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A monolithic coupling scheme for mid-infrared quantum cascade laser arrays is investigated with respect to brightness enhancement. The tree-shaped resonator enables parallel coupling of six laser elements into a single element by means of several Y-junctions. Phase-locking is observed on the basis of far field analysis, and leads to in-phase emission on both sides of the device. The experimental results match calculated far field profiles and demonstrate a high level of modal control when driven far above threshold. Whereas optical power measurements confirm negligible coupling losses, the slope efficiency is below the theoretically expected value, which is attributed to modal competition. Additional evaluation of near fields and spectral characteristics provides background on the modal dynamics of the sophisticated cavity and reveals limitations to coherent beam combining. The findings pave the way to improved coupling efficiency and brightness scaling of a single facet emitting compact quantum cascade laser array.
©2009 Optical Society of America
1. Introduction
Mid-infrared quantum cascade lasers (QCLs) are reliable coherent sources of light at high spectral density and designable wavelength. In this field of research, up-to-date spectral frontiers at 2.8μm [1] and 24μm [2] have been reached, limited by conduction band offset and reststrahlen band of the material system. In particular, watt-level optical output powers, more than 10 % wall-plug efficiency and user friendly hermetical packaging have been demonstrated at a wavelength of 4.6μm in continuous wave operation at room temperature [3, 4]. Independent on further specifications, such as high-temperature performance [3, 5, 6], efficient heat removal [5, 7], single-mode operation [8, 9, 10] and tunability [5, 11], optical power and beam quality are key parameters for laser applications in the mid-infrared frequency range [12, 13]. Essentially, open-path environmental sensing [14, 15], free-space communications [16] and hyperspectral imaging [17, 18] call for laser beams of high intensity while maintaining a homogeneous and well focusable profile.
As high brightness, defined as optical power per unit area per unit solid angle, cannot be realized in wide ridges or non-coupled laser arrays, a series of coherent beam combining concepts have been studied to push forward the performance of semiconductor laser sources [19, 20]. Among these concepts, a monolithic and robust coupling scheme is favorable, which provides a high level of modal stability and control. In this respect, parallel coupling is crucial, where all elements are equally coupled to all others and intermodal discrimination is maximized [21]. Based on standard fabrication techniques of semiconductor lasers, parallel coupling can be integrated within a monolithic active waveguide resonator. Thus, phase-locking has been demonstrated in parabolic bow-tie arrays [22], curved waveguide arrays [23] and tree arrays [24] of diode lasers, and encouraging results were obtained.
Applied to QCLs, phase-locking has been achieved in a two-branch Y-junction resonator, where the suppression of higher order lateral waveguide modes leads to stable supermode operation [25]. The concept yields promising results in terms of power scaling efficiency [26] and has been successfully implemented in a complex Mach-Zehnder-type cavity [27]. In the supermode approach, even if a coupled array is composed of elements of different optical path length, the optical spectrum is expected to self-adjust in order to minimize the overall loss of the resonator [19, 28]. In this respect, more than two branches can be combined in parallel in the same manner by means of several Y-junctions. The outcomes of this tree array QCL with regard to phase-locking and brightness are the scope of this paper.
2. Design and fabrication
2.1. Sample preparation
The QCL structures were grown on n-type InP substrate by gas-source molecular beam epitaxy. The active region consists of 35 cascades of lattice-matched In0.53Ga0.47As/In0.52Al0.48As layers, embedded in a InP vertical waveguide. The epitaxial heterostructure is based on the double phonon resonance design [29] and has been published by R. P.Green et al. [30].
Fabry-Pérot (FP) and tree array (TA) waveguides were defined using reactive ion etching in a SiCl4/Ar atmosphere. Thereafter, a 300 nm thick SiNx insulating layer was deposited by plasma enhanced chemical vapor deposition, which also acts as a lateral waveguide cladding. Windows were opened on top of the ridges, followed by sputtering of Ti/Au (20/500 nm) extended contact pads in combination with a lift-off process. For the back side contact Ge/Au/Ni/Au (15/30/14/150 nm) metal layers were deposited and annealed.
Samples with as-cleaved facets were soldered with indium to a copper heat sink and wire bonded. The lasers were operated in pulse-mode, either at 293 K (room temperature) to allow for flexible and well accessible mounting, or at 78 K in a liquid nitrogen flow cryostat which increases their dynamic range. The QCL structure was designed for λ ~ 10.3μm [30], and central wavelengths between 10.8μm at 293K and 10.0μm at 78K were obtained from regrown wafers.
