In this paper we present recent results obtained in the area of monolithically integrated modelocked semiconductor laser systems using generic InP based integration platform technology operating around 1550nm. Standardized components defined in this technology platform can be used to design and realize short pulse lasers and optical pulse shapers. This makes that these devices can be realized on wafers that can contain many other devices. In the area of short pulse lasers we report design studies based on measured optical amplifier performance data. This work has the ultimate goal to establish a library of widely applicable short pulse laser designs. Such lasers can include components for e.g. wavelength control. An important boundary condition on the laser design is that the laser can be located anywhere on the InP chip. In the area of pulse shaping we report on a 20 channel monolithic pulse shaper capable of phase and amplitude control in each channel. Special attention is given to the calibration of the phase modulator and amplifier settings. Pulse compression and manipulation of pulse generated from modelocked semiconductor lasers is demonstrated using a 40 GHz quantum dash modelocked laser.
Nonlinear polarization rotation in semiconductor optical amplifiers has been the focus of a lot of work in the past decade. A lot of research has been devoted to this phenomenon due to its possible use in all-optical switching. It has been mentioned as a possible competitor to such established switching techniques as cross-gain modulation, cross-phase modulation and four-wave mixing. The speed at which the switching can be performed is determined by the gain dynamics in the device. So far the majority of the work has focused on switching due to the relatively slow carrier density recombination, which limits the switching to the order of tens of gigahertz. If the polarization dependence of ultrafast gain mechanisms such as carrier heating and spectral hole burning can be identified and measured then there is the possibility to increase the switching speed obtainable using this process into the terahertz range. In order to further the understanding of the polarization dependence of the gain of a bulk SOA under tensile strain and to determine the plausibility of ultrafast all-optical switching using nonlinear polarization rotation an experiment is presented based on a four-wave mixing technique.
All-optical regeneration at 40 Gbit/s and beyond appears to be a crucial element for future transparent networks. One solution to achieve the regeneration is an all-optical clock recovery element combined with a Mach-Zehnder interferometer. Among the different approaches investigated so far, a scheme based on a single self-pulsating distributed Bragg reflector laser is of particular interest from practical and cost viewpoints. In this structure at least two longitudinal modes beat together, generating power oscillation at 40-GHz even though the laser is DC-biased. The 40-GHz signal frequency is fixed by the free spectral range of the cavity, and in the case of the self-pulsation, it has been demonstrated that its linewidth is smaller than the sum of the linewidth of the lasing modes. It is believed that the signal benefits from the phase correlation of the optical modes through the interband four-wave mixing (FWM) non-linearity. The FWM results as a modulation of the carrier population, leading to a non-linear gain and refractive index modulation, affecting both the amplitude and the phase of the cavity modes. Based on a four-wave mixing formalism a theoretical model has been developed and it corroborates the experimental linewidth measurement.
A contra-propagation set up is implemented and dynamic pump probe studies of a InGaAsP/InP SOA in the gain regime are undertaken using pulses of 2ps duration. The time resolved amplified probe signal is measured separately for the TE and TM modes of the semiconductor optical amplifier. Different behaviours are observed both in the gain compression and the timescales of the effect, with the TM mode displaying a faster component and a higher gain compression.
All-optical regeneration at 40 Gbit/s and beyond is a crucial element for future transparent networks. One solution to achieve the regeneration is an all-optical clock recovery element combined with a
Mach-Zehnder interferometer. Among the different approaches investigated so far to accomplish the clock recovery function, a scheme based on a single self-pulsating distributed Bragg reflector laser is of particular interest from practical and cost viewpoints. In this structure at least two longitudinal modes beat together, generating power oscillation even though the laser is DC biased. The oscillation frequency is given by the free spectral range of the structure. In order to optimize the clock recovery performance of such a laser, a model based on four-wave-mixing has been developed. It takes into account the evolution of the amplitude and the phase of the complex electricfield of each longitudinal mode. From this model, a stability analysis is derived through the adiabatic approximation. The spectral density of the correlated phases of these modes is calculated and compared to the uncorrelated spectral density of each mode.
We present experimental and theoretical investigations of the temperature dependence of self-pulsation in CD laser diodes. We use a rate equation model to predict the device dynamic behavior over a large temperature range and identify the role of carrier diffusion. We show experimentally and by calculating that the temperature dependence of the threshold current is driven by the carrier diffusion--particularly at low temperature. We experimentally show that for several temperatures the self-pulsation variation with respect to normalized bias current is highly linear. These results call into question whether pulsations in CD laser structures are undamped relaxation oscillations. Our results also suggest that the highly temperature dependent carrier diffusion does not play a first order role in CD laser diode self- pulsation.