U.S. patent application number 11/485737 was filed with the patent office on 2010-11-18 for phase compensator for coupling an electromagnetic wave into an optical condenser.
This patent application is currently assigned to Seagate Technology LLC. Invention is credited to William A. Challener, Edward C. Gage, Chubing Peng, Tim Rausch, Robert E. Rottmayer, Michael A. Seigler.
Application Number | 20100290116 11/485737 |
Document ID | / |
Family ID | 43068295 |
Filed Date | 2010-11-18 |
United States Patent
Application |
20100290116 |
Kind Code |
A1 |
Rausch; Tim ; et
al. |
November 18, 2010 |
PHASE COMPENSATOR FOR COUPLING AN ELECTROMAGNETIC WAVE INTO AN
OPTICAL CONDENSER
Abstract
An apparatus comprising a phase compensator and an optical
condenser in communication with the phase compensator. The phase
compensator provides for phase shifting a portion of an
electromagnetic wave. The optical condenser is shaped to direct the
electromagnetic wave to a focal region of the optical
condenser.
Inventors: |
Rausch; Tim; (Gibsonia,
PA) ; Seigler; Michael A.; (Pittsburgh, PA) ;
Gage; Edward C.; (Mars, PA) ; Challener; William
A.; (Sewickley, PA) ; Rottmayer; Robert E.;
(Wexford, PA) ; Peng; Chubing; (Pittsburgh,
PA) |
Correspondence
Address: |
PIETRAGALLO GORDON ALFANO BOSICK & RASPANTI, LLP
ONE OXFORD CENTRE, 38TH FLOOR, 301 GRANT STREET
PITTSBURGH
PA
15219-6404
US
|
Assignee: |
Seagate Technology LLC
Scotts Valley
CA
|
Family ID: |
43068295 |
Appl. No.: |
11/485737 |
Filed: |
July 13, 2006 |
Current U.S.
Class: |
359/486.01 |
Current CPC
Class: |
G02B 27/286 20130101;
G11B 2005/0021 20130101; G11B 11/10541 20130101; G11B 7/1367
20130101; G02B 19/0028 20130101; G02B 19/0033 20130101; G11B 5/314
20130101; G11B 7/1398 20130101; G11B 2005/0005 20130101 |
Class at
Publication: |
359/489 |
International
Class: |
G02B 27/28 20060101
G02B027/28; G02B 27/44 20060101 G02B027/44 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0001] This invention was made with United States Government
support under Agreement No. 70NANB1H3056 awarded by the National
Institute of Standards and Technology (NIST). The United States
Government has certain rights in the invention.
Claims
1. An apparatus, comprising: a phase compensator configured to
receive an electromagnetic wave which has a first portion and a
second portion, wherein the phase compensator includes a first
section having a first wave propagation characteristic and a second
section having a second wave propagation characteristic, wherein
the first portion and the second portion of the electromagnetic
wave pass through the first section and the second section,
respectively, of the phase compensator so as to provide for phase
shifting of either the first portion or the second portion of the
electromagnetic wave; and an optical condenser in communication
with said phase compensator, said optical condenser shaped to
direct the electromagnetic wave to a focal region of said optical
condenser.
2. The apparatus of claim 1, wherein said phase compensator
includes a first section with a first thickness and a second
section with a second thickness.
3. The apparatus of claim 2, wherein at least a portion of said
first section has a refractive index that is different from a
refractive index of said second section.
4. The apparatus of claim 1, wherein said phase compensator is
integrally combined with an optical component.
5. The apparatus of claim 4, wherein the optical component includes
at least one of an optical flat, an optical fiber, a laser diode, a
lens, a mirror, a half wave plate or a light source.
6. The apparatus of claim 1, wherein the phase compensator provides
for about 180.degree. of phase shifting.
7. The apparatus of claim 1, wherein the optical condenser
comprises a first diffraction grating and a second diffraction
grating, where the first diffraction grating and the second
diffraction grating are offset in a longitudinal direction.
