Raman Amplifier with Integrated Dispersion-Compensating Fiberby Shinya Nagamatsu *, Bera Pálsdóttir *2, Takeshi Hirasawa *3,Akira Fujisaki *4, Ryuji Takaoka *5 and Shigeru Shikii *4

It has been more than 20 years since fiber-optic communication became
practicable, and there has been a continuing stream of technologicalbreakthroughs. The increased diffusion of the Internet has seen the rapid introduction ofdense wavelength division multiplexing (DWDM) systems. With the burgeoning increases incommunications capacity in recent years, attention has been focused on Raman amplifiers, andwork goes forward on applying them to various systems. Among these may be mentioned Raman
amplification technology for dispersion compensating fiber (DCF) modules. This paper reportson the results of integrating a DCF module that can be added to existing fiber-optic transmission
devices with a Raman amplifier (incorporating control circuitry).

ABSTRACTINTRODUCTION

It has been more than 20 years since fiber-optic
communication became practicable, and there has beena continuing stream of technological breakthroughs. The
increased diffusion of the Internet has seen the rapid
introduction of dense wavelength division multiplexing(DWDM) systems. With the burgeoning increases incommunications capacity in recent years 1), attention has
been focused on Raman amplifiers 2), 3), and work goesforward on applying them to various systems. During the1980s much work was done on the technology of Raman
amplification as a prime contender for use as a fiberoptic communications amplifier, but at the practical level
the development and adoption of the erbium-doped fiber
amplifier meant that this never went beyond the research
stage.
However as bit rates progressed from 2.5 Gbps to
10 and even 40 Gbps, it became difficult to design
systems with only EDFAs, and the advantages of
distributed Raman amplification, in which the transmission
path itself serves as the amplification medium, wererecognized. More recently, the use of Raman amplification
while optimizing the dispersion characteristics of thetransmission path has become an indispensable
technology for upgrading transmission capacity andtransmission limits. That is to say in order to upgrade
transmission capacity, technologies for optimizing

dispersion characteristics (dispersion management) haveassumed great importance.
One of these important technologies is Raman
amplification for dispersion compensating fiber (DCF)
modules. This paper reports on the results of integratinga DCF module that can be added to existing fiberoptic transmission devices with a Raman amplifier(incorporating control circuitry).

2.

CHARACTERISTICS OF VARIOUS
OPTICAL FIBERS

Figure 1 shows the relationship between the Ramangain coefficient and fiber length for various fibers. It
can be seen that in comparison to single-mode fiber,
all the others--the highly nonlinear fiber (HNLF),
dispersion compensating fiber (DSC), reverse dispersion
compensating fiber (RDF) and dispersion-flat fiber (DFF)

20
HNLF

Raman gain (dB)

1.

DCF
RDF

10DFF

SMF
*Metal Products Development Center, Metal Research Center*2 OFS FITEL Denmark I/S*3 Thermal Products Dept., Electronic Components Div.
*4 Optical Components Dept., FITEL Products Div.
*5 OptCom Div.

00102030

Fiber length (km)Figure 1

Relationship between Raman gain and fiber lengthfor various fibers.

Furukawa Review, No. 24

2003

13Raman Amplifier with Integrated Dispersion-Compensating Fiber

Depolarizer

40
SMF

Dispersion (ps/nm/km)

200

LDM
WDM

SMF+DCF(Total link dispersion)-20-40

DCF-60-80

Isolator
DCF-100-120
1500

WDM

1550

Figure 3

Wavelength (nm)Figure 2

Wavelength dependence of dispersion compensation
using SMF + DCF link.

Optical configuration.

Shutdown
SPILD

Table 1

ACC, (APC)

L-band characteristics of DCF.
Item

Effective area: Aeff ( µ m2)

Loss coefficient: α (dB/km)
Dispersion (ps/nm/km)
Dispersion slope (ps/nm2/km)
PMD (ps/km1/2)
Rayleigh scattering coefficient (l/m)

Valueat λ c*

25.8at λ p*

15.5at λ c0.4at λ p0.4at λ c-152at λ c-0.4at λ c to λ p0.1at λc0.2 × 10-5(* λ c = 1590 nm, λ p = 1470 nm)--have higher Raman gain coefficients. Figure 2 shows
an example of dispersion compensation with SMF + DSF.
It will be appreciated that this is an extremely effective
approach to upgrading existing SMF links while retaining
their many advantages--low loss, low PMD, low cost, lownonlinearity and high localized dispersion. It also has
disadvantages, however, in terms of the high loss in DCFand the space taken up when accommodated as spools insystem equipment.

3.

IMPROVING DCF CHARACTERISTICS

Removing the two disadvantages referred to above
would make the approach described a highly effective
solution. The first area we focused on was improving
the characteristics of the DCF itself. Table 1 shows the
improved characteristics. It can be seen that DCF loss has
been held to around 0.4 dB for both the amplification band
(1470 nm) and the signal band (1590 nm).
When considering Raman amplification, the importantfactors are: 1) the pumping band loss because it is directly
related to Raman amplification efficiency, and 2) theRayleigh scattering coefficient, which should be as low
as possible since it gives rise to problems such as double
Rayleigh back-scattering noise (multipath interferencenoise, or MPI).

BFM

Externalcontroller

ADC

RS-232 DAC

PWM

TEC
Thermistor
Temp

Figure 4

Block diagram of control system.

It can be seen that the Rayleigh scattering coefficient, at
0.2 × 10-5 l/m, has been improved to a level approaching
the SMF value of 0.1 × 10-5 l/m .

