Fig. 1. Simplified diagram of linearized downconversion link and corresponding signal spectra and transfer characteristics. DE-MZM/DD, dual-electrode Mach–Zehnder modulated direct detection; IMD3, third-order intermodulation distortion; IIE, intelligent impairment equalization; RF, radio frequency; IF, intermediate frequency.

Fig. 2. Equalization-based inverse link transfer function estimation. BP, backpropagation.

Fig. 3. Experimental setup of proposed linearized microwave downconversion link. LD, laser diode; PC, polarization controller; DE-MZM, dual-electrode Mach–Zehnder modulator; LO, local oscillator; EDFA, erbium-doped fiber amplifier; PD, photodetector; IIE, intelligent impairment equalization.

Fig. 4. (a) Dependence of MSE on the number of epochs for the training subsets; inset, the electrical spectrum of two-tone signal with the MSE of 10⁻⁶; (b) DE-MZM/DD-based transmission link and calculated inverse link transfer functions; black solid curve, theoretical inverse transfer function; red dotted curve, IIE-based inverse link transfer function; inset, whole downconversion link transfer characteristic by applying IIE.

Fig. 5. Electrical spectra of the two-tone input tests before and after applying IIE when the frequencies are set to be (a), (b) 2.11 GHz and 2.13 GHz; (c), (d) 2.10 GHz and 2.14 GHz; and (e), (f) 4.11 GHz and 4.13 GHz, respectively.

Fig. 6. Measured SFDR before (solid line) and after (dashed line) IIE.