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The effect of irradiation temperature on the optical attenuation recovery in heavily Ge-doped single mode silica core fibers
Nuclear
Instruments
and Methods
in Physics Research
Nuclear Instruments 8 Methods in Physics Research SC< fIl,rJ 6
R6S (IYYZ) 402-104
North-Holland
The effect of irradiation temperature on the optical attenuation recovery in heavily Ge-doped single mode silica core fibers M. Bertolotti
“, M.A. Mabrouk
The hehaviour km
under
induced
under irradiation
different
irradiation
attenuation
has been
appropriate
“, A. Ferrari
‘, A. Serra “ and G. Viezzoli
of a ainglc mode fiber heavily doped with germanium
temperatures descrihrcl
in the range from
using empirical
equation\,
Optical
t’or different
has hcen investigated time hehaviour
tenqxratureh. at different
An
,Ith
at 0.X5 km and
of the recovery order
kinetics
1.3
of the seems
temperatures.
and after irradiation was measured on-line in order to obtain the induced attenuation and the suhscqucnt recovery. The optical power propagating in the fiber was about 250 and 90 nW for 0.85 and 1.3 km mcasurcmcnts rcspcctivcly, at the begining of the irradiation. The precision of the loss measurement was 0.02 and 0.007 dB l’or high and low values of the rcspcctivcly. Many measurcmcnts wcrc attenuation. pcrformcd bcforc irradiation test5 to ensure system stability. Repeated measurements during and after y-irradintion have high reproducibility. during
l’ibcr
wavcguidcs communication
arc
widely
applied
bccausc of their
wry
for low
attenuation losses as low as 0.12 dB/Km. Howcvcr when exposed to nuclear radiation silica fibers suffer a high increase in optical attenuation dcpcnding on various paramctcrs [i-3] as irradiation dose. doping and temperature (in both low and high H content fibers). The radiation-induced darkening arises as a result of dcfcct gcncration. In thia work the nature of radiation-induced dcfccts in G-doped silica optical fibers has been investigated pcrforming a low tcmpcraturc irradiation and mcasuring the optical induced attenuation during the irradiation and thermal treatment at two different wavelengths 0.X5 and 1.3 km.
2. Experimental The samples wcrc single mode silica fibers heavily doped with germanium, 60 m long, wound on a reel 22 cm in diameter. Two fiber rumples were irradiated at the same time using a “OCO source up to a dose of 52.6 Gy at a dose rate of 32.X Gy/min. The irradiation tests wc’rc pcrformcd on unirradiatcd fibers at diffcrcnt tempcraturcs in the range - 65 o C to + 60 o C and at 0.X5 and 1.3 )*rn wavelengths. A special apparatus was used to obtain ;I stable and automatically controlled temperature. The mcasurcmcnt setup consisted of two sources of 0.85 and 1.3 )*rn wavelength rcspcctivcly, and ;I twochannel powcrmctcr. The optical power transmitted ol~x-sx~x/Y2/sos.oo
o C to 60” C. The
to describe the results and the order of kinetics has hcen dctermincd
1. Introduction
long-distance
-65
’
(“
1YY2
Elsrvier
Science Puhlishrrs
3. Results and discussion The growth of the radiation-induced attenuation at 0.X5 and I.3 IJ-m versus total dose in the fibers is shown in figs. la and lb rcspcctively, for different tempcratura. in the range bctwccn -65°C and +6O”C. A simple inspection shows that the sensitivity to radiation of optical fibers increases with the decreasing of the temperature. The dose depcndenccs show identical behaviour ah :I function of tcmpcraturc when 1.3 and 0.X5 km light sources arc used during y-irradiation. This indicate that the origin of the radiation-induced dcfccts which arc rcsponsihlc for attenuation losses is the same for both wavclcngths but the value of the dcfccts concentration is diffcrcnt (the transient attcnuation measured when using 0.85 pm is about 6 times higher than using I.3 Fm). Thus y-irradiation seems to crcatc the same type of radiation-induced defects in optical fibers probed with I.3 or 0.X5 km light, contemporarily with y-irradiation.
