Influence of birefringence dispersion on a distributed ... - Science Direct

Optical Fiber Technology 15 (2009) 83–89

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Optical Fiber Technology www.elsevier.com/locate/yofte

Influence of birefringence dispersion on a distributed stress sensor using birefringent optical fiber Tianhua Xu a,b,∗ , Wencai Jing a,b , Hongxia Zhang a,b , Kun Liu a,b , Dagong Jia a,b , Yimo Zhang a,b a b

College of Precision Instrument and Optoelectronics Engineering, Tianjin University, Tianjin 300072, PR China Key Laboratory of Optoelectronics Information and Technical Science, Tianjin University, Ministry of Education, Tianjin 300072, PR China

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 8 December 2007 Revised 23 March 2008 Available online 30 June 2008

Distributed stress sensor with a white-light scanning interferometer is used to detect stress distribution by analyzing polarization mode coupling caused by forces exerted on PMFs (polarization maintaining fibers). In measurement of polarization coupling, the birefringence in sensing fiber is usually considered to be wavelength independent. Sensitivity and spatial resolution of the distributed stress sensor are constant for a given optical source spectrum. In practical measurement, however, the birefringence in PMFs is related with optical wavelength. In other words, birefringence dispersion exists in PMFs. Due to birefringence dispersion, the relationship between stress and coupling strength varies with different positions of external forces, and spatial resolution of the distributed stress sensor descends obviously with transmitted distance. In this paper, influences of external-force positions and optical source spectrum on distributed stress sensor are analyzed in consideration of birefringence dispersion and validated with experiments. A method for compensating birefringence dispersion in the sensing system is proposed in the end. © 2008 Elsevier Inc. All rights reserved.

Keywords: Distributed stress sensor Polarization coupling Birefringence dispersion Spatial resolution Dispersion compensation

1. Introduction Over past few years, optical fiber sensors have been applied widely and played important roles in stress and temperature detections [1–3]. Such sensors have advantages over conventional techniques, including immunity to electromagnetic interference, remote sensing, ease of handling, low cost and small size [4,5]. The orientation of birefringence in polarization maintaining fibers can be changed by external transverse forces, which leads to the phenomenon of polarization coupling. In the measurement of stress induced polarization coupling using white light interferometry, the birefringence in PMFs is usually considered to be wavelength independent, namely, the difference between two orthogonal eigenmodes of PMFs is invariable [6,7]. Therefore, the sensitivity and spatial resolution of distributed sensor does not vary with the diversification of force locations. While in practical measurement, the birefringence is relative with transmitted wavelength. The chromatic dispersion coefficients are different in the two orthogonal axes. Therefore, birefringence dispersion exists in PMFs [8–10]. The negative influences of birefringence dispersion on the distributed sensing system cannot be ignored when the transmitted distance is relatively long. The sensitivity of forces de-

*

Corresponding author at: College of Precision Instrument and Optoelectronics Engineering, Tianjin University, Tianjin 300072, PR China. Fax: +86 22 27890672. E-mail address: [email protected] (T. Xu). 1068-5200/$ – see front matter doi:10.1016/j.yofte.2008.06.001

© 2008

Elsevier Inc. All rights reserved.

tection in the system descends obviously along the sensing fiber due to the birefringence dispersion. Meanwhile, the usual relationships between spatial resolution and forces positions together with optical spectrum have to be modified on account of the birefringence dispersion in PMFs. 2. Principle of the distributed stress sensing system The distributed stress sensing system is based on the polarization coupling phenomenon caused by external forces applied at birefringent fiber, which generates a concrete mathematical connection between power coupling strength and external disturbances [11,12]. Hence, the system consists of two parts, the sensing of external forces and the detecting of polarization coupling. 2.1. Model of polarization coupling induced by transverse extern forces Stress-induced polarization mode coupling arises from the changes of birefringent orientation in PMFs caused by transverse forces. Establishing a coordinate system as shown in Fig. 1, the transverse force makes an angle of α with the fast ( y-) axis in birefringent fiber. The input polarized light can be expressed with Jones matrix:

E x (0) cos δ = E0 , E y (0) sin δ exp(i Φ)

(1)

84

T. Xu et al. / Optical Fiber Technology 15 (2009) 83–89

Fig. 1. Model of transversal forces exerted on PMFs.

where E 0 is the electric field intensity of input light, δ is the angle between electric vector and slow (x-) axis, and Φ is the phase difference between the two orthogonal polarization modes. When sensing fiber is disturbed by a transverse force, the output light can be expressed as follows:

− sin θ cos θ

E x (Lf ) cos θ = E y (Lf ) sin θ

× ×

cos θ − sin θ

sin θ cos θ

exp(−ikno L f ) 0 0 exp(−ikne L f )

E x (0) E y (0)

Fig. 2. Relationships between coupling strength and external forces.

