JP2011039141A - Variable wavelength optical filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a variable wavelength optical filter which accurately controls the distance between reflection films. <P>SOLUTION: The variable wavelength optical filter includes: a first substrate 10 having a first movable part 11 which is movable in thickness direction and a second movable part 12 which is movable in thickness direction; a second substrate 20 which forms an internal space 13 between the second substrate 20 and the first substrate 10 and connected to the first substrate 10; a third substrate 30 which is disposed on the side opposite to the second substrate 20 interposing the first substrate 10; a piezoelectric element 50 pinched between the second movable part 12 and the third substrate 30; a liquid 60 which fills the internal space 13; a first reflection film 15 provided on the face 14 of the first movable part 11; and a second reflection film 22 provided on the second substrate. According to the dislocation in the thickness direction of the piezoelectric element 50, the second movable part 12 is dislocated toward one side in the thickness direction, pressure of the liquid 60 is applied on the first movable part 11 to dislocate the first movable part to the other side in the thickness direction, thus the distance G between the first reflection film 15 and the second reflection film 22 varies. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a wavelength tunable optical filter.

  Conventionally, regarding an optical device typified by a wavelength tunable optical filter, a gap (internal space) that allows displacement of the movable portion is formed between the first structure having the movable portion and the first structure. A second structure bonded to the first structure, a first reflective film provided on the movable portion, and a second reflective film provided on the second structure, The first drive electrode and the second structure provided in the movable part are configured to emit light having a wavelength according to the distance between the first reflective film and the second reflective film to the outside. There is known a configuration in which an electrostatic attractive force is generated by applying a voltage between the provided second drive electrode and the movable part is displaced (for example, see Patent Document 1).

JP 2008-116669 A

In the optical device described above, it is important to accurately control the distance between the first reflective film and the second reflective film, but the distance becomes extremely short depending on the wavelength of the target light. (For example, several hundred nm to several μm).
From this, in the optical device, the electrostatic drive between the first drive electrode and the second drive electrode is caused by electrostatic attraction generated by applying a voltage between the first drive electrode and the second drive electrode. Contact and sticking may occur.
As a result, the optical device may have difficulty in accurately controlling the distance between the first reflective film and the second reflective film.

  SUMMARY An advantage of some aspects of the invention is to solve at least a part of the problems described above, and the invention can be implemented as the following forms or application examples.

  [Application Example 1] A wavelength tunable optical filter according to this application example is formed around a first movable part formed to be displaceable in the thickness direction and displaceable in the thickness direction around the first movable part. A first substrate having light transmissivity provided with a second movable portion, and an interior allowing displacement of the first movable portion and the second movable portion between the first substrate and the first substrate. A light-transmitting second substrate that forms a space and is bonded to the first substrate, and is disposed on the opposite side of the second substrate across the first substrate. The bonded third substrate, the piezoelectric element sandwiched between the second movable part and the third substrate, the liquid filled in the internal space, and the first movable part A first reflective film provided on a surface on the second substrate side, and opposite the first reflective film on the second substrate; And the second reflection film is repeatedly reflected by light between the first reflection film and the second reflection film to cause interference, thereby causing the first reflection film and the second reflection film to be interfered with each other. The second movable part is configured to be capable of emitting light having a wavelength according to a distance between the two reflective films to the outside, and in accordance with the displacement in the thickness direction of the piezoelectric element generated by voltage application. The first movable part is displaced to the other of the thickness direction by the pressure applied to the liquid by being displaced in one of the thickness directions, and the distance between the first reflective film and the second reflective film is Changes.

According to this configuration, the wavelength tunable optical filter has the first pressure applied to the liquid by the displacement of the second movable portion in one of the thickness directions in accordance with the displacement in the thickness direction of the piezoelectric element generated by voltage application. The movable part is displaced to the other in the thickness direction, and the distance between the first reflective film and the second reflective film changes.
Thus, the wavelength tunable optical filter can be realized as a wavelength tunable optical filter without contact or sticking between electrodes, which is a problem of the prior art using the opposed drive electrodes.
As a result, the wavelength tunable optical filter can accurately control the distance between the first reflective film and the second reflective film.

