JPH06103605A - Image forming optical system, optical head device and optical information device - Google Patents

Abstract

PURPOSE:To provide an optical head device for which focal position movement and the generation of astigmatism do not occur regardless of wavelength fluctuation and astigmatic difference fluctuation at the time of changing light source output. CONSTITUTION:Light beams 3 emitted from a semiconductor laser 21 are turned to approximately parallel rays of light by a collimating lens B. The light beams are shaped by a wedge-shaped prism 35, then transmitted through a beam splitter 36, made incident on a hologram leans 107 and an objective lens 3 and converged on an information medium 5. The light beams reflected at the information medium 5 follow an original optical path backwards, are reflected at the beam splitter 36, and made incident on a photodetector 7. The hologram lens 107 and the objective lens 4 are integrally driven by a driving device 110. By overcorrecting the chromatic aberration of the objective lens 4, the fluctuation of the focal distance of the collimating lens and the astigmatic difference amount of the semiconductor laser 21 are offset and even when the output of the semiconductor laser is changed, the focal position on the side of the information medium does not change and the astigmatism on the information medium is not generated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head device for recording / reproducing or erasing information stored on an optical medium or a magneto-optical medium such as an optical disk or an optical card, and an optical system thereof, and an information medium. The present invention relates to an optical information device for recording / reproducing / erasing information.

[0002]

2. Description of the Related Art Optical memory technology using an optical disk having a pit pattern as a high-density and large-capacity storage medium includes digital audio disk, video disk,
It has been put to practical use while expanding its use with document file discs and even data files. The mechanism by which information recording / reproducing to / from an optical disk is successfully performed with high reliability via a light beam that is minutely focused is solely due to the optical system. The basic functions of the optical head device, which is the main part of the optical system, are the light condensing function that forms a minute spot of diffraction limit, the focus control (focus servo) and tracking control of the optical system, and the pit signal (information signal). ) Broadly divided into detection.

Focusing on the component parts constituting the optical head device, a light source for generating a light beam, a lens (group) for guiding the light beam to a disk and for guiding the light beam reflected by the disk to a signal detection system, It is composed of an optical component that constitutes a signal detection system, a photodetector, a drive unit that performs servo control, and the like. Of these, for the light source, instead of the gas laser used in the early stages of commercialization of optical discs,
Small-sized and low-cost semiconductor lasers have become the mainstream.

However, when the semiconductor laser is used as a light source for a readable / writable optical disk, the refractive index of the lens changes due to the wavelength fluctuation caused by the change in the optical output at the time of reading and writing, until the focus servo control follows. There are problems that the light spot on the information medium is in a defocused state and information cannot be read out or writing becomes insufficient.

To solve this problem, a method of using a material having a low wavelength dispersion as a lens material (glass material) can be considered, but in general, a glass material having a low wavelength dispersion has a small refractive index, so that the curvature must be increased, It is especially difficult to make a lens with a large NA. The next conceivable means is to combine a plurality of lenses having different wavelength dispersion characteristics to form a combined lens. This is shown in FIG. 1 as a first conventional example.
2 shows.

In FIG. 12, reference numeral 21 is a semiconductor laser light source. The linearly polarized light beam 3 (laser light) emitted from the semiconductor laser 21 is collimated by the lens group 402, shaped by the wedge prism 35, and then transmitted through the beam splitter 36 to pass through the objective lens 4. And is condensed on the information medium 5. The optical beam reflected by the information medium 5 traces the original optical path in the opposite direction and is reflected by the beam splitter 36, and the collimator lens A
The light is condensed by (121), and the servo signal detecting means 34
After the wavefront is converted so that a focus error signal and a tracking error signal can be obtained, the light enters the photodetector 7. By calculating the output of the photodetector 7, the servo signal (focus error signal and tracking error signal) and the information signal can be obtained.
Here, the objective lens 4 must be lightweight in order to be moved at high speed by the driving device 110, and therefore cannot be used as a lens combination of refraction lenses. Therefore, the combined lens 402 overcorrects the chromatic aberration (hereinafter referred to as “color extraction”) to cancel the chromatic aberration of the objective lens.

Next, although not an application example to an optical head device, FIG. 13 shows a second conventional example in which a hologram is used to perform achromatization as a single lens. In FIG. 13, 107 is a diffraction grating type lens (hologram lens), and 401 is a refraction type lens. Assuming that the focal length of the hologram lens 107 when the wavelength is λ0 is fhoe0, the focal length fhoe1 when the wavelength is λ1 is fhoe.
1 = fhoe0 × λ0 / λ1. ． ． It becomes (1). The refractive index of the refractive lens 401 is n (λ) and the focal length is f.
If (λ), then f (λ1) = f (λ0) × (n (λ0) -1) / (n (λ1) -1). ． ． (2)

From the expressions (1) and (2), the condition for achromatization is λ1 / (fhoe1 × λ0) + (n (λ1) -2) / (f (λ0) × (n (λ0) -1)) = 1 / fhoe0 + 1 / f (λ0). ． ． It becomes (3).

Here, as the wavelength becomes longer, the focal length becomes shorter in the formula (1) and becomes longer in the formula (2). Therefore, the positive and negative values of fhoe1 and f (λ0) are set to the same formula (3). If the lens is selected so as to satisfy the above condition, the lens shown in FIG.

