Research On Speckle Suppression Based On Self-mixing Interference Effect In Composite Cavity Semiconductor Laser

Laser speckle seriously affects the contrast and definition of the image,which diminishes the ability of the observer to extract details from the image.For the speckle noise in laser source application,a speckle reduction method by the spectral broadening is proposed,which is based on the composite cavity semiconductor laser system.Firstly,by appropriately tuning the feedback ratio,the state of coherence collapse is obtained using the optical-feedback-induced self-mixing interference and can be harnessed for the realization of the spectrally broadband emission.Then the optimized broadband laser source is utilized in the speckle contrast measurement system.Significant speckle reduction can be achieved by the spectral broadening due to the low temporal coherence properties of lights.Firstly,based on the basic theory and statistic property of laser speckle,the optical methods of speckle suppression are studied in detail,including speckle suppression based on polarization diversity,moving diffuser,wavelength and angle diversity,the reduction of coherence.Theoretical analysis shows that,when the surface roughness is within a reasonable scope,it is theoretically feasible to achieve the desired speckle suppression effect by the spectral broadening.Then,the composite F-P cavity model and the L-K rate equation are used to analyze the self-mixing interference effect respectively.Theoretical analysis shows that,by appropriately tuning the feedback ratio,the state of coherence collapse emerges and can be harnessed for the realization of the spectrally broadband emission.Finally,for the proposed scheme of speckle reduction by the spectral broadening,the composite cavity semiconductor laser system is designed.In experiment,the spectrally broadband emission is achieved in the state of coherence collapse by appropriately tuning the feedback ratio.And the optimized broadband laser source is used for the speckle contrast measurement system.Experimental results show the performance of the proposed approach.When the feedback ratio is-16.3 dB,the extraordinarily high spectral bandwidth of about 6 nm with multimode emission is achieved in the coherence collapsed state.In free-space reflection geometry,the speckle contrast has been successfully reduced down to 0.049 with this broadband laser source.