Optical feedback height control system using laser diode sensor for near-field data storage applications

Jen-Yu Fang, Chung-Hao Tien, Han-Ping D. Shieh, Senior Member, IEEE, Philipp Herget,

James A. Bain, and T. E. Schlesinger, Senior Member, IEEE

Abstract—We demonstrated a laser diode position sensor for

a near-field height control system. The feedback signal of the

laser diode sensor that resulted from self-mixing interferometry

was characterized and modeled by a simplified method. Due to

the fine spatial resolution of the laser sensor, an active height

control system with nanoscale position precision was designed and

realized under a spinning disk for near-field applications.

Index Terms—Data storage, laser sensor, nanoscale active

control, self-mixing interferometry.

I. I

N

EAR-FIELD data storage systems approach the medium

with nanometer spacing to break through the diffraction

limit that conventional focusing systems have done. Therefore,

a precise gap detecting system and a servo control system are

of critical importance to make near-field recording feasible. Air

gap servo systems employing the dependence of

evanescent-wave penetrating efficiency on the gap between the solid

im-mersion lens (SIL) and the disk surface were reported [1]–[5].

However, external optical systems and signal processing

sys-tems are necessary for separating air gap signals from the

reflected optical power from the SIL. In contrast, laser diode

position sensors that employ the dependence of the laser output

on the distance between the laser and the target can detect

the displacement by monitoring the power modulation of the

laser without external optical components [6]–[8]. Therefore,

the advantages of compact package and flexibility of

inte-gration with other systems make laser diode sensors widely

used for displacement, velocity, and acceleration measurement

Manuscript received May 28, 2007.

J.-Y. Fang is with the Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, R.O.C. (e-mail: [email protected]).

C.-H. Tien and H.-P. D. Shieh are with the Department of Photonics and Display Institute, National Chiao Tung University, Hsinchu 300, Taiwan, R.O.C. (e-mail: [email protected]; [email protected]).

P. Herget was with Carnegie Mellon University, Pittsburgh, PA 15213 USA. He is now with the IBM Almaden Research Center, San Jose, CA 95120 USA (e-mail: [email protected]).

J. A. Bain and T. E. Schlesinger are with the Data Storage Systems Center, Carnegie Mellon University, Pittsburgh, PA 15213 USA (e-mail: jbain@ece. cmu.edu; [email protected]).

Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JLT.2007.909349

with millimeter-scale precision. However, less attention was

paid to apply laser diode sensors in the servo control field,

particularly near-field applications. Thus, we demonstrated a

height control system for near-field storage applications with

nanoscale precision by employing a laser diode position sensor

to detect the distance between the sensor and a spinning disk

[9], [10]. In this paper, we characterized the signal as a function

of the distance between the laser and the target. A simplified

method was derived to model the output signals and to compare

them with the measured signals. According to the characteristic

signal, the configuration and design of a servo control system

were presented. The operation of the system under a spinning

disk was demonstrated.

II. S

C

To achieve nanoscale position precision, the feedback signal

has to be characterized as a function of the spacing. Many

studies presented theoretical models, but most of them

empha-sized on large spacing detection; therefore, multiple reflection

was ignored. However, in near-field storage systems, different

assumptions should be considered. The target surface in a

storage system is a disk covered by a metal film with high

re-flectivity. In addition, the spacing between the laser sensor and

the target surface is only within several wavelengths. These two

conditions implied that multiple reflections should be taken into

consideration in the calculation of near-field output modulation.

Consequently, we presented a simplified model to characterize

the laser sensor signal with a near-field gap.

We considered a laser diode of length L with an external

cavity of length d, as shown in Fig. 1(a), where r

, r

, and

r

denote the amplitude reflection coefficients of the laser

facets F

and F

and the external reflector F

, respectively.

If we assume that the reflector is only a few wavelengths

away from the laser, i.e., d

L, the effect of the

external-cavity modes due to the presence of the external external-cavity can

be ignored [11]. The coupling condition from the reflected

light into the laser cavity alters the effective reflectivity of the

laser-emitting facet and then results in the interference with

the field inside the cavity. A complex coupling factor, which

consists of a phase and an amplitude factor, was employed to

describe the coupling condition [12], [13]. The effective

reflec-tivity in terms of reflection coefficients and coupling coefficient

Fig. 1. Configurations of (a) external-cavity laser diode and (b) equivalent laser diode.

was used to present an equivalent laser diode, as shown in

Fig. 1(b). Thus

r

= r

−

(1

− R2

)

r

C

(

−r2

· r3

· e

)

(1)

where the phase shift Φ resulted from the gap d, i.e.,

φ =

4π

· d

λ

(2)

and the power reflectivity of the laser diode facet R

, i.e.,

R2

=

|r2|

.