Since the hologram is a diffractive element as described above, the wavelength dependency of the focal length of the hologram lens formed by using the hologram is opposite to that of the refractive index type lens.
Achromatism can be achieved by combining hologram lenses with positive power and refractive index lenses, or hologram lenses with negative power and refractive index lenses, so that the curvature of the lens can be relatively small. Since the hologram lens is a flat element, it has many advantages such as being lightweight and excellent in mass productivity. The above-mentioned conventional example is, for example, Document 1-D. Faklis and
M. Morris (1991) Photonics Spectra Novenver 205 & De
cember 131 (Dee, Phakris and M, Morris (19
91) Photonics Spectra November issue page 205 and December issue page 131), Reference 2-MAGan et al.
(1991) SPIE Vol.1507 p116 (M, A, Gun, etc. (19
91) S, P, A, E, 1507, 116), Reference 3-P. Twardowski and P. Meirueis (1991) SPI
E Vol.1507 p55 (Pee, Toward Wusuki and Pee, Mail Ace (1991) S, Pee, Eye, E 1507
Volume 55).

Further, as described in Document 1 for the method of manufacturing the hologram lens, as shown in FIG. 14, the photolithography process and the etching generally performed in the manufacturing process of the silicon LSI are performed plural times. By repeating it, the hologram lens can be manufactured as a multilevel hologram having a stepped cross-sectional shape. Also, K. Goto et al. (1987) JJAP Vol.26 Supple
ment26-4 (Kay, Goto et al. (1987) J, J, A, P, Vol. 26, Supplement 26-4) -As shown in Reference 4-, a hologram lens is shown in FIG. It can be manufactured as shown in FIG.

Document 4 is an example in which a hologram lens is applied to an optical head device. The purpose is to combine the spherical lens and the hologram lens to achieve the effect of an aspherical lens (suppression of aberrations such as off-axis aberrations). It is intended, not intended to correct chromatic aberration. Therefore, the focus position movement due to the wavelength variation of the semiconductor laser is not corrected.

As a third conventional example, JP-A-3-15551
Reference is made to JP-A No. 4 and JP-A-3-155515. In the third conventional example, as shown in FIG.
And the objective lens system and the optical system of the optical information recording / reproducing apparatus, which are arranged closer to the light source than the objective lens 4 and are composed of a chromatic aberration correction element having almost no power (no lens action). Although it is described that the objective lens itself uses a hologram lens, the chromatic aberration correction element is composed of a combination of a positive lens 250 and a negative lens 251 having different dispersion values, and an example using a hologram lens is not disclosed. . By utilizing the difference in the dispersion values of the positive lens 250 and the negative lens 251, the chromatic aberration correction element can correct the chromatic aberration of the objective lens although it has almost no lens action (power).

[0014]

The light beam emitted from a semiconductor laser usually contains astigmatism. This is because, when a wide direction of the active layer is set to the horizontal direction on the emission surface of the semiconductor laser, the emission point in the horizontal direction slightly enters the active layer. In order to remove this astigmatism, in the first conventional example, the combined lens 402
Adjust by moving in the direction of the optical axis. At this time, when the elliptical beam correction is replaced by the focal length of the collimator lens (combined lens 402), the image forming relationship from the emission point of the semiconductor laser 21 to the information medium 5 in the vertical direction and the horizontal direction is shown in FIG. (A) Vertical direction, (b) Horizontal direction. In FIG. 17, the combined lens 402 and the wedge prism 3
The collimating lens 12 is a lens composed of 5 (FIG. 12).
It is shown as 3. The meanings of symbols in FIG. 17 are as follows. f0: Focal length of objective lens 4 fc: Combined lens 402 (collimator lens B-122 in FIG. 1 described later) fcv: Vertical equivalent focal length (= fc) of collimator lens 123 fch: Horizontal direction of collimator lens 123 Equivalent focal length (= fc × γ) γ: Elliptic correction factor> 1 δ: Astigmatic difference of semiconductor laser δv: Vertical position of object point (emission point of semiconductor laser 21) position from focal point FC of collimating lens δh: Amount of deviation of the horizontal object point (emission point of the semiconductor laser 21) position from the focus position FC of the collimator lens εv: Amount of deviation of the image point position in the vertical direction from the focus position F0 of the objective lens 4 εh: Objective lens 4 Of the horizontal image point position from the focal point position F0 in FIG. 17A, the compound lens 402 is moved to the right in the optical axis direction and δv> 0. Here, from the relationship of the vertical magnification, εv = δv × (f0 / fcv) ² = δv × (f0 / fc) ² . ． ． (5) εh = δh × (f0 / fch) ² = δh × (1 / γ ² ) × (f0 / fc) ² . ． ． (6)

From equations (5) and (6), in order to eliminate astigmatism on the information medium 5 (εv = εh), δv × γ ² = δh. ． ． (7) so that the combined lens 402 (collimator lens 12
It turns out that it is enough to move 3).

However, if the optical system is adjusted as described above in accordance with the reading of information (low output), the astigmatism on the information medium 5 at the time of writing (high output) is as follows. Aberration is a problem.

Increasing the power of the semiconductor laser increases the wavelength. At this time, since the combined lens 402 is a color-developing lens, the focal length fc becomes short, and δv and δh increase by the amount that fc becomes short, and as a result, δh / δv becomes smaller than γ ² . Further, when the output of the semiconductor laser is increased, δ decreases, so δh / δv also decreases. As described above, there is a problem that the two effects are strengthened to break the relation of the expression (7) and to generate astigmatism on the information medium 5.

Next, regarding the second conventional example, although the correction of chromatic aberration is studied in detail for each lens, there is no application example of this lens to the optical head device.
There are problems in the following points.

1. When the second conventional example is applied to the optical head device, the chromatic aberration of each lens used in the optical head optical system is corrected, so that there is no design freedom.
There is a problem that a change in the amount of astigmatic difference due to a change in the output of the semiconductor laser cannot be compensated, and as a result, astigmatism is generated in the focused spot on the information medium and the information signal is deteriorated.

2. An image forming optical system using a combination of a collimator lens and an objective lens is used in the optical head device. However, when chromatic aberration correction is performed by applying the second embodiment, the chromatic aberration of each lens such as the collimator lens and the objective lens is used. Since all the hologram lenses are used to correct the above, there is a problem that the number of hologram lenses increases and the cost increases.

