William Angerer - Department of Physics and Astronomy - University ...

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output
coupler
r--
'- modelocked
beam
shutter
I
/ L
cwbeam
Figure 3.6: Shutter configuration in the Ti:Ah03 laser. The high intensity of the
pulsed laser activates the Kerr lensing action of the Ti:A1 2 0 3 crystal. This focuses
the modelocked laser pulse (thick lines) through the shutter without loss. In contrast,
the cw beam (thin lines) is not focused through the slit. The shutter mechanism is
very similar to the saturable absorber method used to mode lock dye lasers.
will be focused through the slit without attenuation (see Fig. 3.6).
Thus, the noise
spike proceeds through the cavity unattenuated and is amplified at the expense of the
cw beam until it forms a femtosecond laser pulse with negligible cw background. ~ oise
is introduced into the cavity through the starter galvo which rapidly oscillates with a
small amplitude to slightly alter the cavity alignment. Modelocking was optimized by
observing the laser signal with a fast silicon photodiode terminated by a 50 n resistor
on an oscilloscope. Fig. 3.7 displays a typical modelocked power spectrum for the
Ti:AI 2 0 3 laser. Once the modelocking of the laser was optimized by adjusting the
shutter position and width, the stater galvo was turned off. Typically, we modelocked
the laser at the peak of its tuning curve, ",800 nm. It is advisable to modelock the
laser at the peak of its tuning curve and then tune the birefringent filter to the desired
wavelength rather than attempt to modelock the laser at the desired wavelength.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.