Download Few Mode Transmission Fiber With Low DGD, Low Mode Coupling

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 23, DECEMBER 1, 2012
3693
Few Mode Transmission Fiber With Low DGD, Low
Mode Coupling, and Low Loss
Lars Grüner-Nielsen, Member, IEEE, Member, OSA, Yi Sun, Senior Member, IEEE, Member, OSA,
Jeffrey W. Nicholson, Dan Jakobsen, Kim G. Jespersen, Robert Lingle, Jr., Member, IEEE, and Bera Pálsdóttir
Abstract—A transmission fiber for mode division multiplexing
supporting
and
modes, with low differential group
delay, low mode coupling, and low loss for both modes is presented.
imaging) is
Spatially and spectrally resolved mode imaging (
used for characterization.
Index Terms—Optical fibers, optical fiber measurements.
I. INTRODUCTION
M
ODE division multiplexed transmission in few mode
fibers has recently attracted considerable attention as a
means of increasing the transmission capacity of a single fiber
[1]–[3]. Two different methods for mode division multiplexing
have been demonstrated. In the first approach [1], [3], mode
coupling during transmission is compensated by multiple input,
multiple output (MIMO) processing at the receiver. In the
second approach [2], mode coupling in the transmission path
is minimized, so the transmitted signals can be separated by
detecting each mode separately.
The few mode transmission fiber is obviously a key component. The requirements for such a fiber are similar to that of
single-mode transmission fibers: low attenuation, low nonlinearity (i.e., a high effective area), and a high dispersion coefficient. In addition, the few mode fiber should support a specific
number of well-guided modes, have low differential group delay
(DGD), and low coupling between the modes. The fiber should
also be able to splice to itself with low splice loss for all guided
modes and low cross coupling between the modes. Low DGD
is essential for MIMO processing to reduce the complexity and
power consumption of the MIMO processing. Low mode coupling between the modes in the transmission fiber is essential
for systems without MIMO processing and might be useful for
Manuscript received August 30, 2012; revised October 23, 2012; accepted
October 24, 2012. Date of publication November 16, 2012; date of current version December 07, 2012. This work was supported by the European Union program FP7-ICT MODE-GAP.
L. Grüner-Nielsen, D. Jakobsen, K. G. Jespersen, and B. Pálsdóttir are
with OFS Fitel Denmark, DK-2605 Brøndby, Denmark (e-mail: [email protected]; [email protected]; [email protected]; [email protected]).
Y. Sun and R. Lingle, Jr. are with OFS, Norcross, GA 30071 USA (e-mail:
[email protected]; [email protected]).
J. W. Nicholson is with OFS Laboratories, Somerset, NJ 08873 USA (e-mail:
[email protected]).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JLT.2012.2227243
Fig. 1. Illustration of the refractive index profile.
systems using MIMO processing to help reduce the complexity
of the needed MIMO processing.
Most reported few mode transmission fibers [1], [4], [5] have
been simple step-index fibers, and suffered from high DGD [4],
[5] and high mode coupling [4]. However, the fiber in [1] had an
additional trench, which resulted in low DGD. In this paper, we
present results from a new fiber design using an index profile
based on a graded index core and an outer trench supporting the
and
modes, corresponding to six degenerations. The
new fiber has low DGD, low mode coupling, and low loss for
both modes as well as good splice performance. The new fiber
can also be fabricated with both negative and positive DGD, and
by combining fibers with opposite signs for the DGD, the total
accumulated DGD can be minimized.
II. DESIGN
An illustration of the refractive index profile design is shown
in Fig. 1.
The refractive index profile consists of a graded index (parabolic shaped) core to obtain the same group velocity for the
two modes plus a relatively large difference in effective indices
between the modes. The large effective index difference minimizes distributed mode coupling. The inner cladding is configured to improve the bend loss performance of the modes (es) so that the differential mode attenuation is low.
pecially
and
modes above
The fiber is designed to only guide
1300 nm. Optical properties of the fiber, obtained from the index
profile by solving the scalar wave equation with a finite element
mode solver, are summarized in Table I.
