Near-Field Optical Data Storage
Proefschrift
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus prof. dr. ir. J.T. Fokkema voorzitter van het College voor Promoties,
in het openbaar te verdedigen op maandag 12 november 2007 om 10:00 uur door
Ferry ZIJP
Samenstelling promotiecommissie:
Rector Magnificus, voorzitter
Prof. dr. ir. J.J.M. Braat, Technische Universiteit Delft, promotor Prof. dr. H.P. Urbach, Technische Universiteit Delft
Prof. dr. ir. P. Kruit, Technische Universiteit Delft Prof. dr. T.H.M. Rasing, Radboud Universiteit Nijmegen Prof. dr. C.R. Ronda, Universiteit Utrecht
Prof. dr. T.D. Milster, University of Arizona
Dr. P. Török, Imperial College London, United Kingdom Prof. dr. ir. P.M. van den Berg, Technische Universiteit Delft, reservelid
The work described in this thesis has been carried out at the Philips Research Laboratories Eindhoven, as part of the Philips Research programme.
ISBN 978-90-9022391-9 / NUR 926
Copyright © 2007 by Koninklijke Philips Electronics N.V.
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, mechanical, photocopying, recording or otherwise, without prior written permission of the copyright owner.
Contents
1 Introduction 1
2 Optical disc systems 3
2.1 Optical disc drive architecture 3
2.1.1 Optical discs 3
2.1.1 Mechanics and light-path 5
2.1.3 Focus and tracking error signals 6
2.1.4 Data path, coding and bit detection 8
2.2 Read-out of optical discs 10
2.3 Key parameters of consumer optical disc formats 13 3 Near-field optical data storage with a solid immersion lens and active
air gap control 15
3.1 Principles of near-field optical data storage 15 3.1.1 Basic principles of the solid immersion lens 16 3.1.2 Passive air-bearing sliders versus active air gap control with
an actuator 17
3.1.3 First-surface disc systems versus cover-layer protected disc
systems 19
3.2 Parameters of near-field optical disc systems 20 3.3 Overview of alternative fourth-generation technologies 22 4 Theoretical aspects of near-field optical data storage 25 4.1 Field structure in a homogenous focal region 26 4.1.1 Scalar approximation for a homogenous focal region 28 4.1.2 Vectorial solution for a homogenous focal region 29 4.1.3 Energy ratios for the vectorial field components 35 4.2 Plane waves in the focal region of a solid immersion lens 37 4.2.1 Multilayer transmission and reflection for a single plane wave 37 4.2.2 Total internal reflection and evanescent waves 40
4.2.3 Surface plasmons 44
4.3 Field structure in the focal region of a solid immersion lens 46
4.4 Phase aberrations and amplitude variation 49
4.4.1 Phase aberration correction and compensation for the
amplitude variation 50
4.4.2 The corrected field with rotation symmetry 54 4.4.3 Simplified phase-only correction for circular polarisation 57 4.4.4 Aberration analysis for a solid immersion lens and a
multilayer structure 58
4.4.5 Expansion of the aberrations in Zernike polynomials 58 4.4.6 Aberrations due to a single transition: a comparison with
geometrical optics 61
4.5.2 Approximation of a tilted multilayer structure 71
4.5.3 Surface plasmons on a grating 72
4.6 Conclusion 74
5 Study of the field structure in the focal region of a solid immersion lens 77
5.1 Homogenous focal region 78
5.2 Field structure with a solid immersion lens in air 82 5.3 Focusing with a solid immersion lens through an air gap into a third
medium 87
5.3.1 Field structure without correction 87
5.3.2 Correction fields for perfect correction 89 5.3.3 Field structure with perfect correction 94 5.3.4 Field structure with phase-only correction 95 5.4 Air gap dependence of the spot size for practical optical disc examples 97 5.4.1 CuSi write-once medium with a 3 μm cover-layer and an
NA=1.45 solid immersion lens
97 5.4.2 GeSbTe rewriteable first-surface phase-change medium and
an NA=1.9 solid immersion lens 99
5.5 Conclusion 103
6 Exploratory experiments with a solid immersion lens 105
6.1 Near-field static tester 105
6.1.1 Solid immersion lens manufacturing and alignment 105
6.1.2 Alignment and positioning mechanics 106
6.1.3 Near-field static tester light-path 107
6.2 Reflected irradiance distribution with a flat sample 108
6.2.1 Total internal reflection 109
6.2.2 Large air gap with a glass sample 111
6.2.3 Approaching contact, linearly polarised illumination 112 6.2.4 Approaching contact, circularly polarised illumination 112
6.3 Reflectivity versus air gap 117
6.4 Reflected irradiance distribution with a pregrooved sample 118
6.4.1 Diffraction pattern 118
6.4.2 Wood’s anomalies 122
6.4.3 Push-pull signal 125
6.5 Conclusion 126
7 The gap error signal and central aperture contrast in near-field read-out
129
7.1 Detection of the GES and CA signal 130
7.2 GES and CA signal with linear and circular polarisation 131 7.2.1 Implementation of the GES in a near-field optical disc drive 132 7.2.2 Calibration of the GES in a near-field optical disc drive 132 7.3 GES and CA contrast with a GeSbTe phase-change medium 133 7.4 Alternative methods for detecting the air gap height 137
7.5 Conclusion 140
8 Lens designs 141
8.1 The aplanatic solid immersion lens 141
8.1.2 Sine condition 145
8.1.3 Progress in immersion optics 146
8.2 High refractive index glasses and crystals 147 8.3 Lens design with NA=1.9 for first surface recording 149
8.3.1 Doublet of a plano-aspherical lens and a super-hemispherical
solid immersion lens 150
8.3.2 Chromatic aberration correction 151
8.3.3 Opto-mechanical assembly 154
8.4 Lens design with NA=1.45 for cover-incident recording 156 8.5 Considerations for dual-layer near-field optical recording 159
8.5.1 Spherical aberration 159
8.5.2 Minimum spacer-layer thickness 161
8.6 Conclusion 163
9 Lens manufacturing and testing 165
9.1 Phase-stepped interferometry for lens testing 165
9.2 Interferometry set-up 166
9.2.1 Lens alignment 167
9.2.2 Interferogram with total internal reflection in the NA=1.9 lens
167 9.3 Polarisation-induced aberrations in the NA=1.9 lens 169 9.3.1 Origin of the polarisation induced aberrations 170
9.3.2 Calculations and measurements 170
9.3.3 An additional effect due to the vacuum chamber 173
9.4 SIL manufacturing for the NA=1.9 lens 174
9.4.1 Cone shape 174
9.4.2 Thickness optimisation using Focused Ion Beam milling 175
9.4.3 Thickness error compensation 180
9.5 Characterisation of the non-periodic phase structure 181 9.6 Manufacturing and assembling of the NA=1.45 lens 182
9.7 Conclusion 183
10 Near field read-out and recording experiments 185
10.1 Near-field optical recording set-up 185
10.1.1 Light-path of the recorder 185
10.1.2 Alignment of the lens and the optical disc 188
10.2 Air gap servo system 189
10.2.1 Basic operation principles 190
10.2.2 Actuators 191
10.2.3 Air gap servo requirements 191
10.2.4 Air gap servo dynamic characteristics 192 10.2.5 Experimental GES and its calibration 193 10.2.6 Pull-in procedure and servo performance 195 10.2.7 GES normalisation on the laser power 197
10.2.8 Tracking servo operation 198
10.3 Experiments with the NA=1.9 lens on first-surface discs 199
10.3.1 Read-out of a 50 GB ROM disc 199
10.3.2 Recording on a phase-change disc 201
10.4.2 Tilt error signal principle 204
10.4.3 Tilt error signal characterisation 206
10.4.4 Closed-loop radial tilt control 209
10.4.5 High-density read-out and recording 210
10.5 System robustness against dust 211
10.6 Conclusion 212
11 Conclusion and outlook 215
Chapter 1
Introduction
The work described in this thesis was carried out in a project at the Philips Research Laboratories in Eindhoven, the Netherlands, as part of the Philips Research programme. The objective of this project was to develop a technology option for optical disc products that had the potential to succeed Blu-ray Disc (BD), the most recent optical disc format. To achieve this objective the project developed new optical technologies, scientific insights and intellectual property rights. The greater part of this work was carried out in the Storage Physics group of Philips Research between 2003 and the end of 2006. During this period, the work was part of a collaboration with the Optical Storage Laboratory of Sony Corporation in Tokyo. During this collaboration we shared results of theoretical and experimental research on near-field optical data storage with a solid immersion lens (SIL). This project was not our first effort at Philips Research on near-field optical data storage with a SIL. Earlier experiments with a SIL were carried out in the Optics group in 1999 and 2000, the results of which are included in Chapter 6. Results of our research were frequently reported in (invited) presentations at international conferences, most notably the Optical Data Storage (ODS) conferences and several times at the annual International Symposium on Optical Memory (ISOM).
