6 Direct Femtosecond Laser Writing of Optical Waveguides …
187
In this chapter, we review the fundamentals and new advances of femtosecond
laser-written waveguides in dielectrics. Section 6.2 introduces the fundamentals of
refractive index modification in dielectrics by femtosecond laser pulses. In Sect. 6.3,
the geometries of different waveguides by laser writing are overviewed. In Sect. 6.4,
we summarize, in a brief way, the dielectric materials applied for laser writing of
waveguide devices. Several selected applications of laser-written waveguides are
presented in Sect. 6.5 to indicate the state of the art of the research fields. Finally, in
Sect. 6.6 a summary is given and an outlook of the future topics is presented.
6.2 Femtosecond Lasers Induced Refractive Index Changes
When ultrashort laser pulses are focused inside transparent dielectrics well localized
and, to some extent, controlled material modifications can be induced. The highintensity that can be reached in the focal region and the short temporal duration
of the laser pulse (tens or a few hundreds of femtoseconds) are responsible for a
very complex laser-matter interaction phenomenology. As the laser pulse intensity
reaches 10
12 –10
13 W/cm
2 , values that can be easily achieved with amplified laser
systems, a larger number of electrons can be ionized through the so-called strong-field
ionization processes [16]: multiphoton ionization (more efficient at low intensities) or
tunnel ionization. Then, detached electrons form a plasma that continues interacting
with the laser field, absorbing energy by inverse Bremsstrahlung and releasing more
electrons by collisions (process called avalanche ionization) [17]. However, due to the
short pulse duration, the laser–matter interaction finishes before a significant energy
transfer between electrons and ions took place, and thus negligible thermal effects
are induced: the irradiated area consists of a hot plasma of electrons and a “cold”
lattice of ions. The evolution of such system once the interaction with the laser has
finished is so complex that is still far from being completely understood and depends
strongly on both the plasma as the material properties [18]. Once the equilibrium is
again reached after the irradiation, localized modifications of the material may be
found, directly linked to the ionized electron density that was reached in the material
[19]. In fact, there are clear thresholds [20], in terms of the input pulse energy, to
induce certain modification in each target (depending, of course, on the experimental
conditions as focusing, pulse duration, wavelength, etc.).
For low pulse energies, a weak and smooth material modification is induced in
the focal volume that changes the index of refraction in this area [3]. The first microanalysis studies were done in glasses and suggest that a structural rearrangement of
bonds takes place without destruction of the overall integrity of the glass [21]. This
weak material modification is usually referred to as “Type-I modification” (following
the classification scheme proposed in [22]) and can be associated with a positive
or a negative refractive index change in the focal volume. Positive refractive index
changes associated with Type-I modification are more frequently produced in glasses
than in crystals, provided that densification is expected to be more easily induced in
amorphous materials than in ordered crystalline structures. However, in glasses with
187
In this chapter, we review the fundamentals and new advances of femtosecond
laser-written waveguides in dielectrics. Section 6.2 introduces the fundamentals of
refractive index modification in dielectrics by femtosecond laser pulses. In Sect. 6.3,
the geometries of different waveguides by laser writing are overviewed. In Sect. 6.4,
we summarize, in a brief way, the dielectric materials applied for laser writing of
waveguide devices. Several selected applications of laser-written waveguides are
presented in Sect. 6.5 to indicate the state of the art of the research fields. Finally, in
Sect. 6.6 a summary is given and an outlook of the future topics is presented.
6.2 Femtosecond Lasers Induced Refractive Index Changes
When ultrashort laser pulses are focused inside transparent dielectrics well localized
and, to some extent, controlled material modifications can be induced. The highintensity that can be reached in the focal region and the short temporal duration
of the laser pulse (tens or a few hundreds of femtoseconds) are responsible for a
very complex laser-matter interaction phenomenology. As the laser pulse intensity
reaches 10
12 –10
13 W/cm
2 , values that can be easily achieved with amplified laser
systems, a larger number of electrons can be ionized through the so-called strong-field
ionization processes [16]: multiphoton ionization (more efficient at low intensities) or
tunnel ionization. Then, detached electrons form a plasma that continues interacting
with the laser field, absorbing energy by inverse Bremsstrahlung and releasing more
electrons by collisions (process called avalanche ionization) [17]. However, due to the
short pulse duration, the laser–matter interaction finishes before a significant energy
transfer between electrons and ions took place, and thus negligible thermal effects
are induced: the irradiated area consists of a hot plasma of electrons and a “cold”
lattice of ions. The evolution of such system once the interaction with the laser has
finished is so complex that is still far from being completely understood and depends
strongly on both the plasma as the material properties [18]. Once the equilibrium is
again reached after the irradiation, localized modifications of the material may be
found, directly linked to the ionized electron density that was reached in the material
[19]. In fact, there are clear thresholds [20], in terms of the input pulse energy, to
induce certain modification in each target (depending, of course, on the experimental
conditions as focusing, pulse duration, wavelength, etc.).
For low pulse energies, a weak and smooth material modification is induced in
the focal volume that changes the index of refraction in this area [3]. The first microanalysis studies were done in glasses and suggest that a structural rearrangement of
bonds takes place without destruction of the overall integrity of the glass [21]. This
weak material modification is usually referred to as “Type-I modification” (following
the classification scheme proposed in [22]) and can be associated with a positive
or a negative refractive index change in the focal volume. Positive refractive index
changes associated with Type-I modification are more frequently produced in glasses
than in crystals, provided that densification is expected to be more easily induced in
amorphous materials than in ordered crystalline structures. However, in glasses with