2.2. Resonator geometry
Figure 1 shows a scanning electron microscope (SEM) image of adjacent FP and TA lasers with corresponding dimensions. The sample was tilted to display the 2.6 mm long devices. In a tree-like geometry, six laser ridges (branches) spaced by 60 μm are coupled into a single ridge (stem) by several Y-junctions. Smooth waveguide bending with a bending radius of 8.3 mm increases the device length but therefore minimizes waveguide losses and coupling losses [31]. A ridge width w ≈ 10 μm was chosen for both devices, which results in a relative waveguide width w/λ ≈ 1. Thus higher order lateral modes are strongly suppressed [26]. The current cross-sections of the two types of lasers imply a ratio ATA/AFP = 3.89.
Fig. 1. Tilted SEM image of the fabricated devices with corresponding dimensions: The FP QCL emits light from a single facet (a). The TA QCL shows emission from the stem facet (b) and the branch facet (c). Near field images taken with a micro-bolometer camera illustrate the intensity distribution at high current density (J ≈ 4 × Jth) at the three different facets. The image of the stem facet (b) is defocused to display its concentric beam profile.
In addition to small deviations due to cleaving, the parallel coupled branches have 3 different lengths. Induced by bending, the two inner, middle and outer branches are prolonged by 0.6 μm, 3.1 μm and 6.7 μm, respectively. As a result, the tree-like geometry implies phase shifts of ~ 4π among the coupled beams, which leads to complex requirements for the propagating supermodes. The modal arrangement (i.e. the onset of lasing within each branch) can be observed in the near field by means of a micro-bolometer camera. In Fig. 1 the illustrated QCLs exhibit three different types of emitting facets, labeled a, b, and c, and corresponding near field images are given, which were recorded at high current density values J ≈ 4 × Jth. Whereas the light of a FP laser is emitted via a single facet (a) on both sides of the device, the TA laser emission can be collected from its stem (b) or branches (c). The image of the stem facet (b) is slightly defocused. The evolving interference fringes are concentric and reveal TM00 emission. The six emitters of the branch facet (c) are in focus and well resolved. They indicate, that all branches contribute to the excited supermode.
3. Results and discussion
3.1. Phase-locking
Fig. 2. Comparison of normalized two-dimensional angular far field intensity profiles (interpolated) between the single facet of a FP laser (a) and the branch facet of a TA laser (b). The lasers were operated in pulse-mode (100 ns, 250 Hz) at 293 K, at current densities of 11 kA/cm2 (TA) and 13 kA/cm2 (FP), respectively. The data were recorded in a 8 cm distance with an MCT detector and a resolution of 0.45°. Cross-sectional data plots at central vertical far field angle θV = 0° illustrate the lateral far field distribution of the single facet (c) and the branch facet (d). The measured intensity values (full circles) correspond to calculated far field profiles of phase-locked emission (solid lines), centered at θL = -1°.
In coupled laser arrays, phase-locking is of great importance, as it leads to constructive interference between the coupled fields and enables stable operation. In particular, in-phase coupling is desirable, which produces a power maximum on-axis [19]. Hence, coupling is most efficient and radiation losses are minimized, if all branches contribute to an in-phase supermode. In this respect, far field analysis provides information about the modal arrangement within the resonator [25, 26].
Figure 2 displays two-dimensional far field intensity profiles of FP and TA lasers operated at 293 K. In absence of a cryostat, the sample was mounted on a rotational stage, where the facet of emission is placed at the pivot point of rotation. By tilting the sample, the far field angle is varied from -45° to 45° with a resolution of 0.45° in lateral direction (θL) and vertical direction (θV). The angle dependent intensity values were collected in a 8 cm distance by a mid-infrared mercury cadmium telluride (MCT) detector and normalized.
In Fig. 2(a) the far field of the single facet of a FP laser is plotted in two-dimensional representation. The recorded data show a homogeneous gaussian-like intensity distribution, which emanates from a TM00 mode within the cavity. The beam exhibits full width at half maximum values θL(FWHM) = 50° in lateral direction and θV(FWHM) = 61° in vertical direction, respectively. The findings represent a fairly good match between the cavity’s lateral and vertical dimensions.
