FBG polariser.pdf

7/30/2019 FBG polariser.pdf

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Fiber Bragg Grating PolarizerP. Torres,L. C. G. Valente , L. C. B. Linares, and J. P. v o n der Weid

Abstract - n optical fiber polarizer is constructed with afiber Bragg grating (FBG) based on birefringence effectsinduced by a transverse force applied to a section of thegrating. In this work we present the principle of operation andexperimental results of this device. We show how this fiberpolarizer can he used in transmission mode if the affectedregion of the grating is about one percent of the grating length.When the affected region of the grating is equal to or greaterthan 10 percent, the grating is a good candidate as polarizer i nreflection mode, in a configuration completely different fromthe first one. Both configurations can he spectrally tunedaccording to the applied force andlor pressed region along thegrating.

I ndex Te rms - ratings, local pressure, optical fiberdevices, phase shifts, polarization.1. INTRODUCTION

In many applications, it is important to obtain light in apure state of polarization while, at the same time, keepingthe light guided inside the core of an optical fiber. In oneimportant application, for example,-there is a need to adjustthe polarization of light prior to sending the light to anexternal electro-optical modulator.One known approach to providing linearly polarized lightin an optical fiber is to utilize a special polarizing fiber. Insuch fiber, one state of polarization experiences a very highloss while the other state suffers relatively low losses.According to the manufacturer, a straight, several meterslong, special fiber is capable of obtaining a 20 dB extinctionratio, while coiling the same length in a, few centimeterdiameter, rod provides an extinction ratio of over 40 dB .

Other approaches for providing an in-line polarizeremploy technology utilized to fabricate polished couplers orD-shaped fibers. In these alternative approaches, the flatpart of the fiber is coated with different layers, whichinclude a buffer layer and a metallic absorbing layer.

Manuscript received on March 25, 20 03. This work wa s suppalted inpan by DIME (National University o f Colombia) and ANP (BrazilianPetroleum Agency ).P. Torres is with the Physics School, Univenidad Nacional de ColombiaSede Medellin. A. A. 3840 Medellin. Colombia ([email protected]).L. C. G . Valente is with th e Mechanical Engineering Department,Pontificia Universidade Catdlica do Rio de Janeiro, 22452-970 Ria deJaneiro, Brazil (e-mail: [email protected]).L. C. B. Linares and J . P. Vo n der Weid are with the Center farTelecommunications Studies. Pontificia Univcrsidade Catdlica do Ria deJaneiro, 2245 3-900 Rio de Janeiro, Brazil.

Each of the above-described approaches is not fullysatisfactory. For example, the approach utilizing specialpolarizing fiber implies the use of several meters of a fiberthat is rather expensive. Furthermore, a problem that iscommon to approaches based on the use of special fibersis the coupling efficiency of light from a standard telecommfiber to the polarizing fiber. The approaches that utilizetechnologies developed to fabricate polished fiber couplersor D-shaped fibers suffer from the problem that very well-controlled film deposition and cladding thickness arerequired.Fiber Bragg gratings (FBG) have been under considerableinvestigations recently. Some of the latest applications ofFBG include dispersion compensators [ I , 21 multiplexer/demultiplexer elements for wave division multiplexingsystems (WDM) [3-4], sensors [6], high reflectors for lasercavities [7], and all fiber polarimeter [SI. In this paper theprinciple of operation and experimental demonstration of afiber polarizer based on birefringence effects induced by atransverse force applied to a small section of an FBG ispresented. An optical fiber that incorporates a grating topolarize the propagating light has been presented in 191.which present a single polarization fiberiamplifier thatincludes a non polarization-preserving fiber with a gratingtap incorporated therein. The grating tap is described asbeing oriented at a special angle, with a specific gratingperiod, strength and length. This structure taps out onepolarization while transmitting the second polarization as anoutput light from the optical fiber. Although provideslinearly polarized light. guided in a normal optical fiber,rather than special fiber as described above, polarization isattained by Brewster angle reflection. As a result, there is adisadvantage in that light in th e rejected polarization state iscoupled out of the fiber and cannot be used.

11. FB G POLARIZER PRINCIPLEBased on our previous research work [ IO , I I], it is knownto he possible to have different reflection spectra for twoorthogonal polarization states of light if a FBG is subjectedto a lateral force. When an optical fiber is laterally pressed,refraction index is altered, changing the Bragg reflectionwavelength of a FBG.The Bragg reflection wavelength changes AX a at anypoint of the disturbed FBG caused by transverse appliedloads, under the plane strain approximation, are given by

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0-7803-7824-5/03/S 17.00 02 00 3 IEEE 983 Proceedings SBMO/IEEE MTT-S IMOC 2003


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0.75 I I

1539.9 1540.2 1540.5 1540.8 1541.1Wavelength (nm)

