In order to measure the beam profile after the amplifier #2, we flip up the flip #3 before the mirror M16 ( see setup schematics ) to send the laser beam into a wedge. The laser beam was splitted by the wedge, and small part of laser beam was delivered into a SPRICON LBA-PC laser beam analyzer for diagnostics.
The beam profile, as shown in the right picture, looked very ugly. The diffraction patten implied the Ti:sapphire crystal or the reflected mirror was somehow damaged. After carefully observation, we found the crystal surface has been burnt two spots. Some anti-reflection coating areas probably were stripped by the strong pump laser.
So we replaced the damaged crystal with a new one, the measured beam profile was shown in the left picture. The light distribution looked homogeneous, no any diffraction was found.