Experimental Tests of a Full Analog Fiber Optic Monitoring System Suitable for Safety Application at CERN

In this work, experimental tests of a full analog fiber optic monitoring system are reported. The proposed concept is conceived to satisfy the constraints imposed to be used for safety application, in terms of robustness and reliability, due to full passive optical devices and analog-based circuitry. Moreover, it is compatible with the stringent requirements imposed by the detector safety system (DSS) of the Compact Muon Solenoid (CMS) experiment at CERN. From the optical side, the main device is the arrayed waveguide grating (AWG), with the aim to filter and address the optical signal through a determined output channel, that is then transduced in electrical by means of a photodiode. From the electronic side, the designed board has the purpose to manipulate the signal in order to have an increasing monotone output current, in the range of 4–20 mA, a standard widely employed in a safety system environment. In this way, the circuit gain can be set in order to match the value to the physical quantity under the monitoring threshold. To give proof of its characteristics, the system is subjected to many tests led at CERN, in which two fiber Bragg grating (FBG) sensors are under test in a custom climate box. Step temperature variation, as well as fast oscillating test and long-term stability measurement are reported, confirming its key strength and field-validating it.