We describe performance of a differential spot-size (wax-wane) focus servo. Crosstalk from the tracks are analyzed in the single detector and differential focus circuits. Magnitude of the crosstalk is reduced by a factor of three in the differential circuit. A false focus-error signal (FES) is present when the spot crosses sector marks at an angle

Erwin, K. E.

Froehlich, F. F.

Kann, J. L.

Milster, T. D.

Treptau, J. P.

Wang, M. S.

English

NASA Technical Reports Server

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Differential Spot-Size Focus SeDro9 2J-! 4
I
T.D. Milster, M.S. Wang, F.F. Froehlich, J.L. Kann, J.P. Treptau, and K.E. Erwin
Optical Sciences Center, University of Arizona, Tucson, Arizona, 85721 .f
Abstract /1 "X_"_ _"
We describe performance of a differential spot-size (wax-wane) focus servo. Cross
talk from the tracks are analyzed in the single detector and differential focus circuits.
Magnitude of the cross talk is reduced by a factor of three in the differential circuit. A false
FES signal is present when the spot crosses sector marks at an angle.
2. Introduction
detector 2 becomes smaller.
There are several techniques that can be used for focus-error detection in an optical
data storage device. Astigmatic, knife-edge, critical-angle prism, pupil obscuration, and spo,-
size detection are common techniques (!)(2)(3). These methods sense the focus
error by manipulating reflected light from the disk and creating an electrical focus-error
signal (FES) with sectioned detectors. If a continuously pregrooved disk is used, the
reflected light also contains diffracted orders that are used to provide a tracking-error signal
(TES). It is difficult to completely separate the focus-error information from the tracking-
error information, regardless of the focus-error detection method. The residual amount of
TES observed in the FES is called cross talk. Other kinds of pattern noise, such as
diffraction from sector marks, beam motion, and partial obscuration, can also lead to false
FES signals. Prikryl (4) has modeled the sensitivity of several focus-error detection
methods to sources of cross talk. Cohen (1) and Stahl (5) have modeled cross talk
sensitivity of astigmatic focus-error detection. In this paper, we discuss the characteristics
of a differential spot-size measurement technique, which has better cross-talk rejection than
the single-detector spot-size measurement technique. Similar differential techniques have
been presented in the literature (6), but they have not been analyzed with respect to cross
talk.
Our differential spot-size technique is illustrated in Figure 1. Reflected light from the
objective lens is focused through a polarizing beam splitter onto two quadrant detectois.
Detector 1 is slightly inside focus, and detector 2 is slightly beyond focus. Representations
of spot sizes through focus are sketched in Figure 2. For the in-focus conditien, the spots
are approxin.m, tely the same size, but they are displaced slightly from the center of the
detector. Displacement on detector 1 is opposite from the displacement on detector 2. As
the disk moves outside of focus (farther away from the objective lens), the spot on detector
1 becomes smaller, and the spot on detector 2 becomes larger. As the disk moves inside of
focus (closer to the objective lens), the spot on detector 1 becomes larger, and the spot on
An error signal is generated from :
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Prepr_nt from SPIE Proceedings Volume _ 1499
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Figure 1. Detector optics layout for the
differential spot size technique.
_ OutsideFocus
k.J
m
L_tc,.Ior I I_,¢lettt_ 2
f'_ In
Focus
Inside
Focus
Figure 2. Spot shapes on the quad
detectors for three different focus conditions.
2(A + B)-(C + D)
et: A +B+C+D
(A + 8)-2(C * D)
_= A+B÷C.D
• =I[ 1 -@_ j
[1]
where A, B, C, and D are detector voltages, e x and ¢ z are FES signals for detectors 1 and
2, and e is the differential FES. This method can also be considered as a differential wax-
