Generalised ultrafast dispersion scans of continuum generation induced by sub-50fs chirped pulses in highly nonlinear tapered planar waveguides

摘要

Ultra-high bandwidth continua generated by ultrashort fs pulses have been attracting enormous interest forapplications such as general spectroscopy, Optical Coherence Tomography and metrology. Dispersion engineeringis one of the key aspects of optimised continuum generation in optical waveguides. However in addition, thedispersion of the pump pulse can be continuously adapted to control bandwidth and spectral characteristicsof the generated continua. In this work we report on a systematic investigation of how 2nd ,and 3rd orderdispersion affects the continuum generated in strongly non linear planar waveguides. A ∼30 fs Ti:Sapphiretuned to 800 nm was used as a pump source delivering ∼3 nJ pulses. The chirp of the pulses was controlledcompletely-arbitrarily by an acousto-optic programmable dispersive filter (Dazzler). The power launched intothe structures was kept constant to compare the generated continua as the pulse dispersion is varied. Highrefractive index tantalum pentoxide (Ta2O5) waveguides grown by standard silicon processing techniques wereused. The devices investigated were specially designed tapered ridges with ∼5 mm2 input modal volume and zerogroup velocity dispersion at ∼l–3.7 mm. Self-phase modulation, which is responsible for the spectral broadeningof the continua, is tracked by finely tuning the both 2nd and 3rd order dispersions. The nonlinear propagation isdramatically influenced by the simultaneous presence of these dispersive effects resulting in a change of bandwidthand spectral shape. Pulse widths of up to Dl > 100 nm for launched powers as low as 300 pJ. Spectral peakintensity can also be systematically modulated by simply scanning the 2nd and 3rd order dispersion aroundtheir relative zeros. Specific combinations of high order dispersion contribution are currently targeted as a routeto control and optimise the continua bandwidths and to control dispersion lengths in specifically engineeredwaveguides.