JP2003233054A - Liquid crystal optical switch and method for driving the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polarization controlling type liquid crystal optical switch which is applicable to optical fiber communications, is suitable for being arrayed owing to a simple structure and a simple driving method and furthermore is applicable to an optical attenuator. <P>SOLUTION: The liquid crystal optical switch is provided with a liquid crystal polarization rotator 317 consisting of a plurality of liquid crystal cells and a birefringent plate 331 disposed either between a plurality of the liquid crystal cells or outside thereof. Liquid crystals in a plurality of the liquid crystal cells are aligned so as to make liquid crystal layers, respectively sandwiched between two substrates, uniformly be in an antiparallel or in a parallel state. The ratios of cell thickness to retardation of the liquid crystal layers of a plurality of the liquid crystal cells are almost equal among themselves. Furthermore, respective azimuthal angles of liquid crystal directors adjacent to the two substrates are set to be nearly identical to each other. The birefringent plate is constructed in such a way that an advanced phase axis of the birefringent plate and the azimuthal angles of the liquid crystal directors are made to be nearly parallel to each other. <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 a polarization control type liquid crystal optical switch for optical communication and a method of driving the same, and more particularly to a polarization control type liquid crystal optical switch using an optical fiber and a polarization control type used in an optical network. A liquid crystal optical switch and a driving method thereof.

[0002]

2. Description of the Related Art A polarization control type optical modulator using a nematic liquid crystal has been put into practical use for liquid crystal display applications. A typical example of the polarization rotator used in the polarization control type optical modulator is a 90 ° twisted nematic (TN) type liquid crystal. Although the TN type liquid crystal cell is basically suitable for increasing the contrast ratio,
Since the cell thickness is proportional to the square of the response speed, it is necessary to reduce the liquid crystal cell thickness in order to increase the response speed.
Here, λ is the wavelength, Δn is the anisotropy of the refractive index of the liquid crystal, and d is the cell thickness of the liquid crystal cell.
Since the condition n), that is, the condition that λ / 2 is sufficiently small with respect to Δn · d is not satisfied, the waveguide effect becomes small. As a result, the phenomenon that the contrast ratio deteriorates occurs. To solve this problem, today's fast response TN
The type display is a cell that meets the minimum (first: 1st) minimum condition in a normally black mode of a liquid crystal display in which the Morgan parameter u of the formula d = (λ / 2) · (u / Δn) is set to a cube root. In many cases, parameters are decided and cells are formed.

As an example of applying the above-mentioned TN type liquid crystal adopting the first minimum condition to a 2 × 2 switch for optical fiber communication, a reference document is, for example, (Y. Hakamata, T. Yo.
shizawa and T. Kodaira, “A 1.3 μm Single-Mode 2X
2 Liquid Crystal Optical Switch ”, IEICE Trans. Co
mmun., Vol. E77-B, No.10 October 1994). In this document, a single mode optical fiber is provided with an input / output unit for converting it into parallel light by a collimator, and a prism in which a polarization separator and a total reflection mirror are integrated and a wavelength of 1.3 μm band satisfy the first minimum condition. A 2 × 2 switch composed of a TN type liquid crystal sandwiched between two designed prisms is shown.

[0004]

However, as shown in the example of the above document, the wavelength used in optical fiber communication is in the near infrared region, and the wavelength used in the visible region when used as a display (example: green However, the 1300 nm band and the 1550 nm band are often used. Therefore, since the wavelength used is long, the cell thickness d becomes thick as a result. Therefore, when the conventional TN type liquid crystal cell is used in a polarization control type optical switch for optical fiber communication, there is a problem that the response speed becomes slow. For example, a synchronous optical network (SONE
The standards such as T) and Synchronous Digital Hierarchy (SDH) stipulate that the recovery time is 50 ms or less when a network failure occurs. Therefore, even when the opticalization of the system is advanced in the future, affinity with the conventional network is important, and the response time of the optical switch needs to be shorter than at least 50 msec. The conventional TN type liquid crystal could not be directly applied to the liquid crystal optical switch in such a polarization control type optical modulator conforming to the above standard. The details will be described below.

First, it is considered that the polarization rotator of the optical switch is realized by using the TN type liquid crystal technology. When the center wavelength is 1550 nm, which is widely used in optical fiber communication,
The cell thickness that meets the above-mentioned 1st minimum is ZL
When I-4792 (trade name of Merck Japan) is used and Δn (refractive index anisotropy of liquid crystal) is 0.09,
The liquid crystal cell thickness d is approximately 15 μm. At this time, the rising response time τr is inversely proportional to the square of the magnitude of the electric field applied to the liquid crystal cell. Therefore, even if the cell thickness is large, it is possible to reduce the applied voltage to 50 ms or less by increasing the applied voltage. However, since the fall response time τd is proportional to the square of the cell thickness of the liquid crystal, it is 150 ms near room temperature. As described above, the liquid crystal cell having the conventional configuration is not practically suitable for an optical switch used for optical communication.

When it is considered that the polarization rotator is realized by an optical switch using a liquid crystal cell as a zero-order half-wave plate, a cell for an anti-parallel or parallel alignment liquid crystal cell to become a half-wave plate. The thickness is d = λ / (2 · Δn). Here, Δn of ZLI-4792 = 0.0.
9 and the central wavelength λ is 1550 nm, the cell thickness d is about 8.6 μm. In this case, the fall response time τd becomes faster than that of the TN type liquid crystal cell, and about 60 msec can be obtained, but it is not easy to set it to 50 msec or less.

The object of the present invention is to solve the above-mentioned problems, and it is applicable to optical fiber communication, has a simple structure, is easy to drive, is suitable for arraying, and is also applicable to an optical attenuator. Type liquid crystal optical switch and its driving method.

[0008]

In order to achieve the above object, the present invention basically employs the technical constitution as described below.

That is, the first means for solving the above-mentioned problems in the present invention is a liquid crystal comprising a plurality of liquid crystal cells and a birefringent plate arranged at any position between or outside the plurality of liquid crystal cells. The plurality of liquid crystal cells are provided with a polarization rotator, and the liquid crystal layer sandwiched between the light incident side substrate and the light emitting side substrate is uniformly oriented in either anti-parallel or parallel, and the light emitting side is arranged. The azimuths of the liquid crystal directors contacting the substrate and the substrate on the light incident side are set to be substantially equal to each other, and the birefringent plate has the advance axis of the birefringent plate and the liquid crystal director azimuth angle substantially parallel to each other. It is to be a configuration configured as
The means is to adopt a configuration in which the cell thickness of the plurality of liquid crystal cells and the retardation ratio of the liquid crystal layer are set to be substantially equal.

A third means is that the liquid crystal polarization rotator operates as a half-wave plate for a predetermined wavelength incident on the liquid crystal polarization rotator, and the fourth means is The plurality of liquid crystal cells are configured by n pieces, and the retardation of the liquid crystal layer in each liquid crystal cell is configured to be a 1 / (2n) wavelength condition with respect to the predetermined wavelength. Is a configuration in which a polarization separator is provided on the light incident side and the light emitting side of the liquid crystal polarization rotator, and the sixth means is on the light incident side and the light emitting side of the liquid crystal polarization rotator. The seventh means is to provide a polarization separator and a total reflection mirror, and the seventh means is to divide the electrodes formed in each liquid crystal cell into an arbitrary number and to separately control the electrodes. By configuring the liquid crystal polarization rotator part of Ri, eighth means, the retardation of the birefringent plate, is to at least larger configuration than the residual birefringence component generated upon application of the electrode to the ON voltage of the liquid crystal panel.

