Integrated chirped Bragg gratings for dispersion control.
PhD thesis, University of Glasgow.
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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.
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