In addition, since the wavelength tunable optical filter changes the distance between the first reflective film and the second reflective film via the liquid, it is between the first reflective film and the second reflective film. The distance can be easily controlled as compared with the conventional case where the distance is changed by electrostatic attraction in a space.
As a result, the wavelength tunable optical filter can improve the responsiveness of distance control.

  Application Example 2 In the wavelength tunable optical filter according to the application example, it is preferable that the liquid can be selected from a plurality of types having different refractive indexes.

  According to this configuration, since the wavelength tunable optical filter can be selected from a plurality of types of liquids having different refractive indexes, the first reflective film can be selected by selecting different liquids among the plurality of wavelength tunable optical filters. Without changing the distance between the first reflective film and the second reflective film, light having different wavelengths can be emitted to the outside.

  Application Example 3 In the wavelength tunable optical filter according to the application example, it is preferable that the liquid is configured to be exchangeable with another liquid having a different refractive index.

  According to this configuration, the wavelength tunable optical filter is configured such that the liquid can be exchanged with another liquid having a different refractive index. Therefore, in one wavelength tunable optical filter, the liquid is exchanged with another liquid having a different refractive index. The effect similar to the application example 2 can be acquired by replacing.

  Application Example 4 In the wavelength tunable optical filter according to the application example described above, a first antireflection film is provided on the surface of the first movable portion opposite to the surface on which the first reflection film is provided. It is preferable.

  According to this configuration, the wavelength tunable optical filter includes the first antireflection film on the surface opposite to the surface on which the first reflective film of the first movable portion is provided. For example, light incident from the outside on the first substrate side can be efficiently incident on the first reflective film by suppressing reflection of light on the surface opposite to the surface on which the reflective film is provided. .

  Application Example 5 In the wavelength tunable optical filter according to the application example described above, a second antireflection film is provided on the surface of the second substrate opposite to the surface on which the second reflection film is provided. It is preferable.

  According to this configuration, the wavelength tunable optical filter has the second antireflection film provided on the surface of the second substrate opposite to the surface on which the second reflection film is provided. The light having a wavelength according to the distance between the first reflective film and the second reflective film transmitted from the first substrate side is transmitted to the first surface on the side opposite to the surface on which the second reflective film is provided. Can be efficiently emitted to the outside on the second substrate side.

The schematic diagram which shows schematic structure of the wavelength variable optical filter of this embodiment. FIG. 6 is a schematic cross-sectional view illustrating the operation of the wavelength tunable optical filter.

DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, embodiments of the invention will be described with reference to the drawings.
(Embodiment)
FIG. 1 is a schematic diagram illustrating a schematic configuration of a wavelength tunable optical filter according to the present embodiment. FIG. 1A is a plan view seen from the second substrate side, and FIG. 1B is a cross-sectional view taken along line AA of FIG.

As shown in FIG. 1, the wavelength tunable optical filter 1 receives light and emits only light (interference light) corresponding to a specific wavelength among the wavelengths of the light by interference action.
The wavelength tunable optical filter 1 includes a first substrate 10 having optical transparency, a second substrate 20 having optical transparency, and a third substrate 30.
The wavelength tunable optical filter 1 has a laminated structure in which the second substrate 20 is bonded to one side of the first substrate 10 and the third substrate 30 is bonded to the other side of the first substrate 10. Yes.

The first substrate 10 is formed in a substantially circular planar shape, and is formed so as to be displaceable in an arrow B direction that is a thickness direction (a direction orthogonal to the extending direction), A second movable portion 12 is also provided around the movable portion 11 and formed so as to be displaceable in the thickness direction.
The second substrate 20 has a substantially circular planar shape and is provided with a recess on the first substrate 10 side, so that the first movable portion 11 and the second movable portion can be moved between the second substrate 20 and the first substrate 10. An internal space 13 that allows displacement of the portion 12 is formed, and is joined to one side of the first substrate 10.
The third substrate 30 is formed in an annular shape so as not to overlap the first movable portion 11 but to overlap the second movable portion 12 in plan view, and is formed on the other side of the first substrate 10. It is joined. That is, the third substrate 30 is disposed on the opposite side of the second substrate 20 with the first substrate 10 interposed therebetween. The second substrate 20 and the third substrate 30 are formed to have higher rigidity than the first substrate 10 in order to suppress deformation.

