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Pulser amplitude feedback amplifier (view source)
Revision as of 19:48, 20 June 2014
, 19:48, 20 June 2014→Configuration within the dark box
==Configuration within the dark box==
==Configuration within the dark box==
The reason we created this feedback scheme was to improve the reproducibility of our fiber light yield measurements. A variety of reasons contribute to our difficulty getting reproducible light yields from our fibers. We are seeking to eliminate them one by one. Besides variation in the light intensity from pulse to pulse, we also observed variations in the pulse shape, depending on how the cables connecting the pulse generator to the laser diode module lie in the dark box. To help reduce this variability, we mounted the pulse generator inside a shielded housing and screwed the housing onto the frame of the dark box. Once this was done, the pulse shape is much more reproducible and less dependent on cable routing.
Another source of variability is motion in the position of the optics coupling the laser diode to the fiber bundle. These optics consist of a frosted glass diffuser, followed by a mirror. The diffuser is mounted to the laser diode housing in a fixed frame that is wrapped with dark tape, but this module was held in place using a poorly constructed ring stand arrangement that was easy to bump out of alignment, and the connection to the pulse generator cable and photodiode amplitude feedback board was flexible and loose. To improve this, the laser/diffuser module was rigidly mounted to the roof of the dark box, and the mirror glued to a fixed wooden wedge that holds the correct alignment such that the image of the diffuser disc is directly in line with the fiber axis.
To reduce the variability of the laser pulse intensity, we removed the 50Ohm termination resistor from the end of the pulser cable. The laser has a huge nonlinearity in its termination impedence with input voltage, so it is useless to try to terminate the cable to 50Ohms in any case, and that resistor was sucking current from the pulse. Afterward, we found that the light yields for the brightest fibers were causing overflows in the 12-bit FADC, so we installed a 50% neutral density filter over the diffuser plate. Now the brightest fibers use 70% of the dynamic range of the FADC, but we see no evidence of overflows.