I3.V. All rights reserved
403
Time
Fig. 1. Growth germanium.
tsec)
of the r~idi~tion-induced
The irradiation VU
temperatures
32.8 Gy/min.
~~ttenu~ti[~n at I.3 Km (a) and 0.85 urn fhf in a single mode fiber were
~ 65 3 C (11, - 30 0 C (2).
1300
heavily doped with
+ 30 o C (4) and +hO o C (Sk The dose rate
The first part of the curves was affected hy the increasing of the dose rate.
The recovery after irradiation of the induced attenuation at 0.85 and 1.3 urn is shown in figs. 2a and 2b. It is apparent that cvcn at low temperature the induced attenuation at 0.85 f&m recovers faster than at 1.3 urn. Moreover the induced attenuation at 0.85 pm recovers practically completely even at - 65 o C, while at 1.3 urn a residual damage is cvidcnt at low temperature. The time hehaviour of the rccovcry curves of fig. 2 is rather complex. In the litcraturc there is not yet a definite model to describe this bchaviour nor a clear expIanat~on of the mechanisms responsible for the recovery, so we have used several empirical models which arc listed below. (a) a sum of two time exponentials;
Waveleoqth
- 5 n C (3).
nm
ii
(b) a stretched exponential relation suggested by Jackson and Kalios [4] for a dispersive diffusion model of the form n(r)=.4
exp(-(Bt)“),
where A(l) is the measured attenuation as a function of time, and B and C are suitable parameters: (c) a model suggested by Garlick et al. [S] of the form ln(~(~))=,~(ln(f+~)+~), whcrc A = l/(1 ~ n) with IZ b&g the order of reaction and R and C are quantities depending on the order of reaction. activation energy and temperature;
rj Wavelength
850
“In
160
ONt 120
Fig. 2. Recovery
of the radiation-induced
germanium.
attenuation
-
at 1.3 em (a) and 0.85 pm (b) in a single mode fiber heavily doped with
following an exposure of 52.5 Gy at - 65 ’ C (I).
- 30 ‘C (2). - 5 o C (3). + 30 OC (4) and + 60 ” C (5).
VI. GLASSES
M. Bertolotti
404
Table I Standard
deviation
for different
et al. / Effect
models
of irradiation
on optical
attenuation
recol,ery
for 1.3 km and 0.85 km radiation Model C
II
Model D
12
0.035!,
0.0557
0.029 I 0.0597 0.1347 0.2101
0.0662 0.0120 0.0080 0.0061
16.6 3.9 4.3 4.4 4.x
0.0248 0.0261 0.02 19 0.0629 0.0579
1.9 5.8 4.1 4.0 5.5
_
5.5
0.0660 0.0659 0.3320 0.3668
2.2 3.2 2.5 3.0
Mddel A
Model B
+ 60
0.0262
+30 -S - 30 ~ 65
0.0488 0.0959 0.1243 0.4349
+ 60
0.0361
O.OY60
_
+30
0.0240
05252
0.0557
-5 -30 -65
0.6181 1.2405 3.3314
0.4540 1.1391 l.YllS
0.0183 0.0076 0.01n1
T [“Cl
temperature
1.3 11,~~radiation
0.85 Pam radiation
(d) a model which
/4(t) = (A,,-A,)(1
proposed
by Friebele
et al. [6] for
+Ct)‘~“‘“~“‘+R,,
where C = (2(“-‘) - l)/~, and A,,, A,, the half-life 7. and the order kinetics II are adjustable paramctcrs. All the curves fitted reasonably well with the applied formula except the curve at +60” C, however each model gave a different standard deviation for the fitting. Table 1 reports the standard deviations at different temperatures for the diffcrcnt models and the two wavelengths. One can see that at low temperature the Garlick et al. [5] model seems to fit better the data, while at high temperature the Friebele model seems preferable. The order of kinetics which arc derived from these models arc shown in the table. It can be seen that in the low temperature range from ~ 65 to -5 o C the order of kinetics at 1.3 km dccrcascs slowly from 4.X to 4.3. It than suddenly drops at less than 2 at the high temperature of +60 o C. The results at +30 o C are fitted bcttcr by the Fricbele model with a high kinetics order of 5.8 but could also bc fitted by the Garlick model with a somehow higher standard deviation but a more reasonable order of kinetics around 4. For the 0.85 urn radiation, all points arc best fitted with the Garlick model from -65 to +30 “C with an order kinetics which shows a reverse behaviour increasing from 2.7 at ~ 65 o C up to nearly 4.3 at - 5 ’ C. The point at +30 o C could bc fitted with the Friebele
