Extended tuning of mid-ir quantum cascade lasers

Extended tuning of mid-ir quantum
cascade lasers using integrated resistive
heaters
∗
Alfredo Bismuto,1, Yves Bidaux,1,2,3 Camille Tardy,1 Romain
Terazzi,1 Tobias Gresch,1 Johanna Wolf,2 Stéphane Blaser,1 Antoine
Muller1 and Jerome Faist1
1 Alpes
Lasers SA, 1-3 Passsage Max Meuron, CH-2001 Neuchâtel, Switzerland
for Quantum Electronics, ETH-Zurich, CH-8093 Zurich, Switzerland
3 [email protected]
2 Institute
∗
[email protected]
Abstract: We present single mode quantum cascade lasers including a
microscopic heater for spectral emission tuning. Through the use of a buried
heater element, the active region temperature can be modified without
changing the submount one. Emission frequency tuning in continuous-wave
as large as 9 cm−1 at 1270 cm−1 and 14 cm−1 at 2040 cm−1 are observed,
corresponding to an increase of the active region temperatures of ∼ 90 K.
Due to the proximity of the heaters to the active region, emission can be
modulated at several kHz range and the absence of moving parts guarantees
the mechanical stability of the system. This method can be successfully
applied to all buried heterostructure lasers, becoming an attractive solution for molecular spectroscopy in the IR. Using the presented devices,
molecular absorptions of N2 O have been measured between 1270 cm−1 and
1280 cm−1 and are in agreement with data from the HITRAN database.
© 2015 Optical Society of America
OCIS codes: (140.5965) Semiconductor lasers, quantum cascade; (300.6340) Spectroscopy,
infrared; (140.6810) Thermal effects; (140.3600) Lasers, tunable.
References and links
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with 27% wall plug efficiency,” Appl. Phys. Lett. 98, 181102 (2011).
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such a laser,” (2014). US Patent Application 14/480,812.
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Appl. Phys. 100, 043109 (2006).
#250652
© 2015 OSA
Received 23 Sep 2015; revised 23 Oct 2015; accepted 27 Oct 2015; published 4 Nov 2015
16 Nov 2015 | Vol. 23, No. 23 | DOI:10.1364/OE.23.029715 | OPTICS EXPRESS 29715
10. R. Quay, C. Moglestue, V. Palankovski, and S. Selberherr, “A temperature dependent model for the saturation
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J. M. Flaud, R. R. Gamache, J. J. Harrison, J. M. Hartmann, C. Hill, J. T. Hodges, D. Jacquemart, A. Jolly,
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H. S. P. Mller, O. V. Naumenko, A. V. Nikitin, J. Orphal, V. Perevalov, A. Perrin, E. R. Polovtseva, C. Richard,
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1.
Introduction
Coherent sources in the mid-infrared (Mid-IR) spectral range are of great interest due to the
large number of scientific and industrial applications, e.g. high resolution spectroscopy, industrial control, clinical diagnostic [1, 2]. Quantum cascade lasers (QCLs) [3] are nowadays
probably the most mature source in the Mid-IR range. In fact, both high optical power and low
electrical dissipation devices [4] have been demonstrated across the whole Mid-IR range and
wallplug efficiencies up to 27 % have been observed [5]. In addition to efficient laser designs,
for most of the application stable and tunable monomode emission is required. Typically buried
heterostructure lasers integrating a distributed feedback grating (DFBs) [6] are used, which
provide stable monomode emission. Unfortunately, the electrical tunability of these sources,
typically ∼ 0.1 − 0.2% of the central frequency, limits the capability to detect multiple molecular resonances at the same time or to detect broad resonances. By changing the substrate
temperature the spectral range accessible with DFB QCLs is a few times wider, but the tuning
speed is then strongly reduced, typically to the Hz range.
a)
IH
AR
b)
IH
AR
Fig. 1. (a) Sketch of a buried laser with the integrated semiconductor resistance placed at
few microns far from the active medium. (b) SEM image of one of the fabricated devices.
