a2014_1.pdf CFB3.pdf
Widely Tunable, High Power, Mode-hop Free, CW External Cavity Quantum Cascade Laser at 8.4µm
Gerard Wysockia, Robert F.Curla, Frank K. Tittela, Federico Capassob, Laurent Diehlb, Mariano Troccolic, Gloria Höflerc, Richard Maulinid, Jérôme Faistd a
Rice Quantum Institute, Rice University, 6100 Main St., Houston, TX 77005, USA b Harvard University, 9 Oxford St., Cambridge MA 02138 c Argos Tech, LLC, 3671 Enochs St., Santa Clara, CA 950051 d Institute of Physics, University of Neuchâtel, 1 A.-L. Breguet, CH-2000 Neuchâtel, Switzerland Author e-mail address:
[email protected]
Abstract: An external cavity quantum cascade laser (λ =8.4 µm) is reported. The laser operating at -30oC exhibits a single mode tuning range of 135 cm−1 providing up to 50 mW of CW laser radiation. ©2007 Optical Society of America OCIS Codes: (140.3600) Lasers, tunable; (300.6360) Spectroscopy, laser
The recent advances of quantum cascade lasers (QCLs) fabricated by band structure engineering offer an attractive source option for ultra-high resolution and sensitivity infrared absorption spectroscopy applications [1]. QCLs operating in continuous-wave (CW) mode at room temperatures or temperatures accessible with thermoelectric cooling and emitting over 100 mW optical power were reported by several research groups worldwide [2-5]. This feature makes them suitable for spectroscopic applications which require compact, sensitive, liquid-nitrogen free sources as for example in trace gas analyzers. For high resolution spectroscopic measurements single frequency operation is required, which is usually achieved by introducing a distributed feedback (DFB) structure into the QCL active region. DFB QCLs show high performance and reliability, however wavelength tuning of the emitted light can be performed within a very limited spectral range by varying either temperature of the chip or the laser injection current. Typically the maximum thermal tuning range of DFB-QCLs is ~10 cm-1. Another drawback of this method is a substantial decrease in laser power associated with an increasing operating temperature. The intrinsic spectral gain profile of QCLs is in fact much broader than 10 cm-1 and depending on the particular QCL structure design can provide very wide frequency tuning capability. By employing bound-to-continuum transitions in combination with heterogeneous cascade design an electroluminescence spectrum of 350 cm-1 FWHM (full width at half maximum) at room temperature was observed for a QC structure emitting at λ ≈ 9 µm [6]. Such QCL active region can provide sufficient amplification to achieve laser action within a very broad spectral range. Therefore in order to take advantage of this broadband tunability potential an external cavity (EC) configuration can be implemented for wavelength selection [6,7]. The advances obtained with an improved EC-QCL configuration based on our previous work published in Ref. 7 will be reported. The new spectroscopic source as depicted in Fig. 1 is based on the Littrow type grating coupled EC configuration similar to the previous prototype platform [7]. Several modifications were applied to the EC-QCL source in order to improve its performance. In the present setup the QCL is enclosed in a compact vacuum tight housing equipped with TEC laser temperature stabilization and optional chilled water cooling, which allows laser operation temperatures down to −40oC. The housing itself can be used in any QCL based system. However, its design was optimized to provide maximum functionality and configuration flexibility in EC-QCL systems. To avoid any additional intra-cavity elements (e.g. windows) a sealed X-Y-Z positioning manipulator used for collimating lens alignment, capable of accepting up to 1 inch diameter optics, was designed as an integral part of the housing. Furthermore, the optical configuration of the EC system was improved. In the present arrangement the beam walkoff caused by grating angle tuning, which was observed in the previous system, was eliminated resulting in both direction and position of the laser beam to be completely independent of the laser wavelength tuning process. The present laser utilizes a QCL gain chip operating at λ ≈ 8.4 µm with a gain region fabricated as a buried heterostructure using metal-organic chemical vapor deposition technology (MOCVD). Due to improved thermal management in the active region the gain chip is capable of achieving higher output power and high temperature operation (CW operation up to 130oC). A set of LIV curves measured at every stage of the gain chip preparation for EC system is shown in Fig. 2a. The maximum CW power of ~50 mW was obtained when the EC-QCL operates at −30oC. The maximum frequency tuning range of 135 cm-1 (∆λ ≈ 1µm) was obtained in both pulsed (see Fig. 2b) and CW laser operation. This represents a substantial performance improvement in comparison to our previously reported source [7] operating at λ ≈ 5.2 µm with a maximum output power of 1.1mW and tuning range of 35cm-1, both measured at -30oC. The EC architecture is also equipped with a piezo-activated cavity mode tracking system
©OSA 1-55752-834-9
a2014_1.pdf CFB3.pdf
for mode-hop free frequency tuning. Several specific applications of this EC-QC laser source in high resolution molecular spectroscopy and multi species trace-gas detection will be described.
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Fig. 1 (a) A photograph and (b) a schematic diagram of the EGC QCL laser and the associated measurement system. 0.0
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Fig. 2 (a) LIV curves for the MOCVD grown QCL gain medium operated at -30 oC measured at different stages of chip processing and after its implementation into the EC-QCL. (b) Wavelength tuning range of the EC-QCL together with the corresponding optical power acquired for the QCL operated in a pulsed mode.
Acknowledgements: This work was partially funded by the NSF ERC MIRTHE project. References [1] F.K. Tittel, Y. Bakhirkin, A.A. Kosterev. and G. Wysocki, "Recent Advances in Trace Gas Detection Using Quantum and Interband Cascade Lasers," The Review of Laser Engineering, Vol.34, No.4, 275-284 (2006) [2] L. Diehl, D. Bour, S. Corzine, J. Zhu, G. Hoefler, M. Loncar, M. Troccoli, and F. Capasso, 2006: High-power quantum cascade lasers grown by low-pressure metal organic vapor-phase epitaxy operating in continuous wave above 400K. Appl. Phys Lett., Vol. 88, 201115 (2006) [3] S. R. Darvish, S. Slivken, A. Evans, J. S. Yu, and M. Razeghi, “Room-temperature, high-power, and continuous-wave operation of distributed-feedback quantum-cascade lasers at λ ~ 9.6 µm”, Appl. Phys. Lett. 88, 201114 (2006) [4] C. Mann, Q. Yang, F. Fuchs, W. Bronner, K. Köhler, and J. Wagner, “Influence of Injector Doping Concentration on the Performance of InPBased Quantum-Cascade Lasers”, IEEE Journal Of Quantum Electronics 42, No. 10, 994 (2006) [5] C. Faugeras, S. Forget, E. Boer-Duchemin, H. Page, J.-Y. Bengloan, O. Parillaud, M. Calligaro, C. Sirtori, M. Giovannini, and J. Faist, ” High-Power Room Temperature Emission Quantum Cascade Lasers at λ =9 µm”, IEEE Journal Of Quantum Electronics 41, No. 12, 1430 (2005) [6] R. Maulini, A. Mohan, M. Giovannini, Jérôme Faist, E. Gini, “External cavity quantum-cascade laser tunable from 8.2 to 10.4 µm using a gain element with a heterogeneous cascade”, Appl. Phys. Lett. 88, 201113 (2006) [7] G. Wysocki, R. F. Curl, F. K. Tittel, R. Maulini, J. M. Bulliard, J. Faist, "Widely tunable mode-hop free external cavity quantum cascade laser for high resolution spectroscopic applications", Appl. Phys. B 81, 769-777 (2005)
