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HA0373T Rev Ck May 2017
Chapter 4
One cathode electrode is connected to the back surface, which is covered with
resistive material. When radiation is incident upon the sensor, a photocurrent is
generated at that position and it is propo rtional to the incident light energy. The
photocurrent is pro portionally distributed between the four electrodes on the front
surface relative to th e position of the light spot on the sen sor. A tra nsimpedance
amplifier is used to create measurable voltages. These values are used to calculate
the position using the equations below.
Fig. 4.3 Lateral Effect Sensor
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 right hand quadrant of the detector. In this case,
the XDiff and YDiff signals may be ‘-4’ and ‘2’ respectively, with a sum value of ‘8’.
Note
In the general mathematical convention of drawing graphs, 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.
(B+C) - (A+D)
X Diff =
(A+B) - (C+D)
Y Diff =
C
B
D
A
SUM = A+B+C+D
X-axis sensor: (Q2 + Q3) - (Q1 + Q4), so XDIFF = (1 + 1) - (4 + 2)
Y-axis sensor: (Q1 + Q2) - (Q3 + Q4), so YDIFF = (4 + 1) - (1 + 2)
SUM: (Q1 +Q2 + Q3 + Q4) = 8
1
2
4
1
