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Factors that are important in maintaining zero-order-mode operation at high power in large-cavity GaAs double-heterostructure lasers are described. In the first place, it had previously been shown that the electronic gain of the zero-order mode is larger than that of high-order modes when gain is restricted to two-thirds of the waveguide thickness, and the remaining one-third is kept lossy. In practice, this is realized by the LPE growth of a heteroboundary within the cavity such that (a) by the addition of a small amount of aluminum the band gap of the umpumped region is made larger (≈40 meV) than that of the gain region and (b) the umpumped region is doped with a donor concentration larger than the electron concentration in the gain region at lasing threshold. The second factor that is useful in maintaining zero-order-mode operation at high power levels is the use of a zero-order-mode antireflective (AR) coating that increases the mirror transmission loss for the undesired high-order modes. This zero-order-mode selective AR coating consists of layers of ZnS and Al2O3 whose thicknesses are accurately controlled to provide maximum transmission loss at some prescribed angle of emergence relative to the normal axis. Experimental results obtained on zero-order-mode large-cavity PpnN lasers substantiate these design considerations.
Hakki et al. (1974) studied this question.