High slope efficiency measured from a composite-resonator vertical-cavity laser

Abstract

We report high differential slope of the light versus current (L-I) characteristic in excess of 400% external quantum efficiency from a monolithic dual resonator vertical-cavity surface-emitting laser. The additional optical cavity of the composite resonator can provide gain or loss to the distributed laser mode, depending on the bias conditions. We describe the factors contributing to the internal optical loss, and present a qualitative model for the L-I characteristic. With sufficient current injected into the top cavity, the composite-resonator vertical-cavity laser achieves over 6 W/A of differential slope efficiency from threshold to greater than 1 mW of output power, which may be applicable for analog optical data links.

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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 9, MAY 1, 20061019
High Slope Efficiency Measured From a
Composite-Resonator Vertical-Cavity Laser
D. M. Grasso, K. D. Choquette, D. K. Serkland, G. M. Peake, and K. M. Geib
Abstract—We report high differential slope of the light versus
current (L–I) characteristic in excess of 400% external quantum
efficiency from a monolithic dual resonator vertical-cavity
surface-emitting laser. The additional optical cavity of the com-
posite resonator can provide gain or loss to the distributed laser
mode, depending on the bias conditions. We describe the factors
contributing to the internal optical loss, and present a qualitative
model for the L–I characteristic. With sufficient current injected
into the top cavity, the composite-resonator vertical-cavity laser
achieves over 6 W/A of differential slope efficiency from threshold
to greater than 1 mW of output power, which may be applicable
for analog optical data links.
Index Terms—Coupled cavity, optical link, slope efficiency.
I. INTRODUCTION
V
communications, including high modulation bandwidth, low
power consumption, and relative ease of manufacturing [1].
VCSELs are being considered as a potential low-cost solu-
tion for optical-fiber-fed radio-frequency (RF) links, and data
rates of over 15 Gb/s have been demonstrated with 1 km of
high-bandwidth multimode fiber [2], [3]. A thorough experi-
mental study has been conducted on the microwave properties
of oxide-confined VCSELs under dc injection and direct mod-
ulation [4]. One conclusion of this prior work is that although
multimode VCSELs can provide the necessary bandwidth and
dynamic range to meet requirements for link gain, the differen-
tial slope efficiency of the light–current (L–I) characteristic is a
significantsourceofRFlossinasystemwithdirectmodulation.
This is in agreement with the current gain calculations for a
directly modulated link given in [5]. Therefore, improvement
of the slope efficiency may enable future VCSEL-based link
applications. The increase in efficiency without simultaneous
increase in threshold current is possible through the use of mul-
tielement devices. Previous results on cascade VCSEL arrays
[6] and bipolar cascade VCSELs [7] have shown efficiencies of
180% from a 2
3 array and 130% from a single device, re-
spectively. Similar work using series-connected edge-emitting
lasers has shown slope efficiencies of up to 390% for a 12-stage
device [8].
ERTICAL-CAVITY surface-emitting lasers (VCSELs)
offer many attractive properties for use in short-distance
Manuscript received November 17, 2005; revised February 6, 2006. This
work was supported by National Science Foundation Grant 0121662.
D. M. Grasso and K. D. Choquette are with the Micro and Nanotech-
nology Laboratory, Department of Electrical and Computer Engineering,
University of Illinois at Urbana-Champaign, Urbana, IL 61801 USA (e-mail:
grasso@uiuc.edu).
D. K. Serkland, G. M. Peake, and K. M. Geib is with the Sandia National
Laboratories, Albuquerque, NM 87185 USA.
Digital Object Identifier 10.1109/LPT.2006.873544
Fig. 1. CRVCL fabrication process up to planarization.
In this letter, we show high differential slope efficiency
of over 6 W/A ( 400% quantum efficiency at 850 nm) near
threshold from a single VCSEL with two electrically indepen-
dent, optically coupled active regions. This device is referred
to as a composite-resonator vertical-cavity laser (CRVCL)
[9], [10]. Previous work has analyzed the effect of the second
optical cavity on the threshold and modal characteristics over a
broad operating temperature range [11], [12]. We will discuss
the fabrication of the dual oxide-confined cavity CRVCL, the
continuous-wave operating characteristics, and a qualitative
model of this performance.
