Femtosecond Laser-Based Micromachining of Rotational-Symmetric Sapphire Workpieces

Abstract

Sapphire is a robust and wear-resistant material. However, efficient and high-quality micromachining is still a challenge. This contribution demonstrates and discusses two novels, previously unreported approaches for femtosecond laser-based micromachining of rotational-symmetric sapphire workpieces, whereas both methods are in principal hybrids of laser scanning and laser turning or laser lathe. The first process, a combination of a sequential linear hatch pattern in parallel to the workpiece's main axis with a defined incremental workpiece rotation, enables the fabrication of sapphire fibers with diameters of 50 μm over a length of 4.5 mm. Furthermore, sapphire specimens with a diameter of 25 μm over a length of 2 mm can be fabricated whereas an arithmetical mean height, i.e., S_a parameter, of 281 nm is achieved. The second process combines a constant workpiece feed and orthogonal scanning with incremental workpiece rotation. With this approach, workpiece length limitations of the first process are overcome and sapphire fibers with an average diameter of 90 µm over a length of 20 cm are manufactured. Again, the sapphire specimen exhibits a comparable surface roughness with an average S_a value of 249 nm over 20 cm. Based on the obtained results, the proposed manufacturing method paves an innovative and flexible, all laser-based way towards the fabrication or microstructuring of sapphire optical devices, and thus, a promising alternative to chemical processes.

Keywords: ablation threshold; femtosecond laser; laser lathe; laser turning; micromachining; sapphire.

Figure 1

Schematic of the employed laser setup.

Figure 2

Femtosecond laser-based diameter reduction of sapphire rods by means of an axial scanning process combined with incremental workpiece rotation.

Figure 3

Femtosecond laser-based diameter reduction of sapphire rods over a length of 20 cm by means of an orthogonal scanning process.

Figure 4

Squared diameter of the ablation areas as a function of the applied laser fluence for ultraviolet (λ = 343 nm), green (λ = 515 nm) and infrared (λ = 1030 nm) laser radiation. N is the pulse quantity. Each data point represents the average of three single measurements.

Figure 5

(a,b) Microscopic images of a sapphire fiber. It exhibits a reduced diameter of 50 µm over a length of 4.5 mm. (c) Surface roughness quantification via white-light interferometry.

Figure 6

Microscopic images of a sapphire fiber with a reduced diameter of 25 µm.

Figure 7

Microscopic images of stitching errors.

Figure 8

Sapphire fiber with an average outer diameter of 90 µm over a length of 20 cm, fabricated via orthogonal scanning in combination with incremental rotation of the workpiece and constant workpiece feed. Both zoom-ins show exemplary microscopic images at different positions along the fiber.

Figure 9

(a) Exemplary surface profile acquired via white-light interferometry. (b) Arithmetical mean height, or S_a value, of a micromachined sapphire fiber with an average diameter of 90 µm over a length of 20 cm.