Optical waveguides – Polarization without modulation
Reexamination Certificate
2000-03-01
2002-04-23
Ngo, Hung N. (Department: 2874)
Optical waveguides
Polarization without modulation
C385S024000
Reexamination Certificate
active
06377719
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to optical fiber signal transmission and, in particular, to the generation of polarization-mode dispersion (PMD) to emulate the natural occurrence of PMD in optical fiber; and to use PMD emulation to compensate for PMD generated by optical fiber.
BACKGROUND OF THE INVENTION
Polarization-Mode Dispersion (PMD) is a fiber-optic telecommunication system impairment which can prevent the transmission of high data rates, such as 10 Gb/s and 40 Gb/s. The effect of PMD originates with the inherent, built-in residual birefringence present in all single-mode optical fiber. Over the course of pulse transmission, PMD interacts with a transmitted optical pulse in such a way as to distort the shape of the pulse. The consequences vary with the degree of pulse distortion, from small penalties in transmission fidelity, to complete system outage. Accordingly, in order to transmit optical pulses at rates above 2.5 Gb/s, either the quality of the optical fiber must be sufficiently high so as to not introduce significant PMD or a PMD-compensating apparatus must be inserted in the transmission system, for example, between the end of the fiber-optic transmission line and the input to the optical receiver.
In order to transmit at a rate of 10 Gb/s over legacy fiber (that is, currently installed fiber), a PMD compensator (PMDC) is frequently necessary to recover acceptable system performance. It is generally believed that in order to transmit at a rate of 40 Gb/s and above over the most recently available fiber, a PMDC at the receiver may be essential. Accordingly, a PMDC is a desirable apparatus. A means for laboratory and factory testing of a PMDC is, consequently, a desirable apparatus. Such an apparatus is herein referred to as a PMD Emulator (PMDE).
Indeed a PMDC and PMDE are quite similar because both apparatuses must generate PMD; the former apparatus must generate PMD in order to cancel the accrued fiber PMD, the latter apparatus must generate PMD in order to test a PMDC. However, the generation of PMD for a PMDC and PMDE does have practical differences. A PMDC further requires the generation of a control signal which is used to monitor the PMD cancellation, and further requires a feedback system and control algorithm which automatically corrects for changing PMD. A PMDE further requires the precise and repeatable generation of PMD, and may not require the speed of change which may be necessary for a PMDC. A PMDE further requires the synthesis of the PMD effect so as to approximate the PMD of a real fiber as closely as possible. A PMDE further requires a performance which is both known and repeatable so as to test and verify the performance of a PMDC.
The term “PMDC” will refer herein to an apparatus which consists of: 1) a PMD generating mechanism; 2) a control-signal generating mechanism; and 3) a feedback control mechanism and algorithm which changes the PMD generating mechanism so as to cancel the PMD of the fiber-optical link. The term “PMDE” will refer solely to a PMD generating mechanism, which includes a means to change the state of PMD. It is recognized that a PMDE can be transformed into a PMDC through the addition of a control-signal generating mechanism and a feedback control mechanism and algorithm.
Polarization-mode dispersion is the composite phenomenon of two interleaved effects. One effect is the projection of an input state-of-polarization (SOP) onto a birefringent dielectric system. The other effect is an accrued differential temporal delay between two orthogonal polarization states.
FIG. 1
a
illustrates an input optical pulse
100
with an arbitrary input SOP
120
. The pulse
100
is incident upon an optical birefringent medium
110
with orthogonal birefringent axes fast
121
and slow
122
. The terms “fast” and “slow” refer to the speed of the optical pulse as projected on either axis: the pulse on one axis propagates faster than the pulse of the other axis due to the difference in refractive index, the latter which is due to the inherent birefringence of the fiber. The projection of the input SOP
120
onto the birefringent interface
110
results in the formation of two orthogonally polarized pulses
101
and
102
. The balance of energy on the two orthogonal polarization axes is dictated by the relative orientation of the input SOP
120
and the birefringent axes
121
,
122
at the interface
110
.
FIG. 1
a
illustrates the phenomena of polarization projection at a birefringent interface.
FIG. 1
b
illustrates an example of “simple” PMD. A short section of optical fiber
130
and the effect of PMD on an optical pulse
100
is herein illustrated. The optical pulse
100
has its SOP
120
projected onto the birefringent axes of the fiber
110
, resulting in pulse
101
on fast axis
121
and pulse
102
on slow axis
122
. The birefringence of fiber
130
causes a relative temporal delay between the two pulses
101
and
102
. This temporal delay is referred to as differential-group delay (DGD). At the end of optical fiber
130
pulses
101
and
102
exhibit a DGD of magnitude &Dgr;&tgr;
140
. The magnitude of DGD
140
depends on the magnitude of the birefringence and the length of fiber
130
over which the birefringence does not significantly change. The present instance of a single polarization projection followed by a single differential-group delay stage is denoted as simple, one one-stage, PMD.
FIG. 2
illustrates the concatenation of several simple PMD stages to form a more complex PMD response.
FIG. 2
a
illustrates substantially the same PMD as
FIG. 1
b
but the orthogonal polarization states are not explicitly indicated.
FIG. 2
a
illustrates a pulse
200
input to birefringent fiber segment
130
. The PMD of this fiber segment
130
generates DGD
240
between two output pulses
200
and
201
.
FIG. 2
b
illustrates the concatenation effect of two birefringent fiber segments
130
and
131
possessing dissimilar lengths and birefringent orientation. Fiber segment
130
produces two pulses
200
and
201
with DGD
240
. Fiber segment
131
produces two pulses for each pulse input, resulting in four pulses
200
,
201
,
202
,
203
. The time delay between pulse images
200
and
202
, and
201
and
203
, is the DGD
241
of fiber segment
131
.
FIG. 2
c
adds a third fiber segment
132
with dissimilar length and birefringent axis orientation. Again, each input pulse
200
,
201
,
202
,
203
to fiber segment
132
is copied and each pair
210
,
211
is delayed by DGD
242
, forming four pulse pairs
210
,
211
,
212
,
213
. Note that at each interface between fiber segments, the polarization projection alters the balance of energy between that on the incident SOP and that on the projected coordinates; thus, the variation in pulse amplitudes.
The fiber within a typical fiber-optical link is composed of tens or hundreds of fiber segments joined in series much as those in
FIG. 2
c.
The time-domain representation becomes difficult to extend to such a fiber because of the geometric increase in the number of pulses that is output from a long fiber link. The appropriate alternative representation is in the frequency domain.
FIGS. 3
a
and
3
b
illustrate the customary technical representation of the PMD effect in the frequency domain. The production of multiple pulses with various relative temporal delays is Fourier transformed into the spectrum of DGD,
FIG. 3
a.
The magnitude of DGD
301
is plotted as a function of frequency
300
. The relative energies of the output pulses and their composite state-of-polarization is represented by the Poincare-sphere representation of Principal States of Polarization (PSP). The PSP is used to represent the overall birefringent axes of a whole fiber link at each frequency. If an input sinusoidal optical wave has an SOP which aligns to the PSP of the fiber which corresponds to the frequency of the optical wave, then the energy of the input optical wave is completely transferred to only one PSP axis. Any other input SOP will cause a splitting of the input pulse
Lucent Technologies - Inc.
Moser, Patterson & Sheridan L.L.P.
Ngo Hung N.
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