JPH04220618A - variable wavelength filter - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention applies to 1.3 to 1.55 μm.
This invention relates to a liquid crystal etalon type variable wavelength filter for band wavelength multiplexing and frequency multiplexing communications.

0002

2. Description of the Related Art Optical communication using optical fibers is capable of transmitting large amounts of information at high speed, and has recently been rapidly put into practical use. However, at present, they only transmit light pulses of a specific wavelength. If optical pulses of many different wavelengths can be transmitted, even larger amounts of information can be transmitted. This is called wavelength division multiplexing communication, and is currently being actively researched. In wavelength division multiplexing communication, a variable wavelength filter is required to selectively select only light of an arbitrary wavelength from among optical pulses of many wavelengths.

[0003] There are various variable wavelength filters, but there is one in which the etalon is filled with liquid crystal and the optical gap of the etalon is varied by applying a voltage (Stephen R. Mallinson,
Applied Optics Vol. 23, N
o. 3 (1987) p. 430). However, in this variable wavelength filter using liquid crystal, the loss in the liquid crystal layer and the alignment film are large, and the reflectance of the mirror is as low as 90%, so the half width of the peak is 1 nm, the finesse is 30, and the variable width is 15 nm. The transmittance is 38%, and the performance is poor, making it difficult to apply to wavelength multiplexing and frequency multiplexing. The reason for this is that the scattering and absorption by the liquid crystal and liquid crystal alignment film are large.

[0004] Also, JP-A No. 62-178219 (optical wavelength selection element), JP-A No. 63-29737 (optical wavelength selection element), and JP-A No. 63-5327 (Fabry-Perot resonator).
, JP 58-3524 (Liquid crystal Fabry-Perot interference device), British patent GB2219099A (Tunabl
Patent applications have been filed for inventions relating to liquid crystal entero type variable wavelength filters such as Fabry-Perot Filter. However, the specifications of these inventions do not describe any methods for reducing scattering in the liquid crystal layer, absorption in the transparent electrode, and absorption in the alignment film.

[0005] When a Fabry-Perot etalon is filled with a liquid crystal, an alignment film, and an electrode, the properties of the etalon become extremely poor due to their absorption and scattering (the reason for this will be described later). For example, these inventions have a structure in which a transparent electrode is placed on a dielectric mirror or a structure in which a metal mirror is provided, but the indium tin oxide (ITO) transparent electrode has a thickness of 1.3 to 1.55 μm. Since the band has a large amount of light absorption, the absorption inside the etalon increases, resulting in very poor characteristics. Furthermore, when a metal mirror is used, 1.3 to 1.
In the 55 μm band, the light reflectance cannot be increased to 98% or more, and the characteristics are poor. In addition, the liquid crystal and alignment film absorb and scatter light to a large extent. The above-mentioned specification of the invention only shows the base structure, but does not mention anything about the thickness of the liquid crystal alignment film that reduces scattering and absorption inside the etalon, the rubbing conditions, the material of the transparent electrode, and the film thickness. It has not been.

[0006]

Problems to be Solved by the Invention The present invention has a structure that reduces absorption and scattering inside a liquid crystal etalon and improves the characteristics (finesse, transmittance) of the etalon.
The object of the present invention is to provide a variable wavelength filter for wavelength multiplexing and frequency multiplexing communications in the 5 μm band.

[0007]

[Means for Solving the Problems] The present invention provides a variable wavelength filter filled with liquid crystal with a wavelength of 1.3 to 1.55 μm.
In order to achieve transmittance and finesse applicable to wavelength division multiplexing communications and tunable wavelength lasers, absorption and scattering within the etalon is reduced. That is, a transparent electrode is placed between the glass substrate and the mirror, the transparent electrode is made of indiumtin oxide (ITO) with a thickness of 500 nm or less, the reflectance of the dielectric mirror is 98% or more, and the alignment film is 100 nm or less.
By making the nematic liquid crystal thinner and rubbing it antiparallel to each other and aligning the liquid crystal molecules of the nematic liquid crystal homogeneously, scattering and absorption by the liquid crystal are reduced, the peak width of the liquid crystal etalon type variable wavelength filter is narrowed, and finesse is increased. , increase the transmittance.

