Pump Laser Module for Co-propagating Raman Amplifier
by Yutaka Ohki *, Naoki Hayamizu *, Satoshi Irino *,Hiroshi Shimizu *2, Junji Yoshida * and Naoki Tsukiji *The inner-grating multimode (iGM) laser, which has a new structure, hasbeen proposed and developed as the pump laser for co-propagating Raman
amplification. Among the important characteristics of co-propagating Raman amplification, we
have focused on relative intensity noise (RIN) and stimulated Brillouin scattering (SBS). We haveeffected structural optimization and evaluation, and have devised a driving system.

ABSTRACT1.

INTRODUCTION

In the field of fiber-optic communications, increases intransmission distance and capacity have been effectedusing Erbium-doped fiber amplifiers (EDFAs). Recently,
however, it has become essential to rely not only on
EDFAs but to combine them with Raman amplification totake advantage of its characteristics. The most common
type of Raman amplification currently in use relies on
counter-propagation, that is, pumping light is input in the
direction opposite to the signal.
However as we move to the next generation, withspeeds of 40 Gbps, transmission spans of 100 km, andbroader bandwidth (using L- and S-bands), the key
will be to use co-propagating Raman amplification, in
which pumping light is input in the same direction as
the signal, at the same time as counter-propagating
Raman amplification. This technology results in what is
known as bi-directional Raman amplification. It has beenreported that by means of multi-wavelength pumping
it is possible to achieve flattening of Raman gain andbroadening of bandwidth even with counter-propagating
Raman amplification alone, but that without the use ofco-propagating Raman amplification, flattening of thenoise figure cannot be realized 1), 2).
Here we will discuss the reasons why, in addition to
the 14xx-nm laser diode modules (LDMs) widely used for
EDFA pumping 3), 4), it is necessary to develop a pumplaser of the same wavelength band for co-propagating
Raman amplification. The main requirements are: 1) lower
relative intensity noise (RIN), 2) lower stimulated Brillouinscattering (SBS) and 3) lasing wavelength stability, andthese will be addressed in turn below.

1.1 Need for Lower Relative Intensity Noise (RIN)
RIN is an indicator of the minute changes in laser
light intensity normalized by the total light intensity. In
Raman amplification the lifetime of the pumping levelthat gives rise to gain is short (a few femtoseconds),
so that the presence of noise in the pumping light will,through the amplification process, be turned directly into
signal noise. In the traditional EDFA, on the other hand,
the lifetime of the pumping level is comparatively long(approximately 10 msec), so that this sort of problem does
not occur. In Raman amplification the gain per length ismuch smaller than in an EDFA, but in co-propagating
Raman amplification, where the signal and pumping
light propagate together in the fiber over long distances,
pumping light noise is gradually superimposed onto
signal noise. In a counter-propagating Raman amplifier,where the signal and pump light propagate in opposite
directions, the crossover time between pumping lighthaving a noise component and the signal is short, and theeffect of pumping light noise on the signal is minimal. And
since the pumping light noise is random, any effect thatthe signal might receive will be flattened as it proceeds inthe opposite direction and not give rise to any problem.
As will be understood from the above, a pump laser forTransmission fiber

Isolator
Signal light

Pumpinglight

Pump laser

λ2 ... λna) Co-propagating*2

Development Dept., FITEL Products Div.
Manufacturing Dept., FITEL Products Div.

Figure 1

Wavelength
MUX-DEMUX

Pumping
light

...

λ1

*

Signal light

...

λ1

λ2 ... λnb) Counter-propagating

Basic configuration of co- and counter-propagating
Raman amplifiers.

Furukawa Review, No. 24

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Pump Laser Module for Co-propagating Raman Amplifierco-propagating Raman amplification must be provided
with a low RIN level, such as has not been needed in the
past.
1.2

Need for Lower Stimulated Brillouin Scattering(SBS)
SBS is the third-order nonlinear optical effect whereby
light undergoes scattering in the reverse direction by
acoustic phonons excited in the fiber by light. SBS is
undesirable since, when it occurs, light is scattered in thereverse direction and makes no effective contribution to
Raman amplification. In SBS there is a level of power atwhich this phenomenon arises--the threshold power Pth -which may be represented by the Equations 5)

Pth =21Aeffb
×
GLeff

{

1
Fp /Fp(if Fp <Fb )(if Fp >Fb )(1)

Leff =(1-exp(-αL ))/α(2)

Fb =ηk 2ρ

(3)

