Optical waveguides – With optical coupler – Plural
Reexamination Certificate
2002-04-03
2004-04-20
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
With optical coupler
Plural
C385S123000, C385S127000, C398S147000, C398S148000
Reexamination Certificate
active
06724956
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to optical fibers and, more particularly, to providing very accurate dispersion compensation over an entire range of wavelengths.
BACKGROUND OF THE INVENTION
Dispersion in a glass fiber causes pulse spreading for pulses that include a range of wavelengths, due to the fact that the speed of light in a glass fiber is a function of the transmission wavelength of the light. Pulse broadening is a function of the fiber dispersion, the fiber length and the spectral width of the light source. Dispersion for individual fibers is generally illustrated using a graph having dispersion on the vertical axis (in units of picoseconds (ps) per nanometer (nm), or ps
m) or ps
m-km (kilometer) and wavelength on the horizontal axis. There can be both positive and negative dispersion, so the vertical axis may range from, for example, −250 to +250 ps. The wavelength on the horizontal axis at which the dispersion equals zero corresponds to the highest bandwidth for the fiber. However, this wavelength typically does not coincide with the wavelength at which the fiber transmits light with minimum attenuation.
For example, typical single mode fibers generally transmit best (i.e., with minimum attenuation) at 1550 nm, whereas dispersion for the same fiber would be approximately zero at 1310 nm. The theoretical minimum loss for glass fiber is approximately 0.16 db/km, and that occurs at the transmission wavelength of about 1550 nm. Because minimum attenuation is prioritized over zero dispersion, the wavelength normally used to transmit over such fibers is typically 1550 nm. Also, Erbium-doped amplifiers, which currently are the most commonly used optical amplifiers for amplifying optical signals carried on a fiber, operate in 1530 to 1565 nm range. Because dispersion for such a fiber normally will not be zero at a transmission wavelength of 1550 nm, attempts are constantly being made to improve dispersion compensation over the transmission path in order to provide best overall system performance (i.e., low optical loss and low dispersion).
Many techniques have been used for dispersion compensation, including the design and use of dispersion-shifted and dispersion flattened fibers. Dispersion Compensating Modules (DCMs) have also been used in optical communications systems for dispersion compensation, especially in wavelength division multiplexing (WDM) systems. A number of patents describe various uses of DCMs to compensate dispersion including: U.S. Pat. No. 4,261,639 (Kogelnik et al.); U.S. Pat. No. 4,969,710 (Tick et al.); U.S. Pat. No. 5,191,631 (Rosenberg); and U.S. Pat. No. 5,430,822 (Shigematsu et al.). These patents compensate dispersion by inserting DCMs at appropriate intervals along the transmission path. The DCMs usually contain Dispersion Compensating Fiber (DCF) of an appropriate length to produce dispersion of approximate equal magnitude (but opposite sign) to that of the transmission fiber.
One problem with using the known DCMs to compensate dispersion is that DCF designs are very sensitive to production tolerances. Therefore, if the DCF design is not highly precise, then when the DCF is combined with the transmission fiber, the resulting transmission link may have too much residual dispersion (i.e., dispersion on wavelength channels other than the center wavelength channel being compensated). This is especially true in broadband applications where the transmission rates may be, for example, 40 gigabits per second (Gbit/s). Also, once the DCF is produced, only the length of the DCF can be selected to meet the desired target for dispersion compensation. Moreover, selection of the DCF length (and thus the dispersion of the DCM) should ensure that first order and higher order dispersion are compensated.
When compensating for higher order dispersion, it is very important that the Relative Dispersion Slope (RDS) of the transmission fiber match the RDS of the DCF (and consequently of the corresponding DCM). For a given fiber, the RDS is defined as the ratio of the dispersion slope, S, of the fiber to the dispersion, D, of the fiber. Thus, the RDS for a given fiber is equal to S/D for that fiber. For a DCF combined with a transmission fiber, the total dispersion and the total dispersion slope of the compensated link, D
LINK
and S
LINK
, respectively, can be expressed by Equations 1 and 2, respectively, as follows:
D
Link
=D
TransmFiber
×L
TransmFiber
+D
DCF
×L
DCF
(Equation 1)
S
Link
=S
TransmFiber
×L
TransmFiber
+S
DCF
×L
DCF
(Equation 2)
In Equation 1, D
TransmFiber
corresponds to the dispersion of the transmission fiber, L
DCF
corresponds to the length of the DCF, and D
DCF
corresponds to the dispersion of the DCF. In Equations 1 and 2, L
TransmFiber
corresponds to the length of the transmission fiber and L
DCF
corresponds to the length of the DCF. In Equation 2, S
TransmFiber
corresponds to the dispersion slope of the transmission fiber and S
DCF
corresponds to the dispersion slope of the DCF.
When the dispersion of the system is compensated, i.e., when D
Link
=0 (i.e., when D
LINK
is set equal to 0 for purposes of calculations), the length of DCF needed to compensate for the dispersion slope and the dispersion of the link can be determined by Equation 3. Because the values of the DCF dispersion, the transmission fiber dispersion, and the transmission fiber link are known, the length of DCF needed is given by:
L
DCF
=−(
D
TransmFiber
/D
DCF
)×
L
TransmFiber
. (Equation 3)
In order to compensate the link for the dispersion slope, S
DCF
, of the DCF itself, the RDS for the DCF and for the transmission fiber must be matched such that:
RDS
Trams
.
Fiber
=
S
Trams
.
Fiber
D
Trams
.
Fiber
=
S
DCF
D
DCF
=
RDS
DCF
(
Equation
4
)
However, when producing a DCF, the production tolerances inherently cause variations in the dispersion and in the RDS of the DCF. Dispersion variations at the center wavelength can be compensated by choosing the correct length of DCF, but RDS variations are not compensated. Tolerances on RDS for DCF are typically about ±15%, which can cause significant residual dispersion at the edges of the transmission band, which is undesirable for the aforementioned reasons.
A technique that uses DCM technology for improving dispersion compensation over a center wavelength and for reducing residual dispersion on wavelengths at the edges of the transmission band is disclosed in U.S. Pat. No. 5,781,673 (hereinafter the '673 patent), which is assigned to the assignee of the present invention, and which is incorporated herein by reference in its entirety. This patent discloses a wavelength division multiplexing (WDM) system in which the transmission path between a WDM receiver and a WDM transmitter comprises a transmission fiber of a particular length having a particular dispersion of a particular sign combined with a DCF of a particular length and having a particular dispersion of opposite sign as that of the dispersion of the transmission fiber. This combination ensures that the center wavelength of the channel will have a nominally zero overall dispersion.
In order to compensate for the residual dispersion on the other channels, the '673 patent discloses adding to the link a dispersion slope compensating fiber (DSCF) of a particular length and having a relatively large negative dispersion slope and a nominally zero dispersion. The dispersion slope of the DSCF is calculated as the sum of the residual dispersions on the extreme channels of the transmission path divided by the wavelength difference between the extreme channels.
Although the technique disclosed in the '673 patent improves dispersion compensation in broadband applications, a need to further improve dispersion compensation in various applications, such as broadband applications, still exists.
SUMMARY OF THE INVENTION
The present invention provides a dispersion compensation module (DCM) for compensa
Edvold Bent
Gruner-Nielsen Lars
Jorgensen Lene V.
Reed William A.
Fitel USA Corporation
Gardner & Groff, P.C.
Palmer Phan T. H.
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