the ac sidebands is filtered out and inverse Fourier transformed back into the frequency domain, where the interferometric spectral phase can be extracted: . The final exponential term, resulting from the delay between the two interfering fields, can be obtained and removed from a calibration trace, which is achieved by interfering two unsheared pulses with the same time delay (this is typically performed by measuring the interference pattern of the two fundamental pulses which have the same time-delay as the upconverted pulses). This enables the SPIDER phase to be extracted simply by taking the argument of the calibrated interferometric term: . There are several methods to reconstruct the spectral phase from the SPIDER phase, the simplest, most intuitive and commonly used method is to note that the above equation looks similar to a finite difference of the spectral phase (for small shears) and thus can be integrated using the trapezium rule: . This method is exact for reconstructing group delay dispersion (GDD) and third order dispersion (TOD); the accuracy for higher order dispersion depends on the shear: smaller shear results in higher accuracy. An alternative method us via concatenation of the SPIDER phase: for integer and concatenation grid . Note that in the absence of any noise, this would provide an exact reproduction of the spectral phase at the sampled frequencies. However, if falls to a sufficiently low value at some point on the concatenation grid, then the extracted phase difference at that point is undefined and the relative phase between adjacent spectral points is lost. The spectral intensity can be found via a quadratic equation using the intensity of the dc and ac terms (filtered independently via a similar method above) or more commonly from an independent measurement (typically the intensity of the dc term from the calibration trace), since this provides the best signal to noise and no distortion from the upconversion process (e.g. spectral filtering from the phase matching function of a 'thick' crystal). Alternative techniques Spatially encoded arrangement for SPIDER (SEA-SPIDER) is a variant of SPIDER. The spectral phase of an ultrashort laser pulse is encoded into a spatial fringe pattern rather than a spectral fringe pattern. Other techniques are frequency-resolved optical gating, streak camera with picosecond response times, and multiphoton intrapulse interference phase scan (MIIPS), a method to characterize and manipulate the ultrashort pulse. Micro-SPIDER is an implementation of SPIDER in which the spectral shear