Measurement of the optical fiber numeric aperture ...

Measurement of the optical fiber numeric aperture exposed to thermal and radiation aging Ales Vanderka, Lukas Bednarek, Lukas Hajek, Jan Latal, Radek Poboril, Petr Zavodny, Vladimir Vasinek VSB-Technical University of Ostrava, Department of Telecommunications, 17. listopadu 15/2172, Ostrava-Poruba, Czech Republic ABSTRACT This paper deals with the aging of optical fibers influenced by temperature and radiation. There are analyzed changes in the structure of the optical fiber, related to the propagation of light in the fiber structure. In this case for numerical aperture. For experimental measurement was used MM fiber OM1 with core diameter 62.5 μm, cladding diameter 125 μm in 2.8 mm secondary coating. Aging of the optical fiber was achieved with dry heat and radiation. For this purpose, we were using a temperature chamber with a stable temperature of 105 °C where the cables after two months. Cables were then irradiated with gamma radiation 60Co in doses of 1.5 kGy and then 60 kGy. These conditions simulated 50 years aging process of optical cables. According to European Standard EN 60793-1-43:2015 was created the automatic device for angular scan working with LabVIEW software interface. Numerical aperture was tested at a wavelength of 850 nm, with an output power 1 mW. Scanning angle was set to 50° with step 0.25°. Numerical aperture was calculated from the position where power has fallen from maximal power at e2 power. The measurement of each sample was performed 10 hours after thermal and radiation aging. The samples were subsequently tested after six months from the last irradiation. In conclusion, the results of the experiment were analyzed and compared.

Keywords: Optical fiber, thermal and radiation aging, numerical aperture, 60Co, 1. INTRODUCTION Nowadays, aging of optical fiber is a current issue. In optical networks, traffic raised several times recently. It has the effect more rapid degradation of materials of optical fibers and hence degradation of optical parameters. Therefore, researches are focused on this area. Attenuation of the optical fiber is one of the most important parameters of the optical fiber. However, this area is already examined by other research groups. In this article, the presented research is focused on changes in the numerical aperture. This parameter relates the refractive index of core and cladding and also depends on the optical fiber materials. In operating conditions, research of aging is very lengthy because the optical fibers do not age so fast as would be needed for research. Therefore, the optical fibers were aging by the methods of high temperature and gamma radiation. The presented results show the influence of temperature and gamma irradiation on the parameter of numerical aperture. Four samples of multimode fibers were analyzed. The first sample was the reference (new optical fiber). The second sample underwent by thermal aging (100 °C during 90 days). The third sample underwent by thermal aging and radiation aging (at the dose 1.5 kGy). The fourth sample underwent by thermal aging, radiation aging (1.5 kGy) and radiation aging (at the dose 60 kGy). 1.1 Ageing methods Thermal aging Artificial thermal aging was performed in an electric drying oven from Binder. This drying oven allows the accurate temperature settings in tenths of °C. Another feature of the drying oven is controlled air circulation inside the drying oven (it ensures the same temperature throughout the drying oven). The sample was placed around the circumference of the polymeric cylinder; it ensures homogeneous heating. The set temperature was 100 °C and the total period of the thermal aging was 90 days. *[email protected]; http://optice.vsb.cz/

20th Slovak-Czech-Polish Optical Conference on Wave and Quantum Aspects of Contemporary Optics, edited by Jarmila Müllerová, Dagmar Senderáková, Libor Ladányi, Ľubomír Scholtz, Proc. of SPIE Vol. 10142, 1014224 · © 2016 SPIE · CCC code: 0277-786X/16/$18 · doi: 10.1117/12.2257339 Proc. of SPIE Vol. 10142 1014224-1

Downloaded From: http://proceedings.spiedigitallibrary.org/pdfaccess.ashx?url=/data/conferences/spiep/90819/ on 04/10/2017 Terms of Use: http://spiedigitallibrary.org/ss/term

