JPH03130638A - Interferometer - Google Patents

Abstract

PURPOSE:To observe interference fringes in a broad range clearly even if luminous flux having whatever higher intensity distribution is used and to measure wavefront aberration and the like accurately by providing a light reducing filter wherein variable- density parts for reducing the light in proportion to the intensity distribution of the luminous flux are formed. CONSTITUTION:Laser luminous flux passes through a 1/4-wavelength plate and a 1/2-wavelength plate 102 and 103. The luminous flux is split into two luminous fluxes through a beam splitter 105. The laser luminous fluxes are rotated by approximately 180 degrees with image rotators 106 and 107, reflected from rectangular prism 108 and 109, overlapped and made to interfere. The laser luminous flux is guided on a image sensing surface 112 through a reflecting mirror 110 and an image forming lens 111. The interference fringes are observed with a monitor in real time. The intense part (central part) of the laser luminous flux is made to hit the dense part of a light reducing filter 118, and the intensity is weakened. The weak intensity part (peripheral part) is transmitted through the light part of the filter, and the intensity reduction is small. Therefore, the intensity distribution of the laser luminous flux at the image sensing surface 112 is not governed with Gaussian distribution. The intensity distribution of the interference-fringe part has a sine-curve pattern. The linear interference fringes are uniformly observed on the entire screen.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an interferometer, and particularly to an interferometer suitable for measuring wavefront aberration of an optical information recording/reproducing device such as an optical disk.

[Prior Art] An interferometer is used to measure the wavefront aberration of a laser beam of an optical information recording/reproducing device or the like as a device to be measured. Such interferometers generally measure wavefront aberration by dividing a laser beam into two optical paths using a beam splitter, superimposing the laser beams that have passed through each optical path, causing interference, and analyzing the interference fringes. Conventionally, the laser beams passing through two optical paths were simply superimposed.

[Problems to be Solved by the Invention] However, the intensity distribution of the laser beam generally exhibits a Gaussian distribution, as shown in FIG. This property is particularly noticeable in devices that utilize most of the laser energy, such as optical disks.

Therefore, if the laser beams passing through two optical paths were simply superimposed as in the past, the intensity distribution of the laser beam in the interference fringe area would be dominated by a Gaussian distribution, as shown in Figure 6, for example. distribution.

As a result, when the intensity of the laser beam is adjusted to make it easier to observe the center of the interference fringes, the periphery becomes darker, as shown in FIG. The central part becomes too bright, and in both cases, the visible range of interference fringes becomes extremely narrow.

Moreover, in either case, the boundary line between brightness and darkness of each interference fringe becomes a curve, making it impossible to clearly judge how much the interference fringe is actually curved.

Therefore, it is conceivable to observe the interference fringes without being affected by the Gaussian distribution using, for example, an image processing device, but this would significantly increase the cost of the device. Also, since there is a time lag of 0.5 to 1 second before the interference fringes are output, it is difficult to manually adjust the amount of wavefront aberration of the device under test, for example while observing the interference fringes. The adjustment work is extremely difficult because the changes in the interference fringes are not linked to the movement of the interference fringes.

The present invention eliminates such conventional drawbacks, enables accurate observation of interference fringes over a wide range from the center to the periphery using a low-cost device, and also allows interference fringes to be observed in real time without time lag. The purpose is to provide an interferometer that can perform observation.

[Means for Solving the Problems] In order to achieve the above object, the interferometer of the present invention divides a coherent light beam into two optical paths using a beam splitter,
In an interferometer in which the light beams that have passed through each optical path are superimposed and interfered to form interference fringes, shading is formed that reduces light in proportion to the intensity distribution of one of the light beams split by the beam splitter. A neutral density filter is provided on the optical path where the light beams are superimposed, and the light density filter is formed by a photosensitive material that is exposed and developed by one of the light beams separated by the beam splitter. can be formed.

[Operation] Interference fringes are formed when a coherent light beam is divided into two by a beam splitter and the light beams passing through each optical path are superimposed and interfered with each other. Here, when the superimposed light beams pass through the neutral density filter, the parts with high intensity hit the dark parts of the neutral density filter and their intensity is weakened, and the parts with weak intensity pass through the light parts of the neutral density filter. Since the decrease in intensity is small, it is possible to make the intensity distribution of the light beam uniform in the area where interference fringes are formed.

