Long period fiber grating and high sensitivity refractive index sensor based on hollow eccentric optical fiber

Sensors and Actuators B 188 (2013) 768–771

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Long period fiber grating and high sensitivity refractive index sensor based on hollow eccentric optical fiber Chunying Guan a,b,∗ , Xiaozhong Tian a , Shuqiang Li a , Xing Zhong a , Jinhui Shi a , Libo Yuan a a b

Key Laboratory of In-Fiber Integrated Optics of Ministry of Education, College of Science, Harbin Engineering University, Harbin 150001, China Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, UK

a r t i c l e

i n f o

Article history: Received 14 February 2013 Received in revised form 16 July 2013 Accepted 24 July 2013 Available online 6 August 2013 Keywords: Long-period gratings Optical fiber sensors Microstructured fibers Refractometry

a b s t r a c t We have demonstrated experimentally a long period fiber grating (LPFG) written in a hollow eccentric optical fiber (HEOF). A high-quality LPFG with no structural deformation was fabricated using a highfrequency CO2 laser with a low energy density of 0.896 J/mm2 . The LPFG writing parameters showed only a weak dependence on orientation of the fiber core to the writing beam. The liquid-filled HEOF LPFG has a high refractive index sensitivity of 1005 nm/RIU over a refractive index range of 1.422–1.441. The good performance and easy fabrication of the HEOF LPFG indicate that it is a promising platform for novel in-fiber microfluidic devices. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Refractive index (RI) is an important parameter for quality control in chemical, medicine, biotechnology or food industries. Optical fiber refractive index sensors and biosensors are attractive, owing to their compact size, high sensitivity, flexibility in their design, and cost effective. Long period fiber gratings (LPFGs), which can couple the core mode to the forward propagating cladding modes of a fiber, are widely used in optical fiber sensors and optical communication systems. Since the wavelength and the power of attenuation band of LPFGs are sensitive to the changes of the external environment, LPFGs can be used as multi-parameter sensors for temperature, bend, strain, chemicals, and biological compounds. In addition to conventional single-mode fibers (SMFs), LPFGs have been formed in other fiber types, such as index-guiding photonic crystal fibers (PCFs) [1], air-core photonic bandgap PCFs [2], and microfibers [3]. Compared with other LPFG fabrication methods (e.g. UV exposure, fs laser writing, and etching), the high-frequency CO2 laser pulse technique provides many advantages [4–6], including lower insertion loss, lower cost, and non-H2 -loading fiber. The maximum wavelength shift of the resonance peak in the conventional SMF LPFG which was written by a CO2 laser was ∼8 nm for the

∗ Corresponding author at: Key Laboratory of In-Fiber Integrated Optics of Ministry of Education, College of Science, Harbin Engineering University, Harbin 150001, China. Tel.: +86 045182519850. E-mail address: [email protected] (C. Guan). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.07.086

refractive index range of 1.33–1.45 [7,8]. The chemical etched and thinned cladding LPFGs were proposed to enhance the refractive index sensitivity [9], but these processes will reduce the mechanical strength and durability of sensors. The air holes in some novel fibers can enhance evanescent waves and allow strong light/material interaction around the fiber core, which offers a new platform for developing ultra-sensitive distributed sensors [10–12]. PCFs that include an array of air holes running along the fiber axis, especially the photonic bandgap PCFs, require a certain periodicity of the air holes, which makes PCF fabrication more difficult. The refractive index sensitivity of PCF LPFGs can reach 440 nm/RIU for variations in the external refractive index over the range from 1.42 to 1.45 [13] and 2262 nm/RIU for the refractive index range of 1.33–1.35 when the air channels were filled with aqueous solutions [14], respectively. For PCF LPFGs written by a CO2 laser, the laser energy density must be strictly controlled to avoid completely collapsing of air holes [2,14]. Furthermore, filling liquid into holes of such fibers were slow and the filling length was limited by the small size of the holes. In this manuscript we proposed and demonstrated a LPFG fabricated by a high-frequency CO2 laser in a hollow eccentric optical fiber (HEOF) [15,16]. The HEOF has a large central air hole, which is easy to fill with liquid or gas, and its fabrication is simple [17]. We studied the effects of orientation of the fiber core relative to the writing beam on the grating characteristics. The spectral responses of the proposed LPFG to the refractive indices of both the surrounding and filled liquid were investigated, respectively. The results show that the HEOF LPFG has a high sensitivity when the air hole

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Fig. 1. (a) Cross-section and (b) side view of a fiber sample. (c) Zoomed-in view of dashed box in (a). (d) Cross-section of the fiber sample after CO2 laser irradiation.

