JP2003066450A - Liquid crystal element and optical attenuator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a voltage-variable optical attenuator from which a stable and high optical quenching ratio can be obtained. SOLUTION: This liquid crystal element 100 consists of a liquid crystal cell 110 and a phase plate 120. The cell 110 is formed so that a liquid crystal layer 1 is interposed between translucent substrates 5, 6 provided with transparent electrodes 3, 4 respectively and has the retardation value changing from R1 to R2 with respect to the linear polarized light of λ wavelength which is made incident to the cell 110 and passes through the cell 110 when the voltage of V1 to V2 (V1 ≠V2 ) is applied between the electrodes 3 and 4. The retardation value R of the plate 120 is Rv satisfying R1 >=Rv >=R2 and satisfies R+Rv =λ with respect to the linear polarized light of λ wavelength. This optical attenuator is obtained by preparing the element 100 and furthermore arranging a polarizer on the light entering side or the light exiting side of the element 100.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal element and an optical attenuator, and more particularly to a liquid crystal element and an optical attenuator used for optical communication.

[0002]

2. Description of the Related Art FIG. 6 shows an example of a conventional optical attenuator using liquid crystal. Liquid crystal of nematic liquid crystal in which the alignment direction of the liquid crystal molecules is parallel to the substrate surface between the transparent substrates 5 and 6 on which the transparent electrodes 3 and 4 are formed, and which is aligned in a direction forming an angle of 45 ° with the X-axis direction. The liquid crystal cell 110 in which the layer 1 is sandwiched inside the sealing material 8 provided on the periphery of the substrate, and X is provided on the light emitting surface side thereof.
A polarizer 12 that transmits only linearly polarized light having a polarization direction in the axial direction is arranged.

Here, an AC power source 9 having a rectangular wave output is connected to the transparent electrodes 3 and 4, and when no voltage is applied by this power source,
The thickness dlc of the liquid crystal layer 1 is set so that the retardation value of the liquid crystal cell 110 for linearly polarized light having a polarization direction in the Y-axis direction at the wavelength λ is approximately λ / 2. here,
The reason why the retardation value of the liquid crystal layer 1 is approximately λ / 2 is to minimize the insertion loss of the optical attenuator when no voltage is applied and to function as a λ / 2 plate.

In this optical attenuator, the linearly polarized light which is transmitted through the liquid crystal layer and which is polarized in the Y-axis direction when no voltage is applied between the transparent electrodes becomes linearly polarized light having a polarization direction in the X-axis direction and is transmitted through the polarizer. To do. When a voltage is applied, as the applied voltage increases, the alignment direction of the liquid crystal molecules tilts in the thickness direction of the liquid crystal layer, that is, perpendicular to the substrate. Along with this, the retardation value of the liquid crystal layer decreases, and the light transmitted through the liquid crystal cell 110 becomes elliptically polarized light. As a result, the amount of light transmitted by the polarizer monotonically decreases with an increase in applied voltage. Become a vessel.

[0005]

In an optical attenuator using a liquid crystal element for optical communication in which the wavelength λ of incident light is in the range of 1.3 to 1.6 μm, for example, the retardation value of the liquid crystal cell is λ / In order to obtain 2, it is necessary to make the liquid crystal layer thicker than the optical attenuator in the visible wavelength region. As a result, even if an AC voltage with a voltage amplitude of 10 V or more is applied, the alignment direction of the liquid crystal molecules is not sufficiently aligned in the thickness direction of the liquid crystal layer, and the retardation value of the liquid crystal cell does not become zero. There is a problem that an optical attenuator having a high extinction ratio cannot be obtained due to the presence of the component.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a liquid crystal element and an optical attenuator using the same, which can stably realize a high extinction ratio in view of the above situation.

[0007]

The present invention is a liquid crystal cell in which a liquid crystal layer is sandwiched between transparent substrates with electrodes, and the voltage applied between the electrodes is from V ₁ to V ₂ (V ₁ ≠ V ₂ ) the wavelength λ that is incident on and transmitted through the liquid crystal cell when changed to V ₂ )
Retardation values for linearly polarized light of R ₁ to R
₂ (R ₁ > R ₂ > 0) and a wavelength λ
Has a retardation value R for linearly polarized light of R ₁ ≧ R _v ≧
There is provided a liquid crystal element comprising: R _v having a relationship of R ₂ ; and a phase plate satisfying a relationship of R + R _v = m × λ (m: integer).

