An optical fiber probe (OFP) is an intrusive but useful device for bubble/droplet measurement in gas-liquid two-phase flows. Although previous researchers have invented various types of OFP and established their own probe systems, few techniques have been applied to measurements in practical machinery. In particular, successful examples of OFP for droplet measurement are extremely rare. We invented a single-tip OFP (S-TOP) for size/velocity measurement of submillimeter droplets. The difficulty of using S-TOP measurement in droplet flows, as is true with other OFPs and electric conductivity probes, is how to discriminate touch positions of the S-TOP on the droplet. To solve this difficult problem and improve the accuracy of the S-TOP, signal analysis of S-TOP was performed based on 3D ray tracing. Our ray-tracing simulation successfully revealed a very hopeful characteristic signal (named the post-signal) from a seemingly noisy peak; its intensity corresponded to the S-TOP's touch position. Based on this characteristic of the post-signal, we developed a method of signal processing for practical measurement of droplet velocity, chord length, and number density through the S-TOP. This new method considerably reduced the difference in the measured results between the chord lengths and velocities through the S-TOP and through visualization, to less than 10%.