3 Beijing Academy of Quantum Information Sciences, Beijing 100193, China
Email: liusm@semi.ac.cn
Interband cascade laser arrays with continuous-wave (CW) watt-level output power at room temperature are demonstrated. A three-emitter laser array episide-down bonded on a diamond submount exhibited a CW output power in excess of 1 W at 10℃. The wall-plug efficiency of the laser array is almost the same as that of a single emitter, indicating that there is little heat accumulation caused by the thermal interference between the emitters in the array structure.
Introduction: Quantum cascade lasers (QCLs) [1] and interband cascade lasers (ICLs) [2] are important coherent light sources in the mid-infrared spectral range. In the past few decades, great progress have been made in the output power and wall-plug efficiency (WPE) of QCLs [3-5]. Benefiting from the efficient heat dissipation properties of the regrown InP in the buried ridge geometry, a room temperature output power of 5 W has been achieved from a 40-stage QCL [6] and 22% WPE [7] from a 45-stage one in continuous wave (CW) operation. Unlike QCLs, ICLs are much more sensitive to the temperature of the active regions. The characteristic temperature of a typical ICL is in the range of 45-55 K [8], which is on the order of 200 K for a QCL. The low characteristic temperatures are ascribed to the high vertical thermal resistance of the thick InAs/AlSb superlattice cladding layers [9] and the unrealized regrowth of GaSb-based materials required for the buried ridge geometry. As a consequence, it is not feasible to increase the CW output power of an ICL just by increasing the number of cascaded stages.
In fact, the maximum room temperature CW output power of 592 mW so far was reported from a 7-stage ICL with a ridge width of 32 µm and a cavity length of 3 mm by the Naval Research Laboratory [10], which is higher than that from their 10-stage device [11]. In addition to stacking multiple active regions, another way to increase the output power of a semiconductor laser is to arrange monolithically multiple emitters in parallel to form an array. Using this configuration, a CW optical power near 1 kW was obtained from a 980 nm diode laser array [12]. For long wavelengths, a GaSb-based type-I quantum well laser array emitting at 2.3 μm exhibited 18.5 W quasi-CW output power at 18℃ [13]. In this letter, we demonstrate a 3.4 μm type-II ICL array with CW output power in excess of 1 W at 10℃, using 3-mm long array chips each containing three emitters.
Device design and fabrication: The laser structure was grown on an n-GaSb (001) substrate in a Riber Compact 21 molecular beam epitaxy system. The growth started from an InAs/AlSb superlattice cladding layer, followed by a 7-stage active region sandwiched by two 0.7 μm GaSb separated confinement layers (SCLs). The growth ended with an InAs/AlSb superlattice cladding layer and a 10 nm heavily-doped InAs cap layer. Both SCLs were uniformly n-doped with GaTe to 8×1016cm-3, trading off the serial resistance against the waveguide loss. The thick upper and lower cladding layers of 1.2 μm and 2.5 μm, respectively, were used to prevent optical leakage into the GaSb substrate and the top metallization. The 7-stage active region applied the carrier rebalanced design [14], with necessary adjustments to shift the emission wavelength to 3.4 μm. The wafer was then processed into 16 μm-wide ridges by contact lithography and wet etching. The distance between each ridge was 400 μm, which is necessary to avoid thermal interference according to our thermal simulation. A SiO2 layer of 450 nm was used for electrical insulation. A Ti/Au metal stack was e-beam evaporated as the top contact. Then, the substrate was mechanically thinned and metalized by a Ge/Au/Pt/Au bottom contact. The wafer was then cleaved into either 3-mm-long single emitters or arrays with multiple emitters. Al2O3/Au high-reflection coatings and λ/4 Al2O3 anti-reflection coatings were deposited on the back and front facets of the lasers.
Laser performance: Single ICL emitters were mounted episide-down on AlN heat sinks and tested in CW mode. The emission spectra were recorded by a Fourier transform infrared spectrometer. The output power was measured by a pyroelectric power meter.