SNR Enhancement of up to 9.5 Db Utilizing Four-Wave Mixing for Angle-Modulated Analog Optical Links

Angle modulation, such as phase and frequency modulation (FM/PM), has recently attracted attention for improving the fidelity of analog optical links. This is because angle modulation can enhance the signal-to-noise ratio (SNR) of the original analog message simply by increasing the modulation index or the bandwidth. However, direct generation of a broadband optical angle-modulated signal is not necessarily easy. On the other hand, there is an indirect generation method, where a narrowband angle-modulated signal is first generated and then converted to a broadband signal with the help of nonlinear devices. This method is often used in FM radio systems, but few reports are available on the optical implementation of this indirect method. In this paper, therefore, we propose the optical implementation by using four-wave mixing (FWM) in an optical fiber. Since an optical fiber naturally has a third-order nonlinearity (i.e., the Kerr effect), it should be possible to use fiber nonlinearity for the optical implementation of the indirect method to increase the bandwidth of an optical angle-modulated signal. We propose two types of FWM-based methods. In the first type, an angle-modulated signal is injected into a dispersion-shifted fiber (DSF) with an unmodulated pump wave. Consequently, one of the idlers generated via the FWM has a double modulation index, and the SNR of the original signal can be enhanced by 6 dB. In the second type, the input becomes a conjugate pair of an angle-modulated signal. Consequently, the idler has a threefold modulation index, and therefore, this type enhances the SNR by 9.5 dB. We experimentally confirmed the concept. The results showed that the modulation index can actually be increased almost threefold, and the idler can enhance the SNR of the original signal by approximately 8.8 dB.