JPS6141120A - Optical switch - Google Patents

Abstract

PURPOSE:To speed up an optical switch and to reduce a cross-talk by applying the prescribed orientation processing to the substrate of a liquid crystal cell, and making polarized light incident with the prescribed inclination. CONSTITUTION:The orientation processing by an oblique deposition of SiOX and rubbing is applied to a substrate having a transparent electrode to form a pi cell 19 obtained by twisting the major axis of a pneumatic liquid crystal molecule at pi/2 angle between two substrates. Polarized light prisms 2 and 3 are symmetrically disposed at right and left sides of the pi cell 19. A drive circuit 20 applies AC voltages VL and VH giving maximum and minimum values of transparency, respectively. P and S polarized light components 16 and 17 are separated from the light inputted from an optical fiber 8 by means of the prism 2 through a rod lens 4, pass through the cell 19 under the impression of the voltage VL (in case of A), are synthesized by the prism 3 and inputted to an optical fiber 10 through a rod lens 6. Under the impression of the VH (in case of B) P and S types of polarized light are alternately converted at incident and emitting sides of the cell 19, synthesized by the prism 3 and inputted to an optical fiber 11 through a rod lens 7.

Description

Detailed Description of the Invention [Technical Field of the Invention] This invention uses a liquid crystal cell as an active medium, and by applying stain pressure from the outside to this liquid crystal cell, one or two
This invention relates to an optical switch that can switch light transmitted through one optical fiber to either of two other optical fibers.

[Prior art]

Conventionally, this type of optical switch is shown in Fig. 1 (5) and (
There was something shown in (b). Figure 1 (G) and CB) are
FIG. 3 is a configuration diagram showing two switching states in a conventional optical switch, respectively. In each figure, 1 is a liquid crystal cell, 2
and 3 are first and second polarizing prisms that are a combination of a polarizing beam splitter and a right-angle prism; 4, 5, and 6;
7Fi person, output lens 1 and second focusing rod lens, 8, 9, 10.11 Mori person, output optical fiber.

In general, liquid crystal has a dielectric constant of 6□1 in the direction parallel to the long axis of the liquid crystal molecules, and a dielectric constant of ε in the perpendicular direction, then ε=
ε0, -1↓ are classified as either positive or negative, and the former is called a P-type liquid crystal, and the latter is called an N-type liquid crystal.

The liquid crystal cell 1 is constructed by inserting a spacer 14 made of a material such as a polymer compound between glass substrates 13 on which a transparent electrode 12 such as an InzOs 8n03 film is deposited, as shown in FIGS. 2(5) and 2(b). However, P-type nematec liquid crystal molecules 15 are sealed within the interval formed by the spacer 14. As shown in FIG. 2 (5), when no voltage is applied to the liquid crystal cell 1, the liquid crystal cell 1
As shown in the figure, by vapor deposition of 8i0x and rubbing with cotton cloth.
This constitutes a so-called twisted nematec cell (TN cell) in which the length 11IILQ alignment direction of the P-type nematec liquid crystal molecules 15 is twisted by π/2 between the two glass substrates 13. As shown in (b), when a voltage is applied between the transparent electrodes 12 of the liquid crystal cell 1, the long axes of the P-type nematic liquid crystal molecules 15 are oriented in the direction of the electric field because it is a P-type liquid crystal.

Now, as shown in the first part and (c) above, the light is made to enter the optical fiber 8 and the first converging rod lens 4 having a length of 0.25 pitch to convert it into parallel light. , this parallel light is converted into P-polarized light component (linearly polarized light component parallel to the plane of incidence of the first polarizing prism 2) by the first polarizing prism 2.
and S-polarized component (linearly polarized component perpendicular to the plane of incidence of the first polarizing prism 20) light 17, of which the S-polarized component light 17 is totally reflected by a right-angle prism, and each is sent to the liquid crystal cell 1. Make it incident vertically. As shown in Figure 1, liquid crystal cell 1
When no voltage is applied to P, as shown in the second part,
Since the long axis of the nematic liquid crystal molecules 15 is twisted by π/2 between the glass substrates 13, when linearly polarized light passes through the liquid crystal 1-1, the plane of polarization of this linearly polarized light is rotated by π/2. , the P-polarized light component 16 is converted into the S-polarized light component 17, and the 8-polarized light component 174-1F is converted into the F-polarized light component 16. When the light whose polarization direction has been converted in this way is again incident on the second polarizing prism 3, the P-polarized light component 16 is transmitted, and the S-polarized light component 17 is reflected.
The two P and S polarized light components 16 and 17 are combined, enter the second focusing rod lens 6, and are focused onto the optical fiber 10 and output.

