1480-nm Laser Diode Module with 250-mW Output
for Optical Amplifiers (FOL 1404QQ Series)
by Toshio Kimura *, Naoki Tsukiji *, Junji Yoshida *, Naoki Kimura *2,
Takeshi Aikiyo *, Tetsuro Ijichi * and Yoshikazu Ikegami *1480-nm laser diode modules for pumping optical amplifiers have been developed to realize an optical fiber output of 250 mW at a case temperature of 70°C
by optimizing the laser diode chip and by reviewing the thermal design of the module.

ABSTRACT1.

INTRODUCTION

Accompanied by the widespread use of the Internet anddigital image transmission, WDM (Wavelength Division
Multiplexing) of an optical transmission system is now
going to be used to expand transmission capacity per optical fiber. Increase in multiplexed signals requires higher
power performance of the optical amplifier. High output
power laser diode module (LD module, hereinafter) isused for pumping erbium-doped fibers (EDFs) in opticalamplifiers in the 1.55 µm band. Pump LD modules used
are in the spectrum of either 980 nm or 1480 nm. The
advantages of 1480-nm LD modules are:
1) High reliability
2) High conversion efficiency of EDF, suitable for higher
output of the amplifier
3) Broad absorption bandwidth of EDF, capable of multiplexing many wavelengths4) Availability of a variety of peripheral optical components for this band such as isolator, wavelength multiplexer, and polarization coupler
The 1480-nm LD module, therefore, is used as pumpsource for high power optical amplifiers that combine aplurality of LD module outputs by bi-directional pumping,wavelength multiplexing and polarization multiplexing. 1) In
addition to the EDF pumping, the LD module is expected
to be used as pumping source for Raman amplifier. 2)
Since pump LD modules with high output are demandedby the market, competition among manufacturers for higher power has resulted in new devices reaching an output
power of 200 mW for 1480-nm LD modules.
Also, the requirement for the operating temperature of
the modules is becoming severe; for example, the module
has to work with good stability within a wide range of casetemperatures (Tc) between -20 and +70°C and is also
expected to operate at higher temperatures.

**2

Optical Device Dept., FITEL Products Div.
Thermal Management Team, Yokohama Lab. R & D Div.

Furukawa Electric has developed and is marketing thefollowing modules:
PA series; maximum fiber output power of 160 mW at a
driving current of 600 mA
PB series; maximum fiber output power of 200 mW at a
driving current of 800 mA
In order to meet the need for higher output power, a
new 250-mW module, the QQ series, has been developed
achieving the highest power in the world. Here is a reporton the study and development of the module.

2.

1480-NM PUMP LD MODULE

A pump LD module consists of an LD chip, an optical coupling which couples the laser beam from the LD chip to
optical fiber, and a temperature controller which stabilizes

Figure 1

Thermistor

Appearance of LD module

LD chip

First lensSecond lensSingle-mode
fiber

Peltier module

Figure 2

Structure of 1480-nm LD module292.1 LD Chip
Figure 3 shows the structure of an LD chip, in which
GRIN-SCH-BH (Graded Index- Separate Confinement
Heterostructure-Buried Heterostructure) with compressively strained MQW (Multi Quantum Well) active layer isadopted to realize low-leakage current, stable transverse
fundamental mode and high efficiency operation. 3) On thefront facet of the chip, AR (Anti-reflection) film is coated,and on the rear facet of the chip, HR (High Reflection) film
is coated. The chip is bonded onto the carrier junctiondown via an aluminum nitride heat sink.
2.2 Optical Coupling Portion
Usually, an LD chip emits a beam with a cone angle ofapproximately 30°. The beam is collimated by the firstlens and then is focused by the second lens on the end
facet of the optical fiber thereby effecting optical coupling.
The positioning accuracy of these optical components isrequired to be within 1 to 10 µm, or within 1 µm for fixing
the optical fiber in particular.
The LD chip is spatially coupled to the optical fiber
through these lenses; therefore, it is possible to insert
optical components such as optical isolator and the likebetween the lenses and the fiber.
2.3 Temperature Control Portion
The wavelength and optical output from an LD chipdepend on temperature. Therefore, in order to prevent thefluctuation of characteristics due to self-heating and environmental temperature changes, a thermo-electric cooler
is used in the pump LD module to control the temperature.
A thermistor is also mounted near the LD chip. The temperature of the LD chip is kept constant, usually at 25°C,
by monitoring the thermistor to autonomously control thethermo-electric cooler.

