Tsunami - Beckman Institute Laser Resource Center

Service and Repair
Symptom: Unstable mode locking
Possible Causes
Possible Feedback
Tsunami is not optimized.
Poor beam mode profile.
Improper phase adjustment.
Optics are dirty.
Corrective Action
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.
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.
Inspect the optics for dirt or contamination. If dirty, clean the optics in the following
order, using the cleaning procedure in chapter 9, "Maintenance." This
list orders the optics by influence in power loss. If an optic is dirty, an ir viewer
will show unusual scatter.
1. Input and output Brewster window surfaces
2. Prisms
3. Rod surfaces
4. Bi-fi (ps configuration), outside surfaces only
5. PI, PP
6. M3 (both surfaces)
7. M2
8. M4, M5
9. M7, M8
10. M6, Mg
11. 4 0 7 M1
12. AOM
13. Beam splitter (highly unlikely it will need cleaning)