20-GHz optical analog link using an external modulator

An optical analog link operating at 20 GHz with a 3-dB bandwidth of 5.2 GHz was demonstrated. This link has an intermodulation-free dynamic range of 108 dB-Hz/sup 2/3/ and a maximum signal-to-noise ratio (SNR) of 155 dB-Hz. Previously reported direct modulation analog links have not been able to operate at 20 GHz and, even in the 10-GHz frequency range, have had 20-30-dB-poorer SNRs; externally modulated links are capable of high-frequency operation but have demonstrated 30-50-dB-poorer SNRs than the present link. The link consists of a CW laser, an integrated-optical intensity modulator, and a p-i-n diode detector connected by fibers; it contains no amplifiers. The link performance is evaluated at the RF ports, i.e. it is treated as a two-port RF device. It is noted that by simply using a detector with no rolloff and by reducing the modulator excess optical insertion loss to 3 dB, the RF insertion loss can be improved to -25 dB and the noise figure to 33 dB.<>