The branch facet’s far field of a TA laser (Fig. 2(b)) exhibits a qualitatively different intensity distribution. Whereas its vertical profile remains unchanged, sharp interference fringes are visible in lateral direction. Figures 2(c) and 2(d) illustrate cross-sectional far field intensity values (full circles) recorded in lateral direction at θV = 0° of the FP laser and the TA laser, respectively. Plotted in one-dimensional representation, the measurements are compared to simulations. Whereas the solid line of Fig. 2(c) represents the normalized Gaussian far field profile of a Gaussian near field distribution at the single facet, the solid line of Fig. 2(d) illustrates the calculated result of six interfering Gaussian beams, with a lateral spacing of 60 μm. In both simulations an electric field value is assigned to every point on the lateral axis of the facet, which reflects in-phase emission. The propagating spherical waves are collected on a spherical surface in 8 cm distance, and summed up for all points of emission. Squaring the electric field distribution yields the plotted far field intensity profiles.
Both data sets show a strong agreement between experiment and calculation. The measured far field intensity values match the expected in-phase profiles of monochromatic light at wave-length λ = 10.8 μm, when corrected by -1° lateral offset. In fact, the room temperature spectra of the lasers peak at 10.8 μm. Moreover, a fundamental lateral waveguide mode can be deduced from the single-lobed far field of the FP laser (Fig. 2(c)). Whereas the branch facet’s lateral far field exhibits sharp peaks of constructive interference with a 10.4° spacing (Fig. 2(d)), the envelope of the profile is single-lobed, too. Hence, only the fundamental lateral mode is excited in the branches. However, as not all branches exhibit the exact same output power, also smaller features are visible in between the major maxima. As a result of uneven field amplitudes and a comparably small resolution of 0.45°, the four minor maxima of a six emitter interference pattern are not represented in the measurement.
Mode control is achieved due to a relative waveguide width of w/λ ≈ 1, when lateral waveguide losses of fundamental and first order modes differ by a factor of 4 (Ref. [25]). Based on this fact, the branches are phase-locked and all branches emit in-phase at the branch facet. As all waveguides are covered with gold, fields of neighboring branches do not interact. Hence, the overall phase is locked within the stem and parallel coupling emerges.
3.2. Brightness
When beams of non-coupled lasers are combined, the brightness is no better than that of a single laser element [19]. In turn, the observation of phase-locking in TA QCLs suggests an enhancement of brightness compared to FP QCLs. Experimentally, more diffraction-limited optical power is expected from the stem facet of a TA laser than from the single facet of a FP laser of the same length, as both facets exhibit identical dimensions and therefore the same TM00 beam profile. In this respect, threshold current density Jth and slope efficiency dP/dI provide information about coupling losses and overall performance of the presented parallel coupling scheme.
Figure 3 shows peak power versus current density curves of both types of QCLs measured at 78K with a calibrated deuterated triglycine sulfate (DTGS) detector. The FP laser (dashed line) exhibits a threshold current density of 2.5 kA/cm2, a maximum slope efficiency of 95mW/A and its output power exceeds 250 mW The solid line represents the current dependent output power from the stem facet of the TA laser. If coupling losses and saturation effects are negligible, threshold current density and slope efficiency are expected to be identical to the FP laser. However, only the threshold current density value is comparable, but the evolving slope efficiency is reduced by the ratio of the current cross-sections of the two types of lasers,
Fig. 3. Peak optical power vs current density curves of the single facet of a FP laser (dashed line) and the stem facet of a TA laser (solid line). The data were recorded in pulse-mode operation (100 ns, 5 kHz) with a calibrated DTGS detector at 78K. Near field images of the branch facet illustrate the distribution of light among the branches at different points of operation.
As waveguide losses and mirror losses do not depend on the geometric differences between FP and TA resonators, the proximity of both threshold current density values indicates that coupling losses are small (Eq. 1). In fact, measured thresholds of TA lasers never exceeded 3 kA/cm2. The maximum increase ΔJth = 0.5 kA/cm2 corresponds to an upper bound of 3.2 cm-1 coupling losses.
