JP4014319B2 - Optical pickup device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical pickup device having two or more light sources and capable of reproducing or recording / reproducing at two or more different wavelengths.
[0002]
[Prior art]
2. Description of the Related Art In recent years, various types of optical recording media having different recording densities, substrate thicknesses, substrate materials, recording / reproducing systems, disk sizes, and the like have been provided as the application range of optical recording media has expanded. Therefore, in addition to the basic functions such as recording and reproduction, the optical pickup device is required to have high versatility that can handle different types of media.
[0003]
In the following, a configuration necessary for handling optical recording media having different recording densities, substrate thicknesses, and the like in a conventional optical pickup device and its problems will be described with reference to the drawings. In the present specification, the substrate means an optical medium that exists between the recording surface and the light source and affects incident light. For example, when a protective layer made of resin or the like is provided on the recording surface and recording light or reproducing light is incident through the protective layer, the protective layer functions as a substrate in this specification.
[0004]
FIG. 10 is a schematic diagram showing the configuration of a conventional optical pickup device.
[0005]
In this optical pickup device, the light beam emitted from the semiconductor laser 3 is separated into a main beam (for a focus error signal and a recording / reproduction signal) and a sub beam (for a tracking error signal) by a diffraction grating 5, and then a collimator lens. 6 is incident. The light beam incident on the collimator lens 6 is converted from divergent light into parallel light and incident on the beam splitter 7. The light beam incident on the beam splitter 7 is reflected in the direction of the objective lens 9 and enters the objective lens 9. The light beam incident on the objective lens 9 is changed from parallel light to convergent light, and a spot is formed on the recording surface of the optical recording medium 10.
[0006]
This light beam is reflected by the recording surface, and is modulated in accordance with the recording information on the recording surface. The light beam reflected by the recording surface is converted into parallel light again by the objective lens 9 and enters the beam splitter 7. The light beam incident on the beam splitter 7 travels straight through the beam splitter 7 and enters the condenser lens 11. The light beam incident on the condenser lens 11 is converted from parallel light into convergent light and incident on the anamorphic lens 12. The light beam incident on the anamorphic lens 12 generates astigmatism for detecting a focus error signal and enters the light receiving element 13. The light beam incident on the light receiving element 13 is converted into an electrical signal such as a tracking error signal, a focus error signal, and a recording / reproducing signal.
[0007]
Here, in order to obtain an accurate electric signal, it is necessary to form an appropriate spot on the recording surface. For this purpose, it is necessary to sufficiently reduce the aberration of the light beam. Hereinafter, aberration correction for reducing the aberration of the light beam and its problems will be described.
[0008]
FIG. 11 is an explanatory diagram showing a light beam that passes through the substrate 40 of the optical recording medium and reaches the recording surface.
[0009]
The optical recording medium shown in the figure includes a substrate 40 and a recording surface 41. For example, polycarbonate is used for the substrate 40, and a film that reflects light such as aluminum is used for the recording surface 41.
[0010]
The thickness of the protective layer 40 is, for example, about 1.2 mm, and the surface of the recording surface 41 is provided with fine irregularities (pits) having a depth of 120 nm and a width of about 0.5 μm.
[0011]
When an appropriate spot 42 is imaged on the recording surface 41 of this optical recording medium, the light beam is narrowed down to the diffraction limit. The spot diameter is about 1.4 μm, for example. In such an optical system that narrows the light beam to the diffraction limit, it is necessary to sufficiently reduce the wavefront aberration of the light beam, and a standard called a Marechal standard is generally known. According to this standard, the amount of wavefront aberration is 0.07λ RMS.
[0012]
Here, the fact that the wavefront aberration of the light beam contributing to the imaging is small means that the wavefront, that is, the equiphase surface of the light beam (the surface indicated by the curve connecting the points a and a ′) is centered on the spot 42. It is that it has become a spherical surface.
[0013]
However, when the optical recording medium is irradiated with a light beam that is not properly corrected for aberrations, the equiphase surface of the light beam in the substrate 40 shown in FIG.
[0014]
Thus, the reason why the equiphase surface of the light beam outside the substrate 40 does not have the same curvature as the equiphase surface in the substrate 40 is that the refractive index of the substrate 40 (for example, 1.55 for polycarbonate) and air. This is because the speed of the light beam traveling in the substrate 40 is different from the speed of the light beam traveling in the air (vacuum) due to the difference in refractive index of (vacuum).
[0015]
That is, the speed of the light beam traveling in the substrate 40 having a refractive index of 1.55 (speed between dc) is slower than the speed of the light beam traveling in air (vacuum) (speed between cb ″). The lengths of the line segment bd (or line segment b′d) and the line segment b ″ d having the same optical path length are not equal.
