6
Chapter 2. Connections and controls
2.1.2 Ramp control
The internal ramp generator provides a sweep function for scanning
the laser frequency typically via a piezo actuator, diode injection
current, or both. A trigger output synchronised to the ramp is pro-
vided on the rear panel (
TRIG
).
INT/EXT
Internal or external ramp for frequency scanning.
RATE
Trimpot to adjust internal sweep rate.
BIAS
When
DIP3
is enabled, the slow output, scaled by this trimpot, is
added to the fast output. This bias feed-forward is typically re-
quired 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
MOGL
abs
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 loop variables allow the gain of the proportional, integrator
and differentiator stages to be adjusted. For the integrator 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
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.