Radiation aging Irradiation of samples was performed in two different irradiation shafts. Schematic diagram of the shaft is shown in Fig.1. The samples are stored in the top part of the shaft. The entire shaft is placed in a tunnel which is made of the concrete with a thickness of 2 m. The environment around the shaft is secured with doors for protection against gamma radiation. After closing the door, the source material is ejected from the bottom part of the shaft and irradiation of samples starts. The samples are mounted around the circumference of the polymeric cylinder to achieve the greatest possible homogeneity of irradiation. The radiation source is located in the middle of the cylinder. The radiation source is 60Co which emits two gamma rays with energies of 1.17 MeV and 1.33 MeV. The sizes of these energies are not sufficient to activate the irradiated samples. Total activity of radiation source is approximately 2 TBq. A size of the received dose depends on the storage of the sample relative to the source. Three alanine pills attached to the cylinder at various heights are used to determine the size of the received dose. After 3 hours, the pills are pulled out, and the average hourly dose is determined. The average dose for a sample which received a total dose of 1.5 kGy was 0.02 kGy per hour. Irradiation lasted a total of 75 hours. The average dose for a sample which received a total dose of 60 kGy was 0.5 kGy per hour. The total obtained dose of this sample was 60 kGy, and irradiation took a total of 120 hours.The dose measurement uncertainty is 10 %. The temperature is about 19 °C in the shaft during the irradiation process.

Figure 1:Scheme of irradiation shaft. 1.2 Numerical aperture The numerical aperture (NA) is ability of an optical fiber to receive a light. Numerical aperture can be calculate using the refractive index of the core and cladding or measured using far-field measurement. Far-field measurement is defined in IEC 60793-1-43: 2015. The angular scan method was chosen for the test. Measuring assembly was composed of the radiation source with a wavelength of 850 nm with optical power of 1mW, mode-filter and dark plastic box. In the box, the rotation stage (CR1_Z6) and intensity sensor (S120-C) were placed. The optical fiber was placed in the SC/PC fast connector. The front of fiber was placed above the center of the rotation axis of the detector, whereby circular shift is ensured. Minimal distance of the detector R and end face of optical fiber was calculated by Eq. (1). The used distance was 5 cm.

R≥

d2

λ

,

(1)

where d - the fiber core diameter (62.5 μm), λ - the wavelength (850 nm). Maximal diameter of detector was calculated by Eq. (2)

D = 4 R sin(δ ) ,

(2)

Proc. of SPIE Vol. 10142 1014224-2


where D is the detector aperture diameter (mm), δ - angular resolution (°) and R - distance between the detector and end face fibers. The application was created for purposes of measurement in LabVIEW. Application controlled rotation stage (CR1_Z6) and collected data from the intensity sensor (S120-C). The angle of 80° with a resolution of 0.25° was set for measurement of the spot. Period of one step took 2 seconds. During this time, we wrote data which were averaged (2000 measured data). Figure 4 shows setting in LabVIEW. Scheme of an arrangement of measuring workplace can be seen in the Fig.2. Real scheme of NA measurement can be seen in the Fig.3.

Driver TDC001

PC LabVIEW

Testing fiber

r

Power sensor S120C

Mode filter

Power Meter PM 100 USB

Rotation Stage CR1-Z6

Source 850 nm, 1mW

Figure 2:Scheme of an arrangement of NA measurements. Ova

Figure 3:Real scheme of NA measurements. JIIT' 11

,1>

111i1

A hA

4i

h

J

,7

_

t

I

LI,

t

I

i

e

r

te

IA

t

MT;

; -1

Figure 4: Front Panel in LabVIEW program.

Proc. of SPIE Vol. 10142 1014224-3


2. RESULTS The light spot was measured five times for each sample in two perpendicular axes. These resulting measurements were averaged subsequently. After, only one-half of the curve was created by averaging. The resulting curves are plotted in Fig.5. There was observed a small decrease in peak power for thermal aging sample compared to the reference sample. Angle increased about 0.25° at dropped from a peak value of 5 %. More influenced sample was at 1.5 kGy where the angle increased about 0.35°. The last sample, which was still irradiated 60 kGy, was the most affected. At the last sample, the angle increased about 1.77°.

Figure 5: Measured radiation angle for test samples. According to the following formula, the numerical aperture was calculated for the angle at 5% of the measured maximum power:

NA = sin θ

,

(2)

The numerical aperture increased about 0.0042 compared to the reference for the temperature aging sample. At the irradiated sample of 1.5 kGy, the numerical aperture increased about 0.0059. We recorded an increase in the numerical aperture about 0.0296 for a sample of 60 kGy.

