JP2006317645A - Wavelength variable liquid crystal filter of half-value width control type - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wavelength variable liquid crystal filter that will not change in the half-value width. <P>SOLUTION: A constant half-value width is realized by layering a liquid crystal and a retardation medium and combining wavelength dependence and voltage dependence. The wavelength variable filter comprises a light source, two polarizers disposed on the optical axis of the light source, and a liquid crystal cell and a retardation plate, such as a uniaxial or biaxial birefringence optical film disposed in between the polarizers. The wavelength dependence of retardation through the liquid crystal cell and the birefringent plate is controlled, in such a manner that the gradient of retardation with respect to wavelengths is constant near the maximum of transmittance, regardless of the voltage applied. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to a wavelength tunable liquid crystal filter using a birefringence mode, and relates to a filter capable of changing the wavelength with a constant half width by combining the birefringence wavelength dispersion of a liquid crystal and a birefringent film with respect to the half width.

The tunable liquid crystal filter can be applied to wavelength analysis such as a spectral filter of a two-dimensional image, and can perform various image analysis. In order to perform wavelength analysis of an image, it is necessary to suppress as much as possible the fluctuation of the half width at each wavelength of the liquid crystal filter used for this. In U.S. Pat. No. 2000-267127, “tunable color filter using liquid crystal”, Uchida et al. Devised a plurality of ECB cells or OCB cells between two polarizers arranged in the transmission order with the transmission axes orthogonal to each other. .

Ishibe and others are developing a rio filter, which is a narrow-band filter, and discussing how to keep the full width at half maximum. [1] In the discussion, assuming that the design is such that Δn (λ) ∝λ ² assuming that the birefringence Δn (λ) is proportional to the square of the wavelength dispersion characteristic wavelength λ, the change in the half-width is changed. This shows that it is possible to produce a liquid crystal cell without (constant). In this case, the phase value Δn (λ) ∝λ ² when the retardation value is Δn (λ) d is a linear function with respect to the wavelength, and a constant value with a half width is obtained regardless of the wavelength. Here, d is the thickness of the liquid crystal layer.
JP 2000-267127 A T. T. et al. Ishinab, A. et al. Tsuchiuchi and T. Uchida: Abstracts of 20th ILCC 2004, no. APPL-P112, pp. 312 (2004).

In a conventional wavelength tunable filter, the alignment of liquid crystal molecules changes with the application of voltage. Since liquid crystal molecules have birefringence, the retardation changes with voltage. Therefore, the transmission wavelength is controlled by changing the retardation value of the liquid crystal. Generally when a voltage is applied will not be able to satisfy Δn (λ) αλ ² conditions, there is a possibility that the half width constant condition collapses. Therefore, it is a problem to make a wavelength tunable filter having a constant half width.

(1) A light source and two polarizers are arranged on the optical axis, and further, one liquid crystal cell arranged between them and a retardation plate, for example, a uniaxial or biaxial birefringent optical film. A wavelength tunable filter, in which the wavelength dependence of retardation between the liquid crystal cell and the birefringent plate is constant in the vicinity of the maximum value of transmittance regardless of voltage application. A half-width control type liquid crystal wavelength tunable filter characterized by being adjusted to (2) The wavelength tunable filter according to (1), wherein the liquid crystal uses a mode in which birefringence changes with a change in molecular orientation during voltage application. (3) The wavelength tunable filter according to (2), wherein the two polarizers are arranged at a rotation angle of 0 ° or 90 °. (4) The wavelength tunable filter according to (3), wherein the delay axes of the liquid crystal and the birefringent optical film are arranged at a rotation angle rotation angle of 0 ° or 90 °.

(5) The wavelength λ at which the transmittance is maximum is the main wavelength λ _0, and the refractive index anisotropy and thickness of the liquid crystal and the retardation plate at that wavelength are Δn1, Δn2, d1, and d2, respectively. In the structure obtained by laminating a liquid crystal and retardation plate, .DELTA.n1 with the voltage applied to the liquid crystal is changed, lambda ₀ is moved. At that time, changes in [Delta] nd / lambda ₀ in lambda ₀ the vicinity of the liquid crystal and the retardation plate is complementary to each other, according to results in the slope of [Delta] nd / lambda ₀ in lambda ₀ the vicinity is adjusted to a constant (4) Tunable filter. (6) The wavelength tunable filter according to (5), in which a voltage is applied to the liquid crystal cell and the principal wavelength to be transmitted can be arbitrarily varied in the ultraviolet, visible, and infrared regions, and the principal wavelength is single or plural. (7) A wavelength tunable filter obtained by laminating a plurality of wavelength tunable filters described in (6) having the same main wavelength, thereby narrowing the band.

