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

Hidden nonlinear supersymmetries in pure parabosonic systems

The existence of intimate relation between generalized statistics and supersymmetry is established by observation of hidden supersymmetric structure in pure parabosonic systems. This structure is characterized generally by a nonlinear superalgebra. The nonlinear supersymmetry of parabosonic systems may be realized, in turn, by modifying appropriately the usual supersymmetric quantum mechanics. The relation of nonlinear parabosonic supersymmetry to the Calogero-like models with exchange interaction and to the spin chain models with inverse-square interaction is pointed out.Comment: 20 pages, one reference corrected, to appear in Int. J. Mod. Phys.

Passively modelocked 15, 20 and 40 Ghz bulk InGaAsP lasers

Passively modelocked linear lasers have been fabricated using bulk InGaAsP/InP material. Modelocking in 20 GHz self colliding pulse modelocked lasers and 40 GHz colliding pulse lasers has been demonstrated and the devices have been characterized. Pulse lengths down to 1.6 ps have been observed from a linear device at 20GHz. 15GHz modelocked ring lasers have been fabricated as well. In order to avoid internal reflections in the ring, the design employs successfully adiabatic bends and a directional coupler. Measurements with a 50GHz RF analyzer showed more stable operation than the linear devices, but pulses are highly chirped. The layer stack used for these lasers is compatible with our active-passive integration scheme

On the design of electrically pumped vertical-external-cavity surface-emitting lasers

Vertical-external-cavity surface-emitting lasers (VECSELs) yield an excellent beam quality in conjunction with a scalable output power. This paper presents a detailed numerical analysis of electrically pumped VECSEL (EP-VECSEL) structures. Electrical pumping is a key element for compact laser devices. We consider the optical loss, current confinement, and device resistance. The main focus of our investigation is on the achievement of an adequate radial carrier distribution for fundamental transverse mode operation. It will be shown that a trade off between the conflicting optical and electrical optimization has to be found and we derive an optimized design resulting in guidelines for the design of EP-VECSELs which are compatible with passive mode lockin

Measurement of reflectivity of butt-joint active-passive interfaces in integrated extended cavity lasers

A method and measurement results are presented for the determination of the reflectivity of butt-joint active-passive interfaces in a series of extended cavity Fabry-Pe´rot lasers. The method is based on the analysis of subthreshold laser spectra. The small reflections at the two intracavity active-passive interfaces modify the mode structure of the laser. By fitting the calculated subthreshold mode structure to the recorded data, values of the reflectivities are extracted. An average value of 9.10-5 has been determined. The value of the reflectivity of those interfaces is relevant for photonic integrated circuits and particularly integrated mode-locked lasers

Observation of nonlinear dynamics and transition to chaos in photonic integrated circuits

Photonic Integrated Circuit (PIC) technology has revolutionized the application and fabrication of optoelectronic devices. Most affected by this development is the field of telecommunications, where both active and passive photonic devices are key components in the optical networks. PIC based optical components are cheaper to fabricate than their stand-alone counter parts, multifunctional, low energy consumers and much smaller in size. These qualities make PICs very attractive from a mass-integration point of view and they are generally viewed as the successors of electronic ICs. In this work we demonstrate that the nonlinear dynamics exhibited by the PICs are stable, well-classifiable from a bifurcational theoretical point of view and reproducible from batch-to-batch

Non inverting and non filtered wavelength converter based on an InAs/InP (100) QD ring laser at 1.55 μm

A novel wavelength conversion concept based on InAs/InP(100) quantum-dot ring- laser structure is demonstrated requiring no external laser, optical inversion or optical filtering. Demonstration at 622 Mb/s for a 2 mm ring, suggests applicability for much higher speeds

Modelocked quantum dot vertical external cavity surface emitting laser

We report the first successful modelocking of a vertical external cavity surface emitting laser (VECSEL) with a quantum dot (QD) gain region. The VECSEL has a total of 35 QD-layers with an emission wavelength of about 1060 nm. In SESAM modelocked operation, we obtain an average output power of 27.4 mW with 18-ps pulses at a repetition rate of 2.57 GHz. This QD-VECSEL is used as-grown on a 450 μm thick substrate, which limits the average output powe

A monolithic 20GHz integrated extended cavity mode-locked quantum well ring laser at 1.58µm fabricated in the JEPPIX platform

We report on a passively modelocked InP/InGaAsP quantum well semiconductor ring laser which operates at 20GHz repetition rate and at 1.58µm output wavelength. A number of devices with varying relative positions of the absorbers and amplifiers have been realized using active-passive integration technology in the JEPPIX fabrication platform. The 4mm-long laser ring cavity incorporates a 750µm-long optical amplifier section, a separate 40µm-long saturable absorber section, passive waveguide sections and a passive MMI-type 50% output coupler. We investigate operation regimes of the laser and explore conditions for single mode lasing and mode-locked operation