JPH0816738B2 - Optical Head Device - Google Patents

Description

[Detailed Description of the Invention] Technical Field This invention relates to an optical head device that performs recording/reproducing/erasing on an optical information recording medium, and that eliminates spherical aberration inherent in the optical system and corrects the spherical aberration to obtain good focusing characteristics.

BACKGROUND ART In recent years, optical head devices for writing to and reading from optical disks have been actively developed as optical devices that utilize so-called diffraction-limited optical systems that are nearly aberration-free.

7(a), (b) and (c) are diagrams showing the configuration of the main parts of such an optical head device.

In the figure, (1) is a semiconductor laser (hereafter referred to as
(2) is a condenser lens, (3) is a disk,
(4) is the information recording surface of the disk (3), (5) is the focusing point of the focusing lens (2), (6) is the light beam emission point of the LD (1),
(7) is the light beam emitted from the LD (1), (10) is the beam splitter, (11) is the photodetector, (12) is the information track formed on the information recording surface (4), and (13) is the light condensing point on the photodetector.
(14) is a diffraction grating, and (15) is a cylindrical lens.

Next, the operation will be explained. The divergent light beam (7) emitted from the light beam emission point (6) of the LD (1) is
The light is converted into a convergent beam by a condenser lens (2) and condensed at a point (5) on the information surface (4) through a transparent substrate with a thickness d on the surface side of the disc (3).

The reflected light beam from the information surface (4) passes through the condenser lens (2) again, is separated from the outgoing light beam (7) by the beam splitter (10), and is received by the photodetector (13) to obtain a signal.

The light convergence point (5) on the disk (3) by the condenser lens (2) must always be irradiated onto the information track (12), and therefore the device has focus servo and tracking servo functions.

7 performs focus servo by the astigmatism method and tracking servo by the twin spot method. This will be briefly explained below.

In the astigmatism method, an astigmatism generating means such as a cylindrical lens (15) is placed in the reflected light beam, and astigmatism is given to the reflected light beam. When the focus (5) is correctly on the information track (12), the photodetector (11) is adjusted in the direction of the optical axis so that the circle of least confusion (13) on the photodetector (11) is illuminated. The photodetector is a four-segment detector (11a) as shown in Figure 7(b).
(11b) (11c) (11d).

At this time, with respect to the displacement of the optical disk (3) in the optical axis direction,
The spot shape on the photodetector (11) changes from the circle of least confusion (solid line (13)) to an elongated elliptical spot (dotted line). To obtain this change in spot shape as an electrical signal, a focus error signal is obtained by differentially calculating the sum outputs of the diagonal components of the four-segment detector {((11a) + ((11c)) - ((11b) + (11d))}, and the focusing lens (2) is moved by a focus actuator (not shown) to correct the deviation of the focusing point (5) from the information surface (4) in the optical axis direction.

In the twin-spot method, a diffraction grating (14) placed in the output beam (7) of the laser diode (1) separates the beam into zero-order and ±1st-order beams, which are then focused on the information track (12) as shown in Figure 7(c). The zero-order beam is irradiated exactly at the center of the information track and is used to read and record signals. The ±1st-order beams are positioned slightly off-track, and the lines connecting the three spots are slightly tilted relative to the information track (12).

At this time, the diffracted and reflected light beams of the ±1st order light are detected by a photodetector (11e).
The light is received at (11f), and the difference signal ((11e) - (11f)) is obtained as a tracking error signal, and the focusing lens (2) is moved by a tracking actuator (not shown) to correct the in-plane deviation of the focusing point (5) from the information track (12).

In the optical head device described above, the information density stored on the recording medium is increased as much as possible, and in order to use it as a large-capacity information medium, the pit length and track spacing are made small enough to read the signal when the focusing system from the LD (1) to the focusing lens (2) is in a diffraction-limited state.

That is, in order to irradiate the diffraction-limited focused spot (5) onto the information track (12), the convergent light beam incident on the information surface (4) must be aberration-free. In this case, the standard deviation of wavefront aberration allowed as the diffraction limit is 0.07λ (Mar
chal limit).

As mentioned above, when light is collected through a transparent substrate disk with a thickness d, the fourth-order spherical aberration is as follows: It is known that wavefront aberration occurs as given by the following formula:

Optical disk heads require focusing with nearly no aberration, so the focusing lens (2) is designed with spherical aberration undercorrected by W ₄₀ in advance, so that this cancels out and balances the aberration that occurs when the light beam passes through the disk substrate.

