JP2902417B2 - Interferometer for measuring wavefront aberration - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interference device for measuring wavefront aberration, and more particularly to an optical information recording / reproducing device for recording and reproducing information on and from a storage medium such as an optical disk. The present invention relates to a wavefront aberration measuring interference device suitable for measuring a wavefront aberration of a laser beam irradiated on a recording medium.

2. Description of the Related Art In general, an interference device for measuring wavefront aberration such as an optical pickup optical system of an optical disk device divides a measured light beam from an aperture into two by a beam splitter, and sets a light beam on one of the optical paths as an object light beam. Using the light beam on the other light path as a reference light beam, the object light beam and the reference light beam are superimposed, and the wavefront aberration is measured by observing interference fringes generated there.

It is required that the amount of wavefront aberration of the light beam emitted from the optical pickup optical system of such an optical disk device is extremely small. Therefore, interferometers for which wavefront aberration should be measured include:
A device for analyzing the interference fringes with high accuracy is provided (for example, ZYGO-8100 of Zygo Co., Ltd. for commercial products, and WYKO-LADITE of Waiko Co., Ltd.).

In such an interferometer, a magazine "Optics" (February 1984, p. 58) is used as a method for analyzing interference fringes with high accuracy.
And the like. In this method, a reference mirror in a reference optical path is finely moved (scanned) in the optical axis direction, and the interference light intensity at that time is detected on an observation surface to perform processing, thereby obtaining a wavefront aberration.

[Problems to be Solved by the Invention] However, the above-described conventional interference device performs so-called temporal scanning in which scanning is performed by a change such as finely moving a reference mirror using a piezo element or the like, and thus interference fringes are changed. If so-called disturbances such as vibrations and air fluctuations occur during the measurement, an error occurs in the measurement, and accurate wavefront aberration measurement cannot be performed.

Therefore, for example, the reference wavefront or the wavefront to be measured is tilted (tilted) at a small angle with respect to the optical axis by a reflecting mirror or a reflecting prism introduced in the magazine "Optics" (February 1984, page 61). If a so-called spatial scanning method (hereinafter referred to as "tilt method") that analyzes a change in interference fringes by spatial scanning due to the angle change is used, such a defect can be solved.

For example, as an apparatus for measuring the wavefront aberration of a light beam emitted from an optical disk apparatus, there is an optical system using a radial sharing interference method as shown in FIG. In the figure, reference numeral 901 denotes a device to be measured such as a pickup optical system of an optical disk device. 902 is a cover glass having a thickness equivalent to an optical disk. An objective lens 903 converts a light beam emitted from the device under test 901 into a parallel light beam. Reference numeral 904 denotes an opening which is located at the position of the exit pupil of the objective lens 903.
The wavefront aberration at the position of 04 is measured. A beam splitter 905 splits a light beam into a reference light beam and an object light beam. 906 is a beam splitter for superimposing two light beams, and 907 is a mirror for bending a reference light beam. Reference numeral 908 denotes a mirror for bending the object light beam. In order to perform high-precision fringe analysis, the mirror 908 is slightly moved in the optical axis direction by a piezo or the like to perform a fringe scanning method.

Reference numeral 909 denotes a beam expander for enlarging the light beam diameter. Usually, a magnification m (share ratio) of about 10 is used. An interference fringe observation lens 910 has a conjugate relationship between the aperture 904 and the observation surface 911. FIG. 4 shows the positional relationship between the aperture 904, the mirror 908 for tilting the object light beam, the observation lens 910, and the observation surface 911. The image of the aperture 904 is an imaging lens
910 forms an observation surface 911 at an appropriate magnification M.

M = b / a Here, in order to apply the tilt method, the mirror 908 is set to Δ
Suppose you have tilted by θ. Since the light beam is tilted by Δα when tilted by Δθ, Δα = 2 · Δθ. Then, since the position of the image is shifted laterally by δ on the observation surface, interference fringes cannot be observed.

 Assuming that the distance between the opening 904 and the mirror 908 is L, δ becomes δ = MLLΔα.

Therefore, for example, M = 1.5, L = 200 (mm), Δα =
If 6 × 10 ⁻³ (rad), a large lateral shift amount of δ = 1.8 (mm) occurs.

Thus, simply rotating the reflection mirror or the reflection prism in the optical path causes a lateral shift of the position of the image of the aperture that defines the interference area formed through the optical path, and the interference occurs. The fringes cannot be observed.

The present invention provides a wavefront aberration measurement interferometer capable of solving such conventional drawbacks and analyzing a fringe pattern with high accuracy by a tilt method without causing a lateral shift of an image position of a measured wavefront. The purpose is to do.

