HAL Id: jpa-00249547
https://hal.archives-ouvertes.fr/jpa-00249547
Submitted on 1 Jan 1996
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
The Photorefractive Bragg Gratings in the Fibers for Telecommunications
B. Poumellec, F. Kherbouche
To cite this version:
B. Poumellec, F. Kherbouche. The Photorefractive Bragg Gratings in the Fibers for Telecommuni- cations. Journal de Physique III, EDP Sciences, 1996, 6 (12), pp.1595-1624. �10.1051/jp3:1996204�.
�jpa-00249547�
The Photorefractive Bragg Gratings in the Fibers for Telecommunications
B. Poumellec (*) and F. Kherbouche
Laboratoire CNS (**), Thermodynamique et Physico-chimie des Mat@riaux, Bitiment 415,
Universit4 Paris Sud, 91405 Orsay Cedex, France
(Received 13 March 1996, accepted 6 September 1996)
PACS.78.40.Fy Semiconductors
PACS.78.40.Ha Other nonmetallic inorganics
PACS.42.70.Ce Glasses, quartz
Abstract. This revue article describes photorefractive Bragg gratings (elaboration, inter- est, performances) which are planned to be used in optical fibers for telecommunications. This
application justifies the effort made presently to increase the photosensitivity of the fibers and the stability of the grating inscribed in them. Since Bragg gratings are based on UV induced
refractive index change, underlied mechanisms are described as long as the change of UV ab-
sorption at the origin of photo transformation of the glass. Different models invoked to describe UV induced refractive index changes are extensively presented and criticized, leading to con- clusions. The different directions of research undertaken to enhance the photosensitivity: H2 loading, other dopants, other glasses and other writing wavelengths are explicited. All this leads to a classification of Bragg gratings in terms of the UV induced mechanism involved.
Introduction
A fiber for telecommunications is a cylindrical waveguide composed by three parts if it is made by the Modified Chemical Vapor Deposition (MCVD) process: the supporting tube, the optical cladding, and the core (see Fig. I). The tube is made of synthetic pure silica, the optical cladding is fluorine and phosphorus doped silica and the core (3 6 /tm in diameter)
is made with germania-silica or alumina-silica glasses. This is for presently used fibers. The
composition may vary depending upon the role ensured by the fiber (transport, commutation, amplification) but the base will be silica for the next ten years.
In the beginning, fibers were used only for transport of visible wavelengths. As the manu-
facturing process was improved, the optical loss decreased reaching the theoretical loss defined by Rayleigh scattering and the wavelength moved into the Infra-Red. Thereafter, the main
objective was to increase the bit rate. A first improvement was obtained by modifying the geometry of the fiber, from multimode to polarization preserving single mode fiber. A sec- ond improvement was to use as many channels as possible and to make all signal treatment
functions using all-optic devices. Among these functions are switches, demultiplexers and also
(*) Author for correspondence (**) URA 446 CNRS
© Les kditions de Physique 1996
2250~m
~~~~ ~-/- j ~j_~~
&54-6~m )
-_ ~' "
ultra transparent
core for 1,5 ~m sjjjca tub~
(÷I over 50 km)
binary glass ~~~~sParent °Pt'cal cladding,
sjo2-Geo2 allows the light guiding, The
its index is 1-3 10 '~ lndeX Is lower than
higher than the one the one of the core. Doping:
of the cladding. PI FlGe:SiO ~
Fig. 1. Optical fiber components.
mirrors for amplifiers or laser sources. A number of these functions can be satisfied with the
technology of thin layers on substrates. However, in this case splicing with fibers is always
difficult and leads to additional losses. The discovery by Hill et at. in 1978 [lj of index mod-
ulation gratings in fiber cores spontaneou81y made by interference of contradirectional waves and by Meltz et at. in 1989 [2j of UV writing by side exposure of the fiber, has redesigned the
optical communication system including optical devices in including index gratings.
1. Bragg Grating Technology
1. I. THE DIFFERENT GRATING ELABORATION PROCEDURES. As remarked above, the first
discovery of photorefractivity in fibers was made by Hill using two contradirectional beams.
He was then follo~ved by others (see Tab. I). The laser wavelength laying between 488-580 nm
was launched into a Ge doped core fiber. Power densities approached few MW cm~~, the writing time wa8 around a fe~v minutes to half an hour. The reflectivity obtained was around loo% with a grating length of few tens of cm. However, in this method it is not possible to vary the grating pitch unless one varies the laser wavelength, a dramatic disadvantage for filter
applications which led to the abandon of this procedure.
