methanol ratio as a hybrid solvent on fabrication of polymethylmethacrylate optical fiber sensor

Optics and Laser Technology 123 (2020) 105896

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Effect of acetone/methanol ratio as a hybrid solvent on fabrication of polymethylmethacrylate optical fiber sensor ⁎

T

⁎

Zahra Samavatia, Alireza Samavatia, , Ahmad Fauzi Ismaila, , Noorhana Yahyab, Mukhlis A. Rahmana, Mohd Hafiz Dzarfan Othmana a b

Advanced Membrane Technology Research Centre, Universiti Teknologi Malaysia (UTM), 81310 Johor Bahru, Malaysia Department of Fundamental and Applied Sciences, Universiti Teknologi Petronas (UTP), 32610 Bandar Seri Iskandar, Malaysia

H I GH L IG H T S

highly sensitive optical fiber probe by using an accurate chemical etching method is fabricated. • AAbsorption coefficient, penetration depth and scattering are affected by surface roughness. • A precise refractive index sensor through controlled cladding removal technique is proposed. •

A R T I C LE I N FO

A B S T R A C T

Keywords: Fiber technology Polymer optical fiber Sensors Surface roughness

The key success for fabrication of highly sensitive optical fiber probe is precise control of cladding thickness and surface roughness. An easy, economic and accurate chemical etching technique for fabrication of multimode polymer fibers sensor is investigated. The cladding diameter of multimode optical fiber is reduced up to ~100 nm by immersing in mixture of acetone/methanol. To obtain the required cladding diameter and roughness, the etching time, solvent concentration and etching temperature have been varied. An approach for dynamic monitoring of etching using 850 nm light power transmitted through the fiber is used to determine the optimum solvent concentration in which the sensitivity of the probe is highest. The fabricated sensors are subjected to detect the refractive index changes of saline and crude oil having different concentrations. The maximum sensitivities of ~9.1 and ~24.2 dB/RIU are achieved once the probe immersed in saline and crude oil respectively. The optimum parameters for having highly sensitive sensor are mixture of acetone/methanol in the rate of 40/60 and solvent temperature of 15 °C. Interference of core and cladding mode, scattering, refraction, and absorption of evanescent waves are responsible for light intensity modulation.

1. Introduction

stripping the cladding can be divided into mechanical and chemical methods. Polishing the fiber is the most common mechanical technique to remove the cladding [4]. However, significant disadvantage of this method is that the fiber optic stripper can potentially damage the fiber core and it typically requires more expensive equipment. Furthermore, lasers and precision lenses on laser processing platforms with a moving mechanism as another mechanical method are used for removing purpose, however, the precision lens on the laser processing platform tends to age, which may affect the accuracy of the moving platform and lead to cause experimental errors. Moreover, the problems such as an inability to correctly remove materials, and/or changes in the material properties may occur using the laser [5]. The chemical method like using of various solutions such as hydrofluoric acid (HF) for GOF and organic solvents for POF was employed by different researchers [6–9].

Polymer optical fiber (POF) has many advantages over conventional silica optical fiber, especially in sensing technologies, which include immunity to electromagnetic interference, multiplexing capability, compact, lightweight, and higher sensitivity. The wide application of POFs in sensor design is originated from the low cost competitive system over the glass optical fiber (GOF). Large diameter of multimode POF allows propagation of multiple modes [1,2]. For developing the sensor based polymer optical fiber, the diameter of POF must be reduced by removing the cladding part. Due to refraction of evanescent wave in the cladding and its absorption in surrounding media, the etched region of POF becomes more sensitive [3]. For removing the POF cladding, several ways are existed. The current techniques for ⁎

Corresponding authors. E-mail addresses: [email protected] (A. Samavati), [email protected] (A.F. Ismail).

https://doi.org/10.1016/j.optlastec.2019.105896 Received 30 June 2019; Received in revised form 10 September 2019; Accepted 12 October 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.

