JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 5, MARCH 1, 2012 805

All Single-Mode Fiber Mach–Zehnder InterferometerBased on Two Peanut-Shape Structures

Di Wu, Tao Zhu, Member, IEEE, Member, OSA, Kin Seng Chiang, Member, IEEE, Fellow, OSA, and Ming Deng

Abstract—A peanut-shape fiber structure that can realizethe coupling and recoupling between the fiber core mode and thecladding modes is proposed in this paper. Based on the structure, asimple and low-cost Mach–Zehnder interferometer (MZI) formedby cascading two peanut-shape structures in the single-modefiber is demonstrated. The theory and the experimental resultsshow that the first peanut-shape structure can couple the lightenergy of the core mode into the cladding modes and the secondpeanut-shape structure can recouple the light in the claddingmodes into the core mode. A high-quality interference spectrumwith a fringe visibility of about 13 dB is observed. Moreover, it hasvery good mechanical strength compared with the MZIs basedon the tapers or the offset structures. When the interferometerlength mm, the temperature sensitivity of the device is

pm/ C and the strain sensitivity is pm/ . Such kindof interferometer would find potential applications in communi-cation and sensing fields.

Index Terms—Coupling, fiber optics sensors, Mach–Zehnder in-terferometer (MZI), temperature, strain.

I. INTRODUCTION

O PTICAL fiber sensors have been widely used in sensingapplications of various physical, chemical, and even

biological measurements. So far, the different fiber-optic sen-sors have been proposed, such as fiber Bragg gratings [1],[2], long-period gratings [3], [4], and Fabry–Perot interfer-ometers (FPIs) [5]. These sensors have several advantages,including high sensitivity, fast response, good stability, andimmunity to electromagnetic interference. Recently, thecore-cladding-mode interferometer sensors have been demon-strated in bothMach–Zehnder andMichelson types. Fabricationof a core-cladding-mode interferometer sensor requires a mech-anism to split the input optical signal into two different opticalpaths (the solid fiber core and the cladding) at the first coupler

Manuscript received May 25, 2011; revised September 04, 2011, December21, 2011; accepted December 27, 2011. Date of publication January 02, 2012;date of current version February 03, 2012. This work was supported by the Nat-ural Science Foundation of China under Grant 61007049, the Fundamental Re-search Funds for the Central Universities under Project CDJXS11121144, andProgram for New Century Educational Talents Plan of Chinese Education Min-istry under Grant NCET-08-0602.Di Wu, Tao Zhu, and Ming Deng are with the Key Laboratory of Optoelec-

tronic Technology and Systems, Ministry of Education, Chongqing University,Chongqing 400044, China (e-mail: wudi@cqu.edu.cn; zhutao@cqu.edu.cn;dengming@cqu.edu.cn).Kin Seng Chiang is with Key Laboratory of Optoelectronic Technology and

Systems, Ministry of Education, Chongqing University, Chongqing 400044,China, and also with the Department of Electronic Engineering, City Univer-sity of Hong Kong, Hong Kong, China (e-mail: 20090802006@cqu.edu.cn).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/JLT.2011.2182498

and subsequent recombination into one path at the secondcoupler [6]–[14]. Several fabrication techniques have beenproposed, including long-period fiber gratings (LPFGs) [6],[7], optical fiber tapers [8], misaligned spliced joint [9], [10],core diameter mismatch [11]–[13], CO laser irradiated points[14], partially collapsing the air holes of photonic crystalfiber (PCF) [10], and so on. The LPFGs and the CO laserirradiated point methods require precise (and often expensive)photolithographic alignment equipment and amplitude masks.The fiber taper and the misaligned spliced joint methods have adegraded mechanical strength due to the small waist diameterand misaligned spliced joint. In addition, some PCF-basedsensors require special fibers, and most of the others haverelatively complex sensor structures.In this paper, we propose a novel peanut-shape fiber structure

