Research Article Tunable Optical Filter Based on ...downloads.hindawi.com/archive/2013/415059.pdf · Research Article Tunable Optical Filter Based on Mechanically Induced Cascaded

Hindawi Publishing CorporationJournal of PhotonicsVolume 2013 Article ID 415059 4 pageshttpdxdoiorg1011552013415059

Research ArticleTunable Optical Filter Based on Mechanically Induced CascadedLong Period Optical Fiber Grating

Sunita P Ugale and Vivekanand Mishra

Electronics Engineering Department S V National Institute of Technology Ichchhanath Surat Gujarat 395007 India

Correspondence should be addressed to Sunita P Ugale spueltxgmailcom

Received 28 March 2013 Revised 30 May 2013 Accepted 31 May 2013

Academic Editor Baldemar Ibarra-Escamilla

Copyright copy 2013 S P Ugale and V Mishra This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

We have proposed and demonstrated experimentally a novel and simple tunable optical filter based on mechanically induced andcascaded long period optical fiber gratings In this filter variable FWHM and center wavelength is provided by cascading longperiod and ultralong period optical fiber gratings with different periods in a novel fiber structure We report here for the first timeto our knowledge the characterization of mechanically induced long period fiber gratings with periods up to several millimeters innovel multimode-single-mode-multimode fiber structure We have obtained maximum loss peak of around 20 dB

1 Introduction

Long period fiber grating (LPFG) is the special case of fiberBragg grating (FBG) It was first suggested by Vengsarkar andcoworkers in 1996 [1] LPFG can be formed by introducingperiodic longitudinal perturbations of refractive index alongthe core of a single-mode fiber It can couple light betweenfundamental core mode and copropagating cladding modesat specific resonance wavelength The period of a typicalLPFG ranges from 100 120583m to 1000 120583mThe claddingmodes ofLPFG are absorbed by the polymer coating of the fiber hencethe transmission spectrum consists of a number of rejectionbands at the resonance wavelengths In contrast to the Bragggrating LPFG does not produce reflected light and can serveas spectrally selective absorberTherefore it is also called trans-mission grating LPFGs are relatively easy to produce Theirspectral properties that is resonancewavelength bandwidthand so forth can be varied in a wide range All thesefeatures make LPFG very attractive for many applications intelecommunication laser and sensor system

LPFG can be induced optically or mechanically [2ndash4]Optically induced gratings are permanent whereas mechan-ically induced gratings are reversible LPFGs formed by amechanically induced technique have generated great interestdue to their versatility in the process of fabrication In thesegratings the fiber is subject to periodical stress which results

in alternated regions under compression and stretching thatmodulate the refractive index via the photoelastic effectMechanically induced long period fiber gratings (MLPFGs)induced by pressure need neither a special fiber nor an expen-sive writing device for fabrication These gratings also offeradvantages of being simple inexpensive erasable and recon-figurable and also give flexible control of transmission spec-trum

2 LPFG Mathematical Model

Due to their highly multimode nature cladding modes arelocated closely and multiple cladding modes can satisfy thephase matching condition for a given index modulation ofperiod Λ

2120587119899coeff120582

minus2120587119899

cleff120582

=2120587

Λ (1)

where 119899coeff is the effective index of the core mode and 119899cleff isthe effective index of cladding mode

For a given periodicity Λ one can induce mode cou-pling between the fundamental mode and several differentcladding modes [5] a property that manifests itself as a set ofspiky losses at different wavelengths in the transmission spec-trum In design of optical filters concatenation of gratings

2 Journal of Photonics

is required and the relatively close-spaced resonance peaksof cladding modes can cause serious difficulties to generate adesired spectrum

The coupled mode equations describe their complexamplitude 119860co(119911) and 119860cl(119911)

119889119860co (119911)

119889119911= 119894120581condashco119860co (119911) + 119894

119904

2120581condashcl119860cl (119911) 119890

minus119894

2120575119911

119889119860cl (119911)

119889119911= 119894120581clndashcl119860cl (119911) 119890

minus119894

2120575119911

+ 119894119904

2120581clndashco119860co (119911)

