JPH0862496A - Recording and reproducing optical system for optical information recording medium - Google Patents

Abstract

PURPOSE: To provide a focusing lens composed of a single lens which can ensure a stroke distance and a focusing function and with which variation in focusing position caused by a wavelength is less, by forming ring bands with which the thickness of a lens is micro-steppedly changed as the distance from the optical axis is increased, on at least one surface of the focusing lens. CONSTITUTION: A laser beam emitted from a light source 1 transmits through a cover glass pane 2, is then incident upon a beam splitter 3, and is reflected by a half mirror 3a. Then, the beam is refracted by a focusing lens 4, then transmits through a recording surface protecting layer 5, and is focused onto a recording surface 5a. The light focused on the recording surface 5a is reflected, and reversely transmits through the focusing lens 4. Then, it is incident upon the beam splitter 3, then transmits the half mirror 3a, and enters a signal processing device which therefore reads data on the recording surface 5a. With this arrangement, no collimator lens is used, and both surfaces r5, r6 of the focusing lens 4 are aspherical, and further, at least one of them is formed therein ring bands which increases micro-steppedly the thickness of the lens as the distance from the optical axis O is increased.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical system for recording / reproducing information on an optical information medium, and particularly to an information recording surface of a laser beam from a light source, without using a collimator lens, only by an image forming lens having a finite image forming magnification. The present invention relates to an optical system for recording and reproducing optical information.

[0002]

2. Description of the Related Art As a lens structure of an optical system for recording and reproducing optical information, a collimator lens for making divergent light emitted from a light source into substantially parallel light and an objective for forming the parallel light on an information recording surface. There are a so-called infinite system configuration including a lens and a so-called finite system configuration including one image forming lens having the functions of the collimator lens and the objective lens.

In an optical information recording / reproducing apparatus, focusing is performed by moving an image forming lens in the optical axis direction with respect to surface wobbling of an information recording surface. Therefore, it is desired to reduce the size and weight of the lens. Therefore, in a read-only system such as a compact disc, a single lens using an aspherical surface is used.

On the other hand, a semiconductor laser used as a light source of an optical information recording / reproducing apparatus has a characteristic that the oscillation wavelength shifts due to a change in output power or a change in temperature. Therefore, in an ordinary optical information recording / reproducing apparatus that switches the recording / reproducing operation by directly modulating the output of the semiconductor laser, the oscillation wavelength of the laser changes at the time of switching the operation. Therefore, if chromatic aberration remains in the lens, the focus may be deviated when the operation is switched, and writing and reading of information may be erroneous.

As a method for solving such a problem,
Although there is a method of forming each lens by a lens whose chromatic aberration is corrected, the lens whose chromatic aberration is corrected is about 10 times as heavy as the aspherical single lens.

On the other hand, the above-mentioned type is disclosed in JP-A-62 / 62
No. 2,692,922 proposes a method of excessively correcting chromatic aberration with a collimator lens.
No. 14 shows a method of correcting chromatic aberration that does not increase the load on the focus mechanism by combining a chromatic aberration correction group having almost no power with an objective single lens.

However, in the above type, there is no collimator lens, the distance between the light source and the image forming lens is determined by the image forming magnification, and there is no space for inserting the chromatic aberration correcting element. Therefore, the image forming lens is a single lens. Then, I had to give up correcting chromatic aberration. That is, the type (1) is originally intended to reduce the size and weight by providing the action of the collimator lens and the objective lens in one imaging lens, and increasing the number of lenses to correct chromatic aberration increases the size of the device. Therefore, it was necessary to give up, and it was not possible to obtain good recording and reproducing characteristics with this type.

Further, in Japanese Examined Patent Publication No. 4-5362, a chromatic aberration-corrected cemented aspherical objective lens with two lenses in one group can be seen. When this technique is used for a finite system type, It is difficult to correct the chromatic aberration by this technique because the thickness of the end portion of the positive lens cannot be ensured.

According to the present invention, an optical information recording / reproducing optical system of the above type, that is, a laser beam from a light source is formed on an information recording surface only by an image forming lens having a finite image forming magnification without using a collimator lens. It is an object of the present invention to provide an imaging lens composed of a single lens that secures a working distance and imaging performance in an optical information recording / reproducing optical system for forming an image and has a small change in focus position due to wavelength.