based on simple fiber microlens, which can realize the couplingand recoupling between the core mode and the cladding modes.Hence, a very simple and low-cost MZI based on cascadingtwo peanut-shape structures (CTPS-MZI) in single-mode fiber(SMF) is realized, for the first time to our knowledge. One of thetwo novel peanut-shape structures is used as the beam splitterto couple part energy of the core mode into the cladding, andanother one is used as the combiner to recouple the light fromthe cladding to the core. The distance between the two couplingpoints corresponds to the physical length of the interferometer.In the CTPS-MZI, the optical paths of the two arms are dif-ferent due to the effective refractive index difference betweenthe core mode and the cladding modes. We demonstrate thatthe novel peanut-shape structure can excite high-order claddingmodes and the clear interference patterns are obtained with theCTPS-MZI fiber structure. Finally, the potential applicationsof the proposed interferometer are presented. As a temperaturesensor, it is observed that the interference fringe shifts linearlytoward the longer wavelength direction with a sensitivity of

pm/ C. As a longitudinal strain sensor, it is more robustthan the MZIs based on fiber tapers or offset structures, and thesensitivity is measured about pm/ .

II. FABRICATION AND PROPERTIES OF THE CTPS-MZI

In the fabrication process of the MZI, a high-accuracy op-tical spectrum analyzer (OSA; Si720, Micro Optics, USA) isused to monitor the interference spectrum of the sensor with awavelength resolution and precision of 0.25 and 1 pm, respec-tively. A Furukawa S176 arc fusion splicing machine is used forjoining and fabricating the sensors.

A. Ellipsoidal Microlens Fabrication

After two sections of SMF (SMF-28) are cleaved, the properarc discharges are applied to fabricate the ellipsoidal microlens.

0733-8724/$31.00 © 2012 IEEE

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Fig. 1. Microscopic image of the peanut-shape structure: (a) after the arc dis-charge treatment and (b) after the fusion splicing.

Fig. 2. Simulated mode field distribution along the ellipsoidal microlens:(a) mode field diameter of 50% of the total energy and (b) mode field diameterof 70% of the total energy.

The details of the arc discharge processing strategy are as fol-lows: the arc power is , the prefuse time is ms, andthe arc duration is ms. Since the arc power level of 200is much higher than that of arc power used in the standard SMFfusion splicing (100), the fiber end will become an ellipsoidalstructure, as shown in Fig. 1(a). We know that heating the silicato a temperature below the transition temperature and, then,cooling it down will dandify the silica glass and increase its re-fractive index [15]. In the heating and cooling process, both ofthe cladding and the core will be densified; hence, the whole re-fractive index of the fiber is increased. The parameters of the twofabricated ellipsoidal [as shown in Fig. 1(a)] microlenses are:

m, m, and m, respectively.The mode field distribution along the ellipsoidal microlens canbe simulated by the beam propagation method (BPM) [16]. Asshown in Fig. 2, the mode field remains constant before it en-ters the ellipsoidal-structure section , but increases as thepropagating distance increases to 350 m However, it begins todecrease when the propagation distance is farther than 350 m.This means that the light could be diverged at first and then becondensed later by the microlens. In general, 50% and 70% ofthe total energy will lead to the increase of mode field diameterfrom 5 to 7.45 m, and from 7.74 to 12.51 m, respectively.

B. Peanut-Shape Structure Fabrication

It is noticed that the ellipsoidal microlens can diverge thelight, thus cascading two ellipsoidal microlenses can form aMZI. However, since only a little energy of the core mode iscoupled into the cladding by the first ellipsoidal microlens lessenergy is recombined from the cladding to the core at the secondellipsoidal microlens and the extinction ratio will be very small( dB). So, we splice two ellipsoidal microlenses to form apeanut-shape structure [as shown in Fig. 1(b)] to increase thepower of the mode field in the cladding. The used arc powerlevel is 100 and the insertion loss of the peanut-shape structureis 3 dB.We find that the different parameters of the peanut-shape

