JPS60163238A - Focus detector - Google Patents

Abstract

PURPOSE:To keep high sensitivity by forming a part of a reflecting surface of an optical member as a reflecting mirror. CONSTITUTION:A total reflection part 12 is arranged on a part of a reflecting surface 11a of a critical angular prism 11 and a photodetector 13 is divided into three parts 13a-13c. The total reflection part 12 is located about the center of reflected light flux and arranged so that the light reflected by the total reflection part 12 is irradiated to the central photodetecting part 13c of the three-divided photodetector 13. Rays of light 14a-1, 14a-2 around the reflected light flux 14a are irradiated to respective peripheral photodetecting parts 13a, 13b.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a focus detection method and a focus detection device for detecting a focus shift of an objective lens with respect to a recording medium in an optical disk device.

[Prior art]

FIG. 1 is an explanatory diagram for explaining a focus detection method in a conventional focus detection device. That is, the light emitted from the light source 1 is focused onto the recording medium 6 by the objective lens 5, and the reflective surface 7α is set so that the returned light forms an approximately critical angle with respect to the center ray of the returned light beam. The reflected light incident on the optical member 7 and reflected by the reflective surface of this optical member is straddled, and the scene distribution of the transmitted light from the reflective surface is expressed as 2
Detection is performed by a divided photodetector 8 to obtain a focus error signal. In FIG. 1, 2 collimator lenses, 3 a beam spotter, and 4 let 1/l wave plate are shown, but their explanation will be omitted. Now, here, the reflective surface 7α is set to have a critical angle K slightly smaller than the critical angle with respect to the incident light beam (parallel light flux) in the focused state. If the critical angle is set exactly, the returned light, which is a parallel beam of light, will be totally reflected by the reflecting surface 7α in the state of convergence. When the distance between the recording medium 6 and the objective lens 5 becomes shorter than the focal point, the returning light beam becomes vl, a diverging beam as shown in Figure 1BK, and conversely, the distance between the recording medium 6 and the objective lens 50 becomes When the distance becomes longer than the focal point, the returned beam becomes a convergent beam as shown in FIG. 1CK. That is, when the focus is deviated, the returned light changes continuously around the critical angle except for the central optical axis. Therefore, when it deviates from the in-focus state, the reflection intensity at the reflecting surface 7α changes rapidly near the critical angle, as shown in Figure 2. The situation is opposite when the object is closer than the focal point and when it is further away from the focal point. On the other hand, in the focused state, the light is totally reflected uniformly, so no brightness or darkness of the light appears. 1st
The diagram is a plan view showing the position W relationship between the photodetector and the reflected light spot (8a, 8a).

It is divided into two parts 8b, and is set so as to be located at the center of the nine optical detectors 8. Incidentally, FIG. 2 shows the reflection intensities RP and R8 for P-polarized light and 8-polarized light, respectively, when the refractive index of the reflective optical member 7 is 1.50.

Now, the explanation has been given for the case where the reflection surface 7α in the light is set at exactly the critical angle with respect to the returning light, and the point Xl in FIG. 2 is the operating point. on the other hand.

If the reflective surface 7α is set larger than the critical angle with respect to the returned light, then the point x2 in w, 2 becomes the operating point, so
The distance from point 2 to point Xl becomes a dead zone, so it cannot be made larger than the critical angle. Furthermore, if the reflective surface 7α is set smaller than the critical angle for the returning light, the point X3 in Fig. 2 becomes the operating point and there is no dead zone, but the sensitivity near the in-focus point decreases, and There was a major drawback in that a considerable amount of the scene was transmitted through the focus and did not enter the photodetector.Therefore, in such a focus detection device, the reflective surface 7α must be placed at an extremely critical position with respect to the returning light. It is necessary to set the angle to be slightly smaller than the critical angle, which makes mass production extremely difficult.Furthermore, the main signal is detected by taking the sum of the outputs of the photodetectors 8a, 8b. Therefore, it has the disadvantage that the main signal is also modulated due to the shift in focus.

[Purpose of the invention]

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a focus detection device that eliminates the above-mentioned drawbacks, has good mass productivity, and can accurately detect main signals.

[Structure of the invention]

The present invention will be explained below with reference to the drawings.

Figure 3 shows a specific example of the focus detection device of the present invention.

Although it is the same as the secondary center example mentioned above, in the present invention, the reflective surface 11 of the critical angle prism 11 (
Total reflection part 12 in part of Z? - 13 photodetectors arranged;
It is divided into three parts: 13α, 13b, and 13C. Then the fourth
The figure shows a critical angle prism 11, a total reflection section 12, and a photodetector 1.
FIG. 3 is a perspective view showing the relationship between the amount of light and the reflected light beams 14α and 14b and the light spot 14C on the photodetector. That is, the total reflection section 12 is located near the center of the reflected light beam, and the light reflected by the total reflection section 12 is arranged so as to be irradiated onto the central light receiving section 13C of the three-split photodetector 13. . Then, the peripheral light 14n, -1 of the reflected luminous flux 14σ
, 14a-21d are arranged so as to illuminate the surrounding light-receiving parts 13α and 13b, respectively. Then, as shown in FIG. 5, a focus error signal can be obtained by detecting the main signal from the central light receiving section 13c and taking the difference #l between the signals of the peripheral light receiving sections 13W and 13b. In the present invention, the light near the center of the reflected light is always totally reflected, so
Even if the reflective surface 11α is set to be smaller than the critical angle, there is almost no loss in the amount of light. In FIG. 5, 15α indicates a differential amplifier and 15b#-i amplifier.