[0010]

SUMMARY OF THE INVENTION In order to achieve the above object, an optical system according to the present invention is an aspherical single lens used at a finite magnification, particularly at a reduction magnification, and at least one surface of the aspherical single lens is separated from the optical axis. A feature is that a ring zone group that changes in a minute stepwise direction is provided in the direction in which the lens thickness increases, and chromatic aberration is corrected by the diffraction effect of this ring zone group.

According to another expression of the present invention, in the optical information recording / reproducing apparatus for forming the laser beam from the light source on the information recording surface, the only lens system is provided between the light source and the information recording surface. An imaging lens composed of a single lens is arranged, and at least one surface of the imaging lens composed of the single lens is formed with a group of annular zones in which the lens thickness changes in a minute stepwise manner with distance from the optical axis, and The amount of step in the optical axis direction between the circular center incident surface of the ring group and the outer ring incident surface, and the adjacent ring incident surface is the light incident on each incident surface for the light of the reference wavelength. To the light having a wavelength shorter than the reference wavelength, the light having a wavelength shorter than the reference wavelength is given a phase difference according to the wavelength shift as the wavelength becomes shorter, and the divergence as a whole is increased. For each light with a wavelength longer than the reference wavelength, It is characterized in that determined to give the overall convergence by giving a phase difference corresponding to the deviation of the wavelength in accordance with the wavelength to the light incident on the surface is increased.

Here, when the ring-shaped group is provided only on one surface, the loss of the light amount is smaller when the lens conjugate point is provided on the surface farther than when the lens conjugate point is provided on the surface closer to the conjugate point. Few. On the other hand, if it is provided on the surface where the conjugate point of the lens is close,
Since the radius of curvature is larger than that of the surface on the far side of the conjugate point, processing is easy. Each has the advantage.

In the present invention, a cover glass of the light source, an optical element for branching the optical path (half mirror or diffractive element), and an information recording surface side are provided as an essential optical member for the optical information recording / reproducing apparatus. The optical material for protecting the recording surface is disposed in the recording medium.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing a main configuration of an optical information recording / reproducing apparatus of the present invention. In the figure, 1 is a light source of a semiconductor laser, 2 is a cover glass thereof, 3 is a beam splitter which is an optical element for branching an optical path, 4 is an imaging lens having a function of a collimator and an objective lens, and 5 is an information recording medium. Is a recording surface protective layer that protects the recording surface 5a.

The laser light emitted from the light source 1 passes through the cover glass 2, enters the beam splitter 3, is reflected by the half mirror 3a thereof, and is refracted by the imaging lens 4 to protect the recording surface of the information recording medium. An image is formed on the information recording medium through the layer 5. The light imaged on the information recording medium is reflected, returns in an inverted state of the original optical path, passes through the imaging lens 4 in the opposite direction, enters the beam splitter 3, and enters the half mirror 3
This time, the light passes through a and enters a signal processing device (not shown) to read the information on the information recording medium.

The structural feature of this embodiment is that the light flux of the light source 1 is focused on the information recording surface 5a of the information recording medium 5 only by the imaging lens 4 without using a collimator lens. That is, the imaging lens 4 has a finite imaging magnification and a reduction magnification.

The first to eighth surfaces of each optical element from the light source 1 to the information recording medium 5 are labeled as r1 to r8 as shown in the figure. The fifth surface r5 and the sixth surface r6 of the imaging lens 4 on the beam splitter 3 side are both aspherical surfaces, and at least one of the surfaces becomes thicker in a stepwise manner as the distance from the optical axis O increases. A ring group is formed.

FIG. 2 is an enlarged view of portion II of FIG. 1, and in this embodiment, the ring group is formed on the fifth surface. Then, as described above, the surface having the ring zone group becomes a diffractive surface and chromatic aberration can be eliminated. Therefore, even if the wavelength of the laser light of the light source changes, the position of the image plane does not change, and correct information can always be read.

The surface shape of the surface having the diffraction effect has a point of intersection between the surface r5 or the surface r6 and the optical axis as an origin, an X axis in the optical axis direction, and is orthogonal to the optical axis and is orthogonal to each other. In the orthogonal coordinate system with axes, the YZ plane including the origin and the m-th
Let ΔX (m) be the distance to a point on the ring zone (m = 0, 1, 2, ... 0 on the optical axis)

[Formula 1] ΔX (m) = Δd × m + X (m) (1)

Here, Δd is a distance on the optical axis when the shapes of adjacent ring zones are extended to the optical axis. Also, the above X
(M) is the aspherical shape defined by the following equation (2),

[Formula 2] X = CH ² / {1+ [1- (1 + K) C ² H ² ] ^1/2 } + A4H ⁴ + A6H ⁶ + A8H ⁸ + ... (2) {H = (Y ² + Z ² ) ^1/2 , C = 1 / r, H is the height from the optical axis, K is the conical coefficient} The r and aspherical surface coefficient Ai in this equation (2) are obtained as a function of m. Is.