TABLE IMEASURED DIFFERENT INSERTION LOSS WITH DIFFERENT PARAMETERS OF

THE PEANUT-SHAPE STRUCTURE

structure have the different insertion loss. As shown in Table I,the insertion loss of peanut-shape structures increases with thevalue of .The simulated output mode field distribution of the peanut-

shape structure by using the BPM [16] is shown in Fig. 3(a). Theparameters of the peanut-shape structure [as shown in Fig. 2(b)]are: m, m, m, and

m. The original excitation mode and the meshsize are and 1 m 1 m 1 m, respectively. Whenthe input wavelength is 1550 nm, the simulated modeelectric field at 1000 m is shown in Fig. 3(a). The power dis-tribution of the core and the cladding modes for the differentpeanut-shape structures (the specified parameters are shown inTable I) is shown in Fig. 3(b). The mode decreases slightlybefore it enters the peanut-shape structure section but de-creases monotonically until the light goes through the left ellip-soidal microlens. When the light passes into the right ellipsoidalmicrolens, the mode increases at first and then decreaseslater. Finally, after the light goes through the peanut-shape struc-ture section, the mode almost remains constant. It is worthnoting that every curve has a peak at the range of 400–430 m.This is due to the fact that part of evanescent wave can be cou-pled back to the fiber at the early part of the second ellipsoidalmicrolens. The peanut-shape structure formed by 200 arc powerhas the largest energy loss, which agrees with the values shownin Table I.Meanwhile, the experimental near-field images of the fun-

damental core mode through a 3 dB peanut-shape structure ismeasured by the NanoScan Near-Field Profiler (Photon Inc.) asshown in Fig. 4. The axially symmetric two- and three-dimen-sional images of the near-field are shown in Fig. 4(a) and (b),respectively. The measured mode field diameter at 50% of themaximum power is 245 m, which is much wider than that ofthe standard SMF-28 (13 m). Thus, we can make a conclu-sion that the peanut-shape structure has excited the axially sym-metric cladding modes .

C. CTPS-MZI Fabrication and the Characteristics

A MZI can be realized by cascading two 3 dB peanut-shapestructures (as shown in Fig. 5). The input optical signal is splitinto two optical paths at the first peanut-shape structure andthen is recombined together at the second peanut-shape struc-ture. Fig. 6 shows the interference spectra of the CTPS-MZIswith lengths of 22, 37, and 41 mm, respectively. The typicalinterference fringe has a visibility around 13 dB, which is suf-ficient for the sensing application. The extinction ratio could beimproved by optimizing the detail parameters and of the

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Fig. 3. (a) Simulated electric field after the peanut-shaped structure and(b) simulated mode power distribution along the different peanut-shapestructures.

Fig. 4. (a)Measured two-dimensional near-field image and (b) measured three-dimensional near-field image.

Fig. 5. Schematic diagram of the CTPS-MZI.

peanut-shape structure. The lower value of will couplethe light energy of the core mode into the cladding modes.As can be seen from Fig. 6, the spectra are somewhat inho-

mogeneous since there are more than two modes involved in aninhomogeneous interference pattern. In our experiment, it canbe assumed that practically only one cladding mode is domi-nantly excited. This dominant cladding mode interferes with thecore mode to form the main interference pattern. Other claddingmodes should also be excited; therefore, the interferences be-tween the core mode and other cladding modes will slightlymodulate the main interference pattern. However, the modula-tion effect is very weak.

Fig. 6. Measured transmission spectra of the CTPS-MZIs: (a) mm,(b) mm, and (c) mm.

