Compact Optical Fiber Amplifier
by Isamu Oshima *, Katsuhiko Nishiyama *, Katsuya Kurotori * and Hirokazu Hayakawa *Recently, WDM systems have been increasingly installed. As a result, we havebeen developing optical fiber amplifiers for WDM systems.
Compact optical fiber amplifiers for single channel and small channel applications are needed
for SDH systems and for WDM systems to compensate for insertion losses of optical passive
components. In response, we have developed compact optical fiber amplifiers using a coaxial
pumping laser. However higher output power and higher gain have recently been demanded.
Therefore, compact optical fiber amplifiers using a butterfly pumping laser or a mini-DIL pumping
laser have been developed. These amplifiers are reported in this paper.

ABSTRACT1.

INTRODUCTION

The spread of the Internet is leading to increased communication traffic and requires the evolution of Dense
Wavelength Division Multiplexing (DWDM) technology.
The Erbium Doped Fiber Amplifier (EDFA) developed
using a gain flattening filter technique 1), 2) and an expansion of the signal wavelength range to L-band (approximately from 1570 to 1605 nm) 3), 4) have supported thisprogress in the DWDM communication region.
Compact optical fiber amplifiers for single channel andsmall channel applications are needed. Their output
power and gain are not so high and they are mainly used
in a single channel SDH system as a booster amplifier
and a preamplifier, or in the DWDM system to compensate for insertion losses of optical passive components
such as Optical Multiplexer (OMUX), Optical DeMultiplexer (ODMUX), Optical Add / Drop Multiplexer(OADM), and Optical Switch (OSW). As a compact opticalfiber amplifier, we have developed a name card-sized
amplifier using a coaxial pumping laser. However higher
output power and higher gain have recently been required
because of increasing losses from optical passive components due to increments in the number of signal channels
and complexity of configuring DWDM systems. To achieve
higher output power and gain, we have developed platforms as compact optical amplifiers using a butterfly
pumping laser and a mini DIL pumping laser.
They are reported below.

*

Optical Subsystems Dept., FITEL Products Div.

2.

LINE-UP OF COMPACT OPTICAL FIBERAMPLIFIERS

Generally, optical fiber amplifiers are categorized according to signal wavelength ranges: C-band (approximately
1530~1560 nm) and L-band (approximately 1570~1605nm), or applications: booster amplifier, inline amplifier andpreamplifier.
In the case of compact optical fiber amplifiers, theyshould be categorized by type of pumping laser: butterfly
type and mini-DIL type. Mini-DIL pumping lasers do notneed temperature control because the operating forward
current is not so high and they do not need a coolingdevice. This contributes to reducing power consumption.
On the other hand, butterfly pumping lasers need temperature control because the operating forward current ishigh to achieve a high pumping power. This difference isimportant because it has an impact on the design of theelectrical circuit. (In this paper the butterfly pumping laser
and the mini-DIL pumping laser are called cooled pumpand uncooled pump, respectively.)
Two platforms need to be designed. One includes thecooled pump and the other includes the uncooled pump.
The dimensions are the same, 90(W)×70(D)×12(H) mm.
The height of the compact optical fiber amplifier with theuncooled pump can be reduced to 10.5 mm.
Furthermore in the case of compact optical fiber amplifiers with the cooled pump, if a 1480-nm pumping laser isused, for instance, for L-band amplification, a heat sink isneeded to radiate heat emitted by the laser diode and thecooling device, because the operating forward current ishigher than in the case of using 980-nm pumping laser.
So, a platform with a heat sink was developed.
Table 1 shows a summary of platforms for compact optical fiber amplifiers.
Next, the actual characteristics of compact optical fiber
amplifiers are introduced according to the categories
shown in Table 1.

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Compact Optical Fiber Amplifier

Line-up of compact optical fiber amplifiers.

(Signal wavelength(Pumping laser)

Uncooled pump

Cooled pump
Cooled pump

and application)
Mainly C-band Booster-Low power consumption

and Pre-amplifier

980-nm LD pump

Mainly C-band Booster-High output power

and Pre-amplifier

Mainly 980-nm LD pump

Mainly L-band Booster-Higher output powerwith heat sinkIN

Coupler

Notes

and Pre-amplifier

Isolator

WDM
coupler

Mainly 1480-nm LD pump

EDF
Isolator

Coupler

OUT

Power (dBm/0.1 nm)

Division 2

Division 1

Figure 1

0
-10
-20
-30
-40
-50
950

955

960

965

970

975

980

985

990

995

1000

985

990

995

1000

985

990

995

1000

Wavelength (nm)
25 °C

20
10
0
-10
-20
-30
-40
-50
-60
950

955

960

965

970

975

980

Wavelength (nm)Outputmonitor PD

Optical configuration of amplifier with uncooled
pump LD.