Given an Aeff of 47 μm2, Leff of 55 km, α of 0.21 dB/km,
which were the values for DSF used in the experiments,
together with a G of 4×10-11 m/W and b of 2, this yields avalue for Pth of 2.57 mW.
In the above, Aeff is the effective area of the fiber core;
L eff is the effective length of the fiber as calculated in
Equation (2) using loss α and fiber length L ; G is thecoefficient of Brillouin gain; F p is the linewidth of thepumping light; Fb is the width of the SBS gain band; and b
is a value between 1 and 2 depending on the polarization
state of the pumping light and SBS reflection light.
In Equation (3), k is 2π /λ , ρ is fiber density, and η is aconstant.
If λ is 1480 nm, F b will be approximately 17.5 MHz.
Since F b is less than the longitudinal mode spacing of
the pump laser, SBS will be generated for each of thelongitudinal modes individually. The bandwidth of Raman
gain, on the other hand, is wide (approximately 30 nm),so that gain from each of the longitudinal modes overlaps
and the total output power of the laser contributes to gain.
Accordingly if total output power is the same, the mosteffective way to suppress SBS without reducing Ramangain would be to increase the number of longitudinalmodes and reduce the power per mode.
Looking at Equation (1) from the viewpoint of the fiber,
since both stimulated Raman scattering and SBS are thethird-order nonlinear optical effects, those factors that
increase Raman gain, such as lower loss and reduced
A eff , simultaneously facilitate the generation of SBS. In
fibers such as DSF, NZDSF, DCF and the like, Aeff is smallin comparison with SMF, and SBS readily occurs.
1.3

Need for Wavelength Stabilization and Demand
for a Pump Laser for Co-propagating Raman
Amplification
Pumping wavelength stability is essential for designing
the Raman gain profile, and the wavelength stabilization
technique conventionally adopted for 14xx-nm laser
diode modules was the fiber Bragg grating (FBG). Ina laser diode module with FBG (FBG-LDM), the lightis reflected at the FBG, which is located at a distanceof several tens of centimeters, and fed back to thelaser diode. This situation is referred to as coherence
collapse, and is considered desirable from the standpointof stable operation of the laser diode. However laser
coherence is degraded by reflection from a point situatedat a distance, so the laser linewidth becomes broader
and RIN increases. Referring to Equation (1), broader
linewidth is advantageous, since the threshold power
for the occurrence of SBS becomes greater. Since SBS
presents a problem even in counter-propagating Raman
amplification, an FBG-LDM that satisfies the conditionsof low SBS and wavelength stability can be used as
the pump laser but an FBG-LDM with a high RIN is notsuitable for co-propagating Raman amplification. To
reduce RIN, it is desirable to form a grating inside thelaser diode, as in a distributed feedback (DFB) laser. But
the DFB laser has high coherence, generating singlemode lasing and having a narrow linewidth, so that SBS
presents a problem, making it unsuitable for use as thepump laser for co-propagating Raman amplification.
As can be seen from the above discussion none of theexisting laser diode modules is suitable for co-propagating
Raman amplification, making it necessary to develop a
new pump laser.
In this paper we report on the design of a pump laser
for co-propagating Raman amplification, evaluate its
characteristics, and discuss the driving system.

2.

STRUCTURE OF iGM LASERThe laser diode that we have developed for copropagating Raman amplification has been designated as
an inner grating multimode (iGM) laser. Its primary featureis that it does not, like the DFB laser, have a grating
formed over the whole length of the laser cavity, but
only over a portion, thereby achieving multimode lasing.
The inner grating thereby realizes low RIN and pumping
wavelength stability, and by multimode lasing realizes
SBS reduction. Multimode lasing is also indispensable
in reducing the degree of polarization (DOP), which is so
important in Raman amplification.
The fundamental structure of the laser diode is likethat of 14xx-nm laser diodes, which are of proven highoutput power and high reliability, with a waveguide of theburied heterostructure (BH) type. The layer structure has
a strain compensation multi quantum well active layer andmultistep separate-confinement heterostructure (SCH).
For the purpose of Fabry-Perot mode suppression, ananti-reflection coating is applied to one of the laser diodefacets.
The module structure, like conventional 14xx-nm laser
diode modules, features a 14-pin butterfly package and a2-lens coupling system. An advantage of the iGM laser isthat, since the grating is inside the laser diode, the isolator
can be mounted inside the laser diode module. In the
FBG-LDM, by contrast, the light reflected from the FBG

Furukawa Review, No. 24

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7

Pump Laser Module for Co-propagating Raman Amplifier

has to be fed back to the laser diode, so that the isolator
cannot be accommodated. In configuring an amplifier, it
may be that when the signal is input to the pump laser
module it is reflected back into the transmission path
resulting in signal noise. To avoid such a situation it isadvantageous to provide the isolator inside the LDM.
Providing an isolator eliminates reflected light and the lighttransmitted by other LDMs from entering the laser diode,thereby effecting stable operation of the pump laser.

