The super-thin laser mirror, which has the same 99.9% percent reflective punch as current high-grade mirrors, called distributed Bragg reflectors (DBRs), but is 20 times thinner, was developed by engineers at the University of California, Berkeley and functional in a considerably wider spectrum of light frequencies, and easier to manufacture. All these advantages could present today's smaller integrated optical devices.
"Today's semiconductor lasers demand laser mirrors that can deliver high reflectivity, but without the extra thickness," said Chang-Hasnain, who is also a UC Berkeley professor of electrical engineering and computer science. "When you reduce the thickness of a mirror, you are significantly reducing the mass of the device, which also translates into lower power consumption. The mirror we've developed overcomes the hurdles that have stalled the advancement of certain lasers."
Early semiconductor lasers used crystal for the mirrors, which resulted in only 30% reflection, which is too inefficient for VCSELs. High reflectivity can be achieved with DBRs, in which the light passes through alternating layers of aluminum gallium arsenide and gallium arsenide, each of which has a slightly different refractive index. The difference in the refractive indices allows a small mount of light to be reflected from each pair of alternating layers. The light from the multiple layers adds up to form a strongly reflected coherent beam.
"Reducing the size of the laser's mirror also means a dramatic reduction in weight, which is particularly important for high -speed MEMS devices," said Chang-Hasnain.
"DBRs can reflect 99.9 percent of light, but it can take up to 80 layers of material to achieve this high reflectivity," said Huang, lead author of the paper. "The DBR ends up being a relatively thick 5 micrometers wide. The precision necessary for the layers also requires a complicated manufacturing process. Our mirror is thinner and will be easier to manufacture, which keeps the cost low."
In the effort to overcome the DBR limitations, the researchers incorporated a single-layer, high-index-contrast subwavelength grating (HCG) mirror in the VCSEL (see Fig). They claim it provides efficient optical feedback, control of the wavelength and polarization of the emitted light. Such integration reduces the required VCSEL mirror epitaxial thickness - the interface between a thin film and a substrate - by a factor of two and increases fabrication tolerance, the researchers claim.The HCG reflectivity HCG reflectivity bandwidth is 10 times wider than that of a conventional DBR. The fabrication process for the HCG-integrated VCSEL was similar to a standard VCSEL.
There is a wide range of products based upon laser optics that could benefit with this thinner mirror. They include light emitting diodes, photovoltaic devices, sensors, computer chips and telecommunications equipment.
References:
http://www.innovations-report.de/htm