JP2000356763A - Liquid crystal optical switch - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the deterioration of important characteristics such as an extinction ratio or cross talk and to attain a speed up of the response speed in switching of optical paths. SOLUTION: The liquid crystal optical switch is provided with a polarized light separation means 4 on which incident light L1 is made incident, a liquid crystal cell 1 on which both P polarized light and S polarized light, separated in the polarized light separation means 4, are made incident and a polarized light composition means 5 which combines the P polarized light and S polarized light into outgoing light L2 and emits it to the outside. The liquid crystal cell 1 is constituted so that two 45 deg. twisted nematic liquid crystal cells 2, 3, of which the sum total of cell gaps is equal to the cell gap of a 90 deg. twisted nematic liquid crystal cell are laminated so that rubbing directions c, d of their mutually adjacent insides 2a, 3a coincide with each other or be placed opposite to each other by 180 deg., and the voltages applied to the two 45 deg. twisted nematic liquid crystal cells are simultaneously turned on and off.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal optical switch used for optical information communication, for example, for switching an optical path by utilizing a change in the arrangement state of liquid crystal molecules.

[0002]

2. Description of the Related Art FIGS. 4 and 5 show a conventional liquid crystal optical switch A1. This liquid crystal optical switch A1 is subjected to a rubbing treatment between a polarization separating means 4 comprising a polarizing beam splitter and a polarization synthesizing means 5 so that the twist angle of a nematic liquid crystal having a positive dielectric anisotropy becomes 90 °. The liquid crystal cell 10 having a gap optimized for the wavelength band to be used is arranged and roughly configured. The OFF state and the ON state of the applied voltage of the liquid crystal cell 10 are shown in FIGS. 4 and 5, respectively. The applied voltage of the liquid crystal cell 10 is determined by the switch 1
1 provides an OFF state and an ON state.

According to the liquid crystal optical switch A1, the incident light L1 emitted from the fiber collimator 6 (or 7)
The (infrared wavelength light) is separated by the polarization separation means 4 into S-polarized light S and P-polarized light P whose optical axis directions are perpendicular to each other, and enters the liquid crystal cell 10. The incident light rotates the respective optical axes of the S-polarized light S and the P-polarized light P to the left by 90 ° by turning ON (FIG. 5) and OFF (FIG. 4) the voltage to the liquid crystal cell 10 so that the S-polarized light S To P-polarized light P and P-polarized light P to S-polarized light S
(FIG. 4) or pass as it is (FIG. 5).

As described above, when the voltage is applied to the liquid crystal cell 10,
By controlling the presence or absence of rotation of the optical axis by turning OFF, the output light L2 multiplexed by the polarization combining means 5 again.
Can arbitrarily switch the emission direction to the fiber collimator 8 (FIG. 4) or 9 (FIG. 5). FIG.
5, in FIG. 5, reference numerals 4a and 4b denote polarization separating means 4.
Reference numerals 5a and 5b denote emission portions of the polarization combining means 5.

In the liquid crystal optical switch A1, the optical path switching speed is determined by the rise time (Tr) from when a voltage is applied to the rise of liquid crystal molecules, or when the applied voltage is O.
It is expressed as a fall time (Td) until the liquid crystal molecules are restored to the original state by setting it to FF. As shown in FIG. 6, the rising time Tr and the falling time Td are obtained by applying a voltage when the amount of transmitted light T (%) is taken on the vertical axis with respect to the time t (ms) on the horizontal axis as shown in FIG. The transmitted light amount is 10% to 9
The time required to change to 0% and the time required to change from 90% to 10% are shown.

In the conventional liquid crystal optical switch A1, 1.55
A cell with a gap of about 11 μm optimized in the case of using a liquid crystal having a μm wavelength band light and a refractive index Δn = 0.12 has a rise time Tr of about 10 ms and a fall time Td of about 50.
ms.

[0007] Incidentally, Japanese Patent Application Laid-Open No. 3-148639 discloses that "a wavelength of 1.
The response speed was measured by connecting a 3 μm semiconductor laser to a single mode optical fiber, and the result was 5 to 10 ms. ”(Publication, page 4, lower right column, lines 12 to 15). According to this description, the liquid crystal light changeover switch described in the publication has a cell gap estimated to be 6 to 9 μm, and a rise time Tr of 5 to 10 ms as described in the publication.
The fall time Td is 16 to 3 from the value of the cell gap.
It is estimated to be 5 ms.

