Frequency stabilised laser diodes and their use in length metrology

This thesis is concerned with the use of indexed-guided GaA1As laser diodes in length metrology. Since these lasers have important differences from more commonly use HeNe lasers in interferometry, the thesis begins with a review and investigation of these differences. A key problem is the relatively large linewidth (typically 30 MHz). This is reduced, by using the optical feedback from a resonant confocal etalon, to less than -10kHz.

Potential spectroscopic frequency references are discussed and the choice made of the rubidium (Rb) D lines at 780nm and 795 nm, using either Doppler-free features or the linear absorptions. The optically narrowed laser is then stabilised to the various hyperfine components. By measuring the reproducibility of the difference frequency between two systems, a relative frequency reproducibility of \(\approx\) 1x10\(^{-10}\) is demonstrated. Laser frequency stabilities are measured for a number of different cases, for example with the lasers free-running and offset locked. for two optically narrowed RB-stabilised laesrs at \(\tau\)=10 s, the relative frequency stability is \(\approx\) fx10\(^{-12}\). The hyperfine intervals of the two Rb lines are also measured for both isotopes (\(^{85}\)Rb and \(^{87}\)Rb), representing the most complete study of Rb-stabilised diode lasers to date. From these results, values for the hyperfine constants and isotope shifts are obtained. Finally, the absolute frequencies of both RB-stabilised laser diodes are measured by interferometric comparison with an iodine stabilised HeNe laser at 633 nm with a relative accuracy of \(\pm\) 1x10\(^{-9}\).

To demonstrate the use of a frequency tunable laser diode in distance measurement, a swept wavelength measurement system was developed with a demonstrated accuracy of a few parts per million. The thesis ends with a discussion of longitudinal mode control in laser diodes and some potential future uses in length and optical frequency metrology.