hv
photonics
Article
Engineering Multi-Section Quantum Cascade Lasers
for Broadband Tuning
Steven Slivken and Manijeh Razeghi *
Center for Quantum Devices, Electrical Engineering and Computer Science Department,
Northwestern University, Evanston, IL 60208, USA; [email protected]
* Correspondence: [email protected]; Tel.: +1-847-491-7251
Received: 1 June 2016; Accepted: 17 June 2016; Published: 27 June 2016
Abstract: In an effort to overcome current limitations to electrical tuning of quantum cascade lasers,
a strategy is proposed which combines heterogeneous quantum cascade laser gain engineering with
sampled grating architectures. This approach seeks to not only widen the accessible spectral range
for an individual emitter, but also compensate for functional non-uniformity of reflectivity and gain
lineshapes. A trial laser with a dual wavelength core is presented which exhibits electroluminescence
over a 750 cm´1 range and discrete single mode laser emission over a 700 cm´1 range. Electrical
tuning over 180 cm´1 is demonstrated with a simple sampled grating design. A path forward to even
wider tuning is also described using more sophisticated gain and grating design principles.
Keywords: monolithic tunable laser; quantum cascade laser; sampled grating
1. Introduction
One of the primary appeals of the quantum cascade laser (QCL) is its ability to access a wide
portion of the infrared spectrum (3 < λ < 11 µm) in continuous operation at room temperature [1,2].
High output power and a small form factor are also key characteristics. This makes the QCL a very
important, even enabling, technology for many applications which would benefit from portability,
such as spectroscopy. While initial development concentrated on fixed frequency (or mildly tunable)
applications, recent years have seen a resurgence in QCL development based on wide wavelength
coverage from a single device.
The first step to achieving this is to demonstrate a laser medium with an extremely broad gain
bandwidth. With a QCL, a single, homogeneously broadened emitter can exhibit several hundred
inverse centimeters of bandwidth, especially at shorter wavelengths. This is one advantage of an
intersubband emitter over a mid-infrared interband laser like the interband cascade laser (ICL). The
second advantage of the QCL relies on the intersubband gain lineshape, which has very little absorption
adjacent to the primary gain peak (when properly designed). This allows multiple wavelength emitters
to be combined within the same laser waveguide (a so-called heterogeneous QCL) without significant
cross-absorption. All emitters can emit simultaneously, or a single wavelength can be selected with the
proper feedback.
While a single homogeneous QCL might have a gain bandwidth of 200 cm´1 (24.8 meV), a
properly designed heterogeneous QCL can exhibit significantly broader bandwidth. Our group has
recently demonstrated a room temperature long wavelength infrared (LWIR) DFB laser array capable
of emission wavelengths from 5.9–10.9 µm [3]. This represents a bandwidth of 760 cm´1 (94 meV). The
waveguide core of this wafer was composed of six different emitting stages. This covers a significant
portion of mid-infrared spectral region and will be part of a new generation of tunable infrared sources.
Another, as yet unpublished, initial effort was made to generate an ultra-broadband gain region
used eight emitting stages. This wafer showed electroluminescence that covered the entire 4–12 µm
Photonics 2016, 3, 41; doi:10.3390/photonics3030041
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Photonics 2016, 3, 41
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wavelength region (>1600 cm´1 ), although some additional optimization is necessary to flatten the
gain distribution.
The second step to achieving broadband emission from a laser is to provide a means of adjustable,
wavelength-selective feedback. One of the more common methods of doing this relies on external
cavity feedback via a diffraction grating. Overall, this is very successful, and it has been applied
to heterogeneous QCLs in the past. However, this technique relies on multiple optical components
beyond the QCL gain medium [4]. Precise optical alignment is also required to maintain smooth
operation. Finally, the tuning is traditionally mechanical in nature, which has a limited tuning speed.
Some recent demonstrations, however, have significantly increased the tuning speed using MEMs or
acousto-optic elements, but the other restrictions still apply [5,6].
