Study On Free-Space Optical Hybrid Of Grating

Free-space laser communications have made a rapid development in recent 20 years.However,there are still some kinds of shortcomings such as low sensitivity,short relay distance,and low-bite rates.In order to develop a new generation of free-space laser communication of terminals with high bit rate and lightweight, high sensitivity, coherent optical communication system has become an important research aspect of free-space laser communications. In the coherent optical communication systems,optical hybrid is one of the main components,which connects the signal laser to the local oscillator laser, so that it produces the phase-shift in required, the performance of hybrid has an important effect on the receiving performance of the coherent communication systems.My thesis is mainly about a new type of spatial optical hybrid which is regarding the grating as the main device to design the available coherent optical detection system.With the progress of the grating production technology,the types of gratings have increased day by day, to make full use of the diffraction properties of different grating, it can compose of different spatial optical hybrids. This kind of spatial optical hybrid has the advantages of small size, light weight, easy miniaturization-integration and realization of proper phase relation, etc, and it has potential application in the areas about the laser of coherent communications and the laser of radars.The thesis mainly includes the following aspects:1. A brief introduction of Grating's type and features.Classify and summarize the Grating in different ways, and then illustrate the main features of grating briefly. 2. Research on the modulation of the wave plate on the amplitude and phase of beams separated by the polarizing splitter.This thesis studied the situation of different polarized light which change by the wave plate and polarizing splitter, and the numerical analysis of the features and the regular changes in its amplitude and phase, which is realize the phase function of optical hybrid by combining the features of wave plate with the properties of grating, and it provides references to the design of the space optical hybrid of grating.3. Research on the free-space optical hybrid of differential grating.A new free-spaceoptical hybrid of differential grating is present by multipurpose using the properties of diffraction and phase shift of grating. In the proposal,it realizes diffraction beams splitted and coupled by the differential structure of the grating period, meanwhile,it produces a certain phase output, a single grating can realize the optical hybrids of 2×290°and 180°, the optical hybrids of 2×490°can be realized by combining with the PBS. A reasonable hybrid is designed through the comprehensive analysis.4. Research on the optical hybrid of subwavelength grating.Base on the features of subwavelength polarization grating,another new optical hybrid of free space is proposed.The split and recombination of the signal beam and the local oscillator beam is achieved by applying the features of subwavelength polarization grating,and then the desired phase relation is obtained by rotating the 1/2 wave plate to adjust the angle between the fast axis of the polarization beam splitter and the x direction of the incident circularly polarized light. And make an optimal design of the hybrid according to the actual situation.5. Analysis on property and error of optical hybrids.Analyse the property and the error of optical hybrid of grating that we designed, each error factors is analyzed and calculated in detail separately. Lastly, provide the compensation scheme to the errors.