We present a wavelength-tunable, external-cavity GaSb-based quantum-well laser operating near 3.2 μm. The laser setup consists of an intra-cavity grating in Littman-Metcalf configuration and a cascade pumped GaSb-based gain chip with a narrow-ridge waveguide. The narrow-ridge waveguide has a length of 2 mm and width of 7.5 μm. Cascade pumping is realized with three type-I quantum-wells, using one quantum-well per cascade stage. The laser provides continuous-wave output powers up to 8 mW and slope-efficiencies of 13 % at room temperature. Laser operation is demonstrated over a wavelength range of more than 300 nm, using continuous-wave and pulsed operation regimes

Belenky, Gregory

Chichkov, Nikolai B.

Kipshidze, Gela

Rafailov, Edik U.

Shterengas, Leon

Wang, Meng

Yadav, Amit

Zherebtsov, Evgeny

English

Nikolai B. Chichkov

Amit Yadav

Evgeny Zherebtsov

Meng Wang

Gela Kipshidze

Gregory Belenky

Leon Shterengas

Edik U. Rafailov

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Wavelength-Tunable, GaSb-Based, Cascaded Type-I Quantum-Well Laser Emitting Over a Range of 300 nm

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1041-1135 (c) 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LPT.2018.2873049, IEEE PhotonicsTechnology Letters1Wavelength-tunable, GaSb-based, cascaded type-Iquantum-well laser emitting over a range of 300 nmNikolai B. Chichkov, Amit Yadav, Evgeny Zherebtsov, Meng Wang, Gela Kipshidze, Gregory Belenky,Leon Shterengas, and Edik U. RafailovAbstract—We present a wavelength-tunable, external-cavityGaSb-based quantum-well laser operating near 3.2 µm. Thelaser setup consists of an intra-cavity grating in Littman-Metcalfconfiguration and a cascade pumped GaSb-based gain chip witha narrow-ridge waveguide. The narrow-ridge waveguide has alength of 2 mm and width of 7.5 µm. Cascade pumping is realizedwith three type-I quantum-wells, using one quantum-well percascade stage. The laser provides continuous-wave output powersup to 8 mW and slope-efficiencies of 13 % at room temperature.Laser operation is demonstrated over a wavelength range ofmore than 300 nm, using continuous-wave and pulsed operationregimes.Index Terms—Semiconductor lasers, quantum well lasers, lasertuning, mid-infrared.I. INTRODUCTIONWAVELENGTH tunable, continuous wave (CW) lasersoperating in the mid-infrared (mid-IR) range havebeen actively investigated in the last two decades. A widerange of commercial devices is available and used for variousapplications in industry and science, including medical diag-nostics, molecular spectroscopy, and industrial process control.Tunable mid-IR lasers are most extensively used for trace-gas sensing applications, such as environmental monitoring,clinical breath analysis, and atmospheric sensing [1], [2]. Sev-eral approaches to extend wavelength-tuning ranges, increaseoutput powers, and access new wavelength regions have beendemonstrated, including nonlinear frequency conversion, Cr-doped crystal lasers, fluoride fiber lasers, quantum-cascadelasers, and GaSb-based diode lasers [2]–[9].GaSb-based type-I quantum-well (QW) diode lasers arepromising sources for the realization of tunable mid-IR lasers.They extend the wavelength range accessible with diode-basedlaser systems and allow for straightforward integration intocompact, portable devices for mid-IR applications. The laseremission at room temperatures is adjustable from 1.9 µm to3.7 µm [8], [9] and is suitable for the detection of water vapor,methane, and carbon dioxide. The wavelength range of 2.9 µmThis work was supported by the National Science Foundation, GrantECCS-1408126US and in part by the U.S. Department of Energy, Officeof Basic Energy Sciences, through the Center for Functional Nanomaterials,Brookhaven National Laboratory, under Contract DESC0012704.N.B. Chichkov thanks the Alexander von Humboldt Foundation for thesupport provided by the Feodor Lynen Research