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Wavelength Shift of Cladding Mode Resonances in aMechanically Induced LPFG by Twisting the FiberB. Unnikrishnan Nair a , V. P. Sudeep Kumar a , V. P. Mahadevan Pillai a & V. U. Nayar aa Department of Optoelectronics , University of Kerala, Kariyavattom , Thiruvananthapuram,Kerala, IndiaPublished online: 24 Apr 2007.

To cite this article: B. Unnikrishnan Nair , V. P. Sudeep Kumar , V. P. Mahadevan Pillai & V. U. Nayar (2007) Wavelength Shift ofCladding Mode Resonances in a Mechanically Induced LPFG by Twisting the Fiber, Fiber and Integrated Optics, 26:3, 159-172,DOI: 10.1080/01468030701239501

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Fiber and Integrated Optics, 26:159–172, 2007Copyright © Taylor & Francis Group, LLCISSN: 0146-8030 print/1096-4681 onlineDOI: 10.1080/01468030701239501

Wavelength Shift of Cladding Mode Resonancesin a Mechanically Induced LPFG by

Twisting the Fiber

B. UNNIKRISHNAN NAIRV. P. SUDEEP KUMARV. P. MAHADEVAN PILLAIV. U. NAYAR

Department of OptoelectronicsUniversity of KeralaKariyavattom, ThiruvananthapuramKerala, India

Abstract The long-period fiber grating is mechanically induced over a twisted fiberand its characteristics are investigated. The amplitude as well as the wavelength shiftof the resonance is studied in response to the applied twist and pressures. These res-onances decrease in amplitude and shift to shorter wavelength side as the appliedtwist increases. The spectral responses of a grating assembly formed by two gratingsections in series, one section with a twisted fiber and other with an untwisted fiber,are also investigated. Shearing stress and photo-elasticity causes the fiber to be circu-larly birefringent and the mechanically induced grating formed over the twisted fiberregion causes the appearance of two resonances shifted away from the resonances ofthe untwisted grating section. At higher twist rates, resonant wavelength shift becomesinsensitive to applied pressures, showing a reduction in the induced linear birefrin-gence. The wavelength shift is almost symmetric with respect to the applied twist ratein clockwise and counterclockwise directions.

Keywords circular birefringence in twisted fibers, cladding mode resonance,long-period fiber grating, micro bends, photo-elasticity in fibers, shearing stress infibers

1. Introduction

Long-period fiber gratings (LPFGs) with low insertion loss, low back reflection, highelectromagnetic immunity, electrical isolation, and light weight are finding many applica-tions in fiber-optic communication systems and sensing. The wide variety of applicationsmakes it one of the most stable and versatile technologies in the optical field. The long-period grating has a period in the range of 100 µm to 1 mm. An LPFG promotes thecoupling between the propagating core mode LP01 and co-propagating cladding modes

Received 16 October 2006; accepted 19 January 2007.Address correspondence to V. P. Sudeep Kumar, Department of Optoelectronics, University of

Kerala, Kariyavattom, Thiruvananthapuram, 695581 Kerala, India. E-mail: sdetrgrttc@sancharnet.in

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160 B. U. Nair et al.

resulting in the transmission spectra of the fiber with a series of attenuation notches atdiscrete wavelengths that satisfy the phase matching condition:

λnres = (neff core − neff clad(n))� (1)

neff core and neff clad(n) are the effective refractive indices of the core mode and nthcladding mode at the resonant wavelength λn and the grating period � [1]. Long-periodfiber gratings can be induced mechanically since they have periods of hundreds of mi-crons [2]. The mechanically induced LPFGs are low cost, reproducible, tunable, erasable,and strong since the external stress is applied over the protective coatings.

The resonance wavelength of mechanically induced long-period fiber grating(MLPFG) can be tuned by varying the pressure over the grating, changing the grat-ing period [2], varying the temperature [3], quasi-linear chirping and phase shifting, stepchanging in MLPFG [4], or applying torsion in corrugated grating [5]. In this investiga-tion, we present a mechanically induced long-period grating whose resonant wavelengthis shifted over 20 nm by forming a mechanically induced grating over a twisted fiber.The amplitude as well as the resonant wavelength variation of the resonant dip of theinduced grating is studied in response to the applied twist.

