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

Direct waveguide writing with femtosecond lasers can be divided into two general categories based upon the type of lasers used: amplified systems that emit high pulse energy (>2 μJ) at low repetition rates ( 1 MHz). In this presentation, we report on waveguide writing with a novel commercial femtosecond fiber laser system (IMRA, FCPA μJewel) that bridges the gap between these two regimes, providing sub-400 fs pulses with pulse energies of >2.5 μJ at 100 kHz and >150 nJ at 5 MHz. The laser repetition rate can be varied from 100 kHz to 5 MHz in 1 kHz increments through a computer controlled user interface. The ability to quickly and easily vary the repetition rate of this laser was critical in identifying and optimizing laser processing windows for different target glasses. An overview of laser processing windows and waveguide characteristics are presented for borosilicate and fused silica glasses exposed to fundamental (1045 nm) and second harmonic (522 nm) laser light.

MHz-rate ultrafast fiber laser for writing of optical waveguides in silica glasses

This paper summarizes the results of simulation of a module for side-pumping a Nd:YAG rod. The module consists of three laser diode arrays separated by 120° rotation angle around the laser rod, where each array contains 10 emitters producing a maximum output power of 15 W at 808 nm wavelength. This high power diode pumped solid-state (DPSS) laser system was modeled in both LASCAD and GLAD simulators. LASCAD model was used to simulate the laser output power as a function of the total input power and the output mirror reflectivity. The model predicted an output power of 140 W given 400 W total input pump power with optical efficiency of 35%, in a good agreement with the published experimental results and similar commercially available CW DPSS laser systems. LASCAD was also used to model the temperature distribution inside the rod and to examine the heat load and thermally induced mechanical stress on the rod. Simulation in GLAD enabled a detailed analysis of the beam quality, beam size, and mode stability inside the resonator. GLAD models were used to simulate the pumping light distribution in the Nd:YAG rod for a single diode element, a single diode array, and three diode arrays. The GLAD shows that a stable multi-transverse mode "top hat" beam is formed after 30 passes through the resonator of the adopted high power DPSS laser system.

Analysis and modeling of a high power side diode pumped solid state laser system

Two thulium fiber laser configurations are described providing widely tunable and narrow linewidth output We show that such systems can produce average powers greater than 100 W

Tunable high power thulium fiber lasers

Fiber lasers offer an excellent technology base for production of an industrial-quality tool for precision microfabrication, answering the need to expand the capabilities of laser material processing beyond traditional welding, cutting, and other industrial processes. IMRA's FCPA μJewel TM femtosecond fiber laser has been developed to address the particular need for direct-write lasers for creation of clean and high-quality micron and sub-micron features in materials of commercial interest. This flexible Yb:fiber chirped-pulse amplification architecture, capable of operating at rep-rates between 100 kHz and 5 MHz, balances the need for higher-repetition rate with that of sufficient pulse energy to work at or near ablation threshold, while meeting industrial standards for temperature, shock and vibration. Demonstration of the need for higher-repetition rates for direct write processes will be provided in this paper. Further, results of laser-processing of materials typically used in flat panel displays, photomasks, and waveguide production using the FCPA μJewel TM laser will be presented.

Optimized precision micromachining using commercially-available, high-repetition rate, microJoule femtosecond fiber lasers

Compact and robust high power eye-safe laser sources are required for rapidly deployable free-space optical (FSO) communication networks Such systems have been demonstrated using essentially telecom-based lasers in a relatively narrow bandwidth window around 15 μm Here we discuss additional wavelength transmission bands within the mid-IR Using advanced laser sources to provide illumination across wide wavelength ranges, particularly within the 2-5 μm it may be possible to overcome transmission limitations associated with adverse weather and atmospheric conditions

Lawrence Shah

Papers

MHz-rate ultrafast fiber laser for writing of optical waveguides in silica glasses

Analysis and modeling of a high power side diode pumped solid state laser system

Tunable high power thulium fiber lasers

Optimized precision micromachining using commercially-available, high-repetition rate, microJoule femtosecond fiber lasers

Wideband Lasers on Manned/Unmanned Platforms Under Poly-Environment