Sensing characteristics of long period grating by writing directly in SMF-28 based on 800 nm femtosecond laser pulses

Optics and Laser Technology 121 (2020) 105839

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Sensing characteristics of long period grating by writing directly in SMF-28 based on 800 nm femtosecond laser pulses ⁎

T

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Yani Zhanga,b,d, , Peng Jiangb,d,1, Dun Qiaoc, Yaru Xib,d, Yuyu Zhub,d, Qiang Xub,d, , Chaojin Wangb,d a

School of Arts and Sciences, Shaanxi University of Science & Technology, Xi’an 710021, China School of Physics and Optoelectronics Technology, Baoji University of Arts & Science, Baoji 721016, China c Wireless and Optoelectronics Research and Innovation Centre, Faculty of Computing, Engineering and Science, University of South Wales, Pontypridd CF37 1DL, UK d Baoji Engineering Technology Research Centre on Ultrafast Laser and New Materials, Baoji 721016, China b

H I GH L IG H T S

was fabricated in SMF-28 without hydrogen loading by fs laser direct writing. • ATheLPGself-made shows high temperature, RI, and strain sensitivity. • Its temperatureLPG is about 124 pm/°C with a linearity of 0.989. • Its RI sensitivitysensitivity is −582.5 nm/RIU (refractive index unit) with a linearity of 0.998. • Its strain sensitivity is −9.45 × 10 dB/με with a linearity of 0.98. • −4

A R T I C LE I N FO

A B S T R A C T

Keywords: Femtosecond laser Long Period Gratings (LPGs) Refractive index (RI) Temperature sensitivity Strain sensing

A long period grating is fabricated in standard SMF-28 fibers without hydrogen loading by adopting point-bypoint direct writing based on femtosecond laser pulses with 800 nm wavelength and 100 fs pulse width. Its sensing characteristics is investigated and the sensing sensitivities to temperature, external refractive index (RI) and strain are systematically examined as well. The experimental results show that the sensing sensitivity of the long period grating to temperature, RI and strain is about 124 pm/°C with a linearity of 0.989 within temperature ranging from 20 °C to 800 °C, −582.5 nm/RIU (refractive index unit) with a linearity of 0.998 in the RI ranging from 1.342 to 1.380, and −9.45 × 10-4 dB/με with a linearity of 0.98 in the strain ranging from 0 to 2925 με, respectively. The high sensitivity of prepared LPG will have potential application values in sensing measurement of multi-parameters and in the field of high temperature and harsh environments detection.

1. Introduction Long period gratings (LPGs), whose grating period is approximately 10 to 100 μm [1], as important fiber gratings, fabricated in a singlemode fiber (SMF), have been widely applied as sensor in temperature [2,3], strain [4,5] and refractive index [6–8] sensing because of their high sensitivity to the environment and physical perturbations. They bring a series of coupling between fiber core mode and cladding mode at wavelengths of meeting the phase resonance condition, resulting in a series of attenuation bands in the transmission spectrum [9–11]. Owing to their environmentally-dependent narrow band rejection, LPGs are extensively used in optical sensors and communications applications

[12–14]. Moreover, the ambient perturbation can easily influence the cladding-mode, which makes it possible for the measurement of ambient temperature and external refractive index (RI) [15,16]. Compared with other LPGs fabrication methods, such as high-frequency CO2 laser [17] and the UV exposure [18], infrared femtosecond laser direct writing is one of the most attractive methods in terms of its flexibility [19–21]. The UV exposure methods need extra process to enhance the photosensitivity of the fiber [22], what’s worse, the prepared gratings will degrade at high temperatures. The LPGs fabricated by high-frequency CO2 laser has favorable thermos stability, but its sensing sensitivity is difficult to control accurately at the high temperature [23].

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Corresponding authors at: School of Arts and Sciences, Shaanxi University of Science & Technology, Xi’an 710021, China. E-mail addresses: [email protected] (Y. Zhang), [email protected] (Q. Xu). 1 Co-first author. https://doi.org/10.1016/j.optlastec.2019.105839 Received 6 June 2019; Received in revised form 1 September 2019; Accepted 11 September 2019 0030-3992/ © 2019 Published by Elsevier Ltd.

