Appendix C
39
HA0114T Rev 10 May 2019
C.1.2 How the NanoTrak Maximizes Optical Power
To understand the principles involved in the alignment, consider the operation in a
single axis. When two fibers are aligned, maximum power is transmitted between
them, and if the fiber is moved a few microns to the left (Fig. C.3, position A), the
power is reduced. Furthermore, oscillating the fiber by a few microns causes the
detected power signal to assume an a.c. component whose amplitude is proportional
to the amplitude of the mechanical oscillation (modulating signal).
Fig. C.3 AC component of optical power
The received power signal contains a frequency component at the scanning
frequency, and the phase of this (relative to the circle scan voltages) gives information
on which direction to move.
When the circle is centred in the ideal position, the power signal contains no
modulation at the scanning frequency. However, at position A, the signal has a phase
difference with respect to the modulating signal. The power signal is Fourier analysed
and the result of the computation is passed to an algorithm which drives the circle to
the correct position.'
However, piezo transducers are unipolar and to achieve a positive and negative
adjustment either side of center, the piezo must be at its center of travel when the
error signal is zero. This is achieved by adding an offset voltage to the error signal.
0.0
0.5
1.0
position A
cou
ple
d p
ow
er
cou
ple
d p
ow
er
0
5 µ
m displacement
0.0
0.5
1.0
0
center