Further, a polarizer is further disposed on at least one of the light incident side and the light emitting side of the liquid crystal element, and the intensity of emitted light decreases as the voltage applied to the liquid crystal layer increases. An optical attenuator is provided.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The liquid crystal element of the present invention has the following constitution.
It is a liquid crystal element which has. That is, the liquid crystal element has electrodes.
A liquid crystal cell in which a liquid crystal layer is sandwiched between transparent substrates,
The liquid crystal cell has V₁To V_Two(V₁≠ V_Two)
When a voltage is applied to the liquid crystal cell,
The retardation value for linearly polarized light of wavelength λ is R₁
To R _Two(R₁> R_Two> 0).
Furthermore, the liquid crystal element has a retardation for linearly polarized light of wavelength λ.
The solution value R is R₁≧ R_v≧ R_TwoR in the relationship_vWhen
R + R_v= M × λ (m: integer)
I have it.

By configuring the liquid crystal element of the present invention in this way, the liquid crystal element can obtain a high extinction ratio at a low voltage. Hereinafter, the liquid crystal device of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a side view showing a structural example of a liquid crystal element of the present invention. The transparent electrodes 3 and 4 are formed on one surface of the translucent substrates 5 and 6, respectively, and an alignment film (not shown) subjected to an alignment treatment in the same direction is formed on the transparent electrodes 3 and 4, and the sealing material 8 is formed.
Are made into cells. In addition, the ordinary refractive index n
_o (LC) and extraordinary refractive index _{n e (LC) (n o} (L
A nematic liquid crystal of C) <n _e (LC) is injected to form the liquid crystal layer 1, and a liquid crystal cell 110 in which the alignment directions of liquid crystal molecules are aligned parallel to the substrate is obtained.

Further, after applying a solution for an alignment film to the respective surfaces of the transparent substrates 6 and 7 which face each other, the coating film is subjected to alignment treatment in the same direction to form an alignment film (not shown), which is not shown. Create cells using a sealant. Further, a liquid crystal monomer solution is injected into the cell to form a liquid crystal monomer layer in which the alignment directions of liquid crystal molecules are aligned parallel to the substrate in the cell. The liquid crystal monomer layer is irradiated with ultraviolet rays and solidified to form the polymer liquid crystal layer 2 in which the alignment direction of the liquid crystal molecules is fixed to obtain the phase plate 120. Accordingly, the liquid crystal cell 110 and the phase plate 12
A liquid crystal element 100 in which 0 and 0 are stacked is obtained.

At this time, a liquid crystal composed of a nematic liquid crystal
Slow axis direction of layer 1 (refractive index of extraordinary light n _eGive (LC)
Direction) is the polarization direction of the incident light in FIGS.
45 ° to the Y-axis direction. Ordinary refraction
Rate n_o(PLC) and extraordinary light refractive index n_e(PLC) (n_o
(PLC) <n_e(PLC)) polymer liquid crystal layer 2
Direction of the phase axis of the phase plate (the ordinary refractive index n_o(PLC)
Direction) forms an angle θ with the slow axis direction of the liquid crystal layer 1.
Has been done. The retardation value described in the present invention
Is the optical path length for polarized light in the fast axis direction of the liquid crystal layer 1 and
Shows the difference from the optical path length for polarized light in the slow axis direction of crystal layer 1.
However, there is also a negative retardation value.

Here, it is preferable that the angle θ formed by the fast axis direction of the phase plate and the slow axis direction of the liquid crystal layer 1 is within 45 degrees. By setting the angle θ within 45 degrees, the retardation value R of the phase plate becomes a negative value that cancels out the retardation value R _v existing when a voltage is applied, but when it exceeds 45 degrees, R becomes a positive value and R _v becomes It will be difficult to offset.

