Optical pick-up apparatus for multiple data surface optical disk system

An optical pick-up device includes a laser source which emits multi-mode laser beam consist of varied wavelength in wide spectrum, and an optical filter which eliminates improper ranges of the laser's spectrum and transmits only preferred wavelength domains. Therefore, a diffraction device composed of at least two diffractive elements can produce several focal points dependent upon selected wavelength by the optical filter and diffractive index of the each diffractive element.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to optical data storage systems. More specifically, 
this invention relates to an optical reading and recording apparatus and 
method for optical data storage systems having multiple data storage 
surfaces. 
2. Description of the Prior Art 
Initialized by the vast increase in information that needs to be processed, 
optical data storage system have become most important system particularly 
because of their high storage density per area. Most of the recent optical 
information storage systems rotating single optical disk are used on which 
the information is digitally stored in concentric circular tracks in an 
ordered, predifined manner to allow chronological fast reading and fast 
random access to desired pits of data. 
In order to accomplish the even more storage capacity of optical disk 
systems for the enormous information processing, such as video or picture 
communication like so called video-on-demand service, multiple disk 
systems have been proposed. An optical disk system having two or more data 
layers may in theory be accessed at different disks by changing the focal 
point with moving lens. Example of this type of state-of-the-art include 
U.S. Pat. No. 5,202,875 issued Apr. 13, 1993 to Rosen, et al.; Japanese 
Published Application, 63-276732 published Nov. 15, 1988 by Watanabe, et 
al. 
The serious problem with these prior art systems is an interference lights 
reflected on neighboring layers, that causes difficulty to detect the data 
recorded on certain layer. That cross-talk-signals from the other layers 
may effect critically to reduce the ability to read the data, especially 
with several data layers system. In addition, all of the prior art systems 
requires the moving lens component to modulate the focal point of the 
laser beam. Example of such type of state-of-the-art include U.S. Pat. No. 
5,416,756 issued May 16, 1995 to Takeshita, et al. However, such a 
actuating component including moving lens can not be fabricated on a 
single substrate. That has been considered disadvantage on productivity 
and reliability. An optical reading, and recording pick-up device is 
needed which overcomes these problems mentioned above. 
SUMMARY OF THE INVENTION 
The present invention has for its object to provide an optical reading and 
recording pick-up device for multiple data layers system, in which moving 
mirror is no longer necessary to focus onto selective layer. Another 
significant object of the invention is to provide a reliable optical 
pick-up device for multiple data layers system, which can greatly reduce 
the effect of interference lights reflected on neighboring layers. 
The object of the present invention can be achieved by an optical pick-up 
device for multiple data surface optical disk system, the apparatus 
comprising: a laser source which emits in-coherent laser beam; an optical 
filter which eliminates improper ranges of the laser beam's spectrum and 
transmits at least two part of selected spectrum; and a diffraction device 
composed of at least two diffraction element which produces diffracted 
beam each. 
As for an exemplified structure, the laser beam emitted from laser diode is 
transmitted to the optical filter. Then the spectrum of the laser beam is 
selectively eliminated by the optical filter. For example, predetermined 
domains of .lambda.1 and .lambda.2 are transmitted and the other part of 
the spectrum is eliminated. The each of the spectrum .lambda.1 and 
.lambda.2 is transmitted to an aspheric lens which is comprising two part 
of duplexed lens element. The center part and outer part of the lens has 
different diffraction index. And the each of the spectrum .lambda.1 and 
.lambda.2 is diffracted by the center part and outer part of the aspheric 
lens to produce four focal points. 
For a fuller understanding of the nature and advantages of the present 
invention reference should be made to the following detailed description 
taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Embodiments of the present invention will be explained with reference to 
the drawings. 