The material of the first substrate 10 and the second substrate 20 is not particularly limited as long as it has light transmittance with respect to the wavelength of light to be used. For example, soda glass, crystalline glass, quartz glass, lead glass, Examples include various glasses such as potassium glass, borosilicate glass, sodium borosilicate glass, and alkali-free glass, and crystals.
As the material of the third substrate 30, light transmission is not necessary, but it is preferable to use a material having a linear expansion coefficient approximate to that of the first substrate 10 from the viewpoint of suppressing thermal stress.
In addition, it is preferable to use glass of the same material for each substrate from the viewpoints of optical characteristics in the visible light region, dimensional accuracy, mechanical characteristics, suppression of thermal stress, and the like.

In addition, for joining the first substrate 10 and the second substrate 20, for example, (1) a joining method using low-melting glass, and (2) hydrophilizing the joining surfaces of each other by plasma irradiation, etc. Direct bonding method such as hydrogen bonding by activating and bonding the bonding surfaces together, (3) Bonding method for bonding by ultraviolet or plasma irradiation using a bonding member containing alkoxide, organosiloxy group, etc. (4) A eutectic bonding method using a metal film such as a gold-tin alloy film as a bonding member is used.
Further, for bonding the first substrate 10 and the third substrate 30, a non-conductive adhesive 40 such as epoxy or polyimide is used except for a part described later.

Inside the outer peripheral portion of the first substrate 10, the second movable portion 12 is formed so as to be recessed annularly on the second substrate 20 side, and on the inner side from the outer peripheral portion of the third substrate 30. Thus, a piezoelectric element (piezo element) 50 is sandwiched between the support portion 31 of the third substrate 30 that is formed so as to protrude annularly toward the first substrate 10.
The piezoelectric element 50 is used as an actuator that displaces the second movable portion 12 in the thickness direction (arrow B direction) by the piezoelectric effect.
The piezoelectric element 50 is formed in an annular shape in plan according to the shape of the support portion 31 of the third substrate 30, and has electrodes on the second movable portion 12 side and the third substrate 30 side. Here, for convenience, the electrode on the second movable portion 12 side is referred to as an upper electrode 51, and the electrode on the third substrate 30 side is referred to as a lower electrode 52.

The upper electrode 51 is connected to the lead electrode 51 a wired from the outer periphery of the first substrate 10 to the second movable portion 12, and the lower electrode 52 is connected to the second movable portion 12 from the outer periphery of the third substrate 30. Are connected to the lead electrode 52a wired to the opposite surface.
Note that the extraction electrode 51a is a conductive material obtained by mixing a metal filler with an extraction electrode 51b provided on the outer peripheral portion of the third substrate 30 so as to face the extraction electrode 51a, for example, an epoxy or polyimide adhesive. It is connected via the adhesive 53 or the like.
The extraction electrode 51b and the extraction electrode 52a are connected to an energization circuit (not shown), and the wavelength tunable optical filter 1 can apply a voltage to the piezoelectric element 50 via the upper electrode 51 and the lower electrode 52. It has become.

The material of the piezoelectric element 50 is not particularly limited as long as it has a piezoelectric effect, and examples thereof include zinc oxide, lead zirconate titanate, lithium niobate, lithium tantalate, and quartz.
Examples of the material of the upper electrode 51, the lower electrode 52, the extraction electrode 51a, the extraction electrode 51b, and the extraction electrode 52a include metals such as Au, Cr, Al, Al alloy, Ni, Zn, and Ti.

The internal space 13 is filled with the liquid 60. An inlet (not shown) communicates with the internal space 13, and after the first substrate 10 and the second substrate 20 are joined, the liquid 60 is injected from the inlet. The internal space 13 is sealed with a sealing material (not shown) so that the liquid does not leak, and the sealed state is maintained.
Examples of the type of liquid 60 include alcohols such as acetone, ethyl alcohol, methyl alcohol, and methanol, solutions thereof, water, and aqueous solutions. The liquid 60 is preferably of a type that does not deteriorate the first reflective film 15 and the second reflective film 22 described later.

In the wavelength tunable optical filter 1, the liquid 60 can be selected from a plurality of types having different refractive indexes as described above. As an example of the refractive index, ethyl alcohol is about 1.36, water is about 1.33, and benzene is about 1.50.
Further, the wavelength tunable optical filter 1 is configured such that the liquid 60 can be replaced with another liquid 60 having a different refractive index by attaching and detaching the sealing material.

