Silver metaphosphate glass wires inside silica fibers—a new approach for hybrid optical fibers

Abstract

Phosphate glasses represent promising candidates for next-generation photonic devices due to their unique characteristics, such as vastly tunable optical properties, and high rare earth solubility. Here we show that silver metaphosphate wires with bulk optical properties and diameters as small as 2 µm can be integrated into silica fibers using pressure-assisted melt filling. By analyzing two types of hybrid metaphosphate-silica fibers, we show that the filled metaphosphate glass has only negligible higher attenuation and a refractive index that is identical to the bulk material. The presented results pave the way towards new fiber-type optical devices relying on metaphosphate glasses, which are promising materials for applications in nonlinear optics, sensing and spectral filtering.

Full text

Silver metaphosphate glass wires inside silica
fibers—a new approach for hybrid optical fibers
Chhavi Jain,1,3,* Bruno. P. Rodrigues,2 Torsten Wieduwilt,1 Jens Kobelke,1 Lothar
Wondraczek,2 and Markus. A. Schmidt1,2,3
1Leibniz Institute of Photonic Technology, Albert-Einstein-Str. 9, 07745 Jena, Germany
2Otto Schott Institute of Materials Research (OSIM), Friedrich Schiller University of Jena, Fraunhoferstrasse 6,
07743 Jena, Germany
3Abbe Center of Photonics, Friedrich Schiller University Jena, Max-Wien-Platz 1, 07743 Jena, Germany
*chhavi.jain@leibniz-jena.de
Abstract: Phosphate glasses represent promising candidates for next-
generation photonic devices due to their unique characteristics, such as
vastly tunable optical properties, and high rare earth solubility. Here we
show that silver metaphosphate wires with bulk optical properties and
diameters as small as 2 µm can be integrated into silica fibers using
pressure-assisted melt filling. By analyzing two types of hybrid
metaphosphate-silica fibers, we show that the filled metaphosphate glass
has only negligible higher attenuation and a refractive index that is identical
to the bulk material. The presented results pave the way towards new fiber-
type optical devices relying on metaphosphate glasses, which are promising
materials for applications in nonlinear optics, sensing and spectral filtering.
©2016 Optical Society of America
OCIS codes: (130.3130) Integrated optics materials; (130.7408) Wavelength filtering devices;
(060.4005) Microstructured fibers.
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1. Introduction
One current trend in fiber optics is directed towards fully integrating the functionalities of
planar photonic devices into fibers [1–3], with the vision to establish an all-fiber photonic
platform. This is often achieved by the inclusion of customized materials into silica host
fibers, leading to hybrid optical fibers (HOFs) with unprecedented properties and
functionalities which are compatible with the state-of-the-art silica fiber circuitry. Various
implementation approaches are being currently used to incorporate “unusual” materials into
silica fibers, which include high-pressure chemical vapor deposition (HPCVD) [4–6], direct
fiber drawing [7–9] or pressure-assisted melt filling (PAMF) [10,11]. Due to its flexibility, the
latter approach (details can be found in [10]) has proven useful for placing chalcogenides
[12,13], tellurites [14], semiconductors [15] or even metallic nanowires (NWs) into capillaries
and photonic crystal fibers (PCFs) [16,17]. These resulting devices are used for applications
such as broadband mid infrared supercontinuum generation [18] or ultrahigh extinction
photonic band gap guidance [19].
From the perspective of waveguiding applications, the integration of sophisticated glasses
into silica fiber can impose additional optical attenuation compared to the bulk material due to
a combination of various effects (e.g., Rayleigh scattering, scattering at cracks, bubbles or
interfacial in-homogeneities or chemical reactions at the material boundaries). For this reason,
it is of high importance to seek dielectric glasses that show bulk optical properties and in
particular low optical attenuation when filled into silica fibers. Moreover, low optical
attenuation allows usage of long fiber ‘effective’ lengths which ensure high efficiency of
nonlinear effects in fibers [20]. Here we show that we can incorporate metaphosphate (MP)
wires with diameters below 2 µm into silica fibers using PAMF, that act as high index
contrast and low loss step index fiber (SIF) waveguides.