[0016]
As described above, when convergent light passes through a parallel plate having a refractive index different from that in air or in vacuum, an aberration (hereinafter referred to as an ideal wavefront passing in air or in vacuum). , Expressed as spherical aberration). Therefore, when designing an optical pickup device, it is necessary to perform aberration correction in consideration of this spherical aberration. More specifically, in the optical system, the amount is the same as that of the spherical aberration generated in the substrate 40 but opposite in direction. It is necessary to give aberration (correcting spherical aberration) in advance. FIG. 11 shows an equiphase surface of the wavefront in the system subjected to such aberration correction.
[0017]
By the way, with the diversification of optical recording media as described above, the thickness of the substrate tends to be reduced particularly in an optical recording medium having a high recording density. The reason why the thickness of the substrate is reduced as the density of the optical recording medium is increased is as follows.
[0018]
Generally, when the density of an optical recording medium is increased, the spot diameter required for reading is also reduced. Also, the incident light is a plane wave with ideal astigmatism and uniform intensity distribution, and the beam waist is 1 / e of the center intensity.²The spot radius R is
Formula IR = 0.41λ / NA
Given in. In the above formula I, λ is the wavelength of the light beam. NA is the numerical aperture of the lens,
Formula II NA = n × sinθ
Defined by In the above formula II, n is the refractive index at the wavelength λ, and θ is the angle spanning the exit pupil diameter, as shown in FIG. 12 (a diagram for explaining the numerical aperture of the objective lens 9).
[0019]
As can be seen from these equations, in order to reduce the spot radius R, it is necessary to shorten the wavelength λ of the light beam or increase the numerical aperture NA of the objective lens, that is, θ.
[0020]
However, as the numerical aperture NA of the objective lens increases, the spherical aberration generated on the substrate increases. Further, as shown in FIG. 13, coma aberration generated when the optical recording medium 10 is tilted with respect to the optical axis of the light beam is also increased. The tilt of the optical recording medium with respect to the optical axis of the light beam is mainly caused by the warp of the optical recording medium, and it is not uncommon for the tilt to be approximately 0.7 degrees, for example.
[0021]
As described above, when the numerical aperture NA of the objective lens is increased corresponding to the increase in the density of the optical recording medium and the wavelength λ of the light beam is shortened, the spherical aberration and the coma aberration increase. By reducing the thickness, spherical aberration and coma can be suppressed.
[0022]
Therefore, a driving device that needs to support a plurality of types of optical recording media having different recording densities is required to be able to drive media having different thicknesses and emits light beams having different wavelengths. It is necessary to provide an optical pickup having two or more light sources.
[0023]
However, an optical recording medium having a different substrate thickness, for example, a low-density recording medium having a substrate thickness of 1.2 mm and a high-density recording medium having a substrate thickness of 0.6 mm, have spherical surfaces generated on the substrate. Since the amount of aberration is different, it is necessary to change the amount of spherical aberration for correction previously given to the convergent light beam of the optical pickup device. For this reason, in an optical pickup device that handles optical recording media having different substrate thicknesses, it is usually not possible to share an objective lens between light beams having different wavelengths. In general, Japanese Patent Laid-Open No. 8-102079 discloses means for reproducing optical recording media having different substrate thicknesses with a single objective lens. .
[0024]
14 (a) and 14 (b) are explanatory views showing means for reproducing optical recording media having different substrate thicknesses with a single objective lens. Is a simplified version of the means described in. Here, FIG. 14A shows a case where a high-density recording medium having a thin substrate is reproduced, and FIG. 14B shows a case where a low-density recording medium having a thick substrate is reproduced.
[0025]
In the case of FIG. 14A in which a high-density recording medium is reproduced by this means, the light beam incident on the liquid crystal element 51 is uniformly transmitted through the liquid crystal element 51 and then converged by the objective lens 52. In addition, when passing through the objective lens 52, the light beam is given a reverse spherical aberration that cancels the spherical aberration generated on the substrate 53a of the high-density recording medium. The convergent light beam that has been subjected to aberration correction in this way passes through the substrate 53a and forms a small spot 55a on the recording surface 54a.
[0026]
On the other hand, in the case of FIG. 14B for reproducing a low-density recording medium, the liquid crystal element 51 functions as a circular aperture filter and blocks the peripheral portion of the light beam, so that only the central portion of the light beam is incident on the objective lens 52. And converged light. Further, when passing through the objective lens 52, the same aberration correction as that of the high-density recording medium is performed on the light beam. The convergent light beam thus corrected for aberrations passes through the substrate 53b and forms a spot 55b on the recording surface 54b. However, since θ1> θ2, the numerical aperture is reduced, and the spot diameter is It becomes larger than the case of FIG.
[0027]
Since the substrate 53b of the low density recording medium is thicker than the substrate 53a of the high density recording medium, the aberration correction is insufficient with the same correction amount as in the case of the high density recording medium, but in the case of the low density recording medium. , Θ2 is small, that is, the numerical aperture is small, and the spherical aberration that cannot be corrected is small and can be ignored. Therefore, it can be considered that the spherical aberration is approximately corrected. In other words, the amount of spherical aberration generated is large at the periphery of the light beam (light beam) and extremely small at the center of the light beam (light beam). Therefore, if the light beam at the periphery is blocked, the aberration correction is insufficient. Even so, spherical aberration is hardly a problem.
