IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 22, NO. 24, DECEMBER 15, 2010
1829
Mitigation of Voltage Defect for High-Efficiency InP
Diode Lasers Operating at Cryogenic Temperatures
Paul O. Leisher, Member, IEEE, Weimin Dong, Mike P. Grimshaw, Mark J. DeFranza, Mark A. Dubinskii, and
Steve G. Patterson
Abstract—The power conversion efficiency of cryogenically
cooled InP-based diode lasers is limited by excess electrical voltage
caused by the freeze-out of holes at low temperature. Hall-effect
measurements are performed to determine the ionization energy
of Zn in bulk InP and In0 90 Ga0 10 As0 24 P0 76 (the values obtained are 18.6 and 11.6 meV, respectively). A laser design with
an InGaAsP p-cladding layer shows a large decrease in the 77 K
voltage defect relative to a more traditional InP design. Peak
conversion efficiency of 73% and 10-W maximum power are
reported at 1493 nm from a single 200- m stripe laser operating
at 77 K.
Index Terms—Cryogenic, diode lasers, diode-pumped solid-state
laser, Er : YAG, eye-safe, high efficiency, high power.
I. INTRODUCTION
ESONANTLY diode pumped Er-doped lasers are currently viewed as the most promising path to a highly scalable, eye-safe, bulk solid-state laser source [1], [2]. Considerable effort has recently been expended in improving the performance (power and efficiency) of InP-based broad area diode
pump lasers operating in the 14xx–15xx-nm band [3], [4] to
be used for resonant pumping of Er-doped lasers [1], [2]. Operating solid-state lasers with cryogenically cooled gain media
has recently proven to be a viable path toward significant power
scaling without loss of beam quality due to thermal distortions
[5] in specific military applications, such as directed energy,
where the cost implications are manageable. Er-doped media
are shown to benefit most significantly from cryo-cooling [6].
Thus, in systems which are already equipped to provide cryogenic cooling to the solid-state gain medium, the low marginal
cost and effort of extending the cryogen to the semiconductor
pump source makes it both feasible and desirable to implement,
yielding a system which is overall highly efficient and power
scalable.
R
Manuscript received April 29, 2010; revised September 13, 2010; accepted
October 11, 2010. Date of publication October 18, 2010; date of current version December 02, 2010. This work was supported in part by the United States
Army Research Laboratory under Contract W911QX-09-C-0048 and has been
approved for public release.
P. O. Leisher, W. Dong, M. P. Grimshaw, and M. J. DeFranza are with nLight
Corporation, Vancouver, WA 98665 USA (e-mail: pleisher@ieee.org).
M. A. Dubinskii is with the United States Army Research Laboratory,
Adelphi, MD 20783 USA (e-mail: mdubinskiy@arl.army.mil).
S. G. Patterson was with nLight Corporation, Vancouver, WA 98665 USA.
He is now with is with DILAS Diode Laser, Inc., Tucson, AZ 85747 (e-mail:
spatterson@dilas-inc.com).
Color versions of one or more of the figures in this letter are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2010.2088115
It is well established that cryogenic cooling of diode lasers
can provide great benefit to efficiency and power scaling [7]–[9].
For example, Maiorov et al. report 100% increase in the maximum output power of room-temperature optimized InP-based
diode lasers cooled to 80 K [9]. Here we demonstrate that specific optimization of the laser design for use at cryogenic temperatures offers further benefit to the power conversion efficiency. It has been shown that as the temperature of a diode
laser is reduced, the internal quantum efficiency increases and
the threshold current density is reduced [8], [9]. These improvements are attributed to a dramatic reduction in the nonradiative
losses (Shockley–Read–Hall and Auger recombination) and a
reduction in leakage current associated with thermionic emission of carriers from the quantum well [7], [10]–[13].
The high power diode laser community has typically reported
voltage defect as the difference between the diode electrical
voltage and the photon voltage. Here, this quantity is normalized to the photon voltage in order to draw a parallel to the concept of quantum defect in solid state and fiber lasers [1], [2].
is the measured
The voltage defect is defined in (1), where
is the photon voltage, also defined in (1)
diode voltage and
and where is Planck’s constant, is the speed of light, is the
electron charge, and is the operating wavelength of the diode
laser
(1)
Defined this way, the voltage defect provides a simple means to
calculate the maximum achievable power conversion efficiency
in a diode laser [8]. As temperatures decrease, the freeze out of
excited-state dopant carriers results in reduced electrical conductivity in the bulk [14] and further depletion at the epitaxy
heterobarriers. Here we show that a dramatic rise in the diode
voltage results, making voltage defect the dominant source of
power loss in cryogenically cooled diode lasers.
Design of a diode laser structure which achieves optimal
efficiency at 77 K, therefore, requires mitigation of the voltage
defect. Strategies for reducing this may include increasing
doping density, reducing the energy band offsets at the heterobarriers, or changing materials to reduce the dopant ionization
energy, thereby preserving relatively higher carrier densities as
the temperature is reduced. In this work, the p-cladding material chosen is highly doped InGaAsP. In addition to reducing
the energy band offset between the cladding and waveguide,
the approach is shown to reduce the ionization energy of Zn
(the p-type dopant species). These effects combine to produce
a large decrease in the voltage defect of the laser at 77 K
compared to a reference structure based on a more traditional
InP p-cladding layer.
1041-1135/$26.00 © 2010 IEEE
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 22, NO. 24, DECEMBER 15, 2010
Fig. 1. Measured carrier concentration and mobility versus temperature for Zn
Ga
As
P
.
p-type dopant in InP and In
II. EXPERIMENT
Samples of Zn-doped bulk InP and In Ga As P
are grown by metal–organic chemical vapor deposition
(MOCVD) and assessed by cryogenic Hall-effect measurements. Fig. 1 illustrates the measured carrier concentration and
mobility as a function of temperature for the two materials.
