High Contrast Gratings And Its Applications In Optical Communication System

Optical grating is a research subject with a long history. Through so many years’study, researchers have applied the optical grating in various optoelectronic devices, such as beam splitters, polarizers, sensors, and lasers. Sub-wavelength High Contrast Grating (HCG) has caused wide concern in recent years. Not only do HCGs have the property of high transmittivity and high reflectivity, but HCGs can change the propagation direction of light beam by modulating the phase shift of wave front, which is of great significance to simplify the structure of photoelectric devices.In this dissertation, we focus on the research of phase shift modulation of sub-wavelength High Contrast Gratings and its potential applications in the field of optical communication. The specific research work and innovations are as follows:1. We study two different research methods used to analyzing diffraction characteristics of periodic HCGs—Rigorous Coupled-Wave Analysis and "double-mode" analytical method proposed by professor Connie J. Chang-Hasnain and study the characteristics of periodic HCGs, including broadband high reflectivity, polarization selectivity, wavelength scalability—multiply the dimensions and wavelength by a constant while keep the other parameters the same, the reflection characteristics keeps unchanged. Moreover, we study a method-Finite Element method to analyze the diffraction characteristics of non-periodic HCGs and give a concise description on the calculation principle. Moreover, we introduce the setting method of simulation using COMSOL software. 2. We design a high-transmittivity non-periodic sub-wavelength High Contrast Grating with large-angle (30-degree) beam-steering ability. The properties of the beam steering and transmission are numerically studied with the Finite Element Method. The results show that when the1550nm TM light incidents upon the grating normally, the transmittivity is up to0.91and the steering angle is27.42degrees, which is very consistent with the theoretical30degrees. This work proves that the discrete sub-wavelength grating bars can achieve large-angle beam steering.3. We present a new approach to design high-numerical-aperture focusing reflectors using circular sub-wavelength High Contrast Gratings from the point of mathematical transformation. Using this approach, we design a circular grating reflector with a diameter of29.788μm. Numerical simulation shows that when the1550nm radial polarized light incidents upon the circular grating, most of the light will be reflected and focus on a spot. The reflectivity is0.9163and the Numerical Aperture is0.8302. At the focal plane, the E-field distribution Full Width at Half Maximum is1.5548μm.4. We present another concise approach to the design of focusing reflector using concentric circular sub-wavelength High Contrast Gratings. With this method, we design a focusing reflector with a focal length of5μm. Simulation results show that, when the incident wavelength is1550nm, the reflectivity is0.8538, and the Numerical Aperture is up to0.9213. At the reflection focal plane, the Full Width at Half Maximum of E-field distribution is0.9845urn.5. We study two-dimension blocky gratings, a design method for2D blocky sub-wavelength High Contrast Grating polarization-insensitive focusing lens is put forward. Also, we propose a high-transmittivity, polarization-insensitive focusing lens for1550nm TE and TM mix light using2D blocky sub-wavelength High Contrast Gratings. Properties of focusing and transmission are numerically studied with the Finite Element Method. The numerical simulation results show that, when the incident wavelength is1550nm, the transmittivity is0.8659, and the Numerical Aperture is0.535. At the focal plane, the field distribution of the average Full Width at Half Maximum is1.7515μm. In the case of focal length keeping unchanged, a bigger Numerical Aperture can be reached by increasing the area of gratings. Moreover, two adjacent edges of a block carry the modulating information of TE light and TM light respectively. So by changing the two adjacent-edge lengths of blocks, we can control the TE light and TM light simultaneously. Based on this, we propose the frameworks of power divider, wavelength divider, and polarization beam splitter realized by two-dimension blocky gratings.6. We design two bar-type sub-wavelength High Contrast Gratings with a10-degree beam-steering angle and a20-degree beam-steering angle respectivity, and two bar-type sub-wavelength High Contrast Gratings with a15-μm focal length and20-μm focal length respectivity. Also, we design two concentric circular sub-wavelength High Contrast Gratings with a15-μm focal length and20-μm focal length respectivity. Moreover, the gratings have been fabricated. We study the experimental setup to test the properties of reflected focusing, transmitted focusing, reflected steering, and transmitted steering of sub-wavelength High Contrast Gratings. According to the testing purposes, detailed key process and points for attention are discussed.7. We test a bar-type non-periodic subwavelength grating. The result shows a17.2-degree steering, which is very close to the designed20degrees.