Series FOL0903m Mini-Pump Coolerless Laser Modules
for Optical Amplifiersby Masayuki Minamino *, Yuji Yamagata *2, Tomoya Kato *3,
Takashi Koseki *3 and Toru Fukushima *3

A more compact 980-nm laser module with low power consumption has beendeveloped as a pumping source for optical amplifiers in metropolitan networks.
To cope with miniaturization and low power consumption, a coolerless structure has been newly
employed. To ensure high-temperature operation we have adopted a new diode chip structureand fiber Bragg grating (FBG) parameters. The resulting device has one-fourth the package sizeand one-third the power consumption of the conventional butterfly type. Wavelength stability isachieved over the entire operating temperature range.

ABSTRACT1.

INTRODUCTION

The widespread use of the Internet has drastically accelerated the increase in capacity and data transmission
speeds in backbone systems, leading to the popularity ofdense wavelength division multiplexing (DWDM). As optical systems spread to local area/metropolitan networks,
the devices used, specifically optical amplifiers, should beof low cost, smaller size, and lower power consumption

than in backbone systems.
For example, a gain module containing erbium-doped
fiber (EDF), optical couplers, monitor photodiodes (PD)and pump laser modules for amplification must be of credit card size and priced around US$ 1000. This means thatpump laser modules for optical amplifiers must also be
smaller in size, lower in power and lower in cost.
Here, a coolerless structure was chosen for the laser
module to achieve these requirements. Eliminating the

Possible application

Backbone DWDM
(10 Gbps x 32 wavelengths)
λ1~λ32

Level adjusting amp.

Add/Drop

λ1

Level adjusting
amp.

λ2

λ1
λ2

Metropolitan network(10 Gbps x 1 wavelength)

λ32

λ32

In-line amp.
Level adjusting amp.
PD

LD

Pre-amp. Booster amp.
Access network
Access network

Figure 1

Application of coolerless pump for metropolitan network.

* Purchasing Dept.
MC FITEL Inc.
*3 Design & Engineering Dept., FITEL Products Div.
*2

Furukawa Review, No. 23

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Series FOL0903m Mini-Pump Coolerless Laser Modules for Optical Amplifiers

thermo-electronic cooler in the module was the key concept, as it accounts for a high percentage of component
cost, physical size and electrical power consumption. The
new structure for a 980-nm diode chip has maintained
quantum efficiency and reliability, especially at high temperatures.

2.

MODULE DESIGN

2.1 Optical Design
We chose a two-lens coupling system and a thermal
expansion core (TEC) fiber having good coupling efficiency and wide tolerance. Figure 2 shows the relationshipbetween coupling loss and the offset between the diodechip and the 980-nm fiber with and without TEC processing. The mode field diameter of the 980-nm fiber is smaller
than that of a conventional single-mode fiber. The tolerance width with single mode fiber at a coupling loss of 0.2
dB is only about 1 µm, but with the TEC fiber this increases to about 2 µm, making it better suited to mass production assembly.
2.2 Module Structure
An 8-pin miniature dual in-line (mini-DIL) package wasselected, and the internal components were re-designed
to fit the package.
In a coolerless laser module, the mechanical test conditions for the standard (Telcordia) reliability tests change
significantly. For a module with a built-in Peltier cooler, themaximum acceleration is 500 G, for a coolerless module,
this rises to 1500 G. To satisfy this condition, the internal
components are fixed more rigidly by YAG laser welding.
To prevent degradation in the coupling efficiency withchanges in the relative position of the diode chip and optical system due to ambient temperature changes, materials for the internal components were selected to have similar coefficients of thermal expansion. Furthermore, optimizing the thickness of the solder fixing the sub-mount in
the package prevents solder creep and degradation of
coupling efficiency, which ensures long-term reliability.
Figures 3 and 4 show the appearance and the internal
structure of the coolerless 980-nm LD module.

Figure 3

X With TEC

Appearance of laser module.

