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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The speckle phenomenon produced by coherent waves interfering with each other is undesirable in laser imaging systems. For each of the laser speckle reduction methods in the literature, it is difficult to reduce speckle to an extremely low level (<3%) and also ensure good image quality. Therefore, a compound speckle reduction method based on the combination of a vibrating multimode fiber and a tracked moving flexible DOE loop is proposed and demonstrated for the first time. We have experimentally demonstrated the effectiveness of the proposed compound method, which can reduce the speckle contrast to 1.96% and obtain good spot quality. The relationship between the time-averaging effect of the speckle patterns from a vibrating multimode fiber and from a tracked moving DOE loop is discussed thoroughly. Our experimental results are in good agreement with Goodman's speckle theory. We expect that the compound speckle reduction method we proposed will have promising potential for applications in laser imaging systems.

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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.