Note: Descriptions are shown in the official language in which they were submitted.
r,
CA 02280472 1999-08-31
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
CA 02280472 1999-08-31
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
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~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
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° . -:,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
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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.
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?~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
3
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:
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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.
CA 02280472 1999-08-31
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.
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~: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
CA 02280472 1999-08-31
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.
