Imaging unit adjustment and reference sample measurement with 1310 nm probe laser.

Members: Manuel, Kuroki

We tried adjusting the distances of the parts in the IU to make the sharp image on the detector of a blade at 18mm after the crossing point.
We can make it a bit sharp. It is blurred because of diffraction.
We found the problem which the probe laser saturates the detector signal at 4V.
So, we put a neutral density filter (optical density = 2) in front of the laser and put a optical damp in order to remove the reflected beam by the filter. We measured a photodetector signal DC=2V, about half of the range (with laser current of 130mA)
We measured optical power ,which is 0.48 mW. (Before putting the filter, the power was 37.5 mW.)
After that, We measured a absorption signal of a reference sample.
We get it, but it have a low signal to noise ratio (AC=40mV). Also the phase shape of the scan is not as expected.
We think that imaging unit(IU) should be correctly aligned
Next step is that we should consider how to better align IU.
Maybe we can try a different position of the blade after the crossing point, and see if we get a better signal.