**6. Direct writing**

A typical FSL direct writing system is composed of a laser source, a beam control/ shaping system, a microscope objective, or an aspherical lens (**Figure 5**). A tightly focused spot and a high-precision XYZ translation stage are produced by the lens (which determines the fabrication resolution) and controlled by a computer for 3D translation of the sample. As FSL pulses possess broad spectra, objective microscope lenses are frequently used for minimizing both spherical and chromatic aberrations. For controlling the repetition rate, an electro-optic or acoustic-optic device is employed in the beam control/shaping system, which can also be used for creating burst mode, a spatial or temporal pulse shaper, a tunable attenuator, and a mechanical shutter. For real-time monitoring of the fabrication process, a charge-coupled device camera connected to a computer can be installed above the focusing lens. A tunable

### **Figure 5.** *Schematic image of femtosecond laser direct writing system.*

### *Fundamentals of Femtosecond Laser and Its Application in Ophthalmology DOI: http://dx.doi.org/10.5772/intechopen.106701*

attenuator is another component of the direct writing system, which controls the power and ensures stable operation of laser parameters such as the pulse width, pulse energy, and pointing direction. Autocorrelator devices have also been developed for characterizing the spatiotemporal profiles, including temporal duration or structure of ultrashort pulses [6].

Direct writing has two methods, depending on scanning directions (geometries): horizontal (where the sample is moved perpendicularly to the laser beam, commonly used to fabricate surface structures) and vertical (where the sample is moved along the direction of laser beam irradiation; it can be performed from the upper or lower surface). The greater longitudinal depth of the focal spot intensity distribution than its transverse dimension results in an asymmetric cross-section of the laser modification zone. An objective lens with a larger NA can reduce this difference, resulting in tight focusing and adjust the spatial distribution of pulse intensity. When direct longitudinal writing is processed from top to bottom, the laser beam is affected by the scattering of ablated materials, which reduces fabrication quality [3].

Horizontal direct writing is appropriate for the fabrication of ultra-slip surface porous network structures with excellent performance in liquid repellency and cell proliferation resistance, hierarchical structures, nano grooves, nanoholes, and 3D resonant optical cavities, widely used in applications such as hydrophilic and hydrophobic treatments, optical communication and sensors and biomedicine. Direct writing is also applicable for the fabrication of two-dimensional (2D) and 3D microstructures, either through moving the 3D transform stage (applicable to array machining and applications not requiring high precision) or through a galvanometer combined with the transform stage. In the former method, even by using Bessel Beam, which improves efficiency, rapid and flexible micromachining of complex microstructures cannot be achieved at a large scale. On the contrary, the latter method, scanning galvanometers and piezo has highly-developed high power, high repetition frequency, and miniaturized FSL, which makes it appropriate for high throughput and high-resolution micromachining both axially and laterally; this method is beneficial for the commercialization of FSL micromachining. TPP technique can be used for the construction of complex mesoscale 3D microstructures with nanoscale precision, like entire hollow devices and spiral phase plates. Direct writing also has another great potential, such as large-scale multifunctional smart materials with 3D gradient densities that can be widely employed in the processing of four-dimensional (4D) smart sensors and reconfigurable micromachines, actuators, and soft robots applicable in biomedicines (which use hydrogels; soft materials with high biocompatibility and deformability).

Besides the advantages of FSL direct writing, it also has some limitations, which makes it inappropriate for oil-working conditions used for biomaterials (e.g., cells and tissues). The serial process nature of this method also results in low throughput, although we can increase the scanning speed by replacing the XYZ motion stage with a galvo scanner. The recent ultrafast laser systems have high power and high repetition rate and their pulses are much broader than the pulses generated by the Ti: sapphire systems [9].

### **6.1 Parallel systems**

Parallel FS microprocessing has three typical systems: multifocus laser writing, processing based on multiple-beam interference, and using a hologram. The first system, the most straightforward technique, is based on a microlens array that splits the single focal spot into multiple foci, uniformly distributed in the focal plane. By the combination of a relay lens and an objective lens, a fabrication resolution comparable with that of the objective lens can be realized. This method can be used for the fabrication of structures with arbitrary 3D geometries in each unit cell. In earlier models, multifocus parallel laser fabrication could only be used for microstructures arranged in periodic arrays. Implementation of multifocus TPP technique, based on individuallycontrolled phase modulation, enabled rapid prototyping of symmetric and asymmetric complex 2D and 3D structures. The second technique, parallel processing using multiple-beam interference, is potentially faster than the first method and can be used for the fabrication of periodic 2D/3D structures with a wide variety of interference patterns by only a single laser shot. In this technique, an optical diffraction element splits the incident beam into five diverging beamlets, which are collimated by a lens. The phase and amplitude of each beamlet can be tuned, and the beamlets are refocused into the sample by a second lens to create the interference patterns [6].

Various methods have been developed for efficient and flexible fabrication of complex microstructures, including multi-focus parallel processing through optical modulation and diffraction, structured light, photolithography based on a digital micromirror device (DMD), and a liquid crystal spatial light modulator (LC-SLM). The two latter methods, DMD and LC-SLM based FSL processing, can modulate the graphic fabrication of arbitrary structures dynamically with high speed and flexibility, which makes them applicable in various structures. Cross-scale high throughput processing with submicron resolution can also be achieved through a spatiotemporal synchronization focusing approach.

DMD technique is high-throughput, high-contrast, rapid-response, and easy-touse, appropriate for modulating spatially homogenized flat-headed light sheets into arbitrary 2D patterns, high throughput processing of large-scale microstructural arrays, and single-pulse fabrication of the complex microstructure arrays. It can also be used in combination with digital holography technology for printing various arbitrarily complex 3D structures with high resolution. A larger field of view and patterned area can also be obtained by an objective lens with moderate magnification in applications where super-resolution is not required. A maximum of 44% of laser energy is irradiated on the sample, considering reflection from the DMD surface and energy loss in the gaps between the micromirrors [9].

The second most widely used technique for direct writing is LC-LSM, which modulates the light field phase by a variable distribution of liquid crystals and produces a higher utilization rate of the light energy. This flexible micro-patterning approach was first introduced by researchers from Tokushima University and gained interest in recent years. The accuracy, efficiency, and resolution of LC-SLM-based FSL processing have been improved by achieving the desired patterned beam through holographic algorithm and fabrication approaches. Special microstructures have also been built using structured laser micromachining. It generates Matthew beam, appropriate for fabrication of complex microcages, by regular intensity distribution and the diversification of controllable parameters, as well as Bessel beams, appropriate for fabrication of high aspect ratio microtubes, hollow microhelical structures, and chiral rotating microstructures using the non-diffracting high-quality beam. Both approaches, DMD and LC-SLM based FSL processing, have limitations in achieving high precision and large size simultaneously, for instance, in TPP applications. This limitation has been improved by the spatiotemporal synchronization focusing technology proposed by Cheng et al., which can realize the shortest laser pulse width with the highest laser intensity and improves the efficiency, volume, resolution, and flexibility of FSL fabrication [9].
