44 research outputs found
Semianalytic theory of self-similar optical propagation and mode locking using a shape-adaptive model pulse
A semianalytic theory for the pulse dynamics in similariton amplifiers and
lasers is presented, based on a model pulse with adaptive shape. By changing a
single parameter, this test function can be continuously tweaked between a pure
Gaussian and a pure parabolic profile and can even represent sech-like pulses,
the shape of a soliton. This approach allows us to describe the pulse evolution
in the self-similar and other regimes of optical propagation. Employing the
method of moments, the evolution equations for the characteristic pulse
parameters are derived from the governing nonlinear Schr\"odinger or
Ginzburg-Landau equation. Due to its greatly reduced complexity, this
description allows for extensive parameter optimization, and can aid intuitive
understanding of the dynamics. As an application of this approach, we model a
soliton-similariton laser and validate the results against numerical
simulations. This constitutes a semianalytic model of the soliton-similariton
laser. Due to the versatility of the model pulse, it can also prove useful in
other application areas
Coherent instabilities in a semiconductor laser with fast gain recovery
We report the observation of a coherent multimode instability in quantum
cascade lasers (QCLs), which is driven by the same fundamental mechanism of
Rabi oscillations as the elusive Risken-Nummedal-Graham-Haken (RNGH)
instability predicted 40 years ago for ring lasers. The threshold of the
observed instability is significantly lower than in the original RNGH
instability, which we attribute to saturable-absorption nonlinearity in the
laser. Coherent effects, which cannot be reproduced by standard laser rate
equations, can play therefore a key role in the multimode dynamics of QCLs, and
in lasers with fast gain recovery in general.Comment: 5 pages, 4 figure
Terahertz Frequency Combs Exploiting an On-Chip, Solution-Processed, Graphene-Quantum Cascade Laser Coupled-Cavity.
The ability to engineer quantum-cascade-lasers (QCLs) with ultrabroad gain spectra, and with a full compensation of the group velocity dispersion, at terahertz (THz) frequencies, is key for devising monolithic and miniaturized optical frequency-comb-synthesizers (FCSs) in the far-infrared. In THz QCLs four-wave mixing, driven by intrinsic third-order susceptibility of the intersubband gain medium, self-locks the optical modes in phase, allowing stable comb operation, albeit over a restricted dynamic range (∼20% of the laser operational range). Here, we engineer miniaturized THz FCSs, comprising a heterogeneous THz QCL, integrated with a tightly coupled, on-chip, solution-processed, graphene saturable-absorber reflector that preserves phase-coherence between lasing modes, even when four-wave mixing no longer provides dispersion compensation. This enables a high-power (8 mW) FCS with over 90 optical modes, through 55% of the laser operational range. We also achieve stable injection-locking, paving the way to a number of key applications, including high-precision tunable broadband-spectroscopy and quantum-metrology
Sculpting harmonic comb states in terahertz quantum cascade lasers by controlled engineering
Optical frequency combs (OFCs), which establish a rigid phase-coherent
link between the microwave and optical domains of the electromagnetic
spectrum, are emerging as key high-precision tools for the development
of quantum technology platforms. These include potential applications
for communication, computation, information, sensing, and metrology
and can extend from the near-infrared with micro-resonator combs, up
to the technologically attractive terahertz (THz) frequency range,
with powerful and miniaturized quantum cascade laser (QCL) FCs. The
recently discovered ability of the QCLs to produce a harmonic
frequency comb (HFC)—a FC with large intermodal spacings—has attracted
new interest in these devices for both applications and fundamental
physics, particularly for the generation of THz tones of high spectral
purity for high data rate wireless communication networks, for radio
frequency arbitrary waveform synthesis, and for the development of
quantum key distributions. The controlled generation of harmonic
states of a specific order remains, however, elusive in THz QCLs.
Here, and by design, we devise a strategy to obtain broadband HFC
emission of a pre-defined order in a QCL. By patterning n regularly spaced defects on the top
surface of a double-metal Fabry–Perot QCL, we demonstrate harmonic
comb emission with modes spaced by an (n+1) free spectral range and with an
optical power/mode of ∼270µW.</jats:p
Passive and hybrid mode locking in multi-section terahertz quantum cascade lasers
It is believed that passive mode locking is virtually impossible in quantum cascade lasers (QCLs) because of too fast carrier relaxation time. Here, we revisit this possibility and theoretically show that stable mode locking and pulse durations in the few cycle regime at terahertz (THz) frequencies are possible in suitably engineered bound-to-continuum QCLs. We achieve this by utilizing a multi-section cavity geometry with alternating gain and absorber sections. The critical ingredients are the very strong coupling of the absorber to both field and environment as well as a fast absorber carrier recovery dynamics. Under these conditions, even if the gain relaxation time is several times faster than the cavity round trip time, generation of few-cycle pulses is feasible. We investigate three different approaches for ultrashort pulse generation via THz quantum cascade lasers, namely passive, hybrid and colliding pulse mode locking