The distinctive advantage of phosphate glasses is that they are excellent network-forming
glass structures with low softening and melting points [21]. They can be easily mixed with
alkali, alkaline earth and transition metal oxides for improved chemical stability, tuning their
ionic conductivity and tailoring their dielectric functions [22–24]. They provide higher
solubility for rare earth ions (e.g., Er3+, Yb3+ and Nd3+) than silica without detrimental
clustering effects, which makes them particularly attractive for applications in light
generation, i.e., fiber lasers and amplifiers [25–29]. Furthermore, certain phosphate glasses
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Received 29 Oct 2015; revised 6 Jan 2016; accepted 19 Jan 2016; published 9 Feb 2016
© 2016 OSA
22 Feb 2016 | Vol. 24, No. 4 | DOI:10.1364/OE.24.003258 | OPTICS EXPRESS 3260

exhibit negative thermo-optic coefficients, which can be used to compensate thermal drift
[30,31].
In this work, several centimeters long continuous MP-wires with micrometer diameters
have been integrated into two types of silica fiber hosts, namely MP-SIFs and dual core
directional mode couplers, having one MP-glass and one GeO2-doped silica core. We show
that filled MP-glass has bulk optical properties in terms of refractive index (RI) and optical
attenuation, proving that PAMF does not alter the glass properties inside silica
microstructured fibers. Since the MP-glass is encapsulated within a silica host, it remains
protected from environmental influences [32]. As a result, PAMF represents a promising path
towards making hygroscopic phosphate glasses accessible for photonics.
2. Material and experimental section
We use silver MP-glass (nominal composition: AgPO3) as an example for filling a phosphate
glass into fiber. This binary MP-glass is comprised of corner-linked PO4 tetrahedra covalently
connected via bridging oxygens while the Ag+-ions are located between the phosphate chains,
anchoring the surrounding non-bridging oxygens via mostly ionic bonds. It has a low glass
transition temperature of Tg ~192°C [33]. We estimate the dynamic viscosity to be 0.097 Pa.s
at 700°C from a series of capillary filling experiments (this procedure is described in [34]),
which is in accordance with literature values [35]. At such low temperatures, silica remains
chemically inert, and hence a chemical reaction between the MP-glass and the silica host can
be excluded a priori, which we justify during our studies using Raman spectroscopy and cut-
back measurements. Moreover, silver MP-glasses allow the growth of plasmonically active
nanoparticles inside the glass upon UV irradiation [35] or thermal poling [36], which results
from the high intrinsic ionic conductivity [37]. Higher ionic conductivities can be achieved by
doping AgPO3 glass with monovalent metal iodides (e.g., Li, Na, K, Rb, Cs and Tl, forming
(MI)0.2-(AgPO3)0.8 systems) [38]. For instance, the glass system AgI-AgPO3 has been
extensively studied and showed ionic conductivities of the order of 10−2 S/cm at room
temperature [37].
AgPO3 glass samples were prepared using batches of reagent grade silver nitrate (AgNO3)
and dibasic ammonium phosphate ((NH4)2HPO4), thoroughly mixed and melted at 500°C for
2 hours in Pyrex containers. Bulk samples were prepared via quenching the melt and further
annealing at 180°C for 30 minutes to relax internal stresses, followed by polishing to obtain 2
mm thick glass disks. Since the glass is hygroscopic, all samples were prepared, grinded and
polished using isopropyl alcohol instead of water. The optical attenuation of these disks was
measured using a UV-VIS spectrometer (Lambda 950 Perkin Elmer) taking into account the
Fresnel reflections. The RI dispersion was determined using a variable-angle spectroscopic
ellipsometer (SE850 SENTECH) at two angles of incidence (55° and 65°) at room
temperature. The resulting data was fitted with a one-term Sellmeier function, in accordance
with literature [22] (details of the mathematical model can be found in the Appendix). For
PAMF, fiber-like cylindrical pieces of AgPO3 samples were made by fast pulling the glass
from the melt and cutting out pieces that had diameters between 80 and 120 μm and lengths
between 1 and 5 mm. Raman microscopy was used to investigate the structural aspects of the
prepared bulk and filled samples (Renishaw Raman microscope with an Argon laser source
(excitation wavelength 488 nm).
