The properties of fiber Bragg gratings in hydrogenated fibers under conditions of ultraviolet overexposure were investigated. Abnormal spectral evolution of the regenerated grating following erasure of the initial type I grating was observed in the hydrogenated fibers. The regenerated grating also exhibited less temperature sensitivity and an 18 nm shift in the Bragg wavelength

Bennion, I.

Liu, Y.

Williams, J.A.R.

Zhang, L.

English

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586 OPTICS LETTERS / Vol. 27, No. 8 / April 15, 2002Abnormal spectral evolution of fiber Bragg gratingsin hydrogenated fibersY. Liu, J. A. R. Williams, L. Zhang, and I. BennionPhotonics Research Group, School of Engineering and Applied Science, Aston University, Aston Triangle,Birmingham B4 7ET, UKReceived October 25, 2001We report the observation of abnormal spectral evolution in regenerated fiber Bragg gratings in hydrogenatedB–Ge-codoped and standard telecom fiber with UV overexposure. The behavior of this new type of regen-erated grating, which we have designated type IA, contrasts with that of the previously reported type IIAgrating in nonhydrogenated fiber by exhibiting a large redshift in Bragg wavelength with increasing exposureof as much as 18 nm from a strong (16-dB) regenerated grating in B–Ge fiber. © 2002 Optical Society ofAmericaOCIS codes: 060.2400, 060.2310, 060.2270.Exposing the core of a photosensitive optical f iber toa UV laser interference pattern changes the fiber’srefractive index periodically, producing a fiber Bragggrating (FBG).1 A number of FBG types have beendistinguished, characterized by markedly differentspectral and thermal behaviors, that originate fromthe UV excitation of several distinct physical mecha-nisms. During the typical inscription process, thecentral wavelength of the grating increases with theUV exposure, which is indicative of the inductionof a positive change in refractive index. The re-sultant FBG has come to be referred to as a type Igrating and is the type most commonly fabricatedby either a free-space holographic or a phase-maskexposure technique; however, this FBG is the leaststable at high temperatures of the type reported todate. With further UV exposure the type I gratinggrowth saturates, and formation of the so-called typeIIA grating commences.2 The observed ref lectivitydecreases almost to zero before increasing again and,eventually, saturating; the central refractive wave-length decreases during this stage of the exposure,indicating a negative induced index change. TypeIIA gratings are usually inscribed at a wavelength of193 nm,2 – 4 and, although the underlying mechanismis still not wholly clarif ied, links to stress and defectconcentrations have been identified.5 – 7In all observations reported to date, the type IIAgratings were formed in nonhydrogenated fibers.However, the introduction of hydrogen into the fiberto increase photosensitivity is a commonly adoptedprocedure. In seeking to clarify FBG growth with UVoverexposure in hydrogenated standard telecom andB–Ge-codoped fibers we have observed, for the f irsttime to our knowledge, regenerated grating growthwith an abnormal, i.e., contrary to the behavior ex-pected on the basis of previously reported observation,spectral evolution that is quite distinct from that oftype IIA. We have designated this new grating typeIA and found it to exhibit less temperature sensitivitythan the type I grating.0146-9592/02/080586-03$15.00/0We inscribed the FBGs reported here by two-beamholographic exposure, using a cw 244-nm laser with8-mm beam width and 60-mW power. For refer-ence, we produced a type I FBG in nonhydrogenatedB–Ge-codoped fiber: a maximum 2.2-dB ref lectivitywas obtained in 2 min of UV exposure, as shown inFig. 1(a); the ref lectivity decreased with subsequentexposure and fell to zero after 7 min. A maximumredshift of 2.0 nm in the Bragg wavelength wasrecorded. As the UV exposure was maintained, thegrating ref lectivity increased from zero, accompa-nied by a blueshift in the Bragg wavelength that ischaracteristic of the type IIA grating and a negativeinduced refractive-index change. Limited by the UVwavelength and power, the type IIA grating attained8.7-dB ref lectivity and 0.66-nm blueshift after 37-minexposure. Subsequent annealing of the grating for12 h at 70 ±C induced a further blueshift of 0.55 nmowing to relaxation of the photoinduced positive indexchange; the ref lectivity remained unchanged, indicat-ing good thermal stability, as shown in Fig. 1(b).This fabrication procedure was repeated with a hy-drogenated sample of the same fiber. Type I gratingformation and complete erasure were again observed;however, with continued UV exposure a quite differ-ent behavior emerged that included formation of anFig. 1. Formation of an 8-mm-long type IIA grating innonhydrogenated B–Ge-codoped fiber: (a) ref lectivityand wavelength shift relative to exposure time, (b) trans-mission spectra before and after annealing.