U.S. patent application number 10/039919 was filed with the patent office on 2002-07-04 for system and method for measuring polarization mode dispersion suitable for a production environment.
Invention is credited to Allen, David W., Evans, Alan F., Racki, Jerome G..
Application Number | 20020085195 10/039919 |
Document ID | / |
Family ID | 22428340 |
Filed Date | 2002-07-04 |
United States Patent
Application |
20020085195 |
Kind Code |
A1 |
Allen, David W. ; et
al. |
July 4, 2002 |
System and method for measuring polarization mode dispersion
suitable for a production environment
Abstract
A system for measuring polarization mode dispersion (PMD) in a
fiber using a polarizer controlling the polarization state of light
input to the fiber and a polarization analyzer measuring the
polarization state of light output from the fiber. Jones matrix
analysis is applied to data derived from three input polarization
states and two wavelengths of probing radiation. Performance is
improved by using incoherent light sources such as light emitting
diodes in conjunction with two bandpass filter. However, a laser
source and optical detector are used to align the fiber. The system
is particularly useful in measuring PMD values in short lengths of
fiber and mapping those values with a long fiber from which the
test fiber was cut. Preferably, the PMD is measured for various
values of twist experimentally induced in the test fiber, and the
short-length PMD value is that associated with zero-internal twist
in the fiber as calculated according to a model. The fiber may also
be loaded during measurement.
Inventors: |
Allen, David W.;
(Wilmington, NC) ; Evans, Alan F.; (Beaver Dams,
NY) ; Racki, Jerome G.; (Painted Post, NY) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
|
Family ID: |
22428340 |
Appl. No.: |
10/039919 |
Filed: |
October 29, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10039919 |
Oct 29, 2001 |
|
|
|
09512724 |
Feb 24, 2000 |
|
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60127107 |
Mar 31, 1999 |
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Current U.S.
Class: |
356/73.1 |
Current CPC
Class: |
G01M 11/336
20130101 |
Class at
Publication: |
356/73.1 |
International
Class: |
G01N 021/00 |
Claims
What is claimed is:
1. A polarization mode dispersion measurement system for measuring
a fiber under test comprising: at least one incoherent light source
emitting light at a first wavelength and at a second wavelength; an
optical polarizer adjustable to at least three polarization states;
two bandpass filters passing light respectively at said first and
second wavelengths and being insertable in a light path between
said at least one light source and said optical polarizer, light
output from said polarizer being received by said fiber under test;
and a polarimeter receiving an optical output of said fiber under
test and measuring a polarization state of light received from said
fiber under test.
2. The system of claim 1, wherein the at least one light source
comprises a first and a second light emitting diode emitting
respectively at said first and second wavelengths and the first and
second bandpass filters receive respective optical outputs of the
first and second light emitting diodes, and further comprising a
first light switch having two first switch inputs receiving
respective outputs of said first and second bandpass filter and
selectively connectable to a first switch output, the polarizer
receiving the first switch output.
3. The system of claim 2, further comprising: a second optical
switch comprising a second switch input receiving the light output
from the fiber under test, and at least two second switch outputs
selectively connectable to the second switch input, an output of a
first of the switch outputs being received by the polarimeter; and
an optical detector receiving an output of a second of the second
switch outputs.
4. The system of claim 3, further comprising a laser emitting
within a bandwidth associated with the first and second light
emitting diodes and wherein the first switch includes a third first
switch input receiving an output of the laser and being selectively
connectable to the first switch output.
5. The system of claim 3, further comprising a visible laser
emitting at a visible wavelength and wherein the first switch
includes a fourth switch input receiving an output of the visible
laser and being selectively connectable to the first switch
output.
6. The system of claim 1, further comprising: an optical switch
comprising a switch input receiving the light output from the fiber
under test, and at least two switch output selectively connectable
to the switch input, an output of a first of the switch outputs
being received by the polarimeter; and an optical detector
receiving an output of a second of the switch outputs.
7. The system of claim 1, further comprising: a laser emitting
within a bandwidth associated with the first and second
wavelengths; and a switch including at least two inputs receiving
respective outputs of the laser and the two bandpass filters and
being selectively connectable to an output providing light to the
fiber under test.
8. The system of claim 1, further comprising: a visible laser
emitting light emitting light at a visible wavelength; and a switch
including at least two inputs receiving respective outputs of the
laser and the two bandpass filters and being selectively
connectable to an output providing light to the fiber under
test.
9. The system of claim 1, further comprising a twist unit able to
induce a selected amount of twist along the fiber under test.
10. The system of claim 1, further comprising a load cell able to
induce a selected load upon the fiber under test.
