An all-optical two 2-bit numbers multiplication with the help of Semiconductor Optical Amplifier (SOA)—assisted Sagnac Switch is proposed and described. The paper describes the multiplication using a set of all-optical half-adder and AND gate. Because of the lack of fast, accurate, and large dynamic range analog-to-digital converter, optical implementation of the digital multiplication through analog convolution algorithm yields a slow digital multiplier. By using a set of optical half-adder and optical AND, a new optical fast digital multiplication method is proposed. The new method promises both higher processing speed and accuracy. Numerical simulation result confirming described methods and conclusion are given in this paper.