Independent Control of the Gain and Noise Figure Spectra ofRaman Amplifiers Using Bi-Directional Pumping
by Yoshihiro Emori *, Soko Kado * and Shu Namiki *Conventional multi-wavelength pumped Raman amplifiers normally use backward pumping, because, in comparison to forward pumping, it has less gain saturation, and less RIN transfer from the pumping light to the signal light. Backward pumping has aproblem, however, in that when gain is flattened over a broad bandwidth, the wavelength-dependence of the noise figure becomes pronounced. This paper shows that, in a broadband Raman
amplifier having five pumping wavelengths, it is possible by using only the three shortest wavelengths for bi-directional pumping, to design the gain spectrum and the noise figure spectrum
independently.

ABSTRACT1.

INTRODUCTION

It is more than 20 years since fiber-optic communications
have been in commercial use and new technologicaldevelopments are constantly being made. Several years
ago wavelength-division multiplex (WDM) transmission
using Erbium-doped fiber amplifiers became the mainstream technology for large-capacity long-haul systems,
and more recently, to set new records, it has become
indispensable to make use of Raman amplifiers, optimizing
the dispersion characteristics of the transmission path 1)~4).
During the 1980s the Raman amplifier was extensively
studied as a promising candidate for use in fiber-optic
transmission but with the development of commercially
viable EDFAs, they were allowed to languish for a period.
When bit-rates were rising from 10 Gbps to 40, it was notpossible to design systems that used only discrete amplifiers like EDFAs and the advantages of distributed Raman
amplification, in which the transmission path as a whole isthe amplifying medium, again came to be recognized. We
may further say that the concept of multi-wavelength
pumping, in which high-reliability laser diodes are used as
the pump source to achieve adequate gain over a broad
band, was advanced to extend the applicability of Raman
amplifiers.
In conventional multi-wavelength Raman amplifiers,
backward pumping is the configuration normally used.
This is because, in comparison to forward pumping, thereis less gain saturation, and less RIN transfer from thepumping light to the signal light 5), 6). Backward pumping
has a problem, however, in that when gain is flattenedover a broad bandwidth, the wavelength-dependence of*

Optical Sub-Systems Development Dept., FITEL-Photonics
Lab.

the noise figure becomes pronounced 7 ) . This paper
shows, in a broadband Raman amplifier having five pumping wavelengths, by using, as an example, only the three
shortest wavelengths for bi-directional pumping, that it ispossible to design the gain spectrum and the noise figure
spectrum independently.

2.

PRINCIPLE OF RAMAN AMPLIFICATION

Figure 1 shows a schematic diagram of Raman scattering,using a combination of the hypothetical electron state andthe actual phonon energy state. When the incident lightundergoes Raman scattering, the phonons of the molecules are excited and the energy of the incident light itself
decreases. As a result the scattered light is lower in frequency than the incident light corresponding to the energy
difference of the phonons. When light of the same frequency impinges simultaneously, Raman scattering isstimulated, and optical amplification is achieved. In Figure
1 the pumping light excites the electrons to the hypotheti-

Hypothetical electron state

Pumping light

Raman scattering
Pumping light

Raman scattering

Phonon energy

Figure 1

Schematic diagram of Raman scattering.

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Independent Control of the Gain and Noise Figure Spectra of Raman Amplifiers Using Bi-Directional Pumping

cal state, but since this state is not stable, the electrons
immediately return to the ground state. And since thehigher state is hypothetical, there is virtually no absorption
with respect to the signal light. For this reason when thepumping light decreases the probability of the occurrenceof Raman scattering is reduced and gain is smaller, but atleast the pumping light does not act as a medium ofabsorption. Thus with a Raman amplifier it is possible toachieve a constant low-noise amplification process irrespective of the intensity of the pumping light. This constitutes a major difference from the EDFA, in which, unlessthe laser system is fully inverted, the signal light isabsorbed and the noise characteristic degenerates.
Figure 2 shows the Raman gain efficiency measured for1510-nm pumping of standard single-mode fiber (SMF),
dispersion-shifted fiber (DSF) and dispersion-compensating fiber (DCF). The horizontal axis represents a frequency shift with respect to the frequency of pumping light, andthe vertical axis represents the Raman gain efficiency
when using unpolarized pumping light. Qualitatively
speaking the horizontal axis corresponds to phonon energy and the vertical axis to scattering probability. In silicabased optical fibers, gain efficiency is maximized at a frequency shift of approximately 13 THz; the larger theamount of germanium dopant and the smaller the effective
core area, the higher this efficiency will be. Although thepeak value for efficiency will vary with the pumping wavelength, the shape of the curves shown in Figure 2 will be
substantially the same despite changes in the pumping
wavelength. Thus if measurements of the characteristic
shown in Figure 2 are made at a specific pumping wavelength, it is possible to predict the wavelength-dependence of gain for any desired wavelength.