= exp(−ikno L f ) [cos2 θ + sin2 θ exp(−i2π L f / L b )] E x (0) sin θ cos θ[1 − exp(−i2π L f / L b )] E y (0) [sin2 θ + cos2 θ exp(−i2π L f / L b )] E y (0) sin θ cos θ[1 − exp(−i2π L f / L b )] E x (0)

tan 2θ =

, (2)

where no and ne are refractive index of the fast and slow axes in disturbed PMFs, respectively, θ is the rotating angle of birefringent axis induced by external force, and L b is a variable related with external force and the beat length L b0 of fiber. According to Eq. (2), the power coupling strength h can be calculated as h = h x→ y = h y →x = sin2 (2θ) sin2 (π L f / L b ).

(3)

Analyzing the force-induced polarization coupling model in Fig. 1, the stress tensor T in (x, y ) coordinate plane is given according to elasticity theory, as below: T xx = − T xy = −

f

(1 − 2 cos 2α ),

πr 2f

πr

T yy = −

f

πr

(1 + 2 cos 2α ),

sin 2α ,

(4)

where r is the radius of PMF, f (N/mm) is the value of external force in unit-length. Strain tensor S i j is calculated by Hooke principle: ⎛ S ⎞ ⎛ 1/ E −μ/ E −μ/ E 0 xx 0 ⎜ S y y ⎟ ⎜ −μ/ E 1/ E −μ/ E ⎜ ⎟ ⎜ 0 ⎜ S zz ⎟ ⎜ −μ/ E −μ/ E 1/ E ⎜ ⎟=⎜ 0 0 2(1 + μ)/ E ⎜ S yz ⎟ ⎜ 0 ⎝ ⎠ ⎝ S zx S xy

0 0

⎛T

xx

⎞

0 0

⎜ T yy ⎟ ⎜ ⎟ ⎜T ⎟ × ⎜ zz ⎟ , ⎜ T yz ⎟ ⎝ ⎠

0 0

0 0

0 0 0 0 2(1 + μ)/ E 0

0 0 0 0 0

⎟ ⎟ ⎟ ⎟ ⎟ ⎠

2(1 + μ)/ E

(5)

where E is the modulus of elasticity, μ is the Poisson coefficient. The dielectric constant in fiber has changed and the optical anisotropic phenomenon has occurred due to external forces [13],

n

(7)

2λ L b0 cos 2θ λ λ = = , n ne − no 2λ + (b2 − b1 ) N 3 L b0

(8)

where n0 = λ/ L b0 is the birefringence in undisturbed PMFs, N is the equivalent refractive index of fiber, and λ is the central wavelength of the light propagating in sensing fiber. Substituting Eqs. (7) and (8) into Eq. (3), the polarization coupling strength h can be expressed as

2

2

h = F sin (2α )

sin2 [π ( L f / L b ) 1 + F 2 + 2F cos(2α )] 1 + F 2 + 2F cos(2α )

,

(9)

where F = 2(1 + μ) N 3 L b0 ( p 12 − p 11 )/(π λr E ) · f is the normalized variable of external force, μ is the Poisson coefficient, N is the equivalent refractive index of the fiber, p 12 and p 11 are the elastooptic or Pockels coefficients of the fiber core, r is the radius of the fiber, and E is the modulus of elasticity. The relationship between coupling strength h and external forces f is shown in Fig. 2. Detection of coupling strength h indicates the sensitivity of sensing system. 2.2. Measurement of stress induced polarization coupling

⎞

T zx T xy

1 bi = 2 = p i j S j ,

Lb =

2b6 N 3 , (b1 − b2 ) N 3 − 2n0

(6)

i

where b i is the variance of inverse dielectric tensor, p i j is the optical strain coefficient. Meanwhile, the following expressions are obtained from optoelastic theoretical analysis [13],

