An effective scheme of frequency doubling up-conversion is proposed and demonstrated, which achieves the generation of high-frequency microwave signal and the compensation of chromatic dispersion-induced power fading (CDIPF). The system structure is based on a cascading of a Sagnac loop and a Mach-Zehnder modulator (MZM), where a phase modulator is used in the Sagnac loop. In the loop, only the clockwise propagating light wave is modulated by a small local oscillator signal, leaving the counterclockwise propagating light wave unmodulated because of the velocity mismatch. The output signal is divided into two parts, one traveling into an MZM and the other entering an optical phase shifter. The intermediate frequency signal is supplied to the MZM, which is biased at its minimum point and then the carrier-suppressed double sideband is achieved. With the adjustment of the optical phase shift, the CDIPF can be compensated at any frequency. In the simulation, by comparing the frequency responses of the proposed link with and without dispersion compensation, we successfully demonstrate the performance of power fading compensation for 60-and 80-km link. Consequently, the spurious-free dynamic range is effectively improved by 13.54 dB at 22.6 GHz for 60-km link.
We propose an instantaneous frequency measurement (IFM) scheme featured with unambiguous measurement utilizing a Sagnac loop with an embedded bi-directional phase modulator and an optical filter with a linear frequency response. The frequency of the microwave signal to be measured is instantaneously estimated by calculating an amplitude comparison function (ACF) of photocurrents from the upper and lower branches of the proposed structure. Compared to previous IFM schemes, the ACF in the proposed scheme is monotonic, by which the frequency of the unknown microwave signal can be uniquely confirmed without ambiguity. Only one laser source and a phase modulator are needed, which simplifies the measurement system. In addition, the monotonous ACF can guarantee measurement accuracy in a wide range, from low frequency to high frequency. The IFM system has robustness in optical and RF power shifts. Simulation results show that frequency measurement over the range of 30 GHz with a measurement error of 0.4 GHz is achieved.
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