embodiment of the present invention.

FIG. 7 is a plan view illustrating an interference image of the liquid crystal element according to the second embodiment.

FIG. 8 is a diagram showing the movement of the focal position of the liquid crystal element according to the second embodiment.

FIG. 9 is a schematic diagram showing a light transmission state of the optical coupler of the present invention.

[Explanation of symbols]

 1,16 Liquid crystal microlens (liquid crystal element) 2,3 Optical fiber (other optical element) 4 Semiconductor laser array (other optical element) 5,6 Substrate 7,8,17,18 Perforated pattern division electrode 7a, 8a , 17a, 18a First part 7b, 8b, 17b, 18b Second part 17c, 18c Third part 17d, 18d Fourth part 11 Liquid crystal layer 12, 19 Hole 13 Liquid crystal molecule 14, 20, 21 Slit 22a, 22b, 22c Transmitted light d Diameter of hole t Thickness of liquid crystal layer

Claims (7)

[Claims] 1. An optical coupler for coupling incident light to another optical element by a liquid crystal element having a liquid crystal layer between a pair of electrodes facing each other, wherein each electrode has a circular or elliptical hole. And a hole pattern dividing electrode divided into a plurality of portions by a slit provided continuously with the hole portion. 2. The optical coupler according to claim 1, wherein a voltage is independently applied to a plurality of portions of the electrode divided by the slit. 3. The diameter of the hole or the length of the elliptical major axis is 1 mm or less, and the thickness of the liquid crystal layer is 1/4 to 1 times the diameter of the hole or the length of the elliptical major axis. The optical coupler according to claim 1, wherein the optical coupler is doubled. 4. The device according to claim 1, wherein the holes provided in the pair of electrodes have the same shape and the same size, and are provided at positions facing each other. An optical coupler according to any of the preceding claims. 5. The liquid crystal layer according to claim 1, wherein the liquid crystal layer is made of a nematic liquid crystal or a smectic liquid crystal exhibiting an electroclinic effect.
5. The optical coupler according to any one of 4. 6. The optical coupler according to claim 1, wherein the light coupling efficiency changes continuously when the voltage applied between the electrodes is changed continuously. 7. The light coupling efficiency changes when different voltages are respectively applied to a plurality of portions of the electrode.
An optical coupler according to claim 1.