1. No category

Selective coating of holes in microstructured
optical fiber and its application to in-fiber
absorptive polarizers
X. Zhang,1 R. Wang,1 F. Cox,1 B.T. Kuhlmey,2 and M. C. J. Large1
2
1
Optical Fibre Technology Centre, University of Sydney, Australia
Centre for Ultrahigh-bandwidth Devices for Optical Systems (CUDOS), University of Sydney, Australia
[email protected]
Abstract: An interesting feature of microstructured optical fibers (MOFs)
is that their properties can be adjusted by filling or coating of the holes.
Some applications require selective filling or coating, which has proved
experimentally demanding. We demonstrate selective coating of MOFs with
metal and use it to fabricate an in-fiber absorptive polarizer.
©2007 Optical Society of America
OCIS codes: (060.2310) Fiber optics; (060.2280) Fiber design and fabrication; (240.6680)
Surface plasmons; (999.9999) Photonic crystals.
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1. Introduction
The properties of microstructured optical fibers (MOFs) can be adjusted by filling or coating
of the holes. For example, by injecting high index fluid in the holes, index guiding MOFs can
be turned into photonic bandgap guiding fibers, whose dispersion and transmission bands can
in turn be tuned over a large range [1-3]. Various tunable all-in-fiber devices using the filling
or coating of holes have been demonstrated or suggested, some of which require only selected
holes to be filled or coated [4-8].
While in principle these designs promise a new range of applications in sensing,
polarization control or even routing, filling only selected holes of a MOF has proved to be
experimentally difficult. A few techniques have been suggested in the literature: some rely on
the differential surface tension or capillary forces in holes of different size, and therefore can
only be used when the diameters of filled and unfilled holes are different [9-13]. Other
techniques, such as direct injection using micropipettes [14] or end-face photolithography [15]
have been suggested and demonstrated in the context of planar photonic crystals and
conventional optical fibers respectively. However, it is unclear how far they can be applied for
MOF holes, which by nature have extreme aspect ratios and cross sections of the order of the
micrometer.
The combination of plasmonics and photonics is an emerging field that would benefit
from improvements in coating techniques. This area is attracting growing interest in the MOF
community. Kuhlmey et al [6] have provided a theoretical formalism for metal-coated MOFs
and showed that they exhibit plasmonic resonances usable for filtering, sensing or dispersion
management. Hassani and Skorobogatiy [7,8] analysed in detail designs for a MOF with
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Received 25 Jul 2007; revised 9 Oct 2007; accepted 19 Nov 2007; published 21 Nov 2007
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metallic coatings for a biosensor application. These applications also require the use of
selectively coated holes.
Here we demonstrate a new approach to selectively filling and coating holes of MOFs,
and demonstrate what is to our knowledge the first device based on MOFs having selected
holes coated with metal.
2. Existing techniques
While most of the early work on MOFs focused on optical properties of the microstructure,
there is increasing interest in varying the material properties, and in particular, the role that the
microstructure can play in enhancing material effects.
The first fibers in which different matrix materials were used were produced by the stack
and draw method. Using this technique it is easy to combine different materials within the
structure, provided they can be drawn together. This technique has been used to produce for
example, MOF lasers [16] by using a doped core, all solid bandgap structures [17] and hybrid
guiding structures [18], in which some of the air holes were filled with doped glass.
Liquids have also been used to infiltrate the holes of MOFs after drawing. Filling all of
the holes of the microstructure with a higher index material can convert a regular hexagonal
stacked fiber from an index guiding to photonic bandgap guiding structure [1-3]. If a liquid
crystal is used, this approach allows the properties of the bandgap fiber to be made switchable
[19], although the speed of the response will limit its applicability.
In a recent –and impressive– experiment Sazio et al. [20] showed that various materials
including semiconductors could be incorporated into MOFs using high pressure microfluidic
chemical deposition. Chemical Vapour Deposition (CVD) may also be possible for materials
including silver [21]. While CVD is in principle capable of achieving very high quality films
[22], the very high aspect ratios in fibers make this a difficult task.
