TAPERED LONG-PERIOD GRATING (TLPG) WITH TEFLON AF COATING FOR PRESSURE SENSING APPLICATIONS

Aldona Kos1,2, Jiahua Chen2, Wojtek J. Bock2 Fellow IEEE, and Predrag Mikulic2

1Faculty of Physics, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland

2Centre de recherche en photonique, Département d'informatique et d'ingénierie, Université du Québec en Outaouais, P.O. Box 1250, St. B, Gatineau, Québec, J8X 3X7 Canada

Phone: 819-595-3900 ext. 1623, Fax: 819-773-1638, Email: wojtek.bock@uqo.ca

ABSTRACT This paper presents a simple and easy method of depositing a coating of Teflon AF onto a tapered long-period grating (TLPG) to enhance its pressure sensitivity. The coated TLPG was fixed in a pressure housing so as to assure that no elongation of the TLPG occurs during the process of pressure application, and that only the lateral pressure applied on the TLPG can contribute to the change of output spectrum of the TLPG. The pressure sensitivity of uncoated TLPG fabricated in our lab depends on the conditions of fabrication and was measured to be from 0.0040 to 0,0051 nm/bar. The measurement results show the pressure sensitivity of the Teflon AF coated TLPG is as high as 0.0066 nm/bar. This means that the pressure sensitivity of a TLPG can be enhanced by up to 57% with a suitable Teflon AF coating.

Index Terms— Teflon AF, long-period grating, tapered long-period grating, pressure sensitivity

1. INTRODUCTION

A long-period grating (LPG) is an optical fiber device that usually couples its propagating core mode to one or to several of its co-propagating cladding modes by a region of the fiber in which the refractive index is periodically changed. LPGs can be used in environment monitoring and biochemical sensing since their output spectra are easily affected by changes in the environment, such as temperature or refractive index. LPGs with overlays have been of interest in recent years due to the fact that a thin overlay with suitable thickness and refractive index can enhance and optimize the sensitivity of an LPG [1][2][3][4]. A TLPG is another kind of LPG. Actually it is a segment of an optical fiber in which one part is periodically tapered, ensuring that the coupling between the core mode and a cladding mode will occur effectively. Compared with ordinary LPGs, TLPGs have certain advantages: they can be manufactured in a cost-effective way by use of a simple set-up described in [5], and they can be made from a traditional fiber as well as from a photonic crystal fiber [6] regardless of whether the

optical fiber is photosensitive or not. Recently TPLGs have been successfully used by our group for pressure sensing [5] [6]. The pressure sensitivity of a TLPG depends on the diameter of the minimum part of the grating pitch (the maximum is assumed to be the diameter of the cladding). Decreasing that diameter can increase the pressure sensitivity, but would also increase the difficulty of fabrication and the output loss. For the fabrication set-up in [5], it is difficult to control the discharge current of the electrodes precisely. Adding a coating to the surface of a TLPG would seem to be an easier and more practical way to increase its pressure sensitivity. Teflon is a material currently in wide use in our daily life, for example, as the coating inside cooking pots, lubricants, the material for protective clothes. Teflon solution can be easily purchased on the market. In this paper we describe an experiment of depositing a Teflon coating onto a TLPG in order to enhance its pressure sensitivity.

2. TLPG As discussed in section 1, a TLPG is a segment of fiber in which one part is periodically tapered and causes the coupling of the core mode into the cladding mode to occur. Fig. 1 The outer shape and four parameters characterizing a TLPG

It can be mainly characterized by four parameters as shown in Fig.1: D, the cladding diameter of the fiber that is a standard 125 μm; d, the minimum diameter in a pitch that is determined by the discharge current of the electrodes and the discharge time of the splicer in the fabrication set-up, and the constant fiber pulling tension applied during the process of fabrication; �, the pitch that is controlled by the

�

D d

L

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CCECE/CCGEI May 5-7 2008 Niagara Falls. Canada978-1-4244-1643-1/08/$25.00 � 2008 IEEE

step of the fiber moving part; and L, the length of the grating.

The difference between D and d for a tapered grating made of SMF28 in our lab is usually about 15μm, the pitch grating � can vary from 400 to 1000 micron, and the grating length is 2 to 5 cm.

A scanning electron microscope (SEM) photo of the outer shape of an uncoated TLPG taken with a Hitachi 3000N SEM is shown in Fig. 2.

