Dense-WDM Technologies: Furukawa Electric's Contribution
by Katsuhiko Okubo * and Shintaro Sentsui *2

INTRODUCTION

With the successful development of lasers in the 1960s,
fiber-optics became the technology of the future fortelecommunications, but problems relating to the two mostimportant elements--the light source and the transmission
medium--meant that practical applications remained elusive. By 1970, however, a laser diode capable of continuously lasing at room temperature and an optical fiber witha transmission loss of 20 dB/km at last provided the levelsof performance required. In a little over 10 years, fiberoptic communications underwent remarkable growth. At
that time it was believed that the next generation of systems would be optical coherent communication using heterodyne detection, and many institutions were active inpursuing this line of research. It was in 1989, when hightemperature superconductivity was the rage, that the
erbium-doped fiber amplifier (EDFA), an amplifier using
erbium-doped fiber (EDF) pumped by a 1480-nm laser
diode, forever changed this perception. 1), 2) In no time the
EDFA was in actual use, and has become the key to thetechnologies of wavelength-division multiplexing (WDM).
Furukawa Electric early recognized the possibilities offiber-optics and the later EDFA, and was quick to emphasize research in these areas. In 1974 Furukawa led theworld in laying outdoor optical fiber cable, 3) and in 1989
was in the forefront of research on fiber-optic amplifiers. A
1480-nm pumping laser diode in 1989 2) and an L-band
fiber-optic amplifier 4) are among the important contributions we have made to fiber-optic communications and the
WDM technologies that support it. Another success wasscored with the optical fiber amplifier (OFA) repeater system for WDM-based HDTV (high-definition television) programming for the 1998 Winter Olympics in Nagano. Also,
realizing that WDM would require OFAs of higher output
power, we have been a leader in the pumping laser field inworking to increase diode output.
Furukawa Electric places the highest priority on thedevelopment of WDM products, and by developing thisnew technology is creating business and at the same time
contributing to the advancement of society.
As an aid to understanding the thrust of this "mini-special" on wavelength-division multiplexing, this article provides an outline of WDM transmission systems anddescribes the contributions Furukawa Electric has made in
the field.

2.

WDM TRANSMISSION SYSTEMSWith the explosion of the Internet, telecommunications
capacity is more than doubling every year. This technological revolution has sent line charges plummeting, and the
new demand created a virtuous circle that has further
spurred technical innovation, centered on WDM. Timedivision multiplexing (TDM) and WDM offer the two mainmeans of increasing communications capacity, and as
Figure 1 shows, until the commercialization of OFAs,
progress depended on increasing the capacity per channel.
WDM itself is not a new concept, and even then waseasy to implement. At that time, it was difficult to produce
economically light sources having wavelengths stabilized
within a narrow range, and filters capable of separating
light with narrow channel spacing. The result was WDM
using different spectral windows, such as 1310 and 1550
nm.
Figure 2a) shows the basic structure of a conventional
optical repeater system in which signals from the transmitters (Tx) are propagated along the optical fibers. Whenthe optical signals have become attenuated, they arerelayed by 3R regenerators (repeaters), requiring optoelectrical and electro-optical conversion.
In the WDM system, shown in Figure 2b), signals fromtransmitters of different wavelength are multiplexed andthe multiplexed signal is propagated along the opticalfiber. When the optical signal has become attenuated, it is1000

Transmission capacity (Gbit/s)

1.

10 Gbit/s x 40
10 Gbit/s x 16
2.5 Gbit/s x 32
2.5 Gbit/s x 16

100

WDM
TDM
40 Gbit/s

2.5 Gbit/s x 4

1010 Gbit/s

1565 Mbit/s

2.5 Gbit/s
Single-mode fiber

0.1
100 Mbit/s
Multi-mode fiber
0.01
1980

1985

1990

1995

2000

2005

Year
*
*2

Executive Director, General Manager of R & D Division
R & D Division

Figure 1

Evolution of transmission capacities

1Repeater

RepeaterRxa) Conventional system

EDFA
Tx

Level of sophistication

Tx

Optical router
Opticalcross-connect
Optical add-drop
Point-to-point
WDM
Point-to-pointfiber-optic amplifier

