JPH02166631A - Out-of-focus detector - Google Patents

Abstract

PURPOSE:To easily detect out-of-focus with high accuracy by comprising a detector of a beam splitter whose transmission factor or reflectance is provided with incident angle dependency against specific incident light, and a split photodetector. CONSTITUTION:An out-of-focus detector is comprised of the beam splitter 1 whose transmission factor or reflectance is provided with the incident angle dependency against the specific incident light, and a two-piece photodetector 2 or 21 and 22. At such a case, no output difference occurs in the two-piece photodetectors 2, 21, and 22 if no incident angle dependency exists in the reflectance or the transmission factor of the beam splitter 1 even when the incident light is divergent light or convergent light. However, when a certain degree of incident angle dependency exists, the output difference occurs in the two-piece photodetectors 2, 21, and 22, and (deviation) from parallel incident light i.e. an out-of-focus signal can be obtained. In such a way, it is possible to easily perform optical axis alignment, position alignment, and angle alignment with simple constitution, and to detect the out-of-focus with high accuracy and high sensitivity.

Description

[Detailed Description of the Invention] [Summary] The purpose of the present invention is to detect focal position deviation with high accuracy and ease by receiving converging or diverging light, and to detect a focal position deviation detector with specific incident light. In contrast, the present invention relates to an improvement in a focal position deviation detector using a beam splitter whose transmittance or reflectance is incident angle dependent and a split photodetector.[Industrial Application Field] The present invention relates to an improvement in a focal position deviation detector.

In recent years, the development of optical disk devices that record, reproduce, and erase information using laser light and their recording media, optical disks, has been remarkable.
), and CD-R as a ROM medium for personal computers.
OM has become widespread.

Furthermore, expectations are increasing for optical disk devices as large-capacity storage devices for office automation equipment and other information-related equipment. In particular, rewritable magneto-optical disk devices are being developed as file memories that are expected to cover a larger capacity area than the currently used magnetic disk devices.

Writing, reading, erasing, etc. on a magneto-optical disk are all performed using a semiconductor laser focused on a minute spot smaller than lam and rotating at high speed, so the optical head that is the device for this purpose must be highly functional, compact, and lightweight. That is what is required.

As an example of such a function, for example, it is absolutely necessary to correctly focus the laser beam on the surface of a magneto-optical disk, and for this purpose, the optical head must be equipped with a focal position shift detection function and a feedback function, so that the optical head is always It is desired that the focus be on the disk surface, that is, that the in-focus state be maintained.

[Conventional technology]

Conventionally, the astigmatism method, the Naifezi method, the critical angle method, and the like have been known as methods for detecting focal position deviations in optical disk devices.

The most typical one is the Naifezi method, and FIG. 6 shows a conventional example of an optical head using a focal position shift detector based on the Naifezi method.

The light emitted from the semiconductor laser diode 4 is passed through a collimator lens 5. Beam shaping prism 8. The light passes through the beam splinter 9, enters the objective lens 3, is condensed, focuses on the surface of the magneto-optical disk 10, undergoes Kerr rotation, that is, is reflected with recorded information on it, and passes through the beam splitter 9 again. Then, the polarizing beam splitter 11 separates the light into transmitted light and reflected light.

A signal detection system is arranged along the reflected light path, and 17
By rotating the two-wavelength plate 12, the polarization direction of the signal light is rotated and set so that the signal amplitude is maximized.

The signals detected by the two photodetectors 14 and 15 are in the relationship of reflection and transmission at the polarization beam splitter 11°, that is, the relationship between S waves and P waves, so they are signals with opposite phases, and differential detection is performed. Remove in-phase noise by S
/N can be increased.

Note that the polarizing beam splitter 11 used in this case
．． 11" is polarized light that is virtually independent of the angle of incidence, minimizing variations in reflectance or transmittance due to variations in the angle of incidence of the incident light, so as not to be affected by positional deviation when incorporated into an optical system. A beam splitter is used.

Meanwhile, along the transmitted light path, lens 16. The components up to the knife 17.2-split photodetector 2 are the focal position shift detection system that we are focusing on here.

A detailed explanation will be given below with reference to the drawings. Fig. 7 is a diagram illustrating a focal position shift detection system using the Naifetsu method, and the solid line ■ on the left side of the figure indicates when the surface of the magneto-optical disk is in focus.
In other words, it shows the case where the magneto-optical disk is in the focused position,
Furthermore, the broken line ■ indicates a case where the magneto-optical disk is closer than the in-focus position, that is, in front of the in-focus position.

Correspondingly, the optical paths are also shown by solid lines and broken lines, respectively.

The bright/dark display of ■ and ■ shown on the right side of the figure is when looking at the two-segment photodetector 2 from the incident light side. Since ■ is at the in-focus position, no light enters the top and bottom of the two-segment photodetector 2, making it dark. , no output difference occurs between the two-split photodetector 2.

