JP2001256672A - Optical pickup and optical disk device using the same - Google Patents

Abstract

(57) Abstract: Chromatic aberration for correcting a focal position shift due to a wavelength variation of a semiconductor laser in an optical pickup having a blue semiconductor laser and a high NA objective lens capable of recording and reproducing an optical disk having a capacity of 20 GB to 25 GB. Provided are an optical pickup having a correction element and an optical disk device. SOLUTION: A chromatic aberration correcting element including a diffractive optical element and a concave lens is provided in an optical system between an objective lens and a semiconductor laser. The focal length of the diffractive optical element is from 30 mm to 45 m
m.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION An optical pickup having a semiconductor laser light source and a high NA objective lens, and an optical disk apparatus using the optical pickup, particularly, a chromatic aberration correction element for reducing deterioration of recording / reproducing characteristics due to wavelength fluctuation of the semiconductor laser. The present invention relates to an optical pickup provided and an optical disk device using the optical pickup.

[0002]

2. Description of the Related Art At present, optical discs such as DVDs are becoming widespread as external recording devices for computers or as home video players, video recorders, and audio equipment. An optical pickup used for a DVD uses a semiconductor laser having a wavelength of about 650 nm as a light source,
Recording and reproduction are performed using an objective lens having an NA (numerical aperture) of 0.6 to 0.65. DVD recording capacity is 1 diameter
A 2 cm disk medium has a capacity equivalent to 4.7 GB, but a next-generation DVD requires a disk medium having a diameter of 12 cm to have a capacity of 20 GB to 25 GB. The capacity of an optical disk is substantially determined by the spot diameter formed by the optical pickup. Spot diameter (1/1 for peak value)
e ² ) is approximately expressed as λ / NA when the wavelength of the light source is λ and the numerical aperture NA of the objective lens is used. In the case of DVD, the spot diameter is about 1.1 μm.
In order to achieve 25 GB of the next generation DVD, the spot diameter needs to be about 0.48 μm. To achieve this, the light source wavelength λ must be smaller, or (and)
It is essential to increase the numerical aperture NA of the objective lens.
As a semiconductor laser as a light source, a blue semiconductor laser having a wavelength of about λ = 410 nm has recently been put into practical use. In order to make the spot diameter 0.48 μm using this blue semiconductor laser, the objective lens NA needs to be about 0.85.
The shape of this NA 0.85 objective lens and a recording / reproducing apparatus using this objective lens are disclosed in
7 is disclosed.

[0003]

An optical pickup using a short-wavelength blue semiconductor laser light source and a high-NA objective lens enables high-density recording and reproduction, but the influence of aberrations becomes very large. Particularly serious is the influence of aberration due to wavelength fluctuation of the semiconductor laser. The wavelength variation of a semiconductor laser is
It occurs when the temperature of the operating environment or the emission intensity of the semiconductor laser changes. Generally, at an operating temperature of 25 ° C., 1
When the temperature changes by ° C., the wavelength changes by about ± 0.07 nm, and when the emission intensity changes by 1 mW, the wavelength changes by about ± 0.04 nm. The semiconductor laser of the recordable optical pickup emits a pulse light having a peak power of 30 mW to 50 mW during recording. Therefore, the wavelength instantaneously changes around 1.2 nm to 2 nm. When a wavelength fluctuation of 2 nm occurs instantaneously, a focal position shift of about 0.35 μm occurs with an objective lens of NA 0.85. This focal position shift occurs because the refractive index of the glass material used for the objective lens changes depending on the wavelength, and the focal length of the entire objective lens changes. The depth of focus of the objective lens is represented by λ / (NA) ^{2. If} the focal position shift exceeds ± λ / (2NA ² ), a spot necessary for recording and reproduction cannot be formed. When λ = 410 nm and NA = 0.85, λ / (2
NA ² ) = 0.28 μm, so that when the focus position shifts by 0.35 μm due to a fluctuation of 2 nm, sufficient recording / reproducing characteristics cannot be obtained.

In view of the above problems, an object of the present invention is to provide an optical pickup provided with a chromatic aberration correcting element for correcting a focal position shift caused by a wavelength change of a semiconductor laser, and an optical disk device using the optical pickup. To provide.

