Terahertz time-domain spectroscopy reveals laser diode turn-on behavior
Mode-hopping of a frequency-tunable laser diode can be observed on picosecond timescales.
For many years, it has been predicted that operating frequencies within single-mode frequency-tunable semiconductor lasers stabilize on a timescale of a few nanoseconds and can be changed within a few hundreds of picoseconds. Until now, though, no detector has been capable of measuring and proving this precisely, and the best results have only been achieved on nanosecond timescales, which are too slow to allow really efficient analysis or to be used to develop the most effective new systems.
University of Leeds researchers, working with international colleagues at École Normal Supérieure in Paris and the University of Queensland in Brisbane, have now used terahertz-frequency quantum-cascade lasers and terahertz time-domain spectroscopy to understand this laser-stabilization process by characterizing a laser's ultrafast switch-on dynamics, mode competition, and frequency selection.1
The technique can measure the wavelength of light in periods of femtoseconds, giving unprecedented levels of detail. By knowing the speed at which wavelengths change within lasers, and what happens during that process within minuscule time frames, more efficient devices and systems can be built. Mode-hopping can be observed on picosecond timescales.
"The benefits for commercial systems designers are potentially significant," says Iman Kundu, one of the Leeds researchers. "Terahertz technology isn't available to many sectors, but we believe its value lies in being able to highlight trends and explain the detailed operation of integrated photonic devices, which are used in complex imaging systems which might be found in the pharmaceutical or electronics sectors. Designers can then apply these findings to lasers operating at different parts of the electromagnetic spectrum, as the underlying physics will be very similar."
1. Iman Kundu et al., Nature Communications (2018); doi: 10.1038/s41467-018-05601-x.