Fibre-optic Probe Design with Side-Surface Interface
Makoto Tsubokawa
Waseda University, 2-7 Hibikino, Wakamatsu, Kitakyusyu, Japan
Keywords: Fibre-optic Sensor, Optical Probe, Optical Fibre, Optical Waveguide, Light Concentrator.
Abstract: We proposed a fibre-optic needle probe with a side-surface interface, and evaluated the normalized received
power as the ratio of the signal power to source power by using ray-trace simulation. Our probe with a 1-
mm diameter and 50-mm length demonstrated its performance in the sensing of target objects over a
surrounding space of ~20-mm radius with a normalized received power of 0.01–10%. We also described the
optimized design of the probe for use in water, and a technique used to sense targets distributed along a
fibre axis with simple rotation of the probe.
1 INTRODUCTION
Because of their thin structure and ease of
fabrication, there have been many studies on light
concentrators with acceptance top/side surfaces of
planar waveguides/optical fibres. Most of them are
relevant to the well-known luminescent solar
concentrator (van Sark, 2008; Mcintosh, 2007;
Edelenborsch, 2013); in contrast, simple
concentrator that directly collect illumination light
have not been fully explored because of their low
efficiency (Kim, 2012). However, this simple
scheme appears to be attractive for optical sensor
applications because it may produce versatile
applications by expanding the optical interface from
a point to the surface. As an interesting example, the
integration of optical fibres into textile structures
called photonic textiles has been proposed to realize
multifunctional sensors such as wearable
photodetectors (PDs) or direction sensors of light
beams (Cho, 2010; Rothmaier, 2008; Abouraddy,
2007). We are currently studying this technique to
enable their application as thin optical probes used
to sense the surroundings. In general, such optical
probes need to incorporate micro-lens systems or
several optical fibres, and should sweep over the
target space because a point detector only gets a
point image (Lorenser, 2011; Sampson, 2011; Wang,
2008; Piao, D., 2006; Dam, 2001; Ansari, 1993). If
the detection through the side-surface interface is
applicable, it may not only simplify the sweep
mechanism but also enable us to realize passive
optical probes without the need for precise lens
systems.
In this study, we show a new design of thin fibre-
optic probe with a side surface interface, and
evaluate basic characteristics by performing ray-
trace simulations (LightTools, Synopsys, Inc.). Both
light source (LS) for illuminating target objects and
a PD are attached to the end of an optical fibre, and
the light propagation through a side surface is
adjusted by a Mie scattering layer embedded in the
optical fibre. We show the output signal of the probe
as a function of the size, position and materials
comprising the target object, and finally evaluate the
spatial distribution of the target and its resolution.
2 FIBRE-OPTIC PROBE MODEL
Figure 1 shows the probe model consisting of two
serially concatenated guides and sensor parts. The
parameters used in our simulation for the waveguide
and light source are listed in Table 1. The guide part
has a two-layered structure with an outer coating as
an absorber in which the core guiding input light
from a light source is embedded in the bottom of the
cladding in addition to a normal core. A light source
and PD are attached to the end face of each core.
The sensor part has the scattering part at the bottom
in the cladding, which is connected to the bottom
core in the guide part. Light scattering is observed in
the scattering part (width D) composed of a cluster
of air particles with a volume density ρ. The width D
is defined as the arc length corresponding to the
width of the focus zone for normal incident light and
158
Tsubokawa M..
Fibre-optic Probe Design with Side-Surface Interface.
DOI: 10.5220/0004696301580162
In Proceedings of 2nd International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS-2014), pages 158-162
ISBN: 978-989-758-008-6
Copyright
c
2014 SCITEPRESS (Science and Technology Publications, Lda.)
Figure 1: Fibre-optic probe model.
Figure 2: Normalized received power η
probe
as a function
of distance d
2
and width w
2
/R
cl
.
is calculated using the size and refractive indices of
the optical fibre. The minimal arrangement of the
scattering part effectively generates light
components in the direction of the target and
suppresses excess loss during propagation along the
fibre. Because of reducing D, the high index fibre is
used in the simulation. Input lights are scattered in
the sensor part and a portion of them reaches the
target object owing to the lens effect in cylindrical
optical fibre. Lights reflected at the target partially
re-enter the optical fibre, and are either scattered
again and transmitted to the PD or are escaped to the
outside. As shown in Fig. 1, we assume the square
target object of w
1
2w
2
⨯ δw at distances d
1
and d
2
above the sensor part. Angles ψ is the angular
Table 1: Parameters of fibre-optic probe.
