## 4 Discussion
At the position of 1/2\(\mathbf{H}\), the maximum phase variations are 8.5\({}^{\circ}\), 11.1\({}^{\circ}\), and 15.3\({}^{\circ}\) for the three applied voltages of 75, 100, and 150 V, respectively, as shown in Figure 8a. At the position 3/4\(\mathbf{H}\) with the previous three voltages, Figure 8b presents the phase variations: 22.1\({}^{\circ}\), 27.7\({}^{\circ}\), and 37.8\({}^{\circ}\). The position selections of 1/2\(\mathbf{H}\) and 3/4\(\mathbf{H}\) are just used for comparing the phase response dependence on the initial positions. The adjustable displacement sensitivities are considered for the different dynamic measurement ranges. Because the received power of the refracted light is decreased dramatically when the incident positions over 3/4\(\mathbf{H}\) on the CDS of \(\mathbf{R}\) = 7.1 mm, it means that the initially maximum positions are still limited by the beam divergence effect on the curved surface. The compared results of the phase responses dependent on the different radii of CDS are shown in Figure 9. The smaller radius and larger incident distances can increase phase variations under the same applied voltage. This means that the displacement sensitivity is enhanced along with measurement resolution being increased. Based on simulations as discussed in Refs. [8; 9], use of a smaller radius in a cylindrical lens can achieve higher sensitivity. However, their demonstration experiments used cylindrical lenses with radii of 10 and 25 mm only. The main concern is that the beam spot from the original laser is too large when the lens radius is smaller than 10 mm. Actually, we tried the OLB for measuring the phase response of CDS (\(\mathbf{R}\) = 7.1 mm). However, the results show that the repeatability and displacement sensitivity are both worse compared to the GWB. According to our experimental results, the design of CDS is important and the GWB is essential to improve the measurement sensitivity by employing the small radius of CDS. Moreover, the proposed GWB plays a key role for the repeatable measurements. In previous experiments, the CDS was fabricated using only a single KTP plate (1-KTP) attached to the cylindrical lens since phase variation is also proportional to the thickness of the KTP plate. Furthermore, a CDS made of two adhesive KTP plates (2-KTP) was explored to compare measurement sensitivity with that of a CDS using 1-KTP. Figure 10 shows the relationship between the phase variation and the applied voltage using the CDS with \(\mathbf{R}\) = 7.1 mm and 2-KTP plates. At position 1/2\(\mathbf{H}\), the maximum phase variations are 13.1\({}^{\circ}\), 16.7\({}^{\circ}\), and 22.2\({}^{\circ}\) for the three applied voltages 75, 100, and 150 V, respectively, as shown in Figure 10a. At position  
displacement at the \(3/4\)H position for the different CDSs when the PZT stage has 150 V applied (displacement = 40 \(\upmu\)m). Figure 11: Phase vs. applied voltage at the different incident positions on the CDSs of **R** = 7.1 mm with the single and double KTP plates (1-KTP and 2-KTP). Figure 9: Phase response for the different applied voltages using the GWB and CDS of **R** = 7.1 mm at different incident positions: (**a**) 1/2**H** and (**b**) 3/4**H**. The slope and R-square after a linear fit on these phase curves can be further calculated to compare performance on the different CDSs. The slope is expressed as average displacement sensitivity. In comparing the different radii of 1-KTP CDS probed by the GWB at the position 3/4**H**, according to Equation (10), the simulation values of DS are 0.45 and 0.98\({}^{\circ}\)/um and the experimental values are 0.49 and 0.99\({}^{\circ}\)/um for **R** = 10.2 mm and **R** = 7.1 mm, respectively. The close proximity of values in the simulations and the experiments indicate that the fabrication and application of the 1-KTP CDS are easily achievable. The largest slope of 1.49\({}^{\circ}\)/um in displacement ranging from 0 to 40 um with R-square of 0.9998 was achieved when probed by the GWB at the position 3/4**H** on the 2-KTP CDS of **R** = 7.1 mm.