## 3 Measurement Setup  
To compare the displacement measurement performance depending on the characteristics of a beam spot, therein the OLB from a stabilized He-Ne laser with a wavelength of 632.8 nm (Model R-32734, Newport Corporation, Irvine, CA, USA), and the measurement setup shown in Figure 2 based on the dual-channel homodyne interferometer [17], was used. The OLB with a vertical polarization was obtained using a Glan-Thompson polarizer (GPL). A half-wave plate (HWP) at an azimuth angle of 22.5\({}^{\circ}\) was used to obtain two orthogonal polarizations of equal amplitude. The input optical power was controlled using an attenuator (AT). An electro-optic modulator EOM (Model 4002 New Focus, Newport Corporation, Irvine, CA, USA) was used to modulate the phase delay between the two polarizations. The EOM was driven by a function generator (FG1) and voltage amplifier (AP1). The amplitude and frequency of the applied sinusoidal voltage were 105 V and 1 kHz, respectively. The voltage induces the corresponding phase modulations used in the homodyne interferometry. The incident light was divided into two paths via a non-polarization beam splitter (BS). The reflected light passing through an analyzer (AL1) was received by a photodetector (PD1). The reflected signal was used to monitor the photorefractive induced phase noise from the EOM. The CDS was placed on the PZT stage driven by a function generator (FG2) and a voltage amplifier (AP2). According to the product specifications from the vendor, displacement is around 40 \(\upmu\)m at an applied voltage of 150 V onto the stage. The transmitted light passed through the CDS, and an analyzer (AL2) was then received by a photodetector (PD2). The transmitted light was used to measure the phase variations caused by the displacement. Both the interferometric signals of the reflected and transmitted paths were connected to a multichannel data acquisition module (PXI). A fast Fourier transform (FFT) scheme was adopted to decide the measured phase variations [18], and this phase information was displayed on a personal computer (PC). The common phase noise can be effectively reduced by synchronously measuring the phase variations of both channels [17].  
In comparison with the measurement provided by the OLB, the GWB was formed through the measurement setup shown in Figure 3. The same He-Ne laser was used for the measurements. The laser beam was modified by the setup consisting of a channel waveguide device (WG), and two coupled object lenses (L1 and L2). To fabricate a single-mode waveguide for TE and TM polarizations as developed in our research works [17; 18], a 35 nm Zn film with a predeposition Ni film of 5 nm was deposited over the lithium niobate substrate via e-beam evaporation. Next, the waveguide width of 4 \(\upmu\)m was formed by a lift-off technique. After thermal diffusion of 850 \({}^{\circ}\)C for 120 min, the Zn ions penetrate the host substrate and form a graded index distribution in the lateral and in-depth directions. The substrate end faces were polished for the light coupling. The two lenses L1 and L2 were used to focus the input and output light beams. A pinhole (PH) was used to block the scattering light. The functions of GPL, AT, and HWP are the same as described in Figure 2. In case of the GWB measurements, both of the WG and EOM are the lithium niobate materials. Therefore, the AT is placed in front of the WG. A series of experiments was conducted to compare the measurement performance between the OLB and GWB.  
Figure 3: Measurement setup with the guided-wave beam.  
Figure 2: Measurement setup with the original laser beam.