JP2580726B2 - Optical head device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head device used for an information input / output device that records and reproduces information using light.

(Prior Art) Currently, in an information input / output device for recording and reproducing information by using light, a concentric or spiral track is provided on a disk-shaped recording medium, and a laser light source is provided on the track. A recording pit is generated by condensing the emitted light as a minute spot, the presence or absence of the pit is recorded as information, and further, a small spot is irradiated on this track, and the presence or absence of a pit on the track is detected from the reflected light, There is a method of reading information.

In recent years, with the demand for increasing the recording capacity, it is necessary to increase the recording density in such an apparatus.
Since the recording capacity depends on the number of the recording pits that can be generated on the recording medium, it is necessary to reduce the recording pits, that is, to reduce the spot of the light irradiated on the medium, in order to increase the recording density. It is essential. The size of the minute spot irradiated on the medium depends on the wavelength λ of the laser and the numerical aperture NA of the condenser lens, and is proportional to λ / NA. Therefore, in order to reduce the size of the minute spot,
It is necessary to decrease λ and increase NA. For this reason, developments are being made in the direction of shortening the oscillation wavelength of a semiconductor laser for an optical disk, and the aperture of the condenser lens is as large as possible. However, the size of the minute spot irradiated on the medium depends on the wavelength of the light source and
It cannot be made smaller than the value determined by the numerical aperture of the condenser lens. Accordingly, there is a disadvantage that the recording density cannot be increased beyond the value determined by this value.

On the other hand, as shown in FIG. 2 (a), the intensity of light near the center is reduced in the cross section of the circular or elliptical outgoing light beam 22 from the light source, and the light intensity as shown in FIG. 2 (b) is reduced. By setting the distribution, the intensity distribution of the beam irradiated on the recording medium by the condenser lens becomes as shown by the solid line 24 in FIG. The size of the beam spot on the medium surface can be reduced as compared with the case where no beam spot is provided. This is conventionally known as super-resolution technology. (See, for example, the articles Osterberg and Wilkins, Journal of Optical Society of America (H. Osterberg and J.
See E. Wilkins, Jr., J. Opt. Soc. Am, 39, 553 (1959). (Prior Art) FIG. 3 shows one of the demonstration examples showing the effect of the super-resolution technique. The measurement conditions are as follows: the wavelength of the light source is about 0.83 μm, the diameter of the collimated beam is about 5 mm, and the numerical aperture of the condenser lens is 0.55. FIG. 3A shows the relationship between the light-shielding band width and the beam spot diameter on the medium surface. It can be seen that in order to reduce the beam spot on the medium surface, it is necessary to increase the width of the light shielding band. On the other hand, the ratio of the total light amount on the medium surface to the light-shielding band width and the laser output light amount shown in FIG.
It can be seen from the relationship that the light utilization rate decreases as the width of the light-shielding band increases.

Further, in the super-resolution beam using the light-shielding band, as shown in FIG. 3C, the side lobe intensity of the condensed spot becomes higher as the light-shielding band width Δw is increased. For example, if the design imposes a constraint that the side lobe height is suppressed to about 1/3 or less of the main lobe height, the main beam diameter d will also be about
It cannot be reduced to 1.05 μm or less.

(Problems to be Solved by the Invention) Generally, it is known that the smaller the condensed spot on the medium surface, the more advantageous in increasing the recording density. Therefore, in the optical head device having the above-described configuration, in order to increase the recording density of information, in order to form a smaller condensed spot, the light-shielding band width of the optical modulator is required as shown in FIG. Should be made wider. However, when the width of the light-shielding band is widened as shown in FIG. 3B, the light utilization rate decreases, and the output intensity required for the light source increases accordingly. At present, there is a limit to the output that can be obtained from a semiconductor laser, and therefore there is a maximum value in the width of the light-shielding band that can be set. That is, the conventional light intensity modulator has a drawback that the super-resolution effect cannot be sufficiently used due to restrictions due to the light utilization factor.

An object of the present invention is to eliminate such a conventional limitation, to obtain a super-resolution effect equivalent to the case where a light-shielding band is installed without installing a light-shielding band, and to reduce the amount of light by the amount that no light-shielding band is installed. An object of the present invention is to provide an optical modulator having a high utilization factor and an optical head using the same.

