Femtosecond laser micromachining can be used either to remove materials or to change a material's properties, and can be applied to both absorptive and transparent substances. Over the past decade, this technique has been used in a broad range of applications, from waveguide fabrication to cell ablation. This review describes the physical mechanisms and the main experimental parameters involved in the femtosecond laser micromachining of transparent materials, and important emerging applications of the technology. Interactions between laser and matter are fascinating and have found a wide range of applications. This article gives an overview of the fundamental physical mechanisms in the processing of transparent materials using ultrafast lasers, as well as important emerging applications of the technology.

Femtosecond laser micromachining in transparent materials

Fibre lasers in the mid-infrared regime are useful for a diverse range of fields, including chemical and biomedical sensing, military applications and materials processing. This Review summarizes the different rare-earth cations and host materials used in mid-infrared fibre laser technology, and discusses the future applications and challenges for the field.

Towards high-power mid-infrared emission from a fibre laser

The unique characteristics of ultrafast lasers, such as picosecond and femtosecond lasers, have opened up new avenues in materials processing that employ ultrashort pulse widths and extremely high peak intensities. Thus, ultrafast lasers are currently used widely for both fundamental research and practical applications. This review describes the characteristics of ultrafast laser processing and the recent advancements and applications of both surface and volume processing. Surface processing includes micromachining, micro- and nanostructuring, and nanoablation, while volume processing includes two-photon polymerization and three-dimensional (3D) processing within transparent materials. Commercial and industrial applications of ultrafast laser processing are also introduced, and a summary of the technology with future outlooks are also given. Scientists in Asia have reviewed the role of ultrafast lasers in materials processing. Koji Sugioka from RIKEN in Japan and Ya Cheng from the Shanghai Institute of Optics and Fine Mechanics in China describe how femtosecond and picosecond lasers can be used to perform useful tasks in both surface and volume processing. Such lasers can cut, drill and ablate a variety of materials with high precision, including metals, semiconductors, ceramics and glasses. They can also polymerize organic materials that contain a suitable photosensitizer and can three-dimensionally process inside transparent materials such as glass, and are already being used to fabricate medical stents, repair photomasks, drill ink-jet nozzles and pattern solar cells. The researchers also explain the characteristics of such lasers and the interaction of ultrashort, intense pulses of light with matter.

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Ultrafast lasers—reliable tools for advanced materials processing

High-repetition rate femtosecond lasers are shown to drive heat accumulation processes that are attractive for rapid writing of low-loss optical waveguides in transparent glasses. A novel femtosecond fiber laser system (IMRA America, FCPA muJewel) providing variable repetition rate between 0.1 and 5 MHz was used to study the relationship between heat accumulation and resulting waveguide properties in fused silica and various borosilicate glasses. Increasing repetition rate was seen to increase the waveguide diameter and decrease the waveguide loss, with waveguides written with 1-MHz repetition rate yielding ~0.2-dB/cm propagation loss in Schott AF45 glass. A finite-difference thermal diffusion model accurately tracks the waveguide diameter as cumulative heating expands the modification zone above 200-kHz repetition rate.

Heat accumulation effects in femtosecond laser-written waveguides with variable repetition rate.

Nonlinear Optical Crystals: A Complete Survey

An integrated thulium-doped photonic crystal fiber pump combiner is characterized for potential use in an ultrashort pulse amplifier. Spectral modulations were mitigated with amplification, and further investigations into coupling efficiencies and gain are ongoing.

Integrated All-fiber Thulium-doped PCF Pump Combiner

We review high power and high energy ultrafast fiber amplifiers. An Yb-fiber chirped pulse amplification system employing cubicon pulses producing sub-650 fs pulses at >100 muJ and >15W is presented.

High power ultrafast fiber amplifiers

Summary form only given. It is well-known that micromachining with high intensity femtosecond pulses has significant advantages over nanosecond laser pulses including increased ablation rates although there are many complex interaction processes involved. They have been shown to be effective in creating microstructures in fused silica, particularly gratings. We are investigating the interaction of high intensity (/spl sim/10/sup 14/ W/cm/sup 2/) 110 fs laser pulses with a variety of glasses to determine their potential for detailed structural micro-fabrication in transparent media, and through analysis of the changes in material structure and chemical properties, assess the interaction processes involved.

Femtosecond micro-machining of high index and photo-sensitive glasses in air

A polarization-maintaining (PM), narrow-linewidth, continuous wave, thulium fiber laser is demonstrated. The laser
cavity is formed from two femtosecond-laser-written fiber Bragg gratings (FBGs) and operates at 2054 nm. The
laser output possesses both narrow spectral width (78 pm) and a high polarization extinction ratio of ~18 dB at 5.24
W of output power. This laser is a unique demonstration of a PM thulium fiber system based on a two FBG cavity
that produces high PER without any free-space elements. Such a narrow linewidth source will be useful for
applications such as spectral beam combining which often employ polarization dependent combining elements.

All-fiber single-mode PM thulium fiber lasers using femtosecond laser written fiber Bragg gratings

Until recently, most femtosecond laser systems have required chirped pulse amplification (CPA) schemes to generate sufficient energy for micromachining. Here we present experimental results in precise surface structuring of materials using modified femtosecond laser oscillators which are generally less expensive and less complicated than traditional CPA systems.

Lawrence Shah

Papers

Integrated All-fiber Thulium-doped PCF Pump Combiner

High power ultrafast fiber amplifiers

Femtosecond micro-machining of high index and photo-sensitive glasses in air

All-fiber single-mode PM thulium fiber lasers using femtosecond laser written fiber Bragg gratings

Sub-microjoule kHz/MHz femtosecond laser oscillators for use in surface micromachining