alpes lasers - quantum cascade laserscreated date 2/4/2015 11:57:20 am

High performance, low dissipationquantum cascade lasers across the

mid-IR range

Alfredo Bismuto,∗ Stephane Blaser, Romain Terazzi, Tobias Greschand Antoine Muller

Alpes Lasers SA, 1-3 Passsage Max Meuron, CH-2001 Neuchatel, Switzerland∗[email protected]

Abstract: In this work, we present the development of low consumptionquantum cascade lasers across the mid-IR range. In particular, short cavitysingle-mode lasers with optimised facet reflectivities have been fabricatedfrom 4.5 to 9.2 µm. Threshold dissipated powers as low as 0.5 W wereobtained in continuous wave operation at room temperature. In addition,the beneficial impact of reducing chip length on laser mounting yield isdiscussed. High power single-mode lasers from the same processed wafersare also presented.

© 2015 Optical Society of AmericaOCIS codes: (140.5965) Semiconductor lasers, quantum cascade; (230.1480) Bragg reflectors;(250.5960) Semiconductor lasers; (140.0140) Lasers and laser optics.

References and links1. J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson and A. Y. Cho, ”Quantum cascade laser,” Science

264, 553 (1994).2. 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).3. B. Hinkov, A. Bismuto, Y. Bonetti, M. Beck, S. Blaser, and J. Faist, ”Singlemode quantum cascade lasers with

power dissipation below 1 W,” Electronics Letters 48(11), 646-647 (2012).4. F. Xie, C. G. Caneau, H. P. LeBlanc, N. J. Visovsky, S. Coleman, L. C. Hughes, and C. Zah, ”Room Temperature

CW Operation of Mid-IR Distributed Feedback Quantum Cascade Lasers for CO2, N2O, and NO Gas Sensing,”IEEE J. Quantum Electron. 18, (5), 1605 (2012).

5. R. M. Briggs, C. Frez, C. E. Borgentun, and S. Forouhar, ”Regrowth-free single-mode quantum cascade laserswith power consumption below 1W,” Appl. Phys. Lett. 105, 141117 (2014).

6. J. Faist, D. Hofstetter, M. Beck, T. Aellen, M. Rochat and S. Blaser, ”Bound-to-continuum and two-phononresonance quantum cascade lasers for high duty cycle, high temperature operation,” IEEE J. Quantum Electron.38 (6), 533 (2002).

1. Introduction

In the recent years, significant improvement has been made in the development of efficient highpower quantum cascade lasers [1]. This effort has led to multi-watt emission from quantum-cascade lasers (QCL) using devices that can consume up to 20-50 W of electrical power [2].These devices have an important impact on many applications, e.g. counter-measures, photo-acoustic spectroscopy or cavity ring-down spectroscopy. However, in many of the commercialapplications, optical powers of few tens of mWs are sufficient. The minimization of the powerdissipation in the light source becomes crucial [3–5]. In fact, in order to remove the heat gener-ated by a standard QCL, the power consumption of the cooling system itself can exceed 100W,constraining heavily the packaging options and domains of application of these sources.

#228837 - $15.00 USD Received 10 Dec 2014; revised 19 Jan 2015; accepted 2 Feb 2015; published 23 Feb 2015 (C) 2015 OSA 9 Mar 2015 | Vol. 23, No. 5 | DOI:10.1364/OE.23.005477 | OPTICS EXPRESS 5477

In the present work, we will focus on the development of low consumption single modeQCLs from 4.5 µm to 9.2 µm. In addition, we analyze on the impact of the chip length on thedevice’s price and fabrication yield. Optimizing the chip length and the facet reflectivity ratherthan active region or grating design, allows moreover to fabricate both low dissipation devicesand high optical power devices at the same time.

2. Chip yield

The major cause of failure in QCLs is related to the presence of defects along the laser waveg-uide. These defects can be originated both from the epitaxial growths and from the laser devicefabrication steps. In order to reduce the defect density, fabrication process optimization wascarried out in order to produce buried heterostructure devices with narrow ridges while keepinglow optical losses. The active region etching procedure was optimized resulting in lasers withactive regions as narrow as 2.5 µm, e.g. see Fig. 1. Only wet etching has been used in order tominimize the sidewalls roughness. The lasers were then processed in a buried heterostructureconfiguration using Metalorganic Vapour Phase Epitaxy (MOVPE) for the selective growth ofIron-doped InP. In case of randomly distributed defects, the failure probability follows a Pois-

Fig. 1. SEM picture of a narrow ridge device.

sonian law which depends both on the defect density λ and on the device dimensions:

P =n

∑k=1

λk ek

k!(1)

