Passive mode-locking in semiconductor lasers with saturable absorbers bandgap shifted through quantum well intermixing

Passive mode-locking in semiconductor lasers in a Fabry–Perot configuration with a bandgap blueshift applied to the saturable absorber (SA) section has been experimentally characterized. For the first time a fully post-growth technique, quantum well intermixing, was adopted to modify the material bandgap in the SA section. The measurements showed not only an expected narrowing of the pulse width but also a significant expansion of the range of bias conditions generating a stable train of optical pulses. Moreover, the pulses from lasers with bandgap shifted absorbers presented reduced chirp and increased peak power with respect to the nonshifted case

Integrated chirped Bragg gratings for dispersion control

In this work, the need for an integrated optical dispersive device is discussed, with particular reference to pulse compression of semiconductor mode-locked laser (MLL) pulses that exhibit temporal chirp and therefore, worse than transform limited behaviour. It is shown that current techniques in fibre and integrated dispersion control do not overlap the dispersion regime presented, making it necessary to design a new integrated device for this purpose. A monolithic chirped Bragg grating is presented with dispersion and bandwidth characteristics coinciding with the previously mentioned regimes. The device, based on a deeply etched tapered waveguide design, may be fabricated fully post-growth, lending it a significant advantage over current grating designs that require the pattern to be written into the core material and the upper cladding layers subsequently overgrown. The deeply etched sidewall grating structures provide the requisite high coupling coefficients, and the ability to induce arbitrary apodisation profiles, while the tapered waveguide design allows the same freedom the grating Bragg condition profile. The coupled-mode analysis for a chirped grating structure is presented and used as a basis for a Transfer Matrix Method (TMM) representation of the device. This simulation tool allows modelling of the arbitrary Bragg condition and apodisation profiles for steady state analysis of passive grating devices, Distributed Feedback (DFB) and Distributed Bragg Reflector (DBR) lasers. The fabrication of low loss passive grating devices and DFB lasers is described with particular attention paid to lithography and reactive ion etching methods. In addition, work is presented on a wet chemical oxidation technique for reduction of sidewall roughness in A1GaAs based waveguides. Deeply etched waveguides were shown to exhibit losses reduced by up to 4dBcm[superscript-1] after application of this procedure. The fabricated passive grating devices exhibit transmission and grating phase profiles closely matching those predicted by the simulations, with control shown over both Bragg condition and coupling coefficient. The DFB lasers, again in agreement with simulation, show unique multi-mode behaviour, closely related to the chirped grating modulation profile. Also presented is a method by which sub-100 [m] tapers for transitions between shallow etched and deep etched waveguides may be fabricated for quasi-adiabatic propagation. These tapers provide a means by which integration may be achieved between optical systems with different mode profiles, these being defined by device properties, for example integration of small radius bends and waveguide gain structures. A simulation tool based on teh TMM is derived and a set of optimised tapers are fabricated, their results matched to the simulations. Low loss, low reflectivity tapers are exhibited with properties in close agreement with teh TMM and Finite Difference Time Doain (FDTD) simulations

Job Loss and Effects on Firms and Workers

This paper serves as an introduction and (incomplete) survey of the wide-ranging literature on job loss. We begin with a discussion of job stability in the US and the commitment between firms and workers, and how this has changed in recent years. We then focus on the short and long-term consequences to workers (i.e. wages, health outcomes) following a layoff, and the effect which mass layoffs have on future firm performance. The changing nature of these relationships over the past several decades is a central theme of this paper. We review the common data sources used to examine these questions, and identify many influential papers on each topic. Additionally, we discuss alternative policies to the typical mass layoff, such as worksharing

High-extinction-ratio TE/TM selective Bragg grating filters on silicon-on-insulator

We report on the design and fabrication of TE and TM polarization selective Bragg grating filters in the form of sinusoidal perturbations on the waveguide sidewall and etched holes on the top of the waveguide, respectively. Combining the two geometries on a silicon-on-insulator waveguide resulted in Bragg grating filters with high extinction ratios of approximately 60 dB

High Extinction Ratio Polarization Selective TE/TM Bragg Gratings Filters on Silicon-on-insulator

No abstract available

High-extinction-ratio TE/TM selective Bragg grating filters on silicon-on-insulator

We report on the design and fabrication of TE and TM polarization selective Bragg grating filters in the form of sinusoidal perturbations on the waveguide sidewall and etched holes on the top of the waveguide, respectively. Combining the two geometries on a silicon-on-insulator waveguide resulted in Bragg grating filters with high extinction ratios of approximately 60 dB

Ultrashort Q-switched pulses from a passively mode-locked distributed Bragg reflector semiconductor laser

A compact semiconductor mode-locked laser (MLL) is presented that demonstrates strong passive Q-switched mode-locking over a wide range of drive conditions. The Q-switched frequency is tunable between 1 and 4 GHz for mode-locked pulses widths around 3.5 ps. The maximum ratio of peak to average power of the pulse-train is >120, greatly exceeding that of similarly sized passively MLLs

Ultrafast pulse generation in semiconductor lasers

Integrated semiconductor laser devices are presented as extremely compact generators of ultra-short pulse trains. Control is demonstrated on a wide range of emission parameters including wavelength, pulse duration, repetition rate and emitted power. All device geometries require simple drive electronics, consisting of only constant current injection and reverse bias voltage control

High accuracy transfer printing of single-mode membrane silicon photonic devices

A transfer printing (TP) method is presented for the micro-assembly of integrated photonic devices from suspended membrane components. Ultra thin membranes with thickness of 150nm are directly printed without the use of mechanical support and adhesion layers. By using a correlation alignment scheme vertical integration of single-mode silicon waveguides is achieved with an average placement accuracy of 100±70nm. Silicon (Si) μ-ring resonators are also fabricated and show controllable optical coupling by varying the lateral absolute position to an underlying Si bus waveguide