Generation of high repetition rate pulse bursts via femtosecond readout of a double-passed arrayed waveguide grating

In summary, we have shown that very-high repetition rate pulse bursts can be generated with an increased number of pulses by double-passing a small-FSR AWG. Equivalently, two identical AWG's could be interconnected back-to-back to generate the same functionality in transmission. The output temporal profile is determined by the autoconvolution of the temporal profile of an output channel in single-pass. This scheme shows promise for manipulating ultrafast optical waveforms via hybrid WDM/TDM optical processing. For example, with further engineering of the AWG structure, it may be possible to generate continuous very-high repetition rate pulse trains, as would be desired for a repetition-rate multiplication scheme of a lower-rate source.

Significant development effort has been expended on the Arrayed-Waveguide Grating (AWG) in recent years primarily focused on their use as wavelength-division multiplexed (WDM) channel multiplexers and demultiplexers^{[1], [2]}Time domain application of these devices, in contrast, has seen only limited exploration. For example, modelocked pulse inputs have been spectrally sliced to yield pulses in the tens of picoseconds range at the repetition rate of the modelocked source laser.^[3]supercontinuum sources have been sliced to yield multiple optical wavelengths for high-speed systems studies.^{[4], [5]}and modified AWG devices have been used for Fourier transform optical pulse shaping,^[6]Recently, we demonstrated a mechanism by which an AWG can be used to generate multiple wavelength shifted, spatially separated bursts of short optical pulses with repetition rates on the order of 1 THz using a single lower repetition rate source laser.^[7] Here we report for the first time that these spatially separated high repetition rate bursts may be combined onto a single optical channel, with a variable delay between the bursts, by double-passing the AWG device. Compared to the single-pass configuration, the total number of output pulses is substantially higher. In the experiments reported here. a single femtosecond pulse input into the double-pass AWG produces more than 100 output pulses on a single output fiber in four distinct bursts. These results point to intriguing new possibilities for manipulating ultrafast time-domain signals via mixed time-frequency processing. Fig. 1. Experimental apparatus showing four independently variable free-space delay sections.