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Mach-Zehnder Interferometer and itstemperature based applications


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Mach Zehnder Interferometer and

its temperature based applications


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Basic Concept:

3 dB fiber-to-fiber coupler splits the laser output beam

50 % of the light to single mode sensing fiber

Rest 50% into the reference fiber

Another 3 dB coupler used to recombineCombined beam; detected and phase shift measured.

Phase shift results from variation in length andrefractive index of the sensing fiber.

Based on the path length, the phase and the intensityof the recombined beam are decided.


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All Fiber MachZehnder Interferometer

Based on Suspended Twin-Core Fiber

Presented by : Orlando Frazo, S. F. O. Silva, J. Viegas,Jos M. Baptista, Jos L. Santos, Jens Kobelke, and KaySchuster

Published in : IEEE PHOTONICS TECHNOLOGYLETTERS, VOL. 22, NO. 17, SEPTEMBER 1, 2010


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Introduction The temperature sensitivity of a twin-core fiber sensor

with a single cladding and the circular cores is duealmost entirely to the linear expansion of the core andthe thermo-optic effect.

The coupling between the two cores is increased withthe spin rate in a spun twin-core fiber.

In this work, the authors propose a new sensing head

based on a MachZehnder interferometer (MZI) usinga suspended twin-core fiber.

This sensing head presents different sensitivity whenis subjected to curvature or temperature.


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EXPERIMENTAL SETUP


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Light from a broadbandsource in the window of

1550 nm is linearlypolarized with a polarizerand injected into thesuspended twin-core fiber

with a length of 0.33 m

Polarization controller wasused to induce onlyrotation of thispolarization state.

The interferometric fringepattern observed in theoptical spectrumanalyzer(OSA)


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Working

The angle between the two field orientations(of planepolarised rays) is ~90

The cores diameters are 1.5 m; the cladding is 124 m;and the big/small holes are 10/5 m, respectively.

The distance between the two cores is approximately8.6 m, where it is possible to illuminate the two coressimultaneously using a standard single-mode fiber(SMF 28).

The bridge that connects the two cores is very thin,hence no coupling between the two cores is expected.

The splices were made using a conventional splicemachine


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Temperature Sensitivity

The interference fringe spacing variation of the MZIsfor temperature variation was also characterized.

The fiber was placed in an oven where the temperaturecould be set from room temperature up to 100 C withan error smaller than 0.1 C

because this sensing structure operates in a differentialmode, cores should have common temperature effect

But so doesnt happen. Reason : due to small coredimensions, modal evanescent field into the air holesoccurs, different for x & y plane because ofgeometrical asymmetries


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.

Ifi (i=x,y)are theinterference fringe spacing

variations of theinterferometers for the twopolarizations induced bytemperature T and

curvature C variations,then it is possible to writethe following matrixequation:


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Where D=KT(y)KC(x) KC(Y)KT(x)

Where T and C are in degrees centigrade and m-1 ,respectively, while the wavelengths shifts are innanometres.

The performance of this measurement approach was

tested varying the temperature at constant curvatureand reciprocally, i.e., varying the curvature at constanttemperature.

The experimental determination ofx and y

permitted us to obtain the measured values for andwhich T and C are compared with the effectivelyapplied ones.


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Conclusion

In the present work, it was possible to obtain a new sensinghead based on a twin-core fiber that allows us to obtaindistinct interferometric signals associated with the twoprincipal linear polarization states of the input light to thesensing head.

These signals show substantially different sensitivities totemperature variations,

A feature that is not present when considering theimplementation of a sensing head of this type with standardtwin-core fibers.

This characteristic permits much higher design flexibility,in particular turns fairly easy to perform with good accuracysimultaneous measurement of curvature and temperature.


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High-Temperature Sensing Using Miniaturized

Fiber

In-Line MachZehnder Interferometer

Presented by:Ying Wang, Yuhua Li, Changrui Liao,D. N. Wang, Member, IEEE, Minwei Yang, and PeixiangLu

Published in: IEEE PHOTONICS TECHNOLOGYLETTERS, VOL. 22, NO. 1, JANUARY 1, 2010


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Introduction

A number of optical fiber high-temperature sensorshave been developed, such as fiber Bragg grating(FBG), long-period fiber grating (LPG), andinterferometer-based systems

Recently, fiber in-line MachZehnder interferometers(MZIs) based on interference between the guidedmodes of the fiber, including single-mode fiber (SMF),multimode fiber (MMF), and PCF, have been proposed

for temperature measurement The commonly used splitter/combiners include LPGs,

tapers, sections of MMF or PCF, and other elementsthat may cause core diameter mismatch


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Principle temperature sensitivity originates from the difference of

the temperature coefficients of refractive index betweenthe fiber core and air


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Experimental Setup The input light split into two parts by the micro cavity:

Iin1 and Iin2 ; guided and unguided modes respectively

The guided beam travels through the fiber and the nonguided one through the micro-cavity, and interference

occurs when both the o/p beams recombine

If the cavity length is L; and considering low reflectivity,multiple reflections can be neglected, then intensity ofthe recombined beam can be given as:


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Where the above relation gives index difference between guided andunguided mode.

