Planar proximity lens element and disk drive using same

A planar solid immersion lens has flat surfaces on both the top and bottom. The planar structure is made to function as a solid immersion lens by including compensation for the refraction of light rays entering the top surface of the solid immersion lens other than normal to the top surface. Compensation can be provided by using a radial graded index lens material or by including on the top surface a surface relief diffraction grating. The planar solid immersion lens can be included in a slider or flying head used in an optical disk drive data recording system.

BACKGROUND 
1. Field of the Invention 
The present invention relates generally to optical systems for use with 
visible and near-visible wavelengths of light. More particularly, the 
present invention relates to such optical systems, used in optical data 
storage devices. 
2. Related Art 
Optical systems use a variety of lenses and other elements to manipulate 
light. The basic element used to focus a light beam in a simple optical 
system is referred to hereinafter as an objective lens. A simple optical 
system is illustrated in FIG. 1, in which a light beam 101 passes through 
a first medium 103, typically air, through a first interface 105 into a 
second medium 107 of which the objective lens 108 is formed, and then 
through a second interface 109 back into the first medium 103. The 
objective lens 108 focuses, or causes to converge, rays of light 111, 113 
comprising the light beam 101, so that they form a small spot 115 at a 
target 117. 
The simple optical system of FIG. 1 has an apparent resolving power which 
depends upon the effective numerical aperture achieved by the system 
design. Numerical aperture (NA) is a parameter known to those skilled in 
this art and readily calculated based upon the wavelength of the light 
involved, the indices of refraction (n) of the first and second media, and 
other readily measured physical characteristics of the system. 
Immersion lenses of various types are well-known in the field of optical 
microscopy to improve the apparent resolving power of optical microscopes. 
Common immersion lenses used in optical microscopy include oil immersion 
lenses and solid immersion lenses (FIG. 2, 201), both of which are located 
between the objective lens and the target, thereby increasing the 
numerical aperture of the system. Achievable numerical aperture figures 
include NA&gt;1. Immersion lenses, including solid immersion lenses achieve 
the high NA figures by conducting light to the target through a medium 
having a high index of refraction which is either in contact with or 
evanescent wave coupled to the target. Evanescent wave coupling occurs 
when the wavefront of a light beam extends a microscopic distance past an 
interface surface before being internally reflected at the interface 
surface. Such a wave can couple into adjacent materials, such as the 
target. 
The use of solid immersion lenses has been extended to the field of optical 
data recording, for example as disclosed by Corle et al. in U.S. Pat. No. 
5,125,750, incorporated herein by reference in its entirety. The system 
disclosed by Corle et al. includes a SIL having a flat lower surface 
adjacent the target and a hemispherical upper surface facing an objective 
lens. A laser light beam is shone through the objective lens and the SIL 
onto the target. Corle et al. teach that the hemispherical shape of the 
top surface of the SIL ensures that light rays comprising the light beam 
which has been focused by the objective lens enter the SIL normal to the 
hemispherical top surface of the SIL, while the flat shape of the lower 
surface ensures that the SIL can be located at a uniform distance from the 
target, i.e., a data recording medium. Corle et al. teach that use of 
their system reduces the spot size of the light focussed on the target by 
a factor 1/n, where n is the index of refraction of the SIL material. 
The use of SILs in optical data recording has been advanced beyond the 
teachings of Corle et al. noted above, for example by Mamin et al., as 
disclosed in U.S. Pat. No. 5,497,359, incorporated herein by reference in 
its entirety. Mamin et al. teach that the hemispherical SIL of Corle et 
al. can be improved upon by increasing the thickness thereof at the center 
beyond the thickness a hemispherical shape would have. Marnin et al. 
reduce the spot size of the light focussed on the target by a factor 
1/n.sup.2, where n is again the index of refraction of the SIL material. 
The use in an optical disk flying head of a SIL with a hemispherical top 
and a curved bottom surface is disclosed by Berg et al. in U.S. Pat. No. 
