MediaTest - I
Static test system for optical data storage media
Arno Euteneuer, Dominik Fernandez, Thomas Weber
TOPTICA Photonics AG, Fraunhoferstrasse 14, D-82152 Martinsried/Munich, Germany
Phone: +49 89 / 899969 – 0, Fax.: +49 89 / 899969 – 35, Email: thomas.weber@toptica.com

Introduction
Marks on write-once and rewritable optical
data storage media are typically created by athermal process initiated by a focused laser pulse.
While for phase-change (PC) media the local
heating induces a physical change between theamorphous and the crystalline phase, for magnetooptical (MO) media heating above the Curie
temperature allows the reversal of the magnetic
moment. These changes result in a change of theintensity or the polarization state of the reflected

beam. For development or optimization of such
write-once or rewritable layers a system is helpful,that allows to irradiate the media with a pulsed anddiffraction limited laser spot, the pulse power andduration of which can be modified. Fast
photodetectors allow measuring the change ofintensity and/or polarization during and after theheating process initiated by the laser. These time
resolved measurements reflect the dynamics of themedia and are very important for the developmentof recordable media.

CCD Camera
Detector 423nm
Detector 398nm

   Lens

Detectors
Detector 398nm

Analyzer

Prism

Detector 423nm

Polarizing Beam
Splitter

Lens
Beam Combining

Prism

Lens

Microscope

Beam Splitter
Polarizer

Half Wave PlateWhite Light
SourceDichroic Mirror
MirrorLaser 2 (423nm)

Leaky Beam
Splitter

Dichroic Mirror
Objective andFocus Unit
Tilt - Stage 

Read & Write
Laser

X/Y - Stage
Electro Magnet%'&(*),+-.$/0.$/*#!" $

Figure 1: Schematic of the violet laser static test system.

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Tester Setup
The system is based on a regular industrial
microscope (Fig. 1). The standard part consists ofan eyepiece and full color CCD camera for visualinspection of the sample, a white-light source forirradiation of the field of view, polarizer andanalyzer for polarization microscopy. This partallows visual inspection of the written marks and isnecessary for proper alignment of the instrument.
Connected to an extra port of the microscopea laser and detector unit allows to couple additionallaser light into the microscope and to detect thereflected light. In this special type of tester two
independently controllable light sources are used:
One laser acts as writing source, heating thesample with adjustable pulse intensity and pulse
duration (writing laser). The other laser, which
operates continuously at low power, acts as theprobe, detecting the intensity/polarization changes
in the material (reading laser). The advantage ofusing two lasers is that the measurement of thechanges in the material happens independently of
the pulse shape or pulse intensity of the strong
writing laser. Without that feature often problems
with saturation effects or the linear response of thedetectors occur such that measurements during thewrite pulse are hard to obtain.
In order to simultaneously couple the beamsof read and write laser into the microscope, theirwavelengths are slightly different, enabling the useof dichroic mirrors for the beam superposition. The
optical setup of the MediaTest-I is available withlasers and optics sets in different wavelength
regimes: a red wavelengths version e.g. with aDVD-compatible wavelength combination of
632nm/658nm is as well available as a violet
version with 398nm/423nm for next generation
disks (DVR, Blu-Ray).
The beams of the read and write laser
modules are combined with a first dichroic mirror
and are coupled into the microscope beam pathwith a second dichroic mirror, transmitting most of
the visible range. Both lasers can be operated
independently in cw or pulsed mode and include a
RF circuit to allow low noise reading.
The leaky polarizing beam splitter (LPBS)
transmits 80% of the p-light and reflects 100% ofs-light, allowing PC measurements as well as MO
measurements with the same setup. The liquid

crystal retarder (LCR) is set to quarter-wave
operation for PC media. For MO media it allows tointroduce a controlled phase-shift between the pand s-component of the reflected light. This allows
correcting for any undesired ellipticity introducedby the MO sample, the substrate or any othercomponent.
The beams are focused on the sample by thewell-corrected microscope objective (NA typ. 0.6)
and therefore are able to obtain diffraction-limitedperformance. The objective allows measuring on
surface or through different substrate thicknessesby aligning the cover-plate correction collar.
The light backreflected from the sample isreflected by the dichroic mirror inside themicroscope, passes the LCR and is directed to thedetection section by the LPBS. It first passes a
half-wave plate, which for MO media is used tobalance the intensity on the two differential
detectors. A polarizing beam splitter splits the pand s-component of the reflected light. In theindividual p and s detection paths prisms separate
the two wavelengths and direct them to a split
photodetector. For MO the p- and s-detectors are
combined to obtain the differential signal, for PC
only the one set of detectors is basically needed.
The sample is placed on a motorized XYstage underneath the objective. For MO
experiments a strong cylindrical magnet ismounted under the stage. Magnetic fields of up to8 kOe can be obtained.
The complete system is operated from apersonal computer that controls the laser power
and pulse length, provides the readout from a
digital oscilloscope, acquiring and digitizing thedetector signals at high resolution, and controls theXY-table of the sample. The software supports
automated
experiments
andautomaticallygenerates report files, logging all importantparameters as well as camera images and data filenames. The acquired data is saved in ASCII format
that easily can be imported in all kind of thirdparty data-analysis programs.