(Means for Solving the Problems) According to the present invention, light emitted from a light source is condensed as a minute spot on a recording medium surface by a condensing lens, and light reflected from the condensing point is guided to a photodetector. In an optical head device for recording and reproducing information by a system, a light beam from the light source is divided into a plurality of parallel collimated light beams between the light source and the condenser lens, and the plurality of divided light beams are collected by the condenser. It is characterized by having means for entering the lens.

(Operation) In the embodiment of the present invention shown in FIG.
Is made of a material that transmits light. Consider a case where collimated light is incident on the modulator from the direction of arrow A as shown in FIG. If the refractive index of the material constituting the modulator is n ′, the refractive index of air is n, the thickness of the modulator is d, and the moving amount of the optical axis after exiting the modulator is Δ, from the law of refraction, n sin θ = n ′ sin θ ′ (1) Also, from the geometrical consideration in the figure, From (1) and (2) Becomes Here, if the optical modulator is made of glass having a thickness of 5 mm and the inclination angle θ from the direction perpendicular to the traveling direction of the incident light is 25 °, Δ ≒ 0.5 mm, and since the structure is symmetrical, the second It is possible to obtain the same light intensity distribution as when the light shielding band width is 1 mm in the conventional optical modulator shown in the figure. Therefore, in the condensed spot of the light emitted from the modulator by the condensing lens, a super-resolution effect equivalent to the case of using the conventional modulator can be obtained.

If the light modulator is made of glass, the light transmittance can be about 100%, and if it is made of plastic, the light transmittance can be about 95%. Therefore, it is possible to increase the light utilization factor by about 1.5 times as compared with a conventional super-resolution application head using a light-shielding band. Thereby, the output value required for the laser of the light source can also be reduced.

Embodiment Next, an embodiment of the present invention will be described with reference to FIGS.

FIG. 1A is a diagram showing an optical system according to an embodiment. Light emitted from a semiconductor laser 1 as a light source is collimated by a collimating lens 2 and then transmitted through an optical modulator 3 which is a feature of the present invention, so that the light is split into two light beams.
When this light is condensed by the condensing lens 4, a super-resolution effect equivalent to that obtained when an optical modulator using a light-shielding band shown in FIG. 2A is used at a condensed spot on the surface of the recording medium 5 is obtained. be able to. The reflected light from the recording medium 5 is separated by a beam splitter 7 and then guided to a signal detection system 6. FIG. 1B is a diagram showing the operation principle of the optical modulator 3. This modulator is formed of a material such as glass or plastic and has a property of transmitting light. The shape is such that two parallel flat plates inclined at a fixed angle θ from a direction perpendicular to the optical axis are arranged symmetrically with respect to the optical axis. The collimated light incident from the direction A is obliquely incident on the incident surface 8 to the optical modulator symmetrically with respect to the optical axis in the left-right direction. The light is emitted in a direction parallel to the traveling direction. At this time, the moving amount Δ of the outgoing light can be controlled by the thickness t of the modulator, the incident / outgoing surface tilt angle θ, and the refractive index of the modulator material. Typical values are
As described above, when t = 5 mm, θ = 25 °, and n = 1.5, Δ
≒ 5mm.

FIG. 1 (c) shows an embodiment for realizing a modulator which lowers the side lobe intensity of the condensed spot by transmitting light in the central part of the light-shielding zone in FIG. In the plane of incidence of the collimated light, the central part of the light beam is a plane 9 perpendicular to the optical axis, and the exit plane 9 'is a plane parallel to this. The other parts have the same configuration and shape as in FIG. In this case, the light at the central portion of the incident light is perpendicularly incident on the modulator and travels straight without being refracted.
The other part of the light is split into two light beams according to the same principle as that shown in FIG. 2B, and is emitted again in a direction parallel to the optical axis of the incident light. In this way, the outgoing light having the intensity distribution of the light-shielding region 14, the divided parallel light fluxes 11 and 12, and the central transmitted light 13 is obtained, and this light is condensed by the condensing lens to become the solid line in FIG. The resolution effect shown can be obtained. In the figure, a broken line 42 is a side lobe indicated by 25 in FIG. 2 (c) and is provided for comparison. In this embodiment, the primary side lobe 43 can be made sufficiently small. Although the secondary side lobe 44 is larger than the primary side lobe 43, it can be made smaller than the side lobe 42, thereby solving the problem of erroneous pit formation due to the side lobe.