which pumping light propagates bi-directionally in theamplifier fiber, but in some it propagates in the same
direction as the signal light (forward pumping) or the opposite direction (backward pumping). Generally speaking
with forward pumping the signal-to-noise ratio (SNR) canbe kept high, while with backward pumping the saturation
output power can be increased. In the case of a Raman
amplifier the process of optical amplification takes place
so rapidly that, unless the intensity noise of the forward
pumping light is sufficiently small, the pumping light noise
will be transferred to the signal light resulting in increasedtransmission error. Thus in many cases only backward
pumping is used.
Figure 4 shows the principle of flattening Raman gain byusing multi-wavelength pumping, in an example with fivepumping wavelengths. The gains produced by each of thevarious pumping wavelengths are represented by heavylines, and the gain produced by the aggregate of the four
shortest wavelengths is represented by line A. Line C isan aggregation of the five heavy lines. Line B represents
the Raman gain produced by the single longest pumping
wavelength, and by adjusting lines A and B it is possible
to flatten total gain, as in line C. The Raman gain spectrum for single-wavelength pumping shows no ripples on
the shorter-wavelength side of the gain peak, but ripples
do occur on the longer-wavelength side of the peak. Thus
ripples are readily formed in the gain spectrum producedby the shorter pumping wavelengths (corresponding toline A). There are a number of ways to keep these ripples
small: increase the number of pumping wavelengths so
the gain per wavelength is smaller; allocate pumping

Isolator

3.

MULTI-WAVELENGTH PUMPED RAMANAMPLIFIER

Figure 3 is a schematic showing the configuration of a typical Raman amplifier. Basically pumping light and signal
light are input to a single amplifier fiber and amplification
is effected by means of the stimulated scattering that
occurs in the fiber. Figure 3 shows a configuration in

Signal light

Signal light

Wavelength
MUX-DEMUX
Pump source

Figure 3

Pump source

Schematic configuration of typical Raman amplifier.

71510nm pump

C
DCF

3.0

6

Raman gain (dB)Raman gain efficiency (1/W/km)

Isolator

Wavelength
MUX-DEMUX

3.5

2.5
2.0
1.5
1.0

5
4

A

B

1550

1570

3
2

DSF
SMF

0.5

1

0.0

0
0

-10

-20

1510

-30

Raman gain efficiency measured for 1510-nm pumping.

1530

1590

1610

1630

Wavelength (nm)Frequency shift (THz)

Figure 2

Amplifying fiber

Figure 4

Principle of flattening Raman gain by using multiwavelength pumping.

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Independent Control of the Gain and Noise Figure Spectra of Raman Amplifiers Using Bi-Directional Pumping

wavelengths at the same frequency interval; and arrangethat the gain from each of the wavelengths is approximately equal.

4.

WAVELENGTH-DEPENDENCE OF
NOISE FIGURE

18

4

17

2

Relative signal power (dB)Raman gain/NF (dB)In EDFAs, Raman amplifiers and other optical amplifiers
that use the principle of stimulated emission and stimulated scattering, spontaneous emission and spontaneous
scattering occur irrespective of the presence of signal
light. This light is amplified as it propagates in the amplifier
fiber in the same way as the signal light, and is output
from the amplifier, and is known as amplified spontaneousemission (ASE). In time slots in which there is no signal,
ASE increases the zero level and in slots in which there isa signal, it interferes with the signal light and causes intensity fluctuations. In this way it is the noise figure (NF) of
the optical amplifier that qualitatively defines the phenomenon whereby ASE produced inside the optical amplifier
constitutes an indeterminate factor (noise) with respect tosignal light detection; it is calculated by dividing the SNR
before inputting the signal to the amplifier by the SNR at
the amplifier output.
Since the SNR at the output is worse (smaller), thenoise figure is larger than unity taking a positive value in
decibels. Normally the main factor determining the SNR at
the amplifier output is the noise produced by ASE, and thenoise figure is calculated assuming that noise in the light
input to the amplifier is limited to shot noise only.
Figure 5 shows typical spectra of Raman gain and noise
figure by backward pumping using five wavelengths. Here
we see the results of a simulation assuming a distributed
amplifier and 80 km of SMF as the amplifier fiber. By
adjusting the pumping power to flatten Raman gain a flatness of 0.5 dB was obtained. The flatness of the noise figure, however, was as large as 2.3 dB, and was dramatically worse on the short-wavelength side than on the long.
To assist in the discussion of the wavelength-dependence of the noise figure shown in Figure 5, the change insignal power along the amplifier fiber under backward
pumping is shown in Figure 6. All of the signals were input