Fig. 2 The outer shape of a TLPG

3. TEFLON AF COATED TLPG 3.1. Teflon AF Teflon AF (amorphous fluoropolymer) of grade 400S1-100-1 was bought from DuPont in solution. According to DuPont, Teflon AF polymers have the lowest index of refraction of any known polymer. At 1.21~ 1.31, it is much lower than the refractive index of the cladding of the optical fibers. Teflon AF polymers have high gas permeability, high creep resistance, high compressibility, and low thermal conductivity. Most of these properties are suitable for applying a coating on an LPG for pressure sensing applications. 3.2. Teflon AF coated TLPG

The procedure of coating Teflon AF on a TLPG is simple: just dip the TLPG into the Teflon AF solution for about 4 seconds and then take it out and leave it in the air for about 5 seconds to dry. The Teflon AF coating made in this way is not uniform on the surface of the TLPG it is thicker in some places but thinner in other places. This situation is illustrated in Fig. 3.

Fig. 3 Teflon AF coated TLPG

An SEM photograph of the outer shape of a TPLG coated by Teflon AF is shown in Fig. 4. The non-uniformity of the thickness can be seen by comparing the outer shapes of the uncoated and coated TLPGs in Figs. 2 and 4. It is evident that the Teflon AF coating is thicker in the regions where the diameter of the TLPG is smaller.

Fig. 4 The outer shape of a Teflon AF coated TLPG

The thickness of the Teflon AF coating is estimated to be less than one micrometer. The SEM photograph of the cross-section of a Teflon AF coated TPLG is shown in Fig. 5: the white part is the fiber cladding and the gray part is the Teflon AF coating.

Fig. 5 The cross section of a Teflon AF coated TLPG 3.3. Output spectrum of a Teflon AF coated TLPG

Teflon AF coatingFiber cladding

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The output spectrum of a TLPG coated with Teflon AF

and surrounded by instrument oil is shown in Fig. 6(a) and the output notch displayed in higher resolution is shown in Fig. 6(b). The background of the light source is not eliminated because during the measurement only the minimum position of the output notch is of interest. It should be pointed out that, as is true for ordinary LPGs, the output notch of a Teflon AF coated TLPG could be easily affected by its environment, for example, a change in the refractive index. The shape and position of the output notch will not be the same when the grating is immersed in the instrument oil. Usually the output notch of an immersed Teflon AF coated TLPG will be much shallower than it is in the air, and red shift for about 3 to 4 nm will be observed. Fig. 6 is the output spectrum of a Teflon AF coated TLPG surrounded by the instrument oil.

(a)

(b)

Fig. 6 Output of a Teflon AF coated TLPG dipped into instrument oil: (a) output spectrum; (b) output notch.

4. EXPERIMENTS AND RESULTS The Teflon AF coated TLPG was fixed in a pressure chamber specially designed for testing the grating spectral shift under the application of pressure [5] as shown in Fig. 7. The fiber loop in the figure is used to prevent the TLPG from being elongated by pressure. This arrangement ensures that the only parameter acting on the TLPG is hydrostatic pressure. Fig. 7 Teflon AF coated TLPG fixed in a pressure housing

The lead-in and the lead-out fibers were connected to a broadband light source and an optical spectrum analyzer respectively, and the pressure housing was connected with a pressure generator through an oil pipe. The measurement set-up is shown in Fig. 8. Fig. 8 Instrumentation set-up for measuring pressure sensitivities of Teflon AF coated TLPGs: 1 - broadband optical source EXFO FPMD-5600 with a spectral width from 1500 to 1600 nm, 2 - pressure housing, 3 – instrument oil pipe, 4 - deadweight pressure standard DWT-35, 5 –optical spectrum analyzer ANDO AQ-6315A.

All the TLPGs used in the measurement were fabricated under the same conditions, that is, using the same kind of fiber (SMF28), and setting the same pitch (770 μm) and discharge current. The resolution of the optical spectrum analyzer was set in 0.1 nm. To compare the output properties of the TPLGs with and without Teflon AF coating, a TLPG without Teflon AF coating was first fixed into the pressure housing and then underwent the pressure test. The pressure sensitivity obtained was 0.0040 nm/bar, a reading that agrees well with our previous measurement

4

1 52

3

Fiber loopTeflon AF coated TLPG

Pressure sealing glue

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results. The highest sensitivity for pressure we have ever obtained was 0.0051 nm/bar [5]. The measurement for Teflon AF coated TLPG was repeated several times. Fig. 9 shows the measurement result of a Teflon AF coated TLPG. The directions of pressure increase and decrease are marked in the figure with arrows. It can be seen from Fig. 9 that the output response of a TLPG is nearly linear in the case of both increasing and decreasing pressures. The pressure sensitivities for the increasing and decreasing pressures are 0.0063 and 0.0057 nm/bar respectively.

y = 0.0057x + 1556.4

y = 0.0063x + 1556.21556

1557

1558

0 100 200 300

Pressure (bar)

Wav

elen

gth

(nm

)

Up

Down

Fig. 9 Output curves of a Teflon AF coated TLPG under application of pressure

Fig. 9 shows that there is some degree of pressure hysteresis while cycling the pressure. The reason for this is considered to be that the Teflon AF coating might not be able to resume its former shape as soon as the applied pressure is reduced or released.

