Low Power Consumption Multimode Laser Diode Modulefor 900-nm Band (915~975 nm)
by Naoki Hayamizu * , Jun Miyokawa * , Yuichiro Irie * , Shuichi Tanaka * ,Yasuo Oeda * 2 , Tsuyoshi Fujimoto * 2 and Yoshikazu Ikegami * 2

A low power consumption multimode laser diode module (MM-LDM) for the
900-nm band (915~975 nm) has been developed for use in material processing
and measurement systems.
This LDM has advantages over conventional modules in terms of higher power, smaller sizeand lower power consumption. In this paper, we report how improvement of the characteristicsof the 900-nm band MM-LDM with a standard 14-pin butterfly package was achieved byincreasing the power conversion efficiency of the laser diode chip and the optical coupling
efficiency with multimode fiber, and by optimizing heat dissipation design.

ABSTRACT1.

INTRODUCTION

Gas lasers like CO 2, Ar or excimer lasers, and solidstate lasers like YAG lasers pumped by lamps havebeen used widely in industrial fields including material
processing (cutting, welding, soldering and marking) or
measurement systems. These lasers, however, had somedisadvantages in terms of large power consumption,
frequent maintenance and short lifetime. Recently,
high power laser diodes have been attracting attention
because they have the advantages of smaller size, higher
electrical-to-optical conversion efficiency, higher reliability
and superior productivity, and they have begun to be
used for direct processing and as the pumping light forsolid-state lasers. Especially, cladding pump fiber lasers
pumped by light from laser diodes is rapidly coming into
widespread use because it has a higher beam quality
than gas or solid-state lasers. High-power 808-nm laser
diodes are being used for medical appliances andprinting devices. In telecommunications, high-power
900-nm band laser diodes are used in cladding pump
amplifiers for CATV. High-power 900-nm and 808-nm
band multimode laser diode modules (MM-LDMs) arebecoming important in various applications, but since
they were developed focusing on high output power only,
they have the following problems:
1) large power consumption owing to un-optimized
heat dissipation design;2) incompatibility owing to the large packages andpackage dimension that differ vendor by vendor; and

3) inconvenience owing to an output optical interface
without using an optical fiber.
The large and incompatible packages complicate thedesign of circuit board heat dissipation, and large power
consumption makes it difficult to downsize the package
and reduce running cost.
In this paper, we report approaches that we adopted
in order to develop a 900-nm band MM-LDM with highpower and low power consumption, namely:
1) using a high-power laser diode chip with highconversion efficiency;
2) improving coupling efficiency to the optical fiber;
3) improving the heat dissipation of the module; and
4) using a standard 14-pin butterfly package.

2.

LASER DIODES

2.1 Laser Diode Design 1)
The 900-nm band multimode laser diode (MM-LD) is
being produced by MC-FITEL, a joint venture between
Furukawa Electric and Mitsui Chemicals.
The MM-LD consists of InGaAs/AlGaAs material that is
epitaxitially grown by low-pressure metalorganic chemical
vapor deposition (MOCVD). Figure 1 shows a crosssectional view. Stripe width was controlled to 100 µm by
an n-GaAs current blocking layer buried in a p-type GaAs
cap layer. The front facet had a low-reflectivity coating(4 %) and the rear facet a high-reflectivity coating (96%). The cavity length was 1.8 mm. This MM-LD features
adoption of a decoupled confinement heterostructure(DCH), achieving both higher power and higher efficiency.

* Semiconductor Device Development Dept., R&D Div.
* 2 MC-FITEL Inc.

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Cross-sectional view of MM-LD chip.

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LI characteristics of typical LD chip.

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Figure 2

Comparison of the separate-confinement anddecoupled confinement heterostructures.

2.1.1 Decoupled Confinement Heterostructure 2)
The separated confinement heterostructure (SCH) witha single quantum well is a popular structure in highpower InGaAs/AlGaAs LDs, but makes it difficult toconfine the carrier completely in a narrow active region.
The waveguide layer is thus narrower and therefore must
be made deeper in energy level than the clad layer. The
design of the waveguided mode is limited by the relation
between the bandgap and the refraction index in InGaAs/
AlGaAs LDs.
On the other hand, the DCH confines carriers in theactive region by wide bandgap carrier-blocking layers
located at both sides of the quantum well, making it
possible to design the waveguide structure independentlyof the carrier confinement structure.
2.1.2 Improvement of COD Level 2)
The maximum power of an InGaAs/AlGaAs LD isgenerally limited by the incidence of catastrophic opticaldamage (COD), which may be caused by re-coupling ofcarriers, oxidation of the facet, or increased temperature
due to absorption of emitted light at the facet. Decreasing
the optical density at the facet is an effective way to limit
the incidence of COD, and the DCH decreases the opticaldensity at the facet to improve the waveguided mode
profile as shown Figure 2. Moreover, we have improved
the incident level of COD by setting up a non-current
injection layer near the front facet of the LD.