The detecting principle of polarization coupling is illustrated in Fig. 3. Polarized broadband light is coupled into the employed PMF with only one polarization mode excited. When there is a force exerted on the sensing fiber, a little fraction of light is coupled into the unexcited mode. Due to the modal birefringence nb (λ) in fiber, two polarization modes propagate through the fiber with different group velocities. Thus, the two orthogonally polarized modes have an optical path difference (OPD) of nb (λ)l at the output end of fiber, where l is the distance from the force point to the output end of fiber. After passing thorough an analyzer, the two modes are projected to the same polarization direction. The OPD nb (λ)l is compensated by a scanning Michelson interferometer and white light interferograms are read out during the scanning process. As mentioned above, it is usually considered that the modal birefringence is wavelength independent and nb = nb (λ0 ), where λ0 is the central wavelength of broadband light. When there is only one external force applied at the sensing fiber, the read out interferogram is expressed as


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Fig. 3. Schematic of the white light interferometer for distributed stress sensing. Fig. 5. Scheme of distributed stress sensing system.

The scheme of the distributed stress sensing system is shown in Fig. 5. It consists of three parts: A superluminescent diode (SLD) as the broadband source, the high birefringent sensing fiber of 500 m long, and polarization coupling measurement module. 3. Influence of birefringence dispersion on sensing system

Fig. 4. Interferogram with one external force applied on sensing fiber.

I (d) = I 0 γ0 (d) cos(k0 d)

+

√ hγ0 (nb l − d) cos k0 (nb l − d) ,

(10)

where I 0 is the DC component of the interference, γ0 (x) = exp[−(2x/ L C0 )] is the optical coherence function of light source, d is the OPD of the scanning Michelson interferometer, k0 = 2π /λ0 is the wave-number in free space, and h is the power coupling strength which is described in the above section. Equation (10) displays as three interferential packets modulated by the interferential envelop |γ0 (x)|, as shown in Fig. 4. The central interferential packet is the interference between excited optical waves and has no relationship with the polarization coupling. The other two symmetrical interferential packets are the interference between excited and coupling optical waves, when the OPD nbl is compensated by the scanning Michelson interferometer. The two interferential packets contain the same information of the polarization coupling, including coupling strength h and the external force position l. The coupling strength and the position of external force can be calculated by analyzing the corresponding interferential packet:

h= l=

I coupling I0

d

nb

2

or h (dB) = 20 lg

,

I coupling I0

λ2 LR = = , nb λnb

ω0

β(ω) ≈

c

,

I coupling =

c

Nb − π c

2

+∞

1

(ω − ω0 )2

S (ω) 1 + Re exp i2 −∞

1 1 + ξ2

exp −

× cos k0 (nbl − d) −

ω02

D,

(14)

ω c

d − β(ω)l

1

2( N b l − d)

1 + ξ2

ξ 1 + ξ2

2

L C0

nb − d L C0

2 ,

(15)

where ξ indicates the accumulation of chromatic dispersion along the fiber:

ξ = 2π c D (λ0 )l

λ λ0

2 (16)

.

According to Eq. (15), envelop of alternating current item in the interferogram is expressed as I ac = I 0 4

(13)

where λ is the 3 dB spectrum width of light source. For a ¯ and modal specific birefringent fiber, its operation wavelength λ birefringence nb is predictable. So the spectrum width λ of light source becomes the only determinant factor for spatial resolution. Wider spectrum means higher spatial resolution. When operation wavelength λ = 1310.2 nm, λ = 35.8 nm and birefringence nb = 6 × 10−4 , the spatial resolution can reach 7.99 cm without considering birefringence dispersion.

ω − ω0

= I0 1 + 4

(11) (12)

nb +

where nb is the phase birefringence of the PMFs, N b is the group birefringence of the PMFs. The optical intensity of interferogram can be expressed as

where I coupling is the amplitude of the zero-order fringe in the interference packet, as shown in Fig. 4. Spatial resolution is defined as the minimum distance between two adjacent external force points which can be distinguished by the sensing system. The spatial resolving distance L R is a constant decided by the coherence length of light source and the modal birefringence of PMF: L C0

In above description, the birefringence dispersion of PMFs is neglected. Actually, the propagation constant difference β(ω) is wavelength dependent, and it leads to birefringence dispersion existing in PMFs [14–16]. The OPD generated in birefringent fiber cannot be compensated at all wavelengths using a non-dispersive Michelson interferometer [17,18]. With the increasing of birefringent sensing fiber length, the influences of birefringence dispersion in PMFs become more and more obvious. And the analysis of external forces position and spatial resolution need to be modified. Propagation constant difference β(ω) can be expanded in a Taylor series to the second-order near the frequency of ω0 :

1 1 + ξ2

exp −

1

2( N b l − d)

1 + ξ2

L C0

2 .