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However, the etching quality is difficult to control because a slight error can generate unexpected processing phenomena and affect the sensing quality. Regardless of which method is employed, they all provide a lack of comprehensive post-processing quality control procedures. Based on the above discussion, to minimize the disadvantages of chemical etching we improve the etching process by dilute the solvent with methanol and control the etching temperature. The evanescent wave in fiber optic is a part of light leak in cladding part, whereby the incident between different refractive index of core and cladding is transmitted into the cladding rather than being reflected back based on the principle of total internal reflection [10]. Fiber-optic evanescent wave sensors provide fast and reliable responsive results during real-time monitoring of surrounding refractive index changes thus, they have been widely applied in chemistry [11], biochemistry [12], life sciences [13], and environmental research [14,15]. The sensitivity of fiber-optic evanescent wave sensors is directly affected by attenuation of the evanescent waves on the partially unclad fiber surface [16,17]. For improving the sensitivity of the fiber different types of sensors based on shape variations of unclad part and fiber end region have been fabricated [18,19]. It was discovered that the sensitivity of optical fiber sensors depends on the radius of the fiber, taper waist length, launch angle and surface roughness of the sensing area [20]. Although the light-scattering loss is increased by increasing the surface roughness, the sensors with rough surfaces exhibit higher sensitivity than those with smooth surfaces [21]. While the effects of core roughness after fully etching the cladding, on performance of conventional core-clad structure glass fiber sensors have previously been studied [19,22], the effects of cladding roughness associated with varying organic solvent concentration on sensitivity of partially unclad POF probe have not been investigated extensively. Thus, in this study, to clarify the effects of acetone concentration on surface roughness and performance of the probe we have conducted the theoretical study on reflection, scattering of evanescence wave in cladding-surrounding interface and interference of different modes. Moreover, the experimental dynamic monitoring of transmitted light intensity in different refractive index media is carried out. The purpose of blending acetone which is ketone with methanol is to enhance the reactivity of the solvent towards the fiber surface.

Table 1 Final cladding diameter of POF during etching process with acetone/methanol having 40/60%.

Time (sec) Cladding Diameter (µm)

0 14

100 10

200 7

300 4

400 1

Appropriate time

Core damaging

423 0.1

500 −5

600 −14

After partially removing the cladding, cleaning process is done. The fiber was immediately immersed in deionized water for 30 min to remove the remaining acetone/methanol and prevent further etching. The experimental design is including tunable broad band source with a laser wavelength ranges from 360 to 2800 nm and a power meter (Thorlabs model PM100D) with an InGaAs sensor (Thorlabs model S145C) which are connected to the two end of fiber sensor. The detector is connected with a PC to monitor and collect the power loss spectra due to etching reaction time. For sensitivity testing purpose sodium chloride and crude oil solution having variety of concentration corresponding to different refractive index are applied as surrounding media. The amount of 5, 10, 15 and 20 g sodium chloride per 100 ml in de-ionized water is solved to prepare a solution with the refractive index ranges from ~1.342 to ~1.368. The solution refractive index is measured by KRUSS OPTRONIC DR201-95 refractometer. For the second set of measurand media, different concentration of crude oil from 5% to 100% using hexane as a solvent (99.9% purity, Sigma-Aldrich, USA) is employed. For these solutions the refractive index varied from ~1.382 to ~1.478. Then the fiber probe is fixed in petri dish and the solutions are filled in petri dish one at a time. The weak intermolecular bonding of crude oil molecules and saline molecules and the fiber surface make it possible to be easily removed by washing with hexane and distilled water. It should be noted that the hexane is also can be removed in ambient condition. Therefore, sensor demonstrates the acceptable repeatability. The surface morphology of the samples is characterized using atomic force microscopy (AFM) (SPI3800) built by Seiko Instrument Inc., (SII). To observe acetone/methanol effect on the fiber surface and the formation of pits scanning electron microscope (FESEM, JEOLJSM6380LA) are used.

