受限于空间光调制器(SLM)有限的刷新速率,现有的基于SLM的多模光纤(MMF)成像方法并不能满足对活体生物组织内窥成像的需求。考虑到数字微镜器件(DMD)的刷新速率比SLM高两个数量级,因此提出了一种基于DMD二值振幅调制的MMF出射光斑聚焦扫描技术。理论分析表明,MMF出射端任意聚焦区域内的总光强与DMD子区域的振幅调制系数之间存在二次函数关系,因此,通过DMD对MMF入射波前进行二值振幅调制,可实现对MMF出射光斑的聚焦和扫描。对于给定数目的可调制子区域,该二值振幅调制算法的调制次数是基于纯相位迭代优化算法的1/256,是基于三步移相最优相位算法的1/3。基于该技术实现了对长度为5 m、直径为105 μm的MMF出射光斑在三维空间上的聚焦和扫描。研究表明,该技术具有调制速度快、算法可靠性高、聚焦点均匀性好等优点。

Due to the limited updating rate of spatial light modulator (SLM), the reported methods based on SLM can't satisfy the requirements of biological tissues endoscopic imaging in vivo. Considering the updating rate of digital micro-mirror device (DMD) is two orders of magnitude higher than the fastest SLM, a method is proposed for focusing and scanning light through a multimode fiber (MMF) based on binary amplitude modulation DMD technique. Theoretical analysis shows that there exists a quadratic function between the total light intensity within the focused region at the distal end of the MMF and the amplitude modulation coefficient of the sub-region on DMD. Thus, it can be realized that focusing and scanning light through a MMF by modifying the incident wavefront with the binary amplitude modulation DMD technique. What’s more, for a given number of modulation sub-regions, the number of iterations for this method is 1/256 of that for the phase iterative optimization methods or 1/3 of that for the optimal phase computation optimization methods with three-step phase shift. We demonstrate that focusing and scanning light in three-dimensional space through a 5 m length and 105 μm core diameter MMF with this method. The results show that the proposed method has the advantages of fast modulation speed, high algorithm reliability and good uniformity of focused spots.