JPH0212622A - Optical head device - Google Patents

Abstract

PURPOSE:To obtain the same super-resolution effect as that obtained when a light shielding band is set, and to make the title device at a high light using rate by dividing optical beams from a light source into plural parallel collimated light fluxes between the light source and the condenser lens, and making the light fluxes incident on the condenser lens. CONSTITUTION:The light emitted from a semiconductor laser 1 to be the light source is collimated by a collimator lens 2, and divided into the two light fluxes by transmitting an optical modulator 3. When they are condensed by the condenser lens 4, the same super-resolution effect as that obtained when the optical modulator by means of the light shielding band shown in a figure (a) can be obtained at a condenser spot on the surface of a recording medium 5. Consequently, the higher recording density than that in the normal case can be realized by the super-resolution effect, and the optical modulator and its application at the higher light using rate for the portion without the shielding band can be realized.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical head device used in an information input/output device that records and reproduces information using light.

(Prior Art) Currently, in information input/output devices that record and reproduce information using light, concentric or spiral tracks are provided on a disk-shaped recording medium, and a laser light source is placed on this track. A recording bit is generated by condensing the emitted light into a minute spot, and the presence or absence of the bit is recorded as information6.The minute spot is then irradiated onto this track, and the presence or absence of the bit on the track is detected from the reflected light. , to read information.

In recent years, with the demand for increased recording capacity, it has become necessary to increase the recording density in such devices. Since the recording capacity depends on the number of recording bits that can be generated on the recording medium, making the recording bits smaller, that is, reducing the spot of light irradiated onto the medium, is the key to higher density. It is essential. The size of the minute spot irradiated onto the medium depends on the wavelength λ of the laser and the numerical aperture NA of the condensing lens.1. Proportional to VNA. Therefore, to reduce the size of the minute spot,
It is necessary to make λ small and NA7il-large. For this reason, development is progressing in the direction of shortening the oscillation wavelength of semiconductor lasers for optical disks, and the aperture of the condenser lens is being used as large as possible. but,
The size of the minute spot irradiated onto the medium cannot be made smaller than a value determined by the wavelength of the light source and the numerical aperture of the condensing lens. Therefore, there was a drawback that the recording density could not 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 of the circular or elliptical cross-section of the light beam 22 emitted from the light source is reduced, and the light intensity is reduced as shown in FIG. 2(b). By setting the distribution, the intensity distribution on the medium surface of the beam irradiated onto the recording medium by the condensing lens becomes as shown by the solid line 24 in FIG.
The size of the beam spot on the medium surface can be made smaller than that in the case where 1 is not used. This is conventionally known as super-resolution technology. (For example, see the literature Osterber, Wilkins, Journal of Optical Society of America (H. Osterber)
g and J, E.

Wilkins, Jr., J., Opt., Soc.
, Am, 39.553 (1959)) (Prior Art) FIG. 3 shows one of the demonstration examples showing the effects of super-resolution technology.

The measurement conditions are that the wavelength of the light source is approximately 0.83 pm, the diameter of the collimated beam is approximately 5 mm, and the numerical aperture of the condensing lens is 0.55. FIG. 5A shows the relationship between the width of the shielding zone 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 widen the shading band width. On the other hand, from the relationship between the light-shielding band width and the ratio of the total amount of light on the medium surface to the amount of laser emitted light (hereinafter referred to as the light utilization rate) shown in FIG. It can be seen that this decreases.

In addition, the super-resolution beam using the shading zone is shown in Figure 3(c).
As shown in FIG. 2, the wider the shading band width ΔW is, the higher the sidelobe intensity of the focused spot becomes. For example, if a design constraint is imposed to keep the side lobe height to about 1/3 or less of the main lobe height, the main beam diameter d will also be about 1.
This means that it cannot be made thinner than 0511 m.

(Problems to be Solved by the Invention) It is generally known that the smaller the focused spot on the medium surface, the more advantageous it is in increasing the recording density. Therefore, in the optical head device having the configuration described above, in order to form a smaller focused spot for the purpose of increasing the information recording density, the light-shielding band width of the optical modulator is It would be better to make it wider. However, in the same figure (b)
When the width of the shading zone is widened as shown in , the light utilization efficiency decreases, and the output intensity required for the light source increases accordingly.

