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DOI: 10.1007/s00340-008-3047-x
Appl. Phys. B (2008)
Lasers and Optics
Applied Physics B
g. wysocki
1,u
r. lewicki
2
r.f. curl
2
f.k. tittel
2
l. diehl
3
f. capasso
3
m. troccoli
3
g. hofler
4
d. bour
4
s. corzine
4
r. maulini
5
m. giovannini
5
j. faist
5,6
Widely tunable mode-hop free external cavity
quantum cascade lasers for high resolution
spectroscopy and chemical sensing
1
Electrical Engineering Department, Princeton University, Engineering Quadrangle, Olden Street, Princeton,
NJ 08544, USA
2
Rice Quantum Institute, Rice University, 6100 Main St., Houston, TX 77005, USA
3
School of Engineering and Applied Sciences, Harvard University, 9 Oxford Street,
Cambridge, MA 02138, USA
4
Agilent Laboratories, 3500 Deer Creek Road, Palo Alto, CA 94304, USA
5
Institute of Physics, University of Neuchâtel, 1 A.-L. Breguet, 2000 Neuchâtel, Switzerland
6
ETH Zurich, Wolfgang-Pauli Str. 16, 8093 Zurich, Switzerland
Received: 15 January 2008/Revised version: 17 April 2008
© Springer-Verlag 2008
ABSTRACT Recent progress in the development of room
temperature, continuous wave, widely tunable, mode-hop-free
mid-infrared external cavity quantum cascade laser (EC-QCL)
spectroscopic sources is reported. A single mode tuning range
of 155 cm
−1
(∼8% of the center wavelength) with a max-
imum power of 11.1 mW and 182 cm
−1
(∼15% of the center
wavelength) with a maximum power of 50 mW was obtained
for 5.3 and 8.4 µm EC-QCLs respectively. This technology
is particularly suitable for high resolution spectroscopic ap-
plications, multi species trace-gas detection and spectroscopic
measurements of broadband absorbers. Several examples of
spectroscopic measurements performed using EC-QCL based
spectrometers are demonstrated.
PACS 42.55.Px; 42.60.-v; 42.62.Fi; 07.07.Df
1 Introduction
The invention of quantum cascade lasers
(QCLs) [1], has led to a significant progress in mid-IR spec-
troscopy and its applications to trace gas sensing. High power
(
50–500 mW), room temperature operation with thermoelec-
tric cooling (TEC), and continuously improving wall plug
efficiency of continuous wave (cw) and pulsed QCL de-
signs [2–6], make them suitable field deployable sources for
most demanding real-world chemical sensing applications.
For accurate spectroscopic analysis [7, 8], both single trans-
verse and longitudinal mode operation for QCLs are required.
The latter can be achieved either by embedding within the
QCL active region a periodic structure that provides dis-
tributed feedback (DFB) for a single longitudinal laser mode
at a precisely selected wavelength [6,9], or by using an exter-
nal cavity configuration (EC) [10–14].
The wavelength tunability of DFB QCLs relies on ther-
mal tuning of the refractive index, which limits the fre-
quency coverage to
∼ 10–20 cm
−1
when slow temperature
u Fax: +1-609-258-2158, E-mail: gwysocki@princeton.edu
tuning is applied or to ∼ 2–3cm
−1
when fast Joule heat-
ing by an injection current is used. Thermal tuning strongly
affects the QCL threshold resulting in a decrease of the out-
put power at higher chip temperatures. Thus DFB-QCLs are
typically designed to target absorption lines of one specific
molecule with narrow, well resolved ro-vibrational absorp-
tion lines. Substantially low fabrication yield of DFB-QCLs
operating at precisely selected wavelengths has limited the
availability of commercial QCLs to wavelengths that corres-
pond to absorption by molecules of widespread interest. The
application of an EC configuration allows selection of the
QCL wavelength anywhere within the available QCL spec-
tral gain without changing the chip temperature, thus sig-
nificantly increasing the laser spectral coverage and allowing
much more efficient utilization of the available QCL spectral
gain.
This is especially important for the QCL gain media de-
signed to have intrinsically broader gain profiles. This in-
cludes bound-to-continuum QCLs in which the lower state of
the laser transition is a relatively broad continuum [15, 16]
or heterogeneous QCL structures with ultra-broad optical
gain. The latter design that consists of multiple active re-
gions that support laser transitions at different wavelength was
first demonstrated by Gmachl et al. [17]. Recently frequency
tunability from 961 to
1220 cm
−1
(∼24% of the center wave-
length) of a pulsed QCL was achieved using a heterogeneous
gain medium with a two-wavelength (8.4 and
9.6 µm) active
region in a Littrow type EC-QCL configuration [12].
