JPS62169103A - Beam splitter - Google Patents

Abstract

PURPOSE:To obtain a beam splitter which has substantially no absorption loss, is small in the difference of reflectivity or transmittance between S polarized light and P polarized light and has excellent durability by adequately determining the film thicknesses of respective layers of alternate layers consisting of TiO2 and SiO2. CONSTITUTION:The multi-layered dielectric films C formed on the surface of a substrate B are made into the 6-layered structure formed with the 1st layer 1-6th layer 6 successively from the substrate B side to the air A side. The 1st layer 1, the 3rd layer 3 and the 5th layer 5 are thin high-refractive index films consisting of TiO2. The 2nd layer 2, the 4th layer 4 and the 6th layer 6 are this low-refractive index films consisting of SiO2. The optical film thicknesses of the respective layers are lambda0/r, lambda0/2, lambda0/2, lambda0/4, lambda0/4, lambda0/2 successively from the 1st layer. lambda0 is a design wavelength and is adequately set according to the wavelength of a light source to be used. The absorption loss is thereby substantially eliminated and the difference of the reflectivity or transmittance between the S polarized light and the P polarized light is decreased.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a beam splitter used in an optical pickup head of an optical information device.

[Background technology and its problems]

Conventionally, optical pickup heads efficiently utilize the light emitted from a light source such as a semiconductor laser to prevent fluctuations in the semiconductor laser's oscillation spectrum and output light level due to backtalk of signal light. Therefore, an optical isolator that uses a polarizing beam splitter and a 174-wave plate, which have different reflectance or transmittance depending on the polarization state of the incident light, has been used. However, recently, semiconductor lasers that do not have the above-mentioned backtalk as a problem or whose spectra are stabilized even with a certain amount of backtalk have been obtained. In such situations, it is possible to replace the 1/4 wavelength plate and polarizing beam splitter used in conventional optical pickup heads with a beam splitter, which simplifies the optical system of the optical pickup head and makes it more compact. It can be made lighter and lighter.

An example of the optical system of the above-mentioned optical pickup head is shown in FIG. Emitted light (32) from the semiconductor laser (30) is reflected by a beam splitter (34), and one reflected light (36) is turned into parallel light (40) by a collimating lens (38) and enters an objective lens (42). Objective lens (42)
The reflected light (46) containing information is focused on the disk (44) on which information is recorded by -(27), travels backward through the optical path, passes through the beam splitter (34), and is guided to the signal detection system (48). The optical signal is converted into an electrical signal.In this optical system, when the transmittance and reflectance of the beam splitter (34) are both 50%, the amount of light reaching the signal detection system (48) is 25%; has the highest efficiency.

However, the beam splitters currently used in such optical systems use metal films such as Ag or semiconductor films such as SL in order to equalize the reflectance or transmittance for S-polarized light and P-polarized light. There is. Therefore, 5%
There is a drawback that an absorption loss of ~10% occurs, resulting in a decrease in efficiency. For example, if there is an absorption loss of 10%, 10% of the light will be absorbed each time it is reflected or transmitted by the beam splitter, so the maximum amount of light that will reach the signal detection system will be about 20%. Put it away. Furthermore, metal films and semiconductor films have poor durability compared to dielectric films.

On the other hand, it is conceivable to form a beam splitter using only a dielectric film that has little absorption and high film strength. However, since the dielectric film alone results in a large difference in reflectance or transmittance for S-polarized light and P-polarized light, this type of beam splitter is rarely seen as a beam splitter for optical pickup heads. When there is a large difference in reflectance or transmittance for S-polarized light and P-polarized light.

The reflectance or transmittance varies depending on the polarization state of the light incident on the beam splitter. For example, the reflectance or transmittance changes depending on the magnitude of birefringence of the disk (44).1. Therefore, the amount of light reaching the signal detection system (48) also changes. Therefore, the beam splitter (34) for the optical pickup head must have a small difference in characteristics for S-polarized light and P-polarized light.

[Purpose of the invention]

An object of the present invention is to provide a beam splitter with almost no absorption loss and a small difference in reflectance or transmittance for S-polarized light and P-polarized light.

[Summary of the invention]

The present invention provides a high refractive index thin film made of Tie and S on a substrate.
Low refractive index thin films made of iO□ are alternately arranged in order with an optical thickness of λ. /4. λ, /2. λ, /2. λ. /4. λ. /4
゜λ. It is a beam splitter stacked in such a manner that the beam splitter has a ratio of 1/2, almost no absorption loss, and a small difference in reflectance or transmittance for S-polarized light and P-polarized light.

[Embodiments of the invention]

Hereinafter, one embodiment of the present invention will be described in detail with reference to illustrated embodiments.

FIG. 1 is a longitudinal sectional view showing the configuration of the beam splitter of the present invention, where A is air, B is a substrate, and C is a dielectric multilayer film formed on the surface of substrate B. FIG. The multi-WJ film C has a first layer (G), a second layer (■), and a third layer (■) in order from the substrate B side to the air A side.
, a fourth layer (A), a fifth layer (C), and a sixth layer 0 are formed. The first layer ■, the third layer ■, and the fifth layer ■ are high refractive index thin films made of TiO□, the second layer ■, and the fourth layer 0)
The sixth layer 0 is a low refractive index thin film made of SiO□. What is the optical thickness of each layer?

