High-resolution Interferometric Diagnostics for Ultrashort Pulses

2. BACKGROUNDoeχ (2)2D imagingspectrometerFigure 2.15: Principle of long-crystal CAR-SPIDER: the test pulse forms the o-wave and the ancillaforms the e -wave. The black arrows across the beams indicate the direction of increasingupconversion frequency.Multiple-trace interferometry. It is possible to measure the phase difference between the shearedreplicas using multiple trace methods similar to those discussed in section 2.3.3.2. These obviatethe need for a temporal carrier, so that the replicas are not separated in time. This is essential, forexample, with high-duty cycle arbitrary waveforms. A form of quadrature detection [201], in whichthe interferogram is sequentially measured with absolute phase of 0,π/2,π, and 3π/2, has appliedto such cases. Another approach, two-dimensional spectral shearing interferometry [97], uses adesign similar to a ZAP- or SEA-SPIDER, in which two chirped ancillae are mixed with a singletest pulse. However, the upconverted replicas are detected colinearly and cotemporally on a onedimensionalspectrometer. The phase is uniquely determined by scanning the time delay of oneof the ancilla over a few optical cycles (not enough to appreciably change the shear), producing aphase shift of the corresponding replica.Spectral interferometry resolved in time (SPIRIT). A different approach is to spatially dispersethe test pulse, as in a spectrometer, and then interfere two slightly displaced replicas of the dispersedpulse on a linear detector array. The field incident on any given pixel of the detector isE (ω)+E (ω + Ω), where the shear Ω is produced by the displacement. Of course, under normalcircumstances interference between these two different frequencies does not occur. If however,the resulting signal is then time-gated with a duration less than the temporal stretching caused bythe dispersion (the inverse of the spectrometer resolution), then the phase of the temporal beatingproduced by the frequency-shifted fields may be sampled. This is the basis of SPIRIT [202–204].48