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09 Dec 2004
Sony scientists say etching tiny holes in the top surface of a VCSEL is the key to high-power singlemode operation.
Vertical-cavity surface-emitting lasers (VCSELs) could soon be emitting over 10 mW singlemode thanks to research being carried out at Sony in Japan. (Applied Physics Letters 85 5162)
By etching tiny triangular holes into the top surface of an oxide-confined 850 nm VCSEL, the team generated output powers of up to 7 mW. What's more, it believes its holey VCSEL structure can produce 10 mW in various material systems.
Oxide-confined VCSELs emitting at 850 nm have become a standard technology in applications such as local area and storage area networks. But as Akio Furukawa and his colleagues explain, stable single transverse mode operation, which is important for high-efficiency coupling into optical fibers, is difficult to obtain for output powers of more than a milliwatt or so.
To tackle this issue, Furukawa's team fabricated a pattern of triangular holes on top of the VCSEL using electron beam lithography and reactive ion etching. "The holes are aligned so that they surround the center modal area and the [triangular hole's] tip penetrates the oxide aperture by a few microns."
The first holey VCSEL contained a circular pattern of nine triangular holes with a length, width and depth of 5, 1.25 - 3.75 and 3 microns respectively. The resulting device produced an output power 2 mW at a threshold current of 5 mA and a side-mode suppression ratio (SMSR) of 45 - 50 dB.
The second pattern contained eight holes and the device produced 7 mW at a threshold current of 5 mA and a SMSR of 40 dB. "These results suggest that the emission is not a multimode pattern or a higher order singlemode, but a somewhat deformed fundamental mode," say the authors. "Further analysis is required to clarify the nature of this singlemode emission."
The authors believe that refining the process will lead to better performance. "Further improvements in the threshold current and power output are possible by reducing absorption loss at the top contact layer, optimising the oxide aperture, the number of holes and modal area," conclude the team.
Author
Jacqueline Hewett is technology editor on Optics.org and Opto & Laser Europe magazine.
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