Code-Division Multiple-Access Laser Source For Indirect Time of Flight

Abstract

This thesis describes the development of an optical pulse coded laser source for Indirect Time of Flight (iToF) measurement applications. The developed laser source is intended for use in a system that achieves Few-Pixel Imaging (FPI) by using Code-Division Multiple-Access (CDMA) techniques. A mathematical model of the complete transmission channel is shown, proving the theoretical applicability of such a system. The choice of codes is discussed and important properties of these are derived from the developed system. The presented system enables multiple simultaneous measurements with light pulses at a single optical wavelength. By combining few-pixel imaging with a CDMA-based Time of Flight (ToF) system, this work explores a new 3D-imaging approach that enhances depth sensing performance while reducing hardware costs. The developed laser source consists of multiple transmitters that emit binary pulse sequences (wavelength ≈ 640 nm) simultaneously, forming a CDMA system. Gold-Code (GC) sequences are employed as the transmitted codes due to their favorable cross-correlation and auto-correlation characteristics and efficient generation. Each of the parallel sequences is projected onto a line by an optomechanical system, forming a pattern of parallel light lines. The laser source is designed for use with a receiver that employs a perpendicular optical configuration, together creating a grid of discrete measurement points. Signal generation is implemented using a Field Programmable Gate Array (FPGA) in combination with multiple high-speed serializers, enabling modulation frequencies of up to 1 GHz. The used GC and operation frequency can be changed dynamically during operation of the system. The laser diodes are driven by Vertical-Cavity Surface-Emitting Laser (VCSEL) driver chips. These chips generate the modulated pulsed light signals required for precise ToF measurements. The prototype consists of eight independent transmission channels. The developed laser source can therefore simultaneously emit up to eight different pulse sequences, thus enabling parallel depth measurement with high throughput within the measuring grid. An experimental characterization of the prototype is performed to verify the functionality of the PCB and the system.