JP4375108B2 - Optical head device - Google Patents

Description

  The present invention relates to an objective lens and an optical head device for writing information on an optical disc or reading information from an optical disc (hereinafter, writing information on an optical disc or reading information from an optical disc is referred to as “recording / reproducing of an optical disc”). In particular, the present invention relates to an objective lens and an optical head device for performing recording / reproduction of an optical disc using light beams having at least two different use wavelengths.

  2. Description of the Related Art Conventionally, optical disks known as CDs and DVDs are widely used for storing music information, video information, etc., recording data output from a computer, and the like. With the advancement of advanced information technology in recent years, demands for larger capacity and higher density of optical discs are increasing.

  In order to achieve high recording density on the optical disc, it is necessary to reduce the spot diameter of the light beam collected through the objective lens (hereinafter referred to as spot diameter). In general, this spot diameter is known to be proportional to the wavelength of the light beam used (hereinafter referred to as the used wavelength) and inversely proportional to the numerical aperture (NA) of the optical system. In this case, the wavelength used may be shortened or the numerical aperture of the optical system may be increased.

  Densification from CD to DVD is realized by shortening the operating wavelength from about 780 nm to about 650 nm and increasing the numerical aperture of the optical system from 0.45 to 0.60 or 0.65. Yes. In order to further increase the density in the future, an optical head device using a light beam having a wavelength shorter than 440 nm as a light source has been studied.

These optical head devices are required to be able to record / reproduce conventional optical disks. In order to meet the above requirement, that is, to record / reproduce an optical disk using light beams having a plurality of use wavelengths, aberrations are minimized with respect to one of the two use wavelengths. In addition to the objective lens, there has been a method of providing another element (hereinafter referred to as an aberration correction element) such as a phase correction element that corrects the phase of the luminous flux having the remaining working wavelength and corrects the aberration or the like (hereinafter referred to as an aberration correction element). Patent Document 1).
JP 2003-177226 A

  However, such a conventional technique for recording / reproducing an optical disc has a problem that new aberration is generated due to decentration generated between the objective lens and the aberration correction element. To avoid this, even if the objective lens and the aberration correction element are decentered and held together, the integrated holding increases the weight of the element that is the target of the tracking servo or focus servo, and the above servo The load increases and becomes a problem. This is due to the design philosophy of optimizing the optical characteristics of the objective lens and minimizing aberrations off the optical axis for one used wavelength for recording / reproducing of the optical disk.

  The present invention has been made to solve such problems, and it is necessary to optimize the optical characteristics of the objective lens so as to minimize even aberrations off the optical axis for one wavelength used. In addition, an objective lens and an optical head device capable of recording / reproducing an optical disk using one or two used wavelengths are provided.

Considering the above points, the invention according to claim 1 is capable of recording / reproducing with respect to the first optical disc using a light beam having a wavelength of 390 nm to 430 nm and having a first use wavelength λ _1. recording and reproducing for the second optical disk is an optical head device capable to emit the first light flux of the wavelength used lambda ₁ of using but a second light flux of the wavelength used lambda ₂ of 680nm from 630nm a first light source, an objective for focusing on the second and the second light source emitting a light flux with wavelength used lambda _2, the optical disc determined in advance in accordance with the light beam emitted from the light source to the wavelength used comprising a lens, and a light receiving element for receiving the light beam reflected from the optical disc, the objective lens has two surfaces intersecting with an optical axis thereof, said using the first light flux of the wavelength used lambda ₁ of the first record for the first optical disk When performing raw numerical aperture of 0.6 or more, the wave front aberration on the optical axis RMS with third order off-axis coma in the position of the image height 0.05mm is 0.5 lambda ₁ or more in RMS value It becomes 0.03 ₁ or less in value, and the recording and reproducing on the second optical disk by using the second light flux of the wavelength used lambda ₂ when performing a numerical aperture of 0.6 or more, the image height 0.05mm And a third-order off-axis coma aberration at the position of 0.52 is an RMS value of 0.5λ ₂ or more, and the wavefront aberration on the optical axis is an aspherical shape having an RMS value of 0.03λ ₂ or less. ing.