Although the wavelength tuning speeds available with
an EC configuration are slower than for DFB-QCLs, the
EC-QCLs have already proven to be interesting alternative
spectroscopic sources that can enable new applications such
as agile detection of multiple gases including those with
broad absorption bands [10, 18, 19]. At present the alternative
means for achieving broad spectral coverage is multiple DFB
QCL arrays with a varying DFB grating period [6, 20].
In this work we report the development of a cw, TEC
cooled, EC-QCL technology with specific emphasis on appli-
cations that require broadband, high resolution spectroscopy.
Two EC-QCL sources based on the same EC architecture but
employing two different gain media have been investigated.
Applied Physics B – Lasers and Optics
The QCL chips utilized were fabricated using significantly
different technologies:
1) the QCL operating at
5.3 µm with a bound-to-continuum
design of the active region grown used a molecular beam
epitaxy (MBE) process and was fabricated with a standard
ridge waveguide structure, and
2) the QCL operating at
8.4 µm employed a double-phonon
resonance design of the active region which was grown
using metal organic vapor-phaseepitaxy(MOVPE) process
and fabricated as a buried heterostructure.
Some important aspects of the two technologies will be dis-
cussed below, and the performance and the spectroscopic ap-
plications of the EC-QCL technology will be demonstrated.
2 EC-QCL configuration
The EC-QCL system is an improved miniaturized
version of the source reported in [10]. The optical configura-
tion of the Littrow type EC-QCL is shown schematically in
Fig. 1a. The overall size of the system is
∼ 9in×6in×6in
(∼23 cm×15 cm ×15 cm). It consists of a QCL housing and
an electrically controlled external cavity positioning system
(see Fig. 1b). The present system has proved to be a con-
venient research tool, which allows investigation and testing
of the EC-QCL components using off-the-shelf optics (e.g.
collimating lenses, diffraction gratings and QCL gain chips).
The laser housing is equipped with an integrated TEC cooling
system capable of operating the QCL at temperatures as low
as
−40
◦
C with optional external chilled water cooling. The
housing is vacuum-tight and incorporates a manual 3D lens
positioning system that accepts collimating optics up to
1inch
in diameter. As in the previous system the EC arrangement
consists of three main elements: the QCL chip, a beam colli-
mating lens, and a diffraction grating. The zeroth order reflec-
tion from the grating is reflected from the mirror, M mounted
together with the grating on the same rotary platform pro-
viding the output laser beam. This configuration provides an
FIGURE 1 (a) Schematic diagram of the EC-QCL spectroscopic source configuration (QCL – quantum cascade laser; TEC – thermoelectric cooler; CL –
collimating lens; LB – laser beam; GR – diffraction grating; PP – pivot point of the rotational movement; M – mirror mounted on the same platform with GR).
(b) A 3D model of the EC-QCL source assembly
output laser beam fixed in location and direction during the
wavelength tuning process. This fixed beam is achieved by
precise alignment of the grating and the mirror M in such
a way that the planes collinear with both reflecting surfaces
intersect exactly at the rotation axis of the rotary platform.
The previous system could only provide a fixed direction of
the laser beam, but the beam was subject to parallel walk-
off while the laser wavelength was tuned. The piezo-activated
mode-tracking system that provides independent control of
the EC length and diffraction grating angle for mode-hop free
tuning was adopted from the previous system [10]. The flexi-
bility of this EC architecture makes it compatible with any
QCL gain chip at any mid-infrared wavelength without chang-
ing the EC configuration. The performance of the new system
was demonstrated with two cw, TEC, Fabry–P
´
erot QCL gain
chips operating at
∼5.3 and 8.4 µm, respectively.
