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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 4, FEBRUARY 15, 2008
Emission and Propagation Properties of Midinfrared Quantum Cascade Lasers Kannan Krishnaswami, Bruce E. Bernacki, Member, IEEE, Bret D. Cannon, Nicolas Hô, and Norman C. Anheier
Abstract—We report divergence, astigmatism, and beam propagation factor (M 2 ) measurements of quantum cascade lasers (QCLs) with emission wavelengths of 8.77 m. Emission profiles from the facet showed full-width at half-maximum divergence angles of 62 and 32 6 2 for the fast and slow axes, respectively. Diffraction-limited Ge aspheric microlenses were designed and fabricated to efficiently collect, collimate, and focus QCL emission. A confocal system comprised of these lenses was used to measure M 2 yielding 1.8 and 1.2 for the fast and slow axes, respectively. Astigmatism at the exit facet was calculated to be about 3.4 m, or less than half a wave. To the best of our knowledge, this is the first experimental measurement of astigmatism and M 2 reported for midinfrared QCLs. Index Terms—Laser beams, laser measurements, optical propagation, quantum-well lasers, semiconductor lasers. Fig. 1. Schematic of the apparatus for measuring fast and slows axes divergence of a QCL.
I. INTRODUCTION
VER THE past decade, a significant effort has gone into the development of quantum cascade lasers (QCLs). Their ability to span the mid-infrared (mid-IR) range of 3.5–25 m, overlapping the spectroscopic molecular fingerprint region, has made them an important light source for many chemical sensing applications [1]. However, there is still insufficient information in the literature regarding emission properties of QCLs such as divergence, astigmatism, and the beam propagation factor . Far-field divergence measurements of a QCL along the fast axis are required to specify the minimum numerical aperture necessary for efficient beam collimation as well as for predicting the behavior of the propagating beam as it encounters apertures and other physical objects. For example, the sensitivity of chemand apodization ical sensors can depend heavily on the of the QCL beam. One implementation of laser photoacoustic spectroscopy in our laboratory required focusing a QCL beam through an approximately 300- m-wide gap between the tines of a quartz tuning fork [2]. Stray laser light impinging on the tuning fork tines increased the laser-induced background which in turn diminished the sensitivity of the chemical sensor. Similarly, a multipass absorption cell, such as a Herriott cell, can
O
Manuscript received September 17, 2007; revised November 20, 2007. This work was supported by the Defense Advanced Research Projects Agency (DARPA) under Contract MDA972-01-C, and by the U.S. Department of Energy, Office of Nonproliferation Research and Development (NA-22). Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract DE-AC05-76RLO1830. The authors are with the Pacific Northwest National Laboratory, Richland, WA 99352 USA (e-mail:
[email protected]). Digital Object Identifier 10.1109/LPT.2007.915635
suffer substantial degradation in sensitivity due to weak interference fringes that are attributable to the beam shape and divergence of the propagating beam. In addition, uncorrected astigmatism could result in beam clipping at the entrance/exit ports and astigmatism is essenof the cell [3]. Thus, knowledge of tial to the performance of QCL-based chemical sensors. With the recent development of mid-IR waveguide devices [4] and single-mode mid-IR fibers, both requiring diffraction-limited performance for efficient QCL coupling, knowledge of a QCL’s far-field performance is essential for efficient optical designs. In this letter, we report the results of a series of measurements to of roomdetermine far-field divergence, astigmatism, and temperature QCLs. Measurements presented here are typical for QCLs of the structure described below. The QCLs used in this study were all Fabry–Pérot devices based on a double phonon resonance design with emission wavelengths of 8.77 m at room temperature [5], [6]. The QCL heterostructure comprised 35 periods of InGaAs–AlInAs, yielding a 2.1- m-thick active region grown by molecular beam epitaxy lattice-matched to an InP substrate. The laser structure was then capped with an InP clad layer. Ridge structures were dry-etched to form laser cavities that were 15 m wide and 3 mm long followed by the deposition of a dielectric layer and the formation of a Au contact layer. A high reflectivity coating was applied to the QCL’s back facet prior to mounting them epi-up on modified C-mounts with In solder. II. EMISSION PROFILE OF QCLS Fig. 1 shows a schematic of the experimental setup used to measure far-field emission of QCLs. The apparatus comprised a rotary stage with a step resolution of 0.5 and a fixed liquid
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KRISHNASWAMI et al.: EMISSION AND PROPAGATION PROPERTIES OF MID-IR QCLs
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Fig. 2. Divergence profile of the fast and slow axes of QCLs with (a) 1-mm and (b) 8-mm die setback from the edge of the c-mount.
