CHIRPED FIBER BRAGG GRATING (CFBG) BY ETCHING TECHNIQUE FOR SIMULTANEOUS TEMPERATURE AND REFRACTIVE INDEX SENSING

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1 CHIRPED FIBER BRAGG GRATING (CFBG) BY ETCHING TECHNIQUE FOR SIMULTANEOUS TEMPERATURE AND REFRACTIVE INDEX SENSING Siti Aisyah bt. Ibrahim and Chong Wu Yi Photonics Research Center Department of Physics, Faculty of Science, University of Malaya, 563 Kuala Lumpur, Malaysia. ABSTRACT: This paper presents the fabrication of chirped Fiber Bragg Grating (CFBG) from uniform Fiber Bragg Grating (FBG) by etching technique in order to utilize it as refractive index sensors. Etching is carried out with the presence of a layer of silicone oil on top of the hydrofluoric acid to achieve partial-etching of the FBGs. The partiallyetched FBGs exhibit two different modes: fundamental mode from the FBG region still protected by fiber cladding; modes excited by exposed FBG region due to different effective index. The CFBG has a temperature sensitivity of.nm/ C. Refractive index response is also tested by immersing the CFBG into liquid with refractive index ranging from.38 to.37 and it is found that only modes excited by exposed FBG region respond linearly to refractive index change. This characteristic shows that the CFBG can be used for simultaneous sensing of temperature and refractive index. Keywords: Fiber Bragg Grating (FBG), Chirped Fiber Bragg Grating (CFBG), Refractive index measurement, Temperature sensitivity, Etching technique I. INTRODUCTION Fiber Bragg Grating (FBG) has attracted growing interest[] in recent years in the field of optical sensing due to the advantages such as electromagnetic interference immunity, can resist corrosion, high sensitivity to temperature, strain and etc.[] An optical fiber comprises two layers; the core and the cladding. The core of fiber is made up of dielectric material which is a glass or silica for instances. The layer of material that coats the core is called cladding which have a lower refractive index compared to the core and is generally made up of glass. Several functions of cladding are to reduce scattering of light from the core to the surrounding, protect the fiber from absorbing contaminants.[3] Usually, the diameter of fiber core is about 8 µm and 5 µm for cladding. Basically, FBG is a fiber that has a periodic variation of refractive index along its core. These periodic variations[4] act as a filter for certain wavelength depending on the grating period and effective index of core. A narrow band wavelength will be reflected by the grating and the other wavelengths are transmitted after propagating inside the core and the reflecsimplifyted wavelength is called Bragg wavelength.[5] The grating inside the core is fabricated by exposing the fiber to the UV laser source and based on the principle of photosensitivity the periodically varying refractive index of an in-line grating is formed. There are many techniques used to fabricate the FBG but the most effective and preferable technique is phasemask techniques. This is because of its simple writing setup which reduces cost and simplifies optical alignments.[6] As is well known, the reflected Bragg wavelength condition is shown as [7] λ B = n eff Λ () where, λ B is the reflected Bragg wavelength, n eff is the effective index of fiber and Λ is the grating pitch. The equation above shows that the reflected Bragg wavelength is dependent on effective index and the grating pitch of the fiber. If these two factors experience any changes, the reflected Bragg wavelength is changed as well. There are many types of Fiber Bragg Grating structures such as uniform FBG, chirped FBG, tilted FBG and superstructure FBG, each giving a unique spectral response. In this paper, we are interested in uniform FBG and chirped FBG. Chirped FBGs is the modified grating structure in order to add other aspects such as linear variation in grating period.[8] The incoming signals are reflected at different point with different time delays along the gratings. There are many ways to produce chirped FBGs such as: exposure to UV beams of nonuniform intensity of the fringe pattern[9], incorporating a chip in the inscribed grating and by establishing strain gradient[] for instances. In this experiment by using a normal FBG, we can create a chirped FBG-based sensor using etching technique, thus we can vary the effective refractive index along the normal FBG which results in a change of Bragg wavelength. Spectral responses of chirped FBG to temperature and refractive index change are studied and compared with normal FBG.