Optical fibre Sensor part Guide part
Core radius
R
co
0.5 m
m
Cladding radius
R
c
l
0.55mm
Refractive index (core) 1.83 1.83
(core for input light) 1.86 1.86
(cladding) 1.83 1.46
Particle radius
r
250 nm (air)
Particle density ρ 2%
Width of scattering part
/
core of input light D
0.07 m
m
, 0.3 mm
(central angle 7.5°, 31°)
Length
s
,
g
50 mm
Light source (LS)
Input light powe
r
1 W
Wavelength 550 n
m
Polarization Rando
m
Position
Embedded in the end of
core for input light
Refractive indices of 1.86 (LaSFN9), 1.83 (LaSFN40),
1.46(SiO2) are assumed. (Schott Glass Inc.)
diameter of the target. For simplicity, we assume the
bottom surface of the target plane is a 96% reflector
as a reference. Here we define the normalized
received power η
probe
as the ratio of light power that
reaches the PD’s surface by way of the target plane
to the light power of the source. In our model, η
probe
estimated by the ray-trace simulation gives results
that are equivalent to the case using incoherent light
without interference effects. Under the Mie
scattering model used in this simulation, the
wavelength dependence is almost negligible over a
broadband spectrum such as natural light; therefore,
we use a monochromatic source of 550-nm
wavelength with a random polarisation instead of a
broadband light source.
3 SIMULATION RESULTS
3.1 Normalized Received Power Η
probe
Figure 2 shows η
probe
, which is dependent on the
distance d
2
and width w
2
. Here d
1
= 0 and w
1
= L
s
=
50 mm. The particle density ρ is assumed to be 2%
because η
probe
has a maximum for this model in our
simulation. Larger ρ causes not only larger scattering
efficiency but also excess loss in light propagation
along a fibre. Under this condition, 22% and 36% of
the source power arrive at the target plane through
the sensor part with and without the 96% reflector,
respectively. Although these are due to scattering
and lens effect in a fibre, it is fairly high efficiency
without imaging optics. We can see in Fig. 2 that
L
s
L
g
Cladding with outer absorber
Sensor part Guide part
PD
Core
Absorbed
LS
d
2
w
1
Scattering part
Target objects
D
R
co
R
cl
x
y
z
2w
2
With /without
reflector
2w
2
Escape
Rays
Reflector
d
1
Escape
x
y
R
co
R
cl
D
Sensor part Guide part
Scattering
part
Core for input light
ψ
δw
W
2
/R
cl
= 1 with 96% reflector
W
2
/R
cl
=1
Cluster of water
particles
W
2
/R
cl
= 1 or 2
W
2
/R
cl
= 0.5
W
2
/R
cl
= 0.1
with 96% reflector
W
2
/R
cl
= 0.1
W
2
/R
cl
=1
Lambertian reflector
0
5
10
15
20
10
1
0.1
0.01
Distance d
2
(mm)
Normalized received power η
probe
(%)
w
1
= L
s
= 50 mm
d
1
= 0 mm
Fibre-opticProbeDesignwithSide-SurfaceInterface
159
η
probe
does not vary much for d
2
2 mm, but above 2
mm, it tends to decrease. If the incident angle
distribution to the target object is uniform, the
number of round-trip rays between the sensor part
and the target is roughly proportional to a half of the
angular diameter
/2 , and then

/2 2 tan

2
⁄
. In fact, the downward
curves in Fig. 2 indicate the presence of a slightly
small slope compared to the variation of
/2
because light components with nearly normal
incidence dominate owing to the lens effect in
optical fibre. On the other hand, η
probe
decreases at a
rate smaller than w
2
/R
cl
because light components
other than normal incidence can re-enter the sensor
part when w
2
/R
cl
< 1. Regarding the received power,
for example, η
probe
is approximately 0.3% and 1.8%
for w
2
/R
cl
= 0.1 and 1, respectively, at d
2
= 2 mm,
when the sensor part has no reflector. These values
are much smaller than the rate of light power that
reaches the target plane as mentioned above, which
suggests that the optical loss after reflection at the
target plane is significant in our model. Actually, 3–
4 dB loss is observed only at the boundary between
the sensor and guide parts, which remains an issue.
