John Houlihan

Lasing from semiconductor microring on the end of an optical fiber

Isolated InGaAsP microrings with an outer diameter of 5.8 μm, a width of 1 μm, and a thickness of 0.41 μm were fabricated by epitaxial separation. Individual devices were bonded to multimode optical fiber using Van der Waals forces and optically pumped through the fiber. Lasing around 1505 nm was measured under pulsed and cw pumping at room temperature. The threshold pump power for pulsed operation was estimated to be 38 and 80 μW for cw operation. Multiple radial and azimuthal modes were present due to strong, three-dimensional confinement. Under strong pulsed pumping thermal effects caused the emission wavelength to chirp.

High brightness semiconductor lasers with reduced filamentation

High brightness semiconductor lasers have many applications. The main difficulty associated with high brightness is that, because of COD, high power requires a large aperture. Large apertures result in high order transverse modes, filamentation and spatio-temporal instabilities, all of which degrade spatial coherence and therefore brightness.

Differential refractive index in 1320 nm quantum-dot lasers

In this paper we resolve the separate contributions of resonant and non-resonant carrier populations to the differential index below threshold. We then apply this understanding to the explanation of index shifts above and below laser threshold as a function of device gain saturation.

Delay induced excitability in VCSELs

We analyse the stochastic polarization fluctuations in a vertical cavity surface emitting laser (VCSEL) under the influence of electro-optical feedback and show that the dynamics can be modeled as a bistable system with time-delayed memory. Assuming an asymmetric potential, we show the existence of a regime in which the systems dynamic displays excitability. We calculate the relevant residence time distributions and correlation times and compare our system to a well known discrete model for excitability. Finally, we present experimental data that demonstrates excitable behaviour in the polarization dynamics of a VCSEL and, in particular, show the appearance of coherence resonance.

Dynamics of a self-pulsing laser with delay

We analyse the dynamics of a self-pulsating semiconductor laser with optical feedback. Without re-injection of light the laser displays periodic oscillations. At very weak feedback levels we observe an amplitude instability whose frequency increases with the feedback level until the laser enters the low frequency fluctuation regime commonly observed in cw lasers with optical feedback. We show that such behaviour can be observed within the framework of the Lang Kobayashi equations for self-pulsating semiconductor lasers.

Bistable systems with noise and delay

The polarisation dynamics of vertical cavity surface emitting lasers (VCSELs) in the bistable regime is well described by Kramers theory for noise induced transitions. By employing feedback, a memory mechanism can be introduced, which make the dynamics of the system non-Markovian. Here we analyse theoretically and experimentally the residence time distribution of the bistable systems in the presence of noise and time-delayed feedback, using an opto-electronic feedback cycle for a VCSEL. We demonstrate and explain various non-exponential features of the residence time distribution using a continuous as well as a two-state model. Additionally we compare the results to an electronic Schmitt trigger, which represents an experimental realization of the two-state model.

Mode structure of quantum dot semiconductor lasers

An analysis of the transverse and longitudinal mode structure of broad area quantum dot lasers emitting at 1060 nm is presented. In particular, temperature is shown to play an important role in the stabilisation of the transverse mode structure of the devices. In addition, the investigation of the interaction between these transverse modes, through the measurement of the spatial intensity correlation, shows that the laser retains some modal properties in the unstable regime. Finally, measurements of spectral correlations between longitudinal mode groups display a strong dependency on their respective transverse mode structures indicating the importance of spatial overlap.

Current profiling in broad-area semiconductor lasers

We describe the different mechanisms to generate waves in the transverse section of lasers. Our analysis, based on the Maxwell-Bloch equations, is compared to recent experimental results.