Fabrication of three-dimensional 1 x 4 splitter waveguides inside a glass substrate with spatially phase modulated laser beam

3D Manufacturing of Glass Microstructures Using Femtosecond Laser

The rapid expansion of femtosecond (fs) laser technology brought previously unavailable capabilities to laser material processing. One of the areas which benefited the most due to these advances was the 3D processing of transparent dielectrics, namely glasses and crystals. This review is dedicated to overviewing the significant advances in the field. First, the underlying physical mechanism of material interaction with ultrashort pulses is discussed, highlighting how it can be exploited for volumetric, high-precision 3D processing. Next, three distinct transparent material modification types are introduced, fundamental differences between them are explained, possible applications are highlighted. It is shown that, due to the flexibility of fs pulse fabrication, an array of structures can be produced, starting with nanophotonic elements like integrated waveguides and photonic crystals, ending with a cm-scale microfluidic system with micro-precision integrated elements. Possible limitations to each processing regime as well as how these could be overcome are discussed. Further directions for the field development are highlighted, taking into account how it could synergize with other fs-laser-based manufacturing techniques.

In-system optimization of a hologram for high-stability parallel laser processing

A method for optimizing a computer-generated hologram (CGH) for high-stability laser processing is proposed. The CGH is optimized during laser processing; therefore, unpredicted dynamic changes in the laser processing system, in addition to its static imperfections, are automatically compensated for by exploiting the rewritable capability of the spatial light modulator. Consequently, the short-term and long-term stability are improved, which will contribute to the realization of high-speed, high-precision laser processing. A CGH that generated 36 parallel beams was continuously optimized, and the maximum uniformity reached 0.98, which is higher than reported in previous research. To the best of our knowledge, this is the first demonstration of gradual improvement of parallel laser processing with in-process optimization of the CGH. Furthermore, it was also demonstrated that the performance of the laser processing system against unexpected disturbances was improved.

Adaptive optics in laser processing

Adaptive optics are becoming a valuable tool for laser processing, providing enhanced functionality and flexibility for a range of systems. Using a single adaptive element, it is possible to correct for aberrations introduced when focusing inside the workpiece, tailor the focal intensity distribution for the particular fabrication task and/or provide parallelisation to reduce processing times. This is particularly promising for applications using ultrafast lasers for three-dimensional fabrication. We review recent developments in adaptive laser processing, including methods and applications, before discussing prospects for the future.

High-aspect-ratio microtubes with variable diameter and uniform wall thickness by compressing Bessel hologram phase depth

In this Letter, we present a light field regulation method to form a ring light field with controllable density distribution. This method is to compress the phase modulation depth of Bessel holograms superimposed with blazed gratings and tune the diffraction efficiency of the superimposed holograms by gray scale. The experimental light field generated by the superimposed holograms is consistent with the simulation results. By designing the phase modulation depth of the superimposed holograms with different parameters, ring light fields with suitable intensity distribution are obtained, and the fabrication of ring microstructure with uniform wall thickness is realized. Finally, as a special case of processing, dynamic holographic processing of high-aspect-ratio microtubes with variable diameter and uniform wall thickness is demonstrated.

Spatial light modulator based laser microfabrication of volume optics inside solar modules

Ultrashort pulse laser systems enable new approaches of material processing and manufacturing with enhanced precision and productivity. Time- and cost-effectiveness in the context of the industrialization of ultrashort laser pulse processes require an improvement of processing speed, which is of key importance for strengthening industrial photonics based manufacturing and extending its field of applications. This article presents results on improving the speed of a laser process by parallelization for creating light deflecting volume optics. Diffractive optical elements are fabricated directly inside the encapsulant of solar modules by utilizing a spatial light modulator based parallel laser microfabrication method. The fabricated volume optical elements effectively deflect light away from front side electrodes and significantly reduce the corresponding optical losses.

