Collinear holography

Collinear holography
Hideyoshi Horimai, Xiaodi Tan, and Jun Li
A novel reading and writing technology for a holographic storage system called collinear holography is
developed. With this method, two-dimensional page data can be recorded as volumetric holograms
generated by a reference beam and a signal beam that are bundled on the same axis and irradiated on
the recording medium through a single objective lens. The multiplex recording and reconstructing process
is demonstrated, and it is presented that optical configuration and the dichroic media disk structure are
suitable for a compact system. This method enables us to construct a small volumetric optical disk storage
system with CD and DVD upper compatibility. © 2005 Optical Society of America
OCIS codes: 210.0210, 210.2860, 210.4590, 210.4680, 210.4770, 090.0090.
1. Introduction
In the information era, we are surrounded by digital
products and digital technology. The necessity for
large-capacity storage equipment has increased. The
holographic data storage technology, owing to its large
storage capacities and high transfer rates, is a promising candidate of the next-generation storage techniques. Recently, revival of activity in holographic
storage has resulted from the dramatic developments
in the requisite components for a system, such as laser
technology, spatial light modulators (SLMs), and complementary metal-oxide semiconductor (CMOS) image
sensors. In addition, the limits of the conventional storage technologies involving magnetic recordings and
optical storages loom on the horizon.1
In fact, within the past several years, two U.S. consortia (photorefractive information storage materials
and holographic data storage systems) have developed
unique demonstration platforms with digital volume
holography as well as recording materials for the media. For instance, Stanford University and Siros Technologies (San Jose, California) have demonstrated a
write-once read-many holographic storage system
with high-capacity and a 10᎑Gbit兾s transfer rate.2
Aprilis Incorporated (Maynard, Massachusetts) has
developed a highly sensitive low-shrinkage photopoly-
The authors are with OPTWARE Corporation, 2-5-1 ShinYokohama, Kohuku-ku, Yokohama, Kanagawa 222-0033,
Japan. H. Horimai is also with the Japan Science and Technology
Agency --- Crest, Kawaguchi, Saitama 332-0012, Japan. X. Tan’s
e-mail address is [email protected].
Received 3 October 2004; revised manuscript received 8 February 2005; accepted 10 February 2005.
0003-6935/05/132575-05$15.00/0
© 2005 Optical Society of America
mer material whose polymerization occurs through a
cationic ring-opening mechanism.3
Apart from the recording media, however, these
hardware systems still have essential issues that affect their practicality. First, the reference beam for
the read-out process is separated spatially with an
off-axis optical configuration, which results in a complex and large optical system that prevents the system from miniaturization. Second, for storage density
increase, angle multiplexing,4 phase-coded multiplexing,5 wavelength multiplexing,6 and other methods7,8 are employed normally. Spatial fluctuation of
the storage media should then be strictly controlled,
and a large and heavy damping system is indispensable, which, in turn, prevents the removability and
interchangeability of the media as well as the miniaturization of the system. Third, holographic storage
technology completely differs from other existing
storage systems, such as bit-by-bit storage methods,
and the hologram media cannot be compatible with
conventional storage media.
The collinear holography, a novel holographic optical storage technology that we propose and describe
in this paper, can solve the problems mentioned
above. This technology can produce practical, small
holographic optical storage systems more easily than
conventional off-axis holography. The construction of
the optical setup, the structure of the media being
employed in this technology, and a two-dimensional
(2-D) digital-page-data format used in the collinear
holographic system and their multiplex recording experimental results are presented.
2. Collinear Holography and Disk Structure
Collinear holography as a novel writing and reading
technology for a holographic storage system is very
promising and differs from conventional off-axis holo1 May 2005 兾 Vol. 44, No. 13 兾 APPLIED OPTICS
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Fig. 3. Schematic of a dichroic reflective structure disc media to
record in the collinear holographic system.
Fig. 1. Optical configuration of the collinear holography as a
novel reading and writing technology for a holographic storage
system. The green or blue laser and the red laser mean a laser
beam with wavelengths of green or blue light and red light.
graphic technology. The unique feature of this technology is that 2-D page data are recorded as volume
holograms generated by a coaxially aligned information beam and a reference beam, which are displayed
simultaneously by one SLM and interfere with each
other in the recording medium through a single objective lens.
