JPH05249426A - Wavelength selection filter and transmission wavelength control method - Google Patents

Abstract

PURPOSE:To eliminate the dependence of a Fabry-Perot resonator type filter on polarization. CONSTITUTION:A uniaxial liquid crystal 27 is inserted between two sheets of reflection films 22 which face each other and are parallel with each other. Transparent electrodes 23 which can impress electric fields to the liquid crystal 27 and a temp. control device 29 and temp. control circuit 30 which can control the temp. of the liquid crystal 27 from the outside are provided, by which the wavelength selection filter is provided. The intensity of the electric fields to be impressed to the liquid crystal 27 of this wavelength selection filter and the temp. of the liquid crystal are controlled, by which the resonance peak wavelength of the extraordinary rays of transmitted light and the resonance peak wavelength of ordinary rays are matched with a target wavelength.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength selection filter and a transmission wavelength control method for selecting one target wavelength from signals multiplexed in optical communication.

[0002]

2. Description of the Related Art Conventional wavelength selection filters include diffraction gratings, dielectric multilayer filters, Mach-Zehnder interference type filters, and Fabry-Perot resonator type filters.

By the way, in order to continuously vary the filter wavelength by using a diffraction grating and a dielectric multilayer filter,
It is necessary to mechanically move the filter itself or the optical path before and after it. This example will be described with reference to FIGS. 11 and 12.

FIG. 11 shows an example using a diffraction grating. As shown in the figure, when a wavelength-multiplexed signal is incident from the optical fiber 1 toward the diffraction grating 2, when light of a specific wavelength λ is emitted toward the optical fiber 3, for example, the wavelength λ
_In order to obtain the light of 2, it is necessary to move the diffraction grating 2 and the optical fiber 3 as shown by the broken line.

FIG. 12 shows an example using a dielectric multilayer filter. As shown in the figure, when a wavelength-multiplexed signal is incident from the optical fiber 4 toward the dielectric multilayer filter 5, when light of a specific wavelength λ ₁ is emitted to the optical fiber 6, for example, light of a wavelength λ ₂ is emitted. In order to obtain it, it is necessary to move the dielectric multilayer filter 5 and the optical fiber 6 as shown by the broken line.

A device for continuously varying the wavelength by the method shown in FIGS. 11 and 12 requires mechanical precision, and requires an expensive and large-sized device. Further, in this case, the switching speed of the selected wavelength is slow and the power consumption becomes large.

An example of a Mach-Zehnder interferometer type filter is shown in FIG. As shown in the figure, a Mach-Zehnder interference type filter is usually manufactured on an optical waveguide, and a directional coupler having a branching ratio of 1: 1 is provided between both ends of the first and second optical waveguides 7A and 7B having different lengths. , So-called 3dB coupler 8
It is combined with A and 8B. As a result, the light wave input from the optical fiber 9 is 1: 1 by the 3 dB coupler 8A.
To the first and second optical waveguides 7A and 7A, respectively.
After passing through B, they are multiplexed again by the 3 dB coupler 8B and output to the optical fiber 10. That is, this filter selects a wavelength by splitting a light wave into two to provide an optical path difference, and then overlapping them again to cause interference.

In order to make the filter wavelength variable, a method of changing the optical path difference by changing the refractive index due to the electro-optical effect or the thermo-optical effect is adopted (H. Toba e
t.al., Electron. Lett., 23, 789 (1987)). That is,
As shown in FIG. 13, for example, a thin film heater 11 is provided on the optical waveguide 7A, and when the temperature of the optical waveguide 7A is changed, its refractive index is changed and the optical path difference with the optical waveguide 7B is changed.

However, in the case of changing the filter wavelength as described above, in order to obtain both a wide interference peak interval and a steep filter characteristic by any method, it is necessary to connect filters having different optical path differences in multiple stages. However, there is a problem in that the insertion loss, size, and power consumption increase. Further, since it is a wavelength filter of the optical waveguide, there is a problem in that it is difficult to manufacture, such as connection with an optical fiber, and it becomes expensive.

On the other hand, the Fabry-Perot resonator type filter has two
It is a filter that uses the interference of light that is multiple-reflected between two parallel reflecting surfaces.A transparent substance with an electro-optical effect, such as a liquid crystal, is inserted between the reflecting faces and the interference peak wavelength is changed by applying a voltage to this substance. Although it can be changed, it usually has anisotropy in the direction of change in the refractive index of this substance, and as a result, the transmitted light intensity depends on the polarization state of the incident light.

[0011]

As described above, in the conventional wavelength selection filter, it is not easy to continuously change the filter wavelength, and the transmitted light characteristic depends on the polarization state of the light source. However, there are problems such as size and power consumption, and none satisfying all of these has been realized.

