JP3529699B2 - How to drive a liquid crystal retarder - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of driving a liquid crystal retarder.

[0002]

2. Description of the Related Art A retarder is an optical element that changes the polarization state of light by utilizing the birefringence of a substance having optical anisotropy. Crystals such as quartz and magnesium and films such as polymers and liquid crystals are known as substances used. Among them, the liquid crystal retarder is an attenuator / polarizer because it can electrically change the magnitude of birefringence. Rotor / optical modulator
It is widely used as a light compensation plate.

The structure of a liquid crystal retarder is one in which two parallel ITO glasses that have been subjected to alignment treatment are covered with a spacer having a thickness of several microns, and a nematic liquid crystal is inserted between them to seal them. The orientation state of is a homogeneous orientation in which the molecules are oriented parallel to the glass.
When a voltage is applied to this, the liquid crystal molecules rotate so as to be parallel to the electric field, so the in-plane component of the birefringence becomes smaller.
Is the mechanism.

[0004]

However, a commercially available liquid crystal retarder can change the magnitude of the birefringence (usually expressed by the phase difference between ordinary and extraordinary rays) by voltage control. The relationship between the applied voltage and the phase difference is not linear.

FIG. 5 is a diagram showing the relationship between the applied voltage and the phase difference of such a conventional liquid crystal retarder. The applied voltage V is plotted on the horizontal axis.
_t (V _rms ) and the vertical axis indicate the phase difference (retardation) (radian).

As is apparent from this figure, the applied voltage-phase difference characteristic is not linear but curved.

Therefore, in order to obtain the phase difference from the applied voltage, the correspondence between the voltage and the phase difference measured by the sales company at the time of purchasing the liquid crystal retarder is attached in the form of a graph or a table. Must be converted to the original.
In such a method that requires reading the data sheet at any time, an error easily occurs, and simple proportional voltage control cannot be performed, which is not only inconvenient but also a factor that lowers the accuracy as a retarder. There is.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a liquid crystal retarder driving method capable of eliminating the above problems and performing simple proportional voltage control.

[0009]

In order to achieve the above object, the present invention provides a liquid crystal retarder [1] which electrically changes the polarization state of light by utilizing the birefringence of a substance having optical anisotropy. In the driving method, linear control of the reciprocal of the applied voltage and the phase difference of the liquid crystal retarder is enabled in a region where the applied voltage V to the liquid crystal retarder is higher than the threshold voltage V _th .

[2] In the method for driving the liquid crystal retarder according to the above [1], the phase difference R∝1 of the liquid crystal retarder.
The magnitude of retardation of the liquid crystal retarder is controlled by 1 / CV using a relationship of / CV (where C is the electric capacity of the liquid crystal cell).

[3] In the method for driving a liquid crystal retarder according to the above [1], a passive circuit in which an additional resistance having an impedance (Z) sufficiently larger than the impedance (1 / iωC) of the liquid crystal cell is connected in series to the liquid crystal retarder is provided. And to apply an external voltage V _IN via this passive circuit,
The phase difference of the liquid crystal retarder is controlled only by the reciprocal of the voltage.

[4] In the method for driving a liquid crystal retarder according to the above [1], an active circuit composed of a constant current circuit is provided in series with the liquid crystal retarder, and the phase difference of the liquid crystal retarder is controlled by keeping the current constant by the active circuit. It is characterized by

[5] In the method for driving a liquid crystal retarder according to the above [3], the magnitude of the phase of the liquid crystal retarder which controls the physical property value and structure of the liquid crystal cell as parameters is made variable.

[6] In the method for driving a liquid crystal retarder according to the above [5], the physical property value of the liquid crystal cell is such that the liquid crystal cell has a main dielectric constant in the principal axis and a direction perpendicular thereto, elasticity of the liquid crystal molecule with respect to spreading and bending deformation. It is characterized by being a rate.

[7] In the method for driving a liquid crystal retarder according to the above [ 5 ], the structure of the liquid crystal cell is the thickness of the liquid crystal cell and the surface area of the liquid crystal cell.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

(1) R and 1 / C in the region of V> V _th
Utilization of Linearity of V FIG. 1 is a schematic diagram of a liquid crystal retarder showing an embodiment of the present invention.

In this figure, 1 is a lower ITO electrode (ITO glass), 2 is an upper ITO electrode (ITO glass), 3 is a lower alignment layer (spacer having a thickness of several microns), and 4 is an upper side. Alignment layer (spacer having a thickness of several microns), 5 is a homogeneously aligned nematic liquid crystal cell (film thickness d), and 6 is a liquid crystal molecule.