wane focus scheme. We subtract the combined quadrant signals from each detector to
generate the magneto-optic read-back signal.
The following paragraphs describe modeling and experiment used to evaluate the
differential spot-size technique.
3. Modeling
Our model is a scalar diffracdzn implementation of the servo path from the disk to
the detectors. We use a Fresnel approximation to describe the propagation from disk to
objective lens and from detector lens to detectors. The A, B, C, and D signals are found by
integrating the squared absolute value of
the amplitude over detector quadrants.
An important consideration for servo
design is gain, G, expressed in volts per
micron. If one assumes a uniform beam, an
expression for G in spot-size focus detection
is given by
t3 : a_.Y.e
Az
: G, x 0.976 x( f'" /l
--7- tZ.) '
Table I
Single spot
size servo
Gain 0.36V/_m
(measured)
O.37V/_m
Differential
spot size servo
Gain
(calculated)
Residual FES 0.14V 0.085V
(p-p)
Crosstalk (p-p) 0.3_.m O.12#.m
0.71V/tzm
0.74V//_.m
Jqi l +er Z
- Appendix G 177
14
411
OI
-O |
°O 2
-4 3
-414
dtFFe: • e Lis_
k
3 -I_ -t O I Z
defocus(micron)
Figure 3. Calculated _ curves for single
detector and differential circuits.
4lJJ--
Ill
O,|
-02 I
-44 i
_0_41
• OI 02 03 04
FF.S
Figure 4. Calculated Lissajous envelope for
the single-detector circuit.
where G_. is electronic gain (5), I is distance
from detector to nominal focus (11 ram), fm
is focal length of the detector lens (124 ram),
fo is focal length of the objective lens (4.3
ram), and _tz is disk displacement. Our
calculated G is 0.37 V/_m, which corresponds
well to the measured value of 0.36 V/t_m.
Gain for the differential spot-size focus
technique is twice G, or 0.74 Vh_m. The
measured differential gain is 0.71 V/_m.
Table I summarizes these results.
Figure 3 displays FES versus defocus
for both detectors individually and the
differential FES. Single-detector FES is a
nonlinear function of position. The
differential FES is more linear.
O.4F
o.,eF
-o.- F
-o.,0t
-4111
-0 • Oil -04 -08 • OZ 414
FES
Figure 5. Calculated Lissajous envelope for
the differential circuit.
We studied the interaction of focus and tracking signals by calculating the envelope
of the Lissajous pattern formed between the FE_ and the TES. Figure 4 displays the
Lissajous pattern for the single-detector ease. Due to nonlinear gain of the FES, the TES
signal approaches zero rapidly as the system goes out of focus in one direction. In the
opposite focus direction, the TES falls off more slowly. The envelope of the differential FES
is displayed in Figure 5, which is nearly symmetrical around best focus.
4, Expe_rimental Procedure arid Resolts
The measurement of cross talk on the _ (focus error signal) requires knowledge
of the focus servo gain and the peak-to-peak voltage fluctuation of the FES as a result of
track crossings. The servo gain is measured on the linear region of the open-loop FES. The
actuator-to-disk spacing is varied by translating the optical head with a micrometer screw,
and the change in open-loop FES voltage at a fixed time reference is noted. The time
OP,IGIb ALPAGE IS
OFPoo Qu .u . . 3
178 ,4ppend/xG
Figure 6. Open loop FES signals. A: single
detector, B: differential circuit. X: 10ms/div, Y:
5V/div.
Figure 7. Lissajous patterns. A: single-
detector X-1V/div, B: differential circuit
X=2V/div. X: FES, Y: TES 1V/div.
reference is established relative to the synch
pulse from the disk spindle. The gain is then
computed as the change in open-loop FES
voltage for a given displacement of the actuator and is expressed in volts per micron. The
peak-to-peak voltage fluctuation of the FES is measured directly on an oscilloscope with the
focus servo locked and the tracking servo unlocked. The peak-to-peak cross talk is then
calculated by
Cross talk (umpp) = FES Voltage (Vpp)
Servo Gain (V/urn)
We made most measurements on a
glass substrate magneto-optic disk spinning
at 1800 rpm. Figure 6 displays the open-
loop FES signals for the single-detector and
differential circuits. Figure 7 displays the
Lissajous patterns for single-detector and