2. Theory

The fiber, having a perfectly circular core and cladding with a rotationally symmetricindex profile, will be non-birefringent if free from mechanical and electrical disturbances.The presence of external disturbances such as stress, bends, and twists in the fiber leadsto linear, elliptical, and circular birefringence as well as the rotation of the fast axis.Thus, a non-birefringent fiber can become birefringent. A non-birefringent fiber supportstwo nearly degenerate orthogonally polarized modes and has a very small differencein propagation constant. This degeneracy of the modes are lifted by the geometricalanisotropy of core, residual stress, twist, or local bending [6]. When subjected to anelastic deformation, birefringence can be introduced in the fiber and modify the phasematching condition given by Eq. (1). Therefore, the shift in resonant wavelength λn

reswith respect to twist rate φ can be obtained as:

dλnres

dφ= �

(dneff core

dφ− dneff clad(n)

dφ

)(2)

The effective index change resulting from the photo-elasticity of a single-mode fiberis sensitive to the shearing stress applied to it. The shearing stress under torsion isproportional to the radius of the fiber as well as to the applied twist rate. For a twistedsingle-mode fiber, the shearing stress will be much larger in its cladding compared tothe core. The twist induced changes in effective core indices and its contribution to thewavelength shift of cladding mode resonances of an in-fiber grating is very small evenin photo-induced long-period gratings [5]. Therefore, the effective index change in thecore is much less than that in the cladding. Neglecting effective index change in the corecompared to that in the cladding [7], Eq. (2) can be approximated as:

dλnres

dφ= −�

(dneff clad(n)

dφ

)(3)

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Mechanically Induced LPFG over Twisted Fiber 161

Then the resonant wavelength shift becomes

dλnres = −�(dneff clad(n)) (4)

When a fiber of length L is twisted by an angle φ, a shearing strain is induced in thefiber. The shearing strain and photo-elastic effect cause the fiber to become circularlybirefringent [8, 9]. Since there is no obvious inherent linear birefringence within thefiber, the induced birefringence on twisting will be of circular type [9, 10]. The inducedcircular birefringence per unit length, β, is expressed as:

β = n2co(P11 − P12)

φ

2L(5)

where P11 and P12 are the photo-elastic constants and nco is the refractive index of thecore. By substituting the values of (P11–P12) as 0.15 and nco as 1.46 for silica in Eq. (5),β = −0.16 φ/L rad/m. For small and moderate twists, the birefringence effect will bemainly in the cladding and therefore the effective index modification, dn, with respectto the twist rates can be written as:

dn ≈ dneff clad(n) ≈(

0.16φ

L

)λn

res

2π(6)

Thus, induced circular birefringence modifies the effective indices of the cladding modeseffectively and causes the shift in resonant wavelengths.

3. Experimental

The side view of the mechanical setup is as shown in Figure 1. A grooved plate, 5 cmlong and 0.5 cm wide, in which grooves of 200 µm were carved, was used. The gratingperiod of the grooved plate was 0.7 mm, and it was machined with an accuracy of±5 µm. The standard single-mode fiber, having 9-µm core and 125-µm cladding, withthe protective coating, was positioned above the grooved plate. A flat deformer plate was

Figure 1. Mechanical setup with deformer plates for creating microbend and twist for a singlegrating.

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162 B. U. Nair et al.

fitted on a frictionless vertical beam as shown in Figure 1. A rack-and-pinion arrangementwas provided in the vertical arm so that the rotation of a screw would move the deformerplate up or down with a displacement accuracy of 0.01 mm. This arrangement wouldapply uniform pressure over the whole of the specified length of the fiber. A knownlength of fiber was fixed between two optical stages, one of which had a rotator attachedto it. An unpolarized optical broadband source was used for the measurements. A fiberpatch cord was cut equally into two and a fiber with coating was fusion spliced betweenthe two pieces. The splices were covered and fused in a thermo shrink tube to ensuremechanical strength to the splices and this spliced portion was clamped in the opticalstage mounts. This arrangement ensured no slip to the fiber while twist was applied.Also, the applied twist was distributed along the whole length of the exposed fiber. Amarker was provided in this plastic tube to note the angle of twist applied to the fiber.The optical spectra of the MLPFGs were recorded using an optical spectrum analyzer(OSA) (Anritsu-MS9710C, range 600 nm–1,750 nm).

