Design And Evaluation Of Mach-Zehnder Modulator

Design And Evaluation Of Mach-Zehnder Modulator

Design And Evaluation Of Mach-Zehnder Modulator Ass. Prof. Dr. Sinan Majid Abdul Satar Laser and Optoelectronic Engineering Department, University of Technology. Email: [email protected] Dr.waleed Yassen Hussein Laser and Optoelectronic Engineering Department, University of Technology. Mrs. Noor Waleed Hamodi Laser and Optoelectronic Engineering Department, University of Technology. Email: [email protected] Received on: & Accepted on: ABSTRACT With many advantages compared to traditional sensors, optical fiber sensors have been studied and applied to many different areas. In this paper, design of strain sensors by using two methods of MZM, two Separated Arms MZM and in-line MZM. The in-line MZM is simple and lower cost than separated arms MZM, this in-line MZM is designed by splicing SMF with MMF. Four lengths of MMF have been used which are (90mm,80mm, 70mm and 50mm).Weights from (50 to 2000) g are used and placed on MMF. The sensitivity calculated of these MMF length have been (7.6pm/µε , 6.15pm/µε , 5.45pm/µε and 4.615pm/µε) respectively, best sensitivity was obtained with length of MMF is 90 mm. The experimental results shows that the two separated arms method in the MZM is more sensitive than the in-line MZM, which is (0.315pm/kpa) for two separated arms and (0.09 pm/kpa) for the inline MZM. Keywords: fiber-optic sensor, MZM , strain sensor. ‫تصميم وتقييم المضن ماخ – زندر‬ : ‫الخالصــــة‬ ‫ تم دراسة أجهزة استشعار األلياف‬،‫العديد من المزايا بالمقارنة مع أجهزة االستشعار التقليدية‬ ‫تم تصميم أجهزة استشعار الضغط‬، ‫ في هذه البحث‬.‫البصرية وتطبيقها على العديد من المجاالت المختلفة‬

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Design And Evaluation Of Mach-Zehnder Modulator

‫ ذراعان النفصالن للماخ زندر و الذراعان الملتقيان للماخ‬: ‫ وهما‬، ‫باستخدام طريقتين من الماخ زندر‬ .‫زندر‬ ‫ تم‬,‫الذراعان الملتقيان للماخ زندر هوا بسيطة وأقل تكلفة من الذراعان المنفصالن لالخ زندر‬ ‫ وقم تم‬, ‫تصميمه بواسطه لحم الليف البصري المتعدد النمط مع اثنان من الليف البصري االحادي النمط‬ ‫ وتم تحميله‬, ‫مم‬90 , ‫ مم‬80 , ‫ مم‬70 , ‫مم‬50: , ‫استخدام اربع اطول ختلفه لليف البصري المتعدد النمط‬ ‫فوجدت الحساسية لألطوال االربعه هي‬. )‫ غم‬2000 – ‫غم‬50( ‫بأوزن‬. (7.6pm/µε,6.15pm/µε,5.45pm/µε,4.615pm/µε) ‫ زندر هو اكثر حساسيه من طريقة الذراعان‬-‫تبين من التجارب أن الذراعان المنفصالن للماخ‬ ‫ حيث ان حساسيه‬. ‫زندر‬-‫الملتقيان للماخ – زندر هي أكثر حساسية الذراعان الملتقيان للماخ‬ ‫ بينما حساسية الذراعان الملتقيان للماخ‬0.315 pm/kpa ‫الذراعان المنفصالن للماخ – زندر هي‬ 0.09 pm/kpa ‫زندر هي‬

Introduction A sensor is a device used for determining or sensing the value of a chemical or a physical variable of a system or the environment of the system. There are many types of sensors depending on the principle of their work such as, mechanical sensors, optical sensors, electrical sensors... etc. The optoelectronics industry has brought about such products as compact disk players, laser printers, bar code scanners, and laser pointers. The fiberoptic communication industry has literally revolutionized the telecommunication industry by providing higher performance, more reliable telecommunication links with ever decreasing bandwidth cost. This revolution is rapidly becoming a rout as fiber-optic links move steadily toward fiber to the curb and finally the home bringing about the benefits of high volume production to component users and a true information superhighway built of glass [1]. Intensity sensors are the first fiber optic sensors were developed even before low-loss fibers became available in the 1970s. They used bundles or single fibers to measure the light reflected or transmitted by an object. This technology, which is elementary by today’s standards, nevertheless provided the advantages of fiber optics to a limited number of applications. As new fibers became available, the performance of sensors improved. The availability of durable single-fiber cables allowed efficient optical systems and miniature sensors to be employed. In addition to