Two types of silica host fibers were prepared using in-house fiber fabrication facilities
(outer diameters: 200 µm): (i) silica capillaries with an inner diameter of 2.4 μm and (ii) a
modified graded index fiber (MGIF) with a central graded index GeO2-doped core (diameter
1.9 µm, peak doping level 11 mol%) and a parallel hollow channel (diameter: 2.28 µm),
separated by a defined lateral distance (center-to-center distance: 3.9 µm). Both types of silica
fibers were filled with AgPO3 using PAMF which involves inserting the mentioned
cylindrical AgPO3 pieces into an auxiliary large-hole capillary and splicing this to the fiber to
be filled. This was followed by pressing the MP-glass at 700°C into the empty bore using
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Received 29 Oct 2015; revised 6 Jan 2016; accepted 19 Jan 2016; published 9 Feb 2016
© 2016 OSA
22 Feb 2016 | Vol. 24, No. 4 | DOI:10.1364/OE.24.003258 | OPTICS EXPRESS 3261

Argon gas applied at the open end of the large-hole capillary [10,34]. This technique allows
the creation of large-aspect ratio MP-wires inside the silica host with micrometer diameters
and lengths of several centimeters (i.e., the strands have aspect ratios of ~5·104).
Using the above mentioned host fibers and PAMF, two types of MP-enhanced fibers were
implemented here: (i) a high RI-contrast SIF with an AgPO3 core and silica cladding (referred
as MP-SIF, Fig. 1(a) and 1(b), a dual-core directional mode coupler established by filling
AgPO3-glass into the mentioned silica MGIF (referred as MP-MGIF, Fig. 1(b). Since MP-
glass wets silica (contact angle < 90°), no critical pressure has to be overcome to initiate the
filling process [11,39]. The filling dynamics are determined by the Lucas-Washburn law [40]
with negligible capillary forces, since pressures above 4 bars are applied in all our
experiments (This estimation is based on the surface tension of Ba(PO3)2 (0.250 N/m)) [41],
since the value for AgPO3 is unknown. Due to its low viscosity, very small strand diameters
are feasible under realistic experimental conditions. For instance, 10 cm long filled MP-
strands with diameters below 1 µm can be achieved by applying pressures above 100 bar for
only one hour (Fig. 1(c)). Even strand diameters below 200 nm can be obtained if the pressure
exceeds 1000 bar (which our current system is capable of providing). Moreover, the side
imaging of the filled AgPO3-strand (example of such a strand is shown in Fig. 1(d)) reveals
that the metaphosphate strand is in physical contact with the silica at all points along the fiber
and that no delamination is occurring
Fig. 1. Schematics of (a) the step-index fiber with metaphosphate core and silica cladding and
(b) the dual-core directional mode coupler with GeO2-doped core (green) and high refractive
index AgPO3 strand (red). The purple arrows indicate the input light. (c) Figure-of-merit
calculation showing the minimal hole diameter which can be filled with AgPO3 glass within
one hour for a length of 10 cm as function of applied pressure. Inset: a filled metaphosphate
strand (diameter 2.38 µm). The grainy appearance of the metaphosphate is a result of the gold
sputtered onto the sample to prevent charging. (d) Microscopic side image of a continuous
AgPO3 strand (diameter: 2.4 µm) in silica.