2. Sensor fabrication 2.1. Hollow eccentric optical fiber A photograph of the HEOF used in the experiment is shown in Fig. 1(a). The optical fiber consists of a large central air hole, a quasielliptical core and an annular cladding. The HEOF was fabricated by the modified “suspended core in tube” preform technique [17]. The outer diameter of the fiber is 121.8 ␮m and the diameter of the central air hole is about 71.2 ␮m. The major and minor axes of the quasi-elliptical core are 7.50 and 3.48 ␮m, respectively. A very thin cladding lies between the core and the air hole. The shortest distance between the core and the air hole is 1.91 ␮m. The cutoff wavelength of the lowest high-order mode is about 0.87 ␮m, which was measured by transmitted power method. 2.2. Fabrication of LPFG The LPFG was written by a high frequency pulsed CO2 laser (CO2H10, Han’s Laser), which has a maximum average output power of 10 W. The laser beam was focused into a spot of ∼50 ␮m diameter. The beam was computer-controlled to set the scan speed across the fiber and thus the energy density of the laser radiation on the fiber. Fig. 1(d) shows a photograph of a cross-section view of the HEOF after laser irradiation. With more irradiation, the cladding was hit down more and more but the collapse of the air hole was not apparent. This may because the laser spot size was smaller than the size of the air hole, unlike in PCF LPFGs [2]. A non-H2 -loading fiber sample was fixed to a fiber clamp, which was connected to a 360◦ rotator with 1◦ precision to adjust the orientation of the fiber core. Because the annular cladding is thin, one can clearly see the location of the core from the side of a fiber under a microscope. The exposure angle  between the core orientation and the laser exposure direction as described in Fig. 1(a) was determined by the core location in the side view. The side view of a fiber sample is shown in Fig. 1(b), where the exposure angle is approximately 0◦ and the core is located in the upper side of the air hole due to the image inversion by the microscope. One end of the fiber was butt-coupled to the SuperK supercontinuum white light laser source (NKT Photonics) and the other end was connected to an optical spectrum analyzer (OSA), which was employed to measure the evolution of the transmitted spectrum during the process of fabricating LPFG. A 5 g weight was used to stretch the fiber near the rotated fiber clamp. For our experiments, the pulse frequency and the average output power of the laser were kept at 5 kHz and 0.28 W, respectively. The scan speed was 6.25 mm/s. The laser beam was first scanned transversely across the fiber and then moved longitudinally by a grating step , and then another transverse scan was repeated. This process of making M periods is called one scanning cycle. Fig. 2 shows

the transmission spectra of the LPFGs for different scanning cycles denoted by N. The grating period is 433.5 ␮m and the grating length is 21.24 mm (M = 50). The spectra clearly show the growth of a notch with increasing number of scanning cycles. After 8 cycles, over coupling occurs as the product of the grating coupling constant and length exceeds /2 and the loss of the main peak then decreases. The resonance wavelength is unchanged with increasing number of scanning cycles. A peak attenuation of 20 dB can be easily obtained with only a few (5–8) cycles and the insertion loss is <0.5 dB. The number of scanning cycles obtained the resonance peaks with the same attenuation depth has slight difference due to instability of the laser power for the different samples. The full width at half maximum (FWHM) of the resonance spectrum is ∼7 nm. The deformation of the cladding is actually much weaker than that shown in Fig. 1(d) and the groove on the surface of the LPFG is not observable. The energy density for obtaining a peak attenuation of 20 dB is only about 0.896 J/mm2 , which is much lower than the typical value for LPFGs in SMFs [18] and PCFs [2]. This can be explained as that the core mode has a strong evanescent field due to the thin cladding and the small distance between the core and the air hole, which strengthens the coupling between the cladding modes and the core mode. So a lower energy density is enough to achieve a strong grating. We investigated the effect of the orientation of the fiber core during the writing process on the transmission spectra, and the corresponding results are shown in Fig. 3. The HEOF LPFG can be written in any exposure direction, however, as the exposure angle  increases, the number of scanning cycles also increases in order to obtain the same peak attenuation in the transmission spectra. For example, when the core is oriented opposite to the beam direction ( = 180◦ ), we need 25–30 cycles at an energy density of 1.04 J/mm2 to realize a peak attenuation of about 20 dB, whereas only 5–8 cycles at 0.896 J/mm2 gives the same grating depth for  = 0◦ . The resonance wavelength has a slight red shift with

0

Transmission/dB

was filled with the liquid. The good performance and simple fabrication of the HEOF LPFG indicate that it provides an attractive platform for in-fiber microfluidic and sensing devices.

