JP3201668B2 - Tunable wavelength filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tunable wavelength filter capable of selectively and variably extracting an optical signal of an arbitrary wavelength from a wavelength-multiplexed optical signal in an optical fiber, and capable of changing the wavelength at high speed. .

[0002]

2. Description of the Related Art Optical communication using an optical fiber has recently been rapidly put into practical use because it can transmit a large amount of information at a high speed. However, at the moment, only an optical pulse of a specific wavelength is transmitted. If a large number of light pulses of different wavelengths can be transmitted, a much larger amount of information can be transmitted. This is called wavelength division multiplexing (WDM), and is being actively studied at present. In wavelength division multiplexing communication, a variable wavelength filter is required which selectively selects only light of an arbitrary wavelength from light pulses of many wavelengths.

[0003]

Although various variable wavelength filters have been studied, for example, mechanical grading and mechanical Fabry-Perot etalons have a wide variable width, a wide bandwidth, and a low response. The disadvantage was that the speed was as slow as several hundred msec at most. Further, the acousto-optic (AO) filter has a short response time of several μsec, but has a disadvantage that the bandwidth is wide.

A liquid crystal tunable wavelength filter having a structure in which a nematic liquid crystal homogeneously aligned is sandwiched between an alignment film, a dielectric mirror, a transparent electrode, and a glass plate has a narrow bandwidth (0.1 nm) and a wide variable width (0.1 nm). (100 nm or more), which is excellent as a variable wavelength filter. However, a drawback is that the wavelength sweep speed is as slow as at most several msec.

The present invention has been made in view of the above-mentioned drawbacks, and has as its object to provide a variable wavelength filter capable of changing the wavelength of the liquid crystal variable wavelength filter at a high speed.

[0006]

In order to achieve the above object, a variable wavelength filter according to the present invention comprises a liquid crystal layer sandwiched between a liquid crystal alignment film, a mirror, a transparent electrode and a glass substrate. In a Fabry-Perot type variable wavelength filter for communication that selectively selects only arbitrary light from a pulse, the liquid crystal layer is in a chiral smectic A state.

Hereinafter, the contents of the present invention will be described. here,
In the present invention, the liquid crystal layer in the chiral smectic A state refers to a liquid crystal layer made of a chiral smectic A liquid crystal having a transition temperature to a smectic C phase instead of a conventional nematic liquid crystal.

Here, the chiral smectic A liquid crystal will be described. As the temperature of the smectic A liquid crystal is lowered, the liquid crystal transitions to the smectic C liquid crystal. Transition temperature of 10
In a temperature range up to about ° C., a liquid crystal state called a chiral smectic A layer is taken. The chiral smectic A state has the same orientation as smectic A when no voltage is applied, whereas the chiral smectic A state assumes the smectic C orientation state when a voltage is applied. That is, when no voltage is applied, as shown in FIG. 1A, the orientation vector of the liquid crystal molecules 11 is oriented in the normal direction of the smectic phase and freely rotates around the long axis. When an electric field along the layer is applied, free rotation is hindered. As shown in FIG. 1 (b) when a positive voltage is applied by a plus or minus electric field, and as shown in FIG. 1 (b) when a negative voltage is applied. As shown in FIG. 1C, the liquid crystal molecules 11 are tilted. This tilt angle is linearly proportional to the applied electric field.

This effect is an electroclinic effe
ct) and was discovered by S. Garoffand RBMeyer ("Electroclinic effect a
t the AC Phase change in a Chiral liquid crysta
l, "Phys. Rev., Al9, pp. 338 (1979)). When a voltage is applied to a nematic liquid crystal, the molecules tilt due to the interaction between the dielectric anisotropy of the liquid crystal molecules and the electric field. The force of the electric field applied to the liquid crystal molecules is small, so that the response speed is as slow as several tens of msec.

On the other hand, in the case of the electroclinical effect, the spontaneous polarization of the liquid crystal molecules and the electric field interact with each other, so that the force is large and responds in several μsec to several tens μsec. Therefore, if this is used for a liquid crystal variable wavelength filter, high-speed wavelength sweep can be expected.