2.1 EC-QCL operating at 5.3 µm
2.1.1 Technical details. The gain chip with a ridge waveguide
width of
12 µm andlengthof2.25 mm was used to construct
an EC QCL operating at
5.3 µm. The EC feedback was ob-
tained using a first order diffraction from the Littrow type
diffraction grating with
150 grooves/mm, ∼ 93% reflectiv-
ity and blaze angle optimized for
λ = 5.4 µm (Optometrics,
model: ML401). The QCL chip was fabricated as a stan-
dard ridge waveguide structure and an active region based
on bound-to-continuum design [21]. Since in the bound-to-
continuum scheme the lower state of the laser transition is
essentially a broad continuum the QCLs based on this tech-
nology exhibit a broadened gain profile. A high-reflection
(HR) coating (
Al
2
O
3
/Au 300 nm/100 nm) was deposited on
the back facet and an anti-reflection (AR) coating (a multi-
layer dielectric AR coating [22]) was deposited on the front
facet of the chip. Figure 2 shows the LIV curves measured for
the QCL chip at
−30
◦
C at different stages of the chip fabri-
cation. The characteristic threshold currents densities for the
WYSOCKI et al. Widely tunable mode-hop free EC-QCL for high resolution spectroscopy and chemical sensing
FIGURE 2 LIV curves for the 5.3 µm QCL gain medium operated at
−30
◦
C. The curves were measured at different stages of the QCL chip pro-
cessing
current chip operated at −30
◦
C are: J
th(UC)
=2.72 kA/cm
2
,
J
th(HR)
=2.15 kA/cm
2
, J
th(HR+AR)
=3.82 kA/cm
2
, for an un-
coated chip (with facet reflectivity of
27.4%), a HR coated
back facet (with reflectivity of
95%), and a HR and AR coated
chip respectively. A threshold current of
566 mA measured
with EC feedback at the center of the gain curve yielded
a threshold current density of
J
th(EC)
=2.1kA/cm
2
.Usingthe
relation between the resonator losses and laser threshold cur-
rent [23], these current densities allow the waveguide losses
to be estimated as
α
w
≈5.64 cm
−1
, the total EC optical feed-
back to be estimated as
∼ 31%, and the calculation of the
AR coating reflectance yields
∼0.7%. QCL wavelength tun-
ing was evaluated using a
1/8mmonochromator (CVI model:
CM110).
Figure 3 shows power normalized spectra recorded for
several positions of the diffraction grating within the tun-
ing range of the EC-QCL operated at
950 mA together with
the corresponding optical power. This EC-QCL has a max-
imum tuning range of
155 cm
−1
with an output power of up to
11.1mWat the center of the gain profile.
FIGURE 3 Laser frequency tuning range and corresponding optical power
of the 5.3 µm EC-QCL operated in cw at 950 mA and −30
◦
C
Mode-hop-free laser frequency tuning with a spectral
resolution of
< 0.001 cm
−1
(<∼30 MHz) is primarily limited
by the laser linewidth and can be used for high resolution
spectroscopic applications. A mode-hop-free scan can be per-
formed at any available laser wavelength with a maximum
frequency scan range of
∼2.5cm
−1
limited primarily by the
maximum available excursion of the PZT element (
90 µm)
controlling the EC length. Unlike the previous system [10],
in which the available dynamic range of QCL current tuning
(between laser threshold and power rollover) was the parame-
ter limiting the maximum mode-hop-free tuning range, in this
5.3 µm EC-QCL the dynamic range of 400 mA (at −30
◦
C)
should allow a mode-hop-free scan of
∼ 4cm
−1
(estimated
using the typical FP mode tuning rate of
∼0.01 cm
−1
/mA).
This increased tuning range should be achievable either by
employing a PZT element with a maximum travel of
150 µm
or by a further reduction of the EC length from its current
∼7 to ∼4.7cm. Of course at the extremes of the total tuning
range, the EC-QCL experiences a reduction of the available
laser gain and the mode-hop-free range may become current
dynamic range limited.
Maximum scan rates in opto-mechanical systems are usu-
ally limited by relatively low frequency mechanical vibration
resonances. The EC-QCL system reported in this work con-
sists of commercially available components and the first me-
chanical resonance frequency of the system associated with
cavity length control was found to be at
f
m
=∼68 Hz. Sinu-
soidal signals at frequencies lower than
0.8 f
m
, should provide
optimum operating conditions for EC-QCL wavelength scan-
ning. The electronics used in this work to drive the piezo
actuators allows the maximum piezo travel range (providing
a
∼2.5cm
−1
laser frequency scan) to be achieved with scan-
ning frequencies
≤22 Hz.
