Recent Progress of High Power Semiconductor Lasers
for EDFA Pumping
by Akihiko Kasukawa *, Toshikazu Mukaihara *, Takeharu Yamaguchi * and Jun'jiro Kikawa *Optical fiber communication systems using a WDM (wavelength division multiplexing) system are being introduced in long-haul networks to manage the explosive increase in transmission capacity. Erbium-doped fiber amplifier (EDFA) is one of the key
components to support WDM systems. High power lasers emitting at both 980 nm and 1480 nm
are essential for pumping sources for EDFA. In this paper, state-of-the-art high power pumping
lasers are reviewed. Furukawa Electric has succeeded to release high performance and high reliable pumping lasers. In particular, ultra-high output power of 250 mW-1480 nm modules havebeen commercialized. These high output power laser modules are of great importance for Raman
amplifiers as well as EDFA applications.

ABSTRACTINTRODUCTION

Optical fiber communication systems have realized largetransmission capacity; however, bit rate increase utilizing
traditional TDM (time division multiplexing) cannot manage the explosive demands for larger transmission capacity triggered by data communication and Internet. The
urgent demands for larger transmission capacity have driven system planners to introduce WDM systems insteadof TDM systems to make the best use of installed opticalfibers.
WDM systems are thought to be the most cost-effective
way to handle the increasing transmission capacity using
established transmission technologies. In WDM systems,
which use multi-channel single frequency lasers such as
distributed feedback lasers with slightly different wavelengths determined by ITU (International Telecommunication Union), total throughput transmission capacity can beincreased by the number of lasers (channels).
Transmission capacity trends are illustrated in Figure 1
for both commercial implementations and laboratory
demonstrations. Total transmission capacity of more than
100 Gbps can be easily realized by using the established2.5-Gbps and developing 10-Gbps technology. Extremely
large transmission capacity of more than 1 Tbps will be
realized in the near future utilizing WDM systems.
On the other hand, strict specifications are required for
optical devices in terms of wavelength, since WDM systems utilize the wavelength regime. Single frequency
lasers used in the systems, for example, have to control
the absolute lasing wavelength to meet the ITU grid aswell as wavelength separation. The wavelength separa-tion according to ITU, for example, is set to be 1.6 nm(200 GHz), 0.8 nm (100 GHz ) and 0.4 nm (50 GHz)
depending on the WDM channels (bit rate).
In addition to the strict requirements in wavelength control, signal light sources have to have high output power inorder not only to extend the transmission distance but tocompensate for the insertion loss caused by the many
optical components like the wavelength coupler used in
WDM systems.
In order to amplify the signal light source in an efficientway, optical amplifiers are the key component to support
WDM systems. Erbium-doped fiber amplifiers (EDFAs),
which will be described later, have an advantage over traditional O/E and E/O amplification because of the excellent amplification characteristics such as high speed,
simultaneous amplification of many channels and so forth.
Higher pumping power can make it possible to produce
higher signal output power. Therefore, high power semiconductor lasers are one of the key devices for pumping
erbium-doped fibers (EDFs).

128

Total
Capacity

Commercial
system

1 Tb/s64

WDM Channels

1.

100 Gb/s

Lab demo.
32

10 Gb/s

16
1 Gb/s84'892'80 '83 '85 '86'87 '91

'96

1
0.01

0.1

1

10

100

1000

Channel Data Rate (Gb/s)

Figure 1*

Semiconductor R&D Center, Yokohama R&D Laboratories

Progress of transmission systems speed based on
WDM

23

Er-doped fiber
Signal light
λ: 1550 nm

Coupler
AmplifiedSignal output

Isolator
FilterPumping laser
λ: 980 nm/1480 nm

980 nm1480 nm Signal

AmplifiedsignalAbsorption and emissionof Er-doped fiber

Figure 22.

Configuration of optical fiber amplifier

OPTICAL FIBER AMPLIFIER (OFA)

It is no exaggeration to say that the invention of the opticalfiber amplifier is the innovation making it possible to realize the WDM system. In particular, the EDFA 1) is very
attractive for practical application because light amplification occurs in the wavelength range of the 1500-nm band,
which is the low loss wavelength region of conventional
silica fibers.
Figure 2 illustrates the configuration of EDFA. It consistsof EDF, a pumping laser module, a WDM coupler to couple the pumping light into EDF, an optical isolator to prevent the lasing in the EDF, and an optical fiber.
High power lasers emitting at wavelengths of 980 nm
and 1480 nm are used as pumping sources for effective
pumping of EDF. Efficient power conversion can be realized with a 1480-nm pump 2), while low noise amplification
can be realized with a 980-nm pump. In practical application, 1480-nm pumping is used as a booster amplifier and
980-nm pumping is used as a pre-amplifier.