On the other hand, when the magneto-optical disk 10 in (3) is located in front of the focusing position, the optical path is as shown by the broken line, so the upper half is cut by the knife 17, but the lower half is cut by the optical knife 17.
When the light reaches the divided photodetector 2, the light spot becomes a semicircular bright part as shown in the figure, so by taking the difference in the outputs of the two divided photodetectors 2, a focal position shift signal can be obtained.

Note that the area within the frame indicated by the broken line is the optical system of the optical head for the magneto-optical disk.

[Problem to be solved by the invention]

However, since the conventional astigmatism method and Naifezi method use lenses, high precision is required for adjusting the optical axis and positioning the photodetector, and the critical angle method is sensitive to angular changes, so the optical axis There were problems such as the need to adjust the angle of incidence and careful design consideration for thermal expansion, and solutions to these problems were strongly sought after.

[Means to solve the problem]

FIG. 1 shows the basic components of the focal position shift detector of the present invention.

In the figure, 1 is a beam splitter, and 2, 21, and 22 are split, especially two-split photodetectors (2D-PD).

Figure (A) shows the case where there is one two-part photodetector (I
(X2D-PD type), and the figure (b) shows the case of two (2
X2D-PD type).

(a) shows the upper half region of the incident light, and (b) shows the lower half region of the incident light. θ1 and θ2 are the angles of incidence of the top and bottom rays of the incident light onto the beam splitter functional layer.

A and B are the left and right photodetector areas of the two-split photodetector 2 or 21, which receive the reflected light from the beam splitter 1, while C and D are the upper and lower photodetector areas of the two-split photodetector 22, which receive the transmitted light from the beam splitter l. is arranged to receive light.

Now, if the reflectance or transmittance of the beam splitter 1 has no dependence on the angle of incidence, no difference in output will occur between the two-split photodetectors whether the incident light is diverging light or convergent light.

However, if the reflectance or transmittance of the beam subric l has some kind of dependence on the angle of incidence, there will be a difference in the output of the two-split photodetector regardless of whether the incident light is diverging light or convergent light. A "'deviation" from the light, that is, a focal position deviation signal is obtained.

That is, the above problem can be solved by configuring a focal position shift detector from a beam splitter 1 whose transmittance or reflectance is incident angle dependent for a specific incident light, and a two-split photodetector 2. Can be done.

[Effect]

Generally, the beam splitter functional layer is composed of a well-known dielectric multilayer film, and when light enters there, it undergoes multiple reflections and interference in each layer repeatedly, and is reflected at a certain reflectance. . If the incident angle changes, the optical path length in each layer will change, the phase difference condition will change, and the reflectance, or transmittance, will also change. Therefore, the type of dielectric multilayer film depends on the wavelength of light used.

By selecting the thickness, number of layers, etc., an optimal beam splitter can be configured.

Figure 2 is a diagram showing the incident angle dependence of the transmittance of the beam splitter according to the present invention, in the case of a beam splitter with the most general configuration in which a beam splitter functional layer is sandwiched between two isosceles right angle prisms. It is.

When the incidence is perpendicular to the prism surface, that is, when the incidence is 45° to the beam splitter functional layer, the transmittance is 50%, and the incident angle (θ
) is 45″ or more, the transmitted light decreases and the incident angle (θ)
This is a typical example of the present invention having a linear characteristic in which transmitted light increases when the angle is 45° or less.

FIG. 3 is a diagram explaining the operating principle of the present invention, and is a diagram for explaining the operating principle of the present invention.
A case will be described in which the beam splitter has the same configuration as (b) and the characteristics of the beam splitter are shown in FIG. 2 above.

In the figure, ■, ■, ■ on the left side of the objective lens 3 indicate the position of the surface of the optical disk, and ■ is when the semiconductor laser beam is focused on the disk surface, that is, at the in-focus position. , ■ is when the disk surface moves away from the in-focus position, and ■ is when the disk surface is closer to the in-focus position.

The bright/dark displays (2), (2), and (2) below the two-split photodetector 21 indicate the brightness and darkness of the reflected light from the beam splitter I depending on the amount of light incident on the left and right photodetector areas A and B of the two-segment photodetector 21. Similarly, the bright/dark displays (2), (2), and (2) on the right side of the two-part photodetector 22 indicate the brightness and darkness of the transmitted light depending on the amount of light incident on the two photodetector regions C and D of the two-part photodetector 22.

Now, if the characteristics of the two photodetector areas A and B1 and C and D of both the two-split photodetector 21 and 22 are the same, then in the case of the focus position (■), the incident light to the beam splitter 1 becomes parallel light, Since the transmitted light and the reflected light are each 50% and have a --like light intensity distribution, the two photodetector areas both have a light intensity of 25% of the incident light, and are bright, so there is no output difference between the two photodetectors.

On the other hand, when the disk (■) moves away from the in-focus position, the reflected light from the disk becomes a convergent light after passing through the objective lens 3, so from the transmittance characteristics of the beam splitter 1 shown in FIG. The light intensity is less than 25% of the incident light and becomes dark, and the light intensity of D and A is more than 25% of the incident light and becomes bright, and the two 2-split photodetectors 2
1 and 22 will both produce an output difference.

