Broadband Optical Amplifiers for DWDM Systems
by Haruki Ogoshi *, Seiji Ichino * and Katsuya Kurotori *The spread and expansion of DWDM (Dense Wavelength Multiplexing) systems
are going on keeping pace with various technology developments of opticalamplifiers and, in particular, the bandwidth broadening of EDFAs (Erbium Doped Fiber Amplifier).
In this technology area, gain-equalizing technology has been developed to flatten the gain peaks
around 1532 nm, making available a wide range of wavelengths from 1530 to 1560 nm.
Furthermore, development of gain-shift technology is under way, in which gain is shifted to thelong wavelength band of 1570-1600 nm using long lengths of Erbium doped fibers (EDFs). This
paper reports on the optical amplifier products that Furukawa Electric has developed so far: 1)
modularized optical amplifier for 1530-1560 nm, which permits flexible control using a microprocessor; 2) desktop optical amplifier having a gain band shifted to 1570-1600 nm; and 3) ultra
compact gain module for single channel use, which compensates for the loss due to passive
components in DWDM systems, thus improving flexibility in the system design.

ABSTRACT

Raman

Optical Sub-systems Dept., FITEL Products Div.

Er

GE
L-EDF

Tm

L-EDF

1600
1620

1560
1570

1520
1530

1480

GE: Gain equalization
L-EDF: Long EDFs

1460

GE

Technology

Wavelength (nm)Figure 1*

Er

Fluoride fiber

Tellurite fiber

EDFA + Raman

ErThe rapid increase of communications demand triggered
off by the Internet has strongly supported the development
and practical use of the DWDM systems. Thus, the
DWDM system that had been a subject of discussion atacademic meetings until only a few years ago has comeinto practical applications with a rush, and the number ofmultiplexed channels is reaching a figure as many as 100.
Such spread and expansion of the DWDM systems
have been progressing keeping pace with the technology
developments to broaden the bandwidth of EDFAs.
The band broadening technology of EDFAs based on
silica fibers includes: flattening of the gain vs. wavelengthcharacteristics for 1530-1560 nm (i.e., C-band) using gain
equalizer 1); and shifting gain to longer wavelengths using
long lengths of EDFs with a low population inversion rateensuring a sufficient gain at 1570-1600 nm (i.e., L-band) 2), 3);
and both technologies have been practically used.
Moreover, EDFFA (Er Doped Fluoride Fiber Amplifier) has
been developed and actually used, in which silica fiber as
host glass is replaced by fluoride glass fiber 4), 5). Recently,
EDTFA (Erbium Doped Tellurite Fiber Amplifier) based on
tellurite glass has been developed, achieving excellent
amplifier characteristics over an entire wavelengths of
1530-1600 nm --unattainable with EDFAs 6)~8).
Optical amplification in most of the 1500-nm band has
become practicable, as described above, using opticalfiber amplifiers based on EDFs. On the other hand, the1400-nm band is now drawing attention as a new communications band, since the problem of absorption loss due
to water contamination in fiber has been solved thanks tothe advances in fiber manufacturing technology. TDFA
(Thulium Doped Fiber Amplifier) has been developed andreported to cover the 1400-nm band, which is based on
stimulated emission using Thulium 9), 10). What is more,unlike optical amplification based on stimulated emission,
Raman amplification enables amplification at an arbitrary
wavelength by selecting a suitable pumping wavelength.
Based on this principle, an ultra-broadband amplifier system has been developed, in which Raman amplificationand EDFA are adopted for the 1400-nm and the 1500-nm
band, respectively 11). It is interesting to note that Raman
amplification was a major research concern before EDFA
appeared. It was discarded as impracticable, however,Silica fiber

INTRODUCTION

Stimulated emission type optical amplification

1.

