JPH0467255B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an optical disk device for reproducing information on a disc-shaped recording medium (hereinafter referred to as a disk).

Conventional Structure and its Problems In recent years, high density optical disk devices have been most needed.

In order to perform high-density recording, it is necessary to form pits on a track at a high density, that is, to increase the number of pits per unit length.

On the other hand, in general, in an optical disk device, the upper limit of the reproducible pit recording density is determined by the wavelength λ of the light source used in the optical disk device and the objective lens.
It is said to be determined by NA (numerical aperture). this,
This will be explained using FIG.

Figure 1 shows an ideal lens (aberration-free lens).
It shows MTF (Modulation Transfer Function), where the horizontal axis shows spatial frequency and the vertical axis shows modulation degree. ν ₀ in the diagram
is the cutoff frequency, which indicates the spatial frequency at which the modulation degree is zero.

This ν ₀ is given by the formula ν ₀ =2×NA/λ (1).

In general, this spatial frequency ν ₀ is called the resolution limit, and in conventional optical disk devices, it is not considered to reproduce pits formed at a spatial frequency higher than this. A pit with a length corresponding to 1 is the resolution limit, and has therefore been regarded as the limit of high density.

Therefore, conventionally, efforts have been made to increase the density from the viewpoint of improving the cutoff frequency by shortening the wavelength of the light source and increasing the NA of the objective lens.

However, when it comes to shortening the wavelength of the light source, there is a problem that the lifespan of semiconductor lasers decreases as the wavelength becomes shorter, and when using short wavelength gas lasers such as He-Ne lasers, the equipment becomes larger. I had an issue.

Furthermore, increasing the NA of the objective lens has the problem that the amount of aberration due to uneven thickness and tilt of the disk increases as the NA increases.

Purpose of the invention The present invention solves the above-mentioned conventional problems.
It is an object of the present invention to provide an optical disc device that is capable of reproducing a disc having pits shorter than the pit length, which was conventionally considered to be the resolution limit.

The technical means to achieve this objective are as follows.

In a real optical system, the ideal optical system shown in Figure 1 is
Unlike the MTF characteristics, the frequency at which the MTF becomes 0 ν ₀
An MTF value useful for signal reproduction can be obtained even in a frequency band on the higher frequency side.

Conventional optical disk device technology uses only the frequency band from 0 to ν ₀ for recording and reproduction. As mentioned above, short-wavelength light sources and high NA are needed for high-density recording and playback.
The idea was to use a value lens to shift the frequency ν ₀ further to the higher frequency side and expand the frequency band 0 to ν ₀ to be used.

On the other hand, the technical idea of the present invention is that in an actual optical system, even in a frequency band higher than the frequency ν ₀ at which the MTF is 0, it is useful for signal reproduction.
Focusing on the fact that an MTF value can be obtained, reproduction is attempted in a frequency band on the higher frequency side than this frequency ν ₀ .

Structure of the Invention The present invention provides an optical disk including a laser, an objective lens that focuses a light beam from the laser onto a disc-shaped recording medium, and a photodetector that detects reflected light from the disc-shaped recording medium. This is an optical disk device that performs signal reproduction in a frequency band on the higher frequency side than the frequency ν ₀ .

DESCRIPTION OF EMBODIMENTS The configuration of an embodiment of the optical disk device of the present invention will be described below with reference to FIGS. 2 and 3. FIG. 2 is an overall configuration diagram of an optical disk device according to an embodiment of the present invention.

In FIG. 2, 1 is a disk, 2 is a motor that rotates the disk 1, 3 is a semiconductor laser, 4 is a modulation circuit that modulates the light intensity of the semiconductor laser 3 according to the recording signal, and 5 is a signal from the semiconductor laser 3. The emitted light is made into parallel light, and the disk 1
An optical system that separates incident light and reflected light from
Reference numeral 6 denotes an objective lens that focuses a laser beam on the disk 1, and 7 a photodetector that detects a signal from the reflected light from the disk 1.

FIG. 3 shows the MTF characteristics of the optical system of the device of this embodiment. In FIG. 3, the horizontal axis represents the spatial frequency ν of pits formed on the track, and the vertical axis represents the degree of modulation of the reproduced signal, in other words, the resolution.

Furthermore, the spatial frequency ν (1/mm) of the pit row formed on the track of the disk, and the frequency (Hz) in the time domain of the reflected light reflected from this pit row as the disk rotates. What is
They are related by the following equation (2).