Usually, the fast axis direction of the polymer liquid crystal layer 2 is the same as the slow axis direction of the liquid crystal layer 1 (θ = 0). That is,
The fast axis direction of the polymer liquid crystal layer 2 forms an angle of 45 degrees with the polarization direction of incident light.

Here, the retardation value of the liquid crystal cell 110 is approximately λ when a voltage is not applied from the AC power source 9 with respect to the incident light of linear polarization having the polarization direction in the Y-axis direction at the wavelength λ.
The thickness d (LC) of the liquid crystal layer 1 is set to 0.5 so that
Let λ / Δn (LC). Where Δn (LC) = n
a _{e (LC) -n o (LC} ).

When a rectangular wave AC voltage having a voltage amplitude V is applied from the AC power supply 9 to the transparent electrodes 3 and 4 of the liquid crystal cell 110 thus obtained, the retardation value of the liquid crystal cell 110 decreases but is not zero. It is a finite value R _v . In order to make the retardation value of the liquid crystal element zero by the applied voltage of the voltage amplitude V, the retardation value R (= of the phase plate 120 is set so as to cancel the retardation value R _v.
-R _v) may be adjusted. That is, the polymer liquid crystal layer 2
When the fast axis direction of the liquid crystal layer 1 coincides with the slow axis direction of the liquid crystal layer 1, the thickness d (PLC) of the polymer liquid crystal layer 2 is set to R _v / Δn.
(PLC) may be used. Here, Δn (PLC) = n
a _{e (PLC) -n o (PLC} ). If they do not match, the effective Δn (PLC) becomes a small value, and d becomes thick.

When the retardation value R _v is large, the retardation value of the liquid crystal element when no voltage is applied is R from λ / 2.
_{Since the} value becomes smaller by _v , the retardation value of the liquid crystal cell 110 is previously set to λ / 2 in consideration of the decrease R _v.
It may be + R _v .

The linearly polarized light having the polarization direction in the Y-axis direction at the wavelength λ enters the liquid crystal element 100 thus obtained, and only the linearly polarized light having the polarization direction in the X-axis direction is incident on the light exit side of the liquid crystal element 100. An optical attenuator is provided by arranging a polarizer that transmits light. As a result, most of the light is transmitted when no voltage is applied, and the light is blocked by the polarizer when a voltage is applied (applied voltage V). Therefore, optical attenuation having a high extinction ratio corresponding to the extinction ratio of the polarizer is achieved. Can be realized, which is preferable.

In the above description, the retardation value of the liquid crystal element is approximately λ / 2 when the applied voltage is zero and is zero when the applied voltage is V, but the retardation value is λ / 2. And may be different from zero.

The range of voltage applied to the liquid crystal cell 110 is V ₁
To V ₂ (V ₁ ≠ V ₂ ), at which time the retardation value changes from R ₁ to R ₂ . In order to obtain an optical attenuator having a high extinction ratio at a specific voltage V within the above voltage range, the retardation value R _{v of} the liquid crystal cell 110 generated at the voltage V and the retardation value R of the phase plate 120 are the wavelengths. For incident light of the same linear polarization of λ, R + R _v = m × λ
It suffices to satisfy the relationship of / 2 (m: integer). Where R
_v satisfies the relationship of R ₁ ≧ R _v ≧ R ₂ .

Here, when m is an odd number, the polarization direction of the transmitted linearly polarized light in the polarizer is arranged so as to be orthogonal to the polarization direction of the incident linearly polarized light in the liquid crystal element, and when m is an even number, the transmission linearly polarized light in the polarizer is set. If the polarization direction of the polarized light is made to coincide with the polarization direction of the incident linearly polarized light in the liquid crystal element, the amount of transmitted light becomes the minimum at an applied voltage V in the range from V ₁ to V _2, so that an optical attenuator with a high extinction ratio can To be realized. R + R _v
Since the smaller the absolute value of is, the smaller the wavelength dependence of the extinction ratio is, m = 0 is usually preferable, but m = ± 1 or ± 2 may be set.