Next, focus error detection sensitivity will be explained. In the present invention, as shown in FIG. 6, the light beam 16α near the center enters the total reflection part 121C and is always totally reflected, so it is unrelated to the focus error signal.
The focus error is detected by c. Here, as is clear from Fig. 6 (example when the returned light becomes divergent light),
Whether the returned light becomes diverging light or converging light, the angle of the light flux near the center does not change, and the angle of the light flux around the periphery changes sharply. In other words, it is better to use only the peripheral light flux.

1 is shown in curves 17 and 18 in Figure 7. Curve 17 is
The curve 18 is a curve showing the conventional focus error detection sensitivity, and the curve 18 is a curve showing the focus error detection sensitivity of the present invention in which only the peripheral light flux is used for focus detection. As is clear from FIG. 7, the sensitivity near the focal point is significantly improved in the present invention. FIG. 7 shows the characteristics when the reflecting surface is set at just the critical angle.

As described above, in the focus detection device of the present invention shown in FIGS. 3, 4, and 5, the light beam near the center is always totally reflected, and the focus error caused by the peripheral light beam is reduced. Since this is a detection method, even if the reflecting surface 11cLf of the critical angle setting prism 11 is set to an angle smaller than the critical angle, there is almost no loss in the amount of light, and a more sensitive focus detection device can be obtained.

Also, as is clear from the previous achievements, the total reflection part 1
Since the light reflected by the total reflection part 12 needs to be incident on the central photodetector 13C, the width of the light reflected by the total reflection part 12 is WO1-
t is set to be equal to or smaller than the width WD of the central photodetector 13C. Here, the width WO of the totally reflected light is as follows: If the setting angle of the reflection surface 11α of the width −1 of the total reflection section 12 is θ, then Wo = Wm @Binθ, that is, WD≧Wm*Sinθ. Set.

Figure t48 shows another embodiment of the focus detection device of the present invention.

In other words, the focus detection sensitivity is improved by performing three reflections near the critical angle, and in this case, the total reflection section 20 of each reflecting surface 19α, 19b, 19c is
By providing α, 20b, and 20C, loss of light quantity is eliminated and focus detection sensitivity is significantly improved. Although an example of three reflections has been given here, it goes without saying that the same effect can be obtained by multiple reflections. Further, the total reflection section mentioned above is formed by attaching a metal thin film or a multilayer thin film to one reflection surface.

[Effects of the Invention] By using the focus detection device of the present invention as described above, there is no loss of light quantity even if the reflective surface is made smaller than the critical angle, and the sensitivity can be kept sufficiently high. Since there is no deterioration in performance even if the set angle is slightly deviated, mass productivity is greatly improved. Furthermore, since the main signal KFi focus error signal has no influence at all, the main signal can be detected accurately. Therefore, the original objectives of improving mass productivity and accurately detecting the main signal can be completely achieved.

[Brief explanation of drawings]

Fig. 1 shows a structural diagram showing the conventional focusing compensation, Fig. 1 B, O.
Figure 1 is a plan view showing the relationship between the optical detector and the cover of the reflected light spot, Figure 2 is an explanatory diagram showing the change in reflected luminous intensity due to the angle blindness of the reflective surface, Figure 3 4 is a perspective view of the focus detection device of the present invention; FIG. 5 is an M-line diagram of focus error signal and main signal detection; and FIG. 6 is a diagram of the focus detection device of the present invention. 7 is an explanatory diagram showing the focus detection and main signal detection principles of the present invention, FIG. 7 is an explanatory diagram showing the focus detection sensitivity of the conventional and the present invention, and FIG. be. 1... Semiconductor laser 2... Collimator lens 5... Beam splitter 4... λ/4 plate 5... Objective lens 6... ... Recording medium 11 ... Critical angle setting prism 11α ... Reflection surface 12 ... Total reflection section 13 ... Photodetector 14a, 14 (7, -1, 14a-2, 14b--Return light beam 14c... Optical subotato 15a... Differential amplifier 15b... Amplifiers 16α, 16b, 16c... ...Return light 17,
j8...Focus error signal - focus shift characteristic 1
9...Critical angle setting prism 19rz, 196.196...Reflecting surface 20
α, 20/l, 20C... Total reflection plate Upper applicant Seiko Electronics Co., Ltd. Figure 1 A

Claims (1)

[Scope of Claims] 1) An optical member having a reflecting surface that focuses light emitted from a light source onto an optical disk using an objective lens, and sets the returning light to be at an approximately critical angle with respect to the returning light. In a focus detection device that obtains a focus error signal by detecting a change in the light amount distribution of reflected light reflected by a reflective surface of the optical member or transmitted light from the reflective surface, the A focus detection device characterized in that a part of the surface is a reflecting mirror. 2) The part of the reflection mirror is located approximately in the vicinity of the center of the light beam incident on the reflection surface of the optical member, and the width of the part of the reflection mirror is smaller than the diameter of the light spot. A focus detection device according to claim 1, characterized in that: 3) The focus detection device according to claim 1, wherein the reflecting mirror is configured by attaching a thin film to a part of the reflecting surface. 4) The focus detection device according to claim 1, wherein A patent characterized in that the photodetector for detecting reflected light is divided into at least five parts, and the width of the photodetector in the central part is approximately equal to the width of the reflected light at the part of the reflecting mirror or larger than XFi. B. A focus detection device according to claim 1.