65 °C

20

Power (dBm/0.1 nm)

Inputmonitor PD

10

-60

FBG
Uncooled 980nm
pump LD

0 °C

20Power (dBm/0.1 nm)

Table 1

10
0
-10
-20
-30
-40
-50
-60
950

955

960

965

970

975

980

Wavelength (nm)Figure 3

Figure 2

3.

Appearance of uncooled pump LD (L) and cooled
pump LD (R).

COMPACT OPTICAL FIBER AMPLIFIERWITH UNCOOLED PUMPThe optical configuration of the compact optical fiber
amplifier with an uncooled pump is shown in Figure 1. The
package size is 90×70×12 mm and some optical components such as uncooled pump, Photo Diode (PD), optical
coupler, optical WDM coupler, optical isolator, and EDF
are included in this package. The wavelength of theuncooled pump is around 980 nm.
The appearance of the uncooled pump is shown in
Figure 2. The appearance of the cooled pump is also
shown in the same figure.
As mentioned above the uncooled pump does notinclude a cooling device. So, the oscillation wavelengthchanges according to ambient temperature. This behavior
causes a degradation in the conversion efficiency of theamplifier, because the absorption band of EDF around
980 nm is very narrow. To avoid this phenomenon a Fiber
Bragg Grating (FBG) is set in the output fiber of laser. ThisFBG keeps the oscillation wavelength stable, in spite ofTemperature dependence of output wavelength of
uncooled pump LD with FBG.

the change of ambient temperature. See Figure 3. The
spectra of the output of uncooled pump are shown in thisfigure. They are measured under conditions with ambient
temperature of 0, 25, and 65°C. They show that the wavelength is locked by the FBG in spite of the change of
ambient temperature, although the oscillation wavelengthof the uncooled pump moves. (The curves near the bottom line are the output spectra of the uncooled pump.)
Figure 4 shows the optical characteristics of the compact optical fiber amplifier with an uncooled pump. It is
designed for a higher output power. The upper figure is
the required pumping power vs. signal wavelength, andthe lower figure is the noise figure vs. signal wavelength
when the input power is -10 dBm and the output power isadjusted to +11 dBm. Output power of +11 dBm was
obtained under the condition that the signal wavelength
was from 1530 to 1566 nm and the ambient temperature
was from 0 to 65°C. The noise figure was less than 5.5 dB
and the required pumping power was under 80 mW.
These results are summarized in Table 2.
Figure 5 shows the optical characteristics of the compact optical fiber amplifier optimized as a preamplifier. It is
designed so that the gain is over 22 dB when the input
power is -25 dBm. The noise figure was under 5.7 dB.
These results are summarized in Table 3.
Figure 6 shows a compact optical fiber amplifier with an
uncooled pump.

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Compact Optical Fiber Amplifier

80

6.5

70

6.0

60
50

25°C

5.5
5.0
4.5

65°C
1520

1530

1540

1550

1560

1570

4.0
1525

Signal wavelength (nm)Noise Figure
Pin=-10dBm, Pout=+11dBm

7.0
6.5

NF (dB)

25°C
65°C
0°C

0°C

40
30

1530

1535

1540

1545

1550

1555

1560

1565

Wavelength (nm)Figure 5

Characteristics of compact optical fiber amplifier
with uncooled pump LD (II).

Table 3

Summary of compact optical fiber amplifier with
uncooled pump LD (as a preamplifier).

25°C
0°C

6.0

65°C
5.5
5.0
4.5
4.0
1520

1530

1540

1550

1560

Figure 4

Table 2

Items

1570

Signal wavelength (nm)Characteristics of compact optical fiber amplifier
with uncooled pump LD (I).

Characteristics

Operating temperature

0 to 65°C

Signal wavelength range

1529 to 1563 nm

Input power-40 to -20 dBm

Gain> 22 dB

Noise figure @ Pin=-25 dBm< 5.7 dB

Summary of compact optical fiber amplifier with
uncooled pump LD (as a booster amplifier).
Items

4.

