Abstract

For many applications in opto-electronic integrated circuits (OEIC) and for optical analogue modulation, there is a strong interest in waveguide modulators with linear relation between transmission and applied voltage. In recent years much effort has been devoted to studying the different effects that can be used for intensity modulation in waveguide structures. Most of the suggested devices are based on the quantum-confined Stark effect (QCSE). In our opinion the Franz-Keldysh effect (FKE) is even more attractive for waveguide applications. Compared to the QCSE, the FKE exhibits a lot of advantages and shows only slightly lower absorption changes, which is quite unimportant for waveguide modulators. The absorption changes due to the FKE can be obtained in relatively simple structures (e.g., double hetero (DH) p-i-n structures) that are well-suited for integration in OEICs. The absorption changes due to the FKE extend over a broad wavelength range. Therefore, FK-modulators are quite insensitive to temperature variations. A possible temperature drift can be compensated nearly perfectly by a small dc bias. Furthermore, in such devices there are no problems with trapping photo-generated carriers in quantum wells. This results in much shorter transit times. Saturation problems under high speed and high power operation are largely avoided. In addition, FK-modulators show a weaker polarisation dependence of the switching contrast compared to devices based on the QSCE.¹ Finally, FK-modulators are particularly well suited for analogue optical modulation.

© 1994 IEEE