Therefore, the liquid crystal used in the liquid crystal element is a nematic liquid crystal, and the alignment direction of the liquid crystal molecules when no voltage is applied is a parallel alignment in which the transparent substrates are aligned in a fixed direction, and the phase plate is R + R _v. It is preferable that the liquid crystal element satisfy the relationship of = 0 and the fast axis direction forms an angle within 45 ° with the slow axis direction of the liquid crystal layer.

Further, in FIG. 1, the liquid crystal layer 1 and the polymer liquid crystal layer 2 are shown.
The transparent substrate 6 is interposed between the transparent substrate 6 and the transparent substrate 6.
Alternatively, the transparent electrode 4 and the alignment film may be formed on the polymer liquid crystal layer 2 on the translucent substrate 7 without using, and the liquid crystal layer may be formed after the cell formation.

Although FIG. 1 shows a structural example of a phase plate made of polymer liquid crystal, a phase plate made of a birefringent crystal such as quartz may be used. In that case, instead of the polymer liquid crystal layer 2 and the transparent substrates 6 and 7 of FIG. 1, a crystal wave plate having a transparent electrode 4 formed thereon can be used as a transparent substrate 6 of a liquid crystal cell, and a liquid crystal element is formed. Can be miniaturized. Further, FIG. 1 shows a configuration in which the transparent electrodes 3 and 4 are used as the electrodes of the liquid crystal cell, but one of the electrodes may be a reflective liquid crystal cell using a light-reflecting electrode such as gold or aluminum. In this case, since the light travels back and forth through the liquid crystal layer, the thickness of the liquid crystal layer can be halved as compared with the transmissive type, which leads to lower driving voltage and faster response.

The liquid crystal used is not limited to nematic liquid crystal, but may be ferroelectric liquid crystal, antiferroelectric liquid crystal or the like.
Further, the alignment of liquid crystal molecules may be twisted alignment, vertical alignment, hybrid alignment, etc. other than parallel alignment. Any liquid crystal material can be used as long as the retardation value of the liquid crystal layer changes in response to voltage application, and liquid crystal with orientation can be used. Among these liquid crystals, it is preferable to use a nematic liquid crystal because a stable liquid crystal alignment can be obtained.

Examples of the polarizer include a polarizing glass which transmits linearly polarized light having a specific polarization direction and absorbs linearly polarized light having a polarization direction orthogonal thereto, for example, polarizing glass in which fine metal particles are dispersed in glass. However, when the polarizing glass is used integrally with the liquid crystal element, there is a problem that the temperature of the liquid crystal element rises due to light absorption and the light attenuation rate changes.

In order to reduce the temperature change due to the above light absorption, it is preferable to use a polarization diffraction type polarizer. FIG. 3 shows a configuration example of the polarization diffraction type polarizer 200 used in the optical attenuator of the present invention. Ordinary light refractive index n _o (PLC) and extraordinary light refractive index n _e (PLC) formed on the transparent substrate 12.
Polymer liquid crystal layer 1 of (n _o (PLC) <n _e (PLC))
0 is processed into a diffraction grating having a rectangular cross section by a photo-etching technique. Then, the concave portion of the grating is filled with a homogeneous refractive index transparent material 11 having a refractive index n _{s that} is substantially equal to the ordinary light refractive index n _o (PLC) and sandwiched between the transparent substrates 12 and 13 to form a polarization diffraction type polarizer 200. Form.

The ordinary light polarization incident on the above-mentioned polarization diffraction type polarizer goes straight through without being diffracted, and the extraordinary light polarization is diffracted. Therefore, if a light-collecting element such as a lens is provided on the light exit side, the straight transmitted light is condensed at one condensing point and the diffracted light is condensed at another condensing point. Can be spatially separated. Therefore, if only the straight transmitted light is taken out, a polarizer having a high extinction ratio can be obtained.

Therefore, the polarizer linearly transmits the incident light of the first linearly polarized light, and diffracts the second linearly polarized light having a polarization direction orthogonal to the polarization direction of the first linearly polarized light, which is a polarization diffraction type polarization. The above-mentioned optical attenuator which is a child is preferable.

