Analysis On The Characteristic Of Diffraction From Single Mode Optical Waveguide

With the rapid development of wavelength division multiplexing technology in the optical fiber communication area, the research on wavelength division multiplexer has turned to the domain of integrated optics since the initial stage of 1990s, and the design of many kinds of integrated optics devices is based on the theory of optical waveguide eigen mode, the characteristic of optical waveguide's diffractive field and the non-contact coupling characteristic between end surfaces in optical waveguides. So, the characteristic of diffraction from optical waveguide and the beam parameters of diffractive beam are researched in this paper, it has some significance for the design of integrated optics device.Taken single mode planar waveguide as the object, the diffraction from its end surface is researched thoroughly in theories, including deep Fresnel diffraction region, Fresnel diffraction and Fraunhofer diffraction, and the computer numerieal simulation is taken to testify the theory's validity, at the same time, the characteristic of deep Fresnel diffraction region, Fresnel diffraction and Fraunhofer diffraction is analyzed separately and the difference between them is discussed also.Referred to the Rayleigh-Sommerfield diffraction scalar integral formula and convolution theorem, the calculable formula of diffractive field distribution from the restricted output end surface in single mode dielectric planar optical waveguide is derived. It is described as the convolution between the Dirae function, the Sine function and the Lorenz function and the essence of restricted diffraction is clarified. The characteristic of diffractive field is discussed in detail. More, the connection between the full angle width at half maximum intensity of central bright fringe and the half width of slit is researched and the expression between them is acquired by curve fitting.Through mathematical modeling and the curve fitting method, the relationship between the beam parameters in the diffractive field of single mode optical waveguide and the normalized frequency is discussed minutely. The fitting function between mode field radius in the near field, angular radius of far field divergence, beam propagation factor and the normalized frequency are obtained respectively and has a higher precision; It was illuminated that when the normalized frequency is in a certain scope, the mode field of LP₀₁ mode can be expressed as Gaussian function. The view that the equivalent mode field radius, which is used for Gaussian approximation, can be obtained according to the equivalent matching efficiency was advanced. The formula of the equivalent mode field half radius, which is based on the core layer radius, the normalized standing wave parameter and the normalized evanescent wave parameter, is introduced. For convenience, the relationship between the equivalent mode field radius and the normalized frequency is fitting as a function. These conclusions will provide academic foundation for analyzing or calculating the beam parameters approximately.