Download Monolithic 2D-VCSEL array with > 2W CW and

5
6
SWEENEY, S.J., PHILLIPS, A.F , ADAMS, A R , O‘REILLY, E P., SILVER, M.,
and THIJS, P . J A : “. CLE0’98 Tech. Dig., 1998, Vol. 6, p. 304
ADAMS, A.R., SILVER, M , and ALLAM.
J : ‘Semiconductors and
semimetals’ (Academic Press, 1998), pp. 301-352
PATEL, D., MENONI, c.s , and TEMKIN, H.: ‘Enhanced characteristics
of InGaAsP buried quaternary lasers with pressures up to 1 .SGPa’,
Appl. Phys. Lett., 1993, 62, pp. 2459-2461
8 HIGASHI. T., SWEENEY. S.J , PHILLIPS. A.F.. ADAMS, A R , O’REILLY.E.P.,
UCHIDA. T , and FUJII. r : ‘Observation of reduced non-radiative
current in 1.3pm AlGaInAdInP strained MQW lasers’, submitted
to ZEEE Photonics Technol. Lett.
7
Monolithic 2D-VCSEL array with > 2W CW
and > 5W pulsed output power
D. Francis, H . - L . Chen, W. Y u e n , G. L i and
C. Chang-Hasnain
CW output power in excess of 2W and pulsed output exceeding
5 W were obtained from a monolithic two-dimensional vertical
cavity surface-emitting laser (VCSEL) array. These are the highest
CW and pulsed powers achieved from a monolithic VCSEL
structure to date.
VCSELs in densely packed configurations (60-75 pm centre-tocentre spacing). We mounted each chip p-side down onto an
actively cooled copper block, and AR coated the back side with
Si,N,.
To obtain parallel VCSEL operation, it is important that the
individual devices have uniform characteristics [3]. Our results
from free standing VCSELs show very little variation in both
threshold current and voltages. We did, however, observe a minor
variation, from 1.6to 1.8mW CW, in the maximum output power
of individual emitters in the array. Since the peak power occurs at
a fairly uniform voltage, the array performance was not significantly affected.
We measured the L-I and V-I characteristics of arrays with similar packing densities and total numbers of elements varying from
64 to 1000.Fig. 2 shows typical L-I and V-1 characteristics for a
1000 element array, measured at -1°C heatsink temperature. With
all elements running in parallel, it showed output powers of up to
2.4W CW at 10A. This array at room temperature produced
1.7W CW. The spectral range of the devices was from 944 to
932nm. To demonstrate the scaling of our devices, we compared
the average output per device of arrays with 64 and 1000 lasers.
I
heatsink temperature -1°C
2.0
The ability to fabricate monolithic two-dimensional (2D) diode
laser arrays is a key step towards making high-power lasers at low
cost, because the manufacture of such laser arrays eliminates
labour-intensive fabrication and packaging [ 11. The surface-normal
topology of the vertical cavity surface-emitting laser (VCSEL)
makes it ideally suited to the formation of 2D arrays for high
power applications such as printing, materials processing and the
pumping of solid state lasers. There are two ways to increase the
output power of VCSEL arrays: increase the aperture diameter of
each einitter [2],or increase the total number of emitters packaged
in the arrays. Previous attempts to achieve scalable high power
from VCSEL arrays have been limited by difficulties in thermal
management and non-uniformity between elements of the array.
In this Letter, we report the performance of several VCSEL
arrays. By active cooling of the laser arrays, we were able to
repeatedly generate > 2W CW and >5W pulsed output power at
940nm.These are highest CW and pulsed output powers achieved
from a monolithic VCSEL array to date. To the best of our
knowledge, the highest reported CW output power from a monolithic VCSEL structure was -700mW, while the highest reported
pulsed output was 1.SW.