Primary lensLaser diode
Secondary lensPhoto diodePolarization maintaining fiber

Figure 4

0.2

Fiber offset loss (dB)

2.3 LD Chip
Because the coolerless laser module operates under awide range of ambient temperatures (0~70°C), therequirements for the diode chip are that (1) quantum efficiency should not deteriorate drastically at high temperatures; (2) lasing wavelength should be stabilized over thefull operating temperature range; and (3) high reliability
should be guaranteed in high-temperature operation. In
addition, (4) stability of fiber output is an important characteristic in achieving low noise amplification in an EDFA
with 980-nm pumping. A suitable diode chip, known as the
FF chip, was developed expressly for the coolerless 980nm pump laser module to satisfy these requirements.
The FF chip is ridge-guided with a cavity length of 1800
µm. The purpose of making the cavity longer than that ofa conventional diode chip was to increase kink output
power and to improve reliability by increasing thermal conductivity with a heat sink. The active layer structure of thediode chip was also optimized to ensure stability of lasing
wavelength and output power due to provision of an FBG.
Figure 5 shows the structure of the FF chip. MOCVD
was used to fabricate the epitaxial wafer for the chip, andits active layer consists of an InGaAs single quantum wellwith compression strain, a GaAsP barrier layer and an
AlGaAs separate-confinement-heterostructure (SCH)

Structure of laser module.

With TEC

Y With TEC

0Ridge waveguide-0.2

X Without TEC-0.4

Y Without TEC-0.6
Without TEC

Active layer

-0.8
-1
-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

AlGaAs-clad
AlGaAs-SCH
GaAsP barrier
InGaAs well

GaAs substrate

Offset (µm)

Figure 2

Fiber offset loss.

Figure 5

Structure of 980-nm FF chip.

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Series FOL0903m Mini-Pump Coolerless Laser Modules for Optical Amplifiers

25°C
45°C
75°C

50

400

40

300

30

200

20

100

10

φ 9 CAN sample (Rth~40K/W)
CW operation

Efficiency (%)

Light output (mW)

500

Relative change in current

60

600

120%

110%
105%
100%
95%
90%
85%
80%
0

0

200

400

600

800

1000

2000

3000

4000

5000

Test hours (h)

0

0

N=29
Total device hr~
114.4 khr
No failure

115%

Figure 7

1000

Aging test results for FF chip.

Operating current (mA)

Figure 6

I-L curve of 980-nm FF chip.
20

Pull-in width

10

2.4 Method of Wavelength Locking
At around 980 nm the wavelength dependence of EDF
gain profile increases, so to achieve stable gain profile of
EDFA the pumping wavelength should be locked.
The lasing wavelength of the 980-nm diode chip inFabry-Perot (FP) mode has a wavelength shift coefficientwith temperature change of about 0.34 nm/°C. Thus from0 to 70°C the FP mode lasing wavelength shifts about 24
nm. When the difference between the FP mode lasing
wavelength and reflecting wavelength of the FBG, known
as the detuning value, is too large, the lasing wavelength
cannot be locked at the FBG reflecting wavelength.
The maximum pull-in width is defined, as illustrated in
Figure 8, as the range of wavelengths from the shortest
detuning value to the longest, in both cases the lasing
wavelength is locked at the reflecting wavelength of the
FBG. The shortest and longest detuning values are evaluated when the side mode suppression ratio (SMSR),
which is the intensity ratio of FP mode lasing to FBG
mode lasing, is greater than 13 dB.
To lock the wavelength in the 0 to 70°C temperature
range the maximum pull-in width should be about 30 nm,
to compensate for the temperature dependent wavelength
shift as well as the current dependent shift and production
variations in the wavelength of the diodes.
To achieve this wider maximum pull-in width, the reflectivity of the anti-reflection (AR) coating on the front facet ofthe diode chip and of the FBG was studied. Generally, awider pull-in width can be obtained by decreasing AR

Shortest detuning value

Longest detuning value

0Power (dB)layer. To obtain stable single transverse mode lasing, aridged waveguide approximately 3.5 µ m in width wasformed.
Figure 6 shows the current dependence of light output
at selected ambient temperatures. The kink output
obtained at room temperature was 500 mW and even at
75°C was 400 mW. The maximum conversion efficiency
from electrical power to optical power was 44 % at room
temperature and 40 % at 70°C. Figure 7 shows the resultof an aging test under APC at 250 mW with a junction
temperature Tj of 85°C. Twenty-nine devices were tested
under these conditions, and after 5000 hours no failuresoccurred, confirming that the chip offers sufficient reliability under 70°C operation.