3.

SEMICONDUCTOR MATERIAL AND EPITAXIAL GROWTH

1480-nm lasers can be fabricated using the conventional
GaInAsP quaternary compound on an InP substrate,
which has been used for telecommunication lasers emitting at 1300 nm and 1550 nm. We can use the establishedtechnology. On the other hand, 980-nm lasers cannot be
fabricated by use of lattice matched materials. The wavelength of around 980 nm emitted from semiconductor
lasers has been a forbidden wavelength region which cannot be covered by traditional short wavelength GaAsbased lasers (wavelength of shorter than 900 nm) andInP-based long wavelength lasers (wavelength longer
than 1200 nm). The concept of strained-layer quantum
wells 3), 4), however, made it possible to realize 980-nm
lasers as well as rapid progress of epitaxial growth technique to grow high quality very thin strained-layer material.
980-nm lasers can be fabricated by use of an intentionallylattice-mismatched InxGaAs layer on a GaAs substrate.
The InxGaAs strained-layer has a large lattice constantwith respect to GaAs. If the layer thickness is controlled
within the critical thickness calculated by Mathew's law,
the layer can be grown with high crystalline quality on a

24

GaAs substrate even if the InGaAs layer has a large
amount of strain. This condition is approximately given by
εLz < 20 nm% 5), where ε is the amount of strain and Lz is
the layer thickness.
Strained-layer GaInAsP on an InP substrate is also used
for 1480-nm lasers since the concept of strained-layer
quantum can improve the lasing characteristics drastically.
Let's explain the quantum wells and strained-layer
quantum wells used in high performance 980-nm and1480-nm lasers. Quantum wells are made up of two different materials with a layer thickness less than the de
Broglie wavelength, say 20 nm. High material gain can beobtained with less carriers because carriers are effectively
confined to the quantized state formed by the quantum
well. In particular, the use of compressive strain into thequantum wells can change the modification of valenceband structure in such a way that the effective mass ofheavy-hall becomes light. As a result, the BernardDuraffourg condition (Fermi-level separation larger thanenergy band-gap) can be satisfied with less carrier density. Thus, lower threshold current density and higher quantum efficiency can be obtained in strained-layer quantum
well lasers.
A strained-layer quantum well laser wafer can be grownby metalorganic chemical vapor deposition (MOCVD) for
both 980 nm and 1480 nm.

4.

APPROACH FOR HIGH POWER OPERATION

In a practical application, a laser diode module, so-called
pig tailed module, is used. The laser diode has to be
designed in such a way that high power operation is
obtained with a narrow and circular beam for high coupling into a single mode fiber (SMF). It should be notedthat highly reliable operation under high output power has
to be realized. For these purposes, buried heterostructure(BH) structure is used for 1480-nm lasers and a ridge
waveguide (RWG) structure is used for 980-nm lasers.
The limiting factors for high power operation under acontinuous wave (CW) condition in these narrow stripe
lasers are categorized into two phenomena; one is rollover phenomenon due to the increase of temperature in
the active layer or increase of invalid current, and theother is catastrophic phenomenon mainly due to the optical mirror damage. The former is observed in 1480-nm
lasers, and the latter is the sudden death phenomenon
called catastrophic optical mirror damage (COD) and isobserved mainly in 980-nm lasers. A detailed explanationof the COD is reported in reference 6.
Let's explain the approach of how to attain high power
operation in solitary lasers. The output power from thefront facet, Pf is given by the following equation if COD
does not occur.

P f= η i

1
1-R f
1+
1-R r

Rr
Rf

x

I ac
αm
Θ (T)
(I-I th )I ac +I L
α i+ α m()

