Hitachi overview of fiber-optic commun developments

[David Farber <farber () central cis upenn edu>]

From:
 Dr. David K. Kahaner
 US Office of Naval Research Asia
 (From outside US):  23-17, 7-chome, Roppongi, Minato-ku, Tokyo 106 Japan
 (From within  US):  Unit 45002, APO AP 96337-0007
  Tel: +81 3 3401-8924, Fax: +81 3 3403-9670
  Email: kahaner () cs titech ac jp
Re: Hitachi overview of fiber-optic communication developments
10/08/94 (MM/DD/YY)
This file is named "hitachi.94"


ABSTRACT. Article on fiber optic transmission developments and plans, as
seen by scientists at Hitachi.


The following article appeared in the English language journal
HITACHI REVIEW, April 1994   Vol 43, No. 2 (pp 41-46). For copies of the
complete journal contact the publishers at the following address.


 Editor-in-Chief: Shozaburo Kobayashi
 Hitachi Review
 Hitachi Limited
 6 Kanda Surugadai 4-chome, Chiyoda-ku, Tokyo 101-10 Japan


FIBER-OPTIC TRANSMISSION FOR ENHANCED NETWORK SERVICES


Recent Trends in Fiber-Optic Transmission Technologies for
Information and Communication Networks


Naoki Chinone, D. Eng. (Hitachi Central Research Lab)
Minoru Maeda, D. Eng.  (Fiberoptics Division, Hitachi Ltd)




ABSTRACT:


Fiber-optic transmission technologies have been developing considerably
to satisfy demand for large-capacity digital transmission in public
telecommunication networks.  Gigabit-per-second (Gbit/s) transmission
systems have already been put into practical use for trunk line
networks, and next-generation 10-Gbit/s system are being intensively
developed.  On the other hand, various information services including
voice, data and moving images are becoming indispensable to human
activities in step with recent development of the information society.
Therefore, the broadband Integrated Services Digital Network (ISDN) and
the high-speed multimedia Local Area Network (LAN) are being intensively
developed.  Fiber-optic transmission technologies are expected to be
deployed in various fields, from present trunk lines to subscriber loops
and also in local-area high-speed information networks.  Hitachi is
actively contributing to the evolution of the information society by
promoting development of optical transmission technologies and products.


INTRODUCTION


FIBER-OPTIC transmission technologies utilizing optical fiber
transmission lines support high-speed/large-capacity and long-distance
transmission and have been developing considerably to satisfy demands in
large-capacity digital transmission in public telecommunication
networks.  At first, optical transmission technology with speeds of
several tens of Mbit/s using multimode optical fibers as transmission
lines were developed in the 1970s.  In the 1980s, high-speed transmission
technologies having transmission speeds from several 100 Mbit/s to
Gbit/s were developed based on the development of single-mode optical
fiber technology.


The above technologies have been utilized to realize large-capacity
digital transmission mainly in trunk lines of public telecommunication
networks.  Recently, however, various information services including
voice, data and moving images have become indispensable to human
activities.  Therefore, the broadband ISDN in public transmission
networks and the high-speed multimedia LAN in in-house information
networks are being intensively developed.  Fiber-optic transmission
technologies, as key-technologies to realize a highly-developed
information society, are expected to be deployed in various fields, from
present trunk lines to subscriber loops in public telecommunication
networks and also in local-area high-speed information networks, as
shown in Fig. 1 [omitted, showing trunk line fiberoptic transmission
from central office to switching node, and thence to remote nodes,
including CATV centers, etc.]


Here, we describe new technological trends in optical transmission
toward high-speed/large-capacity transmission and the impact on
telecommunication and information networks.


