Single-mode optical fiber

Details Single-mode optical fiber Single-mode optical fiber

2001-03-02

2003-03-04

Sircus, Brian (Department: 2839)

Optical waveguides

Optical fiber waveguide with cladding

Utilizing multiple core or cladding

Reexamination Certificate

active

06529666

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an optical single-mode fiber having low dispersion for the wavelength division multiplex operation (WDM) of optical transmission paths, which is made of a central fiber core, at least two inner fiber cladding layers, and of an outer fiber cladding layer (triple-clad fiber), the refractive index profile n(r) of the fiber not being constant as a function of the fiber radius r.
BACKGROUND OF THE INVENTION
To be able to transmit ever greater data rates over single-mode fibers, the wavelength division multiplex method (WDM) is increasingly gaining in importance. In WDM operation of an optical transmission path, up to 80 to 100 channels having a spectral bandwidth of &Dgr;&lgr; are transmitted over one fiber. The number of channels that can be transmitted over one fiber of a given length is essentially limited by the fiber attenuation and dispersion at the wavelengths being used. Also, the channel spacing needed to ensure transmission quality means that the fibers must have a large enough spectral width for the transmission.
The fiberglass cables installed in the optical networks of telecommunications companies contain all-silica optical fibers, which are made of a fiber core and a fiber cladding. The minimum attenuation of all-silica fibers is within the third optical window, thus within the spectral region of around 1550 nm. In this wavelength range, powerful optical amplifiers are also available, e.g., erbium-doped fiber amplifiers (EDFA), which are used within the optical network to regenerate the transmission signals following a specific path section. For these reasons, the WDM system currently used is conceived for the third optical window.
In the case of pre-installed glass fibers, one can encounter the problem of dispersion. For normal standard fibers, the zero dispersion wavelength &lgr;
0
, at which no dispersion or only very slight dispersion of optical signals occurs, is &lgr;
0
≈1310 nm. This means that a signal transmitted with a wavelength of about &lgr;
0
is not or only slightly distorted, in particular, the pulse width is retained. However, the attenuation in this range is greater than in the third optical window. The chromatic dispersion D(&lgr;) in the case of standard fibers is substantially wavelength-dependent and, for &lgr;=1550 nm, amounts to about 16 to 17.5 ps/(km*nm). If an optical signal having wavelengths of about 1550 nm is transmitted, the pulse width is enlarged due to dispersion. This effect is an obstacle to a high transmission capacity; a chromatic dispersion of 16 to 17.5 ps/(km*nm) is much too high for ultra-high bit rate systems.
To be able to use laid standard fibers in the third optical window, it is necessary to compensate for the dispersion, which entails considerable outlay. In this regard, one knows of dispersion-compensating fibers, for example, from U.S. Pat. No. 5,568,583, which, at 1550 nm, exhibit a very high negative dispersion of D≈−100 ps/(km nm). These dispersion properties are achieved by raising the refractive index of the fiber core and by lowering the refractive index of a first cladding layer in comparison to the refractive index of the outer fiber cladding, made of silica. For the application, the dispersion-compensating fiber is spliced onto a standard fiber, so that the signal that is separated by positive dispersion when propagating through the compensation fiber is compressed again by the negative dispersion. A dispersion that is high in terms of absolute value is necessary to keep the length of the compensation fibers to a minimum.
It is also known to use special dispersion-shifted DS fibers, which have a zero dispersion wavelength of about 1550 nm, for the third optical window. A DS fiber of this kind is known, for example, from U.S. Pat. No. 5,675,688. In principle, comparably to the dispersion-compensating fibers, the zero wavelengths are shifted through the use of a specific refractive index profile.
However, these DS fibers have decisive disadvantages when used in WDM operation. The dispersion curve D(&lgr;) does, in fact, intersect the wavelength axis at about &lgr;
0
=1550 nm, however, in comparison to the dispersion curve of standard fibers, it is merely shifted toward higher wavelength D values. Thus, near 1550 nm, it has a steep rise angle, i.e., a steep slope angle S(&lgr;
0
), which lies at about 0.09 ps/km*nm
2
. This applies comparably to 1300 nm standard fibers, as well. This means, that for &lgr; values, which differ from &lgr;
0
, one has to expect significant dispersion values, which rise virtually linearly with the spacing from &lgr;
0
. This is, of course, a serious disadvantage, which limits the usable WDM spectrum and, therefore, must be overcome. The second disadvantage of the DS fibers is the relatively small effective surface A
eff
of the fibers, i.e., the small mode field diameter MFD (Petermann II) and MFD
eff
. They increase the nonlinear refractive index (Kerr coefficient) of the fibers and, thus, nonlinear effects (Brillouin und Raman scattering), which degrade the transmission quality.
Furthermore, to overcome the dispersion problem in the third optical window, optical monomode fibers have been developed as a replacement for standard fibers. In the relevant spectral region, the monomode fibers exhibit low chromatic dispersion, as well as low loss. From the company prospectus “TrueWave™ Single Mode Fiber” of AT&T Network Systems, a fiber is known, which, for wavelengths of about 1540 to 1560 nm, exhibits a chromatic dispersion D of 0.8≦D≦4.6 ps/(km*nm), given a mode field radius of 4.2 &mgr;m. Qualitatively, the refractive index profile n(r) shows a triangular core profile, the triangle resting on a broad platform, whose height makes up about one tenth of the height of the triangle. With respect to the silica glass value of n=1.4573 (outer cladding area), only positive n(r) values occur, if one assumes n=1.4573 as the zero level. One forgoes lowering the refractive index level, e.g., through incorporation of fluorine.
A dispersion-shifted fiber is also known from the EP Patent 0851 251 245 A2. For wavelengths of around 1550 nm, it exhibits a dispersion of 1.0 to 4.5 ps
m/km, a dispersion curve gradient of less than 0.13 ps
m
2
/km, and an effective surface of at least 70 &mgr;m
2
. The core of the fiber is subdivided into four layers, each having a different refractive index level. Contiguous to this fiber core is the outer fiber cladding layer. Thus, it is a quadruple-clad fiber. Another quadruple-clad fiber having at least four levels with a flat dispersion curve (0.03 ps
m
2
/km) is known from WO 97/33188. To achieve the desired optical properties, the inner core level must be substantially increased in comparison to the reference refractive index of the outer clad level. In this context, close radius tolerances must be observed, in order to accommodate four layers. It is difficult to produce a refractive index profile with close radius tolerances on a regular basis, in the case where the profile varies considerably within the range of only a few micrometers. For the manufacturing, a plasma CVD process is suited. It enables fine layer structures of this kind to be precisely deposited. This process requires substantial outlay.
The usable spectrum in the third optical window is limited by the spectral operating range of the optical amplifiers (EDFA) used, which is between about 1510 and 1570 nm. However, since glass fibers, once installed, must be available for many years, one should anticipate future technical development and set the usable operating range of the fibers to be much higher, for instance between 1400 and 1700 nm.
From the EP 0 732 119 A1, a fiber is known, whose fiber core is partitioned into three or four layers, each having a different refractive index level, the maximum value of the refractive index deviation occurring within each layer being given by a reference value, and the dispersion within the wavelength range of 1400 to 1700 nm assuming values between −

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