Furukawa Review, No. 19. 2000

Where, ηi : internal quantum efficiency, αi : internal loss,
αm : mirror loss, I: injection current, Ith : thresholdcurrent, Iac: effective current which contributes tothe lasing, IL: leakage current, and Θ(T): term representing the optical power output saturation due
to heat generation.
From this equation, the following measures are thought
to be effective for high power operation.
(a) realization of high internal efficiency(b) introduction of asymmetric coating for low mirror loss(c) realization of low internal loss(d) suppression of leakage current(e) high thermal dissipation
Structural optimizations have to be made because contradictory items are included in the above. The introduction of a long cavity, for example, is effective for low thermal resistance, however, increase of the threshold current
and decrease of external quantum efficiency are observedin long cavity lasers.
Detailed explanations for realizing the above items are
given as follows.
(a) In order to achieve high internal quantum efficiency,
structures of both quantum wells and optical confinement
layers such as composition and thickness have to be optimized carefully. Introduction of strained layer quantum
wells into the active layer is indispensable for this purpose. The use of GRIN-SCH (Graded-Index SeparateConfinement-Structure) is very effective 7) for high internal
quantum efficiency, which will be described in detail later.
(b) Parallel mirror formed by cleavage of semiconductormaterial is used for Fabry-Perot resonator, thus, an equalamount of light is emitted from both sides of the mirror.
The output power from the rear facet is not so importantbecause it is used only for a back-monitor in a practicalapplication. Therefore, asymmetric facet coatings composed of dielectric mirror are used to increase the output
power from the front facet. Low reflective coating (several
percent) is used for the front facet and high reflective coating (95%) is used for the rear facet.
(c) Introduction of quantum well structure, especially astrained-layer quantum well is effective for low internal
loss. A strained-layer quantum well laser can provide low
threshold current and high differential quantum efficiency
operation due to low internal loss even if a long cavity isused.
(d) The reduction of leakage current leads to the suppression of temperature rise of the active layer, thus leading to the high power operation.
(e) It is possible to reduce the temperature rise in theactive layer by the use of a long cavity (low electric andthermal resistance) and junction down bonding on heatsink materials with high thermal conductivity such as diamond and AlN.
Next, the output beam design for achieving high coupling into an SMF is explained. High coupling efficiency
into an SMF is indispensable for fabricating a high power

Figure 3

TEM image of the active region of a 1480-nm laser

laser diode module. Narrow and circular output beams arefavorable for efficient fiber coupling. Output beam characteristics can be controlled by the layer composition andthickness of the GaInAsP layer 7) which form the SCH
layer. In particular, the use of a GRIN-SCH layer can
expand design freedom by not only taking into account thecarrier confinement into the active layer but also opticalconfinement as well. It is theoretically and experimentally
verified that the use of a GRIN-SCH layer compromising athinner and wider bandgap is effective for both output
beam characteristics and low threshold current, highquantum efficiency operations.
It is noted that the packaging design, including couplinglens system, is also very important for high coupling efficiency into an SMF.

5.

CHARACTERISTICS OF 1480 NM
LASERSTransmission electron microscopy of a cross sectionalview of the active layer for a 1480-nm laser is shown in
Figure 3. The active layer is made up of 1% compressively strained quantum wells (4 nm thick) separated by
GaInAsP (10 nm thick) with a bandgap wavelength of 1.2µm. The number of wells is five. The GRIN-SCH layer,
composed of two different GaInAsP, sandwiches thestrained-layer active layer. The use of GRIN-SCH structure is shown to be effective for high performance in thelong wavelength region 9). Very sharp hetero-interfaces are
obtained through MOCVD growth optimization.
BH structure with an active layer width of 2 µm is used
for both electrical and optical confinements. All epitaxial
growth, including selective growth for BH structure using
MOCVD process, ensures a high yield process using a 2inch wafer.
Figure 4 shows the injection current versus light output
power characteristics for an optimized 1480-nm laser. The
cavity length is 1200 µm for high heat dissipation, which islonger than those of conventional lasers having typicalcavity lengths of 300-400 µm used for telecommunication
applications. In order to increase the heat dissipation, the

25

400

600

300

Driving current (mA)

Output power (mW)

35˚C, 160-180 mW
λ=1480 nm
L=1200 µm
AR/HR

Pmax=360 mW

200

100

400

60˚C, 80-110 mW
200

Automatic power control
0
0

500

1000

0

1500

0

Injection current (mA)

Figure 4

Angle (deg.)θParall:20 deg.

Angle (deg.)

Output beam pattern of 1480-nm laser

laser is bonded with a junction down configuration, that is,
the laser is bonded on a heat sink in such a way that theactive layer, which is the main heat generator, lies close to
the heat sink. The facet is coated with an AR/HR film. The
reflectivities are 5% and 95% for front and rear facets,
respectively. Low threshold current of 25 mA was
obtained in spite of a long cavity of 1200 µm. Output
power as high as 360 mW from the front facet wasachieved, which, to the best of our knowledge, is the maximum output power ever reported for transverse-mode
stabilized long wavelength lasers 7). High output power ofover 300 mW was obtained even at a high temperature of
50°C. Extremely high output power was obtained for thisparticular device with a long cavity, however, driving current or power consumption is relatively high for 100-200
mW output power, which is commonly used for practicalapplication. Therefore, a cavity length around 800-1000µm is used for low power consumption operation.
Maximum output power is decreased due to the poor heatdissipation, however, driving current for 100-200 mW canbe decreased because of higher differential quantum efficiency.
Figure 5 shows the far field pattern (FFP) parallel andperpendicular to the junction plane at an output power of
100 mW. Full-width at half-maximums of FFP parallel andperpendicular to the junction plane are 20° and 25°,
respectively. The narrow and circular output beamobtained here is favorable for high coupling into SMF.
Highly reliable operation must be achieved like conventional FP and DFB lasers since optical fiber amplifiers areused for long-haul trunk lines and submarine optical com-26

15,00020,000Figure 6

Reliability test results of 1480-nm laser

100 mW
θVer.
:25 deg.