2.1.3 Reduction of Thermal and Electrical Resistance 2)
In addition to COD, saturation of optical output poweras LD temperature increases is also a major factor
limiting output power. To prevent the increase in LD
temperature, it is useful to use materials of low electricalresistance, and to suppress generation of Joule’s heatand ensure thermal conductivity the use of material of
low thermal resistance is indicated. In InGaAs/AlGaAs
lasers, low thermal and electrical resistance can berealized by reducing the Al content of the clad layers. The
conventional SCH cannot block the overflow of carriers
since reducing the Al content of the clad layers induces adecrease in bandgap energy at the clad layer.
DCH lasers can realize low thermal and electricalresistance without carrier overflow since the waveguidestructure can be designed independently of the carrierconfinement structure.
2.2 LD Chip Characteristics 1)
2.2.1 Optical Output Power
Figure 3 shows the LI characteristic of a typical 900-nmband MM-LD chip at selected temperatures. LD chips witha cavity length of 1.8 mm and DCH achieved high powerand high conversion efficiency at the same time, with amaximum output power of 9 W and a power conversion
efficiency of 62 %.
2.2.2 Far-Field PatternFigure 4 shows the far-field pattern (FFP) of a typical
MM-LD. In the vertical direction the beam intensity
distribution for the active region was almost Gaussian, but
in the horizontal direction was substantially rectangular.
The vertical beam divergence angle was 33 deg. for full
width at half maximum (FWHM) and 70 deg. for 1/e2. The
horizontal beam divergence was about 10 deg, smaller
than the numerical aperture (NA) =0.1 up to 4 W. The
horizontal beam divergence increased slightly with opticalpower because of the thermal lens effect.

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FFP of a typical MM-LD chip.

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Cross-sectional view of MM-LDM.

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Results of accelerated aging test of MM-LD chips.

2.2.3 Reliability
Accelerated aging test of LDs was carried out at 70°C
under auto current control (ACC) of 3 A for 15,000 hours(see Figure 5). At a fiber output power of 1.5 W at 25°C,
the random failure rate can be estimated at 490 FIT andthe median lifetime at 1,400,000 hours.

3.

LASER DIODE MODULE DESIGN

To realize a laser diode module with high power and low
power consumption, it is important to have high LD-tofiber coupling efficiency, stable structure and efficientheat dissipation design.
3.1 Basic Design
Design of the multimode laser diode module (MM-LDM)
follows that of the 980-nm single-mode laser diodemodule (SM-LDM). Figure 6 shows a cross-sectionalview of an MM-LDM. The LD chip was bonded on a CuW
sub-mount in a junction-down configuration. The submount was fixed to a base on which a multi-mode fiber(MMF) and a photodiode (PD) for power monitoring werealso mounted. The optical lensed fiber has a micro lensat the tip of the MMF, and is fixed by YAG welding. LD
temperature is kept constant by a thermoelectric cooler(TEC) fixed under the base.

Figure 7���Configuration of optical coupling between
LD chip and lensed fiber.

3.2 Improving Coupling Efficiency 3)
The design of the lensed fiber is important in achieving
high optical coupling efficiency. The shape of the lensed
fiber was arrived at by taking account of the FFP of
the typical MM-LD shown in Figure 4. We consideredthe vertical and horizontal components of the beam
separately since the shape of the beam emitted from theLD was elliptical. In the horizontal component, the optical
coupling is thought to be butt coupling. As describedin Section 2.2.2, the horizontal beam divergence wassmaller than NA=0.1, which is smaller than that of the
MMF, and the core diameter of the MMF, at 105 µ m, wassubstantially equal to the stripe width of the LD, which
was 100 µ m. Thus the horizontal component of LD-toMMF coupling is not critical, because our MMF had an
NA of 0.15.
We designed the lens shape assuming that the wedge
consisted of two parts: one an inclined plane and theother was a spherical part located at the tip of thewedge. Lens shape could thus be described by apex
angle and tip radius. Figure 7 shows the optical coupling
configuration between the LD chip and the lensed fiber.
The lens was formed at the tip of the optical fiber where
the core diameter was 105 µ m. Because the focus length
was short and the lensed fiber was close enough to the
LD facet, the apex angle and the tip radius were designedon the following two assumptions:

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Figure 8

Apex angle dependence of refraction angle
for incident light of 70 deg. and 40 deg.