5,729,393, incorporated herein by reference in its entirety. The lower 
surface of the SIL disclosed in Berg et al. has a curve which stabilizes 
the flight and performance of the device in a flying optical disk drive 
head. The radius of curvature used is not more than about 10 meters. 
SILs as known in the art present certain problems, which make their use 
difficult and expensive in practical optical disk drive storage systems. 
In combination with a flat bottom surface, the hemispherical shape or 
other bulging shape is difficult to construct, difficult to align with 
other components of an optical system and imposes spacing and tooling 
constraints on the optical system design. 
SUMMARY OF THE INVENTION 
According to one aspect of the invention, a proximity lens element (PLE) 
has two surfaces through which an incident light beam passes, and the PLE 
is substantially flat on both surfaces thereof. 
According to another aspect of the invention, there is provided a method of 
modifying a light beam having an axis and comprising a plurality of light 
rays directed substantially parallel to the light beam axis. The method 
comprises steps of: receiving the light beam from a first medium into a 
second medium at a substantially flat interface therebetween; modifying by 
different amounts, directions through the second medium traversed by 
different light rays of the light beam; and receiving the modified light 
beam from the second medium into a target medium through a substantially 
flat surface of the second medium. 
Finally, according to yet another aspect of the invention, an optical disk 
drive includes a PLE having two surfaces through which an incident light 
beam passes and which is substantially flat on both surfaces thereof.

DETAILED DESCRIPTION 
Construction and use of a number of embodiments of the invention is now 
described in greater detail with reference to the accompanying drawing. 
Briefly described, as shown in FIG. 3, one embodiment of the present 
invention is a lens element 301 having two surfaces 307, 309 through which 
an incident light beam 101 passes and which is substantially flat on 
surface 307 and wherein surface 309 behaves as though optically flat in 
the exit aperture region 310, where the light beam evanescently radiates 
or otherwise exits the lens element 301, referred to hereinafter as a 
"planar proximity lens element" or "planar PLE." In this description, a 
proximity lens shall be an optical element which is located close to or 
within evanescent wave coupling distance of a target and which may have an 
optical power ranging from no focussing power to all the focussing power 
required to focus a beam of light, however incident, to a point. When a 
PLE has no focussing power beyond that required to allow a properly 
focussed beam to pass through the PLE as if the top surface 307 does not 
exist, and is evanescently coupled to the target, then it is operating 
similarly to a conventional solid immersion lens. Several variations of 
this embodiment are also described. 
The term "planar PLE" is also used herein to encompass a PLE, described 
below, in which a top surface 307, i.e. that surface usually facing an 
objective lens 108 or light source (not shown), is substantially flat, but 
a bottom surface 309, i.e. that surface usually facing a target 117 such 
as a recording medium, may be curved or have functional shapes. In such a 
device, it should be noted that the bottom surface 309 still behaves 
optically flat in the exit region 310, which may be a region 1 micron in 
diameter. 
As used herein, a "substantially flat" surface is one which is 
macroscopically flat, as opposed to a hemispherical or parabolic surface. 
A substantially flat surface may contain microscopic features including, 
for example, diffraction gratings, surface relief patterns or other 
curvatures, provided the mean change in height from an absolute mechanical 
flat having negligible height variation as measured using an optical 
interferometer is on the order of the wavelength of the light at which the 
device is intended to operate. Note that a bottom surface 309 that is 
substantially flat and whose curvature, if any, also complies with this 
definition will behave optically flat in the exit region 310. 
Another embodiment of the invention incorporates a planar PLE in an optical 
disk drive having a flying optical head. 