[0028]
As described above, in this prior art, the spot diameter is changed and spherical aberration is approximately corrected by controlling the diameter of the light beam incident on the objective lens.
[0029]
In addition, Japanese Patent Application Laid-Open No. 10-27375 discloses an optical pickup device that controls a light beam by a similar action by using a dye film having a wavelength dependency of transmittance instead of a liquid crystal element. Has been.
[0030]
[Problems to be solved by the invention]
The above-described conventional technology attempts to change the effective numerical aperture of the objective lens in accordance with the reduction in the substrate thickness accompanying the shortening of the light source wavelength. However, whether the liquid crystal element is used to control the numerical aperture or the dye film is used, the transmittance around the objective lens is completely cut off on the long wavelength side and completely transmitted on the short wavelength side. I can't. Even if a wavelength-selective filter having a considerably high performance is used in spite of the high cost, the limit is that the transmittance is about 5% at the long wavelength and about 90% at the short wavelength. As described above, the following problems arise due to the limitation on blocking and transmission.
[0031]
If the transmittance at a long wavelength of the circular aperture filter is lowered in order to suppress spherical aberration when a long wavelength is used (when the substrate is thick), the transmittance at a short wavelength is also lowered accordingly. As a result, the amount of light at the outer periphery of the objective lens that contributes most to the image formation of the minute spot is reduced at a short wavelength, so that the spot diameter is increased, making it unsuitable for recording and reproduction of high-density recording information.
[0032]
On the other hand, if the transmittance at the short wavelength of the circular aperture filter is increased in order to suppress the increase in the spot diameter on the short wavelength side, the transmittance at the long wavelength is also increased accordingly. As a result, the light passing through the outer periphery of the objective lens, which causes spherical aberration, cannot be sufficiently blocked, and the spherical aberration at a long wavelength increases and the spot diameter increases.
[0033]
An object of the present invention is to use a light beam having a shorter wavelength as the thickness of the substrate or the protective layer is thinner than a plurality of types of optical recording media having different thicknesses of the substrate or the protective layer existing on the light beam incident side. In an optical pickup for reproduction or recording / reproduction, the spot diameter formed on the recording surface of the optical recording medium is reduced both at a short wavelength and a long wavelength.
[0034]
[Means for Solving the Problems]
The above object is achieved by the present inventions (1) to (10) below.
(1) An optical pickup device having two or more light sources that generate light beams of different wavelengths and optical means for guiding the light beams emitted from these light sources to an optical recording medium, the light from the light sources In the optical path to the recording medium, there is at least one optical element having a selective transmission region whose transmittance varies depending on the wavelength. When all the optical elements are viewed in the optical path direction from the recording surface of the optical recording medium, In the region corresponding to the beam cross-section, the selective transmission is such that the transmittance of short wavelength light is high on the outer peripheral side and low on the inner peripheral side, and the transmittance of long wavelength light is low on the outer peripheral side and high on the inner peripheral side. An area is providedThe light transmittance on the inner periphery side is 70% or less for short wavelength light and 80% or more for long wavelength light, and the light transmittance on the short outer periphery side is 60% or more for short wavelength light and long wavelength light. Is 10% or less, and when the light beam of short wavelength light is guided to the optical recording medium, the light beam transmitted through the inner peripheral side is attenuated from the light beam transmitted through the outer peripheral side. Added optical super-resolution to the light beam of wavelength.Optical pickup device.
(2) An optical pickup device having two or more light sources that generate light beams of different wavelengths and optical means for guiding the light beams emitted from these light sources to an optical recording medium, the light from the light sources In the optical path to the recording medium, there is at least one optical element having a selective transmission region whose transmittance varies depending on the wavelength. When all the optical elements are viewed in the optical path direction from the recording surface of the optical recording medium, In the region corresponding to the beam cross section, a short wavelength selective transmission region in which the transmittance of short wavelength light is higher than the transmittance of long wavelength light, and a long wavelength in which the transmittance of long wavelength light is higher than the transmittance of short wavelength light. A selective transmission region, and the short wavelength selective transmission region exists outside the long wavelength selection region.The light transmittance in the long wavelength selective transmission region is 70% or less for short wavelength light and 80% or more for long wavelength light, and the light transmittance in the short wavelength selective transmission region is 60% or more for short wavelength light. The long wavelength light is 10% or less, and the transmittance of the short wavelength light in the long wavelength selective transmission region is set lower than the transmittance of the short wavelength light in the short wavelength light selective transmission region. When the light beam of light is guided to the optical recording medium, the light beam transmitted through the long wavelength selective transmission region is attenuated from the light beam transmitted through the short wavelength selective transmission region, so that light of short wavelength light is transmitted. Added optical super-resolution to the beamOptical pickup device.
