An Asymmetric Tapered Long Period Fiber Grating: Fabrication and Characterization 3 l 2 Sheng Chyan Lee , Yun Thung Yong , Kim Ho Yeap
4 Faidz Abdul Rahman
Department of Electronic Engineering Faculty of Engineering and Green Technology Universiti Tunku Abdul Rahman lalan Universiti, 3 1900 Kampar, Perak, Malaysia 2 l
[email protected] ;
[email protected] ;
[email protected]/
Department of Electrical and Electronic Engineering Faculty of Engineering and Science Universiti Tunku Abdul Rahman lalan Genting Klang, Setapak 53300 Kuala Lumpur, Malaysia 4
[email protected]
(Invited paper) Abstract-
An
asymmetric
tapered
Long
Period
Fiber
Grating (LPFG) was fabricated using an ignition coil based electric arcing fabrication system. The angle measurement of LPFG imaging using Matlab confirms the asymmetric tapered LPFG formation. Characterization of the asymmetric tapered Long Period Fiber Grating was done to test its sensitivity with respect to changes in the surrounding refractive index and temperature for LPFG at 900 nm to 1000 nm and 1500 nm to
1600 nm wavelength ranges. The fabricated asymmetric tapered LPFG performance is comparable with LPFG fabricated by fusion splicer but with less gratings and higher transmission loss.
KeywordsIndex.
LPFG,
Electric Arcing, Temperature, Refractive
I.
INTRODUCTION
Long period fiber gratings (LPFGs) have always been receiving favourable feedback as one of the better candidate to be used as fiber sensors. Since its introduction back in 1996 by Vengsakar et at. [1], LPFGs have been used as a band-rejection filter and as sensors. An LPFG is formed by creating periodic perturbations on the fiber. These periodic perturbations enable light coupling between the propagating core and co-propagating cladding modes [2]. Various methods have been reported in fabricating LPFGs. These include the irradiation of UV, periodic relaxation of residual stress, physical deformation, and microbending. Among these techniques, it is found that physical deformation is one of the common techniques used for fabrication of LPFG due to its simplicity. This can be done by using electric arcing.
II.
EXPERIMENTAL SETUP
In our experiment, we use the SM750 specialty fiber, i.e. SM750 for 900nm to lOOOnm and the SMF-28 standard fiber for 1500 nm to 1600 nm wavelength ranges, respectively. A simple electric arc induced LPFG fabrication setup is shown in Fig.I. This setup consists of an ignition coil (Bosch 30KW 12V) with an arcing circuit, a tension meter with weight, a pair of electrode (ER-IO) mounted on the motorised stage and controlled by a computer, a pair of fiber clamp with and without sliders, a light source, and an optical spectrum analyser. During the fabrication process, a constant force is applied to the fiber to create a constant axial tension. In this setup, a grating period is selected based on the notch wavelength for the fabrication. The arcing current is set to 9 rnA with fixed duration for each arcing. During the grating making process, the fiber clamp with slider allows the deformation of the cladding with the help of a constant axial tension. As a result, tapers will be formed at the arcing point. The structure of the tapers depends on the axial tension and arcing discharge time. Once the first grating formation is completed, the motorised stage will move the electrodes to the next point for the formation of subsequent gratings on the fiber. Fig 2 shows the formation of gratings on the fiber. The fabrication parameters setup for LPFG at the specified wavelength range is listed as in Table 1.
Table 1 : LPFG fabrication parameters
Majority of the electric arc induced LPFGs are fabricated using fusion splicers [3-5]. It is noted, however that an LPFG with similar characteristics can also be fabricated using an electric arc-induced system based on ignition coil. In this paper, we give a detail illustration on both techniques. Comparison and analysis are also performed between the LPFG fabricated using the ignition coil based system and fusion splicer. We also highlight some of the critical parameters, in which care must be taken during the fabrication process. These parameters are especially important in ensuring the overall sensitivity of an LPFG.
978-1-4673-6075-3/13/$31.00 ©2013 IEEE 21
Wavelength (nm)
Grating Periods (pm)
Tension (eN)
Seconds (s)
900-1000
350
30
2
1500-\600
630
28
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Figure I
Figure 3
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Cladding diameter, D
Figure 2
Cladding taper diameter,d
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Long period jiber gratings angle measurement using Matlab(SMF-28)
"Vavelength (DDt)
Grating Period, A
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III.
RESULTS AND DISCUSSION
The formation of gratings on the fiber determines the characteristic of the LPFG. Close inspection of the images of the tapered gratings formed on the SMF-28 fibers, we observe that the average grating period, A formed is 753.391lm ±43Ilm. It is found that the grating period differs from the original grating period set during the fabrication process. During the arcing process, the weight tied to the end of the fiber tends to pull the fiber towards the other end, at the opposite direction of the light source. Hence, we attribute the discrepancy found in the grating periods to the tapering effect caused by the constant axial tension force exerted by the weight. The average cladding taper diameter, dis 93.29 11m ±7 11m compared to the original cladding diameter, D of 1251lm. When the angles of the tapered region are measured, it is found to be asymmetric with an angle difference of 0.88560 as shown in Fig.3. Similar finding has been reported by Malki et at. in [6]. According to their finding, the asymmetric deformation has caused changes in the effective refractive index of the core and cladding [6]. With asymmetric tapering, it is possible to further enhance the notch formation as shown in Fig. 4 compared to other symmetric LPFGs. The ±43 11m error of the average grating period, A and ±7 11m error of the cladding taper, d are caused by the inability of the electrodes to maintain a constant arcing power during the fabrication process. Ivanov et at. has suggested that the reproducibility of the electric arcing can be improved by placing the fiber in a region with a larger temperature gradient [7].
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Figure 4 : Long period jiber gratings (SMF-28) produced with thefabrication setup using electric arcing technique.
The same experiment has been repeated for specialty fiber SM750 (as shown in Fig.5) to verify the characteristic ofLPFG in the range of 900 nm to 1000 nm wavelength. The average grating period, A formed is 529.1 11m ±68 11m and the average cladding taper diameter, d is 98.9 11m ±3 Illl. Similarly, an asymmetric formation with an angle difference of 1.030 is also observed in this case.
Wavelength (nm)
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Long Period fiber gratings (SM750) produced with the same technique.
In order to verify the response of the asymmetric tapered LPFG, we characterize the LPFG by varying the surrounding refractive index (using Cargille refractive index oil) and temperature (40°C to 190°C). As shown in Fig. 6, when the refractive index changes from 1.43 to 1.45, a sharp and rapid decrease at the wavelength shift is observed. This is because the applied refractive index at the surrounding of the fiber is very close to the refractive index of the cladding. We also learn from Fig. 6 that, the refractive index for LPFG using SM750 is more sensitive than LPFG using SMF-28. In the study on the temperature sensitivity, we find that the gradient of the LPFG that we have fabricated on the SMF-28 fiber is 0.04 nm/°C. The temperature sensitivity is lower than those reported in [8], which have a temperature sensitivity of 0.06 nmJ°C for temperatures of up to 200°C. Fig. 7 depicts the wavelength shift with respect to the variation of temperature in an LPFG fabricated using the SM750 and SMF-28 fibers. Clearly, the temperature sensitivity measured in the SM750 is less sensitive (0.026nm/°C) than that in the SMF-28 (0.04nmJ°C) fiber. We believe that the temperature sensitivity can be further improved with modification of the grating formation parameters. A summary of the transmission loss (dBm) in the LPFGs, fabricated using electric arc produced by fusion splicer on SMF-28 is listed on Table 2. Refl'active Index, x 2.5 1.5
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