All processes now use UV laser beams and side writing, the most popular one is described in Figure 2 [2j. The beam is focused onto the fiber through a pair of cylindrical lenses which
shape the beam to a cross section of15 x o.3 mm~ oriented along the length of the fiber [3j.
A second type of writing method is derived from photolithography using a phase mask made of silica glass (non-absorbing UV light). An excimer laser beam at normal incidence is
modulated spatially by a phase mask grating. The diffracted light which forms a periodic, high
contrast intensity pattern with half the phase mask grating pitch, photoimprints a refractive
index modulation into the core of a photosensitive fiber placed behind, closely and parallel to the mask [5,6j.
A third type of writing method is to tailor the grating point by point. In this case, the fiber is moved in front of the beam sequentially in order to make individually the index element
forming the grating [7,8j. This method offers the possibility to produce non-periodic and long
Table I. Specifications of grating writing.
An or
wavelength, power or Writ>ng of the of writing reflectivity Ref.
laser and glass energy density frequency pulses process and length of
the
v>sible 5 3
L = 15 cm
514 nm 14 2
514 nm 30 m>n
4 MW 30 x = I cm [12]
fiber of K O. Hill
mm
514 nm L = 53 cm
nm IA mm
5 x 10~~
nm mm
70 ml 6 x 10~~
nm s
L = 26.2 cm nm
nm s
L = 4 cm
488 nm 50 m>n
bulk mult>-
nm m>n
7 x 10~~
nm x
s
pulsed 100 ml
2~d step 690 0.16 MW 5 min -10~5
900 nm, 1.3 mm 5 min -2 x 10~~
60 min < 10~~
249 nm 14 s 2 x
nm M 50
Ge/Si02 6 ml
nm ns
133 ml
~~~~ j~ 40-120 mj 5 ns
G~'mplantation ,~
~~
s'O~
~J/Pulse/
12 ns
' 10 x 30 mm2
~
~)~ '°)~(fj~~
Mev)
ns =
248 nm 500 ml
25 1 nm
nm ns s
250 ml
nm ns s
H2 loaded fiber
30 ~~
160 ml 20 ns 5 min 2 x
H2 loaded fiber P :
nm ns m>n x
sn' Ge. si02 R = 100% L
= 15 mm
M2
, '
,
Ml, M2, M3, Reflectors
, ,
Beam spider , '
,
, Oplicaljibei
,
UV JAS£r , '
beam Ml ',
,
Cyl. L£nS ' ,
, ,
, ,
lnte~fierence padern M3
Fig. 2. Apparatus set to write gratings in the core of optical fibers (after [4]).
grating. This is useful to explore the reflection properties of the grating or to make transmission filters.
1.2. CHARACTERISTICS OF THESE TYPES OF GRATINGS
1.2.i. Bandwidth. Long gratings can be produced ranging from I mm to few cm with loo%
reflectivity. The bandwidth can be monitored and is typically 20 GHz; the minimum attained is soo MHz [13]. The optical loss is lower than o.01 dB at 1.55 /tm compared to o.5 dB for the
other processes.
1.2.2. lbnability, Sensors. A great interest of the in-fiber grating is on one hand the tun-
ability or the use as sensors since the Bragg grating wavelength is sensitive to strain and temperature and on the other hand the ease to place fiber into superstructures. The Bragg wavelength shift is due either to variations of the grating period or of the refractive index of the fiber core.
The Bragg ~vavelength shift, A~B, induced by a temperature change AT is given by [32j
A~B/lB
" (a + ()A T where a is the thermal expansion coefficient of the fiber ~~
(0.55 x
L 0T 10~6 ° C~ for silica) and ( is the thermo-optic coefficient ~~
(6.86 x 10~6 °C~~ for Ge doped
n 0T
silica). The sensitivity is around 10~~ nm °C~~ The Bragg wavelength also varies when an axial strain is applied to the fiber.
The Bragg wavelength shift induced by a longitudinal strain £ is given by [32j A~B/~B
=
(1- pe)£ where pe is an effective photoelastic constant and given by pe = y~2" lp12 v(pi1 + 2
p12)) " 0.2 where vii
" 0.113 and p12
" 0.252 are the components of the photoelastic constant
tensor, v = 0.16 is Poisson's ratio, neR is the effective index of the fiber core (values are given for fused 8ilica measured at 633 nm [33j). This leads to a sensitivity of1.2 nm/10~3 strain
which is very good. The first demonstration was made by Morey et at. [34j who showed that
two 50$lo gratings can be written with a I GHz difference simply by stretching the fiber. This
leads to imagine that it is possible to write few thousands channels over 37 nm around 1.55 /tm
(the working range of Er doped fiber amplifier).
