Megahertz pulse-burst laser systems coupled with megahertz-rate framing cameras have proven (over the last ten years) to be very robust in imaging of high-speed reacting and nonreacting supersonic flows. These Nd:YAG systems produce 20–30 pulses (at variable rates from 500 kHz to 1 MHz) with 50–100 mJ/pulse (λ = 1064nm) and have been used with narrow, spectral-linewidth, iodine, atomic filters to image turbulence in supersonic boundary layers with great success (when operating at λ = 532nm). To extend this pulse-burst capability at other wavelengths (wavelengths outside of the 5–30 GHz tuning range of Nd:YAG: λ = 1064 nm fundamental, and λ = 532 nm second harmonic), two unique, tunable, megahertz-rate alexandrite laser systems were designed and built. This dissertation documents these two systems and discusses the potential for tunable, megahertz, pulse-burst systems that have more tuning range than Nd:YAG. These tunable alexandrite systems substantially extend the wavelength range of pulse-burst laser technology, but, to date, have pulse-energy limitations. Tunable from 710 nm to 800 nm (in the fundamental), these lasers provide researchers one laser to reach multiple molecular or atomic resonances with variable pulse-burst pulse separations. The molecular and atomic species of interest in reacting and nonreacting flows are presented in Chapter 1, providing a road-map for the development of these tunable lasers.
This dissertation presents the design and development of these systems, including mode control, Herriott cell design for pulse separation, and the megahertz-tuning ringmaster-oscillator. Chapter 2 covers the physics of alexandrite as a solid-state, lamp-pumped, tunable medium and compares it to the tunability of Ti:sapphire. Chapter 3 and 4 present the pulse-burst alexandrite systems. The first system, built in Princeton’s Applied Physics group (PAPG) (Chapter 3), produced 1-5 mJ total pulse-packet energy of 20–30 pulses, or approximately 100 μJ per pulse at λ = 761 nm. The second system, built at Princeton Plasma Physics Labs (PPPL) (Chapter 4), produced pulse-bursts of 3–10 pulses with pulse power of 5–10 mJ/pulse at the fundamental wavelength of 758 nm. The spectral linewidths varied throughout the development of the two systems. Two different master-oscillator configurations were used, one linear, with a standing wave, and one ring, with a unidirectional wave. Using a linear, master-oscillator with double inter-cavity Fabry-Perot etalons, the PPPL pulse-burst system achieved 0.3Å linewidth and limited tuning capability (limited by the tuning resolution of the inter-cavity, 9-plate, birefringent tuner). This made the system appropriate for laser induced fluorescence (LIF) studies of plasma turbulence, but, not sufficient for filtered Rayleigh scattering. The linear oscillator for the PAPG system achieved linewidths on the order of 1Å (by way of a 4-plate, birefringent tuner). PAPG’s system was designed with a Sacher diode-seeding system to decrease the linewidth to under 1 GHz (i.e., 0.002Å) by way of cavity seeding, however, the linear oscillator did not reliably mode-lock.
To achieve mode-locked, mode-hop-free tuning on the order of 30 GHz with a 88 MHz linewidth pulse, the master oscillator was configured and built as a mode-locked, diode-injection-seeded, alexandrite ring-cavity with “rapid-ramp” cavity length stabilization (RCLS) technology. Chapter 5 and Chapter 6 present the design and performance of the unidirectional-wave, alexandrite ring laser. The mode-locked, alexandrite, ring laser’s piezo modulation system and driver are presented in this thesis, along with experimental results which focus on spectral linewidth and spectral-purity characterization, using an atomic potassium filter at λ = 766.701 nm and atomic rubidium filter at λ = 780.2445 nm (vacuum wavelengths) for the alexandrite ring in single-pulse mode. These atomic, vapor-cell, experimental-scan results document the reliability and tunability of the ring as a master oscillator for the pulse-burst system.
The final chapter of this dissertation, Chapter 7, presents a new design which was not built, but was inspired by all of the technological advances developed in the process of building the MHz pulse-burst systems. With the proper funding, this author believes, the final system would be capable of producing 3–10 megahertz-repetition-rate pulses with 10–30 mJ/pulse and stable pulse linewidths of 88 MHz or better, as documented in Chapter 6. Furthermore, this system would be tunable from 710–800 nm which provides a range of wavelengths (through harmonic doubling and tripling crystals, and Raman, spectral-shifting gas cells) which would reach a number of the molecular species of interest in reacting and nonreacting high-speed flows, as presented in Chapter 1.