In 1958 Schawlow, the optical region. the first successful together with Charles Townes, showed how to extend the principle of the maser to He shared the 1981 Nobel Prize with Nicolaas Bloembergen. Maiman demonstrated operation of the ruby laser in 1960. 494 The laser is an optical oscillator. It comprises a resonant optical amplifier whose output is fed back into its input with matching phase (Fig. ). In | Fundamentals of Photonics Bahaa E. A. Saleh Malvin Carl Teich Copyright 1991 John Wiley Sons Inc. ISBNs 0-471-83965-5 Hardback 0-471-2-1374-8 Electronic CHAPTER 14 LASERS THEORY OF LASER OSCILLATION A. Optical Amplification and Feedback B. Conditions for Laser Oscillation CHARACTERISTICS OF THE LASER OUTPUT A. Power B. Spectral Distribution C. Spatial Distribution and Polarization D. Mode Selection E. Characteristics of Common Lasers PULSED LASERS A. Methods of Pulsing Lasers B. Analysis of Transient Effects C. Q-Switching D. Mode Locking Arthur L. Schawlow born 1921 Theodore H. Maiman born 1927 In 1958 Schawlow together with Charles Townes showed how to extend the principle of the maser to the optical region. He shared the 1981 Nobel Prize with Nicolaas Bloembergen. Maiman demonstrated the first successful operation of the ruby laser in 1960. 494 The laser is an optical oscillator. It comprises a resonant optical amplifier whose output is fed back into its input with matching phase Fig. . In the absence of such an input there is no output so that the fed-back signal is also zero. However this is an unstable situation. The presence at the input of even a small amount of noise containing frequency components lying within the amplifier bandwidth is unavoidable and may initiate the oscillation process. The input is amplified and the output is fed back to the input where it undergoes further amplification. The process continues indefinitely until a large output is produced. Saturation of the amplifier gain limits further growth of the signal and the system reaches a steady state in which an output signal is created at the frequency of the resonant amplifier. Two conditions must be satisfied for oscillation to occur The amplifier gain must be greater than the loss in the feedback system so that net gain is incurred in a round trip through the feedback loop. The total phase shift in a single round trip must be a multiple of 2ir so that the fedback .
