Aluminum Doped Silica Preform Fabrication using MCVD and Solution Doping Technique: Effects of Various Aluminum Solution Concentrations. S.M. Aljamimi, Z.Yusoff, and H.A.
Khairul Anuar M. S, S.Z. Muhd
N. Tamchek
Abdul-Rashid
Yasin, M.1. Zulkifli, Hanif S.
Faculty of Science
Advance Physical Technologies
Universiti Putra Malaysia, Serdang, Selangor, Malaysia
[email protected]
Faculty of Engineering Multimedia University, Cyberjaya, Selangor, Malaysia
Telekom Research and Development Cyberjaya, Selangor Malaysia
[email protected]
[email protected]
Abstract-
This work is described for solution doping in
Modified Chemical Vapor Deposition (MCVD) used for silica optical
fiber
fabrication.
This
paper
will
concentrate on
aluminum solution doping and the effect of different solution concentrations. The effect of three different concentrations of aluminum (O.3M,O.7M and 1.2M) with the soot undergo heat treatment are studied while the other parameters of MCVD
In MCVD process, precursors such as SiC4 are delivered to quartz substrate tube (through bubbling technique) and these mixtures are oxidized by high temperature (1IOO-21000C). This process creates white powder-like soot on the tube wall. Equation 1 shows the oxidation of SiCl4 [5].
SiC14 (g) + O2 (g)-7 Si02 (5) + Ch (g)
and solution doping are fixed such as deposition temperature, SiCI4 flow, and soaking time. The refractive index profile(RIP) of each doped preform is measured using preform analyzer to investigate aluminum distribution in the core region. Further investigation about Al distribution across the core sintered layer is also examined by EDX techniques.
Keywords-MCVD; solution doping technique; refractive index profile; EPMA (SEM-EDX) 1.
INTRODUCTION
Incorporation of rare earth ions in silica has been studied since 1964 after Koester and Snitzer published the paper on neodymium fiber laser [1]. Generally they are two methods of incorporating aluminum or rare earth in silica matrix using MCVD i.e. vapor phase or non-vapor phase. The vapor phase method is usually much complex due to the fact that desired precursor of dopant (e.g. rare earth chlorides (RECl3)) have relatively low vapor pressure compared to the standard precursor of MCVD at moderate temperature. The second method is non-vapor process i.e. liquid phase solution doping process. This technique first reported by Stone and Burrus et al. [2] and further improved by Pool et al. [3]. In RE doped silica, Ah03 is the most effective dopant as index raiser due to the process simplicity compared to Ge02, the standard index raiser dopant in MCVD. Other reasons include the ability of Al atoms to dilute the RE in silica matrix , high vapor pressure of Al203 and less evaporation compared to Ge02 during process. [4] MCVD with solution doping is a proven technique to fabricate RE doped fiber with example of EDFA success story in telecommunication industry. However, some improvements are still needed to fabricate specialty optical fibers for certain applications like fiber lasers and fiber sensors which have complex designs (material and optical) .For example, high power fiber lasers require high doping concentration and also large core area.
For solution process (soaking or impregnation) there are more parameters which play significant role for example type of solvent, concentration of solution, composition (AliRE ratio) [6], soaking time [7], soaking temperature [8,9], drying process and oxidation temperature. Sintering and collapsing processes will follow the soaking process to complete the process sequence. In this paper we study the effect of different solution concentrations in solution doping process for Ah03 doped silica preform and relative Ah03 distribution around the core of the perform. Other parameters such as deposition temperature, SiCl4 flow, soaking time and drying time are set constant. Three types of concentrations are used in this study. The fabricated preforms are then analyzed using refractive index profiler. Besides that, three techniques of energy dispersive x-ray spectrometry EDX are also performed which are mapping, line scan and point&id to investigate Al distribution across the core layer. Point&id technique is used to scan aluminum content (wt %) point by point along the core diameter. II.
EXPERIMENTAL PROCEDURES
Three Ah03 doped silica preforms were prepared using MCVD with solution doping process. A high quality silica tube (F300) has an inner diameter of 19 and outer 25 mm,was mounted on the lathe as the initial step in the fabrication process. The quartz tubes were cleaned (prior the deposition) in glass working lathe using sulfur hexafluoride SF6 at 1600°C.The fabrication process was followed by deposition of silicon tetra-chloride (SiCI4) and oxygen (02) at high temperature (2120°C) by slowly moving oxygen hydrogen burner (llOmmlmin) in the same direction of reactants flow and fully sintered into transparent glass which make the cladding layer. This process was followed by the deposition of porous core layer (soot) for 40cm in length which consists of the same mixture of SiCl4 and O2 with the same burner speed but at a lower deposition temperature of 1800°C.
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(1)
The obtained performs are scanned using perform analyzer to investigate the refractive index profile for every preform of the intended solutions. Three samples of the collapsed preforms that contain different Al concentration were sent for SEMIEDX to measure aluminum distribution within the core layer quantitatively and qualitatively. III. RESULT AND DISCUSSION
Fig.1: Vertical solution doping apparatus.
