Author's Accepted Manuscript

Collapsed optical fiber: A novel method forimproving thermoluminescence response ofoptical fiber

G. Amouzad Mahdiraji, F.R. Mahamd Adikan, D.A. Bradley

PII: S0022-2313(15)00024-1DOI: http://dx.doi.org/10.1016/j.jlumin.2015.01.021Reference: LUMIN13147

To appear in: Journal of Luminescence

Received date: 25 July 2014Revised date: 28 November 2014Accepted date: 8 January 2015

Cite this article as: G. Amouzad Mahdiraji, F.R. Mahamd Adikan, D.A. Bradley,Collapsed optical fiber: A novel method for improving thermoluminescenceresponse of optical fiber, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2015.01.021

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Collapsed Optical Fiber: A Novel Method for Improving Thermoluminescence

Response of Optical Fiber

G. Amouzad Mahdiraji1, F. R. Mahamd Adikan

1, D. A. Bradley

2,3

1Integrated Lightwave Research Group, Department of Electrical Engineering, Faculty of

Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia 2Department of Physics, University of Surrey, Guildford, GU2 7XH, U.K.

3Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia

*ghafour@um.edu.my / ghafouram@gmail.com

Abstract: A new technique is shown to provide improved thermoluminescence (TL)

response from optical fibers, based on collapsing down hollow capillary optical fibers (COF)

into flat fibers (FF), producing fused inner walls and consequent defects generation. Four

different fused silica preform tubes are used to fabricate in-house COFs and FFs, i.e. ultra-

pure (F300), relatively pure silica (PS), germanium-doped (Ge), and Ge-Boron-doped (GeB).

The optical fibers are then subjected to 6 MeV electron irradiation. While the results show

similar TL response from F300-COF and -FF, the TL response of PS-COF is improved by a

factor of 6 by collapsing it down to a FF. By doping Ge into the F300 tube, the TL response

of the resultant Ge-COF shows an improvement of 3 times over that of F300-COF, while an

improvement of a factor of 12 is obtained by producing a Ge-FF. In GeB preform, by

collapsing the capillary fiber into a FF, an improvement in TL response of 31 times that of

GeB-COF is obtained. TL glow curve analysis shows an additional peak to be generated in

the FFs compared to that observed in the COFs. The TL intensity value of the new peak is

significantly increased in the doped FFs compared to the undoped FFs. The results suggest

that defects generation occurs as a result of the fusing/collapsing technique, providing a TL

response from the optical fibers that can substantially improve upon that of existing TL

system sensitivities.

Keywords: Optical fiber fabrication; Optical fiber application; Radiation dosimeter;

Thermoluminescence; Glow curve.

1. Introduction

Radiation dosimeters based on thermoluminescence (TL) are widely used for offline

monitoring applications, as in for instance in hospitals, nuclear power plants, synchrotron

particle accelerator facilities, as well as other environments in which high-energy radiations

are encountered. The most commonly used TL detector/dosimeter (TLD) is the phosphor-

based TLD-100 (LiF:Mg,Ti), typically used in medical applications, in part because it has an

effective atomic number approaching that required for tissue equivalence. While phosphor-

based TL materials are well-established for dosimetry, there are several notable drawbacks,

including such forms being manifestly hygroscopic [1] and a relatively poor spatial

resolution, up to a few mm [2]. With these restrictions in mind, novel TLD materials based

on doped SiO2 optical fibers are currently being identified, offering characteristics that

provide good potential for extending the applicability of TLD [3, 4].

In recent years, researchers have sought to demonstrate the potential of silica (SiO2)

optical fibers in detecting a variety of ionizing radiations [4, 5]. In attempts to improve the

TL response of SiO2, investigations have been performed using different phosphor additives

and extrinsic dopants in glass and optical fibers, the dopants including aluminum [6],

germanium [4, 6-8], lithium and barium [9], zirconium oxide (ZrO2) [10], copper activated

calcium borate (CaB4O7:Cu) nanocrystals [11], manganese doped calcium tetraborate

(CaB4O7:Mn) nonocrystal [12], lithium potassium borate glass doped with titanium oxide

(TiO2) and magnesium oxide (MgO) [13], di-potassium yttrium fluoride (K2YF5) crystals

doped with samarium (Sm3+

) and terbium (Tb3+

) ions [14].