8. The apparatus of claim 1, further comprising a near field
transducer positioned adjacent the focal region of the optical
condenser.
9. The apparatus of claim 8, wherein the near field transducer is a
metallic pin.
10-17. (canceled)
18. An apparatus, comprising: a phase compensator configured to
receive an electromagnetic wave which has a first portion and a
second portion, wherein the phase compensator includes a first
section having a first wave propagation characteristic and a second
section having a second wave propagation characteristic, wherein
the first portion and the second portion of the electromagnetic
wave pass through the first section and the second section,
respectively, of the phase compensator so for phase shifting of
either the first portion or the second portion of the
electromagnetic wave; and a planar waveguide in optical
communication with said phase compensator.
19. The apparatus of claim 18, wherein the planar waveguide
includes a side shaped to direct an electromagnetic wave to a focal
region of the planar waveguide.
20. (canceled)
Description
FIELD OF THE INVENTION
[0002] The invention relates to a system for confined optical power
delivery and enhanced optical transmission efficiency.
BACKGROUND INFORMATION
[0003] In an effort to increase areal density of magnetic storage
media, it is desirable to reduce the volume of magnetic material
used to store bits of information in magnetic storage media.
Superparamagnetic instabilities become an issue as the grain volume
is reduced. The superparamagnetic effect is most evident when the
grain volume V is sufficiently small that the inequality
K.sub.uV/k.sub.BT>70 can no longer be maintained. K.sub.u is the
material's magnetic anisotropy energy density, k.sub.B is
Boltzmann's constant, and T is the absolute temperature. When this
inequality is not satisfied, thermal energy demagnetizes the stored
bits. Therefore, as the grain size is decreased in order to
increase the areal density, a threshold is reached for a given
material K.sub.u and temperature T such that stable data storage is
no longer feasible.
[0004] The thermal stability can be improved by employing a
recording medium made of a material with a very high K.sub.u.
However, with the available materials current recording heads are
not able to provide a sufficient or high enough magnetic writing
field to write on such a medium. Accordingly, it has been proposed
to overcome the recording head field limitations by employing
thermal energy to heat a local area on the recording medium before
or at about the time of applying the magnetic write field to the
medium. By heating the medium, the K.sub.u or the coercivity is
reduced such that the magnetic write field is sufficient to write
to the medium. Once the medium cools to ambient temperature, the
medium has a sufficiently high value of coercivity to assure
thermal stability of the recorded information.
[0005] Heat assisted magnetic recording allows for the use of small
grain media, which is desirable for recording at increased areal
densities, with a larger magnetic anisotropy at room temperature to
assure sufficient thermal stability. Heat assisted magnetic
recording can be applied to any type of magnetic storage media,
including tilted media, longitudinal media, perpendicular media and
patterned media.
[0006] For heat assisted magnetic recording, an electromagnetic
wave of, for example, visible, infrared or ultraviolet light can be
directed onto a surface of a data storage medium to raise the
temperature of the localized area of the medium to facilitate
switching of the magnetization of the area. Well-known solid
immersion lenses (SILs) have been proposed for use in reducing the
size of a spot on the medium that is subjected to the
electromagnetic radiation. In addition, solid immersion mirrors
(SIMs) have been proposed to reduce the spot size. SILs and SIMs
may be either three-dimensional or two-dimensional. In the latter
case they correspond to mode index lenses or mirrors in planar
waveguides. A metal pin can be inserted at the focus of a SIM to
guide a confined beam of light out of the SIM to the surface of the
recording medium. Commonly assigned U.S. Pat. No. 6,795,630, which
is hereby incorporated by reference, discloses several waveguides
having a metallic pin transducer for concentrating optical energy
into a small spot.
