Demonstration of Broadband Raman Amplifiers:
a Promising Application of High-power Pumping Unit
by Yoshihiro Emori * and Shu Namiki *A high-power pumping unit with a total optical output power of 2.2 W has beendeveloped by connecting a fiber Bragg grating to a pumping laser diode, constricting and stabilizing the lasing wavelength, and performing wavelength-division multiplexingfor 12 wavelength channels between 1405 and 1510 nm, with a Mach-Zehnder interferometer
made with silica-based planar lightwave circuitry. Using this pumping unit, Raman amplification
tests were carried out using three types of transmission link fibers --25-km single mode fiber(SMF), 25-km dispersion shifted fiber (DSF) and 20-km reverse dispersion fiber (RDF)-- as theRaman amplification medium. At an average gain of 2 dB for SMF and 6.5 dB for DSF and RDF,
amplification characteristics of ±0.5 dB gain deviation over a 1520- to 1620-nm band wereachieved.

ABSTRACT1.

INTRODUCTION

At the beginning of the 1990s, the development of erbiumdoped fiber amplifier (EDFA) technology brought to opticalcommunications systems a dramatic leap in transmission
capacity as well as in electrical repeater spacing. However
the explosive growth in the Internet, which began in North
America at about the same period, increased the volume
of public telecommunications traffic to the point where
even greater capacity was required. Luckily the gain bandof EDFAs is comparatively wide, so that wavelength division multiplexing (WDM) using all the optical signals
included in this wavelength band was identified as amethod that could provide a solution to the problem, andwas rapidly implemented. The greater part of the information available on the Internet still consists of still pictures,the importance of video images is expected to increase sothat there will be no slackening in demand for transmission capacity.
WDM technology using EDFAs started from 1550 nm,
where gain flattening is easily accomplished, but in only a
few years spread to the 1530-nm band and has recently
been employed even in the 1580-nm band, where thesmall gain coefficient had made it impracticable. The bandof low fiber loss, however, is wider than the band in which
amplification by EDFA is possible, so that interest in optical amplifiers that operate outside the EDFA bandwidth isincreasing. One likely candidate is the Raman amplifier. In
optical amplifiers using erbium or other rare earth ions as
the medium, the gain wavelength band is determined by
the energy level of the ions, whereas in Raman amplifiers*

it is determined by the wavelength of the pumping light. 1)~3)
This makes it possible to amplify any desired wavelength
simply by selecting the appropriate pumping wavelength.
The principle of Raman amplification in optical fibers
was confirmed by Stolen et al., 4) and verified by experiments carried out by Aoki et al. 5 ) It was applied by
Mollenauer et al. 6) in optical soliton transmission, andEdagawa et al. verified WDM transmission using laser
diodes as the pumping source. 7) At the time, however,obtaining sufficient gain required several hundred milliwatts of pumping power, and there was no alternative tosolid-state lasers. Thus in telecommunications applications priority was given to EDFAs, which would operate
perfectly well on a few tens of milliwatts. Highly efficient
EDFAs, pumped by laser diodes, came into wide use,
while Raman amplifiers were left on the sidelines.
Research on Raman amplifiers continued, however, andtwo broad advantages were identified. The first was thatwhen a Raman amplifier was structured as a distributed
type, noise levels for the system as a whole could be
reduced compared to amplifiers using fibers doped withrare earth ions, which signals are subjected to lumped
amplification in the transmission link. 8) The second advantage was that by broadening the bandwidth of the pumpusing WDM, the gain bandwidth could be broadened in a
way not achievable with EDFAs. 9), 10)
This paper describes the development of a broadband
HPU using WDM technology with a laser diode, andreports on a Raman amplifier using this unit that achieved
extremely broadband amplification characteristics, with again deviation of ±0.5 dB at wavelengths from 1520 to1620 nm.

WDM Team, Opto-technology Lab., R & D Div.

