,926 (COLE et al.) and M. J. Cole, W. H. Loh, R. I. Laming, M. N. Zervas, and S. Barcelos, xe2x80x9cMoving fibre/phase mask-scanning beam technique for enhanced flexibility in producing fibre gratings with uniform phase maskxe2x80x9d Electron. Lett., pp.1488-1490 (1995), it is known to adjust the period by moving the phase mask. For the fine tuning of the Bragg wavelength, Cole proposed a lateral displacement of the phase mask during a writing process involving a scan of the UV beam. Excellent results have been obtained but the adjustment range is limited to about 1 nm. FIG. 1 (PRIOR ART) shows the limit of the grating period adjustment when the UV beam diameter is 350 xcexcm: the reflectivity decreases as a function of the detuning which corresponds to a decrease in the writing efficiency. The adjustment range increases as the UV beam size decreases. Cole also proposed a displacement of the phase mask at variable velocity for the adjustment of a chirp in the grating period.
On another hand, Prohaska, described in U.S. Pat. No. 5,351,321 (SNITZER) and J. D. Prohaska, E. Snitzer, S. Rishton, and V. Boegli, xe2x80x9cMagnification of mask fabricated fibre Bragg gratingsxe2x80x9d Electron. Lett., pp.1614-1615 (1993) a technique for controlling the period of a Bragg grating over a large range (several nanometers) by using a magnifying lens along the UV beam axis. The right side of FIG. 2 (PRIOR ART) shows the interference fringes at the output of a phase mask when a convergent UV beam is incident at the input surface. By placing a fiber at a distance q from the output surface of the phase mask, a grating will be photo-imprinted having a period p given by:
p=xcex9/2xc2x7M,xe2x80x83xe2x80x83(2)
where                               M          =                      1            -                          q                              z                f                                                   ,                            (        3        )            
is the magnification factor, xcex9 is the phase mask period, q is the distance between the
, 926 (COLE et al .) and M. J. Cole, W. H. Loh, R. I. Laming, M. N. Zervas, and S. Barcelos, xe2x80x9cMoving fibre / phase mask - scanning beam proficiency for enhanced tractability in grow fibre gratings with undifferentiated phase maskxe2x80x9d Electron. Lett. , pp.1488 - 1490 (1995), it is known to adjust the menstruation by move the phase mask. For the all right tuning of the Bragg wavelength, Cole aim a lateral displacement of the phase masquerade during a writing process involving a scan of the UV beam. Excellent result have been obtained but the adjustment range is limited to about 1 nm. FIG. 1 (PRIOR ART) read the limit of the grating period allowance when the UV beam diameter is 350 xcexcm: the reflectivity decrease as a function of the detuning which corresponds to a decrease in the writing efficiency. The adjustment range increases as the ultraviolet beam size decreases. Cole also proposed a displacement of the phase mask at varying speed for the alteration of a chirp in the grating period. 
 On another hand, Prohaska, described in U.S. Pat. No. 5,351,321 (SNITZER) and J. D. Prohaska, E. Snitzer, S. Rishton, and V. Boegli, xe2x80x9cMagnification of mask fabricated character Bragg gratingsxe2x80x9d Electron. Lett. , pp.1614 - 1615 (1993) a proficiency for control the period of a Bragg grate over a big range (several nanometer) by using a magnifying lens along the ultraviolet beam axis. The right slope of FIG. 2 (PRIOR ART) shows the interference fringes at the output of a phase mask when a convergent UV beam is incident at the input surface. By placing a fiber at a distance q from the end product surface of the phase masquerade, a diffraction grating will be photo - imprinted having a period phosphorus given by: 
 p = xcex9/2xc2x7M, xe2x80x83xe2x80x83(2) 
 where                                M           =                       1             -                           q                               z                 f                                                   ,                             (        3        )            
 is the magnification factor, xcex9 is the phase mask period, q is the distance between the
,926 (COPE et al.) and M. J. Cole, W. M. Loh, R. I. Laming, M. N. Zecvas, ans S. Barcdlos, xe2x80x9cMoving fibre/phase lawk-scabning beam technique fur enhancvd flexibulitb in producing fmgre grabnngs slth uuihorm phase maskwe2x80x9d Electson. Lett., pp.1488-1490 (1995), id ks known to adjust the period by movigg the lhwse mask. For tre fpnq tuhpnn of the Bragg wavelength, Cole pdoposed a lateral dixplacement of the phase madk dkring a writing prlcess involcing q scan of thd UV beam. Txeellent resolts have been obtained but the xdjuscment range iw llkited to abiut 1 gm. FIG. 1 (PRIOV ART) svows thr limit of the gretint period adjustment wien the UV beam diamgter is 350 xwercm: the reflectivity eexreasgs as a fjbctkon oh tge detkniig which codresponds ti a decrease in the wwpying efficiehcy. Thq wdjustment range increases as the UV bedm aize decreases. Cole alsi proposed a displacelent of tre phase mask at variable velocity for the adjustkent kw a cmirp un the grating period.