[0007] Data storage systems often incorporate optical components to
assist in the recording of information. Such systems may include,
for example, optical recording systems, magneto-optical recording
systems or other thermal or heat assisted type recording systems,
as described herein. An important aspect of such systems utilizing
optical components may include the ability to generate small and
intense optical spots of energy. The optical spots can be used for
various functions in the recording process, such as aiding in the
reading or writing of bits of information.
[0008] Prior to generating the small and intense optical spots of
energy, it is usually necessary to couple an electromagnetic wave
from an energy source into a desired optical condenser, such as a
waveguide. One known structure for coupling the electromagnetic
wave into the optical condenser is a diffraction grating.
Diffraction gratings are generally known components in an optical
system that mutually enhance the effects of diffraction to
concentrate the diffracted electromagnetic wave in specific
directions determined by the spacing of the lines and by the
wavelength of the electromagnetic wave.
[0009] There is an increased emphasis on improving the areal
densities of data storage systems. Thus, all components of a data
storage system are being improved to achieve higher areal
densities. For example, those systems that incorporate optical
components to assist in the recording of information are in need of
the ability to efficiently generate even smaller and more intense
optical spots of energy to support the data storage systems of the
future.
[0010] Accordingly, there is identified a need for improved devices
that overcome limitations, disadvantages, and/or shortcomings of
known devices for coupling electromagnetic waves and generating
smaller and intense optical spots of energy.
SUMMARY OF THE INVENTION
[0011] The invention meets the identified need, as well as other
needs, as will be more fully understood following a review of this
specification and drawings.
[0012] An aspect of the present invention is to provide an
apparatus including a phase compensator and an optical condenser in
communication with the phase compensator. The phase compensator
provides for phase shifting a portion of an electromagnetic wave.
The optical condenser is shaped to direct an electromagnetic wave
to a focal region of the optical condenser.
[0013] Another aspect of the invention is to provide an apparatus
that includes a phase compensator including a first section having
a first wave propagation characteristic and a second section having
a second wave propagation characteristic. The apparatus also
includes an optical condenser in communication with the phase
compensator.
[0014] Another aspect of the invention is to provide an apparatus
that includes a phase compensator and a planar waveguide in optical
communication with the phase compensator.
[0015] These and other aspects of the present invention will be
more apparent from the following description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a pictorial representation of a magnetic disc
drive that can include magnetic heads constructed in accordance
with this invention.
[0017] FIG. 2 is a schematic representation of an apparatus
constructed in accordance with the invention.
[0018] FIG. 3 is an isometric view of an optical condenser.
[0019] FIG. 4 illustrates a planar waveguide in accordance with an
aspect of the invention.
[0020] FIG. 5 illustrates an embodiment of a phase compensator in
accordance with the invention.
[0021] FIG. 6 illustrates an additional embodiment of a phase
compensator in accordance with the invention.
[0022] FIG. 7a is an embodiment of an optical condenser in
accordance with the invention.
[0023] FIG. 7b is a graphical illustration of intensity versus
position for a phase compensated wave in contrast to a non-phase
compensated wave.
[0024] FIG. 8a illustrates an embodiment of a phase compensator
constructed in accordance with the invention.
[0025] FIG. 8b is a graphical illustration of intensity versus time
for a wave passing through the phase compensator illustrated in
FIG. 8a.
[0026] FIG. 8c is a graphical illustration of signal versus time
for a wave passing through the phase compensator illustrated in
FIG. 8a.
[0027] FIG. 9 illustrates an additional embodiment of a phase
compensator constructed in accordance with the invention.
[0028] FIG. 10 is a schematic illustration of diffraction gratings
and a planar waveguide in accordance with the invention.
[0029] FIG. 11 is a schematic illustration of diffractions gratings
and a planar waveguide constructed in accordance with the
invention.
[0030] FIG. 12 is a graphical illustration of filling factor versus
Gaussian FWHM for an embodiment of the invention having a phase
compensator in contrast to an embodiment without a phase
compensator.
