*2.3. Performance Evaluations*

The fabricated device without an attached optical fiber was first tested by applying DC voltages. The device's performance and characterization were observed using a tunable laser with butt-coupling at a wavelength of 1.3. The TE-polarized input light was butt-coupled to the devices, collected at the output by a microscope's objective lens, and focused onto a photo-detector for measurement. TE or TM polarized light was selected by properly adjusting a fiber optic polarization controller. We first observed the single-mode propagation for TE polarization in the 1 × 2 YBB-MZI modulator.

It was observed that when the voltage was not applied, the two outputs of the device were almost the same. The voltage required to switch either output light power from a bar state (maximum intensity) to a cross state (minimum intensity) was measured to be ~-16.6 V, which corresponds to a ~14.7 dB extinction ratio. Figure 5 shows the optical output power versus the DC voltage measured by an optical power meter and shows a slightly asymmetric DC output characteristic, as well as a switching voltage of ~16.6 V, as mentioned previously. The AC modulation responses of the two outputs versus the driving sinusoidal voltage were further measured in Figure 6, where the optical signals are below the sinusoidal curve and the applied ac voltage signal is above the curve, at a frequency of ~1 kHz (5 V/div). The power of the two outputs was confirmed to be nearly equal, and the periodic exchange of output power in the two outputs expresses a good inverse relationship in the output sinusoidal curves. The slightly skewed and flattened shape in the optical response in Figure 6 was observed due to the imperfect single mode waveguide and the out-diffusion that occurred in the diffusion process.

**Figure 5.** Measured optical output power intensity versus the applied DC voltage.

**Figure 6.** The 1 kHz ac modulation responses at the (**a**) upper and (**b**) lower output port, as shown in Figure 1a.
