JPS62299905A - Two-wavelength separation filter - Google Patents

Abstract

PURPOSE:To steepen the rise of a transmittance characteristics and to remarkably improve the transmittance in a transmission band and reflectivity in a reflection band by forming a titled filter into a 40-layered structure by using a high refractive index material and low refractive index material and forming a specific film constitution. CONSTITUTION:A substrate material is one of flint glass and has about 1.66 refractive index. The 40-layered 2-wavelength sepn. filter consisting of the high refractive index material TiO2 and the low refractive index material SiO2 is formed on said substrate. The TiO2 is formed as the layers of the odd numbers counted from the substrate and the SiO2 as the layers of the even numbers in the film constitution. The optical film thicknesses of the respective layers are so determined that the optical film thicknesses of the 3rd layer, 8th layer, 14th layer, 20th layer, 26th layer, 32nd layer and 37th layer are about lambda0/2 and the rest are lambda0/4 when the design wavelength is designated as lambda0. The rise characteristic is thereby steeped and the two-wavelength sepn. filter which is improved in the reflectivity and transmittance in the use wavelength band is obtd.

Description

Detailed Description of the Invention 3. Detailed Description of the Invention Field of Industrial Application The present invention is used for optical disk file devices, etc.
This relates to wavelength separation filters.

2. Description of the Related Art In recent years, optical disk devices have been actively developed and commercialized as large-capacity information storage devices, such as still image disk file devices and correspondence file devices. 2.＼-7' The development of optical heads for optical disk devices having an erasing/rewriting function is becoming more active, and various attempts are being made to further reduce the size and improve the performance of optical heads.

In order to realize such an optical head, the wavelength λ1=780
An optical element that separates a laser beam with wavelength λ2=830 nm from a laser beam with wavelength λ2=830 nm is essential.

Hereinafter, an optical element having such a function will be referred to as a two-wavelength separation filter.

Conventionally, two-wavelength separation filters have been realized using dielectric multilayer films. For example, T* as a high refractive index material
02, attempts to obtain desired characteristics by alternately stacking SiO2 as a low refractive index material.

FIG. 7 shows a conventional film structure in which the number of film layers is 23. Counting from the base, the first layer is TiO2, the second layer is S1
02. The third layer is T102, which are alternately laminated.

If the center wavelength is λO, the optical film thickness is the 4th layer, the 1st layer,
The second layer and the 20th layer are approximately λO/2, and the other layers are 3 P7 λO/4.

The refractive index of T z O2 is n-2.20, and the refractive index of S 102 is n=1.46. FIG. 8 shows an example of transmittance characteristics when the center wavelength is λO=830 nm.

Here, the vertical axis is the transmittance (unit: %), and the horizontal axis is the wavelength (unit: %).
Unit: nm). The dotted line on the left side of the figure is λ1 = 780n
m, and the dotted line on the right side represents λ22-830n.

Problems to be Solved by the Invention Problems that arise when a two-wavelength separation filter having such a film configuration is used in an optical head will be explained.

This two-wavelength separation filter reflects light with a wavelength λ = 780nm and transmits light with a wavelength λ22-830n, or transmits light with a wavelength λ = 780nm, and transmits light with a wavelength λ2-830n.
It reflects light of 30 nm.

At this time, if light that should be transmitted is reflected or light that should be reflected is transmitted, these lights will reach the detection system of the optical head and have an adverse effect.

Therefore, it is strongly desired that the reflectance be as small as possible in the transmission wavelength region and the transmittance as small as possible in the reflection wavelength region.

Generally, the manufacturing wavelength variation of semiconductor lasers is ±1° nm. Furthermore, it is known that after the dielectric multilayer film is formed, the transmittance characteristics shift toward longer wavelengths or shorter wavelengths due to changes over time. According to our experiments, the shift amount is approximately ±8
It was nm.

Therefore, from the point of view of mass production, from the design stage of the thin film system, λ
1=780nm±18nm, λ2=:830nm+18
It is desirable that the characteristics be satisfied over a wavelength range of nm.

However, in the conventional film configuration, λ1=780nm±18nm
Then, about 30 inches of light passes through, and λ2=83onm±18
At nm, about 8 - of light is reflected. By making the rise of the transmittance characteristic steeper from the reflection region to the transmission region, the characteristics in the wavelength region can be improved, but with conventional film configurations, it has been difficult to make the rise even steeper.

The present invention was made in view of this point, and the transmittance characteristic from the reflection region to the transmission region, that is, the rise characteristic, is made steep, λ, = 780 nm ± 18 nm 6 b,
′, the reflectance (transmittance n) is λ2=830 nm±1
The objective is to provide a film configuration that maximizes transmittance (reflectance) at 8 nm.

Means for Solving the Problems The technical means of the present invention for solving the above problems is to have a 40-layer structure using a high refractive index material and a low refractive index material, with odd-numbered layers counting from the base. a high refractive index material layer, an even-numbered layer is a low refractive index material layer, a third layer, an eighth layer, a fourteenth layer,
20th layer, 26th layer.

The optical thickness of the 32nd layer and the 37th layer is approximately λO/2, which is the design wavelength of λO, and the optical thickness of the other layers is approximately λO/4.
The membrane structure is as follows.

Effect: By using the above-mentioned film structure, the present invention has a steep rise characteristic and a low reflectance in the used wavelength band.

A two-wavelength separation filter with improved transmittance can be obtained.

EXAMPLE An example of the present invention is shown in FIG. The base material is BaFlo (trademark 9 and 8t(-
; The refractive index is approximately 1.66.

On this base, a 40-layer two-wavelength separation filter consisting of a high refractive index material T z 02 and a low refractive index material S 102 is formed.

