Optical computers -- computers in which photons replace the electrons of today's machines -- are expected to bring huge improvements in terms of compactness and speed and to yield unimaginable improvements in technology. Optical computing exists today only in science fiction, but recently researchers at the Birla Institute of Technology in Ranchi, India, described an all-optical device that can perform with photons functions similar to those that a conventional transistor performs with electrons.Figure 1. The three ports of the proposed optical transistor are analogous to the base, emitter and collector of an electronic transistor. If a steady input signal is applied to P1, the output from P3 is controlled by the input to P2. Images ©OSA.The concept of the all-optical transistor is straightforward. Two single-mode fibers are spliced to form a Y-connector, which is coupled through a nonlinear medium to a third single-mode fiber (Figure 1). There are two input ports, designated P1 and P2, and one output port, P3. When a strong, steady input signal is applied to P1, the signal emerging from P3 depends on the signal applied to P2. Figure 2. For a specified set of experimental conditions, the output from P3 varies with the input to P2. In this case, the transistor is functioning as a NOT gate.The researchers derived the coupled equations describing how the electric fields that are input interact in the nonlinear medium, and then slogged through a numerical analysis of the resulting equations. The answers they obtained enabled them to predict the output as a function of the inputs and the distance the fields traveled in the nonlinear medium.Figure 3. For a different set of experimental conditions, the output varies with the input. A small change in input signal causes a larger change in output signal. In this case, the transistor is acting as an amplifier.For example, for a particular constant input to P1 and a particular distance between the fiber facets in the nonlinear medium, they predicted that the output at P3 would vary with the input at P2 (Figure 2). When no signal is applied to P2, a signal X emerges from P3. When a signal X is applied to P2, no signal emerges from P3. That is, the optical transistor performs as a NOT gate.Figure 4. In the case described in Figure 3, a small signal input at P2 is amplified and inverted in the output.To exhibit amplification, the transistor must be operated with different values for the input at P1 and the spacing between the fiber facets. For this choice of parameters, the numerical analysis predicted that the output would vary as shown in Figure 3. A small change in the signal at P2 (~3 × 1025 in the units used in the analysis) will produce a change three times larger (~1 × 1024 in the same units) in the output (Figure 4). The small signal input on P2 is amplified and inverted in the output.