11. A method of measuring a birefringent property in an optical
fiber comprising the steps of: (a) passing incoherent light through
a bandpass filter to form probing light; (b) setting a state of
polarization for the probing light; (c) passing the probing light
with set state of polarization through a fiber under test; (d)
detecting a state of polarization for the probing light with set
state of polarization after exiting the fiber; and (e) from
repeated sequences of steps (a) through (d) determining the
birefringent property of the fiber under test.
12. The method of claim 11 further comprising aligning the fiber
under test with coherent light.
13. The method of claim 11, wherein the determining step uses
polarization states measured for two optical wavelengths passed by
two bandpass filters and three sets of polarization states.
14. The method of claim 13, wherein the determining step uses Jones
matrix analysis.
15. The method of claim 11, wherein the birefringent property is a
differential group delay.
16. The method of claim 11 further comprising: (f) twisting the
fiber under test to a twist value, wherein the determining step
determines the birefringent property value corresponding to the
twist value.
17. The method of claim 16 further comprising: (g) repeating steps
(a) through (f) for a plurality of twist values; and (h) selecting
a birefringent property representative of the fiber under test from
the plurality of birefringent property values.
18. The method of claim 17, wherein the selected birefringent
property is a differential group delay.
19. The method of claim 18, wherein the selected differential group
delay is a maximum value determined from the plurality of
differential group delay values.
20. The method of claim 11, further comprising applying a plurality
of loads to the fiber under test and measuring the birefringent
property for each of the loads.
21. The method of claim 11, wherein said birefringent property is a
differential group delay.
22. A method of measuring polarization mode dispersion comprising
the steps of: (a) passing light from an incoherent light source
through a bandpass filter, having a transmission peak at a
transmission wavelength, to a polarization system passing a
selectable polarization state of light; (b) passing light from the
polarization system through a fiber under test; (c) measuring a
polarization state of light output from the fiber under test; (d)
performing steps (a) through (c) at least six times for all
combinations of three polarization states of light of said
polarization system and for two transmission wavelengths; and (e)
calculating a polarization mode dispersion from the six
polarization states measured in step (d).
23. The method of claim 22, including aligning the fiber under test
with a source of laser light incident upon said polarization system
and an optical detector receiving an output from the fiber under
test.
24. A method of qualifying a spool of at least 4 km of fiber
comprising: cutting a length of no more than 5 m of fiber from the
spool, the 5 m of fiber forming a fiber under test; measuring a
polarization mode dispersion of the fiber under test; and
associating the polarization mode dispersion measured for the fiber
under test with the fiber remaining on the spool through an
empirically derived mapping.
25. The method of claim 24 wherein the empirically derived mapping
comprises: (a) from a first fiber wrapped on a first spool
containing at least 4 km of fiber, cutting a length of at least 100
m of the first fiber; (b) measuring a first value of polarization
mode dispersion on the at least 100 m fiber; (c) cutting a length
of no more than 5 m of the first fiber; (d) measuring a second
value polarization mode dispersion on the no more than 5 m of
fiber; (e) associating the first value of polarization mode
dispersion with the second value of polarization mode dispersion;
and (f) repeating steps (a) through (e) on other spools of fiber to
thereby form a mapping between said first and second values of
polarization mode dispersion.
26. The method of claim 25, wherein the step (b) of measuring the
first value of polarization mode dispersion includes externally
twisting the fiber under test to a sequence of twists and measuring
respective twist values of polarization mode dispersion; and
selecting for the first value of polarization mode dispersion a
value derived from the respective twist values associated with a
predetermined amount of twist internally experienced in the
fiber.
27. The method of claim 26, wherein the selected value is a maximum
value derived from the respective twist values.
28. The method of claim 27, where the selecting step includes
fitting the respective twist values to a relationship describable
with a number of parameters less than the number of respective
twist values and calculating the first value of polarization
dispersion from the parameters.
Description
[0001] This application is a Divisional Application of U.S. patent
application Ser. No. 09/512,724, which claims priority to and the
benefit of U.S. Provisional Patent Application No. 60/127,107,
filed Mar. 31, 1999.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to optical measuring
equipment and methods, and in particular to such equipment and
methods for measuring birefringence in such measures as
differential group delay.
[0004] 2. Technical Background
[0005] Optical fiber is the favored transmission medium for
long-distance telecommunication systems because of its very large
bandwidth (that is, data carrying capacity), immunity to noise, and
relatively low cost. Attenuation in silica optical fiber has been
reduced to such low levels that it is possible to transmit data
over hundreds of kilometers without the need for amplifiers or
repeaters. The data carrying capacity of a fiber communication
system over relatively short distances is in large part dictated by
the speed of the electronics and opto-electronics used at the
transmitter and receiver. At the present time, the most advanced
commercially available optical receivers and transmitters are
limited to about 10 gigabits/sec (Gb/s), although 40 Gb/s systems
are being contemplated.
