Design and realization of an optically controlled oscillator for radio over fiber at 5.2 GHz

Abstract

We propose the design of a device directly converting an OOK (on-off keying) baseband optical signal from an optical fiber into a microwave carrier. It is based upon a microwave oscillator, using a Silicon-Germanium (SiGe) Heterojunction Phototransistor (HPT), realized in fully-integrated MMIC 0,35 µm technology. Simulations showed that the oscillation frequency is controlled by an optical illumination of the base-collector junction, increasing the junction capacitance, thus decreasing the oscillation frequency. A FSK (frequency shift keying) modulation is possible. If the frequency is locked by current injection from a reference oscillator, a PSK (phase shift keying) modulation becomes possible. First experimentations on the phototransistor confirmed these simulations.Index Terms -Radio over fiber, phototransistor, photooscillator, WLANs, digital modulations.

SiGe heterojunction bipolar phototransistor for optics–microwaves interfacing

“…The characteristic is rather linear with a slight saturation effect at high illumination. The measured responsivity is R = 0.066 A/W, thus an external efficiency of 15% with respect to its theoretical value [6], this low value is simply due to the non-optimization of the fiber-to-detector coupling. In a previous experiment, we had achieved a higher efficiency ( R = 0.15 A/W) with a sharper fiber end.…”

“…This structure was improved since the first one [6], in order to enhance the efficiency without degrading the transition frequency: the main modification consists of an increase in the transistor length to get a larger illuminated area. Removing a base finger creates an illumination window.…”

“…Removing a base finger creates an illumination window. This structure was improved since the first one [6], in order to enhance the efficiency without degrading the transition frequency: the main modification consists of an increase in the transistor length to get a larger illuminated area. Nevertheless, we chose not to increase the dimension of the illumination window in the other direction to preserve the transition frequency of the device and to remain compatible with commercial RF Foundry services, in contrast with other designs aiming to optimize the responsitivity [7].…”

An 850 nm SiGe/Si HPT with a 4.12 GHz maximum optical transition frequency and 0.805A/W responsivity

“…SiGe HPTs have been used for digital optical receiver circuits [13], photo oscillators [14] and more recently a transmission of a 60 GHz WiFi signal operating at 3 Gbps [15]. They provide a novel approach in order to combine high speed HBTs with low cost microwave phototransistors directly in SiGe bipolar and BiCMOS technologies.…”