Simulation Model
3.1 Simulation Description
In the design process of the ODS system, the physical structure of the optical disc, the data format and the design of pick‐up head has been standardized for the compatibility of all kinds of discs, such as the wavelength of the laser diode, the numerical aperture of the objective lens.
Different composing components are used for various focus servo and tracking servo adopted by different companies. In this thesis, the simulation framework is based on the optical pick‐up head provided by ASUSTek computer Inc. The purposes of the optical pick‐up head are to transmit the laser beam to the disc, to focus the laser beam to a diffraction‐limit spot, and to transmit the information on the disc to the detector as the readout or the servo signals. In Fig. 3.1‐1, the configuration of the DVD pick‐up head system
is demonstrated.
PDIC
Cylinder Cube 1
Cube 2
QWP+GT Mirror
Collimator
Objective Lens
Disc
LD
Fig. 3.1‐1 The configuration of the simulated DVD system
The light of wavelength λ = 660 nm, emitted from a laser diode is brought to the quarter wave plate (QWP) combined with a grating (GT) to generate a zero order and ±1 orders light beams and to change the polarization state from linear to circular polarization. The beams are then collimated and focused on the disc by a collimator and an objective lens, accordingly. The recorded data on the disc will makes the distribution of the reflected light vary with the different reflectivities of the marks and land and this variation of the reflected distribution represents the digital data 0 or 1. Reflected from the disc, the light passes a cylindrical lens and is finally detected by the quad photo‐detector. The readout signal can be generated as a result of the sum of
the quadrant signals.
Based on the pick‐up head system from ASUSTek company Inc., the simulation model is determined. But in this thesis only main beam (zero order) is taken into consideration, the ±1 orders light beams of the diffraction grating are omitted. Such way speeds up the simulation time. The components which just use to change the direction of the light or the split the optical paths of incident and reflected light can be omitted as well, such as folding mirror, cube 1, cube 2. Therefore, in this thesis only the laser diode, the collimator, the objective lens, the optical disc, the astigmatic lens, and the photo‐detector are taken into account, as depicted in Fig. 3.1‐2.
Z λ
Laser Diode Collimator Objective Lens Disc
(a)
Z
Disc Objective Lens Collimator Astigmat Detector
(b)
Fig. 3.1‐2 The unfolded optical path of the simulation model: (a) The incident optical path, and (b) the reflected optical path
3.1.1 Readout Model
In the case of readout modeling, only one track is considered and the pre‐groove structure is ignored to obtain the signal from the on‐track marks.
The readout signal created by the superposition of the sequential marks based on the Babinet principle is presented. The method of superposition of the sequential marks in the tangential direction (x‐axis), sketched in Fig. 3.1‐3, consists of two steps: in the first procedure, two isolated marks (with different lengths) are considered. The total field reflected from the recording layer is decomposed into components that consist of the isolated marks and the background reflection. In the subsequent procedure, the readout signal is then formed by superposition of a sequence of isolated marks of different lengths with varied spacing offset. The readout signal, called RF signal or eye‐pattern, created by superposition of the sequential marks is formed by overlapping the signals of isolated marks more than 300 times to exhibit the eyepattern shown on the oscilloscope.
+ +
rM rL
=
nT mT
rM
nT mT
Background Isolated Mark 1 Isolated Mark 2
X Y
Fig. 3.1‐3 Schematic diagram of the superposition of the sequential marks
3.1.2 Parameters
As mentioned in previous section, the simulation model is constructed by the laser diode, the collimator, the objective lens, the optical disc, the astigmatic lens, and the photo‐detector and the main parameters of these optical components are listed in Table 3.1.
Table 3.1 Parameters of the DVD+R/RW System
L D Pa r a m e t e r s Va l u e Wavelength 660 nm Astigmatism Distance 0.001 mm Astigmatism Degree in X / Y direction 9.5 ° / 17°
C o l l i m a t o r Pa r a m e t e r s Va l u e Numerical Aperture 0.1116 Focal Length 17.8 mm O b j ec t i ve L e n s Par a m e te r s Va l u e N u m e r i c a l A p e r t u r e 0 . 6 5 Focal Length 3.05 mm Astigmatic Lens Parameters Val u e N u m e r i c a l A p e r t u r e 0 . 1 2 8 Focal Length 1 2.112 mm Focal Length 2 2.217 mm P h o t o ‐ d e t e c t o r Pa r a m e t e r s Va l u e Length in X & Y Directions 0.058 mm Division Line 0.004 mm D i s c Pa r a m e t e r s Va l u e Type DVD+R/RW Groove Trapezoidal pre‐groove Refractive I nd e x of t he S u b s tra te 1 . 5 8 Reflection Coefficient – Crystalline / Amorphous 0.55 / 0.38 Servomechanism Type Focus Servo Astigmatism Method Tracking Servo Push‐pull Method
3.1.3 Simulation Tool
Numerical calculations for this study was accomplished by use of DIFFRACT™, a FORTRAN‐based diffraction modeling program that was created as a general purpose tool for optical data storage research. The user has a various functions of which different lenses, polarization optics, media, and detectors can be selected. DIFFRACT™ permits the user to choose different light beam distribution as the initial beam, and then it manipulates the propagation of this beam through the optical system by a plane‐wave decomposition approach and Fourier techniques as mentioned in Chapter 2.
The polarization state, intensity and phase of the propagating beam, as well as detector outputs, are available at any position along the optical path.