Top illuminator design for 2-D parallel readout in a 3-D multilayer
optical data storage system
Wenyi Feng*, Edwin P. Walker, Haichuan Zhang, Yi Zhang, Alexander S. Dvornikov, Sadik Esener
Call/Recall, Inc., 6160 Lusk Blvd. Suite C-206, San Diego, CA 92121
ABSTRACT
To achieve very high data rates in 3-D multilayer optical data storage systems, a novel approach is investigated to read out in
parallel multiple tracks at different layers simultaneously. Data bits at different layers are arranged as titled data pages inside
the disk. A uniform optical beam sheet is generated to illuminate the desired data page from the top of the disk, and a depth
transfer imaging system is used to collect the fluorescence of the written bits within the data page to a detector array. The
performance of the illumination optics has been experimentally evaluated and optimized by aberration compensation and
equalization of irradiance distribution on the entire data page. Other important factors including reflection loss, sensitivity to
disk quality, and servo requirements of disk wobbling are analyzed.
Keywords: illumination optics, parallel readout, multilayer data storage, two-photon
1. INTRODUCTION
Volumetric optical data storage systems with high data capacity (200Gb/disk) and high data rates (250Mb/s) that rely on twophoton absorption are being developed at Call/Recall Inc.1, 2 Data is recorded at desired locations inside a monolithic thick
plastic disk by two-photon absorption and read out by fluorescence of written bits excited by single photon absorption.3 High
data capacity is achieved by recording multiple data layers as well as maintaining the areal density. Fast access speed and
high data rate are important in such a high capacity system. It is necessary to introduce some parallel processing mechanisms
in the design of optical pickups.
Layer 1
Track 1
Track 1
Layer M
Track N
Track N
(a)
(b)
(c)
(d)
Figure 1 Serial readout of single data channel (a), parallel readout of 1×N data channels (b)(c),
and parallel readout of M×N data channels (d)
In conventional CD and DVD optical data storage systems, the number of data channels is one as shown in Figure 1(a). They
are single data channel devices using serial readout. Exploiting the potential of parallelism that exists in optical systems
increases data throughput. One way is to fan out the readout beam into several beams through the use of a diffraction
grating.4 This action results in a linear 1×N array of focused spots oriented radially so that each individual focused spot reads
out a different data track as shown in Figure 1(b), thereby increasing the data throughput by N times that of a single channel
device. However, there is a limit as to how large N can be due to the limited object field of optical systems. An alternative
to the arrangement of 1×N focused spots in a radial line is to arrange them as a two dimensional spot array as shown in
Figure 1(c). This will not increase the data throughput but can reduce some kinds of crosstalk. Employing a large number of
laser beams using a lenslet array or a single high NA annular-field objective lens can generate the two dimensional spot
*
Correspondence: Email: wfeng@call-recall.com; Phone: 1 858 550-0596; Fax: 1 858 550-0917
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Photorefractive Fiber and Crystal Devices: Materials, Optical Properties,
and Applications VII, and Optical Data Storage, Shizhuo Yin, Francis T. S. Yu,
Hans J. Coufal, Editors, Proceedings of SPIE Vol. 4459 (2002) © 2002 SPIE · 0277-786X/02/$15.00
array.5 Moreover, a linear spread beam can also be use to read out the 1×N multi-tracks.6 Heretofore, all these proposed
approaches can generally achieve parallel readout within a single layer. Because the recorded data layers are non-reflective
and can be closely recorded together, two-photon monolithic multi-layer media optical data storage can take advantage of the
2-D object field to increase data throughput. The readout is accomplished by using a class of optical systems known as depth
transfer optics. The concept is to have the 2-D data page array tilted with respect to the optical axis so that multiple tracks
(1×N) within multiple layers (M) can be readout. The data page format is organized with 1×N multiple tracks being read out
at a given depth inside the material with M more layers being readout at the same radial location as shown in Figure 1(d).