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0 L=25 mm

Transmission loss/dB

-2 -4 -6 -8

-10

1250

1300

1350

1400

1450

1500

1550

Wavelength/nm Fig. 1. Schematic diagram of experimental setup for fabrication of LPGs.

Fig. 2. The transmission spectrum of the LPG with Λ = 500 μm and D = 0.5, and inset shows the FWHM.

In this paper, a LPG was fabricated in standard SMF-28 fibers without hydrogen loading by adopting point-by-point direct writing based infrared femtosecond laser. Then, the sensing properties of LPG to temperature, external refractive index (RI) and strain are systematically investigated. The experimental results show that the fabricated LPG has higher sensing sensitivity to high temperature, external refractive index (RI) and strain, which will have potential applications in measurement of multi-parameters and the high temperature environmental monitoring.

2. Experimental setup and fabrication of LPGs The experimental setup for LPG writing is shown in Fig. 1 [19]. A femtosecond laser amplifier system (Spectra-Physics, Inc.) producing 800 nm laser pulses with pulse width of 100 fs and repetition rate of l KHz is used to fabricate LPGs in a standard communication fiber (Corning SMF-28), which is mounted on a computer-controlled 3D axis nano-translation platform with 5 nm resolution (Newport, Inc.). The laser pulse energy is attenuated by a neutral density attenuator, and then focused into the fiber core by a 100 × Oil microscope objective lens (NA = 1.25). A super continuum light source (NKT Photonics, wavelength range: 500–2400 nm) and an optical spectrum analyzer (OSA, YOKOGAWA AQ6370B, wavelength range: 1200–1700 nm) are used with a resolution of 0.06 nm to trace the transmission spectrum of the LPGs during grating inscribing. The single pulse self-focusing critical power is 3.5 μJ. But after attenuated, its energy irradiated in the fiber is estimated to be from 0.5 to 1.3 μJ. The LPGs were fabricated in SMF-28 by point-by-point direct writing inscription method based on femtosecond laser pulses. During the LPGs fabrication, the visualization of inscription process was realized by adopting the horizontal and vertical double CCD video to adjust alignment of fiber. According to the mechanism of femtosecond laser direct writing LPGs [19,24], it can be found that the spectral characteristics of LPGs as well as the transmission loss of resonance peak are closely related to the pulse energy, the scanning speed and grating parameter. In experiment, the 3D-plot moving speed, grating period (Λ) and inscription length (L) were set as 10 μm/s, 500 μm and 25 mm, respectively. After that, the pulse energy of the inscription process was 1.3 μJ measured before the microscope objective. Fig. 2 shows the transmission spectrum of the LPGs with the grating length of 25 mm and the measurement resolution of 0.5 nm [21]. It can be seen that a narrow and strong resonance peak with a coupling strength of 11.4 dB appeared at the wavelength of 1304.9 nm and its FWHM of 15 nm is shown in Fig. 2 Insert. The experimental setup for exploring the sensing sensitivity of the

Fig. 3. The schematic of the measurement system of sensing sensitivity.

self-made LPG is shown in Fig. 3. The measurement system consisted of a broadband light source with a wavelength ranging from 500 to 2400 nm (NKT Photonics, Inc.) and an optical spectrum analyzer with a wavelength ranging from 1200 to 1700 nm (YOKOGAWA, Inc). To eliminate the disturbance from strain, one side of the LPG was fixed on the fiber holder and the other side was hung through the pulley of 5 g weigh to keep it straight all the time. In addition, the sensing sensitivity of RI and temperature can also be measured by using this experimental system. 3. Experimental result and discussion The transmission spectrum of LPGs was formed by coupling light from the forward-propagating fundamental core mode into co-propagating cladding modes at different wavelengths, and its resonant wavelength λ m was determined by the phase-matching condition [25]. The formula is as follows: eff eff λ m = (nco − ncl,m )Λ

eff nco

(1) eff ncl,m

where, is the effective index of the core mode, is the effective index of the m order cladding mode, Λ is the grating period. The cladding modes can escape from the fiber into the atmosphere due to absorption and scatterings. It follows that the transmission spectrum in Fig. 2 can be changed with the temperature, RI and strain due to the fiber birefringence changes of the phase difference in Eq. (1). 3.1. Sensing characteristics of refractive index The refractive sensitivities of resonant wavelength dλ m /dn e can be 2