=2πrnν...Equation (2) where, r: radial position of the track where playback is being performed n: disk rotation speed (rps) Unlike the MTF of the ideal optical system shown in Fig. 1, this example The MTF characteristics of actual optical systems such as equipment include residual aberrations such as spherical aberration and coma aberration that remain uncorrected during the actual objective lens design, aberrations due to the processing and assembly of the objective lens, blurring due to focus control errors, Since there are various aberrations such as aberrations due to uneven thickness of the disk, inclination, etc., the result is as shown in FIG. 3.

In other words, the optical system of the actual optical disk device
The MTF characteristic monotonically decreases in the band m ₀ (0<ν<ν ₀ ). In an even higher frequency band m ₁ , the phase is reversed, that is, the MTF has a negative value and an extremum value A ₁ . Below, the extreme values A ₂ , A ₃ , ..., A _o-1 ,
Bands m2, m3, ..., m _o-1 , m _o , with A _o , A _o+1
At m _o+1 , the MTF periodically repeats positive and negative values.

The meaning of FIG. 3 is as follows.

Assume now that a pit having a length corresponding to a frequency equal to or higher than the cutoff frequency ν ₀ is formed on the optical disk.

For example, suppose that a pit is formed with a pit length corresponding to the band _m1 in which the MTF first becomes negative, as shown in FIG.

Since the band m ₁ is on the higher frequency side than the cutoff frequency ν ₀ , the minimum beam spot of the optical system of the device of this embodiment is naturally larger than the length of the pit corresponding to the band m ₁ , but FIG. With this optical system,
This shows that it is possible to play back an optical disc on which pits are formed with a length corresponding to band _m1 .

In Figure 3, even after the MTF becomes ₀ , MTFs with _negative values and MTFs with positive values appear alternately. This means that it is possible to reproduce a band signal, that is, it is possible to resolve a shape smaller than the beam diameter of the optical system.

Furthermore, the fact that the MTF is negative means that a phase inversion occurs in the reproduced signal.
That is, for example, when a black and white striped pattern is reproduced, the black and white are reversed from the original image.

Note that the cutoff frequency ν ₀ shown in FIG. 3 is generally smaller than ν ₀ shown in FIG. 1.

In conventional optical disk devices, recording and reproduction are performed in the band m ₀ , but in the present embodiment, reproduction is performed in a frequency band equal to or higher than the frequency ν ₀ at which the MTF becomes 0.

Now, the operation of the optical disk device of this embodiment will be explained below.

In the apparatus of this embodiment, pits having a spatial frequency higher than the MTF cutoff frequency ν ₀ shown in FIG. 3 are recorded on the track of the optical disc.

This record is, for example, a short wavelength laser light source and a high
This can be carried out using a recording device equipped with a lens of NA value.

This frequency in the band m ₁ has a wider band than other higher frequency bands m _o , and its extreme value A ₁ is also higher, and has a relatively higher resolution than other bands higher than the cut-off frequency ν ₀ . However, it is convenient for performing reproduction using reflected light from the pit.

In this embodiment, the recorded signal is reproduced by:
The laser 3 is driven with a constant light intensity to irradiate the disk 1 with light through the optical system 5 and the objective lens 6, and the reflected light from the disk 1 is sent to the objective lens 6 and the optical system 5.
This is done by detecting with the photodetector 7 via the photodetector 7.

As mentioned above, the minimum beam diameter of this example device is
Although it is larger than the pit length corresponding to this band, it is nevertheless possible to reproduce an optical disc formed with a pit length of this band, as described above.

Since the MTF is negative in the band _m1 , so to speak, black and white inversion occurs in the reproduced signal. However, unlike cameras and microscopes, in optical disk devices, even if black and white are reversed, the photodetector 7
Since the output signal of can be easily re-inverted using a known inverter circuit, there is no problem in reality.

In addition, in order to perform higher-density signal reproduction by shifting the frequency ν ₀ in the MTF characteristic in Figure 3 to a higher frequency side, for example, "Optical Information Processing" edited by Junpei Tsujiuchi and Kazumi Murata, Asakura Shoten, 1972 Methods such as the diffraction grating scanning method (described on page 128), the apodization method (described on page 144), and the spatial frequency filtering method (described on page 233), which are described in the publication and are already known in the field of optical microscopy. can be used as appropriate.

