Fiber optics communications using liquid crystal based Fourier optical spectrum analyzer without moving parts

ECE

Wavelength division multiplexing (WDM) corresponds to the scheme in which multiple optical carriers at different wavelengths are modulated by using independent electrical bit streams and are then transmitted over the same fiber. The optical signal at the receiver is demultiplexed into separate channels by their wavelengths. WDM has the potential for exploiting the large bandwidth offered by the optical fiber. In this thesis, a method of Fourier analysis of multi-beam interference is developed. It is shown that the total electric field and relative phase delay of each beam form a Fourier transform pair. Thus methods and properties of Fourier analysis are applicable in multi-beam interference analysis and design. Fourier transform based design is presented. Novel devices that apply such design principles are introduced. Principles and structures of novel adaptive attenuators based on various technologies such as segment deformable mirror, liquid crystal, phase modulation array are given. Simulation results for segment deformable mirror based adaptive attenuator are presented. In wavelength division multiplexed (WDM) optical communication networks, signals are amplified periodically by optical amplifiers. Since the gain profiles of optical amplifiers are not flat, equalizers are usually used to maintain signal powers at different wavelengths in equal to avoid crosstalk and data loss. However, fixed attenuation can only compensate fixed input power and amplification. In active network, input power and amplifier gain change with time. Active level compensation at each wavelength is needed. An adaptive attenuator is a device with a chromatically variable transitivity used to equalize channel powers in wavelength-division Multiplexing (WDM) fiber-optic communication lines. We propose and demonstrate a liquid-crystal (LC)-based Fourier optical spectrum analyzer (FOSA). The FOSA consists of a birefringent filter array with an embedded LC phase modulator. The LC phase modulator is used to control the phase difference between two orthogonally polarized beams to avoid mechanical movement in conventional Fourier transform spectrometers. The detailed operation principle of this FOSA is described. A single-LC-based 6-stage FOSA is experimentally demonstrated, and the results obtained using such a FOSA show good agreement with the results measured using a reference spectrometer. We propose a liquid crystal (LC)-based cost-effective Fourier optical spectrum analyzer (FOSA) without moving parts. Unlike normal spectrometers, the LC FOSA retrieves the spectrum's Fourier coefficients instead of the direct spectrum measurement by changing different LC states in a number of stages. Besides the common applications, the LC FOSA can precisely read and recover the envelope of a dense wavelength-division-multiplexing spectrum to act as a spectral monitor and then work with a gain-flattening device to control the optical network instantly and dynamically.

This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.