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Transmission loss/dB

0

dλ m eff eff eff eff = Λ(ξco nco − ncl,m ξcl ) + α (nco − ncl,m ), dT

where, ξco and ξcl are the thermo-optic coefficients of the core and cladding of the fiber, respectively [29], α is the thermal expansion coefficient of the fiber. From Eq. (3), the temperature sensitivity of the resonant wavelength is mainly determined by the thermal expansion effect and the thermal-optic effect of LPG. The mechanism of the thermal expansion effect of LPG is that the ambient temperature changes the grating diameter and length, which causes the change of the grating period and the drift of the resonant wavelength. The effect of LPG thermos-optic effect is mainly caused by the change of the effective refractive index of core mode and cladding mode. In addition, the temperature response of the LPG can be explored by using the experiment system in Fig. 3 to observe the shifts of the resonance wavelength. First, the LPG was placed into the preheated hightemperature tube furnace (Type: OTF-1200X) with its natural state. Then, the temperature was increased from 20 °C to 800 °C at an average rate of 20 °C /min with a step of 100 °C. The transmission spectra of each step were recorded when the temperature was stabilized for 30 min to ensure the establishment of the thermal equivalent. Meanwhile, both a wide band light source and an optical spectrum analyzer were connected to monitor the transmission spectra of heating process. The Fig. 5(a) shows the LPG response of resonance wavelength to temperature. The change of resonance wavelength and resonance peak loss with temperature is shown in Fig. 5(b). It can be seen that the resonance wavelength is red-shifted gradually with the temperature rising from 20 °C to 800 °C, but the resonance peak loss is basically kept stable from 0 °C to 400 °C, when the temperature is above 400 °C, the resonance peak loss increases obviously with temperature. The reason

-5

n=1 n=1.342 -10

n=1.355 n=1.365 n=1.370

-15

n=1.375 wavelength blue-shift

(a)

-20 1200

1225

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1350

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Wavelength/nm 1280

Experment Date Linear Fit of Mean

(b)

Wa v e len g th /n m

1275 1270 1265

(3)

y=2051.5-582.5x 2 R =0.998

1260 1255 1.345 1.350 1.355 1.360 1.365 1.370 1.375 1.380

Refractive Index

-2

Transmission loss /dB

Fig. 4. The transmission spectra of the LPG to external RI (a) and the linear fit of RI sensitivity (b).

obtained by Eq. (2) [26] eff dncl,m (n cl , n e) dλ m = −Λ dn e dn e

(2)

where, ne and n cl are the RI of different liquid material and the fiber cladding, respectively. It can be seen from Eq. (2) that the resonant peak loss with different values of m has different RI sensitivity [27], and the sensitivity of wavelength shift strongly depends on the diffraction order and the cladding-mode number. In this experiment, the RI sensitivity of LPG was characterized by full immersion of LPGs in a series of solution with different refractive index. The range of RI is from 1.342 to 1.380, which is smaller than that of the fiber cladding. In the experiment, the LPG was placed straightly in different solutions with keeping the temperature at 20 °C. And then, the changes of transmission spectra were monitored in real-time with OSA as shown in Fig. 4. It can be seen from Fig. 4(a) that the resonant wavelength presented blue-shifts with the increase of solution refractive indices from 1.342 to 1.380. In order to verify the sensitives of the proposed grating to external physical parameters, three groups of repeated measurements have been conducted and the data of the average fitting are shown Fig. 4(b). The fitting result shows that the mean sensitivity of response wavelength was −582.5 nm/RIU in RI range from 1.342 to 1.380 and its linear fitting rate of R-square is 0.998.

