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626 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 8, APRIL 15, 2008

Refractive Index Sensing With MachZehnderInterferometer Based on Concatenating Two

Single-Mode Fiber TapersZhaobing Tian, Scott S.-H. Yam, Member, IEEE, Jack Barnes, Wojtek Bock, Fellow, IEEE, Patricia Greig,

James M. Fraser, Hans-Peter Loock, and Richard D. Oleschuk

AbstractA novel refractive index (RI) sensor based on a fiberMachZehnder interferometer was realized by concatenatingtwo single-mode fiber tapers separated by a middle section. Theproposed device had a minimum insertion loss of 3 dB and max-imum interferometric extinction ratio over 20 dB. The resolution(0.171 nm) of the two-taper sensor to its surrounding RI change(0.01) was found to be comparable to that (0.252 nm) of similarstructures made from an identical long-period gratings pair, andits ease of fabrication makes it a low-cost alternative to existing

sensing applications.

Index TermsFiber tapers, long-period grating (LPG),MachZehnder (MZ) interferometer, refractive index (RI) sensor.

I. INTRODUCTION

RECENTLY, optical fiber sensors have been intensivelystudied to monitor qualities such as temperature, stress,

gas phases, or refractive index (RI) change in solutions dueto their compact footprints and ability for high resolutiondetection. Different sensor designs have been proposed withconventional optical fibers: e.g., fiber tapering [1], [2], fiber

Bragg gratings (FBGs) [3][5], long-period gratings (LPGs) [6],[7], and core-mismatch-based fiber sensing [8]. MachZehnder(MZ) interferometers have also been constructed from LPGpairs [9] to further enhance sensitivity. However, in termsof RI measurement, all of these approaches have their owndrawbacks. Tapered fiber sensors and core mismatch sensorshave high sensitivity for RI close to 1.45 (RI of fiber cladding),

Manuscript received November 9, 2007; revised December 18, 2007. Thiswork was supported by the Natural Science and Engineering Council of Canada(RGPIN 311817-06), CanadianInstitute for PhotonicInnovationsand PhotonicsResearch Ontario.

Z. Tian and J. M. Fraser are with the Department of Physics, EngineeringPhysics, and Astronomy, Queens University, Kingston, ON, K7L 3N6, Canada

(e-mail: tianzhaobing@gmail.com; fraser@queensu.ca).S. S.-H. Yam is with the Department of Electrical and Computer En-

gineering, Queens University, Kingston, ON, K7L 3N6, Canada (e-mail:scott.yam@queensu.ca).

J. Barnes, H.-P. Loock, and R. D. Oleschuk are with the Depart-ment of Chemistry, Queens University, Kingston, ON, K7L 3N6,Canada (e-mail: jbarnes@chem.queensu.ca; hploock@chem.queensu.ca;oleschuk@chem.queensu.ca).

W. Bock is with the Centre de Recherche en Photonique, DpartementdInformatique et dIngnierie, Universit du Qubec en Outaouais, Gatineau,Qubec, J8X 3X7, Canada (e-mail: Wojtek.Bock@uqo.ca).

P. Greig is with the Advanced Photonic Systems Laboratory, National Mi-croelectronics and Photonics Testing Collaboratory, Canadian MicroelectronicCorporation, Kingston, ON, K7L 3N6, Canada (e-mail: trish@cmc.ca).

Color versions of one or more of the figures in this letter are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LPT.2008.919507

Fig. 1. Structure of two-taper-type interferometer.

yet only limited sensitivity for RI from 1.3 to 1.4, the typicalrange of protein analytes. On the other hand, grating-based(FBG, LPG) sensors are more responsive to a larger rangeof RI, but require precise and often expensive phase masksand stringent photolithographic procedures. In this letter, weproposed a single-mode fiber (SMF)-based MZ interferometerby simply concatenating two fiber tapers separated by a short(2455 mm) middle section. The fiber tapers were fabricatedby a commercially available fusion splicer using an automatedpreset program. Based on this MZ interferometer, an attenua-tion maxima wavelength shift of 0.171 nm was measured for

an RI change of 0.01. The sensitivity of the new sensor wascomparable to that of the LPG pair sensor, while the fabricationprocess was much simpler and faster than that of the LPG pairsensor.

II. PRINCIPLE

Tapered structures in optical fibers have been extensivelystudied as power couplers, sensors, and adddrop multiplexers[10][12]. Previous work on power coupling and filteringfocused mostly on minimization of insertion loss by graduallytapering SMF to a waist diameter of several micrometersunder high heat (flame/oven). Hence, more gradual tapers orsmaller taper angles (as shown in Fig. 1; i.e., a large value of

) are generally preferred. If the taper angle islarge, some light energy in the core will be coupled into thecladding and finally be attenuated by the coating. However,if another similar taper structure follows within centimeters

of the first one, the attenuation of the cladding mode isnegligible, and the cladding mode energy can be coupled backfrom the cladding to the core. Due to the phase differencebetween the core and cladding modes, an MZ interferometer iscreated after the second taper. can be approximated as [13]

(1)

where is the effective RI difference between the core and

cladding modes, and is the input wavelength in vacuum. When

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TIAN et al.: RI SENSING WITH MZ INTERFEROMETER BASED ON CONCATENATING TWO SMF TAPERS 627

light emitted by a broadband source (BBS) propagates throughboth tapers, its interference spectrum can be recorded by an op-tical spectrum analyzer (OSA). The attenuation maxima wave-lengths are characterized by

(2)

where is an integer. The separation of attenuation maximawavelengths is given by

(3)

If the RI of the environment surrounding the SMF increases,for example by submersing the middle section in water, the ef-fective RI of the cladding mode increases by , while thatof the core mode stays almost constant, so decreases by

. With (1) and (2), has to shift to the shorter wavelengthby

(4)

Based on , one can measure RI of an unknown sample,or determine the concentration of a known solution sample.

