1.55 μm integrated modelocked semiconductor lasers

This thesis presents research on, and the realization of compact InP/InGaAsP integrated passively modelocked lasers (MLL) operating in the 1.55 µm wavelength range. The goal of this work was to obtain modelocked laser designs at a repetition rate of several tens of GHz that can be integrated with other devices on a single semiconductor wafer. These modelocked lasers should be usable as optical pulse sources in an all-optical clock recovery application in optical time domain multiplexing (OTDM) systems. The integration of the modelocked laser on a single chip is achieved using the active-passive integration technique. This technique allows one to integrate active components such as optical amplifiers and saturable absorbers, with passive components such as waveguides and optical power splitters. The modelocking mechanism of the integrated lasers is passive modelocking using a slow saturable absorber. The saturable absorber is a short optical amplifier section that is reversely biased. The work was largely concentrated on ring laser type cavities. Such a configuration has many advantages. Firstly it allows one to fix the repetition rate of the laser by photolithography. It also provides better performance thanks to the two counter-propagating pulses which collide in the saturable absorber. Finally the output of the laser can be directly interconnected on the same wafer with other devices such as an all-optical switch or a pulse compressor. From the first realization of integrated ring modelocked lasers (RMLLs) using active-passive integration and a demonstration of a device at 27 GHz, many issues came up and have been addressed in this thesis. First, the understanding of the modelocking mechanism and other dynamics needed to be better understood. To address this issue, a simulation tool of RMLL was developed. Simulation results showed that symmetrical cavities show a much wider operating range for stable modelocking. The transitions from a modelocked state of the laser to another operating regime have been explored with the model. The simulation tool requires parameters describing the gain properties of the material. These have been accurately measured using a new type of high resolution spectrum analyzer. Another important issue which came out from the first RMLL realization was the necessity to reduce all the reflections inside the modelocked laser cavity and in particular the reflections at the active-passive interfaces. Special efforts have therefore been made to characterize the optical losses and reflections at those interfaces and to minimize them to a sufficiently low value of less than -50 dB. To validate techniques of fabrication and materials required to achieve high repetition rate RMLL designs, the realization of more compact devices through the use of deep etching has been investigated in this thesis. Results are presented on, at that time, the world’s most compact AWG using a double-etch technique, and the world’s first InAs/InP quantum dot (QD) lasers employing narrow deeply etched ridge active waveguides in the 1.55 µm wavelength region. Before realizing a final RMLL design on an active-passive wafer, a series of allactive devices has been designed, fabricated and characterized. These all-active chips provided material for the gain measurements and allowed to look further into short pulse laser characterization techniques and to test designs for reducing reflections from other intra-cavity components. The results of the all-active MLLs have been obtained in different configurations. Firstly, 20 GHz and 40 GHz linear all active Fabry-Pérot MLL (FPMLL) lasers have been successfully fabricated. Modelocking has been achieved with these lasers in the colliding pulse modelocked (CPM) and self CPM configurations. Pulse lengths down to 1.6 ps (at 20 GHz) have been observed. A 40 GHz repetition rate was demonstrated in a CPM laser with a Saturable Absorber (SA) positioned in the center of the FP cavity. All-active 15 GHz RMLLs have also been successfully fabricated. These lasers show a relatively good timing stability due to the ring configuration. Measured output pulses are highly chirped and an FWHM bandwidth of up to 4.5 nm was obtained. Such lasers with high bandwidth pulses and compatible with active-passive integration are of great interest for optical code division multiple access applications, where information is coded in the spectrum. Finally, first results from MLLs realized on an active-passive wafer are presented. Passive modelocking has been demonstrated in these integrated Extended Cavity FPMLLs with minimized intra-cavity reflections. Pulses of 2.1 ps duration and with a small pedestal have been observed. The pulses are close to transform-limited. The longer timescale dynamics of the EC-FPMLLs are reduced compared to the all-active FPMLLs, which is understood to be due to the short amplifier section. The use of a MLL at 20 GHz for the all optical clock recovery (AOCR) application and a special RMLL design for AOCR at 40 GHz are presented in the last chapter of this thesis. Many characteristics of the AOCR at 20 GHz could be quantified. The design of the 40 GHz RMLL laser is for an active-passive wafer. The design utilizes all the minimizations of small intra-cavity reflections. For the AOCR application a novel way to couple the optical input signal into the MLL via a separate waveguide is presented. Based on the accumulated results presented in this thesis the timing jitter of the clock recovered from this laser is expected to be sufficiently low to comply with the telecom requirements at 40 GHz.