59

12 channels

MZI-type WDM coupler
PLC

11 MZIs integrated

OutputLD

FBG

Figure 1

PBC

Schematic diagram of 12-channel HPU for WDM

10
9

BROADBAND HIGH-POWER PUMPINGUNIT

The popularity of high-output EDFAs has been greatly
accelerated by the realization of compact pumping laser
modules based on compound semiconductors. 11) Since alarger number of channels requires higher laser power,
the development of EDFAs for WDM has led to advancesin techniques for increasing pumping laser output. This
increase has been particularly evident in 1480-nm lasers,
and outputs as high as 250 mW at the end of an SMF
have already been reached. 12)
To reach the higher outputs that are essential if EDFAs
are to be practicable for broadband WDM transmission,
efficient coupling of the pumping sources must be
achieved. In recent years a high-power EDFA has beendeveloped that multiplexes three polarization multiplexed
wavelengths (for a total of six), for an signal light output of
1.5 W. 13) The absorption band of an erbium-doped fiber(EDF) in the 1480-nm band is from 1450 to 1500 nm, andto multiplex three or more wavelengths, the lasing spectrum of each pumping laser must be constricted and stabilized. This can be achieved during the module fabrication
process by forming a fiber Bragg grating (FBG) on thefiber pigtail to provide an external cavity mirror having narrow-band reflection characteristics. 14)
By using an FBG to narrow and stabilize the lasing
spectrum of the pump laser diode to a few nanometers, it
was possible to get efficient combining using a MachZehnder WDM coupler. Thus Tanaka et al. were able toproduce an 8-wavelength integrated WDM coupler using
silica-based planer lightwave circuit (PLC) technology,
thereby developing an inexpensive and efficient coupler
for a high-density WDM pumping unit. 15) This made it possible to realize a high-power pumping unit (HPU), with an
output exceeding 1 W.

60

8
7

Power (a.u.)

2.

6
5
4
3
2
1
0
1400

1420

1440

1460

1480

1500

1520

Wavelength (nm)Figure 2

3.

Output spectrum of HPU

RAMAN AMPLIFIER WITH FLAT-GAIN
BANDWIDTH OF 100 NM

The 1480-nm WDM pumping unit developed in order toincrease EDFA output can be readily adapted for broadband Raman amplifiers. In Raman amplification, aroundthe 1500-nm band, gain is produced at approximately 110
nm on the long-wavelength side, so that if the pumping
unit is multiplexed around 1450 nm to have a broad bandwidth, Raman amplification also occurs in a broad bandaround 1560 nm. If the signal light and the pumping lightare multiplexed/demultiplexed using a WDM coupler, theapproximately 100-nm wavelength difference between thepumping light and the signal light will be the upper limit of
the bandwidth of Raman gain. Recently the authors havemultiplexed the pumping light over a 105-nm bandwidthand have achieved broadband gain of 100 nm. 16)
Figure 1 shows the optics of a prototype 12-channel
WDM laser diode unit. Each channel is polarization multiplexed by a polarization beam combiner (PBC) laser diodestabilized by an FBG. The multiplexer/demultiplexer is asilica-based PLC, with 11 Mach-Zehnder interferometersintegrated on a single substrate. Polarization multiplexing

Furukawa Review, No. 19. 2000

of the laser diode is designed to increase pumping unit
output, as well as to suppress the polarization dependence of Raman gain. Figure 2 shows the HPU output
spectrum, and Figure 3 shows total output power (the I-L
characteristic). At a drive current of 700 mA per diode,total output was 2.2 W. Channel spacing was approximately 7.5 nm from 1405-1457.5 nm, and approximately
15 nm from 1465-1510 nm. The reason for this asymmetrical channel assignment is to provide asymmetrical compensation for the gain conditions resulting from the factthat short-wavelength pumping light causes Raman amplification of long-wavelength pumping light. Figure 4 showsthe experimental setup.
Coupling of the pumping light and the signal light was
accomplished by a counter-propagating configuration
using a long-wavelength pass filter (LWPF). Total insertion loss for two isolators and the LWPF was 1.6 dB at
1550 nm. Measurements of gain conditions were basicallycarried out using a broadband amplified spontaneousemission (ASE) light source, but for confirmation purposes, measurements were also made using probe light input
in addition to the ASE source using a tunable laser. Figure
5 shows the net gain profiles for Raman amplifiers of
extra-broad bandwidth using three types of fiber--SMF,
DSF and RDF. 17)
By adjusting the pumping power for each channel it waspossible to obtain flat gain over a wide bandwidth, with again deviation of only ±0.5 dB across a bandwidth of 100
nm. Average gain was 2 dB for the 25-km SMF, and 6.5