This study presents a novel method for improving TL response of optical fibers. The

proposed method is based on the collapsing of capillary fibers during the fabrication process

to produce contacting inner wall surfaces (i.e. the inner circumferential surfaces of the hollow

capillary fiber), as carried out in the formation of flat optical fiber [15, 16]. Initially, flat fiber

(FF) was introduced as flexible waveguide for direct UV writing [16]. Its application as a

dosimeter results from the recent fortuitous recognition of significant TL response, initial

observations being reported by Alawiah et al. [17, 18]. With the origin and formation of this

significant TL generation in the FFs being unclear, it has been the interest of present study to

show more detailed observations of the formation of the defects associated with the

additional TL generation in such fibers. In terms of the fabrication process, FF fabrication is

closely related to that of a capillary optical fiber, both being fabricated from the same initial

hollow glass tube. For the FF a vacuum pressure is required to be applied from the top of the

glass tube during the capillary fiber fabrication process, deforming or collapsing down the

cylindrical shape into the flattened fiber. Comparing the TL response and characteristics of

these two fibers, i.e. capillary optical fiber and FF, is expected to be greatly informative in

better understanding the origin of the TL generation, associated as it must be expected to be

with defect center developments in the fiber. To obtain a better initial understanding of TL

characteristics resulting from deformation of fiber shape to create contacting inner surfaces,

the study started with the use of undoped fibers. The study was then extended to doped fibers

to create additional TL. Considering the above, the objective of this study has been to first

demonstrate the effect on TL response of collapsing/fusing fiber inner wall surfaces

compared to that from the pre-collapsed fiber, i.e., capillary fiber. The second objective has

been to show the effect on TL response resulting from the introduction of impurity or dopant

within the collapsing inner surface area of such optical fibers. For this purpose, four sets of

undoped and doped flat and capillary fibers have been fabricated. The TL responses and glow

curves of these fibers are presented and compared under 6 MeV electron irradiation against

the sensitivity of TLD-100. To the best of our knowledge, this is the first report showing that

such fabrication technique (fusing/collapsing) can significantly improve the TL response of

optical fibers.

2. Experimental Details

2.1. Fiber Fabrication

Four sets of capillary optical fibers (COFs) and flat fibers (FFs) have been fabricated for this

study, two sets being made using undoped performs and the other two sets being made using

doped preforms. For the undoped fibers, one used standard F300 ultra-pure fused silica with

low Hydroxyl (OH) content (below 1 part per million). The fibers fabricated using F300 tube

are hereafter referred to as F300-COF and F300-FF. The other set of undoped fiber has been

made using relatively pure fused (PS) silica but with slightly higher OH compared to the

F300. The optical fiber made by this preform are hereafter referred to as PS-COF and PS-FF.

For the two sets of doped fiber, one is fabricated with Ge-doped preform (with 4.5% weight

Ge concentration in the fiber) with large core area (near to that of a multimode fiber core

size), the preform being produced by Telecom Malaysia Research and Development (TM

R&D). The optical fibers obtained from this preform are are hereafter referred to as Ge-COF

and Ge-FF. The other doped fibers are made by using a commercial photosensitive-core

preform with very small core area (single-mode fiber core size), the fibers containing Ge and

Boron with about 8.4% and 2.2% weight concentration, respectively. Both doped fibers are

fabricated using the modified chemical vapor deposition (MCVD) process by using a F300

fused silica tube as the substrate. It should be noted and stressed here that the goal of this

paper is not to investigate the effect of the material types and/or the doping concentrations.