FIG. 4 shows a structural example of the optical attenuator 300 of the present invention in which the above-mentioned polarization diffraction type polarizer 200 is integrated with the liquid crystal element 100. Here, an example in which a polarization diffraction type polarizer 200 is arranged on the light exit side where linearly polarized light is incident on the liquid crystal element 100 is shown. However, only the linearly polarized light component of a specific polarization direction of incident light is incident on the liquid crystal element 100. In this case, a polarization diffraction type polarizer may be arranged also on the light incident side.

As described above, by arranging the polarization diffraction type polarizer on at least one of the light incident side and the light emitting side of the liquid crystal element and integrally forming them, the element can be downsized and stable performance can be obtained. It is preferable because it is possible.

[0033]

EXAMPLE A liquid crystal element 100 of this example will be described with reference to FIG. A nematic liquid crystal having an ordinary light refractive index n _o (LC) = 1.50 and an extraordinary light refractive index n _e (LC) = 1.66 is applied to the translucent substrates 5 and 6 having the transparent electrodes 3 and 4 formed on one surface. A liquid crystal cell 110 was produced in which the liquid crystal layer 1 was sandwiched and the thickness d (LC) of the liquid crystal layer 1 was 5 μm. The slow axis direction of the liquid crystal layer 1 was 45 degrees with respect to the Y-axis direction shown in FIG. 2 and was parallel to the substrate.

When no voltage is applied to the transparent electrodes 3 and 4, the retardation value of the liquid crystal cell 110 for light having a wavelength of 1.55 μm is 0.8 μm (R ₁ ), and linearly polarized light having a polarization direction in the Y-axis direction. However, the polarization direction of the linearly polarized light was the X-axis direction when entering and exiting the liquid crystal cell 110. Further, the retardation value R _v of the liquid crystal cell 110 was 0.128 μm (R ₂ ) when a rectangular wave AC voltage having a voltage amplitude of 5 V was applied.

Further, it is sandwiched between the transparent substrates 6 and 7.
The polymer liquid crystal layer 2 having a normal refractive index n _o(PLC) = 1.
55 and extraordinary light refractive index n_e(PLC) = 1.59 and thick
The phase plate 120 is configured by setting the height d (PLC) to 3.2 μm.
The phase plate 120 and the liquid crystal cell 110 are used to
A child 100 was produced. Here, the fast axis of the phase plate 120
If the direction is aligned with the slow axis direction of the liquid crystal layer 1, that is, θ
= 0, the phase plate 120 composed of the polymer liquid crystal layer 2
Since the retardation value R is -0.128 μm,
Retardation value existing in the liquid crystal layer 1 at an applied voltage of 5V
0.128 μm was offset. That is, m × λ = 0 and m =
Corresponds to 0.

Further, as shown in FIG. 3, the ordinary refractive index n
_o (PLC) = 1.55 and extraordinary light refractive index n _e (PL
A diffraction grating 10 having a grating pitch of 10 μm was manufactured using a polymer liquid crystal having a thickness of 5.2 μm with C) = 1.70. After that, the concave portions of the grating are filled with the transparent material 11 having a uniform refractive index of 1.55, and these are filled with the transparent substrate 1 made of a glass substrate.
A polarizing diffraction type polarizer 200 was produced by sandwiching the polarizing element 200 with the polarizing element 200. Next, as shown in FIG. 4, the polarizer 200 was adhesively fixed to the light emitting surface side of the liquid crystal element 100 to obtain a voltage variable type optical attenuator 300. Here, the polarization direction of the diffracted light of the polarizer 200 is the Y-axis direction.

Optical attenuator 300 manufactured in this way
Then, linearly polarized parallel light having a wavelength of 1.55 μm and a polarization direction in the Y-axis direction was made incident, and the emitted light was condensed on an optical fiber by a condenser lens.

When the amplitude of the voltage applied to the liquid crystal layer of the optical attenuator is changed from 0 to 5 V, it is defined by the light intensity ratio I (V) / I (0) in the light intensity I (V) after the optical fiber transmission. An example of the change in extinction ratio is shown by a circle in FIG. In FIG. 5, the smaller the numerical value on the vertical axis (lower), the larger the extinction ratio. As a comparative example, the change in extinction ratio in the case of a conventional optical attenuator without a phase plate is shown by □.

