Browse Prior Art Database

Multiple Foci Objective Lens

IP.com Disclosure Number: IPCOM000115611D
Original Publication Date: 1995-May-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 2 page(s) / 51K

Publishing Venue

IBM

Related People

Kant, R: AUTHOR

Abstract

High-density optical disk storage utilizes high Numerical Aperture (NA) optics to produce a small, diffraction limited spot. One problem associated with increasing NA is that the depth of focus of the system is inversely proportional to NA**2. Thus decreasing the spot size by increasing NA puts severe requirements on the mechanical servo system used to keep the spot in focus. Disclosed is a means of obtaining small spot sizes over an increased depth of field by making a multi-foci optical element.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 80% of the total text.

Multiple Foci Objective Lens

      High-density optical disk storage utilizes high Numerical
Aperture (NA) optics to produce a small, diffraction limited spot.
One problem associated with increasing NA is that the depth of focus
of the system is inversely proportional to NA**2.  Thus decreasing
the spot size by increasing NA puts severe requirements on the
mechanical servo system used to keep the spot in focus.  Disclosed is
a means of obtaining small spot sizes over an increased depth of
field by making a multi-foci optical element.

      The idea is to produce a phase mask, using well developed
techniques of binary optics, which is used in conjunction with an
objective lens.  The phase mask is divided into 2 or more radial
zones.  The phase mask in each zone introduces a weak focal power
which changes from zone to zone.  Thus the light from each zone is
focussed at a slightly different position along the optical axis as
shown in Fig. 1.  This effectively extends the depth of focus of the
lens.  By choosing the width of the different zones and the
corrective phase function for each zone, a small spot size can be
achieved over an extended depth of focus.

      We consider two examples: for NA of 0.55 and of 0.65
respectively.  In each case divide the wave in three parts containing
equal energy.  To each sector we give a defocus of -0.5, 0.5, and 1
wave respectfully.  This produces a diffraction image as if there
were three different focal length lens...