JP4786841B2 - Liquid crystal optical switch and driving method thereof - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a polarization control type liquid crystal optical switch for optical communication, and more particularly to a polarization control type liquid crystal optical switch used in a wavelength division multiplexing (WDM) communication system / optical network using an optical fiber and a driving method thereof.
[0002]
[Prior art]
The polarization-controlled optical modulator using nematic liquid crystal has been put into practical use for liquid crystal displays. As a typical example of the polarization rotator, a 90 ° twisted nematic (TN) type liquid crystal is given. In the TN type liquid crystal, the response time is proportional to the square of the cell thickness. Therefore, in order to realize a high-speed response cell, it is necessary to reduce the thickness of the cell.
The TN liquid crystal is basically suitable for increasing the contrast ratio, but the liquid crystal cell thickness is reduced in order to increase the response speed.
[0003]
Here, λ is the wavelength, Δn is the refractive index anisotropy of the liquid crystal, and d is the thickness of the liquid crystal cell, so-called Morgan conditions.
λ / 2 is sufficiently small relative to Δn · d
The wave guide effect is reduced because As a result, a phenomenon that the contrast ratio is deteriorated occurs. In order to solve this problem, today's fast response TN type display has the formula
d = (λ / 2) · (u / Δn)
In many cases, the cell parameters are determined in accordance with the first (first: hereinafter referred to as 1st) minimum condition in the normally black mode of the liquid crystal display in which the Morgan parameter u is set to 3 at the square root.
[0004]
As an example of applying TN type liquid crystal to a 2 × 2 switch for optical fiber communication, reference literature is (Y. Hakamata, T. Yoshizawa and T. Kodaira, “A 1.3 μm Single-Mode 2X2 Liquid Crystal Optical Switch”, IEICE Trans. Commun., Vol. E77-B, No. 10 October 1994).
[0005]
[Problems to be solved by the invention]
However, the wavelength used in optical fiber communication is in the infrared region, and the 1300 nm band and the 1550 nm band are often used, unlike the wavelength used in the visible range (eg, 550 nm for green). Therefore, since the wavelength used is long, the cell thickness becomes thick as a result. For this reason, when a conventional TN type cell is used for a polarization control type optical switch for optical fiber communication, there is a problem that the response speed becomes slow.
[0006]
For example, standards such as the synchronous optical network (SONET) and the synchronous digital hierarchy (SDH) stipulate that the return time is 50 milliseconds or less when a failure occurs in the network.
Therefore, even when the opticalization of the system progresses in the future, the compatibility with the conventional network is important, and the response time of the optical switch needs to be shorter than at least 50 milliseconds.
[0007]
For example, consider realizing a polarization rotator of an optical switch using TN liquid crystal technology. When the center wavelength is a wavelength used for optical fiber communication of 1550 nm, the cell thickness that satisfies the 1st minimum condition is when the liquid crystal is ZLI-4792 (a product name of Merck Japan) and Δn is 0.09, The liquid crystal cell thickness d is approximately 15 μm.
[0008]
At this time, the rise response time τr is inversely proportional to the square of the magnitude of the electric field applied to the liquid crystal cell, so even if the cell thickness is large, the applied voltage can be increased to 50 ms or less. Since the fall response time τd is proportional to the square of the cell thickness of the liquid crystal, the fall response time τd becomes 150 msec or more in the vicinity of room temperature and is not suitable for an optical switch used for optical communication.
[0009]
In addition, considering that the polarization rotator is realized by an optical switch using a liquid crystal element as a zeroth-order half-wave plate, the cell thickness for the anti-parallel alignment liquid crystal cell to be a half-wave plate is
d = λ / (2 · Δn)
It becomes. Here, when Δn = 0.09 of the ZLI-4792 and the center wavelength λ is 1550 nm, the cell thickness d is about 8.6 μm.
In this case, the falling response time τd is faster than that of the TN element, and about 60 msec is obtained, but it is not easy to set it to 50 msec or less.
[0010]
The object of the present invention is to solve the above-mentioned problems and to be applicable to optical fiber communication, a simple structure, a simple driving method, suitable for arraying, and also applicable to an optical attenuator. It is to provide a switch and its driving method.
[0011]
[Means for Solving the Problems]
In order to achieve the above object, the polarization control type optical switch using the liquid crystal and the driving method thereof according to the present invention employ the following means.
[0012]
The liquid crystal optical switch of the present invention is a liquid crystal optical switch using liquid crystal, wherein the liquid crystal optical switch has a liquid crystal polarization rotator composed of a plurality of antiparallel or parallel alignment liquid crystal cells, and the cell thickness of each liquid crystal cell And the ratio of the retardation of the liquid crystal layer of each liquid crystal cell is substantially equal, and the light emitting side substrate and the light incident side substrate face each other between the liquid crystal cells. When incident light of linearly polarized light having an azimuth angle β1 = 0 radians inclined by −π / 4 with respect to the liquid crystal director of the liquid crystal molecules at the interface of the light incident side substrate is incident on the liquid crystal polarization rotator, It becomes a linearly polarized outgoing light oscillating at an azimuth angle β2 = π / 2 radians tilted by π / 4 with respect to the liquid crystal director at the interface of the outgoing side substrate. Control It is characterized by that.
[0013]
The liquid crystal polarization rotator of the liquid crystal optical switch of the present invention is a half-wave plate having a predetermined wavelength.
[0014]
The liquid crystal optical switch of the present invention has two liquid crystal cells, and the retardation of the liquid crystal layer of each liquid crystal cell is approximately a quarter wavelength.