The transverse mode structure of the laser was also ex-
amined. The far field emission pattern of the output laser
beam collimated with a
1.89 mm, focal length f/0.52 ZnSe
aspheric lens (provided for this laser by Daylight Solutions)
was imaged with a mid-IR camera (Electrophysics model:
PV320LZ). Since the threshold current of the AR coated chip
was close to the cw power rollover region the far field meas-
urement of the QCL output without the EC was observed in
a pulsed mode just above the lasing threshold. The
5.3 µm
QCL with a relatively wide ridge of 12 µm (∼38 µm in opti-
cal distance, which is equivalent of
∼7λ) operates in higher
order lateral mode, TM
02
as shown in Fig. 4a. When the QCL
was placed in an EC configuration and operated cw both the
TM
01
and TM
00
modes were observed with relative intensi-
ties strongly dependent on the actual EC alignment. A stable
TM
00
operation within the entire EC-QCL wavelength tuning
range could be achieved after introduction of an additional op-
tical diaphragm inside the laser cavity that restricted the laser
operation to a fundamental lateral mode TM
00
.AfarfieldEC-
QCL emission pattern recorded for cw operation at
900 mA
is shown in Fig. 4b. The intensity profile of the laser beam
is slightly elliptical, but a quasi-Gaussian distribution is ob-
served in both axes as shown in Fig. 4b. The collimating lens
results in an output beam diameter of
∼4mm.
Compared to the previous EC-QCL system described
in [10] the reduction of the beam diameter by a factor of 5
while maintaining the other parameters, caused a reduction of
Applied Physics B – Lasers and Optics
FIGURE 4 (a) Laser beam profile recorded for the
5.3 µm QCL FP chip operated in a pulsed mode.
(b) Laser beam profile measured for the same QCL chip
operated cw in EC configuration. The cross-sections
along the dotted lines are shown with a fit by a Gaussian
curve (red lines). The same collimating lens was used
in (a)and(b). The interference fringes visible in the im-
ages result from some parasitic reflections in the optical
system and are not an intrinsic feature of the EC-QCL
the grating resolving power by the same factor. This yields
a grating spectral bandwidth of
∼ 3cm
−1
. With an EC op-
tical length of
∼ 7cm and QCL optical length of ∼ 7mm,
which corresponds to a free spectral range (FSR) of 0.07 and
0.72 cm
−1
respectively, ∼ 42 longitudinal EC Fabry–P
´
erot
(FP) modes and
∼ 4 QCL FP modes are within the grating
spectral bandwidth simultaneously. Since the gain section of
the EC-QCL is adjacent to one of the resonator mirrors, the
EC FP modes located in close spectral proximity to the lasing
mode
ν
0
occupy nearly the same physical space in the active
region and multimode operation caused by spatial hole burn-
ing phenomena will not occur. However for the EC modes at
frequencies separated by at least the QCL FSR hole burning
and thus multimode operation may occur. However in this sys-
tem despite the large grating bandwidth the cavity losses at
frequencies
≤(ν
0
−FSR
QCL
) and ≥(ν
0
+FSR
QCL
) are suffi-
ciently high to maintain a stable single mode operation at
ν
0
.
2.1.2 Example spectroscopic applications. Both features,
wide tunability and the capability of mode-hop-free tun-
ing of the reported EC-QCL system, have a vital role in
spectroscopic applications. Measurement of broad absorp-
tion features (of large molecules in the gas phase, liquids,
and
/or solid samples), as well as simultaneous detection of
multiple trace gas species can greatly benefit from the broad
tunability offered by EC-QCL lasers. The combination of
broadband tuning and high resolution fine tuning in this EC
system can alleviate the problem of obtaining a QCL with
a wavelength that coincides with the optimum absorption
wavelength of a particular trace gas species as mentioned
above. At
5.3 µm, nitric oxide (NO) detection and monitor-
ing can be performed. This capability is of great importance
for biomedical applications (e.g. for breath analysis), environ-
mental applications (NO is an environmental pollutant) [8], or
industrial applications (e.g. combustion process control) [24].
Broad wavelength coverage provides the ability to select a tar-
get NO absorption line that is free of spectral interference
from other species such as water (
H
2
O) or carbon dioxide
(
CO
2
) as shown in Fig. 5a, is a critical requirement in the
aforementioned applications.