(3) The optical pickup device according to (2), wherein the short wavelength selective transmission region and the long wavelength selective region are provided concentrically.
(4) The optical pickup device according to any one of (1) to (3), wherein all the selective transmission regions are provided in one optical element.
(5) The optical pickup device according to any one of (1) to (4), wherein the selective transmission region is formed of a dye film.
(6) The optical pickup device according to (5), wherein the dye film is formed on a transparent substrate, and an antireflection film is formed on the dye film.
(7) The number of the dye films is k, and the refractive index at the shortest wavelength to be used is expressed as n in each dye film._i(I = 1 to k), thickness is d_iWhen (i = 1 to k), n_iAnd d_iThe optical pickup device according to (5) or (6), wherein the product is constant.
(8) One long-wavelength selective transmission region is formed, two short-wavelength selective transmission regions are formed, and one transmission region that transmits both the short wavelength and the long wavelength without attenuation is formed. The long wavelength selective transmission region is arranged in the center, the first short wavelength selective transmission region in a ring shape is arranged outside the long wavelength selective transmission region, and the ring shaped transmission region is formed outside the first short wavelength selective transmission region. The optical pickup device according to any one of (2) to (7), wherein a second short wavelength selective transmission region is formed outside the transmission region.
(9) The long wavelength selective transmission region is formed in a circular shape in the center, and a long wavelength light beam is transmitted between the long wavelength selective transmission region and the short wavelength selective transmission region formed outside the long wavelength selective transmission region. The optical pickup device according to any one of (2) to (7), wherein the third selective transmission region having a high rate and a low transmittance of the light beam of the short wavelength light is formed in a band shape.
(10)The different wavelength light beams have a long wavelength light beam wavelength of 780 nm. The optical pickup device according to any one of (1) to (9), wherein the wavelength of the light beam having a wavelength is 650 nm.
[0035]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1C shows a configuration example of the optical element used in the present invention. In this optical element, a circular long wavelength selective transmission region 101 and a short wavelength selective transmission region 102 surrounding the circular long wavelength selective transmission region 101 are formed on the surface of a square transparent substrate made of optical glass (BK7). Each of these selective transmission regions is composed of an organic dye, and the long wavelength selective transmission region 101 in the central part has a higher transmittance of long wavelength light than that of short wavelength light, and a short wavelength selective transmission region in the outer peripheral part. No. 102 has a transmittance of short wavelength light higher than that of long wavelength light.
[0036]
FIG. 2C shows an optical element having the same configuration as that of FIG. 1C except that the short wavelength selective transmission region 102 is provided on the outer peripheral portion and the transparent portion 104 is a transparent portion at the center. The size of the central transmission region 104 is the same as that of the long wavelength selective transmission region 101 in FIG. This optical element has the same configuration as the circular aperture filter described in JP-A-10-27375.
[0037]
In FIG. 1C and FIG. 2C, the transmittance of the long wavelength selective transmission region 101 is about 95% at a wavelength of 780 nm and about 70% at a wavelength of 650 nm, and the transmittance of the short wavelength selective transmission region 102 is About 5% at a wavelength of 780 nm and about 90% at a wavelength of 650 nm.
[0038]
Next, consider a case where any one of the above optical elements is arranged in front of the objective lens of the optical pickup device, and incident light that has passed through the optical element is imaged as a beam spot on the recording surface by the objective lens. The incident light is laser light having a wavelength of 650 nm having a Gaussian distribution, the numerical aperture of the objective lens is 0.60, the rim intensity of incident light at the outermost periphery of the objective lens is 0.50, and the short wavelength selective transmission region 102 is an aperture stop. Is considered to be 0.45.
[0039]
FIG. 1 (a) shows the intensity distribution of the light beam incident on the objective lens when the optical element of FIG. 1 (c) is provided, and FIG. 1 (b) shows the intensity distribution of the beam spot in that case. FIG. 2A shows the intensity distribution of the light beam incident on the objective lens when the optical element of FIG. 2C is provided, and FIG. 2B shows the intensity distribution of the beam spot in that case. Show. For comparison, FIG. 3A shows the intensity distribution of the light beam incident on the objective lens when neither optical element is provided, and FIG. 3B shows the intensity distribution of the beam spot in that case. Shown respectively. In each figure, the vertical axis is normalized by the center intensity, and in the intensity distribution graph of the incident light beam, the horizontal axis is normalized by the objective lens radius.
[0040]
In FIG. 3B in which no optical element is provided, the spot diameter (1 / e of the center intensity) is obtained.²Is equivalent to 0.961 μm, but in FIG. 2B where a conventional optical element having a circular aperture is provided, the spot diameter increases to 0.976 μm. The increase in the spot diameter is because the intensity of the outer peripheral portion of the light beam that contributes most to the image formation of the minute spot is reduced by about 10% by the short wavelength selective transmission region 102 shown in FIG. In order to suppress an increase in spot diameter under the condition of using this conventional optical element, it is necessary to increase the rim intensity of incident light. For example, when the rim intensity is 0.60, the intensity distribution of the incident light to the objective lens is as shown in FIG. 4A, and as a result, the intensity distribution of the beam spot is as shown in FIG. Thus, a spot diameter equivalent to that shown in FIG. 3B without the optical element is obtained. However, such a change in rim strength is not practical because it involves a significant design change in the optical system.