Further, the ninth means is, in the method of driving the liquid crystal optical switch, analog-modulates the effective value of a predetermined drive waveform applied to the plurality of liquid crystal cells, and continuously modulates the polarization rotation amount and the incident light. The configuration is to control elliptically polarized light.

[Operation] In the polarization control type liquid crystal optical switch of the present invention, at least two unified anti-parallel or parallel alignments in the same optical axis direction functioning as a polarization rotator constituting the optical switch. This is a structure in which a plurality of liquid crystal cells are arranged in multiple stages and a birefringent plate for correcting the phase is inserted in the optical path. When the director azimuth angle of the liquid crystal in contact with the light incident side substrate of the first liquid crystal cell located closest to the light incident side of the plurality of liquid crystal cells is used as a reference, the liquid crystal in contact with the light incident side substrate of the i-th liquid crystal cell The director azimuth angle is set to be substantially equal to the director azimuth angle of the liquid crystal in contact with the light emission side substrate of the (i-1) th liquid crystal cell. Further, the birefringent plate has a fast axis substantially parallel to the director direction of the liquid crystal.

The first to kth liquid crystal cells of the plurality of liquid crystal cells are also provided.
Retardation, which is the product of the effective refractive index anisotropy Δni and the cell thickness d _i of each liquid crystal cell in the initial alignment (up to the first liquid crystal cell), where i is 1 to k. A value obtained by dividing the sum of φ _i by the cell thickness d _i is set to a constant value, and it is preferable to use cells having the same Δn among a plurality of liquid crystal cells.

First, the cell thickness d of the liquid crystal cell applied to the present invention will be considered. As is well known for anti-parallel or parallel alignment type liquid crystal elements, when the liquid crystal cell is used as a half-wave plate capable of switching at a predetermined wavelength λ, the cell thickness condition is d = λ / (2 · Δn) (1) d is the cell thickness, Δn is the refractive index anisotropy of the liquid crystal, and λ is the wavelength. However, in the structure of the present invention, as described above,
Since the birefringent plate is inserted in the optical path of the optical switch, the cell thickness d when the liquid crystal polarization rotator composed of the composite element of the liquid crystal cell and the birefringent plate acts as a half-wave plate is d = λ / (2 · Δn ) + (Δn _B / Δn) d _B (2) is preferably satisfied. Where Δn
_B is the birefringence of the birefringent plate, and d _B is the thickness of the birefringent plate. In the expression (2), the birefringence of the birefringent plate is defined as a value obtained by subtracting the refraction index of the fast axis from the refraction index of the slow axis, so Δn _B is always a positive value.

Here, the operation of a plurality of liquid crystal cells which satisfy the above formula (2) and have the same director azimuth angle of all the liquid crystal cells and are arranged in multiple stages will be described. The liquid crystal optical switch of the present invention is optically equivalent to the half-wave plate condition when no electric field is applied to the electrodes of each liquid crystal cell. Therefore, incident linearly polarized light having an azimuth angle of π / 4 or an odd multiple thereof with respect to the incident side director azimuth angle of the first liquid crystal cell at a predetermined wavelength becomes elliptically polarized light on the exit side of the kth liquid crystal cell. It is possible to rotate the azimuth angle of the outgoing linearly polarized light by π / 2 radian with respect to the azimuth angle of the incident linearly polarized light.

On the other hand, when a predetermined electric field is applied to each of the first to kth liquid crystal cells, the retardation of each liquid crystal cell becomes small, and the residual birefringence of each liquid crystal cell is further reduced by the birefringent plate. Since it can almost disappear, the incident linearly polarized light can be emitted from the k-th cell without rotating the azimuth angle. Therefore, in the polarization control type liquid crystal optical switch of the present invention, by arranging a polarization beam splitter or a polarization beam separator using a birefringent crystal on the exit side, and controlling voltage application or non-application as described above, the incident linear It is possible to change the direction of the polarized light and to change the emitted light intensity in a predetermined emitting direction, which is applicable to an optical switch.

Next, the response speed of the liquid crystal optical switch of the present invention will be described. For simplicity, the liquid crystal polarization rotator of the present invention has two liquid crystal cells having the same cell thickness and the retardation (Δn · d) synthesized by the birefringent plate is a quarter wavelength plate condition. Consider the case of a liquid crystal optical switch of the type.

In the conventional half-wave plate liquid crystal cell consisting of one liquid crystal cell, the cell thickness d is set to d = λ / (2 · Δn) according to the equation (1). In the case of a polarization control type liquid crystal optical switch in which the polarization rotator of the present invention is composed of two liquid crystal cells under the condition of a quarter wavelength plate, since the cell thickness d can be divided into two by two liquid crystal cells, The cell thickness can be approximately d / 2. In a liquid crystal cell under anti-parallel or parallel-alignment half-wavelength conditions, the rise response time τr from when no electric field is applied to when an electric field is applied is approximately τ when the applied voltage is V.
It is known that r is proportional to (d / V) ² . Also, falling response time when the electric field non-application of an electric field applied state (.tau.d) is, .tau.d is known to be proportional to d ² approximation. Therefore, in the case of the polarization control type liquid crystal optical switch in which the liquid crystal polarization rotator of the present invention is composed of two liquid crystal cells under the condition of a quarter wavelength plate and the birefringent plate, compared with the prior art, the same liquid crystal material is used for comparison. Then, the response speed of each can be increased up to about 4 times. Here, if the number of divisions in the thickness direction is increased to reduce the cell thickness per sheet, the speed can be further increased, and the rise response time τr and the fall response time τd, which cannot be achieved by the configuration of the conventional technique, can be obtained. Both can easily be set to 20 msec or less.

Further, the liquid crystal polarization rotator constituting the liquid crystal switch of the present invention is made to approach 0 equivalently by a birefringent plate which inserts the residual birefringent component remaining when a voltage is applied to the liquid crystal cell into the optical path. You can Therefore, it is possible to reduce the crosstalk attenuation of the optical switch.

As is apparent from the above description, in the liquid crystal optical switch and the liquid crystal variable optical attenuator of the present invention and the driving method thereof, a high-performance optical switch and a variable optical attenuator can be provided by a simple structure and a simple driving method. realizable. Further, an object of the present invention is to provide a polarization control type liquid crystal optical switch used for wavelength division multiplexing (WDM) communication using an optical fiber and an optical network, and a driving method thereof, but the scope of the present invention is an apparatus described here. It is needless to say that the present invention can be applied to, for example, a liquid crystal light modulator for a free space optical communication device, without being limited thereto.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION The configurations of a liquid crystal optical switch, a liquid crystal variable optical attenuator, and a driving method thereof in the best mode for carrying out the present invention will be described below with reference to the drawings.

First, the structure of the liquid crystal optical switch according to the embodiment of the present invention will be described with reference to the drawings. FIG. 2 is a structural cross-sectional view for explaining the configuration of the liquid crystal cell 220 that constitutes the liquid crystal optical switch in the embodiment of the present invention.