It was tried to make this thesis a practical text for the experimental optical scientist and engineer. It is therefore as self-contained as possible. Chapter 2 is an introduction into the basic principles of conventional optical disc systems. Chapter 3 introduces the basic principles of the solid immersion lens and near-field optical data storage as well as a brief overview of alternative technologies. Chapter 4 presents theoretical aspects of near-field optical data storage. It introduces the theory that describes the electric field structure in the focal region of a lens, starting with a scalar approximation, then it gives a full vectorial theory for a homogenous focal region followed by a vectorial theory for a multi-layer focal region and a theory describing the illumination of the pupil that is required for perfect focusing in a multi-layer structure. Chapter 4 also presents a theory that describes the aberrations due to focusing in a multi-layer structure and a theory that describes the irradiance distribution of the light that is reflected back into the pupil of the lens by the multi-layer structure. Chapter 5 discusses a study of the electric field structure in the focal region of a solid immersion lens. Since it is impossible to directly observe this focused field structure in detail, this study is entirely based on calculation results. Chapter 5 also addresses the effect on the focused field structure of the air gap height between the SIL and the optical medium. As apposed to the field structure in the focal region, the field that is reflected back into the pupil of a lens can be observed directly in an experimental set-up. Chapter 6 therefore discusses such observations with a SIL and makes comparisons with calculation results. Using these experimental results, the principle of the gap error signal (GES) is introduced. The GES was also used in a closed-loop air gap servo system in our experimental near-field optical recording system. Chapter 7 discusses the properties of the GES and the central aperture read-out signal in more detail. Chapter 8 discusses the properties of the SIL from an optical design point of view and describes two lens designs that were used in the near-field optical recorder. Chapter 9 discusses the manufacturing of prototypes of these lenses and certain aspects of the interferometry that was used during lens assembling to evaluate lens aberrations. Chapter 10 discusses the light-path and servo systems of the near-field optical recorder. Chapter 10 further presents experimental recording and read-out results with this system and it discusses the robustness of the system. In Chapter 11 we summarise the conclusions of the previous Chapters by discussing what could be a realistic commercial near-field optical disc system considering the results presented in this thesis. Chapter 11 further concludes with a general outlook.
Chapter 2
Optical disc systems
This Chapter briefly describes basic system aspects and basic optics of data storage on optical discs. The purpose of this Chapter is to introduce a number of methods, technologies and the associated terminology that will be referred to throughout this thesis. This Chapter does not provide a complete introduction to all aspects of optical data storage. For such introductions, the reader is referred to literature on optical data storage [1-8].
2.1 Optical disc drive architecture
2.1.1 Optical discsConsumer optical discs such as Compact Disc (CD), Digital Versatile Disc (DVD) and Blu-ray Disc (BD) are made of polycarbonate with a total thickness of approximately 1.2 mm. Nearly all disc formats have an outer diameter of 12 cm and a centre hole with a diameter of 15 mm, see Fig. 2.1. Typically, the discs have one clear side from which the data-layer is read-out while the other side is covered with a non-transparent label. The CD standards allow only a single data-layer in the disc. The DVD and BD standards also allow multiple data layers in a single disc. Most DVD-Video and BD-Video discs are dual-layer discs that are read-out from one side. The two data-layers in such disc are separated by a thin transparent spacer-layer and one of the data-layers is semi-transparent. Digital data is stored on optical discs in a physical form that depends on the disc type; read-only memory (ROM) discs, write-once-read-many (WORM, often referred to as Recordable or -R) discs, and rewriteable (-RW) discs. On ROM discs the data is stored on the disc in the form of pits that follow a virtual spiral that
centre hole data area inner radius (24 mm)
data area outer radius (58 mm) data spiral track
disc outer radius (60 mm) centre hole data area inner radius (24 mm)
data area outer radius (58 mm) data spiral track
disc outer radius (60 mm)
usually starts at the inner diameter of the disc’s data area, at a radius of approximately 24 mm, and ends at the outer radius of the disc’s data area at a radius of approximately 58 mm. The length of the CD spiral is approximately 5.5 km, with DVD the spiral length is approximately 12 km and with BD 27 km. On recordable and rewriteable discs, the data is stored on a continuous spiral. This spiral is formed by a shallow continuous depression known as a pregroove. This pregroove is used by the recording system as guidance for the positioning of the data on the disc. A slight transverse wobble in the pregroove is used for timing of the recording process. The radius of the spiral increases constantly with the tangential disc position resulting in a dense periodic structure in the radial direction of the disc. Individual periods of this radial structure are referred to as tracks, although all tracks together form a single spiral. The distance between the tracks is known as the track-pitch. Data is stored on the disc in the form of physical structures that modulate the reflection of light. These structures are typically elongated and oriented parallel to the spiral direction. Their width is limited to a fraction of the track-pitch. The data stored on the disc is coded in the form of discrete lengths of these structures and their intermediate distances, known as run-lengths. Optical discs are manufactured by injection moulding of polycarbonate in a mould. The moulds used for manufacturing ROM discs already contain data in the form of pit structure. The side of the mould that contains this structure is replicated from an original master disc, which is made with a photolithographic process known as laser cutting or mastering [1]. During disc manufacturing, the pit structure in the mould is replicated into the polycarbonate and then covered by a thin metal mirror, see Fig. 2.2a. The colloquial term pits is used to describe the ROM disc structures, however often it would be more accurate to speak of bumps rather than pits. Most recordable discs are based on a dye material that absorbs part of the laser light of the recorder. Data is recorded on the disc by a thermal process in which high-power laser pulses change the chemical and physical structure of the dye layer and the pregroove, see Fig. 2.2b. This process cannot be reversed and therefore one can only record once on a disc track. Recently, a new type of inorganic and more environmentally friendly recordable disc was developed based on a CuSi bi-layer [9,10]. The recording process is based on thermal inter-diffusion of the two layers resulting in recorded marks formed of a CuSi alloy. This process also cannot be reversed. Nearly all rewritable discs are based on so-called phase-change materials. The data-layer in these discs is made of a metal alloy that can be switched from a crystalline phase to an amorphous phase and back using a
(a) (b) (c)
thermal process. An amorphous mark is recorded in a crystalline background with a short high-power laser pulse that melts the phase-change material. When the cooling rate of the disc structure is sufficiently fast, an amorphous mark results, due to quenching of the molten state, after the laser is switched to low-power. Quenching occurs when the time required to cool down to below the material’s glass-transition temperature is shorter than the crystallisation time of the metal alloy. A crystalline mark is recorded by a longer and lower-power laser pulse that heats the phase-change material above the glass-transition temperature without melting it. If the temperature of the material stays above the glass-transition temperature longer than the crystallisation time, the atoms have time to rearrange in a crystalline structure and a crystalline mark is formed. The process of recording data directly over older data is known as direct over-write (DOW). Fig. 2.2c shows an image of data recorded in a phase-change material. Besides the phase-change mechanism a more complex method that allows rewriteability exists that is based on thermal magneto-optical (MO) recording. The MiniDisc system is an example of an MO system [4]. MO recording for consumer applications has now been entirely replaced by phase-change recording, or in the case of MiniDisc by recording on flash-memory in MP3 players. Nevertheless, the MO recording mechanism is further explained in the next Chapter because its physics allows recording at data densities that are higher than for phase-change or dye recording.
