Abstract
An expression for the spontaneous-emission factor β = βgβs is presented and discussed. βg is found to be proportional to the solid angle subtended by the far-field intensity distribution and accounts for the guiding. βs 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. βg for a gain-guided device is not necessarily K times greater than βg 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 βg 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.
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