Figure 1 shows the schematic optical configuration
of the collinear holography, and Fig. 2 shows one kind
of the 2-D page-data patterns. The linear polarizationstate collimated laser beam with a green (or blue)
wavelength [called a green (or blue) laser beam] is used
for both writing and reading holograms. In the write
process, a combined image of the information pattern
(center part) and the reference pattern (outer ring
part) is displayed on the SLM, as shown in Fig. 2(a).
The modulated laser beams by the SLM, with p polarization, pass through the polarizing beam splitter
(PBS) and are then incident upon a quarter-wave plate
(QWP). The laser beams, whose polarizations are converted from the p-polarized state to a circularly polar-
Fig. 2. One kind of 2-D digital page-data pattern used in the
collinear holographic system: (a) write process pattern and (b) read
process pattern are displayed on the SLM.
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ized state by the QWP, are focused and interfere with
themselves in the holographic recording media by an
objective lens. The interference pattern is recorded as
a volume hologram in the media. For the read process,
only the outer ring pattern, as shown in Fig. 2(b), is
displayed on the SLM to be used for creating a reference beam. The p-polarized laser beam passes through
the PBS and is then converted to a circularly polarized
state by the QWP and focused by the same objective
lens as used in the recording process. The circularly
polarized reconstructed information beam, which is
diffracted from the hologram and reflected by the reflective layer in the media, is sent back to the objective
lens and passed through the QWP again. The reconstructed beam, whose polarization is converted from a
circularly polarized state to an s-polarized state, is
reflected by the PBS and detected by a high-speed
CMOS sensor. A red laser with a red wavelength is
used for optical servo control to adjust the focal point of
the objective lens onto a recording media (disk) correctly and to locate the hologram address in a disk. The
servo and address information are preformatted in the
disk as embossed pits. Eliminating the diffraction
noise in the recording media caused by the embossed
pits is possible by use of a dichroic mirror interlayer to
reflect green (or blue) laser beams and pass through
red laser beams. The green (or blue) and red laser
beams are combined to the same axis and separated by
a dichroic mirror layer in the media.
A special holographic disk structure is designed for
realizing the uses mentioned above. Figure 3 shows
the structure schematically. This holographic recording disk has a reflective layer with preformat address
information and a dichroic mirror layer. The holographic material used in the recording layer is a photopolymer, which has a dynamic range 共M兾#兲 of
approximately 10. The disk structure to be used in
collinear holography simplifies the optical systems,
allowing small and compact packaging on the same
side of the disk. This method enables us to construct
a small volumetric optical disk storage system, compatible with existing storage disk systems, like CD
and DVD.
Fig. 4. Page data format encoded from the user data to be used in
the collinear holographic system.
3. Page Data Format
The page-data format based on the subpage is used to
eliminate the problems of illuminated intensity distribution in a page data, of distortion and aberration
of the optical system, of tilting, and of the estimation
error caused by amplification. As shown in Fig. 4, the
size of a subpage (24 ⫻ 24 pixels is used) depends on
the parameters and the magnitude of the inhomogeneity of a system. In each subpage there are 32-byte
data symbols (4 ⫻ 4 pixels) and a synchronous mark
(8 ⫻ 8 pixels) in its center. The synchronous symbol,
which includes a 4 ⫻ 4 pixel rectangular block, is
used to locate the subpage and to provide the necessary coordinate information for data decoding. The
sorting method and correlation technique that differ
from the threshold method are used to distinguish
the ON-pixel and OFF-pixel states from the reconstructed image in the decoding process. In this pagedata format the code rate is 0.5, and the white rate is
approximately 19%.
Fig. 6. Reconstructed 2-D digital page-data pattern image from
the collinear holographic system.
control of focusing and addressing. A digital micromirror device (DMD, Texas Instruments) is used
as the SLM to display 2-D page data and thus create
an information and reference beam. A 2-D reconstructed page-data image is detected by a highspeed CMOS sensor.
In detecting the reconstructed image from a hologram storage system, the pixel-to-pixel matching
method9,10 has both a high decoding speed and a high
4. Experimental Results
To develop the collinear holographic storage technologies, we designed an optical system, as shown
in Fig. 1. A green laser 共532 nm兲 is used for both
writing and reading holograms, and a red semiconductor laser 共650 nm兲 is introduced for optical servo
Fig. 5. Microscope photograph of a hologram stored in the media
of the collinear holographic system.