Pin=-25dBm,Gain=22dB

7.0

NF (dB)

Pump power (mW)Noise Figure

Pump power
Pin=-10dBm, Pout=+11dBm

90

Characteristics

Operating temperature

0 to 65°C

Signal wavelength range

1530 to 1566 nm

Input power> -10 dBm

Output power> +11 dBm

Noise figure @ Pin=-10 dBm< 5.5 dB

COMPACT OPTICAL FIBER AMPLIFIERWITH COOLED PUMPThe optical configuration of the compact optical fiber
amplifier with a cooled pump is shown in Figure 7. It is thesame as that with an uncooled pump with the exception of
the pumping laser. The wavelength of the pumping laser
is around 980 nm. The butterfly pumping laser is in thesame package as the mini-DIL pumping laser. This amplifier includes an input power and an output power monitor.
A higher output power is achieved compared to theuncooled pump optical fiber amplifier.
Figure 8 shows the optical characteristics of the compact optical fiber amplifier optimized as a booster amplifier. It is designed for an output power of over 15 dBm
when the input power is -10 dBm. The upper figure is therequired pumping power vs. signal wavelength, and thelower figure is the noise figure vs. signal wavelength when
the input power is -10 dBm and the output power is adjusted to +15 dBm. An output power of +15 dBm was
obtained under the condition that the signal wavelength
was from 1530 to 1561 nm and the ambient temperature

Figure 6IN

Appearance of compact optical fiber amplifier with
uncooled pump LD.

Coupler

Isolator

Input
monitor PD

Figure 7

WDM
coupler

EDF
Isolator

Coupler

Cooled 980nm
pump LDOUT

Outputmonitor PD

Optical configuration of amplifier with cooled pumpLD.

was from 0 to 65°C. The noise figure was less than 5.0 dB
and the required pumping power was under 150 mW. It
was confirmed that the radiation of heat was good and thecooling device functioned well, even when the ambient
temperature was 65°C.
These results are summarized in Table 4.

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Compact Optical Fiber Amplifier

Pump power (mW)

Pump power
170
160
150
140
130
120
110
100
90
1525

Pout=+15dBm const.

25°C
65°C
-5°C
1530

1535

1540

1545

1550

1555

1560

1565

Wavelength (nm)Noise Figure

Pout=+15dBm const.

6.0

25°C
65°C
-5°C

NF (dB)

5.5
5.0

Figure 9

Appearance of compact optical fiber amplifier withheat sink.

4.5
4.0
3.5
3.0
1525

1530

1535

1540

1545

1550

1555

1560

1565

Wavelength (nm)Figure 8IN

Coupler

Isolator

Summary of compact optical fiber amplifier withcooled pump LD (as a booster amplifier).

Isolator

Cooled 1480nm
pump LDEDF
Isolator

Items

5.

EDF

Characteristics of compact optical fiber amplifier
with cooled pump LD.
Inputmonitor PD

Table 4WDM
coupler

Coupler

OUT

Characteristics

Operating temperature-5 to 65°C

Signal wavelength range

1530 to 1561 nm

Input power> -10 dBm

Output power> +15 dBm

Noise figure @ Pin=-10 dBm< 5.0 dB

COMPACT OPTICAL FIBER AMPLIFIERWITH A HEAT SINKA 1480-nm pumping laser is used for a higher output
power amplifier and a L-band amplifier, because it is
known that the efficiency of pump to signal conversion
when pumped by a 1480-nm laser is higher than that by
980-nm laser. However the operating forward current of
the 1480-nm laser is apt to be higher and this causes anincrement of cooler current. So, a heat sink is needed toradiate heat. A compact optical fiber amplifier platform
with a heat sink was developed for higher output power
amplifiers and L-band amplifiers. The heat sink was optimized by simulating actual conditions, operating forward
current, and cooler current, which depend on the output
power of optical fiber amplifier.
Figure 9 shows an example of a compact optical fiber
amplifier with a heat sink. The dimensions are 90 (W)×70(D)×25 (H) mm.
The characteristics of an optical fiber amplifier using thisplatform are introduced in this section.
Figure 10 shows the optical configuration of the compact optical fiber amplifier for L-band. An isolator is located between two EDFs; here, the ratio of these two EDFs
in length is about 1:10. (This isolator is called a middle
isolator in this paper.)Outputmonitor PD

Figure 10 Optical configuration of amplifier for L-band.

