Characterization of Arc-shaped Side-polished Fiber
H Ahmad, H Hassan, AZ Zulkifli, K Thambiratnam, IS Amiri
Photonics Research Centre, University of Malaya, 50603 Kuala Lumpur,
Malaysia
Corresponding Author: [email protected]
Abstract: In this paper, a characterization of a home-made arc-shaped sidepolished fiber (SPF) is demonstrated. The arc-shaped SPF is made possible by a
polisher design that polishes suspended SMF-28 fibers from the bottom side using
motor wheel technique. Characterization of the fabricated SPFs are made by
applying at different V-bending conditions governed by delta s values to gauge the
SPF power loss and stability towards bending. As delta s increases, the spectral
power shows dropping in a linear manner with slopes of -0.045dB/µm, -0.036
dB/µm and -0.008 dB/µm are attained for different SPF insertion losses of 1.5 dB,
5.27dB and 12.39 dB respectively. In addition, the same SPFs have shown to be
able to recover power up to a level when 200 µm and back-returned of 0 µm
displacements were made. In addition, heat test on an SPF with insertion loss of
11.4 dB has proved that there is no significant spectral change is observed at
different temperature settings up to 60 degrees Celsius. This shows that the
fabricated SPF is highly immune to temperature noise and even when Graphene
oxide (GO) is deposited on a SPF with insertion loss of 1.8 dB. The SPF with the
deposited GO also shows similar result at different V-bending conditions having a
slope of -0.0195 µW/µm with linearity of 0.9984 are obtained.
Keywords: Side-polished fiber, bending, polisher
1.
Introduction
Side-polished fibers (SPFs) are conventional optical fibers that has had its
cladding partially removed on one side, enabling the evanescent field of light
propagating through the fiber to radiate and interact with its surrounding (Ahmad
et al. 2017) This ability to tap into the evanescent field of the propagating beam
has given rise to many of the common devices used in optical system today,
including directional couplers (Bergh et al. 1980), polarizers (Eickhoff 1980) and
even modulators and switches (Markatos et al. 1987). Their core working
principles relies on strong evanescent fields interaction with their surrounding
media either via close-spaced coupling between two fiber cores, metal layered on
polished surface or varying refractive index of a medium layered on a SPF. In
addition to this, the SPF has also been demonstrated as a viable means of
generating ultra-short pulses through the non-linear saturable absorption process
that is made possible when the polished surface is coated with absorptive
materials such as graphene (Song et al. 2010) and graphene oxide, (Jung et al.
2012) carbon nanotubes (Liu et al. 2013; Song et al. 2007) topological insulators
(Kowalczyk et al. 2016) and transition metal dichalcogenides (Sotor et al. 2014;
Mao et al. 2015) and even exotic materials such as black phosphorus (Park et al.
2015). In addition to the afore-mentioned 2- and 3-dimensional materials, the
surface plasmon resonance (SPR) effect can also be induced onto the SPF through
the interaction of free electrons in a semitransparent noble metallic layer and the
light of the signal propagating through the SPF. By inducing the SPR effect onto
the SPF, numerous sensing applications, particularly for biological and chemical
constituents can be realized (Zhao et. al. 2016, Lin et. al 2007 and Esmaeilzadeh
et. al. 2015).
Despite its numerous potential uses however, SPFs are typically not found
in many real-life applications due to their challenging fabrication process. This is
due to the removal of the fiber cladding, which makes the already delicate fiber
mechanically weak and highly brittle (Senosiain et al. 2001; Kurkjian et al. 1989)
and thus prone to breakage during the polishing process. In addressing this issue,
many polishing techniques have been explored in the past, such as etching the
optical fiber while it is still attached to a v-grooved silica block (Tseng,Chen
1992) or the use of a motor wheel to polish one side of the fiber (Hussey,Minelly
1988; Zhao et al. 2015). Of the methods, the motor wheel technique would be the
most desirable as it has a number of crucial advantages over its closest competing
technique. These include the ability to polish in a short duration of time, as well as
a simple and cheap fabrication method. Most importantly however is the that
success rate of this technique is nearly 100%, thus making the fabrication of
devices based on the SPF significantly easier and cost-effective.
