Many research teams have begun pursuing optical micromachining technology in recent years due to its associated noncontact and fast speed characteristics. However, the focal spot sizes and the depth of focus (DOF) strongly influenced the design requirements of the micromachining system. The focal spot size determines the minimum features can be fabricated, which is inversely proportional to the DOF. That is, smaller focal spot size led to shorter DOF. However, the DOF of the emitted visible or near-infrared light beam is typically limited to tens of nanometers for traditional optic system. The disadvantages of using nanosecond laser for micromachining such as burrs formation and surface roughness were found to further influence the accuracy of machined surfaces. To alleviate all of the above-mentioned problems, sub-wavelength annular aperture (SAA) illuminated with 780 nm femtosecond laser were integrated to develop the new laser micromachining system presented in this paper. We first optimized the parameters for high transmittance associated with the SAA structure for the 780 nm femtosecond laser used by adopting the finite difference time domain simulations method. A lateral microscope was modified from a traditional microscope to facilitate the measurement of the emitted light beam optical energy distribution. To verify the newly developed system performance the femtosecond laser was used to illuminate the SAA fabricated on the metallic film to produce the Bessel light beam so as to perform micromachining and process on silicon, PCB board and glass. Experimental results were found to match the original system design goals reasonably well.
Adopting optical technique to pursue micromachining must make a compromise between the focal spot sizes the depth
of focus. The focal spot size determines the minimum features can be fabricated. On the other hand, the depth of
focus influences the ease of alignment in positioning the fabrication light beam. A typical approach to bypass the
diffraction limit is to adopt the near-field approach, which has spot size in the range of the optical fiber tip. However,
the depth of focus of the emitted light beam will be limited to tens of nanometers in most cases, which posts a difficult
challenge to control the distance between the optical fiber tip and the sample to be machined optically. More
specifically, problems remained in this machining approach, which include issues such as residue induced by laser
ablation tends to deposit near the optical fiber tip and leads to loss of coupling efficiency. We proposed a method based
on illuminating femtosecond laser through a sub-wavelength annular aperture on metallic film so as to produce Bessel
light beam of sub-wavelength while maintaining large depth of focus first. To further advance the ease of use in one
such system, producing sub-wavelength annular aperture on a single mode optical fiber head with sub-wavelength
focusing ability is detailed. It is shown that this method can be applied in material machining with an emphasis to
produce high aspect ratio structure. Simulations and experimental results are presented in this paper.