Fig. 7. Reconstructed 2-D page-data images from the hologram
by shifting of the position of the recorded hologram in both radial
and tangential directions. The reconstructed images disappeared
completely with a 3᎑␮m shift in both directions, indicating that the
shift selectivity of the collinear holography allows us to record
holograms by overlapping at a pitch of at least 3 ␮m.
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Fig. 8. Three reconstructed 2-D page-data images from the recording order of the first, tenth, and twentieth with 20 multiplex recording
holograms overlapped at 3 ␮m.
transfer rate, but it requires an extremely precise
optical system and careful alignment. Obtaining a
low bit-error rate requires that the alignment, distortion, and magnification of the lenses be controlled to
approximately 0.01%.1 Currently an oversampling
method with a ratio of 3 is used in this system. It
means that one pixel on the SLM is imaged onto the
CMOS with 3 ⫻ 3 pixels. Simple analysis shows that,
if the oversampling rate between SLM and CMOS
pixels is larger than 2, each SLM pixel covers fully at
least one CMOS pixel, so even a simple decoding
algorithm can be effective.
By using the optical configuration, as in Fig. 1, and
the media shown in Fig. 3, the page-data hologram
with an average diameter of 200 ␮m is clearly recorded. Figure 5 shows the stored hologram image as
observed by a microscope. Figure 6 shows the reconstructed 2-D image, taken from the hologram shown in
Fig. 2(a), and reconstructed by the reference pattern,
as shown in Fig. 2(b). In this case, the stored page data
is set in black and white and is reconstructed clearly by
the collinear holographic technology.
Using the shift-multiplexing method to increase
recording density is suitable for collinear holography,
because the hologram can be recorded continually
with the rotating disk.11 Figure 7 shows the reconstructed 2-D image from a hologram, as in Fig. 5, the
position of which has been shifted in both radial and
tangential directions. The reconstructed image disappeared completely with a 3᎑␮m shift in both directions, indicating that the collinear holography allows
us to record holograms by overlapping at a pitch of at
least 3 ␮m.
The multiplex recording process is demonstrated
with the collinear holographic system. Figure 8
shows three reconstructed 2-D images from 20 multiplex recording holograms at the recording order of
the first, tenth, and twentieth. The intensity distribution analysis of them indicated that the black and
the white pixels are clearly separated. The definition
of the signal-to-noise rate (SNR) is
Fig. 9. SNR of the reconstructed 2-D image from the 20 multiplex
recording holograms with overlapping at a 3᎑␮m pitch. The qualities of them are almost similar.
Fig. 10. Symbol error rate of the reconstructed 2-D image from
the 20 multiplex recording holograms with overlapping at a 3᎑␮m
pitch. The user data can be decoded at these symbol error rates.
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SNR ⫽
␮on ⫺ ␮off
(␴on2 ⫹ ␴off2)1兾2
,
where ␮on and ␮off are the average levels of the ON
pixels and the OFF pixels, and ␮on2 and ␮off2 are the
level variances. The SNR of every reconstructed image is shown in Fig. 9, and the symbol error rates are
shown in Fig. 10. As expected, if the symbol error rate
is less than 10⫺2, the user data can be decoded from
the reconstructed page-data images.12 From these results, it is clear that the qualities of the reconstructed
page-data images are almost similar from the first to
the twentieth hologram, and they can be decoded to
the user data.
5. Conclusion
We have developed a novel reading and writing technology for a holographic storage system called collinear holography. The optical configuration and the
dichroic disk structure suitable for the system are
presented. The multiplex recording process demonstrates that the shift-multiplexing method is suitable
for collinear holography to increase the recording
density and that the shift selectivity is 3 ␮m in any
direction of the media plane. Experimental and theoretical studies suggest that the proposed system and
media structure are indeed very effective for creating
a small optical storage system with huge density.
Further investigations are under way to balance the
data density and the transfer rate by incorporation of
newly designed optical and electronic components.
With this method, 2-D page data can be recorded as
volumetric holograms generated by an information
beam and a reference beam that are bundled on the
same axis and irradiated on the recording medium
through a single objective lens. This method enables
us to construct small volumetric optical disk storage
systems with CD and DVD upper compatibility.
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