Since there is currently a limit to the output that can be obtained from semiconductor lasers, there is also a maximum value for the width of the light-shielding band that can be set. In other words, the conventional light intensity modulator has the disadvantage that the super-resolution effect cannot be fully utilized due to constraints imposed by the light utilization rate.

The purpose of the present invention is to remove such conventional limitations, obtain a super-resolution effect equivalent to that obtained by installing a shading zone without installing a shading zone, and reduce the amount of light by the amount of light without installing a shading zone. An object of the present invention is to provide an optical modulator with a high utilization rate and an optical head using the same.

(Means for Solving the Problems) The present invention is an optical system that focuses emitted light from a light source on a recording medium surface as a minute spot using a condensing lens, and guides reflected light from this condensed point to a photodetector. In an optical head device that records and reproduces information using a system, a light beam from the light source is divided into a plurality of parallel collimated beams between the light source and the condensing lens, and the divided beams are divided into a plurality of parallel collimated beams. It is characterized by having means for inputting the light into the lens.

(Function) In the embodiment of the present invention shown in FIG. 1, it is assumed that the optical modulator 3 is made of a material that transmits light.

Consider the case where collimated light is incident on this modulator from the direction of arrow A, as shown in FIG. 2(b). 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 amount of movement of the optical axis after exiting the modulator is Δ, then
From the law of refraction, and from the geometric considerations in the diagram n5inθ=n'sinθゝ, we have (1) and (2). Here, if the optical modulator is made of glass with a thickness of 5 mm and the tilt angle θ from the direction perpendicular to the direction of propagation of the incident light is 25 degrees, then it becomes Δ-0.5 mm, and since it has a symmetrical structure, In the conventional optical modulator shown in FIG. 2, it is possible to obtain a light intensity distribution equivalent to that when the light-shielding band width is 1 mm. Therefore, at the condensing spot of the light emitted from this modulator by the condensing lens, it is possible to obtain the same super-resolution effect as when using a conventional modulator.

Furthermore, if the material of the optical modulator is glass, the light transmittance can be about 100%, and if it is made of plastic, it can be about 95%. Therefore, it is possible to increase the light utilization efficiency by about 1.5 times compared to a conventional super-resolution head using a light-shielding band. This also makes it possible to reduce the output value required of the laser of the light source.

(Example) Next, an example of the present invention will be described with reference to FIGS. 1 to 9.

FIG. 1(a) is a diagram showing an optical system of one embodiment. Light emitted from a semiconductor laser 1, which is 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, and is divided into three beams. When this light is focused by the condenser lens 4, a super-resolution effect equivalent to that obtained when using an optical modulator with a light-shielding band shown in FIG. 2(a) can be obtained at the focused spot on the surface of the recording medium 5. 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. 1(b) is a diagram showing the operating principle of the optical modulator 3. This modulator is made of materials such as glass and plastic, and has the property of transmitting light. The shape is two parallel flat plates tilted by a certain angle θ from the direction perpendicular to the optical axis, arranged symmetrically with respect to the optical axis. The collimated light incident from direction A is symmetrically centered on the optical axis and is obliquely incident on the incident surface 8 of the optical modulator, so it is split into three beams within the modulator, and the incident beam is re-distributed. is emitted in a direction parallel to the direction of travel.

At this time, the amount of movement Δ of the emitted light can be controlled by the thickness t of the present modulator, the inclination angle θ of the exit surface, and the refractive index of the modulator material. A typical value is t=
5mm, Δ'65mm when θ=25°, n=1.5
It is.

Further, FIG. 1(e) shows an embodiment of a modulator that reduces the side lobe intensity of the condensed spot by transmitting the light in the central part of the light-shielding band in FIG. In the incident plane 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 also a plane parallel to this. The other parts have the same configuration and shape as those shown in FIG. 3(b).

In this case, the central part of the incident light is perpendicularly incident on the modulator, and therefore travels straight without being refracted. The light of the other portions is divided into three beams according to the same principle as shown in FIG. 3(b), and is emitted again in a direction parallel to the optical axis of the incident light. In this way, the output light having the intensity distribution of the light-shielding area 14, the divided parallel light beams 11 and 12, and the central transmitted light 13 is obtained, and by condensing it with a condenser lens, the solid line in Fig. 4 is obtained. The resolution effect shown can be obtained. In the figure, a broken line 42 is a side lobe shown at 25 in FIG. 2(e), and is included for comparison. In this embodiment, it is possible to make the first-order side lobe 43 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, and the problem of incorrect focus caused by the side lobe is solved.

