Silica Optical Fibers Doped with Nanoparticles for Fiber Lasers and Broadband Sources

14 Pages·2016·2.79 MB·English

277 Send Orders for Reprints to [email protected]
Current Nanoscience, 2016, 12, 277-290
Silica Optical Fibers Doped with Nanoparticles for Fiber Lasers and (cid:13)(cid:2)(cid:2)(cid:3)(cid:3)(cid:3)(cid:3)(cid:4)(cid:4)(cid:5)(cid:5)(cid:6)(cid:6)(cid:7)(cid:7)(cid:14)(cid:8)(cid:9)(cid:9)(cid:8)(cid:10)(cid:11)(cid:11)(cid:15)(cid:12)(cid:9)(cid:7)(cid:14)(cid:10)(cid:15)(cid:9)
Broadband Sources (cid:2)(cid:8)(cid:12)(cid:3)(cid:5)(cid:13)(cid:6)(cid:14)(cid:4)(cid:7)(cid:15)(cid:5)(cid:9)(cid:16)(cid:6)(cid:10)(cid:7)(cid:11)
Ivan Kasik*, Pavel Peterka, Jan Mrazek and Pavel Honzatko
Institute of Photonics and Electronics, CAS, Chaberska 57, Prague 8, 182 51, Czech Republic
Abstract: We present a review on recent progress in the research and development of nanoparticle-
containing optical fibers for high-power fiber lasers, amplifiers, and ASE sources. Attention has been
focused on rare-earth-doped silica fibers with nanoparticles in the core. Progress in materials, tech-
nologies, characterization techniques, and achievements has been summarized. Materials of active fi-
bers based on yttrium-aluminium silicates, mullite, protoenstatite, and phosphates doped with Er3+,
Yb 3+, Eu3+, and Tm3+ have been reviewed. The material research in this field has been systematically
investigated by research groups from ORC Southampton, CGCRI Kolkata, LPMC Nice together with
CNR-IOM-OGG Grenoble, nLIGHT (formerly Liekki), and IPE Prague since 2007. The best slope ef-
ficiency achieved with Yb –doped nanoparticle-containing fiber was in the range of 70-80 %.
Keywords: Ceramics, fiber laser, nanocrystal, nanoparticle, optical fiber, rare-earth, silica.
INTRODUCTION resonator to the point of application, which is another impor-
tant competitive advantage to solid state lasers whose output
Fundaments in the Field
beam requires additional optical coupling systems.
Silica optical fibers have been known since the 1960s
New trends in fiber laser applications lead to highly pre-
thanks to the pioneering studies of the groups of Charles. K.
cise low-power systems suitable for laser eye surgery, me-
Kao, Erich Spitz, and Robert D. Maurer [1-3]. Lasers have
trology, or high-powered applications requiring watts to
also been known since the same time period due to the ex-
kilowatts of optical power for metal welding, cutting, splic-
periments of T. Maiman using ruby crystals [4]. These two
ing or other directed energy applications [8-12]. The avail-
inventions were awarded by the Nobel Prize in 2009 and
ability of efficient sources of scalable output optical power
1964, respectively. In 1960 E. Snitzer designed and later
(Fig. 1) in wide spectral range for variety of applications is a
demonstrated the first fiber laser based on a Nd-doped opti-
challenge for today’s laser science and technology [13].
cal fiber as the active medium [5]. This idea stayed relatively
unnoticed until the mid-eighties when D. Payne from ORC 110
100kW
[6], E. Desurvire from Bell Labs [7], and M. Nakazawa from 100
Yd (IPG)
NTT Labs revitalized it for optical amplification. Their Er-
90
doped fiber amplifiers revolutionized the world of telecom- W]
80
munications and enabled the global penetration of high- k
e sppheoetdo niinctse rnmeat.t eTrioadlsa ya’sn dre sfeibaercr hl aasnedr sd ehvaevloe prmiseennt ifnr ommo dtehrins wer [ 6700
o
nc background. put p 4500 Y4b0 (kIPWG)
Fiber Lasers and Amplifiers ut
e o 30
r 1.2kW 1,5kW
i Novel systems of fiber lasers and amplifiers exhibit a se 20 Yb (ORC) Yb (Jena)
c nearly Gaussian, high-quality beam of low divergence, high La 10 3N0Wd 11Y0bW 10kW
s brightness (up to 12 GW/m2) and wide tuneability in a com- Yb (IPG)
0
o pact form. Other significant features include high optical 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014
conversion efficiencies (up to 70 - 90%), effective pumping,
n and an absence of complicated cooling. Apart from a high Fig. (1). Trends in output power.
a beam quality, these features bring crucial benefits for energy
savings. Fiber lasers are also inherently compatible with
N
flexible passive fibers and can deliver the energy from the Spectral Range and Output Power of Fiber Lasers and
Amplifiers – Materials, Structures, Designs

