6 Direct Femtosecond Laser Writing of Optical Waveguides …
197
6.4.3 Ceramics
Transparent ceramics are polycrystalline materials. The monocrystalline partners
of the ceramics are usually with cubic lattice structures. As of yet, rare-earth iondoped YAG and yttria [109] are the investigated ceramics for femtosecond laser
inscribed waveguides. Nd- or Yb-doped YAG ceramic waveguides have been used
for efficient gain cavities to realize waveguide lasing at 1 µm [110], while Tm-doped
YAG ceramics have been ascertained for 2 µm laser generation in waveguide cavity
[111]. Also due to the working wavelength of the waveguides, Nd- or Yb-doped YAG
waveguides can take the geometry of double line or claddings, while the Tm-doped
YAG is only with depressed claddings because of the longer operation wavelength.
As for the waveguide qualities, compared with the single-crystalline partners, the
guiding behaviors and the laser-induced micrometric modifications are very similar in
ceramics. This also results in similar laser performances of waveguide lasers between
the gain cavities of single crystals and polycrystalline ceramics. Nevertheless, in
principle, the ceramics possess the advantage of easy production of larger bulks,
which may lead to larger waveguide wafers for the integration of more functions.
6.5 Selected Applications
6.5.1 3D Waveguide Devices
One of the most interesting features of femtosecond laser irradiation is the possibility to do true 3D micro-processing inside transparent dielectrics: The laser beam
can be focused at any depth of the sample, thus allowing the strongly localized
modification at arbitrary positions. This capability has opened the door to many
novel and exciting applications [4], as nano-surgery [112], or additive/subtractive
3D nano-manufacturing [113]. Photonics and waveguide fabrication, in particular,
have been specially benefited from this feature, provided that other manufacturing
techniques (ion-beam implantation, Silica-on-Silicon or sol–gel) are restricted to
planar or quasi-planar geometries. However, there exists a wide variety of applications that require 3D photonic circuits based on optical waveguides, in different
scientific and technological fields such as astro-photonics or biomedicine.
For the fabrication of 3D photonic circuits, the most straightforward technique is
the use of Type-I modifications (see Sect. 6.2) in such a way that guidance is directly
produced at the focal volume with no further damage at the surroundings. This technique allows the implementation of low losses waveguide splitters/combiners in 3D
[44], or the efficient coupling between neighbor waveguides through evanescent field
[76], having both elements crucial importance for complex 3D devices. For instance,
it has been demonstrated the direct integration of such waveguides in microfluidic
197
6.4.3 Ceramics
Transparent ceramics are polycrystalline materials. The monocrystalline partners
of the ceramics are usually with cubic lattice structures. As of yet, rare-earth iondoped YAG and yttria [109] are the investigated ceramics for femtosecond laser
inscribed waveguides. Nd- or Yb-doped YAG ceramic waveguides have been used
for efficient gain cavities to realize waveguide lasing at 1 µm [110], while Tm-doped
YAG ceramics have been ascertained for 2 µm laser generation in waveguide cavity
[111]. Also due to the working wavelength of the waveguides, Nd- or Yb-doped YAG
waveguides can take the geometry of double line or claddings, while the Tm-doped
YAG is only with depressed claddings because of the longer operation wavelength.
As for the waveguide qualities, compared with the single-crystalline partners, the
guiding behaviors and the laser-induced micrometric modifications are very similar in
ceramics. This also results in similar laser performances of waveguide lasers between
the gain cavities of single crystals and polycrystalline ceramics. Nevertheless, in
principle, the ceramics possess the advantage of easy production of larger bulks,
which may lead to larger waveguide wafers for the integration of more functions.
6.5 Selected Applications
6.5.1 3D Waveguide Devices
One of the most interesting features of femtosecond laser irradiation is the possibility to do true 3D micro-processing inside transparent dielectrics: The laser beam
can be focused at any depth of the sample, thus allowing the strongly localized
modification at arbitrary positions. This capability has opened the door to many
novel and exciting applications [4], as nano-surgery [112], or additive/subtractive
3D nano-manufacturing [113]. Photonics and waveguide fabrication, in particular,
have been specially benefited from this feature, provided that other manufacturing
techniques (ion-beam implantation, Silica-on-Silicon or sol–gel) are restricted to
planar or quasi-planar geometries. However, there exists a wide variety of applications that require 3D photonic circuits based on optical waveguides, in different
scientific and technological fields such as astro-photonics or biomedicine.
For the fabrication of 3D photonic circuits, the most straightforward technique is
the use of Type-I modifications (see Sect. 6.2) in such a way that guidance is directly
produced at the focal volume with no further damage at the surroundings. This technique allows the implementation of low losses waveguide splitters/combiners in 3D
[44], or the efficient coupling between neighbor waveguides through evanescent field
[76], having both elements crucial importance for complex 3D devices. For instance,
it has been demonstrated the direct integration of such waveguides in microfluidic