Service and Repair
11-5
Possible Feedback Feedback occurs when a reflection of the output returns to the laser cavity.
Feedback radiation of a few mW may disrupt mode locking. To test for pos-
sible feedback, set up a means for monitoring the Tsunami pulse before the
suspected feedback source. If stable pulse operation is obtained after
blocking the beam to the suspected source, you have a feedback problem.
Try realigning the suspected device to the Tsunami laser. If this does not
work, install a Faraday isolator before the suspected source of feedback.
When selecting an isolator, be sure it can accommodate the Tsunami
wavelength tuning range.
The matching network is not
connected for >10 ps output,
and/or the coarse phase is not
set to accommodate its pres-
ence in the circuit, and vice
versa.
Connect the matching network in series with the mode locker, then adjust the
coarse and fine phase controls and the GTI position to achieve mode lock-
ing.
Symptom: Unable to mode lock for specific ps wavelengths (after ruling out all possible causes for
general inability to mode lock).
Possible Causes Corrective Action
Wavelength is specific to H
2
O,
CO
2
or O
2
absorption.
There is a gas leak in the purge system:
Set the flow rate to 0.28 m
3
/hr (10 SCFH), then disconnect the purge line
from the Tsunami laser head. The fitting shuts off all flow when discon-
nected. If flow does not fall to <0.02 m
3
/hr (0.5 SCFH) within a few seconds,
inspect the filters and connections inside the Model 3910 for leaks (refer to
Figure 9-5).
The Tsunami laser head gaskets and/or seals are leaky.
Inspect all the gaskets and seals for damage or debris stuck to them.
Look for possible missing gaskets under the Tsunami cover.
There is a water leak inside the Tsunami laser head.
Check the quick-disconnect waterline fittings inside the Tsunami laser head
near the front panel for leaks. Push in the fittings to make sure they are fully
engaged. Also verify there is no moisture around the barbed fittings near
the rod.
The bi-fi is misaligned (or other
mishaps).
There is dirt in the beam path of the bi-fi and/or the GTI. Clean.
The wrong order is present as a result of mis-orienting the bi-fi.
The Bi-fi assembly is not at
Brewster’s angle to the beam.
The beam is not centered on the GTI.
Changing from a 2-plate to a 3-plate bi-fi requires a correction to the Brew-
ster’s angle.
Note the tick marks on the bi-fi Brewster’s angle adjustment bracket, then
loosen the bracket lockdown screw and rotate the bracket 1.5 marks toward
the input bezel. Reverse the direction of rotation when going back to the 2-
plate bi-fi.
The wrong bi-fi is installed. Consult Chapter 10, “Options and Accessories,” Table 10-10, for the appro-
priate bi-fi for the installed optic set.
Symptom: Laser will not mode lock
Possible Causes Corrective Action
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