Ti:sapphire frequency combs for dual-comb distance metrology
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This thesis presents research developing Ti:sapphire lasers for applications in precision absolute distance metrology. Dual Ti:sapphire frequency combs were developed and characterised, each typically generating 64 fs pulses at a wavelength of 780 nm and a pulse repetition rate of 513 MHz. Electronic phase stabilisation using f-to-2f interferometry and direct pump power modulation achieved phase slips of 234 mrad (124 mrad) for the carrier-envelope offset frequencies of the probe (local-oscillator) comb in observation times of 1 second. The comb mode spacings were stabilised with cavity length feedback, achieving a phase noise of 4 mrad in one second for each laser. The developed Ti:sapphire dual-comb system was evaluated for distance metrology. When both combs were fully locked, absolute distance metrology was demonstrated for distances of up to 1.6 m, corresponding to ambiguity ranges of up to order 6. Time-of-flight precision of < λ/4 was achieved in an averaging time of 1 s, allowing handover to interferometric precision, which achieved a precision of 2 nm after an averaging time of 4 s. Cross-calibration of distance measurements using a 100-nm-precision delay stage allowed a direct measurement of the group velocity of air to an uncertainty (0.0026%) consistent with values from established atmospheric dispersion models. Modifying the repetition-rates of probe and local-oscillator combs was shown to potentially extend the effective ambiguity range up to 12.385 km. Methods of reducing the cost and complexity of a Ti:sapphire comb were explored by constructing a Ti:sapphire frequency comb directly pumped by 462 nm and 520 nm laser diodes. The laser was modelocked using a commercial saturable absorber and generated 90 mW average power. Using piezoelectric feedback to the laser cavity length, and current modulation of one of the pump diodes, the laser was fully phase-stabilised to achieve the first example of a directly diode-pumped Ti:sapphire laser with fully stabilised repetition rate and carrier envelope offset frequencies. This comb achieved a phase slip of 860 mrad for the 10 MHz carrier envelope offset frequency and 54 mrad for the 79 MHz repetition frequency, each over 1 second. Pulses with durations of 54 fs were generated at a central wavelength of 803 nm. Single walled carbon nanotubes were explored as potential saturable absorbers. A device was fabricated using spin coating and was shown to achieve modelocking at pump powers as low as 545 mW.