Figure 510,000Aging time (hrs)

I-L characteristics of 1480-nm laser

100 mW

5,000

munication systems. Output power of pumping lasers isextremely high as compared to conventional signal lasers.
Reliability test under high temperature and high output
power has to be investigated. Figure 6 shows the aging
test result at high power condition. The aging test has
been performed at 35°C and 60°C in the automatic power
control mode. The output power is set to be 80% of the
maximum output power at a given temperature, determined by the thermal roll-over, which is 180 mW at 35°C
and 120 mW at 60°C. Stable operation has been confirmed after 20000 hours without appreciable increase ofdriving current. The activation energy of 0.62 eV is derived
from various aging test conditions. Extremely high reliability of 100 million hours as a mean time to failure (MTTF) isestimated at an output power of 150 mW, which corresponds to the module output power of 120 mW (reasonable coupling efficiency into an SMF of 80% is assumed) .

6.

CHARACTERISTICS OF 980-NM
LASERS

The noise figure of fiber amplifiers pumped at 980 nm is
lower than that of fiber amplifiers pumped at 1480 nm,
which is very attractive for practical application.
The material candidate for 980-nm lasers is compressively strained InGaAs quantum wells for the active layer,
while there are two candidates for cladding layer materials. Those are conventional AlGaAs 10)~14) layer and novel
InGaP 15), 16) layers. The use of InGaP cladding layer material might have an advantage over AlGaAs in terms of lessoxidized property and slow surface recombination velocity,which are thought to be effective methods to suppress
COD.
As mentioned previously, transverse-mode stabilized
laser structure is used for efficient coupling to SMF. Ridge
waveguide structure is widely used as shown in Figure 7.
This laser structure is grown by MOCVD. Recently, otherlaser structures like buried ridge waveguide structure andself-aligned-structure (SAS) have been investigated.
Light output power versus injection current characteristics is shown in Figure 8 for a 980-nm RWG laser with 4µm-wide ridge and 800 µm-long cavity. Light output power

Furukawa Review, No. 19. 2000

300

p-AlGaAs cladn-AlGaAs clad

200
1.0

100

0.5

AlGaAsn-GaAs sub.

AR Film

λ=980 nm
25˚C, CW

Efficiency (W/A)

HR Film

p-GaAs

Output power (mW)

Metal

GaAsP

InGaAs

0
0

100

200

300

400

500

Injection current (mA)

Figure 7

Structure of ridge-waveguide type 980-nm laser

400

500

Output power (mW)θparall.
: 9 deg.

λ=980 nm
L=800 µm
AR/HR

400
Output power (mW)

Figure 10 I-L characteristics of 980-nm laser module

300

200

θver.
: 23 deg.

SE: 1W/A

λ=1480 nm
25˚C, CW300
Pf>300 mW
200

100

100

0

0
0

100

200

300

400

I-L characteristics of 980-nm laser

Driving current (mA)

1000

1500

Figure 11 I-L characteristics of 1480-nm laser module

atmosphere and in-situ facet passivation, facet passivation using high thermal conductivity material, and formation of so-called 'window structure' near the facet, which istransparent to the emission wavelength.
Long term aging result is shown in Figure 9 under conditions of 60°C, 250 mW. No appreciable change in driving current has been observed after 5000 hours.

500
λ=980 nm500

Injection current (mA)

Angle (deg.)

Injection current (mA)

Figure 80500

60˚C, 250 mW

400
300
200
Automatic power control
100
0
0

1000

2000

3000

4000

5000

Aging time (hrs)

Figure 9

Reliability test results of 980-nm laser

more than 300 mW was successfully achieved with aslope efficiency as high as 1 W/A. FFPs parallel and perpendicular to the junction plane are also shown in Figure 8
at an output power of 250 mW, showing an elliptical output beam as compared to BH lasers. It is possible toobtain high coupling efficiency by optimizing a coupling
scheme. With a wedge-shaped fiber, which is speciallydesigned for beam shape, coupling efficiency as high as
80% was achieved.
Similar to 1480-nm lasers, highly reliable operation isrequired. One of the key items for high reliability operation
is to overcome COD. Key technology is facet passivation
technology. Several approaches have been reported,
those are, cleavage laser bars in the ultra-high-vacuum

7.

