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I

IEEE PHOTONICS TECHNOLOGY LE'ITERS. VOL. 7, NO. 4, APRIL 1995 391

Polarization Selectivity of Gratings Written

in Hi-Bi Fibers by the External Method

P. N iay , P. B ern ag e , T. T au n ay , M. Douay, E. D elev aq u e , S. Boj, a n d B. P o u m e l l e c

Abstract-Gratings have been written in a commercially avail-able elliptical-core fiber using the transverse holograp hic method.Type I and 11-A dynamics have been observed as being stronglyanisotropic. As a result, the grating reflectivity can be m ade highlypolarization dependent according to the experim ental conditionsof the inscription.

I. INTRODUCTION

INCE the discovery in 1978 by Hill et al . of photosensitiv-S ty of germanosilicate optical fibers, the phenomenon has

received much interest since it has both scientific and practical

significance [11. Although, since that time, significant progress

has been made in the understanding of the phenomena, their

exact mechanisms remain yet to be clarified.

We have recently reported experiments in which gratings

were written by a transverse holographic method using either a

pulsed or a cw laser. Long exposure time leads first to a grating

photoinscription, followed by complete or partial erasure of the

first order spectrum and then by a new spectrum formation [2],

[3]. To give an account of the complex dynamical e volutions of

the spectra, we have suggested that two photorefractive effects,

at least, should be responsible for the grating growth and that

the dynamics of a grating formation results from the kinetics

difference between two reactions [ 2 ] . he first reaction leads to

a positive change in the refractive index, whereas the second,

slower than the first, produces a defect with a negative index

change. This phenomenological model is consistent with the

results of the photobleaching experiments in which we have

shown that long exposure time of a grating by fringeless UVtransverse illumination can first erase the first order grating

transmission spectrum and then build a new grating spectrum

[3]. For convenience, we will distinguish between the two

successive spectra by labeling them type I and type 11-A

grating spectrum respectively [3]. The fibers used in these

studies were manufactured by the CN ET laboratory in Lannion

and were heavily doped with Germanium. To check that the

above related observations are not relevant of some special and

unknown property of the fibers manufactured by the CNET

Manuscript received August 5, 19 94; revised December 28, 1994. This workwas supported in part by a DRET contract no. 92147 and by CNET contracts938B006 and 9388180.

P. Niay, P. Bemage, T. Taunay, and M. Douay are with Laboratoirede Dynamique MolCculaire et Photonique URA C NRS 779, UniversitC desSciences et Technologies de Lille, U.F.R de Physique-Bstiment P 5 , 59655

Villeneuve d'Ascq Cedex, France.E. Delevaque and S . Boj are with LAB/RIORSO, France Telecom CNET,

Route de TrCgastel, 22301 Lannion, Cede x, France.B . Poumellec is with Laboratoire des composes non stoechiomktriques URA

CNRS 446, Universite de Paris Sud, 91405 Orsay, France.IEEE Log Number 9409254.

Laboratory, we have performed Bragg grating inscriptions

for long irradiation time in a commercialy available fiber.

In this letter, we demonstrate that Bragg grating inscription

in an elliptical core fiber with a large corekladding index

difference leads to dynamical evolutions of grating spectra

similar to those previously observed in the CNET fibers.

Furthermore, we show that the grating reflectivities can be

highly polarization-dependent.

11. EXPERIMENTALETAILSAND RESULTS

The method for producing the Bragg gratings was an

extemal writing technique [4] which used a Lloyd's mirror

type interferometer and a 243-nm pulsed source to realize

the UV fringe pattem. Further details about the methods used

for writting the gratings are related in [ 5 ] , [ 6 ] .The spectral

transmissions of the Bragg gratings were recorded in real

time in the course of the photoinscription using a white light

source, a high-resolution spectrometer (theoretical resolving

power: 240 000),and a cooled germanium detector set-up. The

fiber was an elliptical core fiber manufactured by Andrew

Corporation with a D-shaped cross section (core/cladding

index difference w 0.035, cut-off wavelength M 700 nm , beat

length M 5.5 mm, reference number 48280820 S-1). The UV

pump beam was linearly polarized either along the slow axis

(G) or along the fast axis (fa) of the fiber. The spectral

transmissions of the Bragg gratings were recorded using a

probe beam linearly polarized either along s"a or along fa. Runs

were performed in succession as follows. First, the probe beamwas polarized along sa. The transmission spectrum was then

recorded which allowed to measure the grating transmittance

T,, nd thus, assuming that R+T = 1, the gra ting reflectivity

R,,. Afterwards, the state of polarization of the probe beam

was rotated by 90", being now polarized along the fa axis.

