Optical: systems and elements – Polarization without modulation – Depolarization
Reexamination Certificate
2001-07-13
2003-11-11
Chang, Audrey (Department: 2872)
Optical: systems and elements
Polarization without modulation
Depolarization
C359S485050, C359S488010, C359S492010, C359S490020, C359S490020, C359S506000, C349S096000
Reexamination Certificate
active
06646802
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a polarizing element in which a birefringence phenomenon is exploited, such as a Wollaston prism, a Glan-Thompson prism, or the like.
2. Description of the Related Art
It has conventionally been known that, when unpolarized light is incident on an anisotropic crystalline substance, such as calcite (CaCO
3
), quartz (SiO
2
), rutile (TiO
2
), lithiumniobate (LiNbO
3
), or the like, two pencils of refractive light rays are observed. This is called a birefringence phenomenon, and the two refractive light rays are called a normal light ray which is obedient to Snell's law and an abnormal light ray which is disobedient to Snell's law, respectively.
Here, the greater the index of birefringence, i.e., the difference in refractive index between the normal and abnormal light rays, the more notable the birefringence derived from light incident in a direction nonparallel to an optical axis. This results in an increase in the separation angle between the normal and abnormal light rays.
Accordingly, if the separation angle between the normal and abnormal light rays is assumed to be kept constant, by employing a crystal having as great a birefringence index as possible, it is possible to realize an element having a shorter length. Reduction in the size of an element contributes to the miniaturization of apparatuses necessitating a polarizing element, such as the light-reading portion of an optical recording apparatus, an optical isolator, or the like. Particularly, in recent years, as larger and larger capacities are achieved in optical recording apparatuses, the light source wavelengths have come to be made shorter and shorter. For example, a blue-color semiconductor laser having a wavelength of about 400 nm is expected to be used as a light source.
However, conventionally-used crystal materials have the following disadvantages. The optical data of typical birefringent crystal materials will be shown in Table 1. Note that, in the table, symbols no, ne, and &Dgr;n represent the refractive index of a normal light ray, the refractive index of an abnormal light ray, and the birefringence index, respectively, as observed at a wavelength of 633 nm.
TABLE 1
Refractive index (&lgr; = 633 nm)
Blue-light
Crystal
no
ne
&Dgr;n
transmittability
SiO
2
1.54
1.55
0.01
◯
CaCO
3
1.64
1.48
0.16
◯
TiO
2
2.584
2.872
0.29
X
LiNbO
3
2.286
2.200
0.09
&Dgr;
YVO
4
1.993
2.215
0.22
&Dgr;
Li
2
B
4
O
7
1.609
1.552
0.06
◯
For example, as for quartz (SiO
2
), on the positive side, it is a naturally occurring substance and can nevertheless be produced synthetically by the hydrothermal synthesis method. On the negative side, the birefringence index of quartz is about 0.01, which is unduly small compared to other crystals, and therefore its use leads to an unduly long element length. The quartz is a material which exhibits satisfactory transmittability not only for blue-color light but also for ultraviolet light.
As for calcite (CaCO
3
), on the positive side, it has a birefringence index of about 0.16, which is sufficiently large compared to other crystals, and exhibits satisfactory blue-light transmittability. On the negative side, calcite occurs naturally only in nature. This makes it difficult to provide an inexpensive and high-quality element.
Moreover, as for a rutile (TiO
2
) single crystal, on the positive side, it has an extremely large birefringence index of about 0.29 and is thus suitably used in a polarizer designed for an optical isolator or the like at present. On the negative side, rutile can be developed only by the Verneuil's method or floating zone method. The difficulty of obtaining a substance with satisfactory crystallzability causes a problem in that a reduction in yields occurs due to the existence of grains and inner warps, or a large-sized crystal cannot be obtained easily. For this reason, it is inevitable that a rutile-made element becomes expensive. A slight yellowish tint is recognized in rutile even under visual observations. From this fact, it is understood that rutile offers poor blue-light transmittability.
Further, a lithium niobate (LiNbO
3
) single crystal has in recent years attracted attention. This is because, such a large-size crystal as has a crystal diameter of 0.076 to 0.102 m can be developed relatively easy by the Czochralski method. However, lithium niobate has the following disadvantage. For example, just like a process for fabricating a polarizing prism, in a case where a crystal is cut up at a predetermined angle with respect to the C axis and then the polished crystal portions are bonded together, if, as the crystal material, lithium niobate or rutile having a larger refractive index (equal to 2.0 or above) is used, proper adhesive is unavailable that has a refractive index close to that of the material. Therefore, it is essential to suppress reflection occurring in the bonded area and the loss of transmission light by providing, in accordance with the transmission wavelength, a reflection prevention film, such as a dielectric substance, for adhesive in use. This process makes the manufacture complicated, and consequently the finished element becomes expensive.
Note that, to improve the stability of an optical system, in some instances, a plurality of prisms are bonded together to form a single unit of a combined prism. In connection with this, in a case where optical components made of materials having larger refractive indices than in an ordinary glass material are integrated together, similar problems arise. Moreover, lithium niobate offers poor blue-light transmittability.
Yttrium vanadate (YVO
4
) is a positive uniaxial crystal which is developed by the Czochralski method. It offers a significantly large birefringence index of about 0.2 or above in a range from the visible region to the near-infrared region. Yttrium vanadate is mechanically and physically excellent and is thus frequently used as a substitute for calcite or rutile to form an optical polarizer. However, the absolute value of the refractive index of Yttrium vanadate is so large that it has similar disadvantages to those of lithium niobate and rutile from the standpoint of selecting materials including adhesive and combined optical components. Moreover, the blue-light transmittability is poor.
As for lithium tetraborate (Li
2
B
4
O
7
), its single crystal is synthesized by the Czochralski method or other means. Lithium tetraborate has a refractive index substantially the same as that of a normal glass or plastic, and offers satisfactory blue-light transmittability. However, as compared with the above stated Yttrium vanadate and rutile, the birefringence index is small. Since lithium tetraborate is a crystal material, the cost cannot be reduced easily.
FIG. 6
is a perspective view showing an example of conventional wedge-like elements. Japanese Unexamined Patent Publication JP-A 6-59125 (1994) proposes a method of using a liquid crystal polymer instead of the above stated crystal materials. The example shown in the figure is a beam splitter
105
, an example of a wedge-like element formed of a unidirectionally oriented polymer material composed of a setting liquid crystal monomer composition. A manufacturing method therefor will be described below.
Firstly, two rectangular glass plates are coated with a nylon-made orientation layer and are then rubbed with a soft pile cloth or the like in a selected direction parallel to one end of each glass plate. Secondly, the two glass plates are arranged face to face with each other with a wedge-like space secured therebetween such that the rubbing direction of the two glass plates is aligned with a parallel direction. Then, a liquid crystal monomer is charged into the wedge-like space. Lastly, UV-irradiation treatment is conducted to form a solid wedge-like element, and subsequently the glass plates are removed. As a result, the polarized light sensitivity beam splitter
105
shown in
FIG. 6
is realized. As the fill
Birch, Stewart, Kolasch and Birch LLP
Chang Audrey
Curtis Craig
Sharp Kabushiki Kaisha
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