**2. Ultrafast laser inscription**

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

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

*Ep* = *Pout* ⁄ *<sup>f</sup>* (1)

*Pp* = *Ep* ⁄*<sup>t</sup>* (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

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 the peak

laser, the power varies pulse-by-pulse. In this latter case, the pulse energy, *Ep*

compared to conventional light sources [14].

106 Advanced Surface Engineering Research

, can be calculated as:

and dispersion compensation [18–20].

power, *Pp*

matter [2–4, 14].

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 [23–44, 48–67].

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

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, several times higher than the strength of any material. This causes the solid to superheat yielding to the formation of a confined plasma, which explodes and generates a powerful shock wave that expands out of the focal volume and compresses the surrounding material, with pressures over 10 megabar (1 TPa) [45–47]. Therefore, an appropriate combination of the laser processing conditions in terms of numerical aperture (*NA*), diameter at the focal plane (*d*), laser wavelength (*λ*), pulse energy (*Ep* ), pulse-width (*Δt*), repetition rate (*f*), and scanning speed (*V*) can be used to create a wide variety of laser-induced structures for waveguiding. Specifically, the energy delivered by the laser over each focal spot *d* is given by:

$$E\_d = \frac{f \times d \times E\_p}{V} \tag{3}$$

Thus, this waveguide configuration has been widely applied in materials such as Nd:YAG,

The damage and the negative refractive index change originated by the ultra-intense and ultrashort laser radiation can be employed to surround a volume region of the bulk material. This is the origin of Type III or cladding waveguides for which the guiding core is adjusted and tailored by an appropriate position of the low-index tracks, placed few micrometers to each other to confine the light inside. The core diameter normally ranges between 25 and 150 μm so that it is possible the guidance of both monomode and multimode laser radiation for both TM and TE polarizations, from the visible to the IR region. These characteristics allow a better coupling to the waveguides, low propagation losses and a more efficient laser action

Finally, in Type IV, waveguide also referred as ridge waveguides, the ultra-high intensity achieved by the femtosecond laser pulses is used to ablate the surface to produce ridge waveguides on planar waveguide substrates obtained by other methods. Therefore, the guiding

In addition to the ULI technique, the fabrication of optical waveguides has also been reported by other processing routes. Among these techniques, it is worth noting ion exchange (IE), annealed proton exchange (APE), molecular beam epitaxy (MBE), liquid phase epitaxy (LPE), and sol-gel [68]. **Table 1** shows some examples of optical waveguides manufactured by these methods.

Nevertheless, when compared to these fabrication techniques, ULI has demonstrated to be a very powerful and robust technique due to its versatility, capable of manufacturing waveguides in a great variety of materials, from glasses to crystals, and flexible, allowing to inscribe a broad range of 3D structures and a large variety of waveguide configuration. Furthermore,

**Material Active ion Fabrication technique Reference** YAG Nd3+/Yb3+ IE [69] BK7 Nd3+ IE [70] LiNbO3 Nd3+ APE [71] LiTaO3 Nd3+ APE [72] LaF3 Nd3+ MBE [73] CaF<sup>2</sup> Er3+ MBE [74] GGG Nd3+ LPE [75]

<sup>2</sup> Tm3+ LPE [76]

**Table 1.** Some examples of optical waveguides manufactured by ion exchange (IE), annealed proton exchange (APE),

Silicate glass Er3+ Sol-gel [77]

molecular beam epitaxy (MBE), liquid phase epitaxy (LPE), and sol-gel techniques.

)2

, and Nd:Gd3

Ga5

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

O12 [29, 31, 38, 39, 50, 52, 53]

109

http://dx.doi.org/10.5772/intechopen.79577

, Yb:KY(WO4

Nd:YVO4

KY(WO4 ) , LiNbO3

and glasses [54–58].

, Yb:KGd(WO4

in materials doped with Rare Earths (RE) [40–44, 59–62].

features strongly depend on the planar waveguide substrate [63–67].

)2

The repetition rate is a critical parameter since the accumulation of multiple laser pulses over the same point will result in a local temperature increase if the period between consecutive pulses (*1/f*) is shorter than the cooling time:

$$t = d^2 / D \tag{4}$$

where *D* is the thermal diffusion coefficient. Thus, there is a critical repetition rate which determines two processing regimes; non-thermal and thermal regime for repetition rate lower and higher than the critical repetition rate [3]. For dielectric materials, thermal diffusivity is in the range of 10−3cm<sup>2</sup> s−1. For a typical laser diameter at the focal plane of around 1 μm, the critical frequency ranges 100 kHz.

It is generally accepted the classification of waveguides structures into four categories according to the structure generated with the laser-induced filament [34]. In Type I waveguides, the laser beam generates a positive refractive index change, Δ*n >* 0, directly in the irradiated area and the hence, the dimensions of the waveguide can be controlled by the amount of energy delivered on the focal volume. This type of waveguide can be induced in glasses since amorphous structure facilitates a positive refractive index change [3, 4, 23, 36, 37]. On the contrary, the mechanisms involved to be produced in crystals are complicated and entangled and hence, the type of refractive index change and the guiding axis cannot be envisaged. In addition, the damage created by the laser beam in the irradiated area gives rise to a non-desirable strong structural change of the material crystalline network. Furthermore, the waveguiding region is not stable with temperature, and the guidance is only supported in one polarization direction. Therefore, this type of waveguide has been reported in few crystalline materials such as LiNbO3 , ZnSe and YCa4 O(BO3 )3 [35, 48–51].

