Chirped polymer Bragg grating sensors

Carlos A. F. Marques*a,b, L. Pereirab, P. Antunesb, P. Mergoc, D. J. Webbd, J. L. Pintob, P. Andrée aInstituto de Telecomunicações, Campus de Santiago, 3810-193 Aveiro, Portugal bUniversity of Aveiro, Physics Department, Campus de Santiago, 3810-193 Aveiro, Portugal cLaboratory of Optical Fibers Technology, Maria Curie Sklodowska University, Lublin, Poland dAston Institute of Photonic Technologies, Aston University, B4 7ET Birmingham, UK eInstituto de Telecomunicações and Department of Electrical and Computer Engineering, Instituto Superior Técnico, University of Lisbon, 1049-001 Lisbon, Portugal *E-mail: [email protected]

ABSTRACT

We report chirped fiber Bragg gratings (CFBGs) photo-inscribed in undoped PMMA polymer optical fibre (POF) for the first time. The chirped polymer optical fiber Bragg gratings (CPOFBGs) were inscribed using an UV KrF excimer operating at 248 nm. The rectangular gauss laser beam was expanded to 25 mm in horizontal direction along the fiber core by a cylindrical lens, giving a total of 25 mm grating length. A 25 mm long chirped phase mask chosen for 1550 nm grating inscription was used. The laser frequency was 1 Hz with an energy of 5 mJ per exposure, exposing few pulses for each grating inscription. The amplitude spectrum evolution of a CPOFBG is investigated as a function of the applied strain and temperature. Also, some results regarding to group delay are collected and discussed. These results pave the way to further developments in different fields, where POFs could present some advantages preferably replacing their silica counterparts. Keywords: Fiber Bragg gratings; Polymer optical fiber sensors; chirp, sensing.

1. INTRODUCTION FBGs in POF (POFBGs) are claimed to offer an interesting alternative to silica FBGs for sensing applications [1–4]. There are a several distinct types of FBG structures such as: the uniform Bragg grating characterized by a constant grating pitch (spacing between grating planes), the tilted FBG (TFBG) which has the grating planes tilted with respect to the fiber axis, and the chirped Bragg grating (CFBG) that has a non-periodic pitch, displaying a linear variation in the grating pitch, called a chirp [5,6]. CFBGs or also called aperiodic Bragg gratings are known in the literature for gratings with chirp and generally refer to gratings in which the resonance condition varies along its length. If the variation of the periodicity is small, it can be considered locally uniform. Because of this, each part of the grating will reflect different without affecting each other [7]. The most common aperiodic FBGs are gratings with variable period along its longitudinal extension. The sensing element behaves as a layer of FBGs, in which different Bragg wavelengths are encoded along the fiber axis z [6]. The modulation period ᴧ(z) follows a linear profile

()zkz 0 ), for 0 < z < L, where L is the grating length; k is the chirp coefficient, which defines the increase of period per unit of length. Of course, the period of an aperiodic FBG can be expressed by a polynomial of N degree given as ᴧ(z) = ᴧ0 + ᴧ1z + … + ᴧN zN, which is important for optical communications field. The aperiodicity has effect in two properties of Bragg gratings: the bandwidth and group delay. The group delay characteristics of the grating are different from conventional gratings. Since different wavelengths are reflected in different positions on the fiber, it causes a group delay dependent on the . This kind of FBGs and analysis are used in optical communications to compensate the in different situations or others applications [14,15]. For sensing applications, there are many works using silica CFBGs for refractive index and strain measurements, biomedical, splits and transverse cracks in structural health monitoring (SHM) by local pressure analysis, or liquid level monitoring [10,11]. For the aforementioned applications, POFs bring additional benefits, such as robustness, cost-effectiveness, ease of installation and improved biocompatibility. Also, photo-inscription in POFs has matured to such a degree that it now becomes feasible to investigate CPOFBGs fabrication, which requires more efficiency than uniform FBGs.

