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Progress in photonics by monolithic integration for higher functional density, performance and reduced cost faces challenging hurdles due to technological and functional heterogeneities. Advanced local material growth techniques are enabling concepts towards high-density photonic integration, unprecedented performance and multi-functionality and ultimately optical systems-on-a-chip.
For example mode-locked laser diodes (MLLDs) are key devices for ultra-short pulse generation for all-optical Tbit/s communication networks. MLLDs suffer from material compromises and will benefit from the possibility to design the gain, absorber and passive-waveguiding sections independently. We have proposed and demonstrated the integration of a saturable absorber with a fast absorption recovery time based on an InP/InGaAsP uni-traveling-carrier structure (UTC) to achieve pulses below 1 ps with repetition rates up to 40 GHz. The use of the UTC absorber instead of the commonly employed reverse-biased gain material requires however the heterogeneous growth of multiple layer stacks on the same chip with the butt-coupled regrowth technique.Critical for the MLLD performance are the reflections and the optical coupling between the different monolithic integrated layer structures of passive, absorbing and amplifying sections. 2D FDTD simulations of the optical waveguides demonstrate that to minimize reflections an angled interface between the different structures is preferable and can lead to reflection coefficients as low as 10^-6. To obtain an angled interface we used a wet chemical etching process sequence of selective and non-selective etchants, which is sensitive to crystal orientation and yields a 55° tilted interface. In addition we can conclude from our simulations that in order to minimize both, insertion loss and reflections, a bending of the light guiding layers has to be prevented. Bendings can lead to measured losses of 5-7 dB per interface whereas correctly aligned light guiding layers results in losses of 1.5 dB and intensity reflections below 10^-5 per interface. The bendings originate from different growth rates near and far away from masked areas during regrowth due to reactants diffusion on the SiO2 mask. The bending can be minimized by optimizing the mask under etch of the SiO2 mask and low pressure MOVPE growth. We demonstrate operation of mode-locked laser diodes with an integrated UTC absorber and pulse durations below 1 ps.
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