Holographic line-shaped femtosecond processing was developed for large-area machining. It can be performed with high throughput in laser cutting, peeling, grooving, and cleaning of materials. We demonstrated the single-shot fabrication of a line structure in a glass surface using a line-shaped pulse generated by a holographic cylindrical lens displayed on a liquid-crystal spatial light modulator, a line-shaped beam deformed three-dimensionally for showing the potential of holographic line-shaped beam processing, laser peeling of an indium tin oxide film, in-process laser cleaning of debris on the surface of a fabricating sample, and laser grooving of stainless steel.
Arbitrary and variable beam shaping of femtosecond pulses by a computer-generated hologram (CGH) displayed on a spatial light modulator (SLM) have been applied to femtosecond laser processing. The holographic femtosecond laser processing has been widely used in many applications such as two-photon polymerization, optical waveguide fabrication, fabrication of volume phase gratings in polymers, and surface nanostructuring. A vector wave that has a spatial distribution of polarization states control of femtosecond pulses gives good performances for the femtosecond laser processing. In this paper, an in- system optimization of a CGH for massively-parallel femtosecond laser processing, a dynamic control of spatial spectral dispersion to improve the focal spot shape, and the holographic vector-wave femtosecond laser processing are demonstrated.
Holographic femtosecond laser processing is very useful for high-speed processing with low-loss of light. One of
important subjects is to design a computer-generated hologram (CGH) with good performance in the processing system.
We have proposed a CGH is optimized in the processing system. The latest method is the SH optimization method based
on parallel SH generation. The SH method automatically incorporates the pulse duration and spatial beam profile into the
CGH, and therefore gives high quality parallel laser processing. Because of the enhanced processing accuracy, smaller
structures are processed with the smallest energy. We demonstrate the 18 parallel laser pulses performs the parallel
processing on a glass surface with the average diameter of 271nm under the average fluence of 0.88 J/cm<sup>2</sup>.
In holographic femtosecond laser processing, a precise control of the diffraction peaks generated by a computergenerated
hologram (CGH) displayed on a liquid crystal spatial light modulator is very important. We developed some
design methods of the CGH. We developed a method that the CGH was optimized with based on an optical measurement
of the diffraction peak intensities. Recently we also developed the second harmonic optimization based on the second
harmonic generations induced by parallel femtosecond laser pulses. In our presentation, our recent progresses of the
CGH optimization for holographic femtosecond laser processing are demonstrated.
We propose a holographic spatiotemporal lens to improve spatial resolution of two-photon excitation spot as a new
focusing technique of femtosecond laser pulse. Femtosecond laser pulses dispersed by a diffraction grating are irradiated
to a chirped diffractive lens displayed on a spatial light modulator. The chirped diffractive lens has a spatially chirp of
focal length for a design for its corresponding wavelength. The shortest pulse was experimentally obtained only at the
focal plane. The pulse duration was also supported with a computer simulation.
Precise control of diffraction peaks of a hologram is indispensable in holographic femtosecond laser processing. To
obtain the uniform diffraction peaks, an adaptive optimization due to the diffraction peaks measured by an image sensor
was proposed. It used a one-photon absorption. However, the structure processed by a femtosecond laser pulse was based
on multi-photon absorptions. Therefore, a mismatch between the optimized diffraction peaks and the processed
structures was observed. An adaptive optimization method using second harmonics induced by parallel pulse irradiations
to a nonlinear optical crystal is proposed to solve this mismatch.
Femtosecond laser processing acquires futures of high throughput and high light-use efficiency by using a computer-generated
hologram. In the holographic femtosecond laser processing, a precise control of diffraction peaks is
indispensable to fabricate enormous numbers of nanometer-scale structures simultaneously. The computer-optimized
hologram has high uniformity of the diffraction peaks in the computer reconstruction. However, the uniformity decreases
due to spatial and temporal properties of the optical system. We propose some optimization methods of the hologram to
improve the uniformity and demonstrate the processing performance.