## 2 Configuration of the Optical Fiber Probe  
Figure 1 shows the optical system of the fiber probe. The probing system is composed of the stylus shaft and tip made of an optical fiber, two focused beams and two dual-element photodiodes in the X- and Y-directions. The probe shaft and tip are manufactured using a wet etching device and CO\({}_{2}\) laser processing [9]. The axes of two laser beams are inclined \(\theta^{\circ}\) to XY plane. The laser beams are reflected in an upward direction by the prisms. This makes all optical systems install above the stylus and measured surface, and it is possible to avoid the interference between the optical system and measured surface [8]. The intermediate part of the stylus shaft is then irradiated with the focused laser beams in the X- and Y-directions. The length of the stylus shaft from the fixing jig to the stylus tip is 2 mm, and the projecting length of the stylus shaft from the lower end face of the prisms is 1 mm. The stylus tip diameter is within the limit from 1 \(\upmu\)m to several 100 \(\upmu\)m. Therefore, the stylus can be employed for measuring the microstructures, such as micro-holes with a depth of 1 mm and a diameter of a few \(\upmu\)m [10]. In cases where the stylus tip makes contact with the surface being measured, the deflection occurs in the shaft, as shown in Figure 1. Figure 2 shows the section A-A in Figure 1, which shows the laser irradiation position and the locus of the laser beam in the Y-direction. The focused laser beam passes through the inside of the shaft and is detected by the dual-element photodiode. The focus point of the laser beam coincides with that of the stylus shaft as a rod lens. When the diameter of the stylus shaft is 70 \(\upmu\)m, the distance from the focus point of the laser beam to the surface of the stylus shaft is approximately 20 \(\upmu\)m. The diameter of the laser beam is approximately 15 \(\upmu\)m where the laser beam is incident on the surface of the stylus shaft. The laser beam intensities measured by respective region of the dual-element photodiode are converted into voltage signals and are defined as \(I_{\text{PX1}}\) and \(I_{\text{PX2}}\) (V). As indicated in Figure 2a, in cases where the stylus tip does not make contact with the surface being measured, the laser beam intensities detected by respective region of the dual-element photodiode are equal (\(I_{\text{PX1}}=I_{\text{PX2}}\)). In cases where the stylus tip hits on the surface being measured in the positive Y-direction, the stylus shaft is deflected in the same direction  
and the cross-section of the stylus shaft emitted by the laser beam is also displaced, as shown in Figure 2b. Consequently, the refraction angle of the laser beam passing through the inside of the stylus shaft in the X-direction varies and the laser beam is emitted toward the positive Y-direction because of the displacement of the irradiated part of the stylus shaft, and the light intensities of respective regions of the dual-element photodiode become unequal (\(I_{\text{PX2}}>I_{\text{PX1}}\)), as indicated in Figure 2b. The output signals \(I_{X}\) in the X-direction using \(I_{\text{PX1}}\) and \(I_{\text{PX2}}\) are defined in Equation (1). The contact between the stylus tip and the surface of the microstructure in the Y-direction is determined when the output signal \(I_{X}\) exceeds the threshold value. \[I_{\text{X}}\ =\ I_{\text{PX1}}-I_{\text{PX2}} \tag{1}\]  
Similarly, the contact between the tip and the measured surface in the X-direction can be detected by using the laser diode and dual-element photodiode placed in the orthogonal Y-direction. When the tip comes into contact with the horizontal plane in the Z-direction, the stylus shaft is buckled and then deflected.