(3)

The coupling coefficient C

represents the ratio of reflected

light coupling into the laser diode at the nth reflection. Because

the reflectivity coefficient term decreases exponentially and

much more rapidly than the coupling coefficient does as the

reflection increases, we assume that the reflectivity coefficient

term dominates the amplitude and the coupling coefficient is

a function of the spacing d and independent of the reflection.

Then, we can derive the effective reflectivity as follows:

r

≈ r2

−

(1

− R2

)

r2

· C ·



−r2

· r3

· e



= r

+

(1

− R2

)

· C · r3

· e

(1 + r

· r3

· e

)

.

(4)

The coupling coefficient C is in terms of an amplitude factor

and a phase factor. The amplitude factor represents the fraction

of reflected light that can couple back to the cavity, and the

phase factor is a phase shift at the coupling interface. Therefore,

the coupling coefficient can be written as

C = A(d)

· e

_{= C}

·





1 − e



(

(

))





· e

(5)

where C

is a proportional constant.

Fig. 2. Simulated self-mixing interferometric fringe.

If we assume a constant drive current applied to the laser

diode, the output power can be represented in terms of the

applied current I and the threshold current I

, i.e.,

P

∝ (I − Ith

)

∝

P

− ln



1

√

R1

· R



(6)

where the power reflectivities R

and R

are given by

R1

=

|r1|

(7)

R

=

|r

|

(8)

and P

is a constant. In the case of r

= 0.99, r

= 0.9, r

=

0.8, λ = 0.635 µm, and w

= 1 µm, we calculated the power

output as a function of the spacing between the laser and the

target surface according to the previously derived equations, as

shown in Fig. 2.

To measure interferometric signals, a silicon wafer coated

with an aluminum film was placed in close proximity to the

output facet of a laser diode with a wavelength of 635 nm. The

laser diode was fixed in a mount attached to a piezoactuator

that provided a back-and-forth motion to generate the

self-mixing interferometric signal. The experimental setup is shown

in Fig. 3(a). Since the optical misalignment caused signal

distortion, we kept the angular alignment precision between the

two surfaces within 50 µrad.

The monitoring current of the photodiode was converted into

a voltage signal, as shown in Fig. 3(b), at a sampling rate of

10 ks/s. The signal is a periodic function of the distance with

a maximum amplitude of 0.37 V, and the pitch, which is a

complete interferometric fringe, corresponds to a displacement

of λ/2, which is 317.5 nm in this case. Since the absolute

value of the measured signals was highly dependent on the

gain of the entire system, a comparison of the measured

sig-nals and the simulation model was made by normalizing the

measured signal to unity and converting it to a function of the

relative position in terms of wavelength. Fig. 4 clearly shows

that the simplified model agreed well with the measurement.

Then, the slope of the signal, the sensitivity of the signal to

the position variation, as well as the minimum displacement

resolution that the laser sensor can achieve, can be obtained by

finding the derivative of the signal function. According to the

signal amplitude and the pitch, the slope at the half-maximum

Fig. 3. (a) Experimental setup for generating self-mixing inteferometric sig-nals. (b) Measured signal from the photodiode.

Fig. 4. Comparison of the calculated and measured self-mixing interferomet-ric signals.

level of this laser sensor is 6 mV/nm. Compared with the

noise level of this system, an accuracy of less than

±1 nm

can be theoretically achieved, which is competitive with other

control methods.

III. A

H

C

S

A. Configuration of Control System

The laser-sensing control system for near-field

applica-tions consisted of a laser sensor, an actuator, drive circuits,

and a controller. The laser sensor was obtained by modifying a

commercial laser diode with a wavelength of 635 nm. Because

of the compact package, the laser diode sensor was able to

be installed on a conventional biaxial pickup actuator. The

monitoring output power detected by the photodiode inside the

B. Controller Design

The open-loop frequency response of the actuator consisting

of the pickup with the laser sensor and the pickup drive circuit

was measured to characterize the dynamic performance of

the control plant. As shown in Fig. 6(a), the first resonance

frequency is 43 Hz, and the transfer function of the actuator

was modeled accordingly. Then, the step response of the pickup

was calculated, as shown in Fig. 6(b). The settling time was less

than 0.1 s, whereas the overshoot was around 60%.

According to the sensitivity of the feedback signal and the

frequency response of the actuator, a PI controller was designed

for a spinning disk at a rotation speed of 1200 r/min or greater

and with a runout of 20 µm or less. The controller was

digi-tally implemented in a dSPACE system at a sampling rate of

100 kHz. In Fig. 7, the dashed curve shows the designed

open-loop dynamic characteristics of the active height controller

system. From the calculated data, the bandwidth is 6 kHz, and

the phase and gain margins are 52

and

−24 dB, respectively.