The optical characterization of our fibers was performed using a combination of a
broadband light source and a transmission setup (Fig. 2). The polarization of the incident light
(provided by broadband supercontinuum source (450 nm-2.4 µm, NKT superK)) was
controlled using a thin film linear polarizer and a half-waveplate both located in front of the
incoupling objective. This light was coupled into the samples using a 20x and a 40x
microscope objective for the MP-MGIF and MP-SIF samples, respectively. The transmitted
light was out-coupled using another 20x objective and characterized using either an optical
spectrum analyzer (OSA, Yokogawa AQ-6315A) or a CCD camera. Undesired cladding light
was blocked using an iris diaphragm in front of the final objective, which couples the output
light to a multimode fiber (MMF) connected to the OSA.
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Received 29 Oct 2015; revised 6 Jan 2016; accepted 19 Jan 2016; published 9 Feb 2016
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22 Feb 2016 | Vol. 24, No. 4 | DOI:10.1364/OE.24.003258 | OPTICS EXPRESS 3262

Fig. 2. Schematic of the transmission setup used for the optical characterization of the fiber
samples (pol.: polarizer, λ/2: half wave plate, OBJ: objective, CCD: camera, OSA: optical
spectrum analyzer, MMF: multimode fiber). The red arrows indicate the direction of the light
beam. To illustrate the principle of the measurement this schematic includes the MP-MGIF
structure as an example sample.
The modal attenuation of the fundamental mode of the MP-SIF sample was measured by
the cut back technique. For this measurement, light was launched into the AgPO3 filled core
and four cut-back steps were performed on an initial 7 cm long sample. Imaging the output
light onto the camera confirmed that in all our studies, we observe light in the fundamental
mode only. MP-MGIF samples had a total length of about 5 cm with the filled section being 2
cm long. The unfilled section in the first part of the sample has two functions: (i) it ensures
the largest possible modal overlap of the modes in the filled and unfilled sections, i.e. optimal
in-coupling conditions, and (ii) it naturally avoids optically induced damage of the AgPO3 end
face. The measurements were performed taking into account the birefringence of the MP-
MGIF (x-pol.: electric field parallel to the symmetry axis of the MGIF; y-pol.: perpendicular
polarization state) and the data was normalized with respect to an unfilled MGIF, thus
yielding the transmission of the GeO2 mode. In the OSA measurements, we used out-coupling
objective with relatively small numerical aperture (NA = 0.4). Thus, we only detect light from
the GeO2-doped core while the AgPO3 strand modes having high numerical apertures (NA =
0.8) are not captured.
The dispersion of the fundamental HE11 mode of the MGIF sample was simulated using a
commercial finite element solver (COMSOL), whereas the higher order AgPO3-modes were
analyzed by solving the transcendental dispersion equation of a cylindrical SIF [42]. All
simulations include the measured bulk material dispersion of AgPO3 and the RI of doped and
undoped silica from literature [43]. The spatial distributions of the Poynting vector of the
MGIF-structures at resonance have been calculated using COMSOL including the exact
geometry obtained from SEM.
3. Results and discussion
The ellipsometric data show that bulk AgPO3 glass (network structure shown in inset of Fig. 3
(a)) has a RI between 1.68 and 1.69 in the visible (VIS) (Fig. 3(a)), which results from
electronic absorption below 400 nm [32]. Using the one term Sellmeier ansatz (Eq. (1)) to fit
the experimental data (R2 = 0.90) a good overlap with the experimental data was obtained
(details in Appendix). As a result bulk AgPO3 glass exhibits a comparable “mild” material
dispersion and due to its high RI, our AgPO3 strands form high contrast waveguides (C = (nco
- ncl)/(nco + ncl) = 0.0715 at 600 nm) which can support more than 24 modes throughout the
VIS part of the spectrum.