As shown in FIG. 2, the liquid crystal cell 220 of the liquid crystal light modulating device of the present invention has a first electrode in which a signal electrode 211 is formed.
Substrate 203 and second substrate 2 on which common electrode 213 is formed
It is constituted by sandwiching the nematic liquid crystal layer 201 between 05. This nematic liquid crystal layer 201 is
Above the signal electrode 211 of the first substrate 203 and the second substrate 2
The alignment layer 217 formed on the common electrode 213 of No. 05 causes the pretilt angle 209 of the director 207 of the positive (p) type liquid crystal molecule when an electric field is not applied to be 0.5 degrees to 20 degrees.
The anti-parallel orientation is adopted so that the degree becomes high. Here, the alignment layer 217 is formed of polyimide and the liquid crystal molecules are aligned in a predetermined direction by a rubbing method. Although FIG. 2 shows the case of anti-parallel alignment, the director 207 may have parallel alignment.

Although not explicitly shown in FIG. 2, the first substrate 203
The second substrate 205 is fixed with a sealing material around the liquid crystal cell 220 via a spacer so that the nematic liquid crystal layer 201 maintains a predetermined constant thickness of several μm. Although not shown in FIG. 2, the signal electrode 211 and the common electrode 21
In order to prevent the short circuit of 3 on the signal electrode 211, the common electrode 213, or both, tantalum pentoxide (Ta ₂
A transparent insulating film such as O ₅ ) or silicon dioxide (SiO ₂ ) may be formed under the alignment layer 217.

Signal electrode 2 formed on the first substrate 203
11 and the common electrode 213 formed on the second substrate 205.
Is formed with a predetermined pattern made of a transparent conductive film as needed, for example, when forming an array. In addition to ITO, a thin film of indium oxide (In ₂ O ₃ ), tin oxide (SnO ₂ ), zinc oxide (ZnO), or the like can be used as the transparent conductive film.

An antireflection coating 215 is formed on the surfaces of the first substrate 203 and the second substrate 205 made of glass, which are opposite to the nematic liquid crystal layer 201, to prevent reflection at the interface between the air and the substrate, if necessary. To do. Further, although FIG. 2 shows the case where the antireflection coating 215 is formed only on the first substrate 203, it is also formed below the second substrate 205 as necessary.

Next, a case where the liquid crystal polarization rotator of the present invention is constituted by two stages of antiparallel alignment liquid crystal cells having the same cell thickness and pretilt angle of the liquid crystal cell will be described with reference to FIGS. 3 and 4. FIG. 3 is a diagram schematically showing a liquid crystal polarization rotator 317 composed of a first liquid crystal cell 301, a second liquid crystal cell 303, and a birefringent plate 331. In FIG. 3, the first liquid crystal cell 301 is the first glass substrate 3
19 and the second glass substrate 321 form the first liquid crystal layer 305.
The structure is sandwiched between. In addition, the second liquid crystal cell 303
Is a third glass substrate 323 and a fourth glass substrate 325.
The second liquid crystal layer 307 is sandwiched between and. The first cell thickness d ₁ of the first liquid crystal cell 301 and the second cell thickness d ₂ of the second liquid crystal cell 303 are made equal. Furthermore, the first
A birefringent plate 331 is arranged between the liquid crystal cell 301 and the second liquid crystal cell 303. In this embodiment, the first liquid crystal cell 3
01 and the second liquid crystal cell 303, an example in which the birefringent plate is arranged is shown. However, if the birefringent plate is in the optical path of the liquid crystal polarization rotator 317, it is immediately before the first liquid crystal cell 301 or the second liquid crystal cell 301. It may be arranged immediately after the cell 303. Although not explicitly shown in FIG. 3, the first liquid crystal cell 301 and the second liquid crystal cell 305 have the same configuration as the liquid crystal cell 220 described in FIG.

Arrows in the first liquid crystal layer 305 and the second liquid crystal layer 307 in FIG. 3 are for indicating the alignment state of liquid crystal molecules, and schematically show the director of the liquid crystal. Here, in the first liquid crystal cell 301, an interface between the first glass substrate 319 and the first liquid crystal layer 305 is a first interface 309, and similarly, a second glass substrate 321 is used.
The interface between the first liquid crystal layer 305 and the first liquid crystal layer 305 is referred to as a second interface 311. Further, in the second liquid crystal cell 303, an interface between the third glass substrate 323 and the second liquid crystal layer 307 is set as a third interface 313, and an interface between the fourth glass substrate 325 and the second liquid crystal layer 307 is set. This is the fourth interface 315. Where x
In the −yz coordinate system, when the clockwise direction from the positive direction of the x-axis to the positive direction of the y-axis is a positive azimuth angle, the first interface 309 side of the first liquid crystal cell 301 The azimuth angle of the liquid crystal director is π / 4 radians, and the thickness direction is selected to be parallel to the z axis.

Next, each interface 309, 311, 313, 31
The directions of the liquid crystal director and the direction of the fast axis of the birefringent plate at 5 are shown in the plan views of FIGS. Hereinafter, description will be given with reference to FIG. 3 and FIG. 4 alternately. Figure 4
At the first interface 309 shown in (a), the azimuth angle α ₁ measured from the x-axis of the first liquid crystal director 401 is π / 4.
The first liquid crystal cell 301 shown in FIG.
Has an antiparallel orientation, so the second interface 3
In 11, the azimuth α _{2 of} the second liquid crystal director 403 is also π / 4 radian. Further, the azimuth angle α _B of the fast axis 421 of the birefringent plate 331 shown in FIG.
4 radians. Next, the azimuth angle α ₃ of the third liquid crystal director 405 at the third interface 313 in FIG.
In order to make it parallel to the second liquid crystal director 403, π / 4
It becomes radians, and the second liquid crystal cell 3 in FIG.
02 also has an anti-parallel orientation, so that the azimuth angle α of the fourth liquid crystal director 407 at the fourth interface 315 is
₄ becomes π / 4 radians. With such a configuration, the incident linear deflection 410 in FIG. 4A and the outgoing linear deflection 413 in FIG. 4E are rotated by π / 2 radian with no voltage applied to the liquid crystal panel. Can be made. Further, it can be seen that in the voltage applied state, the light can be emitted without rotating the azimuth angle.