The optical fibers were pulled using a conventional drawing tower with a furnace hot

zone of 3.4 cm. The temperature of the furnace was initially set at 2100 °C for the fiber drop

to occur. Then, a fiber capillary cane with a diameter of 2-3 mm was pulled from the bottom

of the furnace at a temperature of 2000 ± 10 °C and drawing speed of 1-2 m/min. In the next

step, the capillary cane was re-pulled to form the desired size capillary and flat fiber. For

obtaining the FF shape, a low vacuum pressure was applied from the top of the preform. The

capillaries and FFs were pulled with a drawing speed of 1-1.5 m/min and a temperature of

1990 ± 5 °C. Fig. 1 shows fiber cross section images of the fabricated fibers samples while

Table 1 shows the size and weight of fiber types.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Fig. 1. Optical fiber sample image: (a) F300-COF, (b) F300-FF, (c) PS-COF, (d) PS-FF, (e)

Ge-COF, (f) Ge-FF, (g) GeB-COF, (h) GeB-FF.

Table 1. Size and weight of fiber types

Fiber types Size (µm) Weight (mg)

F300-COF 145/116* 0.089

F300-FF 300 × 50 0.068

PS-COF 140/106* 0.082

PS-FF 300 × 70 0.380

Ge-COF 146/101* 0.099

Ge-FF 360 × 75 0.172

GeB-COF 163/140* 0.047

GeB-FF 320 × 32 0.160

*Outer diameter/Inner diameter

2.2. Sample Preparation

All the fiber samples were manually cut into 4 ± 1 mm lengths using a diamond cone point,

the choice of size being based on accommodation by the square planchet of the TLD reader.

Next, the samples were wrapped with aluminum foil and annealed at a temperature of 400 °C

for 1 hour, minimizing any record of dose from the environment and restoring the fiber

samples to initial conditions prior to irradiation. The samples were allowed to cool to room

temperature.

2.3. Irradiation

A set of 5-10 samples per fiber type were irradiated with 6 MeV electrons, use being made of

a linear accelerator (LINAC) located at the University of Malaya Medical Center. Five doses

of 0.5, 2, 4, 6, and 8 Gy were applied to the fiber samples through use of a field applicator of

size 20 × 20 cm2 and source to surface distance of 100 cm. To deliver accurate dose to the

fiber samples, bolus thicknesses of 1.5 cm were used as the build-up medium during

irradiation.

2.4. TL Measurement

To ensure well-controlled account of fading, TL measurements were carried out 24 hours

post-irradiation using a Harshaw 3500 TL reader. A flat planchet was used for measuring the

TL response for all irradiated samples, including the capillary fibers, flat fibers, and TLD-

100. Nitrogen gas flow was maintained during measurement of the TL in order to minimize

possible spurious light signals due to triboluminescence. The sample read-out was performed

by setting the TL reader at a maximum temperature of 400 °C, a time temperature profile

with a pre-heat temperature of 50 °C, and an acquired heating rate of 25 °C/s, and acquisition

time of 20 s.

Prior and during TL measurement, the TLD reader background noise has frequently been

recorded. The background noise is measured when there is no dosimeter sample in the TLD

reader. Closely similar background noise levels have been observed from the TLD reader in

each case, with an average background noise of about 3.520 nC and a standard deviation of

0.199 nC. Fig. 2 shows the average TLD reader background noise over the time and

temperature profile recorded during the measurement. The graph shown in this figure is

normalized to the peak noise value.

It should be noted that for the various fiber types their TL response was very low, near to that

of the TLD reader background noise; instead of reading one fiber sample at a time, 5 fiber

samples have been used to be read-out together. This leads to higher optical signal-to-noise

ratio, providing more accurate TL response. The accumulated TL response from the 5

samples has then been normalized to the accumulated mass.

Fig. 2. Average TLD reader background noise versus acquisition time and temperature.

2.5. Mass Normalization

For mass normalization, 10-15 samples from each fiber type were used to measure the mean

weight of individual fibers, use being made of an accurate electronic scale. For simplicity, the

mean mass per fiber type was used to normalize TL response of individual samples.