λ in order from the first layer. /4. λa/2. λ. /2. λ
. /4° FIG. 2 shows the spectral transmittance characteristics of a beam splitter according to an example of the present invention, and shows the case where the incident angle to the substrate is 45 degrees. The curve (c) shows the transmittance for S-polarized light, and the curve (lO) shows the transmittance for P-polarized light. The beam splitter in this example is one in which a multilayer film made of TiO□ and SiO2 is formed on a commonly used optical glass having a refractive index of 1.51.6The design wavelength λ. is 548
nm, the actual thickness of each layer is 60rv+ from the first layer,
188nm, 120nm, 94nm, 60nm
, and 188n+m. Design wavelength λ. 550n
The reason why it is set to m is that the wavelength of the semiconductor laser incident on the beam splitter is 780 nm, and λ. is 550nm
When , the difference in reflectance or transmittance between S-polarized light and P-polarized light at 780 nm becomes the smallest. Here, the refractive index of the Sin used as the low refractive index thin film is 1.46.
It is. In addition, rio, which is used as a high refractive index film,
[The refractive index is 2.25 to 2.3 depending on the deposition conditions such as the substrate temperature, the evaporation rate of the evaporation material, and the atmospheric gas pressure.
It can be changed between about 5, but here it is 2.
It was set at 30. Note that the characteristics do not change much even when the refractive index is 2.25 or 2.30.

FIG. 3 shows spectral transmittance curves when the refractive index of TiO□ is 2.25 and 2.30. Curve (10) shows the transmittance for 89s light when Tie's refractive index power is 12.25, and curve (12)4 shows the transmittance for P-polarized light when Tie's refractive index is 2.25. Curve 1 (14) is the transmittance for S-polarized light when the refractive index of Tie2 is 2.35, and the curve (
16) shows the transmittance for P-polarized light when the refractive index is 2.35.

If the wavelength of the light source used deviates from 780 nn+.

Design wavelength λ. Regarding the variable substrate depending on the wavelength of the light source used, in addition to optical glass with a refractive index of about 1.5, one with a refractive index as high as 58 or as low as 1.4 can also be used. Figure 4 shows a case where the refractive index of the substrate is 1.8 and a case where the refractive index of the substrate is 1.8.
4 shows the spectral transmittance characteristics for case 4.

Curve (18) shows the transmittance for S-polarized light when the refractive index of the substrate is 1.8.Curve (20) shows the transmittance for P-polarized light when the refractive index of the substrate is 1.8. In addition, curve 1 (22) is the transmittance for S-polarized light when the refractive index of the substrate is 1.4.
Curve (24) shows the transmittance for P-polarized light when the refractive index is 1.4.

Furthermore, by appropriately shifting the design wavelength.

Angles of incidence other than 45 degrees can also be used, for example λ for 60 degrees of incidence. =588 nm. The spectral characteristics in this case are shown in FIG. Curve (26) shows the transmittance for S-polarized light, and curve (28) shows the transmittance for P-polarized light.

As a result of using the beam splitter as described above as the beam splitter (30) in the optical system shown in Figure 6, there is almost no absorption loss, so the amount of light reaching the signal detection system increases, resulting in an efficiency of nearly 25%. Ta. In addition, the multilayer film structure has good adhesion and a high film strength of 4 and 5i.
Because of the alternating layers of n2, it has superior durability compared to beam splitters that use metal films or semiconductor films.
ie, the film stress is just right and the distortion of the substrate is small.

Note that the thickness of each layer of the multilayer film is λ. /4 or λ. /
Even if there is a slight deviation from 2, no major change in characteristics is observed, and the film change of each layer is most easily controlled at λ, /4 or λ. /
2, it can be easily deposited using a general deposition apparatus.

〔Effect of the invention〕

According to this invention, by appropriately determining the film thickness of each layer in the alternating layers of T10□ and 5in2, there is almost no absorption loss, the difference in reflectance or transmittance between S-polarized light and P-polarized light is small, and it is durable. A beam splitter with excellent performance can be obtained. Further, by using this beam splitter as a beam splitter of an optical pickup head, the light emitted from the light source can be efficiently and stably guided to the signal detection system.

[Brief explanation of drawings]

FIG. 1 is a sectional view of a beam splitter according to the present invention, FIG. 2 is a spectral transmittance characteristic diagram of a beam splitter according to an embodiment of the present invention, and FIG. 3 is a sectional view of a beam splitter according to an embodiment of the present invention.
Figure 4 shows the spectral transmittance characteristics of the beam splitter when the refractive index of the substrate is 2.25 and 2.35. A spectral transmittance characteristic diagram of the splitter. Figure 5 is a spectral transmittance characteristic diagram of the beam splitter when the incidence is 60 degrees in the embodiment of the present invention. Figure 6 is an optical system of an optical pickup head using the beam splitter. It is a figure showing an example. A...Air, B...Substrate, C...Multilayer film, (1)...First layer. ■...Second layer, (3)...Third layer, 0)
...Fourth layer, ■...Fifth layer, 0...Sixth layer, (30)...Semiconductor laser, (34
)...Beam splitter, (38)...Collimating lens, (42)...Objective lens, (44)...Disc, (48)...Signal detection system. Agent Patent Attorney Noriyuki Ken Yudo Norio Ogo Figure 1 'iL length (Kong) Figure 2 Text length tnm) Figure 3 - & (ym) Figure 4 *- & (yzm) Figure 5 6. Procedures included (self-motivated) Showa, 45. Ha day

Claims (1)

[Claims] Ti is deposited on a substrate with a refractive index in the range of 1.40 to 1.80.
A high refractive index thin film made of O_2 and a low refractive index thin film made of SiO_2 are laminated alternately in order, and the design wavelength is set to λ_0.
In this case, the optical thickness of the first layer, fourth layer, and fifth layer is λ_0/4 on the side closer to the substrate, and the optical thickness of the second layer and the third layer is λ_0/4.
A beam splitter comprising a six-layer multilayer film in which the first layer and the sixth layer have an optical thickness of λ_0/2.