In view of such circumstances, it is an object of the present invention to realize a wavelength selection filter and a control method therefor, by eliminating the polarization dependence in a Fabry-Perot resonator filter. ..

[0013]

A wavelength selection filter according to the present invention that achieves the above object has two reflecting surfaces facing each other and parallel to each other, and a uniaxial property sandwiched between these two reflecting surfaces. And a transparent electrode capable of applying an electric field to the liquid crystal layer, and a temperature control element capable of externally controlling the temperature of the liquid crystal layer.

The transmission wavelength control method according to the present invention is
Two reflective surfaces facing each other and parallel to each other, a uniaxial liquid crystal layer sandwiched between these two reflective surfaces, a transparent electrode capable of applying an electric field to the liquid crystal layer, and a temperature of the liquid crystal layer are set. Using a wavelength selection filter equipped with a temperature control element that can be controlled from the outside, the temperature of the liquid crystal layer is controlled via the temperature control element, and the light that is perpendicularly incident on the reflecting surface passes through the liquid crystal layer. By changing the refractive index in the direction of the ray and the refractive index in the extraordinary ray direction, the resonance peak wavelength of the ordinary ray transmitted through the filter is matched with the target wavelength, and the liquid crystal layer is passed through the transparent electrode. It is characterized in that the resonance peak wavelength of the extraordinary ray transmitted through the filter is matched with the target wavelength by controlling the applied electric field strength and changing only the refractive index in the extraordinary ray direction.

[0015]

In the wavelength selection filter having the above structure, the refractive index of the liquid crystal as the refractive index medium can be controlled by providing the function of controlling the temperature of the filter, and as a result, the resonance peak wavelength of the filter can be controlled. .. At this time, it is possible to control both the resonance peak in the extraordinary ray direction and the resonance peak in the ordinary ray direction when the light perpendicularly incident on the reflecting surface passes through the liquid crystal by temperature. By using this together with the control of only the resonance peak in the light ray direction, it becomes possible to freely control these respective peaks. That is, in the filter, the resonance peak wavelength of the ordinary ray and the resonance peak wavelength of the extraordinary ray can be matched with the desired wavelength by appropriately controlling the temperature and the applied voltage as in the above method. Then, as a result of such an action, it is possible to obtain a filter characteristic that does not depend on the polarization state of the input light.

[0016]

EXAMPLES The present invention will be described below based on examples.

FIG. 1 is a block diagram of a wavelength selection filter according to an embodiment. As shown in the figure, this wavelength filter is a Fabry-Perot resonator type filter, and a light reflection film 22 and a transparent electrode 2 are formed on a smooth surface of a transparent substrate 21.
3 and two substrates 25 in which an alignment film 24 for orienting liquid crystal is sequentially laminated are used, and these two substrates 25 face each other in parallel with the transparent electrode 23 inside, and a distance between the substrates is provided therebetween. 26 for keeping the temperature constant and the uniaxial liquid crystal 2
It has a structure in which 7 is sandwiched. A voltage source 28 is connected to the pair of transparent electrodes 23. Further, a temperature control device 2 such as a Peltier device is provided outside the transparent substrate 21.
9 is provided, and a temperature control circuit 30 is connected to the temperature control device 29.

Here, the transparent substrate 21 may be made of a material such as quartz glass, which is transparent in the intended wavelength range. A dielectric mirror or the like may be used for the reflective film 22, and a material such as ITO (Indium Tin Oxide) may be used for the transparent electrode 23.

The uniaxial liquid crystal 27 is a liquid crystal having no refractive index anisotropy when viewed from a specific uniaxial direction, but having a refractive index anisotropy when viewed from another direction. The type is not limited as long as it has such properties. The liquid crystal 27 is aligned by the alignment film 24, but at least one of the alignment films 24 needs to be subjected to parallel alignment treatment.

The positions of the reflective film 22 and the transparent electrode 23 may be exchanged with each other. Instead of forming the alignment film 24, an alignment treatment such as melting and rubbing (rubbing the substrate surface lightly with a cotton cloth) may be performed. further,
If the distance between the substrates 25 can be kept constant,
The spacer 26 does not necessarily have to be provided.

The incident light 3 enters the wavelength selection filter as described above.
When 1 is incident, the distance between the reflection films 22 is d, and the liquid crystal 27
Let n be the refractive index of n, only light having a resonance wavelength λ that satisfies 2nd = mλ (m is an integer of 1 or more) is emitted as the transmitted light 32. At this time, the refractive index n of the liquid crystal 27 generally changes depending on the temperature, the applied voltage and the like. Therefore, the voltage applied to the transparent electrode 23 is controlled via the voltage source 28, and the temperature control device 29 is controlled via the temperature control circuit 30.
By controlling the temperature of, the refractive index of the liquid crystal 27 can be controlled and the peak wavelength λ of the transmitted light 32 can be varied.