LASER MODULE

Figures 10 and 11 show the light output power versus
injection current characteristics of 980-nm and 1480-nm
laser modules, respectively. Very high coupled powers of
200 mW for a 980-nm laser module 17) and of over 300 mW
for a 1480-nm laser module 18) are obtained.
In particular, coupled power of over 300 mW is the highest power reported for long wavelength laser modules with
SMF. Extremely high coupled power modules of 250 mW
are commercially available.
The failure mode related to the packaging has beenreported in 980-nm laser modules. It is considered thathydrocarbon, contained in epoxy used for package andadhering to the laser facet is the trigger for COD. This failure mode is called PIF (package induced failure). One
solution to avoid PIF is to introduce a small amount ofoxygen into the package to burn out the hydrocarbon
adhered to the laser facet.

27

400
Furukawa

CW Output power (mW)

Innovation

Furukawa

Philips
Mitsubishi

300
NEC

SEI
NEC

BT&D

Optimization

SEI
Oki

Phillips

200

achieved. New technological innovations such as a novel
active layer structure and some consideration for active
layer material can lead to further output power improvement in the future.

Fujitsu

ACKNOWLEDGEMENT

AT&T

NEC

Oki

100
Furukawa

Oki

Bulk
'87

QW
'89

λ=1480 nm
SL-QW

'91

'93

'95

'97

The authors would like to thank Drs. K. Ohkubo, Y.
Suzuki, T. Kamiya for their encouragement and also theentire pump laser team including FITEL products division.

'99

Year reported

Figure 12 Improvements in light output of 1480-nm laser

Several types of laser modules have been fabricated for
WDM application. Wavelength stabilized laser module is
one example 19). Lasing wavelength can be stabilized by
the use of fiber Bragg grating. In general, lasing wavelength of FP lasers shifts to longer wavelengths with atemperature coefficient of 0.5 nm/degree in the 1480-nm
wavelength. By using fiber Bragg grating with a narrowpass band, a stable lasing wavelength for both current
and temperature can be achieved.

8.

FUTURE PROSPECTThe history for power improvement in narrow stripe (transverse-mode stabilized) 1480-nm lasers is shown in Figure
12. Drastic power improvement has been achieved bytechnological innovations, such as, introduction of quantum well active layer and strained-layer quantum wellactive layer. By using a strained-layer quantum well active
layer, maximum output power of about 400 mW was

28

REFERENCES

1) S. B. Poole et al: Electron. Lett., 21 (1985) 737.
2) K. Nishimura : Optical soliton transmission Technical Report ,1995, p.57. (in Japanese)
3) E. Yablonovitch at al: J. Lightwave Technol., LT-4 (1986) 504.
4) A. R. Adams: Electron. Lett., 22 (1986) 249.
5) T. J. Andersson et al.: Appl. Phys. Lett., 51 (1987) 752.
6) I. Mito et al.: Applied Physics Japan, 64 (1995) 2. (in Japanese)
7) A. Kasukawa et al.: IEEE Photon. Technol. Lett., 1 (1994) 4.
8) A. Kasukawa et al.: OPTOELECTRONICS-Device andTechnologies, 9 (1994) 219.
9) H. Hirayama et al.: IEEE J. Quantum Electron., 28 (1992) 68.
10) D. Fekete et al.: Appl. Phys. Lett., 49 (1986) 1659.
11) M. Okayasu et al.: IEEE Photon. Technol. Lett., 2 (1990) 689.
12) S. Ishikawa et al.: IEEE J. Quantum Electron., 29 (1993) 1936.
13) H. Meier: Technical Digest of 20th European Conference on
Optical Communications, Firenze, Sep., 1994, p.974.
14) J. S. Major et al.: Electron. Lett., 27 (1991) 540.
15) S. Sagawa et al.: Technical Digest of 14 t h International
Semiconductor Laser Conference, Maui, Sep., 1994, p.255.
16) M. Ohkubo et al.: IEEE J. Quantum Electron., 30 (1994) 1936.
17) Y. Irie et al.: OFC/IOOC'99, Paper WM15 (1999).
18) T. Kimura et al.: OAA'99, Paper ThD12 (1999).
19) S. Koyanagi et al.: OAA'98, p.35 (1998).
Manuscript received on December 17, 1999.

Furukawa Review, No. 19. 2000