In Type II or double-line configuration waveguides, the damage that can be produced by the laser beam in the focal volume is used to create a stress-induced region and a positive refractive index change between two close parallel tracks, normally between 15 and 30 μm. This type of configuration overcomes the drawbacks of Type I waveguide in crystalline materials so that waveguide structure is stable with temperature, the guidance can be achieved for both laser polarizations and in addition, preserves the luminescence of the bulk material. Thus, this waveguide configuration has been widely applied in materials such as Nd:YAG, Nd:YVO4 , LiNbO3 , Yb:KGd(WO4 )2 , Yb:KY(WO4 )2 , and Nd:Gd3 Ga5 O12 [29, 31, 38, 39, 50, 52, 53] and glasses [54–58].

several times higher than the strength of any material. This causes the solid to superheat yielding to the formation of a confined plasma, which explodes and generates a powerful shock wave that expands out of the focal volume and compresses the surrounding material, with pressures over 10 megabar (1 TPa) [45–47]. Therefore, an appropriate combination of the laser processing conditions in terms of numerical aperture (*NA*), diameter at the focal plane

speed (*V*) can be used to create a wide variety of laser-induced structures for waveguiding.

The repetition rate is a critical parameter since the accumulation of multiple laser pulses over the same point will result in a local temperature increase if the period between consecutive

*t* = *d*<sup>2</sup> /*D* (4)

where *D* is the thermal diffusion coefficient. Thus, there is a critical repetition rate which determines two processing regimes; non-thermal and thermal regime for repetition rate lower and higher than the critical repetition rate [3]. For dielectric materials, thermal diffusivity is

It is generally accepted the classification of waveguides structures into four categories according to the structure generated with the laser-induced filament [34]. In Type I waveguides, the laser beam generates a positive refractive index change, Δ*n >* 0, directly in the irradiated area and the hence, the dimensions of the waveguide can be controlled by the amount of energy delivered on the focal volume. This type of waveguide can be induced in glasses since amorphous structure facilitates a positive refractive index change [3, 4, 23, 36, 37]. On the contrary, the mechanisms involved to be produced in crystals are complicated and entangled and hence, the type of refractive index change and the guiding axis cannot be envisaged. In addition, the damage created by the laser beam in the irradiated area gives rise to a non-desirable strong structural change of the material crystalline network. Furthermore, the waveguiding region is not stable with temperature, and the guidance is only supported in one polarization direction. Therefore, this type of waveguide has been reported in few crystalline materials

[35, 48–51].

In Type II or double-line configuration waveguides, the damage that can be produced by the laser beam in the focal volume is used to create a stress-induced region and a positive refractive index change between two close parallel tracks, normally between 15 and 30 μm. This type of configuration overcomes the drawbacks of Type I waveguide in crystalline materials so that waveguide structure is stable with temperature, the guidance can be achieved for both laser polarizations and in addition, preserves the luminescence of the bulk material.

s−1. For a typical laser diameter at the focal plane of around 1 μm, the

Specifically, the energy delivered by the laser over each focal spot *d* is given by:

), pulse-width (*Δt*), repetition rate (*f*), and scanning

*<sup>V</sup>* (3)

(*d*), laser wavelength (*λ*), pulse energy (*Ep*

108 Advanced Surface Engineering Research

pulses (*1/f*) is shorter than the cooling time:

in the range of 10−3cm<sup>2</sup>

such as LiNbO3

critical frequency ranges 100 kHz.

, ZnSe and YCa4

O(BO3 )3

*Ed* <sup>=</sup> *<sup>f</sup>* <sup>×</sup> *<sup>d</sup>* <sup>×</sup> *<sup>E</sup>* \_\_\_\_\_\_\_*<sup>p</sup>*

The damage and the negative refractive index change originated by the ultra-intense and ultrashort laser radiation can be employed to surround a volume region of the bulk material. This is the origin of Type III or cladding waveguides for which the guiding core is adjusted and tailored by an appropriate position of the low-index tracks, placed few micrometers to each other to confine the light inside. The core diameter normally ranges between 25 and 150 μm so that it is possible the guidance of both monomode and multimode laser radiation for both TM and TE polarizations, from the visible to the IR region. These characteristics allow a better coupling to the waveguides, low propagation losses and a more efficient laser action in materials doped with Rare Earths (RE) [40–44, 59–62].

Finally, in Type IV, waveguide also referred as ridge waveguides, the ultra-high intensity achieved by the femtosecond laser pulses is used to ablate the surface to produce ridge waveguides on planar waveguide substrates obtained by other methods. Therefore, the guiding features strongly depend on the planar waveguide substrate [63–67].

In addition to the ULI technique, the fabrication of optical waveguides has also been reported by other processing routes. Among these techniques, it is worth noting ion exchange (IE), annealed proton exchange (APE), molecular beam epitaxy (MBE), liquid phase epitaxy (LPE), and sol-gel [68]. **Table 1** shows some examples of optical waveguides manufactured by these methods.

Nevertheless, when compared to these fabrication techniques, ULI has demonstrated to be a very powerful and robust technique due to its versatility, capable of manufacturing waveguides in a great variety of materials, from glasses to crystals, and flexible, allowing to inscribe a broad range of 3D structures and a large variety of waveguide configuration. Furthermore,


**Table 1.** Some examples of optical waveguides manufactured by ion exchange (IE), annealed proton exchange (APE), molecular beam epitaxy (MBE), liquid phase epitaxy (LPE), and sol-gel techniques.

the fabrication of integrated photonic devices by ULI has shown to be cost-effective and efficient so that a suitable configuration of the processing parameters may lead to waveguide losses below 1 dB/cm, resulting in a low laser operation threshold.
