JP3088128B2 - How to adjust the objective lens - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for adjusting an objective lens used in an optical system of an optical information recording / reproducing apparatus requiring a high recording density, for example, a laser disk system for high vision.

[0002]

2. Description of the Related Art In an optical disk system in which data required for one image is larger than a conventional laser disk system, such as a laser disk system for a high-definition television, the number of recorded images per disk is the same at the same recording density as the conventional one. And the number of disks per recording time increases. Therefore, it is necessary to improve the recording density of the optical disc to increase the recording capacity.

Since the recording density of an optical disk is inversely proportional to the square of the spot diameter of a laser beam, it is necessary to narrow the light spot for writing in order to improve the recording density. The size of the light spot is the numerical aperture N. A is inversely proportional to A and is proportional to the wavelength used.
A. It is desired that the objective lens be large and can be used at a short wavelength.

When a lens having a large numerical aperture is used to narrow a spot, the accuracy of aberration correction becomes stricter than before, and sufficient performance cannot be obtained only by improving the processing accuracy of the lens alone. It is necessary to correct the aberration also by adjusting.

Conventionally, correction of spherical aberration at the time of assembling a lens is generally performed by adjusting a lens interval by replacing a spacing ring and adjusting a lens thickness by replacing a lens itself.

[0006]

However, in the adjustment method by replacement as described above, the lens which has been once assembled to the lens barrel and whose aberration has been measured is disassembled to some extent, and the distance ring,
Alternatively, since the lens must be replaced, there is a high possibility that aberrations other than the spherical aberration to be corrected are likely to change due to an error in reassembling.

In particular, in the method of exchanging lenses, it is necessary to prepare a large number of lenses having slightly different thicknesses, and the exchanged lenses themselves may have errors in eccentricity and curvature. There is a high possibility that other aberrations will be generated by correction.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and provides an objective lens adjusting method capable of independently correcting only spherical aberration without disassembling a lens once assembled. The purpose is to do.

[0009]

SUMMARY OF THE INVENTION According to the present invention, there is provided an optical information recording / reproducing apparatus comprising:
A method of adjusting an objective lens, comprising: a front group having a light-condensing function; and a reference parallel plane plate provided on the side of a light-condensing point with respect to the front group.
A laser beam spot is formed in the corrected state.
Assemble a measured objective lens and measure its spherical aberration
If the measured spherical aberration is out of tolerance,
Remove the reference plane and replace it with the reference plane.
The thickness and / or refractive index of the face plate are different.
Thickness and bending required to correct the measured aberrations.
A parallel flat plate having a folding ratio is attached .

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings.
1 and 2 show an embodiment of the objective lens according to the present invention.

The objective lens includes, in order from the light source side on the left side in the figure, first to fifth lenses 1 to 5 forming a front group having a light condensing action, and a parallel plane plate 6 forming a rear group. It is arranged and arranged. The plane-parallel plate 6 is provided in the convergent light on the most converging point side of the objective lens, and has a function of correcting spherical aberration.

The cylindrical lens barrel 10 for holding the front group has steps inside for holding the lenses, and the first to fifth lenses engaged with the steps are screwed from the light source side. It is fixed to the lens barrel 10 by a ring screw 11.

An abutting portion 12 perpendicular to the optical axis Ax is formed on the light converging side of the lens barrel 10 so as to be inserted one step from the surroundings. A flat plate portion 21 that is in close contact with the contact portion 12 is formed.

Since the plane-parallel plate 6 is arranged in the optical path converged by the front group, the diameter of the light beam passing through the plane-parallel plate 6 is reduced. Therefore, it is possible to secure a large area of the abutting portion 12 and the flat plate portion 21 provided around the transmission portion.

A screw 13 is formed in a portion of the peripheral wall of the abutting portion 12 of the lens barrel on the light-condensing point side, and a screw 22 screwed to the screw 13 is formed in a peripheral edge of the frame 20. The frame 20 can be press-fitted to the lens barrel 10 by attaching using a screw. Notches 23 are formed on the frame 20 at two locations to engage a jig for rotating the frame during screwing.

[0016] In this embodiment, the abutting portion 12 flat plate portion 21
And a large area, and by crimping these surfaces with screws during assembly,
To prevent coma from occurring due to the inclination of.

Next, a method of adjusting the objective lens configured as described above will be described.

At the time of assembly, lenses 1 to 5 are incorporated into a lens barrel 10 to constitute a front group, and a frame body 20 to which a reference parallel flat plate 6 having a thickness and a refractive index according to design values is fixed is mirror-mounted. It is screwed on the tube 10.