To further investigate the reduced slope efficiency of the TA laser, its near field characteristics were analyzed at the branch facet. Current dependent micro-bolometer images recorded at 3, 6 and 10 kA/cm2 are shown as insets of Fig. 3, and demonstrate the dynamic behavior of the emerging supermode. Slightly above threshold, all light is emitted via only one of the six facets, i.e. the tree array starts lasing in a FP-like sub-cavity. In this regime (J < 3 kA/cm2) most of the biased area ATA does not contribute to lasing and therefore the slope efficiency is reduced by the ratio of the current cross-sections (Eq. (2)). By circumventing the manyfold boundary conditions of parallel coupled branches of different length, lasing favors to evolve in one branch only. Moreover, the outer branch with its maximum length and its minimum out-coupling losses was found to always start lasing first. With increasing current density (J > 3 kA/cm2), more branches start lasing toward a homogeneous distribution of light among the coupled branches. However, the slope efficiency does not increase with more branches contributing to the supermode. Even at high current density values (J > 9 kA/cm2), where all branches equally emit and the phase is locked throughout the whole device (compare Fig. 2), the slope efficiency remains constant. In this regime, the slope efficiency of the TA laser is superior, as the FP laser approaches its thermal roll-over at J = 16.5 kA/cm2. The thermal roll-over of the TA laser is out of reach, as pulse current densities of this type of laser are experimentally limited to ~ 11 kA/cm2. Based on the above comparison of optical power, no enhancement of brightness was found. In TA lasers the slope efficiency suffers from an asymmetric evolution of the resonator supermode and modal competition among branches of different length.
Fig. 4. Comparison between normalized spectra of a FP laser (a) and a TA laser (b) at varying pulse current density. The data were recorded in pulse-mode operation (100 ns, 5 kHz) at 78K with 0.2 cm-1 resolution. All points of operation correspond to the peak power vs current density curves of Fig. 3.
Figure 4 illustrates the spectral behavior of the two different resonators discussed. The normalized spectra were recorded at 78K by means of a Fourier-transform spectrometer with a resolution of 0.2 cm-1. The current density labels correspond to the peak power versus current density curves of Fig. 3. Both types of lasers exhibit typical FP-like spectra, with a large number of longitudinal modes visible. At reasonable current density values (J ≤ 7 kA/cm2) the laser emission peaks at ~ 1000 cm-1 (λ ≈ 10.0 μm). At higher current densities (J > 7 kA/cm2), the gain width of the QCL material increases towards ~ 50 cm-1. In contrast to the FP laser, the longitudinal modes of the TA laser are less pronounced and somewhat broader. In this type of resonator more heat is dissipated at given pulse current density, which leads to a smearing of the spectrum during the duration of the pulse. However, the different lengths of the coupled branches do not influence the spectral behavior. As more than 800 wavelengths fit into the length of the cavity, the modal arrangement flexibly adapts to the geometry of the resonator. As a result, phase-locking is enabled in every longitudinal mode and the optical field can smoothly transit from one branch to the remaining branches at constant slope efficiency.
Although the multi-mode optical field is able to self-adjust in order to minimize the overall loss of the TA resonator, the emergence of supermode operation under manyfold boundary conditions takes place at the cost of strong modal competition within the stem. This results in a reduced slope efficiency comparable to a conventional ridge laser. The undesirable variations in the optical path length among the array elements, known as piston error, can be viewed as being the equivalent of wavefront distortion in a bulk optical element [19]. In this respect, the results are in agreement with outcomes of alternative common-resonator approaches, which also show reduced brightness scaling due to an inefficient superposition of electric fields [22, 23, 24].
4. Conclusion
Six mid-infrared quantum cascade lasers were coherently combined within a monolithic tree-shaped resonator. The parallel coupling scheme was investigated in terms of optical power, beam profile and spectral behavior. Lasing is observed within a broad frequency band of ~ 50 cm-1 and coupling losses are small. The device exhibits in-phase stable supermode operation at room temperature, accompanied with a high level of mode control when driven far above threshold. However, the modal arrangement suffers from phase errors within the sophisticated resonator geometry, which leads to a reduced slope efficiency. In spite of phase-locking, the achieved brightness is comparable to a non-coupled single element laser of identical dimensions. An improved resonator design with carefully chosen branch lengths may eliminate modal competition and enhance the coupling efficiency.
Acknowledgments
The authors would like to thank G. A. Reider for fruitful discussions. Financial support from the Austrian FWF project IRON, the Austrian NANO Initiative project PLATON, the “Gesellschaft für Mikro- und Nanoelektronik” GMe, the German Research Society DFG, the EXIST-SEED program of the German Federal Ministry of Education and Research BMBF, and the New York state award NYSTAR is gratefully acknowledged.
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