to the fiber at the same power, and were output at substantially the same value, but the minimum value of signal
level, reached at a distance of some 60 km, varies considerably with the wavelength. We may thus say that, in thisamplifier, the fiber serves as a loss medium up to the pointof minimum power, that the SNR becomes worse in proportion to the loss, and that from there to the fiber output
SNR degradation occurs due to ASE. Thus the wavelength-dependence of the noise figure is produced by a
combination of the loss up to the point of minimum signal
level and, beyond that point, the wavelength-dependenceof the noise characteristic of the amplifier. Note, however,that since the value of the wavelength-dependence of theminimum signal level in Figure 6 is approximately 2 dB,
this may be considered the dominant factor in the wavelength-dependence of the noise figure in Figure 5.
The following four factors may be identified as contributing to the wavelength-dependence of the noise figure:
1. Wavelength-dependence of fiber loss;
2. Increased absorption due to the inter-signal Raman
effect;
3. Difference in distribution of pumping power (gain)
due to the inter-pump Raman effect; and4. Temperature-dependence of efficiency of production of spontaneous scattering.
Item 2 suggests that since a signal of shorter wavelength acts as the pumping light for Raman amplification
with respect to a signal of longer wavelength, the shorter
the wavelength the greater will be the loss of energy andthe faster the attenuation. Item 3 suggests that, in thesame way as for the inter-signal Raman effect, the imparting and receiving of energy due to the inter-pump Raman
effect causes extreme attenuation in pumping light ofshorter wavelength, limiting amplification of signals of
shorter wavelength to the region close to the output end of
the fiber. Item 4 is based on the fact that the efficiency of
production of spontaneous scattering changes with thewavelength, and at normal temperature spontaneous scattering at wavelengths near that of the pumping light is dramatically larger. The wavelength-dependence of the minimum signal level is due to items 1 through 3 above.

16
NF

15
2.3 dB14
0.5 dB
13

Raman gain

12
80-km SMF
11
1530

1550.101569.6
1589.6-2

1608.3
-4
-6
-8
-10
-12

1550

1570

1590

1610

0

Wavelength (nm)Figure 51531.1

Spectra of Raman gain and noise figure by backward
pumping.

10

20

30

40

50

60

70

80

Distance (km)Figure 6

Change in signal power along amplifier fiber under
backward pumping.

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Independent Control of the Gain and Noise Figure Spectra of Raman Amplifiers Using Bi-Directional Pumping

In an effort to improve the wavelength-dependence characteristic shown in Figure 5, a bi-directional pumping system, shown in Figure 7, was designed. In principle thisshould, by using forward pumping, suppress the degradation in SNR arising near the input of the amplifier andimprove the noise figure of the amplifier as a whole.
However in the hope of applying a wavelength-dependence to this improvement, making it greater for the shorter-wavelength signals, bi-directional pumping was used
only for the shorter-wavelength pumping light.
Figure 8 shows an example of the wavelength-dependence of the gain and noise figure in a Raman amplifier
under bi-directional pumping for shorter wavelengths only,
in a simulation carried out using the same pumping wavelengths and amplifier fiber as in Figure 5. By applying awavelength dependence to the gain due to forward pumping it was possible to obtain the flat noise figure spectrum
shown in the figure. Backward pumping power in this casewas reduced to keep Raman gain flat, achieving flatteningof both gain and noise figure simultaneously.
The change in signal power along the amplifier fiberwith the noise figure flattened under bi-directional multiwavelength pumping is shown in Figure 9. The signal levels at the input and output ends of the fiber are the sameas in Figure 6, but the minimum value occurring at a distance of approximately 60 km was substantially the samefor all the wavelengths. As a result the wavelength-dependence of the noise figure was reduced, and a substantially

Fiber

Signal light

Signal light

Pumping
light41531.1

21550.101569.6
1589.6-2

1608.3

-4
-6
-8
-10

0

10

20

30

1
0

15
14

NF

0.6 dB1312

80-km SMF

Raman gain
1550

1570

1590

50

60

70

80

Figure 9

Change in signal power along amplifier fiber underbi-directional multi-wavelength pumping.