_
II.7 4.3 3.4 2.7
model with n = 2.2 and the point at higher tcmpcrature is better fitted by two simple exponentials. These behaviours from one side confirm the validity of a model of nth order kinetics to fit the annealing results but from the other side do not allow for the moment to draw any precise conclusion about the operating physical mechanisms. The circumstance that the order kinetics has different behaviours for the two wavelengths seems to indicate that diffcrcnt mcchanisms are operating in the two cases, according also with the general behaviour shown in fig. 2. One possibility, which needs however confirmation is that in the cast of the 0.85 Frn wavelength the intensity of the light used to make the measurements is high enough to produce photo-assisted recovery of the damage through some sort of photobleaching effect. More research is however needed to confirm this hypothesis.
References [I] E.J. Friehele, CC. Askins, M.E. Gingerich and K.J. Long, Nucl. Instr. and Meth. Bl (lY84) 355. [2] D.L. Griscom, J. Non-Cryst. Solids 73 (1985) 51. [3] E.J. Friehele and M.E. Gingerich, J. Non-Cryst. Solids 38-39 (1980) 45. [4] W.B. Jackson and J. Kakalios, Phys. Rev. B37 (1988) 1020. [S] G.F.J. Garlick. J.E. Nicholls and A.M. Ozer, J. Phys. C4 (1971) 2230. [6] E.J. Friebele, L.A. Bzambani. M.E. Gingerich. S.I. Hickey and J.R. Onstott, Appl. Opt. 2X (1989) 5138.
Instruments
and Methods
in Physics Research
Nuclear Instruments 8 Methods in Physics Research SC< fIl,rJ 6
R6S (IYYZ) 402-104
North-Holland
The effect of irradiation temperature on the optical attenuation recovery in heavily Ge-doped single mode silica core fibers M. Bertolotti
“, M.A. Mabrouk
The hehaviour km
under
induced
under irradiation
different
irradiation
attenuation
has been
appropriate
“, A. Ferrari
‘, A. Serra “ and G. Viezzoli
of a ainglc mode fiber heavily doped with germanium
temperatures descrihrcl
in the range from
using empirical
equation\,
Optical
t’or different
has hcen investigated time hehaviour
tenqxratureh. at different
An
,Ith
at 0.X5 km and
of the recovery order
kinetics
1.3
of the seems
temperatures.
and after irradiation was measured on-line in order to obtain the induced attenuation and the suhscqucnt recovery. The optical power propagating in the fiber was about 250 and 90 nW for 0.85 and 1.3 km mcasurcmcnts rcspcctivcly, at the begining of the irradiation. The precision of the loss measurement was 0.02 and 0.007 dB l’or high and low values of the rcspcctivcly. Many measurcmcnts wcrc attenuation. pcrformcd bcforc irradiation test5 to ensure system stability. Repeated measurements during and after y-irradintion have high reproducibility. during
l’ibcr
wavcguidcs communication
arc
widely
applied
bccausc of their
wry
for low
attenuation losses as low as 0.12 dB/Km. Howcvcr when exposed to nuclear radiation silica fibers suffer a high increase in optical attenuation dcpcnding on various paramctcrs [i-3] as irradiation dose. doping and temperature (in both low and high H content fibers). The radiation-induced darkening arises as a result of dcfcct gcncration. In thia work the nature of radiation-induced dcfccts in G-doped silica optical fibers has been investigated pcrforming a low tcmpcraturc irradiation and mcasuring the optical induced attenuation during the irradiation and thermal treatment at two different wavelengths 0.X5 and 1.3 km.