4.

DESIGN OF RAMAN PUMPING LASER

Based on the improvements to DCF characteristics
described above, we proceeded to design the simplest
possible Raman pumping laser. Figure 3 shows the opticalconfiguration that was used.
The pumping laser used in this case was of a2-wavelength structure, with one laser diode module foreach of the two wavelengths. And the Raman module
used was entirely contained within the DCF spool, in astructure in which no further space was required to add
the Raman module.
Considering that use could be made of the spaceaccommodating the DCF, a control function was thereforeprovided to enable independent control of the Ramanmodule. Figure 4 is a block diagram of the control system.
As can be seen the control circuitry is accommodated
within the Raman module, and pulse width modulation(PWM) control is utilized to reduce the power consumptionof the amplifier.
Both a serial peripheral interface (SPI) and a RS232
port are provided as communications control interfaces.
This was done to facilitate control from a personal
computer over the RS232, and to support the SPI buses
used for communications control when built into existingtransmission devices.
Figure 5 shows an outside view of the Raman pumpinglaser module and Figure 6 shows the inside.

Furukawa Review, No. 242003

14Raman Amplifier with Integrated Dispersion-Compensating Fiber

The Raman pumping laser module is circular inoutline, from which emerges the communications controlinterfaces. Two types of control are supported: automaticcurrent control (ACC) and automatic power control (APC).
There is also a shutdown function for use when a fault
is detected. Figure 7 shows the final configuration of aRaman amplifier module with integrated DCF. As canbe seen this Raman pumping laser is entirely enclosed
within the DCF module, making it possible to add the laser

Figure 5

Outside view of Raman pumping laser module.

without increasing the space allotted for the DCF module
inside conventional transmitting devices.

5.

EVALUATION OF CHARACTERISTICS

The characteristics of the Raman amplifier describedabove were then evaluated. The evaluation was carriedout with respect to a dispersion and slope compensatingfiber using pumping lasers of two wavelengths--1430 and
1455 nm.
Figure 8 shows the results of measurements of thenoise figure (NF) and multipath interference noise(MPI) with respect to gain for NZ-DSF (TrueWave®) andSMF fibers with a compensating distance of 125 km
and an input power of 5 dBm. In the case of NZ-DSF
compensation the gain was at the 0 dB level and MPI was-50 dB, and for SMF compensation the gain was at the -1
dB level and MPI was -45 dB.
Specifically it was found that for NZ-DSF 125-km
compensation, a completely loss-free condition (gain = 0dB or above ) was obtained at MPI of -50 dB.
Figure 9 shows the results of measurements of relative
intensity noise (RIN). Based on measurements using fourlaser diode modules, results were stable for frequencies
4-30

2Gain of NZ-DSF

MPI of NZ-DSF

Gain of SMF

MPI of SMF-350-45-4

MPI (dB)

Gain (dB)-40-2-50-6-55-8-10
1527

Figure 6

Inside view of Raman pumping laser module.

1537

1547

-60
1567

1557

Wavelength (nm)Figure 8

Measured characteristics of Raman amplifier.

-100
LDM 1
LDM 2
LDM 3
LDM 4

RIN (dB/Hz)

-110
-120
-130
-140
-150
-160

0200

400

600

800

1000

1200

Frequency (kHz)

Figure 9

Figure 7

Raman amplifier module with integrated DCF.

Frequency dependence of relative intensity noise(RIN).

† TrueWave® is a registered trademark of OFS Fitel.

Furukawa Review, No. 24

2003

15

Raman Amplifier with Integrated Dispersion-Compensating Fiber

Table 2

Measured power consumption of Raman amplifier.

Case No. LDM No.
1

2

Operating current (mA)

1416

2404

3435

4422

Power consumption (W)
9.27

9.93

of 200 kHz and above, at -130 dB/Hz or better, confirmingthat no problem exists with respect to control circuit noise.
Table 2 shows the results of measuring power
consumption under the conditions described above:
NZ-DSF 125-km compensation at a gain of 0 dB.
Since this module is installed in a location where theuse of a Raman pumping laser was not envisaged, there
was a strong requirement that the heat emitted by thepumping laser be as small as possible. For this reason
a pulse width modulation (PWM) drive system was used
for the Peltier module of the LDM, which has the greatest
power consumption. As Table 2 shows, consumption washeld to below 10 W.

6.

SUMMARY

A significant improvement has been effected in the lossof dispersion compensation fiber, with a transmission lossof 0.4 dB/km. Success was also achieved in integrating aDCF module with a Raman amplifier (incorporating controlcircuitry).
As a result it was confirmed that for a compensation
distance of 125 km it was possible to use both NZ-DSF
and SMF in actual applications.
Specially noteworthy is that for NZ-DSF 125-km
compensation a loss-free (gain = 0 dB) condition wasachieved at an MPI of -50 dB or below. This technology
has the advantage of facilitating the upgrading of existingtransmission devices, and at the same time enablingtransmission devices to be designed without considering
DCF insertion loss.

REFERENCES
1) Shimojoh et al.; ECOC2001, Th.M.4.8
2) Emori et al.; OFC’99, PD19
3) Namiki; ECOC2001, We.M.2.1
4) L.Grüner-Nielsen et al.; OFC2002, WU15) Nagamatsu et al.; Communications Society Conference 2002,
IEICE, B-10-119 (in Japanese)6) Nagamatsu et al.; Communications Society Conference 2003,IEICE, B-10-152 (in Japanese)

Furukawa Review, No. 242003

16