Short wavelength QCLs reach 400K
Mid-infrared quantum cascade lasers (QCLs) that operate at a temperature of up to a record-breaking 400 K (123 °C) have been fabricated by a team of scientists in Germany. The semiconductor lasers emit pulsed light at a wavelength of 4.5 µm (Applied Physics Letters 86 131107)
"These are the first 4.5 µm QCLs working up to 400 K," explained Quankui Yang from the Fraunhofer Institute for Applied Solid State Physics (IAF) in Freiburg, Germany where the devices were made. "We are currently working towards CW [continuous wave] operation and also trying to make shorter wavelength QCLs."
The lasers consist of 25 periods of GaInAs/AlAsSb active/injection regions which are grown by MBE on InP substrates. The 18 micron wide x 2.8 mm long devices emit up to 750 mW of peak power per facet at room temperature and 30 mW at 400 K when driven by a current of 5.5 A.
QCLs are potentially convenient sources of mid-infrared light and improvements in semiconductor technology now mean that several firms such as Alpes Lasers, Laser Components and Casacade Technologies are now offering commercial versions for applications such as gas sensing.
However, until recently, room-temperature operation of short wavelength (< 5 µm) QCLs has been hard to achieve and hindered their applications in the 3-5 µm atmospheric transparent window. The advent of lasers that operate in this wavelength range at higher temperatures will ease cooling requirements.
According to Yang, there are two main reasons why the IAF lasers can operate at such high temperatures. The first is a high conduction band offset of 1.6 eV between the quantum well and barrier materials used in the laser's active and injection regions and the second is the high quality of its growth process.
The team is now turning its sights to CW operation but it could be a tough challenge. "As the lasers still suffer somewhat from the high threshold current density, CW operation remains at the moment hard to achieve. We are currently working towards the direction of CW operation by optimizing both the design and heat dissipation of the lasers," said Yang.
"Low-temperature CW operation, or at least high duty-cycle (>50%) operation of these short-wavelength QCLs, should be possible in the near future."
Author
Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.