A situation in which target line selection is even more
restrictive is the detection of NO using a Faraday rotation
spectroscopy [25–27]. As predicted by Ganser et al. [26] the
best NO Faraday detection limit should be obtained for the
Q(3/2) transition at 1875.8cm
−1
, which has the highest mag-
netic modulation sensitivity. However targeting of this line
has not been possible to-date using commercially available
DFB QCLs, which are usually optimized to operate either in
a
P-orR-branch of the NO fundamental band. As shown in
Fig. 5a, the
5.3 µm EC-QCL can cover almost entirely the P-,
Q-andR-branches of the NO fundamental absorption band at
∼5.3 µm, and allows performing the Faraday rotation spec-
troscopic detection of NO using the most optimum rotational
component in the
Q-branch (see the measured high resolution
direct absorption spectrum in Fig. 5b). An example Faraday
spectrum of the NO
Q(3/2) and Q(5/2) transitions acquired
for a
10 ppm of NO in N
2
as a buffer gas within a 42 cm
active optical pathlength with a minimum detection limit of
∼5.5 ppb is shown in Fig. 5c. The performance characteris-
tics of a NO EC-QCL based Faraday rotation spectrometer
is currently under investigation and will be reported else-
where [28].
To demonstrate spectroscopic measurements of broad-
band absorbing species a quartz enhanced photoacoustic
spectrum (QEPAS) [29], of a gas mixture containing
7.2%
ethanol and 0.24% H
2
O in N
2
as a buffer gas was measured
using the
5.3 µm EC-QCL. The results along with reference
spectra of ethanol and water vapor are shown in Fig. 6. Coarse
wavelength scanning together with
50% duty cycle direct
laser current amplitude modulation (with
100% modulation
depth) at
∼32 kHz and QEPAS based on AM detection was
used in this measurement [18, 19]. The signal is laser power
normalized to eliminate any laser power fluctuations related
to mode-hoping (up to
3%) that might occur during a coarse
wavelength scan, and to account for an absorption by at-
mospheric water within an open optical path of the system.
Despite a strong QCL chirp during the laser current pulse
combined with a grating spectral bandwidth of
3cm
−1
,which
in this case should reflect the minimum effective resolution
of the system, the narrow water lines can still be clearly iden-
tified in the measured spectrum. The actual resolution of the
system could be estimated to be
∼1.2cm
−1
using a full width
at half maximum measured for several isolated water lines
observed in the spectrum.
QCL frequency chirping can be avoided by using cw op-
eration of the laser and an indirect (external) intensity modu-
lation by means of a beam chopper, an electro-optical modu-
lator or an acousto-optical modulator. However applicability
of a mechanical beam chopper for QEPAS requires a rela-
WYSOCKI et al. Widely tunable mode-hop free EC-QCL for high resolution spectroscopy and chemical sensing
FIGURE 5 (a) Simulation of atmospheric absorption (in blue) over a 286 m path within the 5.3 µm EC-QCL tuning range (H
2
O mixing ratio = 0.6%, CO
2
mixing ratio = 380 ppm, P = 760 Torr, T =276 K). For reference an absorption spectrum of 1 ppm NO at the same working condition is plotted (in red).
Lower panels demonstrate a mode-hop-free spectra of NO acquired as: (b) a direct absorption spectrum of the NO Q-branch recorded at scan rate of 10 Hz for
5% NO in N
2
at reduced pressure and with 10 cm optical path length (red line shows a well matching HITRAN spectrum simulated for the same conditions),
(c) a Faraday rotation spectrum of NO Q(3/2) and Q(5/2) transitions at ∼1875.8cm
−1
for 10 ppm NO in N
2
mixture at 35 Torr and with 42 cm active optical
pathlength (the minimum detection limit estimated as MDL =5.5 ppb), and (d) a QEPAS spectrum recorded for 4.2% NO in N
2
at ∼1903 cm
−1
, pressure of
600 Torr and with 1 cm optical pathlength (the strongest transition in the fundamental NO R-branch)
FIGURE 6 Photoacoustic spectrum of a gas mixture containing 7.2%
ethanol and 0.24% H
2
OinN
2
as a buffer gas measured at atmospheric pres-
sure using a 5.3 µmEC-QCL(red line). Absorption spectra of ethanol (black
line) and water vapor (blue line) obtained from the PNNL database and the
HITRAN database respectively are plotted for reference
tively high chopping frequency, which is not easy to realize in
practice. In our case the
32 kHz amplitude modulation can be
realized up to
∼80% modulation depth using a quartz tuning
fork similar to the one used in the QEPAS based gas sensor
as a chopper. A mode-hop-free spectrum of
4.2% NO in N
2
at
1903 cm
−1
in Fig. 5d clearly demonstrates the high resolution
capability of an EC-QCL based photoacoustic spectrometer. It
should be noted that for the spectra acquired using techniques
that employ a phase sensitive lock-in detection (in our case
both Faraday rotation spectroscopy and QEPAS) the relatively
slow wavelength scanning rate of the EC-QCL is not a limit-
ing parameter. The applied lock-in time constant determines
the time necessary to acquire signal for each spectral point
within the scan.
