Abstract
In standing-wave lasers, spatial hole burning induces a static grating of the population inversion, enabling multimode operation with several independent lasing modes. In the presence of a mode-locking mechanism, these modes may become correlated, giving origin to a frequency comb. Quantum cascade lasers, owing to their ultrafast gain dynamics, are ideally suited to achieve comb operation. Here we experimentally demonstrate that the modes of a quantum cascade laser frequency comb coherently beat to produce time-dependent population inversion gratings, which spatially modulate the current in the device at frequencies equal to the mode separation and its higher harmonics. This phenomenon allows the laser to serve as a phased collection of microwave local oscillators and is utilized to demonstrate quadrature amplitude modulation, a staple of modern communications. These findings may provide for a new class of integrated transmitters, potentially extending from the microwave to the low terahertz band.
Highlights
In a homogeneously broadened standing-wave laser, the first lasing mode arising in the laser will induce, assuming negligible carrier diffusion, a static grating of population inversion with minima corresponding to the antinodes of the mode intensity [Figs. 1(a) and 1(b)] and a period of a half optical wavelength in a process known as spatial hole burning (SHB) [1]
Various studies have focused on the effects of static SHB in semiconductor lasers—mostly distributed feedback (DFB) structures—to investigate the origin of single-mode instabilities, making use of optical techniques that relate the profile of spontaneous emission across the laser to the carrier distribution via the bimolecular recombination rate [2,3,4]
Dynamic population gratings have been studied in different systems: in DFB lasers, dynamic SHB was created by modulating the drive current of the device with a microwave signal [5], while in bare gain media it was shown that intersecting two tunable laser beams produced a moving population grating oscillating at the frequency difference of the two beams [6,7]
Summary
In a homogeneously broadened standing-wave laser, the first lasing mode arising in the laser will induce, assuming negligible carrier diffusion, a static grating of population inversion with minima corresponding to the antinodes of the mode intensity [Figs. 1(a) and 1(b)] and a period of a half optical wavelength in a process known as spatial hole burning (SHB) [1]. We present a conceptually simple microwave measurement capable of probing time-dependent population inversion gratings in quantum cascade laser (QCL) frequency combs.
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