an optical head for a magneto-optic disk test system...optic recording. the decision to build a...
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An optical head for a magneto-optic disk test system
Item Type text; Thesis-Reproduction (electronic)
Authors Bushroe, Frederick Nicholas, 1964-
Publisher The University of Arizona.
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Download date 14/06/2021 15:18:24
Link to Item http://hdl.handle.net/10150/277154
http://hdl.handle.net/10150/277154
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An optical head for a magneto-optic disk test system
BushToe, Frederick Nicholas,'M.S.
The University of Arizona, 1989
U M I 300 N. Zeeb Rd. Ann Arbor, MI 48106
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AN OPTICAL HEAD FOR A MAGNETO-OPTIC
DISK TEST SYSTEM
by
Frederick Nicholas Bushroe
A Thesis Submitted to the Faculty of the
COMMITTEE ON OPTICAL SCIENCES (GRADUATE)
In Partial Fulfillment of the Requirements For the degree of
MASTER OF SCIENCE
In the Graduate College THE UNIVERSITY OF ARIZONA
1 9 8 9
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STATEMENT BY AUTHOR
This thesis has been submitted in partial fulfillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.
Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his or her judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, rermission must be obtained from the author.
SIGNED:
APPROVAL BY THESIS DIRECTOR
This thesis has been approved on the date shown below:
J&nes J. Burke ^Professor of
Date Optical Sciences
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ACKNOWLEDGMENTS
The optical head for the magneto-optic test system was made possible through the advice, cooperation, and efforts of many individuals. Scott DeVore deserves special thanks for his donated time, enthusiasm, and knowledgeable advice. Scott guided the head design from the beginning and worked with us through the alignment and fabrication processes.
I also thank my advisor Professor James Burke for his guidance, moral support, and faith in me, during my time at Optical Sciences. I am grateful to Tom Milster for managing our lab group and pulling the test system together in a short period of time.
I extend my gratitude to the following people and companies for their contributions listed below: Barbara Shula, Henryk Birecki, and John Graves of Hewlett-Packard Labs in Palo Alto, Ca. for the read/write electronics, equipment, and software; IBM, Siemens, and Eastman Kodak for sponsoring the Optical Data Storage Center; David Kay of Eastman Kodak, Glenn Sincerbox and Blair Finkelstein of IBM for their exchange of information on various design issues; Robert Uber for his practical advice and discussions on many aspects of the head; Kevin Erwin our technician for his valuable knowledge of optical and mechanical fabrication techniques; Mark Shi Wang for his willingness to learn the control software and his interfacing abilities; Fred Froehlich and Jim Mueller for their electronic design and fabrication; Bob Trusty for his characterization of the bias coil; Professor Roland Shack for his willingness to answer my questions on optical issues; Dan Vukobratovich for his time spent discussing mechanical issues related to the threaded collimating lens mount and the kinematic head mount; Charlie Brown and Derek Clark for their machine shop skills and cooperation; Tom Walker of OCLI for donation of the partial polarizing beamsplitter; and WYKO Corporation for the use of their Ladite instrument.
Aiding in the conceptualization of an optical head for a test system, were visits during the summer of 1988 to several companies in the optical storage arena. I am grateful to the following people for helping to arrange these visits: Scott Hamilton, Apex Systems Inc., Boulder, CO; Ron Brown, Applied Magnetics Optical Products Division, Monument, CO; Barbara Shula, Hewlett-Packard, Palo Alto, CA; Martin Chen, IBM Almaden, CA; Mr. O'Connor, Information Systems Inc., Colorado Springs, CO; Ralph Poplawsky, Laser Magnetic Storage, Colorado Springs, CO; Di Chen, Optotec, Colorado Springs, CO; Vincent Tarpey, Pencom, Sunnyvale, CA; and Tim Fayram, Verbatim, Sunnyvale, CA.
I would also like to thank our secretary Geri Simons for her help and the use of her laser printer. Finally, I wish to thank my mother, father, and family for their encouragement and support throughout my college education.
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TABLE OF CONTENTS
LIST OF ILLUSTRATIONS 5
LIST OF TABLES 6
ABSTRACT 7
INTRODUCTION 8
OPERATING PRINCIPLES OF THE MAGNETO-OPTIC HEAD 9
DESIGN OF THE MAGNETO-OPTIC HEAD 20
Design Tradeoffs and Alternate Configurations 20
The Semiconductor Laser and Reflected Pulse Phase Delay 25
Beam Truncation and Collimating Lens Selection 29
Beam Shaping and Correction of Laser Diode Astigmatism 33
Optical Path Components 35
Focus and Tracking Servo Optics 37
Mechanical Design 39
ALIGNMENT OF THE OPTICAL HEAD 42
Optical and Mechanical Alignment 42
Focus and Tracking Detector Alignment 44
Alignment of the Beam to the Disk and Related Aberrations 46
RESULTS 50
CONCLUSION 55
APPENDIX A, Commercial Laser Diodes 56
APPENDIX B, Sharp LT024R10 Characteristics 59
APPENDIX C, WYKO Ladite Block Diagram 63
REFERENCES 65
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LIST OF ILLUSTRATIONS
Figure 1. The Write Path and Coil 10
Figure 2. The Write Process 10
Figure 3. The Read Path and Polarization Diagrams1 12
Figure 4. The Complete Head and Read Path 14
Figure 5. Polarization Traces Showing the Effects of the Waveplates 15
Figure 6. Astigmatic Focus Servo Method1 18
Figure 7. Push Pull Radial Tracking1 19
Figure 8. Reflectivity Sensing WORM or CD Head1 21
Figure 9. Alternate Configuration of a Magneto-Optic Head7 24
Figure 10. High Frequency Modulation Timing Diagram11 27
Figure 11. Laser Noise versus Reflected Pulse Phase Delay10 28
Figure 12. Truncation versus Spot Diameter, Power and Irradiance 30
Figure 13. Astigmatism Introduced by Defocus in a 3X Circularizer 34
Figure 14. The Focus Error Signal 38
Figure 15 Threaded Mount and Barrel for Collimating Lens Positioning 40
Figure 16 Typical FES alignment sequence 45
Figure 17 Intensity Distribution of a Focused Spot Containing Coma16 47
Figure 18 Coma versus Tilt Angle Between the Beam and Disk Substrate 48
Figure 19 OPD Wavefront Plot and Beam Wavefront Error Values 51
Figure 20 Intensity Profiles of The Shaped Beam 52
Figure 21 Point Spread Function for the Shaped Beam 53
Figure 22 Photograph of the Optical Head 54
Figure 23 Laser Power versus Drive Current 60
Figure 24 Relative Intensity versus Theta Perpendicular 61
Figure 25 Relative Intensity versus Theta Parallel 62
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LIST OF TABLES
Table 1. Oefocus, Axial Displacement, and Astigmatism 33
Table 2. 780nm Commercial Laser Diodes 57
Table 3. 830nm Commercial Laser Diodes 58
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ABSTRACT
Design and operation of a modular optical head for a magneto-optic test system
are described. Alternate solutions to design problems are discussed. A 30mW semiconductor
laser with an integrated 250MHz oscillator is selected. The oscillator is used to modulate
laser read current for a reduction in laser feedback noise. A collimating lens with an
appropriate focal length is chosen so the beam's truncation at the objective yields the
maximum write power density. Astigmatism associated with the laser diode is reduced to
0.125 waves by defocusing the collimating lens and circularizing with an anamorphic prism
pair. Head components are aligned within several minutes of arc by using alignment
apertures and an autocollimator. Aberrations due to tilt between the disk and beam are
examined and coma is found to be the major contributor.