When the 96% reflector is attached to the sensor part,
η
probe
was more than doubled, i.e. 0.7% and 5% for
w
2
/R
cl
= 0.1 and 1, respectively, at d
2
= 2 mm. In Fig.
2, we also plot curves for different type of targets—
the Lambertian reflector and cluster of water
particles as a Mie scattering object (δw = 0.5 mm, r
= 250-nm). They indicate that η
probe
is smaller and
more steeply decrease as compared to the case of a
simple target with a reflector. These are seen as the
results from the spread of the reflection angle and
lower reflectivity at the target object. Next, the
dependence of η
probe
on the target length w
1
and
position d
1
along a fibre axis is shown in Fig. 3. We
can see that η
probe
is nearly proportional to w
1
and it
exponentially decreases with an increase in d
1
. This
is because the spread of the incidence angle in the y–
z plane has a similar influence on targets of different
w
1
, and an increase in d
1
remarkably reduces the
transmission loss along the sensor part. From these
results, we can estimate the location when the target
size is given, and vice versa. In fact, if we need to
simultaneously sense both size and location without
a sweep operation, an additional scheme is required.
We discuss this issue in the next section.
3.2 Evaluation under Water
Environment
Next, we consider a case involving the use of our
model in water, e.g. probe biological cells in the
body. Because the refraction angle becomes small
under water conditions, we designed another probe
structure with width D calculated to be 0.3 mm
according to the definition. Figures 4(a) and (b)
show the η
probe
as a function of distance d
2
under the
water condition. Here target objects of 1.1 50 mm
are (a) 96% reflector and (b) examples of Mie
models and biological objects with δw = 1 mm.
We see in Fig. 4(a) that η
probe
is obviously
improved at D = 0.3 mm compared to D = 0.07 mm
in the short d
2
region for the case with/without
reflector on the sensor part. For a distance of d
2
> 5
mm, the advantage decreases when using the sensor
part without the reflector. This is because the
number of scattered rays for the target is increased at
an optimum D (= 0.3 mm), but the number of round-
trip rays decrease as d
2
increases owing to the
Figure 3: Normalized received power η
probe
as a function
of location d
1
and length w
1
.
Figure 4: Normalized received power η
probe
as a function
of d
2
under water conditions. The thickness δw of skin
dermis, epidermis and cluster of air particles are 1 mm.
w
1
= 1 mm
w
1
= 5 mm
w
1
= 10 mm
w
1
= 50 mm
Location of target object d
1
(mm)
0
5
1
0.1
0.01
10 20 30 40 50
Normalized received power η
probe
(%)
Sensor part and target
with 96% reflectors
W
2
/R
cl
= 1
d
2
= 2 mm
D = 0.07 mm
D = 0.3 mm
Skin Epidermis
10
1
0.1
0.01
05 15
20
10
01 2 3 45
10
1
0.1
0.01
Without reflector
Sensor part
with 96% reflector
(b)
Skin Dermis
Cluster of air
particles (r = 250
nm, ρ = 10%)
D = 0.3 mm
Sensor part with
96% reflector
Target object with
96 % reflector
(a)
Target object with
96% reflector
Distance d
2
(mm) Distance d
2
(mm)
Normalized received power η
probe
(%)
PHOTOPTICS2014-InternationalConferenceonPhotonics,OpticsandLaserTechnology
160
Figure 5: Fibre-optic probe with spiral-shaped scattering
part.
degradation of the directional property with the
increase in D. The effect of the directional property
is mitigated by putting a reflector on the sensor part.
In Fig. 4(b), we see that the skin dermis shows a
higher reflectivity compared to the epidermis and
Mie model of air particles (LightTools, Synopsys,
Inc.). These results mainly indicate the differences
of the reflectivity of the objects, which are useful
when discerning target objects.
