Oct 4 2007
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.
"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 £gm 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)."