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INTRODUCTION
Magneto-optic recording technology is the most successful erasable optical
scheme for computer disk data storage. The Optical Data Storage Center at the University
of Arizona's Optical Sciences Center carries out research on various aspects of magneto-
optic recording. The decision to build a magneto-optic disk test system for use at the
Optical Data Storage Center was made in March of 1988. The system's proposed capabilities
included media characterization as well as bit error testing. The system has since been
constructed and has demonstrated the abilities to read, write, and erase data. Hewlett-
Packard supplied the read/write electronics, test equipment, computer hardware, and system
software. The optics and mechanics for the magneto-optic test system were designed and
built at the Optical Sciences Center. This thesis is a description of the optical head, its
design, and fabrication.
Design of the optical head was influenced by several factors. The academic
research environment of the system made it necessary to design a modular head to simplify
future modifications. To allow for easy removal or replacement of the head, it was designed
with small components and constructed on a single platform.
Coordinate convention in this manuscript is as follows: the z-axis is the optical
axis, the x-axis is parallel to the plane containing the head components (horizontal), and the
y-axis is perpendicular to this plane (vertical). The following sections explain operating
principles of the head, various design tradeoffs, component selection, optimization, and
alignment procedures.
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OPERATING PRINCIPLES OF THE MAGNETO-OPTIC HEAD
The Write Path
The writing process in magneto-optic recording is a thermal magnetic recording
process. During writing, the focused laser beam heats a small area (Dia. s; Iftm) of a
magnetic disk in the presence of an external magnetic bias field. The disk contains
magnetic moments perpendicular to its plane which align with the polarity of the external
bias field only if the temperature of the media is great enough. Figure 1 shows the write
path and the coil which produces the external bias field. Assume that all magnetic moments
on the disk are pointing in one direction as shown in Figure la. The coil's polarity produces
an external constant bias field (Hf) opposing the direction of the moments on the disk
(Figure 2b). The moments flip and align with the external bias field when the coercivity of
the media is lowered by laser heating. A magnetic domain (bit of information) is frozen
into the media when the laser's thermal energy is removed (Figure 2c), To write a stream of
data, this process is repeated by pulsing the laser rapidly while the disk spins. Typical laser
write powers at the disk are S to lOmW. Reversing the coil's field direction and heating the
media in a continuous path erases data. The moments along a track all point in the same
direction, and the initial state of Figure 2a is achieved. The media must pass under the
write head twice to rewrite over existing data, once to erase and a second time to rewrite.
The Read Path
The read process in magneto-optic recording is a polarization sensing technique.
The laser is on continuously and linearly polarized light is incident onto the magnetic media
as the disk spins. Upon reflection from the media, the polarization state is rotated an
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Write Path
Beam Steering Mirrors
v
Collimating ^ Laser & Lens
1 Prism Pair
HF Modulator [ Taohs Actuator & Objective
Disk
Reference Detector
-Eg
•80:20 Partial Polarizing Beam Splitter
H, Coil
tan —* Hf
Figure 1. The Write Path and Coil
Written "Data' Laser Pulse,
Figure 2. The Write Process
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amount 9* by the Kerr magneto-optic effect. Typical values of 6^ are 0.3 to 0.6 degrees.
The direction of rotation, clockwise or counterclockwise, depends on the magnetization
direction of the written domains in the media (Figure 2c).
A simplified read path is shown in Figure 3 along with corresponding
polarization diagrams. The dashed and solid arrows in the polarization diagrams depict
rotation from oppositely oriented domains (logical one's and zero's) on the disk. Note that
both states do not occur at the same instant in time. The 80:20 partial polarizing
beamsplitter has the following properties of transmittance and reflectance: Tp = 0.8, Rp =
0.2, Ts = 0, Rs «• 1.0. A description of the differential detection scheme is as follows.
Linear p-polarized light is incident upon the disk (a). Its polarization is rotated by ±8fc and
its amplitude is reduced after reflection (b). All of the s-component (the signal) and 20% of
the p-component is reflected by the partial polarizing beamsplitter (c). (A fraction of the p-
component is reflected because it is needed as a reference for the orientation of the s-
component. If Rp was large then Tp would be small, and the beamsplitter's power
throughput from the laser to the disk would significantly reduced write power.) If the
polarization shown in (c) was to go directly into the detection polarizing beamsplitter (A/2
plate removed) the differential amplifier would yield the same data signal for both a logical
one and a logical zero because the magnitudes of the p and ^-components are the same.
However, with the A/2 plate in place (oriented such that its fast axis makes an angle of 22.S
degrees with the horizontal), the plane of polarization is rotated 45 degrees (d) and the
differential amplifier's subtraction of the p-channel from the s-channel now produces a data
signal that is positive or negative depending on the direction of the Kerr rotation.
The partial polarizing beamsplitter used in combination with differential
detection produces an increase in signal by a factor of ISO when compared to the signal
generated by detecting only the .s-component of polarization rotation.1 This boost in signal
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Simpl i f ied Read Path
Data Signal
Partial Polarizing Beam Splitter Tp =0.8, Ts=0
Rp=0.2, Rs = 1.0
0 * (a)'
(c) v/
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is due to a first-order dependence on when using differential detection, as opposed to a
second order dependence on 0A when detecting only the s-component of polarization.
Another advantage of differential detection is common mode rejection, a characteristic of
the differential amplifier which suppress noise caused by intensity fluctuations.
The complete head is shown in Figure 4. The actual read path employs a A/4
plate and servo detection optics as shown. The A/4 plate improves the magneto-optic signal
by compensating for ellipticity in the polarization of the reflected beam. The following
explanation of how the A/4 plate effects the polarization is aided by the polarization traces
o f F i g u r e 5 [ A ] t h r o u g h [ F ] , a n d t h e c o r r e s p o n d i n g l a b e l e d p o i n t s i n F i g u r e 4 . F i g u r e s 5 [ A ] t
[5], and [C] show only the case of a +9* rotation representing a logical one. The initial
polarization state incident on the media is a linear p-component as previously shown in
Figure 3a. Figure 5 [A] shows the polarization components after the partial polarizing
beamsplitter and just before the A/4 plate. In reality |Er | » \Er^ | and Efp and Er^ are out
of phase by A^. Therefore, the resulting polarization is not linear, but slightly elliptical and
rotated from the p-axis by the small angle ±9^. The reflected beam then contains two
forms of information, the Kerr angle Qk and the amount of ellipticity (c, resulting from
The differential detection scheme as described above can only detect the Ken-
rotation, thus the information contained in the ellipticity is lost unless it is converted to a
useful form. Conversion of the ellipticity can be carried out by utilizing a Babinet-Soleil
compensator or by using a A/4 plate in conjunction with a A/2 plate. Both techniques yield
an equal increase in the signal to noise ratio. The Babinet-Soleil compensator retards the
phase of either the p or ^-component such that the two are in phase, producing a purely
linear state in which the 5-component (the signal) has been maximized. The method using
the A/4 plate deserves a more detailed description due to its complexity.
Light incident onto the A/4 plate has the polarization properties shown in
Figure 5 [A] and described above. The fast axis of the A/4 plate is oriented at 45 degrees to
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Head Layout for M-0 Tester
Collimating Lens -i Laser Sc
HF Modulator
Taotis Actuator Sc Objective
Prism Pair Disk Beam
Steering Mirrors Reference
Coil
30:20 Partial Polarizing Beam Splitter
X/4 Plate [0.C.D1
A/2 Plate Quad. Oetectar
Astigmatic Lens Pair
Polarizing Beam Splitter
Data Channel 1 Sc Focus Error Signal Lens
Data Channel 2 Sc Tracking Error Signal
Quad. Detector
Figure 4. The Complete Head and Read Path
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A. S
—E&-
+"e7 S.
t=0
t= l
t=2 t=3
Figure 5. Polarization Traces Showing the Effects of the Quarter and Half-Waveplates
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the p and s-axes. Visualizing the p and ^-components as acted upon separately by the A/4
plate yields the right and left circular polarized traces as show in Figure 5 [£]. Vector
addition of the components which sweep out the opposite handed circles produces the ellipse
shown in Figure S [C]. Note that the major axis of this ellipse is at 45 degrees (ignore the
phase difference A for now) and the ellipticity ratio (the ratio of the minor axis to major
axis) depends only on the magnitude of the Kerr component, |£r |. Superimposed
polarization traces for +0* and -0A signals are shown in Figure 5 [£>]. After the A/4 plate
light passes through the A/2 plate (oriented such that its fast axis makes an angle of 22.5
degrees with the horizontal) which rotates the plane of polarization 45 degrees. For a
maximum difference signal the resulting traces should look like those shown in Figure 5 [F].