[0005]

An optical pickup according to the present invention is an optical pickup including a blue semiconductor laser and an objective lens, wherein the chromatic aberration compensating element constituted by a combination of a concave lens and a diffractive optical element is the blue semiconductor laser. And an optical system between the objective lens.

The chromatic aberration correcting element can be constituted by an optical element in which plane portions of a plano-concave lens and a diffractive optical element formed in a parallel plate are adhered to each other.

[0007] Alternatively, the chromatic aberration correcting element can be realized by using a special plano-concave lens in which a diffractive optical element is formed on the plane portion of the plano-concave lens.

The chromatic aberration correcting element has a focal length of 30 mm or more and 45 mm or less, and the focal length of the concave lens is a negative value of the focal length of the diffractive optical element. realizable.

An optical disk apparatus according to the present invention includes an optical pickup including a blue semiconductor laser and an objective lens, and a chromatic aberration correction element formed by a combination of a concave lens and a diffractive optical element is provided with the blue semiconductor laser and the objective lens. An optical pickup provided in an optical system between lenses is used.

The chromatic aberration correcting element can be constituted by an optical element in which plane portions of a diffractive optical element formed of a plano-concave lens and a parallel plate are adhered to each other.

Alternatively, the chromatic aberration correcting element can be realized by using a special plano-concave lens in which a diffractive optical element is formed on a plane portion of the plano-concave lens.

The focal length of the diffractive optical element is not less than 30 mm and not more than 45 mm, and the concave lens has a focal length that is a negative value of the focal length of the diffractive optical element. realizable.

By using the optical pickup provided with the chromatic aberration correction element according to the present invention, even in an optical pickup using a blue semiconductor laser light source and a high NA objective lens, it is possible to correct the focal position shift due to the wavelength fluctuation of the semiconductor laser, Good recording and reproduction characteristics can be realized. Further, in the optical disk device using the pickup of the present invention, the focal position shift due to the fluctuation of the wavelength of the semiconductor laser can be corrected, and an optical disk device having good recording and reproducing characteristics can be realized.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 shows an optical system of an optical pickup according to the present invention. Laser light from the blue semiconductor laser 101 having a wavelength of 410 nm is collimated by the collimating lens 102 into substantially parallel light. The beam shaping prism 103 shapes the elliptical beam after passing through the collimating lens into a substantially circular beam. The light transmitted through the polarization beam splitter 104 becomes substantially circularly polarized light by the λ / 4 wavelength plate 105, passes through the chromatic aberration correction element 106, and is transmitted through the objective lens 107 (NA 0.85) to the optical disk 10.
8 is focused on the information recording surface. The reflected light from the information recording surface returns to the objective lens 107 again, and the λ / 4 wavelength plate 1
Due to 05, the light becomes linearly polarized light orthogonal to the polarization direction at the time of incidence, is reflected by the polarization beam splitter 104, and is guided to the detection optical system.

The detection optical system detects a focus position error signal, a tracking position error signal, and a reproduction signal. Methods for detecting the focus position error signal include a knife edge method and an astigmatism method. In FIG. 1, the astigmatism method is used. The light guided to the detection optical system is reflected by the beam splitter 109 by about 20 to 30%, guided to the focus position error detection system, transmitted through the condenser lens 110, and then tilted with respect to the 45 ° disk rotation direction. The light passes through the cylindrical lens 111 and is guided to the quadrant detector 112. By taking the sum of the diagonals of the quadrant detector and taking the respective differences, a focal position error signal is obtained. Although not shown in FIG. 1, this focus position error signal is fed back to a two-dimensional actuator that holds an objective lens, focus servo is applied, and recording / reproducing light is always
Focus on the information recording surface of No. 8. Methods for detecting the tracking position error signal include a push-pull method and a DPD method. In FIG. 1, the push-pull method is used. The light transmitted through the beam splitter 109 is condensed on the two-divided detector 114 after passing through the condenser lens 113. The two-divided detector is divided by a straight line passing in the direction of rotation of the disk, that is, in a direction 116 parallel to the recording track, and substantially at the center of the detected light beam. The tracking position error signal is obtained by taking the difference between the outputs of the two-divided detector. This tracking position error signal is
Feedback is provided to the two-dimensional actuator, tracking servo is applied, and control is performed so that the recording / reproducing light always scans the spot substantially at the center of the information track on the optical disk.

