Laser Line Broadening Due to Classical and Quantum Noise and the Free-Electron-Laser Linewidth

摘要

The problem of fundamental laser line broadening due to random spontaneous emission of radiation and amplification of thermal radiation noise is analyzed in terms of a classical fluctuating field phasor model. A general expression is derived for the intrinsic linewidth, given in terms of the spectral power of the radiation noise source, which can be classical or quantum mechanical in nature. In the case of a two level atomic laser, recovered by the use of Einstein relations are the traditional linewidth formula of the Schalow Townes form. In the case of the free-electron laser (FEL), using the explicit expression for the spontaneous emission, calculation of the laser linewidth are presented by purely classical methods. The result agrees with the one obtained in the framework of a quantum-mechanical model. Using extended Einstein relations which are applicable to classical radiators, shows that a Schalow Townes type formula can be obtained for the FEL. The theory predicts extremely narrow intrinsic linewidth (10 to the -7 power Hz) for cw FEL's with parameters similar to those of the FEL experiment of Elias et al.