An all-optical microwave mixer with gain [using semiconductor optical amplifier]

Fiber-optic transmission of microwave signals has attracted significant attention for photonic RF communication systems such as satellite communications and subcarrier multiplexed systems. Recently a novel fiber-optic link architecture has been proposed, where the baseband data is up converted by an external modulator, transmitted through a fiber link, and mixed down to IF by another external modulator at the receiving end. Such a scheme offers the advantage of the infinite RF isolation and also avoids the need for high-speed photodetectors and microwave mixers. However, it also has several problems, specifically at the receiving end. First, an external modulator is intrinsically polarization sensitive, and the optical links already laid out are not polarization maintained. Secondly, the external modulator has additional loss of ~5 dB, potentially requiring optical amplification. In this paper, we propose a novel microwave mixer for photonic microwave systems based on cross-gain saturation in a semiconductor optical amplifier. In the experimental demonstration, an optical signal at 1312 nm carrying a 6-GHz RF signal is down and up converted to 1 GHz and 11 GHz by another optical signal at 1320 nm carrying a 5-GHz local oscillator. A lossless up/down conversion operation is also achieved. Such a mixer offers the advantage of polarization independence and optical amplification