In-fibre LP11-LP21 mode conversion using force-induced long-period fibre gratings with polarization controllers

Mode conversion between higher-order modes is attained in a step-index four mode fibre using an in-line polarization controller followed by a force-induced long period fibre grating. By adjusting the field orientation of the LP11 mode to the antisymmetric fibre grating, the LP11 mode is converted to the LP21 mode with a spatially periodic force of 50 N along the fibre length of 50 mm. The mode conversion is evaluated by observing the far-field and near-field patterns of the fibre output and by comparing the transmitted optical power with and without the additional two-mode fibre. The phase-matching grating period is required of the accuracy of roughly +/- 5 mu m and the radiation loss during the mode conversion is as low as 0.01 dB.