JPH0358081B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical system, and more particularly to an optical system that minimizes the influence of fluctuations in the angle of incidence of light rays on a phase plate.

Conventionally, an optical system as shown in FIG. 1 has been used as an optical pickup device for a video or audio disc player. In this process, a laser beam 2 emitted by a semiconductor laser 1 is linearly formed through a beam splitter 6 in which a polarizing film 3 is sandwiched between prisms 4 and 5, and a 1/4 wavelength plate 7 disposed diagonally in the beam 2 The circularly polarized light is converted into circularly polarized light, reflected by the bits on the surface of the optical disk 8, and the circularly polarized reflected light whose rotation direction is reversed is passed through the quarter phase plate 7.
The linearly polarized light 9 whose polarization plane differs by 90 degrees from the incident light is generated by the polarizing film 3 of the beam splitter 6.
An optical signal reflected by the surface and modulated by the pits on the surface of the optical disk 8 is converted into an electrical signal by a light receiving device 10.

At this time, the reflected light from the surface of the optical disk 8 does not become completely circularly polarized light, mainly because the beam angle of the emitted light from the semiconductor laser 1 has a certain spread. The phase difference that occurs between the ordinary ray and the extraordinary ray that are incident on the plate 7 varies greatly from the desired value, and as a result, the reflected light that reaches the beam splitter 6 also includes a component that passes through the polarizing film 3. There was a defect in that the returned light caused instability in the oscillation of the semiconductor laser 1. Note that 1/4 is one of the measures to prevent unnecessary reflected light from being directed toward the semiconductor laser 1 as much as possible by arranging the wavelength plate 7 at an angle with respect to the beam.

The present invention was made in order to eliminate the above-mentioned defects in the optical system, and the present invention was made in order to eliminate the defects in the optical system as described above. It is an object of the present invention to provide an optical system in which the refraction direction of an extraordinary ray to be determined is made to coincide with any crystal axis other than the optical axis of the crystal plate.

Hereinafter, the present invention will be explained in detail based on drawings illustrating the theory.

FIG. 2 is a diagram for analyzing a generalized wave plate 7, that is, a case where the three axes of the crystal have a constant inclination with respect to the plane of the wave plates. The beam is divided into an extraordinary ray e and an ordinary ray o, which are refracted at angles of refraction β and β', respectively. It is known that the refraction angle β of the extraordinary ray e, unlike that of the ordinary ray o, depends on the angle of incidence α, and when the crystal is quartz, the difference between β and β' is extremely small.

Now, under the conditions shown in Figure 2, the ordinary ray o
The phase difference Г that occurs between and the extraordinary ray e is the incident angle α
Consider how it will be affected by

First, let n _p be the refraction angle of the ordinary ray, n _e be the refraction angle of the extraordinary ray, and the extraordinary ray depends on the incident angle α, in other words, it is a function of the angle θ (assuming that the angle γ is a given condition). The refractive index n _e ′(θ) of is n _e ′(θ) = n _p n _e /√ _p ^{2 2} + _e ^{2 2} ……(1).

On the other hand, if the thickness of the crystal plate 7 is l, the wavelength of the incident light is λ, the phase of the ordinary ray at a certain moment is Γ _p , and that of the extraordinary ray is Г _e , then Г _p = (2π/λ )n _p (l/cosβ′) Г _p = (2π/λ)n _e ′(θ) (l/cosβ) The phase difference Г that occurs between the two and depends on the angle of incidence α
Considering that β≒β' when the crystal plate 7 is crystal, (α) is Г(α)=Г _e −Г _p = (2π/λ) {n _e ′(θ) − n _p }(l/cosβ)
...(2) becomes.

As is clear from equation (2) above, the factors that cause the variation of Г(α) are the angles θ and β, but if the variation of the incident angle α is relatively small, the amount of variation in β (or β′) can be ignored. Therefore, it can be considered that the amount of variation in Г(α) depends exclusively on the amount of variation in n _e ′(θ).

Therefore, the condition for minimizing the variation in n _e ′(θ) with respect to the amount of variation Δβ in the angle of refraction β (≒β′), in other words, the amount of variation in the angle of incidence α, is to minimize the variation in Г(α). It is understood that this is a condition for doing so.

Therefore, referring to Fig. 2 above, since Δβ = Δθ, sin ² (θ + Δθ)≒sin ² θ+sin2θ・Δθ cos ² (θ+Δθ)≒cos ² θ−sin2θ・Δθ ...(3) The above formula (3) Substituting into the above equation (1) and rearranging, n _e ′ (θ + Δθ) ≒ n _p n _e {1 − (n _p ² − n _e ² ) sin2θ・Δ
θ／2(n _p ² sin ² θ＋n _e ² cos ² θ)}／√ _p ^{2 2}
+ _e ^{2 2}
...(4) becomes.