The difference between effective indices of the modes of
is considerably larger than the value of
0733-8724/$31.00 © 2012 IEEE
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JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 23, DECEMBER 1, 2012
TABLE I
PROPERTIES AT 1550 NM FOR THE INDEX PROFILE DESIGN OF FIG. 1
Fig. 2. Schematic of the
setup using a tunable laser and CCD camera.
TABLE II
TYPICAL PROPERTIES AT 1550 NM FABRICATED FIBER. Italic Values Are
Calculated From Index Profile. Non-Italic Values Are Measurements
of the two-mode fiber reported in [4] where a distributed mode
coupling of 18.2 dB for 500 m was measured.
In the
L-band from 1530 to 1620 nm, it is observed that
the DGD is only varying from
to
ps/nm.
A sensitivity study of the parameters in Table I toward small
variations of the index profile revealed that the DGD is extremely sensitive to small index profile perturbations, while the
other parameters are rather robust.
III. RESULTS
A preform with the index profile as shown in Fig. 1 has been
made with the modified chemical vapor deposition technique.
The graded index core is made from silica doped with germania,
where the amount of germania is varied from layer to layer to
obtain the graded index. The trench is made of silica doped with
fluorine, while the outer cladding is made from pure silica. The
preforms was overclad with a pure silica tube, drawn into the
fiber, and characterized.
The mode properties of few mode fibers have been recently
characterized by spatially resolved interferometric techniques
[6]–[8]. In this study, we measure the mode coupling as well as
the DGD between
and
modes using spatially and
spectrally resolved mode imaging ( imaging). Higher order
modes (HOMs) in optical fibers propagate with different group
delays, and therefore, multiple HOMs copropagating in a fiber
produce spectral interference whose beat period
is the inverse of the mode group delay difference DGD [9]
(1)
where is the wavelength, c is the speed of light in vacuum, and
L is the fiber length. The technique is based on spectrally and
spatially resolving the interference pattern of the guided modes
at the output of the fiber. By Fourier transforming the spectra for
each spatial point of the mode, beat amplitude versus DGD is
found [6]. With relatively simple data analysis [10], one obtains
the mode image, HOM power levels, and group delays relative
to the fundamental mode. Here, the setup shown in Fig. 2 based
on a tunable laser at the input and a camera at the output is used
[4], [11], [12]. The tunable laser can be tuned in steps of 0.001
nm. From the aforementioned equation, taking into account that
at least two samples per beat period is needed, it is found that
the step size of 0.001 nm allows for a maximum DGD of 4000
ps to be measured at 1550 nm.
The camera used in these experiments was an InGaAs array
(Ophir-Spiricon model XC-130) with 320 256 pixels. The
laser was a Tunics external cavity laser from Photonetics. The
laser power was set to 3 mW. Multiple neutral density filters
were used to attenuate the optical beam to avoid saturating the
camera. A polarizer could be included to measure the HOM
content for different polarization settings but was not used in
this study.
The properties of the
mode, shown in Table II, were
measured with single-mode fibers (SMFs) spliced to the ends.
From
measurement, it was found that such a splice to SMF
gives better than
dB suppression of
mode.
For the measurement of the properties of the
mode,
the light was launched into the fiber via an in-fiber long period
grating (LPG) mode converter. The LPG was thermally induced
into a step-index two-mode fiber [13]. The conversion efficiency
of the LPG was characterized using the
technique. The results are shown in Fig. 3. For comparison, the measured trans-
GRÜNER-NIELSEN et al.: FEW MODE TRANSMISSION FIBER WITH LOW DGD
Fig. 3. Measured transmission spectrum from
to
, and
power
power calculated from the
measurement for the LPG mode
relative to
converter used. (Inset) Measured mode pattern from the mode converter at 1525
nm.
mission spectrum from
to
, i.e., with modes strippers
on both sides of the grating, is shown in Fig. 3 as well.
A very good agreement between the simple transmission
measurement and the
measurement is observed in Fig. 3.
