Effect of optical scattering on one-way RF frequency transfer over optical fibers

Microwave frequency transfer over optical fibers is a promising technology due to the high bandwidth, low loss, low weight, and EMI immunity of optical fibers. Fiber-induced noise has led to challenges in implementation that are commonly resolved by using two-way signal propagation with active feedback. However, some applications require one-way frequency transfer without the use of active feedback. In this paper, we demonstrate that optical scattering can be the dominant source of fiber-induced noise and can be mitigated without active feedback. When light from a low noise laser propagates through kilometers of optical fiber, light scattering can increase the optical intensity noise by several orders of magnitude. In an RF-photonic link, this intensity noise can be converted to phase noise around the transmitted frequency. Consequently, the scattering-induced noise degrades the stability of the transferred signal. For example, we measured a 14 dB degradation of the Allan deviation after 10 km transmission. We previously showed that the scattering-induced intensity noise can be mitigated by dithering the laser current. We implemented this technique to reduce the scattering-induced frequency instability. We suppressed the Allan deviation that is induced by propagation through 10 km of fiber by 12 dB to within 2 dB of the back-to-back link.