Although the reproduction signal detection optical system may be provided separately from these two optical systems, it is often used in common with the tracking error detection optical system in order to simplify the optical system.
This is the case with the detection optical system of FIG. The reproduction signal usually takes the sum signal of the two-division detector 114. After the sum signal is amplified to an appropriate voltage by an amplifier, data is demodulated by a data demodulation circuit in accordance with a modulation method, and information is reproduced.

FIG. 2 shows the configuration of the chromatic aberration correction element 106. This chromatic aberration correction element is composed of a combination of a concave lens and a diffractive optical element. The diffractive optical element is a so-called Fresnel lens and has a concentric periodic structure centered on the center of the optical axis. Utilizing the diffracted light by this periodic structure, a lens effect is produced. As shown in FIG. 2, in order to increase the utilization efficiency of transmitted light, the phase of the transmitted light is changed periodically instead of periodically, and the efficiency of a specific diffraction order is improved. It is desirable to have a brazing structure.

As shown in the upper part of FIG. 2, the concave lens and the diffractive optical element are combined with each other by combining a plano-concave lens 202 and a diffractive optical element 201 formed on one side of a parallel plate, or a plano-concave lens 203. There are two types of special plano-concave lenses in which a diffractive optical element is previously configured on the plane side. The former is a plano-concave lens 202 and a diffractive optical element 2
01 are separately formed, and the respective plane portions are bonded to each other. In the latter case, the plano-concave lens 203 is formed by glass molding or plastic injection molding, and the periodic structure of the diffractive optical element is previously engraved on a plane portion of a mold, and the plano-concave lens and the diffractive optical element are molded at once.

Next, the effect of this chromatic aberration correction element will be described by specific ray tracing. FIG. 3 shows numerical data of the cross-sectional shape and the surface shape of the objective lens 301 used in the optical pickup of the present invention. The design wavelength of this objective lens is 4
10 nm, the effective light beam diameter is 2.98 mm, and the focal length is 1.75 mm. When information is recorded / reproduced on the recording surface of the optical disk 302 using the objective lens 301, an objective lens actuator (not shown) is operated by an autofocus mechanism so that the focal position is exactly on the recording surface of the optical disk 302. Controls the position of the objective lens in the optical axis direction.

FIG. 4 shows a calculation result of aberrations caused by a focal position shift when this objective lens is used. In general, it is considered that the aberration generated due to the focal position shift needs to be suppressed to 0.03λ or less in rms wavefront aberration value. That is, in this optical pickup, the focus position deviation allowed for the autofocus mechanism is about ± 0.1 μm. If the focal position shifts beyond this value, spot aberrations will increase and sufficient recording / reproducing characteristics cannot be obtained.

Next, FIG. 5 shows a change in the optimum focus position with respect to the wavelength of the objective lens. The optimal focus position is rm
This is the position where the s wavefront aberration is minimized. Although the optimum focus position fluctuates as the wavelength fluctuates, the aberration when the autofocus mechanism follows the focus position so that the recording surface of the optical disc coincides with the focus position is 0.03λ.
Since this value is considerably lower than that of, the influence on the recording and reproducing characteristics is small. However, if the autofocus mechanism cannot follow the fluctuation of the optimum focus position, a position shift occurs between the optimum focus position and the recording surface of the optical disc. Even if the wavelength fluctuates by 1 nm as shown in FIG.
The aberration is much larger than 3λ.

The wavelength fluctuation of the semiconductor laser occurs when the temperature of the operating environment or the emission intensity of the semiconductor laser changes.
Generally, at an operating temperature of 25 ° C., the wavelength changes by about ± 0.07 nm when the temperature changes by 1 ° C., and the wavelength changes by about ± 0.04 nm when the emission intensity changes by 1 mW. The semiconductor laser of the recordable optical pickup has a recording power of 30 mW
Pulse emission with a peak power of about 50 mW is performed. Therefore, the wavelength instantaneously changes around 1.2 nm to 2 nm. When a wavelength change of 2 nm occurs instantaneously, FIG.
According to the result of (1), in the objective lens with the NA of 0.85, the value of 0.1 was obtained.
A focus position shift of about 35 μm occurs. This value is considerably larger than the allowable value of the focus shift of 0.1 μm. The auto-focus mechanism of the optical disc cannot follow such an instantaneous focus position shift. Therefore, the wavelength 4
In order to commercialize a high-density optical pickup using a blue semiconductor laser having a numerical aperture of about 0.85 and a blue semiconductor laser having a numerical aperture of about 0.85, even if the wavelength of the semiconductor laser fluctuates, the focal position shift becomes 0.1 μm or less. It is necessary to make such an optical system.