Assuming the cavity length L is kept constant, the temperature

sensitivity from (1) could be derived as follows, when efffor core andcladding is function of temperature. And deff/dT for core and cavityare the temperature coefficients of refractive index of the fiber core andcavity medium :


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Fabrication

Femtosecond laser pulses (=800nm) of the duration of 120fs and the repetition rate of 1 kHz were focused onto thefiber by a 10Xobjective lens with an NA value of 0.25 and a

working distance of 7mm in the experiment

The microcavity was fabricated by femtosecond pulseablation with a scanning speed of 20 m s along the fiberlength.

After each scanning cycle, the focal spot of the laser beam

was moved closer to the fiber core with a step of 400 nmbefore the next cycle started, until the largest fringevisibility was obtained.

The micro-cavity created was cleaned with methanol afterthe processing was completed.


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Fig. 2(a) shows the transmission spectra at a number of temperaturepoints and it can be clearly seen from the figure that

the dip shifts toward the longer wavelength with the increase oftemperature. Fig. 2(b) presents the variation of dip wavelengthwith the temperature change, the results obtained show repeatabilityfor the heating and cooling processes and a sensitivity of0.046 nm C can be obtained

The FSR of the interference dip of interest is determined bythe cavity length L as

A smaller FSR would improve the accuracy of the measurement,which calls for a larger L. However, a larger L would increasethe insertion loss and degrade the mechanical quality of the device.


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Fiber In-Line Mach

Zehnder InterferometerEmbedded in FBG for Simultaneous Refractive

Index and Temperature Measurement

Presented by- C. R. Liao, Student Member, IEEE, YingWang, D. N. Wang, Member, IEEE, and M. W. Yang

Published in- IEEE PHOTONICS TECHNOLOGYLETTERS, Vol. 22, No. 22, NOVEMBER 15, 2010


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Introduction The in situ measurement of liquid refractive index is of

great value

Used in chemical, biological and many otherapplications

Major concern in determination- Temperature cross-sensitivity which leads to an unreliable measurement

It is desired that temperature and refractive index bemeasured simultaneously and unambiguously


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Recent optical fibers RI sensors have the followingadvantages-

Compact size

Immunity to electromagnetic interference

Capability of multiplexing

Remote sensing

Optical fiber sensors for simultaneous RI andtemperature measurement can be obtained by-

Using single sensing element such as slanted fiber BRAGGgrating Disadvantage-Small RI sensitivity

Adopting a combination of multiple sensor elements such ashybrid structure of FBG and long period grating

Disadvantage-Large system size and inaccurate sensinglocation


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Principle and analysis

Temperature response of FBG- dFBG /dT= FBG .(core+)

core =thermo optic coefficient~8.6*10-6 /C

= thermal expansion coefficient~ 0.55*10-6

By removing part of the core near the core and claddinginterface, a fiber in line MZI can be constructed


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The light wave propagating along the fiber core is split

into two beams, denoted by Iin1 and Iin2 One beam remains propagating in the core while the

other travels through the cavity

When the two output beams, Iout1 and Iout2 , are

recombined in the core, interference takes place due tothe phase difference induced by the effective RIdifference between the fiber core and the cavity

The output intensity is given by I=Iout1+Iout2+2(Iout1Iout2)cos(+o)

= 2L n/ is the phase difference

o = Initial phase

n=neffcore neff

cavity


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Assuming L as constant the temperature response isgiven by

dm(MZI) /dT= m(MZI) /( neffcore -neff

cavity)*( core- cavity)

The RI response is given by dm(MZI) /dT= m(MZI) /( neff

core -neffcavity)*((d neff

core /dn)-(dneff

cavity/dn))

Optical microscope image of the sensor


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Experimental Setup

The FBG inscribed in single mode fiber by fs laserirradiation through phase mask

Laser pulses produced (120fs,1mJ,1khz,800nm) by aTi:sapphire laser system

Laser beam focused onto fiber core by cylindrical lens(f=40mm)

Length of the inscribed grating ~4mm

Microcavity created by using a laser beam focusedonto the FBG by a 10x objective lens(NA=0.25)

On target laser power used ~12mW

Fiber with FBG mounted on a PC controlled three axix

translation stage


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Experiment and Results In principle, the temperature and RI information can

be recovered simultaneously by use of a standardmatrix inversion method

Matrix elements are determined by temperature and

RI responses Temperature performance determined by placing the

sensor in tubular oven

Temperature control range 22-100oc with resolution of0.1 oc

It is observed that both FBG and m(MZI) increaselinearly with time


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To investigate RI performance, the room temperatureof 22oC was maintained, and the sensor was

sequentially immersed into a set of index matchingoils ranging from 1.300 to 1.400 (at 489.3 nm) with astep of 0.005

Isopropanol used after each test to remove the residual

oil and to make it dry It is observed that an increase of the RI value leads to

a blue shift of m(MZI) , while FBG is essentiallyunchanged.

It is observed that m(MZI) is extremely sensitive to theRI change with an almost linear response, whereas FBGremains to be a constant.


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According to twoseparate temperatureand RI measurements,the two equations for

FBG and m(MZI) Thus T and n can be

determined from thematrix by


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