2. Experimental The length of 3 cm polymer Graded-index multimode optical fiber (Jiangxi Dashing POF Co., Ltd) are subjected to be partially unclad by mixture of acetone/methanol to prepare fiber probe. The POF consists of poly (methyl methacrylate) (PMMA) core having 486 μm diameter (RI: 1.492) and a fluorinated polymer as cladding with 14 μm in thickness (RI: 1.402). The etching process is carried out using mixture of acetone/methanol having percentages ratio of 50/50, 40/60, 30/70 and 20/80. After finding the optimum mixture of acetone and methanol value, the etching temperature for that specific concentration varies from 15 °C to 30 °C. To have more effective interaction with evanescent light and outside media the cladding thickness should be as narrow as possible. From another aspect, removing process should not interrupt the total internal reflection and light propagating through the core. Therefore, for finding the appropriate optimum etching time and for preventing core surface corrosion which causes huge loss in propagating light, the dynamic monitoring records on transmission intensity was carried out when the multimode POF was immerse in acetone/ methanol solvent and the results are summarized in Table 1. By using this technique not only the small thickness of cladding remain but also the light intensity lost after partially removing process still is in acceptable level. The appropriate etching time is found to be 423 sec for 40/60 acetone/methanol ratio. Further increasing the etching time causes damaging the core (negative number in Table 1). The corresponding FESEM cross section images of different etching time is presented in results and discussion part.

3. Results and discussion Since sensitivity plays a significant role in determining the performance of optical probes, thus, high sensitivity is always desired. In order to further enhance the sensitivity of the multimode fiber sensor, the remain cladding part should be as small as possible. However, other parameter such as surface roughness of partially removed cladding also plays a predominant role. The FESEM images of the partially removed cladding surface of the samples prepared at different solvent concentration are shown in Fig. 1. The altering in the surface roughness after immersing at different solvent concentration can be observed in the figures. The cylindrical-shaped holes are produced by chemical etching and they become deeper by increasing the solvent concentration to 40/60. Further increasing the concentration of methanol leads to formation of narrower holes results in smoothing the fiber surface. This behavior can be described as bellow. The interaction of PMMA-acetone is strong, since acetone is a good solvent for PMMA. Therefore, etching process becomes uncontrollable and causes breaking the fiber before removing the cladding. A mixture of methanol and acetone should be miscible since both are polar and can interact with each other using hydrogen bonds. Thus, the hydrogen of hydroxyl groups, present in solution, readily bonds with the oxygen in ester groups present on PMMA. Using pure solvent (acetone) provides a strong and deep penetration into polymer fiber, an often destructive phenomenon which leads to break the fiber. Through solving 2

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Z. Samavati, et al.

(b)

(a)

10 μm

10 μm

(c)

(d)

10 μm

10 μm

Fig. 1. Top-view FESEM images of the fiber sensor prepared at acetone/methanol having percentages of (a) 50/50, (b) 40/60, (c) 30/70 and (d) 20/80.

Fig. 2. Schematic of acetone/methanol etching mechanism on polymer optical fiber and corresponding propagating light through original polymer, when acetone/ methanol having percentages of (a) 50/50, (b) 40/60, (c) 30/70 and (d) 20/80 is applied.

The diameter of the fiber cladding in the region exposed to acetone/ methanol, declines and the surface roughness altered based on the reactions dominating for low and high methanol concentration depicted in Fig. 2. The original figure of POF once light propagating through them after etching in different solvent concentrations is illustrated in Fig. 2. It is clearly seen that the evanescent interaction with outside media is higher for the fiber using acetone/methanol with 40/60

the PMMA in acetone/methanol mixtures, the methanol encourages aggregation of PMMA monomers from one side and penetration of acetone into the PMMA molecule by hydrogen bonds from other side leads to having different surface roughness in different concentration of these two polar solutions. The higher roughness is occurred at 40/60 (acetone/methanol) concentration. By increasing the amount of methanol the roughness decreases because of less penetration occurred. 3

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Fig. 3. 3D AFM images of partially removed polymer fiber using acetone/methanol concentration of (a) 50/50, (b) 40/60, (c) 30/70 and (d) 20/80.

