**1. Introduction**

Since its invention by T.H. Maiman in 1960, the laser interaction with matter has been successfully investigated to find and develop new applications in fundamental, applied, technological and industrial research [1]. Laser materials processing is a versatile tool for materials synthesis, modification and structuring involving in many cases multidisciplinary approaches. Laser technology has been studied in the search for novel applications in research fields such as optics, photonics, energy, microelectronics, aerospace and biomedicine [2–12].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The acronym "LASER" stands for "Light Amplification by Stimulated Emission of Radiation" and was first introduced in the late 1950s [13]. Essentially, a laser device is a resonator which emits coherent light originated in a stimulated emission process. A laser resonator or oscillator is usually made up of three main parts: a gain medium which amplifies light by stimulated emission, a pumping source to provide external energy to stimulate the atoms of the gain medium to its excited states, and an optical cavity which supplies feedback to the laser light. The resulting radiation, the laser light, has unique properties such as a high degree of temporal and spatial coherence, and low divergence which confers it of exceptional advantages compared to conventional light sources [14].

Along the last 50 years, laser technology has progressed based on the advances achieved in the development of new gain media, pumping sources and cavity design, leading to many different types of lasers. Attending to the gain medium, lasers can be classified as gas laser, chemical laser, excimer laser, solid-state laser, semiconductor laser, dye laser, free electron laser and metal-vapor laser. According to the laser operation wavelength, the classification can also be as ultraviolet (UV) laser, visible and infrared (IR) laser. In particular, for an operation wavelength between 700 nm and 2 μm, the laser is referred as near-infrared (NIR) laser. Also, the classification can be carried out attending to the operation mode; continuous wave (cw) and pulsed lasers [14]. The output power of a cw laser is constant, whereas for a pulsed laser, the power varies pulse-by-pulse. In this latter case, the pulse energy, *Ep* , and the peak power, *Pp* , can be calculated as:

$$E\_p = \,^p\!\!\!\!\!\/= \,^p\!\!\!\/+\!\/\,\_p\tag{1}$$

Mode-locking is a technique in which a fixed or locked phase relationship between the longitudinal modes of the laser cavity is induced. The longitudinal modes are made to interfere with each other giving rise to the emission of a train of short pulses whose duration may range from few femtoseconds to several picoseconds [21]. The temporal separation between these pulses is the time that a pulse takes to complete one round trip in the resonator cavity. As the duration of a mode-locked laser pulse is very short, according to the time-bandwidth product, its spectrum consists of a broad wavelength range. Mode-locking technique can be active or passive. In active mode-locking, an external signal induces a modulation of the intra-cavity laser radiation by using acousto-optics modulators. Passive mode-locking is based on the introduction in the laser cavity of some elements such as saturable absorbers, dyes or solidstate media which induce self-modulation of the laser light. The most common mode-locked femtosecond lasers are the Ti:Sapphire femtosecond lasers based on the Kerr-Lens Modelocking (KLM) mechanism, in which the Ti:Sapphire gain medium crystal placed inside the cavity acts as Kerr-effect medium [14, 21]. Nowadays, typical commercial femtosecond laser

Ultrafast Laser Inscription of Buried Waveguides in W-TCP Bioactive Eutectic Glasses

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The exceptional characteristics of femtosecond laser pulses have allowed the development of novel applications in the framework of femtosecond laser processing. In particular, the fabrication of two- and three-dimensional permanent structures inside transparent optical materials to be applied in the fields of optics, photonics and telecommunication as key elements such as waveguides, photonic crystals, diffraction gratings, beam splitters, and so on. Since the first report on femtosecond laser written waveguides in glass by Davis et al. [22], different types of integrated optical waveguides have been produced in a great diversity of transparent materials such as glasses, crystals, polycrystalline ceramics and polymers

In the ultrafast laser inscription (ULI) technique, femtosecond laser pulses are tightly focused beneath the surface. During the absorption process of ultrashort laser pulses, a high local spatial and temporal electronic and vibrational excitation densities are produced which together with the laser pulse duration, short compared to the relevant relaxation processes give rise to laser-induced nonlinear processes in the focal volume, such as two-photon or multiphoton absorption, inducing avalanche ionization in a very short time, leading to localized micro or sub-micrometric lattice damage, modifying the local refractive index and forming the so

The permanent refractive index change (Δ*n*) induced in the focal volume has been found to be either positive or negative, depending on the processing parameters and the characteristics of the material. In addition to refractive index modification, a suitable configuration of the femtosecond laser pulses can be used to micromachine small areas by laser ablation or even to create nanovoids inside dielectric materials. The tight focalization in the focal spot can create extreme pressure and temperature conditions, with intensities significantly above the optical breakdown threshold and energy densities of several MJcm−3 into a submicron volume,

systems produce sub-100 femtoseconds.

**2. Ultrafast laser inscription**

called "track" or "filament" [3].

[23–44, 48–67].

$$P\_p = \,^E\_\gamma \!/ \!/ \!/ \!/ \tag{2}$$

where *P*out is the output power, *f* is the repetition rate and Δ*t* the pulse duration. Therefore, an appropriate combination of both repetition rate and pulse duration may allow pulsed lasers to achieve peak powers much higher than cw lasers. To date, the peak power that a pulsed laser can reach is in the range of a Petawatt with extremely high intensities, in the order of 1019 Wcm−2, much higher than the atomic unit of intensity (3.5 1016 Wcm−2). Hence, ultrashort pulsed lasers have recently attracted scientist's attention aiming at developing novel applications in fields such as biochemistry, spectroscopy, medicine, photonics and telecommunications due to the unique properties of the interaction between ultrashort laser pulses and matter [2–4, 14].

After the demonstration of laser action in the ruby laser by Maiman in 1960, for which pumped flash lamp produced a burst of spikes, each several nanoseconds long, that lasted several hundred microseconds, scientists focused their efforts in developing pulsed laser systems with shorter pulse duration. Q-switching was successfully applied in the early 1960s to produce single pulses 10 nanoseconds long [15]. Next, mode-locking technique was first introduced in 1964 for which pulse duration was reduced to 100 picoseconds [16], and by 1981, pulse duration had been reduced down to 100 femtoseconds using the colliding-pulse mode-locking technique [17]. Sub-6 femtosecond pulses were achieved improving passive-mode-locking and dispersion compensation [18–20].

Mode-locking is a technique in which a fixed or locked phase relationship between the longitudinal modes of the laser cavity is induced. The longitudinal modes are made to interfere with each other giving rise to the emission of a train of short pulses whose duration may range from few femtoseconds to several picoseconds [21]. The temporal separation between these pulses is the time that a pulse takes to complete one round trip in the resonator cavity. As the duration of a mode-locked laser pulse is very short, according to the time-bandwidth product, its spectrum consists of a broad wavelength range. Mode-locking technique can be active or passive. In active mode-locking, an external signal induces a modulation of the intra-cavity laser radiation by using acousto-optics modulators. Passive mode-locking is based on the introduction in the laser cavity of some elements such as saturable absorbers, dyes or solidstate media which induce self-modulation of the laser light. The most common mode-locked femtosecond lasers are the Ti:Sapphire femtosecond lasers based on the Kerr-Lens Modelocking (KLM) mechanism, in which the Ti:Sapphire gain medium crystal placed inside the cavity acts as Kerr-effect medium [14, 21]. Nowadays, typical commercial femtosecond laser systems produce sub-100 femtoseconds.
