Extended tuning of midir quantum cascade lasers using integrated resistive heaters

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 N2O 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 1. A. Bauer, K. Rößner, T. Lehnhardt, M. Kamp, S. Höfling, L. Worschech, and A. Forchel, “Mid-infrared semiconductor heterostructure lasers for gas sensing applications,” Semicond. Sci. Tech. 26, 014032 (2011). 2. G. von Basum, H. Dahnke, D. Halmer, P. Hering, and M. Mürtz, “Online recording of ethane traces in human breath via infrared laser spectroscopy,” J. Appl. Physiol. 95, 2583–2590 (2003). 3. J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, “Quantum Cascade Laser,” Science 264, 553–556 (1994). 4. A. Bismuto, S. Blaser, R. Terazzi, T. Gresch, and A. Muller, “High performance, low dissipation quantum cascade lasers across the mid-IR range,” Opt. Express 23, 5477 (2015). 5. Y. Bai, N. Bandyopadhyay, S. Tsao, S. Slivken, and M. Razeghi, “Room temperature quantum cascade lasers with 27% wall plug efficiency,” Appl. Phys. Lett. 98, 181102 (2011). 6. J. Faist, C. Gmachl, F. Capasso, C. Sirtori, D. L. Sivco, J. N. Baillargeon, and A. Y. Cho, “Distributed feedback quantum cascade lasers,” Appl. Phys. Lett. 70, 2670 (1997). 7. A. Bismuto, J. Wolf, A. Muller, and J. Faist, “Tunable laser, a method for making, and a method for operating such a laser,” (2014). US Patent Application 14/480,812. 8. A. Bismuto, T. Gresch, A. Bächle, and J. Faist, “Large cavity quantum cascade lasers with InP interstacks,” Appl. Phys. Lett. 93, 231104 (2008). 9. A. Lops, V. Spagnolo, and G. Scamarcio, “Thermal modeling of GaInAs/AlInAs quantum cascade lasers,” J. Appl. Phys. 100, 043109 (2006). #250652 Received 23 Sep 2015; revised 23 Oct 2015; accepted 27 Oct 2015; published 4 Nov 2015 © 2015 OSA 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 velocity in semiconductor materials,” Mat. Sci. Semicon. Proc. 3, 149 (2000). 11. L. S. Rothman, I. E. Gordon, Y. Babikov, A. Barbe, D. C. Benner, P. F. Bernath, M. Birk, L. Bizzocchi, V. Boudon, L. R. Brown, A. Campargue, K. Chance, E. A. Cohen, L. H. Coudert, V. M. Devi, B. J. Drouin, A. Fayt, J. M. Flaud, R. R. Gamache, J. J. Harrison, J. M. Hartmann, C. Hill, J. T. Hodges, D. Jacquemart, A. Jolly, J. Lamouroux, R. J. Le Roy, G. Li, D. A. Long, O. M. Lyulin, C. J. Mackie, S. T. Massie, S. Mikhailenko, H. S. P. Mller, O. V. Naumenko, A. V. Nikitin, J. Orphal, V. Perevalov, A. Perrin, E. R. Polovtseva, C. Richard, M. A. H. Smith, E. Starikova, K. Sung, S. Tashkun, J. Tennyson, G. C. Toon, V. G. Tyuterev, and G. Wagner, “The HITRAN 2012 molecular spectroscopic database,” J. Quant. Spectr. Ra. 130, 4 (2013).


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.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].

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 10 17 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 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.Using the common relation ∆ν ν = 8 • 10 −5 ∆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   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 N 2 O.In Fig. 8, the transmission spectrum measured with a modulation frequency of 100 Hz is shown and compared with the data from HI-TRAN database [11].Good agreement between the two curves can be observed with a resolution smaller than 0.

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.

Fig. 1 .
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.

Fig. 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 the common relation ∆ν ν = 8 • 10 −5 ∆T .

Fig. 3 .
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.

Fig. 4 .
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 .
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.

Fig. 6 .
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.

1 ]Fig. 7 .
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.