Hybrid Detectors Work Fast

Robert C. Pini

Using a hybrid photomultiplier, spectroscopists now can measure an optical pulse 17 ns after a thousandfold more intense pulse without having to gate the detector to protect it. This development can reduce the complexity of spectroscopy study, making it valuable for applications such as measuring laser-induced fluorescence.
The versatile wide-dynamic-range photon detector works by accelerating a photoelectron from the photocathode through a vacuum tube onto a silicon PIN-diode substrate.
Until now, the complexity of developing flash protection and wiring for a specific application required plenty of time and money before a conventional photomultiplier could be used.
Angelo J. Alfano, an on-site contractor at the US Air Force Research Laboratory, conducted tests of the device as an aspect of his other research. "I saw it as an answer to the need for high speed and high current required by ultrafast lasers to monitor laser-induced fluorescence," he said. Alfano chose to run the tests because "there is very little published about the technology, and DEP [Delft Electronic Products of The Netherlands] couldn't supply the wiring information. But I know people use it for a lot of different applications and keep reinventing the wheel to set it up."
As a result, Alfano's work described the wiring and characterization for gain, impulse response, pulse height and pulse integral linearity of the hybrid photomultiplier, and demonstrated that the device can be used to measure directly a weak optical signal that follows shortly after an intense flash. Such a requirement is typical of an experimental environment in combustion research, according to Alfano.
To perform this feat, the detector must produce a rapid high gain that can be used with standard signal processing devices. Alfano thinks an avalanche photodiode may one day produce even better gain than the PIN diode, "enough additional gain to be used with a fast pulse amplifier" for a single photon. But at this point, tests by the manufacturer with an avalanche photodiode have not produced positive results.
A report in Applied Spectroscopy (52:2:303-307) detailed the testing, which used a commercially available electrostatically focused device produced by Delft.