Lok-to-Clock Electronics
8-13
1. Verify the output of the Model 3932 is 0 dbm into a 50 Ω termination
(0.63 Vpp).
2. With the Model 3930 unlocked, place a photodiode (e.g., an ET2000)
at the source of the experiment.
3. Adjust the optical alignment so that the pulse train for each laser can
be identified.
4. Mode lock the non-LTC laser.
5. Using the Model 3930, set up and lock the LTC Tsunami as outlined in
“Operation Notes” above.
The oscilloscope display should now show two pulse trains with a fixed
delay. The pulse trains are now synchronized but are probably not over-
lapped in time.
6. Verify the time between the pulse trains can be varied by using the
coarse TIMING control on the LTC Tsunami laser.
The timing control provides up to 2 ns of adjustment. If the pulses are
more than 2 ns apart, change the length of the cable between the Model
3930 and the Model 3932 (30 cm of cable corresponds to about 1 ns of
delay). Do not modify the cables between the LTC laser head and its
Model 3955! When the cable length is optimized, activate the sweep
generator (press the TIMING button) on the Model 3930 and observe the
pulses from the LTC Tsunami sweeping back and forth over those from
the mode-locked laser.
To overlap the pulses more precisely in time, focus the two laser output
beams in a nonlinear crystal such as BBO.
*
As you sweep through the
phase-matching angle of the BBO crystal, a frequency-doubled signal
should be observed from each laser. Set the crystal for an intermediate
angle to observe the sum-frequency mixed signal from both lasers. By
activating the sweep generator, the crystal can be easily phase-matched
for observation of this signal. It will be modulated at the 10 Hz sweep
period. Once the sum-frequency mixed signal is observed, deactivate
the sweep generator and use the coarse/fine
TIMING controls to opti-
mize this signal.
A cross-correlation of the two pulses can be generated by displaying
the sum frequency mixed signal on a photodetector and activating the
sweep generator. Alternatively, the cross-correlation signal can be
observed by sending the spatial and temporally overlapped beams into
an autocorrelator, such as the Model 409-08. Figure 8-9 shows a typi-
cal cross-correlation trace for a ps
LTC Tsunami lasers and a second
mode-locked ps laser, each having 1.5 ps pulse widths.
*
The nonlinear crystal must be cut such that it can frequency-double the output of each
laser. Verify this by temporarily placing a white card at the output of the crystal and
changing the phase-matching angle of the crystal while watching the laser output on the
card.
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