Dual focus lens with extended depth of focus

Details Dual focus lens with extended depth of focus Dual focus lens with extended depth of focus

1999-04-08

2001-12-11

Sugarman, Scott J. (Department: 2873)

Optical: systems and elements

Lens

Plural focal length

C359S742000, C359S565000, C369S112040

Reexamination Certificate

active

06330118

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention pertains to lenses. In particular, this invention pertains to lenses that can be used to focus a laser beam on a small spot, or on two spatially separated small spots, but simultaneously provide large depth of focus. Such a lens is particularly useful in optical and magneto-optical disk drives, and drives that read and write to different disks with different overcoat thicknesses.
Referring to
FIG. 1A
, a prior art optical or magneto-optic disk drive typically comprises a laser source
1
for providing a laser beam
2
, an optical or magneto-optic data recording disk
3
, and a lens
4
for focussing the laser on a small spot on disk
3
. A motor
5
rotates disk
3
so that laser beam
2
can scan over a data recording track of the disk. In such applications, the depth of focus is an important parameter which is preferably maximized. In conventionally designed optics, the depth of focus D is given by the following relationship:
D
=0.8&lgr;/(
NA
)
2
where NA is the numerical aperture of the lens and &lgr; is the laser light wavelength.
In optical and magneto-optic data storage, one also generally strives for high data recording density. This means that the spot size produced by the focussing lens should be as small as possible. To reduce the spot size of a given design, one increases the NA of the lens. However, as can be seen from the above equation, this has the detrimental effect of reducing the depth of focus D. This detrimental effect is especially important in the case of drives in which the disk spins rapidly, e.g. 40× drives. (The nomenclature “40×” is part of a numbering system which is well known in optical storage and refers to a rotation rate 40 times that of standard audio CD players. An audio CD player spins at 1×.) Because disk
3
spins rapidly, it can sometimes wobble, e.g. moving in the direction of arrow A, causing disk
3
to move closer to or further from lens
4
, and causing the laser spot to be out of focus. In such drives, a shallow depth of focus puts an unmanageable burden on the frequency response of the auto-focus system of the drive. Accordingly, it would be desirable to provide a lens with a relatively large depth of focus and a small spot size.
There are several types of optical and magneto-optic media known in the art.
FIG. 1B
shows a prior art laser beam
6
passing through a lens
7
to read data from a DVD disk
8
. DVD disk
8
comprises a transparent substrate
9
over a recording layer
10
. Typical transparent substrates
9
on DVD recording media have a thickness T
1
of about 600 microns.
FIG.
1
B′ shows laser beam
6
passing through a lens
11
to read data from a CD disk
12
. CD disk
12
comprises a transparent substrate
13
over a recording layer
14
. Typical transparent substrates
13
on a CD disk have a thickness T
2
of about 1200 microns. Thus, in order to read both CD disks and DVD disks, an optical disk drive must have two lenses—one for reading CD media and one for reading DVD media. Such drives typically include a mechanism for switching lenses. Such a mechanism is cumbersome and adds expense to the disk drive. Further, it is difficult to substitute one lens for the other while continuing to maintain precise spacing between the lens and the media. Accordingly, it would be desirable to provide a lens capable of reading data from (or writing data to) different types of recording media having different substrate thicknesses.
SUMMARY
A lens in accordance with the invention comprises regions having at least two different optical phase functions implementing two different focal lengths, which are spatially multiplexed over the lens aperture. The lens is used to produce from a single incoming laser beam at least two converging beams, but produce only one extended depth of focus spot on a currently inserted rotating data recording disk, for a given overcoat thickness. Thus, even if the disk wobbles, and moves closer to or further from the lens, the lens will continue to focus one aberration-free small spot onto the recording layer of the current disk inserted in the drive.
In one embodiment, a lens in accordance with our invention accommodates types of recording media having transparent substrates thereon of different thicknesses. For example, in one embodiment, the lens has a first region having a first focal length for accommodating recording media having a transparent substrate of a first thickness. The lens also has at least a second region having a second focal length for accommodating recording media having a transparent substrate of a second thickness. Thus, one can use one lens to read data from (or write data to) different types of recording media.


REFERENCES:
patent: 3586412 (1971-06-01), Leith
patent: 5122903 (1992-06-01), Aoyama et al.
patent: 5225858 (1993-07-01), Portney
patent: 5315095 (1994-05-01), Marom et al.
patent: 5351230 (1994-09-01), Takeda et al.
patent: 5446565 (1995-08-01), Komma et al.
patent: 5561558 (1996-10-01), Shiono et al.
patent: 5648951 (1997-07-01), Kato et al.
patent: 0 871 163 (1998-10-01), None
patent: 59-105605 (1984-06-01), None
Sochacki, et al., “Nonparaxial Design of Generalized Axicons”, Applied Optics, vol. 31, No. 25, Sep. 1, 1992, pp. 5326-5330.
Leseberg, “Computer Generated Holograms: Cylindrical, Conical, and Helical Waves”, Applied Optics, vol. 26, No. 20, Oct. 15, 1987, pp. 4385-4390.
Frere, et al., “Computer Generated Holograms of Three-Dimensional Objects Composed of Line Segments”, J. Opt. Soc. Am., vol. 3, No. 5, May 1986, pp. 726-730.
McCleod, “Axicons and Their Uses”, J. Opt. Soc. Am., vol. 50, No. 2, Feb. 1960, pp. 166-169.
Dong, et al., “Iterative Optimization Approach for Designing an Axicon With Long Focal Depth and High Transverse Resolution”, J. Opt. Soc. Am., vol. 13, No. 1, Jan. 1996, pp. 97-103.

Affiliated with

Also associated with

Rating

Dual focus lens with extended depth of focus does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Dual focus lens with extended depth of focus, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Dual focus lens with extended depth of focus will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2586622