Abstract: An infrared emitting semiconductor device having a layer-shaped infrared emitting region with a superlattice structure. The superlattice structure consists of thin alternating narrow and wide wells separated by thin barriers. The narrow well has one quasibound state E'.sub.0 while the wide well has two states E.sub.0 anad E.sub.1 with E'.sub.0 being located between E.sub.0 and E.sub.1. Under proper bias, an electron in state E.sub.0 can resonately tunnel out of the well through the quasibound state E'.sub.0 to the first excited state E.sub.1 of the next wide well. This electron can then relax to the ground state E.sub.0 where it can resonately tunnel to the next wide well and repeat the process. Infrared radiation with its photon energy equal to E.sub.1 -E.sub.0 is emitted as the electrons relax from E.sub.1 to E.sub.0 in the wide wells.

Abstract: At a transmitter of a digital transmission system which uses a redundant transmission code to transmit data, at least one predetermined code combination which occurs randomly in the encoded data is selectively modified, to form a predetermined code violation, in dependence upon an additional signal which is to be transmitted. At a receiver, the predetermined code violation is detected and corrected to reproduce the additional signal and the data. By selection of appropriate code combinations and violations more than one additional signal can be transmitted. The arrangement is particularly suitable for transmission of additional signals, which may comprise protection switching signals or a digital voice channel signal, via fiber optic transmission links using 2AMI encoding.