The film structure is such that the odd-numbered layers counting from the substrate are TiO2°, and the even-numbered layers are Sto2. The optical thickness of each layer is the third layer, assuming the design wavelength is λO.

The optical thickness of the 8th layer, 14th layer, 20th layer, 26th layer, 32nd layer, and 37th layer is approximately λO/2, and the others are λO/4
It becomes.

The transmittance characteristics when light is perpendicularly incident on this multilayer film are
Shown in Figure a. The vertical axis is transmittance and the horizontal axis is wavelength. Note that the design wavelength is λO = 855 nm, and the refractive index of Tio2 is n =
2-22, the refractive index of S102 is set to 1.46.

When λ=780 nm and 18 nm, the transmittance becomes low, and when λ2=830 nm±18 nm, the transmittance increases and the characteristics are significantly improved.

Figure 2 shows the transmittance characteristics when BK7 is used as the base glass. Similar to the case of BaFlo, 7 better results were obtained.

FIG. 3 shows a case where the setting fi-1 wavelength λO-750 nm.

In the case of Figure 2, the long wavelength side (λ2-2-830n) was transmitted and the short wavelength side (λ1-=ysonm) was reflected, whereas the short wavelength side was transmitted and the long wavelength side was reflected. It has become a characteristic of

With this film configuration, such an opposite effect can be easily obtained by simply changing the design wavelength, and good characteristics can also be achieved.

Next, the characteristic fC when light is incident on the multilayer film at an angle of 21° (incident angle -21°) will be described.

Figure 4 shows its transmittance characteristics. Note that the design wavelength at this time is λO-855 nm. Even at an incident angle of 21°, the reflectance at λ1=780nm±18nm is high, and λ22
The transmittance at -830n±18nm is also high, and good characteristics are obtained.

The transmittance characteristics described above are determined by the value of the design wavelength of the thin film system. Therefore, the characteristic values change depending on the value of the design wavelength, especially λ = 780nm + 18n
m=798nm, λ2=830nm-18nm=81
The value at 2 nm is greatly affected. Since the transmittance at these wavelengths greatly affects various detection systems, the design wavelength must be determined after fully considering these effects.

In recent years, the above λ = 798 nm, λ2 = 812 n
There is a trend to change the wavelength of semiconductor lasers because the deterioration of characteristics at m2 has a very negative effect on the detection system. That is, the movement is to shift the semiconductor laser with λ1 = 780 nm to the shorter wavelength side and the semiconductor laser with λ2 = 83 onm to the longer wavelength side.

Thereby, even if the transmittance characteristics change due to aging of the semiconductor laser or thin film system, the characteristics can be sufficiently realized. However, the problem at this time is the width of the transmission band, and the transmission band of this film configuration is approximately 1100 nm, which is sufficient to accommodate changes in the wavelength used by these semiconductor lasers.

In FIG. 6, as an example, λ = 760 nm.

The transmittance characteristics are shown when λ=860 nm and the design wavelength λO=863 nm.

9. There is sufficient margin for wavelength shifts due to wavelength variations in vane semiconductor lasers, changes in thin film systems over time, etc., and it is therefore expected that yields can be significantly improved even in the production of thin film systems.

FIG. 6 shows transmittance characteristics when T*02 is used as the high refractive index material and MgF2 is used as the low refractive index material. The refractive index of TiO is 2.22, the refractive index of M (JF2 is 1.36), the design wavelength is 876 nm, and the incident angle is 21°.
It is said that It exhibits characteristics such as a steep rise, a wide transmission band width, and little ripple.

Effects of the Invention As mentioned above, by using this film structure, the rise of the transmittance characteristics becomes steeper, and the transmittance in the transmission band and the reflectance in the reflection band are significantly improved, resulting in a high-performance two-wavelength separation filter. can be realized and is extremely useful from a practical standpoint.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the configuration of a two-wavelength separation filter in an embodiment of the present invention, and Fig. 2a shows the case where BaFlo is used as the base of the same two-wavelength separation filter and the design wavelength 10 t = -> is λO = 865 nm. Figure 2 is a characteristic diagram showing the transmittance characteristics when BK7 is used on the same substrate, and Figure 3 is a characteristic diagram showing the transmittance characteristics when the same design wavelength is λO = 750n.
Figure 4 is a characteristic diagram showing the transmittance characteristics at m.
10. Figure 5 is a characteristic diagram showing the transmittance characteristics at the same design wavelength λO-853 nm.
Fig. 6 is a characteristic diagram showing the transmittance characteristics when T*02 is used as the high refractive index material and MgF2 is used as the low refractive index material. Fig. 7 is a diagram showing the configuration of the same conventional two-wavelength separation filter. FIG. 8 is a characteristic diagram showing the transmittance characteristics. 1 to 40...layers. Name of agent: Patent attorney Toshio Nakao and 1 other person No. 1
Figure No. x)! ° 1 37 Ti0z 2 ! 32 5tCh 2 Ti / 26 Sing 2 20 δ1o22 7 ′ ′ / . /4 5i(k 2 7.3 / ' / & SiO22 J T'+022

Claims (1)

[Claims] In a thin film system consisting of a multilayer film in which dielectric thin films are alternately laminated on a substrate, the total number of layers is 40, and the odd-numbered layers counting from the substrate are made of a high refractive index material and the even-numbered layers are made of a low refractive index material. material, 3rd layer, 8th layer, 14th layer, 20th layer, 2nd layer
The optical thickness of the 6th layer, the 32nd layer, and the 37th layer is determined by setting the design wavelength to λ.
A two-wavelength separation filter in which _O is about λ_O/2 and the optical film thickness other than the above is about λ_O/4.