Ethanolic solutions were prepared using aluminum chloride hexahydrate (AICI3.6H20) with three concentrations ranged from low concentration (0.3M), middle concentration (0.7M) and high concentration (1.2M).The substrate tube was then removed from the lathe carefully and put vertically in the solution doping apparatus shown in Fig.l for soaking time of 90 minutes. The solution was drained out slowly with a flow rate of 0.08 cm3/min to avoid the soot from peeling off. After soaking, the soot layer was dried for 30 minutes with slow flow of nitrogen stream (O.Sbar) at room temperature to dry the soot network. After solution soaking and drying with nitrogen, the soot was exposed to heat treatment in a furnace, with a ramp up rate of SOC min-l up to 600°C before being cooled to room temperature. Then, the substrate tube was remounted on the lathe and MCVD process is preceded. The soot is subjected to oxidation process at temperature of lS000C with oxygen flow at 1000sccm which is very important to convert the chlorides base to oxides. Several sintering stages were implemented for the porous layer in the presence of oxygen and helium at temperature ranged from 18000C to 20000C and fully sintered at high temperature 2200oC. The temperature in sintering process was increased gradually to ensure the soot is sintered to fine transparent glass. Finally the collapse process was performed at 22000C to obtain the solid preform. Table below is the summary of fabrication parameters. TABLE1 : SUMMARY OF FABRICATION PARAMETERS Preforms fabricated
MCVD+SD parameters Cladding deposition Temp Core deposition
PI
P3
P2
° 2120 C
0 2120 c
0 2120 c
I 800°C
° I 800 C
I 800°C
The refractive index profile (RIP) for each perform is scanned using perform analyzer PK104. Aluminum oxide has a higher refractive index than silica, so the refractive index profile (RIP) across the perform core reflects the aluminum distribution across the deposited soot. The index difference was found uniform without central dip due to the strong retention of Si02 with Al203 during sintering process which prevents Al203 from evaporating compared to Ge02 which evaporates during sintering process [13]. Refractive index difference of Ah03 measured along the collapsed preform length at two radial angles 0 degrees and 90 degrees for each preform is shown in Fig.2.
� ;;:::
70sccm
70sccm o
e
a... x "0 C
0.006 0.005 0.004
>
0.003
�
0.002
t5 cr
0.001 0.000 -0.001
Fig.2: Refractive index difference of
As observed from the Fig.2, when the solution concentration is increased the proportion of aluminum refractive index profile increases appreciably. Different concentration of Al solution shows distinct increment of RIP in each preform. The radial RIP of Al203 has slight variation when RIP was scanned at 0 degree and 90 degree. This variation could be due to the ellipticity of the core layer during the sintering and collapsing process.
Solution
I 900-2 I oo c
I 900-2 I oo c
I 900-2 Ioo c
0 2200 c
0 2200 c
0 2200 c
90mints
90mints
90mints
0.3M
0.7M
I.2M
[ ]
N2 drying
30mints
30mints
30mints
° 600 C
° 600 C
o 6OQ C
Oxidation temp
I 500°C
0 1500 c
I 500°C
Oxidation 02 flow
lOOOsccm
lOOOsccm
lOOOsccm
Furnace
drying
Temp.
Ah03 doped silica preform with
different Al concentrations and different radial angles.
Average Index Difference (I'm).
concentration
o
Sintering Temp
Solution M
-3
TABLE2 : SUMMARY OF SCANNED RIP
Col1apse Temp. Soaking time
-6
Radial position (mm)
70sccm o
--
0.007
Temp. Sic14 flow
Delta-n(O.3M),Odeg Delta-n(O.3M),90deg -- Delta-n(O.7M),Odeg -- Delta-n(O.7M),90deg -- Delta-n(1.2M),Odeg -- Delta-n 1.2M ,90de --
0.008
Odegree
90 degree
O.3M
O.OO18±O.OO2
O.OO12±O.OOO4
O.7M
O.OO28±O.OOO2
O.OO30±O.OOO3
1.2M
O.OO55±O.OOO4
O.OO64±O.OOO4
There is slight vanatlOn for Al203 index difference noticed along the deposited length and in the radial area.
269
Delta-o(t ,2M),OOdeg Delta-o(t,2M),Odeg Delta-o(Q,7M),OOdeg - Delta-o(Q,7M),Odeg - Delta-o(Q,3M),OOdeg - Delta-oiQ,3Mi,Odeo -
Mapping technique was performed to show the basic idea about how aluminum is distributed within the core layer. It can be noticed from the mapping technique, the aluminum distribution is denser for 1.2M sample than 0.7M.The aluminum spreads around the core region non uniformly, and this explains the variation in the refractive index profile.
n (\
"
�
0
Radial oosition (mm) Fig. 3(a): Core diameter changes as aluminum concentration increases.
\.2
�
:a �
!.
" ," . ' ,
.
.
Imm
AI distribution in the preform core
E'1.1 31.0 OJ
.
