## 3 Linear Optical Methods for Pulse Characterization  
### Historical Outlook  
The nature of ultrashort pulses makes it more convenient to examine their properties in the spectral domain instead of the time domain, simply because detectors that are fast enough do not exist yet. However, steady or slowly changing patterns can be created by the means of interferometry. By its nature, it is extremely sensitive to the phase properties of short pulses, which is also very important to know in the temporal reconstruction process. Therefore, the possibility to involve interference methods for ultrafast pulse characterization becomes evident. When interference patterns are recorded in the time domain, multiple shots and pulse scanning are usually required. On the contrary, however, in the spectral domain, it is feasible to have single-shot measurement without moving parts. The spectral distribution of the pulse can be easily measured by spectrographs, and can be transformed to the time domain to calculate the transform-limited pulse duration. Last but not least, some of the special propagation issues may require the detection of the intensity distribution of the pulse in space. The technique called spectrally and spatially resolved interferometry (_SSRI_) fulfills all of these aspects. _SSRI_ employs two beams forming an angle during propagation, and the interferometric pattern is established via spatially tilted phase fronts. Spectral resolution of the produced interference results in such an interferogram, on which the pattern of the interference fringes almost directly reveals the shape of the spectral phase difference function, and thus the magnitude of spectral phase difference can be obtained by visual analysis. Methods using crossed-beam interferometry date back relatively early, well before the age of lasers. [PERSON] published his first measurement results about the anomalous dispersion of oxy-hemoglobin [34] in 1901. Eleven years later, [PERSON] used essentially the same technique to measure the oscillator strengths of atomic transitions in sodium vapor [35]. In his experiments, he placed a test tube containing metal vapor into one arm of a Jamin- or Mach-Zehnder interferometer, and empty tube in the other in order to compensate the dispersion of the end-windows. Near the absorption lines, a hook-like interference pattern emerged, and a high-precision measurement could be obtained by reading off their spectral position. An analytical description of this hook method (_Hakenmethode_) was presented by [PERSON] [36]. After photo plates had been replaced by _CCD_ detectors allowing for a computerized analysis, [PERSON] and [PERSON] established a Fourier transform-based evaluation method that significantly improved the accuracy of the measurements [37]. At present, the _SSRI_ method is preferable for investigating the ranges of normal dispersion rather than for anomalous dispersion. In early years, this method was used to determine the phase dispersion of metal layers [38] and multilayered thin films [39]. Later, researchers turned toward its ultrashort pulse-related utilization through high-precision measurement of the spectral phase shift of dispersion-compensating laser mirrors [40; 41; 42; 43; 44]. The extended versions of _SSRI_ using arbitrary broadband light source (e.g., white light or ultrashort laser pulses) quickly became a widespread technique. In addition to further measurements on the phase shift of dispersion-compensating chirped mirrors [45; 46], _SSRI_ was used among others for pump-and-probe experiments [47], fine adjustment of the stretcher and compressor units of phase-modulated pulse amplifying systems [48], characterization of laser pulses [49; 50; 51], as well as the investigation of devices controlling the temporal shape of laserpulses [52,53]. Most recently, a high-power passive optical resonator was studied by this technique [54]. In addition, a linear interferometric method has even been proposed for characterization of attosecond pulses [55].