This two dimensional architecture offers the potential for much higher data rates compared to serial readout or in-plane
parallel readout. For example, if each individual data channel within the 2-D tilted data page is operating at 1MHz an array
format of 32×32 data channels read out simultaneously provides a total data rate of 1GHz.
Illumination optics
Readout optics
Disk
(a)
Data pages
1× 1
M× 1
1 ×N
M× N
(b)
Figure 2 Formats of top illumination for 2-D parallel readout in a volumetric optical data storage system:
(a) page-orientated readout, (b) possible illumination beam patterns on data pages (M×N bit cells in one data page)
Figure 2(a) illustrates the concept of 2-D data page array on a tilted plane with respect to the optical axis of the readout optics
and also shows the format that how 2-D parallel readout of tilted data pages is achieved. In this format, bit cells at different
layers are arranged to form data pages at a certain tilted angle inside the disk. Illumination modules are designed to generate
desired beam shapes to illuminate data pages from the top of the disk. Depth-transfer imaging readout optics are used to
collect the excited fluorescence of a data page and image the collected fluorescence to a detector array. Each detector
element within the array can be thought of as an individual serial data channel. During the rotation of the disk, data pages fan
into the field of the optical system so that page-orientated parallel readout is achieved. The illumination module and the
readout module then move synchronously to address data pages at different super-layers. Geometrically, four kinds of beam
patterns could be used for illumination as shown in Figure 2(b). From the left to the right, they are a uniform beam sheet that
covers the entire data page, a column vector array that illuminates the data page column by column, a row vector array that
illuminates the data page row by row, and a spot array that illuminates the data page point by point. Beams come into the
disk along a tilted direction in the first two geometries and do not share the same optics as the readout depth transfer imaging
system (non-collinear), in the row by row and point by point illumination geometries they share a collinear path with the
depth transfer readout optics.
The detailed design of the illumination optics to generate a uniform beam sheet is studied in this paper. A top illumination
module based on a single high power laser diode is described in section 2. The requirements imposed by our systems design
on the beam sheet illuminator are first derived. Initial experimental results are given. Section 3 describes how to achieve the
required illumination performance. Spherical aberration and coma introduced by the tilted disk is compensated to maintain
the diffraction-limited width of the beam sheet. Beams from a VCSEL array are combined to improve the uniformity of
irradiance distribution along the length of the beam sheet. An axicon based aspheric lens is designed to enable uniform
illumination along the depth of the beam sheet. Other important factors related to illumination performance, such as
reflection loss, sensitivity to disk surface defects, and servo issues are discussed in section 4. Section 5 presents a summary
of our findings.
2.
TOP ILLUMINATION MODULE BASED ON A SINGLE HIGH POWER LASER DIODE
As shown in Figure 3(a), the objective of illumination module design is to generate a uniform beam sheet to illuminate tilted
data pages inside the disk. Three dimensions of the beam sheet are crucial to achieve optimum illumination performance.
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Illumination beam
(Width)
w
(Depth)
d
l (Length)
Bit cells
Data pages
(a)
(b)
Figure 3 Side view of illumination with a single beam sheet (a) and three-dimensional definition of the beam sheet (b)
w, l and d are the width, length, and depth of the beam sheet as shown in Figure 3(b), where w is related to the data page
pitch, and l×d is related to the data page size. Smaller width w is preferred in the module to achieve lower inter-page
crosstalk. Longer depth d and length l are desired to illuminate larger data pages and increase data throughput.