l45
1
U1
-3
9
-2
-1
0
2
4
current,A
6
8
0
10
Fig. 2 L-I characteristics of 1000 element VCSEL array
light out 940 nm
F
oxide
3
current per VCSEL,mA
1“30131
Fig. 3 Comparison of average output power of 64 and I000 VCSELs
f r o m actively cooled monolithic arrays
2ZO)t
(i) average of‘ 64 VCSELs
Fig. 1 Sclzematic diagram of p-down bottom-emitting V C S E L
(ii) average of 1000 VCSELs
The VCSEL wafer we used is a bottom-emitting structure with
22 bottom distributed Bragg reflector (DBR) mirror pairs of AIAs/
GaAs and 30 top mirror pairs of Al,,Ga,,As/GaAs. Fig. 1 is a
schematic diagram of a VCSEL from the arrays. The active region
consists of three InGaAs quantum wells (QWs). The wafer was
designed for low threshold operation at 950nm and is not optimised for high power operation. Our array elements are oxide
confined VCSELs with 4 5 p mesas and 1 O p i apertures. We
chose the aperture size to maximise the efficiency of the structure
and to minimise potential problems caused by non-uniformity
between array elements. We fabricated arrays with 64 and 256
2132
Fig. 3 shows the comparison. Note that the perfomance of the
individual VCSEL is nearly identical at low input power and starts
to deviate at higher current (>7mA per VCSEL). What appears to
be happening in these cases is that the cooling of the relatively
small, 64 element, laser array occurs laterally as well as vertically
into the broad copper heatsink, while heat from the 1000 lasers
goes primarily straight down into the heatsink, reducing the relative cooling power of the heatsink. The power density of the 64
element array was 161 W/cm2 while the power density of the 1000
element array was 30W/cm2. We believe that the 3OW/cmz figure is
ELECTRONICS LETTERS
29th October 1998
Vol. 34
No. 22
scalable in the present configuration, and with improved cooling,
optimal wafer design, and higher wallplug efficiency, we could
scale up to the 102Wicm2level.
We also tested the laser arrays under pulsed operation. The testing conditions were similar to those of the CW tests. The L-I characteristics for the 1000 element array measured under two
different pulse conditions (100p and 3 0 p pulsewidth, both at
10Hz) are shown in Fig. 4 a Given the configuration of our pulser,
we were unable to drive the lasers below 3A of input current.
T ~ u s our
, L-I characteristics reflect only the output power above
threshold. In QCW mode with 1 0 0 p pulses we see outpIts of
-4W. In shorter pulse operation with 3 0 p pulses, we see outputs
exceeding 5W peak power. We also looked at the effect 'of the
duty cycle on the output performance of the array. Results are
shown in Fig. 4b. As the driving power increases to nearly 50W
(3.1V at 16A of drive current) the array power begins to decrease
as the duty cycle reaches -10'%1, due to rise in the average temperature of the substrate.
6,
,
,
,
,~
....,......,,,........,.,.,
References
1
CATCHMARK, I M.,
ROGERS, L.E ,
MOKGAN. R A ,
ASOM, M T.,
and CHKISTODOULIDES, D N : 'Optical charateristics o f
multitransverse-mode two-dimensional vertical-cavity top surfaceemitting laser arrays', IEEE J. Quantum Electron., 1996, QE-32,
( 6 ) , pp. 986 -995
2 WIPIEJEWSKI, T., PETERS, M G , THIBEAULT, J J , YOIJNG, D.R , and
COLDREN, L.A.: 'Size-dependent output power saturation of verticalcavity surface-emitting laser diodes', ZEEE Photonics Teclinol.
Lett., 1996, 8, ( I ) , pp. 10-12
3 CHOQLlETl~E,K.D , HOU, H Q , G E N . K.M., and HAMMONS. B.E.:
'Uniform and high power selectively oxidized 8x8 VCSEL array'.
I997 Dig. IEEEiLEOS Summer Topical Meetings, Montreal,
Quebec, Canada, 11-15 August 1997, pp. 11-12
GIJ mi, G.D.,
RT pulsed operation of metamorphic VCSEL
at 1.55pm
J. Boucart, C. Starck, A. Plais, E. Derouin, C . Fortin,
E;. G a b o r i t , A. Pinquier, L. Goldstein, D. Carpentier
a n d J. Jacquet
01
0
a
'
'
5
10
'
15
20
current,A
I.....
10.'
b
""""'
.
""""
'
IO0
101
duty cycle,%
~-
43014
Fig. 4 L-I characteristics an pulsed output p o w r from 1000 element
VCSEL arrrry
a L-l characteristics of' 1000 element VCSEL array measured a1 l0Hz
undcr pulse conditions
(i) 3 0 p pulses
(ii) 1 0 0 pulses
~
h Pulsed output power from 1000 element VCSEL array measnrcd at
various duty cycles
100ps pulses driven with up to 3V
(i) 16A
(ii) 13A
The maximum power obtained under pulsed operation IS only
two times higher than that of CW. The relatively small dif'erence
in output powers between CW and pulsed modes is strong evidence that the cooling of VCSEL arrays is very effective arid that
heat generated during operation can be quickly removed from the
active region, unlike (Rack & Stack) EEL arrays, where the cooling of laser structures is much more difficult because its active
region can be as much as lmm away from the heatsink.
