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

Recent years have seen a phenomenal rise in the capabilities of high power fibers lasers. 10 kW is now achievable in single‐mode fiber and the race is on to break the 100 kW mark. This has been enabled with the development of high power 976 nm pump diodes, ideally matched for efficiently exciting the ∼1030–1080 nm transition in Yb‐doped silica fibers. In the shadow of this remarkable development we are now witnessing a similar growth pattern for a sister to this laser, the 2 μm thulium fiber laser. This laser possesses rather unique characteristics. Pumped by 790 nm diodes, the excitation scheme uses a cross‐relaxation process that results in two‐for‐one photon generation with slope efficiencies approaching 70%. Powers approaching the kW level are now achievable. This talk summarizes the development of this unique laser source in the MIR, and speculates on its future applications in the medical, manufacturing and defense arenas. In particular we describe new experiments where we exploit the broad >200 nm spectral bandwidth, and the advantages this laser brings to operating in the pulsed regime. We characterize the capabilities of conventional gratings, fiber Bragg gratings, volume Bragg gratings and guided mode resonant filters to lock the linewidth to as small as 50 pm, within the range from ∼1950 nm to ∼2150 nm. This spectral region covers many sharp atmospheric absorption lines in the atmosphere. We describe the first long range (1 km) atmospheric transmission tests with a tunable high power (200 W) thulium fiber laser. The capability to lock a number of these lasers to specific wavelengths each with <100 pm linewidths opens the option of spectrally combining many lasers within the overall spectral bandwidth. We examine the benefits of this approach to reaching 100 kW power levels. In the pulse regime we describe the generation of nanosecond, picosecond and femtosecond pulses in oscillator and MOPA configurations.

High Power Thulium Fiber Lasers

In a particular embodiment, a relatively high-energy thulium fiber laser operating at the wavelength λ=2 μm may be used to selectively modify a front and/or a back surface of silicon and gallium arsenide wafers. The processing regime was studied in terms of the process parameters variation, and the corresponding modification fluence thresholds were determined. The results revealed considerable differences in morphology between front and back surface modifications, and that the back surface modification threshold of Si is significantly higher than at the front surface. Basic analytic modeling and z-scan measurements were performed to study the absorption mechanisms. In a broader embodiment the processing regime is not specifically limited to a thulium fiber laser.

Laser-based materials processing apparatus, method and applications

PROBLEM TO BE SOLVED: To provide a laser optical source for producing a pulse laser beam comprising a plurality of ultrashort optical pulses whose repetition frequencies are variable. SOLUTION: The laser optical source comprises a fiber oscillator for outputting an optical pulse and a pulse stretcher arranged in order to receive the optical pulse. The optical pulse has an optical pulse width. The pulse stretcher has dispersion for expanding the optical pulse width to produce an extended optical pulse. The laser optical source further comprises a fiber amplifier arranged in order to receive the extended optical pulse. The fiber optical amplifier has a profit for amplifying the extended optical pulse. The laser optical source comprises a grating compressor which has dispersion for reducing the optical pulse width and which can be automatically adjusted. The grating compressor automatically adjusts the dispersion for different repetition frequencies. COPYRIGHT: (C)2008,JPO&INPIT

Pulse laser optical source comprising an adjustable grating compressor

Experimental studies of laser induced filament propagation through atmospheric conditions have been done to understand the interaction of filaments with its propagating medium as they propagate at different pressures and through aerosols.

Laser induced filament propagation through adverse conditions

The first diode-pumped cw red laser through self-frequency doubling is demonstrated in 5% Nd:YCOB crystal cut along phase matching direction. 13.5 mW red output at 666 nm has been generated with 950 mW diode pump power absorbed in the crystal.

Lawrence Shah

Papers

High Power Thulium Fiber Lasers

Laser-based materials processing apparatus, method and applications

Pulse laser optical source comprising an adjustable grating compressor

Laser induced filament propagation through adverse conditions

Diode-Pumped, Self-Frequency Doubled Red Nd:YCOB Laser