In this work, a modified motor wheel polishing technique using a low-cost
6V DC motor, 400 Grit size silicon carbide paper, small polishing wheel diameter
of 1.3 cm and translation stages is demonstrated. The SPF fabricated will exhibit
an arc-shaped rough polished surface with a very short polishing time of less than
40 seconds. The proposed approach takes into account the Δy parameter, which is
the movement of the polishing wheel perpendicular to the fiber being polished. In
this manner, a small amount of pressure is placed on the fiber being polished,
optimizing the contact between the fiber and the polishing wheel and making it
advantageous over similar works such as that done by (Hussey et. al. 1988) and
(Zhao et al. 2015). Moreover, in this work, our SPF will be characterized in terms
of V-bending and temperature sensitivity at different insertion losses. Since the
arc-shaped SPF exhibits a weak and brittle mechanical property, V-bending is
bound to occur at the weakest point along the polished site during rough handling
and also when harsh environment is involved. The SPF will find its practical use
in areas where interactions with the evanescent field is desired, such as in pulse
laser generation, directional coupler and optical sensor.
2.
Fabrication of Arc-Shaped SPF
The polisher setup is illustrated in Figure 1. In this design it is critical that
the single-mode fiber (SMF-28), which is used to fabricate the SPF, be suspended
tightly above the polishing wheel during the polishing process. Two Newport M562-D stage optical alignment with Newport 561-FH fiber holders are used for
this purpose. The SMF-28 is prepared by stripping a 3 cm long portion of the fiber
of its coating, and then suspending it over the polishing wheel as shown in Figure
1, with L0= 5.2 cm, and thus L0/2= 2.6 cm. The SMF-28 is adjusted so that the
center of the stripped portion is located at L0/2 from the edge of either fiber
holders, and cleaned with alcohol to remove dirt and polymer debris. The
polishing wheel was made by wrapping scotch tape around a 6 V DC motor shaft,
followed by double sided tape (3M, 4011M-10 mmx5m/2131009). Then, silicon
carbide paper with grit size of 400 is wrapped around the double sided tape to
create a uniform polishing wheel having a diameter of 1.3 cm. The grit size is
chosen over the other available grit sizes as a lower grit value will make the
surface rougher and affect the SPF. This, a grid size of 400 gives a very smooth
polish. The DC motor used has a maximum speed of 11442 RPM, with a torque of
1.04x 10-3 Nm. The motor and wheel assembly is then secured on a Newport M561D alignment stage such that the polishing assembly lies perfectly
perpendicular to and exactly in between the two translation stages.
Straight
fiber
Bent fiber
Polisher wheel at
∆𝑦 position
∆𝑦
Polisher wheel's position
at a contact with a straight
fiber
Center of the
polishing wheel
Figure 1. Polisher design setup
To polish the fiber, the polisher wheel position is adjusted vertically until
contact can be made between the fiber and the wheel. The movement of the
polishing wheel is done in resolutions of 10 µm, made possible through the use of
Newport SM-13 Vernier micrometer heads incorporated onto the translation stage.
The adjustment was also made in such manner where the fiber is bent where
polishing can be more effective. The amount of bending done to the fiber is
governed by the height from the center of the polisher wheel in respect to the
baseline which is denoted as ∆y as shown in the schematic. Hence, the base line is
defined as an offset center of the polisher height when the fiber is perfectly
straight and touches the polisher.
To monitor the polishing process, a Yokogawa AQ2200-136 tunable laser
source (TLS) is used to launch a signal at 1550 nm and 8 dBm power into one end
of the SMF-28, while the other end of the SMF-28 is connected to a Thorlabs
PM100USB optical power meter (OPM) to measure signal power at the fiber
output. Figure 2 shows transmission loss in the form of a time series during the
polishing process at different ∆y positions. The polishing wheel’s where the
rotation speed is fixed by applying a constant 5 V DC. From Figure 2, it can be
seen that at ∆y= 900 µm, the power of the test signal travelling through the core
begins to drop from the initial value of 0 dBm approximately 15.3 seconds after
the polishing process starts, and quickly reaches – 20 dBm after only 17.2
seconds. At ∆y values of 1000 and 1200 µm, it takes 12.1 seconds and 22.6
seconds for the power of the signal to begin to decrease after the polishing process
begins, and for both cases reaches a power level of -20 dBm after 1.3 and 1.7
seconds respectively. Finally, for the ∆y value of 1400 µm, the power begins to
drop from 0 dBm approximately 32.2 seconds after the polishing process starts,
and is completed at -20 dBm after approximately 34.8 seconds
(a)
(b)
(d)
(c)
(d)
Figure 2. Transmission loss against time polishing at different ∆y positions of a) ∆y= 900 µm,
b) ∆y= 1000 µm, c) ∆y= 1200 µm and d) ∆y= 1400 µm.
