Mach-Zehnder Interferometer Sensor for Acoustic Detection with Optimal Performance

Transcription

1 IOSR Jounal of Electonics and Communication Engineeing (IOSRJECE) ISSN : Volume, Issue 5 (Sep-Oct 01), PP 9-33 Mach-Zehnde Intefeomete Senso fo Acoustic Detection with Optimal Pefomance 1 Pashil M. Junghae, D. Santosh Kulkani, 3 Raghavenda Janawad Depatment of electonics and communication 1,,3 K.S. School of Engineeing and Management, Bangaloe, India Abstact: This pape investigates analytically the pefomance of MZI senso fo acoustic detection in tems of light powe, fibe chaacteistic and detectable acoustically/induced phase in the tems of output cuent at the photomultiplie tube (PMT). In this pape, pessue of 4.7*10-11 Pascal shows the change in the effective length 0.15 mete of fibe and change in the phase of the light path is 6.5*10-4 Radian and output cuent has been obseved fo boosilicate cown glass is 1.68*10^-19 Ampee at pessue of.5mpa. Keywods MZI, Mandel, Sensitivity, PMT I. Intoduction Mach Zhende intefeomete is a device used to detemine the phase shift between two am-senso am & efeence am) fom a coheent light souce. It using light modulation technique, to measue small phase shift in one of the two am caused by a small change in length of one of the paths. Intefeometic fibe optic senso exploits the changes in an optical path length induced by tansvese load in the optical fibes. MZI sensos modulate the phase of the electomagnetic waves popagating within the optical waveguide. One of the majo aeas of application fo MZI is in defense. Thee ae two main appoaches to coupling a load induced phase shift in an optical fibe to the detection pocess; a mandel senso o a coated fibe senso [1,, and 3]. Both of these appoaches ae viable up to ~ 50 kh. In coated fibe sensos the optical fibe is wound in a coil as a sensing element and the sie is compaatively lage. Since the optic coupling of the fibe waveguide is weak, a long fibe is geneally used to incease the induced phase change.. Mandel sensos appea to be ideal fo point sensos even though they have somewhat limited geometical vesatility. II. Pessue Sensitivity Of Mi In a fibe wound mandel senso, a thin jacket fibe is typically wapped aound a sensing mandel and a thin polyme, less than 1 mm thick, is coated ove the fibe as potection duing opeation. The optical fibe measues the pessue-induced stain in the mandel and potecting laye. Mandel sensos ae impotant because they ae easy to poduce and they exhibit a high sensitivity and amenability to spatial shading. Conside the mandel senso shown in Fig. 1(b). A homogeneous cylindical mandel of length L m and adius R m is adially wapped with a single-mode fibe of length l ove a length L. Fig 1(a) Fig 1(b) The molding laye is assumed to be only coated on the fibe laye and has a thickness of less than 1mm. Assuming that the fibe senso esponse is diven solely by the mandel esponse, it is clea that a Cicumfeential change in the mandel couples diectly to a length change in the fibe. The phase of light popagating though an optical fibe is defined as, * l n* k0 * l...(1) whee is β the popa gation constant in the fibe length diection, l is the total fibe length wapped aound the mandel equal to, N- is the numbe of fibe windings ound the mandel, R is the distance fom the cente of the mandel to that of the fibe as denoted in Fig.1 (b), n is the efactive index of the fibe coe, and k 0 is the fee space popagation constant. The change in the optical phase of the fibe may be witten as, 9 Page

