HP 5501A Operating And Service Manual page 92

3.2 SIMPLIFIED THEORY O F OPERATION
A low-power helium-neon laser (Figure
3-7)
emits a coherent light beam composed of two
slightly different optical frequencies, f, and f , , of opposite circular polarizations. After con-
version to orthogonal linear polarizations the beam is expanded and collimated, then directed
to the reference beam-splitter where a small fraction of both frequencies is split off.
The split off portion of the beam is used both to generate a reference frequency and to pro-
vide an error signal to the laser cavity tuning system. The difference in amplitude of f, and f,
is used for tuning while the difference in frequency between f, and f, (about 2 MHz) i s used
for the reference signal.
The major portion of the beam passes out of the 5501A Laser Transducer to an interferometer.
All HP interferometers measure relative displacement of two reflectors by splitting the beam
into f, and f , , directing them to the two reflectors, and returning the resultant signals to a
common point where they are made to interfere with each other. These combined beams are
directed to a photodetector in the receiver.
Relative motion between the reflectors causes a Doppler shift in the difference frequency
which is detected by the receiver. This Doppler shifted difference frequency (f,
-
f,
+
Af) is
the measurement signal.
Each output accessory (e.g., pulse converter, counter, etc.) receives the reference and mea-
surement signals and compares them cycle-by-cycle in a dual output mixer. The mixer pro-
duces an appropriate up or down output pulse whenever one of the signals gets one-half
cycle ahead of or behind the other. Each pulse corresponds to a reflector movement of one-
quarter wave length of light. The pulses are accumulated in either a customer-supplied revers-
ible counter or counters in one of the output accessories.
- -
Figure 3-7. Simplified Block Diagram