12 Chapter 2. Connections and controls
2.1.2 Ramp control
The internal ramp generator provides a sweep function for scanning
the laser frequency through a piezo actuator, diode injection current,
or both. A trigger output synchronised to the ramp is provided on
the rear panel (TRIG).
INT/EXT Internal or external sweep mode.
RATE Trimpot to adjust internal sweep rate.
BIAS When DIP3 is enabled, the slow output is added to the fast out-
put as scaled by this trimpot. This bias feed-forward is typically
required when adjusting the piezo actuator of an ECDL to prevent
mode-hopping. However, this functionality is already provided by
some laser controllers (such as the MOGLabs DLC) and should only
be used when not provided elsewhere.
SPAN Adjusts the ramp height, and thus the extent of the frequency sweep.
FREQ OFFSET Adjusts the DC offset on the slow output, effectively providing a
static shift of the laser frequency.
2.1.3 Loop variables
The gain of each proportional, integrator and differentiator stage
can be adjusted. For integrators and differentiator stages, the gain
is presented in terms of the unit gain frequency, sometimes referred
to as the corner frequency.
SLOW INT Corner frequency of the slow servo integrator; can be disabled or
adjusted from 25 Hz to 1 kHz.
SLOW GAIN Additional single-turn slow servo gain; from −20 dB to +20 dB.
FAST INT Corner frequency of the fast servo integrator; off or adjustable from
10 kHz to 2 MHz.
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