We developed an analytical model to determine the impact of
VCSEL non-uniformity on the output performance of the array.
Similar to EEL arrays, the most important parameter for uniform
operation in VCSEL arrays is its turn-on voltage. However,
VCSEL arrays are not susceptible to thermal runaway and can
tolerate much larger performance variations from individual
devices. This tolerance results from the relatively high differential
resistance (-lCYpmz or 1006Ylaser), mostly from the top DBR
[2, 31. The top DBR acts as a current limiter preventing ihermal
runaway.
It is clear that the ultimate performance of the arrays depends
heavily on the cooling efficiency of the heatsink. In the future,
with the use of commercial microchannel heatsinks, we 3lan to
further increase the cooling capacity of the heatsink and consequently to increase the performance of large area VCSEL arrays.
Aclcnowledgments: This work is supported by ONR & BMDR
grant number N00014-96-1-1200 and by the University of California under the Campus Laboratory Collaborations Program.
I .Septrmhi,i 1998
0 IEE 1998
Electronics L e t t ~ rOnline
.~
No: 19981517
D. Francis, W. Yuen, G. Li and C. Chang-Hasnain (Univc,rsitJ (1j
Culfornin, Berkeley. CA 94720, U S A )
H.-L. Chen (Lawrence Livermore Nutiorial Lahouutoiy, Liveuniure, CA
94550, USA)
ELECTRONlCS LETTERS
29th October 1998
Vol. 34
An all molecular beam epitaxy grown 1 . 5 5 ~vertical cavity
surface-emitting laser is presented which comprises an InPI
TnGaAsP bottom mirror, multiple quantum well active layer and
a GaAlAsiCaAs metamorphic top mirror directly grown on the
InP cavity. This structure takes advantage of the intrinsic optical,
elcctrical and themml properties of GaAlAsiGaAs material and is
compatible with a 2in process. Such an approach will therefore
lead to a drastic reduction in the cost of optical sources and offer
the possibility of a massive development of the optical network.
Introduction: Long wavelength 1 . 5 5 p VCSELs (vertical cavity
surface-emitting lasers) are well suited to a great variety of fibre
communication systems due to inherent advantages of the structure. The output beam has intrinsically low and circular divergence. Consequently, efficient coupling and simplified pigtailing
contribute to the relaxation of positioning tolerances. This advantage, combined with the on-wafer testing capability and the high
density of devices on a wafer, is a key factor towards a large
reduction in the loss or modules, since the test, mounting and
assembly time represent more than half the cost of an optical
source. VCSELs are intrinsically single longitudinal mode sources,
and the fabrication yield is therefore close to 100%; this is of parlkular interest in the case of 2D array fabrication for parallel optical interconnections. Finally, owing to the low volume of the
active region, VCSELs can operate over a wide range of temperam-es with low power consumption.
In recent years, much work has been conducted to achieve room
temperature (RT) continuous wave operation (CW) of long wavellength (1.3 and 1 . 5 5 ~ VCSELs.
)
The greater difficulties experienced in obtaining long wavelength operation arise from the low
index contrast between lattice matched materials on InP. Much
effort has been focused on the design and fabrication of high performance Bragg mirrors [I, 21. The wafer bonding of GaAsi
GaAlAs Bragg reflectors on both sides of an InP based active cavity represented a technical breakthrough for long wavelength
VCSEL fabrication and has led to the best results at 1 . 5 5 [3]
~
and 1 . 3 [4].
~ The advantage of this approach lies in the electrical
conductivity of this material as well as its efficient heat dissipation. This technique, however, can be limited to small areas and
requires the use of two or three substrates, leading to dramatic
increases in cost.
In this Letter we propose an attractive alternative to the wafer
bonding technique and demonstrate an all molecular beam epitaxy
(MBE) grown VCSEL. The structure comprises an InPiInGaAsP
bottom mirror, strained MQW (multiple quantum well) active
layer and GaAs/GaAIAs metamorphic mirror (see Fig. 1) directly
grown on the active stack in a 2in planar process [5, 61. In this
approach, the intrinsic advantages of GaAsiGaAIAs matcrial and
the low cost of VCSELs are exploited. Recently, a similar stmcture has been proposed and demonstrated [7]; the GaAs based
mirror is, however, not electrically active in this case, and lateral
current injection requires a complex process (such as growth on
patterned substrates or AllnAs oxidation).
No. 22
2133