dB for the 25-km DSF and 20-km RDF. Gain in Figure 5
includes fiber loss, and is the gain defined by the ratio of
input to output signals. The transmission loss was approximately 5 dB for each of the fibers.
Figure 6 shows the allocation of launched pumping
power for Raman amplifiers with gain flattened over 100nm bandwidth. As can be seen, due to the Raman effectbetween pumps, launched power at short wavelengths
has to be made greater than at long wavelengths to keep
gain flat. Although SMF had the largest total pumping
power, average gain was less than for the other two types,
so that its gain efficiency was lowest of all. RDF and DSF
had the same average gain, but since the pumping power
required was 790 mW for DSF but only 610 mW for RDF,
we see that RDF was more efficient. This can largely be
explained by the fact that Raman gain is proportional topumping power density, and that mode field diameterdecreases in the sequence SMF, DSF, RDF.
Direct comparisons between EDFAs and distributed
Raman amplifiers are complicated by the difference
between the lumped and distributed types, but if the total
pumping power necessary to get a bandwidth of 100 nm is
600 mW, it can be seen that Raman amplifiers cannot bedismissed as inherently less efficient than EDFAs.

8
7

Net gain (dB)6

Total output power (W)

2.5

2.0

20-km RDF

5

25-km DSF

4

25-km SMF

3
2
1

1.5
0
1520

Transmission loss is about 5 dB in each fiber tested
1540

1560

1580

1600

1620

1.0

Wavelength (nm)Figure 5

0.5

Net gain profiles for Raman amplifiers of 100-nm
bandwidth

0.0
0

200

400

600

800

Drive current per diode (mA)
160

Total output power of HPU

Test fiber

ASE

LWPF
TLS1
TLS2
HPU
Input signals

Launched pumping power (mW)

Figure 325-km DSF: total 0.79 W
120

Experimental setup

20-km RDF: total 0.61 W

100
80
60
40
20
0
1400

1420

1440

1460

1480

1500

1520

Pumping wavelength (nm)Raman amplifier

Figure 425-km SMF: total 1.00 W

140

Figure 6

Allocation of launched pumping power for Raman
amplifiers of 100-nm bandwidth

61

4.

APPLICATIONS OF BROADBAND
RAMAN AMPLIFIERS

Particularly with regard to WDM transmission, research is
being conducted to reduce noise levels in the system as awhole through the use of distributed Raman amplifiers. In
the EDFA method of repeating that was used previously,
the signal light level is high and nonlinearity due to fourwave mixing in particular poses problems, so that it is difficult to achieve WDM transmission over DSF links in which
the zero-dispersion wavelength is included within the signal bandwidth. If, on the other hand, Raman amplificationis built into a WDM non-repeating transmission link using
EDFA, it is possible to reduce the signal level in the transmission link by the amount by which the noise characteristic is improved. Research groups at NTT and Lucent havemade use of this effect, reducing the signal level on DSF
links, in which the zero-dispersion wavelength is in thesignal band, thereby suppressing the nonlinear effect andachieving large-capacity WDM transmission. 18), 19) The useof Raman amplification in SMF-RDF links, which are
attracting attention to their dispersion-compensation properties, has also been reported to have reduced the effectof self-phase modulation on the signal. 20) Eliminating noise
and nonlinearity is a matter of life and death in largecapacity WDM transmission, and distributed Raman
amplifiers are one highly promising approach to a solution.

5.

CONCLUSION

Research on Raman amplifiers has a long history, and in
the days when the search was on for optical amplifiers fortelecommunications, they were regarded as a prime candidate. The great progress made in erbium-doped fiber
amplifiers, however, rather put Raman amplifiers in the
shadows. As EDFA-based WDM transmission technology
has matured, however, renewed attention is being paid to
Raman amplifiers. As the present work has demonstrated,the fact that their gain bandwidth is determined by thepumping wavelength can be utilized to realize a gain
bandwidth of 100 nm, and applications to a wide variety ofsystems are anticipated. A variety of configurations arealso possible--lumped, distributed, or even in combination
with EDFAs, so that the Raman amplifier will doubtless be
indispensable in super-broadband WDM transmission
over the whole optical fiber low-loss band, and in the next
generation of long-haul, high bit-rate, large-capacity WDM
transmission over dispersion-compensated links.

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Furukawa Review, No. 19. 2000