High power laser diode
Collimation lens
Asymmetric optics
for Focusing
20µ
µm
Disk
(a)
(b)
(c)
Figure 4 An illumination module based on a high power laser diode (a), readout of six tracks (20µm track pitch)
at one layer by a CCD camera (b), and readout of three layers (75µm layer pitch) by a spectrum analyzer (c)
An experimental illumination module has been built using a 635nm, 14mW laser diode to verify the feasibility of the format
as shown in Figure 4(a). Since only one-dimension of the beam sheet needs to be tightly focused the elliptical beam does not
need to be circularized. Asymmetric optics, such as two orthogonal cylindrical lenses, can be used to control the size of the
beam sheet to match the data pages. A beam sheet with dimensions of w=20µm, d=160µm, and l=30~200µm (adjustable) is
generated to excite tilted fluorescent data pages for 2-D parallel readout. Figure 4(b) shows fluorescent bit cells on six tracks
at one layer with the track pitch of 20µm. Groups of three tracks are recorded with different frequencies at three layers
separated by 75µm. The excited fluorescence is collected by the readout optical system and detected by a PMT. The data
signal is then analyzed on a spectrum analyzer. Figure 4(c) shows the three detected frequencies. Using top illumination
with a single beam sheet experimentally achieves tilted page parallel readout.
3. METHODS TO SATISFY ILLUMINATION REQUIREMENTS
Although the functionality of the illumination module could be verified with the above system, our experiment also
demonstrated certain weaknesses of the present design. For example, the compensation of spherical aberration and coma
introduced by the index mismatch of the tilted disk interface has not been considered. It results in a loose w that may lower
signal to noise ratio and introduce unacceptable crosstalk. Due to the nonuniformity of irradiance distribution along the
length l of the beam sheet and the variance of the width w along the depth d, fluorescent outputs at different channels are very
nonuniform making it difficult to post-process the signals. So further optimization of the beam sheet generator is necessary
for a better performance.
3.1 Aberration compensation for diffraction-limited width
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Having a small width w is desirable to increase readout efficiency. The illumination optics must be able to compensate for
the change in spherical aberration and coma introduced when addressing data pages at different super-layers inside the disk.
The following describes a mechanism for aberration compensation of the illumination beam.
A multilayer optical disk can be modeled as a plane parallel plate. It is complex to analyze wave-front aberration coefficients
of a tilted plane-parallel plate.7 However, a titled plane-parallel plate generates spherical aberration and coma of the opposite
sign of that introduced by an off-axis spherical surface as shown in Figure 5(a). Larger aberration values are introduced by
the part of the spherical surface farther away from the optical axis. This is used to compensate spherical aberration and coma
at different super-layers inside the disk by a lateral movement of the illumination objective lens as shown in Figure 5(b).
n0
n
Disk
Lens
Lens
Disk
(a)
(b)
Figure 5 Plane-parallel plate and off-axis spherical surface generate spherical aberration and coma of opposite sign (a)
and mechanism of wave-front aberration compensation by a lateral movement of the objective lens (b)
In a ZEMAX simulation, a cylindrical lens focuses a plane wave into the disk. The effective working NA is 0.066, the
incident angle is 73°, and the depth inside the disk is 5mm. The lateral movement of the lens is used to compensate the
wave-front aberration at the depth. Figure 6 shows OPD curves before and after compensation. The peak-to-valley OPD in
the interested dimension is 0.57λ and 0.07λ, respectively. The simulation also shows that a lateral movement of the lens
from 0 to 1.6mm is required to compensate the wave-front aberration through a whole 10mm thick disk to maintain the
diffraction-limited width of the beam sheet. A commercially available cylindrical lens can achieve diffraction-limited
performance when the working NA is small. A custom-designed aspheric cylindrical lens can be used in a high NA system.
(a)
(b)
Figure 6 OPD before aberration compensation (a) and after compensation (b)
3.2 Beam combination of a VCSEL array for uniform illumination along length
Another important factor related to illumination performance is the length l of the beam sheet and its irradiance distribution.
