Review of ultra-high repetition rate laser diagnostics for ......particle image velocimetry, planar Doppler velocimetry, planar laser induced ﬂuorescence, ... available pulsed lasers

IOP PUBLISHING MEASUREMENT SCIENCE AND TECHNOLOGY

Meas. Sci. Technol. 24 (2013) 012002 (22pp) doi:10.1088/0957-0233/24/1/012002

TOPICAL REVIEW

Review of ultra-high repetition rate laserdiagnostics for fluid dynamicmeasurementsBrian Thurow1, Naibo Jiang2 and Walter Lempert3

1 Department of Aerospace Engineering, Auburn University, 211 Davis Hall, Auburn, AL 36849, USA2 Spectral Energies, LLC, 5100 Springfield Ave Suite 301, Dayton, OH 45431, USA3 Department of Mechanical and Aerospace Engineering, The Ohio State University, 201 W 19th Ave,Columbus, OH 43210, USA

E-mail: [email protected]

Received 5 December 2011, in final form 11 April 2012Published 29 October 2012Online at stacks.iop.org/MST/24/012002

AbstractRecent advances in ultra-high repetition rate (100 kHz and above) laser diagnostics for fluiddynamic measurements are reviewed. The development of the pulse burst laser system, whichenabled several of these advances, is described. The pulse burst laser system produces highrepetition rate output by slicing the output of a low power continuous wave laser and passingthe resulting burst of pulses through a series of pulsed Nd:YAG amplifiers. Several systemshave been built with output approaching 1.0 J/pulse over bursts of up to 100 pulses generatedat between 50 and 1000 kHz. Combined with the capabilities of several types of commerciallyavailable high-speed cameras, these systems have been used to make a wide variety of highrepetition rate and 3D flow measurements. Several examples of various high repetition ratelaser diagnostics are described, including flow visualization, filtered Rayleigh scattering,planar Doppler velocimetry, particle image velocimetry, planar laser induced fluorescence,molecular tagging velocimetry and 3D flow visualization.

Keywords: high repetition rate lasers, high-speed imaging diagnostics, flow visualization,particle image velocimetry, planar Doppler velocimetry, planar laser induced fluorescence,molecular tagging velocimetry, 3D flow visualization, turbulence

(Some figures may appear in colour only in the online journal)

1. Introduction

Over the last several decades, the rapid growth of commerciallyavailable pulsed lasers and digital imaging systems has playedan integral role in the development of advanced measurementtechniques for turbulent and reacting flow fields. Today,techniques such as particle image velocimetry (PIV) andplanar laser induced fluorescence (PLIF) are available ascommercially packaged, turnkey systems capable of makingnearly instantaneous flow and species measurements in awide range of facilities and flow environments. A commonlimitation of these modern systems, however, is the limited

repetition rate at which they can operate with standardflashlamp-pumped systems operating on the order of 10 Hz.While recent advances in diode-pumped laser and CMOScamera technology have enabled the offering of ‘high-speed’laser-based imaging systems with repetition rates on the orderof 1000 Hz, in many turbulent and reacting flows, repetitionrates on the order of 104–106 Hz and above are necessaryin order to capture the dynamics that govern the underlyingphysics. Some examples include supersonic/hypersonic flow,combustion, aero-acoustics and plasma physics.

The need for high repetition rate measurements inturbulent flows can be illustrated by considering the

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Meas. Sci. Technol. 24 (2013) 012002 Topical Review

Figure 1. Graph of characteristic frequency of large-scale turbulentmotions as a function of length scale. Measurement systems must beable to operate at frequencies several factors higher than those ofinterest.

characteristic length, l, and characteristic velocity, u, of theturbulent flow under investigation. These values are typicallychosen to represent the largest scales of motion and can becombined to form a characteristic frequency, f large = u/l. Thisrelationship is illustrated in figure 1 where it can be seen thateven modest low-speed wind tunnel experiments (velocities onthe order of 10 m s−1) can exhibit large-scale frequencies wellabove 1000 Hz. For high-speed flow experiments, such as thoseassociated with supersonic and hypersonic flows, frequencieson the order of 104–106 Hz are common. Furthermore, a well-known property of turbulence is the energy cascade wherebysmaller scale motions are generated by large-scale motionsthus forming a spectrum of scales present in the flow at anygiven time. Using Kolmogorov’s theory and expressing interms of the characteristic frequency, it can be shown that

fsmall

flarge=

√Re, (1)

where Re is the Reynolds number (Re = ρul/μ), ρ is the flowstatic density, u is the flow velocity and μ is the kinematicviscosity. With Reynolds numbers encountered in practicetypically much greater than 104, the frequencies associatedwith small spatial scales, albeit at a reduced amplitude, arehigher than the large-scales. Thus, even simple turbulent flowspossess a significant amount of high frequency content across abroad spectrum. As such, a large portion of our understandingof turbulence and the associated cascade process is based onhigh frequency point measurements, such as those providedby hot-wire anemometry. As turbulence is characterizedby fluctuations in both space and time, however, furtherunderstanding will require simultaneous measurement of flowproperties in space and time on the appropriate scales.

Reacting flows present an additional challenge as the timescales associated with various chemical reactions span an evenbroader (and generally higher) range that often overlaps withthose present in the flow. This is particularly true in non-premixed combustion systems where turbulence governs themixing of fuel and oxidizer, which in turn react and can releaseenough heat to significantly alter the underlying turbulence.This coupling between fluid dynamics and chemical kinetics

presents a significant challenge to both experimentalists andcomputationalists such that measurements on the same timescale as the underlying physics are still needed to betterunderstand these complex interaction mechanisms. Combinedwith the Nyquist–Shannon sampling theorem ( f sample >2 f measured) it is clear that flow diagnostics with repetition rateson the order of 104–106 Hz hold a key role in advancing ourunderstanding of turbulent and reacting flows. A more detaileddiscussion of temporal, and spatial, resolution requirements inturbulent flames and jets can be found in Patton et al (2012a,2012b).

There have, to the authors’ knowledge, been threeapproaches which have been utilized to achieve laser-basedimaging diagnostics in this framing rate regime. The firstapproach, reported by Kaminski et al (1999), used fourindividual and independent double-pulsed Nd:YAG lasersto pump a single commercial dye laser. The system wassuccessfully utilized for high-speed OH PLIF imaging.Starting with 270 mJ per individual pulse at 532 nm, Kaminskiet al (1999) were able to generate eight pulses at 282 nm,with an average energy of ∼1 mJ/pulse and a repetitionrate of ∼8 kHz, constrained by the high-intensity pumpingrequirement of the dye laser. Recently, Sjoholm et al (2009)have been able to overcome this limitation by use of an opticalparametric oscillator (OPO) system, in place of the dye laser,similar to the approach to be described in section 4 below.This strategy has also been recently reported by Miller et al(2011), who used a cluster of four commercial Nd:YAG lasersto perform CH imaging measurements in a turbulent diffusionflame. The second approach is based on diode-pumpedNd:YAG and Nd:YLF lasers, which are becoming increasinglyavailable commercially. While the total pulse energy of diode-pumped laser systems is relatively low, such systems have theadvantage that extremely large continuous image sequencescan be obtained, when used with commercial CMOS-basedimaging cameras. As an example of this approach, Kittlerand Dreizler (2007) have reported use of a frequency-doubleddye laser pumped by a 5 kHz Nd:YLF laser to obtain UVlight at 283 nm for turbulent flame studies. They obtained22 μJ per pulse at 283 nm. Other examples include Bork et al(2010), who have successfully demonstrated 1D 10 kHz rateRayleigh scattering thermometry in what is known as DLRFlame A, which has nearly constant total Rayleigh scatteringcross section, using an 80 W, diode-pumped Nd:YAG laser;Cundy et al (2011), who used the fourth harmonic of Nd:YAG(266 nm) for 10 kHz toluene PLIF imaging of a heated jetimpinging on a surface; Steinberg et al (2011), who performedsimultaneous OH PLIF and PIV at 10 kHz in the DLR-Bflame; Heeger et al (2010), who performed simultaneous10 kHz OH PLIF and 20 kHz PIV in a swirling lean pre-mixed flame; and Barbosa et al (2009), who carried out 12 kHzPIV measurements in a swirled propane flame designed tosimulate a gas turbine. A thorough recent review of this rapidlyadvancing area can be found in Bohm et al (2011). Finally,there are also commercially available femto-second kHz lasersused in flow and combustion diagnostics such as in Kulatilakaet al (2011) as well as those discussed in the review article ofRoy et al (2010).