2 d d * l * nl * D * l...() dn dd Whee, D is the fibe diamete [5, 6]. The change in the fibe length using the cylindical coodinate system is witten as, l l *( )...(3) Whee ϵ is the adial stain and ϵ is the axial stain in the fibe with the axis convention with espect to the mandel in Fig.1. Since the efactive indices of the coe and cladding diffe only slightly, one can use the appoximation of d k 0...(4) dn The change in the efactive index n can be witten in tems of the change in the optical indicatix given by the well-known expession [7, 8]. 3 1 (, 1,,3)...(5) Pij j i j n i j 1 Whee p ij ae elements of the stain optic tenso and ϵ j ae pincipal stains. Since fluid pessue is tansmitted to the tansduce only at ight angles to its bounday at evey point, the components containing a shea stain have been assumed to be eo in Eq.(5). A typical optical fibe consists of a coe and a cladding fabicated fom glasses which have simila popeties. Hence, the fibe may be consideed homogeneous and isotopic leading to p11p1 p ij...(6) p1p11 Thus, the nl in the Eq. () fo the light popagating in diection [6] is n n *[( p11 p1)* p1 * )]...(7) The thid tem in Eq. () can be shown to be negligible compaed with the fist two. A pessue P inteacting with the fibe induces a change in phase ϵ/p and is given by k0n0l k0ln0 P Whee the fist tem coesponds to the change in the length of the fibe and the second tem coesponds to the photoelastic effect. Theefoe, the pessue sensitivity of the mandel senso pe unit of ai pessue becomes: n [( P11 P1) P1 ]...(8) Equation (8) means that the phase change of the mandel senso can be found once we detemine the appopiate stain distibution in elation to the unit of applied pessue, which then leads to the analysis of the tansduce pefomance. In this pape, the stain distibution is calculated by using the MATLAB. Eq. (8) is witten in the fom of a summation of the stains distibuted ove all the discete elements in the MATLAB. The following values fo Pockel's coefficients of the unclad fibe ae assumed in the calculations of the pessue sensitivity with the MATLAB []. III. Mathematical analysis of MZI In this pape a mathematical analysis of intefeomete has to be done though MATLAB. Hee afte paamete like phase, fequency fo MZI and fibe optical cable elated paamete like coe-cladding adius, length shows in the following gaph simulated by MATLAB gives the pessue sensitivity. Intefeometic ams diffe by vitue of the petubation in one of the fibe legs, and the phase shift between the two light signals povides the measuement. Because the measuements ae made with the intefeence of two light signals with a small wavelength (typically nm) the level of sensitivity is extemely high. In Fig. Change in length Vs Pessue, analysis indicating pessue inceases with changes in l w..t length of the fibe wapped aound the mandel L will be less. Mathematical analysis of intefeomete, and keeps a pomise to give accuacy in the measuement of all the paamete and highe possibility of sensitivity. 30 Page

3 (a) Fig. Change in Length Vs Pessue (b) Fig3. Vaiation in phase due to change in length Fom Fig 4 anothe paamete also keeping the pomises of moe sensitive like pessue (tansvese load) occus with the change in phase. (a) Fig.4 Change in pessue Vs Change in Phase (b) Radial tansvese load applied to the mandel, slightly inceases diamete of the mandel which in compaison with- without tansvese load applied on mandel of diamete is vey less quantity of measuand, shown in Fig 5. Fig 5 Diamete Vs Minimum Phase Fig.6 Length Vs Numbe of tuns Wapping of the fibe on mandel whee it needed to find out No of Tuns. The eason is educing bending losses. Fig 6 analying this as length inceases, no of tun inceases. Fom all above mentioned the 31 Page

4 gaph shows: All the paamete has to veify with the diffeent wavelength with diffeent paamete and some of the gaph mentioned hee, length, no of tuns, diamete, and min phase ae showing the esult that MZI intefeomete senso is moe sensitive. Following measuements ae fomed in table which gives the highest possibility of detecting the tansvese load. Table 1 Coe Clad Fist coating (soft) Second coating (soft) Rimplast Nylon Polyme laye (Thick) polyuetha ne Composition SiO 95%SiO, 5%B O 3 Silicone ubbe Diamete (μm) Young s modulus (10¹ºdyn/cm²) Poisson s atio P₁₁ 0.16 P₁₂ 0.7 n IV. Photo Multiplie Tube Light detection technology is a poweful tool that povides bette undestanding fo moe sophisticated phenomenon. Photodetecto o light sensos ae divided boadly in thee categoies: extenal photoelectic effect, intenal photoelectic effect and themal types. The extenal photoelectic effect is a phenomenon in which light stikes a semiconducto placed in vacuum, electons ae emitted fom its suface into the vacuum. Photomultiplie Tube (PMT) makes use of detecto as extenal photoelectic effect and is supeio in esponse speed and sensitivity (low level light detection). Block diagam of MZI senso, one am of which (sensing mandel am) contains a fibe optic coil defom in an undewate sound pessue field. Since detectable acoustic pessue poduce vaiations in the optical path length of the sensing mandel. Tansduction mechanism of such devices is diectly dependent on the acoustically/ induced phase modulation of the sensing mandel. Afte modulation of the light path fom the sensing mandel and efeence mandel both ae combined and sent to a photomultiplie tube fo detection. Woking Block Diagam If two light signals ae adjusted so that they ae 90 0 out of phase with each othe, then the signal of the output cuent i s of the photomultiplie is [4] 1 i S G( P P )...(9) s c R Whee β is the homodyne efficiency, S c the photocathode sensitivity, G the photomultiplie gain, P R and P s is the espective powes of the efeence signal and sensing signal light, and Δϵ the acoustically/induced phase [4, 9] diffeence between two ams. Fo a boosilicate cown glass, ϵ is evaluated to yield 18 1 i 8.1*10 S Gk l( P P ) P...(10) s c Whee k 0 is fee space optical wavenumbe, l is the length of the wapped fibe aound the mandel, and P is he applied acoustic pessue (in units of μpa). shown in the below fig.7 0 S R S 3 Page

