MOGlabs DLC202, Manual, Page: 33 / 82

3.2 Laser frequency control 23
One, two, or three piezo elements can be controlled. Typically, only
a single “stack” actuator, such as the Tokin
AE0203D04
(available from
Thorlabs, www.thorlabs.com), will be required. The single stack ac-
tuator allows frequency scanning and frequency offset selection, and
active slow (up to
≈ 100 Hz) feedback. A second actuator, typically
a disc, can be added for faster active feedback control.
From Rev 5, the laser headboard has two stack outputs, driven in
parallel, with a trimpot adjustment of the voltage ratio between the
two. This control allows adjustment of the effective grating pivot
point to optimise the mode-hop-free scan range. The two actuators
can be driven equally so that only the cavity length will vary, with-
out rotation, or with a large ratio so that the grating rotates while
translating. See appendix F for more information.
0V
0V
120V
time
TRIG
STACK
5V
FREQUENCY
SPAN SPAN
Figure 3.1: Stack output voltage and trigger signal, when scanning.
In normal (
SCAN
) mode, a sawtooth ramp is supplied to the the stack,
at frequency of 4 to 70 Hz; see fig. 3.1. At the nominal midpoint of the
sweep, a trigger (low to high) signal is output via the rear panel
TRIG
connection, for synchronising to an oscilloscope or external experi-
ment. The trigger is not precisely at the midpoint of the apparent
frequency sweep, because the piezo stack has limited range and
thus the sweep can saturate (see section A.2 for details). Several
adjustments of the frequency sweep are possible: