Fabrication of Lensed Optical Fibers for Biosensing Probes Using CO2 and Femtosecond Lasers

We propose a new method for precisely fabricating a lensed fiber with a desired focal length by first cleaving a coreless silica fiber using an ultrafast femtosecond laser without thermal effects and subsequently shaping the radius of curvature at the optical-fiber end using a CO2 laser. The precisely cleaved segment of the coreless silica fiber obtained with the femtosecond laser is attached to a long single-mode fiber. The beam-exposure time and laser power of the CO2 laser are adjusted to melt the coreless-fiber end to yield a uniform, consistent, and precise radius of curvature, thereby realizing a lensed optical fiber. The precision of the radius of curvature in this case is greater than those obtained with the conventional arc discharge method with thermal treatment requiring fairly complex processes and yielding relatively low fabrication accuracy. In our study, we observe a difference between the measured and calculated focal lengths of the fabricated lens, possibly because the exact value of the mode field diameter is uncertain. On the other hand, the beam size measured using the knife-edge method matches closely with the theoretical size. Our findings confirm the feasibility of fabricating lensed optical fibers for fiber-based biosensing using CO2 and femtosecond lasers.