However, because of the initial phase difference A, the major axes of the ellipses do not lie
on the p and s-axes (see Figure 5 [£]). The A/2 plate is adjusted accordingly (away from its
22.5 degrees setting) to compensate for this phase difference by rotating the plane of
polarization so the axes of the ellipses lie on the p and s-axes to obtain a maximum signal to
noise ratio (see Figure 5 [F]). A mathematical description of this process as well as results
from increased signal to noise ratios can be found in reference 2 and 3,
Optical Servo Paths
Imperfections in disks and drive motors make it necessary to employ active
servo systems to keep the laser beam in focus and on track as the disk rotates. These servo
systems sense the position of the focused beam with respect to the disk and send electric
control signals to an actuator which supports and positions the objective lens. The actuator
responds by moving the objective in and out along the z-axis to maintain focus, and side to
side along the x-axis to keep the focused beam on track. The focus servo keeps the beam in
focus to ±0.5fim or less as it follows displacements of up to 500ftm. The tracking servo
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maintains the beam's position on track to ±0.1 pm or better and can follow radial disk
runouts of 100/wi. The bandwidths of the tracking and focus servos are on the order of 1 to
5kHz.
The focus servo scheme used in the optical head was the astigmatic focus
method, shown in Figure 6. A spherical lens focuses the beam onto a quadrant detector and
a cylindrical lens introduces astigmatism into the beam. The quadrant detector is placed at
the circle of least confusion when the beam is in focus on the disk. All four quadrants of
the detector are uniformly illuminated and the focus error signal (A+C)-(B+D) is zero at the
in focus condition. When the disk moves in or out of the objective's focal plane the
reflected beam will converge or diverge. This effectively shifts the primary and secondary
image planes of the astigmatic system causing orthogonal ellipses to be imaged onto the
quadrant detector. The focus error signal generated, either positive or negative, is processed
by servo electronics and a control signal is sent to the actuator to focus the objective.
The tracking servo scheme employed was the push-pull radial tracking method,
illustrated in Figure 7. In this scheme diffracted light is reflected from grooves in the
surface of the disk. The grooves in the disk have a pitch (radial period) of 1.6/jm and an
optimum depth of A/8. The reflected beam from the media contains a bright zero-order
beam and two diffracted first-order beams which are equally spaced. Interference of the
first-order beams with the zero-order beam forms the baseball shaped interference pattern
shown in Figure 7. The resulting intensity patterns incident upon the quadrant detector are
shown in the figure. The tracking error signal generated is greater or less than zero
depending on the direction of the radial offset. As with the focus error signal, this signal is
processed and a control signal causes the actuator to correct the objective's radial position.
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ASTIGMATIC FOCUS SERVO METHOD
Astigmatic Lens System Quad Detector Illumination
In focus
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PUSH PULL RADIAL TRACKING METHOD
(A+O) - (B+C) >0 =0
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DESIGN OF THE MAGNETO-OPTIC HEAD
Design Tradeoffs and Alternate Configurations
This section describes some of the reasoning that was done during the design
phase which led to the current head configuration. There are many forms a magneto-optic
head can take in response to several design problems. Advice on various issues was sought
from numerous experienced people in the field of magneto-optic recording. Their different
ideas and sometimes conflicting solutions to problems were appreciated. The following
paragraphs address several engineering tradeoffs and alternatives.
A mechanism was needed to translate the laser beam in x and y at the objective
as well as to control its angle with respect to the z-axis. It was decided that a simple v-
mount for the laser/beam-shaping assembly and two turning mirrors would do the job in a
compact fashion. One alterative was an x-y translation stage for the laser assembly and one
turning mirror for the beam angle adjustment. This would not have been as compact or
stable. The mirror mounts used were very stable flexure type mounts. These flexure
mounts are free from stick-slip problems, have low hysteresis, and are more stable over time
than the traditional ball bearing/spring-loaded thumb screw mounts.
The laser should be isolated from the reflected return beam to decrease laser
noise while reading data. The major source of laser noise is optical feedback into the laser
cavity from reflected light.4 This feedback causes instabilities and random competitive
mode-hopping in the cavity as well as a decreased power output.6*6 WORM and CD heads
employ a A/4 plate between the objective and polarizing beamsplitter to combat this problem
(see Figure 8). After passing through the A/4 plate, circular polarized light is incident on
the disk. Upon reflection the handedness of the light is changed and after a second pass
through the A/4 plate the polarization is rotated 90 degrees. The polarizing beamsplitter
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optical disk
polarizing beam
splitter
circularizing optics collimating / Jens objective lens laser diode / lasi
(NT mirror p— astigmatic lens
quad, detector
Figure 8. Reflectivity Sensing WORM or CD Head1
N)
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then reflects the return light towards the detection channel, isolating the laser from the
return beam. This will not work for a magneto-optic head since it is required that linear
polarized light be incident on the media.
Two options for controlling feedback induced laser noise were considered. In a
magneto-optic scheme, a polarizing filter and a Faraday rotator can be placed after the laser
and before the partial polarizing beamsplitter to yield optical isolation. Linear polarized
light from the laser passes through the polarizer and Faraday rotator, remaining linear while
its plane of polarization is rotated +45 degrees. The plane of polarization of the return
reflected light is rotated another +45 degrees after its second pass through the Faraday
rotator. This 90 degree rotation of the reflected light causes it to be absorbed by the
orthogonally oriented polarizing filter. This method was not used because of four problems
associated with Faraday rotators: cost, size, insertion loss, and isolation ratio.4
The second noise reduction technique examined was high frequency modulation
of the laser's DC drive current while reading data. A delay exist between the time a
semiconductor laser is turned on and the time it reaches single mode operation.6 High
frequency modulation (200MHz to 400MHz) of the laser's drive current from below
threshold to a level above threshold constrains the cavity to multimode operation. Though
the laser is not optically isolated, this method represses mode hopping caused by optical
feedback as well as mode hopping caused by thermal effects. The average optical power out
of the laser is much more constant and thus amplitude noise is reduced while reading data.
This technique is easy to implement (as discussed later) and has been shown to produce good
results.4
A common alternative to using a partial polarizing beamsplitter to reflect part
of the p-polarization towards the data channel, is to use a standard polarizing beamsplitter
rotated slightly in the plane of incidence. Rotation of a standard polarizing beamsplitter
causes the effect of a partial polarizing beamsplitter by reflecting a fraction of the p-
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23
component towards the data channel. However, this technique complicates the alignment
and mechanical design of the head. Varying with the beamsplitter's degree of rotation are
two things: translation of the beam in the aperture of the objective, and the angle of the
reflected beam towards the data channel. The use of a partial polarizing beamsplitter made
the mechanical design straightforward and simple.
The head of Figure 9 illustrates three additional aspects which were considered
during the design period but were rejected. The first aspect shown is the disk mounted
horizontal, parallel to the plane of the head. Horizontal mounting of the disk is common in
most disk test systems. Vertical mounting was decided upon for two reasons. If a glass disk
shatters in a vertical plane it is less likely to hit and injure someone than if it shatters in a
horizontal plane. Also, the head optics are more accessible and easily viewed with a vertical
configuration.