EDFA

MUX

DEMUX

1992

Rx

1994

Comparison of conventional optical repeater system
and WDM system with optical amplifiers for long-haul
applications

amplified by an OFA and, after successive amplifications,is demultiplexed and delivered to the receivers (Rx).
When WDM is applied to the conventional repeaters,
there must be a multiplexer/demultiplexer (MUX/DEMUX)
unit plus a repeater for each signal at every repeater
point, and while this saves on optical fiber it has little further economic advantage and was never widely used. In a
WDM system using OFAs, by contrast, increases in thenumber of multiplexed channels can be accommodated bysimply providing the appropriate number of the previously
developed transmitters and receivers and increasing
amplifier output, resulting in an extremely economical system. Because such a system allows wavelength multiplexing at narrow channel spacing, it is distinguished from earlier WDM systems by the term dense wavelength divisionmultiplexing (DWDM).
Systems employing OFAs have evolved from the first
generation of single-channel systems in which the conventional 3R repeater was replaced by an OFA to the second generation of WDM systems which currently are themainstay for trunk networks, featuring increased capacity
using multiple wavelengths. WDM systems are expected
to evolve as shown in Figure 3, and their application will
be extended from trunk systems to access systems. Some
optical add-drop multiplexers (OADMs), which can add
and/or drop some of the optical channels, are already in
use at nodes.
Optical cross-connects (OXCs) make it possible to separate the optical signals transmitted by WDM from multiple
fibers at the respective demultiplexers and allocate themby channel to output fibers. This makes possible large volume switching of optical signals without opto-electric conversion, resulting in a more economical system configuration.
Topics requiring future research include optical routers
for routing signals in the optical domain and repeaters for
performing the 3R functions on optical signals, where
there are numerous technical problems to be solved
before viable products are available.
The key factor technologies for WDM include lightsources, fiber amplifiers, multiplexers/demultiplexers, andoptical fiber for the transmission medium. Large-capacity

2

1998

2000

2002

2004

Year

b) WDM system

Figure 2

1996

Figure 3

Evolution of photonic networks

OXCs differ in structure according to the type of fiber-optic
components that can be used, but they require integrated
optical switches, and the ability to use wavelength conversion devices will increase system flexibility. Furukawa
Electric is proceeding with the development of all major
products including OXCs.

3.

KEY FACTOR TECHNOLOGIES FOR
WDM

Effective utilization of the bandwidth of OFAs depends on
the performance of the demultiplexer. The demultiplexer
must have filter characteristics such as to remove from themultiplexed signal the single signal for each receiver andassure that there is no crosstalk from other channels. In
the first WDM systems with fewer multiplexed signals,
demultiplexers having dielectric multi-layer filters werecommon, but new technologies based on fiber Bragg gratings (FBGs) and arrayed-waveguide gratings (AWG) 5)
have made possible filtering within a narrow range of
wavelengths, so that multiplexed channel spacing has
been reduced to 50 GHz (approximately 0.4 nm). In thisregion, even if an ideal filter were available, any attempt toincrease the transmission capacity per channel would only
result in a broadening of the optical spectrum due toincreasing modulation frequency in proportion to transmission capacity, and the point at which this reached theadjacent channels would be the limit of transmission
capacity per channel. In actual practice the filter is not
ideal, and, when transmission capacity per channel islarge, the signal deteriorates due not only to filter crosstalk
but also to chromatic dispersion. It is therefore importantthat demultiplexer performance be improved to take thesefactors into account.
A demultiplexer can be used as a multiplexer by simply
reversing the input and output directions, but since there
is no crosstalk problem and insertion loss can be compensated by an OFA, multiplexing can also be provided by an
inexpensive star coupler, provided the number of multiplexed channels is small. Furukawa Electric offers a
MUX/DEMUX using an AWG that has outstanding narrowband filtering performance.
OADMs may be of fixed or variable wavelength or time

