Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
1999-09-03
2002-04-30
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S227170, C356S364000
Reexamination Certificate
active
06380533
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of fiber optic communications and specifically to the measurement of first- and higher-order polarization mode dispersion vectors in optical fibers.
BACKGROUND OF THE INVENTION
Dispersion refers to the tendency of a light beam to spread out in time as it propagates through optical fiber. Several types of dispersion occur in optical fibers. One type is known as polarization mode dispersion.
Polarization mode dispersion 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 of a short fiber section the polarization or state of polarization of the light beam can be thought of as the direction of these right angle vibrations, where the light beam travels in a straight line. 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 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 such as 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”.
PMD affects the polarization 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” or &Dgr;&tgr;. For the simple case of a short fiber section, PMD causes the polarization of the light beam at the output of the fiber section to vary with frequency in a periodic fashion when the polarization of the light beam at the input remains fixed. However, in the general case of PMD, most fibers can be modeled as many such fiber sections whose axes are oriented at random angles relative to each other. Although the behavior is more complex, the PMD effects of this random combination are similar to the simple case above over a narrow frequency range. Instead of two polarization waveguide modes, there are pairs of special polarizations, called the principal states of polarization, both at the input and output, displaying the differential group delay.
A convenient way to represent the effects of PMD caused by a particular optical device or span of optical fiber is using 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. For instance, the positive s
1
axis of the Poincare sphere represents horizontal linear polarization, while the positive s
2
axis represents 45-degree linear polarization, and all linear polarizations are on the equator.
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 describes the axis of the rotation. Finally, the direction of the PMD vector also describes an axis that intercepts the Poincaré sphere at two points on the surface of the sphere. These two intercept points represent the two principal states of polarization for the optical device or fiber.
A principal state of polarization, “PSP”, is a property of an optical device or span of fiber such that if a light beam's polarization is aligned with the PSP at the input of the optical device or fiber, to first order, the light beam's polarization at the output will not change when the light beam's frequency at the input is varied. However, to second and higher orders with frequency, the output polarization does change. In the absence of polarization-dependent loss, each optical device or span of fiber has an 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.
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. Although there are currently many methods for measuring PMD, most of these 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. Determination of the full vector characteristics of PMD is necessary for deducing the effects of higher order PMD. Higher order PMD describes the change of the PMD vector with frequency. Knowledge of the higher order PMD effects is necessary where there are significant changes of the PMD vector across the signal frequency bandwidth.
There are two commonly used methods that provide information on the PMD vector—the Poincaré Sphere Technique, “PST,” and the Jones Matrix Eigenanalysis, “JME.” A general prior art apparatus for measuring PMD that is common to both methods is shown in block diagram form in
FIG. 5. A
light source
100
capable of operating at different frequencies, such as a tunable laser, inputs a light beam of a chosen frequency. A polarizing device
110
, such as one or more linear polarizers, then imparts a chosen polarization state to the light beam. A control block
120
, which could be a computer, controls the frequency of light source
100
and chooses the polarization imparted by polarizing device
110
. The polarization state of the light beam may be represented by a vector in Stokes space and in the Poincaré sphere. The light beam then passes through the device under test
130
which could be a span of optical fiber. A measuring device
140
, such as a polarimeter, measures the polarization state of the light beam at the output of the device under test. The data obtained from the measuring device is then analyzed in analysis block
150
, which could be a computer, to determine the PMD vector characteristics.
The Poincaré Sphere Technique requires the input of at least two di
Jopson Robert Meachem
Kogelnik Herwig Werner
Nelson Lynn Elizabeth
Le Que T.
Lucent Technologies - Inc.
Luu Thanh X.
Moser, Patterson & Sheridan L.L.P.
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