nitrogen-cooled HgCdTe detector, with a detection area of 0.25 mm , that was located about 150 mm from the front facet of the QCL. The QCLs under test were mounted on a precision block such that the emission facet was always located within 0.5 mm of the axis of rotation, with either the fast or slow axis of the QCL parallel to the rotation axis. The lasers were operated with 1- s current pulses, a repetition rate of 32 kHz and peak currents above their approximately 1-A threshold. The QCLs were maintained at a constant temperature of 15 C via a closed loop thermoelectric cooling device. The emission profiles obtained from several identical QCL structures typically showed a full-width at half-maximum divergence of for the fast axis and for the slow axis, as shown in Fig. 2. For the fast axes measurements, 90 and 90 correspond to the bottom and top of the lasers, respectively. The wide fast axis divergence is due to nonparaxial propagation typical of emission from small apertures. The beam is non-Gaussian intensity points is 40 , thus since the off–axis angle at invalidating a Gaussian approximation [7]. Asymmetry in the fast axis is most probably due to the asymmetric waveguide structure itself [6]. Examination of the fast axes profiles showed the presence of a superimposed fringe structure. This was correlated to the QCL die recessed from the edge of the submount, as depicted in Fig. 1, measured by an optical profilometer. Due to the large fast axis divergence, even minimal die recess results in reflections and clipping from the edge of the submount. As a result, the far-field divergence profile shows a superposition of both interference and diffraction effects that result in superimposed fringe structure. Further, the magnitude of fringing was observed to be correlated to the extent of die recess. Fig. 2(a) and (b) shows the divergence profiles of identical QCL structures that are recessed from the edge by about 1 and 8 m, respectively. Here it can be seen that an increased facet recess with respect to the edge of the submount corresponds to a larger fringe magnitude. A simulation of QCL die recess from the submount edge was performed using an implicit finite-difference method (BeamPROP version 7) to calculate far-field distribution. However, due to the nonparaxial nature of beam propagation and limitations of commercial beam propagation algorithms, smaller laser divergences compared to those observed above and, therefore, larger QCL die recesses were simulated. Results of these simulations showed an identical trend in data with larger recesses producing larger fringe magnitudes. Far-field fringing in the fast axes is entirely due to mounting errors of the laser die on the submount. This can be easily corrected by placing the emission facet flush with the edge of the submount.
M
Fig. 3. Spot radii and fit measured at various locations within the focal region. Direction of beam propagation is towards increasing negative values.
III.
