Spectra-Physics Tsunami - Establish Modelocking Process

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Alignment

6-13

Establish Modelocking

1. Push the STATUS mode locker enable button (the ENABLE LED should

be lit).

2. Adjust Pr

and Pr

so that the beam passes through them about

/4 to

from the apex.

3. Set up an optical spectrum analyzer to view the wavelength signal,

then adjust the fine PHASE control on the Model 3955 electronics to

initiate modelocking. When the system becomes modelocked, the sin-

gle wavelength CW component will change into a Gaussian-shaped

spectrum.

If the signal displayed is not a short pulse (about 80 fs), adjust the fine

PHASE control on the Model 3955 electronics module until either a

short pulse appears on the oscilloscope or the autocorrelator signal is

maximized.

If the phase adjustment is far from the optimum setting, the pulses

might be too broad to see individual pulses on the oscilloscope. If this

occurs, see “Symptom: Laser will not modelock” in the troubleshoot-

ing guide in Chapter 11.

If you are using cables other than those shipped with the system, or

you recently converted this system from ps to fs, or changed from

extra-long to standard wavelength, or vice versa, the phase signal

might have moved outside the range of the fine PHASE control and a

coarse adjustment is necessary.

Note: an 80 MHz pulse train corresponds to a pulse every 12.5 ns and

the fine PHASE control provides about 100 ps of phase adjustment. If

you cannot see pulses, adjust the coarse control located inside the

Model 3955. Refer to “Coarse Phase Adjustment...” at the end of this

chapter for instructions on locating and changing the setting of the

coarse control.

Optimizing Modelocking

1. Adjust the prism dispersion compensation control until the spectrum

increases in bandwidth when more prism glass is inserted into the

beam. You should not see, however, the presence of any CW break-

through throughout the spectrum. Adjust for a FWHM bandwidth that

is 1.2 times that of the value calculate based on specified pulse width

and wavelength according to Figure 6-11.

2. Optimize Tsunami output power.

Place a power meter in the Tsunami beam path and adjust

and M

for maximum output power as specified in Step 4 on page 6-10.

The

PHOTODIODE light bar level indicator on the Model 3955 front

panel should indicate maximum for standard wavelengths and >30%

for x-long wavelengths.

Spectra-Physics Tsunami - Establish Modelocking Process

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