Figure 3(b) shows a comparison of the different Raman spectra of pure SiO2 (red), MP-
SIF (blue) and bulk AgPO3 (dark yellow). The two key Raman signals in bulk AgPO3 and in
the MP-SIF are associated with symmetrical stretching vibrations of terminal PO2- group (at
1140 cm−1) and with symmetric stretching vibrations of phosphate backbone O-P-O units (at
675 cm−1), indicating the presence of phosphate groups [33]. All features observed in the
spectrum of bulk SiO2 are also found in that of the MP-SIF sample (signals at 800 cm−1 and
605 cm−1 which arise from Si-O-Si bending and SiO2 defects [44]), since the incoming laser
light propagates through the 100 µm thick silica cladding before reaching the MP-strand, thus
#252667
Received 29 Oct 2015; revised 6 Jan 2016; accepted 19 Jan 2016; published 9 Feb 2016
© 2016 OSA
22 Feb 2016 | Vol. 24, No. 4 | DOI:10.1364/OE.24.003258 | OPTICS EXPRESS 3263

producing residual SiO2 Raman signal. However, we did not observe features present in the
MP-SIF spectra that can be assigned to either Si-O-P bonds or to crystalline silver
metaphosphate, giving strong support for the assumption that both glasses do not react during
the PAMF process.
Fig. 3. (a) Material refractive index of bulk AgPO3 glass measured using ellipsometry. Inset:
2D-schematic of the network of AgPO3 glass. (b) Comparison of the different Raman spectra
of pure SiO2 glass (red), bulk AgPO3 glass (dark yellow) and of the filled AgPO3 strand (blue).
The Raman excitation wavelength is 488 nm.
As a first optical characterization step, the modal attenuation of the fundamental mode of
MP-SIFs (Figs. 4(a) and 4(b)) was measured and compared to the bulk absorption of AgPO3
in the various spectral regimes. In the VIS, the attenuation is only about 7% higher than of the
bulk glass (Fig. 4(a)), which is almost a negligible difference compared to other filled glass
systems [14]. An increasing absorption starting at around 1.5 μm is observed in the bulk
sample (inset of Fig. 4(b)), which is associated with small traces of water contaminants [45]
inside AgPO3. Phosphate glasses are known to be highly reactive to water, especially during
synthesis [46–49]. This can be one origin of the higher attenuation measured in the waveguide
than in the bulk glass, as PAMF represents an additional preparation step in which the sample
is exposed to the atmosphere for at least a short time period. Current experiments aim to
minimize this effect by keeping the samples in a dry environment during all the above
mentioned fabrication steps. Another source of additional losses might be interface roughness,
which results from frozen-in surface-capillary waves at the surface of the silica wall induced
by the high temperature drawing process (2000°C) of the silica fibers [50] However, it was
shown experimentally that the corresponding roughness amplitude is within the nanometer
scale, and hence this effect is presumably irrelevant on the length scales discussed here.
Intuitively, the mismatch of the thermal expansion coefficients between typical
metaphosphate glasses and silica (αmetaphosphate = 4·10−5K−1 [41] and αsilica = 0.5·10−6 K−1 [51])
may suggest the appearance of air gaps between the silica wall and the AgPO3 strand,
imposed by the high temperatures. However, no delamination of the AgPO3 strand from silica
capillary occurs (Fig. 1(d)), which would have otherwise raised the waveguide loss to
immeasurable high values. The lack of delamination has also been observed in chalcogenide-
silica systems, which show an even larger mismatch (for instance see [19]). The gap between
the metaphosphate and the silica observed in Fig. 1(c) is induced by the cleaving process and
does not result from the filling process or a delamination of the AgPO3 from the silica.
Overall, it is evident that the modal attenuation of MP-SIFs is comparable to that of the bulk
MP-glass, which makes PAMF a promising approach for creating waveguides made from
amorphous phosphate materials, especially when taking into account their hygroscopic nature.
#252667
Received 29 Oct 2015; revised 6 Jan 2016; accepted 19 Jan 2016; published 9 Feb 2016
© 2016 OSA
22 Feb 2016 | Vol. 24, No. 4 | DOI:10.1364/OE.24.003258 | OPTICS EXPRESS 3264

Fig. 4. Modal attenuation of the fundamental core mode of the metaphosphate-silica step index
fiber (diameter of the metaphosphate strand: 2.4 µm) compared to the extinction of bulk
AgPO3 glass in (a) the visible and (b) the near infrared spectral domain. The insets in both
figures show the extinction of the bulk metaphosphate glass in an extended spectral interval.