FIG. 1 shows a schematic diagram of a first preferred embodiment of the 
optical pick-up apparatus of the present invention. A laser source 10 
emits multi-mode laser beam which has wide spectrum. The emitted laser 
beam is collimated by a collimating lens 11. An optical filter 12 located 
on the laser beam path to intercept and selectively eliminate improper 
range of the laser's spectrum to cancel the interference noise. The 
functional detail of the optical filter 12 is fully explained lately with 
accompanying FIG. 2. In this exemplified embodiment, only the 
predetermined two domains of the laser spectrum of the wavelength is 
transmitted to an aspheric lens 13, and the laser beam is focused 
according to the laser spectrum's wavelength and diffractive index of the 
aspheric lens 13. The aspheric lens 13 consists of two part of lens 
element 13-a and 13-b. Each part of the lens element 13-a and 13-b has 
different diffractive index, then each of the laser spectrum produces two 
focal points. Thereafter, four focal points are produced with the optical 
filter 12 which transmits two part of the spectrum and the aspheric lens 
which has two part of lens element 13-a and 13-b as described in this 
exemplified embodiment. A multiple data surface optical disk 14 having 
four data surfaces 14-1.about.4 is associating with the optical pick-up 
apparatus of this exemplified embodiment. Each wavelength spectrum of the 
laser beam deflected by each part of the aspheric lens 13 is focused onto 
each data surface 14-1.about.4. Then each reflected laser beam from the 
data surface is deflected by a half-mirror 15. And the deflected laser 
beam is focused by a lens 16 to transmit the laser beam to a detector 17. 
Then, the optical signal detected by the detector 17 can be processed by 
conventional electrical circuits. 
FIG. 2 shows a graph of laser intensity versus wavelength of a laser source 
of the present invention. X axis of the graph denotes wavelength of the 
laser beam, and Y axis of the graph denotes intensity of the laser. A 
dotted line 20 shows laser emitting affairs of the multi-mode laser 
source. As mentioned, the multi-mode laser beam emitted from the laser 
source has wide spectrum, and it is functionally considered that the laser 
source emitting varied wavelength of the laser beam. A solid line 22 shows 
laser beam affairs that some parts of the pre-determined wavelength of the 
laser beam is eliminated and selectively secured by an optical filter. A 
graph 22 shows an example of the particular laser beam affair which 
preferred two parts of the laser beam spectrum .lambda.1 and .lambda.2 are 
secured, in order to focus onto four data layers associating with 
diffraction device consisted of two part of diffractive elements. 
The apparatus of the present invention comprising an in-coherent laser 
source, gratings and an optical filter to vary the focusing positions, 
should be compared with the conventional apparatus that scans the laser 
beam by using a laser source which is tunable to produce different 
wavelength and a grating, for example U.S. Pat. No. 4,918,679 issued Apr. 
17, 1990 to Ophenji, et al. However, in the prior art system, the 
wavelength is tuned by changing the current supply to the laser source. 
That must require complicated electrical circuit. In addition, it is quite 
difficult to tune the wavelength stably with outer electrical circuit. 
The multi-mode laser source in the apparatus of the present invention does 
not require any complicated electrical circuits to drive. The multi-mode 
laser source to emit in-coherent laser beam, Super Luminescent Diode (SLD) 
for example, constantly emits varied wavelength of the spectrum beam. It 
is much more reliable to select the preferred wavelength spectrum by an 
optical filter, than tuning the wavelength itself by changing the current 
supply from the electrical circuit. In addition, in-coherent laser beam 
may hardly interfere one and another, that'll greatly reduce the effect of 
interference lights reflected on neighboring layers. 
FIG. 3 shows a schematic diagram of a second preferred embodiment of the 
optical pick-up apparatus of the present invention. A laser source 30 
emits multi-mode laser beam which has wide spectrum. The emitted laser 
beam is collimated by a collimating lens 31. An optical filter 32 located 
on the laser beam path to intercept and selectively eliminate improper 
range of the laser's spectrum to cancel the interference noise. In this 
exemplified embodiment, only the pre-determined two domains of the laser 
spectrum of the wavelength is transmitted to a hologram lens 33, and the 
laser beam is focused according to the laser spectrum's wavelength and 
diffractive index of the hologram lens 33. The hologram lens 33 consists 
of two part of the engraved holographic grating 33-a and 33-b. Each part 
of the hologram 33-a and 33-b has different diffractive index, the 
hologram 33-a has low diffractive index with the wide grating pitch and 
the hologram 33-b has high diffractive index with the narrow grating 
pitch. Then each of the laser spectrum produces two focal points. 
Thereafter, four focal points are produced with the optical filter 32 
which transmits two part of the spectrum and the hologram lens which has 
two part of holographic grating 33-a and 33-b as described in this 
exemplified embodiment. A multiple data surface optical disk 34 having 
four data surfaces 34-1.about.4 is associating with the optical pick-up 
apparatus of this exemplified embodiment. Each wavelength spectrum of the 
laser beam deflected by each part of the hologram lens 33 is focused onto 
each data surface 34-1.about.4. Then each reflected laser beam from the 
data surface is deflected by a half-mirror 35. And the deflected laser 
beam is focused by a lens 36 to transmit the laser beam to a detector 37. 
