Coherence in Y-coupled quantum cascade lasers

Abstract

A monolithic coupling scheme in which two active waveguides merge into a single waveguide is presented for a GaAs∕AlGaAs quantum cascade laser. The evolving fields interfere and a constant phase is observed in the Y-shaped laser cavity, resulting in a far field pattern of a double slit. The mode distribution is comprehensively derived by matching the far field profiles to simulated values and shows a weak current dependence. The device demonstrates the feasibility of coherent laser resonators with prospective applications in interferometric sensing and high power laser arrays.

Coherent coupling of mid-infrared quantum cascade lasers

“…21 Two spatially separated modes are coupled laterally into the same ridge. Due to the large bending radius of the merging branches, the mode propagation can be treated as one-dimensional.…”

“…θ ≈ 1), two field configurations exist within the Y-shaped cavity. 21 A two-dimensional schematic view is given in Figure 3. The corresponding cavity modes have even or odd symmetry.…”

“…21 Locking the phase maximizes the field amplitude within the coupled ridges and therefore also the output power. As spectroscopic applications cover the full spectral range of QCLs, coupling schemes have to be investigated in respect to their spectral applicability.…”

Power Amplification and Coherent Combination Techniques for Terahertz Quantum Cascade Lasers

“…Investigating the impacts of coupling length and wavelength variations on the coupling had revealed that the phase-locked emission strongly depends on the length of the coupling region and also on the ratio between waveguide width and emission wavelength [31,32]. This understanding of the coupling mechanism is important to exploit Y-coupling in midinfrared QCL arrays with complex structures for high output performance.…”

“…Obtaining the coherence within an array relies on the ability to control the coupling between individual lasers. Inspired by concepts developed in more mature, shorter wavelength diode laser systems, several coupling schemes, including coupling through exponentially decaying fields outside the high index dielectric core (evanescent-wave coupled) [27][28][29], through feedback from external reflectors (diffraction-wave coupled) [30], connecting two ridges to one single-mode waveguide (Y coupled or tree coupled) [31][32][33][34][35], through lateral propagating waves (leaky-wave coupled) [36][37][38] and combining gradedphotonic-heterostructure (GPH) QCLs with a ring resonator [39], have exhibited excellent performance on phase-locking QCL arrays, especially in mid-infrared range.…”