2.2 High power EC-QCL operating at 8.4 µm
2.2.1 Technical details. The EC-QCL operating at 8.4 µm
was constructed using a new MOCVD grown buried het-
erostructure Fabry–P
´
erot QCL gain chip operating at
8.4 µm [5] (with a waveguide width of 5 µm, and length of
3mm) in the same EC-QCL architecture as that used for
5.3 µm. For better performance a grating with 135 grooves/
mm and a grazing angle optimized for the longer wave-
lengths was used (nominal blaze angle optimized for
λ =
10.6 µm and efficiency > 90% between 6.5 and 13.5 µm).
For beam collimation an AR coated (
3–12 µm) f/0.6 ger-
manium aspheric lens with
24 mm-diameter (Optical Solu-
Applied Physics B – Lasers and Optics
tions, model: 4682) was incorporated, which provided an
output beam with a diameter of
∼ 20 mm. Similar to the
5.3 µm QCL chip a high-reflection (HR) coating (Al
2
O
3
/Au
300 nm
/100 nm) was deposited on the back facet and an
AR coating (the same multilayer dielectric AR coating [22]
could also be used in this wavelength region with appro-
priate design of the layer thicknesses) was deposited on
the front facet of the
8.4 µm QCL chip. Figure 7 shows
the LIV curves measured for the QCL chip at
−30
◦
C at
different stages of the chip fabrication. Using the charac-
teristic threshold currents densities for the QCL operated
at
−30
◦
C: J
th(UC)
= 1.37 kA/cm
2
, J
th(HR)
= 1.13 kA/cm
2
,
J
th(HR+AR)
= 2.33 kA/cm
2
and J
th(EC)
= 1.03 kA/cm
2
,the
uncoated facet reflectivity of
27.4% and the HR coated facet
reflectivity of
95%, the estimates of the waveguide losses,
the total EC optical feedback, and the AR coating reflectance
yielded:
α
w
≈ 7.8cm
−1
, ∼ 46.7%,and∼ 0.046% respec-
tively. We attribute such an excellent AR coating quality to the
much higher uniformity of the deposited coating over the area
of the guided mode field at the facetof a buried heterostructure
QCL waveguide. In the ridge waveguide structure the mode
field is adjacent to the side wall edges, which may introduce
AR coating thickness variations, thus decreasing the coating
quality.
The combination of a high performance QCL chip, a high
quality AR coating and strong EC feedback (due to a combi-
nation of a very fast aspheric lens and optimized diffraction
grating) resulted in single mode laser frequency tuning of
182 cm
−1
,whichis∼ 15% of the center wavelength. To our
best knowledge this is the best result reported to-date for a cw
EC system employing a QCL chip with a standard (double-
phonon resonance) design of the active region. If the same
processing steps were applied for a QCL with a bound-to-
continuum transition, an even broader tuning range might be
possible. Figure 8 depicts spectral measurements of the laser
output radiation for several different diffraction grating angles
recorded together with a corresponding laser power measured
at a driving QCL current of
420 mA. The maximum cw power
of
∼50 mW was measured for the 8.4 µm EC-QCL operated
at a heatsink temperature of
−30
◦
C and a driving current of
∼ 750 mA (see Fig. 7). The laser exhibited a stable funda-
FIGURE 7 LIV curves for the 8.4 µm MOCVD grown QCL gain medium
operated at −30
◦
C. The curves were measured at different stages of the chip
processing and after its incorporation into the EC-QCL configuration
FIGURE 8 Laser frequency tuning range and corresponding optical power
of the 8.4 µm EC-QCL operated in cw at 420 mA and −30
◦
C
mental TM
00
lateral mode operation and did not require an
additional intracavity optical diaphragm for transverse mode
restriction.
The
8.4 µm EC-QCL can also perform mode-hop-free
tuning using similar opto-mechanical and PZT components
as for the above described
5.3 µm laser system. The optical
EC length in this system is also
∼ 7cmincluding the QCL
chip optical length of
∼1cm, which corresponds to EC FSR
of
0.07 cm
−1
and FP QCL FSR of 0.5cm
−1
respectively. The
maximum mode-hop-free tuning, which in this case is also
limited by the
100 µm PZT range, can be performed over
up to
∼1.7cm
−1
with a spectral resolution of < 0.001 cm
−1
(∼30 MHz).
