Laser lab testing

This page describes the measurements carried out in the PH-DT detector development lab, maintained by C.Gallrapp and located in the DSF. The setup consists of micron-precision stages inside a climate controlled chamber, with laser setups above and below the sample. Both red and infrared lasers are available.

Calibration of CCPDv3 assemblies

The aim of these tests was to get a relative (or absolute if possible) calibration of the CCPDv3 amplifier. The infrared laser was chosen to penetrate from the back side, due to the prior mounting of the HV-CMOS devices on top of CLICpix ASICs. By varying the intensity of the beam and recording the true intensity with a reference diode, a calibration of the HV-CMOS amplifiers can be performed. Using a bare assembly which has been additionally tested with Fe55 (low penetration depth, front side illumination) we should be able to get an absolute calibration, but at the very least we will have a comparison between the devices tested in beam tests.

Process for measurement taking

Once the assembly is mounted inside the climate chamber (a 3D-printed mount has been produced for this purpose), several steps are required before taking usable data.

• First, find the laser spot. Running the radsource.py script will show the sum of all ToTs across the CLICpix matrix. When the beam lands on the chip, this should increase by ~200 or so counts.
• Move the device until the laser illuminates the pixel of interest (typically 3,0). Positive y in the stage co-ordinate system corresponds to negative x in the CLICpix system. Positive x in the stage system corresponds to negative y.
• Take a rough xy scan to figure out where the centre of the pixel is
• Take a scan in xz in order to find the focal position of the beam. This is the point where most charge is deposited in the pixel and the distribution should be sharpest/narrowest.
• Take a rough xy scan at this focal point to determine the pixel centre.

Now the setup is ready for real data taking. For each device a detailed 2D scan is taken as a function of bias, with the laser intensity at 50%. The laser is then positioned in the centre of the pixel for an intensity scan with increased granularity on the oscilloscope in order to read the reference diode accurately.

Intensity scan

The intensity of the laser does not vary linearly with the intensity setting of the device.

Detailed setup

• Mount the sample after removing the protective window, screw in place and turn on the FPGA
• Check the computer connection by setting the ethernet port address and pinging the FPGA on 192.168.200.16
• Close the climate chamber and turn on the bias voltage
• Update the script radsource.py to use the threshold for the sample mounted (shown below) and take a noise scan by running "./radsource.py laserLabSETXNoiseTest" for sample number X
• Turn on the laser and move the stages such that the sample should be under the laser (take beam spot from previous samples, XY typically 2, 30). Take data at this position by running "./radsource.py laserLabSETXLaserTest"
• Look at the output file, frames/blah_laserLabSETXLaserTest.txt, and there should be a large circle of 10*10 pixels or so - this is the laser spot!
• Either move by hand or, if the spot is roughly in the upper-left / upper-middle region use the XY position of the previous sample to align the laser over pixel 0,3
• At this point, start the script "./ccpd_test.py" on the clic pc, to activate the output of pixel 3,0. This can be used to find the pixel with some small xy steps
• Once there is at least some signal on the oscilloscope, run a coarse XY scan around this point to find the rough pixel centre. +/- 150 um in x and y, in steps of 25 um should be enough.
• Look at the output and move the stage such that the laser is centred on the pixel. Now perform a scan in XZ in order to obtain the focal point. +/- 1000 um in steps of 200 um should be sufficient
• Rough scan XY in focal plane
• Detailed XY scan and intensity scan

-- DanielHynds - 2015-06-22