Super-resolution optical head apparatus

Details Super-resolution optical head apparatus Super-resolution optical head apparatus

1999-12-17

2001-02-06

Tran, Thang V. (Department: 2753)

Dynamic information storage or retrieval

With servo positioning of transducer assembly over track...

Optical servo system

C369S110040, C369S112040

Reexamination Certificate

active

06185168

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a super-resolution optical head apparatus for optically reading information from an information storage surface with a light beam focused thereon. More particularly, it relates to a super-resolution optical head apparatus applicable to an optical head apparatus for optical disks, a bar-code scanner, and an image scanner.
The super-resolution optical head apparatus mentioned above comprises: a focusing means for focusing a coherent beam onto a fine spot on the information storage surface; and a photoelectric converting means for detecting the intensity of the beam reflected from the information storage surface. The above super-resolution optical head apparatus is equipped with various arrangements that have been devised to obtain a fine focal spot equivalent to or smaller than the diffraction limited.
FIG.
20
(
a
) is a schematic view showing the structure of a conventional image-forming optical system using an annular diaphragm, which is well-known as a super-resolution optical system, as a double diffraction optical system. Such a super-resolution optical system using the annular diaphragm or a phase plate has found applications in optical head apparatus, which are reported in the following documents:
(1) “High Density Optical Recording by Super Resolution,” Y. Yamada, Y. Hirose and K. Kubota, Proc. Int. Symp. on Optical Memory. 1989, Jap. J. of Appl. Phys., Vol. 28 (1989) supplement 28-3, pp. 197-200.
(2) “Optical Head with Annular Phase-Shifting Apodizer,” Hideo Ando, Tsuneshi Yokota and Koki Tanoue, Jap. J. Appl. Phys., Vol. 32 (1993) pp.5269-5276, pt. 1, NO. 11B.
As shown in
FIG. 20
, a coherent beam emitted from a coherent light source 50 is turned into parallel-beams upon passing through a collimator lens (a first Fourier transform lens)
51
. The resulting parallel beams are then allowed to pass through apertures
52
a
and
52
b
(slits in one dimension) of an annular diaphragm
52
and converged by an objective lens (a second Fourier transform lens)
53
so as to form an image, thereby providing a super-resolution spot having the intensity distribution I(X), which is shown in
FIG. 20
as the power spectrum of the transmittance of the foregoing annular diaphragm
52
.
The above document (1) discloses an optical head which forms such a super-resolution spot in one dimension and uses only the main lobe thereof obtained by means of knife-edges constituting a slit. The above document (2) discloses a system which uses a plurality of phase distributions and a specified amplitude distribution as the annular diaphragm in order to form a super-resolution spot in two dimensions, thereby suppressing the side lobes on both sides of the main lobe shown in FIG.
20
(
b
). In the system, the conditions for designing the annular diaphragm are optimized to suppress the side lobes.
However, the system for suppressing the side lobes by means of the annular diaphragm is not free from a reduction in intensity of the focused beam. In the case where the peak intensity of the focused beam is reduced to about 50% to 15%, e.g., if the half bandwidth of the main lobe is reduced to 85% of the diffraction limited, the intensity of the side lobe becomes about 7% of the peak intensity of the main lobe.
As described above, if the aperture through which light is incident upon the objective lens is formed into a slit or an annularity, there can be achieved super resolution smaller than the diffraction limited with the side lobes suppressed to a certain extent. On the other hand, there arise the following problems: (1) since the quantity of light reaching an image forming plane is reduced significantly, the quantity of light in the main lobe is also reduced disadvantageously; (2) moreover, since the aperture for shielding the side lobes is provided a higher accuracy is required in adjusting the optical path, while the reliability of the apparatus is lowered because the components of the optical system deteriorate with the passage of time or for other reasons; and (3) furthermore, the half bandwidth of the beam is reduced to about 90% to 80% at most.
SUMMARY OF THE INVENTION
In view of the foregoing, an object of the present invention is to provide a super-resolution optical head apparatus with extremely high performance wherein a simple optical system prevents a significant reduction in the quantity of light and the beam width of the diffraction limited can be reduced to about 70%.
To attain the above object, in a first super-resolution optical head apparatus of the present invention, a primary beam having a peak intensity in the center thereof and a subsidiary beam having peak intensities at least on both sides of the center thereof, both having equal-sized principal portions, are incoherently superimposed upon each other and focused onto an information storage surface and the light beam reflected from the information storage surface is separated into the primary beam and the subsidiary beam such that the light intensities thereof are differentially detected, thereby implementing a super-resolution optical system. Here it mean by “super resolution that the spatial frequency response of optical scanning system is increased. It will become apparent that the cross-talk cancelling effect of the present invention is also greatly appreciated.
The first super-resolution optical head apparatus comprises: a first coherent light source for emitting a first coherent beam serving as a primary beam; a second coherent light source for emitting a second coherent beam which has either a plane of polarization perpendicular (orthogonal) to a plane of polarization of the first coherent beam or a wavelength different from a wavelength of the first coherent beam; a phase plate for receiving the second coherent beam and emitting a subsidiary beam which has an intensity distribution in which peak values are placed at least on both sides of a center thereof on a plane perpendicular to an optical axis and a principal portion thereof is equal in size to a principal portion of the primary beam; a focusing means for superimposing the primary beam emitted from the first coherent light source and the subsidiary beam emitted from the phase plate upon each other and focusing them onto an information storage surface having an optical information storage carrier; a control means for controlling the focusing means such that a light beam composed of the primary beam and the subsidiary beam being superimposed upon each other performs focusing and tracking with respect to the optical information storage carrier of the information storage surface; a light separating means for receiving the light beam reflected from the information storage surface, separating the light beam into the primary beam and the subsidiary beam through polarization separation or wavelength separation, and emitting them; an optical detecting means for individually detecting an intensity of the primary beam and an intensity of the subsidiary beam and outputting a light intensity signal; and a calculating means for calculating, based on the light intensity signal outputted from the optical detecting means, a super-resolution scanning signal and outputting it.
With the first super-resolution optical-head apparatus, the primary beam has the normal Airy-disc pattern or has a peak intensity on the optical axis. On the focal plane, the principal portion of the subsidiary beam connected with a dipped profile or a trough has a beam size equal to about 70% of that of the primary beam. The subsidiary beam has a principal portion equal, or almost equal, in size to that of the primary beam and has a double-humped intensity distribution with peak values placed at least on both sides of the center thereof. The subsidiary beam having the double-humped intensity distribution can easily be generated by means of a stepped phase plate which imparts a relative phase difference varying from 0 to &pgr; with the wavelength of the light transmitted thereby. By irradiating the phase plate with the subsidiary beam having substant

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