Light Transmission Characteristic Analysis Of GRIN Medium Optical Waveguide Coupler

With the development of the micro-nano photonics, the main trend of photonicintegration is how to miniaturize the devices, which requires the miniaturization ofthe guided wave structure. SOI waveguide attracts much attention in this field forthe big refractive index contrast between the cladding and the core layers materials,strong limitation to optical field and the structure size in the sub-micrometer range.However, when light beam is coupled from an optical fiber to a SOI waveguide, itsuffers much coupling loss due to the mismatch of the spot sizes. How to improvethe coupling efficiency between different components in the microstructure opticalsystem and the coupling efficiency between external port and the optical system isstill a problem to be solved. Optical coupler，which effectively improves the opticalsignal transmission efficiency between different components of the optical system，plays a very important role in optical communication, and integrated optical circuit.GRIN medium can expand or compress the beam periodically and may be usedas a beam coupling structure, which can effectively improve the coupling efficiencybetween a optical fiber and a SOI planar waveguide.First of all, the distribution of the mode field is investigated in a sech GRINmedium from the wave equation, and then the ray propagation trajectory equationand formula of the ray focus position are discussed from the perspective ofgeometric optics. With the help of electromagnetic simulation of finite elementmethod, the electric field distribution in GRIN medium is obtained, when theincident wave is a plane electromagnetic wave.Second, The propagation characteristics of the Gaussian beam in the sechGRIN medium are discussed on the condition of the paraxial approximation, andthree variation laws of the waist half width are obtained, the theoreticalexplanations are made by means of the ABCD matrix theory of the ray.Finally, GRIN medium couplers with symmetric and asymmetric refractiveindex of distribution are discussed for the beam coupling between a single-modeoptical fiber and a single-mode SOI planar waveguide. The coupling efficiencyvaries with the focus parameters of the GRIN medium, the angle of inclination ofthe Gaussian beam and the center position of the SOI waveguide core layer areanalyzed.