WDM VCSEL array incorporates tapered wet oxidation

Paula Noaker Powell

The demand for high data transmission rates has been drawing a lot of attention to short-haul local area networks and fiber-to-the-desktop applications. Economical use of both applications, though, depends on how successful companies are at reducing manufacturing costs. Because of their desirable features, vertical-cavity surface-emitting lasers (VCSELs) and wavelength-division-multiplexing (WDM) in tangent offer a promising solution.

In line with this, researchers at the University of California, Santa Barbara, recently demonstrated a compact pie-shaped array of WDM VCSELs direct-coupled to a single multimode fiber. Because the photonic-integrated-emitter VCSEL array delivers the multiple channels through the fiber, there is no need for complex external coupling optics. In return for the simple and efficient optics, however, researcher Larry Coldren and colleagues point out that the structure requires a high integration density of VCSELs within a circle 60 µm in diameter. Unfortunately, they say, varying the physical thickness over a large lateral dimension by changing growth conditions or using a masked MBE growth technique is difficult. Another option, anodic etching of a tuning layer followed by regrowth of the top DBR, is too prohibitive for low-cost device fabrication.

Simple solution

Now the researchers may have found a simple solution in a post-growth wet oxidation technique that may be an attractive method of realizing densely packed WDM VCSEL arrays using purely mask-defined methods after a single epitaxial growth. Their experiments indicate that high-density integration of the arrays is possible with a combination of one-dimensional oxidation and large-scale tapered oxidation. The process integrates eight channels into a 60-µm-diameter circle, seven of which are found to operate as lasers. Wavelengths range from 823 to 836 nm, corresponding to the distance between the VCSEL mesa and the tuning trench.

Although the scientists found the experimentally measured wavelengths of the lasers to be blue-shifted by a large amount compared to the originally designed wavelength, they attributed this to variations in grown material and over-oxidation. Coldren and colleagues also say the bandwidth can be easily increased to 35 nm by adjusting the oxidation speed and optimizing the spread of the tuning trenches. For more details, contact Young-Gu Ju at ygju@xanadu.ece.ucsb.edu.

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