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

Fiber laser technology is leading a revolution in ultrashort-pulse lasers and their applications. Unlike conventional ultrashort-pulse bulk solid-state laser systems, ultrafast fiber amplifiers can be scaled to average powers >100 W without significant beam-quality distortion from thermal lensing. In this paper, we review recent developments in the generation of high-average power and high-pulse energy ultrashort-pulse fiber sources. In particular, we focus upon advances that enhance the practicality of these laser systems. In several application examples, we show how these improvements in laser performance and utility are critical for achieving the high speed and reliability necessary for commercial processing applications.

High-Power Ultrashort-Pulse Fiber Amplifiers

Laser tunability in Yb3+:YCa4O(BO3)3 {Yb:YCOB}

We used guided-mode resonance filters (GMRFs), fabricated using thin-film deposition and chemical etching, as intracavity feedback elements to stabilize and narrow the output spectrum in thulium-doped fiber oscillators operating in the 2 μm wavelength regime, producing linewidths of <700 pm up to 10 W power levels. A Tm fiber-based amplified spontaneous emission source was used to characterize the reflective properties of the GMRFs. Linewidths of 500 pm and a 7.3 dB reduction in transmission were measured on resonances.

Spectral narrowing and stabilization of thulium fiber lasers using guided-mode resonance filters

We report lasing and amplification in a rod type thulium-doped photonic crystal fiber with 80 μm core diameter. The rod is pumped with a 793 nm laser diode and produces more than 20 W output power at a beam quality M(2)<1.3. The laser/amplifier has a slope efficiency of 27.8%/20.1% relative to absorbed pump power with a lasing threshold at 28.6 W. The output wavelength in the lasing configuration can be tuned over 180 nm from 1810-1990 nm.

CW-lasing and amplification in Tm(3+)-doped photonic crystal fiber rod.

We report on a Tm:fiber master oscillator power amplifier (MOPA) system producing 109 W CW output power, with >15 dB polarization extinction ratio, sub-nm spectral linewidth, and M2 <1.25. The system consists of polarization maintaining (PM) fiber and PM-fiber components including tapered fiber bundle pump combiners, a single-mode to large mode area mode field adapter, and a fiber-coupled isolator. The laser components ultimately determine the system architecture and the limits of laser performance, particularly considering the immature and rapidly developing state of fiber components in the 2 μm wavelength regime.

Lawrence Shah

Papers

High-Power Ultrashort-Pulse Fiber Amplifiers

Laser tunability in Yb3+:YCa4O(BO3)3 {Yb:YCOB}

Spectral narrowing and stabilization of thulium fiber lasers using guided-mode resonance filters

CW-lasing and amplification in Tm(3+)-doped photonic crystal fiber rod.

Integrated Tm:fiber MOPA with polarized output and narrow linewidth with 100 W average power