The volumetric removal rate of the fiber cladding against the polishing time can
be described as (Tong et. al. 2006):
𝑑𝑉
= 𝐶𝑝 𝑣𝑠 𝐹𝑁
𝑑𝑡
(1)
whereby CΡ is Preston coefficient, 𝑣𝑠 is the surface speed between the fiber and
polisher and 𝐹𝑁 is the normal grinding force. This pattern is shown in Figure 2,
whereby it takes less time for the power of the test signal to drop when Δy = 1000
μm as compared to when Δy =900 μm. However, above Δy = 1000 μm, the time
taken for the power to begin to drop is longer. This is attributed to the increased
friction between the polishing wheel and the fiber as a higher Δy induces more
pressure onto fiber, thus increasing the friction as well. This in turn results in the
speed of the polishing wheel reducing as a result of the extra loading onto the 6V
DC motor, thus reducing its RPM even though the voltage remains fixed. As the
optimum polishing time is obtained for a ∆y value of 1000 m, thus this setting is
taken to fabricate the SPF that is used further in this work.
3.
Characterization of Arc-Shaped SPF against V-bending and
Temperature
To characterize the arc-shaped SPF, SPF’s with different insertion losses
(ILs) were fabricated. Different SPF insertion loss can be attained by discontinue
the polishing process at different polishing time duration via turning off the DC
motor voltage. Also, the polishing time can be delayed by reducing to a lower DC
voltage; therefore, the desired insertion loss can be attained more accurately. By
shifting the polisher away from the fiber and applying alcohol gently on the polish
site allows an accurate measurement of insertion loss. Noted that, we have
observed that without cleaning the SPF with alcohol, an additional loss as high as
0.89 dB can be obtained. Figure 3 shows a microscope image of a fabricated SPF
with an IL of 5.27 dB and a polarization dependent loss (PDL) of 0.59 dB. Also,
the arc-shaped side of the polished fiber can be observed with its polished site to
be approxiamtely 2.08 mm long.
Figure 3. Image capture from microscope of an arc-shaped SPF with insertion loss of 5.27 dB
The robustness of the fabricated SPF is evaluated by testing its V-bending
and temperature responses. Figure 4 shows the experimental setup used to test the
V-bending response of the SPF. The SPF is placed and secured tightly between
two fiber holders with L0= 5.2 cm and the center of the SPF located at a distance
of L0/2= 2.6 cm. The polished site is located at the bottom part of the fiber for the
purpose of this experiment. To create the V-bending conditions, one translation
stage remains fixed in position while the other one moves horizontally, with the
displacement from the initial state denoted as delta-s. The movement of the stages
is accomplished through the use of the micrometer heads, and an amplified Cband spontaneous emission (ASE) source with an output power of about -7.5 dBm
is launched into the SPF as a test signal. The output from the SPF is measured
using an Anritsu MS9740A optical spectrum analyser (OSA) for different delta-s
values. All the spectral measurements are made at a resolution of 0.03 nm and
span of 20 nm.
