JPH02137132A - Reproducing device for optical recording medium - Google Patents

Abstract

PURPOSE:To improve the degree of modulation by providing a beam splitter between an optical recording medium and a laser diode and detecting reflected light obtained through the beam splitter. CONSTITUTION:A laser light beam from the end face 11a of a laser diode 11 is reflected by the optical recording medium 20 through a lens 12, the beam splitter 13 and a lens 14. A part of the reflected light is separated by the beam splitter 13 through the lens 14 and enters a photodetector 16 through a lens 15. By detecting the change of the intensity of the separated light, the presence or absence of the pit of the optical recording medium 20 is read. Meanwhile, the rest of the light which passes through the beam splitter 13 is fed back to the end face 11a of the laser diode 11 through the lens 12. Self combining effect is generated in the laser diode 11 with the light which is fed back and the light output of the laser diode is increased. Thus, the degree of modulation is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical recording medium reproducing apparatus using a so-called self-coupling optical pickup.

[Summary of the invention]

[Prior Art] As shown in FIG. 3, an optical recording medium (disc) 20 such as a compact disc or an optical video disc has so-called pits 2 on the surface of a substrate 21 made of, for example, polycarbonate.
A series of recesses called 2 are provided in a concentric or spiral track, and information such as music is recorded here as digital data. The optical recording medium 2
0 is a reflective film 2 with aluminum vapor-deposited over the entire surface.
3 is formed. The depth of the pit 22 is set to 1/4 of the wavelength of the laser beam, and the laser beam is irradiated onto the optical recording medium, reflected by the reflective film 23, and the diffraction phenomenon occurring in the pit 22 is utilized. The digital data is read and reproduced by detecting changes in the intensity of the reflected light with a photodetector.

As shown in FIG. 4, the optical pickup for reading the optical recording medium reproducing apparatus described above includes a laser diode 31 as a light source and a quarter-wave plate for converting the linearly polarized light of the laser beam into circularly polarized light. 32, a beam splitter 33 for separating the reflected light from the optical recording medium, a photodetector 34 such as a PIN photodiode for detecting the reflected optical recording medium separated by the beam subrick 33, and various types such as a cylindrical lens. It is composed of lenses 35, 36, and 37. The photodetector 34 detects changes in the intensity of reflected light caused by the presence or absence of pits on the optical recording medium, and reads the data recorded on the optical recording medium. It also has a so-called focusing and tracking signal detection function for controlling the image formation on the track.

[Problem to be solved by the invention]

By the way, when reproducing the focus formed as described above, the response function of the optical system (OTF: 0ptical
Absolute value (M
TF: Modulation transfer
As shown in FIG. 5, the function) decreases as the recording density of the pits increases, and finally reaches zero. This happens because when the pit becomes smaller, the diffraction at the pit increases and the reflected light to the photodetector decreases, and eventually the reflected light deviates from the path of the optical system, making it impossible to read the pit. It is something. That is, the MTF is one measure representing the limit of the recording density of the optical recording medium. For example, the recording density of general optical recording media is such that the MTF value is, for example, about 0.5 or more.

By the way, the above-mentioned optical recording medium is rotated at a predetermined angular velocity. For this reason, the pits of the disk are formed to be longer toward the outer circumference in the circumferential direction, and conversely to become shorter toward the inner circumference. In other words, as shown in Figure 5, the pits are formed at a higher density toward the inner periphery, and the recording density of an optical recording medium that is rotated at a constant angular velocity is determined by the size of the pits on the inner periphery. .

In the above technology, since the output of the laser diode during reproduction is constant, when the length of the pit becomes shorter as described above, the MTF decreases and the reproduction characteristics, for example, C/N (Car
ier to No. 1se Ratio) decreases rapidly.

Note that as a method for improving the MTF, the numerical aperture (NA: Numerical Aper.
There is a method of making the wavelength of the laser beam larger or a method of making the wavelength of the laser beam shorter. However, increasing the NA causes disadvantages such as the depth of focus becomes shallow, the amount of aberration due to the inclination of the optical axis increases, and the price increases. Furthermore, there is a technical limit to shortening the wavelength of a laser diode. Therefore, it is necessary to improve the modulation depth using another method.

By the way, there is a strong demand for smaller and lighter optical pickups, and in response to this demand, so-called self-coupled optical pickups (S
COOP: 5elf Coupled 0pti
cal Pickup) is known. The 5 COOP
The configuration is shown in FIG. The 5coop converts the laser beam reflected by the optical recording medium 20 into a laser diode 41.
The external resonator formed by the optical recording medium 20 and the laser diode 41 generates an external resonance state, and the photodetector 4 provided behind the laser diode 41 generates an external resonance state.
2, this change in resonance state, that is, the laser diode 4
By detecting the change in the rear light output intensity of No. 1, the data recorded on the optical recording medium is read in the same manner as the above-mentioned optical pickup.

According to this technology, since the rear output of the laser diode is detected, the above-mentioned servo control signal detection cannot be performed.
It is necessary to provide another detection means.