FBG mode

-10

SMSR

FP mode-20

FP mode

-30
-40
-50
-60
-70
950

960

970

980

990

1000

1010

Wavelength (nm)Tc= -10°C

Figure 8

Tc=25°C

Tc=80°C

Definition of pull-in width of FBG mode lock.

coating reflectivity or increasing FBG reflectivity 1), but high
FBG reflectivity leads to decreases in quantum efficiency
and kink current. In this work the reflectivity of the AR
coating and the FBG were optimized to achieve both awider pull-in width and a higher kink current.
LD chips are also polarization dependent. When the
returned light from the FBG to the diode chip is TE mode,the highest effective reflectance can be obtained.
Changing the polarization state of the returned lightincreasing the TM mode component is equivalent todecreasing FBG reflectivity, thereby reducing the pull-in
width and unlocking the wavelength. To avoid this phenomenon, a polarization-maintaining fiber was adopted forthe coolerless laser module, and the polarization state of
the light returned from the FBG was stabilized thus pull-in
width can always be kept constant regardless of strain on
the fiber when twisted or being mounted. A single-mode
fiber is fusion spliced to the end of the fiber to facilitate
connection to other optical components such as couplers
on customers’ premises.

3.

SPECIFICATIONS

Table1 shows the specifications of the coolerless laser
module developed here. Operating temperatures werefrom 0 to 70°C, and fiber output power was 80 mW.

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Series FOL0903m Mini-Pump Coolerless Laser Modules for Optical Amplifiers

Table 1

Specifications of 980-nm coolerless pump laser module
Sym.

Min.

Typ.

Max.

Unit

FOL0903mNR-D14- λ

60

-

-

mW

Tc=0~70°C, IfBOL=<280 mA

FOL0903mNS-D14- λ

70

-

-

mW

Tc=0~70°C, IfBOL=<280 mA

Parameter

Condition

Pf

Optical output power

FOL0903mNT-D14- λIthThreshold current

80

-

-

mW

Tc=0~70°C, IfBOL=<280 mA

-

30

50

mA

CW

Threshold current at 70°C

Ith70

-

50

85

mA

Tc=70°C, CW

Kink current

Ikink

IfEOL

-

-

mA

-

LD forward voltage

Vf

-

2

2.5

V

IfBOL

Center wavelength

λc

λ−1.5

λ

λ+1.5

nm

Tc=0~70°C, IfBOL, PeakSpectral width

∆λ

-

-

3

nm

Tc=0~70°C, IfBOL, FWHM

Monitor current

Im

50

-

3000

µA

VrPD=5 V, IfBOL

Monitor dark current

Id

-

-

100

nA

VrPD=5 V

Thermistor resistance

Rth

9.5

10

10.5

kΩ

Ts=25°C

Thermistor B constant

Bth

-

3900

-

K

-

Tc: Case temperature
BOL: Beginning of life
EOL: End of life4.

4.2 Output Spectrum
Figure 13 shows the optical spectrum of the laser module.
Wavelength is locked within a range of 976±1.5 nm from
low-temperature low output operation (Tc = -10°C, injection current 100 mA) to high-temperature high output
operation (Tc = 95°C, injection current 500 mA). The center wavelength can be selected for each EDFA by meansof diode chip and FBG parameters.

PRODUCT CHARACTERISTICS

Figure 9 shows the outline dimensions of the coolerlesslaser module. Compared to a conventional module with a
14-pin butterfly package, the volume is reduced to about
one-fourth.
4.1 Optical Output Power
Figure 10 shows optical output power vs. driving current(L-I curves) at case temperatures Tc of 0, 25 and 70°C.
Figure 11 shows the injection current dependence of fluctuation of fiber output power and lasing wavelength. In
coolerless pump laser module developed here wavelength
fluctuation was less than 0.3 nm and power fluctuation
was less than 0.5 % at driving currents from aroundthreshold current right up to rated current, and the multimode lasing spectrum was stable at all driving currents,
as shown in Figure 12.
5.2±0.30

4.3 Power Consumption
Figure 14 shows the total power consumption of the coolerless and conventional laser modules. At Tc of 70°C andoptical output power of 80 mW, the power consumption of
the coolerless module was less than 0.5 W, about onethird that of a conventional LD module in a 14-pin butterfly
package with a Peltier cooler.

