抄録
Although fiber LDVs have been developed for their flexibility and easy handling, the performance of the fiber LDVs developed and commercialized so far is inferior to that of conventional fixed-type LDVs. The smaller the diameter of the LDV probe is, the worse the quality of its measurement volume becomes. This is because that the diameter of the output beam from the fiber is so small that it requires a beam expander to improve the quality of the measurement volume; however, the use of a beam expander makes the adjustment complex and the probe diameter undesirably large. We have developed a small fiber LDV probe including a beam expander with a perforated lens, successfully improving the measurement volume quality. The SN ratio of the developed FLDV was ten times as much as that of the conventional fiber LDV. In order to achieve a high launching efficiency of the laser beam into a fiber, a single lens having a desirable focal length was adopted. The positioning of the beam spot into the fiber core was done by means of a tilting mechanism of high resolution of 0.3μm. The transmitting efficiency of 80% was performed. The porformance of the LDV probe was examined by measuring both the tangential velocity of the rotating disk and the longitudinal velocity profiles of water in a pipe. The results show an excellent performance of the fiber LDV. The use of the perforated lens as a space filter enables us to measure the velocity close to the wall (0.5mm), even in the condition that the half of the measurement volume is formed in the wall.
