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Design of Al-free and Al-based
InGaAs/GaAs strained quantum well
980-nm pump lasers including
thermal behavior effects on E/O
characteristics
Sergio Pellegrino, M. G. Re, C. Beoni, D. Reichenbach,
F. Vidimari
Sergio Pellegrino, M. G. Re, C. Beoni, D. Reichenbach, F. Vidimari, "Design
of Al-free and Al-based InGaAs/GaAs strained quantum well 980-nm pump
lasers including thermal behavior effects on E/O characteristics," Proc. SPIE
2150, Design, Simulation, and Fabrication of Optoelectronic Devices and
Circuits, (2 May 1994); doi: 10.1117/12.175004
Event: OE/LASE '94, 1994, Los Angeles, CA, United States
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Design of Al-free and Al-based InGaAs/GaAs Strained Quantum Well
980 urn Pump Lasers including thermal behaviour effects on E/O
characteris tics
S .
Pellegrino,M.
G .Re, C .Beoni, D .Reicheabach, F .Vidimari
Alcatel-Telettra Research Center
Via Trento 30, 20059 Vimercate (Mi), Italy
Abstract
A two-dimensional thermal simulator and a model to evaluate
high power lasers characteristics have been developed.
With these models it was possible to optimize cavity length of
InGaAs/GaAs (Multiple) Quantum Well 980 nm lasers realized both
with Al-based and Al-free confining layers.A comprehensive
experimental investigation of the influence of cavity length and
temperature
on
the
laser
emission
wavelength has been
performed.This allows a fine trimming of the devices to match the
Erbium doped fiber absorption bandwidth.
Introduction
High power InGaAs/GaAs 980 nm Lasers have become of great
interest for application as pump sources in Erbium Doped Fiber
Amplifiers (EDFA) .These devices are based on InGaAs/GaAs Single
and Double Quantum Well (SQW,DQW) strained active layers and
AlGaAs Separate Confinement Heterostructure (SCH) .To improve their
performances for high reliability applications (such as undersea
links) Al-free devices based on InGaAsP/InGaP SCH are currently
under investigation .
It
has been demonstrated
[11 ,
that
the
introduction of Indium increases the resistance to dark-line
defect (DLD) motion, being the key to dislocation pinning.
By deliberate introduction of defects by scribing near the active
stripe, a different behaviour has been found from, on one hand,
devices with GaAs QW and, on the other hand, Indium bearing alloys
either in the strained InGaAs active or as constituent of the
unstrained InGaAsP active and InGaP cladding of 800 nm emitting
devices.The former rapidly degrades due to fast growth of DLD
along (100) direction, while the later show a DLD growth velocity
which is two orders of magnitude lower than the GaAs one.
One of the key points in designing these devices is the choice of
cavity length [2], in order to minimize operating current in the
range of 150-200 mW emission power.
Furthermore, the emission wavelength of these devices has been
found to be strongly dependent on the cavity length [31, which is
a well known feature of semiconductor lasers, that might allow the
fine trimming of the lasers to match the relatively narrow
(roughly 10 nm) absorption bandwidth of the Erbium doped fibers.
20 / SPIE Vol. 2150
0-8194-1445-X/94/$6.00
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Results and discussion
The choice of device length is usually done considering only
its influence on threshold current density (Jth) and Slope
efficiency (Se) without taking into account all the effects of the
thermal behaviour of the devices, which are not negligible in the
high power regime.For this purpose a 2-D Thermal Simulator was
developed exploiting Finite Differences Equations, suitable to
evaluate temperature distribution for arbitrary structures and
materials.The structure is assumed to be symmetrical, thus
allowing a reduction in the calculation time, and is divided in
regions of given thermal conductivity.For every block a physical
area and the number of discretization points are defined; the
discretization cell is a five points star which can be completely
asymmetrical both in the discretization step and in the thermal
conductivity [4] .In the frame of such general formulation the
adiabatic boundaries of an area are simply defined by imposing
zero conductivty to the confining regions.
Finally the discretized equations are solved calling the DO3UAF
NAG routine; usually the convergence is fast and requests few
minutes of VAX 8600 computing time.
A typical result which could be achieved for a Ridge Waveguide
(RW) Laser structure is shown in figure 1.
Figure 1: Thermal resistance map for a Al-based, junction-up
mounted RW device with a 3 micrometrs wide ridge, planarized with
a 5 micrometers thick gold heat spreader.
Thermal Resistance Map (K*mm/W)
0.0 1.0
2.0
3.0
4.0 5.0
6.0
7.0
2.0
1.0
0.00.0
3.0
2.0
1.0
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
x (jim)
SPIEVo!. 2150/21
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A wide range of technological options have been simulated, as an
example in figure 2 is depicted the effect of the thickness of a
gold heat spreader on the thermal resistance (Rth) for p-side-up
mounted RW devices with different ridge widths.
Thermal
resistance (K*mm/W)
Figure 2: Thermal resistance dependence
spreader thickness; the thermal resistance
the active layer lateral dimension.
on RW width and heat
value is averaged over
In these two mentioned figures the thermal resistance values are
normalized to 1 millimeter long devices, as Rth scales with l/L.
In order to complete the description of laser characteristics,
threshold and Slope efficiency on several SQW and DQW structures
have been analyzed, both with Al.5GaAs and InGaP cladding
layers .
The
waveguide structure comprises either Al .
2GaAs
and GaAs
spacer, or InGaAsP (Xgap=780-720 nm) and GaAs spacer for Al-based
and Al-free structures, respectively.
Transmission Electron Microscopy investigation has been performed
to extract precisely the waveguide dimensions which allow a
detailed evaluation of gain characteristics.
The gain curves are well fitted by a logarithmic dependence like:
g=ra01n 1tr)
[1]
Transparency
current
values
and
coefficients
are summarized in table 1
22/SPIE Vol. 2150
modal
differential
gain
6
8
10
12 14
Heatspreader thickness (jim)
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