Then, the spherical aberration of the objective lens is measured using a measuring device such as an interferometer. If the aberration is not within the allowable range, the frame 20 is removed, and a frame to which a parallel flat plate having a thickness and a refractive index capable of correcting the measured aberration is fixed, and the measurement is performed again. This operation is repeated until the spherical aberration falls within the allowable range.
The lens barrel 10 and the frame 20 are bonded and fixed.

By changing the thickness and the refractive index of the plane-parallel plate, the order and amount of the aberrations change. Therefore, the change in the thickness of the plane-parallel plate or the refractive index required to correct a predetermined aberration. The minute can be calculated or measured in advance.

According to the above method, since the spherical aberration can be corrected without changing the arrangement of the lens unit having the convergence difference, other aberrations caused by eccentricity can be changed by the correction as in the related art. I will not let you. In addition, since a parallel plane plate can be easily processed with higher precision than a spherical surface, by applying the spherical aberration correction effect to the parallel plane plate, spherical aberration can be more accurately corrected than by using another lens. Can be corrected.

Next, the operation and adjustment of the plane-parallel plate will be described with reference to specific examples. FIG. 3 shows an objective lens according to the example. The specific numerical configuration is shown in Table 1. In the table, NA is the numerical aperture, f is the focal length, ω is the half angle of view, fB is the back focus, λ is the used wavelength, r is the radius of curvature, d is the surface interval, and Nd is d-line (588 nm).
, And Nλ is the refractive index at the used wavelength. The glass material name is a trade name of OHARA CORPORATION. Tλ is the internal transmittance of a hole having a thickness of 10 mm at a wavelength λnm.

[0023]

[Table 1] NA = 0.90 f = 4.00 ω = 1.1 ° fb = 0.54 λ = 351nm Surface number rd Nd ν Nλ Glass material name T350 1 -6.892 5.730 1.59551 39.2 1.64249 F8 0.924 2 -10.450 1.230-3 23.860 3.390 1.53172 48.9 1.56376 LLF6 0.973 4 -23.860 1.700-5 -7.241 1.810 1.62004 36.3 1.67388 F2 0.850 6 38.129 0.222-7 63.688 3.900 1.58313 59.4 1.61064 SK12 0.929 8 -9.260 0.060-9 10.144 4.350 1.58313 59.4 1.61064 SK12 0.929 10 65.187 0.060-11 4.460 4.870 1.77250 49.6 1.817 LaSF016 0.720 12 3.982 0.440-13 ∞ 0.436 1.51633 64.1 1.53844 BK7 0.915 14 ∞--

FIG. 4 shows the spherical aberration and astigmatism of this configuration, and FIG. 5 shows the wavefront aberration. The left side of the wavefront aberration in FIG. 5 indicates meridional and the right side indicates sagittal, and the unit of the vertical axis representing the amount of aberration is wavelength λ.
From these aberration diagrams, it can be understood that aberration correction is favorably performed by the above configuration.

However, at the time of actual manufacturing, the above-described ideal state is not always realized due to an assembly error or the like. In this case, the aberration is adjusted by replacing the parallel plane plate.

In the following, the effect of changes in the thickness and the refractive index of the plane-parallel plate on the aberration will be described. In the following description, based on the configuration shown in Table 1, the thickness of the plane-parallel plate or the refractive index is changed without changing the configuration of the six lenses.

The thickness of the parallel flat plate is reduced by 1 μm,
When it becomes 0.435 mm, as shown in FIG. 6, the peripheral wavefront advances from about 0.20λ axis.

The glass material forming the parallel plane plate is LaSF016 (n =
1.81713 ), and when the thickness is determined so that the wavefront aberration when the numerical aperture NA is 0.90 is 0, the thickness is 0.461 mm. In this case, since the higher-order component of the aberration generated in the plane-parallel plate is reduced, when the higher-order aberration is corrected as shown in FIG. 7, the lower-order aberration is over-corrected, and the wavefront in the intermediate portion is delayed by 1.0λ.

When the change in the refractive index is used to adjust a minute aberration, the glass material of the parallel plane plate is changed to BK3 (n = 1.51921) having a slightly lower refractive index than BK7, and the thickness is made equal. As shown in FIG. 8, the lower order wavefront is advanced and the higher order wavefront is delayed.

If the thickness of the glass material of the plane-parallel plate is BK6 (n = 1.54560) having a slightly higher refractive index than BKK7 and the thickness is not changed, the lower-order wavefront is delayed as shown in FIG. Can proceed.

