JPH03212829A - Optical head of rewriting type optical recorder - Google Patents

Abstract

PURPOSE:To allow the erasing of the recording spots remaining on both sides of information tracks by providing the diffraction grating of an optoacoustic element on the front stage of a beam forming prism and deforming an overwriting erasing beam to an elliptical beam, the major axis of which is in the transverse direction of tracks. CONSTITUTION:The diffraction grating 21 consisting of the optoacoustic element for controlling the incident light of the incident collimated beams of light to the beam forming prism 4 is provided and the beam is projected by deforming the same to the elliptical beam having the major axis in the direction orthogonal with the information tracks at the time of erasing recorded information. Namely, the pulse width and period of the voltage pulses to be impressed to the diffraction grating 12 are corresponded to the timing for switching the recording and erasing in the overwriting, by which the recording beam to the optical information tracks is deformed to the circular beam and the erasing beam to the elliptical beam having the major axis in the direction orthogonal with the tracks. The function to eras the old information spots remaining on both sides of the information tracks 40 without being erased is obtd. in this way.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention relates to an optical head of an optical recording device using a rewritable phase-change optical disk, and in particular to an optical head for recording and reproducing information. This relates to an optical head that uses a beam.

[Conventional technology]

In recent years, there has been a growing demand for higher density and larger capacity information storage, and research and development has been actively carried out both domestically and internationally.In particular, optical disks that use lasers as light sources are about 100% faster than conventional magnetic recording media. It has a recording density 10 to 100 times higher, and also has features such as being able to record and reproduce information without contact between the recording/reproducing head and the recording medium, causing less damage to the recording medium, and having a long life. Therefore, it is promising as a high-density, large-capacity recording method for recording and reproducing huge amounts of information. Depending on the purpose, this optical disc may be a playback-only type or
It can be roughly divided into six types: write-once type and rewrite type. The read-only type is a read-only disk from which information can only be read, and the write-once type allows information to be recorded and reproduced as needed, but the recorded information cannot be erased. On the other hand, rewritable files are capable of recording and reproducing information, as well as erasing and rewriting recorded information, and are desired to be used as data files for computers, making them the most promising option. .

Regarding rewritable disks, two recording methods, a magneto-optical method and a phase change method, are being developed, but both methods still leave room for improvement in terms of recording materials, writing mechanisms, etc. Among these methods, the phase change method generally focuses a laser beam on the recording surface of the disk, heats it, and controls the pulse output and pulse width of the laser beam, thereby causing a phase change in the recording material, that is, from a crystalline state to a non-crystalline state. It causes a transition to a crystalline state or a phase transition, and records and erases information by combining the reflectance in each state.

Although the structure of this phase change type optical disk is omitted from illustration, 9 it usually has a large amount of tracking l [- is provided with zinc sulfide (ZnS) on the surface of a substrate such as polycarbonate.
The structure is such that a protective film made of ceramic, etc. is formed, a recording material, that is, a medium film is provided on the protective film, and a protective film made of ceramic, etc., and a surface protective film of an organic material are sequentially deposited thereon. Furthermore, a cooling film made of aluminum or the like which also serves as a reflective layer is provided between the protective film and the organic surface protective film. The cooling film increases the cooling rate from the molten state when changing from the crystalline state to the amorphous state, and also increases the absorption efficiency of the medium film by reflecting the light that has passed through the medium film. It also works to increase In a typical optical disk, the optical recording material is initially in a crystalline state, and when recording information, a laser is irradiated onto it to melt the irradiated area and then rapidly cooled to form an amorphous spot.

During erasing, this non-crystalline state is cyannealed using a sporatra laser to return it to a crystalline state.

There are two main types of recording methods for phase change optical recording media. When using a recording medium that requires a longer laser beam irradiation time for erasing than the laser beam irradiation time required for recording, a laser beam dedicated for recording and a trap elongated along the track of the recording medium so that the irradiation time is longer are used. Recording and erasing are performed using a total of two modified laser beams dedicated to erasing. This method requires two healds, which complicates the structure, and also causes the head weight to be about twice as much as in the case of one head, which poses a major problem in shortening the head access time.