1.2.3. Thermal Stability. The thermal stability is excellent as it is shown to be more than
one year at room temperature. The thermal resistance depends on the way the grating is written as we will see there are several microscopic processes involved depending on the power
density and the deposited energy. Nevertheless, gratings resist till 300 °C for one type and even
500 to 600 °C for other types [35,36j. Recently [31j, a resistance up to 800 °C was found in
a Sn/Ge/Si02 core fiber. Erdogan et at. [125j show experimentally that medium temperature annealing (several hundreds °C) strengthens the thermal stability by suppressing the weakly
stable part of the grating. They have studied the temperature and time dependence of the
erasure and found that the refractive index followed the law ~~)
~~ , where Ano, n, To are
I + i~t °
constant, T and t are respectively temperature and time. This behavior seems to apply for a great number of cases.
In addition the gratings can be thermally erased at 900 °C and rewritten with the same photosensitivity [37j.
2. Mechanisms
In order to obtain reliability of telecommunication devices over 25 years and to optimize the quality, a deep understanding is required. We list below the different experimental observations which lead us to point out several mechanisms. Then, in the discussion part, we list the possible mechanisms which can be considered to explain the refractive index change and give
conclusions.
2.I. CLAssificATioN oF MECHANISMS ACCORDING To KiNETics. We will describe here
only the time behavior for UV written Bragg gratings without making a distinction between pulsed and CW lasers since Niay et al. showed that they are similar [39j.
Typical results are shown in Figure 3 [40j for an energy density lower than 300 mJ cm~~
The laser used in this case is 244 nm, 12 ns, lo Hz, the observations are made at the Bragg wavelength. For the first pulses, we observe a fast decrease of the transmission of the fiber
corresponding to an increase of reflectivity of the Bragg grating of about 10%. Then, the transmission goes on to decrease more slowly to reach 0% (reflectivity equal to 100% ). However,
if we go on with the irradiation, the transmission then increases, indicating the disappearance
of the index modulation. For a fluence corresponding to the maximum transmission, there is no longer index modulation. Beyond this threshold, another index grating appears. This
complex behavior has been seen in a lot of different fibers so it can be concluded that it does not arise from instability in the writing process.
As it is easier to speak in term of index change than in terms of Bragg wavelength shift and
reflectivity, we define below the mean index and the index modulation of the index grating.
Let's assume the following UV intensity distribution:
1(z,t) = IoIi + V cos 2irz/A) 0 < z < L,
where V is the visibility of the interference pattern, L is the length of the grating and A is the period of the interference pattern. An(z) the index change along the fiber axis can be
expanded up to the three first terms of the Fourier series expansion:
An(z) = Ano + Ani cos(2irz IA + Pi + An2 cos(4irz IA + fl2), 0 < z < L
An(z)
= 0, z > L or z > 0.
. . Hocnce=lMmJlcm2
3 . ~mda=1250 -
O-B mda=12S0
n~ & .
~ ~
*
( O.6 .
.
~ ~
~ .
DA
p~ .
g .
3 ~
0.2 ~
.
.~&
_ .
.- 3 .
0 . O
6o~
mber
o-s
,,a---±---o,, _-o
-e-
DA ,' ~~
§,
'~ ~'~ fl,
~ ~
, Gmdne G> FL= U0wJi<w
~' G2 Gmme G2 FL=330wJi<w2
~
~'
0.2 ~~"""~~"'~"" "~ ~"~~"~~°"~
o-i
o
0 30000 6D000 90000 12000D 150D00
NumberofpuJses
Fig. 4. Bragg wavelength shift in the same fiber than the one use in Figure 3 as a number of pulses (after [40]).
index modulation follows a power law:
r~J t" where a
= 0.44. The Bragg wavelength increases
following a similar law with a
= 0.38 [41j. Xie et at. [42j found 0.3 < a < 0.5 for a pulsed laser
~vith an energy density around 140 240 mJ cm~~ in different types of germanosilicate optical fiber preforms. For a CW laser, a is 0.42 0.52 for power ranging from 18 to 58 W cm~~
Observations performed by Patrick et at. [43j lead to the conclusion that a increases with power
density: 0.25 < a < 0.29 for power ranging 3 46 W m~~ We
can conclude that the previous law of time dependence during writing a type I grating is usually followed but the exponent is
rather spread, from 0.25 to o-s- It can be noted that a stretched exponential leads sometimes to a better fit [44j. Furthermore, if the power density is increased the refractive index increases
dramatically even with a single pulse, we will call this last type: IIb [4, 6, 26,45-49j.