Selective filling and/or closing of holes at the fiber stage has received quite significant
attention, particularly because it is generally needed for index guidance in liquids [23,24].
There are however a number of other applications that require these techniques, including the
production of non-uniform tapers (ie, in which the structure varies with length) [25],
birefringent fibers [1] and surface plasmon effects [6-8].
The least demanding of these applications are those which require the closure of holes
which have a very different size to that of the holes which remain open. Surface tension
preferentially closes smaller holes, an approach that has been used extensively, sometimes
with the addition of pressure [11, 12]. Capillary action has provided an alternative approach,
as larger holes are more quickly filled than smaller ones. Use of curable glue in combination
with differential capillary forces allows to generalize this technique to fill any group of
similarly sized holes [10]. However, this class of techniques cannot be used to fill selected
holes having sizes similar to that of holes having to remain empty.
A more versatile, but experimentally difficult approach is the direct injection of fluid
under a microscope [1, 14, 25]. While Intoni et al have demonstrated selective filling of holes
in planar photonic crystals using direct microinjection, the experimental setup requires
mechanical and optical access to both sides of the sample, so that the technique cannot readily
be applied to MOFs [14]. Kerbage et al filled two out of six holes of a MOF [1] with glue, for
a fiber having holes of 30μm tapered down to 8μm. Witkowska et al managed to fill selected
micrometric holes with glue [25].
3. Experimental method
A two-stage draw process routinely used in mPOF fabrication was used in this work. A
primary preform of 70 mm diameter, made from commercially available
polymethylmethacrylate rods, was drilled with the required hole pattern using a computercontrolled mill. This was drawn to a 1 cm diameter intermediate perform or “cane” (see Fig.
1), which was then drawn to fibre.
The aim of our work was to selectively coat the holes of a MOF with silver, for the
demonstration of potential plasmonic effects. While there are many techniques available for
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Received 25 Jul 2007; revised 9 Oct 2007; accepted 19 Nov 2007; published 21 Nov 2007
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producing metallic coatings, only a few have proved to be suitable for the small scale of
microstructured fibers. One possible coating method involves suction and evaporation of
metal nanoparticle mixtures into the fiber. Another is chemical deposition via precipitation
from a reduction reaction. The chemical deposition method for silver has been widely used in
producing of mirrors, and recently has been used to coat hollow waveguides, because of the
ease with which the quality of the silver deposition can be controlled by adjusting reaction
conditions [26].
The coating method we developed exploits the neck-down region of the fiber preform, as
shown in Fig. 1. Industry grade silicone glue was used to selectively block holes on the
perform end of the neck-down prior to solution coating. The method is similar to that
previously used in MOFs using a fiber taper [1]. However, the holes at the large end of the
neck-down have diameters of the order of the millimeter, making the task of selectively glueplugging holes straightforward. Furthermore, the small end of the neckdown region gives
access to the final fiber, with micrometric holes so that as opposed to Kerbage et al’s
technique no further tapering is required to fill micrometric holes.
Fig. 1. The fiber neck-down region
As shown in Fig. 2, the fiber end of the neck-down is connected to a syringe, allowing
suction of the reaction mixture through the unblocked holes. The reaction mixture was an
aqueous solution of 0.933 M dextrose and 0.675 M silver nitrate. These reactant
concentrations were found to be optimal after reaction rate tests with different solute
concentrations. At the preform end, the reaction mixture was maintained at 5º C to prevent
excess deposition of silver and blockage. The fiber end was heated to increase rate of silver
deposition in that area. Suction was maintained for 24 hours. The residual reaction mixture
was removed by suction of water then air through the neck-down for a further 24 hours.
Details of the physical deposition process are described elsewhere in the literature [26].
