Picosecond laser electronic excitation tagging velocimetry using a picosecond burst-mode laser.

Detailed characterizations of picosecond laser electronic excitation tagging (PLEET) in pure nitrogen (N2) and air with a 24 ps burst-mode laser system have been conducted. The burst-mode laser system is seeded with a 200 fs broadband seeding laser to achieve short pulse duration. As a non-intrusive molecular tagging velocimetry (MTV) technique, PLEET achieves "writing" via photo-dissociating nitrogen molecules and "tracking" by imaging the molecular nitrogen emissions. Key characteristics and performance of utilization of a 24 ps pulse-burst laser for MTV were obtained, including lifetime of the nitrogen emissions, power dependence, pressure dependence, and local flow heating by the laser pulses. Based on the experimental results and physical mechanisms of PLEET, 24 ps PLEET can produce similar 100 kHz molecular nitrogen emissions by photodissociation, while generating less flow disturbance by reducing laser joule heating than 100 ps PLEET.

High-speed 2D Raman imaging at elevated pressures.

Two-dimensional (2D) Raman scattering at 10 kHz in non-reacting flow mixtures is demonstrated by employing a burst-mode laser with a long-duration pulse of about 70 ns and pulse energy of about 750 mJ at 532 nm. To avoid optical breakdown, the pulse width of the laser was varied in the range of 10-1000 ns. The effects of pulse shape, pulse energy, and harmonic conversion on 2D measurements are also studied. The applications of high-speed, single-shot, 2D imaging of CH4 and H2 jets in N2 at elevated pressures are demonstrated. In addition, the scalar dissipation rate of CH4 in N2 at 20 bar is determined, and multi-dimensional, multi-species, high-speed imaging of flows at elevated pressures is demonstrated.

Seedless velocimetry at 100 kHz with picosecond-laser electronic-excitation tagging.

Picosecond-laser electronic-excitation tagging (PLEET), a seedless picosecond-laser-based velocimetry technique, is demonstrated in non-reactive flows at a repetition rate of 100 kHz with a 1064 nm, 100 ps burst-mode laser. The fluorescence lifetime of the PLEET signal was measured in nitrogen, and the laser heating effects were analyzed. PLEET experiments with a free jet of nitrogen show the ability to measure multi-point flow velocity fluctuations at a 100 kHz detection rate or higher. Both spectral and dynamic mode decomposition analyses of velocity on a Ma=0.8 free jet show two dominant Strouhal numbers around 0.24 and 0.48, respectively, well within the shear-layer flapping frequencies of the free jets. This technique increases the laser-tagging repetition rate for velocimetry to hundreds of kilohertz. PLEET is suitable for subsonic through supersonic laminar- and turbulent-flow velocity measurements.

1-kHz two-dimensional coherent anti-Stokes Raman scattering (2D-CARS) for gas-phase thermometry.

Two-dimensional gas-phase coherent anti-Stokes Raman scattering (2D-CARS) thermometry is demonstrated at 1 kHz in a heated jet. A hybrid femtosecond/picosecond CARS configuration is used in a two-beam phase-matching arrangement with a 100-femtosecond pump/Stokes pulse and a 107-picosecond probe pulse. The femtosecond pulse is generated using a mode-locked oscillator and regenerative amplifier that is synchronized to a separate picosecond oscillator and burst-mode amplifier. The CARS signal is spectrally dispersed in a custom imaging spectrometer and detected using a high-speed camera with image intensifier. 1-kHz, single-shot planar measurements at room temperature exhibit error of 2.6% and shot-to-shot variations of 2.6%. The spatial variation in measured temperature is 9.4%. 2D-CARS temperature measurements are demonstrated in a heated O2 jet to capture the spatiotemporal evolution of the temperature field.

High-repetition-rate laser ignition of fuel-air mixtures.

A laser-ignition (LI) method is presented that utilizes a high-repetition-rate (HRR) nanosecond laser to reduce minimal ignition energies of individual pulses by ∼10 times while maintaining comparable total energies. The most common LI employs a single nanosecond-laser pulse with energies on the order of tens of millijoules to ignite combustible gaseous mixtures. Because of the requirements of high energy per pulse, fiber coupling of traditional LI systems is difficult to implement in real-world systems with limited optical access. The HRR LI method demonstrated here has an order of magnitude lower per-pulse energy requirement than the traditional single-pulse LI technique, potentially allowing delivery through standard commercial optical fibers. Additionally, the HRR LI approach significantly increases the ignition probability of lean combustible mixtures in high-speed flows while maintaining low individual pulse energies.

Regenerative amplification and bifurcations in a burst-mode Nd:YAG laser.

An Nd:YAG-based burst-mode regenerative amplifier laser was developed that offers high extraction efficiency at high repetition rates with low seed energies. The regenerative amplification technique, combined with the burst-mode laser technology, shows promise as an efficient method for amplification of femtojoule-nanojoule pulses up to millijoule energies at repetition rates exceeding 100 kHz. Output energies at repetition rates near the inverse upper state lifetime are limited by bifurcations in the pulse energies of the burst. A model is developed and advantages and limitations are discussed.

100-ps-pulse-duration, 100-J burst-mode laser for kHz-MHz flow diagnostics.

A high-speed, master-oscillator power-amplifier burst-mode laser with ∼100 ps pulse duration is demonstrated with output energy up to 110 J per burst at 1064 nm and second-harmonic conversion efficiency up to 67% in a KD*P crystal. The output energy is distributed across 100 to 10,000 sequential laser pulses, with 10 kHz to 1 MHz repetition rate, respectively, over 10 ms burst duration. The performance of the 100 ps burst-mode laser is evaluated and been found to compare favorably with that of a similar design that employs a conventional ∼8 ns pulse duration. The nearly transform-limited spectral bandwidth of 0.15 cm(-1) at 532 nm is ideal for a wide range of linear and nonlinear spectroscopic techniques, and the 100 picosecond pulse duration is optimal for fiber-coupled spectroscopic measurements in harsh reacting-flow environments.

100 kHz, 100 ms, 400 J burst-mode laser with dual-wavelength diode-pumped amplifiers.

The burst duration of an all-diode-pumped burst-mode laser is extended to 100 ms and 100 kHz (10,000 pulses) by utilizing dual-wavelength diode pumping. Total energies of 225 J at 10 kHz and 400 J at 100 kHz are achieved during the 100 ms burst period at 1064 nm. This represents an order-of-magnitude increase in the number of pulses compared with prior work, while maintaining similar or higher pulse energies. Amplitude tailoring of each pulse is used to flatten the burst profile, reducing the standard deviation in pulse energy over the 100 ms burst from 3.7% to 2.1% with a burst-to-burst standard deviation of 0.8%.