Patent Grant
7849713
This application is a national phase application based on PCT/EP2002/014757, filed Dec. 24, 2002, the content of which is incorporated herein by reference.
Present long-haul communication links make generally use of wavelength division multiplexing (WDM) and zero dispersion or low dispersion fibres to increase capacity and to extend distances between signal regeneration. However, the use of zero- or low-dispersion transmission fibres in WDM systems can result in severe performance degradation due to non linear phenomena, such as four-wave mixing (FWM). In order to minimise FWM the zero-dispersion wavelength should be located out of the transmission bands, normally the C-band (1530-1565 nm) and/or the L-band (1565-1620 nm). The resulting fibres with a controlled amount of dispersion and low attenuation in the transmission band are called non-zero dispersion-shifted (NZD) fibres, specified in ITU-T Recommendation G.655. Examples of commercial NZD fibres are the TrueWave® (trademark of Lucent), LEAF® and MetroCor® (trademarks of Corning), and FreeLight® (trademark of Pirelli).
NZD fibres with large effective area, i.e., with effective area larger than about 50 μm2, such as LEAF® and Freelight® fibres, have the potential to carry signals with high optical power while reducing the non-linear optical effects. Large effective area fibres are therefore good candidates for high-capacity WDM networks over long distances.
An optical fibre having a segmented core can provide a large effective area while limiting the non-linear optical effects. U.S. patent application Ser. No. 2002/0044755 discloses a single-mode optical fibre having a relatively large effective area, greater than 45 μm2, together with a substantially flat dispersion slope over the operating range. The disclosed fibre includes a segmented core index profile.
U.S. Pat. No. 6,212,322 discloses an optical fibre having a segmented core index profile, positive total dispersion at 1530 nm and effective area larger than 60 μm2.
In optical systems, such as long-haul communication links, optical fibres normally need to be joined together to make a continuous optical path for light transmission from one fibre length to another. Often, the joined optical fibres are of different type, such as in the case of an optical path in which a transmission fibre is coupled to a dispersion-compensating fibre. Also, the end of the fibre needs to be connected to terminations, such as repeaters, data transmitters or power meters. Joining of two fibres or two groups of fibres is generally referred to as splice. When two optical fibres are spliced, a loss in optical power across the splice point, i.e., a splice loss, may occur. Splicing together two different types of fibres, i.e., hybrid splicing, may lead to an elevated splice loss due to mode field diameter (MFD) and fibre geometry mismatch. Even when splicing two different lengths of the same fibre, i.e., homogeneous splicing, optical power can be lost across the splice because of misalignment of the fibre cores and of intrinsic parameters of the fibres such as the core/clad concentricity or fibre curl.
Fibre curl is the inherent curvature over a specific length of uncoated fibre and is measured by rotating an unsupported fibre end about the fibre's axis and by measuring the deflection from horizontal. Fibre deflection, typically measured in microns, is converted to radius of curvature, usually expressed in meters. Fibre curl is believed to be an important parameter for splice performance, together with the core/clad concentricity, which defines how well the core is centred in the cladding glass region, and the cladding diameter.
A number of techniques have been developed over the years to.limit the detrimental effects of splice loss. Two commonly used techniques are fusion splicing, which involves the melting together of fibre ends, and mechanical splicing, which makes use of a mechanical coupling device to hold the fibre ends together. Permanent joining of single-mode optical fibres is typically performed by fusion splicing.
NZD fibres are typically fabricated with more complex refractive index profiles (RIPs) than standard single mode fibres. The RIP describes the variation of glass refractive index along the optical fibre radius. Due to their more complex physical design, NZD fibres are believed to be more effected by critical parameters such as splicing conditions and procedures. M. Adcox in “Splicing and fiber assembly compatibility for non-zero dispersion-shifted fiber and standard single-mode fiber”, presented at NOC/EC 2000, describes splice studies for single-mode fibre and NZD fibres by using an active alignment splicer. Splice loss is said to be more sensitive to cleave end angle when splicing a NZD fibre compared to standard single-mode fibre. M. E. White and S. A. Cooper in “Splice loss in non-zero dispersion-shifted fibres”, published in the IWCS proceedings, p. 891 (1998), report splice loss studies for LEAF ® fibres manufactured by Corning with MFD between 9.48 and 9.92 μm. LEAF® fibre are said to provide lower loss splices with active alignment systems than other NZD fibre designs.