differential circuits. The single.detector
Lissajous pattern consists of nominally
straight lines with small oscillations.
Vertical lines correspond to small values of
cross talk (7). The nonlinearity of the
gains corresponds to the envelope predicted
in Figure 4. At this time we have no
Hgure 8. Open-loop TES servo signals from
the differential circuit. A: differential FES,
Y=0.1V/div. B: TES, Y-2 V/div. X--2ms/div.
v
AppendLt G 179
Figure 9. Cancellation ofcrosstalkforthe Figure 10. False FES due to diffraction
differential spot size technique. A: detector 1, from the sector marks.
B: detector 2, C: differential circuit. X=I
ms/div, Y--0.1 V/div.
explanation for the oscillations or the skew of the pattern. The differential-detector
Lissajous pattern is more symmetric, but there is still some asymmetry due to a small
imbalance in the gains. The lines making up the patternare nearly vertical, and there are
no observable oscillations.
Oscilloscope traces of servo error signals for the single-detector spot size technique
are shown in Figure 8. The lower trace is the open-loop TES (trackir, g error signal), and
the upper trace is the closed-loop _ showing cross talk. The cross talk was minimized
by rotational and lateral alignment of the quad detector. The cross talk was found to be
0.38
_"_e-- improved cross talk performance for the differential spot size technique is
illustrated in Figure 9. The lower two traces are individual FES signals from quad detectors
1 and 2. Track crossings are in phase on these signals. The upper trace is the differential
FES, which shows cancellation of the track crossings. The residual cross talk was 0.12 #mpp,
which is a factor of three improvement over the single-detector technique.
In the course of measuring the cross talk for the differential technique on a plastic
disk, we discovered an FES signal generated by sector marks. This excitation yields the
focus servo response shown in Figure 10. The response acted like a true focus error, thus
the individual FES signals are out-of-phase, and the response is doubled in the differential
FES. We believe that this response is due to diffractien as the spot crosses the sector mark.
5. Conclusions
We have analyzed a differential spot-size (wax-wane) focus error technique for
sensitivity to cross talk from track crossings. The single-detector circuit exhibited cross talk
of 0.38_mpp. The differential circuit exhibited cross talk of 0.12t, mpp, which is approximately
a factor of three improvement. An undesired signal was discovered as the spot crosses a
sector mark. It is believed that diffraction from the sector mark causes a false FES signal.
We are investigating how to minimize this effect.
180 Appendix G
6. References
°
°
.
.
.
°
o
D. K. Cohen, "Analysis of methods for detecting focus error in optical data storage systems,"
Ph.D. Dissertation, University of Arizona, Tucson, Arizona (1987).
M. lrie, T. Fujita, M. Shinoda, and M. Kondo, "Focus sensing characteristics of the pupil
obscuration method for continuously grooved disks," Proc. Int. Syrup. on Optical Memory,
1987 Japanese Journal of Applied Physics, vol. 26, pp.183-186 (1987) supplement 26-4.
M. Yamamoto, A. Watabe, and H. Ukita, " Optical Pregroove dimensions: design
considerations," Appl. Opt., vol 25, 22, pp. 4031-4034 (1986).
I. Prikryl, "Optical servo cross talk," SPIE 1078 Optical Data Storage Topical Meeting, pp. 244-
255 (1989).
R.P. Stahi, "Modeling optical feedtrough of track crossings into the focus error signal in an
optical storage device," Master's Report, Depar:-aent of Optical Sciences, University of
Arizona, Tucson, Arizona (1990).
M. Oka, A. Fukumoto, IC Osato, and S. Kubota, "A new focus servo method for magneto-
optical disk systems," Pro(:. of the Int. Syrup. on Optical Memory, 1987, Japanese Journal of
Applied Physics, vol. 26, pp. 187-190 (1987) supplement 26-4.
S.L. Grove, K.W. G-etreuer, and D.L. Schell, "Lissajous analysis of focus cross talk in optical
disk systems," to be published in SPIE Proceedings 1499.
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