[Embodiment 1] Tables 1 to 4 show data of each optical member when a surface having a diffraction effect is provided on the side having a long conjugate distance, that is, the r5 surface. The oscillation reference wavelength of the semiconductor laser is 780 nm, the annular lens as the imaging lens 4 has an imaging magnification β = −0.200, a numerical aperture N.V. A = 0.5, and the distance from the semiconductor laser emission point to the first surface (r5 surface) of the annular lens = 15.18 mm.
Radius of curvature r, thickness d, wavelength 780 from r1 to r8
Table 1 shows the refractive index n (780) in the case of nm and the value of ν _{d at} the d line.
Shown in

[0022]

[Table 1] Surface No. rd n (780) ν _d r1 ∞ 0.25 1.51072 64.1 (Semiconductor laser cover glass) r2 ∞ r3 ∞ 4.00 1.51072 64.1 (Beams slitter) r4 ∞ 2.00 r5 *** 1.85 1.53677 55.7 (Diffraction) Effective surface) r6 -2.970 r7 ∞ 1.2 1.57346 29.9 (recording surface protective layer of recording medium) r8 ∞

The *** mark in the r column of r5 indicates that the surface is an aspherical surface having a ring group, and each coefficient in the equation (2) is shown in Tables 2 and 3.

[Table 2] Surface shape of the fifth surface Δd = -0.001453 r (m) = 1.751 + 0.000253 × m K = -0.646 A4 = -4.356 × 10 ^-3 A6 (m) = (-3.275 + 0.0162 × m) × 10 ^-4 A8 = -1.938 × 10 ^-4 A10 (m) = (-7.700 + 0.0100 × m) × 10 ^-5

[Table 3]

R6 is an aspherical surface without a ring group, and Table 4 shows each coefficient in the equation (2).

[Table 4] Aspherical coefficient of the sixth surface K = 0.000 A4 = 4.323 × 10 ^-2 A6 = -1.227 × 10 ^-2 A8 = 2.176 × 10 ^-3 A10 = -1.568 × 10 ^-4

[Embodiment 2] Tables 5 to 8 show data of each optical element in the case where a surface having a diffraction effect is provided on the r6 surface, which has a shorter conjugate distance. The oscillation reference wavelength of the semiconductor laser, the imaging magnification β of the imaging lens 4, the numerical aperture N.V.
A, and the distance from the light emitting point of the semiconductor laser to the first surface (r5 surface) of the annular lens are the same as in the first embodiment.

[0026]

[Table 5] Surface No. rd n (780) ν _d r1 ∞ 0.25 1.51072 64.1 (Semiconductor laser cover glass) r2 ∞ r3 ∞ 4.00 1.51072 64.1 (Beam slitter) r4 ∞ 2.00 r5 1.751 1.85 1.53677 55.7 r6 *** (Diffraction effect surface) r7 ∞ 1.2 1.57346 29.9 (Recording surface protective layer of recording medium) r8 ∞

R5 is an aspherical surface having no ring group, and Table 6 shows the respective coefficients in the equation (2).

[Table 6] Aspherical coefficient of the fifth surface K = -0.646 A4 = -4.356 × 10 ^-3 A6 = -3.275 × 10 ^-4 A8 = -1.938 × 10 ^-4 A10 = -7.700 × 10 ^-5

*** in the r column of r6 in Table 5 indicates that the surface is an aspherical surface having a ring group, and each coefficient in equation (2) is shown in Tables 7 and 8.

[Table 7] Surface shape of 6th surface Δd = 0.001453 r (m) =-2.970 + 0.00447 × m K = 0.000 A4 (m) = (4.323 + 0.0139 × m) × 10 ^-2 A6 (m) = (- 1.227-0.0025 × m) × 10 ^-2 A8 (m) = (2.176-0.0115 × m) × 10 ^-3 A10 (m) = (-1.568 + 1.0280 × m) × 10 ^-4

[0029]

[Table 8]

The second embodiment is different from the first embodiment only in the easiness of processing and the light utilization efficiency.
Is almost equivalent to

Generally, if the diffractive lens surface is formed so that the step difference of one step corresponds to one wavelength, the first-order diffracted light is used, and the deterioration of the wavefront aberration due to the wavelength change can be suppressed. It is possible to prevent the deterioration of the diffraction efficiency and the imaging performance due to the wavelength change.