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This review primarily focuses on the third approach,which utilizes what are known as ‘pulse burst’ laser systemscoupled with recently developed high framing rate imagingcameras. In particular, recent advances in the developmentof pulse burst laser-based flow imaging techniques capableof operating at framing rates in the range of 104–106 Hzare reviewed. In this approach ultra-high repetition rates areachieved by use of the ‘burst mode’ concept wherebythe measurement system is operated at high repetition rate overa short period of time followed by a relatively long period ofinactivity resulting in a low overall duty cycle for the system.In the case of pulsed laser systems, the burst mode conceptis required to overcome thermal loading limitations inherentto the generation of high-energy pulses. Camera systems, onthe other hand, are limited by the high data generation rateswhich require rapid data transfer rates in order to digitally storesuccessive images.

The article is arranged as follows. Section 2 discussesrecent technological advances in high repetition rate lasersystems with a particular focus on the development ofNd:YAG based pulse burst laser systems that are at thecenter of the majority of recent work in this area. Section 3presents key concepts employed in the design of commerciallyavailable high framing rate cameras that overcome traditionallimitations. Lastly, section 4 presents a survey of themeasurement techniques and applications that have exploitedthese advances in high repetition rate lasers and cameras.

2. Pulse burst laser systems

The majority of recent developments in laser-based flowdiagnostics have utilized the capabilities of pulsed lasersystems, which concentrate a large amount of light energyover a short period of time. The advantage of pulsed lasersover continuous lasers is several. For one, the short durationof the laser pulse effectively defines the exposure time of theimaging system enabling virtually instantaneous imaging ofeven the highest speed flows. Secondly, the high intensity ofthe laser pulse (typically measured in MW cm–2) allows forefficient generation of energy at non-fundamental wavelengthsusing nonlinear optics (e.g. harmonic generation, opticalparametric oscillators) expanding the range of techniques thatcan be considered. The most common of these systems arepulsed Nd:YAG lasers where xenon flashlamps are used tooptically pump an Nd:YAG gain medium placed between highreflectivity mirrors forming an optical cavity. Typically, anelectro-optic Q-switch is used to release the built-up laserenergy forming a high-energy, short-duration laser pulse.Commercial systems produce laser pulses with durationbetween 5 and 20 ns and pulse energies ranging from lessthan 1 mJ per pulse to well over 1 J per pulse depending onthe size of the system.

The repetition rate of pulsed laser systems, however, isprimarily limited by the high average power that accompaniesoperation at high repetition rates. To illustrate, considerthe hypothetical scenario of generating 10 mJ pulses at acontinuous repetition rate of 100 000 Hz (or equivalently100 mJ pulses at 10 000 Hz). The output laser power of such

a system would be 1 kW. The input power, however, wouldbe significantly higher as the broadband spectral output of axenon flashlamp possesses a relatively small spectral overlapwith the absorption profile of the Nd:YAG gain medium suchthat the electrical input power is approximately two orders ofmagnitude larger than that extracted. As such, such a systemwould have an energy input on the order of 100 kW or morewith the excess energy being dissipated in the form of heat. Tofacilitate continuous operation, this heat must be efficientlytransferred away from the system. Conventional Nd:YAGsystems employ air or water cooling schemes and are generallyconstrained to cooling capacities on the order of 1–10 kW.Additionally, even with sufficient thermal management, thelarge magnitude of heat transfer can lead to other undesiredeffects in the laser system such as thermal lensing. Thesedemands generally constitute the limitation on the overallrepetition rate and pulse energy of commercial systems withmaximum output power generally constrained to the order oftens of watts at repetition rates on the order of 10–100 Hz.

Recent improvements pushing repetition rate into thekHz regime are the result of advances made in diode-pumped systems, which have a greater pumping efficiency thanflashlamps. In particular, laser diodes constructed of aluminumgallium arsenide (GaAlAs) have an output wavelength of808 nm, which overlaps quite well with the absorption spectraof Nd:YAG. Thus, the amount of residual heat generatedis significantly decreased allowing for operation at higherrepetition rates before thermal loading becomes an issue. Thelimitation of laser diodes, on the other hand, comes from theirrelatively low power output and increased costs as compared totraditional flashlamp-based systems. Commercially availablediode-pumped systems generate on the order of 10 mJ/pulseat 1064 nm and 1 kHz repetition rate.

2.1. Pulse burst laser system concept and design

To achieve very high repetition rates (order 1 MHz), severalNd:YAG based laser systems have been designed and builtaround the concept of a ‘pulse burst’ mode of operation. Inthis mode of operation, a burst (or train) of high-energy laserpulses is generated at high repetition rates over a short periodof time (order 1 ms). The system is then allowed to rest for arelatively long period of time (order 0.1–1.0 s) such that theheat generated has sufficient time to be removed by the coolingsystem. In this fashion, the overall duty cycle of the system iskept low (∼1%) such that the total, time-average power outputis on the same order of that of a conventional low repetitionrate system.

The generation of a large number of high-energy laserpulses at high repetition rates from a single system, however,is not trivial as repetitive Q-switching of a conventional systemwould result in rapid gain depletion within the Nd:YAGmedium such that only a few pulses of significant energy canbe extracted from the oscillator. Thus, while the pulse burstlaser systems described in this work have several similaritiesto conventional Nd:YAG laser systems, there are several keydesign changes made to facilitate high repetition rate output.

The basic design of a pulse burst laser system is describedin the context of figure 2, which is a schematic of a pulse

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Figure 2. Diagram of the Auburn University third-generation pulse burst laser system, configured for output at 532 nm.

burst laser system designed and constructed by the first authorat Auburn University. Several variations on this basic designexist and will be discussed in the next section. Generically, thepulse burst laser system can be classified as a master oscillatorpower amplifier (MOPA) configuration and consists of threefundamental stages:

(1) pulse slicer,(2) power amplifier chain,(3) wavelength conversion.

The function of the pulse slicer is to generate a seriesof low-energy pulses at the desired repetition rate and pulseduration by slicing them from the output of a continuouswave (cw) laser. In this case, the master oscillator of thesystem is a low power (100 mW), single longitudinal mode,cw Nd:YAG ring laser (JDSU Model NPRO-126) operatingat 1064 nm. This type of laser is often used as a seed laserin conventional Nd:YAG laser systems. Individual pulses areformed by ‘slicing’ the cw output using an acousto-opticmodulator (AOM). The AOM (Brimrose Model GPM-400-100-1060) has a rise/fall time of ∼10 ns and is capable ofcreating an arbitrary number of pulses, as short as 10 ns each,with a maximum repetition rate of over 40 MHz and a contrastratio of over 2000:1. The contrast ratio is a critical parameter inpulse burst laser systems as any ‘leaked’ laser light that passesto the power amplifier chain will be amplified depleting theoverall gain available for subsequent pulses. As the pulses gainenergy and the system saturates (i.e. departure from the smallsignal gain regime), the losses due to the residual light presentbetween pulses have a disproportionate effect on the rest ofthe system.

The resulting short-duration, low-energy pulses(1 nJ/pulse) are then passed through a power amplifierchain to increase the pulse energy by 6 to 9 orders ofmagnitude depending on the number of amplifiers and theirvarious settings. In this case, the power amplifier chainconsists of a series of six flashlamp-pumped Nd:YAG rodamplifiers, with rod diameters of 4, 5, 6.3, 9.5, 12.7 and12.7 mm, respectively. The first three amplifiers operate ina double-pass configuration for maximum gain (factor of10–100 per pass) while the pulse energy remains in the smallto moderate signal gain regime. Amplifiers four through sixare single-passed to minimize gain depletion throughoutthe duration of the burst of pulses. Energy is delivered tothe flashlamps using flashlamp drivers (Analog ModulesModel 8800V) that allow the user to adjust both the amountand duration of current delivered to the flashlamps. The2 ms maximum duration of the flashlamp pulse defines thetemporal envelope within which the pulses can be amplifiedand represents a fundamental limitation on the system’sability to generate a finite number of pulses at varyingrepetition rates. Also evident in the schematic are severalFaraday isolators placed between each amplifier stage. Theisolators are critical to system performance as they greatlyattenuate backwards propagation of radiation, thus mitigatinginadvertent cavity formation, which prevents the desired pulseburst amplification.

