Optical waveguides – Planar optical waveguide
Reexamination Certificate
2000-05-11
2003-07-15
Lee, John D. (Department: 2874)
Optical waveguides
Planar optical waveguide
C385S033000, C359S717000
Reexamination Certificate
active
06594430
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
BACKGROUND OF INVENTION
1. Field of Invention
The present invention relates generally to optics and, more particularly, solid immersion lenses for focusing collimated light in the near-field region.
2. Description of the Background
In modern optical data storage systems, data is stored on an optical storage medium in the form of marks carried on a surface of the optical medium. The data may be accessed by focusing a laser beam onto the data surface of the optical medium and analyzing the light reflected by the marks. Storage density of the system may be increased by reducing the size of the beam (called the “spot”) focused on the data surface. In addition to optical data storage applications, reduction of spot size is beneficial for photolithography and microscopy applications as well. For example, in photolithography, smaller spot sizes allow for the exposure of finer features in photoresist.
The diffraction-limited spot diameter obtained from classical scalar diffraction theory is provided by:
d
FWHM
=
λ
2
NA
(
1
)
where d is measured at the full width half maximum (FWHM), &lgr; is the wavelength of the light, and NA is the numerical aperture. The numerical aperture of a lens system, such as the lens system illustrated in
FIG. 1
, is an indication of the focusing power and may be approximated as:
NA≈n
medium2
sin &thgr; (2)
where the definition of the variables of equation 2 are provided with reference to FIG.
1
. The numerical aperture of any lens system cannot exceed the value of the refractive index of the lens at the focal plane. Lenses are typically characterized by the value of the numerical aperture in air. For example, with reference to
FIG. 1
, if &thgr; is 30°, and because n
mediun2
≈1(air), then NA
air
=0.5, and the diffraction limited spot size d
FWHM
≈&lgr;. In optical data storage systems, as discussed hereinbefore, the size of a recorded bit, and hence the aerial density, is proportional to the spot size. From equation 1, it is evident that one way of reducing the diffraction limited spot size is to increase the numerical aperture.
One known lens system used in applications where reduced spot size is critical, such as optical data storage systems, involves using an objective lens
10
in conjunction with a solid immersion lens (SIL)
12
, as illustrated in FIG.
2
. Using the SIL
12
allows for the increase of the refractive index at the focal plane f of the objective lens
10
. In
FIG. 2
, the surface
14
of the SIL
20
is hemispherical. Light from the objective lens
10
is incident normal to the upper surface
14
at all points, and no refraction at the upper surface
14
occurs. Therefore &thgr;, which is determined by the objective lens
10
, will be unchanged and the refractive index of the media at the focal plane f is increased. Instead, the numerical aperture of the system of
FIG. 1
is:
NA=n
SIL
sin &thgr;=
n
SIL
NA
air
. (3)
It is apparent from equation 1 that by using the SIL
12
, the diffraction limited spot size is reduced by a factor of n
SIL
. The optical spot may be evanescently coupled to an optical data storage medium with minor expansion provided that the medium is within the near-field region of the bottom surface
16
of the SIL
12
, i.e., very close, typically within a fraction of a wavelength, or a few nanometers depending on the wavelength. The evanescent coupling effectively allows the small spot size to be “copied” across the gap from the bottom surface
16
of the SIL
12
to the media.
Another known type of lens system using an SIL
12
, referred to as the “super SIL” or “SSIL”, is shown in FIG.
3
. For the lens system of
FIG. 3
, the surface
14
of the SSIL
12
is spherical. In addition, the focal plane f of the objective lens
10
is below the lower surface
16
of the SSIL
12
. The SSIL
12
does some additional focusing of the light from the objective lens
10
and, when the incident angle of the light from the objective lens
10
on the SSIL
12
is 90°, &thgr;′ is also 90°. Therefore, sin &thgr;′=1, and the numerical aperture of the system is:
NA=n
SIL
. (4)
One restriction of the SSIL arrangement of
FIG. 3
is that the numerical aperture of the objective lens
10
must be 1
SIL
for maximum performance.
Additionally, to improve the off-axis performance or other aberrations caused by a hemispherical SIL, the lens system of
FIG. 3
may use an aspheric SIL. A lens system using an aspheric SIL in conjunction with an objective lens to improve off-axis performance, however, may sacrifice spot size.
In all three of these cases, however, the objective lens
10
is separated from the SIL
12
by a spacing. In most near-field applications, the dimensions of the spacings are critical, and consequently must be accurate to within a fraction of a wavelength. Otherwise, if the focal plane deviates slightly from the designed location, the performance of the lens system is severely degraded. In addition, where the objective lens
10
and the SIL
12
are mechanically aligned, their alignment may shift, thereby possibly destroying the precise alignment.
Accordingly, there exists a need in the prior art for a lens system which yield a reduced spot size yet does not require precise mechanical alignment of the objective lens and the SIL. There further exists a need for such a lens system to be adaptable to modem near-field applications, such as optical data storage, photolithography, and microscopy.
BRIEF SUMMARY OF INVENTION
The present invention is directed to a lens for focusing collimated light. According to one embodiment, the lens includes a single, optically transmissive material having an aspherical focusing surface and a second surface, such that collimated light incident on the aspherical focusing surface is focused in a near-field region of the second surface.
According to another embodiment, the present invention is directed to a lens for focusing collimated light, including a first focusing portion having a first refractive index, wherein the first focusing portion includes a focusing surface and a second surface, and a second focusing portion having an aspherical focusing surface and a second surface, wherein the aspherical focusing surface of the second focusing portion is connected to the second surface of the first focusing portion, wherein the second focusing portion has a second refractive index which is not equal to the first refractive index, such that collimated light incident on the focusing surface of the first focusing portion is focused in a near-field region of the second surface of the second focusing portion.
According to another embodiment, the present invention is directed to a lens for focusing collimated light, including a first focusing portion having a first refractive index, wherein the first focusing surface includes a focusing surface and a second surface, a second focusing portion having first and second surfaces, wherein the first surface of the second focusing portion is connected to the second surface of the first focusing portion, wherein the second surface of the second focusing portion defines a cavity, and wherein the second focusing portion has a second refractive index which is not equal to the first refractive index, and a third optically transmissive portion disposed in the cavity defined by the second surface of the second focusing portion, wherein the third optically transmissive portion has a high refractive index relative to a wavelength of the collimated light.
The present invention provides an advantage over prior art lens systems for focusing collimated light in the near-field region in that it provides the focusing power of a solid immersion lens while obviating the need to employ a separate and distinct objective lens. Concomitantly, the present invention obviates the need to precisely orient the spacing between a separate and distinc
Bain James A.
Rausch Tim
Schlesinger Tuviah E.
Stancil Daniel D.
Carnegie Mellon University
Kirkpatrick & Lockhart LLP
Lee John D.
Song Sarah U
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