Coherence saturation for energetic photons (abstract)
Authors: Csonka P. L. 
Published: July 2002

There are two well-known methods to generate coherent x-ray photons: bound state x-ray lasers (BXL) and free electron x-ray lasers (FEXL). Both rely on induced photon emission. Because of the short lifetime of inverted states and high required energy densities for BXLs on the one hand, and strict tolerance specifications for FEXLs on the other, serious technological obstacles have yet to be overcome before a working device will be available for research purposes, particularly in the hard x-ray range. Coherence saturation is proposed here as an alternative method to generate coherent energetic photon beams. It does not make use of stimulated emission, therefore, it is not subject to the same technological and cost restrictions which have so far limited the effectiveness of BXLs and FEXLs in the short wavelength range. In fact, the method does not affect the process of photon emission at all: instead it rearranges the photon after they have been emitted, to achieve an increase in (transverse) coherence. The rearrangement is to be accomplished by dynamical optical means, i.e., by a collection of optical elements at least one of which is nonstationary. The speeds required are well within the range of present day technology. The method was devised primarily to facilitate certain interference experiments (e.g., holography) in the x-ray range. Correspondingly, its aim is to increase (first-order) transverse coherence. The special case will be discussed when only one optical element is nonstationary, and that is a rotating mirror. The transversely coherent intensity, I⊥, is defined as the number of photons crossing the (transverse) coherence area per unit time. Denoting by I the total photon intensity, define the (degree of) transverse coherence as C⊥=I⊥/I. Thus, C⊥≤1, and is saturated when C⊥=1.
For given I, the time required to record a certain interference experiment is smallest when C⊥ is saturated. A device will be described which accomplishes coherence saturation by first decomposing an original beam into components, each of which has C⊥≊1, and then reconstitutes a resultant beam without significantly altering C⊥. In this manner device is able to increase C⊥ by the transverse coherence enhancement factor fc. For photons emitted from the PEP storage ring at Stanford, fc is calculated to be 102–103, depending on a variety of parameters, including wavelength and emittance. The method proposed here can be used in conjunction with any noncoherent photon source, but it should prove most advantageous when (a) the photons are expensive to generate, (b) the photon duty cycle is low, and (c) the photon intensity is one of the principal limiting factors in the experiment.
For high-energy electron synchrotron radiation sources (a) and (b) both hold, while for interference experiments (c) does as well. It appears that the method just described will allow the construction of a novel type of x-ray source, for transversely coherent photon beam generation. There seems to be no fundamental reason why it should not be effective over a wide spectral range, including the hard x-ray region.