On the other hand, if the laser irradiation time required to crystallize an amorphous recording spot is the same as or shorter than the laser beam irradiation time required for recording, recording and erasing can be done with just one laser beam. can be done. When overwriting old records using this method, recording and erasing operations can be performed at the same time. In other words, when a new recording spot is not formed, the laser oscillates with the power required for erasing, erasing the old recording spot.

When forming a recording spot, the shi-boo light is oscillated with a power higher than the erasing power and necessary to melt the recording medium and make it amorphous. In this way, the one-beam method in which writing and erasing is performed using one laser beam requires only one optical head, which has the advantage that the optical head can be made lighter than the two-beam method.

[Problem to be solved by the invention]

In the conventional overwriting method using the one-beam method, a circular beam is used for both recording and reproduction, the recording spot is formed by increasing the power of the circular beam, and the recording spot is erased by controlling the power of the circular beam weakly. .

Figure 6 is an overwriting characteristic diagram obtained using a conventional optical head that always emits a circular beam, and the original disk was made of zinc sulfide (ZnS) with a thickness of 1100 nm on a polycarbonate substrate with a diameter of 5.25 inches. a recording layer made of nokermanium, antimony, tellurium alloy (GetSb2Te5) with a thickness of 30 nm, an underlying dielectric layer made of ZnS with a thickness of 170 nm, and a dielectric layer with a thickness of 1
A 100n aluminum reflective layer was sequentially formed using an RP magneto/sputter device, and then overwritten using a protective UV resin coated using a spin coater. In addition, the linear velocity of the track on the original disk was kept at 15 m/s, and the recording power of the circular beam was 15 m/s.
A signal of 4.94 MH2 was recorded at a constant W, and overwriting was performed by changing the erase power of the circular beam in the range of 2 mW to 3 mW. The erasure ratio was determined from the difference in the size of the old signal remaining after writing.

In the figure, both the CNR and the erasure ratio show a chevron characteristic with a maximum value near the erasure power of 6 mW, and the CNR is approximately 50 dB.
K, the erasure ratio remains at about 30 dB K, indicating that there is a problem that old records cannot be erased sufficiently.

FIGS. 4, 5, and 6 are schematic diagrams showing the phase change of the recording layer that occurs on the track of an optical disk due to the conventional overwriting characteristics. This is a schematic representation of the situation as seen. In the figure, in the low erasing power region of 4 mW or less, as shown in Fig. 4, the amorphous old recording spot 4
Region 4 that can be recrystallized compared to the size of 2
3 is narrow and the erasure ratio decreases. On the other hand, when the erasing power is 5 to 6 mW, as shown in FIG.
Although there is deformation in , the width 43 that can be recrystallized is expanded,
Since the area of the old recording spots 42 that remain erased becomes smaller, the erasure ratio is improved.

As the erasing power is increased to 7 mW or more, as shown in FIG. 6, the recording spot 41 becomes larger due to thermal diffusion pressure, and the width of the remaining recording spot 42 decreases, but the laser beam remains irradiated. Since the central portion exceeds the melting point, it becomes amorphous, and a connecting portion 41B is formed between the amorphous portions of the recording spots, and the region 43 that can be recrystallized becomes increasingly narrow. In this way, in the conventional method, even when the erasing power that maximizes the CNR and erasing ratio is selected, the information track 40
It can be seen that the erased recording spots 42 remain in large numbers on both sides of the disk, and as a result, the erasure ratio in the overwriting characteristic remains at about 3 [1 dB].

In view of the above knowledge, an object of the present invention is to improve the erasing ratio in overwriting by erasing the unerased recording spots remaining on both sides of the information track.