3. The diffraction efficiency of the hologram lens changes depending on the wavelength. Since a design method for obtaining the optimum diffraction efficiency in the optical head has not been shown so far, the diffraction efficiency is lowered at the time of signal reproduction, and the light that is not diffracted as the + 1st-order diffracted light (transmitted light = 0th-order diffracted light or −1st-order diffracted light There is a problem that stray light is generated by the generation of the second-order diffracted light and the + second-order diffracted light, and the S / N ratio of the information signal is reduced.

Further, the third embodiment has the following problems. 1. Since the chromatic aberration correction element consists of a combination of two lenses, a positive lens and a negative lens, material cost, manufacturing cost, cost of the combining process including adjustment, and cost of the laminating process are required, which causes a cost increase.

2. Since the chromatic aberration correction element consists of a combination of two lenses, a positive lens and a negative lens, it is heavy and large. Therefore, the entire optical system is heavy and large.

3. Since the chromatic aberration correction element is composed of a combination of two lenses, a positive lens and a negative lens, and is heavy, it is difficult to perform fine movement integrally with an objective lens that requires fine movement at high speed. Therefore, since the relative position changes due to tracking and focusing, aberrations other than chromatic aberration must be corrected independently.
Therefore, there is no choice but to use an aspherical shape in terms of lens design, especially the objective lens, which has a low degree of freedom, which makes designing and manufacturing difficult and costly.

4. Since the chromatic aberration correction element is composed of a combination of two lenses, a positive lens and a negative lens, and is heavy, it is difficult to perform fine movement integrally with an objective lens that requires fine movement at high speed. For this reason, the holding means for the chromatic aberration correction element is necessary independently of the holding means for the objective lens, which causes a cost increase and also makes the optical system large.

In view of the above problems, the present invention applies a hologram lens to an optical head device to
An object of the present invention is to configure an optical head device that does not move a focal position or generate astigmatism in spite of a wavelength variation and an astigmatic difference variation when a light source output is changed. It is another object of the present invention to configure an optical head device capable of optimizing the diffraction efficiency of a hologram lens and stably reading and writing records.

[0027]

In order to solve the above-mentioned problems, the present invention solves the above-mentioned problems by using a semiconductor laser light source and an imaging optical system (an objective lens and an optical system) that receives a light beam from the light source and converges it into a minute spot on an information medium. (Including collimating lens),
In an optical head device comprising a photodetector which receives an optical beam reflected and diffracted by the information medium and outputs an electric signal according to the amount of light, an objective lens arranged near the information medium is a combination of a refraction lens and a hologram lens. And
An optical head device is characterized in that a light beam shaping means is provided between a hologram lens and a collimator lens.

Alternatively, a semiconductor laser light source, an image forming optical system that receives a light beam from the light source and converges it into a minute spot on an information medium, and a light beam reflected and diffracted by the information medium according to a light amount. In an optical head device including a photodetector that outputs an electric signal, an optical head device is characterized in that a collimator lens arranged near the photodetector is a combination of a refractive lens and a hologram lens.

Alternatively, a semiconductor laser light source, an imaging optical system that receives a light beam from the light source and converges it into a minute spot on an information medium, and a light beam reflected and diffracted by the information medium according to a light amount. In an optical head device in which a photodetector that outputs an electric signal and a hologram lens in the imaging optical system are combined, the refractive index at the wavelength λ of the glass material forming the hologram lens is n (λ), when reading a record. When the light source wavelength λ = λ0, the height H of the irregular shape of the hologram lens is set to H = λ0 / (n (λ0) −1).

Still further, a semiconductor laser light source, an imaging optical system for receiving a linearly polarized light beam from the light source and converging it into a minute spot on an information medium, and an amount of light received by the light beam reflected and diffracted by the information medium. In the optical head device in which a photodetector that outputs an electric signal according to the above and a hologram lens in the imaging optical system are combined, one of the hologram lenses is a polarization anisotropic hologram and a λ / 4.
The optical head device is configured by combining the above.

[0031]

By using the above means, (1) a hologram lens and an objective lens are used in combination, or the hologram lens is directly molded integrally with the objective lens 4 to overcorrect the chromatic aberration of the objective lens and focus the collimator lens. Since the two effects of changing the distance and changing the amount of astigmatism of the semiconductor laser cancel each other out, it is possible to prevent astigmatism from occurring on the information medium even if the output of the semiconductor laser is changed. Moreover, the chromatic aberration of the entire optical system can be removed by using only one hologram lens in the image forming optical system from the semiconductor laser to the information medium. (2) By adding a lens function, a wavefront conversion function, and a dividing function to a hologram close to the photodetector, and also having characteristics as an optical element for generating a servo signal, the number of parts of the optical head device can be reduced, so that the weight can be reduced. It is possible to reduce the number of manufacturing processes, improve reliability, and reduce costs. (3) If the light source wavelength λ = λ0 at the time of reading the record,
By setting the height H of the concavo-convex shape to H = λ0 / (n (λ0) −1), there is no deterioration of the recording signal, and unnecessary diffracted light components do not occur during reading of the recording, so that noise Fewer read signals can be obtained. (4) A hologram lens of polarization anisotropy is used in combination with a quarter wave plate as an objective lens, and X
When linearly polarized light in the direction of incidence is made incident on the hologram lens, it is subjected to the lens effect by diffraction and is also divided into 1/4.
It becomes circularly polarized light by the wave plate, and when it is reflected by the information medium, the rotation direction of the circularly polarized light is reversed, and it becomes 1 again.
Since it enters the / 4 wavelength plate and becomes a linearly polarized light in the direction perpendicular to the beginning (Y direction), it is not diffracted by the hologram lens in the return path, and becomes a spherical wave with a curvature different from that of the beginning. Return to. Therefore, the imaging relationship does not hold between the information medium and the semiconductor laser on the return path,
The return light does not form an image on the active layer of the semiconductor laser, so that the return light hardly enters the active layer. Therefore, the problem of noise of the semiconductor laser due to the returning light can be avoided.