3.

FUNDAMENTAL CHARACTERISTICS OF
THE iGM LASER

Figure 2 shows the light-current (L-I ) characteristics of
the iGM laser diode. Cavity length in 1500 μm, and an
output power of 310 mW @ 1200 mA is achieved. As canbe seen from the spectrum shown in Figure 3, satisfactory
multimode lasing is achieved.
Figure 4 shows the current dependence of pumping
wavelength. As can be seen, a good level of wavelengthstability is achieved. Wavelength shift is somewhat greaterthan for an FBG-LDM, but is satisfactory as a pump laser
for Raman amplification. The wavelength shift in the iGM
laser is smaller than in the DFB laser (not shown), theresult of using a longer cavity, junction-down bonding,and the excellent heat-dispersion performance obtainedby means of a heat sink of high thermal conductivity. In

the iGM laser, the temperature of the grating is controlled
together with that of the laser diode as a whole bymeans of a Peltier device internal to the LDM, so thatthe pumping wavelength does not shift with the ambient
temperature. In the FBG-LDM, on the other hand, theFBG expands and contracts with changes in ambient
temperature resulting in shifts in wavelength. In the iGM
laser, if the driving system is such that the greater thecurrent input, the lower the temperature will become, it will
also possible to compensate for wavelength shifts due tothe driving current.

4.

SBS SUPPRESSION

4.1

Relationship between SBS Suppression andNumber of Longitudinal Modes 6)
We can see from Equation (1) that there are two
approaches to SBS suppression: (1) keep the lightintensity of each longitudinal mode below P th ; or (2)broaden the linewidth and increase Pth .
Figure 5 shows the spectra for fiber input light andreflected light. Reflection of the same wavelength as the
input light is due to Rayleigh scattering, and that of shifted
wavelength is due to SBS. The reflected light due to
SBS shows a shift equal to the Brillouin shift (= 11 GHz =0.08 nm @ 1480 nm) with respect to the original light, asshown in the Equation

1485
1480

300

Wavelength (nm)Fiber-coupled power (mW)

400

200

100

1475
1470
1465

FBG
FP

1460

iGM
0
0

400

800

1200

1455

1600

0

200

400

Forward current (mA)

Figure 2

800

1000

1200

1400

Operating current (mA)

Light-current characteristics of iGM laser.

Figure 4

Current dependence of pumping wavelength.

40

10
If=900 mA
0

30

-10

Intensity (dB)Intensity (dB)

600

-20
-30

Rayleigh scattering

Input light spectrum

SBS

Reflected light
spectrum

20
10
0

-40
-10

-50
-60
1465

1475

1485

1495

1505

1515

1525

1535

-20
1452.5Spectrum of iGM laser.

1453

1453.51454

1454.5

Wavelength (nm)Wavelength (nm)Figure 3

Brillouin
Shift

Figure 5

Spectra of fiber input light and reflected light.

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Pump Laser Module for Co-propagating Raman Amplifier

0

18
Fiber: DSF 55 km

Fiber: DSF 55 km

16-5

Pth per mode

SBS reflection (dB)

14
-10
-15
-20

12
10

6

DFB

4

-25

Rayleigh scattering level

2

-30
0

5

10

15

20

25

0

30

0

Relationship between number of longitudinal modes
and amount of SBS reflection.

νB = 2nVs /λ

5

10

15

20

No. of modes (-10 dB)

No. of modes (-10 dB)