[0008]

However, in the conventional liquid crystal optical switch A1, the rise time Tr,
And it is difficult to shorten the fall time Td, so that it is not possible to achieve a high response speed at the time of optical path switching.
It has the problem of saying.

That is, when examining the response speed of the conventional 90 ° twisted nematic liquid crystal cell 10, it is necessary to consider the following known characteristics.

The following factors are known as factors relating to the response speed.

Liquid crystal layer thickness (cell gap): d ... Tr∝
d ² , Td∝d ² Applied voltage: E… Tr∝E ² Liquid crystal viscosity: η… Tr∝η, Td∝η Spiral pitch: P… d / p Temperature dependence As shown by the above equation, the response speed is increased. Considering the improvement of the liquid crystal layer thickness (cell gap) d which is the most effective and can increase the speed without changing the liquid crystal agent itself.

With respect to the gap d and the transmissivity of a 90 ° twisted nematic liquid crystal cell under crossed Nicols, characteristics such as the following expression 1 are known, and if these are not taken into consideration, sufficient characteristics as an optical switch can be obtained. Cannot be obtained.

[0013]

[Number 1] ^{T = 1/2 · Sin 2} (π / 2√ (1 + U 2)) / (1 + U 2) ...... ( Equation 1) U = 2dΔn / λ Δn = n 0 -n e, n 0: ordinary light Refractive index, _ne : extraordinary light refractive index Here, T (transmittance) is a value directly related to the extinction ratio and crosstalk which are important characteristics of the liquid crystal optical switch. This value is expressed as a function of U in Equation 1. When the same liquid crystal agent is used, the value of U is constant because the refractive index and the wavelength of the light source used are constant. As a function of That is, T (transmittance) can be expressed as a function of the gap d. The optimum gap d as a characteristic such as the response time, the extinction ratio, or the crosstalk of the liquid crystal cell is obtained by substituting an arbitrary numerical value into Equation 1. Obtained
It is the first minimum position (1st.min) in the curve shown in FIG. Therefore, the value of the gap d cannot be reduced under this condition.

From equation (1), it is possible to reduce the gap d by using a liquid crystal agent having a large refractive index Δn, but the effect is small.

Therefore, it is difficult to increase the response speed of the conventional liquid crystal optical switch A1 using the 90 ° twisted nematic liquid crystal cell.

Accordingly, an object of the present invention is to provide a liquid crystal optical switch capable of achieving a high response speed at the time of optical path switching without deteriorating important characteristics such as an extinction ratio and crosstalk. I have.

[0017]

According to a first aspect of the present invention, there is provided a light emitting device, comprising:
Polarization separating means for separating polarized light and s-polarized light, a liquid crystal cell on which both p-polarized light and s-polarized light separated by the polarized light separating means are incident, and both p-polarized light and s-polarized light passing through the liquid crystal cell A polarization synthesizing means for multiplexing and outputting the multiplexed light to the outside;
In the liquid crystal optical switch for switching the light emission portion of the polarization combining means, the liquid crystal cell has a total cell gap substantially equal to a cell gap optimized for a wavelength band used in a 90 ° twisted nematic liquid crystal cell. The two 45 ° twisted nematic liquid crystal cells are stacked so that the inner rubbing directions adjacent to each other are the same or 180 ° opposite to each other, and the voltage applied to the two 45 ° twisted nematic liquid crystal cells is It is characterized in that ON and OFF are performed simultaneously.

Therefore, according to the first aspect of the present invention, the two stacked 45 ° twisted nematic liquid crystal cells can exhibit the same optical axis rotation function as the conventional 90 ° twisted nematic liquid crystal cell. That is, in the two stacked 45 ° twisted nematic liquid crystal cells, the total thickness of the two liquid crystal layers is not changed, the cell gap per cell is halved, and the optical axis is set at 45 ° for each cell. The rotation function can be shared. At this time, the rise time Tr and the fall time Td at the time of switching the optical path are:
Since it is proportional to the square of the cell gap, halving the cell gap results in a quarter of the cell gap, whereby a higher response speed can be achieved.