In addition, a polarization anisotropic hologram lens 1 /
Collimate lens B close to a photodetector together with a four-wave plate
When the linearly polarized light in the X direction is made incident on the hologram lens in the forward path, it is subjected to the lens effect by diffraction, becomes circularly polarized light by the quarter wavelength plate, and is further reflected by the information medium. At that time, the rotation direction of the circularly polarized light is reversed, and when it is incident on the quarter-wave plate again, it becomes a linearly polarized light in the direction perpendicular to the beginning (Y direction), so that it is not diffracted by the hologram lens in the return path,
It becomes a spherical wave with a different curvature from the beginning and returns to the semiconductor laser. For this reason, in the return path, the image formation relationship does not hold between the information medium and the semiconductor laser, the return light does not form an image on the active layer of the semiconductor laser, and therefore the return light hardly enters the active layer. Therefore, the problem of noise of the semiconductor laser due to the returning light can be avoided.

[0033]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of the first embodiment of the present invention. 21
Is a semiconductor laser light source. The light beam 3 (laser light) emitted from the semiconductor laser 21 is made into substantially parallel light by the collimator lens B (122), and is shaped by the wedge prism 35 (the elliptical light amount distribution is biaxial perpendicular to the optical axis. After being subjected to the same direction, that is, shaped into a substantially circular shape (sometimes called elliptical beam correction), the light passes through the beam splitter 36 and enters the hologram lens 107 and the objective lens 4, It is condensed on the medium 5 (in the present application, “condensing” is defined as converging a light beam into a minute spot). The optical beam reflected by the information medium 5 traces the original optical path in the opposite direction, is reflected by the beam splitter 36, is condensed by the collimator lens A (121), and is focused by the servo signal detecting means 34 to a focus error signal or a tracking error. After the wavefront is converted so that a signal can be obtained, it is incident on the photodetector 7. By calculating the output of the photodetector 7, the servo signal (focus error signal and tracking error signal) and the information signal can be obtained. The hologram lens is a phase type diffraction element, and the hologram pattern of the hologram lens is concentric. The center of the hologram pattern preferably coincides with the center of the objective lens within an assembly error in order to reduce off-axis aberration. Here, the objective lens 4 is a driving device 11
Although it is necessary to move the hologram lens 107 at a high speed by 0, the hologram lens 107 is a flat optical element and is therefore lightweight (several tens of milligrams or less). Further, if the hologram lens 107 is used, the condensing power of the objective lens 4 can be small. Since the weight of the objective lens 4 can be reduced by that much, as shown in FIG. 1, the hologram lens 107 and the objective lens 4 can be used in combination to be integrally driven by the driving device 110. Further, as shown in FIG. 12, by integrally forming the hologram lens 107 directly on the objective lens 4, it is possible to achieve further weight reduction and cost reduction. In this embodiment, contrary to the first conventional example, the chromatic aberration of the objective lens 4 is overcorrected, and when the light source wavelength becomes long, the collimating lens B
The long focal length of (122) is compensated. By doing so, when the wavelength increases as the output of the semiconductor laser 21 increases, the collimating lens B (12
The focal length fc in 2) becomes longer, and δv and δh decrease as fc becomes longer, and as a result, δh / δv becomes larger than γ ² . On the other hand, when the output of the semiconductor laser is increased, δ decreases, so δh / δv decreases. Since the two effects cancel each other out in this way, the semiconductor laser 2
Even if the output of 1 is changed, the relationship of the expression (7) can be maintained, and it is possible to prevent astigmatism from occurring on the information medium 5. Moreover, the chromatic aberration of the entire optical system can be removed by using only one hologram lens 107 in the imaging optical system from the semiconductor laser 21 to the information medium 5. Therefore, in contrast to the third conventional example, there is an effect that the optical head device can be downsized and the cost can be reduced. Further, as described above, since the objective lens 4 and the hologram lens 107 can be integrated,
It is not necessary to remove aberrations other than chromatic aberration independently,
The aberrations of the objective lens 4 and the hologram lens 107 may be comprehensively removed. Therefore, the degree of freedom in designing the objective lens 4 is high, and a design that is easy to manufacture is possible. Therefore, cost reduction can be achieved. Furthermore, if the objective lens 4 and the hologram lens 107 are integrated, only one common holding means is required, so that the cost can be reduced and the optical system can be downsized.

As described above, the semiconductor laser 2 of the present invention is used.
In the image forming optical system from 1 to the information medium 5, there is an effect that the shift of the focal length due to the wavelength change can be suppressed and the astigmatism does not occur. The reason for this is that the light beam shaping means is a wedge. Objective lens 5 rather than die prism 35
On the side close to the
This is because 7 is arranged.

Further, since the hologram lens 107 and the objective lens 4 both have a convex lens function, the curvature of the objective lens 4 can be reduced. Further, as the glass material of the objective lens 4, a glass material having a relatively large dispersion value (abbe number of 60 or less) can be used, and the refractive index becomes relatively large, so that the objective lens 4 also has a small curvature. Therefore, the objective lens can be easily manufactured, and the cost can be reduced.

Further, the collimator lens B (122) having a dispersion value that simultaneously satisfies the condition of the removal of chromatic aberration and the expression (7).
When there is no glass material for use, it is possible to combine the collimating lens B (122) with the hologram lens 109 as in the second embodiment shown in FIG. 2 so that the wavelength dispersion characteristic can be freely designed. You can also get it.

