Comparison of an in-line asymmetric directional coupler modulator with distributed optical loss to other linearized electrooptic modulators

The transfer function of an external modulator is the critical factor that determines the spurious free dynamic range, signal power gain, and noise fi gure of a wide-band analog RF-photonic link. We present an in-line asymmetr ic directional coupler modulator with distributed optical loss capable of p roviding a transfer function linearized to the fourth order for multioctave bandwidths. This modulator compares favorably with multiple modulator subo ctave linearization techniques consisting of series symmetric directional c oupler and series Mach-Zehnder modulators and superoctave parallel Mach-Zeh nder, and cascaded sections containing series Mach-Zehnder/symmetric direct ional coupler modulators. The optimum bias point for the asymmetric directi onal coupler modulator is determined by device dimensions and material para meters with relative insensitivity to errors introduced during fabrication, Optical insertion loss imposes significant limitations on modulator perfor mance. An implementation of an in-line asymmetric directional coupler modul ator is discussed that eliminates excess loss due to fiber coupling in orde r to achieve a large RF power gain and competitive noise figure compared to the linearization configurations.