A designed NZD fibre with large effective area and low splice loss is presented in “A new design for non-zero dispersion-shifted fiber (NZ-DSF) with a large area over 100 μm2 and low bending and splice loss” by S. Yin et al, Optics Communications 177 (2000), p. 225. Calculations of splice loss are reported for a splice with conventional single mode fibre.
Minimisation of signal attenuation in the transmission band as light travels along the optical fibre is one of the crucial issues in the design and in the manufacturing of optical fibres. Absorption and Rayleigh scattering are the two major causes of signal loss. Transmission loss is greatly influenced by the tension applied to the optical fibre when making the optical fibre by drawing an optical fibre preform upon heating. During drawing, the preform is heated at the optimum temperature to achieve the desired drawing tension.
Tajima K. in “Low loss optical fibers realized by reduction of Rayleigh scattering loss”, OFC '98 Technical Digest, p. 305, reports an increase of the Rayleigh scattering coefficient with increasing the drawing temperature in GeO2—SiO2 core single-mode fibres. The Rayleigh scattering coefficient of at relatively low temperature, i.e., 1800° C., and without increasing the tension was found to be of 0.78 dB/km·μm4.
U.S. Pat. No. 6,427,044 discloses an optical fibre comprising a Ge-doped core whose maximum value of the relative refractive index, Δmax, is not less than 0.8%. In case of an optical fibre in which the core is doped with high concentration of GeO2, the increase of Rayleigh scattering loss due to GeO2 doping can become problematic. A decrease in transmission loss is reported as the drawing tension increases from 13 to 28.5 kg/mm2 for segmented-core optical fibres having Δmax of 1.5% and of 2.5%. In order to reduce loss due to GeO2 doping, it is suggested that the drawing tension should be increased.
U.S. patent application Ser. No. 2001/0046358 discloses a segmented-core optical fibre with a F-doped core segment and F-doped cladding region, which comprises first and second portions having positive and negative chromatic dispersions, respectively, disposed alternately in the longitudinal direction for dispersion management. Positive and negative dispersion portions are achieved by changing the drawing tension in the longitudinal direction from 90 g to 40 g.
The difference in refractive index between core and cladding of the optical fibre, which is produced by doping with GeO2 or other index-raising dopants, may induce internal stress due to the radial variation of thermal expansion coefficient caused by compositional variation in the fibre. Internal stress is also built in optical fibres during the drawing process because of the viscosity mismatch between the core and the cladding. The internal stress is frozen-in in the optical fibre after cooling and may induce a permanent change in the final refractive index. The internal stress due to viscosity mismatch is proportional to the applied drawing tension.
Small variations in process conditions during manufacturing of optical fibres within product specifications result in produced fibres having physical and geometrical parameters that may vary within a given range. Thus, the splicing of two fibres in product specification may be affected by geometry or MFD mismatch. This type of splicing will be referred to as hybrid splicing, as for the splicing of different types of fibres.
Splicing methods for permanent splices, such as fusion splicing, generally involve heating of the fibre ends, which will be joined together, up to temperatures which are high enough to soften the glass and allow the glass to flow. Inventors have observed that heating may induce a modification of the RIP of the fibre in the region close to the splice point, i.e., in the region affected by the heating. They have found that drastic modifications of the RIP after splicing were present in optical fibres that had been drawn at relatively high tensions. Conversely, fibres drawn at relatively low tensions exhibited a reduced RIP change after splicing.
Inventors presume that the frozen-in stress of the optical fibre in the region close to the splice point is at least partially relieved during heating, thereby leading to a RIP change after fusion splicing. As the RIP change is local, i.e., in the fibre length portion close to the splice point, the spliced fibre will result optically inhomogeneous due to the difference in RIP along the fibre, thereby inducing optical losses because of internal light reflection or diffusion. Inventors suppose that a relatively low drawing tension reduces the frozen-in stress in the fibre and thus limits the RIP change after splicing.
In order to reduce splice losses in an optical fibre, inventors have found that the drawing tension should be not larger than 150 g, preferably not more 100 g. In the most preferred embodiment, the drawing tension is comprised between 60 and 100 g.
