Current wireless or optical technologies alone can not provide economically competitive solutions for the growing demand for communication networks capable of simultaneously ensuring higher capacity, coverage, bandwidth, and mobility in environments such as conference venues, university buildings, airports, hotels, shopping centers, and in the end to small offices and homes. Optical-wireless access solutions based on the integration of radio signals into optical carrier at a central office (CO) that are then transmitted by optical fiber to the wireless access point (AP), known as radio-over-fiber (RoF) networks, are being considered the most promising solution because they can bring the optical network bandwidths closer to the fixed and to the mobile users and at same time allowing the deliver of multiple services.

The next generation wireless-communication systems will use air frequencies in the range of 2-8 GHz. With these frequencies Gb/s data rates are possible only if cell size is reduced considerably. The so-called pico-cellular optical-wireless access with few meters range cells are being considered as a highly promising route for delivering high bandwidth and mobility access for in-building applications. Due to the moderate frequencies and the fiber transmission lengths involved (few km at maximum), the most cost-effective access point (AP) interface transponder solution is to use direct laser diode modulation for uplink and direct photo-detection for downlink. These solutions require the use of high power amplifiers and other RF processing electronics. Solutions employing electro-absorption modulators and semiconductor optical amplifiers can simplify electrical/optical conversion because these devices can perform detector/modulator functions simultaneously. However, they need complex optoelectronics and electronics for signals processing, which increase cost, complexity and power consumption.

Recognizing that a major challenge towards economically viable pico-cellular RoF networks is to implement low cost cell transponders with high efficient electrical and optical functionalities, we propose to investigate electrical optical (E/O) uplink and optical-electrical (O/E) downlink transponder functions using non-linear circuits based on the integration of resonant tunneling diode (RTD) oscillators with laser diodes (LD) and photo-detectors (PDs), respectively. The main advantages of these circuits are their intrinsic RF signals amplification and circuit simplicity, which makes them low-cost and more reliable.

The RTD is a nano-electronic device with a N shaped current-voltage characteristic exhibiting a negative differential resistance (NDR) region that can be used to implement high frequency electrical oscillators. When integrated with optoelectronic devices, such as LDs, can lead to novel optoelectronic functionalities such as optoelectronic voltage controlled oscillators (OVCO). This RTD-LD free-running oscillator when perturbed by radio-frequency broadcasted signals can synchronize and amplify very low power (<-40 dBm) incoming wireless signals, with the laser modulation depth being mainly determined by the NDR extension and not by wireless signal level. The oscillator locking range allows also a dynamic use of the frequency spectra available. This novel OVCO concept works as a wireless-to-optical interface converting a received low power wireless signal into an optical signal sub-carrier.

A complementary circuit is obtained integrating a RTD oscillator with a photoconductive region, the RTD-PD oscillator, that when directly illuminated by a modulated optical signal locks to the optical signal subcarrier. The optical injection locking capacity is used to O/E conversion to implement optical-to-wireless interfaces where the electrical output power is determined by the NDR region extension. The objective of our proposal is to demonstrate simple and low-cost downlink and uplink transponder functions using RTD-LD and RTD-PD oscillators, based on locking to wireless and to optical injected signals, respectively. This proposal foresees a low-cost AP solution with no need of format and frequency conversion or complex optoelectronic and electronic circuitry, such as high power amplifiers.