The second aspect was the separation of the servo channel from the data
channel via an additional beamsplitter. Data and servo channels are sometimes separated to
eliminate feedthrough from the servo signals into the data signal. Feedthrough was not
estimated to be a problem with the servo techniques chosen. The decision to combine the
data channel and the servo channel simplified the head by reducing the number of optical
and electrical components. Another benefit gained was a shorter write path which decreases
laser noise as described later in the discussion of reflected pulse phase delay.
The third aspect illustrated in Figure 9 is the 45 degrees rotation of the data
channel instead of using a A/2 plate to rotate the plane of polarization. Rotation of the data
channel was decided against for mechanical reasons. It is much easier to rotate and adjust a
single A/2 plate than to mount the data channel on a separate rotatable mount and baseplate.
It is also easier to keep the data channel aligned if it is mounted in the same plane as the
other head components. The A/2 plate thus led to a simple, more stable and condensed
design.
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(MO-)Konol
Figure 9. Alternate Configuration of a Magneto-Optic Head7
to
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Another design alternative was to use a Babinet-Soleil compensator instead of
the A/4 plate. This was decided against because of the size and cost of a Babinet-Soleil
compensator.
The choice of astigmatic focus and push pull tracking for the servo systems was
made because these techniques have been researched extensively and are widely used.1*8
They are also fairly simple in principle and implementation.
The Semiconductor Laser and Reflected Pulse Phase Delay
Magneto-optic recording requires the laser diode to have an output power of
20mW to 30mW so that about lOmW is incident onto the media during the write process. It
was decided that the laser diode for the magneto-optic head would be index-guided and
emit at 780nm. Index-guided lasers tend to have better beam properties, sharper turn on,
and more linear output/current characteristics than gain-guided lasers.9 Astigmatism
associated with index-guided lasers is usually 3pm to 5fim, whereas astigmatism associated
with gain-guided lasers is typically 20pm to 60pm. Also, the intensity profile of gain-
guided lasers is less Gaussian than that of index-guided lasers and index-guided lasers are
less prone to feedback.4'9 Index-guided lasers are more desirable for applications that
require a small spot size.
A 780nm laser diode was preferred over an 830nm laser diode because reliable
high power 780nm laser diodes are now available, and areal storage capacity increases as the
square of the wavelength. Lists of characteristics and prices for commercially available high
power laser diodes emitting at 780nm and 830nm can be found in Appendix A.
The laser chosen for the head was an index-guided, 780nm Sharp LT024R10
with a maximum output power of 30mW. Specifications and characterization data for this
laser are included in Appendix B. This laser is packaged with a 250MHz integrated
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26
oscillator which modulates the read current (rDCread) down to a hard turn off below
threshold. This is illustrated in Figure 10; the optical power out (P0) is modulated between
zero and some read level when the modulator control signal (Vcc) is turned on. This high
frequency modulation forces the diode to operate multimode, reducing intensity fluctuations
caused by mode hopping, as previously described.
The relation between read current modulation frequency and the laser-to-disk
optical path length, has a strong effect on laser feedback noise. (Do not confuse the
modulation frequency here with modulation of the laser during writing. This is solely a
read current modulation technique used to reduce noise by modulating the normally constant
laser read current.) More specifically, laser feedback noise is a strong function of the phase
difference between the modulated irradiance leaving the laser and the reflected irradiance
returning to the laser.10 This noise is smallest if the reflected intensity at the laser is
minimum when the laser's emitted power reaches a maximum. If however, the reflected
intensity at the laser is maximum just before the laser's emitted power reaches a maximum,
the laser is unstable and the feedback induced noise is large. An expression describing the
phase delay of the reflected laser read pulse is given by:10
RPPD = /^K360 degrees) [1]
where RPPD is the Reflected Pulse Phase Delay, / is the modulation frequency, L is the
optical path length from the laser cavity to the reflective disk, and c is the speed of light in
air. (The phase delay described here is based on the electrical modulation frequency; it is
not an interferometric phase delay based on the wavelength of light.)
The frequency measured for the modulator of the Sharp LT024R10 was / =
249.4MHz. The optical path length was minimized to yield L = 8 inches. Evaluating the
expression above with these values yielded a reflected pulse phase delay of 122 degrees.
Figure 11 is a plot of the laser noise to shot noise ratio versus the reflected pulse phase
delay. The circles, triangles, and squares represent modulation frequencies of 200, 300, and
-
nruu 20mW
J u HflAA/l/l/yyVWi/lA/IA/M u L
High frequency modulator ON
Figure 10. High Frequency Modulation Timing Diagram11
-
10000
K\ H QV
A nu
180 360 540 720 000
phase delay (degrees)
Figure 11. Laser Noise versus Reflected Pulse Phase Delay10
-
29
400MHz respectively. At 122 degrees the laser noise to shot noise ratio is below 10 on the
logarithmic scale, a respectable value. Note that the minimums in noise repeat every 360
degrees - at 90 degrees, 450 degrees, etc. To obtain a reflected pulse phase delay of 90
degrees, with the fixed frequency of the Sharp modulator, the optical path length required is
about 6 inches. This was too constraining for the small amount of noise reduction gained.
For a reflected pulse phase delay of 450 degrees the path length would need to be 30 inches.
Varying the frequency of the modulator allows easy achievement of a 90 degree
reflected pulse phase delay and relieves the requirement of optimizing the optical path
length. This requires an external variable high frequency oscillator to modulate the laser's
drive current during the read operation. One problem with this is that the cables from the
high frequency oscillator to the laser radiate interference that shows up as noise in the data
detection electronics.13 This problem is minimized with the Sharp LT024R10, because the
oscillator and laser are packaged in a common shielded case and the lead lengths are minute.
The Sharp LT024R10 was an inexpensive, easily implemented off-the-shelf solution to read
current modulation.
Beam Truncation and Collimating Lens Selection
Two factors guided the selection of a lens to collimate the laser's divergent
output the size of the focusing objective's aperture, and the wider divergence angle of the
laser's elliptical output beam (Qperp). Filling the objective lens aperture involves tradeoffs
illustrated by Figure 12. Underfilling the objective considerably with the Gaussian beam's
1/e1 profile effectively reduces the NAobj. This produces a larger spot size and lower power
density on the disk. However, transmitted power is very high. Conversely, if the aperture
is considerably overfilled (uniformly illuminated), the smallest possible spot size is obtained.
In this case the power density and transmitted power are low. At a point between underfill
-
I '
Irradiance
Transmitted Power
CO 0.5
0 0.5 1.0 1.5 2.0
Truncation: DIA,en9 -5- i/e2 DIAgau88jan
Figure 12. Truncation versus Spot Diameter, Transmitted Power and Peak Irradiance1 o
-
31
and overfill the power density reaches a maximum. This occurs when the aperture is
slightly underfilled at the truncation ratio given by:1
_DIA \ e*
objective g 1,12 [2]
• ^^Gaussian
As shown below, this ratio is equivalent to truncating the Gaussian at the 1/e*-6 intensity
level. It was decided that the objective's aperture would be filled to this extent because of
the high power density obtained and small sacrifice in transmitted power and spot size.
The objective's aperture was first matched to the beam's intensity
characteristics. The focal length of the collimating lens to make this match was then
calculated and a collimating lens was selected. This process is outlined as follows. A
radially symmetric Gaussian beam intensity profile is given by:
/(r) » I0e
-
32
R - l°»i TM [ioj
The necessary focal length of the collimating lens (fcou) can then be calculated from
OT '»«" r I")
sinfWj
/coll = U, NA°"I , [12]
Substituting R from equation [10] into the above equation yields
NAnM
1.90 sinjW-j
The objective in the Taohs-PB7 actuator has a focal length of fobj = 4.3mm and a NA of
NAobj = 0.5. The Sharp LT024R10 laser's 9perp is 29 degrees. Evaluating expression [12]
with these values, the desired focal length of the collimating lens was fcoH » 4.52mm. The
only other criterion to meet in selecting the collimating lens was that its aperture be larger
than that of the objective's so truncation by the collimator would riot occur. The objective's
aperture was 4.3mm. The collimating lens selected has a focal length of 4.52mm, an
aperture diameter of 4.3mm, and an NAcon of 0.48. Inspection of the curves in Figure 12
about the dashed optimization line reveals that exact optimization is not critical due to the
flatness of the curves in this region.