Development status of optical amplifiers for DWDM
systems17

because of the fact that it necessitated large pumping
power and that EDFA was invented in the late 1980s.
Recently it is drawing attention again, since in the courseof EDFA development, the pumping laser for the 1480-nm
band has made such an improvement in its output powerthat it now provides a pumping power sufficient for obtaining practical Raman gains. A growing need for broadband
amplification also supported reconsideration of this technology 12).
Figure 1 shows applicable wavelength bands of opticalamplifiers thus developed 13). It can be seen that the 1500nm band and the 1400-nm band are covered by the stimulated emission type amplifiers using Erbium and Thulium
respectively, and that the band for Raman amplification is
located to supplement or to partly overlap the two bands.
It is anticipated that, from now on, such technologies as
EDFA, TDFA, and Raman amplification are integrated topursue practical development of photonic network systems that utilizes the entire wavelength band spanning
200 nm.
Furukawa Electric has been keenly developing EDFAs
for DWDM systems from the late 1990s, and is now developing element technologies for Raman amplification, taking notice of the effectiveness of this technology. This
paper reports on the development of EDFAs for DWDM
systems based on silica fibers, leaving out the introductionof Raman amplification development for a certain paper 14).

2.

DEVELOPMENT OF EDFA FOR 15301560 NM (C-BAND)

Characteristics required for optical amplifiers for DWDM
are flatness in the gain vs. wavelength performance andhigh output power. Of these, since DWDM systems haveto amplify multiple optical signals of different wavelengths
simultaneously, the flatness of gain with respect to wavelength (i.e., gain flatness) is considered to be most important. Development of EDFAs for DWDM is identical to, in
other words, technology development of how to achieve again flatness.
Figure 2 shows the gain coefficient spectrum of EDFs
with population inversion rate as a parameter. When we
pay attention to the 1540-1560 nm band of the spectrum,
it is seen that the spectrum shows a negative slope withrespect to wavelength when the population inversion rate
is high and a positive slope when the rate is low. In thecase of Figure 2, an optimum flatness is obtained when
the population inversion rate is controlled to around 70%.
When it comes to the entire bandwidth of 1530-1560 nm,
however, the steep gain peak at around 1532 nm constitutes an obstacle making it impossible to obtain flat gain
characteristics only by controlling the population inversion
rate. Accordingly, it is normal practice to use gain-equalizing filter to flatten the gain in this band. Figure 3 shows aconceptual diagram of gain flattening using a gain-equalizing filter, in which it is illustrated that the filter is provided
with a loss spectrum that is reversed to the gain spectrumof an EDF, thereby resulting in a flat gain spectrum.

1.0

Relative Gain Coefficient

0.8
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
1440

1460

1480

1500

1520

1540

1560

1580

1600

Wavelength (nm)Figure 2

Gain coefficients of EDFs with population inversion
rate as a parameter

2.1 ErFA3300
ErFA3300 has been developed to respond to various
needs of DWDM systems for inter-office trunk lines, in aneffort to offer EDFAs that are flexible enough in opticalcharacteristics as well as in electrical controllability. Figure
4 and Photo 1 show the configuration and appearance of
ErFA3300, respectively. ErFA3300 allows mounting of
three pumping lasers, i.e., one for the preamplifier and two
for the post-amplifier. A gain equalizing filter is installed
between the preamplifier and post-amplifier, thereby
enabling amplification for the 1530-1560 nm band with again flatness of 1 dB or better. An output power of 20 dBm
maximum is available by using a high-power pumping
laser for 1480 nm in the post-amplifier. Figure 5 shows an

Post-amplifier

Preamplifier
EDFA

OpticalcouplerGain equalizing filter

GEQ

Output spectrum

PD

Gain

Loss

Gain

LD

Microprocessor-based control circuit
Wavelength

Wavelength

Wavelength
GEQ: Gain equalizing filter, PD: Photo diode, LD: Pumping LD

Figure 318

Conceptual diagram of gain flattening using gain
equalizing filter

Figure 4

Configuration of ErFA3300

Furukawa Review, No. 19. 2000

output power spectrum of ErFA3300 with 32 channels ofdifferent wavelengths signals incident on this amplifier,demonstrating that flat gain characteristics are obtained
for the entire band of 1530-1560 nm.
ErFA3300 offers a variety of features and functions
valuable for DWDM EDFAs as follows.
• A microprocessor is provided to permit arbitrary setting for any of the automatic gain control, automaticlevel control, or automatic pumping power control ofeach amplifier stage simply by selecting the program.
• Output power level is adjustable in 64 steps.
• Both the serial and parallel interfaces are provided.
• The dynamic range is 40 dB or greater and the input
and output monitoring function has an accuracy of±0.25 dB on average.