FIG. 5 shows an embodiment in which the optical modulator of the present invention is constructed by combining prisms. In FIG. 5A, collimated light enters a prism 51 (hereinafter referred to as a splitting prism) with its optical axis aligned with the symmetry axis, and is split into three beams upon exit. By transmitting these light beams through a prism 52 (hereinafter referred to as a direction correction prism), the traveling direction is changed to a direction parallel to the incident light, and the output light is divided into divided light beams 53.54.
, an intensity distribution having a light-shielding region 56. By focusing this light, a super-resolution effect equivalent to that of the embodiment shown in FIG. 2 can be obtained at the focused spot. Also, the same figure (b)
1 is a split prism 51 in which the apex of the exit surface in FIG. 5A is cut so that the central portion of the exit surface is parallel to the entrance surface, that is, perpendicular to the optical axis of the incident light. The light that has passed through the main splitting prism 51' travels straight at the center, and the other parts are split via the slopes of the prism. By passing only the latter through the direction correction prism, the light after passing through these systems is divided into a light shielding area 56, a divided parallel light beam 53, and so on.
54, and has an intensity distribution of centrally transmitted light 55, and by condensing this light, a super-resolution effect equivalent to that of the embodiment shown in FIG. 4 can be obtained at the condensed spot.

FIG. 6 shows an embodiment in which a direction correction prism is integrated. FIG. 6(a) shows an embodiment in which the same operation as FIG. 5(a) is performed. The incident collimated light is split into three beams by a splitting prism 61, and after passing through an integrated direction correction prism, the beam 6 travels in a direction parallel to the optical axis of the incident light.
3.64 and the light shielding area 66.

Further, in FIG. 6(b), the apexes formed by the slopes of the dividing prism and the direction correcting prism are shaved to provide a function of allowing the central portion of the incident light to proceed straight. When the collimated light beam passes through the splitting prism 61', the light in the center travels straight without changing direction, and the light exiting through the slope changes direction by refraction and is split into three light beams. When these light beams pass through the direction correction prism 62', they have an intensity distribution consisting of a light blocking area 66, a straight light beam 75 at the center, and divided light beams 63 and 64. With this, it is possible to obtain an effect equivalent to that shown in FIG. 5(b).

FIG. 7 shows an embodiment in which a reflecting mirror is used. The same figure (a
) is a means of dividing the incident collimated light into three beams (hereinafter referred to as
(referred to as a beam splitting mirror pair) and a means for correcting the interval between these three divided beams (hereinafter referred to as a correction mirror pair)
This is an example in which a pair of reflecting mirrors is used for both. The incident collimated light is reflected by reflecting mirrors 73 and 74 arranged obliquely to the optical axis and spaced apart from each other with the optical axis as the axis of symmetry, and is divided into three beams. Mirror 73.
Mirrors 73' and 74', which form a pair with 74, convert the beam into three beams 71' and 72' parallel to the direction of incidence. in general,
When two mirrors are arranged as in this embodiment, the output light beam 71',
Very high precision adjustment is required to keep the distance 72' to about 1 mm. In order to avoid this adjustment, this interval 2Δ
If it is allowed to become wider to some extent, it is necessary to bring these light beams closer together in order to obtain the desired super-resolution effect. The one that realizes this function is the correction mirror pair 75.76.
It is. As a result, the emitted light can have an intensity distribution consisting of a light shielding area 78 of about 1 mm and three divided beams 71 and 72.

By condensing these light beams, a super-resolution effect similar to that shown in FIG. 5(a) can be obtained at the condensed spot. Of course, if the interval 2Δ between the light beams is narrow enough to obtain the desired super-resolution effect, the light beams may be made to enter the condenser lens as they are without using the correcting mirror pair. FIG. 7(C) shows an example of the light beam splitting mirror pair shown in FIG.