[0006] However, over the longer distances typical for
telecommunications, dispersion of various types may limit the
useful bandwidth. A cylindrical optical fiber of fairly large cross
section can transmit a number of waveguide modes exhibiting
different spatial power distributions. The propagation velocity
differs between the fundamental mode and the higher-order modes in
an effect called modal dispersion. An optical signal impressed by a
transmitter on the fiber will typically contain a distribution of
all the modes supportable by the fiber. Because of the modal
dispersion, the different modes after traversing a long section of
fiber will arrive at the receiver at slightly different times. The
transmission rate is limited by the dispersion integrated along the
transmission length.
[0007] In order to avoid modal dispersion, most modem fiber
communication systems intended for long-distance transmission rely
upon single-mode fiber. In the case of a simple fiber with a core
and cladding, the core of a single-mode fiber is so small, taken in
conjunction with the difference of refractive indices between the
core and the cladding, that the fiber will support only the
fundamental mode. All higher-order modes are quickly attenuated
over the distances associated with long-distance telecommunication.
The description is more complicated for a profiled fiber or for a
fiber having multiple cladding layers, but it is well known how to
fabricate and test a fiber such that it is single-moded.
[0008] A circularly symmetric single-mode fiber in fact supports
two fundamental transverse modes corresponding to the two
polarization states of the lowest-order modes. To a fair
approximation, these two lowest-order modes are degenerate in the
circular geometry of a fiber and have the same velocity of
propagation so there is no polarization dependent dispersion.
However, as will be explained later, polarization dependent
dispersion can arise in a realistic fiber.
[0009] In the past, high bit-rate transmission over long distances
of single-mode fiber has been limited by chromatic dispersion, also
characterized as group velocity dispersion. A data signal impressed
on an optical carrier signal causes the optical signal to have a
finite bandwidth, whether it be considered produced by the spectral
decomposition of a pulsed signal or by the data bandwidth of an
analog signal. Generally, the velocity of propagation or
propagation constant of an optical signal, is primarily dependent
upon the refractive index of the core, varies with optical
frequency. As a result, the different frequency components of the
optical signal will arrive at the receiver at different times.
Chromatic dispersion can be minimized by operating at wavelengths
near zero dispersion, about 1300 nm for silica, or by other methods
for compensating dispersion.
[0010] Despite its circularly symmetric design, real optical fiber
is typically birefringent. This means that the two lowest-order
axial modes are not degenerate, and the fiber at any point may be
characterized as having a fast axis and a slow axis. The two modes
traveling along the fiber with their electric field vectors aligned
respectively with the fast and slow axes of the fiber will
propagate relatively faster or slower. As a result, the group
velocity of a signal traversing the fiber is a function of the
polarization state of the optical signal. Birefringence can arise
from internal or external sources. The fiber may have been drawn
with a slight physical non-circularity. The fiber may be installed
such that a bend, lateral load, anisotropic stress, or a twist is
applied to it. The birefringent interaction is complicated by
coupling of the two modes also occurring at fiber twists, bends, or
other causes. The coupling causes energy to transfer between the
orthogonal modes. But even with mode coupling, the group delay
continues to spread out, resulting in a significant polarization
mode delay or dispersion (PMD). The cause of mode coupling is not
completely understood, but it is modeled by a statistical model of
randomly occurring mode-coupling sites with an average distance
between the sites (mode coupling length), which typically assumes a
value between about 5 m and 100 m. The exact mode coupling length
depends on the deployment of the fiber and is not usually
characteristic of the intrinsic fiber birefringence.
[0011] It is estimated that above about 10 Gb/s, polarization mode
dispersion limits fiber bit rates more than other types of
dispersion. Polarization mode dispersion also degrades cable
television (CATV) systems by introducing composite second-order
distortion and signal fading.
[0012] Some fiber manufacturers draw their fiber with a small
continuous twist applied to the fiber so that manufacturing
anisotropies do not allow the fast and slow modes to always be
aligned to a propagation mode. Thereby, the difference in
propagation delay between the two modes is lessened, resulting in
reduced PMD. A further technique for reducing net PMD over a long
distance is to periodically reverse the direction of the
manufacturing twist.
[0013] In the past, polarization mode dispersion has been treated
as a time-dependent quantity requiring a statistical description.
PMD has been typically measured on long lengths (lkm or more) of
fiber wound under low tension about a spool of large diameter. The
bending and stress induced by higher tension winding on a smaller
shipping spool affect the birefringence and mode coupling and,
hence, the average PMD experienced. However, setting up such a test
demands time and resources. Further, the lkm sections of fiber cut
from the shipping spool or the production line cannot be otherwise
used, and the testing represents a loss lkm of fiber, which for a
standard 25km spool is a loss of 4%.
