Theory of the fundamental linewidth of a two-mode laser
01 Jul 2004-Journal of Optics B-quantum and Semiclassical Optics (IOP Publishing)-Vol. 6, Iss: 7, pp 276-282
TL;DR: In this article, the authors investigated the dependence of the phase-diffusion linewidth on the degree of coupling between the two modes of a laser oscillating simultaneously in two modes.
Abstract: Starting from the field master equation for a laser oscillating simultaneously in two modes, we probe the dependence of its fundamental phase-diffusion linewidth on the degree of coupling between the two modes. We find that the heterodyned intrinsic linewidth shows the usual decrease when the output power of each mode increases with an increase in the gain, while the nonlinear self-saturation and cross-coupling coefficients are held constant. The linewidth also decreases with increase in the cross-coupling between the modes for constant gain when the output power of each mode is kept constant by decreasing the self-saturation coefficient. The linewidth, however, increases with increasing cross-coupling when (a) the gain and the self-saturation coefficient are held constant and hence the output power in each mode decreases and (b) the gain is increased keeping the output power and the self-saturation of each mode constant.
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TL;DR: In this article, a theory of the spectral width of a single-mode semiconductor laser is presented and used to explain the recent measurements of Fleming and Mooradian on AlGaAs lasers.
Abstract: A theory of the spectral width of a single-mode semiconductor laser is presented and used to explain the recent measurements of Fleming and Mooradian on AlGaAs lasers. They found the linewidth to be inversely proportional to power and to have a value of 114 MHz at 1 mW per facet. This value is 30 times greater than can be explained by existing theories. The enhanced linewidth is attributed to the variation of the real refractive index n' with carrier density. Spontaneous emission induces phase and intensity changes in the laser field. The restoration of the laser to its steady-state intensity results in changes in the imaginary part of the refractive index \Delta n" . These changes are accompanied by changes in the real part of the refractive index \Delta n' , which cause additional phase fluctuations and line broadening. The linewidth enhancement is shown to be 1 + \alpha^{2} , where \alpha = \Delta n'/\Delta n" . A value of \alpha \approx 5.4 , needed to explain the observed linewidth, is close to the experimental values of a of 4.6 and 6.2.
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