Control of mode behavior in semiconductor lasers

In this paper, aspects important to lateral-mode stability and single longitudinal-mode operation in the index-guided semiconductor lasers are examined and analyzed. These include the modal properties of the guiding structure and the current distribution in the active region. Waveguiding mechanisms in various structures are analyzed and approximate expressions for the number of lateral modes are obtained. Possible modes in rectangular dielectric waveguides are presented and discussed. The problems of nonsatisfaction of the boundary condition and ambiguity in the field polarization are pointed out, which could be important in structures with equally strong guiding and approximately equal dimensions in the transverse and lateral directions. New equations governing the current distribution in the active region are presented. Our work differs from the previous work in that the voltage path-independence condition, in addition to the current conservation condition, is taken into account in the equations. The discussions outlined above prepare the background for a discussion of the factors affecting lateral-mode stability and single longitudinal-mode operation. It is shown that a positive gain difference must be maintained in the lateral direction for lateral-mode stability, even though a positive index difference exists. The effect of nonuniform gain on longitudinal modes is examined. It is shown that a ten percent variation in carrier concentration can result in a shift in peak-gain wavelength about 25 Å. For single longitudinal-mode operation, it is important to keep within a certain limit, and the effect of spatial gain variation on spectral gain profile must be considered. The above ideas for maintaining lateral-mode stability and single longitudinal-mode operation are implemented in the narrow-channel DCC-CSP laser. Experimental results are reviewed in the light of the theoretical considerations. For optimal performance over an extended laser-power range, considerations must be given in the design of an index-guided laser to the combined effects of current spreading, spatial hole burning, and carrier diffusion, so as to maintain a positive but small over the desired operatio- n range.