2.2.2 Example spectroscopic applications. The
8.4 µm EC-
QCL with a tuning range between 7.77 and
9.05 µm (span-
ning over
1.28 µm) and a maximum cw output power
of
50 mW represent an excellent spectroscopic source for
a number of laser spectroscopic applications. Some specific
applications such as remote sensing or photoacoustic spec-
troscopy can directly benefit from the high output power
levels. Several molecules possess strong absorption bands
within the tuning range of the laser. Figure 9a shows ab-
sorption line strengths for several environmentally import-
ant molecules obtained from the HITRAN 2004 database
that are accessible with this EC-QCL source. An absorp-
tion spectrum of a pure nitrous oxide (
N
2
O)sampleat
reduced pressure of
5Torr and with 5cm optical path was
acquired using a mode-hop-free scanned EC-QCL. Figure 9b
shows an excellent agreement between the measured and
database simulated spectrum of Doppler broadened
N
2
O lines
confirming the high resolution capability of the reported
system.
An earlier version of the
8.4 µm EC-QCL was also ex-
tensively tested as a spectroscopic source in a photoacoustic
spectrometer (QEPAS) used for detection and quantification
of broadband absorbers. The experimental results including
concentration measurements (at ppb levels) of Freon 125A
(
C
2
HF
5
) and acetone as well as the quantification of the mo-
lecular concentrations in a mixture of both compounds are
reported in [19]. The performance of this EC-QCL was further
optimized and is also reported in the manuscript.
WYSOCKI et al. Widely tunable mode-hop free EC-QCL for high resolution spectroscopy and chemical sensing
FIGURE 9 (a) HITRAN 2000 based simulation of absorption line strength
for several molecules including: nitrous oxide (N
2
O), methane (CH
4
), sulfur
dioxide (SO
2
), and ammonia (NH
3
) within the tuning range of the 8.4 µm
EC-QCL laser. (b)N
2
O spectrum at ∼1174.9cm
−1
recorded using a mode-
hop-free scan (single spectrum recorded at 22 Hz scan rate) of the EC-QCL
(black dots) together with HITRAN simulated spectrum (red line)
3 Conclusions
The technical details and spectroscopic capabili-
ties of the new EC-QCL system have been described. The
reported EC architecture allows the use of QCL chips at any
wavelength within the mid-IR molecular fingerprint region
(
3–20 µm), as was demonstrated using two QCLs at 5.3 and
8.4 µm respectively. The 5.3 µm EC-QCL is an ideal source
for NO measurements since it covers almost the entire spec-
tral region of the
P-, Q-andR-branches of the fundamental
NO vibration. This allows a convenient access to less common
absorption line targets within the NO spectrum. By targeting
a specific line in the
Q-branch, ultra sensitive Faraday rota-
tion measurements of NO could be performed. The
8.4 µm
EC-QCL, capable of high resolution mode-hop free frequency
tuning with a tunability of up to
15% of the central wavelength
and an output power of up to
50 mW at a TEC achievable
QCL temperature of
−30
◦
C, shows the excellent potential of
the EC-QCL technology for compact, high resolution broad-
band laser based mid-IR spectrometers. A few examples of
molecular spectroscopic applications including direct absorp-
tion spectroscopy, photoacoustic spectroscopy, and Faraday
rotation spectroscopy demonstrate the flexibility of the tech-
nology developed and the benefits of both the wide tunability
and high resolution of the EC-QCL based systems.
ACKNOWLEDGEMENTS The authors acknowledge financial
support from the MIRTHE NSF ERC, a subaward of a DoE STTR Grant
from Aerodyne Research, and the Robert Welch Foundation. The Harvard
group acknowledges partial financial support of the development of the
gain medium at 8.4 µm from the U.S. Army Research Office under Grant
No. W911NF-04-1-0253 and also support from the Center for Nanoscale
Systems (CNS) at Harvard University. Harvard-CNS is a member of the Na-
tional Nanotechnology Infrastructure Network (NNIN). The development of
the gain medium at 5.3 µm and AR-coating technology by ETHZ/University
of Neuch
ˆ
atel team was supported by the NCCR-Quantum Photonics.