3.3 Evaluation of Spatially Distributed
Targets
When an optical needle probe with a point detector
is applied to sense a 3D space, it usually needs to be
swept over the target plane; moreover, the
time/frequency domain immersion or OCT
techniques are usually applied to resolve the depth
information. Here we focus on the approach that is
used to simplify the sweep operation and set aside
the techniques of immersion or OCT. Because our
method can extract the information of an object
arranged linearly along the probe, the linear
distribution of an object may be detected without a
complex sweep operation. Figure 5 shows the sensor
part, which has a spiral-shaped scattering part
embedded in cladding. This is the same structure as
the 360° twisted sensor part in Fig. 1. Because the
sensor part has a sharp directional property for the
target plane in the opposite direction (180°) against
the scattering part, it can highly sense the target
plane that is continuously twisted with angle θ
TP
in
Fig. 5. Here θ
TP
= 360d
1
/L
s
°. From a different
viewpoint, we can detect the distribution of the
target plane positioned above the sensor part
(parallelogram of dash-dotted line) when the optical
fibre is rotated by 0° θ
Fibre
< 360°, where θ
Fibre
=
θ
TP
. Figure 6 shows the normalized received power
η
probe
as a function of θ
Fibre
. The parameters are listed
in Table 1. The reflector is similarly twisted with the
scattering part on the sensor part. The curve with
Figure 6: Normalized received power η
probe
as a function
of rotation angle θ
Fibre
.
Figure 7: Curves of normalized received power η
probe
for
the targets with a small gap. Two target planes where w
1
=
2 mm, w
2
/R
cl
= 1, d
1
= 10 mm and d
2
= 2 mm are arranged
in tandem with a gap δd of 1 and 2 mm.
filled circles indicates η
probe
for the target of w
1
= 50
mm, w
2
/R
cl
= 1, d
1
= 0 mm and d
2
= 2 mm, which
corresponds to the maximum η
probe
at θ
Fibre
and
shows a steep decline with an increase in θ
Fibre
. This
tendency is similar to the results for 1 < w
1
< 5 mm
in Fig. 3. Three curves obtained for targets of w
1
= 2,
5 or 10 mm at d
1
= 10 m are also shown in Fig. 6,
and the angles of peak locations and half widths of
three peaks are roughly estimated to be 80, 90 and
110° and 15, 28 and 60°, respectively. On the other
hand, four angles for locations of target edges are
calculated to be 72, 86, 108 and 144°, as shown in
Fig. 6, and thus we can see that the peak locations
d
2
Target plane
Spiral-shaped scattering part
embedded in cladding
y
x
θ
Fiber
Core for input light
θ
TP
Sensor part Guide part
D
0
0.05
0.1
0.15
0.2
0.25
0.3
72°86°108° 144°
Angle variations for target planes
for w
1
= 5 mm
for w
1
= 2 mm
for w
1
= 10 mm
at d
1
= 10 mm
W
1
= 50 mm (= L
s
), d
1
= 0 mm
W
1
= 10 mm, d
1
= 10 mm
d
2
= 2 mm
W
2
/R
cl
= 1
W
1
= 5 mm, d
1
= 10 mm
W
1
= 2 mm, d
1
= 10 mm
Rotation angle of optical fiber probe θ
Fiber
(deg.)
0 30 60 90 120 150 180 210 240 270 300 330 360
Normalized received power η
probe
(%)
0
0.01
0.02
0.03
0.04
0 50 100 150
δd = 2 mm
2
2
δd
10 mm
2 mm
δd = 1 mm
Rotation angle of optical fiber probe θ
Fiber
(deg.)
Normalized received power η
probe
(%)
Fibre-opticProbeDesignwithSide-SurfaceInterface
161
are in good agreement with each other, and that the
half width also agrees well with the target length w
1
in the region of w
1
5 mm. As the three curves are
modulated by the maximum curve (filled symbols),
its peak and width tend to be fuzzy for large w
1
, but
they are somewhat precise for w
1
5 mm.
Next, we evaluate two targets arranged in tandem
with a small gap δd to check the spatial resolution.