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Design And Evaluation Of Mach-Zehnder Modulator

simple reflective and transmissive systems, fringe tracking, microbending, total reflection, and photoelastic techniques were explored. Progress toward practical fiber optic sensors was rapid [2]. Advantage of Fiber Optic Sensors Compared with other types of sensors, fiber optic sensors offer may benefits as following [3], Sensed signal is immune to electromagnetic interference (EMI) and radio frequency interference (RFI), Intrinsically safe in explosive environments, Highly reliable and secure with no risk of fire/sparks, High voltage insulation and absence of ground loops and hence obviate any necessity of isolation devices like optocouplers, Small volume and light weight, e.g., one kilometer of 200um silica fiber weighs only 70 gm and occupies a volume of about 30 cm3., As a point sensor, they can be used to sense normally inaccessible regions without perturbation of the transmitted signals, and Extreme harsh environments such as temperature >800°C or as low as a few Kelvin.

Theory: Optical modulators are used for electrically controlling the output amplitude or the phase of the light wave passing through the device. To reduce the device size and the driving voltage, waveguide-based modulators are used for communication applications. To control the optical properties with an external electric signal, the electro-optic effect, or Pockels effect, is used, where the birefringence of the crystal changes proportionally to the applied electric field. A refractive index change results in a change of the phase of the wave passing through the crystal. If you combine two waves with different phase change, you can interferometrically get an amplitude modulation [4]. One of the most sensitive arrangements for a fiber optic sensor is the Mach–Zehnder (MZ) interferometric sensor arrangement. Light from a laser is passed through a fiber optic coupler, which splits the incoming light beam into two equal-amplitude beams in the two single-mode fiber arms. One of the arms, called the reference arm, is kept isolated from any external perturbation. The fiber in this arm is sometimes coated with a material to make it insensitive to the parameter of measurement. The other arm, called the sensing arm, is exposed to the perturbation that is to be measured. After traversing through the two arms, the light beams are made to recombine at the output coupler. Any

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Design And Evaluation Of Mach-Zehnder Modulator

external parameter, such as temperature or pressure, affects the sensing fiber by changing either the refractive index or the length of the arm, thus changing the phase difference between the two beams as they enter the output coupler as shown in figure (1). [5,6] V0 In

Out 1

Out 2 50/50

50/50 Mach-Zehnder Interferometer

Directional Coupler

Directional Coupler

Figure 1 : Schematic drawing of the Mach-Zehnder modulator. Strain Optics Effect The polarization characteristics of single-mode optical fibers will become much more important. When an external force is applied to a transparent vitrerious elastic medium like a glass, the medium becomes birefringent. This phenomenon is well known as the photo-elastic (opto-elastic) effect. Where optical anisotropy is produced in an optically isotropic medium such as optical fiber when the external force is applied.When an external force that induces a stress in it is applied. The change in the refractive indices is linearly proportional to the stress induced in the material. Therefore, the relation between the stress components δs and the refractive indices ns (s=x,y and z) in the direction of the s axis, can be expressed as follows [7,8]; nx = no + [C1 δx + C2 (δY + δZ )] ny = no + [C1 δy + C2 (δx + δZ )]

……… (1)

nz = no + [C1 δz + C2 (δx + δy )]

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Design And Evaluation Of Mach-Zehnder Modulator

where ns (x, y, and z ) ,and no are refractive index in the direction of s axis is the stressed state and in the unstresses state, δs represents the principal stress in the direction of s axes , C1 and C2 denote direct opto-elastic constant,and transverse opto-elastic constant, respectively. The relative optoelastic constant C is defined as the difference between the direct opto-elastic constant,and the transverse opto-elastic constant ,(C = C1 - C2) is usually termed as the opto-elastic constant. In the case of applying the external force per unit length (kg/mm) in the direction of y-axis to the optical fiber, and the propagation of the light in the z direction, the principal stress component in the center of the core can be approximately expressed as: δx =