5 Fig.7 Pessue Vs Cuent V. Conclusion In this pape, vaious paametes such as length, adius of mandel, fequency of mandel fo tansvese load sensitivity like pessue, inceases the highest possible value of sensitivity of optical fibe based on intefeomete pinciple As Pessue inceases, changes in light path i.e.phase modulation will occus depending on the tansvese load in the senso am which in compaisons with efeence am will be moe so, the effective changes in the fibe length ( l) will be less and vice-vesa. In this pape, we concluded, pessue inceases with changes in l w..t length of the fibe wapped aound the mandel L will be less. Table Length ( l) In mete Pessue ( P) In Pascal Phase ( ϵ) In Radian *10^ *10^ *10^-11 7*10^ *10^ *10^-4 Table shows the changes in the effective length ( l) = 0.15 at pessue of 4.7*10^-11. Also, vaiation in phase of light path at same effective length obseved is 6.5*10^-4. All above gaph of Length Vs No of Tuns, Diamete Vs Minimum Phase, Change in pessue Vs Change in Phase, Vaiation in phase due to change in length, Change in Length Vs Pessue, At photomultiplie tube, output cuent has been obseved fo boosilicate cown glass is 1.68*10^-19 Ampee at pessue of.5mpa. Studies ae pomising the MZI intefeometic senso is moe sensitive fom othe mechanically stuctue like stain-gauge, diaphagm. Refeence [1] J. A. Bucao, N. Lagakos, J. H. Cole, and T. G. Gialloeni, "Fibe optic acoustic tansduction," Physical Acoustics edited by R. H. Thuston, 16, 385 (198) [] N. Lagakos, E. U. Schnaus, J. H. Cole, J. Jaynski, and J. A. Bucao, "Optimiing fibe coatings fo intefeometic acoustic sensos," IEEE Jounal of Quantum Electonics, 18(4), 683 (198) [3] J. Jaynski, R. Hughes, T. R. Hickman, J. A. Bucao, "Fequency esponse of intefeometic fibe optic coil hydophone," Jounal of Acoustical Society of Ameica, 69(6), 1799 (1981) [4] Pete Shajenko, James P. Flatley, and Mak B. Moffett, On fibe hydophone sensitivity Naval Undewate system Cente, New London Laboatoy, New London, Connecticut [5] R. Hughes and J. Jaynski, "Static pessue sensitivity amplification in intefeometic fibe optic hydophones," Applied Optics, 19(1), 1 ( ) [6] V. S. Sudashanam and K. Sinivasan, "Static phase change in a fibe optic coil hydophone," Applied Optics, 9(6), 855 (1990) [7] J. F. Nye, "Physical Popeties of Cystals,"(Claendon, Oxfod [8] Pess, 1976) [9] G. B. Hocke, Fibe Optic sensing of pessue and tempeatue Applied Optics, 18(9), 1445 (1979). [10] G. A. Massey, An Optical heteodyne Ultasonic Image Convete Poc. IEEE 56, (1968). 33 Page