W. Elsasser

ON/OFF phase shift keying for chaos-encrypted communication using external-cavity semiconductor lasers

Synchronization phenomena of two chaotically emitting semiconductor lasers subject to delayed optical feedback are investigated. The lasers are unidirectionally coupled via their optical fields. Our experimental and numerical studies demonstrate that the relative optical feedback phase is of decisive importance: a characteristic synchronization scenario evolves under variation of the relative optical-feedback phase mediating cyclically between chaos synchronization in conjunction with coherent fields, and uncorrelated states in conjunction with incoherent fields. As a key result, we propose, and numerically demonstrate, a novel ON/OFF phase shift keying method opening up new perspectives for applications in communication systems using chaotic carriers.

Control of the spatiotemporal emission of a broad-area semiconductor laser by spatially filtered feedback

We demonstrate the control of the spatiotemporal emission dynamics of a broad-area semiconductor laser in an external optical feedback configuration formed by a spatially filtering mirror. The emission properties are studied with a single-shot streak camera with temporal resolution of approximately 7 ps and spatial resolution of approximately 5 microm. Our results show a significant reduction of the spatial filamentation and, furthermore, suppression of the spatiotemporal instabilities, which are both intrinsic emission properties of standard high-power broad-area lasers. Associated with the control of the emission dynamics, strong improvement of the beam quality, which is essential for numerous high-power applications, is possible.

Influence of anisotropies on transverse modes in oxide-confined VCSELs

We present a comprehensive fully vectorial model for the cavity eigenmodes of oxide-confined vertical-cavity surface-emitting lasers (VCSELs) with the details of their complex structure. It includes device-inherent symmetry-breaking mechanisms like noncircular geometries and material anisotropies related to the elasto-optic and electro-optic effect. The latter is accounted for in the model starting from the material and doping profiles. We compare these theoretical results with experimental findings of spectrally and polarization-resolved transverse mode nearfields of oxide-confined VCSELs with two different aperture diameters. Within a parametric study of the influence of aperture anisotropies, we are able to calculate frequencies and gains of all transverse mode families, their polarization dependence and their spatial mode profiles which are in good agreement with the experimental findings.

Polarization-switching influence on the intensity noise of vertical-cavity surface-emitting lasers

We experimentally investigate the influence of polarization switching on the intensity noise of transverse single-mode vertical-cavity surface-emitting lasers. We show that a polarization switch always leads to strong noise in the two individual orthogonal polarization modes over a large range of pump current in the vicinity of the switch, whereas the effect on the total emission noise strongly depends on the strength of the coupling between total intensity noise and polarization fluctuations, which is mainly determined by the amount of birefringence. Thus our results demonstrate that the occurrence of a polarization switch can have a strong deteriorating effect on the noise behavior, not only if polarization selection is present, but also on the total emission noise, and therefore has to be considered in future investigations and low-noise applications.

Joint Experimental and Theoretical Investigations of Two-State Mode Locking in a Strongly Chirped Reverse-Biased Monolithic Quantum Dot Laser

The influence of different biasing conditions of a passively mode-locked strongly chirped two-section quantum-dot laser on its ground state (GS) and excited state (ES) emission is investigated both experimentally and theoretically. Numerical simulations performed with a modified time-domain travelling-wave model are in qualitative agreement with experimental results and allow to identify the origin of the experimentally observed mode-locking regimes: the complex time-dependent coupling between the ES and GS absorption bleaching in the saturable absorber section. This coupling, together with the difference in group velocities of ES and GS pulses, is found to be responsible for low-frequency inter-modulation signatures as observed experimentally and verified in the simulations.

Reverse-emission-state-transition mode locking of a two-section InAs/InGaAs quantum dot laser

Reverse-emission-state-transition mode locking in a two-section InAs/InGaAs quantum dot laser is experimentally investigated and confirmed by simulations. Stable mode locking starts on the first excited state (λ=1207 nm) and then, with increasing gain current, a transition to stable simultaneous two-state mode locking on excited state and ground state (λ=1270 nm) takes place. This particular state-transition occurs already at 0 V reverse-bias and at moderate gain-section currents. It is attributed to the strong active region chirping of the gain medium and in particular to a photon pumping process in the saturable absorber section.

Dynamic operation of all-optical Flip-Flop based on a monolithic semiconductor ring laser

A monolithic semiconductor ring laser (SRL) is demonstrated as a fully all-optical flip-flop that can be triggered by optical pulses of 400 ps duration, showing 130 ps response time.

Linewidth Enhancement Factor of Semiconductor Lasers: Results from Round-Robin Measurements in COST 288

Round-robin measurements on the linewidth enhancement factor are carried out within several laboratories participating to EU COST 288 action. The alpha-factor is measured by applying up to 7 different techniques. The obtained results are compared.

Intensity Noise Properties of Mid-Infrared Injection Locked Quantum Cascade Lasers: I. Modeling

In this paper, we numerically investigate the effect of optical injection locking on the noise properties of mid-infrared quantum cascade lasers. The analysis is carried out by means of a rate equation model, which takes into account the various noise contributions and the injection of the master laser. The obtained results indicate that the locked slave laser may operate under reduced intensity noise levels compared with the free running operation. In addition, optimization of the locking process leads to further suppression of the intensity noise when the slave laser is biased close to the free-running threshold current. The main factors that significantly affect the locking process and the achievable noise levels are the injected optical power and the master-slave frequency detuning.

Amplitude-squeezed emission from a transverse single-mode vertical-cavity surface-emitting laser with weakly anticorrelated polarization modes.

We discuss the polarization-resolved intensity noise characteristics of a transverse single-mode vertical-cavity surface-emitting laser (VCSEL). Measurements by a sensitivity-enhanced lock-in amplifier detection scheme yield only an imperfect anticorrelation between the strong and the weak orthogonally polarized fundamental modes. Yet the total emission shows amplitude squeezing of 0.4 dB below the shot-noise level. Unlike for transverse two-mode emission, a perfect anticorrelation between the two polarization modes is not necessary for generation of amplitude-squeezed emission in a transverse single-mode VCSEL, in agreement with theoretical predictions.