F πb ......... (2)

δy =

−3F πb

Where F is the force per unit length (kg/mm) and b is cladding radius. The modal birefringence H between the x-axis and the y-axis can be expressed as [7,9]: H= nx − ny = C (δx − δy ) =

4CF

........ (3)

πb

The strain (as a result of external perturbation) in the fiber introduces strain–induced birefringence. In fact the dynamic , phase change, mainly, corresponding to ultrasonic stress wave induced phase change i.e.[10] ∆β = βx − βy

......... (4)

If the total length of fiber is L, the phase of the light wave after passing through the fiber is: ∅ = βL

......... (5)

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Design And Evaluation Of Mach-Zehnder Modulator

Straining the fiber causes a change in the phase by an amount: ∆∅ = ∆βL = (βx − βy ) L = K O (nx − ny ) L ∆∅ = K O H L = ∆∅ =

8C FL

......... (6)

λb

8CW λb

When specimen is subjected to a modulating effect, an optical phase shift, ∆∅, occurs in the light beam, which is given by [10,11,12] ∆∅= n k o ΔL +L Δnk o

......... (7)

∆∅=β ΔL +L ∆β

......... (8)

The first term represents the effect of the physical change of length due to the strain and can be given in terms of the axial strain εz . For the stress considered, β ∆L = β εz L = - β(1 - 2μ) LP/E

.........(9)

The second term, the change in ∅ due to a change in β = Kо Δn, can come about from two effects: the strain-optic effect where by the strain changes the refractive index of the fiber, and a waveguide mode dispersion effect due to a change in fiber diameter produced by the strain dβ

dβ

L∆β=Ldn ∆n + LdD ∆D

......... (10)

The strain-optic effect appears as a change in the optical indicatrix [13] 1

∆ [n2 ] = ∑6i,j=1 Pij εj

....... (11)

ε −με −με Where εj is the strain vector, εj = 0 0 [ 0 ]

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Design And Evaluation Of Mach-Zehnder Modulator

Where the Pij, is the optical constant (Pockel's constant) .When a material is under isotropic stress with no shear strain then: ε4 = ε5 = ε6 = 0 Only i,j=1,2,3, elements of strain-optic tensor for homogeneous isotropic material are needed to be considered where P11=P22=P33 ,P12= P21 = P13 = P31 = P23 = P32 thus [14,15]; P11 Pij εj = [P12 P12

P12 P11 P12

P12 εX P12 ] [ εY ] P11 εZ

.........(12)

Therefore, the change in the optical indicatrix may be expressed : 1

∆ [n2 ] = P11 εX + P12 εY + P12 εZ

......... ( 13)

The relative variation of the refractive index due to the photoelastic effect are always small compared with unity, which make it possible to write [16] : ∆n = −

n3 2

1

∆ (n 2 )

........(14)

On substituting eq.( 10) in eq. (11), ∆n takes the form: ∆n = −

n3 2

(P11 εX + P12 εY + P12 εZ )

.........( 15)

The last term in Eq. (8) represents the change in the waveguide mode propagation constant due to a change in fiber diameter. The approach followed here is report the induced optical phase shift and birefringence for each modulator type . As an example ,is the case of the isotropic stress due to a pressure P. where no shear components is present , the stress can be written as [11,13]:

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Design And Evaluation Of Mach-Zehnder Modulator

− p − σ(x, y, z) = [ p ] − p

.........( 16)

From Hooks law ,it can be shown that the strain ε may be written as [11]: εX −P(1 − 2μ)/E ε ε = [ Y ] = [ −P(1 − 2μ)/E ] εZ −P(1 − 2μ)/ E

.........(17)

Where:μ is the poisson s ratio and E is the Youngs modulus. Substituting the various expressions above into Eq (8), we obtain ∆∅= -

β(1−2μ)LP E

+

k n3 LP 2E

( 1-2μ)(P11 + 2P12 )