[0015]
The liquid crystal optical switch of the present invention is characterized in that polarization separators are provided on the incident side and the output side of the liquid crystal polarization rotator.
[0016]
The liquid crystal optical switch of the present invention is characterized in that a polarization separator and a total reflection mirror are provided on the incident side and the emission side of the liquid crystal polarization rotator.
[0017]
The transparent electrode provided in the liquid crystal cell of the liquid crystal optical switch of the present invention is divided into a plurality of parts, and the liquid crystal cell is provided with a plurality of liquid crystal polarization rotation element portions.
[0018]
The liquid crystal optical switch driving method of the present invention is characterized in that an effective value of a predetermined driving waveform applied to the liquid crystal cell is analog-modulated to continuously control the amount of polarization rotation and the ellipticity of incident light.
[0019]
[Action]
The polarization control type liquid crystal optical switch of the present invention has a structure in which the polarization rotator constituting the optical switch is a predetermined number of two or more and antiparallel or parallel alignment liquid crystal elements are arranged in multiple stages. For the k-th liquid crystal element, the retardation is the product of the effective refractive index anisotropy Δni of each element at the initial alignment and the cell thickness di (where i is 1 to k) The value obtained by dividing the sum of φi by the cell thickness di is a constant value, and when the director azimuth angle of the liquid crystal in contact with the light input side substrate of the first twisted nematic liquid crystal element is used as a reference, the i th liquid crystal element The director azimuth angle of the liquid crystal in contact with the light input side substrate is arranged to be 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 element.
[0020]
As is well known for anti-parallel or parallel alignment type liquid crystal elements, the condition of the cell thickness when the liquid crystal element is used as a half-wave plate that can be switched at a predetermined wavelength λ is:
d = λ / (2 · Δn)
It becomes. d is the cell thickness, Δn is the retardation, and λ is the wavelength.
[0021]
With the structure of the present invention, when an electric field is not applied to the electrodes of each liquid crystal element, it is optically equivalent to a liquid crystal element under half-wave plate conditions. Therefore, incident linearly polarized light having an azimuth angle inclined by π / 4 or an odd multiple of π / 4 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 output side of the kth liquid crystal cell. Instead, the azimuth angle of the outgoing linearly polarized light is rotated by π / 2 radians with respect to the azimuth angle of the incident linearly polarized light.
[0022]
Here, consider a case where an electric field is applied to the first to kth liquid crystal cells.
Since the retardation structure of each cell can be eliminated by applying a predetermined electric field to each cell, the incident linearly polarized light is emitted from the kth cell without rotating the azimuth angle.
Therefore, in the polarization control type liquid crystal optical switch of the present invention, a polarizing beam splitter or a polarizing beam separator using a birefringent crystal is arranged on the output side, thereby changing the direction of incident polarized light and emitting light in a predetermined output direction. It is possible to vary the intensity.
[0023]
For simplicity, consider the polarization controlled liquid crystal optical switch of the present invention consisting of two quarter wave plate conditions with polarization rotators having equal cell thickness.
[0024]
In a half-wave plate liquid crystal cell consisting of one conventional liquid crystal cell, the cell thickness d is
d = λ / (2 · Δn)
It is said. In the case of the polarization control type liquid crystal optical switch in which the polarization rotator of the present invention is composed of two quarter-wave plate condition liquid crystal cells, the cell thickness d is divided into two so that the cell thickness per sheet is d / 2. can do.
[0025]
In an anti-parallel or parallel-aligned half-wavelength condition liquid crystal cell, the response time τr when no electric field is applied to when the electric field is applied is approximately when the applied voltage is V.
τr is (d / V) ² Proportional to
It is known that In addition, the response time τd when no electric field is applied from the electric field applied state is approximate.
τd is d ² Proportional to
It is known.
[0026]
Therefore, in the case of the polarization control type liquid crystal optical switch in which the polarization rotator of the present invention is composed of two quarter-wave plate condition liquid crystal cells as compared with the prior art, the response speed is 4 when compared with the same liquid crystal material. About twice as fast.
Here, if the number of divisions in the thickness direction is increased and the cell thickness per sheet is reduced, the speed can be further increased.
[0027]
High speed is important for optical switches for optical communication. In general, the response time is desired to be 50 milliseconds or less. However, when the center wavelength is a wavelength used for optical fiber communication of 1550 nm in the prior art, the cell thickness that satisfies the half-wavelength condition is, for example, Δn when using ZLI-4792 (trade name of Merck Japan) for liquid crystal Since it is about 0.09, the cell thickness d is about 8.6 μm.
[0028]
At this time, τr can be reduced to 50 milliseconds or less by increasing the applied voltage, but τd becomes 60 milliseconds or more near room temperature. Therefore, it is not suitable for an optical switch.
However, if a polarization control type liquid crystal optical switch comprising, for example, two quarter wave plate condition liquid crystal cells with two polarization rotators is used in the present invention, the response time τd can be easily reduced to 20 ms or less. .
[0029]
As apparent from the above description, the liquid crystal optical switch and the liquid crystal variable optical attenuator of the present invention and the driving method thereof can realize a highly functional optical switch and variable optical attenuator with a simple configuration and a simple driving method.
The present invention also provides a polarization-controlled liquid crystal optical switch used for wavelength division multiplexing (WDM) communication using optical fibers and optical networks, and a driving method thereof. The scope of the present invention is limited to the apparatus described herein. Needless to say, the present invention can be applied to, for example, a liquid crystal light modulator for a free space optical communication device.
[0030]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a configuration 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 with reference to the drawings.
