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 lambda(0), the zero-held exciton position, The two GaAs-AlAs samples have lambda(0)'s of 833.8 and 842.3 mn, and so cover a range useful for modulators designed to operate near 850 mn 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 lambda(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 (Delta lambda) of 17 nm with local variations of wavelength of +/-1 nm, which corresponds to a temperature variation of 60 degrees C while allowing for variations of laser driver wavelength of +/-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 similar to 5 nm by either using stacked-diode designs or extremely shallow quantum wells.