It was not surprising that the collimating lens' focal length and diameter were
precisely what was needed since the Taohs actuator/objective is one of the most popular
available. The collimating lens, distributed by Optima, is a four element air spaced lens
with wavefront aberration typically less than A/10. It was designed for 785nm and all
surfaces are magnesium fluoride anti-reflection coated. The price is amazingly low at
$25.00 each in small quantities.
-
33
Beam Shaping and Correction of Laser Diode Astigmatism
The circularizer chosen was a Melles Griot anamorphic prism pair with a fixed
ellipticity correction ratio of 3 to 1. This was close to the laser's specified ellipticity ratio of
2.9 to 1. The circularizer stretches the narrow axis of the elliptical beam after collimation,
giving rise to a circular beam. If the input to the circularizer is an elliptically truncated
plane wavefront, the output will also be a plane wavefront, since a plane stretched by three
time in one direction is still a plane. However, if the input is a spherical or astigmatic
wavefront, the circularizer can introduce or cancel astigmatism by stretching one axis of the
wavefront. The following is an explanation of how the laser's inherent astigmatism can be
corrected by defocusing the collimating lens and allowing the beam to pass through the
anamorphic prism pair.
The mechanical requirements of positioning the focal point of the collimating
lens in proximity to the laser diode's facet are stringent. The relation between defocus (lf20)
in waves and the axial distance Az is given by the relationship,
= [13]
For A » 780nm and an NAcotl = 0.48 the values below are obtained.
( * ) A t ( n m ) W „ ( A )
1 6 . 9 0 . 8 9 0 . 1 0 . 6 9 . 0 8 9 0 . 1 4 1 . 0 . 1 2
Table 1. Defocus, Axial Displacement, and Astigmatism
Ignore the inherent astigmatism in the laser diode for now. Astigmatism (Wj2) induced by
the 3X circularizer is related to defocus by W22 = (8/9)W20 as illustrated in Figure 13.
Defocus produces a spherical wavefront and is not as harmful to the system as astigmatism
since the focus servo will keep the scanning spot in focus despite a slight divergence in the
beam. From the table above it is realized that in order to keep astigmatism to about A/10
-
34
Astigmatism = (8/9) Defocus For a 3X Prism Circularizer
OPD Contours - Defocus No Astigmatism
Shape of Laser Diode's Elliptical Beam Truncates Wavefront
OPD Cross Sections Truncated by Ellipse
vY
I V,x 7 " ' T
B
H d/3 |«- h
A=B(1/9)
After 3X Circularizer the Beam Is streched 3X along X A and B are still the Same
Sag = Astigmatism = B-A = B(8/9)
*
B is due to Defocus So Astigmatism=(8/9)Defocus
Figaro 13. Aatigmatism Introduced by Defocus in a 3X Circularizer
-
35
the collimating lens must be positioned within about 1 fim of its optimum location. A
mechanism which allows for this degree of control is described later in the section on
mechanical design. Owing to the relation between defocus and astigmatism as illustrated in
Figure 13, it was possible to correct for the laser's inherent astigmatism (an astigmatic length
of £! 3fim) by adjusting the focus of the collimating lens. As the collimating lens was
defocused the resulting astigmatism was minimized by monitoring its magnitude on a
WYKO Ladite phase shifting interferometer (see block diagram in Appendix C). The results
are presented later.
Optical Path Components
Several optical path components which deserve a brief treatment are: the
magnetic bias coil, the reference detector, the Taohs actuator, the 80:20 partial polarizing
beamsplitter, the waveplates, and the detection polarizing beamsplitter.
A magnetic bias coil can be made by winding magnet wire around a piece of
"soft" steel. An alternative is to use the coil from a 12 volt relay. A relay coil is precisely
wound, very neat and cheap. A coil from a relay was used in the test head. The coil was
calibrated with a Gauss meter to measure the magnetic field a fixed distance away as the
current through the coil was varied. Typical field strength at the disk is 300 Gauss.
The reference detector is a simple silicon detector just large enough to contain
the beam without focusing optics. It was tilted to avoid reflection of light back to the laser.
This external detector can be used to monitor the laser power in a feedback loop to stabilize
the laser's output power. The photodiode built into the laser diode is not used because it can
give false power readings due to the difference in emission between the front and back
facets. Temperature variations of the laser package and noise in the laser caused by
reflections can also effect the internal photodiode's accuracy.
-
36
The Olympus Taohs actuator was chosen mainly because of its proven reliability
over the years.13 Its size is large compared to other actuators but it is still used in WORM
drives today since it is dependable. Another plus was that the electrical engineering support
at Hewlett-Packard was familiar with its electrical drive requirements.
The 80:20 partial polarizing beamsplitter was donated by OCLI. It has
polarization phase control built into its coatings which are non-symmetric. The orientation
of the cube must be correct for it to act as desired with minimal phase retardation of the
polarization. The exact ratio of Tp to Rp is not critical but it should be realized that the
fraction, Rp, of the power incident on this beamsplitter is lost on the way to the disk. This
beamsplitter also cleans up the laser's polarization before the disk, reflecting any unwanted
s-polarization.
The half and quarter-waveplates selected were zero-order optically contacted
waveplates. Zero-order waveplates are less sensitive to wavelength variations, making them
ideal for laser diode applications. A ±2% deviation from the center wavelength results in a
0.01 A retardance error (3.6 degrees). These waveplates are also fairly insensitive to
temperature changes. A typical temperature coefficient is 0.0001A per degree Celsius. The
waveplates, purchased from Karl Lambrecht, are AR coated and mounted in a small barrel.
The detection polarizing beamsplitter is a half inch cube, also from Karl
Lambrecht. Its extinction ratio is 500 or better, and it is AR coated as well. The read path
function of the servo optics is to focus light onto the quadrant detectors. The signals from
the quadrants of each detector are summed for the purpose of the differential data channel.
If the servo channel was separated from the data channel, the data channel detectors would
be single element detectors.
-
37
Focus and Tracking Servo Optics
The servo optics consist of an astigmatic lens pair in the focusing channel and a
single element lens in the tracking channel. The servo function of the astigmatic lens pair is
to introduce astigmatism for focus error detection. These elements are AR coated one inch
optics, purchased from CVI. The spherical lens of the astigmatic lens pair and the single
element lens in the tracking channel both have a focal length of 100mm. The cylindrical
lens has a focal length of 150mm. The active area of the quadrant detectors is 1mm1; each
quadrant is 0.5mm on a side. The gap between quadrants is 10/wi. These detector were
purchased from United Detector Technology.
The focus servo design was done by Scott DeVore with software he had
previously developed.14 The spherical lens is placed before the cylindrical lens in the beam
path. The thin lens separation is 46.75mmt and the distance from the rear principal plane of
the cylindrical lens to the detector plane is 45.22mm. The characteristics expected from the
design model are shown in Figure 14. Figure 14a is the focus error signal, (A+C)-(B+D)
normalized by the sum. The horizontal axis of this plot is the displacement of the disk.
Figure 14b shows the anticipated spot on the detector at five equally spaced increments
centered at the zero crossing of the focus error signal. The focus error signal is not
symmetric about the zero crossing, thus the blurs are not symmetric.
The tracking servo was not simulated since the diffraction pattern is entirely
dependent upon the various groove structures of the different media used. The only
requirement is that the spot is centered on the detector. A spot diameter roughly equal to
the "in focus" spot on the focus servo detector is desired. A 100mm lens was used to project
the spot onto the detector since the same focal length lens was used in the focus servo
channel. This was done to keep the two servo channels as symmetric as possible for the sake
of differential detection.
