Near-field measurements find widespread importance in the characterization of laser sources, particularly semiconductor laser diodes. For optimal coupling to an optical fiber, the spatial mode of the laser beam must be matched to the fiber mode. Furthermore, the quality of a laser beam can be assessed by its near-field distribution (e.g., how close to a Gaussian the beam is, leakage current leading to evanescent fields outside the active area, etc.). Previously, near-field measurements of an optical device could be indirectly made via a pinhole, knife edge, or slit1 scanned in the far-field. However, these techniques are all spatially limited by diffraction and thus do not yield a real measure of the near-field output distribution. This is especially important in lieu of the fact that in many conventional semiconductor lasers, the transverse dimension is significantly less than the emission wavelength. Near-field scanning optical microscopy (NSOM)2 is a microscopic technique that uses a subwavelength-sized aperture brought to the vicinity of the optical near-field of a sample to collect the evanescent wave scattered from the sample. Because the scattered wave is evanescent, it has infinite spatial frequency components; thus the spatial resolution of NSOM is limited only by the size of the aperture (typically 100 nm). Furthermore, force-sensing techniques allow the near-field aperture to be maintained at a fixed distance of less than 10 nm from the sample. In this manner we can measure the near-field distribution at several fixed distances away from the sample and watch the output distribution evolve into the far-field. The topographical force image also reveals heat expansion and surface defects at the laser field.

© 1996 Optical Society of America

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