JPS60191445A - Optical reproducing device - Google Patents

Abstract

PURPOSE:To obtain the optical reproducing device capable of an information recording medium with recording density almost at the optical cutoff frequency of a reproducing lens by providing a spatial frequency filter means which cuts off spatially part of reflected or transmitted light incident to a photodetector. CONSTITUTION:The spatial frequency filter 12 is so arranged as to cut off part of 0-order light except overlap parts of the 0-order light and + or -1-order light, and the width of light shielding depends upon a recording and reproducing system, but is only set so as to obtain an excellent reproduction spectrum. The filter 12 has a tapered edge preferably. Consequently, the light shield width is varied easily by making an up/down adjustment as shown by an arrow A in a figure. Thus, the spatial frequency filter 12 is constituted and arranged to reproduce an information signal with an excellent spectrum at the inner and outer peripheries against variation in spatial frequency caused by the difference in the radius of an information recording medium during reproduction while the filter 12 is fixed.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an optical device for reproducing video discs, PCM audio discs, etc.

Conventional configuration and its problems Traditionally, the minimum bit spatial frequency recorded on a video disc was designed to be around the optical cutoff frequency determined by the numerical aperture (hereinafter referred to as NA) of the reproduction lens. Generally, the NA is about 0.45.

If we consider the case where the recording density is doubled for such an information recording structure, the NA will be approximately 0.9 from the above relationship. However, the relationship between NA and the depth of focus fd of the reproduction lens is generally expressed by the following equation.

−2− fd=ffl−(λ/π·NA) In addition, it can be seen that when J and NA are doubled, the depth of focus 1ifd becomes about 1/4. On the other hand, the maximum gain including the current Focus Geng actuator is approximately 50d.
It is about B. Therefore, the current NA-〇, 4! In the case of NA-09 for i, a focus gain of about 62 dB, which is about 1263 higher, is required to follow the same vibration, but it is currently extremely difficult to achieve this technically. be. Therefore, when using a reproduction lens with NA=0.9, it is necessary to suppress the shake to about (1/4). Furthermore, as the NA increases, the field of view of the reproduction lens becomes narrower and the tracking range also becomes narrower, making stable reproduction extremely difficult.

In the mastering process of a disc, etc., there is a need for inspection by reproducing a mold (hereinafter referred to as F1 stamper) transferred from an iIk- to a master disc. This is because it is a very effective means for evaluating the quality of the stamper at the end of the electrical protection process, or for evaluating the stamper after making several hundred replicas by pressing or the like. However, due to the electroforming process, the stamper inevitably warps and the surface runout increases.

In addition, the stamper used as a press mold is distorted due to stress, resulting in even greater surface runout.

Due to the reasons mentioned above, the current optical reproduction method.

Reproduction of a disc having a spatial structure below the spatial cutoff frequency determined by NA-0.4 to 0.6, especially stamper reproduction, is extremely difficult by simply increasing the NA. However, in order to improve recording density, there is currently a demand for a reproducing optical device having a spatial structure with a spatial frequency as high as possible.

Purpose of the Invention The present invention provides that the spatial frequency of recorded information is NA=0.4-06.
It is an object of the present invention to provide a reproducing optical device (1) which can reproduce an information recording medium having a recording density near or below the optical cutoff frequency of the reproducing lens using a reproducing lens.

Structure of the Invention The reproducing optical device of the present invention includes a light source for reading an information recording medium containing information in an optically detectable -4 form, and a beam condensing device that condenses a beam from the light source to near the diffraction limit. a reproducing lens for guiding the beam from the light source to the reproducing lens; a focusing means for maintaining a constant distance between the reproducing lens and the information recording medium; and a focusing means for following the information track on the information recording medium. (1) A racking means and a photodetector for detecting information from reflected or transmitted light from the information recording medium are provided to reproduce information and detect the reflected or transmitted light incident on the photodetector. It is characterized by providing a spatial frequency filter means for spatially shielding a portion.

