Formulation of diffraction efficiencies of binary phase gratings for array illumination with ultrashort pulse beams.

We present simple formulas for the diffraction efficiencies of a binary phase grating that performs array illumination with ultrashort pulse beams. Using scalar diffraction theory, we formulated the efficiencies as a function of pulse spectral width by Fourier-transforming the complex-modulated frequency spectra of diffracted pulses in the far-field region. From the analytical simulations, we found that pulse array uniformity departs from unity as the spectral width increases, or the pulse duration decreases, thereby limiting the attainable split counts. This finding can be considered in the design of gratings for delivering controlled amounts of pulse energies to diffraction orders of interest.

Achromatic optical system with diffractive-refractive hybrid lenses for multifocusing of ultrashort pulse beams.

We report an achromatic cascade optical system for multifocusing ultrashort pulse beams with a diffractive beam splitter. Distortion compensation requires the removal of pulse front distortions from arrayed pulses, which originate from beam-radius-dependent group delay dispersions. The inclusion of hybrid diffractive-refractive lenses can effectively manage system dispersions. Simple design formulas are derived using the ray-matrix analysis and the designed system is evaluated using 20-fs pulses. We confirm that the hybridized system can remove not only chromatic aberrations but also pulse front distortions, hence improving the system spatio-temporal focusing resolutions. The proposed pulse delivery technique enhances the practicality of materials processing with ultrashort pulses.

Distortion-compensated multifocusing of ultrashort pulse beams using cascade optical system.

We report a cascade optical system for multifocusing ultrashort pulse beams, particularly sub-50-fs pulses. System achromaticity is key to simultaneous compensation of the spatio-temporal pulse distortions. In this system, diffractive and refractive subsystems are optically coupled in cascade to correct chromatic aberrations, which are the primary cause of pulse distortion. We design a prototype system by applying achromatic conditions derived from an ABCD matrix analysis. The designed system is then evaluated using 20-fs pulses, by characterizing the transmitted pulses in terms of beam width and pulse duration; hence, the proposed distortion compensation scheme is validated. This pulse delivery system enables damage-free and high-throughput materials processing using ultrashort pulses.

Design and fabrication of a diffractive beam splitter for dual-wavelength and concurrent irradiation of process points.

We report on a dual-wavelength diffractive beam splitter designed for use in parallel laser processing. This novel optical element generates two beam arrays of different wavelengths and allows their overlap at the process points on a workpiece. To design the deep surface-relief profile of a splitter using a simulated annealing algorithm, we introduce a heuristic but practical scheme to determine the maximum depth and the number of quantization levels. The designed corrugations were fabricated in a photoresist by maskless grayscale exposure using a high-resolution spatial light modulator. We characterized the photoresist splitter, thereby validating the proposed beam-splitting concept.

Subwavelength resist patterning using interference exposure with a deep ultraviolet grating mask: Bragg angle incidence versus normal incidence.

Interference lithography using a deep-ultraviolet (DUV) laser is instrumental in the manufacture of subwavelength patterns used at visible wavelengths. We investigated a grating mask strategy for exposure in terms of how to set and illuminate masks. To obtain high aspect ratio patterns, high fringe visibility, and high exposure uniformity are essential, and for that purpose the use of only two beams with liquid immersion is necessary but not sufficient. It needs to be addressed whether the grating should face air or liquid to achieve index matching without affecting its beam-splitting properties. Currently, the most feasible solution to produce sub-200 nm periods requires the use of a fused-silica grating under Bragg geometry (not normal incidence geometry) and filling the gap between the grating and resist with a high-index liquid.

High-efficiency diffractive beam splitters surface-structured on submicrometer scale using deep-UV interference lithography.

We report highly efficient diffractive beam splitters intended for high-power laser applications. Submicron relief structures that work as an antireflective layer are formed on the surfaces of a splitter to improve its transmitted efficiency. Surface structuring is performed using deep-UV interference lithography and reactive ion etching. As immersed in an index-matching liquid, the resist layer coated on diffractive surfaces is exposed to the interference fringes that are set intersecting the grooves on the surfaces. Rigorously designed structures with a period of 140 nm and a depth of 55 nm are lithographed onto fused-silica splitters. Splitting efficiencies at 266 nm are increased by 8% to compare favorably with a theoretical value, while Fresnel reflections are considerably reduced.

Chromatic-distortion compensation in splitting and focusing of femtosecond pulses by use of a pair of diffractive optical elements.

We propose a simple optical system to compensate for chromatic distortion that occurs during fan-out of femtosecond pulses by diffractive optics. The proposed system comprises a pair of diffractive elements, one for splitting an incoming pulse and the other for focusing the split pulses. With an appropriate separation between the elements, chromatic distortion resulting from the spectral bandwidth of a femtosecond pulse is removed, and an array of focused pulses with the same dimensions can be obtained. The theory has been verified through optical and processing experiments with 100-fs, 800-nm pulses.