CA2280472C - Photoinduced grating in b2o3 containing glass - Google Patents

Abstract

It has been demonstrated that B containing glasses are sensitive to radiation in the band 225-275 nm and, therefore, B2O3 glasses are particularly adapted to receive refractive index modulation, e.g., to make reflection gratings. Glasses containing SiO2 and B2O3, are particularly suitable when the grating is to be localised in the cladding of a fibre. Glasses containing SiO2, GeO2 and B2O3 are suitable when the grating is in the path region of a waveguide, e.g., in the core of a fibre.

Description

r, PHOTOINDUCED GRATING IN B?G3 CONTAINING GLASS
This '_nvention reia~es ~o opt.ca? devices which include =efractive index modulation, a. g. .efl ection gratings.
Reflection gratings are often _mplemented as waveguides which have a oath ;ecion and/or a con=firing region with a :nodal aced refracted '_ndex. '~he waveguiding structure is often in the form cf a _ibre. The modulation preferably takes the form of al ternate regions of higher and lower refractive index. When radiation traverses the modulation, it is selectively reflected. The period of the refractive index modulation is usual 1 y equal to the wavelength to be reflected or to a multiple or sub-multiple of said wavelength. Thus periods in the range 250 to 600 nm preferentially ref'_ect selected wavelengths within the range 800 - 1650 nm.
Reflection gratings have many applications in optical signal 1 ing. For axample, a reflection grating can be associated with a ='_~re ~ aser .n order ~o narrow the lasing bandwidth. When the =efractive index bands are not ner~_ endicular to t he =fibre axis, the crating can be used for the selective removal of unwanted wavelengths. In addition to reflection gratings, refractive index modulation has other applications, e. g. to achieve phase matching in waveguides, to control spot size and/or shape in waveguides and _or storing information. .
Refractive index modulation is conveniently produced by an optical process in which a photosensitive.glass is exposed to radiation whic~. causes an adequate change in its refractive index. ~he radiation has higher and lower intensities cor=es~oncing to the intended pattern of modulation of the =efrac~ive index of the glass. In many commonly used embodiments) the mutual interference of two beams of ~ radiation produces the variation of intensity appropriate for re=1 action ara~'_ngs. In the case or infor:,tation storage) the pattern or radiation relates to the data to he storec.

' : classes a=s wi~el y used in _ Si;'_caigerman_a _ opt_ca_ telecommunications and .t '.~.as peen noticed that these glasses have an opt_cal absorption bane extending app=oximateiy over t'.~.e wave 1 sngt range 225 - 275 nm and exposure to radiation within t~is band increases the .0 refractive i nde~ oz the sil ica/germania composi Lion. The peak of the band occ::rs at a gave? ength which is close to 240 nm. It has, therefore, been proposed to produce refractive index modulation, e. g. to make reflection grate ngs, by exposing sil'_caiger:nan=a glass composite ons to , t 5 radiation within the wavel ength band 225 - 275 nm.

radiation close to 240 nm is part_cularly suitable. High powers of =adia~tion, e. g. above 1:~W continuous, are needed to produce adecuate changes in t~e refractive index and wri tine times o. a few mi nutes to a few hours are 20 aDDro~riate. ' '1086/0'303 ~fasc=ibes the :~rit_ng of phase gratings ir.

ODL=cal fibres or ~raveguides ~y t a application of i nte.nse beams of ultraviolet _ight. =t __ stated that the grating is produced in the core of a wave guide and that the core 25 is preferably a germanium-doped silica or glass filament.