The overall gain of the six-stage amplifier chain isestimated to be on the order of 108 to 109 with single pulse(i.e. low repetition rate) energies on the order of 1 J possible.Operation at higher repetition rates produces pulses with lowerenergies. As an example, operation at 500 kHz produces pulses

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with energy on the order of 50–100 mJ/pulse. Providingmore specific performance characteristics for a given system,however, is difficult due to the large number of user-adjustableparameters in the system. These include the number of pulses,the temporal spacing between pulses, the current (i.e. gain)delivered to each amplifier, the time delay between eachamplifier’s firing and the initial time delay of the first pulse inthe burst relative to the flashlamp pulse. While this providesthe user with significant flexibility in adjusting the propertiesof the burst of pulses, it also necessitates significant experiencewith the system to achieve a certain setting and often employs atrial and error process. In general, the most significant tradeoffis between pulse uniformity and pulse energy with a uniformburst of pulses possessing lower average energy than a non-uniform burst.

Following amplification, the high-energy pulses areconverted to a wavelength that is better suited for theapplication at hand. The most common conversion isfrequency-doubling to 532 nm (green) through the useof a KTP nonlinear optical crystal. Presently, we achieveconversion efficiencies on the order of 30–40% althoughhigher efficiencies may be possible with alternative opticalarrangements. Conversion to the third (355 nm) and fourth(266 nm) harmonic wavelengths is also possible usingadditional harmonic crystals. Tunable wavelength generation,including UV wavelengths, using an OPO and sum-frequencymixing has also been achieved. This will be discussed insection 4.

2.2. System evolution

In addition to the pulse burst laser system described above,high repetition rate laser output has also been achievedusing repetitively Q-switched ruby lasers (Huntley 1994,Grace et al 1998) and by chaining together multiple pulsedlaser systems using a combination of polarizers, waveplatesand beamsplitters (Hult et al 2002, Murphy and Adrian2010). Ruby lasers have demonstrated impressive performanceat high repetition rate as evidenced by the production of350 mJ/pulse at 500 kHz over tens of pulses (Grace et al1998), but have not been widely utilized for fluid dynamicmeasurements. A limiting factor is that ruby lasers arecharacterized by relatively long periods of inactivity betweenbursts making their use difficult in environments requiringfrequent operation. The use of multiple laser systems is limitedby the expense and practicality of acquiring, aligning andoperating multiple systems simultaneously. In addition, thesepolarization-dependent beam combining optics generally limitthe total number of pulses generated by these systems to 8pulses. Thus, recent advances in high repetition rate imaginghave largely capitalized on the success of the pulse burst lasersystem platform.

The first pulse burst laser system was constructed atPrinceton University and is described by Lempert et al(1996) with subsequent descriptions appearing in Lempertet al (1997), Wu et al (2000) and Wu and Miles (2000).In the original system, pulse slicing was achieved using apair of electro-optic λ/4 Pockels cells arranged in a double-pass configuration. The function of the Pockels cell was to

rapidly rotate the polarization of the incident laser beam by90◦ (double-pass) such that individual pulses can be pickedout using a thin film polarizer. A pair of Pockels cells wasnecessary due to the relatively slow fall time exhibited bythe cells where the use of a single cell would have resultedin a pulse with a steep rising edge (∼3 ns), but a relativelylong trailing edge (∼150 ns). The purpose of the secondcell is to negate the polarization induced by the first cellalong the trailing edge such that well-defined, short-durationpulses can be formed. The first pulse burst laser system alsoutilized flashlamp drivers with pulse forming networks derivedfrom standard Nd:YAG systems. These drivers produced aGaussian-like gain profile in time with width on the order of100–150 μs. This limited the overall number of high-energypulses that could be formed and served as a source of non-uniform energy across the burst of pulses.

Following on the experience gained in the constructionof the first system, a second laser system was built at theOhio State University and described in Thurow et al (2004).This second-generation system followed the architecture ofthe first system fairly closely, but implemented several smallimprovements such as a reduction in the overall footprint ofthe laser system and the elimination of some optical elements(e.g. spatial filters, four-pass amplifier configurations, etc) toreduce complexity. The most notable change to the system,however, was the inclusion of a phase conjugate mirror (PCM)after the third amplification stage. The PCM, which operateson the principles of stimulate Brillouin scattering, acts as anonlinear mirror and effectively isolates the high-intensity,short-duration pulses from the low-intensity, long-durationbackground illumination thus mitigating gain depletion in thelatter amplification stages such that more energy is availablefor the desired pulses. This relatively simple addition to thesystem increased the system efficiency by several factorsyielding higher pulse energies after frequency conversion to0.532 μm.

Following the success of these two systems, pulse burstlaser systems have also been built at NASA Glenn ResearchCenter (Wernet and Opalski 2004), University of Illinois(Kastengren et al 2007), University of Wisconsin (Den Hartoget al 2008), Auburn University (Thurow et al 2009) andIowa State University (Miller et al 2009). A new, extremelyhigh pulse energy system (goal of ∼1–2 J/pulse at 532 nmand 10 kHz rate) is also currently under development at theOhio State University (Gabet et al 2011). While all of thesystems have the same basic architecture, the details of eachconfiguration are unique and generally in a constant state offlux as users make changes to the system in order to improveperformance. Perhaps the most significant change from theprevious generation systems that has been adopted in recentincarnations is the use of AOM for cw oscillator pulse slicing;the use of a pulsed oscillator in place of the sliced cw oscillator(Gabet et al 2011); and variable pulse duration flashlamppower supplies, which provide longer, more uniform pulses tothe flashlamps. Compared to Pockels cells, the AOM costs less,comes in a smaller package, provides more flexible triggeringand provides an inherently greater initial contrast ratio for thepulse slicer. A planned near future enhancement to the systems

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is the inclusion of Nd:glass amplifiers for the last stages ofamplification. While providing less single-pass gain than anequivalent Nd:YAG amplifier, Nd:glass is capable of storing alarger amount of energy in the rod so that gain depletion effectsare minimized and a larger amount of overall energy can beimparted into the burst of pulses. This has enabled the design ofa system with anticipated 1 J/pulse energies at repetition ratesbetween 5 and 250 kHz (Den Hartog et al 2008). Anotherenhancement to the systems has been the development oftunable OPO by Jiang et al (2008), which are compatiblewith the high repetition rate output of the system and allowfor conversion to UV wavelengths necessary for laser inducedfluorescence of NO, OH and other combustion species. Thesewill be discussed later.

2.3. Future trends

Several pulse burst laser systems have been built andtheir utility demonstrated for many applications; however,widespread adoption and commercialization of the technologyhave been limited by the complexity, size and expense of thesystem and the requirement for specialized training to maintainand operate the system. Thus, while increased pulse energyat high repetition rates is always needed, it is anticipatedthat future innovations will be largely focused on simplifyingthe optical arrangement in a more compact and stable form,reducing the costs, extending the duration over which high-energy pulses can be formed and designing more user-friendlysystems. To this end, the most substantial improvementswill likely come by replacing the front end of the system(cw laser, pulse slicer, first few amplification stages) witha commercially available high repetition rate laser sourcewith moderate pulse energies, such that only a few, single-pass stages of amplification are necessary to achieve thedesired power output. This is the approach being used atOhio State University in its latest system which is still underdevelopment. Initial results of that work have demonstrated inexcess of 800 mJ/pulse at 10 kHz repetition rate after onlytwo amplification stages (Sutton 2011). This system uses acommercial diode-pumped injection-seeded Nd:YVO4 laser(CrystalLaser, Reno, NV) which outputs ∼4 μJ/pulse atrepetition rates as high as 250 kHz.

A potential opportunity for additional improvement lies inthe recent advances made in diode-pumped fiber lasers, wherefiber optics are used as the gain medium. Fiber optics havemany benefits due to their long length and flexibility, whichallows for the creation of laser cavities with very long pathlengths (i.e. high gain) in a compact package. This arrangementalso allows for efficient pumping with diodes and efficientcooling due to the inherently large surface area of fibers.Adoption of such an approach, however, will require furtherdevelopment as one must consider things such as the needs fornarrow linewidth output, asynchronous triggering capability,short pulse durations and the spectral overlap between the fiberlaser output and the gain bandwidth of Nd:YAG amplifiers.These considerations may force some undesirable tradeoffsin the design of future systems. Other areas of improvementinclude the use of diode-pumped amplifiers to enable longer

trains of pulses, saturable absorber mirrors to mitigateparasitic lasing and ASE, actuated mirrors with closed-loopcontrol to maintain stability and spatial mode beam shapingto more efficiently extract the energy already stored inthe rods.