2. Experimental The samples wcrc single mode silica fibers heavily doped with germanium, 60 m long, wound on a reel 22 cm in diameter. Two fiber rumples were irradiated at the same time using a “OCO source up to a dose of 52.6 Gy at a dose rate of 32.X Gy/min. The irradiation tests wc’rc pcrformcd on unirradiatcd fibers at diffcrcnt tempcraturcs in the range - 65 o C to + 60 o C and at 0.X5 and 1.3 )*rn wavelengths. A special apparatus was used to obtain ;I stable and automatically controlled temperature. The mcasurcmcnt setup consisted of two sources of 0.85 and 1.3 )*rn wavelength rcspcctivcly, and ;I twochannel powcrmctcr. The optical power transmitted ol~x-sx~x/Y2/sos.oo
o C to 60” C. The
to describe the results and the order of kinetics has hcen dctermincd
1. Introduction
long-distance
-65
’
(“
1YY2
Elsrvier
Science Puhlishrrs
3. Results and discussion The growth of the radiation-induced attenuation at 0.X5 and I.3 IJ-m versus total dose in the fibers is shown in figs. la and lb rcspcctively, for different tempcratura. in the range bctwccn -65°C and +6O”C. A simple inspection shows that the sensitivity to radiation of optical fibers increases with the decreasing of the temperature. The dose depcndenccs show identical behaviour ah :I function of tcmpcraturc when 1.3 and 0.X5 km light sources arc used during y-irradiation. This indicate that the origin of the radiation-induced dcfccts which arc rcsponsihlc for attenuation losses is the same for both wavclcngths but the value of the dcfccts concentration is diffcrcnt (the transient attcnuation measured when using 0.85 pm is about 6 times higher than using I.3 Fm). Thus y-irradiation seems to crcatc the same type of radiation-induced defects in optical fibers probed with I.3 or 0.X5 km light, contemporarily with y-irradiation.
I3.V. All rights reserved
403
Time
Fig. 1. Growth germanium.
tsec)
of the r~idi~tion-induced
The irradiation VU
temperatures
32.8 Gy/min.
~~ttenu~ti[~n at I.3 Km (a) and 0.85 urn fhf in a single mode fiber were
~ 65 3 C (11, - 30 0 C (2).
1300
heavily doped with
+ 30 o C (4) and +hO o C (Sk The dose rate
The first part of the curves was affected hy the increasing of the dose rate.
The recovery after irradiation of the induced attenuation at 0.85 and 1.3 urn is shown in figs. 2a and 2b. It is apparent that cvcn at low temperature the induced attenuation at 0.85 f&m recovers faster than at 1.3 urn. Moreover the induced attenuation at 0.85 pm recovers practically completely even at - 65 o C, while at 1.3 urn a residual damage is cvidcnt at low temperature. The time hehaviour of the rccovcry curves of fig. 2 is rather complex. In the litcraturc there is not yet a definite model to describe this bchaviour nor a clear expIanat~on of the mechanisms responsible for the recovery, so we have used several empirical models which arc listed below. (a) a sum of two time exponentials;
Waveleoqth
- 5 n C (3).
nm
ii
(b) a stretched exponential relation suggested by Jackson and Kalios [4] for a dispersive diffusion model of the form n(r)=.4
exp(-(Bt)“),
where A(l) is the measured attenuation as a function of time, and B and C are suitable parameters: (c) a model suggested by Garlick et al. [S] of the form ln(~(~))=,~(ln(f+~)+~), whcrc A = l/(1 ~ n) with IZ b&g the order of reaction and R and C are quantities depending on the order of reaction. activation energy and temperature;
rj Wavelength
850
“In
160
ONt 120
Fig. 2. Recovery
of the radiation-induced
germanium.
attenuation
-
at 1.3 em (a) and 0.85 pm (b) in a single mode fiber heavily doped with
following an exposure of 52.5 Gy at - 65 ’ C (I).
