JPS62299907A - Polarizing beam splitter - Google Patents

Abstract

PURPOSE:To improve reflectivity and transmittance in a use wavelength region as well by deviating the optical film thicknesses of a 2nd layer, 4th layer, (N-2)th layer and N-th layer from lambda0/4. CONSTITUTION:A substrate is one kind of flint glass and the refractive index thereof is 1.66. TiO2, ZnS, and ZrTiO4 are conceivable as a high refractive index material and SiO2 and MgF2 as a low refractive index material; the TiO2 and SiO2 are used in this embodiment. The number of film layers is 20 layers. The optical film thickness of the 2nd layer is 0.45Xlambda0/4, the 4th layer 1.08Xlambda0/4, the 18th layer 0.90Xlambda0/4, and the 20th layer 0.41Xlambda0/4 when the design wavelength is designated as lambda0. A polarizing beam splitter with which the transmittance of P polarized light in the use wavelength band hardly deteriorates is thus obtd. even if there are the dispersion of semiconductor laser wavelength, wavelength shift and dispersion in the production of multi- layered dielectric films.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to improving the performance of polarizing beam splitters used in optical disk file devices and the like.

2. Description of the Related Art In recent years, the development and commercialization of optical disk devices as large-capacity information storage devices, such as still image disk file devices, 9 text file devices, etc., has become active. Miniaturization·
Various attempts have been made to improve performance and reduce cost.

In order to realize such an optical head, it is strongly desired to improve the characteristics of a polarizing beam splitter of a semiconductor laser, which transmits a P-polarized laser beam but reflects an S-polarized laser beam.

Conventionally, polarizing beam splitters have been realized using dielectric multilayer films. For example, desired characteristics are obtained by laminating an even number of high refractive index materials on optical glass such as BK7 (trademark).

Figure 9 shows the conventional film configuration when the number of film layers is 20.Generally, polarizing beam splitters have a thin film system formed on a substrate, and a sandwich structure in which the thin film system is formed using a triangular prism, etc. There is something I did. In order to further reduce the size and cost of the optical three-vane head, it is more advantageous to use a structure in which a thin film is formed on the substrate, so this type of head is recently being adopted.

The refractive index of BK7 is nq-1-51, the refractive index of TiO2 is H-2,22, and the refractive index of Si02 is H=1.4.
6° The angle of incidence from the air side is 68°, the design wavelength is λO = 8
The transmittance characteristics of the above film structure when the wavelength is 56 nm are shown in the 10th
As shown in the figure.

The vertical axis is the transmittance (unit; %), and the horizontal axis is the wavelength (unit;
nm). The dotted line in the figure represents λ-830 nm.

Problems to be Solved by the Invention Problems that arise when a polarizing beam splitter having such a film structure is used in an optical head will be explained.

Generally, manufacturing variations in semiconductor lasers are ±10 nm. Therefore, when a polarizing beam splitter having the above film characteristics is used in an optical head or the like, the transmittance of P-polarized light decreases.

This results in not only lowering various efficiencies but also increasing reflectance. The laser beam reflected here reaches the detection system and detects each signal.

adversely affect processing.

Dielectric multilayer films undergo wavelength shifts due to environmental changes such as temperature and humidity. Wavelength shift refers to the movement of the transmission region and reflection region of the transmittance characteristic toward higher wavelengths and shorter wavelengths. In our experiments, we found that it moves by about ±8 nm, and the transmittance characteristics at that time are shown in FIG.

The solid line shows the ideal characteristics, and the dotted line shows the characteristics when a wavelength shift occurs.

It can be seen that when moving to the long wavelength side, the transmittance at λ2'''830 nm is about 87 inches, and when moving to the short wavelength side, the transmittance decreases to about 92 inches.

Furthermore, the wavelength of the semiconductor laser also varies by ±10 nm, so when moving to the long wavelength side considering the worst case, the transmittance will be λ2 = 820 nm, and when moving to the short wavelength side, the transmittance will be λ2 = 840. It is necessary to consider the transmittance in nm. From the graph, 60% and 85%, respectively.
Therefore, the number of vanes has decreased significantly.

When forming a dielectric multilayer film, it is said that the wavelength control accuracy is approximately ±1- of the design wavelength. Therefore, in the case of this film configuration, the design wavelength is about λO''855 nm, so it can be considered that it varies by about ±9 nm.

If we couple the wavelength with the semiconductor laser, good results can be obtained except for the wavelength shift, but the characteristics are sufficient to improve the yield, maximize the mass production effect, and lower the price. I can't.

The deterioration of the P-polarized light characteristics has been described above, but depending on the incident angle, the deterioration of the reflectance of S-polarized light may become a problem.

The present invention has been made in view of this point, and it is an object of the present invention to provide a film structure in which the P-polarized light transmittance and the S-polarized light reflectance at the semi-zero wavelength are less likely to deteriorate.

6 Means for Solving Problems The technical means of the present invention for solving the above problems is as follows: where the number of film layers is 8 or more, the design wavelength is λO, and the number of film layers is N, counting from the substrate. By making the optical thickness of the second layer, fourth layer, N-2 layer, and N-th layer different from λO/4, and setting the optical thickness of the other layers to approximately λO/4, be.

Function: By using the above-mentioned film structure, the present invention provides a structure in which the P-polarized light transmittance in the used wavelength band is unlikely to deteriorate even if there are semiconductor laser wavelength variations, wavelength shifts, and R-electronic multilayer film manufacturing variations. A polarizing beam splitter can be obtained.