Figure 68(4)where: n is the refractive index of the fiber; and Vs is the
speed of sound in the fiber (5760 m/s).
We can see from Figure 5 that SBS occurs inlongitudinal modes where light intensity is high, but notin those where it is low. Thus we may say with respect
to approach (1) that it is effective to increase the numberof modes and distribute the power among these more
numerous modes.
Figure 6 shows the relationship between the numberof longitudinal modes and the amount of SBS reflection.
SBS is evaluated by the ratio of backward-scattered lightintensity to input light intensity. Since the P th for SBS
is low, we have focused on the number of longitudinalmodes in the region from peak to -10 dB. Looking at
Figure 6 we can see that SBS is suppressed when thenumber of longitudinal modes is about 18 or more, andthat the amount of reflection decreases to the level ofRayleigh scattering.
Figure 7 shows the relationship between the number of
longitudinal modes and SBS threshold power Pth . Since
the Pth in Figure 7 is the SBS threshold value occurring ineach longitudinal mode individually, it should be noted thatthe total power being input to the fiber will be greater thanthat. Using a reflection spectrum similar to that shownin Figure 5, Pth may be found from the intensity ratio ofreflected light due to SBS over each longitudinal mode.
It will be understood that if the number of longitudinalmodes is increased, P th will be higher, suggesting thatlaser linewidth expands due to an increase in the numberof longitudinal modes.
The point at which the number of modes is least is fromdata for a selected DFB laser, with a Pth of 2.8 mW, andthis substantially agrees with an estimate from Equation(1). The other points are from data for iGM lasers, inwhich the number of longitudinal modes is varied by theoperating current and the design of grating. In the iGM
laser the number of longitudinal modes increases with anincrease in the operating current, resulting in a situation inwhich, in the region of high operating current, SBS occurs
with difficulty due to the higher number of longitudinalmodes and the accompanying increase in Pth .

Figure 7

Relationship between number of longitudinal modes
and SBS threshold power Pth.

With a DFB laser, by contrast, the single-mode
characteristics remains unchanged even if operating
current increases, with the result that it is impossible toinput to the fiber a total light power in excess of Pth (=2.8 mW). If, as a result of an increase in the number of
longitudinal modes, the linewidth did not vary and P thremained unchanged, it would be necessary, assuming
a desired input power of, say, 200 mW, to have about
80 longitudinal modes (= 200 mW/2.5 mW), but inpractice this sort of situation does not occur, and is infact impossible. Let it be noted in passing that the direct,
accurate measurement of the linewidth of a multimodelaser is, technically speaking, difficult.
4.2 Devising a Driving System for SBS Suppression
For any LDM and for any range of driving currents, themere measure of increasing the number of longitudinalmodes, taken by itself, is ineffective in completely
suppressing SBS. If it is attempted to obtain a further
increase in the number of longitudinal modes, there will bean adverse effect on wavelength stability, RIN and othercharacteristics. It is the authors' opinion that to eliminate
SBS while maintaining the properties of the iGM laser, itis necessary to make adjustments to the method of use.
In co-propagating Raman amplification there are cases
in which the output power per LDM is small (say 100mW), and in such cases it is possible to eliminate SBS byattenuating the output power outside the LDM to the point
where the intensity of all the longitudinal modes is below
Pth . In this case using a higher value of driving current
will mean a greater number of longitudinal modes and theamount of attenuation required will be less.
We have also given attention to another techniqueknown as dithering, a driving method in which a
modulated signal of minute amplitude is superimposed ona DC driving current. This corresponds to approach (2)to SBS reduction. In laser diodes, internal temperaturerises with the driving current causing changes in the lasing
wavelength. Accordingly the use of dithering can produce
minute changes in the lasing wavelength resulting in a
broadening of the linewidth.

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Pump Laser Module for Co-propagating Raman Amplifier

10
Amplitude: 10 %DC, 500 kHz,and 1,
5 & 10 MHz

0

2 kHz
1 kHz

-20
-30
-40
-50
-60
-70
1545.71545.81545.91546

1546.11546.21546.3

Wavelength (nm)Figure 8

Broadening of DFB spectrum achieved by dithering
at various frequencies.

0
Without dithering

SBS reflection (dB)

-5
5%

-10

2%
Amplitude 1 %

-15
-20

5.

10 %

-25
-30
-35

0.1

1

10

100

1000

10000

Dithering frequency (kHz)

Figure 9

Change in SBS reflection due to dithering frequency.