[Means for Solving the Problems] To achieve the above object, a wavefront aberration measuring interference device of the present invention divides a measured light beam emitted from an aperture to be measured into two optical paths by a beam splitter, The light beam in one of the light paths is used as an object light beam, the light beam in the other light path is used as a reference light beam, and these two light beams are superimposed to form an interference fringe on the observation surface. In the interferometer for measuring wavefront aberration, which is configured to measure the direction in which the traveling direction of the object light beam is inclined with respect to the optical axis in the one optical path, and the position where the object light beam is inclined and the aperture are conjugated with each other. Means for providing an optically conjugated positional relationship between the opening, the position where the object light beam is inclined, and the three positions of the observation surface.

[Operation] When the object light beam is slightly inclined with respect to the reference light beam, a fine tilt fringe is obtained as the interference fringe according to the angle change. The intensity of the interference light obtained by giving the required inclination is instantaneously (for example, 1 /
(30 seconds) If the calculation is taken in the processing device, the wavefront aberration can be measured with high accuracy.

Here, if the observation surface has a conjugate relationship between the aperture that determines the interference area, the position where the object light beam is tilted, and the observation surface, the image of the aperture on the observation surface causes a lateral shift even if the object light beam is inclined. Absent.

Example An example will be described with reference to the drawings.

FIG. 1 shows an interferometer for measuring wavefront aberration using a radial sharing interferometer. In the figure, reference numeral 100 denotes an optical head objective lens provided in the optical unit 101 or the like of the optical information recording / reproducing apparatus, and is used to read or write information by irradiating a storage medium such as an optical disk with a laser beam. This is for focusing a laser beam emitted from a diode (not shown) to a diameter of about 1 μm. In the present embodiment, a case where the wavefront aberration of a laser beam emitted from the optical head objective lens 100 is measured will be described as an example.

Reference numeral 1 denotes an objective lens for converting a laser beam emitted from the optical head objective lens 100 into a parallel beam. Reference numeral 3 denotes a mounting surface of the objective lens 1 and also a reference surface of an interference device for measuring wavefront aberration. The device is set to have an inclination or the like with reference to the reference surface 3. Reference numeral 4 denotes a cover glass having a thickness corresponding to an optical disk or the like, and is arranged in parallel with the reference surface 3.

Reference numeral 5 denotes an aperture at the exit pupil position of the objective lens 1,
In this embodiment, the wavefront aberration of the wavefront (measured wavefront) of the laser beam at the position of the opening 5 is measured. Numerals 7 and 8 denote a quarter-wave plate and a half-wave plate rotatably provided around the optical axis for adjusting the polarization state of the laser beam and the direction of the polarization plane.

Reference numeral 10 denotes a beam splitter, and a semi-transparent surface 10a is formed at an angle of 45 degrees with respect to the optical axis of the light beam incident from the objective lens 1. The laser beam converted into a parallel beam by the objective lens 1 is divided by the semi-transparent surface 10a into two beams: a beam that is transmitted straight and a beam that is reflected in a direction perpendicular to the beam. The reflectivity of the semi-transmissive surface 10a is set to about 10% if the share ratio (enlargement ratio of the light beam) is about 10 in order to prevent a decrease in coherence due to a difference in light intensity between the two optical paths.
Reference numeral 11 denotes a shutter which can be freely opened and closed to block a laser beam transmitted through the beam splitter 10.

Reference numeral 12E denotes a beam expander in which the focal lengths of the short focus lens 12a and the long focus lens 12b are substantially matched to each other, and are arranged so that the short focus lens 12a is arranged on the opening 5 side to enlarge the light beam diameter system. Used as a magnifying optical system (hereinafter referred to as “magnifying optical system 12E”).

Reference numeral 16 denotes a first right-angle prism for reflecting the laser beam passing through the magnifying optical system 12E in parallel to the direction inverted by 180 degrees, and the reflected laser beam enters the beam splitter 10 from the opposite direction. To Penetrate.

On the other hand, the laser beam incident on the beam splitter 10 from the objective lens 1 and reflected by the semi-transparent 10a is transmitted to the reduction optical system 12R.
Incident on. This reduction optical system 12R is the one in which the same beam expander as the enlargement optical system 12E is arranged so that the long focus lens 12b is on the object side (the opening 5 side), that is, the enlargement optical system 12E.
The same optical system as that of E is arranged in the opposite direction to reduce the light beam diameter.

Assuming that the enlargement ratio by the enlarged optical diameter 12E is m, the reduction ratio of the wavefront by the reduced optical diameter 12R is 1 / m. Therefore, the relative enlargement ratio, that is, the share between the light beams passing through both optical systems 12E and 12R The ratio is m ² , for example, m ² = 10.

Reference numeral 22 denotes a second right-angle prism for reflecting the laser beam that has passed through the reduction optical system 12R in parallel to the direction that is inverted by 180 degrees, and has the same optical path length of the two beams split by the beam splitter 10 to provide coherence. In order to improve the performance, a prism having exactly the same shape, material and the like as the first right-angle prism 16 is used, and at the same time, it can be finely moved in the optical axis direction by a mechanism not shown.

