Optics: measuring and testing – For optical fiber or waveguide inspection
Reexamination Certificate
2002-01-28
2004-05-11
Nguyen, Tu T. (Department: 2877)
Optics: measuring and testing
For optical fiber or waveguide inspection
Reexamination Certificate
active
06734955
ABSTRACT:
FIELD OF THE INVENTION
The invention resides in the field of optical telecommunications networks, and is directed in particular to dispersion measurement in optical networks.
BACKGROUND OF THE INVENTION
In optical transmission systems, the user traffic is carried by one or more channels traveling between a transmitter and a receiver in optical format. The receiver task is to convert the optical signal back into an electrical format and to extract the user signal. A channel is defined as a carrier wavelength modulated with a user signal. Ideally, a light pulse (representing a digital “1”) is a surge of light of a certain power at wavelength &lgr;
0
; in fact, the pulse of light has a certain “width” comprised of a small range of wavelengths about the central wavelength, as shown in FIG.
1
A.
The optical fibers used as the transmission medium and most optical components (optical amplifiers, filters) are dispersive, that is, different wavelengths of light travel at slightly different phase velocities V
ph
=&ohgr;/k=c
(&lgr;), where c is the vacuum speed of light. The propagation characteristics of each wavelength depend on the effective mode index n(&lgr;), or the effective propagation parameter k=2&pgr;n(&lgr;)/&lgr;. The mode index changes with wavelength, polarization and mode profile, due to material dispersion and due to the waveguide dispersion of the confined mode. The effective mode index n(&lgr;) shows a non-linear wavelength dependence over an extended spectral domain. As a result, not only the phase velocity, but also the group velocity v
g
=∂&ohgr;/∂k=c/[n−&lgr;(dn/d&lgr;)] changes with wavelength. The group velocity is the speed at which non-uniformities in the field intensity, such as an information-carrying modulated pulse train, move through the medium. As an initially short pulse requires some spectral width as dictated by the fundamental property of Fourier transforms, the wavelength-dependence of the group velocity tends to broaden the pulse as it propagates through the fiber, because different spectral components of the pulse travel at different velocities.
This wavelength dependency of the propagation parameter and consequently of the group velocity is termed chromatic dispersion CD, or intra-modal dispersion.
FIG. 1B
shows a signal ‘100101’ at the input of an optical link, and
FIG. 1C
illustrates how the light pulses representing ‘1's’ widen as the signal travels down the fiber. As a result, the pulse energy of symbols “1” spreads into the neighboring symbols “0” (ISI or intersymbol interference), so that the receiver could interpret the signal correctly as ‘100101’, or erroneously as ‘100111’.
It is evident that reconstructing the user signal from the received optical pulses can pose problems, especially in WDM (wavelength division multiplexed) systems, where a plurality of channels travels over the same link.
The chromatic dispersion parameter D(&lgr;) is defined as:
D
(
λ
)
=
∂
τ
∂
λ
·
1
L
EQ
1
where ∂&tgr; is the differential group delay (DGD) of two pulses, i.e. the variation of the travel time (in picoseconds) from the transmitter to the point of measurement, ∂&lgr; is the differential spectral separation of the two carrier wavelengths (in nanometers) and L is the length of the fiber (in kilometers) over which the dispersion is measured. The target dispersion for a fiber link is defined as:
D
T
(&lgr;)=
D
(&lgr;)·
L
EQ2
For example, for every km of fiber traveled through, two pulses with a 1 nm initial separation of wavelengths will experience a differential group delay of 1 ps, if the dispersion parameter of the fiber is 1 ps/(nm km). Similarly, the two outlying spectral components of a 10 Gb/s pulse with a 0.2 nm spectral width will widen by a whole bit period (100 ps) after some propagation distance, and will then cause bit errors by spreading the pulse energy into the neighboring symbol.
Since the dispersion parameter D is wavelength-dependent, another parameter is defined to characterize dispersion, namely the dispersion slope, given by:
S=∂D/∂&lgr;
EQ3
If we assume a linear dispersion dependence on wavelength in some interval &Dgr;&lgr;, the slope can be expressed as the ratio of change in the dispersion to the change in the wavelength &Dgr;D/&Dgr;&lgr; calculated with respect to a reference wavelength.
Chromatic dispersion can be corrected, or “compensated,” through the use of specially designed optical components (such as fibers, Bragg gratings) inserted at given locations along the transmission path. For a comprehensive compensation, the total dispersion of the compensating component (which could be packaged e.g. as a dispersion compensating module DCM) must have the same value, but opposite sign to the dispersion of the preceding transmission section, which is obtained if the dispersion is −D
T
(&lgr;), namely
D
DCM
·L
DCM fiber
=−D
fiber section
·L
fiber section
. EQ4
With the data rates of optical communication systems increasing through techniques such as dense WDM (DWDM), and network reach increasing through techniques known as ultra long reach (ULR), determination of chromatic dispersion of the fiber and optical components within the systems becomes increasingly important, but also more difficult. Thus, dispersion of each transmission section needs to be determined with as much accuracy as possible to provide accurate compensation, for achieving longer un-regenerated reach and ultimately a less expensive network.
Accurate link dispersion values are particularly useful in wavelength switched network. In these networks, end-to-end physical routes (paths) are dynamically set-up and removed arbitrarily (based on users' requests), without interruption of the co-propagating traffic. Agility requires accurate knowledge of the link parameters, since matching an end-to-end path to a connection request is based, among other rules, on individual link/path performance. The chances of setting-up a connection along a path increase (and the time-to-service decreases) if the selection process uses accurate path performance parameters, which include end-to-end (link) dispersion.
Fiber cable manufacturers provide chromatic dispersion parameters by wavelength windows for each fiber cable type. Also, most device specifications include CD information. A simple way to determine the total dispersion over a link is to multiply the dispersion coefficient for a certain type of fiber by the fiber length in km and to add to this the specified dispersion of the optical components connected in the respective link.
This method is often used in current point-to-point networks, where each span/link is provisioned based on estimated data, using in addition generous engineering margins to ensure that the span/link will successfully carry the traffic over the specified distance. This is clearly not the best way of using network resources. In addition, in many cases the fiber type is not known; there are no reliable methods to detect the type of the fiber buried in early days of the optical networking. Also, this method assumes a uniform dispersion along the entire fiber cable length, which is not generally true. While this assumption can be used in systems with a small channel-count and short links, it is not satisfactory for wavelength switched DWDM (dense WDM) systems.
A more accurate value of dispersion is evidently obtained by measuring the dispersion. Chromatic dispersion can be determined by performing time domain measurements and frequency domain measurements, as described for example by P. J Dean in “Optical Fiber Communications, Principles and Practice”, published in 1985 by Prentice-Hall International, Inc, London, pages 196-202.
However, current dispersion measurement methods cannot be readily used in wavelength switched (agile) networks, for at least the following reasons.
The current networks have a point-to-point architecture th
Du Mei
Sizmann Andreas Franz Ludwig
Solheim Alan Glen
Wight Mark Stephen
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