Epitaxially-stacked multiple-active-region 1.55 /spl mu/m lasers for increased differential efficiency

Semiconductor lasers with external differential efficiencies >1 have been created by monolithically connecting several active regions in series within a single optical waveguide. In contrast to previous reports, we have accomplished this task at the technologically important 1.55 /spl mu/m wavelength. This is accomplished by epitaxially stacking a number of p-i-n multi-quantum well active regions with intermediate n/sup ++/p/sup ++/ back-diodes, which enable the entire terminal current to flow through each active region stage in series. In optimized structures, the stacking of N/sub A/ active regions should result in a multiplication of the external efficiency by N/sub A/ and a reduction of the threshold current by a factor a little less than N/sub A/, with only a slight increase in the threshold spontaneous emission power. Thus, the signal to noise ratio for modulated signals can be reduced. Such lasers should improve low-noise, high-efficiency microwave links and integrated detectors, analog repeaters, amplifying wavelength converters, and lossless signal are possible.