Embodiment FIG. 1 shows an example of the film structure of a polarizing beam splitter of the present invention.

The base is BaF, which is a type of 7-lint glass.

(Trademark) n-1,66. As a high refractive index material,
T 102 + Z n S , Z r T z O
4 Low refractive index materials! :S i02 r Mg F
2 is possible, but here we will explain by taking T 1027 to -7 and 8102 as examples.

FIG. 2 shows the transmittance characteristics with this film configuration when the design wavelength λO = 830 nm and the incident angle is 68 mm. The vertical axis is the transmittance (unit: %), and the horizontal axis is the wavelength (unit: nm)
). Two solid lines in the figure indicate the transmittance characteristics of S-polarized light and P-polarized light, respectively.

P-polarized light transmittance characteristics are significantly improved at λ22-830n.
It is significantly improved over a wide range of ±30 nm.

Figure 3 shows that the total number of layers is 26, and the design wavelength λO = 850n.
m, the incident angle θ=68.9°, and the transmittance characteristics of the second case are shown.

Both the transmittance and reflectance of P-polarized light and S-polarized light are greatly improved.

Figure 4 shows that the total number of layers is 30 and the design wavelength is λO = 785n.
m, and the transmittance characteristics are shown when the incident angle is θ=68.1'. The optical thickness of each layer is.

The dotted line in the figure indicates λ11-78on.

Both the transmittance of P-polarized light and the reflectance of S-polarized light are λ=780nm±
It has good characteristics over a wide range of 30 nm.

The eighth layer is approximately 0.52XλO/4.

At λ2=830nm±30nm, good results are shown, with a P-polarized light transmittance of 94% or more and an S-polarized light reflectance of 95%.

9 ・- In Figure 6, design wavelength λO = 850 nm, incident angle θ = 7
5. Transmittance characteristics when the number of film layers is 40.

When λ=830 nm±30 nm, the transmittance characteristics 2 and the reflectance characteristics are significantly improved.

Figure 7 shows the design wavelength λO = 830 nm and the incident angle θ-
The transmittance characteristics are shown when the angle is 68° and the number of film layers is 20. Until now, the flint special W of 111 = 1.66 was n =
The case where BKy of 1.51 is used is shown.

All other layers are approximately λO/4. λ=830nm±
P-polarized light transmittance and S-polarized light reflectance show good characteristics over 30 nm. It turns out that it is also useful to use BN2 as a base.

1o-.-Although all the film structures described so far are film structures in which a high refractive index material is formed on a substrate, FIG. 8 shows the transmittance characteristics when a low refractive index material is formed.

The design wavelength is λO: 855 nm, and the number of film layers is 30.

The angle of incidence is θ=68°. BN2 is used for the base.

The optical thickness of each layer is approximately 0.9×λO/4° for the second layer, approximately 0.62XλO/4 for the fourth layer, and approximately 0.5° for the 28th layer.
6 XλO/4. The thickness of the 30th layer is about 1.08XλO/4, and the thickness of the other layers is about λO/4.

At λ=830nm±30nm, P polarized light transmittance, S
The polarized light transmittance has good characteristics.

However, as shown in FIGS. 3 and 4, it seems that the P-polarized light transmittance is better when a high refractive index material is provided on the substrate.

Effects of the invention As mentioned above, the second layer, the fourth layer, the N-2 layer,
By shifting the optical thickness of the Nth layer from λO//4, the transmittance/reflectance can be improved even in the wavelength range used, which is extremely effective in terms of mass production and practical use.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the film structure of a polarizing beam splitter in an embodiment of the present invention, and FIG. 2 is a diagram showing the polarizing beam splitter using BaFlo as a substrate and having a design wavelength of λO""8.
Figure 3 is a characteristic diagram showing the transmittance characteristics in the case of 55 nm.The total number of layers is 26, the design wavelength is 850 nm, and the incident angle is 6.
Figure 4 shows the transmittance characteristics when the total number of layers is 30, the design wavelength is 785 nm, and the incident angle is 68.1°. The characteristic diagram, Figure 6, shows that the total number of layers is 8 and the design wavelength is so o nm.
A characteristic diagram showing the transmittance characteristics when the incident angle is 75°, FIG. 6 is a characteristic diagram showing the transmittance characteristics when the number of film layers is 40, the film agent wavelength is 850 nm, and the incident angle is 76°, Fig. 7 is a characteristic diagram showing the transmittance characteristics when the number of the same film layers is 20, the design wavelength is 830 nm, and the incident angle is 68°, and Fig. 8 is a characteristic diagram showing the transmittance characteristics when the number of the same film layers is 30. The design wavelength is 855 nm, the incident angle is 680, and the first
Characteristic diagram showing transmittance characteristics using a low refractive index material in the layer,
FIG. 9 is a diagram showing the configuration of a conventional polarizing beam splitter, FIG. 10 is a characteristic diagram showing its transmittance characteristic, and FIG. 11 is a characteristic diagram when the characteristic is shifted by ±B nm. 1 to 20...layers.

Claims (1)

[Claims] In a thin film system consisting of a multilayer film in which dielectric multilayer films are alternately laminated on a substrate, the total number of film layers is 8 or more, and if the design wavelength is λ_O and the number of film layers is N, then the second The optical thickness of the layer, the fourth layer, the N-2 layer, and the N-th layer is λ_O/
4, and the optical thickness of the other layers is approximately λ_O/
4 polarizing beam splitter.