Broadband class-AB microwave-photonic link using polarization modulation

A novel broadband class-AB (CAB) microwave-photonic link is demonstrated using an AlGaAs-GaAs electrooptic mode-converter-based polarization modulator (PolM). The polarization-modulated signal is split into two paths, each independently optically biased to provide two electrooptic transfer functions operating close to, but on opposing sides of, their nulls. Experimental results at 2 and 4 GHz confirm that detection of the transfer functions in a differential detector provides excellent cancellation of second-order distortion and recreates the same third-order distortion as a quadrature-biased Mach-Zehnder modulator. Biasing the operating points near the null reduces optical carrier power, and hence, carrier-related noise, thereby increasing achievable spur-free dynamic range. The use of the PolM allows the CAB link to be implemented with a single modulator, reducing the matching requirements of dual modulator schemes and allowing third-order distortion-limited broadband operation.