Fellowship.N.B. Chichkov, A. Yadav, E. Zherebtsov, and E.U. Rafailov are with theAston Institute of Photonic Technologies, Aston University, Birmingham, UK(email: n.chichkov@aston.ac.uk).L. Shterengas, M. Wang, G. Kipshidze, and G. Belenky are with theDepartment of Electrical and Computer Engineering, Stony Brook University,Stony Brook, USA.to 3.7 µm covers the stretching region of C-H bonds and is ofparticular interest for spectroscopy. In this wavelength region,cascaded GaSb-based lasers provide significantly higher outputpowers than quantum-cascade laser counterparts [2]. CW out-put powers of up to 960 mW at room temperature have beendemonstrated from GaSb-based, multi-mode edge-emitters [9].The recent demonstrations of cascade pumping in GaSb-based quantum-well lasers have resulted in improved effi-ciencies and a more than two-fold increase in achievableoutput powers [9], [10]. Cascade pumping of GaSb-basedtype-I quantum wells allows for efficient carrier recyclingbetween QW stages and results in internal efficiencies ofmore than 100 %, thereby increasing output powers and slope-efficiencies.In this publication, we investigate the wavelength-tunabilityof a cascade-pumped, GaSb-based, type-I quantum-well laseremitting around 3.2 µm. We demonstrate laser emission overa wavelength range of more than 300 nm under CW andpulsed regimes of operation. This is the broadest wavelength-tuning range achieved with a GaSb-based laser, so far. Itis three times broader than the tuning ranges obtained withcomparable quantum-cascade-lasers emitting at wavelengthsnear 3 µm [6].II. EXPERIMENTAL SETUPThe wavelength-tunable laser setup uses an edge-emitting,cascade-pumped, GaSb-based, type-I QW laser. The het-erostructure of the cascade quantum well laser (CQW) isillustrated in Fig. 1(a). In the cascade pumping scheme, thefree-carrier electrons are recycled after each QW to improvelaser efficiency. This is realized with compositionally gradedAlGaAsSb sections and chirped InAs/AlSb superlattices be-tween the QWs. After each QW, superlattices transfer theelectrons from the valence band of one QW to the conductionband of the following QW. The graded AlGaAsSb sectionsbetween QWs act as electron-barriers and confine electronsinside the QWs. Further details on the heterostructure designand its impact on laser performance are discussed in [9], [10].The layer structure and waveguide design of the CQWlaser are illustrated in Fig. 1(b). The CQW laser is grownon a Tellurium-doped GaSb substrate using solid sourcemolecular beam epitaxy. The waveguide cladding is real-ized with two Al0.80Ga0.20As0.07Sb0.93 layers with thicknessesof 2 µm. The waveguide core consists of a 300 nm thickGaSb layer, three GaInAsSb QWs, and a 350 nm thickAl0.30Ga0.40In0.30As0.28Sb0.72 layer. The QWs have a thickness1041-1135 (c) 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LPT.2018.2873049, IEEE PhotonicsTechnology Letters2of ~13 nm, indium composition of ~55 %, and compressivestrain of ~1.5 %.The straight-ridge waveguide has a length of 2 mm, awidth of 7.5 µm, and is etched down to the interface betweenp-cladding and waveguide-core. An SEM-image of the cleavedwaveguide-facet is shown in Fig. 1(c). One waveguide-facet isanti-reflection (AR) coated with a triple-layer Al2O3/Si/Al2O3coating, resulting in a reflectivity of ~0.3%. The oppositewaveguide-facet is coated with a single Al2O3 layer, providinga reflectivity of ~32 %. The CQW lasers used for this workwere fabricated as described in [7].The external-cavity setup used for investigation of wave-length-tuning is illustrated in Fig. 2. The CQW gain chip ismounted epi-side-up on a temperature stabilized copper heatsink. The reflecting waveguide-facet acts as 68 % output cou-pler. The laser light from both waveguide-facets is collimatedwith aspheric lenses (4.0 mm focal length, 0.56 NA). Thelateral beam diameter after the collimating lens is estimatedto 1.4 mm (assuming a slow-axis