Mode coupling and phase modulation in vibrating waveguides

When coherent light is coupled into a multimode waveguide (such as a fiber), a speckle pattern is formed at the fiber output. When the fiber is vibrated, the speckle pattern is modulated due to mode coupling and phase modulation of the waveguide modes. It is shown that when a highly coherent light source is used, the speckle pattern modulation is due primarily to phase modulation of the modes. When less coherent sources are used (such as low-cost laser diodes), mode coupling is the dominant effect.>