A highly efficient quantum cascade laser (QCL) that operates continuous wave (CW) at room temperature could provide a compact, portable and cost-efficient source of mid-infrared radiation, say researchers in the US. Their QCL emits at 4.5 microns, has a wall plug efficiency of over 9.3% and a CW output power of 675mW. Wall plug efficiencies greater than 18% are also reported for devices at a temperature of 150K, with CW output powers of more than 1W. (Applied Physics Letters 91 071101)
"Our device is unique because no other competing technologies have been able to access this wavelength range," Manijeh Razeghi, a researcher at Northwestern University, told optics.org. "This is the first demonstration of high power, high temperature CW operation."
Razeghi believes that QCLs have several significant advantages over competing mid-infrared laser technologies that could pave the way for new applications. "Applications have been limited by the availability of compact, portable, cost efficient mid-infrared sources," commented Razeghi. "This portable laser could be used in active infrared countermeasures for aircraft, spectroscopy equipment for pollution monitoring and trace chemical sensing of explosives/toxins, as well as medical diagnosis and surgery."
The superior performance achieved by Razeghi's team is due to improved material quality for both the molecular beam epitaxy (MBE) growth and metal-organic chemical vapor deposition (MOCVD) regrowth and the use of a high thermal conductivity diamond submount.
"Our QCL consists of a layered structure of GaInAs quantum wells and AlInAs quantum barriers. All layers are grown on an InP substrate in a single growth step by gas-source MBE," explained Razeghi. "A low-loss InP waveguide was also designed to increase the heat removal from the core due to the high thermal conductivity of InP compared with GaInAs." The buried heterostructure QCL is 3 mm long and 6 µm wide and is capped with a thick electroplated gold layer for heat removal.
Further increases in wall plug efficiency are expected by improving the design, material quality and packaging. "Our goal is to achieve 1 W of output power and 50 % efficiency," concluded Razeghi. "Additionally, we aim to apply the knowledge gained to achieve similar performance across the entire mid- to far-infrared spectrum (approximately 3-20 microns)."