Description of Examples Generally, the principle of reproducing information recorded on an information recording medium with unevenness is based on the phase difference of the uneven structure of light reflected or transmitted from the information recording medium, and the zero-order light of diffracted light determined by the dimensions of the phase structure. It is thought that the reproduction is caused by interference between the first-order light and the higher-order light. Therefore, when an information recording medium with a phase structure with a changed spatial frequency is reproduced with a lens of about NA-0.6, the reflected light is scanned with a slit of about 10 μm, and the reproduced signal spectrum is observed and reproduced. When we measured the distance of the optimal spectrum, it was constant at about 60 μm up to the minimum recording wavelength of about 0.9 μm, and when it exceeded 0.9 μm, the distance suddenly became wider, and at the minimum bit wavelength of about 0.5 μm, it was about 100 μm. It turned out that it was about. The wavelength of the light source used was 0.63 μm. This state is shown in FIG.

Figure 1a is the minimum recording wavelength of 0.9μm1 Figure 1A is 05μm
This is the case of m. In the figure, p indicates the interval between the smaller portions of the diffracted light j having the optimum spectrum. The optical cutoff frequency of the lens is given by 5fc-λ/2・NA, and when NA=0.6, the optical cutoff frequency is 0.53 μm, and the minimum recording wavelength is 0.5 μm.
In this case, it will be below the cutoff.

The principle of reproduction in this case is that reproduction is possible if the contribution of defocus or the zeroth order tip itself contains information components in some form due to bits.

-6- In this way, when reproducing bits smaller than the readout beam diameter, if all the reflected light from the information recording medium is incident on the photodetector as in the conventional case, it is not shown in Figure 2. The spectrum component of the color subcarrier, which is a relatively low frequency component, increases by more than 10 dB compared to the 0 component of the luminance signal during subek [-ram], and the information recording signal cannot be reproduced.

The reason for this is when the spatial frequency structure of the information is large relative to the optical cutoff frequency determined by the NA of the reproduction lens, i.e. when the dimensions of the information structure are at least comparable to the readout light spot. , since the reflected light is affected by only one information structure, no unnecessary spectral components are generated.However, when the information structure size is smaller than that of the reproduction spora 1~, for example, a frequency component twice that of the color beam carrier is generated. In the case where several corresponding bits are included in the reproduction spora [...], the diffraction or phase change due to this subcarrier becomes the maximum change in the reproduction light component, and as mentioned above, the necessary brightness frequency component It is thought that more of the color subcarrier 7-rear component is reproduced than that, resulting in overmodulation, which makes normal reproduction impossible.

However, as described above, when only the preferred state position of the reproduction spectrum is reproduced, the level of this color subcarrier is improved by 10d13 or more, and information can be reproduced normally.

If an attempt is made to remove this subcarrier component with an electrical filter, it is difficult to remove it completely even if a band pulse filter with a fairly narrow band is used, and furthermore, phase changes in the necessary band pose a problem. Furthermore, since the level of the unnecessary spectrum varies depending on the reproduction position, it is necessary to switch the characteristics of the filter.

In order to solve this problem, the present invention shields the low frequency component region of the 0th order pile tip, which has many unnecessary spectrum components shown in FIG. 1, and optically improves and covers the reproduced information quality.

FIG. 3 shows an embodiment of the reproducing optical device of the present invention. Third
In the figure, the light from the laser 1, which is linearly polarized, passes through the total reflection mirrors 2 and 3 and enters the intermediate lens 8-4. 4) The light becomes circularly polarized by the plate 6, passes through the racking mirror 7, and is focused on the information recording medium 10 by the reproducing lens 9 mounted on the voice coil 8. The reflected light that has been phase modulated or diffracted by the information recording medium returns on the same optical path as the incident light, becomes a polarization plane perpendicular to the incident light at the (λ/4) plate 6, is reflected by the polarizing beam splitter 5, and is transmitted to the beam splitter 11.
incident on . Here the light is split into pieces with the desired intensity,
On the other hand, a part of the low frequency spectral component is blocked by the spatial frequency filter 12, and enters the photoelectric conversion element 13, where an information signal is detected. The light reflected by the beam splitter 11 enters the photoelectric conversion element 14 in order to detect the tracking signal from the information signal.

Although the focus detection system is not shown in this figure, it may be of any combination such as an Ophakis system, an Astig system, or a Knifetsu system. In addition, since tracking cuts off a part of the low frequency spectrum of the information reproduction light, examples such as the Jrutsu A bling method and the Pyro 1 signal recording method are shown in which the tracking signal is reproduced from the information signal. ing. In this case, since a part of the low-frequency spectrum of the information signal (1-light) is blocked, the low-frequency spectrum during the information transmission is attenuated in both of the above-mentioned methods. It is preferable to divide the information reproduction signal as shown in FIG. 3 and input it to the trough 4 detection photoelectric conversion element.