In order to realize the optical system, the optical pickup of the present invention employs a chromatic aberration correcting element 70 as shown in FIG.
1 has been introduced. This chromatic aberration correction element is one in which a diffractive optical element is formed on a plane portion of a plano-concave lens indicated by reference numeral 203. The glass material is FCD1, the focal length of the diffractive optical element is 40 mm (at a wavelength of 410 nm), the surface interval is 2 mm, the radius of curvature of the concave lens is 0.0510322 (1 / mm), and the focal length of the concave lens portion is -40 mm. .

FIG. 8 shows the shape of the diffractive optical element.
The incident light of the blue semiconductor laser enters from the direction indicated by 803. 802 indicates the surface shape of the diffractive optical element,
Reference numeral 801 denotes a glass material portion. As shown in the figure,
This diffractive optical element is blazed so that the diffraction efficiency is concentrated on the first-order diffracted light. The depth of the diffractive optical element is 80
4 is based on the design incident light wavelength λ and the refractive index n of the glass material,
λ / (n−1). In the case of this embodiment, λ
= 410 nm and n (FCD1) = 1.5067,
The depth d is 0.81 μm.

Between the focal length f of the diffractive optical element and the pitch P of the diffractive optical element (the period of the repetitive structure), when the radial position of the diffractive optical element is h, P (h) = f
× λ / h holds. In the case of the diffractive optical element of this embodiment, since the effective light beam diameter is 2.98 mm, the outermost repetition pitch is about 10.93 μm.
FIG. 9 shows the relationship between the radial position and the pitch of the diffractive optical element of this embodiment.

In the case of a refractive lens having a positive refractive power, such as a general glass convex lens or a plastic convex lens, the shorter the wavelength, the higher the refractive index of the lens glass material.
As a result, the focal length of the lens is shortened. On the other hand, in the case of a diffractive optical element, the characteristic shows that the focal length becomes longer as the wavelength becomes shorter. This is because as the wavelength becomes shorter, the diffraction angle becomes smaller, and as a result, the focal length becomes longer. Since the objective lens is a glass lens having a positive refractive power, a change in the focal length due to wavelength fluctuation can be canceled by combining the objective lens with a diffractive optical element. Plano-concave lenses are
Since the objective lens is designed to be an infinite system, that is, light incident on the objective lens is designed so that a parallel light beam is incident, the light is inserted so as to become a parallel light beam at a design wavelength (410 nm in this case).

FIG. 10 shows the calculation result of the optimum focus position with respect to wavelength fluctuation when this chromatic aberration correction element is arranged at a position 10 mm before the objective lens. from this result,
It can be seen that even when a wide range of wavelength variation from −5 nm to +8 nm is caused by the chromatic aberration correction element, the shift of the optimum focus position is suppressed to ± 0.1 μm or less. FIG. 11 shows the calculated aberration of the optical system including the chromatic aberration correction element and the objective lens. The amount of aberration itself caused by the wavelength fluctuation is suppressed to 0.03λ or less when the wavelength fluctuation is in the range of −4 nm to +8 nm. 2n when recording
Even if the wavelength fluctuation of m occurs instantaneously and the autofocus mechanism cannot follow, the positional deviation between the optimum focus position and the recording surface of the optical disk is about 0.04 μm, and the wavefront aberration value is also 0.015λ or less. By using this chromatic aberration correction element, good recording / reproducing characteristics can be obtained.