(2)

where 119860co and 119860cl are the slowly varying amplitudes of thecore and cladding modes 120581condashco 120581clndashcl and 120581condashcl = 120581

lowast

clndashcoare the coupling coefficients 119904 is the grating modulationdepth 120575 = 120587(((119899

coeff minus 119899

cleff)120582) minus (1Λ)) and is the detuning

from the resonant wavelengthThe coupling is determined bythe transverse fields of the resonantmodes119864119868 and the averageindex of the grating Δ119899119868

120581119894119895

=1205961205760

119899

4intΔ119899 (119903) 119864

119894

(119903) 119864lowast

119895

(119903) 119889119903 (3)

According to coupled mode theory grating transmissionis a function of coupling coefficient 120581

119894119895

Assuming that the detuning from resonant wavelength is

balanced by the dc coupling simplified expression for gratingtransmission is given by

119879 (119911) = cos2 (120581119911) (4)Cross-coupling coefficient 120581 depends on the grating index

profile and field profiles of the resonant modesThe LPFGs with periods exceeding one millimeter are

called ultralong period fiber gratings (ULPFG) The longperiod size makes fabrication of ULPFG very easy as well ascheap In LPFG the core LP

01

mode is coupled with claddingmodes having the same symmetry namely LP

0119898

modes [6]Whereas in ULPFG the coupling of the fundamental guidedcore mode to the cladding modes of high diffraction orderstakes place [7] The phase matching condition for a highdiffraction order grating is given by the following equation

120582res = (119899coeff minus 119899

cl119898eff )

Λ

119873 (5)

where 120582res is the resonant wavelength and 119899coeff and 119899cl119898eff are

effective indexes of fundamental coremode and119898th claddingmode of 119873th diffraction order respectively Λ is the gratingperiod and119873 is the diffraction order119873 = 1 for LPFG

The resonant wavelength with the variation in the effec-tive indexes of the core and cladding ignoring the dispersioneffect is given by the following equation

1205821015840

res = (119899coeff minus 119899

cl119898eff )

Λ

119873

times [

[

1 +(120575119899

coeff minus 120575119899

cl119898eff ) times (119889120582res119889Λ)

(119899coeff minus 119899

cl119898eff )2

]

]

(6)

where 1205821015840res is the resonant wavelength with variation in theeffective indexes of core and cladding and 120575119899coeff and 120575119899

cl119898eff

are the effective index changes of the fundamental core modeand mth cladding mode of the119873th diffraction order

Broadband light source

Optical spectrum analyzer

Pressure

Glass plate

Fiber

Grooved plate

Figure 1 Experimental setup for inducing MLPFG and its charac-terization

LPFGULPFG

Splice 1Splice 2

625125MM9125 SM625125MM

Figure 2 Multimode-single-mode-multimodes (MSM) fiber struc-ture

3 Experiment and Results

The MLPFG with a period of 600 120583m and length = 70mmis induced in single-mode fiber in multimode-single-mode-multimode (MSM) fiber structure [8 9] Light is launchedfrom a broadband superluminescent LED source to the leadin MMF through the device (MLPFG) to the lead-out MMFand spectrally resolved using an optical spectrum analyzer(OSA) (Prolite60) The experimental setup as shown inFigure 1 was prepared

A schematic diagram of the MSM structure used inexperiment is shown in Figure 2 The sample is preparedby splicing a 15 cm long section of SMF (SMF-28) usinga Sumitomo Type39 Fusion Splicer in between two MMFs(625125) Single-sided loading is done the bottom plate isplain while the upper plate is a specially designed groovedplate on which pressure is applied

The MLPFGs induced in single-mode fiber and multi-mode fibers are also studied separately It is observed thatsingle-mode grating produced resonant loss peaks of up tosim7 dB and multimode grating produced resonant loss peaksof up tosim5 dB Inmultimode fiberwith a large number of coremodes and cladding modes the core-cladding power cou-pling occurs at many wavelengths listed in Table 1 So manyclosely spaced loss peaks limit the tunable range as thereare chances of overlap among different loss peaks While insingle-mode fiber there exists only one coremode (LP

01

) andmany cladding modes (LP

1119898

) and the core-cladding cou-pling occurs at certain specific wavelengths listed in Table 2