AND
ASTIGMATISM
Using the measured divergence data, two Ge aspheric lenses (collimation and focusing lenses) were designed and fabricated using single-point diamond turning. The lenses were then characterized by measuring transmitted wavefront error with a longwave infrared Twyman-Green interferometer in a double-pass configuration at 9.4 m using a tunable CO laser. Interferograms acquired from these lenses were analyzed using Quick Fringe to quantify their performance. The collimation lens was designed to have a numerical aperture of 0.85 with a focal length diamof 1.88 mm admitting fast axis light up to about the eter. Though aspheric lenses with NA 0.85 can be fabricated to reduce hard-edged aperture effects, they are extremely sensitive to misalignments that lead to increased aberrations which . Intersubsequently degrade the beam propagation factor ferograms of the collimating lenses indicated diffraction-limited performance with Strehl ratios 0.94. Similarly, the focusing lenses with clear apertures of 10 mm and focal lengths of 25 mm showed diffraction-limited performance with Strehl ratios 0.99. A confocal system using these two Ge lenses was of the laser, providing a used to measure astigmatism and transverse and longitudinal magnification of approximately 13 and 177, respectively. Spot size data for the fast and slow axes were acquired at several locations within the focal region by scanning a 10- m slit and a HgCdZnTe detector located behind it with 2- m steps. The acquired spot data was analyzed using the second moment method for an arbitrary beam [8] incorporating a 1% clip level to provide beam radii at various positions along the axis of beam propagation. Fig. 3 shows the second moment radii of the propagating beam for both fast and slow axes. The beam radii were then fit to the second moment expression for beam propagation of the propagating laser beam was cal[8] from which the culated to be 1.8 and 1.2 for the fast and slow axes, respectively. Fig. 4(a) and (b) shows the beam profile at focus along the fast and slow axes with measured second moment spot radii of 58.8 and 71.5 m, respectively. Gaussian fits to the same profiles yield spot radii of 38.1 and 70.8 m for the fast and slow axes, respectively, clearly demonstrating a gross underestimation for the fast axis using a naïve Gaussian fitting approach. The focal separation of about 600 m between the fast and slow axes foci, measured using the confocal system with its attendant longitudinal magnification, indicates laser astigmatism
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 4, FEBRUARY 15, 2008
Fig. 4. Beam profiles at focus for (a) fast and (b) slow axes with a radius of 58.8 and 71.5 mm, respectively.
of about 3.4 m, which is less than half a wave for these QCLs. Source astigmatism in semiconductor diode lasers is generally caused by the spatial variations in gain and index across the waveguide structure of the laser [9]. However, there are several differences between diode lasers and QCLs that may result in different levels of astigmatism. Unlike diode lasers, the gain mechanism in QCLs involves only sub-bands within the conduction band, which results in the refractive index being independent of the concentration of conduction band electrons, at least to first order [10]. This independence from carrier concentration reduces refractive index variations and hence astigmatism. Additionally, the absence of holes in the gain region may also increase the effective diffusion rate of charge carriers, which would improve gain uniformity. Both, the improved uniformity in refractive index and gain tend to reduce astigmatism, which is consistent with our measurement for QCLs. Fabry–Pérot diode lasers exhibit an astigmatism of 1–10 m [11] amounting to several waves in the visible and near infrared. In contrast, the measured astigmatism of this mid-IR QCL is about 3.4 m, which is less than half a wave. There are, however, other obvious differences between QCLs and diode lasers such as waveguide designs, optical confinement structures, and cavity lengths which may also impact the amounts of astigmatism and require further study. value of a propagating beam depends on the naThe tive emission characteristics of the laser source, the aberrations of the optical elements in the path, and the truncation of the Gaussian beam by collimating and focusing optics. Since both aspheric lenses used in these measurements exhibited diffraction-limited performance, they should not increase the measured values. However, it is evident from the prominent sidelobes value