[0031]
First, the configuration of a liquid crystal optical switch according to an embodiment of the present invention will be described with reference to the drawings.
FIG. 2 is a cross-sectional view for explaining the configuration of the liquid crystal cell 220 constituting the liquid crystal optical switch in the embodiment of the present invention.
[0032]
As shown in FIG. 2, the liquid crystal cell 220 of the liquid crystal light modulation device of the present invention includes a nematic liquid crystal layer 201 between a first substrate 203 on which a signal electrode 211 is formed and a second substrate 205 on which a common electrode 213 is formed. It is comprised by pinching.
The nematic liquid crystal layer 201 is a director of positive (p) type liquid crystal molecules when no electric field is applied, by an alignment layer 217 formed on the signal electrode 211 of the first substrate 203 and the common electrode 213 of the second substrate 205. The anti-parallel orientation is set so that the pre-tilt angle 209 of 207 is 0.5 to 20 degrees.
Here, the alignment layer 217 is formed of polyimide and aligns liquid crystal molecules in a predetermined direction by a rubbing method. Further, FIG. 2 shows the case of anti-parallel alignment, but the director 207 may be parallel alignment.
[0033]
Although not clearly shown in FIG. 2, the periphery of the liquid crystal cell 220 is sealed with a sealant between the first substrate 203 and the second substrate 205 via a spacer so that the nematic liquid crystal layer 201 maintains a predetermined constant thickness of several μm. Fix it.
Although not shown in FIG. 2, in order to prevent the signal electrode 211 and the common electrode 213 from being short-circuited, tantalum pentoxide (Ta2O5) or silicon dioxide (SiO2) is formed on the signal electrode 211 or the common electrode 213 or both. A transparent insulating film such as may be formed under the alignment layer 217.
[0034]
The signal electrode 211 formed on the first substrate 203 and the common electrode 213 formed on the second substrate 205 form a predetermined pattern made of a transparent conductive film as necessary, for example, when arrayed.
When indium tin oxide (ITO) is used as the transparent conductive film, the film thickness is set to 50 nm or less, and in order to improve the transmittance in the infrared region, the sheet resistance is increased from several hundreds Ω with increased oxygen concentration during film formation. It is desirable to use a film of about 1 kΩ.
[0035]
In addition to ITO, thin films such as indium oxide (In 2 O 3), tin oxide (SnO 2), and zinc oxide (ZnO) can be used as the transparent conductive film.
In this case as well, it is desirable to use a film having a film thickness of 50 nm or less and a sheet resistance number of several hundred Ω to 1 kΩ.
[0036]
A non-reflective coating 215 is formed on the surfaces of the first substrate 203 and the second substrate 205 made of glass opposite to the nematic liquid crystal layer 201 to prevent reflection at the interface between the air and the substrate.
2 shows the case where the non-reflective coating 215 is formed only on the first substrate 203, it is also formed below the second substrate 205 as needed.
[0037]
Next, the 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 will be described with reference to FIGS.
FIG. 3 is a diagram schematically showing a liquid crystal polarization rotator 317 composed of a first liquid crystal cell 301 and a second liquid crystal cell 303.
[0038]
In FIG. 3, the first liquid crystal cell 301 has a structure in which a first liquid crystal layer 305 is sandwiched between a first glass substrate 319 and a second glass substrate 321. The second liquid crystal cell 303 has a structure in which the second liquid crystal layer 307 is sandwiched between the third glass substrate 323 and the fourth glass substrate 325.
The first cell thickness d1 of the first liquid crystal cell 301 and the second cell thickness d2 of the second liquid crystal cell 303 are made equal. 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.
[0039]
The 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 the liquid crystal molecules, and schematically show the director of the liquid crystal.
[0040]
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. Similarly, the interface between the second glass substrate 321 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 a third interface 313. Finally, the interface between the fourth glass substrate 325 and the second liquid crystal layer 307 is defined as a fourth interface 315.
Here, in the xyz coordinate system, the first interface of the first liquid crystal cell 301 is obtained when the clockwise direction from the positive x-axis direction to the positive y-axis direction is azimuthally positive. The azimuth angle of the liquid crystal director on the 309 side is π / 4 radians, and the thickness direction is selected to be parallel to the z-axis.
[0041]
Next, the directions of the liquid crystal directors at the interfaces 309, 311, 313, and 315 are shown in the plan view of FIG. Hereinafter, description will be made with reference to FIGS. 3 and 4 alternately.
At the first interface 309, the azimuth angle α1 measured from the x-axis of the first liquid crystal director 401 is π / 4 radians. Since the first liquid crystal cell 301 has anti-parallel alignment, the azimuth angle α2 of the second liquid crystal director 403 is also π / 4 radians at the second interface 311. Next, the azimuth angle α3 of the third liquid crystal director 405 at the third interface 313 is set to π / 4 radians in order to be parallel to the second liquid crystal director 403.
Further, since the second liquid crystal cell 302 is also anti-parallel aligned, the azimuth angle α4 of the fourth liquid crystal director 407 at the fourth interface 315 is π / 4 radians.
[0042]
Up to this point, the case where the liquid crystal polarization rotator 317 is configured by the two liquid crystal cells 301 and 303 having the same cell thickness has been described, but the liquid crystal cell may be configured by three or more k sheets.
In this case, when the total cell thickness d is from the first cell thickness d1 to the kth cell thickness dk,
d = d1 + d2 +. . . dk (1)
It becomes. When the extraordinary refractive index of the liquid crystal material used is ne and the ordinary refractive index is no, the effective extraordinary refractive index neff is when the pretilt angle of the liquid crystal cell is θ,
neff = (sin ² θ / no ² + Cos ² θ / ne ² ) ^-1/2 (2)
It becomes. Here, assuming that the pretilt angle θ of each liquid crystal cell is equal,
Δn = neff−no (3)
The total cell thickness at a given wavelength λ is
d = λ / (2 · Δn) (4)
To satisfy.
