Remote Upconversion in Microwave Fiber-Optic Links Employing an Unbalanced Mach-Zehnder Interferometer

Abstract

We present a new technique for remote upconversion by inserting a passive all-optical device in the microwave (MW) fiber-optic link. Using a semiconductor laser directly modulated by two MW signals and an unbalanced Mach-Zehnder interferometer (UMZ) to convert optical frequency modulation (FM) into intensity modulation (IM), mixing is achieved after photodetection. Experiments with a UMZ integrated on glass-substrate have demonstrated the feasibility of this optical method for MW mixing with a low cost device of easy fabrication. Temperature control of the device allows optimized mixing performance and stable response. This method permits to overcome the effect of chromatic dispersion in standard singlemode fiber systems operating in the 1.55 μm wavelength window. Due to the presence of high-frequency fundamental components in the optical field, the received power is considerably degraded during transmission and direct detection in conventional systems, using either direct or external modulation in the MW band. With the proposed technique, lower frequency components of the field can be transmitted. The insertion of the simple UMZ generates high frequency only at detection side, therefore the available fiber length is extended. As it is shown by simulation results, this method can be used for the upconversion of MW subcarriers carrying digital signals.