Development of a time calibration system for the KLM upgrade in the Belle II experiment

To meet the stringent time calibration requirements for the Belle II experiment upgrade, particularly for its large-size KL and Muon Detector comprising tens of thousands of scintillator channels with time resolutions better than 100 ps, we developed a compact and high-speed time calibration system. The system utilizes a laser diode as its light source, integrated with a fast pulse laser drive circuit that employs high-speed switching GaN FETs and gate drivers. A prototype was constructed and rigorously evaluated using scintillators, achieving timing resolutions of about 13 ps for a single calibration channel. Furthermore, internal deviations among calibration channels were analyzed, with most measurements remaining within 250 ps. These results highlight the system’s precision, scalability, and suitability for large-scale particle physics experiments.

💡 Research Summary

The paper presents the design, construction, and performance evaluation of a compact, high‑speed time‑calibration system intended for the upgraded KLM (KL and Muon) detector of the Belle II experiment. The upgraded detector will contain tens of thousands of scintillator strips read out by silicon photomultipliers (SiPMs) and must achieve a time‑of‑flight resolution better than 100 ps, with some channels approaching 50 ps. To meet this stringent requirement, the authors developed a laser‑based calibration system that uses a 445 nm Osram PLPT5 447KA laser diode as the light source, coupled with a fast pulse driver built around a Gallium‑Nitride (GaN) field‑effect transistor (EPC2037) and a low‑side gate driver (LMG1020).

The driver architecture begins with an NE555 timer that generates a square wave (1 kHz–10 kHz), which is shaped by a SN74AHC123 monostable to produce adjustable nanosecond‑scale pulses. These pulses are integrated through RC networks and fed to the gate driver, whose output switches the GaN FET. The GaN FET’s sub‑nanosecond turn‑on time and low on‑resistance enable the laser diode to emit short, high‑current pulses. A power‑supply chain provides a stable 5 V LDO for the driver and a boost converter that, together with an LM317 regulator, supplies a tunable voltage up to 24 V for the diode.

The laser‑diode circuit itself forms an under‑damped R‑L‑C discharge network (R = 300 Ω, C = 500 pF) that yields a repetition rate of 1.33 MHz and a pulse width of a few nanoseconds. A parallel clamping diode and resistor protect the diode from reverse currents and voltage spikes. PCB layout minimizes trace inductance to preserve pulse integrity.

A prototype board hosts eight independent laser‑control channels, two trigger‑output channels, and eight power‑delivery channels. Light from the diode is distributed to multiple calibration heads via a 1 × 8 optical splitter and multimode fiber. The authors first validated the driver and diode using two SiPMs directly illuminated by the laser, achieving a time‑difference resolution of 13.52 ± 0.09 ps, which demonstrates the intrinsic jitter of the laser system.

Subsequent tests incorporated 100 cm scintillator bars (2 cm × 4 cm × 100 cm) read out by a 1 × 4 SiPM array on one end and an optical fiber on the opposite end. With one channel serving as a reference, the other two channels showed calibration offsets of –37 ps and +25 ps and individual time resolutions of 17 ps and 19 ps, respectively. This confirms that a single calibration channel can provide ≈13 ps resolution when coupled to the actual detector geometry.

To assess inter‑channel consistency, the eight laser‑control channels were sequentially connected to the same diode while the SiPM output was compared to the driver’s trigger signal. Using the first channel as a reference, the remaining channels exhibited deviations up to 138 ps, with most within 100 ps; the full spread across all eight channels stayed below 250 ps. These variations stem from component tolerances and PCB trace length differences but are small enough to be corrected by software calibration.

The authors conclude that the laser‑diode‑GaN‑FET calibration system meets the Belle II KLM upgrade’s demanding specifications: sub‑15 ps single‑channel timing precision and sub‑250 ps channel‑to‑channel uniformity across a scalable, low‑power, and compact platform. The approach offers advantages over traditional bulkier picosecond lasers (e.g., PicoQuant) by delivering comparable timing performance with reduced size, power consumption, and cost. The paper suggests future work on expanding the number of channels, integrating temperature‑compensation circuitry, and implementing real‑time calibration algorithms to further enhance reliability for large‑scale particle‑physics detectors.