ODU Quality Control

This is documentation on Dan Karmgard's quality control system for the CMS ODU components constructed at Notre Dame. It's not ELabs related but I needed a quick place to put it.

ODUs in the Compact Muon Solenoid

In the CMS detector, optical fibers carry light from scintillator panels to the photomultipliers that convert the signal to an electrical pulse. The Optical Distribution Unit (ODU) sits between the scintillators and the PMs and serves to physically map signal fibers into the appropriate photomultiplier.

Figure 1 shows the basic idea.

Each scintillator panel outputs light into a single fiber. These output fibers are bound into cables of 18 fibers each, though not all 18 fibers in the cable are always used (that is, each cable may carry the signal from fewer than 18 scintillators). 18 such cables are routed into a single ODU, whose internal fiber optic lines route one fiber signal from each input cable into a particular PM. The result is that the signal registered by any given PM represents the total light signal from what is geometrically a "radial stack" of scintillator plates in the detector.

The point at which the fiber cable plugs into the ODU represents a physical break in the fiber line. Light signals can cross from the end of the cable fiber into the corresponding ODU fiber, but there will always be a certain amount of signal degradation. Moreover, if the cable plug and ODU receptor are misaligned, there will be a more dramatic signal loss that might affect the usefulness of the CMS detector's data. The Quality Control (QC) system is intended to identify unacceptable signal loss and quantify acceptable signal loss through each individual ODU.

The QC system

The QC system uses LEDs to simulate the light signals that would arrive at the ODU from the scintillators during normal operation of the CMS detector. Each LED's output is divided along two fiber optic lines: the first goes directly to a photodiode to measure the strength of the LED's signal; the second is routed through the ODU to be tested. Instead of a photomultiplier, the output of the ODU is mounted via a lightguide to a second photodiode. The signals from the two photodiodes are compared; their difference is (naively, at least) the light lost at the cable/ODU junction.

Light Source

A panel of 18 blue-white LEDs serves as the system's light source. A typical 18-fiber cable is split such that one fiber is attached to each LED. The other end of the cable attaches to one of the 18 input ports on the ODU. The LEDs are powered and controlled by a program running on an Arduino and accessed via a touchscreen panel. The program flashes each LED multiple times; the number of flashes is selectable by the user, but typically ~1000 provides a statistically significant measurement. One-by-one, this flashing procedure is repeated for each of the 18 LEDs attached to each of the 18 fibers in the fiber cable.

Components:

• Circuit board schematic (Anders and Dan) This file can be opened with the ExpressPCB program available at the ExpressPCB website. It's written for Windows but works well on Linux using Wine.
• Arduino design (Dan)
• Program (Dan)

-- Main.JoelG - 2017-05-18

Comments

• oduqc_control_circuit.pcb: ExpressPCB schematic of the ODU QC control circuit