With longer illumination beam length, more data tracks can be illuminated resulting in increased data throughput. However,
the length is limited by the input power of the laser beam. If the line is too long, the power distributed to illuminate a single
data channel is smaller and may not excite enough fluorescence for readout. If the power density requirement is 100µW/µm2,
the effective power should be approximately higher than 300mW for a beam sheet with the width w of 15µm and the length l
of 200µm. Another tradeoff is that the irradiance distribution along the beam sheet length is not uniform since it has a
Gaussian distribution profile. More power is concentrated at the center than on the tails. As a result, central data channels
emit more fluorescence and those on the edge of the illumination emit less. The phenomenon can be easily observed in
Figure 7, where the center bits are brighter than the edge bits and the signal amplitude of the edge channel drops by more
than 50% compared to that of the central channel.
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Figure 7 Comparison between output signals of a central channel and an edge channel
One straightforward solution to this effect is to use only the central part of the beam. Another solution is to use a diffractive
optical element as a beam shaper that converts the Gaussian distribution profile to a uniform distribution profile or improving
the uniformity by combining beams from a laser diode array. The latter solution is preferred here because the power of a
single laser diode may not be enough for the application.
12µm
Disk
DOE2
DOE3 Prism
(a)
DOE1
Microlens array
VCSEL array
(b)
Figure 8 Beam combination of a linear VCSEL array: (a) schematic drawing of achieving
uniform irradiance distribution by beam combination; (b) an illumination module based on a VCSEL array
Figure 8(a) shows the schematic drawing of the mechanism of achieving uniform irradiance distribution by combining all the
beams emitted by a linear VCSEL array to a single beam sheet. The output of each VCSEL is assumed to be single mode
having a Gaussian irradiance profile. An optical system is designed to let images of these profiles overlap with each other on
the illumination plane. The final output is the incoherent addition of the beams from all the VCSELs. A uniform irradiance
profile could be achieved by carefully controlling the pitch and the waist width of the beams on the output plane. Figure 8(b)
shows one possible optical setup that can realize the beam combination. It consists of a microlens array for collimation, two
diffractive optical elements (DOEs) for magnification and de-magnification, a prism for deflection, and a DOE for focusing
and aberration compensation. The microlens array is used to collimate circularly symmetric Gaussian beams from the
VCSEL array. An aspheric microlens array is preferred here for a better performance. The tradeoff is tighter tolerance of the
alignment between the VCSEL array and the microlens array. De-magnification is required along the linear direction of the
VCSEL array to get the proper pitch for the final beam sheet. Magnification is required at the orthogonal direction to match
the working NA of the objective lens. The asymmetric magnification optical structure and the objective lens are physically
realized as DOEs for a compact system. The prism is designed to deflect the beams and achieve the required incident angle.
A 1×32 VCSEL array having a pitch of 100µm is used in the ZEMAX simulation. Figure 8(b) shows the footprint of the
final beam sheet with the width of 12µm. It has an approximately uniform irradiance distribution.
3.3 Axicon based aspheric lens design for uniform illumination along depth
The third important factor to the performance is the trade-off between the width w and the depth d of the illumination beam
sheet. It is known that the w is given by 1.22λ/NA and the d is given by λ/NA2. The effective working NA of the objective
lens determines both. The higher the NA is, the smaller the w and d will be. However, smaller w and bigger d are preferred
in the uniform beam sheet illumination system. So there is a tradeoff. Under different requirements of the width w,
diffraction-limited performance of the illumination optics is calculated to get a more accurate relationship between the
working NA and the depth d. A set of curves is obtained as shown in Figure 9. The peak of each curve shows that there is an
optimum working NA that can achieve the longest depth d under the limitation of a particular width w. For example, the
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optimum NA to achieve a beam sheet with the width of 20µm is around 0.06, and the longest depth of the beam sheet is
around 170µm. It is difficult to increase the throughput by extending the depth of the beam sheet as well as maintaining the
required width. A better way is to increase the throughput by enlarging the length of the beam sheet under the limitation of
field of view. So illumination of data pages longer in radial direction and narrower along depth is preferred in the
illumination optics.