This is equivalent to a zero-order half-wave plate made of quartz or the like as is well known. (For example, there are Pochi Yeh and Claire Gu, Optics of Liquid Crystal Displays, Wiley, 1999)
[0043]
In addition, the azimuth angle αk of the liquid crystal director at the first to second interface needs to be all equal to the azimuth angle α1 of the first liquid crystal director. For example
When α1 = π / 4 radians
α1 = α2
α2 = α3
...
α2k-1 = α2k
And so on.
[0044]
In FIG. 3, the total cell thickness d of the liquid crystal cell constituting the liquid crystal polarization rotator 317 is
d = λ / (2 · Δn)
And When the center wavelength λ is 1550 nm and the pretilt angle is 1 °, for example, when ZLI-4792 (trade name of Merck Japan) is used as the liquid crystal material, Δn is about 0.09.
d = 8.6 [μm]
And Therefore, when the liquid crystal polarization rotator 317 is composed of two liquid crystal cells, the cell thickness d1 of the first liquid crystal cell 301 and the cell thickness d2 of the second liquid crystal cell 303 are:
d1 = d2 = 4.3 [μm]
And set.
[0045]
The incident linearly polarized light 410 having an azimuth angle β1 = 0 radians inclined by −π / 4 with respect to the first liquid crystal director 401 of the first interface 309 is arranged on the liquid crystal polarization rotator 317 configured as described above. , The outgoing linearly polarized light 413 exiting the plane of the fourth interface 315 becomes linearly polarized light oscillating at an azimuth angle β2 = π / 2 radians inclined by + π / 4 with respect to the fourth liquid crystal director 407.
[0046]
In other words, the liquid crystal polarization rotator 317 composed of the two first and second liquid crystal cells 301 and 303 has the 0th order rotation of the incident linearly polarized light 410 by rotating the azimuth angle by π / 2 and rotating the polarized light to the outgoing linearly polarized light 413. Acts as a half-wave plate.
FIG. 8 shows a comparison of the isolation characteristics of the liquid crystal polarization rotator characteristic 803 produced under the 0th-order half-wave plate condition and the 90 ° TN type polarization rotator characteristic 801 produced under the 1st minimum condition.
[0047]
In the ideal polarization rotator, the incident linearly polarized light 410 should be completely converted to the outgoing linearly polarized light 413, but in reality, a component orthogonal to the outgoing linearly polarized light 413 is generated and becomes the crosstalk component of the optical switch. . FIG. 8 shows the wavelength dependence of the leaked light component orthogonal to the outgoing linearly polarized light 413.
[0048]
As a practical example, when a wavelength range in which isolation is greater than −30 dB is considered as an operating wavelength range and compared, a 90 ° TN polarization rotator characteristic 801 is obtained when the center wavelength is compared at 1550 [nm]. The −30 dB range 810 of the half-wave plate type liquid crystal polarization rotator characteristics 803 is about 60 [nm] and about 3/4, whereas the −30 dB range 810 is about 80 [nm]. 1/3 of total cell thickness d ^1/2 Since the half-wave plate type can be made thin, the half-wave plate type is suitable particularly when a high-speed switch is required.
[0049]
Next, an operation when an electric field is applied to the first and second liquid crystal cells 301 and 303 will be described.
When a sufficiently large predetermined voltage is applied to the first liquid crystal cell 301 and the second liquid crystal cell 303, the liquid crystal molecules in each cell are aligned in the electric field direction, and thus anisotropy disappears. Therefore, the liquid crystal polarization rotator 317 does not give a phase difference between ordinary light and extraordinary light. As a result, even if the incident linearly polarized light 410 is emitted, the azimuth angle is 0 radians, which is the same as the incident angle.
[0050]
Further, when an electric field is applied to the liquid crystal polarization rotator 317 during driving, the electric field is applied to the first liquid crystal cell 301 and the second liquid crystal cell 303 simultaneously. Further, when the liquid crystal polarization rotator 317 is not applied with an electric field, the electric fields applied to the first liquid crystal cell 301 and the second liquid crystal cell 302 are simultaneously removed.
By adopting such a driving method, since the switching time is determined by a single liquid crystal cell, the liquid crystal polarization rotator 317 can be switched at high speed.
[0051]
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 during polarization rotation (no applied electric field) becomes elliptically polarized light after polarization rotation. In addition, the optical characteristics can be optimized, and the response time when the polarization is not rotated (when an electric field is applied) can be shortened.
[0052]
Next, the entire configuration of the polarization control type liquid crystal optical switch will be described in detail with reference to FIG.
[0053]
FIG. 1 is a view schematically showing a polarization control type liquid crystal optical switch 320 of the present invention. The liquid crystal polarization rotator 317 includes a first liquid crystal cell 301 and a second liquid crystal cell 303.
A first signal source 131 is connected to the first liquid crystal cell 301 via a first switch 135. Similarly, a second signal source 133 is connected to the second liquid crystal cell 303 via a second switch 137. In FIG. 1, for the sake of easy understanding, a method in which the application of a signal to a liquid crystal cell and the non-application of a signal are controlled by a first switch 135 and a second switch 137 will be described. Needless to say, it may be controlled electronically using a diode or IC.
