Note: Descriptions are shown in the official language in which they were submitted.
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Measurement of Polarization Mode Dispersion
This is a divisional of copending patent application
serial no. 08t445,3Z0, filed May l9, 1995, hereby incorporated
by reference.
FIELD OF INVENTION
The present invention relates to a method for testing
optical fibers, and more particularly to a method for
measuring at high resolution Polarization Mode Dispersion
(PMD) values in single mode optical fibers.
BACKGROUND OF THE INVENTION
Single mode optical fibers are used to transmit large
quantities of information over significant distances. In
order to preserve the integrity of such transmissions, it is
desirable to eliminate distortion. It is impossible, however,
to remove all forms of distortion from transmissive media.
Therefore, it is necessary to measure the distortionj either
to determine the suitability of a transmissive medium's
~;~um information capacity, or to determine the most
satisfactory manner of handling the-distortion. For a fiber
optic c~ ~ln;cations system, the bit-error rate is the most
significant specification for determ;n;ng the information
carrying capacity of the system. The bit-error rate is
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increased by, among other factors, the pulse broadening caused
by dispersion in a fiber. Use of a single mode fiber
eliminates modal dispersion, but not chromatic dispersion or
Polarization Mode Dispersion, which is a bandwidth limiting
effect that is present to some degree in all single mode
fibers that are suitable for optical transmissions. It is,
therefore, a potential source of signal distortion in optical
communications systems.
In general, Polarization Mode Dispersion measurement
in~Lr; ~nts are known in the art, and particularly defined in
draft standards of the Telecommunications Industries
Association, headquartered in Arlington, Virginia. These
st~n~ds include Fiber Optic Test Procedure FOTP-113 for
Polarization-Mode Dispersion Measurement for Single-Mode
Optical Fibers by Wavelength Scanning, FOTP-122 for
Polarization Mode Dispersion Measurement for Single-Mode
Optical Fibers by Jones Matrix Eigenanalysis, and FOTP-124 for
Polarization-Mode Dispersion Measurement for Single-Mode
Optical Fibers by Interferometric Method. The detailed
operation and procedure of the Polarization Mode Dispersion
measur t-- ~nt instrument is described in these standards.
Also, a particular prior art circuit means is described
in U.S.- Patent No. 4,i50,833, -issued to R. Jones, hereby
incorporated by referen~e. Polarization Mode Dispersion
mea~u~ -nt instruments include cycle counting, time pulse
methods, relative phase methods or Jones Matrix Eigenanalysis,
as discussed in more detail below. For example, Jones
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describes a known method for measuring dispersion in optical
fibers. In particular, Jones describes a relative-phase
method and apparatus for measuring transmissive dispersion,-
such as chromatic or polarization dispersion. A light source
modulated at a first frequency is synchronously varied at a
lower frequency back and forth to and from a first and a
second value of a transmission parameter, e.g. source
wavelength or polarization state. Relative phases of the
first modulation signal and the light transmitted through the
fiber under test are measured by a phase detector. A lock-in
amplifier compares the phase detector output to the lower
frequency signal to provide a direct current output indicative
of dispersion.
Another method for measuring dispersion in optical fibers
measures time differences. The Jones Matrix Eigenanalysis
method measures DGDA~ as a function of wavelength, where DGD
is known as a differential group delay, and PMD is expressed
as < Ar >~. The relative-phase method and apparatus described -
in Jones proved to be superior in resolution than the method
for measuring time differences.
Other known methods of measuring Polarization Mode
Dispersion in optical fibers include Interferometry, Jones
n Matrix Eigenanalysis, the Wavelength Sc~nn;ng (WS) cycle
cou~ting metnod, and the WS Fourier method. Interferometry
uses the time domain to employ a low-coherence light source
and a Michelson or Mach-Zehnder Interferometer to observe
output in the form of the autocorrelation function of the time
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distribution, and the Polarization Mode Dispersion of the
fiber may be obtained from this data. Interferometry is
limited at the low end by the coherence time, typically 0.15
picoseconds, of the broadband source used.
Jones Matrix Eigenanalysis uses a polarimetric
- determination of the instantaneous polarization transmission
behavior, in the form of a Jones matrix with two eigenstates
called Principal States of Polarization (PSP). By measuring
the wavelength variation of the Jones matrix and hence the
PSPs, the different delays between PSPs may be determined.