III. PRINCIPLE AND ANALYSIS

Since the phase difference between the core mode andthe cladding modes, the MZI could be used to measure manyenvironmental parameters. Here, can be approximated asfollows:

(1)

where is the effective RI difference between the coremode and the th cladding mode, is the interaction length be-tween the two peanut-shape structures, and is the input wave-length. The free spectral range (FSR) can be approximated by

(2)

It can be seen that the FSR will increase as the interferom-eter length decreases. When in (1) equals

the transmission reaches its valley value at thewavelength as follows:

(3)

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Fig. 7. Spatial frequency spectra of the CTPS-MZIs with mm,mm, and mm, respectively.

where is the center wavelength of the interference valleyof the th order. When a strain is applied to the CTPS-MZIor ambient temperature changes, the mode indices and the fiberlength will change, and thus, the transmission valley will shift.By differentiating (3), we have

(4)

where indicates the applied strain. Both the changesof the temperature and the strain contribute to the wavelengthshifts, which mean that the CTPS-MZI can be used as a temper-ature or strain sensor.As we have mentioned earlier, there are more than two modes

involved in an inhomogeneous interference pattern. In order toexamine the previous assumption, the wavelength spectra inFig. 6 are Fourier transformed to obtain the spatial frequency(as shown in Fig. 7). Fig. 7 shows that the CTPS-MZIs withdifferent length have only one dominant peak in the spatialfrequency spectra, which means that there is indeed one dom-inantly excited cladding mode. The interference between thedominant cladding mode and the core mode forms the main in-terference pattern. Other weak interferences between the weakcladding modes and the core mode slightly modulated the maininterference pattern, which could be neglected in our sensingapplications.

IV. POTENTIAL APPLICATIONS

A. Temperature Application

The two CTPS-MZIs with and mm are testedas sensors for temperature change The core of the fiber has ahigher thermo-optic coefficient than the cladding, and thus, thespectrum will move to the longer wavelength when the environ-ment temperature increases. The sensors are put into a tempera-ture furnace, whose temperature error is C for temperaturemeasurement. Fig. 8(a) shows the test results of the CTPS-MZIs

Fig. 8. Temperature responses of the CTPS-MZI with (a) mm and(b) mm.

with mm. As shown in Fig. 8(a), the temperature sensi-tivities of the CTPS-MZI with mm are 46.8 or 46 pm/ Cwhen the transmission dip is or 1550 nm, respectively.Meanwhile, we find that the CTPS-MZI with mm has atemperature sensitivity of 43.4 pm/ C in the first heating cycleand 44.9 pm/ C in the second heating cycle at the wavelengthof nm [as shown in Fig. 8(b)]. It is noted that the sen-sitivity of 46.8 pm/ C of mm is little higher than thesensitivity of 44.9 pm/ C of mm in the same wave-length range, which means that the sensitivity of CTPS-MZIdoes not significantly depend on the length of interferometer.Additionally, the CTPS-MZI has good linearity and repeata-bility in temperature responses. Compared with some previ-ously proposed fiber-optic temperature sensors, such as LPFG(60 pm/ C) [17] and fiber Bragg gratings (13 pm/ C) [1], ourdevice shows comparable temperature sensitivity with simplerfabrication and lower cost.

B. Strain Application

The transmission spectrum of the same CTPS-MZI withmm is monitored when the longitudinal strain is applied. The

CTRS-MZI is draggedwith amicrometer whoseminimum scaleis 10 m. Thus, the error in applying strain is about .Fig. 9 shows the strain responses with a sensitivity ofpm/ in the room temperature. We also find the maximumstrain is 3670 . As the resolution of the OSA used is 0.25pm, a strain resolution of is obtained. The small de-parture from the linearity of the wavelength versus strain curvemay be caused by the measurement error. The sensitivity of the

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Fig. 9. The strain responses of the CTPS-MZI with mm.

CTPS-MZI is comparable with that of the fiber tapered sensorpm/ [8], while the fabrication is simpler and the cost is

lower.

C. Discussions

It is noted that the temperature sensitivity of the CTPS-MZIis dependent on the wavelength and thethermo-optic coefficient . The thermo-opticcoefficient of the fiber is C [18], which affectsthe sensitivity of the sensor slightly. Thus, the sensitivity of theCTPS-MZI is mainly dependent on in the samerange of wavelength. Since the pure silica has a much higherthermo-optic coefficient than that of the air, the temperaturesensitivity of the CTPS-MZI can be increased by splicing onesection of multi-mode PCF (MM-HNA-5) between the twopeanut-shape structures. As a strain sensor, the maximum strainthat can be measured is 3670 , which is higher than that of thetapered sensor. In addition, since the strain measurement rangeof the CTPS-MZI is determined by the waist diameter , wecan optimize the parameters of the peanut-shape structure toincrease the measurement range. It should be noted that there isno fiber gratings involved in the CTPS-MZI fiber configurationand only simple splicing method is used. We can expect theoperation of this device at high temperature [11].

