High-Power Micro-Optical Bench Terahertz Quantum-Cascade Laser

Kurzfassung

Heterodyne receivers for airborne and space applications rely on the availability of the critical components such as the local oscillator. For these compact THz sources with sufficient output power are required. Quantum-cascade lasers (QCLs) offer output power in the milliwatt range in combination with narrow linewidth emission to enable high resolution spectroscopy. We present high-power compact THz micro-optical QCL systems (mQCL) that achieve high optical isolation and beam-shaping by directly integrating the necessary optical elements on a micro-optical bench. By performing optical isolation directly inside the cryocooler the optical feedback caused by residual reflection in the beam path in conjunction with the vibration of the mechanical cryocooler can be significantly reduced. We use THz QCLs based on a hybrid design, mounted on a 22 mm by 8 mm submount suitable for installation in a mechanical cryocooler. A groove in the submount allows for precise positioning of an aspherical lens made out of high-resistivity float-zone silicon. We assembled two mQCLs with different lenses. For one mQCL the aspherical lens is in direct contact with its outcoupling facet, resembling an immersion lens by reducing the refractive index contrast at the laser facet and is called L1. Another mQCL, called L2, uses a free-standing aspherical lens placed at a distance of 1.26 mm from the QCL face. Optical isolation is achieved using a Lambda/4 waveplate made from x-cut quartz at the end of the groove in combination with a wire grid evaporated onto its surface. The other side of the waveplate as well as the surface of both lenses are coated with a 14.3 um thick Lambda/4 anti-reflection layer made from parylene. L1 showed a significant increase of output power up to 26 mW at 40 K from 9 mW. Beam profile measurements show a well collimated beam with very low beam divergence for both mQCLs. L2 achieved a reduction of optical feedback equal to a reduction of 70 % for the back reflected power. Using an integrated immersion lens, we were able to achieve an improved outcoupling efficiency resulting in a significant increase of output power but also an increase in sensitivity to optical feedback. Implementing an aspherical lens at a small distance does not increase the output power but the smaller sensitivity to optical feedback makes this approach better suited for spectroscopy. In conclusion, micro-integration of a QCL with optics on a small bench is a promising approach for applications with the detailed design depending on the particular application.