Stressed VCSEL Displays Stable Polarization
When it comes to polarization, VCSELs, or vertical-cavity surface-emitting lasers, can't make up their minds. The devices are circular and symmetrical, but their constituent semiconductor materials have two crystal orientations, so the laser output polarizes along one of two directions. A group from Avalon Photonics Ltd. in Zurich, Switzerland, has found a way to stabilize polarization: Construct the VCSEL with T-shaped arms and put it under stress.
The addition of T-shaped arms of width (d) to a vertical-cavity surface-emitting laser reduced the probability of polarization flipping. The angle of the arms to the crystal orientation direction (ϑ) determines the introduced stress and, hence, polarization stability. Courtesy of Fabrice Monti di Sopra.
The probability of polarization flipping varies among devices, and companies today must cull VCSELs from a batch to obtain those of a specific polarization or those with improved stability. But the polarization will abruptly switch by 90°, even in the same device, under a higher drive current. As a result, the relative intensity noise swells by several orders of magnitude. The current increase also raises the output frequency by a few gigahertz.
The new 2-mW, 850-nm lasers are like their standard counterparts in all ways but one. "A difference in the strain could be measured, which leads to an improved polarization stability," explained Fabrice Monti di Sopra, a researcher on the project who is on leave from Avalon.
The scientists began by building a VCSEL out of a series of alternating layers of AlGaAs and GaAs in two T-shapes connected to a large, round mesa. They then oxidized the structure. Because of their design, the T-shaped elements oxidized completely, but the mesa did so only partially.
The shrinkage of the T-shapes compared with the mesa led to an overall downward stress on the device, helping to constrain the polarization. In experiments involving 562 devices, the group found that the probability of a polarization flip event decreased from 32 percent for standard VCSELs to approximately 2 percent for the strained T-bar variety.
The work continues, with an eye toward possible commercialization. Although the need for polarization-stable VCSELs is currently low, the increased use of polarization-preserving fibers and the expansion of optical sensing applications should boost demand.
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