DETAILED DESCRIPTION
[0031] This invention encompasses devices that can be used to
produce small optical spots and that can be used in magnetic and/or
optical recording heads for use with magnetic and/or optical
recording media. However, it will be appreciated that the invention
may have utility in other technologies such as, for example, high
resolution optical microscopy, lithography, integrated
opto-electronic devices for telecommunications or other
applications.
[0032] Referring to the drawings, FIG. 1 is a pictorial
representation of a disc drive 10 that can utilize magnetic
recording heads, or other type recording heads such as
magneto-optical or thermal/heat assisted recording heads
constructed in accordance with this invention. The disc drive
includes a housing 12 (with the upper portion removed and the lower
portion visible in this view) sized and configured to contain the
various components of the disc drive. The disc drive includes a
spindle motor 14 for rotating at least one data storage medium 16
within the housing, in this case a magnetic disc. At least one arm
18 is contained within the housing 12, with each arm 18 having a
first end 20 with a recording and/or reading head or slider 22, and
a second end 24 pivotally mounted on a shaft by a bearing 26. An
actuator motor 28 is located at the arm's second end 24, for
pivoting the arm 18 to position the head 22 over a desired sector
of the disc 16. The actuator motor 28 is regulated by a controller
that is not shown in this view and is well-known in the art.
[0033] FIG. 2 schematically illustrates a structure for heating a
magnetic recording medium 16 approximate to where the recording
head 22 applies a magnetic write field to the recording medium 16
(FIG. 1) in, for example, a heat assisted magnetic recording (HAMR)
system. A light source 30 which may be, for example, a laser diode
or other suitable laser light source, is used for generating an
electromagnetic wave, as represented by arrow 32. The
electromagnetic wave 32 passes through an optical component 34
which in turn directs the electromagnetic wave 32 toward an optical
condenser 36. The optical component 34 may include at least one of,
for example, an optical flat, an optical fiber, a laser diode, a
lens, a mirror, or a half wave plate. In accordance with an aspect
of the invention, a phase compensator for phase shifting a portion
of the electromagnetic wave 32 may be positioned along the optical
path of the electromagnetic wave 32. In addition, additional
structure for phase shifting the electromagnetic wave may or may
not be included within the optical condenser 36, as will be
described in more detail herein.
[0034] FIG. 3 illustrates a more detailed view of the optical
condenser 36. Specifically, the optical condenser 36 may include a
planar waveguide 38 which may be, for example, in the form of a
solid immersion mirror (SIM) or a parabolic mirror. The waveguide
38 may have a near field transducer 52 such as, for example, a
metallic pin adjacent to an end near the air-bearing surface (ABS)
of the recording head 22. The waveguide 38 includes a dual input
grating coupler, generally represented by arrow 40 that is
comprised of gratings 42 and 44 separated by a gap 46. A focus spot
or laser spot 33 generated by electromagnetic wave 32 is directed
onto the gratings 42 and 44 and coupled to the waveguide 38 by the
gratings to produce the electromagnetic waves within the waveguide
38, as illustrated by arrows 32a and 32b. The electromagnetic waves
32a and 32b are illustrated as phase shifted by 180.degree. with
respect to the wave illustrated by arrow 32a. Arrows 48 and 50
illustrate the instantaneous electric field of the waves. The waves
are reflected off of the parabolic sides of the waveguide 38 and
the electric field components of the reflected waves add in the
vertical direction at the transducer 52 so that the transducer 52
concentrates the electromagnetic waves near the ABS of the
recording head 22 to heat a portion of the magnetic storage medium
16. The waveguide 38 is shown to be embedded, for example, in a
cladding layer 54 and mounted on a slider 56.
[0035] As described herein, the optical condenser 36 can provide
for phase shifting a portion of the electromagnetic wave 32. For
example, FIG. 4 makes use of a diffraction grating arrangement for
providing the phase shifting. Specifically, FIG. 4 illustrates a
two dimensional planar waveguide 58 in the form of, for example, a
SIM and including first and second diffraction gratings 60 and 62.