In addition, in order to transmit the central part of the incident light in order to obtain the same effect as shown in FIG. 5(b), a pair of beam splitting mirrors 73 and 74
, and the pair of correction mirrors 75 and 76 may be spaced apart by the desired width for transmitting light. FIG. 7(b) shows an embodiment in this case. However, instead of the correction mirror pair, a prism having the same function as the modulator shown in FIG. 1(C) is used for beam interval correction. This prism has a symmetrical shape with respect to the optical axis, and the exit surface near the optical axis is a parallel plate with a plane perpendicular to the optical axis, and the other parts are symmetrical with respect to the direction perpendicular to the optical axis. It functions as a tilted parallel plate. The central portion of the incident collimated light travels straight as a transmitted beam 79, and the other portions are split into beams 71' and 72' by a pair of beam splitting mirrors 73, 74.

When these light beams are transmitted through the prism 77, the emitted light has an intensity distribution consisting of a light blocking area 78, divided light beams 71 and 72, and a transmitted light beam 79. By condensing these lights with a condenser lens, a super-resolution effect equivalent to that shown in FIG. 5(b) can be obtained.

FIG. 8 shows an embodiment in which a diffraction grating or a hologram is used as a means for dividing incident light.

(Hereinafter, this means will be referred to as a dividing grating.) In FIG. 8A, the dividing grating 81 emits most of the incident light as ±1st-order diffracted light, and does not generate a 0th-order component. . When the collimated light is incident perpendicularly to the main dividing grating, ±
They are emitted as first-order diffracted lights 83' and 84', and after passing through the direction correction prism 82, they are emitted as three parallel beams 83 and 84 that proceed in a direction parallel to the direction of incidence. Therefore, the light intensity distribution has a light shielding area 86 at the center and bright areas on both sides thereof. By condensing this light with a condensing lens, a super-resolution effect equivalent to that shown in FIG. 5(a) can be achieved at the condensed spot.

FIG. 8(b) is an example in which an effect equivalent to that of FIG. 5(b) is obtained. It is assumed that the dividing grating 81' emits most of the amount of incident light as 0th-order and ±1st-order diffracted light.

In addition, the direction correction prism 82' is a prism 82 shown in FIG. 8A, in which the apex formed by the slope on which the light beam enters is shaved off, and the central part is made into a parallel flat plate with an entrance/exit surface perpendicular to the optical axis. It is. Collimated light splits into grating 8
When incident perpendicularly to 1', ±1st-order diffracted light 83', 84'
The light is emitted as zero-order diffracted light 85', and after passing through the direction correcting prism 82', it is emitted as three parallel beams 83, 84, and 85 that proceed in a direction parallel to the direction of incidence. Therefore, there is a bright area in the center due to the transmitted light beam 85, and a dark area 8 on both sides.
6. Further, it is possible to obtain an emitted light pattern having a light intensity distribution with bright areas due to the divided light beams 83 and 84 on both sides. By condensing this light with a condenser lens, a super-resolution effect equivalent to that shown in FIG. 5(b) can be achieved at the condensed spot.

FIG. 8(c) shows an embodiment in which the function of a direction correction prism is also realized by a diffraction grating or a hologram (hereinafter referred to as a direction correction grating). It is assumed that the dividing grating 81b and the direction correction grating 81c emit most of the amount of incident light as ±1st-order diffracted light, and do not generate a 0th-order component. When collimated light is incident perpendicularly to the main dividing grating, ±1
The diffracted lights 83' and 84' are emitted as the next diffracted lights, and after passing through the direction correction grating 83c, they are emitted as three parallel beams 83 and 84 that proceed in a direction parallel to the direction of incidence. Therefore, the light intensity distribution has a light shielding area 86 at the center and bright areas due to the light beams 83 and 84 on both sides thereof. If this is focused with a condensing lens, the condensed spot will be as shown in Figure 5(a).
It is possible to achieve a super-resolution effect equivalent to that of .

