JP3299384B2 - Polarizing beam splitter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarizing beam splitter.

[0002]

2. Description of the Related Art Conventionally, various optical devices such as a magneto-optical disk are provided with a polarizing beam splitter that varies the diffraction efficiency depending on the polarization direction. Various proposals have been made for this polarizing beam splitter, and are described in, for example, JP-A-63-26604.

In the polarizing beam splitter described in Japanese Patent Application Laid-Open No. 63-26604, at least a concave portion of a surface irregular lattice formed on a birefringent medium has a refractive index substantially equal to the ordinary or extraordinary refractive index of the birefringent medium. Filled with the substance
It diffracts ordinary or extraordinary light.

[0004]

However, the polarizing beam splitter has the following problems. That is, since the refractive index of the filling material must be substantially equal to the ordinary or extraordinary refractive index of the birefringent medium, the range of choice of the filling material is reduced, and the design flexibility is improved. There is a problem that there is no.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a polarizing beam splitter which allows a wide selection of a filling material and improves design flexibility.

[0006]

According to a first aspect of the present invention, there is provided a polarizing beam splitter comprising a birefringent substrate made of a birefringent material and having a concave and convex periodic grating formed on a surface thereof. The concave portion is filled with an optically isotropic substance, a polarizing beam splitter having a flat surface formed of the birefringent substrate and the optically isotropic substance, The relationship between the refractive index (nc) and the ordinary light refractive index (no) and the extraordinary light refractive index (ne) of the birefringent material is represented by the following formula , whereby the refractive index (n) is obtained.
c) is the ordinary refractive index (no) or the extraordinary refractive index (n)
e) Select an optically isotropic substance except for a substance substantially equivalent to
It is characterized by being filled by filling . nc = no + m (no-ne); (m = ± 1, ± 2, ± 3 ...) = ne + 1 (no-ne); (l = ± 1, ± 2, ± 3 ...)

According to a second aspect of the present invention, in order to achieve the above object, in addition to the first aspect, the birefringent substrate is formed of lithium niobate, and the filling material is formed of soda glass. .

[0008]

According to the polarizing beam splitter of the first and second aspects, the filler and the birefringent substrate, which fill the concave portions of the birefringent substrate and satisfy the above expression, diffract either ordinary light or extraordinary light. Without diffracting the other. Therefore, the range of selection of the filling substance can be broadened as compared with the related art.

[0009]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a longitudinal sectional view of a polarizing beam splitter showing one embodiment of the present invention.

In FIG. 1, reference numeral 1 denotes a birefringent substrate made of, for example, lithium niobate. On the surface of the birefringent substrate 1, a periodic lattice formed by irregularities is formed. The concave portion 1a of the birefringent substrate 1 is filled with a filling material having a refractive index nc satisfying the following formula 7 or 8 between the ordinary light refractive index no and the extraordinary light refractive index ne of lithium niobate (however, ordinary light refraction). For example, in this embodiment, soda glass 2 is filled (except for a substance having a refractive index substantially equal to the refractive index no or the extraordinary light refractive index ne).

Here, the thickness of the birefringent substrate 1 is t, the thickness of the filling material 2 is d2, the refractive index of the birefringent substrate 1 for ordinary light is no, the refractive index of the birefringent substrate 1 for extraordinary light is ne,
The refractive index of the filling material 2 is nc, the wavelength of light is λ, and k = 2π /
Assuming that λ, the phase of the ordinary light passing through the region where the filling material 2 is not filled (the region A in the drawing) is: no · t · k Equation 1 The region where the filling material 2 is filled (see FIG. 1) B region), the phase of ordinary light is {no (t−d2) + nc · d2} · k (2) Therefore, the phase difference OPD (o) of ordinary light with respect to A and B is
From Equations 1 and 2, OPD (o) = (no-nc) · d2 · k 3 equations

On the other hand, the phase of the extraordinary light passing through the region A is given by: ne · t · k 4 The phase of the extraordinary light passing through the region B is given by {ne (t−d2) + nc · d2} · k ... Equation 5 Therefore, the phase difference OPD (e) of the extraordinary light with respect to A and B
From Equation 4-5, OPD (e) = (ne−nc) · d2 · k Equation 6

Here, in order to diffract only one of the ordinary light and the extraordinary light, only one of the expressions 3 and 6 needs to be an even multiple of π. That is, to diffract only ordinary light, OPD (e) = 2pπ, (p = 0, ± 1, ± 2...) To diffract only extraordinary light, OPD (o) = 2pπ, (p = 0, ± 1, ± 2 ...)

However, under the above conditions alone, depending on the setting of d2, there is a case where there is an amount of light that is not diffracted among the ordinary light and the extraordinary light that is diffracted. The polarizing beam splitter of the present invention can be used, for example, in a pickup of an optical disk device.In such an application, one of ordinary light and extraordinary light is diffracted, and the other is not diffracted at all. It is desirable to do. That is, OPD (o) = 2pπ, (p = 0, ± 1, ± 2
...) and OPD (e) = (2q + 1) π, (q = 0, ± 1, ± 2
...) or OPD (o) = (2q + 1) π, (q = 0, ± 1, ± 2
...), and OPD (e) = 2pπ, (p = 0, ± 1, ± 2
...). That is, OPD (o) -OPD (e) = (2i + 1)
π (i = 0, ± 1, ± 2...).