Diffraction gratings are commonly used to inject light into a
planar waveguide. To generate split linear polarization, the two
diffraction gratings 60 and 62 are used with a longitudinal offset
X between them as illustrated. The purpose of the dual grating is
to introduce a relative phase shift of about 180.degree. between
the two halves of the light beam. Arrows 64 and 66 illustrate an
incident electromagnetic wave having an electric field represented
by arrows 68 and 70, respectively, and a transverse electric
waveguide mode having an electric field represented by arrows 72,
74, 76 and 78. As shown by arrows 68 and 70, the electric field of
the incident wave is initially linearly polarized in the plane of
the waveguide for TE modes. Grating 60 is used to launch the wave
into half of the waveguide 58. Grating 62 is used to launch the
wave into the other half of the waveguide 58. The longitudinal
offset and the position of the two gratings causes a 180.degree.
phase shift to occur between the two waveguide modes as shown by
arrows 72 and 74. After reflection from the edges 80 and 82 of the
waveguide 58, the reflected waves as illustrated by arrows 84 and
86 have electric fields that include both longitudinal and
transverse components in the case of a TE polarization. When the
reflected waves meet at the focal point, the transverse components
of the fields cancel and the longitudinal components of the
electric fields add. This excites surface plasmons on the near
field transducer 88, which may be, for example, a metallic pin.
[0036] In contrast to providing for phase shifting the
electromagnetic wave within the optical condenser 36, such as
providing the offset gratings 60 and 62 as described herein and
illustrated in FIG. 4, an aspect of the invention includes
providing for the phase shifting of the electromagnetic wave to
occur prior to the electromagnetic wave reaching the diffraction
gratings. It will be appreciated that in accordance with the
invention, phase shifting may occur prior to the electromagnetic
wave reaching the diffraction gratings and/or phase shifting may
occur once the electromagnetic wave reaches the diffraction
gratings as well.
[0037] FIG. 5 illustrates an embodiment of the invention for phase
shifting the electromagnetic wave 32. Specifically, FIG. 5
illustrates a phase compensator 90 which in this embodiment
includes an optical flat 92 with a thickness t and a refractive
index n.sub.f and having a thin film 94 with a thickness d.sub.1
and a refractive index n.sub.d deposited on the optical flat 92. As
used herein, "optical flat" generally refers to an optical
component formed of a transparent material and having opposing
surfaces wherein each surface has a flatness less than about
one-tenth the wavelength of an electromagnetic wave that would
propagate or pass through the optical flat. The film 94 provides a
split line 96 for the optical flat 92 such that a portion of the
electromagnetic wave 32a that passes through the film 94 and the
optical flat 92 on one side of the split line 96 is phase shifted
relative to the remaining portion of the electromagnetic wave 32b
that passes only through the optical flat 92 on the other side of
the split line 96. Thus, it will be appreciated that the split line
96 acts as a dividing line for determining which portion of the
wave 32 is phase shifted and which portion of the wave 32 is not
phase shifted. The thicknesses and material choices for the optical
flat 92 and the film 94 are selected to provide the desired amount
of phase shifting, which in one embodiment of the invention is
about 180.degree.. To generate split linear light, the optical path
difference between the part of the electromagnetic wave 32a that
passes through both the film 94 and the optical flat 92 and the
part of the electromagnetic wave 32b that passes just through the
optical flat 92 must satisfy the following equation:
( 2 m + 1 ) .lamda. 2 = ( n d - 1 ) d 1 ( Equation 1 )
##EQU00001##
In this equation, m is an integer.ltoreq.0 and .lamda. equals the
wavelength of the electromagnetic wave 32.