A new transmission spectrum was then recorded, from which

the grating transmittance Tf, as measured. Then, the state of

polarization of the probe beam was rotated again by 90" to

begin a new run. This operating process w as repeated several

times up to the end of the grating inscription.

Fig. 1 shows a typical evolution of the first order grating

transmittances Tsa and Tfa as a function of the number of

pulses impinging on the fiber. The Bragg grating, 3.5 mm long,

was written at a fluence per puls e of 290mJ/cm 2, using a pumpbeam linearly polarized along G. The Bragg wavelength was

M IO01 nm. After 4000 pulses, the drop in the type I grating

transmittance T,, as saturated (&, reached M 0.9). Further

U V iradiation led to a complete recovery of the transmittance

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392 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 7,NO . 4, APRIL 1995

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NUMBEROF PULSES NUMBER OF PULSES

Fig. 1 . Dynamics of a grating inscription n an elliptical core fiber. The pump

beam is po larized along the slow axis (grating length =3.5 mm, fluence per

pulse =290 mJ/cm', AB = 1001 nm). Open and black triangles are for the

probe beam polarized along the fast and slow axes respectively.

T,, after 24000 pulses and then to a further drop in the

grating transmittance (a type 11-A grating spectrum). A similar

behavior could be observed with the probe beam polarized

along fa, except that the dynamics of the grating spectrum

erasure was markedly slower and that the saturated type I

reflectivity Rf, reached 0.98. As a consequence, o ne can see in

Fig. 1, that the grating reflectivity Rf, was M 0.8 after 24000

pulses whereas the R,, value was at this time approximately

M 0 to within the experimental sensitivity (0 <R <0.03).After its inscription, the grating was irradiated by UV

fringeless exposure. The UV beam, linearly polarized along

z, was made to impinge on the fiber at a nearly normal

incidence to avoid that any possible residual intemal reflection

on the fiber could create a fringe pattem at a period equal to

this previously realized to write the grating. The evolutions of

the transmittances of the grating as a function of the number

of pulses are shown in Fig 2. Fringeless irradiation led to

a monotonic decrease in the grating transmittance T,, (thus

to an increase in the reflectivity of the grating). Rsa was

saturated at =0.85 after 1.5 . lo5 pulses. W hen the probe

beam was polarized along fa, we observed that the type I

reflectivity Rf, first decreased (Rf, was nearly 0 after lo4

pulses) and afterwards increased. The corresponding type II-A grating reflectivity get a saturated level Rf, 0.7 after

1.5 . lo5 pulses.

In the experiment corresponding to the grating inscription

reported in Fig 1, the lateral exposure by the UV fringe pattem

was stopped after 30000 pulses. In other experiments not

reported here, the inscription of the grating was continued

-

--t

Fig. 2.(fluence per pulse 110 mJ/cm2).

Evolution of the grating transmittances under fringeless U V exposure

further up to the inscription of saturated type 11-A spectra.

Whatever the experimental conditions of these inscriptions

may be (length of the grating L', 2 mm 5 L' 5 7 mm

fluence per pulse F , 150 mJ/cm2 5 F 5 300 mJ/cm2) some

reproducible observations could be made. As the number of

pulses impinging on the fiber was made to increase, it was

observed that the type I grating reflectivity Rsa ot a saturated

value R F efore Rf, -R,, begun to decrease from RFF 'whereas Rf, continued to increase toward its saturated value

R y l - t h e Bragg wavelength increased during the steps of

inscription or erasure of grating I spectrum reaching a maxi-mum near the time of spectrum erasure; afterwards it remained

stationary during the second growth of the grating spectrum.

To within the accuracy of the measurement (f0.05 nm), no

significant change in the fiber birefringence co uld be detected.

When the pump beam was linearly polarized along fa, the

difference between the evolutions of the transmittances T,,

and Tf, with irradiation time was less important than those

observed for a pump polarized along s"a. For example, the

reflectivities R,, and Rf, of a 5-mm-long g rating written

at a fluence per pulse of 240 mJ/cm2 reached a saturation

level (R?, M 0.97, Rf, M 0.98) at the same time after 5000

pulses. Further irradiation first induced a partial erasure of the

type I spectra. Thus, after 24000 pulses, the recovery of the

transmittance T,, was saturated near T,,M

0.9 (R?,M

0.1)whereas Tf, was at this time M 0.6 (Rf, M 0.4). Further UV

exposure led to the saturation of the recovery of Tf, (after

40000 pulses, Tf, M 0.7) and to the formation of type 11-A

spectra, one for each state of polarization of the probe beam.