FIG. 8 is a diagram showing only the light-collecting optical system in _the optical head device (FIG. 7).
l ₂ is the distance from the main surface H and H' of the condenser lens to the light source emission point (6) and the condenser point (5), and these distances l ₁ and l ₂
In the design state of the optical disk head, the emission point (6) of the light source and the convergence point (5) on the disk (3) are set so as to be in a conjugate relationship with respect to the paraxial ray.

In other words, if the focal length of the condenser lens (2) is ₀ , A relationship like this is now established.

In addition, in a positional relationship that satisfies the above formula (2), the parameters of the condenser lens (2) (lens shape, thickness, etc.) are set so that the wavefront distortion of the condensed light beam, i.e., the wavefront aberration, is minimized.

However, in the optical head device described above, there are several factors that cause wavefront aberration, which can degrade the recording/reproducing characteristics. Here, we will consider spherical aberration, which is one of the wavefront aberration components.

The causes of this include disc thickness error, disc refractive index variation, deviation of the refractive surface shape of the focusing lens from the design value, focusing lens thickness error, and refractive index variation of the focusing lens.

For example, in a disk with a polycarbonate substrate with a refractive index N = 1.55, if the numerical aperture (NA) of the focusing lens is sinθ ₂ = 0.5, then for light with a wavelength of 0.78 μm,
If the deviation of the disc thickness from the design center is 50 μm, the change in rms wavefront aberration is calculated using the above formula (1).
In other words, even with such a small thickness error, it reaches 20% of the aforementioned allowable rms wavefront difference of 0.07λ.

In addition, any of the factors mentioned above (1) to (5) cause fourth-order spherical aberration that is symmetrical to the optical axis. The superposition of each aberration factor also results in an aberration component that is symmetrical to the optical axis. Therefore, a fourth-order spherical aberration remains in total. For the normalized radius ρ of the pupil, the wavefront aberration is ω
It can be expressed in the form ₄ ·ρ ⁴ (where 0<ρ<1).

It is a well-known fact that the residual spherical aberration reduces the central intensity of the focal point (converged spot) (5), resulting in an increase in the focal spot diameter.
In an optical head, an increase in the diameter of the light-focusing point (5) leads to a deterioration in the OTF of the optical system, which results in a loss of high density recording/reading of the optical disk head, leading to a problem of performance degradation.

Disclosure of the Invention The present invention has been made to solve the above-mentioned problems, and has as its object to provide an optical head device in which spherical aberration is minimized by very simple position adjustment of parts.

The optical head device of this invention is configured as a unit in which the light source, beam splitter, and photodetector have fixed relative distances from each other, and is configured so that spherical aberration is corrected by adjusting this unit in the direction of the optical axis of the focusing lens, making it possible to correct the spherical aberration of the focusing optical system without moving the focusing spot on the photodetector, thereby achieving a diffraction-limited, aberration-free focusing system while maintaining the focus sensor characteristics as designed.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing an optical head device according to one embodiment of the present invention, FIG. 2 is a block diagram showing a second embodiment of the present invention, and FIGS. 3(a), 3(b), and 3(c) show LDs.
FIG. 4 is a diagram illustrating the configuration of a third embodiment of the present invention; FIGS. 5(a) and 5(b) are diagrams illustrating the mounting and adjustment state of an optical head device; FIGS. 6(a) and 6(b) are cross-sectional views illustrating the internal configuration of an embodiment of the present invention in detail; and
8A, 8B and 8C are diagrams showing the configuration of a commonly used optical head device, and FIG. 8 is a diagram showing the configuration of a light-collecting optical system of a commonly used optical head device.

BEST MODE FOR CARRYING OUT THE INVENTION A first embodiment of the present invention will now be described with reference to Fig. 1, in which the same parts as in Figs. 7 and 8 are designated by the same reference numerals. In Fig. 1, (8) denotes an adjustment means for displacing the LD (1) in the positive or negative direction along the optical axis direction (the Z direction indicated by the arrow).

The adjustment means (8) can shift the position of the LD along the optical axis from the normal position, i.e., the normal distance _l1 between the LD (1) and the condenser lens (2) consisting of a finite conjugate objective lens. The parameters of the condenser lens (2) are shown in the figure.
Distance from (1) to the first main surface H of the condenser lens (2)
It is designed to be optimal when l ₁ is the distance from the second main surface H' to the information surface of the disk (3) and l ₂ is the distance from the second main surface H' to the information surface of the disk (3).

Therefore, by displacing the position of LD (1), LD
If the distance from (1) to the first principal surface H changes from l ₁ to l ₁ +Δ, the optimum design state is deviated from and spherical aberration occurs.