The more robust alternative that many groups are now developing is tuning via the Vernier effect.
In this arrangement, the laser is composed of at least two sections with comb-like reflectivity behavior.
When the comb spacing of the two sections is different, the combs do not overlap perfectly, and only
certain wavelength have strong feedback in both sections. When the grating reflectivity envelope is
properly aligned to the gain bandwidth of the laser, the resultant coupled cavity can exhibit single
mode output. Adjusting current or temperature in one of the sections causes a shift in the overlap,
which can lead to a large, controllable, shifts in the output wavelength.
Though there are many geometries that can exhibit the Vernier effect, one of the most successful
for the QCL at present is the sampled grating distributed feedback (SGDFB) laser. This architecture
is similar to that developed for electrical tuning of telecom lasers [7], and was demonstrated for the
QCL in 2012 [8]. Over an order of magnitude, improvement in tuning was demonstrated compared
to traditional single mode DFB QCLs. Since then, multiple demonstrations of this technology have
been made, leading to mid-infrared lasers with published pulsed and continuous wave tuning ranges
of 243 cm´1 and 120 cm´1 from individual lasers, respectively [9,10]. Single mode power output
has also been increased significantly, with over 5 W (1.25 W) tunable peak (continuous wave) output
power demonstrated from single devices [10]. Even wider tuning is possible with more advanced
grating designs, like the superstructure grating (SSG), with predicted tuning limited only by the gain
bandwidth of the laser itself [11].
Other interesting configurations are also being investigated. While one SGDFB has a broad range,
an array of SGDFB lasers has even a wider range. This was demonstrated in the mid-wavelength
infrared (MWIR, 3 < λ < 5 µm) shortly after the first SGDFB QCL, and showed a pulsed wavelength
coverage of 351 cm´1 at room temperature in pulsed mode [12]. Also, combining the SGDFB with an
additional distributed Bragg reflector section has been used to generate a tunable dual wavelength
laser. This was utilized recently as an on-chip pump source for room temperature, single mode, tunable
continuous wave THz emission [13].
2. Combining Broadband Gain and Electrical Tuning
The next evolutionary step in tunable QCL development is the combination of broadband gain
media with Vernier-type feedback. The first demonstration builds on the 6-core laser design from [3].
An 8-laser SGDFB array, each with a shifted central wavelength, was demonstrated on a similar
broadband wafer that can emit at almost any wavelength within the 6–10 µm wavelength range.
Additionally, an on-chip beam combiner was developed which emits all light out of a single aperture.
The laser was incorporated into a system capable of rapid scanning over the whole tuning range [14].
Though this is impressive, it has a somewhat complex control scheme. If spectral coverage is
preferred over continuous tuning, another approach is under investigation. A standard sampled
grating has a reflectivity envelope width (of the central lobe) that is inversely proportional to the
number of gratings periods (Ng ) within one sampling period. This can lead to very wide envelopes,
with a full width at half maximum (FWHM) up to 500 cm´1 for Ng = 4. The consequence is the need
for a longer cavity (more sampling periods) in order to build up sufficient grating reflectivity for single
mode operation. Also, the reflectivity is peaked, which leads to non-uniform threshold gain for a
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standard QCL. However, this can be efficiently compensated for by using a dual core (dual emitter)
QCL,
whose
modal
gain
can be
engineered
to partially
for the varying
grating
reflectivity.
emitter)
QCL,
whose
modal
gain
can
engineered
partiallycompensate
compensate
varying
grating
emitter)
QCL,
whose
modal
gain
can be engineered
totocompensate
partially
forfor
thethe
varying
grating
The
progression
from
a simplefrom
SGDFB
to this
dual core
approach
isapproach
shown
in
Figure
1. Figure
reflectivity.
The
progression
aa simple
SGDFB
this
dualcore
core
approach
is shown
in Figure
reflectivity.