Furthermore, from FIG. 2, θ=90°+β−γ,
As mentioned above, since Δθ = Δβ, the above formula (4) is n _e ′ (θ + Δθ) = n _e ′ (90° + β − γ + Δβ) = n _p n _e
[1+Cn _p ² −n _e ² )Δβsin2(β−γ) /2{n _p ² cos ² (β−γ)+n _e ² sin ² (β−γ)}]/
√ _p ^{2 2} (−)＋ _e ^{2 2} (−)……
(5) becomes. Therefore, the variation amount Δn _e ′(θ) of n e ′(θ) due to the variation of the incident angle α, that is, the variation of the angle θ, is Δn _{e ′} (θ)=n _p n _e (n _p ² −n _e ² )Δβsin2(β−γ)/
2 {n _p ² cos ² (β-γ) + n _e ² (β-γ)} /√ _p ^{2 2} (-) + _e ^{2 2} (-)...
…(6) becomes.

Therefore, the conditions for Δn _e ′(θ) to be zero are β=γ,
That is, it can be seen that the optical system may be constructed so that the refraction direction of the extraordinary ray e coincides with the Y or X axis of the crystal, that is, as shown in FIG. 3, for example.

Based on the above theoretical analysis, when comparing the optical system of the present invention with the conventional one, for example, the thickness l is 0.4691 mm.
A He-Ne laser beam (wavelength λ633n
m) and obtains a phase difference of 90°, if the angle of incidence changes by ±5°, the phase difference changes by 20 to 24°. However, if the thickness l is 0.0154 mm, In the optical system of the present invention, in which the refraction direction is aligned with the Y axis for a quartz crystal plate tilted at -27.2° with respect to the plate surface, the phase difference does not change even if the incident angle is varied by ±5° from 45°. It was confirmed that the dependence on the incident angle α was approximately ±26° and extremely small.

In addition, in the optical system according to the present invention, the refraction optical path and the optical axis (Z-axis) are perpendicular to each other, so in the case of a phase plate using a crystal with optical rotation such as quartz, the influence of optical rotation can be avoided. can be made almost zero, so that incident linearly polarized light can always be converted into almost completely circularly polarized light.

Although the optical system according to the present invention has been described above using the most popular quarter-wave plate, it is obvious that the present invention is not necessarily limited to this.

Since the optical system according to the present invention is constructed and operates as described above, for example, a beam splitter using a prism is omitted, and a phase plate with only a polarizing film attached to one surface is used as a laser beam splitter and an optical isoter. In a simple optical system such as this, the oscillation beam of the semiconductor laser used has a certain spread, and the angle of incidence must be adjusted extremely strictly to the optimum value due to the relationship between the variation in the laser oscillation wavelength and the characteristics of the polarizing film. However, according to the present invention, it is possible to convert the linearly polarized light that has passed through the polarizing film into almost completely circularly polarized light without performing such strict fine adjustment, so such an optical system is suitable. It exhibits a remarkable effect in maintaining various properties at a high level without variation.

[Brief explanation of drawings]

Fig. 1 is a configuration diagram showing an example of an optical system to which the present invention is applied, Fig. 2 is a diagram showing the relationship between the crystal axis, optical path, and ray ellipsoid in a general phase plate, and Fig. 3 is a diagram showing the relationship between the crystal axis and optical path in a general phase plate. FIG. 3 is a diagram showing an example of the relationship between the crystal axis of the phase plate and the optical path in the invention. 2... Incident light, 7... Crystal plate, Z... Optical axis of crystal, X and Y... Axes other than optical axis of crystal, o...
...ordinary ray, e...extraordinary ray, α...angle of incidence, β...
...Refraction angle, Г...Phase difference.

Claims (1)

[Claims] 1. In an optical system that allows a light ray to enter a crystal plate plane having birefringence or both birefringence and optical rotation at a predetermined angle other than 90°, depending on the angle of incidence of the light ray with respect to the crystal plate, The phase difference between the ordinary ray and the extraordinary ray caused by the variation in the angle of incidence of the ray by making the refraction direction of the extraordinary ray to be determined coincide with any crystal axis other than the optical axis of the crystal plate. An optical system that minimizes the influence of fluctuations in the angle of incidence of light rays on a phase plate, characterized by minimizing the amount of change in the phase plate.