[0038] FIG. 6 illustrates another embodiment of the invention that
includes a phase compensator 90a which comprises an optical flat
92a having a thickness t and a refractive index n.sub.f and a
portion of the optical flat that is etched to create a void,
generally represented by arrow 98 wherein the void has a depth or
thickness illustrated by arrow d.sub.2. Once the electromagnetic
wave 32 passes through the phase compensator 90a, then one side of
the beam 32a will be phase shifted relative to the beam 32b due to
the void 98. The void 98 further defines the split line 96a for
determining how much of the electromagnetic wave will be phase
shifted. The amount of material that must be removed to form the
void 98 must satisfy the following relationship:
( 2 m + 1 ) .lamda. 2 = ( n f - 1 ) d 2 ( Equation 2 )
##EQU00002##
[0039] As illustrated in FIGS. 5 and 6, the phase compensators 90
and 90a may be integrally combined with, for example, the optical
component 34, or the phase compensators 90 and 90a may be formed
separate from or without the optical component 34. The phase
compensator 90 or 90a, or other phase compensators constructed in
accordance with the invention, may be advantageously positioned at
any point along the optical path from the light source 30 to the
optical component 34 to the optical condenser 36 as long as the
placement of the phase compensator allows for phase shifting of the
electromagnetic wave prior to the electromagnetic wave 32 being
coupled into the optical condenser 36.
[0040] The optical flat 92 and 92a illustrated in FIGS. 5 and 6,
respectively, may be formed of glass or any other common
transparent material. In addition, the optical flat 92 and 92a
thicknesses t may be in the range of about 0.1 mm to about 1 cm.
Furthermore, the film 94 may be formed of silicon nitride, aluminum
oxide or any other transparent film. In addition, the film 94 may
have a thickness d.sub.1 in the range of about 100 nm to about 1000
nm. The void 98 illustrated in FIG. 6 may have a depth or thickness
d.sub.2 in the range of about 350 nm to about 1000 nm.
[0041] The phase compensators 90 and 90a may be positioned anywhere
along the optical path from the light source 30 to the optical
component 34 to the optical condenser 36, such as placing the phase
compensator 90 or 90a on the slider 56 portion thereof. The phase
compensators 90 and 90a need to be generally centered on the wave
32 (made up of 32a and 32b) so that the wave intersects the
respective split lines 96 and 96a, i.e., in one embodiment
preferably about half of the wave 32 is phase shifted and about
half of the wave 32 is not phase shifted.
[0042] Electromagnetic modeling of the electromagnetic waves 32a
and 32b that pass through the phase compensator 90, as illustrated
in FIG. 5 indicate that the focus spot will have two maxima with
one maxima phase shifted relative to the other. This is illustrated
in FIGS. 7a and 7b. Specifically, FIG. 7a illustrates an optical
condenser 36a that is similar to the optical condenser 36
illustrated in FIG. 3 and described herein except that as described
the phase shifting of the electromagnetic wave occurs prior to the
wave reaching the condenser 36a and results in focus spots 33a and
33b on the gratings 42a and 44a. FIG. 7b illustrates the intensity
versus position across the width of the gratings for the phase
compensated wave (shown in dotted line) as opposed to the non-phase
compensated wave that is incident upon the gratings without any
prior phase shifting (shown in solid line). It will be apparent
that one advantage of the invention is that due to the gap 46a
being formed between the gratings 42a and 44a, the portion of the
electromagnetic wave in the center portion of the structure that
would otherwise not be coupled into the waveguide is now being
efficiently coupled into the gratings 42a and 44a as a result of
the focus spots 33a and 33b and the intensity profile as
illustrated. This results in the efficient coupling of the
electromagnetic wave into the optical condenser 36a in order that a
smaller and a more intense optical spot may be generated.
