A bipolar LED drive technique for high performance, stability and power in the nanosecond time scale

Abstract

Pulsed light sources are often used to monitor the stability of light detectors such as photomultiplier tubes. Light emitting diodes (LEDs) are suitable for this due to their high specific light yield. While pulsed operation in the region of μs is generally accessible with most LEDs and drivers, the ns time scale often represents a technical challenge. This paper describes a technique of bipolar LED drive that can produce light pulses of a few ns at high stability, reliability and power. The driver also offers control over the properties of the light pulse produced such as shape, intensity and repetition rate.

This approach has been studied in 2003 and implemented in 2004 for two fusion neutron spectrometers at the Joint European Torus (JET) namely the Magnetic Proton Recoil upgrade (MPRu) and the Time Of Flight Optimized for Rate (TOFOR). A driver has been manufactured and connected to the scintillation detectors of each spectrometer through an optical fiber distribution network. Both MPRu and TOFOR have been successfully relying on this system for calibration and performance monitoring for several years, confirming the long-term stability and reliability of this technique.

Introduction

Light sources are commonly used for calibrations of detectors such as photomultiplier tubes (PMT) and for monitoring their stability over time. An absolute reference (primary standard) can be provided by weak long-lived radioactive sources coupled to scintillation detectors. However, in cases when one wishes to distribute the same signal over an array of detectors, a more powerful secondary standard is required. Light emitting diodes (LEDs) [1] represent a valid choice for this purpose due to their high light yield per unit surface and the relatively simple electronic drivers they require. The main limitation in using LEDs comes from the minimum pulse duration attainable which is due to the inherent properties of the junction of the LED. Obtaining pulses as short as a few ns represents a technical challenge especially if high output power is required. Simple drivers often make use of a single transistor stage to inject current directly into the LED anode [2]. This approach yields good results but is not generally suitable for the nanosecond time scale due to the parasitic capacitance of the diode. In these conditions the light signal was found to extend significantly beyond the current pulse; this is here referred to as afterglow. Fig. 1 shows an example of afterglow of a high-power LED of 7 mm diameter [3] in response to a 10 ns current pulse produced through a simple transistor stage. A fast PMT suitable for light pulses as short as few ns was used for detection. It can be seen that the tail extends several μs after the peak.