Optical Readout Channel Analysis Program - ORCAP

The signal from an optical readout channel is simulated using a theoretical approach that has its origins in scanning optical microscopy. Using this approach the field distribution at the focal point of a lens, the Fraunhofer or far-field diffraction pattern, is calculated using scalar diffraction theory. Simply stated, the field distribution at the focal point of a lens is calculated by taking the Fourier transform of the field distribution at the exit pupil of the lens, i.e.

In the optical storage channel the focused field distribution interacts with the disc and then propagates back to the objective lens by the diffraction process, here it is collimated and continues to propagate to the detection arm of the channel.

In the case of the reflectance type arrangement, the reflectance properties of the disc modify the focused field distribution and the resulting change in reflectance is measured as a change of field intensity. In the case of the magneto-optic arrangement, the plane of polarization of the focused field distribution is rotated by the polar Kerr properties of the disc and is measured using a differential detection strategy.

Two modeling approaches have been developed and used successively to predict the readout signal in both reflectance and magneto-optic optical readout channels, these are referred to as the ‘transfer function’ and ‘direct calculation’ approaches. In the direct calculation approach the form of the optical field is calculated through the optical system as the disc is scanned beneath the focused spot. In the transfer function approach the signal from the readout channel is expressed in a form where the spatial frequency properties of the optical channel are distinct from those of the sample, thus enabling the quantitative comparison of the imaging performance of various imaging configurations.

The maximum attainable storage density of the optical channel is governed by its optical cut-off frequency, which is proportional to the wavelength of the incident light and inversely proportional to the lens numerical aperture (NA). Hence, to increase the optical cut-off frequency, and thus data storage density, requires either increasing the NA of the lens or working with a source with a shorter wavelength. The minimum wavelength of solid state lasers is currently limited by the technology available; hence, modern optical storage systems employ an objective lens with a higher NA.

A major disadvantage of the scalar diffraction approach is that it does not predict the effects on the polarization state of the optical field due to propagation through the optical system. The lens introduces a curvature phase factor across the incident plane wave such that it is brought to focus at the focal point of the lens. However, across the aperture of the lens the rays are bent towards the focal point by an amount depending upon the numerical aperture (NA) of the lens and their position of incidence relative to the radius of the lens. The marginal rays, which are incident around the circumference of the lens, are bent most severely by an angle given by asin(NA); this bending being reduced for rays incident closer to the center of the lens. The result of this is that the incident field distribution will no longer maintain its polarization state upon propagation through the lens. This is of little consequence in low NA applications where the angle of rotation is small. However, in large NA systems this problem needs to be addressed if the polarization state of the focused field is to be accurately determined and the imaging of smaller objects is to be more accurately predicted.

The scalar diffraction model has been improved by introducing a pseudo-vector diffraction model to take into account the severe bending of the rays upon propagation through a high NA lens. If a linearly polarized plane wave propagates through a lens, then it can be shown that the resulting field distribution at the focal point of the lens is given by,

which has components of polarization in the x, y and z directions.

The figure illustrates the focused spot distributions calculated using the pseudo-vector diffraction model, for a circular objective lens of NA=0.5 with uniform incident illumination linearly polarized in x (track direction). Also illustrated is the corresponding focused spot distribution calculated using the scalar diffraction model.

The relative intensities of the field distributions illustrated agree closely with results published by other researchers.

the figure below illustrates the step response to a DVDROM bit using both the scalar and vector diffraction based modeling approaches.

It can be clearly seen that the pseudo-vector diffraction model accounts for the high frequency components (small features) that are inaccurately modeled using the scalar diffraction model.

The figure below illustrates a simulated DVD-ROM signal generated using the pseudo-vector diffraction model, the incident uniform is assumed to be Gaussian and linearly polarized in x, wavelength=650nm, NA=0.6.

The model has also been used to model the non-linear signal contributions in high density optical storage systems (such as DVD and more recently Blu-ray), as well as provide input light distributions to a Finite Difference Time Domain (FDTD) model of light interactions with complex media in near-field based storage systems, reference can be found here.

ORCAP Software

The optical readout model has been written specifically for the MATLAB™ graphical user interface (GUI) environment. The optical readout channel can be easily investigated using the routines provided by the model software.

The features of the optical readout model include the following:

• Signal generation for an arbitrary resolution
• Arbitrary illumination characterisitcs:
  • Uniform or Gaussian illumination
  • Arbitrary polarization angle
• Focused spot simulation for any arbitrary aperture pupil
• Investigation of apodization techniques, rectangular and annular shading bands
• Analysis of near-field imaging using a Solid Immersion lens, hemispherical and supersphere
• Propagating field analysis (using movies)
• Generation of arbitrary reflectance, phase or magneto-optic objects
• land/groove object
• arbitrary track width, reflectance and/or Kerr rotation
• arbitrary bit width, length, shape, reflectance and/or Kerr rotation
• arbitrary data patterns (used defined)
• simulation of up to three tracks of data (scan made along the central track)
• Read waveform generation:
  • Both reflectance and magneto-optic objects using scalar diffraction or pseudo-vector diffraction
  • Introducing constant tracking error
  • Multiple signal calculations using arbitrary quadrant photo-detector configurations (single and differential)
  • Across track scanning
• Optical system analysis
• Generation of the optical transfer function, 1D and 2D
• Channel optimization investigations using the optical transfer function
• Investigation of the effects of aberrations, Astigmatism, Coma, Defocus, Spherical and Tilt.
• Full compatibility with the recording and channel simulation models

The software manual is available here. To obtain a copy of the software contact me.