The measured carrier concentration fits well to a logarithmic
function, allowing a straightforward extraction of the dopant
ionization energy [14]. The obtained values for ionization enare 18.6 and
ergy of Zn in bulk InP and In Ga As P
11.6 meV, respectively, and are somewhat lower than previously
reported results (47 meV and 22–38 meV, respectively) [14],
[15]. The discrepancy is attributed to shifting of the valence
band edge with respect to the Zn level, effectively reducing
the ionization energy for holes, due to the high doping levels
cm
of the samples reported herein [16], [17].
Based on these results, two diode laser structures are investigated: a reference structure (utilizing a Zn-doped InP p-clad)
based on a commercial high-power design optimized for high
efficiency at room temperature [4] and an experimental design
which is intended for use at 77 K. This experimental design
p-clad
utilizes a uniformly Zn-doped In Ga As P
cm
to account for
with a 60% higher doping density
the relative decrease in hole mobility with respect to Zn-doped
InP. Also, to achieve equivalent electronic confinement at the
intended operating temperature, the quantum well to waveguide
in the experheterobarrier energy difference was scaled by
imental structure (the InGaAsP waveguide bandgap energy was
reduced from 1.2 to 1.1 eV). The two structures are otherwise
identical, with a 700-nm-thick waveguide surrounding an active region utilizing three 70- compressively strained InGaAsP
quantum wells. The lasers are grown by MOCVD on S-doped
InP substrates and wafers follow a standard manufacturing fabrication procedure. Isolation between the 200- m-wide laser
stripes is provided by proton implantation. Bars are cleaved to
1.5-mm cavity length and rear and front facets coated equivalently. Single emitters are cleaved and bonded junction-down to
Cu c-mounts with In solder. Testing occurs in an evacuated cryostat test chamber and power is measured using a thermopile with
a NIST-traceable calibration. The diode voltage is monitored
Fig. 2. (a) Measured voltage versus current and (b) measured voltage defect
at 4 A versus temperature for 200-m broad-area stripe width, 1.5-mm cavity
length lasers of the two designs.
using a dedicated wire pair separate from the current supply;
the packaging resistance is not subtracted.
III. RESULTS
Fig. 2(a) plots the voltage versus current for the two designs
measured at fixed heatsink temperatures of 300 K and 77 K.
The observed decrease in turn-on voltage is primarily attributed
to the 0.1-eV reduction in waveguide bandgap energy in the
improved design. The series resistance was calculated for data
points between 0.6 and 2.0 A (self-heating causes a dramatic reduction in series resistance at higher operating currents), and the
improved design based on the InGaAsP p-cladding is shown to
offer a 65% improvement. Fig. 2(b) plots the measured voltage
defect at 4 A versus temperature for the two designs. The reference InP p-clad structure shows a room-temperature voltage defect of 32% which steadily increases to 52% at 77 K. This limits
the theoretical maximum conversion efficiency (at 4 A, 77 K)
of the design to 48%. The reference InP p-clad design does not
lase at temperatures below 185 K, due to gain reduction caused
by asymmetric filling of the excessively deep quantum wells.
The improved design shows a room-temperature voltage defect
of 14%, but does not lase at this temperature due to thermionic
emission of electrons from the quantum wells. The observable
kink in the curve at 270 K corresponds to the onset of laser operation, and is due to the abrupt change in the overall recombination rate and carrier density at the onset of lasing. The voltage
LEISHER et al.: MITIGATION OF VOLTAGE DEFECT FOR HIGH-EFFICIENCY InP DIODE LASERS
1831
design which mitigates the rise in diode voltage defect with decreasing temperature due to a low (11.6 meV) ionization energy,
effectively reducing the carrier freeze-out which leads to high
bulk resistivity and depletion of the diode heterobarriers.
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Fig. 3. Power and conversion efficiency versus drive current for the 200-m
broad-area stripe, 1.5-mm cavity length InGaAsP p-cladding laser, measured
continuous wave at 77 K. The peak conversion efficiency is measured to be
73% and the chip produces 10-W maximum power. The inset shows the lasing
spectrum measured at 77 K, 4 A.
>
defect of the improved design is shown to be lower and less sensitive to temperature reduction than the reference. At 77 K, the
voltage defect is 22%—less than half the value of the InP p-clad
reference design, limiting the theoretical maximum conversion
efficiency (at 4 A, 77 K) to 78%.
Fig. 3 plots the measured power and conversion efficiency
as a function of drive current for the InGaAsP p-cladding improved laser design measured at 77 K. The absolute error bars
for efficiency are shown and were calculated based on the uncertainty in the power, voltage, and current measurements. At
1 A, the voltage defect is 12%, the differential quantum efficiency is 85%, and the total power conversion efficiency is 73%.
A maximum power of 10.3 W is measured at 20 A. The thermal
impedance of the heatsink and test fixture is 7.9 K/W, yielding
a junction temperature of 205 K at the 20-A injection condition.
The sublinear dependence of power on current at high injection levels is, therefore, attributed to self heating which causes
a rapid increase in threshold carrier density and the total Auger
recombination rate. The inset of Fig. 3 shows the lasing spectrum measured at 4 A, 77 K heatsink.
IV. SUMMARY
In summary, 73% peak conversion efficiency is achieved from
a 1500-nm InGaAsP-based diode laser operating continuous
wave at 77 K. This value is, to the best of our knowledge, the
highest reported conversion efficiency for an InP-based diode
laser operating in the 14xx–15xx-nm wavelength range. The result was enabled by the use of a Zn-doped InGaAsP p-cladding
0
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