Femtosecond laser written arrayed waveguide gratings with integrated photonic lanterns

We demonstrate for the first time functional arrayed waveguide gratings (AWGs) fabricated using the femtosecond laser direct-write technique. This fabrication technique is a mask-less alternative to lithography enabling design flexibility and rapid prototyping. It is ideal for customized small scale production for new applications. The devices were demonstrated in the visible region at 632.8 nm with a measured free spectral range (FSR) of 22.2 nm, and 1.35 nm resolution. To highlight the advantages of using a 3-dimensional fabrication technique, a 3-port photonic lantern was integrated with an AWG in a single monolithic chip. Integration of this type is not feasible with lithography-based AWG fabrication and can increase the functionality of AWGs for sensing applications.

Fabrication of computer-generated holograms using femtosecond laser direct writing

We demonstrate a single-step fabrication method for computer-generated holograms based on femtosecond laser direct writing. Therefore, a tightly arranged longitudinal waveguide array is directly inscribed into a transparent material. By tailoring the individual waveguide length, the phase profile of an incident laser beam can be arbitrarily adapted. The approach is verified in common borosilicate glass by inscribing a designed phase hologram, which forms the desired intensity pattern in its far field. The resulting performance is analyzed, and the potential as well as limitations of the method are discussed.

Submicrometer photonic structure fabrication by phase spatial-light-modulator-based interference lithography

We present a large-area and single-step fabrication approach based on phase spatial light modulator (SLM)-assisted interference lithography for the realization of submicrometer photonic structures on photoresist. A multimirror beam steering unit is used to reflect the SLM-generated phase-engineered beams leading to a large angle between interfering beams while also preserving the large area of the interfering plane beams. Both translational and rotational periodic submicrometer structures are experimentally realized. This approach increases the flexibility of interference lithography to fabricate more complex submicrometer photonic structures and photonic metamaterial structures for future applications.

Three-dimensional dielectric crystalline waveguide beam splitters in mid-infrared band by direct femtosecond laser writing

We report on the fabrication of three-dimensional waveguide beam splitters in a dielectric Bi(4)Ge(3)O(12) (BGO) crystal by direct femtosecond laser writing. In the laser written tracks of BGO crystal, positive refractive index is induced, resulting in so-called Type I configuration waveguiding cores. The "multiscan" technique is utilized to shape cores with designed cross-sectional geometry in order to achieve guidance at mid-infrared wavelength of 4 μm. The fundamental mode guidance along both TE and TM polarizations has been obtained in the waveguide structures. With this feature, we implement beam splitters from 2D to 3D geometries, and realize 1 × 2, 1 × 3, and 1 × 4 power splitting at 4μm.

Low-loss, efficient, wide-angle 1 × 4 power splitter at ∼1.55 μm wavelengths for four play applications built with a monolithic photonic crystal slab

We exhibit a low-loss, efficient, and wide-angle 1×4 power splitter based on a silicon monolithic photonic crystal slab with triangular lattice air holes. A distinctive power-splitting ratio can be obtained depending on the hole shift in the bending region and the structure adjustment at the junction area with regard to the power splitter designed. Simulation results achieved with a rigorous finite-difference time-domain technique show that the TE-polarized light is designed to ensure single-mode operation and the transmitted power is distributed almost equally, with a total transmission of 93.4% at the 1550 nm optical operation wavelength. Furthermore, we demonstrate ultralow-loss output of the optimized power splitter, with a transmittance above 22.5% (-6.48  dB) achieved in the ranges of 1524-1594 and 1610-1620 nm, which cover the entire C-band and a large portion of the L-band of optical communication.

Dynamic control of spatial wavelength dispersion in holographic femtosecond laser processing

Dynamic control of spatial wavelength dispersion is effective due to a potentially large spectral bandwidth of femtosecond pulses, in particular, when using sub-100-fs pulses. We demonstrate spatial wavelength dispersion control, which drastically reduces focal spot distortion in the reconstruction of a hologram, using a pair of spatial light modulators. The improved diffraction spots had nearly diffraction-limited spot sizes, agreeing well with theoretical predictions. The dynamic control of dispersion is also demonstrated in order to restrain unnecessary processing given by the zeroth-order pulse.

