Excitation engineered pulse train generators

Excitation-engineered pulse train repetition rate multipliers fabricated as a planar lightwave circuit are investigated. The overall temporal response is shown to be influenced by the design of both the splitter and combining regions.

Techniques for generating very-high repetition rate bursts (500 GHz to 1 THz) of short optical pulses using an arrayed waveguide grating (AWG) [1] have recently been reported [2], [3]. Initially, the concept of modifying an AWG structure so that it could be used to generate a high repetition-rate pulse burst from each short input pulse was demonstrated [2]; although the output pulses were separated by a short 1 ps spacing, corresponding to a 1 THz repetition rate, the overall temporal window was Gaussian in shape which is detrimental to the use of the device as a repetition-rate multiplier. This roll-off is caused in part by the Gaussian excitation profile of the various waveguides in the waveguide array. In later work, loss-engineering, the intentional insertion of additional loss on a guide-by-guide basis, was implemented in the various waveguides making up the array, resulting in a flat-topped pulse burst[3]. Utilizing an ~13GHz actively mode-locked short pulse source, and combining two outputs of the AWG permitted the generation of a continuous 500 GHz repetition-rate pulse stream demonstrating the full repetition-rate multiplication scheme [4]. While effective, the loss-engineering scheme obviously leads to increased device insertion loss which could significantly impact potential system use of the technique. In the current work, we investigate the ultrafast time-domain response of two different repetition-rate multipliers which employ excitation-engineering to eliminate the excess insertion loss associated with the loss-engineering scheme. While excitation-engineered AWG's have been previously reported in the frequency domain [5], here for the first time to our knowledge, we report the use of both splitters and combiners as a full time-domain pulse-train generator module.