SUMMARY OF THE INVENTION In view of the above-mentioned conventional problems, it is an object of the present invention to provide an optical recording medium reproducing apparatus that can improve the quality of playback.

[Means to solve the problem]

The present invention is based on the above! In order to solve IB, an optical system is provided that irradiates light from a laser diode onto an optical recording medium and returns the reflected light of the irradiated light to the laser diode, and a beam splitter that takes out a part of the reflected light is connected to the optical system. It is characterized by comprising a photodetector that is provided between the recording medium and the laser diode and detects reflected light obtained through the beam splitter.

[For production]

According to this configuration, changes in the intensity of reflected light from the optical recording medium due to the presence or absence of pits on the optical recording medium are emphasized by the self-coupling effect of the laser diode, and the degree of modulation is improved.

〔Example〕

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an optical recording medium reproducing apparatus according to the present invention will be described below with reference to the drawings.

FIG. 1 shows the configuration of an optical pickup according to an embodiment of the present invention. In FIG. 1, laser light from an end face 11a of a laser diode 11 is reflected by an optical recording medium 20 via a lens 12, a beam splitter 13, and a lens 14. This reflected light passes through the lens 14, and a part of the light is separated by the beam splitter 13, and the lens 1
The separated light enters the photodetector 16 via the light beam 5, and detects the change in intensity of the separated light to read the presence or absence of pits in the optical recording medium. On the other hand, the remaining light that has passed through the beam sprick 13 is returned to the end surface 11a of the laser diode 11 via the lens 12.

This feedback light causes a self-coupling effect in the laser diode 11, and even with the same drive current, the optical output of the laser diode increases compared to a case where there is no self-coupling effect.

FIG. 2 shows the light output characteristics of the laser diode drive 11 when the return light to the laser diode is used as a parameter. As mentioned above, the reflectance of an optical recording medium has two values, high (rl) and low (r2), depending on the presence or absence of pits, so the drive current-output characteristics of this example are as shown in Figure 2. , curve A when the returned light is large (no pits, high reflectance of the optical recording medium), and curve A when the returned light is small (with pits,
It is represented by two curves, curve A2 in the case where the reflectance of the optical recording medium is low). Note that the curve A0 shows the drive current-output characteristic when there is no light returning to the laser diode, that is, the larger the light returning to the laser diode, the larger the output of the laser diode.

For example, for a constant drive current (2), if there are no pits, the optical output of the laser diode will be L2, and if there are pits, the optical output of the laser diode will be L2. This change in laser diode output is reflected by the optical recording medium and detected by the photodetector via the beam splitter. As a result, the width of the intensity of light detected when there is a self-coupling effect as described above becomes larger than the width of the intensity of light detected when there is no self-coupling effect. The ratio of the intensity of light caused by the presence or absence of pits becomes large, and the presence or absence of pits can be determined with ample margin. In other words, the reflected light on the optical recording medium without pits is stronger due to the self-coupling effect than the reflected light in the presence of pits, and the above-mentioned degree of modulation is improved.

Further, the above-mentioned C/N is good because the noise component is constant and only the signal component increases due to the self-coupling effect of the laser diode.

Note that by providing a signal detection function for tracking such as the so-called three-beam method and a signal detection function for focusing using astigmatism using a cylindrical lens inside the photodetector, tracking servo control and Focusing servo control is possible with a single optical system.

In addition, the patent application No. 63-5597 already proposed by the inventor of the present invention
By applying the present invention to the optical recording medium reproducing apparatus disclosed in the specification and drawings of No. 6. This makes it possible to read data while suppressing laser diode noise.

That is, a pulse generator is provided in the drive circuit of the laser diode 11 shown in FIG. By interrupting the laser beam at a predetermined period, it becomes possible to read data while suppressing the noise of the laser diode.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram showing the configuration of an embodiment of a reproducing device according to the present invention, Fig. 2 is a drive current-light output characteristic diagram of a laser diode, and Fig. 3 is a diagram showing the configuration of the main parts of an optical recording medium. 4 is a schematic diagram of an optical pickup of a conventional reproducing apparatus, FIG. 5 is a diagram of modulation degree characteristics of an optical recording medium, and FIG. 6 is a schematic diagram of an optical pickup of another conventional reproducing apparatus. 11...Laser diode 13...Beam splitter 16...Photodetector 20...Optical recording medium (Effects of the invention) As is clear from the description of the above embodiments, according to the present invention, By effectively utilizing the self-coupling effect of a laser diode, it is possible to provide a compact and lightweight optical pickup with good reproduction performance.

Claims (1)

[Claims] It has an optical system that irradiates light from a laser diode onto an optical recording medium and returns reflected light of the irradiated light to the laser diode, and a beam splitter that takes out a part of the reflected light is provided between the optical recording medium and the laser diode. 1. A reproducing apparatus for an optical recording medium, comprising a photodetector provided in the beam splitter for detecting reflected light obtained through the beam splitter.