21
T-Cube Position Aligner
4.2.3
Use of the Sum and Difference Signals for Alignment
As previously described, the difference signals indicate the beam position on the
detector in the X and Y axes. The sum signal is used to ‘normalize’ the difference
signals in relation to beam power. For example, consider the situation below, where
the beam is positioned in the upper left hand quadrant of the detector. In this case,
the XDiff and YDiff signals may be ‘8’ and ‘2’ respectively, giving a sum value of ‘10’.
Fig. 4.4 XDiff, YDiff and Sum Example
If the beam power were to be reduced by half, due perhaps to the fitting of a filter, the
XDiff and YDiff values would then be ‘4’ and ‘1’ respectively. Viewed in isolation, these
values indicate that the spot has moved nearer to the center. It is only when the values
are ‘normalized’ and considered together with the sum value, now ‘5’, that the true
picture becomes apparent.
The sum output signal may be used to provide preliminary alignment of the sensor to
the source light beam. First, the beam position is adjusted for maximum sum output
signal. Second, the beam position is adjusted until the X-Axis (XDIFF) and Y-Axis
(YDIFF) difference signals are at minimum. This procedure results in the beam being
centered on the quad photodiode array. See Section 4.5.3.
for a description of how
this procedure is implemented using the TPA101 position aligner cube.
Note
Throughout this document, references to the XDIFF and YDIFF signals
follow the general mathematical convention of drawing graphs, so the
positive values are on the right hand side and upper half of the X-Y plane.
However, as a voltage coming from the sensor (and measured on the rear
panel SMA output connectors in monitor mode), XDIFF is inverted, so the
voltage becomes more negative as the beam is moving from left to right.
The equations describing the detector heads also show this fact.
X-Dif = 8
Y-Diff = 2
SUM = 10
