On the operational and manufacturing tolerances of GaAs-AlAs MQW modulators

We approach the question of optimization of surface-normal p-i(multiquantum-well, MQW)-n modulators from the viewpoint of investigating their tolerance to variations in wavelength and temperature and errors in manufacture. The reflection characteristics of two high-quality samples are carefully processed to eliminate Fabry-Perot fringes, and then their spectra at any bias are characterized with six phenomenological parameters which depend on /spl lambda//sub 0/, the zero-field exciton position. The two GaAs-AlAs samples have /spl lambda//sub 0/'s of 833.8 and 842.3 nm, and so cover a range useful for modulators designed to operate near 850 nm in the normally reflecting condition, i.e., reflection decreases with field. A linear interpolation of the parameters of these two samples is used to predict the behavior of MQW diodes with /spl lambda//sub 0/'s around this range, and so a fully comprehensive examination of normally reflecting MQW modulators is performed. The performance aspect that is examined is contrast ratio as a function of nonuniformities in the devices or operating conditions given a voltage swing of 3 V. There are two operational modes discussed. If the voltage offset of the bias is allowed to vary via a feedback circuit, a contrast of 2:1 may be maintained over an operating wavelength change (/spl Delta//spl lambda/) of 17 nm with local variations of wavelength of /spl plusmn/1 nm, which corresponds to a temperature variation of 60/spl deg/C while allowing for variations of laser driver wavelength of /spl plusmn/1 nm. If feedback Is not permitted, we determine that, given tolerances to manufacturing errors, a contrast of 1.5:1 may be maintained over a wavelength range of /spl sim/5 nm by either using stacked diode designs or extremely shallow quantum wells.