The free energy density when a longitudinal voltage (z direction) is applied to the homogeneously aligned nematic liquid crystal cell shown in FIG.

[0020]

[Equation 1]

It is represented by Here, φ is the angle between the liquid crystal molecule and the glass surface, d is the film thickness, K ₁ and K ₃ are the respective spreading and elastic moduli for bending deformation, D is the electric flux density,
ε ₁ and ε ₂ respectively represent the principal axis of the liquid crystal molecule and the principal permittivity in a direction perpendicular thereto, θ represents the angle between the liquid crystal molecule and the glass surface on the glass surface, and f represents the interface energy. From here, E
When the energy is minimized by using the uler Lagrange Equation (Euler-Lagrange equation) and the balance of the torque at the interface is taken into consideration, the alignment state of the liquid crystal is given by the following equation.

[0022]

[Equation 2]

Δε = ε ₁ −ε ₂ , α = Δε / ε ₁ , γ =
(K ₃ −K ₁ ) / K ₁ Here, φ _m represents φ at z = d / 2, that is, the maximum rotation angle.

On the other hand, the phase difference between the ordinary light of wavelength λ and the extraordinary light is

[0025]

[Equation 3]

By substituting equation (2) into equation (3), the integral for z is multiplied by the integral form for φ. Here, the applied voltage V is V _th = π (K ₁ / Δε) ^1/2
If it is sufficiently larger than, the liquid crystal molecules are aligned almost completely parallel to the electric field (perpendicular to the glass surface) around Z = d / 2.

According to a concrete calculation, when V = 6V _th ,
π / 2−φ _m <10 ⁻³ will be achieved. Therefore, φ _m = π / 2 is substituted into the equation (2), and D = C
Substituting V / S (C and S are the capacitance and surface area of the liquid crystal cell) into equation (3), the phase difference is

[0028]

[Equation 4]

Will be given by [R₀= 2πd
(N_e-N₀) / Λ]. Where ξ is also given by the integral formula
I also depends on the physical properties of the liquid crystal and the interface orientation.
The phase difference R is linearly dependent only on the reciprocal of CV with respect to the external field
It will be. As a result, V> V _thIn the area of
If you use the
R∝1 / CV is used for the relationship between applied voltage and phase difference
1 / CV control of retardation magnitude
It is possible to

FIG. 2 is a diagram showing the relationship between the reciprocal of the voltage applied to the liquid crystal retarder and the phase difference. The horizontal axis represents the reciprocal of the applied voltage [1 / V _t (V _rms ^-1 ), and the vertical axis represents the phase difference ( Retardation) (radians).

As described above, the phase difference can be linear with respect to the reciprocal of the voltage applied to the liquid crystal retarder. Although not shown, the inclination of this straight line can be made smaller by lowering the temperature from the nature of the liquid crystal itself, and can be made smaller by raising the temperature. Further, when the impedance Z of the external circuit of the liquid crystal retarder, which will be described later, is reduced, the inclination of the straight line increases, and conversely, when the impedance Z is increased, the inclination of the straight line can be decreased.

(2) Acquisition of linearity of R and 1 / V by the current control circuit In order to completely control the phase difference between the liquid crystal retarders, in order to take the influence of the electric capacity of the cell, a current control circuit is used. It is necessary to work together. To do this, use passive or active circuits such as:

Passive Circuit FIG. 3 is a circuit diagram (part 1) of a liquid crystal retarder showing an embodiment of the present invention.

In this figure, 11 is an AC voltage source, and 12
Is an impedance Z and is connected in series to the homogeneously aligned nematic liquid crystal cell 5.

In the circuit shown in FIG. 3, the additional resistance of the impedance Z (12) is the impedance 1 of the liquid crystal cell 5.
When it is sufficiently larger than / iωC, the current flowing through the circuit is

[0036]

[Equation 5]

Therefore, the capacitance of the liquid crystal cell 5 is not affected. At this time, C on the liquid crystal cell 5
V is

[0038]

[Equation 6]

CV, that is, the phase difference R can be accurately controlled only by the reciprocal of the voltage regardless of other physical properties.

Active Circuit FIG. 4 is a circuit diagram (part 2) of a liquid crystal retarder showing an embodiment of the present invention.

In this figure, a voltage control gain variable amplifier 22 is connected in series to one end of an AC voltage source 21, and the output of the voltage control gain variable amplifier 22 is an AC ammeter 2.
3, the liquid crystal cell 5 is connected to the AC ammeter 23, the other end of the liquid crystal cell 5 is connected to the other end of the AC voltage source 21, and a current I is applied to the liquid crystal cell 5.