t
n *Address correspondence to this author at the Institute of Photonics and The wavelength and intensity of the emission of fiber
Electronics, CAS, Chaberska 57, Prague 8, 18251, Czech Republic; lasers and amplifiers is dependent on both the composi-
e Tel: + 420 220922391; E-mail: [email protected]
r
r 1875-6786/16 $58.00+.00 © 2016 Bentham Science Publishers
u
C
278 Current Nanoscience, 2016, Vol. 12, No. 3 Kasik et al.
tion and structure of the active medium as well as the la- Table 1. Phonon energies of host matrices.
ser (amplifier) design. In fiber lasers based on special
doped fibers the emission is usually derived from the
E [cm-1]
presence of rare-earth elements (RE) in the fiber host ma- phonon
trix material. Lasers for high-power applications are pres- Fluorides (ZBLAN) 550
ently based mainly on Yb-doped fibers operating at
SbO 700
around 1.06 – 1.12 (cid:2)m [8, 11, 14, 15], Er-doped fibers 2 3
operating at around 1.52 – 1.55 (cid:2)m [16-18], and recently AlO 870
2 3
also Tm-doped fibers operating at around 2 (cid:2)m [19, 20].
Apart from those, lasers based on Sm-doped fibers [21], SiO2 1100
Ho-doped fibers [20, 22-24], Er/Yb-doped fibers [25],
PO 1320
2 5
Yb/Tm-doped fibers [26], and other RE combinations

have been developed. Since no emission of RE within
1100 -1300 nm can be utilized, Bi-doped fibers operating y
nc SmTmNdYbBiNdTmErTm Ho ErHo
in the 1.0-1.3 (cid:2)m spectral region have been investigated e
ar
(Fig. 2) [27-29]. Raman lasers covering all the near- p
s
infrared (NIR) spectrum, parametric oscillators, and su- n
a
percontinuum sources based on non-linear effects in Tr Chalcogenide - telluride
highly non-linear fibers have also been demonstrated [12]. Fluoride
HMO
y Chalcogenide - As2S3
nc SmTmNdYbBiNdTmErTm Ho ErHo Silica
e
ar Sm 650 nm Silicate
p Tm 800 nm
s Polymers
n Nd 900-950 nm
a
Tr Yb 970-1040 nm VIS NIR IR
Raman Bi 1100-1300 nm
Nd 1320-1400 nm 200 600800 1000 1500 2000 3000 5000 10000
Tm 1470 nm
Wavelength [nm]
Er 1550 nm
Tm 1550-2050 nm
Fig. (3). Matrices and emission of fiber sources based on RE- and
Ho 1950-2250 nm
Er 2800 nm Bi–doped fibers.
Ho 2860,3000 nm
VIS NIR IR
Increases of the laser output optical power can be ob-
200 600800 1000 1500 2000 3000 5000 10000 tained by both modifications of the fiber structure and com-
Wavelength [nm] position as well as through the design of the laser. The con-
Fig. (2). Trends in laser wavelength. cept of coherent beam combination is an example of modifi-
cation of leaser design [38-40]. The formation of double-clad
(DC) fiber structures (Fig. 4a) [41-44], of multicore ribbon
fibers [45], and of large mode area (LMA) fibers (Fig. 4b)
The emissions of fiber lasers are also significantly influ-
[8, 46-49] represent the main trends in the modification of
enced by the composition of the fiber host matrices. Dielec-
fiber structures for high-power lasers.
trics like polymers, soft optical glasses, and silica can serve
as matrix materials hosting the RE ions. In principle, more
intensive emissions (output power) can be achieved with Silica-based Rare Earth-doped Optical Fibers
higher concentrations of uniformly distributed RE ions in a
Silica is a unique material of extremely high transpar-
suitable host matrix. The low phonon energy of the host ma- ency in the range from ultraviolet (UV) to NIR (originally
trix is an important factor for many laser transitions. The predicted by Kao [1-3]). It also has good thermal stability,
high-phonon energy of silica may lead to a significant short- with a minimum thermal expansion coefficient of 6.10-7 K-1
ening of the fluorescence lifetime of the upper laser level of and liquidus 1726°C. Because of these properties it could
the respective RE ion, which prohibits use of such materials be considered a good material for RE-doped optical fibers;
as an active medium. There has been a variety of polymers however, the solubility of RE in this host matrix is limited,
that have been used for hosting RE [30]; however, the solu- and only several tens of ppm of RE will lead to phase sepa-
bility of RE in these matrices (or starting monomers) is lim- ration that destroys the material transparency (Fig. 5).
ited [31]. Good solubility of RE can be obtained in soft opti-
Hence, over last three decades a tremendous effort has
cal glasses (chalcogenide, fluoride and oxy-fluoride, heavy
been devoted to modification of the silica host matrix
metal oxide (HMO) or silicate). The low phonon energies
(mainly with Al O , P O and Sb O ) with the aim to in-
(Table 1) of most of them support the idea of doping of these 2 3 2 5 2 3
crease the solubility of RE in these matrices. This effort has
materials with RE (Fig. 3). Unfortunately, all these materials
been concentrated mainly in research groups at ORC South-
are not ideal for high-power applications because of their
ampton (D.N. Payne, S.B. Poole, J. Townsend, J.K. Sahu et
higher attenuation and poor thermal stability in comparison
al.), FORC Moscow (M.M. Bubnov, E.M. Dianov, V.V.
to silica [32-37].
Dvoyrin, V.P. Gapontsev, M.E. Likhachev et al.), IPHT Jena
Silica Optical Fibers Doped with Nanoparticles Current Nanoscience, 2016, Vol. 12, No. 3 279
a) b)
SM MSF
(cid:2)
n
LD
Brightness transformation
r
trench