Laser
The lasers and their drivers support pulses
down to 5 ns duration with rising times of 2 - 3 ns.
A built in pulse generator can generate single

pulses down to 10ns duration. However, with an
external pulse generator the full capability of thelaser drivers, i.e. shorter pulses or pulse trains, canbe exploited.
The high quality optics of the laser modules
with wavefront errors below
λ/20 assuresdiffraction limited performance and highresolution.

Sample StageThe XY sample stage is equipped withencoders and piezo-driven linear-motors enabling
backlash-free positioning with +/- 100nm
resolution. Furthermore the sample stage is tiltable
to allow optimum conditions with perpendicularincidence.

CD-RWFig.3 shows the results of a series ofmeasurements on a commercial CD-RW,
performed with a red version of the MediaTest-I,
consisting of a read laser with 632nm and a write
laser with 658nm. The write pulses in this example
have a width of 200ns and the write power isvaried between 5mW and 12mW while the read
power is always 0.4mW. The curves were obtainedby averaging over 15 individual marks. The changeof reflectivity is clearly seen in the graph andshows a typical step appearance: The crystalline
material is locally heated by the write laser pulseand due to the temperature change shows adifferent, intermediate reflectivity level during thepulse. After the pulse, the material quickly cools
down, changing the reflectivity to a final, lower
level belonging to the amorphous phase. A certain
power level (5-6mW) is needed to induce thechange of reflectivity and the final signal level
clearly depends on the power level. The recorded
read out signal reflects the reflectivity of thesample, integrated over the finite probing area
defined by the read laser’s spot size.

Figure 2:The TOPTICA logo written with a 398nm
laser on a rewritable DVD. The logo consists ofamorphous marks in a crystalline matrix having anominal grid spacing of 500nm.

read out (arb. units)

1.0

0.8

0.4

0.2

0.0

The picture was taken with the integrated
CCD camera. The images can easily be saved instandard file formats and as an option the imaging
optics of the camera allows variable magnificationof the microscope image.

Typical Results
The results given here are examples formeasurements that can be performed with the testsystem.

5mW
6mW
7mW
8mW
9mW
10mW
11mW
12mW

0.6

-0.2

0.0

0.20.40.6

0.8

1.0

1.2

1.4

time (µs)

Figure 3: Varying laser powers on CD-RW
Figure 4 shows a microscope image of apattern of marks resulting from such an automatedexperiment. For each row of marks a different setof parameters was used (varying write powers andpulse durations) and all marks within one row wereused for averaging. The traces shown in Fig. 3
belong to the rows 12 to 5 (counting from top).
The mark spacing is 2 micrometer.
The imagehelps interpreting thetimeresolved transients since it shows that thedifferent final signal levels in Fig. 3 are the result

of different mark sizes rather than of different
reflectivity values of the armorphous phase.

DVD-RW
Figure 6 compares two phase-change
experiments from the amorphous to the crystalline
state on a commercial DVD-RW.

read out (arb. units)

0.3

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.8

1.0

1.2

1.4

time (µs)

1.0read out (arb. units)

0.8

0.6

0.4

0.2

0.0
-0.2

0.0

0.2

0.4

0.6

time (µs)

Figure 6: Erasing experiment on as-grownamorphous material (top) and on amorphousmarks in a previously annealed area (bottom).

1.4
1.2

read out (arb. units)

0.1

0.0

Figure 4: Mark pattern resulting from an
automated experiment.
Figure 5 shows an example experimentstudying the erasing dynamics of amorphous marks
on a CD-RW. The amorphous marks were firstwritten into the crystalline matrix with a 100ns12mW pulse and were subsequently exposed to aweaker erase pulse of 5mW and varying pulse
duration. While a 50ns pulse is not able topermanently change the materials phase back to the
crystalline state, longer pulses are able to at least
partially erase the marks. The slope of thetransients for the longer pulses visualizes the finite‘crystallization speed’ of the material.

0.2

1.0
0.8

50ns
100ns
200ns
300ns
400ns
500ns
600ns

0.6
0.4
0.2
0.0
-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4time (µs)

Figure 5: Mark erasing with varying pulse
durations on a CD-RW.