An image forming optical system which is reflected by the information medium 5, passes through the objective lens 4 again, is reflected by the beam splitter 36, and is condensed on the photodetector 7 by the collimator lens A (121). Also, in order to avoid the occurrence of the focus error signal offset, it is necessary to correct the chromatic aberration. As described above, in the present invention, since the objective lens 4 is combined with the hologram lens 104 and used as a color-developing lens, the collimating lens A (12
Also in 1), the characteristic that the focal length becomes longer with the increase in wavelength may be left as it is, but by using the hologram 103 as shown in FIG. 3 as the third embodiment, a more complete achromatic characteristic can be obtained. The lens can be obtained or the lens can be formed using a less expensive glass material. Further, by adding a lens action, a wavefront conversion, and a dividing action to the hologram 103, the hologram 103 can also serve as a characteristic as an optical element for generating a servo signal. Examples of such holograms are shown in FIGS.
Shown in.

In this embodiment, a spot size detection method (SSD) is used as a focus error signal detection method.
Method) will be described. SSD method
As disclosed in Japanese Patent Laid-Open No. 2-185722, this is a detection method capable of relieving the assembly tolerance of the optical head device remarkably and obtaining a servo signal stably with respect to wavelength fluctuation.

In order to realize the SSD method, the + 1st order diffracted light on the return path of the hologram is designed to be two types of spherical waves having different curvatures. Each spherical wave is designed so as to have a focal point on the front side e and the rear side f of the plane of the photodetector 7 in FIG. 4, and the interference fringes are actually recorded by using the two-beam interference method, or the computer generated hologram (CGH) is used. ) Is used to construct the interference fringes. Then, as shown in FIG.
The diffracted lights 141 and 142 are received by the 6-division photodetector 71 formed on the photodetector 7. Here, (b) shows the just focus state, and (a) and (c) show the defocus state. Therefore, the focus error signal FE is FE = (S10 + S30-S20)-(S40 + S60-S50). ． ． It is obtained by the operation (8).

FIG. 6 shows an example of realizing a hologram for the SSD method. In FIG. 6, a region 151 generates a spherical wave 141 (FIG. 4) having a focus on the front side of the photodetector, and a B region 152 has a spherical wave 142 having a focus on the rear side of the photodetector.
(Fig. 4) is generated. The far-field pattern of the wavefront diffracted from the hologram pattern as shown in FIG. 4 is partially cut off as shown in FIG. 6 reflecting that the hologram pattern is divided, but this does not affect the focus error signal.

Although it has been stated above that the diffracted light generated from the hologram 103 is designed so as to be two types of spherical waves having different curvatures, as can be seen from FIG. Since the shape changes in the directions are used, the two light beams need only have one-dimensional focal positions in a predetermined direction (focal line in the Y direction) on the front side and the back side of the photodetector, respectively. It is not limited to spherical waves. For example, it may include astigmatism.

The light reflected on the information medium 5 has a diffraction pattern by being diffracted by the track grooves on the information medium 5. Therefore, a change in the relative position between the focused spot on the information medium 5 and the track groove causes a change in the light amount distribution on the hologram. For example, with the X direction in FIG. 6 as the direction parallel to the track groove of the information medium, the + Y direction becomes brighter and the −Y direction becomes darker, or the opposite light amount change occurs.

Therefore, it is desirable that the area division of FIG. 6 is made into several to several tens as shown here. This is because the asymmetry in the + Y direction and the −Y direction is reduced by dividing the hologram area into multiple parts in this way, and the light amount on the hologram due to the relative position change between the focused spot on the information medium 5 and the track groove. This is because it is possible to prevent an offset from occurring in the focus error signal due to the influence of the distribution change. Therefore, if the hologram area is divided into multiple areas, stable focus servo characteristics can be obtained.

Further, in order to extract the change in the light amount distribution on the hologram due to the change in the relative position of the focused spot on the information medium 5 and the track groove as the tracking error signal TE, another diffraction area is provided as shown in FIG. 153 and 1
54 may be provided on the hologram 104. The tracking error signal detecting diffracted light 163 from the diffraction areas 153 and 154 is received by the tracking error signal detecting photodetector 72 (FIG. 7), and the tracking error signal TE can be obtained by the calculation shown in Expression (9). .

TE = S70-S80. ． ． (9) As described above, by adding the lens action, the wavefront conversion, and the dividing action to the hologram 103 so that the hologram 103 also serves as the characteristic as the servo signal generating optical element, the number of parts of the optical head device can be reduced, and thus the weight can be reduced. It is possible to obtain effects such as reduction in the number of manufacturing steps, improvement in reliability, and cost reduction.

Next, control of the diffraction efficiency of the hologram lens (or hologram) 107 will be described as a fourth embodiment.

As shown in FIG. 14, the hologram lens is a hologram as a multi-level hologram having a stepwise cross-sectional shape by repeating the photolithography process and the etching generally performed in the manufacturing process of a silicon LSI a plurality of times. A lens can be made. Further, as shown in Document 4, a hologram lens can be manufactured by using an ultra-precision CNC lathe (see FIG. 15), but in any case, as shown in FIG. 8, the surface of the hologram substrate 29 is shown. A relief is formed on the surface to generate a phase difference of the transmitted light. At this time, the height of this uneven shape (when the hologram lens is formed by the method of FIG. 14, the virtual height on the line connecting the peaks of the unevenness) H, the wavelength λ, and the refractive index n (λ) The amount of phase difference is determined. Of course, it is desirable to design so that the required diffraction efficiency of the diffracted light (+ 1st-order diffracted light) is maximized regardless of the wavelength λ, but in reality, the diffraction efficiency changes depending on the wavelength λ. Since the wavelength of several nanometers is different due to the difference in light source output at the time of reading the record and at the time of recording, an optimum design for the diffraction efficiency is required.