2.1.2 Mechanics and light-path
the order of a micrometer or less, a focus servo system with a lens actuator are used to actively follow the data-layer of the optical disc. Such actuator is based on a voice coil motor that can accelerate a lensholder that is mounted with flexible wire springs to the OPU. An example of such an actuator is depicted in Fig. 3.2b in Chapter 3. The light reflected from the disc has interacted with the data structure on the disc. The optical power that is reflected back into the aperture of the lens is thus modulated according to the data as a result of the reflectivity differences of the two states of the data-layer. After reflection on the disc, the light in the pupil of the lens is still substantially circularly polarised. After passing back through the quarter-wave plate, the light becomes linearly polarised with its direction of polarisation perpendicular to that of the laser. As a result, the PBS reflects the light towards a quadrant photodetector. An astigmatic so-called servo-lens focuses the light on this quadrant detector. The astigmatism allows the generating of a focus error signal (FES), which is used in the active focus servo as described in Section 2.1.3. The quadrant detector is large relative to the light distribution and thus captures essentially all reflected light captured by the aperture of the objective lens. The sum of the four signals from the quadrant detector gives the data read-out signal, which is also known as the central aperture (CA) signal. Two additional signals are derived from differences between signals from the four quadrants. The first is the focus error signal (FES), the second is a tracking error signal (TES) which is used in a tracking servo system that drives the actuator and the sledge in the radial direction to keep the focused spot on the centre of the track.
2.1.3 Focus and tracking error signals
Several methods for generating a focus error signal (FES) exist. A simple method is based on the detection of the shift of the astigmatic focal lines [3] that are caused by the astigmatic servo lens. This principle is illustrated in Fig. 2.4. The resulting FES S-curve has a steep linear slope with a zero crossing where the objective lens focuses free of defocus on the data-layer. The linear part of the S-curve is thus proportional to the defocus distance of the data-layer in the disc. The focus servo controller drives the actuator such that the residual FES is minimised near the zero crossing. The curve has a
(a) (b)
disc tray loader disc clamp
spindle motor sledge
optical pickup unit
disc tray loader disc clamp
spindle motor sledge
optical pickup unit
disc objective lens laser diode collimator lens photo detectors PBS λ/4 folding mirror servo lens disc objective lens laser diode collimator lens photo detectors PBS λ/4 folding mirror servo lens
maximum on one side and a minimum on the other side at a few micrometers defocus distance. At even larger defocus distance the signal decreases again towards zero. The FES is normalised to the sum signal from the detector and therefore insensitive to changes of the laser output power.
A basic tracking error signal (TES) detection method is based on radial differential or push-pull detection. The principle is illustrated in Fig. 2.5. The dense periodic structure of tracks gives rise to discrete diffraction orders in the radial direction of the disc, see also Fig. 2.8a. These diffraction orders mutually interfere where they overlap. With a typical track density, only the 0th and –1st and +1st diffraction orders are captured by the
aperture of the objective lens, resulting in a distinct interference pattern. The irradiance distribution that results from the interference between the diffraction orders depends on their relative phase, which is determined by the position of the focused spot relative to the track centre. When the light is focused off-centre, one overlap area will interfere constructively and the other destructively, resulting in an unbalance in the reflected irradiance distribution in the pupil. When this irradiance distribution is imaged on a quadrant detector, a TES is easily derived from the normalised difference of two sides
Fig. 2.4 Illustration of the principle of detecting the focus error signal (FES) based on the astigmatic focus method.
A1 A2 A3 A4 A1 A2 A3 A4 TES r TES r
of the detector. Near the zero-crossings of the TES, the signal is nearly linear. The tracking servo controller uses this part of the TES. The controller drives the actuator, while in focus, in the radial direction such that the residual TES is minimised. The TES is normalised to the sum signal from the detector and therefore also insensitive to the laser output power.
2.1.4 Data path, coding and bit detection
The data path for digital data storage on an optical disc is schematically depicted in Fig. 2.6. A stream of digital data, known as the user bitstream, is provided by a source. The user bits are scrambled, interleaved and augmented with redundant data according to a Reed-Solomon error correction code (ECC) [1,3]. The ECC increases the amount of data that is to be stored on the disc but it provides protection against errors that are made during data recording and optical read-out. The ratio of the number of user bits to the number of ECC encoded bits is known as the ECC rate. Next, the digital data is binary coded with a channel modulation code that optimises the bitstream for the frequency response of the optical read-out. The channel modulation codes for CD, DVD and BD are known as Eight-to-Fourteen modulation (EFM), EFM+ and 1-7PP, respectively, for reasons that will be discussed shortly. The coding is based on a look-up table in which a data byte is mapped onto a channel modulation codeword. The resulting bits are known as channel bits and are coded to the disc by non-return to zero, inverted (NRZI) signalling. NRZI codes each logical 1 in the original bitstream as a binary transition from 1 to 0, or vice versa, while a logical 0 in the bitstream is coded as no transition. The resulting bitstream written on the disc is therefore a sequence of discrete run-lengths.