3. Results

3.1 TL Response

In addition to measuring the TL response of the various irradiated fiber samples, the TL

response of annealed but unirradiated fiber samples (the control samples) has also been

measured for all such fibers. For all fiber types used in present study, the TL response of the

control samples was the same as that of the TLD background noise, each with the same glow

curve, as shown in Fig. 2. This confirms that all fiber types used in this study have the same

initial TL response, without any effect from their weight value.

Figs. 3 (a) to (d) show the weight-for-weight TL response of the four sets of optical fibers to

6 MeV electron irradiation for 0.5 to 8 Gy delivered doses. All sample types were irradiated

simultaneously, ensuring the same irradiation conditions. The TL response provided by the

F300-COF and F300-FF (Fig. 3 (a)) is closely similar to that of the minimal response, being

just above the noise level, formed as they are from Suprasil F300 ultra-pure silica tube

preform.

The TL response shown by PS-COF (Fig. 3 (b)) is not greatly different from the TL

response of F300 fibers, providing some 7% higher sensitivity when averaged over different

doses of. Conversely, the PS-FF shows a significant increase in TL response; averaged over

different doses, the FF provides a TL yield which is some 6 times higher than that of the PS-

COF fiber.

Since both the PS-COF and PS-FF used in this experiment (Fig. 3) are fabricated from the

same preform, the results point to the generation of new defects in production of the FF to be

the basis of increased sensitivity to irradiation dose. This is suggested to be due to the internal

interface created in collapse of the capillary fiber and subsequent biaxial forces experienced

at the interface. The tetrahedral structural arrangement of amorphous silica has been

described by Salh [19], while application of biaxial compressive stress has been modeled,

with predicted creation of edge-sharing tetrahedron defects at the silica surface [20].

On the other hand, comparison between the first and second experiments suggest that the

presence of small impurity (or dopant) in the fiber or the collapsing surface area produces

additional TL response in the FF. To investigate the further potential for increase in TL

production in FF, the same experiments have been repeated using two sets of doped fibers

instead of a pure silica fiber. It should be noted here that selection of the dopant and

concentration were arbitrary, the intention being to simply demonstrate the proposed concept.

Ge is known to provide for an excellent level of TL. However, an optimum dopant

concentration will be required for optimum TL yield.

0

0.01

0.02

0.03

0.04

0.05

0.06

0 2 4 6 8

TL

resp

on

se (

µC

)/m

ass

(m

g)

F300-FF

F300-COF

0

0.1

0.2

0.3

0.4

0.5

0.6

0 2 4 6 8

PS-FF

PS-COF

0

0.2

0.4

0.6

0.8

1

0 2 4 6 8

TL

resp

on

se (

µC

)/m

ass

(m

g)

Dose (Gy)

Ge-FF

Ge-COF

0

0.5

1

1.5

2

2.5

3

0 2 4 6 8

Dose (Gy)

GeB-FF

GeB-COF

TLD100

Fig. 3. TL response measured using 6 MeV. (a) F300-COF and -FF, (b) PS-COF and -FF, (c)

Ge-COF and -FF, and (d) GeB-COF and -FF. and TLD-100.

Fig. 3 (c) shows the TL response of Ge-COF and Ge-FF. The Ge-COF provides about 2.9

times greater sensitivity compared to PS-COF. Since the Ge-doped fibers are made with a

F300 substrate tube, comparing the TL response of F300-COF with Ge-COF is indicative of

the positive impact of doping on the TL response, of the order of 3.1 times under present

irradiation conditions. However, by collapsing the Ge-COF into the flat form, an additional

TL response (by 4 times) is achieved. In other words, the TL yield of the Ge-FF is about 12

times greater than that of the F300-FF. TL response of F300 and Ge-doped fibers confirms

that the main TL response generated from the doped area (fiber core), while the cladding (if it

is F300) has minor TL generation.