.........(18)

So a simplified expression for ∆∅/PL is: ∆∅

= [− PL

k n (1−2μ) E

+

k n3 2E

(1 − 2μ)(P11 + 2P12 )]

........(19)

The equation (19) for the phase shift is used to calculate the phase change due to the applied pressure . The pressure as a function of the deltaphase is plotted and shown in figure (4)

Experimental work : first method Two separated arms in the MZM, The experimental setup is presented in Figure 2. Before applied pressure, the interference pattern must be observed to obtain fringes of the following requirements are to be satisfied 1- 5-D fiber holder should be moved until got good alignment, and exit of laser from fiber end (1m) with high intensity. 2- Fiber end should be put in fiber splitter. 3- The overlapping of two beams must be observed on a screen.

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Design And Evaluation Of Mach-Zehnder Modulator

The two overlapping beams interfere to form a series of circularly bright and dark fringes. Then gradually compressed sensing arm .The fringes have been obtained for many cases of pressure applied. There is no movement of fringes without applied pressure. The fringes got by applying those pressures are as follows: five fringes for pressure 55.kpa, 20 fringes for pressure 222kpa and 45 fringes for pressure 500 kpa.

Figure(2): Block diagram setup configuration Figure (3) shows the experimental result of using pressure with the number of fringes as shown below: 600

pressure (kpa)

500

y = 11.104x R² = 1

400 300 200 100 0 0

10

20

number 30 of fringes 40

50

60

Figure(3): Experimental result ,relation between number of fringes and applied pressure Figure (4) show the experimental results of pressure with delta phase. These figure shown that the linear behavior is common in all cases. This

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Design And Evaluation Of Mach-Zehnder Modulator

linearity in the behavior is reversible i.e. the behavior is the same in case of the compression and relaxation. 600 500

y = 290.13x + 16.218 R² = 0.9938

pressure(kpa)

400 300 200 100

0 0

0.5

1

1.5

2

∆Ф(rad)

figure(4): Experimental result ,relation between detaphase and applied pressure

Theoretical Results The equation (19) for the phase shift derived is used to calculate the phase change due to the applied pressure which is given as: ∆∅ PL

= [−

Kn(1−2μ) E

kn

+ 2E (1 − 2μ)(P11 + 2P12 )]

………(19)

A computer program was written and run for different input data. The pressure as a function of the deltaphase is plotted and shown in figure (4). Which shown that there is small deviation at low pressure and high deviation at high pressure this due to deformation in the fiber structure.

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Design And Evaluation Of Mach-Zehnder Modulator

pressure (kpa)

600 500

y = 290.2x + 15.90 R² = 0.993

400

y = 271.8x - 0.266 R² = 1

300 200 100 0 0

0.5

1

1.5

2

2.5

-100

∆Ф(rad)

Figure (5): Theoretical and experimental results

Second method : In-Line MZM Results SMF splicing with MMF.A four lengths of MMF have been used: 50mm, 70mm, 80mm and 90mm.Weights from (50 to 2000) g have been used and placed on MMF. The external temperature has been fixed while the weight will change continuous so the effect on the MMF is seen clearly. From monitoring the reflected spectrum of a bare MMF, the shifting wavelength can be measured thus the strain has been achieved. Figures (6,7,8,9), shown the relationship between amounts of the total strain for the weights in range (10g to 2000g) with the wavelength for each MMF length. The change of the weights will effect the amount of shifting in wavelength, relationship between the weights and the shifting become linear. It shown that the amount of shifting in wavelength will decrease with increase the weight; From those figures, the sensitivity calculated of this sensor for each MMF lengths have been: 7.6 pm/µε for 90mm MMF length, 6.15pm/µε for 80mm MMF length, 5.45pm/µε for 70mm MMF length and 4.615pm/µε for 50mm MMF length.