Selective coating was performed on a 5-fold symmetric microstructured fiber design with
relatively large holes. Figure 3 shows the structure, indicating the coated holes (left) and an
example of the coating (right). Elemental analysis confirmed that the coating was highly
localized in the desired holes. The diameter of the smallest hole in which selective coating
was successfully applied is 5 µm. The silver coatings are granular but form a well-defined,
adhering layer on the inner walls of the fiber structure. Lengths of up to 40 cm were coated by
this method. While the coatings in the holes are “continuous” in the sense that the deposition
is uninterrupted along the length of the fibre, they are rough and granular, as shown in Fig. 3,
and not hence necessarily continuous on the nanometric scale. The detailed characterization of
the physical, chemical and optical properties of the coatings inside the fibre is a delicate task
requiring further work.
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Received 25 Jul 2007; revised 9 Oct 2007; accepted 19 Nov 2007; published 21 Nov 2007
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Syringe
Neck down
Dextrose and
silver nitrate
Water bath
Fig. 2. Set-up used for silver deposition.
Fig. 3. A micrograph of the structure, indicating the two coated holes (top). Elemental analysis
shows the presence of silver (dots in the image). An SEM of the silver surface of a coated hole
is shown on the bottom.
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4. The MOF absorptive polarizer
4.1 Theory
The structure (shown in Figs. 3-5) breaks the C5v symmetry. Thus, the degenerate pair
forming the fundamental mode splits into two modes, with orthogonal polarisations. These
two modes interact differently with the absorptive silver coating, and consequently have
different levels of absorption losses.
We calculate the modes and their overall losses (absorptive and geometric) at a
wavelength of 623.8nm using the multipole method [6,27,28]. The background index is
nb=1.492, the silver index is taken as nag=(0.13461,3.9883). The uncoated holes are air-filled
with nair=1. The other parameters used are: radii of the two holes: r1=8 μm, and r2=4.5 μm;
distance from the fiber’s centre to the centre of the holes 16μm and 27 μm respectively,
wavelength of light 632.8nm. The thickness of the silver coating was assumed to be larger
than several skin-depths, so that the coated holes are simulated as bulk silver.
When uncoated, the structure’s fundamental mode forms a degenerate pair, with
confinement loss below 4x10-3dB/km. Higher order modes of the structure are leaky, having
confinement loss of 3dB/km for the LP1,2 mode and over 745dB/km for all other modes.
Figures 4 shows vector plots of the transverse components of the electric field for the xand y-polarised fundamental modes of our structure. Calculated losses for the x- and ypolarised modes are 7.24dB/m and 1.26dB/m respectively. The metal coating increases the
losses of both the fundamental modes. Here, one higher order mode (LP1,2, x-polarised) has
comparable losses (4.3dB/m), but all other higher-order modes have losses at least an order of
magnitude larger than the y-polarised fundamental mode. The lower loss for y-polarised
modes is attributed to the fact that these have an electric field predominantly parallel to the
large silver coated hole, and therefore cannot couple easily to surface plasmonic modes, while
the x-polarised modes have an electric field predominantly orthogonal to the silver surface
and couple more readily to surface plasmons. This phenomenon appears to be only weakly
resonant, with negligible wavelength dependence. While surface plasmon resonances
generally have strong wavelength dependence, modelling indicates the presence of multiple
resonances which smear out to produce an effect that is fairly constant with wavelength. This
effect is currently under further investigation.
y
x
Fig. 4. x-polarized fundamental mode, with 7.24dB/m losses (left) and y-polarised fundamental
mode, with 1.26dB/m losses (right). The arrow’s colour, length and direction reflect the value
and direction of the transverse electric field. Red – highest value, blue – lowest value. Holes in
red are those coated with silver.
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4.2 Polarizer Measurements
The polarizing properties of a 2 cm long section of the coated fiber were characterized by the
setup shown in Fig. 5. The experiment was designed to have no movable components in front
of the fiber to avoid any shift in the beam position which could change the excitation mode
distribution. Circularly polarized light from a HeNe laser (632.8nm line) was launched into
the coated fiber using a quarter wave plate. At the detection end, a pinhole was used to isolate
the core light exiting the fiber. The intensity of core light at different polarizations over 360
degrees was sampled every 10 degrees via the rotation of a polarizer (P2). The optical power
measured by photodiode 2 without the sample fiber was used as a control. To ensure
attenuation measurements were not affected by drift, both the signal and control readings were
normalized to the total power launched into the system, which was recorded by photodiode 1.
