External cavity controlled operation of a semiconductor diode gain element in series with an optical fiber

It is shown that an external cavity can control the lasing of a semiconductor gain element in series with either a single-mode or multimode fiber. Temporally stable single-frequency output with a line width much less than 7.5 MHz is achieved. The polarization is stably maintained at the output of the 14-cm-long fiber for both the single-mode fiber and multimode fiber. It is noted that this stability in polarization is important if gain-element fiber combinations are to be coherently coupled in parallel. For the multimode fiber, only one or a selected number of the many modes of the fiber can participate in the laser action of the external cavity; this, it is believed, is because only these mode(s) can be reflected back into the fiber by the spherical mirror in appropriate phase and angle to participate in the laser emission.