Steady state design of photonic transistor to achieve a switching gain>=3 dB

A recently proposed two-staged photonic transistor that provides switching gain is based on the directional coupler with an active arm and a passive arm in each stage. The manipulation of optical interference through optically-controlled gain caused the switching. In the first stage, a long wavelength input signal pulse depletes carriers to change absorption and switch a short wavelength beam into the second stage. In the second stage, the switched short wavelength beam fills the conduction band with carriers to increase the gain seen by another long wavelength pump beam to switch it as the output signal. Through a suitable design of intensity and wavelength of the interacting beams and the length of each stage, photonic transistor exhibits switching gain and hence can drive multiple stages (high fan-out and cascadability). The smaller the detuning of wavelength between the interacting optical fields or shorter the photonic transistor length, smaller is the cumulative change in linear absorption/gain, manifesting in a smaller switching gain. Since the short wavelength beam fills the conduction band with carriers, its intensity depends on the ground state absorption of the medium, α₀. And, since the long wavelength beam depletes carriers filled by the short wavelength beam, its wavelength depends on the gain of the pumped medium, g₀. In this paper, we show that the operational intensities of photonic transistor must be such that |α₀L₁|>27 and g₀L ₂>3.2 to achieve a gain>3dB, where L₁ and L ₂ are the length of 1^st and 2^nd stages respectively.