TECHNOLOGICAL TRENDS


Optical transmission technologies first commercialized in the early
1980s have been developing considerably.  The transmission speed has
increased an order of magnitude in the past 10 years, as shown in Fig. 2
[omitted. This shows growth of transmission capacity by year, single
mode fiber using 1.3mu-m laser in early to mid '80s giving about
0.1--0.8Gbit/s, to single mode fiber using 1.55mu-m DBF laser in late
80s and early 90s giving 3--5Gbit/s, to dispersion shifted fiber --
using optical amps in the late '90s giving 10Gbit/s or more. This is
explained in more detail in the following text.] 100-Mbit/s optical
transmission systems in the early 1980s were put into practical use
using multimode optical fibers and 0.8mu-m wavelength lasers.  It was,
however, clarified that long-distance transmission of high-speed optical
signals is difficult through multi mode fibers due to mode dispersion,
and that loss of optical fibers is lower in the wavelength range above
1mu-m.  High-speed transmission systems were, therefore, developed using
single-mode optical fibers and 1.3mu-m wavelength lasers.  In the
mid-1980s, the fiber loss was further lowered in the 1.55mu-m wavelength
range and distributed feedback (DFB) lasers, which have superior single
mode characteristics, were developed.  Then, in the late 1980s,
1-2-Gbit/s high-speed transmission systems utilizing these technologies
were put into practical use.  In parallel with the development of
high-speed technologies, international standards for transmission
systems were actively being discussed and the synchronous digital
hierarchy (SDH) was standardized by ITU-T (formerly CCITT) in 1988.
Based on this SDH, 2-4-Gbit/s optical transmission systems were put into
practical use in 1990 and are now the highest-speed commercialized
optical transmission systems. [ITU-T = International Telecommunications
Union-Telecommunications Standardization Sector.]


In the future, 10-Gbit/s systems in the mid-1990s and 40-Gbit/s systems
in the year 2000 are expected to be put into practical use, based on the
prediction shown in Fig. 2 [omitted, see above].  The development of new
transmission technologies for these systems has progressed.  The basic
configuration of optical transmission (a), a conventional method of
optical signal generation and detection (b), and a new method being
developed for high-speed transmission (c) are shown in Fig. 3 [omitted,
but described in the following text].  In the conventional method,
optical signals are generated by direct modulation of a laser and
detected and converted to electronic signals by a detector, such as an
avalanche photodiode (APD).  However, since signal distortion due to
fiber dispersion and reduction of receiver sensitivity must be overcome
to realize higher transmission speeds, new methods were developed: 1) an
external modulation method where the optical output from a laser is
modulated by an optical modulator and 2) an optically-amplified signal
detection method where the optical signal amplified by an optical
amplifier is detected by a detector.  Further, dispersion-shifted fibers
were developed, which have zero dispersion in the wavelength range of
1.55mu-m.


Among the new technologies, progress in optical amplifier technology has
been remarkable.  The optical amplifier can be used not only for
improving receiver sensitivity, but also for enlarging transmitter
optical output, and it can be used as an optical repeater.  Therefore,
this device is expected to reduce restrictions in transmission system
design.  The optical fiber amplifier uses the principle that optical
signals of 1.55mu-m wavelength are amplified through an erbium-doped
fiber excited by a laser light of 0.98mu-m or 1.48mu-m wavelength.  An
example of optical fiber amplifiers is shown in Fig. 4 [omitted but
described in the following text].  In this case the erbium-doped fiber
is excited from both ends for use as an optical repeater.  Front end
excitation is usually used for receivers and back excitation for
transmitters.  Very recently, optical fiber amplifiers are becoming
commercialized for long-distance transmission.  Using the technologies
mentioned above, 10 Gbit/s optical transmission over more than 300 km
was experimentally confirmed utilizing optical amplifier repeaters and
is now being intensively developed for practical use.  To realize larger
capacity transmission, technologies for optically multiplexed signal
transmission are important, as well as higher speed signal transmission
technologies.  There is a possibility that 10-20-Gbit/s transmission
will be realized based upon the new technologies mentioned above.  It
is, however, predicted that the limitation of higher-speed transmission
would be several tens of Gbit/s, limited by the performance of the
optical and electronic devices and also by fiber dispersion.  It is,
therefore, necessary to combine high-speed technologies and optical
multiplexing technologies to realize capacities of several tens of
Gbit/s to Tbit/s.  The optical-frequency multiplexed transmission method
has attracted much attention for large-capacity transmission, by which
multiple optical signals having different wavelengths are independently
modulated and optically multiplexed into a single optical fiber.
Approaches toward realization of Tbit/s transmission speeds are shown in
Fig. 5 [omitted, this shows a plot of transmission speed versus number
of multiplexed channels, and indicating that the use of optical
multiplexing can generate 1Tbit by using 10Gbit/s with 100 multiplexed
channels].  10-Gbit/s transmission can be realized, for instance, by
multiplexing 10 channels of 10-Gbit/s optical signals and further
1-Tbit/s transmission by multiplexing 100 channels.