Due to the wavelength dependence of a tunable liquid crystal filter using a birefringence mode, an element was devised in which liquid crystal and a birefringent film were laminated to combine wavelength dispersion. According to this, the half-value width is constant and the wavelength can be varied in the visible region.

By using the birefringence wavelength dispersion of the liquid crystal cell and the uniaxial film, the change in the half-value width when the applied voltage was changed in the visible region could be made constant. The liquid crystal birefringence mode is widely used for displays, measurement evaluation elements, optical elements, etc., and the dispersion characteristic correction method of this method can be widely applied.

Here, a birefringent liquid crystal display element in which liquid crystal molecules having positive dielectric anisotropy are aligned in parallel as shown in FIG. As shown in FIG. 2, the orientation of the liquid crystal molecules changes vertically with voltage application. Therefore, k is used as a parameter representing the proportion of the effective parallel alignment portion that affects the optical characteristics. Parameter k is the ratio 2d s _{/ d} of the thickness 2d _s in effective parallel orientation portion to the thickness d of the entire liquid crystal layer. When no voltage is applied, all the liquid crystal layers are in parallel alignment, so k = 1, and when a sufficiently high voltage is applied, the liquid crystal molecules become vertical and the birefringence portion disappears, so k = 0.

In a liquid crystal using the birefringence mode, a voltage is applied to change the retardation value, and the transmission spectrum changes accordingly. When a parallel polarizer is used, the transmission spectrum becomes maximum when the retardation value Δn (λ) d / λ normalized by the wavelength is an integer multiple of 2mπ (m = 0, 1, 2,...), And (2m + 1 / 2) Minimum value at π (m = 0, 1, 2,...). For vertical polarizers, the maximum and minimum values are reversed. FIG. 3 shows the wavelength dependence of retardation with respect to wavelength in a nematic liquid crystal. Under the parallel polarizer, the transmittance becomes maximum at PK1, PK2, and PK3 that are integer values.

FIG. 4 shows the wavelength dependence of the transmittance when a parallel alignment cell using a general nematic liquid crystal is arranged with an optical axis of 45 degrees between parallel Nicols and no voltage is applied (k = 1). is there. Three peaks appear, but the half-value width of each peak is greatly different. Since the liquid crystal has a wavelength dispersion characteristic of refractive index anisotropy, the retardation value varies depending on the wavelength. For this reason, the half-value widths of the respective peak wavelengths are different, and the characteristic that the longer wavelength side becomes wider is obtained. This is because a general liquid crystal material has a smaller inclination of chromatic dispersion as the wavelength is longer.

For this reason, in order to keep the change in the half-value width constant, the design must be such that the slope of the chromatic dispersion changes constant. FIG. 5 is a diagram showing the wavelength dependence of the normalized retardation value Δn (λ) d / λ. When Δn (λ) d / λ is an integer value, the transmittance is maximum, and when it is an integer value +0.5, the transmittance is minimum. The liquid crystal exhibits large wavelength dispersion when no voltage is applied (k = 1.0). The slopes in the vicinity of each peak are different, which is the reason why the full width at half maximum is different in FIG. When a voltage is applied to the liquid crystal, the retardation value decreases, and when k = 0, birefringence disappears.

When the applied voltage (k) of the liquid crystal is changed, the peak wavelength is shifted, but the half width is also affected. FIG. 6 shows this situation. Each curve shows the change in half width when one peak shifts to a short wavelength by voltage application. The parameter is a value of Δn (λ) d / λ indicating the maximum transmittance. Since it has an inclination with respect to the wavelength, the full width at half maximum varies. The reason why the half-value width is reduced on the short wavelength side is that the wavelength dependence of birefringence is increased.

The design guideline for the liquid crystal filter using the birefringence mode is to find a condition that the half width is constant along with the shift of the peak wavelength in the voltage application state. FIG. 7 shows the concept of half width control. The upper diagram shows the wavelength dependence of the retardation value, and m is the retardation indicating the maximum transmittance. When a voltage is applied to the liquid crystal cell, the retardation value of the element changes, so that a change as shown by a bold line in the figure occurs. At this time, if it can be controlled so that the inclination near the m value is constant, the full width at half maximum is also constant.