Fig. (4). Examples of (a - left) DC and (b - right) LMA structures.
a) b)

Fig. (5). Illustrative photo (a - left) and a scanning electron microscope image (SEM) (b - right) of a phase-separated core of RE-doped pre-
form.
(J. Kirchhof, S. Unger), FOC Sydney (J.D. Love, M. Sceats tion of lithium silicate nanocrystals in glass [52]; however, at
et al.), LPMC Nice (G. Monnom, B. Dussardier, W. Blanc, that time there was no characterization technique having
G. Vienne et al.), Optacore Ljubljana (B. Lenardic et al.), sufficiently high resolution to interpret these materials. Since
CGCRI Kolkata (R. Sen, A. Dhar, M.C. Paul et al.), Yonsei then, nanotechnologies that allow the formation of nanos-
University (K. Oh et al.), OFS Bell Labs Murray Hill, NJ tructures inside disordered glassy matrices have brought
(D.J. DiGiovanni et al.), GTE Labs Waltham, MA (W.J. groundbreaking progress in the field. By maintaining suit-
Miniscalco, et al.), Lab for Lighthouse Tech. Boston, MA able shapes, sizes, compositions, and distributions of
(F.T. Morse et al.), Stanford (M.J.F. Digonnet et al.), Co- nanoparticles, these new approaches can provide high trans-
lumbia (E. Desurvire et al.), and others including a small parencies of the fiber material, even for higher RE-ion con-
group at IPE Prague (V. Matejec, I. Kasik, P. Peterka et al.). centrations inside the glassy matrix. The dispersion of these
The complete list of engaged scientists and valuable publica- nanoparticles (determining the vicinity of light-emitting RE
tions would be quite long and is outside the scope of this ions with an atomic spatial resolution) influences the lumi-
paper. An exhaustive review on this topic can be found, e.g., nescent properties of the bulk materials. Extensive attention
in [50]. has been given to nano-glassceramic thin layers or bulk ma-
terials widely used for bulk and planar optic components
“Nano-epoch” has brought a unique solution to this
[53-56]. An original nickel-doped nanocrystalline glass-
field: the formation of RE doped nanoparticles or nanocrys-
ceramic optical fiber emitting at around 1250 nm represents
tals in either silica or a modified-silica host matrix. This
a certain special case within this study [57]. In the following
paper is concentrated on silica optical fibers with a
part we deal with noble metal, semiconductor, and ceramic
nanoparticle-containing active core for fiber lasers and
nanoparticle-containing silica or silica-based optical fibers
broadband sources.
doped with RE, which are the focus of this paper.
NANOPARTICLE CONTAINING SILICA MATERI-
Rare Earth-doped Silica Fibers Containing Noble Metal
ALS AND OPTICAL FIBERS
Nanoparticles
Recent progress in the field has come from the knowl-
The effect of gold nanocrystallites of 5-60 nm size
edge and experience acquired with glass-ceramics, defined
formed in a matrix of lead-silicate glasses (ruby) has been
as an optically transparent material with a polycrystalline
known to glassmakers for centuries. May be it was the initial
structure prepared by the controlled crystallization of glass
inspiring point for Lin [58] and Watekar [59], who imple-
[51]. The first photosensitive glasses (actually glassceram-
mented Au-nanoparticles along with Er3+ into a germano-
ics) contained a limited amount of Au, catalyzing the forma-
280 Current Nanoscience, 2016, Vol. 12, No. 3 Kasik et al.
silicate core of an optical fiber. A significant enhancement of lite, protoenstatite, phosphates, titanates (pyrochlores), zir-
the Er3+ emission due to the Au-nanoparticles was observed. con, and alkaline earth fluorides. A survey can be seen in