Next, the influence of the arrangement position of the chromatic aberration correction element was calculated. FIG. 12 shows the arrangement position of the chromatic aberration correction element 50 mm, 20 mm from the first surface of the objective lens.
The rms wavefront aberration with respect to the wavelength variation is calculated under the conditions of 10 mm, 5 mm, and 0 mm.
As can be seen from the results of FIG. 12, even if the arrangement position of the chromatic aberration correction element changes, the characteristics do not change much. Since sufficient characteristics are obtained even when the distance is set to 0 mm, it is also possible to integrate this chromatic aberration correction element with the objective lens and drive it with an actuator. In particular, since the chromatic aberration correction element used in the optical pickup of the present embodiment has a diffractive optical element on the plane portion of the plano-concave lens,
It is lighter than an achromatic doublet lens, and is advantageous in terms of speeding up when it is mounted on an actuator together with an objective lens. By driving integrally with the objective lens in this manner, the occurrence of aberration due to the shift of the objective lens is suppressed, so that better recording / reproducing characteristics can be obtained.

Further, the focal length of the diffractive optical element is set to 45 m
The effects of the chromatic aberration correcting element when the length was m, 40 mm, 35 mm, and 30 mm were examined by ray tracing. The focal lengths of the concave lenses at this time are -45 mm and -40 m, respectively.
m, −35 mm, and −30 mm, and the curvatures are 0.0451896 (1 / mm) and 0.0510322, respectively.
(1 / mm), 0.0573924 (1 / mm), 0.
0688304 (1 / mm), the surface spacing is 2 mm, and the glass material is FCD1. Each result is shown in FIG. The upper diagram in FIG. 13 shows the calculation results for the rms wavefront aberration, and the lower diagram shows the change in the optimum focus position. From this result, it can be seen that when the focal length of the diffractive optical element is 30 mm or less or 45 mm or more, the focal position shift becomes ± 0.1 μm due to the wavelength variation of 2 nm. That is, the focal length of the diffractive optical element of the chromatic aberration correction element used in the optical pickup of the present invention may be in the range of 30 mm or more and 45 mm or less. In this range, as can be seen from the upper diagram of FIG. 13, the wavelength variation of 2 nm satisfies approximately 0.03λ or less.

FIG. 14 shows an embodiment of an optical disk apparatus using the optical pickup. The optical disk device of the present invention performs recording and reproduction by an optical pickup 1401 provided with the chromatic aberration correction element. This optical pickup is moved in the track direction by a servo mechanism 1403,
The light spot is moved to a target track of the optical disk medium 1402. The optical disk medium 1402 is rotated by a spindle motor 1404. Servo mechanism 1403,
The spindle motor 1404 is controlled and driven by a control circuit. A reproduction signal obtained from the optical pickup is demodulated by a signal demodulator and output as user data. The recording data is once modulated by a signal modulator, and finally converted into a recording pulse waveform of a laser driver. The laser emits pulse light by driving the laser driver. By this pulse emission, data is recorded on the recording surface of the optical disk. At the time of recording, a pulse emission of 30 mW to 50 mW at peak power is performed,
At this time, the wavelength of the laser fluctuates about 2 nm instantaneously.
In the optical disk device of the present invention, the optical pickup equipped with the chromatic aberration correction element described in detail in the embodiment of the optical pickup is used. Is about 0.1 μm, and is suppressed with an aberration amount that does not affect the recording / reproducing characteristics, so that good recording / reproducing characteristics can be obtained.

[0032]

According to the present invention, there is provided an optical pickup and an optical disk apparatus capable of removing a focal position shift due to a wavelength variation of a semiconductor laser during recording and reproduction and capable of recording and reproducing a high-density optical disk of 20 to 25 GB on one side. Can be realized.

[Brief description of the drawings]

FIG. 1 is an optical system of an optical pickup according to the present invention.

FIG. 2 is a sectional view of a chromatic aberration correction element used in the optical pickup of the present invention.

FIG. 3 shows a numerical aperture of 0.8 used for the optical pickup of the present invention.
5 is the sectional shape of the objective lens and the lens shape factor.

FIG. 4 shows deterioration of wavefront aberration with respect to a focal position shift of an objective lens having a numerical aperture of 0.85.

FIG. 5 is a diagram showing a change in an optimum focus position due to a wavelength change of an objective lens having a numerical aperture of 0.85.

FIG. 6 shows deterioration of wavefront aberration due to wavelength fluctuation of an objective lens having a numerical aperture of 0.85.

