|A scanning-electron-microscope image shows the silicon-based micro-loop mirror (MLM). Light entering the waveguide from the left is guided around the loop and redirected back into the laser structure. The inset shows the laser spot photographed with an IR camera. (Copyright: A*STAR)|
Singapore--A group at the A*STAR Data Storage Institute has fabricated electrically pumped III-V semiconductor lasers and novel cavity mirrors on top of silicon chips -- a step forward for optical data interconnects.1 The laser cavity consists of silicon-on-insulator (SOI) waveguides and passive SOI so-called micro-loop mirrors (MLMs) with 98% reflectivity. The laser emits single-mode, continuous-wave light at room temperature with a lasing threshold current density of 2.5 kA/cm2.
Active optical fibers with silicon-photonic chips can carry much more information for data interconnects than copper cables. Silicon photonics can also be the material of choice for wiring "lab-on-a-chip" devices. One of the greatest difficulties, however, is the implementation of lasers, because silicon is such a poor light emitter.
“Integrated Si/III-V lasers can take advantage of low-loss silicon waveguides, while addressing the problem of low light-emission efficiency that silicon devices typically have,” says Doris Keh-Ting Ng, one of the A*STAR researchers.
Attaching a Si/III-V laser on top of silicon requires challenging fabrication techniques, and device performances can suffer as a result. Furthermore, any laser requires mirrors to maintain lasing action. Such designs typically rely on the interface between air and the semiconductor facets.; however, such mirrors are not perfect and further reduce operation efficiency.
To improve on the latter aspect, the researchers came up with the MLM design. Light emitted from one end of the laser is guided along the waveguide, around a narrow bend, and then back into the device (see figure). The mirror at the other end of the device is still formed by the interface with air so that laser radiation can exit the device.
More than 30 delicate, high-precision fabrication steps are needed to fabricate the device. The researchers aim to further enhance the laser by miniaturizing the device.
“Further improvements, for example, at the interface between the mirror and the lasing structure itself, could lead to even better performance,” says Ng. “A laser with lower threshold and higher output power can possibly be achieved, leading to a potential solution to develop high-speed and low-cost optical communications and interconnects on electronics chips.”
Yunan Zheng et al., Applied Physics Letters 99, 011103 (2011).