On the other hand, in the state where a voltage is applied to the liquid crystal cell 1 as shown in FIG. 1(b), as explained in FIG. 1(5) above,
Light 16°17 of each of the P and 8 polarization components is perpendicularly incident on the liquid crystal cell 1, but as shown in FIG. Therefore, it passes through the linearly polarized light as it is. Therefore, both the P-polarized component light 16 and the S-polarized component light 17 pass through the liquid crystal cell 1 and enter the second polarizing prism 3 without changing.
The two P, 8 polarized light components 16 and 17 are combined, enter the second focusing rod lens 7, and are focused onto the optical fiber 11 and output.

As described above, when the voltage applied to the liquid crystal cell IK is turned off, the light propagating through the optical fiber 8 is transmitted to the optical fiber 10.
Moreover, when turned on, each switch is switched to the optical fiber 11. Note that, according to the same operating principle, the light propagating through the optical fiber 9 is transmitted to the optical fiber 11 when the voltage applied to the liquid crystal cell 1 is turned off, and is transmitted to the optical fiber 11 when the voltage applied to the liquid crystal cell 1 is turned on.
0 respectively.

Incidentally, the switching time of the above-mentioned main switch is defined by the response time of the liquid crystal cell 1. Generally, the response time t of the liquid crystal cell 1 is given by the following equation (1).

Here, η is the viscosity of the liquid crystal, K is the elastic constant of the liquid crystal, k is the wave number, the difference in dielectric constant between the major axis and minor axis of the Δε liquid crystal molecules, go is the dielectric constant in vacuum, and ■ is the applied voltage. The voltage and d are the thickness of the liquid crystal cell 1. In the configuration of the TN cell described above, the wave number can be considered to be approximately x/d, so the response time tQ rise time 1° and fall time 1 (when the transmitted light intensity changes from 10% of the steady value) Assuming the time until it reaches 90% and vice versa, each tr and tf are calculated according to the following (2).
The equation becomes equation (3).

6' ・・・・・・・・・(3) tf=
Figure 3 is a characteristic diagram showing the relationship between the response time and the thickness of the liquid crystal cell of the liquid crystal cells in Figures 2 (5) and (b) using the applied voltage as a parameter. , LCD SE/I/
1, for example, when Merck JlIZLI-1557 is used, the relationship between the rise time tr and fall time tl of the response time t and the thickness d of the liquid crystal cell 1 at room temperature T = 20°C is as follows. An example of calculation using the applied voltage ■ as a parameter is shown. As is clear from FIG. 3 and Equations (2) and (3) above, in order to switch the optical switch at high speed, a liquid crystal material with low viscosity and large dielectric anisotropy is used, and the liquid crystal cell 1 It is better to make the thickness thinner.

However, if the thickness of 1 liquid crystal cell/I/1 is made thinner, the twisted nematic effect will decrease and the rpstoke will increase. 1 has a maximum thickness of about 6 μm. In Fig. 3, when the thickness d of the liquid crystal cell A/1 is 6 μm, when the applied voltage ■ is 12 V or more,
Startup time of response time t1. However, the fall time tl is constant, independent of the applied voltage (2), and is about 9 m s. At this time, at the falling edge of the pulse response, there is a delay time td of about 10 m5 required for the liquid crystal molecules to be twisted and reoriented by π/2.

Therefore, the total fall time is 'f + '(1=1
It will be 9m5. However, as shown in Figure 4, the viscosity η of liquid crystal has a strong temperature dependence;
Compared to the case of ZLI-1557 manufactured by Merck & Co., Ltd.
The liquid crystal C/L/1 has about 2.7 times higher viscosity. Ninth, at temperature θ℃, the rise time is 2.7-@s,
, fall) time was 51 m5, -ab, and there was a drawback that it was difficult to realize high-speed switching of the optical switch. In addition, in order to reduce the crosstalk of the optical switch to less than -20 dB, it is necessary to make the thickness of the liquid crystal cell 1 6 μm or more.
When changing from m to 8 μm, the response time is about 1.8 times longer.As a result, it is difficult to improve the crosstalk characteristics and switching characteristics at the same time to improve the performance of the optical switch. there were.