[0054]
The polarization control type liquid crystal optical switch 320 of the present invention has a configuration in which the first polarization separator 121 and the second polarization separator 123 sandwich the first region 145 of the liquid crystal polarization rotator 317 from both sides. The first total reflection mirror 125 and the second total reflection mirror 127 sandwich the second region 147 of the liquid crystal polarization rotator 317 from both sides.
The thus configured liquid crystal optical switch 320 includes an input A, an input B, an output C, and an output D.
[0055]
Next, the operation of the liquid crystal optical switch 320 of the present invention will be described.
First, the case where the liquid crystal polarization rotator 317 rotates the polarization without applying a voltage to the first liquid crystal cell 301 and the second liquid crystal cell 303 constituting the liquid crystal polarization rotator 317 will be described with reference to FIG. I will explain.
[0056]
First, consider that the first input light 101 enters the input A. Although not clearly shown in FIG. 1, the first input light 101 is light that is collimated from light emitted from the optical fiber. The first input light 101 is considered by dividing it into a first linearly polarized light 103 that is P-polarized light and a second linearly polarized light 105 that is S-polarized light for the first polarization separator 121.
Hereinafter, P-polarized light is indicated by a vertical or horizontal arrow on the drawing, and S-polarized light is indicated by an oblique arrow on the drawing.
[0057]
The first linearly polarized light 103 that has entered the first polarization separator 121 passes through the first polarization separator 121 because it is P-polarized light, and passes through the first region 145 of the liquid crystal polarization rotator 317. Follow the 141 in the liquid crystal polarization rotator 317. In the first linearly polarized light 103, no voltage is applied to the first liquid crystal cell 301 and the second liquid crystal cell 303 constituting the liquid crystal polarization rotator 317, and although not shown in FIG. Since the angle formed with the azimuth angle of the liquid crystal director on the incident side of the liquid crystal cell 301 is 45 °, when the first region 145 is emitted, the azimuth angle is rotated by 90 ° to become S-polarized light.
The first linearly polarized light 103 that has become S-polarized light is emitted from the output D with the second polarization separator 123 changing the traveling direction to a right angle.
[0058]
On the other hand, the second linearly polarized light 105 entering the input A is changed to a right angle by the first polarization separator 121 because it is an S-polarized light, enters the first total reflection mirror 125, and further turns to a right angle. And the light is propagated through the liquid crystal polarization rotator 317 according to the second optical path 143 passing through the second region 147 of the liquid crystal polarization rotator 317 while maintaining the S polarization. When the first liquid crystal cell 301 and the second liquid crystal cell 303 constituting the liquid crystal polarization rotator 317 are not applied with voltage and the second linearly polarized light 105 exits the second region 147, the liquid crystal polarization rotation is performed. Since the element 317 functions as a half-wave plate, the azimuth angle is rotated by 90 ° and becomes P-polarized light.
[0059]
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 further passes through the second polarization separator 123 without changing its direction. To P-polarized light. Therefore, at the output D, the first linearly polarized light 103 that has propagated through the first region 145 and becomes S-polarized light and the second linearly polarized light 105 that has propagated through the second region 145 and becomes P-polarized light are combined. The first output light 113 is emitted.
Although not shown, the first output light 113 is coupled to an optical fiber via a collimator lens as necessary.
[0060]
Next, consider the second input light 107 incident on the input B.
The second input light 107 is divided into a third linearly polarized light 109 that is P-polarized light and a fourth linearly polarized light 111 that is S-polarized light with respect to the first polarization separator 121. The third linearly polarized light 109 that has entered the first polarization separator 121 passes through the first polarization separator 121 because it is P-polarized light, and is turned at a right angle by the first total reflection mirror 125 to rotate the liquid crystal polarization. Proceed through the liquid crystal polarization rotator 317 according to a second optical path 143 passing through the second region 147 of the child 317. In the third linearly polarized light 109, no voltage is applied to the first liquid crystal cell 301 and the second liquid crystal cell 303 constituting the liquid crystal polarization rotator 317, and although not shown in FIG. Since the angle formed with the azimuth angle of the liquid crystal director on the incident side of the liquid crystal cell 301 is 45 °, when exiting the second region 147, the azimuth angle is rotated by 90 ° to become S-polarized light.
The third linearly polarized light 109 that has become S-polarized light is turned to a right angle by the second total reflection mirror 127, and is incident on the second polarization separator 123. However, since it is S-polarized light, the second polarization separator 123 changes the traveling direction to a right angle and emits it from the output C.
[0061]
On the other hand, the fourth linearly polarized light 111 that has entered the input B is S-polarized light, and is turned at a right angle by the first polarization separator 121, and remains as S-polarized light in the first region 145 of the liquid crystal polarization rotator 317. Propagates through the polarization rotator 317 according to a first optical path 141 passing through. The fourth linearly polarized light 111 is the liquid crystal polarization rotation when the first liquid crystal cell 301 and the second liquid crystal cell 303 constituting the liquid crystal polarization rotator 317 are applied with no voltage, and are emitted from the first region 145. Since the element 317 functions as a half-wave plate, the azimuth angle is rotated by 90 ° and becomes P-polarized light. The fourth linearly polarized light 111 emitted from the first region 145 passes through the second polarization separator 123 without changing its direction, and thus reaches the output C as P-polarized light.
[0062]
Therefore, at the output C, the third linearly polarized light 109 propagating through the second region 147 and becoming S-polarized light and the fourth linearly polarized light 111 propagating through the first region 145 and converted into P-polarized light are combined. The light is emitted as the second output light 115. Although not shown, the second output light 115 is coupled to an optical fiber via a collimator lens as necessary.
