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18 Chapter 2. Connections and controls
Digital control
HD12 is a 10-pin header which provides access to several control
signals for locking and for sample-and-hold of the lock-point. HD12
is located near the
DIP
switches, slightly towards the front and left-
hand side of the unit (see appendix H). The pinout of the header is
described in section G.4. The signals are standard
TTL
-compatible,
> 2 . 4 V
HIGH
and < 0 . 8 V
LOW
. The inputs are
ORed
with the front
toggle switches, such that the signal is activated if either the digital
input is active (i.e.
HIGH
) or the toggle switch is on (down).
Laser ON
HIGH
to switch the laser diode current on, regardless of the state of
the front-panel switch.
LOCK
HIGH
to
SLOW
lock, regardless of the state of the front-panel switch.
LOW
to sweep, if the front-panel switch is up.
FAST
HIGH
to
FAST
lock.
HOLD
HIGH
to enable sample-and-hold. With
HOLD
active, the feedback to
the slow piezo will be fixed by a sample-and-hold circuit. The diode
current can then be modulated via the rear-panel
CURR MOD
input
(with
DIP
switch
6 ON
), to jump the laser frequency quickly, without
the error feedback circuit competing with the external modulation.
Note that
FAST
lock should also be disabled at the same time. To
relock, restore the
CURR MOD
input voltage, and return the
HOLD
input
LOW
; the locking feedback will then be reactivated.
FAST
lock
can then be reactivated.
This ability can be used for auto-locking under computer control, and
also for atom trapping experiments involving sequences with different
detunings for polarisation gradient cooling and for compression.