Narrow-linewidth megahertz-repetition-rate optical parametric oscillator for high-speed flow and combustion diagnostics

Abstract

We demonstrate the ability to generate ultra-high-frequency sequences of broadly wavelength-tunable, highintensity laser pulses using a custom-built optical parametric oscillator pumped by the thirdharmonic output of a "burst-mode" Nd:YAG laser. Burst sequences consisting of 6-10 pulses separated in time by 6-10 Îs are obtained, with average total conversion efficiency from the 355 nm pump to the near-IR signal and idler wavelengths of 33%. Typical individual pulse output energy for the signal and idler beams is in the range of 4-6 mJ, limited by the available pump energy. Line narrowing is demonstrated by means of injection seeding the idler wave using a low-power external-cavity diode laser at 827 nm. It is shown that seeding reduces the time-averaged linewidth of both the signal and idler outputs to 300 MHz, which is near the 220 MHz Fourier transform limit. Line narrowing is achieved without recourse to active cavity stabilization. RightsThis paper was published in Applied Optics and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: https://www.osapublishing.org/ ao/abstract.cfm?uri=ao-47-1-64. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law. We demonstrate the ability to generate ultra-high-frequency sequences of broadly wavelength-tunable, high-intensity laser pulses using a custom-built optical parametric oscillator pumped by the thirdharmonic output of a "burst-mode" Nd:YAG laser. Burst sequences consisting of 6 -10 pulses separated in time by 6-10 s are obtained, with average total conversion efficiency from the 355 nm pump to the near-IR signal and idler wavelengths of ϳ33%. Typical individual pulse output energy for the signal and idler beams is in the range of 4-6 mJ, limited by the available pump energy. Line narrowing is demonstrated by means of injection seeding the idler wave using a low-power external-cavity diode laser at 827 nm. It is shown that seeding reduces the time-averaged linewidth of both the signal and idler outputs to ϳ300 MHz, which is near the 220 MHz Fourier transform limit. Line narrowing is achieved without recourse to active cavity stabilization.

NO PLIF Imaging in the CUBRC 48" Shock Tunnel

“…For this work, the purpose of the PCM is to both improve the on/off contrast of the pulse slicer and to limit ASE in the forward direction. Fluorinert FC-75, which has been used extensively as an SBS fluid [18] and has previously performed well when operating the burst mode laser at less than ~250 kHz [18], was used for this work.…”

“…Note that the trace was obtained with a relatively low sampling rate oscilloscope that required the detector output to be deliberately broadened in time (using a load resistor and the intrinsic cable capacitance). For spectrally narrowed imaging the OPO was operated with injection seeding [18] using an external cavity diode laser (ECDL) manufactured by Sacher, Inc. The OPO "idler" beam is seeded, which results in line-narrowed output for both the idler and signal waves.…”

“…Also such instrumentation could only acquire a single image in one shock tunnel run. We have been developing, over the last several years, an ultra-high frame rate PLIF imaging system (up to 1 MHz frame rate) which combines a "burst mode" Nd:YAGbased tunable OPO system [18,19], with a variety of high frame rate cameras, including CCD, ICCD, and intensified CMOS. In addition to enabling capture of the temporal dynamics of hypervelocity flows, by operation at frame rates of the order of 10-20 kHz, the burst mode imaging systems provides the ability to obtain sets of ~10 full planar images (albeit temporally uncorrelated) in a single short-duration facility test.…”

MHz-Rate NO PLIF Imaging in a Mach 10 Hypersonic Wind Tunnel

“…Fluorinert FC-75, which has been used extensively as an SBS fluid [13] was previously found to perform well when operating the burst mode laser at less than ~250 kHz [4]. However, it was found that operation at frequencies of 500 kHz or higher resulted in poor stability and burst uniformity, indicating, presumably, that the system was being operated at, or near, the SBS threshold [12].…”

“…More recently, solid-state diode-pumped/dye laser technology appears capable of achieving up to 10 kHz of continuous PLIF imaging, but this is still much lower than the bandwidth required for tracking flow phenomena in hypersonic flows [3]. To address this diagnostic need we have been developing, over the last several years, an ultra-high frame rate PLIF imaging system which combines a "burst mode" Nd:YAG-based tunable OPO system [4][5][6], with a framing ICCD camera. In this paper we will demonstrate the utility of the system for obtaining time correlated PLIF movie sequences in a large scale flow facility, the NASA Langley 31" Mach 10 Hypersonic Wind Tunnel [7].…”