High power continuous wave (CW) laser beams can be employed to write information on a variety of materials, since when such beams are focused to small diameter spots, they can produce surface marks on the materials. When arranged into dot matrix patterns, these marks become optically readable digital bit arrays, or recognizable images such as alpha-numeric characters.
In one conventional technique for writing data using a CW laser beam, the beam is scanned using a moving mirror. The mirror is moved to direct the beam to a first spot, and then stopped while the beam writes a bit on the first spot area. Then the mirror is again activated to direct the beam to a second spot, and the mirror is again stopped while the beam writes a bit on the second spot area.
However, this technique suffers from the serious disadvantage that it is very slow, due to the time needed to accelerate the mirror periodically from rest (to redirect the beam) and to decelerate the mirror periodically to zero velocity (so that the mirror remains stationary for the required dwell time at each spot area).
Another conventional technique for writing data using a laser beam employs a continuously rotating mirror to project a pulsed CW laser beam along a linear (or raster) scan path on a sample. Flat rotating mirrors, and polygonal rotating mirrors (having multiple flat facets), have been employed to implement this conventional technique. For example, systems which employ a rotating polygonal mirror for this purpose are described in U.S. Pat. No. 3,750,189, issued Jul. 31, 1973, in U.S. Pat. No. 4,040,096, issued Aug. 2, 1977, and in U.S. Pat. No. 4,433,894, issued Feb. 28, 1984.
However, conventional systems using rotating mirrors have a number of serious limitations and disadvantages. One such disadvantage is that to write clearly defined dots on the sample, short exposure times must be used. This limits the practical applications of the technique to cases in which dots are written on a photosensitive surface, or in which dots are written by an extremely high peak power beam capable of marking the sample after only a very short exposure.
Another conventional technique for dot matrix writing uses an acousto-optic deflector to steer, hold, and modulate a CW carbon dioxide laser beam. Such a technique is described in United Kingdom Patent Application No. 2,133,352A. However, suitable acousto-optic deflectors are very expensive. Furthermore, they are limited in the maximum laser power they can transmit, and result in high insertion power loss (and loss of useful power in an undeflected zero order beam transmitted therethrough).