FIG. 5 shows an embodiment in which the optical modulator of the present invention is configured by combining a prism. In FIG. 7A, the collimated light is incident with its optical axis aligned with the axis of symmetry of a prism 51 (hereinafter, referred to as a divided prism), and is split into two light beams upon exit. By transmitting these light beams through a prism 52 (hereinafter, referred to as a direction correction prism), the traveling direction is converted into a direction parallel to the incident light, and the intensity of the emitted light is divided into light beams 53 and 54 and a light shielding area 56. Distribution. By condensing this, a super-resolution effect equivalent to that of the embodiment shown in FIG. 2 can be obtained at the converging spot. FIG. 2B shows a split prism 51 in which the apex of the exit surface in FIG. 2A is cut away, and the central portion of the exit surface is parallel to the incident surface, that is, perpendicular to the optical axis of the incident light. As for the light that has passed through the main splitting prism 51 ', the light at the central portion goes straight, and the other portions are split via the inclined surfaces of the prism. By passing only the latter through the direction correction prism, the light after passing through these systems has the intensity distribution of the light-shielding region 56, the divided parallel light beams 53 and 54, and the centrally transmitted light 55, which are collected. By emitting light, a super-resolution effect equivalent to that of the embodiment shown in FIG. 4 can be obtained in the condensed spot.

FIG. 6 shows an embodiment in which the direction correcting prism is integrated. FIG. 6A shows an embodiment in which the same operation as that of FIG. 5A is performed. The incident collimated light is split into two light beams by a splitting prism 61, and after passing through an integrated direction correcting prism, travels in a direction parallel to the incident light optical axis.
The intensity distribution is composed of 3, 64 and the light shielding area 66. FIG. 6 (b) has a function of cutting off the vertices formed by the inclined surfaces of the splitting prism and the direction correcting prism, so that the central portion of the incident light goes straight. When the collimated light beam passes through the splitting prism 61 ', the light at the central portion goes straight without changing its direction, and the light emitted through the inclined surface changes its direction by refraction and is split into three light beams. When these light beams pass through the direction correcting prism 62 ', they have an intensity distribution composed of a light shielding area 66, a straight light beam 75 at the center, and divided light beams 63 and 64.
Thereby, the same effect as that of FIG. 5B can be obtained.

FIG. 7 shows an embodiment in which a reflecting mirror is used. FIG. 1A shows a means for dividing incident collimated light into two light beams (hereinafter, referred to as a light beam splitting mirror pair) and a means for correcting the interval between these two split light beams (hereinafter, referred to as a correcting mirror pair). 2) is an embodiment in which a pair of reflecting mirrors is used. The incident collimated light is reflected by the reflecting mirrors 73 and 74, which are arranged obliquely with respect to the optical axis and without any interval with the optical axis as a symmetry axis, and are split into two light beams. Output light to mirror 73, 74
Are converted into two light beams 71 'and 72' parallel to the incident direction by mirrors 73 'and 74' forming a pair. In general, when two mirrors are arranged as in this embodiment, the interval between the emitted light beams 71 'and 72' is 1 mm.
Very high precision adjustment is required to keep it to a minimum. If this interval 2Δ is allowed to be widened to some extent in order to avoid this adjustment, it is necessary to bring these luminous fluxes closer together to obtain a desired super-resolution effect. This function is realized by the pair of correction mirrors 75 and 76. As a result, the emitted light can have an intensity distribution composed of a light-shielding region 78 of about 1 mm and two divided light beams 71 and 72. By condensing these light beams, a super-resolution effect similar to that shown in FIG. Of course, if the interval 2Δ of the light flux is sufficiently narrow to obtain the desired super-resolution effect, the light may be directly incident on the condenser lens without using the pair of correcting mirrors. FIG. 7 (c) shows the light beam splitting mirror of FIG. 7 (a) realized by a stepped mirror.

