21 GHz wideband fiber optic link

A 1-km optical fiber link is reported that uses an electrooptically modulated infrared laser for wideband (i.e. 21-GHz) operation. A stabilized 1.3- mu m laser beam of 250 mu W is modulated by a traveling-wave intensity electrooptic modulator (EOM) with polarization control. A MESFET amplifier is used to drive the EOM. Phase balance of the interferometric EOM branches is achieved using a DC bias adjustment through a 26-GHz bias tree. The 1-km single-mode optical fiber is terminated with a pair of demountable biconical connectors to link the optoelectronic transmitter and receiver. The link was tested from 6 to 22 GHz and showed good wideband response. The measured input VSWR varies between 1.1:1 to 2.1:1 and output VSWR is 1.2:1. System signal-to-noise (SNR) measurement is better than 102 dB/Hz. Two-tone third-order intermodulation products (IMP) were measured, and no IMP was observed.<>