The delay is averaged over~a specific wavelength scan to
establish the fiber Polarization Mode Dispersion value. Jones
Matrix Eigenanalysis is limited by polarimetric accuracy and
resolution to 0.01 picoseconds.
The WS cycle counting and the WS Fourier methods both use
a light power transmission through the fiber using a linearly
polarized source and a polarization analyzer before the light
detector. The fiber gives rise to an oscillation pattern
whose oscillation frequency is related to Polarization Mode
Dispersion. In the WS cycle counting method, the number of
complete oscillations in a given wavelength interval is
counted to determine Polarization-Mode Dispersion. The WS
cycle counting method is limited to a ;n;mum of three cycles
in the wavelength scan, typically 0.09 picoseconds. In the WS
Fourier method, however, wavelength scanning Polarization Mode
Dispersion is determined by a frequency analysis technique on
the oscillation pattern based on a Fourier transform. The
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Fourier method is limited to a minimum of one cycle in the
wavelength scan used, typically 0.03 picoseconds.
One important disadvantage of the known prior art is that
the determination of the Polarization Mode Dispersion value is
adversely influenced by spurious responses that combine with
measured results to distort the Polarization Mode Dispersion
value.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a
method-for measuring with higher resolution Polarization Mode
Dispersion values below 0.1 picoseconds, useful, e.g. with
fibers used for transmission systems operating 5 Gigabits per
second or above.
It is also an object of the present invention to provide
a method for measuring Polarization Mode Dispersion values
between 0.01 picoseconds and 0.1 picoseconds for use, e.g.,
with WS Fourier and Interferometry methods.
It is also an object of the present invention to use a
birefringent or wavelength specific artefact to bias
Polarization Mode Dispersion away from zero, resulting in a
broadened Pol-arization Mode Dispersion peak of the artefact,
in order to obtain improved detection and determination of
very low Polarization Mode Dispersion levels in opt-ical
fibers.
It is also an object of the present invention to
determine the Polarization Mode Dispersion of an optical fiber
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by appropriate data processing of the broadened artefact peak.
It is also an object of the present invention to provide ,~
a method for calibrating wavelength scanning Polarization Mode
Dispersion instruments using a birefringent or wavelength
selective device.
In accordance with the present invention, a method is
provided for improving the measurement of Polarization Mode
Dispersion by incorporating an artefact with a stable known
Polarization Mode Dispersion value in a Polarization Mode
Dispersion measuring instrument, and having a light source
transmit light serially through an optic fiber to be tested
and the artefact. The artefact biases a total Polarization
Mode Dispersion measured by the Polarization Mode Dispersion
measuring instrument away from zero, thus removing the
undesirable influence of any spurious (near-zero) Polarization
Mode Dispersion response from the measurement. The
Polarization Mode Dispersion of the optic fiber may then be
accurately determined by appropriate data processing of the
measured Polarization Mode Dispersion. The artefact may also
be used to calibrate a wavelength sc~nn;ng Polarization Mode
Dispersion instrument.
The invention enables a high resolution measurement of
Polarization Mode Dispersion with at least one order of
magnitude higher, and possibly two orders of magnitude higher,
than that achievable with the prior art relative phase or time
mqasurement system.
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BRIEF DESCRIPTION OF THE DRAWINGS
The invention, both as to its organization and manner of
operation, may be further understood by reference to a dra~ing
(not drawn to scale) which includes Figures 1-4 taken in
connection with the following description.
Figure 1 is a block diagrammatic representation of a
Polarization Mode Dispersion measurement instrument in
accordance with the present invention utilizing a Mach-Zehnder
interferometer.
Figure 2 is a block diagrammatic representation of a
Polarization Mode Dispersion measurement instrument in
accordance with another embodiment of the present invention
suitable for use with wavelength scanning Fourier analysis (WS
Fourier).
Figure 3a is a graph of time versus Polarization Mode
Dispersion value for a prior art Polarization Mode Dispersion
measurement instrument.
Figure 3b is a graph of time versus Polarization Mode
Dispersion value for a Polarization Mode Dispersion
measurement instrument of the present invention.