-5 N=3

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Wavelength/nm Fig. 2. Transmission spectra of LPFGs for increasing number of scanning cycles ( = 433.5 ␮m and  = 0◦ ). All transmission spectra were measured by an OSA with a resolution of 1 nm.

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increasing exposure angles. The mechanism can be explained in terms of frozen-in viscoelasticity [19] and the decrease of refractive index of the exposed side in the fiber. When  = 0◦ the refractive indices of both the core and the cladding are decreased, but only the refractive index of the cladding is decreased for  = 180◦ . The transmission spectra of LPFGs with different grating periods are shown in Fig. 4 as the fibers were irradiated at  = 0◦ . The inset shows the resonance wavelength as a function of the grating period. The resonance wavelength increases approximately linearly with increasing the grating period in the range of 430–450 ␮m. However, the maximum loss and the symmetry of the transmission spectra are slightly reduced. 3. Refractive index sensor The refractive index responses of a HEOF LPFG (written at  = 0◦ ) were investigated in detail. First, the sensor was immersed in NaCl

900 860

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430 440 Period /μm

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Fig. 4. Transmission spectra of the LPFGs with different grating periods for  = 0◦ .

and glycerin solutions of different concentration. Fig. 5(a) and (b) shows the transmission spectra and the resonance wavelength via different refractive indexes, respectively. From Fig. 5(a) and (b), one can see that the resonance wavelength and the amplitude of the LPFG are insensitive to the changes of the refractive index. A float of the resonance wavelength is only ∼0.5 nm for an index range of 1.3325–1.4581. However, the results are significantly different when the air hole was filled by liquid, and the corresponding response of the sensor is shown in Fig. 5(c). The resonance wavelength of the LPFG has a nonlinear relationship with refractive index. The resonance wavelength shifts about 26.2 nm toward shorter wavelengths for the index range of 1.33–1.44, which is much larger than that of a conventional LPFG for the same refractive index range [7]. Especially, when the refractive index varies in the range of 1.422–1.441, the blue shift of 19.1 nm could be achieved and the refractive index sensitivity reached a value up to 1005.3 nm/RIU. As the wavelength resolution of the detector is assumed to be 0.1 nm, the corresponding refractive index 921.0

Resonance wavelength/nm

-40

Transmission/dB

940

Wavelength/nm

Fig. 3. Transmission spectra of LPFGs for different exposure directions ( = 432.3 ␮m). The inset describes the wavelength shift of the transmission dips as a function of .

n=1.0 n=1.3325 n=1.3769 n=1.4110 n=1.4581

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(a) -52 912

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890 1

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Refractive index Fig. 5. (a) Transmission spectra and (b) resonance wavelength as a function of the refractive index when the sensor is immersed in the liquid (with air in the air hole). (c) Resonance wavelength as a function of the refractive index when the air hole is filled by the liquid.