Reference is made in this description to light rays normal to a surface. In 
a classical view of optics, a beam of light can be separated into rays 
whose direction is defined by the direction of wavefront propagation at a 
location in the beam. Although common usage sometimes refers to a beam 
normal to a lens surface as one parallel to the optical axis of the lens, 
this description refers to individual ray directions, except in the case 
of evanescent waves, where this does not apply. Thus a ray normal to a 
surface of an element is not necessarily parallel to the optical axis of 
the element. 
A simple glass or polycarbonate optical flat is not usable as a SIL or PLE 
301. Because it is flat on both top and bottom surfaces, it causes 
undesirable refraction of individual incident rays, by varying amounts, at 
the top surface thereof, when light rays reaching the top surface from the 
objective lens do not arrive normal to the top surface, where "normal" 
means making an angle of 90.degree.. 
In order to use an optical element having a flat top surface 307 and a flat 
bottom surface 309 as a planar PLE 301, light rays 111, 113 which do not 
arrive normal 303, 305 to the top surface 307 need to be compensated so 
they focus at the desired focal point 311. The compensation should bend 
the light rays 111, 113 so that they reach the lower surface 309 of the 
planar PLE 301 at approximately the same angle as though they had passed 
through a hemispherical SIL (FIG. 2, 201, for example), instead. 
A glass or plastic, such as a polycarbonate, Graded Index (GRIN) optical 
component can compensate the light rays as described above. Reference is 
now made to FIG. 4. The focusing effect is achieved as rays of light are 
continuously refracted along a continuously varying index of refraction 
401, referred to as an index gradient, built into the substrate. Producing 
a glass or polycarbonate optical component with an index gradient by 
spatially varying the material composition or impurity levels within the 
body of the optical component is well known. Because the focusing effect 
can be achieved by the index gradient and not by a physical curvature, the 
top surface, i.e., the surface facing the objective lens or light source, 
can be made flat. A ray entering an optically denser medium, i.e., a 
medium having a higher index of refraction, through an interface surface 
at an oblique angle of incidence with the interface surface will tend to 
be refracted, i.e., bent, to a path closer to surface normal. The built in 
index gradient of a GRIN compensated planar PLE is selected to 
substantially offset the effect of refraction at the interface surface. 
Various profiles of the index gradient can be used, depending on the nature 
of the incident light and other requirements. Such index profiles can 
either be radial gradients, where the index is made to vary perpendicular 
to the optical axis, or axial gradients, where the index is varied along 
the optical axis. One example radial profile for the index of refraction 
of a GRIN compensated planar PLE is shown in FIG. 4. The index of 
refraction 401 peaks 403 substantially at the optical axis 405 of the PLE 
and falls off to lower values 407 at the periphery 409 of the PLE. 
Profiles which are commercially available in GRIN materials include 
spherical and parabolic profiles, i.e., profiles which vary as a function 
of r, where r represents distance from the optical axis 405. For PLEs 
employing GRIN, that function similarly to conventional hemispherical top, 
flat bottom SIL elements, and where the incident beam is already properly 
converging, for example, having a spherical wavefront, then the desired 
index profile can be that of a radial GRIN with a spherical index profile 
having the maximum index substantially at the optical axis. Axial GRIN can 
also be designed for the pre-converged case, to focus all light to a 
single point because non-normal rays will experience different amounts of 
local refraction as a function of angle of incidence. For applications 
where the light rays are collimated, i.e., where all rays appear to be 
parallel to the optical axis, then only a properly designed radial GRIN 
with a large index gradient can focus all rays to a single point. 