If the diffraction efficiency is lowered during reading of recording, unnecessary diffracted light such as zero-order diffracted light (transmitted light) is also generated, and these diffracted lights reach the disk and are reflected, causing noise to the photodetector. Since it is carried, the read signal is deteriorated. On the other hand, at the time of writing a record, even if the diffraction efficiency decreases and unnecessary diffracted light such as 0th-order diffracted light (transmitted light) is generated, the diffracted light of an unnecessary order is generated due to the difference in wavelength of several nm. Since the light is within 1% at the most, even if they reach the disc, recording is not performed with light having a certain intensity or less, so that deterioration of the recording signal does not occur. Therefore, it is understood that H should be designed so that the required diffraction efficiency of the diffracted light (+ 1st order diffracted light) is maximized when reading the recording. Therefore, assuming that the light source wavelength is λ = λ0 at the time of reading a record, the height H of the uneven shape is H = λ0 / (n (λ0) −1). ． ． In the case of (10), it is possible to obtain a reading signal with less noise without deterioration of the recording signal.

Further, as a fifth embodiment, an example of reducing the return light noise of the semiconductor laser by using a polarization anisotropic hologram will be described with reference to FIG. In FIG. 9, 108
Is a polarization anisotropic hologram lens, and 15 is a λ / 4 wavelength plate. The polarization anisotropic hologram 108 is disclosed in JP-A-61-1.
As disclosed in 89504, linearly polarized light in a certain polarization direction (X direction) is diffracted to act as a hologram lens, and is converted into linearly polarized light in a direction (Y direction) perpendicular thereto. In contrast, it has the property of not causing diffraction. Therefore, as shown in FIG. 9, this hologram lens 1
08 is used in combination with the objective lens 4 together with the 1/4 wavelength plate, and a linearly polarized light beam 30 in the X direction is used in the forward path
When 2 is incident on the hologram lens 108, it is subjected to a lens effect by diffraction and becomes a circularly polarized light beam 303 by the quarter-wave plate 15, and further, when it is reflected by the information medium 5, the circularly polarized light is rotated. Since the direction is reversed and the light enters the quarter-wave plate 15 again and becomes linearly polarized light in the direction perpendicular to the beginning (Y direction), it is not diffracted by the hologram lens 108 in the return path and has a curvature different from the beginning. The semiconductor laser 2 becomes a spherical light beam 304.
Return to 1. Therefore, on the return path, the image formation relationship is not established between the information medium 5 and the semiconductor laser 21, the return light does not form an image on the active layer of the semiconductor laser, and therefore the return light hardly enters the active layer. Therefore, the problem of noise of the semiconductor laser due to the returning light can be avoided.

Contrary to the first conventional example, in this embodiment, the chromatic aberration of the objective lens 4 is overcorrected, and the longer focal length of the collimating lens B (122) is compensated for when the light source wavelength becomes longer. ing. By doing so, when the wavelength increases as the output of the semiconductor laser 21 increases, the focal length fc of the collimating lens B (122) becomes longer, and δ
v and δh decrease as fc becomes longer, and as a result δ
h / δv becomes larger than γ ² . On the other hand, when the output of the semiconductor laser is increased, δ decreases, so δh /
δv decreases. Since the two effects thus cancel each other out, it is possible to maintain the relationship of Expression (7) even if the output of the semiconductor laser 21 is changed, and prevent astigmatism from occurring on the information medium 5. There is. In addition, the chromatic aberration of the entire optical system can be removed only by using the hologram lens 108 and the quarter-wave plate 15 in the image forming optical system from the semiconductor laser 21 to the information medium 5.
Therefore, in contrast to the third conventional example, there is an effect that the optical head device can be downsized and the cost can be reduced. Further, since the hologram lens 108 and the quarter-wave plate are very lightweight, the objective lens 4 and the hologram lens 108
Since it is possible to integrate the ¼ wavelength plate and the ¼ wavelength plate, it is not necessary to remove aberrations other than chromatic aberration independently, and it is sufficient to remove aberrations of the objective lens 4 and the hologram lens 108 comprehensively. Therefore, the degree of freedom in designing the objective lens 4 is high,
A design that is easy to manufacture is possible. Therefore, cost reduction can be achieved. Furthermore, if the objective lens 4 and the hologram lens 108 are integrated, only one holding means is required in common, so that there is an effect that the cost can be reduced and the optical system can be downsized.

As a sixth embodiment, a hologram lens 108 having polarization anisotropy as shown in FIG.
When used in combination with the collimator lens B (122) together with the wave plate, when the linearly polarized light beam 3 in the X direction is made incident on the hologram lens 108 in the forward path, it is subjected to the lens action by diffraction and also the quarter wave plate 15 Becomes circularly polarized light, and when it is reflected by the information medium 5, the rotation direction of the circularly polarized light is reversed, and when it is incident on the quarter wavelength plate 15 again, a straight line in a direction perpendicular to the beginning (Y direction). Since it becomes the polarized light beam 305, it is not diffracted by the hologram lens 108 on the return path, and becomes a spherical wave having a curvature different from that at the beginning and returns to the semiconductor laser 21. Therefore, on the return path, the image formation relationship is not established between the information medium 5 and the semiconductor laser 21, the return light does not form an image on the active layer of the semiconductor laser, and therefore the return light hardly enters the active layer. Therefore, there is an effect that the problem of noise of the semiconductor laser due to the returning light can be avoided.

Also in this embodiment, the hologram lens 1 is used in the image forming optical system from the semiconductor laser 21 to the information medium 5.
The chromatic aberration can be removed only by using the 08 and the quarter wave plate 15. Therefore, in contrast to the third conventional example, there is an effect that the optical head device can be downsized and the cost can be reduced.

In particular, the hologram lens 1 shown in FIG.
In place of 09, the hologram lenses 108 and 1 of this embodiment
By using the / 4 wavelength plate, it is possible to obtain the effect that the chromatic aberration and astigmatism of the entire image forming optical system can be removed, and that the image forming optical system without the return light noise of the semiconductor laser can be configured. .