[0041]
On the other hand, in the case where the optical element shown in FIG. 1C is provided, the long wavelength selective transmission region 101 is provided in the central portion, so that the intensity of the short wavelength light is reduced in this region. Therefore, for a short wavelength incident beam, a drop in intensity at the outer periphery of the incident beam can be compensated, and an increase in the beam spot diameter can be suppressed. In particular, as in this example, if the transmittance (about 70%) of short wavelength light in the long wavelength selective transmission region 101 is set lower than that (about 90%) of the short wavelength selective transmission region 102, FIG. As shown in (a), the incident beam can be attenuated at the center. As a result, a fine spot exceeding the diffraction limit can be imaged by the optical super-resolution effect. In this example, as shown in FIG. 1B, the beam spot diameter is 0.925 μm, which is smaller than that in FIG. 3B in which no optical element is provided.
[0042]
Further, in the optical element shown in FIG. 1C, the beam spot diameter can be easily changed by changing the transmittance of the short wavelength light in the long wavelength selective transmission region 101, so that the design can be easily changed according to various purposes. It becomes.
[0043]
On the other hand, the transmittance of long wavelength light in the optical element shown in FIG. 1C is as high as about 95% in the long wavelength selective transmission region 101 and as low as about 5% in the short wavelength selective transmission region 102. As for the reduction of spherical aberration by cutting the outer peripheral portion, an effect equivalent to that of the conventional optical element shown in FIG.
[0044]
In the example shown in FIG. 1 (c), the long wavelength selective transmission region 101 has a numerical aperture of 40% to 85% of the numerical aperture of the objective lens, especially 50 when the light is considered to pass through only this region. It is preferable to set so as to correspond to the range of% to 80%.
[0045]
In the example shown in FIG. 1C, the transmission characteristics in each of the long wavelength selective transmission region 101 and the short wavelength selective transmission region 102 take into consideration the imaging performance of the beam at each wavelength, the efficiency of the laser light, and the like. The light transmittance in the long wavelength selective transmission region 101 is preferably 70% or less for short wavelength light and 80% or more for long wavelength light, and the light transmittance in the short wavelength selective transmission region 102 is determined appropriately. The short wavelength light is preferably 60% or more and the long wavelength light is preferably 10% or less. Furthermore, in the region corresponding to the light beam cross section, the transmittance of short wavelength light is high on the outer peripheral side and low on the inner peripheral side, and the transmittance of long wavelength light is low on the outer peripheral side and high on the inner peripheral side, If the light transmittance in each of the long wavelength selective transmission region 101 and the short wavelength selective transmission region 102 is set, as described above, the optical super-resolution effect in short wavelength light and the reduction of spherical aberration in long wavelength light can be achieved. Realize at the same time.
[0046]
In the present invention, in addition to the example shown in FIG. 1C, as shown in FIG. 5, for example, a long wavelength selective transmission region 101 is arranged at the center, and a first short wavelength selective transmission region 102 is formed in a ring shape outside thereof. The transmission region 104 may be formed in a ring shape on the outer side, and the second short wavelength selective transmission region 102 may be further disposed on the outer side. In the configuration shown in FIG. 5, long wavelength light is attenuated in the first short-wavelength selective transmission region 102 in the annular shape, and long wavelength light is not attenuated in the outer transmission region 104, so that the aberration correction is not performed. It is possible to attenuate the transmitted light of the annular portion (corresponding to the first short wavelength selective transmission region 102) where the spherical aberration becomes the largest in the wavelength light, and as a result, the spherical aberration of the long wavelength light can be reduced. On the other hand, for short wavelength light, since the long wavelength selective transmission region 101 exists in the center, the effect of the optical super-resolution described above is realized. As described above, in the present invention, since the light transmittance of an arbitrary concentric region where spherical aberration is continuously distributed can be controlled for each wavelength, a target spot diameter can be easily obtained.
[0047]
In the above description, an example in which all the selective transmission regions are provided in one optical element has been described. However, a plurality of optical elements provided with at least one selective transmission region may be arranged in the optical path. In other words, when all the optical elements are viewed in an overlapped manner from the optical path direction, for example, the arrangement of the selective transmission regions as shown in FIG.
[0048]
Further, in the above description, the transmission characteristics change in steps (discontinuously) between adjacent selective transmission areas. However, the transmission characteristics gradually change from one selective transmission area to another selective transmission area. It may be a thing.