Furukawa Review, No. 19. 2000

by switching. The former type has a fixed-wavelength filter
whereas the latter uses a variable-wavelength filter. There
are several types of filter for the latter, but most of them
are configured using FBGs and optical circulators.
To increase the number of signals multiplexed by WDM,
the amplification band of the OFA is broadened and thegain deviation within the band is reduced. To further maintain constant output on each channel, OFA output must be
proportional to the number of channels multiplexed. To
achieve this, the EDF, gain-equalizing filter and pumping
laser must deliver high performance, and Furukawa
Electric is committed to in-house development of these
key technologies. To achieve flat gain for the OFA, thegain-equalizing filter must be designed in combination withthe gain characteristics of the EDF, so that a proprietary
design is essential. The gain-equalizing filters used with
Furukawa Electric's EDFAs are fabricated using dielectric
multi-layer film, FBGs, etalons, etc., and are matched tothe spectral gain of the EDFA specifications. Further
improvement in OFA functionality additionally requires avariable attenuator to respond to changes in input level,
as well as a variable gain-equalizing filter. And as a measure to reduce OFA size and cost, work went forward on
the development of integrated components comprisingcoupler and isolator for the signal light and pumping light.
With increased OFA output power has come a change
in the performance requirements for the laser diodes used
for pumping. Practicable diodes have been developed forwavelengths of 980 and 1480 nm. In terms of fiber amplifier noise, the value is about 2 dB lower for 980 nm, andthe overall energy conversion efficiency--from the power
driving the diode to the amplified optical signal--is higher,
so that at one time it was assumed that it would be the980-nm laser diodes that would be used for pumping. The
reliability problem has already been solved for 980-nm
diodes, but the conversion efficiency from pumping light tosignal light is low and only a narrow range of wavelengths
is available for use as pumping light, resulting in a limitation on OFA output. The 1480-nm laser diodes, on theother hand, have a broader available range of wavelengths and wavelength-division multiplexing of the pumping light is possible 6) so that for high-output OFAs, thistechnology is used. As things stand today, 1480-nm
diodes are indispensable, and their applications are being
further extended to remote pumping for OFA used in submarine cables as well as to Raman amplifiers. 7) In anticipation of the need for higher-power OFAs because of thelarger number of channels in DWDM systems, Furukawa
Electric has from the earliest stage been involved in developing 1480-nm laser diodes, and is also working on 980nm laser diodes and has solved its reliability problems. It
now markets pumping laser diodes of both wavelengths.
Work on the development of distributed-feedback (DFB)
laser diodes has also been undertaken based on technology acquired with pumping diodes.
If existing systems using single-mode fiber are to be
upgraded by DWDM, compensation must be provided forthe large chromatic dispersion in the optical fiber at 1550
nm. This can be done using either dispersion-compensat-