The image in (a) is the output mode of a 7 cm long sample at a wavelength of 650 nm.
The next step in the optical analysis was the characterization of the MP-MGIF structures,
in which a series of sharp transmission dips are observed in both polarization states. These
dips are associated with a phase-matching of higher-order MP-modes with the fundamental
mode of the GeO2-doped core, which can be seen when comparing the effective index
distributions of all involved modes (Fig. 5(a), plotted is the relative effective mode index (neff
- nsilica)) with the measured transmission spectra (Fig. 5(b) showing dip center wavelengths:
475 nm, 530 nm, 612 nm, 720 nm). Exactly at these positions, the effective indices of the
AgPO3-modes cross the dispersion of the GeO2 core mode (blue dashed line in Fig. 5(a)) and
transmission minima are observed. It is important to note that the dispersions of the AgPO3-
modes are highly sensitive to the material’s RI since slight changes in the index cause a
substantial modification of the respective neff. Since our simulations include the bulk RI of
AgPO3 (Fig. 3(a)), the excellent agreement between the phase-matching wavelengths and
transmission dips indicate that the RI of the MP-strand is identical to that of the bulk glass.
The dispersion of the various higher-order AgPO3-modes are not evenly distributed but are
rather clustered in groups [42], and thus we observe multiple resonances within narrow
spectral intervals.
The maximum resonance extinction is about 25 dB with a FWHM (Full Width Half
Maximum) bandwidth of only 3 nm (resonance at 720 nm). Therefore, the MP-MGIF system
can act as an efficient and narrow bandwidth fiber-integrated notch filter, whereas the
resonance wavelengths can be selected via tuning the strand diameters [52]. The observed
reduction of the dip extinction towards shorter wavelength results from a decreasing overlap
of the evanescent fields of the two involved modes and thus a reducing coupling constant.
#252667
Received 29 Oct 2015; revised 6 Jan 2016; accepted 19 Jan 2016; published 9 Feb 2016
© 2016 OSA
22 Feb 2016 | Vol. 24, No. 4 | DOI:10.1364/OE.24.003258 | OPTICS EXPRESS 3265

Fig. 5. The optical properties of the dual core directional mode coupler consisting of one
continuous metaphosphate strand and one GeO2-doped silica core. (a) Real parts of relative
effective mode indices for the different modes involved (blue dashed line: fundamental GeO2-
doped core mode, solid lines: higher-order AgPO3 strand modes (labels in the plot refer to the
mode nomenclature in [42]). The grey dashed line refers to the situation of the effective index
matching the cladding index, i.e. represents the cut-off line. (b) Spectral distribution of the
transmission of the two Eigenmodes of the MP-MGIF (blue: x-pol., dark yellow: y-pol.). The
colored bars indicate the spectral intervals the transmission dips coincide with the phase-
matching points. Inset: definition of the coordinate system. The red dots (referring to x-pol.)
highlight the wavelengths at which the MP-MGIF transmitted modes were imaged onto the
camera (shown on the right handed side, 605 nm and 710 nm). The corresponding simulations
of the spatial distribution of the Poynting vector (saturated linear scale) are also shown below
the respective experimental image in the right (blue circles indicate the GeO2-doped core
boundary).
In the vicinity of the two long-wavelength resonances, the output light emitted from the
two cores was spectrally filtered using narrow bandpass filters (spectral bandwidth 3 nm) and
projected onto the CCD camera. The resulting near-field images clearly confirm the coupling
between the mode in the GeO2-core and the higher-order modes in the AgPO3-strand (right
handed images in Fig. 5). Here, we choose two wavelengths not exactly at the resonance
minima but rather at a transmission level of −5 dB (605 nm and 710 nm) in order to image the
modes of both cores simultaneously. It is important to note that we observe clean and nearly
distortion-free higher-order mode patterns within the AgPO3 glass, which again confirms the
low attenuation of the MP-strand. Corresponding simulations of the Poynting vector
distributions in x-polarization show identical mode patterns, confirming the aforementioned
phase matching process.
