Near-Field Microscopy Validates VCSEL Models
Vertical-cavity surface-emitting lasers (VCSELs) have been gaining ground in a variety of applications, including telecommunications. Researchers at the Colorado School of Mines in Golden, working in conjunction with Cielo Communications Inc. of Broomfield, Colo., have developed a method that uses a near-field optical scanning microscope to test the lasers' output response as a function of the variation in drive current.
Near-field scanning optical microscopy enables researchers to validate models of the behavior of vertical-cavity surface-emitting lasers by imaging the output power and the change in output power with drive current. Courtesy of William C. Bradford.
University researcher William C. Bradford said that the technique is a useful way to test the validity of models employed to design the microchip lasers.
"The VCSEL is a complicated device whose effective optical structure changes as a function of operating conditions, especially current and temperature distribution." Designers model lasers mathematically before they are built, but he said that there is a need to validate them with real-world measurements.
Conventional microscopy has a place in this work, but it is limited by the diameter of the emitting area of a VCSEL, which often is less than 20 µm. Far-field imaging, therefore, can determine if the laser's overall performance agrees with predictions. Near-field scanning optical microscopy, however, which enables the creation of images with resolutions below the diffraction limit, can capture changes in the laser emission with a resolution of approximately 100 nm.
Bradford and his colleagues used the technique to study a proton-implanted, gain-guided, 850-nm VCSEL from Cielo. Because they were interested in observing the laser's response to changes in the current driving the laser, they operated the laser in DC mode to establish an operating current reference point.
To measure how current changes affected the output, they floated a tiny AC "ripple" on the DC drive current. Typically, DC produces no oscillations -- other than noise -- in the laser output, unlike AC, which produces oscillations that correspond to the AC frequency.
"The AC is small enough that the total power output is essentially DC," said Bradford. "However, by using a lock-in amplifier, which is only sensitive to the much smaller AC changes, we are able to detect the minuscule changes in the laser output caused by the AC component." Oscillations in the output Bradford said the researchers learned that their estimate of the index profile of the laser agrees with the models and other measurements when studied at single-mode cutoff. In other, unpublished work, they have looked at lasers in which the mode and noise structures are not ideal and found that they could correlate changes in the mode pattern with performance.
Suitable for photodiodes
He said the technique could be improved by adding spectrally resolved detection and a detector with lower noise to their custom-built near-field microscope. The group is working to develop a type of near-field tip that does not rely on optical fiber and that could be used with VCSELs operating at a wide variety of wavelengths, he added.
The method itself is suitable in any application in which the emission changes at a nonuniform rate and in which spatial variation occurs on a very small scale. The near-field optical tip also can deliver light to solar cells or photodiodes, enabling researchers to test the responses of these devices to excitement in highly localized areas.
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