Up to this point, the case where the liquid crystal polarization rotator 317 is constituted by the two liquid crystal cells 301 and 303 having the same cell thickness and the birefringent plate 331 has been described. The present invention can also be configured as follows.
In this case, the total cell thickness d is d = d ₁ + d ₂ +., Where the first cell thickness d ₁ to the kth cell thickness d _k . ． ． It becomes d _k (3). When the extraordinary refractive index of the liquid crystal material used is n _e and the ordinary refractive index is n _o , the effective extraordinary refractive index n _eff is n _eff = (sin ² θ / when the pretilt angle of the liquid crystal cell is θ. ^{_{n o 2 + cos 2 θ /}} n e 2) a ^-1/2 (4). Here, since the pretilt angle θ of each liquid crystal cell in the liquid crystal optical switch of the present invention is made equal, Δn = n _eff −n _o (5) is set, and three or more liquid crystal cells at a predetermined wavelength λ are used. The configured total cell thickness satisfies the equation (2) d = λ / (2 · Δn) + (Δn _B / Δn) d _B. Here, Δn _B is the birefringence of the birefringent plate, and d _B is the thickness of the birefringent plate. When the liquid crystal polarization rotator satisfies the condition of the expression (2), the birefringent plate is a zero-order half-wave plate made of an optical crystal such as quartz, as is well known optically. Are equivalent. (Reference: For example, Pochi Yeh and Claire Gu, Optics of Liqui
d Crystal Displays, Wiley, 1999)

Further, similarly to the liquid crystal polarization rotator, which is composed of two liquid crystal cells, the azimuth angle alpha _k of the liquid crystal director at the interface from the first to 2k-th, the azimuth angle alpha ₁ of the first liquid crystal director And all must be equal. For example, when α ₁ = π / 4 radians, α ₁ = α ₂ α ₂ = α ₃ . ． ． Set so that α _2k-1 = α _2k . As described above, a liquid crystal polarization rotator composed of two liquid crystal cells is obtained by satisfying the formula (2) and making the azimuth angles of the directors of the three or more liquid crystal cells substantially the same. Similarly, it has been found that it can be applied to a polarization control type liquid crystal optical switch. It should be noted that, as described in the action column, even if there are three or more liquid crystal cells, if the configuration of the present invention is adopted, the cell thickness can be reduced by that much and the response speed can be increased. In consideration of the response speed, when the liquid crystal deflecting rotator of the present invention is constituted by n liquid crystal cells, it is most preferable that the retardation of each liquid crystal cell is 1 / 2n wavelength.

Next, the specific constitution of the liquid crystal polarization rotator of the present invention will be described in detail. In FIG. 3, the total cell thickness d of the liquid crystal cell forming the liquid crystal polarization rotator 317 is considered to be the sum of the cell thickness d ₀ as the half-wave plate and the correction cell thickness d _n of the birefringent plate, and as described above, d = d ₀ + d _n = λ / (2 · Δn) + (Δn _B / Δn) d _B. Since it is sufficient equivalently canceling residual birefringence component remains when a voltage is applied to the liquid crystal cell, for example, quartz crystal wave thickness d _B is 12μm at one wavelength of the birefringent plate 331 zero order to about 14 minutes A configuration example using a plate will be described. The Δn _B of this quartz wave plate is about 0.009. The liquid crystal cell has a center wavelength λ of 1550 nm,
With a pretilt angle of 1 °, for example, ZLI is used for liquid crystal materials.
When -4792 (trade name of Merck Japan) is used,
Since Δn is about 0.09, according to the above equation, d = 9.8 [μm]. Therefore, when configuring the liquid crystal polarization rotator 317 with two liquid crystal cells of the cell thickness d ₂ of the first cell thickness d ₁ and a second liquid crystal cell 303 of the liquid crystal cell 301, at least, d ₁ = d ₂ It can be seen that it suffices to set = 4.9 [μm]. Actually, d ₁ = d ₂ = 5
Instead of completely applying no voltage at about [μm],
By applying a slight bias, the liquid crystal polarization rotator 31
It is desirable to operate 7 as a half-wave plate. By applying a small bias, this minimizes the change in retardation of the liquid crystal cell due to changes in temperature and the like, and is useful for always operating the liquid crystal polarization rotator 317 as a half-wave plate.

Although the retardation of the birefringent plate in the above-mentioned specific example is about 1/14 wavelength, this retardation is at least higher than the residual birefringent component remaining in the liquid crystal cell under voltage application. If a large one is used, the residual birefringence component of the liquid crystal deflection rotator can be made to approach 0 equivalently. Therefore, a commercially available birefringent plate (for example, 1/4 wavelength condition) can be arbitrarily applied in consideration of the residual birefringence component.

Next, the optimum bias value applied to the liquid crystal cell having the above structure will be described. FIG. 9 is a graph showing drive voltage-retardation (Δn · d) characteristics per liquid crystal cell when the liquid crystal cell thickness is 5 μm. At the center wavelength of 1550 nm, when a small bias is applied, the retardation of the entire liquid crystal polarization rotator 317 needs to be about 0.78 [μm], which is half of 1.55 [μm]. Therefore, a correction of 0.78 / 2 + 0.054 = about 0.44 [μm] is required for each liquid crystal cell. At this time, the liquid crystal cells 301, 30
It is understood from FIG. 9 that a small bias of 1.8 [V] may be applied to the No. 3 transistor.

The liquid crystal polarization rotator 31 constructed as described above
Consider the case where the small bias 1.8 [V] obtained from FIG. The first interface 309 in FIG.
With respect to the first liquid crystal director 401 of
Incident linearly polarized light with azimuth angle β ₁ = 0 radian inclined by 4 4
When 10 is incident, the fourth interface 31 in FIG.
The outgoing linearly polarized light 413 that exits the plane of No. 5 is azimuth β _{2 which is} tilted by + π / 4 with respect to the fourth liquid crystal director 407.
The linearly polarized light vibrating at π / 2 radians can be used. That is, the two first and second liquid crystal cells 301,
Liquid crystal polarization rotator 317 including 303 and birefringent plate 331
Can function as a 0th-order half-wave plate that rotates the azimuth angle of the incident linearly polarized light 410 by π / 2 and rotates the polarized light to the outgoing linearly polarized light 413.

Next, in order to explain the crosstalk attenuation of the optical switch described in the action section, the liquid crystal polarization rotator characteristic 803 produced in the birefringent plate of the present invention in FIG. And a comparison of the isolation characteristics of the 90 ° TN type polarization rotator characteristic 801 produced under the conventional 1st minimum condition. Figure 8
Shows the wavelength dependence of the leaked light component orthogonal to the outgoing linearly polarized light 413. As described above, when a small bias of 1.8 [V] is applied in the configuration of the present invention, if the isolation characteristic functions as an ideal polarization rotator, FIG.
The incident linearly polarized light 410 in FIG. 4 should be entirely converted into the outgoing linearly polarized light 413 in FIG. However, in reality, a component orthogonal to the outgoing linearly polarized light 413 is also generated, and becomes a crosstalk component of the optical switch.

As an example, when considering practically, the wavelength range in which the isolation is larger than −30 dB is regarded as the operating wavelength range and compared, the center wavelength is 1550 [nm].
When compared with, the -30 dB range 810 of the 90 ° TN type polarization rotator characteristic 801 of the prior art is about 80 [nm], whereas the -30 dB range of the half-wave plate type liquid crystal polarization rotator characteristic 803 of the present invention is -30 dB. The range 813 is about 60 [nm] and 3/4
However, since both have sufficient bandwidth, there is no problem in operation as an optical switch. Therefore,
The optical switch of the present invention, which functions as a half-wave plate, has a feature that the total cell thickness d can be reduced by about 1/3 ^{1/2 as} compared with the TN type. It can be seen that the configuration of is suitable.

