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

 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

and width w

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

⨯ 2w

⨯ δw at distances d

and d

above the sensor part. Angles ψ is the angular

Table 1: Parameters of fibre-optic probe.

Optical fibre Sensor part Guide part

Core radius

0.5 m

Cladding radius

0.55mm

Refractive index (core) 1.83 1.83

(core for input light) 1.86 1.86

(cladding) 1.83 1.46

Particle radius

250 nm (air)

Particle density ρ 2%

Width of scattering part

core of input light D

0.07 m

, 0.3 mm

(central angle 7.5°, 31°)

Length

50 mm

Light source (LS)

Input light powe

1 W

Wavelength 550 n

Polarization Rando

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

and width w

. Here d

= 0 and w

= L

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

Cladding with outer absorber

Sensor part Guide part

Core

Absorbed

Scattering part

Target objects

With /without

reflector

Escape

Rays

Reflector

Escape

Sensor part Guide part

Scattering

part

Core for input light

δw

= 1 with 96% reflector

Cluster of water

particles

= 1 or 2

= 0.5

= 0.1

with 96% reflector

= 0.1

Lambertian reflector

0.1

0.01

Distance d

(mm)

Normalized received power η

probe

(%)

= L

= 50 mm

= 0 mm

Fibre-opticProbeDesignwithSide-SurfaceInterface

159

probe

does not vary much for d

≤ 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 









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

because light components

other than normal incidence can re-enter the sensor

part when w

< 1. Regarding the received power,

for example, η

probe

is approximately 0.3% and 1.8%

for w

= 0.1 and 1, respectively, at d

= 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,

probe

was more than doubled, i.e. 0.7% and 5% for

= 0.1 and 1, respectively, at d

= 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

and

position d

along a fibre axis is shown in Fig. 3. We

can see that η

probe

is nearly proportional to w

and it

exponentially decreases with an increase in d

. This

is because the spread of the incidence angle in the y–

z plane has a similar influence on targets of different

, and an increase in d

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

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

region for the case with/without

reflector on the sensor part. For a distance of d

> 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

increases owing to the

Figure 3: Normalized received power η

probe

as a function

of location d

and length w

Figure 4: Normalized received power η

probe

as a function

of d

under water conditions. The thickness δw of skin

dermis, epidermis and cluster of air particles are 1 mm.

= 1 mm

= 5 mm

= 10 mm

= 50 mm

Location of target object d

(mm)

0.1

0.01

10 20 30 40 50

Normalized received power η

probe

(%)

Sensor part and target

with 96% reflectors

= 1

= 2 mm

D = 0.07 mm

D = 0.3 mm

Skin Epidermis

0.1

0.01

05 15

01 2 3 45

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

(mm) Distance d

(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

Fig. 5. Here θ

= 360d

°. 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

. Figure 6 shows the normalized received power

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

2 mm, w

= 1, d

= 10 mm and d

= 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

= 50

mm, w

= 1, d

= 0 mm and d

= 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

< 5 mm

in Fig. 3. Three curves obtained for targets of w

= 2,

5 or 10 mm at d

= 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

Target plane

Spiral-shaped scattering part

embedded in cladding

Fiber

Core for input light

Sensor part Guide part

0.05

0.1

0.15

0.2

0.25

0.3

72°86°108° 144°

Angle variations for target planes

for w

= 5 mm

for w

= 2 mm

for w

= 10 mm

at d

= 10 mm

= 50 mm (= L

), d

= 0 mm

= 10 mm, d

= 10 mm

= 2 mm

= 1

= 5 mm, d

= 10 mm

= 2 mm, d

= 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

(%)

0.01

0.02

0.03

0.04

0 50 100 150

δd = 2 mm

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

in the region of w

≤ 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

, but

they are somewhat precise for w

≤ 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

= 2 mm and w

= 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

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

), 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.

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