The
progression
from
simple
SGDFB
totothis
dual
is shown
in
1. 1.
Figure 1. Schematic evolution of original sampled grating distributed feedback (SGDFB) quantum
Figure 1. Schematic evolution of original sampled grating distributed feedback (SGDFB) quantum
cascade
laser (QCL)evolution
to a broadband
dual core
SGDFB.grating distributed feedback (SGDFB) quantum
Figure
1. Schematic
of original
sampled
cascade laser (QCL) to a broadband dual core SGDFB.
cascade laser (QCL) to a broadband dual core SGDFB.
A MWIR dual core QCL has been designed to test the proposed methodology. The emitters used
MWIR
QCL
been
designed
tototest
the
proposed
methodology.
emitters
used
in
inAA
the
laserdual
core
are
based
on
the
high
efficiency
λtest
= 4.9
μm
laser demonstrated
byThe
our
group.
The
MWIR
dualcore
core
QCLhas
has
been
designed
the
proposed
methodology.
The
emitters
used
the
coredesign
are based
onused
theonhigh
efficiency
λ = 4.9λµm
laser
by our
group.
The
primary
change
tothe
shift
theefficiency
wavelength
is
adjustment
of the 1st
quantum
well
inprimary
theThe
in laser
the laser
core
are
based
high
= the
4.9
μmdemonstrated
laser demonstrated
by our
group.
active
region
from
5
(λ
=
5.2
μm)
to
2
(λ
=
4.2
μm)
monolayers.
These
wavelengths
are
appropriate
design
change
used
to
shift
the
wavelength
is
the
adjustment
of
the
1st
quantum
well
in
the
active
primary design change used to shift the wavelength is the adjustment of the 1st quantum well in
the
considering
the
gain
FWHM
of
this
lasermonolayers.
design is typically
400–500 cm−1. are
Minor
adjustments
(1–2
region
from
5
(λ
=
5.2
µm)
to
2
(λ
=
4.2
µm)
These
wavelengths
appropriate
considering
active region from 5 (λ = 5.2 μm) to 2 (λ = 4.2 μm) monolayers. These wavelengths are appropriate
monolayers)
werethis
alsolaser
madedesign
to a few
other
layers
just to adjust
of the energy(1–2
levels.
These
the
gain FWHM
typically
400–500
cm´1alignment
. Minor adjustments
monolayers)
considering
the of
gain FWHM
of thisislaser
design
is typically
400–500
cm−1. Minor adjustments
(1–2
altered
designs
have
not
yet
been
optimized
for
efficiency.
The
waveguide
core
starts
with
22
periods
were
also
made
to
a
few
other
layers
just
to
adjust
alignment
of
the
energy
levels.
These
altered
designs
monolayers) were also made to a few other layers just to adjust alignment of the energy levels. These
of the longer wavelength emitter and 18 periods of the shorter wavelength emitter. This asymmetry
have
notdesigns
yet been
optimized
for efficiency.
waveguideThe
core
starts with
22 starts
periods
of 22
theperiods
longer
altered
have
not yet been
optimizedThe
for efficiency.
waveguide
core
with
is due the presence of a 450 nm thick GaInAs grating layer, which is also part of the waveguide.
wavelength
emitter
and 18emitter
periods
of theperiods
shorterofwavelength
emitter. This
asymmetry
is due the
of the longer
wavelength
and
the shorter
emitter.
This asymmetry
The layer
structure (as
shown
in 18
Figure 2) was
grown
on n-wavelength
InP by gas-source
molecular
beam
presence
of
a
450
nm
thick
GaInAs
grating
layer,
which
is
also
part
of
the
waveguide.
is due
the without
presence
ofupper
a 450 cladding
nm thickand
GaInAs
grating
layer,two
which
is also
of theThe
waveguide.
epitaxy
the
cap layer.