-4 -6

20ć 100ć 200ć 300ć 400ć 500ć 600ć 700ć 800ć

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wavelength red-shift

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Wavelength /nm

1380 (b)

-15 100

200

300

400

500

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Temperature / ć

3.2. Sensing characteristics of temperature

Fig.5. The transmission spectra of LPG heating process (a), the change of the resonant wavelength (left label) and the resonant peak loss (Right label) with increasing of temperature (b).

According to Eq. (1), it can also derive the temperature sensitivity dλ m /dT of the resonant wavelength as follows [28]: 3

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Temperature up Temperature down Average linear fit

1365 1350

-2

y=1285.85+0.124x 2 R =0.989

1335

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Wavelength /nm

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room temperature of 20 C after annealing 1260 1280 1300 1320 1340 1360 1380 1400

Wavelength /nm

100

200

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400

500

Temperature / ć

600

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800

Fig. 6. The temperature sensitivity of obtained 124 pm/°C with linearity of 0.989. Inset: The transmission spectra of the LPG at room temperature of 20 °C and after annealing.

for this phenomenon is that the edge intensity of laser spot is lower than the damage threshold, and the type I refractive index changes are formed, which will be bleached at higher temperature, resulting in an increase in modulated refractive index of grating. According to Eq. (3), the resonant peak loss of LPG is the coupling between the core-mode LP01 and the cladding mode LP11, so the resonance wavelength of LPG presents higher temperature sensitivity. During the temperature test, the transmission spectrum in the process of temperature decrease was measured as well. The LPG was firstly annealed at 800 °C for 2 h and then cooled down naturally to the room temperature in a step of 100 °C. The Fig. 6 shows the experimental and average fitting results of the repeated measurements, indicating that when the resonant wavelength was λ = 1343 nm and temperature of 500 0C, the repeatability is good enough and the resonant wavelength is very consistent during the process of heating and cooling. And more, the mean temperature sensitivities was 124 pm/°C with the linearity of sensitivity is 0.989. There are about 78 nm skewing of the resonance wavelength during the whole changing process of temperature. Since residual-stress relaxation in the LPG, the resonant wavelength at room temperature shifts from 1304.9 to 1330 nm after annealing as shown in Fig. 6 insert. The resonance wavelength shifts linearly with the change of temperature, which is consistent with the theoretical conclusion mentioned in Eq. (3), so the self-made LPG possess a higher temperature sensitivity than the results reported in literature [30].

Fig. 7. The transmission spectra of LPG to strain (a), the change of resonant wavelength (left label) and the resonant peak loss (Right label) with increasing of strain (b).

Eε =

(5)

the weight of the grating axial load can be converted into the strain variables of the grating. Where, ε = Δl/ l is grating strain, E = 7 × 1010N/m2 is the elastic modulus of the fiber, F is the axial tension of the grating, which is equal to the force of the weight, and S is the cross-sectional area of the fiber, in which outer diameter of the grating position is 125 μm. The Fig. 7(a) shows a typical spectral response of LPG with the increasing of strain. It should be noted that the resonant wavelength shift is not linear with the increasing of loadsensitivity, while the resonant peak loss increases linearly as strain increases, and the maximum resonant peak loss is −12.2 dB. As well, the repeated measurements of strain sensitivity have been operated as shown in Fig. 7(b). The Fig. 8 shows that the average linear fitting of strain sensitivity, which is −9.45 × 10-4 dB/με with the linearity of 0.98.

3.3. Sensing characteristics of strain The strain characteristics of LPG can be obtained from Eq. (1) [31–33]: co cl,m η neff − ηcl neff dλ m dλ = Λ m (1 + co co ) cl,m dε dΛ neff − neff

F , s

(4) 4. Conclusion

where, ηco and ηcl are the elastic-optic coefficients of the core and cladding of the fiber, respectively. Here, the strain sensing of the LPG was studied by applying an axial load to the grating, and the weight was increased by 40 g each time until 260 g along axial force as shown in Fig. 3.The strain response was observed by monitoring the changes of the LPG transmission spectrum, the LPG used here was the grating of annealing for many times, which has a resonant wavelength of 1330 nm with resonant peak loss of 9.4 dB. During the strain experiments, 10 min was needed to the establishment of the force equivalent after the load was applied. Before the next load was applied, the former load should return to initial, revealing that the strain was completely released before the next strain load. According to Hooke's law [34],