The total thickness of the liquid crystal layer (total cell gap) of the stacked liquid crystal cells is set to 90 °
Since the gap is designed to be equal to that of the twisted nematic liquid crystal cell, characteristics such as extinction ratio and crosstalk can be maintained as before.

The invention described in claim 2 is the same as the invention described in claim 1.
Wherein the two 45 ° twisted nematic liquid crystal cells are each configured to have a cell gap of １ ／ of a 90 ° twisted nematic liquid crystal cell.

For this reason, in the second aspect of the invention, the cell gap between the two 45 ° twisted nematic liquid crystal cells is 1: 1, that is, the rotation angle per cell is 45 °. At this time, the rise time Tr and the fall time Td at the time of switching the optical path are reduced to 1/4 since the cell gap is reduced to 1/2, thereby achieving a higher response speed.

[0022]

Embodiments of the present invention will be described below with reference to the drawings. 4 and 5 are denoted by the same reference numerals.

FIG. 1 shows a liquid crystal optical switch A according to an embodiment of the present invention. The liquid crystal optical switch A includes a polarization separating unit 4 that separates light incident from outside (incident light L1) into a P-polarized light P and an S-polarized light S, and the P-polarized light P and the S-polarized light separated by the polarized light separating unit 4. Liquid crystal cell 1 to which S is incident
And polarization combining means 5 for causing the P-polarized light P and the S-polarized light S that have passed through the liquid crystal cell 1 to be both incident and multiplexed, and emitting the emitted light L2 to the outside. ON and OFF, the output light L of the polarization combining means 5
It is configured to switch the two emission points.

At this time, the polarization splitting means 4 and the polarization synthesizing means 5 are each constituted by a polarization beam splitter having two incident parts 4a, 4b and emission parts 5a, 5b. Fiber collimators 8 and 9 are provided at the entrances 4a and 4b, respectively.
a and 5b, respectively.

The liquid crystal cell 1 has a total cell gap equivalent to a cell gap optimized for a used wavelength band of a 90 ° twisted nematic liquid crystal cell (corresponding to the liquid crystal cell 10 in FIGS. 4 and 5). The two 45 ° twisted nematic liquid crystal cells 2 and 3 are connected to the inner 2 adjacent to each other.
The layers are laminated so that the rubbing directions of a and 3a are the same or opposite by 180 °, and the voltages applied to the two 45 ° twisted nematic liquid crystal cells 2 and 3 are simultaneously turned ON and OFF. .

In FIG. 2, two 45 ° twisted nematic liquid crystal cells 2 and 3 are stacked so that the rubbing directions c and d of the inner sides 2a and 3a adjacent to each other are opposite by 180 °. FIG. Note that FIG.
, The symbols b and e indicate the rubbing directions on the outside of each of the two 45 ° twisted nematic liquid crystal cells 2 and 3, and the symbols a and f indicate the P and S polarizations of the incident light L1 and the emitted light L2, respectively. 3 shows the optical axis.

In order to form the two 45 ° twisted nematic liquid crystal cells 2 and 3 in a laminated state, an adhesive (refractive index) having a refractive index close to the refractive index of the glass substrate used for each of the liquid crystal cells 2 and 3 is used. The liquid crystal cells 2 and 3 are fixed to each other by fixing them with a rate matching adhesive) or making the inner sides 2a and 3a provided with an anti-reflection coating designed for the wavelength band to be used face each other.

In this embodiment, the applied voltage of the two 45 ° twisted nematic liquid crystal cells 2 and 3 can be simultaneously turned off or on by the switch 11.

More specifically, the 45 ° twisted nematic liquid crystal cells 2 and 3 are the same as those used for the 90 ° twisted nematic liquid crystal cell in the cells rubbed so that the twist angle of the liquid crystal becomes 45 °. Is formed by injecting levorotatory liquid crystal. As described above, the two 45 ° twisted nematic liquid crystal cells 2 and 3 have the rubbing directions coincident with each other on the inside 2a and 3a of the two cells.
Laminate so as to be 0 ° opposite. As a result, the conventional 90
° The optical axis rotation function similar to that of the twisted nematic liquid crystal cell can be provided.

