Optical rise time and frequency chirping predictions for (forward-biased) optical absorption modulators

The modulation characteristics of the (forward-biased) semiconductor travelling-wave absorption modulator, a prime contender for monolithic-integration with an injection laser, is analysed. It is found that, as a consequence of the distribution of charge carriers along the length of the device, the modulator output rise/fall times are limited for practical devices to greater than ~500 ps, corresponding to a maximum bit-rate of the order of 2 Gbits/s. Moreover, phase modulation in the device resulting from fluctuations in the refractive index (linked to the change in absorption) results in unwanted frequency modulation (chirping) of the order of several GHz. These figures suggest that, although easy to fabricate, this device offers little performance advantage over direct modulation of the laser itself.