All-optical image-reject frequency down-conversion based on cascaded electro-optical modulators

An all-optical image-reject frequency down-converter based on cascaded modulators and a wavelength demultiplexer is presented. The radio-frequency (RF) and the local-oscillator (LO) signal are fed to a phase modulator and a dual-parallel Mach-Zehnder modulator, respectively. Using a wavelength demultiplexer, two down-conversion channels can be obtained. Quadrature phase difference between the two down-converted signals is achieved by properly setting the modulator bias. Image rejection can thus be realized based on Hartley architecture. Experimental results show that image rejection ratios of 32.5 dB in a 10-MHz instantaneous bandwidth and over 50 dB for a single frequency can be obtained. The scheme can be applied to antenna remoting applications, and a 25-km fiber transmission is demonstrated.