Optical waveguides – Polarization without modulation
Reexamination Certificate
1999-12-22
2002-05-07
Spyrou, Cassandra (Department: 2872)
Optical waveguides
Polarization without modulation
C359S199200, C359S199200
Reexamination Certificate
active
06385356
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to compensating for polarization mode dispersion in a fiber transmission system. More specifically, the present invention relates to compensating for higher order polarization mode dispersion and in one illustrative embodiment the compensation is performed at the front end of the fiber transmission system.
BACKGROUND OF THE INVENTION
Optical telecommunications generally involves the use of light modulated with data, transmitted through optical fibers. As the light propagates through the fiber, its signal characteristics may become distorted by the fiber in a number of ways. One type of distortion is known as polarization mode dispersion, or “PMD”.
PMD refers to an effect that an optical device, such as a span of optical fiber, has on the separate polarizations of a light beam. A light beam can be approximated as having electrical components that vibrate at right angles to the direction of travel. In the simple case the polarization or state of polarization of the light beam can be thought of as the direction of these right angle vibrations. In the more general case, these components are superimposed in a more complex way. As shown in
FIG. 1
, within a short optical fiber section
10
, an orthogonal set of two polarized waveguide modes
20
and
30
can be found which have electric field vectors aligned with the symmetry axes of the fiber. The polarization of a light beam propagating through the fiber section can be represented by the superposition of vector components aligned with these polarization waveguide modes of the fiber as shown in FIG.
2
. In
FIG. 2
, the polarization waveguide modes
20
and
30
are shown as two axes. The input polarization
40
is represented as the vector sum of two components
50
and
60
which are aligned with the polarization waveguide modes of the fiber section.
In ideal fiber, which has a perfect circular cross-section and is free from external stresses, the propagation properties of the two polarized waveguide modes are identical. However, imperfections introduced in the manufacturing process may result in fiber that is not perfectly circular. In addition, fiber that has been installed may suffer from external stresses arising from pinching or bending. These manufacturing imperfections and external stresses cause the two polarized waveguide modes to have different propagation characteristics which in turn gives rise to polarization mode dispersion, or “PMD”.
A convenient way to represent the effects of PMD caused by a particular optical device or span of optical fiber uses Stokes space, a three-dimensional geometrical space, and the Poincaré sphere, a sphere within Stokes space where every possible polarization state maps to a specific (and different) point on the sphere. Three axes, S
1
, S
2
, and S
3
, define this three dimensional space and any polarization can be described in reference to these axes, in other words by its S
1
, S
2
, and S
3
components. The S
1
, S
2
, and S
3
components of a polarization are called its Stokes parameters.
PMD affects the propagation of a light beam with respect to both time and frequency. With respect to time, PMD causes the two vector components comprising the polarization of the light beam to propagate down the two polarization waveguide modes at different velocities and thus separate in time as seen in FIG.
3
. In
FIG. 3
, the two components
50
and
60
of input polarization
40
are aligned with polarization waveguide modes
20
and
30
. This time gap is known as the differential group delay, “DGD”, represented as &Dgr;&tgr;. The difference of arrival times of the two pulses results in a broadening of the combined signal. The larger the differential group delay the broader the combined pulse. This in turn restricts the bit rate that can be transmitted through the fiber. With respect to frequency, the output polarization will vary as a function of the optical carrier frequency in a periodic fashion when the polarization of the light beam at the input remains fixed.
In general, an optical fiber will not have identical imperfections such that the two polarized wave-guide modes have the same orientation along the entire length of the fiber. However, most fibers can be modeled as a concatenation of many smaller fiber sections, each of which is considered to have a uniform birefringence and thus impart a uniformly oriented PMD to the light beam travelling through it. Birefringence refers to the difference in indices of refraction or light velocity of the two polarization components of a fiber. The effect over the full span is analyzed by considering the smaller lengths to be joined such that their respective axes are oriented at random angles relative to each other.
Although the behavior of a real length of fiber is more complex than that of a small section, over a narrow frequency range the PMD effects of both the real length and simple short length fibers are similar. However, instead of two polarization waveguide modes, the real length of fiber can be viewed as having orthogonal pairs of special polarizations, called the principal states of polarization (“PSP”) which, in general, vary with frequency. When a pulse is launched into a fiber with some optical power on each PSP, the output will consist of two light pulses separated in time by the differential group delay. In the absence of polarization-dependent loss, each optical device or span of fiber has a different orthogonal pair of PSP's for each frequency. Polarization dependent loss refers to the difference in the amount of loss a light wave can experience with changes in its state of polarization.
As stated above, even for a simple short fiber section, PMD causes the polarization of the light beam at the output of the fiber section to vary with frequency. The frequency effect of PMD can be easily seen when displayed on the Poincaré sphere. As shown in
FIG. 4
, for a light beam having a fixed input polarization
40
, the output polarization
70
of the light beam moves locally in a circle on the surface of the Poincaré sphere as the frequency of the light beam is varied from &ohgr;
1
to &ohgr;
2
to &ohgr;
3
.
Using Stokes space and the Poincaré sphere, the various effects of PMD for a given optical device or span of fiber may be compactly represented using a single, three-dimensional vector referred to as the PMD vector or
&OHgr;
. The magnitude of the PMD vector, |
&OHgr;
|, describes the time effect of PMD and the rate of rotation of the output polarization with respect to frequency. In other words, |
&OHgr;
|=&Dgr;&tgr;. The direction of the PMD vector points along one of the PSPs for the fiber. This can be represented mathematically as
&OHgr;
=&Dgr;&tgr;
q
, where
q
is the unit vector indicating the direction of one of the PSPs.
Since PMD can limit the transmission bandwidth of optical fiber, measurement of the PMD of a span of fiber is necessary to determine the span's data transmission capability as well as to provide information for compensating the PMD in the span. There are many known methods for measuring PMD. Some methods only provide a measurement of the magnitude of PMD, i.e., the differential group delay, and do not provide information on the PMD vector characteristics. A method for measuring PMD vectors is the Poincaré Sphere Technique, or “PST”. For each PMD determination, two different input polarizations are injected into an optical device under test, such as a fiber section, at each frequency of a frequency pair and the output polarizations are measured. Specifically, a light beam having a first input polarization is injected at the first frequency of the frequency pair into an optical device under test and the output polarization measured. Then, a light beam having this same first input polarization is injected at the second frequency of the frequency pair into the device under test and a second output polarization is measured. Third, a light beam having a second input polarization is injected at the same f
Jopson Robert M.
Kogelnik Herwig
Nelson Lynn E.
Assaf Fayez
Lucent Technologies - Inc.
Moser, Patterson & Sheridan L.L.P.
Spyrou Cassandra
LandOfFree
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