µ m)
Beam sheet depth d(µ
1000
λ=635nm
w=50µ
µm
800
40µ
µm
600
400
30µ
µm
200
20µ
µm
0
0.02
0.03
0.04
µm
15µ
0.05
0.06
0.07
0.08
Working NA
Figure 9 Relationship among beam sheet width w, depth d and working NA
As shown in Figure 3(a), the width w of the beam sheet is not uniform along the depth d. The smallest width as well as the
highest peak power is obtained at the waist of the propagating beam where the most fluorescence is excited. The width
expands at both directions off the waist that results in the excitation of weaker fluorescence. So output signals at different
layers along the depth in a same data page are not uniform that leaves difficulty to the post-processing of the signals. The
design of an axicon based aspheric lens is proposed here to alleviate the nonuniformity of illumination. Figure 10 shows the
mechanism of axicon design based on the conservation of energy. The energy is conserved between the annular aperture dr
and the axial distance dz to achieve a uniform irradiance distribution from d1 to d2. The phase function of the element can be
expressed as8
1/ 2
π
φ (r ) = − lnìí2a a2r 4 + (1 + 2ad1)r 2 + d12 + 2a 2r 2 + 2ad1 + 1üý , a = (d2 − d1) R 2
(1)
þ
λa î
[
]
where R is radius of the element.
r
dr
r
d1
z
d2
z
dz
Figure 10 Axicon design based on the conservation of energy
Assuming d1=16mm, d2=17mm, R=3mm, and λ=635nm, equation (1) is expanded in a Taylor series, ignoring the constant
term, and keeping the next four terms. At this point only the performance in one dimension is considered. Figure 11(a)
shows its phase function and the ray tracing around focusing area. A longer uniform axial irradiance distribution is achieved
at the cost of the much bigger spot size. So the axicon cannot be directly used in our system for uniform illumination along
depth. As a comparison, Figure 11(b) shows the phase function and the ray tracing of an aspheric lens with diffractionlimited performance that has the same focal length and working NA. Though 120µm depth is achieved under the limitation
of 15µm width, the focusing spot is too tight to give a uniform irradiance distribution along depth. To address this problem,
the first two coefficients in the axicon design are used providing long depth of focus characteristics. The additional two
coefficients are optimized by ZEMAX for diffraction-limited performance. Figure 11(c) shows the final phase function and
the ray tracing of the element. 117µm depth is achieved under the limitation of 15µm width, which can match the
performance of the aspheric lens shown in Figure 11(b). The focusing is not so tight and the uniformity of irradiance
Proc. SPIE Vol. 4459
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distribution along depth is greatly improved. As a result a uniform illumination along depth and illumination minimized
radially is achieved by optimizing an axicon based aspheric lens.
28µm
15µm
120µm
φ(x)= -309.212x2+1.3756x 4-0.18698x 6+0.01016x 8 φ(x)= -309.212x 2+0.2949x 4-0.0005551x 6+0.00000127x 8
(a)
(b)
15µm
117µm
φ(x)= -309.212x 2+1.3756x 4-0.009754x 6+0.00008637x 8
(c)
Figure 11 Phase functions and ray tracing around focusing area: (a) axicon,
(b) aspheric lens with diffraction-limited performance, and (c) axicon-based aspheric lens
4. OTHER IMPORTANT FACTORS RELATED TO THE PERFORMANCE
Other factors, such as reflection loss, disk surface quality, and disk wobbling are also important to the performance. They are
analyzed in this section.
4.1 Reflection loss and sensitivity to disk surface quality
The index mismatch of the tilted disk interface introduces not only wave-front aberration described above but also reflection
loss. The reflection of s-component and p-component satisfies Fresnel formula
Rs =
sin 2 (θ1 − θ 2 )
tan 2 (θ1 − θ 2 )
, Rp =
2
sin (θ1 + θ 2 )
tan 2 (θ1 + θ 2 )
(2)
where θ1 and θ2 are the incident angle and the refractive angle, respectively. Bigger θ2 is preferred in the readout optics at the
cost of large reflection loss in the illumination optics. For example, 35% of s-component and 8% of p-component are
reflected at the incident angle of 73°. So disk AR coating is necessary to reduce reflection loss. On the other hand, relatively
small working NA makes the illumination format sensitive to disk surface quality. The width of the beam sheet projected on
the disk surface is around hundreds of microns. The light is easy to be scattered by surface defects such as scratch and dust.