Initially,
pieces
werepart
set aside.
first piece
The
layer
shown
in
nby gas-source
molecular
beam
The
layerstructure
structure
(as
shownby
inFigure
Figure 2)
2) was
was grown
grown
on
n- InP
InP
beam
had
diffraction
gratings(as
fabricated
e-beam
lithography
andon
plasma
etching.
Uniform molecular
DFB
gratings
epitaxy
without
the
upper
cladding
and
cap
layer.
Initially,
two
pieces
were
set
The
first
epitaxy
without
the
upper
cladding
and
cap
layer.
Initially,
two
pieces
aside.
The
first
piece
of varying period were defined. Both pieces then had the upper cladding and cap layers depositedpiece
had
diffraction
Uniform DFB
DFBgratings
gratings
had
diffraction
gratingsfabricated
fabricatedby
bye-beam
e-beamlithography
lithography and
and plasma etching. Uniform
by
MOCVD. gratings
ofofvarying
cap layers
layers deposited
deposited
varyingperiod
periodwere
weredefined.
defined.Both
Both pieces
pieces then
then had
had the
the upper
upper cladding and cap
by
byMOCVD.
MOCVD.
(a)
(b)
Figure 2. (a) Schematic layer structure of dual core mid-wavelength infrared (MWIR) QCL; (b) Right
axis: Normalized electroluminescence and Fabry Perot laser emission from the dual core wafer. Left
(a)
(b)
axis: Threshold current density of DFB lasers according to targeted emission wavenumber. Points at
the top
left
represent DFB
lasers
whichof
exhibit
only
multimode
emission.
Figure
(a)Schematic
Schematic
layer
structure
of
dual core
core mid-wavelength
mid-wavelength
infrared
Figure
2.2.(a)
layer
structure
dual
infrared (MWIR)
(MWIR) QCL;
QCL; (b)
(b) Right
Right
axis:Normalized
Normalizedelectroluminescence
electroluminescence and
and Fabry
Fabry Perot
Perot laser
laser emission
axis:
emission from
from the
the dual
dual core
core wafer.
wafer. Left
Left
axis:
Threshold
current
density
of
DFB
lasers
according
to
targeted
emission
wavenumber.
Points
axis: Threshold current density of DFB lasers according to targeted emission wavenumber. Pointsat
at
thetop
topleft
leftrepresent
representDFB
DFBlasers
laserswhich
whichexhibit
exhibitonly
onlymultimode
multimode emission.
emission.
the
Photonics 2016,
2016, 3,
3, 41
41
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of 66
44 of
The first sample was fabricated into several identical arrays of DFB lasers, with some Fabry Perot
first sample
was DFB
fabricated
intoThe
several
identical
of DFB lasers,
with some
Fabry
(FP) The
reference
lasers (no
grating).
second
piece arrays
was fabricated
into circular
mesas
for
Perot
(FP)
reference
lasers
(no
DFB
grating).
The
second
piece
was
fabricated
into
circular
mesas
measurement of intersubband electroluminescence (EL). Figure 2 shows testing data for these pieces
for
measurement
of intersubband
electroluminescence
Figure
2 shows
testing
datais for
these
of the
dual core wafer.
The most noticeable
feature is that(EL).
the EL
shows
two peaks,
which
matched
pieces
of the
core peaked
wafer. The
most noticeable
is that
the EL
shows
twoofpeaks,
which
is
fairly well
bydual
the dual
multimode
emissionfeature
from the
FP laser.
The
FWHM
the EL
is over
−1
matched
fairly
well
by
the
dual
peaked
multimode
emission
from
the
FP
laser.
The
FWHM
of
the
EL
750 cm . Also shown are the threshold current densities from two identical DFB arrays as a function
1 . Also shown are the threshold current densities from two identical DFB arrays as a
is
750 cm´emission
of over
the targeted
wavenumber. The DFB array showed single mode emission over the 4–5.5 μm
function
of
the
targeted
wavenumber.