Fig. 3. (a ) Original transmission spectmm of the used 4-cm ong chirpedFBG. (b) Polarization factor when a region !=0.2 m m s pressed with forceof 335 gf. In (h), the results corresponding to grating hand gap.higher level with a highly photosensitive fiber in order toachieve high coupling in the grating. By scanning thepressure element along the grating with the same strength, itis possible to operate the device through all the grating bandgap. To demonstrate the FBG polarizer in the secondcondition, Le., when the perturbed section is 210% of thegrating length, an unchirped FBG was pressed by using aprobe with piezoelectric activation. In this case, thepolarizing beam splitter cube was replaced by a polarizationcontroller and the ASE emission o f an EDFA was used asthe broadband optical source. Fig. 4 shows the originalspectrum of the grating used and the polarization factorwhen the entire grating length is under an estimated stressof 40 kgficm. Two loss peaks are generated in the spectralresponse, corresponding to the fast and slow axes of theentire grating, with a polarization splitting of -700 pm anda bandwidth of -700 pm each one. The correspondingpolarization factor F indicates a transmission (reflectivity)near to 100% for x polarization and 75% for y polarization.With these results, it can be said that the grating is a goodcandidate as a reflection polarizer long wavelength peak. Asthe position of the secondaly loss peak is a function of theapplied force [ I I],he spectral tuning will be accomplishedby varying the applied force.

IV. CONCLUSIONWe have shown the principle of operation andexperimental results of a FBG polarizer based onbirefringence effects induced by a transverse force applied

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y. -0.3:ihfi-0.6-0.91554 1555 1556 1557 1558Wavelength (nm)Fig. 4. (a) Original transmission spectmm of the used unchirpcd FBG. (b)Polarization Factor when the entire g a t i n g length is under a s t m s of 40kgficm.to a section of the FBG. If the affected region of the gratingis small, on the order of 1% of the grating length, anincrease in the transmission inside the grating band gap canbe obtained for one polarization. When the affected regionof the grating is >IO%, in this case loo%, the grating is agood candidate as a polarizer at the wavelength of thesecond peak. This work also shows that moderate forcescan be used to form this grating device in reflection ortransmission mode.

REFERENCES

[ I ] W. H . Loh, R. 1 Laming,A. D. Ellis, and D. Atkinson, Dispersioncompensated 10 Gbitsir transmission over 700 km of standardsingle mode fiber with 10 cm chirped fiber grating and duobinarytransmitter. in Proc.OAA96, 1996, paper PD30-2.R. Kanhyap, A . Ellis, D.Malyon, H.-G. Frochlich, A. Swanton, andD. 1.A m e s , Eight wavelength x 10Gbis simultaneous dispzrsioncompensation over 100 km single-mode fiber using a single 10nanometer bandwidth, 1. 3 meter long, super-step-chirped fiberBragg grating with a continour delay of 13.5 nanoseconds, in Pruc.ECOC96, 1996, paper ThB.3.2.G. P. Agrawal and S . Radic, Phase-shifted fiber B r a g gratings andtheir application for wavelenglh demultiplexing, IEEE Pho lon .Techno!, L e l l . , vol. 6, pp. 995-997, 1994.B. J. Eggleton, P. A. Kmg, L. Poladian, and F. Oullune, L o n gperiodic S U ~ ~ ~ ~ P J C N Kragg gratings in optical fibres, E!ectrorr.Let.[, vol. 30,pp. 1620-1622, 1994.F. Bilodeau, D.C. Johnson, S. Theriault, B. Malo. J. Alhen, and K.0. Hill, An all-tiher dense wavelength-divisionmult iplexcddemult ipleser using photoimprinted B r a e gratings,IEEEPhoion. Techno!. Lea., 01. 7, pp. 388-390, 1995.A. D.Kersey el. a!., Fiber grating sensors, J. Lighiwave Tecknol. ,01. 15,p p. 1442-1463, 1997.

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[3 ]

[4 ]

[SI

[6 ]

985


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[7] C. R. Giles, "Lightwave applications of tiher Bragg gralings." J . [ I I P. Torres and L. C. G. Valcnte, "Spectral response of ocally pressedLighawve Technol., "01. 15, pp. 1391-1403, 1997. fiber ~ r a g grating,"Opr.c u n ~ n u n . , OI.08 , pp. 285-291,2002.[SI P. S. Westbrook, T. A . Strasser, and T. Erdogan, "In-line [I21 P. Torres and L. C. G. Valentc, "Optical tiher polarizer based onpolarimeter using blazed fiber grating." IEEE Photon. Techno/. locally pressed Bragg gratings," in Bmgg Cmring,Phomsmsil ivi /y,

L e t t . , v o i . 1 2 . p ~ . 3 s 2 - 1 3 5 4 , 2 0 0 0 . and Poling in Glar.s Woveguider, OSA Technical Digest (Optical[91 G. MeltrandG. A. B a k u . S. Patent No . 5,546.481 Society of America, Washington DC. 2001). paper BThC14.[ IO ] C. 1.S. de Matos, P. Torres, L. C. G . Valentc, W. Maryl is, and R.Stubhe. "Fiber Bragg grating (FBG) characterizationand shaping bylocal pressure," J. L i g h n o v e Techno/ . , YOI. 9, pp . 1206-1211,2001

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