To investigate the twist-induced effects on cladding mode resonance of a MLPFG, thefiber was twisted in steps using a rotator. The system was designed in such a way that anytwist rate (turns/cm) could be applied by lifting the deformer plate up, twisting the fiber,bringing the plate down, and suitably applying a constant load over the deformer plate.Mechanically induced long-period fiber grating was thus formed for different twists. Thisprocess was repeated and all the spectra were recorded by applying a constant pressure.The connector to the OSA was re-inserted at regular intervals to avoid twisting of theremaining section of the fiber adjacent to the OSA. The effect of twisting the fiber inclockwise and counterclockwise directions was also investigated. Studies were carriedout in the same set up (grating period 0.7 mm) with different fibers, twist rates, andapplied pressures.

To investigate further the effect caused by the twisting of the fiber, a new gratingG2 of same grating period as the first one (G1) was introduced in between G1 andthe broadband source as shown in Figure 2. A constant pressure was applied in bothgratings while recording the spectra. The twist was applied in steps only to the firstgrating portion G1. The fiber was kept without any twist in the newly introduced gratingsection (G2).

Figure 2. Mechanical setup with deformer plates for creating microbend and twist for the gratingassembly.

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Mechanically Induced LPFG over Twisted Fiber 163

4. Results and Discussion

4.1. Effect of Twist in One MLPFG Section

The spectral response of the long-period grating (0.7 mm) under different twist rates andat constant normal pressure is shown in Figure 3. When the applied grating pressure iszero, there will be no cladding mode resonance. When a constant pressure of 11 N/cm isapplied, two attenuation notches at λ1 (1,570 nm) and λ2 (1,630 nm), corresponding to thecoupling of the core mode with two cladding modes, are observed. The amplitudes of thenotches decrease with the increase of the applied twist. As the applied twist increases, thewavelengths of both resonances shift to shorter wavelength side. The amplitude of the firstresonance at 1,570 nm decreases more rapidly with the twist rate compared to the secondone at 1,630 nm. When the twist rate reaches a value of 0.50 turns/cm and by keepingthe pressure a constant, the peak width of the notches starts to increase as observedin photo-induced LPFGs [5, 11]. It is found that the first resonant notch disappearswhen the twist rate becomes 0.56 turns/cm. If the pressure is increased again, the firstresonance reappears with smaller amplitude, shown in Figure 3. This shows that anincrease of grating pressure at this twist rate results in stronger mode coupling and deepernotches [2]. But no reappearance is observed at twist rates higher than 0.56 turns/cm,even when a still higher pressure is applied. The notch at the shorter wavelength is sharperthan that at the longer wavelength. This is mainly because of the slope of the gratingdispersion curve near the shorter wavelength, which is steeper [12]. Based on circularbirefringence theory, explained earlier, the induced birefringence is calculated. Withinthe limits of experimental errors, the values of induced birefringence are in agreementwith the theoretically evaluated values up to 0.38 turns/cm. For twist rates greater than0.38 turns/cm, the experimentally observed birefringence values are higher than that ofthe theoretically evaluated values.

4.2. Effect of Twist in Two MLPFG Sections

The spectral responses of the MLPFG sections with the simultaneous application ofpressure in both G1 and G2 and with different twist rates in G1 are shown in Figure 4. Theshift in the resonant wavelength with the applied twist is toward the shorter wavelengthside. Comparison of the spectral responses is given in Figure 5.

When the applied pressure is zero, there will be no cladding mode resonance. Whenthe pressure is applied either in G2 or in G1, under zero twists there are two claddingmode resonances at λ1 = 1,575 nm and λ2 = 1,634 nm, respectively. Simultaneousapplication of the same values of pressure on both the gratings (under zero twist), thewavelength corresponding to the cladding mode resonances λ1 and λ2 remain at the pre-viously observed values. However, a 3-dB increase in amplitude is observed for both theresonances with two gratings in series under zero twist when compared to that obtainedwith either G1 or G2. The resonant wavelengths λ1 and λ2 are almost same (±1 nm) forindividual gratings G1, G2, and with G1 and G2 in series. It is also observed that theamplitude of resonance is higher for the second cladding mode λ2 compared to that ofthe first mode. This is because the value of overlap integral with the core mode is largefor this mode [5].

