M.Ing. / The aim of this project is to research the feasibility of an all-fibre laser distance measurement device that utilises a figure-eight fibre laser (F8L), in the nonlinear amplifying loop mirror (NALM) configuration, as a light source and implements pulse compression to improve the accuracy and signal-to-noise ratio of the system. A figure-eight fibre laser in the NALM configuration for use in a laser distance measurement device is described. The theory of fibre lasers is discussed, including mode-locking and Qswitching, and the characteristics of a NALM loop are analysed. By varying the length of the NALM loop from 500 m to 2000 m or inserting highly nonlinear dispersion shifted fibre, a variety of pulses in the picosecond to nanosecond range can be produced. The lengths of the pulses depend on the length of the NALM loop, the pump power and the setting of the polarisation controllers. The figure-eight fibre laser is pumped with a 980 nm laser diode up to 550 mA, which corresponds to 320 mW. Distance measurements are done with short unmodulated and long modulated pulses. Distance measurement with short unmodulated pulses is discussed only briefly and tested with a simple experiment. The focus of this project is distance measurement with long modulated pulses. A low autocorrelated binary sequence is modulated onto one of the long pulses produced by the figure-eight fibre laser by an electro-optic amplitude modulator. The long pulse gives the proposed system a good signal-to-noise ratio (SNR), while the modulation improves the accuracy. A Barker code of length 13 is proposed as modulation code because of its good autocorrelation properties. The Barker code will improve the accuracy 13-fold, with a corresponding increase in SNR. An electro-optic amplitude modulator is used to implement the modulation. The modulated long pulse is then sent to a target. After reflection, the signal is detected and cross-correlated to obtain the time-of-flight for the pulse. The code generation and cross-correlation are implemented with an FPGA via VHDL programming. The distance to a target can be calculated by knowing the time-of-flight and the speed of light in the propagation medium. In this project the resolution, single-shot precision, accuracy, linearity, repeatability and maximum unambiguous distance of the proposed all-fibre laser distance measurement device are examined.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uj/uj:6905 |
| Date | 30 August 2010 |
| Creators | Du Plessis, Jan Harm |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
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