Rotating Diffraction Grating Laser Beam Scanner

Abstract

The resolution, bandwidth, and duty cycle of a laser beam scanner make it useful as an image recorder. Laser image recorders can have either a flatfield or a cylindrical recording surface, the flatfield being preferable in applications where dimensional stability is extremely important . A problem with flat field recording systems is the difficulty in obtaining a constant scan rate. This difficulty is a result of the image position for a distortion-free lens being proportional to the tangent of the field angle, rather than the field angle itself. Hence, if the field angle changes at a constant rate (such as the case for rotating mirror scanners), the scan rate is not a constant. One method of eliminating this problem is to use a so-called f-0 lens, which produces a distorted image by the conventional definition, so the image height is proportional to the field angle. 2 This Letter describes a second method of obtaining a linear scan rate for a flat field recorder. It is shown that by using two counterrotating diffraction gratings a constant angular mechanical motion can be used to give a nonuniform angular scan of a light beam. Under appropriate conditions, the angular scan is sufficiently nonlinear that when used with a distortion-free focusing lens a substantially linear scan rate results. The counterrotating diffraction grating scan system has been described previously by Brameley in U.S. Patent 3,721,486; however, it was not pointed out that for appropriate grating parameters a substantially linear scan rate is obtained over a reasonable scan angle. Also, this paper demonstrates that similar results can be obtained using a single rotating diffraction grating folded back onto itself. The rotating diffraction grating scanner is capable of scanning more than one line at a time without line curvature. A 100% duty cycle can be obtained. The rotating diffraction grating scanner is illustrated in Fig. 1. If two identical blazed diffraction gratings are illuminated with a quasi-monochromatic plane wavefront, as illustrated in the figure, and the gratings are counterrotated, the focus of the principal diffraction order will scan a vertical straight line in the focal plane of the focusing lens. If in the zero angle position the lines in the two gratings are parallel, when one grating rotates an angle α in the clockwise direction and the other grating rotates an angle α in the counterclockwise direction, the beam resulting from the +1 diffraction order of the first grating and the -1 diffracted order of this beam produced by the second grating is deviated by an angle θ, given by the equation where d is the spacing of the grating lines and l is the wavelength. If the lens is distortion-free and has focal length f, the focused spot will move a distance l, given by the equation