Alignment
6-19
3. Adjust the GTI POSITION dispersion compensation control on the Tsu-
nami laser head control panel to initiate modelocking (Figure 6-14, b–f).
4. If modelocking is not evident, adjust the fine PHASE control on the
Model 3955 one full turn and repeat the last step to initiate modelock-
ing (Figure 6-14, b–f).
5. Iterate the last two steps until modelocking occurs.
Modelocking occurs when the oscilloscope signal on the spectrum
analyzer drops in amplitude and becomes slightly broader at the base
or when the autocorrelation signal appears (Figure 6-14, b–f).
6. Repeat Steps 3 through 5 until the pulse remains present. The ampli-
tude will drastically increase and the pulse will be well defined.
If the autocorrelator gain is too high and/or the signal is saturated (Fig-
ure 6-14g), lower the gain until a clean pulse is present and increase the
oscilloscope sweep speed to broaden the pulse for viewing (Figure 6-
14h).
Optimize Modelocking
1. Optimize Tsunami output power.
Place a power meter in the Tsunami beam path and adjust M
10
and M
1
for maximum output power as described in Step 4 under “Cavity
Alignment for a PS System.”
The PHOTODIODE light bar level indicator on the Model 3955 front
panel should indicate maximum for standard wavelengths and > 30%
for x-long wavelengths.
2. Since bandwidth is subject to pump power as well as dispersion com-
pensation in the cavity, vary pump power and the GTI POSITION dis-
persion compensation to obtain the desired output power and pulse
width.
Too much pump power can cause CW breakthrough, which is evident
when there is no increase in the autocorrelation or pulse amplitude
when using a fast photodiode detector.
3. Test for proper modelocking.
Too much bandwidth can make it difficult for the regenerative mode-
lock loop to acquire modelocking from CW operation. In addition,
operating at wavelengths at or near water and oxygen absorption lines
would be either difficult or impossible without sufficient cavity purg-
ing (see Figure 3-11 for details).
a. Temporarily disable the regenerative modelock RF loop (push the
STATUS button on the Model 3955 front panel).
b. Close and reopen the pump laser shutter to take the system off
modelocking.
c. Adjust the fine
PHASE control so that only the bottom bar of the
PHASE LED bar graph glows.
d. Enable the regenerative modelock RF loop (push the STATUS but-
ton again).
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