Better than
dB suppression of the
in the wavelength
range (1540–1560 nm) of the optical time-domain reflectometer
(OTDR) used for attenuation measurement is observed. With
measurements, it was verified that the LPG could be spliced
to the fabricated fiber with a drop in
–
extinction less
than the measurement uncertainty.
Typical measurement results for the fabricated fiber are summarized in Table II.
Dispersion was measured using an EG&G CD400. Modefield diameter and effective area were measured using a Photon
Kinetics PK2210 with a variable aperture unit. Attenuation was
measured using two-way OTDR, and polarization-mode dispersion was measured using the interferometric technique. Table II
also includes values calculated from the index profile measured
on the preform and scaled to the fiber. The properties are found
by solving the scalar wave equation with a finite element mode
solver. Good agreement is found between calculated and measured DGD, dispersion and effective area of the
mode.
This gives good confidence in the values for dispersion and effective area of the
mode which was not measured. It is observed that the attenuation is quite low, 0.198 dB/km for
and even lower. 0.191 dB/km for
. The lower attenuation
for
is attributed to the lower field overlap with the germania-doped core for
compared to
. The attenuation
is measured with the 30 km spooled on a spool with a core radius of 90 mm. The lower loss for
, the most bend loss
sensitive mode, also confirms that no bend loss is induced for
the bend radius of 90 mm.
Fig. 4 shows the sum of Fourier transforms for all spatial
points for an
measurement on the spool listed in Table II
from 1550 to 1551 nm in 0.001 nm steps. The peak at
corresponds to the fundamental mode. Fig. 4 also shows spatial
images obtained from the data analysis for some selected DGDs.
Only images corresponding to
are observed confirming
that this is the only guided mode besides
. Measurement on
shorter 20 m lengths showed the same: the only HOM observed
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Fig. 4. Mode beat versus DGD for 30 km fiber.
Fig. 5. Measured preform profile for preform section giving DGD of
and
ps/m.
is
. This confirms that the cutoff of next HOM is well below
1550 nm.
A clear peak at a DGD of 2270 ps is observed, which is due to
beating with
launched at the splice. The relative power is
found to be
dB. The plateau between DGD of 0 and 2270 ps
is due to distributed mode coupling and by integrating over this
plateau, a distributed mode coupling of
dB is found. The
plateau above 2300 ps is believed to be mainly due to numerical
noise from the Fourier transform due to the limited step size of
0.001 nm.
All spools drawn from the preforms were characterized and
found to have different DGD values, but otherwise similar optical properties. The preform was designed to have a DGD of
zero in average but due to the large sensitivity of the DGD to
small variations of the index profile along the length of the preform, the DGD varied along the length of the preform from
to
ps/nm. In a later second preform, it was possible to control the DGD within
to
ps/nm. To illustrate how precisely the preform profile should be controlled,
Fig. 5 show measured preform profiles yielding DGD of
and
ps/nm, respectively. Only very small fluctuations in
the core layer index differ between the two profiles.
To measure the wavelength dependence of the DGD, the interferometric technique was used [1], [9]. A 20 m length of two-
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Fig. 7. Mode beat versus DGD for the 70 km span.
Fig. 6. Measured transmitted power and calculated DGD for 20 m long twomode fiber.
TABLE III
TYPICAL SPLICE PERFORMANCE FOR SPLICING OF THE FIBER TO ITSELF
TABLE IV
PROPERTIES OF FIBERS USED FOR THE SPAN
mode fiber was offset spliced to an SMF at both ends and connected to a broadband light source and an optical spectrum analyzer. The measured transmission spectrum is shown in Fig. 6.
Using (1), DGD versus wavelength is found. The DGD variation in the
band from 1530 to 1620 nm is found to be only
0.009 ps/m in this case. This result was typical for all measured
samples with DGD at 1550 nm between
and
ps/m.
The splice performance of the fiber to itself was investigated
on short pieces. Typical splice loss and mode couplings (measured with the
technique), when launching into the
mode and
mode, respectively, are summarized in Table III.
