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Model 5501A
Theory of Operation
3-12. The laser beam, containing the vertically polarized f, (the lower of the two frequencies) and the
horizontally polarized f, components, passes through a collimating telescope. This device consists of a
compound lens which spreads the extremely narrow laser beam into a parallel 0.28 inch (7 mm) output
beam.
3-13. A device called a beam splitter diverts a small portion of the output beam and routes this sampletoa
polarizing beam splitter. This splitter partially separates and applies the f, and f, signals to the PZTcontrol
circuit. The PZTcontrol circuit comparesthesignal level of thef, and f,samples. If thelevelsof thesesamples
are not equal, an appropriate (dc)
PZT
control voltage i s generated to tune the laser tube and cause
equalization of the f, and f,components. In addition to providingthisautomatictuningcontrol,thecontrol
circuits extract the difference frequency between f, and f, and generate an electrical reference measurment
signal for use by the transducer accessory equipment.
3-14. Additional circuits within the laser head monitor PZT voltage, laser current, and performance of the
automatic tuning circuits, a s well as external operating power inputs. These circuits drive fault lights and
provide diagnostic signals to accessory equipment. A retune command signal,from accessory equipment or
manually activated at the laser head, interrupts the automatic PZT tuning and forces the laser to tune to the
center of its mechanical range for
3
seconds. Control is then returned to the automaticcontrol circuits. This
retune capability is provided to bring the laser operating frequency within the range of automatic control
when, due to extreme environmental changes, the laser attempts to tune beyond the automatic tuning
range.
3-15.
GENERAL FUNCTIONAL DESCRIPTION
3-16. The following text provides a detailed functional analysis of laser head operation. All descriptions
relate to Figure
7-4
unless otherwise specified. Operation of the laser head can be grouped into the
following functions:
Power Distribution
Laser Current Regulation
Automatic (PZT) Tuning
Control and Diagnostic Monitoring
3-17.
Power Distribution
3-18. When +15 Vdc and -15 Vdc are applied to the laser head, the POWER ON light-emitting-diode
indicator (on the A1 Connector Board) is forward biased and illuminates. The dc input potentials also
produce a virtual ground level at the junction of two 15K ohm resistors. This balanced condition results in an
open circuit at both outputs of the comparator switch. An unbalanced condition exists when either the
positive or negative dc input deviates from 15 volts by approximately 1 volt or more. This offset causes a
corresponding comparator switch output to go to ground, resulting in a lit UNBAL indicator. Table 3-1 lists
the unbalanced dc voltage conditions and the resulting unbalanced indications.
Table
3-7.
DC Unbalanced Failures
3-19.
If either the +I5 Vdcor -15 Vdc two-amp fuse opens, current is diverted through the associated LED
indicator, turning that fuse indicator on.
DC Input Voltage
+I5 Vdc
-15 Vdc
Unbalanced Condition
(See Note)
High (more positive)
Low (less positive)
High (more negative)
Low (less negative)
Unbalanced Indicators
+15V UNBAL -15V UNBAL
ON
OFF
OFF
ON
OFF
ON
ON
OFF
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