Next, a specific method for driving the optical switch of the present invention will be described in detail. First, the operation when a sufficiently large electric field is applied to the first and second liquid crystal cells 301 and 303 will be described. First liquid crystal cell 301 and second
When a sufficiently large predetermined voltage is applied to the liquid crystal cell 303, the liquid crystal molecules in each cell are aligned in the direction of the electric field, so that the anisotropy becomes small. FIG. 9 shows the drive voltage-retardation (Δn) per liquid crystal cell when the thickness of the liquid crystal cell is 5 μm.
-D) is a graph showing the characteristic 901, and the central wavelength 155
At 0 nm, the retardation is 0.108 [μm] when a birefringent plate with a wavelength of about 1/4 is used. Therefore, in the liquid crystal cell, correction of 0.054 [μm] is required for each liquid crystal cell. At this time, the liquid crystal cells 301 and 303
It can be seen that it is sufficient to apply 9 [V] as a sufficiently large electric field from FIG. Therefore, at this time, the liquid crystal polarization rotator 317 does not discriminate between ordinary light and extraordinary light and no phase difference due to birefringence occurs. As a result, even if the incident linearly polarized light 410 is emitted, the azimuth angle can be set to 0 radian, which is the same as that at the time of incidence.

Furthermore, when an electric field is applied to the liquid crystal polarization rotator 317, the first liquid crystal cell 301 and the second liquid crystal cell 30 are used.
3 simultaneously with a sufficiently large electric field (9 in the above example)
[V]) is applied. In addition, the liquid crystal polarization rotator 317 is applied with a small bias (in the above example, 1.8).
Also in the case of [V]), the electric fields applied to the first liquid crystal cell 301 and the second liquid crystal cell 303 are removed at the same time. By adopting such a driving method, the switching time can be arbitrarily determined by a single liquid crystal cell, so that the liquid crystal polarization rotator 317 can be switched at high speed.

As is apparent from the above description, in the polarization control type liquid crystal optical switch using the liquid crystal polarization rotator of the present invention, the incident linearly polarized light at the time of polarization rotation (small bias application) becomes elliptically polarized light after the polarization rotation. It is possible to optimize the optical characteristics without causing the incident linearly polarized light to be elliptically polarized light even when the polarization is not rotated (when an electric field is applied). Furthermore, the response time can be shortened.

(Embodiment 1) Next, the overall configuration of the polarization control type liquid crystal optical switch will be described in detail with reference to FIG. FIG. 1 shows a polarization control type liquid crystal optical switch 3 of the present invention.
It is a figure which shows 20 typically. The liquid crystal polarization rotator 31
Reference numeral 7 is composed of a first liquid crystal cell 301, a second liquid crystal cell 303, and a birefringent plate (not shown here). A first signal source 131 is connected to the first liquid crystal cell 301 via a first switch 135. Similarly, in the second liquid crystal cell 303, the second signal source 133 is connected to the second switch 1
Connect via 37. In FIG. 1, for ease of understanding, control of signal application to the liquid crystal cell and control of application of a small bias are described by corresponding to ON and OFF of the first switch 135 and the second switch 137. It goes without saying that the signal source is electronically controlled by using a transistor, a diode or an IC.

The polarization control type liquid crystal optical switch 32 of the present invention
0 is the first polarization separator 121 and the second polarization separator 12
3, the first region 145 of the liquid crystal polarization rotator 317 is sandwiched from both sides. In addition, the first total reflection mirror 125
And the second total reflection mirror 127, the liquid crystal polarization rotator 317.
The second region 147 is sandwiched from both sides. The liquid crystal optical switch 320 configured as described above includes the inputs A and B, the outputs C and D, and can be configured as a 2 × 2 optical switch. The positions of the deflection separator and the total reflection mirror are not limited to those shown in FIG. 1, and the total reflection mirror 127 is provided on the light emission side of the first region 145 and the second position is provided on the light emission side of the second region 147. The deflection separator 123 may be arranged. With this configuration, all the optical path lengths of the inputs A and B and the outputs C and D incident on the liquid crystal optical switch 320 of the present invention can be made the same.

Next, the operation of the liquid crystal optical switch 320 of the present invention will be described. In the following, the function of the birefringent plate 331 is to remove the residual birefringent component of the liquid crystal cells 301 and 303 and is not essential to the description of the operation of the switching characteristics. .
First, referring to FIG. 1, a case where the liquid crystal polarization rotator 317 acts to rotate polarized light by applying a small bias to the first liquid crystal cell 301 and the second liquid crystal cell 303 which constitute the liquid crystal polarization rotator 317 will be described. explain.

First, consider that the first input light 101 is incident on the input A. Although not clearly shown in FIG. 1, the first input light 101 is light emitted from the optical fiber and made into parallel light by a collimator. The first input light 101 is the first linearly polarized light 1 which is P-polarized light to the first polarization separator 121.
03 and the second linearly polarized light 105 which is S-polarized light will be considered separately. Hereinafter, P-polarized light will be indicated by vertical or horizontal arrows on the drawing, and S-polarized light will be indicated by diagonal arrows on the drawing.

Since the first linearly polarized light 103 incident on the first polarization separator 121 is P-polarized light, the first polarization separator 12
1 through the first region 145 of the liquid crystal polarization rotator 317.
A liquid crystal polarization rotator 317 according to a first optical path 141 through
Go inside. The first linearly polarized light 103 is the liquid crystal polarization rotator 3
Although not shown in FIG. 1, the first liquid crystal cell 301 and the second liquid crystal cell 303 which form 17 are in a state in which a small bias is applied, and the azimuth angle of P-polarized light and the liquid crystal director on the incident side of the first liquid crystal cell 301. Since the angle with the azimuth angle of is 45 °, when the light is emitted from the first region 145, the azimuth angle is rotated by 90 ° to become S-polarized light. First linearly polarized light 10 which has become S-polarized light
The third polarization splitter 123 changes the traveling direction to a right angle and emits the output 3 from the output D.

On the other hand, the second linearly polarized light 10 input to the input A
Since 5 is S-polarized light, the first polarization separator 121 changes its direction to a right angle and makes it incident on the first total reflection mirror 125, and further changes its direction to a right angle and remains as S-polarized light in the liquid crystal polarization rotator 317. It propagates in the liquid crystal polarization rotator 317 according to the second optical path 143 passing through the second region 147. Second linearly polarized light 105
Is the first liquid crystal cell 3 that constitutes the liquid crystal polarization rotator 317.
01 and the second liquid crystal cell 303 are in a small bias application state, and when the light is emitted from the second region 147, the liquid crystal polarization rotator 317 functions as a half-wave plate, so that the azimuth angle is rotated by 90 ° and P polarization is obtained. Become.

The second linearly polarized light 105 emitted from the second region 147 changes its direction at a right angle by the second total reflection mirror 127, and passes through the second polarization separator 123 without changing its direction. , Reaches the output D as P-polarized light.
Therefore, in the output D, the first linearly polarized light 103 that has propagated through the first region 145 to become S-polarized light and the second region 1
The second linearly polarized light 105 that has propagated through 45 and becomes P-polarized light is combined and emitted as the first output light 113. Although not shown, the first output light 113 is coupled with an optical fiber via a collimator lens as necessary.

Next, the second input light 10 incident on the input B is input.
Think about 7. The second input light 107 is the third linearly polarized light 10 which is P-polarized light with respect to the first polarization separator 121.
9 and the fourth linearly polarized light 111 which becomes S-polarized light will be considered separately.