To reconfirm the observation of the previous experiment, a fourth experiment has been

performed by using GeB-doped fibers with thinner core and cladding thickness. Fig. 3 (d)

shows the results of this, compared against that of the commercially available TL detector

TLD-100. The GeB-COF, GeB-FF and TLD-100 were irradiated simultaneously i.e. under

the same conditions. In comparing the TL yield from doped fibers with the F300 fibers it has

been observed that the TL response of the GeB-COF is about 12% greater than that of the

F300-COF, while it is about 35 times greater for the GeB-FF compared to the F300-FF.

The results show that, weight-for-weight, the GeB-FF produces some 31 times the TL

yield of that of GeB-COF, also providing significantly more yield (by around 1.3 times) than

TLD-100 chips. The results further confirm that by collapsing the hollow circumference or

wall surface of a capillary optical fiber, the TL response is improved appreciably over that of

the capillary fiber, and more so if the fiber contains impurity in the collapsing surface area.

3.2 Glow Curve

Figs. 4 (a) to (d) show the glow curves of the four sets of optical fibers for received doses of

4 and 8 Gy under 6 MeV electron irradiation. The glow curve generated by F300-FF (Fig. 4

(a)) has slightly greater TL intensity peak compared to the F300-COF. In addition, the

intensity of the FF peak is at slightly lower temperature (around 135 °C), whereas for COF it

is around 170 °C. The second peak at a temperature of around 400 °C, is indicative of the

TLD reader background noise as shown in Fig. 2. This result confirms that by collapsing the

inner circumferential surfaces of COF, some properties of the fiber are changed, which

results in change of glow curve shape, peak temperature, and TL peak intensity. The changes

suggest the formation of new defect centers in the FF not available in the COF.

The TL intensity peak of PS-FF (Fig. 4 (b)) is more than 30 times that of the PS-COF and

also F300-FF. Further, the intensity of the main peak in PS-FF is increased significantly

compared to the PS-COF; the graphs indicate generation of another peak within a time of 10

to 14 seconds using the present time-temperature profile at a temperature of around 300 to

400 °C; this peak is not observed in COF. This difference between the FF and COF glow

curves is more obvious in Fig. 4 (c). In Ge-FF, the TL intensity of the second peak is

significantly increased, while the magnitude of the first peak is also improved compared to

the PS-FF. It is evident from this graph that no such second peak is generated in the COF.

Fig. 4 (d) reconfirms this observed phenomenon in FFs. The TL intensity peak generated by

GeB-FF is more than two times greater than that of the Ge-FF.

0

100

200

300

400

500

0

1

2

3

4

5

0 5 10 15 20

TL

inte

nsi

ty (

µC

)/m

ass

(m

g)

0

100

200

300

400

500

0

50

100

150

200

250

0 5 10 15 20

TL

inte

nsi

ty (

µC

)/m

ass

(m

g)

Time (s)

0

100

200

300

400

500

0

100

200

300

400

500

0 5 10 15 20

Te

mp

era

ture

(°C

)

Time (s)

0

100

200

300

400

500

0

30

60

90

120

150

0 5 10 15 20

Te

mp

era

ture

(°C

)

Fig. 4. Glow curves obtained for doses of 4 and 8 Gy using 6 MeV electrons. (a) F300-COF

and -FF, (b) PS-COF and -FF, (c) Ge-COF and -FF, and (d) GeB-COF and -FF.

4. Discussion

In the main, the TL response of optical fibers is due to the defect centers or impurities which

exist in the material [21]. In pure silica based optical fibers, defects might be induced during

the drawing process [22]. In doped optical fibers, other factors such as the doping

concentration, the type of material and the MCVD process are effective in generating defect

centers. Since the pulling parameters such as drawing speed and pulling temperature were

kept practically constant for all of the produced flat and capillary fibers, differences in

performance are almost certainly due to other factors. The main difference in fabricating FF

compared to COF fibers is the fusing of the inner wall surfaces of the COF. As explained in

Section 2.1, the FF shape was obtained by applying vacuum pressure from the top of the

capillary cane, resulting in deformation of the circular shape into a flat shape at the furnace

hot zone area. This produces stresses on the fusing area in the neck-down region of the

preform that generate additional structural defects especially with the existence of impurity in

the collapsing region. Hibino and Hanafusa [23] have shown that shear stress applied to the

viscous state induce non-bridging oxygen hole centers (NBOHCs) and breakage of the

strained Si-O bonds in the continuous random silica network defining the amorphous silica.

