The Research Of Fibre Light Source With Low Spatial Coherence Used In Free-Speckle Imaging

In the speckle-free imaging,the speckle problem caused by the high temporal and spatial coherence of the light source has long troubled researchers,and it is necessary to effectively suppress this speckle noise to improve the imaging quality.Reducing the spatial and temporal coherence of the light source in the fiber can effectively suppress speckle,and at the same time can take advantage of fiber optic light sources such as small size,light weight,and high brightness with controllable direction.This thesis designs a special waveguide structure combined with fiber optic light source,which can effectively and conveniently suppress speckle.The research content of this article includes the following three aspects:1.We design a single-trench fiber waveguide structure,analyze its mode characteristics and speckle suppression characteristics,and compare it with commercial multimode fiber.The results show that: on the one hand,the numerical aperture of single-trench fiber is larger than that of multimode fiber,which can support more high-order modes to reduce the spatial coherence of the light source;in addition,the inner cladding design of the single-trench fiber makes the effective refractive index difference between adjacent high-order modes smaller,which is conducive to mode excitation and inter-mode coupling,further reducing the light source Spatial coherence.2.Experiments were conducted to control the spatial coherence of the output of the fiber-type amplified spontaneous emission light source with single-trench fiber and multimode fiber.In the experiment,the mode excitation and mode coupling in the optical fiber with different dimensions are modulated to obtain the effect of power redistribution between modes.This study discovered that the use of winding method can suppress speckle.Winding 80 cm single-trench fiber on a 2cm diameter cylinder,bending 2 turns can reduce the speckle contrast to 0.0855,which is about 35% lower.However,the speckle contrast of multimode fiber becomes larger with the increase of the number of winding turns.The discovery of this new feature provides an effective method for suppressing speckle based on a single groove fiber.3.An optical fiber mode measurement experiment based on spectrum and spatial imaging technology was carried out,and the spatial decoherence mechanism of the light source based on the winding method was analyzed.This research innovatively introduced optical fiber mode measurement technology into the process of analysing speckle suppression.By comparing the spectrum before and after bending and its Fourier transform diagram,it is found that the difference in the average power of the spectrum of the multimode fiber is greater than that of the single-trench fiber.It shows that the power distribution between the modes of multimode fiber after bending is not as uniform as that of single-trench fiber.This is because the latter's second core structure enables the higherorder modes escaped from the central core layer to continue to be transmitted in the fiber;the Fourier transform diagrams of the two further show that the output power of the singletrench fiber is more evenly transferred to the higher-order mode after bending.The overall trend of the curve is an upward trend,and the output power of the higher-order mode(corresponding to a large delay)of the multimode fiber has a certain decrease after bending,which is consistent with the previous analysis.