Performance of Laser Energy Measurement System for LHAASO-WFCTA

The Large High Altitude Air Shower Observatory (LHAASO) is situated in Haizi Mountain, Daocheng County, Sichuan Province, at an average altitude of 4 410 meters. The laser calibration system is a vital component of the Wide-Field-of-view Cherenkov Telescope Array (WFCTA) in LHAASO, utilized to calibrate the absolute gain of WFCTA for the received photons. The energy measurement system in the laser calibration system comprises an energy sensor, an energy meter, and a temperature control system, primarily employed to precisely measure the energy of the pulsed laser beam emitted into the WFCTA field of view. This paper primarily focuses on the relative calibration and performance of the energy sensor, and proposes a temperature control system to ensure the energy sensor's normal operation in the harsh plateau environment. The accuracy of energy measurement about laser pulse can be improved by relative calibration. The performance test results of the energy sensor demonstrate that the non-uniformity is less than 1.5% within the circular area with a diameter of 8 mm centered on the energy sensor. The incident angle of the laser beam has negligible impact on the measurement of the laser pulse energy by the energy sensor, but the reflection of the laser by the energy sensor can damage the laser when the laser beam is vertically incident. Hence, avoiding the vertical incidence of the laser beam on the energy sensor is crucial. Furthermore, the energy sensor exhibits a strong temperature effect. Therefore, an independent temperature control system is developed in this paper to ensure the energy sensor's normal operation, fulfilling the experiment's requirements. These research works serve as the foundation for the laser calibration system's regular operation and functioning.