The behavior of silver nanoparticles incorporated into a ger- Table 2.
mano-silicate optical fiber was also investigated with en-
An overture to this research was done by Tang et al., who
couraging results [60].
studied the nanoscale character of soot during optical fiber
Another approach was applied by Fukushima [61], who fabrication [86]. The principle of the performance of these
applied a thin sol-gel Au-nanoparticle and Er3+ containing ceramic nanoparticles in a glassy host matrix can be seen
layer onto the surface of an optical fiber and observed the from (Fig. 6).
enhancement of the Er3+ emission. Nanoparticles of Au or
An extensive systematic study of nano-engineered Yb3+-
Ag applied onto the surface of optical fibers have frequently
doped fibers based on yttrium-aluminium-garnet (YAG)
been utilized in surface plasmon resonance sensors [62, 63];
nanoparticle- (nanocrystal-) containing cores was completed
however, this issue is beyond the scope of this paper.
by Paul et al. [81]. Materials based on nanoparticles of
Al O , ZrO , MgO (CaO), earth alkaline fluorides, and pyro-
2 3 2
Rare Earth-doped Silica Fibers Containing Semiconduc-
chlores were studied only to a small degree.
tor Nanoparticles
On the basis of these summarized findings, the author of
Initial results using noble metal nanoparticles encouraged this review proposes an idea of a possible structure of a ce-
Moon and Watekar to implement silicon nanoparticles along ramic nanoparticle-containing fiber core material (Fig. 7).
with Er3+ into the core of a silica fiber [64]. A significant
enhancement of the Er3+ emission in both VIS and NIR was
TECHNOLOGY OF PREPARATION OF NANOPAR-
again observed [65, 66].
TICLE-CONTAINING SILICA MATERIALS AND
A study outside of the focus of this paper should also be OPTICAL FIBERS
mentioned, in which a few nm thick semiconductor layer of
The clustering of RE ions in silica and silica-based fibers
Cd P nanoparticles was applied into a piece of a holey core
3 2 has been mentioned above as a general problem impairing
silica fiber, and a non-negligible gain was observed [67].
the fluorescence properties of active fibers. These clustering
effects arise during the high-temperature stage of the prepa-
Rare Earth-doped Silica Fibers Containing Ceramic
ration processes when the host glassy matrix and substrate
Nanoparticles
are in a molten state. Several approaches have been proposed
Nanoparticle-containing silica or silica-based fibers de- and investigated to solve this problem. The first one, men-
veloped up to now have been based on RE-doped (Er, Yb, tioned at the end of introduction, is based on dissolving the
Eu, Tm) nanoparticles of yttrium-aluminium silicates, mul- RE ions in a matrix modified with Al2O3, P2O5, or other
Table 2. Summary of the nanoparticle-containing silica or silica-based Rare Earth doped fibers (materials) prepared to date.
RE Nanostructure Host matrix Reference
MgO (CaO) GeO-SiO
2 2
Er [68-71]
(protoenstatite) GeO-PO-SiO
2 2 5 2
ZrO, AlO BaO-LiO-SiO
2 2 3 2 2
Er [72-74],
(zircon silicate, mullite)
AlO PO-SiO [75]
2 3 2 5 2
Er
(mullite) SiO [76, 77]
2
AlO
2 3
Yb SiO [78, 79]
2
(mullite)
RE PO
4
Yb YO- AlO-PO-SiO [80]
2 3 2 3 2 5 2
(phosphate)
YAG
Yb BaO-LiO-PO-(F)-SiO [81, 82]
2 2 5 2
(YAsilicate)
Eu CaF, BaF, SrF SiO or - GeO - SiO [83]
2 2 2 2 2 2
RE TiO
2 2 7
Eu SiO [84]
2
(pyrochlore)
AlO
2 3
Tm SiO [85]
2
(mullite)
Silica Optical Fibers Doped with Nanoparticles Current Nanoscience, 2016, Vol. 12, No. 3 281
RE
RE
RE RE
RE
RE
RE
RE
RE
RE
RE
RE RE
RE
RE