During read-out, a bit clock is recovered from transitions in the read-out signal. This clock is used in the bit detection and its recovery imposes a maximum and a minimum to the run-length distribution. Both very long and very short run-lengths would decrease the clock recovery accuracy, which would increase the chance of bit detection errors. Long run-lengths decrease the accuracy due to the long time to the next read-out signal transition to which the clock synchronises. Very short run-lengths also decrease the clock recovery accuracy since they are less accurately resolved by the optical system. The channel modulation code is therefore designed with run-length constraints. Such codes are known as run-length-limited (RLL) codes and are characterised by two parameters known as the d and k constraints. The d constraint forbids bit sequences with less than d consecutive logical zeroes between two logical ones. The k constraint
Source (data in) Sink (data out) Error protection Error correction Channel coding Channel decoding Laser pulse modulation Channel signal proc. Physical channel Source (data in) Sink (data out) Error protection Error correction Channel coding Channel decoding Laser pulse modulation Channel signal proc. Physical channel
forbids bit sequences with more than k consecutive logical zeroes between two logical ones. The NRZI signalling results in a run-length distribution with a minimum of d+1 channel bits and a maximum of k+1 channel bits. The (d,k) constraint of EFM and EFM+ are (2,10) and of 1-7PP the constraints are (1,7). Due to the (d,k) constraint more channel bits are required for coding a given number of user bits. With EFM, for example, eight data bits are coded into fourteen channel bits. Three merging bits are augmented to the EFM codeword to prevent (d,k) violations in the cascading of codewords. The merging bits are further chosen to lower the spectral content in the frequency band of the servo systems and even make the code DC-free. This is done to reduce crosstalk from the read-out signal to the focus and tracking servo signals. The ratio of the number of user bits to the number of channel bits is known as the code rate. The channel modulation code thus further increases the number of bits to be stored on the disc. However, it also allows the channel bits to be recorded at a higher density along the spiral since it lowers the spatial frequency of the run-lengths by a factor that is larger than the code rate reciprocal. Ignoring the details of the ECC, the data path from user bits to channel bits on the disc is best illustrated with an example. Suppose the data source transmits 1 Byte of user data, say 10101010. Using the EFM look-up table [11], this user Byte is mapped to the EFM codeword 10010001001001(000), in which (000) are three merging bits. The resulting 17 channel bits are coded to the disc by NRZI signalling as 11100001110001111, or its binary reverse. This bitstream is thus recorded on the disc as a run-length sequence of 3, 4, 3, 3 and 4 channel bits. A run-length of n channel bits is often denoted an In. During the recording of data on an
optical disc, the bitstream is converted to a sequence of short laser pulses. For mastering in photoresist this is usually a simple on–off modulation of the laser beam but with (re-)writeable media a more complex pattern of laser pulses is used. Such laser pulse modulation scheme is known as a write strategy.
2.2 Read-out of optical discs
Laser light that is focused onto the data-layer of a disc interacts with the physical structure in this data-layer. In the case of a ROM disc, the pits act as a phase structure resulting in a reflected diffraction pattern. In an intuitive model, the modulation of the reflected light by the pits can be thought as due to destructive interference between the light that is reflected at the high and low parts of the pit structure. In the case of recordable and rewriteable media, the modulation is due to different reflectivities of the multi-layer stack in the recorder and unrecorded states. For rewriteable phase-change media this results primarily in an amplitude contrast in the data-layer, whereas for most recordable media it is a combination of amplitude and phase contrast. The linear dimensions of the runlenghts and track-pitch are of the order of several optical wavelengths, resulting in strong diffraction of the reflected light. This is illustrated in Fig. 2.8a, which schematically depicts the discrete reflected diffraction orders caused by focusing a coherent beam onto a one-dimensional periodic rectangular structure. In the figure discrete overlapping diffracted cones of orderm= ±0, 1 and 2± are depicted. In fact, an infinite number of orders exists, most of which are of small amplitude and evanescent on the diffraction grating. Evanescent fields are further discussed in Chapter 3. Following the Braat-Hopkins model [1,12], the disc structure can be expanded in a Fourier-series of spatial frequencies. Each spatial frequency may contribute coherently to the detected signal if the associated diffraction cone overlaps at least partly with the aperture of the objective lens. This is illustrated in Fig. 2.8b, which depicts the three discrete reflected diffraction orders caused by focusing a coherent beam of light of wavelengthλ0with a lens of numerical aperture NA n= sinαmonto a one-dimensional
sinusoidal grating of spatial frequency ν =1/ p, with p the grating period. Using basic diffraction grating theory it is not difficult to show that a diffracted cone of order m is found in the pupil plane of the lens, displaced my a distance mRλ0/(p NA)with respect
to the optical axis, in which R is the radius of the entrance pupil of the lens. Therefore, a fraction of them= ±1 diffraction orders is captured by the aperture of the lens only when| | 2ν< NA/λ0, where the absolute value of ν is taken since the disc structure
contributes at both positive and negative values of ν in the Fourier domain.
The optical response to spatial frequency( , )ν ν may be described by the optical x y
transfer function (OTF) [1,13]. Two OTFs are distinguished for two types of signal detection relevant to optical disc read-out. With central aperture (CA) detection a large detector integrates all optical power captured by the aperture of the lens. With push-pull detection, the lens aperture is imaged on a split photodetector and the push-push-pull signal is generated from the difference signal from the two halves of the split detector. This type of detection is used for generating a tracking error signal.
In a classic description of the OTF we assume an aplanatic focusing system with a circular entrance pupil and numerical aperture NA, as depicted in Fig. 2.8b. The entrance pupil is illuminated with a scalar monochromatic time harmonic wave of wavelengthλ and propagation vector 0 k0 with k0=|k0| 2 /= π λ0. We assume pupil
coordinates( , )x y that are normalised on the unit circle at the edge of the pupil. If we ignore the time-dependence of the wave we can describe the illumination field
( , )
( , ) ( , ) ikW x y
E x y =E x y e in the entrance pupil with amplitude distribution ( , )E x y and
wavefront W x y( , ). Moreover, we assume undisturbed transmission between the entrance pupil and focal plane of the focusing system and we assume that the object has a mild phase structure such that the optical power in the 0th diffraction order is
non-zero. Without a formal prove, we deduce from Fig. 2.8b that the OTF for CA detection with this focusing system can be obtained from the integral,
2 2 * 0 0 1 ( , )x y ( x / , y / ) ( , ) CA x y OTF ν ν E x ν λ NA y ν λ NA E x y dxdy + ≤ =
∫∫
− − , (2.1) (a) (b) m=0 m=+1 m=-1 m=+2 m=-2 m=0 m=+1 m=-1 m=+2 m=-2 0 mR p NA λ R m=0 m=+1 m=-1 p αm n sin m NA n= α 0 mR p NA λ R m=0 m=+1 m=-1 p αm n sin m NA n= αwhich describes the autocorrelation of the complex pupil illumination [13]. From the absolute value of the OTF, we obtain a modulation transfer function (MTF) that describes the spatial frequency-dependent modulation attenuation in the detected signal relative to the modulation of the scanned object in the focal region. The MTF is normalised to the absolute value of the OTF at zero spatial frequency. From the argument of the OTF we can derive a phase transfer function (PTF). However, since our detector is sensitive only to optical power and not optical phase, the OTF does not play a significant role.
The OTF and MTF for CA detection become rotation invariant with respect to the origin of the spatial frequency spectrum if we assume, in the absence of wavefront aberrations, a plane wave illumination withE x y( , ) 1= of the circular pupil of the lens. Eq.(2.1) is then proportional to the overlap area between 0th and ±1st diffraction orders
in the pupil of Fig. 2.8b. Hence we are able to find for theMTFCA the well known
solution [1] 2 2arccos 2 1 | | ( ) 0 | | c CA c c c c MTF ν π νν πνν νν ν ν ν ν ⎧ ⎛ ⎞ ⎛ ⎞ ⎪⎪ ⎜⎜ ⎟⎟ ⎜⎜ ⎟⎟ ⎨ ⎝ ⎠ ⎝ ⎠ ⎪ ⎪⎩ − − ≤ = > , (2.2)
in whichνc=2NA/λ0is the cut-off frequency at the end of a low-pass filter
characteristic of the MTF. Based on geometrical arguments it can be shown that the MTF for push-pull detection is given by [1]
( ) ( ) (2 )
PP CA CA
MTF ν =MTF ν −MTF ν . (2.3)
In Fig. 2.9 , the MTF for both CA and PP detection are shown with a plane wave illumination of the pupil, in the absence of wave front aberrations. Also shown are the
MTFCA MTFPP /4 λ /2 λ MTFCA MTFPP /4 λ /2 λ
Fig. 2.9 MTF for central aperture detection (MTFCA) and push-pull detection (MTFPP) for an
aberration-free optical system (black lines). Also shown is the deterioration of the MTFCA for
systems withλ0/ 4andλ0/ 2(peak-to-valley) of spherical aberration at optimal defocus (grey
CA MTFs for systems withλ0/ 4andλ0/ 2peak-to-valley of spherical aberration, at
optimal defocus. The MTF of the system with λ0/ 4(peak-to-valley) of spherical
aberration is just slightly worse than what is referred to as a ‘just-diffraction limited’ optical system. Optical disc systems typically have less thanλ0/ 6(peak-to-valley) of
aberrations.