The IH region has been highlighted in red for clarity.
.
2.
Methods
In order to obtain, at the same time, high tuning speed and wide tuning range, we propose the
implementation of an integrated heater (IH)[7] close to the laser active region (AR). This allows
to control the AR temperature independently from the chip carrier one and from the current
#250652
© 2015 OSA
Received 23 Sep 2015; revised 23 Oct 2015; accepted 27 Oct 2015; published 4 Nov 2015
16 Nov 2015 | Vol. 23, No. 23 | DOI:10.1364/OE.23.029715 | OPTICS EXPRESS 29716
a)
a)
30
Temperature [K]
Distance [ m]
40
20
10
IH
0
0
b)
AR
40
20
Distance [ m]
60
0
Frequency tuning [cm-1]
88
66
44
Integrated
heater
Substrate
heating
22
0
0 -8
1010
10-6
10-4
Time [s]
10-2
Fig. 2. (a): Computed cross-sectional thermal profile of the laser with an IH 4 µm far from
the laser active region (AR). (b): Simulated frequency tuning vs time, for a laser designed
for emission at 1270 cm−1 in the case of heat transmission through the laser substrate and
in the case of current injection in the IH. The emission tuning of a QCL is calculated using
−5
the common relation ∆ν
ν = 8 · 10 ∆T .
injected in the laser. This heater is a semiconductor resistor buried close to the AR, as shown
in Fig. 1. Modulating the current flowing across the resistance, the active region temperature
and therefore the modal refractive index can be modified. In order to add the IH to the optical
cavity, the fabrication process of the buried DFB QCL has been modified. A n-doped InP layer
was regrown on the top of the cladding and InP:Fe insulating layers. A resistive path was then
defined by partially etching this layer close to the laser ridge. Two separate contacts on the top
of the chip allow to drive the laser or the heater region. Using 2-D coupled electric-thermal
simulations, Fig. 2(a), and assuming the Drude model for the layer conductivity, we compared
the emission tuning resulting from the injection of 1.2 A in the IH, placed at a distance of 4 µm
from the AR, and the tuning obtainable changing the heat transmission through the substrate,
see Fig. 2(b). Simulations show clearly that the IH heater has a big potential for high speed
tuning due to the reduced distance from the heating element to the AR. In the calculations, the
anisotropic nature of the thermal transport is accounted for as already shown in [8, 9].
3.
Results
The IH concept has been applied to several DFB QCL structures (IH-QCLs) emitting across
the whole Mid-IR range. In this work, we will focus on IH-QCLs emitting at 7.8 µm and
4.9 µm. Heaters with resistances between 2-10 Ω have been fabricated. The heater lengths
and thickness have been varied respectively from 5 to 10 µm and from 0.5 to 2 µm, while
the distance between the heater and the AR was varied from 1 to 5 µm. In order to avoid
increasing the laser waveguide losses, the n-doping of the integrated heater layers was kept
below 1017 cm−3 . In Fig. 3, performances in continuous-wave (cw) of a IH-QCL, 2.25 mm long
and 9 µm wide, emitting at 7.8 µm are shown. In particular, light-current-voltage characteristics
for the laser with no current injection in the IH are shown in the Fig. 3(a) for various heat-sink
#250652
© 2015 OSA
Received 23 Sep 2015; revised 23 Oct 2015; accepted 27 Oct 2015; published 4 Nov 2015
16 Nov 2015 | Vol. 23, No. 23 | DOI:10.1364/OE.23.029715 | OPTICS EXPRESS 29717
Fig. 3. (a) Current-voltage-light characteristics of the laser emitting at 7.8 µm in CW
for different submount temperatures. The laser is mounted episide-up. (b) Heater currentvoltage characteristics in CW operation. A constant current of 450 mA is injected in the
laser. Voltage increases linearly with current for small electrical fields.
temperatures. The current-voltage characteristics for the IH are instead shown in the Fig. 3(b).
