Wolfgang Elsäßer

Transverse modes in oxide confined VCSELs: Influence of pump profile, spatial hole burning, and thermal effects

We present experimental studies on the transverse mode emission behaviour of oxide-confined Vertical Cavity Surface Emitting Lasers (VCSELs). VCSELs with aperture diameters of 6um and 11um exhibit a wide variety of emission patterns from low order Hermite- Gaussian modes to high order Laguerre-Gaussian modes. We obtain detailed information about the spatial gain distribution by recording spontaneous emission intensity profiles during lasing operation. We conclude from these profiles, that the spatial carrier distribution is primarily governed by the in uence of pump induced current spreading and is only secondarily in uenced by further effects such as spatial hole burning, and thermal gradients in the laser. The combination of these mechanisms causes a strong tendency towards the emission of high order transverse modes.

Dynamics of Semiconductor Lasers Subject to Delayed Optical Feedback

We give experimental and numerical evidence for a new dynamical regime in the operation of semiconductor lasers subject to delayed optical feedback occurring for short delay times. This short cavity regime is dominated by a striking dynamical phenomenon: regular pulse packages forming a robust low-frequency state with underlying fast, regular intensity pulsations. We demonstrate that these regular pulse packages correspond to trajectories moving on global orbits comprising several destabilized fixed points within the complicated phase space structure of this delay system.

Measurements of the alpha factor of a distributed-feedback quantum cascade laser by an optical feedback self-mixing technique.

We demonstrate measurements of the alpha factor of a distributed-feedback quantum cascade laser (QCL) by using a newly modified self-mixing interferometric technique exploring the laser itself as the detector. We find a strong dependence of the alpha factor on the injection current, ranging from -0.44 at 120 mA to 2.29 at 180 mA, which can be attributed to the inherent physics of QCLs.

Intensity noise properties of quantum cascade lasers

We present investigations of the the relative intensity noise (RIN) of a quantum cascade laser (QC) laser in continuous wave operation. We analyze the intensity noise properties in terms of the relative intensity noise (RIN). In contrast to conventional interband semiconductor diode lasers we obtain a different scaling behavior of RIN with increasing optical output power for QC lasers. From a semiclassical noise model we find that this result is due to the cascaded active regions each incorporating three laser levels, and is therefore a particular feature of QC lasers.

Timing jitter reduction of passively mode-locked semiconductor lasers by self- and external-injection: numerical description and experiments.

In this paper the influence of different feedback (FB) and synchronization schemes on the timing phase noise (TPN) power spectral density (PSD) of a quantum-dot based passively mode-locked laser (MLL) is studied numerically and by experiments. The range of investigated schemes cover hybrid mode-locking, an opto-electrical feedback configuration, an all-optical feedback configuration and optical pulse train injection configuration by means of a master MLL. The mechanism responsible for TPN PSD reduction in the case of FB is identified for the first time for monolithic passively MLL and relies on the effective interaction of the timing of the intra-cavity pulse and the time-delayed FB pulse or FB modulation together with an statistical averaging of the independent timing deviations of both. This mechanism is quantified by means of simulation results obtained by introducing an universal and versatile simple time-domain model.

Rainbow refractometry with a tailored incoherent semiconductor laser source

The authors demonstrate within a metrology experiment the applicability of a recently proposed temporally incoherent semiconductor laser source which relies on nonlinear dynamics. The realized spectrally broadband emission with an output power of 110mW and a coherence length of only 120μm is used in a rainbow refractometry experiment for sizing of liquid droplets, representing an important problem in industrial processes. The observed emission characteristics are attractive for implementation of modern imaging and metrology techniques which are based on the properties of well-directed, temporally incoherent light.

Thermally induced local gain suppression in vertical-cavity surface-emitting lasers

Joule heating is one of the dominant mechanisms determining the transverse mode formation in vertical-cavity surface-emitting lasers at high injection currents. We give experimental evidence that in this operation regime, strong heating results in local gain suppression in the center of the laser, which overbalances the confining effect of thermal lensing, and thus favors the formation of high order modes. From our investigations of small aperture devices, we conclude that efficient heat removal is crucial for achieving single mode emission at high injection currents.

Highly nondegenerate four-wave mixing in a tunable dual-mode semiconductor laser

We present experimental investigations of highly nondegenerate four-wave mixing in a tunable dual-mode semiconductor laser. The fundamental interacting waves are two lasing modes selected in an external double Littman–Metcalf cavity configuration. We investigate the conversion efficiency depending on the detuning frequencies up to 1.2THz. We find that the newly generated waves are significantly enhanced due to the cavity resonances. Our investigations allow us to characterize and understand the dynamics of the simultaneous dual-mode operation in the semiconductor laser, which is attractive for the generation of continuous-wave THz radiation by photomixing.

Evolution from modal to spatially incoherent emission of a broad-area VCSEL

Broad-area vertical-cavity surface-emitting lasers (BA-VCSELs) can exhibit a state of spatially incoherent emission, as we recently reported in [M. Peeters et al., Opt. Express, 13, 9337 (2005)]. Here, we experimentally study the evolution of a BA-VCSEL under pulsed operation from well-defined modal emission with a multitude of transverse cavity modes to such spatially incoherent emission. The transition is studied using a high-speed intensified CCD camera and differential image analysis with which single-shot measurements of the imaged nearfield, farfield, spatial coherence, and spectral emission properties are acquired. This combination of experimental characterization tools allows for a detailed description of the BA-VCSELs emission behavior, which is necessary for an in-depth understanding of the processes involved. We find the interplay between the thermal chirp and the build-up of a spatially distributed thermal lens to be decisive for the break-up of the global cavity modes.

Generation and spectroscopic application of tunable continuous-wave terahertz radiation using a dual-mode semiconductor laser

We present a tunable continuous-wave terahertz radiation source based on photomixing of a tunable dual-mode semiconductor laser on a log-periodic antenna. The discontinuously tunable dual-mode emission of the laser is achieved by spectrally filtered feedback in a double-external-cavity configuration. Two different two-frequency lasing regimes have been identified: pure dual-mode and dual-mode-comb emission. We spectrally investigate the laser emission in these two regimes and study the consequences for the generation of terahertz radiation. Under pure dual-mode emission, we achieve higher power, higher coherence and good tunability of the THz radiation. We demonstrate by detecting absorption lines of water vapor and hydrochloric acid gas that our concept for a tunable continuous-wave terahertz radiation source offers attractive properties for spectroscopic applications.