Shape control of elemental distributions inside a glass by simultaneous femtosecond laser irradiation at multiple spots

The spatial distributions of elements in a glass can be modulated by irradiation with high repetition rate femtosecond laser pulses. However, the shape of the distribution is restricted to being axially symmetric about the laser beam axis due to the isotropic diffusion of photo-thermal energy. In this study, we describe a method to control the shape of the elemental distribution more flexibly by simultaneous irradiation at multiple spots using a spatial light modulator. The accumulation of thermal energy was induced by focusing 250 kHz fs laser pulses at a single spot inside an alumino-borosilicate glass, and the transient temperature distribution was modulated by focusing 1 kHz laser pulses at four spots in the same glass. The resulting modification was square-shaped. A simulation of the mean diffusion length of molten glass demonstrated that the transient diffusion of elements under heat accumulation and repeated temperature elevation at multiple spots caused the square shape of the distribution.

Multi foci with diffraction limited resolution

The generation of multi foci is an established method for high-speed parallel direct laser writing, scanning microscopy and for optical tweezer arrays. However, the quality of multi foci reduces with increasing resolution due to interference effects. Here, we report on a spatial-light-modulator-based method that allows for highly uniform, close to Gaussian spots with diffraction limited resolution using a wavelength of 780 nm. We introduce modifications of a standard algorithm that calculates a field distribution on the entrance pupil of a high numerical aperture objective splitting the focal volume into a multitude of spots. Our modified algorithm compares favourably to a commonly used algorithm in full vectorial calculations as well as in point-spread-function measurements. The lateral and axial resolution limits of spots generated by the new algorithm are found to be close to the diffraction limit.

Waveguides fabricated by femtosecond laser exploiting both depressed cladding and stress-induced guiding core

We report on the fabrication of stress-induced optical channel waveguides and waveguide splitters with laser-depressed cladding by femtosecond laser. The laser beam was focused into neodymium doped phosphate glass by an objective producing a destructive filament. By moving the sample along an enclosed routine in the horizontal plane followed by a minor descent less than the filament length in the vertical direction, a cylinder with rarified periphery and densified center region was fabricated. Lining up the segments in partially overlapping sequence enabled waveguiding therein. The refractive-index contrast, near- and far-field mode distribution and confocal microscope fluorescence image of the waveguide were obtained. 1-to-2, 1-to-3 and 1-to-4 splitters were also machined with adjustable splitting ratio. Compared with traditional femtosecond laser writing methods, waveguides prepared by this approach showed controllable mode conduction, strong field confinement, large numerical aperture, low propagation loss and intact core region.

Polarization distribution control of parallel femtosecond pulses with spatial light modulators

A parallel femtosecond pulse irradiation method using a computer-generated hologram displayed on a spatial light modulator provides the advantages of high throughput and high energy-use efficiency. Polarization control of the femtosecond pulse enables some unique properties, for example, selective excitation of an anisotropic molecule, focusing at a size beyond the diffraction limit owing to the longitudinal vector component of a radially polarized beam focused by a high-numerical-aperture objective lens, and fabrication of periodic nanostructures with femtosecond laser light. In this study, we propose a parallel femtosecond laser irradiation system with arbitrary polarization distribution control using a pair of spatial light modulators. By using the system, the interval between the diffraction spots was the closest yet reported by avoiding mutual interference among their side lobes. The interval was improved to half compared with our previous work. We also demonstrated the parallel fabrication of periodic nanostructures with orientation control, which, to our knowledge, is the first reported demonstration of its kind.