Then, the optical signal detected by the detector 37 can be processed by 
conventional electrical circuits. 
FIG. 4 shows a schematic view of a hologram lens which is applied for the 
second preferred embodiment described in FIG. 3. A hologram lens 40 is 
composed of two part of holographic grating element. Part 41 has low 
diffractive index because of the grating element engraved with wide pitch. 
Part 42 has high diffractive index because of the grating element engraved 
with narrow pitch. The transmitted laser beam is partially focused by the 
grating element part 41 to produce far focal point. And the laser beam is 
also partially focused by the grating element part 42 to produce near 
focal point. The position of the focal point is varied with changing the 
diffractive index of the each grating element. The distance from the lens 
to the each focal point is also dependent upon wavelength of the laser 
beam. Then, single laser spectrum produces two focal points with dual 
hologram lens. Thereafter, four focal points can be produced with double 
laser spectrum transmitted from the optical filter. 
FIG. 5 shows a schematic diagram of a third exemplified embodiment of an 
optical pick-up apparatus of the present invention. Especially, in this 
embodiment, all device composed of the present invention are fabricated on 
a single substrate as an optical integrated circuit. A laser diode 50 
emits multi-mode laser beam which has wide spectrum. An optical filter 51 
located on the laser path to intercept and selectively eliminate improper 
range of the laser's spectrum to cancel the interference noise. Only the 
selected domains of the laser spectrum is intercepted by a twin grating 
beam splitter 52, and the laser beam is collimated to enter a dual 
focusing grating coupler 53 efficiently. The dual focusing grating coupler 
53 is consisted of two part of grating, center part grating has high 
diffractive index with narrow pitch grating and outer part grating has low 
diffractive index with wide pitch grating. The functional detail of the 
dual focusing grating coupler 53 is fully explained lately with 
accompanying FIG. 6. The laser beam is deflected and focused by a focusing 
grating coupler 53. The focal points are varied dependent upon the optical 
filter 51 which selectively transmits the certain wavelength spectrum and 
upon the dual focusing grating coupler 53 which focuses the laser spectrum 
with two part of focusing grating. A multiple data surface optical disk 54 
having a data surface 54-1.about.4 is associating with the integrated 
optical pick-up device. Each wavelength spectrum of the laser beam focuses 
on each data surface 54-1.about.4. Then, each reflected laser beam is 
deflected by the dual focusing grating coupler 53. And the twin grating 
beam splitter 52 intercepts the laser beam, and deflects each wavelength 
spectrum of the laser beam for selective distribution into a detector 
56a-1.about.2 and 56b-1.about.2. The optical signal detected by the 
detector 56 can be processed by conventional electrical circuits. 
FIG. 6 shows a cross-sectional view of an optical pick-up apparatus of the 
present invention. This is enlarged view of the dual focusing grating 
coupler which is applied for the present invention. The laser beam 60 is 
the particular laser beam that preferred two parts of the laser beam 
spectrum as .lambda.1 and .lambda.2 in FIG. 2, are secured in order to 
focus on four data layers associating with the aspheric lens or dual 
hologram lens. A dual focusing grating coupler is consist of two part of 
grating coupler 61 and 62, the outer part 61 is an engraved grating which 
has low diffractive index with wide grating pitch and the center part 62 
is an engraved grating which has high diffractive index with narrow 
grating pitch. The dual focusing grating coupler intercepts the laser beam 
60. Then the laser beam 60 is deflected, and focused at four certain 
positions 63-1.about.2 and 64-1.about.2. Each of the four data layer must 
be located at positions between 65. By emitting varied wavelength of the 
laser spectrum, each laser beam's focal point locates on a dotted line 66 
approximately. Each focal point can be controlled by changing the 
grating's pitch of the dual focusing grating coupler and by selecting the 
different part of the wavelength of the laser spectrum by the optical 
filter, by mounting a fixed lens or by changing the position of the 
integrated chip itself, etc. 
Although the invention has been particularly shown and described, it is 
contemplated that various changes and modification may be made without 
departing from the scope of the invention as set forth in the following 
claims.