FIG. 7 is an optical system in which a chromatic aberration correcting element of the optical pickup of the present invention is introduced.

FIG. 8 is a sectional view of a diffractive optical element of the chromatic aberration correction element.

FIG. 9 shows the relationship between the radial position of the diffractive optical element portion of the chromatic aberration correction element and the pitch interval.

FIG. 10 shows a change in an optimum focus position due to a wavelength change when a chromatic aberration correction element is introduced.

FIG. 11 shows deterioration of wavefront aberration due to wavelength fluctuation when a chromatic aberration correction element is introduced.

FIG. 12 shows a change in a chromatic aberration correction effect depending on an arrangement position of a chromatic aberration correction element.

FIG. 13 shows deterioration of wavefront aberration due to wavelength fluctuation when the focal length of the diffractive optical element portion of the chromatic aberration correction element is changed (upper figure) and change in the optimum focal position due to wavelength fluctuation (lower figure).

FIG. 14 is a configuration diagram of an optical disk device of the present invention.

[Explanation of symbols]

101. Blue-violet semiconductor laser 102. Collimating lens 103. Beam shaping prism 104. Polarization beam splitter 105.4 quarter wave plate 106. Chromatic aberration correction element 107. Objective lens 108. Optical disk medium 109. Beam splitter 110. Condensing lens 111. Cylindrical lens 112.4 split detector 113. Condensing lens 114.2 split detector 115. Disc radial direction 116. Disk rotation direction (recording track direction) Diffractive optical element configured as a parallel plate 202. Plano-concave lens 203. 301. Special plano-concave lens in which a diffractive optical element is formed on a plane portion of the plano-concave lens Objective lens 302. Optical disk medium 701. Chromatic aberration correction element 801. Glass part of diffractive optical element 802. 803. Surface shape of diffractive optical element Semiconductor laser beam incidence method 804. Depth of diffractive optical element 1401. Optical pickup 1402 described in the embodiment. Optical disk medium 1403. Servo mechanism 1404. Spindle motor.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H087 KA13 LA01 MA04 NA14 PA02 PA17 PB02 QA02 QA05 QA12 QA21 QA33 QA41 RA05 RA12 RA46 5D119 AA11 AA23 BA01 BB01 BB03 DA01 DA05 EC03 EC08 FA02 FA05 JA09 JB01 BB08 KK08 KK08

Claims (8)

[Claims] 1. An optical pickup including a blue semiconductor laser and an objective lens, wherein a chromatic aberration correcting element constituted by a combination of a concave lens and a diffractive optical element is provided in an optical system between the blue semiconductor laser and the objective lens. An optical pickup characterized by being made. 2. The optical pickup according to claim 1, wherein the chromatic aberration correcting element is an optical element in which plane portions of a diffractive optical element configured as a plano-concave lens and a parallel plate are adhered to each other. pick up. 3. An optical pickup according to claim 1, wherein said chromatic aberration correction element is a special plano-concave lens in which a diffractive optical element is formed on a plane portion of a plano-concave lens. 4. The optical pickup according to claim 2, wherein said diffractive optical element has a focal length of 30 mm.
An optical pickup, wherein the focal length of the concave lens is a negative value of the focal length of the diffractive optical element. 5. An optical disc apparatus having an optical pickup including a blue semiconductor laser and an objective lens, wherein a chromatic aberration correction element formed by a combination of a concave lens and a diffractive optical element is provided between the blue semiconductor laser and the objective lens. An optical disk device comprising an optical pickup provided in an optical system. 6. The optical pickup according to claim 5, wherein the chromatic aberration correcting element of the optical pickup is an optical element in which planar portions of a diffractive optical element formed of a plano-concave lens and a parallel plate are adhered to each other. An optical disk device comprising: 7. An optical disk device according to claim 5, wherein said chromatic aberration correcting element of said optical pickup has an optical pickup which is a special plano-concave lens having a diffractive optical element formed on a plane portion of a plano-concave lens. Optical disk device. 8. The optical disc device according to claim 6, wherein the focal length of the diffractive optical element of the optical pickup is 30 mm or more and 45 mm or less, and the focal length of the concave lens is the focal length of the diffractive optical element. An optical disc device comprising an optical pickup including a chromatic aberration correction element having a negative distance value.