In MSM fiber structure while coupling from multimodefiber (core diameter 625120583m cladding diameter 125120583m) to

Journal of Photonics 3

minus18

minus16

minus14

minus12

minus10

minus8

minus6

minus4

minus2

0

2

1291

61

1332

1

1371

44

1409

62

1446

66

1482

54

1517

26

1550

83

1583

25

1614

52

120582 (nm)

Figure 3 Normalized spectral response of MLPFG in MSM fiberstructure

Table 1 Resonance wavelengths for reversible LPFG in multimodefiber

Grating period Resonance wavelengths (nm)600 120583m 1491 1505 1525 1545 1572 1597 1626

Table 2 Resonancewavelengths for reversible LPFG in single-modefiber

Grating period Resonance wavelengths (nm)600 120583m 1466 1510 1595

single-mode fiber (core diameter 9 120583m cladding diameter125 120583m) out of many core modes in multimode fiber onlyone coremode gets coupled with single-mode fiber In single-mode fiber LPFG couples part of core mode power into aforward propagating cladding mode In the useful commu-nication wavelength band the dominating coupling is takingplace between core mode and the cladding mode at centerresonant wavelength Slight coupling is there with othercladding modes responsible for other loss peaks Due tosecond interface between SMF and MMF further filteringand phase shift action are taking place that will eliminate theslightly coupled wavelengths hence only one loss peakappears (almost at the center wavelength) in transmissionspectra

The transmission spectrum of MLPFG inMSM structureis plotted in Figure 3 and the input power spectrum is alsoshown for comparison purpose The peak loss of around 17-18 dB is obtained which is much greater than MLPFG insingle-mode and multimode fiber

The response of grating is very impressive and has a lot ofapplications in telecommunication as well as sensing In thefurther section the application of this grating is discussed

Optical filters play important role in telecommunicationsystems where a certain range of wavelengths are requiredto be present or absent in the optical signal Due to theirlow loss compact size high frequency response and ease ofinsertion in existing optical fiber communication channels

Splice 1Splice 2

LPFG ULPFG

625125MM 9125 SM 625125MM

Λ = 600120583m Λ = 2400120583mD

Figure 4 Cascading of MLPFG-MULPFG in MSM structure

1450 1490 1530 1570 1610 1650Wavelength (nm)

minus48

minus50

minus52

minus54

minus56

minus58

minus60

minus62

minus64

Pow

er (d

Bm)

LPFG 600120583mCascaded str600ndash1200 120583mCascaded str600ndash2400120583m

Figure 5 Transmission spectrums of a band-rejection filter withdifferent gratings

optical fiber filters are preferred over their electronic andintegrated optical counterparts

The MLPFG in MSM structure with Λ = 600 120583m andlength = 70mm can be used as a band reject filter FromFigure 3 the center wavelength is 1550 nm and the FWHMis 20 nm The response of grating is well within the opticalspectrum used for WDM and DWCD applications (1225ndash1565 nm) and thus the filter will be very useful in commu-nication applications The transmission spectrum in Figure 3can be broadened by a novel structure of cascaded MLPFGand MULPFG shown in Figure 4

The transmission spectrum of single MLPFG and cas-cadedMLPFG-MULPFG is combined and shown in Figure 5By changing the period of gratings and the paring betweengratings of different periods the center wavelength andFWHM of filter can be changed

4 Conclusion

We report here for the first time to our knowledge the char-acterization of mechanically induced LPFGs in MSM fiberstructure with periods up to several millimeters MLPFG inMSM structure gives single transmission dip Resonant losspeak strength is around 17-18 dB which is much greater than


maximum loss of 8 dB in single-mode MLPFG and 5 dB inmultimodeMLPFGTheMLPFG inMSM structure withΛ =600 120583m and length = 70mm can be used as a band rejectfilter and its center wavelength is 1550 nm and the FWHMis 20 nm The transmission spectrum obtained using singleMLPFG can be broadened by a novel structure of cascadedMLPFG and MULPFG By changing the period of gratingsand the paring between gratings of different periods thecenter wavelength and FWHM of filter can be changed

References

[1] A M Vengsarkar P J Lemaire J B Judkins V Bhatia TErdogan and J E Sipe ldquoLong-period fiber gratings as band-rejection filtersrdquo Journal of Lightwave Technology vol 14 no 1pp 58ndash65 1996

