An expression for the spontaneous-emission factor β = β_gβ_s is presented and discussed. β_gis found to be proportional to the solid angle subtended by the far-field intensity distribution and accounts for the guiding. β_s is a function of the parameters that describe the material properties of the active region and gives the fraction of the spontaneous emission that has the mode energy and polarization. β_gfor a gain-guided device is not necessarily K times greater than β_g for an index-guided structure, where K is the enhancement or astigmatism factor. This is because K is defined for devices for which the real part of the spatial mode distribution is identical. In general, the dimensions of the active regions of gain-guided and index-guided lasers are not similar, and hence the real parts of the spatial mode distributions are not the same. It is shown that calculation of β_g and the backscatter factor in optical fibers is similar. Reasons for the differences in the spectral properties of gain-guided and index-guided lasers are discussed.

© 1991 Optical Society of America

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