With this configuration, the shape of the two surfaces intersecting the optical axis of the objective lens has an off-axis coma aberration at an RMS value of 0.5 wavelength or more when the optical disk is recorded / reproduced. As a result, the degree of freedom in designing the objective lens can be improved, and it is necessary to optimize the optical characteristics of the objective lens so that aberrations outside the optical axis are minimized for one wavelength used. Thus, an optical head device that enables recording / reproduction of an optical disk can be realized.
The shape of the two surfaces intersecting the optical axis of the objective lens with respect to the first use wavelength and the second use wavelength is off-axis at an image height of 0.05 mm when recording / reproducing an optical disc. Since the coma aberration is 0.5 or more wavelengths in terms of RMS value, it is necessary to optimize the optical characteristics of the objective lens so as to minimize even aberrations off the optical axis for one wavelength used. In addition, it is possible to realize an optical head device that enables recording / reproduction of an optical disk using two used wavelengths.
The objective lens has two surfaces intersecting the optical axis when the first type optical disc is recorded / reproduced with a numerical aperture of 0.6 or more using a light beam having the first wavelength used. Since the upper wavefront aberration has an aspherical shape with an RMS value of 0.03 wavelength or less, optimization of the aberration at the optical axis for recording / reproducing of the optical disc with respect to the first use wavelength is performed. An optical head device that can be realized can be realized.
In addition, the objective lens has an optical axis whose two surfaces intersecting the optical axis are used when recording / reproduction of the second type optical disc with a numerical aperture of 0.6 or more using a light beam having the second used wavelength. Since the upper wavefront aberration has an aspherical shape with an RMS value of 0.03 wavelength or less, the aberration at the optical axis for recording / reproducing the optical disk is optimized for the second wavelength used. An optical head device that can be realized can be realized.

The invention according to claim 2, in claim 1, wherein the first is the use wavelength lambda ₁ is 405 nm, the second wavelength used lambda ₂ may have a configuration Ru 660nm der.

  In the present invention, the off-axis coma aberration at the position where the image height is 0.05 mm is 0.5 wavelength or more in terms of the RMS value when recording / reproducing an optical disk with the shape of the two surfaces intersecting the optical axis of the objective lens. As a result, the degree of freedom in designing the objective lens can be improved, and it is necessary to optimize the optical characteristics of the objective lens so that aberrations outside the optical axis are minimized for one wavelength used. An objective lens and an optical head device that enable recording / reproduction of an optical disc without any problem can be provided.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a diagram showing a conceptual configuration of an optical head device according to an embodiment of the present invention. 1, the optical head device 100 includes a first light source 1, 2 second light source that emits a light beam of the used wavelength lambda _2, the first beam splitter 3 that emits a light beam using a wavelength lambda _1, the second beam splitter 4, a collimator 5, a reflector 6, an objective lens 7, and a light receiving element 10.

  The light beam emitted by the first light source 1 sequentially passes through the first beam splitter 3, the second beam splitter 4, and the collimator 5, is reflected by the reflecting plate 6, passes through the objective lens 7, and passes through the first lens 7. The light is condensed on the information recording surface of the optical disk 8. Similarly, the light beam emitted by the second light source 2 is condensed on the information recording surface of the second optical disk 9. The light beams condensed on the information recording surface of the first optical disk 8 or the second optical disk 9 are reflected by each information recording surface, transmitted through the objective lens 7, reflected by the reflecting plate 6, and collimator 5. , Is reflected by the second beam splitter 4 and enters the light receiving element 10.

  An output signal from the light receiving element 10 is used to obtain a read signal, a focus error signal, and a tracking error signal for information recorded on the information recording surface of the first optical disc 8 or the information recording surface of the second optical disc 9. It is supposed to be. The optical head device 100 includes a mechanism (focus servo) for moving the lens in the optical axis direction based on the focus error signal, and the lens in a direction substantially perpendicular to the optical axis based on the tracking error signal. A moving mechanism (tracking servo) is included, but is omitted in the configuration shown in FIG.