3. Ultra-high framing rate cameras

The capabilities of the pulse burst laser system and otherhigh repetition rate laser systems are complemented byseveral commercially available ultra-high framing rate camerasystems, the number of which has increased significantlyover the last decade. For moderately high speeds, the mostcommonly used cameras are high-speed CMOS cameras,which can capture megapixel resolution images at framingrates on the order of 10 000 fps. CMOS cameras aredistinguished from CCD cameras by the electronic architectureused to read out the signal from the sensor. In CMOS cameras,each pixel contains the necessary electronics to convert theaccumulated charge to a voltage and amplify it. This allowsfor rapid read out of a signal from the chip, which can bedone in parallel (as opposed to in serial with a CCD camera).This parallel architecture allows the user to select a regionof interest such that the framing rate can be increased byrestricting the read-out portion of the image to a small region.For example, the Phantom v1210 (Vision Research, Inc.)can continuously record images at 12 000 fps with imageresolution of 1280 × 800 pixels or 1000 000 fps at a reducedimage resolution of 128 × 16 pixels. A notable advantage tothis architecture is the ability to continuously record at highframing rates until the camera’s memory is full. For manyapplications, however, the reduced spatial resolution at highspeeds is a significant tradeoff.

For higher resolution at the rates discussed in this work(100 000 to 1000 000 fps), alternative approaches to cameradesign must be employed. In general, CCD image sensors arethe sensor of choice. For cameras that operate at these speeds,the total number of frames that can be acquired is limitedwith a mode of operation similar to the pulse burst conceptdiscussed above. One of the original approaches employed toachieve high framing rates is the use of a gas turbine drivenrotating polygonal mirror (e.g. several models by CordinCamera are still available) to mechanically move the imageacross several image sensors (originally film), each possessingtheir own imaging optics, placed on an arc surrounding therotating mirror. These cameras tend to be bulky and somewhatinflexible due to the mechanical nature of the technique.In addition, for imaging at the highest framing rates, theycan consume large amounts of gas to drive the turbine thatmust be replenished between runs. Their strength, however, istheir speed and image resolution, which can reach as high as25 million frames s–1 with image resolution as high as 2k × 2kpixels for each image. The total number of frames that can becaptured at these high rates is only limited by the number ofCCD sensors that can be fitted around the arc with modelscurrently available with 128 frames.

Other cameras (e.g. Cooke Corp., Cordin, SpecialisedImaging, DRS Hadland, LaVision) also use multiple CCDs to

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acquire high framing rate images, but use beamsplitters insteadof a rotating mirror to direct the light along different pathsonto each sensor. As the light is projected onto each camerasimultaneously, the exposure of each camera is synchronizedto capture images at different times, thus producing highframing rates without the need for moving parts. The use ofbeamsplitters, however, reduces the intensity of the incidentlight at each sensor and necessitates the use of intensifiers tocontrol the exposure and to increase signal intensity. Whileintensifiers increase the signal allowing for low light levelimaging and overcoming losses imposed by the beamsplitters,they also add noise to the images decreasing the overall signal-to-noise ratio and increase the costs of the overall system. Aswith rotating mirror cameras, the total number of frames andtheir image resolution are dictated by the number of imagesensors built into the overall design.

An alternative approach to achieving high framing ratesis to design and manufacture a specialized CCD image sensorwhere each pixel contains a number of on-chip electronicstorage bins located next to the photo-active area. High framingrates can be achieved by rapidly shifting the charge producedat each pixel to the neighboring storage unit until the entiresequence of images is read out from the chip. Variationsof this approach have been used by different manufacturers(e.g. Princeton Scientific Instruments, Shimadzu) to producecameras as described in Kosonocky et al (1997) and Etoh et al(2002). The on-chip storage bins take up a large physical areaof the sensor necessitating very large pixels (order 100 μm ×100 μm) to realize moderate fill factors. As a result, theoverall image resolution is reduced. As an example of a recentcamera built on this concept, the Shimadzu HPV-2 can record100 frames at 1 MHz framing rate with a pixel format of312 × 260. The main advantage of a single sensor high-speedcamera is that it constitutes a compact and robust imagingsystem with a single optical path that is fairly flexible and easyto operate. These cameras, however, are quite expensive as theCCD chips must be custom-built and a limited volume keepsunit prices quite high. The basic architecture, however, has thepotential to see much lower prices in the future.

An interesting variation of this concept (Dalsa 64K1M,now discontinued, and LaVision UltraSpeedStar) is to usea mass produced (i.e. lower cost) high-resolution CCD chipand apply a physical mask on top of the chip to only allowlight to strike a limited number of pixels on the chip. Inthis manner, the active pixels are selectively chosen with themasked pixels serving as the on-chip storage. High framingrates can be achieved by rapidly shifting the charge intothese pixel locations. The major downside of this approachis that the mask severely reduces the fill factor (<3% forthe 64K1M) as most of the incident light falls on the mask.This can potentially be alleviated by the addition of a lensletarray in front of the mask, but, due to development costs, hasnot yet been incorporated into a high-speed camera design.Because many of the components are standardized, this couldlead to high framing rate cameras at a fairly reasonable pricepoint.

4. High repetition rate techniques and applications

4.1. Flow visualization

One of the first applications of the pulse burst laser systemwas flow visualization of a Mach 2.5 turbulent boundarylayer (Lempert et al 1996, 1997, Wu et al 2000, Wu andMiles 2000, 2001). These experiments utilized a particle-basedscattering technique for visualization whereby a fog of smallcarbon dioxide particles are formed through condensationin the cold supersonic free stream of a laboratory-scale(13 mm × 26 mm) supersonic tunnel. Although small(less than λ and in the Rayleigh scattering regime) andwith limited incident laser energy, these particles efficientlyscattered enough incident laser light to be imaged using aPrinceton Scientific Instruments CCD camera. This camerautilizes a specialized CCD chip with on-board storage toacquire 30 images with 360 × 360 pixel resolution at framingrates up to 1 MHz. As the temperature rises within theboundary layer, the particles evaporate producing a strongcontrast in intensity near the edge of the boundary layereffectively highlighting the underlying large-scale structure.Figure 3 shows a sequence of 25 images acquired using thistechnique and presented in Wu et al (2000). The sequence wasacquired at 500 kHz and depicts the shock wave boundarylayer interaction of a Mach 2.5 free stream and a 14◦ wedge.The flow is from right to left and the boundary layer canbe visualized on both the upper and lower walls of the tunnel.The laser pulse energy was only 0.4 mJ/pulse at 532 nmwith the signal enhanced through injection-seeding of gaseouscarbon dioxide (<1% by mass) into the tunnel to produce ahigher concentration of particles.

These image sequences clearly show the large-scale natureof the boundary layer and the thickening of the boundarylayer after it passes through the oblique shock generated bythe wedge. The structures’ influence on the shock is seenas the curvature/shape of the shock fluctuates in accordancewith the structures passing through it. In a subsequent study(Wu and Miles 2001) that also included a 24◦ wedge, thesevisualizations were used to show that large-scale structuresretain their geometric features on passage through the shock,but that those features are compressed in the direction normalto the shock indicating a rapid-distortion type of process. Inaddition, it was shown that the shock’s streamwise positioncan fluctuate on the order of one boundary layer thickness andthat this motion is associated with the passage of large-scalestructures contained in the boundary layer. The unique time-correlated nature of these images revealed that this motion hasa characteristic frequency associated with the large eddies ofUe/δ on the order of hundreds of kHz, which would be difficultto ascertain through other diagnostics. Humble et al (2011)used a similar technique to visualize a Mach 4.9 turbulentboundary layer at 500 kHz as it traversed an expansion corner.They were also able to produce three-dimensional views of theflows by visualizing the structure as it passes through a lasersheet oriented in the cross-stream direction.

A similar flow visualization technique was used byThurow et al (2002a, 2003a) and Hileman et al (2002, 2005)to visualize the evolution of large-scale structures contained

7


Figure 3. Sequence of 25 flow visualization images acquired at 500 kHz of the shock wave boundary layer interaction occurring withM = 2.5 and wedge angle of 14◦. The flow is from right to left. Reprinted from Wu et al (2000) with permission of the American Institute ofAeronautics and Astronautics.

in the free shear layer of a high-speed axisymmetric jetexhausting into the laboratory. In this case, moisture containedin the ambient air is entrained into the jet through the turbulentaction of the shear layer and condensed into a fog upon mixingwith the cold, dry air from the high-speed jet’s core. In thisfashion, large-scale structures in the jet’s shear layer, wheremixing has occurred, are seeded with particles (high intensityin the images) while the high-speed core (dry air) and low-speed (warm) ambient air are not seeded.