Figure 4. SPF V-bending experimental arrangement
Delta-s is increased to from 0.0 µm to 200.0 µm at 100.0 µm steps, and
then decreased back to the initial state of 0.0 µm at 100.0 µm steps again. This
measurement analyses the ability of the SPF to maintain its power transmission
characteristics before and after undergoing V-bending. Figure 5 shows the
obtained spectra for three different SPFs with ILs of 1.5 dB, 5.27 dB and 12.39 dB
respectively. Their corresponding PDLs are also measured to be 0.23 dB, 0.59 dB
and 1.39 dB respectively. In the plots, the designation of “forward” after delta s
values means when delta s is in the increased state while “backward” means when
delta s is in a decreased state. It seems that as the delta s is increased, spectrum
power seems to be dropped. Furthermore, no spectral deviations are observed
when delta-s returns back to a value of 0.0 µm displacement, thus proving that the
SPF maintains its transmission characteristics even after V-bending.
Figure 5. ASE spectra of SPFs at different V-bending conditions for forward and backward
translation with SPF insertion losses of a) 1.5 dB b) 5.27 dB and c) 12.39 dB
The V-bending boundary is further increased by extending the delta-s
values to up to 500.0 µm at steps of 100.0 µm. Figure 6 illustrates the spectral
transmission of the same three different SPFs at different delta-s values, with the
corresponding ASE spectrum peak powers plotted in Figure 7. It can be inferred,
that linearity is only observed above a delta-s position of 100 µm. The transition
of the fiber from a straight manner to its initial V-bending condition provides in
larger spectral power change as compared to their subsequent V-bending
conditions. Linearity slopes of -0.045dB/µm, -0.036 dB/ µm and -0.008 dB/ µm
are obtained for the SPF ILs of 1.5 dB, 5.27 dB and 12.39 dB respectively,
indicating that the slope decreases as the IL of the SPF increases.
We also have conducted temperature response test to three different SPFs
with their insertion losses close to the former and results shows no significant in
power change. Figure 8 shows ASE transmission spectrum of an SPF with an
insertion loss of 11.4 dB at different temperature settings.
Figure 6. ASE Transmission spectra of SPFs at different V-bending conditions from delta s= 0 µm
to 500 µm for SPF insertion loss of a) 1.5 dB, b) 5.27 dB and c) 12.39 dB.
Figure 7. Peak power of ASE spectra of side-polished fibers for different insertion losses at
different V-bending conditions.
Figure 8. ASE spectra of SPF with an insertion loss of 11.4 dB at different temperature settings.
The test was conducted by placing the SPF on a glass slide and a hotplate
to provide heat source below the glass slide. A thermocouple is used to measure
the temperature close to the SPF, while the ASE signal source and OSA settings
are kept the same as was fore-mentioned. The result obtained shows no significant
spectrum change from 27oC up to 60oC, which verifies that our fabricated SPFs is
robust to temperature noise.
4.
Characterization of Arc-Shaped SPF with Graphene Oxide
Coating for V-Bending and Temperature Response
This section demonstrates similar test as conducted in the previous section
except now, graphene oxide (GO) is deposited on our fabricated SPF with an
insertion loss of 1.8 dB. The GO was synthesised from Graphite flakes via
chemical oxidation process. The chemical process begins by pouring 3g of
graphite flakes into concentrated 360 ml sulfuric acid, H2SO4 and 40 ml
phosphoric acid, H3PO4 while under stirring in an ice bath. 18 g potassium
permanganate, KMnO4 was added to the mixture slowly and the solution left for
72 hours under stirring and room temperature. An additional 500 ml water, was
added and followed by a slow addition of 15 ml hydrogen peroxide, H2O2 at 30%
under ice bath until the color of the solution changes from dark brown to yellow.
The mixture was washed with 250 ml hydrogen chloride aqueous solution, HCl
1M and followed washing with deionized water, to get the GO dispersion with the
acidity near to pH=7. The deposition method for obtaining the GO coating on the
SPF is given in Figure 9. In this process, the SPF, after being washed with alcohol
and its IL measured, the polishing wheel is removed from the vertical translation
stage and replaced with a cleaned glass slide.
The stage was vertical translated by rotating Newport, SM-13 Vernier
micrometer head until the glass slide touches the SPF which is at the bottom side
of the fiber. Then, 2.5 L amount of GO is drop-cast on the SPF using a 2.5 L
Eppendorf Research pipette. Figure 10 display the process of the GO deposition
on the SPF from the initial of the drop-cast session until the GO is dried on the
SPF. This process is described using the transmission power real-time monitoring
technique using low power of -27.5 dBm ASE light source. This signal is
intentionally kept at a low power to prevent any possible thermophoresis repulsion
as the GO nanoparticles are negatively charged in the water solvent.
