914 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 45, NO. 8, AUGUST 2009 A Compact Four-Wavelength Quantum-Cascade Laser Source Fatima Toor, Student Member, IEEE, Scott S. Howard, Member, IEEE, Deborah L. Sivco, and Claire F. Gmachl, Senior Member, IEEE Abstract—A four-wavelength quantum-cascade (QC) laser source that operates using a single current channel is presented. The source includes two different heterogeneous cascade QC lasers, one with emission wavelengths of 7.0 m and 11.2 m, and the other with 8.7 m and 12.0 m. For 3.0-mm and 3.5-mm cavity lengths, QC lasers with emission wavelengths of 8.7, 11.2, and 12.0 m have threshold current densities within less than a factor of 2, which allows them to be conveniently driven in series by a single current source. Index Terms—Mid-infrared, multiwavelength lasers, quantum- cascade (QC) laser, semiconductor lasers. I. INTRODUCTION Q UANTUM-CASCADE (QC) lasers can be engineered to emit light of almost any wavelength in the mid- and far-infrared (IR) region from 3 to 24 m [1], [2]. QC lasers are based on intersubband transitions in quantum wells; therefore, by choosing the appropriate well and barrier thick- nesses, the band structure is engineered to emit the wavelength of choice. Moreover, QC laser technology in the mid-IR range has great potential for applications in environmental, medical, and industrial trace gas sensing [3]–[7], since a vast majority of chemical vapors have strong rovibrational frequencies in this range and are uniquely identifiable by their absorption spectra through optical probing of absorption and transmission. There- fore, having a wide range of QC laser wavelengths in a single QC laser source would greatly increase the specificity of QC laser-based spectroscopic systems and make the spectroscopic systems compact and field deployable. Here, we report work on a four-wavelength QC laser source that takes advantage of band-structure engineering to emit four different wavelengths using a single current channel. The four emission wavelengths are m, 8.7 m, 11.2 m, and 12.0 m spread across the second atmospheric transmission Manuscript received October 11, 2008; revised December 08, 2008. This work was supported in part by MIRTHE (NSF-ERC). Current version published July 01, 2009. F. Toor and C. F. Gmachl are with the Department of Electrical Engineering, Princeton University, Princeton, NJ 08544 USA (e-mail: (ftoor@ Princeton.edu; cgmachl@Princeton.edu). D. L. Sivco is with Alcatel-Lucent, Murray Hill, NJ 07974 USA (e-mail: dls@alcatel-lucent.com). S. S. Howard was with the Department of Electrical Engineering, Princeton University, Princeton, NJ 08544. USA. He is now with the School of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853 USA (e-mail: showard314@gmail.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JQE.2009.2014882 window. The QC laser source is comprised of two lasers con- nected in series, with each based on a heterogeneous cascade QC laser design [8], [9] with two different wavelength active cores. Hence, the four wavelength source is based on a 2 2 QC laser module with 2 QC laser emission wavelengths in 2 waveg- uides. The 2 2 QC laser module was designed instead of a 4 1 or a 3 1 QC laser module for several reasons, such as a very thick active core containing three or four different wavelength QC laser stacks results in poor temperature performance since the active core is a bad conductor of heat due to its material heterogeneity [10]. Furthermore, since there is a voltage drop across each active region/injector period, having 100 or more of the periods in one structure results in a very high voltage drop across the structure, resulting in a less reliable design, and the epitaxial growth of a very thick QC laser core can compromise the material quality of the QC laser device. The 2 2 module was comprised of two QC lasers bars, namely D3329, which had QC laser stacks for emission wavelengths of 7.0 and 11.2 m, and D3332, which had QC laser stacks for emission wavelengths of 8.7 and 12.0 m. A theoretical model was used to obtain the number of active region/injector periods for each QC laser wavelength in the two two-wavelength waveguides such that each QC laser will have about equal threshold current density while keeping the operating voltage at or below 16 V. Moreover, D3329 was designed to operate under positive polarity and D3332 under negative polarity so that, when mounted on the same submount, the four QC lasers can be operated in series using a single current channel. This paper is arranged as follows. Section II describes the quantum designs of the four QC lasers, Section III explains the theoretical model for the multiwavelength wave- guide design, Section IV presents the results, and Section V is the summary. II. FOUR QUANTUM DESIGNS Here, we present the four different laser designs used in D3329 and D3332. The layer sequences in Angstroms of one period of active region and injector for 7.0-, 11.2-, 8.7-, and 12.0- m emission wavelengths were: 28/45/14/54/12/60/40/24/26/25/2 4 / 2 4 / 1 8 / 2 6 /20/29/22/31/ 22/35, 24/50/9/63/8/63/7/26/40/35/24/37/2 0 / 3 2 / 1 8 / 3 4 /18/40/ 20/42, 42/14/40/16/3 2 / 1 6 / 3 4 / 1 4 /35/20/35/45/75/10/64/12/56/ 28 and 50/12/48/12/4 0 / 1 4 / 3 8 / 1 4 /42/18/44/45/30/7/75/7/68/ 7/58/24, respectively, where In Al As layers are in bold and In Ga As are in roman, the underlined layers were doped cm . Figs. 1 and 2 illustrate the con- duction band diagrams with calculated moduli square of the 0018-9197/$25.00 © 2009 IEEE