V. CONCLUSION

We show that the peanut-shape structure can excite thecladding modes and recouple the cladding modesto the core mode for the first time to our knowledge. Hence,a simple and low-cost MZI based on the two peanut-shapestructures has been fabricated. The temperature and straincharacteristics are investigated. Experimental results show thatthe CTPS-MZI has a linear temperature sensitivity ofpm/ C and a linear strain sensitivity ofpm/ when the interferometer lengthmm. In addition, the CTPS-MZI also has the advantages ofgood mechanical strength, simplicity, fast, and low-cost fabri-cation process, which makes it attractive for communicationand sensing applications.

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Di Wu was born in Chongqing, China, in 1986. He received the B.S. degree inapplied physics from Nanjing University of Science and Technology, Jiangsu,China, in 2009. He is currently working toward the M.S. degree in optical en-gineering from the Key Laboratory of Optoelectronic Technology and Systems,Ministry of Education, Chongqing University, Chongqing China.His current research interests include fiber optical interference components

and fiber optical sensors.

Tao Zhu (M’06) received the Ph.D. degree in optical engineering fromChongqing University, Chongqing, China, in 2008.During 2008–2009, he worked in Chongqing University, China. During

2010–2011, he was a Postdoctoral Research Fellow at the Department ofPhysics in University of Ottawa, Canada. Since April 2011, he has been aProfessor at Chongqing University, Chongqing, China. He has authored orcoauthored more than 80 papers published in the international journals andthe conference proceedings. His research interests include passive and activeoptical components, optical sensors, and distributed optical fiber sensingsystem.Dr. Zhu is a member of the Optical Society of America

810 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 5, MARCH 1, 2012

Kin Seng Chiang (M’94) was born in Guangdong, China. He received the B.E.(Hons.) and Ph.D. degrees in electrical engineering from the University of NewSouth Wales, Sydney, Australia, in 1982 and 1986, respectively.In 1986, for 6 months, he was with the Department of Mathematics, Aus-

tralian Defence Force Academy, Canberra, Australia. From 1986 to 1993, hewas with the Division of Applied Physics, Commonwealth Scientific and In-dustrial Research Organization, Sydney, Australia. From 1987 to 1988, for 6months, hewaswith the Electrotechnical Laboratory, TsukubaCity, Japan. From1992 to 1993, he worked concurrently for the Optical Fibre Technology Centre,University of Sydney. In August 1993, he joined the Department of ElectronicEngineering, City University of Hong Kong, where he is a Chair Professor. He isconcurrently a Chang Jiang Chair Professor at the University of Electronic Sci-ence and Technology of China. He has authored and coauthored more than 380published papers on optical fiber/waveguide theory and modeling, fiber/wave-guide characterization, fiber/waveguide devices, optical sensors, and nonlinearguided-wave optics.Dr. Chiang is a Fellow of the Optical Society of America and a member of

the International Society for Optical Engineering and the Australian Optical So-ciety. He received the Croucher Senior Research Fellowship for 2000–2001. Heis an associate editor of the JOURNAL OF LIGHTWAVE TECHNOLOGY. He is therecipient of the Japanese Government Research Award in 1987.

Ming Deng received the B.S. and M.S. degrees in communication and informa-tion engineering from the Xi’an University of Science and Technology, Shannxi,China, in 2003 and 2006, respectively. She received the Ph.D. degree in opticalfiber sensing from the Department of Optoelectronic Engineering, ChongqingUniversity, Chongqing, China, in 2009.Since 2010, she has been working in Chongqing University. Her research

interests include fiber-optic passive and active optical sensors.