II. DEVICE STRUCTURE AND FABRICATION
The CRVCL in this study is grown by metal–organic
chemical vapor deposition and is composed of a monolithic
bottom p-type distributed Bragg reflector (DBR) with 35
periods, a middle n-DBR with 11.5 periods, and an upper
p-DBR with 19 periods. Each period of the DBR consists
of Al
Ga As–AlGa
gradingbetweenthem.TheDBRmirrorsseparatetwo1- -thick
optical cavities (where
is the lasing wavelength), each of
which contain five GaAs quantum wells (QWs).
The basic fabrication procedure is shown in Fig. 1. A ring
contact of 150/1500-
Ti–Au is deposited on the top surface,
and a broad-area contact of200/2000-
back side of the sample. An inductively coupled plasma reac-
tive-ionetch(ICP-RIE)ofSiCl –Ar isusedtodefineanapprox-
imately 100 100 m lower mesa. Subsequently, a single wet
oxidation of two Al
Ga As layers is done to define a cur-
rent aperture for each cavity. The oxide layer for the top cavity
is placed in the middle n-DBR. This resulting layer is used as an
etch stop for a second ICP-RIE step that defines a smaller upper
As layers with compositional
Ti–Au deposited onthe
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1020 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 9, MAY 1, 2006
Fig. 2.
labeled on plot. (b) Differential quantum efficiency at threshold versus top
cavity current.
(a) Light output versus bottom cavity current. Top cavity currents
mesa of approximately 40
moved with a 45% wt KOH:H O solution [13], and a middle
ring contact of 400/200/1400-
the layer exposed by the KOH removal. The resulting structure
is shown in Fig. 1, Step 4. The CRVCL is then planarized with
HD Microsystems HD-4001 polyimide to facilitate deposition
of ground–signal–ground coplanar waveguide contacts for the
top cavity. The CRVCL is fabricated in a double mesa struc-
ture to allow independent electrical injection into each cavity.
We find that the upper oxide layer is several micrometers larger
than the lower one, and thus, defines the transverse diameter
of the optical cavity. We attribute the difference in oxide layer
lengths to very slight epitaxial nonuniformity during material
growth. The device used in this study has a lower oxide aper-
ture of approximately 5 m, and an upper oxide aperture of
approximately 10
m. Most of the CRVCLs measured cannot
reach threshold with current only injected into the top cavity;
this may be due to lower current confinement from the oxide
aperture placement in the n-DBR as well as the larger aperture
size.
40 m. The oxidized layer is re-
AuGe–Ni–Au is deposited on
III. RESULTS AND DISCUSSION
Fig. 2(a) shows the measured L–I characteristics of the
CRVCL. The top cavity is held at a fixed current for each curve
in Fig. 2, while the bottom cavity current is varied from 0 to
10 mA. The upper cavity current increases in steps of 2 mA be-
tween each curve. As the current in the top cavity is increased,
the current in the bottom cavity required to reach threshold is
reduced [11]. In addition, the differential quantum efficiency of
the device increases. As shown in Fig. 2(b), if 10 mA is applied
to the top cavity, the slope efficiency becomes larger than the
maximum possible for a single cavity laser. Fig. 3 shows a
close up of the L–I characteristic corresponding to a top cavity
current of 20 mA. Near threshold, the slope is approximately
6.35 W/A, which is greater than 400% differential quantum
efficiency. Above threshold, the slope decreases as the bottom
cavity current is increased. There are variations in the max-
imum slope efficiency achievable from device to device, but
the results of decreasing threshold current and increased slope
efficiency with increasing top cavity current are representative
of all lasers tested. We have confirmed that the large slope
efficiency and decrease in threshold current with increased
upper cavity current persists under pulsed operation, and thus
Fig. 3.
20 mA.
Close-up of L–I characteristic near threshold with top cavity current of
are not the result of thermal lensing effects [14]. In addition,
although the CRVCL has two longitudinal resonances [8], the
device in this study lases in a single longitudinal mode shifting
by approximately 4 nm over the entire current range in Figs. 2
and 3. Multiple transverse modes exist at threshold, and follow
a standard VCSEL modal evolution as the current in either
cavity is increased [11]. Therefore, the improvement in effi-
ciency is not related to additional modes reaching threshold as
the top cavity current is increased. In the analysis that follows,
we describe the gain and loss processes that contribute to the
external quantum efficiency, and develop a phenomenological
description that is consistent with the L–I data shown in Figs. 2
and 3.