Therefore, the brightness of the image plane of the interference fringes becomes uniform, and interference fringes with clear contrast can be observed from the center to the periphery.

〔Example〕

Examples will be described with reference to the drawings.

FIG. 1 shows an interferometer for detecting comatic aberration of an optical system for an optical information recording/reproducing apparatus in order to adjust the inclination of an objective lens 1 with respect to a disk. 1 in the diagram
is an objective lens provided in the optical system main body unit 3 of an optical information recording/reproducing device, and is a laser diode used to read or write information by irradiating a coherent laser beam onto a storage medium such as an optical disk. This is to focus the laser beam emitted from the laser beam (not shown) into a diameter of about 1 μm.

This objective lens l is rotatably provided along a spherical seat 20, and its optical axis can be tilted at any angle. Comatic aberration changes as the angle of the optical axis changes.

Reference numeral 100 denotes an objective lens of the interferometer, which converts a laser beam emitted from the objective lens l of an optical system for an optical information recording/reproducing device, converged, and then incident on the interferometer into a parallel beam.

Reference numeral 119 is a mounting surface of the objective lens 100 and a reference surface of the interferometer, which is formed perpendicular to the incident optical axis of the interferometer.

Reference numeral 101 denotes an optical disk or a transparent master glass plate for adjustment having a thickness equivalent to that of an optical disk.

104 is an aperture for the laser beam entering the interferometer, and is located at the exit pupil of the objective lens 100. Reference numerals 102 and 103 denote a quarter-wave plate and a half-wave plate, which are provided rotatably around the axis independently of each other in order to adjust the polarization state and plane of polarization of the laser beam to predetermined states. be.

Reference numeral 105 denotes a beam splitter for splitting the incident light beam, and the beam splitter 105 divides the laser light beam incident from the objective lens 1 into two light beams: a light beam that passes straight through and a light beam that is reflected at right angles.

Reference numerals 106 and 107 are image rotators for rotating the two beams separated by the beam splitter 105 relative to each other around the optical axis. approximately 90 relative to
It is installed at a different angle. These two image rotators 106 and 107 relatively rotate the two light beams about 180 degrees around the optical axis. These two image rotators 106 and 107 have the same shape and the same material in order to make the optical path lengths of the two beams separated by the beam splitter 105 the same and improve coherence.

108.109 is image rotator 106°107
The first and second rectangular prisms reflect the light flux that has passed through the prism in parallel in a direction reversed by 180 degrees.In order to make the optical path length the same and improve the coherence, the prisms have the same shape and the same material. Further, at least one of the rectangular prisms 108 and 109 is movable in the optical axis direction.

The light beams reflected by the two right angle prisms 108 and 109 pass through the first and second correction plates 120 and 121, respectively. Of these, the second correction plate 121 is two optical wedges,
By relatively rotating it around the optical axis, it has the effect of tilting the light beam transmitted through the beam splitter 105,
To facilitate adjustment of coma aberration, an arbitrary amount of tilt can be given to the observed interference fringes.

The first correction plate 120 is a glass plate made of the same material as the second correction plate 121 and has the same thickness as the two optical wedges combined, and is divided into two by the beam splitter 105.
In order to make the optical path lengths of the two optical paths the same, it is fixedly provided on the optical path passing through the first right angle prism 108.

In this way, the two light fluxes that have passed through the first and second correction plates 120 and 121 are superimposed by the beam splitter 105, and after exiting from the beam splitter 105,
The direction is changed by a reflector 110.

Reference numeral 111 denotes an imaging lens, which is arranged to form an image of the aperture 104 on, for example, an imaging surface 112 of a solid-state image sensor. Then, the wavefront aberration at the aperture 104 position can be observed as interference fringes using a CRT monitor (not shown) or the like.

In addition, two polarizing filters 113 that are rotatable about their axes independently of each other are arranged on the optical path between the imaging lens Ill and the imaging surface 112, and by rotating the polarizing filters 113, the imaging surface The intensity of the laser beam incident on 112 can be adjusted. Note that even with one polarizing filter, the intensity can be adjusted to a certain extent.