  The first light source 1 is composed of, for example, a semiconductor laser, and emits a divergent light beam with linearly polarized light at a wavelength in the vicinity of 405 nm. Similarly, the second light source 2 is composed of, for example, a semiconductor laser, and emits a divergent light beam with linear polarization at a wavelength near 655 nm. Here, the first light source 1 and the second light source 2 are arranged so that the polarization direction of the light beam from the first light source 1 and the polarization direction of the light beam from the second light source 2 are orthogonal to each other. ing.

  The first beam splitter 3 has a role of transmitting the light beam from the first light source 1 and reflecting the light beam from the second light source 2 so as to guide them onto the same optical axis. The second beam splitter 4 transmits the light beams from the first light source 1 and the second light source 2 and reflects the light beams from the information recording surface of the first optical disk 8 and the information recording surface of the second optical disk 9. The light is reflected and guided to the light receiving element 10.

  The collimator 5 has a role of converting light beams from the first light source 1 and the second light source 2 into substantially parallel light. Details of the objective lens 7 will be described later. The light receiving element 10 receives reflected light from the information recording surfaces of the optical discs 8 and 9, and based on the received reflected light, a read signal and a focus error corresponding to information recorded on the information recording surfaces of the optical discs 8 and 9 Signals and tracking error signals are generated and output to the outside.

  FIG. 2 is a sectional view showing a sectional structure of the objective lens 7 constituting the optical head device 100 according to the embodiment of the present invention. In FIG. 2, the objective lens 7 has an aspherical shape on both surfaces of the lens, the wavelength of the light beam from the first light source (hereinafter referred to as the first used wavelength, 405 nm), and It is designed so that off-axis coma aberration occurs with respect to both the wavelength of the light beam from the second light source (hereinafter referred to as the second use wavelength, which is 660 nm). As will be described below, by generating off-axis coma aberration, it is possible to satisfactorily correct the on-axis wavefront aberration for both the first use wavelength and the second use wavelength.

  Hereinafter, the objective lens 7 will be described using specific examples.

  A laser light source having a wavelength of 405 nm is used as a light source for a light beam having a first use wavelength, and a laser light source having a wavelength of 660 nm is used as a light source for a light beam having a second use wavelength. Here, the transparent substrate of the first optical disk 8 is a polycarbonate having a thickness of 0.6 mm, and the transparent substrate of the second optical disk 9 is a polycarbonate having a thickness of 0.6 mm, and the shape of the objective lens is determined. The numerical aperture was set to 0.65 for both the first use wavelength and the second use wavelength.

  Table 1 is a table showing the focal length f of the objective lens 7 and the refractive index n of the lens material of the objective lens 7. Here, λ is the center wavelength of the used wavelength (first used wavelength 405 nm, second used wavelength 660 nm), and the focal length f is relative to the first used wavelength and the second used wavelength, respectively. 3.00 mm and 3.09 mm, and the refractive index n is 1.5208 and 1.5057, respectively, for the first use wavelength and the second use wavelength. The lens center thickness is 1.87 mm.

  The aspheric shape of the collimator 5 was determined based on the following formula (1).

Here, the symbol i is an aspherical order and takes one of 2, 4, 6, 8, and 10. Further, the symbol j is a surface number, the surface on the light source side (surface 71 shown in FIG. 2) is surface number 1, and the surface on the optical disk side (surface 72 shown in FIG. 2) is surface number 2. Further, symbol h is a distance from the optical axis 73 (hereinafter referred to as height), and symbol z _j is a tangent plane of the apex of the j-th surface aspheric surface (generally, in contact with the aspheric surface in the vicinity of the optical axis. To the point of the height h on the aspheric surface in the optical axis direction, and r _j , k _j , a _{i, j} , and j are coefficients for determining the j-th surface It is.

  Table 2 shows the values of the coefficients used in the above equation (1).

In Table 2, “Ep (p is an integer)” means “× 10 ^−p ”.

  Next, the off-axis characteristics of the objective lens will be described. FIG. 3 is a graph showing off-axis characteristics with respect to the first use wavelength (405 nm) of the objective lens 7. The horizontal axis of the graph shown in FIG. 3 is the image height (in mm) indicating the distance from the optical axis, and the vertical axis represents the wavefront aberration in units of wavelength λ and RMS value. In FIG. 3, the solid line indicates wavefront aberration (hereinafter referred to as total wavefront aberration) including all types of aberration, and the broken line indicates third-order coma aberration. Hereinafter, the same applies to FIGS. 4 to 6 unless otherwise specified. When the horizontal axis (image height) shown in FIG. 3 is 0 mm, the value of the total wavefront aberration is 0.008λ in RMS value, which is sufficiently small for recording / reproducing.