3.

PROBLEMS ABOUT HIGH OUTPUT
OPERATION

Factors that determine the driving limits of an LD module
are categorized into those of fiber output power and thoseof operating temperature. The former includes the maxi-

HR coating
Electrodes

mum facet power from the LD chip and the coupling efficiency of the LD to the optical fiber; the latter includes
power consumption of the LD chip, thermal impedance inthe heat dissipation path inside the LD module and theheat absorption performance of the thermo-electric cooler.
Thus, the driving limit of an LD chip having a certain heatdissipation structure and a thermo-electric cooler depends
on the reactive power of the LD chip (i.e., the difference
between the LD driving power and the facet power of the
LD chip, which is mainly converted into heat).
Excluding the issue of the LD driving current limit, andespecially in the case of operation in a high temperatureenvironment, heat dissipation design becomes important.
We decided to overcome this problem as follows:
1) Improving the maximum facet power of the LD chip,and optimizing the structure of the chip in terms ofenergy conversion efficiency (i.e., the ratio of the LD
facet power to the LD driving power).
2) Improving the performance of the thermoelectric cooler to suppress the increase in temperature of the LD
chip due to the reactive power of the device.

4.

IMPROVEMENT OF LD CHIP

Generally, when a current begins to flow into an LD chip,
the facet power of the chip gradually increases. At thesame time, however, heat produced from the chip itself
decreases the facet power of the chip, and finally the facet
power begins to saturate at a point where a larger current
will no longer give a higher facet power. In order to makethe saturating current larger, the cavity length of the chip
was extended, so as to reduce the forward voltage andthe thermal impedance. Given a coupling efficiency of the
LD module of 80%, the facet power of the LD chip must
be more than 310 mW to obtain a fiber-coupled power of
250 mW.
Figure 4 shows the relationship between the LD driving
current and the fiber-coupled power using the cavity
length of the LD chip as a parameter. It is seen that at a
driving current of less than 500 mA, a cavity length of 800µm has the advantage; however, at 700 mA and higher, a1300 µm length has the advantage.

25°C

500

LD fiber-coupled power (mW)the lasing condition of the LD chip. Figure 1 and Figure 2
show the appearance and structure of the LD module,
respectively.

Cavity length
800 µm1000 µm
1300 µm
1500 µm

400

300

200

100

0

AR coating

Current blocking layer0GRIN-SCH-MQW active layer

Figure 330

Structure of LD chip

500

1000

1500

2000

LD driving current (mA)

Figure 4

Relationship between LD driving current and fibercoupled power with cavity length as a parameter

Furukawa Review, No. 19. 2000

25°C
Cavity length
800 µm1000 µm
1300 µm
1500 µm

400
300
200
100

T s /T c =25/70°C

12Cooler power consumption(W)LD fiber-coupled power (mW)

500

PA, PB series

10

QQ series

8
6
4
2
0

0
0

1

2

3

4

0

5

Figure 5

Relationship between LD driving power and fibercoupled power with cavity length as a parameter

Figure 5 shows the relationship between the driving
power and the fiber-coupled power of the LD chip. Notethat energy conversion efficiency is implicitly included in
the figure. In the range of low fiber output power ofapproximately 100 mW already, the cavity length of 1300
µm is seen to be advantageous; and at the point where
the driving power reaches 2 W, or the fiber output power
reaches 350 mW, the cavity length of 1500 µm becomes
advantageous. On the contrary, the advantageous featureof the 800 µm length does not appear even in the low
fiber-coupled power range. Thus, consideration on theenergy conversion efficiency is important to identify greatchanges in the optimum length of the cavity. Other design
parameters such as the carrier density of the clad layer
are optimized based on the concepts described above.
Considering these results mentioned above, the authors
determined the cavity length of the LD chip to be 1300
µm. Also adjustments were made on the reflectivity of the
AR-coated front facet in order to suppress the decrease inexternal differential quantum efficiency due to the degradation in the reflection loss. Thus, the driving voltage of
the LD is restrained to approximately 2 V at a driving current of 1 A. These results have achieved the characteristics of the new LD: LD driving power of 1.6 W, LD facet
power of 320 mW, and fiber output power of 250 mW.
An LD facet power of 490 mW was attained at a driving
current of 2 A with a cavity length of 1500 µm. However,as described hereinafter, a high power operation of suchan LD chip with limited LD module dimensions and at
operating temperatures (Ts/Tc = 25/70°C) --which is thetarget of current development-- results in an excessive driving power. This problem including module dimension
changes must be solved in the near future.