The sensitivit:r of the glass '_s important, and this invention is based upon the unexpected discovery that al asses whic:z contain 3~0, are particularly sensitive to 30 radiation, e. g. radiation close to 240nm, and that these classes are well adapted to carry the necessary refractive index :r~odulation. ~referanly the glass contains at leas t one cf SiO~ and GaO. as well as the B~O;.
:;6 Cempositi ons consisting essential ly of GeO~ and BOO
preferably containing at least 2 mule % of eac:z component, are suitable 'or thin fy'_m optica'~_ devices which are ~a~aDie o~ s ~o;i~c cata -n _:~e =o=:~ or =errac~_ve index 'LOC~L:.L d'C- on.
~om~osit~.ons co.~.sist'_ng essan~=ailv o~ SiO~ anc 3,0., ore===ably co~~a____.~.c a~. =easy ~ ~:ois ~ o. each ~omr~cnenz _ ~ , ., CA 02280472 1999-08-31 ....
are par:=cularl w suitabl a .or car=_;=ng the refractive index modul anion wherei n sale modulatio.~. cons t~ Lutes a reflection wavecuide located _=~ the confining region of an optical waveguide. Glass consis ti ng essenti al 1 _ of SiO~ and Ge0_ .. woul d be particuiar'__~ sui=able .c_- use as the path region o. said waveguide.
Compositions (herein after called ternary compositions) consis ring essentially o= Si0" Gel: and B203 are IO particularly suitable for ~~se is optical devices according the invention. Preferred ternary compositions contain: -2-40 mole ~ of B20"
2-40 mole 5 of Ge0" and at leas t 30 Mole ~ of SiO~.
I5 I t should be noted that B,O, tends to decrease the refracti ve index ,oi a sil_ca glass whereas GeOZ tends to __ ir_crease the refractive index of a silica class.
Since the concentration of B.O, affects the refractive index 20 as stated above, the refractive ir_dex will display a maxima at mini ma B~O~ concentranion and the refractive index will dis~lav a minima at maximum 8.~. concentration. It is standard practice i~~ the preparation of optical waveguides to vary the concentration of a dopant radially through the 25 core region, eg to fabr-cate a graded index multimode fibre. However, it is less convenient (and even impractical ) to produce fine detail longitudinal variation, eg a reflection grating, by varying the concentrations of relevant components.
t :~.as been noti ced ~ha ~ s ome gl as s es are photo-s ensitive whereby exposure to suitable light causes changes in the refractive index and exposure to fine patterns is adapted to produce the desired f._~.=_ detail. ~ t is doubtful that the optical exposure changes the chemical composition of the class and it is more appropriate to postulate that S
s tructural changes, possiL' y including defect centres, play ° . -:,e overa' _ e==ect. Sven though the s a :s LanL_a_ .o_ _ __. ___ :sec'.~.an=s:-~ ? s .~.oc =~___r a~cerstood, the prcduction of =efracLive i.~.ce:c sac=er~s by e:cposure to radiat_on has been GemC.~_SWaLeC ~:C~e=_.'..e~vc___~-~., ar.reaG" ~a°- _=aL~C L =av "' aSS2S '~Tn~C. .C~ ..
vll~a=n are pa=L;cul a=~y :a.-_s_t_ :e Lo =ad~aLion and, as =ndicated above, the re_=acL_'.= _rdex ~aL~eras produced i n accordance Hi h ~'ae inve__L=o a_ _ d..pendenL ,._ ~.
t n ' ~-°_ ° ~' '-'~e boron contanL of _0 the c_ass. Ccnver_i=..~_Ll y the mol 2 raLi os B: Si and B: Ge are cons ~azt i n the _eg_c.~. where the refracL_ve index modula~ion '_s appi_=_d. ~_. :host applications it is appropriate yor bot h rat' os to be constant, eg. the glass :~!as a uniform com~ecsit'_or.. ('There one of the elements Sil iC:rn or ge=man=u:a _s case~L _ _ is convenient to take the rel e-:azL raz_o as _: J. ) '"erna=:r compos_ t' c:a as def'_ ed above have great potential L
Or .'-_Gj ~:Sti :.C t::e _=SDO==3I:t ~=~v_'L'ert=2S Of v.i'le QlaSS aS
p =e~QL==~d. '_'e =?=_3CWT7e =idea .s one of the important 'DrOpe=L_eS ~eC3uSe -_ '_S ~.LSUaI'_i~ Of major importance t0 matCi: the r8=r~CL'_':e '_T.:dlCeS O~ the deVlCe aCCOrdlng ~ t0 the '_~vention to t .e =ef=acLi-re i ndex of adj acent optical comnorents. '.'he deV_ce ''cccOrGlng to the lnVentlOn 1.S Often requi=ed to ~er~0~.~.. a ~aaveQUic.ing function and proper adj us L.,.ent c-_' t'_~_~ ==_Tract~we i_~_dices of the confir_ing reaio~ and ~ he pat =egi or. ar= necessary to get good wavegu_di_~_g p=oper~_as. .n par~icular i t is i:nporzant to adjust she =e'rac~_~:e i::dex di_ference between the path region and the cor.f_::ing reason to a predetermined value.
'"?~.is _~=fare.~.ce is usual__r ca? 1 ed :ln.
t is cossibl a to ad- ust _ he =aL_o of B,O,: Gep, so that the decrease i z =e=racL_:~e ; :dex caused by the B'01 .s balanced ?5 fapprcYimatel~~ or eYact~'r) by =he increase caused by the Gep,_. ~'hus the tar..~.ary compose Lions with B'0~ in excess of the a:~eunt needed ce balance the GeO, will have refractive i ndices 1 owes t.ha:. that :,i pL=°_ si'_.ca whereas ternary compositions pith an excess of Ga0_ ~i 1'_ have refractive indices create= .._._.. ,.h at e_-' pu== s'_'_ica. The ternary com~os~ ions ca~ ~e uses ._. Lithe. ,...e conf'_ning region) or the path regi o~ o~ - : bo t =.
The terms "cor.;inir.c =egio~" and "oath =egion" are used to designate the =egions ef _ower and '_higher refractive index respectively. It will be appreciates that, especially in the case of single mode waveQUides) substantial portions of energy will be transferred i n that part of the confining region which is close to the path region. Thus the energy in the confi: irg =egion -gil l interact with a reflection grating located iz the cor_=-ping regior, whereby gratings in the con=inina =egion can be used either alone or to enhance the eT_ect of gratin as _r. the path =egion. ' It will be appreciated what the waveguiding structures mentioned above maybe either planar waveguiding structures or _'_bres, especial_y single mode __bres. In the case of a _ibre the ~ronfi rang ~-eaion corresponds to the cladding and the path =saior_ corrss~onds to the core. ' In addition tc the essent'_al ingredients as specified above the glasses used to make cptical devices according to the invention may contain conventional additives, e. g. melting point depressants to facilitate processing during the manufacture e. the articles. Melting point depressants for silica glasses include phosphorus, usually present as an oxide, and fluorine.
The ore~aration o' optical devices according to the invention usually __~_cl udes the preparation of the glasses by the oxidation e. the appropriate chlorides using OZ at high temperature as the oxidizing agent. If desired, the glass intended to carry the refracted index modulation may be subi ected ~o :~il d reduction, e. G. by heating in the CA 02280472 1999-08-31 '°'"
absence of o:cyger_. T'~is .s co.~.veniently achieved by heati::c the glass -.. t:ze ~=ese.~.ce or '.~.elium.
The ==_==ac ;.i ve i nde:: -~ocu_ a t_ o:: . s ap~l i ed to the al as s whic:: contains 3.~- ~y sx~osi.~.c said glass to the a~~ro~=late matte=:: o. .ad~at'_on which accesses the absorp;.ion band hav= :g a pea?: close to 240nm. Radiation havir.c wavelengths -.~_ t -'-n t .__ :car-d 225 - 275 nm, e. g. a wavelength which is close to 240nm, is particularly .0 suitable. Radiatio:: whi~~ ~as aoubl a these wavelengths is also effective.
Two refl ection grat_::cs aceercir_c to the invention will now be described by wav o= example. The gratings are located y5 in the core of a ='_bre based on silica glasses and the 'oreoaration of the -'_bre wil'_ be described first. The -- extosure of the fibre to radiation in order to produce the refractive index modulanion wil_ also be described with -- reference to accompancirg drawing.
The _=bre was prepared bar a r"odi_'_cation oz the well-known inside deposition crocess .o. makinc optical fibre. In this process, the appropriate number of layers are denosi ted on the inner surface of a tube which serves as a substrate. Thus the outermost layers are deposited first and the i nnermost layers are deposited last. After all the layers have been deposited, the tube is collapsed into a solid rod, and the solid rod ~.s drawn into fibre.
Individual layers are ~=oduced by passing a mixture of oxygen and SiCl; w. th reagents such as ~ZC13 and GeCla through the tube anc :zeating a small section thereof to temperatures .n the =ange :::00~C - 2000°C. Under these conditions the ".~.lorides are converted into the corresponding oxides whicz initially deposit on the wall of the tube in the for: of a fine "soot" which is immediately fused to give a conso'_idated mass.