Table1: Calculated numerical aperture for test samples. New sample NA (-)

0.27311

100 °C (90 days) 0.27733

Gamma irradiation 1.5 kGy 0.27900

Gamma irradiation 60 kGy 0.30275

Proc. of SPIE Vol. 10142 1014224-4


Figure 6:Change of the numerical aperture for test samples.

CONCLUSION The results show that the thermal aging, which accelerates the aging of the optical fiber, has little influence on the change of numerical aperture. A 60Co irradiation which simulates the radiation aging has a very significant influence. The ratio of refractive indexes of the core and cladding is changed probably. This is caused by changes in the structure of crystalline lattice of the enamel of optical fiber. It is seen that the change of the numerical aperture is not directly proportional to the dose of gamma radiation. The graph shows the increase in the value of numerical aperture at a dose of 60 kGy, but this increase is not great in comparison with a smaller test dose of gamma radiation. However, the question is whether a larger numerical aperture of fiber is harmful or not. The theory suggests that the numerical aperture gives the ability of fiber to tie up light into its core and cladding. Thus, this ability is higher at a larger numerical aperture, and thus we better tie up light. However, irradiation has a huge effect on the attenuation of the optical fiber (see. References). From a communication point of view, irradiation by gamma radiation is destructive for optical fibers, since the attenuations reach values which are not suitable for operation of optical networks. Therefore, it is necessary to carry out further research in the field of radiation aging and the aging of the optical fiber and optical components. The team of authors is already engaged in this research, and the results will be presented in subsequent articles.

ACKNOWLEDGEMENTS The research described in this article could be carried out thanks to the active support of the Ministry of Education of the Czech Republic within the projects no. SP2016/149 and SP2016/151. This article was supported by projects Technology Agency of the Czech Republic TA03020439 and TA04021263. The research has been partially supported by the project no. CZ.1.07/2.3.00/20.0217 (The Development of Excellence of the Telecommunication Research Team in Relation to International Cooperation) within the frame of the operation programme Education for Competitiveness financed by the European Structural Funds and from the state budget of the Czech Republic.

Proc. of SPIE Vol. 10142 1014224-5


REFERENCES [1] HAWN, D. P., PETRIE, C. M., BLUE, T. E. and WINDL W., "In-situ gamma radiation induced attenuation in silica optical fibers heated up to 600°C," Journal of Non-Crystalline Solids. Papers 379(3), 192-200 (2013). [2] JIN, J., XU, R.-M., LIU, J.-X. and SONG, N.-F., "Effect of Radiation Dose on Radiation-induced Attenuation and Temperature Dependence in Optical Fiber," ACTAPHOTONICASINICA. Papers 42(11), 1272-1276. (2013). [3] JINGMING, S., JIANHUAGUO, J. G., WANG, X. W. X. and JIN, J. J. J., "Temperature dependence of radiationinduced attenuation of optical fibers," Chinese Optics Letters. Papers 10(11), 110604-110606 (2012). [4] ALFEELI, B., NARAYANAN, S., SMILEY, D. "Performance of randomly distributed holes optical fibers under low dose gamma-ray irradiation," Proc. SPIE 7934, 1-7 (2011). [5] GILL, K., GRABIT, R., PERSELLO, M., STEFANINI, G. and VASEY, F., "Gamma and neutron radiation damage studies of optical fibres," Journal of Non-Crystalline Solids. Papers 216, 129-134 (1997). [6] SEKULIC, R., SLAVKOVIC, N., SRECKOVIC, M., KOVACEVIC, M. and STAMENOVIC, M., "The influence of gamma radiation on polarization mode dispersion of fibers applied in communications," Nuclear Technology and Radiation Protection. Papers 27(2), 171-177 (2012). [7] LUO, W., XIAO, Z., WEN, J., YIN, J., CHEN, Z., WANG, Z., WANG, T. and PAL, B. P,. "Defect center characteristics of silica optical fiber material by gamma ray radiation," Proc. SPIE 8307, (2011). [8] BEDNAREK, L., MARCINKA, O., PERECAR, F., PAPES, M., HAJEK, L., NEDOMA, J. and VASINEK, V., "The aging process of optical couplers by gamma irradiation," Proc. SPIE 9586, 1-13. (2015). [9] VASINEK, V., SISKA, P., BEDNAREK, L., LATAL, J., KOUDELKA, P. and MARCINKA, O., "Ageing of fiber optical devices," Proc. SPIE 9389, (2015).

Proc. of SPIE Vol. 10142 1014224-6
