22 Chapter 3. Operation
3.2 Achieving an initial lock
The SPAN and OFFSET controls of the FSC can be used to tune the
laser to sweep across the desired lock point (e.g. cavity resonance)
and zoom in on it. The following steps are illustrative of the process
required to achieve a stable lock. Values listed are indicative, and
will need to be adjusted for specific applications. Further advice on
optimising the lock is provided in §3.3.
• Connect the error signal to the “A IN” input on the back-panel.
• Ensure that the error signal is of order 10 mVpp.
• Set INPUT to “∆” (offset mode) and CHB to “0”.
• Set MONITOR 1 to “FAST ERR” and observe on an oscilloscope.
Adjust the ERR OFFSET knob until the DC level shown is zero
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.
• Reduce the FAST GAIN to zero.
• Set FAST to SCAN+P, set SLOW to SCAN, and locate the reso-
nance using the sweep controls.
• Increase FAST GAIN until the error signal is seen to “stretch
out” as shown in figure 3.2. If this is not observed, invert the
FAST SIGN switch and try again.
• Set FAST DIFF to “OFF” and GAIN LIMIT to 30. Reduce FAST INT
to ∼100 kHz.
• Set FAST mode to “LOCK” and the controller will lock to the
zero-crossing of the error signal. It may be necessary to make
small adjustments to FREQ OFFSET and ERR OFFSET.
• Optimise the lock by adjusting the FAST GAIN and FAST INT
while observing the error signal. It may be necessary to relock
the servo after adjusting the integrator.
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If there is no need to adjust the DC level, the COUPLING can be set to DC and
the ERROR OFFSET knob will have no effect, preventing accidental adjustment.
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