In order to transmit the central part of the incident light in order to obtain the same effect as that shown in FIG.
In addition, the pair of compensating mirrors 75 and 76 may be arranged so as to be spaced apart from each other by a width desired to transmit light. FIG. 7 (b) shows an embodiment in this case. However, a prism having a function equivalent to that of the modulator shown in FIG. 1C is used for correcting the light beam interval instead of the compensating mirror pair. This prism has a symmetrical shape with respect to the optical axis. The input / output surface near the optical axis is a parallel flat plate having a plane perpendicular to the optical axis. The other parts are slightly inclined from the direction perpendicular to the optical axis. It functions as a parallel plate. The central portion of the incident collimated light travels straight as a transmitted light beam 79, and the other portions are split into light beams 71 'and 72' by light beam splitting mirror pairs 73 and 74. When these light beams are transmitted through the prism 77, the emitted light has an intensity distribution including a light-shielding region 78, divided light beams 71 and 72, and a transmitted light beam 79. By condensing these with a condenser lens, a super-resolution effect equivalent to that of FIG. 5B can be obtained.

FIG. 8 shows an embodiment in which a diffraction grating or a hologram is used as means for splitting incident light. (Hereinafter, this means will be referred to as a dividing grating.) In FIG.
It is assumed that the light is emitted as a first-order diffracted light and does not generate a zero-order component. When the collimated light is perpendicularly incident on the main splitting grating, it is emitted as ± 1st-order diffracted light 83 ′, 84 ′, and after passing through the direction correcting prism 82, travels in a direction parallel to the incident direction. Emitted as 84. Therefore, the light intensity distribution has a light-shielding region 86 at the center portion and bright portions on both sides thereof. If this light is condensed by a condensing lens, a super-resolution effect equivalent to that shown in FIG.

FIG. 8 (b) is an embodiment in which the same effect as that of FIG. 5 (b) is obtained. It is assumed that the division grating 81 'emits most of the incident light amount as 0th-order and ± 1st-order diffracted light.
The direction correcting prism 82 'is a prism shown in FIG.
At 82, cut the vertex formed by the slope where the light beam enters,
The central portion is a parallel flat plate having an incident / exit surface perpendicular to the optical axis. When the collimated light is perpendicularly incident on the splitting grating 81 ', it is emitted as ± 1st-order diffracted light 83', 84 'and 0th-order diffracted light 85', and these are directed to the direction correcting prism 82 '.
After passing through, the three parallel beams 83 and 8 travel in a direction parallel to the incident direction.
4. Emitted as 85. Therefore, the transmitted light flux 85
Thus, it is possible to obtain an emitted light pattern having a light intensity distribution having a bright portion, a dark portion 86 on both sides thereof, and a bright portion on both sides thereof. If this light is condensed by a condenser lens, a super-resolution effect equivalent to that shown in FIG.

FIG. 8 (c) shows a diffraction grating or a hologram (hereinafter referred to as a direction correcting grating) which also functions as a direction correcting prism.
This is an embodiment of what is realized in FIG. It is assumed that the division grating 81b and the direction correction grating 81c emit most of the incident light amount as ± 1st-order diffracted light and do not generate a 0th-order component.
When the collimated light is perpendicularly incident on the main splitting grating, it is emitted as ± first-order diffracted lights 83 'and 84', and after passing through the direction correcting grating 83c, travels in a direction parallel to the incident direction. Emitted as 84. Therefore, the light shielding area
86, a light intensity distribution having a bright portion due to the light beams 83 and 84 on both sides thereof. If this is collected by a condenser lens,
A super-resolution effect equivalent to that shown in FIG.

The present inventors have invented an optical head having means (hereinafter referred to as an improved optical system) for removing side lobe components of reflected light from a recording medium into a reproduced signal component in order to improve the reproduced signal characteristics. . FIG. 9 shows an embodiment in which the present invention and this improved optical system are combined. The light emitted from the light source is collimated by a collimating lens 2, passed through an optical modulator 3, split into two light beams, and irradiated on a recording medium 5 by a condenser lens 4. At the time of reproduction, after the reflected light from the recording medium passes through the condenser lens, a part thereof is taken out by the beam splitter 96, and these are condensed by the condenser lens 93 (hereinafter, referred to as a re-condensing lens). At the focal plane, an intensity pattern similar to the condensed spot on the recording medium surface is formed and passed through the slit 94, thereby eliminating the side lobe component shown in FIG. Can be. The reflected light transmitted through the beam splitter 96 is guided to a focus error detection system 91 and a track error detection system 92 via the beam splitters 97 and 98.