That is, the total thickness of the liquid crystal layers of the two 45 ° twisted nematic liquid crystal cells 2 and 3 is not changed.
The cell gap per cell is halved, and the liquid crystal cells 2 and 3 are assigned an optical axis rotation function by 45 °. At this time, the distribution ratio of the rotation angle (gap) of the two liquid crystal cells 2 and 3 is set to the inside 2a and 3 of the two liquid crystal cells 2 and 3.
The rubbing directions c and d can be arbitrarily changed if they are overlapped so that the rubbing directions c and d coincide with each other. However, since the response speed follows the thicker cell gap, the response speed is 1: 1.
It is desirable that the rotation angle per sheet be 45 °. In this case, two 45 ° twisted nematic liquid crystal cells 2 and 3
Are configured to have a cell gap of の of a 90 ° twisted nematic liquid crystal cell having a cell gap optimized for each wavelength band used. According to this configuration, since the rise time Tr and the fall time Td are in proportion to the square of the cell gap as described above, they become 1/4 times, thereby making it possible to increase the response speed.

The total liquid crystal layer thickness (total cell gap) of the stacked liquid crystal cells 2 and 3 is set to 90 °.
Since the gap is designed to be equal to that of the twisted nematic liquid crystal cell, characteristics such as extinction ratio and crosstalk can be maintained as before.

In the OFF state of the applied voltage of the liquid crystal cell 1 of the liquid crystal optical switch A thus configured, the infrared wavelength light (incident light L1) emitted from the fiber collimator 6 is polarized and separated from the incident portion 4a. The light enters the means 4 and is separated into P and S polarized lights by the polarized light separating means 4. Next, the rubbing direction b of the incident side liquid crystal cell is set to be parallel or perpendicular to the optical axis direction a with respect to the optical axis direction a of the P and S polarized lights.
5 ° twist nematic liquid crystal cell 2 is arranged,
P, whose optical axis has been rotated 45 ° to the left by this liquid crystal cell 2,
The S-polarized light is further rotated left by 45 ° by the 45 ° twisted nematic liquid crystal cell 3 arranged so that the rubbing direction d coincides with the optical axis direction.

The P and S polarized lights whose optical axes have been rotated by 90 ° in total become the optical axis in the direction f, enter the polarization combining means 5, are multiplexed by the polarization combining means 5, and are emitted from the output light L.
The light is emitted from the emission part 5a as 2 and the fiber collimator 8
Incident on.

In the ON state of the applied voltage of the liquid crystal cell 1 of the liquid crystal optical switch A, the P and S polarized lights remain in the optical axis in the direction a without rotating the optical axis in the liquid crystal cell 1. The light passes through the liquid crystal cell 1 and is incident on the polarization synthesizing means 5, is multiplexed by the polarization synthesizing means 5, exits from the emission section 5 b as emission light L 3, and enters the fiber collimator 9.

Similarly, although not shown, incident light emitted from the fiber collimator 7 is transmitted from the incident portion 4b to the polarization separating means 4
, And then enter the polarization combining means 5 through the liquid crystal cell 1 and are multiplexed by the polarization combining means 5 and emitted as outgoing light. The emitted light has an applied voltage of the liquid crystal cell 1 of O
When the liquid crystal cell 1 is in the FF state, the light is emitted from the emission part 5b of the polarization combining means 5 and is incident on the fiber collimator 9;
When the applied voltage is in the ON state, the light is emitted from the emission section 5a of the polarization combining means 5 and enters the fiber collimator 8.

As described above, the liquid crystal optical switch A is arbitrarily applied to the fiber collimator 8 or 9 by simultaneously turning on and off the application of voltage to the two 45 ° twisted nematic liquid crystal cells 2 and 3 constituting the liquid crystal cell 1. The emission direction (emission location) can be switched in the following manner.

FIG. 3 shows a liquid crystal optical switch A obtained in the experiment.
The performance of a liquid crystal optical switch (comparative product) using (a product of the present invention) and a 90 ° twisted nematic liquid crystal cell is shown.

The liquid crystal used in this experiment was MLC-6019-00.
0 (S009) (Merck), Δn = 0.12 (1.
55 μm wavelength band conversion value) and 9
A fiber spacer of 11.7 μm was used for a 0 ° twisted nematic liquid crystal cell, and a 5.5 μm fiber spacer was used for a 45 ° twisted nematic two-layer liquid crystal cell of the present invention.