[2] X Shu L Zhang and I Bennion ldquoFabrication and characteri-sation of ultra-long-period fibre gratingsrdquo Optics Communica-tions vol 203 no 3ndash6 pp 277ndash281 2002

[3] T Zhu Y Song Y Rao and Y Zhu ldquoHighly sensitive opticalrefractometer based on edge-written ultra-long period fibergrating formed by periodic groovesrdquo IEEE Sensors Journal vol9 no 6 2009

[4] S SavinMDigonnet G Kino andH Shaw ldquoTunablemechan-ically induced long-period fiber gratingsrdquoOptics Letters vol 25no 10 pp 710ndash712 2000

[5] T Erdogan ldquoFiber grating spectrardquo Journal of Lightwave Tech-nology vol 15 no 8 pp 1277ndash1294 1997

[6] T Zhu Y J Rao and Q Mo ldquoSimultaneous measurementof refractive index and temperature using a single ultralong-period fiber gratingrdquo IEEE Photonics Technology Letters vol 17no 12 pp 2700ndash2702 2005

[7] Y Zhao and J C Palais ldquoFiber bragg grating coherencespectrummodeling simulation and characteristicsrdquo Journal ofLightwave Technology vol 16 no 4 pp 554ndash561 1998

[8] J-J Zhu A P Zhang T-H Xia S He and W Xue ldquoFiber-optic high temperature sensor based on thin-core fiber modalinterferometerrdquo IEEE Sensors Journal vol 10 no 9 pp 1415ndash1418 2010

[9] L V Nguyen D Hwang S Moon D S Moon and Y ChungldquoHigh temperature fiber sensor with high sensitivity based oncore diameter mismatchrdquo Optics Express vol 16 no 15 pp11369ndash11375 2008

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is required and the relatively close-spaced resonance peaksof cladding modes can cause serious difficulties to generate adesired spectrum

The coupled mode equations describe their complexamplitude 119860co(119911) and 119860cl(119911)

119889119860co (119911)

119889119911= 119894120581condashco119860co (119911) + 119894

119904

2120581condashcl119860cl (119911) 119890

minus119894

2120575119911

119889119860cl (119911)

119889119911= 119894120581clndashcl119860cl (119911) 119890

minus119894

2120575119911

+ 119894119904

2120581clndashco119860co (119911)

(2)

where 119860co and 119860cl are the slowly varying amplitudes of thecore and cladding modes 120581condashco 120581clndashcl and 120581condashcl = 120581

lowast

clndashcoare the coupling coefficients 119904 is the grating modulationdepth 120575 = 120587(((119899

coeff minus 119899

cleff)120582) minus (1Λ)) and is the detuning

from the resonant wavelengthThe coupling is determined bythe transverse fields of the resonantmodes119864119868 and the averageindex of the grating Δ119899119868

120581119894119895

=1205961205760

119899

4intΔ119899 (119903) 119864

119894

(119903) 119864lowast

119895

(119903) 119889119903 (3)

According to coupled mode theory grating transmissionis a function of coupling coefficient 120581

119894119895

Assuming that the detuning from resonant wavelength is

balanced by the dc coupling simplified expression for gratingtransmission is given by

119879 (119911) = cos2 (120581119911) (4)Cross-coupling coefficient 120581 depends on the grating index

profile and field profiles of the resonant modesThe LPFGs with periods exceeding one millimeter are

called ultralong period fiber gratings (ULPFG) The longperiod size makes fabrication of ULPFG very easy as well ascheap In LPFG the core LP

01

mode is coupled with claddingmodes having the same symmetry namely LP

0119898

modes [6]Whereas in ULPFG the coupling of the fundamental guidedcore mode to the cladding modes of high diffraction orderstakes place [7] The phase matching condition for a highdiffraction order grating is given by the following equation

120582res = (119899coeff minus 119899

cl119898eff )

Λ

119873 (5)

where 120582res is the resonant wavelength and 119899coeff and 119899cl119898eff are

effective indexes of fundamental coremode and119898th claddingmode of 119873th diffraction order respectively Λ is the gratingperiod and119873 is the diffraction order119873 = 1 for LPFG