[0043] FIG. 8a illustrates a phase compensator 190 constructed in
accordance with the invention. Specifically, the phase compensator
190 was fabricated using electron beam evaporation of, for example,
a thin film 194 of alumina onto an optical flat 192. A shadow mask
was used so that the thin film 194 of alumina was only deposited on
one-half of the optical flat 192. The phase compensator 190 was
fabricated for a wavelength of 830 nm, so the target thickness was
638 nm [(830 nm)/2/(1.65-1)]. In the direction perpendicular to the
split line 196, the focused spot has two maxima 195 and 197 (see
FIG. 8b) while in the direction parallel to the split line 196,
there is one maximum 199 (see FIG. 8c). An advantage to using this
technique is that the two maxima 195 and 197 are better suited for
coupling light into the waveguide since no energy is lost in the
gap between the gratings. It will be appreciated that the thin film
coating should not be limited to optical flats only and can be
deposited, for example, on the end of an optical fiber, on the
facet of a laser diode, on a focusing lens or on a slider.
[0044] Although any transparent dielectric material can be used for
the thin film that is deposited, for example, on an optical flat as
described herein, ideally the dielectric materials are made of the
same material as the optical flat. If this is done, then the
reflection losses from the dielectric/air interface will be the
same as at the optical flat/air interface for the remainder of the
optical flat and there will be no reflection losses at the
dielectric/optical flat interface thereby maintaining equal
amplitudes for the transmitted electric fields which correspond to
the best interference between the two beams and the focal plane in
the lowest minimum at the center of the spot. Furthermore, a
dielectric anti-reflection coating may be applied uniformly over
the phase compensator 90, 90a or 190.
[0045] FIG. 9 illustrates an additional embodiment of a phase
compensator 200 constructed in accordance with the invention. In
this embodiment, the phase compensator 200 includes a half wave
plate. In an optical wave plate, there is a fast and slow axis and
they are designed so that a beam traveling on the fast axis will be
shifted by a half wavelength relative to the slow axis. To adjust
the phase of the incident beam, a half wave plate is cut in half
and assembled together as shown in FIG. 9. It should be noted that
two separate half wave plates cannot be used because they will not
have the same total optical path link along their fast or slow
axes. The arrows 202 and 204 refer to the fast axis of the film
once assembled. The slow axes are at a right angle to the fast
axes. A beam of light or wave 32b passing through the phase
compensator 200 is polarized along the fast axis 204 while the
other half of the beam or wave 32a travels along the slow axis that
is at a right angle to fast axis 202. By definition a beam
traveling on the slow axis experiences a 180 degree phase
shift.
[0046] It will be appreciated that a phase compensator constructed
in accordance with the invention (such as, for example, phase
compensator 90 illustrated in FIG. 5, phase compensator 90a
illustrated in FIG. 6, and phase compensator 190 illustrated in
FIG. 8a) provide for the phase compensator to include a first
section or portion having different wave propagation
characteristics than a second section or portion of the phase
compensator. For example, phase compensator 90 illustrated in FIG.
5 includes a first section that includes the thin film 94 and the
optical flat 92 on one side of the split line 96 that provides for
the wave 32a to propagate through this section in a different
manner than how the wave 32b propagates through the optical flat 92
section. Thus, the differing wave propagation characteristics of
the phase compensator 90 allow for the desired phase shifting.
Phase compensator 90a includes a first section having the void 98
on one side of the split line 96a which results in different wave
propagation characteristics than the second section that only
includes the optical flat 92a on the other side of the split line
96a. Also, the phase compensator 190 has different wave propagation
characteristics for the optical flat 192 section in comparison to
the section on the other side of the split line 196 that includes a
thin film 194 deposited on the optical flat 192. Advantageously, by
providing a phase compensator having first and second sections or
portions that have differing wave propagation characteristics, an
incoming wave 32 may be phase shifted as desired depending upon the
particular wave propagation characteristics, e.g., refractive index
differences, thickness differences, material differences or other
optical property differences, that are selected.
