Optical waveguides – Optical fiber waveguide with cladding – With graded index core or cladding
Reexamination Certificate
2000-04-26
2003-07-22
Bovernick, Rodney (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
With graded index core or cladding
Reexamination Certificate
active
06597848
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to dispersion compensating optical fibers that are suitable for use in wavelength division multiplexing (WDM) systems, and to dispersion compensating fibers that are particularly well suited for use in L-band systems that operate at wavelengths longer than 1565 nm. It also relates to dispersion compensated links utilizing such dispersion compensating fibers, and to a process for making the dispersion compensating fibers.
2. Technical Background
Telecommunications systems presently in place include single-mode optical fibers which exhibit zero dispersion at a wavelength around 1300 nm; such fibers are referred to herein as “SMF fibers”. Signals transmitted within such systems at wavelengths around 1300 nm remain relatively undistorted. Signals can be transmitted over such systems at wavelengths around 1550 nm in order to achieve lower loss and to utilize the effective and reliable erbium fiber amplifiers that operate in the 1550 nm window.
Over the past few years telecommunications systems have been upgraded from 2.5 Gbs single channel systems to 10 Gbs WDM systems. The increased bit rate per channel has made these systems dispersion limited. Transmission at 1550 nm over SMF fibers introduces a dispersion of about +17 ps
m·km; such fibers are therefore restricted to about 60 kms uninterrupted transmission at 10 Gbs. The solution put forth to counter this has been to dispersion compensate at regular intervals. For example, a single-mode fiber with a dispersion of +17 ps
m·km at 1550 nm requires a dispersion compensation of 1020 ps
m every 60 km. Therefore, a dispersion compensating (DC) module containing a DC fiber has to be inserted into the system at every amplifier stage that accounts for about 1000 ps
m accumulated dispersion. As this length of DC fiber does not account for any real transmission distance, it is desirable to keep this length as short as possible. This implies that the negative dispersion of the DC fiber must be maximized. However, as the dispersion is made more negative via increasing the role played by waveguide dispersion, the fiber becomes more bend sensitive and the base attenuation of the fiber increases. Therefore, most value is gained by maximizing dispersion (D) while simultaneously keeping attenuation (Attn) as low as possible. Thus, the ratio of |D/Attn|, known as the figure of merit, must be maximized rather than dispersion alone.
Until recently, system and DC fiber designers had considered only one channel (1550 nm). That is, a DC fiber would be used to compensate dispersion at only one wavelength, and hence the dispersion slope of the fiber was not important. However, with the new emphasis on WDM technology, it has become necessary to provide dispersion compensation over all wavelengths of transmission within the erbium fiber amplifier window. This implies that designers are now restricted by the channel that has the worst compensation. An obvious solution to the above quandary is to design a DC fiber such that dispersion is simultaneously compensated at all wavelengths. Thus, there is an added criterion to satisfy, namely, dispersion slope. The figure of merit must be maintained at a large value for all wavelengths at which the DC fiber is to be utilized. As the bend-edge causes increased attenuation at longer wavelengths, DC fibers that have a low bend edge have been limited to use at C-band wavelengths (up to 1565 nm) that are substantially unaffected by this effect.
To examine the effect of dispersion slope on the system assume that a system employs the aforementioned SMF fiber, which has a dispersion of +17 ps
m/km and dispersion slope of about +0.056 ps
m
2
·km at 1550 nm. Consider the effect of five different DC fibers on the system. The dispersion and dispersion slope characteristics of the five fibers are shown in Table 1, wherein dispersion, D is expressed in units of ps
m·km, and dispersion slope, Dslope is expressed in units of ps
m
2
·km.
TABLE 1
Uncompensated
Uncompensated
Distance
Distance
DC
D
Dispersion @ 1530/
Dispersion @ 1565/
(km) @
(km) @
Fiber
D
Slope
1000 km
1000 km
10 Gbs
40 Gbs
1
−85
−0.186
−400
300
~1200
~75
2
−102
−0.186
−540
405
~880
~55
3
−85
−0.28
0
0
>10000
>1000
4
−102
−0.28
−200
150
~2400
~150
5
−85
−1200
900
~400
~25
DC fibers having dispersions of −85 and −102 ps
m·km have been chosen for this theoretical example since a length L of DC fiber having a dispersion of −85 ps
m·km will compensate for a length 5L of SMF fiber having a dispersion of 17 ps
m·km, and a length L of DC fiber having a dispersion of −102 ps
m km will compensate for a length 6L of that SMF fiber.
Using the characteristics of the SMF fiber and the DC fiber, the uncompensated dispersion at the end channels (1530 nm and 1565 nm) of the erbium C band window can be calculated, assuming that all DC fibers are designed for complete compensation at 1550 nm. Calculated values are given in columns 4 and 5 of Table 1. If it is assumed that the system is pulse spectral width limited, then the relationship between dispersion, bit rate and total length is given by equation 1,
B
(|&bgr;
2
|L
)
½
<¼ (1)
where B is the bit rate, &bgr;
2
=(D&lgr;
2
)/2&pgr;c, and L is the length.
Equation 1 can be rewritten in terms of bit rate and the total dispersion accumulated in a given length. Based on the above relationship, given a bit rate and the average accumulated dispersion, one can determine the total length of a system before dispersion becomes a limiting factor, and this length is given for bit rates of 10 and 40 Gbs in columns 6 and 7 of Table 1. DC fibers
1
,
2
,
3
and
4
are theoretical examples which are used herein to demonstrate the effects of various dispersions and dispersion slopes on system length. DC fiber
5
is a commercial fiber that compensates for dispersion at only one wavelength, eg. 1550 nm. Dispersion slope is not listed for DC fiber
5
since dispersion slope was not specified for DC fibers intended for operation at a single wavelength, and dispersion slope could vary between approximately −0.5 and +0.5 ps
m
2
·km without adversely affecting system operation. It is noted that DC fibers
1
,
2
,
3
and
4
are suitable for use in a 10 Gbs system in that their use in such a system enables signal transmission over a distance of at least 600 km. Of the five listed fibers only DC fiber
3
is suitable for use in a 40 Gbs system.
The &kgr; value of a DC fiber is defined herein as
&kgr;=(
D
DC
)/(
D
Slope
DC
) (2)
where D
DC
and Dslope
DC
are the dispersion and dispersion slope of the DC fiber. Relative dispersion slope (RDS), the reciprocal of &kgr;, is sometimes used to characterize a ratio of dispersion and dispersion slope. The ratio of the dispersion to dispersion slope of the SMF fiber is about
303
. DC Fiber
3
is unique, since the dispersion and the dispersion slope of that DC fiber are such that essentially complete compensation can be achieved over all wavelengths. In other words, the &kgr; value of DC fiber
3
is also
303
. This criterion is defined as full compensation. Line
20
of
FIG. 2
is referred to as the line of full compensation, as its slope is
303
. DC fiber
3
is represented by that point on line
20
where dispersion is −85 ps
m·km and dispersion slope is −0.28 ps
m
2
·km. Other fibers falling on line
20
, such as one having a dispersion of −102 ps
m·km and a dispersion slope of −0.336 ps
m
2
·km, for example, would also afford full compensation.
Although DC fiber
3
is superior to DC fibers
1
,
2
and
4
for a 10 Gbs system, it does not add value, as terrestrial systems are designed primarily for a maximum distance of about 600 km. Thus, certain DC fibers which do not provide complete c
Berkey George E.
Jiang Lei
Powers Dale R.
Srikant V.
Bovernick Rodney
Carlson Robert L.
Corning Incorporated
Stahl Mike
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