11

λ nm

Design And Evaluation Of Mach-Zehnder Modulator

847.5 847.45 847.4 847.35 847.3 847.25 847.2 847.15 847.1 847.05 847

y = -0.007x + 847.47 R² = 0.9821

0

20

40

60

µ strain

Figure (6) Shown the relationship between the strain and wavelength shift for 90mm MMF length, the sensitivity =7.6 pm/µstrain

y = -0.006x + 847.44 R² = 0.9907

847.45 847.4

λ nm

847.35 847.3 847.25 847.2 847.15 0

10

20

30

40

50

µ strain

Figure (7) Shown the relationship between the strain and wavelength shift for 80mm MMF length,the sensitivity =6.15pm/µstrain

12

λ nm

Design And Evaluation Of Mach-Zehnder Modulator

y = -0.0052x + 847.21 R² = 0.9889

847.2 847.18 847.16 847.14 847.12 847.1 847.08 847.06 847.04 847.02 0

10

20

30

40

µ strain

Figure (8) Shown the relationship between the strain and wavelength shift for 70mm MMF length, the sensitivity =5.45 pm/µstrain

y = -0.006x + 847.44 R² = 0.9907

847.45 847.4

λ nm

847.35 847.3 847.25 847.2 847.15

0

10

20

30

40

50

µ strain

Figure (9) Shown the relationship between the strain and wavelength shift for 50mm MMF length, the sensitivity = 4.615 pm/µ strain

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Design And Evaluation Of Mach-Zehnder Modulator

SENSITIVITY

Figure (10) has given indication that the maximum sensitivity has been recorded when length of MMF equal 90 mm, i.e. The sensitivity increase with increase the length of MMF, and that because, when laser coupling into single mode optical fiber then entered to multimode optical fiber , many mode are exited and propagate in core and cladding so have different propagation constant , when the interaction length increased this will generate higher phase constant , this illustrates that the relation between the length of fiber and the phase constant , when the length of multimode increase the phase shift increase too, i.e ∆𝜆 increase ,this cause the sensitivity increase

8 7 6 5 4 3 2 1 0

y = 0.0705x + 0.8446 R² = 0.9063

0

20

40

60

80

100

lenght of MMF

Figure (10): Relationship between length of MMF and sensitivity Figures (11) ,(12), (13), (14), represents the relationship between amounts of the total strain for the weights in rang (10g to 2000g) with peak power shifts for each MMF lengths. The change of the weights will effect on power loss, relationship between them become linear. Its appeared that the power loss have been increased with increased the strain.

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Design And Evaluation Of Mach-Zehnder Modulator

15.6 15.5 15.4 15.3 15.2 15.1 15

loss (dB)

y = 0.008x + 15.041 R² = 0.9857

0

20

40

60

µ strain

loss(dB)

Figure (11): Relationship between the strain and the peak power for: 50 mm MMF length 16.9 16.8 16.7 16.6 16.5 16.4 16.3 16.2 16.1 16

y = 0.0146x + 16.093 R² = 0.9688

0

10

20

30

40

50

µ strain

Figure (12): Relationship between the strain and the peak power for: 70 mm MMF length 16.5 y = 0.023x + 15.553 R² = 0.9358

loss(dB )

16.3 16.1 15.9 15.7 15.5 0

5

10

15

20

25

30

35

µ strain

Figure (13): Relationship between the strain and the peak power for: 80mm MMF length

15

Design And Evaluation Of Mach-Zehnder Modulator

17.6 17.5 17.4 17.3 17.2 17.1 17

loss (dB )

y = 0.012x + 17.043 R² = 0.9756

0

10

20

30

40

50

µ strain

Figure (14): Relationship between the strain and the peak power for: 90mm MMF length Figure(15,16,17,18) shown the transmission spectra of this sensor. The results have shown that there is blue shift (decrease in wavelength) of the transmission spectra with strain increasing, and loss in the fiber is increase with increase in strain of this sensor. This is due to the leakage power mode from the core to the cladding.

