High-resolution Interferometric Diagnostics for Ultrashort Pulses

4. EXPERIMENTAL IMPLEMENTATIONS OF MULTIPLE SPECTRAL SHEARINGINTERFEROMETRY4.2.4 DiscussionOne limitation of the present setup arises because the ancillae are derived from the test pulse.Upconversion therefore cannot be achieved with these missing frequencies. This can be somewhatinconvenient — the central upconversion frequency ω up must be carefully chosen to lie inthe centre of a spectral lobe so as to provide the widest possible range of upconversion frequencieseither side. Fortunately, ω up can be modified without altering any of the other parametersby rotating the grating used to disperse the ancillae. This issue can also be alleviated by derivingthe ancillae from an external source, as was done for the SEA-SPIDER in section 4.1. However forgeneral use a self-contained device is preferable.4.3 Summary and outlookThis chapter described multiple shearing spectral interferometry using both sequential and simultaneousacquisition of the shears. For the measurement of a complex pulse without spectralnulls, the addition of a second shear significantly improved the precision. For a pulse with a spectralnull, the use of multiple shears gave a precise and accurate phase reconstruction which wouldhave been impossible using only a single shear.These results prove the principle of multiple spectral shearing interferometry. An interestingapplication would be measurement of the output of hollow fibre compressors for few-cyclepulse generation [60], which can produce highly structured spectra, especially when pushed totheir maximum bandwidth. Single-shear measurements using SEA-SPIDER have already beenpresented [197]. The combination of high spectral resolution and precision offered by multipleshear reconstructions would enable high dynamic range measurements with a longer time windowthan is currently achieveable with a single shear.104