The Spatial Light - Fiber Coupling Technology

Free space optical communication has the advantages of both fiber communication and microwave communication, and now it is a new broadband communications which arouses great interests of people. Developed countries have carried out comprehensive studies on various systems such as inter-orbital, space-to-ground, ground-to-space, and terrestrial free space optical communication systems, and some have already brought forward commercial products. With the progress of free space optical communication technologies, researchers prefer systems making use of mature commercial fiber communication techniques based on 1550 nm that increases capacity and decreases the cost. Free-space-to-fiber coupling technique investigated in this paper is the first challenge of applying fiber communication techniques to free space optical communications.We analyzed the factors that affect the efficiency of free-space-to-single-mode-fiber coupling based on mode-matching theory of electromagnetic fields. Coupling efficiency can be maximized by optimizing the relative aperture of coupling lens. Theoretical analysis indicated that coupling efficiency reaches a maximum of 82.54% for 1310nm laser beams, when the 1/F# of coupling lens is 0.203, and the coupling efficiency reaches a maximum of 82.69% for 1550nm laser beams, when the 1/F# of coupling lens is 0.211. Coupling efficiencies decrease with pointing error, and the curve of relationships between them is similar to that of intensity distributions on fiber end which can be used for engineering estimate of coupling efficiency. Our theory is proved by experiments, and the maximum coupling efficiency achieved in our experiments is 61%. The algorithm of extracting angular errors through nutations was developed. The active coupling modules utilizing the extracted angular error from nutation eliminate sensitivity of coupling efficiency to the offset of focused spots. The nutation mechanism combines the angular error detector and data signal detector, and the cancellation of one position detector eased the optical and electronic designs of free space optical communication systems. With fiber nutaion, a potential mini-type device is expected. The static state error of active coupling module in the case of tracking step signal is below 0.02μm.We designed a assembling and testing bed for passive coupling assembly, which solved the problem of unreasonable long time and low precision of assembling and testing process. The bed are based on piezoelectric ceramic device and controlled by computer. It reaches the goal of high precision, quick assembly, and automatic operation at the same time.Theoretical analysis and experiments indicate that the coupling loss can be lower than 3 dB by optimizing the relative aperture parameter of coupling lens and suppressing pointing error. It is feasible of applying fiber communication technologies in free space optical communication systems.