New approaches to widely tunable semiconductor lasers

Abstract

Widely tunable lasers can signi??cantly reduce the costs and increase the functionality of optical networks. So far, the performance of integrated semiconductor based tunable lasers lagged behind the performance of the ??xed wavelength sources they need to replace. The objective of this thesis is to investigate new widely tunable laser concepts and compare their performance with the alternatives. In this work two new concepts for widely tunable lasers are introduced: the Cascaded Sampled Grating (CSG) laser and the T-MMI (Tunable Multi-Mode Interference section) laser. The tuning element in the Cascaded Sampled Grating (CSG) consists of two sampled grating sections. The re ection spectra of these elements add to give a CSG-re ection spectrum with one dominant re ection resonance. The design of this element is detailed and simulations are presented. Experimental devices were processed and characterized. In these devices an output power of up to 10 mW was achieved with an SMSR of at least 30 dB over a tuning range of 57 nm. Simulations, based on the experimental results, predict a capability to generate > 30 mW output power and an SMSR > 37 dB over a 40 nm tuning range. The tuning element in the T-MMI consists of a sampled grating and a novel tuning element, the T-MMI section. The transmission curve of this element can be shifted over a wide wavelength range by current injection in the center of the MMI section. By shifting the transmission curve of the T-MMI section, subsequent sampled grating resonances are selected. The design of this laser is detailed and simulations on the ??nal design are presented. Experimental devices were demonstrated with up to 7 mW output power. The SMSR was more than 25 dB over a tuning range of 38 nm. Over this tuning range a 10 dB variation in output power was observed. Simulations based on these experimental results predict a capability to generate 30mWoutput power and an SMSR of 40 dB over a tuning range of 40 nm (for a maximum gain section current of 220 mA) for an optimized design. An aspect of tunable lasers that has not been given su??cient attention in the past is the role of absorption of optical power in the tuning sections. In this thesis it was demonstrated that for 30 mW operation the composition of the material in the tuning layer is critical. For a tuning layer with bandgap wavelength close to the laser wavelength, the re ection spectrum of the tuning elements saturates before 30 mW output power is achieved. In the comparison between the CSG and T-MMI with other concepts for widely tunable lasers, it follows that both CSG and T-MMI are similar or better than other concepts in the area of output power and SMSR. An important di??erentiation for the T-MMI is achieved in its simplicity of operation.