Figure 7 shows η
probe
for two targets with δd = 2 mm
and 1 mm, respectively. Here, w
1
= 2 mm and w
2
/R
cl
= 1. Two peaks in the curve for δd = 2 mm are
clearly observed, and the peak interval is estimated
to be 4.2 mm for an angle difference of 30°. Because
the peak location indicates the centre of the target,
the peak interval is approximately given by δd + w
1
and δd is almost the same as real gap of 2 mm. For
the case of δd = 1 mm, we also find two peaks with
an interval of 15°, corresponding to 2.1 mm, which
is slightly smaller than the expected value of 3 mm
(= δd + w
1
), and is caused by the peak shift effect
due to the overlapping of different height pulses.
Consequently, the spatial resolution is estimated to
be a little less than 2 mm in this condition.
4 CONCLUSIONS
We proposed a simple fibre-optic probe with a side-
surface interface against a target space, and
evaluated the basic performance by performing ray-
trace simulations. Our model has a needle shape of
approximately 1-mm diameter and 50-mm length,
which enables us to detect the normalized light
power of 0.01–10% of the source power when target
objects are located in a surrounding cylinder space
with an ~20-mm radius. Under water conditions, the
sensitivity was easily improved by adjusting the size
of the scattering part. Moreover, by using the
directional property in the probe twisted by 360°,
targets distributed along the fibre axis were also
detected with less than 2-mm resolution.
In our future work, we firstly aim to improve the
signal level by optimizing the probe structure and
materials, for example, the shape of the fibre and
particle density distribution of the scattering parts
may bring about desirable effects such as narrow
focus area and smooth sensitivity along a fibre. As
next step aim, we need to incorporate a key
technique such as the OCT technique for our method
to resolve radially distributed targets.
ACKNOWLEDGEMENTS
Part of the research presented in this paper has been
done under JSPS KAKENHI Grant Number
25420346.
REFERENCES
Abouraddy, A. F., 2007. ‘Towards multifunctional fibres
that see, hear, sense and communicate,’ Nature
Materials, vol. 6, pp. 336–347.
Ansari, R. R., 1993. ‘Microemulsion characterization by
the use of a nonvasive backscatter fibre optic probe,’
Appl. Opt., vol. 32, pp. 3822–3827.
Cho, G., 2010. ‘Smart clothing technology and
applications,’ CRC Press, Chap.5.
Dam, J. S., 2001. ‘Fibre-optic probe for noninvasive real-
time determination of tissue optical properties at
multiple wavelengths,’ Appl. Opt., vol. 40, pp. 1155–
1164.
Edelenbosch, O., 2013. ‘Luminescent solar concentrators
with fibre geometry,’ OSA Opt. Express, vol. 21, pp.
503–513.
Kim, J. M., 2012.‘Optical efficiency-concentration ratio
trade-off for a flat panel photovoltaic system with
diffuser type concentrator,’ Solar Energy Materials &
Solar Cells, vol. 103, pp. 35–40.
LightTools, Synopsys, Inc., http://www.opticalres.com/
Lorenser, D., 2011. ‘Ultrathin side-viewing needle probe
for optical coherence tomography,” Opt. Lett., vol. 36,
pp. 3894–3896.
Mcintosh, K. R., 2007. ‘Theoretical comparison of
cylindrical and square-planar luminescent solar
concentrators,’ Appl. Phys. B, vol. 88, pp. 285-290.
Piao, D., 2006. ‘Endoscopic, rapid near-infrared optical
tomography,’ Opt. Lett., vol. 31, pp. 2876–2878.
Rothmaier, M., 2008. ‘Photonic textiles for pulse
oximetry,’ Opt. Express, vol. 16, pp. 12973 -12986.
Sampson, D. D., 2011. ‘Microscope-in-a-needle
technology for deep-tissue 3D imaging,’ International
Conference on Information Photonics (IP), pp. 1–2.
Schott Glass Inc., http://www.schott.com/
Van Sark, W. G. J. H. M., 2008. ‘Luminescent solar
concentrators—a review of recent results,’ Opt.
Express, vol. 16, pp. 21773–21792.
Wang, Q., 2008. ‘Measurement of internal tissue optical
properties at ultraviolet and visible wavelengths:
Development and implementation of a fibreoptic-
based system,’ Opt. Express, vol. 16, pp. 8685–8703.
PHOTOPTICS2014-InternationalConferenceonPhotonics,OpticsandLaserTechnology
162