The first substrate 10 is formed on the surface 14 on the second substrate 20 side of the first movable portion 11 having a cylindrical shape (disc shape) having a thin periphery and a circular planar shape. A first reflecting film 15 having a circular planar shape is provided.
Further, the first substrate 10 has a first reflection having a circular planar shape formed on the surface 16 opposite to the surface 14 provided with the first reflective film 15 of the first movable portion 11. A prevention film 17 is provided.

On the surface 21 on the first substrate 10 side of the second substrate 20, the second reflection having a planar shape formed in a circular shape so as to face the first reflection film 15 of the first movable portion 11. A membrane 22 is provided.
The second substrate 20 is provided with a second antireflection film 24 having a circular planar shape on a surface 23 opposite to the surface 21 on which the second reflective film 22 is provided. Yes.
The distance G between the first reflective film 15 and the second reflective film 22 is appropriately set according to the wavelength of light used, but is usually several hundred nm to several μm.
The wavelength tunable optical filter 1 has a configuration in which the distance G changes according to the displacement of the first movable portion 11 and the second movable portion 12 (details will be described later).

The first reflection film 15 and the second reflection film 22 reflect light with a relatively high reflectance, and the first antireflection film 17 and the second antireflection film 24 function to suppress light reflection. have.
The first reflective film 15 and the second reflective film 22, and the first antireflective film 17 and the second antireflective film 24 are preferably made of a dielectric multilayer film.
More specifically, each reflective film and each antireflective film is preferably formed by alternately laminating a plurality of high refractive index layers and low refractive index layers. Thereby, the optical loss can be reduced and the optical characteristics can be improved at the time of light interference between the first reflective film 15 and the second reflective film 22.

Examples of the material for the high refractive index layer include Ti ₂ O, Ta ₂ O ₅ , niobium oxide, and the like when used in the visible light region and the infrared light region. When used in the ultraviolet light region, For _{example, Al 2 O 3, HfO 2} , ZrO 2, etc. ThO ₂ and the like.
Examples of the material for the low refractive index layer include MgF ₂ and SiO ₂ .
The number and thickness of the high refractive index layer and the low refractive index layer constituting each reflective film and each antireflective film are appropriately set according to desired optical characteristics. In general, when a reflective film is composed of a multilayer film, the number of layers required to obtain normal optical characteristics is 12 layers or more. When an antireflection film is composed of a multilayer film, the normal optical characteristics are The number of layers required to obtain is about 4 layers.

The operation (action) of the wavelength tunable optical filter 1 having such a configuration will be described.
As shown in FIG. 1B, in the wavelength tunable optical filter 1, for example, light L such as visible light passes through the first antireflection film 17 and the first reflection film 15 from the third substrate 30 side. When the light enters the internal space 13, light reflection is repeated between the first reflective film 15 and the second reflective film 22 to cause interference (interference action). Only the light L1 having a wavelength corresponding to the distance G between the second reflection film 22 and the second reflection film 22 and the second antireflection film 24 is emitted to the outside (light of other wavelengths). Is attenuated by the interference action and is not emitted to the outside).

  At this time, in the wavelength tunable optical filter 1, reflection of the light L to the incident side (the third substrate 30 side) is suppressed by the first antireflection film 17, so that the light L is hardly lost and the internal space 13 is not lost. Is incident on. Further, in the wavelength tunable optical filter 1, the second antireflection film 24 suppresses the reflection of the light L1 toward the internal space 13 on the surface 23 of the second substrate 20, and the light L1 is hardly lost and is externally lost. To exit.

FIG. 2 is a schematic cross-sectional view for explaining the operation of the wavelength tunable optical filter.
As shown in FIG. 2, when a predetermined voltage is applied to the piezoelectric element 50 from the energizing circuit via the upper electrode 51 and the lower electrode 52, the wavelength tunable optical filter 1 is applied to the piezoelectric element 50 in the thickness direction (by the piezoelectric effect). A displacement in the direction of arrow B occurs.
In the wavelength tunable optical filter 1, the second movable portion 12 is displaced in one direction of the thickness direction, for example, in the direction of arrow C according to the displacement in the thickness direction of the piezoelectric element 50 generated by the voltage application. At this time, the support portion 31 of the third substrate 30 that overlaps the piezoelectric element 50 has a higher rigidity than the first substrate 10 due to being formed thick or the like, and therefore hardly displaces in the thickness direction.