[0014] Accordingly, it is desired to measure the effects of
polarization mode dispersion expected to be experienced in a
realistic environment with out the need to test long lengths of
fiber. It is further desired to measure the effects of polarization
mode dispersion in an accurate and deterministic fashion.
SUMMARY OF THE INVENTION
[0015] The invention includes a method and apparatus for measuring
polarization mode dispersion in an optical fiber, preferably
quantified as differential group delay between the two fundamental
polarization modes.
[0016] In one aspect of the invention, one or more incoherent light
sources are used in conjunction with optical bandpass filters to
provide light to a polarimeter arranged to measure birefringence in
an optical fiber. The polarimeter measures how the fiber affects
the state of polarization of light passing through it, preferably
by a measurement of polarization mode delay or dispersion.
[0017] Visible laser light may be switched into the fiber for
visual alignment. Laser light of wavelength comparable to that of
the incoherent sources may also be switched into the fiber and
electronically detected to complete the alignment. An optical
switch can be positioned at the output of the fiber under test to
switch the light alternatively to the polarimeter and the alignment
detector without affecting the measurement of polarization mode
dispersion.
[0018] The fiber may be subjected to a selected amount of twist
along its length. The measured twist-dependent polarization mode
dispersion may be used to determine several optical properties of
the fiber. The fiber may also be subjected to a selected amount of
load or otherwise stressed during its testing.
[0019] The value of polarization mode dispersion measured for a
short length of fiber may be empirically mapped to values for
longer fiber, with the polarization mode coupling length being
intermediate the two fiber lengths. The mapping may be used to
measure the mode coupling length.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a schematic illustration of system for measuring
polarization mode dispersion in a short length of optical
fiber.
[0021] FIG. 2 is a graph of the twist dependence of the
differential group delay of a fiber.
[0022] FIG. 3 is graph of the mapping between short-length and
long-length values of the polarization mode dispersion.
[0023] FIG. 4 is an axial cross-sectional view of a load cell.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] The invention enables the measurement of polarization mode
dispersion performed by an improved measurement system on a short
length of fiber. The fiber length is maintained generally to about
1 m, which is usually less than the length over which modes are
randomly mixed by environmental influences, that is, the mode
coupling length. The short-length value can be mapped to much
longer lengths of fiber so as to predict their behavior in the
field.
[0025] In the basic Jones matrix measurement technique, the
differential group delay .DELTA..tau..sub.n between two orthogonal
polarization modes is measured over a range of frequencies between
.omega..sub.1 and .omega..sub.n. Under the normal circumstances
described here, only the two end frequencies .omega..sub.n-1 and
.omega..sub.n bracketing a wide region of interest need to be
measured, for example, wavelengths of 1300 nm and 1550 nm. The
differential group delay .DELTA..tau..sub.n is derived from Jones
matrices T measured for each of the two frequencies. A Jones matrix
T is a 2H2 matrix with possibly complex elements relating the
polarization states of two orthogonal input signals, expressed as
two-component vectors, to the corresponding polarization states of
the output signals after traversing some optical component being
measured. An example of the optical measurement circuit used to
measure the Jones matrices is illustrated in the schematic diagram
of FIG. 1. A fiber under test (FUT) 10 having a length of about 1 m
is laid out on a table in a straight line. Two narrow-band light
sources 12, 14 are selectively switched by a 4H1 optical switch 16
to a single-mode input fiber 18. A first lens 20 collimates the
light from the input fiber through a controllable polarizer 22. A
second lens 24 directs the polarized light from the input fiber 18
to the input end of the FUT 10. One of the lenses 20, 24 may be
eliminated with one lens focused on both fibers 10, 18. The light
output by the FUT 10 is switched through a 1H2 optical switch 26 to
a single-mode output fiber 28 inputting to a polarization analyzer
or polarimeter 30 such as an HP8509B available from Hewlett-Packard
of Palo Alto, Calif. The fiber on both sides of the 1H2 switch 26
will be referred to as the output fiber 28.
[0026] A polarimeter measures the polarization state of a detected
signal, which may be characterized as a point on the Poincare
sphere. The equator of the Poincare sphere represents linear
polarizations, the poles represent the two circular polarization,
and the surface between represents elliptical polarizations. For
each optical frequency, the polarizer 22 is set to three different
angular positions or three differently aligned polarizers 22 are
inserted in the beam path to produce a known sequential set of
linearly polarized state entering the FUT 10. The polarization
analyzer 30 measures the resultant complex output polarization
state vector, which may be represented as h, v, and q. A commonly
used set of angles are 01, 601, and 1201 although 01, 451, and 901
could be easily substituted.
