The far-field spectral anomalies of a short Gaussian pulse incident on an adjustable tilted mirror are studied theoretically and some numerical examples are given to illustrate this effect. Some anomalous behavior (such as a red or blue shift of the peak) of the diffracted pulse can be observed under some conditions. Also an important phenomenon called a "spectral switch" is presented, which can be controlled by adjusting the tilt angle of the mirror and which has potential applications in information encoding and transmission for free space communications. Our method has benefits of easier implementation than other method which modulate some properties of the light source, and of allowing selection of the direction of the transmission as needed. (C) 2009 The Japan Society of Applied Physic