Fig. (6). Principle of the optical performance of ceramic nanoparticles formed in a disordered glass matrix of silica fibers: a-left) RE ions in
low concentration distributed in silica glass matrix produce weak luminescence, b-mid) an increased concentration of RE ions results in for-
mation of clusters decreasing the luminescence, c-right) the presence of nanoparticles in the host glassy matrix results in a higher concentra-
tion of RE ions that are evenly distributed, resulting in the enhancement of the material luminescence properties.

RE mullite
mullite
mullite
mullite
mullite RE
mullite
RE mullite
mullite

Fig. (7). Possible structure of a ceramic nanoparticle-containing fiber core material: core-shells homogeneously dispersed in a silica glassy
matrix.
Control
Liquids
Burner
Doped
MFC
nano-
MFC
particles
Vapours

Fig. (8). Direct nanoparticle deposition principle.
modifiers. Another way is based on an employment of proc- Sol-Gel Based Approaches to Production of Rare Earth-
esses (e.g. sol-gel method) that maintain a homogeneous RE doped Nanoparticle-Containing Silica
distribution in the host matrix. Nano-phase separation fol-
Recent progress in this field has been achieved using
lowed by formation of nanocrystals can also lead to the pro-
the knowledge and experience acquired during the long
duction of nanoparticles in glassy host silica. A completely
history of sol-gel [87]. The name of the method summa-
unique approach is called Direct Nanoparticle Deposition
rizes its principle: a colloidal solution is transformed at
(DND) [78], in which nanoparticles of 10-100 nm size are
room temperature into gel that is then thermally treated into
directly produced and deposited to form a preform (Fig. 8).
a glassy stay. The sol-gel has widely been used to produce
282 Current Nanoscience, 2016, Vol. 12, No. 3 Kasik et al.
RE–doped, highly homogenous, ultra-pure glasses and thin below the liquidus of both mullite and Al O [98]. At these
2 3
layers for optical components [88, 89]. The application of temperatures there is no viscous flow of the silica matrix (or
sol layer containing synthesized nanoparticles onto the in- the substrate tube) and no crystallization processes can be
ner wall of substrate tube results in the formation of a thin observed. Both the collapse of the preform and fiber drawing
layer. After gelling, a RE–doped nanoparticle-containing usually proceed at 2000 °C. At this temperature the silica
preform for fiber drawing can be prepared via gentle sinter- matrix (and substrate tube) are in a molten state and the SiO
2
ing and collapse of the substrate tube. The formation of reacts with the imbedded Al O nanoparticles. At the end of
2 3
nanoparticles (nanocrystals) is regulated via control of the preform or fiber preparation, nanocrystals of mullite are
thermal process. Sols containing nanoparticles can also be formed with a spatial resolution close to the original one. By
applied into a porous layer deposited onto a glassy sub- using this technique, a mullite phase has been identified in
strate by a chemical vapor deposition (CVD) method as preforms while no alumina phase was observed [77]. The
described in the following section. same principle can be applied for the description of other
ceramic nanoparticle doping. When noble metal nanoparti-
A modification of this method has recently been pre-
cles are applied, no interaction between the host matrix and
sented by Romano [90]. Silica tubes were first filled with
the nanoparticles can be expected and original character of
both sol-gel and granular silica-based materials, and then
the nanoparticles homogeneously dispersed in the material
passed through laser-assisted re-melting and drawn into opti-
stays unchaged.
cal fibers. This approach allows the production of high ho-
mogeneity RE- or transition metal-activated micro-sized This approach has been demonstrated with both Al O -
2 3
particles in the fiber core. It is close to powder sintering Er3+-doped silica fibers [77] and Al O -Tm3+-doped silica
2 3
technology [91]. fibers [85]. A key feature of this approach can be considered
its high flexibility: in principle, any metallic or ceramic-
Incorporation of Nanoparticles into Host Silica Matrix forming nanoparticle doped with any RE can be incorporated
into any silica-based host. Control of the parameters of the
Technologies for the preparation of nanoparticle-
solution doping process and limited number of deposited
containing preforms and silica fibers are based from
core layers can be considered as a weaker point of the ex-
knowledge and experience acquired since the time when
tended MCVD aproach [99].
the first graded-index fiber was presented [92]. This ap-
proach is frequently linked to the Modified Chemical Va-
Engineered Phase Nano-crystallization - Formation of
por Deposition (MCVD) method [93] extended with solu-
Nanocrystals in Silica-based Matrices
tion-doping (Fig. 9) [6, 94-96]. This modification was re-
quired because suitable RE-containing starting materials The technologies for preparation of nanoparticle-
for the process were available only in solid state and un- containing preforms and silica fibers have emerged from
suitable for any CVD. knowledge and experience acquired in the field of glass-
ceramics [51, 52]. In this approach, nano-engineered phase
In this approach, a suspension of nanoparticles of Al O
2 3 separation of the glassy preform core of proper composition
or nanoparticle-containing sols are introduced into a porous
is first performed. Then nanocrystals from the separated
silica layer and later on carefully sintered and collapsed (Fig.
nanophase are formed during preform annealing. This results
9) [84, 97]. As a result, core materials containing nanocrys-
in a core material containing nanocrystals inside the silica or
tals inside the silica or modified silica host matrix are ob-
modified silica host matrix, where its character stays un-
tained. The starting nanoparticles must be of suitable size
changed during fiber drawing (Fig. 10).
and sufficiently durable for the high-temperature processing
during preform collapse and fiber drawing. Preforms for fiber drawing are prepared with a core of
specially designed glass composition. Usually the originally
The mechanism of this approach can be described as fol-
homogeneous glass contains a nucleating agent (like ZrO
lows. The sintering of layers usually proceeds up to 1600°C, 2
[73] or P O [81]), glass modifiers Al O and RE O support-
2 5 2 3 2 3
Nanoparticle Porous Nanoparticle Sintering Fiber
synthesis SiO2 soaking + and drawing
layer drying collapse
deposition
heat heat
Nanoparticles Porous Impregnated Preform Fiber
nanocrystals layer porous layer
Fig. (9). Principle of nanoparticle implementation of extended MCVD.
Silica Optical Fibers Doped with Nanoparticles Current Nanoscience, 2016, Vol. 12, No. 3 283
heat heat
Preform
Nano phase Nano
separation crystallization
Fiber
Glass core Phase separated Nanostructured core
core