2.3 Key parameters of consumer optical disc formats
From the previous Section, it is evident that the data storage density and therefore also the storage capacity on an optical disc, is limited by the resolution of the optical pickup unit. It was shown that the modulation transfer function (MTF) has a cut-off at spatial frequency 2NA/λ . Therefore, in order to increase the data storage density with 0
successive optical disc generations, the wavelength was reduced and the numerical aperture was increased. These steps were enabled by progress in semiconductor laser technology and lens manufacturing technology. Figure 2.10 shows scanning electron microscopy images of the pit patterns of the ROM discs of the three generations of consumer optical discs that dominate the market for optical discs; CD, DVD and BD. HD-DVD is a competitor format to Blu-ray Disc and is based on the same blue-violet wavelength as BD. HD-DVD, however, has a smaller data density than BD due to a lower NA. Since BD has the largest data density, we will use the BD parameters as a benchmark for the near-field optical recording system that is described in this thesis. Table 2.1 lists some of the key-parameters of the CD, DVD and BD optical disc systems and their single-data-layer discs. The Table shows that the wavelength decreased from infra-red for CD, to visible red for DVD and finally to blue-violet for BD. Moreover, the NA was increased from a low NA value of 0.45 for CD to a value of 0.85 for BD. Thus the NA of BD is just below the upper limit of unity for lenses in air. The Table further shows that the thickness of the transparent layer through which the discs are read-out has been reduced. DVDs are manufactured from two 0.6 mm thick substrates that are bonded together with the data layer in the middle. BDs are based on a 1.1 mm thick substrate onto which a 0.1 mm thick transparent scratch-resistant cover-layer is applied through which the disc is read-out. The reason for reducing the thickness is that wave front aberrations caused by disc tilt and disc thickness errors scale with large powers of the NA. For example, in lowest order aberration theory,
1 μm
1 μm 1 μm1 μm 1 μm1 μm
coma is proportional to h/λ α0 NA3and spherical aberration is proportional to 4
0
/
h λ NA , with α the disc tilt angle and h the thickness. For high NA optics these
aberrations are proportional to even larger powers of the NA [14-16]. With increasing NA, the thickness of the transparent layer was therefore reduced in order to preserve practical disc tilt and thickness tolerances and to obtain practical objective lens designs. The Table further indicates that, besides the directNA/λ0gain, the density gain from CD to DVD and BD was also achieved by shifting the fundamental spatial frequency of the shortest run-length towards the MTF cut-off, from a value of 0.97NA/λ for CD, to 0
1.36 NA/λ for DVD and finally 1.600 NA/λ for BD. The latter was achieved by 0
reducing the d constraint of the RLL code tod= while the channel bitlength was 1 reduced by a factor smaller than theNA/λ gain from DVD to BD. Evidently, the result 0
of this shift is a reduced modulation of the read-out signal from the smallest run-lengths due to the fall-off of the MTF, see Fig. 2.9. The single-data-layer data storage capacities are listed as 650 MByte for CD, 4.7 GByte for DVD and 25 GByte for BD. For CD this is an approximate number since the CD standard allows a relatively large range for the channel bitlength. For BD three capacities have been defined, 23, 25 and 27 GB. Finally, the Table lists the channel and user bit transfer rate at the constant linear velocity (CLV) speeds for the basic ROM application; audio playback for CD, standard definition video playback for DVD and high-definition video playback for BD. Also listed is the maximum disc rotation frequency required to achieve the data transfer rate at the disc’s inner radius.
Table 2.1 Parameters of the consumer optical disc formats CD, DVD and BD. The storage capacity increases with each generation by reducing the optical wavelength λ0 , by increasing the numerical aperture NA and also by increasing
the density relative to NA/λ0 .
CD DVD BD
Optical wavelength (λ0) [nm] 785 650 405
Numerical Aperture (NA) - 0.45 0.60 0.85
Focused spot FWHM [nm] 1040 557 245
Transparent layer thickness [mm] 1.2 0.6 0.1
RLL code EFM EFM+ 1-7PP
Code constraints (d,k) - (2,10) (2,10) (1,7)
Code rate - 1/2 1/2 2/3
Channel bit length [nm] 300 133 74.5
ECC rate - 0.850 0.850 0.817
User bit length [nm] 0.706 0.314 0.137
Shortest run length freq. [NA/λ0] 0.97 1.36 1.60
Track-pitch [nm] 1600 740 320
User bit density [bits/μm2] 0.89 4.30 22.85
Data Storage Capacity [GByte] 0.650 4.7 25 CLV Channel bit rate [Mbits/s] 4.32 26.16 66.09 CLV User bit rate [Mbits/s] 1.41 11.08 36.0
Chapter 3
Near-field optical data storage with a solid
immersion lens and active air gap control
This Chapter introduces the key-technologies of the experimental near-field optical disc system that is discussed in this thesis. This system was developed as a technology-option for a successor to Blu-ray Disc (BD). The minimum data storage capacity target of such successor was 150 GB per disc. Applications were foreseen e.g. in the areas of future home servers that may store TeraBytes of high-definition video content and for archiving of personal multi-media content from future home-PC platforms. The first three generations of consumer optical disc systems CD, DVD and BD were enabled by semiconductor laser technology with ever-shorter wavelengthλ . Also, higher numerical aperture (NA) objective lenses were used with the successive steps from CD to DVD and BD resulting in an increasing NA/λ ratio for the three generations. This evolutionary path ended with BD. With NA=0.85, BD is already close to the upper limit of unity, so there is not much to gain with conventional lenses. Moreover, cost effective mass-produced semiconductor lasers with significantly shorter wavelength than the blue-violet 405 nm wavelength of BD are not expected in the near future. Besides, (deep) ultra-violet lasers may not be attractive at all due to the limited choice of transparent materials available at these wavelengths. Therefore, over the past decade, a variety of technologies was developed that increase the data storage density beyond that of BD while using the same wavelength. The most successful of these fourth-generation technologies are briefly summarised in Section 3.3. Near-field optical data storage with a solid immersion lens is one of these technologies. Section 3.1 introduces the basics of near-field optical data storage and the solid immersion lens (SIL). Section 3.1 also introduces the slider and actuator technologies for maintaining a constant sub-wavelength air gap between a SIL and an optical disc. Such small air gap is required to achieve the full advantage of a SIL. Section 3.1 further introduces two near-field optical disc system concepts. Section 3.2 discusses the key-parameters of potential near-field optical disc systems and Section 3.3 gives the overview of alternative fourth-generation optical disc technologies.
3.1 Principles of near-field optical data storage
wave guiding or total internal reflection. The existence of evanescent waves near an abrupt transition is implied by requirements of electric field continuity imposed by Maxwell’s equations, as will be shown in the next Chapter. Due to their evanescent decay, evanescent waves are located in a small part of the field in a region that stretches to approximately half a wavelength from the medium transition. The part of the field in which the amplitudes of the evanescent waves are significant is referred to as the near-field. The region where evanescent waves are insignificant is referred to as the far-field. Hence, the far-field consists exclusively of propagating waves.
