Drones have been attractive for many kinds of industries, but limited power supply from batteries has impeded drones from being operated for longer hours. Microwave power transmission (MPT) is one of the most prospective technologies to release them from the limitation. Since, among several types of drones, micro-drone has shorter available flight time, it is reasonable to provide micro-drone with wireless charging access with an MPT system. However, there is no suitable rectenna for micro-drone charging applications in preceding studies. In this paper, an MPT system for micro-drone was proposed at C-band where a lightweight and compact rectenna array with 20-W class output power was developed. Under illumination of a flat-top beam with 203 mW/cm2 of power density, a 16-element rectenna array was measured. The 16-element rectenna was formed with the aid of a honeycomb substrate for lightness and GaAs Schottky barrier diodes for high output. It was 37.5 g in weight and 146.4 mm by 146.4 mm in size. It output 27.0 W of dc power at 19.0 V at 5.8 GHz when radio frequency power of 280 W was generated by the injection-locked magnetron and 134 W was transmitted from the transmitting phased array. The power-to-weight ratio was 0.72W/g. The power conversion efficiency was 61.9%. These numbers outperformed the rectennas in the preceding studies and are suitable for an MPT system to micro-drone.

This paper describes a method to evaluate the modulated waveforms output by a high-speed external phase modulator over a wide wavelength range by using linear optical sampling (LOS) and a wavelength-swept light source. The phase-modulated waveform is sampled by LOS together with the reference signal before modulation, and the modulation waveform is observed by removing the phase noise of the light source extracted from the reference signal. In this process, the frequency offset caused by the optical-path length difference between the measurement and reference interferometers is removed by digital signal processing. A pseudo-random binary-sequence modulated signal is observed with a temporal resolution of 10ps. We obtained a dynamic range of ∼40dB for the measurement bandwidth of 10 nm. When the measurement bandwidth is expanded to entire C-Band (∼35nm), the dynamic ranges of 37∼46dB were observed, depending on the wavelengths. The measurement time was sub-seconds throughout the experiment.

We demonstrate the C-band wavelength conversion unit having functions of automatic wavelength recognition, power equalization, and elimination of original signal and pumping light for the first time, which is based on four-wave mixing (FWM) in semiconductor optical amplifiers (SOA's). The constructed unit automatically detects signal wavelength, sweeps wavelength of a pumping light, and adjusts center wavelengths of band pass filters and gain values of erbium-doped fiber amplifiers (EDFA's), in order to convert the wavelength of the signal to the arbitrary wavelength we set, and eliminate the original signal and pumping light after conversion. Amplification of the signal, pumping, and wavelength-converted lights compensates the detuning dependence of conversion efficiency and its asymmetry in the quantum-well (QW) SOA, to keep the power of the wavelength-converted light constant within the whole C-band region. The switching time of wavelength conversion by the unit is about a second, which is dominated by mechanical movement of the tunable filters. Wavelength-converted 2.5 and 10 Gb/s NRZ signals show clear eye-openings when the detuning is positive (ωp > ωs), and a 2-ps pulse train is also successfully wavelength-converted. To overcome the problem of the asymmetric conversion efficiency in the QW-SOA, we adopted quantum-dot (QD) SOA's. Although the 1.5 µm QD-SOA still shows its asymmetry, which will be improved by optimization of quantum dot structure, wavelength conversion of a 160 Gb/s RZ signal is demonstrated by the QD-SOA's. More improvement of the performance of the wavelength conversion unit should be possible by making switching time faster and installing the optimized QD-SOA's.