However, in the case of a 3-beam type or far field type tracking detection method, it is possible to reproduce the data without any problem by adding the necessary optical system.

FIG. 4 shows the arrangement of the previous h[1 spatial frequency filter 12]. The spatial frequency filter 12 has a diameter of 0 as shown in the diagram.
A portion of the zero-order light other than the overlapping portion of the second-order light and the ±1st-order light is placed in a mirror. ShikJ
, [A suitable reproduction spectrum can be obtained.] ”, just set it to . Further, as a preferable shape of the spatial frequency filter 12, it is desirable to have a tapered shape as shown in the figure.

-10- With such a shape, it is possible to easily change the light-shielding width by vertical adjustment indicated by the arrow in the figure. By arranging the spatial frequency filter 12, the spatial frequency filter 12 is kept fixed, and both the inner and outer peripheries respond to changes in the spatial frequency that depend on the radius of the information recording medium during playback. This makes it possible to reproduce information signals having a good spectrum. Further, as another embodiment of the present invention, if it is absolutely necessary to adjust the light shielding width of the spatial frequency filter 12 in response to the above-mentioned change in spatial frequency at the radial position of the information recording medium, the reproduction position may be changed. For example, an optical bench and a bodension meter are configured to be linked together, and a voltage corresponding to position information is generated by a change in the resistance value of the bodension meter, and a spatial frequency filter 12 is generated.
For example, it is possible to change the width of the spatial frequency filter 12 by moving it in the direction of the arrow A in FIG. 4 using a DC motor. It is sufficient to change the width of the filter 12.Also, it may be useful to move the spatial frequency filter 12 to a desired position using, for example, a limit switch at the start of reproduction.

The reproduction position detecting means and the spatial frequency filter 12 moving means described above are not limited to these.

Effects of the Invention As explained above, since the reproducing optical device of the present invention is provided with the spatial frequency filter means, an information structure having a spatial frequency near or higher than the optical cutoff frequency of NA-04 to 0.6 can be obtained. Achieves high-quality playback of information recording media with stable and good playback speed using a playback lens with an NA of 0.4 to 0.6 that has loose restrictions on tracking, focusing, and surface wobbling. Furthermore, it can also be applied to the reproduction of standby bars, etc., which is extremely useful in the mastering process, which has been difficult in the past.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram showing the state of the overlapping portion of diffracted light with respect to changes in the spatial frequency of information, and Figure 2 shows the spatial frequency structure near or below the optical cutoff frequency of the reproduction lens. FIG. 3 is a diagram of the reproduction spectrum of an information recording medium having the following configuration. FIG.
The figure is an explanatory diagram of the shape and arrangement position of the spatial frequency filter of FIG. 3. 1... Laser light, 2, 3... Total reflection mirror, 4...
・Intermediate lens, 5...Polarizing beam splitter, 6...
・(λ/4) plate, 7...Tracking mirror, 8...
・Voice coil, 9... Reproduction lens, 10... Information recording medium, 11... Beam splitter, 12... Spatial frequency filter agent Yoshihiro Morimoto-13= , Nu Figure 3, 2 Figure playback horse lying down Figure 4 (

Claims (1)

[Claims] 1. A light source for reading an information recording medium having information in an optically detectable form, a reproducing lens that focuses a beam from the light source to near the diffraction limit, and a light source for reading an information recording medium having information in an optically detectable form; an optical means for guiding a beam to the reproduction lens; a focusing means for keeping a constant distance between the reproduction lens and the information recording medium; a tracking means for following an information track on the information recording medium; A photodetector for detecting information from reflected or transmitted light of the light is provided to reproduce the information, and a spatial frequency filter means is provided for spatially blocking a part of the reflected or transmitted light incident on the photodetector. Reproduction optical device. 2. At least 2 times the reflected or transmitted light from the information recording medium
1. The reproducing optical device according to claim 1, wherein the reproducing optical device is configured to divide the beam and guide one of the divided beams to a photodetector. 3. The reproducing optical device according to claim 1, characterized in that the spatially shielded area of the spatial frequency filter means is changed depending on the information reproducing position.