[0027] From these six states of polarization, the Jones matrix can
be calculated to within a multiplicative constant by a method such
as the one now described. A set of complex ratios from the three
measured states are calculated from the x and y values of the
measured state vectors: k.sub.1=h.sub.x/h.sub.y;
k.sub.2=v.sub.x/v.sub.yk.sub.3=q.sub.x/q.sub.y; and
k.sub.4=(k.sub.3-k.sub.2)/(k.sub.1-k.sub.3). To within a complex
scalar multiplier .beta., the transmission Jones matrix T is given
by 1 T = [ k k 4 k 2 k 4 1 ]
[0028] Since this is an eigenvalue analysis, scalar constants such
as .beta. are not important. A linear input polarizer 22 is
preferred, but some type of polarizer is needed to set the input
polarization state while the polarimeter 30 measures the output
polarization state.
[0029] The illustrated polarization analyzer 30 includes an optical
output selectable between two Fabry-Perot lasers emitting at near
1310 and 1550 nm. This laser source can be switched to the FUT 10
through the 4H1 switch 16. However, as described below, other light
sources are desired for the principal measurement. Both optical
switches 16, 26 can be implemented with commercial available
switches, for example, ones based on mechanically movable optical
fibers selectively coupling one port to any of several other
ports.
[0030] The Jones matrices measured at the two frequencies are used
to compute a matrix product
T(.omega..sub.n)T.sup.-1(.omega..sub.n-1), itself a 2H2 matrix,
where T.sup.-1 denotes the matrix inverse, TT.sup.-1=1, where 1 is
the diagonalized unit matrix. The differential group delay is then
calculated as 2 n = Arg ( 1 / 2 ) n - n - 1
[0031] where .rho..sub.1 and .rho..sub.2 are the complex
eigenvalues of the matrix product
T(.omega..sub.n)T.sup.-1(.omega..sub.n-1) and Arg denotes the
argument function
Arg(Ae.sup.i.theta.)=.theta.
[0032] The eigenvalues are the two diagonal elements of a
diagonalized version of the matrix product
T(.omega..sub.n)T.sup.-1(.omega..sub.n-1), where the
diagonalization is performed with eigenanalysis techniques well
known in quantum mechanics and optics.
[0033] The DGD (differential group delay) .DELTA..tau..sub.n is one
measure of the fiber=s birefringence or polarization mode
dispersion for wavelengths within the wavelength range of the
measurement and as normalized for the measured length of fiber.
[0034] In practice, to eliminate the effect of the output fiber 20
and associated components in the output optical path, the path
between the polarizer 22 and the polarization analyzer 30 is
divided in two parts, the path through the FUT 10 having a fiber
Jones matrix F and the output path having a residual Jones matrix
R. One measurement is made of the Jones matrix M for the entire
path including both the FUT 10 and the output fiber 20. The FUT 10
is then removed, and the polarizer 22 and associated optics 20, 24
are brought to the point corresponding to the output end of the FUT
10. The residual Jones matrix R is measured for the output fiber 28
and other parts within the output path. The eigenvalues p 1, P2 are
then calculated for the FUT 10 alone based on the matrix
product
F.sub.1F.sup.-1.sub.2=R.sup.-1.sub.1M.sub.1M.sup.-1.sub.2R.sub.2
[0035] This technique is ascribed to be able to measure
differential group delays of less than 12 femtosecond
(12H10.sup.-15 s) with a resolution of at least 50 attoseconds
(50H10.sup.-18 s).
[0036] The measurement circuit of FIG. 1 is improved in several
ways. Instead of the conventional lasers included in the
polarization analyzer 30, light emitting diodes (LEDs) are used as
the light sources 12, 14. Commercial LEDs are available which emit
at two wavelengths, for example, 1310 nm and 1550 nm. The outputs
of the LEDs 12, 14, which have relatively wide spectra since they
are not lasing, are filtered by respective optical bandpass filters
40, 42, for example, dielectric thin film interference filters with
3 db spectral bandwidths of about 10 nm centered near the optical
output peaks of their respective LEDs 12, 14. Other non-coherent
light sources may be used. A single light source could be used for
the two wavelengths if it emits sufficient light at the two
wavelengths. The combination of LEDs 12, 14 and bandpass filters
40, 42 reduces the problem of coherence noise. Coherence noise
arises at the butt-coupled joint between the FUT 12 and the output
fiber 20 at which the two fibers have two facets separated by a
small gap to reduce reflection. A laser has a coherence length of
about 30 cm. As a result, multiple reflections of a coherent signal
in the gap may constructively or destructively interfere, creating
noise.