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Figure 9

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Tip radius dependence of refraction angle
for incident light of FWHM.

1) emitted light with an FFP narrower than the FWHM
is coupled to the optical fiber through the sphericalpart; and2) emitted light with a wide divergence angle between
the FWHM and 1/e2 is coupled to the optical fiber
through the inclined plane.
When we design the apex angle, the apex angle needs
to be considered only for 1/e2, which is 70 deg. of LD
divergence angle, to obtain high coupling efficiency.
Figure 8 shows the apex angle dependence of therefraction angle for incident light with divergence anglesof 70 deg. and 40 deg. respectively. MMF of NA=0.15 canpropagate light when the refraction angle falls within thegray area. In this result, we could select apex angles from90 to 110 deg. for a 70-deg. divergence angle, and for a
40-deg. divergence angle, the apex angle could be from90 to 130 deg. An apex angle of 110 deg. is therefore
thought to be the optimum angle for coupling light withthe optical fiber at divergence angles between FWHM and1/e2 through the inclined plane.
Figure 9 shows the tip radius dependence of therefraction angle for incident light with FWHM divergenceangle. In this result, a tip radius from 2.5 to 7 µ m can beselected. The actual apex angle was determined as 5 µ mtaking account of reproducibility and ease of processingof the tip radius.
In prototype production, we obtained a maximum

Figure 10 Comparison of coupling tolerances between
MM and SM lensed fibers.

coupling efficiency of 90 % and an average of 87 %.
An investigation of coupling tolerances for a lensed
fiber of optimum design was carried out, and Figure
10 shows a comparison of coupling tolerances for MM
and SM lensed fibers in the vertical direction of the LD.
The SM lensed fiber coupling tolerance was extremely
narrow (1 µ m) since the mode field diameter of the SMF
for 980 nm was only 6 µ m. The MM lensed fiber had awider coupling tolerance than the SM fiber even without
optimization of its lens shape, since the core diameter ofthe MMF is so large (105 µm). However, we obtained aflat tolerance level of 4 µ m to optimize the shape of thelensed fiber according to the studies mentioned above.
The coupling efficiency of the SM lensed fiber
decreased 10 % only by the lensed fiber moving
approximately 0.3 µ m. Our 980-nm SM-LDM suppresses
this decrease in coupling efficiency by precise YAG
welding technique. The MM-LDM, having the same
module structure and wide coupling tolerance, does notshow any decrease in coupling efficiency with changes inambient temperature or long-term usage. Moreover, since
the MM-LDM has a high coupling tolerance, production isvery easy and a high yield can be expected.
3.3 Heat Dissipation Design
To improve the heat dissipation characteristics ofthe module, it is desirable to increase the thermal
conductivity of the structural materials from the LD to
immediately above the TEC and to use a TEC having
highly endothermic characteristics. Attention must be
paid, however, to the decrease in coupling efficiency
caused by changes in ambient temperature and tolong-term reliability, which is a function of differencein the thermal expansion coefficients among thestructural materials used in the module. The total power
consumption of the module will also increase when
we use a TEC with unnecessarily large endothermic
characteristics.
We optimized thermal conductivity and thermal
expansion coefficients among the materials used in themodule. In addition, heat conduction for the MM-LDM is
improved by using a large sub-mount which can dissipate
the heat effectively. Figure 11 shows the LD injection

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Figure 13 Comparison of total power consumption of
two types of TEC.

Figure 11 LD injection current dependence of TEC current.

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Figure 12 Comparison of efficiency of two types of TEC.