As can be understood from the above example, when both the higher-order and lower-order wavefronts are shifted in the same direction, the thickness of the plane-parallel plate is changed so that the higher-order and lower-order components are shifted in different directions. In the case of shifting the refractive index of the plane parallel plate, the refractive index of the plane parallel plate may be changed.

Next, a description will be given of a correction method when a part of the objective lens shown in Table 1 includes a manufacturing error.

[0033] When the thickness of the fourth lens (d7) is thickened 10 [mu] m, as shown in FIG. 10, the wavefront aberration of 1λ occurs. Here, the reference parallel plane plate is 4 μm thicker than this.
Replacing the plane-parallel plate of 0.440Mm, wavefront aberration is corrected as shown <br/> in FIG.

The thickness (d11) of the sixth lens is 10
When the thickness is increased by 1 μm, a wavefront aberration of 1λ occurs as shown in FIG. Here, 0.4 μm thick 0.4 μm as described above.
When replaced with a 40 mm parallel flat plate, high-order aberrations remain as shown in FIG. Therefore, in this case, when the plane-parallel plate is changed to PK1 (n = 1.52428) having a thickness of 0.440 mm, the wavefront aberration is corrected as shown in FIG.

In the above-described example, only an example in which optical glass is used as the plane-parallel plate has been described. However, it is also possible to mold this from plastic. In the case of molding with plastic, the frame portion can also be integrally formed.

[0036]

As described above, according to the present invention, a lens group having a light condensing function and a lens group provided on the side closer to the light condensing point than this front group are provided.
The spherical aberration of the objective lens consisting of the reference parallel plane plate
Measured and the measured spherical aberration is out of tolerance
Since the reference parallel plane plate is replaced, the aberration corresponding to the manufacturing error can be corrected without moving the lens group having high aberration generation sensitivity, and the adjustment becomes easy.

[Brief description of the drawings]

FIG. 1 is a cutaway side view in a state where a lens barrel and a holding frame of a parallel flat plate are separated.

FIG. 2A is a cutaway side view in a state where a lens barrel and a holding frame of a parallel plane plate are joined, and FIG . 2B is a front view from the rear group side.

FIG. 3 is a sectional view of a lens according to an example.

FIG. 4 is a diagram illustrating spherical aberration, a sine condition, and astigmatism of the lens of Example.

FIG. 5 is a wavefront aberration diagram of the lens of the example.

FIG. 6 is a wavefront aberration diagram when the plane-parallel plate of the example is thinned.

FIG. 7 is a wavefront aberration diagram when the refractive index of the plane-parallel plate of the example is increased.

FIG. 8 is a wavefront aberration diagram when the refractive index of the plane-parallel plate of the example is slightly lowered.

FIG. 9 is a wavefront aberration diagram when the refractive index of the plane-parallel plate of the example is slightly increased.

FIG. 10 is a wavefront aberration diagram when the fourth lens of the example is thickened.

FIG. 11 is a wavefront aberration diagram when the fourth lens of the example is made thicker and the parallel plane plate is made thicker to correct the aberration.

FIG. 12 is a wavefront aberration diagram when the sixth lens of the example is thickened.

FIG. 13 is a wavefront aberration diagram when the sixth lens of the example is made thicker and the parallel plane plate is made thicker to correct the aberration.

FIG. 14 is a wavefront aberration diagram in the case where the sixth lens of the example is made thicker, the parallel plane plate is made thicker, and the refractive index is reduced to correct the aberration.

[Explanation of symbols]

 1 to 5: lens (front group) 6: parallel plan view (rear group) 10: lens barrel 12: abutting portion 20: frame body 21: flat plate portion

──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Masahiro Ohno 2-36-9 Maeno-cho, Itabashi-ku, Tokyo Asahi Kogaku Kogyo Co., Ltd. (56) References JP-A-4-237007 (JP, A) Showa 52-72735 (JP, U) Actually open Showa 61-16526 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02B ⁷ /02-7/105 G02B 7/12-7 / 16 G02B 9/00-17/08 G02B 21/02-21/04 G02B 25/00-25/04

Claims (1)

(57) [Claims] 1. An optical information recording / reproducing apparatus for use in an optical system.
This is a method of adjusting the objective lens .
The front group and the reference parallel plane
The spot of the laser beam with the aberration corrected by the plate
A freshly assembled sphere with an objective lens designed to form
The surface aberrations measured, if the measured spherical aberration is outside the permissible range, the removal of the reference plane-parallel plate, at least one of said reference plane parallel plate and the thickness or refractive index is instead Ri Do different, and before
Thickness and refractive index required to correct the measured aberrations
A method for adjusting an objective lens, comprising attaching a parallel plane plate having the following .