[0008]

[Operation] We will briefly explain that an etalon filled with liquid crystal operates as a variable wavelength filter. Figure 1 shows the structure of a tunable wavelength filter. 1 is a glass substrate, 2 is a non-reflective coating film, 3 is a transparent electrode, 4 is a dielectric mirror, 5 is a liquid crystal alignment film, 6 is a liquid crystal, 7 is a spacer, and 8 is a lead wire for applying a voltage to the liquid crystal. A glass substrate 1 (with a non-reflection coating 2 on the back surface) on which a dielectric mirror 4 is deposited is spaced L.
When arranged in parallel, the wavelength dependence of transmittance is expressed by the following equation. T=1/[1+Fsin2 (2
πnL/λ)] (1) where n is the refractive index of the etalon cavity, λ is the wavelength, and F is the F of the etalon. F=4R/(1-R)2

(2) The transmission spectrum is shown in Figure 2. Many sharp peaks appear. The half-width of this peak depends on the reflectance of the mirror, and normally in the case of a 99% mirror, it will be a sharp peak with a fanness of 200 or more. The peak wavelength λres is λres = m/2nL
(m=1,2,....)
(3) It is expressed as When an alignment film is applied on the dielectric mirror 4, rubbed antiparallelly, and filled with nematic liquid crystal, the liquid crystal is aligned as shown in FIG. 3(a). 9 is a liquid crystal molecule. The liquid crystal molecules 9 have large dielectric anisotropy (ne, no), and when the polarization direction of incident light matches the orientation direction of the liquid crystal, this light senses a refractive index of ne. When a voltage is applied to this, the liquid crystal rises as shown in FIG. 3(b), and the refractive index of the polarized light changes from ne to no. Therefore, according to equation (3), the peak wavelength is shifted by applying a voltage to the liquid crystal layer. Equation (1) is for the case where there is absorption and scattering inside the etalon cavity, but if there is a material that absorbs and scatters inside the etalon cavity, the transmittance is T=Tmax /{1+Fsin2 (2πmL /
λ)} (
4) Tmax: Maximum transmittance = {(1-R)2ex
p (-αL)}/{1-Rexp(-αL)}2
(5) F=4Rexp(-αL)/(1-Rexp
(-αL)}2
(6) Finesse = πF/2

(7) FWEM:
Half width=2λo/KπF
(8) Here, α is the absorption coefficient of the cavity, L is the cavity gap, R is the reflectance of the mirror, and m is an integer.

The dependence of the maximum transmittance and the F value of the etalon on the reflectance of the mirror is shown in FIG. 4 (a) using α×L as a parameter.
), (b). Increasing the F value narrows the half width and increases finesse. It can be seen that in order to narrow the half-width, increase finesse, and increase transmittance, it is necessary to increase the reflectance of the mirror to 90% or more and to reduce scattering and absorption of the liquid crystal and alignment film.

The reason why liquid crystal exhibits absorption and scattering is that the axes of liquid crystal molecules are not oriented in uniform directions.

[0011] We investigated conditions for minimizing absorption and scattering of liquid crystals regarding various rubbing treatments for alignment films. The orientation states of the liquid crystals studied are shown in Figures 5(a), (b), and (
Shown in c) and (d). → indicates the direction of the rubbing process.

[Example 1] indicates that the rubbing direction is perpendicular to the paper surface, and is from the front to the back of the paper and from the back to the front, respectively. Among these, the ones with the least scattering were (a) and (b). However, in (a), scattering increased as the voltage was applied. As a result, it was found that the nematic liquid crystals can be narrowed using substrates that have been rubbed so that they are antiparallel to each other, as shown in (b). The absorption coefficient at that time is α=0.5
cm-1.

Further, the absorption and scattering of the alignment film itself is hardly affected by light rubbing treatment, but it tends to increase when the film thickness is increased to 100 nm or more. Here, the alignment film was designed to have a thickness of 100 nm or less.

[0013] The transmittance of the ITO transparent electrode depends on the film thickness.
In the 1.5 μm band, the transmittance drops rapidly with increasing film thickness. Figure 6 shows the dependence of transmittance on film thickness. By setting the film thickness to 50 nm or less, a transmittance of 90% or more can be obtained. According to the inventions reported so far, since the transparent electrode is formed on the mirror, it becomes an absorption layer within the etalon, and the transmittance and finesse of the etalon are significantly deteriorated. Therefore, it is desirable that the transparent electrode be placed outside the etalon resonator, that is, between the glass and the mirror.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows the structure of a variable wavelength filter according to the present invention. The substrate is a 25×20×6 mm quartz substrate. A 40 nm thick layer of ITO was formed on this by sputtering. IT
The O transmittance was 92.3%. Next, TiO2 and Si
A dielectric mirror made of a multilayer film of O2 was formed by a vapor deposition method. The reflectance was 98.9%. Furthermore, an alignment film was applied to a thickness of 60 nm, and rubbed so that they were antiparallel to each other. An 18 μm spacer is sandwiched between this pair of substrates,
After adjusting the substrates so that they were parallel, they were adhered. Manetic liquid crystal was filled and the device fabrication was completed.