ing fiber (DCF) or fiber Bragg gratings (FBGs), 8) each ofwhich had its advantages and disadvantages. Furukawa
Electric is working on both approaches but at present it is
the DCF that is commercially available.
Performance requirements for the optical fiber used as
the transmission medium have been increased by the
OFA. In conventional fiber-optic communications systems,
waveform degradation and noise were eliminated at each
repeater and were thus non-cumulative, but this is not thecase in systems using OFAs. Thus while chromatic dispersion is managed for the system as a whole, a new
problem arises of fiber nonlinearity due to polarized mode
dispersion (PMD) and the Kerr effect. The most problematical of the nonlinear phenomena is four-wave mixing(FWM), which is more severe the lower the chromatic dispersion. When chromatic dispersion is high, on the other
hand, this causes degradation of the signal waveform bygroup velocity dispersion (GVD). A proposal to solve this,a new type of fiber, known as non-zero-dispersion shifted
optical fiber (NZDSF: ITU Recommendation G.655), hasbeen proposed. As OFAs practically remove limitations on
the optical signal level launched into the fiber, it is possible to make the optical signal as strong as possible tolengthen repeater spacing if there is no nonlinear phenomenon in the fiber. But in actuality the input signal levelis limited by the nonlinearity of the optical fiber. Since nonlinearity is greater the higher the optical power per unit
area, consideration was given to enlarging the coreregion, but this would only increase the chromatic dispersion slope. On the other hand there is a tendency toward
expansion of the WDM band, and a demand for adecrease in the chromatic dispersion slope, and this has
led to a number of designs from research institutions
around the world for optical fiber suited to WDM.
At Furukawa Electric we are working to develop NZDSF
with larger core diameter, as well as reverse-dispersion
fiber (RDF) 9) having chromatic dispersion characteristics
that are the reverse of those of single mode fiber. Using
single mode fiber in the high power region and RDF in the
low power region reduces nonlinearity as well as chromatic dispersion for the system as a whole over a wider rangeof wavelengths and thereby configures a transmission
path suited to long-haul, large-capacity DWDM. We havefurther confirmed that no problems are encountered in the
cabling process. The results of tests on a transmission
system using this configuration have been reported, andwe believe that it will become a model for the next generation of transmission links. 10)
Optical cross-connects (OXCs) require integrated optical switches. At present the only reliable types aremechanical, and large switching arrays configured in thisway tend to be bulky, but many designs are being proposed using new technology. Work is in progress on
waveguide-type optical switches that change the refractive
index without moving parts by utilizing electro-optical (EO)
acoustic-optical (AO) and thermal-optical (TO) effects andon mechanical switches 11) based micro-electro-mechanical systems (MEMS), and some of them have advanced
to the on-site testing stage.

3

If wavelength conversion becomes possible, identical
wavelengths from different optical fibers could be converted so that the wavelengths do not overlap, and then sentout over the same output fiber, thereby improving the performance of OXCs. Wavelength conversion devices basedon semiconductor optical amplifiers (SOAs) are underdevelopment, but there are many technological problems
to be overcome before practicable units become available.

4.

CONCLUSIONS

The mini-feature on WDM is presented as a way of summarizing Furukawa Electric's current activities in the WDM
field. We hope it will guide you to the many fine components we have developed for WDM systems.

REFERENCES

1) Y.Kimura, K.Suzuki and M.Nakazawa: Efficient Er3+-doped
optical amplifier pumped by a 1.48-µm high-power laser diode,
OFC'89, TUG6 (1989)
2) M.Suyama, K.Nakamura, S.Kashiwa and H.Kuwahara: OFC'89,
PD-6, (1989)
3) H.Murata, S.Inao and Y.Matsuda: Step index type optical fiber
cable, 1st ECOC, London, p.70 (1975)
4) M.Fukushima, et al.: Flat gain Erbium-doped fiber amplifier in
1,570 nm- 1,600 nm region for dense WDM transmission systems, OFC'97, PD-3, (1997)
5) H.Takahashi, S.Suzuki and I.Nishi: Multi/demultiplexer fornanometer-spacing WDM using arrayed-waveguide grating,
IPR'97, PD1-1, (1991)
6) Koyanagi, et al.: The ultrahigh-power 1,480 nm pump laser
diode module with fiber Bragg grating, OAA'98. MC2 (1998)
7) R.Rotwitt, et al.: A 92 nm bandwidth Raman Amplifier,
OFC'98, PD-6 (1998)
8) K.O-Hill, et al.: Aperiodic in-fiber Bragg gratings for opticalfiber dispersion compensation, OFC'94, PD-2 (1994)
9) K.Mukasa, Y.Akasaka, Y.Suzuki and T.Kamiya,: Novel network fiber to manage dispersion at 1.55 µm with combination of
1.3 µm zero dispersion single mode fiber, vol.1, pp.127-130,
ECOC'97 (1997)
10) K.Yonenaga and Y.Miyamoto: Dispersion-managed high-capacity WDM systems using zero-dispersion-flattened transmissionline, FD-4, OFC'99 (1999)
11) Lightwave: All-optical equipment emerging from the lab to themarketplace, June 1999, p1
Manuscript received on November 5, 1999.

4

Furukawa Review, No. 19. 2000