Abstract
The acoustic birefringence in a highly birefringent (Hi-Bi) two-mode
optical fiber is measured and shown to be about 0.8%. The birefringence is
primarily caused by a cladding ellipticity in excess of 1%. The effect of
the softer stress-inducing elements is small in comparison. The effects of
misalignment of the acoustic and optical axes is studied in detail using
both continuous-wave and pulsed acoustooptic interaction. In the Hi-Bi
fiber,the misalignment is approximately 20°. The acoustic
birefringence and misalignment have a profound effect on the spectral
characteristic of tunable acoustooptic notch-and passband filters,giving
rise to double spectral peaks. The birefringence is not specific to Hi-Bi
fibers. The same effect is demonstrated in a birefringent elliptic core
fiber, and it should be expected also in standard telecom single-mode
fibers. In tunable acoustooptic notch filters based on coupling to
cladding modes in standard telecom fibers with ellipticity of the order of
1% the acoustic birefringence may cause double absorption peaks separated
by more than 4 nm. It is also shown that acoustic birefringence provides
guiding of the spatial orientation of the acoustic flexural waves, even if
the fiber is twisted. This guiding effect eases the construction of
acoustooptic components with long interaction length.
[IEEE ]
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