Figure 8 shows the broadening of the DFB spectrum
achieved by dithering at various frequencies using a sine
wave. The dithering frequency was varied between 1 kHz
and 10 MHz. The spectrum was broadest at the minimum
frequency of 1 kHz, and was similarly narrow when thedriving current was DC and 500 kHz or above. In thehigh-frequency range, laser temperature cannot respondto the changes in current produced by dithering, andincreases in linewidth cease to occur. Since the carrier
lifetime of a laser diode is shorter (a few nanoseconds),
the light intensity modulation responds even in that rangeof frequencies. The spectrum when dithered using a sine
wave becomes an superimposition of a Bessel function
centered on the original wavelength. Since intensity of
the higher mode is greater, the shape will be lower in thecenter as shown in Figure 8, and higher at the edges.
Figure 9 shows the change in SBS reflection due todithering frequency and amplitude 7) . The amplitude
shown in the figure is defined as the proportion between
the DC component of the light and the intensity
modulation component. Looking at Figure 9, it can beseen that approximately -5 dB of SBS reflection that was
originally present is suppressed between frequencies ofapproximately 10 to 100 kHz to the Rayleigh scatteringlevel by the small degree of modulation of only 1 %. The
experiments were carried out using iGM lasers with asmall number of longitudinal modes.

EVALUATING THE RIN OF THE iGM
LASER

Figure 10 plots the RIN of an iGM laser against thelogarithm of frequency on the x-axis. Values for a laser
diode module with an FBG and a Fabry-Perot (FP) laser
without an FBG are also shown for comparison. The iGM
laser achieves the low RIN values of -140 dB/Hz or lessthat are required for co-propagating Raman amplification
over a wide range of frequencies. The FBG-LDM has
peaks at intervals of c /2nl corresponding to the time forthe round trip over distance l to the FBG, and the RIN
values are extremely high--about -120 dB/Hz. The FP
laser shows increased RIN on the low-frequency side,
thought to be due to the fact that it has an extremely
large number of longitudinal modes resulting in mode
competition.
The reason we have focused on low-frequency range
RIN is that even in co-propagating Raman amplification,
the velocity of propagation of the signal light and pumping
light differ due to dispersion in the fiber, so that the higher
frequency components of the pumping light intensity noise
-100
-110

RIN (dB/Hz)

Intensity (dB)-10

As can be seen from Figure 8, linewidth itself becomes
greater the lower the frequency. The limit to SBS
suppression on the low-frequency side is due to the factthat the SBS phenomenon responds to the changes
in wavelength that are produced by dithering. SBS is
produced by the interaction of forward- and backwardpropagating waves, so that if, for example, the dithering
frequency is too slow as compared with the time required
for light to travel back and forth over the effective length of
the fiber Leff , the linewidth broadening that occurs will not
be very effective. If, using Equation (2), an Leff of 20 km isused, we will get c /2nLeff = 5 kHz (where c is the speed oflight), which is of the same order as in Figure 9. The limiton the high-frequency side is determined, as describedabove, by the frequency at which linewidth ceases tobroaden at the temperature response limit. It is believedthat frequencies between 10 and 100 kHz that are suitablefor SBS suppression will be extremely slow relative to the
bit rate, and the amplitude will be small, resulting in little
adverse effect on the signal.

FBG

-120
-130
FP
-140
-150

iGM
-160
0.0001

0.001

0.01

0.1

1

10

100

Frequency (GHz)

Figure 10 RIN spectra for selected types of lasers.

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Pump Laser Module for Co-propagating Raman Amplifier-105

RIN (dB/Hz)-115

Fiber: TWRS 37 km
If=900 mA
FP (after)-125

iGM (after)-135
FP (before)
-145
-155
iGM (before)-165
0.0001

0.001

0.01

0.1

1

10

100

Frequency (GHz)

Figure 11 RIN of iGM and FP lasers before and after
transmission over a 37-km TWRS fiber.
-95
TWRS 37 km
If=900 mA

After (DC)-105

17 GHzRIN (dB/Hz)

-115
-125

After (1%)

After (0.5%)

BeforeAfter (5%)

11 GHz

-135
-145
-155
-165
0.0001

0.001

0.01

0.1

1

10

100

Frequency (GHz)

Figure 12 Increase in RIN after transmission producedby SBS and its suppression by dithering.