[0037] For light from an incoherent source producing light with a
coherence length of less than about 200 .mu.m (twice the smallest
air gap in the system), the light cannot significantly interfere on
multiple reflections. It is preferred to use the LEDs 12, 14 in
place of the source light from the polarization analyzer 30. LEDs
do not lase and so have a very short coherence length. They do have
a relatively wide emission wavelength, but the bandpass filters 40,
42 reduce the bandwidth to an acceptable value to permit accurate
polarization measurements, but the bandpass of the filters 40, 42
must not be so narrow as to lead to coherence noise.
[0038] Another means of reducing coherence noise is to cleave the
output end of the FUT 10 at an angle differing from the facet angle
of the input of the output fiber 28 by at least about 11. It is
unlikely that light will resonate in such a varying gap.
Preferably, the FUT 10 is perpendicularly cleaved, and the input
end of the output fiber 28 is cleaved at about 11, as indicated by
the slanted line in FIG. 1.
[0039] The alignment of different fibers and the alignment required
with measuring the Jones matrix R for the output path is performed
by unillustrated translation stages at the output end of the fiber,
at either end of the FUT 10, and at the input end of the output
fiber 20. The output fiber 28 should be rigidly held so that it
does not introduce variable polarization mode dispersion between
measurements. The rough alignments, usually done after bench
maintenance, are facilitated by switching the output of a visible
laser through the 4H1 optical switch 16 to the FUT 10 or, during
the residual measurement, to the output fiber 28. The visible light
propagates with relatively high loss in the infrared single-mode
fibers 10, 18, 28 and causes the fibers to glow, and either the
glowing or the output light can be visually observed for initial
alignment. The optical intensities of the light output from the
LEDs 12, 14 are relatively low compared to the laser light from the
laser sources in the polarization analyzer 30. For fine alignment,
the 4H1 switch 16 and the 1H2 switch 26 switch light from the laser
source of the polarization analyzer, which is of single-mode
wavelength in the fibers, to an optical power detector 46, and the
stages are adjusted to maximize the signal of the detector 46. It
has been observed that the contribution of the 1H2 optical switch
26 to polarization mode dispersion remains relatively stable so
that once it is accounted for in the residual matrix R it does not
interfere with measuring the Jones matrices F of the FUT 10. It is,
of course, possible to incorporate the detector 46 in the
polarization analyzer 30, which already includes at least one
detector.
[0040] The laser sources in the polarization analyzer 30 can also
be used to detect phase aliasing. This effect arises from the fact
that the measured values are in essence phase angles mapped onto
the Poincare sphere, and these phase angles are ambiguous to within
factors of 1801. To detect possible aliasing, either the analyzer
laser source or another laser having a wavelength somewhat
different than that of the two LEDs 12, 14 is used to measure yet
another Jones matrix. If the three DGD values associated with
wavelength are nearly constant, then the measurement is probably
valid. If the values for the middle wavelength are different, there
is a good possibility that the measured polarization mode
dispersion is artificially low because of aliasing.
[0041] An alternative apparatus to that of FIG. 1 includes, instead
of the HP polarization analyzer, a polarimeter utilizing a rotating
half-wave plate, such Model PA430 commercially available from Thor
Laboratories of Newton, N.J. The input end of the fiber 28 and the
polarimeter are placed on a transversely movable stage. The fiber
28 is directly connected to the optical power meter 46 with no
intervening switch 26. With the stage positioning the fiber 28 at
the output of the FUT 10, the stages at the two ends of the FUT 10
are adjusted to align the FUT 10 with the assistance of the power
meter 46. The transverse stage then moves the polarimeter to
closely face the output of the FUT 10 with free space in between.
The DGD measurement is then performed as described before. The
apparatus offers more stability and eliminates the need to account
for the residual matrix R. Yet other types of polarimeters are
available, for example, ones using optical time domain
reflectometry.
[0042] The effect of fiber twist can be investigated by attaching
one end of the FUT 10 to a twist unit 50 which can rotate about the
longitudinal axis of the FUT 10. The other end of the FUT 10 is
immobilized to twist by an unillustrated clamp. Since the length of
the FUT 10 is short and chosen to be less than the mode-mixing
length, the effect of induced twist on polarization mode dispersion
is deterministic and can be predicted through the photoelastic
effect with minimal effect from mode mixing.
[0043] The twist unit 50 must be designed to minimize anisotropic
forces on the fiber since they would contribute their own
birefringence. A prototype design includes two cylindrical clips
which grip the fiber. A jig holds the clips spaced about 2 cm apart
firmly enough to circumferentially hold the fiber as it is rotated
but gently enough to not induce additional birefringence in the
fiber. One such jig attached to one end of the fiber is fixed while
another such jig attached to the other end of the fiber is mounted
on a rotatable stage that can rotate, for example, five turns in
each direction.
