Optical fibres exhibit a bandwidth of about 50 THz, which enables data transmission in excess of 100 Gbits^-1. This potential for ultrafast optical signal transmission stimulates the research on appropriate functional devices for optical switching and optical signal processing in general. A promising device is the semiconductor laser amplifier (SLA). It enables optical signal processing in the femtosecond range when it is operated under conditions where intraband dynamics with subpicosecond time constants are dominating. A suitable experimental technique for operating the SLA under these conditions is four-wave mixing. Based on this technique, various applications for optical signal processing have been investigated, such as optical switches, demultiplexers, wavelength converters and phase conjugators. In this article we describe these experiments and discuss the advantages and disadvantages of optical signal processing using SLAS.