Figure 9. Deposition method of Graphene oxide on SPF arrangement
GO starts to dry hard
2.5 𝜇𝐿 drop-cast
session
Dried GO gel on SPF
Glass and fiber separation
Glass and fiber has been completely separated
Figure 10. Transmission power real-time monitoring of GO deposition on SFP with insertion loss
of 1.8 dB
A possible explanation for this is that before the drop-casting of the GO solution,
the core of the SPF is exposed, allowing a substantial amount of light to leak out.
At this juncture, the power of the signal recorded is -27.5 dBm. When the GOsolution is drop-casted, the solution forms an artificial cladding around the core,
thus trapping more light within the core. As the GO solution dries, it begins to
form a thin film around the exposed core, which is seen by the rising signal
power. However, once the thin film solidifies around the exposed core, loss is
induced in the fiber as the GO molecules begin to interact fully with the
propagating signal. At this juncture, the power begins to drop. Finally, the SPF is
slowly lifted from the forming GO thin film, and the power stabilizes. In this
manner, the refractive index of the GO layer plays an important role whereby its
index is higher as compared to the core, and will strip the power away from the
core. The inset shows the microscope image of the deposited GO coating on the
SPF, which gives an insertion loss of 3.02 dB and PDL of 3.71 dB. The distance
between the core and the polished surface is approximately 37.3 μm when the
insertion loss is 1.5 dB, and 4.63 μm when the insertion loss is 5.3 dB.
The V-bending characterization of the SPF with the GO coating is done in
the same manner as the SPF without the GO coating. From Figure 11, it can be
observed that similarly, the power can be recovered after undergoing V-bending
up to delta-s = 200.0 µm. No power deviations are observed for all the delta-s
values and the transmission spectrum observed is similar to that observed for the
SPF without the GO coating.
Figure 11. ASE spectrum of SPF with deposited GO at different V-bending conditions for forward
and backward translations
Figure 12 shows the ASE spectra of the GO coated SPF with V-bending up to
delta-s = 500.0 µm, and the corresponding peak powers of the different delta-s
values are shown in Figure 13. A linear curve is obtained with a slope of -0.0195
µW/µm and linearity of 0.9984.
Figure 12. ASE spectrum of the deposited GO on SPF at different V-bending conditions from
delta s= 0.0 µm to 500 µm
Figure 13. Peak powers of ASE spectra of SPF with deposited GO at different V-bending
conditions
The temperature response of the GO coated SPF is given in Figure 14, and
conducted in the same manner as was done for the SPF without the GO coating.
Figure 14. SPF with deposited GO ASE spectra at different temperatures
The ASE spectra was recorded at different temperatures from 35.4oC up to
60.0oC. From the figure there is no significant spectral change which confirms
even after the GO deposition, our SPF is robust to temperature noise. This makes
the proposed SPF highly suited for sensing applications requiring the
measurement of strain with independence from temperature.
5.
Conclusion
In this work, a new polisher design is proposed and demonstrated for the
efficient and quick fabrication of arc-shaped SPFs. The characterization is done
in terms of V-bending and temperature response. Experimental results show that
our fabricated SPF is highly immune to temperature noise. Moreover, via the Vbending test, the SPF is able to recover power when delta s reaches up to 200 µm
displacement and returned back to 0 µm displacement. Also, as the delta s
increases, spectral power shows dropping in a linear manner at different SPF
insertion loss of 1.5 dB, 5.27dB and 12.39 dB. This similar test was also
conducted when GO is deposited on the SPF. Similar positive results can be also
found here. These results have proven that the fabricated arc-shaped SPFs are
highly practical where it can be used for many useful applications such as for laser
pulse generation, directional coupler and optical fiber sensor where harsh
environment and rough handling are involved.
6.
Acknowledgment
This work was supported by the Ministry of Higher Education, Malaysia
under Grant LRGS (2015) NGOD / UM / KPT and the University of Malaya
under the Grants RP 029A – 15 AFR and RU010 - 2016.
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