Photo 1

Appearance of ErFA3300• Interconnection with an external computer enables,
through graphical user interface, monitoring and setting of the input and output power, operating conditions of each pumping laser, and operating conditionsof EDFA. See Figure 6.
2.2 ErFA3100
ErFA3100 has been developed aiming at offering a low
gain-, low output power-, low cost-EDFA to such opticalcommunications systems as metropolitan DWDM systems
and optical LANs which are expected to grow rapidly.
While the amplifier, like ErFA3300, has a flattened gain
spectrum over the 1530-1560 nm band using a gainequalizing filter, it allows mounting of just one pumping
laser and has a rather small maximum saturation output
power of 13 dBm. Photo 2 shows its appearance. It isbased on a simple optical configuration consisting of a single stage amplifier and a gain-equalizing filter at the output side; and the 980 nm band was selected as pumping
wavelength taking noise characteristics into account.
Automatic gain control is adopted for control scheme, inwhich the influence of ASE (Amplified SpontaneousEmission) is automatically compensated for when the optical signal level is low, thereby a specified gain is precisely
maintained. Other electrical specifications are identical tothose of ErFA3300. Figure 7 shows its gain flatness deviation with the input level as a parameter, indicating that theflatness is maintained irrespective of input level changes.

32ch. WDM EDFA
Input power = -25 dBm/ch. Resolution = 0.5 nm
5
0
-5
-10
-15
-20
-25
1525

1530

1535

1540

1545

1550

1555

1560

1565

Wavelength (nm)Figure 5

Output power spectrum of ErFA3300

Photo 2∆ gain (dB)

Output power (dBm)

10

0.6
0.5
0.4
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8
1530

Appearance of ErFA3100

Pin=-4 dBm-10 dBm-7 dBm-25 dBm-19 dBm-1 dBm
16 dBm
13 dBm
22 dBm

1535

1540

1545

1550

1555

1560

1565

Wavelength (nm)Figure 7
Figure 6

Graphical user interface of ErFA3300

Characteristics of ErFA3100: gain flatness deviation
vs. input level

19Ultra-long EDF
ex. 200 m

1570~1600 nm
Gain: 30 dB

High power
1480 nm LD

High power
1480 nm LD

Figure 8

Photo 3

Configuration of L-band EDFA

Appearance of ErFA1419, a desktop optical amplifier
using L-band EDFA

1570-1600 nm dispensing with gain-equalizing filters.
Although foreseeable from Figure 2, this is something thatcan not be achieved by C-band EDFAs. Photo 3 shows an
appearance of ErFA1419, a desktop optical amplifier
developed for R&D purpose using an L-band EDFA. It has
achieved a gain flatness of 1 dB in addition to a saturation
output power of +20 dBm or higher at the 1570-1600 nm
band.

0Output ; +12 dBm/chout-40
Input ; -18 dBm/ch
-60
in
-80
1500

1520

1540

1560

1580

1600

1620

Wavelength (nm)Figure 93.

