626 IEEE SENSORS JOURNAL, VOL. 13, NO. 2, FEBRUARY 2013

Inline Microfiber Mach–Zehnder Interferometerfor High Temperature Sensing

Ali Abdulhadi Jasim, Sulaiman Wadi Harun, Hamzah Arof, and Harith Ahmad

Abstract— A compact inline microfiber Mach–Zehnder inter-ferometer (MMZI) is proposed for high temperature sensing.The MMZI is fabricated using a flame-brushing technique inwhich both transition parts of a microfiber are tapered to reducethe waist diameter and form an interference region. Since therefractive index of the fiber core exhibits a different temperaturecoefficient from that of air, the interferometer is sensitive totemperature variation. The temperature sensitivity of the devicewith a length of 40 mm was measured to be 13.4 pm/°C with anexcellent linearity for temperature measurement up to 800 °C.

Index Terms— Flame-brushing technique, high-temperaturesensor, inline Mach–Zehnder interferometer (MZI), microfiber.

I. INTRODUCTION

THE APPLICATIONS of optical fiber sensors inbiological, chemical and environmental industries have

attracted great attention. These include sensors used in themeasurements of the liquid level, refractive index (RI),temperature, strain and others [1]–[3]. Compared to thealternative techniques that are based on the mechanical andelectrical properties, the optical fiber sensors have manyadvantages, such as electromagnetic immunity, resistance toerosion, high sensitivity and possibility to work in contactwith explosives. For temperature sensor, fiber Bragg gratings(FBGs) [4] are possibly the most commonly used, as they canreach temperatures as high as 1000 °C [5] when inscribedin telecom optical fibers. However, developing sensors withFBGs often requires special fibers, hydrogen loading, acumbersome grating writing equipment, working with toxicgases and post fabrication treatments.

Recently, optical fiber Mach–Zehnder interferometer (MZI)sensors based on microfiber structures have attracted a lotof interest for various physical and chemical sensing appli-cations, such as temperature, strain, and RI, due to theirsimple structure, ease of fabrication, and low cost [6]–[9]. The

Manuscript received August 24, 2012; revised September 26, 2012; acceptedOctober 8, 2012. Date of publication October 11, 2012; date of current versionJanuary 14, 2013. This work was supported by the University of Malaya andMinistry of Higher Education under the High Impact Research Grant SchemeGrant D000009-16001. The associate editor coordinating the review of thispaper and approving it for publication was Prof. Julian Chan.

A. A. Jasim and H. Arof are with the Department of Electrical Engi-neering, University of Malaya, Kuala Lumpur 50603, Malaysia (e-mail:a_a_jasim@yahoo.com; ahamzah@um.edu.my).

S. W. Harun is with the Department of Electrical Engineering, Universityof Malaya, Kuala Lumpur 50603, Malaysia, and also with the PhotonicsResearch Center, University of Malaya, Kuala Lumpur 50603, Malaysia(e-mail: swharun@um.edu.my).

H. Ahmad is with the Photonics Research Center, University of Malaya,Kuala Lumpur 50603, Malaysia (e-mail: harith@yahoo.com).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2012.2224106

conventional microfiber-based MZI structures are fabricated byconcatenating two microfibers [6], which are used to couplethe fundamental core mode to cladding modes or the reverse.When light travels the distance between the two microfibers,a relative phase difference is induced between the core andcladding modes due to the difference in the effective RIs.Consequently, light interference occurs at the output side ofthe microfiber. Since the cladding modes are sensitive tothe surrounding RI, this microfiber based MZI can be usedfor sensing applications. In this paper, a microfiber-basedinline MZI (IMMZI) is demonstrated for the first time forhigh temperature measurement sensor. The new IMMZI isfabricated from a standard single mode fiber (SMF) using aflame-heated-drawing technique. The temperature sensitivityoriginates from the difference of the temperature coefficientsof the refractive index between the core and air, which inducesresonant wavelength shift against the temperature.

II. EXPERIMENT AND WORKING PRINCIPLE

The IMMZI is constructed from a low loss microfiber,which is fabricated from a standard SMF using flame-brushingtechnique. The microfiber is obtained by heating the fiberto its softening temperature, and then pulling the ends apartto reduce the fiber’s diameter down to 10 micron. Then themicrofiber is further tapered to a smaller diameter at twoparts separated by an untapered section. The untapered sectionmaintain its current diameter while sections to its left and rightare further tapered as shown in Fig. 1. The untapered partacts as an interference region, which allows the interferencebetween the core and cladding modes. During the taperingprocess, a broadband amplified spontaneous emission (ASE)light source is launched into the microfiber to monitor theloss and spectral response using an optical spectrum analyzer(OSA) at the output end of the microfiber. The input lightwas split into two portions at the interference region; guidedand unguided modes. The guided mode keeps travelling in thecore and the unguided mode propagates through the cladding.Interference occurs when the two modes recombined at thetapered region A. The total length of the microfiber is about40 mm. The fabricated IMMZI is placed inside a furnace withtemperature ranging from 76 °C to 802 °C. Meanwhile theASE light source is injected at the input end of the fiber and anOSA is connected to the output end of the IMMZI to monitorthe change in the spectrum as shown in Fig. 2.

The transfer function of the IMMZI structure can beexpressed as:

I = I1 + I2 + 2√

I1 I2 cos(ϕ) (1)

1530–437X/$31.00 © 2012 IEEE

JASIM et al.: INLINE MMZI FOR HIGH TEMPERATURE SENSING 627

interferometer region

Tapered region ATapered region B

Fig. 1. Schematic diagram of the fabricated inline microfiber MZI.