-
38
1.00
•f§t ilHUUflMi'ftuii ErMA ilfiWL " ' 117577!?-Univarsllv of AZ Itodla T«*t«r — ISO m cvl 17:2C:1S
/\^
-1.00 -49.00 FOCU ERROR (unl 43.00
res 1IHULAT1W: GETECTOA BUJA vm KPUUt Unlwralty of AZ Had la Tutor — ISO m cvl
il/07/lf 17:30:31
A / YW ' X? ; y ' /
B
Figure 14. The Focus Error Signal (a), and The Intensity on the Detector at Focus and at Two Equally Spaced Locations On Each Side (b)
-
39
Mechanical Design
The mechanical design was the most time consuming aspect of the project.
Since modularity was a key issue, the majority of components were mounted separately
within the limits of practicality. The mounts that were designed and machined accounted
for 65% of the component mounts. Of the mounts that were purchased, 50% required
machining for modifications. Some of the more interesting aspects of the mechanical design
are discussed below.
Micron control of the distance between the collimating lens and the laser's facet
was needed to correct for astigmatism using defocus, as previously mentioned. Two
mechanical parts that provided this control are the mount and threaded barrel of Figure 15.
The threaded barrel that holds the collimating lens is brass and the threaded mount is
aluminum. The threads on the O.D. of the barrel and I.D. of the mount are 80 pitch, class
3A threads. Three angular degrees of rotation is a small rotation to achieve with a steady
hand and a screwdriver, but one degree is difficult without a special lever arm. A three
degree rotation of an 80 pitch thread yields a 2.6/un displacement and a one degree rotation
yields a 0.9tim displacement. A special lever arm was designed and it was decided that
iterative trial and error positioning of the collimating lens would be necessary. Thermal
calculations for a 5mm length of aluminum revealed a displacement of O.lnm/K, an
acceptable value over the expected changes in room temperature. Once in position, the
threaded barrel was held in place by glue. A low viscosity, low shrinkage glue was injected
- into the glue hole. A set screw was not appropriate because tightening the setscrew would
displace the barrel more than a micron. The threaded mount fits into a circular hole in the
barrel of the anamorphic prism beam shaper and is held in place by a set screw. The laser
is attached to the threaded mount via two small screws.
Most of the mounts were machined out of standard 6061-T6 rolled aluminum.
-
40
Mount
Laser Diocle
Threaded Barrel
-Glue Hole
Bean Shaper
•CoUlnating Lens
Figure 15 Threaded Mount and Barrel for Positioning of the Collimating Lens
-
41
The baseplate for the head was made of tooling plate aluminum. This is a cast aluminum (as
opposed to rolled) which is stiffer and has better thermal properties than 6061-T6
aluminum.
The baseplate of the head is attached to the head translation stages through a
semi-kinematic mount. This mount allows removal and remounting of the head without
realignment. The angular repeatable accuracy of the head with respect to the disk was
measured to be 15 seconds of arc.
Many of the mechanical parts were designed on a computer with a mechanical
CAD drawing package. The overlay or layer capabilities of the CAD package proved to be
invaluable, especially for the more complex parts and assemblies. Design error was reduced
through a visual verification of the proper mating of parts.
-
42
ALIGNMENT OF THE OPTICAL HEAD
Optical and Mechanical Alignment
The alignment of the laser and beam shaping assembly was done at WYKO
Corporation with the assistance of the Ladite interferometer. The remaining components
were aligned as they were fastened to the head's baseplate with the aid of an autocollimator
and an alignment aperture. The detectors and actuator were mounted on small xyz-mounts
which were adjusted during operation of the head while viewing signals on oscilloscopes.
The laser was centered in the aperture of the collimating lens by measuring the
beam position at the collimating lens, and then about eight feet away in the +z direction.
Positioning it was not as difficult as trying to compensate for its movement when the two
mounting screws were tightened. Once the laser was centered, the beam was roughly
collimated by inspection of the spot size near and far from the laser. Alignment and
collimation was done at the laser's read power since astigmatism changes with output power
and it is during reading that we are most concerned with beam quality. The following
iterative procedure is outlined with numbered steps.
1) The laser and collimating lens mount were attached to the circularizer and mounted to
the Ladite.
2) The laser and collimating lens mount assembly were rotated with respect to the
circularizer while viewing the intensity pattern in real time. When the intensity pattern was
circular the circularized setscrew fixed the mount's position.
3) Data was taken on the amount of astigmatism and the RMS wavefront error with tilt and
defocus removed.
4) The laser and collimating lens mount were then removed from the circularizer and the
collimating lens barrel was defocused slightly by tapping the threaded barrel to produce a
-
43
small rotation.
5) Go to step one and repeat procedure.
This was done for about 12 iterations until a low value of astigmatism was obtained through
the proper amount of defocus (see results section). The collimating lens position was fixed
by gluing its threaded barrel in place. Tilt and defocus were removed from calculation of
the RMS wavefront error since tilt depends on the alignment of the head with respect to the
disk, and defocus is corrected by the focus servo and actuator.
Rough alignment of the beam on the optical head with respect to various
components was accomplished by passing the beam through an alignment aperture and
watching the reflected beam on the periphery of the aperture. When the beam passed back
through aperture the reflective component was roughly in the right place. The alignment
aperture was a quarter inch diameter rod with a 0.05 inch diameter hole at the optical axis
height. Two alignment apertures were made. The baseplate was designed with mounting
holes for these apertures at proper locations.
The alignment apertures were used to get a component close enough to its
desired position so that a reflection off of the component could be seen in the field of view
of the autocollimator. With the autocollimator, the component could be positioned within
several minutes of arc. Some of the components were shimmed with brass shim stock for
fine angular adjustments. The following list gives the positional accuracy of several head
components. The autocollimator's vertical and horizontal cross hair's angular displacements
are denoted with V and H. WRC implies that the alignment was Within the Resolution of
the Cross hairs (about IS seconds of arc or less). Minutes of arc is denoted by "min".
1) P-polarization out of the laser was ±2 degrees to the horizontal
2) 80:20 PBS - Hi 3 min, V: WRC
3) Actuator mounting flange, over x^-range of travel - H: WRC, V: 3 min
4) A/4 plate - ff: WRC, V: WRC
-
44
5) A/2 plate - H: 3 min, V: 3 min
6) Detector PBS - H: 0.5 rain, V: 0.5 min
7) Disk w.r.t. Beam H: WRC, V: 1 min
Focus and Tracking Detector Alignment
The quadrant detectors were initially aligned in x and y by eye. The focus
servo was aligned by slightly tilting a reflective non-grooved disk and spinning it so it
moved inside and outside of focus. The focus actuator could also have been ramped in and
out of focus with a function generator. With the disk wobbling the actuator was disabled.
All four quadrants of the focus detector were connected to a four channel scope. The
detector's position was varied until four responses of approximately equal magnitude were
seen on the scope. The detector was moved around while trying to get the traces of diagonal
quadrants, (A&C, B&D) to overlap (the cylindrical lens was at 45 degrees to the horizontal).
Overlapping of diagonal quadrants signified that these quadrants were equally illuminated
and that the spot was centered. Quadrant scope traces looked like those shown in Figure 16.
If the diagonal quadrant's traces crossed each other, this signified that the spot was being
translated on the detector as the disk went through focus. This occurs when the beam is not
centered in the objective; the chief ray is displaced from the center of the objective. The
objective was then translated to uncross the traces. This moved the spot on the detector and
the detector was repositioned until the diagonal quadrants' traces overlapped. This method
was iterated until the diagonal's traces overlapped and did not cross. The focus servo was
then able to lock on focus. A detailed and well illustrated method for adjusting an
astigmatic focus servo can be found in reference 15.