The fiber sample being only 2cm long and maintained in a straight line, bend losses or torsion
effects are assumed to be negligible. The transmission results as a function of the rotation of
P2 are shown in Fig. 6.
Fig. 5. Polarizer measurement setup.
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Received 25 Jul 2007; revised 9 Oct 2007; accepted 19 Nov 2007; published 21 Nov 2007
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Fig. 6. Transmission intensity through the coated fiber, as a function of the rotation of the
polarizer P2 (see Fig. 5).
The coated fiber section is observed to reduce intensity of core light in one polarization
by approximately 50%. The resulting polarization behavior of the fiber reflects both, the
differential attenuation of the two polarization states and possibly their different launch
conditions. Having optimized launching conditions, light is predominantly coupled to the two
polarizations of the fundamental mode, with the LP1,2 being only weakly excited. From the
simulations, if light was coupled solely into the two polarization of the fundamental mode,
differential absorption losses of such a short sample would lead to an extinction ratio between
the two polarizations of only 0.12dB, so that 50 cm would be required to achieve a 3dB
polarization ratio. However, the silver coatings seen in Fig. 3 are clearly rough and are likely
to be partially oxidized and impure, so that higher material losses are expected: The presence
of impurities in silver reduces its conductivity, and hence increases its loss.
Simulations show that the experimentally measured extinction ratio of 3dB for the two
centimeter long sample is consistent with the silver coating having ten times higher material
loss than pure silver. With such high material losses, the x-polarized and y-polarized
fundamental modes suffer losses of about 175dB/m and 19dB/m respectively, leading to
3.5dB and 0.38dB loss for the 2cm sample, while higher order modes (except the x-polarized
LP1,2 type mode with about 2dB loss) have losses higher than 30dB for the same sample.
While more absorptive films increase the differential loss between the x and y polarization
states, they do so at the cost of much greater overall absorption.
We note that the roughness of the surface in the fiber will cause coupling into lossy
higher order modes, which will also increase the loss.
5. Conclusions
A selective coating technique for MOFs has been demonstrated. Improvements in coating
length and quality will be required to make the technique more useful, however it does
demonstrate, to our knowledge for the first time, that selective metal coating in MOFs is
feasible, which enables a variety of new applications.
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Received 25 Jul 2007; revised 9 Oct 2007; accepted 19 Nov 2007; published 21 Nov 2007
26 November 2007 / Vol. 15, No. 24 / OPTICS EXPRESS 16277
A criticism that could be made of our approach is that it is difficult to scale up,
particularly as there are generally only two tapered regions per fiber draw (the neck down
region and the drop off). There are however at least two possible solutions to this problem.
The draw process itself could be modified by periodically changing the parameters so that
each draw produced a series of tapered regions, or the fiber ends could be expanded after
drawing. This could produce a useful increase in structure size that would facilitate selective
hole closure. For polymer MOFs such an expansion is easily produced by localized heating
due to relaxation of stress in the drawn fiber (Fig. 7). A similar effect may be feasible in silica
MOFs by the use of localized heating and pressure.
Fig. 7. End-face of a polymer MOF expanded using heat treatment. Photograph courtesy
Thomas Plochberger, OFTC.
Acknowledgements
The authors wish to acknowledge the Australian Research Council for funding. We also wish
to acknowledge Steven Manos for the fiber design, and Steve Bosi and Ross McPhedran for
helpful suggestions, and Thomas Plochberger for providing Fig. 7. We thank Sue Law for her
assistance with the silver deposition.
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Received 25 Jul 2007; revised 9 Oct 2007; accepted 19 Nov 2007; published 21 Nov 2007
26 November 2007 / Vol. 15, No. 24 / OPTICS EXPRESS 16278