Here, we will discuss the fourth apodization method, which is considered to be the most suitable for optical disk devices.
This will be briefly explained using FIGS.

According to this method, the frequency ν ₀ shown in FIG. 3 is shifted to a higher frequency side. Therefore, similarly to the above embodiment, if a frequency band higher than this frequency ν ₀ is used as the band of the reproduced signal, the frequency characteristics can be improved.

In FIG. 4, a shows the pupil function of the objective lens, and a circle 9 corresponds to the aperture of the objective lens.
Figure b shows the point spread intensity distribution. FIG. 5 is an explanatory diagram of an annular aperture filter which is a specific example of the apodization method. The annular aperture filter 10 has a light shielding part 11 in the center.

Generally, convergence of light by an objective lens of an optical disk device can be considered in terms of diffraction optics. Therefore, the contribution of the center and peripheral portions of the objective lens aperture to the point spread intensity distribution can be explained as follows.

The larger the lens aperture, the more the diffracted light gathers in the center, and conversely, the smaller the lens aperture, the more the diffracted light gathers on the outside. Therefore, the center of the lens aperture affects the peripheral intensity of the point spread intensity distribution, and the lens aperture periphery affects the center intensity of the point spread intensity distribution.

On the other hand, the contrast in the low frequency part of the MTF of the objective lens is influenced by the peripheral part of the point spread intensity distribution, and the resolution in the high frequency part is influenced by the central part of the point spread intensity distribution.

As a result, the low frequency part of the MTF of the objective lens is influenced by the peripheral part of the lens aperture, and the high frequency part is influenced by the central part of the lens.

The relationship between the lens aperture and MTF described above can also be explained from another perspective. In other words, since the autocorrelation of the lens aperture is the MTF, the amplitude change at the center of the aperture is calculated by slightly shifting the apertures and overlapping them.
In other words, the low frequency range is greatly affected. Further, the amplitude change in the peripheral area of the aperture has a large effect on the area where the aperture is shifted by a large amount, that is, on the high frequency area.

The apodization method uses the above relationship between the lens aperture and MTF, and is a method to improve the MTF characteristics by giving a distribution to the pupil function, which is the complex amplitude transmission function of the pupil corresponding to the lens aperture. It is. That is, it gives distribution to the amplitude or phase of the center and peripheral parts of the light transmitted through the lens.

Therefore, in order to specifically implement the apodization method in the apparatus of this embodiment, the annular aperture filter shown in FIG. may be placed on the entrance side or the exit side of the objective lens 6.

By using such an annular aperture filter, it is possible to suppress the intensity at the periphery of the point spread intensity distribution and sharpen the central area, thereby improving the characteristics of the high frequency area of the MTF. Furthermore, the characteristics are improved by distributing the thickness of the optical thin film deposited on this annular aperture filter in the radial direction to provide a phase distribution. Of course, it goes without saying that the same effect can be obtained by forming a film directly on the surface of the objective lens 6.

Effects of the Invention The optical disk device of the present invention has a height of the objective lens 6.
Unlike conventional attempts to improve the resolution limit by increasing the NA or shortening the wavelength of the semiconductor laser 3, the length of the signal pit on the disk can be shortened by reproducing a signal with a frequency higher than the cutoff frequency of the MTF. As a result, it is possible to realize a high density optical disk device, which has an extremely large industrial effect.

[Brief explanation of the drawing]

Figure 1 shows the MTF when using an ideal objective lens.
Figure 2 is a configuration diagram of an embodiment of the device of the present invention, Figure 3 is an MTF characteristic diagram of the same embodiment, and Figure 4 is a characteristic diagram.
FIG. 5 is an explanatory diagram of the apodization method. 1...disk, 3...laser, 6...objective lens.

Claims (1)

[Scope of Claims] 1 comprising a laser, an objective lens that focuses a light beam emitted from the laser onto a disc-shaped recording medium, and a photodetector that detects reflected light from the disc-shaped recording medium, An optical disk device characterized in that the reproduced signal uses a signal having a higher frequency than a cutoff frequency of a modulation transfer function determined by the wavelength of the objective lens and the laser. 2. The optical disk device according to claim 1, wherein a signal in a band in which the modulation transfer function first becomes negative is used as the frequency higher than the cutoff frequency used for the reproduction signal. .