The general difference expansion method uses reversible integer transform to embed information into a digital image and perfectly restores the original image after removing the hidden data. In multilayer embedding applications, the operation of difference expansion may seriously degrade the visual quality of the embedded image. In this paper, we propose a reduced difference expansion method to solve this problem. Theoretically, in the worst case, the proposed method gains 6-dB improvement in visual quality of the embedded image compared with the general difference expansion method. Therefore, the proposed method is suitable for multilayer data embedding.

A single-longitudinal-mode erbium-doped fiber laser with a passive multiple-ring cavity (MRC) is proposed for the first time to the authors' knowledge. The laser is fundamentally structured by insertion of three different short ring cavities, which serve as mode filters, into the main cavity. When it is combined with a mode-restricting intracavity fiber Bragg grating, the MRC resonator ensures single-longitundinal-mode laser oscillation. The laser can successfully suppress side-mode frequencies of as much as 1 GHz and provide an output power of 23 mW with a side-mode suppression ratio of 51 dB at 1533 nm. The short-term linewidth of the laser output measured is ~2 kHz . The ability of this fiber laser to act as an AM transmitter source is also demonstrated.

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