A forward pumping configuration (without middle isolator) is one of the normal configurations of optical fiber
amplifiers, but it has the weak point that the backward
Amplified Spontaneous Emission (ASE) wastes pumping
power near the input terminal of the EDF, and this behavior causes a degradation of the output power of the amplifier and noise figure characteristics. Therefore, the middle
isolator is used to correct this. This middle isolator prevents the ASE from going back to the input end of EDF
and wasting pumping power. Although this middle isolator
is designed so that insertion loss is lowest for the C-band,
the pumping and signal light can pass the isolator under
the condition that losses at these wavelengths are only
0.2 or 0.3 dB larger than that of the C-band. For this reason, WDM couplers in front of and behind the middle isolator, which make the pumping light bypass the middle
isolator, are not used.
In the case of the optical fiber amplifier for L-band, it isnecessary to make the EDF much longer than for the Cband to obtain enough gain because the gain coefficientat the L-band is very small. The long EDF must be put into
a compact package. To solve this problem a reduced
cladding EDF was developed. The parameters of this EDF
are shown in Table 5. Because the diameter of the normal
EDF is 250 µm, the volume occupied is reduced to lessthan half. The EDF length was about 60 m. By increasing
the density of Erbium the EDF length can be shortened,even if a higher gain is needed.

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Compact Optical Fiber Amplifier

Table 5

Parameters and characteristics of reduced cladding
EDF.

Items

Characteristics

Items

Characteristics
5.21 µm

Operating temperature

0 to 60°C

Cladding diameter80±2 µm

Signal wavelength range

1570 to 1610 nm

Coating diameter

167±5 µm

Input power> -10 dBm

Cutoff wavelength

867 nmOutput power> +13 dBm

Erbium density

1446 wtppm

Noise figure @ Pin=-10 dBm< 5.7 dB

3.3 wt%Pump power
Pin=-10dBm, Pout=+13dBm

180

Pump power (mW)

Summary of compact optical fiber amplifier for Lband.

Mode field diameter

Al density

170
160
150

6.

25°C
0°C
60°C

140
130
120
110
100
1565 1570 1575 1580 1585 1590 1595 1600 1605 1610 1615

Wavelength (nm)Noise Figure
Pin=-10dBm, Pout=+13dBm

6.5
6.0

NF (dB)

Table 6

5.5

25°C
0°C
60°C

CONCLUSION

The following three platforms for compact optical fiber
amplifiers have been developed.
(1) Compact optical fiber amplifier with uncooled pump
(2) Compact optical fiber amplifier with cooled pump(3) Compact optical fiber amplifier with heat sink
Examples of optical fiber amplifiers using these platforms have been reported.
Various compact optical fiber amplifiers for single channel and small channel applications will be designed foruse with these platforms.

5.0
4.5
4.0

REFERENCES

3.5
1565 1570 1575 1580 1585 1590 1595 1600 1605 1610 1615

Wavelength (nm)Figure 11 Characteristics of compact optical fiber amplifier for
L-band.

The characteristics of the compact optical fiber amplifier
for the L-band are shown in Figure 11. It is designed foran output power of over 13 dBm when the input power is 10 dBm. The upper figure is the required pumping powervs. signal wavelength, and the lower figure is the noise figure vs. signal wavelength when the input power is -10
dBm and the output power is adjusted to +13 dBm. Output
power of +13 dBm was obtained under the condition thatthe signal wavelength was from 1570 to 1610 nm and theambient temperature was from 0 to 60°C. The noise figure
was less than 5.7 dB and the required pumping power
was under 180 mW. It was confirmed that radiation of heat
was good and the cooling device operated adequately
even when the ambient temperature was 60°C and theoperating forward current was over 700 mA.
These results are summarized in Table 6.

1) P.F.Wysocki et al., OFC’97, PD-2, 1997
2) Tachibana et al., Society Convention of IEICE, C-3-43, p.143, 2002
(in Japanese)
3) M. Yamada et al., Electron. Lett., Vol.33, No.8, p.710-711, 19974) M. Fukushima et al., OFC’97, PD-3, 1997
5) Nishiyama et al., Society Convention of IEICE, B-10-53, p.341,
2002 (in Japanese)
6) Aiso et al., General Convention of IEICE, C-3-147, p.312, 2001 (inJapanese)

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