Fig. (10). Principle of engineered nano-phase separation and nano-crystallization.
ing the formation of the crystalline host, Li O serving for absorption spectral characterization. In order to obtain more
2
formation of the glass-ceramic based material, and BaO in- information, the sols are then dried and characterized in a
creasing the glass-formation region. A detailed mechanism powder form using X-ray diffraction (XRD), differential
of the formation of RE-doped Yttrium-Aluminum-Silicates thermal analysis (DTA), differential scanning calorimetry
has been previously described in by Paul et al. [81]. (DSC), Raman spectroscopy, fluorescence spectra (steady-
state and/or time-resolved), or transmission electron micros-
Preforms and fibers doped with CaO [68], MgO [70, 71],
copy (TEM). These characterization processes provide valu-
ZrO2 [72-74], and YAGs [81, 100-103] were prepared by
able information for engineering of thermal processes. Addi-
this approach. The formation of YbPO nanocrystallites was
4 tional information about doped porous layer (when preforms
also observed [80]. The precision of this nano-phase separa-
are prepared by extended MCVD) can be obtained from
tion process control (corresponding to a specific preform
SEM images [104].
composition and suitable for LMA fiber preparation) can be
considered as the main advantage of this approach [81];
Material Characterization of Preforms
however, when the composition of the glassy host matrix is
fundamentally changed for some reason, the preparation Preforms produced by any of the methods described
process has to be modified. above can be primarily characterized by optical microscopy,
refractive index profile (RIP), and electron probe micro-
ANALYTICAL AND CHARACTERIZATION METH- analysis (EPMA). Typical features of preforms prepared by
ODS conventional MCVD and extended with solution doping
from starting chlorides and by solution doping from starting
The characterization of nanoparticles inside thin layers or
Al O nanoparticles and ErCl salt can be seen in Fig. (11).
bulk materials like preforms is a difficult task; the charac- 2 3 3
Different features can also be observed from RIP measured
terization inside small core of final active fibers represents a
with typical radial step of 5 (cid:3)m (Fig. 12).
real challenge. Hence, macroscopic samples of starting mate-
rials (e.g. sols) and preforms are usually characterized at The character of RIP corresponds to concentration profile
first, after which some characteristics of drawn fibers are obtained from EPMA. In the case of Cameca SX-100, the
completed and the quality of the product is finally evaluated limit of detection (LOD) of the analyzed elements is around
on the basis of their output lasing performance. 0.1 wt.% (appx. 1000 ppm). LODs are slightly better when
heavy elements like RE are analyzed. The minimum lateral
Characterization of Nanoparticle-Containing Starting step (spot of analysis) is around 1 (cid:3)m. SEM images taken
Materials from secondary electrons can be obtained along with the
local chemical composition analysis; this can be seen from
When preforms are prepared by the incorporation of
Fig. (13). Difficulties may appear only when radiolumines-
nanoparticles into preforms, the starting sols are the first to
cent components in higher concentration are present in the
be characterized, typically by viscosity measurements and
preform core.

Fig. (11). Illustrative microphoto of core of preforms prepared by solution doping from (a - left) starting AlCl, ErCl salts, and (b - right)
3 3
from starting Al O nanoparticles and ErCl salt.
2 3 3
284 Current Nanoscience, 2016, Vol. 12, No. 3 Kasik et al.

0,010
nano AlO + ErCl
2 3 3
AlCl + ErCl
0,008 3 3
0,006
n
Δ
0,004
0,002
0,000
-5 -4 -3 -2 -1 0 1 2 3 4 5
Preform diameter [mm]

Fig. (12). Illustrative RIP of preforms prepared by solution doping from both starting AlCl and ErCl salts as well as from starting Al O
3 3 2 3
nanoparticles and ErCl salts.
3

5
%] 4
ol
m
n [ 3
o AlO
ati RE2 O3
ntr 2 2 3
e
c
on 1
c
nt
a
p 0,1
o
D
0,0
-0,6 -0,4 -0,2 0,0 0,2 0,4 0,6
Preform diameter [mm]