In conclusion, the LPG with a grating length of 25 mm was successfully inscribed in no-hydrogen-loaded SMF-28 by using 800 nm femtosecond laser with point-by-point direct writing method. The strong resonance peak loss at the wavelength of 1304.9 nm appears maximum value of 11.4 dB with the out-of-band loss less than 3 dB. Then the sensing properties of the LPG to temperature, external refractive index (RI) and strain are studied systematically. The experimental results show that the resonant wavelength is red-shifted when the temperature changed from 20 °C to 800 °C and the temperature sensitivity of self-made LPG reaches 124 pm/°C with a linearity of 4

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Fig. 8. Relationships between transmission loss with strain.

0.989. The resonant wavelength is blue-shifted in the RI change ranging from 1.342 to 1.380, and the refractive index sensitivity is −582.5 nm /RIU with a linearity of 0.998. In addition, the resonant wavelength shows a slight blue-shift and the loss of resonant peak increases obviously with increasing of strain, the linearity and sensitivity are 0.98 and −9.45 × 10−4 dB/με, respectively. Therefore, the self-made LPG has a higher sensitivity to wavelength drift in refractive index and temperature sensing, and a higher sensitivity to resonant peak loss in stress sensing. The results will provide a theoretical and experimental reference for LPGs fabrication by femtosecond laser pulse and the application of LPG as multiple physical sensing in high temperature and hazardous environmental monitoring [35,36]. Acknowledgements This work was supported by the National Science Foundation of China (No. 11647008), the Open Research Fund of State Key Laboratory of Transient Optics and Photonics (No. SKLST201802), the International Science & Technology Cooperation and Exchanges Project of Shaanxi (No. 2018KW-016), the Key Sciences and Technology Project of Baoji City (No. 2015CXNL-1-3), and the science and technology project of Xianyang City (No. 2018K02-60). References [1] X.-Y. Sun, P. Huang, J. Zhao, L. Wei, N. Zhang, D. Kuang, Characteristic control of long period fiber grating (LPFG) fabricated by infrared femtosecond laser, Front. Optoelectron. China 5 (3) (2012) 334–340. [2] V. Bhatia, A.M. Vengsarkar, Optical fiber long-period grating sensors, Opt. Lett. 21 (9) (1996) 692–694. [3] J. Ruan, W.-G. Zhang, H. Zhang, L.-M. Yin, X.-L. Li, P.-C. Geng, J. Ruan, Temperature and twist characteristics of cascaded long-period fiber gratings written in polarization-maintaining fibers, J. Opt. 14 (10) (2012) 105403–105406. [4] G. Rego, P.V.S. Marques, J.L. Santo, H.M. Salgado, Arc-induced long-period gratings, Fiber Integr. Opt. 24 (2) (2005) 245–259. [5] M.J. Kim, Y.H. Kim, G. Mudhana, B.H. Lee, Simultaneous measurement of temperature and strain based on double cladding fiber interferometer assisted by fiber grating pair, IEEE Photon. Technol. Lett. 20 (15) (2008) 1290–1292. [6] H.J. Patrick, A.D. Kersey, F. Bucholtz, Analysis of the response of long period fiber gratings to external index of refraction, J. Lightwave Technol. 16 (9) (2002) 1606–1612. [7] Y. Fan, T. Zhu, L.-L. Shi, Y.-J. Rao, Highly sensitive refractive index sensor based on two cascaded long period gratings with rotary refractive index modulation. 21st International Conference on Optical Fiber Sensors, Int. Soc. Opt. Photon. 50 (23) (2011) 4604–4610. [8] L. Benye, J. Lan, S.-M. Wang, T.-H. Lung, H. Xiao, Femtosecond laser fabrication of long period fiber gratings and applications in refractive index sensing, Opt. Laser Technol. 43 (8) (2011) 1420–1423.

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