After lo5 pulses, the reflectivities of the grating were saturated

-

(R5,M 0.7, Rf, =0.5).



NIAY et U / . : POLARIZATION SELECTIVITY OF GRATINGS WRITrEN IN Hi-Bi FIBERS BY TH E EXTERNAL METHOD 393

111. I ) ISCUSSION AND CONCLUSION

These results show that Bragg gratings transversely written

in Hi-Bi fibers can be used as polarization filters with selectiv-

ity depending on the number of pulses used for the inscription.

Although that, according to the long exposure time, one can

not exclude that small temporal drifts in the position of the

fringe pattem relative to the fiber could occur, the grating

spectrum erasure and reformation can not be understood onlyon the basis of some experimental instability. For example,

let us assume, for a while, that bright fringes take the place

of dark fringes at a time as a result of some drift. Then

the erasures of the type I grating spectra as probed along

$ or fa should start simultaneously. Furthermore, the shift

in the Bragg wavelength should be toward the red during

the type 11 spectrum inscriptions. This is not observed in

our experiment. One also can note that highly polarization

selectivity has already been reported for gratings written by the

internal method within an elliptical-core fiber manufactured by

Andrew C orporation [7 ] . Whatever the polarization of a pump

beam may be (fa or G), he saturated reflectivity corresponding

to the type I spectrum was higher when the probe beam was

polarized along fa than when it was polarized along z.Onthe contrary, the reflectivity measured at saturation of the type

11-A spectrum was higher for the probe beam polarized along

z. tress-birefringence reduction in elliptical-core fibers under

ultraviolet irradiation was recently demonstrated by D. Wong

et al . [8]. The proposed mechanism was a stress relief as a

consequence of the relaxation of the tensioned glass network

initiated by breakage of wrong bonds by the UV light [8].

Our observations are consistent with an anisotropic stress

relief occuring during the inscription of the type I spectra.

We would then suggest that the part of the index modulation

+

+

+

resulting from the stress relief is higher when the probe beam is

polarized along fa than when it is polarized along s”a. Another

possibility would be that the second reaction postulated to

give an account of the type 11-A spectrum formation is also

anisotropic and produces a negative refractive index change

whose absolute value is lower for light polarized along fa than

when it is polarized along sa. This issue is under investigation

in our laboratories.

+

+

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REFERENCES

K. 0. Hill, B. Malo, F. Bilodeau, and D. C. Johnso n, “Photosensitivityin optical fibers,” Ann. Rev. Mafer. Sri ., vol. 13, pp. 125-157, 1993.W. X. Xie, P. Niay, P. Bemage, M. Douay, J. F. Bayon, T. Georges,M. Monerie, and B. Poumellec, “Experimental evidence of two typeso f photo-refractive effects occuring during photoinscriptions of Bragg

gratings within germanosilicate fibers,” Opt. Comm., vol. 104, pp.185-195, 1993.M.Douay, W. X. Xie, E. Fertein, P. Bemage, P. Niay, J. F. Bayon, andT. Georges, “Behaviors o f spectral transmissions of photorefractive fil-

ters written in germania-doped fibers writing and erasing experiments,”in Proc. S.P.I.E.Photosensitivity and Self Organization in Optical Fibersand Waveguides, vol. 2044, pp. 88-1 12, 1993.G. Meltz, W. W. M orey, and W. H. Gle nn, “Formation of Bragg gratingsin optical fibers by a transverse holographic method,” Opt. Lef t . , vol.14, pp. 823-825, 1989.

E. Fertein, S. Legoubin,M.

Douay, S . Canon, P. Bemage, P. Niay, J. F.Bayon, and T. Georges, “Shifts in resonance wavelengths of Brdgg grat-

ings during writing or bleaching experiments by UV illumination withingermanosilicate optical fiber,” Electron. Lett., vol. 27, pp. 1838-1 839,1991.W. X. Xie, M. Douay, P. Bemage, P. Niay, J . F. Bayon, and T. Georges,“Second order diffraction efficiency of Bragg gratings written withingermanosilicate fibers,” Opt. Comm., vol. 101, pp. 85-92, 1993.S. E. Kanellopoulos, L. C. G. Valente, V. A. Handerek, and A. J. Rogers,‘‘Polarization properties of p ermanent and nonpermanent photorefractivegratings in Hi-Bi fibers,” IEEE. Photon. Technol. Lett . , vol. 3 , pp. 45 4 7 ,1991.D. Wong, S . B. Poole, and M. G. Sceats, “Stress-birefringence reductionin elliptical-core fibers under ultraviolet irradiation,” Opt. Lett. , vol. 17,pp. 1773-1775, 1992.

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