3.1.1 Basic principles of the solid immersion lens
The numerical aperture (NA) of a lens is defined asNA n= sinαmwith n the refractive
index of the medium in which the lens is focusing andα the angle of the focused m
marginal ray with respect to the optical axis, see Fig. 3.1a. Hence, with the refractive index of air essentially equal to unity, the NA of a focusing lens in air cannot exceed unity. In optical disc systems, the objective lens focuses inside the optical disc with refractive indexnd, see Fig. 3.1b. Refraction at the flat disc surface reduces the angle
of the marginal ray according to Snell’s law and thus the NA of the focusing lens also cannot exceed unity. The Blu-ray Disc system with NA=0.85 has thus approached this upper limit closely. In principle one can achieve an NA greater than unity by filling the space between the objective lens and the optical disc with an immersion liquid, similar to liquid immersion microscopy. The refractive index of the immersion liquid can increase the NA to values greater than unity, resulting in increased optical resolution. However, the use of a liquid between the objective lens and a rotating optical disc does not appear to be practical for a consumer optical disc system, although it has been used for optical disc mastering and in liquid immersion lithography [17-19]. An alternative to the liquid immersion lens is the solid immersion lens (SIL). A SIL is typically a truncated spherical lens of a high refractive indexns. By focusing inside such high
refractive index medium the NA can also exceed unity. As discussed in detail in Chapter 8, two types of truncated spherical lenses allow aberration-free focusing of a beam inside the lens. The first type is a hemi-spherical SIL placed with its centre of curvature at the focal point of a focusing lens, see Fig. 3.1c. The hemispherical SIL does not refract the focused light and thusNA n= ssinαmwhich can be greater than
unity: 0≤NA n≤ s. The second SIL type is a super-hemispherical SIL that refracts the
light towards the optical axis, see Fig. 3.1d. In Chapter 8 it is shown that such a lens can be aplanatic and that this type of SIL increases the NA of the focusing lens by a factorn2shence NA n= s2sinαmwith0≤NA n≤ s. A SIL thus increases the optical
resolution in the focal plane of the lens. However, when a pencil of rays is focused at the flat surface of an NA>1 SIL, a portion of these rays is focused at angles that exceed the critical angleθ at which c sinθc =1 with respect to the surface normal, see Fig.
3.1c. Only rays that are focused at angles smaller thanθ are transmitted through the c
interface into air. The rays that are focused at angles larger thanθ in the SIL are c
in more detail in the Chapter 4. This principle is referred to as frustrated total internal reflection (FTIR) or evanescent coupling. To achieve the full optical resolution advantage of a SIL with NA>1, the air gap between the SIL and the optical disc must thus be less than a fraction of the wavelength.
3.1.2 Passive air-bearing sliders versus active air gap control with an actuator In the second half of the 1990s, two American start-up companies, TeraStor Inc. and Quinta Corp., drew attention with their concepts of an air-bearing slider for maintaining a sub-wavelength air gap between a SIL and an optical disc. Sliders were developed for use in hard-disk drives but these companies tried to apply them also in prototype optical disc drives. The established optical disc industry quickly adopted their approach in research projects. An example of a slider and focusing system that we
(a) (b) αm 0sin m 1 NA n= α ≤ 0 n αm 0sin m 1 NA n= α ≤ 0 n αm 0sin m 1 NA n= α ≤ 0 n αm 0sin m dsin m 1 NA n= α =n α′ ≤ m α′ nd 0 n αm 0sin m dsin m 1 NA n= α =n α′ ≤ m α′ nd 0 n (c) (d) ns sin 1 s m NA n= α > SIL θ αm c sin 1 s c n θ = ns sin 1 s m NA n= α > SIL θ αm c sin 1 s c n θ = ns αm SIL 2sin 1 s m NA n= α > m α′ ns αm SIL 2sin 1 s m NA n= α > ns αm SIL 2sin 1 s m NA n= α > m α′
developed at Philips Research for magneto-optical (MO) data storage [22] is shown in Fig. 3.2a. The slider was mounted on a flexible metal suspension, which was aligned with an objective lens in a voice coil focusing motor. The glass slider had an etched air bearing profile that was designed to provide lift and bearing stiffness at a fly height of approximately 1 μm above a disc. This particular slider was flying at too large air gap height to work effectively with a SIL but very similar glass sliders were also developed for flying at a sub-wavelength air gap height. By 2001 the industry had mostly abandoned this approach since it was considered to be lacking the robustness required for a consumer product. This was mainly due to difficulties with the required flatness and roughness of the plastic optical discs and due to difficulties with contamination of the air-bearing surface. In our MO research, slider technology was abandoned in favour of a conventional actuator and focus servo system that maintained a 10 μm air gap between a polycarbonate disc and a magnetic field modulation coil integrated in the objective lens. This turned out to be a surprisingly robust approach. It was partly due to this work, and, more importantly, to results reported from 2001 onwards by the field research team of Sony Corporation [23-27], that we came to appreciate that near-field optical recording with a SIL in an actuator might actually be practical. Sony’s near-field optical disc system was based on an air gap servo system to maintain a constant sub-wavelength air gap between a SIL and an optical disc. Instead of a conventional focus error signal, the system was based on a gap error signal (GES) that is easily generated and based on the evanescent coupling between an NA>1 SIL and the disc. The GES is further discussed in Chapter 7. In 2000 we had studied the GES principle and contemplated its use for air gap control, but we had not tested it in practice [28]. The positive tests by the Sony team of their active air gap servo system were, therefore, a breakthrough for near-field optical data storage with a SIL. Consequently, in 2003 we started a new project on near-field optical data storage at Philips Research, which resulted in the work discussed in this thesis. Fig. 3.2b shows one of our first actuators equipped with an NA=1.9 SIL. Most of the lens is mounted inside a lens holder and only a very small part of the SIL can be seen to protrude from the lens holder.
(a) (b) glass slider suspension arm objective lens focus actuator VCM glass slider suspension arm objective lens focus actuator VCM wire springs lens holder solid immersion lens
moving body wire springs lens holder solid immersion lens
moving body
3.1.3 First-surface disc systems versus cover-layer protected disc systems
A key-strength of optical data storage is the inherent robustness of the optical discs. The data-layer is read-out through the substrate (CD and DVD) or through a transparent cover-layer (BD) and therefore the data is protected against small scratches, fingerprints and other surface contaminants. This robustness allows the discs to be stored and sold outside the optical disc drive. A cover-layer also functions as a thermally insulating layer. Without a cover-layer the top surface of the optical disc is heated up to several hundred degrees Celsius during recording. As we reported in [22] this may result in instabilities in the optical read-out signals due to evaporation and burning of surface contaminants. Thermal simulations based on a model for optical media [29,30] showed that a separation of the data storage layer and the air by just 1 μm of polymer cover layer already reduced the temperature at the interface to air to ambient temperature. It is preferable that a near-field optical disc system has the same robustness as conventional optical discs by also using discs that are protected by a cover-layer. Prior to the research described in this thesis it was not evident that this would be feasible in practice. In fact, all earlier published experimental results with near-field optical data storage were based on so-called first-surface optical discs in which the data-layer is situated on the surface of the optical disc.