To align the tracking servo a reflective non- grooved disk was placed on the
spindle. Each quadrant of the tracking detector was connected to a separate channel of a
-
Figure 16 Typical alignment sequence using a quadrant detector. Both FES and quadrant signals are shown. The alignment progresses from a centered detector with a moving blur (a and b) to a stationary blur that is no longer centered (c and d). At this point the detector needs only to be moved to the center of the blur, and the alignment will be complete.15
-
46
four channel oscilloscope. The detector was centered by equalizing the response of all
quadrants when the beam fell on the detector. A grooved disk was placed on the spindle
and the tracking servo locked up after some adjustments in the electronics.
Alignment of the Beam to the Disk and Related Aberrations
It is critical that the scanning beam be normal to the disk. A tilted disk or
misaligned head can cause read back errors due to aberrations introduced by the disk's
protective layer and clear substrate. These layers, treated as one substrate, act as a plane
parallel plate with a typical refractive index of n = 1.57 and a thickness of T = 1.2mm.18
Coma can be introduced by a decentered or tilted objective lens, but it is much more
sensitive to the angle of tilt between the focused beam and the disk substrate. Astigmatism
is also introduced from this tilt but it is negligible compared to the amount of coma
introduced by the same tilt angle. The expressions for coma (1K31) and astigmatism (Wiz) in
terms of waves are given by:16*™
= t14l
n^r(NA)2^ ll5]
where yS is the tilt angle in radians and T is the substrate thickness. If the coma lobe (see
Figure 17) of the spot on the disk is oriented towards neighboring tracks it can cause cross
talk16 as well as tracking errors." A quarter wave of coma is about the limit that the system
can be tolerate.17 A plot of Wai verses /9 in degrees, is shown in Figure 18. Values from
the head, used in the plot were A=780nm, «=1.57, 7=1.2mm, and NAobj=0.5. The plot shows
that a tilt of 0.4 degrees produces a quarter wave of coma. The amount of astigmatism
introduced at this same angle is -0.003 waves, two orders of magnitude less.
Spherical aberration introduced by the plane parallel plate substrate is given
-
47
Figure 17 Intensity Distribution of a Focused Spot Containing Coma18
-
48
yd
7
0.4 0.6 0.8 1.0 1.2 Beta (degrees)
1.4 1.6
Figure 18 Coma versus Tilt Angle Between the Beam and Disk Substrate
-
49
by:10
- T̂(NA>'
The objective lens is designed to correct for this by introducing the same amount of
spherical aberration of the opposite sign. The amount of spherical aberration present
without correction is very large: 4.5 waves for A=780nm, n=1.57, r=1.2mm, and NAobj=0.5.
Although the objective was designed to correct for this, allowed thickness variations in the
substrate of ±0.05mm18 can produce ± 0.2 waves of spherical aberration. The objective lens
in the Taohs actuator was designed for «=1.5, not m=1.57. This difference combined with
the thickness variations above yields a worst case of 0.29 waves of spherical aberration
introduced. This would reduce the peak intensity of the focused spot a considerable amount
(roughly 20%) if no refocusing was done.
The astigmatic focus servo works to reduce this aberration by introducing
approximately -0.65 waves of defocus for every one wave of spherical aberration.16 The
peak intensity after refocusing is only reduced by about 5%.1B The spot quality is then
considerably better than if no refocusing were done. With spherical aberration introduced in
the reflected beam, the focus servo tends to refocusing more towards the marginal side of
focus than the paraxial side.
-
50
RESULTS
An OPD plot and wavefront data from the WYKO Ladite are shown in Figure
19. The astigmatism value of 0.125 waves in the lower half of the figure was the value that
was monitored and minimized while adjusting the defocus. The RMS wavefront error is
0.029 as measured out of the circularizer. If the RMS wavefront error before the objective
is less than 0.05 it does not have a significant effect on the spot size.19 The intensity
profiles and point spread function are shown in Figures 20 and 21. The horizontal axis of
the intensity profiles is scaled such that 1 => 3mm. The vertical axis is not normalized to
unity at the peak intensity and thus the 1/e2 points are not as indicated. The \/el diameter
is larger than that shown in the figure.
The assembled head is shown in Figure 22. The actuator and objective
assembly is the round object just above and to the right of the watch band. Above and to
the right of this, with wires coming off of it, is the laser diode and built-in modulator. On
the left of the figure three electronics boards can be seen. The larger horizontal board to
the left is a data channel board. The two smaller boards are preamplifier boards for the
quadrant detectors.
At the time of this writing the tester has not been calibrated and thus detailed
test results are not available. The optical head has been shown to read, write, and erase data
with a carrier to noise ratio of 50dB. This carrier to noise ratio was obtained without the
A/4 plate in place and without the laser read current modulated. Both the A/4 plate and the
modulation of the read current will increase the carrier to noise ratio. The current
configuration of the laser driver does not allow operation of the modulator.
-
Neui Data
r ois : 0 . 02 9 u / v : 7 0 0 . 0 n m
0 3 : 4 2 0 7 / 0 9 / 3 9
OPD
-a.eg -0.23
T F
p — v : 0 . 3 1 3 pupil: 3 7'<
O r l « n t * t l o n
• r
C T r o n t 1
a.a?
9, SI
a. i?0
o. 093
- a . BBS
-ft.144 Ul 1, tr-*0 Li -O.144
WYKO
LADITE CVer 4.21] Today's Data: 07/09/89
New Data Tina: 63:42 Oats: 67/09/89
SE]DEL ABERRATION COEFFICIENTS OF KWEFRONT: AMT ANGLE TJ LT
FOCUS AST1C com SPHERICAL
2.275 0.177 0.125 0.077 0.023
-259.8
-38.7 -47.9
LONGITUOINAL ASTIGMATISM 0.9 ua
ABERRATIONS REMOVED: TILT FOCUS
X CENTER V CENTER
129.0 108.5
RADIUS
90.8
PUPIL: 97X
OATfl PTS PEAK
23144 0.176
VALLEY P-V -6.144 0.313
RHS 0.029
ftPOOJZED UEOGE STREHL RATIO 1.00 0.979
WYKO
Figure 19 OPD Wavefront Plot and Wavefront Error Values of The Collimated and Circularized Laser Diode Beam
-
New Da ta 03 :42 07 /09 /89
intensity ;:;,m0,s;s f l p t r t u r > H t d t h : t . A S X P R O r i L E 0 . 7 6 0
0 . 3 B 0 -
0. 000 1 . 0 -0.5 0.0 0 .5 1 . 0
R e l a t i v e D i s t a n c e i n P u p i l
MYKO
Neui Da ta 03 :42 07 /09 /09
INTENSITY p^r-9™ B p i r t u r a H t d t h t 1 . H f l Y P R O F I L 0 . 3 7 4
0 . 2 1 9 -
0. 000 0 .5 1 .0 0 .0 1 . 0 -0 .5
R e l a t i v e D i s t a n c e i n P u p i l
MYKO
Figure 20 Intensity Profiles of The Shaped Beam Note; Vertical Scale is Not Normalized
-
N e t u D a t a
S t r e h 1 : 0 . 9 7 3 5 i : s ! 9 . 3 6 u m f l p o d i z a d
0*J: 42 07/09/89
PSF
l /•
T F
u j » / : ? 9 0 . O n m pupil: 3 7
p l o t I C l I t ! Q . 9 7 Q t t r a h l u n i t s
Q d e g 9 0 d e g
- 4 5 d e g
WYKO
N e w D a t a
3s 0) L © C a TJ
-
54
-
55
CONCLUSION
Operating principles of the head were described in terms of the write path, read
path, and servo path. Design alternatives were then reviewed and design decisions were
justified. Key issues of the optical head design were presented. Among these were an
explanation of reflected pulse phase delay, collimating lens selection for optimum beam
truncation at the objective, and correction of laser diode astigmatism. Alignment issues
were then addressed.
The optical head's capabilities to read, write, and erase data were successfully
demonstrated. The carrier to noise ratio obtained without complete optimization of circuitry
and optics is very promising. Meaningful test results will follow the complete calibration
and interfacing of the system's optics, electronics, and software.