[0047] The overall coupling efficiency of a device is the product
of the grating efficiency and the amount of light that hits the
grating. Because of finite size of the grating in the incident
spot, not all the light hits the grating and there is an optimal
spot size for a given grating. This is referred to as the "filling
factor" for the SIM opening. FIG. 10 illustrates this concept
wherein there is provided gratings 242 and 244 with a gap 246
therebetween for coupling light into a planar waveguide 238
constructed in the form a SIM. The total power in the cross track
direction (i.e., the filling factor) that couples into the
waveguide 238 is given by the integral expression as follows:
P av = .intg. - W - a 1 .sigma. 2 .pi. - x 2 2 .sigma. 2 x + .intg.
a W 1 .sigma. 2 .pi. - x 2 2 .sigma. 2 x = erf ( W 2 .sigma. ) -
erf ( a 2 .sigma. ) ( Equation 3 ) ##EQU00003##
wherein x equals a variable unit of length in the same direction as
"a" and "W", erf equals an error function, and the remaining
parameters are as illustrated in FIG. 10.
[0048] The optimal spot size is given by the following
equation:
.sigma. optimal = W 2 - a 2 2 ln ( W a ) ( Equation 4 )
##EQU00004##
[0049] FIG. 11 illustrates a similar configuration as set forth in
FIG. 10 wherein there is provided gratings 242 and 244 with a gap
246 therebetween for coupling an electromagnetic wave into a planar
waveguide 238. In the embodiment illustrated in FIG. 11, the wave
that is coupled into the gratings 242 and 244 is provided by, for
example, the phase compensator 190 set forth in FIG. 8a having the
two maxima separated by a distance 2u as opposed to the single
maximum. For this embodiment, the total power in the cross track
direction (i.e., the filling factor) that couples into the
waveguide 238 is given by the following integral expression:
P av = 1 2 .intg. - W - a 1 .sigma. 2 .pi. - ( x + u ) 2 2 .sigma.
2 x + 1 2 .intg. a W 1 .sigma. 2 .pi. - ( x - u ) 2 2 .sigma. 2 x =
1 2 erf ( W - u 2 .sigma. ) - 1 2 erf ( a - u 2 .sigma. ) (
Equation 5 ) ##EQU00005##
wherein x equals a variable unit of length in the same direction as
"a" and "W", erf equals an error function, and the remaining
parameters are as illustrated in FIG. 11.
[0050] The optimal spot size is provided by the following
equation:
.sigma. optimal = ( W - u ) 2 - ( a - u ) 2 2 ln ( W - u a - u ) (
Equation 6 ) ##EQU00006##
In one embodiment of the invention set forth in FIG. 11, 2 W=50
.mu.m and 2a=6 .mu.m.
[0051] FIG. 12 illustrates the filling factor for the embodiments
set forth in FIG. 10 (without phase compensator) and FIG. 11 (with
phase compensator). FIG. 12 clearly illustrates that with the use
of a phase compensator one can choose the FWHM of the incident
spots and the separation between them in order to achieve a filling
factor of about 1.0. This is not possible for coupling with a
traditional Gaussian profile which FIG. 12 illustrates a filling
factor of approximately 0.75 is achievable.
[0052] Although the phase compensator described herein can be used
in any coupling application where split linear light is required,
it is better suited for a situation where a coupling grating is not
used. For example, some of the light delivery options contemplated
for heat assisted magnetic recording use an end fire coupling
scheme where a phase compensator would be required to generate
split linear light. In addition, the phase compensator might be
useful in other as yet unforeseen applications where, for example,
a prism coupler is used and split linear light is required. This
may occur if the solid immersion mirror is ever used, for example,
in a microscopy application.
[0053] Whereas particular embodiments have been described herein
for the purpose of illustrating the invention and not for the
purpose of limiting the same, it will be appreciated by those of
ordinary skill in the art that numerous variations of the details,
materials, and arrangement of parts may be made within the
principle and scope of the invention without departing from the
invention as described in the appended claims.
* * * * *