(17)

It is found interferential envelop has changed in two aspects: the broadening of envelop width and the loss of interference contrast. The loss of contrast reduces measurement sensitivity of the system, and the broadening of envelop decreases the spatial resolution. The descent of sensitivity in distributed sensing system can be calculated from Eq. (17): h h

=

4

1

1 + ξ2

2 =

1 1 + (2π c Dl)2 (λ/λ0 )4

,

(18)

86


Fig. 6. Variation of coupling strength with same force f = 0.15 N/mm at different positions.

Fig. 7. Relationship between the spatial resolution and the spectrum width of light source.

where h is the coupling strength detected in polarization coupling measurement module considering birefringence dispersion. Equation (18) indicates the measurement sensitivity descends more obviously with the increasing of external force position l and spectrum width λ. The relationship between normalization of extern forces F and coupling strength h needs to be modified as below:

h

2

1 + (2π c Dl) 2

2

= F sin (2α )

λ λ0

4 1/ 2

sin2 [π ( L f / L b ) 1 + F 2 + 2F cos(2α )] 1 + F 2 + 2F cos(2α )

.

(19)

When the same external force f = 0.15 N/mm is inflicted on different locations along the sensing fiber, the actual coupling strength h descends with the increasing of position l, as shown in Fig. 6. In order to distinguish two force spots, the distance between two interferential packets center needs to exceed over 1/e width of envelop. Hence, the spatial resolving distance L RD of distributed sensing system can be calculated as follows:

L RD =

1 + ξ 2 L C0

Nb

= L b (λ0 /λ)2 + (2π c Dl)2 (λ/λ0 )2 ,

(20)

where L b is the beat length of the high birefringent fiber, which is constant for a specific fiber, and l indicates the position of external force along the sensing fiber. Equation (20) shows the relationship between spatial resolving distance and spectrum of light source obeys a hyperbola distribution, as shown in Fig. 7.√When the optical spectrum meets the condition of λ0 /λ = 2π c D L, the spatial resolving distance reaches its minimum. Meanwhile, the spatial resolution decreases with the increasing of external-force position. The variation of spatial resolution with both spectrum width λ and external-force position l is shown in Fig. 8, when the birefringence dispersion is D = 0.03 ps/km nm. 4. Experiments The experimental setup of the distributed sensing system can also be illuminated according to Fig. 5. A superluminescent diode (SLD) emitting at 1310 nm was used as the light source. Its spectrum followed a Gaussian distribution and the 3 dB spectral width is approximately 35.8 nm. An in-line polarizer is fusion spliced in front of the high birefringent sensing fiber. Its polarization orientation is aligned to the slow axis of the PMF, so that only the slow axis is excited. The output light from fiber is collimated, passed

Fig. 8. Variation of the spatial resolution with both spectrum width and external force positions.

through a rotatable Glan-polarizer, and then injected into the scanning Michelson interferometer. The interference signal is detected with an InGaAs PIN diode and then transmitted into a computer by a programmable data acquisition circuit. The external forces are inflicted on the sensing fiber via a series of mechanical devices. In order to testify the influence of birefringence dispersion, we observed the changes of low coherence interferograms with several coupling points caused by the same external force f = 0.15 N/mm located at different fiber positions, as shown in Fig. 9. The phenomenon of interferograms broadening and contrast decreasing are validated obviously in the experimental result. Optical filters of 12.55 and 10.35 nm are adopted to test the influences of birefringence dispersion at different optical spectrum. It is illustrated in Fig. 10 that the detected coupling strength descends with the increasing of force positions under all the three spectrum widths. Meanwhile, the influence of birefringence dispersion on spatial resolution in the distributed sensing system stands out in the experiments. Under different spectral widths, the spatial resolution with respect to external forces on different positions along the sensing fiber is plotted in Fig. 11. It is found that the spatial resolution will deteriorate with the increasing of external-force positions under all the spectral width. Here achieves a good agreement with Eq. (20), where such phenomenon is predicted. Also, when the highest 3 dB spectral width of 35.8 nm is applied, its influence on the spatial resolution is the most obvious. This can be explained with Eq. (16) where a higher spectrum width can lead to more accumulation of birefringence dispersion.