[Summary of the invention]

This invention was made for the purpose of improving the drawbacks of the conventional ones as described above, and by applying a voltage (from an external source) to a liquid crystal cell, light transmitted through one or two optical fibers can be transferred to another optical fiber. In an optical switch that can switch between two optical fibers, the liquid crystal cell is formed by diagonally depositing or rubbing in the same direction on both sides of the glass substrate that encloses the liquid crystal, so that the long axis of the nematic liquid crystal molecules is aligned with the glass substrate. 4. The tubes are oriented so as to form a predetermined chilled angle tube in a reverse tilted state on both sides of the tube. p, when linearly polarized light is incident perpendicularly into a liquid crystal cell, the orientation direction of the nematec liquid crystal molecules and the polarization plane of the linearly polarized light have an inclination of π/4! b-'), generate retardation π on the output side of the liquid crystal cell, and make the polarization plane of the emitted linearly polarized light perpendicular to the polarization plane of the incident linearly polarized light. This provides an optical switch that can simultaneously increase the switching speed of the optical switch and reduce the daily stoke.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 5 is a diagram showing the operating principle of the π cell used in the optical switch of the present invention. As shown in the figure, from two axes z = Q to z =
d) The long axes of the liquid crystal molecules 15 are oriented in a uniform direction with respect to the X-axis, and linearly polarized light 18 whose polarization direction is parallel to the X-axis direction is incident in biaxial directions. At this time, Q
(In z(d, the electric field components in the X-axis and y-axis directions are Ex
, Ey, the following equation (4) is obtained.

However, E@d electric field strength, ω is the angular frequency of light, nsx
m n is the refractive index in the long axis and short axis directions of the liquid crystal molecules 15, respectively. Rewriting the above equation (4) and substituting 1 = π/4, m = for stopping the movement... (6) λ and retardation δ] Straight line As the polarized light 18 propagates in two axial directions, the polarization state changes to elliptical, elliptical, and linear polarization, as shown in FIG. 5, and the direction of linear polarization also changes. change.

Therefore, if the refractive index of the liquid crystal molecules 15 in the long axis and short axis directions (n□, -n) and the thickness d of the liquid crystal cell 1 are set to appropriate values so that The polarization direction of linearly polarized light can be rotated by π/2.

As mentioned above, the liquid crystal cell 1 that produces retardation 4
Therefore, this is called a π cell.

The sixth part is a cross-sectional view showing the configuration of a liquid crystal cell for explaining the alignment state of liquid crystal molecules in the π cell of FIG. As shown in the figure, -kJy is obtained by subjecting the glass substrate 13 to an oblique vapor deposition or rubbing process so as to have a predetermined tilt angle t of several positions, and then forming the liquid crystal cell so that both sides of the glass substrate 130 are in a reverse tilt state. It is composed of For comparison, FIG. 6(b) is a cross-sectional view showing the configuration of a liquid crystal cell for explaining the alignment state of liquid crystal molecules in a conventional TN cell.

Now, as shown in FIG. 6 (5) and (c), when voltage V is applied to each of the π cell and the TN cell, the length of the liquid crystal molecules 15 and In the region away from the axis and the vicinity of the transparent electrode 12, the sixth
Pronation and orientation in the direction of the electric field as shown in (c).
When voltage V is turned off in this state, a restoring force acts in both cells in the directions indicated by the arrows in FIGS. 6) and 6(c). In the TN cell shown in FIG. 6, the glass substrate 13
Since a restoring force acts in the opposite direction to the liquid crystal molecules 15 on both sides, the restoring force is weakened and the orientation state before the voltage V is applied is returned. On the other hand, as shown in Part 6, "In the cell, a restoring force acts on all the liquid crystal molecules 15 in the same direction, so they quickly return to the alignment state before the voltage V was applied. For this reason, the optical switch according to the present invention Compared to conventional optical switches, the switching time can be significantly reduced.

FIG. 7 is a characteristic diagram showing a comparison of response times between the π cell and the TN cell in FIGS. 6) and (b). Figure 7 shows
- As an example, when using the above-mentioned Nematec liquid crystal ZLI-1557 manufactured by Merck & Co., Ltd., a characteristic diagram is shown in which the response time of a TN cell is measured by changing the thickness d of the liquid crystal cell. In the figure, the solid line indicates the π cell, and the broken line indicates the TN- cell. For example, if the thickness d of the liquid crystal cell is 6 μm,
Startup time of response time t1. is approximately 172 times longer for the π cell than for the TN cell, and the falling time Tf is approximately 172 times longer than that for the TN cell. Since there is a delay time of 14 due to twisting, it is necessary to evaluate the fall time in TN-+cell as t, + t4]. Taking this into consideration, π-cell has a delay time of about 14% compared to TN-cell. /10, and high-speed switching of optical switches can be realized.