[Means to solve the problem]

In order to solve the above problem, this button is released! According to AK, the laser beam is emitted from a semiconductor laser and converted into a circular parallel beam by a laser beam collimator lens and a beam shaping prism, and the circular parallel beam transmitted through a polarizing beam splitter λ/4 plate is focused by an objective lens. In an apparatus that records and reproduces information by projecting it as a circular beam onto an information track of a phase change type rewritable optical disk, a diffraction device comprising a photoacoustic element that controls the incident angle of a parallel beam incident on the beam shaping prism. A grating is provided, and when erasing recorded information, the beam is transformed into an elliptical beam having a major axis in a direction perpendicular to the information track and projected.

[Effect]

According to the configuration of the present invention, a diffraction grating made of a photoacoustic element utilizing an inverse piezoelectric effect is provided between a collimator lens and a beam shaping prism that are conventionally arranged on an output optical path of a semiconductor laser. By making the pulse width and period of the voltage pulse applied to the diffraction grating correspond to the switching timing between recording and erasing in overwriting K, the recording beam to the original information track is a circular beam, and the erasing beam is perpendicular to the track. Since it is possible to create an elliptical beam with a long axis of the direction button, the erasing power on both sides of the information track can be increased. Therefore, it is possible to perform overwriting with a high erasure ratio.

〔Example〕

The present invention will be explained below based on examples.

FIG. 1 is a schematic diagram of an optical system showing an optical head of a rewritable optical recording device according to an embodiment of the present invention. The width of the optical path shown in the cross-sectional view and indicated by a pair of solid lines and a pair of dashed lines in the optical system indicates the beam diameter in the direction perpendicular to the information track 4CNC, in other words, the beam diameter in the track width W direction. In the figure,
The optical head 1 is composed of a projection optical system consisting of reference numerals 2 to 7 and 21, and a detection optical system consisting of reference numerals 11 to 17, and includes two diffraction gratings 21A and 21E.
Etc. 21 has a new configuration 1j in this invention! Aya bare,
The other components are the same as those of conventionally commonly used optical systems.

In addition, the diffraction gratings 21A and 21B made of photoacoustic elements are, for example, photoacoustic # elements using an inverse piezoelectric effect.
When recording information, by reducing the voltage applied to the diffraction grating to zero, the laser A < beam conductor is passed, and therefore by applying a beam shaping beam, the refractive index of the light changes due to the inverse piezoelectric effect, and the track 1) It produces a function of widening the major axis of the laser beam in the W direction, and therefore controls switching of the laser beam 20 into a circular laser beam and an elliptical laser beam depending on the width and period of the voltage pulse.
A ability can be obtained.

In the optical head 1 configured as described above, the laser beam 20 emitted from the semiconductor laser 2 is turned into a parallel beam by the collimator lens 2, and the diffraction grating 6 and the diffraction grating 21, which are made up of nine elements and all # elements, remain unchanged during recording pulse irradiation. The beam passes through and enters the beam shaping prism 4, where it becomes a circular beam with a diameter a. - When the erasing pulse is irradiated, the diffraction grating 21A and the diffraction grating 21B cause the beam to have a shallower angle of incidence on the beam shaping prism 4 than when the recording pulse is irradiated. The laser beam that has passed through these beam shaping prisms 4 passes through a deflection beam sinter 5'fc,
It passes through the λ/4 plate 6, is focused into a minute spot by the objective lens 8, and is projected onto the information track 40 of the original disk 10, but at the time of recording, it becomes a circular beam, and at the time of erasing, it becomes a circular beam, and at the time of erasing, it is in the track 1 direction. Long diameter? Controlled by an elliptical beam, the recording and erasing of information is repeated at full speed and alternately.
An electric oath will be taken using the beam method.

10,000, the laser beam reflected from the recording spot of the information track 40 passes through the objective lens 7 and the λ/4 plate 6, is reflected by the polarizing beam smitter 5 toward the detection optical system, and is further focused by the semi-transparent prism 11. The system is separated into an error signal detection system 14 consisting of a lens 12 and a photodiode 15, and a recording signal detection system 17 consisting of a critical angle prism 15 and a photodiode 16, each of which undergoes signal processing by a control circuit (not shown) of the optical head 1.

