Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
2000-08-11
2001-05-22
Edun, Muhammad (Department: 2651)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
C369S044230, C369S044140
Reexamination Certificate
active
06236634
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to optical recording systems. More particularly, the invention relates to optical recording systems having a high storage density.
RELATED ART
Optical recording systems employ a focused beam of light to record and read information from a recording medium, which is typically in the form of a specially coated disk of aluminum, glass or plastic. The beam of light is typically generated by a light source and then focused through a lens or other focusing mechanism onto the recording medium. It should be appreciated that it is generally desirable to maximize the amount of information that can be recorded on a recording medium of any particular size, which is accomplished by maximizing the storage density of the recorded information.
To maximize the storage density of recorded information, it is desirable to minimize the spot size of the focused beam of light that contacts the recording medium to read and write information. The spot size of the focused light beam is proportional to the wavelength of the light divided by the numerical aperture of the environment through which the light beam passes before contacting the recording medium. Numerical aperture is a term of art that relates to the effective aperture through which the light beam passes. This can be analogized to a beam of light passing through a physical aperture (e.g., a pin hole) that is smaller in diameter than the light beam. When this occurs, the light passing through the pin hole is scattered, resulting in the formation of a spot that is significantly larger than the pin hole. However, if the physical aperture is increased to a diameter that exceeds that of the light beam, no scattering occurs, resulting in a minimal spot size for the light beam.
As should be understood from the analogy described above, it is generally desirable to maximize the numerical aperture of the environment through which the focused light passes to minimize the spot size of the light beam. For a lens through which the light beam passes, the numerical aperture is equal to the refractive index of the lens material multiplied by sin &thgr;, where &thgr; defines the cone angle of the lens. The cone angle &thgr; is defined by the highest angle at which the focused light rays exit the surface of the lens that is closest to the recording medium. This is illustrated in
FIG. 1
, which conceptually illustrates a lens element
1
having a flat surface
3
disposed adjacent an optical recording medium
5
. The arrows
7
illustrate a plurality of focused light rays exiting the bottom surface
3
of the lens
1
at an exit point
9
. The cone angle &thgr; of the lens is defined by the highest angle from the dotted vertical line
11
at which a focused ray of light exits the lens
1
.
As mentioned above, to minimize the spot size of the focused light beam, the numerical aperture (equal to the refractive index times sin &thgr;) of the environment through which the beam passes should be maximized. The highest value that sin &thgr; can obtain is 1, when &thgr; is equal to 90°. Therefore, it is desirable to form the lens
1
in such a manner that the highest angle &thgr; at which light rays exit the lens is approximately 90°. While such a lens cannot be formed, lenses with an angle &thgr; approaching 90° (e.g., 71°) have been formed using techniques that are known in the art.
In addition to maximizing the cone angle of the lens, the other factor that controls the numerical aperture (and consequently the spot size) of the optical recording system is the refractive index of the environment through which the light beam passes. The lens
1
and recording medium
5
can be made from conventional materials having relatively high refractive indices, i.e., refractive indices greater than 2. However, the factor that limits the numerical aperture in conventional optical recording systems is the air gap
13
between the bottom surface
3
of the lens and the top of the recording medium
5
. In particular, air has a refractive index equal to 1. Therefore, no matter how high the refractive index is of the lens
1
and the recording medium
5
, the numerical aperture for the recording system is limited by the lower refractive index of the air gap
13
, thereby limiting the extent to which the spot size of the focused light beam can be minimized.
Relatively recent advances in the field of optical recording have attempted to take advantage of a concept known as evanescent coupling to minimize or eliminate the impact of the air gap
13
on the numerical aperture of the recording system. The concept of evanescent coupling is described making reference to
FIG. 2
, which illustrates a light beam
15
exiting the bottom surface
3
of the lens
1
. As should be appreciated by those skilled in the art of optical recording, when a light beam passes through a boundary between two materials having different indices of refraction (e.g., the bottom surface
3
defines such a boundary between the lens
1
and the air gap
13
), some portion of that light beam is reflected as represented in
FIG. 2
by the reflected light beam
15
f
. The components of the light beam that are reflected are the rays at a high angle &thgr; to vertical. The particular angle at which reflection occurs is dependent upon the refractive index of the lens material. As will be appreciated by those skilled in the art, if the lens were not disposed adjacent the recording medium, the high angle rays would be totally internally reflected at the surface of the lens, and would not pass out of the lens. However, when the exit surface of the lens is disposed adjacent the recording medium, the light beam is not reflected precisely at the boundary
3
between the lens and the air gap
13
. Rather, a component of the light beam known as the evanescent wave extends some distance d beyond the boundary (the exit surface
3
of the lens in
FIG. 2
) before being reflected.
The concept of evanescent coupling in conventional systems involves positioning the exit surface
3
of the lens
1
at a small distance c from the recording medium. If the distance c is small enough, the evanescent wave for a number of the high angle rays can travel over a distance d that is greater than the distance c, thereby coupling to the recording medium. The smaller the distance c, the greater the angle &thgr; of rays that can be evanescently coupled. If the distance c is made small enough so that none of the rays within the critical angle of the lens (i.e., the highest angle at which the lens can pass a ray of light even if the exit surface was adjacent a medium having a refractive index equaling that of the lens) are totally internally reflected, then the evanescent component of all of the rays of the light beam that the lens can pass contact the recording medium
5
. As a result, the air gap
13
has no impact on the numerical aperture of the system, because any internal reflection that would otherwise be caused by the air gap is eliminated by positioning the exit surface
3
of the lens close enough to the recording medium
5
so that the recording medium is contacted by the evanescent wave of each ray of light.
Some conventional optical recording systems employ a lens element that is positioned below the recording medium at a fixed distance c from the surface thereof. Such systems are incapable of positioning the exit surface of the lens (i.e., the surface from which the light beam exits when directed toward the recording medium) close enough to the disk to achieve evanescent coupling. Relatively recent advances in the field of optical recording have attempted to mount the lens to a slider that is known in the art as a flying head because of the principles on which it relies to maintain its positioning with respect to the recording medium. In particular, during operation of an optical recording system, the recording medium is typically rotated at a high speed (e.g., 3,600 RPM) which causes an air flow in the direction of rotation near the surface of the disk. The slider or head is pla
Berg John S.
Ho Easen
Lee Neville K. S.
Digital Papyrus Corporation
Edun Muhammad
Wolf Greenfield & Sacks P.C.
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