The DFB
arrayisshowed
single
mode
over
wavelength range (700 emission
cm−1). While
the threshold
behavior
fairly flat
over
thisemission
range, some
´
1
the
4–5.5 µm wavelength
range (700
cmnecessary.
). While the threshold behavior is fairly flat over this range,
improvement
in gain balancing
is still
someBased
improvement
in gain
balancing
is an
stillearly
necessary.
on this initial
wafer
testing,
stage dual core SGDFB was also fabricated from the
Based
on
this
initial
wafer
testing,
an
early
dual acore
SGDFB
also fabricated
fromThe
the
same wafer. Unlike previous SGDFB fabrication,stage
however,
buried
ridgewas
geometry
was utilized.
same
Unlike
SGDFB
fabrication,
however,
a buriedreflectivity
ridge geometry
was utilized.
The
main wafer.
purpose
wasprevious
to try and
produce
a more
ideal grating
by reducing
parasitic
main
purpose
was
try and produce
a more
ideal grating of
reflectivity
reducing
reflections
from
thetowaveguide
sidewalls.
A cross-section
a finishedby
device
(w =parasitic
8.48 μm),reflections
is shown
from
the waveguide
sidewalls. A cross-section of a finished device (w = 8.48 µm), is shown in Figure 3.
in Figure
3.
Figure 3.
3. (a)
Figure
(a) Top
Top view
view schematic
schematic of
of two
two section
section laser
laser with
with sampled
sampled grating
grating design
design specifics.
specifics.
Λ
g
=
grating
period.
Λ
s
=
sampling
period
(b)
SEM
cross-section
of
a
buried
ridge
SGDFB
Λg = grating period. Λs = sampling period (b) SEM cross-section of a buried ridge SGDFB laser.
laser.
(c) Tuning
Tuning characteristic
characteristic of
of aa dual
dual core
core SGDFB
SGDFB laser.
laser.
(c)
Several different
different grating
grating periods
periods and
and ratios
ratios of
of sampling
sampling periods
periods between
between sections
sections were
used to
to
Several
were used
explore
the
tuning
behavior.
All
designs
were
based
on
5
mm
long,
uncoated
cavities,
in
order
to
get
explore the tuning behavior. All designs were based on 5 mm long, uncoated cavities, in order to get as
as much
as possible.
number
of sampling
periods
(Ns)varied
was varied
between
7 and
20 per
much
datadata
as possible.
The The
number
of sampling
periods
(Ns ) was
between
7 and 20
per section,
section,
and the of
number
gratingper
periods
per sampling
periods
varied
from
8 toon
12.the
Based
on
and
the number
gratingofperiods
sampling
periods was
variedwas
from
8 to 12.
Based
section
−1, with theoretical tuning
the
section
lengths,
this
translates
to
comb
spacings
between
5
and
13
cm
´
1
lengths, this translates to comb spacings between 5 and 13 cm , with theoretical tuning ranges from
ranges from
cm−1.
92–180
cm´192–180
.
Lasers were
with
a pulse
width
of 100
ns and
a 5%
Lasers
were tested
tested in
inpulsed
pulsedmode
modeatatroom
roomtemperature
temperature
with
a pulse
width
of 100
ns and
a
duty
cycle
(pulse
width/period).
The
emission
wavelength
is
tuned
by
changing
the
current
density
5% duty cycle (pulse width/period). The emission wavelength is tuned by changing the current
in one section
relativerelative
to the other.
The tuning
behavior
is shown
in Figure
3b as3ba as
function
of the
density
in one section
to the other.
The tuning
behavior
is shown
in Figure
a function
of
difference
in
current
density
(ΔJ).