When a twist rate of 0.06 turns/cm is applied to G1 grating section, keeping thesame pressure valves on both gratings, two new resonant dips at λ′

1 and λ′2, appear in

addition to λ1 and λ2. On releasing the pressure at G1 (twisted region), resonant dips at

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164 B. U. Nair et al.

Figure 3. The transmission spectra of a single grating at constant pressure 11 N/cm and withdifferent twist rates.

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Mechanically Induced LPFG over Twisted Fiber 165

Table 1Comparison of theoretical and experimental results

Induced circular birefringence (10−5)

Resonances dn (λ1) dn (λ2)

Turns/cm λ′1 λ1 λ′

2 λ2 Experiment Theory Experiment Theory

0.00 1,574.7 1,574.7 1,632.3 1,632.3 — — — —0.06 1,573.8 1,573.8 1,633.2 1,633.2 — 0.157 — 0.1630.13 1,572.6 1,575.9 1,632.3 1,634 0.471 0.315 0.243 0.3270.19 1,572.9 1,575.3 1,631.5 1,634.9 0.343 0.473 0.486 0.490.25 1,572.3 1,575.3 1,630.5 1,634.7 0.429 0.63 0.6 0.6540.31 1,570.5 1,574.4 1,628 1,633.2 0.557 0.787 0.743 0.8170.38 1,567.8 1,574.1 1,623.6 1,632.6 0.9 0.944 1.286 0.980.44 1,566.8 1,574.1 1,622.5 1,633.5 1.043 1.102 1.571 1.1430.50 1,565.4 1,575.3 1,620.9 1,632.5 1.414 1.26 1.657 1.3060.56 1,558.5 1,574 1,615 1,633.5 2.214 1.417 2.643 1.470.62 1,554.3 1,574 1,611.5 1,633 2.814 1.574 3.071 1.633

λ′1 and λ′

2 disappear. Similarly, the release of pressure at G2 section will result in dis-appearance of resonances at λ1 and λ2. Therefore, we conclude that the appearance ofboth λ1 and λ2 are due to the induced grating G2 in the untwisted fiber region and λ′

1and λ′

2 are due to grating G1 at the twisted region. Shearing stress and photo-elasticitycause the fiber to be circularly birefringent in the twisted region G1. This causes thechanges in neff core and neff clad(n). The changes in cladding mode indices are more thanin core mode indices [7]. The observed shift in resonant wavelength can be attributed tothe induced circular birefringence in the cladding of the fiber due to the twist introduced.

The notches at λ′1 and λ′

2 shift to the shorter wavelength side with the increase ofapplied twist, while those at λ1 and λ2 remain at the same values. When the fiber twistrate is increased at G1, λ1 = (λ1 − λ′

1) and λ2 = (λ2 − λ′2) are increased. The shift

of resonant wavelengths λ1 and λ2 with respect to the twist rates is shown in Figures 6and 7. No remarkable shift in wavelength is observed with twist for the resonant dipsproduced by the grating section G2, showing that the twist-induced birefringence is notmodifying the effective indices of untwisted fiber section. It is also to be noted thatthe introduction of a twist before or after the grating fiber sections will not result innoticeable shift in resonances. The shift in resonances is observed only when the gratingis formed in the twisted fiber section. The resonant amplitude also decreases linearly withthe applied twist. The coupling between the core mode and the cladding modes decreasesresulting in the decrease of resonant peak amplitudes with the applied twist. Increase inwidth of resonances with twist rate is similar to that observed in photo induced LPFGs[5, 11].