AuniquefeatureoftheCRVCLisabilitytoreducethebottom
cavity current required to reach threshold and simultaneously
increasetheslopeefficiencybyincreasingthetopcavitycurrent.
Thelight output
versuscurrentinjected intothebottom cavity
above threshold is given by [15]
(1)
where
the internal and mirror losses corresponding to a threshold cur-
rent of. From (1), the key parameter changing in Fig. 2 with
additional top cavity current is
in a QW laser can have many contributing mechanisms. How-
ever, we can group the important processes in the CRVCL into
two categories
is the internal quantum efficiency, and andare
. The internal loss coefficient
(2)
wheretheprimedtermdependsonthetopcavitycurrentandthe
term does not. The first term represents distributed optical
loss present in every semiconductor laser. The magnitude of the
primed termis determined bythematerial gainof thetop cavity,
and is negative if current is supplied to provide sufficient gain.
Althoughthe
terminaconventionallasercanalsohaveacur-
rent dependence [16], we ignore these effects. There is an im-
plicit temperature dependence in the
term, since an increase

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GRASSO et al.: HIGH SLOPE EFFICIENCY MEASURED FROM A CRVCL1021
in current corresponds to an increase in temperature. The cur-
rent-dependent term is most important, since we are concerned
with the change in efficiency with changing top cavity current.
In addition, the available modal gain
termined by an effective threshold that includes the overlap of
the distributed optical mode with the material gain of both ac-
tive regions [17]
in the CRVCL is de-
(3)
where
mode with the material gains
In addition, the spectral overlap of the two cavity resonances
and material gain of each cavity can affect the threshold gain.
Considering (1)–(3) together, it is evident that the additional
gain or loss from the top cavity determines the slope efficiency
and
.
In a conventional VCSEL, the differential slope of the
L–I characteristic decreases above threshold and eventually
becomes negative due to spectral misalignment of the cavity
resonance with the material gain of the QWs with increasing
injection current. In addition, the internal quantum efficiency
decreases with increasing temperature. In the CRVCL, there
are two independent currents injected into the device, each
of which can increase the temperature in the active regions
and DBRs. Therefore, we expect the CRVCL to show the
same (and possibly larger) reduction in slope efficiency with
increasing current as a single-cavity VCSEL. It can be seen in
Fig. 2 that as the top cavity current is increased, the change
in slope efficiency is evident over a larger range of the L–I
characteristic. In addition, the reduction in threshold current
is smaller between incremental L–I curves as the top cavity
current is increased and
mA. This phenomena is consistent with the thermal
effects of a conventional VCSEL and temperature-dependent
threshold current. Finally, the current-dependent mechanisms
in [16] can only further increase the optical loss with increasing
temperature and current.
To summarize, the increase in slope efficiency and decrease
in bottom cavity current required to reach threshold is caused
by the additional gain from the top cavity. This corresponds to
reductionin
nearthresholdas
current dependent term in (2). The magnitude of this term is
influencedbythemodalgainavailablein(3).Thecorresponding
increase in slope efficiency cannot be maintained overthe entire
L–I curve, similar to a conventional VCSEL.
andrefer to the spatial overlap of the optical
and of the two cavities.
at maximum decreases for
isincreased,causedbythe
IV. CONCLUSION
We report high differential slope of the L–I characteristic
from a CRVCL in excess of 400% quantum efficiency from
threshold to more than 1 mW of power. The increase in slope
efficiency and decrease in current required to reach threshold is
caused by the additional modal gain from the top cavity. Com-
pared with previous VCSEL work in [6] and [7], the CRVCL
obtainshigherefficiency,butoverasmallerrangeofitsL–Ichar-
acteristic. It may be possible to improve the linearity of the de-
vices through spectral detuning of the two sets of material gains
with respect to the cavity resonance or with one another, thus
achieving lasing over a broader temperature range. In addition,
any improvement to the internal quantum efficiency at elevated
temperatures, if possible, will enhance the linearity. Further im-
provementsofthethermalpropertiesmayenablefutureCRVCL
applications requiring RF link gain.
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