KAGRA MIR (Absorption)
ManuelMarchio - 16:50, Wednesday 28 June 2017 (513)
Imaging unit alignment

Friday, June 23, 2017

Following the following procedure, I aligned the Imaging Unit for the HeNe probe beam.

1) move IU micrometer closer to the end of translation which would give you enough translation range to move the whole IU farther away in case you test thick objects;

2) place front lens of the IU to get the probe beam at its center, then place gold-coated half-ball approximately at the focus of the front lens;
3) with a knife edge half-close the probe beam at a location of a Rayleigh length of perturbation behind the pump/probe crossing point;
4) place a paper screen at the detector position to observe the probe spot reflected by the half-ball (may place the screen at a longer distance to have a larger probe spot);
5) changing the distance between the front lens and half-ball find a moment when you see a sharp image of the knife edge.

That will complete rough alignment of the IU. The fine tuning is done by maximizing AC signal coming from the surface calibration piece. For that, try different micrometer positions around one you started with. For every position you have to center the probe (maximize the DC), maximize AC if needed. The maximum R should be close to the original R for the surface calibration. Then make scan with the bulk calibration piece.

According to the theory the signal is maximum when the detector at the Rayleigh length of the perturbation, experimentally we can check this changing the position of the blade and aligning again the imaging unit and measuring the signal. So, in order to maximize the signal, I repeated the procedure changing the position of the blade from 18mm to 12mm and 6mm but I got a lower signal, so I aligned it back to 18mm.

The absorption signal of the reference is similar to original value (the one we had since we bought the system) even if I changed by the 20% the waists of pump and probe,

Parameters: LD current = 0.8A, power without sample = 33mW

Images attached to this report
513_20170628093935_surfrefjune23.png 513_20170628093940_bulkrefjune23.png 513_20170628094809_screenshotfrom20170624191701.png 513_20170628094813_screenshotfrom20170624191633.png 513_20170628094817_screenshotfrom20170624191608.png 513_20170628094820_screenshotfrom20170624191541.png