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F. Chen and J. R. V. de Aldana
key elements for light frequency conversion, realizing color changes, and new light
sources for diverse applications.
The electrooptic crystal used for laser-written waveguides is LiNbO 3 [6]. Since
LiNbO 3 is a multifunctional crystal, many efforts have been performed on it to
study the guiding properties. Liao et al. reported on the modulation of light based
on femtosecond laser inscribed LiNbO 3 (double-line) waveguides [86]. Other crystals such as LGS may be also used for EO modulation. Nevertheless, cost-effective
modulators require low voltages of applied electrical fields, and in principle, the
transverse dimension should be as compact as possible. Considering this factor,
double-line structures may be better since the cross sections are much smaller than
those of depressed-cladding waveguides [86].
Nonlinear crystals for frequency conversion of laser-written waveguides include
birefringent crystals and periodically poled superlattices. Frequency doubling to
generate second harmonics is one of the most important functions for nonlinear crystals based on Type-I or Type-II phase matching conditions. Laser-written waveguides
in nonlinear crystals, such as KTP [87], BIBO [54, 88], GdCOB [37], LiNbO 3 [23],
KDP [89], have been fabricated in both double-line and depressed-cladding geometries. In addition, by using periodically poled wafers in ferroelectrics, such as PPLN,
PPLT, PPKTP, one can realize broadband SHG in laser-written waveguides. The
mostly investigated wafer is PPLN. All the possible designs have been applied on
PPLN, including single-line or multi-line Type-I, double-line Type-II, and depressed
cladding configurations [90–92]. For PPKTP [93], only double-line structures have
been investigated.
The laser crystals are doped by active ions into suitable bulk materials. The
lasing wavelength depends on the atomic energy levels of specific active ions. For
visible waveguide, lasing can be achieved by using Pr-doped crystals, including
Pr:SrAl 12 O 19 (at 644 nm) [94], Pr:YLF (at 604 nm) [53], Pr:YKF (at 610 and 645 nm)
[95] waveguides. Another solution to generate visible laser is to use self-frequencydoubled crystals, which combine the nonlinear properties of bulk crystals and gain
feature of the action ions in a single crystal, such as Nd:YAB [96], and Nd:YCOB [97].
In the near infrared regime, Ti-doped sapphire is for 700–900 nm tunable wavelength
[98] while Nd- or Yb-doped systems are often used for 1 µm lasing. Nd- or Yb-doped
YAG crystals [99, 100] and vanadates (YVO 4 , GdVO 4 ) [57, 101] are favorite gain
media. Other doped crystals are also investigated, such as Nd:GGG [50], Nd:LGS
[102], Nd:KGW [103], Nd:KYW [104], Nd:LiNbO 3 [105]. The Tm-doped systems
can generate lasing at 1.8–2 µm band. The reported result is Tm:KLuW cladding
waveguides [106]. For typical mid-infrared wavelength regime, it is desirable to use
laser-written waveguides in Cr or Fe-doped ZnSe or ZnS [107, 108], in which tunable
lasing from ~2 to ~3 µm.
F. Chen and J. R. V. de Aldana
key elements for light frequency conversion, realizing color changes, and new light
sources for diverse applications.
The electrooptic crystal used for laser-written waveguides is LiNbO 3 [6]. Since
LiNbO 3 is a multifunctional crystal, many efforts have been performed on it to
study the guiding properties. Liao et al. reported on the modulation of light based
on femtosecond laser inscribed LiNbO 3 (double-line) waveguides [86]. Other crystals such as LGS may be also used for EO modulation. Nevertheless, cost-effective
modulators require low voltages of applied electrical fields, and in principle, the
transverse dimension should be as compact as possible. Considering this factor,
double-line structures may be better since the cross sections are much smaller than
those of depressed-cladding waveguides [86].
Nonlinear crystals for frequency conversion of laser-written waveguides include
birefringent crystals and periodically poled superlattices. Frequency doubling to
generate second harmonics is one of the most important functions for nonlinear crystals based on Type-I or Type-II phase matching conditions. Laser-written waveguides
in nonlinear crystals, such as KTP [87], BIBO [54, 88], GdCOB [37], LiNbO 3 [23],
KDP [89], have been fabricated in both double-line and depressed-cladding geometries. In addition, by using periodically poled wafers in ferroelectrics, such as PPLN,
PPLT, PPKTP, one can realize broadband SHG in laser-written waveguides. The
mostly investigated wafer is PPLN. All the possible designs have been applied on
PPLN, including single-line or multi-line Type-I, double-line Type-II, and depressed
cladding configurations [90–92]. For PPKTP [93], only double-line structures have
been investigated.
The laser crystals are doped by active ions into suitable bulk materials. The
lasing wavelength depends on the atomic energy levels of specific active ions. For
visible waveguide, lasing can be achieved by using Pr-doped crystals, including
Pr:SrAl 12 O 19 (at 644 nm) [94], Pr:YLF (at 604 nm) [53], Pr:YKF (at 610 and 645 nm)
[95] waveguides. Another solution to generate visible laser is to use self-frequencydoubled crystals, which combine the nonlinear properties of bulk crystals and gain
feature of the action ions in a single crystal, such as Nd:YAB [96], and Nd:YCOB [97].
In the near infrared regime, Ti-doped sapphire is for 700–900 nm tunable wavelength
[98] while Nd- or Yb-doped systems are often used for 1 µm lasing. Nd- or Yb-doped
YAG crystals [99, 100] and vanadates (YVO 4 , GdVO 4 ) [57, 101] are favorite gain
media. Other doped crystals are also investigated, such as Nd:GGG [50], Nd:LGS
[102], Nd:KGW [103], Nd:KYW [104], Nd:LiNbO 3 [105]. The Tm-doped systems
can generate lasing at 1.8–2 µm band. The reported result is Tm:KLuW cladding
waveguides [106]. For typical mid-infrared wavelength regime, it is desirable to use
laser-written waveguides in Cr or Fe-doped ZnSe or ZnS [107, 108], in which tunable
lasing from ~2 to ~3 µm.