As is apparent from FIG. 3, the product of the present invention has almost the same values of the extinction ratio and the crosstalk as the comparative product.
As can be seen from the comparison between the rise time Tr and the fall time Td, the response speed of the product of the present invention is about four times that of the comparative product.

Incidentally, if the present invention is applied to a used wavelength of 1.3 μm in the same manner as in the prior art described in the above-mentioned publication, the rise time Tr becomes 1/4 (1.2 times) of 5 to 10 ms.
5 to 2.5 ms) and the fall time Td is 16 to 35 m
A response speed of 1/4 of s (4 to 8.75 ms) can be obtained.

[0041]

As described in detail above, according to the present invention, the following effects can be obtained.

That is, according to the first aspect of the present invention, in the two stacked 45 ° twisted nematic liquid crystal cells, the total thickness of the two liquid crystal layers is not changed as in the prior art, and the cell 1 Since the cell gap per cell can be halved and the optical axis rotation function can be assigned to each cell by 45 °, the rise time and the fall time at the time of switching the optical path can be shortened, and the response time can be shortened. Higher speed can be achieved, and characteristics such as extinction ratio and crosstalk can be maintained as in a conventional 90 ° twisted nematic liquid crystal cell.

According to the second aspect of the present invention, 2
Each of the 45 ° twisted nematic liquid crystal cells has 9
Since the cell gap is configured to be half of that of the 0 ° twisted nematic liquid crystal cell, the rise time and fall time at the time of switching the optical path are 1/4 times that of the 90 ° twisted nematic liquid crystal cell. Can be quadrupled.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a liquid crystal optical switch as one embodiment of the present invention.

FIG. 2 is a schematic diagram showing a relationship between a rubbing direction of a liquid crystal cell of the liquid crystal optical switch of FIG. 1 and an optical axis of light.

FIG. 3 is a table showing performance data of a product of the present invention and a comparative product obtained by experiments.

FIG. 4 is a schematic view of a conventional liquid crystal optical switch when an applied voltage to a liquid crystal cell is OFF.

FIG. 5 is a schematic diagram of a conventional liquid crystal optical switch when a voltage applied to a liquid crystal cell is ON.

FIG. 6 is a graph showing a rise time Tr and a fall time Td of a conventional liquid crystal optical switch when switching an optical path.

FIG. 7 is a graph showing light transmittance characteristics of a conventional liquid crystal optical switch.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Liquid crystal cell 2,3 45 degree twist nematic liquid crystal cell 2a, 3a Inside (adjacent to each other) 4 Polarization separation means 5 Polarization combining means c, d Rubbing direction (inside adjacent to each other) A Liquid crystal optical switch PP polarization S S polarized light

Continued on the front page F-term (reference) 2H088 EA47 GA02 GA17 HA03 HA06 HA20 JA05 KA02 KA11 MA10 MA11 2H089 HA29 HA30 JA05 KA20 LA08 LA19 QA14 QA16 RA05 SA01 SA07 TA04 TA09 TA20 UA03 UA09 2H091 FA10X FA10Z FA24FA07 FA24X37 KA03 KA04 LA16 LA30 2H093 NA25 ND31 ND32 ND60 NE04 NE06 NF05 NG11 NH01

Claims (2)

[Claims] 1. A polarized light separating means for separating light incident from the outside into P-polarized light and S-polarized light, a liquid crystal cell to which both P-polarized light and S-polarized light separated by the polarized light separating means are incident. A polarization combining means for causing both the P-polarized light and the S-polarized light to enter and multiplexing the combined light and outputting the combined light to the outside; In the liquid crystal optical switch that performs switching, the liquid crystal cell includes two 45 ° twisted nematic liquid crystal cells whose total cell gap is substantially equal to a cell gap optimized for a wavelength band used by a 90 ° twisted nematic liquid crystal cell. And laminating so that the rubbing directions of the inner sides adjacent to each other are the same or 180 ° opposite,
A liquid crystal optical switch characterized in that a voltage applied to the two 45 ° twisted nematic liquid crystal cells is simultaneously turned on and off. 2. The liquid crystal optical switch according to claim 1, wherein each of the two 45 ° twisted nematic liquid crystal cells has a cell gap of １ ／ of a 90 ° twisted nematic liquid crystal cell. A liquid crystal optical switch.