0.9

Fiber length
(750)
2.54
12.7± 0.20(22)7.62±0.20

41.5±5.5

7.37± 0.20

φ4.80

FBG
PMF φ0.25

Fusion splicerecoated

1.0m < Fiber length < 2.0m

(300)
(25)

(10)
PMF φ0.25(300)

PuremodeTM
HI1060 f0.25(500<)

Fusion splicerecoated

Figure 9

Dimensions of coolerless pump laser module.

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68

200
180
160
140
120
100
80
60
40
20
0

20
10

Tc=25°C

0

Tc=70°C Iop=500mA

Tc=70°C-10

Tc=95°C Iop=500mA

Tc=25°C Iop=300mA

100

200

300

400

-30
-40

-60

500

-70

Current (mA)955

T=25°C

976.6

T=25 (Pf)

0.5

0.4

976.4
0.3

976.2
976.0

0.2

975.8
mode hopping

975.6

Stability of Pf (%)

976.8

PeakWL

0.1

975.4
975.2

Coolerless, 25°C
Conventional, 25°C

1.6

100

150

200

250

Current (mA)

0.8
0.4
0

100mA
200mA
300mA
400mA

-20

60

80

100

120

140

160

180

Figure 14 Comparison of coolerless and conventional laser
modules.

20

0Coolerless, 70°C
Conventional, 70°C

Fiber output power (mW)

Change in fiber output power (%)

Figure 11 Fiber output power stability.

995

1.2

40
50

985

2

0.0
300

975.0

975

Figure 13 Output spectrum of 980-nm coolerless pumplaser module.

Total power consumption (W)

977.0

965

Wavelength (nm)Figure 10 L-I curve of 980-nm coolerless pump laser module.
Tc = 0°C, 25°C, 70°C

Wavelength (nm)

-20

-50
0

Intensity (log arb. unit)

Tc=-10°C Iop=100mA

Tc=0°CPower (dB)Fiber output power (mW)

Series FOL0903m Mini-Pump Coolerless Laser Modules for Optical Amplifiers

-40

-60

20
Shock test : 1500G 5ms
Vibration test : 2000Hz 20G
n=11

15
10
5
0
-5
-10
-15
-20
before test

after shock

after vibration

Figure 15 Result of mechanical impact and vibration tests.

-80
971

976

981

Change in fiber output power (%)

Wavelength (nm)Figure 12 Multimode spectrum of FBG mode lock at room temperature.

5.

RELIABILITY

To verify the long-term reliability of the coolerless laser
module, the reliability tests specified by Telcordia GR-468
was carried out. Typical mechanical impact and vibration
test results are shown in Figure 15, temperature cycling
test results in Figure 16, and aging test results in Figure
17.
Results of all the reliability tests including the above satisfied all the requirements of Telcordia GR-468, thereby
confirming sufficient reliability.

20
15

Temperature = -40 to 85°C
n =12

10
5
0
-5
-10
-15
-20
0

100

200

300

400

500

600

Number of cycles

Figure 16 Result of temperature cycling test.

6.

CONCLUSION

A compact coolerless module has been developed as a
pumping laser module for optical amplifiers for metropolitan networks. This module has only one-third the power

Furukawa Review, No. 23

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Change in fiber output power (%)

Series FOL0903m Mini-Pump Coolerless Laser Modules for Optical Amplifiers

consumption and one-fourth the size of a conventional
module and realizes a fiber output power of 80 mW atambient temperatures from 0 to 70°C.

30
Tc=70°C ACC=200mA
n=14

20
10
0
-10
-20

REFERENCES

-30
0

500

1000

1500

2000

2500

Test hours (h)Figure 17 Result of aging test.

3000

3500

1) A. Mugino, Y. Irie: “Output Power Optimization in 980-nm
Pumping Lasers Wavelength-Locked Using Fiber Bragg
Gratings”, Furukawa Review, No.19, 2000
(http://www.furukawa.co.jp/english/review/fr019/fr19_08.pdf)

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