When a laser produces a frequency comb, it emits waves of light that repeat themselves periodically in time.
Depending on the parameters of the comb, these waves can either have constant intensity while varying in color, or look like short pulses of light that build and drop in intensity.
Researchers know how combs produce pulses, but how so-called frequency-modulated lasers can maintain a constant intensity in the face of changing frequencies has been a long-lasting puzzle.
The team of researchers, led by Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering, were able to reconstruct on a time scale of a trillionth of a second the waveform emitted by light sources known as quantum cascade lasers, widely used in spectroscopy and sensing.
They found that the lasers choose to emit light waves in a way that not only suppresses the intensity fluctuations —leading to a constant intensity in time — but also maximizes the power output.
(Image courtesy of the Capasso Lab/Harvard SEAS) “We discovered that a frequency-modulated laser can adjust parameters by itself, similar to a DJ turning knobs on a music synthesizer, to minimize fluctuations of the emitted intensity wave,” said Marco Piccardo, a postdoctoral fellow at SEAS and first author of the paper.
In producing a nearly-flat intensity waveform, the frequency-modulated laser has solved a complex optimization problem, performing just like an analog computer.”