The third linearly polarized light 109 that has entered the first polarization separator 121 is P-polarized light, and therefore the first polarization separator 12
1 through the liquid crystal polarization rotator 317 along the second optical path 143 passing through the second region 147 of the liquid crystal polarization rotator 317 after being turned at a right angle by the first total reflection mirror 125. The third linearly polarized light 109 is composed of the first liquid crystal cell 301 and the second liquid crystal cell 303 which constitute the liquid crystal polarization rotator 317.
Is a small bias applied state, and although not shown in FIG. 1, the azimuth angle of P-polarized light and the azimuth angle of the liquid crystal director on the incident side in the first liquid crystal cell 301 are 45 °, the second region When the light exits from 147, the azimuth angle is rotated by 90 ° to become S-polarized light. Third linearly polarized light 10 which has become S-polarized light
9 is a second total reflection mirror 127, which is turned at a right angle,
Further, it is incident on the second polarization separator 123. However, since it is S-polarized light, the traveling direction is changed to a right angle by the second polarization separator 123 and the light is emitted from the output C.

On the other hand, the fourth linearly polarized light 11 input to the input B
Since 1 is S-polarized light, the direction is changed at a right angle by the first polarization separator 121, and the S-polarized light is used as the first liquid crystal polarization rotator 317.
Propagating in the polarization rotator 317 according to the first optical path 141 passing through the region 145 of FIG.

In the fourth linearly polarized light 111, the first liquid crystal cell 301 and the second liquid crystal cell 303 which constitute the liquid crystal polarization rotator 317 are in a small bias application state, and the first region 14
When 5 is emitted, the liquid crystal polarization rotator 317 functions as a half-wave plate, so that the azimuth angle is rotated by 90 ° to become P-polarized light. Fourth linearly polarized light 111 emitted from the first region 145
Passes through the second polarization separator 123 without changing its direction, and reaches the output C as P-polarized light. Therefore, in the output C, the third linearly polarized light 109 that has propagated through the second region 147 and has become S-polarized light and the fourth linearly polarized light 111 that has propagated through the first region 145 and have become P-polarized light 111. The light is emitted as the combined second output light 115. Second output light 1
Although not shown, 15 is coupled with an optical fiber via a collimator lens, if necessary.

Next, the first switch 135 of the first signal source 131 and the second switch 13 of the second signal source 133.
7 is turned on, and a sufficiently large voltage (driving voltage) is applied to the first liquid crystal cell 301 and the second liquid crystal cell 303 forming the liquid crystal polarization rotator 317 to apply the liquid crystal polarization rotator 31.
The case where 7 does not rotate the polarization will be described with reference to FIG. In FIG. 5, the same components as those in FIG. 1 are designated by the same reference numerals. First, consider inputting the first input light 101 to the input A. The first input light 101 is the first linearly polarized light 10 which is P-polarized light to the first polarization separator 121.
3 and the second linearly polarized light 105 that becomes S-polarized light will be considered separately.

Since the first linearly polarized light 103 incident on the first polarization separator 121 is P-polarized light, the first polarization separator 12
1 through the first region 145 of the liquid crystal polarization rotator 317.
A liquid crystal polarization rotator 317 according to a first optical path 141 through
Go inside. The first linearly polarized light 103 is the liquid crystal polarization rotator 3
The first liquid crystal cell 301 and the second liquid crystal cell 303, which form part 17, are in a state in which a drive voltage is applied, and the liquid crystal polarization rotator 3
No rotation of azimuth occurs at 17, so the first area 1
Even when the light exits from 45, it remains P-polarized. Therefore, the first linearly polarized light 103 is converted into the second polarized light separator 123.
Through the output C and output from the output C.

On the other hand, the second linearly polarized light 10 input to the input A
Since 5 is S-polarized light, the first polarization separator 121 changes its direction to a right angle and makes it incident on the first total reflection mirror 125, and further changes its direction to a right angle and remains as S-polarized light in the liquid crystal polarization rotator 317. It propagates in the liquid crystal polarization rotator 317 according to the second optical path 143 passing through the second region 147.

In the second linearly polarized light 105, the first liquid crystal cell 301 and the second liquid crystal cell 303 which form the liquid crystal polarization rotator 317 are in a driving voltage applied state, and the second region 147 is applied.
Even when the light is emitted, it remains S-polarized without changing the polarization state. Therefore, the second linearly polarized light 105 emitted from the second region 147 is turned at a right angle by the second total reflection mirror 127, is turned at a right angle by the second polarization separator 123, and the output C To S-polarized light. Therefore, in the output C, the first linearly polarized light 103 propagating in the first region 145 and being P-polarized light and the second linearly polarized light 105 propagating in the second region 147 and being S-polarized light are combined. It is emitted as the second output light 115. Second output light 115
Is coupled to the optical fiber through a collimator lens, though not shown.

Next, the second input light 10 incident on the input B is input.
Think about 7. The second input light 107 is the third linearly polarized light 10 which is P-polarized light with respect to the first polarization separator 121.
9 and the fourth linearly polarized light 111 which becomes S-polarized light will be considered separately. The third linearly polarized light 109 that has entered the first polarization separator 121 is P-polarized light, is transmitted through the first polarization separator 121, and is changed in direction at right angles by the first total reflection mirror 125 to rotate the liquid crystal polarization. Travel through the liquid crystal polarization rotator 317 according to a second optical path 143 through the second region 147 of the child 317.

In the third linearly polarized light 109, since the first liquid crystal cell 301 and the second liquid crystal cell 303 constituting the liquid crystal polarization rotator 317 are in the driving voltage applied state and the polarization rotation function is lost, Even when the second area 147 is emitted, P
The polarization is maintained. Third exiting second area 147
The linearly polarized light 109 of (1) is turned at a right angle by the second total reflection mirror 127, and then enters the second polarization separator 123. However, since it is P-polarized light, it passes through the second polarization separator 123 as it is and is emitted from the output D.

On the other hand, the fourth linearly polarized light 11 input to the input B
Since 1 is S-polarized light, the direction is changed at a right angle by the first polarization separator 121, and the S-polarized light is used as the first liquid crystal polarization rotator 317.
Propagates in the liquid crystal polarization rotator 317 according to the first optical path 141 passing through the area 145 of FIG. The fourth linearly polarized light 111 is
First liquid crystal cell 301 constituting the liquid crystal polarization rotator 317
Since the second liquid crystal cell 303 is in the driving voltage applied state and the polarization rotation function is lost, the S polarization is maintained even when the light is emitted from the first region 145. The fourth linearly polarized light 111 emitted from the first region 145 is reflected by the second polarization separator 12
It turns at right angles at 3 and reaches the output D as S-polarized light. Therefore, at the output D, the P-polarized third linearly polarized light 109 propagating through the second region 147 and the first region 145
Of the S-polarized fourth linearly polarized light 111 propagated as a first output light 113 and is emitted. First output light 11
Although not shown, 3 is coupled with an optical fiber via a collimator lens, if necessary.