RESEARCH AND DEVELOPMENT OF OPTICAL TRANSMISSION TECHNOLOGIES


As mentioned above, high-speed/large-capacity transmission technologies
are being intensively studied.  Hitachi is also actively involved in
research and development of wide-range fiber-optic transmission
technologies.  Here, two topics are introduced among the new
technologies being developed at Hitachi.


Optical Modulators


For high-speed transmission, external optical modulators have been
intensively studied as mentioned in the previous section.  In the
conventional direct modulation method where the semiconductor laser is
directly modulated by electronic signals, wavelength chirping in
transmission signals is substantially large.  This causes distortion of
the optical pulse shape, due to the fact that the propagation speed of
the optical pulses is different at different wavelengths because of
refractive-index dispersion in the optical fiber.  This distortion
becomes more significant as transmission speed and transmission distance
increase.  On the other hand, by using an external modulator, wavelength
chirping can be drastically reduced.  The optical modulator has long
been studied using LiNbO3 materials.  Recently, however, InP
semiconductor material has attracted much attention, because it enables
integration with other semiconductor devices and has the potential for
high reliability.


Two types of semiconductor optical modulators are being studied at
Hitachi.  One is a Mach-Zehnder type modulator.  In this device, the
electro-refractive effect, where the refractive index of the
semiconductor material changes with the applied voltage, is used to
modulate input laser light.  This device, therefore, enables essentially
zero wavelength chirping, which is necessary to very long distance
transmission.  An InGaAs/InAlAs Multiple Quantum Well (MQW) structure is
introduced for waveguides to reduce the driving voltage below 4.0 V.
Insertion loss of the modulator is about 10 dB, which can be compensated
for by an optical amplifier.  A small-signal frequency bandwidth above
12 GHz was achieved and 10-Gbit/s transmission was successfully
demonstrated.


The other is an electroabsorption (EA) type modulator.  In this device,
the EA effect, where the absorption of input light is changed by the
applied voltage, is used.  When the absorption is changed, the
refractive index is inevitably changed.  This device, therefore, causes
a small amount of wavelength chirping.  Advantages of this device,
however, are a small driving voltage (below 2.0 V) and the ability to be
integrated into semiconductor lasers.  So far, two-step epitaxy of the
semiconductor materials has been employed to fabricate integrated
devices, since the modulator and laser each have a different structure.
This process, however, causes degradation of device characteristics, due
to the difficulty in connecting devices grown with the different step
epitaxy.  A new technology for fabricating the integrated device by
one-step epitaxy was developed at Hitachi.  By employing this
technology, stable device characteristics and low insertion loss below 2
dB were realized.  10-Gbit/s modulation at a pulse voltage of 1.5 V was
confirmed.


Coherent Frequency Division Multiplexed (FDM) Transmission Systems


Optically multiplexed transmission technologies look promising for
large-capacity trransmission systems, as mentioned in the previous
section.  There are several schemes to realize optical multiplexing.
One is multiplexing intensity-modulated different-wavelength optical
signals, which is usually called wavelength division multiplexing (WDM).
One of the other schemes is multiplexing optical-frequency-modulated
different-wavelength signals, which are detected utilizing coherent
technology at the receiver.  In this scheme, wavelength spacing between
signals can be minimized to below 0.1 nm corresponding to a frequency of
10 GHz, because the receiver has high wavelength selectivity.  This
scheme is, therefore, usually called frequency division multiplexing
(FDM).  The configuration of a prototype FDM transmission system
developed at Hitachi is shown in Fig. 6 [omitted, described in the
following text].  In this system, multiple frequency-modulated signals
are coupled into a single-mode fiber by an optical coupler and are
transmitted through the optical fiber.  One of the transmitted signals
is detected by a heterodyne receiver, where the signal is mixed with
laser light from a local oscillator laser whose wavelength is tuned to
that of the transmitted signal.  By this heterodyne method, the receiver
itself has superior wavelength selectivity.  In the system shown in Fig.
6 [omitted, described in the following text], the transmitter has 32
channel lasers and each laser is frequency modulated at 1.244 Gbit/s.
Total transmission capacity is, therefore, 40 Gbit/s.  Wavelength
(optical-frequency) spacing is 0.08 nm (10 GHz) in the wavelength range
of 1.55mu-m.  40 Gbit/s-transmission at a distance of 121 km was
confirmed.  This prototype system demonstrates the possibility of
large-capacity transmission using coherent technology.