On the other hand, the current from the AC ammeter 23 | I |
(Absolute value) is added to the divider 24 and the subtractor 25. A control voltage V is set as a target value in the subtracter 25, and the output of the subtractor 25 is fed back to the voltage control gain variable amplifier 22 via the PID circuit 26.

Then, a constant current I flows through the liquid crystal cell 5. That is, the voltage V (proportional to the phase difference R of the liquid crystal retarder) having the control voltage V as the target value is applied.

Therefore, if the constant current circuit given in FIG. 4 is used, the CV can be determined more accurately with a constant current, and the phase difference of the liquid crystal retarder can be linearly controlled.

The present invention is not limited to the above embodiments, and various modifications can be made based on the spirit of the present invention, and these modifications are not excluded from the scope of the present invention.

[0046]

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

In the conventional liquid crystal retarder, the magnitude of the birefringence can be changed by controlling the voltage, but the relationship between the applied voltage and the phase difference at this time was not linear. On the other hand, according to the present invention, an electric circuit that realizes a linear relationship can be obtained by focusing on the relationship in which the phase difference to the liquid crystal retarder is proportional to the reciprocal of the applied voltage.

This is a basic invention and enables high precision and automatic measurement of an ellipsometer using this.

Further, this application field is not limited to these, and can be applied to fields such as optical communication, optical disks, and optical measurement.

[Brief description of drawings]

FIG. 1 is a schematic view of a liquid crystal retarder showing an embodiment of the present invention.

FIG. 2 is a diagram showing the relationship between the reciprocal of the applied voltage and the phase difference of the liquid crystal retarder showing the embodiment of the present invention.

FIG. 3 is a circuit diagram (part 1) of a liquid crystal retarder showing an embodiment of the present invention.

FIG. 4 is a circuit diagram (No. 2) of the liquid crystal retarder showing the embodiment of the present invention.

FIG. 5 is a diagram showing a relationship between applied voltage and phase difference of a conventional liquid crystal retarder.

[Explanation of symbols]

1 lower transparent electrode (ITO glass) 2 upper transparent electrode (ITO glass) 3 lower alignment layer (alignment film) 4 upper alignment layer (alignment film) 5 homogeneously aligned nematic liquid crystal cell (film thickness d ) 6 liquid crystal molecules 11 and 21, AC voltage source 12, impedance Z 22, voltage control gain variable amplifier 23 AC ammeter 24 divider 25 subtractor 26 PID circuit

─────────────────────────────────────────────────── ─── Continuation of the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) G02F 1/13-1/141

Claims (7)

(57) [Claims] 1. A method of driving a liquid crystal retarder for electrically changing the polarization state of light by utilizing the birefringence of a substance having optical anisotropy, wherein a voltage V applied to the liquid crystal retarder is larger than a threshold voltage V _th. 3. The method for driving a liquid crystal retarder, wherein linear control of the reciprocal of the applied voltage and the phase difference of the liquid crystal retarder is enabled. 2. The liquid crystal retarder driving method according to claim 1, wherein a phase difference R∝1 / CV of the liquid crystal retarder.
(Where C is the capacitance of the liquid crystal cell),
The retardation size of the liquid crystal retarder is 1 /
A method for driving a liquid crystal retarder, which is characterized by performing CV control. 3. The method for driving a liquid crystal retarder according to claim 1, further comprising a passive circuit in which an additional resistor having an impedance (Z) sufficiently larger than the impedance (1 / iωC) of the liquid crystal cell is connected in series with the liquid crystal retarder. A method for driving a liquid crystal retarder, which comprises applying an external voltage V _IN via the passive circuit and controlling the phase difference of the liquid crystal retarder only by the reciprocal of the voltage. 4. The method of driving a liquid crystal retarder according to claim 1, wherein an active circuit composed of a constant current circuit is provided in series with the liquid crystal retarder, and the phase difference of the liquid crystal retarder is controlled by keeping the current constant by the active circuit. The driving method of the characteristic liquid crystal retarder. 5. The driving method of a liquid crystal retarder according to claim 3, wherein the phase size of the liquid crystal retarder is controlled by controlling the physical property value and structure of the liquid crystal cell as parameters. Method. 6. The method of driving a liquid crystal retarder according to claim 5, wherein the physical property value of the liquid crystal cell is a main dielectric constant in the main axis of the liquid crystal molecule and a direction perpendicular to the main axis, and an elastic modulus with respect to spreading and bending deformation of the liquid crystal molecule. A method for driving a liquid crystal retarder, which is characterized in that 7. The liquid crystal retarder driving method according to claim 5 , wherein the structure of the liquid crystal cell is the thickness of the liquid crystal cell and the surface area of the liquid crystal cell.