Development of polychromatic laser beacon fiber coupling system based on photonic crystal fibers
Date of Award
M.S. in Electrical Engineering
Department of Electrical and Computer Engineering
Advisor: M. A. Voron︠t︡sov
The simultaneous characterization of laser beams of different wavelengths propagating through the atmosphere has been shown to be a valuable technique for the study of the effects of atmospheric turbulence and refraction on optical propagation. Previously, experiments were performed using three different single-mode fiber collimators with wavelengths 1.55 æm, 1.064 æm and 0.532 æm as laser beacons. The use of different (spatially separated) fiber collimators introduces uncertainties in the experiments such as the mutual pointing errors between the beacons. The goal of this thesis was therefore to implement a polychromatic laser beacon that transmits the three wavelengths mentioned above out of the same fiber collimator. A key requirement was to transmit only a single transverse mode (the fundamental mode of the fiber) for each wavelength. This was achieved using an endlessly single-mode" photonic crystal fiber. To couple the light from three fiber lasers with different wavelengths and correspondingly different single-mode fibers into the photonic crystal fiber, a free-space fiber coupling system using commercial off-the shelf lenses was designed and optimized for best possible fiber coupling efficiency using ray-tracing software (Zemax). A tolerance analysis was performed to determine the sensitivity of the system to misalignments such as lens tilts, defocus and decenter. The analysis showed that realization of the fiber-coupling system was feasible considering typical manufacturing tolerances and precision of alignment. A prototype of the fiber coupling system based on the optical design was set up in the laboratory. In order to position the fiber tips accurately with sub-micrometer precision, fiber positioners based on piezo-electric actuators, which allow for electronically controlled lateral movement of the focused beam relative to the tip of the photonic crystal fiber, were employed in the fiber collimators. The experimentally observed fiber coupling efficiencies for the three wavelengths were in reasonably good agreement with values calculated with the Zemax ray tracing software. A secondary goal of the thesis was to demonstrate feedback control of the fiber coupling system for all three wavelengths simultaneously. Three parallel control loops were established using the fiber positioners along with commercially available controllers based on stochastic parallel gradient descent algorithm (SPGD) to ensure maximum coupling efficiency even under environmental disturbances such as mechanical stress, temperature changes and vibrations. Tests demonstrated the stability of the system with respect to large-amplitude vibrations at frequencies of several hertz. An off-axis parabolic mirror was used to set up a prototype of the polychromatic fiber collimator and components for feedback signal sensing were incorporated. The fiber coupling system and the fiber collimator were integrated for a laboratory demo of the fully working polychromatic beacon system."
Collimators (Optical instrument) Design and construction, Beam optics Calibration, Photonic crystals, Atmosphere Laser observations, Optics; photonic crystal fiber; laser beacon; fiber coupling
Copyright 2013, author
Sangam, Ramyaa Ramesh, "Development of polychromatic laser beacon fiber coupling system based on photonic crystal fibers" (2013). Graduate Theses and Dissertations. 653.