[0049]
In the above description, the shape of the selective transmission region is circular or annular, but for example, the long wavelength selective transmission region 101 in FIG. 1C may be fan-shaped as shown in FIG. As shown in FIG. 6B, a plurality of fan-shaped long-wavelength selective transmission regions 101 may be arranged side by side across the center of the optical element. In addition, the long wavelength selective transmission region 101 may be rectangular as shown in FIG. 6C, and as shown in FIG. 6D, the long wavelength selective transmission region 101 and the short wavelength selective transmission region 102 are The optical element may be divided so that the center of the optical element is included in the long wavelength selective transmission region 101.
[0050]
In the present invention, the permselective region can be provided by forming a dye film on at least one surface of a transparent substrate made of glass or resin as described above. Organic dyes, especially phthalocyanine dyes and naphthalocyanine dyes, have steep absorption characteristics with respect to wavelength, and the absorption wavelength band can be controlled relatively easily by the production method and composition design. Suitable for the invention. Specifically, it is preferable to use a phthalocyanine dye for the long wavelength selective transmission region 101, and it is preferable to use a naphthalocyanine dye for the short wavelength selective transmission region 102. In order to obtain the wavelength dependence of the desired transmittance, an interference filter composed of a transparent inorganic material multilayer film requires a thickness of several micrometers, whereas an organic dye film has a thickness of several tens of nanometers. The target transmittance and its wavelength dependence can be realized with a thickness of about a meter. In addition, as the organic dye is used in the recording layer of the write-once type optical recording medium, it is excellent in corrosion resistance, and those having excellent light resistance have been developed, so that sufficient durability can be obtained. However, if an antireflection film comprising a dielectric single layer film or multilayer film is provided on the dye film, the durability is further improved. The dye film can be formed by coating or vapor deposition.
[0051]
In the present invention, a phase shift may occur between light beams that have passed through selective transmission regions having different properties. When the selective transmission region is composed of a dye film, the dye film is thin, so the phase shift due to this is small and can be ignored, but strictly speaking, only light with the same phase contributes to the image formation. In order to perform good image formation, it is preferable to align the phase on the short wavelength side that is greatly affected by the phase shift. Specifically, when the number of dye films (selective transmission regions) is k, the refractive index at the shortest wavelength to be used is expressed as n in the i-th dye film._i(I = 1 to k), the film thickness is d_iWhen (i = 1 to k), n_iAnd d_iEach dye film may be designed so that the product of is constant.
[0052]
As an application of the present invention, the selective transmission region can also have a function as a phase shifter. An annular phase shifter disclosed in Japanese Patent Laid-Open No. 10-255305 is provided with a convex portion or a concave portion in a ring shape on a part of the opening surface of the objective lens, and by designing the height of the step to an appropriate value, This shows the function of shifting the phase of light transmitted through the portion with respect to the phase of light passing outside the annular zone. Thus, by shifting the phase in a ring shape within the aperture surface of the objective lens, it is possible to minimize the spherical aberration that occurs when the substrate thickness of the optical recording medium differs from the design value. Further, if this step is optimized, both spherical aberrations can be satisfactorily reduced at two wavelengths.
[0053]
In the above Japanese Patent Laid-Open No. 10-255305, a phase shifter is realized by providing a physical step in the objective lens. In the present invention, a similar function is realized by controlling the phase in the selective transmission region. Can do. An example of this is shown in FIG. The optical element shown in FIG. 7 is the same as the optical element shown in FIG. 1C except that the third selective transmission region 103 is located between the long wavelength selective transmission region 101 at the center and the short wavelength selective transmission region 102 outside. Is provided in a ring shape. This third selective transmission region 103 has a high transmittance for long wavelength light, and its phase changes by a predetermined amount, and has a low transmittance for short wavelength light, and its phase shift amount is a long wavelength selection. This is an area designed to be the same as the transmissive area 101. By using the optical element shown in FIG. 7, spherical aberration that occurs when the substrate thickness of the optical recording medium differs from the design value, or aberration correction is optimized when using short wavelength light (when the substrate is thin) Spherical aberration that occurs when long wavelength light is used (when the substrate is thick) can be reduced as compared with the case of simply limiting the aperture, and imaging performance can be improved. FIG. 8A shows spherical aberration at a wavelength of 780 nm (substrate thickness of 1.2 mm) when an objective lens optimized for a substrate thickness of 0.6 mm and a wavelength of 650 nm is used. FIG. 8B shows the spherical aberration when the aperture is limited by the aperture filter, and FIG. 8C shows the spherical aberration when the optical element shown in FIG. 7 is provided instead of the circular aperture filter. . From these graphs, it can be seen that the spherical aberration is significantly reduced by the annular phase shifter having the configuration shown in FIG.
[0054]
Next, a specific example in which the present invention is applied to an optical pickup device having two light sources will be described.
[0055]
FIG. 9 is an explanatory view showing a configuration of an optical pickup device of two light sources using the optical element.