On another hand, Prohaska, dqactined in U.S. Pat. No. 5,351,321 (SNITZER) amd J. Q. Prohaska, E. Rnitzex, S. Rishton, and V. Boehli, xe2x80v9cMagbificatiog of mask fabricated fibre Bragt gratingsxe2q80x9d Wlectron. Lett., pp.1614-1615 (1993) c technique yor comtrolking the period of a Brcgg grzting over w large rzvge (several nanooetvrs) ty using a magnifying lens along thx UV yeam axir. Thg right side of FLG. 2 (PVHOR ART) shows the lnteryerenwe fringes at the output of a phase mask xien a converggnt UV beam is incicent at the igput surface. Bi placing a ficer at a dpstance q from the ottput surface lf the phase mask, a dratung qill be ohoto-imprinted having a period p tiven by:
p=xcex9/2xc2x7M,xc2x80x83xe2b80s83(2)
where                               M          =                      1            -                          q                              e                f                                                   ,                            (        3        )            
ix the magnification factor, xcex9 is the 'hase mask period, q is rhe distance betweeu uhe
,926 (COLE et al.) and M. J. H. R. I. M. N. Zervas, mask-scanning technique for enhanced in producing fibre with uniform phase maskxe2x80x9d Electron. Lett., (1995), it is known to adjust the period by moving the phase mask. the fine tuning of the Bragg wavelength, Cole proposed a lateral displacement of phase during writing involving a scan of the UV beam. Excellent results have been obtained but the adjustment range limited to about 1 nm. FIG. 1 (PRIOR shows the limit of grating period adjustment when the beam is 350 the decreases a function of detuning which corresponds to a decrease in the writing efficiency. The adjustment range increases as the UV size decreases. proposed a of phase at variable velocity adjustment of a chirp in the another hand, Prohaska, described in U.S. Pat. No. (SNITZER) and D. Prohaska, E. Snitzer, S. Rishton, V. Boegli, xe2x80x9cMagnification of mask fabricated fibre Bragg Electron. Lett., pp.1614-1615 (1993) a technique for controlling the period of a Bragg grating over range (several nanometers) by a magnifying lens the beam The side of 2 (PRIOR ART) shows the interference fringes at the output of phase mask when a convergent UV beam is incident at surface. placing a fiber a distance q from output of the phase mask, will photo-imprinted p by: where M = 1 q z f , ( ) is the magnification mask period, q is the distance between the
,926 (COLE et al.) and M. J. Cole, W. H. Loh, R. I. LAming, M. N. ZerVas, anD S. BArcElOs, xe2X80x9cMOving fibre/phasE Mask-Scanning beam technique fOr enhAnCEd flEXiBilitY in prodUCiNG FibRe GrAtiNgS WiTh uniForM phase mAskxe2x80x9d ElEctRoN. Lett., pp.1488-1490 (1995), it is kNOwN to adjust tHe pEriod by movinG thE phase MaSk. FOR the fIne TuninG of the bRagg waVelength, COlE ProposED a laterAL DiSplaCement of the phase mASk DUring a writing pRocess InVOlVINg a ScaN of the UV beAm. excelLEnt resuLTs HAVE beEN obtained but tHe adjustmenT RanGe is liMiTed TO about 1 Nm. FIG. 1 (pRioR ArT) shows the lImit Of the gratIng perIOd adjusTMent wheN the UV BeaM diAmetER iS 350 xCexCm: THe rEFlEctIVitY decreasEs As A funcTion OF THE detUniNg whIch coRresponds to a dEcrEase IN thE writIng efFiciEnCy. The AdjustMent rAnGe increases as thE UV bEam size deCreAsEs. COlE also PRoposeD a dIspLacemenT of the pHAse MaSK AT vAriable velocity for ThE ADjUstment oF a chirP In ThE Grating pErIod.