[0063]
Next, the first switch 135 of the first signal source 131 and the second switch 137 of the second signal source 133 are turned on, and the first liquid crystal cell 301 constituting the liquid crystal polarization rotator 317 and the second switch 137 are turned on. A case where a sufficiently large voltage is applied to the second liquid crystal cell 303 and the liquid crystal polarization rotator 317 does not rotate the polarization will be described with reference to FIG. In FIG. 5, the same components as those in FIG.
[0064]
First, consider that the first input light 101 enters the input A.
The first input light 101 is considered by dividing it into a first linearly polarized light 103 that is P-polarized light and a second linearly polarized light 105 that is S-polarized light for the first polarization separator 121. The first linearly polarized light 103 that has entered the first polarization separator 121 passes through the first polarization separator 121 because it is P-polarized light, and passes through the first region 145 of the liquid crystal polarization rotator 317. Then proceed through the liquid crystal polarization rotator 317 according to 141.
In the first linearly polarized light 103, the first liquid crystal cell 301 and the second liquid crystal cell 303 constituting the liquid crystal polarization rotator 317 are in a voltage application state, and the azimuth angle does not rotate in the liquid crystal polarization rotator 317. When the light is emitted from the first region 145, it remains P-polarized light.
[0065]
Therefore, the first linearly polarized light 103 passes through the second polarization separator 123 as it is and is emitted from the output C. On the other hand, the second linearly polarized light 105 entering the input A is changed to a right angle by the first polarization separator 121 because it is an S-polarized light, enters the first total reflection mirror 125, and further turns to a right angle. And the light is propagated through the liquid crystal polarization rotator 317 according to the second optical path 143 passing through the second region 147 of the liquid crystal polarization rotator 317 while maintaining the S polarization.
The second linearly polarized light 105 has a polarization state even when the first liquid crystal cell 301 and the second liquid crystal cell 303 constituting the liquid crystal polarization rotator 317 are in a voltage application state and are emitted from the second region 147. The S-polarized light remains unchanged.
[0066]
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 and further turned at a right angle by the second polarization separator 123, and the output C To S-polarized light.
Therefore, at the output C, the first linearly polarized light 103 propagating through the first region 145 and being P-polarized light and the second linearly polarized light 105 propagating through the second region 147 and being S-polarized light are combined. 2 is emitted as output light 115. Although not shown, the second output light 115 is coupled to an optical fiber via a collimator lens as necessary.
[0067]
Next, consider the second input light 107 incident on the input B. The second input light 107 is divided into a third linearly polarized light 109 that is P-polarized light and a fourth linearly polarized light 111 that is S-polarized light with respect to the first polarization separator 121.
[0068]
The third linearly polarized light 109 that has entered the first polarization separator 121 passes through the first polarization separator 121 because it is P-polarized light, and is turned at a right angle by the first total reflection mirror 125 to rotate the liquid crystal polarization. Proceed through the liquid crystal polarization rotator 317 according to a second optical path 143 passing through the second region 147 of the child 317.
In the third linearly polarized light 109, the first liquid crystal cell 301 and the second liquid crystal cell 303 constituting the liquid crystal polarization rotator 317 are in a voltage application state, and the polarization rotation function is lost. P-polarized light is maintained even when 147 is emitted. The third linearly polarized light 109 emitted from the second region 147 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 exits from the output D.
[0069]
On the other hand, since the fourth linearly polarized light 111 entering the input B is S-polarized light, the first polarization separator 121 changes the direction at a right angle so that the first region 145 of the liquid crystal polarization rotator 317 remains the S-polarized light. It propagates through the liquid crystal polarization rotator 317 according to the first optical path 141 that passes.
The fourth linearly polarized light 111 is in the first region because the first liquid crystal cell 301 and the second liquid crystal cell 303 constituting the liquid crystal polarization rotator 317 are in a voltage application state and the polarization rotation function is lost. S-polarized light is maintained even when 145 is emitted. The fourth linearly polarized light 111 emitted from the first region 145 is turned at a right angle by the second polarization separator 123 and reaches the output D as S-polarized light.
[0070]
Accordingly, in the output D, the first output obtained by combining the P-polarized third linearly polarized light 109 propagated through the second region 147 and the S-polarized fourth linearly polarized light 111 propagated through the first region 145. The light 113 is emitted.
Although not shown, the first output light 113 is coupled to an optical fiber via a collimator lens as necessary.
[0071]
As is clear from the above description, when the first input light 101 incident on the input A does not apply a voltage to the first liquid crystal cell 301 and the second liquid crystal cell 303 constituting the liquid crystal polarization rotator 317, The light is emitted from the output D as the first output light 113.
Further, the first input light 101 incident on the input A is applied with a 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 rotates the polarization. When the function is lost, the second output light 115 is emitted from the output C. That is, the first input light 101 becomes output light from the outputs C and D that differ depending on whether or not voltage is applied to the liquid crystal cells 301 and 303.
[0072]
The second input light 107 incident on the input B is the second output light 115 when no voltage is applied to the first liquid crystal cell 301 and the second liquid crystal cell 303 constituting the liquid crystal polarization rotator 317. As shown in FIG.
[0073]
Further, the second input light 107 incident on the input B is applied with a 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 polarized. When the rotation function is lost, the second output light 115 is emitted from the output C.
That is, the second input light 107 is output light from the same output C and output D regardless of whether or not voltage is applied to the liquid crystal cells 301 and 303.
[0074]
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.
Needless to say, if only one of the first input light 101 and the second input light 107 shown in FIG. 1 is used, it can be used as a 1 × 2 optical switch.