FIG. 2 is a nine-overwrite characteristic diagram obtained using the optical head according to the example, in which the vertical axis of the diagram shows the recording spot size CNR and the erasure ratio, and the horizontal axis of the diagram shows the elliptical spot. The axial ratio = major axis / minor axis.

The optical disk used was the same as that already explained in connection with the prior art, the linear velocity of the track was maintained at t-15 m/s, and the recording power was 4.9 mW as a constant circular beam.
A 4 MHz signal is recorded, and the erasing power is controlled so that the short axis of the erase beam matches the traveling direction of the track and the short axis matches the diameter of the recording circular beam. Overwriting was performed by changing the major axis of the beam within a range of 1 to 2 times the minor axis. In the figure, when the CNRd axial ratio is 1.6, it shows a maximum value of about 55 dBi, and shows a tendency to decrease when the axial ratio is 1.6 or more. On the other hand, the cancellation ratio showed a chevron characteristic with a maximum value at an axial ratio of 1.6, and the maximum value was approximately 50 dB.We were able to demonstrate that the cancellation ratio could be significantly improved compared to the conventional method explained in Figure 3. . In addition, the value obtained by dividing the erasing power by the axial ratio is approximately 6 mW at any axial ratio, which is consistent with the erasing power of 6 mW at which the erasing ratio is the maximum value in the conventional technology, and the value is equal to 6 mW as the axial ratio increases. In addition to increasing the number of players on both sides of the track 40, the recrystallizable area 4'5 also expands somewhat, and the CN
It is estimated that the R value was also slightly improved. Note that the reason why the CNR value decreases when the axial ratio is 1.6 or more is considered to be because the erasing power spreads to adjacent tracks.

〔Effect of the invention〕

As described above, the present invention is configured such that a diffraction grating made of an optical acoustic element is provided at the front stage of a beam shaping prism to transform an erasing beam in overwriting into an elliptical beam having a major axis in the width direction of the track.

As a result, it is now possible to erase the old recording spots that remain on both sides of the track width with the erasing power of the elliptical beam that spreads in the track width direction, and the erasure ratio that was previously only 30 dBL can be erased by 5 Q dB. Therefore, it is possible to provide an entire replaceable optical recording device using the one-beam method, which is equipped with an optical head that can improve the overwriting characteristics using the same optical disk.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of an optical system showing an optical head of a rewritable optical recording device according to an embodiment of the present invention, and FIG. 2 is an overlapping characteristics diagram obtained using an optical head according to an embodiment. Figure 3 is an overwriting characteristic diagram obtained using a conventional optical head that emits a circular beam, Figures 4, 5, and 6.
The figure is a schematic diagram illustrating a phase change occurring on a track of an optical disk by a conventional overlapping method for different erasing powers. 1... Optical head, 2... Semiconductor laser 3...
・Collimator lens, Yotsukawa beam shaping prism, 5・
...Polarizing beam splitter, 6...λ/4 plate, 7...
- Objective lens, 10... Optical disk, 11... Semi-transparent mirror 14... Error signal detection system, 17... Recording signal detection system, 21, 21A, 21B... Diffraction grating,
20...Laser beam, 40°...Information track, 41...New recording spot, 42...Remaining records Subokko 1 Figure Z '84th Figure S

Claims (1)

[Claims] 1) The laser beam emitted from the semiconductor laser is converted into a circular parallel beam using a collimator lens and a beam shaping prism, and the circular parallel beam transmitted through a polarizing beam splitter and a λ/4 plate is focused using an objective lens, making it possible to rewrite the phase change shape. An apparatus for recording and reproducing information by projecting it as a circular beam onto an information track of an optical disk, which is equipped with a diffraction grating made of a photoacoustic element that controls the angle of incidence of a parallel beam incident on the beam shaping prism, An optical head for a rewritable optical recording device, characterized in that when erasing information, the optical head transforms into an elliptical beam having a major axis in a direction perpendicular to the information track and projects the beam.