Our low-loss dual-core waveguide system effectively constitutes a directional mode
coupler consisting of two non-identical waveguides, in which the electromagnetic energy is
#252667
Received 29 Oct 2015; revised 6 Jan 2016; accepted 19 Jan 2016; published 9 Feb 2016
© 2016 OSA
22 Feb 2016 | Vol. 24, No. 4 | DOI:10.1364/OE.24.003258 | OPTICS EXPRESS 3266

periodically exchanged between the cores along the longitudinal direction [52,53]. At the
transmission dips, a maximum amount of energy is within one defined mode of the AgPO3-
strand, and thus the MP-MGIF structure allows a gentle transfer of electromagnetic energy
into the MP-glass via mode coupling. Thus, very high levels of intensities can be transferred
into the MP-strand which would cause damage of the MP-strand if end-fire coupling is used,
making the directional coupler concept an attractive approach for nonlinear optical
applications (e.g., all-optical switching).
4. Conclusions and outlook
Here we reveal the main optical properties of micrometer-sized MP-strands in
microstructured silica fibers. Using pressure-assisted melt filling, AgPO3-strands with
diameters of 2.4 µm and below have been integrated into silica fibers. The intrinsic low
viscosity of AgPO3 glass allows the fabrication of centimeter-long, continuous cylindrical
strands which act as low-loss high RI waveguides (aspect ratio > 104). Here we show that the
attenuation of the fundamental mode of the MP-SIF is only slightly higher than that of the
corresponding bulk glass. Raman measurements of strand and bulk material indicate no
chemical interaction between the AgPO3 glass and the silica, which is a substantial advantage
of MP-glass compared to other glass systems. The dual-core directional mode coupling fiber
shows distinct resonances with a maximal extinction of 25 dB and a spectral bandwidth of
only 3 nm. The resonances occur exactly at those locations predicted by simulations, which
shows that the RI of the MP-strand is identical to that of the bulk glass, thus revealing
promising applications as narrow bandwidth fiber-integrated notch filters.
AgPO3-glass represents the best glass for PAMF in terms of optical attenuation reported
so far in literature, since all other types of glass such as chalcogenide or tellurite reveal
substantially higher loss when being confined inside such small bores. PAMF thus opens up a
unique path towards making hydroscopic phosphate glasses accessible for photonics and to
fabricate micro- or even nanometer MP-glass structures. Particularly interesting is the option
of post-tuning the optical properties of filled MP-glass via e.g., plasmonic nanoparticle
growth [36], and thus we expect application of our devices in various fields such as spectral
filtering, nonlinear optics or plasmonics.
Appendix: Sellmeier coefficients for AgPO3
The refractive index of AgPO3 was ellipsometrically determined between 300 nm and 900 nm
using a variable-angle spectroscopic ellipsometer (Sentech, SE850). A one term Sellmeier
equation was used to fit the measured data to obtain the refractive index according to the
following equation [22]:
2
2
22
.
1UV
UV
A
n
λ
λλ
=+ − (1)
The Sellmeier coefficient is denoted by AUV and the effective resonance absorption
wavelength in UV region by λUV respectively. For the MP-composition analyzed here, AUV is
1.805 and λUV is 71.83 nm.
Acknowledgments
This work was partly funded by the German Science Foundation (DFG) via grants no. SCHM
2655/3-2, SCHM 2655/6-1 and WO1220/4-2.
#252667
Received 29 Oct 2015; revised 6 Jan 2016; accepted 19 Jan 2016; published 9 Feb 2016
© 2016 OSA
22 Feb 2016 | Vol. 24, No. 4 | DOI:10.1364/OE.24.003258 | OPTICS EXPRESS 3267