In this research we investigated two near-field optical disc systems with a SIL. The first system used first–surface discs with a super-hemispherical NA=1.9 SIL, see schematic in Fig. 3.3a. At an early stage, this system was similar to the one reported on by Sony [25-27]. Our second system used a hemispherical SIL with a more moderate NA=1.45 and discs that were protected by a cover-layer. This system served as a first proof-of-principle for near-field optical data storage with discs that are protected by a polymer cover-layer. The experiments with this system were part of an investigation into the feasibility of a cover-layer-protected, dual-layer disc system, see schematic in Fig. 3.3b. The advantage of the super-hemispherical SIL system with first-surface discs is that it ultimately allows the largest possible NA and storage density on a single data-layer. A disadvantage of an extremely high NA is that it cannot focus through a
(a) (b)
SIL ∼λair gap0/20
optical disc substrate first-surface
data layer
SIL ∼λair gap0/20
optical disc substrate first-surface data layer SIL ∼λ0/10 air gap optical disc substrate cover layer data layer 1 spacer layer data layer 2 SIL ∼λ0/10 air gap optical disc substrate cover layer
data layer 1 spacer layer data layer 2
protective polymer cover-layer. To allow all rays to propagate through the cover-layer, we need for its refractive indexnc >NA. Hence the NA=1.9 system would require a
cover-layer withnc ≈2.0and such polymers do not exist. All transparent materials with a refractive index greater than 2.0 are inorganic and can only be sputter-deposited or evaporated as a thin layer. Due to their brittleness on plastic disc substrates, their built-in stress and long deposition times, the maximum layer-thickness of these built-inorganic materials is of the order of 100 nm. Such materials thus cannot be used as a cover-layer of a few micrometers thickness and neither as a spacer-layer for multilayer discs. This implies that a system with a super-hemispherical SIL with NA≈2.0will almost certainly consist of a single data-layer without a protective cover-layer.
Homogenous polymer materials that are suitable as a cover-layer are now becoming available with a refractive index as high as 1.7 at 405 nm wavelength [31]. These materials can be spincoated as layers of several micrometers thickness and are cured with UV radiation. A system with a hemispherical SIL of NA=1.6 thus seems feasible with cover-layer protected optical discs. Moreover, it should also be possible to use the same polymer material as a spacer-layer in a multilayer disc. A difficulty with a near-field dual-layer optical disc system is that such system requires an optical device that allows the lens to change focus position from a first data-layer to a second data-layer without a change of air gap and without significant extra aberrations. Such a device may for example be an electrically switchable liquid crystal wavefront compensator. In [32] we showed a concept optical design for such a system with NA=1.5. In [33] an improved lens design was discussed with NA=1.6 and it was shown that it might even be possible to have four instead of two data-layers in a near-field optical disc. It may thus be expected that a dual-layer NA=1.6 system is feasible that achieves a larger storage capacity per disc than a single-layer system with NA≈2.0. Due to the cover-layer, it may also be expected that such NA=1.6 system will have a better data protection than a first-surface disc system.
3.2 Parameters of near-field optical disc systems
recovery of the bit clock. Both codes have identical code rates of 2/3 and for all systems in the Table an error correction code (ECC) rate of 0.817 was assumed. Parameters are listed for conventional bit detection with a slicer and for more advanced Viterbi bit detection. In the latter case, we used the BD parameters reported by Padyi in our extrapolations [35]. Assuming conventional bit detection, the shortest run-length frequency is 1.60 [NA/λ0], with Viterbi bit detection this frequency is 2.25 [NA/λ0],
beyond the cutoff of the MTF.
The NA=1.45 and 1.6 systems can be based on a hemispherical glass SIL and can be combined with protective polymer cover-layer discs that may contain more than one data-layer. The NA=1.9 system has to be based on a super-hemispherical SIL and uses single-layer first-surface discs. We believe that this thesis provides sufficient evidence to assume that an NA=1.6 dual-layer system is feasible with a data storage capacity of approximately two times 125 GB or a quarter-Terabyte in total. Depending on the developments with quadruple-layered BD, one might even consider NA=1.6 systems with four layers resulting in capacities of approximately half a TeraByte. There is sufficient theoretical evidence to show that optics at least allows such a system [33]. The Table further shows that with an NA=1.9 system it should be possible to achieve a data storage capacity between 125 and 175 GB on a CD-size disc. Therefore, as already mentioned in the previous Section, an NA=1.6 dual-layer optical disc system will likely beat the higher-NA single-layer first-surface optical disc systems in terms of data storage capacity per disc.
Table 3.1 Parameters of the BD system and three near-field (NF) optical disc systems of different numerical aperture (NA), all systems are assumed to be based on an optical wavelength λ0 = 405 nm.
BD NF(1.45) NF(1.6) NF(1.9)
Numerical Aperture (NA) - 0.85 1.45 1.6 1.9
Focused spot FWHM [nm] 245 168 152 128
Cover-layer thickness [μm] 100 3 3 -
Multilayer option Yes Yes Yes No
RLL code 1-7PP 1102PC 1102PC 1102PC Code constraints (d,k) - (1,7) (1,10) (1,10) (1,10) Channel bitlength [nm] 74.5 43.7 39.6 33.3 Channel bitlength (Viterbi) [nm] 53.0 31.1 28.2 23.7 User bitlength [nm] 137 80.2 72.7 61.2 User bitlength (Viterbi) [nm] 97.3 57.0 51.7 43.5
Track-pitch [nm] 320 188 170 143
User bit density [bits/μm2] 22.8 66.5 81.0 114.2
User bit density (Viterbi) [bits/μm2] 32.1 93.5 113.8 160.5
3.3 Overview of alternative fourth-generation technologies
In this Section the basic principles are summarised of some of the most successful fourth-generation technologies. A large variety of ideas emerged over the years and this summary is therefore by no means complete. Certainly, physics allows optical data storage at much larger densities than that of BD or even that discussed in this thesis. However, a fourth-generation technology should satisfy a number of criteria in order to become attractive to the optical disc industry. It should enable an increase of data storage density compared to BD with at least a factor of four, while at the same time allowing an increase of the data transfer rate by approximately the same factor. A fourth-generation technology must have tolerances that allow cost-effective mass manufacturing. Moreover, the technology should preferably be backwards compatible with BD, DVD and CD. Finally, the optical data storage industry cannot justify large investments in new technologies and therefore a fourth-generation technology should not deviate much from that presently used in BD. The basic principles of the most dominant technologies that were considered by the industry can be divided into five categories. These categories are: multilayer media, advanced bit-detection, super-resolution media, holographic data storage and near-field optical data storage. Each category is very briefly summarised below.
Multilayer media contain more than one data storage layer. Dual-layer DVD and BD formats already exist that nearly double their single-layer data storage capacity. However, it has been demonstrated that this approach can be extended to as many as six data-layers, which enables ROM and recordable discs with a storage capacity of at least 150 GB with modified BD optics [35]. The attractiveness of this approach lies in the fact that these discs can be used with small changes to a BD drive architecture. A major disadvantage is the complexity of the disc manufacturing and the related difficulties with achieving a sufficiently high disc-manufacturing yield to make the discs competitive with dual-layer BD in terms of costs per GigaByte.