As is clear from the above description, the first input light 101 incident on the input A is the liquid crystal polarization rotator 317.
The first liquid crystal cell 301 and the second liquid crystal cell 30 constituting the
When a small bias is applied to 3 and 1, the first output light 113
Is output from the output D. Further, the first input light 101 incident on the input A is applied with a drive voltage to the first liquid crystal cell 301 and the second liquid crystal cell 303 which form the liquid crystal polarization rotator 317, and the liquid crystal polarization rotator 317 When the polarization rotation function is lost, the second output light 115 is emitted from the output C. That is, the first input light 101 is transmitted through the liquid crystal cell 30.
The output light from the output C and the output D differs depending on whether or not a voltage is applied to the light sources 1 and 303.

In addition, the second input light 10 incident on the input B
When a small bias is applied to the first liquid crystal cell 301 and the second liquid crystal cell 303 that form the liquid crystal polarization rotator 317, the light beam 7 is emitted from the output C as the second output light 115. Further, the second input light 107 incident on the input B is applied with a drive voltage to the first liquid crystal cell 301 and the second liquid crystal cell 303 constituting the liquid crystal polarization rotator 317, and the liquid crystal polarization rotator 317 is applied. When the polarization rotation function is lost, the second output light 115 is emitted from the output C. That is, the second input light 107 becomes output light from the same output C and output D regardless of whether or not a voltage is applied to the liquid crystal cells 301 and 303.

From the above description, it can be seen that the liquid crystal optical switch 320 of the present invention operates as a 2 × 2 optical switch. In addition, the first input light 101 or the second input light 101 shown in FIG.
It goes without saying that if only one of the input lights 107 of 1 is used, it can be used as a 1 × 2 optical switch.

(Embodiment 2) Next, the operation when the input light of the liquid crystal optical switch 320 of the present invention is linearly polarized light from the beginning will be described. First, a case in which the liquid crystal polarization rotator 317 rotates polarized light by applying a small bias to the first liquid crystal cell 301 and the second liquid crystal cell 303 forming the liquid crystal polarization rotator 317 will be described with reference to FIG. To do. In FIG. 7, the same components as those in FIG. 1 are designated by the same reference numerals. First, consider inputting the first input light 101 to the input A. Although not explicitly shown in FIG. 7, the first input light 101 and the second input light 107 described later may be light emitted from the polarization maintaining optical fiber and made into parallel light by a collimator. First
Input light 101 of P
The polarized light is composed of only the first linearly polarized light 103 that is polarized light.

Since the first linearly polarized light 103 incident on the first polarized light separator 121 is P polarized light, the first polarized light separator 12
1 through the first region 14 of the liquid crystal polarization rotator 317.
Liquid crystal polarization rotator 31 according to the first optical path 141 passing through 5.
Go through 7. In the first linearly polarized light 103, the first liquid crystal cell 301 and the second liquid crystal cell 303 that form the liquid crystal polarization rotator 317 are in a state in which a small bias is applied. Since the angle formed by the azimuth of the liquid crystal director on the incident side of the first liquid crystal cell 301 is 45 °,
When the light is emitted from the first region 145, the azimuth angle is rotated by 90 ° to become S-polarized light. The first linearly polarized light 103, which has become the S-polarized light that is the first input light 101, is the second polarization separator 1
At 23, the traveling direction is changed to a right angle, and the output D is emitted as the first output light 113. Although not shown, the first output light 113 is coupled with an optical fiber via a collimator lens as necessary.

Next, the second input light 10 incident on the input B is input.
Think about 7. The second input light 107 is assumed to be the fourth linearly polarized light 111 having an azimuth angle that is S-polarized with respect to the first polarization separator 121. Since the fourth linearly polarized light 111 that has entered the input B is S-polarized light, the direction of the fourth linearly polarized light 111 is changed at a right angle by the first polarization separator 121, and the liquid crystal polarization rotator 317 remains as S-polarized light.
Propagates in the liquid crystal polarization rotator 317 according to the first optical path 141 passing through the first region 145 of

In the fourth linearly polarized light 111, the first liquid crystal cell 301 and the second liquid crystal cell 303 constituting the liquid crystal polarization rotator 317 are in a small bias application state, and the first region 14
When 5 is emitted, the liquid crystal polarization rotator 317 functions as a half-wave plate, so that the azimuth angle is rotated by 90 ° to become P-polarized light. The fourth linearly polarized light 111, which is the second input light 107 emitted from the first region 145, is converted into the second polarization separator 123.
Since the light is transmitted without changing its direction at, it reaches the output C as P-polarized light. Therefore, the output C is emitted as the second output light 115. Although not shown, the second output light 115 is coupled with an optical fiber via a collimator lens, if necessary.

Here, the first signal source 131 and the second signal source 133 are respectively arranged in the first liquid crystal cell 301 and the second liquid crystal cell 303 which compose the liquid crystal polarization rotator 317, and the first signal source 131 and the second signal source 133 are respectively arranged. Consider a case where a predetermined drive voltage is applied to each liquid crystal cell by turning on the switch 135 and the second switch 137. In this case, the liquid crystal polarization rotator 317 loses the effect of rotating the azimuth angle of the incident polarized light by 90 °. Therefore, the first input light 101 is output to the output C and the second output light 11 is output.
5 and the second input light 107 is emitted to the output D as the first output light 113.

From the above description, the liquid crystal optical switch 320 of the present invention can operate as a 2 × 2 optical switch in which the output destination can be selected by switching the drive voltage application and the small bias application to the liquid crystal polarization rotator 317. Recognize. Further, without using the first polarization separator 121 shown in FIG. 7, only one of the P-polarized or S-polarized first input light 101 or the second input light 107 is in the first region 145. It is needless to say that it can be used as a 1 × 2 optical switch if it is made incident on the beam.

(Embodiment 3) Next, the configuration and its driving method when the polarization control type liquid crystal optical switch 320 of the present invention is used as a variable optical attenuator will be described. The liquid crystal deflecting rotator in this configuration is the same as that in the first embodiment, and is different from the first and second switches 135 and 137 configuration only in the driving method.
One specific example will be described.

For example, in FIG. 1, assuming that only the first input light 101 is input to the input A and the first output light 113 that is output and is output from the output D is considered. Next, the first switch 135 and the second switch 137 are short-circuited, respectively, and the first signal source 131 is applied to the first liquid crystal cell 301.
And the second signal source 133 is connected to the second liquid crystal cell 303. At this time, it is considered that the effective voltage value applied to the liquid crystal cells 301 and 303 is changed in an analog manner by changing the amplitude or pulse width of the output from the first signal source 131 and the second signal source 133. .

At this time, the light emitted from the first region 145 and the second region 147 of the liquid crystal polarization rotator 317 is polarized by the liquid crystal polarization rotator 317 according to the modulation applied voltage of the liquid crystal cells 301 and 303. Since the function disappears in an analog manner, the emitted light is generally elliptically polarized light. For this reason, the first output light 113 emitted to the output D is subjected to intensity modulation, so that it is continuously output according to the effective values of the outputs of the first signal source 131 and the second signal source 133. It is possible to control the light intensity arbitrarily. Therefore, with the configuration of this embodiment, the polarization control type liquid crystal optical switch 320 can be used as a variable optical attenuator.