  In order to reduce the axial aberration value with respect to the second wavelength used (660 nm), the value of the third-order coma aberration at an image height of 0.05 mm was set to 0.907λ as an RMS value.

  FIG. 4 is a graph showing off-axis characteristics of the objective lens 7 with respect to the second wavelength used (660 nm). When the horizontal axis (image height) shown in FIG. 4 is 0 mm, the value of the total wavefront aberration is 0.013λ in RMS value, which is sufficiently small for recording / reproducing. Further, for example, most of the total wavefront aberration off-axis with an image height of 0.05 mm is the third-order coma aberration, and its value is 0.660λ in RMS value.

  For comparison between the parameters shown in Table 1 and the objective lens (hereinafter referred to as an example objective lens) specified by the mathematical formula shown in Formula (1) and the shape specified in Table 2, An objective lens having the same numerical aperture and the same focal length as this objective lens, and having an off-axis coma aberration substantially zero with respect to the first wavelength used (405 nm) (hereinafter referred to as a comparative objective lens). The aberration at the second use wavelength (660 nm) was calculated. Table 3 shows coefficients representing the aspherical shape of a comparative objective lens designed for comparison.

  FIG. 5 is a graph showing off-axis characteristics with respect to the first use wavelength (405 nm) of the comparative objective lens. When the horizontal axis (image height) shown in FIG. 5 is 0 mm, the value of the total wavefront aberration is 0.0009λ in RMS value, which is sufficiently small for recording / reproducing. The third-order coma aberration at an image height of 0.05 mm is an RMS value of 0.0003λ, which is smaller than that of the above-described example.

  FIG. 6 is a graph showing off-axis characteristics with respect to the second use wavelength (660 nm) of the comparative objective lens. In the comparative objective lens designed to reduce the off-axis coma aberration at the first use wavelength (405 nm), the RMS value of the on-axis aberration at the second use wavelength different from the first use wavelength increases. It has been shown that recording / reproduction using a light beam having the second used wavelength becomes difficult.

  As described above, in the objective lens and the optical head device according to the embodiment of the present invention, when the shape of the two surfaces intersecting the optical axis of the objective lens is recorded / reproduced on an optical disc, the image height is 0. Since the off-axis coma aberration at the position of 05 mm is 0.5 wavelength or more in terms of the RMS value, the degree of freedom in designing the objective lens can be improved, and even for off-axis aberrations for one wavelength used. The optical disk can be recorded / reproduced without the need to optimize the optical characteristics of the objective lens so as to minimize.

  The shape of the two surfaces intersecting the optical axis of the objective lens with respect to the first use wavelength and the second use wavelength is off-axis at an image height of 0.05 mm when recording / reproducing an optical disc. Since the coma aberration is 0.5 or more wavelengths in terms of RMS value, it is necessary to optimize the optical characteristics of the objective lens so as to minimize even aberrations off the optical axis for one wavelength used. In addition, the optical disk can be recorded / reproduced using two used wavelengths.

  In addition, when two surfaces intersecting the optical axis of the objective lens perform recording / reproduction of the first type of optical disk with a numerical aperture of 0.6 or more using a light beam having the first used wavelength, the optical axis of the objective lens Since the wavefront aberration has an aspherical shape with an RMS value of 0.03 wavelength or less, the optimization of the aberration at the optical axis for recording / reproducing of the optical disk with respect to the first use wavelength is to be achieved. Can do.

  Further, when two surfaces intersecting the optical axis of the objective lens perform recording / reproduction of the first type optical disc with a numerical aperture of 0.6 or more using light beams having the first and second wavelengths, the light is used. Since the on-axis wavefront aberration has an aspherical shape with an RMS value of 0.03 wavelength or less, optimization of aberrations at the optical axis for recording / reproducing optical discs for two wavelengths used is possible. Can be planned.

