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HA0303T Rev A Feb 2014
Chapter 4
4.2.1
PDQ80A and PDQ30C Quadrant Detector Heads
The PDQ80A and PDQ30C are quad photodiode arrays that provide top-minus-
bottom (Y-Axis alignment) and left-minus-right (X-Axis alignment) difference signals.
Additionally, the sensor provides a signal that is the sum of signals (total beam power)
from all four quadrants. The two difference signals are voltage analogs of the light
intensity difference sensed by the pairs of photodiode elements in the array. These
three signals are sent in analog form to the TPA101 position aligner via its rear panel
DETECTOR IN (6-pin HRS) connector.
Fig. 4.2 Photodiode Array Details
4.2.2
PDP90A Lateral Effect Sensor
The 2D lateral effect position sensors are sensitive to the position of the beam
independent of the power density or beam shape. The sensing element has four
anode electrodes bonded to the surface (A, B, C & D below).
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 proportional to the incident light energy. The
photocurrent is proportionally distributed between the four electrodes on the front
surface relative to the position of the light spot on the sensor. A transimpedance
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
X-axis sensor: (Q2 + Q3) - (Q1 + Q4)
Y-axis sensor: (Q1 + Q2) - (Q3 + Q4)
SUM: (Q1 +Q2 + Q3 + Q4)
Q1
Q2
Q3 Q4
(B+C) - (A+D)
X Diff =
(A+B) - (C+D)
Y Diff =
C
B
D
A
SUM = A+B+C+D
