**1. Introduction**

Electric-field sensors that exhibit wide, flat frequency response characteristics are important tools for electromagnetic compatibility and interference (EMC/EMI) measurements, high-frequency electronic circuit analysis, medical equipment field observation, radio-frequency reception, and high-power microwave detection. The importance of these sensors is increasing as mobile multimedia communications develops [1,2]. It is necessary to accurately evaluate the strength and distribution of electromagnetic fields surrounding electronic equipment to estimate electromagnetic compatibility. The requirements for electric-field sensors based on the important applications mentioned above are as follows: their wide frequency bandwidth and large dynamic range; their high spatial resolution and low interference to the original field; and their high stability and accuracy.

Even though a variety of sensing modules have been developed, those with photonic links reduce or eliminate some of the inaccuracies and systematic errors that affect measurement techniques using conventional EM-field sensors. They provide electrical isolation, which eliminates ground loops and common-mode electrical pickup between the sensor head and the electronics module. Moreover, the optical fibers and dielectric components produce minimal field distortion. In addition, they can preserve both the phase and amplitude of high-frequency fields with good fidelity and low losses. The development of optical-fiber and optoelectronic components for the telecommunications industry has made it possible to implement photonic sensors that are accurate and convenient to use.

In particular, titanium-diffused lithium niobate (Ti: LiNbO3) waveguide devices are suitable for electric-field detection since their sensors will not perturb the field to be measured. A linear modulator that is passively biased to the optimal linear operating point is required. This has been demonstrated for asymmetric Mach–Zehnder interferometers (MZIs) and 1 × 2 directional couplers. The former devices have an intrinsic bias of π/2, where a geometrical path length difference of a quarter of a wavelength is required between the two arms [3–7]. However, it is not easy to obtain optimal operation through path length difference alone, because of fabrication tolerances.

Moreover, the latter device is automatically biased to the optimal 3 dB operating point due to its symmetrical structure, and it provides a greater tolerance in the fabrication process than an asymmetric Mach–Zehnder interferometric optical modulator [8–11]. However, the complexity of the transfer function makes it impossible to utilize the sensor in a specific range (namely, the ratio of interaction length to conversion length).

In contrast, a Y-fed balanced-bridge MZI modulator (YBB-MZI) consists of a 3 dB coupler at the output, with two complementary output waveguides [12–19]. This type of modulator provides a well-defined transfer function for the output optical power versus the detected electric-field intensity and can be automatically biased at the optimum 3 dB operating point due to its symmetrical structure, which offers a more tolerant design in the fabrication process than Mach–Zehnder interferometric modulators or 1 × 2 directional couplers, which are asymmetrical. A mono-shield gold electrode structure was applied in YBB-MZI by the Tsinghua University group to detect a very high electric field [12]. Moreover, the minimum electric-field strength that can be sensed is determined by the relative intensity noise (RIN) of the laser diode. Therefore, a YBB-MZI configuration was proposed by a German group to reduce RIN and improve sensitivity with a balanced optical receiver [16].

In this paper, we provide the quantitative theory of a YBB-MZI modulator and report on a fabricated Ti: LiNbO3 YBB-MZI modulator operating at a 1.3 μm wavelength. We also present, in detail, the fabrication process and design parameters, as well as the optical and electrical performances of a packaged electric-field sensor with a diploe patch antenna, including the minimum detectable electric-field intensity, dynamic range, and sensitivity.