The optical fibre of the present invention has a core region and an outer cladding region, wherein the core region has a RIP of the segmented core type, each segment having a particular refractive index profile. The core RIP comprises three core segments and an inner cladding segment. In particular, the fibre core comprises a central segment, a first annular segment adjacent to the central segment, a second annular segment adjacent to the first annular segment, and an inner cladding segment surrounding the second annular segment. The refractive index percent difference of the i-th segment relative to the outer cladding region, i.e., the index delta Δi %, is defined herein by the equation
where ni is the maximum refractive index of the i-th segment and nc is the refractive index of the reference region, which is taken as the uniform outer cladding region, made typically of substantially undoped silica glass. A segment of known shape, e.g., a Gaussian-type segment or step-index segment, can be characterised by its index delta, and, if necessary, by one or more other parameters, such as the centre radius or the curve width. Generally, each point of a segment has an associated relative refractive index.
Each of the core segments has a non negative relative refractive index, the index delta of the central segment, Δ0%, being larger than the relative index of the other segments. Therefore, the index delta Δ0% corresponds to the maximum relative index Δmax % of the fibre. The index delta of the second annular segment, Δ2%, is larger than the index delta of the first annular segment, Δ1 %. The relations among the index delta's of the core segments are:
Δ0%>Δ2%>Δ1%≧0,
Δ2%>Δ3%≧0,
where Δ3% is the index delta of the inner cladding segment.
Whereas splice loss has been found by the inventors to be strongly related to the drawing tension, other important optical properties such as the signal attenuation along the fibre also depend on the tension. Inventors have become aware that in order to achieve low signal attenuation, the maximum positive difference in the refractive index between the core and the cladding, i.e., the maximum relative index, Δmax %, should be lower than a certain value that depends on the drawing tension. They have found that Δmax % should be not more than about 0.84% for optical fibres drawn at tensions not larger than 200 g and not more than about 0.80% for tensions not exceeding 150 g. For drawing tensions not larger than 100 g, Δmax % should be preferably not more than about 0.73%. In particular, for drawing tensions within the range of about 150-200 g, Δmax % should be preferably from about 0.80% to 0.84%; for drawing tensions within the range of about 100-150 g, Δmax % should be preferably from about 0.73% to 0.80%, and for drawing tensions in the range from about 60 g to 100 g, Δmax % should be preferably from about 0.64% to 0.73%.
Core parameters of the optical fibre of the present invention are preferably selected so as to obtain a NZD fibre having zero dispersion wavelength, λ0, comprised between about 1430 and 1510 nm. Inventors have recognised that in order to obtain a segmented core NZD optical fibre drawn at tensions not larger than 200 g, the central segment should have relative index Δ0% within the range from about 0.64% to 0.84% and the centre radius and the width of the second annular segment should be within the range of about 5.5-7.5 μm and of 1.7-3.0 μm, respectively. Preferably, for optical fibres drawn at tensions not larger than 150 g, the centre radius of the external ring should be in the range from about 6.7 to 7.5 μm.
For drawing tensions not larger than 100 g, Δ0% should be preferably within the range from about 0.64% to 0.73%. More preferably, for fibres drawn at tensions not larger than 100 g, the centre radius of the external ring should be within the range from about 7.1 to 7.5 μm.
A reduction of the drawing tension normally implies the increase of the furnace temperature to temperatures of typically more than 1900° C. Inventors have recognised that a relatively high drawing temperature may lead to an increase of structural defects in the optical fibre material, such as non-bridging oxygen centres and peroxy radical defects. Hydrogen penetrating the optical fibre can react with these defects and create hydroxil groups which are detrimental to the optical fibre performance.
Inventors have noted that it is advantageous to dope the inner cladding of the optical fibre with a relative low concentration of germanium, with index delta Δ3% comprised between 0.02% and 0.04%, so as to reduce the hydrogen permeability of the optical fibre. More preferably, the relative index of the inner cladding ranges from about 0.021% to 0.027%.
Optical fibre preforms are drawn preferably at a speed comprised between about 9 to about 16 m/s.
In a preferred embodiment, the optical fibre comprises
a central segment with relative index, Δ0%, in the range 0.64% and 0.80%, centre radius between 5.5 μm to 7.5 μm and width at half height w0 from 3.5 μm to 4.3 μm,
a first annular segment of index delta Δ1% not larger than about 0.1% value, preferably not more than 0.07%,
a second annular segment having relative index, Δ2%, ranging from 0.11% to 0.17%, a centre radius r2 from 5.5 to 7.5 μm, and a width at half height, w2, from about 1.7 to about 3.0 μm,
an inner cladding segment of index delta Δ3%≧0.