As a result, less power couples into the disk and excites less fluorescence. The influence appears remarkable while data
pages are close to the disk surface. So the disk needs to have a high quality surface to avoid scattering.
4.2 Non-collinear illumination beam servo requirements
The non-collinear illumination format described above is sensitive to disk wobble especially accessing to the deep depth or
the outer edge of the disk. Due to disk wobble, the beam sheet cannot always illuminate the selected data pages inside the
rotating disk. Without a servomechanism to maintain the data pages continually covered by the beam sheet, the operation of
parallel readout is unthinkable. Figure 12 shows a model for servo analysis. Coordinates xyz are for the disk and x’y’z’ are
H H H for the data pages. Disk wobble is divided into six freedoms of the disk: x , y , z , x , y , and z . Two of them, disk rotation
H
y and radial movement z , could be considered in the access of different data pages. Others including radial movement x ,
H
axial movement y , longitudinal rotation x , and lateral rotation z will cause data pages wobbling inside the disk and need to
H
H
L
L
be followed by the illumination beam sheet. Four actuations of the illumination module x" , y " , z " and u could be used for
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servo. A series of parameters are defined to simply the description. They are also initially given typical values for
calculation.
Disk: radius R=44.5mm and thickness H=5mm.
Beam sheet: width w=20µm, depth d=160µm, length l=600µm, incident angle θ=61°, and refractive angle θ’=36°;
Data page: width t=12µm, depth u=100µm, length v=300µm, tilted angle θ’=36°, distance to the disk axis r, and distance
to the disk surface h; where t is determined by the tilted angle θ’ and the length of data bits b =20µm: t = b sin θ ' .
y
u
x'
y''
x''
z''
Illumination
module
Data pages y'
z'
x
z
Disk
Figure 12 Model for servo analysis
1.
H
Radial movement x
H
H
As shown in Figure 13(a) (a top view of the disk), the radial movement x of the disk results in a radial movement x ' of the
data page with a same distance ∆x. It has no effect on the illumination beam. Servo may not be required if the tolerance
L
satisfies ∆x ≤ (l − v) / 2 . Otherwise a radial actuation x" of the illumination module with a distance ∆x is required to
H
compensate x . The distance is independent to the incident angle θ and the distance to the disk surface h. The compensation
may not be necessary for a non-removable disk because this wobble appears same to recording and illumination, and it has
been compensated in recording.
2.
H
Axial movement y
H
Due to the axial movement y of the disk, the illumination beam and the data page move to different directions as shown in
Figure 13(b). The offset caused by ∆y between the beam sheet and the data page will be ∆y tan θ , and optical path of the
illumination beam inside the disk will change ∆y /( n cos θ ) . Servo may not be required if the tolerance satisfies
∆y tan θ ≤ ( w − t ) / 2 and ∆y /( n cosθ ) ≤ ( d − u ) / 2 . Otherwise, two actuations of the illumination module are required to
H
L
compensate y : a tangential actuation yL " with a distance ∆y tan θ and an axial actuation z " with a distance ∆y /(n cos θ ) .
The compensation is still required for a non-removable disk because this wobble appears different to recording and
illumination.
y
y'
∆x
x
u
z''
z'
(a)
α
θ
α
z'
θ’
θ1
y''
x''
x'
y'
∆y
θ
x
(b)
u
z''
x''
y''
y''
α
z
x' θ2
(c)
(d)
H
H
Figure 13 Disk wobble divisions and compensation: (a) radial movement x , (b) axial movement y ,
(c) longitudinal rotation x , and (d) lateral rotation z
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3.