Generalized phase diffraction gratings with tailored intensity

We report the generation of continuous phase masks designed to generate a set of target diffraction orders with defined relative intensity weights. We apply a previously reported analytic calculation that requires resolving a single equation with a set of parameters defining the target diffraction orders. Then the same phase map is extended to other phase patterns such as vortex generating/sensing gratings. Results are demonstrated experimentally with a parallel-aligned spatial light modulator.

High-quality generation of a multispot pattern using a spatial light modulator with adaptive feedback

We propose and demonstrate high-quality generation of a uniform multispot pattern (MSP) by using a spatial light modulator with adaptive feedback. The method iteratively updates a computer generated hologram (CGH) using correction coefficients to improve the intensity distribution of the generated MSP in the optical system. Thanks to a simple method of determining the correction coefficients, the computational cost for optimizing the CGH is low, while maintaining high uniformity of the generated MSP. We demonstrate the generation of a 28×28 square-aligned MSP with high uniformity. Additionally, the proposed method could generate an MSP with a gradually varying intensity profile, as well as a uniform MSP consisting of more than 1000 spots arranged in an arbitrary pattern.

Multibeam second-harmonic generation by spatiotemporal shaping of femtosecond pulses

We present a technique for efficient generation of the second-harmonic signal at several points of a nonlinear crystal simultaneously. Multispot operation is performed by using a diffractive optical element that splits the near-infrared light of a mode-locked Ti:sapphire laser into an arbitrary array of beams that are transformed into an array of foci at the nonlinear crystal. We show that, for pulse temporal durations under 100 fs, spatiotemporal shaping of the pulse is mandatory to overcome chromatic dispersion effects that spread both in space and time the foci showing a reduced peak intensity that prevents nonlinear phenomena. We experimentally demonstrate arbitrary irradiance patterns for the second-harmonic signal consisting of more than 100 spots with a multipass amplifier delivering 28 fs, 0.8 mJ pulses at 1 kHz repetition rate.

Second-harmonic optimization of computer-generated hologram

A method of optimizing a computer-generated hologram based on parallel second harmonic generation is proposed for holographic femtosecond laser processing. The method, which we call second harmonic optimization, incorporates the width and spatial profile of the pulse into the hologram design. With this method, we demonstrated parallel laser processing with high quality. Because of the enhanced processing accuracy, smaller structures were processed with a smaller energy than in our previous work. In parallel laser processing with 18 beams on a glass surface, the minimum average diameter of the processed structures was 271 nm when the mean fluence of the beams was 0.88 J/cm(2).

Creating three-dimensional lattice patterns using programmable Dammann gratings

We demonstrate the creation of a three dimensional (3D) lattice of focus spots using a 3D Dammann grating structure. Such a 3D lattice of focus spots can be used for probing 3D structures or for creating 3D photonic crystal structures in optically sensitive media. Experimental results are included where the patterns are encoded onto a programmable liquid crystal display. We demonstrate the generation of five planar arrays each having 6×6 points surrounding another set of four planar arrays each having 5×5 points with a single pattern.

Improved phase hologram design for generating symmetric light spots and its application for laser writing of waveguides

The improved method for calculation of a phase hologram and its application to laser writing of waveguides with a spatial light modulator are presented. It was found that the amplitude and phase distributions of light spots generated by a phase hologram can be distorted compared to those of a focused single beam. The distortion of light spots could be reduced by adding a simple constraint, in which light intensities around a light spot should be as small as possible, to the conventional calculation method of a phase hologram. It was also demonstrated that the improved calculation method can be considered essential for laser writing of waveguides.

Parallel direct laser writing in three dimensions with spatially dependent aberration correction

We propose a hologram design process which aims at reducing aberrations in parallel three-dimensional direct laser writing applications. One principle of the approach is to minimise the diffractive power of holograms while retaining the degree of parallelisation. This reduces focal distortion caused by chromatic aberration. We address associated problems such as the zero diffraction order and aberrations induced by a potential refractive index mismatch between the immersion medium of the microscope objective and the fabrication substrate. Results from fabrication in diamond, fused silica and lithium niobate are presented.