Figure 4 shows a sequence of 7 (out of 17 total) imagesacquired along the jet centerline of a Mach 1.3 axisymmetricjet. The laser output was 4 mJ/pulse at 100 kHz and the imageresolution is 240 × 240 pixels. The flow is from left to rightwith several features highlighted in the image. Of particularinterest is the observation of a structure tearing and pairingevent where the structure labeled ‘B’ is seen throughout thesequence to tear apart into two pieces subsequently mergingwith structures ‘A’ and ‘C’ to form two larger structures, an

event associated with the growth of the shear layer. In similarvisualizations of a Mach 2.0 jet, Thurow et al (2003a) notethat these types of events are suppressed and that large-scalestructures are less distinct resulting in a reduced growth rateassociated with the compressibility of the mixing layer.

The availability of time-resolved data also allowedthe researchers to employ other analytical techniques tocharacterize the turbulence in the jet. Figure 5 presentsensemble average space–time correlation data of large-scalestructures contained within the rectangular box in the firstframe of figure 4. In this analysis, the intensity pattern observedin the first frame of a sequence is cross-correlated with eachsubsequent frame to track the location of the structure and themagnitude of the correlation with time, which can be used asa measure of the coherence. Figure 5(b) is a plot of averageposition of a structure versus time with the slope of the linerepresenting the convective velocity of the structure.

8


Figure 4. Sequence of seven flow visualization images acquired of a Mach 1.3 axisymmetric jet. Reprinted with permission from Thurowet al (2003a). Copyright 2003, American Institute of Physics.

Hileman et al (2002, 2005) performed similar flowvisualization experiments simultaneously with acousticmeasurements made using an array of microphones to isolatethe location of noise producing events within the jet. Thesesimultaneous measurements clearly showed that the noiseproducing events within the jet flow field can be associatedwith the dynamics of the large-scale structures contained in

the shear layer. In instances where the jet was relativelynoisy, events such as structure tearing, cross-mixing layerinteraction and structure formation could be observed inthe image sequences. Conversely, in instances where thejet is relatively quiet, large-scale structures containedin the mixing layer displayed relatively little evolution orinteraction. In related works using the same system, Thurow

9


(a) (b)

Figure 5. (a) Ensemble average space–time correlation of large-scale structures contained in the rectangular box depicted in figure 4. (b) x–tplot of peak correlation showing convective velocity of large-scale structures. Reprinted with permission from Thurow et al (2003a).Copyright 2003, American Institute of Physics.

Figure 6. Ten-frame, 10 kHz image sequence of the temperature field in DLR flame B (Re = 22 800) at an axial position of x/d = 10.Reprinted with permission from Patton et al (2012b). Copyright 2012, American Institute of Physics.

et al (2002b) visualized the formation of large-scale structuresof a Mach 1.3 rectangular jet impinging on a flat plateand Thurow et al (2003b) conducted simultaneous opticalwavefront distortion measurements to better understand theaero-optic influence of large-scale structures contained in theshear layer.

Kastengren et al (2007) used a similar flow visualizationsystem to study the compressibility effects on large-scalestructures contained in a M = 2.46 axisymmetric base flow. Intheir case, the flow was seeded with ethanol which evaporatesin the stagnation chamber upstream of the test section, butcondenses upon expansion to supersonic speeds. In the warmbase flow region, the ethanol particles evaporate marking theinterface between the seeded free stream and the unseededwake region. Similar to above, they made several observationsabout the dynamics of large-scale structures contained in thisflow and were able to measure their convective velocity bycorrelating large-scale features from one frame to the nextthroughout the image sequence. Their results showed that thecompressibility effects on a base flow shear layer are similar tothose observed in free shear layers produced by jets and otherflow geometries.

4.2. Rayleigh and Raman scattering

Recently, Patton et al (2012a, 2012b) and Gabet et al (2010)have demonstrated pulse burst laser-based Rayleigh andRaman imaging, respectively, at 10 kHz framing rate in aturbulent non-premixed flame. Figure 6 shows an exampleRayleigh scattering image sequence obtained from what isknown as the DLR-B flame, using ∼200 mJ/pulse at 532 nm.The turbulent diffusion flame utilizes an 8 mm diametertube with exit flow velocity of the fuel of 63.2 m s−1.The corresponding Reynolds number, based on the exit jetdiameter, is 22 800. The fuel mixture for the DLR-B flame,which is identical to the DLR-A flame, is composed of 22.1%CH4, 33.2% H2 and 44.7% N2, which results in mixture-averaged differential Rayleigh cross section which varies byless than 3% throughout the flame, thus enabling a calculationof the local gas temperature by means of the ideal gas equationwithout a need for measuring the local species concentrations.In this manner, the temperature is derived from

T = TrefIref

IRAY, (2)

where Iref is the reference Rayleigh scattering signal fromair at room temperature (Tref) and IRAY is the measured

10


Figure 7. Flow visualization images enhanced using the filtered Rayleigh scattering technique. The laser is tuned so that the low-velocityflow signal is suppressed. Reprinted from Wu and Miles (2001) with permission of the American Institute of Aeronautics and Astronautics.

intensity at the unknown temperature. Rayleigh scatteringimages were captured using a high-speed CMOS camera. Thehighly turbulent nature of this flame is clearly observed in thetime-correlated image sequence. For example, the upper-leftportion of images 1 and 2 are dominated by pockets of coldgas. Frames 3 and 4 show the transition to a hotter flame front,which is seen to clearly develop and propagate in images 5–10.

Ten-frame 10 kHz image sequences similar to those offigure 6 were also obtained in a non-reacting turbulent propanejet, which exited into ambient air (Patton et al 2012a). Thisprovided a means of obtaining temporally correlated images ofmixture fraction and scalar dissipation rate. Ten-frame 10 kHz,1D Raman imaging has also recently been reported by thesame group (Gabet et al 2010). Instantaneous image sequenceswere obtained from a co-flow geometry in which the inner jet,a 40/60 mixture of CH4/H2 with Re = 9000, issued into alower speed air flow. Single pulse laser energy of ∼200 mJ @532 nm resulted in signal/noise ratios as high as 30, with anobject plane spatial resolution of approximately 170 μm.

4.3. Filtered Rayleigh scattering and planar Dopplervelocimetry

Although a useful tool for delineating the basic flow structure,flow visualization based on recording the raw intensity oflight scattered by particles is limited by the inherentlyqualitative nature of the particle seeding technique. Ingeneral, the resulting signal does not directly correlate witha tangible flow property such as velocity, pressure or density,making interpretation of images difficult. The narrow spectrallinewidth output of the pulse burst laser system, however,allows for the development of techniques that exploit thespectral properties of the scattered light to extract meaningfulquantitative measurements. In particular, scattered light will

experience a Doppler shift in frequency that is a function of thescattering particles’ velocity and optical geometry accordingto

� fD = (−→s − −→o )

λ· ⇀

V , (3)

where −→s and −→o are the unit vectors in the scattered andincident laser propagation directions, respectively, λ is thewavelength of light and ⇀

V is the velocity vector. Relativelyspeaking, the magnitude of the Doppler shift is rather small(order 1 GHz or less); however, several atomic and moleculargas species exhibit sharp transmission and absorption bandswith similar bandwidths such that this level of frequencyshift can be determined by measuring transmission/absorptionof scattered light through a glass cell containing the gas.Combined with an inert gas such as nitrogen, the pressureand temperature (via pressure and temperature broadeningeffects) of the optical cells can be adjusted to fine tune thespectral characteristics of the absorption cell to match theexperiment. The most commonly used gas is iodine, which iseasy to handle and possesses several ro-vibrational transitionsthat overlap with the frequency-doubled output (532 nm) ofNd:YAG lasers.

Wu et al (2000) and Wu and Miles (2001) utilized thisbasic concept to enhance the flow visualization experimentillustrated in figure 3. They tuned the output of the pulseburst laser to coincide with an absorption band of iodine suchthat the Doppler shifted light would be selectively absorbeddepending on the flow velocity. By placing the iodine cell infront of their camera, they were able to bias their images suchthat the low-velocity (minimal Doppler shift) signal, as well asstray scattering from window and wall surfaces, was stronglysuppressed, thus increasing the image quality, and in particularthe contrast between low-speed and high-speed fluid. Figure 7

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Sig. Cam.

Jet Nozzle

Laser Sheet

Beam from PBL

Ref. Cam.

