Efficient conversion of light over a wide spectral range by four-photon mixing in a multimode graded-index fiber

The efficient conversion of 1.06-mum radiation in a graded-index multimode optical fiber oscillator into a small number of spectral components propagating in specific modes of the fiber and falling in the spectral range spanning from the blue to 1.53 mum is reported. The effect is shown to result primarily from phase-matched four-photon mixing. The converted light is found to propagate as individual fiber modes which are identified. The wavelengths of the spectral components of the output of the oscillator are governed primarily by fiber geometry. Analysis of the results also indicates the presence and gives a measure of the fiber-core birefringence sensed by low-order modes of propagation of the graded-index multimode fibers we have studied. The efficiency of the oscillator source and the specific characteristic wavelengths available with common graded-index fibers suggest several possible applications, which are discussed.