Process for making rare earth doped optical fiber

Details Process for making rare earth doped optical fiber Process for making rare earth doped optical fiber

2001-03-06

2004-06-22

Hoffmann, John (Department: 1731)

Glass manufacturing

Processes of manufacturing fibers, filaments, or preforms

With measuring, controlling, sensing, programming, timing,...

C065S390000, C065S397000, C065S398000, C065S399000, C065S417000, C065S419000

Reexamination Certificate

active

06751990

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a Process for Making Rare Earth Doped Optical Fiber.
2. Description of the Related Art
Rare-earth (RE) doped optical fibers have shown great potential for a number of applications including amplifiers, fiber lasers and sensors. Oxides of rare earths are doped into the core of such fibers as the active substance. Lasing and amplification have been demonstrated at several wavelengths with the incorporation of various rare-earths but for telecommunication applications erbium doped fiber (EDF) remains the most important since the operating wavelength matches with the third low loss optical window.
Erbium doped fiber amplifier (EDFA) operating around 1.53 &mgr;m low loss window is playing the key role in the present day high capacity communication systems. It is able to amplify the optical signal directly independent of modulation format. Optoelectronic repeaters so long used in these systems were 3R devices with the limitations of amplifying the signal in discrete wavelengths. EDFA has the capability to amplify simultaneous optical channels in a single fiber, which has enabled the implementation of WDM (wavelength division multiplexing) technology with the potential of increasing the bandwidth of long distance transmission systems from Gb/s to Tb/s ranges. It thus exhibits high gain, large bandwidth, low noise, polarization insensitive gain, substantially reduced cross talk problems and low insertion losses at the operating wavelengths. The success of future high capacity optical networking and transmission systems will depend significantly on the development of efficient EDFA.
Reference may be made to Townsend J. E., Poole S. B., and Payne D. N., Electronics Letters, Vol. 23 (1987) p-329, ‘Solution-doping technique for fabrication of rare-earth-doped optical fiber’ wherein, the MCVD process is used to fabricate the preform with a step index profile and desired core-clad structure while solution doping is adopted for incorporation of the active ion. The steps involved in the process are as follows:
i. A conventional cladding doped with P
2
O
5
and F is deposited within a high silica glass substrate tube to develop matched clad or depressed clad type structure.
ii. The core layers of predetermined composition containing index-raising dopant like GeO
2
are deposited at a lower temperature to form unsintered porous soot.
iii. The tube with the deposit is immersed into an aqueous solution of the dopant precursor (typical concentration 0.1 M) up to 1 hour. Any soluble form of the dopant ion is suitable for preparation of the solution although rare earth halides have been mostly used.
iv. Following immersion, the tube is rinsed with acetone and remounted on lathe.
v. The core layer containing the RE is dehydrated and sintered to produce a clear glassy layer. Dehydration is carried out a temperature of 600° C. by using chlorine. The level of OH
−
is reduced below 1 ppm using Cl
2
/O
2
ratio of 5:2 provided the drying time exceeds 30 min.
vi. Collapsing in the usual manner to produce a solid glass rod called preform.
vii. Fiber drawing is conventional.
Reference may also be made to DiGiovanni D. J., SPIE Vol. 1373 (1990) p-2 “Fabrication of rare-earth-doped optical fiber’ wherein the substrate tube with the porous core layer is soaked in an aqueous or alcoholic solution containing a nitrate or chloride of the desired RE ion. The tube is drained, dried and remounted on lathe. The dehydration is carried out by flowing dry chlorine through the tube at about 900° C. for an hour. After dehydration, the layer is sintered and the tube is collapsed to be drawn to fiber.
Another reference may be made to Ainslie B. J., Craig S. P., Davey S. T., and Wakefield B., Material Letters, Vol. 6, (1988) p-139, “The fabrication, assessment and optical properties of high-concentration Nd
3+
and Er
3+
doped silica based fibers” wherein optical fibers based on Al
2
O
3
—P
2
O
5
. —SiO
2
host glass doped with high concentrations of Nd
3+
and Er
3+
have been fabricated by solution method and quantified. Following the deposition of cladding layers P
2
O
5
doped silica soot is deposited at lower temperature. The prepared tubes are soaked in an alcoholic solution of 1M Al(NO
3
)
3
+various concentrations of ErCl
3
and NdCl
3
for 1 hour. The tubes are subsequently blown dry and collapsed to make preforms in the usual way. Al is said to be a key component in producing high RE concentrations in the core centre without clustering effect. It is further disclosed that Al and RE profile lock together in some way, which retards the volatility of RE ion. The dip at the core centre is observed both for P and GeO
2
.
Reference may also be made to U.S. Pat. No. 5,005,175 (1991) by Desuvire et al., ‘Erbium doped fiber amplifier” wherein the fiber for the optical amplifier comprises a single mode fiber doped with erbium in the core having a distribution profile of the RE ion whose radius is less than 1.9 &mgr;m while the radius of the mode of the pump signal exceeds 3 &mgr;m. The numerical aperture (NA) of the fibers varies from 0.2 to 0.35 and the core is doped with both Al and Ge oxides to increase the efficiency. As the radius of the Er doped core region is equal to or less than the radius of the pump mode of the fiber it is claimed that each atom of erbium in the core cross section is exposed to substantially equal levels of the high intensity portion of the pump mode. The fiber with such design is reported to have increased gain and lower threshold compared to the conventional Er doped fiber amplifiers where the radius of the Er doped core is large compared to the radius of the pump mode so that the erbium atoms at the edge of the core do not see a sufficient flux of the pump photons to yield a net gain.
According to U.S. Pat. No. 5,491,581 (1996) by G. S. Roba, ‘Rare earth doped optical fiber amplifiers’ wherein high germania concentration in the core used to enhance the NA of the fiber is reported to result in generation of residual stress at the core-clad interface due to difference in viscosity and thermal expansion coefficient. Residual stress in turn is believed to produce undesirable increase in background loss of the fiber.
According to U.S. Pat. No. 5,778,129 (1998) by Shukunami et. al., ‘Doped optical fiber having core and clad structure are used for increasing the amplification band of an optical amplifier using the optical fiber’ wherein the porous core layer is deposited after developing the cladding inside a quartz tube by MCVD process and solution doping method is employed to impregnate Er as the active ion into the porous core to be followed by vitrification and collapsing for making the preform. The solution also contains compound of Al, say chlorides, for codoping of the core with Al in order to expand the amplification band. The Er and Al doped glass constitutes first region of the core. Surrounding this are the second and third regions of the core. The third region contains Ge to increase the refractive index. The second region has an impurity concentration lower than both those of first and third regions and consequently low RI also. The second region acts as a barrier to prevent diffusion of the active dopant.
Reference may also be made to U.S. Pat. No. 5,474,588 (1995) by Tanaka, D. et. al., ‘Solution doping of a silica with erbium, aluminum and phosphorus to form an optical fiber’ wherein a manufacturing method for Er doped silica is described in which silica glass soot is deposited on a seed rod (VAD apparatus) to form a porous soot preform, dipping the said preform into an ethanol solution containing an erbium compound, an aluminum compound and a phosphoric ester, and desiccating said preform to form Er, Al and P containing soot preform. The desiccation is carried out for a period of 24-240 hours at a temperature of 60°-70° C. in an atmosphere of nitro

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