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

We report on the performance of a prototype pump combiner for use with thulium-doped photonic crystal fiber (PCF). This platform is attractive for all-fiber hi gh energy and high peak power laser sources at 2 µ m. We will report on the performance of this integrated amplifier in comparison to fr ee space amplification in Tm:PCF. In particular, we carefully look for spectral/temporal modulation resulting from multimode interference between fundamental and higher order transverse modes in the amplifier to eval uate this for ultrashort chirped pulse amp lification. The slope efficiency for the all-fiber amplifier is 22.1 %, indicating the need for further improvement. However, an M 2 < 1.07 demonstrates excellent beam quality, as well as amplified polarization extinction ratios of ~25 dB. Keywords: Thulium, fiber amplifier, all-fiber, photonic crystal fiber 1. INTRODUCTION Fibers have seen superb growth in laser systems due to the architectures high gain and efficient heat extraction. Concurrently, these systems can exhibit diffraction-limited output. Fiber lasers also structurally and mechanically offer compactness, robustness, and environmental stability. Pump combiners using step-index fibers have been commercialized, exploiting the above advantages as well as minimizing free-space components. However, novel fiber designs such as photonic crystal fibers (P CFs) are more difficult to inco rporate into an integrated pump combiner architecture. PCFs allow improved single-mode guidance over conventional large mode area (LMA) step-index fibers while drastically increasing the mode field areas as well [1,2]. With these benefits, PCFs have given rise to record peak powers in ytterbium and thulium-doped sy stems [3,4]. In this paper, a new all-fiber Tm:PCF pump combiner is characterized for prospective use as an ultr ashort pulse amplifier to overcome the multimode interference seen in [5].

Photonic crystal fiber pump combiner for high-peak power all-fiber thulium lasers

We describe a technique for the quick and simple assessment of doped optical materials for use as laser gain media. To demonstrate this technique, referred to as Rapid Thermo-Optical Assessment (RTOA), we analyze a set of ceramic and crystalline Yb3+:YAG samples. RTOA is based on Shack-Hartmann wavefront sensing and thermal lensing to evaluate the media’s thermal response, giving a relative overall quality assessment of the material. The technique is also broadly applicable to optical media considered for high power or thermal loading conditions, and useful for the refinement of fabrication methods.

Rapid thermo-optical quality assessment of laser gain media

Ultrashort pulses were generated by a carbon nanotube mode-locked Tm fiber laser and subsequently amplified. Amplified pulses had an average power of 0.6 W, 2.6 kW peak power, and 13 nJ of energy

Fiber Amplification of 2 µm picoseconds pulses

An in-house drawn chalcogenide fiber sustained 12.2 MW/cm2 CW irradiation on the facet without damage, limited by available laser power. After depositing single-layer, anti-reflection coatings on the fiber facets, 90.6% transmission was achieved with 10.2 W output.

High Power 2053 nm Transmission through Single-mode Chalcogenide Fiber

We report spectral beam combining of three 2-?m Tm fiber lasers each locked to slightly different wavelengths, using a new spectral-limiting device, providing a path forward to beam-combining at the multi-kW level.

Lawrence Shah

Papers

Photonic crystal fiber pump combiner for high-peak power all-fiber thulium lasers

Rapid thermo-optical quality assessment of laser gain media

Fiber Amplification of 2 µm picoseconds pulses

High Power 2053 nm Transmission through Single-mode Chalcogenide Fiber

Spectral Beam Combining of 2 μm Tm Fiber Laser Systems