As shown in FIG. 5, an alternative compensation structure useable in a 
planar PLE 501 according to the invention makes use of diffractive optical 
elements 503 that can be easily fabricated using known planar processes 
onto a glass or polycarbonate slab 505. For example, Fresnel zone plates, 
frequently used in optical projection systems as well as micro-lenses, can 
be used to provide the focusing correction. Ordinarily, parabolic or 
spherical phase front curvatures are the corrections introduced. However, 
if other corrections are desired, for example, wavefront aberration 
correction, holographic diffraction patterns or complex diffraction 
gratings can be used. Parallel to the above discussion on GRIN PLEs, at 
least one design of diffractive PLE will function as a conventional 
hemispherical top, flat bottom SIL does, if the diffractive profiles 
provide just enough modification of the individual incident rays so that 
each ray will pass through the top surface of the diffractive PLE without 
changes to their direction of propagation, such as normally occurs at a 
planar interface. In this case, the function of the top surface of the 
diffractive PLE is essentially indistinguishable from a properly matched 
hemispherical refractive surface, except for possible diffractive losses 
and phase differences. In yet another implementation of the diffractive 
PLE where the incident light is collimated, the diffractive profile will 
not only have to provide the function of a refractive spherical surface, 
but also provide appropriate bending of each light ray to converge at a 
desired point. 
A combination of GRIN and diffractive elements can give added freedom to 
optical designers to achieve specific goals. For example, the index 
gradient in a planar PLE using GRIN compensation can also provides 
focusing power, while a diffractive grating relief pattern fabricated on 
to the top side can further be used to correct spherical aberrations, etc. 
or vice versa. In another example, shown in FIG. 6, a diffraction grating 
can be incorporated into the diffractive relief pattern 601 on the top 
surface 603 which provides multiple diffraction spots. The hologram or 
diffraction grating can thus also redirect reflected light from the target 
117, e.g., a data medium surface, to a detector 607. When the target 117 
is a data recording medium, the detector 607 can be part of the read 
apparatus for detecting microscopic optical data marks and spaces on an 
information recording layer thereof. 
In general, a PLE can be designed for a variety of purposes, depending on 
whether it is evanescently coupled to the target and on how much focussing 
power is built into it. The focussing power of the PLE can be varied so 
some or all of the focussing of an optical system is provided by the PLE. 
Thus, for example, the PLE can be used as a singlet in an optical system 
in which it provides focussing power and also evanescently couples the 
focussed beam into the target. 
The lower side of the planar PLE is now described. It is well-known in the 
magnetic recording industry that providing the lower surface of the slider 
that faces the recording medium with a functional curvature is essential 
to reliable and stable flight of flying heads. In an optical disk drive 
system using a planar PLE to perform near field optical recording, the 
bottom surface of the PLE, i.e., the surface facing the data recording 
medium, is maintained at a specific distance from the data recording 
medium by the generation of an air bearing between the data recording 
medium, e.g., a rotating disk, and a slider body, or flying head, designed 
to support the PLE, the bottom surface of the PLE element will also 
require a functional curvature to ensure stable flight and to improve head 
disk interface reliability. 
Some example of doublets using the planar PLE are now described. 
In a doublet application of the planar PLE, the objective lens element 
provides most of the focusing power. This power can be provided by using 
an aspherical, refractive lens; a substantially flat diffractive element; 
a substantially flat radial GRIN element; or an element combining 
refractive, diffractive and GRIN technologies. The two substantially flat 
elements mentioned above can be flat on both sides. The planar PLE 
provides just enough focussing power using GRIN or diffraction 
compensation or a combination thereof to correct the deleterious effect of 
rays entering the planar PLE at a non-normal angle of incidence. 
Therefore, in a doublet system, NA of the planar PLE can be relatively 
low. Any of the objective lenses mentioned above can be combined with any 
planar PLE, including one using diffractive compensation, GRIN 
compensation or a combination of compensations. 
Several variations of a planar PLE useable in doublet configurations are 
now discussed. The bottom surface of the various planar PLEs (i.e., the 
surface facing the target, e.g., a recording medium), can either be 
optically flat or may be curved to allow for best flying head performance 
and reliability. Since the amount of correction can be made to vary as a 
function of distance from the optical axis (see, e.g., FIG. 4), radial 
GRIN and diffractive patterns are particularly suited to the present 
invention. Either can compensate for a non-flat bottom surface, as well as 
a flat top surface. Some planar PLE designs using GRIN or diffractive 
pattern technologies to achieve the desired focussing include: 
Planar PLE with diffraction grating. Here the diffraction grating surface 
relief pattern causes enough bending of a light ray to counter the bending 
of the light ray as it enters the top of the planar PLE. The diffraction 
grating surface relief pattern can be a multi-level phase grating, or a 
kinoform structure. Additional aberration correction functions can also be 
built into the diffraction grating surface relief structure. 