The second right-angle prism 22 is provided with a ridgeline 22a (first
(Perpendicular to the plane of the drawing) is provided rotatably in a plane perpendicular to the plane of the paper, and the rotation imparts a tilt to the light beam. twenty two
s is the center of rotation.

Reference numeral 22b denotes a surface on the bisector 22c of the second right-angle prism 22, which is arranged so that the image of the aperture 5 is formed thereon by the reduction optical system 12R (hereinafter referred to as "intermediate image forming surface 22b"). ). That is, the intermediate image plane 22b and the position of the aperture 5 are in a conjugate positional relationship by the reduction optical system 12R. When the rotation of the second right-angle prism 22 is a small angle, the intermediate imaging surface 22b in the second right-angle prism 22 can be regarded as a fixed point.

The laser beam reflected by the second right-angle prism 22 and reflected by the semi-transmissive surface 10a of the beam splitter overlaps with the laser beam passing through the first right-angle prism 16 and passing through the beam splitter 10, forming interference fringes. As shown in FIG. 2, the direction can be changed by the reflecting prism 24.

In FIG. 2, reference numeral 23 denotes an observation imaging lens, which forms a reduced image of the wavefront at the position of the aperture 5 formed on the intermediate imaging surface 22b on the imaging surface 26a of the TV camera 26 together. Are located. That is, the imaging surface 26a and the position of the aperture 5 have a conjugate positional relationship, and the imaging surface 26a and the intermediate imaging surface 22b have a conjugate positional relationship in the observation imaging lens 23.

Therefore, a reduced image of the wavefront at the position of the aperture 5 is displayed on the imaging surface 26.
An interference fringe is formed by forming an image on a, and the interference fringe can be observed by a CRT monitor or the like (not shown).

Here, since the imaging surface 26a and the intermediate imaging surface 22b are conjugate, even if the light beam is inclined by inclining the second right-angle prism 22, the imaging surface 26a
Above, the image of the opening 5 does not cause a lateral shift. The magnifying optical system
The luminous flux passing through 12E, of course, reaches the imaging surface 26a of the TV camera 26 through the observation imaging lens 23,
In FIG. 2, a line indicating the light flux is not shown.

28 is a mirror for changing the direction of the laser beam passing through the observation imaging lens 23. Reference numeral 30 denotes two polarizing filters that are rotatably provided around an axis for adjusting the intensity of the laser beam.

Referring back to FIG. 1, reference numeral 42 denotes an auxiliary TV camera for performing alignment for adjusting the two light beams split by the beam splitter 10 so as to be coaxially superimposed, and includes an enlargement optical system 12E and a reduction optical system 12R. A part of the laser beam that has passed through and is divided by the beam splitter 10 is focused on the imaging surface of the auxiliary TV camera 42 by the focusing lens 40 for alignment.
Focused on 42a.

Next, the operation of the wavefront aberration measuring interference device will be described.

First, the optical unit 101 is attached to a predetermined position of the wavefront aberration measuring interference device, a laser beam is emitted from a laser diode (not shown) of the optical unit 101, and focused by the optical head objective lens 100. Passes through the cover glass 4 from the focal point, and the objective lens 1
, And becomes a parallel light beam and enters the wavefront aberration measuring interference device.

Before the wavefront aberration measurement, first, the auxiliary TV camera
The position of the optical system unit 101 is adjusted so that the positions of the convergence points of the two light beams converged on 42 coincide. Then, the laser beams separated by the beam splitter 10 and passed through the enlargement optical system 12E and the reduction optical system 12R are coaxially superposed.

Next, when the shutter 11 is closed, the beam splitter 10
The laser beam that passes through is blocked. Then, only the laser beam reflected by the semi-transmissive surface 10a and passing through the reduction optical system 12R is imaged on the imaging surface 26a by the observation imaging lens 23, and the intensity distribution of the laser beam is monitored by a monitor (not shown) or the like. Can be observed.

After checking the intensity distribution, when the shutter 11 is opened, the laser beam incident from the objective lens 1 and transmitted through the beam splitter 10 is reflected by the first right-angle prism 16 after being expanded by the expansion optical system 12E. Beam splitter
Transmit 10

On the other hand, the beam splitter 10
After being reduced by the reduction optical system 12R, the laser light flux reflected by the laser beam is reflected by the second right-angle prism 22, reflected by the beam splitter 10, and overlaps with the laser light flux passing through the enlargement optical system 12E.

At this time, the wavefront of the laser beam that has passed through the expansion optical system 12E is expanded near the center where there is almost no wavefront aberration, so that the wavefront of the laser beam that has passed through the reduction optical system 12R is regarded as almost a plane wave. And is used as a reference light.