Polarizer

50/50 Beamsplitter Mirror

Iodine Filter

Scattered Light

(Frequency Shifted)

Illuminated Flow Field

k is out of plane j

20° Incident laser sheet direction

s

Figure 8. Schematic of a two-camera PDV system. Reprinted fromBlohm et al (2006) with permission of the American Institute ofAeronautics and Astronautics.

shows an example taken from Wu and Miles (2001) of sixtime-correlated images acquired using this approach. Theflow is from left to right, the wedge angle is 24◦ and thefree stream Mach number is 2.5. The most striking effect ofplacing the iodine filter in front of the camera is that the signalcontrast across the shock is significantly enhanced while thescattering from the stationary wedge surface is significantlysuppressed. This method makes imaging near surfaces easierwhile assisting in interpreting the images as the signal can beloosely correlated with velocity.

Thurow et al (2005) took this concept one step furtherin the development of MHz rate planar Doppler velocimetry(PDV). PDV, also known as Doppler global velocimetry,determines the velocity of the flow field by quantifying theabsorption through the simultaneous acquisition of a filtered(signal) and unfiltered (reference) image, such that the ratioof the two images can be used to measure the local Dopplershift of scattered light. The Doppler shift equation can then beused to determine the magnitude of the velocity componentas described in equation (3). The experimental arrangementfor PDV is more complex due to the need to acquire twoimages (a signal and a reference) along the same optical path.Figure 8 shows the experimental arrangement employed byBlohm et al (2006) to measure the velocity of Mach 1.3 andMach 2.0 rectangular jets. It should be noted that the techniqueis only sensitive to a single component of velocity (see equation(3)); three-component velocity measurements would requiretwo additional measurements viewing the flow from separateangles.

Figure 9 shows an example of the signal and referenceimages acquired with this technique and the resulting velocityfield. In essence, the reference image represents the image thatwould have been acquired with a simple flow visualizationtechnique. The image possesses strong contrast between themixing layer and the jet core and ambient air; however, there islittle contrast within the mixing layer itself. The velocity field,on the other hand, is not marked by the same features. Rather,the velocity gradients are significant within the mixing layerand reveal details not clearly illustrated with flow visualizationalone. For example, the roll-up of a large-scale structure can beclearly seen in the velocity and signal images in the lower halfof the mixing layer at x/h = 10.25 that cannot be visualizedin the reference image.

The implications of these differences can be quitesignificant. Thurow et al (2008) utilized a MHz rate flowvisualization and a MHz rate PDV system to study how themeasurement technique can influence the determination ofthe convective velocity of large-scale structures. When usingjust flow visualization images, space–time correlation analysis(see figure 5) of a Mach 2.0 jet indicated that the convectivevelocity took on a bimodal shape such that structures traveledwith either a high-speed or a low-speed mode relative totheoretical models. Similar results had also been obtained withsimilar flow visualization methods in other compressible flowfields. When the same analysis was applied to velocity images,however, the convective velocity took on a normal distributionthat agreed with theoretical arguments and did not indicatethe presence of fast or slow modes of propagation. In thisinstance, the biased nature of the particle seeding technique,which created strong contrast near the edges of the shear layer,but not within it, clearly affected the ability to determine theconvective velocity of structures contained in the flow.

4.4. Particle image velocimetry

PIV is one of the most common measurement techniquesused in fluid dynamics. PIV determines the local fluidvelocity by measuring the displacement of particles (typicallylarger and more sparsely seeded than previously mentionedparticle seeding methods) contained in the flow acrossa pair of time-correlated images. While PIV systemsutilizing a double-pulsed laser and a single, interline CCDcamera are quite common, these systems only yield asingle velocity measurement per image pair. Time-resolvedmeasurements would allow for the calculation of fluidacceleration, which is strongly associated with local pressurefluctuations, as well as more precise and higher dynamic rangevelocity measurements. Extension of the aforementionedflow visualization systems to PIV is straightforward, butmainly limited by the different seeding requirements andthe spatial resolution of available high-speed cameras. Ashigher resolution cameras become more widely available, theapplication of MHz rate PIV for advanced measurements islikely to increase.

Wernet and Opalski (2004) utilized a pulse burst lasersystem with 75 mJ/pulse output over an eight-pulse trainat 500 kHz and an eight-frame MHz rate camera with

12


(a)

(c)

(b)

7 8 9 10 11 12 13-1.5

-1

-0.50

0 5

1

1.5

0

100

200

300

x/h

y/h

Figure 9. Example PDV experimental data. (a) Reference image. (b) Iodine filtered signal image. (c) Velocity image. Reprinted fromThurow et al (2005) with permission of the American Institute of Aeronautics and Astronautics.

1280 × 1024 resolution (Cooke Corp. HSFC-Pro) to performPIV measurements of an 8.8 mm diameter, Mach 1.3 jet.One challenge they faced was the necessity to calibrateslight misalignments between the four CCDs with separateoptical paths used in their camera. This limited displacementresolution to approximately 0.25 pixels compared to lessthan 0.1 pixels in a conventional system. Nonetheless, theywere able to show the ability to make high quality velocitymeasurements with their system using 32 × 32 pixelsubregions.

More recently, Murphy and Adrian (2010) utilized thesame camera model and a commercially built (Quantel USA)custom eight-pulse laser system to make high-speed PIVmeasurements of the blast waves produced by an explosivedetonator. Their laser system essentially consisted of four PIVlaser systems chained together into a single, eight-head lasersystem such that each laser can be independently triggered.The advantage to this system is the ability to individuallytune each laser pulse so that excellent pulse uniformity canbe maintained across the burst of pulses regardless of thetiming of each individual pulse. Figure 10 shows a sequence offour time-resolved velocity vector fields behind the blast waveproduced by an exploding bridge wire (EBW) and representsthe first time-resolved PIV measurements of a blast wave. Themotion and extent of the blast wave as it travels from theEBW are quite clear. In addition, the strength of the shock asit moves can be assessed from the velocity magnitude beforeand after the shock. The authors also address many of thechallenges associated with time-resolved PIV measurementsof blast waves including difficulties with particle seeding in

the vicinity of the blast, particle lag and the finite motion ofthe shock wave from one frame to the next that affect some ofthe assumptions made in PIV image analysis.

4.5. Planar laser induced fluorescence

PLIF has been widely used for qualitative and quantitativevisualization in a broad variety of flow and combustionenvironments (Kulatilaka et al 2011). For high repetition ratePLIF measurements, a tunable ultra-violet laser source withsufficient (∼mJ/pulse) individual pulse energy is necessary.Although, as described previously, several groups havedemonstrated the ability to pump conventional commercialdye lasers at repetition rates as high as ∼1–10 kHz, repetitionrates greatly in excess of this have only been accomplishedusing OPO-based systems. Optical parametric oscillation is anonlinear process in which a ‘pump’ photon is split into whatare known as ‘signal’ and ‘idler’ photons, with the constraint

np

λp= ns

λs+ ni

λi, (4)

where ni are the indices of refraction at the pump, signaland idler wavelengths, λ, respectively. The OPO processhas the advantage that the dynamics of gain establishmentoccur on essentially instantaneous time scales, even for nsecduration pulses, such that pump pulses can be incident to thenonlinear crystals employed at arbitrarily high repetition rate.Figure 11 shows a custom-built OPO system that has beenused extensively with the OSU Pulse Burst Laser system, atrepetition rates as high as 1 MHz. The OPO is most often

13


Figure 10. Time-resolved velocity field of a blast wave. Reprinted from Murphy and Adrian (2010) with permission from SpringerScience + Business Media.

Nonlinear Crystals

Double Pass 355-nm pump

θ

Rs=Ri≈1

Rs=Ri≈20%

Diode Seed Laser

Optical Isolator

OPO Output

Figure 11. Schematic diagram of the injection-seeded burst mode OPO cavity. Reprinted with permission from Jiang et al (2008).

pumped with the 355 nm output of the pulse burst laser,utilizing either a single- or double-pass pump configuration. Apair of beta barium borate (BBO) crystals, arranged to providewhat is known as walk-off angle compensation, constitutesthe gain medium. The cavity incorporates an output couplermirror that has ∼20% reflectance at both the signal and

idler wavelengths as well as a dual wavelength high reflector.Line narrowing of both the signal and output wavelengths isachieved by injection-seeding with an external cavity diodelaser, typically at the idler wavelength. In some cases the seedbeam is injected into the cavity via an optical isolator, asshown in figure 11. The time-averaged spectral linewidths of

14


Figure 12. 500 kHz time-correlated NO PLIF image sequences obtained from the NASA-Langley 31′′ hypersonic wind tunnel. Images areobtained in the boundary layer of a 20◦ flat plate model with sheet located ∼0.3 mm (left) and ∼2 mm (right) above the model surface.Reprinted with permission from Jiang et al (2011c).

the signal and idler beams, when using ∼10–30 mW of seedpower, are ∼300 MHz, as determined using a commercialFizeau wavemeter. Total, signal + idler, conversion efficiencyis ∼30% for 355 pump energy in the range of ∼30–50 mJ/pulse.