FIG. 8 is a characteristic diagram showing the relationship between transmitted light intensity and applied voltage in the π cell in the sixth garden. Figure 8 shows a characteristic diagram in which the above-mentioned π cell was placed between a polarizer and an analyzer, and the relationship between transmitted light intensity and applied voltage was measured in the parallel Nicols state (solid line) and the orthogonal Nicols state (dotted chain line). It shows. Note that the thickness d of the liquid crystal cell is 6 μm, and the frequency f of the driving voltage is 10 kHz. As is clear from the figure, the transmitted light intensity changes periodically depending on the applied voltage. In the parallel Nicol state, the maximum value of the transmitted light intensity is as explained in FIG.
In the case of 3.Keyaki.), the minimum value of the transmitted light intensity is the retardation J=(2m+1)π, respectively. Assuming that the voltages that give the maximum and minimum values of the transmitted light intensity are VL and V□, an optical switch can be constructed from K by switching the voltages vL and vH. At this time, the crosstalk corresponds to the difference in transmitted light intensity between the parallel Nicols state and the crossed Nicols state at each voltage vL, vH, and it is shown that the frosttalk can be reduced to -20 dB or less in either case.

#I9 Figures ^ and (c) are configuration diagrams respectively showing two states of an optical switch which is an embodiment of the present invention. In each figure, 19 is a π cell and 20 is a drive circuit. Other configurations indicated by numerals 2 to 11 and 16°17 are as follows:
This is the same as that described in connection with the conventional optical switch shown in FIG. 1 above. 9 shows a state in which an AC voltage of stain pressure vL1 frequency f is applied to the π cell 19 by the drive circuit 20,
FIG. 9(C) shows a state in which an AC voltage of a stain pressure V□ and a frequency f is applied to the drive circuit 20 at π/I/19.

In the optical switch according to the present invention, the first and second deflection prisms 2 and 3t-π cells 19 are arranged symmetrically, and the light input from the optical fiber 8 is parallelized by the first convergent rod lens 4. After being converted into light, the first deflection prism 2 separates the light into a P-polarized light component 16 and an eight-polarized light component 17. The nine linearly polarized lights separated in this way are shown in Fig. 9 (5), and as explained in the characteristic diagram of Fig. 8 above, the retardation δ is 2mπ. Since the polarized light is transmitted without changing its polarization direction, both the P-polarized light and the S-polarized light pass through the π cell 19 as they are, and are combined again in the second deflection prism 3 to form the second focusing Rondo lens 6.
The light is input to the optical fiber 10, and is focused on the optical fiber 10 and output. On the other hand, the one shown in FIG. 9(C) corresponds to the state where the retardation a is (2m+1)x, as explained in the characteristic diagram of FIG. 8 above, and the polarization direction is π/I/19. Since it rotates by π/2 between the side and the output side, P-polarized light is converted to 8-polarized light, and S-polarized light is converted to P-polarized light. Thereafter, the lights are combined by the second polarizing prism 3, input to the focusing rod lens 7 of 1III2, and then focused to the optical fiber 11 and output.

Note that when light is input from the optical fiber 9, the light is input to the optical fiber 11 in the state shown in FIG. 9, and the light 7 is input in the state shown in FIG. The signals are output to the AIPA 10 respectively. Here, in order to reduce power consumption as the drive circuit 20, for example, a C-
An oscillation circuit, a gate circuit, a supply voltage stabilizing circuit, etc. are constructed using MOS, and AC voltages of specified voltages vL and vH are generated according to input on/off control signals.

FIG. 10 is a waveform diagram of each part showing switching characteristics in the optical switch of FIGS. 9(5) and (b).

In each figure, 21 represents the drive circuit 200 Å force voltage waveform,
22 is the drive voltage waveform of the π cell 19, and 23 and 24 are the waveforms when light is input from the optical fiber 8 and output to each party fiber 10 and 11, as shown in FIG. 9 (5) and (B). The waveforms of the transmitted light are shown, and it can be seen that high-speed switching in the optical switch can be realized.

In the above embodiment, a 2×2 matrix optical switch was explained, but the optical switch of the present invention is not limited to this, and can be applied to a 1×2 double-throw optical switch and an on/off optical switch. It goes without saying that jin can be fully applied to cases such as these.