[0015] When no voltage is applied, when voltage is applied (V=10V)
The transmission spectra of the element are shown in Figures 7(a) and (b). Sharp peaks were observed, and by applying voltage, peaks A,
It was confirmed that B shifted as shown in the figure. FIG. 7(C) is an enlarged view of FIG. 7(a), and as shown in this figure, the half-width is 0.23 nm. finesse is 210
It is. The transmittance is 70%. The interval between peaks in the 1.5 μm band is about 45 nm. FIG. 8 shows the dependence of peak wavelength on applied voltage. By applying voltage, the peak wavelength changes from 1.53μm to 1.48μm.
Shift to μm. Half width and transmittance are not affected by voltage application.

[0016]

As explained above, the tunable wavelength filter of the present invention differs from the conventional liquid crystal etalon type tunable wavelength filter in that the transparent electrode is disposed between the glass substrate and the dielectric mirror. is injumutin oxide with a thickness of 50 nm or less, the thickness of the liquid crystal alignment film is 100 nm or less, and the alignment film is rubbed so that it becomes antiparallel to each other when the substrates are placed facing each other. By making the dielectric mirror nematic and having a reflectance of 98% or more, it is possible to realize a variable wavelength filter with a narrow half-width, high finesse, and high transmittance. It becomes possible to apply it to wavelength multiplexing communications and wavelength tunable lasers in the 55 μm band.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the structure of a variable wavelength filter.

FIG. 2 is a diagram showing a transmission spectrum of a liquid crystal etalon type variable wavelength filter.

FIG. 3(a) is a diagram showing the alignment state of liquid crystal molecules in a liquid crystal etalon type variable wavelength filter when no voltage is applied. (b) is a diagram showing the alignment state of liquid crystal molecules of the liquid crystal etalon type variable wavelength filter when voltage is applied.

[Figure 4] (a) shows the maximum transmittance (Tma) with αL as a parameter for an etalon with absorption and scattering cavities.
FIG. (b) is a diagram showing the dependence of the F value on the reflectance of the mirror (calculation results) using αL of an etalon having a cavity with absorption and scattering as a parameter.

FIG. 5(a) is a diagram showing the examined liquid crystal alignment state when the liquid crystal alignment films are parallel to each other. (b) is a diagram showing the examined liquid crystal alignment state when the liquid crystal alignment films are antiparallel to each other. (c) is a diagram showing the examined liquid crystal alignment state when the liquid crystal alignment films are at right angles to each other and the direction of the rubbing treatment is from the back to the front of the page. (d) is a diagram showing the examined liquid crystal alignment state when the liquid crystal alignment films are perpendicular to each other and the direction of the rubbing treatment is from the front to the back of the paper.

FIG. 6 is a diagram showing the film thickness dependence of transmittance in the 1.5 μm band of an indium tin oxide (ITO) transparent electrode.

FIG. 7(a) is a diagram showing the transmission spectrum of the liquid crystal etalon type variable wavelength filter of the present invention when no voltage is applied. (b) is a diagram showing the transmission spectrum of the liquid crystal etalon type variable wavelength filter of the present invention when voltage is applied (V=10V). (c) is an enlarged view of the transmission spectrum shown in (a).

FIG. 8 is a diagram showing the applied voltage dependence of the peak wavelength of a liquid crystal etalon type variable wavelength filter.

[Explanation of symbols]

1 Glass substrate 2. Non-reflective coating 3 Transparent electrode 4 Dielectric mirror 5 Liquid crystal alignment film 6.LCD 7 Spacer 8 Lead wire 9 Liquid crystal molecules

Claims (3)

[Claims] 1. A variable wavelength filter having a structure in which a liquid crystal layer is sandwiched between a pair of substrates each having a transparent electrode, a dielectric mirror, and an alignment film for liquid crystal on a glass substrate, wherein the alignment film for liquid crystal covers the substrate. A tunable wavelength filter characterized in that the filter is rubbed so that the filters are antiparallel to each other when they are opposed to each other, and that the liquid crystal filled therein is of a manetic type. 2. The variable wavelength filter according to claim 1, wherein the transparent electrode is disposed between a glass substrate and a dielectric mirror. 3. The transparent electrode is made of indium oxide with a thickness of 50 nm or less, and the thickness of the liquid crystal alignment film is 10 nm or less.
0 nm or less, and the reflectance of the dielectric mirror is 98
% or more.
The tunable wavelength filter described in .