152+! , a reflecting prism for changing the direction of a portion of the laser beam that has passed through each image rotator 106, 107 and is split by the beam splitter 105. The laser beam reflected by this reflecting prism 152 is
The direction of the light is changed by a reflecting mirror 156 shown in FIG. 2, and the light enters an auxiliary TV camera 159 for alignment.

This auxiliary TV camera 159 has a beam splitter 105
This is to perform alignment in which the two light beams separated by
9 is focused on the imaging surface 159a.

Reference numeral 158 denotes two polarizing filters that are rotatably provided around the axis independently of each other, and by rotating one or both of the polarizing filters 158, the intensity of the laser beam incident on the imaging surface 159 can be adjusted. Can be done.

Returning to FIG. 1, a shutter 115 that can be opened and closed is arranged in the optical path between the beam splitter 105 and one image rotator 107. By closing this shutter 115, the optical path of one of the light beams divided by the beam splitter 105 is blocked.

Further, a dark filter l18 is provided on the optical path between the imaging surface 112 and the polarizing filter 113. This attenuating filter 118 is formed with shading for attenuating light in proportion to the intensity distribution of one of the beams split by the beam splitter 105. This makes it possible to attenuate light in proportion to the intensity distribution of the laser beam that enters the interferometer from the objective lens 1. In a state where there are no interference fringes, the intensity distribution of the laser beam that reaches the imaging surface 112 is vague. completely uniform. Such a neutral density filter 118 is, for example, a shutter 1.
15 is closed, and the photosensitive material is exposed and developed using the laser beam that is not blocked by the shutter 115. In that case, by changing the exposure time in many ways, it is possible to obtain a neutral density filter 118 that can perform the most appropriate light attenuation. As the photosensitive material, for example, when a laser is emitted from a laser diode, a film sensitive to near infrared rays (for example, IRF manufactured by Konica, etc.) can be used.

Next, the use and operation of the apparatus of the above embodiment will be explained.

First, an optical disk is attached to the optical system of an optical information recording/reproducing apparatus, or a master glass plate 101 for adjustment is attached to an interferometer. At this time, the optical disk or adjustment master glass 101 is attached so as to be parallel to the reference plane 119 of the interferometer.

Then, when a laser beam is emitted from a laser diode (not shown) of the optical system main body unit 3 and focused by the objective lens l, the laser beam passes through the optical disk or the adjustment master glass plate 101 from the focusing point, The objective lens 100 of the interferometer converts the light into a parallel beam of light and enters the interferometer.

Therefore, first, the mounting position of the optical system main unit 3 was adjusted so that the positions of the convergence points of the two beams converged on the auxiliary TV left camera 95 coincided, and the beams entered from the objective lens 100 were split by the beam splitter 105. The two light beams are coaxially superimposed.

The laser beam incident on the interferometer is passed through a quarter-wave plate 10.
The polarization state and direction of the polarization plane are adjusted by the 2 and 1/2 wavelength plates 103, and the light is split into two beams by the beam splitter 105.

The laser beam divided into two beams is then moved by image rotators 106 and 107 so that the laser beam is approximately 1
The beams are rotated by 80 degrees, reflected by the right angle prisms 108 and 109 in the direction of the semi-transparent surface of the beam splitter 105, and overlap and interfere at the semi-transparent surface of the beam splitter 105. The direction of these overlapping and interfering laser beams is changed by the reflecting mirror 11O, guided onto the imaging surface 112 by the imaging lens 111, and interference fringes are observed in real time by a CRT monitor (not shown) or the like. Ru.

At this time, when the laser beam guided to the imaging surface 112 passes through the neutral density filter 118, the strong part (center part) hits the dark part of the neutral density filter 118 and its intensity is weakened, and the weak part (peripheral area) passes through the pale area of the neutral density filter 118 and the decrease in intensity is small, so
The intensity distribution of the laser beam on the imaging surface 112 is not governed by a Gaussian distribution, and the intensity distribution of the interference fringe portion on the imaging surface 112 has a sine curve shape as shown in FIG. 3A, and for example as shown in FIG. 3B. Interference fringes with linear boundaries between bright and dark are uniformly observed over the entire screen.

As described above, according to this embodiment, high-contrast interference fringes are observed in an accurate shape from the center of the screen to the periphery, and partial bends or the like are observed in the interference fringes due to the presence of aberrations or the like.