?~s an al terza='_ ve _'.'.~.e deposi t=o.~. .s car=led out at a temperature such than the depos~.t remains in a porus state and, a~ a later stace i:. tae (=ocess, t he "soot" is Bused at a '.~_=gher tem~era~~=a ~o ci-: a t he consolidated glass.
This alternative ._ app=opr=ac=_ when '_t is desired to submi t the deposi t ~o cherical ~=eaL:~en ~s wherein the porus state _acili rates the desired =eac t_on, e. g. reduction.
Melting point depressants such as phosphorus and fluorine may be incorporated i n the mixture to =acili rate processing i0 by 10 causing Busing at 1 owes tempera~ures.
The heating is carded out by causing a flame to traverse along the length o= the tube. The .lame heats a short section of the tube so that a portion, about 20 mm long, is .5 heated to the worki:.g temperature. This techniaue of heatinc is used for all s rages oL t he process, i. e. for __ _the depositior_, for consolidating porous layers to solid layers and for the collapse o. the tube. Multiple passes - are used at all stages of the process.
The smarting tube was made of pure sil'_ca. It had an external diameter o. '3 mm and an _nternal diameter of 15 mm.
~laddina Deuosi Lion The deposited claddinc toOK the Lvrm o. SiO~ with phosphorus and '1 ~~orine to reduce its mel ring point. Six layers of cl addin g were deposi red, and the condi Lions used for the deposi Lion of each 1 ayer were as foll ows: -~0 CA 02280472 1999-08-31 ~ ' w-v Oxvaen ~
-__ _=eS
/miw I
_ Helium I -- _ -_ ..res /min I