Due to the displacement of the second movable part 12 in the direction of arrow C, the wavelength tunable optical filter 1 applies a compressive pressure in the direction of arrow C to the vicinity of the second movable part 12 of the liquid 60.
In the wavelength tunable optical filter 1, the liquid 60 in the vicinity of the second movable portion 12 moves to the vicinity of the first movable portion 11 by the compression pressure applied to the liquid 60.
Thereby, in the wavelength tunable optical filter 1, the first movable portion 11 is pressed in the direction of arrow D by the liquid 60 and displaced in the direction of arrow D, which is the other of the thickness direction. The distance G1 between the reflective film 22 and the reflective film 22 becomes longer.

The wavelength tunable optical filter 1 is configured to change the distance G1 between the first reflecting film 15 and the second reflecting film 22 (becomes longer than the distance G), so that the light L2 having a wavelength corresponding to the distance G1 ( Only the light having a longer wavelength than the light L1 passes through the second reflection film 22 and the second antireflection film 24 and is emitted to the outside.
As described above, the wavelength tunable optical filter 1 includes the correlation between the voltage applied to the piezoelectric element 50 and the displacement amount of the piezoelectric element 50, and the first displacement amount via the second movable portion 12. After grasping the correlation with the displacement amount of the movable portion 11 in advance, a predetermined voltage is applied to the piezoelectric element 50, so that the first reflective film 15 and the second reflective film 15 are in accordance with the wavelengths of the desired lights L1 and L2. The distances G and G1 between the reflective film 22 and the reflective film 22 can be changed.

As described above, in the wavelength tunable optical filter 1 of the present embodiment, the second movable portion 12 moves in one of the thickness directions (arrow C direction) in accordance with the displacement in the thickness direction of the piezoelectric element 50 generated by voltage application. By displacing, the first movable part 11 is displaced in the other thickness direction (arrow D direction) by the pressure (compression pressure) applied to the liquid 60, and the first reflective film 15 and the second reflective film 22 are displaced. The distances G and G1 between them change.
From this, the wavelength tunable optical filter 1 can embody the wavelength tunable optical filter 1 without the contact and sticking between the electrodes, which is a problem of the prior art using the opposing drive electrodes.
As a result, the wavelength tunable optical filter 1 can accurately control the distances G and G1 between the first reflective film 15 and the second reflective film 22.

In addition, since the wavelength tunable optical filter 1 changes the distances G and G1 between the first reflective film 15 and the second reflective film 22 via the liquid 60, Control of the distances G and G1 is easier than when the distances G and G1 between the second reflective film 22 are changed by electrostatic attraction in a conventional space.
As a result, the wavelength tunable optical filter 1 can improve the control responsiveness of the distances G and G1 between the first reflective film 15 and the second reflective film 22.

Further, since the wavelength tunable optical filter 1 can select the liquid 60 from a plurality of types having different refractive indexes, the first reflective film can be selected by selecting the liquid 60 different among the plurality of wavelength tunable optical filters 1. Without changing the distances G and G1 between the first reflective film 22 and the second reflective film 22, light of different wavelengths (for example, L1 and L2) can be emitted to the outside.
Further, the wavelength tunable optical filter 1 is configured such that the liquid 60 can be exchanged with another liquid 60 having a different refractive index by attaching and detaching the sealing material. Without changing the distance G between the first reflective film 15 and the second reflective film 22, light of different wavelengths (for example, L1 and L2) is transmitted to the outside before and after the liquid 60 is replaced. It can be emitted.

  The wavelength tunable optical filter 1 includes the first antireflection film 17 on the surface 16 opposite to the surface on which the first reflective film 15 of the first movable portion 11 is provided. The reflection of light on the third substrate 30 side on the surface 16 opposite to the surface on which the first reflective film 15 is provided is suppressed, and the internal space can be efficiently produced with little loss of light L incident from the outside. 13 can be made incident.