Fig. (13). Local chemical composition obtained by EPMA (a) and SEM image of preform (b) prepared by conventional MCVD extended
with solution doping (illustrative figure).
Structural data of the preforms are usually obtained from talline lattice and the distribution of ions inside the nanopar-
XRD, Raman spectroscopy, and TEM or high-resolution ticles. For such analyses the preform core must usually be
TEM (HRTEM). XRD analysis provides basic information extracted and precisely shaped to an extra-thin tapered sam-
about the presence, structure, and size of the nanocrystals. ple (e.g. by Focused Ion Beam - FIB). Examples of such
Rapid progress in instrumentation has been made from Ra- high-quality images can be found in Paul et al. [81] and in-
man spectroscopy: a powerful tool for structural analysis. fluence of presence of nanoparticles on fluorescence proper-
Recently developed commercial devices can be coupled with ties was concluded from these results. However, no clear
optical microscopes such that the Raman spectroscopy can explanation of mechanism of influence of the local structure
be obtained locally with microscopic resolution. Raman of nanoparticles on fluorescence properties of fibers was
spectroscopy provides information about the structure of the given up to now. Core-shell structures were observed and
nanocrystals; however, the excitation by a Raman laser can their behavior described in starting sols but explanation of
induce spontaneous luminescence of the RE incorporated observed phenomena in glassy matrix is still rather difficult
inside the samples, which can overlap the Raman signal. for today’s analytical methods.
Therefore, the Raman spectroscopy results must be carefully
Absorption properties of preforms can be obtained via
interpreted to prevent interference with any possible lumi-
conventional absorption spectroscopy. Measurements of lu-
nescence. Both of these structural characterization methods
minescence spectra and lifetimes are more complex and de-
are relatively fast and inexpensive, yet they are strongly lim-
pend on the excitation corresponding to the characterized
ited by their sensitivity: the LOD of nanoparticles in matrix
RE. They can be performed using a spectral spectrofluoro-
material is about five percent.
meter. A suitable pulsed source is required for lifetime
Information about the existence, shape, and size of measurements. An extensive study on the photolumines-
nanoparticles in a glassy host matrix can be obtained from cence spectroscopy of RE–doped glasses was presented by
TEM. TEM images are used to verify the distribution of the Righini et al. [37]. Among other REs, europium exhibits
nanoparticles inside the matrix. These analyses are usually unique fluorescence properties that can be used to determine
accompanied by HRTEM analyses of individual nanostruc- the local displacement of Eu3+ ions inside the host matrix
tures. HRTEM results give full information about the crys- [83]. The luminescence of Eu3+ ions is caused by the elec-
Silica Optical Fibers Doped with Nanoparticles Current Nanoscience, 2016, Vol. 12, No. 3 285
tronic transitions from the excited state 7D to the energy Characterization of Optical Fibers
0
level of ground states 7F (J = 0 to 6). The ground state 7F
J 0 Optical fibers drawn from the preforms are primarily
and the main emitting level 5D are non-degenerate. This fact
0 characterized by optical microscopy, RIP (Fig. 14), spectral
means that the number of spectral lines in Stark split of
5D (cid:2)7F transition corresponds to the number of sites in host attenuation, and steady-state or time-resolved fluorescence
0 0 spectra. In principle, RIP and optical microscopy images of
matrix with inequivalent local site symmetry. The transition
5D (cid:2)7F is allowed by the magnetic dipole. The intensity of fibers and of preforms are of the same character. This can be
0 1 seen from a comparison of Fig. (14) and the previously pre-
this transition is practically independent on the local envi-
sented RIPs of preforms. Since both characterization tech-
ronment of the Eu3+ ion in the host matrix. The transition
5D (cid:2)7F is allowed by the electric dipole. This transition niques employ visible light and the size of the fiber core is
0 2 only several micrometers, fine perturbations of RIP evident
strongly depends on the perturbations of the crystal field of
Eu3+ ion including the displacement inside the amorphous in Fig. (12) cannot be observed in detail (below the diffrac-
tion limit).
lattice. The local site symmetry of Eu3+ ions can be evaluated
from the asymmetric factor R defined by the equation: Lateral resolution of EPMA is not sufficient for analysis
of nanoparticles in the fiber core; however, Secondary Ion
I(7F2)
R= (1) Mass Spectrometry of high lateral resolution (nanoSIMS) is
I(7F1) a suitable tool for this purpose [107]. Since it is extremely

difficult to analyze the fiber core by nanoSIMS, TEM, and
where I(7F ), I(7F ) represent the integral intensities of
1 2 HRTEM, fiber characterization is usually based on meas-
5D (cid:2)7F and 5D (cid:2)7F transitions, respectively. As the re-
0 1 0 2 urement of spectral characteristics.
sult, the higher is the value of the asymmetric factor the
lower is the site symmetry of the Eu3+ ions. The highest val- Absorption spectra (spectral attenuation) can be deter-
ues can be achieved in glass matrices, in highly distorted mined by the cut back method. In this method, the output
crystalline lattices or on the surface of nanoparticles [105, optical power from a fiber is compared to a different fiber
106]. Hence the employment of europium like a “marker” length (within the proper spectral range) that is excited with
with similar chemistry but spectral properties differing from the same conditions. Since the baseline attenuation of fibers
other REs may contribute to the interpretation of the local is several order lower in magnitude with respect to that of
arrangement in RE-doped nanoparticles. the absorption bands of RE, two sets of entirely different
fiber length are tested (Fig. 15). The presence of nanoparti-
Other supplementary methods like atomic force micros-
cles contributes to a small increase of the baseline attenua-
copy (AFM), dynamic light scattering (DLS), X-ray photoe-
tion of fibers (~10-40 dB/km) [77, 78, 85]).
lectron spectroscopy (XPS), and Fourier transform infrared
spectroscopy (FTIR) can also be used for preform charac- Amplified spontaneous emission (ASE) is a second fun-
terization. damental characterization method of produced fibers. Since
Refractive index profile
0,020
0,015
u.]
Δ n [r.i. 0,010
0,005
0,000
-10 -5 0 5 10
radius [μm]
Fig. (14). Illustrative RIP of fiber prepared by conventional MCVD extended with the solution doping method.