The maximum extinction ratio of -12 dB was obtained in the conventional liquid crystal device without the phase plate, but a high extinction ratio of -40 dB was obtained in the structure of this example using the phase plate.

[0040]

As described above, by using the liquid crystal element of the present invention, the amount of emitted light gradually decreases with an increase in the applied voltage, and the voltage variable type light having a high extinction ratio even at a low voltage of about 5V is obtained. Attenuator can be realized.

Further, in the optical attenuator of the present invention in which a polarization diffraction type polarizer which hardly absorbs light is integrated with a liquid crystal element, the temperature rise of the liquid crystal layer is small even when high intensity light is incident, so that it is stable. An extinction ratio is obtained.

[Brief description of drawings]

FIG. 1 is a side view showing a configuration example of a liquid crystal element of the present invention.

FIG. 2 is a plan view showing the relationship between the slow axis direction of the liquid crystal element of the present invention and the fast axis direction of the phase plate.

FIG. 3 is a side view showing a configuration example of a polarization diffraction type polarizer used in the optical attenuator of the present invention.

FIG. 4 is a side view showing a configuration example of an optical attenuator in which a polarization diffraction type polarizer is integrated with the liquid crystal element of the present invention.

FIG. 5 shows an optical attenuator of the present invention and a conventional optical attenuator,
The graph which shows the relationship of the extinction ratio with respect to the liquid crystal cell applied voltage.

FIG. 6 is a side view showing a configuration example of a conventional optical attenuator.

[Explanation of symbols]

1: Liquid crystal layer 2: Polymer liquid crystal layer 3,4: Transparent electrode 5, 6, 7, 12, 13: translucent substrate 8: Seal material 9: AC power supply 10: Polarizing diffraction grating 11: Transparent material with homogeneous refractive index 12: Polarizer 100: Liquid crystal element 110: Liquid crystal cell 120: Phase plate 200: Polarization diffraction type polarizer 300: Optical attenuator

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 2H049 BA02 BA06 BA42 BA45 BB03                       BC22                 2H079 AA02 AA12 BA01 CA04 DA08                       EB17 HA13 KA08 KA17                 2H088 EA32 EA44 HA15 KA07 MA02                       MA20                 2H091 FA08X FA08Z FA11X FA11Z                       FD06 FD10 GA03 GA06 HA07                       LA17 MA10

Claims (4)

[Claims] 1. A liquid crystal layer is sandwiched between transparent substrates with electrodes.
Liquid crystal cell, and the voltage applied between the electrodes is V₁Or
Et V_Two(V₁≠ V_Two) To the liquid crystal cell
Ritaday for linearly polarized light of wavelength λ that is incident and transmitted
Value is R₁To R_Two(R ₁> R_TwoChange to> 0)
Liquid crystal cell, Retardation value R for linearly polarized light of wavelength λ is R₁≧
R_v≧ R_TwoR in the relationship_vAnd R + R_v= M × λ
A phase plate satisfying the relationship of (m: integer), A liquid crystal element comprising: 2. The liquid crystal used in the liquid crystal element is a nematic liquid crystal, and the alignment direction of liquid crystal molecules when no voltage is applied is a parallel alignment in which the transparent substrates are aligned in a fixed direction, and the phase plate is The liquid crystal element according to claim 1, wherein the relationship of R + R _v = 0 is satisfied, and the fast axis direction forms an angle within 45 ° with the slow axis direction of the liquid crystal layer. 3. A polarizer is further disposed on at least one of the light incident side and the light emitting side of the liquid crystal element according to claim 1, and the intensity of emitted light decreases as the voltage applied to the liquid crystal layer increases. An optical attenuator characterized by: 4. The polarization diffractive polarizer, wherein the polarizer linearly transmits the first linearly polarized light and diffracts a second linearly polarized light having a polarization direction orthogonal to the polarization direction of the first linearly polarized light. An optical attenuator according to claim 3.