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resolution of the proposed sensor is 1 × 10−4 . The measurement results can be interpreted as the resonance wavelength of the grating is derived from coupling between the core mode and a low order mode of the annular cladding. Obviously, the change of the refractive index of the external liquid does not substantially influence the effective index of the low order cladding mode, so the sensor is not sensitive to the external environment. While the liquid samples inside the air hole strongly influence the effective index of the low order cladding mode. Therefore, the HEOF LPFG can be used as sensitive micro-flow sensor through filling liquid into air channel. 4. Conclusion We have demonstrated that high quality CO2 laser-written HEOF LPFGs can be easily fabricated within a few scanning cycles with no visible deformation of the cladding by using very low laser energy density. The results indicate that the LFPG characteristics depend on the orientation of the core relative to the writing beam, though the LPFG can be written in any exposure direction. The sensitivity over the refractive index range of 1.422–1.441 is as high as 1005 nm/RIU when the air hole was filled with liquid. The proposed HEOF LPFG is suitable for analyzing liquids since they are easily filled into the large air hole. The HEOF LPFG will be an attractive platform for fluid sensing devices. Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC) under Grant Nos. U1231201, 61275094, 11104043, 61201083 and 11274077, and in part by the Natural Science Foundation of Heilongjiang Province in China under Grant No. LC201006, by the Special Foundation for Harbin Young Scientists under Grant Nos. 2013RFQXJ099 and 2012RFLXG030, by the 111 project (B13015) to the Harbin Engineering University, and by Fundamental Research Funds for the Central Universities. References [1] G. Kakarantzas, T.A. Birks, P.St.J. Russell, Structural long-period gratings in photonic crystal fibers, Optics Letters 27 (2002) 1013–1015. [2] Y.P. Wang, W. Jin, J. Ju, H.F. Xuan, H.L. Ho, L.M. Xiao, D.N. Wang, Long period gratings in air-core photonic bandgap fibers, Optics Express 16 (2008) 2784–2790. [3] H.F. Xuan, W. Jin, M. Zhang, CO2 laser induced long period gratings in optical microfibers, Optics Express 17 (2009) 21882–21890. [4] Y.P. Wang, J.P. Chen, Y.J. Rao, Torsion characteristics of long-period fiber gratings induced by high-frequency CO2 laser pulses, Journal of the Optical Society of America B 22 (2005) 1167–1172. [5] Y.P. Wang, Review of long period fiber gratings written by CO2 laser, Journal of Applied Physics 108 (2010) 081101. [6] Y.P. Wang, Y.J. Rao, Z.L. Ran, T. Zhu, X.K. Zeng, Bend-insensitive long-period fiber grating sensors, Optics and Lasers in Engineering 41 (2004) 233–239. [7] J.H. Chong, P. Shum, H. Haryono, A. Yohana, M.K. Rao, C. Lu, Y. Zhu, Measurements of refractive index sensitivity using long-period grating refractometer, Optics Communication 229 (2004) 65–69. [8] T. Zhu, Y.J. Rao, Q.J. Mo, Simultaneous measurement of refractive index and temperature using a single ultralong-period fiber grating, IEEE Photonics Technology Letters 17 (2005) 2700–2702.

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Biographies Chunying Guan received the B.S. degree in optoelectronics, the M.Eng. degree in optical engineering, and the Ph.D. in photonics from Harbin Engineering University, Harbin, China, in 2001, 2004, and 2007, respectively. Currently, she works at Key Laboratory of In-Fiber Integrated Optics of Ministry of Education, Harbin Engineering University. She is currently a Professor at College of Science, Harbin Engineering University, Harbin, China. She is doing her research works in optical fiber sensors, microstructured fiber and fiber optic devices. She published over 70 research papers and 9 patents. Xiaozhong Tian received the B.S. degree in optoelectronics from Harbin Engineering University, Harbin, China, in 2010. Currently, he is working toward the M.Eng. degree in microstructured fiber and fiber optic devices at the Harbin Engineering University, Harbin, China. Shuqiang Li received the B.S. degree in optoelectronics from Harbin Engineering University, Harbin, China, in 2012. Currently, he is working toward the M.Eng. degree in fiber optic devices at the Harbin Engineering University, Harbin, China. Xing Zhong received the B.S. degree in optoelectronics from Harbin Engineering University, Harbin, China, in 2012. Currently, she is working toward the M.Eng. degree in fiber optic devices at the Harbin Engineering University, Harbin, China. Jinhui Shi received the B.S. degree in optoelectronics, the M.Eng. degree in optical engineering, and the Ph.D. in photonics from Harbin Engineering University, Harbin, China, in 2001, 2005, and 2007, respectively. Currently, he is an Assistant Professor at College of Science, Harbin Engineering University, Harbin, China. He is doing her research works in microstructured fiber and metamaterials. He published over 50 research papers. Libo Yuan received B.S. degree in physics from Heilongjiang University, Harbin, China, in 1984, the M.Eng. degree in communication and electronic systems from Harbin Shipbuilding Engineering Institute, Harbin, China, in 1990, and the Ph.D. degree in photonics from The Hong Kong Polytechnic University, Hong Kong, in 2003. Currently, he is a Professor at College of Science, Harbin Engineering University, Harbin, China. His general area of research is in-fiber integrated optics, fiber optic devices and components, fiber optic sensors and its applications. He published three books, four book chapters, 30 patents, and over 290 research papers.