Our objective is to improve the accuracy of bubble measurement with single-tip optical fiber probe (S-TOP) in a bubbly flow. The S-TOP measures a point-wise void fraction and a velocity and diameter of a bubble simultaneously, by using only a single wedge-shaped tip. The S-TOP structure is very simple but raises an issue of an unclear touch position/angle between the S-TOP and a bubble. We propose a pre-signal threshold method in order to solve this issue. The pre-signal is one of the noisy optical signals from the S-TOP. First, the S-TOP signals are quantitatively analyzed with a newly developed 3D computational ray tracing method; the simulator can trace enormous ray segment trajectories in an optical fiber and render complicated optical boundary conditions. Moreover, in the simulator, evaluation of the complex output signals is achieved by computing the polarization and energy of every ray. We discovered that the intensity of the pre-signal indicates whether the S-TOP touches normally the center region of the bubble, or does not. We applied this characteristic to practical S-TOP measurement of a bubbly flow. As a result, the difference between the measured chord length via the S-TOP and the bubble minor axis from visualized images was significantly improved from 42% (existing signal process) to 15% (with processing through the method). (C) 2013 Elsevier Ltd. All rights reserved.