In a subset of this embodiment, the external ring has an alpha profile. The preferred alpha profile is that where α ranges from about 4 to about 7.
In another subset of this embodiment, the fibre core comprises an inner cladding segment adjacent to the second annular segment having a positive index delta, Δ3%, comprised between 0.02% and 0.04%, preferably between 0.021% and 0.027%.
This core structure provides an optical fibre having positive chromatic dispersion D at 1550 nm of the fibre within the range 2.0 to 8.5 ps/nm/km, and dispersion slope S at 1550 nm ranges from 0.075 to less than 0.1 ps/nm2/km. Preferably, RIPs are selected to provide an effective area, Aeff, from about 60 to 90 μm2. Zero dispersion is preferably within tbe range from about 1430 to 1510 nm.
The optical fibre was drawn from a preform blank. The optical fibre preform was produced preferably by outside vapour deposition (OVD). In first stage, glass particles were deposited on a rotating cylindrical mandrel, preferable made of ceramic material, such as alumina. Glass particles mainly comprised silicon oxide (SiO2) doped with other compounds, such as germanium oxide (GeO2), which modify the refractive index of SiO2. Reactants flow was varied during deposition, e.g., by changing the concentration or the type of dopant, so as to obtain the desired RIP. The mandrel was then removed, leaving a central hole in the soot preform. The soot preform was dehydrated and then sintered to obtain a consolidated core blank which will ultimately form the core and the inner portion of the cladding, hereafter referred to as the inner cladding, of the optical fibre. Dehydration was carried out in an inert gas atmosphere comprising Cl2 to eliminate hydroxide ions or water molecules from the soot preform. The consolidated core blank having a central hole of smaller diameter than that of the hole after removal of the mandrel was placed in a furnace to melt one of its lower ends. During heating of the consolidated core blank, the central hole was evacuated, thereby causing its walls to collapse, in a similar process as that described in U.S. Pat. No. 4,810,276. The fused glass material cooled off forming a cylindrical elongated member which was stretched downwards by a traction device, preferably of the type described in WO 01/49616. The elongated member produced by the stretching was cut into a plurality of rods, which will be referred to as the core rods. Each core rod served as a substrate for the chemical deposition of the glass soot which ultimately formed the overcladding or outer cladding. This deposition process comprised depositing on the core rod glass particles, mainly comprising SiO2, so as to obtain a soot preform. The soot preform was dehydrated and consolidated in a similar process as that carried out for the vitrification of the core soot preform. The preform was subsequently drawn to form the optical fibre.
Although in a preferred embodiment, preform production was carried out by OVD process, it is to be understood that the invention is not limited to preforms manufactured by the OVD process. The preform can be also made by vapour axial deposition (VAD), modified chemical vapour deposition (MCVD), rod-in-tube method or other methods.
Drawing conditions are determined to a large extent by the temperature of the furnace, the drawing tension and the fibre drawing speed. In order to control drawing conditions, feedback loops between the furnace temperature and the tension or the drawing speed can be implemented by using for example the tension meter and/or the diameter meter and a pyrometer inside the furnace.
The optical fibre of the invention is of NZD type with λ0 preferably comprised between about 1430 and 1510 nm. The optical fibre is designed to have an attenuation low enough to allow long-haul communication by drawing the fibre at tensions not larger than 150 g. Preferably, the optical fibre should exhibit low attenuation and good optical performance with drawing tensions of not more than 100 g. The RIP is of type segmented-core and comprises three core segments and an inner cladding segment. All segments have non-negative relative index and the central segment has a relative index Δ0% which is larger than the relative index of the other segments, and thus corresponds to the maximum relative index Δmax %.
The diagram of
where Δ0%, Δ2%, and Δ3%, are the relative indices of the central segment, centred nearly at the core centreline (r0≅0), the second annular segment, i.e., the external ring, and the inner cladding, respectively. U(r−r3) is a unitary function that is equal to unity for r≧r3, with r extending practically to the outer edge of the core rod (in the example of
The central segment 21 has a relative index, Δ0%, within the range of about 0.64% to about 0.80% and a peak width w0=2σ0√{square root over (2 ln 2)} calculated at half height which is comprised from about 3.5 μm to about 4.3 μm. Although the central segment was modelled by a Gaussian, it is to be understood that small variations in the profile shape of the segment would not significantly influence the properties of the resulting fibre. For instance, a triangular-shaped central segment having suitable index delta and width could be used instead.