2.2. CHANGE IN ABSORPTION AND FLUORESCENCE SPECTRA. The absorption spectrum
in the UV range is shown in the Figure 5. We find that an absorption band exists in the
vicinity of the laser wavelength (240 nm). This band has been extensively studied [17,50-63j
and there is a common opinion that it arises from an oxygen vacancy in the Germania silica network. Irradiation in th.is band causes the production of UV at ~ = 290 nm [64j and blue luminescence at 400 nm [44,65j. In several papers, we find that this absorption band is at least twofold [61,66j. In the course of irradiation, the absorption spectrum changes as is shown in Figure 6 [37j. The 240 nm band decreases together with the fluorescence intensity. At the
same time, the absorption increases both for longer and shorter wavelengths (see Fig. 6).
The time dependent spectrum of an optical fiber core shows that changes are fast and large
at the beginning, then, they are much slower and weaker [36j. Duval et al. [65j have shown that An is proportional to the fluorescence change which is associated with the 240 nm absorption at the beginning but then the photoluminescence levels off whereas An goes on increasing. Also, Mizrahi et at. saw a constant fluorescence whereas An and absorption decreased together [67j.
Several structural defects are thus to be considered [61j.
laser wavelength
~~
( j
1
,
300 ',
before
~ ',
g~ 200
after
~
~~~
o
ioo
190 210 230 250 270 290
wavelength (nm)
Fig. 5. Bleaching of the 240 nm absorption band by 248 nm irradiation (after [50]).
changing in absorption, dB/mm
4 5 6 7
energy (eV) Gaunhncomponmtsoffit
sum of Gausdancompon©ntS
Fig. 6. Six Gaussian fit of the UV induced absorption change (after [37]).
2.3. BIREFRINGENCE VARIATION. Beside the change of birefringence due to the change
of the geometric birefringence because of change of the refractive index, and change of stress induced birefringence, there can be a change of material birefringence [68j. Up to now, there have been several papers on birefringence production or change but from two visible photon
absorption [10,14,15,69j. The birefringence can be as much as 10~~
Poirier shows for the UV induced grating that the defects aligned with the laser is preferably bleached [44,70j. Such an effect was already analysed in reference [71j to explain second harmonic generation in glasses.
hV
defect °~ absorption bleaching
electron migration ionic migration
~~~~~~ ~~~~~~ $ili~$I ~~~~~~c~~~~~~~~~~
mo i ica ion
Fig. 7. Possible transformations in the glass after absorption of a photons.
3. Discussion
With the progress on lasers and non-linear crystals, the use of high power UV lasers has become
common. These lasers allow the production of a structural modification in solid state matter.
One of the first common effects is ablation: a surface of a substrate irradiated in vacuum with UV laser pulse of1 J cm~~ under an angle of 10° is vaporised. The interaction of a laser beam with the electronic structure of solids is complex, especially it implies a lot of non-
linear effects when it leads to partial fusion. If the power density is decreased, these effects disappear and the interaction becomes "finer", it becomes mainly linear. This is the case if the laser wavelength lies in a point defect absorption band. It corresponds to local excitation of the absorbing species which can then transform. The first effect observed is a bleaching of the absorption band but it is followed by a series of reactions. Some of them are described in Figure 7. In insulating refractory materials, the UV irradiation produces migration. The
electronic ones lead to the formation of a very strong electronic field: 106 V m~I The ionic or radical ones contribute to modify the atomic arrangement: production of other defects, change of coordination, molecular rotation. From this point of view, the glass with its long range
disorder but short range order is a suitable structure for allowing these transformations.
3.I. THE COLOR CENTER MODEL. As soon as the evidence was provided that the UV
absorption spectrum changes greatly under irradiation, a model based on color center popu- lation variations was proposed [72j. This model was derived from work on second harmonic generation in fibers. The idea is that under UV, Oxygen Deficient Centre (ODC), (probably
an oxygen vacancy [73j transforms into other defects like E' centre (see Figs. 8). Wrong bonds
or ODC absorbs at 240 nm, the E' centre near 200 nm and others called tentatively Ge(1)
and Ge(2) around 280 nm [74j. Investigations in the deep UV [75j showed also another strong absorption at 165 nm. Starting from these observations, various authors computed the refrac- tive index change from the absorption spectrum change with the help of the Kramers-Kronig transformation [76j. The Kramers-Kronig transformation (KK) is based on physical proper-
ties that complex refractive index satisfies [77j. From the physical basis, both quantities (real
refractive index and absorption) arise from the same microscopic properties of the matter that
is the reason why they are connected.
We have the following relationship:
n(~) = 1+ P /~ ~~~~~'~
~
dk.
~r o jkj
~