In addition, the wavelength tunable optical filter 1 includes a second substrate 20 on the surface 23 opposite to the surface 21 provided with the second reflective film 22 (hereinafter also simply referred to as the surface 23). Since the antireflection film 24 is provided, the light beams L1 and L2 having wavelengths according to the distances G and G1 between the first reflection film 15 and the second reflection film 22 that have been transmitted from the first substrate 10 side. (Hereinafter, also simply referred to as lights L1 and L2) can be efficiently emitted to the outside with little loss by suppressing reflection of the surface 23 toward the internal space 13 side.
Note that the second antireflection film 24 may be provided on the entire surface 23. According to this, the wavelength tunable optical filter 1 can suppress the reflection of the light L1 and L2 on the surface 23 toward the internal space 13 over a wide range.

  In addition, the wavelength tunable optical filter 1 may make the light L incident from the outside enter from the second substrate 20 side in the direction opposite to the direction shown in FIG. 1B and FIG. According to this, the wavelength tunable optical filter 1 transmits the lights L1 and L2 from the third substrate 30 side of the first substrate 10 to the outside in the direction opposite to the direction shown in FIGS. Can be emitted.

Further, the wavelength tunable optical filter 1 does not need to be provided with the first antireflection film 17 and the second antireflection film 24. According to this, since the wavelength tunable optical filter 1 can simplify the manufacturing process, the number of manufacturing steps can be reduced.
In addition, the wavelength tunable optical filter 1 is not limited to a circular planar shape, and may be appropriately set according to applications such as a rectangle, a polygon, and an ellipse.

  DESCRIPTION OF SYMBOLS 1 ... Wavelength variable optical filter, 10 ... 1st board | substrate, 11 ... 1st movable part, 12 ... 2nd movable part, 13 ... Internal space, 14 ... 2nd board | substrate side surface, 15 ... 1st Reflective film, 16 ... surface opposite to the surface provided with the first reflective film, 17 ... first antireflection film, 20 ... second substrate, 21 ... surface on the first substrate side, 22 ... first 2 reflective film, 23... Surface opposite to the surface provided with the second reflective film, 24... Second antireflection film, 30... Third substrate, 31. ... Piezoelectric element, 51 ... Upper electrode, 51a, 51b ... Lead electrode, 52 ... Lower electrode, 52a ... Lead electrode, 53 ... Conductive adhesive, 60 ... Liquid, G, G1 ... First reflective film and second The distance between the reflective film, L: light incident from the outside, L1, L2: light having a wavelength according to the distance between the first reflective film and the second reflective film.

Claims (5)

The first movable part formed so as to be displaceable in the thickness direction and the second movable part provided around the first movable part and formed so as to be displaceable in the thickness direction have light transmittance. A first substrate;
An internal space that allows displacement of the first movable portion and the second movable portion is formed between the first substrate and the second substrate having light transmissivity bonded to the first substrate. A substrate,
A third substrate disposed on the opposite side of the second substrate across the first substrate and bonded to the first substrate;
A piezoelectric element sandwiched between the second movable part and the third substrate;
A liquid filled in the internal space;
A first reflective film provided on the second substrate side surface of the first movable part;
A second reflective film provided on the second substrate so as to face the first reflective film;
By repeating light reflection between the first reflective film and the second reflective film to cause interference, the distance between the first reflective film and the second reflective film is increased. It is configured to emit light of wavelength to the outside,
In response to displacement of the piezoelectric element in the thickness direction generated by voltage application, the first movable portion is moved to the thickness by pressure applied to the liquid by displacing the second movable portion in one of the thickness directions. The wavelength tunable optical filter is characterized in that the distance between the first reflective film and the second reflective film is changed by being displaced in the other direction.   2. The tunable optical filter according to claim 1, wherein the liquid can be selected from a plurality of types having different refractive indexes.   The wavelength tunable optical filter according to claim 1 or 2, wherein the liquid is configured to be exchangeable with another liquid having a different refractive index.   4. The tunable optical filter according to claim 1, wherein a first antireflection is provided on a surface of the first movable portion opposite to a surface on which the first reflective film is provided. A wavelength tunable optical filter comprising a film.   5. The wavelength tunable optical filter according to claim 1, wherein a second antireflection film is provided on a surface of the second substrate opposite to the surface on which the second reflection film is provided. A wavelength tunable optical filter characterized by comprising:

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