Further, as a seventh embodiment, an embodiment of an optical information device constituted by using the optical head device of the present invention is shown in FIG.
Shown in. In FIG. 11, the information medium 5 is rotated by the information medium driving mechanism 405. The optical head device 311 is roughly moved by the optical head device driving device 312 to a track on the information medium 5 where desired information exists.
The optical head device 312 also sends a focus error signal and a tracking error signal to the electric circuit 403 corresponding to the positional relationship with the information medium 5. The electric circuit 403
In response to this signal, sends a signal for finely moving the objective lens to the optical head device 311. In response to this signal, the optical head performs focus servo and tracking servo on the optical disc to read information from, write, or erase information on the information medium 5. The optical information device of this embodiment is the optical head device 311.
As an optical head device capable of obtaining an information signal having a very good S / N ratio as described above in the present invention, there is an effect that information can be reproduced accurately and stably. Further, since the optical head device of the present invention is small and lightweight, the optical information device of this embodiment using the optical head device is also small and lightweight, and has an effect of short access time. Further, since the optical head device of the present invention can suppress chromatic aberration, information can be stably recorded. Therefore, the optical information device of this embodiment using this can also stably perform recording / reproduction with a good S / N ratio. effective. Particularly, in the optical head devices of the first to fifth embodiments, astigmatism caused by the astigmatic difference of the semiconductor laser of the light source can be suppressed, so that the optical information device of the present embodiment using this is particularly stable. There is an effect that recording / reproducing with a good S / N ratio can be performed.

[0055]

As is clear from the above description,
According to the present invention, the following effects can be obtained. (1) A hologram lens and an objective lens are used in combination, or the hologram lens is directly molded integrally with the objective lens 4 to overcorrect the chromatic aberration of the objective lens. Since the two effects of changing the amount of point difference cancel each other out, it is possible to maintain the relationship of formula (7) even if the output of the semiconductor laser is changed, and prevent astigmatism from occurring on the information medium. effective. Moreover, it is possible to eliminate the chromatic aberration of the entire optical system by using only one hologram lens in the image forming optical system from the semiconductor laser to the information medium, and to realize downsizing and cost reduction of the optical head device. effective. In addition, since it is not necessary to use a low-dispersion glass material that is high in material cost and difficult to mold, there is an effect that the cost can be reduced and the aspherical lens can be easily processed.

Further, it is not necessary to use a lens group or the like.
Since it is a configuration that only adds one hologram lens,
The optical head device can be downsized and the cost can be reduced. Further, since the objective lens and the hologram lens can be integrated, it is not necessary to remove aberrations other than the chromatic aberration independently, and it is sufficient to remove the aberrations of the objective lens and the hologram lens comprehensively. Therefore, the degree of freedom in designing the objective lens is high, and a design that is easy to manufacture is possible.
Therefore, cost reduction can be achieved. Furthermore, if the objective lens and the hologram lens are integrated, only one holding means is required in common, so that the cost can be reduced.
There is an effect that the optical system can be downsized. (2) By adding a lens function, a wavefront conversion function, and a dividing function to a hologram close to the photodetector, and also having characteristics as an optical element for generating a servo signal, the number of parts of the optical head device can be reduced, so that the weight can be reduced. It is possible to obtain effects such as reduction in the number of manufacturing steps, improvement in reliability, and cost reduction. (3) If the light source wavelength λ = λ0 at the time of reading the record,
By setting the height H of the concavo-convex shape to H = λ0 / (n (λ0) -1), an unnecessary diffracted light component does not occur during reading of the recording, so that a reading signal with less noise can be obtained. You can get the effect. (4) A hologram lens of polarization anisotropy is used in combination with a quarter wave plate as an objective lens, and X
When linearly polarized light in the direction of incidence is made incident on the hologram lens 108, the lens effect is caused by diffraction, and
It becomes circularly polarized light by the quarter-wave plate, and when it is reflected by the information medium, the rotation direction of circularly polarized light is reversed,
Since it enters the quarter-wave plate again and becomes linearly polarized light in the direction perpendicular to the beginning (Y direction), it is not diffracted by the hologram lens on the return path, and becomes a spherical wave having a different curvature from the beginning. Return to. For this reason, in the return path, the image formation relationship does not hold between the information medium and the semiconductor laser, the return light does not form an image on the active layer of the semiconductor laser, and therefore the return light hardly enters the active layer.
Therefore, there is an effect that the problem of noise of the semiconductor laser due to the returning light can be avoided. Further, when a polarization anisotropic hologram lens is used in combination with a collimate lens B close to a photodetector together with a quarter wavelength plate, and linearly polarized light in the X direction is made incident on the hologram lens in the outward path, diffraction causes the lens action. After being received, it becomes circularly polarized light by the quarter-wave plate, and when it is reflected by the information medium, the rotation direction of the circularly polarized light is reversed, and when it enters the quarter-wave plate again, it is at a right angle to the beginning. Since the light is linearly polarized in the direction (Y direction), it is not diffracted by the hologram lens on the return path, and returns to the semiconductor laser as a spherical wave having a curvature different from that at the beginning. For this reason, in the return path, the image formation relationship does not hold between the information medium and the semiconductor laser, the return light does not form an image on the active layer of the semiconductor laser, and therefore the return light hardly enters the active layer. Therefore, there is an effect that the problem of noise of the semiconductor laser due to the returning light can be avoided. (5) Since the optical information device configured by using the optical head device of the present invention uses the optical head device capable of obtaining the information signal having a very good S / N ratio described above in the present invention, it is possible to reproduce the information. It has an effect that it can be executed accurately and stably. Further, since the optical head device of the present invention is small and lightweight, the optical information device of this embodiment using the optical head device is also small and lightweight, and has an effect of short access time. Further, since the optical head device of the present invention can suppress chromatic aberration, information can be stably recorded. Therefore, the optical information device of this embodiment using this can also stably perform recording / reproduction with a good S / N ratio. effective. Particularly, in the optical head devices of the first to fifth embodiments, astigmatism caused by the astigmatic difference of the semiconductor laser of the light source can be suppressed, so that the optical information device of the present embodiment using this is particularly stable. There is an effect that recording / reproducing with a good S / N ratio can be performed.