Similarly, when the disc (■) approaches the in-focus position, the reflected light from the disc becomes divergent light after passing through the objective lens 3, so contrary to the case (■), the C and B portions are bright, and the D and A portions are bright. becomes dark, and the two two-part photodetectors 21 and 22 both produce a difference in output.

That is, the outputs of the areas A, B, C, and D of the two-part photodetectors 21 and 22 are respectively p, pb,
Assuming that P, , P, the focal position shift information can be obtained as the output difference F between the two two-split photodetectors using the following equation.

F = (P, +P a ) (P b+
P c ) ------= (1), and the transmitted light and reflected light of beam splitter 1 are each 50% of the incident light.
% light amount.

〔Example〕

Using a beam splitter with transmittance characteristics as shown in FIG. 2, it was possible to easily form a focal position shift detector using the configuration shown in FIG. 1 (a) or (b). In particular, in the case of FIG. 1(b), since two two-part photodetectors are used, the sensitivity can be approximately doubled.

Note that the beam splitter film is SiO□, TiO
□.

A dielectric multilayer film of MgF, At2Q, etc. was formed by a known method so as to exhibit desired characteristics.

FIG. 4 is a diagram illustrating another embodiment of the present invention, and FIG. 4 (A) shows the configuration of the optical system. In the figure, 1' is a polarization-dependent beam splitter, and 2 is a two-split photodetector.

The figure (b) shows the reflectance characteristics of the above polarization-dependent beam splitter of 1°, which was fabricated so that 100% of the S wave was reflected and only the P wave had a linear incidence angle dependence. The beam splitter film is SiO□, TiO□.

A dielectric multilayer film of MgF, AIzO, etc. was formed by the same method as in the previous example.

Now, if the incident light is a mixed wave of P waves and S waves, all of the S waves will be reflected, and 50% of the P waves will be transmitted and received by the two-split photodetector 2.

If this configuration were applied to an optical head for a magneto-optical disk, recording information would be loaded onto the incident S wave, and the reflected light path would be used as a recording signal detection system, while the transmitted light path would be used as a focal position shift detection system. can do.

The focal position shift information in this case is obtained as the output difference F between the two CD regions of the two-segment photodetector 2 using the following equation.

F=(PC−P,)−−−−−−−−−−−−−(2
) Figure 5 shows an optical head for a magneto-optical disk using the focal position deviation detector of the present invention, and Figure (a) shows a 2X2D-PD type focal position deviation detector using two two-part photodetectors. This is an embodiment applied in place of the focal position shift detector using the Knifezi method shown in FIG. As can be seen from the figure, the lens 16 is no longer needed, making optical axis adjustment extremely easy.

On the other hand, Figure 5 (b) shows a polarization-dependent beam splitter of 1°.
lX2D-P using one two-part photodetector and
This is an embodiment in which a D-type focal position deviation detector is applied in place of the focal position deviation detector based on the Knifezi method shown in FIG. 6 and the polarizing beam splitter 11. In this example, there are two optical components: 1 polarizing beam splitter
1 and lens 16 are no longer necessary, making it possible to make the optical head smaller and lighter and to simplify adjustment work.

The functional operation of the optical head of this embodiment is basically the same as that of the conventional example shown in FIG. 6, and therefore, redundant explanation will be omitted.

Note that the area within the frame surrounded by broken lines is the optical system of the optical head.

〔Effect of the invention〕

As explained above, the focal position shift detector according to the present invention has a simple configuration, facilitates optical axis alignment, two-position alignment, and angle alignment, and is highly accurate and sensitive, so that it can be used in various optical application devices. It greatly contributes to making optical heads for magneto-optical disks smaller, lighter, and more highly functional.

[Brief explanation of the drawing]

Figure 1 is a basic configuration diagram of the focal position shift detector of the present invention. Figure 2 is a diagram showing the dependence of the transmittance of the beam splitter on the angle of incidence in the present invention. Figure 3 explains the operating principle of the present invention. 4 is a diagram explaining another embodiment of the present invention, FIG. 5 is an embodiment of an optical head for a magneto-optical disk using the focal position shift detector of the present invention, and FIG. 6 is a diagram using the Knifezi method. A conventional example of an optical head using a focal position deviation detector. FIG. 7 is a diagram illustrating a focal position deviation detection system using the Knifezi method. In the figure, l is a beam splinter, 1" is a polarization-dependent beam splitter, 2.21.22 is a two-split photodetector, 3 is an objective lens, 4 is a laser diode, 5.6 and 7.16 are lenses, and 8 is a beam. a shaping prism; 9 a beam splinter; 10 a magneto-optical disk; 11.
11' is a polarizing beam splitter, 12 is a half-wave plate, 13 is a mirror 14, 15 is a photodetector, and 17 is a knife. Diagram 2 showing the nature of the human body's four angles. Diagram 3 illustrating the meaning of the hanging Aoihara kuri of the Aoi μ field. 4 + main beam scatterer (1 reflection)

Claims (1)

[Claims] A focal position shift comprising: a beam splitter (1) whose transmittance or reflectance is incident angle dependent for specific incident light; and a split photodetector (2). Detector.