2 II. METHODOLOGY: A. Splicing process Splicing process is a process to permanently join two fibers by fusing their end tips together. Before undergoing a splicing process, the buffer of the fibers must be striped out about cm and cleave to get smoothly cutting in order to reduce the energy loss during the splicing process. To make sure the striped part does not break, that part is temporarily covered on hard cardboard. Fabrication of FBG 44nm UV laser light is used to inscribe the grating inside the fiber core through a phasemask. The laser source is aligned with the phasemask and the fiber. After the UV laser passing through the phasemask, it is diffracted in order of ± modes and a volumetric interference pattern is formed. The result is a modulation of refractive index according to the interference pattern. The optical fiber is exposed to the laser source until the transmission dip of the spectrum on the OSA shows about -3dB which correspond to 99.9% reflectivity. B. Etching process In order to make the FBG sensitive to the refractive index changes, the effective refractive index must be varied. To vary the effective refractive index, FBG must undergoing etching process using HF and BOE solutions to remove the cladding. The spectrum of the FBG is monitored during the etching process and complete etching of the cladding is observed as emergence of second Bragg modes in the shorter wavelength. The diameter of the fiber core after etching is approximately 6 µm. A layer of silicon oil is added on top of the HF or BOE solutions. The purpose of this silicon oil is to protect the FBG part intended to be preserved, thus creating a half-etched FBG where the etched part is sensitive to the changes of liquids refractive index while the remaining part is utilized as temperature sensor and to offset wavelength response of the etched FBG caused by temperature change. To create a tapered profile, the FBG is raised vertically from the etching solution during the whole etching process. C. Experimental setup of optical fiber sensor The figure below shows the experimental setup for refractive index of liquids sensitivity. The instruments used are broadband light source; optical circulator and optical spectrum analyzer (OSA). The broadband light source is incident to port of the optical circulator which route the light source toward port and the FBG sensor probe. Reflected spectrum from the FBG is then routed by the circulator from port to port 3 towards the OSA. For temperature calibration the FBG sensor probe is placed in a beaker filled with de-ionized water and the temperature of the water is controlled by a hot plate. The spectral response of the FBG with different temperature is recorded. Figure : Experimental setup for refractive index sensing For refractive index measurement, the FBG sensor probe is immersed in liquids with different refractive index. The liquids used are water, methanol, acetone and isopropanol and their refractive index are.333,.38,.35 and.37 respectively. Spectral response of the FBG in air is also recorded for comparison purposes. III. RESULTS AND ANALYSIS 5µm Figure : Micrograph of FBG before etching process

Power, P (dbm) -35-45 -55-65 557 558 559 56 Wavelength, (nm) Figure 3: Reflection spectrum of FBG before etching process. Figure above shows a micrograph of the FBG with a diameter of 5 µm.

Figure 4: Micrograph of tapered FBG after etching process. The FBG spans from the thinned region to the unetched region of the fibre.

3 Power, P (dbm) Wavelength, (nm) Figure 3: Reflection spectrum of FBG before etching process. Figure above shows a micrograph of the FBG with a diameter of 5 µm. The reflection spectral response of this FBG is shown in Figure 3, where only a single resonant Bragg mode is observed. The Bragg wavelength is centered at nm. Figure 4: Micrograph of tapered FBG after etching process. The FBG spans from the thinned region to the unetched region of the fibre µm Water Methanol Isopropanol Aceton Wavelength, λ (nm) Figure 5: Spectral response of tapered FBG after etching process. Air Wavelength, nm Figure 6: Spectral response of tapered FBG in water with different temperature. After etching of the FBG is carried out, a thinned fiber region is observed as shown in Figure 4. The diameter of the fiber core after etching is approximately 6 µm. The tapering length is about 85 µm. The spectral responses of the etched FBG when immersed in different liquid solutions are shown in Figure 5. As the thinned FBG region are exposed to the surroundings and have a refractive index lower than the original cladding material, an additional Bragg resonance peak with shorter wavelength is observed. It is also observed that the original resonance peak does not responds to changing surrounding refractive index and only the new Bragg resonance peak shifts when the FBG is immersed into different liquid solutions. The shifting of the new resonance peak is influenced by the variation of effective index due to the different liquids refractive index. We can see in the equation (), if the grating pitch is constant; the reflected Bragg wavelength is affected by effective index and that the Bragg wavelength is directly proportional to the effective index. Figure 6 shows the spectral response of the FBG probe immersed in water with different temperature. It can be seen that both of the resonant modes shifted with temperature change. The responsitivity of the original resonance Bragg wavelength to only temperature allows offset of the effect of temperature at the etched FBG caused by temperature change.