Most
of
the
lasers
behaved
as
expected,
with
some
exhibiting
tuning
the difference in current density (∆J). Most of the lasers behaved as expected, with some exhibiting
up to 183 cm−1 using a standard SGDFB short period grating design (Ng = 8, Ns = 20). This is the widest
tuning up to 183 cm´1 using a standard SGDFB short period grating design (Ng = 8, Ns = 20). This
tuning reported for a SGDFB QCL with a single grating basis. In this case, the step tuning behavior
is the widest tuning reported for a SGDFB QCL with a single grating basis. In this case, the step
has a spacing of ~12 cm−1. The measuring current was not carefully optimized to minimize the side
tuning behavior has a spacing of ~12 cm´1 . The measuring current was not carefully optimized to
mode suppression ratio in this experiment, but the measured values range from 10–20 dB. As the
minimize the side mode suppression ratio in this experiment, but the measured values range from
grating period reduced, the spectrum gradually became multimode, however, which shows
10–20 dB. As the grating period reduced, the spectrum gradually became multimode, however, which
insufficient reflectivity at shorter wavelength to suppress emission from the longer wavelength peak.
shows insufficient reflectivity at shorter wavelength to suppress emission from the longer wavelength
This indicates, as with the DFB measurement, that broadband (>250 cm−1) tuning will benefit from a
peak. This indicates, as with the DFB measurement, that broadband (>250 cm´1 ) tuning will benefit
Photonics 2016, 3, 41
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more balanced gain lineshape, which will require only minor modification of the number of emitting
periods.
from a more balanced gain lineshape, which will require only minor modification of the number of
emitting periods.
3. Discussion
3. Discussion
One challenge with the dual core method presented above is maintaining alignment between
Onegain
challenge
withthe
thesampled
dual core
method
presented
above The
is maintaining
alignment
between
the laser
curve and
grating
reflectivity
function.
tuning mechanism
of the
laser,
the
laser
gain
curve
and
the
sampled
grating
reflectivity
function.
The
tuning
mechanism
of
the
laser,
while controlled electrically, is actually enabled by local temperature changes in the waveguide
while controlled
electrically,
is actually
enabled
by change
local temperature
changes
the waveguide
which
are related
to the current
density.
As the
in gain curve
peakinposition
is fasterwhich
with
are related tothan
the current
density.
the change
in gain
peak position is
withtotemperature
temperature
the Bragg
peak As
position,
a small
(<10 curve
cm−1) misalignment
is faster
expected
occur over
−1),
thanfull
thetuning
Braggrange
peak of
position,
a small
(<10 cm
misalignment
is to
expected
occur over
the
the laser.
However,
this´1is) small
compared
the gaintoFWHM
(~750the
cmfull
´
1
tuning
thebelaser.
However,
is small
compared
to the gain
FWHM
), and
and
therange
error of
can
corrected
by athis
small
increase
in the grating
period
and (~750
carefulcm
choice
of the
error
can
be
corrected
by
a
small
increase
in
the
grating
period
and
careful
choice
of
the
sampling
sampling periods in the two sections.
periods
in thethe
twoinitial
sections.
Though
goal described above was to a combine a dual core QCL with a simple
Though
the initial
goal described
above
wasrange,
to a combine
core
QCL with
simple sampled
sampled
grating
to produce
a very wide
tuning
there isa adual
more
elegant
path aforward
to reach
grating
to
produce
a
very
wide
tuning
range,
there
is
a
more
elegant
path
forward
to
reach
this architecture’s true potential. Beyond the use of a simple grating basis, a tailored SSG canthis
be
architecture’s
true potential.
the use gain
of a simple
grating
basis, a tailored
SSG
can be adapted
adapted
to compensate
for aBeyond
more general
lineshape.
In addition,
the gain
lineshape
can be
to compensate
for a amore
general
gain gain
lineshape.
In addition,
lineshape
can bedesigns.
tailored In
to
tailored
to produce
nearly
arbitrary
lineshape
with thethe
usegain
of multiple
emitter
produce a nearly
arbitrary gain
lineshape
the
use
multiplewith
emitter
designs.