Based on the data collected by our team in the previous years, we were able to define an effectivedefect density of 4 e−5 defects/mm2 which results in an approximate failure rate of 25 % fora 2.5 mm-long laser. The absolute values are still preliminary and based only on local data,nevertheless the relative trends are independent on the chosen defect density. In the Fig. 2 (rightaxis), the estimated failure probability is plotted as function as the device length. One can seethat a reduction in the device length from 3 mm to 0.75 mm does lead to a 4-fold decrease ofthe failure rate. Therefore the reduction of the chip length has a two-fold impact on the numberof chips per wafer. In Fig. 2 (left axis), the predicted maximum number of available devices per2-inch wafer is shown. It can be seen that values as high as 10000 chips per wafers could beachieved resulting in a marginal fabrication cost per chip already on a 2-inch wafer. In addition,as mentioned in the introduction, the reduced power dissipation has a important impact on thepackaging costs, which constitute a big fraction of the laser system.


Fig. 2. Right axis: failure probability as function of the laser length for a 3.5 µm-wide laser.Left axis: number of lasers available for a 2 inch wafer.

3. Optimisation of facet reflectivity

In order to reduce the laser dissipation in a distributed-feedback (DFB) laser, we decided notto increase the grating coupling or reduce the active region doping levels. Instead, we decidedto focus on the optimization of the laser length and facet reflectivities to obtain low dissipationQLCs without the need for dedicated fabrication [4]. As we will see, this permitted us to fab-ricate devices with electrical dissipations at threshold as low as 0.5 W and still obtain lasersfrom the same processed wafer that could deliver up to 300mW in continuous wave operation.All the lasers were coated using metallic high reflection (HR) coatings on the back facets. Inaddition, partial dielectric coatings (PR) were deposited on the front facets to further reducethe threshold dissipation. The length of the devices used in this work has been selected in orderto avoid an impact of the coatings on the single mode yield. The facet reflectivities have beenoptimized in order to provide an equivalent KL value as for 2-2.5 mm long devices. Thesevalues has been chosen since our grating is generally optimized for 2-2.5 mm long devices. Nosensitive difference has been observed in the single-mode yield.

In Fig. 3, the impact of the facet’s coating is shown for a 750 µm-long and 2.5 µm-wideDFB emitting at 4.5 µm. The red curves show the characteristics of the device with HR coatingof the back facet only; in blue are instead the laser performance after the dielectric front facetcoating. It is interesting to see that the impact of the front coating on the laser threshold isso relevant that the laser emitted power from the front facet is not quenched due to the frontPR coating as expected; actually the emitted power increases resulting in a net increase of thedevice’s efficiency.

4. Low dissipation DFBs

Different spectral regions across the mid-IR range have been chosen due to their commercialinterest and low dissipation QCLs have been fabricated across the range. The various activeregions used in this work are based on the two-phonon resonances or bound-to-continuumdesigns [6] and made by lattice matched and strain-balanced InGaAs/InAlAs on InP substrates.The wafers were grown by either low-pressure metalorganic vapor phase epitaxy (MOVPE)or molecular beam epitaxy (MBE). All the presented laser devices are processed in buriedheterostructure (BH) configuration.


Fig. 3. Light-Voltage-Current characteristics of a 750 µm-long, 2.5 µm-wide DFB laseremitting at 4.5 µm. The curves are shown before (in red) and after (in blue) the frontdielectric coating. Curves of the device before back-facet HR coating are not shown sinceno lasing action was observed. In both cases, laser emission is single mode across the wholerange.

In Fig. 4, all the lasers presented in this work are shown, in particular we focused on 7different active region designs. On the right axis of the figure, the electrical dissipation is plottedat threshold, both at -30C (red markers) and at 20C (orange markers). The rollover dissipation isalso shown (black markers). It can be seen that threshold powers as little as 0.3 W are observedat -30 C and at room temperature, while rollover dissipations as low as 0.7W are shown.

4.1. First atmospheric window

In Fig. 5 (left), the power-current-voltage characteristics of a 3.5 µm-wide and 750 µm-longdevice emitting at 4.5 µm is shown. The device threshold current at room temperature is lessthan 50 mA and the rollover current is smaller than 100 mA. In spite of that, the laser lasesin continuous wave operation up to 50 C and the optical power is higher than 10 mW at roomtemperature.

In Fig. 5 (right), the emitted optical power is plotted as a function of the injected power.We can see that the threshold dissipation is as low as 0.35 W at -30C and 0.5 W at 20C. Inthe inset, due to the limited area, only one spectrum for each submount temperature is shown.Nevertheless, as it can be seen from the shape of the LI characteristic, the laser is single modeacross the whole current and temperature range explored.