[0056]
This optical pickup device has a semiconductor laser 3 a having an oscillation wavelength of 650 nm and a semiconductor laser 3 b having an oscillation wavelength of 780 nm, and a light beam output from the semiconductor laser 3 a or the semiconductor laser 3 b is passed through a beam splitter 4 to a diffraction grating 5. Is incident on. The light beam incident on the diffraction grating 5 is separated into a main beam (for a focus error signal and a recording / reproduction signal) and a sub beam (for a tracking error signal), and then enters a collimator lens 6. The light beam that has entered the collimator lens 6 is converted into parallel light from the divergent light and enters the beam splitter 7. The light beam incident on the beam splitter 7 is reflected in the direction of the objective lens 9 and enters the objective lens 9 via the optical element 8. The light beam incident on the objective lens 9 is changed from parallel light into convergent light, and forms a spot on the recording surface of the optical recording medium 10. The objective lens 9 has a numerical aperture and aberration correction, etc., when the optical recording medium 10 is a high-density recording medium (substrate thickness 0.6 mm) and recording / reproduction is performed using a light beam having a wavelength of 650 nm. Has been optimized. The optical element 8 is provided with a selective transmission region as shown in FIG.
[0057]
The light beam that forms a spot on the recording surface of the optical recording medium is reflected by the recording surface and enters the objective lens 9. The light beam incident on the objective lens 9 is converted into parallel light again, passes through the optical element 8 again, travels straight through the beam splitter 7, and enters the condenser lens 11. The light beam incident on the condenser lens 11 is converted from parallel light into convergent light and incident on the anamorphic lens 12. The light beam incident on the anamorphic lens 12 generates astigmatism for detecting a focus error signal and reaches the light receiving element 13. The light beam reaching the light receiving element 13 is converted into a tracking error signal, a focus error signal, and a recording / reproducing signal.
[0058]
In this optical pickup device, for example, the thickness of the substrate of the high-density recording medium is 0.6 mm as described above, and the thickness of the substrate of the low-density recording medium is 1.2 mm. Good recording when the numerical aperture of the objective lens is 0.6 (when the wavelength is 650 nm) and the substantial numerical aperture of the objective lens is 0.35 when reproducing the low-density recording medium (when the wavelength is 780 nm) Of course, the reproduction characteristics are realized, but the thickness, wavelength, and numerical aperture of the substrate are not limited to these. The present invention can also be applied to an optical pickup device equipped with three or more light sources. The present invention can also be applied to a finite system optical pickup device.
[0059]
In addition, the optical element having the selective transmission region does not need to be provided independently of the components of the optical pickup device as in the illustrated example. For example, as described in JP-A-10-27375, the surface of the objective lens Alternatively, a selective transmission region may be provided on the beam splitter surface, the diffraction grating surface, the collimator lens surface, or the like.
[0060]
【The invention's effect】
In the present invention, the selective transmission region as described above is provided in the optical path of the optical pickup device, so that a single objective lens is used, and the optical recording medium is different for a plurality of types of optical recording media having different substrate thicknesses. When recording or reproducing at a wavelength, the spot diameter formed on the recording surface of the optical recording medium can be reduced at both short and long wavelengths.
[Brief description of the drawings]
FIG. 1A is a graph showing the intensity distribution of a light beam incident on an objective lens when the optical element shown in FIG. 1C is provided. (B) is a graph which shows intensity distribution of the beam spot in that case. (C) is a top view which shows the structural example of the optical element used for this invention.
FIG. 2A is a graph showing the intensity distribution of a light beam incident on an objective lens when the optical element shown in FIG. 2C is provided. (B) is a graph which shows intensity distribution of the beam spot in that case. (C) is a top view which shows the structural example of the conventional optical element which functions as a circular aperture filter.
FIG. 3A is a graph showing an intensity distribution of a light beam incident on an objective lens when no optical element is provided. (B) is a graph which shows intensity distribution of the beam spot in that case.
4A is a graph showing an intensity distribution of a light beam incident on an objective lens when the optical element shown in FIG. 2C is provided and the rim intensity is increased. FIG. (B) is a graph which shows intensity distribution of the beam spot in that case.
FIG. 5 is a plan view showing a configuration example of an optical element used in the present invention.
6A, 6B, 6C, and 6D are plan views showing a configuration example of an optical element used in the present invention.
FIG. 7 is a plan view showing a configuration example of an optical element used in the present invention, which has a function as an annular zone phase shifter.
FIG. 8A is a graph showing spherical aberration of long-wavelength light when using short-wavelength light and using an objective lens with optimized aberration correction when the substrate is thin. (B) is a graph showing spherical aberration when aperture restriction is performed by a circular aperture filter in that case. (C) is a graph showing spherical aberration when the optical element shown in FIG. 7 is provided instead of the circular aperture filter.
FIG. 9 is an explanatory diagram showing a configuration example of an optical pickup device of the present invention.
FIG. 10 is an explanatory diagram showing a configuration example of a conventional optical pickup device.
FIG. 11 is an explanatory diagram for explaining spherical aberration occurring in a substrate.
