Third Day Poster Session, 17 June 2010 - NanoTR-VI

Poster Session, Thursday, June 17
Theme F686 - N1123
Nanodistance Measurement based on CD pick-up Head using Auto Offset Canceller Amplifier
Pezhman Sasanpour 1 , Mohammad Haghnegahdar 2 , Milad Taherian 2 and Bizhan Rashidian 1,2*
1 Institute for Nanoscience and Nanotechnology, Sharif University of Technology, Tehran, Iran
2 Department of Electrical and Electronics Engineering, Sharif University of Technology, Tehran, Iran
Abstract— We have designed and implemented system for detection of linear movement detection below 30 m. In linear
regime focus error signal can be exploited as a criterion of displacement. In this regard optical pick-up head of CD drives and
its quadrant photodiode and laser system can be used as a reliable system. In order to improve operation of system independent
of environment light, we have added an additional system consisting amplifier + offset canceller after the module in order to
omit background light noise and accordingly enhance the sensitivity and resolution of system.
Precise displacement detection is one of important issues
in the nano science and nano technology. There are different
ideas for detecting small scale movement with different levels
of accuracy. Among those, interferometery and capacitance
sensors can be mentioned [1-3]. Using position sensitive
photodiodes and embedded quadrant photodiode system of
used in pickup head of optical disk drives, has found lots of
potential applications [4-6]. Considering cost efficiency and
easily access, in addition to accuracy and sensitivity, we have
implemented and improved a method of movement detection
based on quadrant photodiode system used on optical pick-up
head of CD drives.
The main idea in this method is exactly the same used in
the pickup head of CD/DVD for focus error detection and
correction. In a usual pickup head light beam emitted from a
laser diode after spreading over a grating is divided into 3
separated light beam that are directed on the CD surface. Light
beam reflected from CD surface will then be directed onto the
center of quadrant photo diode. If position of CD is exactly on
focus plane of laser beam, reflected light beam on the quad
photo diode will have shape of circle. Otherwise reflection of
beam on the surface of photo diode will not be a complete
circle and it will be similar to ellipse. This elliptic figure will
produce different levels of power on each of photo diode.
Focus error of CD in this system is defined (A+C)-(B+D).
As it has been shown in figure 1, according to direction,
focus error signal (FES) could be positive or negative. By
using the FES in its linear regime, this system can be used as a
method for detection of displacement in linear direction.
c
a
A
D
B
C
(A+C)
(B+D)
b
(A+C)-(B+D)
FE>0
d
A
D
B
C
(A+C)
(B+D)
A
D
B
C
(A+C)
(B+D)
(A+C)-(B+D)
FE=0
(A+C)-(B+D)
Figure1: (a) Microscopy image of quadrant photodiode (QPD)module
used in optical pick-up head. (b) FES for Object in Focus plane.
(c) FES for Object below focus plane. (d) FES for Object above focus
plane.
Figure 2 shows result of QPD signal obtained by moving
object. As it has been depicted linear region in this system is
30 m.
FE