Bandwidth enhancement of electro-optic sensing using high-even-order harmonic sidebands

We demonstrate that the bandwidth of an electro-optic sensing system can be significantly enhanced by the use of even-order harmonic sidebands (order > 2) produced on an optical-carrier probe beam with two cascaded electro-optic modulators. The sensing frequency range may be routinely expanded by at least four times, with respect to the use of fundamental-harmonic sidebands, by enhancing the nonlinearity of high-order-harmonic electro-optic modulation. The creation of harmonic modulation sidebands up to the sixth order of the drive frequency on a laser-diode output is described analytically, as is photonic heterodyne down-conversion of microwave signals using these high-order even harmonics within an electro-optic sensor crystal. The nonlinear harmonics serve as beneficial local-oscillator modulations for broadband electro-optic detection of microwave fields without nonlinear distortion. Transverse near-field distributions from an 18.5 GHz patch antenna are extracted using the fourth- (sixth-) order-harmonic sidebands at the output of the cascaded electro-optic modulators driven with a 4.6- (3.1-) GHz continuous-wave input.