0.5

1

1.5

2

2.5

3

LD driving power (W)LD driving power (W)Figure 6

5.

Optimization of Peltier cooler and LD driving power

HEAT DISSIPATION CHARACTERISTICS
OF LD MODULE PACKAGE

It is necessary for a pump LD module to guarantee longterm reliability; therefore, the degradation of the module
output --which is caused by the degradation of the LD chipor the optical coupling-- must be considered. Usually, 1.2
times the LD driving current at the BOL (Beginning Of
Life) is defined as the EOL (End Of Life) driving current.
The authors confirmed that the LD operated satisfactorily
with the EOL current under the most severe (in mostcases at high temperatures) environment conditions.
In the PA and PB series, the thermoelectric cooler is
optimized at an LD driving current of 600 to 800 mA, andan LD driving power of approximately 1 W. As shown in
Figure 6, the maximum heat absorption at this operation isapproximately 1.6 W, just a little higher than the LD driving
power at EOL.
In the case of 250-mW operation, however, the thermoelectric cooler is required to dissipate 1.6 W even at the
BOL and, when the increase in the LD drive current
together with deviations in characteristics are taken into
account, 2.5 W at the EOL. Considering the above factors,
a more detailed review of the thermoelectric cooler was
carried out. Generally, in order to increase the maximum
heat absorption, the number of Peltier elements in thethermoelectric cooler must be increased. This leads to a
growth of series resistance to increase the power consumption of the cooler. Thus, there exists an optimum setof design parameters for the maximum heat absorption to
be targeted.
In the QQ series, as shown in Figure 6, the LD driving
was optimized by keeping the maximum heat absorption
high in the range of 1.6 W to 2.5 W. Here, we took into
account the fact that the power source used for the opticalamplifier is mainly rated at 5 V, and that the size of the LD
module should preferably be the same as for the existing
PA and PB series. The design enabled high power operations of LD module at 1000 mA BOL and 1200 mA at EOL
thereby augmenting the maximum LD driving power from
1.6 W to 2.5 W under the severe temperature conditionsof Ts/Tc = 25/70°C.

31

6.

PRODUCT CHARACTERISTICS

Figure 7 shows outer dimensions of the LD module, which
are the same as for the 1480-nm LD module in the PA
and PB series.
Figure 8 shows the light vs. current characteristics of atypical LD module in the QQ series. The figure shows thata fiber-coupled power of 250 mW is achieved at an LD driving current of 900 mA and a case temperature of 70°C.
The maximum fiber-coupled power is more than 330 mW.
Figure 9 shows the output spectrum of the LD module at
250 mW.
Figure 10 and Figure 11 show the current/voltage characteristics of the thermoelectric cooler while the case temperature and the LD driving current are varied. The cooler

15.24

2.8
2.54

Fiber Length(26)

14 - 0.5
7.8 ± 0.2
4.8

4 - φ 2. 7
(6.7)

(42)

(15.2)
12.7 ± 0.15
8.9 ± 0.15

φ 6.2

6 - C 0. 3

8.0 MIN

φ 0.9

0.5
0.2

(5.3)

(30)

20.8
26.0 ± 0.15
30.0 ± 0.2

Unit: mmFigure 7

current is 3 A and the cooler voltage is 3.2 V, at a casetemperature of Tc = 70°C together with an LD driving current of 1200 mA EOL. Optional accessories of the LD
module are:• Built-in optical isolator• Various types of fibers, including polarization maintaining fiber (PMF)• Wavelength locking and spectrum narrowing by Fiber
Bragg Grating (FBG) 4)
The above features may be the same as those of the
PA and PB series. By combining the above features,
extremely high output optical amplifiers become available
by bi-directional pumping, polarization multiplexing, wavelength multiplexing, etc. Table 1 shows the characteristicsof the modules in the QQ series.
Since high output power becomes available with the QQ
series LD modules, a combination of two 140-mW class
LD modules in the PA or PB series can be replaced by
one 250-mW class module in the QQ series. Figure 12
illustrates that, to obtain a multiplexed fiber output powerof 250 mW, the power consumption of one 250-mW module is equivalent to that of two 140-mW modules. The new
LD module helps to save space, the control circuit andother devices become simple, and optical components for
multiplexing become unnecessary. Thus, optical amplifiers
based on the new modules become compact and inexpensive.

