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F. Chen and J. R. V. de Aldana
6.3.3 Other Femtosecond Laser Writing Techniques
Fs-laser irradiation can be focused on the surface of a dielectric material leading
to ultra-fast ablation (see for instance [64] and references therein), that consists
in local material removal with minimal damage in the surroundings. Although the
layer removed with a single laser pulse is typically of a few hundreds of nanometers
[65], the incidence with multiple pulses allows the production of micro-holes or
precise cuts in the sample surface [66]. This micro-machining tool offers a pathway
to fabricate ridge waveguides on planar waveguide substrates (the planar waveguide
previously fabricated by other techniques as ion-beam implantation): The fs-laser
is used to precise micro-cuts in the planar surface, constructing ridges that confine
light propagation in certain regions [67], thus forming channel waveguides with
the desired lateral dimensions (see Fig. 6.5e). The technique has been applied to
different crystals in order to fabricate frequency converters or waveguide lasers [68].
The main drawback of these waveguides is the roughness of the ablation grooves
produced with the fs-laser, which introduces large propagation losses and degrades
the modal profiles: To improve the performance, some post-processing treatments
have been used, as thermal annealing [69] or ion-beam sputtering [67].
6.4 Materials
6.4.1 Glasses
Glasses are maybe the family of materials in which more research work on direct
femtosecond laser inscription has been done. There are several reasons for that,
such as the excellent optical properties and quality of many glasses, the stability
of the material or the relatively low cost, among many others. However, maybe the
most interesting property of glasses that make them very attractive for femtosecond
laser waveguide inscription is the possibility to produce Type-I modifications with
refractive index increase [3] in many of them, working both in the non-thermal
regime (low repetition rate laser systems) and in the thermal regime (high repetition
rate systems). Additionally, some of them show the possibility to produce selective
chemical etching in the areas exposed to femtosecond laser irradiation [70], opens the
door to a wide range of applications that mix several types of laser micro-processing,
such as the fabrication of microfluidic or lab-on-a-chip devices [9].
Among the large amount of available glasses with optimal optical properties, some
of them have concentrated most of the research efforts. Fused silica (amorphous SiO 2 )
is one of the most extensively studied materials regarding femtosecond laser irradiation and waveguide fabrication. It is available with excellent optical quality and
shows a very wide transmission window. The techniques used for the inscription
of optical waveguides are both single scan [3] or multiple scan [40], thus obtaining
very low propagation loss in the visible and near-IR (0.12 dB/cm), or even in the
F. Chen and J. R. V. de Aldana
6.3.3 Other Femtosecond Laser Writing Techniques
Fs-laser irradiation can be focused on the surface of a dielectric material leading
to ultra-fast ablation (see for instance [64] and references therein), that consists
in local material removal with minimal damage in the surroundings. Although the
layer removed with a single laser pulse is typically of a few hundreds of nanometers
[65], the incidence with multiple pulses allows the production of micro-holes or
precise cuts in the sample surface [66]. This micro-machining tool offers a pathway
to fabricate ridge waveguides on planar waveguide substrates (the planar waveguide
previously fabricated by other techniques as ion-beam implantation): The fs-laser
is used to precise micro-cuts in the planar surface, constructing ridges that confine
light propagation in certain regions [67], thus forming channel waveguides with
the desired lateral dimensions (see Fig. 6.5e). The technique has been applied to
different crystals in order to fabricate frequency converters or waveguide lasers [68].
The main drawback of these waveguides is the roughness of the ablation grooves
produced with the fs-laser, which introduces large propagation losses and degrades
the modal profiles: To improve the performance, some post-processing treatments
have been used, as thermal annealing [69] or ion-beam sputtering [67].
6.4 Materials
6.4.1 Glasses
Glasses are maybe the family of materials in which more research work on direct
femtosecond laser inscription has been done. There are several reasons for that,
such as the excellent optical properties and quality of many glasses, the stability
of the material or the relatively low cost, among many others. However, maybe the
most interesting property of glasses that make them very attractive for femtosecond
laser waveguide inscription is the possibility to produce Type-I modifications with
refractive index increase [3] in many of them, working both in the non-thermal
regime (low repetition rate laser systems) and in the thermal regime (high repetition
rate systems). Additionally, some of them show the possibility to produce selective
chemical etching in the areas exposed to femtosecond laser irradiation [70], opens the
door to a wide range of applications that mix several types of laser micro-processing,
such as the fabrication of microfluidic or lab-on-a-chip devices [9].
Among the large amount of available glasses with optimal optical properties, some
of them have concentrated most of the research efforts. Fused silica (amorphous SiO 2 )
is one of the most extensively studied materials regarding femtosecond laser irradiation and waveguide fabrication. It is available with excellent optical quality and
shows a very wide transmission window. The techniques used for the inscription
of optical waveguides are both single scan [3] or multiple scan [40], thus obtaining
very low propagation loss in the visible and near-IR (0.12 dB/cm), or even in the