ASELight source

IMMZIOSA

Furnace

Fig. 2. Experimental setup for the IMMZI-based temperature sensor.

where I is the intensity of the interference signal, I1 and I2are the intensity of the light propagating in the fiber core andcladding respectively, ϕ is the phase difference between thecore and cladding modes which it approximately equal to:

ϕ =(

2π(�nef f )L

λ

)(2)

where the �nef f is defined as (nef fc − nef f

cl ) the differenceof the effective refractive index of the core and the claddingmodes respectively, L is the length of the interferometer regionand λ is the input wavelength. The fringe spacing between twointerference patterns can be expressed as:

� = λ20/(�ne f f )L . (3)

Equation 2 indicates that the phase difference between thecore and cladding modes is strongly dependent on the effectiverefractive index of the core and the cladding modes as wellas the length of the interferometer region. Since the effectiverefractive index of the cladding is sensitive to the refractiveindex of the surrounding medium, the temperature variationleads to shift in the interference fringe spectral. This is due tochanges in the effective refractive index disparity between thecore and the cladding modes, and the effective length of theinterferometer. These are the results of the thermo-optic andthermal expansion effects respectively. The relationship can beexpressed as [10]:

�λ/λ =(

�nef f

ne f f+ �Lef f

Le f f

)T emp = (αT OC + αT EC )�T

(4)where αT OC and αT EC is the thermal-optic coefficient (TOC)and thermal expansion coefficient (TEC) respectively.

1546 1548 1550 1552 1554

Out

put P

ower

(10d

Bm/d

iv)

Wavelength(nm)

143

349

473

562

677

782

802

Fig. 3. Interference fringes of the inline MMZI at different temperatures.

III. RESULTS AND DISCUSSIONS

The inline MMZI with a length of 40 mm was employedto evaluate its sensitivity towards temperature variation. Thelength of tapered region A, interferometer region, taperedregion B are 25 mm, 10 mm and 5 mm, respectively. The inter-ferometer was placed inside a high temperature furnace andthe temperature was gradually increased up to 800 °C whilethe interference spectrum was recorded for the experiment.The recorded interference spectra were analyzed to find thespectral position of the interference dip. An interference diparound 1547 nm at room temperature was monitored duringthe experiment as it had the largest free spectral range (FSR).Fig. 3 shows the recorded interference spectra as the furnacetemperature is varied from 143 °C to 802 °C. It was found thatthe interference dip shifts towards longer wavelength as thetemperature rises. This is attributed to the heat that increasesthe effective index difference between the core and claddingmodes as well as the effective length of the interferometer,which in turn changes the phase difference and wavelengthspacing of the interference spectrum according to Eqs. 2 and 3,respectively.

Fig. 4 shows the wavelength shift versus the temperatureincrease for high temperature measurement ranging from450 °C and 802 °C. It is observed that the linear trend line canbe fitted to the experimental data with a correlation coefficientvalue of r >0.99. The inset of Fig. 4 shows the variation ofdip wavelength with temperature change for lower temperaturemeasurement, which illustrates a similar response compared

628 IEEE SENSORS JOURNAL, VOL. 13, NO. 2, FEBRUARY 2013

Fig. 4. Transmission dip wavelength shift of the inline MMZI againstthe temperature response. Inset shows the temperature response at lowertemperature range.

to that of high temperature. Linear-fitting of the experimentaldata indicate that the temperature sensitivity of the device is13.4 pm/C°. The temperature sensitivity of the device can beattributed to the difference in thermo-optic dependence of thefiber core and air cladding. The effective refractive index of thecore and cladding changes differently when the temperaturevaries, causing a linear phase shift of the interference fringe.The resolution of the sensor is measured to be around 4 °Climited by OSA resolution. The thermal test was repeatedseveral times and the results were quite reproducible. Thesensitivity of the sensor can be improved by optimizing thecoupling mechanism of the IMMZI.

IV. CONCLUSION

A new method to fabricate fiber inline MMZI was reportedusing flame brushing technique. The fabricated interferometerwith a length of 40 mm was used to demonstrate a hightemperature sensor by monitoring the interference dip of therecorded interference spectra. The temperature sensitivity ofthe device is obtained at 13.4 pm/C° with an excellent linearitydue to the difference thermo-optic dependence of the fiber coreand air cladding.

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Ali Abdulhadi Jasim received the B.Sc. degree in electrical engineeringfrom the University of Technology, Baghdad, Iraq, in 2007, and the M.Eng.degree in telecommunication from the University of Malaya, Kuala Lumpur,Malaysia, in 2010, where he is currently pursuing the Ph.D. degree.

His current research interests include optical micro- and nano-fibersphotonic devices and their applications.

Sulaiman Wadi Harun received the B.E. degree in electrical and electronicssystem engineering from the Nagaoka University of Technology, Nagaoka,Japan, in 1996, and the M.Sc. and Ph.D. degrees in photonics from the Uni-versity of Malaya, Kuala Lumpur, Malaysia, in 2001 and 2004, respectively.

He is currently a Full Professor with the Faculty of Engineering, Universityof Malaya. His current research interests include fiber-optic active and passivedevices.

Hamzah Arof received the Ph.D. degree from the University of Swansea,Swansea, U.K.

Harith Ahmad received the Ph.D. degree in laser technology from theUniversity of Swansea, Swansea, U.K., in 1983.

He is currently a Full Professor with the Department of Physics, Universityof Malaya, Kuala Lumpur, Malaysia.