seen in the fast axis beam profiles that the fast axis diameter of the colliis increased by its truncation at the mation optic [12]. In this measurement, uncorrected laser astigmatism and residual spherical aberration from the system optics have negligible contributions [13] due to their small magnitudes. values reported here represent measurements for Thus, the a practically collimated beam as it propagates. IV. CONCLUSION We have fully characterized the emission properties of mid-IR QCLs in combination with their associated optical elements using a suite of metrology tools that we developed. Laser divergence measurement results, especially with respect to the fast axis, are essential to the design of efficient optical
elements in order to deliver desired collimation, beam size, and beam apodization for specific applications. Even though of the laser may be better, as evidenced by the the native slow axis measurements, fast axis beam profiles and are limit both influenced by the fast axis truncation at the of the collimation optic and, depending on the application, may require spatial filtering to suppress the sidelobes. For our room-temperature QCL and custom aspheric lens system, the measurements demonstrated the practical performance limitations inherent in the QCL itself in addition to contributions due to fast axis beam truncation and laser astigmatism. Although our sensor [2] was not affected by the small amount of laser astigmatism observed, it may require correction in other high-performance applications. Finally, fringing in the fast axis profiles highlighted the need for careful QCL mounting on its submount to minimize far-field structure in the beam. This problem could become exacerbated with the recently introduced epi-down mounting techniques for QCLs if the utmost care is not taken to ensure flush mounting of the laser facet with respect to the submount. REFERENCES [1] F. Capasso, R. Paiella, R. Martini, R. Colombelli, C. Gmachl, T. L. Myers, M. S. Taubman, R. M. Williams, C. G. Bethea, K. Unterrainer, H. Y. Hwang, D. L. Sivco, A. Y. Cho, A. M. Sergent, H. C. Liu, and E. A. Whittaker, “Quantum cascade lasers: Ultrahigh-speed operation, optical wireless communication, narrow linewidth, and farinfrared emission,” IEEE J. Quantum Electron., vol. 38, no. 6, pp. 511–532, Jun. 2002. [2] M. D. Wojcik, M. C. Phillips, B. D. Cannon, and M. S. Taubman, “Gasphase photoacoustic sensor at 8.41 m using quartz tuning forks and amplitude-modulated quantum cascade lasers,” Appl. Phys. B, vol. 85, pp. 2394–2398, 2006. [3] R. Kormann, R. Königstedt, U. Parchatka, J. Lelieveld, and H. Fischer, “QUALITAS: A midinfrared spectrometer for sensitive trace gas measurements based on quantum cascade lasers in CW operation,” Rev. Sci. Instrum., vol. 76, pp. 075102–, 2005. [4] N. Hô, M. C. Phillips, H. Qiao, P. J. Allen, K. Krishnaswami, B. J. Riley, T. L. Meyers, and N. C. Anheier, “Single-mode low-loss chalcogenide glass waveguide for the midinfrared,” Opt. Lett., vol. 31, pp. 1860–1862, 2005. [5] D. Hofstetter, M. Beck, T. Aellen, J. Faist, U. Oesterle, M. Ilegems, E. Gini, and H. Melchior, “Continuous wave operation of a 9.3 m quantum cascade laser on a peltier cooler,” Appl. Phys. Lett., vol. 78, no. 14, pp. 1964–1966, Apr. 2, 2001. [6] Z. Liu, D. Wasserman, S. S. Howard, A. J. Hoffman, C. F. Gmachl, X. Wang, T. Tanbun-Ek, L. Cheng, and F. S. Choa, “Room-temperature continuous-wave quantum cascade lasers grown by MOCVD without lateral regrowth,” IEEE Photon. Technol. Lett., vol. 18, no. 12, pp. 1347–1349, Jun. 15, 2006. [7] D. Botez and M. Ettenberg, “Beamwidth approximation for the fundamental mode in symmetric double-heterojunction lasers,” IEEE J. Quantum Electron., vol. 14, no. 11, pp. 827–830, Nov. 1978. [8] A. Siegman, G. Nemes, and J. Serna, “How to (maybe) measure laser beam quality,” DPSS Lasers: Applicat. Issues, OSA TOPS, vol. 17, pp. 184–199, 1998. [9] M. Mansuripur and E. M. Wright, “The optics of semiconductor diode lasers,” Opt. Photon. News, vol. 13, pp. 57–61, 2002. [10] J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, “Quantum cascade lasers,” Science, vol. 264, pp. 553–556, 1994. [11] R. W. Fox and L. Hollberg, “Semiconductor diode lasers,” in Experimental Methods in the Physcial Sciences. Boston, MA: Academic, 1997, vol. 29C, p. 82. [12] P. S. Carney and G. Gbur, “Optimal apodizations for finite apertures,” J. Opt. Soc. Amer., vol. 16, pp. 1638–1640, 1999. [13] A. E. Siegman, “Analysis of laser beam quality degradation caused by quartic phase aberrations,” Appl. Opt., vol. 32, pp. 5893–5901, 1993.
