Methods for narrowing the emission line of an injection laser

Theoretical estimates of narrowing of the emission line of an injection laser with an external resonator are obtained for different mechanisms that determine the line width. If the width of a line emitted by a laser diode without an external feedback ("initial" width) is governed by phase fluctuations because of 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. However, if the initial width is determined by fluctuations of the refractive index of the active medium, then the same dependence is linear. It is shown that external resonator configurations used so far have not been optimal. Ways are suggested 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 ~1–103 Hz without altering the dimensions. An analysis is made of the influence of a wide-band electronic circuit for automatic phase control (based on an external high-Q interferometer) on the emission spectrum of an injection laser. It is shown that the "natural" width of an emission line may be reduced by between four and six orders of magnitude if the proposed method is used.