Uwe Bandelow

Mechanisms of fast self pulsations in two-section DFB lasers

We show theoretically, that the detuning between the resonance frequencies of differently pumped DFB sections gives rise to two different pulsation mechanisms, 1) dispersive self Q-switching of a single-mode and 2) beating oscillations between two modes of nearly equal threshold gain. Our analysis is based on the dynamic coupled wave equations accomplished with carrier rate equations. We demonstrate the existence of certain isolated values of the detuning between both sections, at which two longitudinal eigenmodes become degenerate. In the degeneration point, the longitudinal excess factor of spontaneous emission has a singularity and the system of eigenmodes becomes incomplete. We derive reduced equations governing the dynamics in the vicinity of degeneration points. For an example device, the numerical integration of these equations clearly demonstrates the two different self pulsations with repetition rates of more than 100 GHz.

Improving the Modulation Bandwidth in Semiconductor Lasers by Passive Feedback

We explore the concept of passive-feedback lasers (PFLs) for direct signal modulation at 40 Gb/s. Based on numerical simulation and bifurcation analysis, we explain the main mechanisms in these devices that are crucial for modulation at high speed. The predicted effects are demonstrated experimentally by means of correspondingly designed devices. In particular, a significant improvement of the modulation bandwidth at low-injection currents can be demonstrated

Measurement and Simulation of Distributed-Feedback Tapered Master-Oscillator Power Amplifiers

The spectral and spatial behavior of monolithically integrated distributed-feedback tapered master-oscillator power amplifiers emitting around 973 nm is experimentally and theoretically investigated. We demonstrate a good agreement between experiments and theory and analyze peculiarities of the observed dynamical regimes.

Dispersive self-Q-switching in self-pulsating DFB lasers

Self-pulsations reproducibly achieved in newly developed lasers with two distributed feedback sections and with an additional phase tuning section are investigated. The existence of the dispersive self-Q-switching mechanism for generating the high-frequency self-pulsations is verified experimentally for the first time. This effect is clearly distinguished from other possible self-pulsation mechanisms by detecting the single-mode type of the self-pulsation and the operation of one section near the transparency current density using it as a reflector with dispersive feedback. The operating conditions for generating this self-pulsation type are analyzed. It is revealed that the required critical detuning of the Bragg wavelengths of the two DFB sections is achieved by a combination of electronic wavelength tuning and current-induced heating. The previous reproducibility problems of self-pulsations in two-section DFB lasers operated at, in principle, suited current conditions are discussed, and the essential role of an electrical phase-control section for achieving reproducible device properties is pointed out. Furthermore, it is demonstrated that phase tuning can be used for extending the self-pulsation regime and for optimizing the frequency stability of the self-pulsation. Improved performance of the devices applied as optical clocks thus can be expected.

Model for mode locking in quantum dot lasers

We propose a model for passive mode locking in quantum dot lasers and report on specific dynamical properties of the regime which is characterized by a fast gain recovery. No Q-switching instability has been found accompanying the mode locking. Bistability can occur between the mode locking regime and the nonlasing state.

Calculation of combined lateral and longitudinal spatial hole burning in lambda /4 shifted DFB lasers

A quasi-3-D semiconductor laser model that takes into account both longitudinal and lateral spatial hole burning is presented and used to perform calculations of the longitudinal side-mode-suppression ratio, the lateral side-mode kink power, and the linewidth of devices with coupling up to kappa L=6. Detailed results on the effects of inhomogeneous injection, wavelength detuning, phase shift position, and optical losses on these quantities are discussed and used to draw conclusions on the regions of stable single-mode operation. >

Theory of selfpulsations in two-section DFB lasers

A dynamic theory of asymmetrically pumped two-section distributed-feedback (DFB) lasers is presented, which gives a first explanation for the nature of the recently observed gigahertz-selfpulsations in terms of the relative shift between the feedback spectra of the two sections.<<ETX>>

Dispersive self-Q-switching in DFB lasers-theory versus experiment

For the first time, the single mode theory of high speed self-pulsations is applied to InP ridge waveguide three section DFB lasers. Good agreement with measurements confirms the nature of the self-pulsations as dispersive self Q-switching.

Simulation of static and dynamic properties of edge-emitting multiple-quantum-well lasers

This paper demonstrates simulation tools for edge-emitting multiple-quantum-well (MQW) lasers. Properties of the strained MQW active region are simulated by eight-band kp calculations. Then, a two-dimensional simulation along the transverse cross section of the device is performed based on a drift-diffusion model, which is self-consistently coupled to equations for the optical field. Furthermore, a method is described that allows for an efficient quasi-three-dimensional simulation of dynamic properties of multisection edge-emitting lasers.

Frequency regions for forced locking of self-pulsating multi-section DFB lasers

A method is developed which allows for the calculation of locking regions of self-pulsating multi-section lasers which are exposed to external optical data sequences. In particular, resonant locking is investigated where both wavelength detuning and detuning of the modulation frequency are important.