Fabrication and optimization of large core radial-firing optical fiber tip comprised of conically shaped air-pocket and its optical characteristics

Abstract

The fabrication, optimization, and optical characteristics of a novel large core radial-firing optical fiber (RFF) tip comprised of conically shaped air-pocket were investigated. The large core RFF tip comprised of the conically shaped air-pocket takes advantage of simplified fabrication process and no need for post-process for keeping total internal reflection condition, with satisfying high power laser transmission, easy launching from medical laser source, and more easily operable in vivo compared with conventional flat-end optical fiber. The ray-tracing simulation was carried out to optimize the fabrication parameters of conical angle of the air-pocket and numerical aperture (NA) for the large core RFF tip comprised of conically shaped air-pocket. The simulation results were found that the conical angle of air-pocket of the large core RFF tip was ~ 45 degree and the LCF’s NA was 0.12 and they showed ~ 81 degree maximum firing angle and 76.3% relative firing intensity. More specifically, the maximum firing angle of the RFF tip increased with the increase of the conical angle of air-pocket up to 50 degree. When the conical angle of air-pocket was over than ~ 50 degree, a total internal reflection condition at the surface between the air-pocket and the LCF was not satisfied and thus the guided laser beam was fired toward forward directions. The relative firing intensity increased with the increase of the conical angle up to 39 degree and then decreased with the increase of the conicalangle of air-pocket. Effect of the incident light beam conditions such as the beam profile, the beam size, and the incident angle of the radial-firing optical fiber tip comprised of conically shaped air-pocket was investigated by the same ray-tracing method. Regardless of the different Gaussian profiles of the incident beam, no significant difference in the maximum firing angle but a little increase in the firing power was found with the increase of the axial distance of the Gaussian profile. With the increase of the incident beam size, no significant difference in the maximum firing angle was found but the relative firing power decreased and the extent of the power decrease depended on the numerical aperture (NA) of the fiber. On the other hand, the incident beam angle dependence of the firing power was significant, decreased to 65.9% and 31.9 % for the RFF tip of NA 0.12 and NA 0.22, respectively with the increase of the incident beam angle from 0 deg to 12 deg but the maximum firing angles did not show much decrease, smaller than 5 deg. The large core radial-firing optical fiber (RFF) tip comprised of conical shaped air-pocket was fabricated by the gradual collapse of the cavity of the hollow optical fiber (HOF) which was fusion-spliced to a large core optical fiber (LCF) using the electric arc-discharge of the commercially available fusion splicer. The HOF (inner/outer diameters; 70/125 μm) was fabricated by using a fused silica tube rod (Quartz rod, 214 PO, inner/outer diameters; 9/16 mm) with drawing process (furnace temperature; 1890 °C and capstan speed; 6 mm/min). The fabricated the HOF was fusion spliced with the LCF and the HOF was deformed with intaglio conical-shaped region along the horizontal fiber-axis by an arc- discharge. Then the air-pocket was formed by additional arc-discharge with ~ 800 μm away from the conical shaped region. The large core RFF tip based on the LCF with 0.12 NA and 0.22 NA comprised of conical shaped air-pocket with 35, 45, and 55 degree conical angle was finally fabricated by changing of a number of additional arc-discharge onto the end of air-pocket. The optical characteristics of the large core RFF tip were measured by using a rotation stage with a photodetector (Thorlabs DET10A) and the oscilloscope (Agilent Infiniium oscilloscope). The RFF tip based on the LCF (NA; 0.12) with 45 degree conical angle showed the largest firing angle at 75 degree (3dB bandwidth: 65 ~ 77.5 degree) in a positive direction and 77.5 degree (3dB bandwidth: 65 ~ 80 degree) in a negative direction. And the RFF tip based on the LCF (NA; 0.22) with 45-degree conical angle showed the next highest firing angle at 65 degree (3dB bandwidth: 52.5 ~ 75 degree) in a positive direction and 72.5 degree (3dB bandwidth: 60 ~ 77.5 degree) in a negative direction. The measured results clearly showed that the efficient firing RFF tip was successfully fabricated according to the optimized parameters obtained from the simulation results and the optical characteristics were satisfactory with the large firing angles with high intensities. Lastly, Comparison of the simulation and experimental results has been investigated. The modified simulation resulted in the maximum firing angles of ± 76.3 deg for the RFF of NA 0.12, almost the same as the experimental value. However, the RFF tip of 0.22 NA showed the discrepancy in the maximum firing angle, ± 63.8 deg compared to the experimental results of ± 53.8 deg and the merge of the two firings at the maximum and medium angles was found in the experiments.