Since there is no induced birefringence in the untwisted G2 section, λ1, λ2, and(λ1 − λ2) remain almost at constant values. Also, the difference in resonant wavelengthsof twist-induced grating (λ′

1−λ′2) remains almost at a constant value of ≈57 nm, showing

that the twist-induced birefringence is affecting the two cladding modes uniformly.The shifts of resonant wavelengths λ1 and λ2 with pressure for different twist rates

are shown in Figures 8 and 9. The twisting process makes the distribution of refractive

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Figure 4. The spectral response of the two MLPFG sections at a pressure of 11 N/cm and fordifferent twist rates at G1.

index in the cladding more uniform along the axis of the fiber compared to the twist-freegrating section. In other words, the twisting causes the linear birefringence in the fiberto decrease significantly. If a linear birefringent fiber is highly twisted, the internal bire-fringence can be eliminated in the same way as in a spun fiber, and when the twist issufficiently large there will only be circular birefringence [10]. Therefore, the wavelengthshift is almost insensitive to applied pressures for twist rates grater than 0.62 turns/cm.

The resonance at λ′1 reduces its amplitude and vanishes at 0.62 turns/cm. It reappears

with smaller amplitude when a higher pressure is applied. The increase of pressure makesthe mode coupling stronger and results in deeper notches [2]. But no reappearance is ob-served if the twist rate is more than 0.62 turns/cm even when a still higher pressure is

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Mechanically Induced LPFG over Twisted Fiber 167

Figure 4. (Continued).

applied. The coupled-mode theory may be inefficient to explain the above effect becauseat very high twists the change in index profiles cannot be considered small compared tothe core-cladding index difference. A rigorous solution may be required taking into ac-count the inhomogeneous anisotropic refractive index changes in the fiber under torsionalload combined with microbending [13].

When the experiment is repeated with different SM fibers (G652), supplied by dif-ferent manufacturers, a variation in wavelength of around 10 nm is observed at constantpressure for a twist-free fiber. It is difficult to position the fiber with zero twist at thebeginning of the experiment, as there can be a residual twist in the fiber. The wave-length shift can be due to this residual twist in the fiber. This shows that the actual valueof minimum birefringence does not occur at the applied zero twist and its magnitude

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Figure 5. Comparison of the spectral response of the two MLPFG sections at a pressure of 11 N/cmand with different twist rates at G1.

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Mechanically Induced LPFG over Twisted Fiber 169

Figure 6. The wavelength shift of resonance λ1 with the applied twist.

depends on the initial twist induced when the fibers are positioned over the groovedplate. But no distinguishable difference is seen for the transformation of the spectra withthe applied twist for different fibers. Lifting and repositioning of the fiber to introducea twist rate also results in wavelength shifts of ±1 nm. It is observed that the peakresonant wavelength shifts are almost symmetric with the applied twist in clockwise andcounterclockwise directions. We assume that the fiber twist is symmetric and the initialinherent linear birefringence is negligible. Hence, no elliptical birefringence results dueto twisting [8, 10]. If this is not the case, a significant dependence on wavelength shiftwith respect to the twist direction may have been observed. As the twist rate increases,there will be only induced circular birefringence and therefore the shift increases withthe applied twist.

Figure 7. The wavelength shift of resonance λ2 with the applied twist.

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Figure 8. The wavelength shift of λ1 with the applied pressure for different twist rates (P1 <

P2 . . . < P9).

The values evaluated using Eq. (6) are compared with the experimental values inTable 1 (shown on page 165). Within the limits of experimental errors, the values ofinduced birefringence are in agreement with the theoretically evaluated values up to0.44 turns/cm. For twist rates greater than 0.44 turns/cm, the experimentally observedbirefringence values are twice or even higher than that of the theoretically evaluatedvalues. Considerable deviation at higher twist rates may be because the core modes arealso becoming affected and the induced birefringence contributions of the core modeneed to be considered.

Figure 9. The wavelength shift of λ2 with the applied pressure for different twists rates (P1 <

P2 . . . < P9).

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Mechanically Induced LPFG over Twisted Fiber 171

5. Conclusion

The cladding mode resonances of a mechanically induced long-period grating formedover a twisted fiber are investigated for different twists and pressures. These resonancesdecrease in amplitude and shift to shorter wavelength side as the applied twist increases.An increase in amplitude is observed under constant pressure for the resonances withtwo gratings in series under zero twist when compared to the resonances obtained indi-vidually. Shearing stress and photo-elasticity cause the fiber to be circularly birefringentand the mechanically induced grating formed over the twisted fiber region causes theappearance of two resonances shifted away from the resonances of untwisted gratingsection. At higher twist rates, resonant wavelength shift becomes insensitive to appliedpressures, showing a reduction in the induced linear birefringence in the grating sec-tion. The wavelength shift is almost symmetric with respect to the applied twist rate inclockwise and counterclockwise directions.

