6 Direct Femtosecond Laser Writing of Optical Waveguides …
195
mid-IR [71] (1.3 dB/cm at 3.39 µm), with refractive index changes of the order of 1
× 10
–3 . The excellent performance of the waveguides has allowed the implementation of complex photonic devices in fused silica. Planar Y-splitters [72] or multiple
splitters [73] are the basic elements for the fabrication of complex photonic circuits,
but the potential of femtosecond lasers for 3D inscription was exploited with the
fabrication of nearly equalized 3D splitters [74]. Other 3D structures, as directional
couplers in 3D [75], straight waveguide arrays [76], or fan-out devices [77] have been
demonstrated in this material. Fused silica is one of the glasses that shows selective
chemical etching, and integrated optofluidic devices have been fabricated on it [9].
In addition, the possibility to dope silicate glasses with active ions, such as Nd, has
allowed the manufacturing of active waveguides [78].
Borosilicate glass is also an excellent substrate for the inscription of optical waveguides. For instance, complex 3D beam splitters (photonic lanterns) have been demonstrated in this glass [44] by the multi-scan approach, operating at 1539 nm with very
low losses. Depressed-cladding structures have been also inscribed in borosilicate
glass with a photonic-crystal hexagonal structure that modulates the spectral transmission of the waveguide, thus forming anti-resonant reflecting optical waveguides
[79].
Phosphate glass is very interesting for its spectroscopic properties when it is doped
with Nd, Yb, or Er:Yb. Different active devices such as waveguide lasers have been
manufactured in Er:Yb phosphate glass by direct femtosecond laser inscription [80,
81] emitting at 1533.5 nm. It should be remarked that the waveguides inscribed in
this glass exhibit very low propagation loss (0.24 dB/cm) [82].
Another glass with optimum properties is Foturan, a lithium aluminosilicate
glass doped by silver and cerium oxides. This glass is photosensitive and shows
selective chemical etching. The possibility to fabricate optical waveguides on it
by femtosecond laser irradiation [83] makes it very attractive for manufacturing
optofluidic devices [84]. ZBLAN (heavy metal fluoride glass) has been also used for
the inscription of optical waveguides. Doped with Tm, waveguide lasers based on
depressed-cladding structures have been obtained, emitting at 1.9 µm with prop loss
of 0.22 dB/cm and 50% slope efficiency [85].
6.4.2 Single Crystals
Single crystals play significant roles in optics and photonics. According to their
crystalline structures or functions, one can roughly classify the single-crystals into
a few groups. As of yet, in addition to the normal optical crystals (e.g., quartz),
the direct laser writing has been successfully applied to functional crystals, such
as laser, nonlinear, and electrooptic crystals. Electrooptic crystals utilize the electrooptic effects to modulate the light behaviors in crystals on the phase, amplitude,
and polarizations, engineering the light propagation, and multi-beam interactions for
signal processing of photonic systems. Laser crystals are favorite gain media, which
are widely used for laser generation and signal amplification. Nonlinear crystals are
195
mid-IR [71] (1.3 dB/cm at 3.39 µm), with refractive index changes of the order of 1
× 10
–3 . The excellent performance of the waveguides has allowed the implementation of complex photonic devices in fused silica. Planar Y-splitters [72] or multiple
splitters [73] are the basic elements for the fabrication of complex photonic circuits,
but the potential of femtosecond lasers for 3D inscription was exploited with the
fabrication of nearly equalized 3D splitters [74]. Other 3D structures, as directional
couplers in 3D [75], straight waveguide arrays [76], or fan-out devices [77] have been
demonstrated in this material. Fused silica is one of the glasses that shows selective
chemical etching, and integrated optofluidic devices have been fabricated on it [9].
In addition, the possibility to dope silicate glasses with active ions, such as Nd, has
allowed the manufacturing of active waveguides [78].
Borosilicate glass is also an excellent substrate for the inscription of optical waveguides. For instance, complex 3D beam splitters (photonic lanterns) have been demonstrated in this glass [44] by the multi-scan approach, operating at 1539 nm with very
low losses. Depressed-cladding structures have been also inscribed in borosilicate
glass with a photonic-crystal hexagonal structure that modulates the spectral transmission of the waveguide, thus forming anti-resonant reflecting optical waveguides
[79].
Phosphate glass is very interesting for its spectroscopic properties when it is doped
with Nd, Yb, or Er:Yb. Different active devices such as waveguide lasers have been
manufactured in Er:Yb phosphate glass by direct femtosecond laser inscription [80,
81] emitting at 1533.5 nm. It should be remarked that the waveguides inscribed in
this glass exhibit very low propagation loss (0.24 dB/cm) [82].
Another glass with optimum properties is Foturan, a lithium aluminosilicate
glass doped by silver and cerium oxides. This glass is photosensitive and shows
selective chemical etching. The possibility to fabricate optical waveguides on it
by femtosecond laser irradiation [83] makes it very attractive for manufacturing
optofluidic devices [84]. ZBLAN (heavy metal fluoride glass) has been also used for
the inscription of optical waveguides. Doped with Tm, waveguide lasers based on
depressed-cladding structures have been obtained, emitting at 1.9 µm with prop loss
of 0.22 dB/cm and 50% slope efficiency [85].
6.4.2 Single Crystals
Single crystals play significant roles in optics and photonics. According to their
crystalline structures or functions, one can roughly classify the single-crystals into
a few groups. As of yet, in addition to the normal optical crystals (e.g., quartz),
the direct laser writing has been successfully applied to functional crystals, such
as laser, nonlinear, and electrooptic crystals. Electrooptic crystals utilize the electrooptic effects to modulate the light behaviors in crystals on the phase, amplitude,
and polarizations, engineering the light propagation, and multi-beam interactions for
signal processing of photonic systems. Laser crystals are favorite gain media, which
are widely used for laser generation and signal amplification. Nonlinear crystals are