Accuracy of Petermann´s K-factor in the theory of semiconductor lasers

Petermann has proposed that the classical formula for the linewidth of a laser be multiplied by a factor in the case of gain-guided semiconductor lasers [1]. The concept of power in the mode used by that author, however, is not well defined in a waveguide with gain [2], and his theory is therefore opened to question [3]. The analysis given here avoids this difficulty and nevertheless agrees with Petermann\´s result. This is because spatial mode filtering is strong in oscillating lasers.