180
160 3H4 Tm3+ doped fibers 0,5 Tm3+ doped fibers
140 wwiitthh onuatn noasntrouscttruurcetu, r ΔeL, Δ=L =10 100cm0cm m]0,4 ΔΔLL == 229988 mm
m] 120 B/ ΔL = 398 m
B/ 3H d
Attenuation [d 1680000 1G4 3F2 3F3 5 3F4 Attenuation [00,,23
40
0,1
20
0 0,0
400 600 800 1000 1200 1400 1600 840 860 880 900 920 940 960 980
Wavelength [nm] Wavelength [nm]

Fig. (15). Illustrative figure of the spectral attenuation of short (a - left) and long (b - right) sample of RE-doped fiber.
286 Current Nanoscience, 2016, Vol. 12, No. 3 Kasik et al.
ASE propagates in the fiber sample in both directions, both existence and the structure of the nanoparticles. HRTEM or
the forward and backward ASE spectra are usually character- SEM imaging is widely used to prove the existence of
ized. The forward ASE is more influenced by reabsorption nanoparticles inside the fiber core [73, 81]. However, the
and therefore it is typically shifted to longer wavelengths existence of nanoparticles is frequently deduced on the basis
[108]. An extremely wide spectrum of ASE (more than 800 of indirect symptoms, usually from the luminescence proper-
nm wide) from a Tm-Ho-doped optical fiber with energy ties of nanoparticle doped fibers which are compared with
transfer from thulium to holmium was demonstrated by the the luminescence properties of the optical fibers prepared by
spectral combination of forward and backward ASE from the the conventional solution doping method. For such a case,
active fiber (Fig. 16) [109]. introduced luminescence properties of Eu3+ ions are really
appreciated.
The lifetime is the third basic fiber characteristic. It has
been shown that the contribution of the ASE must be taken
RESULTS AND CONCLUSION
into account for lifetime measurements (the ASE shortens
the measured fluorescence-decay signal), even in the case of Prepared silica optical fibers with nanoparticle-
side detection of the fluorescence [85]. Lifetime characteri- containing cores were primarily doped with Er3+ or Yb3+;
zation methods are customized to individual cases depending first attempts with Tm3+ or Er3+ u3+ have recently been pub-
on the kind of RE. Fabricated fibers are then characterized in lished. The results are summarized in Table 3.
terms of their lasing characteristics (threshold, slope effi-
Silica optical fibers having nanoparticles in the cores
ciency, power conversion efficiency) and photodarkening;
have been investigated since the end of the first decade of the
these issues exceed the scope of this paper.
millenium [58, 68, 69, 103, 110]. Since that time the issue
On the basis of analytical methods, the existence of a has been systematically investigated by scientists from ORC
nanoparticles inside the core can be deduced from (a) an ex- Southampton, CGCRI Kolkata, LPMC Nice together with
tension of lifetime; from (b) the progress of the Raman spec- CNR-IOM-OGG Grenoble, nLIGHT (formerly Liekki) and
tra; from (c) visualisation of the nanoparticles by TEM and IPE Prague with the aim to develop novel fibers for high-
HRTEM, usually from broadening of normalized fluorescence power lasers and ASE sources.
spectra; and from (d) lasing characteristics when conventional
For conventional RE-doped fiber lasers and amplifiers,
and nanoparticle-containing materials are compared.
trends in this field started from Er-doped devices operating
The list of introduced methods may cause impression that around 1.5 (cid:4)m, to Yb-doped devices operating at 1 (cid:4)m, to
the existence of nanoparticles inside the fibers can be easily most likely Tm- or Ho-doped devices operating at around
proved. In fact, the characterization of nanoparticle doped 2 (cid:4)m. This trend could change due to the availability or un-
optical fibers is a really challenging task. Fiber core diameter availability of starting materials on global market.
usually ranges from 10 (cid:4)m, for single-mode fibers, up to
Materials of active fibers are based on nanoparticles of
40 (cid:4)m for multimode fibers. The nanoparticles are not dis-
yttrium-aluminium silicates (due to the doping with YAG),
tributed fully homogeneously inside the fiber core but with
mullite (due to the Al O doping), protoenstatite (due to the
certain radial and longitudinal distribution depending on the 2 3
MgO (CaO) doping), and phosphates (due to the P O dop-
fabrication process. The extraction of the pure fiber core 2 5
ing). On the basis of the results recently achieved it is hard to
from the silica body is extremely difficult in practice. The
forecast what kind of RE-doped nanoparticles (YAG-,
spatial resolution, together with relatively low effective con-
Al O -, titanate- or alkali-earth-fluoride- based) can be con-
centration of nanoparticles inside the silica matrix, strongly 2 3
sidered the most promising one for future.
limits analytical methods which can be used to prove the
Tm3+ Ho3+
10
0
B]
-10
d
m [
u
ctr -20
e
p
S
-30
Forward ASE
Backward ASE
-40
1400 1600 1800 2000 2200 2400
Wavelength [nm]

Fig. (16). ASE of a Ho3+ and Tm3+-doped fiber.