(Embodiment 4) Configuration of an array of polarization control type optical switches using the liquid crystal of the present invention FIG.
Will be explained. FIG. 6 is a diagram showing a configuration of a polarization control type liquid crystal optical switch using an arrayed liquid crystal cell. In the case of forming an array, the liquid crystal polarization rotator 317 including a plurality of cells is divided into the number of arrays in the plane.
Although the specific dividing method is not shown in FIG. 6, the transparent electrode of the liquid crystal cell that constitutes the liquid crystal polarization rotator 317 is divided into predetermined regions in the electrode plane to form a plurality of liquid crystal changing rotator elements. And the liquid crystal deflecting and rotating element section divided into a plurality of sections are arbitrarily controlled. FIG. 6 shows a case where the liquid crystal polarization rotator 317 is divided into a first liquid crystal polarization rotator element section 621 and a second liquid crystal polarization rotator element section 623.

As shown in FIG. 6, the first composite prism portion 631 is composed of the first polarization separator 121 and the first total reflection mirror 125, and the second composite prism portion 633 is composed of the second polarization splitter. And a second total reflection mirror 127. In the arrayed polarization control type optical switch, as described above, the liquid crystal polarization rotator 317 having a plurality of divided liquid crystal polarization rotator elements is provided between the first compound prism section 631 and the second compound prism section 633. It is configured by placing it in. FIG. 6 shows an arrayed element in which two 2 × 2 elements are arranged. The first liquid crystal polarization rotator element 621 controls incident light from the first A input 601 and the first B input 603, and outputs the first C output 611 and the first D output 613.
To be emitted from. Further, in the second liquid crystal polarization rotator element 623, the second A input 605 and the second B input 6 are input.
The incident light from 07 is controlled so as to be emitted from the output 615 of the second C and the output 617 of the second D.

In FIG. 6, the liquid crystal polarization rotator 31 is shown for simplification.
The case of a one-dimensional two-array configuration in which 7 is divided into two is shown.
It is needless to say that the arraying can be expanded in a two-dimensional plane by the same method as described above, and thus the invention is not limited to the one-dimensional two-array.

[0074]

As is apparent from the above description, the liquid crystal optical switch and the driving method thereof according to the present invention can be used as a variable optical attenuator with a simple structure and a simple driving method, and has a high function. An optical switch can be realized. In addition, the configuration of the present invention is wavelength division multiplexing (WDM) using an optical fiber.
The range of the polarization control type liquid crystal optical switch used for communication and optical network and its driving method is not limited by the device described here, and can be applied to, for example, a liquid crystal optical modulator for a free space optical communication device. Needless to say.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a device configuration of a liquid crystal optical switch when used as a polarization control type or variable term attenuator in Example 1 or 3 of the present invention.

FIG. 2 is a schematic cross-sectional view showing the structure of a liquid crystal cell that constitutes a liquid crystal polarization rotator of a polarization control type liquid crystal optical switch according to an embodiment of the present invention.

FIG. 3 is a schematic diagram showing a structure of a liquid crystal polarization rotator of a polarization control type liquid crystal optical switch according to an embodiment of the present invention.

FIG. 4 is a plan view for explaining an azimuth angle of a liquid crystal director at each substrate interface of a liquid crystal cell that constitutes a liquid crystal polarization rotator of a polarization control type liquid crystal optical switch according to an embodiment of the present invention.

FIG. 5 is a schematic diagram showing a configuration in which a signal source is connected to a liquid crystal polarization rotator in an embodiment of a polarization control type liquid crystal optical switch and a variable optical attenuator of the present invention.

FIG. 6 is a schematic diagram showing a configuration of an array of polarization control type liquid crystal optical switches in Example 4 of the present invention.

FIG. 7 is a schematic cross-sectional view showing a configuration of a polarization control type liquid crystal optical switch in a case where incident light can be limited to linearly polarized light in Example 2 of the present invention.

FIG. 8 is a graph for comparing the wavelength dependence of the isolation characteristics between the case of using the TN type and the case of using the half-wave plate type in the liquid crystal cell used in the polarization control type liquid crystal optical switch.

FIG. 9 is a graph showing drive voltage-retardation characteristics of a liquid crystal cell used for a polarization control type liquid crystal optical switch.

[Explanation of symbols]

101 First input light 103 First linearly polarized light 105 Second linearly polarized light 107 Second input light 109 Third linearly polarized light 111 Fourth linearly polarized light 113 First output light 115 Second output light 121 First polarization separator 123 Second polarization separator 125 First total reflection mirror 127 Second total reflection mirror 131 First signal source 133 Second signal source 141 First optical path 143 Second optical path 145 First area 147 Second area 301 First liquid crystal cell 303 Second liquid crystal cell 317 Liquid crystal polarization rotator 320 LCD optical switch 331 Birefringent plate

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 2H088 EA33 EA40 EA45 HA02 HA21                       HA23 JA04 JA05 JA11 KA02                       KA07 KA16 KA17 MA10                 2H091 FA10X FA10Z FA21X FA21Z                       GA02 GA13 HA06 HA07 KA02                       KA04 MA10                 2K002 AA02 AB05 BA06 CA14 DA14                       EA11 HA04

Claims (9)

[Claims] 1. A liquid crystal polarization rotator comprising a plurality of liquid crystal cells and a birefringent plate arranged at a position either between or outside the plurality of liquid crystal cells, wherein the plurality of liquid crystal cells are provided on a light incident side. The liquid crystal layer sandwiched between the substrate and the light emitting side substrate is uniformly oriented in either anti-parallel or parallel, and the liquid crystal director azimuth angles in contact with the light emitting side substrate and the light incident side substrate are almost the same. A liquid crystal optical switch, wherein the birefringent plates are set to be equal to each other, and the birefringent plate has an advance layer axis substantially parallel to the liquid crystal director azimuth angle. 2. The liquid crystal optical switch according to claim 1, wherein the cell thickness of the plurality of liquid crystal cells and the retardation ratio of the liquid crystal layer are set to be substantially equal to each other. 3. The liquid crystal optical switch according to claim 1, wherein the liquid crystal polarization rotator operates as a half-wave plate for a predetermined wavelength incident on the liquid crystal polarization rotator. 4. The plurality of liquid crystal cells are composed of n pieces, and the retardation of the liquid crystal layer in each liquid crystal cell is:
The liquid crystal optical switch according to claim 3, wherein the condition is 1 / (2n) wavelength with respect to the predetermined wavelength. 5. The liquid crystal optical switch according to claim 1, wherein a polarization separator is provided on the light incident side and the light emitting side of the liquid crystal polarization rotator. 6. The liquid crystal optical switch according to claim 1, wherein a polarization separator and a total reflection mirror are provided on the light incident side and the light emitting side of the liquid crystal polarization rotator. . 7. The liquid crystal cell is provided with a plurality of liquid crystal polarization rotator elements which can be separately controlled by dividing an electrode formed in each liquid crystal cell into an arbitrary number. 7. The liquid crystal optical switch according to any one of 1 to 6. 8. The retardation of the birefringent plate is at least larger than the residual birefringence component generated when an on-voltage is applied to the electrodes of the liquid crystal panel. Liquid crystal optical switch described in. 9. The method for driving a liquid crystal optical switch according to claim 1, wherein an effective value of a predetermined drive waveform applied to the plurality of liquid crystal cells is analog-modulated,
A method for driving a liquid crystal optical switch, characterized by continuously controlling the polarization rotation amount and the elliptically polarized light of incident light.