Figure 14 LI characteristics of typical MM-LDM.

current dependence of TEC current. It was reduced by 13% at 4 A of LD current by optimizing the heat dissipation
design.
To achieve lower power consumption, we have toselect the best TEC by considering the operating regionin which it dissipates heat from the LD most effectively.
Figure 12 shows a comparison of the efficiency of two
types of TEC. TEC efficiency is defined by W LD/W TEC,
which shows the ratio of dissipated heat to the total
power consumption of the TEC. The value of WLD/WTEC
means efficient thermal transfer. The Type 1 TEC has
an endothermic quantity Qc of 2.4 W at a temperature
difference (∆ T) between the hot and cold side of the TEC
of 45°C, which is obtained from an LD temperature of 25°C and an ambient temperature of 70°C. The Type 1 TEC
is comparatively more efficient when dissipating a small
quantity of heat. The Type 2 TEC has a Qc of 3.0 W ata ∆ T of 45°C, and is comparatively more efficient when
dissipating a large quantity of heat.
As shown in Figure 12, Type 1 shows higher efficiency
at fiber outputs below 1.5 W, while Type 2 efficiency ishigher at outputs above that value. Figure 13 compares
the total actual power consumption of modules using thetwo TECs, calculated by adding the values for the LD and
the TEC. Total power consumption using the Type 1 TEC
was lower at fiber outputs below 1.5 W, and that using theType 2 TEC was lower at outputs above that value.
Accordingly it was decided to use the Type 1 TEC
at fiber outputs power below 1.5 W and the Type 2 in

the region above 1.5 W for MM-LDM under an ambient
temperature of 70°C and an LD temperature of 25°C.
We have also been able to propose an MM-LDM thathas lower total power consumption for each customer's
specification.

4.

CHARACTERISTICS OF LASER DIODE
MODULE

4.1 Fiber Output Power
Figure 14 shows the LI characteristics of a typical
MM-LDM at an LD temperature of 25°C under continuouswave (CW) operation. The threshold current was
344 mA and the slope efficiency was 0.9 W/A at the
maximum. The maximum fiber output power was 4.7
W at an injection current of 7.7 A, but we regulated thefiber output power down to 2.5 W for production from theviewpoint of overall LDM reliability. The injection current
was 3.35 A and the forward voltage was 1.74 V at a fiber
output power of 2.5 W.
4.2 Optical Output Spectrum
Figure 15 shows a typical optical output spectrum of a975-nm MM-LDM. We have been able to produce an
MM-LDM with a wavelength tolerance of ±3 nm. The
spectrum width was approximately 3 nm.
The output power dependence of the wavelength shiftcoefficient was 1.3 nm/W. This wavelength shift was

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Low Power Consumption Multimode Laser Diode Module for 900-nm Band (915~975 nm)

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���������� ����4.3 Total Power Consumption
As shown in Figure 13, the total power consumption was
9.3 W at a fiber output power of 1.5 W and 17.4 W at afiber output power of 2.5 W using the Type 2 TEC. This
total power consumption is 30 % lower than that of aconventional module with the butterfly package.
Time Stability of Fiber Output Power and Monitor
Current
Figure 16 shows the injection current dependence of thetime stability of fiber output power and monitor current.
The time stability was excellent, with a fluctuation of lessthan 1 % over injection currents ranging from 0.5 to 4 A.
This suggests that this MM-LDM can be utilized as thepump source of cladding optical amplifiers for CATV.

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Figure 15 Typical optical output spectrum of MM-LDM.

induced by raising the temperature of the active region by
supplying electric power to the LD. The wavelength shift
can be controlled by TEC temperature changing since
the LD temperature dependence of the wavelength shiftcoefficient was 0.35 nm/°C.

���Figure 16 Time stability of fiber output power and monitor
current.

5.

CONCLUSION

We have developed a multimode laser diode module
for the 900-nm band for use as the pumping lightsource for fiber and the solid-state lasers, not only inthe telecom field but also in certain industrial fields. We
achieved a multimode laser diode module with 14-pin
butterfly package that has a total power consumption
approximately 30 % lower than that of conventional
LDMs. Moreover, our MM-LDM can be used for opticalamplifiers in the telecom field since it exhibits excellent
time stability.

4.4

REFERENCES
1) Y. Oeda, F. Fujimoto: OAA’02 (2002), OMB1-1
2) K. Muro et al.: Functional Materials, August, 1997 (in Japanese)
3) K. Kono: Fundamentals and Applications of Optics Systems for
Optical Devices, Gendai Kogaku Publisher, (in Japanese)
4) N. Hayamizu, J. Miyokawa, H. Murata, K. Nishikata, Y. Oeda, T.
Fujimoto, Y. Ikegami : OAA’03 (2003), TuB4

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