Micro-structured and Specialty Optical Fibres V, edited by Kyriacos Kalli, Jiri Kanka, Alexis Mendez, Pavel Peterka, Proc. of SPIE Vol. 10232, 102320N · © 2017 SPIE CCC code: 0277-786X/17/$18 · doi: 10.1117/12.2264902

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2. EXPERIMENTAL SETUP T hanks to an optimized setup compared to [ 12 ], we report CFBGs photo - inscription in undoped PMMA step - index POF. The fiber used in this work was manufactured at the M aria Curie - Sklodowska University, Lublin, Poland . It is characterized by a core diameter of 4 μm and a cladding diamet er of 210 μm. The cladding and core are in pure PMMA, no dopant in the core is used. CPOFBGs were inscribed using an UV KrF Bragg Star TM Industrial - LN pulsed KrF operating at 248 nm. The laser has a beam spot of 6 mm in width and 1.5 mm in he ight, with pulse duration of 15 ns. A cylindrical lens shapes the beam before it arrives to the phase mask, designed for 248 nm operation. The rectangular gauss laser beam was expanded to 25 mm in horizontal direction along the fiber core by a cylindrical lens, giving a total of 25 mm grating length. A 25 mm long chirped phase mask customized for 248 nm chosen for 1550 nm grating inscription (Ibsen Photonics) was used. The laser frequency was 1 Hz with an energy of 5 mJ per exposure, exposing few pulses for each grating inscription [ 12 ]. 18 cm long POF sections were placed within two magnetic clamps and kept in strain to avoid undesired curvatures. An interrogation system was used to monitor the grating growth, presenting a wavelength resolution equal to 1 p m. Figure 1 depicts the experimental setup. Following grating inscription, in order to characterize the CFBGs, the POF sections containing the FBGs were UV - glued (Loctite 3936) to one 8° angled silica single - mode fiber pigtails.

4 UV Laser

UV beam path

Mirror r Holder (v- groove).., for silica pigtail

Fig. 1 . Setup used for the inscription of CPOFBG in real time in an undoped PMMA step - index POF by the phase mask technique.

3. EXPERIMENTAL RESULTS Figure 2 (a) depicts the reflected amplitude spectra of 25 mm long CPOFB Gs photo - inscribed using 14 pulses of UV light exposure. The reflected spectrum displays a much broader but weaker reflection band when compared with uniform FBG [23]. Thus, one consequence of this linear variation of the period is a higher bandwidth compa red to an uniform FBG [23]. The full - width half - maximum bandwidth (FWHM) of the FBG ranges from 1579.2 nm to 1583.1 nm (3.9 nm bandwidth). The maximum reflectivity is 16.9 dB for 14 pulses of exposure, while the spectrum shows a 3.8 dB ripple within the re flected bandwidth. Increasing the nu mber of pulses, the reflectivity decreases and we observed more ripple in the reflected bandwidth. We can inscribe silica CFBGs with 31 dB of amplitude, 4.1 nm of bandwidth (similar using POF) and a lower ripple within t he reflected bandwidth using the same phase mask [ 1 3]. Also , we show the first group delay collected from a 25 mm long CFBG photo - inscribed in POF, achieving a dispersion (defined as the rate of change of group delay, τ, with wavelength) of around - 66 ps/n m, displayed in Fig. 2 (b) .

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-52 Reflected spectrum CPOFBG (a) (b) Group delay 600 -55-After 14 shots -56- 4000 -, o Ê -60 - m -0

m a -64 - :°1 o -65- ca û- -200 -68 - -70 400 1575 1582 1589 1576 1580 1584 Wavelength (nm) Wavelength (nm)

Fig. 2 . (a) Reflected spectra for 25 mm long CPOFBGs after 14 pulses. (b) Group delay from a 25 mm long CFBG photo - inscribed in POF.