Further wavelength up-conversion is performed usingsum-frequency mixing. As an example, mixing of residual355 nm pump with the signal output at ∼622 nm has beenshown to produce ∼0.5 mJ/pulse at ∼226 nm. This hasenabled NO PLIF imaging in a small laboratory wind tunnel,at framing rates as high as 250 kHz (Jiang and Lempert2008), in the NASA-Langley 31′′ Mach 10 wind tunnel, atframing rates as high as 1 MHz (Jiang et al 2011c), and inthe Cornell–University of Buffalo Research Center (CUBRC)48′′ shock tunnel, where a sequence of 10 NO PLIF flowvisualization images was obtained at 10 kHz framing rate in asingle facility ‘shot’ (Jiang et al 2011a). Figure 12 shows a pair,out of more than 200 obtained, of time-correlated NO PLIF10–20 frame image sequences obtained from the boundarylayer of a 20◦ flat plate model, in which transition was inducedusing a variety of different shaped protuberances includingcylinders, a triangle and the boundary layer transition-detailedtest object, recently flown on the Space Shuttle. The sequencesshown in figure 12 were both obtained at 500 kHz framingrate, and utilized a 2 mm tall × 8 mm wide triangular trip

(visible near the lower-left side of each image) oriented at45◦ with respect to the principal flow direction. The imagesequence on the left was obtained with the sheet located justabove the model surface (approximately 0.3 mm), whereasthat on the right corresponds to a sheet location approximately2 mm above the model surface. The temporal evolutionand progression of characteristic ‘corkscrew’ vortices can beseen in each image sequence, although it is clearly sharperand more developed in the images obtained at the 2 mmlocation. Image sequences were captured using a PrincetonScientific Instruments intensified framing CCD camera with160 × 160 pixel format.

The system used to obtain the image sequences offigure 12 has also been used to perform OH PLIF imagingat up to 50 kHz framing rate (Miller et al 2009) and CH (Jianget al 2011b) and CH2O (Gabet et al 2012) imaging at 10 kHzframing rate, all in turbulent diffusion flames. The OH imagingwas performed in a hydrogen–air flame and utilized a 532 nmpumped OPO system developed at Iowa State University andsimilar, conceptually, to that shown in figure 11. Individualpulse energies in excess of 1 mJ were obtained at ∼311 nm.The CH imaging was obtained in the DLR-A flame, utilizing∼0.4 mJ/pulse at 390 nm generated by sum-frequency mixingof the OPO signal output with residual 1064 nm pulse burst

15


Figure 13. 500 kHz NO2 MTV image sequence (top left to bottom right) from a Mach 5 flow. The tagging location is 5 mm upstream from a5 mm diameter quartz cylinder. The flow is from left to right with free stream average velocity of ∼719 m s−1. The test gas is 1%NO2 seeded N2 at a total free stream static pressure of ∼92 Pa. Reprinted with permission from Jiang et al (2010). Copyright 2010,American Institute of Physics.

laser output. The CH2O imaging utilized ∼100 mJ/pulse fromthe third harmonic output of the pulse burst laser directly.

As a final example of ultra-high framing rate PLIFimaging, figure 13 shows an example NO2 molecular taggingvelocimetry (MTV) image sequence, obtained at 500 kHzframing rate in a Mach 5 flow (Jiang et al 2010). MTV,similar to what is known as flow tagging velocimetry, refersto a class of velocimetry diagnostic in which a pattern (line,grid, etc) is ‘tagged’ into a flow field by means of an opticalresonance. After suitable time delay the displacement ofthe initially tagged element is ‘interrogated’ using opticalimaging, either PLIF from a second resonant excitation, or,in the case of tracer molecules with sufficiently long radiativelifetime, spontaneous emission. Since the inception of theRaman excitation + laser induced electronic fluorescence(RELIEF) method in the late 1980s (Miles et al 1987) a numberof double resonance approaches have been demonstratedin gas phase flows including techniques based on photo-dissociation of water vapor, forming OH, and photo-chemicalformation of ozone, and NO. Examples of single resonancefluorescent/phosphorescent tracers include biacetyl, acetone,and for hypersonic flows, NO. Jiang et al (2010) contains amore complete set of references for gas phase flow taggingtechniques. A particularly attractive tagging approach forapplication to high speed–low density flows is the NO2 photo-dissociation method, first reported by Orlemann et al (1999),

in which NO2 is photo-dissociated (with near unity quantumyield) by absorption of a single photon at 355 nm:

NO2 + hν (355 nm) → NO + O. (5)

The NO formed is subsequently interrogated by ordinary NOPLIF imaging although as recently demonstrated by Hsu et al(2009), the interrogation step can also be performed usingnascent NO formed in the v′′ = 1 level of the ground electronicstate. To obtain the image sequence shown in figure 13, thepulse burst laser output was divided into two beams. A 355 nmbeam was focused into a line to photo-dissociate seededNO2 molecules. A 226 nm beam was formed into a sheetto perform PLIF imaging of the resulting NO. The flow is alaboratory-scale Mach 5 wind tunnel with a 5 mm diameterquartz cylinder located ∼14.5 cm downstream from the nozzlethroat, which produces a characteristic bow shock. The test gasis 1% NO2 seeded pure N2, and the flow direction is from leftto right. The field-of-view is ∼15 mm (height) × 15 mm (flowaxis). The bow shock is clearly visualized in the upper-left NOPLIF image. The laser beam pair is repeated with a repetitionrate of 500 kHz, resulting in a ‘time line’ of displaced MTVimages. The inferred free stream velocity is ∼719.2 ± 10.2(2σ ) m s−1, obtained from the observed displacement of thelines, determined by least-squares curve fitting, in the 2 μsinterval between the images.

16


(a)

(b)

Figure 14. 3D flow visualization image of a turbulent boundary layer with Reθ = 2500. (a) Iso-surface fit to the boundary between smokeseeded boundary layer and unseeded free stream; (b) exploded view of the interior of the boundary layer.

4.6. 3D flow visualization

In addition to planar imaging, the high repetition ratecapabilities of the pulse burst laser and associated high-speedcameras can be extended to 3D imaging by using a high-speed scanning device to translate the laser sheet through theflow field while images are acquired. The resulting stack ofimages, each corresponding to a different plane in the flowfield, can be reassembled forming a 3D image of the flow. Thetemporal resolution of the 3D image is then a function of thetime it takes to complete the scan. While this technique has

been implemented at much lower speeds using kHz rate lasersand cameras yielding time resolution on the order of severalmilliseconds, the pulse burst laser and high-speed camerasdiscussed here can yield temporal resolution on the orderof tens of microseconds. Several devices, including electro-optic deflectors, acousto-optic deflectors and galvanometricscanning mirrors are capable of providing the requisite speeds.In particular, Thurow and Lynch (2009) used a galvanometricscanning mirror (GSI Lumonics VM500, 6 mm aperture)to scan a laser sheet through the flow field while acquiring

17


68 images at 500 000 fps with a DRS Hadland Ultra68 camera,yielding a temporal resolution for the 3D image of 136 μs.The acquisition speed in this case was actually limited by thecamera, as opposed to the mirror which could have operated atnearly an order of magnitude faster. A similar concept utilizingthe scanning of a single, high-energy laser pulse with longduration (several microseconds) has also been demonstratedin Yip et al (1988), Long and Yip (1988), Patrie et al (1994)and Island et al (1996).

Figure 14 shows several views of a 3D image obtainedin a turbulent boundary layer using this technique. The freestream velocity was 24 ft s−1 (∼7.3 m s−1) (right to left inthe figure) and the boundary layer was estimated to be ∼1.5′′

thick with Reθ = 2500. The pulse burst laser was set to output68 laser pulses at 500 kHz and scanned in the z-direction.Images were acquired at each slice through the flow field with220 × 220 pixel resolution yielding an overall image volumeof approximately 4′′ × 4′′ × 4′′ with 220 × 220 × 68voxel resolution. The image is essentially instantaneous as theflow moved a negligible amount (<0.04′′) over the durationof the scan. Flow visualization was achieved by seeding theboundary layer with smoke injected through a slit locatedseveral feet upstream of the imaging location. This methodof seeding provides a large contrast in intensity between thelow momentum boundary layer fluid and the high momentumfree stream. Figure 14(a) shows a view where an iso-surfacehas been fit to an intensity roughly corresponding to thisboundary such that the influence of the large-scale structurescontained in the boundary layer can be visualized. The 3Dview offered by this technique shows the outer structure of theboundary layer to consist of several large, finger-like bulgesextending out and inclined at about a 45◦ angle with respectto the flow direction. In most cases, the structures are tiltedin the direction of shear; however, in one instance a structureappears to be tilted backwards (circled in the figure). Theavailability of 3D information allows for one to take a closerlook as such a structure. This is shown in figure 14(b), whichshows several slices through the interior of the structure circledin figure 14(a). These slices of the interior show that thelarge structure actually consists of an amalgamation of severalsmaller structures with the overall structure aligning in thedirection of shear. Further study of the 3D characteristics ofturbulent boundary layers is ongoing; however, this exampleillustrates the wealth of information available through simple3D flow visualization.