Dimensions of LD module5
Tc = 25°C

300
4Cooler current (A)Fiber-coupled power (mW)

350

250
200
150
100

Tc = 45°C
Tc = 60°C
Tc = 70°C

3

Tc = 80°C

2

1
50
0
0

500

1000

1500

0
400

2000

600

LD driving current (mA)

Figure 8

800

1000

1200

1400

1600

LD driving current (mA)

L-I curve

Figure 10 Characteristics of TEC current-20

5
Tc = 25°C
Tc = 45°C

4Cooler voltage (V)Output (dB)

-30
-40
-50
-60

1460

1470

1480

1490

1500

1510

Wavelength (mm)Figure 932

Tc = 70°C

3

Tc = 80°C

2

1

-70
-80
1450

Tc = 60°C

Output spectrum of LD module

1520

0
400

600

800

1000

1200

1400

1600

LD driving current (mA)

Figure 11 Characteristics of TEC voltage

Furukawa Review, No. 19. 2000

Specifications of QQ series laser diode module

Parameter

Sym. Min.

Ith

Threshold current (mA)
LD forward voltage (V)Fiber output power (mW),
POL 1404 QQO
LD forward current (mA)(BOL)
LD forward current (mA)
(EOL)

-

Vf

-

Pf

250

IfBOL

-

Typ.

Max.

Condition35

70

Ts=25°C, CW-

2.7

Pf =250 mW,
BOL

IfBOL
=<1000 mA-

350

Fiber-coupled power (mW)

Table 1300
250
PA or PB x 2

200
150
PA series x 2

100

QQ series

PB series x 2
50

QQ series

01000 Pf =250 mW

2

4

6

8

10

12

14

16

18

Total power consumption (W), LD + TEC

IfEOL

Center wavelength (nm)

λc

-

1460

-

-

1200 IfEOL=IfBOL x 1.2

1490

Pf =250 mW
RMS,

Spectral width (nm)

∆λ

-

4

8

Monitor current (µA)

Im

200

-

1500

Monitor dark current (nA)

Id

-

-

100 VrPD=5 V

Pf =250 mW
VrPD=5 V,
Pf =250 mW

Ts=25°C,

Cooler current (A)

Ic

-

3

4

Cooler voltage (V)

Vc

-

3

4

Thermistor resistance (kΩ)

Rth

9.5

10

10.5 Ts=25°C

Thermistor B constant (K)

B

-

3900

Isolation *1 (dB)30

Extinction ratio *2 (dB)13

*1 Type 3 or 4 (Isolator built-in)*2 Type 2 or 4 ( PMF is used)-

Figure 12 Total power consumption vs. fiber output power of
the PA, PB and QQ series

RMS,

Tc=70°C, EOL
Ts=25°C,

7.

CONCLUSION

We have successfully developed QQ series 1480-nm
pump laser modules for single-mode fibers. The modules
are capable of high-temperature operation providing afiber-coupled power of 250 mW, the world's highest.

Tc=70°C, EOL-

REFERENCES

1) Y. Tashiro, S. Koyanagi, T. Aikiyo, and Y. Ikegami : 9th
Topical Meet. of Optical Amplifiers and Their Applications,
Vail, Colorado, paper WC2(1998)
2) M.Tanaka, S. Kinoshita, Y. Sugaya, and T. Tanaka : 10th
Topical Meet. of Optical Amplifiers and Their Applications,Nara, Japan, paper ThA3-1(1999)
3) Kasukawa, T. Namegaya, N. Iwai, N. Nakayama, Y. Ikegami,
N. Tsukiji : IEEE Photonics Tech. Lett. Vol66, No.1, (1994), 4
4) S. Koyanagi, A. Mugino, T. Aikiyo, and Y.Ikegami : 9th
Topical Meet of Optical Amplifers and Their Applications, Vail,Colorado, paper MC2(1998)
Manuscript received on November 11, 1999.

33