Near - eld Analysis of Beam Properties of

We report mode structure and beam propagation analysis of high power strained (In,Ga)As quantum well lasers using the super-resolution capabilities of near-eld scanning optical microscopy (NSOM). We are able to directly measure the optical beam waist and astigmatism by imaging the output of the laser diode at various heights above the device facet. In the near-eld we observe spatial shifts in the position of the optical eld between diierent spectral components of the laser diode emission. Near-eld scanning optical microscopy and spec-troscopy (NSOM) is a recent technique 1], where an aluminum coated tapered optical ber probe is placed within a fraction of a wavelength of a sample and scanned over the surface 2]. The tapered single-mode optical ber provides a tiny aperture through which the light is coupled providing a spatial resolution slightly less than tip size (100 nm). In collection mode spectra can be obtained by coupling the collected signal to a monochromator. The ber tip, used in excitation mode, provides a local optical source allowing for near-eld optical beam induced current (NOBIC) measurements. The application of near-eld spectroscopy and imaging to laser diodes provides sub-wavelength information on device structure, performance , and beam properties 3,4]. In our measurement setup, the laser diode is mounted facet up on a piezo actuated positioner with strain gauge feedback and scanned in the ^ x ? ^ y plane beneath the probe tip which is at the focal point of a reeecting objective on a metallurgical microscope. Simultaneous shear-force topography is used to control the tip height z 10nm in both excitation and collection modes. We studied high power strained (In,Ga)As graded-index separate connnement heterojunction (GRIN-SCH) laser diodes. These devices emit at 980 nm and are designed to pump Erbium doped ber ampliiers. The structure consists of a single InGaAs quantum well sandwiched in a symmetrical waveguide of graded Al-GaAs cladding layers. The cavity facets are dielectric coated for front and rear reeectivities of 0.1 and 0.9, respectively. Figure 1 shows an image of the sample topography and the laser emission as measured in the near-eld. The laser mode intensity is plotted as contours superimposed on the topographical image of the laser facet and mesa structure. The metal contact on top of the mesa slightly protrudes over the facet of the device. This image is typical of the mode structure of the device up to 200 mA. The laser emission proole is …