MOGlabs DLC202, Manual, Page: 34 / 82

24 Chapter 3. Operation
SCAN
/
LOCK
The
SCAN
/
LOCK
switch should be on
SCAN
.
FREQUENCY
Sets the mid-point voltage of the ramp.
SPAN
Sets the height of the ramp (limited at 0 and 120 V
or 150 V); see fig. 3.1.
BIAS
The
BIAS
front-panel trimpot controls a feed-
forward bias injection current which follows the
ramp, to enable wider mode-hop-free scans. The
bias can be adjusted in a trial-and-error manner
to achieve the widest possible scans.
BIAS
is dis-
abled unless internal
DIP
switch
4
is
ON
.
f
sweep
The rear-panel f
sweep
trimpot adjusts the ramp rate
from 4 to 70 Hz.
Note The rapid return of the
STACK
sweep drive can excite mechanical
oscillations in the laser. Slower sweeps are recommended; usually
20 Hz works well but if ringing is observed at the start of the sweep,
reduce fsweep.
3.3 External scan control
An external source can be used to control the laser frequency while
in
SCAN
mode.
1. Connect the external frequency control (ramp, or
DC
) signal to
the rear-panel
SWEEP
external input.
2. Select external signal by setting
DIP
switch
9
to
ON
.
3. Set
SCAN
/
LOCK
to
SCAN
. The
FREQUENCY
and
SCAN
knobs will
be inactive.
3.4 Locking to an atomic transition:
DC
Figure 3.3 shows how an
ECDL
can be locked to an atomic transition
as determined from absorption in a vapour cell. The basic configu-
ration described in
§ 3.2 is extended with the
DLC
photodetector, and