Optical waveguides – With optical coupler – Input/output coupler
Patent
1991-02-21
1992-10-27
Ullah, Akm E.
Optical waveguides
With optical coupler
Input/output coupler
369 4411, G02B 600, G02B 636
Patent
active
051596504
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates particularly to an optical disc apparatus capable of satisfactorily reproducing an extremely high-density recorded signal.
BACKGROUND OF THE INVENTION
A conventional optical disk apparatus and the reproduction of a signal recorded on a disc will be described with reference to the accompanying drawings.
In the conventional optical disk apparatus as shown in FIG. 1, the light emitted from a semiconductor laser 8 is made parallel light by a condenser 9 and passed through a half-mirror 30 and through a focus lens 31 to be focused on a reflecting surface 16 of an optical disk on which signal marks are formed. Part of the incident light to the reflecting surface is reflected from the reflecting surface 16, and the reflected light which includes the information of the signal recorded on the disc is conducted again through the focus lens 31 to the half-mirror 30 and reflected therefrom into a light detector 32, where the recorded signal on the disc is detected. This detected signal is amplified by an amplifier 33.
The density of the recorded signal which can be reproduced by this conventional optical disc apparatus is limited as will be mentioned below.
The disc has a large number of very small signal marks recorded thereon These signal marks can be formed in various modes; for example, very small regions on the recording film are heated to change the phase of the film material and thereby to change an optical constant of the very small regions, very small pits or bumps are formed on the disc, and so on.
The information recorded in these signal marks can be reproduced by irradiating a very small spot of focused laser light on the signal marks and receiving the reflected light therefrom.
In the optical disc apparatus having the arrangement shown in FIG. 1, when the wavelength of the laser 8 is .lambda.=780 nm, the numerical aperture of the focus lens 31 is NA=0.50, and the intensity of the laser is constant within the aperture, then the light intensity distribution I (.xi.,.eta.) of the light irradiated on the reflecting surface ((.xi.,.eta.) coordinates system) of the disc is indicated in FIG. 2 by a curve (a). Here, if u (.xi.,.eta.) is the light amplitude distribution, the light intensity distribution can be expressed as I(.xi.,.eta.)=.vertline.u(.xi.,.eta.).vertline..sup.2.
In the following consideration of reflection from the reflecting surface of the disc, it is assumed that a very small and single encircled mark D is formed on the reflecting surface of the disc and the coordinates of the center, the amplitude reflectivity ratio and the phase difference of the signal mark are represented by (.xi..sub.3, .eta..sub.c), A.sub.s and .phi..sub.s, respectively.
The reflection function is given by the following equation: ##EQU2## The above equation can be rearranged by use of the delta function .delta. as ##EQU3## in the equation (4): ##EQU4## where D is the area of the signal mark D, and n is the number of a group of points (.xi..sub.m,.eta..sub.m) and large enough to cover most of the region D.
When the signal mark D is narrower than the focused beam spot (for example, the diameter ratio is about 1/2 or below), u(.xi..sub.m,.eta..sub.m) (m=1, 2, . . . , n) can be represented by the value at the center (.xi..sub.c,.eta..sub.c) of the signal mark D.
Moreover, the aperture is assumed to be in the range of .alpha..ltoreq..theta..beta. (.theta. is the angle between the optical axis and the light ray) as viewed from the focal point F. Here, if .alpha.=0, the aperture is circular, and if .alpha.>0, the aperture is of a ring band.
The amount of reflected light detected by the light detector (or focused by the focus lens) is given by the following equation: +A.sub.R.sup.*)I( .xi..sub.c,.eta..sub.c)+(2.lambda.I.sub.-1 D.sub.S.sup.2 /.pi..sup.2).vertline.A.sub.R .vertline..sup.2 I(.xi..sub.c,.eta..sub.c)(5) I.sub.n are respectively defined by the following equations: ##EQU5## Thus, the standardized reproduced signal amplitude .DELTA.P (the difference of the amounts of detected li
REFERENCES:
patent: 4978187 (1990-12-01), Minemura et al.
patent: 4983009 (1991-01-01), Musk
patent: 4991919 (1991-02-01), Nishiwaki et al.
Asada Jun-ichi
Nishiwaki Seiji
Uchida Shinji
Matsushita Electric - Industrial Co., Ltd.
Ullah Akm E.
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