Near-field optical microscopy of AlGaInP laser diode emissions and comparison with far-field observation

In situ emission profiles of AlGaInP multiple quantum well laser diodes (LDʹs) observed by a near-field scanning optical microscope (NSOM) probe with an intentionally enlarged aperture shows a single broad elliptic profile, whereas the detected wavelength hops among those of the longitudinal multimodes observed in the far-field. This hopping suggests the near-field coupling between the aperture and the LD cavity, which is proved by the observation of strong modulation of laser intensity by the optical coupling only in the near-field regime. Probes with an intact aperture are found not to couple with the lasing TE00 mode, but, therefore, couple only with a near-field longitudinal mode. Around the lasing region, emissions detected by an intact aperture are ascribed to amplified spontaneous emissions located at wavelengths far from those of the lasing multimode emissions. These near-field modes, with spatial profile different from both the far-field TE- and TM-modes, are detectable only by the NSOM.