Our work has advanced the measurement and control of the carrier-envelope phase-slip frequencies of pulses generated by a femtosecond optical parametric oscillator.  Example applications of such control include the generation of a frequency comb spanning nearly three optical octaves, and the creation of a train of 30 femtosecond pulses via coherent pulse synthesis.

The figure opposite shows the output of a femtosecond OPO dispersed by a prism.  The positions of the infrared outputs are shown for illustration only and were not imaged, but the other outputs were recorded on a digital camera.

A broadband comb was generated by locking the comb offset frequencies of a Ti:sapphire pump laser and a femtosecond OPO.  The black spectra opposite are derived directly from the laser or the OPO and the grey spectrum is the pump super-continuum produced by a photonic crystal fibre.  The spectral range covers ~ 3 octaves from 400 - 2500 nm.

Coherent synthesis is the term applied to generating a single pulse from two “parent” pulses so that the generated pulse is fully coherent across its entire bandwidth, and so is indistinguishable from a pulse created by a single modelocked laser.

In order to artificially construct a new “daughter” pulse by coherently combining two “parent pulses” it is necessary for two conditions to be satisfied.  Firstly, the two parent pulses must be synchronised in time (A), and this condition is equivalent to requiring that the inter-mode spacings of the two pulses are equal.  Secondly, coherent synthesis requires that the fields from the two pulses must interfere, and for this to be possible the modes of the parent pulses must share common frequencies (B).  This condition requires that the comb offset frequencies (equal to the carrier-envelope phase-slip frequencies) of the two parent pulses are equal.

We have achieved coherent synthesis by combining two pulses derived from a single femtosecond OPO. The pulses were co-resonant in the OPO cavity but had different wavelengths and different comb offset frequencies.

  

The figure above (left) shows the XFROG trace used to characterise these pulses, and the pulses themselves are shown above (right), together with the synthesized waveform which was a 30 fs pulse train.

We have also made progress towards coherent pulse synthesis using separated pulses from a femtosecond laser and an optical parametric oscillator.

Pulses from a tunable near-infrared femtosecond optical parametric oscillator and its Ti:sapphire pump laser were phase-locked by matching their carrier-envelope phase-slip frequencies to one quarter of their common pulse repetition frequency. Interferometric second-order cross-correlation (below) and spectral interferometry traces (opposite) demonstrated their mutual coherence for periods of at least 20ms, compared with individual coherence times of 0.1 ms estimated from their phase-noise power spectra.

These results are a prerequisite for versatile coherent pulse synthesis.