concentration as the higher fraction of light can be leaked outside. For further morphological study the AFM analysis is conducted and explained as following. Fig. 3 shows 3D AFM images of the prepared sample surface etched in different acetone/methanol concentration. The rms roughness of cladding surface extracted from AFM for acetone/methanol having percentages of 50/50, 40/60, 30/70 and 20/80 are 5 nm, 25 nm, 12 nm and 10 nm respectively. For further sensitivity investigation of the fiber probe the optical transmission in fiber optics sensor through partially removed cladding fiber having rough surface is considered and discussed as bellow. The total internal reflection through rough surface is schematically shown in Fig. 4 and the transmission of light through an absorbing medium is described by the Lambert–Beer law by following equation,

Iout = Iin

e−ξ (n) L

( (

π

ξ (n) =

cos2 ⎡ 2 − arcsin αλ ⎛ n3 ⎞ ⎣ n + 2π (nr )2 ⎝ (nr )2 ⎠ sin ⎡ π − arcsin ⎣2 ⎜

⎟

n0 nr

n0 nr

) sin θ ) − arctan(R ) ⎤ ⎦

sin θi − arctan(Rr ) ⎤ ⎦ i

r

(2) where α is the bulk decay coefficient, λ is the light wavelength in free space which launched into the fibers, n is the refractive index of the surrounding medium, θi in which i = 1; 2; 3;…n; is the incidence angle of the light rays on the interface between the fiber cladding and the medium, r is the fiber radius in the sensing region, nr is the refractive index of the etched-fiber cladding as shown in Fig. 4 and Rr is the roughness of cladding surface which is defined by following equation [24], N

Rr =

(1)

where Iin is the incident light intensity, L is the length of sensing part (partially unclad), and ξ(n) is the decay coefficient of evanescent waves in the medium which is explained by Eq. (2) [23],

2 ∑i = 1 hi N

∑i = 1 Di

(3)

where hi is the local depth, and Di is the local diameter. The decay coefficient extracted from Eq. (2) is tabulated in Table 2. As can be seen by increasing the roughness the absorption coefficient enhances and lead to diminishing the transmitted light intensity. The output optical intensity is significantly affected by the surface and the losses in the transmitted intensity consist of the losses in optical intensity via scattering, refraction, and absorption of evanescent waves at the rough unclad fiber’s surface. When the fiber’s cladding is partially removed and the unclad fiber has a rough surface, the penetration depth Dp depends on the angle θi, roughness Rr, and refractive index n of the surrounding medium. Thus, it can be expressed by the following equation.

λ

DP = 2π

{

π (nr′)2sin2 ⎡ 2

⎣

− arcsin

(

n0 nr′

)

}

sin θi − arctan Rr ⎤ − n2 ⎦

1 2

(4)

The penetration depth of light is calculated and summarized in Table 2. It increases from 143 nm to 628 nm when acetone/methanol concentration changes from 50/50 to 40/60. Further increasing the methanol concentration causes decreasing the penetration depth. The sensitivity of the fiber probes depends on the attenuation of the evanescent waves, which is affected by their decay coefficient, penetration

Fig. 4. The schematic diagram of the light transition in partially unclad optical fiber. I(1) is the total internal reflection in the core (core mode) and I(2) is the reflection of evanescence (cladding mode) wave. 4

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Table 2 Roughness, absorption coefficient, penetration depth, light intensity and sensitivity of the sensors prepared at different acetone/methanol concentration. Acetone/methanol concentration (v/v percent)

Absorption coefficient, ξ′(n) ×10−6

Roughness, (Rr) (nm) ± 2

50/50 40/60 30/70 20/80

5 25 12 10

Penetration depth, Dp (nm)

1.3 7.4 3.8 3.4

Sensitivity in different concentration of

143 628 271 195

Saline (η) dB/RIU ± 0.5

Crude oil (η′) dB/RIU ± 0.5

η1 = 1.8 η2 = 9.1 η3 = 5.8 η4 = 2.1

η′1 = 10.1 η′2 = 24.2 η′3 = 16.2 η′4 = 12.6

(c)

(b)

