Methods for narrowing the emission line of an injection laser

For various mechanisms that govern the line width, theoretical estimates are made of narrowing the emission line of an injection laser with an external resonator. It is noted that if the width of a line emitted by a laser diode without an external feedback ('initial' width) is governed by phase fluctuations owing to the spontaneous noise, then the width of a line emitted by an injection laser with an external resonator is a quadratic function of the line width of the resonator itself. If, however, the initial width is determined by fluctuations of the refractive index of the active medium, then the same dependence is linear. It is found that external resonator configurations used so far have not been optimal. Methods are proposed for introducing high-Q interferometers into the external part of a composite resonator of an injection laser which should make it possible to reduce the width of the emission line of a laser with an external resonator to approximately 1-1000 Hz without altering the dimensions. In addition, the influence of a wideband electronic circuit for automatic phase control (based on an external high-Q interferometer) on the emission spectrum of an injection laser is analyzed.