20

Forward Raman gain (dB)2
Raman gain (forward only)

16

40

Distance (km)3

17

Raman gain/NF (dB)Figure 11 shows the results of experiments to verify the
independent control of gain and noise figure spectrum
using bi-directional pumping. The amplifier fiber used was
a 76-km SMF comprising spools of approximately 25 km
joined by fusion splicing. Table 1 shows the pumping
power used in the experiments. As Figure 11 shows, sev-

λ 1, λ 2, λ 3, λ 4, λ 5

18

1610

Wavelength (nm)Figure 8

EXPERIMENTAL RESULTS-12

Schematic of a Raman amplifier under bi-directional
multi-wavelength pumping.

11
1530

6.

Pumpinglight

λ 1, λ 2, λ 3

Figure 7flat noise figure spectrum was obtained.
Figure 10 shows the results of simulation using evenhigher pumping power. In this example, by increasing thegain due to forward pumping while keeping total Ramangain flat, a noise figure spectrum was obtained that issmaller on the shorter-wavelength side than on the longer.
In this way the use of a pumping scheme that is both
multi-wavelength and bi-directional makes it possible fornoise figure and gain spectra to be controlled independently as desired. If, however, the gain due to forward
pumping is increased, the high signal level will, as shownin Figure 9, continue over comparatively long distances.
This means that the impact of non-linear effects is great,and measures must be taken to assure that the resultant
degradation in transmission quality does not present a
problem 8).

Relative signal power (dB)

DESIGN OF NOISE FIGURE SPECTRUM
USING BI-DIRECTIONAL PUMPING

Wavelength-dependence of gain and noise figure in a
Raman amplifier under bi-directional pumping for
shorter wavelengths only.

19

Raman gain/NF (dB)

5.

18
NF

17
16
15
14

Raman gain

13
12
11
10
1530

1550

1570

1590

1610

Wavelength (nm)Figure 10 Results of simulation of noise figure spectrum control.

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Independent Control of the Gain and Noise Figure Spectra of Raman Amplifiers Using Bi-Directional Pumping

REFERENCES

18
NF

Raman gain/NF (dB)16
14

Backward
Bi-directional (flat)
Bi-directional (tilted)

12
Raman gain
108

76-km SMF
6
1530

1550

1570

1590

1610

Wavelength (nm)Figure 11 Results of experiments in noise figure spectrum control.

Table 1

Pumping power used in experiments.
Flat NF

Wavelength Backward

Tilted NF

Forward Backward Forward Backward

λ 1 1426.2 nm

149 mW

31 mW

96 mW

78 mW

32 mW

λ 2 1438.5 nm

161 mW

36 mW

108 mW

54 mW

78 mW

22 mW

65 mW

45 mW

λ 3 1451.8 nm

91 mW

λ 4 1466.0 nm

83 mW

105 mW

100 mW

λ 5 1495.2 nm

184 mW

206 mW

210 mW

Total power

668 mW

668 mW

26 mW

623 mW

eral noise figure spectra were realized, while maintaining
a substantially similar Raman gain spectrum. Because
Raman gain was maintained, total pumping power was
virtually unchanged. For the three shorter wavelengths,
increases in forward pumping power were matched bydecreases in backward pumping power. For the longest
pumping wavelength, the Raman effect between pumping
wavelengths decreased with decreases in the shorterwavelength pumping power propagating in the same
direction, so that it was necessary to increase backward
pumping power as forward pumping power was increased.

7.

1) G. Charlet, J.-C. Antona, S. Lanne, P. Tran, W. Idler, M. Gorlier, S.
Borne, A. Klekamp, C. Simonneau, L. Pierre, Y. Frignac, M.
Molina, F. Beaumont, J.-P. Hamaide, S. Bigo, “6.4 Tb/s
(159✕42.7Gb/s) capacity over 21✕100 km using bandwidth-limited phase-shaped binary transmission,” Proc. European
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Mamyshev, M. Kimmitt, K. Springer, D. Gapontsev, V. Ivshin,“Transmission of 40✕42.7 Gbit/s over 5200 km UltraWave® fiberwith terrestrial 100 km spans using turn-key ETDM transmitterand receiver,” Proc. European Conference on OpticalCommunication, (2002), PD Paper 4.4.
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CONCLUSION

With Raman amplification in the spotlight as a mainstream
technology for long-haul large capacity transmission, there
is a need for broadband Raman amplifiers with improved
characteristics. Past developmental activities have been
concentrated on controlling and flattening the gain profile,
but in the realization that broadening the bandwidth ofnoise characteristics was a problem for the future, wehave examined bi-directional pumping schemes. This
paper reports on a simulation of the possibility of controlling the gain and noise figure spectra independently as
desired using a multi-wavelength, bi-directional scheme,
and on its experimental verification.

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