The interference fringes observed in this manner are components obtained by rotating the wavefronts of the two divided light beams by approximately 180 degrees relative to the optical axis center using image rotators 106 and 107, and taking the difference between the two wavefronts. Therefore, the beam splitter 10
Of the wavefront aberrations detected as the difference in wavefront shape of the light beams divided by 9 with respect to the symmetrical defocus component and optical axis
Astigmatism components, which are 0 degree asymmetric components, cancel each other out and are not detected at all.

That is, the solid line in FIG. 4A shows the coma aberration component with respect to the incident source height, and the broken line shows the component obtained by rotating the solid line component by 180 degrees around the optical axis. Therefore, the component shown by the dashed line, which is the difference between them, is as follows: ΔW, = ΔW, -ΔW2 = ΔW1-(-ΔW+) = 2ΔW1, and is detected as twice the size of the solid line component.

Fig. 4B shows the detection sensitivity of comatic aberration with respect to the rotation angle around the optical axis.
The degree-asymmetric coma aberration component appears in interference fringes with approximately twice the sensitivity.

Note that astigmatism is an aberration that has components whose positive and negative values are reversed between cross sections rotated approximately 90 degrees around the optical axis, so if the image rotation angle by the image rotators 106 and 107 is approximately 90 degrees, this device can eliminate astigmatism. Point aberration can be detected.

When coma aberration is detected in this way, the inclination of the optical head objective lens 1 is adjusted while observing the interference fringes so that, for example, the bending of the interference fringes is minimized. Thereby, the optical axis of the objective lens 1 can be made perpendicular to the optical disc or the adjustment master glass plate lO1, and coma aberration including the coma aberration included in the objective lens 1 can be minimized.

In addition, in the above embodiment, the film is used as a neutral density filter 1.
18, but the present invention is not limited to this, and any material may be used as long as it has shading that reduces light in proportion to the intensity distribution of the laser beam incident on the interferometer. For example, it may be used instead of a film. It is also possible to use a dry plate or analyze the intensity distribution of the laser beam using an image processing device or the like, and then manufacture a filter with shading similar to that described above based on the analysis.

Further, although coma aberration was measured in the interferometer of the above embodiment, the present invention can be applied to measurement for any purpose.

[Effects of the Invention] According to the interferometer of the present invention, interference fringes with clear contrast are formed from the center to the periphery no matter what kind of intensity distribution a light beam is used. It can be observed clearly and wavefront aberrations can be accurately measured. Therefore, it is particularly effective in measuring the wavefront aberration of an optical information recording/reproducing device using a laser diode with a prominent Gaussian intensity distribution as a light source. Moreover, since it is a simple configuration that requires simply adding a filter, it can be implemented at extremely low cost.

In addition, since interference fringes can be obtained in real time, when making adjustments to reduce the wavefront aberration of a device under test while observing interference fringes, the changes in interference fringes can be linked to hand movements during adjustment work. can be easily done.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram of an interferometer according to an embodiment of the present invention, Fig. 2 is a partial schematic diagram of the interferometer, and Fig. 3A is a line showing an example of the intensity distribution of interference fringes obtained by the embodiment. Figure 3B is a schematic diagram showing an example of interference fringes obtained by the embodiment; Figures 4A and 4B are diagrams showing the characteristics of coma aberration; Figure 5 is the intensity of a light beam with a Gaussian distribution. Figure 6 is a diagram showing an example of the intensity distribution of interference fringes obtained by a conventional interferometer. Figures 7 and 8 are diagrams showing examples of interference fringes obtained by a conventional interferometer. FIG. 105...-Beam splitter, 115...-Shutter, 118...-Dark filter.

Claims (2)

[Claims] (1) In an interferometer, a coherent light beam is divided into two optical paths by a beam splitter, and the light beams passing through each optical path are superimposed and interfered to form interference fringes. An interferometer characterized in that a neutral density filter is provided on an optical path on which the light beams are superimposed, and has a density filter that performs light attenuation proportional to the intensity distribution of one of the light beams. (2) The interferometer according to claim 1, wherein the neutral density filter is formed of a photosensitive material that is exposed and developed by one of the beams split by the beam splitter.