beam divergence of ~20°).Wavelength-tuning is realized with a 450 grooves/mm intra-cavity grating in Littman-Metcalf configuration (V-shapedcavity with arm lengths of ~25 cm), providing an estimatedspectral feedback bandwidth of ~1.5 nm. The Littman-Metcalfconfiguration has been chosen to suppress side-lobes andmodulations arising from the Fabry-Perot formed by the CQWwaveguide-facets. The wavelength-tuning range of the laserwas extended by variation of the operating temperature from15 °C to 25 °C.The laser spectra have been measured with a Horiba iHR550monochromator and an amplified PbSe-detector in combina-tion with a lock-in amplifier. The CW spectra have beenrecorded with a wavelength increment of 0.1 nm, sub-0.5 nmresolution (~0.5 cm-1), and 30 dB signal-to-noise ratio. Thepulsed operation spectra have been measured with sub-1 nmresolution. In all spectra, the contributions from self-lasing inthe CQW waveguide (around 3200 nm) remained at relativepower densities below -10 dB.III. RESULTSThe LIV-characteristics of the CQW gain chip have beenmeasured in an external-cavity without grating and spectralfiltering. Fig. 3 shows the measured output power and forwardvoltage versus applied current at a constant temperature of20 °C. The threshold for lasing is about 114 mA, output powersaturation is observed for more than 360 mA of current, andthe slope efficiency is 0.05 mW/mA. The CQW laser providesa maximum output power of 8.2 mW at a laser current of360 mA and a voltage of 2.4 V. The inset in Fig. 3 showa typical output spectrum at the laser current of 360 mA.The spectrum has a bandwidth of more than 20 nm and ismodulated with a period of ~0.7 nm. The modulation is causedby the Fabry-Perot filter formed by the waveguide facets.Wavelength-tuning in CW operation is performed at a diodecurrent of 360 mA. Fig. 4 plots the measured laser spectraand output powers over the achieved tuning range. At aconstant operating temperature of 20 °C, the laser wavelengthis tunable from 3065 nm to 3266 nm. The maximum outputn-cladEcEVp-cladAl0.80Ga0.20As0.07Sb0.93GaSbAl0.30Ga0.40In0.30As0.28Sb0.72QW QW QWInAs/AlSbchirped SL(a)(b) 7.5 µmAl0.80Ga0.20As0.07Sb0.93 ~ 2 µmGaSb ~ 300 nm3 x QW ~ 350 nmAl0.80Ga0.20As0.07Sb0.93~ 2 µmAl0.30Ga0.40In0.30As0.28Sb0.72 ~ 350 nm2 mmTe-doped GaAs substrate7.5 µmn-claddingp-claddingwaveguide core(c)Fig. 1. Cascaded GaSb-based QW laser: (a) energy band schematic of theCQW heterostructure, (b) layer structure of the narrow-ridge waveguide, and(c) SEM-image of cleaved waveguide facet.Plane silvermirrorCQW chip4 mm lensesLaseroutputAR coated facet out-couplingGrating450 grooves/mmFig. 2. Experimental setup of wavelength tunable GaSb-based QW laser.power of 8.0 mW is obtained at the wavelength of 3206 nm.The output power drops to 4.6 mW at 3065 nm and to 4.2 mWat 3266 nm. The tuning range is further extended by changingthe operating temperature to 15 °C and 25 °C. At 15 °C, thelaser wavelength can be tuned to 3051 nm with an outputpower of 3.4 mW. At 25 °C, laser emission at 3307 nm withan output power of 1.7 mW is achieved. In all measurements,the self-lasing of the CQW waveguide remained at relativepower densities below -10 dB.Fig. 5 shows the normalized laser spectra at 3056 nm,3206 nm, and 3307 nm. The spectra have a sub-0.5 nmbandwidth (full-width at half-maximum), which is limit by theresolution of the measurement setup. The observed side-lobes1041-1135 (c) 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LPT.2018.2873049, IEEE PhotonicsTechnology Letters3                    & X U U H Q W   $       3 R Z H U   P :   D                     & X U U H Q W   $                    9 R O W D J H   9   E          Fig. 3. LIV-characteristics of external-cavity CQW laser without intra-cavitygrating: output power (a) and applied voltage (b) vs laser current. The insetshows the laser spectrum for a current of 360 mA.                  3 R Z H U  G H Q V L W \   P :  