References

1. Vengsarkar, A. M., Lemaire, P. J., Judkins, J. B., Bhatia, V., Erdogan, T., and Sipe, J. E.1996. Long-period fiber gratings as band-rejection filters. Journal of Lightwave Technology14:58–65.

2. Savin, S., Digonnet, M. J. F., Kino, G. S., and Shaw, H. J. 2000. Tunable mechanically inducedlong-period fiber gratings. Optics Letters 25:710–712.

3. Hwang, I. K., Yun, S. H., and Kim, B. Y. 1999. Long-period fibre grating based upon periodicmicrobends. Optics Letters 24(18):1263–1265.

4. Chen, K., Sheng, Q., Ge, C., Dong, X., Han, J., and Chen, S. 2003. Several mechanicallyinduced long-period gratings by a grooved plate. Chinese Optics Letters 1(8):444–446.

5. Ivanov, O. V., and Wang, L. A. 2003. Wavelength shifts of cladding-mode resonance in cor-rugated long-period fiber gratings under torsion. Applied Optics 42(13):2264–2272.

6. Kaminov, I. P. 1981. Polarisation in optical fibers. IEEE Journal of Quantum Electronics17:15–22.

7. Rao, Y. J., Wang, Y. P., Hu, A. Z., Ran, Z. L., and Zhu, T. 2003. Novel fiber-optic sensors basedlong period fiber gratings written by high-frequency CO2 laser pulses. Journal of LightwaveTechnology 21(5):1320–1327.

8. Ulrich, U., and Simon, A. 1979. Polarization optics of twisted single mode fibers. AppliedOptics 18(13):2241–2251.

9. Smith, A. M. 1980. Birefringence induced by bends and twists in single mode optical fibers.Applied Optics 19(15):2606–2611.

10. Payne, D. N., Barlov, A. J., and Ramskov Hansen, J. 1982. Development of low- and high-birefringence optical fibers. IEEE Journal of Quantum Electronics 18(4):471–482.

11. Lin, C. Y., Wang, L. A., and Chern, G. W. 2001. Corrugated long-period gratings as straintorsion and bending sensors. Journal of Lightwave Technology 19(8):1159–1168.

12. Cho, J. Y., Lim, J. H., and Lee, K. S. 2005. Optical fiber twist sensor with two ortho-gonally oriented mechanically induced long-period sections. IEEE Photonics Technology Let-ters 17(2):453–456.

13. Ivanov, O. V. 2004. Wavelength shift and split of cladding mode resonances in microbendlong-period fiber gratings under torsion. Optics Communications 232(1–6):159–166.

Biographies

B. Unnikrishnan Nair received his M.Sc. degree from the University of Kerala(India) in 1989 and M.Tech. degree in optoelectronics and laser technology from theCochin University of Science and Technology Kochi (India) in 1993. Currently, he is

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working as a senior lecturer in physics at University College, Thiruvananthapuram, India.His research interests include optical fiber components in communication and sensingapplications.

V. P. Sudeep Kumar received his M.Sc. degree and M.Phil. degree from the Uni-versity of Kerala (India) in 1995 and 1997, respectively. Currently, he is an engineer atBharat Sanchar Nigam Limited (BSNL), a telecom company of the government of India.His research interests include optical fiber grating and its applications.

V. P. Mahadevan Pillai received his M.Sc. degree in physics and Ph.D. from theUniversity of Kerala (India) in 1982 and 1996, respectively. He is currently a reader inthe Department of Optoelectronics, University of Kerala. His research interests includephotonic materials, laser spectroscopy, nonlinear optics, and fiber-optic sensors. He haspublished 46 papers in these areas.

V. U. Nayar received his M.Sc. degree in physics from Annamalai University andPh.D. in laser spectroscopy from the University of Kerala (India). He is currently workingas an honorary professor at Department of Optoelectronics, University of Kerala. Hisresearch interests are applications of lasers in engineering, holographic non-destructivetesting, WDM, and laser spectroscopy. He has published 140 works on laser spectroscopyholography and optical communication.

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