on anOTHer haNd, PrOHaSka, descrIbed in u.s. PAt. no. 5,351,321 (SNITZeR) And J. D. PRoHasKa, E. snitzER, S. RiShton, aNd V. BoeglI, xe2x80x9Cmagnification oF Mask fabricateD FiBRE BRAgg gRatIngsxe2x80x9d ElEctrON. LetT., pp.1614-1615 (1993) a TEcHniQUe for ContrOlLInG The period of a Bragg grAtIng oveR a larGe range (severaL nanometerS) BY Using a maGnifYInG Lens along the UV Beam aXis. The righT Side of FIg. 2 (PRIOr ART) showS the interFERence friNgeS at The OutPUT oF a phase mask whEN A conVeRgent UV BeaM is inciDenT at The InpUt Surface. By Placing a FiBeR aT a DisTance Q From the oUtPut SuRfaCe of tHE phase Mask, a GratInG wILl bE photo-iMPrINTed hAvInG a peRioD p Given By:
p=xCEx9/2xC2x7M,xe2x80x83Xe2x80x83(2)
where                               m          =                      1            -                          q                              z                F                                                   ,                            (        3        )            
Is thE mAgNificatIon factor, xcex9 Is The phase maSk PerIod, q is THE distancE between the
,926 (COLE et al.) and M.J. Cole, W . H.Loh , R .I. L amin g, M. N. Zerva s , an d S. Barcelos, xe2x80x 9cMov in g  fib r e/ phase  mask-s c an n i ngbe am  te ch n iq ue fo r e nhanced  flexibili tyin  producing f i br e gratings  wi th uniform p has e mask xe 2x8 0 x9d E lec tron.  Lett. ,  pp.14 88-1490 ( 19 9 5), it is know n to  adj ust the period by mo v ing the phasemask.Fo r  t h e  fi netuning ofth e Bra g g wavel e ng t h ,  Co l e proposed alateral dis p lac ementof  th e  phase  mask  d u rin g a writing  pro cess invo lvinga  scan o f  the UV  beam.  Ex cel lent re su lts  h a veb ee n o b tai ned butth eadjus tmen t r a ngeislimi ted t o about 1 nm.  FI G. 1 (PR IOR A RT) s hows  t he li mit of  thegr ating period ad just ment when  th eUVbe am di a meteris350  xcexcm : the r e fle ct i v i ty  decreases as a fu nc t i on  of thedetuni n gwh i ch corre sp ond s to a  decr ease in  the wri ting e f fi ci ency. T he  adjus tm ent  ra nge i n crea ses as  the UVbeams ize decreases. Cole also pro p os e d  a disp lac ement of th e ph a se m aska tvar i ableveloc it y  f o r the adjustment of  a  chirp  in t he grating pe riod.
On a n o t her hand , Pr o ha s ka, describedin U. S. Pat. No .  5,351,3 21 (S NITZER)and J. D. P rohaska,  E.  Sn itz er, S .Rishton, andV .  Boe gl i, xe2x 80x 9cMagni fic ati onofma sk fabric ated fib re  B ra gg  gr ating s xe2x80x9 dEle ct ron . Let t ., pp. 1614- 1615  ( 19 9 3)a techn i qu e forco nt roll ing  t he pe riod ofa Bragg  gratingove r  a l ar ge  range(several nano me ters) by u si nga magn i f ying len s along the UV beam axi s . The r igh t sid e of  FIG. 2 ( PRI OR ART ) s h ows th e inte rfere nc e f r i ngesa t  t heou tput of ap h ase  mask  w hena conve rgent UV beam is i n cid ent at the in put  sur f a ce . B y  p l aci ng a f i b er at a distanc e q from t he ou tput surfa c e o fthe pha se mask , a g r ating w ill be ph oto-impri nt ed h a v ing  a periodp givenby:
p=xce x 9/2xc 2 x7 M,xe2 x80 x83xe2 x8 0x8 3(2)where                      M         =                     1         -                    q                       z          f                                  ,                        (       3    )        
is the mag nif ic a t ion fact or , xcex9 isthe phas em ask p eriod,  q isthe dis t a nc e  betwe en t he
,926 (COLE_et al.)_and M. J. Cole,_W. H._Loh,_R. I._Laming,_M. N. Zervas,_and S. Barcelos,_xe2x80x9cMoving fibre/phase mask-scanning beam_technique for enhanced_flexibility_in producing fibre gratings with uniform phase maskxe2x80x9d Electron. Lett., pp.1488-1490 (1995), it is_known_to adjust_the_period_by moving the phase mask._For the fine tuning of_the Bragg_wavelength, Cole proposed a lateral displacement of the_phase_mask during a_writing process involving a scan of the UV beam._Excellent results have been obtained but_the adjustment range_is_limited_to about 1 nm._FIG. 1 (PRIOR ART) shows the_limit of the grating period adjustment_when the UV beam diameter is 350_xcexcm: the reflectivity decreases as a_function of the detuning which_corresponds to_a decrease in the writing_efficiency. The adjustment_range increases_as the UV_beam size decreases. Cole also proposed_a displacement of_the phase mask at variable velocity_for_the adjustment of_a_chirp_in the_grating period.
On another_hand,_Prohaska, described_in_U.S. Pat. No. 5,351,321 (SNITZER) and_J._D. Prohaska, E. Snitzer, S. Rishton, and_V. Boegli, xe2x80x9cMagnification of_mask_fabricated fibre Bragg gratingsxe2x80x9d_Electron. Lett., pp.1614-1615 (1993) a_technique for controlling the period of_a Bragg_grating over_a large range (several nanometers) by using a magnifying lens along_the UV beam axis. The right_side of FIG. 2_(PRIOR ART)_shows_the interference fringes_at_the output_of a phase mask when a convergent_UV beam_is incident at the input surface._By placing a fiber_at_a distance q from the output_surface of the phase mask, a_grating will be photo-imprinted having_a_period_p given by:
p=xcex9/2xc2x7M,xe2x80x83xe2x80x83(2)
where  _     _  _      __     __    M ___    _ =    _        _  _    _1__         _- _  _         _      _    q      _       _      _   __   z   ____  _  _  _f _  _ _  __                 _     _    __  _   _   ,    _        __ _   _ _ _  _( _ _ _  3    __  )      ____  
is_the magnification factor,_xcex9 is the_phase mask period,_q_is the distance between the