Figure 4 is a block diagram of an application of the
present invention to calibrate a Polarization Mode Dispersion
instrument.
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DESCRIPTION OF THE BEST MODE OF THE INVENTION
Figures 1 and 2 are block diagrams of two Polarization
Mode Dispersion measurement instruments for high resolution,
polarization dispersion measurement, which are the subject of
the present invention. Figure 1 shows one Polarization Mode
Dispersion measurement instrument using an interferometric
measurement technique, while Figure 2 shows another
Polarization Mode Dispersion measurement instrument using a
wavelength scanning Fourier analysis (WS Fourier) t~chn;que.
In Figure 1, the Polarization Mode Dispersion measurement
i-nstrument has a light source 10 which may be either a light
emitting diode (LED), as shown, or in an alternative
embodiment, a superfluorescent light source (not shown). The
Polarization Mode Dispersion measurement instrument has a
polarizer 12 that responds to light coming from an output of
the light source 10, for providing polarized light. A beam
splitter 14 connected to the polarizer 12 divides the
polarized light for transmission in a first path 16 and a
second path 18. The second path 18 includes a delay line 20
for delaying the transmission of light. The delay line 20 can
be adjusted to alter a relative optical delay between the
first-and second paths 16 and 18. As shown, the delay line 20
is a Mach-Zehnder interferometer, which is known in the ~rt.
A beam splitter 22 receives light transmitted along the first
and second paths 18 and 20 and delivers it to an artefact 28.
The artefact 28.is a device which produces a known,
stable Polarization Mode Dispersion. The artefact 28 will
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assure that the Polarization Mode Dispersion measurement
~ instrument will have a known interference peak level at a
particular time value T, as best shown in Figure 3b. In the
embodiment shown in Figure 1, the artefact 28 is a
birefringent device, which may be a birefringent waveplate,
birefringent fiber or other birefringent device. The time T
is the time difference between the fast and slow polarization
modes, or simply the Polarization Mode Dispersion of the
artefact 28. In the present invention, the artefact 28 serves
to bias the total Polarization Mode Dispersion measured by the
instrument away from zero, removing the influence of any
spurious (near-zero) Polarization Mode Dispersion response
from the measurement, as shown in Figure 3b.
A test fiber 26 receives light from an output of the
artefact 28. The connection between the artefact 28 and the
test fiber 26 is known in the art, and may include a lens
system, a butt splice to a single mode fiber pigtail or an
index-matched coupling. However, the scope of the invention
is not intended to be limited to any particular series
arrangement between the artefact 28 and the test fiber 26.
For example, as shown, the artefact 28 is arranged before the
test fiber 26 in Figure 1, while in Figure 2, the artefact 60
is arranged after the test fiber 56. Such a construction
could also be incorporated in accordance with these teachings
in a Michelson interferometer.
An output of the fiber 26 is delivered to an analyzer 30
that observes interferences between principal orthogonal
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states of polarization, and provides an analyzed signal to a
detector 32, that represents polarization states versus power.
The detector 32 converts an optical signal to an electrical.
signal. In an alternative approach, a polarimeter may be
used. A lock-in amplifier 34 is a synchronized phase and volt
meter which is used to demodulate chopped or modulated optical
signals for signal processing. A computer 36 provides
electronic signal processing and apparatus control functions.
The interference signature versus the setting of the delay
line 20 is determined and stored using a standard computer 36.
Figure 2 shows an alternative embodiment of the present
invention in which the Polarization Mode Dispersion
measurement instrument has an artefact 60 coupled to an output
of a test fiber 56. In Figure 2, a light source 50 delivers
light to a polarizer 52 for coupling through a splice 54 to a
fiber under test 56. A splice 58 couples an output of the
test fiber 56 to the artefact 60. An analyzer 62 analyses the
light polarization state, and an optical spectrum analyzer or
monochromator 64 allows the polarization transmission versus
optical wavelength to be measured. The computer 68 performs a
Fourier analysis and apparatus control functions, and
Polarization Mode Dispersion calculations.
