The Near-Field Scanning Optical Microscope (NSOM) is a tool that combines the spatial resolution of scanning probe microscopy with optical characterization techniques. Using this technique, we have generated high-resolution spatial intensity maps of the output from vertical-cavity surface-emitting lasers (VCSELs) in the near-field region of the facet as a function of operating current. The VCSELs studied were proton implanted, gain guided devices designed to operate at ~850nm. Optical signals that have been spatially imaged include total intensity, the spectrally resolved intensity of individual transverse modes, and the derivative of intensity with respect of operating current. Deviations from expected mode patterns in the devices have been qualitatively linked to unacceptable levels of noise in operating lasers. These deviations can be observed at operating currents below the actual onset of unacceptable noise. We have also found that derivative spectroscopy can be used to sensitively detect the cutoff points of transverse modes. Using the spatial intensity profile at the cutoff point of an allowed mode, a first approximation to the index of refraction profile can be made that is in good agreement with prior work. A series of index profile estimates from the cutoff points of a VCSEL can provide information on the evolution of the index profile and the thermal lens as the power is ramped up.
A prototype adaptive optics system has been developed at Lawrence Livermore National Laboratory for use at Lick Observatory. This system is based on an ITEK 69-actuator continuous-surface deformable mirror, a Kodak fast-framing intensified CCD camera, and a Mercury VME board containing four Intel i860 processors. The system has been tested using natural reference stars on the 40-inch Nickel telescope at Lick Observatory yielding up to a factor of 10 increase in image peak intensity and a factor of 6 reduction in image full width at half maximum. These results are consistent with theoretical expectations.
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