Q P    D      &    &    &                              : D Y H O H Q J W K   Q P           2 X W S X W  S R Z H U   P :   E      &    &    &Fig. 4. Spectrum (a) and output power (b) at different laser wavelengths andoperating temperatures.result from Fabry-Perot modulations of the CQW waveguide,with an estimated modulation period of 0.7 nm.Further temperature changes did not result in increased CWtuning-ranges in our setup. The laser operation range couldbe further extended to shorter wavelengths by use of pulsedoperation. At 12 °C operating temperature, using 400 nspulses with 1.8 A peak current and 10 kHz repetition rate,we obtained laser emission at 2986 nm. The correspondingspectrum is shown in Fig. 5 and has a bandwidth of 1.8 nm.The presented results correspond to a CW wavelength-tuning range of 256 nm and laser operation over a range of320 nm. The wavelength-tuning is limited by the onset of self-lasing in the CQW waveguide, indicating that broader tuning-ranges may be achieved by reducing the reflectivity of theAR-coated CQW waveguide facet.IV. CONCLUSIONThe wavelength-tuning of a cascade pumped, GaSb-basedtype-I QW laser in an external-cavity configuration has beeninvestigated. A CW tuning range of 256 nm from 3056 nm to                            3 R Z H U  G H Q V L W \   D U E   X       P :      &          : D Y H O H Q J W K   Q P      P :      &              P :      &              Q V      &Fig. 5. Normalised spectra at the shortest CW wavelength, maximum outputpower, longest CW wavelength, and in pulsed operation (from left to right).3307 nm, with output powers between 1.7 mW to 8.0 mW, hasbeen demonstrated. Pulsed laser operation has been achievedat wavelengths down to 2986 nm, corresponding to laseroperation (CW and pulsed) over a range of 320 nm. Furtherwavelength-tuning is limited only by the onset of self-lasingin the straight-ridge CQW waveguide. To the best of ourknowledge, this is the broadest tuning range demonstratedfrom an edge-emitting, CQW laser, so far.The achieved results highlight the potential of GaSb-based,cascade pumped, type-I quantum well lasers for the realizationof broadly tunable mid-IR laser devices. Future work will befocused on the use of tilted ridge waveguide designs to reducereflections from the AR-coated waveguide facet. This is ex-pected to suppress self-lasing of the CQW waveguide, reducespectral side-lobes, and further increase the wavelength-tuningrange.REFERENCES[1] T. Risby and F. K. Tittel, “Current status of midinfrared quantum andinterband cascade lasers for clinical breath analysis,” Opt. Eng., vol. 49,no. 11, p. 111123, 2010.[2] R. F. Curl, F. Capasso, C. Gmachl, A. A. Kosterev, B. McManus,R. Lewicki, M. Pusharsky, G. Wysocki, and F. K. Tittel, “Quantumcascade lasers in chemical physics,” Chem. Phys. Lett., vol. 487, no.1-3, pp. 1–18, 2010.[3] D. Brida, C. Manzoni, G. Cirmi, M. Marangoni, S. 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Amann, “Room-temperature 3.73 νm GaSb-based type-I quantum-well lasers with quinternary barriers,” Semicond.Sci. Technol., vol. 27, no. 3, 2012.[9] L. Shterengas, R. Liang, G. Kipshidze, T. Hosoda, S. Suchalkin, andG. Belenky, “Cascade pumping of 1.9-3.3 µm type-I quantum wellGaSb-based diode lasers,” IEEE J. Sel. Top. Quantum Electron., vol. 23,p. 1500708, 2017.[10] L. Shterengas, R. Liang, G. Kipshidze, T. Hosoda, G. Belenky, S. S.Bowman, and R. L. Tober, “Cascade type-I quantum well diode lasersemitting 960 mW near 3 µm,” Appl. Phys. Lett., vol. 105, no. 16, pp.1–5, 2014.

Wavelength-tunable, GaSb-based, cascaded type-I quantum-well laser emitting over a range of 300 nm

https://publications.aston.ac.uk/id/eprint/37343/1/Wavelength_tunable_GaSb_based_cascaded_type_I_quantum_well_laser_emitting_over_a_range_of_300_nm.pdf

Wavelength-tunable, GaSb-based, cascaded type-I quantum-well laser emitting over a range of 300 nm

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