In this way, the superposed laser beam is guided by the observation imaging lens 23 into a light beam passing through the enlargement optical system 12E, that is, a reference light beam, and a light beam passing through the reduction optical system 12R, that is, an object light beam, on the imaging surface 26a of the TV camera 26. Then, interference fringes are observed on a monitor (not shown) or the like.

Here, the reduction optical system 12R is obtained by disposing the same beam expander as the enlargement optical system 12E in the opposite direction, and the respective optical path lengths of the enlargement optical system 12E and the reduction optical system 12R correspond to light of any wavelength. It will be exactly the same. Therefore, even if a plurality of wavelengths of light are mixed in the laser beam, the effect of dispersion by wavelength between the laser beam passing through the expansion optical system 12E and the laser beam passing through the reduction optical system 12R is completely lost. The interference fringes are canceled to form clear interference fringes.

Then, by rotating the second right-angle prism 22 by a small angle around the axis 22s, the image at the position of the aperture 5 formed on the imaging surface 26b through the reduction optical system 12R is changed with respect to the light flux passing through the enlargement optical system. A minute angle tilt (tilt) and a tilt fringe are obtained, and by analyzing these, the wavefront aberration can be measured with high accuracy.

At this time, when the second right-angle prism 22 is rotated, the intermediate image plane 22b is at a fixed point, so that the position of the aperture 5 (wavefront to be measured), which is conjugate with the intermediate image plane 22b, and the image plane 26
The positional relationship with a (interference plane) is not affected by the rotation of the second right-angle prism 22 at all. Therefore, the imaging surface 26
The position of the image at the position of the aperture 5 to be imaged on b does not shift laterally, and the measurement can be performed accurately. The interference fringes are preferably processed by an image processing device (not shown) or the like, and the wavefront aberration is calculated accordingly.

The optical path on the side of the magnifying optical system 12E is transmitted twice through the semi-transmissive surface 10a, whereas the optical path on the side of the reducing optical system 12R is two-passed by the semi-transmissive surface 10a.
Since the light is reflected twice, if the transmittance of the semi-permeable surface 10a is 90%,
The light quantity ratio between the two optical paths is 1:81. On the other hand, reduction optical system
When the relative magnification of the magnifying optical system 12E with respect to 12R is 10, the cross-sectional area ratio of the two light beams is 1: 100. Therefore,
The ratio of the illuminance on the imaging surface 26a of each laser beam that has passed through the enlargement optical system 12E and the reduction optical system 12R is 81 to 100,
Since the illuminances of the two laser beams are almost the same, interference fringes with good contrast can be obtained.

Further, the enlargement / reduction optical system is not limited to the type of the above embodiment, and any optical system may be used.

In the present embodiment, the description has been made using the wavefront aberration measuring interferometer using the radial sharing interferometer, but the present invention is not limited to this, and other types of interferometers are used. It goes without saying that it can also be implemented.

[Effects of the Invention] According to the wavefront aberration measuring interference device of the present invention, even if the image of the wavefront to be measured is tilted, the image does not cause a lateral shift, so that interference fringes can be stably and accurately observed. ,
By applying the tilt method capable of analyzing interference fringes with high accuracy, the present invention has an excellent effect of measuring wavefront aberration with extremely high accuracy.

[Brief description of the drawings]

FIG. 1 is a schematic view of a wavefront aberration measuring interferometer, FIG. 2 is a partial schematic view of a wavefront aberration measuring interferometer, and FIGS. 3 and 4 show a lateral displacement phenomenon of an image by a general tilt method. It is a schematic diagram explaining. 1 ... Objective lens, 5 ... Aperture, 10 ... Beam splitter, 12E ... Enlargement optical system, 12R ... Reduction optical system, 22 ... Second right-angle prism, 22b ... Intermediate image plane, 23 ... Observation imaging lens, 26 ... TV camera, 26a ... Imaging surface.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Makoto Iki 2-36-9 Maeno-cho, Itabashi-ku, Tokyo Asahi Optical Industry Co., Ltd. (56) References JP-A-61-178635 (JP, A) (58 ) Surveyed field (Int.Cl. ⁶ , DB name) G01M 11/02 G01B 11/24

Claims (1)

(57) [Claims] A light beam to be measured emitted from an aperture to be measured is divided into two light paths by a beam splitter, a light beam in one light path is used as an object light beam, and a light beam in the other light path is used as a reference light beam. An interference fringe is formed on the observation surface by superimposing two light fluxes, and the wavefront aberration of the measured light flux is measured by the interference fringes. Means for inclining the traveling direction with respect to the optical axis are provided, and means for making the position where the object light beam inclines and the opening are conjugated are provided. An interferometer for measuring wavefront aberration, wherein a spatially conjugate positional relationship is provided.