Characteristics of L-band EDFADEVELOPMENT OF EDFA FOR 15701600 NM (L-BAND)

It was known that usual silica based EDFs had amplification property at wavelengths longer than 1570 nm. This
fact had been neglected, however, until a strong need for
amplification band expansion arose from the DWDM systems side because of the poor amplification efficiency atsuch long wavelengths. When we review Figure 2 focusing on the band near 1570-1600 nm, it can be seen that
the EDFs have optical gains at the long wavelength bandof 1570 nm and higher. However, the gain available at1570 nm and higher is so small that long lengths of EDFs
together with a large pumping power is needed to obtain again of 25-30 dB --a practical level for an optical amplifier-- by accumulating small gains along the fiber length 13).
Development of EDFAs for the 1570-1600 nm band has
made a rapid progress since it was reported in 1997 2), 3).
The technological background of this rapid development
consists in the fact that element technologies for the optical amplifier development are the same as for the conventional 1530-1560 nm EDFA, and that high power pumping
lasers were already available through development.
Figure 8 shows the configuration of an L-band EDFA
and Figure 9 typical characteristics. L-band EDFAs
require long lengths of EDFs longer than those for conventional C-band EDFAs by as much as five to ten times.
One of the characteristics of L-band EDFAs is that almost
flat gain performance is obtained over the entire band of

20

4.

DEVELOPMENT OF ULTRA COMPACT
GAIN MODULE FOR PASSIVE COMPONENT LOSS COMPENSATION

Passive components having large insertion loss are often
used in such major stages of practical DWDM systems as
optical multiplexer, optical de-multiplexer, optical add-drop
multiplexer, and optical cross connect. If an EDFA is available to compensate for these passive components losses
on a unit channel basis, it will greatly enhance systemdesign flexibility, thus making construction of low-cost systems feasible. ErFA1974, an ultra compact gain module ofthe name card size, has been developed in response tosuch needs. ErFA1974 simply consists of a Peltier-element free coaxial package of the 1480 nm pumping laser
18.00

10.00

17.00

9.00

16.00

8.00

15.00

7.00

14.00

6.00
Gain (65°C)

13.00

NF (dB)-20

Gain (dB)

Output power (dBm)

20

5.00

Gain (25°C)
Gain (0°C)

12.00

4.00

NF (65°C)
NF (25°C)

11.00

3.00

NF (0°C)
10.00

2.00
1530

1540

1550

1560

Wavelength (nm)Figure 10 Gain and noise characteristics of the ultra compact
gain module

Furukawa Review, No. 19. 2000

and a coupler that combines the pumping and signal
lights. It measures 85 x 53 x 8 mm, the name card size,
and allows mounting of an optical isolator if necessary.
Figure 10 shows typical characteristics.

5.

CONCLUSION

We have described the development status of opticalamplifiers for DWDM systems including EDFAs for the1530-1560 nm and 1570-1600 nm bands and an ultra
compact gain module for passive component loss compensation. It would be an unexpected pleasure for us if
such technology developments could make some contribution to the growth of not only DWDM systems of today
and tomorrow but also the photonic network systems in
the future.
We would like to take this opportunity to thank all those
who cooperated and worked with us in the course ofdevelopment.

REFERENCES

1) P. F. Wysocki et al.,OFC'97,PD-2,1997
2) M. Yamada et al.; Electron. Lett.,vol.33,pp.710-711,1997
3) M. Fukushima et al.; OFC'97,PD-3,1997
4) M. Yamada et al.; IEEE Photon. Tech. Lett.,vol.8,No.7,pp.882884,1996
5) M. Yamada et al.; IEEE Photon. Tech. Lett.,vol.8,No.5,pp.620622
6) A. Mori et al.; OFC'97,PD-1,1997
7) A. Mori et al.; Electron. Lett. 34,p.887,1998
8) M. Yamada et al.; OFC'98,PD-7,1998
9) T. Sakamoto et al.; OAA'96,ThC3,1996
10) T. Komukai et al.; IEEE Quantum Electron.,vol.31,pp.18801888,1995
11) A. K. Srivastava et al.; OAA'98,PD-2,1998
12) Haruki Ogoshi; OPTRONICS, No.209, pp.141-144, 1999 (inJapanese)
13) Haruki Ogoshi; Journ. IEICE, Vol.82, No.7, pp.718-724, 1999
(in Japanese)
14) Emori et al.; Demonstration of Raman Amplification, aPromising Application of High Power Units, Furukawa Review,
No.19, April 2000
Manuscript received on January 6, 2000.

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