Planar PLE with radial GRIN compensation. Here the radial change in the 
index of refraction, i.e., decreasing index from the optical axis of the 
lens to its outer periphery, provides enough power to bend the light back 
to a single focus, as if each ray had entered a hemispherical SIL normal 
to the top surface. 
Planar PLE with axial GRIN compensation. Here the axial change in the index 
of refraction combined with the non-uniform incidence of impinging light 
rays, causes each light ray to experience different index profiles and 
provides enough power to bend the light back to a single focus, as if each 
ray had entered a hemispherical SIL normal to the top surface. 
Some advantages of the invention are now briefly discussed. 
Flat elements allow for easier handling by providing a datum surface for 
precise alignment. In an optical doublet having high NA, for example, the 
separation between the two lenses needs to be precisely controlled. Such a 
doublet may include a SIL and an objective lens. A SIL having a flat 
surface or microscopic relief pattern that can be precisely referenced to 
a flat surface facing the objective lens could greatly simplify assembly 
tooling design and facilitate the alignment process, particularly if the 
bottom surface of the first lens facing the SIL is also made flat. 
Specific aberration correction can be built into either the GRIN profile or 
diffractive profile, for example to correct for spherical or astigmatic 
aberration. Also, in near-field optical data recording where evanescent 
coupling is used, or in high-NA optical systems where high angles of 
incidence are involved, inhomogeneous phase delays can be introduced 
inadvertently. An optimally designed optical system should control the 
relative phase of the rays that are converging at or beyond the bottom of 
the SIL in order to obtain the smallest spot size. GRIN or diffractive 
elements can be used to impart such an optimized phase profile, using 
design techniques familiar to the skilled artisan. 
Use of a planar SIL permits construction of a hybrid integrated slider 
optical head having several optical elements integrated or mounted onto a 
slider body used in an optical disk drive, as shown in FIG. 7. Such an 
integration presently requires a 90.degree. bend in the optical path. Such 
a bend is achieved by either a waveguide, or a reflecting surface 
incorporating either a GRIN element or diffractive element. The slider 701 
shown includes a laser light source 703, a collimator 705, a beam splitter 
and detector 707, an objective lens element 709 and a SIL built into a 
90.degree. prism. The use of the planar SIL permits construction of the 
optical system entirely on the slider 701. 
Flat elements leave more room for including additional functional elements, 
such as light shutters, mirrors and polarization elements between an 
objective lens element and the planar PLE. For example, a spatial light 
modulator 801 using micro-shutters made using LCD technology can be 
integrated onto one or more of the flat surfaces of a planar PLE, as shown 
in FIG. 8. A spatial light modulator 801 isolates one region 803 of the 
PLE from another 805 and thus ensures that no cross-talk occurs between 
the two regions 803, 805 during operation. The outer zone 805 focusses 
light to one focal point while the inner zone 803 focusses light to a 
second focal point. The outer shutter 807 and the inner shutter 809 can be 
made to open at different times, responsive to a control signal carried on 
one or more signal wires 811, so that cross-talk is prevented, even though 
light beams intended for the two different focal points may overlap. 
The invention has now been described in connection with a number of 
specific embodiments thereof. Numerous extensions and modifications to 
those embodiments, also contemplated as within the scope of the invention, 
will now be obvious to those skilled in this art. The scope of the 
invention should not be considered to be limited by the foregoing, but 
rather is defined by the following claims and equivalents thereto, when 
properly construed.