In order to improve the reproduction signal characteristics, the present inventors invented an optical head having a means (hereinafter referred to as an improved optical system) for removing the sidelobe component of the reflected light from the recording medium from entering the reproduction signal component. . FIG. 9 shows an embodiment in which the present invention and this improved optical system are combined. The emitted light from the light source is collimated by the collimating lens 2, and the light modulator 3
After passing through the beam, the beam is divided into three beams and is irradiated onto the recording medium 5 by the condenser lens 4. During reproduction, after the reflected light from the recording medium passes through a condenser lens, a portion of it is taken out by a beam splitter 96 and condensed by a condenser lens 93 (hereinafter referred to as a recondensing lens). By forming an intensity pattern similar to the condensed spot on the recording medium surface at the focal plane and passing this through the slit 94, the side lobe components shown in FIG. 2 are eliminated from entering the reproduced signal. I can do it. The reflected light that has passed through the beam splitter 96 is guided to a focus error detection system 91 and a track error detection system 92 via beam splitters 97 and 98.

(Effects of the Invention) By using the present invention, the conventional restrictions can be removed and a super-resolution effect equivalent to that obtained when a light-shielding band is installed without installing a light-shielding band can be obtained. It is possible to realize an optical modulator and an optical head device using the same, which can realize a higher recording density than the above, and have a high light utilization rate since no light-shielding band is provided. This also makes it possible to reduce the output power required of the laser of the light source.

[Brief explanation of the drawing]

FIGS. 1(a) to (C) are diagrams showing one embodiment of the present invention, FIGS. 2(a) to (c) are diagrams showing the principle of super-resolution generation, and FIG.
Figures (a) to (C) are diagrams showing the super-resolution effect in an optical head using a light modulator using a light shielding zone, and Figure 4 is an example of an optical modulator that transmits light in the center of the light shielding zone. FIG. 5 (aXb) is a diagram showing the super-resolution effect, and FIG.
aXb) is a diagram showing an example in which a direction correction prism is integrated; FIGS. 7(a) to (C) are diagrams showing an example in which a reflecting mirror is used in the optical modulator used in the present invention; Figure 8 (
a) to (C) are diagrams showing an embodiment in which a diffraction grating or a hologram is used as a means for splitting incident light, and FIG. 9 is a diagram showing an embodiment in which the present invention is combined with an improved optical system. It is. In the figure, 1... semiconductor laser, 2..., collimating lens, 3
... Optical modulator, 4 ... Condensing lens, 5 ... Recording medium, 6 ... Signal detection system, 7 ... Beam splitter,
8...Incidence surface, 9...Directly transmitted light incident surface, 9'...
- Straight transmitted light exit surface, 11, 12... Divided outgoing parallel light flux, 13...
Center transmitted light flux, 14...Dark area equivalent to the light-shielding area, 2
DESCRIPTION OF SYMBOLS 1... Shading zone, 22... Light beam, 23... Focused beam intensity distribution curve not using super resolution, 24... Focused beam intensity distribution curve using super resolution, 25. ... Side lobe, 41... Main lobe, 42... First-order side lobe due to simple light shielding, 43... First-order side lobe due to transmission through the center, 4480. Secondary side lobe by transmitting the central part, 51, 51'... Luminous flux splitting prism, 52-1 unidirectional correction prism, 53.54... Divided output parallel luminous flux, 55... Center Partially transmitted light flux, 56...Dark area equivalent to the light-shielding area, 61, 61''...Light flux splitting prism, 6
2.62' -8, integrated direction correction prism, 63.6
4... Divided outgoing parallel light flux, 65... Center transmitted light flux, 66... Dark area equivalent to the light shielding area, 71, 71', 72.72'... Divided outgoing parallel light flux, 73.73'. 74, 74'-・, Grating for beam splitting, 73", 74"
...Light beam splitting mirror, 75.76...Correction mirror pair, 7
7... Luminous flux spacing correction prism, 78... Dark area equivalent to the light shielding area, 79... Center transmitted straight light, 81, 81'... Diffraction grating or hologram for beam splitting, 82, 82'.・Direction correction prism, 83.84
...Divided outgoing parallel light beams, 83', 84'...
・±1st-order diffracted and emitted light from the beam splitting grating, 85...
Light traveling straight through the center, 86...Dark area equivalent to the light shielding 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)

[Claims] In an optical head device that records and reproduces information using an optical system that condenses light emitted from a light source as a minute spot onto the surface of a recording medium by a condensing lens and guides reflected light from this condensed point to a photodetector. An optical head characterized by having means between the light source and the condenser lens to divide the light beam from the light source into a plurality of parallel light beams and to input the divided plurality of light beams into the condenser lens. Device.