87

Fig. 9. Interferograms of the same transversal force f = 0.15 N/mm at (a) 10 and (b) 90 m.

Fig. 11. Relationship between spatial resolution and forces positions under different spectral widths. Fig. 10. Experimental descent of coupling strength with increasing of external force positions.

5. Discussion on compensation of the birefringence dispersion In the above sections, it is demonstrated that the influence of birefringence dispersion is critical to the sensitivity and spatial resolution in distributed sensing system. Due to birefringence dispersion, sensitivity and spatial resolution as well as other parameters of the system will deteriorate with the increasing of force positions. However, birefringence dispersion is an intrinsic character in birefringent fibers, of which the infection can only be weakened rather than completely eliminated by reducing the source spectral width. So the external compensation on birefringence dispersion becomes necessary. The classical Michelson interferometer, however, cannot compensate such dispersion at all wavelengths. Here, a method of inserting some dispersive medium in the immovable arm of the scanning Michelson interferometer is proposed, as shown in Fig. 12.

According to the description in Section 3, after inserting dispersive medium, the optical intensity of the interferogram can be expressed as

I (d) = I 0 1 + 4

1 1 + ζ2

× exp −

1

2( N b l − d − 2(nC0 − 1)dC )

1 + ζ2

L C0

× cos k0 nbl − d − 2(nC0 − 1)dC ζ − 1 + ζ2

nbl − d − 2(nC0 − 1)dC L C0

2 ,

(21)

88


Fig. 12. Compensation structure with dispersive medium in Michelson interferometer.

Fig. 13. Variation of spatial resolution with force positions after birefringence dispersion compensation.

where nC0 is the refractive index of the dispersive medium at the center wavelength λ0 , dC is the thickness of the medium. The reflection of birefringence dispersion is calculated as follows:

2 λ ζ = 2π c ( Dl − 2D C dC ) . λ

(22)

When the dispersive medium is replaced by a dynamic compensation device, the birefringence dispersion in PMFs could be compensated entirely according to Eq. (22). In practical measurement, a series of dispersive slices with different dispersion value are applied to equalize the birefringence dispersion in sensing fibers. If we choose a dispersive medium which can compensate half birefringence dispersion of the fiber appropriately, that is D C dC = 1/4 Dl, the relationship between spatial resolution and positions of external forces after compensation is shown in Fig. 13. 6. Conclusions The influence of birefringence dispersion on a distributed stress sensor is illustrated theoretically and experimentally in this paper. With the increasing of sensing fiber length, birefringence dispersion becomes a non-ignorable limiting factor to the distributed sensing system. It is found that sensitivity and spatial resolution decreases obviously with the augment of external-force position. In the end, a method is proposed to compensate the birefringence dispersion in distributed stress sensor. References [1] T.G. Giallorenzi, J.A. Bucaro, A. Dandridge, Optical fiber sensor technology, IEEE Trans. Microwave Theory Tech. 30 (1982) 472–511.

