40
NanoTrak Autoalignment Controller
Now, the range of the piezo is typically 20 to 30 µm, while the fiber is oscillated by 2
or 3 microns. Therefore, it can be seen, that by varying this offset voltage we can
adjust the position at which the oscillation of the fiber takes place - see Fig. C.4
detail A.
Fig. C.4 Offset voltage and fiber position
In reality, the fiber is oscillated both horizontally and vertically, with the modulating
signal for Y phase shifted by 90° with respect to X - see Fig. C.4 detail B, which
produces a circular oscillation. The detected signal is compared with both the
horizontal and vertical phases of the modulating signal which results in both horizontal
and vertical error signals.
As mentioned previously, the polarity of the error signals corresponds to the relative
position of the two fibers and consequently, the signals can be used to drive the
piezos until the fibers are aligned and the error voltages fall to zero.
Once the NanoTrak has found the position of maximum power, if one of the fibers is
moved slightly (due to mechanical creepage or thermal drift), the NanoTrak detects
the subsequent drop in power and corrects for the movement to maintain the
maximum. This is called
tracking
.
piezo drive voltage
0.0
75
offset = 37.5V (mid position)
fiber position in X axis (μm)
mid position (37.5V)
increased offset V
time
piezo drive voltage
0.0
75
time
detail A
detail B
x axis
y axis
0
30
X axis (μm)
Y axis (μm)
0
