Professor Charlie Ironside, School of Engineering, University of Glasgow, Glasgow G12 8LT, UK . email Charles.Ironside@glasgow.ac.uk
Quantum cascade lasers are mid-infrared semiconductor lasers that employ intersubband transitions in quantum wells. Quantum cascade lasers have several applications including tunable semiconductor laser spectroscopy. As part of the QCSense project we have been developing tunable single mode quantum cascade lasers for molecular vibrational spectroscopy - for the region of the mid-infrared (3000-4000nm) which is absorbed by molecules that have a carbon-hydrogen (C-H) bond; for example, hydrocarbon gases such as ethane and methane. The sensing of trace amounts of ethane has been developed by Shell Ltd in a system called Light Touch that's used in oil exploration.
We have developed a technology for producing single mode quantum cascade lasers using both a discrete mode technology and a lateral grating technology.
With the discrete mode technology a stable single mode emission near 3300nm with a sidemode suppression ratio of nearly 25 dB was observed and a tuning coefficient of 0.22 nm/K was obtained in the temperature range of 253 K<T<303 K.
The lateral grating technology allowed the development of room-temperature distributed-feedback quantum-cascade lasers operating in a single mode in the 3340 to 3350 nm wavelength range. First-order lateral gratings with high aspect ratio (the ratio between the grating etch depth and its period) were formed using inductively coupled plasma etching. The as-cleaved lasers emit in pulsed regime with a sidemode suppression ratio of up to 24 dB and a peak single-mode output power of 130 mW from a single facet.
We have also developed ring quantum cascade lasers. Unidirectional ring laser do not suffer from spatial hole burning a noise mechanism that broadens the line width of the output and unidirectional ring lasers can deliver up to twice the output power and quantum efficiency. Coupled ring lasers can be configured to give, easily tunable, single mode output.
We have reported on a unidirectional quantum cascade ring laser operating at a wavelength of around 3400nm at 200 K. Unidirectional operation is achieved by incorporating an S-shaped crossover waveguide in a manner that it couples light from the counterclockwise direction to the preferred clockwise direction. The ring laser unidirectionality is confirmed by measuring the counterpropagating wave suppression ratio (CWSR) as a function of injection current. At 1.5 times the threshold current, the CWSR is 9 that is 90% of the light is emitted in the favored (clockwise) direction.
We also designed, fabricated and characterised coupled ring quantum cascade lasers that were single mode and tunable - continuous wavelength tuning of about 13 nm was observed from one of these devices, at a tuning rate of approximately 0.4 nm/mA - the details of the ring laser work can be found in:-http://theses.gla.ac.uk/2603/
Charlie Ironside, Thomas Slight, Chidi Nshii, Devanth Dhirhe.