(a) Cladding ~ 14μm

Cladding ~ 100 nm

Core

Core

Core 200 nm

20 μm

200 nm

Fig. 5. Cross section FESEM images of POF at etching process time of (a) 0, (b) 423 and (c) 500 sec in acetone/methanol (40/60) solution at 15 °C.

depth, and optical path length on the partially unclad fiber surface. Cross section FESEM images of fiber etched in 40/60 acetone/methanol ratio at 0. 423 and 500 sec are illustrated in Fig. 5. It is clear that few seconds after 423 sec the core start damaging. Considering the penetration depth, the evanescent wave can reach the media if the remaining cladding thickness is ~100 nm. For the diameter less than 100 nm probably more leaky modes has been incorporated and the power of light is lost. At higher thickness also less light is contact with media and sensitivity decreases. Therefore, controlled removing thickness is required to remove the appropriate thickness of cladding. The light intensity as a function of refractive index of the media for all probes prepared using different acetone/methanol concentration is depicted in Fig. 6. According to Mach–Zehnder interference theory, the interference of core mode Iout (1) and cladding mode Iout (2) is determined by the following two-beam interference equation [25]:

Iout = Iout (1) + Iout (2) + 2 Iout (1) Iout (2) cos ⎛ ⎝

2πLneff

⎜

+ φ0⎞ ⎠

It can be seen from Eq. (5) that the max transmission is taken placed when

2πLΔneff λ

(5)

where I is intensity of the total interference signal, ϕ0 is initial phase difference. L is the length of sensing part and λ is the wavelength. Δneff is the difference between effective refractive index of core mode and cladding modes which is explained as following [26]: core clad Δneff = neff − neff

(6)

(a) tan α=η1

-12

β

tan β=η2

-13 -14 -15

-9.8

α

α

γ

(Acetone / Methanol)

(50/50) (40/60) (30/70) (20/80)

1.38

1.40

ϕ

tan γ=η3

Intensity (dBm)

Intensity (dBm)

-11

(7)

and m is an integer number. Therefore, the changes in the refractive core and it is almost index of external environment does not affect the neff clad unchanged, however, the neff varies once the refractive index of the external environment changes. Consequently, any small change in the clad , Δneff refractive index of external environment leads to altering the neff and as a results intensity of propagating light received by detector. Both the experimental data and the predicted results show that the transmitted light intensity decreases with increasing the roughness, especially at high values of roughness. To this end, saline and crude oil solutions are prepared as an absorbing medium. The concentrations of the saline and oil ranged from 5% W∕V (n = 1.342) to 20% W/V (n = 1.372), and 5% V/V (n = 1.382) to 100% V/V (n = 1.478) respectively. The experiments were repeated three to five times for each condition. As can be seen, the transmitted light intensity Iout of sensors with different surface roughnesses decreases non-linearly by increasing the refractive index of both media. But this intensity variant which is the sensing element of the probes is more prominent for the sensor prepared with acetone/methanol (40/60) in saline and oil as well. It can be concluded that the fiber sensor prepared using acetone/

⎟

λ

= nmπ

β

1.44

γ

1.33

1.48

tan γ=η3

(Acetone / Methanol)

-10.2

1.46

tan β=η2

-10.0

tan φ=η4

1.42

(b) tan α=η1

(50/50) (40/60) (30/70) (20/80)

1.34

ϕ

tan φ=η4

1.35

1.36

1.37

Refractive Index

Refractive Index

Fig. 6. Propagating light intensity of sensors prepared at variety of acetone/methanol concentration when expose to (a) saline and (b) crude oil having different refractive index. 5

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Intensity (dBm)

Z. Samavati, et al.

-10

30 0C

-15

25 0C

-20

20 0C

Tem. Time ( C) (sec) 30 270 25 300 20 360 15 423

-25 -30 -35

by intermolecular attractions. On the other hand, the average kinetic energy of the solute molecules also increases with temperature, and it destabilizes the solid state. The increased vibration (kinetic energy) of the solute molecules causes them to be less able to hold together, and thus they dissolve more readily. Higher temperature due to nonhomogeneous etching and uncontrollable removing process is not considered. We found that 15 °C is an appropriate temperature for solvent to control the thickness and roughness of the cladding.