The findings are supported by theoretical predictions [24]. Of course, it must also be

acknowledged that in the creation of the FFs, the formation of new structural bonds between

the surface atoms of the internal walls will depend, to a greater or lesser extent, on the

furnace temperature, drawing speed, vacuum pressure and fast quenching rate.

In the case of F300 fibers, since the preform is ultra-pure with very low impurity, the TL

generated from F300 fibers is very low, the noise power dominating the TL yield. In addition,

since the optical fiber samples were cut manually, there is some variation in TL response as

presented in Figure 3 (a). Due to these influences, the performance difference between the

two F300 fibers, COF and FF, is not clear from their TL response, as presented in Figure 3

(a). However, a more reliable observation is obtained from the glow curves shown in Figure 4

(a), indicating that even small changes in the fiber properties in terms of defect centers can

manifest in the TL emission. Thus, these results confirms formation of new defects in the FF,

related to the collapsing area. The fiber properties differences between COF and FF is more

apparent in PS fibers, where the amount of impurity in the fiber is slightly higher compared

to F300. In Ge-FF, generation of the second peak in the TL glow curve confirms significant

differences between the TL properties of the two fibers, COF and FF. In use of Ge and B

dopants, the defect centers in the internal wall surface of the fibers will now be very much

greater than that in Ge-dope fiber. As such, it is to be expected that there will be amplified

induced defect centers/strained bond rupture in the GeB-FF, with associated increase in TL

response.

Regardless of the type of defect, a matter not within the scope of this study; the results

suggest generation of additional defects in the FFs, causing additional increase in TL

response. This defect was not originally available in the COF, as seen in examination of the

respective glow curves. Further investigation is required in clarifying the above mechanisms

in more detail.

4. Conclusions

A new method for increasing TL response of optical fibers are presented for the first time,

based on collapsing/fusing optical fiber wall surfaces. The TL responses and glow curves of

four sets of COFs and FFs to 6 MeV electron irradiation has been compared and analyzed.

The results show that the TL response of capillary optical fibers can be significantly

improved upon by fusing the internal wall surfaces of the capillary fiber, and even more so if

there is impurity/dopant in the collapsing surface area. It has been shown that the TL

response in COF and FF fabricated using ultra-pure silica tube (F300), is closely similar,

while in pure silica preform with slightly elevated impurity compared to that of F300,

subsequent to collapse the TL response of COF is improved by a factor of 6. The

enhancement of TL response in GeB-FF is about 31 times greater than that of GeB-COF.

Fiber glow curves show secondary peak generation in FFs, a peak that was not originally to

be found in the COFs. The intensity of this new TL peak can be increased by increasing the

impurity in the fiber collapsing area. The results strongly suggest the collapsing technique to

be a promising method for designing and fabricating high sensitivity optical fiber based

irradiation dosimeter sensors.

Acknowledgements

The authors would like to acknowledge the UM-MOHE High Impact Research Grant

numbers A000007-50001 that fully supported this project. We also would like to thank the

University of Malaya fiber fabrication group for fiber fabrication; TM R&D for fabricating

Ge-doped preform; MOHE HIR No. UM.C/625/1/HIR/33 for financially supporting Ge-

doped fiber production; E. Dermosesian for assisting in sample preparation; Dr. S. Hashim

for providing TLD reader; and Dr. U. N. Min for assisting in radiation.

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• A new method for increasing TL response of optical fiber is presented

• By collapsing capillary fiber wall surface, TL response of the fiber increased

• By adding impurity in the collapsing area, TL response significantly improved