REFERENCES
1 J. Faist, F. Capasso, D.L. Sivco, C. Sirtori, A.A. Hutchinson, A.Y. Cho,
Science 264, 4 (1994)
2 J. Faist, Appl. Phys. Lett. 90, 253 512 (2007)
3 J.S. Yu, A. Evans, S. Slivken, S.R. Darvish, M. Razeghi, IEEE Photon.
Technol. Lett. 17, 1154 (2005)
4 A. Evans, S.R. Darvish, S. Slivken, J. Nguyen, Y. Bai, M. Razeghi, Appl.
Phys. Lett. 91, 071 101 (2007)
5 L. Diehl, D. Bour, S. Corzine, J. Zhu, G. Hofler, M. Loncar, M. Troccoli,
F. Capasso, Appl. Phys. Lett. 88, 201 115 (2006)
6 A. Wittmann, M. Giovannini, J. Faist, L. Hvozdara, S. Blaser, D. Hof-
stetter, E. Gini, Appl. Phys. Lett. 89, 141 116 (2006)
7 D.D. Nelson, B. McManus, S. Urbanski, S. Herndon, M.S. Zahniser,
Spectrochim. Acta A 60, 3325 (2004)
8 B.W.M. Moeskops, S.M. Cristescu, F.J.M. Harren, Opt. Lett. 31, 823
(2006)
9 T. Aellen, S. Blaser, M. Beck, D. Hofstetter, J. Faist, E. Gini, Appl. Phys.
Lett. 83, 1929 (2003)
10 G. Wysocki, R.F. Curl, F.K. Tittel, R. Maulini, J.M. Bulliard, J. Faist,
Appl. Phys. B 81, 769 (2005)
11 C. Peng, G. Luo, H.Q. Le, Appl. Opt. 42, 4877 (2003)
12 R. Maulini, A. Mohan, M. Giovannini, J. Faist, E. Gini, Appl. Phys. Lett.
88, 201 113 (2006)
13 M.J. Weida, D. Arnone, T. Day, Laser Focus World 42, 2 (2006)
14 A. Mohan, A. Wittmann, A. Hugi, S. Blaser, M. Giovannini, J. Faist, Opt.
Lett. 32, 2792 (2007)
15 J. Faist, M. Beck, T. Aellen, E. Gini, Appl. Phys. Lett. 78, 147 (2001)
16 A. Wittmann, T. Gresch, E. Gini, L. Hvozdara, N. Hoyler, M. Giovan-
nini, J. Faist, IEEE J. Quantum Electron. QE-44, 36 (2008)
17 C. Gmachl, D.L. Sivco, R. Colombelli, F. Capasso, A.Y. Cho, Nature
415, 5 (2002)
18 M.C. Phillips, T.L. Myers, M.D. Wojcik, B.D. Cannon, Opt. Lett. 32,
1177 (2007)
19 R. Lewicki, G. Wysocki, A.A. Kosterev, F.K. Tittel, Opt. Express 15,
7357 (2007)
20 B.G. Lee, M.A. Belkin, R. Audet, J. MacArthur, L. Diehl, C. Pflugl,
F. Capasso, D.C. Oakley, D. Chapman, A. Napoleone, D. Bour,
S. Corzine, G. Hofler, J. Faist, Appl. Phys. Lett. 91, 231 101 (2007)
21 J. Faist, M. Beck, T. Aellen, E. Gini, Appl. Phys. Lett. 78, 147 (2001)
22 R. Maulini, PhD thesis (University of Neuchâtel, 2006)
23 B.E.A. Saleh, M.C. Teich, Fundamentals of Photonics, Wiley Series in
Pure and Applied Optics (Wiley, New York, 1991)
24 G. Wysocki, A. Kosterev, F. Tittel, Appl. Phys. B 80, 617 (2005)
25 G. Litfin, C.R. Pollock, R.F. Curl Jr., F.K. Tittel, J. Chem. Phys. 72, 6602
(1980)
26 H. Ganser, W. Urban, J.M. Brown, Molec. Phys. 101, 545 (2003)
27 H. Ganser, M. Horstjann, C.V. Suschek, P. Hering, M. Mürtz, Appl.
Phys. B 78, 513 (2004)
28 R. Lewicki, R.F. Curl, F.K. Tittel, G. Wysocki, to be published
29 A.A. Kosterev, F.K. Tittel, D.V. Serebryakov, A.L. Malinovsky, I.V. Mo-
rozov, Rev. Sci. Instrum. 76, 043 105 (2005)
