36
Appendix B. Troubleshooting
B.2
When using modulation input, the fast output floats to a large voltage
When using the
MOD IN
functionality of the
FSC
(
DIP 4
enabled) the
fast output will typically float to the positive voltage rail, around 4V.
Ensure
MOD IN
is shorted when not in use.
B.3
Large positive error signals
In some applications, the error signal generated by the application
may be strictly positive (or negative) and large. In this case the
REF
trimpot and
ERR OFFSET
may not provide sufficient DC shift to
ensure the desired lockpoint coincides with 0 V. In this case both
CH A
and
CH B
can be used with the
INPUT
toggle set to ∆,
CH B
set
to
PD
and with a DC voltage applied to
CH B
to generate the offset
needed to centre the lock point. As an example, if the error signal is
between 0 V and 5 V and the lock point was 2.5 V, then connect the
error signal to
CH A
and apply 2.5 V to
CH B
. With the appropriate
setting the error signal will then be between
−
2
.
5 V to +2
.
5 V.
B.4
Fast output rails at
±
0.625 V
For most
MOGL
abs
ECDLs
, a voltage swing of
±
0.625 V on the fast
output (corresponding to
±
0.625 mA injected into the laser diode) is
more than required for locking to an optical cavity. In some appli-
cations a larger range on the fast output is required. This limit can
be increased by a simple resistor change. Please contact
MOGL
abs
for more information if required.
B.5
Feedback needs to change sign
If the fast feedback polarity changes, it is typically because the
laser has drifted into into a multi-mode state (two external cavity
modes oscillating simultaneously). Adjust the laser current to obtain
singlemode operation, rather than reversing the feedback polarity.
