In-Situ Near-Field Optical Study of Mode Fields within the Laser Diode Cavities in Operation

The cleaved facet of a packaged laser diode in operation was scanned by a near-field optical microscope. This " true" near-field emission profile showed several sub-peaks even though the emission was described as single-mode operation by the manufacturer. Possible correspondence existed in the observation in " conventional" near-field emission profile, when the lens focus of monitor video microscope was adjusted just inside the laser diode cavity. This intentionally " under-focused" image showed several sub-peaks that should correspond to the lateral field distribution inside the laser diode cavity. Since the optical probe was estimated to detect the optical near-field around 100nm apart from the tip (when the probe and the ambient refractive index were n= 1.5 and n= 1.0, respectively) or nearer to the tip (when the refractive index was n= 3, as in the GaAs), it may detect the optical field inside the laser diode cavity when the probe was at the vicinity of the end facet of the cavity. Another possibility was an optical coupling between the optical probe and the laser diode cavity, which might modify the original field distribution inside the laser diode cavity. In both cases the present observation suggests us that a near-field probe works as a novel tool to manipulate cavity EM field. We have further compared the emission profile with the " active region" that was identified by the near-field photo-induced current measurement.