Wavelength (nm) Wavelength (nm) 559.3 559. 559. 559 558.9 558.8 558.7 558.6 558.5 y =.87x + 558.4 R² =.9946 5 5 Figure 7: Sensitivity of Bragg wavelength (Peak ) to temperature change. 554.6 554.

4 Wavelength (nm) Wavelength (nm) y =.87x R² = Figure 7: Sensitivity of Bragg wavelength (Peak ) to temperature change y =.5x R² = Figure 8: Sensitivity of Bragg wavelength (Peak ) to temperature change. Figure 9: Micrograph of Chirped FBG µm Wavelength, nm 565 Figure : Spectral response of Chirped FBG immersed in different liquid solutions with different refractive indices. Air Water Methanol Isopropanol Acetone Figure 7 and Figure 8 above show temperature sensitivity of the FBG with and without cladding. By comparison, the thinned FBG is more sensitive to temperature change compare to the unetched FBG, with temperature sensitivity of 5. pm/ o C and 8.7 pm/ o C, respectively. As the core and cladding material of a fibre have different thermal expansion coefficient (the core being larger than the cladding due to germanium doping), the absence of cladding allows the thinned FBG to have a larger response to temperature change. Micrograph of chirped FBG is shown in Figure 9. It can be seen that the tapering profile is more gradual and the tapering length is much longer (out of range of the micrograph). The gradual tapering of the FBG results in the gradual change of effective index along the FBG, where continuous spectral response instead of distinct peaks is observed, as shown in Figure. The Full Width at Half Maximum (FWHM) of the spectrum is also observed to change when immersed in different liquid solutions, where narrower FWHM results from immersion in liquid with higher refractive index. At the same time, the temperature response of the chirped FBG can be measured by the shift of the original resonance peak (indicated as in Figure ) and the results plotted in Figure. The temperature response shows a linear relation

5 Wavelength, (nm) between 3 o and o, with a sensitivity of. pm/ o C. The higher sensitivity compared to unetched FBG is believed to be due to reduced fibre diameter even at the larger diameter end of the FBG y =.x R² = Figure : The straight line graph for sensitivity of temperature (Peak ) IV. CONCLUSION: A FBG-based refractive index sensor for liquid solutions has been demonstrated. The reflected Bragg wavelengths are modulated by liquids with different refractive index. The experiments show that the higher the refractive index, the longer the reflected Bragg wavelength thus the peaks at point tend to shift to the right. The temperature of the liquids is constant and peaks at point do not affected by a constant temperature. Scientific and Industrial Research, 5. 64(): p [5] Hill, K.O. and G. Meltz, Fiber Bragg grating technology fundamentals and overview. Journal of Lightwave Technology, (8): p [6]. Tahir, B.A., J. Ali, and R.A. Rahman, Fabrication of fiber grating by phase mask and its sensing application. Journal of Optoelectronics and Advanced Materials, 6. 8(4): p [7]. Hill, K.O., et al., NARROW-BANDWIDTH OPTICAL WAVEGUIDE TRANSMISSION FILTERS. Electronics Letters, (9): p [8] Byron, K.C., et al., Fabrication of chirped Bragg gratings in photosensitive fibre. Electronics Letters, (8): p [9] Quintela, A., et al. Arbitrary Chirped Fiber Bragg Grating fabrication technique. 5. [] Wei, Z., et al., Fabrication of high quality chirped fiber Bragg grating by establishing strain gradient. Optical and Quantum Electronics,. 33(): p ACKNOWLEDGEMENT A million thanks to my supervisor, Dr. Chong Wu Yi and all members of Photonics Research Center for giving me ideas and guide me to complete this research paper. REFERENCES: [] Canning, J., Fibre gratings and devices for sensors and laser. Laser and Photonics Reviews, 8. (4): p [] Xu, W., X.G. Huang, and J.S. Pan, Simple fiber-optic refractive index sensor based on fresnel reflection and optical switch. IEEE Sensors Journal, 3. 3(5): p [3]. Erdogan, T., Cladding-mode resonances in short- and long-period fiber grating filters. Journal of the Optical Society of America A: Optics and Image Science, and Vision, (8): p [4] Singh, N., et al., Fibre Bragg grating writing using phase mask technology. Journal of

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