The measured results showed that the controller meets the

design requirements and the measured open-loop frequency

response agrees well with the design.

IV. E

R

The control system was evaluated by means of an actuated

surface test system, as shown in Fig. 8(a). A silicon wafer

coated with an aluminum film was attached onto a piezoactuator

in proximity to the laser sensor and the two surfaces aligned

parallel to each other. The movement of the silicon wafer was

synchronized with the vertical movement of a spinning glass

disk by converting a real-time displacement signal from an

interferometer to a drive signal. The vertical runout at the radial

position of 50 mm was 16 µm at a rotation speed of 1500 r/min,

whereas the actual displacement of the silicon wafer was

4.8 µm due to the gains of the interferometer and the

piezoactu-ator. The experimental result in Fig. 8(b) showed the capability

of the control system using a laser sensor to precisely follow

the motion of the silicon wafer and reduce the residual position

error to

±3 nm when the laser sensor was in proximity to the

target surface within several wavelengths.

We then demonstrated the control system operation under a

spinning disk. As shown in Fig. 9(a), the pickup with the laser

sensor installed was mounted on a stage that can compensate

the angular variation of the disk surface. A 120-mm-diameter

glass disk coated with an aluminum film was clamped on an

air-bearing spindle. By illuminating an open window at the

disk edge, the orientation of the pickup was adjusted until two

reflected beams were parallel, ensuring alignment of the laser

Fig. 5. Block diagram of near-field height control system employing a laser sensor.

Fig. 6. (a) Measured open-loop frequency response of the actuator. (b) Calcu-lated step response of the actuator according to the modeling transfer function.

to the disk surface. Fig. 9(b) illustrates the error signal, pickup

drive signal, and feedback signal when the servo was in

closed-loop operation at rotation speeds up to 1500 r/min, i.e., the

linear velocity at the servo radius of about 7.8 m/s and at the

vertical runout of 16 µm. In this case, the residual position

error was

±9 nm, and the system functioned without a collision

between the laser sensor and the disk. However, because of the

relatively low amplitude of the original signal and the noise

induced by the circuits, the result was inconsistent with the

theoretical accuracy of

±1 nm and even higher than that of ±3

to

±5 nm in air gap servo systems. Since the noise frequency

was much higher than 100 kHz, the residual error was able to be

further reduced to

±4 nm by means of a signal amplifier with a

bandwidth of 100 kHz for filtering out noise and increasing the

signal-to-noise ratio.

Fig. 7. Open-loop frequency response of the active height control system.

Fig. 8. (a) Configuration of an actuated surface test system. (b) Experimental results when the displacement is 4.8 µm at 1500 r/min.

V. C

We demonstrated a laser diode position sensor for a

near-field height control system. The system employing the laser

sensor can approach the surface of a spinning disk within

Fig. 9. (a) Configuration of a spinning disk test system. (b) Experimental results with the runout of 16 µm at 1500 r/min.

a few wavelengths with a position accuracy of

±4 nm. In

addition, the dependence of the output power on the

near-field distance from the laser sensor was also theoretically and

experimentally characterized. A simplified method for

near-field self-mixing interferometry was presented to model the

signal characteristic in the near-field regime. The system we

proposed and realized had advantages of simple configuration

and high flexibility of integration with other systems such as

subwavelength apertures, very small aperture lasers, and

fiber-based light-source module [14]–[18]. Therefore, it opens an

alternative approach to achieve high-precision near-field height

servo control and can be applied in any fields where nanoscale

position precision and gap are required, such as near-field data

storage and lithography.

R

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Jen-Yu Fang received the B.S. and M.S. degrees in mechanical engineering

from the National Taiwan University, Taipei, Taiwan, R.O.C., in 1995 and 1997, respectively. He is currently working toward the Ph.D. degree in electrooptical engineering at the National Chiao Tung University, Hsinchu, Taiwan.

He was with Microbase Technology Co., Ltd., Taiwan, where he worked on microfabrication technology from 1998 to 2000, and with Infodisc Technology Co., Ltd., Taiwan, where he worked on optical storage technology from 2000 to 2002, both as a Senior Engineer. He joined Carnegie Mellon University, Pittsburgh, PA, as a Visiting Research Scholar from 2005 to 2006. His research interests include integrated pickups, near-field optics, and microoptical design and fabrication.

Dr. Fang was the recipient of the International Symposium on Optical Memory Excellent Student Award in 2004.

Chung-Hao Tien received the B.S. degree in communication engineering and

the Ph.D. degree in electrooptical engineering from the National Chiao Tung University, Hsinchu, Taiwan, R.O.C., in 1997 and 2003, respectively.

After his work as a Postdoctoral Research Staff with Carnegie Mellon University, Pittsburgh, PA, he joined the Department of Photonics and Display Institute, National Chiao Tung University, as an Assistant Professor in 2004. His current research interests are in optical data storage and nonimaging optics.