Figure (15):Transmission spectra for 90mm MMF length

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Design And Evaluation Of Mach-Zehnder Modulator

Figure (16):Transmission spectra for 80mm MMF length

Figure (17):Transmission spectra for 70mm MMF length

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Design And Evaluation Of Mach-Zehnder Modulator

Figure (18):Transmission spectra for 50mm MMF length 6.1 Conclusions From the conducted experiments, it could be getting the following conclusions 





Two-separated arm MZM method shown that a single-mode fiber interferometer could be used to detect and measure pressure changes in one of the fiber arms. The sensitivity of such an interferometer for pressure measurement experimentally is 0.315pm/pa. Disadvantages of this technique include the alignment requirements for the interferometer, also it is need to observe the motion of optical fringes. In-line MZM method, has been used for measuring strain based on mode interference in multimode fibers. The in-line Mach-Zehnder structure is a good alternative to overcome the above drawbacks also is simple and low cost. By measuring the central wavelength shift of output spectra, the strain can be determined. For In-line MZM method, the maximum sensitivity and good linearity is achieved in the strain range from(0 to 55 𝜇𝜀 ) is 7.6 pm/𝜇𝜀 for 90mm MMF length, and the maximum sensitivity and good linearity is achieved in the pressure range from(0 to 500 kpa ) is 0.09 pm/Pa for 90 mm MMF length.

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Design And Evaluation Of Mach-Zehnder Modulator



By comparison the sensitivity of sensors made from each methods, its appeared that the sensitivity gained from sensor made by using two separated arm MZM (which is 0.315 pm/ pa) is more sensitive than the sensor made by in-line MZM (which is 0.09 pm / pa) .

Reference [1] E. Udd, “ An overview of fiber-optic sensors”, American Institute of Physics, Vol.66, No.8, pp. 4015-4030, (1995). [2] E. Udd, W. Spillman Jr., “Fiber Optic Sensors: An Introduction for Engineers and Scientists”, Wiley Book, Second Edition, ISBN: 9780470126844 (2011). [3] N. Sabri, S. Aljunid, M. S. Salim, R. B. Ahmad and R. Kamaruddin, “Toward Optical Sensors: Review and Applications”, Journal of Physics: Conference Series 423, (2013). [4] K. Thyagarajan and A. Ghatak, “Fiber Optic Essentials”, Book, Wiley, ISBN: 978-0-470-15255-3, (2008). [5] A. Ghatak and k. Thyagarajan, “introduction to fiber optics”, book, Cambridge university press, ISBN: 0521571200, (1998). [6] K. P. Zetie, S. F. Adams and R. M. Tocknell, “How Does A Mach–Zehnder Interferometer Work”, Physics Department, Westminster School, London SW1 3PB, PII: S0031-9120(00)05127-3, (1999). [7] J. Tsuji, M. Nishida and K. Kawada, “Experimental methods of photoelasticity”, Nikkan Kogyo-sha (in Japanese), (1975). [8] ‫قيس عبد الستار النعيمي "دراسة الخواص البصرية الميكانيكة للبولي ميثيل ميثا اكريليت‬ ‫ قسم الفيزياء‬,‫ كلية العلوم‬,‫ جامعة بغداد‬,"‫ واستخدامه كمحسن للضغط‬PMMA

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Design And Evaluation Of Mach-Zehnder Modulator

[9] L. Yuan, G. Zhang and Q. Li, “Interaction Model Between Fiber Optic Ultrasonic Sensor and Matrix Materials”, Department of Physics, Harbin Engineering University, China (2005). [10] Miao Yu. , “Fiber Optics Sensor Technology”,IMAC XXVI, Orlando, FL, (2008). [11] G. Hocker, “ Fiber Optic Sensing of Pressure and Temperature, Applied Optic”, 18(9):1445, (1979). [12] J. Saprid, “Acousto-Optics”, Book, New York: Wiley– Interscience Publication, (1979). [13] S. Timoshenko and J. Goodier, “Theory of Elasticity”, New York: Mcgraw HILL INC, (1951). [14] B. Lee, Y.Kim, K.Park, J.Eom, M.Kim, B.Rho, and H.Choi, “Interferometric Fiber Optic Sensors”, Sensors (Basel), ISSN 1424-8220, pp. 2467-2468, (2012). [15] G. Agrawal, “applications of nonlinear fiber optics”,book,the Institute of New York Optics University of Rochester, fourth edition,ISBA:13:978-0-1239581-1,(2010). [16] N.Al-Douri, “phase modulation in pressure sensing polymer[PMMA] ”,master thesis,college of science,(1998).

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