Figure 3 (a) is a graph showing the relationship between the amount of spherical aberration and the displacement Δ of _l1 using a certain focusing lens as an example.
Figure 3(b) and (c) show the aberration patterns when Δ = ±1 mm. Figure 3(b) shows the aberration patterns when Δ = -1 mm, and Figure 3(c) shows the aberration patterns when Δ = ±1 mm.
= +1mm.

In this way, by moving the LD (1) closer or farther from the condenser lens (2), the condenser lens generates positive and negative spherical aberration. If spherical aberration exists in the original optical system due to the defects mentioned above,
By adjusting the position of the LD (1), the spherical aberrations can be cancelled out.
By moving it by ±1 mm, the spherical aberration can be adjusted by approximately ±λ/4.

FIG. 2 is a diagram showing a second embodiment of the present invention.
In the figure, a position adjustment means (9) in the direction of the optical axis is added to the condenser lens (2), and the distance between the LD (1) and the condenser lens (2) is adjusted on the condenser lens side.
This is the difference from the first embodiment. In this second embodiment, the principle of reducing the wavefront aberration that causes spherical aberration simply depends on the change in the distance between the LD and the condenser lens.
There is no difference from the first embodiment in that it is possible to reduce wavefront aberration.

It goes without saying that the adjusting means (8) or (9) does not necessarily have to be continuously movable, and that a device that approximately provides the position with the smallest wavefront aberration by changing a mount or the like that determines several discrete positions is also effective in reducing wavefront aberration. Furthermore, aberration correction by the focusing optical device of this invention is not necessarily limited to the focusing optical system of an optical disk head, but can also be applied to spherical aberration correction in optical systems that require optical performance close to the diffraction limit.

1 and 2, an adjustment means for changing the distance between the LD and the focusing lens is added, so that the wavefront aberration caused by the spherical aberration of the focusing optical system can be significantly reduced by changing the distance between the LD and the focusing lens, which is extremely effective in keeping the performance of the focusing optical system within the diffraction limit. As a result, when this optical head device is applied to the focusing optical system of an optical head, the OTF of the optical system is improved by reducing the diameter of the focusing point, and the deterioration of the write/read characteristics caused by spherical aberration can be significantly improved, and the tolerance of spherical aberration among the aberration components of the optical head can be expanded.

However, it has been found that the above two embodiments also have the following problems.

This problem will be explained below with reference to FIG.

FIG. 4 shows the optical head device according to the first embodiment (FIG. 1) described above, and parts that are the same as or correspond to the components in FIG. 2 are given the same reference numerals.

In the figure, U is the distance from the light emitting point (6) of the LD (1) to the conjugate point (5) on the optical disk, b is the distance to the beam splitter, g is the distance to the diffraction grating, and b' is the distance from the beam splitter to the other conjugate point, the photodetector (1
1) The distance to the focal point (13) above.

Adjustment means (8) for LD (1) to correct spherical aberration
When the LD (1) is displaced in the optical axis direction (Z) by changing the distance l ₁ to the objective lens (2) by Δ to become l ₁ +Δ, the distance to the beam splitter also changes to b +Δ.

Therefore, the position of the conjugate point (13) also changes, and b' also changes.

As a result, in order to illuminate the light detector (11) with the circle of least confusion (13), it is necessary to move and adjust the light detector (11) or the optical axis adjustment lens (a lens not shown, but placed in the position of the cylindrical lens (15), for example).

In addition to the wide adjustment range required, the cylindrical lens (1
The astigmatic difference changes due to the difference in the focal position of the reflected light beam incident on the diffraction grating 5), which causes a serious problem that the focus sensor characteristics change from the designed values. In addition, the distance g between the light source emission point 6 and the diffraction grating 14 also changes Δ.

At this time, the spot interval P in Fig. 7(c) changes in proportion to Δ, and the spot interval P' on the photodetector in Fig. 7(b) also changes almost in proportion to Δ. This means that not only is there a risk that the photodetectors (11e) and (11f) formed at predetermined positions will not be irradiated, but also that if P' becomes small, the spot interval P' will not be irradiated.
This causes interference with the next light, affecting both the focus sensor characteristics and the tracking sensor characteristics.

Next, problems with the optical head device according to the second embodiment will be described.

By moving the focusing lens (2) and changing _l1 to _l1 + Δ, the distance _l2 from the focusing lens (2) to the disc also changes according to the above formula (2). Therefore, in order to form a focusing point (5) on the information surface (4), the optical disc (3) must be moved in the optical axis direction. This means that a large-scale adjustment mechanism is required when installing an optical head device in a player, which is not practical.