SiCI; 0. -:~ __...=es /min DOC1 ' 0. _ '_'_ tres /min CCi~FZ I 0. 0005 litres/min In the case of SiCl~ and ?OCI, tz2 flow rates specify the rate of =low of 0, t'~rouah a bubbles thermostated at 24°C.
The working temperatura was approx=mately 1525°C. It is emphas_sed, that alter eac~. _=averse, eacz cladding layer was in the form of _ clear class ~ layer before the next layer was deposited.
The c'_addinc layers could be considered to be uarZ of the substra'e tube upon whic~. she core layers were deposited.
The deposition of ciaddirg layers as described above could be omit~ed. The main purpose of tze cladding layers is to reduce the risk of contamination =rom the original tube affect'_ng core layers.
Core Deposit=on Core was deposited in two layers and the conditions for the deposition of each oz tze two layers were as follows:

J
i O~cycen I _. 0 '_i t=es!mi:!

3C1. I 0. G~ '__ __=s 1mi n , i J=C_ I O. _ _=,.=~c ~i~~:_ . ~ -_ i GeC=. ~ 0. 2 1 i tres ~ min i ' _.! t'_:e case of S=C~. and the GeC'_ she =' ew =ates sDecifv _3t~ OL =~ Ow ~L 0., tnrOUgh a b4bblar v.= .'-.=mOStoted aL
:n ..~° C3Se ~~= 3Cii the =lv'w =at2 ._ ~::at Jf the 0 vapour _ is a 1 = a t ? OOC and L atmos phe re.
The working temperature was or_1 ~r ? 4~0''C but this conso_'_dated t':e core _arers.
f ter L he prepa==_t_on described above, _ he tube was col _apsed into a s.,i;d rod in t'~e convenz'_ora_ -Banner using _ -_-re __,verses cf _~e =' ame.
The - : ~ r is T Y. _o= __bre hac a core :which _ so__~ cd, ..~e pre_o_m , 20 cor_~ained appro:ci.-.,atei_r ~7 mol a ~ S' O_, 25 ,cia 5 B=0; and '3 .ao_e ~ GaO~ gi-r=ng an ~I of _. 402. _'_:e cladding, essentially S=0~, had ari RI of ?.:~58 so that ~n - 0. 004.
The comaosi t_on c. the glass. in the ccre was s sbstantially uni'or:~, ie. the -:pie ratio 5: Si ~,ras i: 2. ~8 ~'.hroughout and 25 the mcle ratio ..: Ge tHas ' : 0. i 2 throughout.
'"he procedure desc=ibed above, apar~ =rpm the use of BCI_ in r t the core, consLi t;aes an essent'_ally conventional preparation of a ;ib.:e pr=form.
J O
'~he V_esorm prepa_ee as descr_bed above Nas crawn into __br= o. ? 20~S:n c'_ameLar at a temoeracure o. 2, COO C. The :fibre .gas produced at a rate of L8 met=es/mi~. This fibre is the precursor of =eflection gratings acco=ding to the 35 ' nvent_on.

