tors, but at present these systems lack the needed sub-fs resolution. Optical-IR bunching in the FEL context has, on the other hand, been measured using coherent transition radiation (CTR) [17]. With CTR, the observed radiation spectrum is essentially that associated with the electron beams temporal profile. One may also use non-destructive radiative methods such as coherent edge radiation (CER [18]) to obtain a spectrum equivalent to that of CTR, as has been recently observed [19]. In these cases, one may deduce bunch length and shape from single-shot measurement of the coherent radiation spectrum. FIG. 3: Simulated coherent edge radiation spectrum from entrance into 1 T magnetic field, calcu- lated by QUINDI for LCLS low charge beam parameters; (inset) far-field angular distribution. In order to illustrate the expected signals in such a measurement, we have simulated, using a UCLA-developed Lenard-Wiechert field code termed QUINDI [19], the radiation spectra for CER emitted from an equivalent Gaussian-profile beam with LCLS parameters, arising during entrance into a 1 T dipole magnetic field. The result of this study shown in Figure 3. The spectrum displayed is a Gaussian-like, with a cut-off frequency near f$_{c}$ = (2 πσ$_{t}$ ) − 1 = 83 THz, as expected. It reflects the beams longitudinal Fourier spectrum because ER like TR is flat in its frequency response. This similarity is also seen in the far-field angular distribution (inset in Figure 3), which displays a TR-like hollow pattern. We note that beyond its spectrum the emitted CER pulse is unique, as high power sub-cycle sources do not exist in this region of the IR. The pulse simulated in Fig. 3 has a peak power > 200 MW, making it an ideal source for synchronized pumping of physical systems that can be probed by the coherent, fs X-ray pulses of the LCLS. The measurement of few-100 σ$_{x}$ transverse sizes presents more formidable challenges. One might consider optical transition radiation, OTR; we expect here also coherent OTR 7