Low-noise-figure photonic links without pre-amplification

Using optical fiber to retrieve signals from remote sensors has several advantages compared to remoting by means of metallic waveguides such as coaxial cable. Fiber-optic retrieval of an RF signal can be achieved by down-converting and digitizing the signal for conveyance by a digital fiber-optic link, or it can be achieved by conveying the RF signal over an analog fiber-optic link before digitization. The latter approach can be realized with a minimum of hardware and dc power required at the sensing site, provided that the analog fiber-optic link has a sufficiently low noise figure without a pre-amplifier. Early demonstrations of “amplifierless” analog fiber-optic links typically reported very high noise figures—in excess of 30 dB. In the last decade or so, several techniques have been developed to improve this situation. We describe five such techniques and show that they have resulted in much lower measured noise figures for amplifierless links. One technique, for example, has yielded noise figures ≪ 5 dB for amplifierless links at frequencies of up to 10 GHz. The existence of amplifierless links with such low noise figures may enable remote sensing of signals in situations where the size, weight, and power (SWAP) of the remote hardware is of primary concern.