[0075]
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, the case where the liquid crystal polarization rotator 317 rotates polarized light without applying a voltage to the first liquid crystal cell 301 and the second liquid crystal cell 303 constituting the liquid crystal polarization rotator 317 will be described with reference to FIG. In FIG. 7, the same components as those in FIG.
[0076]
First, consider that the first input light 101 enters the input A. Although not clearly shown in FIG. 7, the first input light 101 and the second input light 107 to be described later may be light that is collimated from the light emitted from the polarization maintaining optical fiber.
The first input light 101 is polarized so as to consist only of the first linearly polarized light 103 which is P-polarized light with respect to the first polarization separator 121. The first linearly polarized light 103 that has entered the first polarization separator 121 passes through the first polarization separator 121 because it is P-polarized light, and passes through the first region 145 of the liquid crystal polarization rotator 317. Follow the 141 in the liquid crystal polarization rotator 317. In the first linearly polarized light 103, no voltage is applied to the first liquid crystal cell 301 and the second liquid crystal cell 303 constituting the liquid crystal polarization rotator 317, and although not shown in FIG. Since the angle formed with the azimuth angle of the liquid crystal director on the incident side of the first liquid crystal cell 301 is 45 °, when the first region 145 is emitted, the azimuth angle is rotated by 90 ° to become S-polarized light.
[0077]
The first linearly polarized light 103 that has become S-polarized light, which is the first input light 101, is output from the output D as the first output light 113 with the second polarization separator 123 changing the traveling direction to a right angle.
Although not shown, the first output light 113 is coupled to an optical fiber via a collimator lens as necessary.
[0078]
Next, consider the second input light 107 incident on the input B. It is assumed that the second input light 107 is the fourth linearly polarized light 111 having an azimuth angle that is S-polarized light with respect to the first polarization separator 121. The fourth linearly polarized light 111 that has entered the input B is changed to a right angle by the first polarization separator 121 because it is S-polarized light, and passes through the first region 145 of the liquid crystal polarization rotator 317 while remaining S-polarized. It propagates through the liquid crystal polarization rotator 317 according to the first optical path 141.
[0079]
The fourth linearly polarized light 111 is the liquid crystal polarization rotation when the first liquid crystal cell 301 and the second liquid crystal cell 303 constituting the liquid crystal polarization rotator 317 are applied with no voltage, and are emitted from the first region 145. Since the element 317 functions as a half-wave plate, the azimuth angle is rotated by 90 ° and becomes P-polarized light. The fourth linearly polarized light 111 that is the second input light 107 emitted from the first region 145 passes through the second polarization separator 123 without changing its direction, and thus 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 to an optical fiber via a collimator lens as necessary.
[0080]
Here, a predetermined drive voltage is applied to the first liquid crystal cell 301 and the second liquid crystal cell 303 constituting the liquid crystal polarization rotator 317 by the first signal source 131 and the second signal source 133, respectively. Consider a case in which the switch 135 and the second switch 137 are applied.
In this case, the liquid crystal polarization rotator 317 disappears from the action of rotating the azimuth angle of incident polarized light by 90 °. Accordingly, the first input light 101 is emitted as the second output light 115 at the output C, and the second input light 107 is emitted as the first output light 113 at the output D.
[0081]
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 that can select an output destination by applying or not applying a voltage to the liquid crystal polarization rotator 317.
Further, only one of the first input light 101 and the second input light 107 shown in FIG. 7 is used as P-polarized light or S-polarized light in the first region 145 without using the first polarization separator 121. Needless to say, it can be used as a 1 × 2 optical switch.
[0082]
Next, a configuration and 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. For example, assume that only the first input light 101 enters the input A in FIG. 1, and only the first output light 113 emitted from the output D is considered as an output.
The first switch 135 and the second switch 137 are short-circuited, and the first signal source 131 is connected to the first liquid crystal cell 301 and the second signal source 133 is connected to the second liquid crystal cell 303. To do. At this time, the effective voltage applied to the liquid crystal cells 301 and 303 is changed in an analog manner by changing the amplitudes of the outputs of the first signal source 131 and the second signal source 133 or changing the pulse width. Think.
[0083]
At this time, the light emitted from the first region 145 and the second region 147 of the liquid crystal polarization rotator 317 has an analog function of rotating the polarization according to the modulation applied voltage of the liquid crystal cells 301 and 303. In general, it becomes elliptically polarized light.
For this reason, since the first output light 113 emitted to the output D is subjected to intensity modulation, it is continuously output according to the effective values of the outputs of the first signal source 131 and the second signal source 133. The light intensity can be controlled. Therefore, the polarization control type liquid crystal optical switch 320 can be used as a variable optical attenuator.
[0084]
A configuration in the case of arraying polarization control type optical switches using the liquid crystal of the present invention will be described with reference to FIG. FIG. 6 is a diagram showing a configuration of a polarization control type liquid crystal optical switch using an arrayed liquid crystal cell.
[0085]
In the case of forming an array, a liquid crystal polarization rotator 317 composed of a plurality of cells is divided into the number of arrays in a plane. Although not shown in FIG. 6, the dividing method is realized by dividing the transparent electrode of the liquid crystal cell constituting the liquid crystal polarization rotator 317 into predetermined regions within the electrode plane.
FIG. 6 shows a case where the liquid crystal polarization rotator 317 is divided into two parts, a first liquid crystal polarization rotator unit 621 and a second liquid crystal polarization rotator unit 623.
[0086]
As shown in FIG. 6, the first composite prism unit 631 includes a first polarization separator 121 and a first total reflection mirror 125, and the second composite prism unit 633 includes a second polarization separator 123. The second total reflection mirror 127 is used.