While both traces were recorded with 0.4mW read power at 632nm and 500ns-5mW write
pulses at 658nm, the upper trace shows a more orless fruitless experiment on as-grown amorphousmaterial and the lower one the successful erasureof a written data mark The different results suggest
an edge-growth dominated crystallization mode.
Figure 7 shows the microscope image of thearea where the second trace was recorded.

0.03

MO signal (s-p)/(s+p)

0.02

0.01

0.00

1 mW
2 mW
3 mW
4 mW
5 mW
6 mW

-0.01

-0.02

-0.03

02

4

68time (µs)

Figure 7: A circular annealed area with two 5x5mark patterns of a write and an erase experiment.

Figure 8: Timeresolved MO recording withvarying write laser powers

A circular area was first annealed with a
defocused exposure of the write laser. Inside thecircle two 5x5 patterns can be seen. While the left
pattern of dark, amorphous marks is the result of awrite experiment (traces not shown), the hardly
recognizable pattern of slightly bright marks is theresult of an erase experiment where first marks
were written and subsequently were erased (seelower trace of Fig. 6).

Before, a prepulse of 8.5mW and a magnetic
field of 1 kOe in the direction opposite to the 400
Oe field applied during the timeresolved
measurements initialized the material at eachindividual position.
Figure 9 shows a differential polarization
microscope image of the resulting pattern of MO
marks.

Typical MO results
As already mentioned above, also magnetooptical media can be investigated withMediaTest-I. For the detection of the Kerr signal
now the complete detector setup of Fig.1 and thestrong electro-magnet have to be used. The
following paragraphs show results of some typicalmeasurements.

Violet MO recording transients
Figure 8 shows some transients recordedwith a set of violet lasers (read laser at 423nm,write laser at 398nm) on a commercial MiniDisc.
The cw read laser was fixed at only 0.3mW on
sample. The write laser was pulsed with 2µs pulse
duration at six different power levels from 1mW to6mW; additionally a magnetic field of 400 Oe wasapplied.

Figure 9: Differential polarization microscope
image of the mark pattern.
The distance between the different sampling
positions is 5 micrometer. The large, bright,
circular marks, on each of the 10x7 positions are
the result of the initializing prepulse with the 1
kOe field. The dark marks of varying size belong
to the traces in Fig. 8, starting with 1mW write
power in the top row up to 7mW write power inthe bottom row (trace not shown). Clearly visible
is the dependence of mark size on laser power.

Hysteresis loops

OutlookFig.10 shows an example of hysteresis loop
tracing performed on a MiniDisc. For thismeasurement only the read laser at 632nm is used.
It was operated with 1mW in cw mode, firstly todetect the Kerr signal and secondly to adjust by its
power a certain local temperature on the sample.
The magnetic field then was scanned from 0 Oe to+4 kOe then to -4 kOe and so on. The dotted curveshows the originally recorded raw data. As alreadydiscussed in [1] the shape of the hysteresis loop
looks different from conventionally recorded ones.
Possible explanations given in [1] were the nonuniform temperature profile in the laser spot andthe influence of the Faraday effect of themicroscope objective. Measuring such a ‘hysteresis
loop’ on a piece of silicon wafer indeed reveals anMO signal (s-p)/(s+p) which linearly depends onthe magnetic flux of the magnet, supporting theFaraday explanation. The solid curve in Fig. 10
now shows the original data of the MiniDisc (seedotted curve) minus the linear dependence
retrieved from the silicon wafer.

Due to its versatility the system can be used
for other applications like general scanning laser
microscopy. A similar instrument could also beused for characterization of the dynamics ofmagnetic materials via their magneto-opticalbehaviour.
In addition, the implementation of the pulsed
violet laser source with wavelengths in the 400 nm
range offers even more interesting possibilities.
Media manufacturers can optimize their media forthe next generation of optical data storage discs.
Moreover, the smaller spot size may be interestingfor many other applications like optical lithography
and the high photon energy may be advantageous
for all kinds of photoluminescence microscopy.

MO signal (s-p)/(s+p)

0.1

0.0-0.1

original data
faraday effect substracted
-0.2

-4000

-2000

02000

4000

magnetic flux (G)Figure 10: Hysteresis loop on a MiniDiscReferences[1] M. Mansuripur, J.K. Erwin, W. Bletscher, P.
Khulbe, K. Sadeghi, X. Xun, A. Gupta andS.B. Mendes
Static tester for characterization of phasechange, dye-polymer, and magneto-optical
media for optical data storage
Appl. Opt. 38, 7095 – 7104, 1999.