2 to 5 are conceptual views showing an example of a liquid crystal molecule array uniformly aligned in the layer of the liquid crystal 27 sandwiched between the two alignment films 24 in the above embodiment. FIG. 2 is a view seen from the direction perpendicular to the reflection film 22, in which A is the direction of the projected image of the optical axis of the uniaxial liquid crystal molecule on the reflection surface, and B is the direction orthogonal to these. At this time, A and B are the polarization directions of the extraordinary ray and the ordinary ray when light passes through the liquid crystal. 3 and 4 are respectively shown in FIG.
It is a projection view seen from the A and B directions. FIG. 5 shows one molecule of this liquid crystal, and the arrow in the figure is its optical axis.

Assuming that the refractive indices of the liquid crystal 27 in the A and B directions in FIG. 2 are n _e and n _o , respectively, the projection image of the optical axis of the liquid crystal 27 on the reflecting surface is oriented in the A direction at all times. If you are only n _e is changed depending on the applied voltage, n _o
Does not change. Also, the case of changing the temperature of the liquid crystal 27, generally, n _e, n _o both change. Examples showing this state are shown in FIGS. 6 and 7, respectively. 33, 34 in the figure
Indicate the states of n _e and n _o , respectively.

In the transmitted light intensity characteristic of such a Fabry-Perot resonator, the resonance peak wavelength λ _e of the extraordinary ray depending on the refractive index n _e and the resonance peak wavelength λ _o of the ordinary ray depending on the refractive index n _o. And exist. FIG. 8 shows an example of the wavelength characteristic of the transmittance 32 in the filter of the above-mentioned embodiment.
Peaks 35 and 36 in the figure respectively indicate the resonance peak wavelength λ _e of the extraordinary ray and the resonance peak wavelength λ _o of the ordinary ray. In the Fabry-Perot type resonator wavelength selective filter of this embodiment, the resonance peak wavelength λ _e of the extraordinary ray and the resonance peak wavelength λ _o of the ordinary ray can be simultaneously controlled by controlling the voltage and the temperature. Is.

Next, using the Fabry-Perot type resonance wavelength selection filter of the above embodiment, the resonance peak 35 (λ _e ) of the extraordinary ray and the resonance peak 36 of the ordinary ray in FIG.
A procedure for controlling (λ _o ) so as to overlap the target wavelength 37 will be described.

In this embodiment, the resonance peak 35 (λ _e ) of the extraordinary ray and the resonance peak 3 of the ordinary ray in FIG.
The temperature of the filter is first controlled so that the peak 36 overlaps with the target wavelength 37 by utilizing the fact that any of 6 (λ _o ) changes with temperature. This temperature control is performed by controlling the temperature of the temperature control device 29 with the temperature control circuit 30. In this way, when the peak 36 and the wavelength 37 overlap each other as shown in FIG. 9, the peak 35 is then moved by a voltage so that the peak 35, the peak 36, and the wavelength 37 all overlap. This control is performed by controlling the voltage applied to the transparent electrode 23 by the voltage source 28. FIG. 10 shows a state in which the peaks 35 and 36 and the wavelength 37 overlap each other in this way. In this state, both the A direction (extreme ray direction) and the B direction (ordinary ray direction) in FIG. 2 have the same wavelength. Resonance peak appears. That is, by controlling in this way, the intensity of the transmitted light 32 does not change depending on the polarization state of the incident light 31.

As described above, by using the wavelength selection filter having the structure shown in FIG. 1 and controlling as described above, a polarization-independent wavelength tunable filter can be provided so as to be particularly suitable for optical fiber communication and the like. Can be realized.

[0028]

As described above, in the wavelength selection filter according to the present invention, the resonance peak wavelength λ of the extraordinary ray is
_{Both e} and the resonance peak wavelength λ _o of the ordinary ray can be freely set, and by controlling the filter as in the method of the present invention, the transmitted light characteristic depends on the polarization state of the incident light. A filter that does not can be realized.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a wavelength selection element according to an embodiment.

FIG. 2 is a conceptual diagram showing an alignment of liquid crystal molecules as seen from a direction perpendicular to a reflection film of FIG.

FIG. 3 is a projection view from the direction A in FIG.

FIG. 4 is a projection view from the direction B in FIG.

FIG. 5 is an explanatory diagram showing liquid crystal molecules and their optical axes.

FIG. 6 is an explanatory diagram showing applied voltage dependence of refractive indices n _e and n _o in the example.

FIG. 7 is an explanatory diagram showing temperature dependence of refractive indices n _e and n _{o in} the example.

