Fiber‐optic millimeter‐wave subcarrier transmission links using HBTs as photodetectors

Abstract

The authors utilized monolithic microwave integrated circuits (MMIC) compatible devices, GaAs HBT and InP/InGaAs HBT, as photodetectors for a fiber‐optic/millimeter‐wave subcarrier link, and investigated the link loss characteristics, subcarrier‐to‐noise ratio (SNR) characteristics, and IM3 characteristics of the fiber‐optic millimeter‐wave subcarrier link, comparing them with those of the link using a pin photodiode. The link gain characteristics are compared with those of the links which use other MMIC compatible photodetectors (MSM and HEMT) and pin photodetectors (pin‐PD). It is found that for both the common emitter HBT link and the common‐base HBT link, good response and flat frequency characteristics can be seen over several gigahertz in the millimeter‐wave band. The SNR characteristics of the common emitter HBT link governs the noise characteristics of the optical/RF transducer in the low‐frequency region where there is internal gain. However, in the millimeter‐wave region (where there is no gain), it is dependent on the noise characteristics of the LNA connected after the HBT or in the common base HBT link. The input/output characteristics and IM3 characteristics of the HBT link are dependent on the power characteristics of the external optical modulator (EOM) when the optical power is less than ‐ 3 dBm. An optical/RF transducer combining an HBT photodetector, LNA, and horn antenna is designed. Using this optical/RF transducer, an optical fiber/millimeter‐wave radio link is designed and the FM video transmission characteristics are investigated experimentally. It is shown that this link was effective at wavelengths of 0.83 μm and 1.3 μm.