IV. LINK RESULT
To further investigate the potential of the new fiber, a 70 km
span was spliced together. Three spools from different parts of
the preform than the spool of Table II were used. These spools
differ from the spool of Table II by having different values of
DGD. Total DGD and distributed mode coupling measured by
the
setup for the three new spools are shown in Table IV.
The sign of the DGD cannot be measured with the
setup but
is determined from simulations based on the refractive index
profile.
Fig. 8. Mode images at 1530 nm of (a) LP mode and (b) LP
tively, after transmission through a 70 km span.
mode, respec-
The total insertion loss for the 70 km span was measured to
be 14.4 dB for the
mode and 13.9 dB for the
mode
at 1530 nm.
Measured mode beat versus DGD from measurement after
splicing the spools together is shown in Fig. 7. The measurement
is performed for two launch conditions: 1) the light is launched
into the span from an SMF via a normal splice, and 2) via a
splice where the SMF is offset relative to the two-mode fiber to
get more light into the
mode. The measurement direction
is after launching from the SMF: spools 1, 2, and 3. No beat
signal is observed for DGD larger than 1844 ps, corresponding
to the maximum accumulated DGD with same sign. The apparent peak at 2900 ps, which is only seen for the normal launch,
is believed to be a measurement artifact. For the offset launch,
a peak around 444 ps is observed, corresponding to a discrete
mode coupling of
dB, larger than the peak at 1800 ps which
corresponds to a discrete mode coupling of
dB, independent of the launch condition. The 444 ps peak corresponds to
the total accumulated DGD of the three spools. The distributed
mode coupling between 0 and 1800 ps is calculated to
dB.
As with the measurement on the single spool in Fig. 4, the distributed mode coupling is observed to be quite close to the noise
floor of the measurement technique. The reason that a small
peak is observed at 1800 ps but not at 1400 ps is probably that
the mode coupling in the splice between spools 2 and 3 is higher
that the two other splices between the two-mode fibers.
The low mode coupling is confirmed by the stable mode images on the output of the 70 km span when launching light into
either
mode or
mode, as shown in Fig. 8(a) and (b).
GRÜNER-NIELSEN et al.: FEW MODE TRANSMISSION FIBER WITH LOW DGD
V. CONCLUSION
A new two-mode transmission fiber supporting
mode
and
mode with attenuation for both modes below 0.20
dB/km, DGD below 0.1 ps/m, distributed mode coupling for a
30 km spool below
dB, and a good splice performance to
itself for both modes has been reported.
For spans composed of spools with DGD of opposite signs,
the DGD from discrete mode couplings at the input accumulates
linearly (including sign). The distributed mode coupling scales
approximately linearly with the length.
The measurement on a 70 km span with the
technique
represents more than a factor of 100 increase in fiber lengths
previously characterized with the
technique.
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Lars Grüner-Nielsen (M’07) was born in Copenhagen, Denmark, in 1959. He
received the Master’s degree in electrical engineering and the Ph.D. degree in
optical fibers from the Technical University of Denmark, Copenhagen, in 1983
and 1998, respectively.
From 1983 to 1994, he was with the Danish cable manufacture NKT’s R&D
Department for optical cables where in 1991 he became a Section Manager.
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Since 1994, he has been with the R&D Department, OFS Fitel Denmark,
Brøndby, Denmark. At OFS, he has been involved in the development of new
fibers for transmission, optical signal processing, and short pulse lasers, where
since 2000, he has been a Project Manager and became an OFS Fellow in 2008.
He has authored or coauthored more than 200 scientific publications within the
field of fiber optics. He holds 10 patents.
Dr. Grüner-Nielsen is a member of the Optical Society of America. In 2000,
he received the Electro Price from the Danish Society of Engineers. From 2004
to 2007 he was a member of the European Conference on Optical Communication Technical Program Committee. He has also served as a reviewer for several
journals.