(Effects of the Invention) By using the present invention, the conventional restriction is removed, and a super-resolution effect equivalent to the case where the light-shielding band is set without setting the light-shielding band is obtained. It is possible to realize an optical modulator that achieves a higher recording density and has a higher light utilization factor because no light-shielding band is provided, and an optical head device using the same. Thereby, the output required for the laser of the light source can also be reduced.

[Brief description of the drawings]

1 (a) to 1 (c) show an embodiment of the present invention, FIGS. 2 (a) to 2 (c) show the principle of super-resolution generation, and FIG.
4A to 4C are views showing a super-resolution effect in an optical head using a light modulator using a light-shielding band, and FIG. 4 is an embodiment of an optical modulator for transmitting light at the center of the light-shielding band; 5 (a) and 5 (b) show an embodiment in which the optical modulator of the present invention is combined with a prism, and FIGS. 6 (a) and (b) show direction correcting prisms. FIGS. 7 (a) to 7 (c) show an embodiment in which a reflecting mirror is used as an optical modulator used in the present invention, and FIGS.
FIGS. 9A to 9C show an embodiment in which a diffraction grating or a hologram is used as means for splitting incident light, and FIG. 9 shows an embodiment in which the present invention is combined with an improved optical system. FIG. In the figure, 1 ... semiconductor laser, 2 ... collimating lens, 3 ...
Optical modulator, 4 ... Condenser lens, 5 ... Recording medium, 6 ...
Signal detection system, 7 ... beam splitter, 8 ... incident surface,
9: straight transmitted light incident surface, 9 ': straight transmitted light emission surface,
11, 12 ... split outgoing parallel light flux, 13 ... central transmitted light flux, 14 ... dark area equivalent to light-shielding area, 21 ... light-shielding band, 22
…… Light beam, 23 …… Condensed beam intensity distribution curve without using super-resolution, 24 …… Condensed beam intensity distribution curve using super-resolution, 25 …… Side lobe, 41 …… Main lobe, 42
…… Primary side lobes due to simple shading, 43 …… Primary side lobes due to transmission through the center, 44 ……
Secondary side lobes through the center, 5
1, 51 ': light beam splitting prism, 52: direction correcting prism, 53, 54: split outgoing parallel light beam, 55: centrally transmitted light beam, 56: dark portion equivalent to light-shielding region, 61, 61 ´ ……
Light beam splitting prism, 62, 62 ': Integrated direction correcting prism, 63, 64: Split outgoing parallel light beam, 65: Central transmitted light beam, 66: Dark portion equivalent to light-shielding region, 71, 71' , 72,7
2 ': Split outgoing parallel beam, 73, 73', 74, 74 '...
Light splitting mirror pair, 73 ", 74" ... Light splitting mirror, 75, 76 ...
… Correction mirror pair, 77… Flux interval correction prism, 78… Dark part equivalent to the light-shielding area, 79 …… Central transmitted straight light, 81,8
1 '... Diffraction grating or hologram for splitting light beam, 82,82'
... Direction correcting prism, 83, 84... Split outgoing parallel light beam, 83 ′, 84 ′. Dark area equivalent to the area, 91: focus error detection system, 92: track error detection system, 93: refocusing lens, 94: slit,
95 …… Photodetector, 96,97,98 …… Beam splitter

Claims (1)

(57) [Claims] 1. A light beam emitted from a light source is condensed as a minute spot on a recording medium surface by a condenser lens, and information is recorded / reproduced by an optical system for guiding reflected light from the focal point to a photodetector. In the optical head device, between the light source and the condenser lens, there may be provided a unit that divides a light beam from the light source into a plurality of parallel light beams and makes the plurality of divided light beams incident on the condenser lens. Characteristic optical head device.