15 0C

4. Conclusions

-40 0

100

200

300

400

500

600

Sensitive and versatile evanescent wave sensor containing partially unclad optical fiber-based probe using chemical etch technique have been developed for refractive index changes detection. The effects of fiber surface roughness on the sensitivity of the probe against refractive index changes are studied. We revealed that increasing the surface roughness which is consequence of applying 40/60 (acetone/methanol) concentration in etching process leads to increase the absorption coefficient, penetration depth and scattering of propagating light. By increasing the surface roughness the sensitivity of the probe increased initially. The maximum sensitivity is found to be ~9.1 dB/RIU and 24.2 dB/RIU once probe immersed in saline and oil having different concentration respectively. This optimum value of sensitivity is achieved for 40/60 (acetone/methanol) having roughness of ~25 nm. In addition, an approach for accurate cladding removal is carried out in which reducing etching temperature is applied. It is found that 15 °C is the most suitable temperature for accurate removing the cladding. Entire this study indicates that partially unclad multimode optical fiber by chemical etch technique is promising optical structure to produce high performance sensors for detecting the refractive index changes. The performance confirms that the proposed sensors are highly sensitive and commercially viable therefore, no further modification is required.

Time (sec) Fig. 7. The on-line power transmission (dBm) measurement in fiber optics as a function of etching time (min) using (a) 40/60 (acetone/methanol) solution in different temperature.

methanol (40/60) present higher sensitivity compare to the others. To investigate the sensitivity of the probes containing fibers with rough surfaces, the effect of surface roughness on the decay coefficient ξ(n) and penetration depth Dp are considered. The results summarized in Table 2 depict that by increasing the surface roughness these two parameters increase. Therefore, the sensitivity of the probe can be enhanced by increasing the value of Rr. However, the initial effective evanescent wave intensity (transmitted light intensity) decreases with increasing the roughness due to the light-scattering and refraction loss in rough interface, which degrades the sensor sensitivity. The competition of these two conflicting results determines the sensitivity of the sensor. The sensors’ sensitivity is evaluated as the net change in the transmitted light intensity at unit refractive index using following formula [27],

η=

ΔIout Δn

(8)

Declaration of Competing Interest

The sensitivity of the probe at saline and crude oil are summarized in table 2. The sensitivity increases initially and then decreases. For the sensor prepared at 40/60 (acetone/methanol) sensitivity reaches to maximum amount of ~9.1 and ~24.2 dB/RIU for saline and oil respectively. The trend of altering the sensitivity with roughness is attributed to three phenomena. (1): when the partially unclad fiber is immersed in the solution (saline or oil), high-concentration layer of donor ions is coated on fiber surface. It causes enhancing the saline or oil molecule to be hold by the surface of fiber at presence of ions. These saline or oil molecules attached to the surface of the sensing region can induce absorbance and increase the nonlinearity. (2): by increasing the surface roughness the effective surface area increases and it causes to enhance the fixed capacity of the fiber consequently, the absorbance and nonlinearity also increase. (3): the Rayleigh scattering loss increases with increasing roughness, which also increase the absorbance and nonlinearity [28,29]. In addition, increasing the roughness reduces the number of modes propagating within the fiber because of decreasing the θ'i in rough surface (see Fig. 4) and generates stronger evanescent wave decay for rough fibers than smooth ones. The fast etching rate at higher acetone concentration makes the etching process difficult to control therefore, the probe prepared by 40/ 60 (acetone/methanol) which shows higher sensitivity is subjected for further etching temperature study. The results of the etching studies at different temperature for the probe fabricated at 40/60 (acetone/methanol) are shown in Fig. 7. By increasing the temperature the etching time threshold is rapidly reduced from 423 sec for 15 °C to 270 sec for 30 °C. Additional etching continues reduces the fiber diameter results in degradation of the core and light losing power. By increasing the solution temperature, the average kinetic energy of the solution molecules increases. This increment in kinetic energy allows the solvent molecules to more effectively break the solute molecules which are held together

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[20]

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7

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