Longitudinal rotation x
Due to the longitudinal rotation x of the disk, the illumination beam turns to an angle θ1 and the data page rotates to an angle
θ2 as shown in Figure 13(c). A small run-out ∆θ appears between the beam sheet and the data page. The run-out is caused
by the nonlinear angular relationship of refraction. It results in an offset between the beam sheet and the data page, which is
related to the distance to the disk surface h. The deeper the data page is, the bigger the offset will be. If the angle of the
wobble is α, the run-out ∆θ will be
æ sin θ ö
æ sin(θ − α ) ö
÷ − arcsin ç
÷
n
è n ø
è
ø
∆θ = θ 2 − θ 1 = arcsin ç
(3)
Servo may not be required if the tolerance satisfies h ⋅ ∆θ ≤ ( w − t ) / 2 . Bigger θ and smaller h will loosen the tolerance. A
longitudinal rotation x" of the illumination module with an angle ∆θ is required to compensate x , which is difficult to be
achieved by opto-mechanics. The rotation actuation x" could be replaced by a movement actuation yL " if d ⋅ ∆θ ≤ ( w − t ) / 2 .
Here, the change of illumination optical path inside the disk is relatively small, which could not be considered. The
compensation is still required for a non-removable disk.
4.
Lateral rotation z
Assuming wobble angle and data page size are relatively small, it is deemed that the data page wobbles in a plane. Due to the
lateral rotation z , the data page rotates to an angle α and the illumination beam turns to the same direction with an angle
arcsin(sin α / n ) as shown in Figure 13(d). A small offset ∆θ appears between the beam sheet and the data page. It results in
an offset between the beam sheet and the data page, which is related to the distance to the disk surface h. The deeper the data
page is, the bigger the offset will be. The run-out ∆θ is given by
∆θ = α − arcsin(sinα / n)
(4)
Servo may not be required if the tolerance satisfies h(1 − cos ∆ θ ) ≤ ( d − u ) / 2 and h ⋅ ∆ θ ≤ (l − v) / 2 . The latter one is
tighter. Bigger l and d will loosen the tolerance. A lateral rotation u of the illumination module with an angle ∆θ is required
to compensate z , which is difficult to be achieved by opto-mechanics. So it is a good choice to have a big l. The
compensation is still required for a non-removable disk.
The typical values given above are used obtaining two important results. First, servo is not required for a non-removable disk
L
H
if ∆y<3.9µm and a<0.005°; for a removable disk, plus ∆x<150µm. Secondly, only two actuations, y" and z " , are required
for servo if the disk wobble a<5.2° and l is much bigger than v.
5. SUMMARY
A top illumination module based on a single high power laser diode is constructed, which verified experimentally the
feasibility of illumination with a single beam sheet for parallel readout. The feasibility to read out 2-D tilted data pages in
parallel to achieve very high data rate is shown. The lateral movement of the illumination objective lens is shown to
compensate spherical aberration and coma while super-layer addressing inside the disk. Combination of a VCSEL array and
an axicon based aspheric lens to achieve uniform illumination on the whole data page is presented. The non-collinear
illumination methods require uncoupled servo mechanisms between the illumination and readout optical systems. These
techniques enable 3-D multilayer optical data storage systems with high data capacity and high data rate.
ACKNOWLEDGMENTS
The authors would like to thank Xuezhe Zheng, Carlos Caponera and Joannes M. Costa for their contributions to this work.
This effort was supported as part of the Fast Readout Optical Storage Technology (FROST) program, sponsored by the
Defense Advanced Research Projects Agency (DARPA) and administered by the Air Force Research Laboratory (AFRL)
under agreement F30602-98-C-0226. AFRL sponsorship under agreements F30602-95-C-0168 and F30602-98-C-0240 is
also gratefully acknowledged. The US government is authorized to reproduce and distribute reprints for governmental
purposes notwithstanding any copyright annotation thereon.
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