The arrayed polarization control type optical switch is configured by disposing a liquid crystal polarization rotator 317 having the dividing element portion between a first composite prism portion 631 and a second composite prism portion 633. FIG. 6 shows an arrayed element in which two 2 × 2 elements are arranged.
[0087]
In the first liquid crystal polarization rotation element unit 621, incident light from the first A input 601 and the first B input 603 is controlled and emitted from the first C output 611 and the first D output 613.
The second liquid crystal polarization rotator unit 623 controls incident light from the second A input 605 and the second B input 607 so as to be emitted from the second C output 615 and the second D output 617. .
[0088]
In FIG. 6, for the sake of simplicity, the case of a one-dimensional two-array configuration in which the liquid crystal polarization rotator 317 is divided into two parts is shown. However, since the array can be expanded in a two-dimensional plane by a similar method, a one-dimensional two-array It goes without saying that it is not limited to.
[0089]
【The invention's effect】
As is apparent from the above description, according to the liquid crystal optical switch and the driving method thereof of the present invention, a high-performance optical switch that can be used as a variable optical attenuator with a simple configuration and a simple driving method can be realized.
[Brief description of the drawings]
FIG. 1 is a drawing schematically showing a configuration of a liquid crystal optical switch in an embodiment of the present invention.
FIG. 2 is a drawing schematically showing the structure of a liquid crystal cell constituting a liquid crystal polarization rotator of a liquid crystal optical switch in an embodiment of the present invention.
FIG. 3 is a drawing schematically showing the structure of a liquid crystal polarization rotator of a liquid crystal optical switch in an embodiment of the present invention.
FIG. 4 is a plan view for explaining the azimuth angle of a liquid crystal director at each substrate interface of a liquid crystal cell constituting a liquid crystal polarization rotator of a liquid crystal optical switch according to an embodiment of the present invention.
FIG. 5 is a drawing schematically showing a configuration in which a signal source is connected to a liquid crystal polarization rotator of a liquid crystal optical switch in an embodiment of the present invention.
FIG. 6 is a drawing schematically showing a configuration when liquid crystal optical switches according to an embodiment of the present invention are arrayed.
FIG. 7 is a drawing schematically showing a configuration in an embodiment when incident light of a polarization control type optical switch using a liquid crystal of the present invention can be limited to linearly polarized light.
FIG. 8 is a graph showing the wavelength dependence of isolation characteristics when a TN type is used for a liquid crystal cell used for a polarization control type optical switch and when a half-wave plate type is used.
[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
135: First switch 137: Second switch
141: First optical path 143: Second optical path
145: First area 147: Second area
201: Nematic liquid crystal layer 203: First substrate
205: Second substrate 207: Director
209: Pretilt angle 211: Signal electrode
213: Common electrode 215: Non-reflective coating
217: Alignment layer 220: Liquid crystal cell
301: First liquid crystal cell 303: Second liquid crystal cell
305: First liquid crystal layer 307: Second liquid crystal layer
309: First interface 311: Second interface
313: Third interface 315: Fourth interface
317: Liquid crystal polarization rotator 319: First glass substrate
320: Polarization control type liquid crystal optical switch 321: Second glass substrate
323: Third glass substrate 325: Fourth glass substrate
401: First liquid crystal director 403: Second liquid crystal director
405: Third liquid crystal director 407: Fourth liquid crystal director
410: incident linearly polarized light 413: outgoing linearly polarized light
601: 1A input 603: 1B input
605: 2nd A input 607: 2nd B input
611: 1C output 613: 1D output
615: 2C output 617: 2D output
621: First liquid crystal polarization rotation element section
623: Second liquid crystal polarization rotation element unit
631: First composite prism portion 633: Second composite prism portion
Characteristics of 90 ° TN polarization rotator fabricated under 801: 1st minimum conditions
803: Liquid crystal polarization rotator characteristics produced under half-wave plate conditions
810: TN type -30dB range
813: -30 dB range of half-wave plate type

Claims (4)

In liquid crystal optical switches using liquid crystals,
The liquid crystal optical switch has a liquid crystal polarization rotator comprising a plurality of liquid crystal cells of anti-parallel or parallel orientation,
The ratio of the cell thickness of each liquid crystal cell and the retardation of the liquid crystal layer of each liquid crystal cell is substantially equal,
The light emission side substrate and the light incident side substrate face each other between the liquid crystal cells ,
When linearly polarized incident light with an azimuth angle β1 = 0 radians inclined by −π / 4 with respect to the liquid crystal director at the interface of the light incident side substrate enters the liquid crystal polarization rotator, the light emission The output light is linearly polarized light that vibrates at an azimuth angle β2 = π / 2 radians inclined by π / 4 with respect to the liquid crystal director at the interface of the side substrate,
The liquid crystal optical switch, wherein the incident light is controlled depending on whether or not a voltage is applied to the liquid crystal polarization rotator .   2. The liquid crystal optical switch according to claim 1, wherein the liquid crystal polarization rotator is a half-wave plate having a predetermined wavelength. Two liquid crystal cells;
The liquid crystal optical switch according to claim 1 or 2, wherein the retardation of the liquid crystal layer of each liquid crystal cell is approximately a quarter wavelength. A method for driving a liquid crystal optical switch according to any one of claims 1 to 3 ,
A method of driving a liquid crystal optical switch, characterized in that an effective value of a predetermined drive waveform applied to the liquid crystal cell is analog-modulated to continuously control the amount of polarization rotation and the ellipticity of incident light.

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