On the other hand, in the first embodiment described above, since only LD (1) is displaced, the above-mentioned problem occurs when applied to an optical head device, but if the position of the entire optical head device relative to the optical disk is adjusted, the distance between b and g (Figure 4) will not change and the problem may not occur.

That is, in the first embodiment, the spherical aberration is adjusted by changing the distance _l1 , which means that the distance U between the conjugate points of the light spot (6) and the light condensing point (5) on the optical disc is
It is nothing but changing.

Therefore, as another embodiment for correcting spherical aberration, the fifth embodiment is
The structures shown in Figures (a) and (b) are conceivable.

In Figure 5, U is the distance between the light source's light emitting point (6) and the light condensing point (5) on the optical disc. (16) is the optical head device, A is the mounting reference plane of the optical head device (16), and t is the distance from the mounting reference plane A to the optical disc (3).

FIG. 5(a) shows the relative positional relationship with the optical disk in the reference state before spherical aberration correction, and FIG. 5(b) shows the relative positional relationship when the entire optical head device (16) is moved by Δ in the Z direction.

In FIG. 5(b), the distance from the light source emission point (6) to the light condensing point (5) on the disk is U+Δ, and the spherical aberration is corrected by the slight movement Δ.
The focus on the optical disk 5 will shift as the condenser lens 1 or condenser lens 2 moves, but this focus shift is automatically corrected by a focus servo mechanism (not shown).

However, in the embodiment of Figures 5(a) and (b), the distance t from the mounting reference plane A to the optical disc (3) changes to t + Δ, so when this optical head device is applied to a player, a mechanism for changing the mounting height of the optical head device (16) is required, and problems arise in terms of space allocation with other parts inside the player, which is also not desirable.

The present invention includes yet another embodiment (third embodiment) for solving the above-mentioned problems, and the following describes in detail an optical head device that has a means for adjusting the distance between the light source and the objective lens, and that can maintain the focus sensor characteristics and tracking sensor characteristics according to the reference design, thereby forming an aberration-free focusing optical system.

The third embodiment will be explained using Fig. 4, but since the adjustment means (8) shown in Fig. 4 is not used, the following explanation will be based on the assumption that the adjustment means (8) is not present. The focus servo, tracking servo system, and spherical aberration correction principle in this embodiment are exactly the same as those described above, so
The explanation regarding these will be omitted.

In this embodiment, the distances g, b, and b' are fixed. Therefore, to correct spherical aberration, the distance l1 is adjusted by moving the LD (1), diffraction grating (14), beam splitter (10), cylindrical lens (15), and photodetector (11) as a whole in the optical axis direction (Z direction) using the adjustment means ( ₁₇ ). As a result, spherical aberration can be corrected as described above.

However, since the distance b between the LD (1) and the beam splitter remains constant, the distance between the beam splitter and the focal point (13) of the reflected light beam from the optical disk, which is conjugate with the light-emitting point (6) of the LD (1), also remains constant and is equal to b'. Therefore, the focal point (13) remains illuminated on the photodetector (11).

Naturally, the astigmatic difference does not change, so the focus sensor characteristics are almost the same as in the reference state (design value). Also, in this embodiment, the distance g between the LD (1) and the diffraction grating (14) is unchanged, so as previously mentioned, the spot interval P' does not change in Figure 2(b), and the tracking sensor characteristics and focus sensor characteristics do not change, so good servo characteristics can be maintained.

In recent years, injection-molded and press-molded lenses such as plastic molded lenses, glass molded (pressed) lenses, grating lenses, and Fresnel lenses have been devised and some have been put to practical use as objective lenses, with the aim of achieving low cost and light weight. While such lenses are easy to mass-produce, they suffer from aberrations due to manufacturing errors during molding and errors in the design of the mold itself. However, it is known that the variation in aberrations between lenses in the same molding lot is relatively small, and that they have certain aberration characteristics. Therefore, as a fourth embodiment of the present invention, if the amount of wavefront aberration of a large number of lenses in a certain molding lot, for example the average value of spherical aberration, is known,
The spherical aberration correction means (17) as in the third embodiment is not used for each optical head device _.
By preparing several different types of optical barrels with different values and selecting the barrel that is closest to canceling out the average value of the spherical aberration, it is possible to correct the spherical aberration to a large extent by using the same type of barrel for lenses from the same lot.

This will be explained in more detail in FIGS. 6(a) and 6(b).
In the figure, (18) is the optical lens barrel, and (19) is the focus and tracking actuator.
In Fig. 6(a) and Fig. 6(b), the distance from the light emitting point (6) of the LD (1) to the main surface H of the condenser lens (2) is l ₁ a, and in Fig. 6(b) it is l ₁ b. This is an optical head device using a lens barrel (18), and in both Figs. 6(a) and (b), g, b, b'
The distances are equal.