OPD (o) -OPD (e) = {(no-ne)-(ne-nc)}. D2.k = (no-ne) .d2.k = (2i + 1) .pi., (I = 0 , ± 1, ± 2...) As can be seen from this equation, d2 can take an infinite number of values depending on how i is taken, but the minimum d2 is i = 0 or i = −1.
And d2 = π / {k · | no-ne | ＝= λ / {2 · | no-ne |}.

Further, in this d2, as described above, one of the OPD (o) and the OPD (e) must be an even multiple of π and the other must be an odd multiple of π. Nc = no + m (no-ne), (where m = ± 1, ± 2...) 7 and nc = ne + 1 (no-ne), (where l = ± 1, ± 2 ...) ... 8 formulas are selected.

Accordingly, OPD (e) = (ne−nc) · d2 · k = {(ne−nc) / (no−ne)} · π = {− 1 (no−ne) · π} / (no −ne) = − lπ Therefore, when l in the nc = ne + 1 (no-ne) of the filling material is an even number, all extraordinary light is not diffracted, all ordinary light is diffracted, and when l is odd, all extraordinary light is It can be diffracted and all ordinary light is not diffracted.

Therefore, in this embodiment, λ = 0.6
At 33 μm, no = 2.28 for lithium niobate
6, ne = 2.2, nc = no + m (no-ne) = 2.286-9 × 0.086 = 1.512 nc = ne + 1 (no-ne) = 2.2-8 × 0.086 = 1.512, and soda glass is used.

As described above, in the present embodiment, the birefringent substrate 1 made of lithium niobate as a birefringent material and having an uneven periodic lattice formed on the surface thereof, and the concave portion 1a of the birefringent substrate 1 And a filling material having a refractive index nc satisfying the above formula 7 or 8 between the ordinary refractive index no and the extraordinary refractive index ne of the birefringent material of the birefringent substrate 1 (however, the ordinary refractive index or And soda glass 2) (excluding a substance having a refractive index substantially equal to the extraordinary light refractive index), so that it can function as a polarizing beam splitter as described above.

Since the refractive index nc of the filling material 2 only needs to satisfy the above equation (7) or (8), the range of choice of the filling substance can be broadened as compared with the prior art. It is possible to improve.

As a second embodiment of the present invention, an example in which quartz is used as a birefringent substrate material will be described. The crystal is no = 1.52,
ne = 1.48, and m = −1 as m and l.
Assuming that 3, l = −12, nc = no + m (no−ne) = 1.52-13 × 0.04 = 1.0 nc = ne + 1 (no−ne) = 1.48−12 × 0.04 = 1.0 That is, the filling material can be air under the condition of d2 = π / {k · | no-ne |} = λ / {2 · | no-ne |}.

As a third embodiment of the present invention, an example using calcite as a birefringent substrate material will be described. Calcite no = 1.6
4, ne = 1.48, and m = −, where m = −
Assuming that 4, 1 = −3, nc = no + m (no−ne) = 1.64−4 × 0.16 = 1.0 nc = ne + 1 (no−ne) = 1.48−3 × 0.16 = 1.0 That is, the filling material can be air under the condition of d2 = π / {k · | no-ne |} = λ / {2 · | no-ne |}.

Although the invention made by the inventor has been specifically described based on the embodiments, the present invention is not limited to the above-described embodiments, and can be variously modified without departing from the gist thereof. Needless to say, for example, in the above embodiment, the birefringent substrate 1 is made of lithium niobate, quartz, and calcite, and the filling material 2 is made of soda glass. However, the material is limited to the above materials. Not something.

[0024]

As described above, according to the polarizing beam splitters of the first and second aspects, the birefringent substrate which is made of a birefringent material and has an uneven periodic grating formed on a flat surface thereof is provided. A polarizing beam splitter in which the concave portions are filled with an optically isotropic substance, wherein the refractive index (nc) of the optically isotropic substance and the ordinary refractive index (n) of the birefringent material are
o) and the extraordinary light refractive index (ne) have the relationship shown in the following equation, so that the refractive index (nc) is
Absolutely equal to light refractive index (no) or extraordinary light refractive index (ne)
Selected and filled with optically isotropic material
Having characterized the door, nc = no + m (no -ne); (m = ± 1, ± 2, ± 3 ...) = ne + l (no-ne); (l = ± 1, ± 2, ± 3 ...) It is sufficient that the refractive index nc of the filling material satisfies the above expression, and therefore, the range of selection of the filling material can be broadened as compared with the related art,
The degree of freedom in design can be improved.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a polarizing beam splitter showing one embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Birefringent substrate 1a Depression of birefringent substrate 2 Filling substance

Claims (2)

(57) [Claims] 1. A birefringent substrate comprising a birefringent material and having a concave and convex periodic grating formed on a surface thereof, wherein said concave portion is filled with an optically isotropic substance, and What is claimed is: 1. A polarizing beam splitter having a flat surface made of an optically isotropic substance, comprising: a refractive index (nc) of the optically isotropic substance; an ordinary refractive index (no) of the birefringent material; between the refractive index (ne), by that is shown by the following equation, the refraction
The refractive index (nc) is the ordinary light refractive index (no) or the extraordinary light refraction.
Optically isotropic material excluding a material that is approximately equal to the rate (ne)
A polarizing beam splitter characterized by being selectively filled . nc = no + m (no-ne); (m = ± 1, ± 2, ± 3 ...) = ne + 1 (no-ne); (l = ± 1, ± 2, ± 3 ...) 2. The polarizing beam splitter according to claim 1, wherein the birefringent substrate is formed of lithium niobate, and the filling material is formed of soda glass.