A strain characterization was performed in order to show the spec tral dependence of the Bragg reflection peak with strain. From our strain results, it is clearly shown that the CPOFBGs under strain become wider and the peak reflection remains stable and the main reflection band shifts to longer wavelengths. The results are shown in Fig. 3 and as it can be seen the Bragg wavelength shift was linearly red shifted 9.8 nm with 0.6% deformation. After adjusting a linear regression model, the sensitivity obtained was 1.63±0.02 pm/με, which represents more than 35% when compare d with values already reported for tapered silica CFBGs [ 14 ]. From literature [ 15 ], where was done some simulation work using CPOFBGs, the authors achieved a Bragg wavelength shift of ~6 nm with 0.3% deformation, which corresponds 1.96 pm/με of strain sens itivity.

CPOFBG Linear fit:1.63 ±0.02 pm /µs

0A0 0.3 01.6 s (%)

Fig. 3 . Bragg wavelength shifts obtained from the inscribed CPOFBG under different strains.

Additionally, experiments were carried out to explore the temperature respon se of CPOFBGs. The fibers were placed in an environmental chamber under varying temperatures to study their response. The temperature was increased from 22°C up to 47°C with steps of 5°C. In each step, the temperature was kept constant over 30 min to ensur e thermal equilibrium was achieved. There was no control of the humidity level in the chamber. Fig. 4 (a) shows the reflection spectra to variations in temperature. We observed that the ripple in the reflection spectrum is more pronounced from 37°C. The th ermal tuning characteristic of PMMA CPOFBG is shown in Fig. 5 (b). The temperature sensitivity obtained was - 117.4±4.1 pm/°C, after adjusting a linear regression model. It can be seen that 3 nm tuning range can be achieved only with the temperature variati on of 25°C, which is larger than the achieved in silica CFBG by the several hundred - degree temperature variation [ 14 ]. From simulation work using CPOFBG [ 15 ], almost 10 nm tuning range was achieved with the temperature variation of 55°C (grating was heated up from room temperature to 75°C), which corresponds - 180 pm/°C of sensitivity.

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Temperature variation 22 °C 0- (b) Linear fit: -117.4 ± 4.1 pm/°C (a) -48 - 27 °C 32 °C

37 °C 42 °C

47 °C

,

1575 1582 1589 20 25 30 35 401 45 50 Wavelength (nm) Temperature (°C) Fig. 4 . (a) Reflected spectra for different temperatures. (b) Bragg wavelength shifts obtained from the inscribed CPOFBG under different temperat ures.

Up to now, although the ripple within the reflected bandwidth can be slightly significant, we are able to produce high - quality CPOFBGs. Moreover, in order to reduce the ripples of the grating spectra the “raised - cosine” apodization could be included in the grating profile [5]. Also, with continuous improvement of POF photosensitivity in undoped fibers [ 12 ] and optimized photo - writing processes, we believe that this limitation should be soon overcome. Furthermore, the group delay ripple is high and im provements need to be done.

4. CONCLUSIONS In conclusion, we have achieved the very first chirped fiber Bragg gratings photo - inscribed in POFs. The fast growing achieved is related with the high absorption of the PMMA under 248 nm UV light. The CPOFBG was r ecorded in the core of an undoped step - index POF and presents a rejection band of more than 16 dB and an FWHM bandwidth of 3.9 nm. The spectral dependence of the CPOFBG with strain is linearly defined with a sensitivity of 1.63±0.02 pm/με. A CPOFBG was cha racterized to temperature and a sensitivity reaching - 117 . 4±4.1 pm/°C was reported with the temperature variation of 25°C. To conclude, these first results pave the way to further developments of CFBGs in POFs (using even TOPAS or polycarbonate fibers) and to their use for different domains, such as biomedicine and SHM (including in aeronautical structures) where POFs could preferably replace their silica counterparts, with improved bending tolerance and biocompatibility.

Acknowledgment . This work was sup ported by Fundação para a Ciência e Tecnologia (FCT)/MEC through national funds and when applicable co - funded by FEDER – PT2020 partnership agreement under the projects UID/EEA/50008/2013, and UID/CTM/50025/2013. C. A. F. Marques also acknowledge the finan cial support from FCT through the fellowship SFRH/BPD/109458/2015. The research leading to these results has also received funding from the Marie Curie Actions of the European Union's 7 th Framework Programme FP7/2007 - 2013/ under REA grant agreement No. 608 382.

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