MOGlabs DLC202, Manual, Page: 56 / 82

46 Appendix B. Troubleshooting
B.3.3
FAST
does not lock
•
FAST
feedback has wrong polarity. Try reversing the polarity
with the front-panel switch.
• If the laser frequency is close to a mode hop (i.e. intrinsic
diode cavity resonance is half way between two external-cavity
longitudnal modes), the current response can be opposite to
normal. Try adjusting the diode current very slightly.
• Lock signal zero-crossing too far from trigger point.
• Gain too high. Try smaller and smaller gain, but be careful to
ensure that the lock error signal is crossing zero.
B.3.4
FAST
locks only briefly
The
FAST
channel is normally
AC
-coupled (see
DIP
switch
12
), with
a time constant of 0.1 s. Thus with
FAST
feedback only, the laser
will drift off resonance. Normally the
SLOW
channel is used to com-
pensate for very slow drift, but the laser can be locked by current
feedback only with
DIP
switch
10
ON
. With
DC
current feedback, the
feedback saturates at ± 10 mA.