The pressure sensitivity of the Teflon AF coated TLPG is higher than that of the uncoated TPLG by 57% (0.0063/0.0040=1.575) and higher than the best result we have achieved so far by 23% (0.0063/0.0051=1.235). The reason for this enhancement of pressure sensitivity is the high compressibility of Teflon AF - this material can be compressed significantly by pressure, causing the thickness and the refractive index of the compressed Teflon AF to change with the applied pressure and eventually causing the output notch of a Teflon AF coated TPLG to move with pressure. A coated LPG can be considered as a four-layer step index waveguide as shown in Fig. 10 with core, cladding, and coating radii r1, r2, and r3.

Fig. 10 A coated step index LPG

The grating can cause the mode coupling to happen and

the output notch will occur at the wavelength: Λ−= )( i

cladcorei nnλ (1)

where � is the pitch of the grating and ncore and icladn are the

effective refractive indices of the LP core mode and the ith LP cladding mode and they are determined by:

0

)(0)(0)(0000)(14)(13)(13000

0)(0)(0)(0)(000)(13)(13)(12)(120000)(0)(0)(0

000)(12)(12)(11

=

−−

−−−−

−−−−

lKkYkJlKwkYukJu

jYjJiYiJjYujJuiYuiJu

hYhJgJhYuhJugJu

(2) with 11rug = , 12ruh = , 22rui = , 23ruj = , 33ruk = , 34 rwl = and 22 )( i

cladmm nnku −= (m=1, 2, 3)

24

24 )( nnkw i

clad −= where J0, J1, and Y0 and Y1 are the Bessel functions of the first and the second kinds of the orders 0 and 1, and K0 and K1 are the modified Bessel functions of the second kind of the orders 0 and 1, k is the wave number in the vacuum, and n1, n2, n3, and n4 are the refractive indices of the core, the cladding, the coating, and the environment of the fiber.

The numerical analysis [1][4] shows that the output notch of the coated LPG varies with the thickness and the refractive index of the coating: increasing the refractive index moves the notch towards the shorter wavelength and decreasing the thickness moves the notch towards the longer wavelength. In the case of pressure sensitivity, the decreasing of thickness is considered to be the only reason for its enhancement. This can be seen from the start and the end point of the pressure cycle in Fig. 9. Let us specify the refractive indices and thicknesses of the coating, and the output notches at the start and the end points as nstart, nend, tstart, tend, �start, and �end. Due to existence of hysteresis the layer cannot restore its original state as soon as the applied pressure is released so we have:

tend < tstart, nend > nstart.

The first inequality comes from the common fact that the dimensions of a pressed body will be reduced and the second is based on another fact that usually the refractive index of a denser material will be higher. The first inequality implies the notch will move towards the direction of longer wavelength and the second implies the notch will move towards the shorter wavelength. The final notch shift is determined by the combination of the two movements. It is clear from the notch wavelengths corresponding to pressure 0 in Fig. 9 that �end >�start. This implies that in our

r1

r2

r3

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case it is the layer thickness that is responsible for the enhancement of the pressure sensitivity.

Syndiotactic Polystyrene (sPS) was used for depositing nanocoating on TLPGs [7]. A uniform sPS coating can be made by dipping a TLPG into a sPS solution and then controlling the withdraw speed of the TLPG. The coating thickness can be determined by the viscosity of the solution and the speed of withdrawal. The pressure sensitivity of a TLPG could be theoretically enhanced by using such an sPS coating of suitable thickness. However, the pressure measurements with sPS coated TLPGs have still to confirm this observation.

5. CONCLUSION A simple and easy method of coating the surface of a TLPG with Teflon AF was presented and the pressure sensitivity of the Teflon AF coated TLPG was measured. The measurement results show that the Teflon AF coating can enhance the pressure sensitivity of uncoated TLPG up to 57%. Experiment results also show that the responses of the Teflon AF coated TLPG are different under the conditions of increasing and decreasing pressure.

6. ACKNOWLEDGEMENTS The authors gratefully acknowledge support for this project from the Natural Sciences and Engineering Research Council of Canada and from the Canada Research Chairs Program.

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Sensors, Napoli, Italy, SPIE Vol. 6619, pp. 66192P-1-66192P-4, 2007.

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