IMPACT ON TELECOMMUNICATION NETWORKS


For in-house information networks such as LANs in offices and factories,
transmission speeds have increased almost an order of magnitude from
10-Mbit/s Ethernet to 100-Mbit/s fiber distributed data interface
(FDDI), as shwon in Fig. 8 [omitted, showing the evolution from
Ethernet, Token ring, FDDI, to Gbit/s LAN, HIPPI, FFOL, etc].  further
FDDI follow-on LAN (FFOL), high performance parallel interface (HIPPI)
for high speed computers and also Gbit/s LANs including those
technologies are being intensively studied, especially in the U.S.A.,
towards commercialization in the late 1990s.


The Gbit/s LAN shown in Fig. 9 [omitted, showing a Gbit/s fiber-optic
LAN connecting workstations, supercomputers, mainfram computers, storage
systems, and via bridges to WANs and FDDIs] connects computer
mainframes, storage systems and workstations at speeds above 1 Gbit/s
and is also connected to high-speed public telecommunication networks to
build up a wide area network (WAN).  This network enables multimedia
information processing, including moving images information and also
high-volume diverse data processing.  The fundamental fiber-optic
transmission technologies for those networks have been already developed
for public telecommunication networks.  They should, however, be
optimized for implementation in LAN environments and made more
economical.


CONCLUSIONS


Fiber-optic transmission technologies and their impact on
telecommunication and information networks were described.


Optical transmission technologies have developed remarkably in the past
10 years to satisfy demand in large-capacity digital transmission in
public transmission networks.  Gigabit-per-second range systems have
already been put into practical use in trunk lines, and next generation
10-Gbit/s systems and also fundamental technologies toward future Tbit/s
systems have substantially progressed.  Fiber-optic transmission
technologies are expected to be deployed in various high-speed networks,
not only in trunk lines, but also in subscriber loops in public
telecommunication networks and also in in-house information networks.


Hitachi is actively involved in research and development of wide-range
fiber-optic transmission technologies, such as optical and electronic
semiconductor devices, optical transmitters and receivers, and also
full-range transmission systems.  Hitachi is contributing to the
development of an information society by implementing; these
technologies in practical applications.




REFERENCES


(1)  T. Shimada, "Trends and Future Prospects on Research and
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(2)  M. Maeda and N. Chinone, "Recent Trends in Fiber Optic Transmission
Systems Technologies," Hitachi Review 40, 161-168 (1991).


(3)  K. Nakagawa et al., "A bit-Rate Flexible Transmission Field Trial
Over 300 km Installed Cable Employing Optical Fiber Amplifiers," Topical
Meeting on Optical Amplifiers and their Applications, PdP11 (1991).


(4)  H. Toba et al., "Optical Frequency Division Multiplexing Systems,"
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(5)  H. Tsushima et al., "1.244-Gb/s 32-channels Transmission Using a
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(6)  M. Aoki et al., "High Speed (10 Gb/s) and Low Driving Voltage (1V_{
p-p}) InGaAs/InGaAsP MQW Electroabsorption Modulator Integrated DFB
Laser with Semiinsulating Buried Heterostructure," Electronics Letters
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(7)  H. Sano et al., "A High Speed InGaAs/InAlAs MQW Mach-Zehnder Type
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(1993).


(8)  H. T. Kung, "Gigabit Local Area Networks:  A Systems perspective,"
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(9)  H. Ishio, "Next Generation Lightwave Communications Technologies,"
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