The artefact 60 produces a known, stabie Polarization
Mode Dispersion. It will assure that ~he Polarization Mode
26 Dispersion measurement output will have a known peak level at
a particular time value T. In the embodiment in ~igure 2, the
artefact 60 may be a birefringent device, which may be a
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birefringent waveplate, birefringent fiber or other
birefringent device. The time T is the time difference
between the fast and slow polarization modes, or simply the
Polarization Mode Dispersion of the artefact 60. In
alternative embodiments, the artefact 60 may also be a
reflective or trAn~ ive device which provides a known
stable sinusoidal response of power versus wavelength
indicative of the insertion loss spectrum of the artefact. An
example of either artefact 60 is a Fabry-Perot etalon,
including an interferometer. The sinusoidal response of power
versus wavelength will give an apparent Polarization Mode
Dispersion peak at time T, corresponding to the known, stable
insertion loss spectrum of the artefact.
A comparison of the results of the graphs in Figures 3a
and 3b illustrates how the addition of the artefact 28 (Figure
1) or the artefact 60 (Figure 2) of the present invention
greatly improves the resolution of Polarization Mode
Dispersion measurement instruments shown in Figures 1 and 2.
In both Figures 3a and 3b, the abscissa is time, and the
ordinate is Polarization Mode Dispersion value. As shown,
spurious responses are illustrated in dotted lines, and
measured results are illustrated in solid lines. As
Polarization Mode Dispersion values approach zero, there are
many effects that can produce a greater error than the value
of the Polarization Mode Dispersion. Spurious responses are
due to optical losses and other optical imperfections, or
source coherence. Such responses provide results that combine
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with a low level of Polarization Mode Dispersion and distort
it. As shown in Figure 3a, in a prior art Polarization Mode
Dispersion measurement instrument (not having the artefact.28
or 60), the width, e.g. Root Mean Square width, of the
signature (solid line) is calculated to determine Polarization
Mode Dispersion of the fiber under test. However, the
spurious responses have combined with the measured results to
distort the Polarization Mode Dispersion value.
In contrast, as shown in Figure 3b in the Polarization
Mode Dispersion measurement instrument of the present
invention, the basic Polarization Mode Dispersion signature is
transposed by the artefact 28 or 60 from zero to time T. The
spurious effects do not interact with the measured dispersion.
As shown in Figure 3b, the spurious responses do not affect
measurement of Polarization Mode Dispersion during a
calculation of the width, e.g. Root Mean Square width, of the
measured peak.
Figure 4 shows a calibration of a Polarization Mode
Dispersion in accordance with the present invention. A test
fiber is not included in the light path. An artefact 106 is
included, which receives light transmitted from a broadband
source 100 through a polarizer 102 coupled to a splice 104. A
splice 108 couples the output of the artefact 106 to an
analyzer 110 whose output is analyzed by a measuring means 112
comprising an optical spectrum analyzer or monochromator and
~uLation means 114 operating in-accordance with the
principles described with respect to Figure 2. The artefact
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104 will bias the measured Polarization Mode Dispersion away
from zero. The measured Polarization Mode Dispersion is
compared to the known Polarization Mode Dispersion value of
the artefact 104. The result of this can be viewed in a
number of ways. The system of Figure 4 is thus calibrated so
that a particular electrical output corresponds to the known
value of the artefact 104. Further, this operation provides
an update to factory calibration or periodic field
calibration. This method is applicable to a WS Fourier method
and WS cycle counting measuring methods, discussed above.
Additionally, there are further calibration techniques
available for wavelength scanning methods, which may use
wavelength transmissive or reflective devices for an artefact.
In such a case, the Polarization Mode Dispersion apparatus
lS produces a Polarization Mode ~ispersion signature at time T
equivalent to the known, stable insertion loss spectrum of the
artefact in accordance with the principles described with
respect to Figure 2. The above teachings will enable those
skilled in the art to provide many different embodiments of
high resolution measuring means which can employ wavelength
transmissive devices for the artefact.
It will thus be seen that the objects set forth above,
and those-made apparent from the preceding description, are
efficiently attained and, since certain changes may be made in
the above construction without departing from the scope of the
invention, it is intended that all matter contained in the
above description or shown in the accompanying drawings shall
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be interpreted as illustrative and not in a limiting sense.
It is also to be understood that the following claims are r
intended to cover all of the generic and specific features of
the invention herein described and all statements of the scope
of the invention which, as a matter of language, might be said
to fall therebetween.