We can see that for currents smaller than 1 A the resistance shows an Ohmic behavior with R
∼ 8 Ω which is nearly independent of the submount temperature. For high currents, the heater
impedance increases due to carrier velocity saturation effects. We analyzed the impact of both
the temperature and the electric field on the electron velocity using analytic models [10]. The
increase in the resistance is mainly due to the high electric field which reaches values higher
than 10 kV/cm, making saturation velocity effects relevant.
Figure 4(a) demonstrates the current tuning effect of the DFB QCL when no current is injected in the heater contact; setting the heat-sink temperature to -20◦ C and varying the laser
current, the frequency tuning is 3.6 cm−1 . In the Fig. 4(b) the tuning induced by the IH is shown
for a constant laser current and submount temperature, i.e. 0.45 A and -20◦ C. The emission frequency tunes by 6.8 cm−1 with power dissipated in the heater with a slope of 0.74 cm−1 /W.
−5
Using the common relation ∆ν
ν = 8 · 10 ∆T we can deduce that the temperature rate of change
◦
with power is 7.4 C/W.
In order to prove the generality of the presented method we also implemented the IH concept
on BH QCL DFBs emitting at 4.9 µm. In this case we also reduced the length of the heater
element in order to reduce its resistance by roughly half. In Fig. 5, performances in CW of a
IH-QCL, 1.5 mm long and 4.5 µm wide, emitting at 4.9 µm are shown. Light-current-voltage
characteristics for a laser with no current injection in the IH are shown in the Fig. 5(a) for
various heat-sink temperatures. The current-voltage characteristics for the IH are instead shown
in the Fig. 5(b), R ∼ 4.5 Ω. Also in this device the IH impedence tends to increase for large
fields due to the decreasing mobility.
The DFB tuning as a function of the laser current is shown in Fig. 6(a) for a submount
temperature of -10◦ C and allows a spectral coverage of 4 cm−1 . The tuning induced by the IH
is shown in Fig. 6(b). Setting the heat-sink temperature to -10◦ C and the laser current to 0.28 A,
the frequency tuning of the DFB QCL is 12.8 cm−1 with power dissipated in the heater with
a slope of 1.5 cm−1 /W. We can deduce that the temperature raise induced is 9.4 ◦ C/W in this
case. Even if an extensive study of the impact of the IH on the device lifetimes has not yet
#250652
© 2015 OSA
Received 23 Sep 2015; revised 23 Oct 2015; accepted 27 Oct 2015; published 4 Nov 2015
16 Nov 2015 | Vol. 23, No. 23 | DOI:10.1364/OE.23.029715 | OPTICS EXPRESS 29718
Fig. 4. (a) Current tuning of the laser, shown in Fig. 3, for a fixed submount temperature,
i.e. -20◦ C. No current is injected in the IH contact. (b) Tuning induced by the IH for fixed
laser current and temperature, i.e. 0.45 A and -20◦ C. For each spectrum the optical power
was measured using a calibrated detector and is presented in black squares.
Fig. 5. (a) Current-voltage-light characteristics of the laser emitting at 4.9 µm in CW
for different submount temperatures. The laser is mounted episide-up. (b) Heater currentvoltage characteristics in CW operation. A constant current of 0.28 A is injected in the
laser. Voltage increases linearly with current for small electrical fields.