If the wavelength used is narrow, or the ring width is narrow and it is difficult to manufacture, chromatic aberration can be corrected even if the step difference in the stairs is set to twice the wavelength or an integral multiple of three times or more. . Shift amount (step) t between adjacent ring zones
Satisfies the following conditions. 0.8 ≦ t (n−1) / λ ≦ 10, where n is the refractive index of the medium forming the lens, and λ is the wavelength of the laser light used. If the upper limit of the above conditions is exceeded and the wavelength is 10 wavelengths or more, the configuration becomes equivalent to that of the conventional Fresnel lens, and the phase matching deviation due to the manufacturing error of the step amount is likely to be large, and the wavelength away from the design wavelength is used. The efficiency of the diffractive lens decreases with respect to the incident light.

When the value is below the lower limit of the above condition, phase matching as a diffractive lens cannot be achieved, and the diffractive lens does not substantially function. FIG. 3 shows the first embodiment.
6 is a diagram showing spherical aberration of the imaging lens 4 shown in FIG. r5
It can be seen that the spherical aberration is almost corrected by making r6 an aspherical surface. FIG. 4 is a diagram showing the axial chromatic aberration of the imaging lens 4 shown in Example 1, in which the horizontal axis represents the wavelength and the vertical axis represents the focus position shift amount. The solid line A is the axial chromatic aberration of the imaging lens 4 of the first embodiment, and the dotted line B is the axial chromatic aberration of the conventional lens having the same configuration as that of the first embodiment and not forming a ring zone. As can be seen from this figure, according to the present invention,
Even if the wavelength of the laser light source changes, the amount of focus shift is very small.

[0034]

According to the present invention as described above,
In the optical system for optical information recording / reproducing, the chromatic aberration is corrected well, so that even if the wavelength of the semiconductor laser fluctuates, the focus position change is small and an error in information recording / reproducing can be prevented. Further, since it is a single lens and no collimator lens is used, the optical system for recording / reproducing optical information can be downsized.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a recording / reproducing optical system of an optical information recording medium of the present invention.

FIG. 2 is an enlarged view of a portion II of FIG. 1 and an exaggeratedly drawn step of an annular zone.

FIG. 3 is a diagram showing spherical aberration of Example 1.

FIG. 4 is a diagram showing axial chromatic aberration of the first example.

[Explanation of symbols]

 1 semiconductor laser light source 2 semiconductor laser cover glass 3 beam splitter 3a half mirror 4 imaging lens 5 recording surface protective layer of information recording medium 5a information recording surface

Claims (4)

[Claims] 1. An optical information recording / reproducing optical system having an image forming lens having a finite image forming magnification, wherein the image forming lens has a step-like shape in which at least one surface has a lens thickness that becomes minute as the distance from the optical axis increases. An optical system for recording / reproducing an optical information recording medium, which is an aspherical single lens having an annular zone group that changes. 2. The recording / reproducing optical system for an optical information recording medium according to claim 1, wherein the ring zone group is provided on a surface of the lens farther from the conjugate point. 3. The recording / reproducing optical system for an optical information recording medium according to claim 1, wherein the ring-shaped group is provided on a surface closer to a conjugate point of a lens. 4. An optical information recording / reproducing apparatus for forming an image of a laser beam from a light source on an information recording surface, wherein an image forming lens composed of a single lens is arranged between the light source and the information recording surface as the only lens system. Then, on at least one surface of the imaging lens composed of this single lens, an annular zone group in which the lens thickness changes minutely in a stepwise manner with distance from the optical axis is formed. For the light of the reference wavelength, the step difference in the optical axis direction between the outer ring-zone incident surface and the adjacent ring-zone incident surface is set to 2π for the light incident on each incident surface.
For a light with a wavelength shorter than the reference wavelength, a phase difference corresponding to the wavelength shift is given to the light incident on each incident surface, and the overall divergence is given. For light with a wavelength longer than the reference wavelength, it was determined that the light incident on each incident surface will have a phase difference according to the wavelength shift as the wavelength becomes longer, and that the light will converge as a whole. An optical system for recording / reproducing an optical information recording medium characterized by the above.