technology lasers
QCLs can be mounted in butterfly packages containing thermoelectric coolers and collimation optics
reduced thanks to the introduction of several parallel extraction paths.
MBE or MOCVD?
The first QCLs were produced by MBE, a technique that is adept at producing precise growth of thin layers with abrupt heterointerfaces. This form of epitaxy dominated the growth of QCLs for a decade, but notable improvements to MOCVD technology during the 1990s have enabled process engineers to now have a choice of deposition techniques. MOCVD’s potential advantages include a faster growth rate - a particular cost advantage for the very thick QCL structures - and nominally lower reactor maintenance.
The first MOCVD-grown QCL was demonstrated in 2005 by researchers at the University of Sheffield, UK, and since then this approach has been gaining traction. As of today there is no consensus in the QCL field regarding fundamental superiority of MBE or MOCVD, and we keep an open mind, producing lasers with both techniques.
We have produced a portfolio of high-quality, QCL epi- structures for emission in the medium-wave infrared by optimizing our growth process for strained structures containing hundreds of nanometer-thick layers. QCL quality is normally assessed through measurements of the spontaneous emission spectrum’s full-width at half- maximum: our 4.6 µm structures have a value of just 26 meV at room temperature, 20 percent less than that of previous growths of the same design.
To simplify systems integration of our QCLs, we have developed advanced, high-reliability, self-contained packages that employ well-proven telecom practices. These require only electrical power and heat sinking to operate.
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www.compoundsemiconductor.net April/May 2010
QCLs run in CW mode generate a substantial amount of heat – typically 10 MW/cm3 – and we have addressed these thermal issues with a buried-heterostructure geometry. The epitaxial laser structure is etched to form near-vertical ridges defining the side-walls of the laser cavity, and valleys are overgrown with a material providing superior thermal conductivity to that of the active region superlattice. This additional material, MOCVD-deposited iron-doped InP in the case of InGaAs/InAlAs QCLs, is transparent to the lasing wavelength and electrically insulating. At the package level, we have pioneered the use of epi-side mounting of QCLs for efficient thermal management. Thanks to optimized thermal management, we have realized a ratio of pulsed-to-CW output power of just 1.5 for a 3W QCL attached to a diamond submount. This type of submount is widely used to report results, because it is very efficient at extracting heat from QCLs, but its thermal expansion mismatch to the thermal expansion of the QCL material impairs long-term reliability. In the case of diamond substrates, to prevent damage to the laser, QCLs are soldered to the submounts with soft indium solder, but this leads to solder electro-migration at high temperatures and/or high currents densities.
We circumvent all these issues by: utilizing AlN submounts with a thermal coefficient similar to that of the laser; bonding the submount to the QCL with hard AuSn solder; and optimizing device geometry and facet coatings for room-temperature, CW operation. This has enabled a maximum CW output of 2.9W at 293K. We have also studied the performance of our QCLs without thermoelectric cooling (often called “uncooled” operation) and found that they produce a maximum average power of 1.2W, and a CW power in excess of 1W. Recently, thanks to further improvements in thermal management, we have raised the bar for average power output for “uncooled” operation to 2.0W.
It is worth noting that our output power and wallplug efficiency figures are given for single-ended emission. As with all edge-emitting semiconductor lasers, as-cleaved QCLs emit light equally from both facets, and many researchers report the combined output from both facets as the output power. But the vast majority of applications demand single-ended output, a requirement that is fulfilled by depositing a high reflectivity coating on one of the facets. This is a daunting task for high-power QCLs – optical power density on the facet of a 2W laser can exceed 10 MW/cm2. However, we have risen to the challenge of producing a reliability coating operating in the mid-infrared and developed QCLs emitting 1W or more that can deliver many thousands of hours of degradation-free operation (see Fig.3).
To facilitate the integration of our QCL chips into various applications and ensure long-term reliability, these devices are installed into custom-designed butterfly type packages containing a thermoelectric cooler and collimation optics.
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