Imm
\,3
... "
® .
with (0. 7M) concentration
AI distribution in the preform core
with (1 ,2M) concentration
I
Fig. 4: Mapping technique of AI samples (The circle is the core of the
0.9
perform)
0.8 0.7
o U 0.6
0.4
0,6
0,8
Al concentration
[M]
Fig. 3(b): Core diameter as a function of aluminum concentration in
Further investigation about aluminum distribution around the core layer was performed using line scan technique. The EDX line scan measurement carried out at two different circumferential positions for each polished sample of the collapsed preform.
ethanolic solution.
Fig.3 (a) and 3(b) show the core diameter for different aluminum concentrations. The RIPs have been offset for visual purpose in Fig.3 (a). It can be observed that the higher concentration of aluminum leads to wider core diameter and less depression in the cladding region. For the low aluminum concentration (0.3M), the depression in the cladding layer is distinguished and this depression decreases when the concentration is increased to 0.7M.For the higher aluminum concentration (1.2M), the index difference is high and the depression cladding region is shallow. The reson for this phenomenon however is under further investigation but it could be related to the increment of the core soot thickness due to additional accumulation of the soot during solution draining and drying process or related to the diffusion of aluminum from the core to the cladding layer. Furthered future solution doping experiments will able to clarify on these possibilities. From the measurement by the preform analyzer (PK104), the average core diameters for collapsed preform for 0.3M, 0.7M and 1.2M concentrations of aluminum are 0.57mm, 0.90mm and 1.2mm respectively. Energy dispersive X-ray spectrometry (EDX) has been used to analyze the aluminum's distribution across the core layer of collapsed preform. Three EDX techniques are performed which are mapping, line scans, and points&id. These three techniques provide detailed investigation about aluminum distribution in the core layer with different aluminum concentrations. During our EDX measurement, detector is able to detect and measure the aluminum distribution for the concentrations 0.7M and 1.2M.However, the sample with the lowest concentration of 0.3 was not detected.
-1,5
-1.0
-0.5
0.0
0.5
1.0
1.5
Radial position Cmm) Fig. 5: EOX line scans results of aluminum distribution along the core.
From the line scan measurement, aluminum distribution around core region has the similar pattern for refractive index profile where aluminum increased gradually and reaches the maximum values (peak) then decreasing gradually as well. The aluminum concentration in the core of the preform soaked in 1.2M aluminum solution is higher compared to preform core soaked in 0.7M aluminum solution. We can also notice the core diameter of 1.2M preform is also found wider than 0.7M. Higher aluminum concentration leads to wider core diameter. As discussed previously, the diameter increment is due to aluminum diffusion from the core layer to the cladding layer during sintering and collapse process. To verify about the amount of aluminum content incorporated to the core layer, points &id measurement is performed. This technique provides the weight percentage of Al retained in the core layer after collapse. The samples had been analyzed at X5000 magnification image. The size
270
for every image is about 261-lm for forty points to cover the core layer length of 1.04mm.
ACKNOWLEDGMENT
The authors would like to thank Telekom Research & Development for providing the fund (RDTC 11076) to run this work. REFERENCES
[1]
2.0 1.8 �
�
�
1.6
[2]
1.4 1.2
......, 1.0 C Q) 0.8
C o
()
[3]
0.6
_
0.4
«
0.2
[4] .(J.6
.(J.4
.(J.2
0.0
0.2
0.4
Radial position (mm)
0.6
Fig . 6: EDX point&id results of aluminum distribution along the core layer.
Fig.6 provides detail about Al content in weight percentage versus the core diameter for both samples. It can be observed the Al content around the core for both samples start with small values of weight percentage and gradually increased to maximum value about 1.97wt% for 1.2M sample and 0.97wt% for 0.7M sample. This pattern of AI wt% is also in agreement with the RIP and line scan pattern. From the Al line scan and points&id measurement, we can conclude that, Al distribution is uneven around the core layer. The RIP and EDX measurements have provide the basic concept about aluminum uneven distribution in our collapsed preforms with the different intended solution concentrations. It is also clear that, higher aluminum concentration leads to higher RIP and wider core diameter. IV. CONCLUSION Solution doping with MCVD is very well technique used to incorporate Al203 and other dopants into the core region of silica perform. The Si02 soot deposited at 18000C on the substrate tube has been soaked in three different aluminum concentrations. Ab03 doped Silica host preform refractive index profile was measured. The Ah03 index profile across the core layer indicates no 'central dip' in all aluminum concentrations (0.3M, 0.7M and I.2M). The preforms refractive index profiles are relatively uniform with a little variation in the longitudinal axial of each preform due to variation of process parameter. Increment in the aluminum solution strength leads to increment of preform refractive index difference and wider diameter. The EDX measurement techniques indicate uneven distribution of Al around the core layer. The result and analyses of this study are important for further enhancement of solution doping procedures, fabrication parameters, and the effect of different solution concentration on RIP, core diameter and aluminum distribution in the core layer that can produce improved quality of the fabricated performs.
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