〔Effect of the invention〕

As explained above, this invention allows light transmitted through one or two optical fibers to be switched to either of the other two optical fibers by applying a voltage to the liquid crystal cell from the outside. In the optical switch that can be used, the liquid crystal cell is formed by performing oblique vapor deposition or rubbing in the same direction on both sides of the glass substrate that encloses the liquid crystal, so that the long axes of the nematec liquid crystal molecules are tilted in the opposite direction on both sides of the glass substrate to form a predetermined tilt angle. When linearly polarized light is made perpendicular to the liquid crystal cell, the alignment direction of the nematic liquid crystal molecules and the polarization plane of the linearly polarized light are made to have an inclination of π/4. Retardation π is generated on the output side of the liquid crystal cell so that the polarization plane of the output linearly polarized light is perpendicular to the polarization plane of the input linearly polarized light. In comparison, the switching speed of the optical switch can be increased simply by controlling the applied voltage without increasing the thickness of the liquid crystal panel, and the switching speed can be simultaneously reduced. It is.

[Brief explanation of the drawing]

Figures 1 (5) and (b) are block diagrams showing two switching states in a conventional optical switch, and Figure 2 (5) is a block diagram showing two switching states in a conventional optical switch, respectively.
) and (b) are cross-sectional views showing the structure of the liquid crystal cell in the optical switch shown in FIG. 1 (5) and (b), respectively, and FIG. A characteristic diagram showing the relationship between the response time and the thickness of the liquid crystal cell using the applied voltage as a parameter. Figure 4 shows the temperature dependence of the viscosity of the liquid crystal in the liquid crystal cell shown in Figure 2 (5) and proviso. 5 is a diagram of the operating principle of the π cell used in the optical switch of the present invention, and FIG. 6 (5) shows the configuration of the liquid crystal cell for explaining the alignment state of liquid crystal molecules in the π cell of FIG. 5. 6(B) is a sectional view showing the configuration of a liquid crystal cell for explaining the alignment state of liquid crystal molecules in a conventional TN cell, and FIG. 7 is a sectional view showing the sixth direction and (B). A characteristic diagram showing a comparison of response times between a π cell and a TN cell, FIG.
Figure (5) is a characteristic diagram showing the relationship between the transmitted light intensity and the applied voltage in the π cell, and Figures (5) and (c) respectively show two states of the optical switch which is an embodiment of the present invention. The configuration diagram shown in FIG. 10 is a waveform diagram of each part showing the switching characteristics in the optical switch in the ninth and (b). In the figure, 1...liquid crystal cell, 2 and 3...first and second deflection prisms, 4, 5 and 6.7-...first
and second focusing rod lens% 819F10.11
... Optical fiber, 12 ... Transparent electrode, 13 ... Glass substrate, 14e ... Spacer, 15 ... P-type nematic liquid crystal molecule, 16 ... P-polarized light component, 17 ...
・S-polarized light, 18...Linearly polarized light, 19...
- π cell, 20... Drive circuit. In each figure, the same reference numerals indicate the same or equivalent parts. Surrogate Masao Iwa (B) Fig. 2 (A) (B) Fig. 3 T = 20'C Surface crystal T = Q thickness dp Mechanism Fig. 4 Temperature T ('C) wJ5 Fig. 6 (A) (B) Figure 7 T-20'C Liquid crystal 'C) l/1f) 4J-1-d (P7FL) Figure 8 mark 1XJ voltage ff) (B) 1012th Penalty Ginlock

Claims (1)

[Claims] In an optical switch that switches light input from one or two optical fibers to either of the other two optical fibers in response to an external control signal, the output of the one or two optical fibers A first convergent rod lens is provided at the end of the optical fiber and converts the light input from the optical fiber into parallel light, and the parallel light converted by the first convergent rod lens is converted into P-polarized light and S-polarized light. Oblique vapor deposition or rubbing is performed in the same direction on both sides of a first polarizing prism that separates into linearly polarized light and a glass substrate that encloses liquid crystal, so that the long axes of nematic liquid crystal molecules are set in a reverse tilted state on both sides of the glass substrate. The plane of polarization of the linearly polarized light is incident at an angle of π/4 with respect to the alignment direction of the nematic liquid crystal molecules, and the linearly polarized light is a liquid crystal cell that transmits linearly polarized light while rotating the plane of polarization by π/2; a drive circuit that supplies the liquid crystal cell with a predetermined voltage generated by an input on/off control signal;
A second polarizing prism that recombines the linearly polarized light that has passed through the liquid crystal cell, and a second polarizing prism that is provided at the input end side of the other two optical fibers, and that combines the light that has been combined by the second polarizing prism with the An optical lens comprising a second focusing rod lens that focuses light onto two other optical fibers.