Optical: systems and elements – Light interference
Reexamination Certificate
2003-01-09
2004-11-02
Boutsikaris, Leo (Department: 2872)
Optical: systems and elements
Light interference
C359S260000, C356S454000, C356S506000
Reexamination Certificate
active
06813081
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an etalon used for an optical filter and so forth.
RELATED ART
Optical filters for taking lights of a specific wavelength region out of lights having continuously or discontinuously distributed wavelength characteristics are used in various fields. Among such optical filters, filters utilizing etalons have been widely used from old days for refractometry of gases, precision measurement of length and interference spectroscopes. Moreover, etalons have been used basically as interference filters for forming a series of sharp transmission peaks in the fields of other measurement instruments, research of light scattering, performance analysis of lasers, development of laser lines of narrower band, astronomy and so forth (for example, Kudo and Uehara, Basic Optics, 3rd edition, pp.336-338, 1999, Gendai Kogakusha; and Oh, Optronics, No. 9, pp.167-171, 2001).
The etalon means two of multiple reflection planes of which gap is fixed with a spacer made of invar or the like used in, for example, a Fabry-Perot interferometer. In particular, an etalon of this type is called a Fabry-Perot's etalon. Because it is difficult to maintain one reflection plane parallel to the other, the gap between the planes is fixed (Dictionary of Physics, Ed. by Editorial Committee of Dictionary of Physics, 1992, Baifukan).
There is also an etalon consisting one piece of transparent plate having two parallel surfaces so that the two surfaces of the transparent plate should serve as two of reflection planes. Such an etalon may be called a solid etalon. In such an etalon comprising one transparent plate, highly reflective coatings may be provided on the both surfaces of the transparent plate. The transparent plate of the etalon used in this case consists of optical glass, quartz glass, air or the like.
A transmission wavelength peak of light transmitting such an etalon as described above is observed when phases of light multiply reflected in the etalon become uniform at the outgoing plane of the etalon.
Meanwhile, demands for thinner etalons are increasing in various applications of etalons including measurement instruments. According to Kudo and Uehara, Basic Optics, 3rd edition, 1999, Gendai Kogakusha, transmission of the aforementioned solid etalon constituted with one transparent plate is given by the following equation.
T=
1/(1
+H
sin
2
&dgr;) (1)
In the formula:
H=
4
R
/(1
−R
)
2
(2),
&dgr;=(&pgr;/&lgr;)2nd cos
i−&phgr;
(3),
T: Transmission of etalon,
R: Reflectance of etalon,
&lgr;: Wavelength,
n: Refractive index,
d: Thickness of transparent plate,
i: Angle of incidence,
&phgr;: Phase change due to reflection in etalon.
As for the characteristics of T, the maximum transmission is obtained at the time of sin &dgr;=0, i.e., &dgr;=m&pgr; (m=1, 2, 3 . . . ). In the case of the angle of incidence i=0, the phase change &phgr; should be &phgr;=0. In this case, &dgr; is defined by the following equation.
&dgr;=(&pgr;/&lgr;)2nd (4)
Realization of a thinner etalon, which is one of the objects of the present invention, is equivalent to making the thickness d of the transparent plate in the equation (4) smaller. Since &dgr; is &dgr;=m&pgr; and &lgr; is determined by the desired wavelength, the way of making the thickness d of the transparent plate smaller, i.e., obtaining thinner etalon, is using a larger n.
Although a thinner etalon can be attempted by producing the etalon by using a material exhibiting a larger refractive index as described above, high thermal stability must be simultaneously attained. This can be expressed that, in other words, the value of nd in the equation (4) must not vary as far as possible, even though temperature variation occurs. To realize this is another object of the present invention. There is a tendency that conventional material exhibiting a high refractive index show poor thermal stability, and thus smaller thickness of etalon and higher thermal stability could not be satisfied simultaneously.
SUMMARY OF THE INVENTION
As described above, an object to be achieved by the present invention is to provide an etalon consisting of a material that simultaneously satisfies the following two of requirements.
(1) Larger Refractive index n
(2) Smaller variation of product of refractive index n and thickness d of transparent plate due to temperature variation.
The present invention was accomplished in order to solve such a problem, and an object of the present invention is to provide a solid etalon consisting of one transparent plate, which has a small thickness and exhibits high thermal stability.
The present invention accomplished in order to achieve the aforementioned objects provides an etalon comprising at least one transparent plate, wherein the transparent plate consists of lithium tantalate single crystal.
According to the present invention, the transparent plate constituting the etalon is made from lithium tantalate single crystal as described above. Lithium tantalate has a refractive index of 2.13 for an ordinary ray at a wavelength of 1550 nm, which satisfies a requirement that the refractive index should be 2 or more, and exhibits high thermal stability as represented by peak variation of 2.5 pm/° C. for transmission wavelength due to temperature variation. Therefore, by preparing the transparent plate constituting the etalon from lithium tantalate single crystal, an etalon having a small thickness and exhibiting high thermal stability can be obtained.
The present invention also provides an etalon comprising at least one transparent plate, wherein the transparent plate consists of lithium niobate single crystal.
According to the present invention, the transparent plate constituting the etalon is also made from lithium niobate single crystal. Lithium niobate has a refractive index of 2.22 for an ordinary ray at a wavelength of 1550 nm, which satisfies the requirement that the refractive index should be 2 or more, and exhibits high thermal stability as represented by peak variation of 5 pm/° C. for transmission wavelength due to temperature variation. Therefore, by preparing the transparent plate constituting the etalon from such a material, an etalon having a small thickness and exhibiting high thermal stability can be obtained.
In the aforementioned etalons, an angle between a normal of light incidence and outgoing plane of the transparent plate and a c-axis of the single crystal constituting the transparent plate according to the representation for hexagonal system is preferably 0° to 10°.
As described above, both of lithium tantalate and lithium niobate satisfy the requirement of refractive index of 2 or more, and exhibit high thermal stability as represented by peak variation of less than 5 pm/° C. for transmission due to temperature variation. When the direction of the c-axis of the single crystal and forward direction of light (referred to as “optical axis” hereinafter) are parallel to each other, in other words, when the normal of light incidence and outgoing plane of the etalon conforms to the optical axis and an angle between the optical axis and the c-axis of lithium tantalate or lithium niobate is 0°, good thermal stability can be obtained.
However, when an etalon is used, the median wavelength may be controlled by providing a certain angle between the optical axis and the light incidence and outgoing plane. Because lithium tantalate and lithium niobate show anisotropy, a larger angle between the optical axis and the normal of the light incidence and outgoing plane causes a larger peak variation of transmission wavelength due to temperature variation. This is caused by influences of the temperature dependency of refractive index, different coefficients of thermal expansion for different crystal orientations and abnormal light refractive index. However, if the angle is 0° to 10°, desirably 0° to 5°, it is possible to maintain high temperature stability even in such a case. Therefore
Boutsikaris Leo
Hogan & Hartson LLP
Shin-Etsu Chemical Co. , Ltd.
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