FIG. 8 is an explanatory diagram showing a process of controlling a transmission wavelength in one example.

FIG. 9 is an explanatory diagram showing a process of controlling a transmission wavelength in one example.

FIG. 10 is an explanatory diagram showing a process of controlling a transmission wavelength in one example.

FIG. 11 is an explanatory diagram showing a conventional wavelength selection method using a diffraction grating.

FIG. 12 is an explanatory diagram showing a wavelength selection method for a conventional dielectric multilayer filter.

FIG. 13 is a configuration diagram showing a conventional Mach-Zehnder interferometer type filter.

[Explanation of symbols]

21 Transparent Substrate 22 Reflective Film 23 Transparent Electrode 24 Alignment Film 25 Substrate 26 Spacer 27 Uniaxial Liquid Crystal 28 Voltage Source 29 Temperature Control Device 30 Temperature Control Circuit 31 Incident Light 32 Transmitted Light 35 Extraordinary Ray Resonance Wavelength Peak (λ _e ) 36 Normal Resonance wavelength peak of light (λ _o ) 37 Target wavelength

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[Procedure amendment]

[Submission date] November 26, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0028

[Correction method] Change

[Correction content]

[0028]

As described above, in the wavelength selection filter according to the present invention, the resonance peak wavelength λ of the extraordinary ray is
_{Both e} and the resonance peak wavelength λ _o of the ordinary ray can be set freely, and the transmitted light characteristic depends on the polarization state of the incident light by controlling the filter as in the method of the present invention. A filter that does not can be realized.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Name of item to be corrected] Brief description of the drawing

[Correction method] Change

[Correction content]

[Brief description of drawings]

FIG. 1 is a configuration diagram of a wavelength selection element according to an embodiment.

FIG. 2 is a conceptual diagram showing an alignment of liquid crystal molecules as seen from a direction perpendicular to a reflection film of FIG.

FIG. 3 is a projection view from the direction A in FIG.

FIG. 4 is a projection view from the direction B in FIG.

FIG. 5 is an explanatory diagram showing liquid crystal molecules and their optical axes.

FIG. 6 is an explanatory diagram showing applied voltage dependence of refractive indices n _e and n _o in the example.

FIG. 7 is an explanatory diagram showing temperature dependence of refractive indices n _e and n _{o in} the example.

FIG. 8 is an explanatory diagram showing a process of controlling a transmission wavelength in one example.

FIG. 9 is an explanatory diagram showing a process of controlling a transmission wavelength in one example.

FIG. 10 is an explanatory diagram showing a process of controlling a transmission wavelength in one example.

FIG. 11 is an explanatory diagram showing a conventional wavelength selection method using a diffraction grating.

FIG. 12 is an explanatory diagram showing a wavelength selection method for a conventional dielectric multilayer filter.

FIG. 13 is a configuration diagram showing a conventional Mach-Zehnder interferometer type filter.

[Explanation of Codes] 21 Transparent Substrate 22 Reflective Film 23 Transparent Electrode 24 Alignment Film 25 Substrate 26 Spacer 27 Uniaxial Liquid Crystal 28 Voltage Source 29 Temperature Control Device 30 Temperature Control Circuit 31 Incident Light 32 Transmitted Light 35 Resonant Wavelength Peak of Extraordinary Ray ( λ _e ) 36 Resonant wavelength peak of ordinary ray (λ _o ) 37 Target wavelength

Claims (2)

[Claims] 1. Two reflective surfaces facing each other and parallel to each other, a uniaxial liquid crystal layer sandwiched between these two reflective surfaces, and a transparent electrode capable of applying an electric field to the liquid crystal layer, A wavelength selection filter comprising: a temperature control element capable of controlling the temperature of a liquid crystal layer from the outside. 2. A pair of reflecting surfaces facing each other and parallel to each other, a uniaxial liquid crystal layer sandwiched between these two reflecting surfaces, a transparent electrode capable of applying an electric field to the liquid crystal layer, Using a wavelength selection filter equipped with a temperature control element capable of controlling the temperature of the liquid crystal layer from the outside, the temperature of the liquid crystal layer is controlled through the temperature control element, and the light which is vertically incident on the reflecting surface passes through the liquid crystal layer. By changing the refractive index in the direction of the ordinary ray when passing and the refractive index in the extraordinary ray direction, the resonance peak wavelength of the ordinary ray transmitted through the filter is matched with the target wavelength, and the transparent electrode is It is characterized in that the resonance peak wavelength of the extraordinary ray transmitted through the filter is matched with the target wavelength by controlling the electric field intensity applied to the liquid crystal layer via the filter to change only the refractive index in the extraordinary ray direction. Transmitted wave Long control method.