The wavelength λ at which the transmittance is maximum is the main wavelength λ _0, and the refractive index anisotropy and thickness of the liquid crystal and the retardation plate at that wavelength are Δn1, Δn2, d1, d2), respectively. In the structure obtained by laminating a liquid crystal and retardation plate, .DELTA.n1 with the voltage applied to the liquid crystal is changed, lambda ₀ is moved. At that time, changes in [Delta] nd / lambda ₀ in lambda ₀ the vicinity of the liquid crystal and the retardation plate is complementary to each other, resulting in constituting a tunable filter slope of [Delta] nd / lambda ₀ is adjusted to be constant at lambda ₀ near To do. This realizes a wavelength tunable filter in which the wavelength of transmitted light is constant.

Since the time when the birefringence is to be controlled is limited only by the liquid crystal, this condition was realized by laminating a uniaxial birefringent film. Consider a simplified case. In order to make the full width at half maximum constant, when a voltage is applied in a state where the liquid crystal and the birefringent film are combined, the same inclination must be obtained at any voltage. If this characteristic is linear and the slope is constant regardless of the applied voltage, no change in the half width occurs at any applied voltage. However, since all materials have wavelength dispersion characteristics, a new design is required.

Actually, Δn (λ) d / λ becomes a curve with respect to the wavelength of both the liquid crystal and the birefringent film. In addition, the wavelength dependency changes with voltage application. Furthermore, the liquid crystal layer is required to be thinner in consideration of response recovery characteristics. Therefore, the design of the optical filter using the birefringence mode will be described.

When laminating the liquid crystal and the birefringent film, it is conceivable that the optical axis is made parallel or perpendicular. In the former case, both retardation values are added. In general, both have absorption in the ultraviolet region, so that birefringence is reduced on the long wavelength side. Therefore, it becomes inconvenient for making the inclination constant. On the other hand, the latter is canceled out with each other, and the wavelength dependence may be relaxed. Therefore, this configuration is adopted. Furthermore, it can have a characteristic of rising to the right and falling to the right. In a configuration with a rising right angle, the wavelength dependency tends to be convex, and there is a possibility that the fusion of peak wavelengths and the shift direction cannot be determined. From the above, the liquid crystal and the birefringent film have a configuration in which the optical axes are orthogonal to each other and have a right-sloping characteristic.

In the case of this element, when k = 1, the birefringence between the liquid crystal and the film cancels each other. The half-value width of the wavelength showing the peak is determined by the Δn (λ) d / λ characteristics on both short wavelengths when k = 1, and the characteristics on the long wavelength side of the film are reflected as k decreases. When k = 0, the birefringence of the film itself determines the characteristics. The results obtained in consideration of these characteristics are shown in FIG. Here, a general polycarbonate is used as the optical film. It shows a downward-sloping characteristic in any state of voltage application. Further, FIG. 9 shows the wavelength dependence of the half width. It can be seen that the change in the half-value width is smaller and the change in the half-value width is smaller than the characteristic of FIG. 4 used for the liquid crystal alone. Note that the wavelength dependency is rising to the right because the wavelength dependency of polycarbonate is large. Improvement can be made by using a material with less wavelength dispersion.

With the two-dimensional wavelength spectrum of the image in mind, the configuration of the Rio filter (Non-Patent Document 2) shown in FIG. 10 was examined. Three elements having a thickness ratio of 1: 1.5: 2 were stacked with the thickness of the liquid crystal layer set to 1. (Non-patent document 3) Each of the parallel polarizer, the orthogonal polarizer, and the parallel polarizer was configured. The transmission characteristics are shown in FIG. 11, and the half width characteristics are shown in FIG. It can be seen that the full width at half maximum can be narrowed, and the variation in the full width at half maximum is suppressed to about 20% as compared with the case of liquid crystal alone.
B. Lyot: Comptes Rendus, vol. 197, p. 1593 (1933), Ann. Astrophys, vol. 7, p. 31 (1944). K. Sato, N .; Kato, S .; Kano, Y .; Hanazawa and T.H. Uchida: Japan display, p. 392-395 (1989).