Similar curves are presented in Fig. 6 for a device emitting at 5.25 µm. In this case, therollover dissipation is lower than 0.75 W resulting in a device without the need for active cool-ing and that can be mounted in low dissipation packages generally used for SWIR diode lasers.Optical powers of few mWs are still observed. Also in this case the spectral emission is single-mode across the whole range. It is important to mention that in this work we concentrated onthe optimization of the process and the facet reflectivity. For this reason, we have decided toapply the concept also on non-optimal active region designs.


Fig. 4. Selected spectra of the low dissipation devices fabricated in the framework of thepresent work. Electrical power dissipations at threshold for -30C are shown as red markersand at room temperature as orange markers. Roll-over dissipations are shown as blackmarkers.

Fig. 5. Left: Light-Voltage-Current characteristics as a function of the temperature for a 750µm-long, 3.5 µm-wide DFB laser emitting at 4.50 µm. Right: Optical power vs electricalpower dissipation. In the inset, some spectra are shown at different submount temperatures.

4.2. Second atmospheric window

In Fig. 7 (left), the power-current-voltage characteristics of a 10.5 µm-wide and 750 µm-longdevice emitting at 7.85 µm is shown. The device threshold current at room temperature isless than 100 mA and the rollover current is smaller than 240 mA. In spite of that, the laserlases in continuous wave operation up to 50 C and the optical power is higher than 50 mW atroom temperature. In the Fig. 7 (right), the emitted optical power is plotted as a function of theinjected power. We can see that the threshold dissipation is as low as 0.55 W at -30C and 0.75 Wat 20C. In the inset, due to the limited area, only one spectrum for each submount temperature


Fig. 6. Left: Light-Voltage-Current characteristics as a function of the temperature for a 750µm-long, 6.6µm-wide DFB laser emitting at 5.26 µm. Right: Optical power vs electricalpower dissipation. In the inset, some spectra are shown at different submount temperatures.

Fig. 7. Left: Light-Voltage-Current characteristics as a function of the temperature for a 750µm-long, 10.5µm-wide DFB laser emitting at 7.82 µm. Right: Optical power vs electricalpower dissipation. In the inset, some spectra are shown at different submount temperatures.

is shown. As it can be seen from the shape of the LI characteristic, the laser is single modeacross the whole currents and temperatures range explored. In Fig. 8, the performances of a 1mm-long and 12.4 µm-wide laser emitting at 8.4 µm are show. It can be seen that in this casethe power dissipation is sensibly bigger than for the other devices and further improvement isstill needed on the active region design. Devices at 9.2 µm are being tested as shown in the Fig.4, but optimisation of the front dielectric coating is still ongoing and the data reported are froma device where the front facet is left uncoated while the back facet is coated with a metallic HRcoating.

5. High output power dissipation DFBs

As mentioned above, the choice of optimising the facet reflectivities, rather than the activeregion doping or DFB grating coupling, allows to obtain from a single process a wide spectrumof optical powers and electrical dissipations. In order to show it, we present here two highoptical power devices at 4.57 µm (Fig. 9) and 7.72 µm (Fig. 10) obtained from the sameprocessed wafers as the low dissipation devices. The only major difference is the device length


Fig. 8. Left: Light-Voltage-Current characteristics as function of the temperature for 1 mm-long, 12.4µm-wide DFB laser emitting at 8.40 µm. Right: Optical power vs electricalpower dissipation. In the inset, some spectra are shown at different submount temperatures.

Fig. 9. Left: Light-Voltage-Current characteristics as a function of the temperature for a2.25 mm-long, 8.5 µm-wide DFB laser emitting at 4.57 µm. Right: Optical power vselectrical power dissipation. In the inset, some spectra are shown at different submounttemperatures.

and the absence of the coatings. It is important to remind that both the devices are mounted epi-side up and that single-mode spectral emission is obtained across the whole range of operation.As it can be seen, total optical powers of more than 300 mW were obtained at both extremes ofthe spectral range.

6. Conclusion

In the present work, we present low dissipation quantum cascade lasers from 4.5 to 9.2 µmobtained by optimising the laser facets reflectivities. The impact of device length on laser’sfailure rate is also presented showing that the light source yield increase with decreasing chiplength. High power DFB lasers from the same processed wafers are also presented with opticalpowers as high as 300 mW for epi-side up mounted devices.


Fig. 10. Left: Light-Voltage-Current characteristics as a function of the temperature fora 2.25 mm-long, 10 µm-wide DFB laser emitting at 7.72 µm. Right: Optical power vselectrical power dissipation. In the inset, some spectra are shown at different submounttemperatures.

Acknowledgments

The research leading to these results has received funding from the European Union SeventhFramework Programme (FP7/2007-2013) under grant agreement n317884, the collaborativeIntegrated Project MIRIFISENS.


alpes lasers - quantum cascade laserscreated date 2/4/2015 11:57:20 am

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