Abstract

Exposition of a multimode Ge-doped optical fiber to high power cw argon laser light leads to the creation of a permanent mode-coupling grating in the fiber core. This grating has the right periodicity and transverse index structure to induce codirectional mode coupling and is written through the photosensitivity of the fiber.¹ The experiments are done as follows. First the fiber is exposed to 600-mW cw argon laser light at 514 or 488 nm for 30 min or more. Afterward, the fiber transmission is measured as a function of wavelength around the irradiating wavelength, using a white light source and a monochromator. The fiber is placed between two polarizers. If the input and output polarizers are crossed, one observes narrow peaks (0.1 nm) at well-defined wavelengths that depend on the fiber used. This occurs since the fundamental mode polarization does not rotate, but that of the higher-order modes does. If more strippers are installed at the input and output of the fiber and the polarizers are parallel, one observes narrow dips in the transmission spectrum. A number of fibers have been tested and more than 50% coupling has been observed in 1-m fiber strands.

© 1989 Optical Society of America