A method of speckle suppression without any active device is expected for pico-projectors. The effectiveness of the passive method of speckle reduction using a single multimode fiber and a multimode fiber bundle was actually measured and theoretically analyzed. The dependences of the speckle contrast and speckle suppression coefficient on the parameters of multimode fiber and projection systems were investigated. Our results shown that the efficiency of speckle suppression was limited because only the radial direction of the objective lens aperture was used. An improvement using both of the radial and azimuthal directions of the objective lens aperture is required.

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Speckle-free imaging using a multimode fiber has been widely used for imaging systems. Generally, previous work has assumed that all the propagating modes of the fiber are uniformly excited, but the modal power distribution is actually affected by excitation conditions. Here, we propose the utilization of a modal analysis method to study the dependence of the speckle contrast on the modal power distribution by changing the tilt angle of the Gaussian beam and on the group delay time difference caused by different fiber lengths. The results of numerical simulations and experiments show that, with an increase in the tilt angle of the Gaussian beam, the modal power is transferred to higher-order modes and the maximum delay difference between excitation modes becomes larger. Therefore, the inter-mode interference effect is effectively weakened, and the speckle contrast is significantly reduced. The increase in fiber length will also make the delay difference between excitation modes larger and thus the speckle contrast is decreased. For the larger tilt angle of the Gaussian beam, only a shorter optical fiber is required to reduce the speckle contrast significantly. Our work further promotes the use of a multimode fiber to produce speckle-free patterns in laser imaging systems.

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