  In the above description, an example in which a light source having a wavelength of 405 nm is used as the light source has been described. However, the present invention is not limited to this wavelength, and a light source having a wavelength of about 390 nm to 430 nm is used. An effect is obtained.

In consideration of compatibility with the optical system for DVD, the second wavelength used has been described as being a 660nm, or in light having any wavelength of between 630nm instead of 660nm to 680 nm. Further, considering compatibility with the optical system for CD, a light source having any wavelength between 760 nm and 820 nm may be used.

  The objective lens 7 has been described as a lens for both wavelengths of 405 nm and 665 nm. However, the objective lens 7 may be a lens for performing recording / reproduction using a light beam having a wavelength of only 405 nm. Moreover, although the example used by the structure which makes what is called an infinite optical system using the collimator 5 was demonstrated, this invention is not necessarily restricted to application in an infinite system, The so-called collimator 5 is not used. It may be configured to form a finite optical system.

  Further, although the thickness of the protective layer of the optical disk that performs recording / reproduction using a light beam having a wavelength of 405 nm is 0.6 mm, the present invention is not limited to this. Moreover, although the case where a synthetic resin is used as the material of the objective lens has been described, the present invention is not limited to the synthetic resin, and glass may be used. In particular, the objective lens of the present invention does not need to retain the functions of a phase shifter, a diffraction grating, etc., and the objective lens can be formed in a smooth shape as described above, so that it can be easily formed by simply press-molding glass. It is suitable for glass lenses.

  The objective lens and the optical head device according to the present invention do not require optimizing the optical characteristics of the objective lens so as to minimize even the aberration off the optical axis for one use wavelength. Alternatively, the present invention can be applied to applications such as an objective lens and an optical head device that have the advantage of being able to record / reproduce an optical disk using two used wavelengths.

The figure which shows the notional structure of the optical head apparatus which concerns on embodiment of this invention. Sectional drawing which shows the cross-section of the objective lens 7 which comprises the optical head apparatus based on Embodiment of this invention The graph which shows the off-axis characteristic with respect to the 1st use wavelength of an objective lens The graph which shows the off-axis characteristic with respect to the 2nd use wavelength of an objective lens The graph which shows the off-axis characteristic with respect to the 1st use wavelength of a comparison objective lens The graph which shows the off-axis characteristic with respect to the 2nd use wavelength of a comparison objective lens

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 2 Light source 3, 4 Beam splitter 5 Collimator 6 Reflecting plate 7 Objective lens 71 Light source side surface 72 Optical disc side surface 73 Optical axis 8, 9 Optical disc 10 Light receiving element 100 Optical head device

Claims (2)

Recording / reproduction can be performed on the first optical disk using a light beam having a _first use wavelength λ _{1 having} a wavelength of 390 nm to 430 nm, and a light beam having a second use wavelength λ ₂ having a wavelength of 630 nm to 680 nm is used. An optical head device capable of recording / reproducing with respect to the second optical disc ,
A first light source that emits a light beam having the first use wavelength λ ₁ ;
A second light source that emits a light beam having the second use wavelength λ ₂ ;
An objective lens for condensing the light beam emitted from the light source onto the optical disc determined in advance according to the use wavelength;
A light receiving element for receiving a light beam reflected from the optical disc,
The objective lens, when two surfaces intersecting with the optical axis thereof, recording and reproducing for the first optical disc performed by the numerical aperture of 0.6 or more using the first light flux of the wavelength used lambda _1, The third-order off- axis coma aberration at an image height of 0.05 mm is an RMS value of 0.5 λ ₁ or more, and the wavefront aberration on the optical axis is an RMS value of 0.03λ ₁ or less. In the case where recording / reproduction with respect to the second optical disk is performed at a numerical aperture of 0.6 or more using a light beam having a working wavelength λ ₂ of 2, the third-order off-axis coma aberration at an image height of 0.05 mm is an RMS value. in the optical head device comprising a Turkey which have a non-spherical shape the wavefront aberration on the optical axis is 0.03 ₂ or less in RMS value with a 0.5 [lambda ₂ or more. The first used wavelength lambda ₁ is 405 nm, the second wavelength used lambda ₂ is an optical head device according to claim 1 Ru 660nm der.

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