HP 5501A Operating And Service Manual page 104

1
Laser Tube
1 Laser
Beam
-
SUPP~Y~
I
Regulators
I
Tuning1
Control
-
Diagnostics
[
I
LED Status
Indicators
-
Diagnostic
-
Status Lines
Figure 3-8. 5501A Laser Transducer Block Diagram
The laser system measures displacements by looking at the Doppler shift induced by the motion
of the displacements. Both frequencies of laser light come from the laser head to the interfero-
meter, where they are separated optically. One of the two frequency components is directed
to the object whose motion is being measured. There
it
is reflected by a mirror or retroreflec-
tor (cube-corner) and returned to the interferometer. The effect of the motion is to cause a
Doppler shift in the frequency of the reflected component. The two laser light components
now differ in frequency by the original frequency split plus or minus the Doppler frequency
shift. By integrating the total positive and negative excursions in the difference frequency,
the system can find the net displacement in wave lengths of laser light.
The primary advantage of the two-frequency "ac" measurement system is its large dynamic
range. For example, in the laser transducer system, the nominal output power is 120 micro-
watts, but the power returned to the laser receiver can be a s low a s four microwatts and the
laser will continue to operate the system. The 5501A Laser Transducer System takes advant-
age of
this
wide dynamic range by moving the receiver from inside the laser head, where
space considerations would limit the number of receivers, to the outside world. As a result,
a single laser head can drive up to six linear measurement axes. Furthermore, the remote
receivers can be mounted wherever most convenient; the laser light need not be returned to
the laser head.
The laser head provides TTL-logic-level diagnostic signals that indicate the status of its auto-
tune circuit and supply voltages to the system controller. Eight rear-panel light-emitting
diodes give the same information in visible form. A RETUNE pushbutton provides for manual
tuning whenever the laser exceeds its auto-tune range. The same function can also be com-
manded by an external controller.
For additional information and schematics, refer to the 5501A Laser Transducer Operating
and Service Manual.