[0011]

【Example】

Example 1 FIG. 2A is a structural diagram of a liquid crystal variable wavelength filter of the present invention.
FIG. 2B shows an enlarged view of a main part thereof. Here, the chiral smectic A liquid crystal used was the MDH of BDH.
764E. Since this liquid crystal has a transition point between smectic A and smectic C near 28 ° C., in this example, this element was operated by heating from 28 ° C. to 38 ° C. In FIG. 2A, reference numeral 12 denotes a glass substrate, 13 denotes a non-reflective coating, 14 denotes a transparent electrode, 15 denotes a dielectric mirror (reflectance is 99%), 16 denotes a liquid crystal alignment film, and 17 denotes a liquid crystal alignment film.
Is a chiral smectic A liquid crystal, 18 is a spacer, 19
Shows lead wires for applying a voltage to the liquid crystal.

The orientation of the liquid crystal molecules 11 is parallel to the substrate 12,
Both vertical and vertical directions are possible, but here, they are oriented vertically to the substrate. For parallel alignment, rubbing treatment may be performed in an antiparallel manner as in the case of normal nematic liquid crystal alignment. For vertical alignment, a temperature gradient method may be used. In this example, the thickness of the liquid crystal phase was 4 μm.

FIG. 3 shows a transmission spectrum of the liquid crystal variable wavelength filter in the 1.5 μm band. Bandwidth is about 1n
m, FSR was 150 nm, finesse was about 150, and transmittance was 60%.

FIG. 4 shows the tuning characteristics of the wavelength when a DC voltage is applied to the variable wavelength filter. As shown in the figure, the wavelength shifts to the longer wavelength side with the applied voltage. Further, the wavelength tuning characteristic greatly depends on the temperature (T (temperature) 29 ° C., 32 ° C., 37 ° C.), and the tuning width is widest near the transition point, and it is possible to change the wavelength by about 50 nm.

FIG. 5 shows the response speed (μ
2 shows the temperature dependence of the second embodiment. This response speed hardly depends on the tuning direction and the width of the wavelength, but greatly depends on the temperature. It can be seen from the figure that the higher the temperature, the lower the response speed. In the vicinity of 29 ° C. where the variable width is the widest, the response speed is several μsec, which is larger than that in the case of using a nematic liquid crystal.
It was confirmed that the response speed was three orders of magnitude faster.

[0016]

As described above, the present invention uses a chiral smectic A liquid crystal in place of the conventional nematic liquid crystal, thereby increasing the wavelength sweep speed of the liquid crystal variable wavelength filter by three orders of magnitude. It is possible to be faster.

[Brief description of the drawings]

FIG. 1 shows the orientation of chiral smectic A. In FIG. 1, (a) shows the state when no voltage is applied, and (b) shows the state.
(C) shows the state at the time of voltage application.

FIG. 2 shows a structural diagram of a liquid crystal variable wavelength filter.

FIG. 3 is a transmission spectrum of the variable wavelength filter of the present invention.

FIG. 4 shows tuning characteristics of the variable wavelength filter of the present invention.

FIG. 5 is a diagram showing the temperature dependence of the response speed of the variable wavelength filter of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Liquid crystal molecule 12 Glass substrate 13 Anti-reflection coat 14 Transparent electrode 15 Dielectric mirror 16 Liquid crystal alignment film 17 Cameral smectic liquid crystal 18 Spacer 19 Lead wire which applies voltage to liquid crystal

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G02F 1/13 505 G02F 1/13 500 G02F 1/137

Claims (1)

(57) [Claims] 1. A liquid crystal layer of the liquid crystal alignment film for the mirror, it across the transparent electrode and the glass substrate, selectively Fabry to pick out only any light from the light pulses of multiple wavelengths
A variable wavelength filter for Perot type communication , wherein the liquid crystal layer is in a chiral smectic A state.