Also, A is the reference mounting position of the optical head device, the distance to the reference position of the optical disk being a constant value t, (a),
(b) Both figures are at the same position t from the optical disk.

For example, if it is known that a large number of objective lenses obtained from a certain lot have a statistically certain spherical aberration, and if the lens barrel (18) of Figure 6(a) with a distance of l ₁ a is used for this spherical aberration value, and the spherical aberration correction effect is greater than in the case of the lens barrel (18') of Figure 6(b), then all of the lenses in this lot can be mounted on the lens barrel (18) of Figure 6(a). In this way, by preparing several types of lens barrels (18) with slightly different l ₁ values, it is possible to correct spherical aberration with considerable precision. In this case, since there is no need for an adjustment means as in the first embodiment, a low-cost optical head device can be constructed, and the b, g, and
Since the condition for keeping b' constant is satisfied, the same effect as in the first embodiment can be obtained. Furthermore, in the third embodiment, the distance t from the mounting surface A to the disk can be kept constant, so when using the optical head device in a player, there is no need to change the disk mounting position or the optical head device mounting position, which is convenient. Note that in this embodiment, an astigmatism method focus servo and a twin spot method track servo are used, but
This method can also be applied to servo methods such as the Foucault method, knife edge method, critical angle method, heterodyne method, and push-pull method.In other words, in an optical system in which a beam splitter separates the reflected light beam to the sensor system and the emitted light beam from the light source, the object of the present invention can be achieved by changing the distance from the light source to the objective lens without changing the relative positions of the light source, beam splitter, and photodetector.

As described above, according to the present invention, an optical head device comprising at least a semiconductor laser as a light source, focusing optical means for focusing a light beam emitted from the semiconductor laser on an information recording medium, a beam splitter element for separating a reflected light beam from the information recording medium and a light beam emitted from the semiconductor laser, and a photodetector for receiving the separated reflected light beam, the device comprises a first optical element group including the semiconductor laser, beam splitter element, and photodetector, and a second optical element group including the focusing optical means, wherein the relative positions of the optical elements in the first optical element group are fixed and an adjustment means is provided for adjusting the distance in the optical axis direction between the first and second optical element groups. Therefore, while keeping the positional relationship between the light emission point of the light source and the focusing point of the reflected light beam (position of the photodetector), which are conjugate via the beam splitter means, unchanged, the relative spacing between the focusing optical means constituting the second optical element group and the first optical element group can be adjusted, resulting in the effect of correcting spherical aberration of the focusing optical system with almost no change in the focus sensor characteristics from the reference state.

INDUSTRIAL APPLICABILITY The present invention is applicable to optical heads used for optical disks, compact disks, and the like.

──────────────────────────────────────────────────── Continued from the front page (72) Inventor: Eiichi Miyade 1 Babazusho, Nagaokakyo City, Kyoto Prefecture, Japan, Mitsubishi Electric Corporation Electronic Product Development Laboratory (56) References: JP 59-216116 (JP, A) JP 50-156945 (JP, A) Utility Model Application Publication No. 1985-128320 (JP, U) Kogoro Yamada, "Optical Machinery and Apparatus" (1944- 8-20), Seibundo Shinkosha, pp. 204 and 217

Claims (5)

[Claims] 1. A light source comprising: a semiconductor laser as a light source; a focusing optical means for focusing a light beam emitted from the semiconductor laser on an information recording medium; and a light beam splitter element for separating a light beam reflected from the information recording medium from a light beam emitted from the semiconductor laser.
In an optical head device comprising at least a first optical element group including the semiconductor laser, a beam splitting element, and a photodetector for receiving the split reflected beam, the device comprises a second optical element group including the light collecting optical means.
and a second optical element group, wherein the relative positions of the optical elements in the first optical element group are fixed, and an adjustment means is provided for adjusting the distance in the optical axis direction between the first and second optical element groups. 2. The optical head device according to claim 1, wherein the adjusting means displaces the first optical element group in the direction of the optical axis. [Claim 3] An optical head device as described in claim 1, characterized in that each element constituting the first and second optical element groups is attached to a lens barrel, several types of lens barrels are prepared with gradually different distances in the optical axis direction between the groups, and the lens barrel that produces the smallest spherical aberration is selected and used. 4. The optical head device according to claim 1, wherein the first optical element group has a diffraction grating means. 5. The optical head device according to claim 1, wherein the second optical element group comprises a finite conjugate objective lens.