192
F. Chen and J. R. V. de Aldana
case, there exist two approaches to get light confinement that has been extensively
studied in crystals provided that, in most of them, Type-I modification with refractive
index increase cannot be produced.
Stress-induced waveguides. The first one consists of making use of the refractive
index increase that is produced at the surroundings of the damage track, caused
by the mechanical stress and compression induced after the irradiation. In order to
increase the refractive index change in this region and obtaining a better confinement
of the light, two parallel laser scans are done along the sample at certain separation
(~15–25 µm; see Fig. 6.4a) so that the waveguide is formed between the two damage
tracks: this is the so-called double-scan technique [23, 29, 48]. A very interesting
property of this fabrication approach is that the guiding region has not been directly
exposed to the laser irradiation, and then the physical characteristics of the bulk
are almost unaffected: This is crucial in many applications, particularly in crystals,
that are based on the nonlinear or spectroscopic features of the material [49, 50].
Moreover, in comparison with waveguides based on Type-I modification, stressinduced waveguides are very stable even at high temperatures [51], making them
very suitable for high-power applications. In addition to the simple “double-scan”
technique, some approaches with more complexity have been developed in order to
gain better confinement of the guided mode [52, 53] or to engineer the stress-field
geometry [54].
Depressed-cladding waveguides. An alternative approach for the fabrication of
optical waveguides based on Type-II modifications consists of the inscription of
many parallel damage tracks forming a modified cladding and a central unexposed
core where the light propagates through [55] (see Figs. 6.5a–c). In order to get a
good optical performance, the separation between damage tracks must be very small
(2–3 µm) and the scanning velocity very large in order to minimize the stress induced
in the surroundings of the tracks. In this way, the refractive index profile is basically a
cladding with decreased index (severe damage) and the central core keeps the index
of the bulk. In principle, this configuration supports only leaky modes [46] what
means that only continuum radiation modes exist. However, when the width of the
lower-index region is large, propagating modes very similar to confined modes are
supported.
Efficient depressed-cladding waveguides have been successfully fabricated in
many crystalline materials [6] even with very low propagation losses. In similar
manner to the behavior of “double-scan” waveguides, they have been demonstrated
to preserve the spectroscopic properties at the waveguide core [56]. In addition, in
most of the crystals, guidance features are independent of light polarization. Both
properties make cladding waveguides very attractive for constructing active components as waveguide lasers [55, 57]. But the main advantage of cladding waveguides
is that they can be designed with any arbitrary geometry and dimensions in order to
tailor the modal behavior for the required wavelength, even in the mid-infrared [58],
optimizing the coupling with external devices (i.e., optical fibers): In this sense, the
most convenient geometry is circular, with typical diameters between 30 and 150 µm.
In addition to the “standard” cladding waveguides described above, different
improvements have been introduced in the cladding designs. For instance, complex
F. Chen and J. R. V. de Aldana
case, there exist two approaches to get light confinement that has been extensively
studied in crystals provided that, in most of them, Type-I modification with refractive
index increase cannot be produced.
Stress-induced waveguides. The first one consists of making use of the refractive
index increase that is produced at the surroundings of the damage track, caused
by the mechanical stress and compression induced after the irradiation. In order to
increase the refractive index change in this region and obtaining a better confinement
of the light, two parallel laser scans are done along the sample at certain separation
(~15–25 µm; see Fig. 6.4a) so that the waveguide is formed between the two damage
tracks: this is the so-called double-scan technique [23, 29, 48]. A very interesting
property of this fabrication approach is that the guiding region has not been directly
exposed to the laser irradiation, and then the physical characteristics of the bulk
are almost unaffected: This is crucial in many applications, particularly in crystals,
that are based on the nonlinear or spectroscopic features of the material [49, 50].
Moreover, in comparison with waveguides based on Type-I modification, stressinduced waveguides are very stable even at high temperatures [51], making them
very suitable for high-power applications. In addition to the simple “double-scan”
technique, some approaches with more complexity have been developed in order to
gain better confinement of the guided mode [52, 53] or to engineer the stress-field
geometry [54].
Depressed-cladding waveguides. An alternative approach for the fabrication of
optical waveguides based on Type-II modifications consists of the inscription of
many parallel damage tracks forming a modified cladding and a central unexposed
core where the light propagates through [55] (see Figs. 6.5a–c). In order to get a
good optical performance, the separation between damage tracks must be very small
(2–3 µm) and the scanning velocity very large in order to minimize the stress induced
in the surroundings of the tracks. In this way, the refractive index profile is basically a
cladding with decreased index (severe damage) and the central core keeps the index
of the bulk. In principle, this configuration supports only leaky modes [46] what
means that only continuum radiation modes exist. However, when the width of the
lower-index region is large, propagating modes very similar to confined modes are
supported.
Efficient depressed-cladding waveguides have been successfully fabricated in
many crystalline materials [6] even with very low propagation losses. In similar
manner to the behavior of “double-scan” waveguides, they have been demonstrated
to preserve the spectroscopic properties at the waveguide core [56]. In addition, in
most of the crystals, guidance features are independent of light polarization. Both
properties make cladding waveguides very attractive for constructing active components as waveguide lasers [55, 57]. But the main advantage of cladding waveguides
is that they can be designed with any arbitrary geometry and dimensions in order to
tailor the modal behavior for the required wavelength, even in the mid-infrared [58],
optimizing the coupling with external devices (i.e., optical fibers): In this sense, the
most convenient geometry is circular, with typical diameters between 30 and 150 µm.
In addition to the “standard” cladding waveguides described above, different
improvements have been introduced in the cladding designs. For instance, complex