[0044] The twist unit 50 can be used for a number of different
purposes. It can measure the effect of twist stored in a fiber and
incurred during spooling. Previous attempts to do this have used
100 m lengths of fiber. It can be used as an alignment tool if the
installation of the FUT 10 inadvertently induces a twist, as often
happens in a production environment. As will become evident in our
discussion, it can be used to separate twist-induced birefringence
from the intrinsic birefringence of the fiber, sometimes reported
as beat length.
[0045] It is believed that a PMD value associated with zero twist
and measured on a short fiber is the best predictor of PMD for a
long fiber. The net zero-twist value in a low twist region can be
derived in the face of both manufacturing and experimentally
induced twist by using a model for the polarization mode dispersion
.DELTA..tau..sub.twist induced or present in a short length of
unspun fiber (fiber without significant twist in the drawing
process) as a function of twist angle .theta. 3 twist = 0 1 + 4 ( -
0 ) 2
[0046] where .DELTA..tau..sub.0 is the net zero-twist DGD value,
.theta..sub.0 is a twist offset angle, and .DELTA..beta. is the
inherent birefringence of the fiber, which is inversely
proportional to the beat length L.sub.B. An example of the measured
polarization mode dispersion as a function of applied twist is
shown in the graph of FIG. 2, where one turn is 3601 of twist. The
experimental data are marked by the solid circles. The data have
been fit to curve 60 with the above equation according to the two
parameters .DELTA..tau..sub.0 (the peak of the curve 60) and the
inherent birefringence .DELTA.k, which corresponds to a beat length
of L=9.75 m. However, the effective induced twist is assumed to be
equal to 0.92 that of the actual mechanical twist where the
difference is due to a photoelastic effect in the opposite
direction. The interpolation provided by the curve fitting to the
above equation provides a more accurate value of the net zero-twist
polarization mode dispersion .DELTA..tau..sub.0.
[0047] In this curve, the twist offset .theta..sub.0 induced
experimentally or otherwise present has already been aligned out.
The internal twist may be induced by the operator, and values of
0.75 turns/m are not unusual. The fiber winding operation may twist
the fiber, and the twist is not reversed by the operator. Values of
0.3 turns/m are typical. The fiber manufacturing may inadvertently
introduce a net unidirectional spin. It is not untypical for a
fiber that is manufactured with a spin oscillation (clockwise then
counterclockwise) with an amplitude of about 3 turns/m to have a
net unidirectional spin of 0.1 turns/m. Spin differs from twist in
that there is no restoring photoelastic force for spin induced
during the drawing process.
[0048] To account for the induced twist in determining an intrinsic
birefringence of the fiber, the following procedure may be
followed. After the intensity alignment mentioned previously have
been performed, the polarization mode dispersion should be measured
for a number of values of twist. Between each measurement, the
input side of the FUT 10 is realigned to compensate for any
rotation offset. The twist angle .theta. exhibiting the maximum
value of polarization mode dispersion, as measured with the
polarization analyzer, is taken as the net zero-twist position
.theta..sub.0. It is not unusual that 901 of twist needs to be
compensated, and at least part of this is believed to be induced
during fiber mounting. Using the initial measured value for
polarization mode dispersion would normally result in too low a
value according to the dependence shown in FIG. 2.
[0049] It is of course appreciated that the repetitive measurements
required for polarization mode testing and the twisting experiments
can be easily automated. Furthermore, the twist equation can be
generalized to the unknown angular offset .theta..sub.0 so as to
combine twist alignment and generation of the twist data. It is
also appreciated that the residual polarization mode dispersion,
that is, the residual Jones matrix R, needs to be tested only
infrequently since it is assumed to be independent of the fiber
used as the FUT 10.
[0050] The twist dependence predicted by the above equation and
experimentally observed as in FIG. 2 assumes that the photo-elastic
effect is relatively small so that the twisting does not induce
significant stress in the fiber. Expressed alternatively, the
inherent birefringence is assumed to be large compared to the
photo-elastic effect. A more complete version of the equation
incorporating stress effects is given by 4 twist = 0 2 + ( g - 2 )
( - 0 ) 2 g ' 2 0 2 + ( g - 2 ) 2 ( - 0 ) 2
[0051] where g is the photoelastic constant and gN is its
derivative with respect to frequency, Mg/M.omega.. Any negative
values of .DELTA..tau. should be changed to positive values. This
equation also takes advantage of the relationship
.DELTA..beta...omega..tau..sub.0. Typical values for silica are
g.0.14 and gN.1.036H10.sup.-17 when the angles .theta. are
expressed in rad/m, .DELTA..tau. in s/m, and .omega. in rad/s. For
very small values of inherent birefringence .DELTA.k, the observed
twist dependence .DELTA..tau..sub.twist starts with a very low
value and increases monotonically with the twist difference angle
(.theta.-.theta..sub.0) for both positive and negative values of
the difference angle. For fibers with such low inherent
birefringence that its non-twisted DGD cannot be measured, the
zero-twist DGD can be calculated from the slope of the larger
values on the sides. In intermediate ranges of inherent
birefringence and photo-elastic effect, the peak of FIG. 2 is
surrounded by sharply rising tails.
