Beam Combining For High Power Quantum Cascade Laser Arrays

High-power quantum cascade laser (QCL) arrays emitting in the mid and long-wave infrared atmospheric windows (3 to 5 μm and 8 to 12 μm, respectively) with nearly diffraction-limited beam quality will be of immediate use to several military applications, including infrared countermeasures (IRCMs) and laser radar (LADAR), and to several civilian applications such as free space optical communications. The performances of today’s state-of-the-art QCL already allow the realization of multiwatt arrays. We propose to develop application-ready high power QCL arrays to fulfill this market need. In phase I, we will evaluate three different techniques for beam combining of QCL arrays: external cavity spectral beam combining (SBC), monolithic phased-locked ‘tree’ arrays, and monolithic phased-locked antiguide arrays. SBC has been applied very successfully to diode laser arrays and our preliminary analysis shows that it is also compatible with QCLs. Its only drawback is the use of external optical elements. For this reason we will also investigate monolithic phased-locked arrays. Previous work on diode laser arrays has shown that robust phase-locking requires parallel coupling of the array elements. We will evaluate two schemes that provide such coupling: Y-junctions coupled tree arrays and leaky-wave coupled antiguide arrays.