Adaptive Optical Beam Control for Rangefinders and Delay Lines

Abstract

In this thesis, the use of adaptive optics to exercise control over the fundamental TEM00 Gaussian laser beam is employed for applications in optical metrology and photonic signal processing. Split into two parts, this treatise explores the use of adaptive optics to introduce novel design solutions in both of these important areas of optics. Two novel designs of motionless Variable Optical Delay Lines (VODLs) are proposed with the respective proofof-concept implementations. The first VODL design employs multiple Electronically Controlled Tunable Lenses (ECTLs) to vary the optical path of a beam between two fixed locations. The design provides a repeatable digitally controlled short variable signal delay as well as a small signal delay step size for short delay measurements. The second Variable Fiber-Optic Delay Line (VFODL) design imparts long variable delays by switching an input optical/RF signal between Single Mode Fiber (SMF) patch chords of different lengths through a pair of Electronically Controlled Tunable Lenses (ECTLs) resulting in polarization independent operation. Moreover, two laser rangefinder designs are also proposed and experimentally demonstrated. Various existing target ranging techniques are limited in terms of the dynamic range of operation and measurement resolution. These limitations arise as a result of a particular measurement methodology, the finite processing capability of the hardware components deployed within the sensor module and the medium through which the target is viewed. Generally, there is a trade-off between the sensor resolution and dynamic range. First, we propose a novel design of a triangulation-based optical rangefinder which improves both the sensor longitudinal resolution and its dynamic longitudinal range through adaptive electronic control of beam propagation parameters. We present the theory and working of the proposed sensor and clearly demonstrate a longitudinal range and resolution improvement. The second divergence-based displacement sensor design works by tuning the spot size of a Gaussian Beam from a Laser Source (LS) at the plane of the target. The beam spot size tuning is achieved through the use of an Electronically Controlled Tunable Lens (ECTL). Using a carefully aligned sensor assembly, the corresponding beam diameters are recorded at the plane of an imaging detector for different ECTL focal length settings. This dataset is then used to estimate the distance of the target from the ECTL. The proposed rangefinder is compact, requires minimal data acquisition and processing resulting in a fast response time compared to its predecessor. The estimation of target distance through a measurement dataset also ensures that the proposed method is robust to errors associated with estimates which are based on the use of a single measurement data point. Experimental results demonstrate an excellent agreement with theory.