We have analyzed experimentally and theoretically the modal properties of a semiconductor ring laser and the
wavelength jumps that occur in connection with directional switching above threshold. A transfer matrix analysis
allow us to explain the transfer function measurements when amplified spontaneous emission in the cavity is
accounted for. Moreover the transfer matrix analysis permits to determine the threshold condition for the laser
modes, which split in two branches due to the symmetry breaking imposed by the output coupler and output
waveguides. The wavelength jumps displayed by the device above threshold are interpreted with the frequency
splitting and threshold difference between these two branches of solutions, together with the redshift of the
material gain.
We analyze a rate equation model in the Langevin formulation for the two modes of the electric field and the
carrier density, modelling the spontaneous emission noise in a semiconductor ring laser biased in the bidirectional
regime. We analytically investigate the influence of complex backscattering coefficient when the two modes
are reinterpreted in terms of mode-intensity sum (I-Spectrum) and difference (D-spectrum). The D-spectrum
represents the energy exchange between the two counterpropagating modes and it is shaped by the noisy precursor
of a Hopf bifurcation influenced mainly by the conservative backscattering. The I-Spectrum reflects the energy
exchange between the total field and the medium and behaves similarly to the standard relative intensity noise
for single-mode semiconductor lasers. Good agreement between analytical approximation and numerical results
is found.
The optical spectrum of monolithic Semiconductor Ring Lasers (SRLs) is measured simultaneously for both lasing
directions with a grating-based OSA, in the regimes of bidirectional and unidirectional operation. In the unidirectional
operation regime the SMSR is larger than 25 dB, and the directional extinction ratio (i.e., the ratio of the power emitted
in the two opposite directions) is larger than 20 dB. The influence of the current injected in the active output waveguides
that act as SOAs is outlined. In the unidirectional regime the linewidth of the SRL is measured by an heterodyne
technique, revealing linewidth values around 2 MHz.
We review theoretical results on the dynamics of solitary single longitudinal mode and single transversal mode
semiconductor ring lasers. These analyses are based on a rate equation model for the slowly varying envelopes
of the counter-propagating fields in the ring cavity which has been proposed by Sorel et al. [Opt. Lett. 27, 1992
(2002); IEEE J. Quantum Electron. 39, 1187 (2003)]. The model shows several operating regimes. The lasers
are found to operate bidirectionally up to twice the threshold, where unidirectional operation starts. Just above
threshold, the lasers operate in a regime where the two counterpropagating modes are continuous wave, while
as the injected current is increased, a regime appears where the intensities of the two counterpropagating modes
undergo alternate sinusoidal oscillations. To understand these dynamical features, we discuss a reduction of this
basic rate equation model derived by Van der Sande et al. [accepted for publication in J. Phys. B (2008)]. The
reduction has been achieved using asymptotic methods based on the typical relative scaling of the dynamical time
scales of the system. Physical conditions for the emergence of the operating regimes are assessed quantitatively in
terms of nonlinear (saturation processes) and linear coupling (backscattering) between the counter-propagating
modes.
We review three two-mode models for different semiconductor laser structures: Vertical-Cavity Surface-Emitting Lasers (VCSELs), Twin-Stripe Semiconductor-Lasers (TSSL), and Semiconductor Ring Lasers (SRL). The VCSELs model and TSSL model display rich dynamic behavior when a saturable absorber is embedded in the cavity. VCSELs with saturable absorber showed polarization chaos, which found applications in encoded communications; TSSLs with saturable absorber show coherent locked states as well as chaotic behavior; and SRLs show a complex two-mode dynamics giving rise to bidirectional operation, alternate oscillations and spontaneous symmetry breaking toward quasi-unidirectional bistable solutions, with potential applications to all-optical switching.
We have theoretically investigated the bifurcation scenario that leads to the emergence of a bistable regime in a two-mode model for a Semiconductor Ring Laser. The bistability takes place between two quasi-unidirectional solutions for the electric field, which are selected as stable solutions via gain-crossaturation, for well-above threshold operating conditions. Furthermore, we analyzed the switching properties of a single Semiconductor Ring Laser (SRL) operating in the bistable regime, under coherent optical pulse injection, in view of the possible implementation of a single SRL
an optically adressable memory element. The result is that the response time and the minimum switching energy respectively attain values the order of a few tenth of ps, and 1 fJ. Those values are espected to scale down with the device radius, due to the consequent decreasing of the cavity flight time. We have observed that the fast switching dynamic is due to an energy redistribution process between the two counterpropagating modes, that does not involve the (slow) carrier density through field-medium energy exchange processes. This allows to attain time scales much faster than the typical limit represented by the inverse of relaxation oscillation frequency.
Theoretical investigation and device measurements are reported to demonstrate the strict fabrication requirements of small diameter shallow etched semiconductor ring lasers. A very accurate control over the dry etching depth is crucial to both minimise the bending losses and achieve very precise control of the coupling ratio in directional couplers. A reactive ion etching process was developed on Aluminium-quaternary wafer structures, showing selectivity greater than 30 between the AlInAs core layer and the InP upper cladding. The process proved very effective in providing a complete and controllable etching of directional couplers with 500nm wide gaps. Assessment on the effect of the bending losses and on the minimum ring radius was performed through characterisation of half ring lasers. A minimum current threshold of 34mA is reported on 150μm ring radius devices emitting at 1300nm.
In this work we numerically calculate the linewidth of a single-mode monolithic Semiconductor Ring Laser (SRL) operating in the unidirectional regime. A new expression for the SRL linewidth is derived from the conventional Henry's formula, and the importance of different physical parameters is discussed. In particular, the linewidth is mainly determined by the SRL diameter, because the waveguide bending loss have a very strong dependence on the diameter. We show that circular SRLs with diameter smaller than 180 μm are unlikely to be operated CW. As a general rule, the linewidth decreases for increasing SRL diameter, and it varies from 15 MHz for 200 μm diameter down to 1 MHz for 2000 μm diameter.
In this work we present experimental results on the auto-correlation and cross-correlation properties of the two counterpropagating modes of a monolithic semiconductor ring laser. The ring laser can operate in a bidirectional regime where the two modes have equal power, and also in a unidirectional regime where one of the modes is almost suppressed. Auto-correlation measurements, that are carried out using an unbalanced Mach-Zehnder fiber interferometer, allows to determine the coherence length and linewidth of the ring laser. Cross-correlation measurements are carried out using a modifed interferometric set-up, and they reveal that the two counterpropagating modes are phase-locked.
In this contribution, we analyze the synchronization properties of two chaotic optical field generated by two VCSELs in presence of a saturable absorber. The vectorial nature of the chaotic field requires a continuous control scheme to achieve a high-quality synchronization between the two sources. We proposed a novel encoding scheme, exploiting a two contact VCSEL structure, which allows an easy and direct modulation of the transmitter. The added degree of freedom constituted by the chaos in polarization allows a reduction of the probability error (or enhancement of the transmission capacity) at the receiver.
We develop a model that describes the polarization dynamics of VCSELs with an absorbing region surrounding the active zone. We find that the dynamical behavior of the optical field is organized in four regions: stable linearly-polarized operation, intensity pulsations of a linearly-polarized component, pulsations of both total intensity and polarization, and polarization self-pulsations with constant total intensity. We characterize the four regions by computing the polarization resolved optical and power spectra. The predicted scenario agrees with recent experimental results.
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