-
APPENDIX A
Commercial Laser Diodes
-
HIGH POWER LASER DIODES 780 nm
Mfg. G=Gain Max 0*@ x Astig Price Pt.# I=Index P™ 6° 0£ Pout A (urn) (U.S.Dol)
Guided (raW) (mW) fom) 1 pc.
HITACHI 7838G I 20 10 26 20 780 146.00
MITSUBISHI ML6702A ! 30 12 23 30 780 175.00
6412N/C I 30 12 21 30 780 175.00 6411A/C 20 12 30 10 780 81.70
TOSHIBA Told
121/122 I 20 10 27 20 780 package 127 I 20 10 27 20 780 121T I 30 9 20 30 780
SHARP LT024 I 30 10 29 20 780 3 260.00 LT024R10 I HF modulation built in 389.00
SONY SLD201U-3 G 50 14 28 40 780 10 180.00 SLD201V-3 G 50 14 28 40 780 40 180.00 SLD201U G 20 13 28 15 780 4-10 SLD20IV G 20 13 28 15 780 40-60
Table 2 780nm Commercial Laser Diodes
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HIGH POWER LASER DIODES - 830 nm
Mfg. & Mndex Max fif@ Pries Part# G-Gain P„ 9® Si P« A Astig (U.S.Dol)
Guided (mW) (mW) (nm) (/ttm) 1 pc.
NEC NDL3I10 30 12 30 830 2 S
hitachi 8351E 50 9 27 50 830 8.5 1,030.00 8318E 40 11 25 40 830 4 560.00 8312E 20 10 27 10 830 256.00 8312G 20 10 27 10 830 256.00 83143E/G 30 10 27 30 830 307.00 83J5E 20 10 27 10 830 1,030.00
qrtel XL303 30 30 30 30 835 1,150.00
MITSUBISHI ML5702A I 30 12 21 30 820 175.00
5412N I 30 12 23 30 820 175.00 5415N I 30 11 26 30 825 407.50
toshiba Told
131/132 I 35 10 27 35 830 131TR I 40 9 20 40 830 131T I31TV I 35 9 20 35 830 13SA I 30 10 27 25 830 136 A I 25 10 27 20 830 137 I 40 10 27 35 830 151/152 I 35 10 27 30 810 153/154 I 25 10 27 20 810 155 A I 25 10 27 20 810 157 I 35 10 27 30 810
sharp LT015 I 40 10 27 30 830 LTO15RI0 I 40 10 27 30 830 LT016 I 40 810
PHILIPS amperex CQL61A G 20 35 40 20 820 20 CQL62A G 40 30 38 40 820 20
SONY SLD202U-3 G 50 13 28 40 820 SLD202V-3 G 50 13 28 40 820 7 SLD20V G 25 13 28 20 820 SO
Table 3 830nm Commercial Laser Diodes
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APPENDIX B
Sharp LT024R10 Characteristics
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60
Optical Power Out VS. Input Current Sharp LT024R10 Laser # 3
o. ro
if).
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61
Sharp LT024R10 Laser Diode No. 3
1.00 -I
oo
— 0.50 -Ld
o.oo TTT ' ' • I | I -40.0-30.0-20.0-10.0 0.0 10.0 20.0 30.0 40.0
THETA PERPENDICULAR
Figure 24 Relative latensity versus Theta Perpendicular
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62
Sharp LT024R10 Laser Diode No. 3
1.00 H
cn
L±J
^ 0.50 -LLI
h ;
0.00 -10.0 -5.0 0.0 5.0 10.0
THETA PARALLEL
Figure 25 Relative Intensity versus Theta Parallel
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APPENDIX C
WYKO Ladite Block Diagram
-
DETECTOR
FOUNNQ ynttvt «T FOCUSN6 LOIS
BEAM SPUTTER ft
FUP-IM BEAU BLOCK| SHUTTER k—if
4?=Q= REFERENCE ARM
COUJUMMB (£N3 FOCUSMO LENS
77ZZZZZZA
TOWAlV
ACCESS APERTURE 'sssssss/y/s/rsssA
Q BEAU BLOCK
TEST ARM
EXTERNAL̂ ALIGNMENT MIRROR
EXTERNAL l/j CENTERHO MIRROR
M k UK ID
FOLDING MIRROR |2
"ilUP-W ^CENTERING LENS
BEAM REIMAGMC LENS H
FUP-IH ALIGNMENT LENS
W BEAM REniACWO LENS |2
VAJOFLE ATTENUATOR
COUJMATMO LENS
rZT MIRROR
REIMAGING ARM
SEAM SHJTIEH f 2
ENTRANCE PUPfL (THEORETICAL POMT I INCH . „«. ™OU EDGE OF INTERFEROMETER) LASER SOURCE POSITION
Figure 26 LADITE Block Diagram20 o •Ct
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65
REFERENCES
1. Sincerbox, G.T., "Miniature optics for optical recording", Gradient-Index Optics and Miniature Optics, Proc. SPIE 935, pg 63-76, (1988)
2. Wolniansky, P., Chase R., Rosenvold, M., Ruane, Mansuripur M., "Magneto-optical measurements of hysteresis loop and anisotropy energy constants on amorphous Tbx Fej_x alloys" J.Appl.Phys.60(l), (1986)
3. Milster, T.D., "Characteristics of Phase-compensation Techniques in Magnetooptical Read-back Systems", for publ. in Polarization Considerations for Optical Systems, SPIE, San Diego, CA (1989)
4. Finkelstein, B.I., "Laser-feedback noise in magneto-optical recording", Optical Storage Technology and Applications, Proc. SPIE 899, pg 69-76, (1988)
5. Ojima, M., Arimoto, A., "Diode laser noise at video frequencies in optical videodisc players", App. Opt. 25(9), 1404, (1986)
6. Arimoto, A., Ojima, M., "Optimum Conditions for the High Frequency Noise Reduction Method in Optical Videodisc Players", App, Opt. 25(9), 1398, (1986)
7. Keppler, and Goldman, "Analysis of Magneto-optic Media for Optical disk drives", Optical Storage technology and Applications, Proc. SPIE 899, pg 69-76 (1988)
8. Mansuripur, M., "Analysis of astigmatic focusing and push-pull tracking error signals in magnetooptical disk systems", App. Opt. 26(18), pg 3981-3986 (1987)
9. Hecht, J., The Laser Guidebook, McGraw-Hill, USA, pg 246-270 (1986)
10. Call, D.E., and Finkelstein, B.I., "Dependence of laser-feedback noise on optical-path length", Optical Data Storage Topcial Meeting SPIE Vol. 1078, pg 272-278 (1989)
11. Sharp Laser Diode User's Manual, Sharp Corporation, Osaka, Japan (1988)
12. Private communication with Blair I. Finkelstein, University of Arizona, Optical Sciences Center, (1989)
13. Private communications with Scott B. Hamilton, Apex Systems, Inc., Boulder, CO (1988)
14. Private communications with Scott DeVore, University of Arizona, Optical Sciences Center. (1989)
15. DeVore, S.L., "Simulation Methods for Optical Disk Drive Functions", Ph.D. Dissertation, University of Arizona (1988)
16. Braat, J., "Read-out of Optical Discs", in Principles of Optical Disc Systems, ed. G. Bouwhuis, Adam Hilger, Bristol (1985)
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66
17. Private communication with Scott L. DeVore, Tom D. Milster, Blair I. Finkelstein, University of Arizona, Optical Sciences Center, (1989)
18. Zajaczkowski, J.S., "Proposed American National Standard, Unrecorded Optical Media Unit for Digital Information Interchange - 130mm Reversible Optical Disk Cartridge", Revision X3B1I/88-094R2, (1988)
19. Private communications with Tom Milster, University of Arizona, Optical Sciences Center. (1989)
20. Instruction Manual for LADITE Laser Diode Tester, WYKO Corporation, Tucson, AZ (1987)