The resonant wavelength with the variation in the effec-tive indexes of the core and cladding ignoring the dispersioneffect is given by the following equation

1205821015840

res = (119899coeff minus 119899

cl119898eff )

Λ

119873

times [

[

1 +(120575119899

coeff minus 120575119899

cl119898eff ) times (119889120582res119889Λ)

(119899coeff minus 119899

cl119898eff )2

]

]

(6)

where 1205821015840res is the resonant wavelength with variation in theeffective indexes of core and cladding and 120575119899coeff and 120575119899

cl119898eff

are the effective index changes of the fundamental core modeand mth cladding mode of the119873th diffraction order

Broadband light source

Optical spectrum analyzer

Pressure

Glass plate

Fiber

Grooved plate

Figure 1 Experimental setup for inducing MLPFG and its charac-terization

LPFGULPFG

Splice 1Splice 2

625125MM9125 SM625125MM

Figure 2 Multimode-single-mode-multimodes (MSM) fiber struc-ture

3 Experiment and Results

The MLPFG with a period of 600 120583m and length = 70mmis induced in single-mode fiber in multimode-single-mode-multimode (MSM) fiber structure [8 9] Light is launchedfrom a broadband superluminescent LED source to the leadin MMF through the device (MLPFG) to the lead-out MMFand spectrally resolved using an optical spectrum analyzer(OSA) (Prolite60) The experimental setup as shown inFigure 1 was prepared

A schematic diagram of the MSM structure used inexperiment is shown in Figure 2 The sample is preparedby splicing a 15 cm long section of SMF (SMF-28) usinga Sumitomo Type39 Fusion Splicer in between two MMFs(625125) Single-sided loading is done the bottom plate isplain while the upper plate is a specially designed groovedplate on which pressure is applied

The MLPFGs induced in single-mode fiber and multi-mode fibers are also studied separately It is observed thatsingle-mode grating produced resonant loss peaks of up tosim7 dB and multimode grating produced resonant loss peaksof up tosim5 dB Inmultimode fiberwith a large number of coremodes and cladding modes the core-cladding power cou-pling occurs at many wavelengths listed in Table 1 So manyclosely spaced loss peaks limit the tunable range as thereare chances of overlap among different loss peaks While insingle-mode fiber there exists only one coremode (LP

01

) andmany cladding modes (LP

1119898

) and the core-cladding cou-pling occurs at certain specific wavelengths listed in Table 2

In MSM fiber structure while coupling from multimodefiber (core diameter 625120583m cladding diameter 125120583m) to


minus18

minus16

minus14

minus12

minus10

minus8

minus6

minus4

minus2

0

2

1291

61

1332

1

1371

44

1409

62

1446

66

1482

54

1517

26

1550

83

1583

25

1614

52

120582 (nm)










Splice 1Splice 2

LPFG ULPFG

625125MM 9125 SM 625125MM

Λ = 600120583m Λ = 2400120583mD


1450 1490 1530 1570 1610 1650Wavelength (nm)

minus48

minus50

minus52

minus54

minus56

minus58

minus60

minus62

minus64

Pow

er (d

Bm)






4 Conclusion




References















FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics




minus18

minus16

minus14

minus12

minus10

minus8

minus6

minus4

minus2

0

2

1291

61

1332

1

1371

44

1409

62

1446

66

1482

54

1517

26

1550

83

1583

25

1614

52

120582 (nm)










Splice 1Splice 2

LPFG ULPFG

625125MM 9125 SM 625125MM

Λ = 600120583m Λ = 2400120583mD


1450 1490 1530 1570 1610 1650Wavelength (nm)

minus48

minus50

minus52

minus54

minus56

minus58

minus60

minus62

minus64

Pow

er (d

Bm)






4 Conclusion




References















FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics





References















FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics








FluidsJournal of


Journal of










Superconductivity




GravityJournal of








Journal of






Volume 2014


PhotonicsJournal of


Journal of

Biophysics



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Research Article Tunable Optical Filter Based on ...downloads.hindawi.com/archive/2013/415059.pdf · Research Article Tunable Optical Filter Based on Mechanically Induced Cascaded