In combination,
an
combination,
an extremely
broad
spectralwith
range
can
beofaccessed
minimal
variation
in threshold
extremely
broad spectral range can be accessed with minimal variation in threshold or power output.
or
power output.
The main
maingoal
goalwill
willbebetotoachieve
achieve
gain
reflectivity
across
tuning
range
of interest.
The
flatflat
gain
andand
reflectivity
across
the the
tuning
range
of interest.
The
The general
is sketched
in Figure
4. Besides
improving
power
uniformity,
this architecture
will
general
idea idea
is sketched
in Figure
4. Besides
improving
power
uniformity,
this architecture
will also
also make
alignment
thecomponents
two components
and reflectivity
curves)
less critical
asare
there
no
make
alignment
of theoftwo
(gain (gain
and reflectivity
curves)
less critical
as there
no are
sharp
sharp
features.
The
first
task
is
to
develop
the
SSG
basis
using
discretized
grating
periods
and
phase
features. The first task is to develop the SSG basis using discretized grating periods and phase shifts.
shifts.is This
is not trivial,
but
the approach
can
be adapted
related
efforts
[15].
Optimization
This
not trivial,
but the
approach
can be
adapted
fromfrom
priorprior
related
efforts
[15].
Optimization
of
of the
gain
curve
must
then
be achieved
which
now
been
investigated
extensively
in LWIR
the LWIR
the
gain
curve
must
then
be achieved
which
hashas
now
been
investigated
extensively
in the
for
for heterogeneous
QCLs.
Some
optimization
required
arriveatatthe
theproper
proper shape
shape without
heterogeneous
QCLs.
Some
optimization
willwill
be be
required
totoarrive
efficiency, however,
sacrificing device efficiency,
however,which
whichwill
willfollow
followthe
theprocedure
proceduredeveloped
developedin
in[3].
[3].
Figure
Figure 4.
4. Evolution
Evolution of
of electrically
electrically tunable
tunable QCL
QCL design
design for
for extreme
extreme spectral coverage.
4.
4. Conclusions
Conclusions
In
conclusion, this
thispaper
paperhas
haspresented
presented
a plan
to increase
electrical
tuning
range
accessible
In conclusion,
a plan
to increase
thethe
electrical
tuning
range
accessible
with
with
QCLs
by
optimizing
the
design
of
both
gain
and
reflectivity
lineshape
within
a
SGDFB
QCLs by optimizing the design of both gain and reflectivity lineshape within a SGDFB architecture.
−1 and DFB laser
1 and750
architecture.
A dual
wafer is demonstrated
with an
FWHM
of´over
cm
A dual core wafer
is core
demonstrated
with an EL FWHM
ofEL
over
750 cm
DFB
laser
single mode
−1
´
1
single
mode
emission
over (4
700<cm
λ < 5.5Electrical
μm). Electrical
tuning
is also
demonstrated
over
180cm
cm´−11
emission
over
700 cm
λ < (4
5.5< µm).
tuning
is also
demonstrated
over
a a180
range
single grating
grating basis.
basis. With
range with
with aa single
single SGDFB
SGDFB laser
laser with
with aa single
With further
further refinement,
refinement, this
this method
method
shows promise for a much wider spectral range, which can potentially cover the entire MWIR or
LWIR spectral range.
Photonics 2016, 3, 41
6 of 6
shows promise for a much wider spectral range, which can potentially cover the entire MWIR or LWIR
spectral range.
Acknowledgments: This work is partially supported by the Department of Homeland Security Science and
Technology Directorate (grant HSHQDC-13-C-00034), National Science Foundation (grants ECCS-1505409 and
ECCS-1306397), Naval Air Systems Command (grant N68936-13-C-0124), and an Early Stage Innovations grant
from NASA’s Space Technology Research Grants Program (grant NNX13AT10G). The authors would like to
acknowledge the encouragement and support of all the involved program managers.
Author Contributions: S.S. and M.R. conceived and designed the experiments; S.S. performed the experiments.
S.S. wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.
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