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TECHNOLOGY VCSELs


The evolving GaN VCSEL


Building a GaN VCSEL is far harder than making one from GaAs, but progress is being made through the introduction of different types of mirrors, alternative current injection schemes and new crystal orientations.


BY DANIEL FEEZELL FROM THE UNIVERSITY OF NEW MEXICO.


THE VERTICAL-CAVITY SURFACE- EMITTING LASER (VCSEL) has several advantages over its edge-emitting cousin. Superiorities include a lower threshold current, direct modulation at high speeds, a circularly symmetric output beam, wafer-level testing and the option to form densely packed, two-dimensional arrays. Thanks to the geometry of this class of laser, monolithic processing of large batches of devices is possible, and there is no need for cleaving, facet coating and bar handling. Consequently, the VCSEL can combine relatively low manufacturing costs with great performance.


Origins of the device can be traced back to 1977, when Kenichi Iga from Tokyo Institute of Technology first proposed this class of laser. Commercialisation


followed in the 1990s, and the VCSEL is now serving a wide range of applications, including fibre optic communication networks, optical interconnects, data storage, sensing and laser printing.


As the VCSEL has evolved, researchers have expanded the range of material systems that can be used to produce this device, cut threshold currents to sub-milliamp levels and increased output powers. They are now in excess of 7 mW for single-lateral-mode devices and beyond watt-level for multi-lateral-mode devices. Error-free serial data rates have also increased significantly, with recent demonstrations exceeding 50 Gbit/s.


These impressive figures suggest that the VCSEL has no weaknesses. But


44 www.compoundsemiconductor.net January / February 2014


that’s not true. Although this chip delivers very high levels of performance when emitting at the standard wavelengths of 850 nm and 980 nm, it has been very challenging to stretch emission to 1310 nm or 1550 nm, the wavelengths used for long-haul data transmission in optical fibre. Shifting the output to shorter wavelengths has also been tricky, and this has prevented the VCSEL from being used in high-resolution printing, high- density optical data storage, head-up displays, backlighting and chemical/ biological sensing (see Figure 1).


Here, we will look at the challenges of fabricating VCSELs that are based on the III-N materials system and span 400 nm to 550 nm. Some impediments to forming this device are very similar to those for


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