20120519 Knife Edge Measurements

Want to measure profile / propagation of laser through focus in LWIP.

• Camera pixels too big
• Special beam profiles don't work with pulsed beams

To measure profiles on the order of a micron, a knife edge is used and in the case of commercial profiles, these are actually slits considerably wider than this that are mounted in a wheel. The wheel rotates at a well defined speed and therefore the position can be well know as a function of time.

Anyway, the only way to measure this is through a manual knife edge measurement. This can be done with the OTR screen if it is rotated to be perpendicular to the laser beam and then translated away from the notch where there is considerable damage to the edge. Far from the notch the edge of the screen is sharp. Additionally, the manipulator must be set to a low speed to achieve the necessary accuracy in position.

Laser Preparation

The laser divergence and beam profile is known to vary with pulse energy and so the laser must be run at full power, however the screen would be easily damaged by this. Therefore the first and second turning mirrors after the laser aperture were replaced with a 50% and 10% reflectance mirrors respectively. Several ND filters were added to reduce the intensity further. The laser was run with a flash lamp delay of 90 microseconds.

Post-LWIP

The post-ip setup was in the laser-wire mode and the energy meter was replaced with a turning mirror so that the laser (not visible on paper) was allowed to propagate and eventually focussed by the post-ip recollimating lens. An amplified photodiode was placed here and connected to the CAMAC DAQ upstairs. This was rewired in place of the laserpd photodiode pre-ip. The laser was further attenuated until the full beam created a voltage of around 1.2V on the photodiode and was unsaturated. 12dB of attenuation were used before the signal was inverted and put into the gated ADC. The gate was left the same for the laserpd photodiode, which covered the peak of the voltage pulse from the photodiode. The pulse decayed over several microseconds. The gate was 200ns.

With these settings, the ADC value was around 12000.

*With the laser blocked, the ADC pedestal subtraction script was run - records 100 pulses, and puts pedestal into epics database.

• For simplicity, the signal was put into the laserpd channel in the ADC to avoid changing the EPICS database. In the following data files, the photodiode signal is 'laserpd' variable.

• 100 pulse log for background ../dat/raw/20120520_1423_log.dat
• 100 pulse log with manipulator out ../dat/raw/20120520_1429_log.dat