FIG. 12 is an explanatory diagram for explaining a numerical aperture.
FIG. 13 is an explanatory diagram showing a case where the optical recording medium is tilted with respect to the optical axis.
FIGS. 14A and 14B are diagrams for explaining a difference in image formation when the diameters of light beams incident on an objective lens are different. FIGS.
[Explanation of symbols]
101 Long wavelength selective transmission region
102 Short wavelength selective transmission region
103 3rd selective transmission area
104 Transmission area
3, 3a, 3b Semiconductor laser
4 Beam splitter
5 Diffraction grating
6 Collimator lens
7 Beam splitter
8 Optical elements
9 Objective lens
10 Optical recording media
11 Condensing lens
12 Anamorphic lens
13 Light receiving element
40 substrates
41 Recording surface
42 spots

Claims (10)

An optical pickup device having two or more light sources that generate light beams of different wavelengths and optical means for guiding the light beams emitted from these light sources to an optical recording medium,
In the optical path from the light source to the optical recording medium, there is at least one optical element having a selective transmission region having different transmittance depending on the wavelength,
When all the optical elements are overlapped in the optical path direction from the recording surface of the optical recording medium, the transmittance of the short wavelength light is high on the outer peripheral side and low on the inner peripheral side in the region corresponding to the light beam cross section. The selective transmission region is provided so that the transmittance of long wavelength light is low on the outer peripheral side and high on the inner peripheral side ,
The light transmittance on the inner circumference side is 70% or less for short wavelength light and 80% or more for long wavelength light, and the light transmittance on the short outer circumference side is 60% or more for short wavelength light and long wavelength light. 10% or less,
When a light beam of short wavelength light is guided to the optical recording medium, the light beam transmitted through the inner peripheral side is attenuated from the light beam transmitted through the outer peripheral side, thereby reducing the light beam of short wavelength light. An optical pickup device in which optical super-resolution acts . An optical pickup device having two or more light sources that generate light beams of different wavelengths and optical means for guiding the light beams emitted from these light sources to an optical recording medium,
In the optical path from the light source to the optical recording medium, there is at least one optical element having a selective transmission region having different transmittance depending on the wavelength,
Short wavelength selection where the transmittance of short wavelength light is higher than the transmittance of long wavelength light in the area corresponding to the cross section of the light beam when all the optical elements are overlapped in the optical path direction from the recording surface of the optical recording medium There is a transmission region and a long wavelength selective transmission region in which the transmittance of long wavelength light is higher than the transmittance of short wavelength light, the short wavelength selective transmission region exists outside the long wavelength selection region ,
The light transmittance in the long wavelength selective transmission region is 70% or less for short wavelength light and 80% or more for long wavelength light, and the light transmittance in the short wavelength selective transmission region is 60% or more for short wavelength light, The long wavelength light is 10% or less, and the short wavelength light transmittance of the long wavelength selective transmission region is set lower than the short wavelength light transmittance of the short wavelength light selective transmission region,
When the light beam of the short wavelength light is guided to the optical recording medium, the light beam transmitted through the long wavelength selective transmission region is attenuated from the light beam transmitted through the short wavelength selective transmission region, so that the short wavelength light Pickup device in which optical super-resolution acts on the light beam of   The optical pickup device according to claim 2, wherein the short wavelength selective transmission region and the long wavelength selective region are provided concentrically.   The optical pickup device according to claim 1, wherein all the selective transmission regions are provided in one optical element.   The optical pickup device according to claim 1, wherein the selective transmission region is formed of a dye film.   6. The optical pickup device according to claim 5, wherein the dye film is formed on a transparent substrate, and an antireflection film is formed on the dye film. The number of the dye film is k pieces, in each dye layer, n _{i (i} = _1~k) the refractive index at the shortest wavelength to be used, when the thickness was d _{i (i} = 1~k) , _Ni and d _i are constant, the optical pickup device according to claim 5 or 6. The long wavelength selection transmission region is one formed, the short-wavelength selective transmission region is two formed, transmissive region transmitting are one formed without any short wavelength and long wavelength attenuation,
The long wavelength selective transmission region is arranged in a circular shape in the center, and a first short wavelength selective transmission region in a ring shape is arranged outside the long wavelength selective transmission region, and the transmission in a ring shape is formed outside the first short wavelength selective transmission region. The optical pickup device according to claim 2, wherein a region is formed and a second short wavelength selective transmission region is formed outside the transmission region.   The long wavelength selective transmission region is formed in a circular shape in the center, and the transmittance of a long wavelength light beam is high between the long wavelength selective transmission region and the short wavelength selective transmission region formed outside thereof. The optical pickup device according to claim 2, wherein the third selective transmission region having a low transmittance of the light beam of short wavelength light is formed in a band shape.   10. The optical pickup device according to claim 1, wherein the different wavelength light beams have a long wavelength light beam of 780 nm and a short wavelength light beam of 650 nm.

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