Research Of Coherent Light Realtime Transformation Based On Liquid Crystal Spatial Light Modulator

Nowadays, with the increasing requirement of high precision, high response and high integration, the traditional optical scanning system is becoming to less competent in the applications of complicated wavefront transformation due to its mechanical inertia reason. How to change a laser beam into arbitrary beams is becoming to a critical problem in the wavefront transformation field.The effective refractive indices of liquid crystal devices can be changed by applying electrical volts on the Liquid Crystal (LC) cell. Because there is no mechanical inertia involved in the process of controlling LC refractive indices, the LC device acts as an agile and variable Diffractive Optical Elements (DOE). Therefore, it is a practical way to adopt Liquid Crystal-Spatial Light Modulator (LC-SLM) as the beam steering elements to generate programmable beam shapes and programmable patterns.When a plane wave is modulated by an LC-SLM and forms a certain expected diffractive pattern in the far field image plane, the phases distribution of the SLM pixels should satisfy some constraints. The phase constraints can be obtained by an optimal algorithm. However, for some inherent reasons, the phases retrieval algorithm is still in the state of off-line computing. It is difficult to realize the real-time phases retrieval by current algorithms, so there are few researchers who are studying the real-time wavefront transformation by phase-only LC-SLMs. It is necessary to develop a fast phase retrieval algorithm to realize the real-time wavefront transformation.In this thesis, a new type of dynamical beam steering system is designed with a phase-only LC-SLM, which has a high response time. The far-field arbitrary diffractive pattern approaching by non-mechanical laser beams is resolved by an in-line phases retrieval algorithm. Besides, on the base of our dynamical beam steering system, we design a moving objects tracing system with agile multiple beams pointing to the objects dynamically. The main contributions of this dissertation are as follows:Firstly, a new method is proposed which can perform the real-time wavefront transformation by a phase-only LC-SLM. This new method can transform a plane wave into arbitrary beams based on the liquid crystal theory of electrically controlled birefringence and the algorithm of real-time phase retrieval. The traditional viewpoint of off-line phase retrieval is broke through and a new field of using the LC-SLM as an in-line programming diffractive optical element is developed.Secondly, a modified fast phase retrieval algorithm is proposed, which adopts the pseudorandom phases encoding technology to initialize GS (Gerchberg–Saxton) algorithm. The modified algorithm is a robust and fast constringency one which has the merits of decreasing the sensitivity to initial phases and of decreasing the twinborn images effects.Thirdly, a new system of tracing moving objects by phased array beams is proposed and presented. A high speed camera is used to capture the moving objects information from the reflected image. According to the feedback information of the camer, the SLM generates another frame of diffractive pattern to focus the laser energy onto the objects dynamically. Results show that the tracing system has the position error of less than 0.032°and the response time is less than 200ms for one tacking frame. It is more capable of dynamically scanning and pointing the moving objects than the traditional scanning methods.