[0052] The polarization mode dispersion measured for a short length
of fiber, that is, one significantly shorter than the mode coupling
length, needs to be somehow associated with a value for a long
fiber for which the unmeasured mode coupling has a significant
effect. The association can be performed with an empirically
developed map. A 1 km length of fiber is tested for polarization
mode dispersion, for example, according to the conventional process
described before. The long-length measurement is performed under
some set of predetermined conditions of temperature, diameter of
the fiber reel, tension of the fiber on the reel, and the type of
cable in which the fiber is embedded. A 1 m length of fiber is cut
from one end of the 1 km fiber (or possibly both with replication
of the procedure) or from the same spool, and that short length is
tested for polarization mode dispersion according to the method of
the invention described above. Preferably, any spun-on twist
introduced during manufacturing is removed by the twist alignment,
although the mapping can be performed without zeroing out the
twist. The measured short-length polarization mode dispersion
coefficient .DELTA..tau..sub.SHORT is then paired with the measured
long-length dispersion coefficient .DELTA..tau..sub.LONG. In
practice, the short-length DGD is normalized to the length of the
fiber being measured while the long-length DGD is normalized to the
square root of the length of the fiber since these are the observed
dependencies of the differential time delay in the two regimes. A
large number of samples are measured, perhaps 200 to 1000 samples
for each map. Each sample is taken from a unique shipping reel of
fiber. The normalized long- and short-length differential group
delays are expected to be related by 5 LONG = L MLC L LONG L SHORT
SHORT
[0053] where L.sub.MCL is the average mode-coupling length. As a
result, the mapping in large part simply quantifies the average
mode coupling length for a particular type of deployment as long as
the deployment conditions do not additionally change the
short-length DGD. The relationship of the above equation is
expected to hold for lengths longer than 1 km. A preliminary
mapping for 15 fibers is presented in the data marked by solid
circles in FIG. 3. The indicated linear fitting of this data
corresponds to an average mode-coupling length L.sub.MCL of 2.9 m.
This value satisfies the conditions that the short length LSHORT of
fiber is less than the mode-coupling length L.sub.MCL and the long
length L.sub.LONG is longer than the mode-coupling length. The
mapping demonstrates the validity of the relationship of the above
equation.
[0054] Subsequent spools of fiber, at least manufactured with the
same general manufacturing techniques, are tested only for a
short-length value. The empirical mapping is used to predict the
fielded behavior dependent upon the long-length value.
[0055] The apparatus for measuring polarization mode dispersion
shown in FIG. 1 can also be used to measure the effects of cabling
in a fielded environment. The FUT 10 is placed between a load jig,
illustrated schematically in FIG. 4, comprising a table 70 and a
load block 72. A variable load L is applied to the load block 72 to
impose a lateral load on the fiber 10, and the apparatus of FIG. 1
is used to measure the DGD, that is, .DELTA..tau..sub.0. The
experiment is repeated for a number of different values of the load
to demonstrate the effect of loading.
[0056] Such a measurement was performed for three fibers exhibiting
low, medium, and high DGD with no load. When the load was increased
to 2400 g/m, the low-DGD fiber exhibited a very large relative
increase, the medium-DGD fiber exhibited only a modest increase,
and the high-DGD fiber exhibit a decrease.
[0057] The fiber lengths mentioned in the examples are illustrative
only. Although a lm length for the fiber under test is preferred,
the experimental equipment may be extended to 5 m without undue
inconvenience. Lengths shorter than 1 m are possible, but introduce
difficulty in measuring small values of polarization mode
dispersion. Attempts to use 30 cm lengths have proven difficult
because of the small measured values. Lengths less than 2 m are
conveniently sized, 1 m being preferred. Although conventionally, 1
km lengths of fibers have been measured for polarization mode
dispersion, in many circumstances adequate polarization mode mixing
can be achieved in lengths of greater than 100 m. These lengths are
to be compared with typical spool lengths of 25 km, although spool
lengths may range from 4 km to 50 km.
[0058] Thus it is seen that the invention provides an effective and
simple apparatus and method for measuring birefringent properties,
such as differential group delay, in an optical fiber. The
invention also provides a method of predicting the birefringent
behavior of long lengths of fiber without having to measure the
long lengths of fiber.
[0059] It will be apparent to those skilled in the art that various
modifications and variations can be made to the present invention
without departing from the spirit and scope of the invention. Thus,
it is intended that the present invention covers the modifications
and variations of this invention provided they come within the
scope of the appended claims and their equivalents.
* * * * *