Abstract
The implementation of optical of wireless communications (OWC) requires the use of a light-emitting-diode (LED) or laser diode (LD). Due to significant path loss exhibited by these sources in an outdoor environment, an unobstructed point-to-point link must be maintained in order to increase the signal-to-noise ratio (SNR) at the receiver. This thesis considers a solution to alleviate the fundamental limitations of the OWC channel in an outdoor environment by investigating optical beam steering (OBS) to increase the signal strength in the desired direction. Conventional methods to implement on OBS use a microelectromechanical (MEMS) mirror or a spatial light modulator (SLM) which both suffer from low switching frequency. A high frequency OBS device can be created by using optical phased array (OPA). An electro-optic modulator (EOM) such as LiNbO3 can be used to create an OPA but can not be directly integrated in silicon. For monolithic silicon-on-insulator (SOI) solutions, previous literature uses thermo-optic couplers on SOI to implement the OPA, however this introduces the issue of thermal cross-talk. Therefore, this thesis focuses on the use of silicon as the EOM for use in an OPA to create a high frequency monolithic OBS. Our contributions consist of providing a design method for a OBS SOI device which exhibits minimum internal cross-talk and provides propagation in free-space with high directivity and a wide steering range. Additionally, propose the use of an internal heterodyne optical phase locked loop (PLL) to stabilize the OBS instead of an external signal processor for phase correction. This optical PLL reduces beam jitter, minimizes beam squint, and provides active tracking for the output beam towards the receiver. We have also characterized a shadowing scenario in an OWC channel which OBS has the potential to alleviate. Moreover, we simulated the optical far-field radiation pattern from a SOI waveguide to free-space which has not been demonstrated in previous research. Finally, our simulation results of a SOI OPA demonstrates the coherent combining capability of OBS using MEEPTtm and the Optiwavetm suite.
Highlights
Radio frequency (RF) wireless connectivity has been used for several decades as it allows for indoor and outdoor links to be established without any physical connection
We have presented the use of optical beam steering (OBS) to provide a LOS condition in an outdoor optical wireless communications (OWC) link
Our study on shadowing in an OWC link illustrates the importance of the use of OBS in mitigating the effect of shadowing and reducing the bit-error rate (BER)
Summary
List of Symbols βT βmn λ γT γ σ12 η φ τ ∆ne ∆nh ∆Lmn ∆f ωif θc μ Ar Amn B0 c D Ep Er Ez F (φ, θ) F (φ, θ) f h I0 ish ith J1 k K ke Li N (x, y) xe n nth, nef f nr nx(t) Ne Nh. Total extinction coefficient Phase of the signal received element Wavelength Channel attenuation Viscosity Strength of power fluctuation indicator Quantum efficiency of the photodiode Angle of irradiance Time to return to a no-voltage state Change in refractive index resulting from change in free electron carrier concentrations Change in refractive index resulting from change in free hole carrier concentrations Waveguide delay section of length Spectral range of a source Intermediate frequency Critical angle Transverse power distribution coefficient Active area of the photo detector Amplitude Brightness of the source Speed of light in a vacuum Lens diameter Maximal ratio combining Received signal energy Electric field along the z-direction Angular dependence Angular dependence Focal length Plancks constant Waveguide height Intensity of the incident radiation Photocurrent shot-noise Thermal noise of the receiver First order Bessel function Wavenumber Wave vector Elastic constant Interaction length Spatial dependence Photoelectrons Order of the radiating source Decision threshold Effective index for the TE-like fundamental mode Refractive index Noise in the x polarization direction Electron carrier concentrations Hole carrier concentrations xiii.
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