- 30 ‘C (2). - 5 o C (3). + 30 OC (4) and + 60 ” C (5).
VI. GLASSES
M. Bertolotti
404
Table I Standard
deviation
for different
et al. / Effect
models
of irradiation
on optical
attenuation
recol,ery
for 1.3 km and 0.85 km radiation Model C
II
Model D
12
0.035!,
0.0557
0.029 I 0.0597 0.1347 0.2101
0.0662 0.0120 0.0080 0.0061
16.6 3.9 4.3 4.4 4.x
0.0248 0.0261 0.02 19 0.0629 0.0579
1.9 5.8 4.1 4.0 5.5
_
5.5
0.0660 0.0659 0.3320 0.3668
2.2 3.2 2.5 3.0
Mddel A
Model B
+ 60
0.0262
+30 -S - 30 ~ 65
0.0488 0.0959 0.1243 0.4349
+ 60
0.0361
O.OY60
_
+30
0.0240
05252
0.0557
-5 -30 -65
0.6181 1.2405 3.3314
0.4540 1.1391 l.YllS
0.0183 0.0076 0.01n1
T [“Cl
temperature
1.3 11,~~radiation
0.85 Pam radiation
(d) a model which
/4(t) = (A,,-A,)(1
proposed
by Friebele
et al. [6] for
+Ct)‘~“‘“~“‘+R,,
where C = (2(“-‘) - l)/~, and A,,, A,, the half-life 7. and the order kinetics II are adjustable paramctcrs. All the curves fitted reasonably well with the applied formula except the curve at +60” C, however each model gave a different standard deviation for the fitting. Table 1 reports the standard deviations at different temperatures for the diffcrcnt models and the two wavelengths. One can see that at low temperature the Garlick et al. [5] model seems to fit better the data, while at high temperature the Friebele model seems preferable. The order of kinetics which arc derived from these models arc shown in the table. It can be seen that in the low temperature range from ~ 65 to -5 o C the order of kinetics at 1.3 km dccrcascs slowly from 4.X to 4.3. It than suddenly drops at less than 2 at the high temperature of +60 o C. The results at +30 o C are fitted bcttcr by the Fricbele model with a high kinetics order of 5.8 but could also bc fitted by the Garlick model with a somehow higher standard deviation but a more reasonable order of kinetics around 4. For the 0.85 urn radiation, all points arc best fitted with the Garlick model from -65 to +30 “C with an order kinetics which shows a reverse behaviour increasing from 2.7 at ~ 65 o C up to nearly 4.3 at - 5 ’ C. The point at +30 o C could bc fitted with the Friebele
_
II.7 4.3 3.4 2.7
model with n = 2.2 and the point at higher tcmpcrature is better fitted by two simple exponentials. These behaviours from one side confirm the validity of a model of nth order kinetics to fit the annealing results but from the other side do not allow for the moment to draw any precise conclusion about the operating physical mechanisms. The circumstance that the order kinetics has different behaviours for the two wavelengths seems to indicate that diffcrcnt mcchanisms are operating in the two cases, according also with the general behaviour shown in fig. 2. One possibility, which needs however confirmation is that in the cast of the 0.85 Frn wavelength the intensity of the light used to make the measurements is high enough to produce photo-assisted recovery of the damage through some sort of photobleaching effect. More research is however needed to confirm this hypothesis.
References [I] E.J. Friehele, CC. Askins, M.E. Gingerich and K.J. Long, Nucl. Instr. and Meth. Bl (lY84) 355. [2] D.L. Griscom, J. Non-Cryst. Solids 73 (1985) 51. [3] E.J. Friehele and M.E. Gingerich, J. Non-Cryst. Solids 38-39 (1980) 45. [4] W.B. Jackson and J. Kakalios, Phys. Rev. B37 (1988) 1020. [S] G.F.J. Garlick. J.E. Nicholls and A.M. Ozer, J. Phys. C4 (1971) 2230. [6] E.J. Friebele, L.A. Bzambani. M.E. Gingerich. S.I. Hickey and J.R. Onstott, Appl. Opt. 2X (1989) 5138.