© 2002 Optical Society of AmericaApril 15, 2002 / Vol. 27, No. 8 / OPTICS LETTERS 587Fig. 2. Transmission spectra for 8-mm-long gratings in-scribed in hydrogenated f ibers, illustrating abnormal spec-tral evolution with increasing UV exposure (the directionof the large, bold arrow): (a) in B–Ge-codoped fiber and(b) in standard fiber.Fig. 3. Formation of four 8-mm-long gratings (G1)– (G4)in hydrogenated B–Ge-codoped fiber: (a) ref lectivity and(b) Bragg wavelength shift, plotted relative to exposuretime.abnormal grating spectrum and a significant redshift,as illustrated by Fig. 2(a). Similar behavior was ob-tained with hydrogenated standard fiber, as shownin Fig. 2(b). In Fig. 3 we have plotted the measuredref lectivities and wavelength shifts relative to UV ex-posure time for a series of four gratings, (G)– (G4),inscribed in the hydrogenated B–Ge fiber. The be-havior is similar in all cases: After 15 min the typeI gratings are completely erased, and after 60 minthe new spectra have formed and the regenerated grat-ings (type IA) have attained their maximum ref lec-tivity, 16 dB, with corresponding redshifts as highas 18 nm. With longer exposure (we continued for 4and 6.5 h), no significant further changes in eitherthe ref lectivity or the wavelength shift were observed.For this fiber, we calculate that the 18-nm redshiftcorresponds to a positive refractive-index change ofDn  1.68 3 1022.It is noteworthy that the calculated value of Dn forthe exposed fiber core yields an increase in the numeri-cal aperture of the fiber by a factor 2.6. In Fig. 4 ahigher-order core mode is apparent, with 6-dB ref lec-tivity, separated from the primary Bragg resonanceby 5.9 nm. (Higher-order modes are also apparent inFig. 2.) After annealing for 12 h at 70 ±C, the regen-erated grating exhibited a Bragg wavelength shift toa shorter wavelength of 2–3 nm and a signif icant de-crease in coupling strength, also shown in Fig. 4.Figure 4 also shows the overall ref lection spec-trum of the regenerated grating. Two plateaus areapparent; the first, commencing at l1  1545 nm,corresponds to the remnant type I region, and the sec-ond, commencing at l2  1550 nm, is indicative of thetype IA region. The higher-order core mode is clearlyvisible in the spectrum at l3.Finally, we investigated the temperature sensitivityof the type IA grating compared with the initial typeI grating by measuring the Bragg wavelength shiftsas the sample temperature was raised; the results forone sample are shown in Fig. 5. Whereas the type Igrating shows a Bragg wavelength shift of 8.9 pmK,the regenerated grating exhibits the somewhat lowersensitivity of 6.4 pmK.In conclusion, the properties of fiber Bragg gratingsunder conditions of UV overexposure in B–Ge-codopedand standard fibers have been investigated experi-mentally. In nonhydrogenated fiber, thermallystable type IIA gratings have been observed. Inhydrogenated fiber, significantly different behaviorwas observed, with abnormal spectral evolution ofregenerated grating following erasure of the initialtype I grating. A substantial shift of the Braggwavelength by as much as 18 nm to a longer wave-length, implying a refractive-index increase as largeas 1.68 3 1022, is in marked contrast to the shift toshorter wavelength and the negative index change ofthe type IIA grating. The regenerated grating alsoexhibits less temperature sensitivity than the type Igrating.Fig. 4. Ref lection (R) and transmission (T) spectra of aregenerated grating after 1-h exposure in hydrogenatedB–Ge-codoped fiber.Fig. 5. Bragg wavelength shifts measured in type I andregenerated (type IA) gratings inscribed in hydrogenatedB–Ge f iber plotted versus temperature. Symbols, experi-mental points; solid curves, best fits to the data.588 OPTICS LETTERS / Vol. 27, No. 8 / April 15, 2002This research was carried out with the supportof the UK Engineering and Physical Sciences Re-search Council. We acknowledge the provision ofB–Ge-codoped fiber by Nortel, UK. Y. Liu’s e-mailaddress is liuy@aston.ac.uk.References1. R. Kashyap, Fibre Bragg Gratings (Academic, London,1999).2. I. Riant and F. Haller, J. Lightwave Technol. 15, 1464(1997).3. W. X. Xie, P. Niay, P. Bernage, M. Douay, J. F. Bayon, T.Georges, M. Monerie, and B. Poumellec, Opt. Commun.104, 185 (1993).4. M. Douay, W. X. Xie, T. Taunay, P. Bernage, P. Niay,P. Cordier, B. Poumellec, L. Dong, J. F. Bayon, H.Poignant, and E. Delevaque, J. Lightwave Technol. 15,1329 (1997).5. J. Canning, D. Moss, M. Aslund, and M. Bazylenko,Electron. Lett. 34, 366 (1998).6. L. Dong, W. F. Liu, and L. Reekie, Opt. Lett. 21, 2032(1997).7. W. X. Xie, P. Niay, P. Bernage, M. Douay, T. Taunay,J. F. Bayon, E. Delevaque, and M. Monerie, Opt. Com-mun. 124, 295 (1996).

Abnormal spectral evolution of fiber Bragg gratings in hydrogenated fibers

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Abnormal spectral evolution of fiber Bragg gratings in hydrogenated fibers

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