Figure 15 shows a 3D image acquired in the near-fieldof a Re = 10 200 turbulent jet. The jet flow was seededwith smoke and the image shows an iso-surface of intensityrepresenting the boundary between the seeded jet flow andunseeded ambient flow. In this case, the images were acquiredwith a Shimadzu HPV-2 camera yielding 3D intensity valuesover a volume with 312 × 260 × 100 voxel resolution. Theimage is effectively instantaneous as the jet core flow movedless than 1 pixel over the 100 μs acquisition time of the system.This image illustrates the rich and complex nature of thisrelatively simple flow field. The image depicts a flow which isinitially characterized by ring vortices formed near the nozzleexit. As expected, the formation of the ring vortices begins as

Figure 15. 3D image of a Re = 10 200 turbulent jet acquired using ahigh-speed scanning technique.

a low amplitude perturbation of the shear layer that quicklygrows into a large-scale vortex. The most striking feature ofthese images, however, is the onset of azimuthal instabilities,which appear as fingers of fluid elongated in the streamwisedirection and are periodically spaced around the peripheryof the jet. These structures are thought to be the result of asecondary instability that occurs in the braid region betweenring vortices resulting in pairs of counter-rotating streamwisevortices. Observations based on numerous 3D visualizationsshow that the azimuthal frequency of these disturbancesincreases over the Reynolds number range considered here. Inaddition, the images show that these instabilities grow in thestreamwise direction and have a streamwise extent that spansacross several ring vortices. Further downstream, the complex3D interaction of the ring vortices with the streamwise vorticesultimately leads to a more chaotic structure of the flow and theeventual break down to the fully developed turbulent flow inthe far field.

In an effort to produce high repetition rate, 3D flowvisualization images, Medford et al (2011) performedstereoscopic NO PLIF flow visualization in NASA Langley’sMach 10 wind tunnel. In these experiments, NO PLIF wasgenerated at 500 kHz in a similar manner as discussed earlier(see figure 12); however, in this case, the 226 nm laser beamwas formed into a thick laser sheet to provide volumetricexcitation of the NO being injected into the flow field. Aseries of collection mirrors were used to modify the opticalpath to the camera such that two images of the flow couldbe acquired side-by-side on a single high-speed image sensor

18


(a) (b) (c) (d )

(e) (f ) (g) (h)

Figure 16. Eight sequential MHz NO PLIF anaglyphs for flow over a 2 mm tall by 4 mm wide cylinder with a seeded NO flow of 300 sccm,P0 = 4.96 MPa. The images were acquired consecutively at a rate of 500 kHz. The flow is from top to bottom. Each image measures 41 mmfrom top to bottom. Reprinted with permission from Medford et al (2011).

with viewpoints separated by a 15◦ angle. By combining thetwo NO PLIF images into a single image with red/blue colors,the stereoscopic images shown in figure 16 were produced.The flow is from top to bottom and depicts the turbulentflow produced by a 2 mm tall by 4 mm wide cylindrical trip,which can be seen at the top, center of the image. Viewed withanaglyph (red/blue) glasses, the visualized structures take ona 3D shape providing more realism to the corkscrew-shapedstructures that are observed in this flow. These structuresappear to rotate counterclockwise as viewed from downstream,and propagate downstream where they devolve into smallermore irregular flow structures.

4.7. Thomson scattering

While this review focuses on ultra-high framing rate imagingmeasurements applied to studies of turbulent fluid mechanicsand combustion, some recent developments in pulse burstlaser technology at the University of Wisconsin–Departmentof Physics should also be noted. This work focuses on thedevelopment of new Thomson scattering-based diagnosticsfor determination of electron number density and electrontemperature in the Madison Symmetric Torus (MST) plasmafusion facility. Thomson scattering is similar to molecular

Rayleigh scattering except that the scattering is from freeelectrons, rather than from bound electrons in atoms ormolecules. While the scattering cross section of free electronstypically exceeds that of molecular Rayleigh scattering byapproximately three orders of magnitude, the number densityof free electrons is typically quite low, necessitating highindividual pulse energies if measurements are to be madeduring the typical steady-state facility run time of ∼20 ms.

Two pulse burst laser systems have been reported from theU. Wisconsin group. The first employs a pair of commercialNd:YAG lasers in which the Q-switch drivers and flashlamppower supplies have been extensively modified (Den Hartoget al 2010). In that system the original Q-switch drivers havebeen replaced by drivers capable of switching the Pockels cellat a repetition rate of approximately 30 kHz (30 μs). In additionthe flashlamp power supplies, which originally had fixed pulseduration of approximately 150 μs and could be fired at 10 Hz,were replaced with power supplies that could deliver variablepulse width, in the range of 150–390 μs at a repetition rate of1 kHz. By interleaving two such systems, trains of up to 30pulses at 2 kHz repetition rate and individual pulse energy of2 J at 1064 nm have been reported.

The second system employs a MOPA design, similar tothe systems previously described (Den Hartog et al 2008,

19


Harris et al 2010). The oscillator is a diode-pumped pulsedNd:YVO4 laser very similar to that being used in the newestsystem at Ohio State University discussed previously. A totalof six variable pulse duration flashlamp-pumped amplifiers areplanned, the first four stages of which are Nd:YAG and the lasttwo stages of which are 16 mm diameter Nd:glass. As of thewriting of this review the system has been operated with fourof the six planned amplifier stages. Eighteen pulse trains, withinterpulse spacing of 100 μs (10 kHz) and individual pulseenergy of ∼0.53 J, have been reported. The authors anticipateachieving ∼1–2 J per pulse at 250 kHz repetition rate uponaddition of the final two Nd:glass amplifier stages.

5. Conclusions

The past two decades have seen several notable advancementsin ultra-high repetition rate laser diagnostics for measurementsin turbulent and reacting flows. In particular, the developmentof the pulse burst laser system and the availability of severaldifferent models of high-speed cameras have proven to beeffective for high repetition rate and 3D measurements in awide variety of facilities ranging from small-scale laboratoryenvironments to large-scale industrial wind tunnels. Thesesystems have matured and been sufficiently demonstratedto the point where techniques such as MHz rate flowvisualization, PIV and PLIF can be considered somewhatroutine and should be considered as potential, viablemeasurement techniques for any future studies involving high-speed, turbulent and/or reacting flow fields.

In addition, while the early focus of researchers has beenon the high repetition rate implementation of traditionally lowrepetition rate techniques, it is expected that future works willbetter exploit the novel capabilities of the pulse burst lasersystem. For example, Miller et al (2010) exploits the frequencytuning capabilities of pulse burst laser-based systems to enablekHz rate wavelength scanning over the temperature-broadenedOH transition, capturing a 2D image sequence of the PLIFexcitation spectra. The combination of the pulse burst lasersystem with coherent anti-Stokes Raman scattering is alsoexpected to be an interesting research area for high repetitionrate temperature and species concentration measurements.These sort of innovative ideas are enabled by the flexiblearchitecture of the pulse burst laser system, which allowsresearchers to make relatively small modifications to thelaser, yet achieve significant improvements in capability andperformance. As laser and camera technology continues toimprove, particularly in regard to cost and system complexity,and the systems become more widely available, the advancesdiscussed in this article are expected to be rapidly superseded.

Acknowledgments

The authors would like to acknowledge the Air Force Officeof Scientific Research, Air Force Research Laboratories–Propulsion Directorate, Army Research Office, NASA andNational Science Foundation for supporting the majority ofthe research activities discussed in this article. The authorsare also grateful to the many students who have contributed

to the works discussed in this article. In addition, the authorsacknowledge Jeffrey A Sutton, for many useful discussionsand continuing development of advanced pulse burst lasersystems.

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