Transformation, spectrum, and amplification of light modulated by a low frequency traveling wave

The behavior of electromagnetic waves in a medium modulated in time and space, largely investigated decades ago, has attracted emerging current interest. Here, we address one of the key questions of this research: can an optical wave be amplified by a slowly modulated realistic photonic circuit? To solve this problem, we investigate the propagation of light with an initial phase velocity v₀ and frequency ω₀ propagating through an optical waveguide and resonator modulated by a traveling wave with phase velocity vₚ and low frequency ωₚ << ω₀. Close to the synchronous condition, vₚ =v₀, the modulation effects can significantly grow. Consequently, the bandwidth of the optical frequency comb spectrum can be dramatically enhanced by a synchronous traveling wave with vₚ ≈ v₀ compared to the bandwidth generated by an instantaneous wave with vₚ=∞. However, for realistic optical waveguides, the light amplification remains small due to its essentially broadband character. In contrast, we show that the modulation of a racetrack optical resonator by a traveling wave with a small phase velocity vₚ <<v₀ (rather than with synchronous vₚ≈ v₀) can significantly amplify light remaining within a relatively small bandwidth.