[Brief description of drawings]

FIG. 1 is a schematic sectional view of an optical head device according to a first embodiment of the present invention.

FIG. 2 is a schematic sectional view of an optical head device according to a second embodiment of the present invention.

FIG. 3 is a schematic sectional view of an optical head device according to a third embodiment of the present invention.

FIG. 4 is a schematic cross-sectional view for explaining a state of diffracted light (spherical wave) obtained from a hologram and a method for designing a hologram in a third embodiment of the present invention.

5A is a plan view for explaining a defocused state of diffracted light on the photodetector of the third embodiment of the present invention, and FIG. 5B is a light of the third embodiment of the present invention. A plan view for explaining a focused state of diffracted light on a detector is a plan view for explaining a defocused state of diffracted light on a photodetector of the third embodiment of the present invention.

FIG. 6 is a plan view showing a hologram pattern of a hologram according to a third embodiment of the present invention.

FIG. 7 is a schematic perspective view of essential parts (hologram pattern and photodetector) of an optical head device according to a third embodiment of the present invention.

FIG. 8 is a schematic sectional view of a hologram lens which is a requirement of the fourth embodiment of the present invention.

FIG. 9 is a schematic cross-sectional view of a polarization anisotropic hologram lens, a quarter-wave plate and an objective lens, which are requirements of the fifth embodiment of the present invention.

FIG. 10 is a schematic cross-sectional view of a polarization anisotropic hologram lens, a quarter-wave plate and a collimator lens, which are requirements of the fifth embodiment of the present invention.

FIG. 11 is a schematic sectional view of an optical information device according to an embodiment of the present invention.

FIG. 12 is a schematic sectional view of a conventional optical head device.

FIG. 13 is a schematic sectional view of an achromatic lens using a conventional hologram lens.

FIG. 14 is a schematic explanatory view of a conventional example and a production example of a hologram lens which is a requirement of an example of the present invention.

FIG. 15 is a schematic explanatory view of a conventional example and an example of producing a hologram lens which is a requirement of the example of the present invention.

FIG. 16 is a schematic sectional view of a conventional achromatic optical system.

FIG. 17 is a diagrammatic explanatory view showing a part of a trajectory of a light beam in the optical system of the conventional example and the example of the present invention.

[Explanation of symbols]

 3 light beam 4 objective lens 5 information medium 7 photodetector 21 semiconductor laser 34 servo signal detecting means 35 wedge prism 36 beam splitter 107 hologram lens 110 driving means 121 collimating lens A 122 collimating lens B 301 light spot

(72) Inventor Hidehiko Wada 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. (72) Seiji Nishino, 1006 Kadoma, Kadoma City, Osaka Matsushita Electric Industrial Co., Ltd.

Claims (13)

[Claims] 1. A semiconductor laser light source and an imaging optical system for converging a light beam emitted from the light source into a minute spot on an information medium, wherein an objective lens arranged near the information medium is a refraction type. The hologram lens is a phase type diffractive element, the hologram pattern of the hologram lens is concentric, and the center of the hologram pattern substantially coincides with the center of the objective lens. An imaging optical system characterized in that 2. The hologram lens has a convex lens function, and the longer the wavelength λ of the semiconductor laser light source, the shorter the total focal length f of the hologram lens and the objective lens. Imaging optics. 3. A collimator lens B is provided between the hologram lens and the semiconductor laser light source, and a light beam shaping means is provided between the hologram lens and the collimator lens B. The imaging optical system according to 1 or 2. 4. The imaging optics according to claim 1, wherein the hologram lens is a combination of a polarization anisotropic hologram and a quarter-wave plate. system. 5. The image forming optical system according to claim 1, wherein the hologram lens and the objective lens are integrated. 6. The image forming optical system according to claim 1, wherein the hologram lens is formed on a surface of the objective lens. 7. The image forming optical system according to claim 3, wherein the collimator lens B comprises a refracting lens C and a hologram lens D. Optical system. 8. An image forming optical system according to any one of claims 1 to 7, and a photodetector for receiving an optical beam reflected and diffracted by the information medium and outputting an electric signal according to a light amount. Optical head device. 9. The optical head device according to claim 8, wherein:
It comprises a branching means for branching the light beam reflected from the information medium, and a collimator lens A for focusing the light beam branched by the branching means on the photodetector, wherein the collimator lens A is a refraction type lens E. An optical head device comprising a combination of a hologram and a hologram. 10. The optical head device according to claim 9, wherein the hologram, which is a constituent element of the collimator lens, is composed of a plurality of divided hologram patterns. 11. The optical head device according to claim 9 or 10, wherein the hologram, which is a constituent element of the collimator lens, is composed of a hologram pattern that is not concentric. 12. An imaging optical system according to any one of claims 1 to 3 or claim 5 and a light beam reflected and diffracted by the information medium, and outputs an electric signal according to a light quantity. And a light source wavelength λ at the time of reading a record, wherein the refractive index at the wavelength λ of the glass material forming the hologram lens is n (λ).
An optical head device characterized in that the height H of the concave-convex shape of the hologram lens is H = λ0 / (n (λ0) −1), where λ0. 13. A drive mechanism for an information medium, and claims 8 to 12.
9. An optical information device comprising at least the optical head device according to any one of claims 1 to 3, a focus servo mechanism and a tracking servo mechanism, an electric circuit for realizing the servo mechanism, and a connection part for a power source or an external power source.