The half-width control type tunable liquid crystal filter of the present invention can control the half-width to be constant according to the transmission wavelength region. And it is an invention that can quickly perform spectral analysis of two-dimensional images and videos, and has high industrial utility value.

Basic configuration of devised half-width control type tunable filter. Sectional drawing of the liquid crystal cell orientated in parallel. When a voltage is applied, the liquid crystal molecules change their alignment from around the center of the cell. The surface layer represents a portion where the orientation does not change effectively. Wavelength dependence of retardation with respect to wavelength in nematic liquid crystal. Under the parallel polarizer, the transmittance becomes maximum at PK1, PK2, and PK3 that are integer values. Wavelength dependence of transmittance exhibited by the birefringent liquid crystal mode. As the part where the transmittance shows a peak becomes a longer wavelength, the full width at half maximum increases. Wavelength dependence of retardation Δn (λ) d / λ normalized by wavelength. Although the ratio of the surface layer to the cell thickness is k, k decreases as voltage is applied, and when k = 1, the liquid crystal is vertically aligned and there is no retardation. The wavelength dependence of the half-value width exhibited by the birefringent liquid crystal mode. The wavelength can be varied by the voltage, but the full width at half maximum changes greatly. The concept of half-width control in a birefringent variable wavelength filter. By finding the condition that the inclination of the value obtained by dividing the retardation in the liquid crystal and the birefringent filter by the wavelength is constant, the half width can be made constant. Wavelength dependence of retardation in an element in which a birefringent film and liquid crystal are laminated. The slope of the curve across the part where the transmittance is maximum is made constant. Wavelength dependence of the half-value width in an element in which a birefringent film and liquid crystal are laminated. Compared to FIG. 4, the change of the field is suppressed in the half ratio. Configuration of narrowband wavelength tunable rio filter. Wavelength dependence of transmittance in a rio filter using an element in which a birefringent film and liquid crystal are laminated. There is no change in wavelength even if the wavelength is varied. Wavelength dependence of the half-value width in a rio filter using an element in which a birefringent film and liquid crystal are laminated. The lamination of the birefringent film has a great effect on the constant half width.

Explanation of symbols

LCa, LCb --- liquid crystal cells RFa, RF1b, RF2b, RF3b --- retardation plates P1a, P2a, P1b, P2b, P3b --- polarizers La, Lb --- incident light PSa, PSb --- power supply

Claims (7)

A wavelength tunable filter comprising a light source, two polarizers on the optical axis, and a liquid crystal cell disposed between them and a retardation medium, for example, a uniaxial or biaxial birefringent optical film The retardation dependence between the liquid crystal cell and the birefringent plate is adjusted so that the retardation gradient with respect to the wavelength is constant in the vicinity of the maximum transmittance regardless of voltage application. A half-width control type liquid crystal wavelength tunable filter, characterized in that The wavelength tunable filter according to claim 1, wherein the liquid crystal uses a mode in which birefringence changes in accordance with a change in molecular orientation when a voltage is applied. The wavelength tunable filter according to claim 2, wherein the two polarizers are arranged at a rotation angle of 0 ° or 90 °. 4. The wavelength tunable filter according to claim 3, wherein the delay axes of the liquid crystal and the birefringent optical film are arranged at a rotation angle of 0 ° or 90 ° with respect to each other. The wavelength λ at which the transmittance is maximum is the main wavelength λ _0, and the refractive index anisotropy and thickness of the liquid crystal and the retardation plate at that wavelength are Δn1, Δn2, d1, d2), respectively. In the structure obtained by laminating a liquid crystal and retardation plate, .DELTA.n1 with the voltage applied to the liquid crystal is changed, lambda ₀ is moved. At that time, changes in [Delta] nd / lambda ₀ in lambda ₀ the vicinity of the liquid crystal and the retardation plate is complementary to each other, according to claim 4, resulting in the slope of [Delta] nd / lambda ₀ in lambda ₀ the vicinity is adjusted to be constant Tunable filter. The wavelength tunable filter according to claim 5, wherein a voltage is applied to the liquid crystal cell and the principal wavelength to be transmitted can be arbitrarily varied in the ultraviolet, visible, and infrared regions, and the principal wavelength is single or plural. A wavelength tunable filter in which a plurality of wavelength tunable filters according to claim 6 having the same dominant wavelength are stacked to achieve a narrow band.

Images