#250652
© 2015 OSA
Received 23 Sep 2015; revised 23 Oct 2015; accepted 27 Oct 2015; published 4 Nov 2015
16 Nov 2015 | Vol. 23, No. 23 | DOI:10.1364/OE.23.029715 | OPTICS EXPRESS 29719
Fig. 6. (a) Current tuning of the laser, shown in Fig. 5, for a fixed submount temperature
of -10◦ C. No current is injected in the IH contact. (b) Tuning induced by the IH for a fixed
laser current and temperature, i.e. 0.28 A and -10◦ C. For each spectrum the optical power
was measured using a calibrated detector and is presented in black squares.
been done, some of the IH-QCLs were characterized over more than 100 hrs and 5000 thermal
cycles without any measurable impact on the spectral stability. The IH-QCL performances,
e.g. threshold and slope efficiency, have been compared with the ones of standard DFB QCLs
fabricated on the same wafer and the IH presence does not impact measurably the laser losses.
As mentioned above one of the main advantages of the developed device is the possibility for
fast and broad electrical tuning. In fact, for many applications a broader tuning could provide
an important enhancement in the lower detection limit and could also increase the selectivity
to other substances and reduce the impact of false positives. We studied therefore the tuning
obtained by modulating the two devices shown above using a sinusoidal current in the IH while
keeping constant the laser current. Results are shown in Fig. 7 and compared with the results
obtained by our 2D thermo-electric model. The relative emission tuning for the lasers presented
above as a function of the modulation frequency is shown. One can see that despite the different
laser wavelengths the relative tuning of the two devices as function of the modulation frequency
is similar and in good agreement with the simulation results.
In order to show the mode-hop free nature of the devices shown, we used one of the presented devices to measure the transmittivity of N2 O. In Fig. 8, the transmission spectrum measured with a modulation frequency of 100 Hz is shown and compared with the data from HITRAN database [11]. Good agreement between the two curves can be observed with a resolution smaller than 0.1 cm −1 .
#250652
© 2015 OSA
Received 23 Sep 2015; revised 23 Oct 2015; accepted 27 Oct 2015; published 4 Nov 2015
16 Nov 2015 | Vol. 23, No. 23 | DOI:10.1364/OE.23.029715 | OPTICS EXPRESS 29720
7.6 12.2
0.5
6.3 10.2
0.4
5.0 8.1
0.3
3.8 6.1
0.2
2.5 4.0
]
0.6
✁ ✁
[cm
[%]
1
✄
✁
0.1
0.0 -3
10
1274 [cm✂1 ]
2043 [cm✂1 ]
Simulation
-2
-1
1.2 2.0
0
10
10
10
Frequency [kHz]
10
1
0.0 0.0
Fig. 7. Relative tuning as function of the modulation frequency for the two devices presented above. In black comparison with the simulated tuning is presented.
Fig. 8. Red: Optical transmission simulated for N2 O gas from HITRAN. Black: N2 O transmission measured with a modulation frequency of 100 Hz using the IH-QCL emitting at
7.8 µm. (P=0.02 atm, L=10 cm)
#250652
© 2015 OSA
Received 23 Sep 2015; revised 23 Oct 2015; accepted 27 Oct 2015; published 4 Nov 2015
16 Nov 2015 | Vol. 23, No. 23 | DOI:10.1364/OE.23.029715 | OPTICS EXPRESS 29721
4.
Conclusions
To summarize, we have demonstrated that an integrated resistive heater buried on the side of the
AR of a DFB QCL can be used to change independently and fast the laser temperature and to
increase the electrically accessible tuning range of DFB QCLs. This method can be successfully
applied to all the buried heterostructure lasers, becoming an attractive solution for molecular
spectroscopy in the IR.
Acknowledgments
The research leading to these results has received funding from the European Union Seventh
Framework Programme ( FP7/2007- 2013 ) under grant agreement n 317884, the collaborative
Integrated Project MIRIFISENS. The authors want to acknowledge Dr. Lorenzo Lugani for
careful reading of the paper.
#250652
© 2015 OSA
Received 23 Sep 2015; revised 23 Oct 2015; accepted 27 Oct 2015; published 4 Nov 2015
16 Nov 2015 | Vol. 23, No. 23 | DOI:10.1364/OE.23.029715 | OPTICS EXPRESS 29722