Krassimir Panajotov1,
Mikel
Arizaleta Arteaga1,2, Miguel Valencia1, Marc
Sciamanna3,
Manuel Lopez-Amo2, Hugo Thienpont1
1Department
of Applied Physics and
Photonics, Vrije Universiteit Brussel,
2Department
of Electrical and Electronic Engineering,
Public
3Supelec,
LMOPS CNRS,
Feedback from an extremely-short
external-cavity in VCSELs and
experimental observation of coherence resonance
We first discuss the effect of
isotropic optical feedback from an extremely short external cavity
(ESEC) on
the emission properties of Vertical-Cavity Surface-Emitting Lasers
(VCSELs).
When changing the external cavity length a modulation of the total
power and
the wavelength emitted by the VCSEL is observed with a period of half
the emission
wavelength. By making use of a two modes rate equation VCSEL model we
develop a
map of bistability to investigate the parametric dependence of
polarization
properties of VCSELs in such configuration, finding out a periodic
dependence
of the polarization switching currents on the ESEC length. By
increasing the
external mirror reflectivity we can make this periodic dependence
stronger and
strongly asymmetric providing the possibility to prevent PS for any
injection
current and thus, achieving polarization stabilization in VCSELs.
Further
numerical simulations with isotropic and non isotropic feedback show
how
parameters as gain compression coefficients, frequency splitting
between the
linearly polarized modes, differential gain and the mirror
reflectivity, affect
the map of bistability. The theoretical results are supported by
extensive
experimental mapping. The switching currents and the hysteresis width
can be
widely tuned by varying the external cavity length. The amplitude of
modulation
of the polarization switching current with the external cavity length
is
experimentally confirmed to be proportional to the external mirror
reflectivity, proving its key role in achieving polarization control of
such
lasers using optical feedback. Moreover, by a proper choice of the
optical
feedback parameters, the emission can be stabilized in any of the two
linearly
polarized modes. Finally, by making use of the fine-polarization tuning
provided by the ESEC feedback we report on the experimental observation
of
coherence resonance in a bistable system with delay. Our system
consists of a
VCSEL subject to time-delayed optical feedback simultaneously from a
long and
from extremely short external cavity.
Coherence resonance is experimentally proven by analysis of the
residence
time distribution of the polarization mode-hopping regime and of the
signal to
noise ratio in the power spectrum.