are averaged during transmission and have only a small
effect on the signal light.
The authors have also measured a phenomenon
whereby RIN increases after fiber transmission (Figure11), which is thought to be due to mode partitioning
noise (MPN) 8). This is a phenomenon that occurs in
multimode lasers, whereby the ratio in which laser gain
is distributed among the longitudinal modes fluctuates
over time so that the output power of each mode changes
but total output power does not. At the point of input thechanges due to MPN are canceled out among the various
modes, but after transmission over long distances thearrival times differ under the influence of fiber dispersion
so that they are not canceled out, and the total output
power fluctuates. For that reason RIN is greater after
transmission (gray lines) than before (black lines). In
an FP laser the large number of longitudinal modes
means an extraordinary increase in RIN. In an iGM too,
the multimode lasing results in an increase in RIN after
transmission, but about 20 dB less than the maximum
values for the FP.
It was also discovered that RIN also increases as aresult of the occurrence of SBS 9). Figure 12 shows the
RIN of an iGM laser before transmission (black) andafter transmission (gray) at an If of 900 mA over a 37-km
TWRS fiber. The increase in RIN is greater on the lowfrequency side, and the amount of the increase is greaterthan that due to MPN. In experiments, the occurrence of
SBS was suppressed by changing the dithering amplitude(in parentheses in Figure 12). The increase in RIN was
suppressed with SBS, indicating that the phenomenon is
attributable to SBS. Increases in RIN due to MPN are stillseen even after SBS suppression. Since it was an iGM
laser with a small number of longitudinal modes, such that
SBS would occur, that was used in the experiments, theincrease in RIN due to MPN was smaller than that of theiGM laser in Figure 11. Furthermore in the increase in RIN
due to MPN a peak was seen, whereas when due to SBS,
no peak was seen in the frequency range observed, andthere was a tendency to increase the lower the frequency.
When SBS occurred, two sharp peaks were observedin the high-frequency region. These were determined,
respectively, by the difference in frequency between thelongitudinal mode giving rise to SBS and the SBS light(that is to say, by the Brillouin shift, approximately 11GHz), and by the difference in frequency between the
single longest-wavelength longitudinal mode giving rise to
SBS and the SBS light (approximately 17 GHz).
Since the DFB is a single-mode laser, there is no
increase in RIN due to MPN even after transmission
but since SBS occurs readily, an increase in RIN due to
SBS is observed. In the FP laser, as can also be seen in
Figure 4, the wavelength shifts due to operating current
are large, but consideration is being given to using themin co-propagating Raman amplification by stabilizing the
wavelength by constant current drive and varying theoutput power using a variable optical attenuator (VOA).
Nevertheless, as has been described above, the RIN of
the FP laser increases in the low-frequency range or after
transmission, raising concerns about the effect they would
exert when used in co-propagating Raman amplification.
The FP lases in a large number of longitudinal modes, but
SBS is not suppressed in all of them, and still occurs in
some.

6.

CONCLUSION

A new type of laser has been developed as a pumplaser for co-propagating Raman amplification, having an
inner grating and known as the iGM. It has been shownthat SBS can be reduced through the use of multimodelasing and dithering. In comparison with other types oflaser, there are major problems of RIN in the FBG-LDM,
wavelength stabilization in the FP, and SBS in the DFB.
More detailed investigation of RIN showed that increases
occurred in the FP in the low-frequency range and after
fiber transmission, and in the DFB after transmission
when SBS occurred, so that, taken overall, the noisecharacteristics of the iGM were superior. The above leads
us to conclude that the iGM is the most suitable to use asa pump laser in co-propagating Raman amplification.

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Pump Laser Module for Co-propagating Raman Amplifier

REFERENCES
1) S. Kado, Y. Emori, S. Namiki, N. Tsukiji, J. Yoshida, and T.
Kimura, ECOC’01, (2001), PD Paper 1.8.
2) Y. Emori et al., Independent Control of the Gain and Noise
Figure Spectra of Raman Amplifiers Using Bi-Directional
Pumping, Furukawa Review, No.23, 2003, pp.11-15
3) T. Kimura et al., 1480-nm Laser Module with 250-mW
Output for Optical Amplifiers, Furukawa Review, No.19,
pp.29-33
4) N.Tsukiji, J. Yoshida, T. Kimura, S. Koyanagi, and T.
Fukushima, ITcom’01, vol 4532, (2001), pp.349-360.
5) R. G. Smith, Appl. Opt., vol 11, (1972), pp.2489-2494.
6) N. Tsukiji, N. Hayamizu, H. Shimizu, Y. Ohki, T. Kimura,
S. Irino, J. Yoshida, T. Fukushima, and S.Namiki, OAA’02,(2002), OMB 4.
7) J. Yoshida, N. Tsukiji, T. Kimura, M. Funabashi, and T.
Fukushima, ITcom’02, vol 4870, (2002), pp.169-182.
8) R. H. Wentworth, G. E. Bodeep and T. E. Darcie, J.
Lightwave Technol., vol 10, (1992), pp.84-89.
9) Y. Ohki, N. Hayamizu, H. Shimizu, S. Irino, J. Yoshida, N.
Tsukiji, and S.Namiki, OAA’02, (2002), PD Paper 7.

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