Multichannel High-Current LED Calibration Light Source for the Intensified Camera of the HERD Calorimeter

In the High Energy cosmic-Radiation Detection facility (HERD), each intensified camera of the calorimeter (CALO) reads out 7,489 wavelength-shifting (WLS) fibers. Given the 10-year operational lifetime of HERD, precise calibration of the gain for fiber pixels and the crosstalk between pixels across a wide dynamic range is critical. To achieve these goals, a novel multi-channel high-current light emitting diode (LED) calibration light source is presented. The calibration system features cascaded shift registers with AND-gate buffering to independently control multiple LED channels using a shared trigger signal, enabling precise adjustment of LED activation duration and frequency. A single-pole double-throw analog switch (SPDT) converts two direct current (DC) voltages into a pulsed signal for LED driving. The combination of a digital-to-analog converter (DAC) and a DC-to-DC converter (DC/DC) ensures accurate brightness adjustment. Moreover, energy storage capacitors are incorporated to supply high transient currents during LED activation. A 138-channel prototype has been developed and tested, exhibiting a wide light intensity range equivalent to 0.7 to 2×10⁶ minimum ionizing particles (MIPs), with a uniformity exceeding 99% and negligible electrical crosstalk. The intensified camera was tested for linearity and inter-pixel crosstalk using the LED calibration light source, and the results were consistent with previous work. The modular design allows for channel expansion without additional inter-board cabling or increased power supply capacity.