Han-Ping D. Shieh (S’79–M’86–SM’91) received the B.S. degree in physics

from the National Taiwan University, Taipei, Taiwan, R.O.C., in 1975 and the Ph.D. degree in electrical and computer engineering from Carnegie Mellon University, Pittsburgh, PA, in 1987.

He joined the National Chiao Tung University (NCTU), Hsinchu, Taiwan, as a Professor with the Institute of Electro-Optical Engineering and the Mi-croelectronics and Information Research Center in 1992, after his work as a Research Staff Member with the IBM T. J. Watson Research Center, Yorktown Heights, NY, in 1988. He is currently the Dean of the College of Electrical and Computer Engineering and the AU Optronics Chair Professor with NCTU. He founded the Display Institute at NCTU in 2003: the first such kind of graduate academic institute in the world dedicated to display education and research. He has been holding a joint appointment as a Research Fellow with the Center for Applied Sciences and Engineering, Academia Sinica, Taipei, since 1999. In 2004, he was appointed as a co-Principal Investigator of the “Display Science and Technology Large-Scale Project,” which is a national project to drive Taiwan display into new era. He is the author or coauthor of more than 100 papers published in international journals. He is the holder of more than 30 patents. His current research interests are in display, optical MEMS, nanooptical components, and optical data storage technologies.

Prof. Shieh has been a Fellow of the Society for Information Display (SID) since 2005 and a Fellow of the Optical Society of America since 2006 for his many novel applications of optics and microoptics for projection and liquid-crystal displays and for his contributions to display education. He currently serves as a Director of the SID and has served as the Program Chair and Committee Member of organized conferences in major data storage (e.g., International Symposium on Optical Memory, Magneto-Optical Recording International Symposium, International Magnetics Conference, Optical Data Storage Conference, and Asia–Pacific Data Storage Conference) and display (e.g., Society for Information Display International Symposium, International Display Research Conference, Asian Symposium on Information Display, and Flat Panel Display Exposition).

Philipp Herget received the B.S. degree in electrical engineering from

Worcester Polytechnic Institute, Worcester, MA, in 1997 and the M.S. and Ph.D. degrees in electrical and computer engineering from Carnegie Mellon University, Pittsburgh, PA, in 1999 and 2004, respectively, where he worked in the fields of optical and magnetic data storage with the Data Storage Systems Center.

He is currently with the IBM Almaden Research Center, San Jose, CA, where he is working on advanced magnetic tape head technology.

James A. Bain received the B.S. degree from the University of Pennsylvania,

Philadelphia, in 1988 and the M.S. and Ph.D. degrees from Stanford University, Stanford, CA, in 1991 and 1993, respectively, all in materials science and engineering.

He is a Professor with the Department of Electrical and Computer Engineer-ing, Carnegie Mellon University (CMU), Pittsburgh, PA. He also holds a cour-tesy appointment with the Department of Materials Science and Engineering and is an Associate Director of the Data Storage Systems Center (DSSC). The aforementioned departments and DSSC are part of the College of Engineering, CMU. He currently leads the tape recording research thrust within the DSSC. He has coauthored more than 40 papers in the field of magnetic thin films and magnetic recording. His research interests are in the areas of thin-film magnetic materials and devices, and magnetic recording systems.

Dr. Bain is a member of the Materials Research Society and the IEEE Magnetics Society.

T. E. Schlesinger (M’87–SM’91) received the B.Sc. degree in physics from

the University of Toronto, Toronto, ON, Canada, in 1980 and the M.S. and Ph.D. degrees in applied physics from the California Institute of Technology, Pasadena, in 1982 and 1985, respectively.

He is a Professor and the Head of the Department of Electrical and Computer Engineering, Carnegie Mellon University (CMU), Pittsburgh, PA. Prior to this, he was the Director of the Data Storage Systems Center and was the Founding Co-Director of the General Motors Collaborative Research Laboratory, CMU. He is also currently the Director of the Center for Memory Intensive Self-Configuring Integrated Circuits, CMU. His work and the work of his students are of direct interest to a number of industrial partners. He has published more than 200 archival journal publications and invited and contributed conference presentations. He is the holder of ten patents. His research interests are in the areas of solid-state electronic and optical devices, nanotechnology, and information storage systems.

Dr. Schlesinger is a Fellow of the International Society for Optical Engineers (SPIE). He was the recipient of a number of awards and honors, including the R&D 100 Awards in 1999 and 1998 for his work on nuclear detectors and electrooptic device technology, the Carnegie Science Center “Scientist” Award in 1998, and Benjamin Richard Teare Award for Teaching from the Carnegie Institute of Technology in 2001.