Transverse mode control of junction lasers optically coupled to single-mode fibers

Preliminary experimental results are reported to demonstrate that fundamental transverse mode operation can be obtained from a multimode double-heterostructure (DH) junction laser while simultaneously coupling its output into a single-mode optical fiber. Mode selection has been achieved by three different methods: 1) incorporating the fiber into the resonant cavity; 2) using the reflections off the diode-fiber interface; and 3) by a combination of the aforementioned methods. In all approaches the diode-fiber interface effectively becomes a mirror with a reflectivity which is higher for the fundamental mode than for all other modes of the DH waveguide. By using the reflections off the diode-fiber interface, a peak pulsed power of 80 mW has been obtained from the uncoated end of a single-mode fiber butted against a P-p-n-N junction laser. This laser radiated a peak power of 165 mW in a first-order mode when operated without coupling to the fiber.