The Electro-Optical (EO) effect refers to the change in the optical properties of a material in response to a slowly varying external electric field compared to optical frequencies. This effect is key in EO modulators and beam steerers, which utilize the medium's linear or quadratic EO effect to modulate or steer light. A traveling-wave Mach-Zehnder modulator (TW-MZM) is used in optical communications for rapid and efficient light modulation, with a longer interaction region ideal for high-speed applications compared to lumped-element versions. On the other hand, the EO beam steerers use either an optical phased array (OPA) based on an EO thin film or a bulk EO crystal. Despite advantages like speed, robustness, and compactness, the EO steerers have limited angles, apertures, and high production costs. OPAs also struggle with low diffraction efficiency and dispersion, while bulk crystals are crystal polarization-dependent and require high voltages. To overcome or mitigate some of those drawbacks, this dissertation aims to tackle the following five key questions that have remained unresolved in the field of EO beam steerers for decades.
How can bulk EO crystals be used for fast deformable mirrors, various types of varifocal lenses, and nonmechanically steer a large aperture light to a wide angle?
How to increase the efficiency of OPAs and overcome the dispersion?
How to make the OPAs manufacturable due to the very large number of electrodes?
How to steer a large aperture continuously to a wide angle with OPAs?
How can light be steered in a mechanical and non-mechanical hybrid manner to direct it mechanically and scan within a small area non-mechanically, or to perform back scanning non-mechanically? This approach is particularly relevant in space applications, where back scanning may be important to account for delays caused by the speed of light.
The first section of the research provides a detailed exploration of a bulk SBN75 single crystal - known for its high linear EO effect, and a polycrystalline PMN-PT - noted for its strong quadratic (Kerr) effect. It involves designing, simulating, and testing a laser beam steering device with these EO crystals under various conditions. The SBN steerer is thoroughly evaluated by measuring the first and second EO effects and all linear EO coefficients (r_13, r_33, r_51) based on laser beam deflection angles, with comparisons to literature values. To overcome the need for poling the EO crystal and to provide a larger EO medium, a polarization-independent EO steerer with PMN-PT ceramic has been designed. This design steers light from visible to mid-IR wavelengths, evaluating the quadratic effect's dispersion and temperature dependency.
This research further branches into the design and optimization of a TW-MZM for six diverse EO crystals - Lithium Niobate (LNB), Potassium Niobate (KNB), Lithium Titanate (LTO), Beta Barium Borate (BBO), Cadmium Telluride (CdTe), and Indium Phosphide (InP). Broad performance metrics, including optical and radio frequency (RF) loss, applied voltage, and modulation bandwidth, are estimated and compared to single out potential alternatives that can outperform LNB, the most commonly employed EO crystal in modern EO modulators.
The second section of this dissertation presents three innovative approaches to addressing the five major questions alluded to above.
How can the light be expanded and steered simultaneously and defy the aperture expansion law?
How can a light steerer handle any type of incoming and outgoing light?
How to design a novel light scanner that overcomes the disadvantages of current mechanical steerers, such as single thread, blind spot, dispersion, large number of elements, and inertia due to the large size of the moving parts?
The three innovative approaches outlined provide the means to address the aforementioned five critical questions, aiming to improve the efficiency, manufacturability, and affordability of the EO steerers, potentially paving the way for their deployment in demanding applications such as the military, laser communication, and space applications.