A laser oscillator apparatus is disclosed having a laser medium flowing in one direction in a chamber, an unstable resonator including a concave mirror and a convex mirror that face each other in order to generate a laser beam transmitted in a direction perpendicular to the direction of flow of the laser medium, an output window provided in the chamber and rotation means for rotating the laser beam provided between the unstable resonator and the output window.

A tuning arrangement (10) for a tunable laser comprises a single holographic grating (12) and two flat surface reflective mirrors (13 and 14). The beam (15) from the laser cavity is incident on the grating at a grazing angle for optimum beam expansion. The diffracted beam propogates from the grating to the first mirror (13), therefrom to the second mirror (14) and is reflected at the Littrow angle to the grating, whereat it is diffracted a second time and returned to the second mirror (14) for reflection to the first mirror (13). Therefrom it is reflected back to the grating. After undergoing a third diffraction it is directed back into the cavity for further amplification.

An unstable optical resonator having a gain region that has an elliptic or other non-circular cross section still has the benefits of a total collimated cavity Fresnel number that is azimuthally symmetric by the combination of several cavity regions of azimuthally non-uniform Fresnel number of appropriate size.

In an FEL, unwanted sidebands in the laser pulses are suppressed by the introducing of a temporal dispersion of the sideband with respect to the main wavelength, causing a time lag between the main wavelength and the sideband. This is preferably accomplished by the use of diffraction gratings in a grating rhomb within a ring resonator. The first of the gratings in the rhomb has a zero order diffraction which can be used for outcoupling. Tuning in real time is achieved by adjusting the positions of the elements in the ring resonator so that the desired wavelength will be in spatial overlap with the electron pulses in the wiggler.

Optical systems, using volume holographic elements (gratings) having geometries which tailor the spatio-temporal dispersion of the optical pulses for the system. The input optical pulse is characterized by a frequency variation across the temporal profile of the pulse. The various frequency components of this pulse are first dispersed by at least one grating which may be of the blazed reflection or holographic volume transmission type. The resultant dispersed light is then diffracted by a holographic volume grating which imparts the desired temporal dispersion characteristics to the pulse. The shape of the holographic element will vary according to the input pulse frequency profile as formed by varied chirping techniques. A grating stage may then be repeated, preferably with additional elements in mirror symmetry to the first or by retro-reflection, in order to recombine the spatially dispersed pulse components into an exiting pulse which may be of vastly compressed temporal profile. In optical dispersive delay lines, the grating geometry provides temporal dispersion which is a desired function of wavelength of the optical pulses.