[2] A.D. Kersey, A. Dandridge, Applications of fiber-optic sensors, IEEE Trans. Compon. Hybrids Manuf. Tech. 13 (1990) 137–143. [3] A.D. Kersey, A review of recent developments in fiber optic sensor technology, Opt. Fiber Technol. 2 (1996) 291–317. [4] C.I. Merzbacher, A.D. Kersey, E.J. Friebele, Fiber optical sensors in concrete structures, Smart Mater. Struct. 5 (1996) 196–208. [5] A.D. Kersey, A. Dandridge, Distributed and multiplexed fibre-optic sensor systems, J. Inst. Electron. Radio Eng. 58 (1988) 99–111. [6] M. Tsubokawa, T. Higashi, Y. Negishi, Mode couplings due to external forces distributed along a polarization-maintaining fiber: An evaluation, Appl. Opt. 27 (1988) 166–173. [7] S. Chen, B.T. Meggitt, A.W. Palmer, An intrinsic optical-fiber position sensor with schemes for temperature compensation and resolution enhancement, J. Lightwave Technol. 15 (1997) 261–266. [8] P. Hlubina, W. Urbanczyk, Dispersion of the group birefringence of a calcite crystal measured by white-light spectral interferometry, Meas. Sci. Technol. 16 (2005) 1267–1271. [9] P. Hlubina, W. Urbanczyk, T. Martynkien, Spectral-domain interferometric techniques used to measure the intermodal group dispersion in a two-mode bowtie optical fibre, Opt. Commun. 38 (2004) 313–318. [10] P. Hlubina, T. Martynkien, W. Urbanczyk, Measurement of birefringence dispersion and intermodal dispersion in a two-mode elliptical-core optical fibre using an interferometric method, Int. J. Light Electron. Opt. 15 (2004) 109–114. [11] K. Okamoto, Y. Sasaki, N. Shibata, Mode coupling effects in stress applied single polarization fibers, IEEE J. Quantum Electron. 18 (1982) 1890–1899. [12] T.H. Chua, C.-L. Chen, Fiber polarimetric stress sensors, Appl. Opt. 28 (1989) 3158–3165. [13] S.P. Timoshenko, J.N. Goodier, Theory of Elasticity, McGraw–Hill, New York, 1970. [14] N. Shibata, A. Nakazono, Y. Inoue, Interference between two orthogonally polarized modes traversing a highly birefringent air-silica microstructure fiber, J. Lightwave Technol. 23 (2005) 1244–1252. [15] D.A. Flavin, R. McBride, J.D.C. Jones, Dispersion of birefringence and differential group delay in polarization-maintaining fiber, Opt. Lett. 27 (2002) 1010–1012. [16] F. Tang, X.-Z. Wang, Y. Zhang, Distributed measurement of birefringence dispersion in polarization maintaining fibers, Opt. Lett. 31 (2006) 3411–3413. [17] F. Tang, X.-Z. Wang, Y. Zhang, Characterization of birefringence dispersion in polarization maintaining fibers using white light interferometry, Appl. Opt. 46 (2007) 4073–4080. [18] P. Pavlicek, J. Soubusta, Measurement of the influence of dispersion on white light interferometry, Appl. Opt. 43 (2004) 766–770.

Tianhua Xu was born in Hei Longjiang province in China. He received his B.Eng. degree in optoelectronics information engineering in 2005 and his M.Eng. degree in optical engineering in 2007, both from Tianjin University, China, where he is pursuing his Ph.D. study. He is engaged in fiber sensing systems, optoelectronics and white light interferometry. Wencai Jing received his Ph.D. degree in optical engineering in 1998 from Tianjin University, China, where he is currently a professor in College of Precision Instrument and Optoelectronics Engineering. His research interests include optical detection, optical interconnection and fiber sensing. Professor Jing has published more than 70 papers in scientific and technology journals and proceedings. He is a member of SPIE and Chinese Optical Society.


Hongxia Zhang received her Ph.D. degree in optical engineering from Tianjin University, China, in 2005. She is currently an assistant professor in college of Precision Instrument and Optoelectronics Engineering. Her principal interests are fiber optic sensing, photoelectric detection, interferometry and optical design.

Kun Liu received his M.S. degree in optical engineering in 2006 from Tianjin University, China, where he is pursuing his Ph.D. degree. He is working on programs in fiber sensing systems and optoelectronics.

89

Dagong Jia received his Ph.D. degree in optical engineering in 2004 from Tianjin University, China. From 2004 to 2006, he was employed as a Post Doctor within Department of Precision Instrument at Tianjin University. He jointed Tianjin University as associate professor of optical engineering in 2006. His research interests include optical interconnection, optical sensing and passive optical component. Yimo Zhang graduated from Tianjin University in 1955 and finished postgraduate study there in 1958. He has been managing a number of programs in applied optics, optoelectronic technology, and optical signal processing. He is the President of Academic Committee of Precision Instruments and Optoelectronic Engineering College, Tianjin University. Mr. Zhang is a Fellowship of SPIE and a member of Standing Committee in Chinese Optical Society. He is also the President of Reprographic Scientists and Engineers Society of China.