*3.2. Test Results and Discussions*

Figure 8 shows the spectrum-analyzer outputs for an input of 20 dBm to the TEM cell at frequencies of 10, 50, and 70 MHz, respectively. The rf power detected at the photodetector was measured to be −101.5, −110.9, and −122.2 dBm, as shown in Figure 8, and the noise floor was measured to be about −130 dBm at the same frequencies. The internal electric field of 29.8 V/m in the TEM cell produced an SNR of 28.5, 19.1, and 7.8 dB at each frequency. Therefore, the minimum detectable electric-fields are ~1.12, ~3.3, and ~12.13 V/m, respectively, at those three frequencies, based on the equation Emin = 29.8 <sup>×</sup> <sup>10</sup>(-SNR/20) <sup>V</sup>/m.

**Figure 8.** The detected rf spectra of (**a**) 10 MHz, (**b**) 50 MHz, and (**c**) 70 MHz rf input signals into the TEM cell, with a power level of 100 mW.

Figure 9 shows the sensitivity curves at rf frequencies of 10 MHz, 50 MHz, and 100 MHz. We can confirm that the graph shows almost linear response characteristics from the applied electric-field intensity from 0.293 V/m to 23.2 V/m. Even though some data are off the linear response line, they remain very close. The device also shows a dynamic range of about ~22, ~18, and ~12 dB at frequencies of 10, 50, and 100 MHz, respectively. Figure 10 shows the photodetector power at different electric-field intensities.

**Figure 9.** The photo-detected signal power versus the electric-field strength in the TEM cell at different frequencies.

**Figure 10.** The photo-detected signal power versus frequency at different electric-field strengths in the TEM cell.

Figure 11 illustrates the frequency response of the sensor measured with 20 dBm of rf input power applied to the TEM cell. This figure shows a nearly flat frequency response from 1 MHz to ~50 MHz. The cut-off high frequency of the device is derived from the series-coupled time constant of the electrode resistance and the structural and packaged capacitances of the device. Therefore, a much higher cutoff frequency can be expected when a metal material with a higher coefficient of conductivity, such as gold instead of aluminum, is applied to an electrode.

So far, the theoretical analysis and experimental results have confirmed that an electric-field sensor based on YBB-MZI exhibits a superior 3 dB optical bias and simple sinusoidal transfer characteristics. Regardless of the refractive index of the optical waveguide, a 3 dB optical bias was obtained because of the perfect symmetry of the two arms that make up the YBB-MZI. However, in the case of a conventional MZI, a 3 dB optical bias can be realized by the optical path difference between the two arms. Moreover, the optical bias depends on both the optical path difference and the effective refractive index of the waveguide, which is especially affected by fabrication parameters, such as titanium thickness, diffusion time, temperature, and ambience. Therefore, the YBB-MZI structure allows much better control of the optical bias than does a conventional MZI.

**Figure 11.** The frequency response of the sensor.

#### **4. Conclusions**

We have demonstrated a photonic electric-field sensor utilizing a 1 × 2 electro-optic Ti: LiNbO3 Y-fed balanced bridge Mach–Zehnder Interferometric modulator, which provides the unique characteristic of an intrinsic 3 dB operating point, due to its symmetrical geometry. The theoretical analysis demonstrates that the YBB-MZI structure inherits advantages from both conventional MZI and directional coupler structures: namely, a sinusoidal transfer function and a better optical bias control. The sensors were designed and fabricated with a 49 × 15 × 1 mm size and operated at a wavelength of 1.3 μm. We observed a dc switching voltage of ~16.6 V and an extinction ratio of ~14.7 dB. The minimum detectable electric-field strengths for this device were ~1.12 V/m and ~3.3 V/m, corresponding to a dynamic range of about ~22 dB and ~18 dB at frequencies of 10 MHz and 50 MHz, respectively. The sensor exhibits a nearly linear response to an applied electric-field intensity from 0.29 V/m to 29.8 V/m.

In the future, further work on electric-field sensors will be needed to improve sensitivity, operational stability, response speed, detectable frequency range, and encapsulation. To realize a high sensitivity, it is necessary to suppress the noise in the laser diode and the photodetector as much as possible while improving the performance efficiency of the YBB-MZI modulator. The sensitivity limited by shot-noise can be improved by suppressing the relative intensity noise (RIN) of the laser diode as much as possible in the photodetector, and it is possible to configure the balanced detection receiver by combining a YBB-MZI modulator and a balanced photodetector. Since the sensitivity of electric-field sensors utilizing various Ti: LiNbO3-integrated optical modulators is greatly affected by the structures of electrodes and antennas, the performance of sensors based on various electrode structures and antennas (such as dipole antennas, loop antennas, and segmented patch antennas) should be compared and discussed together.

**Funding:** This research was supported by the Basic Science Research Program through the national Research Foundation of Korea (NRF: 2018049908) and funded by the Ministry of Education, Science and Technology.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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