The first annular segment 22, adjacent to the central segment 20, has non negative relative index Δ1% not larger than 0.1%, preferably not larger than 0.07%. The second annular segment 23, adjacent to the first annular segment, also referred herein to as the external ring, has a relative index Δ2% within the range of about 0.11% to about 0.17%, while its radius r2 ranges from about 5.5 μm to 7.5 μm. The external ring was modelled with a super-Gaussian having an α ranging from about 4 to 7. The width of the external ring, w2=2σ2(2 ln 2)1/αcalculated at the half height, is of about 1.7 μm to about 3.0 μm for α in the range 4-7. Curve spread σ2 of the external ring is from about 0.8 to about 1.4 μm.
It is to be understood that small variations in the profile shape of the external ring will not notably affect the fibre properties. For instance, the external ring could be modelled by a step-like profile.
The inner cladding segment 24 has an index delta Δ3% such that Δ2%>Δ3%≧0. Preferably, the inner cladding segment 24 has an index delta Δ3% such that ranges from about 0.02% to 0.04%, more preferably from 0.021 to 0.027%. The raised inner cladding region can be included in the core design to reduce the permeability of the fibre to hydrogen. Preferably, the width of the inner cladding, taken as the difference between the outer radius of the inner cladding and outer radius of the external ring r2+σ2, is comprised in the range of about 2-4 μm. It is to be understood that small deviations from a step-like shaped inner cladding will not modify the optical properties of the resulting fibre. For example, a slightly curved step profile could be envisaged for the inner cladding segment.
A dip in the index profile may be present at the centreline of the fibre core, the dip being at least partially due to the diffusion of dopants occurring mostly during dehydration and consolidation processes.
The RIP of
Table 1 reports the optical properties, calculated using a computer model, of an optical fibre having the profile parameters according to
Optical fibres manufactured with a RIP shape exhibited on average a good hydrogen resistance, as demonstrated by testing hydrogen ageing loss.
Bending performance of optical fibres was estimated by calculating microbending according to the IEC standard 62221 TR3 Ed. 1, Method A. Microbending of the optical fibres according to the invention was not larger than 10 (dB/km)/(g/mm).
An optical fibre was modelled using the profile shape illustrated in
An optical fibre was modelled using the profile shape illustrated in
An optical fibre was modelled using the profile shape illustrated in
An optical fibre was modelled using the profile shape illustrated in
An optical fibre was modelled using the profile shape illustrated in
An optical fibre was modelled using the profile shape illustrated in
The diagram of
Optical fibres with parameters centred around the values given above are considered as comparative example. The optical properties of these optical fibres are given in Table 8.
In
As shown in
Table 9 reports experimental measurements of the average splice loss measured at 1550 nm for splicing of different types of NZD fibres. For comparison, measurements of the average splice loss of commercial LEAF® fibres are given. LEAF® fibres were randomly selected on a fibre population in which the MFD varied between 9.5 and 9.9 μm, core/clad concentricity was on average 0.19 μm, having a maximum value of 0.32 μm, and curl was of 40 m on average, having a minimum value of 11 m.
NZD-A and NZD-B fibres had an index profile shape illustrated in
The NZD-C fibres had a RIP compliant to that shown in
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP02/14757 | 12/24/2002 | WO | 00 | 4/5/2006 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2004/059355 | 7/15/2004 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 3823995 | Carpenter | Jul 1974 | A |
| 4810276 | Gilliland | Mar 1989 | A |
| 6212322 | Ma et al. | Apr 2001 | B1 |
| 6247987 | Moore | Jun 2001 | B1 |
| 6427044 | Yanada et al. | Jul 2002 | B1 |
| 6535677 | Tanaka et al. | Mar 2003 | B1 |
| 20010046358 | Sasaoka et al. | Nov 2001 | A1 |
| 20020044755 | Li | Apr 2002 | A1 |
| Number | Date | Country |
|---|---|---|
| 1 239 312 | Sep 2002 | EP |
| WO-0149616 | Jul 2001 | WO |
| WO-02051762 | Jul 2002 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 20070003200 A1 | Jan 2007 | US |