III. EXPERIMENT

To characterize the MZ interferometer response, light froma JDS Uniphase BBS (15201610 nm) was injected into SMFand the transmission spectrum was recorded by an ANDOOSA (AQ6317B). The attenuation spectrum of the devices wasobtained by subtracting the sources spectrum from the trans-mission spectrum of the device. An Ericsson fusion splicer,model FSU 995FA with a built-in taper program, was used toget the 3-dB tapers. For a given piece of SMF, a 3-dB taper was

made and the attenuation spectrum was recorded. A second3-dB taper was then made a few centimeters further along thesample, and the fiber device was mounted and straightenedon positioning stages, with the attenuation spectrum recorded.The fiber coating in the middle section between the two taperswas stripped off, and the attenuation spectrum was recorded.Lastly, the BBS was injected from the opposite direction, andthe reverse attenuation spectrum was recorded.

As shown in Fig. 2, the image of one 3-dB taper was takenby a NIKON charged coupled device camera through a stereo-zoom trinocular microscope. The length of the tapered region

measured was 707 m, while the waist was 40 m. Asshown in Fig. 3, three interferometers with length ,

, and mm were made and measured. Strong interferenceswere observed in all three interferometers, whereas the atten-uation spectrum of one single taper was almost flat across thewhole wavelength window, which means that attenuation due toone taper was wavelength-independent. The devices containinga coating also showed interference but the extinction ratio wassmaller compared to that in which the coating was stripped off.This is due to the strong absorption of the cladding mode bythe higher RI coating layer between two tapers. The interferom-eter containing a stripped middle section showed nearly iden-tical attenuation spectrum regardless of the propagation direc-tion of the light, indicating attenuation of the interferometer isdirectionally independent. The maximum extinction ratio was

7.8 dB mm , 23.4 dB mm , and 14.0 dBmm , and the separation of was 21.6, 15.4, and

Fig. 2. One 3-dB taper region with L = 7 0 7 m, D = 4 0 m.

Fig. 3. Attenuation spectrum for different length L of (a) a single taper, (b) oftwo tapers separated by SMF with coating, (c) of the two tapers separated bySMF without coating, and (d) same setup as (c) with reversed propagation di-rection.

10.6 nm, respectively. This result is consistent with (3) since

varies inversely with . With (3) and separation of at-tenuation maxima wavelength , we deduced that

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628 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 8, APRIL 15, 2008

Fig. 4. Attenuation maxima wavelength shift due to different solution.

is 0.0048 mm , 0.0044 mm , and 0.0043mm , which is very close to the RI difference (0.005)

between the core and cladding of SMF. The extinction ratio de-pendence with the wavelength could be due to more than onecladding mode participating in the interference.

IV. SENSOR APPLICATION

The three interferometers were tested as sensors for RIchange. An adjustable stage was placed below the fiber to sup-port a microscope glass slide, which had test sample solutionon its surface. The sensor was then lowered onto the glass slideso that the fiber between the two tapers was immersed in theRI standard solution while the output spectrum was recorded.After the measurement, a syringe was used to remove thesolution, and the device was cleaned with distilled water and

compressed air. A different RI standard was then applied, andthe procedure was repeated.

Nine DMSO solutions with different concentrations (0.0%,4.0%, 8.0%, 12.0%, 16.0%, 20.0%, 24.0%, 28.0%, and 32.0%)were used in this experiment. The corresponding RIs (calcu-lated from Fresnel reflection using water as reference at 25 C)are 1.315, 1.3208, 1.3266, 1.3324, 1.3382, 1.3441, 1.35, 1.3559,and 1.3618, respectively. Fig. 4 shows the attenuation maximawavelength shift with the change of RI of the sample. Fora 0.01 RI change, 0.094 nm mm , 0.121 nm

mm , 0.171 nm mm shifts were observed, re-spectively. These results are consistent with (4) since the sen-sitivity of the sensor is linearly related with . The sensitivity

mm is comparable with that of LPG pair sensor(0.259 nm) [14], which has 62-mm (including length of twoLPGs) interaction length with solution. Since the measurementprinciple is the same for both approaches, one expects an iden-tical sensitivity. We attributed the differences to the SMF, whichis normal transmission fiber SMF-28 in our experiment, whilethat of [14] is photosensitive fiber, which is also more expensivethan SMF-28. We note that the fabrication of two-taper structureis simpler compared to that of the LPG pair and manufacture ofone 3-dB taper normally takes less than 1 min. For RI 1.31.4,which is the typical range of protein analytes, sensitivity of thesensor based on concatenating two SMF tapers should be uni-form. Based on (4), increasing the separation length of two ta-

pers could linearly increase the sensitivity. Another possible

way to increase the sensitivity of the sensor is to use smallerdiameter cladding SMF, which increases for the same RIchange of tested sample.

V. CONCLUSION

A new type of MZ interferometer based on a two-taper struc-ture in SMF was constructed. The maximum interferometer ex-tinction ratio was large (more than 20 dB), while the insertionloss was small (3 dB). The RI sensors based on the two-taper in-terferometer demonstrated comparable sensitivity with that ofthe LPG pair sensor. This novel, low-cost MZ interferometermay find application in chemical sensing but may also be usedas a sensor for stress or temperature.

ACKNOWLEDGMENT

The authors would like to thank the Canadian Microelec-tronics Corporation for their experiment assistance. Some mea-

surements were made with equipment from the National Micro-electronics and Photonics Testing Collaboratory.

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