ABSTRACT
A laser is an oscillator that operates at optical frequencies. These frequencies of operation lie within a spectral region that extends from the very far infrared to the vacuum ultraviolet (VUV) or soft x-ray region. At the lowest frequencies at which they operate, lasers overlap with the frequency coverage of masers, to which they are closely related, and millimetre wave sources using solid state or vacuum tube electronics, such as TRAPATT, IMPATT and Gunn diodes, klystrons, gyroklystrons and travelling wave tube oscillators, whose principles of operation are quite different [2]. In common with electronic circuit oscillators, a laser is constructed using an amplifier with an appropriate amount of positive feedback. The acronym LASER, which stands for light amplification by stimulated emission of radiation is in reality, therefore, a slight misnomer.