La Figure 1 illustrates apparatus for producing reflection gratings in optical fibre. Short lengths of the fibre described above were converted into reflection gratings using the technique illustrated in the drawing. In each short length of fibre the core had a uniform composition, ie. as specified for its preform. Before exposure as described below the refractive index of the core was uniform.
A shoe port=on I4 oT t'_~_e =ibr=_ I5 Was =lluminated by a source I0. This =adeation was, i = the 'e rs t instance, IO produced by an Ar" ? aser, freguenct doubled to give output at a wavelength o. 244 nm. Th=_ beam =rpm the source IO was directed onto a spietter II so that two beams were directed onto mirrors i2 and i3. The mi==o=s I2 and I3 caused the beams to converge onto ~'r!e tarc=_ t s ec ti on, I ~. Thus an 1J enterference part=_r_~_ is produced with alternate ng regions of hegher and _ower intensity. 3ecause the fibre 15 is _ photosensitive, the region is (whereon the beams are ~ocused) is a=fected by the beams and the refractive index' - a s encreased in the areas o. high intense ty. Thus a 20 r=_=1 ection grating ' s produced _r. the region I4.
t will be ap~rec=aged that the spacing of the interference ~ the two beams pattern is a=fected by the ang? s ar. whicL
intersect one another, a~d hence the spacing of the grating 25 can be adi us red by adj us ring the rel ateve position of the splitt~r II and the mirrors I2 and 13.
Two specimens of this fibre were subj acted, to . an aster=erence pattern ~o produce reflection gratings A and 30 3. Far com~ar=son) a rsflecteon gratiag was prepared from a conventional _ibre, e. e. without the boron. This comparative cra~i:~c .s _denti=eee as grating . Important rteasuramencs o:: these gratings ease Their _e bra waveguides are given '_n she .o' lowing gab? e.

~:R?TI NG ~ ~ Gtr--TT ~ GRATI NG X1 ~G B

Length I 2mm I :mm ~ 2mm RI Core I _. :02 ~ ' . =c2 ~. I. 463 pn ~ . 004 I . 004 I . 005 I ndex Modulatior, _ x 10'~ 7 x _0" 3. 4 x 10'' Gr ati ng Reflectivity 9. 5% 67% 1.2%

i0 RIC ' 25% I 18% ~ 0. 68%

Input Energy ~ o0J ( X83 ~ 192J

_.. The "RI C" i s the relative i ndex chance and it is calculated as ((index modulazion)/~n)j Y .00 (to convert to :: - percentage).
(In optical tech: ology, ref=acn_ve _ndex matching of components is ofte_~_ important to avoid unwanted reflections . from componer_t inter=aces. mhus rs=lection gratings need 20 to be refractive _ ndex-matched to adjacent components and this limits the _~eedom to adjus~ the composition to maximise the photo sensitivity and the crating properties.
It is usually easi~= to obtain index modulation in fibre which has high ~n a::d the RIC takes ~:Zis circumstance into := account ) . .
The properties of _~at~ng Z can be compared directly with grating A because both Gratings are 2mm long. The most important properti o. the orating =s reflectivity and in ~0 this key parameze= grazing ~ is ver_; much better than grating X ( 99. 5% as against _. 2% ) . = t will be appreciated that 'the 1 eng th o:: s grati ng has a s t=ong a f f ect upon i is reflectiv~.ty and t'_~_= ' onger a era ti~c (other things being eQUal) the ~e~~er _~s r°=lact_~:.-) :~ is, therefore important than ho'~. grat=ng a and :. ave the same length.
Grating E has only ~al' the 1 eng~ = hL~ i is reflectivity is .. s til'_ 07% whic__ is co.~_siderabl_- hez~er than grating :~ even though Brat=: g X ' s '_o nger. ~ha '_ndex modulations of gratings a and 3 are si::i'_ar ( 10 ~: :G-' as compared with 7 x 10'1). Gra~'_ng :: as a muclower :~odulation (0. 34 x 10~
°) which is a clear _ndicati on t hay the boron, containing IO the glasses are more p hoto sensitive. Grating 'X has a slightly higher ~n (0.005 against 0.004) so the RIC values emphasise the su~eriorit:~ of the gratings according to the invention.

Claims (7)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A photosensitive optical fibre having a photosensitive core and a cladding, said cladding consisting of a first silica glass and said core consisting of a second silica glass having a refractive index substantially 0.004 higher than said first silica glass wherein said second silica glass contains:
a) at least 30 mole% of SiO2 b) 2 - 40 mole% of GeO2 c) 2 - 40 mole% of B2O3.

2. A photosensitive optical fibre according to claim 1, wherein the second silica glass composition is a ternary composition.

3. A photosensitive optical fibre according to claim 2, wherein the second silica glass consists of substantially:
a) 57 mole% of SiO2 b) 18 mole% of GeO2 c) 25 mole% of B2O3.

4. A photosensitive optical fibre according to claim 1, wherein the first silica glass composition consists essentially of SiO2.

5. A photosensitive optical fibre according to claim 4, wherein the first silica glass composition contains combined P and/or F to reduce its melting point.

6. A photosensitive optical fibre according to claim 1, wherein the first silica glass composition consists essentially of SiO2 and B2O3 whereby the first silica glass composition is photosensitive.

7. A photosensitive optical fibre according to claim 1 wherein said cladding consists essentially of 2 -98 mole% SiO2 and 2 - 98 mole% B2O3.