Advanced bit-detection reduces the channel bitlength and relies on a form of Viterbi bit detection to correctly detect the bits from a central aperture (CA) signal in which the modulation of the smallest run-lengths is essentially zero. Viterbi bit detection is not based on a slicer to detect the logical ones and zeroes in the CA signal but instead estimates the most likely bitstream from the sampled CA signal. It has been shown that Viterbi bit detection allows a storage capacity of 35GB on a single data-layer of a conventional BD, a gain by a factor 1.4, without loss of system margins [36]. A related approach, known as two-dimensional optical storage (Two-DOS), allows an increase of the data storage density by a factor of 2 compared to BD by applying a two-dimensional channel modulation code, a modified disc structure and a Viterbi bit detection method known as a stripe-wise bit detector [37]. TwoDOS also greatly increases the data transfer rate by using up to 11 focused beams on the disc and an array of photodetectors. A disadvantage of this approach is the increased drive complexity, especially due to the electronics that is used to detect and process all signals simultaneously. The associated increase of the power dissipation in the optical pickup unit adds to the difficulties with this approach.
Super-resolution (SR) media generate a read-out signal even when data is stored at spatial frequencies beyond the cutoff frequency of the MTF of the optical system. Most SR methods are based on magneto-optical (MO) media that enable magnetic domain expansion. The three dominant domain expansion methods are magnetic super-resolution (MSR), Magnetically AMplifying Magneto-Optical System (MAMMOS) [38] and Domain Wall Displacement Detection (DWDD) [39]. In these media data is stored in an MO layer as run-lengths with a magnetisation that is perpendicular to the MO layer. The run-lengths are, therefore, said to be perpendicularly magnetised in an ‘up’ or ‘down’ direction. Data is recorded by locally heating the MO layer with a focused, pulsed laser while an external magnetic field is applied. Due to a large magnetic coercivity of the MO layer, data is reliably stored at room temperature. Heating the MO layer with the focused laser reduces the coercivity, which allows a reversal of the magnetisation with the aid of the external magnetic field. During recording this field is modulated according to a channel modulation code. This recording mechanism is known as laser-pulsed magnetic field modulation (LP-MFM). MO media allow LP-MFM recording of magnetic domains that are much smaller than the diameter of the focused spot. Such small magnetic domains are recorded by pulsing the laser and modulating the external magnetic field such that a recorded domain largely overlaps the previous domain of reverse magnetisation. Hence, the smallest run-lengths are crescent shaped with a fundamental spatial frequency that can be larger than the MTF cutoff frequency of the focusing system. Recorded data is read-out using linearly polarised light. The direction of polarisation rotates slightly upon reflection at the MO layer due to the polar magneto-optical Kerr effect of a perpendicularly magnetised MO layer. The read-out polarisation state thus depends on the local up or down magnetisation direction of the MO layer. A read-out signal is generated by detecting the polarisation state of the reflected light in a polarisation-sensitive light path. Super-resolution read-out of MAMMOS and DWDD is based on a rapid expansion of the magnetic domain to approximately the size of the focused spot, followed by a collapse of the expanded domain. During read-out the MO medium is slightly heated and a fine balance of the thermal profile, magnetic fields and media magnetisation drive the domain expansion. An overview of the physics and technology of MO recording is given in [2], an overview of MSR, DWDD and MAMMOS is given in [7]. A description of a MAMMOS system that we developed can be found in [22, 40-43]. MO domain expansion technologies never made it into a successful product due to a lack of industrial support. The disc technology and recording and read-out processes were considered to be too complex and the drive architecture deviated too much from mainstream drive architectures. An alternative to the domain expansion method is known as the super-resolution near-field structure (SuperRENS) [44]. SuperRENS is based on linear and non-linear effects of temperature on the optical constants of the multilayer structure in a SuperRENS disc. The density gains that have been reported with SuperRENS are, however, modest in comparison to the other technologies, especially with random data tests.
recording density with a system that is capable of recording data at a surface-equivalent density of 515 Gbit/in2 at a wavelength of 407 nm [46]. This density translates to 800
bits/μm2 or approximately 34 times the BD single-layer density. An advantage of
holographic data storage is that it allows a roadmap to data storage capacities up to a few terabytes per CD-size disc. A disadvantage is the complexity and cost of the holographic data storage drive. The commercial drives that have been announced by InPhase are roughly the size of a shoebox and it appears difficult to develop such a drive with a bill of materials that is acceptable for a consumer product.
Chapter 4
Theoretical aspects of near-field optical data
storage
This Chapter presents the theory that describes the structure of the electric field in the focal region of focusing systems with arbitrary numerical aperture (NA), including solid immersion lens systems with an NA larger than unity. We will make a comparison between the distribution in a homogeneous focal region of a low-NA lens and that of a high-NA lens. This comparison is based on the classical scalar theory resulting in the Airy formula [13] and the classical vectorial theory of Richards and Wolf [50, 51]. After this, a theoretical analysis is presented that describes the structure of the vectorial field in a focal region that comprises a multilayer structure. This theory adequately describes the field in the focal region of a solid immersion lens with an NA larger than unity that focuses through an air gap into such a multilayer structure. This study is based on theoretical work by Van de Nes et al. [52] and Ichimura et al. [53] and is related to earlier work by Flagello et al. [54, 55] and Török et al. [56-59]. The description of the field structure by Van de Nes et al. is of a general nature. To make an easy comparison in this Chapter with the results by Richards and Wolf for a homogenous focal region and the integrals found by Ichimura et al., we have further worked out the formulae of Van de Nes et al. so that the resulting integrals may be regarded as modified Richards and Wolf integrals.
All theory presented in this Chapter pertains to the focusing of light in media that do not contain grooves, pits or other structures that are present in optical discs. Theory and numerical tools for the rigorous treatment of vector diffraction effects are still under development [20, 21, 60]. The impact of such highly sophisticated tools on the design of near-field optical discs systems has so far been relatively small.
Throughout this Chapter, we have chosen to follow a notation based on propagation vector components in a cylindrical coordinate system in a way that is identical to that used by Van de Nes et al. . In their work, Richards and Wolf use angles in real space and spherical coordinates which leads to less compact formulae.
4.1 Field structure in a homogenous focal region
Consider an infinite-conjugate focusing lens with rotational symmetry with respect to the optical axis. We assume the focusing of the lens to be aplanatic. i.e. the lens obeys Abbe’s sine condition [13] and focuses without of wavefront aberration within the field of our study. It is further assumed that the entrance and exit pupil diameters of the lens are large compared to the wavelength of the illuminating light and that the lens is free of absorption or reflection losses. The medium of the focal region on the image side is assumed to be homogenous, isotropic, nonmagnetic with permeabilityμ μ= 0and
transparent.
We assume that the entrance pupil of the lens is illuminated by a monochromatic time harmonic plane wave with frequency ω and electric field E r( , ) Re{ ( )t = E r e−i tω}in the stationary state, with r a position vector in the Cartesian (x,y,z) coordinate system and field amplitude E with components in this coordinate system E=(E E Ex, y, z),
see Fig. 4.1. The modulus of the propagation vector in vacuum follows from
2 2 2
0 0 0 0
k = k ≡μ ε ω withε the permitivity of vacuum and the wavelength in vacuum 0 0 2 /k0
λ = π . We assume that the system is illuminated by a plane wave propagating along the positive z-axis, such that for its components k0=(k0x,k0y,k0z) we
havek0x =k0y =0. The transition from the disc Ω’ in the entrance pupil to the convex
Fig. 4.1 Illustration of the notation used for the field vectors and coordinates in the entrance pupil plane Ω’, exit pupil surface Ω and the focal region of the studied optical system.
x y
α z
entrance pupil exit pupil
y x focal region ϕr Ω’ Ω f kr kϕ h E U E kr kϕ x y α z
entrance pupil exit pupil
