AbstractA spatial light modulator GaAs‐pin/ferroelectric liquid crystal spatial light modulator (GaAs‐pin/FLC‐SLM) was developed in which a GaAs photodiode is used as a photodetector, and a ferroelectric liquid crystal is used as a modulator. In the present device, write‐in with visible light and a transmission‐type readout at infrared are possible. Other characteristics of this device include a high‐speed operation due to the use of a ferroelectric liquid‐crystal and no need for erasing light due to the use of a pin diode.In this paper, the configuration of the SLM is presented and the operating principle is explained by means of an operation model. Next, experimental examples as a switching device are described. Also, several examples of the system applications are presented.The first example is a self‐routing crossbar switch. In this switch, the 1.3‐μm signal light is switched by providing a header at a different wavelength to a connected output port. the present SLM is used as an optical switch section. the experimental results of the present switch with 2 × 2 ports are reported.The second example of the system application is an optical parallel processor which is based on the accumulative threshold characteristic of the present SLM. By means of this characteristic, logic operation is possible for time series parallel optical signals. It is presented that the sequential operation system can be constructed in a much simpler configuration. the experimental results for configuration of the