A device is proposed which is the optical analog of the transistor wherein a weak incoming optical signal directly controls a strong outgoing optical signal. The basic principle utilized in the proposed device is the strong dependence of second harmonic generation on the phase-matching condition. A weak incoming signal can perturb the indices of refraction in a birefringent material so that the second harmonic power derived from a pump beam is significantly affected. Large incremental power gain is shown to be possible. Tellurium is considered as an example.