Yi Sun (M’06–SM’10) received the B.S. and M.S. degrees in astronomy from
Peking University, Beijing, China, in 1992 and 1995, respectively. She received
the second M.S. degree in electrical engineering (in optical communication program) and the Ph.D. degree in physics from Northwestern University, Evanston,
IL, in 1999 and 2001, respectively.
From 1995 to 1996, she was at Funder Corporation for digital printing system.
Since 2001, she has been with Bell Labs, and then OFS Labs as a member of
Technical Staff in the Fiber and System R&D Department, Norcross, GA. She
has many publications in peer-reviewed journals and conferences and holds several patents in optical fibers. Her current research interests include the areas of
novel fiber design and system transmission.
Dr. Sun is a member of the Optical Society of America. She has served as
a reviewer for IEEE journals and session chairs in APC. She has received the
2001 Spirit of Endeavor Award from the Technology Association of Georgia
and TechAmerica of Georgia.
Jeffrey W. Nicholson was born in Louisville, KY, in 1969. He received the
B.S. degree in physics from the University of Houston, Houston, TX, in 1991,
and the Ph.D. degree in optical sciences from the University of New Mexico,
Albuquerque, in 1997.
After graduation, he was a Postdoctoral Researcher with Los Alamos National Laboratory, in the area of short pulse propagation and with Directed Energy Solutions, Albuquerque, in the area of high-power gas lasers. Since 2000,
he has been with Bell Labs, and then OFS Labs as a member of Technical Staff
in the Optical Fiber Research Group, Somerset, NJ. His current interests include
fiber lasers and amplifiers and nonlinear propagation effects in fibers.
Dan Jakobsen was born in Copenhagen, Denmark, in 1966. He received the
Bachelor’s degree in chemical process engineering in 1991.
After graduation, he was with R&D Department, OFS Fitel Denmark,
Brøndby, Denmark, primarily with the modified chemical vapor deposition
process.
Kim G. Jespersen was born in Fredericia, Denmark, in 1971. He received the
Master’s degree in terahertz physics from Aarhus University in 1998. After a
short stay in Telecom working with wireless networks, he returned to scientific
research and received the Ph.D. degree in optics at The National Research Lab,
Copenhagen, Denmark, in 2003.
After graduation, he was a Postdoctoral Researcher in the Department of
Chemical Physics, Lund University, Lund, Sweden, in the area of femtosecond
spectroscopy of organic solar cell materials and at NKT Research in the area
of short-pulsed fiber lasers and supercontinuum generation. Since 2006, he has
been with the R&D Department, OFS Fitel Denmark, Brøndby, Denmark, where
he was involved with dispersion-managed fibers for short-pulsed fiber lasers and
other specialty optical fibers.
Robert Lingle, Jr. (M’02) received the Ph.D. degree in physics from Louisiana
State University, Baton Rouge.
He is the Director of Fiber Design and Systems Research Group, OFS, Norcross, GA, as well as an Adjunct Professor of electrical and computer engineering at the Georgia Institute of Technology, Atlanta. He has a research background in short pulse lasers and their application to fundamental processes in
liquids and interfaces. He was a Postdoctoral Researcher in surface physics at
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the University of California, Berkeley. At Bell Labs and currently OFS, he was
involved in sol-gel materials chemistry and managed the development and commercialization of new optical fibers for fiber-to-the-home, long haul, and submarine networks. His current research interests include ultralarge area fibers,
few-mode fibers, and the confluence of optical and electrical methods for mitigating impairments in transport.
Copenhagen, Denmark, in 1990, and the Ph.D. degree from Aarhus University,
Aarhus, Denmark, in 1994.
She was with the R&D Department of Lycom and Lucent Technologies. Since
1991, she has been with OFS Fitel Denmark, Brøndby, Denmark, where he is
currently the Manager of the Incubation Center. Her research interests include
erbium-doped fibers and amplifiers, Yb- and Tm-doped fibers, Raman amplification, and components for ultrafast fiber lasers.
Bera Pálsdóttir received the B.Sc. degree from the University of Iceland, Reykjavik, Iceland, in 1986, the M.Sc. degree from the University of Copenhagen,