Tsunami - Beckman Institute Laser Resource Center

Tsunami
Symptom: Unable to mode lock for specific ps wavelengths
Possible Causes
Tsunami is not optimized.
Poor beam mode profile.
Improper phase adjustment.
Wavelength on or near
water or oxygen absorption
lines.
Gas leak in purge
system.
Water leak inside the
Tsunami laser.
Possible oxygen or
moisture contamination
from the purge source.
Leaky Tsunami laser
head gaskets and seals.
Dirty hi-fi
Possible Feedback
assembly'
Wrong optical components
in the system.
Corrective Action
Refer to chapter 7, "Operation: Optimizing Laser Output,"
With the beam directed to far field (e.g., displayed on a wall), adjust M2 to
obtain a solid, slightly elliptical mode with high contrast. There should be no
spikes on the outer edges of the beam profile.
Press the STATUS mode locker enable button on the Model 3955. lncrease
the gain on the autocorrelator and angle-tune the doubling crystal to search for
the autocorrelator signal. Adjust the fine PHASE control for the highest signal,
then adjust the GTI POSITION control to obtain the pulse. If there is no noticeable
change in the autocorrelator signal, center the PHASE LED display using
the PHASE fine control. Increase the autocorrelator signal by adjusting the
coarse phase control inside the Model 3955 (refer to "Coarse Phase Adjustment
..." in chapter 6). Test for proper mode locking (refer to "Test for Proper
Mode Locking" in chapter 6). Lower the pump power and repeat the above if a
pulse still does not appear.
Refer to Figure 7-1 to see if your operating wavelength is on or near the
absorption region. If so, purging is necessary. Note that more intense absorption
lines require larger quantities of purge gas for longer periods of time. For
operation near 900 to 980 nm, a purge rate of > 0.1 7 m3/hr (6 SCFH) for >4
hours is essential for problem-free mode locking.
Set the flow rate to 0.28 m3/hr (10 SCFH), then disconnect the purge line from
the Tsunami laser head. The fitting shuts off all flow when disconnected. If flow
does not fall to < 0.02 m3/hr (0.5 SCFH) within a few seconds, inspect the
filters and connections inside the Model 391 0 for leaks (refer to Figure 9-5).
Check the coolant line quick disconnect fittings inside the Tsunami laser head
near the front panel for leaks. Push the fittings in to make sure they are fully
engaged. Also verify there is no moisture around the barbed fittings near the rod.
Use only dry, oil-free, Electronic Grade 5 nitrogen gas from a cylinder. Avoid
boil-off tanks which might contain oxygen or water vapor. Check the color of
the desiccants in the molecular-sieve assembly in the Model 391 0 and make
sure they are not pink. If they are, replace the assembly (refer to chapter 9).
Inspect all the gaskets and seals for damage. Look for possible missing
gaskets under the Tsunami cover and, when in doubt, replace them.
Remove the bi-fi filter stack assembly and clean the outside surfaces. DO
NOT disassemble the stack. Replace the filter stack assembly if dirt is found
between the plates or if misalignment of the plates is suspected (refer to
chapter 9).
Feedback occurs when a reflection from a flat surface (such as a mirror, fiber,
or the plano surface of a lens) enters the laser cavity. Feedback radiation of a
few mW may disrupt mode locking. To test for possible feedback, set up a fast
photodiode (or the Model 409 autocorrelator) to monitor the Tsunami pulse
using a beam splitter between the Tsunami and 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, try installing a Faraday or
other isolator in your optical system. When selecting an isolator, be sure it can
accommodate the Tsunami wavelength tuning range.
Verify all the optics in the system are correct according to the optical listing
tables in chapter 10.